Changes in electronic structure of the electrochemically Li-ion deintercalated LiNiO₂ system investigated by soft X-ray absorption spectroscopy

The electronic structures of Li_1-xNiO₂ system after electrochemically deintercalated to various "x" values have been investigated using soft X-ray absorption spectroscopy (XAS) for oxygen K-edge and Ni L_II,III-edge. By comparing the O K-edge XAS spectra of Li_1-xNiO₂ electrode collected simultaneously by the partial electron yield (PEY) mode and florescence mode (FY), it was found that the surface electronic structure of Li_1-xNiO₂ electrode is different from the bulk. Therefore, if the PEY mode, which is a surface-sensitive technique, is used alone, the reliability of the results is limited to the surface structures only. Ni L_II,III-edge XAS results of Li_1-xNiO₂ electrode at different charge states, in both the FY and PEY modes, show that Ni²⁺ ions at the surface are mostly oxidized to Ni³⁺ ions during Li deintercalation, whereas the process in the bulk is the oxidation of Ni ions from Ni³⁺ to Ni⁴⁺.