Molecular simulations of understanding the Na ion structure, dynamic and thermodynamic behavior in ionic liquids: Butyl ammonium hydrogen bisulfate and tri-butyl ammonium hydrogen bisulfate.

This manuscript presents the all-atom molecular dynamics simulations to investigate intermolecular structure and solvation thermodynamics of Na ion in two different ammonium-based protic ionic liquids (1) Butyl Ammonium hydrogen bisulfate [BA][HSO], (2) Tri-butyl ammonium hydrogen bisulfate [TBA][HSO]. The ionic liquid [BA][HSO] show a more coordinated behavior when compared to [TBA][HSO], which is observed over the temperature range from 278 K to 348 K. Hydrogens of the cations show a hydrogen bonding interaction with oxygens of anions. The cationic [TBA] molecules show more solvation behavior with anions when compared to the [BA]. The Na ion show a strong coordination structure with [HSO] in [TBA][HSO] when compared to the [BA][HSO]. We further calculate the detailed solvation free energy (ΔG) calculations using thermodynamic integration. We found that the ΔG of Na is more favorable in [TBA][HSO] when compared to [BA][HSO] in the temperature range varying from 278 K to 348 K. With the temperature rise, we observe the more favorable solvation of Na in both ionic liquids. On the other hand, the solvation of Cl becomes less favorable. Overall, this manuscript provides detailed molecular level structural and thermodynamic origins of Na in protic ionic liquids useful for designing and developing sustainable electrolytes for Na battery applications.