Fundamental Investigation of AlCl3-Amide Electrolyte for Application in Aluminum Ion Battery

Abstract

In recent years, aluminum-based batteries have received increasing attention because of the high abundance and high volumetric and gravimetric capacity of aluminum and the growing need for secondary batteries. Aluminum and its alloys are extensively utilized for surface coating because of their excellent corrosion resistance, high electric conductivity, and low density. Thus, the study of electrodeposition of aluminum and its alloys has received increasing attention, especially at room temperature. The electrodeposition of aluminum is only possible in organic solvents or in ionic liquids because of its high reactivity and high negative reduction potential (–1.67 V vs. SHE). The application of organic solvents for electrodeposition of aluminum is also limited because of their narrow electrochemical window, very low solubility of Al salts, and low thermal stability. Ionic liquids have the ability to dissolve a large variety of compounds because of their strong electrostatic interactions and the ability to remain liquid at room temperature as well as good thermal stability, low volatility, and low toxicity. A new class of ionic liquids, knowns as ionic liquid analogs (ILAs) or deep eutectic solvents (DESs) have received increasing attention as an alternative to traditional ionic liquids because of their low price, higher solvability of metal chlorides and better largescale viability. The ILAs are derived from a mixture of AlCl3 and an oxygen donor amide (urea or acetamide) ligand. Despite their wide employment in research especially for new Al-ion batteries, there is lack of fundamental data on their structure/property relationship, including speciation of the ionic moieties at different molar compositions. In the present study, physicochemical properties of a room temperature ILA composed of AlCl3 and urea is investigated by varying the molar ratio of AlCl3/urea from 1.0 to 1.5. The effect of temperature on the electrodeposition of aluminum and its structure is studied using cyclic voltammetry and scanning electron microscopy. Different additives are utilized to improve the ionic conductivity and to reduce the viscosity of AlCl3/urea electrolyte and their effect is investigated. The results indicate that the addition of these additives improves the electrolyte properties, but reduces the voltage stability window of the electrolyte. Future work is underway to further investigate the effect of temperature on dynamic equilibrium of various ionic species present in the electrolyte to improve the properties of AlCl3/urea electrolyte.