Ionic Liquids and Ammoniates as Electrolytes for Advanced Sodium-Based Secondary Batteries

This review aims to provide an up-to-date report on the state of the art of electrolytes based on (quasi-)ionic liquids for sodium batteries. Electrolytes based on conventional ionic liquids are classified into one-anion- and two-anion-type electrolytes according to the number of different anions present in the media. Their application for sodium-based batteries is revised, and the potential advantages of two-anion-type electrolytes are highlighted and rationalized based on the higher tunability of interactions among the different electrolyte components enabled by the presence of two different anionic species. Next, the synthesis and properties of liquid ammonia solvates (aka liquid ammoniates) are presented, with a focus on their use as alternative electrolytes. Attention is paid to some of the outstanding properties of ammoniates, notably, their high conductivity and sodium concentrations, together with their ability to sustain dendrite-free sodium deposition, not only on sodium but also on copper collectors. Finally, the prospects and limitations of these electrolytes for the development of new sodium-based batteries, including anode-less devices, are discussed.