A positive-electrode material composed of LTO (80%) and the SGCNT (20%)
A positive-electrode material composed of LTO (80%) and the SGCNT (20%)
[Click to enlarge image]
An electrode made of only a composite material and an aluminum alloy current collector
An electrode made of only a composite material and an aluminum alloy current collector
[Click to enlarge image]

Japanese researchers developed a lithium-ion (Li-ion) capacitor whose capacity and discharge properties at a high rate were enhanced by using a composite material of single-layer carbon nanotube and lithium titanate (Li4Ti5O12, LTO) for the negative electrode.

The capacitor's energy density and output density per volume are 45Wh/L and 17,000W/L, respectively, which are about 4.5 and 3.8 times higher than those of an electric double layer capacitor using activated carbon. The capacitor was developed by the Naoi Laboratory at the graduate school of the Tokyo University of Agriculture and Technology and the participants of the "Capacitor Technology Lecture" at the school.

In March 2009, the graduate school announced that it developed a Li-ion capacitor with negative electrodes made of a composite material of carbon nanofiber (CNF) and LTO and achieved an energy density about three times higher than that of an electric double layer capacitor using activated carbon (See related article). And Nippon Chemi-Con Corp announced that it will start shipping samples of a Li-ion capacitor using the technology in the spring of 2011.

Li-ion capacitors using LTO have the following advantages. First, dendrite is less likely to be generated at their negative electrodes. Second, they have excellent low-temperature properties. Third, they enable to select an electrolyte from a variety of choices.

Furthermore, Li ions do not have to be doped in the manufacturing process, making it possible to use the same process for existing electric double layer capacitors, said Kenji Tamamitsu, chief of the Functionality Materials Lab at the Basic Research Headquarters of Nippon Chemi-Con.

The most distinctive feature of the new Li-ion capacitor is that it can secure an electronically conductive path despite its high content percentage of LTO (80-85%). Therefore, the capacity maintaining rate is high even the capacitor is discharged at a high rate of 300C, said Katsuhiko Naoi, professor at the graduate school of the Tokyo University of Agriculture and Technology.

For the negative electrode, a single-layer carbon nanotube made by "super-growth method" (SGCNT) is used instead of CNF. When the SGCNT, whose surface area is very large, and LTO are used with the ultracentrifugal processing technology developed by K&W, a Japan-based venture firm, it is possible to firmly attach LTO with a particle diameter as small as 1 to 10nm to both the inner and exterior walls of the SGCNT.

Moreover, a sheet electrode that does not contain a conductive auxiliary agent or a binder has already been made.

Because a metal oxide with a very small particle diameter can be attached to carbon by using the ultracentrifugal processing technology, the Naoi Laboratoty and Nippon Chemi-Con are expecting that the technology will be used for positive-electrode materials and electrode materials for Li-ion rechargeable batteries.