Figure 4: Chemical evolution at the surface of NiO nanosheets.

(a) XAS/AEY spectra for electrodes at different SOC, and the voltage-time profile is provided in parallel to indicate the states where data collection was accomplished. XAS/AEY is a surface detection mode and the Ni L-edge signal disappears at the fully discharged state due to a thick coating of AEI on the surface. (b) XAS/TEY O K-edge spectra for electrodes in the pristine state (grey) and in the 50% SOC (red), and the assignment of peaks is indicated in the figure. (c) STEM image of a NiO nanosheet during a discharging process, and NiO, Ni and AEI are indicated with arrows in the image. The scale bar is 50 nm. (d) EELS Li K-edge, C K-edge and O K-edge spectra for the AEI layer, and the assignment of peaks is indicated in the figure. A reference Li K-edge spectrum of Li₂CO₃ (ref. 43) is provided in the figure.