Room Temperature Chloroaluminate Deep Eutectic Solvents As an Electrolyte in Aluminium Rechargeable Batteries.

Abstract

Novel chemistry of Deep Eutectic Solvents (DES) is a matter of current interest as the unusual properties of these solvents make them a versatile medium for a variety of reactions. In particular, they provide media with a wide potential window and high ionic strength for the redox chemistry of metals. A wide range of type II DES (Metal halide (MX) + Hydrogen Bond Donor (HBD)) with AlCl₃ (MX) have been synthesised, where hydrogen bond donors (HBD) included acetamide, methyl-, dimethyl-, trimehylacetamide, urea, di- and tetramethylurea. The ratio of AlCl₃ - HBD was maintained at 1.5:1 throughout. Viscosity and conductivity of these novel DES, have been related to the melting point of HBD as well as to the availability of proton exchanging groups. Mechanisms of aluminium deposition/dissolution from room temperature chloroaluminate melts were studied in order to elucidate the effects of amines type on the properties of these novel electrolytes. Mass transport and speciation were investigated by means of Electrochemical Quartz Crystal Microbalance (in dispersive mode), UV-Vis and NMR. It was found that although the higher deposition rate was proportional to the higher conductivity of the respective DES, the rate of the stripping process was related to the type of HBD used. Creation of an aluminium based battery requires a balance between current efficiency and deposition rate and this can be achieved by a choice of suitable HBD. The influence of surface tension of DES on their electrochemical properties, and ultimately availability of proton exchanging groups to participate in liquid formation is currently under investigation. Figure 1