English

• 0.   Introduction

  With the development of wind energy and solar energy, energy storage devices have been paid more and more attention^[1-3]. As a new type of energy storage device, lithium-ion batteries (LIBs) have a series of significant advantages, such as high working voltage, large specific capacity, light weight, small size, long cycle life, and no memory effect^[4-6]. However, the high price of lithium compounds limits the widespread use of lithium-ion batteries. By contrast, sodium, a member of the same family as lithium, is more widely distributed and cheaper than lithium^[7]. Therefore, exploring sodium-ion batteries (SIBs) is a meaningful direction in the field of energy storage.

  Enormous efforts have been dedicated to the development of new electrodes for SIBs, particularly for cathode materials, two type of materials, sodium layered oxides and polyanionic materials, have attracted considerable attention^[8-10]. Layered oxides are known by their high capacity, but they suffer from poor cycling stability^[11]. Compared with layered oxides, polyanion compounds have been extensively studied for their structural stability and open frames that promote sodium ion extraction and insertion^[12]. The stable 3D host framework leads to better thermal stability, longer cycle life, and better safety features^[13-14]. Such excellent characteristics have made it fairly popular in SIB applications. Among all polyanionic compounds, phosphates particularly gained a great deal of interest, because of their excellent structural stability, which arise from the strong P-O chemical bond^[15]. For example, Na₃V₂(PO₄)₃ (NVP) with a NASICON-type structure is a promising candidate, which satisfies the requirements of being a high-performance cathode^[16]. Its electrochemical activity is related to the redox couple V³⁺/V⁴⁺ at around 3.4 V vs Na/Na^{+ [17]}. Besides, NASICON-type structure can accommodate a wide selection of transition metals, which offer the replacement of V³⁺ in NVP by other cheaper transition metals towards developing new cost-effective NASICON-type cathode materials^[18]. In previous research, several NASICON-type cathode materials were reported such as Na₃MnTi(PO₄)₃^[19] and Na₄MnV(PO₄)₃^[20]. Na₄MnV(PO₄)₃ has a corner-shared Mn/VO₆ octahedral and PO₄ tetrahedral units to establish the anion framework [MnV(PO₄)₃]^4-. This stable 3D frame structure can provide a large channel for the transmission of sodium ions. Inspired by these works, we attempted to synthesize the analogous Na₄FeV(PO₄)₃ by replacing Mn²⁺ with the more environmentally-friendly and cheaper Fe²⁺ in Na₄MnV(PO₄)₃. Finally, we synthesized the single crystal of Na₄FeV(PO₄)₃ by high temperature molten salt method and obtained its structural information by single crystal XRD analysis.

  1.   Experimental

  1.1   Syntheses

  Polycrystalline powder of Na₄FeV(PO₄)₃ was prepared by the sol-gel method using analytically pure Na₂CO₃, FeC₂O₄·2H₂O, NH₄VO₃ and NH₄H₂PO₄ in a molar ratio of 2∶1∶1∶3. Firstly, 5 mmol of Na₂CO₃, 7.5 mmol of NH₄H₂PO₄ and 20 mL of deionized water were put into the beaker to form solution A. Secondly, 2.5 mmol of NH₄VO₃ was dispersed in deionized water (20 mL) under vigorous stirring at 80 ℃ to obtain a yellow solution. After the heat source was turned off, 1.5 g of citric acid was added into the solution and stirred until the color of the solution gradually changed blue to form solution B. Thirdly, the mixture of A and B was kept at 70 ℃ to evaporate water until it turned into a viscous gel. The viscous gel was heated in a muffle furnace at 300 ℃ for 1 h to obtain the dry gel. Finally, 2.5 mmol of FeC₂O₄·2H₂O were added into the above-mentioned dry colloid and ground uniformly, then the mixture was put in a tube furnace and sintered at 800 ℃ for 36 h in reducing atmosphere (5% H₂ and 95% N₂, V/V). The final product was obtained.

  Crystals of Na₄FeV(PO₄)₃ were grown by employing the NaCl as flux in a vacuum quartz tube. The powder samples of Na₄FeV(PO₄)₃ and NaCl in the weight ratio of 1∶4 were melted in a vacuum quartz tube. The samples were heated to 940 ℃ from room temperature at a rate of 7 ℃·min^-1, held for 2 h to ensure that the samples turned into a homogeneous liquid solution, and then slowly cooled at a rate of 0.1 ℃ ·min^-1 to 700 ℃, followed by furnace-cooling to room temperature. The high quality crystals can be selected under an optical microscope (Fig. 1).

