Table of contents

There is a clear need to develop dense, lithium intercalation oxides with > 200 mAh/g practical capacity. Today, nearly all high energy density cathodes for rechargeable lithium batteries are well-ordered materials where lithium and other cations occupy distinct sites. Cation-disordered materials are generally disregarded because lithium diffusion tends to be limited in them. The recently demonstrated performance of Li_1.211Mo_0.467Cr_0.3O₂, achieving close to 300 mAh/g, shows that lithium diffusion can be facile in disordered materials [1] and made us revisit the question of how Li diffuses through rocksalt-like materials. We have combined ab initio computations of local Li migration barriers with percolation modeling to develop a unified understanding of Li diffusion in close-packed oxides. The theory explains the high capacity of layered and spinel-like materials, and the lack of reversible capacity in γ-LiFeO2. More surprisingly, the new percolation theory also clearly supports that Li-excess is needed to achieve high capacity in partially or fully disordered materials. We can now give very specific guidelines for the amount of Li-excess needed in order to achieve a particular reversible capacity, and open up a new direction for finding very high capacity cathodes.

[1] J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries, Science, 343 (6170), 519-522 (2014)

Vanadium-based materials have received considerable attention for lithium ion batteries due to the facts that vanadium is the fifth most abundant transition metal in the earth's crust, its relatively low atomic mass, its multiple oxidation states. The redox operating voltage (typically < 4.5 V) of vanadium-based materials is typically within the stability limit of conventional electrolyte, and vanadium oxides have rich crystal structures [1]. The ability of layered V₂O₅ for Li⁺ insertion has been well characterized. Intercalation of one Li⁺ per formula unit corresponds to a specific capacity of 147 mAh g^-1. Further Li⁺ ions (x > 1 in Li_xV₂O₅) insertion may cause irreversible structural transformations [2].

In this work, a new high-capacity (> 400 mAh g^-1) and high energy density (> 1000 Wh kg^-1) cathode material comprising of lithium metal oxyfluoride with a representative composition of Li₂VO₂F has been synthesized through a mechanical ball-milling method. In comparison with the prior art, several remarkable features of this new compound Li₂VO₂F are (i) two-electron reaction based on V³⁺/V⁵⁺ is accessible per transition metal, (ii) the initial composition is at lithiated state, (iii) it has a high theoretical specific capacity of 462 mAh g^-1. Furthermore, in comparison with the start-of-the-art commercial cathode materials, this new material offers the opportunity to reach higher capacity and energy density.

References:

[1] N. A. Chernova, M. Roppolo, A. C. Dillon, M. S. Whittingham, J. Mater. Chem. 2009, 19, 2526

[2] C. Delmas, H. Cognac-Auradou, J. M. Cocciantelli, M. Ménétrier, J. P. Doumerc, Solid State Ion. 1994, 69, 257.

Only a small number of Li-containing cathode materials groups are considered for practical use in Li-ion battery systems. The possible candidates of cathode material were limited to the crystals that contain both redox-active element and lithium ion in the open framework. This concept has been conventionally considered as a standard for searching the cathode materials. But, it could be constraints to restrict the choices of materials for cathode in Li-ion battery systems. To expand the sight for seeking new positive electrode material, we suggested a novel strategy to use various kinds of Li-free transition metal ionic compounds (MX, M = transition metal, X = anion or polyanion group) as a positive electrode material by blending with a Li ionic compound (LiY, Y = anion or polyanion group) in nanoscale.^[1] MX and LiY provided a redox couple and lithium ion supply for an electrochemical reaction. This concept is unconventional with general system of electrode material (Li ions and transition metal ions are in the same crystal system). In this case, transition metal ion and Li ion in the nanocomposite do not exist in the same crystal system and spatially separated as a mixture of MX and LiY.

In this paper, we will introduce and discuss about the transition metal oxide system as a redox couple among the infinite possible combinations of lithium ionic compounds and metal ionic compounds. Transition metal oxides such as FeO, MnO were considered as promising anode materials due to the earth abundance and large capacity from conversion reaction.^[2] Although they have redox potential below 1 V as anode material,^[3][4] we will show that this transition metal oxides can be applied to the cathode materials (3 V-class) by making nanocomposite with lithium ionic compound. To better understand the mechanisms during electrochemical cycling we have performed XPS, XANES/EXAFS analysis, allowing us to study the local environment change of Fe or Mn during the charge and discharge reaction. FeO and MnO showed different aspects during the electrochemical reaction in terms of electrochemical activity and local environment change. We will discuss about the reaction mechanism in detail.

[1] S.-W. Kim, K-Y. Nam, D.-H. Seo, J. Hong, H. Kim, H. Gwon, and K. Kang, Nano Today, 7, 168 (2012)

[2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, and J.-M. Tarascon, Nature, 407, 496 (2000)

[3] M. Gao, P. Zhou, P. Wang, J. Wang, C. Liang, J. Zhang, and Y. Liu, J. Alloy. Compd., 565, 97 (2013)

[4] X. Li, D. Li, L. Qiao, X. Wang, X. Sun, P. Wang, and D. He, J. Mater. Chem., 22, 9189 (2012)

Due to its high theoretical specific capacity, silicon has been the subject of intense interest and research as a high energy density anode for lithium-ion batteries. In the last several years, nanostructured silicon has been shown to be very effective for relieving lithiation induced strain and avoiding pulverization, which can lead to capacities > 3000 mAh/g, a 10X improvement over the commercially used graphite anode. However, despite this impressive proof-of-principle, nanostructured silicon can still suffer from degradation over long cycling times due to several reasons such as collapse of the nanostructure, increased porosity, structural instability, and insufficient surface passivation. The work to be presented here will introduce our investigations into the lithiation of silicon clathrates, (fullerene-like materials where silicon atoms are arranged in cage-like structures) that have been shown, through first principles calculations, to be able to allow lithium insertion without a large volume change or pulverization [1].

In our group, we have investigated silicon clathrate materials, which have cage-like structures, as an anode material of Li-ion battery. Our studies on type II clathrate based on Na₂₄Si₁₃₆ show that lithium insertion is energetically feasible inside silicon cages that are fully occupied by other guest atoms. However, large amounts of lithium insertion cause a transformation between from the clathrate structure to an amorphous silicide and eventually Li₁₅Si₄ [2].

This presentation discusses the structure and electrochemical performance of ternary Ba-X-Si type-I clathrate (where X = Cu, Al). Polycrystalline silicon clathrate materials are synthesized using thermal annealing and arc-melting methods. Electrochemical and structural analysis show these cage-like silicon structures have the ability to allow lithium insertion and removal without a large volume change or pulverization. Lithiation/delithiation of silicon clathrate anodes will be presented comparing to that of diamond cubic and amorphous silicon. Ex-situ X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic resonance are also employed to understand the structural changes upon lithiation and delithiation.

References:

[1] K.S. Chan, C.K. Chan, W. Liang, Silicon clathrate anodes for lithium ion batteries. U.S. Patent Application no. 12/842,224 (2010)

[2] N.A. Wagner, R. Raghavan, R. Zhao, Q. Wei, X. Peng, C.K. Chan. Electrochemical cycling of sodium-filled silicon clathrate. ChemElectroChem. 1, 347-353 (2014).

• Introduction

  Research focusing on iron-based cathode materials for lithium ion batteries tends to favor materials that operate on the (de) intercalation of one Li per Fe atom (based on Fe²⁺/Fe³⁺redox couple); thereby imposing an intrinsic limitation on the energy density of the cathode material. To drastically enhance the energy density, intercalation materials with more than one electron per Fe atom are needed. In commonly used cathode materials, however, more than one electron reaction has not been utilized for charge-discharge reaction. For the development of cathode materials which achieve more than one electron reaction, it is essential to investigate the reaction mechanism.

  This study investigates reaction mechanism of more than one electron reaction in cathode materials. Li_2-xFeTiO₄ is selected as an appropriate model compound, as both the Fe²⁺/Fe³⁺ as well as Fe³⁺/Fe⁴⁺ redox couples are expected to be utilized to achieve high capacity. The possibility of de (intercalation) of two lithiums from the spinel-type Li_2-xFeTiO₄ is examined and the mechanism underlying the Li⁺ (de) insertion mechanism in Li_2-xFeTiO₄is discussed.

• Experimental

  Spinel-type LiFeTiO₄ was synthesized via the conventional solid state ceramics route. Electrodes prepared from this material were examined in two-electrode coin-type cells, using lithium metal as a counter electrode. Electrodes were prepared from LiFeTiO₄ to which carbon black was added and ball-milled at 400 rpm for 6 hours. Polytetrafluoroethylene (PTFE) binder was thereafter added. The weight ratio of LiFeTiO₄, carbon black and PTFE was 6:3:1. The electrolyte used was a 1 M solution of LiClO₄ in ethylene carbonate / diethyl carbonate. Initial lithium extraction from LiFeTiO₄ was carried out based on the capacity of one lithium per Fe atom. Cells were then cycled between 1.6V and 4.2V at various current densities. The electrochemical measurements were conducted at 55.0^oC. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements were conducted at SPring-8 and at SR Center, Ritsumeikan University.

• Results and Discussion

  The pristine LiFeTiO₄ was characterized by Rietveld analysis. As shown in Fig. 1, all the reflections of the XRD patterns of the pristine LiFeTiO₄ are fully indexed to a cubic structure (space group Fd3m). The rietveld analysis shows that all cations are randomly distributed on the tetrahedral (8a) and octahedral (16d) sites. The refined lattice parameters are consistent with the previously reported values [1-3].

  The spinel-type LiFeTiO₄ can be electrochemically delithiated below the voltage of 4.2 V, to obtain FeTiO₄. The subsequent discharge and charge measurements show that a reversible capacity of about 250 mAhg^-1can be obtained under C/20 rate at an average voltage of 2.5 V with good capacity retention.

  For the discharged and charged Li_2-xFeTiO₄, XRD and XAS measurements were conducted to elucidate the crystal and electronic structural changes underlying the high capacity in spinel-type LiFeTiO₄. Fig. 2 shows XRD pattern of charged and discharged Li_2-xFeTiO₄. The spinel structure of LiFeTiO₄ is maintained during both charge and discharge reaction. Additionally, changes in the lattice constants obey Vegard's law, indicating a solid-solution behavior during the charge and discharge reaction in Li_2-xFeTiO₄.

• References

• M. A. Arillo, M. L. Lopez, E. Perez-Cappe, C. Pico, M. L. Veiga, Solid State Ion.,107, 307-312 (1998).

• M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, E. Rodriguez-Castellon, J. Alloys Comp.,160-163, 317-318 (2001).

• M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, M. L. Vega, Chem. Mater.,17, 4162-4167 (2005).

For many years, challenges to develop lithium ion secondary batteries with high energy density have been extensively investigated. Li₂MnO₃-based positive electrodes such as LiNi_1/3Mn_1/3Co_1/3O₂ have attractive cell performances which are suitable for the electrical vehicles using lithium ion batteries. These materials include a large amount of Li which is essential for large capacity as a positive electrode. However, Li₂MnO₃ itself without Co or Ni has a poor electrochemical activity except for those of prepared at lower temperatures such as 773 K. Recently, we have found out the way to activate Li₂MnO₃ electrochemically by coexisting with CuO, not by substitution [1]. We try to clarify the reason why the existence of CuO induces the electrochemical activity of Li₂MnO₃ focusing on the crystal structure by SR-XRD, XAFS and ab-initiocalculation. In addition, a positive material with new composition exhibiting good electrochemical performance will be presented.

EXPERIMENTAL

The samples were prepared by combining coprecipitation and solid state reaction. The compositions of them are x = 1/5, 1/4, 3/8, 1/2 in (1-x)Li₂MnO₃-xCuO respectively. After dissolving CuSO₄ and Mn(CH₃COO)₂ into the distilled water, the co-precipitates were obtained by changing pH from 10 to 12. Dry precipitates and LiOH·H₂O were mixed and after calcination at 743 K, sintered at 973 K for 12hr under O₂ flow. Electrochemical testing was carried out using coin-type cells with Li/1M LiPF₆ in EC:DMC(3:7)/samples. X-ray diffraction (XRD) measurements using both CuKα radiation and a synchrotron radiation source were performed and for the latter on BL02B2 and BL19B2 at SPring-8. Structural refinements were carried out by Rietveld analysis using the RIETAN-FP program[2]. X-ray absorption measurements of above samples at the Mn and Cu K-edges by transmission method were performed on BL14B2 at SPring8. The first principle calculations were carried out using the WIEN2k and VASP program packages.

RESULTS AND DISCUSSIONS

To clarify the reason why coexisitng with CuO enhances the electrochemical activity of Li₂MnO₃, at first we fouced on the crystal structure of Li₂MnO₃ and carried out Rietveld analysis using SR-XRD for all the investigated samples, based on the structural model of monoclinic Li₂MnO₃, S.G. C2/m. It was found that Li and Mn locate on both 2b and 4g sites of Wyckoff positions. A disordering of Li and Mn in the structure of Li₂MnO₃ increased by CuO contents. The results of neutron diffraction patterns supports those of XRD. These disordering behaviour probably may correlate closely with the electrochemical activity of Li₂MnO₃. In addition, a significnat difference of electron diffraction patterns was observed between x = 1/5 which showed discharge capacity of 160 mAhg^-1 and x = 1/2 that of 355 mAhg^-1. A distinguished streaks along c* axis appeared for x = 1/2 suggesting a stacking fault, wheares some diffraction spots did for x = 1/5. A similar results for Li(Ni, Mn, Co)O₂ are rescribed in ref. [3]. The coexsisting with CuO intoroduce a stacking fault into Li₂MnO₃. Next, we considered an effect of partial substiontion of Cu. The ab-initio electronic structure calculation allows us to know more about the electrical property. Density of States (DOS) of our assumed supercell, Li₁₆(Mn₇Cu₁)O₂₄ indicated a new sharp band which expect to show higher electrical conductivity than that of Li₂MnO₃ itself. Hence, the partial replacing Mn by Cu suggests an improvement of the electrochemical property. Next, we focused on CuO coexisting with Li₂MnO₃. Whereas a pure CuO is an insulator, Li-doped CuO could be a semiconductor. Therefore, it is reasonable that the kinetic barrier of the electrode reaction for Li₂MnO₃was depressed by coexisting with CuO.

We will present a new way to activate Li₂MnO₃ electrochemically by coexisting CuO, not by substitution and discuss the mechanism why coexisitng with CuO enhances the electrochemical activity of Li₂MnO₃based on both experimental and calculation results.

ACKNOWLEDGEMENT

This research was supported by Japan Society for the Promotion of Science (25410255).

REFERENCES

1 Y. Arachi, K. Hinoshita and Y. Nakata, ECS Transactions, 41(29), 1-7(2012).

2 F. Izumi and K. Momma, Solid State Phenom., 130, 15-20(2007).

3 A. Boulineau, L. Croguennec, C. Delmas, F. Weill, Solid State Ionics., 29, 1652 -1659(2010).

The current lithium-ion technology is based on insertion-compound cathodes and anodes. Layered and spinel oxides and olivine phosphates are the leading insertion-cathode hosts while graphite is the sole insertion-anode host at the present time. Each of the three cathode systems in play offers advantages and disadvantages while exhibiting rich but complex structural/chemical behaviors, and their adoption in practical cells is determined largely by the type of application. This presentation will focus on the challenges and prospects of transitioning from insertion-compound electrodes to next-generation, high-capacity, conversion-reaction electrodes.

First, an overview of how low-temperature synthesis and processing (e.g., chemical lithium extraction and microwave-assisted solvothermal synthesis) approaches have assisted to develop a fundamental understanding of some of the complex behaviors of the three classes of cathodes. For example, with the use of low-temperature synthesis processes, the roles of the position of the transtion-metal:3d energy relative to the top of the oxygen:2p band, degree of metal-oxygen covalence, morphology and surface crystal planes, surface compositions, isovalent and aliovalent cationic and anionic doping on the electrochemical performances (e.g., capacity, irreversible capacity loss in the first cycle, rate capability, and cycle life) will be pointed out with representative examples.

Second, the potential of alternative, conversion-reaction, nanocomposite alloy anodes to overcome the safety issues of the graphite anode will be presented. Specifically, the use of facile mechanochemical reactions to realize nanoengineered alloy anodes both for lithium-ion and sodium-ion batteries will be discussed.

Third, novel electrode architectures as well as innovative cell designs to overcome the persistent problems of the conversion-reaction sulfur cathode will be presented. Specifically, approaches to enhance the electrochemical utilization of sulfur and suppress the migration of dissolved polysulfide ions to the anode will be discussed both with lithium-sulfur and ambient-temperature sodium-sulfur cells. Also, approaches to stabilize the lithium-metal anode surface in a polysulfide-rich environment will be outlined.

Fourth, to overcome the formidable challenges of aprotic lithium-air cells, the possibility of hybrid lithium-air cells in which a solid electrolyte separates the lithium-metal anode in an aprotic electrolyte from the air electrode in an aqueous catholyte will be briefly presented. Specifically, the design and development of inexpensive, efficient catalysts for the oxygen reduction reaction and oxygen evolution reaction will be discussed.

Introduction

The safety concern of Li-ion batteries due to the potential leakage and the flammability of the electrolyte severely hinders applications. The emergence of all-solid-state batteries, consisting of solid state electrodes and solid state ionic conductor electrolyte, will solve this safety problem because it eliminates the problematic liquid electrolytes.

One key issue that needs to be addressed in constructing such a battery is the contact between electrode and electrolyte. Different from liquid electrolytes that can submerge the cathodes, it is difficult for solid state electrolyte to have a sufficient touch with electrodes. A common cathode-side fabrication method is to mix the electrolyte powder with cathode powder and electronically conductive materials, yielding a point-to-point contact. This contact configuration limits the number of effective pathways of li-ion diffusion, therefore hurts the performance of the battery.

solves the aforementioned problems. Starting with precursor solutions, reactants are homogenously mixed at a molecular level, and are much easier to get into the scaffold than powders. Additionally, in-situ synthesis allows the high voltage cathode material to adhere to the surface of the electrolyte scaffold, yielding a face-to-face contact. This enlarges contact area, creating more pathways for Li-ion diffusion and improves the conductivity between the cathode and the electrolyte.

Experimental Methods

The high voltage cathode Li₂FeMn₃O₈ (LFMO) is synthesized by glycine-nitrate combustion. Precursor solution containing metal nitrates and glycine with an optimized ratio is infiltrated into the porous garnet electrolyte scaffold. After drying at 300℃ combustion takes place. The pellet is subsequently annealed at 700℃ to achieve the desired LFMO phase. Finally, carbon nanotube ink is infiltrated to the scaffold to form a conductive network over the cathode.

Results

Fig. 1a shows a schematic of the cathode. With the conductive network and improved contact of cathode and electrolyte, it is expected to have an improved performance. As is shown in Fig. 1b, the cathode adheres to the surface of the scaffold, forming a face-to-face contact.

The LFMO cathode has been tested in LFMO/Li coin cell with LiPF₆/ FEC electrolyte. Fig.2 a) gives the XRD spectrum of the cathode prepared by glycine-nitride combustion method. The spectrum matches with JCPDS#48-0258, indicating a Li₂FeMn₃O₈ phase. Fig.2 b) shows the cycling performance of the cathode. It can be seen from the voltage profile of the tested cell that LFMO has two plateaus at around 4.1V and 4.9V, with capacity being 104mAh/g.

Iron pyrite (p-FeS₂) has been widely utilized as a commercial cathode material for lithium ion batteries (LIBs) for 30+ years, due to its high charge capacity, natural abundance, low cost, and non-toxicity. Industrialized versions include both non-rechargeable Li/FeS₂ batteries at ambient temperatures (-40 – 60 °C) and rechargeable Li/FeS₂ batteries at high temperatures (400 – 450 °C). However, FeS₂ cathodes suffer from very poor cyclability at room temperature. Four specific reasons have been identified for this problem: 1) Volume fluctuations during cycling, resulting in pulverization of large particles and a subsequent loss of contact to the current collector; 2) poor electrical conductivity of the lithiation product, lithium sulfide; 3) detrimental reactions between the electrolyte solution and the active materials (FeS₂and its subsequent derivatives); 4) the loss of materials due to the formation of soluble lithium polysulfides.

In this presentation, we will outline our strategy to address all of the above challenges for FeS₂ through the encapsulation of FeS₂ nanoparticles in an elastic carbon (EC) matrix. Two carbon sources are explored to produce an ideal EC matrix, which is chemically and mechanically stable, elastic, and conductive. These unique properties allow accommodation of the volume fluctuation, enhance the charge transfer, and protect the FeS₂ from damaging chemical reactions. The obtained FeS₂@EC composites present significantly improved cyclability over bare FeS₂ nanoparticles. Scanning electron microscopy, Raman spectroscopy, electrochemical impedance spectroscopy, and cyclability studies are utilized to confirm the structure-performance relationship.

We developed a new electrochemically active Li storage material based on a polyanionic framework. By replacing a portion of Mn with Fe and Mg in monoclinic lithium manganese borate (LiMnBO₃), a phase-pure LiMn_0.5Fe_0.4Mg_0.1BO₃ was successfully synthesized. A C/50-rate galvanostatic discharge delivered a near theoretical capacity of 201 mAh g^-1 at room temperature with much improved rate capability, 120 mAh g^-1 at a 1C discharge, compared to other borate chemistries. We explained this superior Li storage activity in relation to the topological effect of substitution, which leads to facile Li diffusion and stabilizes the structure. Combining with the inherent stability of the borate group against oxygen evolution, we believe that the enhanced electrochemical properties achieved in this study makes the LiMn_0.5Fe_0.4Mg_0.1BO₃ compound a strong contender as a new cathode material for Li-ion batteries and will bring more attention to the lithium transition metal borate chemistry.

Over the last decade the technological advancements which have led to products such as smartphones, tablet computers and electric vehicles have been outstanding. Currently, battery technology is just about keeping up with consumer demands. In order to prevent lithium ion batteries from impeding future technological advancements every aspect of lithium ion batteries must be examined to determine how future improvements can be made. There is a tremendous amount of research being done into possible cathode materials to replace the most commonly currently used cobalt oxides [1, 2].

Vanadium oxide nanotubes (VONTs) were first reported in 1998 by Spahr et al. [3] and since then there has been a great deal of research into how to fully optimise VONTs as a cathode material for lithium ion batteries [4, 5]. Typically VONTs are prepared by hydrothermal treatment of a vanadium oxide precursor mixed with a primary amine [6]. The presence of amine molecules are crucial to the formation of the VONTs as they maintain the vanadium oxide layers which scroll to form the nanotube structure [7]. While the amines are vital in the synthesis of the VONTs they are unfortunately detrimental in their electrochemical performance. It has been proposed that amine molecules occupy the majority of the possible lithium intercalation sites within the VONTs and hence are responsible for the poor cycling performance which has been reported for as-synthesised VONTs [8].

In this work we investigate two alternative methods to remove amine molecules from the as-synthesised VONTs, using the nanotube structure as a 'backbone' or 'starting structure' for other polymorphs. The first method consists of annealing as-synthesised VONTs to high temperatures (≈ 600 ^oC) in an effort to evaporate the amines out of the vanadium oxide structure. This method results in polycrystalline vanadium oxide nanorods (poly-NRs). The second method is a two-step process. First, the as-synthesised VONTs undergo an ion exchange treatment, partially substituting amine molecules with Na⁺ ions to form Na-VONTs and secondly, the resulting Na-VONTs are also annealed to 600 ^oC.

We confirm the removal of amine molecules by monitoring the inorganic and organic phase changes and decomposition, respectively, using IR spectroscopy, electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy analyses. Through detailed electrochemical investigations we compare the electrochemical performance of as-synthesised VONTs, poly-NRs, Na-VONTs and Na-VONTs after thermal treatment. We confirm that the heavy functionalization of VONTs by amine molecules can impede the intercalation of lithium ions, and that their removal results in a significant improvement in electrochemical characteristics. We will show that poly-NRs exhibit the most promising results for practical use as a cathode material. High rate capability tests demonstrate their stability at high charging rates (1C) and long term cycle life testing indicates a high level of capacity retention over 500 cycles.

References

[1] J.W. Fergus, Journal of Power Sources 195 (2010) 939-954.

[2] M.S. Whittingham, Chemical Reviews 104 (2004) 4271-4302.

[3] M.E. Spahr, P. Bitterli, R. Nesper, M. Müller, F. Krumeich, H.U. Nissen, Angewandte Chemie International Edition 37 (1998) 1263-1265.

[4] S. Nordlinder, L. Nyholm, T. Gustafsson, K. Edström, Chemistry of Materials 18 (2005) 495-503.

[5] M.L.-Q. CHEN Wen, XU Qing, PENG Jun-Feng, ZHU Quan-Yao, Chemical Journal of Chinese Universities 25 (2004) 904-907.

[6] M.E. Spahr, P. Stoschitzki-Bitterli, R. Nesper, O. Haas, P. Novák, Journal of The Electrochemical Society 146 (1999) 2780-2783.

[7] F. Krumeich, H.J. Muhr, M. Niederberger, F. Bieri, B. Schnyder, R. Nesper, Journal of the American Chemical Society 121 (1999) 8324-8331.

[8] A.I. Popa, E. Vavilova, C. Täschner, V. Kataev, B. Büchner, R.d. Klingeler, The Journal of Physical Chemistry C 115 (2011) 5265-5270.

Acknowledgements

This publication has emanated from research conducted with the financial support of Science Foundation of Ireland under Grant No: 06/CP/E007.

Introduction

James and Goodenough first introduced layer-structured Li₂MoO₃ as a lithium-battery cathode material [1]. It has a disordered NaFeO₂ structure (; a = 2.884 Å, c = 14.834 Å) consisting of a cubic close-packed oxygen cations with basal planes of octahedral sites alternatively filled with Li ions (3a sites) and a randomly distributed mixture of 1/3 Li (3b sites) and 2/3 Mo (3b sites) forming a Li-Mo layer(with a 1:2 Li:Mo ratio), in which the Mo ions forming disordered Mo₃O₁₃ clusters rather than isolated Mo ions.[1-5] The theoretical capacity of the Li₂MoO₃ reaches 339 mAh g^-1 based on the Mo⁴⁺/Mo⁶⁺ redox reaction alone (i.e. without oxidation of the O^2- anions) and the Mo⁴⁺/Mo⁶⁺ redox reaction potential is much lower than that of the oxygen release in Li₂MnO₃. Therefore, Li₂MoO₃ is expected to be a good component in building a new layer-structured xLi₂MoO₃·(1-x)LiMO₂ system as high-capacity cathode materials. Here we report the structural studies and charge compensation of Li₂MoO₃ during the initial charge and discharge. The close to fully reversible structural changes and Mo ion migration, originated from the charge compensation of Mo ions in both the Mo-O and Mo-Mo covalent bonds in the Mo₃O₁₃ cluster, make the Li₂MoO₃ an appropriate alternative of Li₂MnO₃ in constructing new xLi₂MoO₃·(1-x)LiMO₂ cathode materials, which have less irreversible transition metal migration and oxygen evolution. The findings in this work will also shed light on the fundamental understandings of the relationships between the performance and structure changes, as well as on the new approaches in developing lithium-rich cathode materials with both high energy density and long cycle life.

Results and discussion

To understand the structural changes of Li₂MoO₃ during lithium extraction, in situ x-ray diffraction (XRD) and X-ray absorption (XAS) spectroscopy at Mo K-edge were used to study the crystal structure and valence state as well as local structural changes of Mo ions in charging process. The X-ray absorption near edge structure (XANES) spectra of the Mo K-edge during charge show a continuous increase of the pre-edge peaks indicates the increased distortion of Mo-O₆ octahedral. The white line of the K-edge shifted to the higher energy gradually, suggesting the increasing oxidation state of Mo ions upon charge. Compare with the Mo K-edge XANES data of the MoO₂ and MoO₃ references, it can be estimated that the Mo ions were oxidized from Mo⁴⁺ to average oxidation state close to Mo⁶⁺. More detailed results will be discussed in the presentation.

Acknowledgement

This work was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-AC02-98CH10886. Use of the National Synchrotron Light Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The work at Institute of Physics, Chinese Academy of Sciences was supported by the National Natural Science Foundation (No. 51372268) of China and the National 973 Program of China (2009CB 220100).

References

[1] A. C. W. P. James and J. B. Goodenough, J. Solid State Chem., 1988, 76, 87..

[2] S. J. Hibble and I. D. Fawcett, Inorg. Chem., 1995, 34, 500.

[3] S. J. Hibble, I. D. Fawcett and A. C. Hannon, Acta Cryst., 1997, B53, 604.

[4] S. J. Hibble, A. C. Hannon and I. D. Fawcett, J. Phys.: Condens. Matter, 1999, 11, 9203

[5] W. H. McCarroll, L. Katz and R. Ward, J. Am. Chem. Soc., 1957, 79, 5410.

Towards the establishment of low-carbon emission society, one urgent issue is to standardize an efficient method to store unused energy. A promised candidate is an electrochemical storage such as lithium-ion batteries. However, it is still hindered as a major problem that the lithium-ion batteries do not conduct with sufficiently high energies capabilities and stable operational cycles. In particular, it is remained as an inefficient barrier of the lithium-ion (de)intercalation at the interface between electrode and electrolyte. It is therefore necessary to determine a crucial factor which influence to the homogeneity in ion transport across the interface by in-situ measurement with nanometer resolution.

In this study, the homogeneity in local ion transport of LiFePO₄ thin electrodes is investigated by a nanoscale meniscus cell microscope (NMCM). It is a newly established scanning probe microscopy with a nano-size pipet with electrolyte and a reference-electrode.¹ As the pipet is in proximity of the LiFePO₄ electrode, a meniscus is created. Then, ionic current is only measured through the meniscus when the voltage is applied between reference and LiFePO₄electrodes, as shown in Figure 1(a). When the pipet is scanned through the interface in nanometer step, a mapping of the ion current distribution in lithium (de)intercalation is obtained with topographical information in Figure 1 (b). Further, as the pipet was pinned at specific area, it allows us to study the localized ion transport by cyclic voltammetry or charge /discharge process. This new analytic technique will reveal a bottleneck of the ion transport in the materials. leading to the creation of electrode with homogeneous ion transport.

Reference:

(1) Y. Takahashi, et al., under review.

Future Li-ion batteries are expected to reach high energy density by the use of high voltage cathode materials. At the high operating voltages (≥ 4.5 V vs. Li/Li⁺) most of the common alkyl carbonate electrolytes start to decompose followed by an evolution of gases upon cycling. In recent studies, Tsiouvaras et. al. and McCloskey et. al. showed that CO₂ is the main gas evolved by the anodic oxidation of water free alkyl carbonate based electrolytes (e.g. PC) at high electrode potentials (≥ 4.5 V vs. Li/Li⁺) [1][2]. A recent study by our group [3], demonstrated that CO₂ is already evolved at lower potentials if the electrolyte contains traces of water. An accumulation of gas in a sealed battery cell after several cycles is a potential safety threat and its origins need to be understood.

In this work we employ on-line electrochemical mass spectrometry (OEMS) to analyze the gas evolution in battery test cells over a wide electrode potential window (2 - 6 V vs. Li/Li⁺). CO₂ is a known degradation product, but it can be either due to the electrochemical oxidation of the electrolyte or the carbon in the cathode through the reaction:

C + 2H₂O → CO₂ + 4H⁺ + 4e^- (1)

In order to deconvolute these two processes, isotopically labeled ¹³C carbon (99 atom% ¹³C, Sigma-Aldrich) is used to prepare a model electrode that consists exclusively of ¹³C carbon. This allows to assign the evolved CO₂ isotopes to either the degradation of the electrode or the electrolyte. In the context of Li-O₂ batteries, this strategy was used to analyze the formation of Li₂CO₃ in presence of Li₂O₂ [4].

Furthermore, the effect of water on the degradation of carbon and electrolyte is investigated. By adding a defined amount of water (4000 ppm) to the electrolyte one can mimic the effect of trace water that could unintentionally be introduced to the cell through the active materials. Joho et. al. studied the irreversible charge loss due to water reduction in graphite based Li-ion cells, however no correlation between water content and cycling stability could be found [5]. To clarify whether water promotes exclusively the degradation of either the carbon electrode or the electrolyte or the degradation of both components, isotopically labeled ¹⁸O-water (98 atom% ¹⁸O, Sercon) is added to the electrolyte.

Figure 1 features a linear potential sweep and the corresponding gas evolution in OEMS for a ¹³C carbon cathode and LP30 electrolyte containing 4000 ppm H₂¹⁶O. An evolution of the ¹³C¹⁶O₂ isotope of carbon dioxide is detected together with ¹²C¹⁶O₂ at potentials of 4.6 V vs. Li/Li⁺. The first isotope is likely to originate from the electrode corrosion, while the latter could result from the oxidation of the electrolyte. The signals at channels 48 and 49, corresponding to the ¹²C¹⁸O₂ and ¹³C¹⁸O₂ isotopes, respectively are not detected, since this system contains no labeled ¹⁸O-water.

References:

[1] N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, J. Electrochem. Soc., 160, A471 (2013).

[2] B. D. McCloskey, D. S. Bethune, R.M. Shelby, G. Girishkumar, and A. C. Luntz, J. Phys. Chem. Lett., 2, 1161 (2011).

[3] R. Bernhard, S. Meini, and H. A. Gasteiger, J. Electrochem. Soc., 161, A497 (2014).

[4] M. M. O. Thotiyl, S. A. Freunberger, P. Zhangquan, P. G. Bruce, J. Am. Chem. Soc., 135, 494 (2013).

[5] F. Joho, B. Rykart, R. Imhof, P. Novák, M. E. Spahr, A. Monnier, J. of Power Sources., 81-82, 243 (1999).

Acknowledgment:

Support of BASF SE in the framework of its scientific network on electrochemistry and batteries is acknowledged by TUM.

In recent years, the crucial role of solid electrolyte interphases (SEIs) in Li-ion batteries (LIBs) has been well recognized by extensive studies. In spite of all available information, there are still issues relevant to the SEI formation, composition, and mechanical properties that have to be addressed. We have recently utilized Electrochemical Quartz Crystal Microbalance with Dissipation (EQCM-D) to study how SEI develops on tin thin film electrodes and probed the mass gain/loss in electrolytes with different amounts of fluoroethylene carbonate (FEC) additives upon electrochemical cycling. As shown in Figure 1, the detailed processes of the decomposition of the electrolyte on the electrode surface and Li alloying/dealloying with Sn were characterized quantitatively by measuring the decrease/increase of the resonance frequency in the cell. The effects of FEC on SEI formation and lithiation/delithiation cycles are also carefully studied and presented. The results suggest that varying the concentration of FEC is important for adjusting the SEI mass and thickness on the anodes.

The key points of employing EQCM-D in this work are the extension of measurements to higher harmonics and incorporation of dissipation (D). The capabilities enable the detection of the properties of interfacial films such as rigidity and viscoelasticity. The final modeling results from EQCM-D demonstrate that the SEI layer is dynamically shifting in the mass, thickness and viscoelastic properties during every cycle. Therefore, given the unique advantage of EQCM-D, it is expected that the full picture of SEI formation, including mechanic properties and compositional change, can be reflected.

Active materials for rechargeable lithium-ion batteries are generally coated with polymer binder and conductive carbon on a current collector and evaluated as a porous composite electrode. However, the use of composite electrode makes it difficult to understand the intrinsic electrochemical properties of active material since the electrode structural factors such as porosity and thickness are included in the electrochemical response of composite electrode. In order to prevent such misunderstanding, single particle measurement is one of useful tools. In the case of single particle measurement, a micro current collector is used to establish electrical connection to an active material particle. Thus, the structural factors of composite electrode can be neglected, resulting in precise understanding the intrinsic electrochemical properties of active materials. In this study, LiCoO₂ was focused and its electrochemical properties, particularly the cycleability at high temperatures was investigated by single particle measurement. Furthermore, the cluster particle of LiCoO₂ composite electrode was also evaluated in order to investigate the effects of PVdF binder and conductive carbon on the cycleability of LiCoO₂. As shown in Figure 1, the rate capability of cluster particle was low compared with LiCoO₂ single particle, namely the discharge capacity was decreased at high discharge rates. This difference is expected to be due to the blocking effect of PVdF binder on Li⁺ diffusion to LiCoO₂ particle from an electrolyte solution. In contrast, the capacity retention in charge-discharge cycle test (Figure 2) was significantly improved in the cluster particle. This result suggests that LiCoO₂ is stabilized by PVdF binder and conductive carbon.

In the research field of Li ion batteries is an increasing request to characterize complete battery cells and study Li ion batteries in operando. At the Heinz Maier-Leibnitz Zentrum in Garching near Munich (Germany), neutrons are provided as a probe to carry out a variety of diverse and complementary in situ experiments with Li ion batteries. Neutrons have the unique property to penetrate deeply into materials (including metals) because the scattering power is unrelated to the Z number. In addition neutrons are quite sensitive to light elements and even neighboring elements can have excellent contrast to each other (important for example to discern Mn, Fe, Co and Ni). As neutrons are electrically neutral with typical low energy transfers in the meV range no ionization or local heating occurs. Therefore the neutron methods with their possible large beam cross sections (up to cm2 is feasible) are non-destructive.

In the last years neutron scattering in particular with neutron diffraction started to be established as a method to follow in situ phase transformations in batteries as function of temperature and/or charge state. Many experiments characterize the intercalation process of lithium in graphite including the different phases of LiCx formed during the charging and discharging process. Here, neutrons are an ideal probe to monitor the intercalation process due to the fact that they are sensitive to LiCx compounds. Due to the high penetration depth even spatially resolved measurements at various positions within a battery are possible. Measurements on Li ion batteries (NMC/graphite) will be presented to show how neutrons can monitor the intercalation during charging and discharging under different temperatures in detail (Fig. 1). In addition the process of Li plating can be observed and has been described in dependence to relaxation processes of LiCx phases.

Recently further neutron techniques have been employed to extract additional information on the mechanism in Li ion batteries. For example small-angle neutron scattering was applied to identify nanoscaled structures (1 – 300 nm) in Li ion batteries. As the method works in transmission mode thin Li ion batteries of pouch bag format were used in these experiments. A measurement of a single NMC-cathode was fitted to a mass fractal model yielding a spherical particle radius of ~ 85 nm. Further we applied the prompt gamma activation analysis technique (PGAA) which uses the neutron capture in nuclei of the sample material and the subsequent detection of prompt gamma rays emitted during de-excitation of the compound nuclei. The method determines elemental composition and concentration of samples down to the ppm range. Thus PGAA can detect even trace amounts of elements on electrodes. This method was used to study cation dissolution (Mn, Co, Ni) and transport to the anode quantitatively, giving an insight into cell ageing. Finally, to study lateral structures on the nanometer scale another method called time-of-flight grazing incidence small-angle neutron scattering (GISANS) has been used. In this method, the neutron beam impinges on the sample surface at grazing incidence and the scattered neutron pattern enables the description of objects laterally organized in a layered structure near the surface. An example of self-organized anodic titanium oxide nanorods will be presented.

In this presentation a comprehensive overview on the potential of the available neutron scattering methods to study Li ion batteries will be given.

Achnowledgement: The authors gratefully acknowledge the funding by the German Federal Ministry of Education and Research (BMBF) under the auspices of the project ExZellTUM with grant number 03X4633A and the funding by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology under the auspices of the EEBatt project. J. Kunze-Liebhäuser thanks the DFG (project KU 2397/3-1) for financial support.

Introduction

Electrode/electrolyte interfaces have a great influence on performance of lithium-ion batteries, e.g., rate capability and cycle durability. However, structures of the electrode surfaces are not yet clear on the atomic scale. Recently, we have found using a surface sensitive XAS technique and thin-film model electrodes that Co ions at the surface of LiCoO₂are reduced just by immersion into organic electrolyte [1]. This suggests that the electrode surfaces are likely to be modified in the real batteries by chemical reactions with organic electrolyte components. Therefore, it is necessary to consider stability of the surface structures including changes in compositions and arrangements under the environment in the real batteries.

In this study, first-principles calculations have been carried out on several models of LiCoO₂(104) surface. The stoichiometric (104) surface is reported to be one of the low energy surfaces [2]. The (104) planes have the stoichiometric composition as in the bulk, and thus the naturally cleaved (104) surface is in stoichiometry. In addition to the stoichiometric surface model, surface models with modification on the topmost ions, i.e., addition, removal, and substitution, have been examined. Most of the modified surface models have nonstoichiometric compositions, and thus the stability of the surface models is discussed considering the environment of the real batteries.

Calculation method

The surface models used in this study are slabs consisting of 15–19 layers of the (104) planes with sufficiently thick vacuum regions. Inversion symmetry is assumed to avoid artificial dipole effects. The plane-wave basis PAW method is used for the first-principles calculations with the GGA+U exchange correlation functional.

Surface energy is estimated as a function of the environment, i.e., chemical potentials, μ_i, as

E_s = (E(slab) – Σ_iN_iμ_i) / S,

where E(slab) is the energy of the slab model, N_i is the number of atoms of species i in the slab model, and Sis the surface area, respectively. The chemical potentials have a constraint of

μ_Li + μ_Co + 2 μ_O = E(LiCoO₂) ,

where E(LiCoO₂) is the energy of LiCoO₂ bulk. Assuming that LiCoO₂ coexists with Li₂O, i.e., 2 μ_Li + μ_O = E(Li₂O), the surface energy is estimated as a function of μ_O.

Results and discussion

The stoichiometric surface model has the topmost Co ions whose oxidation state is +3 with the intermediate spin configuration (d_up⁴, d_down²), whereas the spin configuration of the other Co ions is the low spin configuration (d_up³, d_down³). The electronic states of the stoichiometric (104) surface are consistent with the literature [3].

Here, we show one of the nonstoichiometric surface models, in which Co ions are substituted for the topmost Li sites. The oxidation state of the topmost Co ions is +2 with the high spin configuration (d_up⁵, d_down²), whereas the oxidation state of the other Co ions is +3 with the low spin configuration. Thus, this Co substitution surface model has a similar structure to rocksalt CoO.

Surface energy of the stoichiometric model is independent from μ_O. On the other hand, surface energy of the Co substitution model decreases as m_O decreases. It becomes smaller than the surface energy of the stoichiometric model at μ_O < -1 eV versus the standard state of O₂ gas at 300 K. This suggests that the surface of LiCoO₂ is modified at low m_O in the organic electrolytes by the chemical reaction with an electrolyte component E

LiCoO₂ + E → Co_Li + Li₂O + EO.

The suggested Co reduction at the surface is consistent with the experimental observation by the surface sensitive XAS measurements [1].

Acknowledgement

This work was supported by RISING battery project from NEDO, Japan.

References

[1] D. Takamatsu, et al., Angew. Chem.-Int. Edit., 51, 11597 (2012).

[2] D. Kramer and G. Ceder, Chem. Mater., 21, 3799 (2009).

[3] D. N. Qian, et al., J. Am. Chem. Soc., 134, 6096 (2012).

