Long-term cycle stability at a high current for nanocrystalline LiFePO4 coated with a conductive polymer

Highly uniform hierarchical-microstructured LiFePO4 particles with dumbbell- and donut-shape and individual LiFePO4 nanocrystals were prepared by a hydrothermal method utilizing citric acid or a triblock copolymer (Pluronic P123) as a surfactant. The cathode composed of the individual nanocrystalline LiFePO4 particles exhibited higher specific capacity than the cathodes composed of the hierarchically assembled microparticles. Coating a conductive polymer, poly-3,4-ethylenedioxythiophene (PEDOT), on the surface of LiFePO4 particles improved the battery performances such as large specific capacities, high rate capability and an improved cycle stability. The nanocrystalline LiFePO4 particles coated with PEDOT (20 wt%) exhibited the highest discharge capacities of 175 and 136 mAh g−1 for the first battery cycle and 163 and 128 mAh g−1 after 500 battery cycles, with a degradation rate of 6–7%, at the rates of 1 and 10 C, respectively.


Introduction
Lithium ion batteries are one of the great successes of electrochemical energy sources for not only modern portable electronic devices but also future electric vehicles including plug-in hybrid electric vehicles [1][2][3]. An olivine structure material, LiFePO 4 , has emerged as one of the most promising cathode materials with good structural stability, lower toxicity and relatively low cost, which was first reported by Padhi et al [4]. LiFePO 4 has a theoretical capacity of 170 mAh g −1 with a stable plateau voltage at 3.45 V versus Li/Li + . However, the application of LiFePO 4 is always accompanied with the large cathodical polarization, particularly for the high rate demand, because of the low electronic conductivity and the low Li + diffusion rate in the LiFePO 4 particles [5][6][7]. Improvement of the intrinsic conductivity of LiFePO 4 by doping with cations or metals into the crystalline lattice seems to be challenging to achieve [8][9][10]. It has been demonstrated that the improved performance was able to obtain by reducing the particle size to sub-micrometer and by enhancing the particle crystallinity, since electrode reaction occurs at the surface and requires the ions' transport into the electrode lattices [5][6][7][8][9][10][11][12][13][14][15]. Nanodimensional materials provide large surface area and short diffusion length. Hydrothermal synthesis is an effective method to obtain well-crystallized materials with well-defined morphologies, since the hydrothermal process is carried out under high pressure [11][12][13][14][15]. The particle size and morphology can be controlled by a careful choice of surfactants, solvents and reaction parameters in hydrothermal synthesis [11][12][13][14][15][16].
Surfactant molecules can act as a template or be adsorbed onto the particle surfaces during particle growth, which determines the particle size and morphology of the product.
Considerable efforts have been made to improve the conductivity and the charge/discharge rate by surface coating with conductive materials [2,[8][9][10][11][12][13][14][15][16]. Since the interface characteristics is a key factor for the electrode performance, coating of the LiFePO 4 particles with conductive materials, such as carbon, conductive polymers, ceramics or metals, is expected to improve the electrode performance. Conductive polymers, such as poly-3,4-ethylenedioxythiophene (PEDOT), are attractive materials in covering inorganic crystallite surfaces in terms of improving not only electrical conductivity but also mechanical flexibility [8][9][10]16]. Conductive polymer coated on the nanoparticles can accommodate small volume expansion/contraction during discharging/charging process, which will reduce cracking of the electrode with ageing. Another advantage of conductive polymers is that they can be coated under mild processing conditions compared to carbon coating. In addition to their electrical conductivity and mechanical flexibility, conductive polymers can also improve lithium ion accessibility and diffusion pathways within battery cathodes.
In this work individual LiFePO 4 nanoparticles and donutand dumbbell-shaped LiFePO 4 microstructures hierarchically constructed with the nanoplates were synthesized by a hydrothermal method, using various surfactants to controllable size and shape of LiFePO 4 particles. Then, in order to increase the conductivity of the electrode surfaces, these particles were coated with PEDOT. The electrochemical performances of these conductive polymer-coated LiFePO 4 nanocrystalline particles and microstructures were examined in terms of specific capacity, cycle stability and rate capability as the cathodes of Li + ion batteries.

