A sodium iron fluorophosphate positive electrode material with a wide interlayer spacing two-dimensional laminar structure and a preparation method thereof

Abstract

The application belongs to the field of sodium ion batteries, and particularly relates to a sodium iron fluorophosphate positive electrode material with a wide interlayer spacing two-dimensional layer sheet structure and a preparation method thereof. The preparation method of the sodium iron fluorophosphate positive electrode material with a wide interlayer spacing two-dimensional layer sheet structure comprises the following steps: ball milling a sodium source, an iron source and a phosphorus source in a solvent, calcining the mixture in a protective atmosphere after drying to obtain NaFePO4; ball milling the obtained NaFePO4, a NaF precursor and a carbon source in a solvent, calcining the mixture in a protective atmosphere after drying to obtain a sodium iron fluorophosphate Na2FePO4F / C / N positive electrode material coated with nitrogen-doped carbon. The process involves a transformation process from a three-dimensional framework structure (NaFePO4, NFP) to a two-dimensional layered structure (Na2FePO4F, NFPF), and the interlayer spacing of Na2FePO4F in the finally obtained Na2FePO4F / C / N is significantly larger than that of a product prepared by a conventional method. The increased interlayer spacing significantly reduces the diffusion resistance of Na ions between the layers, and is also beneficial to electron transmission, thereby synergistically improving the specific capacity and rate performance of the material. + diffusion resistance of Na ions between the layers, and is also beneficial to electron transmission, thereby synergistically increasing the specific capacity and rate performance of the material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, specifically relating to a sodium fluorophosphate cathode material with a wide interlayer spacing and a two-dimensional layered sheet structure, and its preparation method. Background Technology

[0002] Against the backdrop of a profound global energy structure transformation and a surge in demand for large-scale energy storage, sodium-ion batteries, with their resource advantages and electrochemical performance potential, are becoming an important alternative technology to lithium-ion batteries in the field of large-scale energy storage. According to geological survey data, sodium has an abundance of 2.75% in the Earth's crust, far exceeding that of lithium (0.0065%), giving sodium-ion batteries a significant cost advantage in raw material supply. Furthermore, sodium-ion batteries, like lithium-ion batteries, utilize a "rocking chair" charge-discharge mechanism, and their production process is partially compatible with existing lithium battery manufacturing equipment, significantly lowering the capital investment threshold in the early stages of industrialization. In terms of safety, sodium-ion batteries exhibit a higher thermal runaway initiation temperature and stronger overcharge tolerance, effectively reducing the safety risks caused by thermal runaway in large-scale energy storage systems.

[0003] In sodium-ion battery cathode material systems, polyanionic materials are linked by strong covalent bonds to [XO4]. n- (X = P, S, Si, etc.) or [XO4F] n- Polyhedra serve as structural building blocks, constructing a highly stable three-dimensional framework. These materials typically exhibit lattice parameter changes of less than 1% during charge-discharge cycles, thus demonstrating excellent structural stability and long cycle life.

[0004] Sodium ferric fluorophosphate (Na2FePO4F), as a typical representative of the polyanion family, possesses high operating voltage and energy density. Based on the current trend towards simplified process design, those skilled in the art tend to simplify complex synthetic steps when synthesizing Na2FePO4F or Na2FePO4F / C. For example, a one-step method is often used to prepare the target product, thus simplifying the process. However, in practical applications, it has been found that in the traditional one-step synthesis of Na2FePO4F or Na2FePO4F / C products, due to the small spacing of the two-dimensional lamellar structure, Na... + The high diffusion resistance and slow migration rate of ions in the two-dimensional structure limit the charge-discharge capability of Na2FePO4F at high current densities, i.e. its rate performance. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for preparing sodium iron fluorophosphate cathode material with a wide interlayer spacing and a two-dimensional layered sheet structure, which solves the technical problem of poor charge-discharge performance and rate performance of traditional Na2FePO4F under high current density.

[0007] (II) Technical Solution

[0008] In a first aspect, the present invention provides a sodium fluorophosphate cathode material with a wide interlayer spacing and a two-dimensional sheet structure, characterized in that the sodium fluorophosphate cathode material Na2FePO4F / C / N is a two-dimensional sheet material.

