Lithium extraction using ionic liquids: Insights from quantum chemical and molecular dynamics simulations

Abstract

Lithium-ion batteries (LIBs) dominate the modern energy infrastructure in scalable power storage and electric mobility. Lithium recovery is crucial for the emergence of a circular economy, and the supply of spent LIBs has increased due to their widespread usage. This work presents the comprehensive evaluation of lithium binding energies and reduced density graph analysis with ionic liquids (ILs) using density functional theory (DFT) calculations. In addition, lithium extraction mechanisms from the aqueous solution using ILs are probed using molecular dynamics (MD) simulations, revealing molecular-scale selectivity. We compared the four ionic liquids (tetra-butylammonium mono-2-ethylhexyl (2-ethylhexyl) phosphate ([N₄₄₄₄] [EHPMEH]), tetra-butylammonium bis(2-ethylhexyl) phosphate ([N₄₄₄₄][DEHP]), tetrabutylphosphonium bis(2-ethylhexyl)phosphate ([P₄₄₄₄] [DEHP]), and tetrabutylphosphonium dodecanoate ([P₄₄₄₄][C₁₁COO]) to extract lithium. Furthermore, from these MD studies, we investigated the extraction mechanism, structural and dynamic properties, such as density analysis, trajectory density contours, and diffusion coefficients. The detailed analysis of structural properties has yielded critical insights into the interfacial interaction of lithium between the aqueous and the ionic liquid phase; the lithium-ion mobility along the different phases was analysed from computed diffusion coefficients. Our results explain the atomistic mechanism of selected ILs and the superior performance of ([N₄₄₄₄] [EHPMEH]) IL in comparison to the other ILs based on localized lithium in the IL phase and binding energies.