Preparation method of sodium iron phosphate pyrophosphate positive electrode material, positive electrode material and electrochemical device

Abstract

The application provides a preparation method of a sodium iron phosphate pyrophosphate positive electrode material, a positive electrode material and an electrochemical device. The preparation method comprises the following steps: mixing a sodium source, an iron source, a phosphorus source and a first carbon source, performing one-time grinding and drying, and then performing one-time sintering to obtain a sodium iron phosphate pyrophosphate precursor; dispersing the sodium iron phosphate pyrophosphate precursor, the first carbon source and an organic acid in a solvent, performing two-time grinding, and then drying to obtain a powder, wherein the organic acid comprises at least one of glyoxylic acid, phthalic acid, maleic acid, malic acid, oxalic acid, citric acid, acetic acid, squaric acid and tartaric acid; and performing two-time sintering on the powder to obtain the sodium iron phosphate pyrophosphate positive electrode material. The sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method has the advantages of low surface residual alkali content, good air stability and excellent electrochemical performance.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to a method for preparing sodium iron pyrophosphate cathode material, the cathode material, and an electrochemical device. Background Technology

[0002] Sodium iron pyrophosphate (NFPP) has attracted much attention as a cathode material for sodium-ion batteries due to its stable three-dimensional framework structure, excellent safety performance, resistance to overcharge and over-discharge, good thermal stability, and small volume change during charge and discharge cycles.

[0003] However, NFPP also faces challenges in practical applications. One of the most prominent issues is its poor air stability. NFPP readily reacts with moisture, carbon dioxide, and oxygen in the air, forming sodium carbonate and other residual alkaline substances on the material surface. These byproducts not only trigger defluorination and degradation of the polyvinylidene fluoride (PVDF) binder during slurry preparation, leading to slurry gelation, but also cause the loss of active sodium, damaging the structural integrity of the material. Furthermore, during charge-discharge cycles, residual alkali interacts with the electrolyte, forming a thicker positive electrolyte interphase (CEI) layer, accelerating electrolyte decomposition, increasing interfacial resistance, and thus reducing the material's electrochemical performance. Summary of the Invention

[0004] In view of this, in order to solve at least one of the above technical problems, this application provides a method for preparing sodium iron pyrophosphate cathode material.

[0005] In addition, this application also provides a cathode material prepared by the aforementioned preparation method and an electrochemical device using the cathode material.

[0006] In a first aspect, embodiments of this application provide a method for preparing a sodium iron pyrophosphate cathode material, comprising: mixing a sodium source, an iron source, a phosphorus source, and a first carbon source, followed by a first grinding and drying, and then a first sintering to obtain a sodium iron pyrophosphate precursor; dispersing the sodium iron pyrophosphate precursor, a second carbon source, and an organic acid in a solvent, followed by a second grinding and drying to obtain a powder, wherein the organic acid includes at least one of glyoxylic acid, phthalic acid, maleic acid, malic acid, oxalic acid, citric acid, acetic acid, squaric acid, and tartaric acid; and performing a second sintering of the powder to obtain the sodium iron pyrophosphate cathode material.

[0007] Based on the first aspect, in some embodiments of this application, the organic acid accounts for 1.0% to 6.0% of the mass of the sodium iron pyrophosphate precursor; and / or, the organic acid includes at least one of oxalic acid, citric acid, and maleic acid.

[0008] Based on the first aspect, in some embodiments of this application, the organic acid includes oxalic acid.

[0009] Based on the first aspect, in some embodiments of this application, the temperature of the first sintering is 300℃~500℃ and the time is 5h~10h; and / or, the temperature of the second sintering is 450℃~600℃ and the time is 5h~10h.

[0010] Based on the first aspect, in some embodiments of this application, the D50 of the powder is 0.2 μm to 0.8 μm.

[0011] Based on the first aspect, in some embodiments of this application, the molar ratio of the sodium source, iron source, and phosphorus source is 4:2.7 to 3.3:4; and / or, the first carbon source includes at least one of glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin, and the first carbon source accounts for 6% to 12% of the mass of the sodium iron pyrophosphate precursor; and / or the second carbon source includes at least one of glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin, and the second carbon source accounts for 6% to 12% of the mass of the sodium iron pyrophosphate precursor.

[0012] Based on the first aspect, in some embodiments of this application, the sodium iron pyrophosphate precursor further includes a doping element, which includes at least one of titanium, magnesium, zirconium, niobium and molybdenum.

[0013] Based on the first aspect, in some embodiments of this application, the mass of the dopant element accounts for 0.05% to 0.45% of the mass of the sodium iron pyrophosphate cathode material.

[0014] Based on the first aspect, in some embodiments of this application, the heating rate of the first sintering is 2℃ / min to 5℃ / min; and / or, the drying is carried out by spray drying, with an inlet air temperature of 180℃ to 240℃ and an outlet air temperature of 85℃ to 110℃.

[0015] Secondly, embodiments of this application provide a sodium iron pyrophosphate cathode material, which is prepared by the aforementioned method for preparing sodium iron pyrophosphate cathode materials.

[0016] Thirdly, embodiments of this application provide an electrochemical device, including a positive electrode, a negative electrode, and an electrolyte located between the positive electrode and the negative electrode, wherein the positive electrode is made of the aforementioned sodium iron pyrophosphate positive electrode material.

[0017] In the preparation method of sodium iron pyrophosphate cathode material provided in this application, an organic acid with moderate to strong acidity, strong reducing ability, and metal ion complexing ability is selected. When the sodium iron pyrophosphate precursor is mixed and ground with a carbon source, the organic acid can neutralize the unreacted sodium source remaining on the surface of the precursor particles, converting the surface residual alkali into the corresponding organic acid salt; at the same time, the reducing and complexing ability of the organic acid can reduce the incompletely reduced ferric iron to ferrous iron, and form a complex with the free ferrous iron, thereby making the element distribution in the slurry more uniform and avoiding local element enrichment during the drying process.

[0018] Based on this, the second carbon source on the surface of the precursor particles undergoes graphitization transformation under secondary sintering conditions, forming a continuous, uniform, dense carbon coating layer with a high degree of graphitization. This carbon coating layer effectively blocks direct contact between moisture and carbon dioxide in the air and the surface of the material particles, thereby further improving the air stability of the sodium iron pyrophosphate cathode material and mitigating side reactions during storage and slurry preparation. Furthermore, this carbon coating layer provides a good electronic conductivity network for the material particles, improving the conductivity of the sodium iron pyrophosphate cathode material. Simultaneously, because the carbon coating layer is continuous, uniform, and dense, it effectively reduces the exposure of residual alkali on the surface of the material particles, thus reducing the risk of defluorination reaction between residual alkali and the binder during slurry preparation, preventing slurry gelation, improving slurry flowability and coating uniformity, and enabling the sodium iron pyrophosphate cathode material to achieve excellent processing and electrochemical performance.

