A sodium ferric sulfate composite cathode material, its preparation method and application

Abstract

This invention provides a sodium ferric sulfate composite cathode material, its preparation method, and its applications. The sodium ferric sulfate composite cathode material has a porosity of 0.1%–10% and a compaction density of 2.1–3 g / cm³. 3 Furthermore, the specific surface area of ​​the sodium ferric sulfate composite cathode material is 5–12 m². 2 / g, wherein the sodium-rich impurity phase in the sodium ferric sulfate composite cathode material has a mass percentage content of 0.1% to 1%. This invention helps to improve the charge-discharge capacity and initial coulombic efficiency of the battery.

Description

Technical Field

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

[0002] Limited by the existing preparation process of sodium ion cathode materials, the bonding between conductive carbon and sodium iron sulfate in sodium ion cathode materials is not tight enough, which is not conducive to the performance of the materials.

[0003] Sodium-ion battery cathode materials include three types: layered oxides, Prussian white, and polyanionic materials. Among them, polyanionic materials have attracted widespread attention due to their structural stability, high operating voltage, good safety performance, high cycle stability, and high specific capacity. Iron-based sulfates are a resource-rich polyanionic material containing Fe. 3+ / Fe 2+ Reversible redox couples and SO4 2- Due to its strong electronegativity and inductive effect, polyanionic materials have the advantages of low production cost and high operating voltage (3.0-3.8V), making them suitable as cathode materials for sodium-ion batteries. However, polyanionic materials themselves have poor conductivity and often require carbon coating to obtain sodium-ion battery cathode materials with excellent electrochemical performance.

[0004] However, due to limitations in existing manufacturing processes, carbon-coated iron-based sulfate and other sodium-ion battery cathode materials have low compaction density, large specific surface area, and unsuitable porosity, which degrades their initial coulombic efficiency, charge-discharge specific capacity, and other performance characteristics.

[0005] For example, among existing methods for preparing sodium ferric sulfate, such as solid-state method, solvothermal reaction method, anti-solvent method, spray drying method, and freeze drying method, the solid-state method is simple and easy to operate, and therefore has been studied most extensively. In the solid-state method, ball milling is often used to crush and mix the materials. However, the limitations of ball milling can lead to insufficient mixing uniformity. Furthermore, since the sintering temperature for preparing sodium ferric sulfate cannot be too high, generally not exceeding 350℃, atoms are difficult to diffuse at lower sintering temperatures, resulting in insufficient reaction between phases, poor product consistency, low compaction density, and high impurity content in the prepared sodium ferric sulfate, thus leading to lower electrochemical performance.

[0006] Therefore, a composite cathode material of sodium iron sulfate with excellent porosity, compaction density and specific surface area is relatively scarce in the field. Summary of the Invention

[0007] This invention provides a sodium ferric sulfate composite cathode material, its preparation method, and its application. The sodium ferric sulfate composite cathode material has excellent porosity, compaction density, and specific surface area, and low sodium-rich impurity phase content, which helps to improve its charge-discharge capacity and first coulombic efficiency.

[0008] This invention provides a sodium ferric sulfate composite cathode material, wherein the porosity of the sodium ferric sulfate composite cathode material is 0.1% to 10%, and the compaction density is 2.1 to 3 g / cm³. 3 Furthermore, the specific surface area of ​​the sodium ferric sulfate composite cathode material is 5–12 m². 2 / g, the mass percentage of sodium-rich impurity phase in the sodium ferric sulfate composite cathode material is 0.1% to 1%.

[0009] Optionally, in an atmosphere with a humidity of 0.1–0.5 ppm, the rate of increase in water content of the sodium ferric sulfate composite cathode material is less than 5.5 ppm / h; in an atmosphere with a humidity of less than 0.1 ppm, the rate of increase in water content of the sodium ferric sulfate composite cathode material is less than 2.5 ppm / h; and / or, the mass percentage of trivalent iron in the sodium ferric sulfate composite cathode material is less than 1%; and / or, the mass percentage of impurities in the sodium ferric sulfate composite cathode material is less than 5%.

[0010] Optionally, the chemical formula of the sodium ferric sulfate composite cathode material is Na. 2+2x Fe 2-x (SO4)3@C, where 0≤x<2.

[0011] Optionally, the primary particles of the sodium ferric sulfate composite cathode material have a particle size of 50–1000 nm, and some of the primary particles agglomerate to form secondary particles with a particle size of 1–10 μm; in the sodium ferric sulfate composite cathode material, the mass percentage of the secondary particles is 15%–40%, and the mass percentage of the remaining primary particles is 60%–85%.

[0012] Optionally, the sodium ferric sulfate composite cathode material further includes conductive carbon; in the secondary particles, some of the secondary particles are formed by the agglomeration of the primary particles through the conductive carbon, the mass percentage 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%.

[0013] This invention provides a method for preparing the sodium iron sulfate composite cathode material as described above, comprising the following steps: pulverizing an iron source and a sodium source to a particle size of less than 1 μm; mixing the pulverized iron source, the pulverized sodium source, conductive carbon, and an antioxidant to obtain a precursor; drying the precursor under vacuum conditions; sealing the dried precursor to ensure it is under vacuum conditions; and then isobaric sintering the sealed precursor at a pressure of 1–500 MPa, a temperature of 300–400 °C, and a time of 1–24 h to obtain the sodium iron sulfate composite cathode material.

[0014] Optionally, the process of mixing the pulverized iron source, the pulverized sodium source, the conductive carbon, and the antioxidant to obtain the precursor includes: premixing the pulverized iron source, the pulverized sodium source, the conductive carbon, and the antioxidant, then pulverizing and mixing them again using an air jet mill or a rolling ball mill, and finally performing solid solution mixing using a planetary ball mill or a vibratory ball mill to obtain the precursor; and / or, the isobaric sintering includes hot isostatic pressing or spark plasma sintering; and / or, during the isobaric sintering process, the heating rate is 1–5 °C / min.

[0015] Optionally, the iron source includes ferrous sulfate; and / or, the sodium source includes sodium sulfate; and / or, the conductive carbon includes one or more of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, reduced-redox graphene, acetylene black, Ketjen black, carbon nanofibers, and activated carbon; and / or, the antioxidant includes one or more of ascorbic acid, citric acid, tea polyphenols, and glucose; the mass ratio of the antioxidant to the iron source is 0.01% to 10%.

[0016] The present invention also provides a positive electrode sheet, the positive electrode sheet comprising the sodium ferric sulfate composite positive electrode material as described above or the sodium ferric sulfate composite positive electrode material prepared according to the preparation method described above.

[0017] The present invention also provides a sodium-ion battery, the sodium-ion battery comprising the positive electrode sheet as described above.

[0018] This invention provides a sodium ferric sulfate composite cathode material, its preparation method, and its application. The sodium ferric sulfate composite cathode material has excellent porosity, compaction density, and specific surface area, and a low mass percentage of sodium-rich impurity phase, which helps to improve its charge-discharge capacity and first coulombic efficiency, thereby improving its electrochemical performance. Attached Figure Description

[0019] Figure 1 The charge-discharge curves for Examples 1-5 are shown below.

[0020] Figure 2 The charge-discharge curves for Examples 1, 6, and 7 are shown below.

