New Na‐Ion Solid Electrolytes Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33) for All‐Solid‐State Na‐Ion Batteries

Sulfide Na‐ion solid electrolytes (SEs) are key to enable room‐temperature operable all‐solid‐state Na‐ion batteries that are attractive for large‐scale energy storage applications. To date, few sulfide Na‐ion SEs have been developed and most of the SEs developed contain P and suffer from poor chemical stability. Herein, discovery of a new structural class of tetragonal Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33) with space group I41/acd is described. The evolution of a new phase, distinctly different from Na4SnS4 or Na3SbS4, allows fast ionic conduction in 3D pathways (0.2–0.5 mS cm−1 at 30 °C). Moreover, their excellent air stability and reversible dissolution in water and precipitation are highlighted. Specifically, TiS2/Na–Sn all‐solid‐state Na‐ion batteries using Na3.75Sn0.75Sb0.25S4 demonstrates high capacity (201 mA h (g of TiS2)−1) with excellent reversibility. A new structural class of Na‐ion solid electrolytes Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33), distinctly different from Na4SnS4 or Na3SbS4, is discovered. The new structure is fully identified by ab initio X‐ray structural analysis, revealing fast 3D ionic conduction (0.2–0.5 mS cm−1). With excellent air‐stability and aqueous‐solution processability for Na4−xSn1−xSbxS4, a promising electrochemical performance of TiS2/Na–Sn all‐solid‐state batteries is demonstrated.