Electrochemical Behavior of Emeraldine Salt Polymer Coated LixFePO4 Cathode Materials with Excess Lithium Doping

Abstract

The LixFePO4 cathode material with x = 1.00, 1.05, 1.10, 1.15, and 1.20 has been synthesized by chemical precipitation method. The particles of LixFePO4 material are further in-situ coated with emeraldine salt (ES) polymer. These ES coated LixFePO4 (LixFePO4-ES) particles have been characterized by XRD, FE-SEM, TEM, and DTA/TG analyses. The XRD analysis of LixFePO4 indicates the formation of pure single phase of LiFePO4 (for x = 1.00, 1.05, and 1.10). Excess of lithium have no influence on the crystal structure of LixFePO4 upto x = 1.10 and no other phase containing Li+ is formed. The material LixFePO4 (for x = 1.15) shows low intensity peaks corresponding to Li3PO4 phase at 2θ = 22.31° and 23.19°. The peaks are found significant for the composite material for x = 1.20. The micrograph of composite powder LixFePO4-ES shows good coverage of LixFePO4 particle by PANI. The materials LixFePO4 and LixFePO4-ES (x = 1.00–1.20) are tested for discharge capacity and electrochemical impedance measurements for cathode materials. The electrochemical impedance spectra of LixFePO4-ES (x = 1.00 to 1.20) show low charge transfer resistance (Rct), as compared to LixFePO4 indicating polyaniline incorporation have offered good conductivity. The highest discharge capacity of 131 mAh g–1 is obtained at the current rate of 2C for LixFePO4-ES (x = 1.10) cathode. The impurity phase that appears in the XRD pattern reduces the discharge capacity of LixFePO4-ES (x = 1.15 and 1.20).