Electrochemical properties of single-phase LiFePO4 synthesized using LiF as Li precursor and hydrogen and carbon gel as reducing agents

LiFePO₄ samples have been synthesized by mixing stoichiometric amounts of (NH₄)₂HPO₄, FeC₂O₄·2H₂O, and LiF. During synthesis, carbon gel was used as the carbon source. Single-phase LiFePO₄ can be formed when the heating temperature ranges from 650 to 800 °C and it is decomposed into Li₄P₂O₇, Li₃PO₄, Fe₂P, and Li₃P₇ when the temperature comes to 850 °C. We find that the ratio of the lattice parameter (a/c) decreases with the increasing temperature, thereby increasing the Li⁺ diffusion channel length. Both the decrease of a/c and the abrupt crystal growth are expected to contribute to the monotonic decrease of the initial capacity of the samples. The sample heated at 650 °C with a smaller uniform particle size and relative higher specific surface area (8.2 m²/g) shows an excellent electrochemical performance. The initial specific capacity of 156.7(3) mAh/g is obtained at the rate of C/10.