High-capacity ferric sodium pyrophosphate material as well as preparation method and application thereof

Abstract

The invention discloses a high-capacity ferric sodium pyrophosphate material and a preparation method and application thereof, and belongs to the technical field of battery materials, the high-capacity ferric sodium pyrophosphate material comprises the following raw materials: a phosphorus source, an iron source, a sodium source, a first carbon source, a second carbon source, a coating agent and a dispersing agent. According to the high-capacity ferric sodium phosphate pyrophosphate material as well as the preparation method and the application thereof, through a core-shell gradient carbon coating and low-temperature reducing atmosphere sintering synergistic process system, the problem that the high compaction density and the long cycle life of the ferric sodium phosphate pyrophosphate material cannot be considered at the same time is successfully solved; common raw materials are utilized to construct spherical particles with internal porous nanostructures and external carbon content gradient, and low-temperature controllable reduction sintering is combined, so that the intrinsic conductivity, the structural stability and the phase purity of the material are improved while the energy consumption is remarkably reduced; the prepared material has high specific capacity close to a theoretical value, excellent rate capability and ultra-long cycle life under an extreme compaction condition.

Description

Technical Field

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

[0002] As the global energy structure transitions towards cleaner and lower-carbon energy, large-scale energy storage technology has become crucial for supporting the development of renewable energy. Sodium-ion batteries, due to the abundance and wide distribution of sodium resources, their low cost, and their similar electrochemical principles to lithium-ion batteries, are considered one of the most promising candidate technologies for large-scale energy storage. Among numerous sodium-ion battery cathode materials, polyanionic compounds, especially sodium iron pyrophosphate pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) with a NASICON (sodium superionic conductor) crystal structure, have attracted widespread attention due to their stable three-dimensional framework structure, moderate operating voltage, high theoretical specific capacity, and environmentally friendly, safe, and non-toxic properties.

[0003] However, the industrial application of NFPP materials still faces a series of key technical challenges: First, its intrinsic electronic conductivity is low, which severely limits the full utilization of capacity and rate performance; second, the structural stability of the material is tested during cycling, especially under high compaction density, resulting in rapid capacity decay, i.e., there is a common industry problem that it is difficult to balance high compaction density and long cycle life; third, traditional preparation processes mostly adopt high-temperature solid-state methods (sintering temperature is usually higher than 700℃), which not only consumes a lot of energy, but also easily causes sodium volatilization, iron oxidation and the formation of impurity phases (such as NaFePO4, Fe2O3, etc.), which damages the phase purity and electrochemical performance of the material.

[0004] Carbon coating is a commonly used and effective method to improve conductivity. However, existing technologies often use a single carbon source (such as glucose or sucrose) for simple coating, resulting in a simple carbon layer structure that cannot simultaneously meet the multiple requirements of high electronic conductivity, strong interfacial bonding, and effective buffering of volume changes. While a uniform, thick carbon layer can improve conductivity, it hinders ion transport and reduces volumetric energy density; conversely, a thin or incomplete carbon layer offers limited protection. Furthermore, the coupling of traditional coating processes with high-temperature sintering processes may lead to excessive graphitization of the carbon layer at high temperatures or adverse reactions with active materials. Summary of the Invention

[0005] The purpose of this invention is to provide a high-capacity sodium iron pyrophosphate material, its preparation method, and its applications. By employing a synergistic process system of core-shell gradient carbon coating and low-temperature reducing atmosphere sintering, the problem of simultaneously achieving high compaction density and long cycle life in sodium iron pyrophosphate materials is successfully solved. Furthermore, spherical particles with an internal porous nanostructure and an external carbon content gradient are constructed using common raw materials. Combined with low-temperature controllable reduction sintering, energy consumption is significantly reduced while greatly improving the intrinsic conductivity, structural stability, and phase purity of the material. The prepared material exhibits high specific capacity close to theoretical values, excellent rate performance, and ultra-long cycle life under extreme compaction conditions.

