Dissolution of Spent Lithium-Ion Battery Cathode Materials: Overlooked Significance of Aluminum Impurities

Impurities are pivotal in determining the quality of the critical metal products derived from spent lithium-ion batteries (LIBs), but there is a lack of understanding of their potential impacts. The mechanisms by which Aluminium (Al) impurities permeate the active crystals of spent ternary nickel─cobalt─manganese oxide (NCM) cathode materials and interact with critical metal sites are elucidated. During frictional contact, the substitution of transition metals by Al impurities disrupts the bonding between critical metals and oxygen, leading to the formation of more stable Al─O covalent bonds. Al can preferentially substitute for Co, altering the local coordination and electronic structure of transition metals. Owing to the strong interaction of Al─O, the Fermi level shifts downward, thus increasing the stabilization of lattice oxygen in the NCM cathode materials and consequently delaying the dissolution of NCM cathode materials. In different extraction environments, the invasion of Al retards the release of lattice oxygen and inhibits the dissolution of NCM in formic acid but enhances those in ammonia solution and shows mixed results in deep eutectic solvents. These crucial findings will help to elucidate the mechanisms of Al impurities in the recycling industry chain of retired LIBs, thereby enhancing the recovery of high-quality critical metal products.