Interfacial Stability and Dry Room Compatibility of Na3PS4 Solid-State Electrolyte Dry Films

Sodium solid-state batteries (NaSSBs) employing nonflammable sulfide solid-state electrolytes (SSEs) are a promising solution for large-scale energy storage due to their safety, low cost, and high volumetric energy density. However, the state-of-the-art pellet type NaSSBs face challenges such as inadequate humidity stability and poor mechanical properties, resulting in nonnegligible ohmic losses, limited crucial current density, and low energy density. Reducing the thickness of the SSE layer and improving the anti-moisture stability is crucial to realize the scalable production and practical applications of NaSSBs. To address these challenges, a dry process was developed to fabricate Na3PS4 (NPS) solid electrolyte film with high retention of intrinsic ionic conductivity, and increased flexibility by incorporating only 0.2wt% of polytetrafluoroethylene (PTFE) binders. Notably, it is found that PTFE with hydrophobic groups improves moisture stability by reducing the available adsorption sites for H2O on the surface. After 24 hours exposure in a dry room condition (RH 0.1%), the NPS film exhibits more than 90% conductivity retention and stable cycling for up to 100 cycles, representing a significant step towards the scalable production and practical use of low-cost NaSSBs. Our findings may also guide the development of robust lithium-based sulfide electrolyte films, such as Li6PS5Cl.