[0006] To achieve the above objectives, the present invention provides a high-capacity sodium iron pyrophosphate material, comprising the following raw materials: a phosphorus source, an iron source, a sodium source, a first carbon source, a second carbon source, a coating agent, and a dispersant, wherein the atomic molar ratio of the phosphorus source, iron source, and sodium source is Na:Fe:P = 4:3:5, and the first carbon source, second carbon source, coating agent, and dispersant account for 2.5-5.5%, 1.0-2.4%, 0.4-2.1%, and 0.1-0.5% of the total mass of the phosphorus source, iron source, sodium source, first carbon source, second carbon source, coating agent, and dispersant, respectively.

[0007] Preferably, the sodium source is one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium sulfate; the phosphorus source is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; the iron source is one of ferrous sulfate and ferric nitrate; the first carbon source is one of citric acid and tartaric acid; the second carbon source is one of glucose and sucrose; the coating agent is one of L-ascorbic acid and maltodextrin; and the dispersant is one of polyethylene glycol and polyvinyl alcohol.

[0008] This invention also provides a method for preparing a high-capacity sodium iron pyrophosphate material, comprising the following steps: Step 1: Preparation of precursor slurry: Weigh sodium source, phosphorus source and iron source to prepare precursor slurry; Step 2, spray drying and granulation: Add the second carbon source to the precursor slurry, stir evenly to obtain a mixed slurry, and spray dry the mixed slurry to obtain spherical precursor powder; Step 3: Sinter the spherical precursor powder to obtain the sintered product; Step four: The sintered product is crushed and sieved to obtain high-capacity sodium iron pyrophosphate material.

[0009] Preferably, the specific operation of step one is as follows: S1. Dissolve the sodium source and phosphorus source in deionized water and stir until clear to obtain a sodium-phosphorus mixed solution; S2. Dissolve the iron source, the first carbon source, the coating agent and the dispersant in deionized water to obtain a mixed solution of the iron source; S3. Under nitrogen protection, the iron source mixed solution is added dropwise to the sodium-phosphorus mixed solution, the reaction pH is controlled, and the reaction is stirred to obtain the precursor slurry.

[0010] Preferably, in S1, the temperature of the deionized water is 60-70℃; In S3, the dropping rate of the iron source mixed solution is 0.5-2 mL / min, the reaction pH is 4.5-5.5, the reaction temperature is 65-75℃, and the reaction is stirred for 3-5 h.

[0011] Preferably, in step two, the parameters for spray drying are: using a pressure-type or centrifugal spray dryer, controlling the inlet temperature to 200-230℃ and the outlet temperature to 95-110℃.

[0012] Preferably, in step three, gradient sintering is used, which includes three stages: First stage: Increase the temperature from room temperature to 300-350℃ at a rate of 3-5℃ / min, and hold at this temperature for 0.5-1h under an argon atmosphere; Second stage: After holding at the temperature, increase the temperature to 570-590℃ at a rate of 2-3℃ / min, switch to argon-hydrogen mixture, and hold at the temperature for 2-4 hours; Third stage: After the heat preservation is completed, switch to argon gas and cool the furnace to room temperature.

[0013] Preferably, in the second stage, the hydrogen gas fraction in the argon-hydrogen mixture is 1-3%.

[0014] Preferably, in step four, the sieve opening is 300-400 mesh.

[0015] This invention also provides an application of a high-capacity sodium iron pyrophosphate material, which is used to prepare sodium-ion batteries.

