Improving Electrolyte Sustainability for Sodium‐Ion Capacitors by Combining a Bio‐Based Solvent With a Low‐Fluorine Salt

This work focuses on improving the sustainability of electrolytes for sodium‐ion capacitors (SICs). Through the combination of a low‐fluorinated salt, namely sodium difluoro(oxalato)borate (NaDFOB), and the bio‐based solvent γ ‐Valerolactone (GVL), a new electrolyte formulation (1 mol L −1 NaDFOB in GVL) is being studied for application in SICs. Remarkably, the performance of the SIC full‐cells is very comparable to the most commonly used formulation of sodium hexafluorophosphate in ethylene carbonate:propylene carbonate (1 mol L −1 NaPF 6 in EC:PC). Furthermore, presodiation strategies were compared for the novel electrolyte system. The in situ oxidation of a sacrificial salt (sodium squarate, Na 2 C 4 O 4 ) incorporated into the positive electrode yielded comparable results to the ex situ electrochemical approach. X‐ray photoelectron spectroscopy studies revealed that depending on the presodiation strategy, the solid‐electrolyte‐interphase composition varies significantly.