Molecular-level investigation of the performance of ionic liquids in Co extraction

Cobalt (Co), a highly valuable transition metal with unique physical properties, has been widely used as a battery material and, significantly, it satisfies the demand for the rapid development of electric vehicles. However, the scarcity of available sources of Co²⁺ hinders its sustainable development and the extraction and recovery of Co²⁺ is facing severe challenges. Compared to traditional extraction processes, ionic liquids (ILs) have been widely studied as advanced solvents due to their excellent properties. In this study, we investigated the Co²⁺ extraction performance of several ILs, including tri(2,4-methylpentyl)phosphinate ([C₈H₁₇NH₂][Cyanex272]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄MIM][NTF₂]) with tributyl phosphate (TBP), [C₄MIM]citrate ([C₄MIM][C₆H₇O₇]) with di(2-ethylhexyl)phosphoric acid (D2EHPA), 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF₆]) with histidine-2-ethylhexylamide (H2EHA), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄MIM][NTF₂]) and trioctyl phosphine oxide (Cyanex923). We found that the number of H₂O molecules impacts the cluster structures, while the different types of ILs determine the Co²⁺ extraction efficiency. The extraction performance follows the order of [C₄MIM][C₆H₇O₇]/D2EHPA > [C₈H₁₇NH₂][Cyanex27₂] > [C₄MIM][NTF₂]/Cyanex923 > [HMIM][PF₆]/H2EHA > [C₄MIM][NTF₂]/TBP. The structures with the highest formation energy were selected for analysis of their intra-cluster interactions using an independent gradient model based on Hirshfeld partition (IGMH), electrostatic potential and Natural Bond Orbital (NBO) analysis. Unexpectedly, the extraction effects of five cluster structures on Co²⁺ were all better than those on Li⁺, indicating that the extraction performance of ILs significantly differs among different metals. Additionally, we found that the overall extraction efficiency of four ILs for extracting Co²⁺ and Li⁺ was identical, among which [C₄MIM][C₆H₇O₇ exhibited the best performance. Using the same IL [C4MIM][NTF₂], we compared the extraction performance in different co-solvents, Cyanex923 and TBP, and found that Cyanex923 was more effective in extracting metal ions than TBP. This work provides theoretical evidence for the application of ILs in liquid-liquid extraction and for the design of highly efficient extractants in metal recovery.