Synergistic Promotion by Acidity and Reducibility for the Mild and Efficient Leaching of Lithium Cobalt Oxide in Deep Eutectic Solvents: Mechanism Elucidation

The green recycling of spent lithium-ion battery cathodes is crucial to alleviating energy and environmental challenges. Deep eutectic solvents (DESs) have emerged as promising green alternatives to conventional hydrometallurgy for cathode material recovery. In this work, a series of organic-acid-based DESs were synthesized using choline chloride as a hydrogen bond acceptor. Their acidity was measured by the Hammett method, and their reducibility was measured by the open-circuit potential. The dissolution of lithium cobalt oxide (LCO) was examined in these DESs at 60 °C for 12 h. Additional acid or reductant was supplied for the poorly soluble systems. A “solubility–acidity–reducibility” relationship was established to interpret the dissolution mechanism. The results show that these methods accurately quantify the DES acidity and reducibility. The strong acidity and reducibility together enable a high LCO solubility in DESs. The mechanistic study indicates that H + displaces Li +, breaks Co–O bonds, and raises the reduction potential of CoO 2 – /Co 2+ . Organic acids act as reductants, converting insoluble Co(III) in LCO into soluble Co(II) complexes while themselves being oxidatively decarboxylated to CO 2 and small organic acids. This study clarifies the synergy of acidity and reducibility in DES leaching of LCO, providing guidance for solvent design and optimization and facilitating the industrial application of green solvents in battery recycling.