Figure 4. Changes in electronic structures for Li_1.3−xNb_0.3Fe_0.4O_2−δ on initial charge/discharge and second charge.

Changes in the O K-edge (a) Fe L_{II, III}-edge XAS spectra (b), Nb L_III-edge XAS spectra (c), and the points where XAS spectra have been collected in (d). XAS spectra of KO₂ (superoxide) and Li₂O₂ (peroxide) are also shown in (a) for comparison. Niobium is not responsible for charge compensation process (other data sets of Nb are shown in Supplementary Fig. 10). (e) TEM images of Li_1.3Nb_0.3Fe_0.4O₂ and Li_1.3Nb_0.3Mn_0.4O₂ particles before and after the electrochemical cycle at 50 °C. Oxygen loss for the Fe system results in the formation of nanosized grains in the single particle, and not for the Mn system. Lattice fringes are clearly observed after electrochemical cycle for the Mn system. (f) Surface structures observed by X-ray photoelectron (XPS) spectroscopy after discharge to 1.5 V. Oxygen molecules released on charge result in the electrochemical reduction on discharge, leading to the formation of surface deposits because of chemical reaction between active superoxide with electrolyte, as shown in our previous literature¹¹.