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) requiresolid-state electrolytes (SSEs) with fast Li-ion conduction, wideelectrochemical stability window, low cost, and low mass density.Recent density functional theory (DFT) simulations have suggestedthat lithium thioborates are a particularly promising class of materialsfor high-performance SSEs in Li batteries, but these materials havenot been studied extensively experimentally due to synthesis difficulty.Particularly, their electrochemical properties remain largely underexplored,limiting their further development and application as SSEs. In thiswork, we report the successful synthesis and a comprehensive electrochemicalperformance study of single-phase, crystalline Li6+2x [B10S18]-S- x (x approximate to 1). We find cold-pressed samples ofLi(6+2x )[B10S18]-S- x (x approximate to 1) to exhibita high ionic conductivity of 1.3 x 10(-4) S cm(-1) at room temperature. Furthermore, Li6+2x [B10S18]-S- x (x approximate to 1) shows an electrochemical stabilitywindow of 1.3-2.5 V, much wider than most sulfide SSEs. SymmetricalLi-Li cells fabricated with a Li6+2x [B10S18]-S- x (x approximate to 1) pellet were cycled up to a current densityof 1 mA cm(-2) and exhibited good long-term cyclingstability for more than 140 h at 0.3 mA cm(-2). Theseresults suggest Li6+2x [B10S18]-S- x (x approximate to 1) as a promising choice of SSE for high-performance ASSBs for energystorage.