Influence of Counterion Chemical Structure on Ionic Conductivity in Polymerized Ionic Liquids

Interest in a new class of electrolytes called polymerized ionic liquids has been growing in recent years, and the current goal in scientific literature is to explore mechanisms of ion motion and probe ways to increase the diffusivity and conductivity. In this work, molecular dynamic simulations were implemented in order to explore the effect of anion (or counterion) chemical structure on three important properties of ion mobility and ion transport: diffusivity (proportional to the conductivity), ion-association lifetime, and structural relaxation time. The eight counterions were bistrifluoromethylsulfonylimide, bispentafluoroethylsulfonylimide, tetrafluoroborate, tetrachloroaluminate, pentafluoroethylsulfate, trifluoromethylsulfate, and the chloride and bromide ions. The cation was poly(1-butyl-3-vinylimidazolium). Tetrachloroaluminate has the highest diffusivity and shortest structural relaxation time and ion-association lifetime. There is a negative correlation between structural relaxation time and diffusivity, and the physical meaning of the slope of this fit is explored.