Closed-loop regeneration of battery-grade FePO4 from lithium extraction slag of spent Li-ion batteries via phosphoric acid mixture selective leaching

FePO4 regeneration from lithium extraction slag (LES) is a key link in the closed-loop recycling of LiFePO4, but this link has not yet been effectively achieved. This study presents a selective leaching treatment for removing impurities from LES, and then the battery-grade FePO4 is recovered. The leaching efficiency of impurity elements such as Ni, Cu, and Mn can reach up to 99.53%, 98.08%, and 97.65%, respectively, while the main element of Fe and P remain as low as 0.3% and 1.6%, and recovered FePO4 (R-FePO4) conformed to China Chemical Industry Standard for battery-grade FePO4 (HG/T 4701–2014). The leaching experiment shows that H3PO4 treatment contributes to obtaining excellent crystalline FePO4, while the addition of HCl mainly enhanced the leaching performance of metal impurities. The optimal conditions for recovering battery-grade FePO4 were: 1.5 mol/L H3PO4, H3PO4/HCl molar ratio of 3:1, a liquid–solid ratio of 10 mL/g, leaching at 90 °C for 3 h. The thermodynamics of the leaching reactions was analyzed to determine the feasibility of leaching impurities and crystallizing FePO4 from LES. The composition and structure of the regenerated FePO4 were restored, and the electrochemical performance reached the same level as commercial materials. In addition, the cost of R-FePO4 is calculated to be $1.35 kg−1, which is about 32.5% lower than the cost of commercial FePO4 (C-FePO4, $2.0 kg−1). This work presents a potential “waste-to-wealth” approach to guide the efficient and environmentally friendly recycling of spent Li-ion batteries (LIBs).