Integrating Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolytes in combination with a high-energy cathode remains a major challenge in developing all-solid-state batteries. In particular, diffusion processes and solid-state reactions at the cathode-electrolyte interface during the co-sintering of the oxide materials at elevated temperatures result in high interfacial impedances. In this work, we study the performance of lithiated Nb, Al, and Ti metal oxide interlayers as diffusion barriers to prevent the formation of deleterious interphases at the cathode-electrolyte interface during fabrication, thus enabling easy Li-ion transfer between LiCoO₂ and LLZO. Specifically, we characterize the impact of the different interlayers on the morphology and elemental distribution at the interface and evaluate their influence on the electrochemical behavior of the battery stacks after the high-temperature process. We find that the mixing of Co/La cations at the interface is reduced by using the metal oxide diffusion barriers. It is shown that the interfacial impedance can be reduced from 8 kΩ cm² to 1 kΩ cm² and that the electrochemical performance of all cells with interlayers exceeds that of the battery without interlayer. In particular, the Li-Nb-O modification outperforms the other metal oxide interlayers in terms of the discharge capacities achieved.