High-compaction composite sodium ferric phosphate positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a preparation method and application of a high-compaction composite sodium ferric phosphate positive electrode material. One of the purposes of the invention is to provide a preparation method of a high-compaction composite sodium ferric phosphate positive electrode material. The composite sodium ferric phosphate positive electrode material shows a novel microstructure: a sheet structure is used as a support framework, granular substances are tightly filled between the sheet structure and the support framework, and the sheet structure and the granular substances are interspersed, stacked and interwoven to form an internally compact three-dimensional network. Compared with the prior art, the composite sodium ferric phosphate prepared by the technical route of the prior art generally only presents a single granular morphology. Gaps among particles are difficult to effectively eliminate in the densification process of such materials, and more micro-pores exist, so that the compaction density improvement space is limited, and the materials become one of key factors for restricting the battery performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and specifically relates to a preparation method and application of a high-density composite sodium iron phosphate positive electrode material. Background Art

[0002] Sodium-ion batteries have achieved unprecedented development in the field of energy storage due to their low cost, low temperature resistance, long life and high safety. The positive electrode material is crucial to the energy density, cycle life and safety of sodium-ion batteries. At the same time, the positive electrode material accounts for up to 40% of the cost of battery cell manufacturing, which has attracted many researchers to focus on the research and development of sodium battery positive electrode materials. Among the many positive electrode material candidates, composite sodium iron phosphate positive electrode material (with the general structural formula of Na 3+x Fe 2+x (PO4) 1+x (P2O7), 0≤x≤1, hereinafter referred to as NFPP) has taken the lead in realizing the transformation from laboratory basic research to large-scale commercial application due to its regular three-dimensional sodium ion channels, excellent structural stability, good safety and relatively mature synthesis process route. It is regarded as one of the sodium battery positive electrode systems currently closest to large-scale industrialization.

[0003] The development of NFPP materials is driven by a relentless pursuit of performance. Early NFPP research focused on verifying its electrochemical activity and structural stability. As industrialization progressed, increasing energy density became a key bottleneck. NFPP itself has a relatively low theoretical true density (approximately 3.2 g / cm³), and its practical application is as a composite NFPP / C cathode powder with conductive carbon.

[0004] However, the compaction density of NFPP / C materials produced by current mainstream preparation methods is generally low, with most products reaching only approximately 2.0 g / cm³ or even lower. The compaction density of electrode materials is a crucial factor in improving the energy density of sodium-ion batteries. A higher compaction density allows for more active material to be packed into the same volume, allowing for thinner and more compacted electrode sheets. Thinner electrode sheets not only significantly improve the volumetric and gravimetric energy densities of individual cells, but also facilitate rapid electrolyte penetration and infiltration into the electrodes, thereby reducing the battery's internal resistance and improving rate capability and reaction kinetics. Therefore, despite the many advantages of NFPP, its relatively low compaction density has become a key limitation hindering further improvements in the energy density of related sodium-ion batteries. Current common preparation methods face the challenge of striking a balance between ensuring good electrochemical activity (which typically requires appropriate particle morphology and size) and achieving dense packing to achieve a high compaction density when pursuing high-performance NFPP. Traditional solid-phase methods often produce irregular and oversized particles, making them difficult to compact. Furthermore, some processes that improve activity (such as carefully controlling particle morphology or reducing primary particle size) can introduce excessive porosity or cohesive effects, which can lead to reduced powder packing properties and make increasing compaction density extremely difficult. Developing a novel synthesis strategy that can effectively achieve high compaction density NFPP / C cathode materials while maintaining excellent electrochemical performance, while also being simple and amenable to large-scale production, is of great technical and economic significance for accelerating the industrialization of high-performance sodium-ion batteries. Summary of the Invention

[0005] In order to overcome the defects of the prior art, the present invention provides a high-density composite sodium iron phosphate positive electrode material, a preparation method and an application thereof, which can solve the technical problems of low compaction density and low capacity of the currently prepared sodium ion battery positive electrode materials.

