A sodium ferric pyrophosphate-carbon composite positive electrode material, a preparation method and application

Abstract

The application provides a sodium iron pyrophosphate phosphate-carbon composite positive electrode material, a preparation method and application, and the preparation method comprises the following steps: uniformly mixing a soluble iron source, a hardly soluble iron source, a sodium source, a phosphorus source, a carbon source, a complexing agent, a dispersing agent and a solvent, reacting to obtain a sol, drying the sol to obtain a xerogel, and sintering to obtain the sodium iron pyrophosphate phosphate-carbon composite positive electrode material; and the molar ratio of the soluble iron source and the hardly soluble iron source is (2.3-2.7):1. The application adopts a double iron source, namely, the soluble iron source and the hardly soluble iron source are matched. The soluble iron source can quickly dissociate to form a large number of uniform crystal nuclei in the initial stage of the reaction, realizes nucleation, the hardly soluble iron source can slowly deposit on the surface of the crystal nuclei and grow gently, effectively inhibits excessive coarsening and agglomeration of particles, so that the effects of optimizing particle size, regulating particle size distribution and improving particle morphology are achieved. In combination with the addition of the carbon source, the structure stability and excellent electrochemical performance of the obtained sodium iron pyrophosphate phosphate-carbon composite positive electrode are realized.

Description

Technical Field

[0001] This invention relates to the field of sodium battery cathode material technology, and in particular to a sodium iron pyrophosphate-carbon composite cathode material, its preparation method, and its application. Background Technology

[0002] The rapid development of renewable and clean energy sources (such as solar, wind, tidal, and geothermal energy) is of great significance to the sustainable development of human society. However, their intermittency and volatility make efficient energy storage technology a critical issue that urgently needs to be addressed. Currently, lithium-ion batteries (LIBs) dominate the secondary battery field, but their long-term large-scale application faces significant challenges due to the limited and uneven distribution of global lithium resources and their high cost. Sodium-ion batteries (SIBs), as a highly promising alternative, have received widespread attention in recent years. Sodium resources are abundant (approximately 2.74% of the Earth's crust), inexpensive, and their working principle is similar to that of lithium-ion batteries, making it advantageous to leverage existing technologies to develop electrode materials suitable for sodium storage. However, achieving high-performance sodium-ion batteries still presents many challenges, among which developing structurally stable and electrochemically superior sodium cathode materials is particularly crucial.

[0003] Currently, research on cathode materials for sodium-ion batteries mainly focuses on three categories: transition metal oxides, Prussian blue analogs, and polyanionic compounds. Polyanionic compounds have attracted significant attention due to their advantages such as strong structural tunability, high thermal stability, small volume change during charge and discharge, and stable three-dimensional framework. They can provide robust structural support for the reversible insertion and extraction of sodium ions and achieve high operating voltages. Among them, composite pyrophosphates have a smaller molecular weight, which improves ionic conductivity, reduces viscosity, and lowers cost. Furthermore, this mixed phosphate structure exhibits enhanced cycle stability and sodium ion diffusion kinetics, with an operating voltage of 3.1V (Na₂O₃). ＋ (Na), surpassing most single polyanionic systems, thus becoming one of the most promising commercial cathode materials.

[0004] The core bottlenecks faced by existing sodium iron pyrophosphate cathode materials in sodium-ion battery applications mainly include two aspects: First, their intrinsic electronic conductivity is poor. This is mainly due to the strong covalent bond between [FeO6] octahedra and [PO4] tetrahedra in the polyanionic framework, forming a wide band gap that hinders the free migration of electrons. This results in severe polarization of the material during high-rate charge and discharge, making it difficult to fully utilize its actual capacity. Second, the theoretical specific capacity of the material is limited by the three-electron reaction mechanism. In actual charge and discharge processes, the diffusion kinetics of sodium ions in the rigid crystal structure are relatively slow. In addition, the utilization rate of some active sites is insufficient, which often results in the actual reversible capacity being lower than the theoretical value, limiting further improvement in battery energy density. These problems together restrict the large-scale commercial application of sodium iron pyrophosphate materials.

[0005] Existing research indicates that by coating the surface with a carbon layer and rationally controlling the particle size, particle size distribution, and morphology of the cathode material, the sodium ion diffusion path can be effectively shortened, diffusion resistance reduced, and active site utilization improved, thereby effectively improving ion diffusion kinetics. However, existing synthesis processes typically cannot effectively guarantee particle size and uniformity, resulting in irregular morphologies, easy agglomeration, and complex steps or expensive precursors, which in turn increases production costs. These factors severely hinder the commercialization of this material system.

[0006] Therefore, how to provide a method for preparing sodium iron pyrophosphate-carbon composite cathode material, and provide an effective material solution for developing sodium-ion batteries with high energy density, long life and wide temperature range applicability, is a technical problem that urgently needs to be solved. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a sodium iron pyrophosphate-carbon composite cathode material, its preparation method, and its applications. This invention utilizes a dual iron source and a carbon source as raw materials, combining a soluble iron source and a sparingly soluble iron source. Combined with a sol-gel preparation process, the soluble iron source rapidly dissociates in the early stages of the reaction to form numerous uniform crystal nuclei, achieving nucleation. The sparingly soluble iron source slowly deposits and gently grows on the surface of the crystal nuclei, effectively inhibiting excessive particle coarsening and agglomeration. This optimizes particle size, regulates particle size distribution, and improves particle morphology. Simultaneously, this invention utilizes a liquid-phase reaction environment to achieve uniform mixing of raw materials at the molecular level, allowing for further precise control of particle size, particle size distribution, and microstructure. This results in excellent structural stability and electrochemical performance of the obtained sodium iron pyrophosphate-carbon composite cathode material, enabling sodium-ion batteries prepared using this cathode material to possess advantages such as high energy density, long lifespan, and wide temperature range applicability.

