Electronic structural changes of the electrochemically Li-ion deintercalated LiNi_0.8Co_0.15Al_0.05O₂ cathode material investigated by X-ray absorption spectroscopy

In situ hard X-ray absorption spectroscopy (XAS) at metal K-edges and soft XAS at O K-edge and metal L-edges have been carried out during the first charging process for the layered Li_1-xNi_0.8Co_0.15Al_0.05O₂ cathode material. The metal K-edge XAS results show that the major charge compensation at the metal site during Li-ion deintercalation is achieved by the oxidation of Ni ions, while the cobalt ions remain mostly unchanged in the Co³⁺ state. Ni L_II,III-edge and O K-edge XAS results in both the fluorescence yield (FY) and partial electron yield (PEY) modes show that substantial amount of Ni ions at the surface of LiNi_0.8Co_0.15Al_0.05O₂ powders exist as Ni²⁺, whereas most of Ni ions in the bulk are in the form of Ni³⁺. Therefore, if the PEY mode, which is a surface-sensitive technique, is used alone, the interpretation of the results is limited to the surface structures only. In order to get the full picture of both the surface and the bulk, the FY mode and PEY mode should be used simultaneously.