Cation-aromatic nitrogen interaction as a design principle for sodium deep eutectic electrolytes

Abstract

We report a combined experimental and ab initio molecular dynamics investigation into the role of nitrogen hybridization (sp2 vs sp3) and ring aromaticity in the sodium-based deep eutectic electrolyte formation and their potential use in sodium-ion battery electrolytes. Mixtures of NaFSI with three amines at 1:4 molar ratio reveal a sharp dependence on amine electronic structure: aromatic 1,2-dimethylimidazole (DMIm) and 1-methylimidazole (MIm) readily dissolve NaFSI, whereas non-aromatic N-methylpyrrolidine (MPy) leads to phase separation. AIMD simulations of NaFSI/DMIm, NaFSI/Im, and NaFSI/MPy demonstrate that in aromatic systems Na+ forms directional coordination with the sp2 lone pair of the imidazole –N=C– nitrogen, while in MPy the sp3 nitrogen fails to stabilize Na+, leaving it ion-paired with FSI− and driving salt clustering. Composition-dependent AIMD of NaFSI/DMIm (1:1, 1:4, 1:8) shows a monotonic exchange of Na…O for Na…N contacts as DMIm content increases, with coordination numbers confirming a genuine enthalpic preference for imidazole nitrogen over FSI oxygen. The NaFSI/DMIm (1:4) electrolyte exhibits moderate viscosity (49.3 cP) and ionic conductivity (1.24 ± 0.02 mS cm−1), with Walden plot analysis showing a higher ionicity than for most conventional DES. These results identify the sp2 hybridization of the coordinating nitrogen as the key design principle for deep eutectic electrolytes formation, providing a guide for the rational design of safer and efficient sodium-ion battery electrolytes.