Electronic states in oxidized Na_xCoO₂ as revealed by X-ray absorption spectroscopy coupled with ab initio calculation

Layered cobalt oxides are promising cathode materials for sodium ion secondary batteries (SIBs). By combined study of the X-ray absorption spectroscopy (XAS) around the O K-edge and ab initio calculation, we investigated the electronic state of the Na_xCoO₂ with different oxidization state, i.e, in O3-Na_0.91CoO₂ (CoO₂ ^−0.91) and P2-Na_0.66CoO₂ (CoO₂ ^−0.66). The O K-edge spectra in the pre-edge (529–536 eV) region shows significant change with oxidization of Na_xCoO₂. In O3-Na_0.91CoO₂, the spectra shows an intense band (B band) at 531 eV. In P2-Na_0.66CoO₂, the spectral weight of the B band increases and a new band (A band) appears at 530 eV. These spectral changes are qualitatively reproduced by the calculated partial density of states (pDOSs) of O3-NaCoO₂ and P2-Na_1/2CoO₂. These results indicate that the electrons are partially removed from the O 2p state with oxidization of Na_xCoO₂.