Diffusion of transition metals across the cathode–electrolyte interface is identified as a key challenge for the practical realization of solid-state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials. Herein, the hypothesis that formation of interphases is associated with the incorporation of Co into the Li₇La₃Zr₂O₁₂ lattice representing the starting point of a cascade of degradation processes is investigated. It is shown that Co incorporates into the garnet structure preferably four-fold coordinated as Co²⁺ or Co³⁺ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 per formula unit, whereby concentrations beyond this limit causes a cubic-to-tetragonal phase transition. Moreover, the temperature-dependent Co diffusion coefficient is determined, for example, D_{700 °C} = 9.46 × 10⁻¹⁴ cm² s⁻¹ and an activation energy E_a = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al₂O₃ coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid-state batteries.