The Nature of Interface Interactions Leading to High Ionic Conductivity in LiBH4/SiO2 Nanocomposites

Complex metal hydride/oxide nanocomposites are a promising dass of solid-state electrolytes. They exhibit high ionic conductivities due to an interaction of the metal hydride with the surface of the oxide. The exact nature of this interaction and composition of the hydride/oxide interface is not yet known. Using H-1, Li-7, B-11, and Si-29 NMR spectroscopy and lithium borohydride confined in nanoporous silica as a model system, we now elucidate the chemistry and dynamics occurring at the oxide scaffold has a significant effect on the ionic conductivity. A previously unknown silicon site was observed in the nanocomposites and correlated to the LiBH4 at the interface with silica. We provide a model for the origin of this silicon site which reveals that siloxane bonds are broken and highly dynamic silicon-hydride-borohydride and silicon-oxide-lithium bonds are formed at the interface between LiBH4 and silica. Additionally, we discovered a strong correlation between the thickness of the silica pore walls and the fraction of the LiBH4 that displays fast dynamics. Our findings provide insights on the role of the local scaffold structure and the chemistry of the interaction at the interface between complex metal hydrides and oxide hosts. These findings are relevant for other complex hydride/metal oxide systems where interface effects leads to a high ionic conductivity.

Automated summary

Complex metal hydride/oxide nanocomposites are potential solid-state electrolytes with high ionic conductivities. This study uses NMR spectroscopy to investigate the chemistry and dynamics of lithium borohydride confined in nanoporous silica, revealing the presence of a previously unknown silicon site in the nanocomposites. The thickness of the silica pore walls was also found to correlate with the fraction of lithium borohydride exhibiting fast dynamics. These findings provide insights into the interaction between complex metal hydrides and oxide hosts, which is crucial for understanding high ionic conductivity in these systems.