Preparation method of sodium ferric sulfate material for sodium ion battery with high-stability structure

Abstract

The invention discloses a preparation method of a sodium ferric sulfate material for a sodium-ion battery with a high-stability structure, which comprises the following steps: S1, preparation of a precursor solution: carrying out wet mixing and temperature control dissolution on a sodium source, an iron source, sulfuric acid and an antioxidant to form the precursor solution; s2, preparation of an amorphous Na2Fe2 (SO4) 3 precursor: performing low-temperature evaporation to obtain the amorphous Na2Fe2 (SO4) 3 precursor; s3, preparation of a local crystalline state Na2Fe2 (SO4) 3 crystal: carrying out low-temperature calcination to form the local crystalline state Na2Fe2 (SO4) 3 crystal; s4, preparation of Na2Fe2 (SO4) 3 / C slurry: dispersing the local crystalline state Na2Fe2 (SO4) 3 crystals and a conductive agent in an organic solvent for wet grinding to form the Na2Fe2 (SO4) 3 / C slurry; and S5, carrying out temperature-controlled evaporation to form the Na2Fe2 (SO4) 3 / C composite material with a high-stability structure. The preparation method of the sodium ferric sulfate material for the sodium ion battery with the high-stability structure has the characteristics of high phase purity, high conductivity and excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a method for preparing a sodium iron sulfate material for a sodium ion battery with a high-stability structure. Background Art

[0002] The performance of cathode materials significantly influences the energy density and electrochemical performance of sodium-ion batteries, directly determining their energy storage capacity, cycle life, and safety. Sodium-ion battery materials suitable for large-scale production must not only meet basic requirements such as low cost, high performance, and ease of processing, but also possess good structural stability, high ionic conductivity, and excellent rate capability to meet the needs of diverse application scenarios.

[0003] However, among the currently available sodium-ion battery positive electrode materials, such as layered oxides, Prussian blue systems and polyanion materials, although each has its own advantages, their performance is uneven and it is difficult to fully meet the needs of practical applications.

[0004] For example, although layered oxides have a high specific capacity, they are prone to structural phase transitions during the cycle process, resulting in capacity decay. For example, although the synthesis process of Prussian blue materials is simple and the cost is low, its crystalline water content is difficult to control, which affects the electrochemical performance. In contrast, although polyanion materials have a stable structure and a high voltage platform, their specific capacity is relatively low, and the preparation process of some materials is complex. Therefore, sodium-ion battery positive electrode materials with excellent comprehensive performance and suitable for large-scale production are still relatively scarce. It is urgent to break through the existing bottlenecks through material design, process optimization and the development of new synthesis methods to promote the commercialization of sodium-ion battery technology.

[0005] Sulfate materials are an important type of positive electrode materials for polyanion sodium ion batteries. They have the advantages of abundant resources, simple preparation process, high voltage platform and high energy density. Currently, they are mainly used in the field of low-cost power. However, the edge-sharing arrangement of FeO6 octahedrons in the structure of sulfate materials leads to the 3+ -Fe 3+ The Coulomb repulsion between them is large, the structure is in a metastable state, and it is difficult to synthesize it in a one-step method. To solve this problem, Fe defects are usually introduced into the structure to reduce the Fe 3+ -Fe 3+ The Coulomb repulsion between the two elements stabilizes the structural framework and facilitates large-scale preparation. However, this method reduces the amount of transition metal available for redox, resulting in a decrease in the material's specific capacity and hindering the improvement of energy density. Furthermore, a high iron defect content also reduces the densification of the material structure, affecting the improvement of compaction density. Summary of the Invention

[0006] The present invention aims to provide a method for preparing a sodium ferric sulfate material for a highly stable structure sodium ion battery, which has the characteristics of high phase purity, high electrical conductivity and excellent electrochemical performance.

