Charge Transport and Dynamics in Imidazolium Chloroaluminate Ionic Liquids

Abstract

Imidazolium based chloroaluminates were the first widely studied room-temperature ionic liquids. However, work in these systems has waned over the past 25 years in favor of ionic liquids with less reactive anions. Researchers are beginning to rediscover chloroaluminates because they possess some very advantageous properties that hold significant promise for applications such as energy storage, electrodeposition, and chemical synthesis. Chloroaluminates are not simple mixtures of a single cation with a single anion. It is more appropriate to think of these systems as binary mixtures of two ionic liquids which share the same cation. The binary nature of the chloroaluimates leads to highly tunable physical and chemical properties including liquidus range, electrochemical stability window, dc ionic conductivity, zero-shear viscosity, and static dielectric permittivity. However, the binary nature also leads to significant experimental challenges with respect to characterization of the structure and anion speciation in the chloroaluminates. The majority of work characterizing the imidazolium based chloroaluminate was conducted more than 25 years ago and focused on physical property measurements across limited compositions and/or narrow temperature/frequency ranges. The data from these early studies revealed important aspects of chloroaluminate physical properties; however, due to their limited scope they failed to make significant progress in understanding underlying structure-property relationships which are critical to realizing the full potential of chloroaluminate ionic liquids as electrolyte systems. Our current work seeks to reexamine imidazolium chloroaluminate ionic liquids leveraging the improved experimental tools and capabilities available today. In this presentation we will discuss our initial studies of imidazolium chloroaluminate ionic liquids using broadband dielectric spectroscopy, oscillatory shear rheology, and Raman spectroscopy. We will present preliminary work on the impact of changes in cation structure and ionic liquid composition on anion speciation, transport properties, and dynamics.