Establishment of an empirical force-field for crystalline and amorphous Li2S−SiS2 electrolytes

An empirical force-field of Buckingham type is derived for crystalline and glassy ${\mathrm{Li}}_{2}\mathrm{S}\text{\ensuremath{-}}{\mathrm{SiS}}_{2}$ binary alloys using fractional charges, short-range repulsion, and long-range attractive dispersion interactions. Parameters are fitted on low-temperature equilibrium polymorphs ${\mathrm{Li}}_{2}\mathrm{Si}{\text{S}}_{3}$, and permit to recover typical bond distances, cell lengths, and to predict elastic properties. We then use classical molecular dynamics to study the structural properties of corresponding glasses and a very good agreement is found with corresponding data obtained from neutron and x-ray scattering. The resulting glasses are composed of fourfold tetrahedral Si and twofold sulfur, whereas Li atoms experience a more complex environment, partially in defective octahedral or tetrahedral geometry with a coordination number close to three. Unlike the base ${\mathrm{SiS}}_{2}$, the resulting glass network does not contain edge-sharing tetrahedra and displays a rather broad distribution of tetrahedral species that are quantified using the usual ${Q}^{n}$ formalism.