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

Abstract

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 SnS₄^4– 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.