The Influence of Reorientational and Vibrational Dynamics on the Mg²⁺ Conductivity in Mg(BH₄)₂·CH₃NH₂

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(BH₄)₂·CH₃NH₂) 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 BH₄ ^- anion compared to Mg(BH₄)₂. This is likely part of the explanation for the high Mg-ion transport observed for Mg(BH₄)₂·CH₃NH₂. Furthermore, while the dynamics of both the BH₄ ^- anion and the CH₃ group of the methylamine ligand is rapid, the NH₂ 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 Ų for NH₂, 1.503 Ų for CH₃, and 1.856 Ų for BH₄ ^- 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.