Important Technical Elements to Enhance Electrochemical Capacitor Performance
Authors: Ishikawa Masashi , Yamagata Masaki , Soeda Kazunari , Uchida Satoshi 
Published: February 2020

Electrochemical capacitors (ECs) are essential energy storage devices, and their revision is regarded as important goals of scientific and industrial development. Generally, ECs storage much higher energy density than conventional dielectric capacitors, and provide higher power density than lithium-ion batteries. Moreover, ECs have long cycle life (>106 cycles) that makes them maintenance-free energy storage devices. Therefore, ECs have been widely studied for various applications of electric equipment, e.g., automobiles, tramways, buses, cranes, forklifts, and wind turbines. We have to, however, improve their performances more for various advanced applications.
We found that specific power density of EDLC can be improved by applying various natural polymers gel electrolytes, e.g., alginate acid (Alg) and chitosan, as a matrix of gel electrolytes [1-4]. Moreover, we attempted to apply various natural polymers to EDLC as a binder for activated carbon electrodes, and improved their performances [5]. The cell with Alg binder shows high discharge capacitance and excellent rate capability when compared to the other cells with CMC and PVdF binders. This result suggests that the electrode with Alg has a lower internal resistance due to higher affinity between activated carbon and the natural polymer, which reduces a reaction resistance at the electrode/electrolyte interface. This property is consistent well with our previous affinity test [1].
Recently, we also applied gelatin material to binder in EDLCs. Our gelatin derivatives can keep electrode materials such as activated carbon and conductive materials steady as a important function of electrode binder. Resulting electrode films with gelatin are very strong enough to protect themselves against a manual scratching. An internal electric resistance in the gelatin-applied electrode sheets is somewhat higher than that in CMC-SBR-based electrode sheets. However, not only their mechanical strength but also their resistivity against oxidation, which we found in cell-cycling and cell-floating tests, are quite attractive. The applied electrode can be prepared in aqueous media including gelatin, conductive auxiliaries, and activated carbons. Thus the gelatin-based preparing process can provide us with environmentally friendly, safe and low-cost production for EDLC electrode sheets.
Moreover, to decrease an electronic resistance between a current collector and active materials, we applied diamond-like carbon (DLC) coating to the aluminum current-collector surface. Generally, DLC is a insulator although it can reduce a mechanical friction. Thus we doped some heteroatoms into DLC to enhance the electric conductivity. The resulting DLC layer contributes to increase power of EDLC by reducing the interfacial resistance between aluminum foil and activated carbon. This was proved by ac impedance measurements. Moreover, the DLC was found to enhance the durability and cycle ability of EDLCs under some severe conditions: typically, high-voltage operation or long-term high-voltage floating. So we successfully applied DLC to the current collector surface; this is beneficial to enhance EDLC’s power, energy and durability. We also applied a three-dimensional (3D) current collectors to both the positive and negative electrodes in lithium-ion capacitors (LICs). By utilizing such thick electrodes, we increased mass loading of activated carbon and graphite at both the electrodes, which can enhance the energy of LIC without decreasing the power capability.

References

[1] M. Yamagata, K. Soeda, S. Yamazaki, M. Ishikawa, Electrochem. Solid-State Lett., 14, A165 (2011).
[2] K. Soeda, M. Yamagata, M. Ishikawa, J. Power Sources, 280, 565 (2015).
[3] M. Yamagata, K. Soeda, S. Ikebe, S. Yamazaki, M. Ishikawa, Electrochim. Acta, 100, 275 (2013).
[4] K. Soeda, M. Yamagata, S. Yamazaki, M. Ishikawa,Electrochemistry, 81, 867 (2013).
[5] M. Yamagata, S. Ikebe, K. Soeda, M. Ishikawa, RSC Advances, 3, 1037 (2012).