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

Sodium thioantimonate (Na₃SbS₄) and its W-substituted analogue Na_2.88Sb_0.88W_0.12S₄ 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 Na₃SbS₄ and Na_2.88Sb_0.88W_0.12S₄ 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 Na₃SbS₄ following the ball mill, resulting in an increase in Na⁺ conductivity. In contrast, the Na⁺ conductivity was reduced in mechanochemically treated Na_2.88Sb_0.88W_0.12S₄ due to the formation and accumulation of a WS₂ phase. In addition, mechanochemical treatment alters the surface morphology of densified Na_2.88Sb_0.88W_0.12S₄ pellets, providing intimate contact at the solid electrolyte/Na interface. This phenomenon was not observed in Na₃SbS₄. This work reveals the structural and morphological origin of the changes seen in these materials' electrochemical performance and how mechanochemical synthesis can introduce them.