Ion Dynamics and Charge Transport in Imidazolium Chloroaluminate Ionic Liquids

Abstract

Imidazolium-based chloroaluminates were the first widely studied room-temperature ionic liquids. Now, following 25 years of intense study of ionic liquids with less reactive anions, researchers are beginning to rediscover chloroaluminates, and other halometallate ILs, because they possess potentially advantageous properties that hold significant promise for applications such as energy storage, electrodeposition, and chemical synthesis. The majority of work characterizing the imidazolium-based chloroaluminates was conducted more than 25 years ago and focused on physical property measurements across limited compositions and/or narrow temperature/frequency ranges. In this study, we employ broadband dielectric spectroscopy, rheology, and differential scanning calorimetry to investigate ion dynamics and charge transport in a series of 1-alkyl-3-methylimidazolium chloroaluminates over much broader frequency and temperature ranges than these prior studies. The influence of cation chemical structure on the time-scales of ion-hopping, structural relaxation, and interfacial polarization associated with mesoscale solvophobic aggregates will be compared with other classes of aprotic imidazolium ionic liquids to highlight important differences observed for chloroaluminates. Finally, the influence of the relaxation time-scales on transport properties including dc ionic conductivity, zero-shear viscosity, and static dielectric permittivity will be presented. These new, in-depth studies, are focused on developing underlying structure-property relationships that are critical to realizing the full potential of chloroaluminate ionic liquids as electrolyte systems.