Nanocrystalline LCO/LLZO Composite Cathode Films for Solid State Batteries

Scalable fabrication of high-performing composite cathodes remains a key hurdle for oxide-based all-solid-state batteries (ASSBs). Li-garnet (Li₇La₃Zr₂O₁₂, 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 LiCoO₂/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 LiCoO₂ 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.