Preparation method of composite iron phosphate precursor with different ratios of phosphate radicals to pyrophosphate radicals

Abstract

The invention discloses a preparation method of composite iron phosphate precursors with different ratios of phosphate radicals to pyrophosphate radicals. The preparation method comprises the following steps: preparing a ferric iron solution, a phosphate solution, a pyrophosphate solution and a sodium hydroxide solution; adding the base solution into a reaction kettle, adding a ferric iron solution and a phosphate solution, and reacting to obtain slurry containing amorphous iron phosphate; carrying out centrifugal treatment on the slurry to obtain solid amorphous iron phosphate; adding a base solution into a reaction kettle, taking solid amorphous iron phosphate as a seed crystal, and putting the amorphous iron phosphate into the reaction kettle to serve as a composite iron phosphate precursor inner core; adding a ferric iron solution and a pyrophosphate solution for reaction to obtain amorphous composite iron phosphate precursor slurry taking iron pyrophosphate as an outer core; and carrying out solid-liquid separation, washing and drying on the prepared composite iron phosphate precursor slurry to obtain composite iron phosphate precursor powder. The composition and microstructure of the material can be accurately regulated and controlled, and the prepared composite sodium ferric phosphate positive electrode material has high specific capacity and excellent cycle stability.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a method for preparing a composite iron phosphate precursor with different phosphate to pyrophosphate ratios. Background Technology

[0002] Sodium-ion battery materials, as an important supplement to lithium-ion battery materials, have shown broad application prospects in various fields such as energy storage, start-stop power supplies, low-speed electric new energy vehicles, and special vehicles due to their advantages such as abundant sodium resources, long cycle life, high safety, low cost, and significant low-temperature performance. Currently, widely studied sodium-ion battery cathode materials mainly include layered / tunnel transition metal oxides, polyanionic compounds, Prussian blue analogs, and organic materials. However, these materials generally suffer from significant drawbacks: layered / tunnel transition metal oxides often experience drastic volume changes and complex phase transitions during charge and discharge, and are highly sensitive to air, severely restricting their commercial application; Prussian blue analogs generally suffer from low utilization of active materials, poor coulombic efficiency, and poor cycle stability, which are closely related to their inherent poor thermal stability and abundant lattice defects; organic compounds are limited by large molecular weight and low sodium content, resulting in lower theoretical capacity, while also facing challenges such as poor rate performance, slow reaction kinetics, and easy solubility.

[0003] In contrast, polyanionic compounds exhibit significant comprehensive advantages, including lower cost, higher structural stability, and smaller volume changes during charge and discharge, making them more suitable for the practical needs of large-scale energy storage systems. Among them, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) is particularly noteworthy as an iron-based polyanionic cathode material. It not only possesses a theoretical capacity as high as 129 mAh / g but also a capacitance of approximately 3.1 V (vs. Na2PO4). + It exhibits an operating voltage of PO4 / Na and also demonstrates an extremely low volume expansion rate (approximately 4%). Various synergistic modification strategies can be employed to further enhance its electrochemical performance, such as by adjusting the PO4 / Na content. 3- / P2O7 4- The proportions and preparation of iron-deficient phases work synergistically to optimize material properties.

[0004] Conventional synthesis methods for composite sodium iron phosphate cathode materials mainly include solid-phase and liquid-phase methods. Solid-phase methods are simpler and suitable for industrial production; liquid-phase methods are more complex, but yield products with superior performance. Currently, research on composite sodium iron phosphate cathode materials is still in its early stages, lacking standardized precursors. Because solid-phase methods struggle to achieve uniform solution, improvements in material performance are significantly limited.

[0005] Therefore, developing a precursor synthesis process suitable for this material has become an important research direction for the future, and the preparation of composite sodium iron phosphate precursor by co-precipitation method is particularly urgent. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] Methods for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios include:

[0009] Step 1: Prepare ferric iron solution, phosphate solution, pyrophosphate solution, and sodium hydroxide solution;

[0010] Step 2: Add a base liquid to the reaction vessel, wherein the base liquid is pure water;

[0011] A ferric iron solution and a phosphate solution were added dropwise to a reaction vessel to carry out a synthesis reaction. The pH of the reaction was adjusted by sodium hydroxide solution. After the ferric iron solution and the phosphate solution had reacted completely, a slurry containing amorphous FePO4 was obtained.