[0019] Furthermore, the preparation method of this application, which combines two sintering processes and the addition of the aforementioned organic acid during the second grinding, can effectively reduce the residual alkali content on the surface of the sodium iron pyrophosphate cathode material, improve the air stability of the material, and thus improve the processing performance of the material in the slurry preparation process. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the preparation method of sodium iron pyrophosphate cathode material provided in the embodiments of this application.

[0021] Figure 2 This is a scanning electron microscope image of the sodium iron pyrophosphate cathode material prepared in Example 1 of this application.

[0022] Figure 3 This is a scanning electron microscope image of the sodium iron pyrophosphate cathode material prepared in Example 4 of this application.

[0023] Figure 4 The X-ray diffraction pattern of the sodium iron pyrophosphate cathode material prepared in Example 1 of this application and its exposure to air for 24 hours.

[0024] Figure 5The image shows the sodium iron pyrophosphate cathode material prepared for Comparative Example 1 of this application and its X-ray diffraction pattern after being placed in air for 24 hours.

[0025] Figure 6 This is a comparison of the X-ray diffraction patterns of sodium iron pyrophosphate cathode materials prepared in Example 1 and Comparative Example 3 of this application.

[0026] Figure 7 The sodium iron pyrophosphate cathode material prepared in Example 1 of this application and its 0.1C charge-discharge curve after being placed in air for 24 hours.

[0027] Figure 8 The image shows the sodium iron pyrophosphate cathode material prepared for Comparative Example 1 of this application and its 0.1C charge-discharge curve after being placed in air for 24 hours. Detailed Implementation

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of this application pertain. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of this application. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturer of reagents or instruments is not specified, they are all conventional products that can be purchased commercially. The terms "first" and "second" appearing in this application are for descriptive purposes only, used to distinguish different components or process steps, and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. For example, both the first carbon source and the second carbon source may be selected from at least one of glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin, and may be the same or different.

[0029] The following describes some embodiments of this application in detail. Unless otherwise specified, the embodiments and features described below can be combined with each other.

[0030] For this purpose, please refer to Figure 1 As shown in the embodiments of this application, a method for preparing sodium iron pyrophosphate cathode material is provided, comprising: mixing a sodium source, an iron source, a phosphorus source and a first carbon source, grinding and drying them once, and then sintering them once to obtain a sodium iron pyrophosphate precursor; dispersing the sodium iron pyrophosphate precursor, a second carbon source and an organic acid in a solvent, grinding and drying them a second time to obtain a powder, wherein the organic acid includes at least one of glyoxylic acid, phthalic acid, maleic acid, malic acid, oxalic acid, citric acid, acetic acid, squaric acid and tartaric acid; and sintering the powder a second time to obtain the sodium iron pyrophosphate cathode material.

[0031] Specifically, this application provides a method for preparing a sodium iron pyrophosphate cathode material, comprising: Step S1: Provide sodium iron pyrophosphate precursor.

[0032] The preparation method of the sodium iron pyrophosphate precursor includes: mixing sodium source, iron source, phosphorus source and first carbon source, grinding once, drying and then sintering once to obtain sodium iron pyrophosphate precursor.

[0033] This step of preparing the precursor can effectively remove most of the volatile substances in the raw materials and initially reduce ferric iron to ferrous iron, thereby forming a precursor with a preliminary crystalline structure. This precursor has high crystallinity and low impurity content, providing a good structural basis for achieving complete crystal transformation and forming a uniform and dense carbon coating layer during subsequent sintering.

[0034] In some embodiments, when preparing the sodium ferric pyrophosphate precursor, the molar ratio of sodium source, iron source, and phosphorus source is 4:2.7 to 3.3:4. Controlling the molar ratio of sodium source, iron source, and phosphorus source within this range facilitates sufficient matching of the elements during the reaction process, reduces unreacted sodium source residue, thereby lowering the residual alkali content on the precursor surface and suppressing the formation of impurity phases. Exemplary molar ratios may be 4:2.7:4, 4:2.8:4, 4:2.9:4, 4:3.0:4, 4:3.1:4, 4:3.2:4, 4:3.3:4, or any value within the range of any two of the above values.

[0035] In some embodiments, the general formula of the sodium iron pyrophosphate precursor is Na4Fe3(PO4)2P2O7.

[0036] In some embodiments, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium oxalate, sodium citrate, monosodium glutamate, and sodium dihydrogen phosphate and its related hydrates, disodium hydrogen phosphate and its related hydrates, sodium pyrophosphate, and disodium dihydrogen pyrophosphate; the iron source is selected from at least one of ferric phosphate, iron oxide, iron tetroxide, ferric hydroxide, and ferrous oxalate dihydrate; the phosphorus source is selected from at least one of ferric phosphate, phosphoric acid, sodium dihydrogen phosphate and its related hydrates, disodium hydrogen phosphate and its related hydrates, sodium pyrophosphate, disodium dihydrogen pyrophosphate, monoammonium phosphate, and ammonium dihydrogen phosphate. The above raw materials are widely available, cost-controllable, and exhibit good reactivity during sintering, which is beneficial for the formation of the target product.

[0037] In some embodiments, the first carbon source includes at least one selected from glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin. The first carbon source provides a reducing atmosphere during pyrolysis, effectively promoting the reduction of ferric iron and forming a preliminary carbon coating layer on the precursor surface, laying the foundation for obtaining a complete carbon-coated structure.

[0038] In some embodiments, the first carbon source accounts for 4% to 8% of the iron source mass. By controlling the amount of the first carbon source within the above range, it is beneficial to ensure sufficient reduction of ferric iron while avoiding excessive residual carbon or particle agglomeration in the precursor due to excessive carbon source. The mass ratio of the first carbon source to the iron source can be 4%, 5%, 6%, 7%, and 8%, or any value within the range of any two of the above values.

[0039] In some embodiments, the temperature for the first sintering is 300°C to 500°C, and the time is 5 hours to 10 hours. Controlling the temperature and time of the first sintering within these ranges facilitates the complete removal of volatile substances and promotes the initial crystallization of the precursor, while also helping to suppress excessive particle growth and impurity phase formation, thus maintaining the precursor in a good crystalline state. The exemplary first sintering temperature can be 300°C, 350°C, 400°C, 450°C, 500°C, or any value within the range of any two of the above values; the exemplary first sintering time can be 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any value within the range of any two of the above values.

