Sodium iron sulfate-based composite positive electrode material, and preparation method therefor and use thereof

Abstract

The present application relates to the technical field of sodium‑ion batteries, and provides a sodium iron sulfate-based composite positive electrode material, and a preparation method therefor and a use thereof. The sodium iron sulfate-based composite positive electrode material has a porosity of 0.1%-10% and a compacted density of 2.1-3 g / cm3; in addition, the sodium iron sulfate-based composite positive electrode material has a specific surface area of 5-12 m2 / g, and a sodium-rich impurity phase in the sodium iron sulfate-based composite positive electrode material has a mass percentage content of 0.1%-1%. The present application helps to improve the performance of batteries, such as the charge / discharge capacity and the initial coulombic efficiency.

Description

Sodium ferric sulfate composite positive electrode material and preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 202411390338.2, filed on September 30, 2024, and entitled "Sodium ferric sulfate composite positive electrode material and preparation method and application thereof", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of sodium ion batteries, in particular to a sodium ferric sulfate composite positive electrode material and a preparation method and application thereof. BACKGROUND

[0003] Due to the limitations of the existing preparation process of sodium ion positive electrode materials, the combination of conductive carbon and sodium ferric sulfate in the sodium ion positive electrode material is not tight enough, which is not conducive to the performance of the material.

[0004] Sodium ion battery positive electrode materials include three types of materials: layered oxides, Prussian blue, and polyanionic materials. Among them, polyanionic materials have stable structure, high working voltage, good safety performance, high cycle stability and high specific capacity, and have attracted widespread attention. Iron-based sulfate is a resource-rich polyanionic material, which contains Fe 3+ / Fe 2+ reversible redox couples and SO4 2- strong electronegativity and induction effect, has the advantages of low production cost and high working voltage (3.0-3.8V), and can be used as a sodium ion battery positive electrode material. However, polyanionic materials have poor electrical conductivity, and often need to be coated with carbon to obtain sodium ion battery positive electrode materials with excellent electrochemical performance.

[0005] However, due to the limitations of the existing preparation process, the carbon-coated iron-based sulfate sodium ion battery positive electrode material has low compaction density, large specific surface area and unsuitable porosity, which deteriorates its first coulombic efficiency, charge-discharge specific capacity and other properties.

[0006] For example, in the existing solid phase method, solvent thermal reaction method, anti-solvent method, spray drying method, freeze drying method and other methods for preparing sodium ferric sulfate, the solid phase method is simple in process and easy to operate, so the research on the solid phase method is the most in-depth. In the solid phase method, ball milling is often used to achieve the crushing and mixing of materials. However, due to the limitations of ball milling, the mixing uniformity of the materials is not sufficient. In addition, the sintering temperature for preparing sodium ferric sulfate cannot be too high, basically not exceeding 350℃. Therefore, at a lower sintering temperature, atoms are difficult to diffuse, resulting in insufficient reaction of each phase, poor product consistency, low compaction density, and high impurity content of the prepared sodium ferric sulfate, which further reduces the electrochemical performance.

[0007] Therefore, a sodium ferric sulfate composite positive electrode material with excellent porosity, compactness and specific surface area is relatively scarce in the field. SUMMARY

[0008] The application provides a sodium ferric sulfate composite positive electrode material, a preparation method and application thereof. The sodium ferric sulfate composite positive electrode material has excellent porosity, compactness and specific surface area, and a low content of sodium-rich impurities, which helps to improve the charge and discharge capacity, the first coulombic efficiency and other performances.

[0009] The application provides a sodium ferric sulfate composite positive electrode material, which has a porosity of 0.1% to 10%, a compactness of 2.1 to 3 g / cm 3 , and a specific surface area of 5 to 12 m 2 / g, and a mass percentage of sodium-rich impurities of 0.1% to 1%.

[0010] Optionally, in an atmosphere with a humidity of 0.1 to 0.5 ppm, the water content of the sodium ferric sulfate composite positive electrode material has a rising rate of less than 5.5 ppm / h; in an atmosphere with a humidity of less than 0.1 ppm, the water content of the sodium ferric sulfate composite positive electrode material has a rising rate of less than 2.5 ppm / h; and / or, the mass percentage of trivalent iron in the sodium ferric sulfate composite positive electrode material is less than 1%; and / or, the mass percentage of impurities in the sodium ferric sulfate composite positive electrode material is less than 5%.

[0011] Optionally, the chemical formula of the sodium ferric sulfate composite positive electrode material is Na 2+2x Fe 2-x (SO4)3@C, wherein 0≤x<2.

[0012] Optionally, the particle size of primary particles of the sodium ferric sulfate composite positive electrode material is 50 to 1000 nm, and part of the primary particles are agglomerated to form secondary particles with a particle size of 1 to 10 μm; in the sodium ferric sulfate composite positive electrode material, the mass percentage of the secondary particles is 15% to 40%, and the mass percentage of the remaining primary particles is 60% to 85%.

[0013] Optionally, the sodium ferric sulfate composite positive electrode material further comprises conductive carbon; part of the secondary particles are agglomerated by the primary particles through the conductive carbon, and the mass percentage of the part of the secondary particles in the secondary particles is 15% to 40%, and the mass percentage of the remaining secondary particles in the secondary particles is 60% to 85%.

[0014] The application provides a preparation method of the sodium ferric sulfate composite positive electrode material, and the method comprises the following steps: crushing an iron source and a sodium source respectively to a particle size of less than 1 μm; mixing the crushed iron source, the crushed sodium source, conductive carbon and an antioxidant to obtain a precursor; drying the precursor under vacuum conditions, sealing the dried precursor to ensure that the precursor is in the vacuum condition, and then performing isobaric sintering on the sealed precursor, wherein the isobaric sintering is performed at a pressure of 1-500 MPa, a temperature of 300-400 ℃ and for 1-24 h, to obtain the sodium ferric sulfate composite positive electrode material.

[0015] Optionally, the process of mixing the crushed iron source, the crushed sodium source, the conductive carbon and the antioxidant to obtain the precursor comprises the following steps: premixing the crushed iron source, the crushed sodium source, the conductive carbon and the antioxidant, then performing secondary crushing and mixing by using an airflow mill or a rolling ball mill, and finally performing solid solution mixing by using a planetary ball mill or a vibration ball mill to obtain the precursor; and / or the isobaric sintering comprises hot isostatic sintering or spark plasma sintering; and / or the speed of temperature rise during the isobaric sintering is 1-5 ℃ / min.

[0016] Optionally, the iron source comprises ferrous sulfate; the sodium source comprises sodium sulfate; the conductive carbon comprises one or more of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, redox graphene, acetylene black, ketjen black, carbon nanofibers and activated carbon; and / or the antioxidant comprises one or more of ascorbic acid, citric acid, tea polyphenols and glucose; and the ratio of the mass of the antioxidant to the mass of the iron source is 0.01%-10%.

[0017] The application further provides a positive electrode sheet, which comprises the sodium ferric sulfate composite positive electrode material or is prepared by the preparation method.

[0018] The application further provides a sodium ion battery, which comprises the positive electrode sheet.

[0019] The application provides a sodium ferric sulfate composite positive electrode material, a preparation method and application thereof. The sodium ferric sulfate composite positive electrode material has excellent porosity, compactness and specific surface area, and has a low mass percentage of sodium-rich impurities, which helps to improve the charge-discharge capacity, the initial coulombic efficiency and other performances, and improves the electrochemical performance. BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a charge-discharge curve of Examples 1-5.

[0021] Figure 2 is a charge-discharge curve of Example 1, Example 6, Example 7;

[0022] Figure 3 is a charge-discharge curve of Example 1, Comparative Example 1-3;

[0023] Figure 4 is an XRD pattern of sodium ferric sulfate (Na 2.56 Fe 1.72 (SO4)3) in Example 1;

[0024] Figure 5 is an XRD pattern of sodium-rich impurity phase (Na6Fe(SO4)4) in sodium ferric sulfate in Comparative Example 1. DETAILED DESCRIPTION

[0025] In order for those skilled in the art to better understand the scheme of the present application, the present application is further described in detail below. The following specific embodiments are only to describe the principles and characteristics of the present application, and the examples are only used to explain the present application, but not to limit the scope of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0026] The embodiments of the present application provide a sodium ferric sulfate composite positive electrode material, the porosity of the sodium ferric sulfate composite positive electrode material is 0.1% to 10%, the tap density is 2.1 to 3 g / cm 3 , and the specific surface area of the sodium ferric sulfate composite positive electrode material is 5 to 12 m 2 / g, and the mass percentage of sodium-rich impurity phase in the sodium ferric sulfate composite positive electrode material is 0.1% to 1%.

