Electrochemical Properties of NaPSiSO Na₂S-SiS₂-P₂S₅-NaPO₃ Glasses: Toward Low-Cost, Stable and Easily Processable Moderate-Temperature Solid Electrolytes for All-Solid-State Sodium Batteries

All solid-state sodium batteries (ASSSBs) are attracting significant research interest in an effort to decrease the raw materials competition and environmental damages caused by harvesting limited amounts of lithium, particularly for grid-scale energy storage applications. ASSSBs offer the advantages of being a more cost-effective and environmentally friendly solution to energy storage needs. To enable these ASSSBs, significant efforts have been made to develop new sodium solid-state electrolytes (SSEs). Glassy solid-sate electrolytes (GSEs) have been found to be particularly promising due to their high conductivities and processability. Here, the mixed oxy-sulfide (MOS) GSE, 0.59Na2S + 0.30SiS2 + 0.05PS5/2 + 0.06NaPO3 (NaPSiSO), has been found to exhibit excellent electrochemical performance at usable current densities while maintaining low interfacial impedances and high reductive and oxidative stabilities in contact with Na-metal (−0.5 V–3 V). At 80 °C, the NaPSiSO GSE exhibited an ionic conductivity of 4.8 × 10–4 (Ω cm)−1, a critical current density (CCD) of 0.85 mA cm–2 and stable cycling at 0.1 mA h cm–2 for over 900 h without shorting. Compared to its pure sulfide analogue, NaPSiS, the relatively small oxygen doped NaPSiSO, O/(O + S) ratio of 0.12, GSE shows significantly improved cycling performance and ionic conductivity. For these reasons, this new GSE is a promising candidate SSE for use in ASSSBs.