Toward Sustainable Sodium-Ion Capacitors: Investigating Imide-Based Electrolytes for Enhanced Performance and Environmental Safety

To date, sodium hexafluorophosphate (NaPF 6 ) in carbonate solvents is the state-of-the-art electrolyte in sodium-ion capacitors (SIC). However, it possesses serious safety issues owing to its high fluorine content, poor chemical and thermal stability, resulting in the formation of corrosive and hazardous byproducts. For this reason, its replacement with less fluorinated salt is considered of great importance for the development of sustainable SICs. In this work, we investigate the use of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI), which are more sustainable and contain less mass percentage of fluorine compared to NaPF 6 (68 wt% F in NaPF 6 > 38 wt% F in NaTFSI > 19 wt% F in NaFSI). The chemical-physical properties of electrolytes containing these salts dissolved in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) is studied and compared to that of the state-of-art electrolyte. The use of these electrolytes in combination with hard carbon (HC) and activated carbons (AC) electrodes, in lab-scale SIC is thoroughly analysed. SICs containing these imide-based electrolytes exhibit far superior long-term stability compared to the state-of-the-art system. Furthermore, the degradation processes occurring in these innovative devices are investigated by X-ray photoelectron spectroscopy (XPS). The result of this study indicates that it is possible to realise high performance SICs containing low fluorinated salts.