Sodium iron pyrophosphate phosphate and a preparation method and application thereof

Abstract

The application discloses sodium iron pyrophosphate phosphate and a preparation method and application thereof, and the preparation method comprises the following steps: S1, mixing a phosphorus source and an iron source, adding the mixture into pure water and stirring to obtain a pretreatment solution; S2, adding a sodium source into the pretreatment solution to obtain a high-pH solution; S3, adding first phosphoric acid into the high-pH solution, then adding hydrogen peroxide, and then adding second phosphoric acid to obtain a pre-synthesis solution; S4, performing heat treatment on the pre-synthesis solution, and performing filtration and drying to obtain a sodium iron pyrophosphate phosphate precursor; and S5, sintering the sodium iron pyrophosphate phosphate precursor after mixing the sodium iron pyrophosphate phosphate precursor with a carbon source to obtain the sodium iron pyrophosphate phosphate. According to the application, a high-pH protection environment is constructed by adding sodium after phosphorus, phosphoric acid is added twice and is interlaced with oxidation operation, iron-phosphorus ratio is calibrated by adding phosphorus again after oxidation, and the problems of uneven precipitation, out-of-control iron-phosphorus dissolution ratio and easy generation of iron hydroxide impurities are solved in a one-pot reactor, so that the problems of deviation of stoichiometric ratio of a product and unstable electrochemical performance are solved.

Description

Technical Field

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

[0002] Sodium-ion batteries, with their abundant sodium resources, outstanding safety performance, and significant cost control potential, have become a key strategic supplement to lithium-ion batteries. Among them, polyanionic materials stand out due to their unique structural advantages, becoming a core choice for mainstream applications such as energy storage. By introducing polyanionic groups such as phosphate and sulfate, a stable three-dimensional covalent bond framework is constructed. The volume change during the insertion / extraction process is small, with almost no lattice stress or microcracks, resulting in excellent stability and ultra-long cycle life. Simultaneously, their voltage platform and energy density performance are balanced, enabling stable operation over a wide temperature range.

[0003] Among numerous polyanionic cathode materials, sodium iron pyrophosphate (SOP) stands out due to its lower cost, more stable charge-discharge performance, and strong market competitiveness. The mainstream approach for SOP utilizes iron phosphate as a raw material, achieving a grinding effect by controlling the slurry particle size. However, this approach is heavily influenced by iron phosphate, resulting in lower compaction and limitations in the process. Preparing SOP precursors can effectively overcome the material's performance limitations. Precise control of raw material ratios can also significantly improve the compaction and electrochemical performance of the finished product, making it an advanced preparation process that the market is currently exploring.

[0004] However, current methods for synthesizing sodium iron pyrophosphate suffer from uneven precipitation, uncontrolled iron-phosphorus dissolution ratio, and easy formation of ferric hydroxide impurities, leading to deviations in the stoichiometry of the product and unstable electrochemical performance. Therefore, a further technical solution is needed to address these issues. Summary of the Invention

[0005] In view of this, this application provides a sodium ferric pyrophosphate, its preparation method and application, to solve the problems of uneven precipitation, uncontrolled iron-phosphorus dissolution ratio and easy formation of ferric hydroxide impurity phase in the one-step wet synthesis of sodium ferric pyrophosphate, which lead to deviation of the stoichiometric ratio of the product and unstable electrochemical performance.

[0006] To achieve the above technical objectives, this application adopts the following technical solution: In a first aspect, this application provides a method for preparing sodium iron pyrophosphate, comprising the following steps: S1. Mix the phosphorus source and iron source solutions, add them to the pure water base solution and stir to obtain the pretreatment solution; S2. Add a sodium source to the pretreatment solution to obtain a high pH solution; S3. Add the first phosphoric acid to the high pH solution, then add hydrogen peroxide, and then add the second phosphoric acid to obtain the pre-synthesized solution; S4. The pre-synthesized solution was heat-treated, filtered and dried to obtain sodium iron pyrophosphate precursor; S5. After mixing the sodium iron pyrophosphate precursor with a carbon source, sintering is performed to obtain sodium iron pyrophosphate.

