Nanocrystalline LCO/LLZO Composite Cathode Films for Solid State Batteries

ABSTRACT Scalable fabrication of high‐performing composite cathodes remains a key hurdle for oxide‐based all‐solid‐state batteries (ASSBs). Li‐garnet (Li 7 La 3 Zr 2 O 12 , LLZO) electrolytes provide high ionic conductivity and compatibility with lithium metal, yet, combining them with high‐voltage layered oxides is often limited by high‐temperature ceramic processing that complicates manufacturing and promotes interfacial side reactions. Here we report a LiCoO 2 /LLZO composite cathode prepared by single‐solution spray deposition that co‐deposits all constituents at a maximum processing temperature of 750°C. The process produces dense, crack‐free, micrometer‐thick layers with nanograined, phase‐separated microstructures. The two phases of cubic LLZO and layered LiCoO 2 crystallize independently, only minor secondary phases form, and no cobalt migration into the LLZO substrate is detected. Consistent with a low cathode‐electrolyte interfacial resistance observed after the first charge, full cells with lithium metal exhibit 111.7 mAh g −1 CAM initial discharge capacity and retain 93% of the capacity over 100 cycles. Capacity fade is dominated by progressive growth of cathode‐electrolyte interfacial impedance, whereas post‐mortem microscopy reveals preserved adhesion and morphology, suggesting the nanograined architecture accommodates cathode volume changes without severe cracking or delamination. Overall, single‐solution co‐deposition offers a practical, low‐thermal‐budget route to oxide composite cathodes and a scalable platform for extending LLZO‐based ASSBs to broader cathode chemistries.