Ternary Ionic Liquid Analogues for Ambient and Low-Temperature Rechargeable Aluminum Batteries

Many issues surround lithium, such as scarcity, safety, and ethical extraction. One promising alternative to lithium-ion batteries are aluminum batteries, which offer the earth abundance, inherent safety, high theoretical capacity, and low cost of aluminum metal. Additionally, batteries for transportation, space, or defense applications may often need to operate at low temperatures, where electrolytes exhibit lower ionic conductivity and are prone to freezing. The state-of-the-art electrolytes for aluminum batteries, Lewis acidic AlCl 3 (aluminum chloride)-[EMIm]Cl (1-ethyl-3-methyl-imidazolium chloride) ionic liquids (ILs), are not cost effective and exhibit low ionic mobilities and conductivities below 0 o C. Their electrochemical and thermophysical properties can be tuned by adding a third species, such as urea. The AlCl 3 -urea ionic liquid analogue has also been investigated as a lower cost alternative. However, little research has been done on ternary mixtures of AlCl 3 -urea-[EMIm]Cl, including investigations of their physical properties and ability to reversibly electrodeposit Al metal from ambient to low temperatures. In this work, AlCl 3 -urea-[EMIm]Cl mixtures with molar ratios of 1.3:X:(1-X) molar ratios were synthesized, where X = 0, 0.125, 0.25, 0.5, 0.75, and 1. Physical and electrochemical properties were characterized from ambient to low temperatures (down to -70 o C) with spectroscopic, thermoanalytical, and electrochemical measurements. Differential scanning calorimetry (DSC) was utilized to observe the thermal behavior of the electrolytes and determine their liquid-phase temperature windows. Quantitative liquid-state 27 Al and 1 H single-pulse nuclear magnetic resonance (NMR) experiments revealed how the types and populations of the polyatomic aluminum complexes changed based on urea concentration, as well as understand how the local environments of the EMIm + cations and urea change across different compositions. The Al electrodeposition capability of the electrolyte mixtures were tested from 25 to -70 o C, both galvanostatically (two-electrode cells) and potentiodynamically (three-electrode cells). Subsequently, the most promising electrolyte compositions were tested in Al-graphite batteries to determine their technological feasibility and electrochemical properties, particularly at lower temperatures. The results indicate that AlCl 3 -urea-[EMIm]Cl in a 1.3:0.25:0.75 molar ratio can be used to enhance the electrochemical performance,