It has been known that LiBH₄ has a hexagonal crystal structure with a high ionic conductivity (~ 10^-3 S/cm) above ~ 110 ℃, although it undergoes a reversible phase transformation to an orthorhombic crystal structure with a low ionic conductivity (~ 10^-8 S/cm) below the phase transition temperature. So, many researchers attempted to extend the stability region of the high temperature phase up to room temperature by making a solid solution of LiBH₄ and lithium halides. Another route to increase the ionic conductivity of LiBH₄ is nanoconfinement. The confinement of LiBH₄ in the mesoporous scaffolds amorphizes the structure raising the ionic conductivity as high as ~ 10^-4 S/cm even at room temperature. However, it is still unclear which mechanism, i.e. nanoconfinement or generation of interface between LiBH₄ and SiO₂, governs the ionic conductivity enhancement of infiltrated LiBH₄.

We therefore systematically investigate the ionic conductivity of LiBH₄ and a mesoporous silica (here MCM-41) composite system. In order to separate the effect coming from nanoconfinement and interface, as-purchased MCM-41 was hand-mixed with LiBH₄ in one case while in the other case mesopores of MCM-41 was intentionally destroyed by high-energy ball-milling. Interestingly, ionic conductivity of ball milled LiBH₄ – MCM-41 mixture turned out to be ~ 10^-5 S/cm, which is smaller than that of LiBH₄ infiltrated into MCM-41 but it is 1000 times higher than that of pure LiBH₄. The result highlights the importance of interface effect and indicates that significant enhancement in ionic conductivity can be achieved through interface engineering. To further exploit the interface effect, volume fraction of LiBH₄ was systematically varied, and other kinds of compounds were tested. We believe that such approach can be useful for designing a fast ionic conductor for lithium ion battery.