Thermodynamics analysis of LiFePO4 pecipitation from Li–Fe(II)–P–H2O system at 298 K

Thermodynamics of the precipitation from Li–Fe(II)–P–H2O system at 298 K was investigated. The results demonstrate that LiFePO4 can be formed at room temperature under pH value of 0–11.3, and the impurities Li3PO4 and Fe(OH)2 will be yielded at pH value above 11.3 and 12.9, respectively. The optimum pH value for LiFePO4 precipitation is 8–10.5. Considering the low rate of phase transformation kinetics, metastable Li–Fe(II)–P–H2O system was also studied. The results indicate that equimolar ratio of co-precipitation precursor Fe3(PO4)2·8H2O and Li3PO4 cannot be obtained at the initial molar ratio 1:1:1 and 1:1:3 of Li:Fe:P. In contrast, equimolar ratio of the co-precipitation precursor can be yielded by adjusting the pH value to 7–9.2, matching the molar ratio 3:1:1 of Li:Fe:P, meaning that Li+-excess is one of the essential conditions for LiFePO4 preparation by co-precipitation method.