A sodium ferrous sulfate cathode material, its preparation method, and a sodium-ion battery

Abstract

This application provides a sodium ferrous sulfate cathode material, its preparation method, and its application, belonging to the field of sodium-ion battery technology. The preparation method of the sodium ferrous sulfate cathode material includes the following steps: mixing an iron source, a sodium source, an inorganic carbon source, and a solvent to obtain a slurry; drying the slurry to obtain a first powder; sintering the first powder under an inert gas to obtain a second powder; and pulverizing the powder. The sintering process includes a preheating stage, a first sintering stage, and a second sintering stage performed sequentially. The preheating stage involves heating to 80℃~100℃; the first sintering stage employs multi-stage heating, with each stage increasing by 20℃~40℃ and holding for 2h~4h until the temperature reaches 210℃~230℃; the second sintering stage involves heating to 380℃~410℃. The sodium ferrous sulfate material obtained by this application has no white spots on its surface, and the product exhibits good consistency and stability.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a sodium ferrous sulfate cathode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries, due to their richer and more uniform distribution of essential components, better safety, and similar ion extraction and insertion working principle to lithium-ion batteries, hold promise for large-scale supplementation or replacement of lithium-ion batteries in the field of energy storage. Sodium ferrous sulfate (NFS) exhibits unique advantages in numerous fields. As a cathode material, it boasts low cost, which reduces production costs to some extent, providing an economic advantage for related industries. It possesses a high-voltage platform, providing more stable and robust power support for related equipment, playing a significant role in energy applications. Its superior rate performance allows for more efficient energy release during charging and discharging, meeting the demand for rapid energy conversion, thus gaining widespread popularity.

[0003] Currently, the mass production method of sodium ferrous sulfate materials involves mixing and sintering the raw materials. The sintered sodium ferrous sulfate materials have a high content of impurity phases, and are particularly prone to white spots on the material surface (mainly a mixture of trivalent and divalent ferrous salts). The batch stability of the products is poor, and their performance needs to be improved. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides a sodium ferrous sulfate cathode material, a preparation method and a sodium-ion battery, aiming to solve the technical problems of high impurity content in the sintered material, easy appearance of white spots on the material surface and insufficient batch stability of the product during the batch preparation of sodium ferrous sulfate material.

[0005] In a first aspect, embodiments of this application provide a method for preparing sodium ferrous sulfate cathode material, comprising the following steps: Iron source, sodium source, inorganic carbon source and solvent are mixed to obtain slurry; The slurry is dried to obtain the first powder; The first powder was sintered under an inert gas to obtain the second powder; The second powder was pulverized to obtain sodium ferrous sulfate cathode material; The sintering process includes a preheating stage, a first sintering stage, and a second sintering stage, which are carried out sequentially. The preheating stage raises the temperature to 80℃~100℃; the first sintering stage uses multi-stage heating, with each stage raising the temperature by 20℃~40℃ and holding it at that temperature for 2h~4h until the temperature reaches 210℃~230℃; the second sintering stage raises the temperature to 380℃~410℃.

[0006] In the technical solution of this application embodiment, the three-stage sintering process, consisting of a preheating stage, a multi-stage first sintering stage, and a second sintering stage, effectively accelerates the removal of moisture from the raw materials, reduces the impact of moisture accumulation on the material during sintering, and minimizes the formation of impurities by moisture. In particular, it inhibits the oxidation of ferrous ions by moisture, preventing the formation of ferric double salts, thereby reducing white spots on the material surface. This effectively solves the technical problem of high impurity content and easy appearance of white spots on the surface of sodium ferrous sulfate materials during preparation. Simultaneously, the smooth heating process and precise temperature control ensure the consistency of the material's phase and morphology, improve batch stability, and adapt to the needs of mass production.

[0007] In some embodiments, the iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, ferrous sulfate pentahydrate, anhydrous ferrous sulfate, and ferrous oxalate. And / or, the sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium carbonate, and sodium oxalate; And / or, the inorganic carbon source includes at least one of graphene oxide, CNT, graphene, and Ketjen black.

[0008] In this embodiment, the specific types of iron source, sodium source, and inorganic carbon source can meet the matching selection of different types of raw materials, better meet the requirements of slurry dispersion and reaction sufficiency, and avoid problems such as uneven slurry and poor product performance caused by poor raw material compatibility, thus providing raw material guarantee for the preparation of high-performance sodium ferrous sulfate cathode materials.

[0009] In some embodiments, the molar ratio of the iron source to the sodium source is 1:(1.3~1.6). And / or, the molar ratio of iron source to inorganic carbon source is 1:(0.02~0.04).

[0010] In this embodiment, the sodium-to-iron ratio is 1.3 to 1.6. This ratio ensures sufficient sodium source, promotes the full formation of the sodium ferrous sulfate phase, improves the phase composition of the final product, and optimizes the material's performance.

[0011] Inorganic carbon sources are mixed in the above proportions; an appropriate amount of carbon source can improve the conductivity of the material.

[0012] In some embodiments, an antioxidant is added during the preparation of the slurry. The antioxidant includes at least one of vitamin C, vitamin E, citric acid, pyrrole, and sodium sulfite. The amount of antioxidant added is 0.1% to 5% of the total mass of the iron source and the sodium source.

