Kazuyoshi Tatsumi

Capacity-Fading Mechanisms of LiNiO2-Based Lithium-Ion Batteries II. Diagnostic Analysis by Electron Microscopy and Spectroscopy

We used a suite of transmission electron microscopy (TEM) and associated electron spectroscopy methods to examine the local structure and changes in the electronic structure of LiNi 0.8 Co 0.15 Al 0.05 O 2 positive electrode material. We found a scattered rock-salt phase near grain surfaces and grain boundaries, where Ni 3+ turned to Ni 2+ , deduced from relative intensity ratios and fine structures of the L 2,3 white-line peaks of the transition metals. The spatial distribution of the degraded phase throughout the secondary particle was found using a scanning TEM-electron energy loss spectroscopy spectral imaging technique and multivariate analysis. The degradation process and its relationship to the surface reactions with electrolytes is discussed based on the spatial-distribution map of the degraded phases.

Capacity-Fading Mechanisms of LiNiO2-Based Lithium-Ion Batteries I. Analysis by Electrochemical and Spectroscopic Examination

I. Analysis by Electrochemical and Spectroscopic Examination Tsuyoshi Sasaki,* Takamasa Nonaka, Hideaki Oka, Chikaaki Okuda, Yuichi Itou, Yasuhito Kondo, Yoji Takeuchi, Yoshio Ukyo,* Kazuyoshi Tatsumi, and Shunsuke Muto* Toyota Central Research and Development Laboratories, Incorporated, Nagoakute 480-1192, Japan Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan

Hardness of cubic silicon nitride

We report that polycrystalline cubic-Si 3 N 4 with a spinel structure and low oxygen concentration (<0.5 wt%) shows Vickers hardness of 43 GPa when measured with the indentation load of 10 mN. The hardness decreases with the increase of the indentation load, which can be ascribed to the presence of weak grain boundaries. The high hardness can be well explained by its large shear modulus as predicted by first-principles calculations.

Quantitative characterization of nanoscale polycrystalline magnets with electron magnetic circular dichroism

Electron magnetic circular dichroism (EMCD) allows the quantitative, element-selective determination of spin and orbital magnetic moments, similar to its well-established X-ray counterpart, X-ray magnetic circular dichroism (XMCD). As an advantage over XMCD, EMCD measurements are made using transmission electron microscopes, which are routinely operated at sub-nanometre resolution, thereby potentially allowing nanometre magnetic characterization. However, because of the low intensity of the EMCD signal, it has not yet been possible to obtain quantitative information from EMCD signals at the nanoscale. Here we demonstrate a new approach to EMCD measurements that considerably enhances the outreach of the technique. The statistical analysis introduced here yields robust quantitative EMCD signals. Moreover, we demonstrate that quantitative magnetic information can be routinely obtained using electron beams of only a few nanometres in diameter without imposing any restriction regarding the crystalline order of the specimen.

New algorithm for efficient Bloch-waves calculations of orientation-sensitive ELNES.

We discuss in detail the Bloch waves method for the calculation of energy and orientation dependent scattering cross-section for inelastic scattering of electrons on crystals. Convergence properties are investigated and a new algorithm with superior timing and accuracy is described. The new method is applied to calculations of intensity of weakly excited spots, maps of magnetic signal, and tilt series from zone axis orientation towards three-beam orientation.

Discovery of the Last Remaining Binary Platinum‐Group Pernitride RuN2

The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.

Mapping of Heterogeneous Chemical States of Lithium in a LiNiO2-Based Active Material by Electron Energy-Loss Spectroscopy

It is difficult to analyze the local concentrations and chemical states of lithium in lithium-ion secondary battery electrodes by microanalysis techniques based on transmission electron microscopy because the core excitation spectra of transition metals invariably overlap with the absorption/emission spectra of Li-K. We propose a promising analysis method that enables the spatial distribution of lithium with different chemical states from the original phase in a LiNiO 2 -based positive electrode to be visualized. It employs a suite of spectrum imaging techniques including scanning transmission electron microscopy, electron energy-loss spectroscopy, and multivariate curve resolution. This method is successfully applied to a cross-sectioned positive electrode.

Sparse modeling of EELS and EDX spectral imaging data by nonnegative matrix factorization.

Advances in scanning transmission electron microscopy (STEM) techniques have enabled us to automatically obtain electron energy-loss (EELS)/energy-dispersive X-ray (EDX) spectral datasets from a specified region of interest (ROI) at an arbitrary step width, called spectral imaging (SI). Instead of manually identifying the potential constituent chemical components from the ROI and determining the chemical state of each spectral component from the SI data stored in a huge three-dimensional matrix, it is more effective and efficient to use a statistical approach for the automatic resolution and extraction of the underlying chemical components. Among many different statistical approaches, we adopt a non-negative matrix factorization (NMF) technique, mainly because of the natural assumption of non-negative values in the spectra and cardinalities of chemical components, which are always positive in actual data. This paper proposes a new NMF model with two penalty terms: (i) an automatic relevance determination (ARD) prior, which optimizes the number of components, and (ii) a soft orthogonal constraint, which clearly resolves each spectrum component. For the factorization, we further propose a fast optimization algorithm based on hierarchical alternating least-squares. Numerical experiments using both phantom and real STEM-EDX/EELS SI datasets demonstrate that the ARD prior successfully identifies the correct number of physically meaningful components. The soft orthogonal constraint is also shown to be effective, particularly for STEM-EELS SI data, where neither the spatial nor spectral entries in the matrices are sparse.

Elastic constants and chemical bonding of LaNi5 and LaNi5H7 by first principles calculations

Elastic constants of LaNi5H7 and LaNi5 are calculated by a first principles pseudopotential method using plane-wave basis sets. Some extra calculations using model clusters were made in order to discuss the magnitude of chemical bondings. Inner displacements associated with all deformation modes are taken into account. Elastic constants are smaller and more isotropic in the hydride than those in the host. The electronic mechanism to determine the change in the elastic properties is investigated from the viewpoint of chemical bonding. Strong Ni–H bonds are formed in LaNi5H7 at the expense of Ni–Ni bonds. They play key roles in determining the elastic properties. The isotropic distribution of the Ni–H bonding charge in LaNi5H7 should be responsible for the isotropic elastic constants. Hydrogen atoms are found to relax considerably during the deformation to maintain the Ni–H bond length. When the inner displacements are ignored, the elastic constants of LaNi5H7 are as large as those of LaNi5. However, the remarkable displacement of hydrogen atoms during the elastic deformation plays an essential role in softening by hydrogenation.

Local electronic structure analysis by site-selective ELNES using electron channeling and first-principles calculations

In this paper, we review our recent analyses of electron energy loss near edge structure (ELNES) of particular crystalline sites, exploiting dynamical electron diffraction effects, or electron channeling, whereby the excitation weights of the Bloch waves propagating in a crystal can be controlled systematically by adjusting the diffraction conditions. A state-of-the-art data processing technique, multivariate curve resolution (MCR), can restore purely site-specific spectral profiles and their compositions from the experimental data set. Another technique, the Pixon deconvolution method, effectively removes the statistical noise, which enables us to compare the spectral fine structures with those calculated by first principles and discuss the site-specific local atomic and electronic structures. We demonstrate typical case studies in model materials and then an advanced chemical state analysis in a real material. Finally, some remarks toward further refinement of the method are made.