XPS was adopted to investigate the oxidation states of elements
and the chemical compositions of MgCo2O4 NCAs. Except the C1s
peaks for calibration, the XPS spectra in Fig. 2c shows distinct peaks
and two weak peaks, which are on account of the existence of Mg,
Co, Ni and O elements in the MgCo2O4 NCAs. The atomic ratio of Mg
to Co is close to 1:5 in the MgCo2O4 NCAs. The binding energy of Mg
1s is around 1302.8 eV (Fig. 2d), which indicates the existence of Mg
element in the composite as previously reported [36]. The two
peaks of Co 2p at 780.0 and 795.5 eV (Fig. 2e) correspond to Co 2p1/2
and Co 2p3/2 respectively, suggesting the oxidation state of Co3þ
[37]. The O 1s spectrum in Fig. 2f consists of two oxygen contributions
at 529.5 and 531.1eV, which are attributed to the metaleoxygen
bonds and numerous defect sites with low oxygen
coordination, respectively [38].