Synergistically Competitive Coordination for Modulating Electrolyte Solvation Structures Toward High-Performance Low-Temperature Sodium Metal Batteries.

Abstract

With low melting points and viscosities, linear ether-based solvents effectively lower the Na⁺ desolvation energy barrier in low-temperature sodium metal batteries. Among them, 1,2-diethoxyethane (DEE) is considered a promising solvent due to its relatively weak solvating ability at low temperatures; however, its two oxygen atoms remain electronically isolated, forming quasi-chelating bidentate coordination structures with Na⁺ and still triggering a high desolvation energy barrier under extremely cold conditions. Herein, a novel electrolyte based on the concept of synergistic-competitive coordination is designed by introducing dimethoxymethane (DMM) as a cosolvent into the DEE-based electrolyte, where the lone-pair electrons on oxygen atoms in DMM are partially delocalized, thus reducing its electron-donating capability toward Na⁺ and reconstructing the Na⁺ solvation structure. Molecular dynamics simulations reveal that DMM competes with DEE for Na⁺ coordination sites, thereby weakening the Na⁺-DEE interaction, lowering the desolvation energy barrier, and promoting anion-involved coordination under severe cold conditions. Consequently, Na||Na symmetric cells run stably for over 3500 h at -40°C, while Na||Cu cells show 99.7% coulombic efficiency over 200 cycles at -20°C. Moreover, NaFe_1/3Ni_1/3Mn_1/3O₂||Na full cell retains 78.7% capacity after 200 cycles at -20°C, while Na₃V₂(PO₄)₃||Na full cell maintains an impressive 99.2% reversible capacity over 300 cycles at -40°C.