We have lately reported 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 performance [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 an 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 very 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 an 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, our 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 the ionic liquid electrolyte that is FSI anion-based EMImFSI to EDLC. In our previous work, we found that an EDLC cell containing EMImFSI cannot work well beyond 2.0V due to a significant leak current over 2.0V [6]. In this work, we successfully identified the origin of leak current, mended it, and improved the cell system containing an EMImFSI electrolyte. As a result, we realized durable operation of an EDLC cell with EMImFSI in at least normal operation voltage range, 0-2.5V.

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).

[6] N. Handa, T. Sugimoto, M. Yamagata, M. Kikuta, M. Kono, M. Ishikawa, J. Power Sources, 185, 1585 (2008).