Balancing Fast Li-Ion Transport Kinetics and Mechanical Strength in PEO-Based Composite Solid Electrolytes through Minimal Dinitrile Incorporation

Poly(ethylene oxide) (PEO)-based composite solid electrolytes (CSEs) suffer from poor ionic conductivity because strong Li+–ether oxygen coordination restricts Li+ transport. Herein, we develop a competitive coordination strategy by introducing a minimal amount of hexanedinitrile (HDN; 2.30 wt %) into PEO CSEs. Its flexible tetramethylene spacer allows nitrile groups to compete with PEO for Li+, forming mobile Li(HDN)x+ species while largely preserving the polymer backbone. This design accelerates local Li+ migration and promotes interfacial Li+ exchange, mitigating the trade-off between ionic conductivity and mechanical strength. The optimized CSE-HDN achieves an ionic conductivity of 4.87 × 10–4 S cm–1 (60 °C) and a tensile strength of 1.08 MPa. Li||Li symmetric cells cycle stably for 1000 h at 0.2 mA cm–2, while Li||LiFePO4 cells retain 94.8% capacity after 250 cycles at 3C. These results identify competitive Li+–nitrile coordination as a strategy for designing high-performance PEO-based CSEs.