Halides have high Li-ion conductivity and possess good stability against high-voltage cathodes but are limited by their instability against Li metal anodes. Oxide-based Li-ion conductors, such as Li7La3Zr2O12 and variants (LLZO; see structural details in Figure ) belong to the most promising candidates, due to their unique feature of combining electrochemical stability with relatively high Li-ion conductivities (up to 1.3 mS/cm) with the potential to solid-state batteries with theoretical densities of up to >400 Wh/kg and >1000 Wh/L. Further advantages of LLZO are its processibility in air, and improved safety even at high operating temperatures compared to other solid electrolytes.