Synthesis and Application of an Aromatic Sulfonate Sodium Salt for Aqueous Sodium‐Ion Battery Electrolytes

The two-step synthesis of sodium (2,3,5,6-tetrafluorophenoxy) diethane sulfonate (Na-TFP) is reported, starting from 2,3,5,6-tetrafluorohydroquinone as the precursor. Compared with conventional aqueous electrolytes such as 0.5 m NaClO$_4$ and Na$_2$SO$_4$, the 0.5 m aqueous solution of Na-TFP provides higher ionic conductivity over a wide range of temperature (e.g., >60 mS cm$^{−1}$ at 20 °C and >70 mS cm$^{−1}$ at 30 °C) and a wider electrochemical stability window. Electrochemical cells with Na$_2$VTi(PO$_4$)$_3$ as the positive electrode active material and a carbon-based negative electrode provide a stable capacity for more than 450 cycles in 0.5 m Na-TFP. Additionally, symmetric cells with Na$_2$VTi(PO$_4$)$_3$ as the active material for the negative and positive electrode are studied. The cells employing 0.5 m Na-TFP exhibit a good cycling stability at a high dis-/charge rate of 5C for more than 50 cycles, which is superior to the performance of the cells employing 0.5 m aqueous solutions of NaClO$_4$ and Na$_2$SO$_4$ as the electrolyte. The design of this bifunctional salt may trigger new ideas for the development of water-based sodium-ion battery electrolytes.