[0021] Figure 3 The charge-discharge curves for Example 1 and Comparative Examples 1-3 are shown below.

[0022] Figure 4 Sodium ferric sulfate (Na) in Example 1 2.56 Fe 1.72 XRD pattern of (SO4)3;

[0023] Figure 5 The image shows the XRD pattern of the sodium-rich impurity phase (Na6Fe(SO4)4) in sodium ferric sulfate in Comparative Example 1. Detailed Implementation

[0024] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention provides a sodium ferric sulfate composite cathode material with a porosity of 0.1% to 10% and a compaction density of 2.1 to 3 g / cm³. 3 Furthermore, the specific surface area of ​​the sodium ferric sulfate composite cathode material is 5–12 m². 2 / g, the mass percentage of sodium-rich impurity phase in the sodium ferric sulfate composite cathode material is 0.1% to 1%.

[0026] According to the inventors' research and analysis, the porosity of the sodium ferric sulfate composite cathode material in this invention is 0.1% to 10%, avoiding excessive porosity that could limit the electrochemical performance of the sodium ferric sulfate composite cathode material. Furthermore, the compaction density of this sodium ferric sulfate composite cathode material is 2.1 to 3 g / cm³. 3 Specific surface area is 5-12 m² 2The sodium-rich impurity phase ( / g) helps improve its initial coulombic efficiency and charge / discharge specific capacity, thus improving its electrochemical performance. In addition, the sodium-rich impurity phase helps promote sodium ion transport, possibly because the sodium-rich impurity phase can form a heterojunction with sodium ferric sulfate, acting as a fast ion channel to promote sodium ion transport. However, if the content of the sodium-rich impurity phase is too high, it will inhibit the normal capacity of sodium ferric sulfate and also lead to a significant reduction in the electrochemical performance of the sodium ferric sulfate composite cathode material. In the sodium ferric sulfate composite cathode material of the present invention, the mass percentage of the sodium-rich impurity phase can be 0.1% to 1%. A lower content of sodium-rich impurity phase helps improve the electrochemical performance of the sodium ferric sulfate composite cathode material.

[0027] The sodium ferric sulfate composite cathode material of this invention exhibits weak water absorption. Specifically, in an atmosphere with humidity of 0.1–0.5 ppm, the rate of increase in water content of the sodium ferric sulfate composite cathode material is less than 5.5 ppm / h; in an atmosphere with humidity less than 0.1 ppm, the rate of increase in water content is less than 2.5 ppm / h. This contributes to improving the stability of its electrochemical performance and avoids the problem of electrochemical performance degradation caused by the strong water absorption of sodium ferric sulfate.

[0028] In some embodiments, after the above-mentioned sodium ferric sulfate composite cathode material is exposed to a drying room with a humidity of 0.01 to 0.1 ppm for one week, its moisture content is less than 1000 ppm.

[0029] In some embodiments, the mass percentage of impurities in the above-mentioned sodium ferric sulfate composite cathode material is less than 5%, that is, the purity of the sodium ferric sulfate composite cathode material is high.

[0030] The aforementioned impurities generally include manganese, zinc, calcium, and ferric iron. These impurities originate from sources such as the raw materials used in the preparation of sodium ferric sulfate composite cathode materials, which may contain manganese, zinc, and calcium. Furthermore, during the preparation process, wear and tear on equipment and oxidation can introduce impurities accounting for less than 3% of the mass content of the sodium ferric sulfate composite cathode material.

[0031] Furthermore, during the preparation process (sintering process), a sodium-rich impurity phase (Na6Fe(SO4)4) is generated in the sodium ferric sulfate composite cathode material. This sodium-rich impurity phase helps promote sodium ion transport, possibly because it can form a heterojunction with sodium ferric sulfate, acting as a fast ion channel to facilitate sodium ion transport. However, excessively high levels of this sodium-rich impurity phase can inhibit the normal capacity utilization of sodium ferric sulfate and significantly reduce the electrochemical performance of the sodium ferric sulfate composite cathode material. Therefore, it is necessary to control the content of the sodium-rich impurity phase at a low level. In the sodium ferric sulfate composite cathode material of this invention, the mass percentage of the sodium-rich impurity phase can be 0.1% to 1%. A lower content of this sodium-rich impurity phase helps improve the electrochemical performance of the sodium ferric sulfate composite cathode material.

[0032] In the process of preparing the sodium iron sulfate composite cathode material, ferrous iron is easily oxidized to ferric iron. When the content of ferric iron is too high, it will reduce the electrochemical performance of the sodium iron sulfate composite cathode material. However, in the sodium iron sulfate composite cathode material of the embodiment of the present invention, the mass percentage content of ferric iron can be less than or equal to 1%, for example, 0.1% - 1%. The influence of the lower content of ferric iron on the electrochemical performance of the sodium iron sulfate composite cathode material can be negligible.

[0033] In some embodiments, the chemical formula of the sodium iron sulfate composite cathode material is Na 2+2x Fe 2-x (SO4)3@C, where 0 ≤ x < 2. Combining sodium iron sulfate and conductive carbon helps improve the conductivity of the sodium iron sulfate composite cathode material, solves the problem of poor conductivity of sodium iron sulfate itself, and can also increase the specific surface area (BET) of the formed sodium iron sulfate composite cathode material and improve its electrochemical performance.

[0034] 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., 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, chlorides, nitrates or carbonates 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, z = x / 2 + y; Na x M y (SO4) z N n represents the sulfate, where x, y, z, 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, 0.3 ≤ a / b ≤ 2.8.

[0035] In some embodiments, the sodium ferric sulfate composite cathode material is granular, formed by combining conductive carbon and sodium ferric sulfate. The primary particles of this sodium ferric sulfate composite cathode material (conductive carbon composite sodium ferric sulfate) have a particle size of 50–1000 nm, and some of the primary particles agglomerate to form secondary particles with a particle size of 1–10 μm. That is, the primary particles of the above-mentioned sodium ferric sulfate composite cathode material can be divided into two parts: one part agglomerates to form secondary particles, and the remaining part does not agglomerate to form secondary particles and exists independently.

[0036] Various forms of conductive carbon exist in sodium ferric sulfate composite cathode materials, 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). Zero-dimensional conductive carbon is dot-like conductive carbon, such as Super-P and carbon black; one-dimensional conductive carbon is linear conductive carbon, such as carbon nanofibers and carbon nanotubes; and two-dimensional conductive carbon is sheet-like conductive carbon, such as graphene and graphene oxide. Free conductive carbon refers to the distribution state of conductive carbon. Any conductive carbon that does not form a definite connection with the sodium ferric sulfate matrix is ​​considered free conductive carbon. The free conductive carbon and the matrix material only have physical contact. The position of free conductive carbon in the sodium ferric sulfate composite cathode material is not fixed; stirring and ultrasonication during the preparation process can change its position.

[0037] It is evident that the surface of the primary particles is coated with conductive carbon in various forms.

