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

Abstract

A new solid-state Li ion conductor composed of LiBH 4 and Al 2 O 3 was synthesized by a simple ball-milling process. The element distribution map obtained by transmission electron microscopy demonstrates that the LiBH 4 and Al 2 O 3 are well mixed and form a large interface after ball-milling. The ionic conductivity of the mixture reaches as high as 2 × 10 –4 S cm –1 at room temperature when the volume fraction of Al 2 O 3 is approximately 44%. The ionic conductivity of the interface between LiBH 4 and Al 2 O 3 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 LiBH 4 . 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.