[0040] In some embodiments, the heating rate of the first sintering is 2°C / min to 5°C / min. Controlling the heating rate of the first sintering within this range facilitates uniform heating of the precursor and avoids damage to the particle structure due to excessively rapid heating or excessive volatile emission. Exemplary heating rates for the first sintering can be 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min, or any value within the range of any two of the above values.

[0041] In some embodiments, when preparing the sodium iron pyrophosphate precursor, spray drying is employed, with an inlet air temperature of 180°C to 240°C and an outlet air temperature of 85°C to 110°C. Spray drying allows for rapid drying of the slurry into spherical or near-spherical particles, which is beneficial for improving particle uniformity and flowability during subsequent sintering. The inlet air temperature for spray drying can, for example, be 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, or any value within the range of any two of the above values; the outlet air temperature can, for example, be 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, or any value within the range of any two of the above values.

[0042] In some embodiments, the sodium iron pyrophosphate precursor further includes a dopant element, which includes at least one of titanium, magnesium, zirconium, niobium, and molybdenum. The dopant element can be derived from corresponding compounds, such as titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, niobium pentoxide, or molybdenum trioxide, or any one or more of these. In preparing the sodium iron pyrophosphate precursor, the dopant compound is mixed with a sodium source, an iron source, a phosphorus source, and a first carbon source. After one grinding, drying, and sintering process, the dopant element is incorporated into the precursor's crystal lattice structure. Introducing the dopant element can alter the material's crystal lattice structure, improve its conductivity and structural stability, thereby improving its rate performance and cycle life.

[0043] In some embodiments, the mass percentage of the dopant element is 0.05% to 0.45% of the mass of the sodium iron pyrophosphate cathode material. By controlling the doping amount within the above range, the dopant element can uniformly substitute within the precursor lattice. Since the introduction of the dopant element alters the electronic structure within the lattice, it enhances the electronic conductivity of the sodium iron pyrophosphate cathode material, enabling it to maintain a high capacity during high-current charge-discharge cycles, thereby improving the rate performance. Simultaneously, the uniform distribution of the dopant element in the lattice enhances the stability of the crystal structure, effectively suppressing volume changes in the sodium iron pyrophosphate cathode material during charge-discharge processes. This reduces particle cracking and structural damage caused by volume expansion and contraction, effectively suppressing capacity decay during long-term cycling, and extending the cycle life of the material. The exemplary mass percentage of the dopant element can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, or any value within the range of any two of the above values.

[0044] Step S2: Disperse the sodium iron pyrophosphate precursor, the second carbon source, and the organic acid in a solvent to obtain a slurry. Grind the slurry twice and dry it to obtain a powder. Then, sinter the powder twice to obtain the sodium iron pyrophosphate cathode material. The organic acid includes at least one of glyoxylic acid, phthalic acid, maleic acid, malic acid, oxalic acid, citric acid, acetic acid, squaric acid, and tartaric acid. In the preparation method of sodium iron pyrophosphate cathode material provided in this application, the selected organic acid is a moderately strong acid, containing only C, H and O elements, and has strong reducing power and metal ion complexing ability. When this type of organic acid is added during the mixing of sodium iron pyrophosphate precursor and carbon source, the organic acid can neutralize the unreacted sodium source remaining on the surface of precursor particles during the grinding process, converting the surface residual alkali into the corresponding organic acid salt. At the same time, by utilizing the reducing power and metal ion complexing ability of the organic acid, the organic acid can neutralize the unreacted sodium source remaining on the surface of precursor particles during the grinding process, reducing the incompletely reduced ferric iron to ferrous iron and forming a complex with the free ferrous iron, thereby making the element distribution in the slurry more uniform and avoiding local element enrichment during the drying process.

[0045] Furthermore, the preparation method of this application, which combines two sintering processes and the addition of the aforementioned organic acid during the second grinding, can effectively reduce the residual alkali content on the surface of the sodium iron pyrophosphate cathode material, improve the air stability of the material, and thus improve the processing performance of the material during slurry preparation. Based on this, the second carbon source on the surface of the precursor particles undergoes graphitization transformation under secondary sintering conditions, forming a continuous, uniform, dense carbon coating layer with a high degree of graphitization. This carbon coating effectively blocks direct contact between airborne moisture and carbon dioxide and the surface of the material particles, thereby further improving the air stability of the sodium iron pyrophosphate cathode material and mitigating side reactions during storage and slurry preparation. Furthermore, the carbon coating provides a good electronic conductivity network for the material particles, enhancing the conductivity of the sodium iron pyrophosphate cathode material. Simultaneously, the continuous, uniform, and dense carbon coating effectively reduces the exposure of residual alkali on the particle surface, thus lowering the risk of defluorination reaction between residual alkali and the binder during slurry preparation, preventing slurry gelation, improving slurry flowability and coating uniformity, and enabling the sodium iron pyrophosphate cathode material to achieve excellent processing and electrochemical performance. The solvent includes water, ethanol, or mixtures thereof.

[0046] In some embodiments, the second carbon source includes at least one selected from glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin. During sintering, the carbon source can further coat the particle surface, forming a conductive network and improving the electronic conductivity of the sodium iron pyrophosphate cathode material. The second carbon source accounts for 6% to 12% of the mass of the sodium iron pyrophosphate precursor. By controlling the amount of the second carbon source within this range, a uniform and continuous carbon coating layer can be formed on the particle surface, effectively improving the conductivity of the material while maintaining an appropriate thickness and density of the carbon layer. The proportion of the second carbon source to the precursor mass can, for example, be 6%, 7%, 8%, 9%, 10%, 11%, 12%, or any value within the range of any two of the above values.

[0047] In some embodiments, the organic acid includes at least one of oxalic acid, citric acid, and maleic acid. These organic acids contain only C, H, and O elements, thus preventing the introduction of other impurity elements during the secondary sintering process. Furthermore, their pKa is greater than 0, indicating moderate acidity. This allows them to gently react with the residual sodium source on the surface of the sodium iron pyrophosphate precursor during the secondary grinding process, converting the residual alkali into the corresponding organic acid salts. Simultaneously, the reducing and metal ion complexing abilities of these organic acids help to further reduce incompletely reduced ferric iron and form complexes with free ferrous iron, resulting in a more uniform elemental distribution in the slurry. This facilitates the formation of a continuous, uniform, and dense carbon coating layer during subsequent secondary sintering, thereby improving the conductivity and air stability of the resulting sodium iron pyrophosphate cathode material.