[0038] In sodium ferric sulfate composite cathode materials, the mass percentage of the aforementioned secondary particles can be 15% to 40%, for example, 15%, 20%, 30%, 40%, or any combination thereof. The mass percentage of the remaining primary particles (primary particles that do not agglomerate to form secondary particles and exist alone) can be 60% to 85%, for example, 60%, 70%, 80%, 85%, or any combination thereof. The mass percentage of free conductive carbon can be 0.1% to 3%. This helps to improve the porosity, compaction density, and specific surface area of ​​the sodium ferric sulfate composite cathode material, thereby improving its electrochemical performance such as initial coulombic efficiency and charge / discharge specific capacity.

[0039] Secondary particles are mainly formed by the agglomeration of primary particles. These secondary particles can be divided into two parts according to their connection methods. One part consists of primary particles agglomerated through conductive carbon connections. These conductive carbon connections include the entanglement and bonding of one-dimensional conductive carbon, such as carbon nanotubes, and the coating and bonding of two-dimensional conductive carbon, such as graphene. This results in conductive carbon not only being distributed at the interfaces between primary particles, but also extensively coating the surface of the secondary particles. This coating is consistent with the form of two-dimensional conductive carbon coating the surface of primary particles. This portion of secondary particles constitutes 15%–40% of the total mass of the secondary particles. The remaining secondary particles are formed by the agglomeration of primary particles mainly through mechanical intercalation and van der Waals bonds. This remaining portion of secondary particles constitutes 60%–85% of the total mass of the secondary particles. This process helps improve the porosity, compaction density, and specific surface area of ​​the sodium ferric sulfate composite cathode material, thereby improving its electrochemical performance, such as initial coulombic efficiency and charge / discharge specific capacity.

[0040] This invention also provides a method for preparing the above-mentioned sodium iron sulfate composite cathode material, comprising the following steps: pulverizing an iron source and a sodium source to a particle size of less than 1 μm; mixing the pulverized iron source, the pulverized sodium source, conductive carbon, and an antioxidant to obtain a precursor; drying the precursor under vacuum conditions; sealing the dried precursor to ensure it is under vacuum conditions; and then isobaric sintering the sealed precursor at a pressure of 1–500 MPa, a temperature of 300–400 °C, and a time of 1–24 h to obtain the sodium iron sulfate composite cathode material.

[0041] According to the inventors' research and analysis: pulverizing the iron and sodium sources to a particle size D50 of less than 1μm helps promote uniform and thorough mixing of the raw materials; sealing the precursor under vacuum conditions for isobaric sintering not only prevents the oxidation of ferrous iron to ferric iron, but also ensures sufficient reaction during isobaric sintering by applying pressure to the precursor uniformly throughout the precursor, thus guaranteeing a purity of over 99% for the sodium ferric sulfate composite cathode material; furthermore, applying uniform pressure to the precursor during isobaric sintering allows for a tight bond between sodium ferric sulfate and carbon, improving its porosity, compaction density, and specific surface area. It also addresses the issue of sodium ferric sulfate's high water absorption, effectively reducing the water absorption of the sodium ferric sulfate composite cathode material. Furthermore, improving the bonding between carbon and sodium ferric sulfate further enhances the conductivity of the sodium ferric sulfate composite cathode material, thereby improving its initial coulombic efficiency, charge-discharge specific capacity, and other performance characteristics.

[0042] In specific implementation, mechanical milling and / or air jet milling can be used to pulverize the iron and sodium sources to a particle size of less than 1 μm (controlled within 1 μm). When pulverizing the iron source, it should be carried out under inert gas protection. This embodiment of the invention does not specifically limit the pressure and time of the air jet milling, as long as the particle size of the pulverized iron and sodium sources meets the requirements. For example, the pressure and time of the air jet milling can be 1.4–1.5 MPa and the time can be 8–10 min. The inert gas may include nitrogen and / or argon.

[0043] Generally, the iron and sodium sources can be vacuum dried before pulverization, for example, vacuum drying at 200–350°C for 1–15 hours. Before mixing various raw materials, the iron and sodium sources are pre-crushed (pulverized) to ensure that the D50 of the pulverized iron and sodium sources is less than 1 μm, which helps to improve the uniformity of subsequent mixing.

[0044] The process of obtaining a precursor by mixing pulverized iron source, pulverized sodium source, conductive carbon and antioxidant includes: premixing the pulverized iron source, pulverized sodium source, conductive carbon and antioxidant, then pulverizing and mixing again using an air jet mill or a rolling ball mill, and finally performing solid solution mixing using a planetary ball mill or a vibrating ball mill to obtain the precursor.

[0045] Understandably, inert gas protection is required during the process of obtaining the precursor.

[0046] In practice, the pulverized iron source, pulverized sodium source, conductive carbon, and antioxidant can be premixed in a high-speed mixer (high-speed mixer) to achieve a relatively uniform macroscopic state. Next, the materials are further pulverized and mixed using an air jet mill or a rolling ball mill. This further pulverization (crushing) of the premixed materials makes the mixture more uniform, achieving a higher degree of uniformity at the microscopic level, which helps reduce component segregation. Finally, a planetary ball mill or a vibratory ball mill is used for solution mixing. Planetary or vibratory ball mills promote the pre-reaction of the iron and sodium sources. Specifically, under the high energy generated by high-energy ball mills such as planetary or vibratory ball mills, the iron and sodium sources can undergo a pre-reaction, achieving solid solution (essentially atomic diffusion) of the iron and sodium phases. This facilitates the formation of a solid solution of the iron and sodium sources, reducing some diffusion barriers for subsequent sintering reactions. Through the above premixing, air jet milling, or rolling ball milling... The synergistic effect of ball milling for further crushing and mixing, planetary ball milling, or vibratory ball milling for solid solution mixing, combined with the uniform pressure provided by isobaric sintering, promotes atomic diffusion, allowing all materials to react fully. This, in turn, promotes the formation of the target phase (sodium ferric sulfate composite cathode material), thereby increasing the purity of the sodium ferric sulfate composite cathode material to greater than 99%. It also helps improve the porosity, compaction density, specific surface area, initial coulombic efficiency, and charge / discharge specific capacity of the sodium ferric sulfate composite cathode material. This avoids the environmental costs associated with organic solvent recovery and treatment, as well as the degradation of cathode material performance caused by organic solvent residues, which are common with wet ball milling using organic solvents. Furthermore, it solves the problem in existing technologies where insufficient crushing of raw materials to the submicron level limits the uniformity of mixing and the degree of reaction. It also avoids the problem of unreacted material residues leading to performance segregation in the sodium ferric sulfate composite cathode material due to insufficient reaction.

[0047] The premixing time of the above-mentioned high-speed mixer can be 5-10 min; the airflow pressure of the airflow 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 is (15-25):1, the rotation speed is 500-600 rpm and the time is 22-26 h; the ball-to-material ratio of the planetary ball mill is (8-12):1, the rotation speed is 500-700 rpm and the time is 4-8 h; the vibration frequency of the vibrating ball mill is 1400-1500 r / min and the time is 4-6 h.

[0048] The aforementioned isostatic sintering may include hot isostatic pressing (HIP) or spark plasma sintering (SPPS), with HIP being preferred.