[0008] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material, the method comprising the following steps: A soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a carbon source, a complexing agent, a dispersant, and a solvent are mixed evenly and reacted to obtain a sol. The sol is dried to obtain a dry gel, and then sintered to obtain a sodium iron pyrophosphate-carbon composite cathode material. The molar ratio of the soluble iron source to the insoluble iron source is (2.3-2.7):1, for example, 2.3:1, 2.35:1, 2.4:1, 2.45:1, 2.5:1, 2.55:1, 2.6:1, 2.65:1 or 2.7:1, etc.

[0009] This invention uses a dual iron source and a carbon source as raw materials. The soluble iron source can rapidly dissociate in the early stage of the reaction to form a large number of uniform crystal nuclei, achieving nucleation. The insoluble iron source can slowly deposit and grow on the surface of the crystal nuclei, effectively inhibiting excessive particle coarsening and agglomeration, thereby optimizing particle size, controlling particle size distribution, and improving particle morphology. During sintering, the decomposition characteristics of the carbon source are utilized to construct a uniform, highly graphitized, conductive carbon layer in situ on the surface of sodium iron pyrophosphate, further improving the conductive network. This invention employs the sol-gel method, utilizing the liquid-phase reaction environment to achieve uniform mixing of raw materials at the molecular level. The combination of the soluble and insoluble iron sources further optimizes particle size, controls particle size distribution, and improves particle morphology.

[0010] This invention regulates the molar ratio of soluble iron source to sparingly soluble iron source to be (2.3-2.7):1, which can precisely balance the nucleation rate and crystal growth rate, achieving optimal control over particle size, particle size distribution, and particle morphology. This ensures that the material possesses small particle size, narrow distribution, low agglomeration, and high structural stability, thereby significantly improving electrochemical performance. If the molar ratio is too high, i.e., the content of soluble iron source is too high, it will lead to excessive nucleation and rapid growth of crystal nuclei, causing excessive coarsening of particles, severe agglomeration, and a widening of particle size distribution. At the same time, it will disrupt the uniform coating of the carbon layer, reducing the electronic conductivity and ion diffusion efficiency of the material. If the molar ratio is too low, i.e., the content of sparingly soluble iron source is too high, it will lead to insufficient nucleation, slow and uneven crystal growth, irregular particle morphology, and poor dispersibility. Similarly, it will result in a wide particle size distribution, low compaction density, and deterioration of battery rate performance and cycle stability.

[0011] It should be noted that, in this invention, soluble iron sources refer to iron sources that are soluble in deionized water, while sparingly soluble iron sources refer to iron sources that are sparingly soluble in deionized water. Specifically, in 100 mL of deionized water at 25°C, iron sources with a solubility of <0.2 g are considered sparingly soluble iron sources, while those with a solubility of ≥5 g are considered soluble iron sources.

[0012] As a preferred embodiment of the present invention, the soluble iron source includes at least one of ferrous sulfate, ferric sulfate, ferric nitrate, ferrous nitrate, ferric chloride, ferric citrate, or ferrous chloride.

[0013] Preferably, the insoluble iron source includes at least one of ferrous oxalate, ferric phosphate, ferrous phosphate, ferric hydroxide, or iron(II,III) oxide.

[0014] In this invention, ferrous oxalate not only plays a role in slow deposition and gentle growth on the surface of crystal nuclei, effectively inhibiting excessive particle coarsening and agglomeration, but also thermally decomposes to produce CO during sintering, which in-situ reduces Fe. 3＋ For Fe 2＋ This can further ensure the purity of the target phosphate ferric sodium pyrophosphate phase.

[0015] Preferably, the sodium source includes at least one of disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, or sodium pyrophosphate.

[0016] Preferably, the carbon source includes at least one of sucrose, glucose, starch, or citric acid.

[0017] Preferably, the phosphorus source includes at least one of disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, white phosphorus, yellow phosphorus, black phosphorus, phosphoric acid, phosphorous acid, hypophosphorous acid, phosphine, phosphorus trioxide, phosphorus pentoxide, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate.

[0018] Preferably, the complexing agent includes at least one of citric acid, EDTA, tartaric acid, oxalic acid, or glycine.

[0019] Preferably, the dispersant includes at least one of ethylene glycol, propylene glycol, glycerol, or polypropylene glycol.

[0020] Preferably, the solvent includes deionized water.

[0021] As a preferred technical solution of the present invention, the reaction includes first mixing a soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a complexing agent, a dispersant and a solvent evenly to carry out a first reaction, and then adding a carbon source to carry out a second reaction.

[0022] Preferably, the mixed raw materials also include a dispersing agent.

[0023] Preferably, the dispersing agent includes at least one of polyethylene glycol, polyvinylpyrrolidone, or sodium dodecylbenzenesulfonate.

[0024] Preferably, the reaction includes first mixing a soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a complexing agent, a dispersant, the dispersing aid, and a solvent evenly to carry out a first reaction, and then adding a carbon source to carry out a second reaction.

[0025] The carbon source of this invention is added slowly, and preferably separately from other reaction raw materials. This can prevent the viscosity of the reaction system from becoming too high, which is more conducive to the sol-gel reaction and ensures the uniformity and stability of the sol system.

[0026] Preferably, the reaction time is 2h-3h, such as 2h, 2.2h, 2.5h, 2.8h or 3h.

