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 tra nsition at similar to 7727.8 eV for the pristine LiCo0.85Al0.15O2 have shif ted effectively to higher energy regions of similar to 0.6 eV and similar t o 2.5 eV for the higher Li ion extraction, respectively, which shows that t he average oxidation state of Co ion increases gradually with the extractio n. The systematic variations of peak intensities for the 1s --> 3d transiti on and 1s --> 4p transition result from Co 3d and 4p orbital mixing by the local structure distortion around Co atoms. In particular, the abrupt decre ase of peak intensity for 1s - 4p transition with shakedown process by liga nd to metal charge transfer (LMCT) represents the transfer of the hole stat e 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 el ectrochemical 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 intera tomic distances of bond pairs decrease for the Li ion extraction. The chemi cal and electrochemical extractions have a significant effect on Fourier tr ansform (FT) magnitude, which decreases linearly with the extraction. Since single and multiple scatterings with Co atoms have predominantly contribut ed 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.