Solubility of lithium, nickel, and cobalt oxides in deep eutectic solvents

Waste valorization plays a vital role in the circular economy by enabling the recovery of critical metals from spent lithium-ion batteries (LIBs). Deep eutectic solvents (DESs) have emerged as green, tunable alternatives to conventional leaching agents, offering selective dissolution of LIB cathode materials with minimal environmental impact. However, the lack of comprehensive solubility data for LIB-relevant oxides in these solvents has hindered their broader implementation. To this end, this study systematically investigates the solubility of five key oxides (Li 2 O, NiO, Ni 2 O 3 , CoO, and Co 3 O 4 ) in 10 DES systems across a broad temperature range (40–220 °C). The selected DESs span a range of acidity, polarity, reducing capacity, and complexation ability to probe their influence on dissolution behavior. Comprehensive physicochemical characterization, including density, viscosity, FTIR, TGA-DSC, and 1 H NMR, was performed to evaluate solvent structural integrity and thermal stability. Solubility trends were found to strongly correlate with solvent properties such as Brønsted acidity, redox activity, and ligand complexation strength. Most oxides showed increasing solubility with temperature. Li 2 O solubility peaked in ChCl:Urea (1,2) owing to robust hydrogen bonding and efficient ion stabilization, while nickel and cobalt oxides exhibited enhanced solubility in DESs containing highly acidic, reducing, and/or complexing components. Notably, each DES exhibited distinct solvating behavior, and thus, strategically choosing or designing DESs has great potential to enable sequential or simultaneous recovery of critical metals from spent LIBs. By establishing an extensive solubility database for LIB-relevant oxides in DESs, this study provides mechanistic insights into solvent design for efficient LIB recycling. • Comprehensive solubility dataset for Li, Ni, and Co oxides in DESs over 40–220 °C. • Solubility is governed by DES acidity, reducing strength, and complexation ability. • Li 2 O showed maximum solubility in ChCl:Urea (1:2) due to strong hydrogen bonding. • Ni and Co oxides showed distinct solubility trends dependent on oxidation state. • This work enables rational design of green DESs for Li-ion battery recycling.