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.