Anion‐Synergistic Regulation of Solvation‐Layer Chemistry in Nonflammable Electrolytes Enables Highly Safe and Durable Sodium Metal Batteries

Uncontrolled dendrite growth, accompanied by thermal runaway in sodium metal, remains a critical bottleneck for practical applications of sodium metal batteries (SMBs). Here, we develop a phosphate‐ester‐based electrolyte and further reconstruct Na ⁺ solvation environment by introducing FSI ⁻ into an initially single‐anion solvation shell. This design overcomes limitations of conventional single‐anion‐derived interphases, enabling intrinsic nonflammability as well as uniform Na deposition. The electron‐rich, multi‐atom sites of FSI ⁻ confer strong coordinating ability, facilitating effective cooperation with to construct a stable − Na ⁺ − FSI ⁻ solvation shell and induce layered ion‐rich and ion‐poor domains. Multiple characterization techniques confirm that anion‐synergistic solvation chemistry drives in situ formation of a composite interphase enriched with NaF and Na ₃ N on the Na anode surface, which homogenizes ion flux, enhances ion transport, and effectively suppresses dendrite growth. The Na ₃ V ₂ (PO ₄ ) ₃ ||Na full cell exhibits 90.5% capacity retention after 1700 cycles at 2 C and 90.7% retention after 500 cycles at 5 C under 60 °C, demonstrating excellent interfacial and thermal stability. Overall, this study offers a general framework for anion‐synergistic regulation of interfacial chemistry toward intrinsically safe and high‐temperature‐tolerant SMBs.