Phosphonate-based electrolytes with the merits of low cost and intrinsic nonflammability are promising candidates to realize the safe operation of sodium-ion batteries. However, they generally suffer from poor interfacial chemistry because of the solvent-dominated solvation structure induced by the strong ion-dipole interactions between cations and phosphonate molecules. Herein, we report an electrolyte design strategy that selectively improves the competitive coordination of low-solvating-power molecules, achieving stable interfacial chemistry with a non-flammable, low-cost and fluorine-free electrolyte. By improving the ion-ion interaction between cation and anion, weakly coordinated molecules can enter the Na