Rechargeable sodium-ion batteries have received growing attention as promising alternatives to lithium-ion batteries because of the abundance of Na resources as well as its high theoretical capacity of 1165 mAh/g, and low electrochemical potential (-2.71 V vs. standard hydrogen electrode). However, the safety issues remain as the most critical problems for its application. The rise of sulfide-based solid state electrolytes (SSEs) with high Na-ion conductivity have brought about new opportunities for the development of sodium metal batteries. Compared to organic electrolytes, the inorganic SSEs have an increased Young's modulus, which suppress stress points in the metal anode to prevent dendrite formation. Additionally, the SSEs are non-flammable, which addresses the safety concern posed with the organic electrolyte.
Despite the high ionic conductivities of sulfide-based SSEs, most of them are not stable with a metallic sodium anode. The recently investigated tetragonal Na3SbS4 having a superior Na-ion conductivity up to 3 mS/cm at room temperature is a good candidate for SSE, but it also suffers from poor chemical stability with Na. Based on theoretical calculation and experimental results, we find Na3SbS4 is not stable with Na metal and will decompose into Na2S and Na3Sb, resulting in a significant increase of interfacial resistance. Our approach for stabilizing the Na3SbS4/Na interface is to introduce a cellulose-supported poly(ethylene oxide) (CPEO) layer between Na3SbS4 and Na. Owing to the high stability of the CPEO layer and the enhanced interfacial contact with Na, the Na/CPEO/Na3SbS4/CPEO/Na symmetric cell shows a stable Na stripping/plating over 800 hours under 0.1 mA/cm2 at 60 oC. This simple and scalable polymer protecting approach provides a solution to improve the anode stability of sulfide-based electrolyte.