(Invited) Hybrid Supercapacitors: Their Evolution to Second and Third Generations

To meet growing demands for electric automotive and regenerative energy storage applications, researchers all over the world have sought to increase the energy density of electrochemical capacitors. Hybridizing battery capacitor electrodes can overcome the energy density limitation of the conventional electrochemical capacitors because they employ both the system of a battery-like (redox) and a capacitor-like (double-layer) electrode, producing a larger working voltage and capacitance. However, to balance such asymmetric systems, the rates for the redox portion must be substantially increased to the levels of double-layer process, which presents a significant challenge. An in situ material processing technology called “ultracentrifugation (UC)” has been used to prepare a novel ultrafast Li4Ti5O12 (LTO) nanocrystal electrode for capacitive energy storage. The present paper describes an extremely high-performance supercapacitor that utilizes highly optimized “nano-nano-LTO/carbon composites” prepared via the UC treatment. The UC-treated LTO nanocrystals are grown as either nanosheets or nanoparticles, and both have hyperlinks to two types of nanocarbons: carbon nanofibers and supergrowth (single-walled) carbon nanotubes. Using this ultrafast material, we assembled a hybrid device called a “nanohybrid capacitor” that consists of a Li-intercalating LTO electrode and a non-Faradaic AC electrode employing an anion adsorption desorption process. The “nanohybrid capacitor” cell has demonstrated remarkable energy, power, and cycleability performance as an electrochemical capacitor electrode. The new-generation “nanohybrid capacitor” technology produced more than triple the energy density of a conventional electrochemical capacitor. Moreover, the synthetic simplicity of the high-performance nanostructures makes it possible to scale them up for large-volume material production and further applications in many other electrochemical energy storage devices. References [1] K. Naoi, Fuel Cells, 10, 825 (2010), [2] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science, 306, 1362 (2004), [3] K. Naoi, S. Ishimoto, J. Miyamoto, W. Naoi, Energy & Environmental Science, 5, 9363 (2012), [4] K. Naoi, W. Naoi, S. Aoyagi, J. Miyamoto, T. Kamino, Accounts Chemical Research, 46,1075(2013).