Reactivity-Guided Interface Design in Na Metal Solid-State Batteries

Solid-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 Na3SbS4 solid-state electrolyte and sodium metal. First-principles calculations identify a protective hydrate coating for Na3SbS4 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 Na3SbS4 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.