[0027] Preferably, the reaction time is 30-40 minutes, such as 30 minutes, 32 minutes, 35 minutes, 38 minutes, or 40 minutes.

[0028] As a preferred embodiment of the present invention, the molar ratio of iron to phosphorus in the soluble iron source, the sparingly soluble iron source, the sodium source, and the phosphorus source is (0.72-0.78):1, for example, 0.72:1, 0.73:1, 0.74:1, 0.75:1, 0.76:1, 0.77:1, or 0.78:1, etc.

[0029] Preferably, the molar ratio of iron to sodium in the soluble iron source, the sparingly soluble iron source, the sodium source, and the phosphorus source is (0.72-0.75):1, for example, 0.72:1, 0.73:1, 0.74:1, or 0.75:1.

[0030] Preferably, the mass-volume ratio of the total iron in the soluble iron source and the sparingly soluble iron source to the solvent is (40g-45g):1L, for example, 40g:1L, 41g:1L, 42g:1L, 43g:1L, 44g:1L, or 45g:1L, etc.

[0031] Preferably, the molar amount of the complexing agent is 1 to 2 times the total molar amount of the soluble iron source and the insoluble iron source, for example, 1, 1.2, 1.5, 1.8 or 2 times.

[0032] Preferably, the amount of dispersant added is 8wt%-12wt% of the mass of the solvent, such as 8wt%, 9wt%, 10wt%, 11wt% or 12wt%.

[0033] Preferably, the amount of carbon source added is 10wt%-15wt% of the target mass of sodium iron pyrophosphate, such as 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%.

[0034] Preferably, the amount of the dispersing agent added is 0.5wt%-1wt% of the mass of the solvent, for example, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt% or 1wt%.

[0035] As a preferred technical solution of the present invention, the reaction temperature is 75℃-85℃, such as 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, 81℃, 82℃, 83℃, 84℃ or 85℃.

[0036] Preferably, the stirring speed of the reaction is 300rpm-600rpm, such as 300rpm, 350rpm, 400rpm, 450rpm, 500rpm, 550rpm or 600rpm.

[0037] Preferably, the reaction is carried out under water bath conditions.

[0038] Preferably, the drying process includes a primary drying and a secondary drying, performed sequentially.

[0039] Preferably, the temperature of the secondary drying is higher than the temperature of the primary drying.

[0040] The present invention employs a gradient drying method, first dehydrating at a relatively low temperature and then curing at a relatively high temperature, in order to prevent the gel from cracking, bubbling and breaking down, and to obtain a uniform, loose and crack-free dry gel.

[0041] Preferably, the temperature of the first drying step is 85℃-95℃, such as 85℃, 86℃, 87℃, 88℃, 89℃, 90℃, 91℃, 92℃, 93℃, 94℃ or 95℃, and the drying time is 2h-5h, such as 2h, 3h, 4h or 5h.

[0042] Preferably, the temperature of the secondary drying is 95℃-105℃, such as 95℃, 96℃, 97℃, 98℃, 99℃, 100℃, 101℃, 102℃, 103℃, 104℃ or 105℃, and the time of the secondary drying is 5h-9h, such as 5h, 6h, 7h, 8h or 9h.

[0043] This invention requires that the temperature of the secondary drying be higher than that of the primary drying. Therefore, when the temperature of the primary drying is 95°C, the temperature of the secondary drying needs to be greater than 95°C.

[0044] Preferably, the drying process further includes grinding and sieving the resulting dry gel once to obtain precursor powder.

[0045] Preferably, the grinding time for one grinding cycle is 10 min to 40 min, such as 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, or 40 min.

[0046] Preferably, the mesh size of the sieve used in the first sieving is 80-120 mesh, such as 80 mesh, 85 mesh, 90 mesh, 95 mesh, 100 mesh, 105 mesh, 110 mesh, 115 mesh or 120 mesh.

[0047] As a preferred embodiment of the present invention, the sintering is carried out under an inert atmosphere.

[0048] Preferably, the inert atmosphere comprises high-purity nitrogen and / or a helium-nitrogen mixture.

[0049] Preferably, the sintering includes sequentially performing a first-stage sintering, a second-stage sintering, and a third-stage sintering.

[0050] Preferably, the heating rate of the sintering section is 1℃ / min-3℃ / min, for example, 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min or 3℃ / min.

[0051] Preferably, the holding temperature of the sintering section is 250℃-300℃, such as 250℃, 255℃, 260℃, 265℃, 270℃, 275℃, 280℃, 285℃, 290℃, 295℃ or 300℃.

[0052] Preferably, the holding time for the first sintering stage is 1.5h-2.5h, such as 1.5h, 1.8h, 2h, 2.2h or 2.5h.

[0053] Preferably, the heating rate of the two-stage sintering is 2℃ / min-5℃ / min, such as 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min or 5℃ / min, etc.

[0054] Preferably, the holding temperature for the two-stage sintering is 450℃-500℃, such as 450℃, 455℃, 460℃, 465℃, 470℃, 475℃, 480℃, 485℃, 490℃, 495℃ or 500℃.

[0055] Preferably, the holding time for the two-stage sintering is 3h-6h, such as 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h.

[0056] Preferably, the heating rate of the three-stage sintering is 4℃ / min-7℃ / min, for example, 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min or 7℃ / min.

[0057] Preferably, the holding temperature for the three-stage sintering is 620℃-650℃, such as 620℃, 625℃, 630℃, 635℃, 640℃, 645℃ or 650℃.

