A spherical monocrystal-like sodium iron pyrophosphate material, a preparation method, application and sodium ion battery thereof

Abstract

This invention belongs to the field of sodium-ion batteries, specifically relating to a spherical single-crystal sodium iron phosphate pyrophosphate material, its preparation method, application, and sodium-ion batteries. The preparation method comprises the following steps: refining and slurrying metered sodium, iron, phosphorus, and carbon sources, then spraying to form precursor particles; calcining and cooling at 350-450 °C to obtain a first-stage material, which is then mixed with two or more MCl₂... n The composite sintering aid is mixed and sintered in two stages at 500-600 °C. The finished product is obtained by washing with a saturated sodium salt solution and drying. M is selected from at least two of Li, Na, K, Al, and Zn. The material of this invention has a spherical, near-single-crystal morphology with few grain boundary defects, excellent ion diffusion and electronic conductivity; the particles are dense, resistant to compression and not easily pulverized, with a compacted density of 2.2-2.5 g / cm³. 3 This can improve the volumetric energy density of the electrode and solve the problem of insufficient energy density of existing sodium-ion battery cathode materials.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery materials technology, specifically relating to the field of positive electrode active materials for sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries have significant application prospects in large-scale energy storage, low-speed electric vehicles, and portable electronic devices due to the abundant and widely distributed nature of sodium resources and their low cost. The cathode material, as the core component of sodium-ion batteries, directly determines the battery's energy density, cycle life, and safety performance.

[0003] Sodium iron phosphate pyrophosphate (chemical formula Na4Fe3(PO4)2P2O7) is a novel composite sodium phosphate-ion battery cathode material, combining the structural stability of pyrophosphate with the high ion conductivity of phosphate. It boasts a high theoretical specific capacity, moderate redox potential, and low raw material cost, making it an environmentally friendly material and a research hotspot in recent years. However, sodium iron phosphate pyrophosphate materials prepared by traditional methods suffer from several drawbacks: firstly, the products are mostly polycrystalline aggregates with uneven particle size distribution and numerous grain boundary defects, leading to slow ion diffusion rates and poor electronic conductivity; secondly, the material has a low compaction density (typically below 2.0 g / cm³). 3 This makes it difficult to improve the volumetric energy density of battery electrodes, severely limiting their large-scale application in high-energy-density sodium-ion batteries.

[0004] Single-crystal materials not only have fewer grain boundary defects, effectively improving ion diffusion efficiency and electronic conductivity, but also have a dense structure, making them less prone to fine powder formation under pressure, thus significantly improving the compaction characteristics of the material and consequently increasing the volumetric energy density of the battery. Therefore, preparing single-crystal sodium iron pyrophosphate is a key direction for solving the aforementioned problems. For example, the preparation methods of single-crystal sodium iron pyrophosphate disclosed in patents such as CN117228646A, CN117810414, CN118943336A, and CN120717430A either rely on the crushing of polycrystalline materials or require the selection of highly soluble raw materials or specific precursors. The production process is characterized by high equipment costs, high energy consumption, and poor raw material compatibility. Moreover, the single-crystal sodium iron pyrophosphate materials prepared by existing methods have irregular morphologies and sharp edges, which can easily lead to excessive local stress during electrode rolling, causing microcracks and dark cracks in the aluminum foil, affecting the later electrical performance of the battery.

[0005] Therefore, developing a rounded, high-purity sodium iron phosphate pyrophosphate material, along with a corresponding controllable and scalable preparation method, is of great significance for promoting the industrialization of sodium-ion batteries. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a method for preparing spherical single-crystal sodium iron pyrophosphate material, aiming to obtain spherical single-crystal sodium iron pyrophosphate material with a rounded spherical single-crystal structure and excellent phase purity and electrochemical performance.

[0007] The second objective of this invention is to provide a spherical single-crystal sodium iron pyrophosphate material prepared by the aforementioned method and its application in sodium-ion batteries.

[0008] A third objective of this invention is to provide a sodium secondary battery comprising the aforementioned spherical monocrystalline sodium iron pyrophosphate material.

[0009] A method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material involves refining and slurrying stoichiometric sodium, iron, phosphorus and carbon sources and then spraying them to obtain precursor particles.

