Mechanochemistry in Sodium Thioantimonate Solid Electrolytes: Effects on Structure, Morphology, and Electrochemical Performance.

Sodium thioantimonate (NaSbS) and its W-substituted analogue NaSbWS have been identified as potential electrolyte materials for all-solid-state sodium batteries due to their high Na conductivity. Ball milling mechanochemistry is a frequently employed synthetic approach to produce such Na-conductive solid solutions; however, changes in the structure and morphology introduced in these systems via the mechanochemistry process are poorly understood. Herein, we combined X-ray absorption fine structure spectroscopy, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy characterization techniques to provide an in-depth analysis of these solid electrolytes. We report unique changes seen in the structure and morphology of NaSbS and NaSbWS resulting from ball milling, inducing changes in the electrochemical performance of the solid-state batteries. Specifically, we observed a tetragonal-to-cubic crystal phase transition within NaSbS following the ball mill, resulting in an increase in Na conductivity. In contrast, the Na conductivity was reduced in mechanochemically treated NaSbWS due to the formation and accumulation of a WS phase. In addition, mechanochemical treatment alters the surface morphology of densified NaSbWS pellets, providing intimate contact at the solid electrolyte/Na interface. This phenomenon was not observed in NaSbS. This work reveals the structural and morphological origin of the changes seen in these materials' electrochemical performance and how mechanochemical synthesis can introduce them.