[0016] Therefore, the present invention, by employing the above-mentioned high-capacity sodium iron pyrophosphate material, its preparation method, and its application, has the following beneficial effects: (1) Through the unique core-shell gradient coating and low-temperature atmosphere sintering synergistic process, the NFPP material prepared by this invention maintains high specific capacity while also possessing excellent cycle stability and high electrode compaction density; experiments show that the material has a discharge specific capacity of up to 125 mAh / g at a 0.1C rate, close to the theoretical value; especially at 2.5 g / cm³. 3 Under high density, the capacity retention rate is still ≥98% after 500 cycles at 1C rate, which effectively solves the key technical problem of NFPP material being difficult to balance high density and long cycle life, and provides a key material basis for manufacturing sodium-ion batteries with high volumetric energy density and long life. (2) Through a composite carbon source system consisting of a first carbon source (inner layer) and a second carbon source (outer layer) and a corresponding two-step process, a gradient coating layer with a gradual change in carbon density and structure from the inside to the outside was successfully constructed. The dense carbon in the inner layer effectively isolates the electrolyte, suppresses side reactions, and buffers volume changes. The highly conductive carbon in the outer layer constructs a fast electron transport network. This gradient structure not only ensures excellent ion transport (manifested as high rate performance, 5C retention rate ≥95%), but also significantly enhances interface stability, which is the fundamental guarantee for long cycle life. (3) The sintering temperature was significantly reduced from the traditional >700℃ to 570-590℃, effectively suppressing sodium volatilization and the formation of high-temperature impurity phases (such as NaFePO4, Fe2O3), resulting in a material phase purity of over 99%. Simultaneously, a reducing atmosphere of trace amounts of hydrogen (1-3%) was introduced, introducing controllable oxygen vacancy defects in situ within the crystal lattice, thereby improving the intrinsic electronic conductivity of the material and further optimizing the electrochemical kinetics. Furthermore, the introduced hydrogen reducing atmosphere not only protected the Fe... 2+ It also significantly enhances the mobility of ions in the crystal lattice, thereby promoting the formation and growth of crystal structures at lower temperatures; (4) The co-precipitation-spray drying-low temperature sintering process adopted in this invention uses all common industrial raw materials, which is inexpensive; the equipment involved is all conventional chemical equipment, which is easy to scale up production; the data of the examples show that high-performance products can be obtained in a wide range of sintering temperatures of 570-590℃, which proves that the process has good robustness and operating window, laying a solid foundation for large-scale industrial production. (5) With the help of the dispersant's morphology control effect, the precursor is spray-dried to form secondary particles with good sphericity and uniform particle size distribution (1-5 μm), and its tap density can reach 1.28-1.35 g / cm³. 3 This spherical morphology not only helps to improve the compaction density of the electrode, but also makes the powder flow well, the electrode coating processing performance excellent, and it is easy to prepare uniform, high-load electrodes. (6) The material is based on elements such as iron, sodium and phosphorus, which are abundant and non-toxic, ensuring the high safety and environmental friendliness of the battery system from the source.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 The figures are test results of the high-capacity sodium iron pyrophosphate material of the present invention, its preparation method and application examples 1-5 and comparative examples 1-3, wherein (a) is a bar chart of the first discharge specific capacity test results at 0.1C; (b) is a bar chart of the first coulombic efficiency test results; and (c) is a bar chart of the tap density test results. Figure 2 The figures are test results of the high-capacity sodium iron pyrophosphate material of the present invention, its preparation method and application examples 1-5 and comparative examples 1-3, wherein (a) is a bar chart of 1C discharge specific capacity test results; (b) is a bar chart of 1C cycle 500 cycles capacity retention rate test results; and (c) is a bar chart of 5C / 2C rate retention rate test results. Detailed Implementation

[0019] This invention provides a high-capacity sodium iron pyrophosphate material, comprising the following raw materials: a phosphorus source, an iron source, a sodium source, a first carbon source, a second carbon source, a coating agent, and a dispersant. The atomic molar ratio of the phosphorus source, iron source, and sodium source is Na:Fe:P = 4:3:5. The first carbon source, second carbon source, coating agent, and dispersant account for 2.5-5.5%, 1.0-2.4%, 0.4-2.1%, and 0.1-0.5% of the total mass of the phosphorus source, iron source, sodium source, first carbon source, second carbon source, coating agent, and dispersant, respectively.

[0020] In this invention, the sodium source is one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium sulfate; the phosphorus source is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; the iron source is one of ferrous sulfate and ferric nitrate; the first carbon source is one of citric acid and tartaric acid; the second carbon source is one of glucose and sucrose; the coating agent is one of L-ascorbic acid and maltodextrin; and the dispersant is one of polyethylene glycol and polyvinyl alcohol.

