A sodium iron phosphate pyrophosphate composite material and its preparation method

Abstract

This invention provides a sodium iron phosphate pyrophosphate composite material and its preparation method. The preparation method includes controlling the zinc and potassium content in hematite slag, mixing and dissolving the hematite slag with sodium, phosphorus, and carbon sources, and then drying the mixture to obtain a mixed solid powder. The mixed solid powder is then sintered under a protective atmosphere to obtain the sodium iron phosphate pyrophosphate composite material. The composite material has a core-shell structure, with the core being zinc and potassium-doped sodium iron phosphate pyrophosphate, and the coating layer is a carbon material. This invention allows for the direct use of hematite slag as an iron source to prepare the sodium iron phosphate pyrophosphate composite material, realizing the direct synthesis of battery materials from hematite slag produced by hydrometallurgical zinc refining.

Description

Technical Field

[0001] This invention relates to the field of battery materials, and more specifically, to a sodium iron phosphate pyrophosphate composite material and its preparation method. Background Technology

[0002] In hydrometallurgical zinc refining, to improve the leaching rate of non-ferrous metals such as zinc, copper, and germanium from zinc ore, neutral leaching residue is often subjected to hot acid leaching treatment, thereby increasing the leaching rate of non-ferrous metals. Along with the leaching of zinc, copper, and germanium, most of the iron in the ore also enters the leachate. To improve the quality and efficiency of electrolytic zinc production, it is necessary to separate and precipitate the iron in the leachate. Currently, commonly used methods for iron precipitation in the leachate include the vanadium-iron method, the goethite method, and the hematite method. Among these, the hematite method has gained attention and favor in the hydrometallurgical zinc refining industry due to its advantages such as high recovery rate of associated valuable metals and the resource utilization of iron slag. However, the widespread application of the hematite method generates a large amount of hematite slag, and how to achieve the resource reuse of this slag has become a major concern in the industry.

[0003] Among various sodium-ion battery cathode materials, polyanionic compounds possess advantages such as high operating voltage, structural stability, and small volume change, making them more suitable for the practical needs of energy storage systems. Among polyanionic compounds, the mixed iron-based phosphate pyrophosphate sodium iron phosphate exhibits 3D Na... + Transport path, PO4 suppresses crystal structure distortion 3- and P2O7 4- The advantages of dimers and phosphate / pyrophosphates. Furthermore, sodium iron phosphate pyrophosphate also possesses a moderate theoretical specific capacity (129 mAh g). -1 Sodium iron pyrophosphate (SFP) has advantages such as high average operating voltage (above 3.1V), excellent cycle stability (volume change less than 4%), low cost, and environmental friendliness. However, the intrinsic low electronic conductivity and ion diffusion capacity of SFP hinder its electrochemical performance and practical application. Summary of the Invention

[0004] In view of the shortcomings of the prior art, one of the objectives of this invention is to solve one or more problems existing in the prior art. For example, one objective of this invention is to provide a method for preparing sodium iron phosphate pyrophosphate composite material by directly using hematite slag as an iron source, realizing the direct synthesis of battery materials from hematite slag produced by hydrometallurgical zinc refining.

[0005] One aspect of the present invention provides a method for preparing a sodium iron phosphate pyrophosphate composite material, which may include the following steps: controlling the zinc and potassium content in hematite slag; mixing and dissolving the hematite slag with a sodium source, a phosphorus source, and a carbon source, and then drying the mixture to obtain a mixed solid powder; sintering the mixed solid powder under a protective atmosphere to obtain a product with the chemical formula Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C is a sodium iron phosphate pyrophosphate composite material, wherein the sodium iron phosphate pyrophosphate composite material has a core-shell structure, the core is zinc and potassium doped sodium iron phosphate pyrophosphate, x>0, y>0 and 0.01<x+y<0.5.

[0006] Furthermore, by mass percentage, the zinc content in hematite slag can be greater than 1%, the potassium content can be greater than 0.03%, and the total content of zinc and potassium can be less than 13.5%.

