In situ XAFS study on cathodic materials for lithium-ion batteries

Ni and Co K-edge X-ray absorption spectra of LiNi_0.8Co_0.2O₂ have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO₆ octahedron.