Mechanochemical Synthesis of Potassium–Ion Conductor K₃SbS₄ (Supporting Information)

<div>Antimony-based sulfide solid electrolytes exhibit high conductivity for alkaline cations. In this study, we synthesized K₃SbS₄ potassium-ion conductors using the mechanochemical method for the nominal compositions with <i>x</i> mol% excess K₂S (<i>x</i> = 0, 5, 10, and 15) to compensate for the chemical impurities in the K₂S reagent. The mechanochemically prepared samples showed X-ray diffraction patterns similar to β-K₃SbS₄ in all the compositions. Raman bands attributed to the SbS₄³⁻ unit were observed in all the samples. The ionic conductivities at 25 °C showed a positive correlation with increasing <i>x</i>, reaching a maximum ionic conductivity of 3.6 × 10⁻⁶ S cm⁻¹ at 10 mol% excess K₂S. Subsequent heat-treatment further enhanced the ionic conductivity, achieving 1.2 × 10⁻⁵ S cm⁻¹ at 25 °C. This improvement is attributed to the nominal composition being close to that of K₃SbS₄ by adjusting the excess amount of K₂S and the increased crystallinity of β-K₃SbS₄.</div>