[0058] Preferably, the holding time for the three-stage sintering is 6h-9h, such as 6h, 6.5h, 7h, 7.5h, 8h, 8.5h or 9h.

[0059] In this invention, sintering includes sequential sintering at 250℃-300℃, 450℃-500℃, and 620℃-650℃. First, pre-decomposition occurs at a low temperature of 250℃-300℃. Temperatures that are too high or too low will cause the gel to crack and the structure to collapse. Then, an amorphous quasi-crystalline stage is performed at 450℃-500℃. If the temperature is too low, effective crystal nuclei cannot be formed, resulting in a material with many fixed shapes, low crystallinity, low capacity, and poor cycle life. If the temperature is too high, crystal nuclei will grow prematurely and too quickly, resulting in coarse grains and difficulty in controlling the particle size. The third stage at 620℃-650℃ is the crystal regularization and phase formation stage. Omitting this stage will lead to insufficient grain growth, internal looseness, and severe secondary particle agglomeration, making it impossible to guarantee the material's density, stability, and particle size uniformity. If the holding temperature during the third sintering is too low, the crystal structure will be irregular and locally agglomerated, the particle size distribution will be widened, the target phosphate iron sodium pyrophosphate phase will be incomplete, there will be more impurities, and the crystallization will be poor, which will lead to low electrical conductivity, low capacity and large polarization of the material. If the holding temperature during the third sintering is too high, it will lead to abnormal grain growth, hard agglomeration, coarsening of particles, and the formation of impurity iron phosphate, which will lead to rapid cycle decay.

[0060] As a preferred embodiment of the present invention, the sintering environment is further purged with inert gas before sintering.

[0061] Preferably, the inert atmosphere comprises high-purity nitrogen and / or a helium-nitrogen mixture.

[0062] Preferably, the flow rate of the inert atmosphere is 50 mL / min to 100 mL / min, such as 50 mL / min, 55 mL / min, 60 mL / min, 65 mL / min, 70 mL / min, 75 mL / min, 80 mL / min, 85 mL / min, 90 mL / min, 95 mL / min, or 100 mL / min.

[0063] Preferably, the purging time is 20-40 minutes, such as 20 minutes, 25 minutes, 30 minutes, 35 minutes, or 40 minutes.

[0064] Preferably, the sintering process further includes secondary grinding and secondary sieving.

[0065] Preferably, the secondary grinding time is 40-60 minutes, such as 40 minutes, 45 minutes, 50 minutes, 55 minutes, or 60 minutes.

[0066] Preferably, the mesh size of the sieve used in the secondary sieving is 180-220 mesh, such as 180 mesh, 185 mesh, 190 mesh, 195 mesh, 200 mesh, 205 mesh, 210 mesh, 215 mesh or 220 mesh.

[0067] Preferably, the secondary sieving process further includes vacuum drying.

[0068] It should be noted that the present invention does not impose specific requirements or special limitations on the vacuum drying conditions after the second sieving. As long as the raw materials can be completely dried, it is acceptable. Those skilled in the art can make adaptive selections and adjustments according to actual conditions.

[0069] Preferably, the preparation method further includes vacuum drying the solid raw materials before the mixture is homogenized.

[0070] It should be noted that the present invention does not impose specific requirements or special limitations on the vacuum drying conditions of solid raw materials. As long as the raw materials can be completely dried and the raw materials are not decomposed, it is acceptable. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, the drying temperature can be 55℃-70℃, and ferrous oxalate needs to be dried at ≤60℃ to avoid decomposition.

[0071] In a second aspect, the present invention also provides a sodium iron pyrophosphate-carbon composite cathode material, wherein the sodium iron pyrophosphate-carbon composite cathode material is prepared according to the preparation method described in the first aspect, and the sodium iron pyrophosphate-carbon composite cathode material includes sodium iron pyrophosphate and a carbon coating layer located on the surface of the sodium iron pyrophosphate.

[0072] As a preferred technical solution of the present invention, the median particle size D50 of the sodium iron pyrophosphate-carbon composite cathode material is 1μm-5μm, such as 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm or 5μm, preferably 1μm-2.5μm.

[0073] Preferably, the thickness of the carbon coating layer is 1nm-5nm, such as 1nm, 1.5nm, 2nm, 2.5nm, 3nm, 3.5nm, 4nm, 4.5nm or 5nm.

[0074] Thirdly, the present invention also provides a sodium-ion battery, the sodium-ion battery comprising a sodium iron pyrophosphate-carbon composite cathode material prepared by the preparation method described in the first aspect, or a sodium iron pyrophosphate-carbon composite cathode material as described in the second aspect.

[0075] Compared with the prior art, the present invention has at least the following beneficial effects: The method for preparing sodium iron pyrophosphate-carbon composite cathode material provided by this invention uses dual iron and carbon sources as raw materials, namely, a combination of soluble and insoluble iron sources, and a sol-gel preparation process. The soluble iron source can rapidly dissociate in the early stage of the reaction to form a large number of uniform crystal nuclei, thus achieving nucleation. The insoluble iron source can slowly deposit and grow on the surface of the crystal nuclei, effectively inhibiting excessive particle coarsening and agglomeration, thereby optimizing particle size, controlling particle size distribution, and improving particle morphology. At the same time, this invention utilizes the liquid-phase reaction environment to achieve uniform mixing of raw materials at the molecular level, which can further precisely control particle size, particle size distribution, and micromorphology, thereby achieving excellent structural stability and electrochemical performance of the obtained sodium iron pyrophosphate-carbon composite cathode material. As a result, sodium-ion batteries prepared using the cathode material provided by this invention have the advantages of high energy density, long life, and wide temperature range applicability. Detailed Implementation

[0076] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0077] The scope of this invention can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form an effective range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this invention. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this invention. In this invention, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0078] In this invention, "a combination of at least two" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or a combination of at least two" means that any one of the listed items can be selected, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this invention. In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" means a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this invention, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0079] In this invention, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond the listed elements unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this invention, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this invention. The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various locations throughout the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this invention can be combined with other embodiments that do not conflict with the technology. The ordinal numbers "first," "second," "third," and "fourth," etc., used in the expressions "first aspect," "second aspect," "third aspect," and "fourth aspect" in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly specifying the importance or quantity of the indicated technical features. They serve only as a non-exhaustive enumeration and do not constitute a closed limitation on quantity.

