Experimental Discovery of a Fast and Stable Lithium Thioborate Solid Electrolyte, Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1)

High-performance, practical all-solid-state batteries (ASSBs) require solid-state electrolytes (SSEs) with fast Li-ion conduction, wide electrochemical stability window, low cost, and low mass density. Recent density functional theory (DFT) simulations have suggested that lithium thioborates are a particularly promising class of materials for high-performance SSEs in Li batteries, but these materials have not been studied extensively experimentally due to synthesis difficulty. Particularly, their electrochemical properties remain largely underexplored, limiting their further development and application as SSEs. In this work, we report the successful synthesis and a comprehensive electrochemical performance study of single-phase, crystalline Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1). We find cold-pressed samples of Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1) to exhibit a high ionic conductivity of 1.3 × 10^-4 S cm^-1 at room temperature. Furthermore, Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1) shows an electrochemical stability window of 1.3-2.5 V, much wider than most sulfide SSEs. Symmetrical Li-Li cells fabricated with a Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1) pellet were cycled up to a current density of 1 mA cm^-2 and exhibited good long-term cycling stability for more than 140 h at 0.3 mA cm^-2. These results suggest Li_6+2x[B₁₀S₁₈]S_x (x ≈ 1) as a promising choice of SSE for high-performance ASSBs for energy storage.