Oxygen contribution on Li-ion intercalation-deintercalation in LiCoO₂ investigated by O K-edge and Co L-edge X-ray absorption spectroscopy

To investigate the electronic structure of the electrochemically Li-ion deintercalated Li_1-xCoO₂ system, soft X-ray absorption spectroscopy (XAS) for the oxygen K-edge and the Co L_II,III-edge has been carried out intensively with compositional x value variation, compared with Co K-edge X-ray absorption near edge structure (XANES) spectroscopy. To get reasonably good XAS spectra for the electrochemically Li-ion deintercalated Li_1-xCoO₂ system, we made a binder-free LiCoO₂ film electrode using the electrostatic spray deposition (ESD) technique. The oxygen K-edge XAS for Li_1-xCoO₂ shows more effective spectral change with respect to Li-ion content than the Co L_II,III-edge XAS. The dependence of the absorption peak feature on the Li content is described in terms of the ground state of the Co and O atoms, showing the systematic variation of the hole-state site distribution between Co and oxygen atoms. From the Co L_II,III-edge XAS, it is found that the broad peak shift to higher energy with the Li-ion deintercalation is due to rehybridization between Co and O atoms under the local structural distortion of CoO₆ octahedra, which is also confirmed by the formation of two additional absorption peaks below the threshold energy corresponding to the oxygen 2p hole state hybridized with the 3d orbital of Co ion in the distorted CoO₆ octahedral site. In the O K-edge XAS spectra for the deintercalated Li_1-xCoO₂ film, the shoulder absorption peak in the energy region higher than the threshold energy could be assigned to the higher oxidation state of the oxygen site on Li deintercalation. From the Co L_II,III-edge and O K-edge XAS results for the Li-ion deintercalated Li_1-xCoO₂ film, it is concluded that the charge compensation for the Li-ion deintercalation process could be achieved mainly in the oxygen site and Co metal atomic site simultaneously. OK-edge and Co L_II,III-edge XAS results for cycled LiCoO₂ film show that the capacity fading of the LiCoO₂ system is related to the decrease of Co-O bond covalency by the local structural distortion of CoO₆ octahedra remaining in the cycled LiCoO₂.