[0009] Secondly, the present invention provides a method for preparing a sodium iron fluorophosphate cathode material with a wide interlayer spacing and a two-dimensional layered sheet structure, comprising the following steps:

[0010] S1. Sodium source, iron source and phosphorus source are ball-milled in solvent, dried and calcined in a protective atmosphere to obtain NaFePO4;

[0011] S2. The NaFePO4 obtained in S1, along with the NaF precursor and a nitrogen-containing carbon source, is ball-milled in a solvent, dried, and then calcined in a protective atmosphere to obtain nitrogen-doped carbon-coated sodium iron fluorophosphate Na2FePO4F / C / N cathode material.

[0012] Optionally, in S1, the sodium source is selected from at least one of sodium carbonate, sodium oxide, sodium hydroxide, sodium acetate, and sodium oxalate;

[0013] The iron source is selected from at least one of ferrous oxalate, ferric oxide, ferrous phosphate and ferrous acetate;

[0014] The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, pyrophosphate, and sodium pyrophosphate.

[0015] Optionally, in S1, the molar ratio of Na, Fe, and P in the sodium source, the iron source, and the phosphorus source is 1:(1-1.1):(1-1.1).

[0016] Optionally, in S1, the ball milling speed is 350-400 rpm and the ball milling time is 5-8 h; the calcination adopts a two-step calcination process, wherein the first step sintering temperature is 300-350℃ and the time is 4-6 h, and the second step sintering temperature is 600-700℃ and the time is 6-8 h.

[0017] Optionally, in S2, the NaF precursor includes a sodium source and a fluorine source;

[0018] The nitrogen-containing carbon source is a nitrogen-containing organic compound or a combination of a nitrogen-containing organic compound and a carbon source;

[0019] The nitrogen-containing organic compound is polyvinylpyrrolidone, polyaniline, or urea;

[0020] The carbon source is selected from at least one of ascorbic acid, glucose, sucrose, citric acid, graphene, reduced graphene oxide, and carbon nanotubes.

[0021] The sodium source is selected from at least one of sodium carbonate, sodium oxide, sodium hydroxide, sodium acetate, and sodium oxalate;

[0022] The fluorine source is selected from at least one of sodium fluoride, ammonium fluoride, potassium fluoride, and sodium fluoroborate. Optionally, in S2, the molar ratio of NaFePO4 to NaF is 1:(1-1.2).

[0023] The amount of carbon source added is 15-45% of the total mass of sodium iron phosphate and sodium fluoride.

[0024] Optionally, in S2, the ball milling speed is 350-400 rpm, the ball milling time is 2-4 h, the calcination temperature is 600-650℃, and the time is 2-4 h; the heating rate is 3-10℃ / min.

[0025] Optionally, in S1 and S2, the solvent is selected from anhydrous ethanol, acetone and deionized water;

[0026] The protective atmosphere is selected from either nitrogen or argon.

[0027] Secondly, the present invention provides a sodium fluorophosphate cathode material prepared by the method described above.

[0028] Thirdly, the present invention provides a sodium-ion battery, including a positive electrode sheet, wherein the positive electrode sheet contains the sodium fluorophosphate positive electrode material described in the second aspect.

[0029] (III) Beneficial Effects

[0030] Experiments have shown that, compared with products prepared by existing one-step methods, the product of this invention has a larger cell volume and greater interlayer spacing in its two-dimensional layered structure, and the increased interlayer spacing significantly reduces Na... + The diffusion resistance of ions between layers facilitates electron transport, thereby improving the specific capacity and rate performance of the material. Therefore, this unique process and the high-performance Na₂FePO₄F / C / N material prepared from it demonstrate significant potential application value.

[0031] This invention effectively inhibits the decomposition of fluorine sources and avoids the generation of toxic gases such as HF and NH3 in traditional processes by using staged temperature-controlled calcination (pre-crystallization at 350℃ and crystallization at 700℃). The ball milling media are anhydrous ethanol or acetone, resulting in a high solvent recycling rate and no acidic or alkaline waste liquid discharge. The raw material utilization rate is >98%, and the reaction byproducts are only trace amounts of CO2 / H2O (absorbed by the tail gas treatment system), with no solid hazardous waste generated. The entire preparation process of this invention is carried out in a closed inert atmosphere (N2) system, achieving zero emission of toxic substances.

[0032] Compared with similar cathode materials for lithium batteries, the raw material cost of this invention is reduced.