[0021] In this invention, the use of excess phosphorus effectively compensates for the loss of phosphorus due to volatilization caused by the thermal decomposition of the phosphorus source, thereby achieving high phase purity. The first carbon source acts as a chelating agent alongside the inner carbon source; during the co-precipitation stage, its carboxyl groups react with Fe... 2+ The formation of stable, soluble complexes can effectively inhibit the hydrolysis and oxidation of iron ions, ensuring that iron is preserved as Fe. 2+ Upon entering the precursor, its pyrolytic carbon layer structure becomes dense, forming an inner protective barrier. The second carbon source, acting as the outer carbon source, forms a carbon network with superior conductivity and a relatively loose structure after pyrolysis. This network primarily functions to construct continuous electron conduction channels, forming a functionally graded carbon coating layer with a dense inner layer and conductive outer layer together with the first carbon source. The coating agent can continuously stabilize Fe in a solution environment. 2+ To prevent it from being oxidized to Fe 3+ As a surfactant, the dispersant can effectively reduce the surface energy of the particles and prevent the hard aggregation of nanoparticles during the formation process, laying the foundation for obtaining a regular spherical morphology in subsequent spray drying.

[0022] This invention also provides a method for preparing a high-capacity sodium iron pyrophosphate material, comprising the following steps: Step 1: Preparation of precursor slurry: Weigh sodium source, phosphorus source and iron source to prepare precursor slurry; Step 2, spray drying and granulation: Add the second carbon source to the precursor slurry, stir evenly to obtain a mixed slurry, and spray dry the mixed slurry to obtain spherical precursor powder; Step 3: Sinter the spherical precursor powder to obtain the sintered product; Step four: The sintered product is crushed and sieved to obtain high-capacity sodium iron pyrophosphate material.

[0023] In this invention, the specific operation of step one is as follows: S1. Dissolve the sodium source and phosphorus source in deionized water and stir until clear to obtain a sodium-phosphorus mixed solution; S2. Dissolve the iron source, the first carbon source, the coating agent and the dispersant in deionized water to obtain a mixed solution of the iron source; S3. Under nitrogen protection, the iron source mixed solution is added dropwise to the sodium-phosphorus mixed solution, the reaction pH is controlled, and the reaction is stirred to obtain the precursor slurry.

[0024] In this invention, in step S1, the temperature of the deionized water is 60-70°C; In S3, the dropping rate of the iron source mixed solution is 0.5-2 mL / min, the reaction pH is 4.5-5.5, the reaction temperature is 65-75℃, and the reaction is stirred for 3-5 h.

[0025] In step one, nitrogen protection is used to isolate oxygen, ensuring that the iron source is Fe. 2+ It participates in the reaction and enters the crystal lattice. Furthermore, under mild acidic conditions, it facilitates the formation of uniform, fine amorphous or non-crystalline precursors, avoiding coarse crystals or impurity phases caused by localized supersaturation. Its slow dropping rate ensures homogeneity of mixing at the ionic level.

[0026] In this invention, in step two, the parameters for spray drying are: using a pressure-type or centrifugal spray dryer, controlling the inlet temperature to 200-230℃ and the outlet temperature to 95-110℃.

[0027] The homogeneous precursor slurry is instantaneously dried to directly form secondary microspheres with controllable particle size and high sphericity in a one-step process. This morphology has the highest packing density, which can greatly improve the tap density and compaction density of the final cathode material, thereby increasing the volumetric energy density of the battery. At the same time, the spherical particles have good flowability, which is beneficial for electrode slurry coating.

[0028] In this invention, step three involves gradient sintering, comprising three stages: The first stage involves heating from room temperature to 300-350℃ at a rate of 3-5℃ / min and holding the temperature under an argon atmosphere for 0.5-1h. This low-temperature process removes water of crystallization, decomposes ammonium salts and some organic matter, and avoids the destruction of particle structure caused by the generation of a large amount of gas due to rapid heating.

