(435a) Ternary Ionic Liquid Analogues As Electrolytes for Low-Temperature Rechargeable Aluminum Batteries

Due to the many issues surrounding lithium such as scarcity, safety, and ethical extraction, many researchers have been focused on finding alternatives to lithium-ion technology. One promising alternative is rechargeable aluminum batteries due to the earth abundance, inherent safety, high theoretical capacity, and low cost of aluminum metal¹. Additionally, batteries for space or defense applications often need to operate at extreme temperatures, but at low temperatures electrolytes often exhibit low ionic conductivity and are prone to freezing. The state-of-the-art electrolytes for aluminum batteries, the Lewis acidic AlCl₃-[EMIm]Cl (1-ethyl-methyl-imidazolium chloride) ionic liquids (ILs), are effective but expensive and exhibit low ionic mobilities below 0 °C. Their electrochemical and thermophysical properties can be tuned by adding a third species, such as urea. The AlCl₃-urea ionic liquid analogue (ILA) has been considered as a lower cost alternative to the AlCl₃-[EMIm]Cl IL electrolyte, though little research has been done on solvation in ternary mixtures of AlCl₃-urea-[EMIm]Cl ILAs, or their corresponding electrochemical properties. Previous investigations have shown improved ionic conductivity and Coulombic efficiency of the Al-graphite battery system using ternary mixtures of chloroaluminate ILs with urea², while another study has shown unique electrodeposition morphology in ternary chloroaluminate ILs³; however, these studies both explored a restricted compositional design spaced and focused on battery performance and deposition morphology at ambient temperatures.

Here, we synthesized AlCl₃-urea-[EMIm]Cl mixtures with 1.3:X:(1-X) molar ratios, where X = 0, 0.125, 0.25, 0.5, 0.75, 1, and characterized their physical and electrochemical properties at both ambient and low temperatures with spectroscopic, thermoanalytical, and electrochemical measurements. Quantitative solution-state ²⁷Al and ¹H single-pulse nuclear magnetic resonance (NMR) experiments were used to reveal how the types and populations of the polyatomic aluminum complexes vary with urea content, as well as understand how the local environments of the EMIm⁺ cations and urea change across different compositions. Differential scanning calorimetry (DSC) was utilized to observe the phase behavior of the electrolytes and determine their liquid-phase temperature windows. The Al electrodeposition capability of the ILA electrolytes were tested from 25 to -70 °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 AlCl₃-urea-EMIm[Cl] ILA electrolytes can be used to enhance the electrochemical performance, reduce the cost, and expand the operating temperature window of rechargeable aluminum-graphite batteries.

References

1. Zhang, Yu, et al 2018 Advanced Materials 30.38: 1706310.
2. Junfeng Li et al 2017 J. Electrochem. Soc. 164 A309
3. Tsuda, Tetsuya et al 2022 J. Electrochem. Soc. 169(9) 092520