Interfacial Effects on Structure and Ion Dynamics in Polymer–Organic Ionic Plastic Crystal Composites

Abstract Organic ionic plastic crystals (OIPCs) have emerged as promising solid-state electrolytes owing to their intrinsic ionic conductivity and plasticity. To further enhance their mechanical stability and transport properties, polymer–OIPC composites have been developed. However, the role of polymer–OIPC interfaces in governing ionic transport at the molecular level remains elusive. Herein, we employ atomistic molecular dynamics simulations to investigate the interfacial effects on structure and ionic transport in polymer–OIPC composites. Polymer–OIPC composites based on diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]) are constructed with two representative polymers, poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO). Our results reveal that both PVDF and PEO induce similar interfacial structures, yet the mobility of OIPC ions is enhanced in PEO–OIPC composites. Upon the introduction of Li+ ions as dopants, we find that Li+ ions follow distinct diffusion mechanisms in the two composites: anion-mediated diffusion in PVDF–OIPC and polymer-mediated diffusion in PEO–OIPC. The faster relaxation of Li+–PF6– coordination shells in PVDF–OIPC allows Li+ ions to exhibit normal diffusion along the interfacial plane, whereas Li+ ions in PEO–OIPC remain subdiffusive due to the slower relaxation of Li+–O(PEO) coordination shells. These findings highlight the importance of understanding ion diffusion mechanisms at the molecular level for the rational design of solid-state electrolytes.