[0027] According to the research and analysis of the inventors: the porosity of the sodium ferric sulfate composite positive electrode material of the embodiments of the present application is 0.1% to 10%, which avoids that the large porosity limits the performance of the sodium ferric sulfate composite positive electrode material, and the tap density of the sodium ferric sulfate composite positive electrode material is 2.1 to 3 g / cm 3 , and the specific surface area is 5 to 12 m 2 / g, which helps to improve the first coulombic efficiency and the charge-discharge specific capacity, and improves the electrochemical performance. In addition, the sodium-rich impurity phase helps to promote the transmission of sodium ions, which may be because the sodium-rich impurity phase can form a heterojunction with sodium ferric sulfate, as a fast ion channel, to promote the transmission of sodium ions. However, too high content of sodium-rich impurity phase will inhibit the normal performance of the capacity of sodium ferric sulfate, and will also cause a significant decrease in the electrochemical performance of the sodium ferric sulfate composite positive electrode material. The mass percentage of sodium-rich impurity phase in the sodium ferric sulfate composite positive electrode material of the embodiments of the present application can be 0.1% to 1%, and a lower content of sodium-rich impurity phase helps to improve the electrochemical performance of the sodium ferric sulfate composite positive electrode material.

[0028] The porosity of the sodium ferric sulfate composite positive electrode material refers to a ratio of a volume of pores in the sodium ferric sulfate composite positive electrode material to a total volume of the sodium ferric sulfate composite positive electrode material.

[0029] The sodium ferric sulfate composite positive electrode material has weak water absorption. Specifically, in an atmosphere with humidity of 0.1-0.5 ppm, the sodium ferric sulfate composite positive electrode material has a water content increase rate of less than 5.5 ppm / h; in an atmosphere with humidity of less than 0.1 ppm, the sodium ferric sulfate composite positive electrode material has a water content increase rate of less than 2.5 ppm / h, which helps to improve the stability of the electrochemical performance of the sodium ferric sulfate composite positive electrode material. Thus, the problem of deterioration of the electrochemical performance of the sodium ferric sulfate composite positive electrode material due to strong water absorption of the sodium ferric sulfate composite positive electrode material is avoided.

[0030] In some embodiments, the sodium ferric sulfate composite positive electrode material has a water content of less than 1000 ppm after being exposed to a dry room with humidity of 0.01-0.1 ppm for 1 week.

[0031] In some embodiments, the sodium ferric sulfate composite positive electrode material has a mass percentage of impurities of less than 5%, that is, the sodium ferric sulfate composite positive electrode material has high purity.

[0032] The impurities generally include manganese, zinc, calcium, trivalent iron, etc. The sources of the impurities include that the raw materials for preparing the sodium ferric sulfate composite positive electrode material carry impurities such as manganese, zinc, and calcium, and in addition, in the preparation process, impurities with a mass percentage of less than 3% in the sodium ferric sulfate composite positive electrode material are introduced due to abrasion and oxidation of instruments and equipment.

[0033] In addition, in the preparation process (sintering process), a sodium-rich impurity phase (Na6Fe(SO4)4) is generated in the sodium ferric sulfate composite positive electrode material. The sodium-rich impurity phase helps to promote the transmission of sodium ions, which may be because the sodium-rich impurity phase can form a heterojunction with sodium ferric sulfate to serve as a fast ion channel and promote the transmission of sodium ions. However, a too high content of the sodium-rich impurity phase will inhibit the normal performance of the capacity of sodium ferric sulfate and also cause a significant decrease in the electrochemical performance of the sodium ferric sulfate composite positive electrode material. Therefore, it is necessary to control the content of the sodium-rich impurity phase at a low level. The mass percentage of the sodium-rich impurity phase in the sodium ferric sulfate composite positive electrode material according to the embodiments of the present application can be 0.1%-1%, and a low content of the sodium-rich impurity phase helps to improve the electrochemical performance of the sodium ferric sulfate composite positive electrode material.

[0034] In the process of preparing the sodium ferric sulfate composite positive electrode material, divalent iron is easily oxidized to trivalent iron. When the content of trivalent iron is too high, the electrochemical performance of the sodium ferric sulfate composite positive electrode material is decreased. The mass percentage of trivalent iron in the sodium ferric sulfate composite positive electrode material according to the embodiments of the present application can be less than or equal to 1%, for example, 0.1%-1%, and a low content of trivalent iron has negligible effect on the electrochemical performance of the sodium ferric sulfate composite positive electrode material.

[0035] In some embodiments, the sodium ferric sulfate composite cathode material has a chemical formula of Na 2+2x Fe 2- x (SO4)3@C, where 0≤x<2. The sodium ferric sulfate and the conductive carbon are compounded, which helps the sodium ferric sulfate composite cathode material to have conductivity, solves the problem of poor conductivity of the sodium ferric sulfate itself, and also increases the specific surface area (BET) of the sodium ferric sulfate composite cathode material and improves the electrochemical performance thereof.

[0036] In addition, the above chemical formula can also be replaced by one or more of Formula 1, Formula 2, Formula 3, and Formula 4. Formula 1 is NaFe 1-x M x SO4F@C, where M = Ni, Co, Mn, etc., and 0 < X≤1; Formula 2 is Na x Fe y (PO4) a (SO4) b-z M c O d @C, where 0 < x≤4, 0 < y≤4, 0 < z≤0.5, 0 < a≤4, 0 < b≤3, 0 < c≤4, 0 < d≤5; M is at least one of oxides, hydroxides, chloride salts, nitrate salts, or carbonate salts of Ti, V, Cr, Mn, Fe, Co, Cd, Ni, Cμ, Zn, Al, Ag, Mg, Ca, Sn, or Se; Formula 3 is Na x Fe y (SO4) z , where 1 < x≤10, 1 < y≤10, and z = x / 2 + y; Formula 2 is Na x M y (SO4) z N n , where x, y, z, and n are all positive real numbers, M includes Hf, Fe, and rare earth elements, and N includes Se and Cl; Formula 4 is Na a Fe b (SO4) c @C, where a + 2b = 2c, and 0.3≤a / b≤2.8.

[0037] In some embodiments, the sodium ferric sulfate composite cathode material is in a granular form, is formed by combining conductive carbon and sodium ferric sulfate material, and has a primary particle size of 50-1000 nm, and part of the primary particles are agglomerated to form secondary particles, and the secondary particles have a particle size of 1-10 μm. That is, the primary particles of the above-mentioned sodium ferric sulfate composite cathode material granules can be divided into two parts, one part is agglomerated to form secondary particles, and the remaining part exists alone without agglomeration to form secondary particles.

[0038] There are various forms of conductive carbon in the sodium ferric sulfate composite positive electrode material, including zero-dimensional conductive carbon, one-dimensional conductive carbon, and two-dimensional conductive carbon. These conductive carbons are partially distributed in a free state (free conductive carbon). Among them, the zero-dimensional conductive carbon is point-like conductive carbon, such as super-p, carbon black, etc.; the one-dimensional conductive carbon is linear conductive carbon, such as carbon nanofiber, carbon nanotube; the two-dimensional conductive carbon is sheet-like conductive carbon, such as graphene, graphene oxide. The free conductive carbon is a conductive carbon distribution state. Any conductive carbon that does not form a certain connection with the sodium ferric sulfate matrix is free conductive carbon. The free conductive carbon only has physical contact with the matrix material. The free conductive carbon has a fixed position in the sodium ferric sulfate composite positive electrode material. Stirring and ultrasonic in the preparation process can change its position.

[0039] As can be seen, the surface of the primary particles is coated with conductive carbon in various forms.

[0040] In the sodium ferric sulfate composite positive electrode material, the mass percentage content of the above-mentioned secondary particles can be 15% to 40%, for example, 15%, 20%, 30%, 40%, or a range formed by any two of them. The mass percentage content of the remaining part of the primary particles (primary particles that are not agglomerated to form secondary particles, and exist alone) can be 60% to 85%, for example, 60%, 70%, 80%, 85%, or a range formed by any two of them. The mass percentage content of the free conductive carbon can be 0.1% to 3%, which helps to improve the porosity, tap density, and specific surface area of the sodium ferric sulfate composite positive electrode material, and further improve its first coulombic efficiency, charge and discharge specific capacity, and other electrochemical properties.

