fig4

Figure 4. Sodium storage mechanism at 25 °C. Ex-situ XRD patterns of the (A) Co_0.7Ni_0.3HCF and (B) CoHCF electrodes at different states during the first cycle. The dots in different colors in the charge-discharge curves (right) represent the states tested by ex-situ XRD. (C) Schematic phase evolution as a result of Na⁺ extraction/insertion from/into the Co_0.7Ni_0.3HCF.