Impact of additives on stability of high-voltage sodium-ion capacitors containing NaTFSI-based electrolytes

Abstract A key objective for the sodium-ion capacitors (SICs) is to increase the energy density while maintaining their inherent high-power capability, which can be achieved by widening the operating voltage window. Imide-based salts such as sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) offer good chemical and thermal stability over conventional sodium hexafluorophosphate (NaPF 6 ) based systems. However, at elevated potentials, these imide-based electrolytes trigger pitting corrosion of aluminium current collector (anodic dissolution), limiting voltage expansion. Herein, we employ sodium difluoro(oxalato) borate (NaDFOB) and fluoroethylene carbonate (FEC) as functional additives to address the issue. The optimised formulation enables high-voltage SIC operating from 1 V to 4.2 V, extending the upper cutoff potential by 400 mV over the additive free system. This device delivers higher energy density and good stability, retaining ~65 % of its initial capacity after 100 h of floating at elevated potential of 4.2 V. This work demonstrates a tailored electrolyte design that effectively overcomes voltage limitations in electrochemical energy storage systems when employing imide-based electrolytes.