Na3SbS4: A Solution Processable Sodium Superionic Conductor for All-Solid-State Sodium-Ion Batteries

All‐solid‐state sodium‐ion batteries that operate at room temperature are attractive candidates for use in large‐scale energy storage systems. However, materials innovation in solid electrolytes is imperative to fulfill multiple requirements, including high conductivity, functional synthesis protocols for achieving intimate ionic contact with active materials, and air stability. A new, highly conductive (1.1 mS cm−1 at 25 °C, Ea=0.20 eV) and dry air stable sodium superionic conductor, tetragonal Na3SbS4, is described. Importantly, Na3SbS4 can be prepared by scalable solution processes using methanol or water, and it exhibits high conductivities of 0.1–0.3 mS cm−1. The solution‐processed, highly conductive solidified Na3SbS4 electrolyte coated on an active material (NaCrO2) demonstrates dramatically improved electrochemical performance in all‐solid‐state batteries. Dissolve and solidify: A new and highly conductive (1.1 mS cm−1), dry air‐stable sodium superionic conductor, tetragonal Na3SbS4, was developed. Importantly, its solution processability using methanol or water allows direct coating onto active materials. The solution‐processed highly conductive (0.1–0.3 mS cm−1) solidified Na3SbS4 coating on NaCrO2 solves the poor ionic contact problem in all‐solid‐state sodium‐ion batteries.