Enhancement of ionic conductivity in Li argyrodite solid electrolytes with bromide and borohydride anions for all-solid-state batteries

Abstract

All-solid-state batteries (ASSBs) containing solid electrolytes can offer high energy density and enhanced safety compared to conventional Li-ion batteries. Among solid electrolytes, sulfide-based materials have garnered significant attention owing to their high ionic conductivity and facile processability. In this study, Li argyrodite solid electrolytes containing both BH₄⁻ and Br⁻ anions were prepared using a straightforward two-step milling method without heat treatment. The synthesized materials exhibited an impressive ionic conductivity of up to 14.4 mS cm⁻¹ at room temperature, which surpassed previously reported values of borohydride-substituted argyrodite electrolytes. To elucidate the effect of Br⁻ anion replacement in the thiophosphate local structure, Raman and ³¹P solid-state nuclear magnetic resonance spectroscopic analyses were performed. The results showed that the total proportion of the PS₄³⁻ units in the argyrodite phase, including both BH₄⁻ and Br⁻ anions, determined the Li ionic conductivity of the solid electrolytes. The prepared solid electrolyte samples were stable up to 5 V vs. Li⁺/Li without any notable side reactions. The rate performance of the ASSBs, including both the Br⁻ and BH₄⁻-substituted solid electrolytes, was superior to that of the cell with the BH₄⁻-substituted solid electrolyte.