[0010] The precursor particles are first calcined and then cooled to obtain a first-stage material; the first-stage material is then mixed with a composite sintering aid and subjected to a second-stage sintering treatment, followed by washing in a saturated sodium salt solution and then drying to obtain the spherical monocrystalline sodium iron pyrophosphate material.

[0011] The temperature for the first stage of roasting is 350~450℃;

[0012] Composite sintering aids include those with the chemical formula MCl. n Two or more compounds; wherein M includes two or more of Li, Na, K, Al, and Zn; n is the valence of M; the composite sintering aid has a melting point temperature below 500 ℃ and does not decompose during the sintering process;

[0013] The second stage of roasting is carried out at a temperature of 500~600 ℃.

[0014] There are no reports on spherical, single-crystal sodium iron phosphate pyrophosphate materials in the current technology. To fill the gap in the preparation of this material and solve the problem of simultaneously constructing spherical, single-crystal-like structures, this invention previously attempted to use MCl4. n Inorganic salts were used to assist in the nucleation and sintering of materials, but early in the research and development, it was discovered that MCl... n The introduction of [a specific ingredient] facilitates crystallization and nucleation, but it also easily leads to lattice hybridization, affecting the phase purity of the material and failing to achieve the desired preparation effect. Addressing the challenges of simultaneously achieving spherical, single-crystal-like structures and controlling impurities in the preparation of the spherical, near-single-crystal sodium iron pyrophosphate material described in this invention, this invention innovatively involves pre-spraying the raw materials, first-stage calcination, followed by a second-stage high-temperature sintering with the aid of the composite sintering aid, and subsequent washing with a saturated sodium salt solution. This approach successfully constructs a material with high sphericity and a near-single-crystal structure. Furthermore, it also solves the problem of [a specific issue related to MCl]. nThe problem of impurity phases caused by hybridization. The material prepared by the method described in this invention has the advantages of processability, energy density and long-term cycling stability.

[0015] In this invention, the chemical formula of the sodium iron phosphate pyrophosphate material is Na. x Fe y (PO4) 4.3-2z (P2O7) z Where 2.8≤x≤4.3, 2≤y≤3, 1≤z≤1.5.

[0016] In this invention, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium sulfate;

[0017] Preferably, the iron source is one or more of ferric phosphate, ferric phosphate dihydrate, ferric pyrophosphate, ferrous oxalate, ferric oxide, ferrous oxide, and magnetite;

[0018] Preferably, the phosphorus source is one or more of sodium pyrophosphate, trisodium phosphate, ammonium dihydrogen phosphate, and phosphoric acid;

[0019] Preferably, the carbon source is one or more of polyvinylpyrrolidone, polyethylene glycol, citric acid, glucose, starch, and sucrose;

[0020] Preferably, the amount of carbon source used is 0.2~15wt% of the total mass of sodium source + iron source + phosphorus source; more preferably, it can be 5~12wt%.

[0021] Preferably, the refining and slurrying is performed by one of ball milling, grinding, or sand milling;

[0022] Preferably, the solvent for refining and slurrying includes water;

[0023] Preferably, the solid content of the refined pulp is less than 70%, more preferably 15-50%; the particle size D50 in the pulp is controlled to be less than 500 μm, more preferably 100-350 μm.

[0024] Preferably, the D50 of the spray-dried material is controlled to be less than 20 μm, more preferably 2~16 μm; the moisture content of the spray-dried material is less than 10wt%, more preferably 1~6wt%.

[0025] In this invention, the atmosphere for the first stage of roasting is one of nitrogen, rare gas, hydrogen, or argon.

[0026] In this invention, the temperature of the first stage of roasting is 360~430℃; it can further be 380~410℃.

[0027] Preferably, the roasting time for the first stage is 3 to 8 hours; more preferably, it can be 4 to 5 hours.

