The effect of nanoscaffold porosity and surface chemistry on the Li-ion conductivity of LiBH₄-LiNH₂/metal oxide nanocomposites

Solid-state electrolytes are crucial for the realization of safer batteries with improved capacity. Lithium-based complex hydrides, for instance LiBH4, display promising characteristics as solid-state electrolytes. However, increasing their low room temperature conductivity (10-8 S cm-1 for LiBH4) is a prerequisite for application. Partial ionic substitution of BH4- with NH2- followed by nanoconfinement in mesoporous oxide scaffolds increases the conductivity to 5 × 10-4 S cm-1. Here, we show that the conductivity of LiBH4-LiNH2/metal oxide nanocomposites is strongly influenced by the chemical and physical nature of the scaffold material. By tuning both the surface chemistry and the pore structure, the conductivity can be varied by three orders of magnitude at room temperature. Unexpectedly, even though a significant influence of the scaffold surface chemistry is observed, the nanocomposite conductivity is largely dictated by the scaffold pore volume. This is in contrast to nanoconfined pure LiBH4, where the conductivity is governed by the chemical nature of the mesoporous scaffold. For nanoconfined LiBH4-LiNH2, the conductivity improvement is attributed to stabilization of a highly conductive phase inside the scaffold pores, rather than the formation of a conductive interfacial layer at the oxide/hydride interface as observed for nanoconfined LiBH4. These findings could be applicable to other cation- and anion-substituted nanocomposites and provide a useful tool to develop novel solid-state electrolytes with excellent ionic conductivities.