“Double-Salt” Electrolytes for High Voltage Electrochemical Double-Layer Capacitors

Abstract

The electrolyte is the key component in high-voltage electrochemical double-layer capacitors (EDLCs). Ionic liquids (ILs) represent one of the most promising classes of electrolytes for the development of these devices. However, as a consequence of their high viscosity, ionic-liquid based EDLCs usually display a relatively low power performance compared to those based on conventional electrolytes (propylene carbonate or acetonitrile based ones), especially at and below room temperature. We showed in previous work that the use of mixtures of ILs and organic solvents as electrolyte for high voltage EDLCs appears to be a promising strategy to overcome the power limitations of IL-based EDLCs, as the increased operative voltage of the latter can be retained in their mixtures with such solvents. However, the use of bis(trifluoromethanesulfonyl)imide-based ILs in these mixtures introduced a new obstacle, namely the limited oxidative stability of the Al current collector of the positive electrode in the presence of conventional solvents. Hence, in this work, we suggest using double-salt electrolytes containing a second salt with a $$ {\text{BF}}_{4}^{ - } $$ or $$ {\text{PF}}_{6}^{ - } $$ anion. These anions are known to effectively passivate Al and thus to prevent its anodic dissolution. These double-salt electrolytes allow the development of EDLCs with good power performance and higher operative voltage than that of conventional EDLCs, while overcoming the stability issue of the Al current collector.