Mechanical and Electronic Properties of Bulk and Surface Li6PS5Cl Argyrodite: First-Principles Insights on Li-Filament Resistance

Abstract

Different Li-filament growth patterns have been experimentally observed in numerous solid electrolytes (SEs) with high ionic conductivity such as garnet Li₇La₃Zr₂O₁₂ (LLZO) and argyrodite Li₆PS₅Cl (LPSC). Herein, we probed the mechanical and electronic properties of LPSC, using density functional theory calculations, and compared with other SEs to determine the relevant descriptors for predicting Li-filament resistance. LPSC has a complicated structure that can incorporate S^2–/Cl^– inversion and has Li⁺ distributed among two Wyckoff sites (24g and 48h). A representative bulk structure that incorporates both phenomena was determined via systematic structure sampling. The lowest energy bulk structures had a majority of Li⁺ in 48h sites after relaxation, agreeing with experimental studies. The Young’s modulus and shear modulus of bulk LPSC are low, ∼10–30 GPa, and the fracture energy of cleaving along the (100)-Li₂S-deficient surface is also low, 0.20 J/m², suggesting poor mechanical resistance to filament growth. The crack surfaces and pore surfaces in LPSC have a similar bandgap and excess electron distribution compared to bulk LPSC, suggesting that these internal defects will not trap electrons to reduce Li⁺ to Li-metal. Thus, LPSC is likely to experience “dry” cracks, with a mechanical crack opening up first, followed by a Li-filament filling the crack. This is opposite to LLZO, which has a high fracture energy and experiences electron localization at internal defects (e.g., crack surfaces, pore surfaces, and grain boundaries). LLZO has been experimentally observed to suffer “wet” cracks.