[0033] The preparation method of this invention is directly compatible with existing ball mills and roller kilns in the lithium battery industry, with low equipment modification costs and high single-batch production capacity.

[0034] With a high active substance loading (1.2 mg / cm³) 2 Even under certain conditions, the electrode prepared from the cathode material of this invention still exhibits stable electrochemical performance. It demonstrates excellent stability in both capacity retention during charge-discharge cycles and rate performance at different rates. This means that the material can operate stably in practical battery devices, meeting the performance requirements of various electronic devices and energy storage systems for electrode materials. Attached Figure Description

[0035] Figure 1 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 1 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0036] Figure 2 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 2 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0037] Figure 3 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 3 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0038] Figure 4 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 4 at a 0.1C rate in the voltage range of 2.0-4.0V are shown.

[0039] Figure 5 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 5 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0040] Figure 6 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 6 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0041] Figure 7 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 7 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0042] Figure 8 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 8 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0043] Figure 9 The first three charge-discharge voltage curves of the Na2FePO4F / C / N cathode material in Example 9 at a 0.1C rate within a voltage range of 2.0-4.0V are shown.

[0044] Figure 10 XRD patterns of Na2FePO4F / C / N samples from Examples 3 and 9: (a) XRD pattern at 10°–70°; (b) magnified XRD pattern at 25°–45°.

[0045] Figure 11 The cycling performance of Na2FePO4F / C / N in Examples 3 and 9 is shown in the graphs at a 0.1C rate and a voltage range of 2.0-4.0V.

[0046] Figure 12 The graph shows the rate performance of Na2FePO4F / C / N in Examples 3 and 9 at different rates within a voltage range of 2.0-4.0V. Detailed Implementation

[0047] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] This invention innovatively employs a stepwise synthesis strategy: first, a three-dimensional framework structure of sodium phosphotenite-type NaFePO4 is prepared; then, the target product Na2FePO4F / C / N is obtained by mixing it with a fluorine source, a sodium source, and a nitrogen-containing carbon source, followed by ball milling and oxygen-free calcination. In this process, the sodium source and fluorine source act as structure regulators, continuously embedding themselves into the sodium phosphotenite-type NaFePO4, transforming the three-dimensional framework structure (NaFePO4, NFP) into a two-dimensional layered structure (Na2FePO4F, NFPF), ultimately yielding a Na2FePO4F / C / N product with a large interlayer spacing.

[0049] The interlayer spacing and particle size of the products obtained by the existing one-step method can be characterized by the unit cell parameters a1, b1, and c1, among which... Unit cell volume

[0050] The interlayer spacing and particle size of the Na2FePO4F / C / N product obtained in this invention can be characterized by unit cell parameters a, b, and c, wherein... Unit cell volume Existing technologies use a one-step method to mix all precursors together, followed by a single ball milling and calcination to obtain the product, but this method lacks Na. + Similar to the embedding process of structure regulators; however, in the two-step method of this invention, the second-step embedding process increases the cell volume of sodium iron phosphate, thereby increasing the interlayer spacing. The increased interlayer spacing significantly reduces the Na... + The diffusion resistance of ions between layers, while facilitating electron transport, synergistically enhances the specific capacity and rate performance of the material. Therefore, this unique process and the high-performance Na₂FePO₄F / C / N material prepared by it demonstrate significant potential application value.

[0051] Example 1

[0052] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0053] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. The resulting mixture was dried and then calcined in a two-step process under a nitrogen atmosphere: first at 350 °C for 5 h, then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 was mixed uniformly with NaF in a molar ratio of 1:1 and ball-milled in anhydrous ethanol at 400 rpm for 4 h. The dried mixture was then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the final product NFPF / C / N-1.

[0054] like Figure 1 As shown, when the material is charged and discharged at a rate of 0.1C, its first-cycle discharge specific capacity is 71.9 mAh g. -1 (Set 1C = 124mAg) -1 ).

[0055] Example 2

[0056] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0057] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 20 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 4 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-2.

[0058] like Figure 2 As shown, the material was tested at a charge-discharge rate of 0.1C, and its first-cycle discharge specific capacity was 102.8 mAh g⁻¹. -1 .

[0059] Example 3

[0060] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0061] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 25 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 4 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-3.

[0062] like Figure 3 As shown, the material was tested at a charge / discharge rate of 0.1C, and its first-cycle discharge specific capacity was 116.1 mAh g. -1 .

