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)−224 Synthesiology - English edition Vol.1 No.4 (2009) The example of synthesis of titania nanoporous electrode is shown as specific example. Ethanol solution of titanium alkoxide was added to PS colloid crystal, and the solution was mixed with titania nanoporous body consisting of anatase crystal by firing at 450 ºC, and followed by niobium alkoxide, to obtain TixNb1-xO2 nanoporous electrode (Fig. 5). Because these electrodes had sequential structure of three-dimensional framework and had regular arrangement of nanopores and active materials, it was possible to obtain high ion conductivity, electronic conductivity, and lithium storage capacity.Next, AIST conducted synthesis and structural evaluation of high-power battery electrode material using these nanocrystals followed by electrochemical property evaluation, and investigated the physical chemical mechanism of high-speed charge transfer property for how such innovative energy property could be obtained from this nanomaterial technology[1].In actual material development, the electrochemically active nanocrystal titania was set as model material for high-power electrode, and anatase and rutile structures were prepared in size ranges 6 nm ~ 100 nm, to conduct systematic evaluation of size dependency of lithium battery electrode property[2][3].In nanocrystal titania, specific surface area was large since the particle diameter was in nanosize range, and surface energy storage property was readily available, and surface and interior of solid were thought to be two phases in coexistence as region that reacted electrochemically with lithium. That is, there were two different lithium storage mechanisms, where former was surface lithium adsorbed accompanied by charge transfer on surface of nanocrystal, while latter was lithium stored by intercalation inside the nanocrystal. The faradaic capacity by surface adsorption manifested as pseudo-capacity, and stored lithium inside the solid manifested as intercalation capacity. These two different lithium storage mechanisms were shown clearly in charge-discharge curve, as shown in Fig. 6. In nanocrystal titania, charge-discharge curve differed greatly from that of bulk material. The intercalation capacity in ordinary titania solid was normally 168 mAh/g for equilibrium composition Li0.5TiO2, whereas this increased to about 230 mAh/g in the research result. This capacity increased with smaller crystal size. It was thought that increase in capacity occurred due to lithium stored on the nanocrystal surface as pseudo-capacity on the surface in addition to intercalation capacity.In fact, as shown by the charge-discharge curve, lithium was stored at certain site potential up to Li0.5TiO2, which is theoretical capacity of lithium composition, so discharge curve showed constant voltage of about 1.75 V. When this lithium composition was surpassed, pseudo-capacity discharge curve appeared with gradual decline in voltage along with increased capacity. Large pseudo-capacity that appeared in region below 1.75 V was apparently oxidative-reductive capacity of titania, and might represent the capacity of lithium that was electrochemically adsorbed to the surface of active material. These did not have constant site potential, and the voltage was thought to decrease, as the surface turned metallic as lithium concentration on titania surface increased.On the other hand, the most important issue when designing high-power active material was question of most appropriate nanosize. From the result of size dependency of Fig. 6, when crystal size of titania was 100 nm and 30 nm, the electrode property increased, and the effect of nanosizing became apparent for active material of 30 nm size. Moreover, nanocrystal property became dominant in 6 nm size active material, different from the bulk material. From these results, there was great possibility in using active material with size less than several 10 nm when utilizing innovative nanocrystal property.For charge-discharge mechanisms using pseudo-capacity on titania surface, high-speed charge transfer became possible since they did not involve slow diffusion process inside the solid. As shown in Fig. 7, intercalation inside the nanocrystal showed good electrochemical property for battery material since plateau voltage occurred in two-phase coexistence. However, this storage mechanism was not suitable for high-speed charge-discharge since it accompanied slow charge transfer process due to diffusion within solid. On the other hand, surface pseudo-capacity mechanism by adsorption and desorption of ion on the surface may enable charge transfer and lithium storage in second order since they did not accompany diffusion process. In fact, the change of discharge capacity when charge-discharge current density of titania crystal with anatase structure of 6 nm and 30 nm were changed is shown in the figure. In both cases, intercalation capacity tended to decrease as the current density was raised, and while capacity decreases severely in titania of 30 nm (4)−0501001502002501.01.52.02.53.03.5Capacity (mAh/g)Voltage (Li+/Li)6 nm100 nm30 nm15 nmSurface pseudo-capacityBulk capacityLi0.5TiO2 (168 mAh/g)Fig. 6 Charge-discharge curve of nanocrystalline titania.

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