Synergy‐Driven Electrolyte Design: Fluoroethylene Carbonate and Succinonitrile Co‐Solvation in a Eutectic Electrolyte for Stable Sodium Metal Batteries

Abstract Sodium metal batteries (SMBs) are promising alternatives to lithium‐ion batteries due to their high energy density and abundant sodium resources. However, high reactivity of sodium metal anodes (SMAs) presents significant challenges for practical application. Herein, we report a multifunctional ternary eutectic electrolyte, termed NSF‐5, comprising sodium bis(fluorosulfonyl)imide (NaFSI), succinonitrile (SN), and fluoroethylene carbonate (FEC) in a 1:1:5 molar ratio, which enhances the cycling stability of SMAs. Combined experimental and computational studies confirm a synergistic effect between FEC and SN, where FEC integrates into the Na + solvation sheath, regulates the desolvation process, and promotes the formation of a robust, inorganic‐rich solid electrolyte interphase, thereby suppressing sodium dendrite growth. Consequently, SMBs with NSF‐5 demonstrate desirable cyclability under demanding conditions. Full cells coupled with Na 3 V 2 (PO 4 ) 3 achieve over 1650 cycles with 90.5% capacity retention at 0.5 C and 3715 cycles with 80.2% retention at 10 C. Even at 0 °C, the cell maintains 87.9 mAh g ‒1 capacity after 3500 cycles with 81.4% retention. Moreover, this versatile electrolyte formulation can be adapted to other alkali metal battery systems, underscoring its broad applicability. This work highlights the importance of electrolyte engineering in regulating the electrode−electrolyte interphase, offering valuable insights into achieving long‐term cyclability of SMBs.