Importance of reorientational dynamics for the charge transport in ionic liquids

TL;DR

This study demonstrates that reorientational dynamics of dipolar ions significantly influence charge transport in ionic liquids, revealing a close coupling that is independent of viscosity and highlighting an overlooked mechanism.

Contribution

It provides a comprehensive analysis of reorientational dynamics in ionic liquids and establishes their direct impact on ionic charge transport, a previously underappreciated factor.

Findings

Reorientational relaxation times align with previous light-scattering and dielectric studies.

Reorientational motions are closely coupled with charge transport across various temperatures.

The coupling mechanism is likely direct, not mediated by viscosity, suggesting a revolving-door process.

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 Tg. 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.