Effects of Oxide Morphology on Lithium-Ion Conductivity of LiBH₄–Al₂O₃ Composites

The ionic conductivity of LiBH4–Al2O3 composites was investigated, focusing on the effects of oxide morphology and surface functional groups. Composites were prepared by mixing mesoporous Al2O3 (wormhole), gamma Al2O3, and nanofiber Al2O3. All composites showed an improvement in ionic conductivity, with the 55 wt % mesoporous composites showing the highest ionic conductivity (4.6 × 10–5 S cm–1 at 30 °C) and lowest activation energy for ionic conduction (0.53 eV for 30–100 °C). Notably, 7Li static nuclear magnetic resonance revealed that only the mesoporous composites exhibited a motionally narrowed line, indicative of highly mobile lithium ions. The proportion of highly mobile lithium ions in all composites was found to correlate with the specific surface area and specific pore volume of each Al2O3 type. Multiple structural analyses indicated the existence of fast lithium-ion conduction layers at the interface between LiBH4 and Al2O3. Additionally, the influence of mesoporous Al2O3 surface functional groups on the ionic conductivity was explored. This study highlights the potential of using different oxide morphologies to better understand and visualize ionic conductivity enhancement interfaces.