Sodium halide solid electrolytes for all-solid-state sodium batteries: advances, challenges, strategies, and prospects

Abstract

All-solid-state sodium batteries (ASSSBs) have emerged as promising options for next-generation energy storage systems owing to high energy density, superior safety, rich sodium resources, and low cost. To promote the practical application of ASSSBs, advanced solid electrolytes (SEs) that are well matched with metallic sodium anode and high-voltage cathodes are of urgently needed to be developed. Sodium halides, which rationally combine the merits of considerable ionic conductivity, wide electrochemical window, and excellent mechanical flexibility, show great promise as ideal SEs for use in ASSSBs. However, due to the nature of sodium halides, several bottlenecks still need to be overcome, such as high sensitivity to moisture, limited methods for efficient production, unideal interface stability with electrodes, and complex assembly process for full batteries. In this review, the characteristics of sodium halides SEs, including crystal structure, ionic transport mechanism, and synthesis routes are well summarized. Meanwhile, the issues of high moisture sensitivity and unstable interface are focused, and associated optimization strategies are overviewed. In addition, the assembly and electrochemical performance of ASSSBs are outlined. For future research directions, several constructive suggestions are proposed. This work may provide a relatively comprehensive understanding of promising sodium halides SEs, helping to the exploration of innovative electrolyte design and utilization solutions for advanced ASSSBs.