Table of contents

Lithium-ion batteries (LIB) are today the main energy storage devices from portable electronics devices to electric vehicles. LIB have a high energy density, low self-discharge, and low memory effect allowing for large number of charging cycle without degrading storage capacity. Performance, cost and safety are the main factors driving research and different chemistry are investigated in order to achieve better performance at a lower cost and increase safety.

Research, development and quality control of LIB can benefit much from X-ray Computed Tomography (CT) and Focused-Ion Beam – Scanning Electron Microscopy (FIB-SEM).

MicroCT allows for complete non-destructive acquisition of a complete battery cell, allowing to control the production process and to monitor the changes occurring in the structure during multiple charging cycles. Different visualization techniques are proposed on a 7µm microCT acquisition of a Lithium Nickel Manganese Cobalt Oxide cell 18650 (NMC) which allow for a visual inspection of the internal structure of the cell, including techniques of unfolding to look at rolled electrodes and measurement such as electrode thickness, height, or length.

Macroscopic properties of a battery, including mechanical, thermal and electrical, find their origins at a nano-scale. Using FIB-SEM, high resolution images can be acquired which can reveal the structure of the electrodes or the separator. Besides the volume fraction or surface areas of the different phases, the connectivity of pores and/or particles, their surface of contacts, path tortuosity or constrictivity, permeability or molecular diffusivity, become accessible. We will present simulation results on permeability and tortuosity on this sample.

We will also present correlative experiment, performing first a microCT scan of a cathode foil sample on its aluminium collector, spotting sites of interest in it, and acquiring those volumes with FIB-SEM.

Solid-state lithium-ion batteries (SSLIBs) can simultaneously improve both energy density and safety over current state-of-the-art lithium-ion batteries (LIBs). These critical advantages are expected to enable next generation battery-based energy storage for both commercial applications such as consumer devices and transportation and greater utilization of renewable energy technologies. The key component to SSLIBs is the solid-electrolyte, ideally allowing fast ion conduction, possessing electrochemical stability with metallic lithium to enable higher energy density, and exhibiting chemical stability under ambient conditions. Lithium conducting garnets, in particular doped lithium lanthanum zirconate (LLZO) possess all of these advantageous characteristics. While lithium garnets have demonstrated exceptional properties under laboratory conditions, only a few groups have demonstrated a robust, scalable process to integrate them into SSLIBs. Some of the main challenges to incorporating garnets such as LLZO are the relatively high processing temperatures required and brittle nature of devices based on these sintered polycrystalline ceramics. Processing schemes amenable to roll-to-roll type processing such as tape-casting have been demonstrated, but generally require very fine powders to produce thin electrolyte membranes and subsequently low cell resistance. However, lithium garnets are most commonly synthesized via solid-state reactions (SSR), generally requiring high temperatures (> 900 °C), long reaction times (often in excess of 8 h), and concomitant high energy cost. Further, the resultant particle sizes of LLZO synthesized via SSR are relatively large, and fine powders are only obtained after further processing via high energy milling. Molten salt synthesis (MSS) has recently been demonstrated as an alternative strategy to obtain phase-pure crystalline LLZO, with the added benefit of generally shorter reaction times (4 h or less), lower temperatures, and in some cases inherently submicron particle sizes, while maintaining expected high ionic conductivity. Design principles for formation of doped and co-doped LLZO are discussed, wherein salt and reagent composition have direct effects on the formation temperature, particle size, particle size distribution, and electrochemical performance of the as-synthesized material. By engineering inherently fine LLZO powders, a more direct route to practical processing methods such as tape-casting becomes accessible. As such, MSS may prove to be a crucial method to allow LLZO and SSLIBs based on it to transition to a scalable solution for safer, more efficient battery-based energy storage.

Li-S technology stands out as a potential alternative for Li-ion batteries due to the improved energy density, particularly in applications where the specific energy density is important [1,2]. OXIS Energy is currently developing Li-S batteries that outperform other battery technologies available in the market nowadays. Among the different challenges to obtain a high-performance Li-S battery, the issues related to metallic Li electrode are a limiting factor [3]. Among others, the continuous degradation reactions on Li surface and dendrite formation result in poor cycle life and arise safety hazards. Different approaches have been proposed to overcome these effects, such as the use of additives on the electrolyte, the development of structured composite Li anode or the formation of an artificial solid electrolyte interphase (SEI) [3,4]. Direct deposition of protective coatings on the surface of Li electrodes by means of Magnetron Sputtering is an easy and potentially scalable method to successfully produce thin and homogeneous layers with upgraded properties as an artificial SEI [5]. In this work current research activities in sputter deposited coatings being developed at OXIS Energy will be described, focusing both on the deposition process of coating materials and the electrochemical response of coated electrodes.

[1] Manthiram, Arumugam, et al. "Rechargeable lithium–sulfur batteries." Chemical reviews 114.23 (2014): 11751-11787.

[2] Fotouhi, Abbas, et al. "Lithium-Sulfur Battery Technology Readiness and Applications—A Review." Energies 10.12 (2017): 1937.

[3] Cheng, Xin-Bing, Jia-Qi Huang, and Qiang Zhang. "Li metal anode in working lithium-sulfur batteries." Journal of The Electrochemical Society 165.1 (2018): A6058-A6072.

[4] Cheng, Xin‐Bing, et al. "A review of solid electrolyte interphases on lithium metal anode." Advanced Science 3.3 (2016): 1500213.

[5] Li, Juchuan, et al. "An artificial solid electrolyte interphase enables the use of a LiNi0. 5 Mn1. 5 O4 5 V cathode with conventional electrolytes." Advanced Energy Materials 3.10 (2013): 1275-1278.

SnS₂/PB anode material was synthesized via two steps. First, three-dimensional Prussian Blue (PB) nanostructures were derived from a simple hydrothermal synthesis. After etching with hydrochloric acid (HCl), the PB nanocubes were used as a template for the next step. Two-dimensional SnS₂ nanosheets were grown on the PB nanocubes through a facile hydrothermal synthesis, giving rise SnS₂/PB hybrid nanoarchitecture. The as prepared SnS₂/PB is further employed as the anode of sodium ion batteries (SIBs). The SnS₂/PB nanoarchitecture exhibits high capacity, long cycle life and exceptional rate capabilities as an anode of SIBs. In specific, it delivers a capacity of 725.7 mA h g^-1 at 50 mA g^-1. When cycled through 200 cycles, it achieved a stable cycling capacity of 400 mAh g^-1 at 200mA g^-1 This enhanced SIB performance further proves that SnS₂ has the capabilities and potential as an anode material for SIBs. The stable Na⁺ storage properties of SnS₂/PB was attributed to the synergistic effect among the conductive PB carbon, used as the template in this work. These results obtained potentially paves the way for the development of excellent electrochemical performance with stable performance of SIBs.

Keywords: Prussian Blue, Carbon Nanocubes, Tin disulfide, Sodium ion batteries

Figure 1

Lithium ion secondary batteries (LIBs) have been widely used as energy-storage system for variety of power devices. It is necessary to further develop LIBs toward high-functional devices, such as electric vehicles and mobile electronics. Nowadays, all solid-state LIBs have been much interested because of a variety of potential advantage, including energy densities, cost, size, safety, and operating temperature. However, there are serious problems to be solved toward practical uses. For example, diffusion of lithium ions at interface between different solid materials, including active materials and electrolytes, is still poor to operate charge/discharge in batteries. Our group has studied high-quality crystals for applications as energy and environmental materials by using a flux method. Flux method is a nature-mimetic liquid-phase crystal growth technique. Fluxes accord to salts, which work as solvent at temperature over their melting and / or eutectic points. By using fluxes, it is possible to construct specific crystal-growth field at any temperature with facile setup, and give designed crystals regarding to shape, including crystal faces, which has never achieved using other methods like solid state reaction.

Recently, we have applied the flux method to battery materials to create "all-crystal (solid)-state LIBs". We have expected that flux crystal growth gave (I) crystal-shape control of active materials, (II) construction of good interfaces in electrodes among cathodes, solid electrolytes, and anodes. The second topics would be possible if electrode materials can be dissolved and densely recrystallized on their surfaces. As a result, smooth ionic transportation through bulks and their interfaces would be realized in all-crystal (solid)-state LIBs. Our concept using flux method would provide new aspect to lead an innovation in all solid state LIBs as next-generation energy storage. In the presentation, the details of interfacial and crystal designs of a variety of battery materials will be introduced, coupled with their crystallographic, ionic, and battery properties.

Acknowledgement

This work was supported by JST CREST Grant Number JPMJCR1322 in Japan and Program for Building Regional Innovation Ecosystems of MEXT.

The fast growing use of natural energy sources such as solar and wind to generate electricity, requires the development of reliable, affordable, and efficient electric energy storage systems [1]. Thus, electrochemical capacitor systems can offer excellent devices showing high power (>10 kW kg^-1), high rate capability and long cycle life (>1 000 000 cycles) [2]. Many kind of materials have been widely investigated as supercapacitor electrodes, such as carbon materials, graphene-based composites of metal hydroxides and metal oxides. In this work, new pseudocapacitive materials were obtained by thermal descomposition of cyanometalates (PBAs) obtained by precipitation over different carbon structures (graphene oxide, carbon nanotubes and mesoporous carbon). The materials were characterized using XRD, Raman, ATR-FTIR, SEM, TEM and XPS. Furthermore, the electrochemical performance of the materials was tested by cyclic voltammetry, step potential electrochemical spectroscopy, electrochemical impedance spectroscopy and charge-discharge curves at different current rates in 1 M KOH. These techniques were applied in order to understand the double layer and pseudocapacitive mechanism of K ion storage. The material formed in presence of graphene oxide exhibited the best electrochemical behavior, maintaining almost constant its initial capacitance after 500 cycles at 1 A g^-1.

References

1. Yang, J. Zhang, M. C.W. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon, J. Liu, Electrochemical Energy Storage for Green Grid. Chem. Rev., 2011, 111, 3577-3613.

2. S. Aric, P. Bruce, B. Scrosati, J.-M. Tarascon, W. V. Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater., 2005, 4, 366 – 377.

Aqueous alkali metal-ion batteries have attracted much attention, because they have three major advantages, namely non-flammability, high ionic conductivity and low cost. In addition, water can dissolve various salts in large amounts to provide a variety of electrolytic solutions. On the other hand, the 1.2 V theoretical electrochemical window of water restricts the selection of the cathode and anode active materials. However, three years ago, this issue was first addressed by U.S. Army Research Laboratory [1]. They successfully expanded the cell voltage of the aqueous Li-ion battery more than 2 V by introducing highly concentrated electrolytes, which effectively suppressed undesired electrolysis of water. To confirm the electrochemical window expansion of the concentrated electrolyte even in Na system, we also tried to realize a high voltage aqueous Na-ion battery by concentrated aqueous electrolyte. In preliminary measurement, 1.4 V average discharge voltage was obtained in the combination of Prussian blue-type Na₂Mn[Fe(CN)₆] (NMHCF) cathode and NASICON-type NaTi₂(PO₄)₃ (NTP) anode with concentrated (17 mol/kg) NaClO₄ aqueous electrolyte [2]. However, according to a CV measurement, the electrochemical window was almost 2.8 V [2]. Therefore, in order to further raise the cell voltage, instead of NTP anode based on Ti⁴⁺/Ti³⁺ redox, Prussian blue-type KMn[Cr(CN)₆] (potassium manganese hexacyanochromate; KMHCC) anode based on Cr³⁺/Cr²⁺ redox was adopted. Then, 2 V class aqueous Na-ion battery was successfully obtained [3].

As this analogy, we have newly found the rechargeable operation of the aqueous K-ion battery in the combination of Prussian blue-type K₂Mn[Fe(CN)₆] (potassium manganese hexacyanoferrate; KMHCF) cathode and KMHCC anode with highly concentrated potassium triflate (19 mol/kg KSO₃CF₃; KOTf) aqueous electrolyte. The average cell voltage was approximately 1.5 V and the specific capacity was 118 mAh/g-KMHCF and 60 mAh/g-KMHCC, respectively. However, the two similar full cells suffered from capacity fading caused by degradation of Prussian blue-type structure induced by side reaction such as electrolysis of water solvent or electrolyte salt decomposition. The mechanism of degradation and our trial to improve the cyclability by changing the condition such as charging rate, cell configuration, electrolyte or its additives will be discussed in presentation.

References

[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, and K. Xu, Science, 350 (2015) 938-943.

[2] K. Nakamoto, R. Sakamoto, M. Ito, A. Kitajou, and S. Okada, Electrochemistry, 85(4) (2017) 179-185.

[3] K. Nakamoto, R. Sakamoto, Y. Sawada, M. Ito, and S. Okada, Small Methods, (2018) 1800220.

Lithium Sulfur battery is supposed to be one of the most promising candidates in next generation rechargeable batteries. Owing to the high theoretical specific capacity (~1675 mAh/g) and abundance of sulfur, lithium-sulfur battery can achieve high energy density with relatively low cost. However, the practical energy density is much smaller than the theoretical value. The operation of Li-S batteries under lean electrolyte condition was revealed by investigating Li-S pouch cell under different electrolyte/sulfur ratio. The cathode interfacial resistance was figured out in testing Electrochemical Impedance Spectroscopy (EIS) during discharging for different E/S ratio. The electrolyte resistance, interfacial resistance as well as the charge transfer resistance were keeping changing in same trend for different E/S ratio. The changing was observed in both DOL/DME and DMSO electrolytes for all E/S ratios we tested. Such resistance behaviors were also confirmed by enforced Galvanostatic Intermittent Titration Technique (GITT). By applying enforced GITT onto the lean electrolyte cell, it can even get as high specific capacity as flood electrolyte cells did. We propose that the cathode interfacial resistance can be one of the reasons which is challenging the Li-S battery to achieve its theoretical specific capacity under lean electrolyte condition. Such mechanistic understanding of the Li-S battery under lean electrolyte condition can provide guidance in developing high energy density Li-S battery.

The current state-of-the-art batteries comprising graphite-based anodes are facing its performance limit for the use in automotive applications, thus urging to develop electroactive materials with high capacity and low operating potentials [1]. Si has shown great promise as a next-generation anode, meeting the above requirements in natural abundance. However, unavoidable large volume change upon lithium insertion/extraction causes particle fracture and disintegration of the electrode, resulting in fast degradation of battery along with poor electric conductivity being the main hurdle for fast-chargeable battery design [2]. Recently, interesting breakthroughs have been proposed principally in nanostructuring of Si, composite formation with conductive carbons and mechanically stable binders, which achieved long-term cycle stability and rate capability [3]. Yet, these approaches lead to lowering the electrode density, initial Coulombic efficiency and energy density of battery and thus utilizing the Si microparticles can be rather feasible toward a practical system while stress-driven particle degradation becomes more serious. The existing technologies have focused on coalescing the fractured particles, not preventing itself, and have not considered ionic diffusion of Li-ion across the microparticles as well as the electronic conduction.

Herein, we propose the material design concept of Si microparticles by integrating the chalcogen component into the bulk structure through a low temperature doping strategy. The low electric conductivity of intrinsic Si (~10^-4 S m^-1) with a near-insulator property undergoes a transition into the metallic state via this new method unlike other dopants of boron or phosphorous that only gives extra charge carriers without insulator-to-metal transition. In addition to electronic conduction, the chalcogen chains restrict the saturation of Si dangling bonding during recrystallization process of low temperature doping and create the internal channels inside the crystalline Si lattice. The incorporation of Li-ion channels further increases Li-ion diffusivity without any barriers in case of undoped Si microparticles due to the interfaces of lithium silicide and amorphous Si. Interestingly, chalcogen chains can sustain its internal structure with high flexibility and robustness which is directly corroborated by microscopy analysis, and lithium-induced intermediate can maintain the metallic nature at the interfaces that enhance the Li-ion diffusion over the cycles. Further, the porous but minimized surface area of Si structures increases the initial reversibility, extends the cycle life of battery up to hundreds of cycles with a high structural stability and facilitates fast-charging ability.

[1] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nat. Mater. 2005, 4, 366.

[2] H. Wu, Y. Cui, Nano Today 2012, 7, 414.

[3] J. Ryu, T. Chen, T. Bok, G. Song, J. Ma, C. Hwang, L. Luo, H.-K. Song, J. Cho, C. Wang, S. Zhang, S. Park, Nat. Commun. 2018, 9, 2924

Prevailing demand for lithium-ion batteries with higher energy density draws more researcher's attentions on the use of lithium metal as the anode material. An order of magnitude higher specific capacity and much lower reduction potential of lithium metal compared to graphite makes lithium metal suitable anode for the next-generation lithium-sulfur and lithium-air batteries. However, nonuniform plating or dendrite formation of lithium metal is a major issue for shorting batteries. Stable interface between lithium metal and electrolyte is the key to the uniform growth of lithium. Vinylene carbonate (VC) is a widely used additive in commercial electrolyte for lithium-ion batteries, and the effect of VC on stabilizing lithium metal has been reported. VC is proposed to polymerize on lithium surface to form flexible thin coating layer, which regulates the lithium-ion flux at the interface and put a mechanical pressure on the growing lithium dendrite. The interface layer is composed of multiple decomposition products from organic solvents and fluorinated salt and extremely complex to analyze.

Here we propose a gel electrolyte composed of vinylene carbonate and lithium iodide (LiI) to form polymer coating layer on the lithium surface. FTIR, NMR, and GC-FID analysis confirm polymerization of vinylene carbonate is triggered by lithium iodide through decarboxylation and forms poly(vinylene catbonate). Because iodide is electrochemically irreducible anion, no decomposition occurs on the lithium surface and simplifies the interface composition. Without the formation of inorganic compounds in VC-LiI gel electrolyte, Li//Li symmetric cell cycles for 1000 hours at 1 mA cm^-2 and 1 mAh cm^-2, with a practical electrolyte thickness of 25 μm (separator free). Coulombic efficiency of lithium plating/stripping in VC-LiI gel electrolyte is 98.6%, which is significantly higher than conventional carbonate-based electrolyte. Cross-sectional SEM image of cycled lithium metal reveals dense polymer coating and dendrite free morphology. Therefore, the simple all-organic interface, differing from conventional heterogeneous inorganic/organic mixture, can stabilize the lithium surface. We present a new perspective on the design of electrolyte and lithium interface.

LiCoO₂ is one of the most widely commercialized cathode materials for Li-ion batteries. Recently, LiCoO₂ has been revisited to improve its reversible capacity by increasing the upper cut-off voltage to 4.55 V (vs. Li/Li⁺). However, LiCoO₂ suffers from determental Co-dissolution and electrolyte decomposition at high voltages and high temperatures, resulting in poor electrochemical performance.[1,2] In this connection, various coated LiCoO₂ powders have been investigated to reduce these irreversible surface reactions.[3]

Coated LiCoO₂ powders are usually obtained via sol-gel synthesis, dry coating, and atomic layer deposition. These coating techniques are pursuing to uniform coating covering the whole surface of LiCoO₂ powders. However, the full coverage of coating layers interrupts the charge-transfer of Li⁺ ions, resulting in the poor rate capability of LiCoO₂. This implies that we can improve the rate performance of LiCoO₂, if we are able to coat specific facets of LiCoO₂ powders except for the charge-transfer planes.

In this presentation, we introduce, for the first time, a new plane-selective coating of LiCoO₂. For example, we demonstrate that metal oxides coating layers are grown plane-selectively on the LiCoO₂(001) surface. We show synthesis details for the plane-selective coating of LiCoO₂. Scanning transmission electron microscopy (STEM) analysis was carried out with a high-angle annual dark field (HAADF) mode to observe the plane-selective coatings. The electrochemical performance of the plane-selectively coated LiCoO₂ was better than that of the conventionally coated LiCoO₂. Moreover, we discuss the origin of the plane-selective coating of LiCoO₂.

References

[1] B. Xu, D. Qian, Z. Wang, Y. S. Meng, Materials Science and Engineering: R: Reports 2012, 73, 51

[2] W. Li, A. Dolocan, P. Oh, H. Celio, S. Park, J. Cho, A. Manthiram, Nat Commun 2017, 8, 14589

[3] Y.-K. Sun, C. S. Yoon, S.-T. Myung, I. Belharouak, K. Amine, Journal of The Electrochemical Society 2009, 156, A1005

The electrochemical stability of an electrolyte solution, its so-called "potential window", is simply determined by the oxidative potential and reductive potential of the solvent, if solutes dissolved in the electrolyte solution are electrochemically stable within the potential window of the solvent. Water is the most conventional solvent in the field of electrochemistry, and its potential window is as narrow as 1.23 V, thermodynamically. Practically, however, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) need certain overpotentials, which extends the potential windows of aqueous solutions. Besides the catalytic activities of the electrode materials, the properties of aqueous solutions such as hydration structures and salt concentrations also affect the potential window.Recently, aqueous rechargeable lithium batteries (ARLBs) have attracted much attention due to their low cost and safety. However, the working voltage of ARLBs is limited to below 1.5 V due to the narrow potential window of water. Recently, a few papers have discussed the potential window of concentrated aqueous solutions with neutral pH. However, there are still fundamental unanswered questions regarding the pH-dependence of the potential window of water. In this study, the potential window of concentrated electrolyte solutions with neutral pH and the expansion of the potential window were investigated from the viewpoint of the local pH change and water concentration. Here, we shed light on the dependence of the potential window of water on local pH changes and water concentrations.

Next, we focused on the water concentration, since the potential windows are determined by the water electrolysis reactions (OER and HER). The potential windows are shown in Fig. 1 as a function of the water concentrations. In this study, the onset potential of the OER/HER was defined when the current density was ± 0.1 mA cm^-2 in CVs at 1 mV s^-1. There are two clear tendencies in Fig. 1. First, the windows in neutral pH electrolyte solutions (closed symbols in Fig. 1) were obviously wider than those in acidic/alkaline electrolyte solutions (open symbols in Fig. 1), even at a dilute salt concentration (ca. 55 M water concentration). Second, the windows in neutral pH electrolyte solution (closed symbols in Fig. 1) were not affected by the kind of the electrolyte salt, but depended linearly on the water concentration. The windows were expanded when the water concentration decreased, i.e., the electrolyte salt concentrations increased.

Potential windows of aqueous solutions were investigated systematically using various salts at different concentrations. Cyclic voltammetry measurements of Pt electrodes revealed two important points. First, the potential window in unbuffered neutral pH solution was broader than that in acidic/alkaline solutions. This expansion of potential windows can be explained by the shift in the reaction potential with local pH changes in the vicinity of the electrode. Second, the potential windows were not affected by electrolyte salts, but rather depended linearly on the water concentration. The difference for OER overpotentials was much larger than that for HER overpotentials. While HER overpotentials were derived from a local pH change, OER overpotentials were derived from both a reduced water concentration and local pH change. This study highlights the importance of these two main factors (water concentration and local pH change) in determining the potential windows of concentrated electrolyte solutions.

Figure 1

Biogenic oxidation of water-soluble metal ions into insoluble oxides has been taking place for millions of years leaving its signature all around us. Nature's evident success in producing nanophase metal oxides is inspiring scientists to understand and learn from these processes.

The creation of nanostructured electrode materials for energy storage may represent one of the most attractive strategies and a path forward to dramatically improving the performance of both Li-ion and beyond Li-ion systems. Short diffusion length associated with the nanoscale dimensions effectively reduces the distance that ions and electrons must travel during cycling relative to the equivalent bulk material. Enhanced kinetics significantly improves capacity and rate capability, suppression of phase transformations improves electrochemical reversibility, and the use of defects and high surface area produces high capacity for intercalation. Furthermore, nanomaterials can offer a possible solution to excellent electronic and/or ionic conductivity required for unhindered charge flow with their ability to connect materials and build up structures from the molecular level.

Electrochemical synthesis is a facile way for preparation of nanoscale architectures. Utilization of electrochemical deposition brings a high level of control to the structure, morphology, and uniformity of electrodes by adjusting the crucial parameters such as applied current, potential, electric pulses, as well as the temperature and concentration of the electrolyte. Most of the known conventional bulk battery materials may start out as highly crystalline materials but upon many repeated cycles can pulverize overtime leading to non-electrochemically active, and electrically disconnected particles. Our previous work has shown that by starting from amorphous, low-crystalline materials by electrochemical cycling we can create nanostructured electrodes with self-optimized crystalline phases (cycling of TiO₂ amorphous nanotubes in Li batteries converted them into cubic titania¹). Self-organization of materials during electrochemical cycling could be an easy general approach for synthesis of new, optimized crystalline forms of materials for energy applications.

Manganese oxide-based materials are an ideal platform for understanding the underlying properties that determine capacity for a broad spectrum of energy storage chemistries due to the availability of multiple valence states for charge storage and the relative ease and low cost with which they can be synthesized. MnO₂ exists in various crystallographic polymorphs, namely α-, ß-, γ, δ-, λ-, and ε-type. These highly crystalline, structurally disordered or amorphous phases show unique electrochemical activity as a result of complex factors such as: structural intergrowths, lattice defects, cation vacancies, random octahedral-unit (MnO₆) distribution, proton diffusion, conductivity within the oxide particles.

In order to accelerate the implementation of nanostructured systems in beyond Li-ion batteries functional links must be developed between nanoscale materials structure and the resulting performance characteristics. My recent results showed that it is possible to extend reversible ion-insertion chemistry of MnO₂ from monovalent Li to ions with higher charge: Zn ²⁺. Starting from low-crystalline layered manganese oxide upon electrochemical cycling new phase was formed at the nanoscale². One of the most important prerequisites to understanding this process is the knowledge of the atomic-scale structure of its product. Furthermore, combined with a metal anode, understanding the underlying intercalation mechanism of these systems in a multivalent metal cell and the underlying principles behind reversible intercalation of multivalent ions into a cathode (e.g., intercalation vs conversion and possible structural transformation compared to an aqueous system) is a valuable asset for multivalent cell design.

Developing new synthetic electrochemical techniques to better control the nature and structure of nanostructured oxide materials and modify environment of layered oxide materials to tune ability to host various ions will ultimately enable the design of optimal structures that can operate with multiple transporting ions with higher capacity and dramatically improved cycle life.

Acknowledgement This work was supported by the U. S. Department of Energy, US DOE-BES, under Contract No. DE-AC02-06CH11357.

References

1. Self-Improving Anode for Lithium-Ion Batteries Based on Amorphous to Cubic Phase Transition in TiO2 Nanotubes Hui Xiong, Handan Yildirim, Elena V. Shevchenko, Vitali B. Prakapenka, Bonil Koo, Michael D. Slater, Mahalingam Balasubramanian, Subramanian K. R. S. Sankaranarayanan, Jeffrey P. Greeley, Sanja Tepavcevic, Nada M. Dimitrijevic, Paul Podsiadlo, Christopher S. Johnson, and Tijana Rajh The Journal of Physical Chemistry C 2012 116 (4), 3181-3187

2. Mechanism of Zn Insertion into Nanostructured δ-MnO2: A Nonaqueous Rechargeable Zn Metal Battery Sang-Don Han, Soojeong Kim, Dongguo Li, Valeri Petkov, Hyun Deog Yoo, Patrick J. Phillips, Hao Wang, Jae Jin Kim, Karren L. More, Baris Key, Robert F. Klie, Jordi Cabana, Vojislav R. Stamenkovic, Timothy T. Fister, Nenad M. Markovic, Anthony K. Burrell, Sanja Tepavcevic, and John T. Vaughey Chemistry of Materials 2017 29 (11), 4874-4884

The depletion of fossil fuel and strict environmental regulation on carbon footprints has prompted a greater reliability on sustainable energy sources. Batteries represent an important energy storage option due to their wide range of applicability and portability, however, for certain applications such as transportation, a higher energy density battery is required. Beyond the conventional Li-ion battery system, current research is focusing on next-generation cost effective devices. Amongst them, the Li-S battery technology is very promising due to its high theoretical capacity and energy density [1]. However, the chemistry involved in the Li-S battery is a complex process with multiple consecutive electrochemical reactions. Despite having many advantages, its application is still limited by several issues, e.g., the dissolution and diffusion of intermediate polysulphides, unstable plating and stripping of Li metal, both resulting in rapid capacity fading [2]. Previously, we reported the application of a novel electrolyte system composed of N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, C₃mpyrFSI, ionic liquid (IL) and 1,2 dimethoxyethane (DME) in the presence of varying saturated lithium bis(fluorosulfonyl)imide, LiFSI which predominantly suppressed the polysulphide dissolution and diffusion while showing improved lithium plating and stripping behavior [3]. In order to expand our understanding on the role of DME and the lithium salt concentration; in this work, we have considered a similar ionic liquid based electrolyte system but at a constant concentration of LiFSI salt. The ionic interactions among different species of the electrolyte system have been extensively studied by nuclear magnetic resonance (NMR) spectroscopy using ¹D chemical shift, PFG-NMR and Heteronuclear Overhauser Effect SpectroscopY (HOESY) measurements. We found that, when increasing the DME concentration in the system, a strong association of Li⁺ cation with the DME occurs due to the electronegative oxygen atoms present in the DME molecules which leads to strong coordination. This leads to a change of the chemical shift of ⁷Li nuclei due to them being increasingly de-shielded. Interestingly, we found that the diffusivity of both Li⁺ and DME species are very similar, which also confirms that a Li-DME complex is present with FSI anion associated in the next coordination sphere to counterbalance the positively charged Li complex. MD simulations are underway to better elucidate the molecular shell structure of the system. We have also carried out electrochemical characterizations of these hybrid electrolytes to determine their potential applicability in a lithium sulphur battery. It was apparent using cyclic voltammogram (CV) performed in a three electrode set-up, that an increasing amount of DME increases the ionic conductivity but decreases the lithium plating-stripping efficiency. A coin-cell study of Li/Li symmetrical cycling showed an excellent plating-stripping behaviour for 70%IL-30%DME composition. We have carried out a full cell cycling against sulphur cathode and the optimised electrolyte composition obtained a promising first discharge capacity of 1100mAh/g. Additionally, we have investigated the sulphur speciation in the presence of this ionic liquid in a three-electrode system where platinum mesh has been used as working electrode coupled with in-situ UV-vis spectroscopy. Finally, a time dependent bulk electrolysis study revealed the plausible intermediates formed during the charge-discharge cycle of sulphur redox reaction. From these studies it appears that the composition of this novel hybrid electrolyte system can be tuned to provide a system with improved transport, electrochemical and sulphur speciation properties which lend themselves to a higher performance, next generation lithium sulphur battery technology.

References:

[1] Ji, X.; Nazar, L. F., Advances in Li-S batteries. Journal of Materials Chemistry 2010, 20 (44), 9821-9826.

[2] Scheers, J.; Fantini, S.; Johansson, P., A review of electrolytes for lithium–sulphur batteries.Journal of Power Sources 2014, 255, 204-218.

[3] Pal, U.; Forsyth, M; Improved Li-Ion Transport by DME Chelation in a Novel Ionic Liquid Based Hybrid Electrolyte for Li-S Battery Application. The Journal of Physical Chemistry C, 122 (2018) 14373-14382.

Figure 1

Direct liquid fuel cells (DLFCs) are promising power sources for future portable, wearable or implantable electronic devices owing to their high energy density and low emissions. Large efforts have been given for the development of DLFCs since 1990s, however the wide spread commercialization of DLFCs is still limited by the high cost coming from the expensive noble metal catalysts and proton exchange membranes. Furthermore, the safety risks related to organic fuels (methanol, ethanol and formic acid), such as toxicity and flammability, bring up tremendous consideration in fuel cell design, fuel distribution and transportation ^[1]. Finally, the emission of CO₂ is inevitable by using organic fuels, which escalates to Green House Effect.

In this work, we proposed a new category of inorganic fuels for DLFCs. Hypophosphite is a type of food additive, therefore no safety hazards at all, and is also a strong reductant whose electrochemical oxidation reactivity is strongly correlated to metallic palladium ^[2]. Moreover, the inorganic fuels could realize "Zero Emission" after "Burning". From thermodynamic calculations, the theoretical cell voltage and volumetric energy density at 1M are 2.05 V and 188 Wh/L at standard conditions, which are comparable and even better than the data of DMFCs (Direct Methanol Fuel Cells).

The unique electrochemical oxidation behavior of hypophosphites on Pd catalyst brings advantages in simplifying fuel cell structure and broadening our selections of cathodic catalysts for ORR (Oxygen Reduction Reaction). Herein, we report the investigation of catalyst design and cell design of a novel membrane-free Direct Hypophosphite Fuel Cell (DHPFC), which showed an open circuit voltage of 1.0 V and a maximum power density of 32 mW cm^-2 under air flow at 25 ^oC. The oxidation mechanism of hypophosphites on Pd was also studied in this work.

This work is financially sponsored by Solvay and "Shanghai Rising-Star Program" (16QB1404600).

Reference:

[1] G. L. Soloveichik, Beilstein J. Nanotechnol. 2014, 5, 1399-1418.

[2] N. Fujiwara, Z. Siroma, T. Ioroi, K. Yasuda, J. Power Sources 2007, 164, 457-463.

Metal-ion batteries based on aqueous electrolyte are attracting widespread attention in the field of large-scale energy storage due to the low cost, high safety and environmental friendliness. However, the development of the most studied aqueous Li⁺ and Na⁺ batteries are plagued by the low energy density, which mainly derived from the low capacity of electrode materials (<150 mAh g^-1) and the limited voltage window of aqueous electrolyte (<1.8 V). The latter could be conquered by the water-in-salt electrolyte, and the operating voltage > 3V could be obtained¹. But the capacity of electrode materials still needs to be enhanced.

Due to the high capacity of metal zinc anode (820 mAh g^-1), aqueous Zinc-ion battery based on Zn²⁺ storage cathode and metal Zn anode holds immense potential to achieve high capacity. But the capacity of cathode still has much room for improvement. As the most used cathode materials for zinc-ion battery, manganese oxides exhibit high theoretical capacity (308 mAh g^-1), low cost and low toxicity. Various manganese dioxides, including the α-MnO₂, β-MnO₂, γ-MnO₂, δ-MnO₂, etc. have been reported as host materials for Zn²⁺/H⁺ insertion in the mild aqueous electrolyte. However, all the crystalline form of manganese oxides undergo structure transformation to layered structure with interlaminar water molecules during zinc-ion insertion, leading to capacity fading of the cell. And the cycling stability of manganese oxides decreases dramatically when cycled at high depth of discharge².

In this work, we proposed novel polyaniline-intercalated MnO₂ nanolayers as a high performance cathode material for zinc-ion battery, which is prepared by a simple one-step inorganic/organic interface reaction³. With the typical nano-size (approximately 10 nm), expanded interlayer space, uniform meso-structure and polymer-reinforced layered structure, the polyaniline-intercalated MnO₂ nanolayers show a high rate performance and excellent cycle stability at high charge/discharge depth (200 stable cycles with capacity of 280 mAh g^-1, corresponding 90.9% utilization of theoretical capacity of 308 mAh g^-1), which is much superior to previous reports. The polyaniline-reinforced layered structure efficiently eliminate the hydrated H⁺/Zn²⁺-insertion-induced phase transformation and the subsequent structure collapse, which is important to achieve long cycle life and high utilization simultaneously. In addition, a H⁺/Zn²⁺ co-insertion process in the layered MnO₂ was proposed, and a self-regulating mechanism of electrolyte involving generation/dissolution of flake-like zinc hydroxide sulfate was clarified.

Reference:

1 C. Yang, J. Chen, T. Qing, X. Fan, W. Sun, A. von Cresce, M.S. Ding, O. Borodin, J. Vatamanu, M.A. Schroeder, N. Eidson, C. Wang, and K. Xu, Joule, 1, 122 (2017).

2 J. Huang, Z. Guo, Y. Ma, D. Bin, Y. Wang, and Y. Xia, Small methods, 1800272 (2018).

3 J. Huang, Z. Wang, M. Hou, X. Dong, Y. Liu, Y. Wang, and Y. Xia, Nature Communications, 9, 2906 (2018).

The urge for electrochemical energy storage devices with high gravimetric and volumetric energy density is imminent and the lithium−sulfur (Li-S) battery has been poised as a major next generation battery concept candidate. While it has a (very) high theoretical energy density, low cost and non-toxicity of starting materials,[1] its practical application has been hampered by several obstacles; the active material(s) elemental sulfur (S₈) and lithium sulfide (Li₂S) are both electronic insulators, polysulfides dissolve into the electrolyte and diffuse between the cathode and anode –generating an active material loss, there are dendrites formed at the lithium metal anode, etc.[2,3]

To address (some of) these problems, the solubility properties of the electrolyte are of utmost importance [4] and here we use semi-solid electrolytes to try to hinder the dissolution of sulfur and possibly also mitigate the creation of lithium dendrites. These systems are composed of a deep eutectic electrolyte (DEE), LiTFSI and/or LiFSI based, confined within a porous silica framework – creating an eutectogel.[5] We here report on the initially assessed physico-chemical properties, basic electrochemical properties such as ion conductivity (Figure 1), before progressing to feasibility tests in Li-S battery cells.

This work was funded by "Batterifondsprogrammet" of the Swedish Energy Agency.

Figure 1. Arrhenius plot of a DEE and a DEE confined in a SiO₂ framework.

References

[1] Q. Pang, X. Liang, C.Y. Kwok, L.F. Nazar, Nat. Energy 1 (2016) 16132.

[2] X. Judez, H. Zhang, C. Li, G.G. Eshetu, J.A. González-Marcos, M. Armand, L.M. Rodriguez-Martinez, J. Electrochem. Soc. 165 (2018) A6008–A6016.

[3] J. Scheers, S. Fantini, P. Johansson, J. Power Sources 255 (2014) 204–218.

[4] S. Drvarič Talian, S. Jeschke, A. Vizintin, K. Pirnat, I. Arčon, G. Aquilanti, P. Johansson, R. Dominko, Chem. Mater. 29 (2017) 10037–10044.

[5] B. Joos, T. Vranken, W. Marchal, M. Safari, M.K. Van Bael, A.T. Hardy, Chem. Mater. 30 (2018) 655–662.

Figure 1

As we approach the limits of lithium-ion batteries, new alternative chemistries are required^[1]. Those that involve the sulfur conversion reaction at the positive electrode are among the most promising to move beyond Li-ion^[2]. The conversion reaction of sulfur to form lithium sulfide is characterized by a high theoretical specific capacity of 1672mAh g^-1, translating in a theoretical energy density of c.a. 2500 Wh kg^-1, a value 2.5 times higher than that of commercial state-of-the-art Li-ion cells, using transition metal oxide based cathodes^[3].

There are well-documented challenges associated with the traditional Li-S system which have been demonstrated to restrict the practical energy density and the cycle life^[4]. Soluble polysulfides are generally formed, resulting in the shuttling mechanism, a major issue causing low coulombic efficiency, high self-discharge rates and negative electrode corrosion^[4]. This reaction is also associated with large volume changes. This can lead to cell failure, due to the alteration of the cathode and anode structure and the favoured formation of lithium dendrites^[5].

Strategies to enhance cycle life, while maximizing practical energy density, require a holistic approach. This includes optimization of the positive electrode (e.g., selection of conversion reaction catalyst, use of polysulfide adsorbents, binder selection); the negative electrode (e.g., lithium coating, functionalization and protection); the separator (e.g. functional barriers and polymeric membranes); the electrolyte (e.g. use of alternative solvents and salts, additives).

This work will focus on the recent negative electrode advances at OXIS Energy Ltd, the pioneer in the research and development of large-scale, high-energy, longer cycle life lithium-sulfur batteries.

[1] D. A. J. R. Ronald M. Dell, Lithium Batteries, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2013.

[2] B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci.2011, 4, 3287.

[3] M. S. Whittingham, Proc. IEEE2012, 100, 1518.

