Abstract
<jats:p> Sulfide-based lithium-ion solid electrolytes are promising candidates for solid-state batteries due to their high ionic conductivities, although they typically exhibit high moisture sensitivity. Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> has recently attracted scientific interest because it exhibits higher moisture resistance than most other sulfide electrolytes. Two orthorhombic Li4SnS4 polymorphs (α and β) are known, yet their temperaturedependent phase behavior remains unexplored. Differential scanning calorimetry and temperaturedependent powder X-ray diffraction revealed that α-Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> transitions to β-Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> at 96 °C. Furthermore, we identified a second, previously unknown reversible phase transition from β-Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> to a new γ-polymorph at 196 °C. Other phase transitions from −150 °C to 1000 °C were not identified. All polymorphs crystallize in space group <jats:italic toggle="yes">Pnma</jats:italic> (no. 62) and feature identical zig-zag-like arrangements of SnS <jats:sub>4</jats:sub> <jats:sup>4-</jats:sup> tetrahedra. The main structural differences lie in the ordering of the lithium substructure. By combining temperature-dependent powder X-ray and neutron diffraction with high-temperature <jats:sup>7</jats:sup> Li and and <jats:sup>119</jats:sup> Sn NMR as well as Raman spectroscopy, we show that Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> transitions from the lithium-vacancy-disordered α-phase to the ordered β-phase and finally to the disordered γ-Li <jats:sub>4</jats:sub> SnS <jats:sub>4</jats:sub> . Using electrochemical impedance spectroscopy, we measured ionic conductivities from 10 <jats:sup>−6</jats:sup> S cm <jats:sup>−1</jats:sup> to 10 <jats:sup>−1</jats:sup> S cm <jats:sup>−1</jats:sup> between 25 °C to 220 °C. Conductivity changes are gradual, consistent with the subtle structural variations among all polymorphs. </jats:p>