XAFS study on cathode materials for Li-ion batteries using x-ray microbeam

We attempted to investigate whether the Ni valences change homogeneously in a grain of LiNi_0.8Co_0.2O₂ during charging/discharging Li-ion batteries. In situ XAFS measurements using x-ray microbeam with the size of about 1–2 μm were applied to measure position dependencies of Ni (Co) valences in a grain. In order to investigate the electronic changes caused by capacity fading, three types of coin cells for in situ XAFS measurements were prepared, the cell after one charge/discharge cycle, after 100 cycles at 60 °C and after aging at 60 °C for 21 days, respectively. Two-dimensional fluorescence maps of Ni and Co were obtained to determine measuring points in a grain (the center of a grain, the edge of a grain, etc.). Ni and Co K-edge XAFS spectra were collected in transmission mode for each point. As a result, no apparent differences depending on positions were observed for all types of the cells. It was concluded that the changes of Ni valences during charging/discharging batteries occur homogeneously in a grain, not depending on the degree of capacity fading.