Molecular Weight Blending of Poly(ethylene succinate) for Enhanced Performance of Solid Polymer Electrolytes

Solid polymer electrolytes (SPEs) based on biodegradable polyesters present a promising alternative to conventional polymers, yet their ionic conductivity remains a challenge. Herein, we propose a molecular weight blending strategy to fabricate high-performance SPEs using poly(ethylene succinate) (PES). By incorporating low-molecular-weight PES10k (10 kg·mol–1) into a high-molecular-weight PES104k (104 kg·mol–1), we synergistically enhance segmental mobility for ion transport while maintaining mechanical integrity for freestanding membranes. The optimized blend (PES60@SPE, 60 wt % PES104k) exhibits a significantly improved ionic conductivity of 1.62 × 10–5 S·cm–1 at 70 °C, a high lithium ion (Li+) transference number of 0.91, and an extended electrochemical stability window up to 5.3 V vs Li+/Li. These enhancements are attributed to the increased amorphous content and reduced crystallinity confirmed by WAXD and DSC. When assembled into Li//LiFePO4 full cells, the PES60@SPE enables superior rate capability and exceptional cycling stability with 99.0% capacity retention after 100 cycles at 0.1 and 70 °C. Furthermore, molecular weight information analysis reveals that the in situ degradation of PES during cycling, particularly pronounced in the blended system, contributes to long-term performance by dynamically refreshing the electrolyte structure. This work provides a facile and effective molecular weight engineering approach for developing sustainable, high-performance SPEs for solid-state lithium–metal batteries.