[0028] In this invention, the composite sintering aid includes one or more of the following: LiCl-ZnCl2 composite aid a, NaCl-AlCl3 composite aid b, KCl-AlCl3 composite aid c, LiCl-ZnCl2 composite aid d, NaCl-ZnCl2 composite aid e, KCl-ZnCl2 composite aid f, LiCl-NaCl-AlCl3 composite aid g, LiCl-KCl-ZnCl2 composite aid h, LiCl-NaCl-ZnCl2 composite aid i, NaCl-KCl-ZnCl2 composite aid j, LiCl-KCl-AlCl3 composite aid k, LiCl-NaCl-AlCl3 composite aid m, LiCl-KCl-MgCl2 composite aid n, LiCl-NaCl-KCl-MgCl2 composite aid o, and LiCl-NaCl-KCl-ZnCl2 composite aid p.

[0029] Preferably, the weight percentage of each component in the composite sintering aid is the average content ± 7.5 wt%.

[0030] In this invention, the mass percentage of the composite sintering aid in the primary material and the composite sintering aid is 1-15%, preferably 5-10%.

[0031] Preferably, the atmosphere for the second stage of roasting is one of nitrogen, rare gas, hydrogen, or argon.

[0032] Preferably, the second roasting time is 6-18 hours; more preferably, it can be 8-12 hours.

[0033] In this invention, the saturated sodium salt solution is a saturated sodium carbonate solution.

[0034] The present invention also provides a spherical single-crystal sodium iron pyrophosphate material prepared by the aforementioned preparation method.

[0035] The spherical single-crystal sodium iron phosphate pyrophosphate material of this invention has a D50 of less than 20 μm, a carbon content of less than 10 wt%, and a powder compaction density of 2.2~2.5 g / cm³. 3 Specific surface area is 3~11m² 2 / g.

[0036] The present invention also provides the application of the spherical single-crystal sodium iron pyrophosphate material prepared by the above preparation method, which is used as a positive electrode active material for the preparation of sodium secondary batteries.

[0037] In this invention, sodium iron phosphate pyrophosphate material described herein can be used to make sodium secondary batteries based on existing principles and methods.

[0038] The present invention also provides a sodium secondary battery, including a positive electrode, wherein the positive electrode contains a spherical single-crystal sodium iron pyrophosphate material prepared by the preparation method described above.

[0039] In this invention, the sodium secondary battery, apart from containing the spherical single-crystal sodium iron pyrophosphate material described in this invention, may have other known components and structural relationships.

[0040] Beneficial effects

[0041] 1. The spherical single-crystal sodium iron phosphate pyrophosphate material of the present invention has few grain boundary defects and a dense structure, which enhances the ion diffusion rate and electronic conductivity, and can improve the capacity release, cycle and rate performance of the material.

[0042] 2. The spherical monocrystalline sodium iron phosphate pyrophosphate material of the present invention has a more rounded morphology than that of monocrystalline sodium iron phosphate pyrophosphate, which can achieve uniform stress distribution during rolling, reducing damage to the current collector. The compaction density of the material powder is 2.2~2.5 g / cm³. 3 This effectively improves the volumetric energy density of battery electrodes, solving the problem of insufficient energy density in existing materials.

[0043] 3. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material according to the present invention has the advantages of wide compatibility of raw materials, low energy consumption, and low risk of introducing impurity phases. Attached Figure Description

[0044] Figure 1 XRD patterns of the finished products prepared in Example 1 and Comparative Examples 1 to 5;

[0045] Figure 2 SEM images of the materials finally obtained in Example 1 and Comparative Examples 1-3 are shown below. (a) is the SEM image of the finished product prepared in Example 1; (b) is the SEM image of the finished product prepared in Comparative Example 1; (c) is the SEM image of the finished product prepared in Comparative Example 2; and (d) is the SEM image of the finished product prepared in Comparative Example 3.

[0046] Figure 3 The charge-discharge curves of the finished products prepared in Example 1, Comparative Example 1, and Comparative Example 6 are shown.

[0047] Figure 4 The images show the coin cell cycle test results of the finished products prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0048] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0049] In this invention, the amount of carbon source used is based on the total weight of sodium source + iron source + phosphorus source.

[0050] The washing process can be rinsing or solid-liquid separation after slurrying the solid to be washed and the washing liquid. The washing process can involve multiple washes, and there are no special requirements for the amount of washing liquid used in a single wash. Considering efficiency and cost, it can be 2~3 mL / g.

