Composite Cathodes for Oxide-Based Solid-State Batteries: Processing, Interfacial Mechanisms, and Performance Engineering

Composite cathodes are a key component in oxide-based solid-state batteries (SSBs), enabling electrochemical activity through the integration of cathode active materials, solid electrolytes, and conductive additives within a single composite architecture. However, their performance is strongly influenced by complex interfacial phenomena and chemo-mechanical effects arising from heterogeneous solid-solid contacts, which often result in high interfacial resistance, limited percolation of ionic and electronic pathways, and progressive degradation during processing and cycling. This review aims to clarify how composite cathode designs govern the performance of oxide-based SSBs, focusing on the interplay between material selection, fabrication routes (wet and dry processes), interfacial structure evolution, and transport efficiency. Emphasis is placed on critical challenges such as co-sintering-induced interfacial degradation, space-charge layer formation, poor contact, and an imbalance between ionic and electronic transport, as well as on strategies developed to address these issues, including buffer layers, co-sintering additives, and microstructural engineering. Advanced characterization techniques are also discussed for probing buried interfaces and correlating structure with performance. Finally, we identify critical gaps in current understanding and outline prospective research pathways aimed at formulating robust design guidelines for oxide-based composite cathodes with enhanced mechanical integrity and electrochemical stability, ultimately advancing high-performance oxide-based SSBs.