In this paper, we report on the structural, electrochemical and conductivity properties of the NASICON phase NaFe2(PO4)(SO4)2. The crystal structure was solved by the Rietveld method using powder x-ray diffraction (PXRD) data. SAED and HRTEM experiments confirmed the proposed structural model. The electrochemical performances were examined by galvanometric cycling, cyclic voltammetry, and impedance spectroscopy. The stability of the NASICON structure during cycling was also followed by ex-situ PXRD experiments.
NaFe2(PO4)(SO4)2 was synthesized via a solid state synthesis route from stoichiometric mixtures of NaNO3, Fe(NO₃)₃.9H₂O, (NH4)2SO4, and NH4H2PO4. The starting raw materials, dissolved in an aqueous medium, were stirred at 80oC until evaporation of water. The resulting orange powder was calcined at 550oC for 12h under air, then a pure green powder of NaFe2(PO4)(SO4)2 was obtained. TG-DTA-MS experiments resulted in a 30% weight loss which corresponds to the departure of two SO2 molecules above 600oC, and hence confirm the chemical composition of NaFe2(PO4)(SO4)2. The crystal structure of the latter was solved using the structure of the NASICON NaTi2(PO4)3 as a starting structural model. The Rietveld analysis of the x-ray powder diffraction data led to the reliability factors, Rp = 0.5%, wRp = 0.8%, RB = 9.17%, which corroborates the NASICON crystalline structure of NaFe2(PO4)(SO4)2. When compared to Na4Zr2(SiO4)3, the formula can be rewritten as Na1□3Fe2(PO4)(SO4)2 with the 6b and 18e atomic positions occupied by sodium and vacancies, respectively (□ stands for vacancy). Sodium atoms and vacancies are located in two type of interstitial positions within 3D-interconnected channels formed by the covalent skeleton [Fe2P3O12]- built of FeO6 octahedra and XO4 tetrahedra (X= S, P). To further confirm the x-ray structural model, high resolution electron microscopy (HREM) study was performed along the  and [2-21] zone axes. In the case of the  zone axis, the corresponding HREM micrograph shows an apparently well-ordered NASICON material with d-spacings of 6.2 and 4.3 Å, corresponding to d10-2 and d104, respectively. The cycling performances of NaFe2(PO4)(SO4)2 were studied in cells embedding (NaPF6:EC:FEC) electrolyte and Na-metal. During the first discharge at a rate of C/20, the title compound delivers a capacity of 89 mAh/g indicating that 70% of the theoretical specific capacity (127mAh.g-1) was achieved (Fig. 1). The material was cycled at the C/5 rate and a 96% capacity retention was obtained after 30 cycles. After the first cycle, the powder XRD pattern was very similar to the one of the initial phase indicating a stable Nasicon structure during cycling.
Figure 1. Charge/discharge curves of NaFe2(PO4)(SO4)2 at the rate of C/5, C/10, and C/20 vs. Na+/Na (a), capacity retention (b), and cyclic voltammetry curves at the scan rates of 0.1, 0.25, and 0.5mV/s (c).
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