[0041] The secondary particles are mainly formed by agglomeration of primary particles. The secondary particles can be divided into two parts according to the connection form. Part of the secondary particles are agglomerated by primary particles through conductive carbon connection. The above-mentioned conductive carbon connection includes the winding and bonding of one-dimensional conductive carbon such as carbon nanotube, and the coating and bonding of two-dimensional conductive carbon such as graphene, so that the conductive carbon is not only distributed at the interface between the primary particles, but also a large amount of conductive carbon is coated on the surface of the secondary particles. This coating is consistent with the above-mentioned form of two-dimensional conductive carbon coating on the surface of the primary particles. The mass percentage content of this part of the secondary particles in the secondary particles can be 15% to 40%. The remaining part of the secondary particles is mainly formed by mechanical intercalation and van der Waals bond connection of the primary particles. The mass percentage content of this remaining part of the secondary particles in the secondary particles can be 60% to 85%, which helps to improve the porosity, tap density, and specific surface area of the sodium ferric sulfate composite positive electrode material, and further improve its first coulombic efficiency, charge and discharge specific capacity, and other electrochemical properties.

[0042] The application further provides a preparation method of the sodium ferric sulfate composite positive electrode material, comprising the following steps: crushing the iron source and the sodium source respectively to a particle size less than 1 μm; mixing the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant to obtain a precursor; drying the precursor under vacuum conditions, sealing the dried precursor to ensure that the precursor is under vacuum conditions, and then performing isobaric sintering on the sealed precursor, wherein the pressure of the isobaric sintering is 1-500 MPa, the temperature is 300-400 ℃, and the time is 1-24 h, to obtain the sodium ferric sulfate composite positive electrode material.

[0043] According to the research and analysis of the inventor: crushing the iron source and the sodium source to a particle size D 50 less than 1 μm helps to promote uniform and sufficient mixing of raw materials; the isobaric sintering of the precursor under vacuum conditions not only avoids the oxidation of divalent iron into trivalent iron, but also uniformly applies pressure to the precursor during sintering, thereby ensuring that the reaction is fully carried out during isobaric sintering and the purity of the sodium ferric sulfate composite positive electrode material is higher than 99%; in addition, the uniform and consistent pressure applied to the precursor during isobaric sintering enables the sodium ferric sulfate and the carbon to be tightly combined, which can improve the porosity, the compaction density and the specific surface area of the sodium ferric sulfate composite positive electrode material, solve the problem of high water absorption of the sodium ferric sulfate, effectively reduce the water absorption of the sodium ferric sulfate composite positive electrode material, and further improve the conductivity of the sodium ferric sulfate composite positive electrode material by improving the combination of the carbon and the sodium ferric sulfate, thereby improving the first coulombic efficiency, the charge-discharge specific capacity and other performances of the sodium ferric sulfate composite positive electrode material.

[0044] In specific implementation, the iron source and the sodium source can be crushed to a particle size less than 1 μm (controlled within 1 μm) by mechanical grinding and / or air flow grinding, wherein the crushing of the iron source should be carried out under the protection of inert gas. The pressure and time of the air flow grinding are not particularly limited in the application, and the particle size of the crushed iron source and the sodium source can meet the requirements, for example, the pressure and time of the air flow grinding can be 1.4-1.5 MPa and 8-10 min, and the inert gas can include nitrogen and / or argon.

[0045] Generally, the iron source and the sodium source can be dried under vacuum before being crushed, for example, vacuum drying at 200-350 ℃ for 1-15 h. Before mixing the various raw materials, the iron source and the sodium source are respectively pre-crushed (crushed), and the D 50 less than 1 μm of the crushed iron source and the sodium source helps to improve the uniformity of subsequent mixing.

[0046] The process for obtaining the precursor by mixing the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant includes: pre-mixing the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant, and then performing secondary crushing and mixing by using an air flow mill or a rolling ball mill, and finally performing solid solution mixing by using a planetary ball mill or a vibration ball mill to obtain the precursor.

[0047] It can be understood that, in the above process for obtaining the precursor, inert gas protection is required.

[0048] In specific implementation, the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant can be pre-mixed in a high-speed mixer (high-speed mixing machine) to preliminarily achieve a relatively uniform state on a macroscopic level; then secondary crushing and mixing are performed by using an air flow mill or a rolling ball mill, so that the material mixing is more uniform, and a higher uniformity on a microscopic level is achieved, which is beneficial to reducing composition segregation; finally, solid solution mixing is performed by using a planetary ball mill or a vibration ball mill, which can promote the pre-reaction of the iron source and the sodium source. Specifically, under the action of high energy generated by the planetary ball mill or the vibration ball mill, the iron source and the sodium source can pre-react to realize solid solution (essentially atomic diffusion) of the iron source and the sodium source and other phases, which is beneficial to forming a solid solution of the iron source and the sodium source, reducing part of the diffusion energy barrier for the subsequent sintering reaction, and promoting atomic diffusion through the synergistic effect of the above pre-mixing, secondary crushing and mixing by using the air flow mill or the rolling ball mill, solid solution mixing by using the planetary ball mill or the vibration ball mill, and the uniform pressure provided by the isostatic sintering, so that the materials can fully react to promote the generation of the target phase (sodium ferric sulfate composite positive electrode material), thereby improving the purity of the sodium ferric sulfate composite positive electrode material to more than 99%, and helping to improve the porosity, the compact density, and the specific surface area of the sodium ferric sulfate composite positive electrode material, as well as the first coulombic efficiency, the charge-discharge specific capacity, and other performances, avoiding the problems of organic solvent recovery and treatment, organic solvent residue, and deterioration of the performance of the positive electrode material caused by wet ball milling using an organic solvent, and solving the problems of insufficient crushing of raw materials in the prior art, which cannot achieve a sub-micron level, thereby limiting the mixing uniformity and the reaction degree, and avoiding the problem of performance segregation of the sodium ferric sulfate composite positive electrode material caused by insufficient reaction.

[0049] The premixing time of the high-speed mixer can be 5-10 min; the air flow pressure of the jet mill can be 0.5-1.5 MPa, and the time can be 3-8 min; the ball-to-material ratio of the rolling ball mill can be (15-25):1, the rotation speed can be 500-600 rpm, and the time can be 22-26 h; the ball-to-material ratio of the planetary ball mill can be (8-12):1, the rotation speed can be 500-700 rpm, and the time can be 4-8 h; the vibration frequency of the vibration ball mill can be 1400-1500 r / min, and the time can be 4-6 h.

[0050] The isostatic sintering can include hot isostatic sintering or spark plasma sintering, and preferably hot isostatic sintering.

[0051] It can be understood that the sodium-rich impurity phase is difficult to completely remove, and although it can be kept at a low level by adjusting the raw material ratio and process parameters (for example, reducing the iron content), the phase is prone to appear in the process of preparing sodium ferrous sulfate with a high iron content according to a conventional method, and in the preparation method system provided in the embodiments, the isostatic sintering, preferably hot isostatic sintering, can be used to prepare sodium ferrous sulfate composite positive electrode material with a low content of sodium-rich impurity phase.

[0052] In addition, since the temperature in the sintering process for preparing sodium ferrous sulfate cannot be too high, and basically does not exceed 350℃, atoms are difficult to diffuse at a lower sintering temperature, resulting in problems such as a long reaction time required for preparing sodium ferrous sulfate, insufficient reaction between phases, poor consistency of sodium ferrous sulfate, low electrochemical performance, and low tap density. The hot isostatic pressing technology can be used to appropriately reduce the sintering temperature, shorten the sintering time, improve the production efficiency, reduce the energy consumption, improve the porosity of the sodium ferrous sulfate composite positive electrode material, and further improve the tap density and specific surface area of the sodium ferrous sulfate composite positive electrode material, so that the electrochemical performance of the sodium ferrous sulfate composite positive electrode material such as the initial coulombic efficiency and the specific charge capacity is improved.

[0053] In the process of hot isostatic sintering, the precursor can be loaded into a mold and dried under vacuum conditions, which can effectively reduce the moisture in the precursor and effectively inhibit the oxidation of divalent iron in the sintering process.

[0054] The vacuum condition can be a vacuum degree of 0.001-0.1 Pa, the drying temperature can be 250-300℃, and the time can be 0.5-2 h.

[0055] After the precursor is loaded into the mold, it can be vibrated to not less than 1.0 g / cm 3 , and then subsequent drying treatment and other processes are performed.