[0012] The conditions for the synthesis reaction include: stirring speed of 300-900 rpm, pH value of 1.5-2 during the reaction process, and reaction temperature of 70-90℃;

[0013] After the reaction was completed, the slurry containing amorphous FePO4 was centrifuged to obtain solid amorphous FePO4.

[0014] Step 3: Add a base liquid to the reaction vessel, wherein the base liquid is pure water;

[0015] Solid amorphous FePO4 was used as a seed crystal and added to the reactor to serve as the core of the composite iron phosphate precursor.

[0016] Subsequently, ferric iron solution and pyrophosphate solution were added dropwise to the reactor for synthesis reaction, and the pH of the reaction was adjusted by sodium hydroxide solution. After the ferric iron solution and pyrophosphate solution had reacted completely, an amorphous composite ferric phosphate precursor slurry with Fe4(P2O7)3 as the outer core was obtained.

[0017] The conditions for the synthesis reaction include: stirring speed of 300-900 rpm, pH value of 6-7 during the reaction process, and reaction temperature of 60-80℃;

[0018] Step 4: Perform solid-liquid separation, washing and drying on the prepared composite iron phosphate precursor slurry to obtain composite iron phosphate precursor powder.

[0019] In the above scheme, step three can be carried out in another reactor or in the original reactor after it has been cleaned.

[0020] Further technical solutions may include step five, which involves grinding, spraying, drying, and calcining the composite iron phosphate precursor powder with a sodium / carbon source to obtain a composite iron phosphate sodium cathode material.

[0021] In a further technical solution, in step four, the formula for the obtained composite iron phosphate precursor is (FePO4). x Fe4(P2O7)3, and 1≤x≤2; 0.33≤PO4 3- / P2O7 4- ≤0.67, 0.71≤Fe / P≤0.75;

[0022] The precursor has a core of FePO4 and an outer shell of Fe4(P2O7)3;

[0023] And the precursor meets the requirement of 1g / cm 3 <TD <1.5g / cm 3 5m 2 / g<SSA<10m 2 / g, the phase structure is amorphous.

[0024] In a further technical solution, in step one, the concentration of the ferric solution is 0.5-1.5 mol / L, the concentration of the phosphate solution is 0.5-1.5 mol / L, the concentration of the pyrophosphate solution is 0.1-0.3 mol / L, and the concentration of the sodium hydroxide solution is 1-4 mol / L.

[0025] In a further technical solution, in step one, the iron source in the ferric solution is at least one of ferric sulfate, ferric nitrate, ferric acetate, and ferric chloride;

[0026] The phosphate in the phosphate solution is at least one of phosphoric acid, monoammonium phosphate, diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, and trisodium phosphate.

[0027] The pyrophosphate in the pyrophosphate solution is at least one of H4P2O7, Na4P2O7, K4P2O7, and Na2H2P2O7.

[0028] A further technical solution involves, in step three, ensuring that the amount of solid amorphous FePO4, as well as the amounts of ferric iron solution and pyrophosphate solution, satisfy 0.33 ≤ PO4. 3- / P2O7 4- ≤0.67, and 0.71≤Fe / P≤0.75.

[0029] In a further technical solution, in step two, when synthesizing amorphous FePO4 solid, the molar ratio of phosphate ions to iron is (1-1.3):1.

[0030] In a further technical solution, in step three, when synthesizing the composite iron phosphate precursor, the molar ratio of pyrophosphate ions to iron is (1.5-2):1.

[0031] In a further technical solution, in step two, the primary particle size of the FePO4 particles is 100-500 nm.

[0032] In a further technical solution, in step four, the composite iron phosphate precursor has a spherical morphology and a primary particle size of 100-500 nm.

[0033] Furthermore, this invention also discloses the application of a high-performance composite iron phosphate precursor, which combines the different PO4 groups... 3- / P2O7 4- The method for preparing a composite iron phosphate precursor by ratio involves mixing the composite iron phosphate precursor with a sodium source and a carbon source, followed by sand milling, spraying, and sintering to obtain sodium iron phosphate cathode material.

[0034] A further technical solution is provided, wherein the sodium source is one of sodium carbonate, sodium acetate or sodium bicarbonate, and the carbon source is one of citric acid, sucrose or glucose, wherein the amount of carbon source added is 3-15 wt% of the amount of precursor added.

[0035] The terms “include,” “including,” and “have” used in this article are all open-ended, meaning they include but are not limited to.

[0036] Unless otherwise specified, the terms used herein generally have their ordinary meaning in the context of the art, the subject matter, and the specific context. Certain terms used to describe this case will be discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing this case.

