Optimization of reaction condition for solid-state synthesis of LiFePO4-C composite cathodes

We optimized synthesis condition of LiFePO4-C composites by solid-state reaction of LiH2PO4 and FeC2O4·2H2O in the presence of carbon powder. The preparation was conducted under a N2 flow through two heating steps. First, the starting materials were thoroughly mixed in a stoichiometric ratio and decomposed at 350-380 °C to form the precursor. Second, the resulting precursor was heated at a high temperature to form the crystalline phase LiFePO4. For formation of the precursor, the optimized temperature was 350 °C for LiFePO4 and 380 °C for LiFePO4-C composites, respectively. For formation of crystalline phase composites, the optimized condition was to heat the precursor in a pelletized form at 800 °C for 5 h, and the optimized content of carbon was 3-10 wt.%. In composites, the carbon not only increases the rate capability, but also enhances capacity stability. We found that capacity of the composites increases with specific surface area of carbon. The best result was observed from a composite made of 8.7 wt.% of black pearls BP 2000 having a specific surface area of 1500 m2 g−1. At room temperature and low current rate (0.02 C), such a composite shows a specific capacity of 159 mAh g−1. Electrochemical properties and cycling performance of the optimized composite also were evaluated.