[4] A. Manthiram, Y. Fu, S. H. Chung, C. Zu, Y. S. Su, Chem. Rev.2014, 114, 11751.

[5] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Adv. Mater.2017, 29, 1601759.

Here we study the molten salt chemical cell using Nd magnet scraps and Cl₂ or O₂ gas electrodes in order to achieve new rare earth recycle process. The anodic polarization of Nd magnet scraps and cathodic polarization of O₂ gas electrodes were investigated in molten LiCl-KCl systems at 723 K. The anodic polarization was carried out by potentiostatic electrolysis of Nd magnet scraps electrode from 0.69-2.39 V(vs. Li⁺/Li) at every 0.050 V interval. The large anodic current was observed at 1.80-2.00 V due to anodic reaction of dissolution of rare earth elements. On the other hand, the cathodic polarization was carried out by potentiostatic electrolysis of O₂ gas electrode using porous Ni from 1.20-2.30 V at every 0.10 V interval in a molten LiCl-KCl added Li₂O(0.50 mol%) system. The large cathodic current was observed from 1.40-1.90 V due to cathodic reaction from O₂ to O^2-.

Sodium-ion batteries (SIBs) have been considered as a promising candidate for large-scale energy storage applications, because of the low cost of sodium element and a broad choice of cathode materials which do not contain the expensive raw materials. However, a lack of promising anode materials still hinders the development of SIBs technology. Herein, we for the first time report a one dimensional tunnel-structure titanosilicate sitinakite compound with an ideal formula of Na_1.68H_0.32Ti₂O₃SiO₄·1.76H₂O employed as a new type anode material in SIBs. This material can deliver a reversible capacity of 110 mAh g^-1 at a current density of 20 mA g^-1 and with an average working voltage of 0.4 V vs. Na⁺/Na. The structure changes of this material during discharge/charge processes were investigated by using in-situ laboratory X-ray diffraction. The results indicated that sodium insertion proceeds via a topotactic intercalation pathway. We also identified pre-dehydration as an effective avenue to further improve the capacity of Na_1.68H_0.32Ti₂O₃SiO₄·1.76H₂O anode (reversible capacity of 131 mAh g^-1 at a current density of 20 mA g^-1 after pre-dehydration process). In addition, cation Nb doped Na_1.68H_0.32Ti₂O₃SiO₄·1.76H₂O compounds have been synthesized and systematically investigated the electrochemical performances in SIBs.

Reference

[1] Liu, Y.; Xia, Y., Na_1.68H_0.32Ti₂O₃SiO₄·1.76H₂O as a Low-Potential Anode Material for Sodium-Ion Battery. ACS Applied Energy Materials 2018, doi:10.1021/acsaem.8b01412.

In January 2017, Samsung announced the reason why the Note 7 explodes and losses of more than 5 billion US dollar. According to their explanations, it was caused by the internal short circuit problem in the lithium ion battery. Currently, there is no effectively solution for eliminating the internal short circuit problem owing to the sudden accident.

In this research, a new technology has been developed, which can be used to terminate the thermal runaway and promise the safety performance of lithium ion battery. High voltage Li-excess and Ni-rich layer-type cathode material is employed and combined with this safety electrode additive for investigation. In terms of the results, the new technology LIVING^@additive significantly enhances the cycle performance at 60^oC and high voltage. In addition, the following figures illustrated that the battery containing LIVING^@technology is stable and passed the nail penetration test. On the other hand, the battery without LIVING^@ cannot be used when the short problem is taking place. The LIVING^@contains self-polymerized hyper branch structure in order to insulate the directly contact between anode and cathode. This electrode additive not only provides high thermal stability on electrochemical reaction, but the columbic efficiency of charge-discharge is also enhanced.

Keywords: Lithium ion battery, safety, cathode electrolyte interphase, Ni-rich

Closo-borates represent a promising but yet under-explored alternative class of electrolytes for all-solid-state batteries. We recently reported ionic conductivities of 1 mS/cm and an electrochemical stability window of 3 V at room temperature for Na₄(B₁₂H₁₂)(B₁₀H₁₀).[1] The mixed-anion configuration stabilizes the ion-conducting high-temperature phase at room temperature.

We further demonstrated stable cycling for a 3V class all-solid-state battery based on this electrolyte consisting of a sodium metal anode and a NaCrO₂ cathode.[2] The cathode composite can be assembled by simple cold pressing, but cycling performance is enhanced through a preliminary solvent-based impregnation step of the cathode particles by a thin electrolyte coating. This coating guarantees intimate contact between cathode particles and electrolyte resulting in reversible and stable cycling with a capacity of 85 mAh/g at C/20 and 80 mAh/g at C/5 with more than 90% capacity retention after 20 cycles at C/20 and 85% after 250 cycles at C/5. We also investigated the effect of cycling outside the electrochemical stability window and observed that electrolyte decomposition leads to faster though not critical capacity fading.

We will further present routes to translate these results to lithium analogues and our efforts to establish low-cost synthesis routes for these non-toxic materials. Our results demonstrate that owing to their physical properties and processability, closo-borate-based electrolytes could play a significant role in the development of a competitive all-solid-state battery technology.

[1] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Comm. 53, 4195 (2017)

[2] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy & Environmental Science 10, 2609 (2017)

Figure 1

Lithium-rich layered oxides (LRLO) have lately emerged as leading candidates for replacing the classical stoichiometric insertion oxides owing to their staggering increase in energy densities provided by the oxygen redox activities.¹ Despite such a positive attribute, LRLO still await commercial success due to practical obstacles that appear during the first cycle and continue upon subsequent cycles (i.e., irreversible structural changes associated with oxygen gas release leading to unfavorable electrochemical properties such as hysteresis and progressive voltage decay).² Though the origin of voltage fade has been discussed from various structural perspectives (i.e., cation capturing through tetrahedral sites, formation of partial dislocations and/or microstructure defects), a consensus has been reached on the strong coupling between structural dynamics and oxygen redox chemistry.^3-6 Nonetheless, establishing an explicit structure-properties scenario has proved challenging due to characterization limitations as well as the inherent complexity of the coupled transition metal and oxygen redox process itself. In this work, we investigated both in-situ and ex-situ the structure evolution of a typical Li-rich material during its initial cycles based on a newly developed oxygen analytical tool, laboratory and synchrotron X-ray diffraction (XRD), coupled with electron microscopic imaging technique. The average and local structure change associated with cation migration, gaseous oxygen loss and solid liquid interfacial reactions will be discussed. Furthermore, a metastable phase transformation pathway induced by the lattice oxygen redox activities will be proposed and its implication for the applicability of such Li-rich oxides will be shared.

References:

• J. Wang, et al.Advanced Energy Materials,6 (21), 1600906 (2016).

• G. Assat and J.M. Tarascon, Nature Energy,3 373-386 (2018).

• M. Sathiya, et al.Nature Materials,14 230-238 (2014).

• A. Singer, et al.Nature Energy,3 (8), 641-647 (2018).

• E. Hu, et al.Nature Energy, (2018).

• W. E. Gent, et al.Nature communications,8 (1), 2091 (2017).

Rechargeable lithium-ion batteries are necessary for our life to use as a mobile phone, electric vehicle and so on. However, there is a limit to the capacity of lithium-ion batteries. Therefore it is necessary to develop batteries which has large capacity. Lithium-sulfur (Li-S) battery is expected for next generation rechargeable battery owing to have high capacity (1,645 mAh/g) compared with conventional Li-ion batteries. However, Li-S battery has serious problem that lithium polysulfide (Li₂S_x) which is an intermediate product dissolves into electrolyte. If we can suppress the dissolution of Li₂S_x, the battery life should be extended. To suppress the dissolution of Li₂S_x, we use solvate ionic liquid (SIL) electrolytes. SIL is mixture of 1:1 complex from low-molecular weight ether and Li salt, which have high thermal / electrochemical stabilities owing to strong interaction of between ether oxygen and Li cation. SIL electrolytes have low solubility of Li₂S_x owing to their low Lewis acidity / basicity and suppress dissolution of Li₂S_x into electrolyte. Recently, it was found that high Li salt concentration more than 1:1 SIL electrolyte is effective for high performance lithium-ion batteries and Li-S batteries. Fig. 1 shows cycle performance of LiNi_1/3Mn_1/3Co_1/3O₂ | [Li(G3)_x]TFSA | Li cell. Excess Li salts achieved high cycle performances and stable charge-discharge operations. Li salt excess SIL electrolyte contributes to extension of cycle life for Li-ion batteries. According to these previous researches, it is shows that local composition changes of SIL electrolytes of glyme and Li salt near the electrode during the charge and discharge process. Li₂S_x should be dissolved in the released free glyme. Therefore, we considered that Li salt excess SIL electrolyte can suppress the formation of temporary free glyme. Therefore, the purpose of this study is to clarify the influence of the composition of SIL electrolyte on the solubility of Li₂S_x and the performance of Li-S batteries. All preparations and measurements were carried out in an inert atmosphere Ar-filled glove box and a sealed cell. SILs, [Li(G3)_1.25]TFSA, [Li(G3)_1.11]TFSA, [Li(G3)]TFSA, [Li(G3)_0.9]TFSA and [Li(G3)_0.8]TFSA (composition ratio of triglyme (G3) and LiTFSA is 10:8, 10:9, 10:10, 10:9 and 10:8), were prepared, respectively. Viscosity, density and thermal stability of SIL electrolyte samples were measured. Then, dissolution tests of Li₂S_x were carried out. Lithium polysulfide is an unstable substance, so S₈ and Li₂S were mixed with S₈ and Li₂S=7:8, we defined as Li₂S₈ were prepared. Prepared SIL electrolyte and Li₂S₈ were mixed for saturation. SILs were each diluted 50 times (molar ratio) with SIL electrolyte, and the absorbance was measured by UV-vis spectrometer. Fig. 2 shows appearances of five SIL electrolytes with saturated Li₂S₈. It was visually observed that the amount of free glyme decreased in the Li excess SIL electrolyte and the dissolution of Li₂S_x into SIL electrolyte was suppressed with LiTFSA concentration. From this result, improvement of Li-S battery performances were expected by using Li salt excess SIL electrolytes. Charge and discharge tests were carried out using [S|[Li(G3)_x]TFSA|Li] cells, and cycle performances were shown (Fig. 3) . All initial capacities were more than 800 mAh/g. The figure is the normalized cycle characteristics as deterioration rate with the initial capacity set to 100%. At 25 cycles [Li(G3)_x]TFSA x=0.8 (Li salt excess SIL electrolyte) showed a sufficient capacity retention about 10% higher than x=1 (equimolar). From the above results, the Li salt excess SIL electrolyte can conclude as effective electrolyte design for Li-S batteries. In the presentation, we will comprehensively report the thermal and transport properties of each SIL and the results of UV-vis measurement results.

Figure 1

Water-in-salt electrolytes enable the operation of batteries at higher voltage than previously possible with conventional aqueous electrolytes.[1,2] High-voltage aqueous batteries are potential candidates for stationary electricity storage where low total cost of ownership is more important than high energy density.

To date, most water-in-salt studies have focused on enabling aqueous lithium-ion batteries. However, considering the high salt content of water-in-salt electrolytes, sodium-ion batteries appear particularly interesting for this approach as sodium salts are typically significantly less expensive than their lithium analogues, but typically exhibit insufficient solubility due to the lower charge density of Na⁺ compared to Li⁺.[3]

Recently, we discovered that NaFSI displays very high solubility in water, enabling a wide electrochemical stability window.[4] Although S-F bonds present in the FSI anion are known to be relatively weak and hence make FSI prone to hydrolysis, we demonstrate a 2 V class NaTi₂(PO₄)₃/Na₃(VOPO₄)₂F cell with an excellent capacity retention of 85% after 500 cycles at 1C. To the best of our knowledge, the specific energy of our cell of 65 Wh/kg (based on the active masses of both electrodes) is the highest reported to date for an aqueous sodium-ion battery. Using strategies to prevent electrolyte crystallization, we also demonstrate stable cycling at temperatures as low as -10 °C.

[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 350 (2015) 938.

[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy 1 (2016) 16129.

[3] D. Reber, R.-S. Kühnel, C. Battaglia, Sustainable Energy Fuels 1 (2017) 2155.

[4] R.-S. Kühnel, D. Reber, C. Battaglia, ACS Energy Lett. 2 (2017) 2005.

Introduction: There have been increasing attention to anion-intercalation behavior and materials. In lithium ion battery field, intercalation property of PF₆⁻ into graphitic carbon was studied for a hybrid capacitor. In ion-exchange field, selective removal of toxic anionic species by anion-exchangeable layered compounds has been focused. Layered double hydroxides (LDHs) was representative of anion-exchangeable layered compounds, represented by a general formula of [M²⁺_1−xM³⁺_x(OH)₂]^x+[A_nx/n]^x−mH₂O consisting of octahedral brucite-like host-layers (M²⁺/M³⁺: divalent and trivalent metal cations), charge-balancing anions (Aⁿ⁻) and interlayer water molecules. Their ion-exchange properties, selectivity and capacity, have been widely studied. Miyata previously reported the order of affinity of various anions to Mg-Al LDHs; OH⁻ > F⁻ > Cl⁻ > Br⁻ > NO₃⁻ > I⁻. Although other researchers have tried to control the affinity by changing composition of metal cations (M²⁺/M³⁺) to control charge density of host layers, significant change in the order was hardly achieved. Looking back to intrinsic crystalline structure of LDHs, host-layers, optimal positions of interlayer anions, and water molecules is supposed to be determined with hydrogen-bonding interaction among them. In other words, the affinity of anion to LDHs should be controlled by tuning strength of hydrogen-bonding in addition to charge density of host layers. In this presentation, we tried to prepare F-substituted Mg-Al LDHs (F-LDHs), OH groups belonging to host layer of LDHs was substituted by F atoms, and anion-exchange selectivity was investigated.

Experiment: Based on traditional co-precipitation method with solution pH = 10, F-LDHs was prepared by using AlCl₃, NaAlF₆, and MgCl₂·6H₂O as a precursor. Mg/Al ratio was controlled as 2.5，3.0，3.5，and 4.0 by changing ([AlCl₃]+[NaAlF₆])/[MgCl₂] ratio and F-substitution ratio was controlled by changing [AlCl₃]/[NaAlF₆] ratio. Crystal structure, morphology, chemical composition, local structure of Al and F atoms of obtained samples were analyzed by XRD, FE-SEM (EDS), XPS, solid-state NMR. Next, anion-exchanging properties of obtained F-LDHs was tested. Firstly, as a pre-treatment, LDHs samples was immersed in acidic NaCl aqueous solution for 24 hours at room temperature and interlayer "contaminated" carbonate was exchanged by chloride ions as much as possible. Seconds, by using chloride-exchanged samples, anion-exchange test was carried out with 1.2~1.5 mM NaNO₃, Na₂SO₄, Na₂HPO₄, NaF, and NaI aqueous solutions for 24 h, where solid to liquid ratio was fixed to 1.0 gL^-1. After 24 h, exchanged amount was analyzed by ion-chromatography.

Results and discussion: XRD patterns of all samples exhibit successful formation of LDHs without byproducts even after chloride-exchange treatment. Successful inclusion of F atoms into host-layer of LDHs was confirmed by solid-state NMR and, in all samples, 80% of interlayer anions was exchanged by chloride ions, which is confirmed by compositional analysis with EDS and XPS. XRD patterns also revealed that the basal spacing, d₀₀₃, of F-LDHs tend to decrease comparing to original LDHs, which implies that host-layers strongly interacts with interlayer anions and water molecules. In addition, F-substitution amount was found out to be limited by about 10%, LDHs can be obtained without byproducts. Next, distribution coefficient, useful indicator of the affinity of respective anions to LDHs, was calculated from the result of ion-exchange test. It clarified that the order of affinity was NO₃⁻ > HPO₄²⁻ > Br⁻ > F⁻ > SO₄²⁻ > I⁻, which is totally different from previous reports. Furthermore, the affinity of NO₃⁻ ions comparing to SO₄²⁻ ion and F⁻ ions was increased depending on F-substitution amount. In conference, detailed formation mechanism of F-LDHs and anion-exchanging mechanism was presented.

Introduction

Lithium-sulfur battery has been attracted attention as a next-generation battery of large capacity. Lithium-sulfur battery has reversible theoretical capacity of 1,672mA h g^-1, which is 10 times value compared with conventional positive electrode materials such as LiCoO₂ using Li-ion battery. Also, sulfur is obtained as by-products of petroleum, and low-cost batteries can be produced. One serious problem of lithium sulfur battery is dissolution of lithium polysulfide (Li₂S_x) as reaction intermediate into the electrolyte solution during charge/discharge reaction. This causes degradation of the charge/discharge cycle characteristics and coulombic efficiency of the battery. In order to solve this problem, "solvate ionic liquid (SIL)" is proposed as new electrolyte, because dissolution of Li₂S_x can be suppressed owing to weak Lewis acidity/basicity. SIL is consisted of weak Lewis base/acid ion, which is coordinated Li cation with solvent, such as complex cations. SIL has physicochemical properties similar to conventional ionic liquid, which melting point lower than room temperature, thermal stability in wide temperature range and negligible vapor pressure. For example, significant improvement for stability of electrolyte solution has been reported by high-salt concentration (ether series^[1]/acetonitrile^[2]) owing to strong interaction between Li salt and solvent molecule. When SIL electrolyte applied to Li-S batteries, high coulombic efficiency and long cycle performance are reported^[3]. However, if SIL was actually used as electrolyte, 1: 1 molar ratio of SIL was locally destroyed in vicinity of the electrode during charge/discharge reaction.Free glyme exists into electrolyte solution with electrochemical process, which is not temporarily coordinated between lithium cation and oxygen of glyme. As a result, Li₂S_x can be dissolved into free glyme, easily. In order to suppress of free glyme formation, we propose to suppress of Li₂S_x dissolution and improve performance of Li-S battery by using excess Li salt concentration of SIL. However, high-concentration electrolyte exhibits large viscosity, and has risks of rate performance for high rate charge/discharge operations. Therefore, in order to obtain low viscosity electrolyte, we proposed to add low viscosity dilute solvent into electrolyte^[4]. Most important demands for dilute solvent are,

• Improvement of rate performances for batteries by low viscosity of electrolyte solution

• Stabilization of solvate structure between Li salt and solvent molecule with/without dilute solvent.

Experiments

In this study, we investigated physicochemical effects for understanding compatibility of two effective approaches of super-concentrated Li salt electrolyte and non-interactive dilute solvent.

In this study high Li salt concentrated SIL sample, [Li_1.25(G4)₁]TFSA (G4: CH3-O-(C₂H₄O)₄-CH₃, TFSA:N(SO₂CF₃)₂) was prepared in Ar-filled glovebox. Molar ratio of [Li_1.25(G4)₁]TFSA was G4:LiTFSA=1:1.25. In addition, we added given predetermined amount of 1,1,2,2, - tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (HFE) dilute solvent into SIL, and measured temperature dependence of viscosity and density for prepared sample.

Results & Discussion

Fig.1(a) showed the composition dependence of the viscosity from 10 to 80℃ (r) for SIL/HFE mixtures. We confirmed that diluting HFE decreased viscosity [Li_1.25(G4)₁]TFSA. When diluting HFE and SIL to about 1: 1, the viscosity decreased to about 1/10. HFE is very effective as a diluting solvent to lowering the viscosity of SIL. Fig.1(b) showed the composition dependence of the density from 10 to 80℃ (r) for SIL/HFE mixtures. r values of HFE were larger than that of SIL ones owing to the difference of fluorine density between SIL and HFE. In mixing SIL and HFE, the density didn't take intermediate value and was mainly lower. To analyze this reason, we assumed SIL as one molecule, excess densities (E_ρ) were expressed as following,

E_ρ=ρ-(x_ρSIL+(1-x)_ρHFE)

Where x, ρ_SIL and ρ_HFE are mole fraction of SIL, ρ of neat SIL and HFE, respectively.

Fig.1(c) shows mole fraction dependence of E_r for SIL-HFE mixtures. E_r always shows a negative value in the temperature range below 30℃. Therefore, by mixing SIL and HFE, and suggested that the density decreasing. Mixture of SIL and HFE indicates possibility of expansion / repulsion etc. The mixture of SIL-HFE was inferred to be a liquid state like phase separation significant interaction. This result correlated with spectroscopic method that coordination structure of SIL and G4 doesn't change significant in dilute HFE^[5]. In this presentation, relationships between lithium-sulfur battery performance (rate properties) and composition of [Li_1.25(G4)₁]TFSA/HFE will be reported, and precise transport properties will be discussed.

References

[1] K. Yoshida et al, J. Am, Chem. Soc. 2011, 133, 13121.

[2] Y. Yamada et al, J. Am. Chem.Soc. 2014, 136, 5039.

[3] S. Seki et al, Electrochemistry2017, 85, 680.

[4] K. Dokko et al, J. Electrochem. Soc. 2013, 160, A1304.

[5] S. Saito et al, J. Phys. Chem. B2016, 120, 3378.

Figure 1

High capacity electrode material has been studied over the years due to the constant demand for high energy density applications in li ion batteries. Among others, silicon has been considered a more promising candidate for its high theoretical capacity of ~ 3600 mAh/g, abundant resources, low cost and low toxicity. However, silicon-based electrodes face rapid degradation due to the extensive volume variation (~300%) in the charge discharge process. Binders used in the electrode fabrication plays a crucial role in these high-performance electrodes since it can reduce the mechanical fracture in the cycling process.

Utilization of polymeric material as binders to hold the active material has been the most common approach used in the li ion battery electrode preparation. Recent studies carried out by Nguyen et. al have shown the use of small molecular carboxylic acids as binders which showed an improvement in the cell performance for silicon electrodes. In this study we introduce a cheap and environmentally friendly alternative that could be used as a non-polymeric binder for silicon electrodes. Further, the electrode preparation can be done in water medium which reduces the introduction of toxic organic solvents such as NMP to the environment. Casein is a milk protein found in bovine milk rich in amine groups and carboxylic acid groups which can form bonds with the silanol groups in silicon. A comparative study conducted between PVDF and Casein as binders have shown that when casein was used as binder, it shows better performance compared to PVDF. It has 12% higher capacity retention compared to PVDF with a 2200 mAh/g capacity after 50 cycles. Surface morphology and Solid electrolyte interface was analyzed using electron microscopy techniques and spectroscopic methods and the results will be discussed.

Increasing deployment of large scale electric storage applications would benefit from the development of inexpensive batteries. Na-ion batteries (NIBs) are of great interest since their cathodes can be made from inexpensive and abundant materials such as sodium, iron, and manganese. However, existing iron and manganese oxide cathodes lack the electrochemical performance, including capacity retention, required for these NIB applications and significant improvement is needed.

Prior work on lithium ion battery cathodes demonstrates that meso-structured materials can improve electrochemical performance for batteries. Incorporating spherical meso-structured materials in lithium ion battery cathodes was shown to improve capacity retention. To emulate this approach in NIB cathode materials, a synthesis method for producing pure phase spherical meso-structured sodium iron manganese oxide is first needed. No literature has been found that describes a method for this synthesis. Key synthesis parameters to be controlled include the choice of method, material precursors, pH of reaction, calcination time, and cooling rate.

In this work we demonstrate a modified co-precipitation method to synthesize spherical meso-structure pure phase P2-Na_0.67Fe_1/4Mn_3/4O₂ (see Figure 1) and we report initial characterization of the electrochemical performance of this material. The key parameter found to control the formation of a spherical meso-structure is the cooling rate applied during synthesis. The slow cooled material forms spherical meso-structures while the quenched material has no ordered meso-structure. Additionally, the spherical meso-structured material shows increased capacity, improved cyclability, and reduced polarization. The electrochemical performance enhancement is attributed to higher surface area, reduced surface concentration of sodium carbonate, and lower internal resistance during cycling. Further exploring the synthesis and mechanisms of meso-structured materials offers a path for improving Na-ion battery cathodes.

Figure 1

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the lowest negative electrochemical potential. Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings, electrolyte additives.^{1, 2} In addition, a 3D host can be beneficial due to the reduced volume change.³ We have recently focused on designing novel electrolyte solution chemistry that enables dendrite free Li metal cycling. The structure, physical and chemical properties of the designed electrolyte solution are well in accordance with the quantum chemistry calculations. The XPS analysis of the SEI (solid electrolyte interface) components on the Li metal surface reveals that a LiF rich protection layer forms. As a result, a high average coulombic efficiency of 99.4% has been achieved at 0.5 mA cm^-2 for 1 mAh cm^-2 over 900 cycles. Even at a high current density of 10 mA cm^-2, the coulombic efficiency of Li metal in this novel electrolyte is above 98.5%. A Li metal full cell with PANS (Poly(acrylonitrile) Sulfur) as cathode realizes a capacity retention of >98% over 900 cycles. This study offers a promising approach to enable highly stable Li metal battery applications.

Acknowledgements

This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility.

• H. Liu, X. Wang, H. Zhou, H.-D. Lim, X. Xing, Q. Yan, Y. S. Meng and P. Liu, ACS Applied Energy Materials, 2018, DOI: 10.1021/acsaem.8b00348.

• H. Liu, H. Zhou, B.-S. Lee, X. Xing, M. Gonzalez and P. Liu, Acs Applied Materials & Interfaces, 2017, 9, 30635-30642.

• H. Liu, X. Yue, X. Xing, Q. Yan, J. Huang, V. Petrova, H. Zhou and P. Liu, Energy Storage Materials, 2018, DOI: https://doi.org/10.1016/j.ensm.2018.09.021.

Concerned about the environmental pollutant such as greenhouse gas emission caused by extensive use of diesel, gasoline vehicle operation, being increased. Implementation of rechargeable battery in EV application because of EV's zero emission has become very popular. Lithium ion battery (LIB) is one of the successfully commercialized system due to their high operating potential, high energy density and long cycle life [1]. However, high price and limited resources of lithium still makes hard to produce for reasonable price of EV. In contrast of LIB issue, sodium-ion batteries (SIBs) have drawn a considerable attention as an alternative to LIBs in the EV applications because of its relative abundance in the earth crust, global distribution, and drastically lower cost [2]. However, the fundamental differences between sodium and lithium make it challenging to develop a suitable anode material to host Na⁺ ions such as well-known graphite intercalation affair in NIB system [3]. Moreover, the higher reduction potential of sodium (–2.71 V vs. S.H.E.) as compared to lithium (–3.04 V vs S.H.E) inherently reduces the energy density of battery system. Therefore, there is still considerable efforts underway to develop a potential anode material that allows to host a large amount of Na⁺ ions at a low voltage potential. Among the various electrode material, sodium titanates (NTOs) have considered one of promising anode materials for SIBs because of their low starting material cost, environmental friendly, and abundance [4]. Among various type of sodium titanate, layered Na₂Ti₃O₇ is one of the most promising phase. It can uptake two Na⁺ ions per formula unit into its interlayer space at a low average potential of 0.3 V vs Na/Na⁺ , which could deliver a high theoretical capacity of 177 mAh g^-1 [5]. It makes particularly promising to design an anode material with high energy density. However, the poor electronic conductivity of Na₂Ti₃O₇ associated with its large bandgap (3.7 eV) and structural distortion upon Na⁺ ion uptake leds to sluggish Na⁺ ion diffusion and cycling stability [6]. In this study, ultrathin and uniform carbon layer-coated, layered Na₂Ti₃O₇ and tunnel Na₂Ti₆O₁₃ hybrids anode materials synthetic route was successfully developed using facile and fast supercritical methanol and subsequent carbon coating with low viscosity liquid carbon dioxide as a coating solvent. The deficiency of each material, e.g., poor rate performance and cyclability caused by sluggish Na⁺ ion diffusion and structural distortion of Na₂Ti₃O₇ and the low capacity of Na₂Ti₆O₁₃, could be overcome in the hybrid by taking advantages of low volume expansion and high electronic conductivity of Na₂Ti₆O₁₃ and high capacity of Na₂Ti₃O₇ with enhanced conductivity by carbon coating. Through the HR-TEM technique, conformal, uniform and ultrathin carbon layers on the NTO surface with an average thickness of 15 nm was observed. Moreover, significantly decreased charge transfer resistance was confirmed by way of the EIS measurement. A careful analysis of the cyclic voltammetry profiles of this sodium titanate hybrids revealed that the existence of two different Na⁺ ion diffusion pathways in the layered structure of Na₂Ti₃O₇ phase. Among two different Na⁺ diffusion pathways, one is kinetically more favorable but energetically less favorable site and the other is kinetically less favorable but energetically more favorable site. Under the high discharge–charge condition, some of Na⁺ ion uptake in the layered structure of Na₂Ti₃O₇ and structural integrity of tunnel structure of Na₂Ti₆O₁₃ resulted in excellent high-rate performance and long-term cyclability in the hybrid.

ACKNOWLEDGEMENTS

This research was supported by a National Research Foundation of Korea (NRF) grant provided by the Korean Government (MSIP) (No. 2016R1A2B3008800, NRF-2018R1A6A3A01012498).

REFERENCES

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The driving range and cost of electric vehicles is severely limited by the energy density of the state of art Li-ion batteries. The energy density of Li-ion batteries can be improved by replacing graphite anodes with lithium metal anode. The US Department of Energy targets for lithium metal batteries are cost < 100 $/kWh, gravimetric energy density > 500 Wh/kg, volumetric density > 800 Wh/L and have a charging time of 15 min. Lithium metal anodes suffer from dendrite/mossy formation and low cycling efficiency. It has been shown that forming stable SEI layers on top of lithium metal anodes can stop dendrite formation.^[i],[ii] Electrolyte components play a crucial role in formation of stable SEI and also the SEI morphology, as the SEI components primarily comprise of decomposition products of the solvent, the salt and the additives in the electrolyte.

In this work, we will discuss a new approach of forming a stable SEI layer with sufficient ionic conductivity and mechanical strength to suppress dendrites . The battery under cycling conditions will unavoidably form cracks in the SEI layer. Our approach is to provide a persistent source for self-healing of these cracks formed in the self-formed SEI during battery cycling. This is achieved by choosing appropriate solvents, salt anions and additives in the battery, which will form the same SEI layer during a forming step and during cycling of the battery. Using the appropriate solvent, salt and additives in the desired proportion leads to a spontaneously formed SEI layer comprised of specific components such as LiF, LiOH, Li₂O, Li₂CO₃, Li₂SO₃, Li₂S. The design principles associated with the selection of the electrolyte additives and salt components will be discussed. The effectiveness of the different SEI components will also be compared. We will quantify the amount of electrolyte components required for self-healing during entire cycle life. An important consideration is that the dissolved species in the electrolyte should also be stable to against high voltage Li-ion battery cathodes. Thus the high voltage stability of the electrolytes will also be shown. We will also explore the ion conduction pathways in SEI through the interfaces formed by the different SEI components. We will also discuss formation of stable SEI in the context of thin lithium foils (< 20 mm) and anode free batteries. Lastly the effect of the current collector in the context of anode free batteries will be discussed.

[i] Xin-Bing Cheng, Rui Zhang, Chen-Zi Zhao, and Qiang Zhang. Toward safe lithium metal anode in rechargeable batteries: a review. Chemical reviews, 117(15):10403–10473, 2017.

[ii] Dingchang Lin, Yayuan Liu, and Yi Cui. Reviving the lithium metal anode for high-energy batteries. Nature nanotechnology, 12(3):194, 2017.

All solid-state batteries have become the focus of next-generation Li-ion battery research and development to meet the ever-growing demands on safe and high-energy-density storage systems. Replacing conventional electrolytes with solids mitigates issues associated with flammable organic liquid and makes safer battery systems. Solid electrolytes which are stable against lithium and resistant to Li dendrite penetration could enable the use of metallic lithium anode, offering a promising pathway to deliver lithium-ion cells with energy densities that significantly exceeds 350 Wh/kg. While each main class of solid electrolytes has their intrinsic challenges, integrating different group of solid electrolytes such as polymers and ceramics has the potential of bringing their individual advantages together and overcoming their drawbacks. In this study, we fabricated a group of hybrid polymer/ceramic electrolytes made from commercially available materials. Their ionic conductivity, electrochemical stability, interfacial stability towards metallic lithium, thermal property, processability and mechanical strength were systematically investigated. Finally, we develop a metrics to screen these hybrid solid electrolytes and identify promising candidates which are most relevant to electric vehicles (EV) applications. Our study illustrates the importance of component integration in developing high-performance solid-state electrolyte. This work serves as a guide to select the appropriate solid electrolytes that can power future electric vehicles.

Room-temperature metal–sulfur batteries have attracted extensive interest because of their advantages of high theoretical capacity, high elemental abundance, and low cost. Towards improving the electrochemical performances of the sulfurized polyacrylonitrile (SPAN) composite cathode in potassium–sulfur batteries (KSBs), an advanced electrode design has been developed by applying a polyacrylic acid (PAA) binder to the SPAN electrode. By integrating the merit of the SPAN composite cathode and PAA binder, the proposed SPAN cell generates a high reversible capacity of 1,050 mA h g^-1 and has excellent cycling stability after 100 cycles (95% retention of the initial cycle) at a high current density of 837.5 mA g^-1 . Ex situ Raman spectra show that the PAA binder is evidently more effective at improving the structural stability of the SPAN electrode than the PVdF binder during cycling. Despite the large volume changes during reduction/oxidation steps in the wide voltage window of 0.1–3.0 V, the SPAN electrode with the PAA binder gave an excellent electrochemical performance in KSBs. To better understand the electrochemical reaction mechanism of SPAN in KSBs, XPS analysis was further performed in a wide discharge cut-off voltage range.

As the production of current-generation energy storage technologies is being scaled up for widescale implementation in electric vehicle and large-scale grid storage, cost and sustainability are becoming more limiting factors for battery utilization than the capacity, which is already sufficient for many applications. Here, the future supply of Li has been pointed out as potentially problematic, [1] with in particular Na, with its similar size, reduction potential and chemistry, but with far greater abundancy and wider global distribution, highlighted as a natural candidate for more sustainable "beyond-Li" energy storage [2].

The notable similarities between Li and Na translate into similarities in the battery chemistries such that the research on Li-ion batteries can be used as a starting point for the development of Na-ion batteries. Unfortunately, these similarities also mean that the combination of high energy densities, high voltages and flammable organic electrolyte solvents that is cause for major safety concerns in Li-ion batteries [3] is also deeply concerning for Na-ion batteries. This can be mitigated by using solid polymer electrolytes, with negligible vapor pressure and inherent mechanical stability, in place of the traditional liquid electrolytes to improve battery safety and stability. These electrolytes are traditionally based on polyethers, in particular poly(ethylene oxide) (PEO) as the ion-solvating and -transporting host material. Unfortunately, for both Li and Na, this material shows poor ionic conductivity at room temperature because of crystallization, in combination with low cation transference numbers [4], requiring elevated temperatures in order to sustain cell cycling.

There are, however, a large range of potential host materials that extends beyond the polyether paradigm and that has been surveyed for Li-based energy storage [5] but has yet to be implemented for Na-ion batteries. We have focused on materials that can coordinate to Na⁺ by means of carbonyl groups. Using the host material poly(trimethylene carbonate) in combination with NaTFSI, fully amorphous electrolytes were obtained that could sustain Na⁺ transport in Na metal cells at 60 °C [6]. Improved stability was seen with NaFSI salt, where also notably high ionic conductivities were attained at high salt concentrations, but at reduced cycling stability. Instead, at moderate salt concentrations, stable cycling over >80 cycles was seen. Finally, using a polyester–polycarbonate copolymer host material, ionic conductivities on the order of 10⁻⁵ S cm⁻¹ were attained in combination with a cation transference number close to 0.5, allowing for room-temperature cycling of solid-state Na-ion full cells.

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In the quest for ever greater storage capacity to satisfy the increasing demand for high-energy-density storage in electric vehicles and large-scale grid storage, anodes comprising Si have the promise of significantly increasing the energy density of Li-ion batteries [1]. Either as a stand-alone active material or as a minor component in a more traditional graphite matrix, Si can through alloying incorporate far more Li than can be intercalated in graphite [1]. This, however, comes at significant volume expansion of the Si particles, resulting in cracking and disintegrating during repeated cycling [2]. This can be counteracted by using nm-sized Si particles which minimizes the impact of severe volume changes during cycling [2].

We have been studying the processing of environmentally benign water-based slurries for Si-containing anodes and note that the Si particle size not only affects the cycling performance, but also significantly affect the processing characteristics. Notably, the rheological properties, i.e., the shear stress and apparent viscosity at the shear rate of electrode coating and the viscoelastic properties, are all determined by the Si particles – even when these are only present as a minor component in a graphite matrix. Whereas µm-sized Si particles result in low-viscosity slurries that are characterized by liquid-like behavior, the slurries containing nm-sized Si particles show both higher viscosity and notable viscoelasticity. These differences are most pronounced at low shear rates, which in the context of electrode processing will affect how the slurry will either flow under its own weight or retain its overall geometry between application and drying. These vastly differing properties appear to be related to the surface area of the Si particles within the slurry, where the polar surface groups of the native oxide on the particles interact strongly with the surrounding water, leading to the observed differences in rheological behavior.

• Thackeray MM, Wolverton C, Isaacs ED. Energy Environ. Sci. 2012; 5: 7854

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Magnesium rechargeable batteries (MRBs) have attracted attentions as next-generation secondary batteries. Mg metal is a promising negative-electrode material because it has a high volumetric capacity (3830 mAh cm^-3) and a low standard potential (-2.37 V vs. NHE). In addition, MRBs ensure a higher safety compared to lithium metal rechargeable batteries, since Mg metal forms no dendrite. We previously reported that g-butyrolactone (GBL)-based electrolyte solutions allowed the electrochemical deposition/dissolution reactions of Mg and the insertion of Mg²⁺ into Chevrel phase Mo₆S₈ positive-electrodes. Mo₆S₈ is a prototypical active material that has a wide path for rapid diffusion of Mg²⁺. However, it works at extremely low potentials (1.1 V vs. Mg²⁺/Mg), and has a small theoretical capacity (129 mAh g^-1). On the other hand, MoS₂, a popular member of metal dichalcogenides, has a larger theoretical capacity (223.2 mAh g^-1) at a higher working voltage around 1.8 V ¹⁾. In this work, we synthesized MoS₂ nanoparticles with a short diffusion path of Mg²⁺, and investigated the charge and discharge properties of AZ31|MoS₂ cells using 0.3 mol dm^-3 Mg(CF₃COO)₂ in GBL as the electrolyte solution.

MoS₂ nanoparticles were synthesized by a solvothermal method ²⁾. Mo(CO)₆ powder and sulfur powder were mixed into 120 ml 2-propanol as a solvent. The solutions were stirred under heating at 80^oC for 10 h, and then the solution was cooled to room temperature naturally. The dark powders were anneled at 800^oC for 2h under Ar atmosphere. The resultant powder was characterized by an X-ray diffraction and Raman spectroscopies. The morphology was observed with a scanning electron microscope. Electrochemical measurements were conducted at a constant current of ca. 5-7 mA cm^-2 at 100^oC using a three-electrode cell. The counter and reference electrode were AZ31 foil (Mg alloy). After the initial discharge and charge, the MoS₂ electrode was analyzed by Raman spectroscopy.

The resultant MoS₂ was impurity-free, and spherical in shape with a diameter of 80-100 nm. The lattice fringe of 0.623 nm corresponds to the spacing of the MoS₂ (002) planes. The Raman bands at 378 cm^-1 and 405 cm^-1 were clearly observed, corresponding to in-plane vibrational E¹_2g mode and out-of-plane vibrational A_1g mode, respectively ³⁾. The initial discharge capacity of AZ31|MoS₂ cell was ca. 180 mAh g^-1. After the intial discharge, the Raman bands shifted to higher frequencies of 382 cm^-1 and 407 cm^-1, which is quite similar to those for the insertion of Li⁺ into MoS₂ ⁴⁾. After initial charge, each Raman band returned to its original position. These results indicated that the insertion/extraction reactions of Mg²⁺ at the MoS₂ occur in the GBL-based electrolyte solutions.