[0007] The present invention can be achieved through the following technical solutions:

[0008] The present invention discloses a method for preparing a sodium iron sulfate material for a sodium ion battery with a high-stability structure, comprising the following steps:

[0009] S1. Preparation of a precursor solution: mixing a sodium source, an iron source, sulfuric acid, and an antioxidant in a wet process and dissolving them under controlled temperature to form a precursor solution;

[0010] S2. Preparation of amorphous Na2Fe2(SO4)3 precursor: In a protective atmosphere, the precursor solution is evaporated at low temperature to obtain an amorphous Na2Fe2(SO4)3 precursor;

[0011] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: calcining an amorphous Na2Fe2(SO4)3 precursor at low temperature in a protective atmosphere to remove bound water while allowing the amorphous Na2Fe2(SO4)3 to bond with each other to form partially crystalline Na2Fe2(SO4)3 crystals;

[0012] Preparation of S4 and Na2Fe2(SO4)3 / C slurry: In a protective atmosphere, partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground to make the conductive agent evenly adhere to the surface of the Na2Fe2(SO4)3 crystals to form a Na2Fe2(SO4)3 / C slurry;

[0013] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is evaporated under controlled temperature to remove the organic solvent, thereby forming a Na2Fe2(SO4)3 / C composite material with a highly stable structure.

[0014] The preparation method of sodium iron sulfate (Na2Fe2(SO4)3 / C) composite material for sodium ion batteries is based on freeze-drying. Taking into account the slow movement of atoms under low temperature conditions, the bonding between atoms is ensured to be more uniform and stable, thereby achieving an orderly arrangement of atoms in the structure. This method can not only effectively avoid Fe 3+ -Fe 3+The Coulomb repulsion between the two can also ensure that there are no iron defects in the material structure, and the iron site occupancy rate is as high as 100%, thereby significantly improving the theoretical specific capacity of the material to more than 120mAh / g. At the same time, the sodium iron sulfate material prepared by this method has high phase purity, good crystallinity, dense structure, and high compaction density, which can meet the needs of large-scale production. In terms of electrochemical performance, the material exhibits excellent cycle stability and rate performance, and can achieve efficient sodium ion insertion and deinsertion at a high voltage platform, significantly improving the energy density and cycle life of sodium-ion batteries.

[0015] Furthermore, in step S1, the temperature is controlled at 25-100°C. Within this temperature range, the iron source can dissolve quickly. At the same time, the antioxidant will not become ineffective due to high temperature, thereby causing oxidation of divalent iron ions in the solution and affecting the formation of subsequent materials.

[0016] Furthermore, in step S2, the temperature of the low-temperature evaporation is <0°C, the purpose of which is to freeze the solution, reduce the activity of ions in the solution, make them move within a shorter distance, and avoid uneven distribution among ions.

[0017] Furthermore, in step S3, the temperature of low-temperature calcination is 80-250°C, and the time is >0.01H; when the temperature is lower than 80°C, part of the bound water in the material cannot be completely removed, affecting subsequent crystallization, and when the temperature is higher than 250°C, the melting of the precursor is intensified, the ion activity increases, and element segregation is easily generated, affecting the phase purity and electrochemical properties of the material.

[0018] Furthermore, in step S5, the evaporation temperature is controlled to be 60-200°C, the purpose of which is to remove the volatile organic solvent in the slurry, so that the conductive agent and the Na2Fe2(SO4)3 material can fit more closely, thereby realizing the construction of an electronic conductive network at the material interface and improving the rate performance of the material; when the temperature is >200°C, the Na2Fe2(SO4)3 material will melt due to the high temperature, the elements will be redistributed, and phase separation will occur, affecting the material properties.

[0019] Furthermore, in step S2, the moisture content in the amorphous Na2Fe2(SO4)3 precursor is <0.2wt%, the purpose of which is to avoid excessive moisture in this step, which will lead to melting of the material during the subsequent low-temperature calcination process, and then produce uneven ion distribution due to the repulsive force between transition metals.

[0020] Furthermore, in step S4, the grinding method is one or more of sand milling, ball milling, and high-speed dispersion, and the dispersion between materials is achieved in the form of shear force or mechanical force.

[0021] Furthermore, the protective atmosphere is one or more of the non-oxygen-containing atmospheres of argon, helium, nitrogen, carbon dioxide, and hydrogen. In different steps, the protective atmospheres may be the same or different, and other non-oxygen-containing gases may also be used as alternative protective atmospheres.

