In silico navigation of Na+ ion solvation in deep eutectic solvents to propel the next wave of sodium-ion battery electrolytes

Abstract

Sodium-ion batteries (SIBs) are increasingly recognized as viable, cost-effective alternatives to lithium-ion batteries (LIBs), particularly for large-scale stationary energy storage. Deep eutectic solvents (DESs) have emerged as promising electrolyte media, offering key advantages such as low cost, high ionic conductivity, and improved electrochemical stability compared to conventional organic solvents. In this study, we combine molecular dynamics (MD) simulations and density functional theory (DFT) calculations to systematically investigate the solvation behavior of sodium hexafluorophosphate (NaPF₆) and sodium bis(trifluoromethanesulfonyl)imide (NaTf₂N) salts in five representative DESs, composed of cholinium chloride [ChCl] as the HBA and urea ([U]), ethylene glycol ([Eg]), glycerol ([Gly]), acetamide ([AC]), or 2,2,2-trifluoroacetamide ([TFA]) as hydrogen bond donors (HBDs), denoted as [ChCl:U], [ChCl:Eg], [ChCl:Gly], [ChCl:AC], and [ChCl:TFA]. Our results reveal that Na⁺ ions are stabilized within NaPF₆@[DES] and NaTf₂N@[DES] clusters via a dense network of noncovalent interactions, confined to the first coordination shell (FCS). Atoms-in-molecules (AIM) and noncovalent interaction (NCI) analyses demonstrate the predominantly electrostatic nature of these interactions, while natural bond orbital (NBO) analysis highlights Cl⁻⋯Na⁺ and HO/CO⋯Na⁺ interactions as key charge-transfer pathways. Natural population analysis (NPA) identifies [ChCl:TFA] as the most effective electron donor, transferring up to 0.415e to Na⁺ in NaPF₆@[ChCl:TFA]. Thermochemical analyses confirm the spontaneous and exothermic formation of the clusters, underscoring their inherent stability. Our results indicate that [ChCl:TFA] demonstrates the most favorable ΔG values in the gas phase, −320.17 kcal mol⁻¹ for NaPF₆ and −290.61 kcal mol⁻¹ for NaTf₂N. In the solution phase across a broad range of dielectric constants, the NaPF₆@[ChCl:Eg] and NaTf₂N@[ChCl:TFA] clusters exhibit superior solubility relative to other clusters, with ΔG_(sol) values spanning from −36.43 kcal mol⁻¹ to −60.02 kcal mol⁻¹ and from −45.60 kcal mol⁻¹ to −74.57 kcal mol⁻¹, respectively. Moreover, our MD simulations reveal a strong correlation between solvation free energy and Na⁺ diffusion, suggesting a mechanistic link between thermodynamic stabilization and transport dynamics. These findings suggest that the highest stability and solubility are achieved when NaPF₆ and NaTf₂N salts are dissolved in [ChCl:Eg] and [ChCl:TFA] DESs, respectively. These insights position [ChCl:Eg] and [ChCl:TFA] as promising candidates for high-performance SIB electrolytes, offering an optimal balance between thermodynamic robustness and efficient ion transport.