Development of a two-dimensional imaging system of X-ray absorption fine structure

A two-dimensional imaging system of X-ray absorption fine structure (XAFS) has been developed at beamline BL-4 of the Synchrotron Radiation Center of Ritsumeikan University. The system mainly consists of an ionization chamber for I₀ measurement, a sample stage, and a two-dimensional complementary metal oxide semiconductor (CMOS) image sensor for measuring the transmitted X-ray intensity. The X-ray energy shift in the vertical direction, which originates from the vertical divergence of the X-ray beam on the monochromator surface, is corrected by considering the geometrical configuration of the monochromator. This energy correction improves the energy resolution of the XAFS spectrum because each pixel in the CMOS detector has a very small vertical acceptance of ∼0.5 µrad. A data analysis system has also been developed to automatically determine the energy of the absorption edge. This allows the chemical species to be mapped based on the XANES feature over a wide area of 4.8 mm (H) × 3.6 mm (V) with a resolution of 10 µm × 10 µm. The system has been applied to the chemical state mapping of the Mn species in a LiMn₂O₄ cathode. The heterogeneous distribution of the Mn oxidation state is demonstrated and is considered to relate to the slow delocalization of Li⁺-defect sites in the spinel crystal structure. The two-dimensional-imaging XAFS system is expected to be a powerful tool for analyzing the spatial distributions of chemical species in many heterogeneous materials such as battery electrodes.