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In this study, in situ time-resolved dispersive X-ray absorption fine structure (DXAFS) analysis of BaTiO 3 LiCoO 2 (BTLC) composites for lithium ion batteries was performed to characterize the cobalt ion valence shift between oxidized and reduced states of driven cells, in an attempt to better understand the contribution of ferroelectric solid electrolyte interfaces (SEIs) to chargedischarge rates. Two types of artificial SEIs, ferroelectric BT and paraelectric Al 2 O 3 , were compared. The magnitude of the shift in the X-ray absorption energy at the peak of the white line, E 1 , during charging and discharging at a 10 C rate, increased in the order of bare LC (0.264 eV) < Al 2 O 3 1 mol % (0.497 eV) < BT 1 mol % (1.15 eV); the corresponding discharge capacities of the laminated cells at 10 C were as follows: bare LC (11.6 mAh/g) < Al 2 O 3 (41.8 mAh/g) < BT (95.1 mAh/g). The increase in E 1 , i.e., the oxidation of Co during charging, intensified under a higher applied potential for the BT-decorated composite compared with that of the Al 2 O 3 -coated specimen. The stronger oxidation of Co for BTLC under application of a large electric field was attributed to the strengthened polarization due to the larger permittivity of BT.

Apparatus for a technique based on the dispersive optics of X-ray absorption fine structure (XAFS) has been developed at beamline BL-5 of the Synchrotron Radiation Center of Ritsumeikan University. The vertical axis of the cross section of the synchrotron light is used to disperse the X-ray energy using a cylindrical polychromator and the horizontal axis is used for the spatially resolved analysis with a pixel array detector. The vertically dispersive XAFS (VDXAFS) instrument was designed to analyze the dynamic changeover of the inhomogeneous electrode reaction of secondary batteries. The line-shaped X-ray beam is transmitted through the electrode sample, and then the dispersed transmitted X-rays are detected by a two-dimensional detector. An array of XAFS spectra in the linear footprint of the transmitted X-ray on the sample is obtained with the time resolution of the repetition frequency of the detector. Sequential measurements of the space-resolved XAFS data are possible with the VDXAFS instrument. The time and spatial resolutions of the VDXAFS instrument depend on the flux density of the available X-ray beam and the size of the light source, and they were estimated as 1 s and 100 µm, respectively. The electrode reaction of the LiFePO4 lithium ion battery was analyzed during the constant current charging process and during the charging process after potential jumping.

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