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.

We use all-atom molecular dynamics simulations to study the effect of polymer polarity, as quantified by the dielectric constant, on the transport properties of lithium bis­(trifluoromethylsulfonyl)­imide (LiTFSI) doped polyethers. Our results indicate that increasing the host dielectric constant leads to a decrease in ionic cluster sizes and reduction in correlated motion of oppositely charged ions. This causes the ionic conductivity to more closely approach the Nernst-Einstein limit in which ionic conductivity is only limited by the diffusivities of Li+ and TFSI–. We compare our results to recent experimental observations which demonstrate similar qualitative trends in host polarity.

Using all atom molecular dynamics and trajectory-extending kinetic Monte Carlo simulations, we study the influence of Al2O3 nanoparticles on the transport properties of ions in polymer electrolytes composed of poly­(ethylene oxide) (PEO) melt solvated with LiBF4 salt. We observe that the mobility of Li+ cations and BF4 – anions and the overall conductivity decrease upon addition of nanoparticles. Our results suggest that the nanoparticles slow the dynamics of polymer segments near their surfaces. Moreover, the preferential interactions of the ions with the nanoparticles are seen to lead to an enhancement of ion concentration near the particle surfaces and a further reduction in the polymer mobilities near the surface. Together, these effects are seen to increase the residence times of Li+ cations near the polymer backbone in the vicinity of the nanoparticles and reduce the overall mobility and conductivity of the electrolyte. Overall, our simulation results suggest that both the nanoparticle-induced changes in polymer dynamical properties and the interactions between the nanoparticles and ions influence the conductivity of the electrolyte.

We report the results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-vinylimidazolium-hexafluorophosphate ionic liquids, studying the influence of the polymer molecular weight on the ion mobilities and the mechanisms underlying ion transport, including ion-association dynamics, ion hopping, and ion-polymer coordinations. With an increase in polymer molecular weight, the diffusivity of the hexafluorophosphate (PF) counterion decreases and plateaus above seven repeat units. The diffusivity is seen to correlate well with the ion-association structural relaxation time for pure ionic liquids, but becomes more correlated with ion-association lifetimes for larger molecular weight polymers. By analyzing the diffusivity of ions based on coordination structure, we unearth a transport mechanism in which the PF moves by "climbing the ladder" while associated with four polymeric cations from two different polymers.