Ionic liquids (ILs) are organic salts typically with melting points below 100°C [1]. They exhibit unique properties, such as negligible vapor pressure, low flammability, high thermal and chemical stability, and tunability, making them attractive for applications in organic synthesis, CO₂ capture, and liquid-liquid extraction [2].
Liquid-liquid extraction using hydrophobic room-temperature ILs (RTILs) has been extensively explored over the past two decades for metal recovery, due to its advantages over traditional organic solvents. The tunability of ILs, by modifying their cation and anion, enables their design as tailored solvents for specific metal extractions. In such applications, ILs can serve as diluents, extractants, or both [3].
Lithium (Li) is a key element in several industries, particularly in rechargeable batteries, ceramics, glass production, and lubricants [4]. The transition towards electric vehicles has significantly increased the global lithium demand, leading to its classification as a critical raw material by the European Commission [5].
Currently, Li is mainly extracted by evaporative brine processing [6] and hard rock mining [7], both of which raise environmental concerns, including high water consumption (85-95% loss) in arid regions, extensive use of strong acids, and elevated greenhouse gas emissions [8]. IL-based extraction methods present a promising alternative, offering potential solutions to these environmental challenges. Additionally, ILs could be used for Li recovery from secondary sources, such as spent batteries.
Recent studies on Li extraction using ILs have primarily employed ILs as diluents for conventional extractants like TBP [9]. While imidazolium-based ILs have demonstrated their ability to extract anionic metal complexes (e.g., [AuCl4]- [10], [TlCl4]- [11]), their direct application for Li (I) extraction has shown limited efficiency. For instance, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][Tf₂N]) exhibited low Li extraction performance [12].
In this study, we investigate the Li extraction capability of the hydrophobic RTIL 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ([C₄mim][NfO]). The IL was synthesized in a single-step reaction and tested for Li extraction without additional extractants. The influence of key parameters, including aqueous phase pH, initial Li concentration, stirring time, and temperature, was systematically evaluated. Furthermore, a mechanistic study was conducted to elucidate the extraction mechanism and the nature of the extracted Li species.
Our results highlight a significant enhancement in Li extraction efficiency (~50%) using pure [C₄mim][NfO], demonstrating the crucial role of IL anion selection in tuning extraction performance. A proposed extraction mechanism, based on equilibrium studies, provides new insights into Li recovery via ILs.