Inactive diluent-modified deep eutectic solvents for highly selective extraction of transition metals from waste...

Developing green and efficient methods for recycling waste lithium-ion batteries (LIBs) is increasingly important, and deep eutectic solvents (DESs) have emerged as a promising alternative to conventional acid leaching, but their high viscosity and the difficulty of separating multiple chemically similar metals remain challenges. Herein, we propose an inactive diluent-modified DES for the gradient extraction of transition metals from waste LIBs, in which dimethyl carbonate (DMC) is introduced to regulate the properties of the choline chloride (ChCl)-oxalic acid (OA) system. Based on our previous study, the synthesized DES is applied to leach transition metals from the OA leaching residue obtained after Li recovery. Under the optimized conditions (2 h, 70 °C, 40 g/L, 10 wt% DMC), it achieves leaching efficiencies of Co and Mn exceeding 99%, while Ni is barely leached and remains enriched in the solid residue. While Co and Mn account for more than 99% of the dissolved transition metals, demonstrating the high selectivity of the gradient separation. Mechanistic analysis reveals that the modified DES retains its structural consistency, with abundant ChCl-OA clusters dispersed in DMC, while lowering the viscosity and modulating the competitive coordination environment. OA provides protons while ChCl facilitates complexation, and both promote the redox processes that drive metal dissolution and precipitation. This approach realizes efficient and environmentally compatible stepwise metal recovery for the green, value-added utilization of retired batteries.