We find, depending on the oxygen fugacity, Co Thereafter, we measured the Co diffusion coefficient for a wide temperature range to determine the corresponding time for a given temperature until a critical Co concentration is reached. This temperature-depended diffusion coefficient is determined and shows that crossdiffusion into LLZO and the associated degradation of LLZO is unavoidable for relevant processing conditions. To mitigate cross-diffusion the introduction of a protecting layer Al2O3 has been studied. Based on FEA studies we propose a process diagram that allows to select processing conditions, i.e., coating thickness, temperatures, time, to enable the efficient interface engineered cell design without degradation and a minimum compromise in electrochemical performance, which is key to enable high energy dense solid-state batteries.