A polyanionic positive electrode material, a positive electrode sheet, a battery, and a preparation method thereof

Abstract

The application belongs to the technical field of sodium ion batteries, and more particularly relates to a polyanion positive electrode material, a positive electrode sheet, a battery and a preparation method thereof. The particle size distribution curve of the polyanion positive electrode material has a first peak and a second peak, wherein the peak particle size of the first peak is 5-10 mu m, and the peak particle size of the second peak is 0.1-3 mu m. The dual-peak particle size distribution characteristics form a synergistic grading effect between particles, realize close packing between particles, significantly improve the compaction density, and thus realize the synergistic optimization of high compaction performance and good electrochemical performance. The sodium ion battery assembled based on the polyanion positive electrode material has excellent electrochemical performance, and effectively improves the endurance of the battery.

Description

Technical Field

[0001] This application belongs to the field of sodium-ion battery technology, and more specifically, relates to a polyanion cathode material, cathode sheet, battery and its preparation method. Background Technology

[0002] Sodium-ion batteries are considered a core technology in the field of electrochemical energy storage because of their abundant crustal resources, low cost, and similar "rocking chair" electrochemical reaction mechanism to lithium-ion batteries. Their process system can be adapted, modified, and reused with lithium-ion batteries.

[0003] Among the cathode materials for sodium-ion batteries, polyanionic cathode materials stand out due to their superior structural stability. (The text abruptly ends here, so the translation also ends here.) 3- P2O7 4- (etc.) construct a rigid three-dimensional crystal framework, which can effectively suppress lattice distortion during charging and discharging, so that the material has both excellent electrochemical cycling stability (the capacity retention rate of some systems exceeds 90% after 2000 cycles) and extremely high thermal stability (the thermal decomposition temperature is generally higher than 500℃).

[0004] However, the intrinsic true density of polyanionic sodium cathode materials is only about 3.2 g / cm³. 3 Compared to lithium iron phosphate, a polyanionic lithium-ion battery cathode material in the same system (theoretical true density 3.6 g / cm³), 3 0.4g / cm 3 This difference directly leads to difficulties in increasing the electrode compaction density of sodium-ion battery cathode materials, reducing the amount of active material per unit volume, and ultimately lowering the volumetric energy density of the battery cell, significantly limiting its application in scenarios requiring high volumetric energy density. Secondly, polyanionic sodium-ion battery cathode materials are mostly solid solution structures, with a narrower phase formation temperature range compared to single-phase materials. During conventional high-temperature solid-state sintering, electrochemically inert impurities (such as NaFePO4) are inevitably generated, further reducing the material's energy density and electrochemical performance.

[0005] Therefore, developing polyanionic cathode materials that combine high solid density, high phase purity, and excellent electrochemical performance is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this application is to provide a polyanionic cathode material, cathode sheet, battery and preparation method thereof, which aims to solve the problems of low compaction density, easy generation of impurity phases during sintering, and difficulty in synergistic optimization of compaction density and electrochemical performance of existing polyanionic cathode materials.

[0007] To achieve the above objectives, in a first aspect, this application provides a polyanionic cathode material, wherein the particle size distribution curve of the polyanionic cathode material has a first peak and a second peak, wherein the peak particle size of the first peak is 5~10μm and the peak particle size of the second peak is 0.1~3μm.

[0008] Preferably, the above-mentioned polyanionic cathode material includes a polyanionic cathode matrix material and a carbon coating layer optionally disposed on the surface of the above-mentioned polyanionic cathode matrix material; The general chemical formula of the above-mentioned polyanionic cathode matrix material is Na. a M b (PO4) c P2O7, wherein 3.6≤a≤4.2, 2.6≤b≤3, 1.6≤c<2.1, and M is selected from at least one of Fe, Mn, Mg, Cr, Cu, Mo, Zn, Zr, Ti, B and Al.

[0009] Preferably, the compaction density of the above-mentioned polyanionic cathode material is 2.1~2.3 g / cm³. 3 .

[0010] Secondly, this application provides a method for preparing the above-mentioned polyanionic cathode material, comprising the following steps: S1. Sodium source, doped metal M source, phosphorus source, carbon source and solvent are mixed in a closed environment to carry out pre-reaction to obtain precursor slurry; S2. Spray dry the above precursor slurry to obtain precursor powder; S3. The above precursor powder is pre-calcined to obtain pre-calcined particles; S4. The above-mentioned pre-burned particles are sieved, and the micron-sized pre-burned particles and submicron-sized pre-burned particles obtained by sintering are separated to obtain the first polyanion cathode material and the second polyanion cathode material. The sintering temperature of the micron-sized pre-sintered particles is higher than that of the submicron-sized pre-sintered particles. S5. The first polyanionic cathode material and the second polyanionic cathode material are mixed to obtain the polyanionic cathode material.