  Figure 1

  

  Figure 1.  Single crystal picture of Na₄FeV(PO₄)₃

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  1.2   Characterization

  The powder X-ray diffraction (PXRD) data were collected over the 2θ angle range of 10°~70° by a Bruker D8 advance (tube voltage was 40 kV, tube current was 40 mA) diffractometer using Cu Kα radiation (λ = 0.154 178 nm) with a step size of 0.02° and a constant counting time of 0.1 s. The chemical composition was determined by an energy dispersive X-ray (EDX) detector (HITACHIS-4800).

  1.3   X-ray crystallography

  Crystals were selected under an optical microscope equipped with a polarizing light attachment and mounted on a glass fiber with epoxy for X-ray diffraction studies. Diffraction data were collected on a Bruker Smart APEX CCD diffractometer using a graphite monochromatic Mo Kα radiation (λ=0.071 073 nm) at room temperature. Absorption corrections were performed with the SADBAS program^[21]. The structure was solved by direct methods using the SHELEX-97 program and refined on F² by full-matrix least-squares techniques^[22]. The final refined structural parameters were corrected by PLATON program^[23]. The crystallography data are listed in the Table 1~2, respectively.

  Table 1

  

  Table 1.  Crystal data and structure refinement for Na₄FeV(PO₄)₃

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  Empirical formula	Na4FeV(PO4)3		V/nm3	1.449 31(18)
  Temperature/K	296(2)		Z	6
  Wavelength/nm	0.071 073		Calculated density/(g·cm-3)	3.325
  Diffractometer	Bruker Smart APEX CCD		Absorption coefficient/mm-1	3.218
  Crystal system	Hexagonal		F(000)	1 404
  Space group	R3c (167)		Goodness-of-fit on F2	1.152
  a/nm	0.878 17(4)		Final R indices [I>2σ(I)]	R1=0.036 6, wR2=0.096 6
  b/nm	0.878 17(4)		R indices (all data)	R1=0.037 6, wR2=0.097 6
  c/nm	2.170 1(2)		Largest diff. peak and hole/(e·nm-3)	0.000 718 and-0.000 792

  Table 2

  

  Table 2.  Selected bond lengths (nm) for Na₄FeV(PO₄)₃

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  Na1-O1ⅰ	0.260 9(4)	Fe/V-O1	0.198 0(3)	Na2-O2ⅹ	0.249 1(3)	P-O1	0.152 6(3)
  Na1-O1ⅸ	0.260 9(4)	Fe/V-O1ⅰ	0.198 0(3)	Na2-O2ⅹⅰ	0.249 1(3)	P-O1ⅵ	0.152 6(3)
  Na1-O1ⅳ	0.285 3(6)	Fe/V-O1ⅱ	0.198 0(3)	Na2-O2ⅹⅳ	0.249 1(3)	P-O2	0.153 4(3)
  Na1-O1ⅶ	0.285 3(6)	Fe/V-O2ⅲ	0.204 6(3)	Na2-O2ⅹⅱ	0.249 1(3)	P-O2ⅵ	0.153 4(3)
  Na1-O2vⅱ	0.240 6(4)	Fe/V-O2ⅳ	0.204 6(3)	Na2-O2ⅹⅲ	0.249 1(3)
  Na1-O2ⅳ	0.240 6(4)	Fe/V-O2ⅴ	0.204 6(3)
  Na1-O2ⅷ	0.247 0(6)
  Na1-O2ⅲ	0.247 0(6)
  Symmetry transformations used to generate equivalent atoms: i -x+y, -x, z; ⅱ -y, x-y, z; ⅲ -x+y-1/3, y-2/3, z-1/6; iv -y+2/3, -x+1/3, z-1/6; v x-1/3, x-y+1/3, z-1/6; vi -x, -x+y, -z+1/2; vⅱ -x+2/3, -y+1/3, -z+1/3; vⅲ y-1/3, -x+y-2/3, -z+1/3; ix -x+1/3, -x+y-1/3, -z+1/6; ⅹx-y+2/3, x+1/3, -z+1/3; ⅹⅰ -x+2/3, -y+4/3, -z+1/3; ⅹⅱ -x+y, -x+1, z; ⅹⅲ -y+1, x-y+1, z; ⅹiv x, y+1, z.

  CCDC: 2048805.

  1.4   Electrochemical measurements

  Firstly, the samples were mixed with super P (9∶1, w/w) by ball milling at 300 r·min^-1 for 12 h. Secondly, the mixture of the milled powder, super P and polyvinylidene fluoride (PVDF) in a weight ratio of 7∶2∶1 was dissolved in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was uniformly coated on an Al foil and dried at 120 ℃ for 12 h. Finally, the electrochemical properties were measured with CR2025 coin cells assembled in a glove box filled with pure argon gas. In Na⁺ half cells, sodium metals were used as counter electrodes, a 1 mol·L^-1 solution of NaPF₆ in vinyl carbonate (EC)-propylene carbonate (PC) (1∶1, V/V) as the electrolyte, and a glass fiber as a separator. Charge/discharge tests were performed at ambient temperature on a Land 2100 testing system.

