NASICON compounds have been well investigated for their luminescence and gas sensor-properties, and as potential hosts for radioactive waste family [1-3]. Owing to their high ionic conductivity, these compounds were extensively studied as solid electrolytes since the mid-70s [4]. When incorporated with transition metals such nickel, manganese and iron cations, the materials regained tremendous attention as possible electroactive materials for sodium-, lithium-, and magnesium-ion batteries [5-7].

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 80^oC until evaporation of water. The resulting orange powder was calcined at 550^oC 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 600^oC, 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%, R_B = 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 [010] and [2-21] zone axes. In the case of the [010] 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 d_10-2 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).

References:

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