First-principles simulations to understand the structural and electrolyte properties of idealized Li7.5B10S18X1.5 (X=Cl, Br, I)

Recently, researchers at the University of Waterloo (Canada) and Oak Ridge National Laboratory (Oak Ridge, TN) reported a new family of lithium thioborate halide electrolytes with the composition ${\mathrm{Li}}_{7.5}{\mathrm{B}}_{10}{\mathrm{S}}_{18}{X}_{1.5}$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ having very impressive ionic room-temperature conductivity of magnitude $\ensuremath{\sigma}=1 \mathrm{mS}/\mathrm{cm}$. The researchers characterized the structures of the three materials in terms of a well-defined thioborate framework with a void structure containing fractionally occupied Li and X sites. The space group of the materials was identified to be monoclinic ($C2/c$, No. 15). We report the results of first-principles simulations of these materials focusing on understanding the idealized ground-state structures, the mechanisms of Li ion migration, and the overall stability of the materials. A systematic search of many possible stoichiometric crystalline configurations found ordered ground-state realizations of the materials for each of the three halides, $X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I}$. Molecular dynamics simulations based on the initially ordered structures at various temperatures show significant Li ion hopping within the void channels of the structures at temperatures as low as T = 400 K. Simulations of possible decomposition products suggest that these electrolytes are also chemically stable. Overall, the simulations are consistent with the experimentally reported findings indicating that these materials are very promising solid electrolytes for possible use in solid-state Li ion batteries.