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

LiFePO powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·HO, FeCl·4HO and HPO 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 N + 3% H gas atmosphere. Raman spectroscopy revealed that α-FeO thin layers were formed on the surface of as-prepared LiFePO powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO powders, LiFePO/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g at 0.1 C and 84 mAh g at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO/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 LiFePO powders were also discussed.