Lithium Makes the Difference─Evolution of Lithium Vacancy Disorder during Temperature-Induced Phase Transitions of the Li-Ion Conductor Li4SnS4

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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. Li4SnS4 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 temperature-dependent phase behavior remains unexplored. Differential scanning calorimetry and temperature-dependent powder X-ray diffraction revealed that α-Li4SnS4 transitions to β-Li4SnS4 at 96 °C. Furthermore, we identified a second, previously unknown reversible phase transition from β-Li4SnS4 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 Pnma (no. 62) and feature identical zig-zag-like arrangements of SnS44– 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 7Li and 119Sn NMR as well as Raman spectroscopy, we show that Li4SnS4 transitions from the lithium-vacancy-disordered α-phase to the ordered β-phase and finally to the disordered γ-Li4SnS4. Using electrochemical impedance spectroscopy, we measured ionic conductivities from 10–6 S cm–1 to 10–1 S cm–1 between 25 °C to 220 °C. Conductivity changes are gradual, consistent with the subtle structural variations among all polymorphs.