Delicately Tailored Ternary Phosphate Electrolyte Promotes Ultrastable Cycling of Na₃V₂(PO₄)₂F₃-Based Sodium Metal Batteries

High-voltage sodium metal batteries are a highly intriguing battery technology in view of their resource sustainability, cost efficiency, and ultrahigh energy density. However, developing a high-performance electrolyte, compatible with both high-voltage cathodes and highly reactive sodium metal anodes, is extremely challenging. In this work, we delicately formulate a ternary phosphate electrolyte, composing of a cost-effective sodium bis(trifluoromethane sulfonyl) imide salt, a nonflammable triethyl phosphate (TEP) solvent, and a fluoroethylene carbonate (FEC) co-solvent. By rationally tailoring the TEP/FEC ratio, the ternary phosphate electrolyte displays a well-balanced performance, not only enabling highly efficient sodium deposition (an average Coulombic efficiency of 95.7% for Na//Cu cells) but also inheriting the intrinsic anodic stability (≥4.5 V vs Na+/Na) and nonflammability of phosphates. As a consequence, high-voltage Na3V2(PO4)2F3 cathode-based sodium metal cells (Na3V2(PO4)2F3//Na) deliver remarkable cyclic stability (97.9% capacity retention after 300 cycles), which is among the best for Na3V2(PO4)2F3-based batteries. This work may guide the electrolyte design principles and is highly enlightening in developing high energy density sodium-based batteries.