[0007] Preferably, in step S1, the phosphorus source includes monoammonium phosphate solution and / or diammonium phosphate solution; the iron source includes one or more of ferrous sulfate solution and ferrous ammonium sulfate solution; the molar ratio of phosphorus source to iron source is 3.2-3.6:3; and the phosphorus content in the phosphorus source is 4.5-6 wt%.

[0008] Preferably, in step S2, the molar ratio of sodium source to phosphorus source is 4:3.2-3.6.

[0009] Preferably, in step S3, the molar ratio of the first phosphoric acid to the iron source is 0.2-0.6:3, and the concentration of the first phosphoric acid is 80-85 wt%.

[0010] Preferably, in step S3, the hydrogen peroxide is 60-70% of the molar mass of the iron source; the concentration of the hydrogen peroxide is 26-28 wt%.

[0011] Preferably, in step S3, the molar ratio of the second phosphoric acid to the iron source is 0.2:3, and the concentration of the second phosphoric acid is 80-85 wt%.

[0012] Preferably, in step S3, the pH value of the pre-synthesized solution is 1.5-2.5.

[0013] Preferably, in step S5, the sintering procedure is as follows: under inert gas protection, the temperature is increased to 560-600℃ at a rate of 3-5℃ / min, and then held at that temperature for 9-12 h.

[0014] Secondly, this application provides a sodium iron pyrophosphate.

[0015] Thirdly, this application provides a cathode material, including sodium iron pyrophosphate. The beneficial effects of this application are as follows: This invention provides a process for synthesizing precursors by one-step co-precipitation of iron, sodium, and phosphorus sources, which greatly reduces the number of steps in precursor preparation and simplifies the process. This invention enables precise control of solution pH, effectively preventing the formation of ferric hydroxide impurities during the process, and ensuring the effective synthesis of the coprecipitation reaction with the subsequent addition of phosphoric acid. This invention adds phosphoric acid to the reaction system in two stages, which can effectively control the dissolution ratio of iron and phosphorus elements in the reaction system and achieve precise control of the iron-phosphorus ratio of the finished product to meet the stoichiometric ratio of sodium iron pyrophosphate. The technical solution provided by this invention is simple, low-cost, and stable, and is suitable for large-scale production. This method is also applicable to the chemical synthesis of other sodium electrode materials. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0017] Figure 1 The image shows the XRD pattern of a sodium iron pyrophosphate sodium cathode material provided in Example 1.

[0018] Figure 2 This is a SEM image of a sodium iron pyrophosphate sodium cathode material provided in Example 1.

[0019] Figure 3 The charge-discharge curves of the sodium iron pyrophosphate sodium cathode materials provided in Examples 1-5 at a rate of 0.1C.

[0020] Figure 4 The charge-discharge curves of the sodium iron pyrophosphate sodium cathode materials provided in Examples 6-8 at a rate of 0.1C. Detailed Implementation

[0021] The embodiments of the technical solution of this application will be described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0022] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0023] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0025] Unless otherwise explicitly defined and specified herein, all technical and scientific terms used in this application shall have the generally accepted meanings understood by one of ordinary skill in the field of chemical and chemical materials technology (including but not limited to polymer chemistry, inorganic chemistry, organic synthesis, catalysis chemistry, materials processing, and chemical unit operations) based on their professional knowledge and conventional practice. The use of any terminology herein is intended to describe the specific embodiments of this application in the clearest and most accurate manner, so as to fully disclose the technical solution. Such use shall not in any way be construed as a limitation on the scope of the claims, nor does it imply the exclusion of equivalent technical solutions that could be reasonably known by one of skill in the art based on the concept of this application.

