Solvent–Solvent Anchoring Decouples Non‐Flammability and Interfacial Stability in Sodium Metal Batteries

Abstract

Non‐flammable phosphate electrolytes are attractive for safe sodium metal batteries, but their strong coordination with Na + and continuous interfacial decomposition impose a fundamental safety‐reactivity trade‐off. Here, we develop a solvent‐centric molecular anchoring strategy to regulate electrolyte chemistry through solvent‐solvent interactions rather than conventional ion‐centered solvation control. Fluoroethylene carbonate (FEC) establishes directional intermolecular interactions with trimethyl phosphate (TMP), anchoring TMP within the solvent network, weakening Na + –TMP coordination and facilitating Na + desolvation. Meanwhile, preferential FEC reduction promotes the formation of thin, robust, and NaF‐rich electrode‐electrolyte interphases, suppressing continuous phosphate decomposition. Spectroscopy, molecular dynamics, and density functional theory collectively reveal the molecular origin and solvation consequences of this anchoring effect. The optimized electrolyte is completely non‐flammable and enables Na||Na 3 V 2 (PO 4 ) 3 cells to retain 83.4% capacity after 1000 cycles at 1 C, nearly 80% capacity after 3800 cycles at 5 C, and 80.2% capacity after 500 cycles at 40°C. Moreover, the anchoring effect can be extended to TEP‐based electrolytes, demonstrating its applicability beyond the TMP–FEC system. This work establishes solvent‐network engineering as a design strategy for decoupling electrolyte safety from interfacial instability in sodium metal batteries.