Unveiling dissolution mechanisms of Li-Ion battery cathode materials in halide–glycol systems toward a regenerable leaching strategy

Deep eutectic solvents (DESs) based on ethylene glycol and choline chloride were among the first systems proposed for the leaching of Li-ion battery cathode materials, enabling the broader adoption of DESs in battery recycling. However, despite extensive efforts on optimizing the leaching efficiencies, the underlying dissolution mechanisms remain poorly understood, limiting control over solvent consumption, decomposition, and circular reuse. Here, systems based on alternative choline halides (Br⁻, I⁻) are applied both on LiNi 1/3 Mn 1/3 Co 1/3 O 2 and black mass revealing, via electrospray ionization the in-situ formation of trihalide species. Spectroscopic and chromatographic analyses show that, under leaching conditions, both ethylene glycol and choline decompose, forming decomposition products that can release protons, either directly or through subsequent reactions. Computational analysis supports a mechanism in which trihalide-mediated oxidation drives proton generation, enabling metal dissolution even in nominally proton-free systems. Ethylene glycol and trihalides are identified as the minimal key components, allowing simplification of the system by replacing choline with inorganic KI salts. Electrochemical regeneration of trihalides is demonstrated, with in-operando Raman spectroscopy revealing an onset potential approximately two times lower than that of an acidic aqueous system. While hazardous decomposition products require further assessment, this approach shows promise for a new circular leaching technology.