[0037] The working principle and advantages of this invention are as follows:

[0038] This invention provides a method for preparing a composite iron phosphate precursor using a co-precipitation method, achieving a uniform mixing of iron and phosphorus elements at the atomic level. The method employs a stepwise synthesis strategy: first, iron phosphate (FePO4) is synthesized as the core under low pH conditions, and then used as a seed crystal in a reaction vessel; Fe4(P2O7)3 is further synthesized in a near-neutral environment at pH 6–7, allowing it to epitaxially grow on the iron phosphate surface. This neutral reaction environment significantly inhibits the hydrolysis of pyrophosphate, ensuring precise control over the composition.

[0039] By precisely controlling the synthesis ratio of FePO4 to Fe4(P2O7)3, this invention achieves the control of PO4 in the precursor. 3- / P2O7 4- The synergistic regulation of the molar ratio and the iron-to-phosphorus ratio (Fe / P) enables the directional preparation of iron-deficient composite iron phosphate materials. Furthermore, this method can control the size of the precursor primary particles within the range of 100–500 nanometers, which helps improve the milling efficiency of the final cathode material.

[0040] This invention effectively solves the problems of uneven element distribution and low compaction density in existing technologies, achieving precise control over the material composition and microstructure. The composite sodium iron phosphate cathode material prepared based on this precursor exhibits high specific capacity and excellent cycle stability in sodium-ion batteries. Attached Figure Description

[0041] Appendix Figure 1 This is a schematic diagram of the preparation process of the composite iron phosphate precursor according to an embodiment of the present invention;

[0042] Appendix Figure 2 This is a schematic diagram of the composite iron phosphate precursor structure according to an embodiment of the present invention;

[0043] Appendix Figure 3A SEM of the core iron phosphate of Example 2 of this invention Figure 1 ;

[0044] Appendix Figure 3B SEM of the core iron phosphate of Example 2 of this invention Figure 2 ;

[0045] Appendix Figure 4A SEM of the composite iron phosphate precursor in Example 2 of this invention Figure 1 ;

[0046] Appendix Figure 4B SEM of the composite iron phosphate precursor in Example 2 of this invention Figure 2 ;

[0047] Appendix Figure 5 SEM image of the composite sodium iron phosphate cathode material prepared in Example 2 of this invention;

[0048] Appendix Figure 6 The XRD patterns of the composite sodium iron phosphate cathode materials prepared in Example 2 and Comparative Example 1 of this invention are shown below.

[0049] Appendix Figure 7 The above are charge-discharge curves of the composite sodium iron phosphate cathode materials prepared in Example 2 and Comparative Example 1 of this invention. Detailed Implementation

[0050] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0051] The present invention will be clearly described below with illustrations and detailed description. Any person skilled in the art who understands the embodiments of the present invention can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0052] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of this work. Singular forms such as “a,” “this,” “this,” “the,” and “the” as used herein also include plural forms.

[0053] Example 1: Refer to Figure 1 The preparation process is as follows:

[0054] (1) Dissolve 3.73 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to prepare a 0.78 mol / L ferric solution; take 0.764 kg of 85% phosphoric acid and add an appropriate amount of water to prepare a 1 mol / L phosphoric acid solution. Add 10 L of pure water to the reactor, and simultaneously add the ferric solution and phosphoric acid solution. Control the pH of the reaction at 1.5 with sodium hydroxide solution, control the temperature at 80℃, and stir at 600 rpm. When the ferric solution and phosphoric acid solution are used up, the reaction ends, and ferric phosphate slurry is obtained. Perform solid-liquid separation on the prepared ferric phosphate slurry to obtain the ferric phosphate precursor.

[0055] (2) Dissolve 3.73 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to prepare a 0.78 mol / L ferric solution; dissolve 1.76 kg of anhydrous sodium pyrophosphate in an appropriate amount of water by stirring to prepare a 0.24 mol sodium pyrophosphate solution. Add 10 L of pure water to the reactor, then add the ferric phosphate precursor to the reactor, stir to 600 rpm, control the pH of the reaction to 6-7 with sodium hydroxide solution, and stop the reaction when the ferric solution and sodium pyrophosphate solution are used up, to obtain a composite ferric phosphate precursor slurry (FePO4)Fe4(P2O7)3. Separate the prepared composite ferric phosphate precursor slurry into solid and liquid components, wash, and dry at 180 degrees for 12 h to obtain a composite ferric phosphate precursor powder (refer to Figure 2 ).

