Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO4 powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H2O, FeCl2·4H2O and H3PO4 stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N2 + 3% H2 gas atmosphere. Raman spectroscopy revealed that α-Fe2O3 thin layers were formed on the surface of as-prepared LiFePO4 powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO4 powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g−1 at a 0.1 C charge–discharge rate. To improve the electrochemical properties of the LiFePO4 powders, LiFePO4/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO4/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g−1 at 0.1 C and 84 mAh g−1 at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO4/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO4 powders were also discussed.