Ion transport in ionic liquid/poly(vinylidene fluoride) system under electric fields: A molecular dynamics simulation

Abstract

Molecular dynamics simulations were applied to explore the ion transport in five ionic liquid/poly (vinylidene fluoride) (IL/PVDF) systems under different electric fields where the cation is 1-ethyl-3-methylimidazolium (EMI+) and the anions are Cl−, Br−, BF− 4, PF− 6 and trifluoromethanesulfonate (TfO−) separately, and each system contains about 40 wt% of IL. The structural properties were analyzed by the following calculations as electric fields increase. The pair correlation function reveals that the local spatial structure of ion pairs can be split into two types, which tend to disorder; the dihedral angle and mean square radius of gyration show that PVDF chains tend to straighten. The ion transport properties were studied by the ion-pair intermittent time correlation function and continuous time correlation function, the diffusion coefficient and mobility of anions and cations as electric fields increase. The simulation results illustrate that ion transport is intensified. The transference numbers of anions and cations show the relative contributions of anions to charge transport shows an upward trend as electric fields increase, and under an applied electric field, contribution of anions to charge transport is closely related to anions volume. In addition, the relationship between diffusion coefficient and the electric fields can be fitted into an exponential function, and the fitting parameters are strongly related to the local structure of the ion pairs, and the relationship between ideal conductivity and the electric fields has the same law. Specially, the anions mobility is determined by the anions volume under the stronger electric fields, and the smaller the anion volume, the greater the mobility.