[0013] In this embodiment, adding a certain amount of antioxidant during the preparation of sodium ferrous sulfate material can further reduce impurities such as ferric double salts generated due to the oxidation of ferrous ions, thereby improving the purity of the material. Controlling the amount of antioxidant added within a certain ratio range of the total mass of the iron and sodium sources not only effectively prevents the oxidation of raw materials and improves the crystallinity of the product, but also avoids the increase in specific surface area and reduced compaction of the material due to excessive addition, thus preventing a decrease in performance.

[0014] In some embodiments, the heating rate of the sintering process is 4°C / min to 6°C / min.

[0015] In this embodiment, the heating rate of the entire sintering process is controlled at 4℃ / min~6℃ / min. This ensures heating efficiency, avoids excessively long production cycles, and maintains stable temperature changes, reducing temperature fluctuations and ensuring orderly raw material reactions and sufficient moisture removal. This heating rate is compatible with the three-stage sintering process, effectively avoiding problems such as high impurity content and white spots on the surface caused by excessively rapid heating, or increased energy consumption caused by excessively slow heating, further improving batch stability and production economy.

[0016] In some embodiments, the second sintering stage employs a multi-stage heating process, with each stage increasing the temperature by 30°C to 80°C and holding it for 2 to 4 hours, until the temperature reaches 380°C to 410°C and is held for 4 to 8 hours.

[0017] In this embodiment, the second sintering stage also employs a multi-stage heating and heat preservation design, which enables a smoother temperature rise, avoids inaccurate temperature display in different temperature zones, and promotes thorough crystal growth of the material. Furthermore, because the temperature difference between each temperature zone in this three-stage sintering process is small, adjacent temperature zones can utilize residual heat flow to maintain temperature stability, thereby reducing the operating frequency of heating equipment, lowering energy consumption, and further reducing production costs.

[0018] In some embodiments, when preparing the slurry, mixing is carried out by stirring for 1 to 4 hours at a frequency of 40 Hz to 80 Hz.

[0019] In this embodiment, the above method ensures that all components are fully and uniformly mixed, improving the uniformity and stability of the slurry. In some embodiments, when preparing the first powder, spray drying is used, and the spray drying temperature is 85°C to 95°C.

[0020] In this embodiment, rapid drying can be achieved, effectively reducing component segregation and maintaining good powder dispersibility and morphological consistency.

[0021] In some embodiments, when preparing sodium ferrous sulfate cathode material, the second powder is pulverized using an air jet milling method.

[0022] In this embodiment, the powder particle size distribution can be effectively controlled, large particle agglomeration can be reduced, and the processing performance and electrochemical performance of the material can be improved.

[0023] Secondly, embodiments of this application provide a sodium ferrous sulfate cathode material, which is prepared using the method for preparing sodium ferrous sulfate cathode material of the first aspect.

[0024] In the technical solution of this application embodiment, the obtained material has high purity, low impurity content, no obvious white spots on the surface, and good batch stability and consistency.

[0025] In some embodiments, the particle size D50 of the sodium ferrous sulfate cathode material is 3μm~6μm, D100≤28μm, moisture content is less than 520ppm, and purity is above 84%.

[0026] In this embodiment, the sodium ferrous sulfate cathode material has small and uniform particle size, low moisture content, and high purity, exhibiting excellent physical and chemical properties, making it suitable as a cathode material for sodium-ion batteries and meeting practical application requirements.

[0027] Thirdly, embodiments of this application provide a sodium-ion battery, including the sodium ferrous sulfate cathode material of the second aspect.

[0028] In the technical solution of this application embodiment, sodium ferrous sulfate cathode material is used as the cathode material of sodium-ion battery, which gives full play to the advantages of low cost, high voltage platform and excellent rate performance of the material, and is highly compatible with the application requirements of sodium-ion battery.

[0029] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0030] 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.

[0031] Figure 1 This is a flowchart illustrating the preparation method of the sodium ferrous sulfate cathode material of this application; Figure 2 Here is a SEM image of the material from Example 1; Figure 3 Here is a cross-sectional SEM image of the material from Example 1; Figure 4 The XRD pattern of the material in Example 1; Figure 5 The charge-discharge curve of a battery made from the material of Example 1; Figure 6 The discharge rate curve of the battery made from the material of Example 1. Detailed Implementation

[0032] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0038] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0039] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0040] In existing technologies, the mass production of sodium ferrous sulfate typically involves mixing raw materials followed by sintering and reacting at high temperatures to obtain the target product. Mixing is mainly divided into two types: dry mixing and wet mixing. Dry mixing involves directly adding raw materials to a mixing tank for dispersion and rotary mixing without adding solvent; wet mixing involves mixing raw materials with a solvent until the raw materials dissolve in the solvent. Sintering involves reacting the raw materials at a specific temperature. Dry mixing results in inconsistent product quality, with variations between different batches and production stages. While wet mixing achieves more uniform mixing and higher yields, the resulting product still suffers from inconsistencies.

[0041] The consistency issue of the product mainly manifests in the high content of impurities in the sintered sodium ferrous sulfate material, particularly the tendency to generate white spots of ferric double salts on the material surface. These problems restrict the widespread application of sodium ferrous sulfate, and the product performance needs improvement. The applicant's research revealed that the above problems arise because moisture in the system cannot be quickly expelled, easily causing ferrous ions to oxidize to ferric ions, resulting in a high content of impurities in the material, especially the further formation of ferric double salts, which leads to the white spot phenomenon.