[0006] One of the objectives of the present invention is to provide a method for preparing a high-density composite sodium iron phosphate positive electrode material, which comprises the following steps: S1: adding iron powder to a phosphoric acid solution, stirring until the iron powder is fully dissolved, and then mixing in a sodium source, wherein the sodium source is selected from one or more of sodium hydroxide, sodium carbonate, and sodium oxide; S2: After sufficient stirring, a supplementary iron source and a carbon source are added, wherein the supplementary iron source is selected from one or more of ferrous oxalate, ferrous oxide, ferrosoferric oxide, and ferrous carbonate; S3: ball milling to homogenize the slurry to obtain a slurry with dual particle size distribution characteristics: the first particle size distribution is between 0.1 and 0.6 μm, and the second particle size distribution is between 1 and 3 μm; S4: The slurry is dried, crushed, and sieved in sequence to obtain a precursor powder; S5: The precursor powder is placed in an inert gas and sintered to obtain a high-density composite sodium iron phosphate positive electrode material with a D50 particle size of 5~10 μm.

[0007] According to a preferred embodiment, after adding the supplementary iron source, the molar ratio of Na:Fe:P in the mixture is (3+x):(2+x):(3+x), where 0≤x≤1.

[0008] According to a preferred embodiment, the amount of carbon source added is 5-15% of the total weight of the positive electrode material, preferably 7-10%.

[0009] According to a preferred embodiment, in S1, the ratio of the iron source to the phosphoric acid is 1:(2.0-3.0), and the concentration of the phosphoric acid is 100-300 g / L.

[0010] According to a preferred embodiment, the first particle size distribution in the precursor powder is between 0.3 and 0.6 μm.

[0011] According to a preferred embodiment, the second particle size distribution in the precursor powder is between 0.5 and 2 μm.

[0012] Preferably, the first particle size distribution in the precursor powder is 0.3 μm, and the second particle size distribution is 1 μm.

[0013] Preferably, the first particle size distribution in the precursor powder is 0.5 μm, and the second particle size distribution is 2 μm.

[0014] Preferably, the first particle size distribution in the precursor powder is 0.45 μm, and the second particle size distribution is 0.5 μm.

[0015] According to a preferred embodiment, in S4, the drying method can be air drying or spray drying, wherein the air drying temperature is 70~100°C, and the air drying time is 2~10 h; the inlet air temperature of the spray drying is 150~250°C, the outlet air temperature of the spray drying is 60~120°C, and the feed rate of the spray drying is 0.5~30 mL / min.

[0016] According to a preferred embodiment, the inert gas is one or more of nitrogen, argon, argon and hydrogen.

[0017] According to a preferred embodiment, in S5, the sintering temperature is 300-600°C, the sintering time is 6-15 h, and the sintering heating rate is 1-10°C / min.

[0018] One of the purposes of the present invention is to provide a high-density composite sodium iron phosphate positive electrode material, which is prepared based on the above-mentioned high-density composite sodium iron phosphate positive electrode material preparation method. Preferably, the high-density composite sodium iron phosphate positive electrode material has the general structural formula of Na 3+x Fe 2+x (PO4) 1+x (P2O7), where 0≤x≤1.

[0019] According to a preferred embodiment, the high-density composite sodium iron phosphate positive electrode material is configured as a sheet-shaped and granular in-situ co-embedded structure.

[0020] According to a preferred embodiment, iron powder and phosphoric acid are prepared in a molar ratio of 1:(2-3), preferably a molar ratio of iron powder to phosphoric acid of 1:(2.2-2.5). The chemical reaction involved in the process is: Fe+2H3PO4——>Fe 2+ +2H2PO4 - +H2O.

[0021] According to a preferred embodiment, the sodium source is mainly sodium hydroxide, sodium carbonate, sodium citrate or sodium acetate. Preferably, the sodium source is sodium hydroxide and sodium carbonate. The chemical reaction involved in this process is: Fe 2+ +2H2PO4 - +H2O——>Fe3(PO4)2 . xH2O.

[0022] One of the purposes of the present invention is also to provide a sodium ion battery positive electrode, which is a high-density composite sodium iron phosphate positive electrode material prepared using the above-mentioned preparation method of the high-density composite sodium iron phosphate positive electrode material.

[0023] One of the purposes of the present invention is also to provide a sodium ion battery, using the high-density composite sodium iron phosphate positive electrode material prepared by the above-mentioned preparation method of the high-density composite sodium iron phosphate positive electrode material.

[0024] Beneficial effects of this technical solution: The innovative technology proposed in this paper utilizes a flaky ferrophosphide source and a granular iron source as precursors. A grinding process is used to uniformly mix the two materials at the micro-nanoscale. High-temperature sintering is then used to successfully facilitate the in-situ composite reaction of the flaky ferric phosphate and granular sodium ferric phosphate. The key lies in the in-situ construction of a unique interwoven flaky-particle stacking composite microstructure. The directional arrangement of the flaky skeleton during the pressing process and the efficient packing of particles enable a high-order "bottom-up" synergistic effect at the microscale, forming a highly cross-linked, dense structure.