[0029] The second stage: After holding at this temperature, the temperature is increased to 570-590℃ at a rate of 2-3℃ / min, switching to an argon-hydrogen mixture, and held for 2-4 hours. Compared to the traditional solid-state method, the second stage significantly reduces the temperature, resulting in a substantial decrease in energy consumption. It also suppresses nano-volatilization and impurity phase formation, and prevents excessive particle growth through sintering. The introduction of trace amounts of hydrogen creates a weakly reducing atmosphere, which protects the Fe... 2+ It is not oxidized by residual oxygen and can also introduce controllable oxygen vacancy defects into the crystal lattice, thereby improving intrinsic electronic conductivity.

[0030] Third stage: After the heat preservation is completed, switch to argon gas and cool the furnace to room temperature. Cooling under an inert atmosphere can prevent the high-temperature products from being oxidized.

[0031] In this invention, in the second stage, the hydrogen gas fraction in the argon-hydrogen mixture is 1-3%.

[0032] In this invention, in step four, the sieve opening is 300-400 mesh.

[0033] This invention also provides an application of a high-capacity sodium iron pyrophosphate material, which is used to prepare sodium-ion batteries.

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0035] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0036] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0037] In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0038] Example 1 This invention provides a high-capacity sodium iron pyrophosphate material, wherein the raw materials are prepared by metering as follows: phosphorus source, iron source, and sodium source are weighed according to the atomic molar ratio of Na:Fe:P=4:3:5. The first carbon source, the second carbon source, the dispersant, and the coating agent account for 3.31%, 1.26%, 0.23%, and 0.46% of the total mass of the phosphorus source, iron source, sodium source, first carbon source, second carbon source, dispersant, and coating agent, respectively.

[0039] The phosphorus source is selected from ammonium dihydrogen phosphate, the sodium source is selected from sodium carbonate, the iron source is selected from ferrous sulfate heptahydrate, the first carbon source is selected from citric acid, the second carbon source is selected from glucose, the dispersant is selected from polyethylene glycol-400, and the coating agent is selected from L-ascorbic acid.

[0040] Its preparation method includes the following steps: Step 1: Preparation of precursor slurry: S1: Dissolve anhydrous sodium carbonate and ammonium dihydrogen phosphate in deionized water at 65°C and stir until clear to obtain a sodium-phosphorus mixed solution. S2: Dissolve ferrous sulfate heptahydrate, citric acid, L-ascorbic acid and polyethylene glycol-400 in deionized water to obtain a mixed iron source solution; S3: Under nitrogen protection, the iron source mixed solution was added dropwise to the sodium-phosphorus mixed solution at a dropping rate of 1.0 mL / min. The pH of the reaction system was maintained at 5.0 and the temperature was maintained at 70℃. The reaction was stirred continuously for 4 hours to obtain a uniform precursor slurry.

[0041] Step 2: Spray drying and granulation: Add glucose to the precursor slurry and stir until homogeneous. Use a centrifugal spray dryer, controlling the inlet temperature at 220℃ and the outlet temperature at 100℃, to obtain spherical precursor powder.

[0042] Step 3, Low-Temperature Gradient Sintering: Place the precursor powder in a tube furnace and sinter according to the following procedure: First stage: Increase the temperature from room temperature to 320℃ at a rate of 5℃ / min, and hold at this temperature for 0.5 hours under an argon atmosphere; Second stage: Increase the temperature to 580℃ at 3℃ / min, switch to an argon-hydrogen mixture containing 2% hydrogen, and hold for 3 hours; Third stage: After the heat preservation is completed, switch to pure argon gas and let the furnace cool naturally to room temperature.

[0043] Step 4, Post-processing: The sintered product is pulverized by air jet milling and passed through a 300-mesh sieve to obtain the final product, high-capacity sodium iron pyrophosphate material, denoted as NFPP-1.

[0044] Example 2 The only difference between this embodiment and Example 1 is that the first carbon source, the second carbon source, the dispersant, and the coating agent account for 2.85%, 1.03%, 0.29%, and 0.40% of the total mass of the phosphorus source, the iron source, the sodium source, the first carbon source, the second carbon source, the dispersant, and the coating agent, respectively. All other conditions are the same, and the resulting product is denoted as NFPP-2.