[0011] Preferably, in step S1, the above-mentioned mixed slurry also includes a surface activator.

[0012] Preferably, in step S1, the solid content of the precursor slurry is 30% to 50%.

[0013] Preferably, in step S2, the inlet air temperature of the spray dryer is 130°C to 180°C, and the outlet air temperature is 70°C to 120°C.

[0014] Preferably, in step S3, the pre-firing temperature is 180℃~350℃, and the pre-firing time is 1h~5h.

[0015] Preferably, in step S4, the particle size of the above-mentioned micron-sized pre-burnt particles is 1μm~20μm.

[0016] Preferably, in step S4, the particle size of the submicron-sized pre-calcined particles is <1 μm. Preferably, in step S4, the sintering temperature of the above-mentioned micron-sized pre-sintered particles is 500℃~600℃.

[0017] Preferably, in step S4, the temperature difference between the sintering temperature of the micron-sized pre-sintered particles and the sintering temperature of the submicron-sized pre-sintered particles is 50°C to 100°C.

[0018] Preferably, in step S4, the sintering time of the micron-sized pre-sintered particles and the submicron-sized pre-sintered particles is 4h to 8h each independently.

[0019] Preferably, in step S5, the median particle size of the first polyanionic cathode material is 5~10 μm.

[0020] Preferably, in step S5, the median particle size of the second polyanionic cathode material is 0.1~1μm.

[0021] Preferably, in step S5, the mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20 to 95:5.

[0022] Thirdly, this application provides a positive electrode sheet, which includes a positive current collector and a positive active material layer located on at least one side of the surface of the positive current collector; the positive active material layer includes a binder and the above-mentioned polyanionic positive electrode material or a polyanionic positive electrode material prepared by the above-mentioned preparation method.

[0023] Fourthly, this application provides a sodium-ion battery, including the above-mentioned polyanionic cathode material or the polyanionic cathode material prepared by the above-mentioned preparation method or the above-mentioned cathode sheet.

[0024] In summary, the technical solutions conceived in this application have the following main technical advantages compared with the prior art: (1) The polyanionic cathode material provided in this application has a particle size distribution curve with a first peak and a second peak, wherein the peak particle size of the first peak is 5~10 μm and the peak particle size of the second peak is 0.1~3 μm. This bimodal particle size distribution characteristic forms a synergistic gradation effect between particles. On the one hand, it avoids the drawback of sacrificing electrochemical activity by simply pursuing large particle compaction; on the other hand, it also avoids the processing difficulties and increased side reactions caused by simply pursuing small particles. More importantly, it achieves close packing between particles, thereby significantly improving the compaction density of the material, and thus significantly improving the volumetric energy density of the battery cell.

[0025] (2) The preparation method provided in this application separates submicron-sized and micron-sized pre-calcined particles by particle size sieving after pre-calcination, and adopts differentiated sintering processes—that is, micron-sized pre-calcined particles are sintered at relatively high temperatures, while submicron-sized pre-calcined particles are sintered at relatively low temperatures, so that pre-calcined particles of different sizes can achieve pure phase transformation under their respective optimal sintering conditions. Compared with the traditional one-time sintering process, the method of this application can effectively suppress the formation of electrochemically inert impurity phases, ensuring that the final product is a highly crystalline solid solution pure phase, fundamentally guaranteeing the intrinsic sodium storage capacity and energy density of the material. At the same time, since the sintering temperature of submicron-sized pre-calcined particles is lower, the preparation method provided in this application also has the industrial advantage of significantly reducing production energy consumption.

[0026] (3) When the mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20 to 95:5, the second polyanionic cathode material can more effectively fill the gaps formed by the stacking of the first polyanionic cathode material, forming an optimized multi-scale stacking structure and further improving the particle stacking efficiency. Under this preferred ratio, the compaction density of the polyanionic cathode material can reach 2.3 g / cm³. 3 This achieves synergistic optimization of high-pressure compaction performance and good electrochemical performance. Attached Figure Description

[0027] Figure 1 This is a particle size distribution curve of the polyanionic cathode material prepared in Example 1 of this application. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0029] In the description of this application, it should be understood that the term "and / or" describes a relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The symbol " / " in this document indicates that the related objects are in an "or" relationship; for example, A / B means A or B.

