Importance of reorientational dynamics for the charge transport in ionic liquids

Abstract

Most ionic liquids contain at least one rather complex ion species exhibiting a dipolar moment. In the present work, we provide a thorough evaluation of broadband dielectric spectra of 12 ionic liquids taking into account the often neglected reorientational dynamics of these ions. We confirm that this dynamics leads to a clear relaxational signature in the spectra, a fact that so far only was considered in few previous works. The obtained reorientational relaxation times are well consistent with earlier inelastic light-scattering and high-frequency dielectric investigations. Evaluating our dielectric spectra in terms of reorientational motions reveals a close coupling of the ion-rotation dynamics to the ionic charge transport in a broad temperature range from the low-viscosity liquid above room temperature deep into the high-viscosity supercooled state close to T g . This coupling does not seem to be mediated by the viscosity but probably is of more direct nature, pointing to a revolving-door mechanism as also considered for plastic-crystalline ionic conductors. Our results show that the reorientational motion of the dipolar ions plays a significant and so far widely overlooked role for the ionic charge transport in ionic liquids.

Translational and reorientational dynamics in deep eutectic solvents

“…Interestingly, in Ref. 24, some of the present authors reported that the dielectric spectra of 12 ionic liquids could be well described by a sum of a CD function and a dc-conductivity contribution. This observation has been interpreted in terms of dominant reorientational relaxations without invoking any additional ac contributions arising from hopping charge transport.…”

“…Similar behavior was also reported for ionic liquids. 23 However, in an earlier work, some of the present authors showed that the RBM can be applied to the rheological response of supercooled liquids as well. 39 More recently, it was demonstrated how the single-particle meansquare displacement can be derived from complex viscosity spectra.…”

“…Or does it occur directly, via a "revolvingdoor" mechanism, as considered for plastic-crystal electrolytes [21][22][23] and also proposed for ionic liquids? 24 In the latter scenario, the reorientation of the asymmetric molecules within the material is assumed to open up transient gaps, which are expected to enhance the ionic mobility, and thus the conductivity, compared to the value expected for a particle diffusing through a viscous medium. On the other hand, for the purely viscosity-driven scenario, it is intuitively clear that the translational motion of particles through a viscous medium should be related to its viscosity, just as the rotational motion of asymmetric molecules within this medium.…”

Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules

“…To unequivocally deduce  dc (T) and (T) of both relaxation processes from the dielectric spectra, we have simultaneously fitted '() and "() applying an equivalentcircuit approach. 48,52 As previously demonstrated for various ionically conducting materials, 48,53,54,55 including PC electrolytes, 18,32,33 a distributed RC circuit connected in series to the bulk sample provides a good formal description of electrode polarization. To account for the  relaxation, we used the phenomenological Cole-Davidson (CD) function 56 as previously done for pure CNA.…”

“…Non-Arrhenius behavior is typical for glassy dynamics and found, e.g., for the structural relaxation of glassforming liquids, where it is usually well fitted by the empirical Vogel-Fulcher-Tammann (VFT) equation. 74,75,76,77 For the ionic dc conductivity in materials exhibiting glassy freezing, 53,54,78,79 including PCs, 18,32,33 such non-Arrhenius behavior also is often found. (One should note that a sufficiently broad temperature range is necessary to detect the deviations from Arrhenius.)…”