High temperatures, however, result in mutual diffusion and interfacial reactions between the solid electrolytes and the electrode leading to the formation of an insulating degradation layer, which is detrimental for the performance of a SSB. For example, the state-of-the-art cell composed of composite cathode based on LLZO, LCO and a sinter additive has been shown to suffer from very high interfacial resistance (270 Ω cm 2 at 100 °C), which is predominantly attributed to the high processing temperature of 700 °C. Such values are too high to achieve a target cycling at 1C with more than 90 % efficiency; values as low as 40 Ω cm 2 are required. The underlying processes taking place at the interface during cell fabrication were intensively studied previously. For example, Kim, et al. deposited 100 nm thick LCO films on LLZO by pulse laser deposition at 650 °C for 1 h and observed La/Co cross diffusion along the interface between LLZO and LCO accompanied by the formation of La2CoO4 phase. Vadar, et al. additionally observed the formation of other poor conducting phases, such as Li2CO3, La2Zr2O7, and LaCoO3. Park, et al. mixed LLZO and LCO powders to study the phase stability as a function of temperature via XRD observing a structural degradation by the formation of the tetragonal LLZO phase. Since the tetragonal phase has a lower Li-ion conductivity a detrimental effect on the interfacial transport properties can be expected. The authors relate the lowering in symmetry to a loss of Al during the heat treatment (Note: supervalent cations are needed to stable the high contacting cubic phase of LLZO at RT, hence, the loss of Al will consequently lead to the tetragonal phase). The formation of the tetragonal phase can also be associated with the loss of Li in LLZO at elevated temperatures leading to the formation of non-conductive phases, i.e., La2Zr2O7 and La2O3. These decomposition products have also been computationally predicted to form when LLZO is in contact with delithiated LCO. Recently, Ren et al. highlighted that the disagreement on the reaction products and their onset formation temperature could originate from different specific processing conditions and thus cannot be accurately predicted solely by thermodynamic considerations. Authors suggest that kinetics must be considered to understand the reactions occurring at the interfaces. This requires detailed knowledge on cross diffusion of ions and formation of the passivation layer between cathode and LLZO at a given temperature. Such knowledge is, however, not available so far.