Yoshiharu Hirose

Synchrotron radiation (SR) is a tunable, highly oriented, intense X-ray. Its tunability has led to XAFS being used as a common analytical tool, and is particularly useful for examining local structures. In fact, its use is indispensable to the examination of not only amorphous materials or solutions but also multi-component systems. Thanks to the high intensity of SR, it is possible to determine the structure or composition of very small areas or monolayers. Researchers who are faced with difficulties caused by the limits of conventional experimental techniques would be well advised to consider X-ray analysis using SR.

The electronic states and local structures of cathode materials (LiNi_0.8Co_0.2O₂) for lithium ion batteries are studied by means of in situ XAFS (X-ray absorption fine structure) method. Ni and Co K-edge XAFS spectra of LiNi_0.8Co_0.2O₂ have been obtained using newly developed in situ coin cells. To investigate the electronic and structural changes that accompany capacity fading due to electrochemical cycling and keeping the batteries at high temperatures, cells with different cycling states and operating conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted to a higher energy, and a good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. We have also performed first-principles molecular orbital calculations using a discrete variational Xα method to reproduce Ni-K XANES spectra. From quantitative analysis of EXAFS data and the results of molecular orbital calculations, capacity fading was found to be closely related to Jahn-Teller distortion of the NiO₆ octahedron.

Tatsuo Noritake, Masakazu Aoki,
Shin-ichi Towata, Yoshiki Seno,
Yoshiharu Hirose

Magnesium is considered one of the most promising materials for reversible hydrogen storage, because it has high storage capacity. However, the high thermodynamic stability of magnesium hydride is unfavorable for dehydrogenation processes. Understanding the bonding nature of Mg and H is essential for improving its dehydrogenation performance. Therefore, the charge density distribution in MgH₂ was measured. Charge density is typically investigated by X-ray diffraction, but the diffraction intensity from hydrogen atoms is very weak. So far, analyzing the hydrogen in metal hydrides by X-ray diffraction has been difficult. We have overcome this difficulty with precise powder diffraction measurement by synchrotron radiation, which is a highly-brilliant X-ray source. The charge density was analyzed by the MEM/Rietveld method from the measurement data. The results show weak covalent bonds between Mg and H as well as between H and H. The charge density in the interstitial region is extremely low, which denies the existence of metallic bonding. As a result of estimation of the number of electrons within the sphere around the Mg and the H atoms, the ionic charge in MgH₂ was represented as Mg^1.91+H^0.26-. We experimentally revealed that the crystal of MgH₂ is stabilized by ionic and weak covalent bonding. We consider that its ionic bonding must be made weaker in order to improve the dehydrogenation performance of MgH₂.