An investigation about the use of mixtures of sulfonium-based ionic liquids and propylene carbonate as electrolytes for supercapacitors

Abstract

This study describes the use of mixtures of the ionic liquid (IL) trimethyl-sulfonium bis[(trifluoromethyl)sulfonyl]imide (Me₃STFSI) and propylene carbonate (PC) as electrolytes for carbon-based electrochemical double layer capacitors (EDLCs). Two different mixtures have been investigated. The first one contained the maximum amount of Me₃STFSI (3.8 mol L⁻¹) soluble in PC at room temperature (RT). The second one contained 1.9 mol L⁻¹ of Me₃STFSI, which is the concentration featuring the highest conductivity among all possible mixtures between PC and Me₃STFSI at RT. The physicochemical properties, including conductivity, viscosity, and electrochemical and thermal stability of both mixtures, have been initially investigated. Afterward, the use of these mixtures as electrolytes for EDLCs has been considered. This study showed that by using these innovative electrolytes, it is possible to develop EDLCs with an operative voltage as high as 3.0 V. It was clearly demonstrated that the EDLC cycle life strongly depends on the concentration of Me₃STFSI present in the mixture. Moreover, it was also proved that mixtures containing high concentrations of IL are able to suppress anodic oxidation of the Al current collector at high potential. When a solution with a high salt concentration is used, an EDLC with high energy, high cycle life and a broad temperature range of operation can be realized.