Anomalous Ionic Conductivity Boost in the Thermally Treated Sulfide Solid Electrolyte Li4SnS4 via Phase Coexistence

The sulfide solid electrolyte Li₄SnS₄ exhibits significantly higher moisture stability against hydrolysis than other sulfide solid electrolytes and has attracted attention as a promising material for overcoming the inherent challenges of sulfide solid electrolytes. Li₄SnS₄ is known to exhibit two crystal phases: a hexagonal phase that is stable at low temperatures and an orthorhombic phase that forms upon thermal treatment at elevated temperatures. In this study, we investigated the effect of heat treatment at various temperatures on lithium ionic conductivity and found that, within a specific temperature range, the conductivity shows a distinct maximum, which coincides with the coexistence of hexagonal and orthorhombic phases. Synchrotron X-ray diffraction analysis revealed that nanosized orthorhombic crystallites are present within the overall hexagonal matrix near 250–300 °C, where the anomalous conductivity maximum appears. Further heat treatment at higher temperatures led to the growth of these orthorhombic crystallites, eventually resulting in a single-phase orthorhombic structure, during which the lithium ionic conductivity sharply decreased. We demonstrate that the characteristic nanoscale mixed-phase state (phase coexistence) serves as the key structural origin for the anomalous enhancement of lithium ionic conductivity in Li₄SnS₄, offering a specific insight into the design principles of heat-treatment protocols for sulfide solid electrolytes.