[0056] Next, the precursor after the drying treatment is sealed to ensure that the precursor is in a vacuum state during the subsequent sintering process, thereby effectively avoiding the oxidation of ferrous iron during the subsequent treatment process. On the other hand, the precursor can also be prevented from contacting water, thereby effectively reducing the water absorption of the sodium ferric sulfate composite positive electrode material. In a specific implementation, the mold can be sealed or closed to ensure that the precursor is in a vacuum state.

[0057] The sealed precursor is then subjected to isobaric sintering, for example, the sealed precursor can be placed in a hot-pressing furnace for isobaric sintering to obtain a sodium ferric sulfate composite positive electrode material. During the isobaric sintering process, the material is fully pressed, the pressure on the precursor is uniform, and the pressure exists throughout the reaction process, which can effectively ensure the full reaction and ensure that the sodium ferric sulfate and carbon are tightly combined together, so that the carbon tightly covers the sodium ferric sulfate, which can effectively reduce the water absorption of the sodium ferric sulfate composite positive electrode material. The problem of water absorption of the sodium ferric sulfate composite positive electrode material cannot be effectively avoided due to the inability of the prior art to effectively combine carbon and sodium ferric sulfate raw materials, so that the moisture content of the sodium ferric sulfate composite positive electrode material remains below 1000 ppm after being exposed to a dry room with a humidity of 0.01-0.1 ppm for one week. The uniform and constant pressure can make the sodium ferric sulfate and carbon directly and tightly contact, which can reduce the gap between the sodium ferric sulfate and carbon, avoid the excessive gap that degrades the performance of the sodium ferric sulfate composite positive electrode material, and avoid the problem of excessive specific surface area caused by the presence of carbon, so that the porosity, tap density, and specific surface area of the sodium ferric sulfate composite positive electrode material are effectively improved.

[0058] Further, the heating rate during isobaric sintering can be 1-5°C / min, for example, 1°C / min, 1.8°C / min, 2.0°C / min, 2.2°C / min, 3°C / min, 4°C / min, 5°C / min, or a range formed by any two of them.

[0059] Under the preparation system of the embodiments of the present application, by adjusting the pressure, temperature, and time of the isobaric sintering process, the porosity of the sodium ferric sulfate composite positive electrode material can be effectively controlled, the porosity is 0.1%-20%, and the crystallization performance of the sodium ferric sulfate composite positive electrode material is better, the tap density of the sodium ferric sulfate composite positive electrode material is 1.5-3 g / cm 3 , and the specific surface area of the sodium ferric sulfate composite positive electrode material is 5-20 m 2 / g, which can avoid the problem of excessive specific surface area reducing the initial coulombic efficiency and affecting the performance of the sodium ferric sulfate composite positive electrode material, and can also improve the water absorption, initial coulombic efficiency, charge-discharge specific capacity, and other properties of the sodium ferric sulfate composite positive electrode material.

[0060] In order to protect the equipment and the mold, the sealing-treated precursor can be placed under an inert gas for isostatic sintering (at this time, the precursor is still in a vacuum state under a closed condition), after the isostatic sintering is completed, the pressure of the reaction system is released, and then the reaction product is cooled to 50-150℃, and then the reaction product is crushed and sieved to obtain the sodium ferric sulfate composite positive electrode material.

[0061] The prior art vacuum hot-pressing sintering process has poor airtightness, and the sintered material has the risk of being oxidized, so it is necessary to maintain a vacuum during sintering, which increases the production cost. The precursor of the present application is subjected to drying treatment and vacuum sealing treatment, so that the precursor cannot be oxidized, thereby avoiding the oxidation of divalent iron to trivalent iron.

[0062] The iron source used in the present application can include ferrous sulfate, such as one or more of ferrous sulfate heptahydrate, ferrous sulfate tetrahydrate, ferrous sulfate monohydrate, and anhydrous ferrous sulfate; the sodium source can include sodium sulfate, such as one or more of anhydrous sodium sulfate and sodium sulfate decahydrate; the conductive carbon can include one or more of conductive carbon black (Super-P), single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, redox graphene, acetylene black, Ketjen black, carbon nanofibers, and activated carbon; and the antioxidant can include one or more of ascorbic acid, citric acid, tea polyphenols, and glucose. In the raw material of the sodium ferric sulfate composite positive electrode material, the mass percentage content of the conductive carbon can be 0.1%-10%, and the mass ratio of the antioxidant to the iron source can be 0.01%-10%.

[0063] The present application also provides a positive electrode sheet, which comprises the above-mentioned sodium ferric sulfate composite positive electrode material or the sodium ferric sulfate composite positive electrode material obtained by the above-mentioned preparation method. The positive electrode sheet has the technical effects corresponding to the above-mentioned sodium ferric sulfate composite positive electrode material, which will not be described here.

[0064] The positive electrode sheet of the present application specifically comprises a positive electrode current collector and a positive electrode active layer formed of the above-mentioned sodium ferric sulfate composite positive electrode material on the surface of the positive electrode current collector.

[0065] In the preparation of the positive electrode sheet, for example, the sodium ferric sulfate composite positive electrode material of the present application, a conductive agent, and a binder can be dispersed in an appropriate amount of N-methyl pyrrolidone (NMP) solvent, and stirred thoroughly to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and then subjected to drying, rolling, and slitting to obtain the positive electrode sheet. In a specific embodiment, the positive electrode active layer comprises 70-99wt% of the sodium ferric sulfate composite positive electrode material, 0.5-15wt% of the conductive agent, and 0.5-15wt% of the binder, and further comprises 80-98wt% of the sodium ferric sulfate composite positive electrode material, 1-10wt% of the conductive agent, and 1-10wt% of the binder.

[0066] The material of the positive electrode current collector can be at least one of an aluminum foil and a nickel foil; the conductive agent can be at least one of carbon black, acetylene black, graphene, Ketjen black, carbon fiber, carbon nanotube, and conductive graphite; and the binder can be at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, an oxirane-containing polymer, polyvinylpyrrolidone, and polyurethane.

[0067] The application also provides a sodium ion battery including the positive electrode sheet. The sodium ion battery has the technical effects of the positive electrode sheet or the sodium ferrous sulfate composite positive electrode material, which will not be described herein.

[0068] It is conceivable that the sodium ion battery of the application includes a negative electrode sheet, an electrolyte, and a separator in addition to the positive electrode sheet.

[0069] The application is not strictly limited to the negative electrode active material in the negative electrode sheet, which can be at least one of a hard carbon, a soft carbon, a titanium-based material, a metal oxide, and a sulfide commonly used in a sodium ion battery.

[0070] The application is not strictly limited to the selection of the electrolyte, which can include one or more of the solvents commonly used in the electrolyte of a sodium ion battery and the electrolyte sodium salt commonly used in a sodium ion electrolyte. For example, the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, gamma-butyrolactone, etc.; and the electrolyte can be one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium fluorotri-fluoromethylsulfonylimide.

[0071] The application is not strictly limited to the material selection of the separator, which can be one of a polypropylene separator (PP), a polyethylene separator (PE), a polypropylene / polyethylene double-layer composite separator (PP / PE), a polyimide electrospun separator (PI), a polypropylene / polyethylene / polypropylene three-layer composite separator (PP / PE / PP), a cellulose non-woven separator, and a separator with a ceramic coating.

[0072] In the preparation of sodium ion battery, the positive plate, the separator and the negative plate are wound or laminated to obtain a bare battery cell, and the bare battery cell is packaged into an aluminum-plastic film bag which is pre-punched and formed. After the packaged battery is dried at 85℃, the electrolyte is injected into the dried battery, and the battery is completed after standing, formation and secondary sealing.

[0073] The application will be further described below by specific examples and comparative examples. Unless otherwise specified, the reagents, materials and instruments used in the following are conventional reagents, conventional materials and conventional instruments, which are commercially available. The reagents and materials involved can also be synthesized by conventional synthesis methods.