[0051] Example 1

[0052] Step 1: Weigh sodium carbonate, ferric phosphate, and ammonium dihydrogen phosphate in a molar ratio of 2:3:1 to obtain a mixed raw material. Then add 5 wt% polyethylene glycol (based on the total weight of sodium, iron, and phosphorus sources) and 5 wt% glucose (based on the total weight of sodium, iron, and phosphorus sources) as carbon sources. Mix all raw materials with water to prepare a slurry with a solid content of 25 wt%. Then, grind the slurry into fine particles with a particle size D50 < 200 μm. Dry the finely ground slurry with a spray drying agent to obtain spherical dry powder with a particle size D50 = 5.1 μm and a water content of 2.8 wt%.

[0053] Step 2:

[0054] The dried powder was heated from room temperature to 400 ℃ at a rate of 2 ℃ / min and held for 4 h in a furnace with nitrogen as the protective gas. After cooling in the furnace, it was taken out to obtain the precursor (first stage material).

[0055] Step 3:

[0056] The precursor and the composite sintering aid LiCl-NaCl-KCl-ZnCl2 (25:25:25:25 wt%) were mixed uniformly in a high-speed mixer at a mass ratio of 90:10 wt% to obtain a mixture. The mixture was heated from room temperature to 550 ℃ at a rate of 2 ℃ / min and held for 10 h in a furnace with nitrogen as the protective gas. After cooling in the furnace, a sinter was obtained. The sinter was washed 5 times with a saturated sodium carbonate solution and then washed 2 times with anhydrous ethanol. After drying, a spherical single-crystal Na4Fe3(PO4)2P2O7 material was obtained.

[0057] Example 2

[0058] Sodium acetate, ferric phosphate, and phosphoric acid were weighed in a molar ratio of 4.1:2.9:1.1. Polyethylene glycol (6 wt% of the total weight of sodium, iron, and phosphorus sources) and glucose (4 wt% of the total weight of sodium, iron, and phosphorus sources) were added as carbon sources. All raw materials were mixed with water to prepare a slurry with a solid content of 25 wt%. The slurry was then milled to a particle size D50 < 150 μm. The milled slurry was dried and shaped using a spray dryer to obtain spherical dry powder with a particle size D50 = 3.5 μm and a water content of 2.4 wt%. The dried powder was heated from room temperature to 400 ℃ at a rate of 2 ℃ / min and held for 4 hours in a furnace with nitrogen as the protective gas. After cooling in the furnace, the precursor was obtained. The precursor and the composite sintering aid NaCl-KCl-ZnCl2 (weight ratio 1:1:1) were mixed in a mass ratio of 95:5. The wt% of the mixture was uniformly mixed in a high-speed mixer to obtain a mixture; the mixture was heated from room temperature to 560 ℃ at 2 ℃ / min and held in a furnace with nitrogen as the protective gas for 8 h, and then cooled with the furnace to obtain a sinter; the sinter was washed 5 times with a saturated sodium carbonate solution, then washed 2 times with anhydrous ethanol, and dried to obtain spherical single-crystal Na. 4.1 Fe 2.9 (PO4)2P2O7 material.

[0059] Example 3

[0060] Sodium acetate, ferric phosphate dihydrate, and trisodium phosphate were weighed in a molar ratio of 0.6:2.85:1.15. Polyethylene glycol (6 wt% of the total weight of sodium, iron, and phosphorus sources) and glucose (4 wt% of the total weight of sodium, iron, and phosphorus sources) were added as carbon sources. All raw materials were mixed with water to prepare a slurry with a solid content of 25 wt%. The slurry was then milled to a particle size D50 < 200 μm. The milled slurry was dried and shaped using a spray dryer to obtain spherical dry powder with a particle size D50 = 4.2 μm and a water content of 2.6 wt%. The dried powder was heated from room temperature to 430 ℃ at a rate of 2 ℃ / min and held for 4 h in a furnace with nitrogen as the protective gas. After cooling in the furnace, the precursor was obtained. The precursor and the composite sintering aid LiCl-KCl-AlCl3 (weight ratio 1:1:1) were mixed in a mass ratio of 95:5. The wt% of the mixture was uniformly mixed in a high-speed mixer to obtain a mixture; the mixture was heated from room temperature to 580 ℃ at 2 ℃ / min and held in a furnace with nitrogen as the protective gas for 8 h, and then cooled with the furnace to obtain a sinter; the sinter was washed 5 times with a saturated sodium carbonate solution, then washed 2 times with anhydrous ethanol, and dried to obtain spherical single-crystal Na. 4.05 Fe 2.85 (PO4)2P2O7 material.