[0025] In terms of particle stacking and electrode densification, the large, cuboid-shaped primary particles provide a strong skeletal support for the material, effectively maintaining structural stability and reducing overall deformation during the pressing process. Simultaneously, the small, granular primary particles efficiently fill the gaps between the large particle skeletons and the various gaps created by the stacking of secondary particles. This synergistic effect of primary particles of complementary size and morphology promotes a tighter graded packing between particles, significantly reducing the material's porosity. This has the direct effect of significantly increasing the compaction density of the precursor powder, laying a solid physical foundation for achieving higher electrode areal density and volumetric energy density in the subsequent preparation of lithium iron phosphate cathode materials.

[0026] In terms of powder processing performance and electrode processing, this secondary particle structure inherently improves material flowability and reduces dust generation. More importantly, the coexistence of large, cuboid-like particles and small, granular particles optimizes the particle size distribution of the precursor powder. This optimized distribution effectively mitigates the stratification (segregation) that can occur with single-sized, spherical particles during electrode slurry coating. It also improves the slurry's rheological properties and stability, resulting in a more uniform, dense, and crack-resistant electrode coating, thereby enhancing electrode processing yield and consistency.

[0027] In terms of electrochemical performance, although large primary particles provide a high-density structural basis, the granular small primary particles, with their smaller size and larger specific surface area, greatly increase the active sites and significantly shorten the solid-phase diffusion path of lithium ions. Large particles ensure the mechanical strength and high density of the electrode structure of sodium-ion batteries, while small particles contribute excellent kinetic active points. This synergistic effect jointly improves the lithium-ion diffusion kinetics of the material, allowing the final positive electrode material to exhibit better rate charge and discharge performance while maintaining high energy density. In addition, the secondary particle structure formed by the reasonable combination of large and small particles has pores retained after stacking that are more conducive to sufficient infiltration of the electrolyte, ensuring good electrochemical accessibility.

[0028] Based on the above preparation method, the composite sodium iron phosphate positive electrode material of the present invention exhibits a novel microscopic morphology: a lamellar structure serves as a supporting skeleton, with granular materials densely filling it, and the two are interspersed, stacked and interwoven with each other to form an intrinsically dense three-dimensional network. In comparison, the composite sodium iron phosphate prepared by the traditional technical route based on iron phosphate or ferrous oxalate usually only presents a single granular morphology. During the densification process of such materials, the gaps between the particles are difficult to effectively eliminate, and there are many microscopic pores, resulting in limited room for improvement in its compaction density, which has become one of the key factors restricting battery performance.

[0029] The composite sodium iron phosphate positive electrode material synthesized by the present invention combines the two major factors of cost and performance, and can comprehensively improve the competitiveness of composite sodium iron phosphate products. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Scanning electron microscope images of Example 1 at different preparation stages; Figure 2 XRD patterns of the NFPP / C composite cathode materials prepared in Examples 1, 2, 5 and 6; Figure 3 XRD patterns of the NFPP / C composite cathode materials prepared in Comparative Examples 1 and 2; Figure 4 The charge and discharge curves of the button half-cells prepared in Examples 1, 3, and 5 at a current of 0.1 C are shown; Figure 5 The charge and discharge curves of the button half-cells prepared in Comparative Examples 1 and 2 at a current of 0.1 C are shown; Figure 6 This is a process flow chart of the positive electrode material provided by the present invention. DETAILED DESCRIPTION

[0031] The experimental methods in the following examples are conventional methods unless otherwise specified. The experimental materials used in the following examples are purchased from commercial channels unless otherwise specified.

[0032] The present invention will be further described below with reference to the accompanying drawings.