[0026] The terms "comprising," "including," "having," "containing," and any grammatical variations or similar expressions used in the specification and claims of this application are all open-ended and non-exhaustive descriptive terms. Their purpose is to explicitly describe the existence of technical features, components, steps, or parts, while explicitly allowing and covering the possibility that other features, components, steps, parts, or any combinations thereof not explicitly listed may exist or be added to the technical solution, as long as such additions do not destroy the integrity and inventiveness of the original technical solution.

[0027] When the terms "embodiments," "some embodiments," or "specific embodiments" are mentioned in the specification, they refer to examples that, in conjunction with the specific parameters, materials, steps, and results described in that section, constitute one or a group of examples for implementing the technical solutions of this application. These embodiments are used for full disclosure and illustrative purposes, not for exhaustive enumeration. Those skilled in the art should understand that, without departing from the overall inventive concept of this application, the various technical features disclosed in different embodiments can be combined, substituted, modified, or deleted to form other implementation methods that are not listed one by one in the specification but also fall within the protection scope of this application.

[0028] Unless otherwise expressly specified and limited, all terms related to chemical process operations, material preparation, processing and analytical testing involved in this application shall be interpreted in the broadest sense based on the conventional understanding of those skilled in the art.

[0029] Regarding performance testing and structural characterization, all testing and characterization methods involved in this application, unless otherwise specified, refer to conventional methods known in the art. Specific testing conditions may be selected and adjusted according to the sample properties and relevant national standards, international standards, or industry-standard methods. Test items may include mechanical properties (such as tensile, bending, and impact strength), thermal properties (such as DSC and TGA analysis), and chemical stability (such as solvent resistance and acid / alkali corrosion resistance). Structural characterization methods may include FT-IR, NMR, XRD, SEM, TEM, and BET. All test results should be understood to be within the allowable range of conventional experimental errors.

[0030] Regarding numerical values ​​and ranges, all parameter ranges expressed in this application in the form of "from a certain value to a certain value" should be understood as explicitly disclosing the endpoints of the range, each specific numerical point between the endpoints, and all sub-ranges formed by any two numerical points within the range. For example, "30℃ to 80℃" discloses 30, 31, ..., 80℃, as well as sub-ranges such as 30-50℃, 45-70℃, etc. When a numerical value is preceded by "about," "approximately," or similar words, it indicates that the numerical value is allowed to have reasonable errors recognized in the art under the measurement or control conditions, which can generally be understood as the deviation allowed by relevant standards or a normal fluctuation range of ±5% or ±10%.

[0031] This application provides a method for preparing sodium iron pyrophosphate, comprising the following steps: S1. Mix the phosphorus source and iron source solutions in a certain proportion, add them to the pure water base solution and stir to obtain the pretreatment solution; S2. Add a sodium source to the pretreatment solution to obtain a high pH solution; S3. Add the first phosphoric acid to the high pH solution, then add hydrogen peroxide, and then add the second phosphoric acid to obtain the pre-synthesized solution; S4. The pre-synthesized solution was heat-treated, filtered and dried to obtain sodium iron pyrophosphate precursor; S5. After mixing the sodium iron pyrophosphate precursor with a carbon source, sintering is performed to obtain sodium iron pyrophosphate.

[0032] The technical problem this application aims to solve is the deviation of the stoichiometric ratio and instability of electrochemical performance in the one-step wet synthesis of sodium iron pyrophosphate due to uneven precipitation, uncontrolled iron-phosphorus dissolution ratio, and easy formation of ferric hydroxide impurities. The causes of this problem are as follows: iron ions readily hydrolyze to form ferric hydroxide colloidal precipitate at pH values ​​above 2-3. In traditional wet synthesis, if all raw materials are mixed at once or in an improper order, the pH of the solution fluctuates drastically, causing iron to preferentially hydrolyze and fail to effectively combine with phosphate ions. Simultaneously, the dissolution ratio of iron and phosphorus is highly sensitive to the pH of the reaction system; a single uncontrollable pH change will cause the actual iron-phosphorus molar ratio to deviate from the required 3:4 stoichiometric ratio of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O4). Furthermore, if the sodium source is added later, sodium ions cannot participate in the co-precipitation lattice construction throughout the process, resulting in uneven sodium distribution in the precursor and affecting charge balance.

