Anion Control of the Electrolyte Na_3–xSbS_4–xBr_x Extends Cycle Life in Solid-State Sodium Batteries

Among the solid electrolytes studied for all-solid-state sodium batteries (ASSBs), Na3SbS4 (NAS) appears particularly promising due to its unique combination of high ionic conductivity and compatibility with humid air, which lowers the cost of processing and facilitates large-scale industrial applications. Considering that partial substitution of sulfur with halide ions increases the number of mobile sodium vacancies, we developed a low-cost structural modification to NAS by integrating the doping step with the aqueous synthesis. Here, we focus on the effect of bromide doping of NAS on its ionic conductivity and kinetic electrochemical stability. The extent and location of Br doping are clarified by X-ray diffraction analysis. The impact of doping on the ion transport properties is studied by static energy landscape analyses and molecular dynamics simulations. Our Br-doped NAS exhibits greatly reduced activation energy down to 0.12 eV. Therefore, the total ionic conductivity is more than doubled from 0.13 to 0.31 mS cm–1, allowing for solid-state batteries with lower overpotentials. Stable cycling profiles of the doped NAS versus both sodium metal anodes and sodium–tin alloy anodes evidenced a major enhancement of the electrochemical stability and thus a path toward longer cycle life ASSBs.