[0022] Furthermore, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium peroxide.

[0023] Furthermore, the iron source is one or more of iron powder, ferrous oxide, ferric oxide, ferric oxide, ferric hydroxide, ferrous hydroxide, ferrous sulfate, and ferric sulfate.

[0024] Furthermore, the antioxidant is one or more of ascorbic acid, riboflavin, cysteine, glutathione, phytic acid, anthocyanin, and tea polyphenols.

[0025] Furthermore, the conductive agent is one or more of carbon black and its derivatives, carbon nanotubes and their slurry, carbon fibers, graphene and its slurry, and flake graphite.

[0026] Furthermore, the organic solvent is one or more of an ether compound, a ketone compound, and a benzene compound; the ether compound is one or more of tetrahydrofuran, dimethoxyethane, trihydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethyl ether, and 2-methyl-tetrahydrofuran; the ketone compound is one or more of 9-fluorenone b, benzophenone, acetone, and cyclohexanone; and the benzene compound is one or more of 4-methylbiphenyl, toluene, xylene, and phenol.

[0027] The present invention provides a method for preparing a sodium iron sulfate material for a sodium ion battery with a high stability structure, which has the following beneficial effects:

[0028] First, the phase purity is high. The present invention uses freeze-drying technology to limit the ion activity to prepare a uniform and ionically cross-linked precursor compound. The low-temperature calcination promotes the local crystallization nucleation of the material to form a nanocrystalline Na2Fe2(SO4)3 material with high phase purity. This material has not been treated at high temperature, has less ion segregation, and has high phase purity.

[0029] Second, the electronic conductivity is high. Na2Fe2(SO4)3 and inorganic carbon can form an efficient conductive network inside the material through grinding, significantly improving the efficiency of electron transmission. The addition of inorganic carbon not only improves the contact of the material interface and reduces the interface resistance, but also changes the electronic structure of the material through the doping effect and increases the carrier concentration. In addition, the chemical modification of inorganic carbon can further optimize its surface properties and enhance its compatibility with the matrix material, thereby comprehensively improving the electronic conductivity of the material.

[0030] Third, the ionic conductivity is high. The grain size of Na2Fe2(SO4)3 nanocrystals is small, and they have more grain boundaries. Grain boundaries serve as fast ion transport channels, which can significantly reduce the activation energy of ion migration and increase the ion diffusion rate. In addition, the defects and stress fields in the grain boundary region of small-grained materials also help to enhance ion migration ability, thereby improving the overall ionic conductivity of the material.

[0031] Fourth, the electrochemical performance is excellent. In the present invention, the Na2Fe2(SO4)3 material crystal prepared by freeze-drying technology has fewer factors such as defects and dislocations that affect the ion content and deintercalation rate. Therefore, it has a higher phase purity, which corresponds to a higher sodium storage content and unobstructed sodium ion diffusion channels in the structure, thereby enabling the material to exhibit a higher reversible capacity and excellent rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the SEM image of the Na2Fe2(SO4)3 / CNT material obtained by freeze drying in Example 1;

[0033] Figure 2 This is the SE diagram of the liquid phase Na2Fe2(SO4)3 / CNT material of comparative example 1. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention is further described in detail below with reference to embodiments.

[0035] The present invention discloses a method for preparing a sodium iron sulfate material for a sodium ion battery with a high-stability structure, comprising the following steps:

[0036] S1. Preparation of precursor solution: Sodium source, iron source, sulfuric acid and antioxidant are mixed and dissolved by wet process under controlled temperature to form a precursor solution;

[0037] S2. Preparation of amorphous Na2Fe2(SO4)3 precursor: In a protective atmosphere, the precursor solution is evaporated at low temperature to obtain an amorphous Na2Fe2(SO4)3 precursor;

[0038] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: calcining an amorphous Na2Fe2(SO4)3 precursor at low temperature in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals;

[0039] Preparation of S4 and Na2Fe2(SO4)3 / C slurry: In a protective atmosphere, partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground to form a Na2Fe2(SO4)3 / C slurry;

[0040] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure.

