First-Principles Investigation of Mechanical Properties and Anisotropy of Argyrodite Li₆PS₅Cl Crystal Electrolytes

Argyrodite Li6PS5Cl crystals are expected to serve as solid electrolytes for all-solid-state batteries owing to their high ionic conductivity. In this study, its detailed mechanical properties were evaluated using first-principles calculations. This approach offers a new pathway for evaluating the intrinsic mechanical properties of solid electrolytes, and, when combined with macroscopic assessments, enables more precise characterization of all-solid-state battery materials. Spacious visualization of the elastic properties revealed that the argyrodite Li6PS5Cl crystal structure has isotropic elasticity. The stress–strain curves obtained by first-principles calculations showed that it has wide elastic regions, indicating high durability under uniaxial tensile strains. Anisotropy in strength and durability was also observed at uniaxial strains exceeding 20%. Investigation of its strained structures suggested that the observed anisotropy was attributable to the ease of chemical bond dissociation. The oxidation states of its individual atoms varied under uniaxial strains, with P showing the largest charge fluctuation. In contrast to uniaxial strain, its shear deformation was weakly resistant, and the crystal structure collapsed under a low strain of 0.7%. The primary cause of its ease of rupture under shear deformation was found to be the movement of Cl atoms toward Li atoms, leading to the formation of Li4Cl units and changes in the atomic charges. This brittleness against shear deformation is considered to contribute to the brittleness observed in actual powder compacts.