[0080] In this invention, the order in which the steps are written in the methods described in each embodiment does not imply a strict execution order. The actual execution order of each step should be determined based on its function and possible internal logic. Unless otherwise specified, all steps of this invention can be executed in the order they are written, or in any order without technical conflict. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) executed sequentially, or it may include steps (b) and (a) executed sequentially. If the method also includes step (c), then step (c) can be added to the method in any order without conflict, including but not limited to the execution order of steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), etc.

[0081] Example 1 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material, the method comprising the following steps: (1) Place FeSO4, NaH2PO4, sodium pyrophosphate, sucrose and citric acid in a vacuum drying oven and dry at 65℃ and vacuum degree -0.07MPa for 10h. Dry FeC2O4·2H2O at 58℃ and vacuum degree -0.07MPa for 10h and set aside.

[0082] (2) Weigh out FeSO4, FeC2O4·2H2O, NaH2PO4 and sodium pyrophosphate according to n(FeSO4) / n(FeC2O4·2H2O)=2.5:1, Fe, P molar ratio 0.75:1, Fe, Na molar ratio 0.75:1. The mass-volume ratio of total iron in FeSO4 and FeC2O4·2H2O to deionized water is 40g:1L. Add citric acid 1.3 times the total molar amount of FeSO4 and FeC2O4·2H2O, ethylene glycol 10wt% of the total mass of deionized water, and polyethylene glycol 0.8wt% of the total mass of deionized water. After mixing the above raw materials evenly, place them in an 80℃ constant temperature water bath and stir at 450rpm for 2.5h to form a transparent sol. Then slowly add sucrose 12wt% of the target amount of sodium ferric pyrophosphate and continue stirring for 35min to obtain the sol.

[0083] (3) The sol was dried at 95℃ for 4 hours (first drying) and then at 100℃ for 7 hours (second drying) to form a dry gel. After 20 minutes of grinding and sieving (100 mesh), the precursor powder was obtained. The precursor powder was spread on a quartz boat and placed in a tube furnace. After purging with high-purity nitrogen (flow rate 70 mL / min) for 25 minutes, the temperature was increased to 280℃ at 2.0℃ / min and held for 2 hours (first stage sintering), then increased to 480℃ at 4.0℃ / min and held for 3.5 hours (second stage sintering), and then increased to 630℃ at 5.5℃ / min and held for 7 hours (third stage sintering). After naturally cooling to room temperature, the material was ground and sieved (200 mesh) twice for 45 minutes to obtain sodium iron pyrophosphate-carbon composite cathode material.

[0084] Example 2 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material, the method comprising the following steps: (1) Place FeCl2, NaH2PO4, sodium pyrophosphate, glucose and citric acid in a vacuum drying oven and dry for 12 h at 60 °C and vacuum degree -0.08 MPa. Dry FeC2O4·2H2O at 55 °C and vacuum degree -0.08 MPa for 12 h and set aside.

[0085] (2) Weigh FeCl2, FeC2O4·2H2O, NaH2PO4 and sodium pyrophosphate according to n(FeCl2) / n(FeC2O4·2H2O)=2.3:1, Fe, P molar ratio 0.72:1, Fe, Na molar ratio 0.73:1. The mass-volume ratio of total iron in FeCl2 and FeC2O4·2H2O to deionized water is 42g:1L. Add 1.2 times the total molar amount of FeCl2 and FeC2O4·2H2O of citric acid and 8wt% of the total mass of deionized water of ethylene glycol. After mixing the above raw materials evenly, place them in a 75℃ constant temperature water bath and stir at 400rpm for 3h to form a transparent sol. Then slowly add 10wt% of glucose based on the mass of the target sodium pyrophosphate and continue stirring for 30min to obtain the sol.

[0086] (3) The sol was dried at 90℃ for 4 hours (first drying) and then at 100℃ for 8 hours (second drying) to form a dry gel. After 25 minutes of grinding (with a small amount of anhydrous ethanol as a grinding aid) and sieving (100 mesh), the precursor powder was obtained. The precursor powder was spread on a quartz boat and placed in a tube furnace. After purging with a helium-nitrogen mixture (volume ratio 1:3, flow rate 60 mL / min) for 30 minutes, the temperature was increased to 250℃ at 1.5℃ / min and held for 2.5 hours (first stage sintering), then increased to 450℃ at 3.5℃ / min and held for 4 hours (second stage sintering), and then increased to 620℃ at 5℃ / min and held for 8 hours (third stage sintering). After naturally cooling to room temperature, the material was ground twice and sieved twice (200 mesh) for 40 minutes to obtain the sodium iron pyrophosphate-carbon composite cathode material.

[0087] Example 3 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material, the method comprising the following steps: (1) Place Fe(NO3)3·9H2O, NaH2PO4, sodium pyrophosphate, sucrose and citric acid in a vacuum drying oven and dry at 70℃ and vacuum degree -0.075MPa for 8h. Dry FeC2O4·2H2O at 60℃ and vacuum degree -0.075MPa for 8h and set aside.