Materials and synthesis of Li Fe P O 4 and Li Fe P O 4 -PEDOT coated cathode materials
All chemicals and materials were obtained from the Sigma-Aldrich corporation and used without further purification. All aqueous solutions were prepared with ultra-pure water deionized by a millipore milli-Q system (18 MW cm −1 ). LiOH (99%), FeSO 4 · 7H 2 O (99+%) and H 3 PO 4 (85%) were used as starting materials. Citric acid, C 6 H 8 O 7 (99%), or a triblock copolymer (P123), poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) was used as a surfactant to control the particle growth and surface morphology.
LiFePO 4 crystalline particles were synthesized by a hydrothermal method from the stoichiometric mixture of LiOH, FeSO 4 · 7H 2 O and H 3 PO 4 (3 : 1 : 1 molar ratio). L-ascorbic acid (C 6 H 8 O 6 , 99%) was added as a reducing agent to the stoichiometic mixture of the starting materials (Fe 2+ : C 6 H 8 O 6 = 4 : 1 molar ratio). Ethylene glycol was added into the aqueous solution to disperse the inorganic salts (1 : 1 volume ratio). The resulting gray solution was transferred into a 125 ml teflon-lined stainless steel autoclave and heated for 12 h at 160 • C. After the hydrothermal reaction was completed, the precipitate was completely washed and dried at 80 • C.
Conductive polymer, PEDOT, was prepared by polymerizing 3,4-ethylene dioxythiophene monomer and a dopant, para-toluene sulfonic acid (p-TSA), at a 2 : 1 mole ratio by using anhydrous FeCl 3 as an oxidizing catalyst [16,17]. The PEDOT was dissolved in DMSO or cresol. PEDOT was coated on the LiFePO 4 particles by simply dispersing the LiFePO 4 powder in the conductive polymer solution at room temperature and evaporating the solvent in a vacuum oven at 60 • C.

Characterization
Powder x-ray diffraction profiles were recorded on a Philips PW1050 diffractometer using graphite-monochromated CuKα radiation (λ = 1.540 56 Å). Data were collected over the 2θ range from 10 • to 100 • for 4 s in each 0.02 • step at ambient temperature. The structural parameters were refined by using the Rietveld program FULLPROF [18]. Scanning electron microscopes (FESEM JSM-6700F and SEM JSM-6380, JEOL, Japan) were used to examine the particle morphology. The electrochemical measurements were carried out using coin cells (CR 2032 type) assembled in a dry room. The cathodes were prepared by compressing and drying after spreading a mixture slurry of the active material (LiFePO 4 /PEODT coated), acetylene black (AB) and polyvinylidene fluoride (PVDF) in NMP. The electrolyte was 1 M LiPF 6 in a 1 : 1 mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) and the separator was a porous polyethylene film (Celgard 2300 µm). The cells were cycled with a constant current mode (galvanostatically) in a potential range of 2.0-4.3 V using a multichannel battery test system (Maccor Series 4000, USA).

Structure and morphologies
The observed, calculated and difference XRD patterns from the Rietveld refinement for the as-synthesized nanocrystalline LiFePO 4 are shown in figure 1. All diffractions for the nanocrystalline LiFePO 4 in the powder XRD patterns were indexed by an orthorhombic structure with the Pnma space In order to improve the intrinsic problem of low ion diffusivity for LiFePO 4 , it is essential to minimize defect concentration and increase the crystallinity, because the Li + transport is one-dimensional in olivine structure. Li + mobility has been simulated to be high in the tunnels along the [010] direction only and Li + hopping between tunnels is very unlikely in olivine structure [6,7,[19][20][21].
Minimizing the crystallite size down to submicron level is also essential. Various LiFePO 4 nanostructures and complex hierarchical architectures with well-defined sizes and morphologies are of great interest [5][6][7][11][12][13][14][15][16]. Figure 2 shows the SEM images of the individual LiFePO 4 nanoparticles and the LiFePO 4 microstructures hierarchically assembled with the nanoplates that were synthesized by the hydrothermal method. Surfactant molecular size and its quantity used for the preparation have a significant influence on the particle morphology. The well-dispersed individual LiFePO 4 crystals formed with the P123, block copolymers, surfactant displayed rectangular-or rhombus-shapes with particle sizes that were approximately 500 nm in length, and 100-200 nm in width and in thickness (nanoplates), as shown in figure 2(a). When the surfactant was changed from P123 to citric acid, the LiFePO 4 nanoplates were hierarchically organized into microstructures. The molecular surfactant, citric acid, can be adsorbed onto particle surfaces during the particle growth, and in turn can organize the nanoscale building blocks into complex hierarchical architectures via a self-assembly process. The LiFePO 4 hierarchical architectures with well-defined dumbbell-like or donut-like microstructures were successfully synthesized by using citric acid as the surfactant. Dumbbell-like microstructures of LiFePO 4 with lengths of about 8 mm were hierarchically constructed with the nanoparticles (90-120 nm thick), as shown in figure 2(b). When quite an excess amount of the molecular surfactant was used, growth of the nanoplates of crystalline LiFePO 4 seemed to be inhibited. The particles were considerably thinner than those without the excess amount of the surfactant added in the reactor. The thinner LiFePO 4 nanoplates (about 50 nm thick) were hierarchically organized into donut-like microstructures, as shown in figure 2(c). Hierarchical microstructures formed through the face-to-face packing of nanoplates. The large surface of each nanoplate is considered to be (010) plane, which has the lowest surface energy calculated by Fisher and Islam [19] and Ceder et al [20,21]. The (010) surface is normal to the most facile pathway for lithium ion conduction along the [010] channel.

