Electrochemical impedance spectroscopy characterization of LiFePO4 cathode material with carboxymethylcellulose and poly-3,4-ethylendioxythiophene/polystyrene sulfonate

Abstract

Novel cathode material compositions based on lithium iron phosphate (LFP) were prepared using conducting polymer dispersion poly-3,4-ethylenedioxythiopene/polystyrene sulfonate (PEDOT:PSS) and water-based carboxymethylcellulose (СМС) as a binder solely and in mixture PEDOT:PSS/СМС. The electrochemical properties of materials in lithium-ion batteries were investigated by galvanostatic charge-discharge curves and by electrochemical impedance spectroscopy and the results were compared with conventional PVDF-bound material. Our best materials consisting of 92wt% of C-LiFePO4, 4wt% of carbon black and 4wt% of conducting polymer binder exhibited excellent rate capability with discharge capacity 148mAhg−1 (at 0.2C, normalized by the electrode mass), 143mAhg−1 at 1C and 128mAhg−1 at 5C as well as good cycling stability at 1C (less than 1% decay after 100 cycles).Impedance spectra of batteries with different compositions were measured and analyzed. Comparison of kinetic parameters obtained for different electrodes revealed main factors responsible for significant improvement of electrochemical performance of LFP-based cathode materials modified with conducting polymer in comparison with conventional electrode. The transition from conventional PVDF-bound LFP-based cathode composition to modified by conducting polymer PEDOT:PSS/CMC was found very effective. The electrode with optimal composition showed substantial decrease of interfacial charge transfer resistance for 30 times, and decrease of Warburg diffusion resistance.The mechanism of positive influence of conducting binder on electrochemical properties of cathode material is discussed.