The structural and chemical change of positive and negative electrodes during overcharge of Li-ion batteries is a primary concern of battery developers and users, because the overcharge will lead to thermal runaway in the worst case. Over the past decades, a considerable number of studies have been conducted on this issue by thermometric and/or gasometric analyses^[1]. However, the reactions between active materials and electrolytes during overcharge are extremely complicated, therefore this issue is still in the heat of the argument. To explore the behaviour under such abusive conditions, analyses "after" overcharge would be not enough, analyses "during" overcharge should be necessary because the active materials are obviously unstable and in a non-equilibrium state at the high potential. From these viewpoints, in situX-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) are perfectly suited methods to investigate the overcharge behaviour of positive and negative electrode materials.

LiNiO₂-based positive electrodes were prepared by coating a dispersion composed of LiNi_0.75Co_0.15Al_0.05Mg_0.05O₂^[2], carbon black (as a conducting agent), and polyvinylidene fluoride (PVDF) (as a binder, Kureha Corp. Japan) (85:10:5 weight ratio) in N-methyl-2-pyrrolidone (NMP) on aluminium foil. Graphite-based negative electrodes were prepared using the same procedure as for the positive electrodes. The mixture of graphite and PVDF (90:10 weight ratio) was coated on a copper foil. Both electrodes were dried at 120 ºC under vacuum for at least 10 h before construction of the electrochemical cell. The electrolyte used in the experiments was 1.0 M LiPF₆ dissolved in a solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (30/40/30 volume ratio, respectively). Laminate-type pouch cells were used for the in situ XAFS and XRD measurements. The cells were charged under constant voltage to 4.1 V and discharged at 0.2 C rate to 3.0 V before in situ experiments. The electrochemical overcharge operation was carried out galvanostatically up to 10 V at 2 C or 10 C on BL33XU beamline (Toyota beamline) at SPring-8^[3].

Figure 1 shows the charge and overcharge voltage curves at 2C during in situ measurements at 25 ºC and the corresponding Ni-K absorption edge energy. The energy at half-step height (where normalized absorbance = 0.5) was used as an index of Ni valence. The horizontal axis at 30 min is corresponding to SOC 100%. Only Ni was oxidized with the increase of SOC until 100% (until 30 min). On the other hand, only Co was oxidized above SOC 120%. Both of Ni and Co valence was not changed above 5.5 V, therefore in this region, every charge current was consumed by side reaction on positive and negative electrodes. At elevated temperature, 50 ºC and 80 ºC, the side reaction was accelerated, especially on the negative electrode side. The relationship between charge reactions, side reactions and degradation effects on positive and negative electrodes will be discussed with temperature dependency.

References

[1] R. A. Leising, M. J. Palazzo, E. S. Takeuchi, K. J. Takeuchi, J. Electrochem. Soc.,148, A838 (2001).

[2] H. Kondo, Y. Takeuchi, T. Sasaki, S. Kawauchi, Y. Itou, O. Hiruta, C. Okuda, M. Yonemura, T. Kamiyama, and Y. Ukyo, J. Power Sources, 174, 1131 (2007).

[3] T. Nonaka, K. Dohmae, T. Araki, Y. Hayashi, Y. Hirose, T. Uruga, H. Yamazaki, T. Mochizuki, H. Tanida, and S. Goto, Rev. Sci. Instrum.83, 083112 (2012).

Figure 1. Charge and overcharge voltage curve of the laminate-type pouch cell at 2C during in situ measurements and the corresponding Ni-K absorption edge energy.

While the performance of lithium ion batteries (LIBs) has steadily increased over the past two decades, improving energy density, rate capability, life, and safety are still challenges, particularly for the deployment of LIBs in electric vehicles. In this talk, we will (1) identify physical origins of processes that limit battery performance at both the materials and microstructural level, and (2) use these findings to develop design guidelines and new strategies to address these fundamental performance limitations.

Battery performance is governed by electrical, chemical, physical, and mechanical processes, which are hard to decouple. Understanding these interrelated phenomena in a quantitative way is now becoming possible due to advances in imaging technologies and numerical techniques. We perform synchrotron radiation x-ray tomographic microscopy at the TOMCAT beamline of the Swiss Light Source to obtain spatially resolved chemical and structural information of lithium ion batteries.

Novel, high energy density materials such as those undergoing conversion and/or alloying reactions typically suffer from short lifetime due to large volume expansion and contraction during battery operation leading to particle fracture. Understanding and mitigating these processes is a key area of research. The fast acquisition times on the order of minutes possible at the TOMCAT beamline enable operando imaging of LIBs during electrochemical operation. Using tin-(II)-oxide (SnO) as a model system, we observe a core-shell process, quantify volume expansion, witness particle fracture, and correlate crack initiation and growth to crystallographic plane defects in crystalline particles. The insights into electrode structure and material degradation during cell operation highlight the problems this class of materials faces and guides synthesis of novel materials and optimization of electrode preparation techniques.

LIBs with high electrode loading (i.e. thick electrodes) are favorable because the fraction of electrochemically inactive material is kept to a minimum and the energy density of the cell is maximized while the cost is minimized. However, the rate performance of LIBs depends on electrode thickness, porosity and tortuosity. Manufacturers must thus compromise between energy density and speed. Using a combination of experimental evidence based on 3D reconstructions of electrodes from tomographic data and simulations, we show that electrode tortuosity (a parameter that measures the effective path length of ion transport in the electrolyte) plays a key role in governing the rate performance of natural graphite electrodes where the platelet shape of natural graphite causes strong tortuosity anisotropy and large tortuosity values in the direction perpendicular to the current collector. To overcome these limitations, we present a method to engineer the microstructure of electrodes to control and reduce tortuosity. The improved effective lithium ion transport through the porous electrode can be leveraged to create faster batteries, or to fabricate thicker, cheaper electrodes without compromising speed.

The properties of the interface between electrode active materials and electrolyte are known to have profound effects on the performance of Li-ion batteries (LIBs). A large amount of research has so far been devoted to modifying the surfaces of active materials in order to enhance the overall performance of the electrodes. Majority of the research has resorted to inorganic oxide coatings, presumably due to their chemical stability, and different beneficial effects have been claimed for both cathode and anode materials. Relatively far less progress has been reported for polymeric coating. Compared with the inorganic ones, the polymeric coatings typically require much lower processing temperature, and when used water as solvent, can be considered as more environmentally-friendly alternative. In addition, the rich chemistry of the polymeric coating intrinsically possess a greater flexibility for dealing with wide varieties of active materials and electrolytes for maximum performance.

For LIB anodes, there are two important issues to deal with for achieving high-energy and high-power performance. First, decomposition of electrolyte at the anode surface takes place at sufficient low potential leading to the formation of a passivation film, known as solid electrolyte interphase (SEI). Formation of the SEI consumes Li ion inventory and hence reduces the overall capacity of the battery cell. Therefore, the amount of the stable SEI layer should theoretically kept at a minimum. Secondly, the power densities for the state-of-the-art graphite electrodes remain insufficient.

In this presentation, we will report our progress in developing polymeric coatings for LIB anodes, particularly for graphite and graphite/Si composite. It is demonstrated that the properties of the anodes in different aspects, such as chemical/electrochemical, mechanical, and wetting behaviors, can be dramatically changed with the combination of polymer chemistry. In particular, it is shown that both the cycle life (Fig. 1) and even the power performance (Fig. 2) of the anodes can be remarkably enhanced by simple polymer coating.

Two multilayered Cu/Si thin films (flat and well-aligned structured) were prepared by using an electron-beam evaporation deposition. An oblique angle deposition was used to form a well aligned, multi-layered Cu/Si nanorods array in the thin film. Galvanostatic half-cell measurement showed that the well aligned multi-layered Cu/Si nanorods containing thin film anode exhibited a moderate capacity for 100 cycles due the particularities in its structure and morphology: The structural analysis demonstrated that Cu in the films buffered the mechanical stress occurred during lithiation and enhanced the electrical conductivity in the film. The morphological observation revealed the importance of the inclined nanorods since they increased the contact area of the anode with Li, decreased the polarization and enhanced the mechanical tolerance against the volumetric changes due to the homogenously distributed nanosized interspaces among them.

Compared with graphite, silicon is a favourable alternative candidate due to a high specific capacity (3572 vs. 372 mAh g^-1) and specific volumetric capacity (2081 vs. 779 mAh cm^-3). However, large capacity fading and low cyclability of Si electrodes is a major issue. Several strategies have been undertaken to overcome these problems. One way is optimising the binder in order to cope at the molecular scale with the expansion and contraction of Si upon alloying and dealloying with Li. Since early works on CMC, several other works proposed alternative binders to CMC, most of them being polysaccharides and/or bearing COOH functional groups. Another way to improve the cycle life is using electrolytes containing a film-forming agent such as VC and/or FEC.

Besides, electro-conductive additives also display an important influence for the performance of nano Si-based electrodes with high active mass loadings of about 2.5-3.3 mAh per cm². We found a significant improvement of the electrochemical performance (2000 mAh g ^-1 after 100 cycles for 2.5 mg of Si cm ^-2) by using reduced graphene oxide (rGO) or exfoliated graphite nanoplatelets instead of carbon black as the conductive additive [1,2]. The influence of electro-conductive additives is not only to play on the electronic conductivity but also on the micromechanics (stress distribution) of the composite films.

In a further work, we aimed to design the formulation of these nano Si-based electrodes with high active mass loadings to wind some cylindrical cells. This way, Poly (acrylic-co-maleic) acid (PAMA) is used as a dispersant to improve the stability of electrodes slurries. Sedimentation test, electrical measurement, SEM-EDX observations as well as rheological measurements show that a more homogeneity distribution of carbon black (CB) inside the stack of Si particles is reached with presence of PAMA. However, there is an optimal amount of PAMA due to the competition in the adsorption of PAMA and Carboxylmethyl cellulose (CMC) at the surface of the CB particles. Upon cycling with capacity limitation (1200 mAh per g of Si), the optimized electrode formulation at lab scale could achieve more than 400 cycles with surface capacity ~2.5-3.3 mAh cm^-2. At the pilot scale, the improvement of adhesion of the tape to the current collector by using Styrene-co-Butadiene rubber copolymer latex (SB) helps to maintain long cycle life while by calendaring is detrimental to electrochemical properties. [3] On the whole, the pilot-made electrodes show a lower cycle life than the labe-made electrode as a consequence of poorer CB distribution, likely due to the larger volume and longer processing time involved.

Acknowledgements

Financial support provided by the European Commission (EC), through the project EuroLiion (NMP3-SL-2010-265368) is gratefully acknowledged.

References

[1] B.P.N. Nguyen, J. Gaubicher, B. Lestriez, Electrochimica Acta, 2014, 120, 319.

[2] B. P. N. Nguyen, N. A. Kumar, J. Gaubicher, F. Duclairoir, T. Brousse, O. Crosnier, L. Dubois, G. Bidan, D. Guyomard, B. Lestriez, Adv. Energy Mater., 2013, 3, 1351.

[3] B. P. N. Nguyen, S. Chazelle, M. Cerbelaud, W. Porcher, B. Lestriez, submitted to J. Power Sources.

A wide variety of a nano-structured Si electrodes have been proposed in the recent years that avoid pulverization of both the Si active nanoparticles and the overall electrode structure. It has been shown that nano-Si particles (dia. < 150 nm) can accommodate the strains associated with full lithiation without fracture (1). Additionally, encapsulation of nano-Si in clever carbon structures provides the volume needed to accommodate Si's expansion without bulk expansion of the overall electrode (2). Unfortunately, many of these nano-structured electrodes are not suitable for commercialization because the complicated fabrication processes are too expensive. Previous work has identified mesoporous Si (mp-Si) as a scalable, commercially viable nanostructured anode material for advanced Li-ion batteries. mp-Si combines ease of manufacturability with the advantages of a Si nanostructure. The internal porosity of mp-Si has been shown to largely accommodate the strain of Si lithiation in order to avoid pulverization of the electrode structure.

An electrochemically etched mp-Si material developed by Sailor is utilized in this study (Figure 1) (3, 4). Porous Si can be carbonized to prevent SEI formation and provide fast electronic transport. Carbonization is typically accomplished by the thermal decomposition of acetylene gas. The resulting carbon coating is strongly bonded to the Si surface via Si-C bonds, but it is brittle so it cannot provide mechanical resiliency nor protect mp-Si's surface from electrolyte decomposition. The degree to which mp-Si's internal structure pulverizes with cycling is also not known. With this in mind, a flexible coating is ideal because it can impart the porous structure with mechanical resiliency for improved cycling performance.

Stabilized polyacrylonitrile (PAN) was found to be a promising conductive binder for nano-Si (5). PAN coatings on nano-Si (dia. < 50 nm) are conformal, elastic, and < 10 nm thick. Cyclizing and dehydrogenating PAN introduces delocalized sp² π bonding for good intrinsic electronic conductivity, but avoids full carbonization to maintain the material's polymeric elasticity. This process cannot be applied to mp-Si because the macromolecules of PAN (Mw > 100,000 g mol^-1) are too large to infiltrate the mesopores. In another work, nano-Si electrodes were prepared by the in-situ polymerization of polyaniline (PANi) (6). PANi is a popular electronically conductive polymer, but it has not been applied to the mp-Si electrode material. PANi also acts as both the binder and the conductive additive.

Here, we propose that either the in-situ polymerization of PANi or PAN are ideal for mp-Si electrodes. We have already demonstrated the feasibility of PAN:mp-Si electrode by the in-situ radical polymerization of PAN in DMF. Our PAN:mp-Si electrode delivers a specific capacity of 1500 mAh/g-Si. We will also discuss the feasibility of a PANi:mp-Si electrode and the appropriate functionalities needed for stable adhesion of the polymer to Si's surface.

Figure 1: a) Etched mesoporous Si particle. b) SEM micrograph of mesoporous Si's porous structure (3, 4).

References

1. X. H. Liu, L. Zhong, S. Huang, S. X. Mao, T. Zhu and J. Y. Huang, Acs Nano, 6, 1522 (2012).

2. N. Liu, Z. Lu, J. Zhao, M. T. McDowell, H.-W. Lee, W. Zhao and Y. Cui, Nature nanotechnology, 9, 187 (2014).

3. T. L. Kelly, T. Gao and M. J. Sailor, Advanced Materials, 23, 1776 (2011).

4. C. K. Tsang, T. L. Kelly, M. J. Sailor and Y. Y. Li, ACS nano, 6, 10546 (2012).

5. D. M. Piper, T. A. Yersak, S.-B. Son, S. C. Kim, C. S. Kang, K. H. Oh, C. Ban, A. C. Dillon and S.-H. Lee, Advanced Energy Materials, 3, 697 (2013).

6. H. Wu, G. Yu, L. Pan, N. Liu, M. T. McDowell, Z. Bao and Y. Cui, Nature communications, 4 (2013).

Introduction

Si is attractive for use in high energy density anode materials due to its high volumetric capacity of 2194 Ah/L (corresponding to Li₁₅Si₄) [1]. The volume expansion of Si can be diluted to improve cycle life by the use of active/inactive composite electrode materials [2, 3]. However, electrode processing conditions also plays an important role in performance improvement. For commercial energy cells, the electrode stack should have high energy density. Therefore, electrode compression or calendering is widely practiced in industry to increase energy density [4]. Si-based materials are typically hard and brittle and cannot be readily calendered. Therefore such coatings can have high porosities, which can result in low energy density.

Here, we present a facile and viable method for the compression of Si alloy electrodes while maintaining their high volumetric capacity, low volume expansion and good cycling performance.

Experimental

Electrode slurries were made by mixing specific ratios of 3M L-20772 Si alloy [5] and LiPAA (Polyacrylic acid) solution in distilled water with/without the addition of SFG6L graphite (28% by weight). The slurries were coated on Cu foil using a 0.004 inch gap coating bar and dried at 120^oC in air for 1 h. The electrode foils were then passed through a calender for calendering to ~20% prorosity. The electrode pore volume was the difference of the total coating volume minus the solids volume. Electrodes were assembled into 2325 size coin-type cells using 1M LiPF6 dissolved in EC:DEC:FEC (3:6:1 vol%) solution. Two Celgard separators and a lithium foil counter/reference electrode were used. Electrode thicknesses were measured to within ± 1μm with a Mitutoyo 293-340 precision micrometer. The morphology of electrodes was studied using the Phenom G2 pro desktop SEM.

Results

The porosity of an uncalendered Si alloy / LiPAA 91/9 w/w electrode is about 56%. When it is fully lithiated coin cell is disassembled, it was found the entire coating had expanded by 96% and the porosity was calculated to be 57%. Therefore the alloy does not expand into the available porosity in the coating. Instead, as shown in Figure 1, as the alloy expands by 96%, the pores expand by the same amount. The large volume fraction porosity in such electrodes makes their volumetric capacity relatively low (624 Ah/L). To increase the volumetric capacity, electrodes with various formulations were calendered to ~20% porosity.

Figure 2 shows the cycling performance of some of these electrodes. Clearly, calendering has a detrimental effect on the cycling performance of Si alloy electrodes when graphite is not present. By adding graphite in the electrode, the cycling excellent cycling can result. Moreover the volumetric coating capacity is increased to 957 Ah/L while overall volume expansion is reduced to only 64%.

In this manner high volumetric capacity, low volume expansion alloy electrodes with excellent cycling characteristics can be obtained. Mechanisms of volume expansion in alloy coatings and methods of improving volumetric capacity and lowering volume expansion will be discussed.

References

[1] M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 7, (2004) A93.

[2] M.N. Obrovac, L. Christensen, Dinh Ba Le and J. R. Dahn, J. Electrochem. Soc., 154, (2007) A849.

[3] O. Mao, R. L. Turner, I. A. Courtney, B. D. Fredericksen, M. I. Buckett, L. J. Krause and J. R. Dahn, Electrochem. Solid-State Lett., 2, (1999) 3.

[4] T. Marks, S. Trussler, J. Smith, D. Xiong and J. R. Dahn, J. Electrochem. Soc., 158, (2011) A51.

[5] L. Christensen, D. Ba Le, J. Singh and M.N. Obrovac, 3M Alloy Anode Materials, 27th International Battery Seminar & Exhibit, Ft. Lauderdale FL, March 15-18, 2010. http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSufSevTsZxtUo8mv4x_1evUqevTSevTSevTSeSSSSSS--&fn=AnodeTechPaperPowerConf.pdf

Silicon is a very promising anode material for next generation lithium ion battery due to its high lithiation capacity. Despite the intense research that has focused on silicon, this material has yet to demonstrate a performance that is compatible with commercial applications. The main shortcomings obstructing its commercializing are (a) the large volume change during lithiation/delithiation which leads to failure of the material, (b) its poor charge transport properties and (c) the formation of an unstable and/or insulating solid-electrolyte interphase on the active material surface. We propose a novel processing technique that allows addressing these problems using a simple and scalable manufacturing protocol. Silicon nanoparticles with an average particle size of ≤10 nm were produced by plasma enhanced chemical vapor deposition (PECVD) and then functionalized with 12-carbon long aliphatic chains. The resulting silicon ink was composed with polyvinylpyrrolidone (PVP) and carbon nanotubes (CNTs) to give a printable dispersion which was then coated onto copper foil and annealed to produce a binder-free anode for lithium ion battery. This unique electrode structure has an average charge/discharge capacity of ~1000 mAh/g (normalized over the total weight of the coated material) for >200 deep cycles (from 1.5 to 0V) and coulombic efficiency exceeding 99.6% after few cycles. Control experiments for the same anode without PVP suggests that the presence of the polymer during thermal annealing inhibits the particle size growth and covers the silicon particles with a layer of carbon contained material, thus enhancing the stability of the anode structure. Preliminary results regarding the extension of this approach to other materials (conductive polymers and graphene flakes) will also be discussed.

Methods and criteria for assessing the commercial viability of Si-based materials are discussed and demonstrated with the 3M V6 alloy and 60 nm nano Si powder. These materials are firstly evaluated through the cycling of neat electrodes containing only alloy and binder to characterize the capacity, first cycle efficiency, binder compatibility, and microstructure stability of the material. The alloy displays higher first cycle efficiency, lower fade, and a more stable amorphous microstructure compared to the nano Si, which displays a variable microstructure with a rate dependent presence of crystalline Li₁₅Si₄.

Figure 1 shows the dQ/dV of neat electrodes containing (a) only nano Si and LiPAA binder for which the Li15Si4 peak gradually decreases, and (b) Si-based alloy (V6Neat) and LiPAA binder with a dQ/dV characteristic of amorphous Si.

The materials are then evaluated in graphite-containing composite electrodes having high areal capacities (> 2 mAh/cm²). In a well designed composite electrode including carbon nanotubes, 3M V6 material was found to cycle with little fade and high coulombic efficiency (~99.8%) while maintaining a stable dQ/dV. A composite electrode of equivalent volumetric capacity with nano Si powder shows similar capacity retention over 50 cycles but an unacceptably low coulombic efficiency (~99.2%). High precision coulometry and calorimetry results show surface area as the dominant factor in levels of parasitic reactions with Si based materials.

A review of failure modes for various Si implementations and their impact on full cell cycling will also be presented.

In lithium-ion batteries, the electrochemical reaction between Li and Si causes structural changes in the negative electrode. The dynamics of lithiation of Si can be further complicated by the crystalline-to-amorphous phase transition. In situ TEM experiments show that a sharp interface, known as phase-boundary, is formed in between c-Si and a-LixSi during initial lithiation. Despite intensive study of the mixing mechanism during lithiation of Si negative electrode, the atomistic investigation of the formation and propagation of phase boundary for different orientation of Si remains unclear. We, therefore, performed molecular dynamics simulations to characterize the structural evolution of the phase boundary with a newly developed reactive force field (ReaxFF) potential for Li-Si. Our results confirm the phase boundary formation in between c-Si and a-LixSi. Structure and dynamics of the phase boundary depend on the crystalline phase of the Si. In particular, the location of the (111) plane plays a key role in crystal-toamorphous phase transformation. A relatively thick phase boundary is developed at the (100) surface, while an atomically sharp interface of negligible thickness is formed at the (111) surface. An amorphous phase of lithiated Si is developed beyond the phase boundary, in which the ratio of lithium to silicon atoms is steady at 0.8. Partial RDF studies revealed that the structures of the phase boundary and the lithiated Si region are c-LiSi and a-Li15Si4, respectively.

Having high capacities and voltages, lithium ion rechargeable batteries are widely used and still are the subject of intensive investigation. In comparison with commercially used graphite carbon, tin-based materials have emerged as prospective anode materials because of their much higher theoretical specific capacity. Most of the various tin-based materials having been researched for inventing high energy anode materials can be subdivided into two groups, pure tin(Sn) metal based and tin oxide(SnO_x) based compounds. Tin metal and its oxides are both based on reversible alloying reaction forming Li_xSn alloys. But they are different in that tin oxide additionally undergoes conversion reaction, known as almost irreversible, forming lithium oxide and reduced tin metal before alloying reaction occurs in the first charging process. Although the lithium oxide formed by the conversion reaction consumes plenty of lithium ions, it is normally considered as an advantage because it acts as mechanical buffer layers and therefore it contributes the capacity retention.

However, there is still a lack of understanding on the reaction mechanism of tin oxide. Particularly, there has been a debate on the partial reversibility of the conversion reaction of tin oxide. The unexpected experimental peak of differential capacity curves and the additional capacity over the theoretical prediction are considered as the evidences of partial reversibility of the tin oxide conversion reaction. Recently, Hu et al. revealed the origin of extra capacities in transition metal oxides using RuO₂ material. But, tin oxide should be distinguished from the transition metal oxides which are mostly operated by reversible conversion reaction in the lithium ion batteries. Therefore, we have traced single SnO₂ particle by ex-situ transmission electron microscopy(TEM) research. We dispersed SnO₂ particles on a carbon film deposited copper mesh TEM grid and the grid was used as a working electrode in a coin cell. The coin cell including the TEM grid was dis/charged toward specific voltages and it was disassembled to analyze the sample which is directly dis/charged on the TEM grid. Repeating this process with the same sample grid enabled us to observe the transformation of the single tin oxide particle and its surroundings. We observed the lithium hydroxide on the specific voltage state, which contributes the additional capacities of tin oxide electrode. The detail explanations on the contribution of lithium hydroxide and the reversibility of the tin oxide conversion reaction will be presented.

Furthermore, we performed the same experimental process with pure tin particles. Eliminating the conversion reaction step, pure tin particles give us better understanding on the reaction mechanism of tin oxide. By comparing with corresponding reaction steps between them, we can figure out where the lithium hydroxide originated from; electrolyte decomposition or lithium oxide formed by the tin oxide conversion reaction. In addition, we can ensure whether the reversible conversion reaction of tin oxide is possible. Including the above experimental results, the reaction mechanisms of tin and tin oxide will be discussed. We expect that these ex-situ TEM studies will provide fundamental knowledge of various tin-based materials.

Lithium-ion batteries (LIBs) are the most popular power source not only for portable electronics but also for upcoming electric vehicles. So far, various materials such as graphitic carbon, Si, Ge, MoO₃, NiO, Fe₂O₃, and SnO₂ have been exploited as the anode materials for LIBs. Among these candidates, SnO₂ is one of the promising materials for anode electrode in LIBs due to its high theoretical capacity (782 mAh g^-1). However, the main difficulties for using the alloy-based materials are their dramatic volume expansion and contraction during Li insertion and extraction. This volume change can generate a large internal stress, leading to pulverization of the electrode and electrical detachment of the active particles. Moreover, formation of Li₂O, related to irreversible capacity, results in a low efficiency and waste of Li⁺. If the conversion reaction of Li₂O, in SnO₂ system, can be induced, the reversible capacity could be increased. Recently, SnO₂-transition metal oxide composites, such as MoO₃, Fe₂O₃, and NiO, etc., have been suggested to induce the synergistic effects between Li₂O, from SnO₂, and transition metal oxide [1-3]. However, most of current studies of these composite materials showed the poor cyclablity and the synergistic effects diminished in the long-term cycling. We believed that these results are realated to the structual instability due to the large volume change during lithium insertion/extraction. Among various nanosturcutres, hollow sphere is the most promising structure because it can accommodate the large volume change and the synthesis process is relatively simple. Thus, we try to synthesize the hollow composite structrue with SnO₂ and transition metal oxides.

In this study, we synthesized multi-layered hollow spheres, composed of SnO₂ and transition metal oxides (Co₃O₄ and Fe₂O₃), by a simple sol-gel based process. Even though cobalt (Co) is well-known catalyst metal for the reversible reaction of Li₂O, it is one of expensive materials. So, we also synthesize the SnO₂@Fe₂O₃ hollow spheres with the similar synthesis procedure. The size of synthesized SnO₂ hollow sphere is about 60 nm and the shell thickness is about 5 nm. After Co₃O₄ and Fe₂O₃ coating, the size was not significantly changed (Fig. 1). The SnO₂@Co₃O₄ and SnO₂@Fe₂O₃@C electrodes showed much higher reversible capacity than SnO₂ hollow sphere electrode (Fig. 2) and further cycling test is also examined. The synergistic effects between SnO₂/Co₃O₄ (Fe₂O₃) and the reaction mechanism observed by TEM and electrochemical analyses will be discussed in this presentation. Furthermore, we also prepared various electrodes such as commercial SnO₂ nanopowder, SnO₂@C hollow spheres, SnO₂@ Fe₂O₃@C solid spheres and the electrochemical performances will be compared. We believe that our studies might suggest the design strategy for the next-generation LIBs, and it can be applied to other SnO₂-metal oxides composites.

Fig. 1 SEM images of (a) SnO₂@Co₃O₄ hollow spheres and (b) SnO₂@Fe₂O₃ hollow spheres.

Fig. 2 Cycling performance of SnO₂, Co₃O₄/SnO_2, and SnO₂@Fe₂O₃@C hollow sphere electrodes.

[1] X. -Y. Xue, Z. -H. Chen, L. -L. Xing, S. Yuan, Y. -J. Chen, Chem. Commun. 47(2011) 5205-5207.

[2] W. Zhou, C. Cheng, J. Liu, Y. Y. Tay, J. Jiang, X. Jia, J. Zhang, H. Gong, H. H. Hng, T. Yu, H. J. Fan, Adv. Funct. Mater. 21(2011) 2439-2445.

[3] M. F. Hassan , M. M. Rahman , Z. Guo , Z. Chen, H. Liu, J. Mater. Chem., 20(2010) 9707-9712.

Lithium-ion (Li-ion) battery chemistries have driven the portable electronics market for almost 25 years and are now poised to significantly impact the light-duty transportation and grid storage sectors as well. The versatility and proven record of Li-ion systems makes them an irresistible choice from a practical standpoint. However, each application continuously demands more from every new generation of battery products; the highly correlated parameters, energy, power, safety, cycle life, and cost, are constantly being pushed to their limits. Although much of the current effort in energy storage research is aimed at transformational gains, novel Li-ion systems, yet to be explored, have the potential of making continual and significant incremental impacts on next-generation technologies.

This presentation will discuss insights into the atomic-level structure and transformation mechanisms of lithium-transition-metal-oxide cathode materials. These mechanisms explain, in part, the failure modes of layered cathodes at high states of delithiation. Furthermore, a discussion on a variety of elemental and structural compositions will be given. For example, unique layered, layered-layered, spinel, and layered-layered-spinel structures and chemistries will be presented that are pushing the boundaries of current Li-ion technology.

Future directions and research in further enabling these new systems will also be discussed.

Acknowledgment

Support from the Vehicle Technologies Program, Hybrid and Electric Systems, in particular, David Howell, Peter Faguy, and Tien Duong at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged.

The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Li₂MnO₃is a critical component in the family of the so-called `Li-excess' materials, which are attracting attention as advanced cathode materials for Li-ion batteries. We present first-principle calculations to investigate the electrochemical activity and structural stability of stoichiometric Li<sub>x</sub>MnO<sub>3</sub>(0 < x < 2) as a function of Li content. We find that the Li<sub>2</sub>MnO<sub>3</sub> structure is electrochemically activated above 4.5 V on delithiation and that charge neutrality in the bulk of the material is mainly maintained by anion oxidization. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O migration remain high throughout the Li composition range, impeding any significant oxygen release from the bulk of the compound. Furthermore, we show that, defect layered structures, where some Mn resides in the Li layer, become thermodynamically favorable at lower Li content (x < 1), indicating a strong tendency towards local spinel-like domain transformation. Concurrently, the calculated energy barriers for Mn migration from the Mn-layer into the Li-layer suggests a Li<sub>2</sub>MnO<sub>3</sub> structural instability for x < 0:5. Based on our observations, we suggest a critical phase transformation path for forming nuclei of spinel-like domains within the matrix of the original layered structure. We also show that formation of defect layered structures during the first charge manifests in a significant depression of the voltage profile on the first discharge, providing a possible background for the observed `voltage fade' of the Li-excess materials.

Positive electrode materials combining the advantages of nickel, manganese and cobalt such as higher reversible capacity, lower cost, and less toxicity [1–4] are currently in the focus of research for lithium ion batteries. The relevant electrochemical performance of NMC type cathode materials (LiNi_xMn_yCo_1−x−yO₂) has been widely discussed in literature. Al, Mg and Cr seem to be promising candidates for the partial substitution of Co in these materials [5-9] in order to further improve the electrochemical performance by decreasing the cation mixing in the structure.

The current study discusses three NMC type materials obtained by cationic substitution of cobalt with aluminum and/or iron in the starting material LiNi_0.6Mn_0.2Co_0.2O₂. The three compounds with the following compositions LiNi_0.6Mn_0.2Co_0.15Al_0.5O₂ (NMCA), LiNi_0.6Mn_0.2Co_0.15Fe_0.5O₂ (NMCF) and LiNi_0.6Mn_0.2Co_0.15Al_0.025Fe_0.025O₂ (NMCAF) were synthesized by the self-combustion method using sucrose as fuel. These materials belong to α-NaFeO₂-type structure (space group R-3m) with hexagonal ordering. Rietveld refinement analysis of the XRD patterns revealed a very low cationic mixing for the double substituted material NMCAF (5%) compared to the non-substituted material (13%) suggesting a higher structural stabilization for the double substituted compound. Galvanostatic cycling measurements indicate improved electrochemical performance, good cycling stability and higher reversible capacity preferentially for NMCAF (190 mAh.g^-1), NMCF (167 mAh.g^-1), NMCA (145 mAh.g^-1) then NMC (140 mAh.g^-1).

References:

[1] I. Belharouak, Y.K. Sun, J. Liu, K. Amine, J. Power Sources 123 (2003) 247.

[2] S. Patoux, M.M. Doeff, Electrochem. Commun. 6 (2004) 767.

[3] B.J. Hwang, Y.W. Tsai, D. Carlier, Chem. Mater. 15 (2003) 3676.

[4] T. Ohzuku, Y. Makimura, Chem. Lett. (2001) 641.

[5] T. Ohzuku, A. Ueda, M. Kouguchi, J. Electrochem. Soc. 142 (1995) 4033.

[6] T. Ohzuku, A. Ueda, N. Nagayama, Y. Iwakoski, H. Komori, Electrochim. Acta 38 (1993) 1159.

[7] Zhang B, Li L, Zheng J, J. Alloy. Compd. 520 (2012) 190.

[8] Idemoto Y, Kitamura N, Ueki K, Vogel SC, Uchimoto Y, J. Electrochem. Soc. 159 (2012) A673.

[9] Luo W, Zhou F, Zhao X, Lu Z, Li X, Dahn JR, Chem. Mater. 22 (2009) 1164.

Lithium secondary battery is promising as a power backup for energy storage system. In this respect, studies towards improvement in performances of electrode are being progressed to assure long cycling performances, thermal stability, safety, and so on. In particular, positive electrode materials are need to be further investigated to accomplish the above- mentioned properties. Several kinds of 4 V class positive electrode materials are commercially available. Among them, LiCoO₂ is one of the most common electrode materials in rechargeable lithium batteries. Although the LiCoO₂ has many advantages, this material has poor capacity retention because of dissolution of Co and structural instability at deeply charged state. To improve the nature of active material, surface modification is effective to overcome those limited properties of active materials. Actually, Metal oxides as coating materials have been extensively studied. In this study, Metal phosphate coatings are applied on the surface of Li[Ni_0.7Co_0.2Mn_0.1]O₂. We, here, report the resulting structural, electrochemical and thermal properties of the surface-modified Li[Ni_0.7Co_0.2Mn_0.1]O₂.

H₃PO₄ and metal salts were selected as starting materials for surface modification of Li[Ni_0.7Co_0.2 Mn_0.1]O₂. A solution anhydrous ethanol containing metal phosphates was stirred at 30 ^oC for 5 hours. Then, active materials were added into the solution. And the solution was evaporized at 80 ^oC in air and the resulting precipitates were then heated at 500 ^oC in air. Also, the coated powders were characterized by XRD, SEM, and HR-TEM. Electrochemical properties of coated powders were examined by galvanostatic cycle test and electrochemical impedance spectroscopy.

Metal phosphates coatings shows uniform coating layers, as observed by TEM. TG and DSC analyses were conducted for the materials. Electrochemical test with half cells in voltage range of 3 - 4.3 V at 25 ^oC indicates that coated materials have better capacity retention and coulombic efficiency, rate capability and thermal properties. Thin coating layers were effective in improvement of the electrochemical, structural and thermal properties. Also, metal phosphates coatings give decrease in the residual lithium content and byproduct on the surfaces of particles, as confirmed by ToF-SIMS. And the materials were analyzed into XPS and ToF-SIMS to confirm surface byproducts of the materials itself at high temperature. Details will be mentioned at the conference site.

The high-energy-density Li-rich layered materials are promising cathode materials for the next-generation high-performance lithium-ion batteries¹. They have attracted a lot of attentions due mainly to their high reversible capacity of more than 250 mAh·g^-1 at low charge-discharge current. However several drawbacks still hinder their applications, such as the poor rate capability².

To conquer this critical issue, the present study is focused on surface modification on Li-rich layered cathode materials to improve their rate capability as well as maintain the high capacity retention of the pristine material. Surface treatment is conducted on Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ using NH₄F by thermal annealing at low temperature. Material characterizations reveal that the modification process triggers fluorine doping and phase transition from layered phase to spinel phase at the particle surface, as shown in Figure 1. Figure 2 shows the rate performances of the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and the modified materials. The pristine material could deliver a high reversible capacity of about 250 mAh·g^-1 at 0.1C (25 mA·g^-1). However, its discharge capacity is about 109 mAh·g^-1 at 1C, which is only 43% of the capacity at 0.1C. Compared with the pristine material, both the materials modified by 5 wt.% and 10 wt.% NH₄F can deliver a discharge capacity over 140 mAh·g^-1at 1C, which is more than 70% of their discharge capacities at 0.1C. Particularly, the material modified by 20 wt.%

NH₄F has a discharge capacity as high as 172 mAh·g^-1 at 1C, which is over 87% of its capacity at 0.1C. Moreover at even higher rate like 5C, the discharge capacity of the material modified by 20 wt.% NH₄F still can reach 126 mAh·g^-1 while the discharge capacity of pristine one is only 41 mAh·g^-1. Generally, the NH₄F modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ exhibits greatly improved rate performance and satisfactory cycling stability compared to the pristine material, which can be attributed to the modified particle surface. Firstly, the spinel shell of the particle provides three-dimensional Li⁺ ion diffusion paths³, which creates fully opened surface, enabling fast Li⁺ ion transfer at the electrode/electrolyte interface. Secondly, the formation of spinel shell prevents the Ni segregation at the surface, thus suppressing its negative effect on Li⁺ion diffusion. Finally, the fluorine doped spinel surface improves the surface stability during wide-voltage-range charge-discharge process, resulting in improved cycling stability.

The enhancement in the electrochemical properties of modified materials as a function of the NH4F amount are comprehensively investigated using Power X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical impedance spectroscopy and electrochemical tests.

References:

1. Whittingham, M. S., Lithium batteries and cathode materials. Chemical Reviews 2004,104(10), 4271-4301.

2. Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A., Li₂MnO₃-stabilized LiMO₂ (M = Mn, Ni, Co) electrodes for lithium-ion batteries. Journal of Materials Chemistry 2007,17(30), 3112.

3. Song, B.; Liu, H.; Liu, Z.; Xiao, P.; Lai, M. O.; Lu, L., High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries. Scientific reports 2013,3.

We report a new synthesis method, termed self-sustaining combustion reaction, that resulted in a lithium-rich MNC (LLMNC) cathode material with high rate capability and excellent capacity maintenance during cycling compared to the lithium-rich metal oxide cathode materials previously reported in the literature ^(1-4). This porous, sponge-like composite metal oxide of the composition, 0.5Li₂MnO₃. 0.5LiMn_0.5Ni_0.35Co_0.15O₂, was characterized by means of its X-ray diffraction (XRD) pattern, FESEM (Field Emission Scanning Electron Microscopy) images and electrochemical data. The interconnected pore structure of the material with enhanced surface properties (figure 1, inset) compared to its previously known counterpart synthesized from the co-precipitation method provides higher rate capability with low capacity fade during long-term cycling.

The XRD pattern of the material, with the Li₂MnO₃ region magnified, presented in Figure 1, perfectly matches with the literature data⁽³⁾ for the layered Li-rich MNC material of similar composition. We obtained discharge capacities of approximately 300, 250, 200 and 150 mAh/g at C/20, C/4, and C and 2C discharge rates for electrodes having an active material loading of approximately 7 mg/cm². Remarkably, excellent capacities with practically zero capacity fade has been observed for hundreds of cycles at the C discharge rate as shown in Figure 2 for a Li half-cell cycling between 2-4.9V at room temperature. Detailed structural and electrochemical properties of the material characterized using cyclic voltammetry (CV), XRD, X-ray Absorption Spectrometry (XAS), FESEM and High Resolution Transmission Electron Microscopy (HRTEM) will be discussed.

References:

1. M. N. Ates, Q. Jia, A. Shah, A. Busnaina, S. Mukerjee and K. M. Abraham, Journal of The Electrochemical Society, 161, A290 (2014).

2. Y. Chen, G. Xu, J. Li, Y. Zhang, Z. Chen and F. Kang, Electrochimica Acta, 87, 686 (2013).

3. A. Ito, D. Li, Y. Ohsawa and Y. Sato, Journal of Power Sources, 183, 344 (2008).

4. F. Amalraj, D. Kovacheva, M. Talianker, L. Zeiri, J. Grinblat, N. Leifer, G. Goobes, B. Markovsky and D. Aurbach, Journal of The Electrochemical Society, 157, A1121 (2010).

Single phase LiMn_0.3Co_0.3Ni_0.3Ti_0.1O₂ materials are prepared using a self-propagating combustion method. The structure and morphology of the materials were characterized using X-Ray powder diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and X-Ray photoelectron Spectroscopy (XPS). The electrochemical performances of the materials were characterized by means of galvanostatic charge-discharge test on the fabricated cells. XRD results showed that the materials are impurity-free and single phase with well ordered hexaganol layered structure of R-3m space group. The compounds annealed at 700 °C at different annealing times showed variations in discharge capacities. The discharge capacities are between 143 and 144 mAhg^-1. The compound annealed at 700 °C for 24 h exhibits the highest first cycle capacity of 143.72 mAhg^-1 over the voltage range of 2.5 to 4.2 V. The 30^th cycle, however, revealed that the materials annealed for 72 h shows the highest specific discharge capacity of 136 mAhg^-1. The capacity fading is only about 4% compared to 23% for the 24 h sample. The longest annealing time of 72 h suggests that highly crystalline LiMn_0.3Co_0.3Ni_0.3Ti_0.1O₂ materials have more stable crystal structure and exhibit good cycling behaviour compound to the 24 h annealed sample. The results suggest that the self-propagating combustion method is a promising technique for the preparation of LiMn_0.3Co_0.3Ni_0.3Ti_0.1O₂ cathode material for lithium-ion batteries.

The transportation landscape is changing and vehicle electrification continues to gain traction in the market. The trend towards electric mobility (e-mobility) has shown a significant rise in recent years along with advances in technology and government subsidies, but there is still a long way to go for the future of e-mobility to be realized. The heart of e-mobility is the battery and its performance is fundamental to the success of e-mobility.

The cathode has a large stake in the key characteristics of the battery, such as energy density, power capability, safety, and cost. Advancements of cathode materials directly result in the improved battery performance with higher significance. BASF has focused on the development of next generation cathode materials (high energy Li-Mn-rich NCM and high voltage spinel) as well as advanced surface modification techniques.

In this presentation, we will report the electrochemical behaviors of a few of our latest cathode materials for advanced lithium-ion batteries. Three-electrode cell testing results will reveal in-depth information regarding early stage electrochemical formation process as well as cycling performance. Subsequently performed post-mortem investigation will provide analysis of capacity fade and failure mechanism during/after cycling procedure. Also, insights on the path forwards to further improve battery performance will be shared.

Introduction

Development of high voltage Li-ion cells is critical for the improvement of energy density of Li-ion cells [1]. Electrolyte additives are used to improve the properties and performance of Li-ion cells [2]. However, the way that electrolyte additives function and the impedance changed that occurs with voltage in Li-ion cells have not been well-explained in the literature.

A Maccor 4000 series charger, combined with a frequency response analyzer (Maccor FRA 0356), was used to investigate the effects of electrolyte additives on cell impedance changes with voltage in Li[Ni_0.42Mn_0.42Co_0.16]O₂ (NMC)/graphite and LiCoO₂(LCO)/graphite pouch cells.