[0042] To address the technical problems of high impurity phase content, easy white spot formation on the material surface, and poor batch stability in the mass production of sodium ferrous sulfate materials, this application provides a sodium ferrous sulfate cathode material, its preparation method, and a sodium-ion battery. This application utilizes an iron source, a sodium source, and an inorganic carbon source to react and generate carbon-coated sodium ferrous sulfate, with antioxidants added as needed to effectively prevent the oxidation of ferrous ions. During the preparation process, a gradient heating procedure is employed for the sintering treatment, including a preheating stage, a multi-stage first sintering stage, and a second sintering stage. The first sintering stage uses multi-stage heating to gradually and completely remove the water of crystallization from the iron source; the second sintering stage promotes crystal growth. This three-stage sintering process effectively accelerates the removal of moisture from the raw materials, reducing the negative impact of moisture accumulation on the material during sintering, thereby significantly reducing the content of impurities generated by moisture, especially reducing the formation of ferric double salts due to moisture oxidation of ferrous ions, which leads to white spots on the material surface. Therefore, this application greatly solves the problem of impurity phase formation caused by the difficulty in removing moisture from raw materials, ensuring product consistency and batch stability, and realizing the large-scale application of sodium ferrous sulfate cathode material.

[0043] Firstly, such as Figure 1 As shown in the embodiments of this application, a method for preparing sodium ferrous sulfate cathode material is provided, including the following steps: S10. Mix the iron source, sodium source, inorganic carbon source and solvent to obtain a slurry; S20. The slurry is dried to obtain the first powder; S30. The first powder is sintered under an inert gas to obtain the second powder; S40. The second powder is pulverized to obtain sodium ferrous sulfate cathode material; In step S30, the sintering process includes a preheating stage, a first sintering stage, and a second sintering stage performed sequentially. The preheating stage raises the temperature to 80℃~100℃. The first sintering stage uses multi-stage heating, with each stage raising the temperature by 20℃~40℃ and holding it at that temperature for 2h~4h until the temperature reaches 210℃~230℃. The second sintering stage raises the temperature to 380℃~410℃.

[0044] In this embodiment of the application, the sintering process is divided into three stages of heating: a preheating stage, a first sintering stage, and a second sintering stage. The temperature of the preheating stage is from room temperature to a preset temperature T1 (80℃~100℃), the temperature of the first sintering stage is from T1 to a preset temperature T2 (210℃~230℃), and the temperature of the third sintering stage is from T2 to T3 (380℃~410℃). For example, T1 includes, but is not limited to, 80℃, 85℃, 90℃, 95℃, 100℃ or any intermediate value between two of the above; T2 includes, but is not limited to, 210℃, 215℃, 220℃, 225℃, 230℃ or any intermediate value between two of the above; the first sintering stage can be divided into three, four, or even more stages; the temperature rise of each stage in the first sintering stage is 20℃, 25℃, 30℃, 35℃, 40℃ or any intermediate value between two of the above; and the holding time after each temperature rise is 2h, 3h, 4h or any intermediate value between two of the above; T3 includes, but is not limited to, 380℃, 385℃, 390℃, 395℃, 400℃, 405℃, 410℃ or any intermediate value between two of the above.

[0045] In the technical solution of this application embodiment, an iron source, a sodium source, and an inorganic carbon source are reacted to generate carbon-coated sodium ferrous sulfate (i.e., sodium ferrous sulfate cathode material). A three-stage sintering process is employed, consisting of a preheating stage, a multi-stage first sintering stage, and a second sintering stage. In the first sintering stage, multi-stage heating gradually removes and slowly drains the water of crystallization from the iron source until it is completely removed. At the critical end of this stage, the iron and sodium sources begin to melt gradually and initially form the target phase of sodium ferrous sulfate. At this point, it is necessary to ensure that the raw materials are in an anhydrous state to guarantee the smooth growth of the crystal form during the second sintering stage; the second sintering stage promotes crystal growth. The multi-stage heating treatment in the first sintering stage effectively accelerates the removal of moisture from the raw materials, reduces the impact of moisture accumulation on the material during sintering, and reduces the content of impurities generated by moisture. In particular, it reduces the white spots formed on the material surface due to the oxidation of ferrous ions by moisture to form ferric double salts, thus effectively solving the technical problem of high impurity content and easy appearance of white spots on the surface of sodium ferrous sulfate materials during preparation. An inert gas atmosphere prevents oxidation of raw materials and products, further ensuring material purity and laying the foundation for subsequent material performance optimization. Simultaneously, a smooth heating process and precise temperature control guarantee the consistency of material phase and morphology, improving batch stability and adapting to the needs of mass production.

[0046] In the embodiments of this application, the iron source refers to a divalent iron source. Further, in some embodiments, the iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, ferrous sulfate pentahydrate, anhydrous ferrous sulfate, and ferrous oxalate; optionally, the iron source includes ferrous sulfate heptahydrate, taking into account both cost and process requirements.

[0047] Furthermore, in some embodiments, the sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium carbonate, and sodium oxalate; optionally, the sodium source includes anhydrous sodium sulfate.

[0048] Furthermore, in some embodiments, the inorganic carbon source includes at least one of graphene oxide, CNT, graphene, and Ketjen black.

