To investigate the redox activity of Co incorporated LLZO, cyclic voltammetry was conducted between 1.0 to 4.5 V using a Biologic MPG-03. The scan rate was set to 25 μV s -1 . For the preparation of the cathode electrodes, Co incorporated LLZO crystals were pre-ground using a mortar and a pestle. Conductive carbon (Super C65) (4 wt.%) and binder (Kynar Flex 2801) (6 wt.%) were added to ground LLZO (90 wt.%). For a better homogenization during the following milling step, the mixture was dissolved in N-methyl-pyrrolidone (NMP). After milling at 400 rpm for a total amount of time of 40 min, the slurry was tape-casted onto a preetched Al foil with a thickness of 100 μm. Subsequently, the cast was pre-dried overnight at 60 °C in a heat drying oven. Afterwards, electrode discs with a diameter of 3 mm were punched out of the cast and dried at 60 °C in a vacuum oven (Büchi) overnight again. Resulting cathodes were transferred into an Ar-filled glovebox, in which the whole cell assembly was conducted.