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Zhang et al. Energy Mater 2023;3:300008 https://dx.doi.org/10.20517/energymater.2022.71 Page 9 of 13

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 Ni HCF and CoHCF at different redox states including as-prepared electrodes (Co Ni HCF/CoHCF-Pristine),
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electrodes charged to 4.2 V (Co Ni HCF/CoHCF-Charged) and electrodes charged to 4.2 V and then discharged to 2.0 V (Co Ni 0.3
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HCF/CoHCF-Discharged).
change determines the cycling stability. Since the two-phase transition is caused by sufficient sodium ions
de-/insertion, phase transition always relates to high capacity. Therefore, considering both the capacity and
cycling stability, it is of particular significance to control the lattice distortion within a small range while
ensuring more sodium ions are de-/intercalated. Here, Ni substitution in CoHCF has been successfully
achieved.
Furthermore, XRD patterns and FTIR spectra of Co Ni HCF and CoHCF electrodes collected before and
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after 500 cycles are shown in Supplementary Figure 9. After 500 cycles, the peak corresponding to the 200
plane of CoHCF shifts from 16.897° to 16.940° (0.043° offset), indicating that the Na content of CoHCF
electrode is reduced after 500 cycles, while the peak of Co Ni HCF mainly does not shift, suggesting
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robust structural stability during long-term cycling. The Rietveld refinements of Co Ni HCF and CoHCF
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after 500 cycles are shown in Supplementary Figure 10. According to the lattice parameters of Co Ni HCF
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and CoHCF before and after 500 cycles [Supplementary Table 4], the volume change rates before and after
500 cycles of Co Ni HCF and CoHCF are 0.04 % and 1.12 %, respectively. According to the occupancy of
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sodium ions in the lattice, reduced percentages of sodium ions of Co Ni HCF and CoHCF before and
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after 500 cycles are 0.79% and 4.01%, respectively. Therefore, it can be preliminarily judged that
Co Ni HCF has a more stable structure.
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It is also confirmed by the results of FTIR tests at different cycles. The absorption peaks at ~2075 cm
corresponding to the stretching vibration of C≡N of Co Ni HCF and CoHCF shift to higher wavenumbers
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(4 cm and 9 cm offset, respectively) after 500 cycles, indicating that the average valence state of transition
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metals in CoHCF is higher than that in Co Ni HCF [48,49] . This result indicates that the percentage of
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sodium retained in Co Ni HCF after long-term cycling is higher than that in CoHCF compared with
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pristine electrodes.
As shown in Figure 5A, the multi-peak from 23° to 25° indexed as -211, 020, 002 and 211 of monoclinic
structure transform into a single peak corresponding to the 220 plane of cubic phase around 25°, indicating
the phase transition from monoclinic to cubic has occurred at -30 °C in charged Co Ni HCF, which is
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owing to the extraction of enough sodium ions from the lattice. While discharge from 4.2 to 2 V at -30 °C,