Environment protection and sustainable energy development have recently become a growing industry, in which energy storage devices are such important components that they play crucial roles. Energy storage devices include batteries, for example, lithium-ion batteries, nickel-metal hydride batteries, and lead-acid batteries, which have high energy densities. The batteries are under vigorous study for further increasing their energy density so that they can power electric vehicles. On the other hand, electric double-layer capacitors (EDLCs), which are high-power energy storage devices capable of effectively utilizing energy, have been also studied and are practically used in tracks, buses, elevators, and in heavy-duty construction and railway usage such as forklifts, yard cranes, and bullet trains [1].However, since EDLCs generally have low energy densities, their uses are limited and they cannot fully meet the various performance demands of the recent markets as shown in Figure 7.1. Particularly in the field of automobiles, new energy devices are strongly desired to have hybrid characteristics between lithium-ion batteries and EDLCs, and, thereby, can be suitably employed in idle reduction systems. Accordingly, it is expected for them to form a large market [2]. In order to satisfy the performance demands, it is often suggested that the energy density of EDLCs be enhanced to 20-30 Wh l −1 , which is approximately twice or more that of the present EDLCs, namely 5-10 Wh l −1 . To realize this high energy density, hybrid capacitor systems comprising nonaqueous redox materials are being dynamically researched and developed in recent years [3][4][5][6][7][8][9][10][11]. This chapter deals with the recent contributions to get this high energy density by focusing on two major hybridized cell configurations in organic media.
Voltage Limitation of Conventional EDLCsAs described, increasing the energy density is one of the most crucial matters. For conventional EDLC systems, designed with two symmetrical activated carbon