[0049] In the technical solutions of this application, the specific types of iron sources, sodium sources, and inorganic carbon sources that can be selected cover a variety of commonly used and readily available raw materials, improving the flexibility and practicality of the process. The appropriate selection of different types of raw materials can better meet the requirements for slurry dispersion and reaction sufficiency, avoiding problems such as uneven slurry and poor product performance caused by poor raw material compatibility, thus providing a raw material guarantee for the preparation of high-performance sodium ferrous sulfate cathode materials.

[0050] Furthermore, in some embodiments, the molar ratio of the iron source to the sodium source is 1:(1.3~1.6). And / or, the molar ratio of iron source to inorganic carbon source is 1:(0.02~0.04).

[0051] In the technical solution of this application embodiment, the sodium-iron ratio is 1.3~1.6. This ratio can ensure sufficient sodium source, promote the full formation of sodium ferrous sulfate phase, improve the phase of the final product, and optimize the performance of the material.

[0052] Inorganic carbon sources are mixed in the above proportions; an appropriate amount of carbon source can improve the conductivity of the material.

[0053] Furthermore, in some embodiments, during step S10, an antioxidant is added when preparing the slurry. The antioxidant includes at least one of vitamin C, vitamin E, citric acid, pyrrole, and sodium sulfite. The amount of antioxidant added is 0.1% to 5% of the total mass of the iron source and the sodium source, optionally 0.5% to 2%. Exemplarily, the amount of antioxidant added is 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, 5% of the total mass of the iron source and the sodium source, or any intermediate value between the above two values.

[0054] In the technical solution of this application embodiment, adding a certain amount of antioxidant during the preparation of sodium ferrous sulfate material can effectively prevent the oxidation of ferrous ions and further reduce impurities such as ferric double salts generated due to the oxidation of ferrous ions, thereby improving the purity of the material. Controlling the amount of antioxidant added within a certain ratio range of the total mass of the iron and sodium sources not only effectively prevents the oxidation of raw materials and improves the crystallinity of the product, but also avoids the phenomenon of increased specific surface area and decreased compaction density due to excessive addition, which would lead to a decline in electrochemical performance.

[0055] Furthermore, in some embodiments, in step S30, the heating rate of the sintering process is 4°C / min to 6°C / min. During the sintering process, the heating rate for each stage can be the same or different. For example, the heating rate for each stage of the sintering process is 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min, 6°C / min, or any intermediate value between the above two values.

[0056] In the technical solution of this application embodiment, the heating rate of the entire sintering process is controlled at 4℃ / min~6℃ / min. This ensures heating efficiency, avoids excessively long production cycles, and maintains stable temperature changes, reducing temperature fluctuations and ensuring orderly raw material reactions and sufficient moisture removal. This heating rate is compatible with the three-stage sintering process, effectively avoiding problems such as high impurity content and white spots on the surface caused by excessively rapid heating, or increased energy consumption caused by excessively slow heating, further improving batch stability and production economy.

[0057] Furthermore, in some embodiments, the second sintering stage employs a multi-stage heating process, with each stage increasing the temperature by 30°C to 80°C and holding for 2 to 4 hours, until the temperature reaches 380°C to 410°C, and then holding for 4 to 8 hours. For example, each stage of the second sintering stage may involve heating at 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, or any intermediate value of the above two values, followed by holding for 2 hours, 3 hours, 4 hours, or any intermediate value of the above two values ​​after each heating stage, and then holding for 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, or any intermediate value of the above two values ​​after reaching the preset temperature T2.

[0058] In the technical solution of this application embodiment, the second sintering stage also adopts a multi-stage heating and holding design, which makes the temperature rise smoother, avoids the problem of inaccurate temperature display in the temperature zone, and promotes thorough crystal growth of the material. Sufficient holding time can ensure the full formation and crystal integrity of the sodium ferrous sulfate phase, improve the purity and electrical properties of the material, and adapt to the needs of continuous production, avoiding fluctuations in material properties caused by sudden temperature changes.

[0059] Furthermore, in some embodiments, in step S10, when preparing the slurry, the mixing is carried out by stirring for 1 to 4 hours and at a frequency of 40 Hz to 80 Hz.

[0060] In the technical solution of this application embodiment, controlling the time and frequency of mixing and stirring can ensure that the raw materials are fully dispersed and form a uniform slurry, providing a good foundation for the subsequent drying and sintering process.

[0061] Furthermore, in some embodiments, in step S20, when preparing the first powder, the drying is carried out by spray drying, and the spray drying temperature is 85°C~95°C.

[0062] In the technical solution of this application embodiment, controlling the temperature during spray drying can effectively remove moisture from the slurry, obtain a first powder with uniform particle size and appropriate moisture content, avoid powder agglomeration or moisture residue caused by improper drying, thereby ensuring the consistency and stability of the material's performance after subsequent sintering and improving production efficiency.

[0063] Furthermore, in some embodiments, in step S40, when preparing the sodium ferrous sulfate cathode material, the second powder is pulverized using an air jet milling method. Optionally, the frequency of the corresponding air jet classifier is 25Hz~28Hz.

[0064] In the technical solution of this application embodiment, an airflow pulverization method is adopted, with the classifier wheel frequency controlled at 25Hz~28Hz. This allows for precise control of the particle size distribution of the material, obtaining particle sizes that meet the requirements and avoiding the impact of excessively large or small particle sizes on the material's compaction density and electrochemical performance. These pulverization parameters are adapted to the material's characteristics, reducing secondary agglomeration during pulverization and ensuring particle size uniformity. Simultaneously, the pulverization process is easily controlled, adaptable to mass production, and provides particle size assurance for the material's subsequent application in sodium-ion battery cathodes.