Experimental

Machine-made Li[Ni_1/3Mn_1/3Co_1/3]O₂/graphite and LiCoO₂/graphite dry pouch cells (402030 size, 220 mAh) were supplied by reputable manufacturers and were filled and sealed at Dalhousie University. Cells were filled with 1 M LiPF₆in ethylene carbonate (EC):ethylmethyl carbonate (EMC) (3:7 by weigh, BASF) as control electrolyte. Vinylene carbonate (VC, BASF, 99.97%) was used as an electrolyte additive. After electrolyte filling and vacuum sealing (MTI Corporation, MSK-115A) in an argon-filled glove box, a 24 h hold at 40.0 ± 0.1°C and 1.5 V was used to ensure complete wetting of the cell coil. The first charge cycle (called the formation process here) consisted of charging at 11 mA (corresponding to C/20 current) to 3.5 V. Then cells were degassed in the glove box and vacuum sealed again. Subsequently, cells were charged to 4.5V using the same current, followed by degassing and vacuum sealing in the glove box.

The cells were cycled using a Maccor 4000 series charger between 2.8 and 4.7 V at 40.0 ± 0.1°C using currents corresponding to C/20 while the cell impedance was measured at every 0.1 V interval between 3.6 and 4.7 V with a Maccor frequency response analyzer (FRA 0356). The FRA unit and cells during testing were in temperature controlled environments (21°C for NMC/graphite pouch cells, and 30°C for LCO/graphite pouch cells) with variations in temperature of an amplitude less than 2°C. AC impedance spectra were collected from 10 kHz – 10 mHz with an amplitude of 2 mV. Ten data points per decade were measured.

Results and discussion

Figure 1 shows selected Nyquist plots for NMC cells containing 2 % VC during the first charge and the first discharge. The charge transfer resistance (R_ct) was marked in Figure 1a and taken to be the diameters of overlapping semicircles from the Nyquist plots [3]. Cells with a small R_ct are much more desirable. Figure 2 shows summary of R_ctversus voltage for NMC/graphite and LCO/graphite pouch cells with 2 % VC. The impedance of NMC/graphite cells increases a lot as the cells are charged above 4.3 V. This impedance change is almost reversible over one cycle but the impedance slowly increases cycle by cycle. By contrast the LCO materials do not show any real impedance increase after charging to 4.4V. There are many interesting things to note which will be discussed in the lecture.

References

[1] M. Hu, X. Pang and Z. Zhou, J. Power Sources, 237, 229 (2013).

[2] S.S. Zhang, J. Power Sources, 162,1379 (2006).

[3] D.Y. Wang, N.N. Sinha, R. Petibon, J.C. Burns, and J.R. Dahn, J. Power Sources, 251, 311 (2014).

Li-ion batteries have yet to reach their full application potential due to their inherent cycling performance issues, such as voltage instability and capacity fading, while fundamental questions remain unanswered regarding the mechanisms responsible. However, Mn-based layered oxides continue to be promising candidates for cathodes in high-energy-density Li batteries given their high voltage and high discharge capacities. A significant impediment to gaining insight into these layered oxides lies in their complexity and inhomogeneity. Thus, the focus of the present contribution will be on the parent material, Li₂MnO₃, which shows cycling characteristics similar to its more complex offspring. Scanning transmission electron microscopy (STEM) and spectroscopy are used to characterize structural and electronic properties of both pristine and cycled material.

STEM-based methods are quickly becoming the most promising characterization tools for these and similar materials, owed largely to the wide-range of techniques available on advanced STEM instruments, including the direct imaging of both heavy and light elements, and both energy-dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies. Imaging modes such as high/low angle annular dark field (H/LAADF) and annular bright field (ABF) are exploited to image heavy atomic columns, strain contrast, and light atomic columns, respectively. Additionally, electron energy loss spectroscopy along with calculations based on density functional theory are used to probe the local electronic structure by monitoring the O K- and Mn L-edges, which can be used to track changes to both the O content and the Mn valence. Thus, the focus will remain on the structural and electronic evolution of the pristine layered oxide explored by combining spectroscopy and atomic-scale imaging with various in situ microscopy techniques and ex situ electrochemical cycling. Specifically, features such as the atomic ordering of Mn/Li atoms, O vacancy evolution, and Mn valence will be of particular interest.

Introduction

Lithium-rich mixed transition metal (TM) oxide positive electrode materials such as Li[Li_0.2Ni_0.2Mn_0.6]O₂ and Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ are attractive due to their high reversible capacities. However, they suffer from some problems such as high irreversible capacity loss¹, poor rate capability² and voltage fade³. There is no clear understanding in the literature how the overall metal composition of the Li-rich material affects their electrochemical properties including the above mentioned issues. Hence we embark, in this study, on studying the effect of metal composition on the electrochemical properties of the Li-rich positive electrode materials.

Experiment

A series of Ni(II)_aMn(II)_bCo(II)_cCO₃ precursors where a + b + c = 1 were made with co-precipitation synthesis using a continuously-stirring tank reactor (CSTR). Ni(II)_aMn(II)_bCo(II)_cCO₃ precursors were mixed with required amounts of Li₂CO₃and made into Li-rich positive electrode materials of the desired composition using solid-state synthesis at 900ᵒC in air. All the prepared materials were characterized by X-ray diffraction and their true densities were measured using a helium pycnometer. Coin-type cells were made from the synthesized positive electrode materials and Li metal anodes, which were then electrochemically tested under galvanostatic conditions at constant temperature, and their electrochemical properties were compared.

Results and Discussion

A Li-rich positive electrode material comprised of Li, Ni²⁺, Mn⁴⁺ and Co³⁺ can be made from a Ni(II)_aMn(II)_bCo(II)_cCO₃ precursor. By knowing the exact composition of Ni(II)_aMn(II)_bCo(II)_cCO₃ precursor, the theoretical formula of a Li-rich material can be calculated. For example, the Ni_0.25Mn_0.75 composition can be used to make Li_1.2Ni_0.2Mn_0.6O₂. Thus, the Ni-Mn-Co compositions that can be used to make all the possible Li-rich positive electrode materials were determined. Details results showing how the electrochemical behavior varies with overall metal composition will be presented.

References

1. J. H. Kim, C. W. Park, and Y. K. Sun, Solid State Ionics, 164, 43 (2003).

2. B. Xu , C. R. Fell , M. Chi , Y. S. Meng, Energy Environ. Sci.2011, 4 , 2223

3. Debasish Mohanty, Athena S. Sefat, Jianlin Li, Roberta A. Meisner, Adam J. Rondinone, E. Andrew Payzant, Daniel P. Abraham, David L. Wood, Claus Daniel, Phys.Chem.Chem.Phys., 2013, 15, 19496--19509

It has been long recognized that dissolution of transition metals, particularly Mn, at the positive electrode is the starting point for a major performance degradation mechanism in Li-ion batteries (LIBs).^1-4 We present recent X-ray absorption spectroscopy data on components from Li_xMn₂O₄ spinel – graphite (LMO-GR) cells with 1M LiPF₆in an ethylene carbonate : diethyl carbonate (1:2 v/v) electrolyte, after 20 days of cycling at 50 °C with 100% depth of discharge and C/5 rate.

Results from the Mn K edge (6539 eV) XANES analysis indicate average oxidation states near +3 for Mn cations in the graphite electrodes and separators from cells that were subjected to a 48 hour stand (in either discharged or charged state) subsequent to the high-temperature galvanostatic cycling, see Fig. 1. This suggests that Mn metal or in oxidation state +2 can only be minor fractions of the Mn existing outside the positive electrode of a Li-ion battery. Our results run counter to the prevailing view that Mn²⁺ cations arising from a disproportionation reaction of two Mn³⁺cations at the positive electrode, followed by dissolution into and migration through the electrolyte, and ending with deposition at the negative electrode, are widely responsible for the performance degradation observed in graphite-LMO LIBs. We will discuss the possible sources of discrepancy between our results and previously published data, as well as the implications of our results for measures to mitigate the Mn dissolution related LIB performance degradation mechanism, see Fig. 2.

References

1. D. H. Jang, & S. M. Oh, J. Electrochem. Soc.144, 3342 (1997).

2. Y. Xia, Y. Zhou, & M. Yoshio, J. Electrochem. Soc.144, 2593 (1997).

3. G. Amatucci, C. N. Schmuts, A. Blyr, C. Sigala, A. S. Gozdz, D. Larcher, & J.-M. Tarascon, J. Power Sources69, 11 (1997).

4. N. Kumagai, S. Komaba, Y. Kataoka, & M. Koyanagi, Chem. Lett.1154 (2000).

Acknowledgements

Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

Li-rich layered oxides Li_1+xM_1-xO₂ with M = Mn, Co and Ni may be used as positive electrode in Li-ion batteries. Mechanisms at the origin of the large capacity exhibited by these Li-excess compounds compared to that obtained for stoichiometric ones (300 mAh/g vs 200 mAh/g, respectively) have already been extensively described in literature [1-2]. These materials can be viewed as an intergrowth between [LiO₆]_∞ and [MO₆]_∞ octahedral layers. Overlithiation induces a Li₂MnO₃-type order into [MO₆]_∞slabs which facilitates oxygen ions oxidation at high potential (>4.4 V), a phenomenon responsible for the extra-capacity observed.

The average structure of these compounds is well-known but the complexity of the local structure needs further understanding. In fact, the local environments govern activation barriers and can affect notably Li ion transport. At the nanoscale, Li_1+xM_1-xO₂ can be described as LiMO₂-Li₂MnO₃nanocomposites with a complex microstructure which depends on the nominal composition. In addition, structural changes and redox processes occurring during cycling depend not only on the initial local structure but also on the cycling conditions (rate and temperature). Thus, a better understanding of the relationship between local structure and electrochemical properties is needed for improving performances of these materials.

In this work two electrodes with different starting microstructures and activated up to 4.6 V at two different temperatures were studied. The electrochemical phenomena observed for each compound during galvanostatic cycling were found to be quite different, especially in discharge.

Structural and redox mechanisms taking place during cycling were studied in detail and compared. In order to follow changes in electronic and crystal structures under battery operation, in-situ time-resolved experiments were performed at the synchrotron SOLEIL using x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) in operando mode. The evolution of microstructure (bulk and surface) during cycling was also monitored by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and x-ray photoelectron spectroscopy (XPS) in ex-situmode.

Li₂MnO₃-type order, probably responsible for extra-capacity in these materials, is detected in both starting compounds; however its evolution is found to be highly dependent on the cycling conditions. During charge, there is a progressive attenuation of the Li₂MnO₃-type order at high potential, even disappearing at the end of the first charge under severe activation conditions. Many stacking faults were observed by HRTEM, especially after activation at high temperature. In addition, spinel-type defects are visible at the edges of crystals (see figure) and are probably the cause of the voltage decrease observed during cycling. Indeed, redox reactions associated with lithium ion insertion into "special" sites, like spinel defects, may require to overcome a larger energy barrier. The analysis of the surface composition by XPS shows a surface enrichment in manganese which is in good agreement with the formation of spinel-type defects (LiMn₂O₄ vs. LiMO₂). Furthermore, we also monitored the effect of temperature and microstructure on the redox mechanisms involved during cycling, in ex-situ and operando mode. The evolutions of Mn-K, Co-K and Ni-K edges were studied by XAS. While the activation temperature has an influence on the Co-K and Ni-K edges, the influence of the microstructure is only noticeable on the Mn K-edge. Finally, in order to get information on redox processes at the nanoscale, O-K and Mn-L₂,₃ edges were studied by EELS at different states of charge. Modifications of the fine structures of these two edges were observed during the entire activation cycle and quantitative information was deduced from L₃/L₂ratios analyze.

All these results will be discussed in detail.

References:

[1] H. Koga et al., Journal of the Electrochemical Society, 160 (2013) A1

[2] H. Yu et al., the Journal of Physical Chemistry Letters, 4 (2013) 1268

Introduction

Materials such as LiNi_0.4Mn_0.4Co_0.2O₂ have been proposed as good candidates for high energy density cells. Even though these types of materials show reversible cycling up to 4.7 V, the stability of typical carbonate based electrolytes towards such high voltage proves to be a problem. Electrolyte additives can improve capacity retention at high voltage, however very little is known about how these additives work. The effect of three additive combinations on the cycling of LiNi_0.4Mn_0.4Co_0.2O₂/graphite cells cycled to different upper voltage cut-offs has been studied using electrochemical impedance spectroscopy (EIS).

Experimental

Dry pouch LiNi_0.4Mn_0.4Co_0.2O₂/graphite cells balanced for 4.7 V with a nominal capacity of 240 mAh were obtained from a reputable cell manufacturer. The pouch cells were filled with ~0.9 g of 1M LiPF₆ EC:EMC (3:7) based electrolyte with different additive combinations. The additive combinations tested were 2% vinylene carbonate (VC), 2% prop-1-ene-1,3-sultone (PES), and 2% VC + 1% Methylene Methanedisulfonate (MMDS) + 1% additive A. Cells were vacuum sealed in an argon-filled glove box. The cells then underwent a formation protocol during which they were opened and re-vacuum sealed at 3.5 V and 4.5 V to remove any gas generated during the first charge. Cells were then moved to a charger to be cycled at 40°C to various upper potential cut-offs. After a period of 400 – 500 h of cycling, the impedance of the cells was measured at 3.8 V and at 10°C.

Results and discussion

Figure 1 shows the discharge capacity as a function of time for cells containing various additive combinations and cycled to various upper voltage cut-offs. Figure 1 shows that all cells exhibit capacity loss when cycled to potentials above 4.4V. Cells containing 2% VC + 1% MMDS + 1% A have encouraging cycling performance at both 4.4 and 4.5 V. Figure 1 also shows that cells containing any of the additive combinations show substantial capacity fade when cycled at to 4.5 V and above.

Figure 2 shows the Nyquist representation of the impedance of LiNi_0.4Mn_0.4Co_0.2O₂/graphite cells cycled for 400 - 500 h to different upper voltage cut-offs. Figure 2 shows that the additives have a dramatic impact on the impedance of cells cycled to high voltage. Figure 2 shows that the additive combination, 2% VC + 1% MMDS + 1% A, yields a very small impedance compared to other more conventional additives such as 2% VC. Figure 2 also shows that the shape of the EIS spectrum of cells containing any additive combination dramatically changes when increasing the higher voltage cut-off. Other measurements (not shown) showed that the EIS spectra of cells cycled to upper cut-offs of 4.0 V to 4.3 V are very similar. This seems to indicate the activation of various surface reactions when increasing the upper voltage to 4.4 V and above. These surface reactions seem to be dramatically hindered when the electrolyte additive ,2% VC + 1% MMDS + 1% A is used

Conclusion

The effect of different additive combinations on the cycling performance of LiNi_0.4Mn_0.4Co_0.2O₂/graphite cells cycled to various upper voltage cut-offs and up to a maximum of 4.7 V has been evaluated. Results showed that 2% VC which is a very useful additive in 4.2 V cells yields very poor cycling performance for high voltage applications. Other additives combinations such as 2% VC + 1% MMDS + 1% A yield dramatically improved performance. This study also showed that the impedance spectra of cells cycled to 4.4 V and above dramatically change as the upper cutoff potential is increased..

Reference

1. J. C. Burns et al., J. Electrochem. Soc., 160, A1668–A1674 (2013).

Introduction

LiNi_0.5Mn_1.5O₄ (LNMO) spinel is a promising cathode material with high voltage for lithium ion batteries because of its high energy density. However capacity of LNMO/graphite cell fades during cycles and high temperature storage. The capacity fading of LNMO cell was explained by transition metal dissolution and active lithium ion loss through continuous solid-electrolyte interface (SEI) formation prompted by transition metal on the graphite surface [1]. Many additives which make stable SEI on the electrode surface were proposed to avoid the capacity fading [2-5].

In this work, we investigated vinyl cyclosiloxane derivatives as additives to improve the durability of LNMO/graphite batteries with high voltage.

Experimental

Electrolyte additives were evaluated with coin cell of LNMO/graphite. Fig. 1 shows the chemical structure of 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (MVCTS) as typical vinyl cyclosiloxane. As a base electrolyte, 1M LiPF₆ was dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (3/4/3 by volume). The coin cells were cycled at 1 C rate between 3.5 V and 4.9 V at 60°C. The electrodes after cycling were analyzed by X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Results and discussion

The cycle performances of LNMO/graphite cell with and without 0.5 wt% MVCTSare shown in Fig. 2. The addition of MVCTS to the electrolyte is clear to improve cycling characteristics. The capacity retention is 50% for the cell containing MVCTS after 100 cycles at 60°C, while that is 41% for the cell without MVCTS. The analogues of vinyl cyclosiloxane such as alkyl substituted cyclosiloxane and vinyl substituted disiloxanes were also investigated as additives to reveal the influence of vinyl group and cyclosiloxane skeleton of MVCTS. From the results, it was shown that only cyclosiloxane having vinyl group lead to improvement of cycle performance.

XPS, EIS and ICP-AES measurement were performed for the electrodes after cycling to make clear the mechanism of improvement of cycle performance with MVCTS. To determine the loss of active lithium ions at the anode, we measured ICP-AES of discharged anode after cycling. The loss of active lithium ions of the electrolyte containing MVCTS is lower than that of the electrolyte without additives. XPS results show that Si, Mn and Ni exist on the surface of graphite. As shown in Fig. 3, one additional Ni 2p peak appears for the anode recovered from the battery with MVCTS. These results suggest that the additives with combination of vinyl group and cyclosiloxane form suitable SEI to suppress the capacity fading by decreasing the reactivity of transition metal on top of the graphite anode surface.

References

[1] J. -H. Kim, N. P. W. Pieczonka, Z. Li, Y. Wu, S. Harris, B. R. Powell, Electrochim. Acta, 90 (2013) 556-562

[2] H. Lee, S. Choi, S. Choi, H. -J. Kim, Y. Choi, S. Yoon, J. -J. Cho, Electrochem. Commun., 9 (2007) 801-806

[3] A. von Cresce, K. Xu, J. Electrochem. Soc., 158 (2011) A337-A342

[4] S. Dalavi, M. Xu, B. Knight, B. L. Lucht, Electrochem. Solid-State Lett., 15 (2012) A28-A31

[5] V. Tarnopolskiy, J. Kalhoff, M. Nádherná, D. Bresser, L. Picard, F. Fabre, M. Rey, S. Passerini, J. Power Sources, 236 (2013) 39-46

Conventional Li-ion batteries containing flammable and volatile liquid electrolytes are challenged to deliver safe and stable performance for practical energy storage applications. Alternatively, solid polymer electrolytes (SPEs), which intrinsically eliminate the use of organic solvents, display good mechanical stability and flexibility against physical deformation of battery pack and volume retention of electrode materials. The use of SPEs with high thermal and electrochemical stability would contribute to the design and manufacture of safe and cost-effective Li-ion batteries, especially for large-scale applications under elevated temperatures [1].

Host materials for most SPEs have yet been dominated by polyethers with ethylene oxide (EO) and propylene oxide (PO) as repeating units. As an interesting replacement to the low-molecular-weight cyclic and linear carbonates, a new class of polymer hosts based on polycarbonates has been considered as Li⁺-conducting electrolytes [2]. The incorporation of polar carbonate units in the polymer host could effectively facilitate the separation of ion clusters and allow high salt doping in SPEs.

In this work, we have developed and explored poly(trimethylene carbonate) (PTMC) as a polymer host alternative for all solid-state Li-ion batteries. High-molecular-weight PTMC was synthesized via bulk ring-opening polymerization. Electrochemical and thermal properties of the PTMC/LiTFSI complexes with varied salt concentration were systematically investigated using electrochemical impedance spectroscopy (EIS) and thermal analysis (DSC/TGA). The as-prepared SPEs are amorphous materials and thermally stable up to 200 °C. The best-performing electrolyte could display useful ionic conductivity at elevated temperatures and was also found electrochemically stable towards 5.0 V vs. Li⁺/Li. The cycling performance of Li | SPE | LiFePO₄ half cells has been tested at elevated temperatures and the cells display long-term cycling stability with retained high capacity up to 153 mAh/g. The gradual increase in capacity might be due to improved interfacial contacts during cycling and storage [3].

Acknowledgements

This work has been supported by the STandUP for Energy project and KIC InnoEnergy.

References

[1] B. Scrosati, J. Garche, J. Power Sources, 195 (2010) 2419.

[2] a) Smith, M. J. et al. Solid State Ionics, 140 (2001) 345; b) Silva, M. M. et al. J. Power Sources, 111 (2002) 52; c) Silva, M. M. et al. Electrochim. Acta, 49 (2004) 1887; d) Silva, M. M. et al. Solid State Sci., 8 (2006) 1318; e) Barbosa, P. C. et al. Solid State Ionics, 193 (2011) 39.

[3] B. Sun, J. Mindemark, K. Edström, D. Brandell, Solid State Ionics (2013), DOI: 10.1016/j.ssi.2013.08.014.

Lithium-ion batteries (LIBs) are widely used energy storage systems in the sector of electric devices and are already utilized in electric vehicles (EVs) and hybrid electric vehicles (HEVs). Especially in the latter case, safety plays a crucial role. For this reason, in the last decades, extensive efforts have been made to reduce the safety concerns related to the use of organic carbonate-based liquid electrolytes. State-of-the-art liquid electrolytes show a high conductivity and good electrochemical performance but they also display low flash and boiling points and are prone to leakage. As alternative solid polymer electrolytes (SPEs)¹ were introduced. Consisting of a polymer matrix containing a lithium salt without solvents, these electrolytes avoid the danger of leakage and show a high mechanical stability, which allows their simultaneous use as separator. However, they display a low conductivity at room temperature.

A possibility to combine the advantages of the two above-mentioned systems is the use of gel polymer electrolytes (GPEs)². Consisting of a polymer matrix in which the liquid electrolyte is immobilized, GPEs show conductivities in the mS cm^-1 range while having a sufficient mechanical stability to work as separator. Furthermore, in the absence of "free solvent" the danger of leakage can be avoided.

In this paper, we report on GPEs based on non-commercial amphiphilic block copolymers with a norbornene backbone. The use of monomers with different side chains and a living polymerization method (ring opening metathesis polymerization, ROMP)³ allow the tailoring of the properties of the polymer matrix according to the desired application. Additionally, the obtained polymers show a low PDI. When gelled with the liquid electrolyte 1 M LiPF₆ in EC:DMC 1:1 (w:w) the resulting GPEs display conductivities up to 2.5 mS cm ^-1 and a broad electrochemical stability window comparable to that of the liquid electrolytes. Within the investigations of the GPEs, particular attention was laid on the interactions between the lithium ions and the other components of the GPE system as well as on the lithium ion transport properties. For these investigations Raman spectroscopy⁴ and pulsed field gradient (PFG)-NMR were used⁵. Besides the influence of the side chains of diblock copolymer-based host systems, the difference between GPEs based on diblock copolymers and triblock copolymers comprising an additional block containing cyclic carbonate moieties was studied.

1. Allcock, H.; Prange, R.; Hartle, T., Poly(phosphazene−ethylene oxide) Di- and Triblock Copolymers as Solid Polymer Electrolytes. Macromolecules 2001,34 (16), 5463-5470.

2. Isken, P.; Winter, M.; Passerini, S.; Lex-Balducci, A., Methacrylate based gel polymer electrolyte for lithium-ion batteries. J Power Sources 2013,225 (0), 157-162.

3. Bielawski, C.; Bielawski, R. H.; Grubbs, C., Living ring-opening metathesis polymerization. Prog Polym Sci 2007,32 (1), 1-29.

4. Edman, L., Ion association and ion solvation effects at the crystalline-amorphous phase transition in PEO-LiTFSI. J Phys Chem B 2000,104 (31), 7254-7258.

5. Adebahr, J.; Forsyth, M.; MacFarlane, D. R.; Gavelin, P.; Jacobsson, P., Li-7 NMR measurements of polymer gel electrolytes. Solid State Ionics 2002,147 (3-4), 303-307.

Introduction

Electrolyte additives can be used to improve the lifetime of a Li-ion cell [1]. Electrolyte additives are believed to function by forming or modifying a solid electrolyte interface (SEI) layer on the surface of the positive or negative electrode thus impacting the cycle life, calendar life and safety of Li-ion cells.

Ethylene sulfite (ES) has been widely studied by many researchers and has been regarded as an effective SEI-forming additive, especially in PC based electrolytes. Wrodnigg et al. [2] found the introduction of 5 vol % ES to a PC-based electrolyte could suppress or even prevent PC co-intercalation into graphite. Ota et al [3] suggested that when ES was used as an electrolyte additive, the SEI film on the graphite anode contained both inorganic materials like Li₂SO₃ and organic materials like ROSO₂Li. The bulk of the studies on ES have focused upon its effect on the carbon electrode.

In this presentation, a detailed study of ES and/or VC as electrolyte additives for Li[Ni_1/3Mn_1/3Co_1/3]O₂/graphite pouch cells was investigated using UHPC [4] and a precision storage system at Dalhousie University [5]. Gas evolution during formation and cycling, coulombic efficiency (CE) and charge endpoint capacity slippage during cycling as well as charge transfer resistance before and after cycling were examined and compared.

Experimental

Dry Li[Ni_1/3Mn_1/3Co_1/3]O₂ (NMC)/graphite pouch cells (225 mAh) were obtained from Whenergy (Shandong, China). Before electrolyte filling, the cells were cut just below the heat seal and dried at 80°C under vacuum for 14 h to remove any residual water. Then the cells were transferred immediately to an argon-filled glove box for filling and vacuum sealing.

Cells cycled using the UHPC were tested between 2.8 and 4.2 V at 40.0 ± 0.05 °C using currents corresponding to C/20 for 15 cycles where comparisons were made. Electrochemical impedance spectroscopy (EIS) measurements were conducted on NMC/graphite pouch cells before and after cycling on the UHPC.

Results and discussion

Figure 1 shows data collected from the UHPC during cycling. Figure 1 shows cells containing 2% VC + ES (1 % or 2 %) can provide similar performance in delta V, coulombic efficiency and charge endpoint capacity slippage to cells containing 2% VC.

Figure 2 shows the Nyquist plots for NMC/graphite pouch cells with different amounts of ES and/or VC after formation and after UHPC cycling measured at 3.80 V and 10°C. Figure 2 shows that cells containing only ES show obvious impedance growth during cycling. When ES (1% or 2%) is used in combination with VC, the impedance was dramatically decreased both before and after cycling. Cells with 2% VC + 2% ES have the lowest impedance after cycling, only half of that of cells with 2% VC. Therefore, there appear to be significant benefits of the combination of VC and ES for high power cells.

References

1. S. S. Zhang, J. Power Sources, 162, 1379 (2006).

2. G. H. Wrodnigg, J. O. Besenhard and M. Winter, J. Electrochem. Soc.,146, 470 (1999).

3. H. Ota, T. Akai, H. Namita, S. Yamaguchi and M. Nomura, J. Power Sources, 119-121, 567 (2011).

4. T. M. Bond, J.C. Burns, D.A. Stevens, H.M. Dahn, and J.R. Dahn, J. Electrochem. Soc., 160, A521 (2013).

5. N. N. Sinha, T. H. Marks, H. M. Dahn, A. J. Smith, D. J. Coyle, J. J. Dahn and J. R. Dahn, J. Electrochem. Soc.,159, A1672 (2012).

One of the major concerns in lithium ion battery is the composition of the electrolytes as it governs some of the vital physicochemical properties of the electrolytes such as viscosity, ionic conductivity, thermal stability and wettability. Most important properties of lithium ion batteries are related to the properties of the solid electrolyte interface (SEI) layer formed on the electrode surface. The SEI in turn is mainly dependent on the electrolyte composition [1]. Therefore, understanding the nature, mechanism and property of the SEI formation in different electrolyte composition is crucial in selecting electrolytes. In this work, we report a direct comparison of the reduction of propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC) as a single, binary and ternary solvent systems. Battery performance test, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscope (SEM) is employed to study the nature of the surface films formed.

The reduction products are determined by using ex-situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), in the glove box, after employing linear sweep voltammetry (LSV) to certain potential regions. EC/PC/DEC based electrolyte shows better capacity retention and less impedance than the EC/PC system. In all systems, the FTIR results indicate the formation of (CH₃CH₂OCO₂Li)₂ and Li₂CO₃ due to the reduction of PC. EC is reduced to (CH₂OCO₂Li)₂ and Li₂CO₃ in both the binary and ternary solvent system. The FTIR result at the potential where DEC reduction occurs shows the formation of (CH₂=CH-OCO₂CH₂CH₃)Li which is less stable and forms a less-passivating layer in the first cycle. However, this product can further react with another species in the subsequent cycles and form a stable SEI.

[1] A.J. Gmitter, J. Gural, G.G. Amatucci, J. Power Sources, 217 (2012) 21-28.

[2] K. Xu, J. Electrochem. Soc., 156 (2009) A751-A755.

[3] G.V. Zhuang, H. Yang, B. Blizanac, P.N. Ross, Electrochem. Solid-State Lett, 8 (2005) A441-A445.

Introduction

Solid state batteries offer increased safety, high voltage stability, and simpler electrolyte chemistry than organic electrolytes. Their inherent protection from dendrite growth gives the capability to use metallic lithium anodes. Recent advances have improved conductivity to within an order of magnitude of organic electrolytes at room temperature. However, interfacial resistance between electrodes and the electrolyte remains a pressing concern. In a typical planar battery, this high resistance per area is magnified by low interfacial surface area.

Elemental composition has a significant effect on the conductivity of a garnet electrolyte. The conductivity of the garnet fluctuates an order of magnitude based on a lithium stoichiometry of 6 to 7 in the Li_7-xLa₃Zr_2-xTa_xO₁₂series. It has also been demonstrated that aluminium composition at the grain boundaries associated with higher temperature sintering increases conductivity. It stands to reason lithium activity extremes at the electrode interfaces effects would cause local variations in electrolyte composition and affect the conductivity.

This study aims to address some of these concerns using a systematic study of the anode-electrolyte interface and the cathode-electrolyte interface. We investigate the elemental composition of the interface, as well as the effect of greatly increased surface area through a highly microporous electrolyte scaffold containing the cathode and anode material.

Experimental Procedure

Li₇La₃Zr₂O₁₂ garnet was produced in a conventional solid state method. 10% excess LiOH-H₂O and stoichiometric amounts of La₂O₃ and ZrO₂were ball milled in ethanol for 24 hours. The milled materials were added to an alumina crucible, calcined at 900 ºC for 4 hours, and milled again.

A colloidal deposition of this slurry is prepared with polyvinyl butyral (PVB), benzyl butyl phthalate (BBP), and Solsperse dispersant in ethanol.

Calcined powder is added to toluene and ethanol solvents with a binder system containing PVB, BBP, and menhaden fish oil as a dispersant. This preparation is milled for two days, then degassed in preparation for tape-casting.

After tape-casting, tapes are drop-coated with the colloidal slurry and another tape is applied on top. This structure is held at 500ºC for 1 hour to burn off binders then heated to 1100 ºC for 1 hour. This produces a three layer porous-dense-porous scaffold of electrolyte that will maintain electronic isolation between electrodes.

Symmetric cells with two cathodes are produced by infiltrating a glycine/metal-nitrate solution into the pores of each side and combusting. For two-anode symmetric cells, lithium is lightly pressed into both sides at 350 ºC.

Electrochemical impedance spectroscopy was performed between room temperature and 300 ºC using a 10 mV amplitude between 10 MHz and 100 mHz.

Electron energy loss spectroscopy was performed on a 2100F TEM equipped with a Gatan Tridiem post-it column energy filter.

Results

Figure 1 shows a cross section of a lithium-penetrated Li-garnet scaffold showing the fracture plane of the garnet(light) fully coated by the lithium (dark). Figure 2 shows the Nyquist plot of a full cell made with a lithium-penetrated scaffold. The bulk conductivity shown in the inset is fairly low at 20 Ω compared to the 1000 Ω overall cell impedance.

The ceramic solid-electrolyte Li₇La₃Zr₂O₁₂ (LLZO) with a garnet crystal structure has developed as a promising solid electrolyte material. Positive properties: high ionic conductivity (4x10^-4 Scm^-1at 25ºC) [1], electrochemical stability with Li metal, and thermal and chemical stability. However, the conductivity of this material remains about two orders of magnitude lower than that of a common liquid organic electrolyte. The main disadvantage is related with the loss of lithium produced during the sintering process up to 1200ºC to obtain the cubic structure, responsible of produce the high conductivity. From this point of view, using a solution process for the development of this kind of electrolyte materials is a very attractive alternative in order to obtain lower thermal treatment. In this work, we explore the use of sol-gel process to obtain Lithium garnet-type oxides Li₇La₃(Zr_2−X, Nb_X)O₁₂(LLZNbO, X=0–1) at low temperatures. The partial substitution of Nb in the typical LLZO improves the ion conductivity based in their dependence on the lattice parameter [2].

The sol-gel synthesis was developed using lithium nitrate, lanthanum nitrate, zirconium butoxide and niobium ethoxide as precursors. Ethylacetoacetate was used as a stabilizing agent for the alkoxides and ethanol as solvent. Firstly, the lithium and lanthanum salts were dissolved in ethanol. Separately, the zirconium and niobium alkoxides were reacted with stabilizing agent. The molar relation between Zr/Nb and stabilizing agent was adjusted in order to prevent the fast reaction with the Li/La solution. After 1 hour, the solutions were mixed and then, the final solution was stirred at 25ºC during 1 hour until a white gel was obtained. The gel was dried at 80ºC for 24 hours. Further, the powder was ground and calcined at 700ºC for 5 hours. The calcined powders were attrition milled with 4 mm diameter ZrO₂balls in a toluene media at 300 RPM for 6 hours. The powder were pressed into pellets and sintered at 900ºC for 10 hours.

The powders obtained after calcination showed wide particle size distribution between few microns until ~10µm, and some agglomerates were observed with sizes of ~20µm. The incorporation of niobium in the pristine LLZO produces bigger particles with regular shape; the effect was more evident with higher concentration of niobium, as shown in Figure 1. Pellet using unmilled powder produce materials with low relative density, 45% and 55% for LLZO and LLZNbO, respectively. The reduction of the particle size using ZrO₂ ball milling resulted in an increase of the relative density upto 75% for Li₇La₃(Zr_1.75, Nb_0.25)O₁₂. Figure 2A shows the small and regular particle shape obtained for the LLZO after milling (compare Fig.1A). However, the individual particle observed after sintering is an evidence of lack of pellet densification. Large particles and open pores were observed in the surface view, Figure 3A. On the other hand, the incorporation of Nb in the garnet crystal structure produces the binding of the particles, Figure 2B-D. Figure 2B displays the good sintering of the sample Li₇La₃(Zr_1.75, Nb_0.25)O₁₂ with low presence of pores. Sintering is also verified on the surface of the pellet, Figure 3B, where the presence of individual particles was minimum. Higher Nb doped concentration presented low relative density (~60%). XRD patterns of undoped LLZO displayed tetragonal phase, while all LLZNbO compositions showed cubic phase and a small peak assigned to La₂Zr₂O₇ was observed in Li₇La₃(Zr_1.75, Nb_0.25)O₁₂. The Lattice parameter varied from 12.9463Å to 12.9203Å with the increase of Nb concentration (12.9682Å for LLZO sintered at 1230ºC[1]). The conductivity obtained was in the order of 10^-6 to 10 ^-7S/cm at 50ºC.

[1] R.Murugan, V.Thangadurai, W.Weppner, Angew. Chem. 2007,119, 7925– 7928

[2] H.Imagawa, S.Ohta, Y.Kihira, T.Asaoka, Solid.State.Ionics 2013, http://dx.doi.org/10.1016/j.ssi.2013.10.059

Among the many challenges to enable large scale commercialization of Li-ion batteries in plug-in hybrid electric vehicles (PHEVs) and full electric vehicles (EVs), formation of high quality solid electrolyte interphase (SEI) is one critical issue to achieve long term battery durability and safety. As a nano-meter scale thin film which natively forms on the negative electrode due to electrolyte reduction, SEI is a complex system composed of various lithium salts and organic reduction products. Most published works have focused on identifying the SEI composition, but the understanding of correlation between SEI properties and battery performance is still lacking.

Previously, we have identified that SEI impedance is closely related with its composition. Briefly, the inorganic components (e.g. Li₂O and Li₂CO₃, etc) constitute a low impedance and less porous part of SEI, while the organic reduction products tend to be more porous and resistive.¹ In this talk, we will advance the findings and evaluate characteristic Li salts using isotope exchange method. Typical SEI components, e.g. Li₂CO₃, LiF, etc. are found to behave differently in passing Li. With these understandings, we will also demonstrate that the SEI composition can be modified by surface functionalization and coating on graphite electrode. By designing the SEI chemistry, the electrode/electrolyte interface is more stable and the cycling efficiency can be improved.

Figure. Isotope profiles of three Li salt thin films after soaked in 1M ⁶LiClO4 (DMC) solution for 30s. The natural abundance of ⁶Li:⁷Li is ~0.08. The different isotope ratios in the films indicate that SEI components may behave differently in passing Li.

Reference 1. Lu, P., Li, C., Schneider, E. W., Harris, S. J., J. Phys. Chem. C, 2014, 118, 896-903

In the organic electrolyte for Li-ion batteries, mixed solvents containing a high-permittivity solvent such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) are essential to form a favorable solid-electrolyte interface (SEI) layer. The mixed solvents should also contain 30 – 50 vol.% high-permittivity solvents in order to obtain high ionic conductivity by sufficient dissociation of Li salts such as LiPF₆ and LiBF₄. On the other hand, lithium bis(fluorosulfonyl)imide (LiFSI) dissolves easily in a low-permittivity solvent such as dimethyl carbonate (DMC) because LiFSI has very weak interaction between Li⁺ and FSI^-. In the FSI-based systems, therefore, the role of high-permittivity solvents becomes SEI formation only. In this study, we prepared LiFSI-based organic electrolytes with low EC content and found that the Li-ion cells assembled with our electrolytes exhibit excellent charge and discharge rate performance by reduction of charge-transfer resistance associated with a solvation state of EC to Li⁺.

1.0 mol dm^-3 LiFSI dissolved in mixed solvents of EC with DMC (1:9 v/v) was prepared and used as the electrolyte with low EC content (denoted by LiFSI/EC:DMC = 1:9), while 1.0 mol dm^-3 LiFSI or LiPF₆ dissolved in mixed solvents of EC and DMC (1:1 v/v) were used as reference electrolyte (denoted by LiFSI/EC:DMC = 1:1 and LiPF₆/EC:DMC = 1:1, respectively). Graphite or LiNi_1/3Mn_1/3Co_1/3O₂ composites containing conductive additive and binder were used as negative and positive electrodes, respectively. The respective mass of active materials in the negative and positive electrodes were ca. 4.5 and 9.5 mg cm^-2. We assembled Li/LiNi_1/3Mn_1/3Co_1/3O₂, Li/graphite and graphite/LiNi_1/3Mn_1/3Co_1/3O₂ cells with each electrolyte and evaluated their battery performances.

Table 1 shows the ionic conductivities and viscosities of each electrolyte. LiFSI/EC:DMC = 1:1 shows higher ionic conductivity and slightly lower viscosity than those of the LiPF₆/EC:DMC = 1:1. LiFSI/EC:DMC = 1:9 exhibits the lowest ionic conductivity among the tested electrolytes probably because EC content is extremely small in this electrolyte. Nevertheless, the conductivity is still sufficiently high. These characteristics should be ascribed from excellent dissociation ability of LiFSI in spite of low-permittivity conditions.

Figure 1 shows the charge-discharge rate performances of Li/graphite cells assembled with each electrolyte. From the ionic conductivities of each electrolyte, it can be expected that the Li/graphite cell containing LiFSI/EC:DMC = 1:1 shows the highest rate performance. In spite of that speculation, however, it is surprising that the Li/graphite cell containing LiFSI/EC:DMC = 1:9 which has the lowest ionic conductivity shows the highest rate performance. In the graphite/LiNi_1/3Mn_1/3Co_1/3O₂ cell systems, LiFSI/EC:DMC = 1:9 shows the highest rate performance , which is similar to the Li/graphite cell system. In order to explain these unexpected results, we carried out AC impedance measurement of Li/graphite cells containing each electrolyte. As a result, the Li/graphite cell containing LiFSI/EC:DMC = 1:9 has significantly smaller charge-transfer resistance of the electrode/electrolyte interface when compared to cells containing the other electrolytes. Furthermore, we found that the reduction of charge-transfer resistance is attributed to a specific solvation state of EC to Li⁺. Raman spectra of each electrolyte show that the solvation number of EC on Li⁺ reduces significantly in LiFSI/EC:DMC = 1:9. From these results, we consider that the rate performances of the cells containing LiFSI/EC:DMC = 1:9 are improved by the low-energy desolvation process at the Li⁺ insertion into the electrode by the reduction of the solvation number of EC on Li⁺.

The potential tunability for Fe³⁺/Fe²⁺ redox couple at unusually high voltage region of 3.5 – 4.1 V will be reviewed based on the new materials discovered in my group over the past 3 years. Overall energetics will be given by decomposing the thermodynamic definition into electronic and structural factors, leading to the guidelines to develop new high voltage cathode materials. In searching new materials, as often missed by the ground state screening, experimental identification of metastable functional phase by entropy gain (including disorder and/or defects) at finite temperature is important and effective. Very promising new cathode materials led by this concept will be introduced as time permits.

LiMPO₄ (M=Fe, Co) materials are known as possible high voltage materials leading to high energy density batteries. LiFePO4 is electrochemically active below 4V vs. Li/Li⁺ with a theoretical capacity of 170 mAh/g, and it is well known for its good stability upon cycling [1]. Conversely, LiCoPO₄, which works at higher potential (4.85 V), exhibits poor performance on cycling in common alkyl carbonate electrolytes [2]. In the literature, two hypotheses can be found which try to explain the low stability of LiCoPO₄: the Li salt is corrosive for the material [3] and/or the electrolyte oxidation at high voltage plays a role. Tsiouvaras et. al. [4] have shown that CO₂ is the main gas evolved by the anodic oxidation of water free alkyl carbonate based electrolytes (e.g. PC) at high electrode potentials (≥ 4.5 V vs. Li/Li⁺). Moreover, a recent study by our group [5], has demonstrated that CO₂ is already evolved at lower potentials if the electrolyte contains traces of water.

In this work we will investigate the evolution of CO₂ at high electrode potentials (≥ 4.5 V vs. Li/Li⁺) for LiFePO₄/C and LiCoPO₄/C by on-line electrochemical mass spectrometry (OEMS). It can be due to the electrochemical oxidation of the electrolyte and/or the carbon, according to the well-known reaction:

C + 2H₂O → CO₂ + 4H⁺ + 4e^- (1)

Both materials are synthetized in house by a solid state route for LiCoPO₄ and a solvothermal route for LiFePO₄.

Firstly, a preliminary experiment on LiFePO₄/C is done in order to study the stability of the carbon coating of LiFePO₄ at high potentials. Glucose ¹³C is used as carbon precursor in the LiFePO₄ synthesis to obtain an isotopically labelled carbon coating. In this case, the signal from the electrolyte decomposition and from the carbon of the electrode can be deconvoluted. By adding a defined amount of water (4000 ppm) to the electrolyte one can mimic the effect of trace water that could unintentionally be introduced to the cell through the active materials.

