In the past few years, Na–ion batteries have been considered as promising candidates for large-scale energy storage system instead of Li–ion batteries because of the natural abundant resources of sodium.  Owing to the excellent cyclability and high rate performance, sodium iron hexacyanoferrate (NaFeHCF) with the general chemical formula Na_xFe[Fe(CN)₆]_y·nH₂O has been researched widely as positive electrode material for Na–ion batteries.  However, off–stoichiometry of NaFeHCF, defects of [Fe(CN)₆] and Na, would deteriorate its reversible capacity.  Therefore, much effort has been devoted to prepare stoichiometric Na₂Fe[Fe(CN)₆] without crystal defects.  For example, Guo and his co–workers have synthesized NaFeHCF with few defects of [Fe(CN)₆] and Na by a precipitation method with HCl solution [1].  During the precipitation synthesis, however, hazardous HCN and NaCN are inevitably produced on the decomposition reaction of Na₄Fe(CN)₆as a starting material in HCl solution.  For safety and environmental concerns, other synthesis method should be used to prepare low–defects NaFeHCF.

  To avoid the HCN or NaCN generation and the defects of final product, control of nuclei formation and crystal growth process of synthesis without HCl is required in general.  Because chelate has the ability to suppress the both process, trisodium citrate (Na₃C₆H₅O₇) has been used as chelator for the preparation of Prussian blue analogues such as NaCoHCF and NaNiHCF [2,3].  Recently, Liu et al.reported the precipitation chelate synthesis of NaFeHCF with trisodium citrate [4].  However, the NaFeHCF shows insufficient electrochemical property because it would have lower crystallinity than that of NaFeHCF synthesized with HCl.  In this study, we optimize the synthesis condition with using trisodium citrate to obtain low–defects and highly redox-acitive NaFeHCF and systematically investigate the synthesis process, crystal structure, particle morphology and electrochemical properties of NaFeHCF.

  Na₄Fe(CN)₆ and FeCl₂ were separately dissolved in 0.2 M trisodium citrate or NaCl solutions.  These two solutions were slowly mixed together with stirring under N₂ atmosphere.  Then, the precipitate was centrifuged and washed with deionized water and ethanol.  The final products synthesized with citrate or NaCl (hereafter denoted as Cit–NaFeHCF or NaCl–NaFeHCF, respectively) were obtained after drying in vacuum oven for 24 h.  Electrochemical measurements were carried out using 2032 coin cells with NaPF₆ dissolved in EC:DEC (1:1) solution as electrolyte and sodium metal as a negative electrode.  Positive electrodes were prepared by mixing 70 wt.% active material, 20 wt.% Ketjen black carbon and 10 wt.% PVdF binder.  The charge and discharge measurements were carried out at room temperature at a current density of 60 mA g^–1 in the voltage range of 2.0 – 4.2 V vs. Na/Na⁺.

  Figure 1 shows the XRD patterns of Cit–NaFeHCF and NaCl–NaFeHCF.  Diffraction lines of Cit–NaFeHCF can be indexed with a space group of P2₁/n and no diffraction lines from impurity phases were observed, which is similar to those of NaFeHCF synthesized with HCl solution in the literature [1].  On the other hand, the diffraction peaks at 24 and 38° for NaCl–NaFeHCF were a single line without splitting, which indicates that the crystal structure of NaCl–NaFeHCF is cubic and the diffraction lines can be indexed with a space group of Fm–3m.  This structural difference between two samples implies that NaFeHCF having fewer defects of [Fe(CN)₆] and Na was successfully prepared with trisodium citrate compared to those of the material synthesized with NaCl.

  Figure 2(a) shows the initial charge/discharge curves of Cit–NaFeHCF and NaCl–NaFeHCF in Na cells.  Initial charge capacity of NaCl–NaFeHCF was lower than that of Cit–NaFeHCF because of the large number of Na defects.  In contrast, initial charge capacity of Cit–NaFeHCF was slightly higher than initial discharge capacity, which indicates few Na defects in Cit–NaFeHCF.  Additionally, the discharge capacity of Cit–NaFeHCF was higher than that of NaCl–NaFeHCF and reversible capacity of 156 mAh g^–1is obtained as high as that for the material synthesized with HCl [1].  Furthermore, Cit–NaFeHCF exhibits better capacity retention over 50 cycles than that of NaCl–NaFeHCF.  The structural and morphological differences based on the synthesis process will be discussed in detail.

  References:

[1] Y. You and Y. Guo et al., Nano Res., 8(1), 117 (2015)

[2] X. Wu and H. Yang et al., ChemNanoMat., 1(3), 188 (2015)

[3] Y. Chiang and Y. Yamauchi et al., Eur. J. Inorg. Chem., 18,3141 (2013)

[4] Y. Liu and Y. Huang et al., Nano Energy, 12, 386 (2015)