Structure and Dynamics in Single-Ion Conducting Polymer Blends for Sodium-Ion Electrolytes

Solid polymer electrolytes (SPEs), especially poly(ethylene oxide) (PEO)-based polymer electrolytes, remain under extensive investigation for next-generation storage systems due to their favorable mechanical properties, wide electrochemical stability windows, and ease of fabrication and handling in thin films. However, the semi-crystalline nature of PEO at ambient temperature limits segmental motion and thereby suppresses ionic conductivity, motivating strategies that reduce crystallinity and enhance chain dynamics. Single-ion conducting (SIC) polymers, in which anions are covalently bonded to the polymer backbone, offer distinct advantages including suppressed ion aggregation, high transference number, and stability. In this study, to combine the complementary advantages of PEO and SIC polymers, PEO with different chain lengths was blended with sodium poly(styrene trifluoromethanesulfonyl imide) (PSTFSI-Na) and sodium poly(methacryloyloxy trifluoromethanesulfonyl imide) (PMTFSI-Na) at various ratios. The blends were systematically characterized by differential scanning calorimetry, broadband dielectric spectroscopy, and X-ray scattering to evaluate their glass transition temperatures, ion transport properties, relaxation processes, and morphology. The blends exhibited enhanced ionic conductivity with increasing EO/Na ratio and shorter PEO chains, reaching values on the order of 10^-4 S cm^-1 at 100 ℃. Moreover, X-ray scattering combined with dielectric analysis revealed that ion transport was partially decoupled from polymer segmental motion due to the formation of localized ion-conducting pathways. These results elucidate the structure-property relationships in PEO–polyanion blends and provide valuable insights for designing high-performance solid polymer electrolytes for sodium-ion batteries.