Diffusion Coefficients of the \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) Anion in the Low-Temperature Chloroaluminate Melt Based on Triethylamine Hydrochloride

Abstract—Since the demand for renewable energy sources is still increasing, the main research in the battery industry is focused on the development of safe and high-capacity energy storage systems capable of sustaining high current loads and made of inexpensive and readily available materials. An aluminum-ion battery (AIB) using metallic aluminum as the anode, carbon materials as the cathode, and chloroaluminate ionic liquids as the electrolyte is considered to be among the most promising systems. A low-temperature chloroaluminate melt based on triethylamine hydrochloride (Et₃NHCl) is one of inexpensive electrolytes for AIB. This melt can reversibly precipitate/dissolve metallic aluminum due to the presence of the \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) ion in it. However, the diffusion of \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) ions in the Et₃NHCl–AlCl₃ system has not been studied previously. In this work, the concentration dependences of the diffusion coefficients of the \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) anion are studied using chronopotentiometry in the concentration range N = 1.3–1.95 (where N is the molar ratio of aluminum chloride to organic salt). The diffusion coefficients are shown to increase with an increase in the aluminum chloride content in the melt: from 1.71 × 10^–7 (N = 1.3) to 4.50 × 10^–7 cm² s^–1 (N = 1.95). A similar behavior can be caused by a decrease in the viscosity of the melts with an increase in the \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) concentration. The obtained results show that Et₃NHCl–AlCl₃ with N = 1.95 is the most suitable electrolyte for operating in AIB. In addition, the electrochemical reduction of the \({\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{7}^{ - }\) ion on the aluminum electrode surface is found to be complicated by the nucleation process, which has the lowest overvoltage at N = 1.95.