All-Fluorinated Low-Solvation Electrolytes for High-Voltage Sodium Metal Batteries with Appealing Stability

High-voltage sodium-metal batteries (SMBs) are considered a promising battery system due to their abundant sodium source and high energy density. However, the instability of electrolytes, notorious reactivity of Na metal, and severe dissolution of solid electrolyte interface (SEI) film impede their long lifespan and high safety. Herein, a low-solvation modulating strategy is proposed to design an all-fluorinated electrolyte consisting of 1 m NaPF6 in fluoroethylene carbonate (FEC), methyl 2, 2, 2-trifluoroethyl carbonate (FEMC), and lithium difluoro(oxalato)borate (LiDFOB), realizing high working voltage and excellent cycling stability of SMBs. Theoretical calculations and experimental characterizations confirm that the presence of FEMC significantly improves the antioxidant property of the electrolyte and contributes to the formation of high-quality interfacial phases on both anode and cathode. While LiDFOB benefits to optimize the energy bandgap and the solvated structure and stabilizing the FEMC. Consequently, excellent oxidative stability up to 5 V and ultra-stable cycling of Na||Na3(VOPO4)2F battery (with a capacity retention of 82.9% after 1800 cycles at 4.4 V) are obtained, exhibiting superior stability to its counterparts. The proposed strategy provides a new insight to design SMBs operated at high voltages. The all-fluorinated low-solvation system is constructed by low polarity solvent FEMC and LiDFOB additive, realizing the regulation of electrochemical window, solvation structure, and electrode-electrolyte interphase. High oxidation resistance is guaranteed and the dissolution of SEI is suppressed, contributing to the ultra-stable cycling of sodium-metal batteries at high-voltages. image