[0074] Example 1

[0075] The present embodiment provides a preparation method of a sodium iron sulfate composite positive electrode material, comprising:

[0076] 1) Drying ferrous sulfate heptahydrate at 350℃ under vacuum for 10h to obtain anhydrous ferrous sulfate; drying sodium sulfate at 200℃ under vacuum for 10h to obtain anhydrous sodium sulfate; using an air flow mill to crush the anhydrous ferrous sulfate and the anhydrous sodium sulfate to a particle size D 50 of 0.8μm, wherein the pressure of the air flow mill is 1.5MPa and the time is 10min, and nitrogen is used for protection when crushing the anhydrous ferrous sulfate;

[0077] 2) Taking 1211g of anhydrous ferrous sulfate, 809g of anhydrous sodium sulfate, 60g of Super-P and 121g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.7, first using a high-speed mixer to premix the above materials for 10min, and using nitrogen for protection during the premixing process; then crushing and mixing the premixed materials using an air flow mill, the air flow pressure of the air flow mill is 1MPa and the time is 5min, and the atmosphere of the air flow mill uses nitrogen; then loading the crushed and mixed materials into a planetary ball mill, the ball-to-material ratio is 10:1, the rotation speed is 600rpm, and the ball milling is carried out under nitrogen protection for 6 hours to obtain a precursor (a positive electrode material precursor);

[0078] 3) Compacting the precursor to 1.4g / cm 3 Then loading the precursor into a hot isostatic pressing mold, and then baking the precursor together with the hot isostatic pressing mold to 300℃ under vacuum 0.001Pa for 0.5h;

[0079] 4) Then sealing the hot isostatic pressing mold, and then loading it into a hot isostatic pressing machine, heating to 350℃ at a rate of 2℃ / min, and then pressurizing to 10MPa at 350℃, and keeping the temperature for 12h, and then cooling to 150℃, and then taking out the material from the hot isostatic pressing mold, crushing and sieving to obtain a sodium iron sulfate composite positive electrode material.

[0080] The sodium ferric sulfate composite positive electrode material of Example 1 has a porosity of 3.72%, a compacted density of 2.48 g / cm 3 , a specific surface area of 7.65 m 2 / g, a mass percentage of sodium-rich impurities of 0.32%, a moisture content of 136.11 ppm, a moisture content increase rate of 4.35 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, a moisture content increase rate of 2.35 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, a purity of 99.53%, a mass percentage of ferric ions of 0.22%, a primary particle size of 382 nm, and a secondary particle size of 4.68 μm. In addition, in the sodium ferric sulfate composite positive electrode material of Example 1, the mass percentage of secondary particles is 28.13%, the mass percentage of primary particles that exist alone without agglomerating to form secondary particles is 69.96%, and the mass percentage of free conductive carbon is 1.91%. In the secondary particles, the mass percentage of secondary particles that are formed by agglomeration of primary particles through conductive carbon is 28.64%, and the mass percentage of the remaining secondary particles is 71.36%.

[0081] Example 2

[0082] This example is basically the same as Example 1, except that:

[0083] In Step 4), the pressure of the hot isostatic pressing is adjusted from 10 MPa to 20 MPa, and the other conditions remain unchanged.

[0084] The sodium ferric sulfate composite positive electrode material of Example 2 has a porosity of 3.12%, a compacted density of 2.65 g / cm 3 , a specific surface area of 6.37 m 2 / g, a mass percentage of sodium-rich impurities of 0.24%, a moisture content of 132.36 ppm, a moisture content increase rate of 4.31 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, a moisture content increase rate of 2.26 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, a purity of 99.62%, a mass percentage of ferric ions of 0.19%, a primary particle size of 489 nm, and a secondary particle size of 5.62 μm. In addition, in the sodium ferric sulfate composite positive electrode material of Example 2, the mass percentage of secondary particles is 31.44%, the mass percentage of primary particles that exist alone without agglomerating to form secondary particles is 67.21%, and the mass percentage of free conductive carbon is 1.35%. In the secondary particles, the mass percentage of secondary particles that are formed by agglomeration of primary particles through conductive carbon is 31.27%, and the mass percentage of the remaining secondary particles is 68.73%.

[0085] Example 3

[0086] This example is basically the same as Example 1, except that:

[0087] In Step 4), the pressure of the hot isostatic pressing is adjusted from 10 MPa to 5 MPa, and the other conditions are kept unchanged.

[0088] The porosity of the sodium ferric sulfate composite positive electrode material of Example 3 is 4.83%, the tap density is 2.23 g / cm 3 , the specific surface area is 9.78 m 2 / g, the mass percentage of sodium-rich impurity phase is 0.35%, the moisture content is 129.58 ppm, the moisture content increases at a rate of 3.57 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, the moisture content increases at a rate of 2.33 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 99.29%, the mass percentage of trivalent iron is 0.21%, the particle size of the primary particles is 361 nm, and the particle size of the secondary particles is 4.16 μm; in addition, in the sodium ferric sulfate composite positive electrode material of Example 3, the mass percentage of secondary particles is 19.75%, the mass percentage of primary particles that exist alone without agglomeration to form secondary particles is 77.63%, and the mass percentage of free conductive carbon is 2.62%; in the secondary particles, the mass percentage of secondary particles that are agglomerated by the primary particles through the conductive carbon is 23.55%, and the mass percentage of the remaining secondary particles is 76.45%.

[0089] Example 4

[0090] This example is basically the same as Example 1, except that:

[0091] In Step 4), the temperature of the hot isostatic pressing is adjusted from 350°C to 300°C, and the other conditions are kept unchanged.

[0092] The porosity of the sodium ferric sulfate composite positive electrode material of Example 4 is 4.03%, the tap density is 2.37 g / cm 3 , the specific surface area is 8.84 m 2 / g, the sodium-rich impurity phase has a mass percentage of 0.41%, the moisture content is 156.27 ppm, the moisture content rises at a rate of 4.32 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and at a rate of 2.45 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 99.45%, the trivalent iron mass percentage is 0.25%, the primary particle size is 357 nm, and the secondary particle size is 4.37 μm; in addition, in the sodium ferric sulfate composite cathode material of Example 4, the secondary particle mass percentage is 27.86%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 70.29%, and the free conductive carbon mass percentage is 1.85%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 26.51%, and the remaining secondary particles have a mass percentage of 73.49%.

[0093] Example 5

[0094] This embodiment is basically the same as Embodiment 1, except that:

[0095] In step 4), the hot isostatic pressing time (holding time) is adjusted from 12h to 8h, while other conditions remain unchanged.

[0096] The porosity of the sodium ferric sulfate composite cathode material in Example 5 was 3.91%, and the compaction density was 2.44 g / cm³. 3 The specific surface area is 8.21 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.28%, the moisture content is 146.16 ppm, the moisture content rises at a rate of 4.43 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and at a rate of 2.41 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 99.37%, the trivalent iron mass percentage is 0.24%, the primary particle size is 374 nm, and the secondary particle size is 4.45 μm; in addition, in the sodium ferric sulfate composite cathode material of Example 5, the secondary particle mass percentage is 27.61%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 70.32%, and the free conductive carbon mass percentage is 2.07%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 26.78%, and the remaining secondary particles have a mass percentage of 73.22%.

[0097] Example 6

[0098] This embodiment is basically the same as Embodiment 1, except that:

[0099] In step 2), "weighing 1244 g of anhydrous ferrous sulfate (FeSO4), 776 g of anhydrous sodium sulfate (Na2SO4), 60 g of Super-P, and 123 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.75" is used to replace "weighing 1211 g of anhydrous ferrous sulfate, 809 g of anhydrous sodium sulfate, 60 g of Super-P, and 121 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.7", and other conditions remain unchanged.

[0100] The porosity of the sodium ferrous sulfate composite positive electrode material of Example 6 is 3.85%, the tap density is 2.51 g / cm 3 , the specific surface area is 7.85 m 2 / g, the mass percentage of sodium-rich impurity phase is 0.53%, the moisture content is 131.65 ppm, the moisture content increases at a rate of 3.86 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, the moisture content increases at a rate of 2.28 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 99.51%, the mass percentage of trivalent iron is 0.31%, the particle size of the primary particles is 396 nm, and the particle size of the secondary particles is 4.87 μm. In addition, in the sodium ferrous sulfate composite positive electrode material of Example 6, the mass percentage of secondary particles is 29.33%, the mass percentage of primary particles that exist alone without agglomeration to form secondary particles is 68.83%, and the mass percentage of free conductive carbon is 1.84%. In the secondary particles, the mass percentage of secondary particles that are agglomerated by the primary particles through the conductive carbon is 27.49%, and the mass percentage of the remaining secondary particles is 72.51%.

[0101] Example 7

[0102] This example is basically the same as Example 1, except that:

[0103] In step 2), "weighing 1244 g of anhydrous ferrous sulfate (FeSO4), 776 g of anhydrous sodium sulfate (Na2SO4), 60 g of Super-P, and 123 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.75" is used to replace "weighing 1211 g of anhydrous ferrous sulfate, 809 g of anhydrous sodium sulfate, 60 g of Super-P, and 121 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.7", and other conditions remain unchanged.