[0033] To address the aforementioned technical problems, this application proposes an improved one-step wet synthesis process, the specific steps of which and their mechanisms of action are as follows: Step S1 ensures that iron ions and phosphate ions form a homogeneous soluble complex or mixed system before the addition of an alkaline source, preventing the iron source from directly contacting the subsequently added alkaline solution and undergoing instantaneous hydrolysis; In Step S2, although it is generally accepted that a highly alkaline environment will lead to the hydrolysis of iron ions, in Step S2, since the phosphorus source is already present in the solution, phosphate ions have a strong coordination stabilizing effect on iron ions, which can generate a soluble ferrous phosphate complex, thereby effectively inhibiting the formation of the ferric hydroxide impurity phase. At the same time, the early introduction of the sodium source ensures that sodium elements are uniformly dispersed in the reaction system from the beginning, providing a sodium environment for the subsequent formation of pure-phase sodium ferric pyrophosphate; In Step S3, firstly, primary phosphate is added to slowly neutralize the high pH solution to a weakly acidic or near-neutral range. At this time, iron still exists in the divalent form, forming a homogeneous ferrous phosphate precursor complex with phosphate ions. Then, hydrogen peroxide is added to oxidize the divalent iron to the trivalent iron. During the oxidation process... While it consumes hydrogen ions, the binding ability of ferric iron to phosphate is much stronger than that of ferrous iron, enabling the rapid formation of dense ferric phosphate or ferric pyrophosphate precipitates. This avoids the hydrolysis of ferric iron alone to generate ferric hydroxide. After all the iron source is oxidized, the second phosphoric acid is added last. At this point, all the iron in the solution exists in the ferric form. The purpose of adding the remaining phosphoric acid is to precisely adjust the iron-phosphorus dissolution ratio and pH, ensuring that the iron-phosphorus molar ratio in the final precursor strictly conforms to the stoichiometric ratio of Na4Fe3(PO4)2P2O7. By adding phosphoric acid in two stages and oxidizing it with hydrogen peroxide dropwise, the dissolution ratio of iron and phosphorus elements in the reaction system can be effectively controlled, achieving precise control of the iron-phosphorus ratio in the finished product. In step S4, the obtained pre-synthesized solution is heat-treated to promote precipitation crystallization, then filtered and dried to obtain a homogeneous sodium iron pyrophosphate precursor without impurities. In step S5, the carbon source is pyrolyzed to form amorphous carbon coating on the surface of the material particles, significantly improving electronic conductivity. At the same time, the optimized temperature range can maintain the structural stability of sodium iron pyrophosphate and prevent excessive particle growth. In summary, this application constructs a high-pH protective environment by first adding phosphorus and then sodium, and then uses phosphoric acid added in two stages and interspersed with the oxidation operation. After oxidation, phosphorus is added again to calibrate the iron-phosphorus ratio. In a one-pot reactor, this solves the problems of uneven precipitation, uncontrolled iron-phosphorus dissolution ratio, and easy formation of ferric hydroxide impurity phase, which lead to deviation of product stoichiometry and unstable electrochemical performance.

[0034] In some embodiments, in step S1, the phosphorus source includes monoammonium phosphate solution and / or diammonium phosphate solution; the iron source includes one or more of ferrous sulfate solution and ferrous ammonium sulfate solution; the molar ratio of phosphorus source to iron source is 3.2-3.6:3; and the phosphorus content in the phosphorus source is 4.5-6 wt%.