[0049] Understandably, sodium-rich impurities are difficult to completely remove. Although their content can be kept at a low level by adjusting the raw material ratio and process parameters (e.g., reducing the iron content), this phase is very likely to appear during the preparation of sodium ferric sulfate with high iron content using conventional methods. However, in the preparation method system provided in the embodiments of the present invention, isobaric sintering, preferably hot isostatic pressing, can be used to prepare sodium ferric sulfate composite cathode materials with a low content of sodium-rich impurities.

[0050] Furthermore, since the sintering temperature during the preparation of sodium ferric sulfate cannot be too high, generally not exceeding 350°C, atoms are difficult to diffuse at lower sintering temperatures. This results in the preparation of sodium ferric sulfate requiring a long reaction time and also leading to problems such as insufficient reaction between phases, poor consistency of sodium ferric sulfate, low electrochemical performance, and low compaction density. The embodiments of this invention employ hot isostatic pressing (HIP) technology to appropriately reduce the sintering temperature and shorten the sintering time, thereby improving production efficiency, reducing energy consumption, and improving the porosity of the sodium ferric sulfate composite cathode material. This, in turn, increases the compaction density and specific surface area of ​​the sodium ferric sulfate composite cathode material, thus improving its electrochemical performance, such as initial coulombic efficiency and charge-discharge specific capacity.

[0051] During hot isostatic pressing (HIP) sintering, the precursor can be placed in a mold and dried under vacuum conditions, which can effectively reduce the moisture in the precursor and effectively suppress the oxidation of ferrous iron during sintering.

[0052] The vacuum conditions described above can be a vacuum degree of 0.001 to 0.1 Pa, a drying temperature of 250 to 300 °C, and a drying time of 0.5 to 2 h.

[0053] After loading the precursor into the mold, it can be compacted to a density of not less than 1.0 g / cm³. 3 Then, subsequent drying and other processes are carried out.

[0054] Next, the dried precursor is sealed to ensure it remains in a closed state throughout the subsequent sintering process. This ensures the precursor is under vacuum, effectively preventing the oxidation of ferrous iron during later processing. It also prevents contact between the precursor and moisture, effectively reducing the water absorption of the sodium ferric sulfate composite cathode material. Specifically, the mold can be sealed or airtight to ensure the precursor is under vacuum.

[0055] Then, the sealed precursor is subjected to isobaric sintering. For example, the sealed precursor can be placed in a hot press furnace for isobaric sintering to obtain sodium ferric sulfate composite cathode material. During isobaric sintering, the material is fully compressed, and the pressure on all parts of the precursor is uniform and consistent. This pressure is maintained throughout the reaction process, effectively ensuring the full progress of the reaction and ensuring that sodium ferric sulfate and carbon are tightly bonded together. This allows carbon to tightly coat the sodium ferric sulfate, effectively reducing the water absorption of the sodium ferric sulfate composite cathode material. This effectively solves the problem of water absorption in sodium ferric sulfate composite cathode materials caused by the inability of existing technologies to effectively bond carbon and sodium ferric sulfate raw materials. After being exposed to a drying room with a humidity of 0.01-0.1 ppm for one week, the moisture content of the sodium ferric sulfate composite cathode material remains below 1000 ppm. At the same time, the uniform and consistent pressure allows sodium ferric sulfate and carbon to directly and tightly contact each other, reducing the voids between sodium ferric sulfate and carbon. This prevents excessive voids from degrading the performance of the sodium ferric sulfate composite cathode material and also avoids the problem of excessive specific surface area caused by the presence of carbon. As a result, the porosity, compaction density, and specific surface area of ​​the sodium ferric sulfate composite cathode material are effectively improved.

[0056] Furthermore, during the isobaric sintering process, the heating rate can be 1 to 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 any combination thereof.

[0057] In the preparation system of this invention, by adjusting the pressure, temperature, and time of the isobaric sintering process, the porosity of the sodium ferric oxide composite cathode material can be effectively controlled, achieving a porosity of 0.1% to 20%. Simultaneously, the crystallinity of the sodium ferric oxide composite cathode material can be improved, ensuring a compaction density of 1.5 to 3 g / cm³. 3 Furthermore, the specific surface area of ​​this sodium ferric sulfate composite cathode material is 5–20 m². 2 / g, to avoid excessive specific surface area reducing the initial coulombic efficiency and affecting the performance of sodium ferric sulfate composite cathode material, and can also improve its water absorption, initial coulombic efficiency, charge and discharge specific capacity and other properties.

[0058] To protect the equipment and molds, the sealed precursor can be placed under an inert gas for isobaric sintering (at this time, the precursor is still in a vacuum state under sealed conditions). After the isobaric sintering is completed, the pressure of the reaction system is released, and the reaction product is cooled to 50-150°C. Then, it is crushed and sieved to obtain sodium iron sulfate composite cathode material.

[0059] The vacuum hot pressing sintering process in the prior art has poor sealing, and the material to be sintered is at risk of being oxidized. It is necessary to maintain vacuum during the sintering process, which increases the production cost. However, the precursor of the present invention is dried and vacuum sealed, which can ensure that the precursor will not be oxidized, thereby avoiding the oxidation of ferrous iron to ferric iron.

[0060] The iron source used in this invention embodiment may 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 may include sodium sulfate, such as one or more of anhydrous sodium sulfate and sodium sulfate decahydrate; the conductive carbon may include one or more of conductive carbon black (Super-P), single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, reduced-redox graphene, acetylene black, Ketjen black, carbon nanofibers, and activated carbon; the antioxidant may include one or more of ascorbic acid, citric acid, tea polyphenols, and glucose. In the raw materials of the sodium ferric sulfate composite cathode material, the mass percentage of conductive carbon may be 0.1% to 10%, and the mass ratio of the antioxidant to the iron source may be 0.01% to 10%.

[0061] This invention also provides a positive electrode sheet, which comprises the above-described sodium ferric sulfate composite positive electrode material or the sodium ferric sulfate composite positive electrode material prepared according to the above method. This positive electrode sheet possesses the technical effects corresponding to the above-described sodium ferric sulfate composite positive electrode material, which will not be elaborated further here.

[0062] The positive electrode sheet of this invention specifically includes a positive current collector and a positive active layer formed of the above-mentioned sodium ferric sulfate composite positive electrode material disposed on the surface of the positive current collector.

[0063] In the specific preparation of the positive electrode sheet, for example, the sodium ferric sulfate composite positive electrode material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and slitting, the positive electrode sheet is obtained. In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% sodium ferric sulfate composite positive electrode material, 0.5-15 wt% conductive agent, and 0.5-15 wt% binder, and further comprises 80-98 wt% sodium ferric sulfate composite positive electrode material, 1-10 wt% conductive agent, and 1-10 wt% binder.

[0064] The positive electrode current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, carbon fiber, carbon nanotubes, and conductive graphite; the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0065] This invention also provides a sodium-ion battery, which includes the aforementioned positive electrode sheet. This sodium-ion battery possesses the technical effects corresponding to the aforementioned positive electrode sheet or the sodium iron sulfate composite positive electrode material, which will not be elaborated further here.

[0066] As can be imagined, the sodium-ion battery of this embodiment of the invention, in addition to the above-mentioned positive electrode, also includes a negative electrode, an electrolyte, and a separator.

