Dr. Andrea Balducci "Strategies for the development of high performance supercapacitors"

Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, are today advancing as one of the most promising energy storage technology. In EDLCs the charge is electrostatically stored at the electrode-electrolyte interface and, because of this storage mechanism, these devices can be charged and discharged within seconds. Currently, the commercial available EDLCs contain activated carbon as active material and quaternary ammonium salts in propylene carbonate (PC) or acetonitrile (ACN) as electrolyte. These EDLCs have operative voltages in the order of 2.7 – 2.8 V, they display high power (up to 10 kW kg^-1) and an extremely high cycle life (>500,000). Because of these characteristics, these systems are nowadays conveniently used in a large number of applications where rapid charge-discharge capability and reliability are required.

The specific energy of EDLCs is defined by the equation E=1/2CV2, where C and V are the capacitance and operative voltage of the EDLC, respectively. Considering this expression, it is obvious that the in order to improve the performance of DLCs the enhancement of the operative voltage is the most effective way to increase the energy of EDLCs. Taking into account the charge-discharge storage mechanism of these devices, it appears evident that, in order to realize systems with high operative voltage, the use of electrolytes with wider electrochemical stability windows (ESWs) compared to the state-of-the-art electrolytes is necessary. For this reason, several types of electrolytes have been proposed in the last years as alternative to conventional electrolytes for the realization of high voltage EDLCs.

Generally, the alternative electrolytes proposed so far can be divided in two main categories: ionic liquid (IL) and organic solvent based electrolytes. Regarding IL based electrolytes, several studies showed that by using ILs as electrolytes, EDLCs with an operative voltage as high as 3.5 - 3.7 V and high cycling stability can be realized. However, because of the relative high viscosity of these electrolytes, the performance of these IL-based EDLCs is often limited at room temperature (RT). Particularly, the equivalent series resistance (ESR) of these devices is considerably higher than that of EDLCs with conventional electrolytes, limiting their power at RT. Regarding organic solvent based electrolytes, some groups proposed the use of mixture of low-viscosity linear carbonates and PC for the realization of EDLCs with an operative voltage as high as 3V. In parallel, new types of solvents have been proposed, e.g. sulfone based electrolytes. Using these electrolytes it is possible to realize EDLCs with an operative voltage of 3.3 V, high performance and high cycling stability. These reports show that IL based electrolytes allow higher operative voltages with respect to organic solvent based electrolytes whereas latter are superior for the realization of systems with low ESR at RT due to their lower viscosity. Therefore, electrolytes enabling high operative voltages like IL based electrolytes while displaying viscosities close to that of conventional electrolytes are extremely attractive for the realization of high voltage and high performance EDLCs.

In this presentation, the development of high voltage EDLCs containing two different types of electrolytes will be discussed. The first type of electrolyte is based on a mixture of PC and the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI). The second type of electrolyte considered in this presentation is based on the solvent adiponitrile (AND). The use of both these electrolytes allows the realization of EDLCs able to display at RT an operative voltage of 3.5 V. Moreover, since the influence of the carbon properties on the performance of ADN-based EDLCs has not been investigated in detail, the development of carbon tailored for this promising electrolyte will be also considered.
 

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