Ag-Substituted Borohydride Argyrodite Sulfide Electrolytes for Superior Interfacial Stability in All-Solid-State Batteries

Sulfide-based solid electrolytes for all-solid-state batteries (ASSBs) have garnered significant attention due to their high ionic conductivity and excellent mechanical properties. However, poor interfacial stability with Li metal remains a critical barrier to their practical implementation. Among these materials, borohydride (BH₄^-)-based argyrodite solid electrolytes offer higher ionic conductivity than halide-substituted Li argyrodites. They are regarded as some of the most promising candidates for sulfide-based electrolytes. In this study, BH₄^--based argyrodite electrolytes were synthesized via partial substitution of Ag at the Li site using a two-step ball-milling process, without any heat treatment. Among the synthesized compositions, Li_5.5Ag_0.1PS_4.6(BH₄)_1.4 achieved a high ionic conductivity of 13.7 mS cm^-1 at 30 °C. Li symmetric cells incorporating the synthesized electrolytes demonstrated stable, long-term Li plating/stripping cycles and a high critical current density of 2.9 mA cm^-2. The Li/solid electrolyte interface was investigated using X-ray photoelectron spectroscopy, revealing that Ag promotes uniform Li deposition by forming a Li-Ag alloy at the interface. This alloy formation effectively suppresses dendrite growth and significantly improves interfacial stability with Li metal. ASSBs utilizing the Ag-substituted solid electrolyte showed enhanced cycling stability and rate performance, highlighting their potential for practical application.