Effect of Temperature on the Diffusion of the $${\text{A}}{{{\text{l}}}_{2}}{\text{Cl}}_{7}^{ - }$$ Anion in a Low-Temperature Triethylamine Hydrochloride–Aluminum Chloride Melt

Abstract—Aluminum-ion batteries (AIBs) are of interest for scientific community due to their low cost, fire safety, and high aluminum content in the Earth’s crust. A low-temperature chloroaluminate melt (or ionic liquid (IL)) based on triethylamine hydrochloride (Et3NHCl) is considered as one of the promising electrolytes for use in AIB. In this work, the diffusion coefficients of the $${\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }$$ ion ( $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ ) have been determined by chronopotentiometry in the temperature range from 322 to 413 K at the AlCl3-to-EtNhCl molar ratio from 1.1 to 1.95. The transport process of $${\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }$$ to the Al electrode surface proceeds according to the model of linear semi-infinite diffusion over the entire temperature and concentration ranges. The $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ diffusion coefficient depends on the aluminum trichloride content in the melt, and the temperature dependences of $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ do not obey the Arrhenius law at T = 323–344 K. However, when the temperature exceeds 344 K, the dependences of $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ in the Arrhenius coordinates are linear, and $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ does not depend on the $${\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }$$ ion concentration at a fixed temperature. This behavior is likely to be caused by the glass-forming nature of the IL at low temperatures. The obtained values of the ideal glass transition temperature are in good agreement with the Et3NHCl–AlCl3 phase diagram presented in the literature. The activation energy of $${{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ calculated from the linear portion of the dependence $$\ln {{D}_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{{\text{-}}}}}}$$ –1000T–1 is 13.4 ± 0.8 kJ mol–1 at T > 344 K.