Ionic Liquid Analogs of AlCl₃ with Urea Derivatives as Electrolytes for Aluminum Batteries

The ionic liquid analog, formed through the mixture of urea and AlCl₃, has previously shown to serve as a low-cost electrolyte for an aluminum-graphite battery, while maintaining good performance and achieving high Coulombic efficiency. Undesirable are the relatively high viscosity and low conductivity of this electrolyte, when compared to chloroaluminate ionic liquids with organic cations. In this work, the fundamental changes to the electrolyte resulting from using derivatives of urea (N-methyl urea and N-ethyl urea), again mixed with AlCl₃, are examined. These electrolytes are shown to have significantly lower viscosities (η = 45, 67, and 133 cP when using N-ethyl urea, N-methyl urea, and urea, respectively, at 25 °C). The associated batteries exhibit higher intrinsic discharge voltages (2.04 and 2.08 V for N-methyl urea and N-ethyl urea electrolytes, respectively, vs 1.95 V for urea system@100 mA g⁻¹ specific current for ≈5 mg cm⁻² loading), due to changes in concentrations of ionic species. Aluminum deposition is directly observed to primarily occur through reduction of Al₂Cl₇ ⁻ when AlCl₃ is present in excess, in contrast to previously suggested cationic Al-containing species, via operando Raman spectroscopy performed during cyclic voltammetry.