A key requirement for achieving high-energy density in solid-state batteries is highly efficient cycling of an alkali metal anode in a safe manner. Herein, we combine first-principles calculations and experimental characterizations to identify a protective hydrate coating for Na 3 SbS 4 that leads to a passivating interface and greatly enhanced stability. The buried interface is characterized using postoperando synchrotron X-ray depth profiling. This finding identifies hydrates as promising for improving the metal/electrolyte interfacial stability and suggests a general strategy for interface design. SUMMARYSolid-state batteries provide substantially increased safety and improved energy density when energy-dense alkali metal anodes are applied. However, most solid-state electrolytes react with alkali metals, causing a continuous increase of the cell impedance. Here, we employ a reactivity-driven strategy to improve the interfacial stability between a Na 3 SbS 4 solid-state electrolyte and sodium metal. First-principles calculations identify a protective hydrate coating for Na 3 SbS 4 that leads to the generation of passivating decomposition products upon contact of the electrolyte with sodium metal. The formation of this protective coating, a newly discovered hydrated phase, is achieved experimentally through exposure of Na 3 SbS 4 to air. The buried interface is characterized using post-operando synchrotron X-ray depth profiling, providing spatially resolved evidence of the multilayered phase distribution in the Na metal symmetric cell consistent with theoretical predictions. We identify hydrates as promising for improving the metal/electrolyte interfacial stability in solid-state batteries and suggest a general strategy of interface design for this purpose.

Superionic lithium conductivity has only been discovered in a few classes of materials, mostly found in thiophosphates and rarely in oxides. Herein, we reveal that corner-sharing connectivity of the oxide crystal structure framework promotes superionic conductivity which we rationalize from their distorted lithium environment and reduced interaction between lithium and non-Li cations. By performing a high-throughput search for materials with this feature, we discover 10 novel oxide frameworks predicted to exhibit superionic conductivity-from which we experimentally demonstrate LiGa(SeO3)2 with a bulk ionic conductivity of 0.11 mS/cm and activation energy of 0.17 eV. Our findings provide new insight into the factors that govern fast lithium mobility in oxide materials and will accelerate the development of novel oxide electrolytes for all solid-state batteries.