Molecular-to-polymeric crossover in ion diffusion in glyme-based electrolytes: from vehicular to hopping transport

Abstract

Ion transport in glyme-based electrolytes arises from a complex interplay between solvation structure, ion correlations, and polymer chain length. Here, combining pulsed-field gradient nuclear magnetic resonance (PFG-NMR), ionic conductivity measurements, and molecular dynamics (MD) simulations, we investigate the diffusion of monovalent cations (Li$^+$, Na$^+$, Cs$^+$) and TFSI$^-$ anions across a wide molecular-weight range, from monoglyme to long poly(ethylene oxide) (PEO) chains up to 4000~g/mol, corresponding to $n$ up to 88, where $n$ is the number of ethylene oxide repeat units. We identify a crossover region at $n \approx 8$ separating two transport regimes. For short chains, ion motion is consistent with a vehicular mechanism, accompanied by pronounced ion correlations. For longer chains, ion transport decouples from polymer motion and proceeds via rapid coordination exchanges within a slowly relaxing matrix. This transition is accompanied by reduced ion clustering and enhanced anion mobility, leading to increasingly anion-dominated charge transport. Overall, our results provide a molecular picture of ion transport across the molecular-to-polymeric transition and highlight the central role of solvation shell dynamics and polymer relaxation in governing ion dynamics in glyme-based electrolytes.