Physicochemical properties of triethylamine hydrochloride-based chloroaluminate ionic liquid

Abstract

The use of chloroaluminate ionic liquids (ILs) based on triethylamine hydrochloride as electrolytes for aluminum ion batteries (AIBs) can reduce the cost of the device, while at the same time preserving its performance characteristics. However, the ionic composition of the system and the physicochemical properties required for the optimization of the electrolyte composition have not been systematically studied in literature. The present paper reports on the studies into the ionic composition, viscosity, density and melting temperatures of the IL across wide temperature and concentration ranges. Raman spectroscopy revealed the presence of AlCl₄⁻, Al₂Cl₇⁻ and Et₃NH⁺ ions in the electrolyte, and the trends of their concentration changes are similar to the behaviors reported for other chloroaluminate ILs. Visual polythermal method and differential scanning calorimetry (DSC) were used to obtain the phase diagram of the electrolyte. The melting temperature of the system increases with the ratio of aluminum chloride to the organic salt (N) and reaches its maximum at N = 1.0 followed by its decrease down to 234 K at N = 1.7. The IL with N = 1.95 displays only glass transition at cooling. The shape of the DSC curves for the ILs is similar to that of the DSC curves obtained at heating isothermally crystallized polymers. The temperature and concentration dependences of density are described by a linear function; however, the concentration dependences contain a bend at N = 1.0. The viscosity of the ILs decreases with the growth of temperature and aluminum chloride concentration. The temperature dependence of viscosity is non-linear and can be described by the Vogel-Fulcher-Tammann equation. The results obtained indicate that the ILs with N > 1 undergo a structural change due to the formation of Al₂Cl₇⁻ ion. ILs with a high content of Al₂Cl₇⁻ anion (N > 1.5) are the most viable choice for electrolytes of AIBs, since they offer a relatively low melting temperature and their viscosity is comparable with that of other chloroaluminate ILs.