At lower temperatures the spinal phase is formed leading to lower Co diffusion coefficient and vice-versa. The Co diffusion coefficient for both Al2O3 and CoAl2O4 phases is rather similar, and the Co will first be used to form the spinel phase when a certain Co concentration is reached, which will at least slow down the build-up of a critical Co concentration. In respect of the desired performance of the solid-state battery the resistive passivation layer must be kept at a minimum to minimize any contribution to interfacial resistance. In our model we introduced an Al2O3 layer with the boundary condition that when a certain Co concentration is reached (i.e., spinel phase is formed) it stays constant. In Fig. diffusion profiles for a fixed Al2O3 thickness of 10 nm is shown. It can be seen that for a given temperature up to 1000 °C hold for 3h no cross-diffusion takes place indicating that Al2O3 can protect LLZO from degradation.