Ionic liquid electrolytes for sodium-ion batteries to control thermal runaway

Sodium-ion batteries are expected to be more affordable for stationary applications than lithium-ion batteries, while still offering sufficient energy density and operational capacity to power a significant segment of the battery market. Despite this, thermal runaway explosions associated with organic electrolytes have led to concerns regarding the safety of sodium-ion batteries. Among electrolytes, ionic liquids are promising because they have negligible vapor pressure and show high thermal and electrochemical stability. This review discusses the safety contributions of these electrolyte properties for high-temperature applications. The ionic liquids provide thermal stability while at the same time promoting high-voltage window battery operations. Moreover, apart from cycle stability, there is an additional safety feature attributed to modified ultra-concentrated ionic liquid electrolytes. Concerning these contributions, the following have been discussed, heat sources and thermal runaway mechanisms, thermal stability, the electrochemical decomposition mechanism of stable cations, and the ionic transport mechanism of ultra-concentrated ionic liquid electrolytes. In addition, the contributions of hybrid electrolyte systems consisting of ionic liquids with either organic carbonate or polymers are also discussed. The thermal stability of ionic liquids is found to be the main contributor to cell safety and cycle stability. For high-temperature applications where electrolyte safety, capacity, and cycle stability are important, highly concentrated ionic liquid electrolyte systems are potential solutions for sodium-ion battery applications.