Advances in sodium secondary batteries utilizing ionic liquid electrolytes

The development of Na secondary batteries that exhibit both sustainability and high energy density as potential successors to lithium-ion batteries for certain large-scale applications has received considerable research interest in recent years. However, although the importance of the electrolyte in such systems has long been largely overlooked, it is becoming increasingly recognized as a key consideration (along with the electrode material) for the non-incremental improvement of Na secondary batteries. Among the candidate electrolytes in this context, ionic liquids (ILs), which are liquids consisting entirely of ions, offer many unique advantages. In this review, the fundamental properties of ILs and the design strategies employed to facilitate their application in batteries are introduced. Comprehensive summaries of the recent advances in the development of positive and negative electrode materials for Na secondary batteries are then presented. Most of the IL-based systems discussed exhibit remarkably enhanced performances compared to those of batteries based on conventional electrolytes. Furthermore, reversible capacity, rate capability, recyclability, and coulombic efficiency are synergistically enhanced by combining IL electrolytes and elevated temperature conditions. Finally, the practical prospects and future challenges associated with the development of electrode materials fabricated from cheap, abundant elements; the efficient utilisation of Na metal as a negative electrode material; and considerations related to the solid-electrolyte interphase are also discussed. This review summarizes the use of ionic liquids in Na secondary batteries and discusses their electrochemical performance with various electrode materials.