Precise Ni/Co separation from spent Li-Ion batteries: Revealing the pivotal role of H2O in deep eutectic solvents

The sustainable recovery of valuable metals from end-of-life lithium-ion batteries (LIBs) has become essential for relieving the supply-demand contradiction of key resources. However, the high similarity in physicochemical properties of transition metals poses a fundamental challenge for the selective recycling. Here we proposed a facile strategy for defining the demarcation line of precipitation/dissolution behavior for precise Ni/Co separation. A malonic acid-based deep eutectic solvent (DES) is developed, emphasizing the H₂O-mediated competitive coordination and differentiated solubility. We discovered the distinctive formation of [Ni(H₂O)₆]²⁺ and [CoCl₄]^2- in the DES-20 % H₂O system. Furthermore, the K_sp differences of the Ni-complex and Co-complex in the C₃H₂O₄^2--rich environment was effectively magnified. These significant differences synergistically enable the generation of C₃H₂NiO₄·2H₂O precipitation (∼ 99.9 % purity), achieving a remarkable Ni/Co separation factor of 125, which guarantees sequential separation of all metals. This universal methodology along with in-depth mechanism understanding provides a guideline toward value-added recovery of LIBs.