Symmetry Breaking in Argyrodite-Type Li6PS5X Solid Electrolytes and Its Implications for Thermodynamic Modeling

Argyrodite Li 6 PS 5 X (LPSX; X = Cl, Br, and I) solid electrolytes are highly promising for all-solid-state lithium batteries. While defect engineering in these materials is commonly investigated via computational methods, ideal high-symmetry crystal structures are typically selected as representative models without critically assessing their suitability as thermodynamic references. Here, density functional theory calculations combined with machine-learning-accelerated phonon analyses are employed to examine structural symmetry breaking in the LPSX series and its consequences for thermodynamic modeling. Structural relaxation lowers the energy of all three argyrodites, yielding a maximum stabilization of 0.86 eV f.u. -1 for LPSC. Phonon calculations confirm that this distorted LPSX (d-LPSX) configuration represents a dynamically stable local minimum, with the distortion-induced stabilization originating from the formation of additional Li–X interactions. Importantly, replacing the ideal high-symmetry LPSC reference with the true relaxed d-LPSC qualitatively alters the predicted energetics in two representative cases: In/O co-doping and S/O compositional mixing. Furthermore, utilizing d-LPSX as the energetic reference significantly improves the accuracy of the constructed quaternary phase diagrams. These results demonstrate that selecting the correct structural reference is a critical prerequisite for the reliable first-principles prediction of defect, compositional, and phase stability in argyrodite solid electrolytes.