Molecular Dynamics Simulations for Understanding the Structure and Dynamics of Na+ Ions in Water-Mixed Ionic Liquid Electrolytes: The Role of Anions

Abstract

In this paper, we have performed all-atom molecular dynamics simulations to understand the structure, dynamics, and thermodynamic behavior of Na⁺ ions in water-mixed ionic liquids. We have considered seven different combinations of water-mixed ionic liquids having common cation 1-ethyl-3-methylimidazolium [EMIM⁺] along with seven different anions such as acetate [ACT]⁻, formate [FRM]⁻, trifluoromethyl-sulfonate [TFS]⁻, benzoate [BEZ]⁻, nitrate [NO₃]⁻, hexafluorophosphate [PF₆]⁻, and tetrafluoroborate [BF₄]⁻. Two different water mole fractions (x) are considered: 0.55 to 0.71. Various structural and dynamic properties are investigated such as radial distribution functions, ion self-diffusion coefficients, and ionic conductivity. We understand that hydrophilic anions interact more with water, which would lead to enhanced mobility of the Na⁺ ions in the neat IL [EMIM]⁺ [TFS]⁻ that presents higher ionic conductivity values; on the other hand, water-mixed IL presents higher conductivity values for the [EMIM]⁺ [BF₄]⁻ and [EMIM]⁺[NO₃]⁻. The self-diffusion coefficient values of Na⁺ ions present higher values in [EMIM]⁺ [BF₄] and [EMIM]⁺ [NO₃]⁻ in water-mixed ILs compared to other ionic liquid–water combinations. Overall, the results presented in this manuscript will help in understanding the molecular-level behavior of imidazolium-based electrolytes for battery applications.