Diffusion coefficients of Al<sub>2</sub>Cl<sub>7</sub><sup>–</sup> 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.