Soft X-ray absorption spectroscopy studies on the chemically delithiated commercial LiCoO2 cathode material

Variation of electronic structure for chemically delithiated LiCoO2 was investigated by X-ray absorption spectroscopy (XAS) at Co L-edge, K-edge and O K-edge. The commercial LiCoO2 powders were de-intercalated by an oxidizing agent, Na2S2O8, in an aqueous solution. The soft XAS was performed at total electron yield as well as fluorescence modes. It was found that the changes in the spectral feature obtained from total electron yield mode are different from that obtained from fluorescence mode at the oxygen K-edge. The electronic transition from oxygen ls state to 2p hole state (threshold energy) decreases at the surface region but not in the bulk. The shoulder absorption peak at the energy higher than the threshold energy contributed to the higher oxidation state of the oxygen site appears at total electron yield mode but does not appear at fluorescence mode, indicating the oxidation state of oxygen and the electronic transitions on the surface and in the bulk of the delithiated compounds are different. It implies that the chemically delithiated process takes place inhomogeneously and starts from the surface of the LixCoO2 compounds.