4 V Na Solid State Batteries Enabled by a Scalable Sodium Metal Oxyhalide Solid Electrolyte

All-solid-state sodium batteries (ASSSBs) are viable candidates for large scale energy storage that could vie with lithium. Ductile solid catholytes for such cells that can be prepared without extensive ball milling and directly paired with high voltage sodium cathodes are lacking, however. We report a new amorphous fast Na-ion conducting metal oxychloride that meets these criteria, synthesized through a scalable and low-cost route based on a spontaneous solid-state reaction with simple short mixing and 100 °C annealing. It has an ionic conductivity of 1.2 mS·cm^–1 and low activation energy of 0.31 eV. Due to its dual O^2–/Cl^– framework, it exhibits a high anodic potential of 4 V vs Na⁺/Na and good chemical/electrochemical compatibility with high voltage sodium cathode materials. ASSSBs consisting of the oxychloride solid electrolyte paired with a high voltage P2–Na_2/3Ni_1/3Mn_2/3O₂ cathode showed stable long-term cycling with a 4.0 V vs Na₃Sn cutoff potential and even to 4.3 V.