Two-step complexation-ion exchange for separation of iron, aluminum and lithium from spent LiFePO₄ materials

Traditional methods for recovery metals from spent lithium iron phosphate (LFP) leachate often suffer from insufficient selectivity, Li loss and incomplete Al removal. To address these challenges, this study proposes a two-step complexation-ion exchange synergistic strategy for the selective separation of Fe, Al and Li. In the first step, a 717-type Cl^--form anion exchange resin is used to preferentially adsorb [FeCl₄]^- formed in spent LFP leachate mixed with NaCl solution, with a Fe adsorption efficiency of 99% and almost no adsorption of Al and Li. In the second step, according to the significant stability difference of the anionic oxalate-complex with Al and Li, oxalic acid is introduced in the Fe-free solution to convert Al into [Al(C₂O₄)₂]^- and/or [Al(C₂O₄)₃]^3- proved by density functional theory calculation, molecular dynamics simulation, analysis of mass spectra and infrared spectra. The aluminum-oxalate complexes are subsequently adsorbed by 717-type OH^--form anion exchange resin with good cycling stability, leading to the 98% Al removal with no Li loss. The rapid Al/Li separation is demonstrated by kinetics study, which reveals Al adsorption conforms to pseudo-first-order model and is jointly controlled by liquid film and intraparticle particle diffusion. The proposed method shows strong potential for efficient separation of metals in spent LFP battery materials.