Effect of solvents on electrochemical performance of polypyrrole coated LiFePO₄/C cathode materials for Li-ion battery

The effect of the nature of solvent on polypyrrole (PPy) synthesis by in situ coating on the prior synthesized carbon coated LiFePO₄ for electrochemical properties of composite materials designated as LiFePO₄/C-PPy has been investigated. An in situ chemical oxidation method is used for the polymerization of pyrrole by using three different solvents e.g. water, ethanol and acetonitrile (ACN). The structure and morphology of PPy grown as coating is characterized by Raman spectroscopy and scanning electron microscopy respectively. X-ray diffraction patterns of the composites confirm the presence of Li_0.05FePO₄ compound as a minor phase along with major LiFePO₄ phase. The ACN assisted synthesized PPy coating shows best electrochemical performance as cathode material among selected solvents used for the synthesis. Electrochemical impedance spectroscopy indicates lowest electrode impedance and highest Li-ion diffusion for the composite LiFePO₄/C-PPy synthesized in ACN solvent as compared to the water and ethanol derived samples. The higher rate capability (82 mAh g⁻¹) of the composite LiFePO₄/C-PPy synthesized in ACN solvent as compared to LiFePO₄/C (42 mAh g⁻¹), is attributed to the morphology and structure of the polymer coating.