[0035] In this embodiment, the lower the phosphorus content of the phosphorus source, the worse the electrochemical performance of the resulting sodium ferric pyrophosphate product.

[0036] In some embodiments, in step S1, the dropping rate of the iron source and phosphorus source solutions is 50-70 mL / min; wherein the pure water bottom solution accounts for 30-40% of the mass of the iron source solution; and the solution stirring speed is 800-1000 rpm. In some embodiments, in step S2, the molar ratio of sodium source to phosphorus source is 4:3.2-3.6.

[0037] In this embodiment, the sodium source concentration plays an important role in the synthesis reaction and the dissolution of iron and phosphorus, affecting the overall pH of the solution, which in turn changes the dissolution of iron and phosphorus and affects the formation of the reaction precipitate.

[0038] In some embodiments, the sodium source includes one or more of sodium hydroxide solution and / or sodium carbonate solution; the concentration of the sodium source is 3-5 wt%; the amount of sodium source added meets the molar ratio of phosphorus source and iron source, sodium source: iron source: phosphorus source = 4:3:3.2-3.6.

[0039] In some embodiments, in step S3, the molar ratio of the first phosphoric acid to the iron source is 0.2-0.6:3, and the concentration of the first phosphoric acid is 80-85 wt%.

[0040] In some embodiments, in step S3, the hydrogen peroxide is 60-70% of the molar mass of the iron source; the concentration of the hydrogen peroxide is 26-28 wt%.

[0041] In some embodiments, in step S3, the molar ratio of the second phosphoric acid to the iron source is 0.2:3, and the concentration of the second phosphoric acid is 80-85 wt%.

[0042] In this application, the pre-synthetic solution exhibits different synthesis pH values ​​under different phosphoric acid addition ratios, leading to changes in the electrochemical performance of the finished product. The molar ratio of the first phosphoric acid to the iron source is 0.2-0.6:3, which has the advantage of effectively adjusting the initial iron-phosphorus ratio to control the morphology of the finished product. The molar ratio of the second phosphoric acid to the iron source is 0.2:3, which has the advantage of reducing the solution pH by adding trace amounts of phosphoric acid, thus preventing precipitation during the oxidation process.

[0043] In some embodiments, after adding the second phosphoric acid in step S3, the pH of the pre-synthesized solution is 1.5-2.5.

[0044] In some embodiments, in step S4, the pre-synthesized solution is heated to 60-70°C, kept at that temperature for 4-6 hours, and then filtered and dried at 80-90°C to obtain sodium iron pyrophosphate precursor. In some embodiments, the sintering procedure in step S5 is as follows: under inert gas protection, the temperature is increased to 560-600°C at a rate of 3-5°C / min and then held at that temperature for 9-12 h.

[0045] This application provides a sodium iron pyrophosphate.

[0046] This application provides a sodium electrode material, including sodium iron pyrophosphate. The following specific embodiments further illustrate this solution.

[0047] Example 1 A method for preparing sodium iron pyrophosphate includes the following steps: S1. Weigh 465 g of FeSO4 solution with an iron content of 6.0% and 300 g of monoammonium phosphate solution with a phosphorus content of 5.5% and add them together to 300 ml of pure water at a rate of 50 mL / min, ensuring that the molar ratio of phosphorus source to iron source is 3.2:3. Stir at a speed of 800 rpm. After all the solution has been added, the pretreatment solution is obtained. S2. Weigh 50 g of a 4% sodium hydroxide solution with a sodium source to phosphorus source molar ratio of 4:3.2 and slowly add it to the pretreatment solution to obtain a high pH solution with a pH of 5.5. S3. Add 11.5g of 85wt% primary phosphoric acid solution to a high pH solution, with a molar ratio of primary phosphoric acid to iron source of 0.6:3. Stir for 10 min, then add 45.8g of hydrogen peroxide (26% hydrogen peroxide content). After all the ferrous iron in the solution is oxidized (tested by the acidic potassium permanganate method, no color fading), stir for 20 min. Then slowly add 3.8g of 85wt% secondary phosphoric acid, with a molar ratio of secondary phosphoric acid to iron source of 0.2:3, to obtain the pre-synthesized solution with a pH of 2.2. S4. Heat the pre-synthesized solution to 70℃ and keep it at that temperature for 6 hours. Then, filter and dry the resulting solution to obtain a filter cake. Place the filter cake in a forced-air drying oven and keep it at 90℃ to obtain the dried filter cake, which is the sodium iron pyrophosphate precursor. S5. The sodium ferric pyrophosphate precursor was mixed with 10g of carbon source and placed in a tube furnace. Under an inert atmosphere, the temperature was increased to 560℃ at a rate of 3℃ / min and held for 12 h to synthesize sodium ferric pyrophosphate. Its XRD pattern is shown below. Figure 1 As shown, the SEM image is as follows: Figure 2 As shown.

