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Page 10 of 13 Zhang et al. Energy Mater 2023;3:300008 https://dx.doi.org/10.20517/energymater.2022.71

Figure 6. First-principles calculations. Electronic density of states of (A) CoHCF and (B) Co Ni HCF at different Na concentrations,
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+
including Na-1.5, Na-1, and Na-0.5. (C) The schematic of the calculated Na migration paths within the lattice of the Co Ni HCF and
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+
CoHCF model structures. (D) Migration energy barriers of the Na -ion diffusion within the lattice of the Co Ni HCF and CoHCF.
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the phase transition occurs reversibly from cubic to monoclinic phase, indicating the extracted sodium ions
can reversibly insert into the lattice even at -30 °C. This reversible two-phase transition is similar to that
occurring at room temperature of Co Ni HCF, demonstrating a fast ion transfer kinetics in Co Ni HCF.
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In contrast, Figure 5D presents no phase transition for charged CoHCF, which maintains the monoclinic
phase during the charging and discharging process at -30 °C, suggesting that too few sodium ions are
extracted from the framework of CoHCF to induce a phase transition. This result can be ascribed to the fact
that sodium ions in CoHCF are difficult to migrate at -30 °C and most sodium ions are still preserved in the
[50]
lattice. The XPS test results of Co 2p at -30 °C in Figure 5B and E show that the binding energy of Co 2p
peak in CoHCF almost does not change during cycling at -30 °C, while the peak of Co 2p exhibits a
significant peak shift towards higher bind energy from Co Ni HCF-Pristine to Co Ni HCF-Charged,
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revealing obvious electronic migration has occurred in the cobalt atoms of Co Ni HCF-Charged. The XPS
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data of Fe 2p at -30 °C are shown in Figure 5C and F for Co Ni HCF and CoHCF samples. Co Ni HCF-
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Charged can be fitted with an obvious Fe 2p peak located at 710.1 eV and 723.7 eV, which is not found in
III
CoHCF-Charged. It is clear that the electrochemical activity of transition metals in CoHCF is severely
inhibited. The valence change of transition metals shows that Co Ni HCF has higher redox activity at low
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temperatures.
According to the partial density of states (PDOS) of Co Ni HCF and CoHCF, the bandgap of
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Co Ni HCF is obviously reduced after the replacing of Co by Ni, leading to the enhanced electronic
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conductivity, which is beneficial for redox kinetics. Figure 6A shows the PDOS of CoHCF with different
sodium content during sodium extraction process. The Co e orbital of Na CoFe(CN) is close to the Fermi
g
1.5
6
level, which means Co is redox active. With the extraction of sodium ions, the t orbital of Fe gradually
2+
2g
approaches the Fermi level, indicating that Fe participates in the redox reaction when further charging. The
above analysis of electronic density of states reveals the charge-transfer mechanism of CoHCF, which
matches well with the CV measurements. While in Co Ni HCF, both Co e and Fe t orbitals locate at
2g
g
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Fermi level in the Na-rich and Na-poor samples, as shown in Figure 6B. It can be seen that the charge-
transfer mechanism of each transition metal ion in Co Ni HCF may be different from CoHCF, although
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[51]
the CV curves of CoHCF and Co Ni HCF are two similar peaks . In addition, it can be found that Ni has
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involved in charge transfer along frameworks throughout the full redox reaction; the electrons around Ni
have delocalized due to the variation of coordination environment, which is also of great benefit to obtain