fig5

Figure 5. Sodium storage mechanism at -30 °C. (A and D) Ex-situ XRD patterns. (B and E) Ex-situ XPS Co 2p spectra and (C and F) Ex-situ XPS Fe 2p of Co_0.7Ni_0.3HCF and CoHCF at different redox states including as-prepared electrodes (Co_0.7Ni_0.3HCF/CoHCF-Pristine), electrodes charged to 4.2 V (Co_0.7Ni_0.3HCF/CoHCF-Charged) and electrodes charged to 4.2 V and then discharged to 2.0 V (Co_0.7Ni_0.3HCF/CoHCF-Discharged).