References:

1) Liang, et al., Adv. Mater., 23, 640-643 (2011).

2) Tao, et al., CrystEngComm, 14, 3027-3032 (2012).

3) Lee, et al., ACN Nano, 4 (5), 2695-2700 (2010).

4) Wang, et al., PNAS, 49, 19701-119706 (2013)

Silicon is a promising anode material for Li-ion batteries¹. Silicon's ability to alloy with lithium results in a theoretical capacity ten times higher than that for graphite. However, since this high capacity involves several lithium ions per silicon atom, silicon will expand during lithiation and contract during delithiation. The volume changes causes the solid-electrolyte interphase (SEI) to crack, leading to continuous SEI formation. This eventually results in electronic insulation of the Si particles and large amounts of consumed lithium². Hence, there is a need for an electrolyte composition that forms an SEI with increased ability to withstand the volume changes.

By changing the electrolyte salt, the reactions at the interphase between electrode and electrolyte also changes. Inspired by the work of Philippe et al.³, we have evaluated lithium bis(fluorosulfonyl)imide (LiFSI) as an alternative electrolyte salt to the commercial lithium hexafluorophosphate (LiPF₆). Promising results were achieved with LiFSI as electrolyte salt in half-cells with micron-sized silicon as active material against circular Li foil as counter electrode.

The silicon based anodes used are made of 60 wt% Si (Silgrain®, e-Si 400, a commercially available battery grade silicon from Elkem), with an average particle size of 3 µm, 10 wt% graphite (Timcal, KS6L), 15 wt% carbon black (Timcal, C-Nergy C65, CB) and 15 wt% Na-CMC binder (Sigma Aldrich Mw ~90000). Slurries were cast onto dendritic copper foil and the electrodes were cycled in 2016 coin cells. The reference electrolyte composition is 1M LiFSI in EC:PC:DMC (1:1:3) + 5 wt% FEC and 1 wt% VC.

In this work, the performance of the silicon anodes are evaluated in full-cells with NMC as cathode. Performance in the reference electrolyte is compared to when LiPF₆ is the electrolyte salt. In addition, the effect of increasing the LiFSI concentration is evaluated. The results include electrochemical performance and post mortem characterization of the silicon electrodes. The post mortem characterization includes XPS and cross sectional analysis to investigate how the electrolyte compositions affects the SEI. Cross sectional analysis is performed using focused ion beam in combination with scanning electron microscopy.

[1] J. Ling et al. Journal of the Electrochemical Society, 154 (3), A156-A161, 2007.

[2] H. Wu et al. Nano Today, 7, (5), 414-429, 2012.

[3] B. Philippe et al. Journal of the American Chemical Society, 135, 9829-9842, 2013.

Aqueous metal-air secondary batteries, such as zinc-air secondary batteries, are attractive power sources for large scale energy storage systems since these battery systems potentially satisfy high energy density, high safety standard and low cost. However, large overpotential in bifunctional air electrodes hinders practical applications of the systems.¹ In order to reduce the overpotential, plenty of effective reaction sites is necessary as well as highly active bifunctional electrocatalysts. Though it is widely known that relatively many reaction sites are formed in porous gas diffusion electrodes composed of hydrophobic gas diffusion layer and mildly hydrophilic catalyst layer, the overpotentials in the porous gas diffusion electrodes are not low enough for the practical application. Further reduction of the overpotential in the porous gas diffusion electrodes requires not only empirical knowledge but also construction guidelines based on fundamental properties of the reaction sites. However, the microstructures of the porous gas diffusion electrodes are too complicated to be analyzed in detail.

In this work, we use a partially immersed electrode system as a model of the porous gas diffusion electrodes to investigate the fundamental properties of the reaction sites. This model system consists of a smooth planer or cylindrical electrode and an electrolyte solution covering the electrode.^2-4 Owing to the simple structure of this system, mass transports of the ions and the gases can be easily simulated. In addition, we applied a platinum segmented electrode (Fig. 1(a)) and homemade multichannel current-voltage converter to the partially immersed electrode system to measure current distribution on the electrode directly.

First, we measured AC impedance between adjacent segments to characterize electrolyte solution covering the electrode. While blocking electrode behavior was observed at CHs 1-3, transmission line-type frequency dependences were observed at CHs 4-10. This indicates that CHs 1-3 were covered with relatively thick electrolyte solution, so-called meniscus, and CHs 4-10 covered with thin liquid electrolyte film. Curve fittings indicated that ionic resistance of the film was almost same on CHs 4-10, and the thickness of the film was calculated to be around 3 μm from the conductivity of the electrolyte solution. Figure 1(b) shows local current distributions on the partially immersed platinum segmented electrode with constant current (±0.400 mA) applied for oxygen reduction reaction (ORR) and oxygen evolution reaction(OER). In the case of ORR, larger currents were observed at CHs 4-6. On the other hand, relatively lower currents were observed at CHs 4-10 for OER. These results show that electrochemically active regions were different for ORR and OER. Therefore, two different regions, hydrophobic region and hydrophilic region, should be formed in the catalyst layer of the porous gas diffusion electrodes to reduce the overpotential for both ORR and OER.

Based on this finding, porous gas diffusion electrodes are prepared and the electrochemical properties will be presented at the meeting.

References

1.H. Arai, S. Müller, and O. Haas, J. Electrochem. Soc., 147, 3584 (2000).

2.F. G. Will, J. Electrochem. Soc., 110, 145(1963).

3.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, J. Electrochem. Soc.,162, A1646 (2015).

4.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, Electrochem. Commun.,84, 53 (2017).

5.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, Chem. Lett., 47, 171 (2018).

Figure 1

The main roles of the secondary battery separator are the active movement of lithium-ion through pores, prevention of physical contact between anode and cathode, and prevention of electrical isolation. The currently used separator is PE film. PE films have the advantage of being cheap, but they are weak at high temperatures. By the way, Polybenzimidazole is well known for its superior heat resistance and mechanical properties. Due to an excellent various property, PBI has been used in a lot of fields such as gas separation, OSN and secondary battery membrane. This work is focused on fabrication of Polybenzimidazole(PBI) using Electrospinning and modified the PBI membrane as hydrophilic using 4-(chloromethyl) benzoic acid(CMBA) and Sulfuric acid. The membrane was crosslinked by α,α'-Dibromo-p-xylene. To observe morphology, scanning electron microscope(SEM) was used and to check the hydrophilicity, contact angle test was performed. In case of separator properties, cycling test of those membranes was conducted.

Development of advanced high energy density lithium ion batteries is important for promoting electromobility. Making electric vehicles attractive and competitive compared to conventional automobiles depends on the availability of reliable, safe, high power, and highly energetic batteries whose components are abundant and cost effective. Nickel rich Li[Ni_xCo_yMn_1-x-y]O₂ layered cathode materials (x > 0.5) are of interest because they can provide very high specific capacity without pushing charging potentials to levels that oxidize the electrolyte solutions. However, these cathode materials suffer from stability problems. We discovered that doping these materials with tungsten (1 mol%) remarkably increases their stability due to a partial layered to cubic (rock salt) phase transition. We demonstrate herein highly stable Li ion battery prototypes consisting of tungsten-stabilized Ni rich cathode materials (x > 0.9) with specific capacities > 220 mAh g^-1. This development can increase the energy density of Li ion batteries more than 30% above the state of the art without compromising durability.

• R. Dahn, U. von Sacken, M. W. Juzkow, and H. Al-Janaby, J. Electrochem. Soc., 1991, 138, 2207.

• -U. Woo, B.-C. Park, C. S. Yoon, S.-T. Myung, J. Prakash, and Y.-K. Sun, J. Electrochem. Soc., 2007, 154, A649.

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Following up the great success of lithium ion batteries (LIBs) in the field of consumer electronics in the recent decades, LIBs are today also widely used as energy storage devices for various types of electric vehicles (EVs). However, it is believed that a substantial breakthrough of the electromobility can only take place, if EVs are able to achieve driving ranges of more than 500 km and, therefore, gaining greater consumer acceptance. To achieve these driving ranges, the energy content of the used LIBs needs to be increased to values of 350 Wh/kg and 750 Wh/L at the cell level. These improvements can be achieved by replacing the state-of-the-art graphite anode by high-capacity anode materials like silicon (Si), which is able to provide a nearly 10 times higher specific capacity compared to graphite.[1] However, due to huge volume changes of up to 300% upon lithiation/de-lithiation, Si-based anodes often suffer from poor cycling performance related to cracking and pulverization of the Si particles and continuous solid electrolyte interphase (SEI) re-formation.

Typically, these issues are addressed by decreasing the size of the active material particles to the nanoscale (i.e. nanoparticles, nanowires or thin films) or the fabrication of composites like intermetallic Si phases or Si/carbon composite materials to decrease the overall volume changes.[2] Although these strategies are reported to be effective to improve the performance of Si electrodes, it was revealed that the dominating failure mechanism of full cells containing Si-based negative electrodes lies in the consumption active lithium from the cathode related to continuous electrolyte reduction at the Si negative electrode. The addition of appropriate electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) can significantly improve the capacity retention and Coulombic efficiency of Si-based full cells by forming a more stable SEI on the Si negative electrode.[3, 4] Recently, Krause et al. demonstrated that the addition of carbon dioxide in the form of dry ice significantly improved the performance of Si-based full cells and even outperformed the most commonly used additive FEC.[5] However, the exact working mechanism of the carbon dioxide additive and especially the reductive decomposition reactions including the resulting morphology and/or chemical composition of the formed SEI layer are still uncertain.

Within this study, thin film Si electrodes prepared via magnetron sputtering are employed as model electrodes (i.e. no binder or conductive additive) therefore, possible effects of the carbon dioxide can be correlated to the Si active material. These Si thin film electrodes are coupled with NMC-111 positive electrodes to Si/NMC-111 full cells, which are used for the electrochemical investigations. As the solubility of carbon dioxide in the organic carbonate based electrolytes is rather low, diethylpyrocarbonate is added to the baseline electrolyte, as pyrocarbonates are known as carbon dioxide generators and, therefore, the carbon dioxide is released in-situ within the assembled full cell. The addition of the diethylpyrocarbonate electrolyte additive leads to significantly improved performance of the Si/NMC-111 full cells in terms of Coulombic efficiency and capacity retention full cells during prolonged cycling compared to the baseline electrolyte. The chemical composition of the SEI layer formed on the Si negative electrodes is investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS investigations reveal that the improved performance of the carbon dioxide containing cells is related the improved passivation of the negative electrode by the formation of mainly lithium carbonate at the Si surface.

References:

[1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198.

[2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502.

[3] R. Nölle, A.J. Achazi, P. Kaghazchi, M. Winter, T. Placke, Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells, ACS Applied Materials & Interfaces, 10 (2018) 28187-28198.

[4] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li_1.03(Ni_0.5Co_0.2Mn_0.3)_0.97O₂/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887.

[5] L.J. Krause, V.L. Chevrier, L.D. Jensen, T. Brandt, The Effect of Carbon Dioxide on the Cycle Life and Electrolyte Stability of Li-Ion Full Cells Containing Silicon Alloy, Journal of The Electrochemical Society, 164 (2017) A2527-A2533.

Lithium-ion batteries (LIBs) are emerging as one of the most promising power sources for portable devices, as well as electric vehicles (EVs) and large energy storage systems (ESS), owing to their high energy density and excellent cycle life. However, the application of LIBs in EVs to reduce CO₂ emissions in internal combustion engine vehicles (ICEVs) involves some critical technical challenges, such as the driving range, safety, and cost concerns due to the properties of LIBs.^1-3 For this reason, research interests and industrial efforts have focused on developing low-cost, high-energy, and safe cathode materials, which are the main component determining LIB performance and cost. Transition metal layered oxides belonging to the LiNi_xCo_yAl_zO₂ (NCA) or LiNi_xCo_yMn_zO₂ (NCM) family, which can increase their intrinsic capacity (above 200 mAh g^-1) by increasing Ni content, are emerging as promising new cathode materials.⁴ However, the intrinsic drawbacks of Ni-rich cathodes, such as low Li intercalation stability over extended cycles, should be improved. In this study, we present the first successful boron doping strategy for a Ni-rich cathode material with very high Ni content. For the boron doping, we used a simple method where only a small amount of B₂O₃ was added during the calcination process. In addition, the structural and morphological stability of the boron-doped Ni-rich NCM cathode material was investigated before and after testing to evaluate the improved electrochemical performance; specifically, the significantly enhanced cycle performance. Thus, we conclude that the boron doping strategy is an effective and simple way to mitigate the poor stability problem of Ni-rich cathode materials for high-energy and stable LIBs.

References

• D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater.2017, 29, 5048.

• J. H. Lee, C. S. Yoon, J.-Y. Hwang, S.-J. Kim, F. Maglia, P. Lamp, S.-T. Myung, Y.-K. Sun, Energy Environ. Sci.2016, 9, 2152.

• S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett.2017, 2, 196.

• K.-S. Lee, S.-T. Myung, K. Amine, H. Yashiro, Y.-K. Sun, J. Electrochem. Soc.2007, 154, A971.

Lithium-ion batteries (LIBs) are considered to be the most promising power sources for electric vehicles (EVs) because of their high energy densities and long cycle lives. However, major impediments for general public acceptance of EVs are their cost, durability, and driving range.^1,2 Following the successful application of the hybrid scheme to the NCM cathodes,^3-6 we developed a core–shell with concentration gradient Li[Ni_0.865Co_0.120Al_0.015]O₂ cathode material (CSG NCA) with a Ni-rich core to maximize the discharge capacity and a Co-rich particle surface to provide structural and chemical stability. Compared to the conventional NCA cathode with a uniform composition, the gradient NCA cathode exhibits improved capacity retention and better thermal stability. Even more remarkably, the gradient NCA cathode maintains 90% of its initial capacity after 100 cycles when cycled at 60 °C, whereas the conventional cathode exhibits poor capacity retention and suffers severe structural deterioration. The superior cycling stability of the gradient NCA cathode largely stemmed from the gradient structure combines with the Co-rich surface, which provides chemical stability against electrolyte attack and reduces the inherent internal strain observed in all Ni-rich layered cathodes in their charged state, thus providing structural stability against the repeated anisotropic volume changes during cycling. The high discharge capacity of the proposed gradient NCA cathode extends the driving range of electric vehicles and reduces battery costs. Furthermore, its excellent capacity retention guarantees a long battery life. Therefore, gradient NCA cathodes represent one of the best classes of cathode materials for electric vehicle applications that should satisfy the demands of future electric vehicles.

References

• U.S. Department of Energy, EV Everywhere Grand Challenge Blueprint, 31 January 2013.

• S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett.2017, 2, 196.

• Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak, K. Amine, Nat. Mater.2009, 8, 320.

• Y.-K. Sun, Z. Chen, H.-J. Noh, D.-J. Lee, H.-G. Jung, Y. Ren, S. Wang, C. S. Yoon, S.-T. Myung, K. Amine, Nat. Mater.2012, 11, 942.

• B.-B. Lim, S.-J. Yoon, K.-J. Park, C. S. Yoon, S.-J. Kim, J. J. Lee, Y.-K. Sun, Adv. Funct. Mater.2015, 25, 4673.

• D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater.2017, 29, 5048.

Investigation of solid electrolyte interphase (SEI) film thickness on a mesophase graphite electrode in the lithium-ion battery is successfully demonstrated by the ex-situ small-angle neutron scattering (SANS) technique for the MGP and the FMGP anodes, after the first charge state and analyzing it precisely by the Guinier–Porod model. SANS data revealed a stable, maximum (150 nm) bi-layer SEI formed on MGP anode (in EC/DMC) at a capacity of 50 mAhg^-1 and sluggish above 100 mAhg^-1. The SEI formed on the FMGP with and without 3% FEC (in EC/DMC) shows the thickness of 220 nm and 245 nm, respectively, whereas the solvent contains only EC/DEC produce thin SEI of 140 nm. Our primary results make evident that SANS could be employed to better understand the complex microstructure SEI formation and its accurate thickness, on a mesophase graphite anode.

Lithium diffusion phenomenon in negative electrodes for Li-ion batteries were studied by using a cold triple-axis spectrometer in-operando neutron scattering. The study on the lithium diffusion mechanism of Li⁺ diffusion in LiC₆ , LiC₁₂ of the MGP and the electrolyte additives (as venylene carbonate, VC) at low temperature (253K or 268K) were very importance for the lithium plating and long cycle. Therefore, SIKA is the instrument good to conduct an experiment at low temperature to understand the Li⁺ diffusion effect in the cell of NMC/graphite LIB at different temperature in order to give an ideal to improve the battery design and to resolve the lithium plating issue in the low temperature which regarding to the safety. From the qualitative analysis of the phase transition as a function of time and temperature, considering the (00l) peak intensities of graphite to extract the diffraction peaks corresponds to the relative fractions of LiC₆ and LiC₁₂ which in turn were related to the amount of Li diffusion through the SEI and the lithium intercalation takes place in the anode.

From the spectra, we obtained the intensity of LiC₆ diffraction plane is increased with increasing charging capacity yielded with the value of 2064 and 1757 mAh at 298 and 268 K, respectively. While, the intensity of graphite phase is firstly shift from 40º to 39.5º then LiC₁₂ and LiC₆ phases were observed with obvious increased intensity. The integrated intensity of LiC₆, and LiC₁₂ reflections as a function of time varies with different temperature. At initial charging condition in Fig. 1, the intensity of pure graphite plane centred at 40º is large and also observed the generation of small amount of LiC_x (37.1º, LiC₆ and 38.9º, LiC₁₂) compounds.

This process is observed to be reverse in the case of discharging the cell at 298 and 268 K. At 298 K, decrease in the voltage (3.99 to 2.80 V), the amount of LiC₆ is slowly disappears and the amount of LiC₁₂ is maximum, finally observed the graphitic peak with small low angle shift, which confirms the presence of small Li⁺ into the graphite layer/surface, in the discharge condition. In the case of discharging at 268 K, the LiC₆ plane is quickly fading from 3.80 V and disappear. The ratio of capacity of the cell was 86% for 268 K compare with 298 K, which was not consistence with the quantity of LiC₆ plane. We also establish the neutron diffraction analysis for the samples prepared with different additives and the results were discussed in detail. In the future, we will also pay attention to the formation of lithium plating which is highly related to the battery safety.

The chemical etching of silicon has been investigated thoroughly during the past 40 years to achieve a fast and controlled process for texturing surfaces. Nonetheless, still little is known about the electrochemical reaction of silicon with KOH for the use in batteries. The high theoretical capacity density of 3817 mAh cm^-2 and potential of 2.09 V make the silicon-air system a promising candidate for energy storage. However, a crucial constraint to the use of an aqueous alkaline electrolyte, such as KOH, is related to the fact that the system discharges only once. Overpotentials reduce the practical potential to 1.4 V, and the competing chemical corrosion of the silicon leads to a strong self-discharge, resulting in a low conversion efficiency. The discharge ends with a sudden potential drop, due to the passivation of the silicon surface before the complete consumption of the electrode. We believe that understanding the mechanisms behind that potential drop and the processes that lead to it are essential to the further improvement of this silicon-air battery. Therefore, we studied cells with different KOH concentrations and with flat phosphor-doped silicon (100) wafers as the Si-electrodes. To investigate the potential drop and increase the conversion efficiency of silicon electrodes, we performed electrochemical measurements in a full cell setup. Galvanostatic discharge tests revealed that the ratio of electrochemical to chemical reaction is influenced by the KOH concentration when the battery is discharged till the passivation. While the highest etching rate for silicon in KOH solution reaches its maximum around 6 mol L^-1 KOH, the electrochemical reaction seems to have a peak at 2 mol L^-1 KOH in our experimental conditions. We compared the performance of cells with various concentrations of pre-dissolved silicon in KOH and we found a correlation between the silicon content in the electrolyte and the time before the potential drop occurred. The conductivity of the solution decreases with increasing silicon content, probably as a result of the presence of a network of silicate species, which might hinder the diffusion of the Si(OH)₄ species. To prove this hypothesis, we tested different volumes of the electrolyte with the same cell parameters. The use of an excess of electrolyte solution enabled the complete consumption of the 500 μm Si-electrode in any KOH concentration used. During these experiments, the silicate species do not reach the critical concentration, and the reaction products can dissolve into the bulk electrolyte, not leading to the formation of a passivation layer.

Figure 1

Interest in solid-state electrolytes has grown rapidly in recent years owing to the desire to utilize lithium metal anode for improved specific energy density and the inherent safety advantage of a non-flammable solid state electrolyte. A practical solid electrolyte for energy storage must be a fast ion conductor, have negligible electronic conductivity and adequate chemical and electrochemical stability with electrodes. For ease of manufacturing and potentially higher power capability we have focused research on oxide ceramic electrolytes. This work will cover our work on NASICON, perovskite, garnet and other potential structural families including how to stabilize the most conductive phase and maximize the ionic conductivity through substitutional chemistry. Our main focus on garnet will include how to understand important mechanical properties, the air and moisture stability and stability with cathode materials, improvement of critical current density and future challenges and opportunities for garnet based fast lithium ion electrolytes and potential new materials.

In recent years, reversible redox activities of both transition-metal (TM) cations and oxygen anions were found to be feasible in a number of Li-excess transition-metal oxides, enabling significant enhancement in charge storage capacity of lithium-ion battery (LIB) cathodes. [1-3] Most high-capacity cathode materials reported in the literature are either layer-structured similar to the well-studied Li- and Mn-rich (LMR) oxides, or cation-disordered rock-salts. For the layered LMR, repeated cycling at high voltages is known to cause significant voltage and capacity fade, hysteresis, and impedance rise. Understanding the origin of these issues has been on-going for well over a decade, and various degradation mechanisms have been proposed. [4-5] Likewise, obtaining high capacity in cation-disordered rock-salt oxides often comes at the expense of cycling stability that becomes progressively worse with either deeper oxidation of oxygen at higher potential or extended cycling involving oxygen redox. It is unclear, however, what properties and/or processes may have caused the performance issues in this newer class of cathode materials with 3D Li migration pathways.

In this presentation, we use cation-disordered Li_1.3Nb_0.3Mn_0.4O₂(LNMO), a compound recently reported to have an impressive discharge capacity of ca. 300 mAh/g at 60 ^oC [6], as a baseline system to investigate the chemical and structural origins of performance deterioration. We show that extensive reduction of the redox active TM occurred both in the bulk and on the surface of the cycled oxide particles. In contrast to what was reported on the layered LMR, TM reduction was not accompanied by phase transition due to cation site migration. We further propose a cathode degradation mechanism and explore design strategies to balanced capacity and stability. As both Li and TM cations share the same crystallographic sites in cation-disordered rock-salts, the oxide chemistry was manipulated to influence the contribution of TM and O redox.The effect of cation and/or anion substitutions in stabilizing the oxide cathodes will be discussed.

References

• J. Lee, A. Urban, X. Li, D. Su, G. Hautier and G. Ceder, Science 2014, 343, 519.

• M. Sathiya, G. Rousse, K. Ramesha, C. P. Laisa, H. Vezin, M. T. Sougrati, M. L. Doublet, D. Foix, D. Gonbeau, W. Walker, A. S. Prakash, M. Ben Hassine, L. Dupont and J. M. Tarascon,Nat. Mater.2013, 12, 827.

• P. E. Pearce, A. J. Perez, G. Rousse, M. Saubanere, D. Batuk, D. Foix, E. McCalla, A. M. Abakumov, G. Van Tendeloo, M.-L. Doublet and J.-M. Tarascon, Nat. Mater.2017, 16, 580.

• 4.J. R. Croy, M. Balasubramanian, K. G. Gallagher andA. K. Burrell, Acc. Chem. Res.2015, 48, 2813.

• W. E. Gent, K. Lim, Y. Liang, Q. Li, T. Barnes, S.-J. Ahn, K. H. Stone, M. McIntire, J. Hong, J. H. Song, Y. Li, A. Mehta, S. Ermon, T. Tyliszczak, D. Kilcoyne, D. Vine, J.-H. Park, S.-K. Doo, M. F. Toney, W. Yang , D. Prendergast and W. C. Chueh, Nat. Commun. 2017, 8, 2091.

• N. Yabuuchi, M. Takeuchi, M. Nakayama, H. Shiiba, M. Ogawa, K. Nakayama, T. Ohta, D. Endo, T. Ozaki, T. Inamasu, K. Sato and S. Komaba, Proceedings of the National Academy of Sciences2015, 112, 7650.

Acknowledgment

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

The class of Li-Mn-rich layered transition metal oxides (LMR-NCM) is one of the most promising candidates for cathode materials in advanced lithium ion batteries (LIBs) due to their high specific discharge capacity of up to 300 mAh/g and specific energy up to 1000 Wh/kg at material level.^[1-4] Furthermore, those materials contain a high content of low-cost manganese due to the addition of a Li₂MnO₃ component compared to standard layered transition metal oxides, which at the same time enables the access of additional capacity. Nevertheless, a strong capacity and voltage fade as well as a poor rate capability plagues the class of LMR-NCM materials. The pronounced reversible oxygen redox activity (causing the voltage hysteresis), the transition metal migration and a phase transition to a spinel-like phase (causing the voltage fade) add to an even stronger fade of the specific energy and inhibit the commercialisation of these cathode materials so far.

To closely study the underlying phenomena, LMR-NCM oxides with different morphologies are synthesized. Therefore, the syntheses via coprecipitation in a Couette-Taylor flow reactor and in a conventional flask are compared. The constant monitoring of critical parameters during the synthesis by means of the reactor leads to dense spherical particles with a narrow particle size distribution. These characteristics offer a spherical cathode material with enhanced capacity retention due to a more compact structure with a lower surface area. Also, taking the voltage fade into consideration allows a more objective evaluation of LMR-NCM oxides materials especially in direct comparison with state-of-the-art NCM-based cathodes.

[1] E. M. Erickson, F. Schipper, T. R. Penki, J.-Y. Shin, C. Erk, F.-F. Chesneau, B. Markovsky, D. Aurbach J. Electrochem. Soc. 2017, 164, A6341 – A6348.

[2] B. Qiu, M. Zhang, L. Wu, J. Wang, Y. Xia, D. Qian, H. Liu, S. Hy, Y. Chen, K. An, Y. Zhu, Z. Liu, Y. S. Meng Nat. Commun. 2016, 7, 12108.

[3] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny J. Mater. Chem. A 2015, 3, 6709 – 6732.

[4] T. Placke, R. Kloepsch, S. Dühnen, M. Winter J. Solid State Electrochem. 2017, 21, 1939 – 1964.

Over recent years, our group has demonstrated that the incorporation of a spinel (S) component into 'layered-layered' (LL) composite electrode structures serves, as a stabilizing unit, to slow transition metal migration during electrochemical cycling.[1] However, unraveling the mechanism by which the S component operates in actual 'layered-layered-spinel' (LLS) materials is a challenging task because of the highly complex, inhomogeneous arrangements of the cations (typically Li, Mn, Ni, and Co) within multiple nano-domains. Moreover, relatively little is known about the structure and electrochemical properties of Co-based spinel materials, which were first reported in early 1990s[2,3], compared to Mn-based spinels. The lithiated spinel Li₂Co₂O₄ is particularly attractive as a stabilizing agent in LLS composite electrodes for two reasons. First, cobalt has a lower propensity to migrate during electrochemical Co^3+/4+ redox reactions and, second, lithium extraction from Li_2−zCo₂O₄ (0 ≤ z ≤ 1), occurs at a potential (∼3.6 V) that is significantly higher than that of its lithiated manganese-oxide spinel analogue, Li₂Mn₂O₄ (∼2.9 V). In this presentation, we will discuss the structure and electrochemical properties of substituted lithiated spinel materials that are easier to produce as a single phase, thereby eliminating the propensity for Li₂Co₂O₄ samples to be contaminated by some layered LiCoO₂.

References

[1] M.M. Thackeray, J.R. Croy, E. Lee, A. Gutierrez, M. He, J.S. Park, B.T. Yonemoto, B.R. Long, J.D. Blauwkamp, C.S. Johnson, Y. Shin, and W.I.F. David, "The quest for manganese-rich electrodes for lithium batteries: strategic design and electrochemical behavior," Sustainable Energy & Fuels2, 1375-1397, (2018).

[2] R.J. Gummow, M.M. Thackeray, W.I.F. David, S. Hull, "Structure and electrochemistry of lithium cobalt oxide synthesized at 400^oC," Mater. Res. Bull.27, 327-337, (1992).

[3] E. Lee, J. Blauwkamp, F.C. Castro, J. Wu, V.P. Dravid, P. Yan, C. Wang, S. Kim, C. Wolverton, R. Benedek, F. Dogan, J.S. Park, J.R. Croy, M.M. Thackeray, "Exploring Lithium-Cobalt-Nickel Oxide Spinel Electrodes for ≥3.5 V Li-Ion Cells," ACS Appl. Mater. Inter.8, 27720-27729, (2016).

The application and performance of Li based batteries is often hindered by their electrolytes. These electrolytes have seen a relatively small amount of change since their commercialization and often limit both the energy density and low-temperature operation of devices. Here we use electrolytes based on solvent systems which are typically gaseous under standard conditions and show dendrite-free long-term cycling with high coulombic efficiencies of Li-metal anodes, as well as enhanced compatibility with high voltage cathodes. Importantly, the performance of the electrolyte is well maintained over a wide temperature range down to -60^oC. Finally, we demonstrate full cell performance of systems utilizing the liquefied gas electrolytes in standard 18650 jelly-roll form factors with safety features unique from conventional Li-ion systems.

Active cathode materials in lithium ion batteries are often based on transition metals like cobalt or nickel. High cost and toxicity as well as the mining and fabrication under ethically questionable conditions are serious drawbacks of these metals used in state-of-the-art materials. On the route towards a "greener" and more sustainable lithium ion technology, alternatives such as organic and hybrid inorganic/organic active materials obtained increasing attraction in the last years. [1] One promising hybrid material class are metal-organic frameworks (MOFs). Due to their high surface area and well-defined porosity as well as their flexible and designable structure MOFs have received increasing attention over the last decades for different potential applications like gas storage or catalysis. More recently, this promising class of materials has also been investigated for energy storage devices as electrode active material. [2]

MOFs consist of inorganic metal-oxo clusters (called secondary building unit, SBU in short) coordinated to multivalent rigid organic molecules (often referred as "linker"), which can be modified with a variety of functional groups forming a crystalline porous structure. The well-defined pore size is tailorable by the length of the linker molecules leading to a high flexibility accessible for reversible insertion and removal of guest molecules. The highly porous crystalline structure, especially their large pore size, make them interesting for reversible cation or anion storage, e.g. in the dual-ion battery concept. [3] Furthermore, multivalent metal ions in the SBU as well as organic linker molecules can act as redox-active sites, leading to a promising active material. [4]

Porphyrin-based organic derivates, which occur. e.g. in human blood and vitamin B12, are well known for their catalytic- and redox-activity. In the present work, we synthesize various porphyrin-based MOFs with different coordinated metals, which are successfully applied as an energy storage material in a lithium metal cell and characterized with respect to the structural and surface properties. Combining the redox-active porphyrin derivate, Tetrakis(4-carboxyphenyl)porphyrin (TCPP), and a redox-active metal(oxo-)cluster, a non-toxic and environmentally friendly cathode material was achieved. Constant current cycling and cyclic voltammetry studies reveal a high and reversible redox activity. Using suitable methods such as X-ray diffraction (XRD), the redox reaction behavior of the metal-organic framework and the structural properties of the MOFs were investigated upon charge/discharge operation. Furthermore, the influence of different conductive salts and solvents on the electrochemical performance were analyzed.

References:

[1] D. Larcher; J.-M. Tarascon; Towards greener and more sustainable batteries for electrical energy storage. Nature Chemistry 2015; 7; 19-29.

[2] Wang, L.; Han, Y.; Feng, X.; Zhou, J.; Qi, P.; Wang, B., Metal–organic frameworks for energy storage: Batteries and supercapacitors. Coordination Chemistry Reviews 2016, 307, 361-381.

[3] Aubrey, M. L.; Long, J. R., A Dual−Ion Battery Cathode via Oxidative Insertion of Anions in a Metal–Organic Framework. Journal of the American Chemical Society 2015, 137 (42), 13594-13602.

[4] D'Alessandro, D. M., Exploiting redox activity in metal-organic frameworks: concepts, trends and perspectives. Chemical Communications 2016, 52, 8957-8971.

One of the most pressing challenges modern society faces is how to provide an energy source for a variety of applications, ranging from small portable devices to electric vehicles (EVs) and a large grid-scale system to store energy from intermittent solar or wind-driven devices. Recently, enough technological advancement has been made that Li-ion batteries (LIBs) are considered as the most promising solution to solve this problem. Over the years, diverse synthetic methods have been developed to synthesize battery cathode materials, such as: solid-state, co-precipitation, sol-gel, hydrothermal, and molten salt syntheses. Each of the synthesis process used to make the cathode oxide material proved to be useful, but simultaneously accompanies some major drawbacks. In the traditional solid-state method, the mixing of multiple metal sources is done by grinding or ball milling. This results in a microcrystalline product with long Li diffusion pathways as well as inhomogeneous morphology and metal distribution. Co-precipitation compensates for some of these shortcomings, but it also requires a careful control of pH in the carbonate method or an inert atmosphere to minimize the undesired impurities in the hydroxide method. Previous studies show that the hydrothermal method is an effective way to synthesize cathode material with high crystallinity, but it requires a complex experimental set up which utilizes a stainless steel autoclave to withstand high vapor pressure during its lengthy reaction time. Furthermore, all the methods require a long post annealing process, typically lasting more than 12 hours at high temperature (>800ºC).

Herein, we developed a novel polyol synthetic process to prepare the most promising cathode materials in layered, spinel, and olivine structure. Developed by Fievet group in 1985, polyol-mediated synthesis has been widely used for the past decades, but its scope has been mostly limited to the synthesis of metals and metal oxides. In this work, our group has extended this synthesis method to develop more complex metal oxide used for battery cathode materials. All three of the synthesized materials are monodispersed nanoparticles with dispersive morphology and competitive electrochemical performance. Using a combination of powder x-ray diffraction (XRD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS), we also confirmed its high crystallinity and uniform elemental distribution for all three polyol-synthesized cathode materials. Finally, HAADF-STEM image and electron energy loss spectroscopy (EELS) for cycled NMC material was analyzed to confirm its structural stability even after charge-discharge process. We anticipate that these findings lead to the plethora of new cathode materials that can be explored using this synthetic method.

Figure 1

To simultaneously address the inherent limitations of the capacity and cycling instability of Na[Ni0.5Mn0.5]O2 cathodes, it is necessary to concurrently stabilize particle surfaces and crystal structure. Encouraged by previously conducted research activities in this field, we have adopted an effective strategy for simultaneous MgO coating and Mg doping to produce substantially improved high voltage stability compared to the previously reported results in the literature on Na[Ni0.5Mn0.5]O2 cathodes. The MgO coating layer effectively suppressed the unfavorable side reactions during cycling while the partial Mg doping into the bulk Ni sites improved the structural stability by moderating the extent of the irreversible multiphase transformation. As a result, the combination of a MgO coating with Mg doping provides enhanced electrochemical performance and structural stability of Na[Ni0.5Mn0.5]O2 within the voltage range of 2.0–4.2 V. The practical acceptability of the simultaneous MgO coating and Mg doping of the Na[Ni0.5Mn0.5]O2 cathode was obviously verified using scaled-up pouch-type full cells with hard carbon anodes. Compared with previously reported similar cathode materials, the proposed MgO-NM55 cathode showed great competitiveness for high-voltage stability in SIBs. Moreover, the use of earth's abundant and inexpensive Mg and Na elements, and a simple practical strategy are highly desirable for developing high-energy and low-cost SIBs. Although the practical use of the MgO-NM55 cathode in SIBs will require further work, the methodology used in this study will be helpful in developing an efficient design of high performance cathode materials for SIBs.

References

• J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Soc. Rev., 2017, 46, 3529–3614.

• J.-Y. Hwang, C. S. Yoon, I. Belharouak and Y.-K. Sun, J. Mater. Chem. A, 2016, 4, 17952.

• P.-F. Wang, H.-R. Yao, X.-Y. Liu, J.-N. Zhang, L. Gu, X.-Q. Yu, Y.-X. Yin and Y.-G. Guo, Adv. Mater., 2017, 29, 1700210.

• S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa and I. Nakai, Inorg. Chem., 2012, 51, 6211.

An integral part of promoting the large-scale use of new renewable energy sources is the development of cost- effective energy storage technologies.^1,2 Recently, potassium-ion batteries (KIBs) have attracted widespread interest as a low-cost, alternative energy storage medium to replace the widely used lithium-ion batteries (LIBs) because potassium has higher natural abundance (the reserve of potassium is close to that of Na in the Earth's crust) and a lower standard redox potential than other metallic elements (E°(Li/Li⁺): -3.04 V; E°(K/K⁺): -2.93 V; E°(Ca/Ca²⁺): -2.87 V; E°(Na/Na⁺): -2.71 V; E°(Mg/Mg²⁺): -2.27 V versus the standard hydrogen redox potential).³

Room-temperature metal–sulfur batteries have attracted extensive interest due to their advantages, which include high theoretical capacities, high elemental abundances, low costs, and environmental friendliness.³ We proposed a different type of room-temperature K–S battery composed of a solution-phase potassium polysulfide (K₂S_x) catholyte and a 3D freestanding carbon-nanotube-film (3D-FCN-flm) electrode.⁴ Based on the reversible conversion reactions, K₂S_x (5 ≤ x ≤ 6) → K₂S₃ (discharge) → K₂S₅ (charge), the proposed K-S battery delivered a high discharge capacity of ∼400 mAh/g at 0.1 C-rate with stable cycle retention (94% after 20 cycles) and good rate capability up to 2 C-rate. In addition, instead of an explosive and highly reactive potassium metal electrode, a full cell consisting of an electrochemically potassium-impregnated hard carbon and the K₂S_x (5 ≤ x ≤ 6) catholyte was constructed to demonstrate the feasibility of a safe K-S battery system free of metallic potassium.

References

• J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Chem. Soc. Rev., 2017, 46, 3529–3614.

• J.-Y. Hwang, J. Kim, T.-Y. Yu, S.-T. Myung, and Y.-K. Sun, Energy Environ. Sci., 2018,11, 2821-2827.

• J.-Y. Hwang, S.-T. Myung, and Y.-K. Sun, Adv. Funct. Mater. 2018, 28, 1802938.

• J.-Y. Hwang, H. M. Kim, C. S. Yoon, and Y.-K. Sun, ACS Energy Lett. 2018, 3, 540-541.

Introduction

To increase the energy density of the Li-ion rechargeable batteries, a use of high-voltage cathode materials as well as their stable operating systems are of importance. However, the issues represented by degradation of the cathode materials, electrolytes, and other cell components have obstructed the development of a high-voltage system over past decade. Here, we report that concentrated LiBF₄ in a mixed solvents, propylene carbonate (PC) and fluoroethylene carbonate (FEC), allows a stable cycling 5 V-class Li₂CoPO₄F/graphite full-cell for more than 600 cycles. In the presentation, the optimized cell components with special attention to the compatibility between the carbon conductive agents and electrolytes, will be discussed.