[0033] Example 1 like Figure 6 As shown, a high-density Na4Fe3(PO4)2P2O7 / C composite material is prepared. The specific steps are as follows: S1: Add 38.00 g of Fe (98 wt.%) and 192.16 g of H3PO4 (85 wt.%) to 1000 mL of deionized water, controlling the Fe / P molar ratio to 1:2.5. Stir continuously until the Fe powder is basically dissolved, and remove the insoluble magnetic material with a demagnetizer to obtain ferrophosphorus liquid. S2 Add 161.60 g of NaOH to 1000 mL of deionized water and stir until the sodium hydroxide is completely dissolved. While stirring, slowly pour the sodium hydroxide solution into the ferrophosphorus solution and continue stirring to obtain a green solid-liquid mixture (ferrophosphorus precursor). S3: 444.04 g of anhydrous ferric phosphate, 40 g of anhydrous glucose, and 10 g of polyethylene glycol were added to the green solid-liquid mixture in S2 in sequence; S4: Add 8 mm diameter zirconium balls at a ball-to-solid ratio of 5:1 and ball-mill at 300 RPM for 8 h to obtain a green precursor slurry with a particle size D50 of approximately 0.4 μm. Transfer the milled green slurry to an 80°C oven for drying to completely remove moisture. After drying, crush the material using a high-speed grinder and sieve it through a 300-mesh screen to collect the first precursor powder. S5: The precursor is placed in a crucible and sintered in a nitrogen atmosphere. Pre-sinter at 350°C for 4 h at a heating rate of 5°C / min. Then, heat to 500°C for 8 h at a heating rate of 3°C / min. Finally, heat to 580°C for 2 h at a heating rate of 3°C / min. After cooling naturally, high-density Na₄Fe₃(PO₄)₂P₂Oₐ / C is obtained.

[0034] Example 2 Preparation of a high-density Na4Fe3(PO4)2P2O7 / C composite material. The specific operating steps are basically the same as those in Example 1. The differences from Example 1 are: In S1, the amount of Fe powder was adjusted to 47.49 g, the amount of phosphoric acid was adjusted to 211.37 g, and the Fe / P molar ratio was controlled to be 1:2.2.

[0035] In S3, the amount of anhydrous ferric phosphate was adjusted to 330.16 g, and the other parameters remained unchanged.

[0036] In S4, the ball milling time was adjusted to 7 h, and the other parameters remained unchanged.

[0037] Example 3 Preparation of a high-density Na4Fe3(PO4)2P2O7 / C composite material. The specific operating steps are basically the same as those in Example 1. The differences from Example 1 are: In S1, the amount of Fe powder was adjusted to 47.49 g, the amount of phosphoric acid was adjusted to 211.37 g, and the Fe / P molar ratio was controlled to be 1:2.2.

[0038] In S3, anhydrous ferric phosphate was adjusted to dihydrate ferric phosphate, the amount of dihydrate ferric phosphate was 408.95 g, and the other parameters remained unchanged.

[0039] Example 4 A high-pressure Na 3.6 Fe 2.6 (PO4) 1.6 Preparation of P2O7 / C composite material. The specific operation steps are basically the same as those in Example 1. The difference from Example 1 is: In step 2, the amount of NaOH was adjusted to 145.44 g, and the other parameters remained unchanged.

[0040] In S3, the amount of anhydrous ferric phosphate was adjusted to 294.61 g, the amount of anhydrous glucose was adjusted to 30 g, and the other parameters remained unchanged.

[0041] Example 5 A high-pressure Na 3.6 Fe 2.6 (PO4) 1.6 Preparation of P2O7 / C composite material. The specific operation steps are basically the same as those in Example 1. The difference from Example 1 is: In S1, the amount of Fe powder was adjusted to 93.26 g, the amount of phosphoric acid was adjusted to 415.06 g, and the Fe / P molar ratio was controlled to be 1:2.2.

[0042] In step 2, NaOH was adjusted to Na2CO3, and the amount of Na2CO3 was adjusted to 194.62 g, while the other parameters remained unchanged.

[0043] In S3, anhydrous ferric phosphate was adjusted to ferrous oxalate dihydrate, the amount of ferrous oxalate was 176.89 g, the amount of anhydrous glucose was adjusted to 10 g, and the other parameters remained unchanged.

[0044] The remaining parameters remain unchanged.

[0045] Example 6 A high-pressure Na 3.5 Fe 2.5 (PO4) 1.5 Preparation of P2O7 / C composite material. The specific operation steps are basically the same as those in Example 1. The difference from Example 1 is: In S1, the amount of Fe powder was adjusted to 82.89 g, the amount of phosphoric acid was adjusted to 415.06 g, and the Fe / P molar ratio was controlled to be 1:2.2.

[0046] In S2, NaOH was adjusted to Na2CO3, the amount of Na2CO3 was adjusted to 172.99 g, and the other parameters remained unchanged.

[0047] In S3, anhydrous ferric phosphate was adjusted to ferric oxide, the amount of ferric oxide was 59.53 g, the amount of anhydrous glucose was adjusted to 30 g, and the other parameters remained unchanged.