  2.   Results and discussion

  2.1   PXRD analyses

  Polycrystalline powder of Na₄FeV(PO₄)₃ was characterized by PXRD at room temperature. Fig. 2 shows the observed powder XRD pattern of Na₄FeV(PO₄)₃ together with that calculated from the single-crystal diffraction data for comparison. The observed XRD pattern is in good agreement with the theoretical one except for the pattern of trace impurities.

  Figure 2

  

  Figure 2.  Experimental PXRD pattern (down) and calculated PXRD pattern (up) based on single-crystal XRD analysis of Na₄FeV(PO₄)₃

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  2.2   EDX

  The chemical composition was analyzed by EDX in a selected crystal (Fig. 3 and Fig. 4). It can be seen that the elements of Na, Fe, V, P and O were distributed uniformly in the crystal with an approximate mole ratio of 4∶1∶1∶3∶15. The excess O element may originate from the adsorptive O₂.

  Figure 3

  

  Figure 3.  EDX spectrum capturing signals of constituent elements

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  Figure 4

  

  Figure 4.  Representative crystal of as‑synthesized samples (a); Elemental mappings of Na (b), Fe (c), V (d), P (e) and O (f) atoms of the representative crystal

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  2.3   Crystal structure description

  Na₄FeV(PO₄)₃ crystallizes in hexagonal R3c space group with a=0.878 17(4) nm, b=0.878 17(4) nm, c=2.170 1(2) nm, Z=6, V=1.449 31 nm³. Its structure belongs to the typical NASICON-type structure. In the structure of Na₄FeV(PO₄)₃, the atoms of V and Fe were considered to disorder on a same site with 50% occupancy for each. A Fe/V atom is coordinated with six oxygen atoms to form octahedron. A P atom is coordinated with four oxygen atoms to form tetrahedron, Fe/VO₆ octahedra and PO₄ tetrahedra form three-dimensional (3D) [FeV(PO₄)₃] framework by sharing the common corners. The framework offers 3D tunnels for Na⁺ migration. Two different types of Na⁺ are located in the gap of the frame, Na1 belongs to 8-coordination and Na2 belongs to 6-coordination (Fig. 5).

  Figure 5

  

  Figure 5.  (a) Structural illustration of the Na₄FeV(PO₄)₃; (b, c) Location of sodium atoms; (d~f) Coordination of Na1, Na2 and Fe/V with surrounding oxygen atoms

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  2.4   Electrochemical properties

  The electrochemical performance of the asprepared Na₄FeV(PO₄)₃ as cathode material for sodiumion batteries were examined by galvanostatic cycling in half-cell with sodium metal as counter electrode. Fig. 6a shows the SEM image of the as-prepared Na₄FeV(PO₄)₃ sample, it is composed of irregular particles, the particle size was about 0.5~2 μm. Fig. 6b shows its charge-discharge profiles at a rate of 0.1C, respectively. Three plateaus were observed at about 3.4, 2.5 and 1.6 V, which corresponds to the redox couples of V⁴⁺/V³⁺, Fe³⁺/Fe²⁺ and V³⁺/V²⁺. In the voltage range of 1.5 to 4.3 V, the capacity was only 140 mAh·g^-1 during the first charging process, which is due to the fact that less sodium ions are released from the positive electrode. The battery capacity started to increase since the second charge because more sodium ions are inserted into the positive electrode after the first discharge. The maximum discharge capacity for the first time was 171 mAh·g^-1, which is higher than the theoretical capacity for insertion of more Na ions, similar phenomenon was also found in Na₃V₂(PO₄)₃^[24]. Fig. 6c shows the rate performance of Na₄FeV(PO₄)₃ at various current densities of 0.1C, 0.2C, 0.5C, 1C, 2C and 5C. As the current density increased, the battery capacity decayed quickly, which may be caused by the larger particle size or clustering together (Fig. 6a). Even so, when the current density was different, it can still be reversibly cycled 5 times without structural damage, showing excellent rate performance.