[0030] In the specification and claims of this application, the terms “first” and “second” are used to distinguish different objects, rather than to describe a specific order of objects, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated.

[0031] In the description of the embodiments in this application, the words "exemplary" or "for example" are used to indicate that they are examples, illustrations, or descriptions. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design options. Specifically, the use of the words "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0032] In the description of the embodiments in this application, unless otherwise stated, "multiple" means two or more.

[0033] This application provides a polyanionic cathode material, the particle size distribution curve of which has a first peak and a second peak, wherein the peak particle size of the first peak is 5~10μm and the peak particle size of the second peak is 0.1~3μm.

[0034] In actual production, the polyanionic cathode material with a bimodal particle size distribution curve can be formed by mixing a first polyanionic cathode material and a second polyanionic cathode material. The median particle size of the first polyanionic cathode material is 5~10μm, the median particle size of the second polyanionic cathode material is 0.1~1μm, and the mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20~95:5.

[0035] Through experiments, the inventors discovered that the aforementioned bimodal particle size distribution characteristic creates a synergistic gradation effect between particles. Larger particles (the first polyanion cathode material) ensure the material's processability and structural stability, while smaller particles (the second polyanion cathode material) provide a shorter sodium ion solid-phase diffusion path, which is beneficial for rate performance and capacity utilization. This synergistic gradation avoids both the drawbacks of sacrificing electrochemical activity by simply pursuing large particles and high compaction, and the processing difficulties and increased side reactions caused by simply pursuing very small particles. It achieves close packing between particles, significantly improving the material's compaction density, thereby significantly improving the cell's volumetric energy density. This results in a high-compact-density polyanion cathode material with a bimodal particle size distribution curve, exhibiting excellent electrical performance.

[0036] In some embodiments, the aforementioned polyanionic cathode material includes a polyanionic cathode matrix material and, optionally, a carbon coating layer disposed on the surface of the polyanionic cathode matrix material. In some embodiments, the general chemical formula of the aforementioned polyanionic cathode matrix material is Na. a M b (PO4) c P2O7, wherein 3.6≤a≤4.2, 2.6≤b≤3, 1.6≤c<2.1, and M is selected from at least one of Fe, Mn, Mg, Cr, Cu, Mo, Zn, Zr, Ti, B and Al.

[0037] On the other hand, this application also provides a method for preparing the above-mentioned polyanionic cathode material, comprising the following steps: S1. Sodium source, doped metal M source, phosphorus source, carbon source and solvent are mixed in a closed environment to carry out pre-reaction to obtain precursor slurry; S2. Spray dry the above precursor slurry to obtain precursor powder; S3. The above precursor powder is pre-calcined to obtain pre-calcined particles; S4. The above-mentioned pre-burned particles are sieved, and the micron-sized pre-burned particles and submicron-sized pre-burned particles obtained by sintering are separated to obtain the first polyanion cathode material and the second polyanion cathode material. The sintering temperature of the micron-sized pre-sintered particles is higher than that of the submicron-sized pre-sintered particles. S5. The first polyanionic cathode material and the second polyanionic cathode material are mixed to obtain the polyanionic cathode material.

[0038] In some embodiments, the sodium source, doped metal M source, phosphorus source, and carbon source mentioned above in step S1 are added according to the stoichiometric ratio of the polyanion cathode matrix material. It should also be noted that this application does not impose specific limitations on the selection of the types of raw materials used in the preparation process; all types of raw materials that can be used in the art to prepare polyanion sodium-ion battery cathode materials are applicable to this application.

[0039] For example, the sodium sources mentioned above include, but are not limited to, one or more of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium nitrate, sodium sulfate, sodium phosphate, sodium carbonate, sodium bicarbonate, sodium bioxalate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, disodium ethylenediaminetetraacetate, sodium pyrophosphate, and sodium hydrogen pyrophosphate.

[0040] For example, the doping metal M source described above is a salt, oxide, or sulfide of metal M. For instance, when the doping metal is Fe, the doping metal M source can be one or more of the following: ferric sulfate or its hydrate, ferric nitrate or its hydrate, ferrous oxalate, ferrous chloride, ferric acetate, ferrous sulfate, ferric phosphate, and ferrous citrate.

