Diffusion coefficients of Al₂Cl₇^– in low temperature chloroaluminate melt based on triethylamine hydrochloride

With the growing demand for renewable energy sources, much of the research in the battery industry is focused on creating safe and high-capacity energy storage systems that can handle high current loads using inexpensive and readily available materials. The aluminum-ion batteries (AIB) are considered as one of the most promising systems. Such materials as aluminum metal, carbon materials and chloroaluminate ionic liquids are used as anode, cathode and electrolyte, respectively. A low-temperature chloroaluminate melt based on triethylamine hydrochloride (Et₃NHCl) is promising and inexpensive electrolytes for AIBs. This melt has the ability to reversibly precipitate/dissolve aluminum metal due to the presence of the Al₂Cl₇^– ion in it. However, the diffusion of Al₂Cl₇^– ions in the Et₃NHCl–AlCl₃ system has not been studied previously. In the presented work, the concentration dependence of the diffusion coefficients of the Al₂Cl₇^– anion was studied using chronopotentiometry in the concentration range N = 1.3–1.95 (where N is the molar ratio of aluminum chloride to organic salt). It was shown that diffusion coefficients increase with aluminum chloride content growth in the studied melt: from 1.71·10^–7 (N = 1.3) to 4.50·10^–7 cm²·s^–1 (N = 1.95). This behavior can be caused by the viscosity decrease of the melts with Al₂Cl₇^– concentration growth. Based on the obtained results it can be concluded that Et₃NHCl–AlCl₃ with N = 1.95 is the most suitable electrolyte for AIB. Moreover, it was established that the electrochemical reduction of the Al₂Cl₇^– on the surface of the aluminum electrode is complicated by the nucleation process, which has the lowest overvoltage at N = 1.95.