Effect of Preparation Methods on the Interface of LiBH₄/SiO₂ Nanocomposite Solid Electrolytes

Nanocomposites of complex metal hydrides and oxides are promising solid state electrolytes. The interaction of the metal hydride with the oxide results in a highly conducting interface layer. Up until now it has been assumed that the interface chemistry is independent of the nanoconfinement method. Using ²⁹Si solid state NMR and LiBH₄/SiO₂ as a model system, we show that the silica surface chemistry differs for nanocomposites prepared via melt infiltration or ball milling. After melt infiltration, a Si···H···BH₃ complex is present on the interface, together with silanol and siloxane groups. However, after ball milling, the silica surface consists of Si- H sites, and silanol and siloxane groups. We propose that this change is related to a redistribution of silanol groups on the silica surface during ball milling, where free silanol groups are converted to mutually hydrogen-bonded silanol groups. The results presented here help to explain the difference in ionic conductivity between nanocomposites prepared via ball milling and melt infiltration.