Mechanism and Condition Optimization of Acid Leaching Recovery of FePO4 from Iron–Phosphorus Slag after Lithium Extraction from LiFePO4

The rapid expansion of lithium iron phosphate (LiFePO4) applications has led to a surge in battery decommissioning, making comprehensive recovery of all components from spent LiFePO4 batteries critical to the advancement of the industry. In this study, iron–phosphorus slag remaining after lithium extraction was recovered through washing, acid leaching, and precipitation. The resulting iron phosphate was characterized by SEM, EDS, and XRD to analyze its crystal structure, morphology, and chemical composition. Under optimized conditions, hydrochloric acid concentration of 7 mol/L, temperature of 70 °C, liquid-to-solid ratio of 15 mL/g, and leaching time of 5 h, the leaching rates of Fe and P exceeded 80%, with corresponding activation energies of 93.62 kJ/mol and 85.40 kJ/mol, respectively. Precipitation yielded iron phosphate with good crystallinity. Recycled and commercial-grade iron phosphate were each converted into lithium iron phosphate via carbothermal reduction and assembled into batteries for performance evaluation. The results demonstrated that precise control of the microstructure and morphology of the recycled material is essential for improving its electrochemical performance. This study provides guidance for the wet-process recovery of iron–phosphorus and LiFePO4 from solid waste residues.