(Invited) New Brands of High Energy Electrochemical Capacitors with Hybridized Carbon Electrodes
Authors: Beguin Francois , Przygocki Patryk , Pan Xuexue , Chojnacka Agnieszka 
Published: December 2020

The main disadvantages of electrical double-layer capacitors – EDLCs, are the relatively low amount of energy stored and the high cost of the devices. For new applications, as powering of e.g., electric vehicles, the concept of hybrid capacitor (HC) appears as a promising approach enabling to enhance the energy output. HCs behave as EDLC (e.g., linear galvanostatic charge/discharge, rectangular cyclic voltammogram, low time constant), while combining capacitive and battery-like electrodes. Since the battery-type electrode operates in a narrow potential range, the potential excursion of the EDL electrode is almost twice larger than in a symmetric EDLC, leading to approximately doubling of the discharge capacitance [1]. Hence, for the same voltage range, the specific energy of the HC is twice higher than for an EDLC. In the last decade, HCs using a redox active electrolyte such as the iodine/iodide system [2] and lithium-ion capacitors (LIC) [3] have emerged as very promising technologies, essentially owing to the fact that carbon electrodes are highly conductive and relatively inexpensive.

LICs implement a lithiated graphite negative electrode operating ca 100 mV above the lithium deposition potential and an activated carbon positive electrode using a wide potential range, allowing the charge storage capacity to be approximately doubled, while the maximum cell voltage can reach 4 V. Hence, the specific energy of a LIC is around 4 to 5 times higher than in an EDLC using the same AC at comparable specific power. However, as it is mandatory to pre-intercalate lithium in the negative graphite electrode before operating a LIC, it has been proposed to include an auxiliary metallic lithium electrode [4], what complicates the cell construction, while excess lithium might cause also safety issues. Therefore, we have recently implemented a new strategy based on a sacrificial lithiated material (for example 3,4-dihydroxybenzonitrile dilithium salt – Li2DHBN [5]) added in the positive AC electrode, and from which lithium is irreversibly extracted and intercalated in the graphite anode. In case of Li2DHBN, we have demonstrated that the DOBN oxidized derivative is soluble in the electrolyte, leaving only AC at the positive electrode after lithium extraction. At 1 kW kg-1, the specific energy of the LIC reaches 50 Wh kg-1 against only 12 Wh kg-1 for the EDLC built with the same AC. We will show that this concept can be extended to high performance and low cost sodium-ion capacitors [6].

For AC-based hybrid capacitors realized by the implementation of redox active electrolytes, aqueous solutions of iodides have lately prevailed over others, owing to well-separated EDL and battery-like charging mechanisms [1]. However, as the voltage of AC/AC cells with alkali iodides is limited to only 0.8-1.2 V [2], we have proposed to use bi-functional aqueous electrolytes including the redox component (e.g., KI) and a supporting neutral electrolyte (e.g., Li2SO4), enabling to extend the potential range of the negative electrode. The capacitance of the HC is doubled as compared to a typical EDLC, whereas the voltage can reach 1.5 – 1.6 V [7]. Recently, an AC/AC cell based on a mixture of choline iodide and choline nitrate demonstrated the same energy and power performance as a commercial EDLC using an organic electrolyte, while being able to perform down to -40°C [8].

Acknowledgements: The Foundation for Polish Science is acknowledged for financially supporting the HYCAP project (research grant TEAM TECH/2016-3/17).

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