Knowing the temperature dependency of Co, as well as the approximate Co concentration that led to detrimental effects on performance we can simulate the impact of different processing parameters, such as temperature and time, on the diffusion profile, hence, the temperature that will lead to high interfacial impedances. In order to determine the diffusion coefficient of Co we analyzed the diffusion profiles of LCO-sputtered LLZO annealed in air at temperatures between 600 °C to 700 °C via STEM and TOF-SIMS and as shown in Figures S9, and The Diffusion coefficients for the individual temperatures are plotted in Figure comparing with values from literature. Significant variations can be observed, even for similar temperatures. For example, these variations can be associated with, e.g. microstructure (i.e., we used single crystals without grain boundaries) or the oxygen fugacity (e.g., annealed in air or in Ar; cf Figure ).