Challenges and advances of sulfide solid electrolytes for high-energy-density sodium metal batteries

With high ionic conductivity and excellent mechanical properties, sulfide solid electrolytes (SSEs) represent a leading candidate electrolyte system for sodium metal batteries (SMBs), offering enhanced energy density and improved safety. However, the development and practical deployment of SSEs are hindered by intrinsic air instability, poor interfacial compatibility with electrodes, and demanding synthesis conditions. This review provides a comprehensive summary aimed at accelerating the adoption of SSEs in high-energy-density sodium metal batteries. We systematically examine the crystal structures and properties of sulfide sodium-ion conductors, followed by critical challenges related to air stability, electrode-electrolyte interfaces, and synthesis techniques. Subsequently, effective strategies for realizing high ionic conductivity, improved chemical/electrochemical stability, and enhanced interfacial compatibility have been thoroughly summarized and discussed. When coupled with different cathodes, the interfacial challenges and long-term cycling stability of SSE-based sodium metal batteries (SSE-SMBs) were further discussed. Besides, the advantages and challenges of different cathodes at high energy densities are also mentioned. Finally, taking the cathode, SSEs, and anode into account, we identify the challenges of these electrode materials and the areas for improvement to achieve high energy densities.