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

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• Primitive Model Electrolytes in the Near and Far Field: Decay Lengths from DFT and Simulations
• Conductivity of Concentrated Electrolytes
• Charge Oscillations in Ionic Liquids: A Microscopic Cluster Model
• Anomalous Underscreening in the Restricted Primitive Model
• Long-range fluctuation-induced forces in driven electrolytes
• Conductance of concentrated electrolytes: multivalency and the Wien effect
• Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids
• Electrostatic correlation induced ion condensation and charge inversion in multivalent electrolytes
• Electrolytes structure near electrodes with molecular size roughness
• Transient fluctuation-induced forces in driven electrolytes after an electric field quench
• Length scales in electrolytes
• Bridging Electrostatic Screening and Ion Transport in Lithium Salt-Doped Ionic Liquids
• Structural and Dynamical Crossovers in Dense Electrolytes