[0065] Secondly, embodiments of this application provide a sodium ferrous sulfate cathode material, which is prepared using the method for preparing sodium ferrous sulfate cathode material of the first aspect.

[0066] The technical solution of this application inherits the advantages of this preparation process, resulting in high material purity, low impurity content, no obvious white spots on the surface, and good batch stability and consistency. Its excellent physical and chemical properties allow it to fully leverage the advantages of sodium ferrous sulfate, such as low cost, high voltage platform, and superior rate performance, when used as a cathode material for sodium-ion batteries. This improves the battery's charge / discharge capacity, cycle stability, and rate performance, meeting practical application requirements.

[0067] Furthermore, in some embodiments, the particle size D50 of the sodium ferrous sulfate cathode material is 3μm~6μm, D100≤28μm, moisture content is less than 520ppm, and purity is above 84%.

[0068] In the technical solutions of this application embodiment, the particle size, moisture content, and purity of the material meet specific requirements, and these parameter standards provide clear guarantees for the material performance. A suitable particle size distribution can improve the compaction density and ion transport efficiency of the material, low moisture content can avoid adverse effects on battery performance, and high purity can ensure the electrochemical stability of the material, thereby ensuring the consistency and reliability of sodium-ion battery performance and facilitating industrial application.

[0069] Thirdly, embodiments of this application provide a sodium-ion battery, including the sodium ferrous sulfate cathode material described in the second aspect.

[0070] In the technical solution of this application embodiment, sodium ferrous sulfate cathode material is used as the cathode material for sodium-ion batteries, fully leveraging the advantages of this material such as low cost, high voltage platform, and excellent rate performance, which is highly compatible with the application requirements of sodium-ion batteries. Its excellent performance can improve the overall performance of sodium-ion batteries, reduce battery production costs, promote the industrialization of sodium-ion batteries, and provide high-performance, low-cost battery material solutions for new energy storage, electric vehicles, and other fields.

[0071] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0072] Example 1 This embodiment provides a carbon-coated sodium ferrous sulfate material, the preparation process of which is as follows: S10. Iron source, sodium source and inorganic carbon source are mixed in a molar ratio of 1:1.529:0.03 to obtain a mixture. The iron source is ferrous sulfate heptahydrate, the sodium source is anhydrous sodium sulfate, and the inorganic carbon source is a mixture of graphene oxide and CNT (carbon nanotubes) in a mass ratio of 1:1. Then, deionized water is added to the mixture and it is dispersed and stirred at a frequency of 60Hz for 4 hours to obtain a slurry.

[0073] S20. The slurry is spray-dried at a temperature of 90°C to obtain the first powder.

[0074] S30. The first powder is placed in a sintering furnace filled with inert gas for sintering treatment. The specific sintering treatment is as follows: S301, Preheating stage: Heat from room temperature to 90℃ at a rate of 5℃ / min, and hold for 3 hours; S302. The first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature is increased from 90℃ to 120℃ at a rate of 5℃ / min and held for 3 hours. During the pre-vaporization stage, ferrous sulfate heptahydrate loses its water of crystallization and becomes ferrous sulfate tetrahydrate, and the moisture in the raw materials is slowly discharged. The temperature is increased from 120℃ to 150℃ at a rate of 5℃ / min and held for 3 hours. Ferrous sulfate tetrahydrate further loses its water of crystallization and becomes ferrous sulfate monohydrate. During this stage, the holding time can be extended as needed to completely remove the water of crystallization. The temperature was increased from 150℃ to 180℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 180℃ to 210℃ at a rate of 5℃ / min and held for 3 hours. Ferrous sulfate monohydrate completely lost its last water of crystallization and became anhydrous ferrous sulfate. At around 210℃, the iron source and sodium source began to gradually melt and initially form the sodium ferrous sulfate phase. At this time, both were in anhydrous state. S303, the second sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 210℃ to 250℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 250℃ to 330℃ at a rate of 5℃ / min and held for 3 hours. This was the transition temperature range, designed to make the temperature rise smoother and avoid the temperature rising too quickly, which could lead to incomplete crystal growth. The temperature was increased from 330℃ to 400℃ at a rate of 5℃ / min, and then held for 4 hours to obtain the second powder.

[0075] The second powder was pulverized using an air classifier at a frequency of 25 Hz to obtain carbon-coated sodium ferrous sulfate material.

[0076] Example 2 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that an antioxidant is added during the ingredient preparation process in this embodiment. The amount of antioxidant added is 0.1% of the total mass of the iron source and the sodium source.

[0077] Example 3 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that an antioxidant is added during the ingredient preparation process in this embodiment. The amount of antioxidant added is 5% of the total mass of the iron source and the sodium source.

[0078] Example 4 This embodiment provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Embodiment 1 in that: in this embodiment, the iron source is ferrous sulfate pentahydrate and the sodium source is anhydrous sodium acetate.

[0079] Example 5 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that in this embodiment, the iron source, sodium source, and inorganic carbon source are mixed in a molar ratio of 1:1.3:0.02.

[0080] Example 6 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that in this embodiment, the iron source, sodium source, and inorganic carbon source are mixed in a molar ratio of 1:1.6:0.04.

