Highly selective and near-complete electrochemical recovery of cobalt and nickel from spent batteries through multifunctional deep eutectic solvent

Abstract

Electrochemical recovery presents a sustainable route for battery recycling, yet it is hindered by a trade-off between achieving purity and yield. This challenge arises because, as the target metal depletes during electrodeposition, mass transport limitations reduce its availability, thereby shifting the electrochemical environment in favor of co-deposition of competing metal - particularly during prolonged deposition intended for near-complete recovery. Here, we report a strategy that leverages a multifunctional deep eutectic solvent (DES), ethaline, where ethylene glycol preferentially coordinates with nickel while chloride stabilizes cobalt as tetrachlorocobaltate complexes. Even at elevated temperatures, where nickel undergoes a partial thermochromic transition to chloride coordination, the system maintains a broadened Ni-Co separation window of similar to 0.3 V at 85 degrees C. By fine-tuning the applied potential and utilizing the intrinsic chlorine redox activity of the DES, self-purification was achieved during electrodeposition, yielding a Ni/Co separation factor >3000 and >97 % nickel recovery in a single-step electrodeposition from a synthetic Ni/Co mixture. Building upon this binary separation, we developed a sequential strategy to recover nickel, cobalt, and manganese from real battery leachates. Applied to real NMC leachates, our process enabled the sequential recovery of nickel, cobalt, and manganese with purities of 99.1 %/96.3 % (NMC111) and 99.2 %/98.8 % (NMC811) for nickel and cobalt, respectively, all with >95 % recovery. For NMC111, >97 % nickel purity and >93 % cobalt purity were retained over repeated reuse of the DES, enabling minimal wastewater discharge, with Cl-2-assisted refining enhancing purity to >99.9 %. A technoeconomic analysis validated the economic feasibility and revealed further potential through thermal optimization.