[0041] For example, the phosphorus source mentioned above includes, but is not limited to, one or more of phosphoric acid, sodium hypophosphite, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0042] For example, the carbon source described above is one or more of organic and inorganic carbon sources. In some embodiments, the inorganic carbon source includes, but is not limited to, one or more of carbon nanotubes, acetylene black, graphene, graphene oxide, carbon nanotubes, carbon fibers, activated carbon, and conductive carbon black. The organic carbon source includes, but is not limited to, one or more of citric acid, oxalic acid, ascorbic acid, and tartaric acid.

[0043] For example, the solvent mentioned above is one or more organic solvents such as water and alcohols. Among them, the water solvent can be ultrapure water, double-distilled water, deionized water, pure water, distilled water, etc.

[0044] In some embodiments, the precursor slurry further includes a surfactant that can improve the dispersibility of the material in a solvent system. The surfactants include, but are not limited to, one or more of the following: oleic acid, sodium oleate, stearic acid, sodium stearate, sodium citrate, sodium alginate, citric acid, ascorbic acid, polyvinylpyrrolidone, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, ammonium dodecyl sulfate, dodecyl phosphate, hexadecyltrimethylammonium bromide, sodium diisooctyl succinate sulfonate, and polyoxyethylene stearate.

[0045] In some embodiments, the solid content of the precursor slurry is 30% to 50%, which helps to improve the reaction efficiency between components in the solvent system.

[0046] In some embodiments, in step S2, the inlet air temperature of the spray drying process is 130°C to 180°C and the outlet air temperature is 70°C to 120°C. This can control the evaporation rate of moisture during the drying process and prevent the evaporation of moisture from being too fast, which would increase the porosity of the precursor powder obtained from the drying process and thus reduce the compaction density.

[0047] In some embodiments, in step S3, the pre-calcination temperature is 180°C to 350°C and the pre-calcination time is 1h to 5h, which can remove the residual moisture in the precursor powder, and at the same time, the phosphate ions are partially dehydrated and polymerized into pyrophosphate ions to obtain anhydrous amorphous products, i.e., pre-calcined particles.

[0048] In some embodiments, in step S4, the particle size of the micron-sized pre-sintered particles is 1~20μm; the particle size of the submicron-sized pre-sintered particles is <1μm. By sintering the micron-sized and submicron-sized pre-sintered particles separately, this application can effectively avoid the formation of impurity phases, ensure that the sintered product is a solid solution pure phase, and help improve the energy density of polyanionic cathode materials.

[0049] In some embodiments, in step S4, the sintering temperature of the micron-sized pre-sintered particles is 500℃~600℃. Specifically, the sintering temperature of the micron-sized pre-sintered particles can be 500℃, 520℃, 550℃, 560℃, 580℃, 600℃, etc.

[0050] In some embodiments, in step S4, the temperature difference between the sintering temperature of the micron-sized pre-sintered particles and the sintering temperature of the submicron-sized pre-sintered particles is 50°C to 100°C. Compared with the prior art, the sintering temperature of the submicron-sized pre-sintered particles in this application is significantly lower than that of the micron-sized pre-sintered particles, which can effectively reduce sintering energy consumption and facilitate industrial production.

[0051] In some embodiments, the sintering time of the above-mentioned micron-sized pre-sintered particles and the above-mentioned submicron-sized pre-sintered particles is independently 4h to 8h.

[0052] In some embodiments, the atmosphere used for pre-firing and sintering is an inert atmosphere. In some embodiments, the inert atmosphere may include one or more of nitrogen, helium, neon, argon, krypton, xenon, and radon. That is, the inert atmosphere can be an atmosphere formed by a single gas or a mixed atmosphere formed by multiple gases.

[0053] In some embodiments, the median particle size of the first polyanionic cathode material is 5-10 μm. In some embodiments, the median particle size of the second polyanionic cathode material is 0.1-1 μm. In some embodiments, the mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20-95:5.

[0054] In some embodiments, the mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20 to 95:5. This application mixes the first polyanionic cathode material and the second polyanionic cathode material of suitable particle size in a specific ratio. This allows the second polyanionic cathode material to more effectively fill the gaps formed by the stacking of the first polyanionic cathode material, forming an optimized multi-scale stacking structure and further improving particle stacking efficiency.