[0048] In some embodiments, the organic acid includes oxalic acid. Oxalic acid exhibits superior acidity, reducing power, and metal ion complexing ability, enabling it to react more effectively with unreacted sodium sources remaining on the precursor surface, converting residual alkali on the surface into sodium oxalate. Simultaneously, it reduces incompletely reduced ferric iron to ferrous iron and forms a stable complex with free ferrous iron, resulting in a more uniform elemental distribution in the slurry. This leads to the formation of a continuous, uniform, dense, and more graphitized carbon coating layer during subsequent sintering, further reducing the residual alkali content on the surface of the sodium iron pyrophosphate cathode material, improving the material's conductivity and air stability, and obtaining a sodium iron pyrophosphate cathode material with superior electrochemical performance.

[0049] In some embodiments, the organic acid accounts for 1.0% to 6.0% of the mass of the sodium iron pyrophosphate phosphate precursor. Controlling the amount of organic acid within this range allows it to fully react with the residual sodium source on the precursor surface, effectively converting residual alkali into organic acid salts. Simultaneously, it exerts reducing and complexing effects, promoting the uniformity of elemental distribution in the slurry. This helps reduce the residual alkali content on the surface of the final sodium iron pyrophosphate phosphate cathode material, improves the continuity and density of the carbon coating layer, enhances the conductivity and air stability of the sodium iron pyrophosphate phosphate cathode material, and yields a sodium iron pyrophosphate phosphate cathode material with excellent electrochemical performance. The mass percentage of the organic acid can, for example, be 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, or any value within the range of any two of the above values.

[0050] In some embodiments, the organic acid accounts for 2.5%-4% of the mass of the sodium iron pyrophosphate precursor. This dosage achieves an optimal balance between reducing surface residual alkali, promoting uniform elemental distribution, and maintaining the integrity of the carbon layer.

[0051] In some embodiments, the particle size D50 of the powder is 0.2 μm to 0.8 μm. By controlling the particle size within this range, the powder can exhibit high reactivity during subsequent secondary sintering, which is beneficial for the complete sintering reaction. Simultaneously, it results in a more uniform particle size distribution of the final sodium iron pyrophosphate cathode material, thereby improving the compaction density and electrochemical performance of the sodium iron pyrophosphate cathode material. The particle size D50 of the powder can, for example, be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, or any value within the range of any two of the above values.

[0052] In some embodiments, the secondary sintering temperature is 450℃~600℃, and the time is 5h~10h. By controlling the secondary sintering temperature and time within the above range, the sodium iron pyrophosphate cathode material can complete its final crystallization, forming a complete crystal structure. Simultaneously, the graphitization degree of the carbon layer on the particle surface is improved, forming a continuous, uniform, dense, and firmly adhered carbon coating layer, thereby enhancing the conductivity and structural stability of the sodium iron pyrophosphate cathode material. Furthermore, controlling the secondary sintering temperature within the range of 500℃~550℃ helps reduce the formation of impurity phases, resulting in a higher purity sodium iron pyrophosphate cathode material. When the primary sintering temperature is 400℃ and the secondary sintering temperature is 520℃, the matching effect is optimal. At this temperature, oxalic acid effectively reduces the local enrichment of elements during the secondary grinding and spray drying process, improving the utilization efficiency of the sodium source during secondary sintering, thereby further reducing the residual alkali content on the material surface. The secondary sintering temperature can be, for example, 450℃, 480℃, 500℃, 520℃, 550℃, 580℃, 600℃ or any value within the range of any two of the above values; the secondary sintering time can be, for example, 5h, 6h, 7h, 8h, 9h, 10h or any value within the range of any two of the above values.

[0053] In some embodiments, the heating rate for secondary sintering is 2–5 °C / min. By controlling the heating rate within this range, the powder can be heated uniformly during the heating process, which is beneficial for the uniform formation of the carbon layer and ensures that the carbon coating layer has good quality and consistency, thereby improving the conductivity and electrochemical performance of the sodium iron pyrophosphate cathode material. The heating rate for secondary sintering can, for example, be 2 °C / min, 2.5 °C / min, 3 °C / min, 3.5 °C / min, 4 °C / min, 4.5 °C / min, 5 °C / min, or any value within the range of any two of the above values.

[0054] In the preparation of the powder, spray drying is employed, with an inlet air temperature of 180–240°C and an outlet air temperature of 85–110°C. Spray drying rapidly dries the slurry into spherical or near-spherical particles. This drying condition helps maintain the integrity of the particle morphology, preventing agglomeration and thus improving the powder's flowability and particle uniformity during subsequent sintering. The inlet air temperature for spray drying can, for example, be 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, or any value within the range of any two of these values; the outlet air temperature can, for example, be 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, or any value within the range of any two of these values.

[0055] Compared with the prior art, the preparation method of sodium iron pyrophosphate cathode material provided in this application has the following beneficial effects: 1. In the preparation process, this application adds an organic acid containing only C, H, and O elements during the grinding step to neutralize the residual alkali on the surface of the precursor particles and convert the residual alkali into organic acid salts, thereby reducing the surface residual alkali content. Simultaneously, the reducing properties of the organic acid are used to reduce incompletely reduced ferric iron to ferrous iron, and the iron element is evenly distributed through complexation, avoiding localized element enrichment during the drying process. This creates favorable conditions for the formation of a continuous, uniform, dense, and highly graphitized carbon coating layer during secondary sintering, resulting in a sodium iron pyrophosphate cathode material with low surface residual alkali content, good conductivity, and superior air stability.

[0056] 2. In this application, oxalic acid is added during the secondary grinding step. This neutralizes the residual alkali on the surface of the precursor particles, converting it into sodium oxalate, effectively reducing the residual alkali content on the precursor particle surface. Simultaneously, utilizing the reducing and complexing properties of oxalic acid, incompletely reduced ferric iron is reduced to ferrous iron and forms a stable complex with free ferrous iron, resulting in a uniform distribution of iron and preventing localized element enrichment during the drying process. These effects provide favorable conditions for the formation of a continuous, uniform, dense, and highly graphitized carbon coating layer during secondary sintering, resulting in a final sodium iron pyrophosphate cathode material with low surface residual alkali content, high conductivity, and good air stability.