Secondly, in order to compare the stability of carbon coating from LiFePO₄ and LiCoPO₄, isotopically labeled ¹³C electrolyte (1M LiPF₆ in labelled DMC) is used to deconvolute the formation of CO₂ from the carbon and the electrolyte. To distinguish carbon from the electrode and from the coating of particles, the electrode is designed with ¹³C carbon instead of the common Super P (Timcal) as conductive additive. The potential limit of the charge is fixed to 5.5 V to allow comparison on carbon coating stability between both, LiCoPO₄ and LiFePO₄.

References:

[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough – J. Electrochem. Soc. 144, 1188 (1997)

[2] N. Bramnik, K. Bramnik, T. Buhrmester, C. Baehtz, H. Ehrenberg, H. Fuess - J. Solid State Electrochem. 8, 558 (2004).

[3] E. Markevich, R. Sharabi, H. Gottlieb, V. Borgel, K. Fridman, G. Salitra, D. Aurbach, G. Semrau, M. A. Schmidt, N. Schall, C. Bruenig – Electrochem. Comm. 15 (2012) 22.

[4] N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, J. Electrochem. Soc., 160, A471 (2013).

[5] R. Bernhard, S. Meini, and H. A. Gasteiger, J. Electrochem. Soc., 161, A497 (2014).

Acknowledgements:

This work is financially supported by BMW AG.

Introduction

Yao et al. invented "relaxation analysis" to make transition of electrode materials from kinetically preferred state to equilibrium state clear. Olivine-type cathode materials are included in it^1-3. These materials are promising, having both large theoretical capacity and fine stability⁴.

Previously, Park et al. found that the amount of LiFePO₄ decreased and that of FePO₄ increased at the relaxation process after the termination of lithium insertion by use of relaxation analysis². They considered that the LiFePO₄ including lithium defects preferable for Li diffusion formed during lithium insertion process and that the defective LiFePO₄ separated to LiFePO₄ without defects and FePO₄ at the relaxation process.

Synchrotron X-ray absorption near edge structure (XANES) method is a powerful tool to investigate the structural/electronic properties. XANES enable to measure the kinetically-preferred state directly during a reaction. In this study, we measured the in situ Fe K-edge XANES of LiFePO₄ cathode during lithium insertion for the purpose of investigating the kinetically-preferred structure and comparing the result with that of relaxation analysis obtained by using the XRD-Rietveld method².

Experimental

Cathode was prepared by mixing LiFePO₄, carbon black, and polyvinylidenefluoride with a weight ratio of 70:15:15 in N-methlpyrrolidone solution. The slurry was spread onto aluminum foil current collector and dried in a 120°C vacuum oven. The cathode and lithium metal anode were separated by a polypropylene membrane separator. LiPF₆ (1 M) in ethylene carbonate/diethyl carbonate (EC/DEC) (3:7 v/v) was used as the electrolyte. The 2032-type coin cell with X-Ray window was assembled in an argon-filled glove box. We first charged the cell to 4.0 V at a rate of 0.19 C and took 12 minutes for rest when the voltage got to 4.0 V. We subsequently discharged the cells at a rate of 0.32 C to 2.65 V.

In situ Fe K-edge XANES measurement was performed in transmission mode at beamline 5S1 of Aichi Synchrotron Radiation Center. We took 3 minutes measurement and 1 minute interval per scan; the scan range was 150 eV before the Fe K absorption edge and 1000 eV after that. We merged three continuous patterns into one pattern to decrease noise and to improve the analysis precision. The composition x in Li_(1-x)FePO₄ of each merged pattern was calculated by integrating the current. We carried out a two-components analysis based on the linear combination of the last merged pattern, called Li-rich phase hereafter, and the merged pattern during the rest time before lithium insertion, called Li-lean phase hereafter, for either end by using Athena⁵program.

Results and Discussion

The composition of Li-lean phase and Li-rich phase were calculated as Li_0.09FePO₄ and Li_0.85FePO₄ respectively by using the integrating current. Figure 1 shows an example of two-components analysis for the composition of x = 0.28 in Li_(1-x)FePO₄ . The XANES pattern was well fitted with a low χ²value of 0.15%. It was found that the relative amount of Li-rich phase from two components analysis, 89.6%, was larger than the amount, 82.9%, calculated from the composition x in Li_(1-x)FePO₄. The large amount of Li-rich phase out of linear relationship may indicate that Li-rich phase with lithium defects formed in large amount during lithium insertion process. This is consistent with the result of the "Relaxation Analysis"²using the XRD-Rietveld method.

[1] S. Park, M. Oda and T. Yao, Solid State Ionics 203(2011) 29-32.

[2] S. Park, K. Kameyama and T. Yao, Electrochem. solid-state lett. 15(2012) A49-A52

[3] Y. Satou, S. Komine, S. Park and T. Yao, Solid State Ionics, In Press

[4] A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrchem. Soc. 144(1997) 1188-1194

[5] B. Ravel and M. Newville, J. Synchrotron Rad. 12(2005) 537-541

LiFePO₄ is the most promising cathode material, especially for electric vehicle (EV) because of its inexpensive cost, superior thermal and structural safety, and non-toxicity. Although LiFePO₄ has been believed to be poor conductor due to its low electronic conductivity and 1D lithium diffusion, nano-sized LiFePO₄ shows the fastest electrochemical reaction during charge and discharge.

To understand these intriguing behaviors, we have to consider delithiation behavior of particles in the electrode which consists of 10¹⁰ ~ 10¹⁷ particles in addition to the transport properties in the material because a particle has a non-monotonic chemical potential^[1]. The non-monotonic chemical potential of single particle in multi-particle electrode leads to characteristic electrochemical behaviors such as a flat potential in voltage curve of the electrode and a sequential phase transformation between particles. Thus, delithiation behaviors in a particle in the electrode should be understood.

In this study, we will explore delithiation behaviors of LiFePO₄ with broad particle size distribution. For this purpose, chemical delithiation and followed centrifuge method were applied to understand the delithiation behavior of LiFePO₄ depending on particle size. This simple method shows phase transformation behaviors more directly. We synthesized LiFePO₄ with broad particle size distribution by solid state reaction. And then, we can easily separate chemically delithiated small and large particles by centrifuge method. In this talk, we will introduce simple centrifuge method and will discuss about delithiation behavior of LiFePO₄ with broad particle size distribution during chemical delithiation.

[1] W. Dreyer et al 2010 The thermodynamic origin of hysteresis in insertion batteries Nature Mat. 9, 448.

Rechargeable lithium ion batteries are recently installed in hybrid electric vehicle (HEV) and electric vehicle (EV). Polyanionic cathodes such as olivine type LiCoPO₄, LiNiPO₄ and Li₂CoPO₄F have attracted much attention as high voltage cathodes for high energy batteries and high power batteries. Especially LiCoPO₄ shows high redox potential 4.8 V vs. Li/Li⁺ and a high theoretical capacity 170 mAh/g^[1]. However this material has poor cycle performance. It was reported that the capacity decreases to 50% of initial discharge capacity during an initial few dozen cycles^[2]. Therefore it is necessary to improve the cycle performance of LiCoPO₄ for commercial applications. We analyzed the bulk structure of LiCoPO₄ before and after the charge-discharge process to consider the mechanism of capacity degradation.

LiCoPO₄ was prepared by electrostatic spray deposition (ESD) method. ESD method is a thin film preparation method. Fine spray of a precursor solution is generated and deposited onto the heated substrates. High voltage around 15 kV are applied between spray nozzle and the substrate during deposition. We prepared precursor solution to dissolve lithium ethoxide, cobalt nitrate and phospholic acid into the mixed solution of ethanol and n-butyl carbitol. The deposition samples on Pt foil were annealed at 600 ºC in air to increase the crystallinity of LiCoPO₄.

LiCoPO₄ thin film consisted of 300 nm particles was fabricated by ESD method. Thin film obtained was measured by XRD. From the results of XRD, the profiles were in good agreement with the orthorhombic, olivine-like structure. Fig. 1 shows the charge-discharge profiles of the LiCoPO₄ thin films. The charge profiles give 2 plateaus at around 4.77 and 4.86 V respectively. 2 plateaus also appear the 4.8 and 4.7 V respectively during discharge. This sample shows low coulomb efficiency for initial few cycles because of electrolyte decomposition in the charge. Discharge capacity decreased during the charge-discharge cycles. TEM observation was performed in order to consider the cause of this capacity degradation. Fig. 2 shows HAADF-STEM (High-angle Annular Dark Field – Scanning Transmission Electron Microscopy) images for the bulk of the LiCoPO₄ electrode after charge-discharge cycle. It was observed that the cobalt and phosphorus ordered regularly and anti-site defects were not observed in the pristine sample. On the other hand, the anti-site defects are observed in the sample after charge-discharge test for 5 cycles. In addition, amount of anti-site defects at the surface were more than at the bulk. From these results, we think the discharge capacity might decrease with the charge-discharge cycle as the anti-site defects might prevent Li-ion diffusion in the LiCoPO₄during the discharge. Therefore the anti-site defects should be suppressed to improve the cycle performance.

References;

[1]: K. Phadhi, K.S.Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188

[2]: H.H. Li, J.Jin, J.P. Wei, Z. Zhou, J. Yan, Electrochem. Commun. 11 (2009) 95-98

There is an ongoing need for higher energy batteries that operate with the highest safety margin. LiCoPO₄ is a 4.8 V cathode material with potentially better abuse tolerance and energy density than the current state of art Li-ion battery chemistries. The intense development of high voltage electrolytes¹ and strategies that improve the cycle life², discharge capacity² and rate capability³have increased the interest in this material.

This paper will review our recent efforts on LiCoPO₄ electrode development for improving the energy and abuse tolerance of Li-ion batteries. We will discuss synthesis methods, transport measurements,³ substitutional chemistry of LiCoPO₄ and performance enhancements that result from reformulating the electrolyte.

References

• A. V. Cresce, A.V and K. Xu, J. Electrochem. Soc. 158, A337, 2011.

• J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 2011, 196, 8656.

• J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow and J. Wolfenstine, J. Power Sources 254, 204 (2014).

In past two decades, tremendous research efforts have been devoted to understanding the phase transformation behavior of olivine type cathode materials, especially LiFePO₄ (LFP). It is well known now that insertion/extraction of lithium in LFP is accompanied by a first-order phase transition between lithiated and delithiated states. Various reports have shown that there is 20~30mV voltage gap between charge and discharge within two-phase range. This voltage hysteresis is attributed to the energy barrier to form intermediate compositions on the single particle level and sequential particle-by-particle transition behavior on the multi-particle level. [1-8]

In comparison, LiMn_yFe_1-yPO₄ (LMFP), which is isostructural to LFP, shows strikingly different phase transition behavior. Recent operando Synchrotron Radiation Powder X-Ray Diffraction (SR-PXD) study from our group revealed the formation of metastable solid solutions covering an extended composition range in LiMn_yFe_1-yPO₄ (LMFP, y=0.1, 0.2, 0.4), indicating continuous transition behavior on single particle level and simultaneous transition behavior on multi-particle level. Under this scenario, the voltage hysteresis should be vanishingly small, if not zero. [9, 10]

In order to determine voltage hysteresis (or zero-current gap), the open-circuit voltage (OCV) of LMFP within the Fe(II)/Fe(III) phase transition region was measured after charging and after discharging to the same composition. The zero-current gap is the difference in OCV under these two conditions. We found that there is no zero-current gap or thermodynamic hysteresis in LMFP (y=0.1, 0.2, 0.4), as shown in Figure 1. This result indicates that the energy barrier for LMFP to accommodate intermediate composition on single particle level is negligible, resulting in continuous phase transition behavior despite the fact that the composition is well within the two-phase field. Also this result supports the hypothesis that phase transition in LMFP (y=0.1, 0.2, 0.4) takes place in many particles simultaneously.

This result may be correlated with the observation that LMFP (y<0.6) has better rate capability than LFP at high rates. Under extremely high rates, surface reaction/transport rather than bulk transport/transition may become rate-limiting. For similar materials, the exchange current density should be similar, hence reaction rate may be correlated with the effective surface area. With the phase transition taking place simultaneously in many particles, LMFP (y<0.6) should have higher active surface area and thus superior rate capability under high rates.

Nowadays, demands lithium ion batteries which are mounted in portable devices increase day by day. As a result, research towards positive and negative electrode materials being intensively progressed.

Most commonly used positive electrode is LiCoO₂ because it has good electrochemical property. However, Co is rare material and it is placed in limited area. Therefore, price of Co increases more and more. To substitute Co, plenty of transition metal material is studied such as Fe, Ni, Mn and V. Among them polyanion phosphate material such as LiMPO₄ (M = Fe, Co, Mn and etc) and Li₃V₂(PO₄)₃ are considered as possible candidate materials because of their structure, thermal stabilities and low price. However, those polyanion material, LiMPO₄ (M = Fe, Co, Mn) has low energy density, causing low energy density. Li₃V₂(PO₄)₃ has good safety as LiFePO₄ but higher energy density than LiFePO₄ because of its high redox potential, V^2+/5+. Also, Li₃V₂(PO₄)₃ has high reversible capacity and high theoretical capacity (197 mAh g^-1). In this paper, physical and electrochemical properties of Li₃V₂(PO₄)₃ is investigated.

Li₃V₂(PO₄)₃ is synthesized by solid state method. Stoichiometric amount of Li₂CO₃, NH₄H₂PO₄, and V₂O₅ powders were grind and mixed by ball mill. The mixed powders were calcined at 500 ^oC in air atmosphere. The calcined powders were pelletized and sintered at 800 ^oC for 5 h in reduction atmosphere again. The calcinations, heated pellets were grind in Ar filled glove box to avoid air exposure. The synthesized Li₃V₂(PO₄)₃ powders were identified by X-ray diffraction (XRD) with Cu Kα radiation and analyzed by Rietveld refinement. Electrochemical test were carried out in coin type cell. Galvanostatic electrochemical charge and discharge test were made between 3.0 V and 4.8 V at 10 mA g^-1 current at room temperature.

The XRD pattern of the product shows single phase Li₃V₂(PO₄)₃ were obtained without impurity. The galvanostatic charge and discharge performances of Li₃V₂(PO₄)₃ as an positive electrode are tested in voltage range of 3.0 V and 4.8 V. Also to understand the mechanism during charge and discharge, ex-situ XRD, XPS and ToF-SIMS measurement were carried out at the first cycle. Furthermore to understand the affect the thermal stability high temperature XRD and TGA were carried out. Details will be discussed in the conference site.

Li-rich cathode materials (Li₂MnO₃-LiMO₂) have been reported as promising cathode materials since they can deliver high specific capacity and perform with a very good rate capability [1]. Considering the relatively high content of Mn and low content of Ni and Co, Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂which is a solid solution between Li₂MnO₃ and LiMn_0.4Ni_0.4Co_0.2O₂ with mole ratio of 5:5, fulfills the requirements of a next-generation cathode material very well: low cost, higher energy density and safe. However, high cut-off voltages of up to 4.6-4.8 V are required during charge to obtain high specific capacity as well as high energy.

The morphology of a material has obvious influence to its electrochemical properties, especially for nanostructure materials. In this work, the specific morphology of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂, has been successfully synthesized by high temperature annealing assisted by molten salt. XRD characterization proved that it belongs to the R-3m layered structure with the monoclinic Li₂MnO₃-like (C2/m) super lattice. The electrochemical experiments proved that the specific structure like spherical (Figure 1A) or porous (Figure 1B) could increase the rate capability and improve the cycle performance [2]. The morphology control during synthesis is a significant way to improve the performance of Li-rich cathode in lithium-ion batteries, and is a promising candidate for the realization of high voltage lithium and lithium-ion batteries.

Reference

[1] J. Li, R. Klöpsch, M. C. Stan, S. Nowak, M. Kunze, M. Winter, and S. Passerini. J. Power Sources, 196 (2011) 4821

[2] X. He, J. Wang, R. Klöpsch, S. Krueger, H. Jia, H. Liu, B. Vortmann, and J. Li. Nano Research, 7 (2014) 110.

[3] J. Wang, X. He, R. Klöpsch, S. Wang, B. Hoffmann, S. Jeong, Y. Yong and J. Li. Energy Technology, 2 (2014) 188

The over-lithiated-oxides (OLOs), a composite of layered structures of Li₂MnO₃ and LiMO₂ (M = Mn, Fe, Co, Ni), have shown much higher storage capacity than the traditional layered oxides for Li ion battery cathode because of the Li₂MnO₃ phase. However, Li₂MnO₃ is not stable after the 1st charge-discharge cycle and will partly transform into layered LiMnO₂, which indicates that the practically used phase is a mixture of both Li₂MnO₃ and LiMnO₂. During the subsequent cycles, the OLO voltage decreases due to the phase transition of layered LiMnO₂ into spinel. Experimentally, the effective dopants satisfying multiple cathode materials requirements of thermodynamic stability, optimized voltage and improved kinetics based on ionic and electronic conductivities are investigated to overcome the voltage degradation and to improve the power capacity. In this work, redox potential, lithium ion diffusion and charge carrier transportation of both phases are examined in details using the ab initio density-functional theory (DFT) simulations. The calculations find, due to the Jahn-Teller effect of Mn³⁺ atoms, Li vacancy migration in LiMnO₂ has special behaviors and hole polaron and electron polaron will form in LiMnO₂ and Li₂MnO₃ phases, respectively. Based on the understanding of the pure phase properties, the effects of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) and 2 anionic (N, F) dopants on the redox potential, ionic and electronic conductivity are investigated. These DFT findings could provide conceptual guidance in the experimental search for the effective dopants enabling the practical application of OLO cathodes.

The LiNi_0.5Mn_1.5O₄ (LNMO) high voltage spinel has been recognized as a tantalizing option for the cathode in next generation of Li-ion batteries. The combination of high operating voltage (4.75 V vs Li) and excellent rate capability, which arises from three-dimensional Li⁺ ion diffusion, makes this material particularly attractive for automotive applications. Recent studies, however, suggest that the most critical barrier for the commercialization of LNMO in Li-ion batteries is electrolyte decomposition and the concurrent degradative reactions at electrode/electrolyte interfaces, which consume active Li⁺ ions and reduce cycle life.[1,2] Elemental substitution for Ni and/or Mn in LNMO has been the most widely accepted strategy to control the crystallographic properties and stabilize the performance of high voltage spinel/Li half-cells. Despite demonstrating excellent half-cell performance, LiNi_0.5-xM_xMn_1.5O₄/graphite full-cells (x = 0.05 and 0.1, M = Fe, Co, Cu, Al, Ga, and Mg) deliver poor cycle live, similar to that of the pristine LNMO; i.e., capacity retention after 100 cycles falls to the range of 45 – 53 %, and shows similar rates of capacity fading ( Fig. 1).

In contrast, Noguchi et al.[3] reported that Ti-substitution for Mn in LNMO improves its cycle life in full-cells paired with amorphous carbon anodes. They showed that the capacity retention of the LiNi_0.5Mn_1.5-xTi_xO₄ full-cells improved with increasing Ti content in the range from x = 0 to 0.19. In our recent work[4], this improvement was also observed for higher Ti contents, up to x = 0.4, as shown in Fig. 2. The LiNi_0.5Mn_1.5-xTi_xO₄/graphite full-cells also showed less oxidation of the electrolyte during cycling compared with that of Ti-free LNMO, as evidenced by higher Coulombic efficiency and lower self-discharge rates.[4] However, the improvement mechanism of the LiNi_0.5Mn_1.5-xTi_xO₄full-cells has not been determined.

In an attempt to identify the improvement mechanism operant in the LiNi_0.5Mn_1.5-xTi_xO₄ full-cells, various surface and bulk analyses of cycle-aged and/or HF-etched cathodes were undertaken, including TEM, TOF-SIMS, FT-IR, and ICP in combination with AC-impedance analysis. Figure 3 shows that LiNi_0.5Mn_1.5-xTi_xO₄ particle surfaces are Ti and O rich after etching in 1 wt% HF solution, and Mn and Ni deficient. In contrast, Ti-free LNMO does not show any elemental gradient after etching under the same conditions. This result suggests that the Ti-rich surface, which is formed as a result of the Mn dissolution, may passivate the LiNi_0.5Mn_1.5-xTi_xO₄ during prolonged cycling. The detailed post-mortem analyses results for LiNi_0.5Mn_1.5-xTi_xO₄will be discussed in comparison with those for Ti-free LNMO to support the above hypothesis.

Since the LiNi_0.5Mn_1.5-xTi_xO₄spinel is a relatively promising new material, its electrochemical performance needs to be optimized by tuning chemical compositions and synthesis parameters. Therefore, we will also discuss what influence these factors have on crystal structure, phase purity, and electrochemical performance.

References

[1] D. Aurbach et al., J. Power Sources, 162, 780–789 (2006).

[2] N. P. W. Pieczonka, Z. Liu, P. Lu, K. L. Olson, J. Moote, B. R. Powell, J.-H. Kim, J. Phys. Chem. C, 117, 15947–15957 (2013).

[3] T. Noguchi, I. Yamazaki, T. Numata, and K. Utsugi,

Electrochem. Soc. Meet. Abstr., 1378 (2011).

[4] J.-H. Kim, N. P. W. Pieczonka, Y. K. Sun, and B. R. Powell, J. Power Sources, in press

Rechargeable batteries can store energy in the form of chemical energy and facilitate it with a high conversion rate when needed. Moreover, rechargeable batteries are used in almost all kinds of portable consumer electronics, hybrid and pure electric vehicles. Thus, the development of advanced battery technologies is a major field of scientific focus. Li-ion technology has been shown to be superior compared to other battery concepts in performance, cycling stability, self-discharge and expected lifetime.[1] However, commercially used cathode materials (LiMO₂) exhibit low capacities compared to the graphite based anodes (~300 mAh/g) and thus limits the battery performance. In the last decade polyanion based materials gained interest and in 2005 Nyten et al. reported on Li₂FeSiO₄as a new Li-battery cathode material [2]. Lithium transition-metal silicate based materials are promising candidates for next generation's Li-ion batteries since they allow Li extraction/insertion beyond one Li ion per formula unit. Furthermore, they consist of cheap, non-toxic and abundant elements [3].

This work focuses on Li₂MnSiO₄ (LMS), which can theoretically deliver two Li-ions per formula unit since the transition metal ion possesses two redox couples (Mn³⁺/Mn²⁺ + Mn⁴⁺/Mn³⁺). Synthesis of phase pure LMS material has, however, turned out to be challenging. Here, synthesis of LMS with high phase purity was demonstrated by an acidic, PVA assisted sol-gel method using metal nitrates and TEOS as precursors, which is also suitable for upscaling. The dried precursor was pre-calcined and then mixed with a given amount of corn-starch as carbon source. To obtain the desired phase and coat the material in a single step, the powder/starch mix was then carbothermally reduced at 700 °C for 10 h. The atmosphere and starch content are seemingly crucial in both heat treatments to achieve high phase purity in the final cathode powder. Best results were achieved when both heat treatments were carried out in 95% Ar 5% H₂ and with starch contents ≥ 25 wt-%. Figure 1 shows powder XRD patterns of LMS with optimized parameters and different starch contents and a full pattern refinement.

The synthesized materials were micro- and meso-porous powders with a thin uniform carbon coating (confirmed by TEM), offering high external surface areas of more than 30 m²g^-1 (excluding micropore area which is inaccessible to the electrolyte). In addition to pure Li₂MnSiO_4, this work also focuses on the synthesis and electrochemical performance of Fe and V substituted LMS. Fe substitution should provide increased cycling stability since Li₂FeSiO₄ has shown relatively good long-term stablility [2,3]. The doping of V on either the Mn or the Si site of LMS is currently under investigation. V doped LMS is believed to offer improved electrochemical performance since V offers 3 redox couples in the accessible voltage window of the electrolyte of a Li-ion battery [4].

[1] Tarascon, J. M., Armand, M, Issues and challenges facing rechargeable lithium batteries, Nature, 2001, 414, 359-367.

[2] Nyten A., Abouimrane A., Armand M., Gustafsson T., Thomas J. O., Electrochemical performance of Li2FeSiO4 as a new Li-battery cathode material, Electrochemistry Communications, 2005, 7, 156-160.

[3] Saiful Islam M., Dominko R., Masquelier C., Sirisopanaporn C., Armstrong A. R., Bruce P. G., Silicate cathodes for lithium batteries: alternatives to phosphates?, J. Mater. Chem., 2011, 21, 9811-9818

[4] Li, Y, Cheng, X, Zhang, Y, Achieving High Capacity by Vanadium Substitution into Li₂FeSiO₄, Journal of The Electrochemical Society, 2012, 159 (2) A69-A74.

This paper presents a novel approach to an explanation of apparently different character of the discharge/charge curve in Li_xCoO₂ (monotonous curve) and Na_xCoO₂ systems (step-like curve). Comprehensive experimental and theoretical studies on crystallographic, electronic and electrical transport properties of the Li_xCoO₂ battery cathode material are reported. During electrochemical deintercalation of lithium we observe an increase of the electrical conductivity by 6 orders of magnitude. The thermal activated character of electrical conductivity for Li_xCoO₂ is detected in the region 0.95 ≤ x ≤ 1. However, only for stoichiometric Li₁CoO₂ the activated character of electrical conductivity indicates clearly semiconducting behavior. Based on the calculated DOS from the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method allowing to take into account Li- and O-deficiency in self-consistent way, it is found that for 0.99 ≤ x ≤ 0.95 the Fermi level is situated inside the tails of the valence states and its activated character corresponds to activated character of electron hole mobility [1]. The electrical conductivity of Li_xCoO₂ samples with x ≤ 0.94 does not depend on temperature and the values tend to increase with lowering of the lithium concentration. On the whole, we postulate that the observed insulator-metal transition in Li_xCoO₂ can be interpreted on the basis on the Anderson type transition. Drastic evolution of the top valence band features (the relative spectral weight of the Co atom orbitals e_g/t_2g) well corroborates crystallographic data as well as the particular effect of the oxygen octahedron distortion on computed DOS shape [2]. We suppose that irregular behavior of the positional parameter z (O sites) with Li concentration is a reason of significant discrepancies in observed crystallographic parameters, transport properties and phase diagram of Li_xCoO₂ system as reported in different works.

In the case of Na_xCoO_2-y it was evidenced that the origin of the observed step-like character of the discharge/charge curve of Na_xCoO_2-y is due to the specific features of the electronic structure, arisen from the presence of the oxygen vacancies and sodium ordering. Performed comprehensive studies of the structural, transport and electronic specific heat properties of Na_xCoO_2-y cathode material in the characteristic points of the discharge curve i.e. on the pseudo-plateaus and on the potential jumps, show non-monotonous variations of its transport properties (i.e. a sequence of alternative, metallic-like or semiconducting-like, behaviours) during sodium intercalation, what suggest that the density of states near the Fermi level is spiky. This effect is coherently supported by KKR-CPA calculations of DOS in Na_xCoO_2-y, accounting for chemical disorder, i.e. oxygen vacancy defects as well as variable occupancy of two sodium sites. We concluded that these unusual electronic structure features lead to an abrupt changes in the position of E_F, experiencing the oxygen-defect DOS peaks upon intercalation, and finally resulting in step-like character of the discharge curve of Na/Na⁺/Na_xCoO_2-y cell.

Acknowledgements

The project was funded by the National Science Centre Poland (NCN) on the basis of the decision number DEC-2011/02/A/ST5/00447.

This work is supported by the Polish-Swiss Research Programme under grant no. 080/2010 LiBeV (Positive Electrode Materials for Li-ion Batteries for Electric Vehicles).

References:

[1] A. Milewska, K. Świerczek, J. Toboła, F. Boudoire, Y. Hu, D.K. Bora, B.S. Mun, A. Braun, J. Molenda, The nature of the nonmetal-metal transition in Li_xCoO₂ oxide, Solid State Ionics (2014)

[2] J. Molenda, D. Baster, K. Świerczek, J. Toboła, Anomaly in electronic structure of Na_xCoO_2-y cathode as a source of its step-like discharge curve, Physical Chemistry Chemical Physics (2014)

As applications of rechargeable lithium ion batteries are gradually increasing from small electronic devices to automobiles or power storages, there is increasing demand for electrode materials for use in rechargeable batteries having various advantageous characteristics including high safety, extended cycle life, high energy density and high output characteristic.

To develop rechargeable lithium ion batteries with high energy density, intensive studies have focused on silicone anode materials due to their high theoretical capacity (4200mAh/g). However, the commercial use of Si-based material is still hindered because of severe volume expansion by up to 300% during the repeated charge and discharge cycles, which results in pulverization and capacity fading.

In order to alleviate the volume changes, various nanostructured Si electrodes have drawn attention. Although the pulverization issue can be resolved by using the nanometer-scale materials, an effective low-cost solution to the volume-change problem remains elusive.

In the past, polymeric binder has played only passive roles in the performance of lithium ion batteries. But recent works show that the choice of binder is very important to stabilize the cycling performance of Si-based negative electrodes for LIB, and the functions of binders have become critical.

So we synthesized the new semi-interpenetrating network (semi-IPN) binder that can endure expansion of the Si-based active material. The term semi-IPN refers to a network structure of a linear polymer and a cross-linking polymer. Such a semi-IPN polymeric binder includes two kinds of polymers in a chain shape to form a network structure, and has high mechanical strength and excellent stability in the liquid electrolyte compared with general polymer. Accordingly, a rechargeable lithium ion battery including this binder can effectively control the expansion of a Si-based active material and exhibit improved cycling performance.

Titanium oxide (TiO₂) has received much attention as the most promising alternative to the conventional graphite anode of Li-ion batteries for high energy and power. The performance of TiO₂ anode for Li-ion batteries depends strongly on the crystalline phase, the morphology, and the porosity of the structure. The nanostructured TiO₂ materials, such as nanoparticles, nanorods, nanowires, and nanotubes have been studied to improve the performance of TiO₂ anode. Recently, it was reported that the hydrogenated TiO₂, called a 'black TiO₂' nanostructures are more attractive for photovoltaics, photocatalysis, and supercapacitors owing to their narrower bandgap (less than typical 3 eV value) and relatively high electrical conductivity.

The smooth and well-ordered TiO₂ nanotubes were synthesized on a Ti disk successfully by anodization in a non-aqueous solution containing fluoride ions. The as-prepared TiO₂ nanotubes were annealed to obtain crystalline anatase (A-TiO₂ NTs) and hydrogenated TiO₂ nanotubes (H-TiO₂ NTs) at 450 °C for 2 h in air and hydrogen atmosphere, and then their electrochemical performances were investigated as alternative anode materials for Li-ion batteries.

The initial discharge capacity of the H-TiO₂ NTs (0.117 mAh cm^-2) was superior to that of the A-TiO₂ NTs (0.110 mAh cm^-2) and the discharge capacities of the H-TiO₂ NTs and A-TiO₂ NTs maintained nearly 72 and 44 % at a current density of 10 mA cm^-2. In addition, the H-TiO₂ NTs (89 %) exhibited much higher the capacity retention than the A-TiO₂ NTs (70 %) at the current density of 1 mA cm^-2 (~10 C-rate) after 300 cycles, as shown in Figure 1. The H-TiO₂ NTs presented smaller the crystallite size and charge transfer resistance (R_c) compared with the A-TiO₂ NTs and the oxygen vacancies were formed in the H-TiO₂ NTs during hydrogenation, which was proved by the presence of Ti³⁺ from the XPS analysis.

These results indicate that the insertion and extraction of Li⁺ through the H-TiO₂ NTs were preferable to those through the A-TiO₂ NTs, which were probably attributed to the short diffusion length for Li⁺, innumerable reaction sites, and relatively high electrical conductivity. Therefore, the H-TiO₂ NTs exhibited vastly superior the rate capability and capacity retention property during cycling to the A-TiO₂ NTs at high current density as anode materials for Li-ion batteries.

Introduction

Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most well-known additive for Li-ion batteries and has been shown to be useful during the solid electrolyte interphase (SEI) formation [1], which can result in longer calendar life and reduced irreversible capacity (IRC). However, it has been shown that the performance of cells containing VC decreases at high temperatures and high voltages. Sulfur-containing additives have been studied in hopes to overcome the temperature sensitivity of VC. Prop-1-ene-1,3-sultone (PES) was found to have smaller IRC than VC-containing cells and was suggested to be an SEI forming additive [2]. In this study, we show that PES is a useful additive for Li-ion pouch cells and suggest that PES, when used in combination with other additives, may be more effective than VC at achieving long lifetimes and high cycling performance.

Experimental

Li[Ni_1/3Mn_1/3Co_1/3]O₂ (NMC)/graphite pouch cells made with 1M LiPF₆in EC:EMC 3:7 (by weight) and varying amounts of PES were studied using ultra high precision coulometry and storage experiments. In addition, gas evolution measurements employing Archimedes principle and electrochemical impedance spectroscopy (EIS) measurements were performed. The PES-containing cells were compared to the same control electrolyte containing 2% VC.

Results

Figure 1 shows a preliminary set of results from both storage and cycling experiments of PES-containing cells compared with control electrolyte and 2% VC-containing cells. The dotted line in Figures 1a, d, and e indicates the value for 2% VC. Figure 1a shows the gas evolution during formation at 40°C and Figures 1b and c show gas evolution during storage at 60°C and 40°C, respectively. Cells containing 2, 4 and 6% PES show smaller gas during formation than that of 2% VC and all concentrations of PES studied show significantly smaller gas during 40°C and 60°C storage than VC-containing cells. The most prominent advantage of PES compared to VC is the extremely low gas evolution during high temperature storage. Figure 1d shows the voltage drop during storage from 4.2 V at 40°C for 500 hours. Figure 1e shows the average coloumbic inefficiency (CIE) during the last three cycles of cycling between 2.8 V and 4.2 V at C/20 and 40°C. Through the voltage drop and CIE, it is clear that 2% PES appears to be as good as 2% VC.

This work suggests that PES is a viable alternative to VC and is superior during high temperature storage. PES may also prove to be beneficial in combination with other additives.

References

[1] M. Broussely, Ph. Biensan, F. Bonhomme, Ph. Blanchard, S. Herreyre, K. Nechev and R.J. Staniewicz, J. Power Sources, 146, 90 (2005).

[2] B. Li, Y. Wang, H. Rong, Y. Wang, J. Liu, L. Xing, M. Xu and W. Li, J. Mater. Chem. A, 1, 12954 (2013)

Cobalt oxide, Co₃O₄, one of the most important metal oxides and p-type semiconductor, has been found wide spread applications in many important fields, including solar cell photo-catalysts and supercapacitors or lithium-ion batteries. Usually, properties of the material depends on its special morphology. Different morphologies of cobalt oxide such as cubes, sheets, wires and tubes have been synthesized^[1][2].

In this work, a novel flower-like Co3O4 has been produced by calcining the precursor, which was obtained through a simple hydrothemal method. The unique hierarchical architecture is able to shorten Li⁺diffusion paths and accommodate the volume change of the materials during charge/discharge process. Electrochemical evaluation demonstrates its potential application as anode material for lithium ion batteries. Fig. 1 shows the cycle life of cobalt oxide, the inserted picture a and b are the TEM images of precusor and cobalt oxide, respectively.

references：

[1] Lou X W, Deng D, Lee J Y, et al. Advanced Materials, 2008, 20(2): 258-262.

[2] Tian L, Zou H, Fu J, et al. Advanced Functional Materials, 2010, 20(4): 617-623.

Li-ion batteries using Li₄Ti₅O₁₂(LTO) anode have been developed for not only automotive but also stationary power applications due to its long-life, safety, high-power performance[1]. Considering cathodes practically used, LiFePO₄(LFP) shows excellent life and safety properties compared to 4V-class cathodes such as LiCoO₂, Li(NiCoMn)O₂, and LiMn₂O₄, although LiFePO₄ has low working potential of 3.4V. In order to enhance the energy density, long-life, and safety performance of Li-ion battery using the LTO anode, LiMnPO₄(LMP) has been developed as a novel 4V-class cathode. It has been recently suggested that LMP/LTO battery system is promising for load-leveling application[2]. However, the electrochemical kinetics of LMP is slower than that of LFP due to its low electronic and ionic conductivity. It has been previously reported that Fe and Mg co-substituted LMP cathode improved the electrochemical properties[3]. This paper reports on hydrothermally synthesized Mg doped LiMn_0.85Fe_0.1Mg_0.05PO₄(Mg-LMFP) as the cathode and the performance of Mg-LMFP/LTO batteries for stationary power applications.

Mg-LMFP and LiMn_0.85Fe_0.15PO₄(LMFP) were synthesized by using a hydrothermal process. Lithium, manganese, iron, and magnesium sulfates, diammonium hydrogen phosphate, and carboxymethylcellulose(CMC) were used as starting materials. The materials with purified water in the autoclave were heated at 200°C for 3 hours. The obtained powder was pulverized by a ball-milling process. The milled powder was heated at 700°C under a flow of Ar containing 3vol % H₂ for 1 hour. The hydrothermal process is a promising method for synthesizing high crystalline and fine particles. The Mg-LMFP and LMFP electrodes after charging at cut-off voltage of 4.25V vs. Li/Li⁺ showed the discharge capacity of 158 and 145 mAh/g at 0.1 C rate at 25°C, respectively. Figure 1 shows I-t curves obtained from potential step chronoamperometry(PSCA) of Mg-LMFP and LMFP electrodes on the anodic step from 80% state of charge(SOC) to 4.25V. Open circuit voltages of Mg-LMFP and LMFP electrodes at 80% SOC were about 4.1V. Current response of Mg-LMFP was larger than that of LMFP, although particle size of both samples was almost same at 80 nm. The fast charging kinetics of Mg-LMFP would be attributed to fast phase boundary movement due to reduction of the lattice mismatch between lithiated and delithiated phases of Mg-LMFP owing to non-active Mg²⁺. Laminated Mg-LMFP/LTO batteries were constructed to demonstrate the performance of discharge, cycle life, and safety. A mixture of propylene carbonate(PC) and diethyl carbonate(DEC) solvent containing LiPF₆ was used as the electrolyte. The nominal capacity, the nominal voltage, and the energy density of 3Ah-class Mg-LMFP/LTO battery were 3Ah, 2.5V, and 90Wh/kg, respectively. The energy density of a large-scale Mg-LMFP/LTO battery with 25Ah was estimated to be 100Wh/kg comparable to that of conventional LiMn₂O₄/carbon Li-ion batteries. The Mg-LMFP/LTO battery showed high-rate discharge performance at 25°C as shown in Figure 2. The capacity retention of the battery was 96% even at 10 C rate. Figure 3 shows discharge curves of the Mg-LMFP/LTO battery at various cycle numbers under a high temperature condition of 60°C. We noted almost no increase in the overpotential and a high capacity retention of 95% at 500 cycles during the high temperature cycling. Little amount of Mn and Fe was deposited on the LTO anode after the cycle test. Mg could suppress Mn and Fe dissolution from Mg-LMFP. Large-scale Mg-LMFP/LTO batteries can be applied for stationary power applications because of the long-life, high rate, and safety performance. The results of abuse tests of Mg-LMFP/LTO batteries will be also reported.

References

[1]N. Takami et al, J. Power Sources, 244, 469 (2013)

[2]S. K. Martha et al, J. Electrochem. Soc., 158, A790 (2011)

[3]C. Hu et al, Electrochem. Commun., 12, 1784 (2010)

A unique characteristic of lithium batteries, as compared to other cell chemistries, is the thermal behavior exhibited during cycling. There are several sources of heat within a lithium cell but they ultimately can be categorized as being either irreversible (always exothermic) in nature or as being reversible. The contribution of the entropic (reversible) heat source alternates between endothermic or exothermic in behavior depending on the state-of-charge (SOC). Past researchers have attributed this to a phase change of the active material.¹The entropic coefficient itself is not a constant factor and its value varies with different SOC levels. This makes heat generation within a cell a rather complicated study but one that must be taken. The amount of heat generated within a cell affects its temperature and, in turn, affects its performance.

The role of the entropic coefficient in calculating the quantity of reversible heat, Q_rev, is described as follows:

Q_rev = T_cell * (dV_oc/dT) * I (1)

where T_cell is the temperature of the cell, dV_oc/dT is the change in the open circuit voltage of the cell that results in a change of cell temperature, and I is the current in amps. dV_oc/dT is the entropic coefficient. It is determined by measuring the open circuit voltage (OCV) of the battery at a set SOC value and noting its change as the temperature of the cell varies a given amount in an environmental chamber.

The Varying Effects of Entropic Heat

The impact of entropic heat on the overall heat generation within a cell runs the gamut from being negligible to significant depending on the chemistry. Kim et al.² found a strong influence of entropic heating at the 1C-rate for a LiMn₂O₄ spinel coin cell. In contrast, Lu and Prakash³ found nearly no effect at the same 1C rate for the C/Li half-cell that utilized mesocarbon microbeads (MCMB).

In this study, the researchers focused on quantifying the entropic coefficient for the Nickel Manganese Cobalt (NMC) lithium-ion electrochemistry. Leading battery manufacturers are focusing more intently on integrating the NMC cell into powertrain applications. By finding the right combination of these three metals for the cathode, they can offer a good balance between power, safety, and life. As a result, this particular type of cell is commercially available in rather large capacities for both pouch and prismatic formats.

The objective of this research will be to fully map the varying entropic coefficient values of the NMC cell. This value will be calculated from data taken at different SOC levels as well as different temperatures. This map will be compared to one compiled for a lithium iron phosphate (LFP) pouch cell done previously by the authors.

Experimental Setup

The SOC levels range from full charge to full discharge in 5% increments. The temperature levels will vary from 55 ⁰C to -20⁰C in 5⁰C increments. A commercially available 12Ah lithium-ion Kokam pouch cell will be used. Eventually the entropic map will be composed of 315 data points (21 SOC levels and 15 different temperature settings). In order to accelerate the data collection process, three cells will be set at SOC levels that are 5% apart and all will be simultaneously exposed to the full range of temperature changes. Each new temperature setting will dictate that the cells have 8 hours of soak time to stabilize its OCV. The effects of self-discharge on OCV will also be quantified and removed from the measured data. SOC levels and data acquisition is to be performed by a Maccor System Model 4200 10-channel cycler.

Entropic Coefficient Map for LFP

The data acquired by the authors in previous work has shown entropic behavior not uncovered by previous literature on the subject. Other studies that have used larger SOC and temperature increments found the entropic coefficient to be almost linear and independent of temperature.⁴ However, a study by the authors on a LFP cell found the entropic coefficient to be influenced by temperature at extreme SOC levels.⁵

References

[1] R. Tamamushi, Electrochemistry, second ed., Maruzen, Tokyo, 2001.

[2] J.-S. Kim, J. Prakash, and J.R. Selman, Electrochemical Solid State Letters, 4, A141 (2001)

[3] W. Lu and J. Prakash, Journal of the Electrochemical Society, 150, A262 (2003)

[4] K. Onda, H. Kameyama, T. Hanamoto, and K. Ito, Journal of the Electrochemical Society, 150 (2003) A285.

[5] S. Bazinski and X. Wang, The Influence of Cell Temperature on the Entropic Coefficient of a Lithium Iron Phosphate Pouch Cell across the Full State-of-Charge Spectrum, 221^st ECS Meeting, Seattle WA, May 2012.

