To understand the Co-incorporation mediated degradation processes taking place at the LLZO|LCO interface, cross-sectional imaging of annealed LCO-sputtered LLZO was performed using Scanning Transmission Electron Microscopy (STEM) high-angle annular dark-field (HAADF) imaging (see Figure ). The LCO layer could clearly be discerned showing a thickness of approximately 150 nm and exhibiting a nano crystalline morphology with lateral crystallite sizes in the range of 30-100 nm (see Figure ). Measurements revealed a high degree of distortion of the LLZO crystal close to the LLZO|LCO interface within a depth of approximately 1.3 µm from the surface, with polycrystalline whiskers reaching even deeper depths. One of these whiskers is highlighted in Figure with corresponding elemental EDX analysis shown in Figure revealing an increased Zr and La content and a decreased O content within the whisker. Most likely these phases can be assigned to LaLi0.95Co0.26O2.0 and La-deficient tetragonal LLZO as revealed by SCXRD. In general, this region is governed by a rather polycrystalline, yet porous structure with several extended voids (see Figure ). These pores seem to be filled with a carbon containing species as revealed by Electron Energy Loss Spectrometry (EELS) analysis (cf Figure ) which could be assigned to LixC. It is interesting that the amorphization of LLZO enhances towards the bulk before approaching the single crystalline region (see Figure ). We propose that small amounts of Co (<0.1 Co pfu) can already build up residual stresses high enough to cause the fracture in LLZO. The fracture in LLZO leads to an increased surface area promoting Co cross diffusion, hence, fostering the degradation of LLZO (formation of extra phases). The phases formed may crystallize and result in grain growth at given temperatures (large single crystals of extra phases have been found herein supporting this hypothesis, see section 2.3), which could explain the more crystalline nature of these regions (i.e., sharper Bragg peaks). This could also explain whisker-like structures considering that crack expansion could promote locally extended degradation processes.