[0048] Comparative Example 1 Na4Fe3(PO4)2P2O7 / C was prepared using conventional iron phosphate technology. The specific steps are as follows: S1: Add 45.71 g of ferric phosphate, 14.20 g of anhydrous disodium hydrogen phosphate, 10.81 g of sodium carbonate, and 8.0 g of anhydrous glucose into a ball mill. Add zirconium beads with a diameter of 3 mm at a ball-to-material ratio of 10:1 and add 130 mL of deionized water. S2 was ball-milled at 300 rpm for 8 h, and the D50 of the ball-milled material was controlled between 0.3 and 0.4 μm. The slurry obtained by ball milling was transferred to an 80°C oven for drying to completely remove the moisture. After drying, the material was crushed using a high-speed grinder and sieved using a 300-mesh screen to collect the finer precursor powder. S3: The precursor is placed in a crucible and sintered in a nitrogen atmosphere. Pre-sinter at 350°C for 4 hours at a heating rate of 5°C / min. Then, heat to 500°C for 8 hours at a heating rate of 3°C / min. Finally, heat to 580°C for 2 hours at a heating rate of 3°C / min. After cooling naturally, high-density Na4Fe3(PO4)2P2O7 / C is obtained.

[0049] Comparative Example 2 Na4Fe3(PO4)2P2O7 was prepared using conventional ferrous oxalate technology, and the specific operating steps were similar to those in Example 1.

[0050] S1: Add 43.16 g of ferrous oxalate, 23.24 g of ammonium dihydrogen phosphate (NH4H2PO4), 26.87 g of sodium pyrophosphate (Na4P2O7), and 11.48 g of citric acid into a ball mill. Add zirconium beads with a diameter of 3 mm at a ball-to-material ratio of 10:1 and add 130 mL of deionized water. S2 is consistent with Comparative Example 1; S3 is consistent with Comparative Example 1.

[0051] Test Example 1 The cathode composite materials obtained at different preparation stages of Example 1 were observed under a scanning electron microscope. Figure 1 It can be seen that before ball milling, the precursor exhibited two morphologies: flaky and granular. The flaky material is likely the ferrous phosphate product generated by the reaction of iron powder and phosphoric acid, while the granular material is likely the added ferric phosphate. After ball milling, the precursor was ground into a nanoscale state, and the flaky and granular morphologies could be vaguely distinguished. After high-temperature sintering, both the flaky and granular structures in the precursor were transformed into granular structures. From the perspective of secondary particles, the material exhibits the effect of a stacking of large and small particles. From the perspective of the magnified structure of the particle surface, the primary particles of the material also exhibit the effect of a stacking of large and small particles.

[0052] Test Example 2 The positive electrode composite materials prepared in Examples 1, 2, 5, 6 and Comparative Examples 1 and 2 were subjected to X-ray diffraction tests. Figure 2 It can be seen that the samples of Example 1, Example 2, Example 5 and Example 6 have no obvious impurity phase, indicating that the purity of Na4Fe3(PO4)2P2O7 in different examples is relatively high. Figure 3 It can be seen that there is an obvious impurity phase in Comparative Example 1, while there is no obvious impurity phase in Comparative Example 2.

[0053] Test Example 3 The positive electrode composite materials of Examples 1 to 6 and Comparative Examples 1 to 2 were used to prepare electrode plates, and the compaction density of the materials and the compaction density of the electrodes were tested. As shown in Table 1, the compaction density of the materials prepared by the technical route provided by the invention was significantly improved. The compaction density of the powders prepared in Examples 1 to 5 was generally higher than 2.3 g / cm 3 The powder compaction of the materials prepared in Example 6 is basically higher than 2.4 g / cm 3 , which is significantly higher than that of Comparative Examples 1 and 2. Under the condition of high areal loading, the pole pieces of Examples 1 to 6 also have obvious advantages in compaction. The pole piece compaction prepared in Example 1 is higher than 2.40 g / cm 3 The electrode compaction of Examples 2 to 5 is higher than 2.35 g / cm 3 The electrode compaction prepared in Example 6 is higher than 2.50 g / cm 3 Compared with Example 1 prepared by conventional iron phosphate technology, its powder compaction and electrode compaction are 1.93 g / cm 3 and 2.04 g / cm 3 ; Compared with the conventional ferrous oxalate technology route prepared in Example 2, the powder compaction and the electrode compaction were 1.85 and 1.91 g / cm 3 .

