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

Li2B12H12 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 Li2B12H12. The proposed potential model reveals structural and dynamical properties of Li2B12H12 that are consistent with first-principles molecular dynamics simulation and experimental results. The mechanism of Li+-ion transport is studied systematically. The low-temperature a phase exhibits negligible diffusivity within a timescale of a few nanoseconds, whereas the high-temperature beta phase with a similar crystal structure and larger lattice parameter exhibits entropy-driven high Li+-ion diffusion assisted by anionic reorientation. We explicitly demonstrate the role of closo-borane anionic reorientational motion in the 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 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).