[0007] Another aspect of the present invention provides a method for preparing a sodium iron pyrophosphate composite material, which may include the following steps: controlling the zinc and potassium content in hematite slag; mixing and dissolving the hematite slag with a sodium source, a phosphorus source, and a carbon source, and then drying the mixture to obtain a mixed solid powder; sintering the mixed solid powder under a protective atmosphere to obtain the sodium iron pyrophosphate composite material, wherein the sodium iron pyrophosphate composite material has a core-shell structure, with the core being zinc and potassium-doped sodium iron pyrophosphate; and by mass percentage, the zinc content in the hematite slag is greater than 1%, the potassium content is greater than 0.03%, and the total content of zinc and potassium is less than 13.5%.

[0008] Furthermore, the chemical formula of the sodium iron pyrophosphate composite material can be Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C, where x>0, y>0 and 0.01<x+y<0.5.

[0009] Furthermore, sintering under an inert protective atmosphere may include pre-firing at 300℃~400℃ followed by sintering at 480℃~600℃.

[0010] Furthermore, the hematite slag, sodium source, and phosphorus source can be mixed in a molar ratio of iron, sodium, and phosphorus of (2.9–3.1):(3.9–4.1):(3.9–4.1), and the hematite slag and carbon source can be mixed in a molar ratio of iron to carbon of 1:(4–12).

[0011] Furthermore, the coating layer of the sodium iron pyrophosphate composite material can be a carbon material, the coating layer thickness can be 3-10 nm, and the carbon content can be 5%-15% of the composite material mass.

[0012] Furthermore, the sodium source may include at least one of sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium acetate.

[0013] Furthermore, the phosphorus source may include at least one of sodium pyrophosphate, pyrophosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0014] Furthermore, the carbon source may include at least one of citric acid, glucose, sucrose, oxalic acid, graphene, and carbon nanotubes.

[0015] Another aspect of the present invention provides a sodium iron phosphate pyrophosphate composite material, wherein the sodium iron phosphate pyrophosphate composite material is Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C has a core-shell structure, with the core being zinc- and potassium-doped sodium iron pyrophosphate and the coating being carbon material, where x>0, y>0 and 0.01<x+y<0.5.

[0016] Another aspect of the present invention provides a sodium-ion battery cathode material comprising the sodium iron phosphate pyrophosphate composite material described above.

[0017] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0018] (1) The method of the present invention can directly use hematite slag as an iron source to prepare sodium iron phosphate pyrophosphate composite material, realizing the direct synthesis of battery materials from hematite slag produced by wet zinc smelting. While comprehensively recycling hematite slag, the prepared composite material has good crystallinity, uniform size and excellent electrochemical performance due to carbon coating. It has the advantages of low production cost and short process flow.

[0019] (2) This invention uses hematite slag as an iron source. Hematite slag contains trace elements Zn and K, which not only enables the one-step preparation of electrode materials from wet zinc smelting slag, but also effectively utilizes the Zn and K elements in the hematite slag. By controlling the Zn and K content in the hematite slag and using Zn and K to directly dope the Fe sites in the generated composite material, the crystal structure of the sodium iron phosphate pyrophosphate composite material is not destroyed during the preparation process, but its electrochemical performance is improved. The trace elements Zn and K enter the sodium iron phosphate pyrophosphate bulk phase through mixing and sintering during the preparation process, and then enter the Fe sites to form the chemical formula Na4Fe. 3-x-y Zn x K yThe composite material of (PO4)2P2O7@C, compared with the pure phase Na4Fe3(PO4)2P2O7@C, Na4Fe 3-x-y Zn x K y The (PO4)2P2O7@C composite material improves ion diffusion rate and electronic conductivity due to the doping of Zn and K, thereby enhancing the material's cycle stability and significantly improving its electrochemical performance.

[0020] (3) The composite material of the present invention has a uniform nanoscale carbon coating layer on its surface, which can avoid direct contact between the composite material and the electrolyte, thereby suppressing interfacial side reactions and metal ion dissolution.

