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

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 S2–/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.