Ultra-low viscosity betaine hydrochloride-formic acid deep eutectic solvent for leaching critical metals from spent NCM lithium-ion batteries

With the rapid development of the electric vehicle market and increasing demand for lithium-ion batteries (LIBs), spent LIBs recycling has attracted extensive attention worldwide. Being environmentally friendly, deep eutectic solvents (DESs) have great potential for critical metal recycling from spent LIBs. In this study, a new type of DES, consisting of betaine hydrochloride (BeCl) and formic acid (FA) in a 1BeCl:9FA molar ratio, was designed to leach critical metals from spent LIBs cathode materials. The study found that the as-prepared DESs exhibited extremely low viscosities and strong coordination abilities, which are suitable for metal leaching. By properly regulating and controlling the leaching parameters, the Li, Mn, Co, and Ni leaching efficiencies from spent LiNi0.8Co0.1Mn0.1O2 LIBs reached 98.03, 96.01, 94.19, and 92.35%, respectively, whereas those of critical metals were all > 99.5% from either LiCoO2 or LiMn2O4. The leaching kinetics revealed that the leaching process was controlled by the interfacial chemical reaction, and the leaching mechanism indicated that the highly efficient critical metal leaching was the result of the strong coordination capacity (Cl-), acidity (H+), and reduction capacity (FA) synergistic effect in the DES. This study provides a novel, environmentally friendly, and efficient strategy for critical metals recovery from spent LIBs.