[0041] Furthermore, in step S1, the temperature of the temperature control is 25-100°C.

[0042] Furthermore, in step S2, the temperature of the low-temperature evaporation is <0°C.

[0043] Furthermore, in step S3, the temperature of the low-temperature calcination is 80-250° C., and the time is >0.01H.

[0044] Furthermore, in step S5, the evaporation temperature is controlled to be 60-200°C.

[0045] Furthermore, in step S2, the moisture content in the amorphous Na2Fe2(SO4)3 precursor is <0.2wt%.

[0046] Furthermore, in step S4, the grinding method is one or more of sand milling, ball milling, and high-speed dispersion.

[0047] Furthermore, the protective atmosphere is one or more of the non-oxygen atmospheres of argon, helium, nitrogen, carbon dioxide, and hydrogen.

[0048] Furthermore, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium peroxide.

[0049] Furthermore, the iron source is one or more of iron powder, ferrous oxide, ferric oxide, ferric oxide, ferric hydroxide, ferrous hydroxide, ferrous sulfate, and ferric sulfate.

[0050] Furthermore, the antioxidant is one or more of ascorbic acid, riboflavin, cysteine, glutathione, phytic acid, anthocyanin, and tea polyphenols.

[0051] Furthermore, the conductive agent is one or more of carbon black and its derivatives, carbon nanotubes and their slurry, carbon fibers, graphene and its slurry, and flake graphite.

[0052] Furthermore, the organic solvent is one or more of an ether compound, a ketone compound, and a benzene compound; the ether compound is one or more of tetrahydrofuran, dimethoxyethane, trihydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethyl ether, and 2-methyl-tetrahydrofuran; the ketone compound is one or more of 9-fluorenone b, benzophenone, acetone, and cyclohexanone; and the benzene compound is one or more of 4-methylbiphenyl, toluene, xylene, and phenol.

[0053] It should be noted that, although there is no special description of the ratio of the sodium source, sulfuric acid, and iron source in step S1, based on the fact that the final sodium ferric sulfate material has a clear molar ratio for different elements, the molar ratio of the corresponding components in the sodium source, sulfuric acid, and iron source is naturally common knowledge. Similarly, the addition amount of the antioxidant and the addition amount of the conductive agent can be conventionally added, and there is no clear and special restriction on this in the present invention.

[0054] Example 1

[0055] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0056] S1. Preparation of precursor solution: A sodium source, an iron source, sulfuric acid, and an antioxidant are wet-mixed and dissolved under controlled temperature to form a precursor solution. Specifically, the sodium source is sodium carbonate; the iron source is ferrous oxide, ferric oxide, or ferric oxide; and the antioxidants are ascorbic acid and riboflavin. The temperature is controlled at 100°C.

[0057] S2. Preparation of an amorphous Na2Fe2(SO4)3 precursor: The precursor solution is evaporated at low temperature in a protective atmosphere to obtain an amorphous Na2Fe2(SO4)3 precursor. Specifically, the low-temperature evaporation temperature is -5°C, the moisture content of the amorphous Na2Fe2(SO4)3 precursor is less than 0.2 wt%, and the protective atmosphere is a non-oxygen atmosphere of argon or helium.

[0058] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: Low-temperature calcining of an amorphous Na2Fe2(SO4)3 precursor in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals. Specifically, the low-temperature calcination temperature is 250°C, the time is >0.01 h, and the protective atmosphere is a non-oxygen atmosphere such as argon or helium.

[0059] Preparation of S4 and Na2Fe2(SO4)3 / C slurries: Partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground in a protective atmosphere to form a Na2Fe2(SO4)3 / C slurry. Specifically, the conductive agent is carbon black and its derivatives, carbon nanotubes, and their slurries; the organic solvent is an ether compound, such as tetrahydrofuran, dimethoxyethane, trihydrofuran, or ethylene glycol dimethyl ether; the grinding method is sand milling, and the protective atmosphere is a non-oxygen atmosphere such as argon or helium.