[0056] (3) Dissolve 0.1 mol of (FePO4)Fe4(P2O7)3, 49.3 g of sodium carbonate, and 21.5 g of citric acid in an appropriate amount of deionized water, and then sonicate the solution. The solution is then spray-dried at a low injection rate to obtain a spherical powder intermediate. The intermediate is first heat-treated at 300℃ for 5 hours in a N2 atmosphere, and then annealed at 450-500℃ for 10 hours to finally obtain Na4Fe 2.856 (PO4)2(P2O7) / C.

[0057] Example 2: As Figures 3A-5 As shown,

[0058] (1) Dissolve 4.66 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric solution; take 0.955 kg of 85% phosphoric acid and add an appropriate amount of water to prepare a 1 mol / L phosphoric acid solution. Add 10 L of pure water to the reactor, and simultaneously add the ferric solution and phosphoric acid solution. Control the pH of the reaction at 1.5 with sodium hydroxide solution, control the temperature at 80℃, and stir at 600 rpm. When the ferric solution and phosphoric acid solution are used up, the reaction ends, and ferric phosphate slurry is obtained. Perform solid-liquid separation on the prepared ferric phosphate slurry to obtain the ferric phosphate precursor.

[0059] (2) Dissolve 3.73 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric iron solution; dissolve 1.76 kg of anhydrous sodium pyrophosphate in an appropriate amount of water by stirring to obtain a 0.24 mol sodium pyrophosphate solution. Add 10 L of pure water to the reactor, then add the ferric phosphate precursor to the reactor, stir to 600 rpm, and control the reaction pH to 6-7 with sodium hydroxide solution. When the ferric iron solution and sodium pyrophosphate solution are used up, the reaction ends, and a composite ferric phosphate precursor slurry (FePO4) is obtained. 1.25 Fe4(P2O7)3. The prepared composite iron phosphate precursor slurry was subjected to solid-liquid separation, washed, and dried at 180 degrees for 12 h to obtain composite iron phosphate precursor powder.

[0060] (3) Add 0.1 mol of (FePO4) 1.25 Fe4(P2O7)3, 51.2 g of sodium carbonate, and 22.4 g of citric acid were dissolved in an appropriate amount of deionized water and subjected to ultrasonic treatment to form a solution. The solution was then spray-dried at a low injection rate to obtain a spherical powder intermediate. This intermediate was first heat-treated at 300℃ for 5 hours in a N2 atmosphere, followed by annealing at 450-500℃ for 10 hours to finally obtain Na4Fe 2.896 (PO4)2(P2O7) / C.

[0061] Example 3:

[0062] (1) 5.60 kg of ferric sulfate nonahydrate was dissolved in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric solution; 0.115 kg of 85% phosphoric acid was added to an appropriate amount of water to obtain a 1 mol / L phosphoric acid solution. 10 L of pure water was added to the reactor, and the ferric solution and phosphoric acid solution were added simultaneously. The pH of the reaction was controlled at 1.5 with sodium hydroxide solution, the temperature was controlled at 80℃, and the stirring speed was 600 rpm. When the ferric solution and phosphoric acid solution were used up, the reaction ended, and ferric phosphate slurry was obtained. The prepared ferric phosphate slurry was subjected to solid-liquid separation to obtain the ferric phosphate precursor.

[0063] (2) Dissolve 3.73 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric iron solution; dissolve 1.76 kg of anhydrous sodium pyrophosphate in an appropriate amount of water by stirring to obtain a 0.24 mol sodium pyrophosphate solution. Add 10 L of pure water to the reactor, then add the ferric phosphate precursor to the reactor, stir to 600 rpm, and control the reaction pH to 6-7 with sodium hydroxide solution. When the ferric iron solution and sodium pyrophosphate solution are used up, the reaction ends, and a composite ferric phosphate precursor slurry (FePO4) is obtained. 1.5 Fe4(P2O7)3. The prepared composite iron phosphate precursor slurry was subjected to solid-liquid separation, washed, and dried at 180 degrees for 12 h to obtain composite iron phosphate precursor powder.

[0064] (3) Add 0.1 mol of (FePO4) 1.5 Fe4(P2O7)3, 53 g of sodium carbonate, and 23.2 g of citric acid were dissolved in an appropriate amount of deionized water and subjected to ultrasonic treatment to form a solution. The solution was then spray-dried at a low injection rate to obtain a spherical powder intermediate. This intermediate was first heat-treated at 300℃ for 5 hours in a N2 atmosphere, followed by annealing at 500℃ for 10 hours to finally obtain Na4Fe 2.932 (PO4)2(P2O7) / C.