[0057] 3. This application utilizes a two-calcination process to generate a more pure and stable NFPP phase. In the first calcination stage, sodium, iron, and phosphorus sources are mixed in stoichiometric ratios and heat-treated at high temperature under an inert atmosphere to complete the initial solid-phase reaction and form the initial target phase. Subsequently, a second grinding process breaks up the sintered agglomerates formed in the first calcination, ensuring thorough mixing of unreacted materials, intermediate phases, and the initial target phase. The second calcination stage further eliminates residual pores and volatile impurities, resulting in a denser, purer material with a more complete crystal structure. This two-calcination process effectively promotes the completeness of the reaction, resulting in a denser material structure, more complete grain growth, and fewer impurities, thereby achieving higher purity, better crystallinity, and a more stable phase structure.

[0058] 4. This application introduces doping elements during the preparation of the sodium iron pyrophosphate precursor, enabling the dopant elements to uniformly substitute within the precursor lattice, altering the electronic structure within the lattice, thereby enhancing the electronic conductivity of the sodium iron pyrophosphate cathode material and improving its rate performance. Simultaneously, the uniform distribution of the dopant elements within the lattice enhances the stability of the crystal structure, effectively suppressing volume changes during charge and discharge, reducing particle cracking and structural damage, and thus extending the material's cycle life. Furthermore, during the subsequent secondary grinding process, the complexation effect of the organic acid helps the dopant elements distribute more uniformly in the slurry, further improving its conductivity and structural stability.

[0059] Secondly, this application provides a sodium iron pyrophosphate cathode material, which is prepared by the aforementioned method for preparing sodium iron pyrophosphate cathode materials. This sodium iron pyrophosphate cathode material has a low surface residual alkali content and is not prone to side reactions with moisture and carbon dioxide when placed in air, thus exhibiting good air stability; it demonstrates excellent processing performance during slurry preparation and is not prone to gelation; simultaneously, it achieves high initial coulombic efficiency and reversible capacity, exhibiting excellent electrochemical performance.

[0060] Thirdly, this application provides an electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte located between the positive and negative electrodes, wherein the positive electrode is made of the aforementioned sodium iron pyrophosphate positive electrode material. This electrochemical device, due to the use of sodium iron pyrophosphate positive electrode material with low surface residual alkali content and good air stability, exhibits high energy density and good cycle stability.

[0061] The present application will be further described below with reference to specific embodiments and comparative examples.

[0062] Note: In the following examples and comparative examples, the amount of each raw material added is based on the theoretical calculation value. The mass ratio of each component in the sodium iron pyrophosphate precursor or sodium iron pyrophosphate cathode material is the theoretical value calculated based on the amount of material fed. The content of each component in the actual material may deviate slightly due to factors such as volatilization and reaction during the preparation process, but all are based on the theoretical calculation value.

[0063] The theoretical value of sodium iron pyrophosphate cathode material is calculated as follows: based on the molar amount of the main element in the feed, the theoretical molar amount of sodium iron pyrophosphate (NFPP) is converted, and then multiplied by the molar mass of sodium iron pyrophosphate to calculate the theoretical mass of sodium iron pyrophosphate cathode material.

[0064] Example 1 Step S1: Preparation of sodium iron pyrophosphate precursor: 7.13 kg sodium carbonate, 1 kg glucose, and 135 g titanium dioxide were dispersed in 61 kg water, and then 4.87 kg sodium dihydrogen phosphate was slowly added. The mixture was stirred continuously (using a pneumatic stirrer with an inlet pressure of 0.2 MPa) for 30 min. After the reaction was complete, 20 kg iron phosphate was added to obtain a preliminary mixture. The main element ratio was Na:Fe:P = 4:3:4.

[0065] The above mixture was subjected to secondary grinding using zirconium beads with a particle size of 0.6 mm, until the slurry particle size D50 was approximately 1 μm. The ground slurry was then spray-dried at an inlet air temperature of 230°C and an outlet air temperature of 90°C to obtain a dry powder. The dry powder was then sintered once under an inert gas atmosphere (specifically nitrogen) at a heating rate of 3.5°C / min and a sintering temperature of 400°C for 4 hours, followed by cooling to obtain a sodium iron pyrophosphate phosphate precursor with a carbon content of approximately 0.1%.

[0066] Step S2: Preparation of sodium iron pyrophosphate cathode material: 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 was dispersed together with 1.44 kg of glucose and 0.48 kg of oxalic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounted for 9.0% of the mass of the sodium iron pyrophosphate precursor, and oxalic acid accounted for 3.0% of the mass of the sodium iron pyrophosphate precursor.

[0067] Step S3: The above mixture is ground once using zirconium beads with a particle size of 0.3 mm until the slurry particle size D50 is approximately 0.5 μm. The ground slurry is then spray-dried at an inlet air temperature of 230℃ and an outlet air temperature of 90℃ to obtain a dry powder. The dry powder is then sintered a second time under inert gas protection at a heating rate of 3.5℃ / min and a sintering temperature of 520℃ for 6 hours. After cooling, a sodium iron pyrophosphate cathode material with a carbon content of approximately 1.8% is obtained.

[0068] Example 2 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.6 kg of glucose and 0.16 kg of oxalic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 10.0% of the mass of the sodium iron pyrophosphate precursor, and oxalic acid accounts for 1.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0069] Example 3 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.2 kg of glucose and 0.96 kg of oxalic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 7.5% of the mass of the sodium iron pyrophosphate precursor, and oxalic acid accounts for 6.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0070] Example 4 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.12 kg of glucose and 0.48 kg of citric acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 7.0% of the mass of the sodium iron pyrophosphate precursor, and citric acid accounts for 3.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0071] Example 5 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.6 kg of glucose and 0.48 kg of acetic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 10.0% of the mass of the sodium iron pyrophosphate precursor, and acetic acid accounts for 3.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0072] Example 6 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.64 kg of glucose and 0.08 kg of oxalic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 10.25% of the mass of the sodium iron pyrophosphate precursor, and oxalic acid accounts for 0.5% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0073] Example 7 The difference between this embodiment and Embodiment 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.04 kg of glucose and 1.28 kg of oxalic acid in 26.88 kg of water and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 6.5% of the mass of the sodium iron pyrophosphate precursor, and oxalic acid accounts for 8.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Embodiment 1.

[0074] Example 8 The difference between this embodiment and Embodiment 1 is that no doping element (titanium dioxide) is added in step S1. The remaining steps are the same as in Embodiment 1.

[0075] Comparative Example 1 The difference between this comparative example and Example 1 is that in step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.68 kg of glucose in 26.88 kg of water without adding any organic acid, and stirred for 10 min to ensure uniform dispersion. Glucose accounts for 10.5% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Example 1.