[0088] (2) Weigh out Fe(NO3)3·9H2O, FeC2O4·2H2O, NaH2PO4 and sodium pyrophosphate according to n(Fe(NO3)3·9H2O) / n(FeC2O4·2H2O)=2.7:1, Fe, P molar ratio 0.78:1, Fe, Na molar ratio 0.72:1. The mass-volume ratio of total iron to deionized water in Fe(NO3)3·9H2O and FeC2O4·2H2O is 45g:1. Add 1.5 times the total molar amount of citric acid (Fe(NO3)3·9H2O and FeC2O4·2H2O), 12 wt% ethylene glycol (total mass of deionized water), and 1 wt% polyethylene glycol (total mass of deionized water). Mix the above raw materials evenly and place them in an 85°C constant temperature water bath. Stir at 500 rpm for 2 hours to form a transparent sol. Then slowly add 15 wt% glucose (total mass of the target sodium ferric pyrophosphate) and continue stirring for 40 minutes to obtain the sol.

[0089] (3) The sol was first dried at 90℃ for 4h (first drying) and then dried at 100℃ for 6h (second drying) to form a dry gel. After 30min of first grinding and first sieving (100 mesh), the precursor powder was obtained. The precursor powder was spread on a quartz boat and placed in a tube furnace. After purging with high-purity nitrogen (flow rate 90mL / min) for 20min, the temperature was first increased to 300℃ at 2.5℃ / min and held for 1.5h (first stage sintering), then increased to 500℃ at 4.5℃ / min and held for 3h (second stage sintering), and then increased to 650℃ at 6℃ / min and held for 6h (third stage sintering). After naturally cooling to room temperature, it was ground and sieved twice (200 mesh) for 50min and then vacuum dried (60℃, 2h) to obtain sodium iron pyrophosphate-carbon composite cathode material.

[0090] Example 4 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and Example 1 is that FeC2O4·2H2O is replaced with FePO4·2H2O, which is used together with FeSO4 as an iron source. After the replacement, the amount of phosphorus source added is adjusted adaptively to maintain the Fe:P molar ratio at 0.75:1. The amounts of other raw materials added are also adjusted adaptively according to the raw material addition relationship in Example 1. The rest of the preparation methods and parameters are consistent with those in Example 1.

[0091] Example 5 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and Example 1 is that FeC2O4·2H2O is replaced with an equal amount of Fe3O4, which is used together with FeSO4 as an iron source. After the replacement, the amount of other raw materials added is adaptively adjusted according to the relationship between the amount of raw materials added in Example 1. The rest of the preparation methods and parameters are consistent with those in Example 1.

[0092] Example 6 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and that of Example 1 is that the three-stage sintering is omitted. The material is first heated to 280°C at a rate of 2.0°C / min and held for 2 hours (first-stage sintering), then heated to 480°C at a rate of 4.0°C / min and held for 3.5 hours (second-stage sintering), followed by direct natural cooling. The remaining preparation methods and parameters are consistent with those of Example 1.

[0093] Example 7 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and that of Embodiment 1 is that the temperature of the tertiary junction is 610°C, while the rest of the preparation method and parameters remain the same as those of Embodiment 1.

[0094] Example 8 This embodiment provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and that of Embodiment 1 is that the temperature of the tertiary junction is 660°C, while the rest of the preparation method and parameters remain the same as those of Embodiment 1.

[0095] Comparative Example 1 This comparative example provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this preparation method and Example 1 is that only a soluble iron source is used, and FeSO4 and FeC2O4·2H2O are replaced with only FeSO4. After using only FeSO4, the amount of other raw materials added is adaptively adjusted according to the relationship of the amount of raw materials added in Example 1. The rest of the preparation methods and parameters are consistent with those in Example 1.

[0096] Comparative Example 2 This comparative example provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this preparation method and Example 1 is that only a sparingly soluble iron source is used, and FeSO4 and FeC2O4·2H2O are replaced with only FeC2O4·2H2O. After using only FeC2O4·2H2O, the amount of other raw materials added is adaptively adjusted according to the relationship of the amount of raw materials added in Example 1. The rest of the preparation methods and parameters are consistent with those in Example 1.

[0097] Comparative Example 3 This comparative example provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this method and Example 1 is that the Fe:P molar ratio is maintained at 0.75:1, the Fe:Na molar ratio is maintained at 0.75:1, the ratio of n(FeSO4) / n(FeC2O4·2H2O) is adjusted to 2.1:1, and the amount of other raw materials added is adaptively adjusted according to the raw material addition relationship in Example 1. The rest of the preparation methods and parameters are consistent with those in Example 1.

[0098] Comparative Example 4 This comparative example provides a method for preparing a sodium iron pyrophosphate-carbon composite cathode material. The difference between this preparation method and Example 1 is that the Fe:P molar ratio is maintained at 0.75:1, the Fe:Na molar ratio is maintained at 0.75:1, the n(FeSO4) / n(FeC2O4·2H2O) ratio is adjusted to 2.9:1, the amount of other raw materials added is adaptively adjusted according to the raw material addition relationship in Example 1, and the rest of the preparation methods and parameters are consistent with those in Example 1.

[0099] The median particle size D50 of the sodium iron pyrophosphate-carbon composite cathode materials provided in Examples 1-8 and Comparative Examples 1-4 was measured using a laser particle size analyzer, and the carbon coating thickness was measured using SEM. The specific test results are shown in Table 1.

