X-ray absorption fine structure and neutron diffraction analyses of de-intercalation behavior in the LiCoO2 and LiNiO2 systems

Variations of electronic and local structures of Ni and Co in Li1 − xNiO2 and Li1 − xCoO2 as a function of x were clarified for the first time by in situ X-ray absorption fine structure analysis (XAFS). Chemical shifts of the X-ray absorption near edge sructure spectra of the Ni K-edge in Li1 − xNO2 as a function of x were continuous while an abrupt change was observed for the Co K-edge spectra of Li1 − xCoO. Radial structure functions obtained by Fourier-transform of the Ni K-edge EXAFS of LiNiO2 exhibited abnormally low height of the Ni-O peak. This phenomenon is explained by the Jahn—Teller distortion of the NiO6 octahedron due to the low spin Ni3+ (d7) ion. De-intercalation of the Li ion caused oxidation of Ni3+ (d7) to Ni4+ (d6) and reduced the distortion, hence the Ni-O peak increased with increasing x value. The crystal structures of LiNiO2 treated with D2O, and electrochemically de-intercalated Li0.34NiO2 were solved by Rietveld analysis of neutron diffraction data.