Recent progresses on insertion materials for lithium-ion batteries are described. Understanding of solid state chemistry and electrochemistry of LiCoO2, LiNiO2, LiMn2O4, LiC6, and so forth was a first step and their modification in terms of solid solution among the same crystal family was a second step. However, materials innovation is restricted because of the limitation of chemical species unless new concept is introduced. Old, but quite new concept is so-called “superlattice” model with which one can design hypothetical lithium insertion materials, calculate electronic and crystal structures by using computational methods, and develop effectively in an empirical basis. To demonstrate the new concept, systematic researches on materials based on superlattice models in a phase triangle of LiCoO2-LiNiO2-LiMnO2 leading to novel binary or ternary oxides of lithium with transition metal elements, i.e., LiNi1/2Mn1/2O2, LiCo1/3Ni1/3Mm1/3O2, and Li[Ni1/2Mn3/2]O4, are highlighted.
Recent progress in proton conducting membrane materials for high temperature polymer electrolyte fuel cells (PEFCs) is reviewed. Perfluorinated ionomers, hydrocarbon ionomers, and other miscellaneous materials are included. As a state-of-the-art material, perfluorinated ionomers have been mostly studied. The current research has focused on reinforcement (with porous support, fibril, fiber, or fabric) and chemical modification (by copolymerzation or cross-linking). Considerable improvement has been achieved in the thermal, chemical and mechanical stability as well as the proton conductivity. The perfluorinated ionomer dispersed with nanoparticles of metal oxides and platinum has proved its potential availability as an electrolyte membrane for non-humidified operating PEFCs. As alternative membranes, non-fluorinated hydrocarbon materials have been a great challenge. Aromatic ionomers such as sulfonated polyimides and polyethers, or acid-doped polybenzimidazoles have been developed, Ionic liquids, organic/in-organic hybrids, and fullerene derivatives have also been described as a novel class of anhydrous proton conducting materials.
The electrolytes currently investigated for high temperature solid oxide fuel cells are reviewed in their electrical and mass transfer properties. First, these features are summarized and compared for the yttria stabilized zirconia, doped ceria and LaGaO3-based oxide. It has been clarified that the high electron conductivity in doped ceria makes this material unfavorable as electrolyte, whereas the LaGaO3-based oxides hold as good electrolyte even at low temperatures. Detailed behavior in the electrical properties for the fluorite oxides are reviewed with an emphasis on the dopant dependence in rare earth doped systems and also on the host dependence in zirconia-ceria solid solutions. The following topics are discussed in details: (1) Correlation of water solubility with hole conductivity; (2) Roles of ceria as the oxide component in anode and as the interlayer between electrolyte and electrode; (3) For the LaGaO3-based oxides, the electrical properties in its relation to the Co doping; (4) Cation diffusivity in LSGM from different techniques are summarized. Finally, effects of nano-crystalline size are briefly discussed concerning their enhancement in electron conduction.
A non-foam type nickel electrode has been developed to improve the power density of a Ni-MH battery. For the positive electrode, a new substrate with a low cost has been developed and substituted for the conventional foam-type substrate For the negative electrode, Mg-added MmNi5-based alloys have been applied to reduce the cobalt content in the electrode material. A new current collector and lead terminal have been developed to reduce their internal resistances. In addition to 30% of the cost reduction as a total, these materials have realized the increase of the power density up to 1055 W/kg, more than twice that of the conventional battery, 480 W/kg. The HEV mode cycle life evaluated is more than 100,000 km.
A new electron acceptor molecule, 11,11,12,12-tetracyanoanthraquinodimethane (TCNAQ)-disulfide, has been prepared and characterized by cyclic voltammetry. Self-assembled monolayers of TCNAQ-disulfide or anthraquinone (AQ)-disulfide are formed by exposure of polycrystalline gold electrode to an acetonitrile solution of TCNAQ-disulfide or a dichloromethane solution of AQ-disulfide. The electrochemical study of self-assembled monolayers with TCNAQ-disulfide indicates that self-assembled monolayers of TCNAQ-disulfide on a gold electrode are stable under repeated potential cycling of ten times.
