The energy storage devices show a rapid rise due to global technological development and increased demand for energy storage devices. The current Lithium-ion batteries can be found in various applications due to their outstanding properties. The alternating metal ion batteries shown a progressive rise due to the limited lithium resources as well as due to the rise in the demand of the battery market, limited resources of Lithium. The Na+ion batteries have been considered sustainable, and alternating energy storage devise effective alternatives for Li+ion battries [1,2]. Because the Na+ and Li+ possess an electron that is loosely paired in its valence shell, which can further form a first oxidation [3]. On the other hand there is only very small difference exist on the standard hydrogen electrode values given as -3.04 V for Li+ and -2.71 V for Na+ [4]. However, there are limitation exists concerning Na+-ion batteries such as ionic radii, molar mass, low power density and energy density when compared to Li+-ion batteries. Therefore, there is a significant need for the development of cathode and anode materials, separators, and electrolytes to achieve highly efficient Na-ion batteries for large-scale energy storage applications.

The electrolyte plays an important role in designing high-performing Na+ ion batteries [5]. Various properties such as energy density, Coulombic efficiency, and cyclic stability for Na+ ion batteries are influenced by the electrolyte [6]. Among the various electrolyte studies, aqueous electrolyte presents good attributes such as eco-friendly, lower cost, and safer to operate. Ponrouch et al. [7] presented the use of based on the optimization of the viscosities and ionic conductivities of the mixture of organic electrolytes such as ethylene carbonate, propylene carbonate, and dimethyl carbonate, present a high energy density of 78 Whkg−1. On the other hand, various organic electrolytes have been used for the Na+ ion batteries such higher cell voltage being. 3.7 V [8]. Michalska et al. [9] presented 4,5-dicyano-2-(pentafluoroethyl) imidazolate sodium salt and 2-(triflouroethyl)imidazolate sodium salt presets the higher ionic conductivities at room temperature. However, the organic electrolytes possess limitations such as high volatility and high flammability.

The Ionic liquids (ILs) possess the promising class of electrolytes for the Na+ ion batteries due to their low volatility, low flammability, higher temperature stability. Ding et al. [10] have utilized the mixture of Na-Bis fluorosulfonyl amide and N-methyl-N propylpyrrolidinium ILs, which have shown temperature stability in the range of 253–363 K. On the other hand, an increase in Na ion concentration leads to an increase in viscosity and a decrease in Ionic conductivities. However, ionic conductivities values are higher when compared to conventional organic electrolytes [11]. Andreeva & Chaban [12] presents the molecular dynamics simulation on four different room temperature ILs as electrolytes for the salts of sodium tetrafluoroborate (NaBF4) and sodium nitrate (NaNO3). Our recent work [13], also presents the Na+ ion structure and dynamics in butyl ammonium hydrogen bisulfate and tri butyl ammonium hydrogen bisulfate ionic liquids, it is observed that alkyl chain on the cation favorably influences the Na+ ion solvation. The intermolecular structure and hydrogen bonding interactions between cation and anion play a major role in understanding the Na+ ion structure and dynamics.

Balducci et al. [14,15] have demonstrated water in protic ILs as the fascinating class of electrolytes for energy storage applications. The addition of water in ILs can greatly enhance the intermolecular structural properties of ILs and water and thereby influence the metal ions present in the systems [[15], [16], [17]]. Feng et al. [17] have utilized two ILs 1-methyl-3-butyl-Imidazolium hexafluoro phosphate [BMIM] [PF6] and [BMIM] bistriflimide [Tf2N] for the water in ILs at the electrified interfaces, showed the enrichment of water near the electrode surface with the rise in surface charge density. Ferdousi et al. [18] have utilized water as an additive for the super concentrated IL electrolytes for the Na ion batteries, which presents the enhanced ionic conductivity for the N−methyl-Npropylpyrrolidinium bisfluorosufonyl amide ILS. Recent work from our group [19] performed MD simulation of water in 1-benzyl-3-methyl imidazolium [ZMIM] [BF4] and [ZMIM] [PF6] for understanding the Na ion structure and dynamics, we found that enhanced ionic conductivities and Na ion diffusion with the rise in water concentration. While very few studies exist to understand the structure and dynamics of Na ions in water-mixed ionic liquids, there are a few issues that remain open. For instance, what are the different driving forces that govern the overall development of Na+ ion batteries, such as role electrolytes, and what are their intermolecular structural and dynamic properties?

In this manuscript, we perform MD simulations on the understanding of the role of two water in IL electrolytes considering ILs as [BMIM] [BF4] and [BMIM] [PF6] with an increase in water content. We present various intermolecular and inter-atomic structural properties, diffusion, and ionic conductivities.