Electrochemical performance of the Li Fe P O 4 coated with PEDOT
Nanocrystals have the benefit of enlarging the specific surface area with the decreased diffusion length for Li + ions. In this work, these particles were coated with a conductive polymer, PEDOT, in order to increase the charge transport kinetics. Figure 3 shows a typical cyclic voltammogram of a Li cell with the cathode composed of nanocrystalline LiFePO 4 coated with PEDOT (10 wt%). The pair of the Fe 2+ /Fe 3+ redox couple with an oxidation peak at 3.55 V and a reduction peak at 3.33 V corresponds to the charge/discharge reaction of the battery. The peak shape is almost symmetric and the potential separation between the two peaks is small. The first charge/discharge curves of the lithium batteries assembled with the LiFePO 4 -PEDOT (10 wt%) coated cathodes of three different morphologies are shown in figure 4(A). All the curves show reversible lithium extraction/insertion during the charging/discharging processes with varying specific capacities. The charge and discharge profile shown in figure 4(A) demonstrates that the individual LiFePO 4 nanoparticle cathode shows the highest specific capacity of 170 mAh g −1 . The dumbbelland donut-shaped microstructure LiFePO 4 cathodes show capacity values of about 120 mAh g −1 . The increase in the specific capacity certainly reflects the increase in the specific surface area. In addition, the smaller the particle is, the more densely the particles can be packed with smaller interstitial volumes in the cathode. The tap density of the positive electrode composed of the nanocrystalline LiFePO 4 powder can be improved with better packing with smaller void volumes. High tap density leads to a high volumetric specific capacity. The tap densities of the positive electrode composed of individual LiFePO 4 nanocrystals, dumbbelland donut-shaped hierarchical microstructures are 1.56, 1.20 and 1.06 g cm −3 , respectively. As the nanocrystalline LiFePO 4 possesses the highest tap density among the various morphologies, a high volumetric specific capacity (260.5 mAh cm −3 ) was achieved. The cycle performances of the individual nanocrystalline particles, dumbbell-shaped and donut-shaped hierarchical microstructures of LiFePO 4 coated with PEDOT (10 wt%) were tested. The discharge specific capacities for 100 battery cycles are shown in figure 4(B). Specific capacities of 153, 107 and 102 mAh g −1 after 100 charge-discharge cycles at a rate of 1 C, with degradation rated of approximately 10, 11 and 15%, respectively, for the individual nanocrystals, dumbbell-shaped and donut-shaped  Optimization of the amount of PEDOT coated on the LiFePO 4 nanoparticles was required to achieve the long cycle life and the high capacity. Figure 5 shows the battery performances of LiFePO 4 nanocrystals coated with 10, 20 and 30 wt% of PEDOT at a rate of 1 C for the consecutive 100 charge-discharge cycles. As shown in figure 5, the nanocrystals LiFePO 4 coated with PEDOT 20 and 30 wt% exhibited higher specific capacities than the theoretical capacity of LiFePO 4 only; the first discharge capacity of around 190 mAh g −1 for 30 wt% PEDOT coated LiFePO 4 cathode and 175 mAh g −1 for 20 wt% PEDOT coated LiFePO 4 cathode. The capacity loss for the LiFePO 4 coated with 30 wt% of PEDOT, 15% for 100 cycles, was the largest among the three cathodes, especially during the first five cycles. The cycle performance of LiFePO 4 coated with 20 wt% of PEDOT was superior to the cathodes of  Long-term cycle stability at the standard 1 C rate and at a higher discharge current rate of 10 C for the individual nanocrystalline LiFePO 4 coated with PEDOT (20 wt%) was investigated and is shown in figure 6. The specific capacity at a rate of 10 C was 136 mAh g −1 for the first battery cycle and 128 mAh g −1 for the 500th cycle, with a degradation rate of approximately 6%. The specific capacity was 163 mAh g −1 after 500 battery cycles at the rate of 1 C, with a degradation rate of 7%. The nanocrystalline LiFePO 4 particles coated with PEDOT (20 wt%) exhibited the high discharge capacities and the long-term cycleability. We believe the mechanical flexibility of the conductive polymer can give room for structural expansion and contraction modification caused by Li insertion and extraction, which results in the long-term cycle stability for the nanocrystalline LiFePO 4 coated with PEDOT cathode.

Conclusions
Nanocrystalline LiFePO 4 olivine crystals were prepared by a hydrothermal process. Uniform individual LiFePO 4 nanoplates and the LiFePO 4 microstructures hierarchically assembled with the nanoplates were synthesized by using P123 block copolymer and molecular surfactants, respectively. The well-dispersed individual LiFePO 4 nanoparticles coated with a conductive polymer, PEDOT, exhibit excellent battery performances in terms of a high rate capability and an improved cycle stability. The nanocrystalline LiFePO 4 coated with PEDOT (20 wt%) exhibited the best cycle performance with the largest specific capacity among the cathodes studied in this work. The specific capacity at a rate of 10 C was 136 mAh g −1 for the first battery cycle and 128 mAh g −1 for the 500th cycle, with a degradation rate of only about 6%.