Molecular dynamics simulation on current-voltage characteristics of room temperature ionic liquids under strong electric field

Abstract

In this work, molecular-dynamics simulations were performed on the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) to investigate the influence of external electric fields on the ion-transport properties. In the low-field regime (0-0.2 V/nm), the two-state model effectively analyzes the deviation from the Nernst-Einstein relation by quantifying the contribution of bound ions to self-diffusion and estimating the range of the free-ion fraction. In the high-field regime (0.2-1.8 V/nm), both mobility and diffusivity exhibit nonlinear growth, accompanied by ionic dipole alignment and field-induced local redistribution of counterions, as revealed by normal and angular-dependent radial-distribution functions (RDFs and ARDFs). We observed saturation of both dipole alignment and radial-distribution functions in the highest fields, which contrasts with the continued increase in mobility and diffusivity. In further analysis of ionic dynamics, we found that the mean residence time of bound ions decreases exponentially with the field strength. This suggests that accelerated ion dynamics might be the key factor driving the continued enhancement of diffusivity and mobility.