Method

The LiBF₄-based electrolytes were prepared by mixing a certain amount of salt with solvents in an Ar-filled glove box. The Raman measurement was conducted to study the solution structure of the electrolytes. The linear sweep voltammetric (LSV) was performed to evaluate the anodic limits of the electrolytes with three-electrode cells consisting a Pt plate as a working electrode and Li metals as reference and counter electrodes. The carbon compatibility with the electrolytes was estimated using cyclic voltammetric (CV) with two-electrode cells of carbon electrode and Li metal. The transition metals deposited on the cycled graphite electrodes in the LCPF/graphite full-cells after charge and discharge test were analyzed by an Energy-dispersive X-ray spectroscopy (EDS).

Results

As a result of LSV measurement, the anodic current started to flow from 5.8 V (vs. Li/Li⁺) in the concentrated 1:1.8 LiBF₄:PC/FEC (5/5,n/n), which was far higher than that in the commercial electrolyte of 1M LiPF₆ EC/DMC (1/1,v/v). The Raman spectroscopy analysis presented that the concentrated LiBF₄ PC/FEC electrolyte had an aggregate (AGG)-predominant solution structure. An upshift of bands corresponding to the anion and free solvent molecules was observed with an increase of salt concentration, denoting strengthened coordination of anion and solvents with Li⁺. Therefore, it was considered that the highest occupied molecular orbital (HOMO) level of the anion and solvents was decreased since the Li⁺ ion functioned as a strong Lewis acid that resulted in partial electron donation from the anion and solvents to the Li⁺ ion.

Moreover, it was revealed that the main causes of cell deterioration at a high potential were (i) decomposition of electrolyte, (ii) anion intercalation into the carbon additives, and (iii) transition metal dissolution from cathode material. Especially, the carbon conductive agents with a low graphitization degree was selected to increase carbon compatibility with the electrolyte. It was because the anion intercalation occurred easily to the carbon additives with a high graphitization degree, leading to the damage of the graphene layers by continuously exposing the sites where were highly reactive with the electrolyte. Also, it was obvious that an intensified coordination between the anion and Li⁺ in the highly concentrated electrolyte helped to impede the anion intercalation reaction.

The concentrated LiBF₄ PC/FEC electrolyte and the optimized cell design including carbon conductive agents with a low graphitization degree were applied to a 5 V-class Li₂CoPO₄F/graphite system. As a result, a stable charge and discharge for more than 600 cycles was obtained. Besides, it was revealed that the concentrated electrolyte contributed to the stable operation of the high voltage battery by suppressing the transition metal dissolution from the cathodes. In conclusion, on the basis of the various design factors as noted above; (i) improvement of oxidation stability, (ii) impeding the anion intercalation, and (iii) suppression of transition metal dissolution, we succeed to obtain a stable reversible operation of the 5 V-class Li₂CoPO₄F/graphite full-cell for the first time.

Figure 1

The demand for solid-state lithium ion electrolytes has increased owing to the safety concerns regarding the use of lithium ion batteries, which contain flammable organic solvent electrolytes. The liquid electrolyte can give rise to serious problems such as leakage and gas explosions when the operating temperature rises.

All solid-state batteries using organic or inorganic solid electrolytes have a higher energy density than lithium-ion batteries and can reduce the number of parts and packaging space, thereby saving weight and volume while using the same amount of power. To increase the size and, thus, the energy density of the lithium secondary battery while maintaining stability, the use of a solid electrolyte is required in place of the conventional flammable organic liquid electrolyte.

NASICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) ceramics are typical lithium ion conducting materials, with ion conductivities as high as 10^-4 S·cm^-1 at room temperature. In addition, LATP is chemically stable in humidified air or carbon dioxide and is expected to exhibit a high oxidative potential (4.21 V). In this regard, LATP has attracted significant attention as a promising solid electrolyte for all-solid-state lithium batteries.

In addition to high ionic conductivity and chemical stability, the solid electrolyte needs to possess high strength and toughness to allow production of large area thin electrolyte sheets. The poor mechanical properties of ceramic-based solid electrolytes such as LATP or Li₇La₃Zr₂O₁₂ limit their practical application. Incorporating organic fillers with lithium ion conductivity in a ceramic-based electrolyte is an effective way to give not only flexibility but also enhanced mechanical properties.

Dry polymer-based solid electrolytes present various advantages, such as excellent interfacial stability with lithium, the formation of complexes with lithium salts, and flexibility. Various types of polymer-based solid electrolytes exist, such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polyvinylidene fluoride (PVDF). Among them, the PEO-based polymer has attracted considerable attention, owing to its flexible skeleton and good interfacial stability with lithium electrodes. However, the commercialization of PEO is difficult because its room-temperature ionic conductivity is as low as 10⁻⁷–10⁻⁶ S·cm⁻¹. Recently, we found that the solid polymer electrolyte with PEO:PMMA=8:1 and 8 wt% silica aerogel exhibited the high lithium-ion conductivity (1.35 × 10⁻⁴ Scm⁻¹ at 30 °C) and good mechanical stability.

In this study, a polymer blend of PEO and PMMA containing silica aerogel particles was incorporated to the LATP matrix. The PEO/PMMA solid polymer which was dispersed in the LATP skeleton permit flexibility and good bonding between the LATP. The LATP powder was synthesized by the sol-gel technique from lithium nitrate, aluminum phosphate, and titanium iso-propoxide to induce a rapid gelation reaction without further dissolution. The lithium ion conductivity of LATP/polymer composite electrolytes was measured from a symmetrical cell consisting of the LATP electrolyte and Pt blocking electrodes. The effects of the LATP:polymer ratio and the structure of the LATP skeleton such as porosity and grain size on the conductivity were investigated.

Rechargeable magnesium batteries have the advantages of low cost and high theoretical capacity without dendrite formation. However, the Mg batteries have critical problem of sluggish kinetic by the slow solid-state diffusion of highly polarizing divalent Mg-ions in most intercalation hosts. This problem can be overcome by hybrid batteries combining a Mg-metal negative electrode and fast lithium intercalation hosts when the Li⁺ ions are used with Mg²⁺ ions. Although two kinds of cations exist in the electrolyte, the Li ions can be easily intercalated into the host positive electrode materials and the Mg ion can be electroplated and stripped at the negative electrode because of its relatively higher standard reduction potential. However, the Mg metal electrode still have problem of the formation of the blocking layer between the magnesium metal surface and electrolyte. In case of using the Mg(TFSI)₂ and glyme-based electrolyte, the huge overpotential occurs in the Mg stripping reaction during discharging process. We found that the electrolyte additives containing the halogen anion such as Cl^- and Br^- reduced the overpotential of the Mg stripping reaction.

The Li₄Ti₅O₁₂ (LTO) and mechanically polished Mg disk were used the electrode materials and the 1.50 M LiTFSI and 0.15 M Mg(TFSI)₂ in glyme:diglyme=1:1 solution (standard electrolyte) was used as dual salt electrolyte. This cell showed a negative discharging voltage (about -1.0 V) due to the huge Mg stripping overpotential while the charging reaction occurs at 0.8 V. After the addition of 0.1 M LiCl, 0.1 M LiBr, or 0.5 M MgCl₂, the discharging voltage increased to +0.5 V from the 2^nd cycle after activation process during 1^st cycle while the charging voltage was almost close to the halogen-free electrolyte. In spite of the increase of the halogen anion concentration, the further improvement was not detected.

When the HMDS (hexamethyldisilazane) was added to the electrolyte with the LiCl, the overpotential of Mg stripping reaction was sharply decreased from the first cycle without activation process. But the overpotential did not decrease in case of the HMDS addition to the standard electrolyte composition (halogen-free).

Therefore, the Mg/LTO hybrid cell with dual salt electrolyte containing halogen anion and HMDS showed good electrochemical performances with low polarization from the initial cycle.

the capacity of Li-rich layered cathode powder was strongly affected by its particle size. By using sol-gel process, particle size of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ powder obtained is around 200 nm. On the other hand, Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place.

Furthermore, the TEM diffraction analysis of cycled Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ shows the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.

The practical specific capacity of conventional positive electrodes, such as LiCoO₂, remained less than the half of graphite negative-electrodes. Hence, increasing the specific capacity of positive-electrodes effectively improves the energy density of lithium-ion batteries (LIBs). A layer-structured LiNi_0.5Co_0.2Mn_0.3O₂ positive-electrode can deliver a high discharge capacity of ca. 200 mAh g^-1 by charging to around 4.6 V. However, the severe decomposition of conventional electrolyte solutions is a problem at such a high potential. The stability of electrolyte solutions against oxidation can be improved by increasing the concentration, and there are three major problems to be solved toward the practical use; 1) high costs due to an extensive use of lithium salts, 2) high viscosities which make it virtually impossible to inject the electrolyte solutions into cylindrical and laminated cells in the production processes of practical LIBs, and 3) low rate capabilities due to the low ionic conductivity of highly viscous electrolyte solutions. These problems can be settled by diluting the highly concentrated electrolyte solution with an appropriate diluent.

We previously reported that propylene carbonate (PC)- and γ-butyrolactone-based concentrated electrolyte solutions showed high stability against oxidation at LiNi_0.5Co_0.2Mn_0.3O₂ and 5-V class LiNi_0.5Mn_1.5O₄ [1-3]; irreversible capacities due to the oxidative decomposition of electrolyte solution decreased with increasing the electrolyte concentration, while the discharge capacities increased. In particular, in the nearly saturated 7.25 mol kg^-1 LiBF₄ /PC electrolyte solution, LiNi_0.5Co_0.2Mn_0.3O₂ retained a high discharge capacity of ca. 186 mAh g^-1 even after 50 charge/discharge cycles at a C/10 rate. The polarization on charge/discharge reactions remained small even at the very high concentration [2]. Considering the wide operating temperature range of LIBs, the electrolyte solution needs to be stable even at elevated temperatures. 7.25 mol kg^-1 LiBF₄ /PC allowed charge/discharge cycles of LiNi_0.5Mn_1.5O₄ between 3.5 and 5.0 V at an elevated temperature of 50 °C, whereas not in the nearly saturated 4.27 mol kg^-1 LiPF₆ /PC [4]. In electrolyte solutions of moderate concentrations, the dissolution of active materials become more of a problem at elevated temperatures. The highly concentrated LiBF₄/PC suppressed the dissolution of manganese ions from a LiNi_0.5Mn_1.5O₄ electrode more effectively than LiPF₆ /PC [4].

Highly concentrated LiBF₄ /PC can be diluted with a diluent to reduce the high viscosity and concentration. Fluorinated solvents have relatively low donor number and high stability against oxidation because they have electron-withdrawing fluorine atoms, and are suitable as diluents for concentrated LiBF₄ /PC systems. We explored various kinds of fluoroalkyl ethers [5,6] and esters as a diluent, of these, bis(2,2,2-trifluoroethyl) carbonate (TFEC) has a relatively low HOMO energy, and could hardly dissolve LiBF₄. Unfortunately, however, the miscibility between TFEC and concentrated LiBF₄ /PC was not high, and hence tris(2,2,2-trifluoroethyl) phosphate (TFEP) was introduced as a co-solvent of PC to prepare 2.03 mol kg^-1 LiBF₄ /PC + TFEP (1:2 by volume, PC/Li⁺ molar ratio = 1.35). This concentrated electrolyte solution was diluted with 50 vol.% TFEC to obtain 1.00 mol kg^-1 LiBF₄ /PC+TFEP+TFEC (1:2:3 by volume, PC/Li + molar ratio = 1.35). The initial discharge capacity reached 192 mhA g^-1 and 98.0% of it was retained even at the 50th cycle (Fig.1) [7]. Thus, the degradation of a LiNi_0.5Co_0.2Mn_0.3O₂ electrode was extremely suppressed even though the upper cut-off voltage was as high as 4.6 V. The initial Coulombic efficiency (84.2%) was low, while the average Coulombic efficiency from the 2nd (97.6%) to 50th cycle (99.1%) was high at 98.7%. Thus, the present electrolyte solution has a low viscosity (ca. 7 mPa s), moderate lithium salt concentration, and high stability against oxidation, and is therefore suitable for higher-voltage operating of LIBs.

This research has been supported by "Advanced Research Program for Energy and Environmental Technologies" from NEDO, Japan.

Reference

[1] T. Doi et al., Electrochim. Acta, 209 (2016) 219-224.

[2] T. Doi et al., J. Electrochem. Soc., 163 (2016) A2211-A2215.

[3] T. Doi et al., ChemElectroChem, 4 (2017) 2398-2403.

[4] T. Doi et al., ChemistrySelect, 2 (2017) 8824-8827.

[5] T. Doi et al., J. Electrochem. Soc., 164 (2017) A6412-A6416.

[6] T. Doi et al., Sustain. Energy Fuels, 2 (2018) 1197–1205.

[7] T. Doi et al., Curr. Opin. Electrochem., 9 (2018) 49-55.

Figure 1

In secondary batteries, the anode/electrolyte interphase plays a key role in the electrochemical performances. As the liquid organic electrolyte undergoes degradation in the electrochemical potential window of a cycling battery, a Solid Electrolyte Interphase (SEI) is formed upon cycling. This interphase layer leads to a double-edged problematic: the formation of the SEI lowers the coulombic efficiency and causes irreversible capacity loss, but it also passivates the electrode from the electrolyte and prevents further aging processes. Knowing this, any modification of the SEI should be performed with parsimony as it could break the balance between the positive and negative aspect of the SEI. By synthetizing a chemisorbed thin fluorinated layer upon anode material, we managed to improve the passivating power of the SEI leading to enhanced electrochemical performances. We also determine that very low quantities of fluorine on the active electrode material surface leads to several beneficial effects.

The chemical nature of the surface layer was describe by the mean of the XPS (figure 1), as well as the fluorine distribution on the surface with both AES and SAM. The fluorine has been quantified around 10 at. % of the extreme surface of the Li₄Ti₅O₁₂ (LTO) material, without diffusion in particles bulks. The bulk and sub-surface properties of fluorinated LTO (LTO-F) were also investigated by coupling XRD, Raman Spectroscopy and NMR ¹⁹F, showing no modifications of the crystallographic structure. The influence of the surface fluorination on the electrochemical performance was investigate by galvanostatic cycling and by coupling XPS and SAM on cycled electrodes. We had a specific attention to the impact of the fluorination on the SEI thickness and stability in charge and discharge. Indeed, LTO-F exhibit a new reactivity toward the electrolyte, leading to a thinner and stabilized SEI. Finally, the gas generation of the LTO-F electrodes has been investigate by Gas Chromatography – Mass Spectrometry (GC-MS), as gassing is known to be a roadblock to the commercialization of LTO^1,2. We demonstrate that the CO₂ outgassing is reduced by the surface fluorination. The strategy implemented in this work, from synthesis to thorough characterization, allow to propose new solutions to improve SEI for Lithium ion batteries.

(1) He, Y.-B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q.-H.; et al. Gassing in Li(4)Ti(5)O(12)-Based Batteries and Its Remedy. Sci. Rep.2012, 2, 913.

(2) Zhang, L.; Zhang, S.; Zhou, Q.; Snyder, K.; Miller, T. Electrolytic Solvent Effects on the Gassing Behavior in LCO||LTO Batteries. Electrochimica Acta 2018, 274, 170–176.

Figure 1

The increasing miniaturization of electronic devices and the needs for stand-alone systems require adapted power sources in terms of dimensions and energy. With these new demands, new constraints come out like flexibility, limited thickness or tighter security requirements. To achieve these requests, both battery core materials and packaging need to be redesigned.

In this work, we developed simultaneously a new gel polymer electrolyte (GPE) material and a new battery design to reach both security (no leakage concern, no flammability, thermal stability) and application requirements (flexibility, thin system (<400µm)). We worked on a quaternary GPE based on a bi-components UV-curable polymer network giving the mechanical strength combined with a binary liquid phase composed of ionic liquid and Li salt. The GPE was optimized in terms of mechanical and electrochemical properties by varying the nature and the proportion of each components (polymer network, ionic liquid and Li salt) and the UV-curing parameters. The optimized GPE exhibits a good conductivity at room temperature (0.375mS.cm^-1) and a Li⁺ transference number of 0.299 (NMR measurements) which is relatively high for this GPE type (Fig.a).

Our new battery design is composed of LiCoO₂ cathode realized by standard coating processes. The liquid solution of GPE precursors is then deposited on the LiCoO₂ cathode and directly polymerized by UV-curing (Fig.b). This step allows a good soaking of the electrolyte in the electrode pores. This system was then integrated in our thin battery design with Li metal foil as anode material (Fig.c). By optimizing each component dimension and sealing parameters (sealing conditions and adhesive type), we succeeded in producing a flexible, less than 400µm thick working battery (Fig.d) with interesting electrochemical performances (more than 2mAh.cm^-2).

This work was done as part of the project EnSO. EnSO has been accepted for funding within the Electronic Components and Systems For European Leadership Joint Undertaking in collaboration with the European Union's H2020 Framework Programme (H2020/2014-2020) and National Authorities, under grant agreement n° 692482

Figure 1

Safety is a highest priority issue for large-scale applications of lithium-ion batteries. Commercial lithium-ion batteries use organic electrolytes, which are highly volatile and flammable to cause serious fires and explosions. An effective approach for minimizing such risks is to replace the flammable organic electrolyte for nonflammable (or fire-extinguishing) one. Organic solvents containing phosphorous or fluorine have been extensively studied as flame retardant additives. However, they cannot passivate carbonaceous negative electrodes; thus, using those additives generally degrades charge-discharge cycleability. As a result, there has been a trade-off between non-flammability and cell cycleability in designing organic electrolytes. Recently, superconcentrated electrolytes are widely studied as a new class of liquid electrolyte for battery applications, which enables salt anion-derived excellent passivation of negative electrodes.¹ Our group applied this strategy to LiN(SO₂F)₂ (LiFSA) / trimethyl phosphate (TMP) electrolytes to achieve both non-flammability (and even fire-extinguishing function) and outstanding cycling performance of carbonaceous negative electrodes.² However, such superconcentrated electrolytes have several disadvantages of high viscosity, low ionic conductivity, and high cost for practical application.¹ To circumvent these issues, Watanabe and coworkers proposed the dilution of superconcentrated electrolytes (specifically solvate ionic liquids) with low-polar solvent ("diluent").³ They demonstrated that diluted solvate ionic liquids retains their original functions but have much lower viscosity and higher ionic conductivity. In this work, we studied a fire-extinguishing LiFSA/TMP superconcentrated electrolyte diluted with low-polar 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) which is also a flame retardant. We investigated the effect of dilution on passivation abilities toward graphite negative electrodes, and discuss the passivation mechanism from the viewpoint of local coordination structure.

We studied three electrolyte compositions: LiFSA/TMP (1:1.3 by molar ratio, "concentrated"), LiFSA/TMP:HFE (1:1.3:8 by molar ratio, "concentrated + diluted"), and LiFSA/TMP:HFE (1:8:1.3, "dilute"). The last one is named as "dilute" electrolyte, because it has an excess amount of high-polar solvent (TMP) as with conventional dilute electrolytes. Figure 1 shows charge-discharge curves of graphite in the three electrolytes. In "dilute" electrolyte, plateaus appeared at around 1.0 V on charging and then almost no discharge capacity was observed, which is due to the co-intercalation of TMP. On the other hand, "concentrated" electrolyte enables highly reversible Li⁺ intercalation reaction due to the formation of FSA anion-derived inorganic solid electrolyte interphase (SEI).² Notably, the high reversibility could be retained by diluting the concentrated electrolyte with low-polar HFE ("concentrated + diluted"). Upon electrode surface analysis with X-ray photoelectron spectroscopy (XPS), we found FSA anion-derived SEI film analogous to that formed in "concentrated" electrolyte, indicating that the unusual passivation ability could be retained even after dilution. Raman spectroscopy and Fourier transformed infrared spectroscopy (FT-IR) of the electrolyte solutions revealed that "concentrated + diluted" electrolyte had specific local coordination structure similar to "concentrated" electrolyte. These results suggest that passivation ability can be controlled by tuning the local coordination structure in electrolyte solutions.

References

• Y. Yamada et al., J. Electrochem. Soc., 161, A2406 (2015)

• J. Wang et al., Nature Energy, 3, 22 (2018)

• K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013)

Figure 1

Magnesium batteries could offer high energy density without compromising safety due to the use of non­dendritic Mg metal anode. However, the charge­ dense divalent Mg²⁺ also makes cation ingress into and diffusion within cathode materials kinetically sluggish. It is therefore intriguing that recently organic cathodes were reported to deliver high energy and power even at room temperature. Herein we reveal that previous organic cathodes likely all operate on an MgCl ­storage chemistry sustained by a large amount of electrolyte that significantly reduces cell­ level energy. We go on to demonstrate Mg batteries featuring an Mg²⁺­ storage chemistry using quinone polymer cathode, Cl­ free electrolytes, and Mg metal anode. Under lean electrolyte conditions, the organic cathode in cells on Mg ­storage chemistry deliver the same energy while using ca. one tenth of the amount of electrolyte needed for the MgCl ­based counterparts. With the right combination of organic cathodes and chloride ­free electrolytes, the observed specific energy (up to 243 Wh kg­^-1), power (up to 3.4 kW kg^-­1), and cycling stability (up to 87%@2500 cycles) of Mg ­storage cells consolidate organic polymers as promising cathode candidates for high­ energy Mg batteries.

Lithium-rich layered rock-salt type Li₂MnO₃ is one of the most attractive cathode materials for lithium batteries because it achieved the high discharge capacities of 250−300 mAh g^-1 after activation during the first cycling [1]. However, the structural deterioration in the following cycles at the cathode/liquid electrolyte interface leads to severe capacity fading. We reported that the Li₂MnO₃ films on solid system exhibit a high capacity of 250 mAh g^–1 and better cycle stability, although liquid system showed severe capacity degradation [2]. To elucidate the activation process and high capacity phase contributing to high cycle stability, crystal structure changes at activation process were analyzed using the different initial structure of Li₂MnO₃. The initial structure was controlled by stacking solid electrolyte using two types of synthesis methods.

The Li₂MnO₃ electrode films were synthesized on SrRuO₃/SrTiO₃(111) substrates by pulsed laser deposition (PLD). Amorphous Li₃PO₄ as the solid electrolyte was synthesized by PLD and magnetron sputtering. Lithium metal was used as the negative electrode, respectively. The crystal structure changes at activation process were analyzed by in situ X-ray diffraction (XRD) measurements.

The Li₂MnO₃ structure had layered rock-salt type structure after stacking Li₃PO₄ by using PLD. On the other hand, in case of stacking by using magnetron sputtering, the Li₂MnO₃ changed disordered structure with the transition metal (TM) layer disordering of lithium and manganese ions in the honeycomb lattice. For the Li₂MnO₃ with layered rock-salt type structure, the discharge capacity increased continuously in the following cycles with persisting plateau region for several cycles. On the other hand, the Li₂MnO₃ with disordered structure showed about 300 mAh g^–1 at the first cycle. The XRD intensity ratio of I₀₂₀/I₀₀₁ was zero after activation at both of initial structures [3]. Since the 020 peak arises from a superlattice structure by a honeycomb-type ordered arrangement of Li and Mn atoms in the TM layer, atomic arrangements in the TM layer transformed to a disordered state. This high capacity phase could contribute to its high capacity and cycle stability.

Acknowledgment: This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2) of the New Energy and Industrial Technology Development Organization (NEDO).

References: [1] M. Sathiya et al., Nat. Mater., 2013, 12, 827-835.[2] K. Hikima et al., The 57th battery symposium in Japan, 2016, 2G22. [3] K. Hikima et al., The 58th battery symposium in Japan, 2017, 2C22.

Sulfide-based solid electrolytes have attracted much attention due to their high conductivities, which are far beyond those of oxide-based solid electrolytes.^[1,2] However, They (Li₂S-P₂S₅ system, i.e., LPS) have been normally synthesized by solid state synthesis such as mechanical ball milling. These methods require rigorous control of reaction environment as well as high temperature heat treatment and repeated pelletizing steps. In contrast, solution-based synthesis methods can induce chemical reaction among precursor particles (Li₂S and P₂S₅) at low temperatures resulting in the formation of conductive phases of β-Li₃PS₄ and Li₃P₇S₁₁ with only moderate thermal treatment.^[3,4] The method deserves great attention since it simplifies synthesis process, yields products of great purity, and may facilitate the fabrication of composite electrodes with improved interfaces.

In this work, we have developed an efficient method to form a thin solid electrolyte layer directly on Li metal using the liquid coating techniques. The formation of LPS (Li₂S-P₂S₅) based electrolyte is achieved by rational design of the solvent and the Li, P, and S precursor ratios. The solution electrolyte can be directly coated and formed on Li metal through the in-situ formation of the solid electrolyte layer, which does not require the complex synthesis process and high temperature sintering step. Layers of thickness of < 50 mm can be fabricated and electrochemical cycling of lithium is achieved. This liquid-phase coating is a simple and straightforward technique for making a thin solid electrolyte and can be applicable to anode surface with complex contours. The new liquid coating technique holds the promise to overcome the limitations of current state solid electrolytes.

[1] Kamaya, N. et al. A lithium superionic conductor. Nature Mater. 10, 682-686 (2011).

[2] Yamane, H. et al. Crystal structure of a superionic conductor, Li₇P₃S₁₁. Solid State Ion. 178, 1163-1167 (2007).

[3] Ito, S. et al. A synthesis of crystalline Li₇P₃S₁₁ solid electrolyte from 1,2-dimethoxyethane solvent. J. Power Sources 271, 342-345 (2014).

[4] Liu, Z. et al. Anomalous High Ionic Conductivity of Nanoporous β-Li₃PS₄. J. Am. Chem. Soc. 135, 975-978 (2013).

The increasing deployment of intermittent energy source such as solar and wind has raised the need of large storage device to improve grid reliability and power quality. Redox flow batteries (RFBs) are regarded as one of the most promising candidates, because of simple design, good scalability, good cycle efficiency, and reliable long lifespan. Among them, zinc-bromine flow battery (ZBB) is considered as one of the most promising candidates for large-scale energy storage due to the high solubility of Zn ion and low cost which is attributed to the abundance of zinc compounds. Combined with high cell voltage (1.84V), ZBB shows up to 440Wh/kg with a practical energy density of around 65-75 Wh/kg. However, widespread application of ZBB has been impeded due to its low power density, zinc dendrite, or hydrogen evolution. In this work, we investigate the effect of zinc powder-based anode modification on the electrochemical characteristics of zinc-bromine flow battery.

Li metal batteries (LMBs) have been revived as promising rechargeable battery chemistry in the last few years due to their high potential energy densities. However, there is still a long way to go for commercialization of LMBs because of the limited cycle life and potential safety concerns of LMBs, which are caused by the formation of high surface (porous) Li and low Coulombic efficiency (CE) during repeated charge/discharge processes. Developing advanced electrolytes is crucial to develop practical LMBs with high energy density. In this work, we report a promising electrolyte approach based on the use of a new electrolyte solvent, which can be worked as the diluent for high concentrated electrolyte (HCE) to get a localized high concentrated electrolyte (LHCE). Although with an apparent salt concentration of 1M, this electrolyte generates salt decomposition dominant solid electrolyte interphases (SEIs) on Li metal anode, which is robust and conductive. Therefore, it shows high Li Coulombic efficiency and great stability in Li||NMC811 cells. The details together with post-cycling analysis will be discussed at the presentation.

A composite of Lanthanum manganese oxide-carbon (LaMnO₃/C) has been also synthesized by a facile electrospinning approach followed by controlled heat treatment, in which the carbon form a continuous conductive network connecting the electrocatalyst LaMnO₃ nanoparticles together to facilitate good electrochemical performance. Based on a correlation between theoretical (DFT) and experimental analysis on the effect of the surface atomic arrangement, the electrocatalyst (Mn-terminated LaMnO₃) with uniform Mn surface termination show favourable rechargeability, and good phase and morphology stability in lithium oxygen batteries compared to La rich surface termination. Excellent cycling performance is also demonstrated, in which the terminal voltage is higher than 2.4 V after 100 cycles at limited capacity of 1000 mAh g^-1 (based on composite).

Figure 1

Energy storage technology for lithium-ion batteries (LIBs) has made rapid progress, and has been applied in a wide range of applications such as portable electric devices, hybrid electric vehicles (HEVs) or electric vehicles (EVs) even in large-scale energy storage system (ESS). Nevertheless, the electrode materials still require the enhancement on its energy or power densities for satisfying the needs of global market. Meanwhile, research on high-capacity anode materials beyond the low theoretical capacity limit of 372 mA h g^-1 of commercialized graphite materials is performed actively in numerous research groups. As one of the alternative candidates, transition metal oxides have strong advantages, which can substitute the graphite materials, such as high theoretical capacity (≥500 mA h g^-1), earth abundant and simple preparation process. However, the charge/discharge process of "conversion reaction", which is typically shown in oxide-based anode materials, causes lower cyclability resulted by repetitive volume change in the reaction process. Also, oxide materials generally exhibit lower electrical conductivity than carbon or metal-based anode materials. Recently, metal ferrites (MFe₂O₄, M = Mn, Co, Ni, Zn) show higher level of electric conductivity and chemical stability from the modified electronic structure which is resulted by doping of foreign metal atoms. Among the various metal ferrites, MnFe₂O₄ is attractive anode material, because it shows high theoretical capacity of 917 mA h g^-1, and it consists of low cost Mn, Fe elements. In our work, to compensate for the limitations of oxide materials using MnFe₂O₄ nanoparticles, we have introduced two representative technologies of nano-structuring and carbon composite to improve the rate capability and cycle performance of the ferrite system. In particular, we also have introduced polydopramine (PDA) coating technique using versatile properties of the polydopamine such as high hydrophilicity and adhesive ability from rich functional groups (e.g. –OH catechol, –NH₂ amine). Mussel-inspired PDA is well known as its strong adhesion to organic/inorganic surfaces, good environmental stability, biocompatibility, and excellent dispersibility in water. In this regard, we applied the PDA coating layer to carbon nanotubes (CNTs) to mitigate the strong inner-tube van der Waals interactions, and it helps to fabricate coaxial nanocable structures with increased dispersibility in the precursor solution of the MnFe₂O₄. In our process, metal ions in the solution are attracted by –OH catechol groups in PDA layer, and MnFe₂O₄ nanoparticles are co-precipitated at low temperature (98 ^oC) around CNT surfaces. Eventually, the nanoparticles are attached on the CNT by PDA layer. As a result, well defined coaxial nanocable structures, which is composed of MnFe₂O₄ nanoparticles and CNT core, were successfully prepared without a particle aggregation after our synthesis process. These coaxial nanocable anode materials effectively supply electrons through the CNT core with well contacted MnFe₂O₄ nanoparticles on the surface by the aid of the strong adhesive ability of the polydopamine coating layer. Besides, increased surface area for a facile electrolyte penetration provides better Li⁺ ion kinetics with an enhanced ionic conductivity. Furthermore, homogeneous CNT matrix can accommodate volume changes in the repetitive conversion reaction, and improves the cyclic stability of anode materials.

The lithium ion batteries have been a dominant power source for a wide range of portable electronic devices such as mobile phones, laptops and power tools since SONY commercialized the first lithium ion battery in 1991.

The development for higher capacity electrode material has been becoming more crucial due to expansion of lithium ion battery applications for advanced energy storage. The lithium and manganese rich cathode materials are one of the most investigated cathode materials among a variety of novel electrode materials with decent capacity. The lithium and manganese rich cathode oxides are considered as the solid solutions or nanocomposites between layered monoclinic Li₂MnO₃ and layered rhombohedral LiMO₂ (M = Ni, Co, Mn, etc.). The complex 'layered-layered' structure has inherent shortcomings which include; large initial irreversible charge capacity, poor rate performance, capacity fade and voltage decay capacity fade upon prolonged cycling.

Many efforts have been devoted to overcome these drawbacks, such as fabrication of nano-sized materials, optimized preparation methods, crystal-plane tuning, doping and surface modification. Nonetheless, the development of this type composite cathode material for lithium ion battery is still the challenge for meeting future energy storage requirements.

Herein, the investigation on the performance of the lithium and manganese rich cathode using polyacrylic acid as binder is reported. The electrode using polyacrylic acid binder shows an enhanced performance compared to that of poly(vinylidene difluoride) based electrode.

The formation and decomposition of the discharge products (Li₂O₂) during the charging and discharging processes in Li-oxygen batteries are the main reactions that drive this technology, initiated at a solid-liquid-gas phase boundary involving Li⁺, O₂ and e^- in the cathode compartment. However the mechanistic contribution of each of these component to the electrochemical reactions in a given cell remains unknown.

In this study, we seek to understand the role of Li⁺ and e^- at the phase boundary by developing a dual-cathode system containing a lithium conducting electrode(Li₃N) and an electron conducting electrode(LiI) separated by nickel mesh. It was observed that, the LiI has the tendency to form reduced oxygens on the electrode surface via I^-/I₃^- redox couple, while Li-ions migrate to the electrode surface by the Li-ion conductive effect of the Li₃N in the reactions leading to the discharge product formation. The discharge products on the Li₃N electrode were found to be oxygen-rich, while those on the LiI electrode surface were found to be oxygen-deficient.

Electrochemical analyses revealed that the cells could discharge up to a capacity of 3.74mAh/cm² at a current density of 0.1mA/cm² with an over potential of 0.59V upon charging. At the same current density the cells were able to discharge-charge up to 184 cycles at a cut-off capacity of 0.2mAh/cm².

In the search for alternative anode materials for Li-ion batteries with high power densities, titanium(IV) oxide has emerged as an attractive candidate in recent years. Pristine and Fe-doped titanium dioxide nanoparticles were synthesized using a solvothermal route employing titanium(IV) tert-butoxide and acetic acid as the capping agent. The use of acetic acid provides a scalable synthesis medium that avoids difficulties in removing the entraining agent, often encountered with longer-chain carboxylic acids. The synthesis incorporates a one-pot carbon- and copper-coating strategy designed to improve the electronic conductivity of the composites. Fe-doping and coating strategies both demonstrated improved electrochemical performance in Li-ion half-cells over extended cycling.

Over the past decade, sodium ion batteries (SIBs) as a highly promising candidate for alternative battery technologies beyond Li-ion batteries (LIBs), have raised much attention for grid-level applications considering the sustainability of SIBs. However, compared with the multitudinous choices of anode materials for LIBs, their sodium counterparts are relative rare, which makes the electrochemical reactions in SIBs more difficult. In this work, we have investigated GeP₃ with two-step protection as an anode material for SIBs. Through involving conductive carbon and graphene nano-sheets , GeP₃/C@rGO possesses high electrical conductivity (5.89*10⁻¹ S·cm⁻¹ ) and high specific area (167.85 m² g⁻¹). Severing as a novel anode for SIBs, GeP₃/C@rGO delivers an unprecedented reversible capacity of 800 mAh g⁻¹ with repeating 400 cycles at 0.2 A g⁻¹. Furthermore, the two-step protection sample (i.e., GeP₃/C@rGO) could maintain a high discharge capacity of 473 mAh g⁻¹ even at a high current density of 5 A g⁻¹, which is higher than those of both pure GeP₃ and one-step protection samples (i.e., GeP₃/C, GeP₃@rGO). Ex-situ XRD and XPS tests demonstrate that GeP₃/C@rGO exhibits the highest reaction activation and highest composition stabilization to resists the damages from huge volume change and electrolyte corrosion with co-contribution of both conductive carbon and graphene nano-sheets components. All these properties suggest GeP₃/C@rGO could sever as a promising anode material for SIBs, and the strategy of two-step protection is meaningful for other alloying-type anodes in the next-generation energy storage applications.

Cobalt-free layered lithium-rich nickel manganese oxides, Li[Li_xNi_yMn_1−x−y]O₂, are promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. Utilization of the oxygen anionic redox in this series of materials enables realization of a high capacity beyond that achieved via the conventional transition metal cationic redox when charging above 4.5 V vs. Li/Li⁺. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. The undesirable voltage decay has been proposed to be linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. Herein, we demonstrate that the voltage decay of the electrode is correlated to the activation of Mn⁴⁺/Mn³⁺ redox and subsequent cation disordering, which is able to be remarkably suppressed via simple compositional tuning to induce the formation of Ni³⁺ in the pristine material. By implementing our new strategy, an alternative redox reaction involving the use of this pristine Ni³⁺ as a redox buffer, which has been designed to be widened from Ni³⁺/Ni⁴⁺ to Ni²⁺/Ni⁴⁺, subdued the Mn⁴⁺/Mn³⁺ reduction without compensation for the capacity in principle. Negligible change in the voltage profile of the modified lithium-rich nickel manganese oxide electrode is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. Based on these findings, we propose a general strategy to suppress the voltage decay of Mn-containing lithium-rich oxides to achieve long-lasting high energy density from this class of materials.

Palladium-copper (PdCu) alloys have two representative crystal structures; one is body-centered cubic (bcc), the other is face-centered cubic (fcc) even though both Pd and Cu originally have fcc crystal. The fcc PdCu alloys have disordered structure within which Pd and Cu have solid-solution in fcc lattice, whereas the bcc PdCu alloys have ordered structure which consists of alternative layers with either Pd or Cu atoms.

In particular, bcc PdCu alloys have occasionally shown superior performance to fcc PdCu alloys since the unique ordered structure of bcc has isolated Pd on the surface. Most of bcc PdCu alloys have been synthesized for structural transformation by annealing or seed-growth method of fcc PdCu alloys with inevitable grain growth, uneven surface structure and particle size distribution. Despite these limitations, the Pd on the bcc surface which is provided charge flow from Cu serves as an active site for catalytic reaction, which is highly favorable for lithium-oxygen battery. However, the same size of fcc and bcc PdCu alloys is quite difficult to be obtained since the crystallites larger than 20 nm favor the ordered bcc structure with lower symmetry. Thus, bcc PdCu alloys in nanoscale have been rarely reported, consequently fcc and bcc PdCu nanoparticles (NPs) have never been properly compared until now.

In this study, we successfully synthesize fcc and bcc PdCu alloys in nanoscale through precisely adjusting the driving force for reducing organometallic complex. The bcc PdCu NPs with higher surface energy govern the growth thermodynamics of discharge product and greatly improved battery performance based on density functional theory calculation and experimental proof. This study provides critical descriptor on material design in the perspective of modulating surface structure via crystal structure to tune its intrinsic properties.

The interest in secondary Li/O₂ batteries has grown rapidly over the past two decades, as they exhibit a practically achieveable specific energy of about 1,700 Wh/kg, which equals that of gasoline and is well beyond those of conventional lithium ion, Ni metal hydride and Zn/air batteries.^1,2 The large energy density is due to the very light-weight cell components: a Li metal anode, a porous C cathode (gas diffusion electrode, GDE) and gaseous O₂ as active cathode material. During discharge LiO₂ and Li₂O₂ (as well as Li₂O) are formed via the oxygen reduction reaction (ORR) in the porous carbon matrix. These products are oxidized again upon recharge.

This seemingly simple system, however, has some major drawbacks due to the decomposition of the organic electrolytes during battery cycling – which is caused by the reaction of the respective electrolyte with either the Li anode, different components of the GDE and/or even the desired ORR products. The decomposition products are deposited in the GDE pores, leading to a minimized active electrode surface, decreasing capacities and ultimately cell failure upon continued cycling.³ There are two major reaction mechanisms leading to the formation if Li₂O₂, and the prevailing mechanism depends on the electrolyte. Li₂O₂ toroids are a common discharge product of the so-called solution growth pathway proceeding via an EC mechanism⁴: O₂ is reduced to O₂^-, which is dissolved and forms LiO₂. The solvated LiO₂ intermediates subsequently disproportionate to solid Li₂O₂ particles with different morphologies (which depend on the nature of the electrolyte)⁵. The ORR proceeds primarily via this mechanism when high DN electrolyte solvents are used and/or discharge parameters, such as low discharge current densities and low overpotentials, are applied⁶. With low DN solvents and/or high current densities and overpotentials, the discharge reaction proceeds via the surface growth pathway.

