Development of New Dispersive XAFS Instrument for Time- and Space-Resolved Analysis of Electrode Reactions

The new instrument based on the dispersive optics for the X-ray absorption fine structure (XAFS) was developed at BL-5 of the SR Center. The vertical axis of the synchrotron radiation is used to disperse the X-ray energy in the newly developed instrument, named vertically dispersive XAFS (VDXAFS) instrument. The vertical energy dispersion is achieved using a cylindrical polychromator, and the horizontal axis of the synchrotron radiation is used to the spatially resolved analysis. The VDXAFS instrument is designed to analyze the dynamic change of the inhomogeneous electrode reaction of secondary batteries during the charge and discharge processes. The line shaped X-ray transmits the electrode sample, and the transmitted X-ray intensities are detected using a two-dimensional detector. The one-dimensional array of XAFS spectra is obtained at the linear footprint of the X-ray on the sample. The VDXAFS system is possible to perform the sequential measurements of the XAFS data array. The time resolution depends on the flux density of the X-ray beam, and the size of the light source affects to the spatial resolution. The latter was estimated to ca. 100 μm at BL-5 of the SR Center. The VDXAFS instrument was used to measure the dynamic process of the electrode reaction of LiFePO4 cathode for the charge and discharge processes under the constant current and the charge process under the constant voltage. The obtained results suggest that the reaction distribution depends on not only the electric conductance but also the diffusion process around the reacting particle of LiFePO4.