Selective Lithium Recovery from Spent Lithium Iron Phosphate Batteries via Acid-Free Leaching

Currently, the electric vehicle industry is expanding rapidly, leading to challenges in the large-scale retirement and resource recovery of lithium-ion batteries. The black mass from spent LiFePO₄ batteries has a complex composition. In this study, an acid-free leaching process based on sodium persulfate was developed to treat the black mass. This method achieves selective lithium recovery and enables cathode material regeneration. Key parameters were optimized via single-factor experiments. At ambient temperature, lithium was selectively leached from the complex black mass by adjusting the leaching time to 40 min, the LiFePO₄/Na₂S₂O₈ molar ratio to 2.0:1.2, and the solid-liquid ratio to 50 g/L. A lithium leaching efficiency of 86.4% was achieved, while the dissolution of Fe, Cu, and Al was minimized. Notably, temperatures above 35 °C led to a substantial increase in the leaching rates of Cu and Al, and temperatures ≥65 °C promoted the dissolution of Fe. Therefore, ambient temperature operation saves energy and ensures leaching selectivity. The mechanism was investigated using multiple characterization techniques. X-ray diffraction (XRD) revealed that the majority of LiFePO₄ was converted into FePO₄, following the reaction: 2LiFePO₄ + Na₂S₂O₈ → 2FePO₄ + Li₂SO₄ + Na₂SO₄. Fourier transform infrared spectroscopy (FTIR) showed characteristic peak shifts and a new peak corresponding to the bending vibration of the PO₄ ³⁻ group. Scanning electron microscopy (SEM) images demonstrated that the graphite and LiFePO₄ particle structures were preserved after leaching, confirming the mildness of the process. Based on the shrinking-core model, the incomplete lithium leaching was attributed to in-situ retained FePO₄, which obstructed the contact between Na₂S₂O₈ and internal LiFePO₄. After impurity removal and leachate concentration, lithium was precipitated using a saturated Na₂CO₃ solution. High-purity Li₂CO₃ was obtained through purification. Regenerated LiFePO₄ (RLFP) was prepared via carbothermal reduction using recycled Li₂CO₃ and FePO₄ as raw materials, with 20% glucose added as the carbon source, followed by roasting at 700 °C for 10 h under an N₂ atmosphere. RLFP exhibited properties comparable to those of commercial LiFePO₄ (CLFP): XRD confirmed its standard crystal phase; X-ray photoelectron spectroscopy (XPS) verified that Fe was in the Fe²⁺ valence state; and Raman spectroscopy showed distinct D and G peaks with an I_D/I_G ratio < 1, indicating a high degree of graphitization. Electrochemical tests demonstrated that RLFP delivered a specific discharge capacity exceeding 155 mAh/g at 0.1 C and maintained a capacity retention of 99.2% after 100 charge-discharge cycles at 0.5 C. Our mild, acid-free, short-flow leaching and regeneration strategy enables efficient selective lithium recovery from black mass and the regeneration of high-performance LiFePO₄. This work offers a practical pathway for the recycling of spent LiFePO₄ batteries and demonstrates promising potential for industrial-scale applications.