Mechanisms Underlying Ion Transport in Polymerized Ionic Liquids

Abstract

We report the results of atomistic molecular dynamics simulations informed by quantum-mechanically parametrized force fields, which identify the mechanisms underlying ion motion and diffusivities in poly(1-butyl-3-vinylimidazolium-hexafluorophosphate) polymerized ionic liquid (polyIL) electrolytes. Our results demonstrate that anion transport in polyILs occurs through a mechanism involving intra- and intermolecular ion hopping through formation and breaking of ion-associations involving four polymerized cationic monomers bonded to two different polymer chains. The resulting ion mobilities are directly correlated to the average lifetimes of the ion-associations. Such a trend is demonstrated to contrast with the behavior in pure ILs, wherein structural relaxations and the associated times are dominant mechanism. Our results establish the basis for experimental findings that reported ion transport in polyILs to be decoupled from polymer segmental relaxations.

Ion transport mechanisms in salt‐doped polymerized zwitterionic electrolytes

“…In past studies on polymerized ionic liquids, we established correlations between ion‐association relaxation timescales and ion mobilities . In particular, the ion‐association structural relaxation time τ C represents the average time required for two ions to permanently separate from each other.…”

“…In our past studies of polymerized ionic liquids, we examined the probabilities of ionic coordination states as a function of coordination number and identified key aspects of ion‐transport mechanisms through such a characterization. Coordination was determined in the present study by direct atomic coordination using cutoff distances informed by the radial distribution functions, such as those displayed in Figure .…”

“…Specifically, we use atomistic molecular dynamics (MD) simulations to probe the ion mobilities, and the underlying structural features in Motifs B and C at various concentrations of Li + ‐TFSI − to understand the origins of the differences noted in their conductivities. Within such a context, we also seek to clarify the ion‐transport mechanisms in salt‐doped polyZIs and examine parallels to the physics identified in our recent studies of polymerized ionic liquids …”

“…In recent studies, Shao et al reported extensive set of MD investigations on ZI/salt solubility and association phenomena . We adopt the optimized potential for liquid simulations (OPLSs) used by Shao et al, which has also been used in other studies on ionic liquids and polymerized ionic liquids . The simulations are effected at temperature of 600K to facilitate faster ion mobilities which can be probed in reasonable timescales in simulations.…”

Influence of side chain linker length on ion‐transport properties of polymeric ionic liquids

“…We quantify the structural organization in such systems through the radial distributions and the average intermolecular separation between cationic groups. Inspired by our recent findings in the context of ion transport mechanisms in polyILs, we use an analysis tool for identifying ionic association/dissociation processes to elucidate the connection between the interchain ion‐hopping mechanism and the diffusion, and thereby identify the origins underlying the influence of side chain linker length.…”