[0060] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure. Specifically, the temperature-controlled evaporation temperature is 200° C., and the protective atmosphere is a non-oxygen atmosphere of argon or helium.

[0061] Example 2

[0062] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0063] S1. Preparation of precursor solution: Sodium source, iron source, sulfuric acid, and antioxidant are wet mixed and dissolved under controlled temperature to form a precursor solution. Specifically, the sodium source is sodium sulfate and sodium peroxide; the iron source is ferric hydroxyhydroxide; and the antioxidants are cysteine and tea polyphenols. The temperature is controlled at 60°C.

[0064] S2. Preparation of an amorphous Na2Fe2(SO4)3 precursor: The precursor solution is evaporated at low temperature in a protective atmosphere to obtain an amorphous Na2Fe2(SO4)3 precursor. Specifically, the low-temperature evaporation temperature is -3°C, the moisture content of the amorphous Na2Fe2(SO4)3 precursor is less than 0.2 wt%, and the protective atmosphere is a non-oxygen atmosphere of nitrogen or carbon dioxide.

[0065] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: Low-temperature calcining of an amorphous Na2Fe2(SO4)3 precursor in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals. Specifically, the low-temperature calcination temperature is 80-250°C, the time is >0.01 h, and the protective atmosphere is a non-oxygen-containing carbon dioxide atmosphere.

[0066] Preparation of S4 and Na2Fe2(SO4)3 / C slurry: In a protective atmosphere, partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground to form a Na2Fe2(SO4)3 / C slurry. Specifically, the conductive agent is carbon fiber, graphene, and their slurry; the organic solvent is a ketone compound and a benzene compound; the ketone compound is 9-fluorenone b and benzophenone, and the benzene compound is 4-methylbiphenyl and toluene; the grinding method is ball milling, and the protective atmosphere is a non-oxygenated carbon dioxide atmosphere.

[0067] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure. Specifically, the temperature-controlled evaporation temperature is 100° C., and the protective atmosphere is a non-oxygen atmosphere of carbon dioxide.

[0068] Example 3

[0069] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0070] S1. Preparation of precursor solution: Sodium source, iron source, sulfuric acid, and antioxidant are wet mixed and dissolved under controlled temperature to form a precursor solution. Specifically, the sodium source is sodium hydroxide or sodium sulfate; the iron source is ferroferric oxide, ferric hydroxide, ferrous hydroxide, or ferrous sulfate; and the antioxidant is anthocyanidin or tea polyphenols. The temperature is controlled at 30°C.

[0071] S2. Preparation of an amorphous Na2Fe2(SO4)3 precursor: The precursor solution is evaporated at low temperature in a protective atmosphere to obtain an amorphous Na2Fe2(SO4)3 precursor. Specifically, the low-temperature evaporation temperature is -8°C, the moisture content of the amorphous Na2Fe2(SO4)3 precursor is less than 0.2 wt%, and the protective atmosphere is a non-oxygen-containing nitrogen atmosphere.

[0072] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: Low-temperature calcining of an amorphous Na2Fe2(SO4)3 precursor in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals. Specifically, the low-temperature calcination temperature is 80°C, the time is >0.01 h, and the protective atmosphere is nitrogen, a non-oxygen atmosphere.

[0073] Preparation of S4 and Na2Fe2(SO4)3 / C slurries: Partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground in a protective atmosphere to form a Na2Fe2(SO4)3 / C slurry. Specifically, the conductive agent is carbon black and its derivatives, or graphene and its slurry; the organic solvent is an ether compound or a benzene compound. The ether compound is tetrahydrofuran, dimethoxyethane, or trihydrofuran, and the benzene compound is xylene or phenol. The grinding method is high-speed dispersion, and the protective atmosphere is nitrogen, a non-oxygen atmosphere.

[0074] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure. Specifically, the temperature-controlled evaporation temperature is 70° C., and the protective atmosphere is a non-oxygen atmosphere of nitrogen.

