Role of Ion Interactions in Molecular Design Rules for Li + and Na + Solid Polymer Electrolytes

Solid polymer electrolytes (SPEs) are promising candidates for next-generation all-solid-state batteries owing to their safety and mechanical flexibility; however, their low ionic conductivity remains a significant bottleneck. In this work, atomistic molecular dynamics simulations systematically elucidated how polymer chemistry governs the transport mechanisms of Li + and Na + ions in chemically diverse host matrices using bis(trifluoromethanesulfonyl)imide (TFSI – ) as a common counterion. Six representative host matrices were analyzed, featuring ether- and carbonyl-oxygen (poly(ethylene oxide) (PEO), poly(ethylene carbonate) (PEC), and poly(propylene carbonate) (PPC)), nitrile (polyacrylonitrile (PAN)), halogenated (poly(vinylidene fluoride) (PVDF), poly(vinyl chloride) (PVC)) functional groups. These functional groups were studied under external electric fields to evaluate field-driven migration. Furthermore, the glass transition temperature ( T g ), radial distribution function (RDF), and free-energy were calculated to elucidate the correlation between polymer segmental dynamics, ion-solvation stability, and the resulting ionic mobility across the different functional groups. The results demonstrate that PEO and PAN facilitate efficient, polymer-mediated conduction pathways by promoting extensive salt dissociation. Instead, halogenated polymers like PVC and PVDF favor an intercluster hopping mechanism within aggregated ionic structures. The high charge density of lithium ions results in rigid coordination shells, restricting their translational motion. Conversely, sodium ions exhibit weaker binding affinities, enabling superior field-responsive mobility. This phenomenon is particularly pronounced in PVDF, where the Na + diffusion coefficient increases by 2 orders of magnitude under an applied electric field (from 6.4 × 10 –8 to 2.1 × 10 –6 cm 2 /s), significantly outperforming the modest increase observed for Li + (from 9.2 × 10 –8 to 3.7 × 10 –7 cm 2 /s). Free-energy analyses further confirm that Li + is strongly stabilized in oxygen- and nitrogen-rich environments, whereas Na + transport benefits from moderate polymer affinity and a more flexible coordination shell. These findings establish fundamental molecular design rules linking polymer functionality, solvation shell dynamics, and field-driven transport. This study provides a theoretical framework for the rational design of polymer hosts optimized for high-rate cation conduction in solid-state energy storage devices.