Oblique angle deposition method was used to fabricate inclined nanocolumnar structured CuSi thin films on Cu substrates. The reversible cyclability of those Si based composite anodes is greatly improved by optimizing the atomic ratio of Cu/Si in the thin films. The thicknesses, surface morphologies, and electrochemical behavior of the films deposited are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV), impedance spectroscopy (EIS), and galvanostatic charge/discharge measurements. The galvanostatic test results show that the best result is achieved when the composite thin film having of 10%at. Cu- 90%at. Si is used as anode in half cell. It is possible to explain this remarkable performance by the amorphous structure of the thin film and its enhanced physical and mechanical properties due to the Cu content of the film. This study proves that electron beam evaporation can be an alternative method to fabricate electrodes used in lithium ion batteries, because this environmentally friendly process enables one to make production in one step without using any binders or conductive additives.

Lithium-ion batteries longevity is of major concern for electric transportation applications. It is therefore necessary to understand the performance decay of a battery subject to aging conditions. Degradation mechanisms of the electrodes are generally divided into three families: (i) irreversible loss of active material, (ii) parasitic reactions leading to a loss of cyclable lithium or electrons, (iii) increase in resistance due to passive films formation and loss of contact [1, 2].

Lab-scale Li₄Ti₅O₁₂/LiFePO₄cells were submitted to accelerated aging during 4 to 5 months. All the cells exhibit capacity fade whose extent is not correlated with the aging condition. In order to understand aging phenomena, cells were disassembled at the end of cycle life and the recovered electrodes were analyzed using electrochemistry, electron microscopy, XRD and MAS-NMR. Positive and negative electrodes show no loss in active material and no change in electrochemical activity, active material structure and composite electrode structure. This rules out any irreversible electrode degradation. Lithium stoichiometry estimated by both XRD and electrochemistry is unexpectedly low in the positive electrode when the aging is stopped at full discharge. That indicates a loss of cyclable lithium or electrons leading to cell balancing evolution [3]. That loss has been caused by parasitic reactions occurring at both electrodes, in accordance with their rich surface chemistry evidenced by MAS-NMR.

This surface chemistry is fully characterized by MAS-NMR both qualitatively and quantitatively, using an original calibration method (see Fig. 1) [4]. Correlations are drawn between the quantity of diamagnetic lithiated species deposited on the surface of both electrodes (mainly LiF) and the loss of cyclable lithium or electrons undergone by the cell. This study attempts to develop a parasitic reaction pathway that explains the loss of cyclable lithium or electrons encountered under these aging conditions.

References:

[1] J. Vetter, P. Novák, M. Wagner, C. Veit, K.-C. Möller, J. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, Journal of Power Sources 147 (2005) 269.

[2] A. Barré, B. Deguilhem, S. Grolleau, M. Gérard, F. Suard, D. Riu, Journal of Power Sources 241 (2013) 680.

[3] M. Kassem, C. Delacourt, Journal of Power Sources 235 (2013) 159.

[4] M. Cuisinier, J. F. Martin, P. Moreau, T. Epicier, R. Kanno, D. Guyomard, N. Dupré, Solid State Nuclear Magnetic Resonance 42 (2012) 51–61.

Figure 1: Calibration of NMR-MAS ¹⁹F signal with LiF-electrode mixtures.

For lithium ion batteries (LIBs) to take their place in widespread commercialization of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and full electric vehicles (EVs), system cost must still be reduced by 2-3× to about $200 kWh.^[1] Two important ways to achieve significant system cost reduction are to: 1) lower the electrode processing cost (elimination of costly organic solvents, increasing the cathode thickness, and reduction of primary solvent drying time); and 2) reduce the formation time associated with the anode solid electrolyte interface (SEI) layer.

Purchasing and handling the N-methylpyrrolidone (NMP) solvent used in much of the electrode formulation and coating steps add unnecessary manufacturing cost to the LIB pack. In addition, more processing energy (heated air flow) is required to remove NMP during drying of the electrode coatings than other solvents (such as water and lower alcohols) with much lower boiling points and higher vapor pressures. NMP recovery involves significant capital expense since multiple condensers or distillation towers are needed, and its use adds to the cost of the coating line equipment in making it explosion proof. Cathode thicknesses, which currently limit the cell specific capacity (mAh/g on a total-unit-cell weight basis) and energy density and indirectly add to cell cost, should also be significantly increased, so much fewer inactive components (current collectors and separators) are required per cell.

Electrode wetting and formation cycling also add significant process energy cost per kWh of usable energy, which is often neglected in LIB pack cost calculations. Furthermore, the wetting and formation steps are a huge process time bottleneck and add substantial capital cost to a LIB production plant. In a large-scale LIB manufacturing plant, the footprint associated with the wetting and formation steps can be as large as 20-25% of the entire layout.

There have been two useful LIB cost studies presented recently by Argonne National Laboratory (ANL)^[2] and TIAX, LLC,^[3]but these studies consider the entire 18650 cell production process without much granularity on individual processing and fabrication steps. These cost models are also heavy on contributions from material, labor, and capital equipment without detailed consideration of process energy requirements. This paper will give a more in depth review of the process energy consumption associated with electrode processing and formation cycling, two particularly costly elements of lithium-ion cell production, and how manufacturing cost savings could be realized.

The cell fabrication steps mentioned above contribute significantly to the current overall pack cost of $400-600/kWh and will be the focus of the calculations in this presentation. Formulation, coating, and drying aspects will be considered with respect to electrode processing cost, and wetting time and low-rate cycling will be addressed with respect to total cell formation time. This cost assessment approach will show the critical link between electrode processing aides, process energy consumption, and LIB pack cost. Initial calculations show that about 20% of the total battery pack costcan be saved by switching to aqueous electrode processing and doubling the electrode thicknesses.

Acknowledgment

This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).

References

[1] D. Howell, "U.S. Battery R&D Progress and Plans," DOE Annual Merit Review, May 14, 2013.

[2] K.G. Gallagher, D. Dees and P. Nelson, "PHEV Battery Cost Assessment," DOE Annual Merit Review, May 9-13, 2011.

[3] B. Barnett, J. Rempel, C. McCoy, S. Dalton-Castor, and S. Sriramulu, "PHEV and LEESS Battery Cost Assessment," DOE Annual Merit Review, May 10, 2011.

Introduction

Recently, electrolyte additives have attracted much attention because they can help improve the lifetime of lithium-ion batteries^1,2. Among them, sulphur-containing electrolyte additives are very promising. For example, 1,3,2-dioxathiolane-2,2-dioxide (DTD)¹ and prop-1-ene-1,3-sultone (PES)² can help increase coulombic efficiency (CE) while 1,3,2-dioxathiane 2,2-dioxide (TMS)¹ can limit gas production during formation. However, electrolyte additives may have an effect on Li-ion battery safety because these additives modify the interfaces between the electrode materials and the electrolyte.

In this work, the reactivity of lithiated graphite or delithiated Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) with control electrolytes (1M LiPF₆ ethylene carbonate (EC):ethyl methyl carbonate (EMC) 3:7 wt% ratio) or electrolytes containing several sulphur-containing additives was studied, respectively, using accelerating rate calorimetry (ARC).

Experimental

The ARC sample preparation process was similar to that reported before³. 2325 coin type pellet cells were made using control electrolyte and charged to 4.2 V for the NMC electrodes or discharged to 0.0 V for the graphite electrodes using the protocol described in Reference 3. The ratios between charged electrode materials and electrolyte were 94 mg:30 mg and 140 mg:140 mg for NMC electrodes and graphite electrodes, respectively. The single-point BET surface areas of the graphite and NMC powders were measured with a Micromeritics Flowsorb 2300 instrument.

Results and Discussion

Table 1 shows the specific surface area results for the NMC and graphite materials used in this experiment.

Figure 1 shows the molecular structures of the electrolyte additives were used in this experiment.

Figure 2 shows the self-heating rate (SHR) versus temperature for the reaction of lithiated graphite or delithiated NMC with different electrolyte additives. Although there is a short-lived exothermic observation at 50°C, 5% DTD decreases the SHR for lithiated graphite. TMS causes a small exothermic peak at around 75°C but the SHR is very small. Both 5% DTD and 5% TMS help eliminate the exothermic peak at around 100°C resulting from the decomposition of metastable solid electrolyte interface (SEI)⁴. Furthermore, 2% PES does not dramatically increase the reactivity of the delithiated NMC with electrolyte from the starting temperature to around 250°C and PES helps decrease the SHR compared with the control electrolyte after 250°C. Further experiments showing the impact of these additives in combination with other additives like VC will be reported.

In summary, some sulphur-containing electrolyte additives which show good electrochemical performance, such as DTD, TMS etc., should not compromise the safety of lithium-ion batteries.

References

1. J. Xia, N. N. Sinha, L. P. Chen, and J. R. Dahn, J. Electrochem. Soc., 161, A264–A274 (2014).

2. J. Xia, L. Ma, C. P. Aiken, K. J. Nelson, L. P. Chen and J. R. Dahn, submitted for publication.

3. J. Jiang, K. W. Eberman, L. J. Krause, and J. R. Dahn, J. Electrochem. Soc., 152, A1879–A1889 (2005).

4. M. N. Richard and J. R. Dahn, J. Electrochem. Soc., 146, 2068–2077 (1999).

Lithium anode an Li-ion batteries are chemical power sources of high energy and power density. In other words, such batteries can be light, small and relatively efficient. Therefore, they can be used in several power-demanding devices, like laptops, tablets, smartphones as well as cameras. Despite of this commercial success, the scientific works oriented on development of these power sources are still conducted. The most important problem, which still needs to be solved, is stability of the battery and battery life length. This is related to the stability of the electrolyte-electrode interfaces.

Due to this, the importance of the research on novel salts for batteries' electrolytes grew more and more important. The focus was put on looking for novel lithium salts which dissociate well in aprotic solvents and are electrochemically and thermally stable.

In the present study, spectroscopic and structural properties of the solvates of the lithium salts with fluorine-free heteroaromatic anion are researched. The main impact is put on complexes of the salts with dimethyl ethers of ethylene glycol oligomers (glymes) and organic carbonates.

Correlation between crystal structures, obtained from X-ray diffraction experiments on the single crystals with vibrational spectroscopy data (FTir and Raman) should make the prediction of the cation-anion-solvent molecule interactions in the liquid systems possible.

The discovering and understanding of phenomena related to the organization of such systems in the solid state is crucial for the elaboration of novel electrolytes. Gained knowledge will be helpful in the design of novel compounds with defined properties and in optimization of the working systems.

Silicon is the most attractive candidate to upgrade carbon as negative electrode material in lithium-ion batteries, due to its large specific charge and low reaction potential. However, the volume changes of silicon upon lithiation and delithiation lead to mechanical stress and poor performance, hindering the commercial use of silicon particles in lithium-ion battery negative electrodes. Many attempts to reduce the volume changes and achieve stable cyclability for silicon electrodes involve the use of carbon and/or graphite as electrochemically active matrix. Here we study the effect of silicon addition to graphite based electrodes and explore the differences in electrochemical response by means of in situ electrochemical dilatometry.

Electrodes based on TIMCAL graphites, TIMREX® KS6 and TIMREX® SLP30, and silicon nanoparticles have been prepared by a simple mixture procedure and coated on Cu foil by standard doctor blade coating technique. Electrochemical performance and cycling stability of the composite electrodes was explored in coin type cells cycled in EC:DMC 1:1w:w, 1M LiPF₆ electrolyte using metallic lithium as counter electrode. Different parameters such as the nature of the binder, the effect of graphite morphology, and the use of electrolyte additives are studied in long term cycling experiments.

The expansion of electrode layers was measured in a special dilatometry setup using three-electrode configuration, with both counter and reference electrodes of metallic lithium. A similar cycling program was used to explore the electrode's expansion, however only during the first few cycles. Differences observed in the expansion of the composite electrode layers help us to understand the distinct electrochemical behaviour of each graphite based system, with and without silicon particles.

The surface of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode material was coated with diamond like carbon (DLC) by chemical vapor deposition method, in order to prevent the side reaction at the interface for sulfide based all-solid state lithium-ion batteries. The DLC coated material showed a higher capacity with good cyclability and high rate performance. The interface resistance between the cathode active material and the solid electrolyte significantly decreases by DLC coating onto the NCA powder. The discharge capacity of DLC coated sample at 0.05C was 113 mAhg⁻¹ while that of bared one was about 102 mAhg⁻¹. The charge and discharge cut off potentials were 4.0 and 2.8 V, respectively. The thickness of the DLC coated layer was verified by transmission electron microscope (TEM), and was about 3`5 nm. The sp2 bonding ratio of the DLC coated LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> powder was estimated from the C-KELNES spectra to be 50`55%. DLC coating could avoid formation of an inactive layer when surface impurities of NCA react with the sulfide electrolyte to form at irreversible passivation byproduct. This layer of impurities obstructs the Li-ion transport at the interface, and therefore increases the interfacial resistance and decreases the rate capability for bare samples.

In this work, the diamond like carbon (DLC) coating were proposed for the first time as buffer coating layers for all-solid state battery. We demonstrated that the buffer layer is not needed to have a lithium conductivity to reduce the interface resistance between cathode material and sulfide based electrolyte.

As the need for nonpolluting individual mobility is rising, the production processes of lithium-ion batteries are gaining more and more attention. Although a lot of research for new battery materials or even for alternative battery technologies is done, the state of the art production process for large scale automotive lithium-ion batteries is still not completely understood. To get as close as possible to the theoretical performances of the used materials, an optimized process which includes a deep understanding of its influences on electrode structure and in the end on cell performance is essential. For this reason the drying process as one of the key-process-steps in the electrode production process is examined.

Since drying fixes the coating structure by evaporation of the solvent, the sedimentation of active material particles and the formation of the binder and carbon black matrix is influenced in a crucial way. In the present work some of the various drying parameters such as air temperature, outlet speed, air flow rate and residence time are varied and their influence on electrode structure is examined. A pilot-plant continuous convective dryer with three individual adjustable segments and a total length of six meters is basis of these investigations.

In order to analyze the resulting electrode structure measurements like mercury porosimetry (to determine the porediameter distribution of the coating), electrode conductivity (to describe the network of the conductive additives) and adhesion force measurements (to characterize the binder distribution) were carried out. As an example, a strong impact on the coating's adhesion force to the current collector with increasing drying intensity (or faster solvent evaporation, respectively) and coating thickness could be found and explained by changes in electrode's structure. Deeper insights in the processes affecting electrode structure can be obtained by determination of the electrochemical cell performance, thus allowing the establishment of process-structure-property relations.

Besides offline measurements to determine the electrode structure the drying process itself is monitored online by detecting the shift from constant rate drying period to falling rate period via infrared temperature sensors which measure the surface temperature of the coating. If the coating speed is kept constant the point of drying period change can be detected within a shorter distance from the dryer inlet when the air temperature is increased (see Fig. 1). The period change is identified for different drying parameters and electrode thicknesses as thicker electrodes tend to show a more distinct influence by the drying process.

In summary recent investigations show that electrode properties (e.g. adhesion force) resulting from solvent evaporation and structure fixation are clearly dependent on drying parameters and thus enabling a way for enhancing the battery's performance.

An important step often overlooked or rarely investigated in lithium-ion battery manufacturing is the formation process. The formation process is the first full charging cycle of a lithium ion battery, which activates the cells before the lithium-ion cells can be used. The study focuses on using novel thermal measurement tool to monitor heat flow during the first charging/discharging cycle and its subsequent cycles of newly manufactured 18650 cylindrical cells. Thermal models and formulations were developed to identify the complexity of heat influence when three cells were tested simultaneously. A heat flow profile versus formation voltage window was discovered to provide important information to the study. Figure 1 shows a clear voltage window from 2.0 V to 3.7 V was identified to use as a reference range for designing alternative formation current rates. The formation process can then be altered to allow the preferential formation current rates to obtain the highest formation efficiency and improved cell performance. The impact of the lithium-ion battery formation process on battery performance such as capacity and cycle life were also examined in this study.

Figure 1: Heater Power Change as a Function of Cell Voltage during C/10 Formation Current Rate

Olivine C-LiFePO₄ has been intensively studied since 1997 ^[1] due to its high thermal stability and the expected material cost reduction compared to usual cobalt based oxides. However, the energy density loss induced by the low operating voltage of LiFePO₄, 3.45V, leads to consider LiMnPO₄, 4.0V, as a promising candidate. But without dopants this material is kinetically limited: iron substitution seems to be a solution to make its utilization possible.

Several publications^[2],[3],[4] present promising results on this type of material with good rate capabilities, good cycle life, but these results are mainly obtained in half cells vs Li° with low loaded electrodes.

Our work is focused on C-LiMn_2/3Fe_1/3PO₄which has shown an excellent cyclability in half cells vs Li°.

Prototype cells vs graphite have been assembled using high loaded electrodes, with a design close to industrial cells. Results obtained show:

- excellent rate capability: >90% of capacity is recovered at 5C

- excellent cycle life: 80% of capacity is still remaining after 900 cycles

The study of ageing mechanisms by post-mortem analyses will be discussed.

Acknowledgment: All results presented here have been obtained on a material supplied by Clariant

[1] A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, Journal of The Electrochemical Society 1997, 144, 1188-1194.

[2] S.-M. Oh, H.-G. Jung, C. S. Yoon, S.-T. Myung, Z. Chen, K. Amine and Y.-K. Sun, Journal of Power Sources 2011, 196, 6924-6928.

[3] B. Z. Li, Y. Wang, L. Xue, X. P. Li and W. S. Li, Journal of Power Sources 2013.

[4] L. Hu, B. Qiu, Y. Xia, Z. Qin, L. Qin, X. Zhou and Z. Liu, Journal of Power Sources 2014, 248, 246-252.

Graphitic carbon, the most used anode in lithium-ion batteries (LIB), will not meet the ever increasing energy density requirements due to a low theoretical specific capacity (372 mA.h.g^-1 for LiC₆). To further increase the energy density of LIB, high-capacity anode materials have been intensely studied. Silicon appears as an ideal and abundant material for carbon replacement due to its high theoretical specific capacity (3579 mAh.g^-1 for the lithiated phase Li_3,75Si) and its low discharging potential against Li/Li+ reference (approximately 0,4V). However, huge volume change of Li_xSi_yalloys upon cycling together with SEI formation induces poor cycling stability and rapid capacity fading. The volume change can be partially counteracted by decreasing silicon particles to the nanosize where mechanical effects appear less severe and/or by limiting the expansion using a protecting shell.

In this context, we consider, as active material, nanoparticles covered by a carbon shell. Silicon coating techniques often imply two or more steps and the manipulation of nanopowders. Moreover, the low quantity of final product in most Si@C synthesis cannot meet mass production requirement. In the context to overcome these limitations, we developed a reactor where core-shell silicon carbon nanoparticles (Si@C Nps) are synthesized using the laser pyrolysis technique in a configuration with two reaction stages. The reactor is composed of two successive reaction zones: silicon cores are synthetized at the first stage and the carbon coating is achieved at the second. Silane is used as silicon precursor of the core and ethylene as carbon precursor of the shell. The Si core is not exposed to air before shell deposition preventing from SiO₂formation around silicon. Moreover the formation of silicon carbide, which is very detrimental to electrochemical properties, is avoided by complete decomposition of silane in the first stage. In this configuration, we can control the core diameter in the range 20 to 200 nm, the core organization (amorphous or polycrystalline), the shell thickness and its organization (turbostratic or graphitic).

We investigated both the effect of size reduction and the effect of carbon shell. Using pure 30 nm diameter Si Nps, we confirm that reducing the size (by comparison with a commercial grade of Si, 200 nm diameter) leads to a better stability of the electrode. In order to investigate the effect of the carbon shell we synthetized a control sample of 30 nm diameter Si Nps without insertion of ethylene. The same silicon core was covered with a 3 nm thick carbon shell by addition of ethylene in the second stage. We confirmed by Auger electron spectroscopy and high resolution transmission electron microscopy that Si@C nPs are well covered with carbon. XRD shows that the carbon shell is mainly turbostratic. Within this small size range, the beneficial effect of the carbon shell is observed compared to pure Si Nps. Without the carbon shell, half of the capacity is lost after 40 cycles while a good stability and high capacity (higher than 2000 mA.h.g^-1) is observed in the batteries elaborated from Si@C material. The origin of this beneficial effect will be discussed by comparison with literature.

To improve the safety of Li ion battery (LIB) in high energy applications, numerous efforts have been made in terms of choosing materials to be used.[1-2] However, these attempts have been limited by thermal stability and durability of separator including electrolyte. In this study, anodic aluminum oxide (AAO) film was used as a separator in Li ion battery for superior performance over commercialized polyolefin-based separator.

The AAO film has been prepared by two-step anodization method which is consisted of etching and anodizing processes. The as-prepared AAO film is around the 20 um thick and they showed perpendicular nanopore arrays having a diameter of 40nm.

The porosity of the AAO film is calculated about 58.79 %. [3] That value is higher than that of the commercial separator (55%, celgard 2500)

The wettability of the AAO film was analyzed using contact angle measurement with liquid electrolyte (1M of LiPF6 in EC:DMC:DEC=1:1:1 (v/v/v)). The AAO film shows higher wettability with smaller contact angle as value of 0^o than commercial separator (36^o). Small contact angle means low cell resistance which is suitable for high power LIBs. [3]

The test for electrolyte absorbance property of the AAO film was conducted. The electrolyte uptakes of the AAO film and the commercial separator are determined to be 126.88 wt% and 264.28 wt% respectively. [3] The density of the AAO film (3.98g/cm3) is smaller than the commercial separator (14.23g/cm3), so they required less electrolyte to operate LIB. Although less amount of electrolyte is used, the AAO film shows higher ionic conductivity of 2.06 mS/cm with SUS electrode than the commercial separator (0.66 mS/cm). This can be explained by low contact angle, which is agreed with low bulk resistance and higher permittivity of AAO film, it contributes to ionic conductivity. (Dielectric constant of AAO film : 28.59, relative permittivity of commercial separator: 9.06 at 1KHz)

The interfacial compatibility of lithium metal with separator was investigated by AC impedance with a lithium metal anode. It could be observed that the interfacial resistance was 118.33 Ω for the AAO and 141.92 Ω for the commercial separator, respectively. It means that the AAO film provide better interfacial characteristics for LIB.

Capacity of the AAO explored with half-cell performance, while LiFePO₄was applied as cathode from 2V to 4.2V. The AAO film and commercial separator showed similar capacity at 0.2C, the highest values were 123.0 mAh/g for AAO film and 122.6 mAh/g for commercial separator. Capacity recovery test was evaluated to confirm the correspondence with high energy LIB. Commercial separator was degraded from 1C rate and did not recovered. However, in case of AAO film, the capacity maintained 110.9 mAh/g at 0.5C, 88.4 mAh/g at 1C rate and recovered 121.1 mAh/g after investigation. And the AAO film showed high columbic efficiency over 98% at 0.5C rate. These excellent electrochemical characteristics related to Li dendrite. The AAO film can mechanically suppress the formation of Li dendrites, and homogeneous current distribution derived from well-ordered nanopores of the AAO film helps prevent Li dendrite growth.

In conclusion, the AAO film is a promising separator for LIB, especially in high energy performance, and durability test is needed for further study in the near future.

Reference

• Bruce, P. G.; Scrosati, B.; Tarascon, J. M., Angew. Chem. Int. ed., 47, 2930 (2008).

• D. Bansal, B. Meyer, M. Salomon, J. Power Sources 178(2), 848 (2008).

• J. Fang, A. Kelarakis, Y.-W. Lin, C.-Y. Kang, M.-H. Yang, C.-L. Cheng, Y. Wang, E.P. Giannelis, L.-D. Tsai, Phys. Chem. Chem. Phys. 13, 14457 (2011).

During the last decade Lithium metal oxides or polyanionic compounds containing two or more different transition metal ions (e.g. LiNi_0.5Mn_1.5O₄ or LiNi_1-x-yCo_xMn_yO₂) are in the focus of research and commercial interest as positive electrodes for Li-ion batteries. Lithium transition metal fluorides are very promising materials compared to common oxide materials with corresponding electrochemically active cations because the more electronegative fluorine atoms increase the redox potential leading to a higher specific energy. However, no reports are given about the electrochemical properties of quaternary lithium transition metal fluorides as positive electrode. In this study a novel sol-gel synthesis route was applied to synthesize several quaternary lithium transition metal fluorides LiMFeF₆ (M²⁺ = 3d transition metal) without toxic chemicals like HF, LiF or F_2(gas). After the structural and morphological characterization the fundamental electrochemical properties are characterized. It could be shown that up to 1 eq. Lithium can be inserted fully reversible into the LiMFeF₆ host structure with a notable cycling stability and a remarkable rate performance. Furthermore an in-situ x-ray powder diffraction and x-ray absorption investigation reveals that the host structure is an insertion material. The electrochemical active redox couple Fe^3+/2+ was confirmed by Mößbauer analysis.

It is well known that a flat plateau at 3.4 V appears in charging/discharging curve of olivine-type LiFePO₄ cathode. The constant voltage indicates that the extraction/insertion proceeds under two-phase reaction in which the electrochemical potential of lithium ions keeps a constant value independent of the state of charge. Thus, in the two-phase reaction, it is considered that the lithium ions can be extracted from the interface between the endmembers, namely, LiFePO₄ and FePO₄. The two-phase interface has been clearly observed on the surface of platelet nanoparticle by using transmission electron microscopy by several groups. The typical interface has a finite thickness about 10 to 20 nm and locates parallel to bc-plane. Delmas et al. explained the mechanism of electrochemical charging by means of "domino cascade model" in which the lithium ions are continuously extracted from the cascade of the interface region.

However, the lithium distribution in the interface is not clearly understood. The first-principles density functional theory (DFT) is a powerful tool to study such a small region from an atomic scale. In the present work, we theoretically investigated the interface region in LiFePO₄ compounds by using DFT. The crystal and the electronic structures are determined by using spin-polarized generalized gradient approximation with short-range Coulomb interactions (GGA+U). In this work, we employed selfconsistently obtained value of U = 4.3 eV which is sufficient to reproduce localized electronic structure. The wavefunction is expanded by planewaves with a kinetic-energy cutoff of 500 eV. The interface model is composed of three regions: fully lithiated (LiFePO₄), fully delithiated (FePO₄), and interface region. The interface region is assumed to be parallel to bc-plane, and is sandwiched between the endmembers. It is considered that the lattice parameters in the interface gradually changes along a-axis. The total energy of the structure in the interface can be determined by giving the intermediate value of the lattice parameters to the unit cell at a microscopic area in the interface. We calculated mixing energies of the endmembers to form partially delithiated structures at the microscopic area by considering possible configurations of lithium ions and localized electrons. The mixing energy of the partially delithiated structure, F, is defined as F = E(x) − [(1−x) E(x=1) + x E(x=0)], where E(x) is the total energy of LixFePO4, and x is concentration of lithium ions. The positive value of F means that the two-phase separation is favored, and the negative value of F means that the solid-solution structure of LixFePO₄ is likely to appear.

The mixing energy of the partially delithiated structure LixFePO₄, (x=0, 0.25, 0.50, 0.75, and 1) is calculated. From the calculated mixing energy, it is found that the curves of the mixing energy are different among the three regions. In the interface, the mixing energy shows negative value with lithium concentration of 0.5 and 0.75. It means that the solid-solution phase is spontaneously formed in the interface region parallel to bc-plane.

SAFT has successfully applied Li-ion electrochemistry to defense, space, and commercial applications which require very high power. In fact, by optimizing both the electrochemical and electro-mechanical cell design, Saft has developed a range of Li-ion products that can deliver up to 10 kW/kg of power for 10 seconds and over 30 kW/kg of power for a fraction of a second. There are, however, still inherent limitations for high power applications with traditional Li-ion chemistry. Particularly, even with an optimized cell design, cycle life can be limited for applications which require very rapid charging due to both Lithium plating and electrolyte decomposition on the graphitic negative electrode. Lithium titanate oxide (LTO), due to operation above both the lithium plating and solvent reduction potentials, minimizes these phenomenon and can potentially provide extremely long cycle life. Saft is working, with DOE/USABC funding, to industrialize LTO based technology for high power applications. Results will be presented including power capability, high temperature impedance growth, capacity fade, and gas generation. In particular, impedance growth, which is a major obstacle to large scale use of LTO, will be discussed including mechanistic understanding and methods of mitigation.

Introduction

Cycle life test methods for batteries used in electric vehicles (EVs) have been proposed by several organizations, e.g., the International Organization for Standardization (ISO), the International Electrotechnical Commission (IEC), and the Society of Automotive Engineers (SAE). In China, national industry standards (QC/T) including standards covering the test procedure for evaluating cycle life of batteries used in EVs have been established. However, there are few published results concerning evaluations of battery life using these standard test methods, and their adequacy remains uncertain. In this study, three types of cycle life tests were applied to high-capacity lithium-ion cells at 45°C. The degradation phenomena caused by each test method were evaluated, and their features and issues were considered.

Experimental

A commercial 40Ah class lithium-ion cell composed of a LiFePO₄cathode and a graphite anode was used in this study. The cycle life of the cell was evaluated by three types of standard test methods, IEC62660-1 [1] and two types of QC/T (constant current and dynamic current profile, denoted as QC/T-1 and QC/T-2, respectively) at 45°C.

The IEC 62660-1 cycle life test has two power-controlled dynamic discharge profiles: dynamic discharge profiles A and B shown in Figs. 1(a) and 1(b), respectively. In these profiles, the cell energy (Wh (at 25°C)) × 3 (1/h) was used as the maximum power. After fully charging at 1/3C in constant current-constant voltage (CC–CV) mode, the cell was discharged following the dynamic discharge profile A until the discharge capacity reached 50% ± 5% of the initial value. After the dynamic discharge profile B was applied once, the dynamic discharge profile A was reapplied until the discharge capacity reached 80% of the initial value. Then, the cell was discharged at 1/3C to the lower voltage limit. These procedures were repeated for 28 days. The cell performance, capacity (1/3 C), and internal resistance (DC) were periodically measured at 25°C.

In QC/T-1, the cell was discharged at a constant current of 1 C to the lower voltage limit after fully charging. The cell performance was evaluated every 100 cycles. The procedure for QC/T-2 was somewhat more complex. The cell was discharged following the dynamic current profile (Fig. 2) until the state of charge (SOC) reached 20% and then charged at 1/3C in CC-CV mode to the upper voltage limit. This procedure was repeated four times; the expended time was defined as x h. Then, the cell was stored for 24 − x h at 100% SOC. The value of x depends on cell specifications; in this study, x was approximately 4 h. The entire procedure described above was repeated. The cell performance was evaluated at monthly intervals following IEC standards.

Results and discussion

The cycle life test was conducted for approximately 6 months, and the capacity retention results for three types of standard cycle life tests were compared. Three cells were evaluated by all three standard methods; average results are shown in Fig. 3. Cumulative discharge capacity was used as the horizontal axis to compare the results obtained from different cycle conditions. An equivalent cycle number, which was defined as the ratio of cumulative discharge capacity to the initial capacity, was added to the second horizontal axis to indicate cycle number.

Rapid capacity decay was observed for QC/T-1 compared to IEC 62660-1 and QC/T-2. Similar results were obtained for internal resistance. Even though the maximum value of the C-rate in QC/T-1 is lower than other methods, the average was highest (Table 1). A high C-rate condition is thought to induce increased cell temperature and deformation of the crystal structure of electrode active materials. These phenomena can cause rapid degradation in QC/T-1. From the perspective of SOC, QC/T-2 has longer duration under high SOC conditions; however, capacity decay in QC/T-2 was not severe compared to other methods. The cathode potential of the cell used in this study is lower than other types of cathode materials, e.g., spinel-type lithium manganate or layered transition metal oxides. The low cathode potential is thought to weaken the impact on degradation under high SOC conditions. Other types of lithium-ion batteries have also been evaluated, and the results will be reported at the conference.

References

[1] IEC62660-1, "Secondary lithium-ion cells for the propulsion of electric road vehicles - Part 1: Performance testing", edition 1.0 (2010)

INTRODUCTION

In last few decades, lithium transition-metal oxides with layered structures, such as Li(Ni,Mn)O₂ and Li(Mn,Ni,Co)O₂, have drawn much attention as a cathode material for the lithium-ion battery. Electrochemical properties of these oxides are supposed to be affected by the local cation configurations in the crystals, and thus the local structures have been studied actively by various methods, like NMR, extended X-ray absorption fine structure (EXAFS) technique and pair distribution function (PDF) fitting.

In this study, we perform the Reverse Monte Carlo (RMC) modelling using the Bragg reflections and total scatterings simultaneously for an analysis on atomic configurations of crystalline lithium transition-metal oxides. Based on the result, we try to determine average (periodic) and local (non-periodic) atomic configurations at the same time.

EXPERIMENTAL

In this work, a lithium transition-metal oxide LiNi_0.5Mn_0.5O₂ was synthesized. The sample was identified preliminarily by laboratorial X-ray diffraction measurement, and the metal composition was evaluated by the inductively-coupled plasma (ICP) technique. Neutron total scattering pattern of the sample was recorded with HIPPO installed at LANSCE, USA. From the pattern, the normalized reciprocal space data, F(Q), and the real space data, G(r), were obtained. By using these data and the neutron Bragg reflections, we performed the RMC modelling with RMCProfile. A rhombohedral simulation box of 2400 atoms, Li₆₀₀Ni₃₀₀Mn₃₀₀O₁₂₀₀, was used in the modelling, and then the atomic configuration was simulated.

RESULTS AND DISCUSSION

From laboratorial X-ray diffractions, it was confirmed that the LiNi_0.5Mn_0.5O₂ had the layered rock-salt structure (space group: R-3m). The ICP analysis demonstrated that the analytical molar ratio of Li and the transition-metal ions was in good agreement with the nominal composition.

For the sample, we measured neutron total scattering pattern and then performed the RMC simulation using the Bragg reflection, F(Q) and G(r). In the analysis, F(Q) was degraded by convolution in order to take the finite cell size into account, which is denoted as F_conv.(Q) in this work. Fig. 1 shows the RMC simulation result for the F_conv.(Q)_. as an example. This result demonstrates that the RMC simulation was successfully carried out by using the data set mentioned above. Fig. 2 shows partial radial distribution functions, g(r), of Li-O, Ni-O and Mn-O. It is found that average bond distances of Ni-O and Li-O are the same essentially, but that of Mn-O is shorter than the others. This tendency can be explained well by differences in the ionic radii of Li⁺, Ni²⁺ and Mn⁴⁺ with 6-fold coordination. From distribution of the bond angles, it is suggested that Ni and Mn forms a local ordering in the transition-metal layer.

Silicon is candidate as an anode material for high energy Li ion batteries. Among Si materials, Si alloys attract great interests from a viewpoint of high durability and high coulombic efficiency. We found that sputter-deposited some of amorphous Si alloys (ex. Si-Sn-Ti ternary alloy) showed high electrochemical performance from the combinatorial study. In the present study, we synthesized amorphous Si alloys by using a Melt Spinning process and investigated the potential toward volume production as well as the electrochemical performance.To obtain amorphous Si alloys, the ease of mixing between Si and added metals or the chemical mixing enthalpy is an important factor. In the case of Si-Sn-Ti alloy, the mixing enthalpy decreases by increasing Ti/(Ti+Sn) ratio and it is expected for easier preparation of the amorphous alloys at higher Ti/(Ti+Sn) ratios (Fig. 1). The SEM/EPMA elemental mapping of several Si-Sn-Ti alloy indicated that the Si atoms in Ti rich composition alloy (Si/Sn/Ti=60/10/30 wt%) is more uniformly and finely dispersed than Sn rich composition alloy (Fig. 2). The electrochemical performance will be reported in the presentation. This work was partially supported by the project "Research and Development of Lithium ion Battery Application / Research and Development of High Capacity Si-based Alloy Anode" of the New Energy and Industrial Technology (NEDO).

Meeting increasing energy demands, storage requirements and energy portability will be expedited through an ability to directly image lithium battery material nano/micro structures at high resolutions in 3D. The performance of the battery is dependent on nano/micro-structure as important reactions often occur at porous electrode interfaces. Furthermore, during processing or operation microstructural evolution may degrade electrochemical performance.

Despite their importance, battery electrode microstructures remain poorly understood, limiting current understanding of their failure. Tomographic techniques allow for the direct 3-D imaging and chracterisation of complex microstructures from millimetres down towards nanometers.

Here we present results from 3D x-ray and FIBSEM tomography of battery materials (e.g. Figure 1) down to resolutions of tens of nanometers. Quantitative analysis of critical parameters important for battery materials is evaluated, and these are linked to simulation tools to relate structure to performance characteristics. Both x-ray and FIBSEM tomography can provide this information, facilitating analysis of micro/nano structure, morphology and provide exciting opportunities to study fabrication effects or operating conditions that could cause electrode degradation and failure.

We find this approach is effective in understanding how lithium ion batteries function at high resolutions and consequently that tomography coupled with modeling/experiments can provide new insights into degradation mechanism.

Figure 1 – 3D lithium ion anode structure with MCMB particles

Li[Li_1/3Ti_5/3]O₄ (LTO) is an ideal negative electrode material for long-cycle life lithium-ion battery (LIB), because of its "zero-strain" character [1]. In the LTO crystal lattice with Fd-3m space group, Li⁺ ions occupy both tetrahedral 8a and octahedral 16d sites, while Ti⁴⁺ ions sit the 16d site. We, thus, expect a solid solution compound Li_1/2+1/2xFe_5/2–3/2xTi_xO₄ (LFTO) between LTO and Fe[Li_1/2Fe_3/2]O₄ via an intermediate spinel compound Li_1/2Fe_1/2[Li_1/2Fe_1/2Ti]O₄. Fe[Li_1/2Fe_3/2]O₄ is known to be one of the ferrites, which exhibit a soft ferromagnetic behavior. Here we report the results of powder X-ray diffraction (XRD) and electron diffraction (ED) measurements, and Raman spectroscopy analyses on the LFTO samples with 0 ≤ x ≤ 1.666.

According to XRD measurements, all the LFTO samples were assigned as a spinel-framework structure. For the x = 0 sample, the extra diffraction lines were clearly observed in addition to the normal diffraction lines in the Fd-3m. The extra diffraction lines are attributed to the superlattice diffraction lines due to the ordering between Li⁺ and Fe³⁺ ions at the octahedral site. As x increases from 0.25, such superlattice diffraction lines disappear, but, surprisingly, superlattice diffraction lines appear again at x > 1.2. The crystal structure at x £ 0.25 and 1.2 < x < 1.6 are assigned as the P4₃32 space group. Combining with the result of x dependence of cubic lattice parameter (a_c), the crystal structure of LFTO is classified into the four regions; (I) regular spinel structure of Fd3-m with x > 1.55, (II) superlattice spinel structure of P4₃32 with 1.2 ≤ x ≤ 1.55, (III) regular spinel structure of Fd3-m with 0.25 < x < 1.2, and (IV) superlattice spinel structure of P4₃32 with x ≤ 0.25.

Acknowledgement

This work was supported by Grant-in-Aid for Scientific Research (C), 25410207, MEXT, Japan.

References

[1] T. Ohzuku, A. Ueda, and N. Yamamoto, J. Electrochem. Soc., 142 (1995) 1431.

One of several challenges in improving the performance of lithium-ion batteries is the enhancement of energy density. Apart from the development of ever better active materials, considerable potential for improvement can be found in the complex manufacturing process. The manufacturing of lithium-ion battery cells exhibits a long, mostly sequential process chain with numerous interdependent parameters and quality-determining characteristics. Among slurry mixing and coating, calendering is the most challenging process step and considered of superior importance to the quality of the electrode. Calendering refers to compressing the porous coating layers of the electrode to a predetermined thickness. To obtain the desired energy density and to prepare a homogeneous electrode thickness it is necessary to densify the coating of the electrodes. In this, more electrochemically active material can be accommodated in every unit of electrode volume. In essence, this defines the potential energy density of the cell. As for high-energy battery cells energy density is one of the most important optimization criteria to achieve ambitious requirements for automotive applications calendering is of crucial importance.

However, compression of the electrode is not merely beneficial to the overall performance of the cell. It is a trade-off between achieving high energy density and, on the contrary, damaging the electrode's structure by applying high mechanical forces. An effect rarely taken into account is mechanical strain induced to the electrode's metal foil underlying the coating, which can lead to serious issues in manufacturing and malfunction of the battery cell.

This paper addresses macroscopic changes in electrode's metal foils resulting from intense calendering procedures of high-energy NMC cathodes. Figure 1 exemplarily depicts an area-resolved longitudinal strain map showing irreversible deformation from calendering. The left green section reflects an uncoated edge of the electrode which is not deformed during calendering. The coated section of the specimen is deformed significantly. The data is derived from before/after comparison of the electrode's surface, conducted with a non-contact optical 3D deformation measuring system. To minimize undesired strain inducement, multi-calendering strategies are assessed which gently apply loads and at the same time still provide for highly compressed electrodes.

Introduction

Recently, metal phosphides have received much attention as anode material for lithium ion batteries due to high reversible capacity at relatively low potential.[1] However, most MP_x suffers from a relatively large irreversible capacity due to the decomposition of Li_xM, Li_xP, and metal during cycling. In order to improve the electrochemical properties, ternary M-M'-P system have been suggested and some of them prove to be effective in improving the cycle performance. [2] Cho et. al. reported the electrochemical properties and reaction mechanism of nanoparticle cluster MoP₂ anode. [3] It showed high reversible capacity and good cycle performance at limited voltage range. However, upon increasing voltage, the capacity decreases rapidly, suggesting that MoP₂ decomposed to Mo and LiP_nphase. Here, we developed a novel Si-substituted MoP₂ (Mo_0.8Si_0.2P₂) alloy to increase the reversible capacity and enhance the cycle performance. We investigated the differences in physical and electrochemical properties between MoP₂ and Mo_0.8Si_0.2P₂ anode, examined using powder X-ray diffraction, SEM, and TEM, and galvanostatic charge-discharge test. We also investigated the volume expansion of MoP₂ and Mo_0.8Si_0.2P₂ alloys during cycling, considering the influence of substitution of Si into MoP layers in MoP₂alloy.

Experimental

Orthorhombic Molybdenum Phosphide (MoP₂) and Si-substituted Molybdenum Phosphide (Mo_0.8Si_0.2P₂) were synthesized by mechanical-ball milling method. The physical properties were examined by powder XRD, SEM, HR-TEM and EDX. The electrochemical characterizations were performed using a CR2032 coin-type cell. The electrodes were prepared by mixing 75 wt% active material, 10 wt% Super P conducting agent, and 15 wt% PAA binder dissolved in NMP. The cells were made of a lithium metal anode separated by polypropylene film. The electrolyte was a mixture of 1M LiPF6-ethylene carbonate (EC)/diethylene carbonate (DEC)/dimethyl carbonate (DMC) (3:5:2 by vol) with 10 wt% fluroethylene carbonate (FEC). The charge and discharge rate was both 0.1C (0.3 mA/cm²) with a cut-off voltage of 5 ~ 1200 mV.