[0067] The embodiments of the present invention are not strictly limited to the negative electrode active material in the negative electrode sheet. It can be the negative electrode active material commonly used in sodium-ion batteries, such as at least one of hard carbon, soft carbon, titanium-based materials, metal oxides and sulfides.

[0068] The embodiments of the present invention do not strictly limit the selection of electrolyte, and may include one or more of the solvents commonly used in sodium-ion battery electrolytes, as well as the electrolyte sodium salts commonly used in sodium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene 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, γ-butyrolactone, etc.; the electrolyte may be one or more of sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium fluorotrifluoromethanesulfonylimide.

[0069] The embodiments of the present invention do not strictly limit the choice of membrane material. It can be one of the membrane materials commonly used in sodium-ion batteries, such as polypropylene membrane (PP), polyethylene membrane (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun membrane (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven membrane, and membrane with ceramic coating.

[0070] In the preparation of sodium-ion batteries, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the sodium-ion battery.

[0071] The present invention will be further described below through specific embodiments and comparative examples. Unless otherwise specified, the reagents, materials and instruments used below are all conventional reagents, materials and instruments, all of which are commercially available, and the reagents and materials involved can also be synthesized by conventional synthetic methods.

[0072] Example 1

[0073] This embodiment provides a method for preparing a sodium ferric sulfate composite cathode material, including:

[0074] 1) Ferrous sulfate heptahydrate was dried under vacuum at 350℃ for 10h to obtain anhydrous ferrous sulfate; sodium sulfate was dried under vacuum at 200℃ for 10h to obtain anhydrous sodium sulfate; the above anhydrous ferrous sulfate and anhydrous sodium sulfate were pulverized to a particle size D50 of 0.8μm using an air jet mill, wherein the pressure of the air jet mill was 1.5MPa and the time was 10min, and nitrogen protection was used when pulverizing anhydrous ferrous sulfate;

[0075] 2) Weigh 1211g of anhydrous ferrous sulfate, 809g of anhydrous sodium sulfate, 60g of Super-P, and 121g of ascorbic acid, with a sodium-to-ferrous molar ratio of 1:0.7. First, premix the above materials for 10 minutes using a high-speed mixer under nitrogen protection. Then, crush and mix the premixed materials using an air jet mill at an air pressure of 1MPa for 5 minutes under nitrogen atmosphere. Finally, load the crushed and mixed materials into a planetary ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 600rpm. Ball mill for 6 hours under nitrogen protection to obtain the precursor (positive electrode material precursor).

[0076] 3) Compact the precursor to 1.4 g / cm³. 3 Then, it is placed into a hot isostatic pressing mold, and the precursor, together with the hot isostatic pressing mold, is heated to 300°C under vacuum of 0.001Pa and baked for 0.5 hours.

[0077] 4) After sealing the hot isostatic pressing mold, place it in a hot isostatic press and heat it to 350°C at 2°C / min. Then pressurize it to 10MPa at 350°C and hold it at that temperature for 12 hours. After cooling it to 150°C, remove the material from the hot isostatic pressing mold, crush and sieve it to obtain sodium ferric sulfate composite cathode material.

[0078] The porosity of the sodium ferric sulfate composite cathode material in Example 1 was 3.72%, and the compaction density was 2.48 g / cm³. 3 Its specific surface area is 7.65 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.32%, the moisture content is 136.11ppm, the moisture content rises at a rate of 4.35ppm / h in an atmosphere with a humidity of 0.1-0.5ppm, and at a rate of 2.35ppm / h in an atmosphere with a humidity of less than 0.1ppm, the purity is 99.53%, the trivalent iron mass percentage is 0.22%, the primary particle size is 382nm, and the secondary particle size is 4.68μm; in addition, in the sodium ferric sulfate composite cathode material of Example 1, the secondary particle mass percentage is 28.13%, the primary particles that do not agglomerate to form secondary particles have a mass percentage of 69.96%, and the free conductive carbon mass percentage is 1.91%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 28.64%, and the remaining secondary particles have a mass percentage of 71.36%.

[0079] Example 2

[0080] This embodiment is basically the same as Embodiment 1, except that:

[0081] In step 4), the pressure of the heat isobaric process is adjusted from 10 MPa to 20 MPa, while other conditions remain unchanged.

[0082] The porosity of the sodium ferric sulfate composite cathode material in Example 2 was 3.12%, and the compaction density was 2.65 g / cm³. 3 Its specific surface area is 6.37 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.24%, the moisture content is 132.36 ppm, the moisture content rises at a rate of 4.31 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and at a rate of 2.26 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 99.62%, the trivalent iron mass percentage is 0.19%, the primary particle size is 489 nm, and the secondary particle size is 5.62 μm; in addition, in the sodium ferric sulfate composite cathode material of Example 2, the secondary particle mass percentage is 31.44%, the primary particles that do not agglomerate to form secondary particles have a mass percentage of 67.21%, and the free conductive carbon mass percentage is 1.35%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 31.27%, and the remaining secondary particles have a mass percentage of 68.73%.

[0083] Example 3

[0084] This embodiment is basically the same as Embodiment 1, except that:

[0085] In step 4), the pressure of the heat isobaric process is adjusted from 10 MPa to 5 MPa, while other conditions remain unchanged.

[0086] The porosity of the sodium ferric sulfate composite cathode material in Example 3 was 4.83%, and the compaction density was 2.23 g / cm³. 3 Its specific surface area is 9.78 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.35%, the moisture content is 129.58 ppm, the moisture content rises at a rate of 3.57 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and 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 trivalent iron mass percentage is 0.21%, the primary particle size is 361 nm, and the secondary particle size is 4.16 μm; in addition, in the sodium ferric sulfate composite cathode material of Example 3, the secondary particle mass percentage is 19.75%, the primary particles that do not agglomerate to form secondary particles have a mass percentage of 77.63%, and the free conductive carbon mass percentage is 2.62%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 23.55%, and the remaining secondary particles have a mass percentage of 76.45%.

[0087] Example 4

[0088] This embodiment is basically the same as Embodiment 1, except that:

[0089] In step 4), the temperature of the heat isobaric process is adjusted from 350℃ to 300℃, while other conditions remain unchanged.

[0090] The sodium ferric sulfate composite cathode material in Example 4 had a porosity of 4.03% and a compaction density of 2.37 g / cm³. 3 The specific surface area is 8.84 m². 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%.

[0091] Example 5

[0092] This embodiment is basically the same as Embodiment 1, except that:

[0093] In step 4), the heat treatment time (heat holding time) is adjusted from 12h to 8h, while other conditions remain unchanged.

[0094] 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%.

[0095] Example 6

[0096] This embodiment is basically the same as Embodiment 1, except that:

[0097] In step 2), replace “weigh 1211g anhydrous ferrous sulfate (FeSO4), 809g anhydrous sodium sulfate (Na2SO4), 60g Super-P, and 123g ascorbic acid, with a sodium-iron molar ratio of 1:0.7” with “weigh 1244g anhydrous ferrous sulfate (FeSO4), 776g anhydrous sodium sulfate (Na2SO4), 60g Super-P, and 121g ascorbic acid, with a sodium-iron molar ratio of 1:0.7”, while keeping other conditions unchanged.