[0021] (4) The sodium iron phosphate pyrophosphate composite material prepared by the method of the present invention can be used in sodium-ion batteries to effectively solve the problems of low electronic conductivity, slow ion diffusion rate and unstable cycling of iron-based cathode materials. Attached Figure Description

[0022] The above and other objects and features of the present invention will become clearer from the following description taken in conjunction with the accompanying drawings, in which:

[0023] Figure 1 This is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Example 1 of the present invention.

[0024] Figure 2 The image shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Example 1 of this invention.

[0025] Figure 3 This is a charge-discharge curve of a button cell assembled from the sodium iron phosphate pyrophosphate composite material prepared in Example 1 of the present invention.

[0026] Figure 4 This is a charge-discharge cycle diagram of the sodium iron phosphate pyrophosphate composite material prepared in Example 1 of the present invention assembled into a coin cell.

[0027] Figure 5 The image shows the charge-discharge curves of the sodium iron phosphate pyrophosphate composite material prepared in Example 2 of this invention assembled into a coin cell.

[0028] Figure 6 This is a charge-discharge cycle diagram of the sodium iron phosphate pyrophosphate composite material prepared in Example 2 of the present invention assembled into a coin cell.

[0029] Figure 7 The image shows the charge-discharge curves of the sodium iron phosphate pyrophosphate composite material prepared in Example 3 of this invention assembled into a coin cell.

[0030] Figure 8This is a charge-discharge cycle diagram of the sodium iron phosphate pyrophosphate composite material prepared in Example 3 of the present invention assembled into a coin cell.

[0031] Figure 9 This is a charge-discharge curve of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2 of the present invention assembled into a coin cell.

[0032] Figure 10 This is a charge-discharge cycle diagram of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2 of the present invention assembled into a coin cell. Detailed Implementation

[0033] In the following, a sodium iron phosphate pyrophosphate composite material and its preparation method according to the present invention will be described in detail with reference to the accompanying drawings and exemplary embodiments.

[0034] One aspect of the present invention provides a method for preparing a sodium iron phosphate pyrophosphate composite material. In some embodiments of the method for preparing the sodium iron phosphate pyrophosphate composite material, the following steps may be included:

[0035] S01, control the zinc and potassium content in hematite slag, mix and dissolve hematite slag with sodium source, phosphorus source and carbon source and then dry to obtain mixed solid powder;

[0036] S02, the mixed solid powder is sintered under an inert protective atmosphere to obtain sodium iron phosphate pyrophosphate composite material, wherein the sodium iron phosphate pyrophosphate composite material has a core-shell structure and the core is zinc and potassium doped sodium iron phosphate pyrophosphate.

[0037] In some implementations, controlling the zinc and potassium content in the hematite slag is to ensure that the zinc (Zn) and potassium (K) content in the prepared sodium iron pyrophosphate composite material is within a suitable range. By mass percentage, the zinc content in the hematite slag can be greater than 1%, the potassium content can be greater than 0.03%, and the total zinc and potassium content can be less than 13.5%. If the zinc and potassium content is too low, it is difficult to achieve the desired doping effect on sodium iron pyrophosphate, and the ion diffusion rate, electronic conductivity, and cycle stability of the composite material cannot be effectively improved. If the total zinc and potassium content is greater than 13.5%, excessive zinc and potassium content will damage the original crystal structure of sodium iron pyrophosphate and deteriorate its electrochemical performance. Therefore, controlling the zinc and potassium content in the hematite slag to a zinc content greater than 1%, a potassium content greater than 0.03%, and a total zinc and potassium content less than 13.5% allows the prepared composite material to have better ion diffusion rate, electronic conductivity, and cycle stability. For example, the zinc content can be greater than 2% and less than 10%, and the potassium content can be greater than 0.5% and less than 3%; or the zinc content can be greater than 3% and less than 9%, and the potassium content can be greater than 1% and less than 4%; or the zinc content can be greater than 5% and less than 12%, and the potassium content can be greater than 0.2% and less than 1.5%; or the zinc content can be greater than 6% and less than 8%, and the potassium content can be greater than 3% and less than 5.5%; or a combination of the above ranges. The iron content in hematite slag can be greater than 65%, for example, greater than 70%. Furthermore, the iron content can be 68%, 72%, 75%, or 80%.

