Selective cobalt recovery from battery black mass enabled by eutectic effect in choline chloride-based deep eutectic solvents

The selective recovery of critical metals from end-of-life lithium-ion batteries is essential for closing material loops and reducing reliance on primary resources. Deep eutectic solvents (DESs) have emerged as promising alternatives to conventional hydrometallurgical systems, yet the extent to which their eutectic structure governs metal dissolution and selectivity remains unclear. Herein, we present a systematic solvometallurgical study addressing this knowledge gap through a direct comparison between choline chloride-based DESs and composition-matched aqueous analogues in the leaching of battery black mass. A microwave-assisted water treatment removed over 80% of lithium, phosphorus, and aluminium, yielding a residue enriched in cobalt, nickel, and manganese. Subsequent leaching experiments using choline chloride-based DESs (choline chloride:ethylene glycol, choline chloride:glycerol, and choline chloride:Urea) revealed that these systems show markedly different behaviour than their aqueous counterparts, suggesting that enhanced extraction may be associated with eutectic effects, as the individual components and their aqueous analogues fail to reproduce the observed performance. Among the investigated systems, choline chloride:ethylene glycol achieved the highest cobalt extraction and an exceptional cobalt selectivity of 97.1% over nickel and manganese, corresponding to a synergy factor of 19 relative to aqueous analogues. Choline chloride:glycerol also exhibited strong cooperative effects, with high cobalt selectivity (87.2%) and enhanced manganese dissolution, whereas choline chloride:Urea showed limited cobalt extraction and poor selectivity. Aging and alkaline treatment tests further demonstrated that DES composition critically controls metal-solvent interactions and solution stability. ChCl:EG and ChCl:GLY form highly stable metal-DES complexes, with less than 2% spontaneous cobalt precipitation after five days;under alkaline conditions, no precipitation is observed for ChCl:GLY even at pH 14. In contrast, ChCl:Urea undergoes extensive and non-selective metal precipitation, leading to co-recovery of cobalt, nickel, and manganese. Overall, this work provides evidence that DESs may influence metal dissolution beyond simple solvent effects. To complement the extraction and mechanistic results, the sustainability of the three DES systems was evaluated using the LEACH framework, which assesses solvometallurgical processes across economic, technical, and safety dimensions. This analysis revealed clear differences in overall sustainability between the DESs, highlighting the critical role of solvent composition, stability, and recyclability in determining process viability.