Reorientational motion and Li+ -ion transport in Li2B12H12 system: Molecular dynamics study

Abstract

${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ and its derivatives are promising solid electrolytes for solid-state batteries. In this work, a potential model is proposed, and an extensive classical molecular dynamics study is performed to understand the origin of the fast ion conduction in ${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$. The proposed potential model reveals structural and dynamical properties of ${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ that are consistent with first-principles molecular dynamics simulation and experimental results. The mechanism of ${\mathrm{Li}}^{+}$-ion transport is studied systematically. The low-temperature $\ensuremath{\alpha}$ phase exhibits negligible diffusivity within a timescale of a few nanoseconds, whereas the high-temperature $\ensuremath{\beta}$ phase with a similar crystal structure and larger lattice parameter exhibits entropy-driven high ${\mathrm{Li}}^{+}$-ion diffusion assisted by anionic reorientation. We explicitly demonstrate the role of closo-borane anionic reorientational motion in the ${\mathrm{Li}}^{+}$-ion diffusion and explain how the cell parameter facilitates the anionic reorientational motion. In addition, enhancing the degree of H freedom (by changing the B-B-H angular force parameters) results in significantly high cationic diffusivity at low temperature. Further insight into ${\mathrm{Li}}^{+}$-ion transport is obtained by constructing a three-dimensional density map and determining the free-energy barrier, and the factors affecting cationic diffusion are thoroughly investigated with high precision using long simulations (5 ns).