[0055] This application also provides a positive electrode sheet, which includes a positive current collector and a positive active material layer located on at least one side of the surface of the positive current collector; the positive active material layer includes a binder and the aforementioned polyanionic positive electrode material or a polyanionic positive electrode material prepared by the aforementioned preparation method.

[0056] This application also provides a secondary battery, which includes the above-described polyanionic cathode material or the polyanionic cathode material prepared by the above-described preparation method or the above-described cathode sheet.

[0057] The polyanionic cathode material provided in this application has high compaction density and high energy density. When applied to sodium-ion batteries, it can endow sodium-ion batteries with excellent electrochemical performance.

[0058] It should be understood that materials of the same or similar type, model, quality, properties, or function as the reagents and instruments used in the following embodiments can be used to implement this application. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods. Unless otherwise specified, the materials and reagents used in the following embodiments are commercially available.

[0059] The following are examples and comparative examples: Example 1 The method for preparing the polyanionic cathode material provided in this embodiment includes the following steps: Step 1: Prepare the ingredients according to the stoichiometric ratio of Na4Fe3(PO4)2P2O7. Place sodium orthophosphate (phosphorus source), FePO4 (iron source), sodium carbonate (sodium source), citric acid (carbon source), and water (solvent) in a closed stirring tank for full pre-reaction to obtain a mixture with a solid content of 30wt%. Then, mill the mixture to obtain the precursor slurry.

[0060] Step 2: Spray dry the above precursor slurry at a feed rate of 10 rpm, an inlet air temperature of 130°C, and an outlet air temperature of 70°C to obtain precursor powder.

[0061] Step 3: Place the above precursor powder under an argon atmosphere for pre-calcination at a temperature of 300°C for 2 hours, and then allow it to cool naturally to obtain pre-calcined particles.

[0062] Step 4: The pre-sintered particles are classified using an air classifier to obtain micron-sized pre-sintered particles with a particle size range of 1~20μm and submicron-sized pre-sintered particles with a particle size <1μm. The micron-sized pre-sintered particles are sintered at 600℃ for 6h in an argon atmosphere to obtain a micron-sized spherical solid solution pure phase product with a median particle size (D50) of 10μm, which is the first polyanion cathode material; the submicron-sized pre-sintered particles are sintered at 480℃ for 5h in an argon atmosphere to obtain a pure phase single particle product with a median particle size (D50) of 0.7μm, which is the second polyanion cathode material.

[0063] Step 5: Mix the first polyanionic cathode material and the second polyanionic cathode material at a mass ratio of 95:5 to obtain a polyanionic cathode material.

[0064] Figure 1 The particle size distribution curve of the polyanionic cathode material prepared in this embodiment shows that the particle size distribution of the polyanionic cathode material has a first peak and a second peak. The compaction density of the polyanionic cathode material prepared in this embodiment, determined using GB / T 44330-2024, is 2.1 g / cm³. 3 .

[0065] Example 2 The preparation method of the polyanionic cathode material provided in this embodiment is the same as that in Embodiment 1, except that in step 5, the mass ratio of the first polyanionic cathode material and the second polyanionic cathode material is 80:20.

[0066] The compaction density of the polyanionic cathode material prepared in this example, determined according to GB / T 44330-2024, is 2.3 g / cm³. 3 .

[0067] Comparative Example 1 The preparation method of the polyanionic cathode material provided in this comparative example includes the following steps: Step 1: Prepare the precursor solution according to the method provided in Step 1 of Example 1; Step 2: Prepare precursor powder according to the method provided in Step 2 of Example 1; Step 3: The above precursor powder is placed in an argon atmosphere for one-time sintering. First, it is pre-sintered at 300℃ for 2 hours, and then sintered at 470℃ for 8 hours to obtain polyanionic cathode material.

[0068] The compaction density of the polyanionic cathode material prepared in this comparative example, determined according to GB / T 44330-2024, is 1.98 g / cm³. 3 .

[0069] Comparing Comparative Example 1 and the Examples, it was found that the polyanionic cathode material prepared by conventional one-time sintering in Comparative Example 1 contained impurity phases and had a compaction density of only 1.98 g / cm³. 3 However, when used to assemble sodium-ion batteries, it cannot effectively improve electrochemical performance. In this application, by mixing a first polyanion cathode material and a second polyanion cathode material of suitable particle size at a specific mass ratio, a high compaction density of 2.3 g / cm³ can be achieved in the polyanion cathode material. 3 This helps improve its performance in electrochemical sodium storage applications. In practical applications, sodium-ion batteries assembled based on this polyanionic cathode material exhibit excellent electrochemical performance, effectively improving the battery life of sodium-ion batteries.

