Understanding the Molecular-Level Structure and Dynamics of Sodium Ions in Water in Ionic Liquid Electrolytes by Molecular Dynamics Simulations

In this article, we perform all-atom molecular dynamics simulations to investigate the structure and intermolecular interactions of Na+ ions in water-in-ionic liquids. We have considered two ionic liquid systems with the same cation and different anions as (1) 1-benzyl-3-methyl imidazolium [ZMIM+] tetrafluoroborate [BF-4] and (2) [ZMIM+] hexafluorophosphate [PF6-]. We have analyzed the inter-molecular structure and dynamics of Na+ ions in the ionic liquid electrolyte with an increase in water (H2O) mole fraction (x) ranging from 0.33 to 0.71. The neat ionic liquid [ZMIM+][BF4-] and [ZMIM+][PF6-] coordination structure shows a decrease, with an increase in x. We have observed that Na+ ions show significant interaction with ionic liquid [ZMIM+][PF6-] when compared to [ZMIM+][BF4-] over the entire range of x considered. The coordination structure of Na+-[PF6-] is more significant when compared to the coordination structure between Na+ and [ZMIM+]. The Na+ ions coordinated more with H2O and showed a higher intensities in the presence of [ZMIM+][PF6-] when compared to [ZMIM+][BF4-]. The diffusion of Na+ ions showed an increase in both the electrolyte solutions with the rise in x. The faster diffusion of Na+ is seen in the presence of [ZMIM+] [BF4-]. The ionic conductivity of ionic liquids is higher for [ZMIM+][BF4-] when compared to [ZMIM+][PF6-]. Overall, this article provides detailed molecular-level insights into Na+ ions in the presence of water in ionic liquids.