Novel electrolytes for next-generation hybrid supercapacitors - Investigating the interaction between electrolyte and electrode

Licentiate thesis, 2018

The storage of electrical energy is of outmost importance in today’s society, ranging from cell phones to large scale energy storage of intermittent electricity sources. Batteries, that are most commonly used, struggle with low power density and limited cycle lifetime. Supercapacitors are seen as an alternative to batteries with their high-power density and almost unlimited cycle
lifetime. However, supercapacitors only use the surface of the electrode which reduces the energy content compared to batteries. The low energy density limits their use in different applications where they otherwise would have been suitable. To increase the energy density metal oxides, such as MnO2, RuO or VO2, with higher energy content than carbon are often added to the electrodes. The total energy is also proportional to the voltage window, squared, in which the device operates. The normal limiting factor of the voltage window is the electrolyte, all electrolytes breaks down if polarized to sufficiently high or low potentials. Choosing an electrolyte with a large voltage window will also increase the energy content of the cell. This thesis focuses on combining these two approaches to increase the energy density even further by investigating novel, highenergy, electrolytes and their interactions with MnO2 and VO2 based electrodes. Electrochemical measurements and physical characterization of the electrolytes are used to determine important parameters for optimal performance of the electrodes. The
results show that by using protic ionic liquids the contribution of MnO2-electrodes to the total energy content can be increased while the potential window is widened. Highly-concentrated aqueous NaTFSI electrolytes present a promising, cheap, alternative with a larger potential window compared to standard aqueous electrolytes. VO2-electrodes performs well but with a limited cycle lifetime in the NaTFSI electrolyte and have higher energy density combined with retained power density.