Deep Eutectic Solvent-Aqueous Two-Phase Leaching System for Direct Separation of Lithium and Critical Metals

Abstract

Developing an effective recycling process for reclaiming valuable metals from lithium-ion batteries is an urgent issue owing to increasing battery waste from electric vehicles. In this study, we developed a leaching method that enables the direct separation of lithium from other critical metals, namely, nickel and cobalt, using a two-phase system that consists of a deep eutectic solvent (DES) and water. The DES consisting of 4,4,4-trifluoro-1-phenyl-1,3-butadione and tri-n-octylphosphine oxide showed the highest leaching performance when combined with water. Several operational parameters, such as the aqueous fraction, solid-liquid ratio, reaction time, and operation temperature, were evaluated. The optimum results in the two-phase direct leaching system were obtained using a 1:1 DES-water ratio and 10 g/L solid-liquid ratio, reacted at 80 °C for 24 h. An in-situ stripping phenomenon was observed, revealing that lithium transferred from the DES phase to the aqueous phase. In the application of black mass leaching, the aqueous phase significantly enhanced Co, Ni, and Mn extraction into the DES phase and thus plays an important role in separating lithium from other metals. The efficiency of direct lithium leaching from the black mass reached 99% within 24 h.