Results and Discussion

Fig. 1 shows the typical XRD results of MoP₂ and Mo_0.8Si_0.2P₂ alloy powders prepared by mechanical milling method. All peaks of MoP₂ and Mo_0.8Si_0.2P₂ could be indexed as the Orthorhombic MoP2 phase of Cmc21 (JCPDS#00-016-0499). The lattice constants of Mo_0.8Si_0.2P₂ show smaller than those of pristine MoP₂ because of substitution of smaller Si ion. The broad peak at 2θ = 22^oindicated the residue amorphous red P. Fig. 2 shows the cycle performance of the Li/MoP₂ and Li/Mo_0.8Si_0.2P₂ half cells at 0.1C rate with voltage region from 1200 to 5 mV. The Mo₀P₂ anode showed a specific capacity of 655 mAhg^-1 in the first charge process and showed a capacity retential rate of 88 % after 100 cycles. Conversely, the Mo_0.8Si_0.2P₂exhibited much higher initial capacity of 783 mAhg^-1better cyclic retention rate of 93.2 % after 100 cycles.

Acknowledgement

This study was supported by a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2011-C1AAA001-0030538).

References

1. D. C. S. Souza, V. Pralong, A. J. Jacobson, L. F. Nazar, Science296, 2012 (2002).

2. G. Park, C. Lee, J. Lee, J. Choi, Y. Lee, S. Lee, J. Alloys compd.585, 534 (2014).

3. M. Kim, S. Lee, C. Cho, J. Electrochem. Soc.156, A89 (2009).

Introduction

There is an increasing demand for the performance and lifetime predictions of lithium-ion batteries under real operating conditions for their application into electric vehicles and energy storage systems. In order to establish a degradation model, the degradation factors, such as decomposition of electrolyte, formation of solid electrolyte interface (SEI), Li metal deposition, etc..., should be evaluated. In this paper, we conducted cycle tests using commercial 18650-type lithium ion cells at the conditions of three temperatures (0 °C, 25 °C, and 45 °C) and two current rates (1 C and 2 C). The electrodes and electrolytes of the degraded cells were also investigated by the several analytical methods.

Experimental

The active materials of cathode and anode are Li(Ni_1/3Mn_1/3Co_1/3)O₂ and graphite, respectively. The electrolyte is composed of EC, PC, EMC, and DMC, containing LiPF₆. The discharge capacities and internal resistances were periodically measured at 25 °C during the cycle tests. To understand the degradation mechanism, we disassembled the degraded cells and took out the electrodes and the electrolytic solution. The obtained electrolytic solutions were investigated by ¹H- and ¹⁹F-NMR, and GC-MS. The electrodes were investigated by ICP-MS, XRD, XPS, ⁷Li-NMR, and SEM. Moreover, the solid electrolyte interfaces (SEIs) of the electrodes were extracted by solvent and investigated by ¹H- and ¹⁹F-NMR.

Results and discussion

Fig. 1 shows the discharge capacity measured at 25 °C vs. cumulative discharge capacity plots during the cycle life tests. Surprisingly, the discharge capacities rapidly decreased at 0 °C conditions regardless of the cycle rates. As for 25 °C and 45 °C conditions, the discharge capacity decreased faster at 45 °C at 1 C rate, while discharge capacity decreased faster at 25 °C at 2 C rate. Fig. 2 shows the increase in the internal resistances measured at 25 °C, and again, the internal resistances increased rapidly at 0 °C, and the internal resistance at 25 °C increased faster than that at 45 °C at 2 C rate. At 1 C rate, the internal resistance at 25 °C increased faster than that at 45 °C before the drastic increase at around 1500 Ah.

The electrolyte analyses revealed that the decomposition of LiPF₆, which causes increase in internal resistance, occur faster at higher temperature conditions at both 1 C and 2 C cycle tests. As a consequence, the amount of inorganic metal fluoride SEI investigated by ¹⁹F-NMR was larger at higher temperature conditions. ¹H-NMR showed that the organic SEI formations also progressed at high temperature. These SEI formations consume active lithium for charge/discharge, and hence, the discharge capacities decrease. On the contrary, ⁷Li-NMR of anode showed that the metal Li tended to deposit at low temperature and high cycle rate. The surface SEM images drastically changed at high cycle rate, indicating the SEI formations of the anode proceed fast. In addition, XPS spectra showed that the component of the SEIs was different, that is, the ratio of carbonates was high at low temperature, and the ratio of phosphate was high at high temperature.

In summary, the observed cell degradations were caused by the several degradation factors which had different temperature/cycle rate dependences.

Estimating the rate of heat generation in a Li-ion battery is very critical for designing a battery thermal management system. Much research has been done that establishes the relationship between the rate of heat generation of a cell to its temperature, state-of-charge (SOC), and rate of charge or discharge. However, experimental studies of the thermal behavior of these cells have focused mainly on the small coin and cylindrical cell formats and at low C-rates.¹ Available literature on the heat generation of large format cells at high C-rates are scant and was the impetus for this study.

The lumped capacitance method (LCM) is a useful technique that can greatly simplify an otherwise complicated problem in transient heat transfer. However, the validity of LCM results hinges on how well certain assumptions and conditions hold. The criterion in this case is to have spatial temperature uniformity throughout the cell. This condition becomes tenuous as the cell is inherently anisotropic and is subjected to increasingly higher C-rates. Often times, computer simulation models summarily assume pouch cells have negligible temperature difference across their thicknesses regardless of size or C-rate.² They have also considered thermal properties to be isotropic and independent of temperature.³

The objective of this research is to determine how well the LCM predicts the rate of heat generation in a large format pouch cell at several different C-rates of full constant-current discharge. The rate of heat generation will be also measured by a battery calorimeter. The LCM data will be compared to the calorimetric data.

Experimental Setup

The initial step in this investigation was to determine the average convection coefficient of the cell geometry when oriented as a vertical plate in natural convection. This was accomplished by first having the 14Ah cell self-heat during a 5C discharge. An infrared video camera recorded the cell surface temperature as it naturally cooled to ambient room temperature. Using the lumped capacitance model (LCM), an average convection coefficient was found from curving the LCM model. The predicted cooling rate was found to have an extremely good correlation to the measured data. This was due in large part to the high area-to-volume ratio of the cell format. The derived value of the average convection coefficient was then used in an energy balance equation. It was then possible to model the rate of heat generation of the pouch cell under different rates of discharge by simply knowing the cell surface and ambient temperatures as well as some thermodynamic properties.

This same cell then underwent various rates of discharge while having its surface temperature recorded by the infrared video camera at ambient room temperature. From this data, the rate of heat loss through convection and stored within the bulk mass was calculated and summed to equate to the internal rate of heat generation. Precautions were taken to minimize heat loss through conduction.

Preliminary Results

During a full discharge at a C/5 constant-current rate, the heat generation predicted by LCM did extremely well in matching the calorimetric data. However, during a 1C discharge, the rate of heat generation predicted by LCM tended to be in lower than that measured by the calorimeter for most of the time. The average rate of measured heat generation was 2.82W while LCM predicted 2.13W. This computes to LCM being 24% lower than the calorimetric data on average.

The major contributor to the discrepancy between the LCM calculations and calorimetric data is the rate of temperature change as a function of time. This value is used to calculate the rate of heat storage within the cell. Heat storage (and heat generation for that matter) are volumetric phenomena. It is very difficult to accurately calculate this rate of temperature change based on surface measurements. In addition, as the C-rates become increasingly higher, temperature gradients become more pronounced throughout the cell core. This has a detrimental effect on being able to apply LCM.

References

[1] Bandhauer, T.M., Garimella, S., and Fuller, T.F., "A critical review of thermal issues in lithium-ion batteries," Journal of Electrochemical Society, pp. R1-R25 (2011).

[2] Kim Yeow, Ho Teng, Marina Thelliez, and Eugene Tan, "3D Thermal Analysis of Li-ion Battery Cells with Various Geometries and Cooling Conditions Using Abaqus", SIMULIA Community Conference, Providence, RI, May 15, 2012

[3] S. Al Hallaj, H. Maleki, J. Hong, and J. Selman, Journal of the Electrochemical Society, 83, 98 (1998)

Electrochemical impedance spectroscopy (EIS) is powerful tool to evaluate a performance of lithium-ion battery because time constants in electrochemical signals can be distinguished with no damage to the electrode. ^1,2

Separation of time constants of anode and cathode is desired in the interpretation of impedance spectra of a lithium-ion battery. Multi -EIS measurement is an analysis method in which simultaneous measurements of three impedance spectra (cathode-reference electrode, anode-reference electrode and anode-cathode) are carried out using a suitable reference electrode. The time-constants of anode and cathode from full cell impedance spectra can be separated by using Multi-EIS measurement.

On the other hand, in the case of in-situ EIS measurements for lithium-ion battery, impedance spectra were measured successively during charge or discharge sequence. The in-situ EIS measurement is very useful for monitoring the degradation process of lithium-ion battery during the charge-discharge cycles. ³

In the present study, we developed a multi-in-situ EIS measurement system in which simultaneous measurements of three impedance spectra were carried out during the charge-discharge cycle. The system is based on a newly developed potentiostat which has an integrated high-performance frequency response analyser and data processing software.

The electrode reactions in rechargeable batteries do not satisfy the time stability during the charge–discharge cycles and the low frequency components of the impedance might contain significant errors due to the time variation. In order to compensate the impedance spectrum deviated by the variation of reaction resistance in the charge or discharge, the impedance spectra were measured successively and plotted on the three-dimensional (3D) complex diagram, which has a time axis. The plots were connected by the spline under tension function at each frequency. The cross-section of 3D impedance shell perpendicular to the time axis gives the instantaneous impedance at an arbitrary time, which can be given automatically by using the software. We investigated the influence of SOC and C-rate on impedance spectra of both cathode and anode.

Acknowledgement

This work was supported by JST A-STEP (AS2421666K).

References

1. M. Itagaki, in Nanoscale Technology for Advanced Lithium Batteries, T. Osaka and Z. Ogumi, Editor, p.123, Springer Science+Business Media, New York (2014).

2. T. Osaka, T. Momma, D. Mukoyama, H. Nara, J. Power Souces 205 (2012) 483.

3. M. Itagaki, N. Kobari, S. Yotsuda, K. Watanabe, S. Kinoshita, M. Ue, J. Power Souces 148 (2005) 78.

Various portable electronic devices are wildly used in the world due to the remarkable progress of information technology since 2000. It leads to the increasing demand for rechargeable batteries. As a result, Lithium Ion batteries (LIB) are intensively investigated by many researchers. Currently graphite-based anode materials have widely used. In order to extend to rapidly increasing demand and wider applications as electric vehicles, we have to develop a new anode material. Si-based alloy anode material is one of the strongest candidates because it has many advantageous properties such as high capacity, high safety, good electrochemical stability, and low production cost. M. Kim. et al.[1] has developed a good Si-base alloy anode material and they reported the structural and electrochemical properties[1]. The Si-base alloy is consisted of finely dispersed active silicon crystals and surrounding inactive metal matrix. The main role of inactive-matrix in Si-based alloy is suppressing the volume change and it helps the Si-based alloy maintain good cycle performance.

In this study, in order to enhance the cycle performance, we have added Sn metal which has theoretical capacity of 972mAh/g. Si and Sn nanocrystallites embedded in Si-Ti-Fe matrix were developed using arc melting followed by a rapid quenching method which could produce a large scale at one time. To identify the reaction mechanism, the ex-situ XRD, SEM, TEM, analyses were employed. Si and Sn nanocrystallites of approximately 100nm were dispersed in matrix composed of Si2TiFe. Consequently the Si/Sn/Si2TiFe composite showed a good cycle performance with 1000mAh/g over 50 cycles.

[1]Journal of electroanalytical Chemistry 687 (2012)84-88

Recent work has shown good correlation between short term measurements of the coulombic efficiency and long term capacity retention during relatively slow cycling (greater than 20 hours per cycle) and high temperature (above room temperature) [1-3] using the High Precision Charger built at Dalhousie University which is able to measure coulombic efficiency to < 0.01% error [4]. Smith et al. [5] studied how the charge rate impacts the coulombic efficiency in commercial cylindrical cells of different positive electrodes and graphite negative electrodes at different temperatures. This work showed that the departure of the coulombic efficiency from the ideal value of 1.00000 was dependant on the time of one cycle and independent of charge rate for slow rates and elevated temperatures. This work could not investigate the dependence of coulombic efficiency on high charge rates due to limitations of the equipment.

Smith et al. [5] showed that as the charge rate of Li-ion cells was increased from ~C/100 to ~C/20, the coulombic efficiency (CE) increased such that (1 – CE) = k*t, where k is a constant that increases with temperature and t is the time of one charge-discharge cycle. Extending this idea to higher cycle rates means that at very high rates the coulombic efficiency approaches 1.00000 since there is very little time per cycle for degradation. However, it is well known that lithium plating can occur at high charge rates with graphite negative electrodes [6, 7] and this would lower the coulombic efficiency. It has been shown that the coulombic efficiency of plating and stripping metallic lithium from graphite electrodes is 0.97-0.98 [8] compared to the coulombic efficiency of intercalating and de-intercalating lithium from a graphite electrode which is > 0.995 [9]. Therefore as the charge rate continues to increase past the onset of lithium plating, the coulombic efficiency should begin to depart further from 1.00000 as a larger fraction of the lithium is involved in plating/stripping instead of intercalation/de-intercalation reactions.

Figure 1 shows CE versus charge rate results for two temperatures that shows the time dependent degradation regime at low rates on the left (low rates) and the regime governed by lithium plating on the right (high rates).

This study will show results of high charge rate cycling on Li[Ni_1/3Mn_1/3Co_1/3]O₂ (NMC)/graphite cells at different temperatures to more thoroughly understand the shape of these curves and the impact of lithium plating on precision measurements of coulombic efficiency and endpoint capacity slippage. This behavior needs to be well understood as many applications for Li-ion batteries, including electrified vehicles, desire high charge rates.

References:

[1] J.C. Burns, G. Jain, A.J. Smith, K.W. Eberman, E. Scott, J.P. Gardner, and J.R. Dahn, J. Electrochem. Soc., 158, A255-A261 (2011).

[2] J.C. Burns, N.N. Sinha, D.J. Coyle, G. Jain, C.M. VanElzen, W.M. Lamanna, A. Xiao, E. Scott, J.P. Gardner, and J.R. Dahn, J. Electrochem. Soc., 159, A85-A90 (2012).

[3] A.J. Smith, H.M. Dahn, J.C. Burns, and J.R. Dahn, J. Electrochem. Soc., 159, A705-A710 (2012). [4] HPC

[5] A.J. Smith, J.C. Burns, and J.R. Dahn, Electrochem. Solid-State Lett., 13, A177-A179 (2010).

[6] S.S. Zhang, J Power Source, 161, 1385-1391 (2006).

[7] W. Lu, C.M. Lopez, N. Liu, J.T. Vaughey, A. Jansen, and D.W. Dees, J. Electrochem. Soc., 159, A566-A570 (2012).

[8] L.E. Downie, L.J. Krause, J.C. Burns, L.D. Jensen, V.L. Chevrier, and J.R. Dahn, J. Electrochem. Soc., 160, A588-A594 (2013).

[9] A.J. Smith, J.C. Burns, X. Zhao, D. Xiong, and J.R. Dahn, J. Electrochem. Soc., 158, A447-A452 (2011).

Introduction

Considering the size of a battery pack in a full electric vehicle, the importance of a safe and reliable state of health assessment of the single cell is obvious. With increasing number of commercialised metal oxide and metal phosphate based cathode materials, new ways to monitor the cell's state of health have to be developed to allow a save operation at all times. Especially non-uniform aging, caused by state of charge (SOC) and temperature gradients are a crucial risk for the cell's reliability^1–5. One approach under consideration uses the thermodynamic properties of Li-Ion cells. It has been shown to be a suitable, nondestructive tool to access further properties of the cell, allowing a closer look into the electrochemical processes in the different electrodes^6–9. Measurements of open circuit voltage at different temperatures allow the calculation of the differential charge/discharge entropy according to equation (1)

where is the open circuit voltage of cell at equilibrium state and the Faraday constant. The variation of as a function of the Li content x in the electrode yields to characteristic "change of entropy" profiles, indicating structural changes in the electrodes during charge and discharge. Furthermore, the full cell's entropy profile can be calculated as a sum of the entropy profiles of anode and cathode, given in equation (2)

As a result, irreversible structural changes within the electrodes as well as de-balancing between anode and cathode with ongoing aging are reflected in the full cell profile^10,11.

Experimental

Different types of commercial 18650 cells were stored at 60°C at 100% SOC to simulate accelerated calendar aging or were cycled at 25°C with a 1C rate respectively until the cell's nominal capacity C_N measured at 0.2C dropped down to 80% of its initial value_. At this defined end of life, the cells were opened in a glove box under argon atmosphere. Different sections of the electrodes were used to rebuild coin cells for local entropy measurements as well as XRD measurements to investigate structural changes.

Results and Discussion

Figure 1 shows the changes in the entropy profiles of the two different aging treatments on the example of a high power cells with a NCA-based cathode as well as a graphite anode in comparison to the profile of a new cell. The capacity loss during the aging process causes a shrinking of the curve on the x-axis. Beside this expected effect, changes in the profile highlights become visible; the maximum at 900mAh vanishes while new a maximum appears at 450mAh for the heat treated cell and at 600mAh for the cycled cell, respectively. Changes in the entropy profile during ongoing aging need to be interpreted carefully as different effects influence the full cell profile.

In this work, local entropy measurements from different sections of the electrodes in combination with the respective XRD patterns are presented. Therefore, the effects of the different aging treatments on anode and cathode, the influence of irreversible structural changes in the cathode as well as the effect of electrode debalancing on the entropy profile are separated. To conclude the work, we will discuss in how far entropy profiles can be used beneficially as a supplement to existing methods for SOC and State-of-Health monitoring.

References:

1. S. Paul, C. Diegelmann, H. Kabza, and W. Tillmetz, J. Power Sources, 239, 642–650 (2013) http://linkinghub.elsevier.com/retrieve/pii/S037877531300116X.

2. B. Stiaszny et al., J. Power Sources, 258, 61–75 (2014) http://linkinghub.elsevier.com/retrieve/pii/S037877531400202X.

3. B. Stiaszny, J. C. Ziegler, E. E. Krauß, J. P. Schmidt, and E. Ivers-Tiffée, J. Power Sources, 251, 439–450 (2014) http://linkinghub.elsevier.com/retrieve/pii/S0378775313019150.

4. A. Barré et al., J. Power Sources, 241, 680–689 (2013) http://linkinghub.elsevier.com/retrieve/pii/S0378775313008185.

5. M. Dubarry et al., J. Power Sources, 196, 3420–3425 (2011) http://linkinghub.elsevier.com/retrieve/pii/S0378775310012127.

6. A. THOMPSON, Phys. B+C, 105, 461–465 (1981) http://dx.doi.org/10.1016/0378-4363(81)90295-3.

7. K. E. Thomas, C. Bogatu, and J. Newman, J. Electrochem. Soc., 148, A570 (2001) http://link.aip.org/link/JESOAN/v148/i6/pA570/s1&Agg=doi.

8. Y. Reynier, R. Yazami, and B. Fultz, J. Power Sources, 119-121, 850–855 (2003) http://dx.doi.org/10.1016/S0378-7753(03)00285-4.

9. Y. Reynier et al., Phys. Rev. B, 70 (2004) http://prb.aps.org/abstract/PRB/v70/i17/e174304.

10. K. Maher and R. Yazami, J. Power Sources, 247, 527–533 (2014) http://linkinghub.elsevier.com/retrieve/pii/S0378775313014018.

11. K. Maher and R. Yazami, J. Power Sources (2014) http://linkinghub.elsevier.com/retrieve/pii/S0378775314000767.

The high-energy-density Li-rich layered materials are promising cathode materials for the next-generation high-performance lithium-ion batteries¹. They have attracted a lot of attentions due mainly to their high reversible capacity of more than 250 mAh·g^-1 at low charge-discharge current. However several drawbacks still hinder their applications, such as the poor rate capability².

To conquer this critical issue, the present study is focused on surface modification of Li-rich layered cathode materials to improve their rate capability as well as maintain the high capacity retention of the pristine material. Surface treatment is conducted on Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ using NH₄F by thermal annealing at low temperature. Material characterizations reveal that the modification process triggers fluorine doping and phase transition (Figure 1) from the layered phase to a spinel phase at the particle surface. Figure 2 shows the rate performances of the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and the modified materials. The pristine material could deliver a high reversible capacity of about 250 mAh·g^-1 at 0.1C (25 mA·g^-1). However, its discharge capacity decreases to 109 mAh·g^-1 at 1C, which is only 43% of the capacity at 0.1C. Compared with the pristine material, both the materials modified by 5 wt.% and 10 wt.% NH₄F can deliver a discharge capacity over 140 mAh·g^-1at 1C, which is more than 70% of their discharge capacities at 0.1C. Particularly, the material modified by 20 wt.% NH₄F has a discharge capacity as high as 172 mAh·g^-1 at 1C, which is about 87% of its capacity at 0.1C. Moreover at even higher rate like 5C, the discharge capacity of the material modified by 20 wt.% NH₄F still can reach 126 mAh·g^-1 while that of pristine one is only 41 mAh·g^-1. Generally, the NH₄F modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ exhibits greatly improved rate performance and satisfactory cycling stability compared to the pristine one, which can be attributed to the modified particle surface. Firstly, the spinel shell of the particle provides three-dimensional Li⁺ ion diffusion paths³, which creates fully opened surface, enabling fast Li⁺ ion transfer at the electrode/electrolyte interface. Secondly, the formation of spinel shell prevents the Ni segregation at the surface, thus suppressing its negative effect on Li⁺ ion diffusion. Finally, the fluorine doped spinel surface improves the surface stability during wide-voltage-range charge-discharge process, resulting in improved cycling stability.

The enhancement in the electrochemical properties of the modified materials as a function of the NH₄F amount is comprehensively investigated using powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and electrochemical tests.

References:

1. Whittingham, M. S., Lithium batteries and cathode materials. Chemical Reviews 2004,104(10), 4271-4301.

2. Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A., Li₂MnO₃-stabilized LiMO₂ (M = Mn, Ni, Co) electrodes for lithium-ion batteries. Journal of Materials Chemistry 2007,17(30), 3112.

3. Song, B.; Liu, H.; Liu, Z.; Xiao, P.; Lai, M. O.; Lu, L., High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries. Scientific reports 2013,3.

Si-based alloy[1] is one of the most promising anode materials for substituting graphite due to its excellent characteristics such as high gravimetric capacity exceeding 1000mAh/g, high safety, good electrochemical stability and low production cost, etc. Currently, graphite anode material is widely used with water-based SBR binder in commercial Li-Ion Batteries and its electrode capacity is about 350mAh/g. In order to increase the energy density of anode electrode there have been many attempts of blending small (less than 10%) quantity of Si-based anode materials with graphite without changing water-based SBR binder system.

In this study, we made Si-alloy/Graphite/Single-walled Carbon nanotubes(SWCNTs) blending electrode and evaluate the electrochemical properties. In case of 8% of Si-alloy / 89% of Graphite / 0% SWCNT / 2% of SBR /1% of CMC electrode, the initial discharge capacity is 387mAh/g, initial the coulombic efficiency is 86.2% and the capacity retention after 50^th cycle is 80.7%. In case of 8% of Si-alloy/89.7% of Graphite/ 0.3% SWCNT/ 2% of SBR /1% of CMC electrode, the initial discharge capacity is 393.5mAh/g, the initial coulombic efficiency is 88.4% and the discharge capacity retention after 50cycle is 99.2%.

This improvement in the electrochemical properties of the SWCNT added electrode was mainly attributed to the good electric contact between Si-alloy particles and graphite particles during cycles. In other words, the main cause of initial capacity lose in Si-alloy/Graphite blending electrode is to lose electric contact due to the volume change of Si-alloy. This result gives many hints to improve Si-based anode materials.

[1] M. Kim et al. Journal of Electroanalytical Chemistry 687 (2012) 84-88

Recently, in terms of Lithium ion battery market has been widely growing up from mobile IT devices into Electrical Vehicle market. The battery is the most important part in xEV than any other parts. Anode materials in lithium ion battery, those price evaluated about 10 % in composition of typical lithium ion battery. However, the price of anode parts including carbon or silicon materialss were very unstable. It absolutely relies on importation in korea. The silicon which has superior theoretical specific capacity than commercial anode materials (Si = 4200 mAh/g, Carbon = 372 mAh/g), To develop silicon based anode materials for lithium ion battery, raw material of silicon should be priceless and stable supply chain.

To figure out these unstable raw material market, we had been focused on waste silicon sludge which were gathered by process for silicon wafer back grinding. The waste silicon sludge were discarded over 2000 ton/year, all of them are currently buried in korea.

Waste silicon sludge were consists of wafer grinding materials, silicon, and water/oils. Herein, we recovered the silicon from waste silicon sludge and purified the silicon for lithium ion battery anode starting materials. Not only recovred silicon, also recovery silicon had been pulverized to nano-sized by beads-milling. these nano-sized recovery silicon induced that shorten lithium ion paths and large surface area. Malic acid was a chosen media for carbon coating on nano-sized recovery silicon surface. We analyzed among recovered silicon properties and its electrochemical performance.

For the last 10 years, a tremendous amount of work has been published to solve the problem of capacity fade of silicon-based electrodes which prevents their utilization in commercial lithium-ion batteries. The use of Si nanoparticles/nanowires to better accommodate large strain without cracking has developed and is very popular in the academic community. By playing on the nano-architecturing effect or tailoring the composite electrode formulation, several groups have reached up to 1000 cycles in half-cells versus lithium metal [1,2]. However, a careful look at the papers shows that in all studies the active mass loading is very low, typically less than 1 mg per cm², and thus the practical surface capacity of the corresponding electrodes is low, typically less than 1 mAh per cm². This is much lower than that of the state of the art graphite-based negative electrode, which reaches up to 5 mAh per cm² in cellular phones for example. As a consequence, although silicon is much more attractive than graphite due to its very high gravimetric capacity (3572 mAh g^-1versus 372 mAh g^-1 for graphite) and volumetric capacity (2249 versus 779 mAh cm^-3 for graphite), Si-based composite electrodes show lower practical surface capacity, as a matter of fact.

The point is that the cycle life of Si-based electrodes dramatically decreases as the active mass loading increases, e.g. 1000 cycles at 0.5 mg per cm² vs. 50 cycles at 4 mg per cm² (Figure 1). We demonstrated that using copper foam instead of copper foil as current collector shows a great advantage in the cycle life and power performance. More than 400 cycles at an impressive Si loading of 10 mg cm^-² could be reached, i.e. with a surface capacity of 10 mAh cm^-2 [3]. The thinness of the composite coating on the foam walls favors a better preservation of the electronic wiring upon cycling and fast lithium ion diffusion. A higher coulombic efficiency in half cells with lithium metal as the counter electrode is achieved by using carbon nanofibers (CNF) rather than carbon black (CB). The possibility to reach in practice higher surface could allow a significant increase of both the volumetric and gravimetric energy densities by 23% and 19%, respectively, for the Cu foam-Silicon//LiFePO₄ stack compared to the Graphite/LiFePO₄ stack of traditional design.

Acknowledgements

Financial funding from the Agence Nationale de la Recherche (ANR) of France (BASILIC project) and the Natural Science and Engineering Research Council (NSERC) of Canada is acknowledged. The authors thank D. Pilon (Metafoam Inc.) for supplying the Cu foams.

References

[1] L. Hu, F. La Mantia, H. Wu, X. Xie, J. McDonough, M. Pasta, Y. Cui, Adv. Energy Mater., 1, 1012 (2011).

[2] I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 334, 75 (2011).

[3] D. Mazouzi, , D. Reyter, M. Gauthier, P. Moreau, D. Guyomard, L. Roué, B. Lestriez, Adv. Energy Mater., DOI: 10.1002/aenm.201301718.

Figure 1. (a) Surface SEM images of a Cu foam filled with 5 mg of Si/CNF/CMC/Buffer composite electrode (4.6 mg Si per cm²). (b) Cycle life as a function of the active mass loading for Foil-Si/CB/CMC/Buffer and Foam-Si/CNF/CMC/Buffer electrodes (Si//Li half-cell with LP30+2%VC+10%FEC, capacity limitation of 1200mAh per g of Si).

Si alloy anodes were produced by mechanical milling in this study. They were prepared by ball milling the pieces of pre-alloyed melt spun ribbon. Our study was focused on the effect of a milling time on the electrochemical performance of Si alloys and microstructural evolution. As a result, it appears that the longer milling time, more cycle stability, the finer crystal size. In addition, we performed XRD, TEM analysis to characterize microstructure and intermetallic phases. This study leads to the best understanding to date of the electrochemistry of a Si alloy anode with an inactive matrix

Li containing binder for improved the first coulombic efficiency and cycleability of Li-ion Batteries

Jun-Hwan Ku,^a Seung Sik Hwang,^a Min-Sang Song,^a Jeong-Kuk Shon,^a Sang-Min Ji,^a Jae-Man Choi,^a

^a Energy Lab, Samsung Advanced Institute of Technology, Electronic Materials Research Complex 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, Republic of Korea

Lithium-ion batteries occupy a large and increasing share of the energy storage device market as a result of their excellent performance in terms of long-life, energy density and high safety. Among the various electrode materials for LIBs, graphites which are representative of carbon materials have been the most commonly used negative electrode materials due to their low working potential close to lithium metal anode, and remarkable cycling performance. As lithium-ion cells typically operate beyond the thermodynamic stability of organic electrolytes, the reduction products arising from chemical reactions during the first few cycles form passivating films on the carbon anode surface, as we call it 'solid electrolyte interphase (SEI)'.^[1] Such SEI, which is comprised of electrochemically insulating layer, plays important roles of preventing the further electrochemical reactions between electrode surface and electrolyte and enabling only lithium ions tunnel through the layer. It is well known that the formation of SEI layers is a determinant factor on the performance of LIBs, affecting cycle-life, life time, power capability and even safety.^[2]While the SEI plays an essential part in delivering the best performance in cells, the development of SEI layers are caused by the irreversible reaction accompanying an electrolyte decomposition, which makes the Coulombic efficiency to decrease during the first few cycles. When the stable SEI is not formed, any accidental misuse such as overcharge, high temperature exposure, and mechanic impact might damage the already formed SEI, resulting in more irreversible reaction during charging. The new anode surface, exposed to the electrolyte, immediately reacts with it to form a fresh thin protective film, which eventually leads to a poor cycleability and other undesirable properties in LIBs. Thus, to improve the performance of the cell, not only minimization of the electrolyte degradation but also thinner SEI film which provides an excellent passivating roll is required because thick and resistive SEI film is not favorable to battery operation.

Many research studies are thus focused on the improvement of the chemical nature and morphology of SEI. For the modification to SEI films having superior properties, several previous papers reported the polymer binder including the SEI ingredients and their electrochemical effects of LIBs.^[3,4]The effects of the surface modification of the graphite electrode by the functional polymer binder, such as poly(acrylic acid), have been reported in several papers. Concretely, it has been demonstrated that the polyion complex layer, which have oxygen species as functional groups, could play the role like artificial SEI to assist the facile penetration of Li ions. Obviously, the stable and efficient operation of LIBs is closely connected with ingredients of surface films, morphology, and coverage feature.

Here we report that artificial pre-SEI, which is driven by Li ions containing polymer binder with functional group (-COOLi), can enhance the Coulombic efficiency and cycleability (Fig. 1). Furthermore, electrochemical performance is compared for SEI films that are produced from some different binders, in which different degree of lithium quantity is included in ISOBAM polymer.

Fig. 1. (a) 1H-NMR spectra substituted ISOBAM polymer by 20, 50, 80, 100 amounts of Li ions, respectively. (b) Cycleability of SFG6 graphites with all investigated binders.

Reference

[1] E. Peled, J.Electrochem. Soc., 1979, 126, 2047.

[2] E. Peled, in: J. P. Gabano (ED.), Lithium Batteries, AP, 1983, 43

[3] K. Xu, Chem. Rev.(Washington, D.C.), 2004, 104, 4303.

[4] S. Komaba, N. Yabuuchi, T. Ozeki, K. Okushi, H. Yui, K. Konno, Y. Katayama, and T. Miura, J. Power Sources, 2010, 195, 6069.

Thin film fabrication is very important especially for device fabrication. For an all solid-state thin film battery, very low dimensional (of the order of sub-micron), high quality thin films are needed and the fabrication technique is a key criteria for producing good quality ultra-thin films. Pulse Laser deposition (PLD) is a suitable method for producing high quality finish of metals and metal oxides. Controlling the PLD deposition parameters can affect the characteristics of thin films such as crystal orientation, thickness and roughness which in turn influence the physical and chemical properties of the thin films. Producing thin film metal oxides with simple stoichiometries such as ZnO, MgO and Al₂O₃, may not be such a problem to fabricate. However, producing thin films of complex metal oxides such as LiCo_0.3Ni_0.3Mn_0.3Cr_0.1O₂ for Li-ion battery cathodes can be a problem. The thin film deposition technique must be appropriate so that the thin film material produced will not suffer a change of phase, stoichiometry or purity. Not all gas phase or plasma based methods are suitable for deposition of this type of material. Reactive type thin film fabrication method may not produce the needed stoichiometry or phase and may produce multiphase samples with impurities. PLD may be the choice method for producing thin films of very complex stoichiometries.

This work explores the thin film method of PLD in the fabrication of thin film LiCo_0.3Ni_0.3Mn_0.3Cr_0.1O₂ material. Characteristics such as phase, stoichiometry, crystal growth orientation, and surface roughness, are studied. For this type of complex metal oxide, it is very important for the thin film to be of the right phase and stoichiometry because this will affect the electrochemical behavior of the energy device that will then be fabricated. It is found that thin films of LiCo_0.3Ni_0.3Mn_0.3Cr_0.1O₂ deposited by PLD have good qualities such as mirror finish, dimensionally thin and of correct stoichiometry and high smoothness. Cyclic voltametry result shows that the cathode film is electrochemically active.

Understanding the ageing phenomena in the Li-ion batteries is of great importance nowadays for their successful commercialization. Li-ion degradation under storage conditions has been widely studied and the causes are well established. However, ageing during cycling remains still unsolved nowadays. Mechanical degradation is recognized as the main ageing mechanism of the battery due to volume changes of the electrodes during the cycling process. So far, many investigations have been published where the stresses on single particles have been analyzed [1]. Others, in more recent publications, have modelled the propagation of the fracture surface and combined it with the chemical degradation in order to explain the break and repair effect [2] and the electrical isolation of the active material [3].

In this research, S-N curve fatigue approach has been followed to describe the fracture of the active material. By means of this method, gradual decrease of the particle size has been reproduced. Furthermore, a parametric analysis has been carried in order to clarify the role of the stress factors (i.e. DOD, C_rate, SOC_mean, etc...) as well as the design parameters on the cycling ageing mechanisms.

The coupled mechanical-physicochemical model

The mechanical degradation model has been integrated in a one dimensional physicochemical model of a Li-ion battery [4], all implemented in Matlab/Simulink®. Figure 1 shows the scheme of the mechanical model.

The stresses are calculated based on the diffusion induced stress phenomenon (DIS) [1]. Due to the cyclic nature of the stress profile, during the cycling of the battery, a fatigue approach is employed to predict the fracture of the active material. The stress profile is first processed using a rain flow cycle counting method. Subsequently, a probabilistic S-N curve [5] is applied, which describes the relationship between the stress and the number of cycles to failure. Finally, damage accumulation is computed using Palmgren-Miners' rule [5] to estimate the fracture of the active material particle.

The model shows that most of the mechanical degradation takes place in the first cycles, later the effect becomes smaller and smaller, a fact that has been verified experimentally by other authors [6]. Using the simulation model, this behavior has been identified to be due to the initial larger particle size which produces larger stresses. As particle size decreases due to particle fracture the induced stress decreases (see Figure 2).

The use of the mechanical degradation model is a big advantage as it enables to analyze the fracture and the mechanical degradation of the electrode under different working conditions.

This work is an attempt to qualitatively study the mechanical degradation and its effect on the performance degradation of Li-ion batteries.

Figure 1: Flow chart of the mechanical degradation model.

Figure 2: The stresses in the graphite in the area close to the separator versus cycle number. The evolution of the particle size is shown in the graph. Smaller particles produce smaller stresses.

Acknowledgments:

The project upon which this publication is based is funded by the Impuls- und Vernetzungsfond der Helmholtz-Gemeinschaft e.V. through the framework of the research initiative HGF Energie Allianz.

References:

[1] J. Christensen, J. Electrochem. Soc., 2010, 157 (3) A366-A380

[2] R. Deshpande, M. Verbrugge, Y. T. Chen, J. Wang, P. Liu, J. Electrochem. Soc, 2012, 159 (10) A1730-A1738

[3] R. Narayanrao, M. M. Joglekar, S. Inguva, J. Electrochem. Soc., 2013, 160 (1), A125-A137

[4] John Newman, William Tiedemann, Porous-Electrode Theory with Battery Applications, AlChE Journal 21 No. 1, 25-41, 1975

[5] S. Suresh, Fatigue of materials, 1998, 2^ndedition, Cambridge university press, ISBN 0-521-57847-7

[6] K. Rhodes, N. Dudney, E. Lara-Curzio, C. Daniel, J. Electrochem. Soc, 2010, 157 (12) A1354-A1360

INTRODUCTION

Positive electrode material, Li(Ni,Co)O₂ system, of Li-ion battery has been attractive because it has the advantage in low environmental load due to low Co content. There are many previous works about the Li(Ni,Co)O₂ and the aluminum-substituted one because of high heat stability. However, detail structural changes during electrochemical processes are still ambiguous for site occupancy, local structures, and electronic structures even though these would play important roles on the battery characteristics. In this research, we used several quantum beam techniques to reveal the crystal structures, electronic and nuclear densities, and local structures of LiNi_0.8Co_0.2O₂ during charge process.

EXPERIMENTAL

To synthesize the LiNi_0.8Co_0.2O₂, the precursor was prepared by co-precipitation method. The co-precipitated material was annealed at 800 °C in air for 15 h. The product was identified by powder XRD and ICP-OES. To study change of crystal and electronic structure of this cathode material during electrochemical process, cathode materials were prepared by four charge depths, and were investigated using the synchrotron X-ray diffraction (BL02B2, SPring-8) and neutron diffraction (BL20, J-PARC). The data was analyzed with the Rietveld technique using Rietan-FP and Z-Rietveld, and by the MEM using the Dysnomia and Z-MEM programs. To examine the local structures, X-ray absorption spectroscopy (BL14B2, SPring-8) and X-ray total scattering (BL04B2, SPring-8) were measured.

RESULTS AND DISCUSSION

The neutron diffraction patterns were measured for the cathode after 2nd discharge process as well as the pristine electrode. It was demonstrated that the crystal structure analysis by ex-situ measurement could be successfully performed even for the electrode with an amount of about 10 mg. The values of 3b-6c bond length decreased and λ², which expresses the distorted degree of the Li-O₆ and M-O₆ octahedra, were increased with increasing charge depth. Synchrotron-based experiments using high energy X-ray total scattering and pair distribution function (PDF) analysis (Fig. 1) indicated that the oxygens arranged to crystallographic a-b plane were disturbed in the charge state of 4.3 V after 2nd cycle (Fig. 2).

ACKNOWLEDGMENTS

This research was supported by the RISING-project of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Lithium-ion batteries (LIBs) are becoming more and more important for mobile energy storage devices, with respect to their energy density. For large size applications such as in electric vehicles (EVs) or hybrid electric vehicles (HEVs), both higher energy density and low-cost materials are required also taking environmental friendliness into account. In order to enhance the performance of lithium-ion batteries, researchers and battery-manufacturers are trying to create new electrode materials and new electrolyte compositions. Nevertheless, battery efficiency strongly depends on the electrode engineering and additionally on the production process with the optimization of each individual step.^[1,2]

Polyvinylidene fluoride (PVDF) based electrodes on the cathode as well as on the anode side are widely used. One major drawback of PVDF based electrodes is the usage of volatile organic solvents that are often toxic (like N-methyl-2-pyrrolidone (NMP)) and are difficult to dispose at the end of the production process. Alternatively, water soluble natural and/or synthetical based binders have been proposed which also cope the increasing demand of a more environmentally-friendly production process and recycling of LIBs. Processing of active materials for negative as well as positive electrodes is possible using carboxy methyl cellulose (CMC) and/or styrene-butadiene (SBR).^[3]

In our study we present the results of our experiments to substitute NMP/PVDF with a water based CMC/SBR binder solution. Moreover, the work comprises the development and adaption of mixing processes, coating techniques, and appropriate drying procedures. To optimize the mixing process for water based slurries we had to figure out ideal substitution grades, mixing ratios between CMC/SBR, and the right pH-value. With the focus on viscosity, mixer types, and shear rates we figured out an optimized slurry configuration for the adaption to our battery pilot plants including the manufacturing of different cell types. Furthermore, the adjustment of coating techniques is one essential issue towards large scale production processes. Nevertheless, we also had to pay attention to the challenging production step, the drying procedure by the combination of different drying duration and temperatures as well as the adapted drying technique.

This work demonstrates the difficulties and production steps going from lab scale to large scale manufacturing.

We kindly thank the European Regional Development Fund for the funding this project "ProLiBat" (EM-1041H) and also the project partners for support and cooperation.

[1] M. Broussely, J. Power Sources, 81-82 (1999), 140.

[2] X. Zhang, et al., J. Electrochem. Soc., 148 (2001), A463.

[3] Lux, S. F., et al J. Electrochem. Soc., 157.3 (2010), A320-A325.

Electrification with Li-ion batteries is a global trend that has been on-going for many years across sectors, driven by a strong decrease in battery prices, increasing energy density and increasing fuel costs. Studies by DNV of the electrification of ships (e.g. Fellowship) have shown a large potential of reduction in energy consumption and emissions of CO₂, NO_x and particulate matters through electrification [1]. In addition, cost calculations have shown the possibilities for return of investment through electrification within 3 years for some applications, more recent data states even 2 years for other applications [1].

It is well-known that the safety of Li-ion batteries is an important issue. E.g. during over-charge of a Lithium-ion battery, the battery may eventually overheat and cause a thermal runaway which can ignite neighboring cells which have not been overcharged. Internal shorting of the electrodes is also reported to create thermal runaway situations. Based on the large amounts of energy in a 1 MWh Li-ion battery, it is absolutely vital that the safety of the battery system is assured. The degradation and ageing of Li-ion batteries will contribute to additional instability and affect the safety performance of the batteries as well. Hence, knowledge and the ability to predict capacity decay and the battery state of health will be vital information to enable safe and long-life operation of marine battery systems.

In this study commercial Li-ion cells were studied with the entropy spectroscopy technique [2, 3] to establish a State-of-Health indicator for aged cells. The cells tested were Li-ion pouch cells which have been aged for both calendar and cycle life up to 3 years.

For cycle life, the Li-ion cells were evaluated at various temperatures and charge and discharge rates. The calendar life was scrutinized at various temperatures and at different state of charge. The cells were characterized, based on continuous monitoring of capacity and energy content. In addition, periodic capacity and impedance data was recorded for all cells at the same temperature (25 °C) and discharge rate (1C).

1. Vartdal, B.-J. and C. Chryssakis, Potential Benefits of Hybrid Powertrain Systems for Various Ship Types, in International scientific conference on hybrid and electric vehicles, RHEVE 20112011: Rueil-Malmaison, France. p. 12.

