Rotational disorder in lithium borohydride

LiBH₄ has been discussed as a promising hydrogen storage material and as a solid-state electrolyte in lithium-ion batteries. It contains 18.5 wt% hydrogen and undergoes a structural phase transition at 381 K which is associated with a large increase in rotational disorder of the [BH₄]^- anion and the increase of [Li]⁺ conductivity by 3 orders of magnitude. We investigated the [BH₄]^- anion dynamic in bulk LiBH₄, in LiBH₄-LiI solid solutions and in nano-confined LiBH₄ by quasielastic neutron scattering, complemented by DFT calculations. In all cases the H-dynamics is dominated by thermally activated rotational jumps of the [BH₄]^- anion in the tera hertz range. The addition of LiI as well as nano-confinement favours the disordered high temperature phase and lowers the phase transition below room temperatures. The results are discussed on the basis of first principles calculations and in relation to ionic conductivity of [Li]⁺. (authors)