  Figure 6

  

  Figure 6.  (a) SEM image of the as-prepared Na₄FeV(PO₄)₃; (b) Charge-discharge curves of Na₄FeV(PO₄)₃ at a 0.1C rate (1C=100 mAh·g^-1); (c) Capacity retention rate of Na₄FeV(PO₄)₃ under different discharge current densities

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  3.   Conclusions

  In conclusion, the Na₄FeV(PO₄)₃ single crystal was synthesized by high temperature molten salt method and the single crystal diffraction analysis demonstrated that it crystallizes in the typical NASICON-type structure. The pure polycrystalline samples of Na₄FeV(PO₄)₃ can be synthesized using a sol-gel method. In Na⁺ half cells, Na₄FeV(PO₄)₃ as the cathode materials, the maximum discharge capacity was 171 mAh·g^-1, and the capacity decayed by 14.2% during the 2nd to 20th cycles. It can reversibly cycle 5 times at different magnifications without structural damage, indicating the NASICON type Na₄FeV(PO₄)₃ can be the potential cathode candidate for sodium ion battery.

  
  
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• 

  Figure 1  Single crystal picture of Na₄FeV(PO₄)₃

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  Figure 2  Experimental PXRD pattern (down) and calculated PXRD pattern (up) based on single-crystal XRD analysis of Na₄FeV(PO₄)₃

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  Figure 3  EDX spectrum capturing signals of constituent elements

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  Figure 4  Representative crystal of as‑synthesized samples (a); Elemental mappings of Na (b), Fe (c), V (d), P (e) and O (f) atoms of the representative crystal

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  Figure 5  (a) Structural illustration of the Na₄FeV(PO₄)₃; (b, c) Location of sodium atoms; (d~f) Coordination of Na1, Na2 and Fe/V with surrounding oxygen atoms

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  Figure 6  (a) SEM image of the as-prepared Na₄FeV(PO₄)₃; (b) Charge-discharge curves of Na₄FeV(PO₄)₃ at a 0.1C rate (1C=100 mAh·g^-1); (c) Capacity retention rate of Na₄FeV(PO₄)₃ under different discharge current densities

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  Table 1.  Crystal data and structure refinement for Na₄FeV(PO₄)₃

  Empirical formula	Na4FeV(PO4)3		V/nm3	1.449 31(18)
  Temperature/K	296(2)		Z	6
  Wavelength/nm	0.071 073		Calculated density/(g·cm-3)	3.325
  Diffractometer	Bruker Smart APEX CCD		Absorption coefficient/mm-1	3.218
  Crystal system	Hexagonal		F(000)	1 404
  Space group	R3c (167)		Goodness-of-fit on F2	1.152
  a/nm	0.878 17(4)		Final R indices [I>2σ(I)]	R1=0.036 6, wR2=0.096 6
  b/nm	0.878 17(4)		R indices (all data)	R1=0.037 6, wR2=0.097 6
  c/nm	2.170 1(2)		Largest diff. peak and hole/(e·nm-3)	0.000 718 and-0.000 792

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  Table 2.  Selected bond lengths (nm) for Na₄FeV(PO₄)₃

  Na1-O1ⅰ	0.260 9(4)	Fe/V-O1	0.198 0(3)	Na2-O2ⅹ	0.249 1(3)	P-O1	0.152 6(3)
  Na1-O1ⅸ	0.260 9(4)	Fe/V-O1ⅰ	0.198 0(3)	Na2-O2ⅹⅰ	0.249 1(3)	P-O1ⅵ	0.152 6(3)
  Na1-O1ⅳ	0.285 3(6)	Fe/V-O1ⅱ	0.198 0(3)	Na2-O2ⅹⅳ	0.249 1(3)	P-O2	0.153 4(3)
  Na1-O1ⅶ	0.285 3(6)	Fe/V-O2ⅲ	0.204 6(3)	Na2-O2ⅹⅱ	0.249 1(3)	P-O2ⅵ	0.153 4(3)
  Na1-O2vⅱ	0.240 6(4)	Fe/V-O2ⅳ	0.204 6(3)	Na2-O2ⅹⅲ	0.249 1(3)
  Na1-O2ⅳ	0.240 6(4)	Fe/V-O2ⅴ	0.204 6(3)
  Na1-O2ⅷ	0.247 0(6)
  Na1-O2ⅲ	0.247 0(6)
  Symmetry transformations used to generate equivalent atoms: i -x+y, -x, z; ⅱ -y, x-y, z; ⅲ -x+y-1/3, y-2/3, z-1/6; iv -y+2/3, -x+1/3, z-1/6; v x-1/3, x-y+1/3, z-1/6; vi -x, -x+y, -z+1/2; vⅱ -x+2/3, -y+1/3, -z+1/3; vⅲ y-1/3, -x+y-2/3, -z+1/3; ix -x+1/3, -x+y-1/3, -z+1/6; ⅹx-y+2/3, x+1/3, -z+1/3; ⅹⅰ -x+2/3, -y+4/3, -z+1/3; ⅹⅱ -x+y, -x+1, z; ⅹⅲ -y+1, x-y+1, z; ⅹiv x, y+1, z.

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