[0100] Table 1 Application Example 1-8 and Comparative Application Example 1-4 The sodium iron pyrophosphate-carbon composite positive electrode material, acetylene black, and PVDF provided in Examples 1-8 and Comparative Examples 1-4 were mixed in a mass ratio of 92:3:5. NMP was added to prepare a slurry, which was then coated onto the surface of aluminum foil. After drying and rolling, a positive electrode sheet was obtained. Hard carbon, conductive carbon black, SBR, and CMC were mixed in a mass ratio of 95:2:2:1. Deionized water was added to prepare a slurry, which was then coated onto the surface of copper foil. After drying and rolling, a negative electrode sheet was obtained. 1.0 M NaPF6 + EC / DEC (volume ratio 1:1) + 5% FEC was used as the electrolyte, and a glass fiber separator was used to assemble a pouch cell. These correspond to Application Examples 1-8 and Comparative Application Examples 1-4, respectively.

[0101] The pouch cells provided in Application Examples 1-8 and Comparative Application Examples 1-4 were tested in a voltage range of 1.5V-4.2V. The reversible capacity at 0.1C at room temperature (25℃), the capacity retention rate after 2000 cycles at 5C at room temperature, the capacity retention rate at 0.1C at -40℃, and the capacity retention rate after 3500 cycles at 1C at -40℃ are shown in Table 2.

[0102] Table 2 The test results in Tables 1 and 2 show that: (1) As can be seen from Examples 1-5 and Application Examples 1-5, the present invention uses dual iron sources and carbon sources as raw materials, that is, soluble iron sources and insoluble iron sources are combined. Combined with the sol-gel method, the soluble iron source can quickly dissociate in the early stage of the reaction to form a large number of uniform crystal nuclei, thereby achieving nucleation. The insoluble iron source can slowly deposit and grow on the surface of the crystal nuclei, effectively inhibiting excessive coarsening and agglomeration of particles, thereby achieving the effects of optimizing particle size, controlling particle size distribution, and improving particle morphology. At the same time, the present invention utilizes the liquid phase reaction environment to achieve uniform mixing of raw materials at the molecular level, which can further precisely control particle size, particle size distribution and micromorphology, and achieve excellent structural stability and electrochemical performance of the obtained sodium iron pyrophosphate-carbon composite cathode material. Thus, the sodium-ion battery prepared using the cathode material provided by the present invention has the advantages of high energy density, long life and wide temperature range applicability.

[0103] Specifically, the median particle size D50 of the sodium iron pyrophosphate-carbon composite cathode material is 2.5 μm or less, the Span value is 0.78-0.87, the particle size distribution is narrow, and the uniformity is relatively high. When assembled into a soft-pack battery, the reversible capacity at 0.1C is 127.5 mAh·g. －1 -129.1mAh·g －1 After 2000 cycles at 5°C, the capacity retention rate is 93.5%-94.5%; at -40°C and 0.1°C, the capacity retention rate is 75.3%-77.2%; and at -40°C and 1°C, the capacity retention rate is 86.9%-89.1% after 3500 cycles.

[0104] (2) As can be seen from Examples 1 and 6-8, and Application Examples 1 and 6-8, the sintering in this invention includes a first sintering at 250℃-300℃, a second sintering at 450℃-500℃, and a third sintering at 620℃-650℃. This invention first pre-decomposes at a low temperature of 250℃-300℃; excessively high or low temperatures will cause the gel to crack and the structure to collapse. Then, an amorphous crystal segment is formed at 450℃-500℃; if the temperature is too low, effective crystal nuclei cannot be formed, resulting in a material with many fixed shapes, low crystallinity, low capacity, and poor cycling. If the temperature is too high, the crystal nuclei will grow prematurely and too quickly, resulting in coarse grains and difficulty in controlling the particle size later. Finally, the third sintering at 620℃-650℃ is the crystal growth phase formation segment, ensuring the compactness and stability of the material. Therefore, the setting of the three sintering stages and the control of the sintering holding temperature at 620℃-650℃ ensure the complete phase of the material and its superior electrochemical performance.

[0105] (3) As can be seen from Example 1 and Comparative Examples 1-2, Application Example 1 and Comparative Application Example 1-2, if only one iron source is used in this invention, that is, only a soluble iron source or a sparingly soluble iron source is used, it is impossible to optimize the particle size and control the particle size distribution of the sodium iron pyrophosphate-carbon composite cathode material. The particle size distribution is relatively wide, and the cathode material prepared is used in soft-pack batteries, resulting in a significant reduction in the overall electrochemical performance.

[0106] (4) As can be seen from Example 1 and Comparative Examples 3-4, Application Example 1 and Comparative Application Examples 3-4, the present invention regulates the molar ratio of soluble iron source to insoluble iron source to be (2.3-2.7):1, which can accurately balance the nucleus generation rate and crystal growth rate, achieve optimal control of particle size, particle size distribution and particle morphology, and ensure that the material has small particle size, narrow distribution, low agglomeration and high structural stability, thereby significantly improving electrochemical performance. If the molar ratio of the two is too high, that is, the content of soluble iron source is too high, it will lead to excessive nucleus generation and excessively fast growth, resulting in excessive coarsening of particles, severe agglomeration, and widening of particle size distribution. At the same time, it will destroy the uniform coating of carbon layer and reduce the electronic conductivity and ion diffusion efficiency of the material. If the molar ratio of the two is too low, that is, the content of insoluble iron source is too high, it will lead to insufficient nucleus generation, slow and uneven crystal growth, irregular particle morphology and poor dispersion, which will also result in wide particle size distribution and low compaction density, deteriorating the rate performance and cycle stability of the battery.

