Electrochemical Stability Windows of Imidazolium-Based Ionic Liquids for Aluminum Batteries: Computational Insights into Cation Functionalization and Fluoride-Containing Anions

Understanding and tuning the electrochemical stability window (ECW) of ionic liquids (ILs) are essential for advancing energy storage technologies. In this study, density functional theory combined with the thermodynamic cycle method is employed to systematically investigate the ECWs of imidazolium-based cations paired with a range of fluorinated and chlorinated anions with potentials referenced to an aluminum electrode. A broad set of cation structures, including alkyl, methoxy-ethoxy, vinyl, and alkyl-bridged derivatives, is explored alongside common and hydrogen fluoride-containing anions, [F(HF)n]⁻ (n = 0 - 3). The results show that while simple alkyl substitution has minimal redox impact, electron-donating and π-conjugated groups lower oxidation potentials via HOMO delocalization. Fluorinated anions confer high redox stability, whereas HF-containing anions limit both the oxidative and reductive boundaries. Notably, [im⁺-C₃-im]⁺[BF₄]⁻ presents the widest ECW (5.813 V), while HF-containing anions yield narrower ECWs due to the coexistence of [F]⁻ and [F(HF)]⁻ entities. Accurate ECW estimation further requires proper consideration of anion redox pathways as the choice of reaction mechanisms strongly influences predicted stability limits. Comparative analysis with the HOMO-LUMO and adiabatic AIE-AEA methods confirms that the thermodynamic cycle approach delivers superior accuracy while remaining computationally efficient, making it well-suited for high-throughput screening. Furthermore, the solvent dielectric constant is found to significantly modulate redox boundaries, emphasizing the importance of solvation effects in predictive modeling. These insights provide a robust foundation for the design of ILs with tailored electrochemical performance in high-voltage rechargeable batteries.