Enhanced Ionic Conductivity and Electrochemical Properties of Li$_2$B$_{12}$H$_{12}$/ZrO$_2$ Nanocomposites for All-Solid-State Lithium Metal Batteries

Solid-state electrolytes play a key role in the development of safe and high-capacity all-solid-state batteries. Complex hydrides such as Li$_2$B$_{12}$H$_{12}$ are attractive as solid electrolytes due to their low weight and good electrochemical stability, but suffer from low conductivities at room temperature. Herein, we report a three-order-magnitude increase in the ionic conductivity of Li$_2$B$_{12}$H$_{12}$ upon nanocomposite formation with ZrO$_2$ via mechanochemical treatment, reaching 2.9 × 10$^{–4}$ S cm$^{–1}$ at 30 °C. Results from infrared spectroscopy, X-ray Raman scattering and electron microscopy coupled with electron energy loss spectroscopy suggest that the increased ionic conductivity is due to strong interfacial interaction/reaction between Li$_2$B$_{12}$H$_{12}$ and ZrO$_2$. This leads to a highly defective interphase region where the Li, B, Zr, and O chemical environments are distinctively different from the bulk Li$_2$B$_{12}$H$_{12}$ and ZrO$_2$. The improved ionic conductivity of the nanocomposite compared to the pristine material enabled the realization of all-solid-state batteries with a Li metal anode and both TiS$_2$ and LiFePO$_4$ cathodes. We demonstrate the suitability of the nanocomposite at various charging rates up to C/2 (0.34 mA cm$^{–2}$) for over 170 cycles at 40–60 °C (Li|Li$_2$B$_{12}$H$_{12}$/ZrO2|TiS$_2$).