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 NaPF₆ in EC:PC). Furthermore, presodiation strategies were compared for the novel electrolyte system. The in situ oxidation of a sacrificial salt (sodium squarate, Na₂C₄O₄) 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.