Thermodynamically Guided Three‐Stage Separation for Deep Aluminum Removal from Delithiated LiFePO ₄ Slag Toward Battery‐Grade FePO ₄ Regeneration

ABSTRACT The recovery of high‐purity iron phosphate (FePO ₄ ) from lithium extraction slag (LES) is crucial for achieving closed‐loop recycling of spent LiFePO ₄ /C. However, this process is significantly challenged by the difficulty in removing aluminum impurities due to the similar chemical behavior of Fe ³⁺ and Al ³⁺ . Guided by thermodynamic principles, we developed a three‐stage purification process for deep aluminum removal, consisting of “selective leaching‐precipitation‐dissolution/recrystallization.” In the first stage, highly selective aluminum leaching (96.3% Al vs. 21.6% Fe) is achieved under mild acidic conditions, leveraging the thermodynamic preference for aluminum dissolution over FePO ₄ . During the second stage, by utilizing the tendency of Fe ³⁺ to precipitate more readily as FePO ₄ ·xH ₂ O than Al ³⁺ , precise pH control enables 95.5% iron precipitation, with only 4.02% aluminum coprecipitation. The third stage involves an in situ dissolution/recrystallization, which promotes efficient iron recrystallization (95%) while further removing residual aluminum, reducing its content by over 50% from an already low level. This enhancement is attributed to the higher crystallization tendency of FePO ₄ ·2H ₂ O compared to AlPO ₄ ·2H ₂ O. Ultimately, battery‐grade FePO ₄ with an aluminum content below 300 ppm is obtained. This work provides an industrially viable strategy for the deep removal of aluminum and the high‐value utilization of LES derived from spent LiFePO ₄ /C.