[0075] Example 4

[0076] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0077] S1. Preparation of precursor solution: Sodium source, iron source, sulfuric acid, and antioxidant are wet mixed and dissolved under controlled temperature to form a precursor solution. Specifically, the sodium source is sodium carbonate or sodium bicarbonate; the iron source is ferric sulfate; and the antioxidants are ascorbic acid and tea polyphenols. The temperature is controlled at 80°C.

[0078] S2. Preparation of an amorphous Na2Fe2(SO4)3 precursor: The precursor solution is evaporated at low temperature in a protective atmosphere to obtain an amorphous Na2Fe2(SO4)3 precursor. Specifically, the low-temperature evaporation temperature is -4°C, the moisture content of the amorphous Na2Fe2(SO4)3 precursor is less than 0.2wt%, and the protective atmosphere is a non-oxygen atmosphere of nitrogen or carbon dioxide.

[0079] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: Low-temperature calcining of an amorphous Na2Fe2(SO4)3 precursor in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals. Specifically, the low-temperature calcination temperature is 150°C for >0.01 h in a protective atmosphere of nitrogen or carbon dioxide, a non-oxygen atmosphere.

[0080] Preparation of S4 and Na2Fe2(SO4)3 / C slurry: Partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground in a protective atmosphere to form a Na2Fe2(SO4)3 / C slurry. Specifically, the conductive agent is carbon fiber, graphene and its slurry, or flake graphite; the organic solvent is a benzene compound, such as 4-methylbiphenyl, toluene, xylene, or phenol; the grinding method is sand milling or ball milling, and the protective atmosphere is a non-oxygen atmosphere of nitrogen or carbon dioxide.

[0081] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure. Specifically, the temperature-controlled evaporation temperature is 100° C., and the protective atmosphere is a non-oxygen atmosphere of nitrogen or carbon dioxide.

[0082] Example 5

[0083] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0084] S1. Preparation of precursor solution: A sodium source, an iron source, sulfuric acid, and an antioxidant are wet mixed and dissolved under controlled temperature to form a precursor solution. Specifically, the sodium source is sodium carbonate or sodium bicarbonate; the iron source is ferric oxide, ferric oxide, or ferric hydroxide; and the antioxidants are ascorbic acid and riboflavin. The temperature is controlled at 40°C.

[0085] S2. Preparation of an amorphous Na2Fe2(SO4)3 precursor: The precursor solution is evaporated at low temperature in a protective atmosphere to obtain an amorphous Na2Fe2(SO4)3 precursor. Specifically, the low-temperature evaporation temperature is -50°C, the moisture content of the amorphous Na2Fe2(SO4)3 precursor is less than 0.2 wt%, and the protective atmosphere is argon, a non-oxygen atmosphere.

[0086] S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: Low-temperature calcining of an amorphous Na2Fe2(SO4)3 precursor in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals. Specifically, the low-temperature calcination temperature is 150° C., the time is >0.01 h, and the protective atmosphere is argon, a non-oxygen atmosphere.

[0087] Preparation of S4 and Na2Fe2(SO4)3 / C slurry: Partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground in a protective atmosphere to form a Na2Fe2(SO4)3 / C slurry. Specifically, the conductive agent is carbon black and its derivatives, carbon nanotubes and their slurry; the organic solvent is a ketone compound, such as 9-fluorenone b, benzophenone, or acetone; the grinding method is sand milling, and the protective atmosphere is argon, a non-oxygen atmosphere.

[0088] S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure. Specifically, the temperature-controlled evaporation temperature is 110° C., and the protective atmosphere is argon, a non-oxygen atmosphere.