[0076] Comparative Example 2 The difference between this comparative example and Example 1 lies in step S1, where 7.13 kg of sodium carbonate, 1 kg of glucose, 135 g of titanium dioxide, and 0.48 kg of oxalic acid are dispersed in 61 kg of water, and then 4.87 kg of sodium dihydrogen phosphate is slowly added while stirring continuously for 30 minutes. After the reaction is complete, 20 kg of ferric phosphate is added to obtain a preliminary mixture. The main element ratio is Na:Fe:P = 4:3:4, the Ti element doping amount is 0.3% of the mass of the sodium iron pyrophosphate material, the glucose addition amount is 3% of the mass of the ferric phosphate, and the oxalic acid addition amount is 3.0% of the mass of the sodium iron pyrophosphate material.

[0077] In step S2, 16 kg of the sodium iron pyrophosphate precursor obtained in step S1 is dispersed together with 1.6 kg of glucose in 26.88 kg of water without adding any organic acid, and stirred for 10 min to ensure uniform dispersion. The glucose constitutes 10.0% of the mass of the sodium iron pyrophosphate precursor. The remaining steps are the same as in Example 1.

[0078] Comparative Example 3 This comparative example was prepared using a one-stage calcination process, with the following specific steps: 7.13 kg of sodium carbonate, 2.7 kg of glucose, 0.81 kg of oxalic acid, and 135 g of titanium dioxide were dispersed in water. Then, 4.87 kg of sodium dihydrogen phosphate was slowly added, and the mixture was stirred continuously for 30 minutes. After the reaction was completed, 20 kg of ferric phosphate was added to obtain a preliminary mixture. The main element ratio was Na:Fe:P = 4:3:4, the Ti element doping amount was 0.3% of the mass of the sodium iron phosphate pyrophosphate material, the glucose addition amount was 13.5% of the mass of the ferric phosphate, and the oxalic acid addition amount was 3.0% of the mass of the sodium iron phosphate pyrophosphate material.

[0079] The above mixture was ground using zirconium beads with a particle size of 0.3 mm until the slurry particle size D50 was approximately 0.5 μm. The ground slurry was then spray-dried at an inlet air temperature of 230°C and an outlet air temperature of 90°C to obtain a dried powder. The dried powder was sintered under inert gas protection at a heating rate of 3.5°C / min, first held at 400°C for 4 hours, then increased to 520°C at a rate of 3.5°C / min and held for 6 hours, followed by cooling to obtain a sodium iron pyrophosphate cathode material with a carbon content of approximately 1.8%.

[0080] Comparative Example 4 The difference between this comparative example and Comparative Example 3 is that 0.81 kg of oxalic acid was replaced with 0.81 kg of citric acid, and the amount of glucose added was adjusted to 2.0 kg (accounting for 12.5% ​​of the mass of ferric phosphate). The remaining steps are the same as those in Comparative Example 3.

[0081] Comparative Example 5 The difference between this comparative example and Comparative Example 3 is that 0.81 kg of oxalic acid was replaced with 0.81 kg of acetic acid, and the amount of glucose added was adjusted to 2.32 kg (accounting for 14.5% of the mass of ferric phosphate). The remaining steps are the same as those in Comparative Example 3.

[0082] Comparative Example 6 The difference between this comparative example and Example 1 is that in step S2, 0.48 kg of oxalic acid is replaced with 0.48 kg of phosphoric acid, and the amount of glucose added is adjusted to 1.68 kg (accounting for 10.5% of the mass of the sodium iron pyrophosphate precursor). The remaining steps are the same as in Example 1.

[0083] The following tests were performed on the sodium iron pyrophosphate cathode materials prepared in Examples 1-8 and Comparative Examples 1-6: Carbon content testing: The carbon content of the sodium iron pyrophosphate cathode material was tested using infrared analysis. The specific testing procedure is as follows: Using a carbon-sulfur analyzer, the sodium iron pyrophosphate cathode material sample was burned under high temperature and oxygen-rich conditions. The carbon elements in the sample were oxidized into carbon dioxide. The carbon dioxide entered the infrared detector with the carrier gas. By quantitatively analyzing the changes in the intensity of the infrared absorption wavelength of the carbon dioxide signal, the carbon content in the sample was calculated.

[0084] Test of residual alkali content on powder surface: This was performed using a Metrohm automatic potentiometric titrator. A certain mass of sodium iron pyrophosphate cathode material was weighed and added to 100 ml of pure water. The residual carbonate and hydroxide ions on the surface of the sodium iron pyrophosphate cathode material were fully dissolved by magnetic stirring, followed by filtration. A certain amount of the filtrate was aliquoted and titrated with hydrochloric acid standard solution using an automatic potentiometric titrator in an equivalence titration manner. The content of residual carbonate and hydroxide ions in the sodium iron pyrophosphate cathode material was calculated by observing the jump points on the titration curve and the volume of hydrochloric acid standard titration solution consumed. The results are expressed as Na+. + count.

[0085] Scanning electron microscopy (SEM) testing: The microstructure and structural features of the sodium iron pyrophosphate cathode material were observed using a Zeiss Sigma 360 scanning electron microscope at a magnification of 30Kx.

[0086] X-ray diffraction analysis: The phase composition of sodium iron pyrophosphate cathode material was analyzed using an X-ray diffractometer. The test step size was 0.02°, the scanning speed was 2° / min, and the scanning angle range was 5° to 60°.

[0087] Particle size testing: A Malvern 3000 laser particle size analyzer was used for testing. During testing, sodium iron pyrophosphate cathode material powder was added to ethanol and ultrasonically dispersed for 3 minutes before the test began. The particle size D50 value of the sodium iron pyrophosphate cathode material was recorded.

[0088] Electrochemical performance testing: Preparation of the positive electrode sheet: The prepared sodium iron pyrophosphate positive electrode material was sieved through a 200-mesh sieve. The material remaining on the sieve was vacuum dried at 120℃ for 12 hours before preparing the positive electrode slurry. The positive electrode slurry formula was as follows: sodium iron pyrophosphate positive electrode material 90% by mass, acetylene black as a conductive agent 5% by mass, and polyvinylidene fluoride as a binder 5% by mass. The above three materials were added together to N-methylpyrrolidone solvent according to the formula, and the slurry was mixed evenly by ball milling to obtain the positive electrode slurry. The positive electrode slurry was evenly coated onto an aluminum foil current collector, dried at 100℃, and then cut to obtain positive electrode sheets of appropriate size. The weight of the electrode sheets was weighed and the mass of the active material sodium iron pyrophosphate was calculated for subsequent specific capacity calculations.

