To answer these questions, we chose two approaches to incorporate Co, (1) large LLZO single crystals has been embedded into LCO powders or (2) LCO has been deposited by RF magnetron sputtering followed by heat treatments. A continuum of characterization techniques has been applied to gain structural (i.e., powder (P) and single crystal (SC) X-ray diffraction (XRD), electron diffraction, UV VIS spectroscopy, Co Emission Mössbauer spectroscopy, X-ray photo emission spectroscopy (XPS)), chemical (e.g., inductive coupled plasma optical emission spectroscopy (ICP OES), time-of-flight secondary ion mass spectroscopy (TOF SIMS)), microstructural (e.g., scanning electron microscopy (SEM), transmission electron microscopy (TEM)), electrical (i.e., impedance spectroscopy, polarization experiments) and electrochemical (i.e., CV) insights supported by DFT and finite element analysis. Finally, we have been able to show that Co can incorporate into the garnet structure with significant impact on the structural, microstructural, and electrochemical properties. Moreover, the temperaturedependent Co diffusion coefficient in LLZO has been determined which allowed for the creation of a process diagram suggesting the optimal Al2O3 coating thickness for any conditions that keeps degradation during processing at a minimum to negate the impact on cell performance by introducing high impedances.