Importance of reorientational dynamics for the charge transport in ionic liquids

Most ionic liquidscontain at least one rather complex ionspecies exhibiting a dipolar moment. In the present work, we provide a thorough evaluation of broadband dielectric spectra of various ionic liquidstaking into account the often neglected reorientational dynamics of these ions. We provide evidence that this dynamics leads to a clear signature in the spectra. The obtained reorientational relaxation times are well consistent with earlier inelastic light scattering and high-frequency dielectric investigations. We find a close coupling of the detected ion-rotation dynamics to the ionic charge transport. This coupling does not seem to be mediated by the viscosity but probably is of more direct nature, pointing to a revolving-doormechanism 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 overlooked role for the ionic charge transport in ionic liquids.