[0063] Example 4

[0064] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0065] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 30 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 4 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-4.

[0066] like Figure 4 As shown, the material was tested at a charge-discharge rate of 0.1C, and its first-cycle discharge specific capacity was 112.3 mAh g⁻¹. -1 .

[0067] Example 5

[0068] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0069] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in acetone at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 25 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in acetone at 400 rpm for 4 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-5.

[0070] like Figure 5 As shown, the material was tested at a charge / discharge rate of 0.1C, and its first-cycle discharge specific capacity was 114.6 mAh g. -1 .

[0071] Example 6

[0072] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0073] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 25 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 2 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-6.

[0074] like Figure 6 As shown, the material was tested at a charge / discharge rate of 0.1C, and its first-cycle discharge specific capacity was 97.6 mAh g⁻¹. -1 .

[0075] Example 7

[0076] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0077] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 and NaF were mixed uniformly in a molar ratio of 1:1, and 25 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of NaFePO4 and NaF was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 4 h, dried, and then calcined at 600 °C for 2 h under a nitrogen atmosphere to obtain the product NFPF / C / N-7.

[0078] like Figure 7 As shown, the material was tested at a charge / discharge rate of 0.1C, and its first-cycle discharge specific capacity was 100.3 mAh g⁻¹. -1 .

[0079] Example 8

[0080] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0081] First, sodium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were mixed in a molar ratio of 0.5:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined in two steps under a nitrogen atmosphere: first at 350 °C for 5 h, and then at 700 °C for 8 h, to obtain NaFePO4. Subsequently, the obtained NaFePO4 was mixed uniformly with NH4F and Na2CO3 in a molar ratio of 1:1:0.5, and 25 wt.% of polyvinylpyrrolidone (PVP) relative to the total mass of the NaFePO4, NH4F, and Na2CO3 mixture was added. The mixture was ball-milled in anhydrous ethanol at 400 rpm for 4 h, dried, and then calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-8.

[0082] like Figure 8 As shown, the material was tested at a charge / discharge rate of 0.1C, and its first-cycle discharge specific capacity was 113.6 mAh g. -1 .

[0083] Example 9

[0084] The preparation method of the wide-interlayer-spacing two-dimensional layered sheet structure sodium fluorophosphate cathode material in this embodiment includes the following steps:

[0085] First, sodium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and sodium fluoride were mixed in a molar ratio of 0.5:1:1:1 and ball-milled in anhydrous ethanol at 400 rpm for 5 h. After drying, the mixture was calcined at 350 °C for 5 h to obtain the Na₂FePO₄F precursor. Subsequently, the Na₂FePO₄F precursor was ball-milled with 0.25 g of PVP in anhydrous ethanol at 400 rpm for 4 h. After drying, the mixture was calcined at 600 °C for 4 h under a nitrogen atmosphere to obtain the product NFPF / C / N-9.

[0086] like Figure 9 As shown, the material was tested at a charge-discharge rate of 0.1C, and its first-cycle discharge specific capacity was 102.9 mAh g⁻¹. -1 .

[0087] As shown in the figure, among all embodiments, Embodiment 3 exhibits the highest first-cycle discharge specific capacity, at 116.1 mAh g⁻¹. -1 Comparing Examples 1, 2, 3, and 4, it is evident that the optimal addition amount of polyvinylpyrrolidone (PVP) is 25 wt.%, which, during calcination, pyrolyzes to form a uniform nitrogen-doped carbon layer coating the surface of Na₂FePO₄F particles. This carbon layer plays a crucial role: firstly, the nitrogen-doped carbon layer significantly improves the electronic conductivity of Na₂FePO₄F material, effectively overcoming its inherent poor electronic conductivity and ensuring full utilization of the active material; secondly, nitrogen-containing functional groups such as pyridine nitrogen help optimize the electrode / electrolyte interface and promote the absorption of Na₂FePO₄F by the active material. +Interface migration; finally, a carbon layer of moderate thickness provides an effective conductive network and physical protection (isolating electrolyte corrosion, inhibiting transition metal dissolution, and buffering volume changes) without excessively hindering Na+ interface migration. + The diffusion pathway. The amount of PVP added was too low (e.g., 20 wt.% in Example 2, capacity 102.8 mAh g). -1 If the carbon layer is not fully covered or is too thin, the conductivity and protection will be insufficient; if the amount of PVP added is too high (such as 30 wt.% in Example 4, with a capacity of 112.3 mAh g), the carbon layer will be incomplete or too thin. -1 If the carbon layer is too thick, it will hinder ion diffusion. Therefore, an addition of 25 wt.% achieves the optimal balance between electronic conductivity and ion accessibility, which is key to high specific capacity.

