Charge compensation and capacity fading in LiCoO₂ at high voltage investigated by soft x-ray absorption spectroscopy

In order to obtain an in-depth insight into the mechanism of charge compensation and capacity fading in LiCoO₂, the evolution of electronic structure of LiCoO₂ at different cutoff voltages and after different cycles are studied by soft x-ray absorption spectroscopy in total electron (TEY) and fluorescence (TFY) detection modes, which provide surface and bulk information, respectively. The spectra of Co L_2,3-edge indicate that Co contributes to charge compensation below 4.4 V. Combining with the spectra of O K-edge, it manifests that only O contributes to electron compensation above 4.4 V with the formation of local O 2p holes both on the surface and in the bulk, where the surficial O evolves more remarkably. The evolution of the O 2p holes gives an explanation to the origin of O₂^- or even O₂. A comparison between the TEY and TFY of O K-edge spectra of LiCoO₂ cycled in a range from 3 V to 4.6 V indicates both the structural change in the bulk and aggregation of lithium salts on the electrode surface are responsible for the capacity fading. However, the latter is found to play a more important role after many cycles.