Highly Stable Composite Solid Electrolytes Based on Caffeic Acid Molecular Bridges for Long Lifespan Lithium Metal Batteries

Solid‐state lithium metal batteries are promising next generation energy storage devices and composite solid electrolytes (CSEs) with high ionic conductivity and mechanical stability have attracted widespread attention. However, the slow ion transport kinetics at the heterogeneous interface between ceramic fillers and polymer matrices severely limits the performance of CSEs. This work reports a facile surface modification using biogenic caffeic acid (CA) to functionalize LLZTO. CA monomers anchor on LLZTO via Lewis acid‐base interactions and react with surface lithium carbonate, during which in‐situ polymerization evolves to form a uniform 5 nm poly(caffeic acid) (PCA) coating. After incorporating PCA@LLZTO into the PEO matrix, hydrogen bonding between PCA polar groups and PEO chains promotes filler dispersion and reduces interfacial resistance. The PCA@LLZTO/PEO CSE exhibits significantly enhanced ionic conductivity of 2.19 × 10 −4 S cm −1 , a reliable Li + transference number of 0.64, excellent mechanical robustness and superior Li metal compatibility, along with a stable electrochemical window of 5.1 V and long‐term stable cycling over 3200 h in Li||Li symmetric cells. This superior performance stems from the sequential reactions of PCA that homogenize Li + distribution, suppress Li dendrites, and stabilize the electrolyte/electrode interface, presenting a green strategy for practical solid‐state lithium metal batteries.