Fundamental Insights into Electrical and Transport Properties of Chloroaluminate Ionic Liquids for Aluminum-Ion Batteries

Around the world, there is a critical need for sustainable energy storage systems for grid-level stationary energy storage. In the search for such systems, aluminum (Al)-ion batteries have recently attracted considerable attention due to their positive attributes including low cost, high abundance of raw materials, and long cycling life. Among all, chloroaluminate ionic liquid (IL) composed of aluminum chloride (AlCl3) and a Lewis basic organic chloride (XCl) is the most widely employed electrolyte system in Al-ion batteries. In many existing Al-ion battery chemistries, chloroaluminate IL simultaneously functions as a medium for ion transportation and as an electroactive anode material. To provide fundamental insights into designing better performing Al-ion batteries, herein, we performed a systematic investigation of the electrochemical and transport properties of four AlCl3-XCl ILs, where X is EMIM (1-ethyl-3-methylimidazolium), BMIM (1-butyl-3-methylimidazolium), TMAH (trimethylammonium), or TEAH (triethylammonium), using a combination of density functional theory calculations and experimentation. Results show that the electrochemical stability window, ion transference number, conductivity, and ionicity of Lewis acidic chloroaluminate ILs are fundamentally governed by the (i) AlCl3/XCl molar ratio (r), which dictates the concentration of ionic species, and (ii) XCl type, which affects the overall degree of interactions among ionic species.