[0089] Application Example 1 Synthesis of Na2Fe2(SO4)3 / CNT Material by Freeze-Drying Method and Its Electrochemical Performance

[0090] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0091] Step 1: Sodium carbonate, atomized elemental iron powder, and sulfuric acid are mixed with water in a molar ratio of 1:2:3, and heated in an 80°C water bath to dissolve. At the same time, ascorbic acid is added as an antioxidant in an amount equivalent to the molar amount of elemental iron powder to react to form a uniform solution;

[0092] Step 2: freeze-dry the above solution at -40°C in a nitrogen atmosphere until the moisture content is less than 0.2 wt% to obtain a dry amorphous Na2Fe2(SO4)3 precursor;

[0093] Step 3: In a nitrogen atmosphere, the amorphous precursor is calcined at 180°C for 5 hours to further remove the water of crystallization in the crystal and simultaneously bond the amorphous Na2Fe2(SO4)3 to each other to form partially crystalline Na2Fe2(SO4)3 crystals;

[0094] Step 4: In a nitrogen atmosphere, Na2Fe2(SO4)3 crystals and CNTs were added to a tetrahydrofuran organic solvent in a mass ratio of 1:0.005, and ground using a planetary ball mill to allow the conductive agent to evenly adhere to the surface of the Na2Fe2(SO4)3 crystals to obtain a Na2Fe2(SO4)3 / C slurry;

[0095] Step 5: Under a nitrogen atmosphere, the Na2Fe2(SO4)3 / CNT slurry is evaporated at 150°C to remove the organic solvent and form a Na2Fe2(SO4)3 / CNT composite material with high structural stability.

[0096] Figure 1 This is an SEM image of the Na2Fe2(SO4)3 / CNT composite material, which is agglomerated particles of about 5um, with a relatively uniform morphology and an overall size smaller than that of Comparative Example 1. The main reason is that the low-temperature drying process hinders the agglomeration between the particles, resulting in a loose distribution, which is conducive to increasing the contact area with the electrolyte and thus improving its ion diffusion rate. The results in Table 1 show that the internal resistance of the material powder is only 3~5Ω, which is much lower than that of the material in the comparative example. This shows that the Na2Fe2(SO4)3 / CNT composite material achieves CNT coating by grinding, with high uniformity, which is conducive to reducing the interfacial internal resistance of the material. In addition, XRD refinement can be used to identify the phase purity of the Na2Fe2(SO4)3 / CNT composite material as high as 99.3%, indicating that the internal defects and dislocations of the structure affect the crystallinity of the material less. It also proves that the material prepared by the freeze-drying method has excellent process feasibility, while the phase purity of the materials in Comparative Examples 1 and 2 is lower, which is related to the large Coulomb repulsion in the material structure in the one-step synthesis method and the low degree of mixing uniformity between the raw materials.

[0097] The Na2Fe2(SO4)3 / CNT composite, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. After grinding, the black slurry was coated on aluminum foil using a 150μm four-sided preparation device. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used as the separator. CR2016 button cells were assembled in a glove box.

[0098] The button cell was subjected to constant current charge and discharge tests at a current density of 0.1C (1C=120mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the electrode has a reversible specific capacity of 116.8mAh / g, which is significantly improved compared to Comparative Examples 1 and 2. This is due to the high phase purity of the material and the high content of active transition metal ions in the structure. In addition, the results in Table 1 show that at 10C, the capacity retention rate of the material is 96.5% of that at 0.1C, showing excellent rate performance, which is consistent with the fact that the material has a high ionic conductivity (10 -9 S / m) are consistent, outperforming the performance of the materials in Comparative Examples 1 and 2. This demonstrates that the freeze-dried material has a small grain size, relatively fast ion migration kinetics, high transition velocities at high currents, and a low energy barrier. Ultimately, the electrode maintained a capacity of 98.2% after 500 cycles at a 1C rate, demonstrating excellent structural and cyclic stability compared to Comparative Examples 1 and 2. This demonstrates that the freeze-dried material exhibits more uniform volume expansion anisotropy during the deintercalation process. Furthermore, the relatively intact crystal structure reduces the degree of side reactions between the material and the electrolyte, effectively inhibiting dissolution at the material interface and thus improving its cyclic stability.

[0099] Application Example 2 Synthesis of Na2Fe2(SO4)3 / graphene material by freeze drying method and its electrochemical properties

[0100] This embodiment relates to a sodium iron sulfate material for a sodium ion battery with a high stability structure, and a preparation method thereof comprises the following steps:

[0101] Step 1: Sodium hydroxide, ferrous oxide, and sulfuric acid are mixed with water in a molar ratio of 2:2:3, and heated in a water bath at 100°C to dissolve. At the same time, ascorbic acid is added as an antioxidant in an amount equivalent to the molar amount of ferrous oxide to form a uniform solution.

