A Mesoporous Nanocomposite of TiO₂ and Carbon Nanotubes as a High‐Rate Li‐Intercalation Electrode Material

Abstract

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...

Self-Assembled TiO2–Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion

“…25 The plateaus are related to the phase transition between the tetragonal and orthorhombic phases with Li insertion into anatase TiO 2 . Similar to rutile TiO 2 ϪFGS, specific capacity of the anatase TiO 2 ϪFGS hybrid is enhanced at all www.acsnano.org chargeϪdischarge rates (Figure 4e).…”

“…The high rate properties obtained from the graphene nanocomposite materials are comparable to some of the best results reported in the literature using RuO 2 or carbon nanotubes with a higher carbon (Super P) additive content. 24,25 The simple self-assembly approach, and the potential low manufacturing cost of graphene may provide a new pathway for large scale applications of novel hybrid nanocomposite materials for energy storage.…”

“…For example, hybrid nanostructures, such as V 2 O 5 -carbon nanotube (CNT) or anatase TiO 2 ϪCNT hybrids, LiFePO 4 ϪRuO 2 nanocomposite, and anatase TiO 2 ϪRuO 2 nanocomposite, 22Ϫ25 combined with conventional carbon additives (e.g., Super P carbon or acetylene black), have demonstrated an increased Li-ion insertion/extraction capacity in the hybrid electrodes at high charge/discharge rates. While the hybrids or nanocomposites offer significant advantages, some of the candidate materials to improve the conductivity, such as RuO 2 and CNTs, 24,25 are inherently expensive. In addition, conventional carbon additives at high loading content (20 wt % or more) are still needed to ensure good electron transport in fabricated electrodes.…”

“…In addition, conventional carbon additives at high loading content (20 wt % or more) are still needed to ensure good electron transport in fabricated electrodes. 24,25 To improve high-rate performance and reduce cost of the electrochemically active materials, it is important to identify high surface area, inexpensive, and highly conductive nanostructured materials that can be integrated with electrochemical active materials at nanoscale.…”

Interfacial phenomena between lithium ion conductors and cathodes

“…efficient energy consumption and effective usage of renewable energies. Lithium ion batteries (LIBs) and related batteries are supposed to be one of promising energy storage devices [1][2][3][4][5][6]. LIBs used for those purpose would be much larger than conventional LIBs, and all solid state batteries that employ solid electrolytes (especially inorganic ceramics electrolytes) attract interests because of their advantages that are hardly achieved by conventional batteries with liquid organic electrolytes [7][8][9].…”