In this work, galvanostatic cycling and CV measurements were conducted with 1 M mixtures of LiTFSI/DMSO and LiTFSI/TEGDME, with and without additions of the redox mediator dimethylphenazine, DMPZ⁷, to analyze the electrolyte influence on the cycling behavior. At different states of charge, the GDE surface and pores were investigated by SEM and XRD to obtain information on the nature, the morphology and the distribution of the product deposits on and in the carbon structure.

Galvanostatic cycling measurements show a larger discharge capacity and lower discharge overpotential with LiTFSI/DMSO as well as a better recharge behavior in the first cycle, although both electrolytes exhibit very high recharge potentials. The TEGDME-based electrolyte on the other hand exhibits a higher stability over the first five cycles, whereas the discharge capacity of LiTFSI/DMSO decreases drastically already upon second discharge. CV measurements confirm these absolute and relative trends of the discharge capacities.

A SEM investigation of the GDE surfaces after first discharge reveals a considerable difference in discharge product morphology depending on the electrolyte. This is in agreement to previously published results: with DMSO as solvent the product consists of toroidal particles with diameters of approximately 200 nm, whereas discharge with the TEGDME-based electrolyte yields a product layer on the porous cathode surface.^5,8 Differential capacity analysis confirmed the difference in discharge product formation: with LiTFSI/TEGDME two reduction processes were detected to contribute to the discharge capacity, whereas discharge with LiTFSI/DMSO contains only one reduction process. X-ray diffraction after the first discharge with LiTFSI/DMSO shows the presence of crystalline Li₂O₂ in the GDE and the absence of crystalline LiOH. The discharge with the TEGDME-based electrolyte, on the other hand, did not yield any crystalline Li₂O₂ (or LiOH). FIB-SEM measurements after the first discharge with each electrolyte furthermore showed that the GDE pores were almost completely blocked by bulk product.

The addition of the DMPZ significantly reduced the charging potential, both for the DMSO and the TEGDME electrolyte. The cycling performance was, however, only improved for DMSO, and not for TEGDME.

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Graphitic carbon is the anode of choice in lithium ion (Li-ion) batteries due to its low cost, availability and cyclability. However, silicon is emerging as a promising candidate to replace/complement conventionally used anodes with a potential to store ten times more lithium (3579 mAh/g) compared to graphitic carbons (372 mAh/g) [1]. But, as is often the case, great performance in one parameter comes with great costs in other parameters. The large lithiation capacity of silicon comes with extreme volume change during lithiation and delithiation, which severely impacts the materials cyclability. Typical mitigation methods for this involve using nanostructured silicon [2-3] , optimisation of binders [4] and adding appropriate SEI forming electrolyte additives [5].

Silicon as anode material has now grown to a mature field, and many degradation phenomena has been explored in detail, in particular on nanostructured silicon using powerful in-situ TEM studies [6]. For current Li-ion batteries, improvement of the anode capacity beyond approximately 1200 mAh/g has negligible impact on the overall capacity cell. Thus, composite anodes containing both silicon and a conventional anode material such as graphite, are sufficient for most batteries. While degradation mechanisms of pure nano-silicon structures have been studied in detail, similar phenomena specific to composite silicon-carbon electrodes have not. In our work we have performed post-mortem FIB-SEM and TEM studies to investigate degradation occurring over a large number of cycles, examining the effect and interplay between the graphite and the silicon in composite anodes. In this work, we have explored different degradation phenomena observed in silicon-carbon composite, including electrode thickening, silicon migration, electrochemical sintering, dendritic surface formation, inhomogeneous lithiation and dependence of electrode thickness on lithiation level. Electrochemical performance of silicon-carbon composite and graphitic carbons have also been studied for comparison.

The composite silicon-carbon and graphite anodes are based on industrial battery grade silicon and carbon produced at Elkem, a world leading company for environment-friendly production of silicon and carbon. The electrochemical performance was cycled in half cells where the working electrode was made by mixing a silicon-carbon composite /graphitic carbon powder with an organic binder in an aqueous slurry and coated on a dendritic Cu-foil. Structural properties and degradation mechanisms were examined by electron microscopy (SEM, FIB-SEM, TEM) and XRD. Support for this work was provided through the ENERGIX program of the Research Council of Norway.

Hydrogen (H₂), which has the highest gravimetric energy density and zero carbon content, is widely regarded as a promising energy carrier to fulfill our needs cleanly and sustainably in the future.(1-3) Central to the electrocatalysis are efficient and robust electrocatalysts composed of earth-abundant elements, which are urgently needed for realizing low-cost and high-performance energy conversion devices. Earth-abundant MoS₂ has emerged as a promising hydrogen evolution reaction (HER) catalyst with high activity and durability. However, such a high HER performance is only limited to acidic media, the kinetic becomes rather sluggish in alkaline media. Besides the hydrogen (H_ad) adsorption energy, there should be a second descriptor for gauging the HER catalytic activity in alkaline media—the binding of hydroxyl species, because here H has to be discharged from water instead of from hydronium ions in acidic media. Here, we demonstrate a dramatic enhancement of HER kinetics in base by judiciously hybridizing vertical MoS₂ sheets with another earth-abundant material, layered double hydroxide (LDH) (Figure 1a). The resultant MoS₂/NiCo-LDH hybrid exhibits an extremely low HER overpotential of 78 mV at 10 mA/cm² and a low Tafel slope of 76.6 mV/dec in 1 M KOH solution (Figure 1b). At the current density of 20 mA/cm² or even higher, the MoS₂/NiCo-LDH composite can operate without degradation for 48 hours (Figure 1c). Benefiting from the desirable structural characteristics, the MoS₂/LDH interfaces synergistically favor the chemisorption of H (on MoS₂) and OH (on LDH), and thus can effectively accelerate the water dissociation step and thus the overall HER catalysis (Figure 1a). This work not only brought forth a cost-effective and robust electrocatalyst, but more generally, it opened up new vistas for developing high performance electrocatalysts in unfavorable media recalcitrant to conventional catalyst design.

Figure captions

Figure 1. (a) TEM images of the MoS₂/NiCo-LDH composite, showing their interfaces. Inset: Schematic illustration of the HER in MoS₂/LDH interface in alkaline environment. The synergistic chemisorption of H (on MoS₂) and OH (on layered double hydroxide) benefits the water dissociation step. (b) Polarization curves of the carbon fiber paper substrate, bare NiCo-LDH, MoS₂ and MoS₂/NiCo-LDH composite catalysts in 1 M KOH solution at a scan rate of 5 mV/s. (c) Polarization curves recorded from MoS₂/NiCo-LDH composite at a scan rate of 5 mV/s before (solid curve) and after (dotted curve) the chronopotentiometry test at -20 mA/cm² of 48 hours. Inset: chronopotentiometry responses (η ~ t) recorded from MoS₂/NiCo-LDH composite at high current densities of -20 mA/cm² and -50 mA/cm².

References

• D. Strmcnik et al., Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nat. Chem.5, 300-306 (2013).

• Y. Jiao, Y. Zheng, M. T. Jaroniec, S. Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev.44, 2060-2086 (2015).

• J. Hu et al., Increasing stability and activity of core–shell catalysts by preferential segregation of oxide on edges and vertexes: oxygen reduction on Ti–Au@Pt/C. J. Am. Chem. Soc.138, 9294-9300 (2016).

Figure 1

Sodium ion batteries (SIBs) continue to receive a tremendous exposure due to its natural abundance (2.6 wt. %, compared to lithium (0.06%)), cost effective and similar chemical and electrochemical properties to the lithium ion batteries (LIBs). However SIBs are still suffering with low energy density and long cycle life which impedes the wide applications and its development of SIBs. These drawbacks such as higher redox potential (-2.71 V vs. SHE), higher atomic mass (23 g mol^-1) and large ionic radii (1.02 Å) of sodium metal facing a challenge for the choice of the electrode materials for the SIBs. Therefore, it's necessary to use suitable cathode materials in order to occupy the sodium ions during insertion and de-insertion process [1-2].

In this case, numerous cathode materials have been investigated for SIBs such as layered transition metal oxides, fluoride-based sulfates and phosphates, olivins, and NASICONs [3-5]. However, their electrochemical performance (specific capacity, long-term cyclability and rate capability performance) as reported is not satisfactory. Thus, the finding of suitable electrode materials is a key challenge for the development of high-performance SIBs [6].

Manganese dioxide is known for large open tunnels, which can provide interstitial spaces for Na-ion storage and transport. Also, manganese dioxide is highly abundant, low cost and environmental benign. Therefore, in this work, α-MnO₂ nanoplatelets/OLC composite was explored as cathode material and has been synthesized by microwave irradiation method using electrolytic manganese oxide (EMD). The physical and chemical characterizations of materials reveal single phase, Mn valence state and its nanoplatelet morphology. The electrochemistry of the fabricated cells was examined by cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. The α-MnO₂ nanoplatelets/OLC composite exhibits highly stable and better capacity retention compared to pure α-MnO₂ nanoplatelets cathode materials. In this discussion, the explored detail of the material and its preparation, physical and electrochemical properties of α-MnO₂ nanoplatelets and its composite cathode materials for the application of Na-ion batteries will be presented.

References:

[1] D. Su, et al., J. Mater. Chem. A, 1, 4845-4850 (2013)

[2] J. Y. Hwang, et al. Chem. Soc. Rev., 46, 3529-3614 (2017)

[3] S. Guo, et al. Angew. Chem. Int. Ed., 54, 5894-5899 (2015)

[4] Funeka P. Nkosi, et al. J. Electrochem. Soc., 164 (13), A3362-A3370 (2017)

[5] F. Yang, et al., Small Methods, 1, 1700216 (2017)

[6] D. Su, et al., Npg Asia Materials, 5, e70 (2013)

Redox flow batteries (RFBs) are regarded as one of the most attractive systems for large-scale electrochemical energy storage because of simple design, good scalability, and good cycle efficiency. Among various flow batteries, zinc-bromine flow battery (ZBB) has a lot of benefits due to the high solubility of Zn, which enables high practical energy density of around 65~75 Wh/kg. Additionally, low cost which comes from the abundance of zinc compounds has thrown light on the popularization of ZBB. Until now, however, widespread application of ZBB has not been realized due ot its drawback.

Zinc dendrite growth has negative effect on Zn/Br performance because of membrane damage, which allows self-discharge. Therefore, to get rid of dendrite formation after prolonged cycling, zinc dendrite strip cycling should run in the process of cycling.

In this work, we introduced Ti-mesh interlayer on the negative electrode to suppress the formation of zinc dendrite. We compared the effect of mesh interlayer using strip cycling during cycling.

The silicon–oxygen binary system (SiO_x) has attracted attention as a negative electrode with a reasonably high energy density and good cyclability. The oxygen-content dependence of the reversible capacity and cycle performance of SiO_x electrodes have been reported.^1,2 However, effects of oxygen content on morphology changes in SiO_x electrodes upon cycling have not been clarified well in spite of their importance in understanding the capacity fading mechanism, and to improve the cycle performance of SiO_x electrode. Therefore, we investigated the cycle performance and morphology changes in SiO_x films with various oxygen contents.

SiO_x thin films were prepared by RF magnetron sputtering on oxygen-free Cu foil, in which the deposited masses of Si were constant. The flow rate of Ar gas was fixed at 20 sccm, while that of O₂ gas was varied in the range of 0-0.20 sccm to change the oxygen content in SiO_x. The RF power and processing pressure were optimized to obtain amorphous SiO_x films without any crystalline phases. After deposition, the SiO_x film was transferred to an Ar-filled glovebox without exposure to air, and a coin-type half-cell was assembled. Solutions of 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) (3:7 by vol.) with and without 10 wt.% VC were used as electrolytes. O/Si atomic ratio was determined from energy dispersive X-ray spectroscopy (EDX) analysis.

The discharge capacity of lightly O-doped SiO_x films (x = 0.21 and 0.48) did not change appreciably compared to that of pure-Si film, meaning that almost all Si atoms in the SiO_x film contributed to the Li alloying/dealloying reactions. The cyclability of the lightly O-doped SiO_x films was improved little. On the other hand, an irreversible fraction of Si in SiO_x at high oxygen contents (x = 1.09 and 1.78) increased owing to the formation of an inactive Li₄SiO₄ phase.³ In return for the decrease of reversible capacity, the capacity retention of the heavily O-doped SiO_x films improved owing to the buffer effect of Li₄SiO₄ matrix against volume changes. Moreover, the capacity retention of the SiO_x films was improved substantially by VC addition, especially for the heavily O-doped SiO_x films.

The morphology changes with cycling in the SiO_x films were affected significantly by the oxygen content and presence of VC. For the pure-Si film, a sponge-like porous structure appeared after cycling owing to repeated crack formation and inhomogeneous volume changes, and resulted in massive electrode swelling. For the heavily O-doped SiO_x films, the morphology change was suppressed by the volume buffer of Li₄SiO₄ matrix. We found that a relatively high oxygen content is required to obtain a stable Li₄SiO₄ phase because SiO_x decomposes to LiO₂ and Li-Si alloy in the lightly O-doped SiO_x. The morphology change was further suppressed by VC addition because the thin and uniform SEI layer derived from VC inhibited the electrolyte decomposition and allowed for uniform Li alloying/dealloying. The volume buffer of Li₄SiO₄ and the uniform SEI are important to achieve a long-term cycle life of SiO_x negative electrode for lithium-ion batteries.

References

[1] M. A. Al-Maghrabi, et al, J. Electrochem. Soc., 160, A1587 (2013).

[2] H. Takezawa, et al, J. Power Sources, 324, 45 (2016).

[3] K. Yasuda, et al., J. Power Sources, 329, 462 (2016).

The ever-increasing demands for safe, energy dense and low cost energy storage systems have been driving interests in beyond Li-ion batteries such as those based on Li metal, magnesium metal and all solid state battery systems^[1,2]. These high energy density batteries suffer from several challenges, several of which are related to the instability or incompatibility of the electrolytes^[1,3,4]. Our group has been pioneering the development of electrolytes that enabled overcoming several key challenges in magnesium batteries and created a new family of highly performing and practical electrolytes ^[3,4]. One challenge with electrolytes for batteries in general is the high flammability and volatility, that can be overcome with the use ionic liquid solvents. However, available ionic liquids suffer from several key draw backs associated with insufficient anodic stability and incompatibility with reactive metallic anode.

Herein, we outline the classes of electrolyte materials we developed and how we overcome current challenges with ionic liquids for Mg and Li metal batteries.^[5]

References

[1] Choi J. W. Aurbach D. Nature Reviews Materials 1, 2016, 16013 1-16.

[2] Manthiram A., Yu X., Wang S. Nature Reviews Materials 2, 2017 16103.

[3] Mohtadi R., Orimo S., Nature Reviews Materials, 2016, 2,16091. 1311.

[4] Mohtadi R., Mizuno F. Beilstein J. Nanotechnol. 2014, 5, 1291–

[5] Kar, M., Tutusaus O., Doug R. MacFarlane, Mohtadi R., Energy Environ. Sci., 2018, Advance Article

As well known, the increasing consumption of fossil fuel and the consequent environment problems, energy storage has become a global concern. The rechargeable batteries with high capacity, low cost and long cycling life is highly desired for large-scale applications in the future. ¹ In the past decade, rechargeable lithium ion batteries (LIBs) have attracted considerable attention for their various advantages, such as high specific capacity, high voltage, long cycle life, and low self-discharge. These features make LIBs to be ideal power source for portable electronic devise and electric vehicles (EVs). ²Nevertheless, LIBs are encountering the next-level energy storage demand on further increase of energy density, operation safety and cycle life for EVs and smart grid energy technologies. ¹ One hinder is the low specific capacity of the traditional graphite anode materials. Great efforts have been made to develop suitable anode materials with both high capacity and safety. Recently, transition metal sulfides have emerged as promising electrode materials for LIBs because of their high specific capacity, low cost, and other unique properties. Particularly, molybdenum disulfide (MoS₂), a representative 2D material which delivers a similar structure to graphene, has been actively investigated owing to its high specific capacity (669 mAh g^-1) from the four-electron transfer reaction, MoS₂+4Li+4e→Mo+2Li₂S. However, MoS₂ still suffers from the inherent low electrical conductivity and large inherent volume change, which will result in poor cycling stability and rate property. ³ To address these drawbacks, one of the most effective strategies is to combine MoS₂ with conductive carbon-based materials, such as porous carbon, carbon nanotubes and graphene. ⁴

Herein, the MoS₂ nanosheets has been successfully grown on reduced graphene oxide (r-GO) aerogel with a one-pot hydrothermal method. The prepared MoS₂ nanosheets show a strong interaction with the r-GO aerogel substrate which could provide strong combination and facile electron transport between the MoS₂ and r-GO substrate. The unique structure of the composite which could remain the high capacity of the MoS₂ and the high conductivity of the r-GO aerogel. This synergy effects makes the MoS₂/r-GO composite exhibits excellent lithium storage performance with high capacity (950 mAh g^-1 at 0.05 A g^-1), good cyclic stability ( about 600 mAh g^-1 at 200 cycles).

Figure captions

Figure 1. (a) Charge-discharge profiles of MoS2/r-GO composites. (b) Cycling performances at the current density of 0.05 A g^-1 and 100 mA g^-1.

References

• Zhang, Z.; Zhao, H.; Teng, Y.; Chang, X.; Xia, Q.; Li, Z.; Fang, J.; Du, Z.; Świerczek, K., Carbon-Sheathed MoS2 Nanothorns Epitaxially Grown on CNTs: Electrochemical Application for Highly Stable and Ultrafast Lithium Storage. Advanced Energy Materials 2017,8 (7), 1700174.

• Qin, W.; Li, Y.; Teng, Y.; Qin, T., Hydrogen bond-assisted synthesis of MoS2/reduced graphene oxide composite with excellent electrochemical performances for lithium and sodium storage. Journal of Colloid and Interface Science 2018,512, 826-833.

• Zhang, X. Q.; Li, X. N.; Liang, J. W.; Zhu, Y. C.; Qian, Y. T., Synthesis of MoS2@C Nanotubes Via the Kirkendall Effect with Enhanced Electrochemical Performance for Lithium Ion and Sodium Ion Batteries. Small 2016,12 (18), 2484-2491.

• Xia, S.; Wang, Y.; Liu, Y.; Wu, C.; Wu, M.; Zhang, H., Ultrathin MoS2 nanosheets tightly anchoring onto nitrogen-doped graphene for enhanced lithium storage properties. Chemical Engineering Journal 2018,332, 431-439.

Figure 1

Despite a decade of extensive research, silicon, as material for anodes of lithium ion batteries, still has not become a practical solution. Suffering from enormous expansion/contraction during lithiation/delithiation cycles, silicon undergoes rapid self-destruction resulting in battery failure. To counteract the degradation several options has been explored over the years. Those include downsizing of the material to nanoparticles, doping and coating with organic or inorganic shells.

An alternative approach was proposed to mitigate the structural destruction of silicon, which is preparation of the alloyed materials. In the present work we demonstrate the use of nanostructured amorphous substoichiometric silicon nitride as such alternative anode material. We demonstrate that upon lithiation, silicon nitride forms lithiated subunits of silicon and ternary phase of lithium silicon nitride, which serves as a matrix material. Such phase separation leads to remarkable stability: the nanostructured thin films of SiNx allowed fabrication of electrodes exhibiting capacities above 1500 mAh/g for 2000 cycles. To achieve sufficient capacities for the practical applications, we demonstrate the application of this principle to particle-based composite electrodes, using SiNx nanoparticles fabricated through a CVD process. Such amorphous substoichiometric nanoparticles demonstrated reversible capacities above 1000 mAh/g and excellent cycling stability over several hundred cycles. We also demonstrate the effects of chemical composition on initial and long-term cycling stability of nanostructured silicon nitride.

Silicon in a form of nanoparticles has attracted a significant interest in the field of lithium-ion batteries, due to the enormous capability of lithium intake. While attracting substantial attention for next-generation lithium-ion batteries, the large volume changes due to lithiation/delithiation during charge/discharge significantly restrict wide application of those materials. To counteract the high-volume expansion associated with lithiation, silicon nanoparticles emerged as materials engineered to extend cycle life. However, correlating particle morphology, size and structure with performance in and lifetime of battery is still needed to make use of the diverse approaches toward predictable tunability of nanoparticle properties. For instance, various sources and various pathways for nanoparticles preparation of silicon lead to different battery performances resulting in large discrepancy of the results. Herein, we examine silicon nanoparticles prepared through pyrolysis of silane gas, compare them to conventional crystalline samples and correlate their morphological characteristics with the battery performance and cycle life. We demonstrate that temperature and silane concentration during synthesis influence the size and morphology of the silicon nanoparticles. For instance, relatively low temperatures typically produce a mixture of small and large round particles with smooth surfaces while relatively high temperatures produce particles with rougher surfaces with a narrower size distribution. These differences are correlated to crack formation and propagation in the electrode during formation cycles, as evidenced by post-mortem analysis. Different degradation pathways of the battery anodes resulting in different rates of capacity fading are demonstrated depending on the starting nanoparticle structure. In addition, in the present work we demonstrate the characterization of silicon nanoparticles with small angle neutron scattering (SANS) as a complementary method to conventional approaches for characterization such as microscopy, which allows the analysis of nanoparticles as an ensemble.

With regard to current trends, the so-called all-solid-state battery (ASSB) is getting enormous attention in academia and industry as a potential next-generation battery technology replacing the state-of-the-art lithium ion battery (LIB). Especially the high energy density (Wh/L) and specific energy (Wh/kg) are appealing properties of the ASSB [1] potentially enabling longer driving ranges for electric vehicles compared to the state-of-the-art lithium ion technology. However, the breakthrough of the ASSB technology is facing practical challenges in terms of processing, rate capability and cycle life. [2] In addition to high energy densities and a long cycle life, significantly enhanced safety properties are considered key for future battery technologies. Regarding the latter, the ASSB benefits from the absence of flammable liquid electrolyte compared to LIBs. However, only very few studies were published showing the evaluation of the safety properties of lithium metal batteries. [3] Therefore, this study aims on giving first insights into the factors influencing the thermal stability, hence the safety properties of different lithium metal battery setups.

As a starting point, lithium metal batteries based on liquid electrolyte were investigated in order to understand the influence of the lithium metal deposition behavior on the thermal stability of the cell. Therein, it has to be considered that during charging lithium metal does not deposit homogeneously but forms dendritic or mossy structures often referred to as high surface area lithium (HSAL). [1, 4] At elevated temperatures, the high reactivity of HSAL in presence of liquid electrolyte can lead to fatal consequences (e.g. fire, explosion). Thus, the controlled deposition of lithium metal is crucial in order to guarantee safe operation of lithium metal batteries. This can be achieved by either modifying the charging process or the lithium metal surface (mechanically/chemically). Beyond that, lithium metal batteries based on either ceramic or polymer-based electrolytes are investigated. Therein, the cells are cycled to different sates of charge (SOC) and/or states of health (SOH) before analyzing the thermal stability by differential scanning calorimetry (DSC). This way, it is possible to determine distinct differences in the onset temperature for exothermic reactions and the evolving heat as a measure for the intensity of the reactions that might lead to a thermal runaway in large-scale cells. Moreover, the thermal stability of the cell components are characterized individually and in presence of each other. In combination with comprehensive post-mortem analysis, it is possible to determine the factors influencing the thermal stability most. In summary, this study presents first results on the thermal stability of various lithium metal battery types to unravel the advantages/disadvantages compared to LIBs.

The authors acknowledge the BMBF for funding the project "BCT" (03XP019I).

[1] T. Placke, et al., J Solid State Electrochem2017, 21, 1939-1964.

[2] K. Kerman, et al., J Electrochem Soc2017, 164, A1731-A1744.

[3] T. Inoue, et al., ACS Appl Mater Interfaces2017, 9, 1507-1515.

[4] M. Winter, et al., Adv Mater1998, 10, 725-763.

Rechargeable battery such as Li-ion battery will be the main focus worldwide for decades to come. However, energy storage beyond the capabilities of Li-ion technologies are becoming increasingly important to satisfy society's future energy storage needs. One such alternative is aprotic Li-air (O₂) battery with its theoretical specific energy several times higher than that of Li-ion. However, many challenges including early cell death, high overpotential and poor cycle life need to be addressed before its commercialisation. Fundamental studies of the processes at the positive electrode have shown that this is a result of passivating Li₂O₂ film at the electrode surface.

It is generally accepted that solution growth of discharge product Li₂O₂ is required to achieve high rates and capacities. One way to achieve solution mechanism is use of high donor or acceptor number electrolytes, however, such electrolytes are less stable towards the reactive oxygen species in aprotic Li-O₂ cell. Here, we discuss our new strategies to use redox mediators as soluble catalysts, to reduce O₂ to Li₂O₂ by halving the overpotential on discharge, without forming passivating films and with less side reactions in ether solvents, while on charge Li₂O₂ can be oxidized rapidly to O₂ even when the former is not in contact with the electrode surface, Fig. 1. As a result, cycling of an aprotic Li-O₂ cell has been demonstrated with rates and capacities of several mA and mAh cm^-2 respectively. The advantages and disadvantages of redox mediators during cycling and decomposition of carbon electrode will be discussed.

Figure 1: Schematics of positive electrode reactions on discharge and charge in the presence of dual mediators.

Figure 1

Recently, great advance of the Li-S battery technology enables its penetration to the power source of the mid- and large-sized devices, which require high energy and power density that cannot be achieved in the Li-ion battery. While the most successful sulfur cathode could be designed by the optimization of the composite structure with carbonaceous materials, the binder system has been recently considered another important factor because the electrode structure of the sulfur cathode suffers huge structural change during the repeated electrochemical cycles. We studied the structural and electrochemical performance of the sulfur cathodes prepared by two different binders, the water-soluble SBR/CMC mixture binder and the traditional PVdF binder. The enhanced battery performance was observed in the SBR/CMC-based electrode and its origin was investigated by post mortem analysis about the electrodes, which confirmed the better mechanical integrity in the electrode compared with the PVdF-based electrode.

Layered cathode materials, including Ni-rich and Li-rich lithium transition metal oxides, are one class of the most promising cathode materials for high energy density Li-ion batteries because of their high capacity and low cost. However, the Ni-rich and Li-rich cathode materials face two same problems of (1) oxygen evolution at high potentials, and (2) significant amount of Li residues in the forms of Li₂O, LiOH, and Li₂CO₃ on the particle surface. Most of known concerns with these two types of cathode materials can be attributed to these two problems, such as irreversible phase transition from hexagonal to cubic phase, intergranular cracking with repeated cycling, transition metal ion dissolution, as well as electrolyte degradation that consequently results in impedance increase and volumetric swelling of the batteries. Since these two problems are the intrinsic properties of the layered lithium transition metal oxides, there are no perfect solutions that can completely solve these two problems. What one can do is to minimize the negative effects caused by these two problems. By selecting Ni-rich LiNi_0.80Mn_0.10Co_0.10O₂ (NMC811) as an example, in this presentation we demonstrate two simple strategies: (1) removing Li residues in the cathode slurry-coating step, and (2) in-situ eliminating the released active oxygen in the battery. Both strategies are simple without need of changing the existing procedures, and show different degree improvements on the cycling stability and rate capability of the Li/NMC811 cells. Our presented strategies are believed to be applicable to all other Ni-rich and Li-rich layered cathode materials.

Present energy demand is hard to sustain due to increasing energy consumption. Thus new energy storage and supply technologies are required which are reliable, safe, inexpensive and show high energy and power densities. Among various battery chemistries, Na based batteries have been identified as potential efficient stationary energy storage technologies. Electrolytes are an integral component of a sodium battery spatially ionic liquid based electrolytes have attracted great interest¹. The use of additives during electrolyte preparation is often carried out to achieve better transport performance, tune the composition of the solid electrolyte interphase, SEI, and alter other characteristics^1–3. Recently, electrolyte additives for Na batteries is an emerging area for researchers³

The effect of water on the properties of sodium salt solutions of ionic liquids (ILs) was investigated in order to design electrolytes for sodium battery applications using water as an additive. Water amounts ranging from 100 to 10000 ppm were added to a superconcentrated NaFSI salt (50 mol%) solution in the ionic liquid N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C₃mpyrFSI)⁴. While the thermal properties (glass transition temperature) are little dependent on the water content, the viscosity and, in particular the ionic conductivity (fig1a), are strongly affected. Whereas addition of the NaFSI salt to the IL significantly increases viscosity with a concommitant decrease in conductivity, adding water up to 10000 ppm (0.99 wt%) remarkably restores the values close to those of the neat ionic liquid (i.e., reversing the detrimental effect of Na salt addition on conductivity). Na|Na symmetrical cell cycling performance is strongly dependent on the applied current density as well as on the water content (fig1b). At higher current densities (1.0 mAcm^-2) the polarisation profiles show a water dependence suggesting that water is being actively involved in the formation of a solid electrolyte interphase (SEI) for high water content samples (1000 – 5000 ppm). The work shown here suggests that water may be a convenient and inexpensive additive in high concentrated ionic liquid electrolytes for sodium which can improve device performance.

Reference:

• Armand, M., Endres, F., MacFarlane, D. R., Ohno, H. & Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater.8, 621–9 (2009).

• Yan, Y. et al. Roles of Additives in the Trihexyl(tetradecyl) Phosphonium Chloride Ionic Liquid Electrolyte for Primary Mg-Air Cells. J. Electrochem. Soc.161, A974–A980 (2014).

• Komaba, S. et al. Fluorinated ethylene carbonate as electrolyte additive for rechargeable Na batteries. ACS Appl. Mater. Interfaces3, 4165–4168 (2011).

• Forsyth, M. et al. Novel Na+ Ion Diffusion Mechanism in Mixed Organic-Inorganic Ionic Liquid Electrolyte Leading to High Na+ Transference Number and Stable, High Rate Electrochemical Cycling of Sodium Cells. J. Phys. Chem. C120, 4276–4286 (2016).

Figure 1

The development of cost-effective lithium-ion batteries depends on the discovery of high-energy-density cathode materials composed of nonprecious elements. Traditionally, lithium-ion cathodes are based on two categories, i.e., layered oxides and polyanionic compounds. However, most polyanionic compounds have low specific capacity due to their heavy polyanions. Layered oxides, though more favorable for energy density purposes, often contains the element cobalt, the supply of which is scarce and not sustainable.

The discovery of cation-disordered rocksalts and their percolation rules unlocked an unprecedented chemical space for the exploration of high-capacity lithium-ion cathodes which do not contain Co and have high capacity[1]. The facile Li diffusion in these materials is enabled through a network of Li-rich environments (so-called 0-TM channels) created by excess Li. Following this insight, many new high-energy-density cathode materials that involve mostly earth-abundant elements have been developed, such as Li_1.2Mn_0.4Ti_0.4O₂[2], Li_1.2Ni_1/3Ti_1/3Mo_2/15O₂[3], as well as their fluorinated variants[4, 5].

A prevailing assumption when studying disordered rocksalt cathodes is that all the cation species are randomly distributed. However, we will demonstrate that even minor deviations from randomness, not detectable by typical X-ray diffraction (XRD), can have profound influence on performance. We employ a combination of thorough experimental characterization and multi-scale computer simulations to reveal that cation short-range order is ubiquitous in these long-range disordered materials. More importantly, the short-range order controls Li transport by altering the distribution of local environments and the connectivity between them. By considering a variety of different chemistries, we explain the microscopic origin of short-range order and identify general guidelines for local-structure manipulation for the benefit of Li transport. This breakthrough in the fundamental understanding of structure-property relationship in disordered Li-ion cathodes sets an exciting new direction for future optimization.

[1] J. Lee, et al, Science 343 (2014) 519-522.

[2] N. Yabuuchi, et al, Nature communications 7 (2016) 13814.

[3] J. Lee, et al, Energy & Environmental Science 8 (2015) 3255-3265.

[4] R. Chen, et al, Advanced Energy Materials 5 (2015).

[5] J. Lee, et al, Nature communications 8 (2017) 981.

Silicon is considered a promising anode material for Li-ion batteries due to the very high theoretical capacity (ca 3600 mAh/g). In fact, commercial batteries today might have up to 5-10 wt% silicon added to the anode. The extensive expansion of the material is a challenge for the mechanical integrity of the electrode, and leads to continuous exposure of fresh surface of silicon to the electrolyte and thereby continuous formation of solid electrolyte interphase (SEI). Thus, the choice of electrolyte, i.e. an electrolyte with good passivating properties, is crucial for the performance of the anode. Common additives for electrolytes to be used in combination with silicon anodes, are fluoroethylene carbonate (FEC) and vinylene carbonate (VC), both known to reduce prior to the solvent (i.e. ethylene carbonate).

Furthermore, replacing the lithium hexafluorophosphate (LiPF₆) salt, known to decompose to PF₅, which again form HF upon reaction with trace amounts of H₂O, could improve the performance. More stable salts are particularly advantageous for silicon electrodes, as HF might attack the native surface oxide (SiO₂) [1]. Salts known to be more stable are lithium bis(trifluoromethanesulfonylimide) (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), and silicon anodes have been shown to perform better with electrolytes containing LiFSI [2]. These salts are furthermore highly soluble in a wide number of solvents, and have therefore frequently been applied in concentrated electrolytes, which are showing great promise as Li-ion battery electrolytes [3]. In spite of increased viscosity and reduced conductivity, a significant change of interfacial reactions at high concentrations might outweigh these drawbacks. Improved performance of Silicon electrodes (nanowires) in highly concentrated electrolytes has been demonstrated [4].

In this work, the electrolyte was optimized by increasing the concentration of LiFSI salt (1M, 3M and 5 M), combined with the electrolyte additives FEC, as well as the anion receptors tris(hexafluoroisopropyl) (THFIPB) and tris(pentafluorophenyl) borane (TPFPB). The silicon based anodes were made of 73 wt% Si (Silgrain®, e-Si 400, a commercially available battery grade silicon from Elkem), with an average particle size of 3 µm, 11 wt% carbon black (Timcal, C-Nergy C65, CB) and 16 wt% Na-CMC binder (Sigma Aldrich Mw ~90000). Slurries were cast onto dendritic copper foil. The electrodes were cycled in 2016 coin cells in a half cell configuration with metallic lithium as the counter electrode.

The impact of the concentration on the solvation of the salt was investigated by FTIR. The surface of the cycled electrodes was investigated post mortem by FTIR, XPS and SEM/EDX as well as FIB-SEM. By increasing the electrolyte concentration, the initial capacity increased, and the SEI layers were found to contain more salt reduction products, and also the SEI appeared to be thinner than for 1M concentration. However, the capacity of the 5M was fading more rapidly than the other electrolytes. The best capacity over 200 cycles was obtained for cells cycled with 3M LiFSI in combination with 10 wt% FEC and 2 wt% THFIPB. Based on the post mortem investigations, the SEI layer of this electrode was found to be rich in LiF, and with only small amounts of salt reduction products, comparable to the 1M electrolyte.

[1] B. Philippe, R. Dedryvere, M. Gorgoi, H. Rensmo, D. Gonbeau, and K. Edström, Chemistry of Materials, 25(3) (2013) 394

[2] B. Philippe, R. Dedryvere, M. Gorgoi, H. Rensmo, D. Gonbeau and K. Edström, Journal of the American Chemical Society, 135 (2013) 9829

[3] Y. Yamada and A. Yamada, Journal of the Electrochemical Society, 162 (14) (2015) A2406

[4] Z. Chang, J. Wang, Z. Wu, M. Gao, S. Wu and S. Lu, Chem Sus Chem, 11 (2018) 1787

Lacking viable electrolytes is one of the fundamental obstacles preventing the realization of rechargeable aluminum batteries. To date, the only electrolytes which can enable allowing reversible Al deposition-stripping with excellent chemical and electrochemical stability are the chloroaluminate ionic liquids (ILs). However, these kind of ILs are extremely corrosive due to the high concentration of chloride. To demonstrate the importance of the cathode/electrolyte interfacial stability in emerging rechargeable aluminum (Al) batteries, chemical compatibility between vanadium(V) oxide (V₂O₅), a widely studied cathode material for Al batteries, and the most common chloroaluminate ionic liquid electrolyte are studied. The potential reactions between V₂O₅ and Lewis acidic species (Al₂Cl₇^-) and Lewis neutral species (AlCl₄^-), respectively, and the resulting electrochemical properties are investigated with electrochemical analysis, spectroscopic characterizations including Raman and nuclear magnetic resonance (NMR) spectroscopy, supported by computational studies using methods based on density functional theory (DFT). Our studies clearly demonstrate that V₂O₅ reacts to both Al₂Cl₇^- and AlCl₄^-, and the reaction mechanisms are proposed and validated. We also developed new organic electrolyte for Al deposition at room temperature based on AlCl₃/g-butyrolactone (GBL) mixture in benzene. It is found that the solubility of AlCl₃ in GBL can be significantly enhanced by adding benzene as diluent. Besides, the coordination structure between AlCl₃ and GBL would change when the molar ratio of AlCl₃/GBL increased above 1:1, which lead to the generation of active species for Al deposition.

The layered transition metal oxides (TMOs) have been investigated as cathode materials for Li-and Na-ion batteries because of their high specific capacity and rate capability.^[1-3] In this respect, researchers have recently studied the layered TMOs as cathode materials for K-ion batteries, and they have so far exhibited only moderate specific capacity and rate capability.^[4-9] However, all the layered K-TMOs reported to date are K-deficient phases (x ≤ 0.7 in K_xTMO₂),^[4-9] which limits their use in practical rocking-chair batteries because in a typical alkali-intercalation battery system all the alkali is brought in through the cathode. The use of K-deficient phases in cathodes requires a pre-potassiation process of the electrodes in order to insert enough K in the cells. Therefore, it is vital to understand the factors that destabilize (or stabilize) the layered structure of K_xTMO₂ (x = 1) and then design a stoichiometric K_xTMO₂ (x = 1) cathode material for K-ion batteries.

In this work, we find that the strong electrostatic repulsion between K ions due to the short K⁺-K⁺ distance destabilizes the layered structure in a stoichiometric composition of KTMO₂.^[10] The stoichiometric KCrO₂ is thermodynamically stable in the layered structure despite a short K⁺-K⁺ distance unlike other KTMO₂ compounds that form non-layered structures. The unique stability of layered KCrO₂ is attributable to the unusual ligand field preference of Cr³⁺ in octahedral sites that can compensate for the energy penalty from the short K⁺-K⁺ distance. Therefore, we develop the stoichiometric layered KCrO₂ cathode material for KIBs and investigate its K-storage properties. In K-half cells, the KCrO₂ cathode delivers a reversible specific capacity of ~90 mAh/g with an average voltage of ~2.73 V (vs. K/K⁺). The practical feasibility of a KCrO₂ cathode is confirmed in a full-cell system using a graphite anode. In-situ diffraction and electrochemical characterization further demonstrate multiple phase transitions via reversible topotatic reactions occurring as the K content changes.

