An Oxychloride Solid Electrolyte with Superior Na-Ion Conductivity

All-solid-state sodium batteries (ASSSBs) have attracted significant attention due to the abundance and lower cost of sodium compared to lithium as well as their potential for higher energy density and improved safety. The solid electrolyte (SE) is a critical component of ASSSBs, as it governs the rate of ion transport and thus determines the battery's power delivery. However, sodium (Na)-ion SEs typically exhibit lower ionic conductivity than their lithium counterparts. In this study, we report a series of Na-ion oxychloride SEs with the composition Na_3xTaO_3xCl_5-3x, which demonstrate a high room-temperature ionic conductivity of 2.6 mS cm^-1 and a low activation energy of 0.26 eV. These Na-ion oxychloride SEs are synthesized using NaTaO₃ as a sustainable ternary oxygen source. X-ray photoelectron spectroscopy (XPS) confirms the oxygen bonding environment in the oxychloride structure, while X-ray absorption spectroscopy (XAS) reveals the complex local coordination around Na⁺ ions. The synergistic structural features of Na_3xTaO_3xCl_5-3x facilitate Na⁺ migration, making it a promising candidate for high-performance ASSSBs. Reversible charge-discharge cycling of all-solid-state sodium-iodine batteries is demonstrated using the oxychloride solid electrolyte.