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

In this study, to balance the high specific capacity and good cycle stability of PBAs, in the synthesis strategy
of Co Ni HCF, we used chelate and sodium salt-assisted coprecipitation crystallization method to obtain
0.7
0.3
the low-defect and Na-enriched PBA to ensure its high specific capacity. Moreover, we chose Ni to replace
Co partially to further stabilize the lattice structure, improving the cycling stability. The results of
inductively coupled plasma-optical emission spectrometer (ICP-OES) and thermogravimetric analysis
(TGA) indicate that Co Ni HCF has a Na-enriched and low-defect structure. Ex-situ X-ray diffraction
0.7
0.3
(XRD) reveals that Co Ni HCF has smaller volume change and better phase transition reversibility during
0.7
0.3
charging and discharging process than CoHCF. Furthermore, the first-principles calculations show that the
Ni substitution of CoHCF can improve the conductivity and reduce the sodium-ion migration barrier.
Galvanostatic intermittent titration technique (GITT) tests also exhibit that Co Ni HCF has a higher Na
+
0.3
0.7
diffusion coefficient. Both of them explain why the synthesized Co Ni HCF has good electrochemical
0.3
0.7
performance at low temperatures and high rates. Co Ni HCF shows excellent electrochemical
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0.3
performance in a wide temperature range (-30 to 60 °C). It exhibits an ultrahigh specific capacity
(142.2 mAh g at 0.2 C), high rate capability (126.2 mAh g at 5 C) and excellent cycling stability with 80.9%
-1
-1
capacity retention after 500 cycles at 5 C at room temperature. At -30 °C, Co Ni HCF can still provide
0.3
0.7
109 mAh g without an activation process. In addition, Co Ni HCF also shows stable electrochemical
-1
0.3
0.7
performance at higher temperatures of 45 °C and 60 °C due to a stable framework. This work confirms that
the low-defect and Na-enriched synthesis methods have obtained CoHCF with low structural defects, and
the introduction of Ni further improves the structural stability. Also, Ni substitution improves the
conductivity and diffusion kinetics of the materials, making Co Ni HCF a powerful candidate for the
0.3
0.7
cathode of all-climate high-energy sodium-ion batteries.
EXPERIMENTAL
Materials synthesis
A series of Na Co Ni [Fe(CN) ]•yH O (Co Ni HCF) (x = 0, 0.1, …, 1.0) PBAs were synthesized by
1-x
x
2
1-x
6
x
2
coprecipitation method. Solution A was formed by dissolving 2x mmol CoCl ·6H O, 2(1-x) mmol
2
2
NiCl ·6H O, 10 mmol trisodium citrate and 4 g NaCl in 50 mL DI water. Solution B was formed by
2
2
dissolving 2 mmol Na Fe (CN) ·10H O in 50 mL DI water. Then, solution A was dropped into solution B
2
6
4
simultaneously by a peristaltic pump with magnetic stirring for 4 h and aged for 24 h at 30 °C. The
precipitated products were collected by centrifugation and washed thoroughly with deionized water and
ethanol several times. Finally, after drying at 105 °C in a vacuum oven for 24 h, samples were collected and
named Co Ni HCF (x = 0, 0.1, …, 1.0). CoHCF and Co Ni HCF were synthesized as described process
0.7
x
0.3
1-x
above when x = 1 and x = 0.7, respectively.
Materials characterization
XRD measurement was performed by Cu Kα radiation (λ = 1.541874 Å) in a scan range (2θ) of 10-80° on
Panalytical X’pert PRO MRD. The scanning electron microscope (SEM, Japan) was used to collect FESEM
images to display the morphology of the samples at an acceleration voltage of 30 kV. The transmission
electron microscopy (TEM, Tecnai G2 F30 S-TWIN, FEI) was used to obtain TEM images of the cathode.
The thermogravimetry measurements were performed using a NETZSCH-STA449c/3/G analyzer from 30
to 500 °C at a heating rate of 10 °C/min in N . The Fourier transform infrared (FTIR) spectra were obtained
2
by A VERTEX 70 FT-IR spectrometer (4000-500 cm region). The composition of PBAs was measured by
-1
ICP-OES (IRIS Intrepid IIXSP, Thermo Elemental, USA). X-ray photoelectron spectroscopy (XPS) analysis
was conducted through a Thermo ESCALAB 250XI instrument.
Electrochemical measurement
For the electrochemical performance tests of PBAs, CR2025 coin cells were assembled in a glove box filled
with pure Ar gas. The working electrodes were made by painting the uniform slurry which contained the