References

• Blomgren, G. E. The Development and Future of Lithium Ion Batteries. J. Electrochem. Soc. 164, A5019 (2017)

• Nitta, N. et al. Li-ion battery materials: present and future. Nano Today 18, 252 (2015)

• Clement, J. R. et al. Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials. J. Electrochem. Soc. 162, A2589 (2015)

• Vaalma, C., et al. Non-aqueous K-ion battery based on layered K_0.3MnO₂ and hard carbon/carbon black. J. Electrochem. Soc. 163, A1295 (2016)

• Kim, H. et al. K-ion batteries based on a P2-type K_0.6CoO₂ cathode. Adv. Energy Mater. 7, 1700098 (2017)

• Hironaka Y. et al. P2- and P3-K_xCoO₂ as an electrochemical potassium intercalation host. Chem. Commun. 53, 3693 (2017)

• Kim, H. et al. Investigation of potassium storage in layered P3-type K_0.5MnO₂ cathode. Adv. Mater. 29, 1702480 (2017)

• Wang, X. et al. Earth Abundant Fe/Mn-based layered oxide interconnected nanowires for advanced K-ion full batteries. Nano Lett. 17, 544 (2017)

• Liu, C. et al. K_0.67Ni_0.17C_0.17Mn_0.66O₂: A cathode material for potassium-ion battery. Electrochem. Commun. 82, 150 (2017)

• Kim, H et al. Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries. Chem. Mater. DOI: 10.1021/acs.chemmater.8b03228 (2018)

Lithium-ion batteries (LIBs) play a crucial role in today's consumer goods and transportation market due to their superior ability to store energy. While the energy and power density has been strongly increased since its commercialization in 1991, LIBs will soon reach a limit, which cannot be overcome with conventional liquid electrolyte based systems [1]. However, solid-state batteries (SSB) based on solid electrolytes (SE) offer the possibility to further enhance the energy and power density [2].

Sulfide-based SEs show superior conductivities of several mS/cm at room temperature. Their ductile nature allows for cold pressing and results in good electrode contacting and hence lower interfacial resistance compared to oxide-based SEs [3]. However, sulfide-based SEs are not stable against lithium and the formation of an interphase has been shown in theory and experiment. The degradation of argyrodite-type Li₆PS₅X (X = Cl, Br, I) in contact with lithium has been investigated by Wenzel et al. using in-situ XPS. They reported Li₃P, Li₂S and LiX as decomposition products, which is in accordance to a thermodynamic analysis based on first-principle calculations carried out by Zhu et al. [4,5].

A promising approach to prevent the lithium/Li₆PS₅X interface from decomposition is the introduction of an interlayer. While many examples about sputtering thin interlayers onto the SE or lithium have been reported, only few reports can be found about thin polymer interlayers. However, the latter could be more attractive for commercial applications as they can be applied by less expensive techniques than sputtering.

By the introduction of a polymer interlayer between lithium and Li₆PS₅X, an extra electrolyte/electrolyte interface in addition to the lithium/electrolyte interface is formed. While the lithium/electrolyte interface has already been intensively studied for several electrolytes, only little is known about the electrolyte/electrolyte interface especially in case of the polymer/solid electrolyte interfaces. In previous studies, the specific resistances for the La_0.55Li_0.35TiO₃\PEO₂₀:LiCF₃SO₃ (2200 Ωcm², 80°C) [6], the Ohara\PEO₁₆:LiCF₃SO₃ (32 Ωcm², 80°C) [7], the Ohara\PEO₁₀:LiTFSI (47 Ωcm², 40°C) [7], and the Li₇La₃Zr₂O₁₂/PEO₂₀:LiClO₄ (9000 Ωcm², 70°C) [8] interfaces have been reported. However, these studies use oxide-based SEs while to the best of our knowledge no comprehensive study on the interface between polymer electrolyte and sulfide-based SE exists.

In the present study, we electrochemically and analytically investigate the interface between argyrodite-type Li₆PS₅Cl and poly(ethylene oxide): lithium bis(trifluoromethanesulfonyl)imide (PEO:LiTFSI). The PEO:LiTFSI electrolyte is prepared solvent-free in order to exclude any solvent-related decomposition reactions at the Li₆PS₅Cl/PEO:LiTFSI interface. In the first part, the Li₆PS₅Cl/PEO:LiTFSI interface is studied via electrochemical impedance spectroscopy (EIS). Two separated processes, which occur at different characteristic frequencies, are identified using an in-house developed four-point measurement setup (Figure 1a). The development of the cell impedance is monitored over 10 days at 80°C and reveals the rise of a medium-frequency process over time. In order to study the interface analytically, the polymer is removed and X-ray Photoelectron Spectroscopy (XPS) measurements are carried out after different ageing times. A clear evolution of decomposition products at the Li₆PS₅Cl/PEO:LiTFSI interface is observed (Figure 1b). These observations are confirmed via Time-of-Flight secondary ion mass spectrometry (ToF-SIMS). Possible decomposition pathways are discussed.

References

[1] J. Janek, P. Adelhelm, in: Lithium-Ion Batteries: Basics and Applications (ed. R. Korthauer), Springer, pp. 187–207 (2018).

[2] A. L. Robinson, J. Janek, MRS Bull.,39(12), 1046 (2014).

[3] J. Janek, W.G. Zeier, Nat. Energy, 1(9), 1167 (2016).

[4] Y. Zhu, X. He, Y. Mo, ACS Appl. Mater. Interfaces, 7(42), 23685 (2015).

[5] S. Wenzel, S.J. Sedlmaier, C. Dietrich, W.G. Zeier, J. Janek, Solid State Ionics, 318, 102 (2018).

[6] T. Abe, M. Ohtsuka, F. Sagane, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc., 151(11), A1950 (2004).

[7] W.E. Tenhaeff, K.A. Perry, N.J. Dudney, J. Electrochem. Soc.,159(12), A2118 (2012).

[8] F. Langer, M.S. Palagonia, I. Bardenhagen, J. Glenneberg, F. La Mantia, R. Kun, J. Electrochem. Soc.,164(12), A2298 (2017).

Figure 1

A metal hydride (MH)/air secondary battery using an alkaline aqueous solution as the electrolyte operates with water generation during discharge and water decomposition during charge. This battery has a potential that high energy density and high safety are compatible, since one of the active masses is oxygen in air and the materials of the electrolyte and the electrodes are non-flammable. In this unique system, the metal hydride electrode works for hydrogen supply and storage and the air electrode provides the reaction sites for OER and ORR. The discharge product is water so that no solid product is generated during discharge and no plugging of the air electrode occurs, which are quite different from the lithium/air and other air batteries producing solid discharge product in the air electrode.

We have been developing the MH/air secondary battery, in which super lattice structure (A₂B₇) of hydrogen storage alloy is used in the negative electrode, and the air electrode comprising pyrochlore type oxide, e.g., bismuth ruthenium oxide, as the bi-functional oxygen catalyst, loaded on the conductive material, e.g., graphite particles, mixed with PTFE [1]. The materials, composition, structure, and preparation procedure of the air electrode and the lab-scale cell have been modified to reduce the polarization of charge and discharge and to improve the cell capacity, the power density, the energy density, and the cycling performance. In this paper, we present and compare the charge-discharge performance of two types of MH/air secondary batteries, i.e., sealed and unsealed types of cells.

The unsealed cell was assembled in a PTFE container, in which from the bottom to the top, there were an MH electrode, a separator with 6 mol/L KOH solution, and an air electrode, as those described elsewhere [1]. In the sealed cell, the electrodes and the separator with the electrolyte were thermally sealed and packed in polyethylene film having an open area on the air electrode. A water repellent film was attached to the air electrode to prevent the flooding of the electrolyte. The sealed and unsealed types of cells were operated with constant current at room temperature without air or oxygen flow to the air electrode.

The test results of the unsealed cell demonstrated that the cell capacity, the output power, and the energy density were 2.48 Ah, 368 mW (232 W/L), and 897 Wh/L, in which the energy density is higher than the theoretical one of lithium ion secondary batteries, and the maximum current for discharge was 1600 mA (136 mA/cm²). The unsealed cell using the air electrode attached with the water repellent film was operated over 250 cycles for charge and discharge. The sealed cell showed very stable charge and discharge voltages for 15 hours and the energy density of 772 Wh/L in the data of 20 charge-discharge cycles. More results on the performance of two types of MH/air batteries will be shown in this paper.

This work was supported by "Advanced Low Carbon Technology Research and Development Program (ALCA), Grant No. JPMJAL 1204" of Japan Science and Technology Agency (JST). The authors acknowledge FDK Corporation for supplying the MH negative electrode.

Reference

1. C. Baba, K. Kawaguchi, and M. Morimitsu, Electrochemistry, 83, 855 (2015).

There is significant interest in the development of rechargeable high-energy density batteries which utilize lithium metal anodes. Recently, fluoroethylene carbonate (FEC) and lithium difluoro(oxalate)borate (LiDFOB) have been reported to significantly improve the electrochemical performance of lithium metal anodes. This investigation focuses on exploring the synergy between LiDFOB and FEC in carbonate electrolytes for lithium metal anodes. In ethylene carbonate (EC) electrolytes, LiDFOB is optimal when used in high salt concentrations, such as 1.0 M, to improve the electrochemistry of the lithium metal anode in Cu||LiFePO₄ cells. However, in FEC electrolytes, LiDFOB is optimal when used in lower concentrations, such as 0.05 – 0.10 M. From surface analysis, LiDFOB is observed to favorably react on the surface of lithium metal to improve the performance of the lithium metal anode, in both EC and FEC-based electrolytes. This research demonstrates progress towards developing feasible high-energy density lithium-based batteries.

Despite their advances and widespread use in various applications for the last two decades, Li-ion batteries continue to exhibit occasional failures from Li plating on the anode. Li plating can have immediate effects on performance and more devastating effects upon the safety, if the Li plating occurs in a dendritic form that could pierce through the separator causing an internal short. Even in a benign form, plated Li—lacking the protective SEI that protects the carbon anode —tends to react with electrolyte which results in a rapid growth in interfacial impedance and capacity loss during cycling. Li plating will manifest whenever intercalation kinetics (into carbon) are slower compared to lithium plating over carbon, despite the lower reversible potential of the latter. Poor cell design factors, i.e., improper matching of anode and cathode in terms of electrochemical capacities and geometric aspects with localized hotspots in current distribution, and inappropriate electrolyte, can cause Li plating on the anode. The nature of the anode, electrolyte and SEI are among the most important factors that can influence Li plating.¹

Apart from the design/chemical abnormalities, lithium plating is also triggered by the operational conditions, such as fast charge rates and low operating temperatures relevant to planetary space missions. In our earlier study, we have demonstrated that the choice of the electrolyte is critical in determining the propensity towards lithium plating.² Specifically, these studies indicate that the Li intercalation kinetics are mainly dictated by the anode surface films (SEI), which in turn could be controlled by a judicious selection of electrolyte solution, i.e., both solvent and salt. We have established a correlation between the poor lithium intercalation kinetics and the propensity towards lithium plating using different electrolytes with different solvent blends and different electrolyte additives.

Recently, Li plating was observed in cells upon extended cycling at ambient temperature in a Low Earth Orbit (LEO) satellite simulation condition, involving 30 minutes of discharge and 60 minutes of charge continuously, possibly due to impeded intercalation kinetics at the anode. Likewise, exposure to high intensity radiation environments may have led to Li plating in prototype 18650 cells upon subsequent cycling. In this paper, we will discuss the Li plating phenomenon in these specific examples, and with reference to anode/cathode interfacial kinetics at low temperatures in different electrolytes.

ACKNOWLEDGEMENT

The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA).

REFERENCES

• B. V. Ratnakumar and M. C. Smart, ECS Trans. 25 (36), 241 (2010).

• M. C. Smart and B. V. Ratnakumar, J. Electrochem. Soc.2011, 158 (4), A379.

Graphene fibers (GF) have received great interest in wearable electronics applications because of their excellent mechanical flexibility and superior electrical conductivity. Herein, an all-in-one graphene and MnO2 composite hybrid supercapacitor fiber device has been developed. The unique coaxial design of this device facilitates large-scale production while avoiding the risk of short circuiting. The core backbone of the device consists of GF that not only provides mechanical stability but also ensures fast electron transfer during charge−discharge. The introduction of a MnO₂ (200 nm in length) hierarchical nanostructured film enhanced the pseudocapacitance dramatically compared to the graphene-only device in part because of the abundant number of active sites in contact with the poly(vinyl alcohol) (PVA)/H₃PO4 electrolyte.

The entire device exhibits outstanding mechanical strength as well as good electrocapacitive performance with a volumetric capacitance of 29.6 F cm⁻³ at 2 mv s⁻¹. The capacitance of the device did not fade under bending from 0° to 150°, while the capacitance retention of 93% was observed after 1000 cycles. These unique features make this device a promising candidate for applications in wearable fabric supercapacitors. Furthermore, the materials were applied to fabricate Li ion battery and the initial data is promising and more analysis is underway.

Manganese dioxide (MnO₂) and zinc (Zn) are one of the most abundant, safest and cheapest materials available. Together, they are found in common household batteries like Duracell, Energizer, etc. as small cylindrical alkaline cells. These cells or batteries are used as primary batteries, i.e., as single use batteries, where the entire capacity of the battery is delivered once and then discarded. The disadvantage of primary batteries is that it takes a lot of energy to produce the battery than the energy that can be actually obtained from it, and also, it creates environmental waste. However, the manufacturing of primary cells has still been rampant due to the cost of manufacturing MnO₂-Zn cells being very cheap. In terms of improving the overall energy efficiency, reducing waste and maintaining its cost advantage, it makes good sense, economically and environmentally, to make MnO₂-Zn cells rechargeable. However, the main deterrent to this direction has been the fundamental material and chemical problems of the main raw components, i.e., MnO₂ and Zn.

Manganese dioxide can theoretically deliver a capacity of approximately 617mAh/g. It delivers this capacity through a 2 electron electrochemical reaction (each electron providing around 308mAh/g). MnO₂ has been found to be rechargeable when the capacity has been limited to around 5-10% of the 617mAh/g. It suffers a crystal structure breakdown as more of the capacity is accessed, and it inherently forms electrochemical irreversible phases. If the entire 2 electron capacity can be accessed then theoretically it can reach energy density numbers near lithium-ion batteries. Similar problems are associated with the zinc electrode, where higher utilization of its capacity causes dendrite formation, shape change and formation of inactive zinc oxides that ultimately lead to electrode failure. These are the main deterrent to a cheap and safe battery that could be a disruptive technology in the energy storage field.

In this presentation, we report the breakthrough of reversibly accessing the 2^nd electron capacity of MnO₂ by using its layered polymorph called birnessite mixed with bismuth oxide (Bi-birnessite) and intercalating the layers with Cu ions (1). Bi-birnessite's undergo conversion reactions in alkaline electrolyte and ultimately form electro-inactive hausmannite (Mn₃O₄) because of its poor charge transfer characteristics. Intercalating the layers of Bi-birnessite with Cu ions is shown to improve its charge transfer characteristics dramatically and regenerate its layered structure reversibly for thousands of cycles. We also present a case of Cu-intercalated Bi-birnessite's applicability in practical batteries by cycling the material at high areal capacities (10-29mAh/cm²) for thousands of cycles at C-rates that are of interest in the battery community.

For true applicability in practical energy dense batteries its pairing with a Zn anode is essential. The use of Zn anodes has also presented problems as it is the source of zincate ions in electrolyte that react with the cathode, MnO₂, to form electro-inactive phase called haeterolite (ZnMn₂O₄). The best reported cycle life data for high depth-of-discharge (DOD) birnessite cathodes with Zn anodes had been 50 cycles till our recent publication, which showed over 90 cycles achieving 140Wh/L. In this presentation, we also report the effect of zincate ions on the Cu-intercalated Bi-birnessite cathodes beyond 100 cycles (2). The Cu-intercalated Bi-birnessite cathodes when paired with Zn anodes are shown to deliver 160Wh/L and cycle reversibly for over 100 cycles. The Cu ions play an important role in mitigating the detrimental effect of zincate ions in the 100 cycles; however, the zincate ions eventually poison the cathode to form ZnMn₂O₄. The mechanism through which ZnMn₂O₄ is formed is presented in detail with the aid of electroanalytical and spectroscopic methods. A solution of trapping the zincate ions is also presented, where the membrane that is used successfully traps the zincate ions from interacting with the cathode and thus, extend cycle life to over 900 cycles. This is the best reported cycle life data with a manganese dioxide cathode accessing the near 2^nd electron capacity paired with Zn anodes.

1] Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S. "Regenerable Cu-intercalated MnO₂ layered cathode for highly cyclable energy dense batteries" Nat. Commun. 8, 14424 (2017).

2] Yadav, G. G.; Wei, X.; Huang, J.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Secor, J.; Banerjee, S., J. Mater. Chem. A, 2017, 5 (30), 15845-15854

Lithium-ion batteries (LIBs) are competent energy storage devices that have been used for electronic devices. However, for large-scale applications, there are concerns about the cost and lithium reserves. The market requires batteries which have high energy density, high safety, low cost, and environmental benignity [1-2]. Recently, the sodium-ion batteries (SIBs) have received much attention because of their similar electrochemical properties with LIBs. Na₃V₂(PO₄)₂F₃ has been developed as a cathode material for SIBs due to high voltage, high thermo-stability, and high capacity [3].

Na₃V₂(PO₄)₂F₃/C (NVPF/C) with a NASICON structure has a Na⁺ intercalation potential of 3.76 V. However, the NVPF/C has a low electronic conductivity which hinders the capacities and rate capability. Carbon coating and particle size reduction can improve the electrochemical performance. In addition, this work use Mg²⁺ doping attempting to increase the bulk conductivity of NVPF. Na₃V_2-xMg_x(PO₄)₂F₃/C cathodes are successfully synthesized using a sol-gel method with citric acid as the reducing agent and carbon source. X-ray diffraction (XRD) analysis confirms that Mg has been doped into NVPF/C. The XRD peak shifts indicate a lattice contraction due to the smaller atomic size of Mg as compared to V. The Mg-doped NVPF/C cathodes show promising properties for SIB applications.

Reference:

[1] M. Arumugam and Y. Ya. Advance Energy Mater., 2018, (8), 1-11

[2] W. Feixiang, Z. Chenglong, C. Shuangqiang, L.Yaxiang, H. Yanglong, H. Yong-sheng, M. Joachim and Y. Yan, Material Today. doi.org/10.1016/j.mattod.2018.03.004, 2017

[3] Z. Liu, Y.-Y. Hu, M. T. Dunstan, H. Huo, X. Hao, H. Zou, G. Zhong, Y. Yang and C. P. Grey. Chem. Mater., 2014, (26), 2513–2521.

For large-format high-energy lithium-ion battery (LIB), formation of internal short circuit (ISC) often results in catastrophic thermal runaway, leading to fire and/or explosion. It imposes touch challenges to system safety and robustness for electric vehicles, personal electronics, large-scale energy storage systems, to name a few. It is desirable to prevent thermal runaway at the electrodes level, through cell engineering.

Recently, we developed "super-safe" LIB cells with functional current collectors (FCC). Independent of the active materials layers, FCC contains physical features that can favorably control the damage mode of electrodes. Once ISC is formed, FCC works as an effective "fuse" that shuts down the internal short current immediately, before heat generation even begins.

The advantages of FCC include:

• FCC is highly effective. Our experiments on FCC-based pouch cells showed only mild temperature increase upon severe impact penetration of metal; reference cells exploded instantaneously under the same abuse testing condition.

• FCC does not affect the cell cycle life, since the thermal runaway mitigation process is non-chemical and decoupled from the electrochemical reactions.

• FCC does not affect the mass production of active materials and other cell components. The failsafe mechanisms are carried entirely by the current collectors.

• Similar FCC technique can be applied to a wide variety of battery chemistries, including future high-energy batteries.

• FCC adds little mass or volume to battery cell, and is cost efficient.

• FCC can be produced through a roll-to-roll procedure.

The separator is an essential inactive component in liquid-electrolyte based batteries, with the function of separating the positive electrode from the negative electrode to avoid the electronic flow but enable the free ionic transport. The major commercial separators are thin, porous polymeric membranes with single layer or multilayers, and the polymer materials are typically polyolefins, polyesters, and so on. In recent years, separators with surface modifications have also been reported and produced for lithium (Li)-ion batteries, with the purposes of improving the mechanical strength, wettability, cell performance and safety tolerance under various abuse conditions. A lot of positive results have been achieved in Li-ion batteries. In recent years, with the demand for higher energy density than that from the state-of-the-art Li-ion batteries, the research and development of rechargeable Li metal batteries has been revived. However, the effects of the separators on the stability of Li metal anode and the performance of Li metal batteries have seldom reported. In this work, we have comprehensively studied the chemical and electrochemical stabilities of commercially available separators with Li metal anode in two kinds of liquid electrolytes, and found that the separators do have different stabilities with Li metal anode especially in conventional LiPF₆-based electrolytes. The details of the investigations will be reported at the presentation.

Silicon is one of the most promising anodes for lithium-ion (LIB) and sodium-ion (NIB) batteries due to its high theoretical capacity, 3590 mAh/g for Li₁₅Si₄^[1] and 954 mAh/g for NaSi.^[2] Nevertheless, its practical application is hindered by a series of obstacles. For lithium (Li), the access to such high lithiated phases causes extreme volume expansion (310%), resulting in a rapid capacity fade. For sodium (Na), the slow kinetics and the ionic radius restrict the sodiation of c-Si. In an attempt to address these problems, recent attention has been given to the two-dimensional 2D silicon structures, comprising calcium silicide (CaSi₂), polysilane (Si₆H₆) and siloxene (Si₆O₃H₆), due to their potential ability to buffer the electrode volume changes during cycling and their facile synthesis through soft-chemical methods. In this work, the lamellar siloxene was obtained via topotactic deintercalation of Ca from CaSi₂ and its electrochemical performance was evaluated with Li, Na and K. The results show the versatility of siloxene as anode for LIB, NIB and KIB, with delivered reversible capacities of 2300, 311 and 203 mAh/g for Li, Na and K, respectively. The material exhibits a noticeable structural stability after several cycles, deriving in a good capacity retention and coulombic efficiency.

The electrochemical mechanism taking place upon cycling is highlighted on the basis of ex situ Raman characterization combined with IR spectroscopy, SEM and TEM. The results are unsuccessful in explaining all the observed phenomena by means of a merely silicon alloying mechanism, therefore a possible alkali intercalation alternative has been proposed. Preliminary evidence of this approach can be found in the siloxene structural integrity after several cycles, the presence of Na in the discharged compound as observed by EDX, the absence of any of the diffraction peaks from NaSi/Li₁₅Si₄ (both crystalline and proper from the alloying mechanism) by XRD, and the lack of their respective Raman vibration bands ^[3-4], at the end of the discharge. Indeed, by Raman spectroscopy it was possible to observe a reversible shift of the main Si-plane vibration band for the discharged and charged siloxene, accompanied by a loss of the –OH and Si-H vibrations observed in the pristine siloxene. Undoubtedly, these two last ones are likely related to a change in the bond nature of the Si-planes with the substituent group producing a different interlayer separation, probably the electrochemical cycling induces an exchange between –OH and –H with Li/Na. In fact, the intercalation of Na/Li into a layered Si-based materials has been theoretically predicted for a single layer of siloxene (silicene), with no experimental record. The calculations foresee a high coverage of the silicene with alkali ions like Na, Li and K due to the nature of their interactions. The full sodiated/lithiated state of silicene corresponds to X₁Si₁ (X=Li/Na), the predicted binding energies and diffusion barriers indicate that their intercalation is achievable without the kinetic limitations (higher diffusion coefficient for silicene), structure degradation and volume expansion of bulk Si. ^[5–9] This feasibility for alkali intercalation with such high structural stability introduces siloxene as a potential anode for LIB, NIB and KIB batteries. Nevertheless, a better understanding of its electrochemical mechanism is necessary to develop its maximum performance. To the best of our knowledge, it is the first time that a lamellar Silicon based material shows such high stable capacity without volume expansion, representing a real breakthrough for the batteries field and particularly for NIB.

References

[1] M. T. McDowell, S. W. Lee, W. D. Nix, Y. Cui, Adv. Mater.2013, 25, 4966.

[2] C. Y. Chou, M. Lee, G. S. Hwang, J. Phys. Chem. C2015.

[3] B. G. Kliche, M. Schwarz, Angew. Chemie Int. Ed. English1987, 26, 349.

[4] T. Gruber, D. Thomas, C. Röder, J. Kortus, C. Röder, F. Mertens, J. Kortus, J. Raman Spectrosc.2013, 44, 934.

[5] X. Lin, J. Ni, Phys. Rev. B - Condens. Matter Mater. Phys.2012, 86, 1.

[6] J. Zhuang, X. Xu, G. Peleckis, W. Hao, S. X. Dou, Y. Du, Adv. Mater.2017, 1606716.

[7] H. Oughaddou, H. Enriquez, M. R. Tchalala, H. Yildirim, A. J. Mayne, A. Bendounan, G. Dujardin, M. Ait Ali, A. Kara, Prog. Surf. Sci.2015, 90, 46.

[8] B. Mortazavi, A. Dianat, G. Cuniberti, T. Rabczuk, Electrochim. Acta2016, 213, 865.

[9] V. V Kulish, O. I. Malyi, M.-F. Ng, Z. Chen, S. Manzhos, P. Wu, Phys. Chem. Chem. Phys.2014, 16, 4260.

Figure 1

Transition metal oxides (TMOs) represent a crucial source for realizing electrode materials for Li-ion batteries (LIBs) and, as such, have been investigated intensively for this purpose. Among several transition metal oxide compounds, a class of ternary oxides known as "Delafossites" is worthy of note due to a characteristic layered structure and the presence of two different polytypes^1,2 (i.e. rhombohedral and hexagonal) depending on the particular orientation of the layers. AgFeO₂ is a typical Delafossite prototype and has attracted attention in LIB studies, as it can be synthesized easily at low temperatures,^3,4 while a good control of Ag/Fe ratios can also be achieved to produce several Ag_xFeO_y compositions.^5-7

However, AgFeO₂ and Ag_xFeO_y have mainly been considered for possible application in positive LIB electrodes so far, despite the fact that this Delafossite structure is not entirely stable towards insertion of Li⁺ ions. Indeed, moderate Li insertion into AgFeO₂ (e.g. one mole per formula unit) causes an irreversible structural change^3-7 at 1.7 V vs. Li⁺/Li with a concomitant reduction of Ag⁺ to Ag⁰, which is progressively extruded from its original lattice in the form of nanoparticles. The latter cannot be re-oxidized to Ag⁺ upon Li⁺ removal, thereby causing an irreversible charge loss. Conversely, the formation of Ag⁰ in these compounds has been reported to have a very positive impact on the overall resistance of the resulting electrodes, in which a massive decrease of the resistivity^6,7 was noticed upon additional Li⁺ incorporation. Therefore, extensive lithiation of AgFeO₂ at low potentials vs. Li⁺/Li is clearly attractive for possible use of this compound as alternative negative electrode. Further Li storage can be achieved in this active material via combined conversion reaction⁸ of its FeO₆ units and Li-alloying⁹ of these in situ-formed Ag⁰ nanoparticles. With these ideas in mind, we have synthesized AgFeO₂ nanoparticles via a simple precipitation route at room temperature and included them in composite electrodes with Na-alginate binder and carbon black conductive additive.¹⁰

A synergy between Li-Ag alloying and a characteristic pseudocapacitive behavior,¹¹ due to the formation of Fe⁰/Li₂O boundaries, is observed at low voltages vs. Li⁺/Li, promoting convenient charge storage. This combined mechanism is also employed in full cells having deeply lithiated AgFeO₂ as negative electrode and LiFePO₄ as positive counterpart to realize an entirely Fe-based rechargeable charge storage device.^10,12 Results from the cycling of both Li half-cells (Figure 1) and full cells will be presented, highlighting current challenges and opportunities for this intriguing Fe-based compound.

Figure 1. (a) Characteristic voltage profiles of a composite electrode containing AgFeO₂ nanoparticles, Na-alginate and carbon black cycled in a Li half-cell at a current density of 0.05 mAcm^-2 between 0.05 and 2.80 V vs. Li⁺/Li with a standard LP40 electrolyte. (b) Discharge and charge capacities for the same cell in correspondence of increasing cycle numbers and associated Coulombic efficiencies.

Acknowledgements

The funding by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) via the personal grant no. 245-2014-668 is acknowledged together with the Knut and Alice Wallenberg (KAW) Foundation for providing the electron microscopy facilities at Stockholm University. StandUp for Energy is acknowledged as well.

References

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Figure 1

LiNi_1-x-yCo_xAl_yO₂ (NCA) was developed from LiNiO₂ by partially substituting Ni with Co and Al, and it has been successfully commercialized and used in electric vehicles by Panasonic and Tesla, respectively. Because Co has a relatively high price of $29.98 USD/lb as of Aug. 3, 2018, while the prices of Ni and Al are only $6.00 and $0.92 USD/lb, respectively, as reported on InfoMine (http://www.infomine.com/investment/metal-prices), reducing the Co content in NCA materials has become a priority.^1,2

For NCA materials, substitution of Al for Ni was shown to improve the thermal stability and safety.^3,4 Partial replacement of Ni with Co was thought to improve structural stability by hindering the mixing between Ni²⁺ and Li^+5,6, and suppressing the multiple phase transitions during charge and discharge. Partially substituting Ni with other elements such as Mn and Mg has been investigated as well.^7,8 However, it is hard to make a head to head comparison between the different substituents because of various synthesis conditions and analysis methods chosen by different researchers. With the increasing demand for reducing Co content, it is important to go "back to basics" and systematically study the impact of different cation substitutions.

In this work, cations including Al, Co, Mn, and Mg, were selected for investigations. Li_xNi_1-nM_nO₂ (M=Al, Co, Mn or Mg, n=0.05 or 0.1) were synthesized and studied with differential capacity versus voltage (dQ/dV vs. V) methods. In-situ X-ray diffraction (XRD) measurements were carried out on selected samples, and the unit cell parameters and unit cell volumes were carefully measured versus x. Figure 1a shows the clipped dQ/dV vs. V of LiNiO₂, in which the peaks corresponding to the phase transitions have been circled. Figures 1b – 1d show that 5% Al, 5% Mn, or 5% Mg substitutions diminish these dQ/dV vs. V peaks, suggesting an effective suppression of the multiple phase transitions observed in LiNiO₂. Figure 1e shows dQ/dV vs. V of 5% Co substitution, and it shows almost identical peak features as LiNiO₂ (Figure 1a), indicating the existence of multiple phase transitions. However, the dQ/dV curves for the 5% Co and LiNiO₂ samples are partially off-scale in Figures 1e and 1a, respectively. Therefore, Figures 1f – 1g show the same dQ/dV vs. V with a larger y-axis scale. Figures 1f and 1j show that both LiNiO₂ and LiNi_0.95Co_0.05O₂ have similar sharp and intense dQ/dV peaks compared to the other samples. The conclusions from dQ/dV vs. V analysis were supported by in-situ XRD measurements. The studies on 5% and 10% cation doped series showed trends in the changes in material structure and specific capacities. Based on the observed trends, a preliminary theory of how the various cations impact LiNi_1-xM_xO₂ has been developed and will be reported.

References

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Figure 1. Cell voltage as a function of specific capacity (V vs. Q) of LiNiO₂ (A), LiNi_0.95Al_0.05O₂ (B), LiNi_0.95Mn_0.05O₂ (C), LiNi_0.95Mg_0.05O₂ (D), and LiNi_0.95Co_0.05O₂ (E); Differential capacity as a function of cell voltage (dQ/dV vs. V) of 2^nd charge and discharge of LiNiO₂ (a), LiNi_0.95Al_0.05 O₂ (b), LiNi_0.95Mn_0.05O₂ (c), LiNi_0.95Mg_0.05O₂ (d), and LiNi_0.95Co_0.05O₂ (e); The same dQ/dV vs. V curves with larger Y axis scale (f – j).

Figure 1

An electrolyte system based on a sulfone solvent will be presented, outlining a newly discovered synergy between solvent and salt that simultaneously addresses the interfacial requirements of coupling a graphitic anode with a high voltage spinel cathode (LNMO). At the anode, a LiF-rich interphase generated by early-onset reduction of the salt anion effectively suppresses solvent co-intercalation and subsequent graphite exfoliation, enabling unprecedented and highly reversible graphite cycling in a pure sulfone system. Under aggressively oxidative conditions, QC calculations predict that high salt concentration promotes formation of complexes/aggregates which slow the decomposition of the solvent and leads to polymerizable rather than gaseous byproducts--a fundamental improvement over conventional electrolytes. These predictions are corroborated by an in-depth analysis of these functional interphases, as well as stable, long term operation of high voltage (4.85V) LNMO-graphite full cells.

Rechargeable batteries with fluorides as charge transporting ions, which we refer to as fluoride shuttle butteries (FSBs, see Fig. 1), potentially offer a high electrochemical energy storage capability that overwhelms those of the state-of-the-art lithium-ion batteries (LIBs). This is due to the unique properties of monovalent fluoride ions, which are lightest of halides, hardly oxidized, and yet capable of reacting with various metals to form mono- and multi-valent fluoride salts. They thus allow anion-based, multi-electron transfer, redox reactions under wide potential window. Besides, FSBs as such no longer require host lattices (prerequisite in LIBs), which further adds to the high energy density.

The fundamental concepts and some successful examples of FSB systems that had been developed in the RISING battery project were addressed by one of the present co-authors (Z. Ogumi) in IBM2017 and IBA2018. We here focus on the recent advance in liquid-type FSBs that came out through the continued RISING2 battery project. It should be noted that none of alkali fluorides are soluble in common buttery-oriented organic solvents no matter how large static dielectric constants they have. Used as alternative electrolytes have thus typically been organic fluoride salts (such as alkylammonium or substituted alkylammonium fluorides) in combination with preferably ionic liquids. Another, more-or-less sophisticated, way out is to utilize so-called anion receptors that make even alkali fluorides substantially soluble in common organic solvents.

We here show nonaqueous plain liquid fluoride electrolytes, which, according to our comprehensive and careful analyses, are apparently free from any addenda but somehow solvated Cs⁺ (or K⁺) cations and F^- anions at a molar concentration of ~50 mM to give total ionic conductivity of typically ~0.8 mS/cm. The simple electrolytes exhibit versatile performance to make a variety of combinations of electrodes (ranging from Au/AuF₃ to at least Zn/ZnF₂) work reversibly in terms of the expected conversion reaction between the respective metal and its fluoride counterpart.

Acknowledgement

This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery 2 (RISING2), financially supported by New Energy and Industrial Technology Development Organization (NEDO).

Figure 1

Among the cathodes of the sodium ion batteries, the manganese and vanadium based oxide materials present many advantages due to their high energy density, low-cost and low-toxicity. In particular, numerous layered materials have been reported in the system Na-Mn-O^1,2. These materials are interesting because they show weak interlayer interactions with free space allowing sodium diffusion. In search of new phases, we present two lamellar compounds of this system with interesting electrochemical properties as well as the first sodium extraction/insertion from Na₂V₃O₇.

The Kagome network of Na₄Mn₂O₅ is made of layers of corner-sharing square-pyramids of MnO₅³. This material was synthesized by a conventional solid-state method starting from Mn₂O₃ and Na₂O and it shows a first charge capacity of 380 mAh/g and a reversible capacity of 130 mAh/g. By mechanical alloying and from the same precursors, a nanostructured material shows a different structure, close to NaMnO₂. The reversible capacity is then improved to 220 mAh/g.

Here, we also report for the first time the electrochemical activity of another lamellar phase of this system: Na₂Mn₃O₇. This material, obtained via a simple synthesis from cheap sodium and manganese salts, consists of Mn-vacancy-[Mn₃O₇]^2- layers built up with edge-sharing MnO₆ octahedra, separated by NaO₆ and NaO₅ polyhedra. Starting from this phase, a reversible capacity of 2 Na/f.u. (160 mAh/g) is obtained through a plateau at 2.1 V with a low polarization of 100 mV⁴. Thus, the electrochemical process allows a reduced phase Na₄Mn₃O₇ to be obtained, which cans intercalate/de-intercalate two sodium per f.u., reversibly⁵. Interestingly, an additional reversible redox process, corresponding to the extraction of 1.5 Na⁺, is observed on oxidation at 4.1 V due to the oxygen redox activity, consistent with DFT calculations⁶. Based on the theoretical explanation given by Ceder et al.⁷, this oxygen redox activity is explained by the presence of ☐-O-Na axes due to the Mn vacancies in the [Mn₃O₇]^2- layers⁸. Therefore, by cycling this material in the potential range 4.7-1.5 V, the reversible specific capacity reaches 200 mAh/g.

In comparison, no oxygen activity has been observed for another Na₂M₃O₇ phase: Na₂V₃O₇. We report the sodium extraction from Na₂V₃O₇ which is a tunnel type structure built of [V₃O₇]^2-_∞ nanotubes hold by sodium ions⁹. In this case a reversible charge capacity of 80 mAh/g at 2.8 V vs Na⁺/Na is due to the V⁴⁺/V⁵⁺ redox activity. Moreover, no oxygen-redox reaction was observed in the vanadium (5+) oxide Na₄V₂O₇.

The mechanism of extraction as well as the structures of the as-prepared and oxidized phases will be discussed in this presentation.

1 X. Ma, H. Chen and G. Ceder, J. Electrochem. Soc., 2011, 158, A1307–A1312.

2 J. Billaud, R. J. Clément, A. R. Armstrong, J. Canales-Vázquez, P. Rozier, C. P. Grey and P. G. Bruce, J. Am. Chem. Soc., 2014, 136, 17243–17248.

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9 E. Adamczyk, M. Gnanavel and V. Pralong, Materials, 2018, 11, 1021.

For effective use of renewable energy it must be coupled to energy storage devices with high efficiencies. One such technology is a battery, where the stored chemical energy can be converted into the electrical energy through the electrochemical reactions. We started the Li battery studies since 2002. Currently, our research covers the Li-ion batteries, Li-S batteries, Li-O₂ batteries, and all-solid-state batteries. Here, I will present some results relating to the electrode materials, electrolyte, and electrode achitectures in our group.

We reported the Li-ion battery based on LiFePO₄ cathode_, which exhibits a good cyclability of 5,000 cycles and a considerable capacity retation of 90 % at -40℃. This novel type battery has been industied and applied for electric vehicles in the low temperature region. The NMC ternary battery and high-voltage battery based on LiNi_0.5Mn_1.5O₂ are also introduced. Besides the cathode, some promising results relating to the anode part will be also introduced, including the graphene and VPO₄/rGO. For Li-S battery, we firstly present a novel cathode of Mo₂C nanorods-S composite, which exhibits a much lower capacity decay of 0.058 % per cycle at 2 C over 500 cycles. Additionally, the carbon-based separator decorated by red phsphorus nanoparticles for Li-S battery will be also introduced, which displays a good rate performance and a considerable cyclability of 500 cycles with 730 mAh/g. For Li-O₂ battery, we reported the yolk-shell Co₂CrO₄ nanospheres as highly active catalysts. Based on the experimental results and DFT calculations, a direct evidence of Co₂CrO₄ employment being linked to the Li₂O₂ morphology was firstly provided and a catalytic mechanism was proposed. In addition, a 3D foam-like composite composing of Mo₂C nanorods decorated by different amount of N-doped carbon was directly employed as the O₂ electrode without applications of any binder and current collector. The fundamental information about the key factors and steps involved in the Li₂O₂ formation and decomposition was revealed. We will also introduce a flexible, self-standing, and binder-free O₂ electrode by growing the flower-like MoS₂ microspheres with sulfur deficiencies onto the CTs (Def-MoS₂@CTs). Futhermore, I will introduce a novel electrolyte of a binary mixtures of highly concentrated tetraglyme electrolyte (HCG4) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) for the Li-O₂ battery, which exhibits good wettability, enhanced ionic conductivity, considerable non-flammability, and high electrochemical stability. For all-solid-state battery, we will introduce a solid polyer electrolyte based on poly(ethylene oxide), which shows a superior electrochemical property with a decent lithium transference number of 0.35, a wide electrochemical stability window above 5 V vs. Li⁺/Li, and a low interfacial resistance. In addition, a triblock copolymer polystyrene-poly (ethylene glycol)-polystyrene (PS-PEG-PS) as the polymer electrolyte for all-solid-state battery will be also introduced, which displays a highly electrochemical stability and a considerable cyclability. Besides some fundermental results, I would like to introduce some industry-level developments in our pilot-line of batteries, because we cooperated with some enterprises of electric vehicles in China since 2009.

