E-pH Diagrams for the Li-Fe-P-H2O System from 298 to 473 K: Thermodynamic Analysis and Application to the Wet Chemical Processes of the LiFePO4 Cathode Material

The wet chemical processes of LiFePO4, hydrothermal synthesis and hydrometallurgical recovery, are of great importance during the life cycle of LiFePO4. To analyze these two processes, E-pH diagrams for the Li-Fe-P-H2O system are plotted from 298 to 473 K in this study. The E-pH diagrams can well explain the practical operating conditions of hydrothermal synthesis and hydrometallurgical recovery and provide thermodynamic basis for them. Besides, suitable conditions for the hydrothermal synthesis of LiFePO4 are obtained from the E-pH diagrams, including high temperature, low redox potential, optimum pH 7.8–8.4, and excess stoichiometric lithium. As found in the E-pH diagrams, LiFePO4 will change to ferric phosphate by promoting the redox potential, while lithium will be extracted to the aqueous solution. Based on the above results, a method is proposed for the selective leaching of lithium from spent LiFePO4, which is successfully verified by leaching experiments. At room temperature (298 K), neutral pH (7.0), and low liquid–solid ratio (5:1), 95.4% of lithium can be extracted using 2.7 M H2O2 as the oxidant, while iron remains in the residue. This method shows a promising commercial value as it can realize the selective extraction of valuable lithium from spent LiFePO4 and avoid using large amounts of acid and alkaline.