[0048] Example 2 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that in step S1, the mass of monoammonium phosphate solution added is 319g to ensure that the molar ratio of phosphorus source to iron source is 3.4:3, in step S3, the amount of first phosphoric acid added is 7.68g to ensure that the molar ratio of first phosphoric acid to iron source is 0.4:3, and the amount of second phosphoric acid added remains unchanged.

[0049] Example 3 A method for preparing sodium iron pyrophosphate is the same as that in Example 1, except that in step S1, the mass of monoammonium phosphate solution added is 337g to ensure that the molar ratio of phosphorus source to iron source is 3.6:3, in step S3, the amount of first phosphoric acid added is 3.84g to ensure that the molar ratio of first phosphoric acid to iron source is 0.2:3, and the amount of second phosphoric acid added remains unchanged.

[0050] Example 4 A method for preparing sodium iron pyrophosphate is the same as that in Example 1, except that in step S2, a sodium hydroxide solution with a concentration of 3.5% is added, the molar ratio of sodium source to phosphorus source is 4:3.2, and the amount added is still 50g.

[0051] Example 5 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that in step S2, a sodium hydroxide solution with a concentration of 3% is added, the molar ratio of sodium source to phosphorus source is 4:3.2, and the amount added is still 50g.

[0052] Example 6 A method for preparing sodium iron pyrophosphate is the same as that in Example 1, except that the concentration of monoammonium phosphate solution added in step S1 is 4 wt%, corresponding to a solution mass of 412 g.

[0053] Example 7 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that the concentration of monoammonium phosphate solution added in step S1 is 3.5 wt%, corresponding to a solution mass of 470 g.

[0054] Example 8 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that the concentration of monoammonium phosphate solution added in step S1 is 3wt%, corresponding to a solution mass of 550g.

[0055] Comparative Example 1 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that hydrogen peroxide is added after the first phosphoric acid and the second phosphoric acid are added simultaneously.

[0056] Comparative Example 2 A method for preparing sodium iron pyrophosphate is the same as in Example 1, except that after adding hydrogen peroxide, first phosphoric acid and second phosphoric acid are added simultaneously.

[0057] Testing and Evaluation Button batteries were prepared using sodium iron pyrophosphate as the positive electrode material in different embodiments: Positive electrode material, carbon black (Super P), and polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 90:5:5. These three materials were dispersed in N-methylpyrrolidone (NMP) and stirred at high speed for 3 hours to form a conductive slurry. This slurry was then uniformly coated onto aluminum foil and vacuum dried to obtain an electrode sheet with an areal density of 2.0 mg / cm³. 2 The negative electrode is made of metallic sodium. The positive electrode, negative electrode, separator and electrolyte are assembled into a sodium-ion battery. Then the electrochemical performance is tested and the results are shown in Table 1.