2. Reynier, Y., R. Yazami, and B. Fultz, The entropy and enthalpy of lithium intercalation into graphite. Journal of Power Sources, 2003. 119: p. 850-855.

3. Viswanathan, V.V., D. Choi, D. Wang, et al., Effect of entropy change of lithium intercalation in cathodes and anodes on Li-ion battery thermal management. Journal of Power Sources, 2010. 195(11): p. 3720-3729.

Widely researched in the recent years, Si offers one of the possibilities for significant increase of the capacity of Li-ion battery anode. During its lithiation Si undergoes series of phase transitions related to formation of a number of Li-Si alloy phases with a total capacity of about 4200 mAhg^-1[1-3]. Consequently, the electrochemical lithiation of Si is associated with a drastic volume increase, usually resulting in rapid capacity fade and mechanical decomposition of the structure [2]. A number of different approaches, including nanostructuring [3], application of amorphous Si [1,2,4-8], limitation of state of charge of Si anodes by means of capacity or voltage control [8-10] composites with carbon [2] and more recently with TiO₂[11-12], has been undertaken in order to improve the cycling stability of Si based electrodes.

This paper will discuss recent results from the author's labs on the application of TiO₂nanotube arrays as templates for magnetron sputtering of Si. Tuning of the deposition parameters (power and time) allowed obtaining a specific particulate Si morphology with sufficient free space, suitable for application as anode material in Li ion batteries (Fig.1).

Fig1. Scanning electron microscopy (SEM) image of the Si-modified TiO2 nanotube layer.

The electrochemical performance of the Si containing structures elucidates the importance of the Si surface morphology for the cycling stability of the anodes. The structures deposited at 50 W are favorable for Si stabilization during electrochemical cycling in Li ion electrolyte.

The nanostructured Ti/TiO₂layers were electrochemically tested for Li-ion exchange in 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl) sulfonylimide ([BMP][TFSI]) containing 1M Li[TFSI]. The results show that the ionic liquid [BMP][TFSI] is a promising electrolyte for batteries with silicon anodes.

The substrate type significantly influences the long term galvanostatic cycling of the samples. It was found that the structures deposited at 50 W on amorphous TiO₂ exhibit a superior constant current cycling, finishing the 200^th galvanostatic cycle with discharge capacity value of 1150 mAh g^-1, with a tendency for further stabilization of the cycling. The good electrochemical performance of this sample type was attributed to the specific morphology of the Si deposit and structural stability of the amorphous TiO₂ nanotubes.

References

[1] S. Bourderau, T. Brouse, D.M. Schleich, J. Power Sources, 81-82 (1999) 233-236.

[2] U. Kasavajjula, C. Wang, A. J. Appleby, J. Power Sources, 163 (2007) 1003- 1039.

[3] M. Green, E. Fielder, B. Scrosati, M. Wachtler, J. S. Moreno, Electrochem. Solid-State

Letters, 6 (2003) A75-A79.

[4] T.L. Kulova, A.M. Skundin, Y.V. Pleskov, E.I. Terukov, O.I. Konkov, J. Electroanal, Chem.600 (2007) 217–225.

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1. Introduction – Solid state batteries overcome many of the essential drawbacks of liquid electrolyte batteries, such as liquid electrolyte and leakage issues [1]. Another major issue is the degradation of the Li based cathode from continuous cycling due to a change in its microstructure [2]. Nanoscale coatings are thought to prevent the degradation of the cathode by acting as a protective layer on its surface [3, 4]. This paper studies the performance of lithium cobalt oxide (LCO) based cathodes with and without protective coatings of Al₂O₃ and TiO₂.

2. Fabrication procedures – Fabrication procedures entail the following stages: (i) Making the liquid lithium lanthanum tantalite (LLTO) electrolyte by a sol gel technique from its precursors and then vaporizing the liquid by exposing it to elevated temperature. The paste left behind is thoroughly dried and finely ground and is included as the solid state electrolyte. (ii) The cathode is made by thoroughly grinding lithium cobalt oxide (LCO) powders and then making slurry by mixing them thoroughly with PTFE binder, carbon powder, and alcohol. Pasted on to a stainless steel foil, this whole system acts as the cathode. (iii) The Atomic Layer Deposition (ALD) technique is used to deposit thin films (~6 nm) on the surface of the LCO cathodes to compare them with the pristine cathodes. (iv) The anode includes Li metal foil. Once this is complete, the three separate components are assembled into a coin cell.

3. Experimental steps and results – The characterization procedures are as follows:

(i) XRD and SEM study to check the phase of the solid state LLTO electrolyte, pristine LCO cathode, and cathodes with Al₂O₃ and TiO₂ coatings and the surface morphology.

Figure 1 shows the XRD data of the LCO cathode with TiO₂ deposition. The main Al₂O₃ peaks are seen and no TiO₂ peaks are obtained, indicating the amorphous nature of the deposition.

(ii) TEM and XPS analysis to study the nature and actual thickness of the coating. This will give an idea about the actual depth of the deposition as well as the nature of electronic bonding at the surface.

From Figure 3 it may be seen that reduction potential shifts more to the right for the TiO₂ coated cathode. The greater ΔV value exhibited by the coated cathode indicates greater energy needed for the reduction reaction owing to the deposition, and TiO₂ has the highest ΔV value.

Figure 2 represents the XPS data of the LCO cathode coated with Al₂O₃, showing clearly the Al 2s and 2p and the O 1s peaks.

(iii) Cyclic voltammetry is performed within a voltage range of 3 V to 4.5 V and the applied current is 5 mA/s.

(iv) Cyclical performance of the cell.

Figure 4 shows that open circuit voltages are obtained of slightly less than 3 V for a current density of 10 mA/cm², compared with 3.259 V and 3.312 V, respectively, for i = 30 mA/cm² and 40 mA/cm² for the LCO pristine cathode.

4. Conclusion – Amorphous transition metal oxide coatings have been tried out. XPS data have shown the formation of the thin film. A rightward shift in the reduction potential demonstrated by the CV data indicates greater energy necessary which is a measure of the protective nature of the coating. Future work includes studies of the cycling performance of the cell with the coated cathodes.

References:

[1] M.V. Tyufekchiev, S. Hur, Developing a Low-Cost Methodology for Fabricating All-Solid-State Lithium-Ion Battery, in: Chemical Engineering, Worcester Polytechnic Institute, 2013.

[2] M. Ogawa, K. Yoshida, K. Harada, Environment, Energy and Resources, 88-90.

[3] X. Li, J. Liu, X. Meng, Y. Tang, M.N. Banis, J. Yang, Y. Hu, R. Li, M. Cai, X. Sun, Journal of Power Sources, 247 (2014) 57-69.

[4] H. Zhao, L. Gao, W. Qiu, X. Zhang, Journal of Power Sources, 132 (2004) 195-200.

With the demand for increasing the energy and power, a lot of attention has been attracted by the LiNi_0.5Mn_1.5O₄ (LNMO) spinel cathode material for lithium ion batteries, as it can be operated at a high voltage of ~ 4.7 V (versus Li⁺/Li) with offering 3-dimensional lithium-ion diffusion channels, allowing fast intercalation/deintercalation of lithium ions. However, it still remains challenging to get excellent rate capacity and cycling stability for LNMO material, especially at elevated temperature, due to the complex influencing factors and the undesired side reactions with the electrolytes. ^[1-5]

Various synthesis techniques, material modifications, and morphology control for high-voltage spinel LNMO have been reported to improve the electrochemical performance.^[4-7] Such as cation doping, surface coating, and creating nanostructures. Meanwhile, the particle morphology of the material, especially the surface crystallographic planes, can also have an obvious influence on the electrochemical properties of material itself.

Herein, a new synthesis approach, allowing shorter material processing, will be presented, which leads to an achievement of special morphology of LNMO cathode material. The obtained LNMO material was characterized by means of X-rays diffractions (XRD), Raman spectrum, electron microscopy (SEM), and electrochemical measurements (CV, EIS, and galvanostatic charge-discharge test). The special morphology enables high-rate performance and cycle life simultaneously. The electrochemical performance was improved remarkably. Detailed discussion will be presented.

References

[1] Goodenough, J. B.; Park, K. S. J. Am. Chem. Soc. 2013, 135, 1167.

[2] Xiao, J.; Chen, X.; Sushko, P. V.; Sushko, M. L.; Kovarik, L.; Feng, J.; Deng, Z.; Zheng, J.; Graff, G. L.; Nie, Z.; Choi, d.; Liu, J.; Zhang, J. G.; Whittingham, M. S. Adv. Mater. 2012, 24, 2109.

[3] Duncan, H.; Duguay, D.; Abu-Leddeh, Y.; Davidson. I. J. J. Elecrochem. Soc.2011, 158, A537.

[4] Chemelewski, K. R.; Lee, E.; Li, W.; Manthiram, A. Chem. Mater.2013, 25, 2890.

[5] Zhang, X.; Cheng, F.; Yang, J.; Chen, J. Nano Lett. 2013, 13, 2822.

[6] Shaju, K. M.; Bruce, P. G. Dalton Trans.2008, 5471.

[7] Zhou, L.; Zhao, D.; Lou, X. Angew. Chem., Int. Ed. 2012, 51, 239.

The rechargeable non-aqueous lithium-air (O₂) battery system is a promising to apply the large scale storage system such as electric vehicle. It can store much higher theoretical energy density than regular Li-ion batteries. Nevertheless this advantage, Li-O2 cell has the problems such as polarization of the oxygen reduction reaction (ORR), decomposition of the organic electrolyte and dendrite growth on the Li metal anode. Li dendrite can make dead Li which significantly drop the cycle retention. Also Li dendrite growth can occurs the unstable solid electrolyte interface (SEI) layer, it is promote the electrolyte decompose. In this study we use Li powder electrode (LPE) instead of the Li metal anode.

The Li powder was made by droplet emulsion technique (D.E.T) (the diameter of lithium powder is 10μm). A Li powder was revealed effective to reduce dendrite growing by the previous study of our group. To make the LPE, we loaded 20mg Li powder on the 15 Φ Sus-mesh and pressed 20kgcm^-2. To compose the O₂ cathode, Poly (vinylidene fluoride) (PVdF), Ketjen black (KB) carbon and MnO₂ catalysts were mixed by 2: 4: 4 wt. %. The current collector of O₂cathode was Ni foam and punched 14 Φ. The electrolyte was used by a 1M LiTFSI in TEGDME. The morphology of electrode was pictured by field emission scanning electron microscope (FE-SEM).

To estimate the galvanostatically electrochemical properties we analyze the impedance data, voltage profile and cycle data. The rate of current is 100mAg-1 and limited the time at 10 hours in charge and discharge respectively. In voltage profile we check there a stable cycle process until 40cycles. Li powder has much stable interface reaction than Li metal, it can possible to reduce the over potential at the charge/discharge and make good cycle ability. To analyze the morphology and chemical reaction after cycle was estimated by the FE-SEM, TEM, XPS and FT-IR.

Reference to a journal publication

[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke, J. Phys. Chem. Lett.1 2193-2203. (2010)

[2] S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Barde and P. G. Bruce, Angew. Chem. 50, 8609-8613. (2011)

[3] J. G. Zhang, D. Wang, W. Xu, J. Xiao and R. E. Williford, Journal of Power Sources 195, 4332-4337. (2010)

[4] S.K Kong, B.K. Kim and W.Y. Yoon, Journal of the Electrochemical Society 159, A1551-A1553. (2012)

With Lithium-ion battery production increasing every year, a lot of efforts are involved in research and development of new advanced materials¹ capable of addressing the challenges that arise from the most interesting applications , such as hybrid electric vehicles (HEVs). In this work, the electroless metallization technique has been used to obtain a coating on the surface of active material particles used in the preparation of cathodes and anodes for Li-ion batteries.

One of the most promising candidates in the field of energy storage is the olivine-type LiFePO₄ (LFP). This particular material, which was first proposed by Padhi et al.², is characterized by high energy density, low cost and chemical stability. However, this material is presenting a major drawback in its low electronic conductivity due to its intrinsic resistance, and several routes are being investigated to mitigate this issue. Most common approaches are applying a Carbon coating on the surface of LFP^3–5, reduce particle size⁶ and particles doping⁷.

In this work, copper-based coating has been applied on LFP particles in order to enhance its electronic conductivity and eventually to increase the resistance to Fe dissolution during cycling, which is recognized as one of the main causes of capacity fading during cycling^8–10. The coating has been obtained with autocatalytic deposition, with a two-step process: (1) Pd-based particles activation and (2) copper plating through deposition bath containing copper ions and a reducing agent that allows the reduction of metal ions from the solution to the surface of the particles. Positive electrodes have been prepared using PVDF latex as polymeric binder. The resulting cathodes show improved electrical conductivity. Electrochemical characterization has been carried out to assess the nature of the coating and its impact on the performances of the electrode in working conditions.

Acknowledgments:

This work has been financed with the contribution of the LIFE financial instrument of the European Community. Project n° LIFE12 ENV IT 000712 LIFE+ GLEE.

References:

1. B. Scrosati and J. Garche, Journal of Power Sources, 195, 2419–2430 (2010)

2. A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, Journal of Electrochemical Society, 144, 1188–1194 (1997).

3. H. Huang, S.-C. Yin, and L. F. Nazar, Electrochemical and Solid-State Letters, 4, A170 (2001)

4. K. Amine, J. Liu, and I. Belharouak, Electrochemistry Communications, 7, 669–673 (2005)

5. C.-K. Park, S.-B. Park, S.-H. Oh, H. Jang, and W.-I. Cho, Bulletin of the Korean Chemical Society, 32, 836–840 (2011)

6. C. Delacourt, P. Poizot, S. Levasseur, and C. Masquelier, Electrochemical and Solid-State Letters, 9, A352 (2006)

7. T.-F. Yi et al., Ionics, 18, 529–539 (2012)

8. W. Porcher, P. Moreau, B. Lestriez, S. Jouanneau, and D. Guyomard, Electrochemical and Solid-State Letters, 11, A4 (2008)

9. K. Zaghib et al., Journal of Power Sources, 185, 698–710 (2008)

10. L. Castro et al., Journal of The Electrochemical Society, 159, A357 (2012)

Fast development of portable electronic devices (such as a smart phone, iPad, and Galaxy Note) and electrified vehicles (such as EVs and HEVs) require better and smaller lithium ion batteries as their energy storages. What is needed for the cathode of the lithium ion battery is a material capable of a higher volumetric energy density as well as gravimetric energy density. However, not only the development of cathode material but also the commercialization of high energy density anode material has been limited due to cathode material's much lower energy density compared to anode material. In this regard, Li-rich material (Li₂MnO₃-LiMO₂, M=Ni, Mn and Co) that has 150 % higher energy density than commercialized LiCoO₂ is the unique next generation cathode material to solve the problem of cathode material's low energy density. In the past decade, many researches have focused on the surface stabilization method of this material to improve its intrinsic problems of poor cycle/ rate capability and such surface treatment methods have been demonstrated in a few examples, including AlF₃ coating and spinel heterostructures, yielding noticeable improvements in rate and cycle ability.

Although such surface treatments showed improved rate and cycle performances, achieving the high volumetric energy density and long-term cycle life which are more critical factors for the commercialization of Li-rich material still remained unsatisfied. In general, Li-rich material's reversible capacity is related with its particle size since the Li₂MnO₃ phase in large Li-rich material, which is a major component to realize high energy density, is not fully activated in low voltage range below 4.6 V. For this reason, many previous papers reported their results with only small primary particle size below 100nm. The high surface area induced by the small particle size increases side reactions with electrolyte resulting poor long-term cycle life as well as lowers the volumetric energy density. Note that Li-rich material's deteriorations are generated from its surface and continue to affect its cycling capability and the deteriorations can be divided into two mechanisms: one is side reactions with electrolyte, and another is the phase collapse. The side reactions with electrolyte at its high working voltage yield active material attack speiceses such as HF and electrolyte exhaustion causing Li-rich material's fast discharge capacity decay during cycles. Furthurmore, it is hardly show long-term cycle life with over 200 cycles since the phase transition from layered structure to spinel-like and NiO rock salt is generated on the surface of active material, and followed by expention of this transition into the bulk. For instance, AlF₃ coated Li-rich LNCMO has abrupt capacity fade after 100 cycles, although it showed good rate and cycle capability within 100 cycles. Hence, new active material design, rather than simple surface modification methods such as coating or doping, is required to solve above previous limits.

The new material design needs to start with a simple question: how to minimize the damages of surface deteriorations such as side reasction with electrolyte and phase trasnsition on bulk material and maximize volumetric energy density. The most effective solution for the question is increasing the primary particle size with the stable activation method, since the volumetric energy density of cathode material is maximized from synthesis of micron secondary particles consisted of primary particles by using co-precipitation method in industry. Here we demonstrate a novel approach for lithium storage, which is a material design of a secondary structure which consists of large flake shaped primary particle (hundred nanometers x mictrometers) with a novel activation method using simple chemical treatment to achieve superior long-term cycle life with high volumetric energy density. In this design, the large primary particle effectively reduced its surface area producing markedly decreased surface instability reaction as well as high tap density. Interstingly, the chemical approach activated only surface Li₂MnO₃ phase of large primary particle and the very surface activation effectively overcame the activation problem, which is limit of large primary particle have. This novel concept is very meaningful in that it is the first and unique method to achieve cathode material's high volumetric energy density with long-term cycle life. As a result, this novel designed material affords remarkable battery performance with extremely high cycle retention during 400 cycles and high volumetric energy density ever reported before.

A simply coupled electrochemical and thermal model is validated and presented for a 15 Ah large format prismatic lithium iron phosphate cell. Understanding the thermal response of lithium-ion cells is essential because it would allow the batteries to be operated more efficiently. Two important issues regarding the thermal response are "cold start" and "thermal runaway."[1] Use of a validated simulation can aid in enhancing the performance and safety of these batteries.

To minimize the number of essential parameters, the simplest models are carefully chosen to depict the battery responses. At least two physical models are necessary for describing an electrochemical system: an electrochemical model and a local energy balance. For the electrochemical model, we select the Newman and Tobias model, which provides an analytic solution with an assumption that the ion concentration gradient is negligible.[2] For the energy balance [3,4], three different heat sources are necessary to match our experimental data: (1) Joule heating of the cell materials (simple IR heating), (2) an additional Joule heating generated by the interfacial resistance between electrode and electrolyte, and (3) the reaction heating from reversible entropy change. Joule heating is always exothermic; however, the reversible heat depends on the electrochemical reaction and can be endothermic or exothermic.

Even though the minimal physics are chosen for simplicity, the number of physicochemical parameters in the completed model was more than ten; however, the number could be decreased to five after rigorous dimensional analysis. Therefore, not only do these parameters suggest important factors for the battery response, but they also help to control the battery in efficient ways, since this test shows that many parameters have the same effect in the electrical/thermal response. Comparison to target experimental data characteristics of the battery (Fig. 1) allows iterations of the dimensionless parameters to increase accuracy through multidimensional Newton-Raphson method. Finalized parameters will guide simulations to help control the battery in efficient ways.

[1] T. M. Bandhauer, S. Garimella, and T. F. Fuller, Journal of The Electrochemical Society 158, R1 (2011).

[2] J. S. Newman and C. W. Tobias, 109(12), 1183 (1962).

[3] D. Bernardi, E. Pawlikowski, and J. Newman, J. Electrochem. Soc. 132, 5 (1985).

[4] J. Newman and K. E. Thomas-Alyea, Electrochemical Systems, 3rd ed. Wiley, 2004.

In this work, a dual approach of metal doping and carbon coating was employed to improve the poor ion diffusion rate and low electrical conductivity of LiFePO₄, respectively. LiFePO₄ and LiFe_0.96Pt_0.04PO₄ materials were synthesized using a sol-gel method followed by carbon coating with sucrose. The effect of platinum doping on the physical-chemical properties and the electrochemical performance of LiFePO₄/C material were investigated by XRD, XPS, SEM, carbon analyzer, charge/discharge testing, cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical results showed that the specific discharge capacities of the material significantly increased by platinum doping. This improvement will be discussed considering the expansion of the lattice parameter, reducing the particles size with uniform distribution and pillar effect of platinum element for stabilizing the crystal structure.

Ex situ activation of Li-excess layered cathode material (Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂) has been carried out via proton exchange in hydrochloric acid (HCl), followed by chemical exfoliation in tetrabutylammonium hydroxide (TBA·OH) solution and post-heat treatment, in which morphologic and structural reconstructions of pristine cathode material simultaneously take place. Exquisite nanoflower-shaped derivative is achieved with significant increased surface area and uniform pore size distribution after protonation and TBA·OH exfoliation. Sintering nanoflower-structured derivative contributes to hierarchical mesoporous structure and cubic spinel phase within final converted cathode material. The chemical activation of Li-excess layered cathode material results in phase transition from original layered to refactored spinel structure. The activated cathode material in spinel phase and hierarchical porous structure shows significantly improved electrochemical performance, which reveals Coulombic efficiency of 99.93% in the initial cycle at 1C (1C=250 mA/g) and retains high specific capacity of 200.8 mAh/g after 100 cycles, in comparison with 59.16% and 58.1 mAh/g of pristine Li-excess layered cathode materials, respectively. Remarkable enhanced rate capability has also been accomplished in activated cathode, delivering initial discharge capacities of 313.6, 267.2, and 126.3 mAh/g at 0.1, 0.5, and 5C, respectively. Such considerably enhanced cycleabilitiy and rate capability can be attributed to ex situ activation of Li₂MnO₃ component, phase transition, and/or morphologic and structural reconstructions of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂. This work opens up various routes to achieve excellent electrochemical performance from high-capacity Li-excess layered cathode materials.

Today, most of the battery pack simulations are conducted for design and analysis of the cooling channels to address non-uniform temperature distribution that lead to loss of performance and life. This non-uniformity could lead to different ageing of the cells depending on their location within the pack. This could subsequently trigger undesirable behavior like undercharge or overcharge conditions. In all these simulations, an equivalent circuit model is used to represent electro-chemistry as the detailed model would become computationally expensive. Incorporating degradation and aging mechanisms into these equivalent circuit models are effective for on-board diagnostic devices where time is critical, but a detailed electro-chemical model is necessary to understand balancing of cells in a Li-Ion battery pack. In this paper, we will present an efficient way to simulate battery packs that utilizes high performance computing to demonstrate the importance of detailed electro-chemical model on temperature and potential gradients in a battery pack that lead to cell imbalance. The 3D multi-physics model for the individual Li-Ion pouch cell used to construct the battery pack has been validated earlier with the experimental data at various discharge rates [1]. The battery pack under consideration has modules connected in series. Each module has four cells connected in series and parallel. The Voltage and Temperature profiles of the battery pack with two modules at the end of discharge are shown in Figure 1. Convective cooling boundary condition is imposed on the exterior of the battery pack.

With increasing demanding of energy, the development of advanced materials to improve battery performance has become increasingly important. Among the batteries on the forefront of the latest technologies, lithium-ion batteries (LIB) are the most popular rechargeable batteries systems. However, LIBs are reaching its limit in specific energy capability by the electrochemical materials used. For example, graphite, the current start-of-the-art anode in LIBs has a limited capability to store Li since the theoretical capacity of graphite is 372 mAh/g [1]. NASA is developing high energy and high performance lithium-ion (Li-ion) cell designs and batteries for future exploration mission under the NASA Space Power System project [2]. To meet NASA aerospace/space applications, rechargeable lithium-ion batteries with higher specific energy and energy density and improved safety are desired.

To achieve higher capacity and energy density, and to improve safety for current LIBs, the nanostructed metal oxide/carbon nanotubes (CNT) as an anode for the LIBs has been investigated. Metal oxides such as Fe₂O₃ are considered as promising anode active materials since Fe₂O₃ displays many attractive features such as high theoretical capacity (1007 mAh/g, which is ~ three times of that of graphite), safe, cost-effective, and environmentally friendly [3]. However, poor electronic conductivity of metal oxides and volume changes during the charge and discharge process results in rapid capacity fade of the metal oxide. CNTs are promising candidates also for use as anode material in lithium-ion batteries since CNTs possess unique structural, mechanical, and electrical properties. However, when CNTs are used alone as anode, they lack stable voltage performance and exhibit high irreversible capacity loss [4]. The nanostructured metal oxide on CNTs, with CNTs serving as backbone/host matrix in the anode, not only provides excellent electronic conductivity to overcome the low conductivity issue of metal oxides, but also acts as an effective buffering component for alleviating the degradation of structural integrity that results from the volume changes associate with the charging and discharging process. In addition, the CNTs are also part of active materials in the anode, resulting in additional capacity and improved energy density for the anode.

In this work, the metal oxide is uniformly attached to CNTs. The electrochemical properties of the developed metal oxide/CNTs as anodes in LIBs have been studied by various electrochemical techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic/galvanodynamic techniques. Li-ion cells with metal oxides/CNT anodes and coupled with NASA advanced non-flammable electrolyte were constructed and cycled. The electrochemical constants, such as reversible capacity, irreversible capacity loss, and coulombic efficiency have been characterized. The rate capability and life cycling performance have been evaluated. The impact of electrolyte type and binder type on cycling performance has also been investigated. In addition, the lithiation/delithiation processes and possible mechanisms during the charge/discharge cycling of the developed metal oxide/CNTs anodes in LIBs have been investigated and will be discussed in this presentation.

References:

[1] Dahn J. R.; Zheng T.; Liu Y.; Xue J. S. Science1995, 270, 590

[2] Mercer C. et al., "Energy Storage Technology Development for Space Exploration", NASA/TM—2011-216964

[3] Brandt, A.; Balducci, A. J. Power Sources2013, 230, 44-49.

[4] Casas C. d. l.; Li W. J. Power Sources2012, 208, 74-85

Microbattery devices are strongly equipped with onboard power systems which consists of one important component, that is, solid-state electrolytes. The solid-state electrolyte thin films used in microbatteries could be perfect and free pinholes to aviod short circuites. In this study, lithium phosphorous oxynitride(Lipon) thin film has been fabricated successfully by E-beam reactive evaporated sintering Li₃PO₄ with the insertion of nitrogen ions. Scanning Electronic Microscopy(SEM) showed that well-controlled film thickness and composition depend on the deposition parameter. Lipon thin films were examined by X-ray photoelectron spectroscopy (XPS), and the obtained results showed that the method of nitrogen ion implantation for the increase of nitrogen content in membrane is obvious and the decrease of the activation energy. Moreover, the lithium ion conductivity of the as-deposited Lipon thin films was investigated by impedance spectroscopy. It was found that the thin film exhibited high lithium ion conductivity. Due to three-coordinated nitrogen atoms, a higher cross-linked structure directly affects the ionic conductivity.

On the way to environmentally friendly and sustainable mobility lithium-ion batteries are in focus of various research works. Fundamental studies regarding the synthesis of new active materials or improvements of other important components, e.g. electrolytes or binders, are providing essential progress towards efficient e-mobility. In addition to all the material based developments, it is indispensable to gain a more detailed understanding of the main leverages which improve the electric and ionic transport processes inside the electrode and cell. One crucial property is the electrode's inner structure. Since not only the materials but also the production processes have a considerable impact on the electrode structure, knowledge of the relationships between processes and structures is essential for a targeted electrode manufacturing.

Electrode's inner structure is basically predetermined by the chosen components, particularly by their particle size distributions and aggregation status (active material, additives) or molecular weight (binder). In addition, some process steps in the manufacturing chain for lithium-ion battery electrodes, such as dispersing and calendering, show crucial impact on the resulting electrode structure. The process modalities while dispersing the powdery materials in a solvent control the desagglomeration degree of the carbon blacks and thus significantly affect the structure in the subsequently produced electrode. Changes in pore diameter distribution (see figure 1), porosity and tortuosity of the pores can be achieved by a more/less intensive or longer/shorter dispersing of the components in the solvent. To predict these structural alterations it is necessary to learn more about the suspension properties, especially about its structure.

Colloidal suspensions are commonly analyzed regarding their rheological characteristics in terms of flow behavior to estimate the properties for further processing steps like coating. In addition, rheological measurements can provide indirect structural information of battery suspensions, for instance concerning the fragmentation degree of the carbon black and the structural changes in the suspension while dispersing. Figure 2 exemplarily depicts the observed differences between suspensions with an aggregated particle-network (solid/gel like: phase angle 0°) to a more liquid-like (phase angle increasing up to 90°) suspension structure. A notable structural decrease can be identified after 45 minutes of dispersing compared to a shorter dispersing time of 10 minutes.

The correlation between suspension structure, analyzed via rheological measurements, and the resulting electrode structure, characterized via mercury porosimetry, is in the focus of the presented work. Additional investigations on the interrelations between structures and electric or ionic transport processes as well as electrochemical cell performances provide a basis for targeted electrode manufacturing.

12V microhybrid (MHEV) technology has potential to deliver 5-10% fuel economy improvement over traditional gasoline vehicles without adding excessive cost to the consumer. Parallelizing the traditional lead acid battery with a high-power lithium ion battery has the potential to enable critical microhybrid function such as regenerative braking. Enabling a low-cost passive connection between the lead-acid and lithium-ion batteries requires compatibility between the two batteries, including design factors such as battery size, resistance, and open-circuit voltage (OCV).

In this work we have developed a dual-battery model that couples an electrochemical model of a lithium ion battery and an equivalent circuit model of a lead acid battery in order to perform design optimization of a passively-connected dual-battery MHEV system. We analyzed the dynamic responses of the two batteries upon high power charge and discharge pulses, with and without simulated vehicle accessory loads. The current split between the two batteries is explored in detail as a function of pulse characteristics, operating voltage, and accessory load. Finally, a simulated vehicle regulatory drive cycle, specifically the New European Driving Cycle (NEDC), is implemented for analysis of contributions of two batteries.

Recently, the importance of various secondary batteries is increasing remarkably, because it is expected as the energy storage devices for renewable energy, such as solar and wind, as the basic technology for energy security and as the power storage for automotive. Especially, in order to increase the high-rate performance, Innovative design of electrode structure on the basis of kinetics and transport phenomena of ion and electron is needed. In our previous study, we focused on the binder and examined the effect of binder distribution and porous structure on the discharge performance by numerical analysis. In this study, actual porous electrode structure was obtained by FIB-SEM and the effective electrical and ionic conductivities were estimated, and these were compared with experimental results. Moreover, the effect of structure on the discharge performance and reaction rate distribution by 3D direct simulation. As a result, the correlation model of the relationship between porosity and relative conductivity of Li⁺ and electron could be obtained. And the remarkable difference between heterogeneous structure and uniform particle packed model could be confirmed. The validity of this simulation method could be confirmed by the comparison of tortuosity and relative conductivity between estimation and experimental results. From the electrochemical reaction simulation, in the case of reducing binder to 1/3 of actual condition, overall current could increase twice. So it was found that the optimization of structure of sub-material, which consists of binder and conductive material, is very important to increase high-rate discharge performance. In addition, we examined the effect of binder adhesion model and migration in the direction of through-plane.

The LiV₃O₈ compounds were synthesized by Solid-state method and calcination at different temperature (with heat treatment temperature ranging from 300 °C ∼ 600 °C) in order to find the relationship between heating temperature and electrochemical properties of LiV₃O₈.

The effects of precursor and heat treatment conditions on the structure, morphology, and electrochemical properties of the LiV₃O₈ were investigated by the implementation of X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and energy dispersive spectroscopy (EDS). In order to find proper heat treatment temperature, TGA/DSC was performed too. All cells were cycled 50 times at a C-rate of 0.2 with the cut-off voltage ranging from 1.8 to 4.0V (versus Li/Li⁺). The result was analyzed by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV).

As the heat treatment temperature is increased, LiV₃O₈ has better crystalline phase but its primary particle size is grown bigger. For the sake of better electrochemical performances, especially high capacity retention, LiV₃O₈should have a proper size and size distribution.

[1] J. Feng, X. Liu, X. X. Zhang, J.Jiang, Z. Zhao, and M. Wang, J. Electrochem. Soc,156, A768 (2009)

[2] H. Yang, J. Li, X.G. Zhang, Y.L. Jin, J. Mat. pro. tec, 207, 265 (2008)

[3] D. Wang, L. Cao, J. Huang, J. Wu, Ceram. Int, 38, 2647, (2012)

[4] Y.Q. Qiao, J.P. Tu, X.L. Wang, J. Zhang, Y.X. Yu, and C.D. Gu, J. Phys. Chem, 115, 25508 (2011)

The public demand for high power energy storage systems is on a continuous ascending path. Efforts are directed towards the development of advanced lithium-ion electrodes, e.g. modified surface architectures. Direct laser patterning of thin and thick film electrode materials is a fairly new technical approach which enables an increase of capacity retention especially for high charging and discharging rates (Figure 1). It is assumed that an appropriate three dimensional surface topography influences the diffusion kinetics of Li-ions in the electrode materials through the electrolyte, whereas critical mechanical tensions during charging/discharging can be avoided and ohmic losses are significantly reduced inside the electrochemical cell.

The goal of this work was to enable quantitative studies of lithiation/delithiation rates which contribute to a better understanding of electrochemical intercalation/deintercalation processes in laser modified electrodes. The simplest quantitative approach to determine the rate of Li-ion insertion in the active material and the rate of Li-ion transport in the electrolyte is expressed by diffusion coefficient values. For this purpose, one of the most common coulometric titration technique, the galvanostatic intermittent titration technique (GITT) has been involved.

Electrochemical measurements were performed using the Swagelok® cell design for both, full and half-cell types, using graphite and metallic lithium as counter electrodes, respectively. The results of Li-ions diffusion rates are presented for laser structured and unstructured lithium -metal oxide cathode materials. The focus is set on composite thick film electrode materials containing binder, conductive carbon and layered intercalation lithium cobalt oxide (LiCoO₂), which is the main cathode material used nowadays in many commercial lithium-ion cells. A main challenge for GITT was to evaluate suitable measurement parameters such as current pulse length, charge/discharge rate and relaxation time for each specific cell system. The results obtained were evaluated and compared to literature data.

In this study, we report the effect of metal coated silicon nanowires (SiNWs) on electrochemical performance as a negative electrode material. We fabricated metal coated SiNWs by using metal assisted chemical etching process. Transmission electron microscopy (TEM) images show that SiNW arrays with silver (Ag) coating on a Si (100) substrate have been successfully prepared in a one-pot process. To investigate the electrochemical performances of Ag-coated SiNWs, coin cells using SiNWs as negative electrodes by the slurry coating method were assembled with Li metal foil as the counter electrode. We show that the rate capability of Ag coated SiNWs is greatly improved compare to bare NWs. Also, the good cycling performances of the Ag-coated SiNWs were clearly observed. The enhanced electrochemical performance is attributed to the improvement of electrical conductivity of SiNWs. Furthermore, we suggest that the Ag layer is exposed to the electrolyte instead of bare Si, and it can prevent solid electrolyte interface (SEI) formation and deterioration of active materials.

This paper examines the health-conscious optimal control of lithium-ion batteries. The paper focuses specifically on the optimizing battery control policies to minimize solid electrolyte interphase (SEI) layer growth. However, its approach is applicable to other degradation mechanisms. The paper exploits the fact, established by the mathematical control literature^1–4, that Fick's second law of diffusion is differential flat for both linear and nonlinear diffusion problems. Differential flatness only applies to each diffusion medium in the battery separately. In the single particle model (SPM), for instance, each electrode's diffusion sub-model alone is differential flat. Ensuring differential flatness for the entire battery is a two-step process. First, one must truncate redundant integrators from the two electrodes' diffusion sub-models. Second, one must then relate the truncated integral state variable to the remaining state variables through an affine transformation. The differential flatness of the resulting battery model makes it possible to solve the health-conscious battery management problem efficiently using pseudospectral methods. This exploitation of the structure (i.e., differential flatness) of lithium-ion battery dynamics for more computationally efficient trajectory optimization is novel and significant. The existing literature already examines the problem of health-conscious optimal battery charging and discharging^5,6. However, the optimal control tools used in this literature are not tailored to exploit the structure (e.g., differential flatness) of lithium-ion battery models. This can lead to higher computational burdens compared to the novel and more efficient approach we present here. The approach has the added advantage that it can be used for both offline trajectory optimization and online model-predictive control.

The paper demonstrates the above ideas using the SPM. The optimization problem is formulated by equation [1-6] and the SPM with film growth model. The cost function in Equation [1] aims to track the reference state of charge (SOC) and at the same time minimize the film growth rate, which is defined by equation [7]-[14] in Ramadass's paper⁷. The weight β represents a tradeoff/balance between aggressive charging and battery degradation. Constraints [2-6] place limits on battery charge/discharge current, reflecting battery management hardware capabilities. Furthermore, these constraints limit battery SOC to prevent over-charging and over-discharging. Finally, these constraints also place bounds on battery state variables contributing to damage phenomena such as lithium plating and mechanical degradation. For instance, we bound the concentration gradients in the battery cell and the overpotentials driving the lithium plating side reaction^8–10. The optimization approach presented in this paper transforms these equations and constraints from a dynamic programming problem to a nonlinear programming (NLP) problem. Solving this NLP problem using traditional optimization methods leads to a health-conscious battery management policy. The value of this paper lies not in this policy, but rather in the computational efficiency with which it is obtained. Specifically, to the best of the authors' knowledge, this paper represents the first attempt to exploit lithium-ion battery model structure for more efficient solution of the health-conscious optimal management problem.

ACKNOWLEDGMENTS

The research was funded by ARPA-E AMPED program grant # 0675-1565. The authors gratefully acknowledge this support.

REFERENCES

1. M. FLIESS, J. LÉVINE, P. MARTIN, and P. ROUCHON, Int. J. Control, 61, 1327–1361 (1995).

2. B. Laroche, P. Martin, and P. Rouchon, Int. J. Robust Nonlinear Control, 10, 629–643 (2000).

3. M. Fliess and R. Marquez, Int. J. Control, 73, 606–623 (2000).

4. T. Meurer, Automatica, 47, 935–949 (2011).

5. R. Klein and N. Chaturvedi, Am. Control ... (2011).

6. V. Boovaragavan and V. R. Subramanian, J. Power Sources, 173, 1006–1011 (2007).

7. P. Ramadass, B. Haran, P. M. Gomadam, R. White, and B. N. Popov, J. Electrochem. Soc., 151, A196 (2004).

8. J. Christensen and J. Newman, J. Solid State Electrochem., 10, 293–319 (2006).

9. X. Zhang, A. M. Sastry, and W. Shyy, J. Electrochem. Soc., 155, A542 (2008).

10. N. Chaturvedi, R. Klein, J. Christensen, J. Ahmed, and A. Kojic, IEEE Control Syst. Mag., 30, 49–68 (2010).

Introduction

Lithium titanium oxides such as spinel-type Li₄Ti₅O₁₂ and ramsdellite-type Li₂Ti₃O₇ have been extensively investigated as electrode materials for rechargeable Li-ion batteries. In recent, another metastable phase of Li₂Ti₃O₇ with Na₂Ti₃O₇-type layered structure have been reported with the structural details and electrochemical Li-ion insertion properties [1]. On the other hand, NaLiTi₃O₇ has been investigated as electrode materials for rechargeable Li-ion batteries [2]. This Ti⁴⁺/Ti³⁺ electrode showed lower operating voltage compared to Li₄Ti₅O₁₂. In addition, NaLiTi₃O₇ has a three-dimensional tunnel structure which is thought to be favorable for fast Li-ion transfer. This compound can be prepared by a conventional solid state reaction. Li₂Ti₃O₇ with the NaLiTi₃O₇-type structure may be prepared by ion exchange of NaLiTi₃O₇ (Fig. 1). Higher Li-ion insertion capacities similar to that observed in Li₂Ti₃O₇ with Na₂Ti₃O₇-type layered structure can be expected. However, to our knowledge, synthesis, the structural details and electrochemical Li-ion insertion reactions in this compound have not been reported yet.

In the present study, we have successfully synthesized single-phase sample of Li₂Ti₃O₇ with the NaLiTi₃O₇-type structure and determined its crystal structure by Rietveld analysis. Furthermore, the Li-ion insertion properties of this compound have been clarified for the first time.

Experimental

The precursor NaLiTi₃O₇ was first prepared by a conventional solid state reaction by using a method similar to that reported previously [3]. A mixture of Na₂CO₃ (99.9% pure), Li₂CO₃ (99.99% pure) and TiO₂ (99.99% pure) in a molar ratio of 1.01:1.03:3 was heated at 950°C for 24 h in air. Then, the lithiated Li₂Ti₃O₇ samples were prepared from NaLiTi₃O₇ via ion exchange at a low temperature. Sodium/lithium ion exchange experiments were performed using the molten salt of LiNO₃. Ion exchange reaction was performed at 400°C for 6h in air. After heat treatment, the reaction mixture was washed with ethanol and then dried at 80°C for 1 day in air. Further Li-ion exchange treatment was accomplished by heating the as prepared Li₂Ti₃O₇ in molten LiNO₃at 380°C for 6 h in air.

The phase purity and crystal structure of the obtained samples were characterized by powder X-ray diffraction (XRD) using a Bruker AXS D8 ADVANCE diffractometer with Cu Kα radiation source filtered by a Ni thin plate (Cu Kα radiation, operating conditions: 40 kV, 55 mA). The particle morphology and chemical composition were verified by scanning electron microscopy equipped with energy dispersive X-ray spectrometer (SEM-EDX; Keyence VE-8800). The chemical and structural characteristics were evaluated using ICP, DTA and FTIR measurements.

Electrochemical lithium insertion/extraction experiments were performed using lithium coin-type cells (CR2032-type). The working electrode were made of 80wt% active materials, 10wt% carbon black (Super-P) as a conductive agent, and 10wt% poly(vinylidene difluoride) as a binder. Copper foil was used as a current collector, and the area of the electrode was a diameter of 14 mm. The counter electrode was a Li foil having a diameter of 16 mm. The separator was a microporous polypropylene sheet. A solution of 1 M LiPF₆ in a 1:2 mixture of ethylene carbonate (EC) and dimethycarbonate (DMC) by volume (Mitsubishi Chemical Co., Ltd.) was used as the electrolyte. Cells were constructed in a dry room, and electrochemical measurements were carried out with a current density of 10 mA g⁻¹between 0.5 and 2.0 V at 25°C after standing 6h under an open circuit condition.

Results and discussion

Figure 2 presents the XRD patterns of the NaLiTi₃O₇ and Li₂Ti₃O₇ samples. These data suggested that the products were single-phase samples of NaLiTi₃O₇ and Li₂Ti₃O₇. Chemical analysis confirmed that the removal of Na was not complete at the present experimental condition.

The electrochemical measurements revealed that the initial Li insertion capacities were 152 mAh g⁻¹ and 243 mAh g⁻¹ for NaLiTi₃O₇ and Li₂Ti₃O₇, respectively, which were equivalent to 1.62 and 2.45 electron transfer per each formula unit. In addition, reversible Li-ion insertion and extraction reactions were observed in these samples, although a capacity loss of approximately 50 mAh g⁻¹ and 75 mAh g⁻¹ for the first cycle was observed in NaLiTi₃O₇ and Li₂Ti₃O₇, respectively.

Acknowledgement

This work was supported by the "Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING project)" of the New Energy and Industrial Technology Development Organization (NEDO; Japan).

References

[1] K. Chiba et al., Solid State Ionics, 178, 1725 (2008).

