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)−225 Synthesiology - English edition Vol.1 No.4 (2009) crystal size, titania with 6 nm crystal size showed little decrease under condition of large current density of 10 A/g, and we found that high-speed intercalation was taking place within the crystal.In charge-discharge curve, capacity decreased severely in the flat part of voltage in 30 nm crystal, and it was found that the intercalation of lithium ion inside the crystal was inhibited under condition of large current density. On the other hand, when pseudo-capacity mechanism of nanocrystal surface was used, the pseudo-capacity at surface did not decrease much even when the current density was raised. This showed that charge transfer at surface reaction occurred at extremely high speed. In 6 nm nanocrystal, the pseudo-capacity content of the surface did not decrease even at high-speed charge-discharge to 40 A/g, and this implied that lithium ion possessed reversible high-speed storage property. That is, in nanocrystal active material, pseudo-capacity appeared due to large specific surface area, and lithium storage at over stoichiometric composition became possible. Moreover, new electrode property where high-speed lithium storage mechanism appeared at surface without intercalation into the solid was discovered. Using this energy storage property unique to nanocrystal, it would be possible to realize innovative high-capacity high-power lithium battery electrode material.In industry-academia-government vertical collaboration development, it was necessary to synthesize nanocrystals using active materials, for which the battery manufacturer was considering product realization, and to investigate their high output property. If nanocrystals were synthesized using active materials used in the products and their high-capacity, high-power, and high-cycle properties were demonstrated, it should lead directly to practical use. To utilize the concept of nanocrystal active material in product development of batteries for power tools by Hitachi Maxell, one of the participant of this project, we developed the fabrication process of nanoporous structure and nanocrystal synthesis of lithium titanate Li4Ti5O12 from which high output property could be expected among titanium oxide materials[4][5]. Li4Ti5O12 active material is electrode active material with negligible level of crystal structure change in insertion and extraction of lithium ions, and therefore has drawn attention as electrode material with excellent charge-discharge cycle property. Therefore, AIST added polymer as dispersing agent for introducing mesoporous structure when synthesizing the active material, fabricated nanoporous structure electrode composed of framework of sequential mesopore and nanocrystal active material, and conducted evaluation of high output property.To introduce mesopore with high ion diffusability to nanocrystal electrode body, template polymer P123 (Pluronic; EO20PO70EO20) was added during sol-gel synthesis of Li4Ti5O12 electrode to promote dispersal of nanocrystal Li4Ti5O12. After adding polymer to primer, and firing in air for 6 h at 400 ºC and then for 2 h at 750 ºC, we fabricated nanoporous structure electrode where Li4Ti5O12 particles with size about 60 nm were linked in highly dispersed manner. When the electrode properties were evaluated, as can be projected from the concept of low-resistance, high-ion diffusion electrode, we obtained sufficient electrode capacity at high charge-discharge current density for Li4Ti5O12 electrode with nanoporous structure, and the cycle property was good.According to AIST research, it was found that the output (5)−Charge-discharge current density (A/g)Capacity (mAh/g)Surface pseudo-capacity (30 nm)Intercalation capacity (30 nm)Intercalation capacity (6 nm)Surface pseudo-capacity (6 nm)A6-CbcA30-CabA30-CbcA6-CabSurface pseudo-capacity increases by nanosizing0.1110204060801001201400Surface pseudo-capacityIntercalation capacityVoltageFast charge transfer processSlow charge transfer processCabFig. 7 Pseudo-capacity mechanism of nanocrystalline titania surface and its high-rate charge-discharge property.

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