Cobalt Dissolution from Metal Oxides and Battery Cathode Materials with Acetic Acid-Based Deep Eutectic Solvents

Recovery of critical metals with alternative solvents beyond those in traditional pyrometallurgy and hydrometallurgy is needed in consideration of environmental challenges and the growing demand for metals in energy technologies. Deep eutectic solvents (DESs), composed of hydrogen bond acceptors and donors (HBA/HBD), have emerged as sustainable alternatives for solvometallurgy in metal separation and recovery. In this study, HBAs including choline chloride (ChCl), acetylcholine chloride (AChCl), and betaine (Bet) were investigated for their effectiveness when paired with acetic acid (AA) as the HBD for the dissolution of cobalt from cobalt oxide (CoO), lithium cobalt oxide (LiCoO2), and lithium nickel manganese cobalt oxide (LNMC). Based on the spectroscopic analysis of the metal dissolution and coordination, Bet:AA was found to provide the highest solubility for CoO (0.33 M) in the form of an octahedral complex. On the other hand, ChCl:AA solvent was more effective at dissolving LiCoO2 with 0.04 M Co+2 corresponding to 17% dissolution efficiency and LNMC with 0.06 M Co+2 corresponding to 62% dissolution efficiency at 50 oC, compared to Bet:AA (9% for LiCoO2 and 31% for LNMC). Although the solubilities of LiCoO2 and LNMC have not significantly improved in ChCl:AA, this difference in effectiveness between the solvents clearly reveals the role of the HBA in solubilization. The coordination synergy between the chloride and the –OH moiety facilitates the breakdown of the LiCoO2 driven by the alteration of the solvent polarity. Cobalt in these solutions was found dominantly as a tetrahedral [CoCl4]2–. A chemical separation of cobalt oxalate from a mixed-metal oxide system based on Co, Fe, and Ni was also demonstrated, confirming the potential of these solvents for practical metal recovery.