Hiroki Moriwake

First‐Principles Calculations of Lithium‐Ion Migration at a Coherent Grain Boundary in a Cathode Material, LiCoO2

Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO(2) . This study suggests that internal interfaces in LiCoO(2) strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all-solid-state Li-ion batteries.

Real-time direct observation of Li in LiCoO2 cathode material

The direct observation of light elements such as Li is a challenge even for state-of-the-art electron microscopy techniques because such elements scatter electrons only weakly. Using the annular bright field scanning transmission electron microscopy imaging technique, we have simultaneously visualized columns of Li, O, and Co ions in the lithium-ion battery cathode material LiCoO2, which is one of the most important cathode materials for industrial applications. The annular bright field image exhibits a good signal-to-noise ratio and the image contrast is not reversed as the specimen thickness changes. The direct visualization of light elements in real time with this method represents an important breakthrough in characterizing the active materials in solid-state electrochemical devices.

Particle-Size Dependence of Crystal Structure of BaTiO3 Powder

The particle size dependence of BaTiO3 powder was investigated from the viewpoint of the crystal phase. The crystal phase of BaTiO3 powder was transformed from tetragonal to cubic at a critical particle size of 0.1 µm at room temperature. With decreasing particle size, the Ba ions in the BaTiO3 structure were absent at the surface, and these disordered surface areas increased in small-particle-size BaTiO3 powders and exhibited particle-size dependence.

Ferroelectricity Driven by Twisting of Silicate Tetrahedral Chains

Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan. Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan. Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan. Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan. Department of Energy Science, Sungkyunkwan University, Jangan-Gu, Suwon 440-746, Korea. Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan. RIKEN Harima Institute, Sayo, Hyogo 679-5148, Japan. Frontier Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan. Faculty of Science, Gakushuin University, Toshima-ku, Tokyo 171-8588, Japan.

Domain boundary structures in lanthanum lithium titanates

Perovskite-type lanthanum lithium titanate (LLTO) is attracting extensive interest because of its high intrinsic ionic conductivity. The material exhibits a complex microstructure with domains of various sizes and orientations that vary with the lithium content. Based on a systematic examination of both Li-poor and Li-rich LLTO compounds using state-of-the-art scanning transmission electron microscopy (STEM), we reveal the structures and composition of the domain boundaries (DBs) and consider their effect on Li-ion mobility and ionic conductivity, in the process positing the origin of the microstructural variations. DBs in this material are shown to consist essentially of two types: frequently occurring 90° rotation DBs and a much less common antiphase-type boundary. It is found that the 90° DBs are coherent interfaces consisting of interconnected steps that share La sites, with occupancies of La sites higher than in the domain interiors. The origin of microstructural variations in the two compounds is associated with different degrees of lattice mismatch strain at DBs in Li-poor and Li-rich materials. The lattice strain and associated O vacancies, as well as the high La occupancies, at DBs are expected to result in lower interdomain Li-ion mobility, which will have a deleterious effect on the overall ion conductivity.

First-Principles Calculation of Solution Energy of Alkaline-Earth Metal Elements to BaTiO3

Quantitative analysis of the solution energy of alkaline-earth metal elements to perovskite-type BaTiO3 was carried out by a first-principles calculation combined with thermodynamics theory. The solution energies of neutral solute and a compensated solute with an oxygen vacancy were systematically calculated. They were obtained for two cation sites and four thermodynamical conditions with different chemical potentials of constituent atoms. Both Ca and Sr preferably occupy the Ba site of BaTiO3. On the other hand, Mg occupies the Ti site. This corresponds well to the widely accepted experimental findings regarding site preference. Moreover, under the condition of coexising BaO, CaO and BaTiO3, energy difference between the Ba-site solution and O-vacancy compensated Ti-site solution of Ca ions has been found to be smaller than that of Sr. Under this condition, the O-vacancy compensated Ti-site solution of Ca should be favorable compared with that of Sr. The same number of oxygen vacancies as Ca ions occupying Ti sites can be introduced. This also explains well experimental feature of the Ca-doped BaTiO3-based nonreducible multilayer ceramics capacitor (MLCC) materials regarding solution site of the Ca ion and abundance of O-vacancy.