[2] S.Y. Yin et al., Electrochem. Commun., 11, 1251 (2009).

[3] L.M. Torres-Martínez et al., Solid State Sci.8, 1281 (2006).

High power and energy density lithium-ion batteries (LIBs) with high charge/discharge rate are required for electric vehicles. Although it is inevitable to clarify the appropriate electrode manufacturing methods to enhance the performance of LIBs, there have been many uncertain points. In the present study, we investigate the influence of positive electrode mixing conditions on the performance of LIBs at 30 °C. Test cells were assembled with positive electrodes of LiCoO₂ mixed with acetylene black as conductive filler and PVDF binder, electrolyte solution of ethylene carbonate (EC)/dimethylcarbonate (DMC) (1:1 vol.) containing 1 M LiPF₆, and negative electrodes of graphite mixed with PVDF binder.

We employed fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS) to analyze the internal resistances under discharge to take advantage of its short measurement time since measurable decrease in the state of charge (SOC) cannot be avoided during EIS measurements with the conventional frequency response analyzer (FRA). Cross-sectional scanning electron microscope (SEM) observations of the positive electrodes were also carried out.

Pre-mixing of LiCoO₂ particles and acetylene black under an appropriate mill rotation speed is effective for the performance enhancement. In both cases without the pre-mixing and with pre-mixing under too high mill rotation speed, the discharge capacities decrease.

The impedance spectrum by FFT-EIS exhibits two semi-circles at 1000 mA/g. In the case without the pre-mixing, the high frequency arc becomes significantly large, while both high and low frequency arcs become significantly large in the case with pre-mixing by too high mill rotation speed.

The appropriate pre-mixing possibly thus decreases the resistance of electron transfer, i.e. contact resistance, between the LiCoO₂ particles and acetylene black by good dispersion increasing the contact between them. This view corresponds to the assignment that the high frequency arc is attributed to the surface insulating layer of LiCoO₂ particles[1,2] contacting with acetylene black. The SEM observations support this view.

On the other hand, the pre-mixing by too high mill rotation speed possibly increases the contact among LiCoO₂ particles, decreasing the contact between the LiCoO₂ particles and acetylene black. This also decreases the contact between the LiCoO₂ particles and the electrolyte. This view agrees with the assignment that the high frequency arc is ascribed to the surface insulating layer of LiCoO₂ particles described above while the low frequency arc represents the Li ion charge transfer at the interface between the LiCoO₂ particles and the electrolyte[1,2]. The SEM observations support this view also in this case.

References

[1] E. Barsoukov, in Impedance Spectroscopy, Theory, Experiment, and Applications, 2^nd ed., E. Barsoukov and J. R. Macdonald, Editors, John Wiley & Sons, Hoboken, NJ (2005).

[2] Advances in Lithium-Ion Batteries, W. A. V. Schalkwijk, and B. Scrosati, Editors., Kluwer Academic/Plenum Publishers, New York (2002)

Si has gained much attention as anode materials for lithium-ion battery due to its high theoretical capacity (3,580 mAhg^-1) and low standard oxidation potential (0.4 V vs. Li/Li⁺). Despite these features, Si anode materials still have limits to commercial uses by poor cycle performance associated with their mechanical pulverization of Si due to severe volume change during alloying/dealloying with Li. To overcome this problem, various approaches have been suggested. In particular, the composites, which are composed of active Si and inactive buffer material, showed promising approaches for reducing the mechanical strain during cycles. On the other hand, nanostructuring of active materials would be helpful for further improvement of cycle performance. After heat treatment of Si/SiO_x precursor at 1200 °C under inert atmosphere, we obtained Si nanocrystals embedded SiO_x nanospheres with a size of 200 nm, in which about 10 nm sized Si nanocrystals are embedded in amorphous SiO_x matrix. In order to improve electrical conductivity of Si/SiO_x nanocomposites, carbon layer was coated on the surface of Si/SiO_x nanocomposites.

The Si/SiO_x nanospheres and C-coated Si/SiO_x nanospheres were characterized by a field emission scanning electron microscope (FESEM), a high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) patterns, solid state ²⁹Si magnetic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray photoelectron spectroscopy (XPS) analyses.

The C-coated Si/SiO_x composites showed a reversible capacity of 950 mAh/g^-1 at current density of 200 mAg^-1 with stable cycle performance over 100 cycles and high rate capabilities. In this presentation, the electrochemical properties of the C-coated Si/SiO_x nanocomposites will be discussed in more detail.

Lithium-ion batteries are a promising technology for automotive application, but limited performance and lifetime is still a big issue. The aim of this work is to study and address degradation processes which affect LiFePO₄ (LFP) cathodes - one of the most common cathodes in commercial Li-ion batteries.

In order to evaluate how the LFP cathode is affected by C-rate a LFP working electrode, Lithium metal foil counter electrode and Lithium metal reference electrode was tested in a 3-electrode setup with a standard 1M LiPF₆ in 1:1 EC/DMC electrolyte and glass fiber separator. The working electrode/counter electrode was subjected to several charge/discharge cycles between 3.0 V and 4.0 V at different discharge rates. Figure 1 shows the voltage profile of the LFP electrode (solid line) and full battery (dotted line) during charge/discharge process. It is seen that the higher the C-rate, the higher is the polarization furnished by the counter electrode which reduces the capacity.

In Figure 2, the discharge capacity [mAh/g] is plotted vs the number of charge/discharge cycles. Series of 10 cycles at a given C-rate was applied to the battery. Each series was followed by a C/10 cycle (green points). A linear fit has been applied to the first series (omitting first two cycles where instability of the system is observed), in order to calculate the degradation rates.

High C-rates are seen to affect the discharge capacity, but the capacity is almost completely recovered (green points) and only a limited degradation occurs.

Impedance spectroscopy has been also applied to investigate the LFP cathode degradation. Figure 3 shows the imaginary part of the impedance measured at 50% State-of-Charge after each series of cycles. The relative increase in the impedance arc around 1 KHz (assumed to be associated with charge transfer resistance at the LFP particle surfaces) is seen to gradually decrease with increasing number of series. This indicates that more cycles per series is needed to establish a convincing relation between C-rate and degradation.

The degradation studies will be coupled with FIB/SEM analysis in order to observe changes in the pore structure or micro cracks that would affect electronic percolation. Figure 4 displays an example of a fresh LFP cathode after FIB cutting. White particles are LFP grains while the black area contains carbon particles and pores, which are difficult to distinguish from each other. Substitution of the epoxy resin with a silicon resin increases the contrast between pores and carbon particles [1] and this will be used in the forthcoming FIB/SEM analysis.

References

[1] M. Ender et al, Journal of The Electrochemical Society, 159 (7) A972-A980 (2012)

Although the performance of LiFePO₄ has been improved significantly, the underlying mechanism of lithium ion insertion and de-insertion in FePO₄/LiFePO₄ (FP/LFP) is still not clearly understood^1-4. There are two basic mechanisms occurring during charge/discharge process: solid-solution reaction mechanism and two-phase reaction mechanism. In the first case – the solid solution reaction – only a single phase is involved, and as a result the lattice parameters and unit cell volumes would be expected to change continuously during the charge/discharge cycle. In the second case – associated with a first order phase transition – the unit cell volumes of both phases remain nearly constant, with the unit cell volume difference of two phases varying by only DV=6.5%. The distinct nature of these two mechanisms is thus apparent using X-ray diffraction, and as a result, in operando synchrotron HEXRD technique is particular amenable to revealing the underlying mechanisms.

In this study, we have investigated the structural changes that occur in LiFePO₄ electrodes in commercial 18650 cells during the charge/discharge process using in operando synchrotron HEXRD.The 18650 cell (APR18650M, 1.1Ah) was provided by A123 Systems with a graphite anode, a LiFePO₄ cathode, and a 1.20 M LiPF₆ in EC: EMC electrolyte. No special modification was needed for the 18650 cell before characterization. Four equivalent cells have been cycled and in situ characterized under different cycling rates from 0.1 C, 1C, 3C to 5 C.

The results show that (1) the phase fractions of both LiFePO₄ and FePO₄ (Fig. 1 a and b) change with SOC at both low rates (i.e. 0.1 C) and high rates (i.e. 1 C), suggesting that the LiFePO₄ electrode is undergoing a two-phase reaction mechanism in the entire flat voltage plateau; (2) the unit cell volume of both LiFePO₄ and FePO₄ (Fig. 1 c and d) changes with the SOC at both low rates (i.e. 0.1 C) and high rates (i.e. 1 C), indicating that the electrode is experiencing the dual-phase solid-solution reaction mechanism, and (3) the difference (DV ) in the unit cell volume between the LiFePO₄ phase and the FePO₄ phase reduces as the rate increases (not shown here due to the page limit), implying that the region over which the dual-phase solid-solution exists will be compressed with increased rates and, eventually, may be diminished at extremely high rates⁵. On the other hand, our results suggest that the insertion/deinsertion process is rate dependent and the two different mechanisms, two-phase and solid-solution, co-exist during the charge/discharge process. At very low over-potential (i.e. very low rate charge/discharge), the process is dominated by the two-phase mechanism while it might be dominated by the solid solution mechanism at very high over-potential (i.e. very high rate charge/discharge). Between the two extremes, the process is controlled by both mechanisms with different ratios depending on the rate. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO₄ in commercial cells.

Electrochemically stable phosphates promise high reversible capacity, high operating voltage, and good ion mobility and have the potential to provide electrochemically superior, environmentally friendly (Co-free) and affordable battery materials. Li₃V_(2−2x/3)Mg_x(PO₄)₃/C (x=0, 0.15, 0.3, and 0.45) composite phosphate cathode materials show high reversible capacity, high operating voltage and good cycling stability. In order to understand the functioning of this cathode at the molecular level, we have determined the local vanadium structure as a function of charge state using in-situ x-ray absorption spectroscopy (XAS). The XAS results verify for the charge compensation involves V cycling between V³⁺ and V⁵⁺ during charge and discharge and shows that these changes in oxidation/reduction are accompanied by modest changes in V-O bond length but little apparent change the overall local structure of the V and its neighboring atoms. In contrast, there is a significant change in local structure if the cut-off potential is increased from 4.5V to 4.8 V. Once the electrode has been charged to 4.8V it undergoes an irreversible conversion to a vanadyl-like structure. While the vanadium is still redox active, it is not converted back to the starting form, even at potentials as low as 2.0 V cut off region. Although the details of the behavior depend on the level of Mg doping, similar results are found for all samples. These data provide for the first time a molecular level explanation for the observation that vanadium phosphate batteries suffer irreversible capacity loss if charges above 4.5 V.

We have previously reported that a high potential negative electrode (TiO₂(B)) is promising as a negative electrode material for large-scale Li-ion batteries (LIB), which require high capacity (> 300 mAh g^-1), high efficiency, long life, low cost and high safety [1]. However, its morphology, needle-like structure, is a problem for attaining a high volumetric energy density of the electrode. Ramsdellite type lithium titanium oxide (TiO₂ (R)) also has a high theoretical capacity (335 mAh g^-1) than spinel type lithium titanium oxide (175 mAh g^-1) [2, 3]. It crystalizes without any specific anisotoropy, and hence a high packing density of the electrode and a high energy density will be achieved. In the present study, we prepared TiO₂(R) from Li₂CO₃ and TiO₂(anatase) as raw materials, and investigated the charge/discharge properties as a high potential negative electrode for lithium ion batteries.

Ramsdellite type LiTi₂O₄was synthesized according to the following processes using solid-state reactions.

Li₂CO₃ + 4TiO₂(anatase)→Li₂Ti₄O₉ [750 ^oC, 12 h, air]

Li₂Ti₄O₉ → 2LiTi₂O₄ [1000 ^oC, 18 h, Ar/H₂(10%)]

TiO₂(R) was obtained from LiTi₂O₄ by ion exchange in 1 M HCl for 3 days. Charge/discharge properties of the TiO₂(R) were investigated at C/30-10C rates between 1.2 and 3.0 V using a two-electrode coin-type cell with a composite working electrode(TiO₂(R): Ketjenblack: PVDF = 8:1:1), a Li metal counter electrode, and 1 M LiPF₆dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) (1:2, by vol.).

XRD pattern of the resultant TiO₂ (R) powder is shown in Fig. 1. The pattern indicated that single-phase TiO₂(R) was obtained. The color of LiTi₂O₄ powder was black, while the TiO₂(R) was white. These should result from the changes in formal oxidation number of Ti from +3.5 to +4. The diameter of TiO₂(R) powder was estimated to be 2.5-10 mm from the SEM image of the TiO₂ (R) powder shown in the inset in Fig. 1. Moreover, the tap density of TiO₂ (R) powder was evaluated to be as high as about 1.33 g cm^-3. Therefore, high-energy density is expected.

Charge and discharge curves of obtained for a TiO₂(R) composite electrode at C/6 are shown in Fig. 2. A couple of plateaux due to the insertion/extraction of lithium ion were observed at around 1.3 V and 2.2 V. This fact suggests that two types of insertion/extraction sites exist in TiO₂ (R). One is an energetically stable site and the other is relatively unstable. The TiO₂ (R) electrode showed an initial discharge capacity as high as 205.8 mAh g^-1. In addition, the cycleability was high; 98.4 % of the initial capacity was retained in the 50th cycle. The electrode density was determined to be 1.24 g cm^-3, and therefore the gravimetric capacity was converted to a volumetric capacity of 255.2 mAh cm^-3.

The rate performance for a TiO₂(R) composite electrode was shown in Fig. 3, together with that for TiO₂(B) as a reference. The initial discharge capacity reached as high as 229.7 mAh g^-1 at C/30. The capacity retention was higher than that of TiO₂(B), particularly at > 3 C rate. The discharge capacity in the 25th cycle (202.7 mAh g^-1), which was obtained after charge/discharge operation at a high rate of 10 C, was almost the same as that in the 7th cycle (204.1 mAh g^-1). These results suggest the high durability of TiO₂(R) electrodes.

Recently, solid solution system, xLi₂MnO₃·(1-x)LiMO₂ (M = Co, Ni, Mn etc.), which has been classified as a lithium rich layered oxide, has drawn much attention due to its extraordinarily large capacity[1]. However, there are a lot of problems to overcome for commercialization such as low rate capability due to small electronic and ionic conductivities, and low cyclic performance attributed to insufficient structural stability. Over the past year, extensive effort has been made to enhance the rate capability of the lithium-rich layered oxide. As an approach, the composition with carbon-based materials with high electronic conductivity has been tried to compensate for low conductivity of the lithium-rich layered oxide [2,3]. In this study, the nanoparticles of lithium-rich layered oxide (Li[Ni_0.2Li_0.2Mn_0.6]O₂) were composited with graphene. Small particle-size of Li[Ni_0.2Li_0.2Mn_0.6]O₂ may facilitate the movement of lithium ions and electrons during cycling due to high surface area of the cathode. Graphene could be applied as a base-matrix for the composition with cathode nanoparticle because it has high electronic conductivity, wide surface area, and good mechanical flexibility. The Graphene/Li[Ni_0.2Li_0.2Mn_0.6]O₂ nanocomposite is expected to offer high rate capability because of high surface area of cathode nanoparticles and good electronic conductivity attributed to the graphene.

References

1.J. Lin, D. Mu, Y. Jin, B. Wu, Y. Ma, F. Wu. J. Power Sources 230(2013)76-80

2.A. Yamada, H. Koizumi, S.I. Nishimura, N. Sonoyama, R. Kanno, M. Yonemura, T. Nakamura,

Y. Kobayashi, Nat. Mater. 5 (2006) 357-360.

3.Y.G. Wang, Y.R. Wang, E.J. Hosono, K.X. Wang, H.S. Zhou, Angew. Chem. Int. Ed. 47 (2008)

7461-7465.

LiMn₂O₄ (LMO)-Li₇La₃Zr₂O₁₂(LLZO) composite film electrode was successfully fabricated without any heat treatments by using aerosol deposition (AD) method. Mixture of 75wt%-LMO and 25wt%-LLZO powders with the averaged particle sizes around 1 μm was directly used as raw material. The mixed LMO and LLZO powders were sprayed onto a stainless steel substrate in deposition chamber evacuated to a low vacuum state below 10 Pa with N₂ carrier gas for several 10 minutes. Mass flow of N₂ carrier gas was set to be 20 L min^-1 to form LMO-LLZ composite film via room temperature impact consolidation (RTIC). From both SEM observation and XRD measurement, it was found that as-deposited film has very dense structure without pores and is consisted from fractured LMO and LLZO crystalline particles with the sizes of several 10-100 nm. The thickness of composite film is 4 μm and LLZO content in the film was estimated to be around 21wt%. LMO-LLZO composite film electrode showed good electrochemical property in liquid electrolyte and higher discharge capacity than LMO film without including LLZO. The discharge capacities reduced by the weight of LMO-LLZO composite film electrode was 70 mAh g^-1, 62 mAh g^-1 and 55 mAh g^-1 at different discharge current rates of 0.07C, 0.7C and 3C (1C = 0.11 mA cm^-2). They are corresponding to the specific capacity per LMO in the film of 90 mAh g^-1, 80 mAh g^-1 and 71 mAh g^-1, respectively. In addition, the LMO-LLZO composite film electrode showed good capacity retention during cycle testing, which is attributed to strong adhesion between the composite film and substrate by AD. The present results suggest that AD method can potentially be used for electrode layer composed of active material and solid electrolyte for all-solid state lithium-ion battery.

Elkem is one of the world's leading companies for environment-friendly production of metals and materials. Among its principal products are metallurgical silicon, solar grade silicon, and different forms of carbon. For the coming generation of Li-ion batteries, silicon is a very attractive candidate to replace graphite as anodic material due to its remarkable energy density (3.6 Ah/g) and volumetric capacity (8.3 Ah/ccm). To tackle the massive cracking and degradation typically observed during electrochemical cycling of silicon anodes, we are developing composite nanostructured materials based on existing production of silicon and carbon at Elkem. The goal is to use silicon of optimum purity and doping w.r.t. production costs and performance mixed with carbon materials through milling techniques in a commercially viable way resulting in a product that can be used as anode material in Li-ion batteries.

Results show that performance can be substantially improved by using appropriate milling methods. The current presentation focuses on silicon-carbon nanostructures obtained by high-energy ball-milling in argon atmosphere. Silicon-carbon composites showed improved cycle life-time with ball-milling time, though usually accompanied with a loss of initial capacity due to high-surface area and extensive SEI formation. More pronounced improvements were achieved by co-milling silicon with graphite. Further enhancements were obtained by using additives in the electrolyte and by controlling the delithiation step during cycling.

The electrochemical performance was tested in half cells where typically the working electrode was made by mixing a silicon-carbon composite powder with an organic binder in an aqueous slurry and coated on a Cu-foil. Lithium metal was used as the counter electrode. Structural properties and degradation mechanisms were examined by electron microscopy (SEM, FIB-SEM, TEM) and XRD.

Figure txt: A) Charge (delithiation) capacity retention for selected Si-C based materials. The electrodes were cycled with C/20 rate for the three first and the last cycle. Other cycles were performed at C/10 rate (C-rate was based on the mass of Si assuming full capacity of 3.6 Ah/g_Si). The curves labeled 1 was for materials only treated by low-energy milling of Si. A rapid decrease in capacity is seen already from cycle one. This is in contrast to composite materials made by mixed milling of graphite and silicon by high-energy ball milling (curves labeled 2 represents material milled for 20 minutes at 800 rpm, curves labeled 3 were milled at 5 minutes) where a rapid degradation zone appears first after ca. 10 cycles. FIB-SEM cross-sectional images of the cycled electrodes (cycle 8 – just before rapid degradation sets in, cycle 15 – middle of rapid degradation zone, and "dead" electrode after 55 cycles) show that the electrodes gradually become clogged both by SEI formation and Si-intergrowth and lose porosity. This is accompanied with a thickness increase, most dramatically seen after the rapid degradation zone. B) Discharge-charge characteristics for a composite Si+C electrode restricting the charge capacity to 1200 mAh/g_Si pr. cycle (material: 80wt.% Si and 20wt.% graphite ball-milled for 5 minutes at 800 rpm). All electrodes were tested as half-cells with a Li-counter electrode and consisted of 60wt.% silicon, 30wt.% graphite, 2 wt.% carbon black and 8wt.% CMC. Typical loading of the active material was 0.8-1.0 mg/cm².

LiVPO₄F has high charge-discharge plateau and good safety, making it a promising high-voltage cathode material for lithium ion batteries (LIBs).^{1, 2} Nevertheless, LiVPO₄F suffers low electronic and ionic conductivity, which brings poor rate capability as well as cycle performance. Moreover, the reaction at the electrode/electrolyte interface at high voltage with high oxidative V⁴⁺ causes low cycle property and low initial coulombic efficiency.³ Therefore, mastering the electrode/electrolyte interface is of great significance for application of LiVPO4F. Surface coating and bulk doping are usually used to overcome these issues in some well-known electrode materials. Coating with highly disperse graphene is able to effectively improve the electronic conductivity of the materials.^{4, 5} Li₃PO₄ is a fast ionic conductor and is also used as inorganic coating material to enhance the ionic transmission of the electrode materials for LIBs.⁶ Also, Li₃PO₄ is expected to act as a barrier that restrains the side reaction at the electrode/electrolyte interface at high voltage.⁷ In this report, we construct the ionic/electronic 3D conductive framework to enhance the comprehensive performance of LiVPO₄F. As shown in the schematic diagram (Fig. 1), in this ideal architecture, the nano Li₃PO₄ are dispersed in the space of thin graphene layers, and the LiVPO₄F particles are well wrapped by graphene. We expect the Li₃PO₄ and graphene co-coated LiVPO4F to deliver impressive electrochemical performance, including superior cycle performance, good rate capability, and high initial coulombic efficiency.

Reference

1. J. Wang, X. Li, Z. Wang, H. Guo, Y. Li, Z. He and B. Huang, J. Alloy. Compd., 581,836 (2013).

2. J. Wang, X. Li, Z. Wang, H. Guo, Y. Zhang, X. Xiong and Z. He, Electrochim. Acta, 91,75 (2013).

3. X. Sun, Y. Xu, M. Jia, P. Ding, Y. Liu and K. Chen, J. Mater. Chem. A, 1,2501 (2013).

4. J. Wang, X. Li, Z. Wang, B. Huang, Z. Wang and H. Guo, J. Power Sources, 251,325 (2014).

5. H. Li and H. Zhou, Chem. Commun., 48,1201 (2012).

6. L.-X. Yuan, Z.-H. Wang, W.-X. Zhang, X.-L. Hu, J.-T. Chen, Y.-H. Huang and J. B. Goodenough, Energ. Environ. Sci., 4,269 (2011).

7. X. Li, R. Yang, B. Cheng, Q. Hao, H. Xu, J. Yang and Y. Qian, Mater. Lett., 66,168 (2012).

One of the most promising anode material for Li-ion battery is silicon since it can insert large amounts of lithium and deliver a very high theoretical capacity of 3580 mAh.g^-1 compared to commercial graphite anodes (372 mAh.g^-1). However, its drawback is to undergo huge volume variations during lithium insertion (around 300%), leading to morphological damage and loss of capacity upon cycling. The use of nanostructures such as silicon nanowires (SiNWs) has already proved to enhance the performances of such electrodes thanks to a better accommodation of volume changes and a direct electrical contact with the current collector^1,2. For further improvement of the cycle life, more complicated structures have been proposed, like coated nanowires³ or silicon nanotubes⁴ that both require a several steps synthesis. Here, however, the electrochemical investigation is made on bare silicon nanowires without any structure or surface optimization and the stability is improved by working on the cycling conditions. The electrolyte used in this study is LiPF₆ (1M) in a standard mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), with fluoroethylene carbonate (FEC) as an additive⁵. The capacity is limited to an intermediate value of 900 mAh.g^-1and a high current rate of 1C is used. The influence of the end of delithiation cut-off voltage is investigated and proves to be of great importance for the capacity retention of the electrode. An increasing number of cycles is obtained when lowering this upper cut-off voltage, from 660 cycles with 2 V, to more than 2300 cycles with 0.6 V. Moreover, for the more stable cycling, an excellent coulombic efficiency, above 99.5 %, is reached after only 20 cycles. It then keeps increasing to stabilize around 99.8 % after 500 cycles.

SEM images of the electrode were made in the delithiated state, at different stages of the cycling, to investigate the evolution of the electrode morphology and thickness. It appears that the reduced delithiation cut-off voltage leads to the formation of a more compact Solid Electrolyte Interphase (SEI) at the beginning of the cycling and less cracks are formed in the material at the end of the delithiation. A partial dissolution or reoxydation of the SEI layer probably occurs at higher potential, which is detrimental for the reversibility of the cycling and for the mechanical strength of the electrode. In the case of a reduced cut-off voltage, however, once the thick layer of SEI is formed, the morphology of the electrode remains rather stable. This SEI probably works as a mechanical support for the electrode and prevent further electrolyte degradation, which leads to the exceptional electrochemical stability.

References

1 C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zang, Y. Cui, Nat. Nanotechnol. 3, 31 (2008)

2 B. Laïk, D. Ung, A. Caillard, C.-S. Cojocaru, D. Pribat, J.-P. Pereira-Ramos, J. Solid State Electrochem. 14, 1835 (2010)

3 Y. Nguyen, M. Zamfir, L. Duong, Y. Lee, P. Bondavalli, D. Pribat, J. Mater. Chem. 22, 24618 (2012)

4 H. Wu, G. Chan, J. Choi, I. Ryu, Y. Yao, M. McDowell, S. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Nat. Nano. 7, 310 (2012)

5 V. Etacheri, O. Haik, Y. Goffer, G.A. Roberts, I.C. Stefan, R. Fasching, D. Aurbach, Langmuir 28, 965 (2012)

Lithium-ion batteries have rapidly become the dominant power sources for portable electronic devices and electric vehicles, due to their high energy density and long cycle life [1,2].However, safety issues still prevent full utilization of these batteries owing to the use of flammable liquid electrolytes, and safety problems have become a significant concern especially in large capacity applications such as electric vehicles and energy storage systems. In this respect, fabrication of all solid-state lithium batteries by using solid electrolytes may give a fundamental solution for the safety issue of lithium batteries [3,4]. Glass ceramic electrolytes present potential advantages, such as absence of electrolyte leakage, large electrochemical stability window, absence of problems relating to vaporization of organic solvents and high thermal stability. However, sheet manufacturing, especially using thin-film technologies for making large-scale batteries, is considered to be difficult because of hard and brittle ceramic materials. In this work, solvent-free hybrid solid electrolytes composed of lithium aluminum germanium phosphate and poly(ethylene oxide) (PEO) were prepared in the form of flexible film, and their electrochemical characteristics were investigated. Their morphological properties were examined using SEM, EDS and XRD. The ionic conductivities and electrochemical stability of hybrid solid electrolytes were much higher than those of PEO-based polymer electrolyte. The optimized hybrid solid electrolyte was applied to the solid-state Li/LiFePO₄ cell, and its electrochemical performance was evaluated.

References

1. J.M.Tarascon, M.Armand, Nature, 414, 359 (2001).

2. P.G.Bruce, B.Scrosati, J.M.Tarascon, Angew. Chem. Int. Ed, 47, 2930 (2008).

3. N.Kamaya, K.Homma, Y.Yamakawa, M.Hirayama, R.Kanno, M.Yonemura, T.Kamiyama, Y.Kato, S.Hama, K.Kawamoto, A.Mitsui, Nat. Mater, 10, 682 (2011).

4. Y.Li, J.B.Goodenough, Electrochem. Commun., 13, 1289 (2011).

Pure layered compound of overlithiated Li_1+xNi_0.8Co_0.2O₂ (x=1,1.05,1.1) samples were successfully prepared by a combustion method. Structural studies of the XRD results showed that when lithium was doped into the LiNi_0.8Co_0.2O₂ cathode materials, the structural parameters of the materials changed slightly when more lithium is doped, that is the cell parameter decreased. Scanning electron microscopy (SEM) revealed that the morphology of the particle c0.2hanged a little from rounded polyhedral-like particle to sharp edged polyhedral crystals when more lithium is doped. Energy dispersive X-ray spectroscopy (EDX) showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. X-ray photoelectron spectroscopy (XPS) studies showed that the binding energy of Li 1s is decreased for the doped Li_1.05Ni_0.8Co_0.2O₂ compound followed by Li_1.1Ni_0.8Co_0.2O₂ and LiNi_0.8Co_0.2O₂. This implies that the Li⁺ ions can be more easily extracted from Li_1.05Ni_0.8Co_0.2O₂ materials than the other stoichiometries. The XPS results are supported by the electrochemical performance of the samples. Li_1.05Ni_0.8Co_0.2O₂ showed the best results with a specific capacity of 113.29 mAh/g compared to Li_1.1Ni_0.8Co_0.2O₂ with a specific capacity of 112.32 mAh/g and LiNi_0.8Co_0.2O₂ with a specific capacity of 94.71 mAh/g. Li_1.05Ni_0.8Co_0.2O₂ also showed the best capacity retention of 7.8 % over 10 cycles.

Reference

[1] R.Santhanam, B.Rambabu, "Improved High Rate Cycling of Li-rich Li_1.1Ni_1/3Co_1/3Mn_1/3O₂

Cathode for Lithium Batteries" Int.J,Electrochem.Sci.,4 (2009) pp. 1770-1778

Silicon has proven to have a great potential as anode material in lithium-ion batteries due to its high theoretical electrochemical capacity. However, silicon anodes deteriorate quickly during cyclic charging and discharging, rendering them useless in only a few cycles [1]. This has been attributed to the stresses induced by the large volume change of the material during cycling. Numerous attempts have been made to reduce these stresses, e.g. by using nanoparticles, nanorods, nanowires, thin films and porous structures, with a varying degree of success [2, 3]. While using finely structured materials aids in the intercalation of lithium by reducing the necessary diffusion distance, it also has the unfortunate effect of greatly increasing the specific surface area of the silicon. When using nano-sized materials, silicon's ability to form a thin and stable solid electrolyte interphase (SEI) therefore becomes increasingly important. There are a number of factors that have a large influence on the SEI formation, making the process notoriously difficult to analyze, but also makes it possible to manipulate. Coating silicon with different nitrides and oxides, e.g. TiN and TiO, have previously been shown to enhance the cycling stability and Coulombic efficiency of the material [4, 5]. In this project the effect of coating the surface of silicon thin films with a thin (< 4-5 nm) layer of silicon nitride is investigated, as well as the effect of varying the stoichiometry of the silicon nitride. By itself silicon nitride has been shown to function as a conversion electrode material, forming elemental silicon and lithium nitride during the initial cycle, with thin films exhibiting capacities of up to 1800 mAh/g [6].

40 nm silicon thin films were deposited by PECVD on copper foil using silane as precursor. The nitride was formed by addition of ammonia to the gas flow in the late stages of the deposition, and different stoichiometries were obtained by changing the ratio of these gases. By varying the flow rate of ammonia, coatings with four different compositions were made; pure silicon (A), stoichiometric Si₃N₄(D) and two intermediate compositions (B and C). Three electrodes were punched from each of the resulting films and mounted in coin cells with lithium metal counter electrodes and cycled at a current rate of C/3 for 150 cycles. During cycling, all the cells exhibited an initial increase in capacity, peaking at close-to-theoretical capacity within 10-40 cycles before beginning a slow decline. The average charge capacity for each series after 50, 100 and 150 cycles is presented in the figure, showing that the capacity retention increases with increasing nitrogen content for series A, B and C, and then decreases for series D. This indicates that a nitrogen coating has a positive effect on the deterioration mechanisms of the electrode, and that this effect increases with increasing nitrogen content of the coating, as long as a stoichiometric nitride is not formed.

1. Kasavajjula, U., C. Wang, and A.J. Appleby, Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells. Journal of Power Sources, 2007. 163(2): p. 1003-1039.

2. Wu, H. and Y. Cui, Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today, 2012. 7(5): p. 414-429.

3. Ge, M., et al., Scalable preparation of porous silicon nanoparticles and their application for lithium-ion battery anodes.Nano Research, 2013: p. 1-8.

4. Memarzadeh Lotfabad, E., et al., ALD TiO2 coated silicon nanowires for lithium ion battery anodes with enhanced cycling stability and coulombic efficiency. Physical chemistry chemical physics : PCCP, 2013. 15(32): p. 13646-57.

5. Kohandehghan, A., et al., Silicon nanowire lithium-ion battery anodes with ALD deposited TiN coatings demonstrate a major improvement in cycling performance. Journal of Materials Chemistry A, 2013. 1(41): p. 12850-12861.

6. Suzuki, N., et al., Silicon nitride thin film electrode for lithium-ion batteries. Journal of Power Sources, 2013. 231(0): p. 186-189.

The development of anode material with high specific capacity is the key issue in present lithium ion batteries (LIBs) technology. Recently, transition metal oxides have attracted much attention as an anode material for LIBs due to its high theoretical capacity (~700 mA h/g), long cycle life and high rate performances. Among various metal oxides, cobalt oxide (Co₃O₄) has been intensively investigated as a promising anode material for LIBs owing to its high theoretical capacity (890 mAh g^-1), which is nearly two times higher than that delivered by conventional carbonaceous anodes. However, Co₃O₄ can suffer from poor cyclability and low rate capability, due to a large volume change and serious aggregation during charge/discharge cycling. Different proposals have been suggested to address this issue based on the tailored morphology, formation of nanocomposites with conductive additives and nano-size fabrication of Co₃O₄ electrode materials. In addition, various Co₃O₄ materials including nanotubes, nanorods, nanoneedles, nanospheres, platelets and nanowires have been synthesized and showed significantly improved lithium ion storage properties compared to their bulk counterparts. In this study, the electrochemical properties of flower-like Co₃O₄ synthesized by simple urea-assisted chemical co-precipitation strategy with porous structure were investigated.

A facile urea-assisted template free, surfactant less chemical co-precipitation method and subsequent calcination (400-600 ^oC) for 2 h was pursued to obtain pure cobalt oxide (Co₃O₄) sample. The synthesized sample exhibited the cubic spinel structure and flower-like morphology. The obtained morphology is assembled by nanorods rising from common center, and the nanorods were comprised of interconnected particles with porous structure. The changes in the surface area and porosity of the flower like structure have been induced by manipulating the calcination temperature, resulting in significant impact on the electrochemical properties. Electrochemical investigation showed that the flower-like Co₃O₄ exhibited excellent cycling stability (1452 mA h g^-1 at 0.5 C up to 300 cycles) and superior rate capability (reversible specific capacity of 880 mA h g^-1 at 10 C). The obtained flower-like Co₃O₄ materials showed excellent cycle life and high rate capability, making it promising for applications in high-power LIBs.

Germanium (Ge) is the potential candidate to replace graphite anode material for lithium ion batteries (LIB). Graphite has been used until now as a main anode material in LIB. However, its limited specific capacity cannot meet the world's growing demand for energy. The volumetric capacity of Ge (7366 Ah/l) is relatively smaller than that of Si (8344 Ahl^-1), but Ge has more advantage over Si: the diffusion coefficient of Li in Ge is approximately two orders of magnitude greater than that in Si, intrinsically electrical conductivity of Ge is four order higher than that of Si due to its smaller band gap, and the volume expansion during lithium insertion-deinsertion into Ge is smaller compared with Si. Despite these of promising characteristic, the capacity fading of Ge after some charge/discharge cycles is main problem to be addressed. To improve the cyclability of Ge most studies so far have focused on the use of buffer layer or reducing particle size of Ge. The use of buffer layer has negligible effect on specific capacity of Ge but it will improve significantly the performance of Ge at high rate of charge-discharge owing to improvement in mechanical properties, contact between active material with electrode, and diffusivity of Li.

In this study, we introduced a simple method for synthesis of Ge nano particles interconnected by carbon (GEC). The GEC electrode showed a high specific capacity up to 1339 mAh/g at the rate of C/2, after 50 cycles the specific capacity retained at value of 1321 mA h/g. Even with increasing the charge-discharge rate up to 10C, the specific capacity only decreased about 30 % compared with that at the rate of C/10. The excellent electrochemical performance of GEC electrode was not only due to the buffer layer of carbon but also the interface layer of Ge-carbon. The formation of Ge-C bonding at interface layer is believed to improve the contact between Ge and C, increase the stability during charge-discharge process and reduce stress tension during volume expansion.

The cyclic life of GEC electrode is under-investigation and it can be observed from its initial data that the electrode shows steady performance till the obtained data. However, further cycling data is necessary for making any stamen about the stability in its performance. The initial results demonstrate that the synthesized GEC can be used as an active Li storage material for lithium ion batteries.

Carboneous materials have attracted great interest and are used as negative electrodes for lithium ion batteries due to more advantageous than lithium metal negative electrodes in terms of cycle performance by easy mobility of Li ion and safety. Recently, there has been a considerable demand for the development of long-life lithium secondary batteries for energy storage systems. In order to increase energy density lithium alloys with Sn and Si are mainly studied as negative electrode, which exhibit three times of that of the graphite. However, these metal negative electrodes suffer significant mechanical disintegration due to the drastic volumetric changes during lithium insertion and extraction. Carbide-derived carbons (CDCs) possess tunable pore structures and narrow pore size distributions in the 0.5-2 nm range that can be formed through selective etching of crystalline metal carbides. CDC can be modified by activation and high temperature vacuum process before and after CDC, which affords high specific areas, large pore volumes, and diverse structural changes. In this study, we introduce modified carbide-derived carbon (CDC) with tunable pore size, uniform pore structure and structural changes, as electrodes for energy storage devices.

Two-dimensional distribution of potential and current density in planar electrodes of pouch-type lithium-ion batteries are investigated numerically and analytically. A concentration-independent polarization expression, obtained experimentally, is used to mimic the electrochemical performance of the battery. By numerically solving the charge balance equation on each electrode in conjugation with the polarization expression, the battery behavior during constant-current discharge processes is simulated. The numerical analysis shows that reaction current density between the electrodes remains approximately uniform during most of the discharge process, i.e., when depth-of-discharge varies from 5% to 85%. This observation suggested simplifying the electrochemical performance of the battery such that the charge balance equation on each electrode can be solved analytically to obtain closed-form solutions for potential and current density distributions. The analytical model shows fair agreement with experimental and numerical data at modest computational cost. The model is applicable for both charge and discharge processes, and its application is demonstrated for 20 Ah and 75 Ah prismatic lithium-ion batteries. The analytical model is used to describe electrical conduction in the electrodes, and to investigate the effects of tab design on voltage drop. It is demonstrated that constriction/spreading resistance in current collectors of the considered battery is fairly small; about 10% of the total cell resistance but it is larger than the contribution of bulk resistance which is about 3%. The model confirms that constriction/spreading increase with: decrease in the aspect ratio of the current collector, decrease in the tab width, and increase in the tab eccentricity.

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the energy and power of common Li-ion batteries.^1,2 Many prospective applications of batteries require improving both energy density and rate capability. The simultaneous improvement of these two parameters is not sufficiently addressed by state-of-the-art approaches. High capacity lithium intercalation materials often exhibit poor ionic and electronic conductivity which limit rate capability. Battery electrodes based on polymers bearing organic free radicals allow for exceptionally high rates but fall short in meeting energy density requirements.^3,4

Here, we combine the advantages of both storage principles, to form battery electrodes capable of delivering the capacity of the high-energy intercalation material and intermittently the high rate capability of the radical material. First, we must understand the processes that occur in the cell on discharge and charge to fully take advantage of the potential of the concept.

We will discuss recent results on the detailed elucidation of the charge transfer mechanism in the so formed dual radical/intercalation electrodes, materials aspects, thermodynamic and kinetic considerations. We combine synthetic, spectroscopic and advanced electroanalytical methods to gain insight into the governing processes.

1. B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci., 4, 3287 (2011).

2. M.M. Thackeray, C. Wolverton, E.D. Isaacs, Energy Environ. Sci., 5, 7854 (2012).

3. K. Oyaizu, H. Nishide, Adv. Mat., 21, 2339 (2009).

4. T. Janoschka, M.D. Hager, U.S. Schubert, Adv. Mat., 24, 6397 (2012).

Despite the highest theoretical capacity of 3579 mAh/g in lithium–ion batteries,^[1,2] silicon (Si) anodes have been still suffered from severe capacity fading, because of two kinds of drawbacks such as (i) dramatic volume expansion called pulverization leading to electrical disconnection as well as (ii) formation of solid–electrolyte interface on their surfaces.

Herein, we prepared an organic/inorganic hybrid anode of polyvinyl alcohol (PVA)/Si nanoparticles (NPs)/graphene nanoribbons (GNRs) nanofibers via electrospinning process. Since PVA polymer could be dissolved in water, our hybrid nanofibers have been made via water‒based electrospinning, which means that toxic solvents like N–Methyl–2–pyrrolidone (NMP) do not need to be used for preparing the electrodes. Furthermore, the PVA polymer as a good dispersant can effectively help to disperse high‒concentrated GNRs (20 mg/ml) under water, which are unzipped from multiwall carbon nanotubes for better dispersion ability. The battery cells using the hybrid PVA/Si/GNRs nanofibers exhibited an initial high discharge capacity of over 5000 mAh/g at 0.1C because of the contribution of well–dispersed GNRs and Si NPs, while only Si NPs have the first discharge capacity of ca. 2500 mAh/g. At even 1C (3.6 A/g), they showed a high reversible capacity of around 1800 mAh/g owing to highly–conductive GNRs within the hybrid nanofibers. Moreover, the nanofibers retained very stable cycle retention of over 90% during 200 cycles. Such excellent cycle retention should be attributed by cross–linked and stabilized PVA nanofibers covering Si NPs, available to not only prohibit the volume expansion but also alleviate the formation of SEI layers. In terms of electrochemical properties, the hybrid nanofibers represented much less charge transport resistance from electrochemical impedance spectroscopy and higher peak current density from cyclic voltammetry than only Si NPs, because the GNRs as electrical conductors in the electrodes must be well–distributed between Si NPs. As a result, we demonstrated an outstanding battery performance through a novel hybrid PVA/Si/GNRs nanofibers directly fabricated via water–based spinning.

References

[1] H. Wu, Y. Cui, Nano Today2012,7, 414.

[2] Wu H.; Chan G.; Choi J. W.; Ryu I.; Yao Y.; McDowell M. T.; Lee S. W.; Jackson A.; Yang Y.; Hu L.; Cui Y. Nature Nanotech.2012, 7, 310.

This talk will present the formation of highly aligned nanotubular TiS_x electrodes consisting of self-organized TiO₂ nanotubes formed by thermal heat treatment in H₂S gas. Depending on the annealing time and temperature several mixtures of titanium oxy-sulfides can be formed. Even a complete transformation into TiS₂ is possible under the right conditions. The formed TiS_x layers show a slightly modified tubular structure compared to the original TiO₂ nanotubes, but still offer a high surface area in form of tubular structure. The formed TiS_x were characterized by SEM, XRD, XPS and tested regarding their battery performance as a cathode material for lithium ion batteries.

References concering high temperature treatments in reactive gas envoirment:

Y.-C. Nah, I. Paramasivam, R. Hahn, N. K Shrestha and P. Schmuki, Nanotechnology 21 (2010) 105704

R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y. Song, J. Kunze, V.-P. Lehto and Patrik Schmuki, Angew. Chem. Int. Ed. 48 (2009) 7236 –7239

Introduction