[0098] The sodium ferric sulfate composite cathode material in Example 6 had a porosity of 3.85% and a compaction density of 2.51 g / cm³. 3 The specific surface area is 7.85 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.53%, the moisture content is 131.65ppm, the moisture content rises at a rate of 3.86ppm / h in an atmosphere with a humidity of 0.1-0.5ppm, and at a rate of 2.28ppm / h in an atmosphere with a humidity of less than 0.1ppm, the purity is 99.51%, the trivalent iron mass percentage is 0.31%, the primary particle size is 396nm, and the secondary particle size is 4.87μm; in addition, in the sodium ferric sulfate composite cathode material of Example 6, the secondary particle mass percentage is 29.33%, the primary particle mass percentage that exists alone without agglomeration to form secondary particles is 68.83%, and the free conductive carbon mass percentage is 1.84%; among the secondary particles, the mass percentage that is formed by the agglomeration of primary particles connected by conductive carbon is 27.49%, and the remaining secondary particles have a mass percentage of 72.51%.

[0099] Example 7

[0100] This embodiment is basically the same as Embodiment 1, except that:

[0101] In step 2), replace “Weigh 1211g of anhydrous ferrous sulfate (FeSO4), 809g of anhydrous sodium sulfate (Na2SO4), 60g of Super-P, and 129g of ascorbic acid, where the sodium-iron molar ratio is 1:0.7” with “Weigh 1294g of anhydrous ferrous sulfate (FeSO4), 726g of anhydrous sodium sulfate (Na2SO4), 60g of Super-P, and 121g of ascorbic acid, where the sodium-iron molar ratio is 1:0.7”, while keeping other conditions unchanged.

[0102] The porosity of the sodium ferric sulfate composite cathode material in Example 7 was 3.82%, and the compaction density was 2.54 g / cm³. 3 The specific surface area is 7.96 m². 2 / g, the sodium-rich impurity phase has a mass percentage content of 0.47%, the moisture content is 135.91ppm, the moisture content rises at a rate of 3.69ppm / h in an atmosphere with a humidity of 0.1-0.5ppm, and at a rate of 2.26ppm / h in an atmosphere with a humidity of less than 0.1ppm, the purity is 99.23%, the trivalent iron mass percentage is 0.38%, the primary particle size is 403nm, and the secondary particle size is 4.91μm; in addition, in the sodium ferric sulfate composite cathode material of Example 7, the secondary particle mass percentage is 30.28%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 67.98%, and the free conductive carbon mass percentage is 1.74%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 27.02%, and the remaining secondary particles have a mass percentage of 72.98%.

[0103] Example 8

[0104] This embodiment is basically the same as Embodiment 1, except that:

[0105] In step 4), replace “4) Then seal the hot isostatic pressing mold, place it in a hot isostatic press, heat it to 400℃ at 2.2℃ / min, pressurize it to 1MPa at 400℃, hold it at 24h, then cool it to 150℃, remove the material from the hot isostatic pressing mold, crush and sieve it to obtain sodium ferric sulfate composite cathode material” with “4) Then seal the hot isostatic pressing mold, place it in a hot isostatic press, heat it to 350℃ at 2℃ / min, pressurize it to 10MPa at 350℃, hold it at 12h, then cool it to 150℃, remove the material from the hot isostatic pressing mold, crush and sieve it to obtain sodium ferric sulfate composite cathode material”, with other conditions remaining unchanged.

[0106] The sodium ferric sulfate composite cathode material in Example 8 had a porosity of 5.36% and a compaction density of 2.13 g / cm³. 3 Its specific surface area is 10.23 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.68%, the moisture content is 176.36 ppm, the moisture content rises at a rate of 4.52 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and at a rate of 2.48 ppm / h in an atmosphere with a humidity of less than 0.1 ppm, the purity is 97.14%, the trivalent iron mass percentage is 1.67%, the primary particle size is 768 nm, and the secondary particle size is 5.52 μm; in addition, in the sodium ferric sulfate composite cathode material of Example 8, the secondary particle mass percentage is 17.39%, the primary particles that do not agglomerate to form secondary particles have a mass percentage of 79.64%, and the free conductive carbon mass percentage is 2.97%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 18.38%, and the remaining secondary particles have a mass percentage of 81.62%.

[0107] Example 9

[0108] This embodiment is basically the same as Embodiment 1, except that:

[0109] In step 4), replace “4) Then seal the hot isostatic pressing mold, place it in a hot isostatic press, heat it to 300℃ at 1.8℃ / min, pressurize it to 500MPa at 300℃, hold it for 1 hour, then cool it to 150℃, remove the material from the hot isostatic pressing mold, crush and sieve it to obtain sodium ferric sulfate composite cathode material” with “4) Then seal the hot isostatic pressing mold, place it in a hot isostatic press, heat it to 350℃ at 2℃ / min, pressurize it to 10MPa at 350℃, hold it for 12 hours, then cool it to 150℃, remove the material from the hot isostatic pressing mold, crush and sieve it to obtain sodium ferric sulfate composite cathode material”, with other conditions remaining unchanged.

[0110] The sodium ferric sulfate composite cathode material in Example 9 had a porosity of 2.43% and a compaction density of 2.86 g / cm³. 3 The specific surface area is 5.23 m². 2 / g, the sodium-rich impurity phase has a mass percentage of 0.98%, the moisture content is 138.23ppm, the moisture content rises at a rate of 3.73ppm / h in an atmosphere with a humidity of 0.1-0.5ppm, and at a rate of 2.24ppm / h in an atmosphere with a humidity of less than 0.1ppm, the purity is, the trivalent iron mass percentage is 0.53%, the primary particle size is 935nm, and the secondary particle size is 8.74μm; in addition, in the sodium ferric sulfate composite cathode material of Example 9, the secondary particle mass percentage is 38.51%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 61.06%, and the free conductive carbon mass percentage is 0.43%; among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 39.12%, and the remaining secondary particles have a mass percentage of 60.88%.

[0111] Comparative Example 1

[0112] This comparative example provides a method for preparing a sodium ferric sulfate composite cathode material, including:

[0113] 1) Ferrous sulfate heptahydrate was dried under vacuum at 350℃ for 10h to obtain anhydrous ferrous sulfate; sodium sulfate was dried under vacuum at 200℃ for 10h to obtain anhydrous sodium sulfate; the above anhydrous ferrous sulfate and anhydrous sodium sulfate were pulverized to a particle size D50 of 0.8μm using an air jet mill, wherein the pressure of the air jet mill was 1.5MPa and the time was 10min, and nitrogen protection was used when pulverizing anhydrous ferrous sulfate;

[0114] 2) Weigh 1211g of anhydrous ferrous sulfate, 809g of anhydrous sodium sulfate, 60g of Super-P, and 121g of ascorbic acid, with a sodium-to-ferrous molar ratio of 1:0.7. First, premix the above materials for 10 minutes using a high-speed mixer under nitrogen protection. Then, crush and mix the premixed materials using an air jet mill at an air pressure of 1MPa for 5 minutes under nitrogen atmosphere. Finally, load the crushed and mixed materials into a planetary ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 600rpm. Ball mill for 6 hours under nitrogen protection to obtain the precursor (positive electrode material precursor).

