Extraction of Lithium from an Aqueous Solution Using a Hydrophobic Deep Eutectic Solvent: A Combined Experimental and Classical Molecular Dynamics Study

Lithium–ion batteries (LIBs) are increasingly prevalent as primary energy storage solutions, particularly in electric vehicles, due to their superior voltage capacity, energy density, and minimal discharge rate. The availability of spent LIBs has increased due to their extensive utilization, which implies that lithium recovery is essential for the development of a circular economy. In this scenario, extraction processes using green solvents such as hydrophobic deep eutectic solvents (HDES) are gaining popularity. In this work, a HDES with DL-menthol and pyruvic acid in a 1:2 molar ratio was formulated and thereafter employed for lithium extraction from an aqueous solution. Formulated HDES has efficiently extracted 80% of the lithium from an aqueous solution at an optimum condition of 25 and 50 mg/L LiCl solution and an organic-to-aqueous (O/A) phase volume fraction of 0.5. The extractant phase was separated and subsequently characterized by using Fourier transform infrared spectroscopy (FTIR) to confirm lithium extraction. Further, pK_a calculations were performed using density functional theory (DFT) to investigate the lithium extraction mechanism using the menthol-based HDES. Classical molecular dynamics (MD) simulations were employed to calculate structural and dynamical properties, elucidating the atomistic extraction mechanism, lithium mobility, interfacial interactions, and the concentration dependence of HDES on lithium extraction. Finally, this study examines potential green HDESs for the recovery of lithium from aqueous solutions at 28 ± 2 °C.