Kinetics of Ion Transport in Ionic Liquids: Two Dynamical Diffusion States

TL;DR

This study uses molecular dynamics simulations to analyze ion mobility in ionic liquids, revealing two dynamic states—bound and free ions—and their impact on conductivity and diffusion, with implications for ionic semiconductor models.

Contribution

It introduces a dynamic two-state model of ion transport in ionic liquids and quantifies the free ion fraction and its temperature dependence.

Findings

Free ions constitute 15-25% of total ions and increase with temperature.

Conductivity calculations match experimental data.

Small energy gap (~0.026 eV) allows easy transition between states.

Abstract

Using classical molecular dynamics simulations, we investigate the mobility of ions in [Bmim][TFSI], a typical room temperature ionic liquid. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound, clustered states, and when the ions move quasi-freely. Using this information, we evaluate the average portion of free ions that dominate conductivity. The amount of thus defined free ions comprises 15-25%, monotonically increasing with temperature in the range of 300-600 K, with the rest of the ions being temporarily bound, moving rather in local potentials. The conductivities as a function of temperature, calculated from electric current autocorrelation functions, reproduce reported experimental data well. Interestingly, for free ions the Nernst-Einstein relationship between the mobility and diffusion coefficient holds fairly well. In analogy with electronic semiconductors, one can speak about an ionic semiconductor model for ionic liquids with valence (or excitonic) and conduction band states for ions, separated by an energy gap. The obtained band gap for the ionic liquid is, however, very small, about 0.026 eV, allowing for easy interchanges between the two dynamic states.