Electrochemical Behavior of Emeraldine Salt Polymer Coated Li_xFePO₄ Cathode Materials with Excess Lithium Doping

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 2 theta = 22.31 degrees and 23.19 degrees. 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 (R-ct), 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).