Influence of Polymer Interfacial Protective Layer Thickness on the Stability of Lithium‐Metal Batteries

Abstract Protective layers formed of polymers with conjugated structures have been demonstrated to improve the cycling stability of lithium (Li)‐metal batteries by reducing Li dendrite growth. However, the influence of important film parameters, such as thickness and ionic conductivity, remains largely unexplored. The present work addresses this issue by investigating the influence of film thickness for electrochemically active polymethyl methacrylate (PMMA) protective layers on the growth of Li dendrites and the long‐term cycling stability of Li‐metal batteries. The results demonstrate that the cycling stability of cells employing PMMA/Cu electrodes attains a maximum value at an optimal polymer film thickness of 93 nm. The results of detailed analysis indicate that the optimal PMMA layer thickness improves the cycling stability of Li‐metal batteries by eliminating the uneven electric fields on the surface of the Cu electrode leading to Li dendrite growth, and by reducing the interfacial impedance due to the formation of intramolecular cyclopentanedione during the discharge/charge cycling. Accordingly, the results provide effective guidance in the future design of polymer protective layers by demonstrating that film thickness is a key factor influencing the cycling stability of Li‐metal batteries.