[0045] Example 3 The only difference between this embodiment and Example 1 is that the first carbon source, the second carbon source, the dispersant, and the coating agent account for 4.06%, 1.69%, 0.17%, and 0.51% of the total mass of the phosphorus source, the iron source, the sodium source, the first carbon source, the second carbon source, the dispersant, and the coating agent, respectively. All other conditions are the same, and the resulting product is denoted as NFPP-3.

[0046] Example 4 The only difference between this embodiment and Embodiment 1 is that in step three, the sintering temperature in the second stage is adjusted to 570℃, while all other conditions are the same. The resulting product is denoted as NFPP-4.

[0047] Example 5 The only difference between this embodiment and Embodiment 1 is that in step three, the sintering temperature in the second stage is adjusted to 590℃, while all other conditions are the same. The resulting product is denoted as NFPP-5.

[0048] Comparative Example 1 The only difference between this comparative example and Example 1 is that no dispersant was added; all other conditions were the same, and the resulting product was designated NFPP-C1.

[0049] Comparative Example 2 The only difference between this comparative example and Example 1 is that the sintering step in step three of the preparation method is changed to: under a pure argon atmosphere, the temperature is directly increased from room temperature to 750°C at a rate of 5°C / min and held for 10 hours, followed by furnace cooling. All other conditions are the same, and the resulting product is denoted as NFPP-C2.

[0050] Comparative Example 3 The only difference between this comparative example and Example 1 is that the first carbon source is not added, and the second carbon source, dispersant, and coating agent account for 4.29%, 0.23%, and 0.46% of the total mass of phosphorus source, iron source, sodium source, first carbon source, second carbon source, dispersant, and coating agent, respectively. The sintering atmosphere in step three is pure argon gas, without hydrogen gas. All other conditions are the same, and the product obtained is denoted as NFPP-C3.

[0051] The products obtained in Examples 1-5 and Comparative Examples 1-3 were used to make electrode sheets (material: conductive agent: binder = 92:4:4). CR2032 button batteries were assembled using sodium sheets as the counter electrode assembly and tested within a voltage range of 1.5-3.8V. The test results are shown in Table 1.

[0052] Table 1 Test Results

[0053] At 2.5 g / cm 3 The properties of Examples 1-5 and Comparative Examples 1-3 were tested under compacted density, and the test results are shown in Table 2.

[0054] Table 2 Test Results

[0055] From Table 1, Table 2, Figure 1 and Figure 2 It can be seen that the sodium iron pyrophosphate materials prepared in Examples 1-5 all exhibit ultra-high reversible specific capacity (0.1C ≥ 117 mAh / g) close to the theoretical value, and excellent first-pass coulombic efficiency (≥ 92%). Particularly noteworthy is the high efficiency achieved at 2.5 g / cm³. 3 Under high compaction density, the material of this invention can still maintain a 1C specific capacity of over 114 mAh / g, and the capacity retention rate is over 98% after 500 cycles, successfully solving the industry problem of NFPP materials being unable to balance high compaction density and long cycle life.

[0056] Comparative experimental data strongly demonstrate the synergistic effect and indispensability of the various technical features of this invention: Comparative Example 1 (lacking a dispersant for morphology control) suffers from severe degradation in tap density and high-pressure performance due to the inability to form a spherical structure; Comparative Example 2 (using conventional high-temperature sintering) exhibits a significant decrease in overall performance due to impurity phase formation and excessive particle growth; Comparative Example 3 (using a single carbon source and without a hydrogen atmosphere) shows significantly inferior cycle stability and rate performance compared to the embodiments of this invention due to the lack of a gradient carbon layer and intrinsic defect control. Furthermore, the results of Examples 4 and 5 demonstrate that the low-temperature sintering process of this invention can obtain high-performance materials within the temperature range of 570-590℃, exhibiting good process robustness.

