Modulating ion–dipole interactions in nonflammable phosphonate-based electrolyte for safe and stable sodium-ion pouch cells

ABSTRACT 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+ solvation shell, thereby promoting more adjustable and advantageous interfacial chemistry. As a result, the fluorine-free Prussian blue||hard carbon pouch cell, with a high cathode mass loading of ∼20 mg cm−2, reaches a high capacity retention with an energy density of over 221.7 Wh kg−1 based on electrode mass and 115.1 Wh kg−1 based on battery mass.