[0115] 3) The precursor was placed in a tube furnace for sintering at a temperature of 350°C for 12 hours. After cooling to 150°C, it was crushed and sieved to obtain sodium iron sulfate composite cathode material.

[0116] The porosity of the sodium ferric sulfate composite cathode material in Comparative Example 1 was 8.87%, and the compaction density was 1.89 g / cm³. 3 The specific surface area is 15.76 m². 2The sodium-rich impurity phase has a mass percentage of 5.85%, a moisture content of 1487.69 ppm, a moisture content increase rate of 9.81 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and a moisture 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 ferric iron mass percentage of 1.61%, a primary particle size of 326 nm, and a secondary particle size of 3.95 μm. In addition, in the sodium ferric sulfate composite cathode material of Comparative Example 1, the secondary particle mass percentage is 16.97%, the primary particles existing alone without agglomeration to form secondary particles are 78.36%, and the free conductive carbon mass percentage is 4.12%. Among the secondary particles, the mass percentage formed by the agglomeration of primary particles connected by conductive carbon is 7.36%, and the remaining secondary particles account for 92.64%.

[0117] Comparative Example 2

[0118] This comparative example provides a method for preparing a sodium ferric sulfate composite cathode material, including:

[0119] 1) Ferrous sulfate heptahydrate was dried under vacuum at 350℃ for 10h to obtain anhydrous ferrous sulfate; sodium sulfate was dried under vacuum at 200℃ for 10h to obtain anhydrous sodium sulfate; the above anhydrous ferrous sulfate and anhydrous sodium sulfate were pulverized to a particle size D50 of 0.8μm using an air jet mill, wherein the pressure of the air jet mill was 1.5MPa and the time was 10min, and nitrogen protection was used when pulverizing anhydrous ferrous sulfate;

[0120] 2) Weigh 1211g of anhydrous ferrous sulfate, 809g of anhydrous sodium sulfate, 60g of Super-P, and 121g of ascorbic acid, with a sodium-to-ferrous molar ratio of 1:0.7. First, premix the above materials for 10 minutes using a high-speed mixer under nitrogen protection. Then, crush and mix the premixed materials using an air jet mill at an air pressure of 1MPa for 5 minutes under nitrogen atmosphere. Finally, load the crushed and mixed materials into a planetary ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 600rpm. Ball mill for 6 hours under nitrogen protection to obtain the precursor (positive electrode material precursor).

[0121] 3) Compact the precursor to 1.4 g / cm³. 3 Then, it is loaded into a hot isostatic pressing mold, heated to 350℃ at 2℃ / min, pressurized to 10MPa at 350℃, held for 12h, and then cooled to 150℃. The material is then removed from the hot isostatic pressing mold, crushed and sieved to obtain sodium ferric sulfate composite cathode material.

[0122] The porosity of the sodium ferric sulfate composite cathode material in Comparative Example 2 was 6.31%, and the compaction density was 2.23 g / cm³. 3 Its specific surface area is 11.83 m². 2 The sodium-rich impurity phase has a mass percentage of 1.14%, a moisture content of 655.28 ppm, a moisture content increase rate of 5.72 ppm / h in an atmosphere with a humidity of 0.1–0.5 ppm, and a moisture content increase rate of 3.67 ppm / h in an atmosphere with a humidity of less than 0.1 ppm. The purity is 98.34%, the ferric iron mass percentage is 0.81%, the primary particle size is 376 nm, and the secondary particle size is 4.71 μm. In addition, in the sodium ferric sulfate composite cathode material of Comparative Example 2, the secondary particle mass percentage is 28.25%, the primary particle mass percentage existing alone without agglomeration to form secondary particles is 69.87%, and the free conductive carbon mass percentage is 1.88%. Among the secondary particles, the mass percentage of secondary particles formed by agglomeration of primary particles connected by conductive carbon is 28.57%, and the remaining secondary particles have a mass percentage of 71.83%.

[0123] Comparative Example 3

[0124] This comparative example is basically the same as Example 1, except that:

[0125] Step 2) is adjusted to: "Weigh 1211g of anhydrous ferrous sulfate, 809g of anhydrous sodium sulfate, 60g of Super-P, and 121g of ascorbic acid, wherein the sodium-iron molar ratio is 1:0.7. First, premix the above materials for 10 minutes using a high-speed mixer, with nitrogen protection during the premixing process; then crush and mix the premixed materials using an air jet mill, with an air pressure of 1MPa and a time of 5 minutes, with nitrogen atmosphere used in the air jet mill; to obtain the precursor (positive electrode material precursor)"; other conditions remain unchanged.

[0126] The porosity of the sodium ferric sulfate composite cathode material in Comparative Example 3 was 3.95%, and the compaction density was 2.41 g / cm³. 3 Its specific surface area is 8.65 m². 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%.

[0127] Comparative Example 4

[0128] This comparative example is basically the same as Example 1, except that:

[0129] 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.

[0130] 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%.

[0131] Experimental Example 1

[0132] The following parameters of the sodium ferric sulfate composite cathode materials in each embodiment and comparative example were tested:

[0133] 1) Porosity: Porosity can be detected by specific surface area (BET) detection combined with porosity analysis.

[0134] 2) Compacted density: The compacted density is tested using an automatic compacted density testing device.

[0135] 3) Specific surface area: The specific surface area was determined using the Bester method.

[0136] 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.

[0137] 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.

[0138] 6) Percentage of ferric iron by mass: determined by chemical titration.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] Test results

[0144] Table 1

[0145]

[0146] Table 2

[0147]

[0148]

[0149] Data Analysis:

[0150] Figure 4 Sodium ferric sulfate (Na) in Example 1 2.5 Fe 1.75 XRD pattern of (SO4)3; Figure 4 The horizontal axis represents 2Theta (degree) (i.e., the diffraction angle 2θ), and the vertical axis represents Intensity (au) (i.e., the intensity of the diffraction peak).

[0151] Figure 5 The XRD pattern of sodium-rich impurity phase (Na6Fe(SO4)4) in sodium ferric sulfate in Comparative Example 1; Figure 5 The horizontal axis represents 2Theta (degree) (i.e., the diffraction angle 2θ), and the vertical axis represents Intensity (au) (i.e., the intensity of the diffraction peak); from Figure 5 As can be seen, the area within the dashed box represents the sodium-rich impurity phase Na6Fe(SO4)4. This impurity phase is currently difficult to completely remove, but its content can be kept at a low level by adjusting the raw material ratio and process parameters. It is certain that excessively high Na6Fe(SO4)4 content will significantly reduce the electrochemical performance of the sample. However, some literature indicates that this phase can form a heterojunction with the target sodium ferric sulfate phase, promoting sodium ion transport, but this requires further verification. Furthermore, this phase readily forms during the conventional preparation of high-ferric sodium sulfate. Using hot isostatic pressing (HIP), sodium ferric sulfate with a lower content of the sodium-rich impurity phase can be prepared.

