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

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 Na₃V₂(PO₄)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.