Stabilizing Interfaces of All‐Ceramic Composite Cathodes for Li‐Garnet Batteries

The development of all‐solid‐state lithium metal batteries employing oxide electrolytes is limited due to chemical incompatibilities and elevated interfacial resistance. In particular, high‐temperature co‐firing of composite cathodes leads to secondary phase formation at the interface between the active material and the electrolyte (catholyte), which in turn degrades electrochemical performance. Here, the cation interdiffusion mechanism is investigated during the co‐firing of Li‐garnet (Li7La3Zr2O12) and LiCoO2 under varying partial pressures of lithium and oxygen at elevated temperatures. At reduced partial pressures of lithium and oxygen, the formation of a minor secondary phase, LaCoO3 is observed; however, this phase is suppressed under higher partial pressures, resulting in a substantial increase in electrical conductivity by several orders of magnitude. Highly dense, secondary‐phase‐free composite cathodes are successfully fabricated that deliver the highest reported areal discharge capacity of 3.48 mAh cm⁻2 at room temperature under a current density of 0.25 mA cm⁻2, demonstrating the cathode's outstanding performance. These findings offer promising insights for the development of all‐oxide solid‐state battery prototypes incorporating thin oxide electrolytes and lithium metal anodes.