Molecular dynamics simulations of ionic liquid/poly(vinylidene fluoride) systems: Ion transport with different anions

MH Qu and SS Li and J Chen and YQ Xiao and JJ Xiao, SOLID STATE IONICS, 375, 115798 (2022).

DOI: 10.1016/j.ssi.2021.115798

Molecular dynamics (MD) simulations were applied to explore the ion transport in six ionic liquid/poly(vinylidene fluoride) (IL/PVDF) systems where the cation is 1-ethyl-3-methylimidazolium (EMIM) and the anions are Cl, Br BF4, PF6, trifluoromethanesulfonate (TfO) and bis(trifluoromethanesulfonyl)imide (NTf2) separately, and each system contains about 40 wt% IL. The glass transition temperature (T-g = 204 K) of EMIM NTf2/PVDF system is good agreement with the experimental value (200 K). Transference number of ions, ideal conductivity and viscosity can be calculated based on diffusion coefficient. The power law shows mobilities of anions and cations are seen to exhibit a "superionic " behavior when considering all the anions (cations) together. Overall, for the same IL/PVDF system, with the increase of temperature, both the ion-pair relaxation times and lifetimes decrease, while diffusion coefficients of ions increase, so ion motion is intensified. The transference numbers of anions and cations show upward and downward trends respectively, indicating that the diffusion coefficients of cations decrease faster than that of the anions. The conductivity gradually increases, and the viscosity gradually decreases. For different IL/PVDF systems, the chemical properties of ion pairs are the main factors affecting ion-pair relaxation times and lifetimes, diffusion coefficients, the conductivity and viscosity at lower temperatures. However, the influence gradually weakens as the temperature increases, and the IL/PVDF systems with a large Tg will have smaller the diffusion coefficients, conductivity, as well as higher viscosity at higher temperatures.

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