Direct observation of reversible charge compensation by oxygen ion in Li-rich manganese layered oxide positive electrode material, Li1.16Ni0.15Co0.19Mn0.50O2

•Ni and Co ions reversibly participate in the charge compensation.•Mn ion stayed unchanged in the in the Mn4+ state even at the charged states.•The reversible redox reaction of oxygen ion was confirmed by O K-edge XAS spectra.•The peroxide phase exists stably in the crystalline network.

The reversible redox reaction of oxygen ion was directly observed by soft X-ray absorption spectroscopy for Li1.16Ni0.15Co0.19Mn0.50O2, the Li-rich manganese layered oxide. The O K-edge spectra indicated that a peroxide phase (namely the formation of O22−) appeared and disappeared reversibly by charging and discharging. It was simultaneously observed both in the total electron yield mode and in the partial fluorescence X-ray yield mode, indicating that the peroxide phase exists stably in the crystalline network. These species should play an important role in the high capacity positive electrode.