(549g) Ternary Ionic Liquid Analogues for Rechargeable Metal Aluminum Batteries

Rechargeable aluminum batteries are promising alternatives to lithium-ion technology because of their high theoretical capacity, earth abundance, inherent safety, and the low cost of aluminum metal¹. However, there are few electrolytes that enable the reversible electrodeposition of aluminum at room temperature. The current gold-standard electrolytes for aluminum electrodeposition are chloroaluminate ionic liquids comprised of Lewis acidic mixtures of AlCl₃ and an imidazolium-based chloride salt (e.g., [EMIm]Cl), though they are corrosive and high in cost. Deep eutectic solvent (DES) or ionic liquid analogue (ILA) electrolytes have been investigated as alternatives, which are more environmentally friendly and lower cost. Such electrolytes, however, generally exhibit lower ionic conductivities and modestly lower electrochemical stability windows. Previous work has shown improved ionic conductivity and Coulombic efficiency of the Al-graphite battery system using ternary mixtures of chloroaluminate ionic liquids and other organic compounds known to form a DES with AlCl₃, such as urea², though these studies explored a restricted compositional design space and focused on battery performance at ambient temperatures.

Our recent research efforts focus on developing ILA electrolytes with varied anion-cation compositions to increase disorder, enhancing ionic conductivity and disrupting crystallization to improve performance under both ambient and low temperatures. Here, we study ternary electrolyte compositions of AlCl₃-urea-EMIm[Cl] ILAs for use in rechargeable aluminum metal batteries, characterized by electrochemical, spectroscopic, and thermoanalytical measurements. The ILAs were synthesized such that the molar ratio of AlCl₃ was kept constant in a ratio of 1.3:X:(1-X) where X is the mole fraction of urea in the binary urea-EMIm[Cl] composition space (0<X<1).

Quantitative liquid state 27Al and 1H nuclear magnetic resonance (NMR) spectroscopy was used to determine the ionic speciation of polyatomic aluminum complexes and understand how the local environments of the EMIm⁺ cations and urea changes with composition. Differential scanning calorimetry (DSC) was employed to understand the phase behavior of the electrolytes and determine their liquid-phase temperature windows. The ILA electrolytes were tested for their ability to reversibly electrodeposit aluminum metal, both galvanostatically (2-electrode symmetric cells) and potentiodynamically (3 electrode cells), including at low temperatures. Subsequently, the most promising electrolytes were tested in Al-graphite batteries to determine their technological feasibility and electrochemical properties, particularly at lower temperatures and fast charging/discharging rates. The overall results provide scientific insights into the design of AlCl₃-urea-EMIm[Cl] ILA electrolytes for rechargeable aluminum metal batteries, for enhanced electrochemical performance, reduced cost, and expanded temperature window.

References

1. Zhang, Yu, et al 2018 Advanced Materials 30.38: 1706310.

2. Junfeng Li et al 2017 J. Electrochem. Soc. 164 A309