Charge Storage mechanisms and interactions of hybrid supercapacitor electrode materials with next-generation electrolytes

Doctoral thesis, 2020

The storage of electrical energy is of outmost importance in today’s society for a wide range of applications. Batteries, that are most common for electrical energy storage today, struggle with low power density and limited cycle lifetime. As an alternative to batteries, supercapacitors have a high-power density and almost unlimited cycle lifetime. However, the lower energy content of supercapacitors limits their use in different applications.
 Two properties determine the energy content of supercapacitors: the capacity of the electrodes and operating voltage of the device. Metal oxides have a high capacity compared to standard carbon electrodes. In this thesis MnO2, VO2 and TiO2 are investigated together with novel electrolytes. Previously these materials have been mostly studied in standard aqueous electrolytes. Ionic liquids (ILs) is a class of novel solvents which can be more stable than aqueous electrolytes and mitigate problems associated with organic electrolytes. Another electrolyte concept receiving increasing interest is highly concentrated electrolytes (HCEs) in which the high salt concentration makes the electrolyte electrochemically stable. The electrode-electrolyte interaction is governed by the properties of the choice of electrolyte but also the morphology of the electrode
In this work I present findings that could facilitate the development of next-generation hybrid supercapacitors with improved energy density as a result of high-capacity electrodes and novel electrolytes. By choosing appropriate electrolytes a higher capacity of the electrode could be obtained together with an increased voltage window, increasing the energy density further. I also present findings regarding the morphology and structure of the electrode. Examples of new findings include the role of protic ionic liquids in the charge-storage mechanism of MnO2, which enables redox reactions in the absence of Li-ions. The mitigation of the capacity fade in TiO2 microbeads through the interaction with an ionic liquid electrolyte.