Interacted Ternary Component Ensuring High‐Security Eutectic Electrolyte for High Performance Sodium‐Metal Batteries

Abstract Due to the intrinsic flame‐retardant, eutectic electrolytes are considered a promising candidate for sodium‐metal batteries (SMBs). However, the high viscosity and ruinous side reaction with Na metal anode greatly hinder their further development. Herein, based on the Lewis acid‐base theory, a new eutectic electrolyte (EE) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), succinonitrile (SN), and fluoroethylene carbonate (FEC) is reported. As a strong Lewis base, the ─C≡N group of SN can effectively weaken the interaction between Na + and TFSI − , achieving the dynamic equilibrium and reducing the viscosity of EE. Moreover, the FEC additive shows a low energy level to construct thicker and denser solid electrolyte interphase (SEI) on the Na metal surface, which can effectively eliminate the side reaction between EE and Na metal anode. Therefore, EE‐1:6 + 5% FEC shows high ionic conductivity (2.62 mS cm −1 ) and ultra‐high transference number of Na + (0.96). The Na||Na symmetric cell achieves stable Na plating/stripping for 1100 h and Na||Na 3 V 2 (PO 4 ) 3 /C cell shows superior long‐term cycling stability over 2000 cycles (99.1% retention) at 5 C. More importantly, the Na||NVP/C pouch cell demonstrates good cycling performance of 102.1 mAh g −1 after 135 cycles at 0.5 C with an average coulombic efficiency of 99.63%.