[0104] The porosity of the sodium ferrous sulfate composite positive electrode material of Example 7 is 3.82%, the tap density is 2.54 g / cm 3 , the specific surface area is 7.96 m 2 / g, the mass percentage of sodium-rich impurity phase was 0.47%, the moisture content was 135.91 ppm, the moisture content increased at a rate of 3.69 ppm / h in an atmosphere with humidity of 0.1-0.5 ppm, the moisture content increased at a rate of 2.26 ppm / h in an atmosphere with humidity less than 0.1 ppm, the purity was 99.23%, the mass percentage of ferric iron was 0.38%, the particle size of the primary particles was 403 nm, and the particle size of the secondary particles was 4.91 μm. In addition, in the sodium ferric sulfate composite positive electrode material of Example 7, the mass percentage of the secondary particles was 30.28%, the mass percentage of the primary particles that existed alone without agglomeration to form secondary particles was 67.98%, and the mass percentage of the free conductive carbon was 1.74%. In the secondary particles, the mass percentage of the secondary particles that were formed by agglomeration of the primary particles through the conductive carbon was 27.02%, and the mass percentage of the remaining secondary particles was 72.98%.

[0105] Example 8

[0106] This example is basically the same as Example 1, except that:

[0107] In step 4), the phrase "then the hot isostatic pressing mold was sealed, and then was placed in a hot isostatic pressing machine, and was heated to 400°C at a rate of 2.2°C / min, and then was pressurized to 1 MPa at 400°C for 24 h, and then was cooled to 150°C, and then the material was taken out of the hot isostatic pressing mold, and was crushed and sieved to obtain the sodium ferric sulfate composite positive electrode material" was replaced with "4) then the hot isostatic pressing mold was sealed, and then was placed in a hot isostatic pressing machine, and was heated to 350°C at a rate of 2°C / min, and then was pressurized to 10 MPa at 350°C for 12 h, and then was cooled to 150°C, and then the material was taken out of the hot isostatic pressing mold, and was crushed and sieved to obtain the sodium ferric sulfate composite positive electrode material", and the other conditions remained unchanged.

[0108] The sodium ferric sulfate composite positive electrode material of Example 8 had a porosity of 5.36%, a tap density of 2.13 g / cm 3 , a specific surface area of 10.23 m 2The sodium sulfate iron composite positive electrode material of Example 8 has a purity of 97.14%, a mass percentage of sodium-rich impurities of 0.68%, a moisture content of 176.36 ppm, a moisture content increase rate of 4.52 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, a moisture content increase rate of 2.48 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, a mass percentage of trivalent iron of 1.67%, a primary particle size of 768 nm, and a secondary particle size of 5.52 μm. In addition, in the sodium sulfate iron composite positive electrode material of Example 8, the mass percentage of secondary particles is 17.39%, the mass percentage of primary particles that exist alone without agglomeration to form secondary particles is 79.64%, and the mass percentage of free conductive carbon is 2.97%. In the secondary particles, the mass percentage of secondary particles that are formed by agglomeration of primary particles through conductive carbon is 18.38%, and the mass percentage of the remaining secondary particles is 81.62%.

[0109] Example 9

[0110] This example is basically the same as Example 1, except that:

[0111] In step 4), "then the hot isostatic pressing mold is sealed, and the mold is placed in a hot isostatic pressing machine, heated to 300°C at a rate of 1.8°C / min, and then pressed to 500 MPa at 300°C for 1 h, and then cooled to 150°C, and the material is removed from the hot isostatic pressing mold, crushed and sieved to obtain the sodium sulfate iron composite positive electrode material" is substituted for "4) then the hot isostatic pressing mold is sealed, and the mold is placed in a hot isostatic pressing machine, heated to 350°C at a rate of 2°C / min, and then pressed to 10 MPa at 350°C for 12 h, and then cooled to 150°C, and the material is removed from the hot isostatic pressing mold, crushed and sieved to obtain the sodium sulfate iron composite positive electrode material", and the other conditions remain unchanged.

[0112] The sodium sulfate iron composite positive electrode material of Example 9 has a porosity of 2.43%, a tap density of 2.86 g / cm 3 , a specific surface area of 5.23 m 2 / g, the mass percentage of sodium-rich impurity phase is 0.98%, the moisture content is 138.23 ppm, the moisture content increases at a rate of 3.73 ppm / h in an atmosphere with humidity of 0.1-0.5 ppm, the moisture content increases at a rate of 2.24 ppm / h in an atmosphere with humidity less than 0.1 ppm, the purity is, the mass percentage of ferric iron is 0.53%, the particle size of the primary particles is 935 nm, and the particle size of the secondary particles is 8.74 μm. In addition, in the sodium ferric sulfate composite positive electrode material of Example 9, the mass percentage of the secondary particles is 38.51%, the mass percentage of the primary particles that exist alone without agglomeration to form secondary particles is 61.06%, and the mass percentage of the free conductive carbon is 0.43%. In the secondary particles, the mass percentage of the secondary particles that are formed by agglomeration of the primary particles through the conductive carbon is 39.12%, and the mass percentage of the remaining secondary particles is 60.88%.

[0113] Comparative Example 1

[0114] The present comparative example provides a method for preparing a sodium ferric sulfate composite positive electrode material, comprising:

[0115] 1) Drying ferrous sulfate heptahydrate at 350°C under vacuum for 10 h to obtain anhydrous ferrous sulfate, and drying sodium sulfate at 200°C under vacuum for 10 h to obtain anhydrous sodium sulfate; using an air flow mill to crush the anhydrous ferrous sulfate and the anhydrous sodium sulfate to a particle size D 50 of 0.8 μm, wherein the pressure of the air flow mill is 1.5 MPa and the time is 10 min, and nitrogen is used for protection during crushing of the anhydrous ferrous sulfate;

[0116] 2) Taking 1211 g of anhydrous ferrous sulfate, 809 g of anhydrous sodium sulfate, 60 g of Super-P, and 121 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.7, first using a high-speed mixer to premix the above materials for 10 min, and using nitrogen for protection during the premixing process; then crushing and mixing the premixed materials using an air flow mill, wherein the air flow pressure of the air flow mill is 1 MPa and the time is 5 min, and the atmosphere of the air flow mill uses nitrogen; then loading the crushed and mixed materials into a planetary ball mill, wherein the ball-to-material ratio is 10:1, the rotation speed is 600 rpm, and the ball milling is performed under nitrogen protection for 6 hours to obtain a precursor (a positive electrode material precursor);

[0117] 3) Placing the precursor into a tube furnace to perform sintering, wherein the sintering temperature is 350°C and the time is 12 h, and then cooling to 150°C, crushing, and sieving to obtain a sodium ferric sulfate composite positive electrode material.

[0118] The sodium ferric sulfate composite positive electrode material of Comparative Example 1 has a porosity of 8.87%, a tap density of 1.89 g / cm 3 , and a specific surface area of 15.76 m2 The sodium sulfate ferrite composite cathode material has a mass percentage of 5.85% of sodium-rich heterogeneous phases, a water content of 1487.69 ppm, a water content increase rate of 9.81 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, a water content increase rate of 6.93 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, a purity of 92.36%, a mass percentage of 1.61% of ferric iron, a particle size of 326 nm of primary particles, and a particle size of 3.95 μm of secondary particles. In addition, in the sodium sulfate ferrite composite cathode material of Comparative Example 1, the mass percentage of secondary particles is 16.97%, the mass percentage of primary particles that exist alone without agglomeration to form secondary particles is 78.36%, and the mass percentage of free conductive carbon is 4.12%. In the secondary particles, the mass percentage of secondary particles that are agglomerated by the primary particles through the conductive carbon is 7.36%, and the mass percentage of the remaining secondary particles is 92.64%.