[0057] Therefore, this invention employs the aforementioned high-capacity sodium iron pyrophosphate material, its preparation method, and its application. Through a synergistic process system of core-shell gradient carbon coating and low-temperature reducing atmosphere sintering, it successfully solves the problem of simultaneously achieving high compaction density and long cycle life in sodium iron pyrophosphate materials. Furthermore, it utilizes common raw materials to construct spherical particles with an internal porous nanostructure and an external carbon content gradient, and combines this with low-temperature controllable reduction sintering, significantly reducing energy consumption while greatly improving the intrinsic conductivity, structural stability, and phase purity of the material. The prepared material possesses a high specific capacity close to the theoretical value, excellent rate performance, and an ultra-long cycle life under extreme compaction conditions.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-capacity sodium iron pyrophosphate material, characterized in that: The product includes the following raw materials: phosphorus source, iron source, sodium source, first carbon source, second carbon source, coating agent, and dispersant. The atomic molar ratio of phosphorus source, iron source, and sodium source is Na:Fe:P = 4:3:

5. The first carbon source, second carbon source, coating agent, and dispersant account for 2.5-5.5%, 1.0-2.4%, 0.4-2.1%, and 0.1-0.5% of the total mass of phosphorus source, iron source, sodium source, first carbon source, second carbon source, coating agent, and dispersant, respectively.

2. The high-capacity sodium iron pyrophosphate material according to claim 1, characterized in that: The sodium source is one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium sulfate; the phosphorus source is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; the iron source is one of ferrous sulfate and ferric nitrate; the primary carbon source is one of citric acid and tartaric acid; the secondary carbon source is one of glucose and sucrose; the coating agent is one of L-ascorbic acid and maltodextrin; and the dispersant is one of polyethylene glycol and polyvinyl alcohol.

3. The method for preparing a high-capacity sodium iron pyrophosphate material according to any one of claims 1-2, characterized in that: Includes the following steps: Step 1: Preparation of precursor slurry: Weigh sodium source, phosphorus source and iron source to prepare precursor slurry; Step 2, spray drying and granulation: Add the second carbon source to the precursor slurry, stir evenly to obtain a mixed slurry, and spray dry the mixed slurry to obtain spherical precursor powder; Step 3: Sinter the spherical precursor powder to obtain the sintered product; Step four: The sintered product is crushed and sieved to obtain high-capacity sodium iron pyrophosphate material.

4. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 3, characterized in that: The specific steps for step one are as follows: S1. Dissolve the sodium source and phosphorus source in deionized water and stir until clear to obtain a sodium-phosphorus mixed solution; S2. Dissolve the iron source, the first carbon source, the coating agent and the dispersant in deionized water to obtain a mixed solution of the iron source; S3. Under nitrogen protection, the iron source mixed solution is added dropwise to the sodium-phosphorus mixed solution, the reaction pH is controlled, and the reaction is stirred to obtain the precursor slurry.

5. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 3, characterized in that: In S1, the temperature of the deionized water is 60-70℃; In S3, the dropping rate of the iron source mixed solution is 0.5-2 mL / min, the reaction pH is 4.5-5.5, the reaction temperature is 65-75℃, and the reaction is stirred for 3-5 h.

6. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 3, characterized in that: In step two, the parameters for spray drying are as follows: a pressure spray dryer or a centrifugal spray dryer is used, and the inlet temperature is controlled at 200-230℃ and the outlet temperature at 95-110℃.

7. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 3, characterized in that: In step three, gradient sintering is used, which includes three stages: First stage: Increase the temperature from room temperature to 300-350℃ at a rate of 3-5℃ / min, and hold at this temperature for 0.5-1h under an argon atmosphere; Second stage: After holding at the temperature, increase the temperature to 570-590℃ at a rate of 2-3℃ / min, switch to argon-hydrogen mixed gas, and hold at the temperature for 2-4 hours; Third stage: After the heat preservation is completed, switch to argon gas and cool the furnace to room temperature.

8. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 7, characterized in that: In the second stage, the hydrogen gas fraction in the argon-hydrogen mixture is 1-3%.

9. The method for preparing a high-capacity sodium iron pyrophosphate material according to claim 3, characterized in that: In step four, the sieve opening is 300-400 mesh.

10. An application of a high-capacity sodium iron pyrophosphate material, characterized in that: The high-capacity sodium iron pyrophosphate material according to any one of claims 1-2 is used to prepare sodium-ion batteries.

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