Vol.1 No.4 2009

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Research paper : Development of high power and high capacity lithium secondary battery based on the advanced nanotechnology (I. Honma)−223 Synthesiology - English edition Vol.1 No.4 (2009) to diffuse inside the active material of width 5 nm is 1 sec at most. When the diffusion coefficient of ion in electrolyte within the pore is estimated to be about 10-6 cm2/s, the time required for diffusion inside pore with length 10 μm can be estimated as 1 sec at most. Therefore, as shown in Fig. 3, if fast ion diffusion (k1) in electrolyte within nanopore and ion diffusion rate (k3) in solid at nanometer level within active material are used, charge and discharge can be realized in second order even if the particle size of nanoporous structure electrode is larger than micrometer. However, this is theoretical consideration under condition that electronic conduction (k4) of active material framework and surface reaction of active material (k2) are sufficiently fast in the rate determining process shown in Fig. 3. On the other hand, since specific surface area is large in nanoporous electrode, there is interest in energy property unique to nano material such as lithium storage caused by electrochemical reaction on the surface. If the surface is utilized, there may be possibility of storing lithium above stoichiometric composition.Various nano-size electrode active materials were made based on migration kinetics at AIST to check the high capacity electrode property of nanoporous crystal material, and high-speed charge-discharge property was evaluated. In principle, if nanoporous electrode could be fabricated, charge-discharge in 36 sec, which was required as power source of HEV, would be possible. To accomplish this, it was necessary to integrate the elemental technologies of the frontier of nanotechnology fields such as solution process, molecular template synthesis technology, self-assembling process, and mass synthesis process for nanocrystals, and then to check the efficacy of innovative electrode active materials at battery cell level jointly with manufacturers. The competition for development is becoming fierce around the world since such high-power battery is long awaited in the industry and the market scale is great.3 Execution and result of R&DI shall describe the outline of development of high-power lithium secondary battery through interdisciplinary fusion and industry-academia-government vertical collaboration conducted as NEDO project, and discuss whether the strategy for “shortening distance” to innovation was effective. This R&D was a vertical collaboration project by four organizations, Nagasaki University, AIST, Hitachi Maxell, Ltd., and Fuji Heavy Industries Ltd., as “Research and Development of High Capacity Secondary Battery by Low Resistance, High Ion Diffusion Nanoporous electrode” under the R&D for Practical Utilization of Nanotechnology and Advanced Materials conducted in FY 2005~2007. This project was conducted to develop high-power lithium secondary battery for HEV using advanced nanotechnology, and it was original because it used nanotech in energy technology effectively. It was characteristic that the end user automobile manufacturer was included in the project from commencement, and it was also distinct that the use of electrode technology of university and AIST as power source of HEV in short period was placed as central topic of R&D.Nagasaki University, which was located upstream, investigated the inorganic chemical synthesis process of nanoporous electrode material from which high output property could be expected, from standpoint of basic chemistry, and developed new synthesis method that could be applied to practical electrode. Mesoporous material using molecular template such as surfactants could be applied to amorphous structure such as silica but were difficult to apply to crystalline active material such as LiCoO2 and LiFePO4 that were positive electrode materials for lithium secondary battery. Therefore, a process to fabricate inverse-opal form electrode framework using template structure with colloid polystyrene (PS) was developed, as shown in Fig. 4.(3)−Rate-limiting step of charge transfer process in mesoporous electrode Ion diffusionk2 : Surface reaction (pseudo-capacity)k3 : Intercalation (bulk capacity)5 nmk4 : Electronic conductivityNanoporeLi+1 µmLi+Li+Li+Li+Li+k1 : Li+e-Fig. 3 Charge transfer process and rate-controlling step within electrode.Polystyrene colloidal crystalIntroduction of inorganic sourceComplexHeat treatmentNanoporeFig. 4 Potential of university: nanoporous electrode synthesis process. 2 µm0.5 µm0.11 µm52 nmFig. 5 Titania nanoporous electrode.

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