We investigated the optoelectronic memories and switches based on the charge trapping/detrapping effect for the binary liquid crystal R10DA composed of N-(4-n-decyloxybenzylidene)-4-dimethylaminoaniline (R10D) as an electron donor, and N-(4-n-decyloxybenzylidene)-4-nitroaniline (R10A) as an electron acceptor. The smectic phase of the liquid-crystalline R10DA led to a large improvement of the reversible charge trapping/detrapping effect This photoconductive liquid crystal may have a promising possibility and new insight toward practical application for information storage and switching devices.
Under vertical high gravity field, multi-layered two-dimensional (2D)-dendritic growth of silver-substitution plating was examined. As a result, in the vertical gravity fields up to 145 G, it was clarified that: 1) when the electrode is set in upward-horizontal mode, the dendritic growth is greatly suppressed by increasing gravitational acceleration, yielding a high-density-dendrite layer, 2) such high-density layer is self-organized on the electrode surface together with gravitational-convection cells, and in front of the high-density layer, a low-density layer is formed. 3) when the electrode surface is oriented downward horizontally, suppression of the deposit is hardly observed; a conventional low-density-dendrite layer grows, and 4) when the gravitational acceleration changing with time is applied, a characteristic dendrite layer is observed, which is alternatively stacked by high-density and low-density layers. These results were ascribed to the fact that the convection cells induced by the hydrodynamic instability in a vertical gravity field interact with nonequilibrium fluctuations accompanying dendritic growth.
In situ FTIR measurements were performed to analyze the electrochemical oxidation of propylene carbonate with 1.0 mol dm-3 LiClO4 on LiMn2O4 cathode used in rechargeable lithium batteries. A thin film electrode of LiMn2O4 was prepared by rf-sputtering method. The prepared LiMn2O4 film had a high quality as an electrode for the in situ FTIR measurement. The subtractively normalized interfacial FTIR (SNIFTIR) spectra were obtained at various electrode potentials from 3.8 V to 4.8 V vs. Li/Li+. The peaks in the SNIFTIR spectra corresponding to decomposition products of propylene carbonate showed that the electrochemical oxidation of propylene carbonate started at 4.4 V. The oxidation products of propylene carbonate were assigned to compounds having carboxylic groups. Furthermore, from the comparison of in situ FTIR spectra for LiCoO2 and LiMn2O4 cathodes, it was shown that final products depend on the kind of transition metal oxides.
Measurement and analysis were performed on the series resistance (Rs) in a dye-sensitized solar cell which was composed of a photoelectrode, a I3-/I- redox electrolyte, and a Pt-coated counter electrode. The photoelectrode was a porous TiO2 film which was formed on a fluorine-doped SnO2 (FTO) substrate and sensitized by a ruthenium dye. Rs was determined from current-voltage curves measured at different light intensities. Rs was about 40 Ω and almost independent of the illumination area. Measurement of the conductivity of electrolyte solution showed that the effect of solution resistance was small. The sheet resistance of the FTO substrate was measured after heat-treatment at different temperatures (350-550℃) with and without formation of TiO2 thereon. The sheet resistance increased by heat treatment at temperatures above 400℃. Digital simulation of potential and current distributions in the transparent electrode indicated that the resistance due to the FTO substrate was 14.3 Ω. Such resistance must have occurred also at the counter electrode. Geometry and arrangement of current collectors are very important to minimize the potential drop in the conducting substrate.
We quantitatively evaluated the effect of a thin Pt film sputtered on the polymer electrolyte on the methanol crossover. The Pt film brought about around 15 and 30% reduction in the methanol crossover for 6-nm and 30-nm thick films respectively, depending on its thickness. With the aid of FE-SEM, a relatively dense structure with many slender cracks of the Pt-sputtered film has been identified and ATR-FTIR analysis for Pt-sputtered membrane revealed a little change in chemical structure of polymer membrane after sputtering Pt onto the Nafion® membrane. Although the Pt film as a barrier for the mass transport caused an increase in the ohmic resistance of the electrode, it also prevented the methanol crossover in a DMFC.