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 Li1 - x CoO2 system, soft X-ray absorption spectroscopy (XAS) for the oxygen K-edge and the Co LII,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 Li1 - x CoO2 system, we made a binder-free LiCoO2 film electrode using the electrostatic spray deposition (ESD) technique. The oxygen K-edge XAS for Li1 - x CoO2 shows more effective spectral change with respect to Li-ion content than the Co LII,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 LII,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 Li1 - x CoO2 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 LII,III-edge and O K-edge XAS results for the Li-ion deintercalated Li1 - x CoO2 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 LII,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.

Electron delocalization in cyanide-bridged coordination polymer electrodes for Li-ion batteries studied by soft x-ray absorption spectroscopy

“…Thus, by Li-ion insertion, the π/π* orbitals should be reduced in accompany with the Fe t 2g orbital, suggesting the electron deloalization on the hybridized Fe t 2g and CN π/π* orbitals. As for the O K-edge XAS studies of TM-oxide such as Li 1−x CoO 2 and Li 1−x FePO 4 , their spectral changes were much larger than those in the present C and N K-edge XAS [22][23][24] . This may be due to the difference in the or- bital hybridization mechanism.…”

Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries

“…1e-g), which suggest an ensemble-averaged chemical homogeneity in the NMC material. Finally, the formal oxidation states of Ni, Mn and Co are determined to be þ 2, þ 4 and þ 3, respectively, as expected 31,[35][36][37] . Furthermore, the formal oxidation state of Ti is þ 4, and the shape of L-edge verifies the Ti substitution in the R 3m layered lattice ( Supplementary Fig.…”

Three-dimensional phase field based finite element study on Li intercalation-induced stress in polycrystalline LiCoO2

“…The Li diffusion-induced stresses within electrode materials can lead to their fracture and failure which results in battery capacity loss and power fade. For LiCoO2, it is found that there exists the capacity fade about 2.2% and 6.5% for exchange of 0.5 Li per CoO2 after 10 and 50 chargedischarge cycles, and the decrease of Co-O bond length was observed by using X-ray absorption spectroscopy [2]. A previous transmission electron microscopy (TEM) study showed that 20% of the LiCoO2 particles were indeed fractured after 50 cycles at 0.2C between 2.5 and 4.35 V [3].…”

“…There are three assumptions for the diffusion model in this study: (1) the diffusion obeys the Fick's 2nd law in grain and grain boundary; (2) No segregation effect in grain boundary is considered, and (3) the diffusivity of Li ion in grain boundary is smaller than that in the grain.…”