[0061] Example 4

[0062] Sodium bicarbonate, ferrous oxalate, and sodium pyrophosphate were weighed in a molar ratio of 2:3.2:0.4. Polyethylene glycol (6 wt% of the total weight of sodium, iron, and phosphorus sources) and glucose (4 wt% of the total weight of sodium, iron, and phosphorus sources) were added as carbon sources. All raw materials were mixed with water to prepare a slurry with a solid content of 25 wt%. The slurry was then milled to a particle size D50 < 200 μm. The milled slurry was dried and shaped using a spray dryer to obtain spherical dry powder with a particle size D50 = 4.2 μm and a water content of 2.2 wt%. The dried powder was heated from room temperature to 360 ℃ at a rate of 2 ℃ / min and held for 5 h in a furnace with nitrogen as the protective gas. After cooling in the furnace, the precursor was obtained. The precursor and the composite sintering aid LiCl-KCl (50:50 wt%) were mixed in a mass ratio of 90:10. The wt% of the mixture was uniformly mixed in a high-speed mixer to obtain a mixture; the mixture was heated from room temperature to 550 ℃ at 2 ℃ / min and held in a furnace with nitrogen as the protective gas for 12 h, and then cooled with the furnace to obtain a sinter; the sinter was washed 5 times with a saturated sodium carbonate solution, then washed 2 times with anhydrous ethanol, and dried to obtain spherical single-crystal Na. 3.6 Fe 3.2 (PO4)2P2O7 material.

[0063] Comparative Example 1

[0064] Compared with Example 1, the only difference is that no composite sintering aid was added; all other operations and parameters are the same as in Example 1.

[0065] Comparative Example 2

[0066] Compared with Example 1, the only difference is that an equal weight of single NaCl is used to replace the composite sintering aid, while the total amount of calcination aid and other operations and parameters are the same as in Example 1.

[0067] Comparative Example 3

[0068] Compared with Example 1, the only difference is that the composite sintering aid is replaced with an equal weight of nitrate sintering aid LiNO3-NaNO3-KNO3-ZnNO3 (25:25:25:25 wt%). The total amount of calcining aid and other operations and parameters are the same as in Example 1.

[0069] Comparative Example 4

[0070] Compared with Example 1, the only difference is that the composite sintering aid is not mixed with the precursor in step 2, but is mixed with sodium carbonate, ferric phosphate and ammonium dihydrogen phosphate raw materials in step 1 and participates in the sand milling and subsequent steps. All other operations and parameters are the same as in Example 1.

[0071] Comparative Example 5

[0072] Compared with Example 1, the only difference is that the first stage of calcination in step 2 was not performed. Instead, the spherical dried powder in step 1 was directly combined with the composite sintering aid in step 3 and then processed. All other operations and parameters were the same as in Example 1.

[0073] Comparative Example 6

[0074] Compared with Example 1, the only difference is that purified water is used as the washing solution in step 3. All other operations and parameters are the same as in Example 1.

[0075] Test Example 1

[0076] X-ray diffraction (XRD) was performed on the sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Examples 1, 2, 3, 4, and 5. Experimental conditions: copper target (λ = 0.1518 nm), 2θ angle range 10–60°. XRD patterns are shown below. Figure 1 As shown;