[0081] Example 7 This embodiment provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Embodiment 1 in that: in this embodiment, the first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 90℃ to 130℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 130℃ to 170℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 170℃ to 210℃ at a rate of 5℃ / min, and held for 3 hours.

[0082] Example 8 This embodiment provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Embodiment 1 in that: in this embodiment, the first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 90℃ to 110℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 110℃ to 130℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 130℃ to 150℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 150℃ to 170℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 170℃ to 190℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 190℃ to 210℃ at a rate of 5℃ / min, and held for 3 hours.

[0083] Example 9 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that in this embodiment, the second sintering stage involves heating from 210°C to 400°C at a rate of 5°C / min, followed by a holding time of 4 hours.

[0084] Example 10 This embodiment provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Embodiment 1 is that in this embodiment, the second sintering stage adopts multi-stage heating, with the following specific parameters: heating from 210℃ to 260℃ at a heating rate of 5℃ / min, and holding for 2 hours. The temperature was increased from 260℃ to 310℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 310℃ to 360℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 360℃ to 400℃ at a rate of 5℃ / min, and then held for 4 hours to obtain the second powder.

[0085] Comparative Example 1 This comparative example provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Example 1 is that in the first sintering stage of this comparative example, the temperature is raised from 90°C to 210°C at a rate of 5°C / min and held for 3 hours.

[0086] Comparative Example 2 This comparative example provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Example 1 in that: in this comparative example, the first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 90℃ to 150℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 150℃ to 180℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 180℃ to 210℃ at a rate of 5℃ / min, and held for 3 hours. That is, compared with Example 1, this comparative example omits the 120°C heat preservation stage.

[0087] Comparative Example 3 This comparative example provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Example 1 in that: in this comparative example, the first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 90℃ to 120℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 120℃ to 180℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 180℃ to 210℃ at a rate of 5℃ / min, and held for 3 hours. That is, compared with Example 1, this comparative example omits the 150°C heat preservation stage.

[0088] Comparative Example 4 This comparative example provides a carbon-coated sodium ferrous sulfate material, the preparation process of which differs from that of Example 1 in that: in this comparative example, the first sintering stage employs multi-stage heating, with specific parameters as follows: The temperature was increased from 90℃ to 120℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 120℃ to 150℃ at a rate of 5℃ / min, and held for 3 hours. The temperature was increased from 150℃ to 210℃ at a rate of 5℃ / min, and held for 3 hours. That is, compared with Example 1, this comparative example omits the 180°C heat preservation stage.

[0089] Comparative Example 5 This comparative example provides a carbon-coated sodium ferrous sulfate material. The difference between its preparation process and that of Example 1 is that in this comparative example, the second sintering stage starts directly from 180°C, i.e., the temperature is increased from 180°C to 250°C at a rate of 5°C / min and held for 2 hours; the temperature is increased from 250°C to 330°C at a rate of 5°C / min and held for 3 hours; the temperature is increased from 330°C to 400°C at a rate of 5°C / min and held for 4 hours. That is, compared with Example 1, this comparative example omits the 210°C heat preservation stage.

[0090] Comparative Example 6 In this comparative example, an antioxidant was also added during the ingredient preparation process. The amount of antioxidant added was 0.05% of the total mass of the iron source and sodium source.

[0091] Comparative Example 7 In this comparative example, an antioxidant was also added during the ingredient preparation process. The amount of antioxidant added was 6% of the total mass of the iron and sodium sources.

[0092] The following tests were performed on the embodiments and comparative examples.

[0093] I. Physical parameter testing: (1) Microscopic morphology detection: The microscopic morphology of the material was observed using a scanning electron microscope (model: Zeiss Sigma 300).

[0094] (2) Particle size detection: The D50 and D100 of the material were detected by a laser particle size analyzer (model: Malvern Mastersizer 3000).

[0095] (3) Compacted density (PD) test: The compacted density of the material was tested using a UTM7305 battery powder compaction density tester provided by Shenzhen Sansi Zongheng Technology Co., Ltd. The test pressure was 3T and the pressing time was 30s.

[0096] (4) Specific surface area (BET) test: The specific surface area of ​​the material was tested using a BELSORP MaxII specific surface area analyzer manufactured by Japan-McQKBY.

[0097] (5) Moisture content detection: The moisture content of the material was detected using a Karl Fischer moisture analyzer (model: Metrohm 831 KF).

[0098] (6) Purity detection: The purity of the material is detected by combining X-ray diffraction (model: Bruker D8 Advance) and chemical titration.

[0099] The main results are shown in Table 1 below: Table 1. Physical parameter results for different materials The microstructure analysis results of Examples 1-10 show that the material particles in Examples 1-10 are regular, near-spherical, well-dispersed, and without obvious agglomeration. Taking the material of Example 1 as an example, according to... Figure 2 The SEM image of Example 1 shown shows that the material is spherical, according to Figure 3 The cross-sectional SEM image of Example 1 shown indicates that the material has good density. According to... Figure 4 The XRD pattern of Example 1 shown indicates that the main component of the material is sodium ferrous sulfate.

