Effects of iron sources on structure and electrochemical properties of LiFePO_4/C prepared by sol-gel method

Ferrous and ferric iron precursors were used to synthesize LiFePO4/C by an aqueous sol-gel method, using ethylene glycol as chelating agent and carbon source. The phase structure, morphology and carbon content of LiFePO4/C products were characterized by XRD, SEM and elemental analysis, respectively. Electrochemical impedance spectroscopy and cyclic voltammetry were applied to study the reaction kinetics of the LiFePO4/C cathode material. The results show that considerable pure LiFePO4 phase is obtained when iron source of Fe(Ⅲ) is used, but when the iron precursor of Fe(Ⅱ) is used, besides the main phase of LiFePO4, a little amount of FeP forms. The existence of FeP phase favors the overall electrochemical properties of LiFePO4/C. The LiFePO4/C prepared from ferrous iron exhibits discharge capacities of 142 and 112 mA·h/g at rates of 0.1C and 1C (1C=170 mA/g, voltage between 2.5 V and 4.2 V), respectively, which are higher than those of LiFePO4/C prepared from ferric iron.