[0065] Example 4:

[0066] (1) Dissolve 7.46 kg of ferric sulfate nonahydrate in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric solution; take 1.53 kg of 85% phosphoric acid and add an appropriate amount of water to prepare a 1 mol / L phosphoric acid solution. Add 10 L of pure water to the reactor, and simultaneously add the ferric solution and phosphoric acid solution. Control the pH of the reaction at 1.5 with sodium hydroxide solution, control the temperature at 80℃, and stir at 600 rpm. When the ferric solution and phosphoric acid solution are used up, the reaction ends, and ferric phosphate slurry is obtained. Perform solid-liquid separation on the prepared ferric phosphate slurry to obtain the ferric phosphate precursor.

[0067] (2) 3.73 kg of ferric sulfate nonahydrate was dissolved in an appropriate amount of water by stirring to obtain a 0.78 mol / L ferric solution; 1.76 kg of anhydrous sodium pyrophosphate was dissolved in an appropriate amount of water by stirring to obtain a 0.24 mol sodium pyrophosphate solution. 10 L of pure water was added to the reactor, and then the ferric phosphate precursor was added to the reactor. The mixture was stirred to 600 rpm, and the pH of the reaction was controlled to 6-7 with sodium hydroxide solution. After the ferric solution and sodium pyrophosphate solution were used up, a composite ferric phosphate precursor slurry (FePO4)2Fe4(P2O7)3 was obtained. The prepared composite ferric phosphate precursor slurry was subjected to solid-liquid separation, washed, and dried at 180 degrees for 12 h to obtain composite ferric phosphate precursor powder.

[0068] (3) Dissolve 0.1 mol of (FePO4)2Fe4(P2O7)3, 56 g of sodium carbonate, and 25 g of citric acid in an appropriate amount of deionized water and sonicate to form a solution. Then, spray dry the solution at a low injection rate to obtain a spherical powder intermediate. The intermediate is first heat-treated at 300℃ for 5 hours in a N2 atmosphere, and then annealed at 450-500℃ for 10 hours to finally obtain Na4Fe3(PO4)2(P2O7) / C.

[0069] Comparative Example 1:

[0070] 0.1 mol of FePO4, 202 g of Fe(NO3)3·9H2O, 99.7 g of Na4P2O7, 49 g of sodium carbonate, and 26.3 g of citric acid were dissolved in an appropriate amount of deionized water and subjected to ultrasonic treatment to form a solution. The solution was then spray-dried at a low injection rate to obtain a spherical powder intermediate. This intermediate was first heat-treated at 300°C for 5 hours in a N2 atmosphere, and then annealed at 500°C for 10 hours to finally obtain Na4Fe 2.896 (PO4)2(P2O7) / C.

[0071] Table 1. Indicators of composite iron phosphate precursors obtained from different embodiments

[0072]

[0073] Table 2 Electrochemical data of Examples 1-4 and Comparative Example 1

[0074]

[0075] Table 1 shows the various properties of the composite iron phosphate precursors prepared in different embodiments. The data indicate that the physical properties (such as particle size, tap density, and specific surface area) of the precursors in the four embodiments are generally similar, with the main differences lying in the chemical properties. The morphology of the precursor is as follows: Figure 4A , Figure 4BAs shown, the primary particles are spherical in shape, with a size between 100 and 500 nanometers. The shorter diffusion path helps improve sodium ion migration efficiency, thereby enhancing the battery's kinetic performance. Furthermore, the secondary particles formed by the uniform stacking of primary particles have a narrower particle size distribution, further ensuring a high tap density and stability and consistency during electrode fabrication.

[0076] Combine Table 2 and Figure 7 The test results show that the composite sodium iron phosphate cathode material prepared by the precursor method exhibits superior electrochemical performance compared to the material prepared by the solid-state method. Furthermore, the material prepared by the co-precipitation precursor method also demonstrates higher phase purity than the material obtained by the solid-state method. Figure 6 As shown, Comparative Example 1 exhibited a small number of NaFePO4 impurity peaks in the range of 32.5–33.5°. These results indicate that the co-precipitation method for synthesizing composite iron phosphate precursors can achieve uniform mixing of iron and phosphorus at the atomic level, thereby reducing the formation of the impure phase NaFePO4 and improving the electrochemical performance of the cathode material. Furthermore, comparisons of Examples 1–4 reveal that the material exhibits optimal electrochemical performance when the Fe / P ratio is 0.724, suggesting that the electrochemical performance can be improved by adjusting the PO4 ratio. 3- / P2O7 4- The ratio further optimizes the electrochemical performance of the composite sodium iron phosphate cathode.