First-Principles Calculations of Electronic Structure and Solution Energies of Mn-Doped BaTiO3

Doping with 3d transition metals, particularly Mn, is thought to play an important role in determining the reliability of dielectrics used in multi-layer ceramic capacitors (MLCCs). However, a detailed examination of the electronic structure, solution energies and compensation mechanisms of these systems is lacking. In this paper, the quantitative analysis of the substitution of Mn in perovskite-type BaTiO3 using first-principles calculations in combination with chemical thermodynamics is reported. The solution energies of dopants with vacancy and n-type and p-type charge compensations have been systematically calculated. Substitution onto the two crystallographically different cation sites in cubic BaTiO3 under four different thermodynamic conditions with different chemical potentials is also examined. Mn is found to be stable on Ti sites under all conditions examined, although its charge state varies. In the oxidizing limit, Mn substitutes for Ti as a Mn4+ ion, but in the reducing limit, Mn substitutes for Ti as a Mn2+ ion compensated by the formation of an O vacancy. Depending on the Fermi level of the system, the valence state of Mn varies from Mn4+ under p-type conditions, to Mn2+ under n-type conditions. Mn3+ is not found to be stable. These results agree well with the experimentally determined site preferences and valence states of Mn, and help to further elucidate the features of Mn-doped BaTiO3 at the atomic level.

Quantitative Evaluation of Electrochemical Potential Windows of Electrolytes for Electric Double-Layer Capacitors Using Ab Initio Calculations

The electrochemical potential windows of seven organic liquid electrolytes for electric double-layer capacitors calculated using ab initio molecular orbital theory are reported. Four types of models were used to investigate the effect of intermolecular interactions: (i) a single-ion in vacuo model, (ii) a single-ion-in-solvent model, (iii) an ion-pair in vacuo model, and (iv) an ion-pair-in-solvent model. For all the calculations, the Hartree-Fock level of theory using the 6-31 + G(d,p) basis set was used. Solute-ion interactions were treated by considering several cation-anion pair confirmations, and solute-solvent interactions were introduced by applying the isodensity polarizable continuum model. The ion-pair-in-solvent model quantitatively reproduced the experimental electrochemical potential windows with high accuracy. This demonstrates that in actual electrolytes intermolecular interactions, particularly cation―anion and solute-solvent, play an important role in determining electrochemical potential windows.

The electric field induced ferroelectric phase transition of AgNbO3

Coexistence of two phases of AgNbO3 is shown to explain the experimentally observed polarization–electric field hysteresis loop better than either phase in isolation, based on detailed first-principles calculations of the structural changes and stabilities of different phases of this compound. Calculations confirm a ferroelectric phase transition, whereby the symmetry of the AgNbO3 crystal switches from antiferroelectric Pbcm to ferroelectric Pmc21, under an electric field of 9 MV/cm. The calculated spontaneous polarization (0.61 C/m2) under this field compares well with the experimental value of 0.52 C/m2. After transforming, the structure remains in the ferroelectric state even after the electric field is removed, despite the structure being energetically metastable. As the energy difference between the antiferroelectric and ferroelectric phases is only +0.5 meV/f.u. and the potential energy barrier between them (∼40 meV/f.u.) is comparable to thermal fluctuation energies, it is possible for these two p...

Synthesis of Disordered Ba(Zn1/3Ta2/3)O3 by Spark Plasma Sintering and Its Microwave Q Factor

Dense Ba(Zn1/3Ta2/3)O3(BZT) with disordered perovskite structure (Pm3m) was synthesized by spark plasma sintering (SPS), and its electronic properties at microwave frequency were studied. Its Q factor, crystal structure and microstructure were compared with those of disordered and ordered BZT synthesized by conventional solid-state reaction (SSR). By SPS, disordered BZT with high density was obtained in an extremely short sintering time for 5 min between 1150 and 1300 °C under 30 MPa. The disordered BZT with high density (=7.62 g/cm3) exhibited a significantly high Qf (=53,400 GHz). It was approximately 5 times higher value than that of low density samples (5.0–6.0 g/cm3) synthesized by conventional SSR. For the high-Qf samples, their crystallization were improved simultaneously with densification. On the other hand, there was no significant improvement in the Q factor observed for BZT grains synthesized by SPS. These results suggest that crystallization with densification in BZT plays a major role in Q factor improvement rather than the effects of structural ordering and grain boundary.