Insight into the structural and electrochemical properties of the interface between a Na6SOI2 solid electrolyte and a metallic Na anode

Solid electrolytes (SE) have attracted a great deal of interest as they can not only mitigate the safety issues related to currently used liquid organic electrolytes but also enable the introduction of a metallic Na anode with extreme energy density in sodium-ion batteries. For such application, SE should exhibit high interfacial stability against metallic Na as well as high ionic conductivity, and Na6SOI2 with a Na-rich double anti-perovskite structure was recently identified as a promising SE candidate. In this work, we performed first principles calculations to investigate the structural and electrochemical properties of the interface between Na6SOI2 and a metallic Na anode. Our calculations revealed that interfaces could be formed safely, keeping the ultra-fast ionic conductivity of the bulk phase near the interface. Through the electronic structure analysis of the interface models, we found the change of upward valence band bending at the surface to downward band bending at the interface, being accompanied by electronic charge transfer from a metallic Na anode to Na6SOI2 SE at the interface. This work provides valuable atomistic insight into the formation and properties of the interface between SE and alkali metal for enhancing battery performance.