Selective separation of critical metals from lithium-ion batteries in a two-phase leaching system based on a hydrophobic deep eutectic solvent and H2O2 solution

Abstract

An efficient two-phase leaching system using a hydrophobic deep eutectic solvent (DES) composed of 2 mol of 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione and 1 mol of tri-n-octylphosphine oxide, with H₂O₂ as the aqueous phase, was developed for the selective recovery of Li, Co, Ni, and Mn from spent lithium-ion batteries. Under optimized conditions (80 °C, 30 min, 500 rpm, 10 mg per mL black mass loading, DES-to-aqueous ratio 1 : 1, and 5 M H₂O₂ as the aqueous phase), the two-phase leaching system achieved leaching efficiencies of 98% for Li, 94% for Co, 94% for Ni, 98% for Mn, with 73% Cu, 54% Al, and 76% Fe in the DES phase and minor amounts of Cu and Al in the aqueous phase, demonstrating pronounced phase-selective partitioning in a single-step process. The equilibrium pH (pH_eq) of the aqueous solution controlled the metal transfer behavior from the DES to the aqueous phase: Li was fully stripped at pH_eq ≤ 4, while Co, Ni, and Mn migrated to the aqueous phase at pH_eq < 2. Furthermore, the DES exhibited stable performance over three successive cycles without regeneration or stripping, confirming its recyclability and operational robustness.