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 Li 7 La 3 Zr 2 O 12 (LLZO) and argyrodite Li 6 PS 5 Cl (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)-Li 2 S-deficient surface is also low, 0.20 J/m 2, 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.