[0088] like Figure 10 As shown in Figure a, the XRD patterns show that the products of Examples 3 and 9 match the standard, with no impurity peaks.

[0089] like Figure 10 As shown in b, the XRD pattern reveals that the diffraction peaks of the sample in Example 3 are significantly shifted to the left (towards a smaller angle) compared to the sample in Example 9 and the standard card, directly demonstrating a significant increase in the interplanar spacing of the sample in Example 3. This structural change stems from its stepwise process: during the second step, when the three-dimensional framework-structured NaFePO4 precursor reacts with NaF to transform into a two-dimensional layered structure Na2FePO4F, lattice reconstruction occurs, leading to a widening of the interplanar spacing. This structural "corridor widening" effect directly reduces the Na... + The resistance to interlayer diffusion increases the ion diffusion coefficient. This not only helps to reduce polarization at high rates and maintain a high capacity output (e.g., Figure 12 As shown, the sample obtained in Example 3 has a discharge specific capacity of 78.1 mAh g at a 5C rate. -1 The sample obtained in Example 9 had a specific capacity of only 38.2 mAh g at a 5C rate. -1 ), and also for Na during charging and discharging. + The insertion and extraction provide more space, buffering volumetric strain and thus significantly improving cyclic stability (e.g., Figure 11 (As shown).

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium fluorophosphate cathode material with a wide-interlayer-spacing two-dimensional layered sheet structure, characterized in that, Sodium iron fluorophosphate cathode material Na2FePO4F / C / N is a two-dimensional layered sheet material; The preparation method of the sodium iron fluorophosphate cathode material includes the following steps: S1. Sodium, iron, and phosphorus sources are ball-milled in a solvent, dried, and then calcined in a protective atmosphere to obtain NaFePO4. The ball milling speed is 350-400 rpm and the ball milling time is 5-8 h. The calcination adopts a two-step calcination process, in which the first step sintering temperature is 300-350℃ and the time is 4-6 h, and the second step sintering temperature is 600-700℃ and the time is 6-8 h. S2. The NaFePO4 obtained in S1 is ball-milled with NaF and a nitrogen-containing carbon source in a solvent, dried, and then calcined in a protective atmosphere to obtain nitrogen-doped carbon-coated sodium iron fluorophosphate Na2FePO4F / C / N cathode material; wherein the ball milling speed is 350-400 rpm, the ball milling time is 2-4 h, the calcination temperature is 600-650℃, and the time is 2-4 h; the nitrogen-containing carbon source is polyvinylpyrrolidone.

2. The sodium fluorophosphate cathode material with a wide interlayer spacing two-dimensional layered sheet structure according to claim 1, characterized in that: In S1, the sodium source is selected from at least one of sodium carbonate, sodium oxide, sodium hydroxide, sodium acetate, and sodium oxalate; The iron source is selected from at least one of ferrous oxalate, ferric oxide, ferrous phosphate and ferrous acetate; The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, pyrophosphate, and sodium pyrophosphate.

3. The sodium fluorophosphate cathode material with a wide interlayer spacing two-dimensional layered sheet structure according to claim 1, characterized in that: In S1, the molar ratio of Na, Fe, and P in the sodium source, the iron source, and the phosphorus source is 1:(1-1.1):(1-1.1).

4. The sodium fluorophosphate cathode material with a wide interlayer spacing two-dimensional layered sheet structure according to claim 1, characterized in that: In S2, the molar ratio of NaFePO4 to NaF is 1:(1-1.2). The amount of nitrogen-containing carbon source added is 15-45% of the total mass of sodium iron phosphate and sodium fluoride.

5. The sodium fluorophosphate cathode material with a wide interlayer spacing two-dimensional layered sheet structure according to claim 1, characterized in that: In S1 and S2, the solvent is selected from one of anhydrous ethanol, acetone and water; The protective atmosphere is selected from nitrogen or argon.

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