[0119] Comparative Example 2

[0120] The present comparative example provides a preparation method of a sodium sulfate ferrite composite cathode material, comprising:

[0121] 1) drying ferrous sulfate heptahydrate at 350°C under vacuum for 10 h to obtain anhydrous ferrous sulfate, drying sodium sulfate at 200°C under vacuum for 10 h to obtain anhydrous sodium sulfate, and using an airflow mill to crush the anhydrous ferrous sulfate and the anhydrous sodium sulfate to a particle size D 50 of 0.8 μm, wherein the pressure of the airflow mill is 1.5 MPa and the time is 10 min, and nitrogen is used for protection when the anhydrous ferrous sulfate is crushed;

[0122] 2) weighing 1211 g of anhydrous ferrous sulfate, 809 g of anhydrous sodium sulfate, 60 g of Super-P, and 121 g of ascorbic acid, wherein the molar ratio of sodium to iron is 1:0.7, first using a high-speed mixer to premix the above materials for 10 min, and using nitrogen for protection during the premixing process; then crushing and mixing the premixed materials using an airflow mill, wherein the airflow pressure of the airflow mill is 1 MPa and the time is 5 min, and the atmosphere of the airflow mill uses nitrogen; then loading the crushed and mixed materials into a planetary ball mill, wherein the ball-to-material ratio is 10:1 and the rotation speed is 600 rpm, and ball milling for 6 hours under nitrogen protection to obtain a precursor (a cathode material precursor);

[0123] 3) compacting the precursor to 1.4 g / cm 3 , then loading the compacted precursor into a hot isostatic pressing mold, then heating to 350°C at a rate of 2°C / min, then pressurizing to 10 MPa at 350°C, and holding for 12 h, then cooling to 150°C, then taking the material out of the hot isostatic pressing mold, then crushing and sieving to obtain a sodium sulfate ferrite composite cathode material.

[0124] The porosity of the sodium iron sulfate composite cathode material of Comparative Example 2 was 6.31%, the tap density was 2.23 g / cm 3 , the specific surface area was 11.83 m 2 / g, the mass percentage of sodium-rich impurity phase was 1.14%, the water content was 655.28 ppm, the water content increased at a rate of 5.72 ppm / h in an atmosphere with humidity of 0.1-0.5 ppm, the water content increased at a rate of 3.67 ppm / h in an atmosphere with humidity less than 0.1 ppm, the purity was 98.34%, the mass percentage of trivalent iron was 0.81%, the particle size of primary particles was 376 nm, and the particle size of secondary particles was 4.71 μm. In addition, in the sodium iron sulfate composite cathode material of Comparative Example 2, the mass percentage of secondary particles was 28.25%, the mass percentage of primary particles that did not agglomerate to form secondary particles was 69.87%, and the mass percentage of free conductive carbon was 1.88%. In the secondary particles, the mass percentage of secondary particles that were formed by agglomeration of primary particles through conductive carbon was 28.57%, and the mass percentage of the remaining secondary particles was 71.83%.

[0125] Comparative Example 3

[0126] This comparative example was basically the same as Example 1, except that:

[0127] Step 2) was adjusted to "weigh 1211 g of anhydrous ferrous sulfate, 809 g of anhydrous sodium sulfate, 60 g of Super-P, and 121 g of ascorbic acid, wherein the molar ratio of sodium to iron was 1:0.7. First, the above materials were premixed for 10 min using a high-speed mixer, and nitrogen was used for protection during the premixing process. Then, the premixed material was broken and mixed using an air jet mill, the air pressure of the air jet mill was 1 MPa, and the time was 5 min, and nitrogen was used as the atmosphere of the air jet mill. A precursor (cathode material precursor) was obtained.", and other conditions remained unchanged.

[0128] The porosity of the sodium iron sulfate composite cathode material of Comparative Example 3 was 3.95%, the tap density was 2.41 g / cm 3 , the specific surface area was 8.65 m 2The sodium-rich impurity phase has a mass percentage of 4.57%, a moisture content of 133.85 ppm, a moisture content increase rate of 4.63 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and a moisture content increase rate of 2.29 ppm / h in an atmosphere with a humidity of less than 0.1 ppm. The purity is 93.67%, the ferric iron mass percentage is 2.23%, the primary particle size is 1363 nm, and the secondary particle size is 15.32 μm. In addition, in the sodium ferric sulfate composite cathode material of Comparative Example 3, the secondary particle mass percentage is 13.68%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 82.46%, and the free conductive carbon mass percentage is 3.21%. Among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 14.39%, and the remaining secondary particles have a mass percentage of 85.61%.

[0129] Comparative Example 4

[0130] This comparative example is basically the same as Example 1, except that:

[0131] In step 1), the anhydrous ferrous sulfate and anhydrous sodium sulfate are pulverized to a particle size of 1.5 μm using an air jet mill.

[0132] The porosity of the sodium ferric sulfate composite cathode material in Comparative Example 4 was 3.87%, and the compaction density was 2.56 g / cm³. 3 Its specific surface area is 8.01 m². 2 The sodium-rich impurity phase has a mass percentage of 3.31%, a moisture content of 136.83 ppm, a moisture content increase rate of 4.52 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and a moisture content increase rate of 2.32 ppm / h in an atmosphere with a humidity of less than 0.1 ppm. The purity is 95.82%, the ferric iron mass percentage is 1.38%, the primary particle size is 1867 nm, and the secondary particle size is 19.58 μm. In addition, in the sodium ferric sulfate composite cathode material of Comparative Example 4, the secondary particle mass percentage is 14.76%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 82.03%, and the free conductive carbon mass percentage is 3.21%. Among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 14.39%, and the remaining secondary particles have a mass percentage of 85.61%.

[0133] Experimental Example 1

[0134] The following parameters of the sodium ferric sulfate composite cathode materials in each embodiment and comparative example were tested:

[0135] 1) Porosity: Porosity can be determined by measuring specific surface area (BET) in conjunction with porosity analysis. Specifically, the porosity ε of the sodium iron sulfate composite cathode material can be calculated using the following formula: ε = V p / (V p +1 / ρ 骨架 )

[0136] Where ε is the porosity of the sodium ferric sulfate composite cathode material, and V p This refers to the pore volume per unit mass of sodium ferric sulfate composite cathode material, expressed in cm³. 3 / g, ρ 骨架 Density of the framework (excluding pores) in the sodium ferric sulfate composite cathode material, expressed in g / cm³. 3 .

[0137] In the specific testing process, porosity can be obtained by measuring specific surface area (BET) and combining it with porosity analysis. This includes measuring the pore volume Vp and the density ρ of the framework (excluding pores) in a unit mass of sodium ferric sulfate composite cathode material. 骨架 The porosity of the sodium ferric sulfate composite cathode material can then be calculated using the formula described above.

[0138] Furthermore, the pore volume Vp in a unit mass of sodium ferric sulfate composite cathode material can be obtained by the following process: Test the maximum nitrogen adsorption volume Vads of a unit mass of sodium ferric sulfate composite cathode material under standard conditions at a relative pressure (P / P0) = 0.995. Then, Vp = Vads / 22400 × 10⁻⁶. -3 The density ρ of the framework (excluding pores) in the sodium ferric sulfate composite cathode material was measured using the helium specific gravity bottle method. 骨架 .

[0139] 2) Compacted density: The compacted density is tested using an automatic compacted density testing device.

[0140] 3) Specific surface area: The specific surface area was determined using the Bester method.

[0141] 4) Moisture content and rate of increase of moisture content: Moisture content was detected using the calorimeter method, and samples were taken every day for testing, for a total of 7 times, and the average rate of increase of moisture content was calculated.

[0142] 5) Chemical formula and purity of sodium ferric sulfate composite cathode material: The chemical formula of sodium ferric sulfate composite cathode material was obtained by analyzing the content of various elements through inductively coupled plasma atomic emission spectrometry (ICP analysis); the purity of sodium ferric sulfate composite cathode material was calculated using the content of various elements.

[0143] 6) Percentage of ferric iron by mass: determined by chemical titration.

[0144] 7) Mass percentage of sodium-rich impurity phase: The content of sodium-rich impurity phase was calculated using the refined X-ray diffraction (XRD) pattern and Jade software.

[0145] 8) Particle size of secondary particles and primary particles: The particle size of secondary particles and primary particles are directly measured from the scanning electron microscope (SEM) images.

[0146] 9) In the sodium ferric sulfate composite cathode material, the mass percentage of secondary particles, the mass percentage of primary particles existing alone without agglomeration to form secondary particles, and the mass percentage of free conductive carbon: First, the sample (sodium ferric sulfate composite cathode material) is centrifuged to separate the free conductive carbon. The mass of the free conductive carbon is weighed, and the mass percentage of free conductive carbon is calculated. Then, the number of primary particles and secondary particles is counted in the scanning electron microscope (SEM) image, and their particle size is measured. The volume of all primary and secondary particles can then be calculated. Since the density of primary and secondary particles is approximately equal, the volume ratio is calculated first, and then the mass percentage can be calculated.