[0058] Table 1 Test Results

[0059] Figure 3 The charge-discharge curves of the sodium iron pyrophosphate sodium cathode materials obtained in Examples 1-5 at a rate of 0.1C are shown. Figure 4 The charge-discharge curves of the sodium iron pyrophosphate sodium cathode materials provided in Examples 6-8 at a rate of 0.1C.

[0060] Data from examples and comparative studies show that different phosphorus source ratios result in varying synthesis pH values ​​in the mixed solution, leading to changes in the electrochemical performance of the final product. This indicates that pH and sodium hydroxide mass play crucial roles in the one-step synthesis reaction and the dissolution of iron and phosphorus. Furthermore, lower phosphorus content in the monoammonium phosphate solution is detrimental to the overall solution synthesis, ultimately resulting in poorer electrochemical performance of the generated sodium iron pyrophosphate product. This application proposes a more effective and feasible technical solution to improve the electrochemical performance of cathode materials, enhance process stability, and precisely control the iron-phosphorus ratio. In this application, the phosphorus source is added in three stages. The first stage involves the addition of the main phosphorus source, aiming to provide initial reaction conditions for uniform mixing of sodium, iron, and phosphorus sources. This stage influences the morphology of the final product and offers considerable adjustability. The second stage involves the addition of phosphoric acid primarily to lower the overall reaction pH and prevent impurities during oxidation, thus allowing for trace control. The third stage involves the addition of phosphoric acid to control the synthesis reaction, regulate the dissolution of iron and phosphorus in the solution, and consequently affect the stoichiometric ratio of the sintered product. The electrochemical performance of the final product is adjusted through the addition of phosphorus source in these three stages.

[0061] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing sodium ferric pyrophosphate, characterized in that, Includes the following steps: S1. Mix the phosphorus source and iron source solutions, add them to the pure water base solution and stir to obtain the pretreatment solution; S2. Add a sodium source to the pretreatment solution to obtain a high pH solution; S3. Add the first phosphoric acid to the high pH solution, then add hydrogen peroxide, and then add the second phosphoric acid to obtain the pre-synthesized solution; S4. The pre-synthesized solution is heat-treated, filtered and dried to obtain sodium iron pyrophosphate precursor; S5. After mixing the sodium iron pyrophosphate precursor with a carbon source, sintering is performed to obtain the sodium iron pyrophosphate.

2. The preparation method according to claim 1, characterized in that, In step S1, the phosphorus source includes monoammonium phosphate solution and / or diammonium phosphate solution; the iron source is one or more of ferrous sulfate solution and ferrous ammonium sulfate solution; the molar ratio of the phosphorus source to the iron source is 3.2-3.6:3; and the phosphorus content in the phosphorus source is 4.5-6 wt%.

3. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of the sodium source to the phosphorus source is 4:3.2-3.

6.

4. The preparation method according to claim 1, characterized in that, In step S3, the molar ratio of the first phosphoric acid to the iron source is 0.2-0.6:3, and the concentration of the first phosphoric acid is 80-85 wt%.

5. The preparation method according to claim 1, characterized in that, In step S3, the hydrogen peroxide is 60-70% of the molar mass of the iron source; the concentration of the hydrogen peroxide is 26-28 wt%.

6. The preparation method according to claim 1, characterized in that, In step S3, the molar ratio of the second phosphoric acid to the iron source is 0.2:3, and the concentration of the second phosphoric acid is 80-85 wt%.

7. The preparation method according to claim 1, characterized in that, In step S3, the pH value of the pre-synthesized solution is 1.5-2.

5.

8. The preparation method according to claim 1, characterized in that, In step S5, the sintering procedure is as follows: under inert gas protection, the temperature is increased to 560-600℃ at a rate of 3-5℃ / min, and then held at that temperature for 9-12 h.

9. A sodium ferric pyrophosphate obtained by the preparation method according to any one of claims 1-8.

10. A positive electrode material, characterized in that, Includes sodium iron pyrophosphate as described in claim 9.

Images