[0077] from Figure 1 The XRD patterns in the sample show that the sodium iron phosphate pyrophosphate material prepared in Example 1 belongs to the orthorhombic crystal system (PDF standard card number: PDF#89-0579), has high phase purity, and no obvious NaFePO4 and Na2FeP2O7 impurity phases. This indicates that the chloride system sintering aid in the implementation scheme will not interfere with the pure phase synthesis of sodium iron phosphate pyrophosphate during the high-temperature sintering process in step 3. In Comparative Example 3, the sintering aid used is a decomposable nitrate system, which interferes with the pure phase synthesis of sodium iron phosphate pyrophosphate during the sintering process in step 3, thus preventing the acquisition of a pure phase. In Comparative Example 4, the sintering aid is directly introduced in step 1, interfering with the required element ratio of sodium iron phosphate pyrophosphate, thus preventing the acquisition of a pure phase. In Comparative Example 5, step 2 is omitted, and the sintering aid directly participates in the thermal solid solution reaction of the raw materials, thus preventing the acquisition of a pure phase.

[0078] Test Example 2

[0079] The morphology of the sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Examples 1, 2, and 3 was observed under a field emission scanning electron microscope (SEM); experimental conditions: copper target (λ = 0.1518 nm), 2θ angle range of 10–60°; XRD patterns are shown below. Figure 1 As shown; accelerating voltage is 3kV, working distance is 3mm, small aperture condenser lens, SEI mode.

[0080] from Figure 2 -a is the sodium iron phosphate pyrophosphate material prepared in Example 1, which has a morphology of near-single crystal spheres, a dense surface, and fused grain boundaries; Figure 2 -b is the sodium iron phosphate pyrophosphate material prepared in Comparative Example 1, which has a polycrystalline spherical morphology, a loose surface, and obvious primary particles; Figure 2-c shows that the sodium iron phosphate pyrophosphate material prepared in Comparative Example 2 could not provide a molten pool environment during the sintering process in step 3 and thus did not obtain a spherical single crystal morphology; Figure 2 -d is the sodium iron phosphate pyrophosphate material prepared in Comparative Example 3. During the sintering process in step 3, gas is generated, which causes pores. Although the grain boundaries of the prepared sodium iron phosphate pyrophosphate material are fused, the structure is not as dense as that of the material obtained in Example 1.

[0081] Test Example 3

[0082] The sodium iron phosphate pyrophosphate materials prepared in Example 1, Comparative Examples 1 and 6 were used to assemble coin cells for charge-discharge testing. The experimental conditions were: charge-discharge rate of 0.1 C and 1 C (1 C = 129 mAh / g), charge-discharge voltage range of 2~3.8 V, and ambient temperature of 25 ℃.

[0083] Figure 3 The figures show the charge-discharge curves of sodium iron pyrophosphate prepared in Example 1 and Comparative Examples 1 and 6 at a rate of 0.1 C. It can be seen that the monocrystalline sodium iron pyrophosphate material prepared in Example 1 has a higher charge specific capacity (118 mAh / g) and discharge specific capacity (101 mAh / g) compared to the polycrystalline sodium iron pyrophosphate material prepared in Comparative Example 1. In Comparative Example 6, the charge-discharge capacity is lower because pure water was used instead of saturated sodium carbonate solution to wash the sintered material in step 3, resulting in the dissolution of active sodium ions in the washing solution.

[0084] Figure 4 These are the charge-discharge cycle curves of sodium iron pyrophosphate prepared in Example 1 and Comparative Example 1 at 1 C. It can be seen that the monocrystalline sodium iron pyrophosphate material prepared in Example 1 has better cycle performance, with a capacity retention of more than 90% after 9000 cycles.

[0085]

[0086] As shown in Table 1, the capacities of the monocrystalline sodium pyrophosphate materials prepared in Examples 1-4 are all much higher than those prepared in Comparative Examples 1-6, which fully demonstrates the versatility and universality of the preparation method of the present invention.

[0087] Test Example 4

[0088] The materials prepared in each case were subjected to compaction density testing. Experimental conditions: compaction pressure 3 T, sample weight 1 g.

[0089]

[0090] As shown in Table 2, the compaction density of the monocrystalline sodium iron pyrophosphate materials prepared in Examples 1-4 is much higher than that of the polycrystalline sodium iron pyrophosphate prepared in Comparative Example 1, which fully demonstrates the versatility and universality of the preparation method of the present invention.

