The Influence of Reorientational and Vibrational Dynamics on the Mg Conductivity in Mg(BH)·CHNH.

Reorientational dynamics in solid electrolytes can significantly enhance the ionic conductivity, and understanding these dynamics can facilitate the rational design of improved solid electrolytes. Additionally, recent investigations on metal hydridoborate-based solid electrolytes have shown that the addition of a neutral ligand can also have a positive effect on the ionic conductivity. In this study, we investigate the dynamics in monomethylamine magnesium borohydride (Mg(BH)·CHNH) with quasielastic and inelastic neutron scattering, density functional theory calculations, and molecular dynamics simulations. The results suggest that the addition of methylamine significantly speeds up the reorientational frequency of the BH anion compared to Mg(BH). This is likely part of the explanation for the high Mg-ion transport observed for Mg(BH)·CHNH. Furthermore, while the dynamics of both the BH anion and the CH group of the methylamine ligand is rapid, the NH group of the methylamine ligand exhibits much slower reorientations, as confirmed by both experimental and computational investigations. Notably, molecular dynamics calculations reveal mean square displacements of 0.387 Å for NH, 1.503 Å for CH, and 1.856 Å for BH using a trajectory of 10 ps. This study confirms the simultaneous presence of fast dynamics and high ionic conductivity in a metal borohydride-based system and can function as an experimental foundation for future studies on dynamics in hydrogen-rich solid electrolytes.