The quest for zero carbon economy promotes the use of lithium-ion batteries (LiBs) with a projected global increase of almost three folds from 250 million units in 1998 to 700 million units in 2030. This magnifies the challenge of e-waste increase and shortage of essential materials used in LiBs (e.g., Li and Co). Hence, recycling of LiBs is a dual solution to manage waste while establishing circular economy by recovering metals and reducing the need for extensive mining. Among the prevalent methods for LiB recycling, hydrometallurgy is the most effective, although it involves the use of harsh acids and hazardous chemicals. The use of Deep Eutectic Solvents (DESs) as a lixiviants provides a new environmentally benign pathway for LiBs recycling. Although several such DES systems with varying efficiencies have been reported for LiB cathode recycling, their leaching mechanism remains elusive. In this work, we investigate the mechanism of LiB cathode metals dissolution in an ethylene glycol (EG): choline chloride (ChCl) DES with commonly used cathode materials NMC and NCA. A combination of spectroscopic methods and DFT calculations reveals that while EG acts as a proton donor and assists in weakening the transition metal-oxygen bond, Cl- from ChCl attacks the metal center to form tetrachlorometalates, which assists in the dissolution. Hence, we observe increased leaching efficiencies with increasing Cl- concentration in the DES. Owing to its small size, Li does not form a chloride complex, but instead is surrounded by Cl- ion to form a van der Waal’s type interaction. This is consistent with the faster leaching rate of Li compared to the transition metals that can also be attributed to its intercalated position in the crystal structure of NMC and NCA. In summary, this study allows understanding the leaching process that enables designing effective DES systems and establishing scale-up pathways.

Mechanistic study of lithium-ion battery cathode recycling using deep eutectic solvents

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