[0089] Assembly of the coin cell: A sodium sheet is used as the negative electrode, a polypropylene microporous membrane as the separator, and 1 mol / L sodium perchlorate and a mixed carbonate-based organic solvent as the electrolyte. These components, along with gaskets, spring contacts, and positive and negative electrode shells, are assembled into a coin cell in a glove box. The mixed carbonate-based organic solvent is prepared by mixing ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate in a volume ratio of 5:3:2.

[0090] Button cell performance testing: Charge-discharge tests were conducted on LAND or Newwell testing systems, with a charging cutoff voltage of 4.0V and a discharging cutoff voltage of 2.0V. The nominal specific capacity was 129mAh / g. The 0.1C charge-discharge specific capacity and initial coulombic efficiency of the sodium iron pyrophosphate cathode materials prepared by each method were tested.

[0091] Air stability assessment: 1 kg of sodium iron pyrophosphate cathode material prepared by each method was evenly placed in a tray and left exposed to air for 24 h. Then, the residual alkali content on the surface and the 0.1C charge-discharge capacity were tested and compared with the test results before placement to assess the air stability of the material.

[0092] The test results of the sodium iron pyrophosphate cathode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 5 are shown in Table 1.

[0093] Table 1 Results Analysis: The difference between Comparative Example 1 and Example 1 is that no organic acid was added in step S2 of Comparative Example 1. Based on the test results in Table 1, it can be seen that the sodium iron pyrophosphate cathode material prepared in Comparative Example 1 has a high residual alkali content on its surface, and the alkali content further increases after being placed in air for 24 hours, resulting in a significant decrease in electrochemical performance. In contrast, the sodium iron pyrophosphate cathode material prepared in Example 1 has a significantly lower residual alkali content on its surface, and the increase in alkali content after being placed in air for 24 hours is smaller, maintaining good electrochemical performance. Figure 4 and Figure 5 The XRD patterns of the samples show that the sodium iron pyrophosphate cathode material prepared in Example 1 maintained its crystal structure after being exposed to air for 24 hours, while the sodium iron pyrophosphate cathode material prepared in Comparative Example 1 showed impurity phase peaks, indicating that it underwent side reactions with moisture and carbon dioxide in the air. Figure 7 and Figure 8 The comparison of the charge-discharge curves shows that the sodium iron pyrophosphate cathode material prepared in Example 1 maintains a relatively stable charge-discharge curve and exhibits minimal capacity decay after being exposed to air for 24 hours. In contrast, the sodium iron pyrophosphate cathode material prepared in Comparative Example 1 shows a significant change in its charge-discharge curve and a substantial capacity decay after being exposed to air for 24 hours. This demonstrates that adding oxalic acid during the initial grinding step can effectively reduce the residual alkali content on the material surface and improve its air stability.

[0094] The difference between Comparative Example 2 and Example 1 is that oxalic acid was added in step S1 when preparing the precursor in Comparative Example 2, while no organic acid was added in step S2. According to the test results in Table 1, the residual alkali content on the surface of the sodium iron pyrophosphate cathode material prepared in Comparative Example 2 was still relatively high, and the improvement in air stability was limited; while the residual alkali content on the surface of the sodium iron pyrophosphate cathode material prepared in Example 1 was significantly lower, and the air stability was superior. This indicates that adding organic acid in a single grinding step can more effectively neutralize the unreacted sodium source remaining on the precursor surface, while simultaneously exerting reduction and complexation effects, resulting in a more uniform elemental distribution and thus achieving better alkali reduction and carbon coating quality.

[0095] The difference between Comparative Example 3 and Example 1 is that Comparative Example 3 uses a one-time sintering process, i.e., all raw materials are mixed, ground, and sintered in one step. According to the test results in Table 1, the sodium iron pyrophosphate cathode material prepared in Comparative Example 3 has a higher residual alkali content on its surface and poorer electrochemical performance; while the sodium iron pyrophosphate cathode material prepared in Example 1 has a significantly lower residual alkali content on its surface and significantly improved electrochemical performance. Figure 6 The XRD patterns of Comparative Example 3 show a distinct characteristic peak of sodium iron phosphate with impurities around 34.3°, while Example 1 does not exhibit this impurity peak, indicating a more complete crystal structure. This demonstrates that the two-stage sintering process can effectively improve the phase purity of the material, resulting in a sodium iron phosphate cathode material with higher crystallinity and fewer impurities.

[0096] The difference between Comparative Example 4 and Comparative Example 3 is that oxalic acid in Comparative Example 4 was replaced with citric acid; the difference between Comparative Example 5 and Comparative Example 3 is that oxalic acid in Comparative Example 3 was replaced with acetic acid. Based on the test results in Table 1, it can be seen that the residual alkali content on the surface of the sodium iron pyrophosphate cathode materials prepared in Comparative Examples 4 and 5 is still relatively high, and the improvement effect on electrochemical performance is limited.

[0097] The difference between Comparative Example 6 and Example 1 is that oxalic acid was replaced with phosphoric acid in step S2 of Comparative Example 6. According to the test results in Table 1, although the sodium iron pyrophosphate cathode material prepared in Comparative Example 6 had a low initial alkali content, its electrochemical performance was poor, and its performance deteriorated significantly after being placed in air for 24 hours. This is because phosphoric acid introduces additional phosphoric acid (P), disrupting the original elemental balance, leading to changes in the phase formation process and a decrease in product consistency and structural stability. Therefore, this application, by selecting a moderately strong organic acid containing only C, H, and O elements and possessing both reducing and complexing abilities, can neutralize residual alkali on the precursor surface while avoiding the introduction of other impurity elements. This improves the continuity and density of the carbon coating layer, prevents the formation of impurity phases and structural instability caused by elemental imbalance, and effectively suppresses the significant increase in alkali content and the degradation of electrochemical performance after the material is placed in air. The resulting sodium iron pyrophosphate cathode material possesses low residual alkali content, high air stability, and excellent electrochemical performance.

[0098] The difference between Examples 2 and 3 and Example 1 lies in the amount of oxalic acid used. In Example 1, the oxalic acid content was 3.0%; in Example 2, it was 1.0%; and in Example 3, it was 6.0%. Based on the test results in Table 1, it can be seen that when the oxalic acid content is controlled within the range of 1.0% to 6.0%, the resulting sodium iron pyrophosphate cathode materials all exhibit better overall performance. Specifically, when the oxalic acid content is 3.0%, the surface residual alkali content of the obtained material is relatively low, and the electrochemical performance is better; when the oxalic acid content is 1.0%, the alkali reduction effect is relatively weak; when the oxalic acid content is 6.0%, although the initial alkali content is low, the alkali content increases relatively quickly after being placed in air for 24 hours. This indicates that controlling the oxalic acid content within the range of 1.0% to 6.0% can achieve a good balance between reducing surface residual alkali, promoting uniform elemental distribution, and maintaining the integrity of the carbon layer.