[0054] Test Example 4 The cathode composite materials prepared in Examples 1, 3 and 5 were tested for specific capacity at a voltage of 2.0-4.0 V. The results are shown in FIG. Figure 4 As shown. Examples 1, 3, and 5 show similar charge-discharge curves, with obvious discharge platforms at 3.2 V and 2.9 V, which correspond to the gradual release of sodium from Na4Fe3(PO4)2P2O7. At the same time, the first charge capacities of Examples 1, 3, and 5 at 0.1 C are 113.3, 117.4, and 117.5 mAh / g, respectively, and the first discharge capacities at 0.1 C are 105.0, 106.5, and 107.1 mAh / g, respectively. Further observation Figure 5The capacity of Comparative Example 1 is significantly lower, likely due to the presence of a larger number of impurities. The capacity of Comparative Example 2 is significantly higher, almost comparable to that of the Example. However, the discharge curve exhibits a slope in the late discharge phase (2.7-2.0 V), indicating poor kinetic performance.

[0055] Table 1 Physical and chemical properties of various cathode composite materials

[0056] In the description of the present invention, it should be understood that the terms "upper", "lower", "bottom", "top", "front", "back", "inside", "outside", "left", "right", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention.

[0057] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be employed in conjunction with other described embodiments.

Claims

1. A method for preparing a high-density composite sodium iron phosphate positive electrode material, characterized in that: The following steps are involved: S1: adding iron powder to a phosphoric acid solution, stirring until the iron powder is fully dissolved, and then mixing in a sodium source, wherein the sodium source is selected from one or more of sodium hydroxide, sodium carbonate, and sodium oxide; S2: After sufficient stirring, a supplementary iron source and a carbon source are added, wherein the supplementary iron source is selected from one or more of ferrous oxalate, ferrous oxide, ferrosoferric oxide, and ferrous carbonate; S3: ball milling to homogenize the slurry to obtain a slurry with dual particle size distribution characteristics: the first particle size distribution is between 0.1 and 0.6 μm, and the second particle size distribution is between 1 and 3 μm; S4: The slurry is dried, crushed, and sieved in sequence to obtain a precursor powder; S5: placing the precursor powder in an inert gas and sintering it to obtain the high-density composite sodium iron phosphate positive electrode material with a D50 particle size of 5-10 μm.

2. The method for preparing a high-density composite sodium iron phosphate cathode material according to claim 1, characterized in that: In the S1, the ratio of the iron source to the phosphoric acid is 1:2.0-3.0, and the concentration of the phosphoric acid is 100-300 g / L.

3. The method for preparing a high-density composite sodium iron phosphate cathode material according to claim 1 or 2, characterized in that: The first particle size of the precursor powder is distributed between 0.3 and 0.6 μm.

4. The method for preparing a high-density composite sodium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that: The second particle size distribution in the precursor powder is between 0.5 and 2 μm.

5. The method for preparing a high-density composite sodium iron phosphate positive electrode material according to claim 1, characterized in that: In said S4, the drying method can be air drying or spray drying, wherein, The air drying temperature is 70-100°C, and the air drying time is 2-10 h; the inlet air temperature of the spray drying is 150-250°C, the outlet air temperature of the spray drying is 60-120°C, and the feed rate of the spray drying is 0.5-30 mL / min.

6. The method for preparing a high-density composite sodium iron phosphate cathode material according to claim 1, characterized in that: The inert gas is one or more of nitrogen, argon, argon and hydrogen.

7. The method for preparing a high-density composite sodium iron phosphate cathode material according to claim 1, characterized in that: In S5 , the sintering temperature is 300-600° C., the sintering time is 6-15 h, and the sintering heating rate is 1-10° C. / min.

8. A high-density composite sodium iron phosphate positive electrode material, characterized in that: The high-density composite sodium iron phosphate positive electrode material is prepared based on the preparation method of the high-density composite sodium iron phosphate positive electrode material according to any one of claims 1 to 7.

9. A sodium ion battery positive electrode, characterized in that A high-density composite sodium iron phosphate positive electrode material prepared using the preparation method of the high-density composite sodium iron phosphate positive electrode material according to any one of claims 1 to 7.

10. A sodium ion battery, characterized in that: A high-density composite sodium iron phosphate positive electrode material prepared using the preparation method of the high-density composite sodium iron phosphate positive electrode material according to any one of claims 1 to 7.

Images