[0100] Based on the results in Table 1, it can be seen that the materials in Examples 1-10 have uniform particle size distribution, and their compaction, BET, and moisture content all meet the relevant requirements, indicating high purity. Taking the material in Example 1 as an example, its D50 is 4.5 μm and D100 is 24 μm, indicating uniform particle size distribution. Furthermore, the particle size meets the requirements of D50 (3 μm-6 μm) and D100 (≤28 μm), satisfying the application requirements as a cathode material for sodium-ion batteries. The compaction of this material is 2.21 g / cm³. 3 BET is 3.4m 2 / g, meeting the application requirements as a positive electrode material for sodium-ion batteries; the moisture content of the material is 310ppm, which is less than the excellent standard of 400ppm; and the purity of the material is 94.5%.

[0101] The materials in Comparative Examples 1 to 7 had low purity and high impurity content, failing to meet application requirements. Taking the material in Comparative Example 1 as an example, its purity was 60.4%, and its impurity content was high. Analysis revealed that the main impurity was ferrous sulfate.

[0102] In addition, six batches of materials were prepared in accordance with the preparation process of Examples 1 to 10 above, and the deviation values ​​of particle size, compaction density, specific surface area, moisture content and purity of different batches of materials were measured to test the batch stability of the preparation method of this application.

[0103] According to the corresponding batch stability test results, the batch stability of Examples 1 to 10 is good. Taking the batch stability test results of Example 1 as an example, it shows that the D50 deviation of each batch of material is ≤2%, the moisture content deviation is ≤50ppm, the purity deviation is ≤1%, and the electrochemical performance deviation is ≤3%, demonstrating excellent batch consistency.

[0104] Based on the above results, it can be seen that the carbon-coated sodium ferrous sulfate material prepared by the preparation method of this application meets all the requirements in terms of parameters and has good batch stability, which fully demonstrates the advantages of the preparation method of this application.

[0105] II. Application Performance Testing: (1) The material is made into a positive electrode sheet and assembled into a battery.

[0106] Preparation of positive electrode sheet: (1) Raw material ratio: The sodium ferrous sulfate positive electrode material (D50=4.2μm, purity 94%), conductive agent Super P, and binder PVDF prepared in this invention are mixed in a mass ratio of 85:10:5. (2) Slurry preparation: Weigh 8.5g of sodium ferrous sulfate positive electrode material, 1.0g of Super P, and 0.5g of PVDF, add an appropriate amount of N-methylpyrrolidone (NMP), and stir in a planetary stirrer for 4h at a stirring speed of 3000r / min to obtain a uniform positive electrode slurry. (3) Coating and drying: Coat the positive electrode slurry uniformly on a 15μm thick aluminum foil with a coating surface density of 15mg / cm², and then place it in a vacuum drying oven and dry at 120℃ for 12h to remove solvent and moisture. (4) Rolling and cutting: Roll the dried electrode sheet to control the compaction density of the electrode sheet to 2.1 g / cm³, and then cut it into circular electrode sheets with a diameter of 14 mm for later use.

[0107] Sodium-ion battery assembly: (1) Battery model: CR2032 button cell. (2) Negative electrode: Sodium metal sheet, purity ≥99.9%, thickness 0.2mm, diameter 15mm. (3) Separator: Glass fiber separator (model: Whatman GF / D), thickness 200μm, diameter 16mm. (4) Electrolyte: 1mol / L NaPF6 dissolved in EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl methyl carbonate) mixed solvent (volume ratio 1:1:1), electrolyte moisture content ≤20ppm. (5) Assembly process: In an argon-protected glove box (oxygen content ≤0.1ppm, water content ≤0.1ppm), assemble the button cell in the order of "positive electrode shell → positive electrode sheet → separator → electrolyte → sodium sheet → gasket → spring sheet → negative electrode shell". After assembly, seal with a button cell sealing machine.

[0108] (2) Battery performance test: (1) Charge and discharge test: The battery test system (model: Neware CT-4008) was used, with a test voltage range of 2.0V~4.5V.

[0109] Taking Example 1 as an example, the charge-discharge curve of the material in Example 1 is as follows: Figure 5 As shown, by Figure 5 It can be seen that the material has a first charge capacity of 101 mAh / g at 0.1C and a first discharge capacity of 95 mAh / g. The discharge rate curve of the material in Example 1 is shown below. Figure 6 As shown, by Figure 6 It can be seen that the material can maintain a discharge capacity of 86mAh / g under a high current density of 10C.

[0110] For the materials in other embodiments and comparative examples, the data for the first charge at 0.1C and the first discharge at 0.1C were obtained using the same charge and discharge test method.

[0111] (2) Cyclic stability test: Cyclic test was performed at 1C rate.

[0112] The specific results are shown in Table 2 below.

[0113] Table 2 Battery performance results As shown in Table 2, compared with Comparative Examples 1-5, the batteries of Examples 1-10 all showed improved performance in 0.1C first charge, 0.1C first discharge, and cycle retention. This indicates that the embodiments of this application use a specific preparation method: mixing and drying the raw materials, followed by sintering and pulverizing. The sintering process includes a preheating stage, a first sintering stage with multi-stage heating under specific conditions, and a second sintering stage. Each stage is heated to a specific temperature, which can improve the electrochemical performance of the material.