[0038] In some implementations, controlling the zinc and potassium content in hematite slag can include adjusting process parameters of the hematite slag precipitation process after hydrometallurgical zinc smelting to obtain the set zinc and potassium content. For example, this can be achieved by controlling process parameters such as reaction time, temperature, and pH value in the hematite slag precipitation process. Additionally, if a high-zinc and high-potassium sodium ferric phosphate pyrophosphate composite material is required, zinc nitrate or potassium nitrate can be added to supplement the zinc and potassium content when the zinc and potassium content in the hematite slag precipitation is low. It should be noted that for hematite slag precipitation directly obtained from existing hematite slag precipitation processes, if the zinc and potassium content is within the set range, the hematite slag precipitation can be used directly as raw material without further treatment to control the zinc and potassium content.

[0039] In some implementations, controlling the zinc and potassium content in hematite slag to a level where zinc is greater than 1%, potassium is greater than 0.03%, and the total zinc and potassium content is less than 13.5% yields a product with the chemical formula Na₄Fe₂O₃. 3-x-y Zn x K yThe (PO4)2P2O7@C composite material of sodium iron phosphate pyrophosphate, wherein x>0, y>0 and 0.01<x+y<0.5, exhibits good ion diffusion rate, electronic conductivity, and cycling stability. For example, combinations of 0.05≤x+y≤0.4, 0.1≤x+y≤0.3, 0.15≤x+y≤0.25, or higher. Furthermore, x can be 0.2 and y can be 0.24; or x can be 0.1 and y can be 0.35; or x can be 0.3 and y can be 0.12.

[0040] In some embodiments, in step S01 above, hematite slag can be crushed and ball-milled to obtain hematite slag powder. Then, the hematite slag powder, sodium source, phosphorus source, and carbon source are dispersed in deionized water and stirred to obtain a mixed solution. Finally, the mixed solution is dried to obtain a mixed solid powder.

[0041] In some embodiments, the hematite slag, sodium source, and phosphorus source can be mixed in a molar ratio of iron, sodium, and phosphorus of (2.9–3.1):(3.9–4.1):(3.9–4.1). For example, they can be mixed in a molar ratio of iron, sodium, and phosphorus of (2.95–3.05):(3.95–4.05):(3.96–4.07), (3.0–3.02):(4.0–4.02):(4.0–4.06), or a combination thereof. The hematite slag and carbon source can be mixed in a molar ratio of iron to carbon of 1:(4–12). For example, they can be mixed in a combination of 1:(5–11), 1:(6–10), 1:(7–9), or a combination thereof.

[0042] In some embodiments, the sodium source may include at least one of sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium acetate.

[0043] In some embodiments, the phosphorus source may include at least one of sodium pyrophosphate, pyrophosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0044] In some embodiments, the carbon source may include at least one of citric acid, glucose, sucrose, oxalic acid, graphene, and carbon nanotubes.

[0045] In some embodiments, in step S02 above, the mixed solid powder can be pre-calcined at 300°C to 350°C under an argon protective atmosphere, and then sintered at 480°C to 600°C under an argon protective atmosphere to obtain a sodium iron phosphate pyrophosphate composite material. Regarding the second sintering temperature after pre-calcination, if the sintering temperature is less than 480°C, the crystal structure and phase of sodium iron phosphate pyrophosphate will be incomplete, affecting the battery's capacity retention; if the sintering temperature is greater than 600°C, the sodium iron phosphate pyrophosphate will decompose. Therefore, the second sintering temperature after pre-calcination is set at 480°C to 600°C. For example, it can be pre-calcined at 320°C to 340°C and then sintered at 520°C to 580°C.

[0046] In some implementations, the pre-firing time can be 2 to 4 hours, and the sintering time can be 8 to 14 hours. For example, the pre-firing time can be 3 hours, and the sintering time can be 12 hours.

[0047] Another aspect of the present invention provides a sodium iron phosphate pyrophosphate composite material, wherein the chemical formula of the sodium iron phosphate pyrophosphate composite material can be Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C has a core-shell structure. The core is zinc- and potassium-doped sodium iron pyrophosphate, and the coating is a carbon material. In this case, x>0, y>0, and 0.01<x+y<0.5.

