Crafting Uniform Cross-Linking Structures via RAFT Polymerization for Enhanced Solid Polymer Electrolyte Performance

Solid-state lithium batteries require solid-state electrolytes that are both ionically conductive and mechanically robust. Solid polymer electrolytes (SPEs) based on poly(ethylene oxide) are promising owing to their excellent lithium salt compatibility and processability. However, their crystallinity severely impedes ion transport, and achieving optimal ionic conductivity and mechanical strength remains challenging. This work used monomers bearing pendant polyethylene glycol to inhibit crystallization, and chemical cross-linking structures were employed to enhance mechanical robustness. Compared to conventional free radical polymerization (FRP), SPE membranes synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization exhibited a more homogeneous cross-linking network, as evidenced by solid-state NMR. Compared to SPE-FRP, the uniform chemically cross-linking network constructed by SPE-RAFT not only enhanced tensile strength and elongation at break by more than 3-fold and 9-fold, respectively, but also facilitated lithium-ion transport, leading to a 45% increase in ionic conductivity and 84% improvement in the cycling stability of lithium symmetric cells. Consequently, the all-solid-state LiFePO4//SPE-RAFT//Li battery delivered a high discharge capacity of 150.0 mAh g–1 after 180 cycles at 0.5 C, while the FRP counterpart exhibited a lower discharge capacity of 97.3 mAh g–1 after 200 cycles at 0.1 C. These findings underscore significant advantages of a uniform cross-linking network achieved by RAFT cross-linking polymerization.