[0114] Comparative Examples 1-5 did not employ the first sintering stage treatment specific to the embodiments of this application, resulting in poor electrochemical performance of the prepared materials. Specifically, Comparative Example 1 did not employ multi-stage heating in the first sintering stage. Although Comparative Examples 2-4 employed multi-stage heating in the first stage of sintering, one stage of heating was excessively high: Comparative Example 2 omitted the 120°C holding stage in the first stage of sintering, directly heating from 90°C to 150°C (a 60°C increase); Comparative Example 3 omitted the 150°C holding stage in the first stage of sintering, directly heating from 120°C to 180°C (a 60°C increase); Comparative Example 4 omitted the 180°C holding stage in the first stage of sintering, directly heating from 150°C to 210°C (a 60°C increase); and Comparative Example 5 omitted the 210°C holding stage in the first stage of sintering, i.e., it directly proceeded to the second sintering stage without heating to the specific temperature. The results showed that the materials prepared in the above comparative examples had poor electrochemical performance.

[0115] As can be seen from Examples 1-3, the electrochemical performance of the material can be effectively improved by not adding antioxidants or adding a certain proportion of antioxidants to the raw materials and by using the specific preparation method of this application. As can be seen from Examples 2-3 and Comparative Examples 6-7, if antioxidants are added to the raw materials, the amount of antioxidants added needs to be controlled to 0.1% to 5% of the total mass of the iron source and sodium source, which is beneficial to improving the electrochemical performance of the material.

[0116] As can be seen from Examples 1 and 4, using different iron and sodium sources as raw materials, and employing the specific preparation method of this application, can all improve the electrochemical performance of the material.

[0117] As can be seen from Examples 1 and 5-6, controlling the molar ratio of the raw material iron source, sodium source, and inorganic carbon source to 1:(1.3~1.6):(0.02~0.04) and using the specific preparation method of this application can improve the electrochemical performance of the material.

[0118] As can be seen from Examples 1 and 7-8, controlling the temperature rise by 20°C to 40°C per stage during the first sintering stage can improve the electrochemical performance of the material.

[0119] As can be seen from Examples 1 and 9-10, different heating methods in the second sintering stage can all improve the electrochemical performance of the material.

[0120] Based on the above results, it can be seen that when the sodium ferrous sulfate cathode material prepared in this application is used as a cathode material for sodium-ion batteries, it can effectively improve the charge and discharge capacity, rate performance, cycle stability and safety of the battery, reduce the battery production cost, provide strong support for the industrial application of sodium-ion batteries, and can be widely used in new energy vehicles, energy storage power stations and other fields.

[0121] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing a sodium ferrous sulfate cathode material, characterized in that, Includes the following steps: Iron source, sodium source, inorganic carbon source and solvent are mixed to obtain slurry; The slurry is dried to obtain a first powder; The first powder was sintered under an inert gas to obtain the second powder; The second powder is pulverized to obtain sodium ferrous sulfate cathode material; The sintering process includes a preheating stage, a first sintering stage, and a second sintering stage performed sequentially. The preheating stage raises the temperature to 80°C to 100°C. The first sintering stage employs multi-stage heating, with each stage raising the temperature by 20°C to 40°C and holding it at that temperature for 2 hours to 4 hours, until the temperature reaches 210°C to 230°C. The second sintering stage raises the temperature to 380°C to 410°C.

2. The method for preparing sodium ferrous sulfate cathode material according to claim 1, characterized in that, The iron source includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, ferrous sulfate pentahydrate, anhydrous ferrous sulfate, and ferrous oxalate. And / or, the sodium source includes at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium carbonate, and sodium oxalate; And / or, the inorganic carbon source includes at least one of graphene oxide, CNT, graphene, and Ketjen black.

3. The method for preparing sodium ferrous sulfate cathode material according to claim 1 or 2, characterized in that, The molar ratio of the iron source to the sodium source is 1:(1.3~1.6). And / or, the molar ratio of the iron source to the inorganic carbon source is 1:(0.02~0.04).

4. The method for preparing sodium ferrous sulfate cathode material according to claim 1, characterized in that, When preparing the slurry, an antioxidant is also added, which includes at least one of VC, VE, citric acid, pyrrole and sodium sulfite; and the amount of antioxidant added is 0.1% to 5% of the total mass of the iron source and the sodium source.

5. The method for preparing sodium ferrous sulfate cathode material according to claim 1, characterized in that, The heating rate of the sintering process is 4℃ / min to 6℃ / min.

6. The method for preparing sodium ferrous sulfate cathode material according to claim 1, characterized in that, The second sintering stage employs a multi-stage heating process, with each stage increasing the temperature by 30°C to 80°C and holding it for 2 to 4 hours, until the temperature reaches 380°C to 410°C and is held for 4 to 8 hours.

7. The method for preparing sodium ferrous sulfate cathode material according to claim 1, characterized in that, When preparing the slurry, the mixing is carried out by stirring, the stirring time is 1h to 4h, and the stirring frequency is 40Hz to 80Hz. And / or, when preparing the first powder, the drying is carried out by spray drying, and the spray drying temperature is 85℃~95℃; And / or, when preparing sodium ferrous sulfate cathode material, the second powder is pulverized using an air jet milling method.

8. A sodium ferrous sulfate cathode material, characterized in that, It is prepared using the method for preparing sodium ferrous sulfate cathode material as described in any one of claims 1 to 8.

9. The sodium ferrous sulfate cathode material according to claim 8, characterized in that, The sodium ferrous sulfate cathode material has a particle size D50 of 3μm~6μm, a D100≤28μm, a moisture content of less than 520ppm, and a purity of not less than 84%.

10. A sodium-ion battery, characterized in that, Including the sodium ferrous sulfate cathode material as described in claim 8 or 9.

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