Challenges and Strategies in the Use of Ionic Liquids for Sodium-Ion Batteries

Sodium-ion batteries (NIBs) represent a promising alternative to lithium-ion technology due to the greater abundance and sustainability of sodium. However, the choice of suitable electrolytes is a major limiting factor for performance, safety, and operational life. Conventional organic electrolytes provide high ionic conductivity but suffer from flammability, volatility, and limited thermal stability. In this context, ionic liquids (ILs) emerge as promising alternatives due to their very or extremely high flame-retardant properties, very low vapor pressure, good to high power solvent, and wide electrochemical/thermal window. Despite these advantages, the use of ionic liquids in NIBs is hampered by their high intrinsic viscosity due to strong ion interactions. These characteristic limits the mobility of Na+ cations and reduces ionic conductivity, especially in low-temperature conditions, resulting in increased internal resistance and worsened performance at high current rates. To overcome these limitations, different strategies have been proposed based on the target selection of cation/anion pairs, the use of additives, and blending with low-content organic compounds, as well as the optimization of the electrolyte-electrode interface. Thus, the intent of authors in this review is to highlight the progress performed in the last decade, trying to realize a coherent and linear discussion on strategies in the use of ionic liquids for sodium-ion batteries.