I would like to apply an oral presentation if possible. Thanks for your consideration.

Figure 1

Holing of lithium iron phosphate (LiFePO₄, LFP) cathodes with a pico-second pulsed laser, in which the average hole diameter and hole opening rate were 20-30 mm and 1-2%, respectively, enabled to retain the high-rate discharging performance even in the LFP cathodes composed of the having the LFP layer with the thickness of over 40 mm on an aluminum current collector. The conventional and flat LFP cathode exhibited the degradation of discharge retention at the high-rate discharge because of the low utilization of LFP materials in the case of the thick cathode layer. On the other hand, in the case of "through-holed" and "non-through-holed" LFP cathodes, there can be a more efficient insertion/de-insertion of Li⁺ ions to/from the LFP materials through the holes formed in the LFP layer, resulting in retaining the high-rate charging/discharging performance even in thick LFP cathodes. The electrochemical impedance spectroscopy analysis confirmed that the formation of through-holes in the thick LFP layer is significantly effective to improve the high-rate discharging performance as a result of the decreased charge-transfer resistance of the LFP discharge process. The decrease in the charge-transfer resistance results from the increase in the area available in the LFP discharge process because the sidewalls of the holes can also take part in the Li⁺ ion transfer during the discharge process.

Figure 1

Na₃PS₄ has been demonstrated to be a promising solid electrolyte material to be used in all-solid-state sodium batteries. Although, much research has been conducted on this electrolyte, there is no consensus on the optimal synthesis protocols that form similar material properties from the electrolyte. This work investigates the present synthesis parameters and variations and determines the critical parameters used to synthesize highly conductive Na₃PS₄. By using only ball milling, a single-step and scalable synthesis method was determined to form Na₃PS₄ from its starting precursors.

Rechargeable Li-ion batteries with higher energy density are in urgent demand to address the global challenge of energy storage. In comparison with anode materials, the relatively low capacity of cathode oxides, which exhibit classical cationic redox activity, has become one of the major bottlenecks to reach higher energy density. Recently, anionic activity, such as oxygen redox reaction, has been discovered in the electrochemical processes, providing extra reversible capacity for certain transition-metal oxides. Consequently, a more complete understanding and precise controlling on anionic electrochemical activity in these high-capacity oxides have become a flourishing, yet challenging subject.

Compared to TM cations, oxygen anion electrochemical activity is more challenging to experimentally prove and quantify for the following reasons: First, oxygen participates in the electrochemical activity at highly charged states when the partially delithiated oxides are typically very sensitive to high energy electron beam or X-ray source exposure. Second, peroxo-like species and O-holes resulted from oxygen oxidation are extremely reactive and unstable in air and carbonate based liquid electrolytes. Third, M(nd)−O(np) metal−ligand hybridization and rehybridization make it intricate to quantify and differentiate cation and anion contributions to electronic structure changes as well as their contributions to the extra capacity. Based on the above considerations, a multimodal characterization approach must be established for oxygen anion electrochemical activity.

Here, we select Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ as the model compound and systematically study its structure response to the oxygen redox activity by combing neutron pair distribution function, X-ray absorption spectrum, resonant inelastic X-ray scattering and DFT calculation. An obviously increased short O-O pairs accompanying oxygen redox reactions is for the first time detected, and which should be ascribed to the shrink of the interlayer O-O distance. Also, theoretical analysis indicates that the selective decrease of O-O distance is originated from the different oxygen chemical coordination environment. Further, a coupling relationship between the oxygen redox and transitions metals migration is experimentally demonstrated, which reveals the structural origin of the stable lattice oxygen redox reactions. Based on these understandings in our work, an optimization guidance for anionic acitvity is unambiguously presented: designing a robust layered structure with high-covalency TMs while constructing a flexible local structure with high-ionicity TMs to achieve the reversible high energy density.

Figure 1

Development of high-power density battery is one of important issues for managing high-performance energetic applications including electric vehicles, rescue robots, elevating machines, and so on. Lithium cobalt oxide, LiCoO₂, is one of the most popular active materials in lithium ion secondary batteries. LiCoO₂ consists of LiO₂ layers and CoO₂ layers stacking along to c axis, resulting into a rock-salt layered structure with hexagonal system. The layer structured LiCoO₂ potentially provides efficient Li⁺ transport along a,b-axes, exhibiting high-output power as electrodes. For application of LiCoO₂ as high-output batteries, there are still some issues to be solved. Usually, LiCoO₂ was prepared by solid-state reaction, which provided a few micron-size polycrystals with inhomogeneous distribution in shapes. At high-speed Li⁺ transportations of the usual LiCoO₂ particles, it is presumed that the LiCoO₂ undergoes irreversible phase transition and cracks of the particles, which should be caused by local overcurrent and volume changes during lithiation / delithiation. Since these electrochemical overloads to the usual LiCoO₂ particles lead serious degradation of the cycle abilities, improvements of LiCoO₂ are inevitable.

There are two kinds of approaches for the electrochemical improvement. The first one is modification of chemical compositions, including doping with other elements, coating with inactive materials, and the second one is control of crystallographic characteristics, such as crystal habits, particle dispersibility, and sizes. Combining them, synergetic improvement of LiCoO₂ toward high-output battery would be possible. Recently, we have grown LiCoO₂ single crystals by using flux method, which is one of liquid-phase crystal growth techniques.[1] Exhibiting submicron-size, low-aspect ratio, and high crystalline natures, the LiCoO₂ single crystals would show less amounts of grain boundaries, shorter diffusion pathways, and more homogeneous lithiation / delithiation at the interface between LiCoO₂ and conductive materials, compared to the usual particles. It is expected that these advantages of the LiCoO₂ single crystals inhibit the unfavorable electrochemical changes.

In this study, we examined crystallographic effects on Li⁺ transportation properties of LiCoO₂ at fast electrochemical load. The flux grown LiCoO₂ single crystals were applied as an active materials for high-rate batteries. The high-rate cycle abilities up to 20C rate were examined. Their electrochemical degradation characteristics would be discussed by investing macroscopic and microscopic particle features and resistence at each electrochemical cycling. These results would be summalized to suggest dominant crystallographic factors for developing active materials applicable to high electrochemical load.

References

[1] Teshima, S. Lee, Y. Mizuno, H. Inagaki, M. Hozumi, K. Kohama, K. Yubuta, T. Shishido, S. Oishi, Cryst. Growth Des., 2010, 10, 4471-4475.

Acknowledgement

This work was supported by JST CREST Grant Number JPMJCR1322 in Japan and Program for Building Regional Innovation Ecosystems of MEXT.

Lithium metal has been regarded as the most promising anode material for future high energy density lithium ion battery due to its high theoretical capacity (~3860mAh/g) and the lowest electrochemical potential (-3.04V vs the standard hydrogen electrode). The limitations for lithium metal to be the ideal anode are mainly the uncontrollable dendritic lithium growth and low coulombic efficiency. This work uses the lab-made fresh thin lithium foil with the thickness of 50 µm to eliminate the thickness influence on cycle life. The following research mainly focus on understanding the failure mechanism of lithium metal anode from aspect of salt and solvent in electrolyte. Two and three electrode symmetric lithium metal coin cells were made using different electrolyte (lithium hexafluorophosphate –LiPF₆, Lithium bis(trifluoromethanesulfonyl)imide –LiTFSI and Lithium bis(trifluoromethanesulfonyl)imide – LiFSI in Ethylene Carbonate and Diethyl Carbonate with volume ratio 50:50) with different concentration to test long cycle stability. The cycle life data shows the decreasing cycle life with the increasing mole concentration of LiFSI and LiTFSI electrolyte, while the cycle life of the cells using LiPF₆ electrolyte increases when using higher concentration. We believe both solvent and salt decomposition reactions influence the composition of Solid Electrolyte Interphase – SEI on lithium metal when cycling. Fourier-transform infrared spectroscopy (FTIR) and Nuclear magnetic resonance (NMR) will be used to realize the decomposition product of solvent and X-ray photoelectron spectroscopy (XPS) will be used to figure out the salt decomposition product. The evidence of lithium dendrite pierce the separator has been found by using Focused Ion Beam (FIB) with Scanning Electron Microscopy (SEM). Also the thickness of dead lithium will growth with cycles to fulfill the available space in the cells. Both coin cells and Electrochemical Quartz Crystal Microbalance (EQCM) were made to calculate the columbic efficiency by using different electrolyte mentioned above. Electrochemical impedance spectroscopy (EIS) was used to analysis the SEI and charge transfer resistance change during cycles to identify which part mostly contribute the overpotential. In this work, we will figure out the decomposition reactions of solvent and salts to analysis the failure mechanism.

Lithium-ion batteries (LIBs) using organic liquid electrolytes have been applied in portable devices such as laptops and cellular phones owing to its wide electrochemical stability, high voltages, and excellent energy density. LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode have attracted significant interest as the cathode materials for LIBs owing to their high capacity, excellent rate capability and low cost. NCM523 cathode materials have a disadvantage of structural instability, which caused by the reduction of Ni ions releases oxygen from the crystal structure at the high temeprature or high charged state, which can lead thermal runaway by reacting with the flammable electrolyte. The use of organic liquid electrolytes in LIBs raises safety issues due to its flammable character and the impact on the explosion is greater in particular for relatively large systems such as electrical vehicles and grid storage. To overcome the safety issues, solid state batteries (SSBs) using a non-combustible solid state Li-ion conductor are regarded as the realistic alternative to prohibit the leakage of liquid electrolytes and the resultant fire hazards. However, due to the large resistance in the electrode-electrolyte interface, the capacity retention and cycle effieciency of the SSBs become worse. Previous studies on SSBs have been mainly focused on developing solid state electrolytes with high ion conductivity. Despite growing interest in stability, the systematic studies of safety of SSBs depending on the type of electrolyte has never been evaluated so far. It is necessary for understanding structural deformations on the cathode material in the effects of the reactions beween the electrode and various types of electrolytes, which is essential for ensuring the safety of the batteries.

In this study, we investigate the structural degradation and thermal stability on NCM523 cathode materials by taking an advantage of in situ transmission electron microscopy (TEM) in various electrolyte conditions, including conventional liquid electrolyte, poly(ethylene oxide) (PEO) complexes with LiClO₄ (PEO-based solid electrolyte), and PEO/LiClO₄/Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) composite electrolyte (PEO-inorganic composite electrolyte). Since the LATP oxide electrolyte has brittleness and the PEO has flexibility, it adopts a composite electrolyte having inorganic and organic phases with good interface contact with the cell electrode. A cell composed of each electrolyte was prepared and charged with cut-off voltages of 3.9 V and 4.3 V at a rate of 0.05 C after the second formation process for stabilzing the cell. The capacity at the same cut-off voltage decreases in the order of conventional liquid electrolyte, PEO-based solid electrolyte, and PEO-inorganic composite electrolytes. To understand the thermal stability and the degradation mechanism, modifications in selected-area electron diffraction (SAED) and electron energy-loss (EEL) spectra of oxygen K-edge and transition metal (Ni, Co, Mn) L-edges of each of charged NCM523 cathode materials are monitored in real time at the range from room temperature to 300°C. Our work demonstrated that contact resistance at the interface between the electrode and electrolyte is important factor. This work provides important information on the relationship with structural deformation and thermal stability of the cathode materials, which is an essential part of the rational design to develope for high engergy densities and safe SSBs. All the details will be available at the meeting.

Acknowledgement

This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Program (Project 2E28142). This work was also supported by the National Research Foundation of Korea (NRF) grant (No. 2018R1A2B2005205).

Impedance in lithium ion cells grows over the lifetime of the cell and can be a large contributing factor to cell failure. This study attempts to understand impedance growth by looking at the factors that contribute to cell impedance. Separating these factors and understanding the origins of impedance in cells provides insight into how and why cells fail and could lead to improvements in cell cycling and lifetime. The positive and negative electrode solid electrolyte interphase (SEI) layer, contact resistances, degradation of electrical conductivity in electrodes, and loss of ionic conductivity in electrolyte are all contributing factors to overall cell impedance. However, separating these factors can be difficult in impedance spectroscopy measurements.

In this study, Li[Ni_0.5Mn_0.3Co_0.2]O₂/artificial graphite pouch cells were used with 1.2M LiPF₆ in EC:DMC 3:7 (w:w) electrolyte containing 2 wt% vinylene carbonate (VC), 1 wt% lithium difluorophosphate (LiPF₂O₂ – called LFO here), or no additive as a control electrolyte. Cells were formed to 4.2 V or 4.4 V and were either left at top of charge or discharged to 3.8 V before disassembly. See Table 1 for a full list of electrolytes and voltages of the cells used in this study. Following pouch cell formation, symmetric cells were made from two positive electrodes or two negative electrodes harvested from the full lithium ion cell to study each electrode separately. Full coin cells were also made with one positive and one negative electrode. Electrochemical impedance spectroscopy (EIS) was performed at a range of temperatures to facilitate the separation of impedance factors. Measurements were taken at -10°C, 0°C, 10°C, 20°C, 30°C, and 40°C. Spectra were measured at 10°C at the beginning, middle, and end of the experiments to ensure repeatability. Figure 1 is an example of measurements taken from one disassembled pouch cell. Note that only -10°C, 10°C, 30°C, and 40°C are shown in this figure for simplicity. The left column in Figure 1 shows the Nyquist plots for negative symmetric cells at the various temperatures. The middle and right columns show the positive symmetric cells and the full cells respectively. The middle, positive symmetric cell column contains two spectra, one for a cell made with aluminum hardware and the other with steel hardware, labelled (A) and (S) respectively. Data from one symmetric cell is shown per type of symmetric cell, however several cells of each type were made from each pouch cell to ensure repeatability of the data.

The low frequency (right side) semicircular feature in these Nyquist plots may be attributed to charge transfer resistance between the electrolyte and the electrodes. The charge transfer resistance used here lumps together ion desolvation, ion transfer through the SEI and combination with an electron at the inner SEI surface. The high frequency (left side) semicircular feature may be attributed to contact resistance. The steel and aluminum hardware positive symmetric cell data have very similarly sized low frequency features – representing charge transfer resistance, which should not change at all with hardware. However, the high frequency feature varies substantially between the two symmetric cells, indicating that this feature is in fact due to contact resistance. In general, for all spectra, charge transfer resistance increases dramatically in magnitude with decreasing temperature, while the contact resistance remains almost constant. Therefore, Figure 1 strongly suggests that researchers interested in studying charge transfer resistance with minimal confusion should make their measurements at low temperature. The full cells have three features, which represent a combination of charge transfer and contact resistances from the negative and positive electrodes in combination. This data shows that the majority of cell impedance growth as temperature decreases originates from the positive electrode SEI.

Equivalent electric circuit models have been used to model the impedance behavior of the symmetric cells. Circuit models include resistance factors representing charge transfer resistance, contact resistance, and solution resistance, as well as imperfect capacitances (constant phase elements) representing electrochemical double layers. Impedance spectra have been fitted using these simple circuit models. Using this technique, charge transfer resistances, contact resistances, solution resistances, and double layer capacitance values can be obtained for positive and negative electrodes/SEI layers separately as a function of temperature. Activation energies for the charge transfer between SEI and electrolyte have been extracted from charge transfer resistance measurements as they follow an Arrhenius temperature dependence. Capacitance values associated with double layers at the SEI/electrolyte interface were also be obtained through these measurements and these will be discussed.

Figure 1

The performance of lithium-ion batteries have been substantially improved in almost all aspects since the initial commercialization.[1] However, cycling stability and safety is still one of the main concerns. In this regard, electrolytes play an important role. Although carbonate electrolytes are reduced at the anode electrode, they are able to form a stable passivating layer called solid electrolyte interphase (SEI). This layer is able to suppress continuous electrolyte degradation improving the cyclability of the batteries. SEI-forming additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are found to be particularly effective to stabilise this SEI layer.[2,3] However, K. Kim et al. have recently reported the thermal instability of FEC in LiPF₆-based electrolyte for Li-ion batteries.[4] They show that FEC can be defluorinated by Lewis acids such as PF₅, generating HF and other acids that are detrimental to the battery.

Herein, we have studied the effect of FEC in LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/Li cells at room- and elevated-temperatures. In addition, we have investigated the possible mechanism of such fast degradation of LiPF₆- and FEC-based electrolytes and approaches to inhibit it.

At room temperature, NMC/Li cells containing FEC additive in the electrolyte presented longer cycle life than the FEC-free electrolytes. However, both electrolytes showed poor electrochemical performance when cycled at 55°C (Fig. 1a). Interestingly, we observed that the LiPF₆-based electrolytes which also contains FEC undergo color change when stored at 55°C precipitating a brown solid (Fig. 1b). Shelf life of LiPF₆ in FEC solution was investigated by nuclear magnetic resonance (NMR) confirming the hydrolysis of LiPF₆ to phosphoric acid. Furthermore, the obtained solid was identified as a fluorine-based crosslinked polymer derived from FEC. Incorporation of a moisture scavenger compound, such as lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), in the electrolyte formulation can slow down the degradation process (Fig. 1b) and improve the battery performance at 55°C (Fig. 1c).

[1] J. Electrochem. Soc. 2017, 164, A5019-A5025. [2] ACS Energy Lett. 2017, 2, 1337−1345. [3] J. Power Sources, 2018, 400, 147–156. [4] Electrochim. Acta, 2017, 225, 358–368.

Figure 1

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, these materials are suffering severe capacity fade caused by surface reconstruction, unstable cathode-electrolyte interface (CEI) formation and micro crack formation during cycling, especially when a high cut off charge voltage is applied (such as 4.6 and 4.8V). To understand the origin of this capacity fade, state-of-the-art diagnostic tools are applied to understand their structure, chemical and morphological properties of LiNi_0.94Co_0.06O₂ (NC) and LiNi_0.92Co_0.06Al_0.02O₂ (NCA) cathode materials during cycling. Electrochemical measurement results show that the NCA cathode delivered higher capacity and stable cyclic performance even cycled at a high cut of voltage of 4.8V. In situ X-ray absorption spectroscopy (XAS) results of NC and NCA electrodes measured during the first and 51^st charge/discharge cycling between the voltage range of 2.8-4.4V showed that both Ni and Co contribute to the capacity by going through the Ni³⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺ redox couples. The Ni-K edge XAS spectra of NC cathode charged to 4.8V showed slightly reduction of Ni⁴⁺ during the high voltage charging. While, XAS spectra of NCA cathode did not showed reduction of Ni⁴⁺ when the cell charged to the higher voltage. More interestingly, C, F, O, Ni and Co soft XAS spectroscopy of NC and NCA electrodes at the different state of charge provided more comprehensive insight into surface and bulk chemical properties. The results showed that much thicker CEI layer formed on the NC surface compared with the NCA cathode, indicated that Al-doping effectively reduced the side reaction of cathode materials with the electrolyte. We have also compared the morphological and chemical evolution of NC and NCA secondary particles using full-filed in-situ transmission X-ray microscopy (TXM) during the initial cycle. Results will be presented at the meeting.

Acknowledgement

This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704.

Sometimes lithium-ion cells show a very insidious type of failure where they display close to 100% of their capacity for about 1000 charge-discharge cycles and then lose most of their capacity in only 100 cycles or so with very little warning to the user. This is called "rollover failure" [1]. Experimental observations show that the likelihood of rollover failure increases with upper cut-off potential of lithium-ion cells and decreases with LiPF₆ concentration in a reasonable range.

Since increasing the upper cut-off potential is essential to increase the energy density of lithium-ion cells, a full understanding of the causes of rollover failure is essential, but this is proving very difficult to attain.

The phenomenon of rollover failure during long-term cycling will be discussed based on a comparison among Li(Ni_0.5Mn_0.3Co_0.2)O₂/graphite pouch cells with different electrolyte and electrode designs undergoing different testing protocols. A few facts can be gleaned from the data:

• For cells charged to the same upper cut-off potential, those showing the highest rates of electrolyte oxidation at the positive electrode (due to electrolyte or cell chemistry changes) are most prone to rollover failure.

• Any cell is more prone to rollover failure if charged to a higher potential. This increases the rate of electrolyte oxidation at the positive electrode.

• When rollover occurs, the impedance at the positive electrode always increases significantly while the impedance at the negative electrode side is relatively stable.

• Lithium metal plating at the negative graphite electrode surface is not always observed during the initial stages of rollover failure.

• Increasing the LiPF₆ concentration properly can delay the occurrence of rollover failure. For example Figure 1 shows the impact of increasing salt concentration from 1.2 M to 1.5 M.

Based on these and other observations, some simple models that integrate electrolyte oxidation, impedance growth and lithium ion diffusion can be postulated but further experimental studies using a variety of methods are required for full understanding.

[1] J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, J. R. Dahn, J. Electrochem. Soc., 160, A1451-A1456 (2013).

Figure 1. Capacity (top panels), normalized capacity (middle panels) and ΔV (bottom panels) versus cycles of Li[Ni_0.5Mn_0.3Co_0.2]O₂/graphite pouch cells filled with 1.2 M LiPF₆ or 1.5 M LiPF₆ in EC:EMC:DMC (25:5:70 vol.%) and 2 wt.%VC+1 wt.%DTD. Cycling was performed between 3V and 4.1V or 4.3V at 20 ^oC with charging/discharging rates at 1C/1C.

Figure 1

The energy density of conventional Li-ion batteries (LIBs) has eventually reached their theoretical limit. Increasing worldwide efforts have been made towards the next generation of energy storage systems including batteries using Li metal anode in combination with various cathodes, including intercalation compounds (such as NMC) or conversion materials (such as oxygen (O₂) and sulfur (S)). For a full demonstration of new battery chemistries, the interfacial stability is essential to long cycle lifespan and battery performance. In general, batteries suffer from "cross-talk" processes upon cycling by the crossover of parasitic chemicals through the electrolyte media [1]. Indeed, "cross-talk" phenomena in Li-ion batteries, such as Mn²⁺ crossover from the cathode and contaminate graphite anode has been well established [2]. Li-S and Li-O₂ batteries also have the shuttle issues of soluble redox-active materials, polysulfides and superoxide radical anions, respectively. However, the similar characteristics in Li metal batteries (LMBs) with conventional Li ion intercalation cathodes have rarely been studied. Therefore, an in-depth understanding of how a Li metal anode affects the cathode interface chemistry is critical to ensure the long-term cycling stability of LMBs.

Here, we report the cathode failure triggered by the chemical "cross-talk" between the electrode pair in rechargeable LMBs. In sharp contrast to LIBs, the cathode in LMBs suffers more significant and irreversible capacity fade during cycling, and its capacity cannot be fully recovered in spite of repeated replacement with new Li metal in the successive cycling. In-depth characterizations of the cathode surface reveal severe deterioration of cathode electrolyte interphase related to the significant accumulation of highly resistive polymeric components and lithium fluoride. Extensive salt-anion decomposition at Li metal surface can cause the chemical aging of the electrolyte allowing the migration of soluble byproducts toward the cathode side, resulting in the severe deterioration of cathode and separator surfaces. A selective Li-ion permeable separator with polydopamine coating has been developed to mitigate the detrimental chemical crossover and enhance the cathode stability.

[1] Mikhaylik, Y. V.; Akridge, J. R., Polysulfide Shuttle Study in the Li/S Battery System. J. Electrochem. Soc. 2004,151 (11), A1969-A1976

[2] Vetter, J.; Novák, P.; Wagner, M. R.; Veit, C.; Möller, K. C.; Besenhard, J. O.; Winter, M.; Wohlfahrt-Mehrens, M.; Vogler, C.; Hammouche, A. Ageing mechanisms in lithium-ion batteries. J. Power Sources 2005,147 (1), 269-281.

Figure 1

In this work, we seek to answer the following question: "How long will a battery last if it is discharged at its maximum possible power?" Various primary battery chemistries (Zn-MnO₂, Ag-AgO, Zn-air) and secondary battery chemistries (Ni-MH, Li-ion) are investigated. The maximum power point tracking (MPPT) algorithm, which has been used in photovoltaic devices so far, is applied to draw the maximum power possible from these batteries.

In the MPPT algorithm used here, a fixed magnitude of current is applied and the new value of power is calculated. If the new value of power is greater than the previous one, the current is varied in the same direction until the maximum power is reached. This algorithm is simple to implement and consumes low computing power. However, the magnitude of a current step should be optimized. If the current step is very small, the time taken to reach the maximum power will be too long. And if the current step is very large, there will be a large oscillation near the power maxima.

We present the effect of current steps in reaching the maximum power on batteries. The optimal current step is different from one battery to another. For fair comparison, the current steps are normalized to the discharge rate to achieve 1 C capacity. For rechargeable batteries, the effect of charging rates on the maximum power obtainable is also investigated.

The information obtained from this study can be a useful guide to designing a battery management system that gets the maximum performance out of any battery.

References

• Selvan, P. Nair and Umayal, "A review of phot voltaic MPPT algorithms," International Journal of Electrical and Computer Engineering, 6, 2016, 567-582.

• A. B. Vieira and A. M. Mota, "Maximum power point tracker applied in batteries charging with PV panels," 2008 IEEE International Symposium on Industrial Electronics, Cambridge, 2008, 202-207.

• Kim, S. Mohan, J. B. Siegal and A. G. Stefanopoulou, "Maximum power estimation of lithium-ion batteries accounting for thermal and electrical constraints," Proceedings of the ASME 2013 Dynamic Systems and Control Conference, California, 2013.

Silicon alloying anodes form an attractive technology in the ongoing evolution of Li-ion batteries. Their easy of availability, low cost, large theoretical specific capacities and low alloying potentials versus lithium (Li) would allow easy commercial adoption with ~10-15 wt% silicon already in use in the industry. To increase the available areal energy density, the mass loading of silicon needs to be increased. In this respect, the volume expansion in silicon (of ~ 300%) during cycling needs to be mitigated, to prevent mechanical failure and loss of electrical contact, both of which severely impede the capacity retention. While traditional techniques such as the use of intermetallics, conformal carbon coatings and tailoring the morphology, shape and size of the silicon anodes have attained reasonable success, polymeric organic binders and ionic liquid electrolyte have recently gained popularity. Polyacrylic acid-based binders (PAA) are ideal due to their high mechanical flexibility while ionic liquids form more stable SEI species compared to carbonate-based electrolytes. However, interactions between the binder and the ionic liquid salts as well as its influence on the SEI has not been considered, due to the traditional held view of the binder being a soft backbone matrix. Herein we report on the effect of PAA and CMC binder on the rate, composition and depth distribution of the SEI species in silicon anodes. We use STEM-EDX (scanning transmission electron microscope-energy dispersive x-ray spectroscopy), EELS (electron energy loss spectroscopy) amd XPS (X-ray photoelectron spectroscopy), to study and observe the distribution of LiF and sulphides/sulphates in the SEI. Based on our observations we propose a mechanism for better capacity retention in electrodes with PAA binder, due to formation of passivating sulphides close to the silicon surface which is a direct consequence of faster LiFSI decomposition to LiF in presence of PAA binder.

All-solid-state batteries have attracted great attention because of their high safety owing to the use of non-flammable inorganic solid electrolytes. Solid electrolytes with an extremely high ionic conductivity have already been discovered, and therefore the discovery of new and efficient electrode materials is key for the construction of high energy density all-solid-state batteries. In the electrode layers for conventional all-solid-state batteries, mixtures composed of active material, solid electrolyte and carbon conductive additive powders are often applied to secure lithium ionic and electronic conduction pathways. Moreover, most of all-solid-state batteries reported so far comprise a thin electrode layer with a small amount of active materials and a thick electrolyte layer as the separator. Therefore, the energy density of all-solid-state batteries at the present stage is considerably lower than that of typical lithium ion batteries. To enhance the energy density of all-solid-state batteries, high capacity electrode materials should be developed and the active material content should be increased in the composite electrode layers.

With consideration to the demands and features described above, we have successfully developed a novel Li₂Ru_0.8S_0.2O_3.2 (80Li₂RuO₃·20Li₂SO₄ in mol%) positive electrode material for all-solid-state batteries. Mechanochemical treatment with Li₂SO₄ imparts ionic conductivity and favourable ductility to the Li₂RuO₃ active material. Because of the favourable formability and high electronic and ionic conductivities, we achieved the fabrication of all-solid-state cells, where the positive electrode layer was composed of the active material without any conductive additives. The all-solid-state cell using a monolithic Li₂Ru_0.8S_0.2O_3.2 positive electrode functioned as a secondary battery showing a high reversible capacity of about 270 mAh g^-1. In this study, the detail charge-discharge mechanism of the Li₂Ru_0.8S_0.2O_3.2 positive electrode was investigated by XRD, TEM, XPS, and XAFS measurements.

All-solid-state batteries are a potentially safer and more energy dense alternative to the modern lithium ion battery, replacing flammable organic liquid electrolytes and enabling lithium metal anodes. While solid electrolytes have been developed with ionic conductivities far in excess of modern liquid organic electrolytes, electrolyte-electrode interfacial resistance remain an issue. In spite of their fundamental importance, the nature of these interfaces remains obscured, primarily due to the limited methods of characterization of both lithium metal and buried interfaces alike. Recently, cryogenic microscopy techniques—traditionally employed by biological field—have been employed to lithium ion batteries, elucidating the nature of solid-electrolyte interfaces and lithium metal morphologies alike. Of similar difficulty, observation of solid-solid interface dynamics is limited to very few techniques, and has only been demonstrated recently, primarily via in situ transmission electron microscopy (TEM) methodologies.

We present developments in the application of cryogenic focused ion beam (FIB) and in situ TEM to enable fundamental characterization of solid-solid interfaces for lithium ion battery applications. Use of cryogenic temperatures is shown to clearly reduce damage induced in the lithium metal, preserving the morphology and structure, as demonstrated by both three-dimensional FIB slice-and-view and the observation of crystallinity in cryogenic-TEM. This technique is further extended to polymer and sulfide electrolytes, overcoming instability during FIB milling and permitting further characterization via cryo-TEM. Such cryo-prepared samples enable lithium metal for in situ TEM cycling of metal-anode-based solid-state batteries. Further, progress in in situ characterization of solid-state interfaces by TEM will be presented, showing developments in the extraction of electrochemically active nanobatteries for in situ testing applications, providing a platform for dynamic solid-solid interface characterization.

Si is a promising negative-electrode material for next-generation lithium-ion batteries due to its high gravimetric and volumetric capacities. A solid electrolyte interphase (SEI) forms on the Si negative electrode owing to reductive decomposition of an electrolyte solution in the initial charging process. Although the SEI layer strongly affects the charge/discharge performance, details of its formation process have not been clarified yet. Therefore, understanding of the SEI formation process is important to improve the cycle performance of Si negative electrodes.

In recent years, glyme-based concentrated electrolyte solutions have attracted attentions because of their striking features; dissolution of polysulfide anions is suppressed in Li-S batteries, and dendrite formation is suppressed in Li metal batteries [1, 2]. In this study, the SEI formation process on Si-thin-film model electrodes was observed by in situ atomic force microscopy (AFM) coupled with cyclic voltammetry (CV) in lithium bis(fluorosulfonyl)imide (LiFSI)/tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/G4 electrolyte solutions.

Si thin films (100 nm) were deposited on mirror polished Cu substrates by RF magnetron sputtering. In situ AFM coupled with CV was performed using a liquid immersion sample stage designed for electrochemical measurements, consisting of the Si-film working electrode, and Li-wire counter and reference electrodes, at a sweep rate of 0.5 mV s^-1 between 1.9 and 0.02 V (vs. Li⁺/Li). The electrolyte solution used was 4.5 M LiFSI or LiTFSI dissolved in G4. AFM scan was conducted in the contact mode at room temperature, and typical scan area was 5 × 5 mm². In situ AFM observation was conducted in an Ar-filled glovebox with a dew point of about -60°C.

Small cathodic currents were observed at potentials below about 1.8 V during the initial potential sweep in both LiFSI/G4 and LiTFSI/G4. The cathodic currents suddenly increased at around 1 V due to the reductive decomposition of the electrolyte solutions. The cathodic current in LiFSI/G4 was larger than that in LiTFSI/G4, indicating that the LiFSI/G4 is more vulnerable to reduction. In fact, in situ AFM observation revealed that some precipitates appeared on the Si surface at around 1 V in LiFSI/G4, which suggests the formation of SEI. In contrast, no morphological change was seen in LiTFSI/G4 during the initial charging process. These results suggest that the decomposition products of LiTFSI/G4 can hardly form a stable SEI on the Si electrode in the initial cycle. The surface cracked after the 2nd potential cycle in both the electrolyte solutions. The crack progressed with repeated potential cycling, which was more significant in LiTFSI/G4. The redox currents due to the alloying/dealloying reaction of Si with Li decreased more rapidly with potential cycling in LiTFSI/G4. Based on the above results, the formation of a stable SEI in the initial cycling in LiFSI/G4 largely contributes to the suppression of crack formation and the superior performance of charge/discharge cycles.

The effect of a fluoroethylene carbonate (FEC)-derived SEI on morphological changes of the Si film electrode will also be presented in our poster [3].

Reference

[1] H. Wang et al., ChemElctroChem., 2, 1144-1151 (2015)

[2] K. Yoshida et al., J. Am. Chem, Soc., 133, 13121-13129 (2011).

[3] M. Haruta et al., J. Electrochem. Soc.,165, A1874-A1879 (2018)

Lithium-sulfur (Li-S) batteries are a promising technology to reach a target of 500-700 Wh kg^-1 as a replacement for current commercial Li-ion batteries in a number of sectors. They also offer lower environmental impact with reduced costs for the cathode materials. However, the level of currently achievable performance is still below expectations. This is due to a lack of understanding of the fundamental electrochemical processes and mitigating the undesired reactions during Li-S battery cycling. In particular the so-called redox shuttle effect, which arises from the solubility of polysulfides (PSs, Li₂S_x) species, and their potential to reduce capacity retention of the battery. Modifying electrolytes with additives and new solvents change the solubility of these intermediates and potentially improve the cycling performance of the battery.

Herein, the effects of using LiNO₃ as an additive as well as C₄mpyr-based ionic liquid electrolyte on the performance of Li-S cells are analysed using electrochemical, in situ X-ray powder diffraction (XRD) and ex situ soft X-ray absorption spectroscopy (sXAS) techniques. Whilst both LiNO₃ and C₄mpyr-based IL participate in forming a protective stable SEI layer on the lithium anode, our studies have provided further evidence for the suppression of Li₂S deposition on the electrodes when using LiNO₃ salt as an additive in the electrolyte, leading to higher capacity and better capacity retention compared to the additive-free electrolyte. In addition, based on XRD data, different species of Li₂S_x were detected during cycling of cells with organic and IL-based electrolytes, which indicates that different (electro)chemical reactions occur in these environments. Overall, cells with electrolytes containing ionic liquids and LiNO₃ showed more stable cycling.

LiFeSO₄F can be synthesised in two different polymorphs, tavorite- and triplite-type, depending on the synthesis conditions¹. We have used a solvothermal route to synthesise the tavorite-type, which offers a decent energy storage capacity with an open crystal framework providing fast solid-state Li-ion transport². It's functionality is strongly affected by surface modification. It shows minimal polarization during electrochemical cycling when coated with p-doped PEDOT³. This coating alleviates a kinetic barrier for its lithium insertion/extraction reactions⁴. The redox activity of this material has been studied with S and Fe K-edge X-ray Absorption Near Edge Spectroscopy (XANES) together with Raman and ⁵⁷Fe Mössbauer spectroscopies. The results indicated a stable p-doping of the PEDOT coating in the entire potential window for the electrochemical cycling. The iron-ligand electronic interaction increased during delithiation.

Fig. 1. Linear combination fit of S K-edge XANES spectrum for LiFeSO₄F-PEDOT conditioned at 2.5 V vs. Li⁺/Li.

Ref: 1. N. Recham et al. Nat. Mater. 9 (2010) 68–74. 2. R. Tripathi et al. Chem. Mater. 23 (2011) 2278–2284. 3. A. Sobkowiak, et al., Chem. Mater. 25 (2013) 3020–3029. A. Blidberg, et al. ChemElectroChem 4 (2017) 1896–1907.

Figure 1

To solve the low-energy efficiency problem of Li-O₂ batteries, many kinds of redox mediators (RMs) have been studied. However, systematic research looking into the problems of RMs in these systems are insufficient. We compare herein effects and problems of RMs in Li-O₂ batteries by applying unique methodology, based on two types of cells, comparison between argon and oxygen atmospheres and combining electrochemistry in conjunction with spectroscopy. Using systematic electrochemical measurements, representative RMs in Li-O₂ battery prototypes were thoroughly explored with respect to oxygen presence, voltage ranges and scan rates. By this comparative, multi-parameters study we reached valuable insights. We identified possible routes for RMs degradation in Li-O₂ batteries related to the cathode side, using bi-compartments cells with solid electrolyte that blocks the crossover between the cathode and the Li metal sides. Based on comparative research, we confirmed that degradation of the RMs activity was caused by intrinsic decomposition of the RMs in the electrolyte solution at the cathode part, even before further reactions with reduced oxygen species. This work provides a realistic view of the role of important RMs in Li-O₂ batteries and suggests guidelines for selecting suitable RMs, mandatory additive in Li-O₂ batteries electrolyte.

References

• W.-J. Kwak, H.-G. Jung, D. Aurbach, and Y.-K. Sun, Adv. Energy Mater. 2017, 7, 1701232.

• W.-J. Kwak, J.-B. Park, H.-G. Jung, and Y.-K. Sun, ACS Energy Lett. 2017, 2, 12, 2756.

• H.-G. Jung, J. Hassoun, J.-B. Park, Y.-K. Sun, and B. Scrosati, Nat. Chem. 2012, 4, 579.

Detailed analysis of the microstructural changes during lithiation of a full-concentration-gradient (FCG) cathode with an average composition of Li[Ni_0.75Co_0.10Mn_0.15]O₂ was performed starting from its hydroxide precursor, FCG [Ni_0.75Co_0.10Mn_0.15](OH)₂ prior to lithiation. Transmission electron microscopy (TEM) revealed that a unique rod-shaped primary particle morphology and radial crystallographic texture were present in the pre-lithiation stage. In addition, TEM detected a two-phase structure consisting of MnOOH and Ni(OH)₂, and crystallographic twins of MnOOH on the Mn-rich precursor surface. The formation of numerous twins, driven by the lattice mismatch between MnOOH and Ni(OH)₂, resulted in crenellated surfaces. Furthermore, the twins persisted in the lithiated cathode; however, their density decreased with increasing lithiation temperature. Cation disordering, which influences cathode performance, was observed to continuously decrease with increasing lithiation temperature with a minimum observed at 790 °C. Consequently, lithiation at 790 °C (for 10 h) produced optimal discharge capacity and cycling stability. Above 790 °C, an increase in cation disordering and excessive coarsening of the primary particles led to the deterioration of electrochemical properties. It is conjectured that the twins in the FCG cathode precursor promoted the optimal primary particle morphology by retarding the random coalescence of primary particles during lithiation, effectively preserving both the morphology and crystallographic texture of the precursor. Furthermore, the effects of the precursor microstructure and subsequent final primary particle morphology on the optimal lithiation schedule and cation disordering were determined.

• Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak, K. Amine, Nat. Mater.2009, 8, 320.

• J. H. Lee, C. S. Yoon, J.-Y. Hwang, S.-J. Kim, F. Maglia, P. Lamp, S.-T. Myung, Y.-K. Sun, Energy Environ. Sci.2016, 9, 2152.

• U.-H. Kim, E.-J. Lee, C. S. Yoon, S.-T. Myung, Y.-K. Sun, Adv. Energy Mater.2016, 6, 1601417.

• C. S. Yoon, K.-J. Park, U.-H. Kim, K. H. Kang, H.-H. Ryu, Y.-K. Sun, Chem. Mater.2017, 29, 10436.

In the search for high-energy Li-ion positive electrodes, the layered lithium NMC oxides have been extensively studied. Despite wide interest in this system, the phase diagram remained poorly understood until recently, which resulted in many conflicting reports in the literature. Combinatorial synthesis, coupled with X-ray diffraction was used to determine the phase diagrams under various synthetic conditions [1-2]. The unexpected complexity seen in this system included multiple 3-phase regions that transform during cooling along with boundaries to single phase regions which also shift during cooling. Even with a deepened understanding of the structural phase diagram, there is still minimal knowledge of how the electrochemical properties evolve across these complex phase spaces. Herein, we adapt a high-throughput electrochemical testing system wherein 64 samples are cycled simultaneously in order to measure the cycling of mg-scale powder combinatorial samples. The methodology involves using a solution-dispensing robot to make the samples by co-precipitation synthesis, followed by high temperature annealing. The samples are subsequently mounted in the combinatorial electrochemical cell and cycled simultaneously. Figure 1 shows cyclic voltammograms obtained for identical LiCoO₂ samples (weighing at most 2.5 mg) in the combinatorial cell. A remarkable level of consistency is achieved in comparison to published cyclic voltammograms for LiCoO₂ [3]. The described methodology allows for the determination of specific capacity, with an RSD approaching 10%. Given that these combinatorial samples are powders, synthesized using methods comparable to those used commercially, the results scale-up very well. The proof-of-concept of this novel high-throughput electrochemical technique will be presented, exploring the level of precision that can be achieved for important electrochemical metrics (redox potential, specific capacity, irreversible capacity, voltage during storage experiments, etc.). Preliminary results from this combinatorial electrochemistry methodology using the Li-Mn-Ni-O system will be displayed, and the ramifications of electrochemically probing this critical composition space will be examined.

[1] McCalla, E., Rowe, A. W., Shunmugasundaram, R., & Dahn, J. R. (2013). Structural study of the Li–Mn–Ni oxide Pseudoternary system of interest for positive electrodes of Li-ion batteries. Chemistry of Materials, 25(6), 989-999.

[2] Brown, C. R., McCalla, E., Watson, C., & Dahn, J. R. (2015). Combinatorial study of the Li–Ni–Mn–Co oxide pseudoquaternary system for use in Li–Ion battery materials research. ACS combinatorial science, 17(6), 381-391.

[3] Cho, J., Kim, Y.J., Park, P. (2001). LiCoO2 cathode material that does not show a phase transition from hexagonal to monoclinic phase. Journal of the Electrochemical Society, 148(10), A1110-A1115

Figure 1: Cyclic voltammetry of LiCoO₂ performed in the high-throughput electrochemical testing system

Figure 1

As the loading level or thickness of LIB composite electrodes increases, it becomes more difficult to uniformly distribute all components within the electrode. Because electric conductors are very small and light, they tend to rise upwards during drying process. Especially, to speed up the electrode fabrication process, not only drying temperature but also coating speed is maximized for reducing production costs. In this harsh condition, the polymeric binder tends to be unevenly distributed depending on the depth of electrode. Nevertheless, due to limited analysis tools, it had been left as research area to explore. Meanwhile, since a new tool, surface and interfacial cutting analysis system (SAICAS), was developed, the adhesion properties of the composite electrodes could be measured deliberately while changing the depth.

Herein, we attempted to unveil the polymeric binder distribution by measuring the adhesion strength at different depths. In order to increase the reliability of the analytical results, composite electrode samples having different binder compositions and distributions were prepared and used for this study. At the same time, the compositional analysis with x-ray photoelectron spectroscopy were also conducted to confirm the binder distribution as a function of electrode depth. Finally, we investigated the effect of binder distribution on the electrochemical properties as well.

After many charge-discharge cycles of showing little to no capacity fade, Li-ion cells can undergo rapid degradation in capacity that occurs over relatively few cycles [1]. We refer to this sudden, accelerated capacity loss as "rollover" failure. "Rollover" failure should be concerning to manufacturers and academics alike, because it can be difficult to predict when it will occur and can require years of cycling to verify. In some instances, "rollover" failure can be caused by impedance growth during cycling that eventually limits the available lithium inventory at a particular charging current due to an ohmic voltage drop. This impedance growth is followed by true loss of lithium inventory by lithium plating.

We show that the impedance growth of the cell during cycling is, among many other factors, strongly tied to the concentration of salt used in the electrolyte. Using more salt (up to a reasonable limit) reduces cell impedance at all frequencies, provides better impedance control during cycling and extends the number of cycles until "rollover" failure. Ultra-High Precision Coulometry and Electrochemical Impedance Spectroscopy combined with post failure electrolyte analysis by Li-ion Differential Thermal Analysis, Gas Chromatography Mass Spectrometry and Inductively Couple Plasma Mass Spectrometry provide clues of how the cell and electrolyte change with varying salt concentration, and as the cell fails.

Finally, we propose a means of testing to accelerate "rollover" failure. Cycling protocols with long constant voltage segments at the top of charge are shown to accelerate impedance growth and "rollover" failure. Using this cycling protocol on cells with electrolytes containing low salt concentrations can reduce the time to "rollover" to a few months. With traditional cycling and good cells, containing electrolytes with 1M – 1.2M salt concentrations and good electrolyte additives, this can take years. We believe that cycling with long periods at high potential, of cells with low salt concentration electrolytes is an accelerated means of testing to quickly screen electrolyte additives, electrode coatings and other cell material choices.

Figure 1 shows the discharge capacity and ΔV (difference between average charge and discharge voltages) versus cycle number of cells the follow our prescribed method to accelerate "rollover". The cells contained electrolytes with varying salt concentrations and spend 24 hours at 4.4V every second charge-discharge cycle. Figure 1 clearly shows that lifetime is increased with increased LiPF₆ concentration. Similarly, Figure 1 shows that use of the electrolyte additive LiPO₂F₂ (LFO) extends lifetime when compared to the combination of fluoroethylene carbonate (FEC) and dioxathiolane-2,2-dioxide (DTD). Cells with longer lifetimes show better impedance control, as evidenced by ΔV. Figure 2 shows discharge capacity and ΔV versus cycle numbers for cells that are tested using typical CCCV cycling to 4.3V. The comparison between FEC and DTD versus LFO is the same as in Figure 1, but the data in Figure 2 took 8 months to collect and distinguish the two additive systems. This is eight times longer than it took to distinguish the two additive systems using cells with 0.2 M LiPF₆ that were held at 4.4V for 24h every second charge, as shown in Figure 1.

"Rollover" failure can be prevented by ensuring that cell impedance remains constant. Increasing LiPF₆ concentrations appears to control impedance, as does avoiding extended times at high voltage. Doing the opposite results in a high throughput screening method than can quickly distinguish the lifetime benefit of small changes in cell chemistry, like electrolyte additives.

[1] J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, J. R. Dahn, J. Electrochem. Soc., 160, A1451-A1456 (2013).

Figure 1

High-Energy Nickel Cobalt Manganese Oxide (HED-NCM) is a state-of-art positive electrode material which achieves a high specific capacity (~250 mAh/g). Overall cycling life is limited by electrode degradation, stemming mainly from oxygen release as well as transition metal dissolution. To mitigate the latter problem, a thin layer of aluminum oxide (Al₂O_­3­) can be applied to the active material by atomic layer deposition (ALD) to form an artificial cathode-electrolyte interphase (CEI). Testing has shown that the Al₂O₃ coating improves overall reversible capacity, most notably at high temperature (40^oC). However, effect of the Al₂O₃ on the CEI and SEI formed upon cycling has yet to be understood. The cycling performance and surface chemistry Al₂O₃ coated HED-NCM materials cycled at room temperature and 40°C were investigated via ATR-FT-IR and XPS spectroscopies, and ICP-MS after the first and 50^th cycles. Overall, electrodes made with Al₂O₃ coated material had lower concentrations of polyethylene carbonate and fluorine species, and Al₂O_­3­ coating helps in preventing transition metal dissolution.

Operando analytical techniques for lithium ion batteries (LIBs) have proven to be valuable tools to gain insight into the processes ongoing during battery cycling. Among these techniques, operando (or online) electrochemical mass spectrometry (OEMS) is a very versatile tool for tracking reactivity of cell components during its operation.

Mass spectrometry made a first appearance as analytical technique for LIBs about two decades ago in the course of studies about the mechanisms of electrolyte reduction on graphitic surfaces (1). However, the then-used experimental cells required large amounts of electrolyte and had rather moderate detection limits. In recent years the cell design and overall experimental setup underwent significant overhaul, thus allowing the identification and quantification of various gases down to ppm level. This, in turn, allowed the more extensive application of OEMS to investigate a broad variety of LIB-related processes and it has been used by us and other groups to study: cell-component reactivity and electrolyte decomposition (2), SEI formation on graphite/silicon composite anodes (3), gas release from cathode active materials due to irreversible anionic oxygen redox (4), surface reconstruction (5) and surface impurities (6).

In this contribution we present the current OEMS setup at PSI, which can be used for measurements down to the ppm level, and highlights from our recent research. One example is the application of the electrolyte additive TMSPa as chemical probe: it allows to monitor the formation of inorganic fluorides such as HF and LiF during cell operation, which in turn made it possible to gain insight into detrimental side-reactions taking place during cell operation (7). Another example is the application of OEMS to study the mechanisms in beyond Li-ion technologies, such as sodium batteries. The study covers the surface reactivity of the graphite anode which acts as a ternary intercalation compounds in the investigated system.(8) The gas release here showed significant differences compared to the Li system stemming from the (electro)chemical reactivity of the conduction salts as well as the sodium anode. The obtained data allowed to draw conclusions for the further optimization of the Na-ion batteries as well as for future OEMS studies in the field of sodium ion batteries.

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(1) R. Imhof, P. Novák, J. Electrochem. Soc. 145,1081-1087 (1998).

(2) A. Guéguen, D. Streich, M. He, M. Mendez, F. F. Chesneau, P. Novák, E. J. Berg, J. Electrochem. Soc. 163 A1095-A1100 (2016); M. Metzger, B. Strehle, S. Solchenbach, H. A. Gasteiger, J. Electrochem. Soc. 163 A1219-A1225 (2016).

(3) R. Jung, M. Metzger, D. Haering, S. Solchenbach, C. Marino, N. Tsiouvaras, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc. 163 A1705-A1716 (2016).

(4) E. Castel, E. J. Berg, M. El Kazzi, P. Novák, C. Villevieille, Chem. Mater. 26, 5051-5057 (2014); K. Luo, M. R. Roberts, R. Hao, N. Guerrini, D. M. Pickup, Y. S. Liu, K. Edstrom, J. Guo, A. V. Chadwick, L. C. Duda, P. G. Bruce, Nat. Chem. 8, 684-691 (2015).

(5) D. Streich, C. Erk, A. Guéguen, P. Müller, F. Chesneau, E. J. Berg, J. Phys. Chem. C 121, 13481-13486 (2017); R. Jung, M. Metzger, F. Maglia, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc. 164 A1361-A1377 (2017).

(6) S. E. Renfrew, B. D. McCloskey, J. Amer. Chem. Soc. 139, 17853-17860 (2017).

(7) C. Bolli, A. Guéguen, M. Mendez, E. J. Berg, Chem. Mater. submitted.

(8) G. Mustafa, C. Bolli, E. J. Berg, P. Novák, K. Pollok, F. Langenhorst, M. v. Roeder, O. Lenchuk, D. Mollenhauer, P. Adelhelm, Adv. Energy Mater. 8, 1702724 (2017).

Li-ion all-solid-state batteries promise to enhance safety and performance compared to conventional liquid based batteries. Nevertheless, only few investigations have been carried out on the effect of the solid electrolyte's ionic conductivity on the electrochemical performance of batteries in order to visualize non-uniform fields of transport.

Neutron Imaging (NI) is a non-intrusive technique that offers the possibility of tracking light elements such as the ⁶Li isotope during all phases of battery operation. Because it helps visualizing lithium's concentration profiles, NI allows quantifying Li-ion transport and enables us to model solid state batteries. We therefore expect NI to help us explain solid state batteries' poor rate capabilities. A reliable electrochemical cell was successfully designed and utilized for an operando test of an all-solid-state battery using PSI's high-resolution neutron imaging detector ('Neutron Microscope') at the POLDI beamline of Swiss Spallation Neutron Source SINQ (Paul Scherrer Institut). A solid state battery with a composite electrode TiS₂:⁶Li₃PS₄(⁶LPS) and a ⁶Li-In alloy counter electrode have been investigated (Figure 1) and the results will be discussed.

Figure 1

Fast charging protocols for lithium-ion batteries are critical for widespread adaptation of electric vehicles. However, a limited understanding of degradation modes during fast charging and the large manufacturing variability of commercial lithium-ion batteries are major challenges to the development of high-performing fast charging protocols. In this work, we optimize a six-step charging protocol for commercial 18650 lithium-ion batteries that achieves 80% state of charge in ten minutes. We employ two key elements to reduce the optimization cost: early prediction of failure, which uses cycling data from the first 100 cycles to predict cycle lives that reach up to 1200 cycles, and adaptive Bayesian optimal experimental design, which reduces the number of experiments required. We identify promising fast charging protocols with identical charging times but high lifetimes out of a candidate pool of 224 protocols. This method can be extended to accelerate development of other tasks in battery manufacturing and deployment, such as formation cycling and state-of-health estimation.

High energy nickel-rich NMC (LiNi_xMn_yCo_zO₂; x+y+z≈1, x≥y+z) cathodes offer promising theoretical high capacity cell metrics. As the nickel content increases, the cathode exhibits a decrease in capacity retention, structural stability, and thermal stability. Herein, the thermal stability of chemically delithiated NMC-811 is investigated as a function of lithium content. A myriad of synchrotron characterization techniques are used to understand the role of the transition metals on structure stability at elevated temperatures. The in-situ heating experiments capture both chemical and morphological changes within the material that yield key particle size information on thermal stability. The bulk and surface sensitive techniques capture intermediate states between the transition metal 3d-oxygen 2p bonds at low temperatures, which inevitably can lead structural changes. This work demonstrates the importance in developing robust cathode materials for lithium ion batteries.

For functional materials used for lithium ion batteries, their electrochemical properties are highly dependent on their defect structures. For examples, the reversible capacity of layered cathode such as LiCoO₂ is limited by its highly defective layered structure when state of charge greater than 50%. On the other hand, capacity of spinel or olivine cathodes show higher delithiation rate without noticeable phase change. Furthermore, with oxygen defects formed in Li₂MnO₃, the capacity of Li-rich layer-structured cathode formulated as xLi₂MnO₃-(1-x)LiMO₂(M = Mn, Ni, Co, etc.) was able to provide high reversible capacity (>250 mAh/g) for Li-ion battery applications. Similarly, the zero-strain anode materials Li₄Ti₅O₁₂ shows high interface polarization and low rate capability due to low electronic defects. Hopefully, the rate capability may be improved by introduction of more electronic defects on Ti cation sublattice.

In additions, the properties of solid state Li ion conductors for all-solid-state lithium batteries is strongly affected by their defect structures as well. Due to its better safety characteristics, solid-state lithium battery may use more reactive anode, such as Li. Thus, it is extremely crucial to find a stable solid electrolyte when the high-capacity Li anode is used. In our study, the stability of perovskite-based La_0.50Li_0.50TiO₃, NASICON-based Al-doped LiTi₂(PO₄)₃, and garnet-based Li₇La₃Zr₂O₁₂ against metallic Li were investigated. After La_0.50Li_0.50TiO₃ reacted with Li, the electron injection accompanied by the incorporation (insertion) of Li ion into vacant cation sites was likely to take place. The apparent reduction of tetravalent Ti into trivalent Ti was observed in La_0.50Li_0.50TiO₃. On the other hand, Al-doped LiTi₂(PO₄)₃, and garnet-based Li₇La₃Zr₂O₁₂ are much more stable when Li anode is used. For these solid electrolytes, the difference in stability against metallic Li may be rationalized based on their defect structures and corresponding atomic arrangements.

Nickel-rich lithium transition metal oxides have been recently considered as one of most promising cathode materials for high energy density lithium-ion batteries. However, the instability of the cathode electrolyte interface has been the major technological barrier for the development of nickel-rich cathodes. The early research has simply assigned this interfacial instability to the electrochemical oxidation of the commonly used carbonate solvents without much discussion on the nature of the parasitic reactions. To shed light on the chemical insight of the parasitic reaction, a proprietary high precision electrochemical system was built in-house to quantitatively measure the rate and kinetics of the side reactions between the delithiated cathode and the non-aqueous electrolyte. Our results clearly indicated the dominant chemical reaction within the working potential window is the chemical, not electrochemical, reaction between the intermediate phase of cathode and the electrolyte, generating locally concentrated protons at the surface of the cathode materials. Detailed investigation demonstrates that the generated proton is the chemical connection between the interfacial stability and the electrochemical performance of the then cathode material.

Na-ion batteries have attracted increasing attention as large-scale energy storage devices. Hard carbon is a standard anode material owing to its large capacity and good cycling stability^{1, 2}. However, sodium storage mechanism of hard carbon has not been fully understood due to its low crystallinity and diversity of microstructure. This study aimed to systematic understanding of the relationship between microstructure and sodium storage behavior with special attention to the diffraction signal from sodium cluster confined in nanopores.

Hard carbons were synthesized by hydrothermal treatment of sucrose and subsequent high-temperature carbonizations at T = 1000-1900 °C (denoted as HC-T). The charge-discharge tests were carried out using half-cells. As the heat-treatment temperature increased, the capacity of the high potential region (> 0.2 V) decreased while the plateau region (< 0.2 V) become dominant. HC-1400 showed the highest overall capacity (347 mAh g^-1) and initial coulombic efficiency (95.2%). Reaction mechanism analyses were performed on three representative samples, HC-1000, HC-1400, and HC-1900.

Ex-situ small angle X-ray scattering (SAXS) measurements revealed (i) the nanopores become larger with decreasing their number for higher annealing temperatures, (ii) insertion of sodium into the nanopores proceeded under ca. 0.07 V. When electrodes were overcharged exceeding the sodium deposition voltage (ca. –0.01 V), the sodium density in the nanopores became comparable to that of bulk bcc sodium. Besides, apparent smaller capacity of HC-1900 was analyzed to be a kinetic observation with larger cathodic polarization.

Careful ex-situ X-ray diffraction measurements have detected smaller mean interlayer distances in order of HC-1900 < HC-1400 < HC-1000. Expansion of interlayer distance upon sodium insertion was observed for HC-1000 and HC-1400, but not at all for HC-1900, suggesting the existence of a threshold interlayer distance to allow sodium insertion. Importantly, for the first time, we have succeeded to detect a broad peak appeared around 29° as a signature of sodium insertion into the nanopores, and its origin was analyzed to be diffraction from sodium clusters in the nanopores using DFT-MD simulations.

Based on these analyses, criteria for better hard carbon will be discussed in the poster.

Figure 1

Recently, all-solid-state lithium batteries have attracted global attentions as next-generation batteries because of their higher safety due to the use of nonflammable inorganic solid electrolytes instead of flammable organic liquid electrolytes. Bulk-type all-solid-state batteries employ composite electrodes consisting of electrode active materials and solid electrolytes. There are a lot of solid-solid interfaces in composite electrode layers resulting in inhomogeneous reaction distributions. In order to achieve higher battery performance, it is important to investigate electrochemical reaction mechanisms at the solid-solid interfaces and evaluate reaction distributions in the electrode layers.

A graphite negative electrode is used in commercial lithium-ion batteries. However, there are few papers regarding all-solid-state batteries using graphite composite electrodes.[1] It is worth studying reaction mechanisms of graphite composite electrodes in all-solid-state lithium batteries for practical use in the near future. Colors of graphite particles change from black via dark blue and red to gold during a lithiation process.[2] Observation of color changes enables us to evaluate reaction distributions in a graphite electrode layer. In our previous paper, we compared reaction distributions of composite negative electrodes consisted of graphite and sulfide solid electrolytes with weight ratios of x : 100-x (x = 50, 60 and 70) by ex-situ optical microscopy.[3] The cell using the x = 50 electrode showed the highest reversible capacity of more than 250 mAh g^-1 and homogeneous reaction distributions. In this study, to monitor forming of reaction distributions during charge-discharge cycles, operando optical microscopy was conducted for a graphite electrode layer in an all-solid-state cell.

Composite electrodes were prepared by mixing graphite and 75Li₂S·25P₂S₅ (mol%) glass electrolyte particles with weight ratios of 50 : 50. 75Li₂S·25P₂S₅ glass and lithium-indium alloy were used as a solid electrolyte separator and a counter electrode, respectively. The cell (Li-In/75Li₂S·25P₂S₅ glass/Graphite) was cut to obtain flat cross-sectional observation areas. Operando optical microscopic observation was conducted for the cross-section of the graphite electrode layer at room temperature under a current density of 0.068 mA cm^-2. The cell was mounted under a low confined pressure of ca. 70 kPa in an Ar-filled vessel during optical microscopy.

Optical micrographs for the graphite electrode layer showed that lithiation and delithiation proceeded preferentially for the graphite particles near the solid electrolyte layer. The color changes in the graphite particles were quantitatively evaluated to compare SOC values. During the initial lithiation process, almost all the graphite particles in the electrode layer changed their colors from black to gold. However, after the 3rd lithiation process, only the graphite particles near the electrolyte separator layer showed color changes. The SOC value for the graphite near the separator layer side was more than twice as high as that near the current collector side. This suggested that inhomogeneous reaction distributions were formed in the graphite electrode layer, which resulted in degradation of cycle performances. In addition, the thickness changes of the graphite electrode layer during charge-discharge tests were also examined.

Acknowledgement:

Optical microscopic observation was supported by Lasertec Corp..

References:

[1] K. Takada, T. Inada, A. Kajiyama, H. Sasaki, S. Kondo, M. Watanabe, and R. Kanno, Solid State Ionics, 158 (2003) 269–274.

[2] S. J. Harris, A. Timmons, D. R. Baker, and C. Monroe, Chem. Phys. Lett., 485 (2010) 265–274.

[3] M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, Solid State Ionics, 323 (2018) 123-129.

Yazami¹ reports on the thermodynamics of lithium ion battery. The understanding on heat generation of battery during charge and discharge and on heat dissipation is very important to maintain within appropriate temperature range^2-4. Especially, it is highly important in the case of the battery of large capacity.

Here, drastic discharge with external resistance was studied with experimental and computational approach applying evaluated parameters of entropy, internal resistance, specific heat and open circuit cell potential. The coulombic capacity of used cell was ca 100 Ah. A case of external short was presented as below figure.

Acknowledgment: This study was supported by the KERI R&D program of MSIP/NST (18-12-N0103-01) and 18-02-N0108-02.

References:

• Kazunori Ozawa, Lithium Ion Rechargeable Batteries, Ch. 5, Wiley-VCH Verlag GmbH & Co. KGaA, 2009.

• K. Maher, R. Yazami, Electrochimica Acta, 101 (2013) 71.

• K. Jalkanen, T. Aho, K. Vuorilehto, Journal of Power Sources 243 (2013) 354.

• D.H. Jeon, S.M. Baek, Energy Conversion and Management 52 (2011) 2973

• Dong Hyup Jeon, Current Applied Physics 14 (2014) 196.

Figure 1

In the development of lithium-ion batteries (LIBs), cathode materials are being further improved to achieve not only higher energy capacity but also longer cycle life. Analyzing the valence of the cathode active materials during charge and discharge cycles is one of the practical methods to understand the redox mechanism and degradation mechanism, which can help in achieving higher energy capacity and longer life. The valence has been evaluated using X-ray absorption fine structure (XAFS) measurements with a synchrotron X-ray source. To conduct in-house analyses of valence, we developed a polychromatic simultaneous wavelength dispersive X-ray fluorescence (PS-WDXRF) spectrometer, which is comprising an X-ray tube, a slit, a flat analyzing crystal, and a silicon strip detector (SSD) [1]. The X-ray dispersed by the crystal is simultaneously detected by the SSD, and the detected signals at each channel provide intensity information at corresponding energy. Because there are no moving parts in the optical setup, the spectrometer is possible to detect the chemical changes, including valence changes, of 3d transition metals with high precision.

Two PS-WDXRF spectrometers were developed. One spectrometer comprises a 1280-channel SSD and a Ge (220) crystal to detect X-rays in the range around 5.38 - 6.52 keV, which is designed for manganese. Another spectrometer comprises same components with another one and the range of detecting X-ray energy is 6.27 – 7.89 keV, which is designed for cobalt and nickel.

The first step was to obtain and analyze data of pressed powders of manganese oxides using the spectrometer for manganese. Thus, powders of MnO (II), Mn₂O₃ (III), MnO₂ (IV), and KMnO₄ (VII) were placed in the evacuated chamber of the experimental setup. An X-ray tube voltage of 20 kV and a current of 100 mA was applied, and the target was made from Rh and no filter was used. A 5-min measurement was repeated five times for each sample. The PS-WDXRF analysis showed that the specimens achieved clearly different peak energies which belong to more fine electron transition fluorescence lines. This means that the WDXRF has sufficient energy resolution, and the valence information is obtained by the peak energy shift derived from the chemical shift [2, 3].

The second step was to obtain and analyze data of pressed powders of cobalt oxides and nickel oxides using the spectrometer for cobalt and nickel. Thus, powders of CoO(II), LiCoO₂(III), NiO(II) and LiNiO₂(III) were placed in the evacuated chamber of the experimental setup. An X-ray tube voltage of 20kV and a current of 100 mA was applied, and the target was made from W and no filter was used. This experiment demonstrated the discriminability of fluorescence peak energy shift derived from the chemical shift, in spite of the worse channel resolution. These results indicate that there is a potential for valence evaluation of cathode materials in a short time with highly precise energy resolution.

These spectrometers were applied to the evaluation of actual cathode active materials in LIBs which is including manganese, cobalt or nickel. Some different states of cathode materials were prepared and the valence estimation was performed using the PS-WDXRF spectrometers. The relationship between the valence result and state of charge/discharge was confirmed, which shows that the valence number of LIBs cathode material is in accordance with the reaction formula.

The recent results of valence evaluation on actual cathode active materials in LIBs will be presented in detail.

[1] K. Sato, A. Nishimura, M. Kaino, S. Adachi, X-Ray Spectrom. 46, 330–335 (2017).

[2] J. Kawai, M. Takami, C. Satoko, Phys. Rev. Lett. 65, 2193–2196 (1990).

[3] K. Sakurai, H. Eba, Nucl. Instr. Meth. Phys. Res. B199, 391–395 (2003).

Energy storage media based on different technologies have gained in importance for a wide field of applications ranging from supplying portable devices to large electric vehicles. In recent decades the energy storage technology, based on lithium ions, dominates to the market due to the best compromise between energy and power densities (both gravimetric and volumetric), low self-discharge when not in use, tiny memory effect etc. Despite the overall success of Li-ion technology, a further progress permanently demands for lower-cost, longer-life, higher energy/power density batteries resulting in active development and research to the field. Nowadays modern Li-ion batteries are sophisticated electrochemical devices, possessing numerous degrees of freedom (chemical, morphological, transport) along with complicated geometries of the electrode integration. This along with the need to minimize the risks for possible materials oxidation, electrolyte evaporation, cell charge changes etc. requires new dedicated experimental techniques capable to reveal "live" information about processes occurring inside the cell. In such instance neutron scattering is already a well-established experimental technique for the characterization of Li-ion batteries¹. Neutron scattering methods when combined with electrochemical characterization undergo an increasing relevance for studies of lithium-ion based electrochemical energy storage systems on different length scales, e.g. neutron imaging, reflectometry, small-angle neutron scattering, quasielastic neutron scattering and powder diffraction. Nowadays in situ experiments with neutrons are performed on different self-developed/special test cells and commercial Li-ion cells of diverse designs depending on the research targets and needs.

Simple in principle, but complicated in practice, designs of modern Li-ion batteries may result in spatial inhomogeneity of current, lithium or electrolyte distribution, which are often difficult to quantify, but they will surely affect performance, cycling stability and/or safety. Despite the increasing popularity of neutron scattering studies of batteries at their operating conditions the problem of cell non-uniformity (indirectly pointed by electrochemical measurements) and its effect is often not properly accounted in literature. Here a combination of three neutron-based experimental techniques, namely computed neutron tomography, high-resolution neutron powder diffraction and spatially-resolved neutron powder diffraction, applied in situ for studies on commercial Li-ion cells of the 18650-type will be presented along with results of electrochemical studies. The details of the cell organization on different length scales and its evolution on various factors like state-of-charge, temperature and fatigue will be presented in light of 3D lithium distribution in cylinder-type^2-3 and prismatic⁴ Li-ion batteries.

References:

[1] H. Ehrenberg, A. Senyshyn, M. Hinterstein, H. Fuess, in: E.J. Mittermeijer & U. Welzel (Eds.), Modern Diffraction Methods, Weinheim: Wiley-VCH, 2012, pp. 491-518.

[2] A. Senyshyn, M. J. Mühlbauer, O. Dolotko, M. Hofmann, H. Ehrenberg, Sci. Rep. 5 (2015) 18380

[3] M.J. Mühlbauer, O. Dolotko, M.Hofmann, H.Ehrenberg, A.Senyshyn, J. Power Sources 348 (2017) 145-149

[4] M.J. Mühlbauer, A. Schökel, M. Etter, V. Baran, A. Senyshyn, J. Power Sources 403 (2018) 49-55

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, earth abundant metal. It has a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg/HgO) in concentrated alkaline electrolyte and a high gravimetric discharge capacity of 820 mAh/g. During discharge, Zn oxidizes and forms a dissolved complex known as the zincate ion, Zn(OH)₄^2-, from which zinc oxide (ZnO) precipitates as a solid. On charge, Zn(OH)₄^2- is reduced to metallic Zn. Motivation to study the Zn electrode in rechargeable batteries is driven by a target overall battery energy density of 200 Wh/L that is sustainable for 500 or more charge-discharge cycles. This corresponds to a Zn electrode discharge density in the range of 650 mAh/mL.

The cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease with increasing percentage of discharge capacity of Zn accessed. The percentage of discharge capacity of Zn accessed is known as the depth of discharge (DOD). Zn electrodes were cycled in the range of 1% to 15% Zn DOD in zinc-manganese dioxide (Zn-MnO₂) batteries and in the range of 15% to 30% Zn DOD for zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the zinc electrodes for the tests done on Zn-Ni batteries. Additives used were surfactant cetyltrimethylammonium bromide (CTAB), bismuth oxide (Bi₂O₃), synthetic layered silicate Laponite, and calcium hydroxide (Ca(OH)₂). The potential values of the Zn electrodes vs. Hg/HgO reference electrodes were recorded.

During healthy galvanostatic discharge, Zn electrodes displayed a nearly constant potential vs. Hg/HgO at approximately -1.35 V. Tests that failed due to Zn exhibited a shift from this equilibrium potential that corresponded to a loss of cell voltage. This shift may be attributed to concentration overpotential as ZnO is precipitated, thereby inhibiting the hydroxide ions from reaching the reacting metal interface. The shift may also be due to formation of a passivated layer on the electrode which effectively blocks the remaining active material. There are generally two regimes observed in the potential curves of the Zn electrode vs. Hg/HgO prior to cell failure. Beyond the healthy plateau at -1.35 V, the first regime consists of a sloping potential while the second regime is an additional plateau. The second plateau shifts with increasing cycle number and corresponds to cell death. See the attached figure for discharge curves of a Zn electrode cycled at 5% Zn DOD. The second plateau is believed to be due to passivation of the Zn electrode. The additives were observed to modify the discharge potential curve of the Zn electrode vs. Hg/HgO.

Figure 1

The degradation and ageing of Li-ion batteries will in many cases contribute to reduced thermal stability which potentially affects the safety performance of the batteries. The fact that aging of lithium-ion cells leads to a reduced capacity and cell life, is extensively covered in the literature by several research groups, e.g. Vetter[1]. The safety effects of ageing are far less studied, with only a handful of empirical studies published, e.g. [2-4].

This poster presents ageing data and safety aspects of large commercial Li-ion cells. The cells have been aged and cycled at 5, 25 and 45 °C. Several diagnostic tools have been applied to characterise the ageing mechanisms. These tools include high power pulse characterisation, entropy spectroscopy, incremental capacity analysis and impedance spectroscopy.

The ageing mechanisms are however different at low and high temperatures and this will affect the thermal stability of the aged cells. The thermal stability of cells which have been aged with different ageing mechanisms was characterized with an Accelerated Rate Calorimeter (ARC). It was e.g. observed that a cell cycled at 5 °C for 3000 cycles reaching 70% State-of-Health showed a reduced thermal runaway limit from 240 to 150 °C compared to the uncycled cell. This was also lower than the exotherm onset temperature for the uncycled cell.

References

• Vetter, J., et al., Ageing mechanisms in lithium-ion batteries. Journal of Power Sources, 2005. 147(1-2): p. 269-281.

• Fleischhammer, M., et al., Interaction of cyclic ageing at high-rate and low temperatures and safety in lithium-ion batteries. Journal of Power Sources, 2015. 274: p. 432-439.

• Gilljam, M., et al., 7E. Effect of electrical energy and aging on cell safety, in Safety of Lithium Batteries, J. Garche and K. Brandt, Editors. 2017, Elsevier.

• Friesen, A., et al., Influence of temperature on the aging behavior of 18650-type lithium ion cells: A comprehensive approach combining electrochemical characterization and post-mortem analysis. Journal of Power Sources, 2017. 342: p. 88-97.

Although several crystal structures have been demonstrated and shown reversible capabilities during lithiation/delithiation processes in cathode of Li ion batteries; oxides with layered structure still remain the ones that provide higher capacities.

However, higher charging capacity means the creation of more ionic/electronic defects in both cation and/or anion sublattices of layered structure. Higher defect concentration in either cation or anion sublattices tend to destabilize its crystal structure. As a result, the phase change is frequently observed in repeatedly cycled cathode. Furthermore, the phase-changing layered cathode tends to show the capacity fading after cycling tests. Based on defect consideration, the stability of highly defective materials may be enhanced by using dopant and/or better processing control. In this study, the electrical/electrochemical/structural properties of NMC 811 and LiNi_0.8Co_0.15Al_0.05 will be investigated. Since Ni content is fixed at 80% in the transition metal ion layers in these cathode materials, the variation in electrochemical properties will be caused by the minor elements/dopants such as Mn and Al. First, the electrical measurement of NMC 811 and LiNi_0.8Co_0.15Al_0.05 are conducted using sintered disc. Assembled cells using above-mentioned cathode and Li anode with desired liquid electrolyte will be tested and cycled. XPS, SEM and XRD analyses will be conducted on as-assembled as well as cycled cells. The analyzing results will be illustrated based on the chemical characteristics of doping ions and the accompanying defects created.

Use of fluorinated ether (TFEE) based electrolyte in lithium-sulfur (Li-S) batteries enables high-energy Li–S cells at low electrolyte loading by significantly reducing the polysulfide shuttle phenomenon. The molecular level origin for the change in polysulfide solubility was found to be poor Li⁺ ion solvation ability of the fluorinated ethers as analyzed by COSMO-RS computations for several electrolytes.¹

There are still several questions open about the mechanism of operation of such cells. How is impedance and its limitations different from the conventional polysulfide solvating electrolytes? Which process during the operation of such cells is the bottle-neck for cell performance? What is the mechanism of Li₂S deposition? Is lithium stripping and deposition limited by the poor Li⁺ ion solvation ability?

In order to understand the cell performance with TFEE electrolyte, an impedance spectroscopy study was conducted on simplified cell geometry model systems. The basic principles elucidated through that approach were then used to understand the workings of the conventional mesoporous carbon cathode cell setup. Charge transfer reaction and diffusion contributions were evaluated and the change due to Li₂S deposition analyzed. Finally, Li metal stripping and deposition analysis was performed, which showed beneficial effects of the fluorinated solvent on the SEI.

References:

¹ S. Drvarič Talian, S. Jeschke, A. Vizintin, K. Pirnat, I. Arčon, G. Aquilanti, P. Johansson, R. Dominko, Chem. Mater. 29 (2017) 10037–10044.

Figure 1

Lithium iron phosphate LiFePO₄ has been extensively studied as a promising electrode material for lithium ion batteries, owing to its low cost and high safety [1]. It has been shown that it mostly undergoes a biphasic reaction during the charge-discharge processes and various analyses have been applied to elucidate the phase transition behavior. Since the phase transition in typical nanoparticles is fast, particle-to-particle (not concurrent) transitions are expected and thus macroscopic reaction inhomogeneity can occur. For example, ex-situ XAFS analysis has shown such reaction inhomogeneity in the LiFePO₄ electrode, particularly in the cross-sectional direction [2,3]. However, laboratory observation of reaction distribution during battery operation (operando) is not easy; XRD has too low spatial resolutions and Raman too low time resolution.

Optical analysis has widely been used to capture macroscopic changes in the electrode and phase transitions can also be analyzed when color changes occur as shown in the graphite electrode [4]. Here we report phase transition behavior of LiFePO₄ electrodes analyzed by color confocal optical system. Reaction distribution in the cross-section of the electrode was observed in operando. We also detected a metastable intermediate "LxFP", which is stabilized at low temperatures [5].

A layer of the LiFePO₄ composite electrode, separator and lithium foil was cut and the cross-section was observed by color confocal optical system (ECCS B320, Lasertec Corporation) after the electrolyte was immersed. The reflective images of the electrode were recorded as a color video and the time changes of the whole (averaged) or local RGB brightness were compared with charge-discharge data. The blue brightness change (centered at 436 nm) can reasonably be correlated to the state of charge, with FePO₄ having higher reflection intensity as shown in Fig. 1(a). At 0.1C at room temperature, the change occurred faster than the linear correlation, probably because the observed surface was filled with electrolyte and thus the reaction proceeded fast in the observed area under ion-transfer limitation conditions.

The inhomogeneity in the cross-sectional direction of the electrode was clearly observed in operando using the optical system. As shown in Fig. 1(b), the separator side reacted faster than the current collector side, in good agreement with the result analyzed by XAFS [3].

At a low temperature (-10 deg.), the sample in the fully charged state (FePO₄) was able to be discharged only ca. 75%, which implies the slow kinetics of "LxFP" to form LiFePO₄ [5]. The reflection intensity of "LxFP" was much lower than that at room temperature. When the temperature was increased to room temperature, the intensity was recovered to the originally observed value, suggesting the disproportionation of metastable "LxFP" to LiFePO₄ and FePO₄ [5]. These results clearly indicate that "LxFP" is optically captured as well as LiFePO₄ and FePO_4.

References

[1] K. Padhi et al., J. Electrochem. Soc., 144, 1188 (1997).

[2] H. Tanida et al., J. Phys. Chem. C, 120, 4739 (2016).

[3] Y. Orikasa et al., Sci. Rep., 6, 26382 (2016).

[4] P. Maire et al., J. Electrochem. Soc., 155, A862 (2008).

[5] Y. Koyama et al., Chem. Mater., 29, 2855 (2017).

Figure 1