[0147] 10) The mass percentage of secondary particles formed by the agglomeration of primary particles connected by conductive carbon, and the mass percentage of remaining secondary particles: Select secondary particles, perform cross-sectional analysis, and the quantity of the two types of secondary particles can be directly counted by transmission electron microscopy (TEM). Then, the result can be calculated based on statistical principles.

[0148] Test results

[0149] Table 1

[0150] Table 2

[0151] Data Analysis:

[0152] Figure 4 shows sodium ferric sulfate (Na) in Example 1. 2.5 Fe 1.75 XRD pattern of (SO4)3); In Figure 4, the horizontal axis is 2Theta (degree) (i.e., the diffraction angle 2θ) and the vertical axis is Intensity (au) (i.e., the intensity of the diffraction peak);

[0153] Figure 5 is an XRD pattern of the sodium-rich impurity phase (Na6Fe(S04)4) in the sodium iron sulfate of Comparative Example 1; in Figure 5, the abscissa is 2Theta (degree) (i.e., diffraction angle 2θ), and the ordinate is Intensity (a.u.) (i.e., intensity of the diffraction peak); as can be seen from Figure 5, the sodium-rich impurity phase Na6Fe(S04)4 in the dashed box is currently difficult to completely remove, and its content can be kept at a low level by adjusting the raw material ratio and process parameters. It can be determined at present that its content is too high, which will greatly reduce the electrochemical performance of the sample, but literature indicates that this phase and the target sodium iron sulfate phase will form a heterojunction, which will promote the transmission of sodium ions, but further verification is needed. And in the process of preparing sodium iron sulfate with high iron content by conventional methods, this phase is extremely easy to appear. By using hot isostatic pressing, sodium iron sulfate with high iron content and low sodium-rich impurity phase content can be prepared.

[0154] Test Example 2

[0155] After the sodium iron sulfate composite positive electrode materials of the examples and comparative examples were prepared into positive electrode sheets, the positive electrode sheets, negative electrode sheets, electrolyte and separators were assembled into button cells according to the following method. The method comprises:

[0156] Each sodium iron sulfate composite positive electrode material was mixed with conductive carbon black (SP) and PVDF at a weight ratio of 80:10:10, and a positive electrode slurry was obtained by dispersion. The positive electrode slurry was coated on an aluminum foil current collector and roll-pressed to prepare a positive electrode sheet with a surface density of 7.8 g / cm 3 The positive electrode sheet was then punched into a small round sheet with a diameter of 12 mm using a film tool, dried and weighed, and then assembled into a button cell in an Ar-protected glove box, using a 2025 button cell shell, a metal sodium negative electrode, an electrolyte solvent of ethylene carbonate and propylene carbonate solvents at a volume ratio of 1:1, and an electrolyte salt of 1 mol / L sodium hexafluorophosphate.

[0157] After the obtained button cells were placed at 25°C for 4h, the first charge-discharge capacity test was performed, and the test conditions were as follows: 0.1C charging to 4.5V, constant voltage charging to 0.025C cutoff, standing for 3min, 0.1C discharging to 2.0V, obtaining the charge-discharge curve, and recording the first charge capacity C0 and the first discharge capacity D0 of 2-4.5V, respectively, and calculating the first coulombic efficiency according to D0 / C0.

[0158] Test results

[0159] Table 3

[0160] Data analysis was performed in combination with the above table, Figures 1, 2 and 3:

[0161] The sodium ferric sulfate composite positive electrode material has good electrochemical performance; the precursor of Comparative Example 1 is sintered at normal pressure in a tube furnace, the contact between the materials is not sufficient during the sintering process, the atomic diffusion energy barrier is high, the reaction between the phases is insufficient, and even part of the reaction cannot be carried out, resulting in poor electrochemical performance of the sodium ferric sulfate composite positive electrode material; the precursor of Comparative Example 2 is pre-pressed (compacted) before sintering, which increases the contact between the materials to a certain extent, but when the pressure is released, the materials will rebound, reducing the tightness of the contact between the materials, and in the subsequent sintering process, the water and carbon dioxide generated by the decomposition of the additives and the water remaining in the materials will be discharged, reducing the tightness of the contact between the materials; even if hot-pressing sintering is used, the materials can be in close contact during the sintering process, the diffusion energy barrier is reduced, and the reaction between the materials is facilitated, but the pressure on the materials is unidirectional, and the pressure on the materials at different positions is inconsistent, resulting in poor electrochemical performance of the sodium ferric sulfate composite positive electrode material.

[0162] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A sodium ferric sulfate composite cathode material, characterized in that, The porosity of the sodium ferric sulfate composite positive electrode material is 0.1% to 10%, the compacted density is 2.1 to 3 g / cm 3 , and the specific surface area of the sodium ferric sulfate composite positive electrode material is 5 to 12 m 2 / g, and the mass percentage of the sodium-rich impurity phase in the sodium ferric sulfate composite positive electrode material is 0.1% to 1%.

2. The sodium ferric sulfate complex cathode material of claim 1, wherein, The sodium ferric sulfate composite positive electrode material has a water content increase rate of less than 5.5 ppm / h in an atmosphere with a humidity of 0.1-0.5 ppm, and a water content increase rate of less than 2.5 ppm / h in an atmosphere with a humidity of less than 0.1 ppm; The sodium ferric sulfate composite positive electrode material has a mass percentage of ferric iron of less than 1%; The sodium ferric sulfate composite positive electrode material has a mass percentage of impurities of less than 5%. 3.The sodium ferric sulfate composite cathode material of claim 1, characterized in that, The chemical formula of the sodium ferric sulfate composite positive electrode material is Na 2+2x Fe 2-x (SO4)3@C, wherein 0≤x<2. 4.The sodium ferric sulfate composite cathode material of claim 1, characterized in that, The primary particles of the sodium ferric sulfate composite positive electrode material have a particle size of 50-1000 nm, and part of the primary particles are agglomerated to form secondary particles, and the secondary particles have a particle size of 1-10 μm; In the sodium ferric sulfate composite positive electrode material, the mass percentage of the secondary particles is 15%-40%, and the mass percentage of the remaining primary particles is 60%-85%.

5. The sodium ferric sulfate complex cathode material of claim 4, wherein, The sodium ferric sulfate composite positive electrode material further comprises conductive carbon; In the secondary particles, part of the secondary particles are agglomerated by the primary particles through the conductive carbon, and the mass percentage of the part of the secondary particles in the secondary particles is 15%-40%, and the mass percentage of the remaining secondary particles in the secondary particles is 60%-85%.

6. A method for producing the sodium ferric sulfate composite positive electrode material according to any one of claims 1 to 5, characterized by, The method comprises the following steps: The iron source and the sodium source are respectively crushed to have a particle size of less than 1 μm; The crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant are mixed to obtain a precursor; The precursor is dried under vacuum, the dried precursor is sealed to ensure that the precursor is in a vacuum condition, and then the sealed precursor is subjected to isobaric sintering, the isobaric sintering is performed at a pressure of 1-500 MPa, a temperature of 300-400 ℃, and a time of 1-24 h, to obtain the sodium ferric sulfate composite positive electrode material.

7. The method for preparing the sodium ferric sulfate composite positive electrode material according to claim 6, wherein The process of mixing the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant to obtain the precursor comprises: pre-mixing the crushed iron source, the crushed sodium source, and the conductive carbon and the antioxidant, and then performing secondary crushing and mixing by using an airflow mill or a rolling ball mill, and finally performing solid solution mixing by using a planetary ball mill or a vibration ball mill, to obtain the precursor; The isobaric sintering comprises hot isostatic pressing sintering or spark plasma sintering; In the process of the isobaric sintering, the temperature is raised at a speed of 1-5 ℃ / min.

8. The method for preparing the sodium ferric sulfate composite positive electrode material according to claim 6, wherein The iron source comprises ferrous sulfate; The sodium source comprises sodium sulfate; The conductive carbon comprises one or more of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, redox graphene, acetylene black, ketjen black, carbon nanofibers, and activated carbon. And / or, the antioxidant comprises one or more of ascorbic acid, citric acid, tea polyphenol, glucose; the ratio of the mass of the antioxidant to the mass of the iron source is 0.01% to 10%.

9. A positive electrode sheet characterized by comprising: The positive sheet comprises the sodium ferrite composite positive material in any one of claims 1 to 5 or the sodium ferrite composite positive material obtained according to the preparation method in any one of claims 6 to 8.

10. A sodium-ion battery, characterized in that, The sodium ion battery comprises the positive sheet in claim 9.

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