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Understanding the interfacial reactions between sodium metal (SM) and the solid-state electrolyte (SSE) Na3PS4 (NPS) and its oxygen-doped derivatives, Na3PS4–x O x (NPSO), will help develop a strategy to stabilize the SM–SSE interface. Previous reports have demonstrated that NPS is a promising SSE due to its high ionic conductivity, but it is known to be unstable against SM. This chemical instability and hence reactivity are critical problems in most sulfide materials, and in this work, we report one of the very first detailed studies of the reaction between SM and NPSO SSEs. It was discovered that the reaction between SM and Na3PS4 is facilitated simply by contact and is not driven by a forced potential difference. A combination of powder X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy was used to identify the main reaction product as the reduced phosphide Na3P. Additionally, the reaction is significantly slowed but not completely eliminated by the addition of oxygen in NPSO oxy-sulfide SSEs. We find that NPS is unstable because the SM-NPS reaction layer product is heavily exfoliated, allowing further sodium reaction between the newly created sheets. This degradation mechanism results in further chemical reaction until either the metallic sodium or NPS SSE is fully consumed, whichever is the limiting reagent in the reaction. As oxygen is added, x > 0, the SSE surface remains dense and is slower to react, making it more difficult for the SM to react through the NPSO SSE. The central finding here is that in our work, we find that the x = 1 Na3PS3O SSE remains unreacted with SM over periods of months at room temperature and so far appears to be one of the very few sulfide-based SSEs that is stable against SM and as such is a highly promising SSE for all solid-state sodium batteries (ASSSBs).

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