[0152] Experimental Example 2

[0153] After the sodium ferric sulfate composite positive electrode materials of the examples and comparative examples were respectively fabricated into positive electrode sheets, they were assembled with negative electrode sheets, electrolytes, and separators according to the following method to obtain coin cells. The method includes:

[0154] Each sodium ferric sulfate composite cathode material was mixed with conductive carbon black (SP) and PVDF at a weight ratio of 80%:10%:10%, and dispersed to obtain a cathode slurry. This cathode slurry was then coated onto an aluminum foil current collector and rolled to obtain an areal density of 7.8 g / cm³. 3The positive electrode sheet was then punched into small round pieces with a diameter of 12mm using a film-making tool. After drying and weighing, the positive electrode sheet was assembled into a coin cell using a 2025 coin cell case, with metallic sodium as the negative electrode, and the electrolyte solvent being ethylene carbonate and propylene carbonate in a volume ratio of 1:1. The electrolyte salt was 1 mol / L sodium hexafluorophosphate.

[0155] After each coin cell was left to stand at 25°C for 4 hours, its first charge-discharge capacity was tested. The test conditions were: 0.1C charging to 4.5V, constant voltage charging to 0.025C cutoff, left to stand for 3 minutes, and then discharged at 0.1C to 2.0V. The charge-discharge curves were obtained, and the first charge capacity C0 and the first discharge capacity D0 from 2 to 4.5V were recorded respectively. The first coulombic efficiency was calculated according to D0 / C0.

[0156] Test results

[0157] Table 3

[0158] serial number 0.1C charging specific capacity mAh / g 0.1C discharge specific capacity mAh / g First Coulomb efficiency % Example 1 108.13 99.08 91.63 Example 2 106.16 97.50 91.84 Example 3 107.74 98.52 91.44 Example 4 108.49 98.39 90.69 Example 5 106.92 97.85 91.52 Example 6 109.54 101.24 92.43 Example 7 113.16 104.31 92.18 Example 8 98.67 89.69 90.89 Example 9 102.38 93.46 91.29 Comparative Example 1 93.21 83.51 90.67 Comparative Example 2 96.13 87.12 90.62 Comparative Example 3 72.95 65.52 89.81 Comparative Example 4 86.39 71.23 82.45

[0159] Based on the table above, Figure 1 , Figure 2 and Figure 3 Conduct data analysis:

[0160] The sodium ferric sulfate composite cathode material of this invention exhibits good electrochemical performance. In Comparative Example 1, the precursor was sintered at atmospheric pressure in a tube furnace. During sintering, insufficient contact between materials resulted in a high atomic diffusion barrier, incomplete reactions between phases, and even partial failure to react, leading to poor electrochemical performance of the sodium ferric sulfate composite cathode material. In Comparative Example 2, even though the precursor was pre-pressed (compacted) before sintering, which increased the contact between materials to some extent, the materials rebounded after the pressure was released, reducing the tightness of the contact. Furthermore, during subsequent sintering, residual moisture and moisture and carbon dioxide from the decomposition of additives were released, further reducing the tightness of the contact between materials. Even with hot-pressing sintering, which allows for closer contact and lowers the diffusion barrier, facilitating reactions, the pressure on the materials is unidirectional and inconsistent across different locations, resulting in poor electrochemical performance of the sodium ferric sulfate composite cathode material.

[0161] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium ferric sulfate composite cathode material, characterized in that, The porosity of the sodium ferric sulfate composite cathode material is 0.1%–10%, and the compaction density is 2.1–3 g / cm³. 3 Furthermore, the specific surface area of ​​the sodium ferric sulfate composite cathode material is 5–12 m². 2 / g, the mass percentage of sodium-rich impurity phase in the sodium ferric sulfate composite cathode material is 0.1% to 1%.

2. The sodium ferric sulfate composite cathode material according to claim 1, characterized in that, In an atmosphere with a humidity of 0.1 to 0.5 ppm, the rate of increase in water content of the sodium ferric sulfate composite cathode material is less than 5.5 ppm / h; in an atmosphere with a humidity of less than 0.1 ppm, the rate of increase in water content of the sodium ferric sulfate composite cathode material is less than 2.5 ppm / h. And / or, the mass percentage of trivalent iron in the sodium ferric sulfate composite cathode material is less than 1%; And / or, the mass percentage of impurities in the sodium ferric sulfate composite cathode material is less than 5%.

3. The sodium ferric sulfate composite cathode material according to claim 1, characterized in that, The chemical formula of the sodium ferric sulfate composite cathode material is Na. 2+2x Fe 2-x (SO4)3@C, where 0≤x<2.

4. The sodium ferric sulfate composite cathode material according to claim 1, characterized in that, The primary particles of the sodium ferric sulfate composite cathode material have a particle size of 50-1000 nm, and some of the primary particles agglomerate to form secondary particles with a particle size of 1-10 μm. In the sodium ferric sulfate composite cathode material, the secondary particles comprise 15% to 40% by mass, and the remaining portion comprises 60% to 85% by mass of the primary particles.

5. The sodium ferric sulfate composite cathode material according to claim 4, characterized in that, The sodium ferric sulfate composite cathode material also includes conductive carbon. In the secondary particles, a portion of the secondary particles are formed by the agglomeration of the primary particles connected by the conductive carbon, and the mass percentage of the secondary particles in the secondary particles is 15% to 40%, while the mass percentage of the remaining secondary particles in the secondary particles is 60% to 85%.

6. A method for preparing the sodium ferric sulfate composite cathode material according to any one of claims 1 to 5, characterized in that, Includes the following steps: The iron source and sodium source were pulverized to a particle size of less than 1 μm, respectively. The pulverized iron source, the pulverized sodium source, conductive carbon, and antioxidant are mixed to obtain a precursor; The precursor is dried under vacuum conditions, and the dried precursor is sealed to ensure that the precursor is under vacuum conditions. Then, the sealed precursor is subjected to isobaric sintering at a pressure of 1-500 MPa, a temperature of 300-400 °C, and a time of 1-24 h to obtain the sodium iron sulfate composite cathode material.

7. The method for preparing the sodium ferric sulfate composite cathode material according to claim 6, characterized in that, The process of obtaining the precursor by mixing the pulverized iron source, the pulverized sodium source, the conductive carbon and the antioxidant includes: premixing the pulverized iron source, the pulverized sodium source, the conductive carbon and the antioxidant, pulverizing and mixing them again using an air jet mill or a rolling ball mill, and finally performing solid solution mixing using a planetary ball mill or a vibrating ball mill to obtain the precursor; And / or, the isostatic sintering includes hot isostatic pressing or spark plasma sintering; And / or, during the isobaric sintering process, the heating rate is 1 to 5 °C / min.

8. The method for preparing the sodium ferric sulfate composite cathode material according to claim 6, characterized in that, The iron source includes ferrous sulfate; And / or, the sodium source includes sodium sulfate; And / or, the conductive carbon includes one or more of the following: conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphene oxide, reduced graphene oxide, acetylene black, Ketjen black, carbon nanofibers, and activated carbon. And / or, the antioxidant includes one or more of ascorbic acid, citric acid, tea polyphenols, and glucose; the mass ratio of the antioxidant to the iron source is 0.01% to 10%.

9. A positive electrode plate, characterized in that, The positive electrode sheet comprises the sodium ferric sulfate composite positive electrode material according to any one of claims 1 to 5 or the sodium ferric sulfate composite positive electrode material obtained by the preparation method according to any one of claims 6 to 8.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode as described in claim 9.

Images