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

The electronic structures of Li1−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 LII,III-edge. By comparing the O K-edge XAS spectra of Li1−xNiO2 electrode collected simultaneously by the partial electron yield (PEY) mode and florescence mode (FY), it was found that the surface electronic structure of Li1−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 LII,III-edge XAS results of Li1−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+.