Claims

1. A method for preparing a spherical single-crystal sodium iron phosphate pyrophosphate material, characterized in that, Precursor particles are obtained by refining and slurrying sodium, iron, phosphorus and carbon sources by stoichiometry and then spraying them. The precursor particles are first roasted and then cooled to obtain a first-stage material. The first stage material and composite sintering aid are then mixed and subjected to a second stage of sintering. After washing in a saturated sodium salt solution and drying, the spherical monocrystalline sodium iron pyrophosphate material is obtained. The temperature for the first stage of roasting is 350~450℃; Composite sintering aids include those with the chemical formula MCl. n Two or more compounds; wherein M includes two or more of Li, Na, K, Al, and Zn; n is the valence of M; the melting point temperature of the composite sintering aid is below 500 °C; The second stage of roasting is carried out at a temperature of 500~600 ℃.

2. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, The chemical formula of sodium iron phosphate pyrophosphate is Na. x Fe y (PO4) 4.3-2z (P2O7) z Where 2.8≤x≤4.3, 2≤y≤3, 1≤z≤1.

5.

3. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium sulfate; Preferably, the iron source is one or more of ferric phosphate, ferric phosphate dihydrate, ferric pyrophosphate, ferrous oxalate, ferric oxide, ferrous oxide, and magnetite; Preferably, the phosphorus source is one or more of sodium pyrophosphate, trisodium phosphate, ammonium dihydrogen phosphate, and phosphoric acid; Preferably, the carbon source is one or more of polyvinylpyrrolidone, polyethylene glycol, citric acid, glucose, starch, and sucrose; Preferably, the amount of carbon source used is 0.2~15 wt% of the total mass of sodium source + iron source + phosphorus source; Preferably, the refining and slurrying is performed by one of ball milling, grinding, or sand milling; Preferably, the solvent for refining and slurrying includes water; Preferably, the solid content of the refined pulp is less than 70%, more preferably 15-50%; the particle size D50 in the pulp is controlled to be less than 500 μm, more preferably 100-350 μm. Preferably, the D50 of the spray-dried material is controlled to be less than 20 μm, more preferably 2~16 μm; the moisture content of the spray-dried material is less than 10wt%, more preferably 1~6wt%.

4. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, The atmosphere for the first stage of roasting is one of nitrogen, rare gas, hydrogen, or argon. Preferably, the roasting time for the first stage is 3 to 8 hours.

5. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, The composite sintering aids include one or more of the following: LiCl-ZnCl2 composite aid a, NaCl-AlCl3 composite aid b, KCl-AlCl3 composite aid c, LiCl-ZnCl2 composite aid d, NaCl-ZnCl2 composite aid e, KCl-ZnCl2 composite aid f, LiCl-NaCl-AlCl3 composite aid g, LiCl-KCl-ZnCl2 composite aid h, LiCl-NaCl-ZnCl2 composite aid i, NaCl-KCl-ZnCl2 composite aid j, LiCl-KCl-AlCl3 composite aid k, LiCl-NaCl-AlCl3 composite aid m, LiCl-KCl-MgCl2 composite aid n, LiCl-NaCl-KCl-MgCl2 composite aid o, and LiCl-NaCl-KCl-ZnCl2 composite aid p. Preferably, the weight percentage of each component in the composite sintering aid is the average content ± 7.5 wt%.

6. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 5, characterized in that, In the primary material and composite sintering aid, the mass percentage of composite sintering aid is 1-15%, preferably 5-10%; Preferably, the atmosphere for the second stage of roasting is one of nitrogen, rare gas, hydrogen, or argon. Preferably, the second roasting time is 6 to 18 hours.

7. The method for preparing spherical single-crystal sodium iron phosphate pyrophosphate material as described in claim 1, characterized in that, The saturated sodium salt solution is a saturated sodium carbonate solution.

8. A spherical single-crystal sodium iron pyrophosphate material prepared by the preparation method according to any one of claims 1 to 7.

9. The application of a spherical single-crystal sodium iron phosphate pyrophosphate material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, It was used as a positive electrode active material in the preparation of sodium secondary batteries.

10. A sodium secondary battery, comprising a positive electrode, characterized in that, The positive electrode comprises a spherical single-crystal sodium iron pyrophosphate material prepared by the preparation method according to any one of claims 1 to 7.

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