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Page 4 of 13 Zhang et al. Energy Mater 2023;3:300008 https://dx.doi.org/10.20517/energymater.2022.71

active material (70%), Ketjen black (conductive agent, 10%), Super P (conductive agent, 10%), sodium
carboxymethylcellulose (NaCMC, binder, 5%) and styrene butadiene rubber (SBR, binder, 5%) [31,32] onto an
aluminum foil and then drying in a vacuum oven at 105 °C for 12 h. The mass loading of active materials
-1
was about 1.5-3 mg cm for the electrodes. The metallic sodium anode is used as the counter electrode. The
composition of the electrolyte was 1 mol L NaClO in ethylene carbonate (EC)/ propylene carbonate (PC)
-1
4
(1:1 v/v) with 2% fluoroethylene carbonate (FEC) as an additive (by volume). The separator was glass fibers
(GF/D) from Whatman. Cyclic voltammetry (CV) was conducted on an electrochemical workstation
(CHI760E) between 2.0 and 4.2 V versus Na /Na at a scan rate of 0.1 mV s . Galvanostatic intermittent
-1
+
titration technique (GITT) testing of the charging and discharging process was performed on a Land battery
-1
system at a current density of 15 mA g from 2.0-4.2 V, in which the coin cell was alternately charged for
10 min, followed by an interval of 60 min.
Calculation method
The first-principles calculations were performed within the density functional theory framework by using
the projector augmented wave (PAW) method, as implemented in the Vienna ab initio simulation package
(VASP) [33,34] . The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) was used
to treat the exchange-correlation interactions . The energy and force convergence values were chosen as
[35]
10 eV and 0.01 eV Å , respectively. The Kohn-Sham orbitals were expanded in plane waves with a kinetic
-1
-5
energy cut-off of 500 eV. The Brillouin zone integration and k-point sampling were performed with a
[36]
Monkhorst-Pack scheme of 3 × 3 × 3 grid for all calculations . The GGA + U correlation method was used
and the U value of Fe, Co and Ni atoms were set as 4, 3.4 and 6 eV, respectively . The diffusion barrier
[37]
[38]
energies of sodium ions were explored using the climbing image nudged elastic band (CI-NEB) method .
RESULTS AND DISCUSSION
Structure analysis
XRD patterns of a series of Co Ni [Fe(CN) ] PBAs with different ratios of Co and Ni (x = 0, 0.1, …, 1.0)
6
1-x
x
show monoclinic phase with a space group of P21/n [Supplementary Figure 1A], suggesting high Na
content of series of samples [15,39] . The refined results for Co Ni HCF [Figure 1A] and CoHCF
0.3
0.7
[Supplementary Figure 1B] were from Fullprof software [40-45] , and it demonstrates the lattice parameters of
Co Ni HCF with a = 10.3656(5) Å, b = 7.4286(4) Å, c = 7.2269(4) Å, α = γ = 90°, β = 92.805(3)°. About
0.7
0.3
CoHCF, a = 10.4118(7) Å, b = 7.4573(5) Å, c = 7.2608(5) Å, α = γ = 90°, β = 92.730(4)°, as shown in
Supplementary Table 1. The crystal structure of Co Ni HCF [Figure 1B] indicates the Fe-C-N and
0.7
0.3
+
C-N-Co (Ni) bond angles are deflected from the typical 180° because of the large accommodation of Na .
In the PBAs lattice, different types of water are removed at different temperatures. The water of Prussian
blue analogues (PBAs) can be divided into absorbed water, interstitial water and coordinated water, which
4-
occupy the surface, lattice gap and Fe(CN) vacancy of PBAs, respectively. Because the adsorbed water is
6
on the surface of the materials, it can be removed by simple heating and drying. The interstitial water can be
forcibly removed by increasing the heating temperature. Removal of coordinated water is even more
difficult and this may also lead to the destruction of the material structure . The occupancy of R m in the
[27]
cubic lattice of PBAs was detected by neutron diffraction and extended X-ray absorption fine structure
[30]
(EXAFS). The lattice water can occupy sites of 8 c, 32 f and 48 g . It is not difficult to find that the occupied
sites of water molecules coincide with the occupied sites of sodium ions. As shown in the TGA curves
[Figure 1C], the content of water of Co Ni HCF and CoHCF is very low due to very small amounts of
0.3
0.7
vacancies formed inside the crystal structure during the effectively controlled coprecipitation process. From
room temperature to 105 °C, the weight loss of Co Ni HCF and CoHCF is about 0.4% and 0.7%,
0.7
0.3
respectively, which can be ascribed to the loss of the adsorbed water on the surface. From 150 °C to 250 °C,
the weight loss of interstitial water in the open frameworks of Co Ni HCF and CoHCF is 10.3% and 10.4%,
0.7
0.3