Enhanced Li Ion Conductivity in LiBH4-Al2O3 Mixture via Interface Engineering

A new solid-state Li ion conductor composed of LiBH4 and Al2O3 was synthesized by a simple ball-milling process. The element distribution map obtained by transmission electron microscopy demonstrates that the LiBH4 and Al2O3 are well mixed and form a large interface after ball-milling. The ionic conductivity of the mixture reaches as high as 2 X 10(-4) S cm(-1) at room temperature when the volume fraction of Al2O3 is approximately 44%. The ionic conductivity of the interface between LiBH4 and Al2O3 was extracted by using a continuum percolation model, which turns out to be about 10(-3) S cm(-1) at room temperature, being 10(5) times higher than, that of pure LiBH4. This remarkable rise in conductivity is accompanied by the lowered activation energy for the Li ion conduction in the mixture, indicating that the interface layer facilitates Li ion conduction. Near-edge X-ray absorption fine structure analysis reveals the presence of B-O bondings in the mixture, which was not detected by X-ray diffraction. This disruption of the chemical bondings at the interface may allow an increase in carrier concentration and/or mobility thereby resulting, in the pronounced enhancement in conductivity. This result provides a guideline for designing fast Li ion conductor through interface engineering.