[0107] In summary, this invention uses dual iron and carbon sources as raw materials, combining soluble and insoluble iron sources. By employing the sol-gel method, the soluble iron source rapidly dissociates in the early stages of the reaction to form numerous uniform crystal nuclei, achieving nucleation. The insoluble iron source slowly deposits and gently grows on the surface of the crystal nuclei, effectively inhibiting excessive particle coarsening and agglomeration. This optimizes particle size, regulates particle size distribution, and improves particle morphology. Simultaneously, this invention utilizes a liquid-phase reaction environment to achieve uniform mixing of raw materials at the molecular level, allowing for precise control of particle size, particle size distribution, and microstructure. This results in excellent structural stability and electrochemical performance of the obtained sodium iron pyrophosphate-carbon composite cathode material. Consequently, sodium-ion batteries prepared using the cathode material provided by this invention possess advantages such as high energy density, long lifespan, and wide temperature range applicability.

[0108] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a sodium iron pyrophosphate-carbon composite cathode material, characterized in that, The preparation method includes the following steps: A soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a carbon source, a complexing agent, a dispersant, and a solvent are mixed evenly and reacted to obtain a sol. The sol is dried to obtain a dry gel, and then sintered to obtain a sodium iron pyrophosphate-carbon composite cathode material. The molar ratio of the soluble iron source to the insoluble iron source is (2.3-2.7):

1.

2. The preparation method according to claim 1, characterized in that, The soluble iron source includes at least one of ferrous sulfate, ferric sulfate, ferric nitrate, ferrous nitrate, ferric chloride, ferric citrate, or ferrous chloride. Preferably, the insoluble iron source includes at least one of ferrous oxalate, ferric phosphate, ferrous phosphate, ferric hydroxide, or iron(II,III) oxide.

3. The preparation method according to claim 1 or 2, characterized in that, The reaction includes first mixing a soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a complexing agent, a dispersant, and a solvent evenly to carry out a first reaction, and then adding a carbon source to carry out a second reaction; Preferably, the mixed raw materials further include a dispersing agent; Preferably, the reaction includes first mixing a soluble iron source, a sparingly soluble iron source, a sodium source, a phosphorus source, a complexing agent, a dispersant, the dispersing aid, and a solvent evenly to carry out a first reaction, and then adding a carbon source to carry out a second reaction.

4. The preparation method according to claim 3, characterized in that, In the soluble iron source, the sparingly soluble iron source, the sodium source, and the phosphorus source, the molar ratio of iron to phosphorus is (0.72-0.78):

1. Preferably, in the soluble iron source, the sparingly soluble iron source, the sodium source and the phosphorus source, the molar ratio of iron to sodium is (0.72-0.75):1; Preferably, the total iron content in the soluble iron source and the sparingly soluble iron source, and the mass-to-volume ratio of the solvent, are (40g-45g):1L; Preferably, the molar amount of the complexing agent is 1 to 2 times the total molar amount of the soluble iron source and the insoluble iron source; Preferably, the amount of the dispersant added is 8wt%-12wt% of the mass of the solvent; Preferably, the amount of carbon source added is 10wt%-15wt% of the target mass of sodium iron pyrophosphate. Preferably, the amount of the dispersing agent added is 0.5wt%-1wt% of the mass of the solvent.

5. The preparation method according to any one of claims 1-4, characterized in that, The reaction temperature is 75℃-85℃; Preferably, the stirring speed of the reaction is 300 rpm to 600 rpm; Preferably, the drying process includes a primary drying and a secondary drying performed sequentially. Preferably, the temperature of the secondary drying is higher than the temperature of the primary drying; Preferably, the temperature of the first drying step is 85℃-95℃, and the drying time is 2h-5h; Preferably, the temperature of the secondary drying is 95℃-105℃, and the time of the secondary drying is 5h-9h.

6. The preparation method according to any one of claims 1-5, characterized in that, The sintering includes sequentially performed first-stage sintering, second-stage sintering, and third-stage sintering; Preferably, the holding temperature for the first sintering stage is 250℃-300℃; Preferably, the holding time for the first sintering stage is 1.5h-2.5h; Preferably, the holding temperature for the two-stage sintering is 450℃-500℃; Preferably, the holding time for the two-stage sintering is 3-6 hours. Preferably, the holding temperature for the three-stage sintering is 620℃-650℃; Preferably, the holding time for the three-stage sintering is 6h-9h.

7. The preparation method according to any one of claims 1-6, characterized in that, Prior to sintering, the sintering environment is purged with inert gas. Preferably, the preparation method further includes vacuum drying the solid raw materials before the mixture is homogenized.

8. A sodium iron pyrophosphate-carbon composite cathode material, characterized in that, The sodium iron pyrophosphate-carbon composite cathode material is prepared according to any one of claims 1-7, wherein the sodium iron pyrophosphate-carbon composite cathode material comprises sodium iron pyrophosphate and a carbon coating layer located on the surface of the sodium iron pyrophosphate.

9. The sodium iron pyrophosphate-carbon composite cathode material according to claim 8, characterized in that, The median particle size D50 of the sodium iron pyrophosphate-carbon composite cathode material is 1μm-5μm, preferably 1μm-2.5μm; Preferably, the thickness of the carbon coating layer is 1nm-5nm.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes a sodium iron pyrophosphate-carbon composite cathode material prepared by the preparation method according to any one of claims 1-7, or a sodium iron pyrophosphate-carbon composite cathode material according to claim 8 or 9.

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