Suppressing the formation of Fe2P: Thermodynamic study on the phase diagram and phase transformation for LiFePO4 synthesis

Impurity of iron phosphide (Fe2P) may weaken the cycle performance of the LiFePO4 battery cell, and it is mandatory to get rid of this impurity in actual production. In order to suppress the formation of Fe2P, the phase transformation and thermodynamic analysis of Li2CO3-Fe2O3-NH4H2PO4-C/CO system for LiFePO4 synthesis were studied in this paper. The phase diagram for LiFePO4 synthesis indicated that LiFePO4 will be respectively decomposed to form FeP, Fe2P, and Fe3P at 718, 776, and 836 °C, corresponding with the insulating Li3PO4 being yielded. Considering the practical synthesis, the sintering temperature should be controlled below 776 °C to avoid the formation of Fe2P impurity in theory. The thermodynamic analysis successfully explained why the sintering temperature for pure LiFePO4 preparation currently controlled to be 650–750 °C. Besides the sintering temperature, adding excess of Li resource is an effective measure to avoid the formation of Fe2P and other impurities.