Oxygen Contribution on Li-Ion Intercalation−Deintercalation in LiCoO2 Investigated by O K-Edge and Co L-Edge X-ray Absorption Spectroscopy

Abstract

To investigate the electronic structure of the electrochemically Li-ion deintercalated Li 1 - x CoO 2 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 - x CoO 2 system, we made a binder-free LiCoO 2 film electrode using the electrostatic spray deposition (ESD) technique. The oxygen K-edge XAS for Li 1 - x CoO 2 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 CoO 6 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 CoO 6 octahedral site. In the O K-edge XAS spectra for the deintercalated Li 1 - x CoO 2 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 - x CoO 2 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. O K-edge and Co L II,III -edge XAS results for cycled LiCoO 2 film show that the capacity fading of the LiCoO 2 system is related to the decrease of Co−O bond covalency by the local structural distortion of CoO 6 octahedra remaining in the cycled LiCoO 2 .