High-quality Fe4[Fe(CN)6]3 Nanocubes Preparation: As Cathode Material for Aqueous Sodium-ion Battery

WANG Wu-Lian. High-quality Fe4[Fe(CN)6]3 Nanocubes: Synthesis and Electrochemical Performance as Cathode Material for Aqueous Sodium-ion Battery. Journal of Inorganic Materials[J], 2019, 34(12): 1301-1308 doi:10.15541/jim20190076

Part 2: Structure Characterization of Fe4[Fe(CN)6]3 Nanocubes

Figure 1(a) shows the XRD patterns of HQ-FeHCF and LQ-FeHCF. It can be seen from the figure that all the diffraction peaks of HQ-FeHCF are consistent with the JCPDS NO. 01-0239 card. It shows that the synthesized HQ-FeHCF has a face-centered cubic (fcc) structure, which belongs to the fm-3m space point group, a=b=c=0.51 nm, α=β=γ=90°. There were no other peaks, indicating that the synthesized HQ-FeHCF was of high purity. Its sharp characteristic peaks also indicate that the HQ-FeHCF nanomaterials synthesized slowly by adding PVP have excellent crystallinity and have a typical Fe4[Fe(CN)6]3 crystal structure. The diffraction peaks of LQ-FeHCF prepared by rapid precipitation are not sharp, indicating that its crystallinity is poor. The illustration in the upper right corner of Figure 1(a) is a schematic diagram of the unit cell structure of HQ-FeHCF, which is composed of an open three-dimensional framework, Fe1 is connected to six nitrogen atoms, and Fe2 is surrounded by carbon atom octahedrons coordinated with cyanide. There is a large interstitial site in the middle of this open framework structure, which provides a large enough space for the insertion/extraction of Na+. In order to determine the content of water of crystallization in the synthesized materials, thermogravimetric analysis tests were carried out on HQ-FeHCF and LQ-FeHCF. Under N2 atmosphere, the results measured at a heating rate of 10 °C/min are shown in Fig. 1(b).  The weight loss at 30-200 ℃ corresponds to the removal of crystal water; the weight loss at 200-400 ℃ corresponds to the decomposition of [Fe(CN)6].  It can be seen from Figure 1(b) that the content of HQ-FeHCF crystallization water is 13%, and that of LQ-FeHCF crystallization water is 18%. HQ-FeHCF contains less water of crystallization than LQ-FeHCF, which also indicates that HQ-FeHCF has fewer [Fe(CN)6] vacancy defects than LQ-FeHCF.  In order to further accurately test the content of [Fe(CN)6] vacancy defects in the material, HQ-FeHCF and LQ-FeHCF were refined by XRD, as shown in Table 1 and Table 2. In HQ-FeHCF, the Fe2/Fe1 atomic ratio is 0.91, indicating that there are 9% [Fe(CN)6] vacancy defects. In LQ-FeHCF, the Fe2/Fe1 atomic ratio is 0.74, indicating that the [Fe(CN)6] vacancy defect content is 26%.

Fig. 1   (a) XRD patterns and (b)TG curves of HQ-FeHCF and LQ-FeHCF with inset in (a) showing crystal structure of HQ-FeHCF

Table 1  Fractional coordinates of HQ-FeHCF determined from Rietveld method

Table 2  Fractional coordinates of LQ-FeHCF determined from Rietveld method

Figure 2(a~b) are SEM photos of HQ-FeHCF at different magnifications, and it can be clearly seen that HQ-FeHCF is a cube structure with a side length of about 500 nm. The surface of the cube is regular and complete, and the sample particles are well dispersed, uniform in size, and without serious accumulation. Figure 2(c~d) are SEM photos of LQ-FeHCF at different magnifications, it can be seen that LQ-FeHCF is in irregular granular shape. This is because the rapid precipitation process makes LQ-FeHCF not have a complete and regular structure morphology. Moreover, there are a large number of disordered [Fe(CN)6] vacancy defects and crystal water, which will also lead to poor electrochemical performance of LQ-FeHCF.

In order to further observe the microscopic morphology of HQ-FeHCF and LQ-FeHCF, the materials were characterized by TEM. As shown in Figure 3(a), each HQ-FeHCF nanocubic particle has a smooth edge and a complete shape without obvious defects, which also shows that the synthesized HQ-FeHCF has good crystallinity and high quality. As shown in Figure 3(b), LQ-FeHCF has different particle sizes and irregular structural features, which is consistent with the SEM photo of LQ-FeHCF in Figure 2, indicating that LQ-FeHCF has poor crystallinity, low quality, and many defects.

Fig. 2   SEM images of (a-b) HQ-FeHCF and (c-d) LQ-FeHCF

Fig. 3   TEM images of (a) HQ-FeHCF and (b) LQ-FeHCF

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