Neutron Scattering Studies of Destabilized Lithium Borohydride

One of the most promising materials for hydrogen storage is lithium borohydride, LiBH$_{\mathrm{4}}$, due to its high hydrogen mass fraction. However, applications require destabilization of the material in order to reduce the temperature and pressure required for hydrogen cycling. One possible avenue for destabilization has been via the use of mixed crystals, for example, LiI and LiBH$_{\mathrm{4}}$, in which the relatively large iodide anion expands the crystal lattice of bulk LiBH$_{\mathrm{4}}$. Here we present neutron scattering results comparing BH$_{\mathrm{4}}^{\mathrm{-}}$ anion reorientational dynamics for bulk LiBH$_{\mathrm{4}}$ and the destabilized LiI-LiBH$_{\mathrm{4}}$ system. Quasielastic neutron scattering spectroscopy shows that at temperatures below room temperature, the reorientational dynamics for hexagonal LiI-LiBH$_{\mathrm{4}}$ is very similar to that of the high-temperature (380 K and above) hexagonal phase of LiBH$_{\mathrm{4}}$ instead of its low-temperature orthorhombic phase, which exhibits different dynamics. This is consistent with the behavior found using NMR spectroscopy.