Characterization of Na-based phosphate as electrode materials for electrochemical cells
Highlights
► Electrochemical features of both maricite and olivine NaFePO4. ► Full insertion of Na achieved at 1.7 V, capacity retention of 147 mAh g−1 in olivine. ► Na insertion/de-insertion operates in a two-phase process. ► XRD and EDX analysis of intermediate compositions.
Introduction
Because the low cost and abundance of sodium, there is a renewed interest in the development of Na-based electrodes, especially those that can be synthesized by hydrothermal method. In the past, several compounds have been investigated such as NaxCoO2 exhibiting different phases depending of its synthesis conditions [1]. Sodium insertion in vanadium oxides, i.e. the one-channel structure (β-NaxV2O5) and two layered structures (Na1+xV3O8 and α-V2O5), have been studied with regard to their use as cathode materials in solid-state sodium batteries. Both Na1+xV3O8 and β-NaxV2O5 battery cycle lives are very poor. The structure of α-V2O5 changes after the first discharge, but the new phase exhibits excellent capacity retention upon cycling [2]. Electrochemical insertion of sodium into phosphates Na3Fe2(PO4)3 has shown that Na+ ions can be accommodated into this host [3]. More recently, a highly reversible capacity of ca. 120 mAh g−1 has been reported for NaCrO2 in Na cell, with suitable capacity retention [4]. Liu et al. [5] have synthesized by hydrothermal route NaV6O15 nanorods that performed stable Na-ion insertion–extraction. However, the poor reversibility and the low capacity of all these materials are far from the expected targeted performance for Na batteries.
Similarly to LiFePO4 [6], [7], NaFePO4 might be a good candidate as electrode materials in electrochemical cells. Known for their excellent redox abilities and thermal stability, transition metal polyphosphates have been studied in great detail. To date, NaFePO4 is not well-documented or characterized in Na cells, and some workers have stated that it is not viable as a cathode material. The reason evoked by Ellis et al. is that the closed maricite framework results in entrapment of Na+ and no reversible redox behaviour [8]. However, such claims have never been verified by substantial experiments. Recham et al. [9] have shown the Na-cells with Na2FePO4F positive electrode display charge–discharge profiles in the 3-V region. Moreau et al. [10] have reported the structure and phase stability of Na intercalated FePO4 olivine.
In this work, we have synthesized NaFePO4 materials using the hydrothermal route. Various conditions have been considered, such as chemical nature of precursors, molar stoichiometry, and pH of the starting solution. The structural characterization of Na-based electrode materials includes X-ray diffraction (XRD), scanning electron (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX) and Raman scattering experiments. Electrochemical properties such as galvanostatic discharge–charge and electrochemical impedance spectroscopy, have also been evaluated as a function of the synthetic conditions.
Section snippets
Synthesis
We used two different routes to prepare the NaFePO4 phases. First, the hydrothermal route using iron sulphate, sodium hydroxide and H3PO4 as precursors. FeSO4 + NaOH were mixed in aqueous solution for 6 h followed by incorporation of H3PO4 to adjust a solution at pH 8. Powders crystallized in the maricite phase were grown in autoclave at 165–200 °C for 7 h (p ≈ 200 bars). Second, the electrochemical insertion of sodium into the FePO4 heterosite was made after chemical delithiation of the triphylite
The maricite phase
Fig. 1 shows the X-ray diffraction pattern of NaFePO4 synthesized by hydrothermal method. The powders are crystallized in the orthorhombic structure (Pnma space group of the primitive cell) of the maricite mineral (Fig. 2), in agreement with prior results [11], [12]. Lattice parameters calculated by a least square fitting method are a = 9.001(1) Å, b = 6.874(2) Å, c = 5.052(5) Å and V = 312.6(3) Å3. We note that the elementary cell volume is slightly larger than for LiFePO4 olivine structure (V = 291.2 Å3) as
Conclusion
We have successfully synthesized the maricite phase of NaFePO4 that is the most stable phase for this material. When optimization of the hydrothermal reaction was done, prismatic particles with average size 400 nm × 800 nm were obtained after subsequent calcination. Raman spectroscopy confirms the well-developed local environment of iron Fe2+O6 sublattice in the orthorhombic phase. Sodium iron phosphate does not show any significant electrochemical activity for the Na insertion in this maricite
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