Progress in safe nano-structured electrolytes for sodium ion batteries: A comprehensive review

Sodium ion batteries (SIBs) have resurfaced into the spotlight, given the supply chain uncertainties and the soaring demand for lithium-ion batteries (LIBs). Although, even now, their lower energy density may stall their commercialization in the portable sector, they are considered prime candidates for large scale electrochemical energy storage applications. Accordingly, advancing, establishing, and maintaining the safety of SIBs is crucial to prevent catastrophic thermal runaways and colossal financial losses to garner the trust of concerned authorities. Electrolytes play a pivotal role in the safety of batteries. Considering the above, this paper presents a comprehensive review of the progress in safe electrolytes for SIBs. It explains the various approaches employed to enhance the safety of high-risk based electrolytes and the electrochemical performance of intrinsically safe electrolytes. Moreover, a state-of-the-art review of the assembled cells/half cells employing different classes of electrolytes is also presented. Particular attention has been devoted to specifying the techniques and results, if available, of thermal stability and safety tests besides highlighting the electrochemical characteristics and performance, such as the cell capacity and cyclability, and electrolyte ionic conductivity and electrochemical stability window (ESW) of the electrolyte. Finally, challenges and future research directions have been summarized and recommended. This review concludes that solid state electrolytes with high conductivity are among the practical and safe electrolytes for SIBs.