Firstly, 100 nm of LCO were magnetron sputtered onto one side of different LLZO single crystals acting as a thin film cathode. Each of these crystals were tempered at various temperatures between 600 °C -700 °C for 48 h under air atmosphere to simulate different annealing times resulting in yellow-colored crystals. In the second preparation method, the aim was to simulate an extreme case of Co cross diffusion. Therefore, transparent, single crystalline LLZO (A ~ 12 mm², h ~ 0.5-1.0 mm) and LCO powder were placed inside a Cu tube. After densifying the powder to guarantee a good LCO|LLZO contact, the Cu tube was sealed on both sides and tempered at 500 °C and 900 °C for 5 days under Ar flow in a ceramic tube oven. After heating, the remaining LCO powder was removed and the LCO incorporated LLZO crystals were polished with a sand-paper with 4000 grit. This extreme case will further be referred to as route 2. If not stated elsewhere, both route 1 and route 2 were used for all sample preparations.