Fluorinated Aromatic Diluent Enabling Stable and Non-flammable Phosphate Electrolytes for Sodium Metal Batteries

The development of non-flammable electrolytes for sodium metal batteries (SMBs) presents a promising path toward enhancing energy density and safety, especially for large-scale grid storage applications. While high-concentration phosphate electrolytes have been reported to mitigate side reactions with sodium metal, their high viscosity, poor wettability, and high cost hinder practical implementation. To overcome these limitations, we introduce 1,2-difluorobenzene (DFB) as a weakly solvating functional diluent to achieve localized high-concentration electrolyte effects in a trimethyl phosphate (TMP)-based electrolyte system. Contrary to the conventional assumption that diluents are electrochemically inert, we show that DFB participates in interfacial reactions and contributes to the formation of stable electrode interphases. Combined spectroscopic and surface analyses reveal an anion-enriched solvation structure and the formation of a thin, homogeneous interphase, including a bilayer solid electrolyte interphase (SEI) on sodium metal and a compact cathode electrolyte interphase (CEI) on the cathode. Consequently, the NaFSI-TMP-DFB electrolyte delivers an average coulombic efficiency of 98.95% in Na||Cu cells, and Na||Na₃V₂(PO₄)₃ full cells retain 85% of their capacity over 250 cycles at C/2. This work highlights the potential of functional diluents for enabling stable and inherently safe SMB electrolytes.