X-ray absorption spectroscopic study of chemically and electrochemically Li ion extracted LiyCo0.85Al0.15O2 compounds

The local structure refinements for chemically and electrochemically Li ion extracted LiyCo0.85Al0.15O2 compounds have been investigated by Co K-edge X-ray absorption spectroscopy. In the X-ray absorption near-edge structure (XANES), the 1s --> 3d transition at similar to 7709.9 eV and 1s --> 4p transition at similar to 7727.8 eV for the pristine LiCo0.85Al0.15O2 have shifted effectively to higher energy regions of similar to 0.6 eV and similar to 2.5 eV for the higher Li ion extraction, respectively, which shows that the average oxidation state of Co ion increases gradually with the extraction. The systematic variations of peak intensities for the 1s --> 3d transition and 1s --> 4p transition result from Co 3d and 4p orbital mixing by the local structure distortion around Co atoms. In particular, the abrupt decrease of peak intensity for 1s - 4p transition with shakedown process by ligand to metal charge transfer (LMCT) represents the transfer of the hole state from the oxygen to Co atom and the localization at the Co atomic site as a form of Co-IV ion by structural distortion. The XANES features for the electrochemical extraction have shown that the electrochemical redox reaction is always not reversible in the Li ion extraction/insertion process. From the extended X-ray absorption fine structure (EXAFS) refinement, the interatomic distances of bond pairs decrease for the Li ion extraction. The chemical and electrochemical extractions have a significant effect on Fourier transform (FT) magnitude, which decreases linearly with the extraction. Since single and multiple scatterings with Co atoms have predominantly contributed to the FT magnitudes, the systematic decrease of FT magnitude is closely related to the static disorder of two different oxidative Co-III and Co-IV ions. This fact is consistent with the increase of the Debye-Waller factor for each bond pair.