[0048] In some embodiments, the particle size of the sodium iron phosphate pyrophosphate composite material can be from 100 nm to 10 μm. For example, the particle size can be a combination of 110 nm to 500 nm, 300 nm to 8000 nm, 800 nm to 5000 nm, 900 nm to 2000 nm, 200 nm to 1000 nm or more.

[0049] In some embodiments, the coating layer of the composite material can be amorphous carbon, and the coating layer thickness can be 2 nm to 10 nm. For example, the coating layer thickness can be a combination of 5 nm to 8 nm, 3 nm to 9 nm, 4 nm to 7 nm, or more.

[0050] In another aspect, the present invention provides a sodium-ion battery cathode material, which may comprise the sodium iron phosphate pyrophosphate composite material described above.

[0051] To better understand the present invention, specific examples are provided below to further illustrate the content of the present invention, but the content of the present invention is not limited to the examples below.

[0052] Example 1

[0053] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0054] S1. Take 1.5g of hematite slag, crush and ball mill it. The hematite slag contains 2% Zn and 0.1% K by mass, with a total Zn and K mass percentage of 2.1%. The ball milling speed is 350 r / min, and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.02 mol sodium dihydrogen phosphate and 0.02 mol citric acid, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain a solid powder.

[0055] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the powder to 300°C at a rate of 2°C / min and hold for 3 hours to obtain the precursor powder.

[0056] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 500℃ at a rate of 2℃ / min and sinter for 10h to obtain carbon-coated sodium iron phosphate pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1ppm and test the electrochemical performance of the coin cells.

[0057] Figure 1 The scanning electron microscope image of the sodium iron phosphate pyrophosphate composite material prepared in this embodiment shows that its particle morphology is irregular and the particle size is between 100 nm and 10 μm. Figure 2 The image shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in this embodiment. Figure 3 This is a charge-discharge curve of the sodium iron phosphate pyrophosphate composite material prepared in this embodiment. From... Figure 3 It can be seen that the 0.2C reversible capacity of the button half-cell assembled from the sodium iron phosphate pyrophosphate composite material prepared in this embodiment reaches 96 mAh / g. Figure 4 This is a charge-discharge cycle diagram of the sodium iron phosphate pyrophosphate composite material prepared in this embodiment assembled into a coin cell. From... Figure 4 It can be seen that the 10C reversible capacity of the button half-cell assembled from sodium iron phosphate pyrophosphate composite material reaches 80 mAh / g, and the capacity retention rate is more than 87% after 500 cycles.

[0058] Example 2

[0059] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0060] S1. Take 1.5g of hematite slag, crush and ball mill it. The mass percentage of Zn in the hematite slag is 2%, the mass percentage of K is 0.1%, and the total mass percentage of Zn and K is 2.1%. The ball milling speed is 350r / min, and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.02mol sodium dihydrogen phosphate and 0.02mol citric acid, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain solid powder.

[0061] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the powder to 300°C at a rate of 2°C / min and hold it at that temperature for 3 hours to obtain the precursor powder.

[0062] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 550°C at a rate of 2°C / min and sinter for 10 hours to obtain carbon-coated sodium iron phosphate pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1 ppm and test the electrochemical performance of the coin cells.

[0063] like Figure 5 As shown, the coin cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this embodiment achieves a 0.2C reversible capacity of 87 mAh / g. Figure 6 As shown, the 10C reversible capacity of the button cell reaches 75mAh / g, and the capacity retention rate is over 90% after 500 cycles.

[0064] Example 3

[0065] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0066] S1. Take 1.5g of hematite slag, crush and ball mill it. The mass percentage of Zn in the hematite slag is 2%, the mass percentage of K is 0.1%, and the total mass percentage of Zn and K is 2.1%. The ball milling speed is 350r / min, and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.02mol sodium dihydrogen phosphate and 0.02mol citric acid, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain solid powder.

[0067] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the powder to 300°C at a rate of 2°C / min and hold it at that temperature for 3 hours to obtain the precursor powder.

