Theoretical Investigation of the Na⁺ Transport Mechanism and the Performance of Ionic Liquid-Based Electrolytes in Sodium-Ion Batteries

Ionic liquids are a promising alternative to common organic electrolytes in sodium-ion batteries that offer a unique combination of physical and chemical properties, leading to the development of high-performance batteries. To improve our atomistic understanding of the Na+ transport mechanism, aggregation effects, and electrolyte performance, we report a theoretical investigation based on the combination of classical molecular dynamics simulations based on the CL&P force field and density functional theory calculations for 25 different ionic liquids (ILs), which includes anions based or related to the [Tf2N]− anion, while the cations are based on the imidazolium and ammonium-based ones. From our molecular dynamics (MD) simulations and analyses, we found that the Na+ aggregation is a result of multiple Na+–anion interactions in the systems and the consequent improvement of the Na+ transport number due to the hopping diffusion mechanism. Although all studied systems have shown X[anion]x–y aggregates, there is a direct correlation between the anion structure and the size of the aggregates, in which flexible anions with a large degree of charge delocalization yield larger aggregates. The electrochemical windows estimated from first-principles calculations exceed 4.0 eV, which indicates the good performance of the systems for electrolyte applications.