Co K-edge XANES of LiCoO2 and CoO2 with a variety of structures by supercell density functional calculations with a core hole

Co K-edge x-ray absorption near edge structures (XANES) of LiCoO2 and CoO2 with a variety of structures, and their orientation dependence, have been evaluated by density functional theory calculations using supercells with a core hole. The spectrum of the layered rocksalt LiCoO2, which is available by experiment, is well reproduced. The effects of the stacking sequence of layered structures and those of Li/Co arrangement in ordered rocksalt polymorphs on XANES are systematically investigated. The spectral shape is sensitive to the stacking sequence in the layered LiCoO2 polymorphs, while it is insensitive in the corresponding CoO2. On the other hand, the ordered-rocksalt polymorphs of LiCoO2 with different Li/Co arrangements exhibit similar spectra, while strong dependence on the manner of the ordering is found in CoO2. Although XANES is rich in information, the use of theoretical fingerprints is desirable for reliable analysis of the oxidation states and identification of the stacking sequence and the manner of ordering.