[0068] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 600°C at a rate of 2°C / min and sinter for 10 hours to obtain carbon-coated sodium iron phosphate pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1 ppm and test the electrochemical performance of the coin cells.

[0069] like Figure 7 As shown, the 0.2C reversible capacity of the coin cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this embodiment reaches 89 mAh / g. Figure 8 As shown, the 10C reversible capacity of the coin cell half-cell reaches 81mAh / g, and the capacity retention rate is over 89% after 500 cycles.

[0070] Example 4

[0071] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0072] S1. Take 1.5g of hematite slag, crush and ball mill it. The mass percentage of Zn in the hematite slag is 2%, the mass percentage of K is 0.1%, and the total mass percentage of Zn and K is 2.1%. The ball milling speed is 350r / min, and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.005mol sodium pyrophosphate, 0.01mol diammonium hydrogen phosphate and 0.02mol glucose, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain solid powder.

[0073] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the powder to 300°C at a rate of 2°C / min and hold it at that temperature for 3 hours to obtain the precursor powder.

[0074] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 500°C at a rate of 2°C / min and sinter for 8 hours to obtain carbon-coated sodium iron phosphate pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1 ppm and test the electrochemical performance of the coin cells.

[0075] The button cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this embodiment has a 0.2C reversible capacity of 85 mAh / g and a 10C reversible capacity of 72 mAh / g, and a capacity retention rate of over 84% after 500 cycles.

[0076] Example 5

[0077] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0078] S1. Take 1.5g of hematite slag, crush and ball mill it. The mass percentage of Zn in the hematite slag is 2%, the mass percentage of K is 0.1%, and the total mass percentage of Zn and K is 2.1%. The ball milling speed is 350r / min, and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.01mol sodium carbonate, 0.02mol ammonium dihydrogen phosphate and 0.02mol sucrose, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain solid powder.

[0079] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the powder to 400°C at a rate of 2°C / min and hold it at that temperature for 3 hours to obtain the precursor powder.

[0080] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 500°C at a rate of 2°C / min and sinter for 14 hours to obtain carbon-coated sodium iron phosphate pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1 ppm and test the electrochemical performance of the coin cells.

[0081] The button cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this embodiment has a 0.2C reversible capacity of 86 mAh / g and a 10C reversible capacity of 74 mAh / g, and a capacity retention rate of over 86% after 500 cycles.

[0082] Comparative Example 1

[0083] A method for preparing a sodium iron phosphate pyrophosphate composite material may include the following steps:

[0084] S1. Take 1.5g of hematite slag, crush and ball mill it. The mass percentage of Zn in the hematite slag is 2%, the mass percentage of K is 0.1%, and the total mass percentage of Zn and K is 2.1%. The ball milling speed is 350r / min and the ball milling time is 12h. Then, disperse the ball-milled powder in deionized water, add 0.02mol sodium dihydrogen phosphate and 0.02mol citric acid, stir to obtain a mixed solution, then evaporate to dryness at 120℃ for 2h, and vacuum dry at 80℃ for 10h to obtain solid powder.

[0085] S2. Grind the solid powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 300°C at a rate of 2°C / min and hold it at that temperature for 3 hours to obtain the precursor powder.

[0086] S3. Grind the precursor powder and place it in a crucible. Under a pure argon atmosphere, heat the mixture to 450°C at a rate of 2°C / min and sinter for 10 hours to obtain carbon-coated sodium iron pyrophosphate composite material. Assemble coin cells in a glove box with a water and oxygen content of less than 0.1 ppm and test the electrochemical performance of the coin cells.

[0087] The coin cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this embodiment achieved a 0.2C reversible capacity of 80 mAh / g and a 10C reversible capacity of 37 mAh / g. After 500 cycles, the reversible capacity decayed to 7 mAh / g, with a capacity retention rate as low as 19.7%. The lower 10C reversible capacity of the coin cell in this embodiment compared to Example 1 is due to the lower sintering temperature of 450°C used in step S3. This lower sintering temperature resulted in an incomplete sodium iron phosphate pyrophosphate crystal structure and phase, leading to a lower capacity retention rate compared to the battery prepared in Example 1.

