The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

There is growing interest in electrical/electrochemical energy-storage devices with both high power and high energy densities for possible application as auxiliary-power sources for electric and/or hybrid-electric vehicles. [1,2] Although lithiumion batteries are attractive power-storage devices with high energy density, their power density is generally low because of a large polarization at high charging-discharging rates. The large polarization is thought to be due to slow lithium diffusion in the solid active material and increases in the resistance of the electrolyte and in the electric resistance of the active materials upon increasing the charging-discharging rate. Therefore, in order to obtain high performance with both high power and high energy densities, it is important to design and fabricate nanostructured electrode materials that provide interconnected nanopaths for electrolyte-ion transport and electronic conduction. Mesoporous materials are quite attractive hosts for Li intercalation because of their large surface area, which decreases the current density per unit surface area; their thin walls, which shorten the Li-diffusion length in the solid phase; and their pores, which enable electrolyte ions to be transported smoothly. Actually, it has recently been reported that control of the porous structure of the active materials is effective in increasing the capacity of Li-intercalating electrode materials, even at high charging-discharging rates. [3][4][5][6] Porous materials are often considered to have the disadvantage of having low volumetric energy density, but this is not always the case for high-rate use: because of the low diffusion coefficient in the solid phase (10 -11 -10 -13 cm 2 s -1 ), only the thin surface layer of the host material is available for Li intercalation at high charging-discharging rates for bulk materials. On the other hand, hosts for Li intercalation generally have a low electronic conductivity, and thus electronic conduction paths are also required in the host material to decrease the polarization. Although conducting additives, such as acetylene black can be mechanically mixed with the host material in conventional Libattery electrodes, it is difficult to mix such large-sized conducting additives with mesoporous host materials, because the wall of the mesoporous structure is easily destroyed by conventional mixing techniques.As a new approach, we have synthesized single-walled carbon nanotube (SWNT)-containing mesoporous TiO 2 by a bicontinuous microemulsion-aided process using a dispersed aqueous solution of cut SWNTs (c-SWNTs) as the water phase of a water/surfactant/oil ternary bicontinuous microemulsion. Although there are some reports on surface modifications of carbon nanotubes with metal oxides, [7][8][9][10] this study is the first attempt to prepare a nanocomposite material with a mesoporous structure consisting of anatase TiO 2 and c-SWNTs. We also demonstrate that the Li-intercalation capacity at high charging-discharging rates increases dramatically for c-SW...

Porous carbons with large meso/macropore surface areas were prepared by the colloidal-crystal-templating technique. The porous carbons exibited extremely high specific electrochemical double layer (EDL) capacitance of 200−350 F g-1 in an aqueous electrolyte (1 M H2SO4). The pore structure dependence of the capacitance was studied mainly by means of cyclic voltammetry and is discussed in detail. From the sweep rate dependence of the series resistance and capacitance, it was found that the ion-penetration depth at the porous electrode surface was finite and decreased with an increasing sweep rate. Peaks around the point of zero charge, which were observed in addition to typical rectangular voltammograms, were explained well by the potential drop in pores. The surface area dependence of the capacitance revealed that the contribution of the meso/macropore surface is as great as that of the plane electrodes and that only the part of the micropore surface adjacent to the opening mouths is effective.

Interface structure between Li and garnet-type Li6.5La3Zr1.5Ta0.5O12 (LLZT) solid electrolyte was investigated by means of electrochemical impedance spectroscopy (EIS) on symmetric cells of Li | LLZT | Li. Charge transfer resistance (RCT) between Li and LLZT was investigated using LLZT pellets with various roughness. RCT and activation energy (Ea) obtained on the flat interface is as high as 746 Ω cm2 and 0.51 eV at 25°C, respectively, indicating that the charge transfer reaction at Li | LLZT (grit number: #8000) interface is a kinetically slow process, which may suppress rate capability of all solid-state batteries. Although the lowest RCT of 363 Ω cm2 was obtained by heating up to the melting point of Li for the LLZT pellet polished with an emery paper (#400), it is supposed that electrochemically effective contact area was saturated for rougher surfaces of LLZT. To prepare interfaces with large effective contact area, Li electrodes were deposited on a LLZT pellet by vacuum-evaporation, which exhibited further low RCT of 69 Ω cm2. The interface with large effective contact area is also a key to prevention of short circuit, and a high critical current density of 0.4 mA cm−2 was demonstrated.

The silver ionic conductivity in heterogeneous systems of AgBr:Al2O3 and AgI:Al2O3 is highly enhanced by utilizing mesoporous Al2O3 as the insulating phase. The highest Ag+ conductivity of 3.1 × 10–3 Ω–1 cm–1 (at 25 °C) has been obtained for the AgI:Al2O3 composite with an Al2O3 volume fraction of 0.3. For AgBr:Al2O3, the enhancement of the conductivity is satisfactorily explained in the framework of the ideal space‐charge model, while in the case of AgI:Al2O3 stacking disorder is also considered to contribute to the ionic conductivity.