[0070] Compared to existing preparation processes for polyanionic cathode materials, this application embodiment separates the micron-sized and submicron-sized pre-sintered particles by sieving the pre-sintered particles. The sintering temperature of the submicron-sized pre-sintered particles is significantly lower than that of the micron-sized pre-sintered particles. Then, the first and second polyanionic cathode materials obtained by sintering are mixed. This significantly reduces sintering energy consumption while ensuring that the resulting polyanionic cathode material is a solid solution pure phase with good crystallinity, significantly improving the compaction density of the material and enhancing its electrochemical performance.

[0071] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A polyanionic cathode material, characterized in that, The particle size distribution curve of the polyanionic cathode material has a first peak and a second peak, wherein the peak particle size of the first peak is 5~10μm and the peak particle size of the second peak is 0.1~3μm.

2. The polyanionic cathode material according to claim 1, characterized in that, The polyanionic cathode material includes a polyanionic cathode matrix material and a carbon coating layer optionally disposed on the surface of the polyanionic cathode matrix material; The general chemical formula of the polyanion cathode matrix material is Na. a M b (PO4) c P2O7, wherein 3.6≤a≤4.2, 2.6≤b≤3, 1.6≤c<2.1, and M is selected from at least one of Fe, Mn, Mg, Cr, Cu, Mo, Zn, Zr, Ti, B and Al.

3. The polyanionic cathode material according to claim 1 or 2, characterized in that, The compaction density of the polyanionic cathode material is 2.1~2.3 g / cm³. 3 .

4. A method for preparing a polyanionic cathode material as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Sodium source, doped metal M source, phosphorus source, carbon source and solvent are mixed in a closed environment to carry out pre-reaction to obtain precursor slurry; S2. Spray dry the precursor slurry to obtain precursor powder; S3. The precursor powder is pre-calcined to obtain pre-calcined particles; S4. The pre-burned particles are sieved, and the micron-sized pre-burned particles and submicron-sized pre-burned particles obtained by sintering are separated to obtain the first polyanion cathode material and the second polyanion cathode material. The sintering temperature of the micron-sized pre-sintered particles is higher than that of the submicron-sized pre-sintered particles. S5. The first polyanion cathode material and the second polyanion cathode material are mixed to obtain the polyanion cathode material.

5. The preparation method according to claim 4, characterized in that, In step S1, the mixed slurry also includes a surface activator.

6. The preparation method according to claim 4, characterized in that, In step S1, the solid content of the precursor slurry is 30%~50%; and / or, In step S2, the inlet air temperature of the spray dryer is 130℃~180℃, and the outlet air temperature is 70℃~120℃; and / or, In step S3, the pre-firing temperature is 180℃~350℃, and the pre-firing time is 1h~5h.

7. The preparation method according to claim 4, characterized in that, In step S4, the particle size of the micron-sized pre-calcined particles is 1μm~20μm, and / or, The submicron-sized pre-calcined particles have a particle size of <1 μm; and / or, The sintering temperature of the micron-sized pre-sintered particles is 500℃~600℃; and / or, The temperature difference between the sintering temperature of the micron-sized pre-sintered particles and the sintering temperature of the submicron-sized pre-sintered particles is 50℃~100℃; and / or, The sintering time for the micron-sized pre-sintered particles and the submicron-sized pre-sintered particles is independently 4h to 8h.

8. The preparation method according to claim 4, characterized in that, In step S5, the median particle size of the first polyanionic cathode material is 5~10 μm; and / or, The median particle size of the second polyanionic cathode material is 0.1~1 μm; and / or, The mass ratio of the first polyanionic cathode material to the second polyanionic cathode material is 80:20 to 95:

5.

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer located on at least one side of the surface of the positive current collector; the positive active material layer includes a binder and the polyanionic positive electrode material according to any one of claims 1 to 3 or the polyanionic positive electrode material prepared by the preparation method according to any one of claims 4 to 8.

10. A sodium-ion battery, characterized in that, Includes the polyanionic cathode material as described in any one of claims 1 to 3, or the polyanionic cathode material prepared by the preparation method as described in any one of claims 4 to 8, or the cathode sheet as described in claim 9.

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