[0088] Comparative Example 2

[0089] The comparative method is the same as the preparation method of Example 1, except that Zn(NO3)2 and KNO3 are added to the hematite slag to make the total mass ratio of Zn and K in the raw material reach 14%.

[0090] like Figure 9 As shown, the 0.2C reversible capacity of the coin half-cell assembled using the sodium iron phosphate pyrophosphate composite material prepared in this comparative example is only 90 mAh / g. Figure 10 As shown, the 10C reversible capacity of the coin cell is 45 mAh / g, and the capacity retention rate is as low as 24% after 500 cycles. Compared with the total Zn and K content controlled below 13.5%, the battery assembled from this composite material has a smaller reversible capacity and significantly reduced cycle stability.

[0091] In summary, the method of this invention can yield a sodium iron phosphate pyrophosphate composite material with good crystallinity and stable structure, exhibiting excellent electrochemical performance.

[0092] Although the present invention has been described above in conjunction with exemplary embodiments, those skilled in the art will understand that various modifications and changes can be made to the exemplary embodiments of the present invention without departing from the spirit and scope defined by the claims.

Claims

1. A method for preparing a sodium pyrophosphate ferric phosphate complex material, characterized by, The method comprises the following steps: controlling the content of zinc and potassium in the hematite iron precipitation residue, mixing and dissolving the hematite iron precipitation residue, a sodium source, a phosphorus source and a carbon source, and then drying to obtain a mixed solid powder; sintering the mixed solid powder in a protective atmosphere to obtain a sodium pyrophosphate iron phosphate composite material, wherein the sodium pyrophosphate iron phosphate composite material has a core-shell structure, and the inner core is zinc and potassium doped sodium pyrophosphate iron phosphate; the content of zinc in the hematite iron precipitation residue is greater than 1%, the content of potassium is greater than 0.03%, and the total content of zinc and potassium is less than 13.5% by mass percentage.

2. The method for preparing sodium iron pyrophosphate composite material according to claim 1, characterized in that, The chemical formula of the pyrophosphoric iron phosphate sodium composite material is Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C, wherein, x>0, y>0 and 0.01 3. The method for preparing sodium iron pyrophosphate composite material according to claim 1 or 2, characterized in that, The sintering in the protective atmosphere comprises pre-sintering at 300-400 DEG C and then sintering at 480-600 DEG C.

4. The method for preparing sodium iron pyrophosphate composite material according to claim 1 or 2, characterized in that, The hematite iron precipitation residue, the sodium source and the phosphorus source are mixed in a molar ratio of iron element, sodium element and phosphorus element of (2.9-3.1):(3.9-4.1):(3.9-4.1), and the hematite iron precipitation residue and the carbon source are mixed in a molar ratio of iron element and carbon element of 1:(4-12).

5. The method for preparing sodium iron pyrophosphate composite material according to claim 1 or 2, characterized in that, The particle size of the sodium pyrophosphate iron phosphate composite material is 100 nm-10 μm, the coating layer is a carbon material, the thickness of the coating layer is 2 nm-10 nm, and the carbon content is 5%-15% of the mass of the composite material.

6. The method for preparing sodium iron pyrophosphate composite material according to claim 1 or 2, characterized in that, The sodium source includes at least one of sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate and sodium acetate; the phosphorus source includes at least one of sodium pyrophosphate, pyrophosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate; and the carbon source includes at least one of citric acid, glucose, sucrose, oxalic acid, graphene and carbon nanotubes.

7. The sodium pyrophosphate ferric phosphate complex material prepared by the method according to any one of claims 1 to 6, characterized in that, Ferric pyrophosphate sodium composite material Na4Fe 3-x-y Zn x K y (PO4)2P2O7@C is a core-shell structure, the inner core is zinc, potassium doped ferric pyrophosphate sodium, the coating layer is carbon material, wherein x>0, y>0 and 0.01 8. A sodium-ion battery cathode material, characterized in that, The sodium pyrophosphate iron phosphate composite material prepared by the method of any one of claims 1-6 or the sodium pyrophosphate iron phosphate composite material of claim 7.

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