Selective separation of Co(II) and Li(I) in chloride media by a β-diketone-based bifunctional ionic liquid

Efficient separation of Co(II) and Li(I) from spent lithium-ion batteries remains challenging because of their distinct chemical properties. In this work, a β-diketone-based bifunctional ionic liquid (IL), [DOC 4 mim][TTA], was employed to construct a selective Co/Li separation system in chloride media. The extraction behavior of Co(II) and Li(I) was systematically investigated by evaluating the effects of operating parameters, while the extraction mechanism was elucidated through equilibrium pH analysis, slope analysis, and FT-IR characterization. The results revealed that [DOC 4 mim][TTA] possessed excellent selectivity toward Co(II), whereas Li(I) remained predominantly in the aqueous phase because of its strong hydration characteristics. A maximum Co/Li separation factor of 12,227 was obtained under the optimized conditions. Mechanistic studies suggested that Co(II) extraction proceeded through a coordination–ion association synergistic pathway, in which the β-diketone anion served as the primary coordination site and the imidazolium cation stabilized the extracted species through ion-association interactions. In addition, [DOC 4 mim][TTA] exhibited good regeneration capability and recycling stability. The applicability of the proposed system was further verified using actual LiCoO₂ leachates. After two extraction stages, a cumulative Co(II) removal efficiency of 99.54% and a Co/Li separation factor of 11,319 were obtained. These findings demonstrate the considerable potential of the β-diketone-based bifunctional IL for selectively recovering valuable metals from spent lithium-ion battery materials and provide useful guidance for the rational development of bifunctional ILs for transition-metal separation in chloride media.