[0077] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios, characterized in that: include: Step 1: Prepare ferric iron solution, phosphate solution, pyrophosphate solution, and sodium hydroxide solution; Step 2: Add a base liquid to the reaction vessel, wherein the base liquid is pure water; A ferric iron solution and a phosphate solution were added dropwise to a reaction vessel to carry out the synthesis reaction. The pH of the reaction was adjusted by sodium hydroxide solution during the process. After the ferric solution and phosphate solution have reacted completely, a slurry containing amorphous FePO4 is obtained. The conditions for the synthesis reaction include: stirring speed of 300-900 rpm, pH value of 1.5-2 during the reaction process, and reaction temperature of 70-90℃; After the reaction was completed, the slurry containing amorphous FePO4 was centrifuged to obtain solid amorphous FePO4. Step 3: Add a base liquid to the reaction vessel, wherein the base liquid is pure water; Solid amorphous FePO4 was used as a seed crystal and added to the reactor to serve as the core of the composite iron phosphate precursor. Subsequently, ferric iron solution and pyrophosphate solution were added dropwise to the reactor for synthesis reaction, and the pH of the reaction was adjusted by sodium hydroxide solution. After the ferric iron solution and pyrophosphate solution had reacted completely, an amorphous composite ferric phosphate precursor slurry with Fe4(P2O7)3 as the outer core was obtained. The conditions for the synthesis reaction include: stirring speed of 300-900 rpm, pH value of 6-7 during the reaction process, and reaction temperature of 60-80℃; Step 4: Perform solid-liquid separation, washing and drying on the prepared composite iron phosphate precursor slurry to obtain composite iron phosphate precursor powder.

2. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step four, the formula for the obtained composite iron phosphate precursor is (FePO4). x Fe4(P2O7)3, and 1≤x≤2; 0.33≤PO4 3- / P2O7 4- ≤0.67, 0.71≤Fe / P≤0.75; The precursor has a core of FePO4 and an outer shell of Fe4(P2O7)3; And the precursor meets the requirement of 1g / cm 3 <TD <1.5g / cm 3 5m 2 / g<SSA<10m 2 / g, the phase structure is amorphous.

3. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step one, the concentration of the ferric iron solution is 0.5-1.5 mol / L, the concentration of the phosphate solution is 0.5-1.5 mol / L, the concentration of the pyrophosphate solution is 0.1-0.3 mol / L, and the concentration of the sodium hydroxide solution is 1-4 mol / L.

4. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step one, the iron source in the ferric solution is at least one of ferric sulfate, ferric nitrate, ferric acetate, and ferric chloride; The phosphate in the phosphate solution is at least one of phosphoric acid, monoammonium phosphate, diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, and trisodium phosphate. The pyrophosphate in the pyrophosphate solution is at least one of H4P2O7, Na4P2O7, K4P2O7, and Na2H2P2O7.

5. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step three, the amount of solid amorphous FePO4 added, as well as the amounts of ferric iron solution and pyrophosphate solution, must satisfy 0.33 ≤ PO4. 3- / P2O7 4- ≤0.67, and 0.71≤Fe / P≤0.

75.

6. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step two, when synthesizing amorphous FePO4 solid, the molar ratio of phosphate ions to iron is (1-1.3):

1.

7. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step three, when synthesizing the composite iron phosphate precursor, the molar ratio of pyrophosphate ions to iron is (1.5-2):

1.

8. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step two, the primary particle size of the FePO4 particles is 100-500 nm.

9. The method for preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios according to claim 1, characterized in that: In step four, the composite iron phosphate precursor has a spherical morphology and a primary particle size of 100-500 nm.

10. An application of a high-performance composite iron phosphate precursor, characterized in that: The composite iron phosphate precursor prepared by the method of preparing composite iron phosphate precursors with different phosphate to pyrophosphate ratios as described in any one of claims 1 to 9 is mixed with a sodium source and a carbon source, and then the sodium iron pyrophosphate cathode material is obtained by sand milling, spraying and sintering.

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