[0099] The difference between Examples 4 and 5 and Example 1 lies in the type of organic acid used. Example 1 used oxalic acid, Example 4 used citric acid, and Example 5 used acetic acid. Based on the test results in Table 1, Example 1, using oxalic acid, is superior to Examples 4 (using citric acid) and 5 (using acetic acid) in terms of surface residual alkali content, electrochemical performance, and air stability. Figure 2 and Figure 3The SEM images show that the carbon coating layer on the surface of the sodium iron pyrophosphate cathode material particles prepared in Example 1 is continuous, uniform, dense, and complete, while the uniformity of the carbon coating layer on the surface of the sodium iron pyrophosphate cathode material particles prepared in Example 4 needs further improvement. This is because oxalic acid has appropriate acidity, good reducing power, and metal ion complexing ability, which can effectively neutralize the unreacted sodium source remaining on the surface of the sodium iron pyrophosphate precursor, converting the surface residual alkali into organic acid salts. At the same time, it further reduces the incompletely reduced ferric iron to ferrous iron and forms a stable complex with the free ferrous iron, making the element distribution in the slurry more uniform and creating favorable conditions for the subsequent formation of a continuous, dense, and highly graphitized carbon coating layer.

[0100] The difference between Examples 6 and 7 and Example 1 lies in the amount of oxalic acid used. In Example 1, the amount of oxalic acid was 3.0%; in Example 6, it was 0.5%; and in Example 7, it was 8.0%. Based on the test results in Table 1, it can be seen that when the amount of oxalic acid is controlled within the range of 1.0% to 6.0%, the resulting sodium iron pyrophosphate cathode material exhibits good overall performance, with the best effect observed at 3.0%. When the amount of oxalic acid is below 1.0%, the neutralization effect on residual alkali on the surface is relatively insufficient. When the amount of oxalic acid is above 6.0%, although the initial alkali content is low, the alkali content increases relatively significantly after being exposed to air. The main mechanism is that excessive organic acid corrodes the precursor material after primary sintering, forming a porous structure, significantly increasing the specific surface area of ​​the material, thereby enhancing its water absorption and promoting side reactions between the material surface and moisture and carbon dioxide in the air, leading to a significant decrease in its stability in air. Therefore, although a high dosage of oxalic acid can result in a lower residual alkali content on the surface of freshly prepared materials, the residual alkali content will significantly increase after being left in air. This demonstrates that controlling the oxalic acid dosage within the range of 1.0% to 6.0% can effectively neutralize surface residual alkali, promote uniform elemental distribution, and maintain the integrity and density of the carbon coating layer, thereby achieving optimal overall material performance.

[0101] The difference between Example 8 and Example 1 is that Example 1 added a dopant element (titanium dioxide) in step S1, while Example 8 did not add any dopant element in step S1. According to the test results in Table 1, the sodium iron pyrophosphate cathode material prepared in Example 8 still exhibits low surface residual alkali content and good electrochemical performance. Therefore, adding dopant elements during the preparation of the sodium iron pyrophosphate precursor is beneficial for further optimizing the material's crystal structure, improving the conductivity and cycle stability of the sodium iron pyrophosphate cathode material, and thus improving the rate performance and cycle life, resulting in a sodium iron pyrophosphate cathode material with superior overall performance.

[0102] In summary, this application, by adding an organic acid containing only C, H, and O elements with both reducing and complexing capabilities during the secondary grinding step, and combining it with a secondary calcination process, can effectively reduce the residual alkali content on the surface of sodium iron pyrophosphate cathode material, improve the quality of the carbon coating layer, thereby improving the conductivity and air stability of the material, and ultimately obtaining a sodium iron pyrophosphate cathode material with excellent electrochemical performance.

[0103] The above description describes some specific embodiments of this application, but in actual applications, the application should not be limited to these embodiments. For those skilled in the art, other modifications and alterations made based on the technical concept of this application should fall within the protection scope of this application.

Claims

1. A method for preparing a sodium iron pyrophosphate cathode material, characterized in that, include: Sodium source, iron source, phosphorus source and first carbon source are mixed, ground and dried once, and then sintered once to obtain sodium iron pyrophosphate precursor. The sodium ferric pyrophosphate precursor, the second carbon source, and the organic acid are dispersed in a solvent, and then subjected to secondary grinding and drying to obtain a powder. The organic acid includes at least one selected from glyoxylic acid, phthalic acid, maleic acid, malic acid, oxalic acid, citric acid, acetic acid, squaric acid, and tartaric acid. The powder is subjected to secondary sintering to obtain the sodium iron pyrophosphate cathode material.

2. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The organic acid comprises 1.0% to 6.0% of the mass of the sodium iron pyrophosphate precursor; and / or, The organic acid includes at least one of oxalic acid, citric acid, and maleic acid.

3. The method for preparing the sodium iron pyrophosphate cathode material according to claim 2, characterized in that, The organic acids include oxalic acid.

4. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The sintering temperature for the first sintering is 300℃~500℃, and the time is 5h~10h; and / or, The secondary sintering temperature is 450℃~600℃, and the time is 5h~10h.

5. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The D50 of the powder is 0.2μm to 0.8μm.

6. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The molar ratio of the sodium source, iron source, and phosphorus source is 4:2.7 to 3.3:4; and / or, The first carbon source includes at least one selected from glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin, and the first carbon source accounts for 6% to 12% of the mass of the sodium iron pyrophosphate precursor; and / or, The second carbon source includes at least one of glucose, sucrose, starch, citric acid, polyethylene glycol, and maltodextrin, and the second carbon source accounts for 6% to 12% of the mass of the sodium iron pyrophosphate precursor.

7. The method for preparing the sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The sodium iron pyrophosphate precursor also includes doping elements, which include at least one of titanium, magnesium, zirconium, niobium and molybdenum.

8. The method for preparing the sodium iron pyrophosphate cathode material according to claim 7, characterized in that, The mass of the dopant element accounts for 0.05% to 0.45% of the mass of the sodium iron pyrophosphate cathode material.

9. A sodium iron pyrophosphate cathode material, characterized in that, The sodium iron pyrophosphate cathode material is prepared by the method for preparing sodium iron pyrophosphate cathode material according to any one of claims 1 to 8.

10. An electrochemical device, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte located between the positive electrode and the negative electrode, wherein the positive electrode is made of sodium iron pyrophosphate positive electrode material as described in claim 9.

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