High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

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.

Poly(diallyldimethylammonium) chloride (P) and sodium decatungstate (W) were layerby-layer self-assembled onto quartz, mica, and ITO-electrode substrates (S). The selfassembled films, S-(P/W) n , were characterized by absorption spectrophotometry, reflectivity, cyclic voltammetry and scanning force microscopy (AFM). The electrochemical properties of the S-(P/W) n films were found to differ from those in which the polyelectrolyte remained at the outermost layer, i.e., S-(P/W) n /P. Photoelectrochemical measurements provided evidence for the electrochromic and photoelectrochromic behavior of these films.

An outstanding compression function for materials preparation exhibited by nanospaces of single-walled carbon nanohorns (SWCNHs) was studied using the B1-to-B2 solid phase transition of KI crystals at 1.9 GPa. High-resolution transmission electron microscopy and synchrotron X-ray diffraction examinations provided evidence that KI nanocrystals doped in the nanotube spaces of SWCNHs at pressures below 0.1 MPa had the super-high-pressure B2 phase structure, which is induced at pressures above 1.9 GPa in bulk KI crystals. This finding of the supercompression function of the carbon nanotubular spaces can lead to the development of a new compression-free route to precious materials whose syntheses require the application of high pressure.

Nanoporous composite electrodes for lithium ion batteries were successfully fabricated by coating porous carbon with V 2 O 5 gel, and their high rate capability was demonstrated. The porous carbons prepared by using SiO 2 colloidal crystals as templates exhibited ordered three dimensionally interconnected pores. The porous carbons were immersed in V 2 O 5 sol under reduced pressure repeatedly to obtain V 2 O 5 /carbon composites. The nanoporous V 2 O 5 /carbon composites thus obtained exhibited large capacity of more than 100 mAh (gcomposite) -1 and good rate capability of 80% at 5.0 A (g-composite) -1 . The good performance is explained by electric double layer capacitance of large surface area and high rate lithium insertion to V 2 O 5 gel. According to a calculation, very high-power energy sources with 177 mAh (g-composite) -1 at 100 A (g-composite) -1 are expected by using the nanoporous composite electrodes.

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