Solvate ionic liquids: past, present and future

Abstract

Solvate ionic liquids (SILs) represent an expanding subclass of ionic liquids (ILs), formed through the chelation of the cationic species, typically by an oligoether, and an array of charge diffuse anions. Since their first report, SILs have garnered significant attention for their characteristic ionic liquid physicochemical properties, including high thermal stability, negligible vapour pressure, and tailored solvation dynamics, while being simple to synthesise and cost effective. These factors position SILs as promising candidates for next-generation energy storage applications, including lithium-ion (Li-ion), lithium–sulfur (Li–S), lithium–air (Li–air), lithium-redox, all-solid-state lithium batteries (ASLBs), as well as electric double layer transistors (EDLTs), thermoelectrochemical systems, piezoelectric generators and capacitor/supercapacitor devices. This review traces the evolution of SILs, from foundational studies on their structural and dynamic properties to contemporary advancements in their synthesis and application. SILs modular design potential, through ligand, cation, and anion modifications is evaluated. The multifaceted roles of SILs across various systems are explored, including their applications in electrodeposition and metal extraction, their function as reaction media in organic synthesis, their use as pharmaceutical delivery agents, and their role as constituents and curing catalysts in polymer composites. Furthermore, critical challenges such as optimising ion transport, understanding coordination dynamics, and mitigating environmental impacts are outlined. By integrating experimental insights with computational modeling, this review provides a comprehensive framework to guide future investigations, paving the way for the sustainable development of SILs across scientific and technological domains. SILs reported in the literature predominantly consist of oligoethers G3 (triglyme) or G4 (tetraglyme) chelating a lithium salt such as LiTFSI/LiTFSA/LiNTf₂, lithium bis(trifluoromethanesulphonyl)imide.