Ionic liquids as potential electrolytes for sodium-ion batteries: an overview

In this study, we present an overview on the use of ionic liquids (ILs) as electrolytes in sodium-ion batteries (SIBs). The development of SIBs has gained traction over the last few years because Na is cheaper and more abundant than Li. In this case, although great efforts have been devoted to finding high-capacity and high cell potential materials for SIBs, electrolyte safety is an important factor in producing more competitive and reliable devices. Specifically, the intrinsic volatility of the organic solvent-based electrolytes commonly used in commercial systems is a safety risk during the operation of batteries, and thus replacing them with ILs is an alternative that should be considered. This family of electrolytes is more thermally stable than organic solvents, but they suffer from poor transport properties. Herein, we discuss these properties, considering neat ILs, effects of cations and anions, and effect of salt concentration. Moreover, the strategies to overcome the transport limitations are highlighted. Then, the recent applications of mixtures containing sodium salts and ILs as electrolytes for the negative and positive electrodes in SIBs are presented. Finally, the use of Na-IL mixtures in solid-state electrolytes is discussed.