Electrolytes for Lithium (Sodium) Batteries Based on Ionic Liquids: Highlighting the Key Role Played by the Anion

Research since 2000 has clearly shown that ionic liquids (ILs) and their metal salt containing mixtures have good potential to be considered as electrolytes (ILELs) in lithium and other electropositive metal (ion) batteries. This outcome is particularly relevant for the operation of such devices in elevated temperature regimes where ILELs would have a significant advantage over the conventional organic carbonate solvent/LiPF6 electrolytes that, due to their limited thermal stability and flammability, cause concerns about the safety of current LIB technology. There is evidence from a number of review articles that physicochemical properties can be tailored such that ILELs could meet requirements for operating in batteries at lower temperature regimes. By drawing on a broad range of cation/anion combinations, there is further evidence from these papers that within a particular family of cations, the physicochemical properties of ILs and ILELs are largely defined by the anion component. Despite the key role of the anion there are few reviews that have sections dedicated to this aspect. This review surveys the physicochemical, transport and structural properties of ILs (mainly pyrrolidinium salts) and ILELs employing prominent and representative anion classes.