[0102] Step 2: freeze-dry the above solution at -60°C in an argon atmosphere until the moisture content is less than 0.2 wt% to obtain a dry amorphous Na2Fe2(SO4)3 precursor;

[0103] Step 3: In an argon atmosphere, the amorphous precursor is calcined at 200°C for 3 hours to further remove the water of crystallization in the crystal and simultaneously bond the amorphous Na2Fe2(SO4)3 to each other to form partially crystalline Na2Fe2(SO4)3 crystals;

[0104] Step 4: In an argon atmosphere, Na2Fe2(SO4)3 crystals and graphene are added to an acetone organic solvent in a mass ratio of 1:0.005, and ground using a high-energy ball mill to uniformly adhere the conductive agent to the surface of the Na2Fe2(SO4)3 crystals to obtain a Na2Fe2(SO4)3 / graphene slurry;

[0105] Step 5: Under an argon atmosphere, the Na2Fe2(SO4)3 / graphene slurry is evaporated at 180°C to remove the organic solvent, thereby forming a Na2Fe2(SO4)3 / graphene composite material with high structural stability.

[0106] The results in Table 1 show that the internal resistance of the Na2Fe2(SO4)3 / graphene composite material powder is only 4~7Ω, which is significantly lower than that of the materials in the comparative examples. This indicates that the uniform coating of graphene achieved by grinding can effectively reduce the electron transmission resistance, thereby improving the conductive properties of the material. This low internal resistance characteristic is not only conducive to the rapid transmission of electrons in the material, but also significantly enhances the rate performance of the material, enabling it to maintain excellent electrochemical performance at high current density. In addition, XRD refinement analysis found that the phase purity of the Na2Fe2(SO4)3 / graphene composite material was as high as 99.5%, indicating that the impurity content in the material is extremely low, and there are few imperfections such as internal structural defects and dislocations, further verifying its excellent structural stability. This result also proves that the composite material prepared based on the freeze-drying method has extremely high process feasibility and structural controllability. In contrast, the phase purity of the materials in Comparative Examples 1 and 2 is lower, which is mainly attributed to the large Coulomb repulsion inside the material in the one-step synthesis method, resulting in poor mixing uniformity between the raw materials, thereby affecting the overall performance and structural integrity of the material. Therefore, Na2Fe2(SO4)3 / graphene composite materials show significant advantages in preparation process and structural optimization, providing a solid foundation for their application in the field of energy storage.

[0107] The Na2Fe2(SO4)3 / graphene composite, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. After grinding, the black slurry was coated on aluminum foil using a 150μm four-sided preparation device. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used as the separator. CR2016 button cells were assembled in a glove box.

[0108] The button cell was subjected to constant current charge and discharge tests at a current density of 0.1C (1C=120mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is 114.3mAh / g, and the capacity utilization is higher than that of the material in the comparative example. This is related to the higher phase purity of the material. The higher the phase purity of the material, the higher the transition metal content per unit mass, the more sodium ions available for redox, the fewer factors affecting ion diffusion in the crystal, and the higher the capacity utilization rate. In addition, the results in Table 1 show that the capacity retention rate of the material at 10C is 94.1% of that at 0.1C, showing excellent rate performance, which is consistent with the fact that the material has a higher ionic conductivity (10 -10 The S / m) ratio was consistent with that of the material in the comparative example, demonstrating that the freeze-dried material had smaller size and faster ion diffusion kinetics. Ultimately, the electrode maintained a capacity of 96.5% after 500 cycles at a 1C rate, demonstrating superior structural and cyclic stability compared to the material in the comparative example. This indicates that the freeze-dried material exhibits more uniform volume expansion anisotropy during the deintercalation process, effectively suppressing interfacial dissolution caused by repeated volume expansion, thereby improving its cyclic stability.

[0109] Comparative Example 1 Synthesis of Na2Fe2(SO4)3 / CNT Materials by Liquid Phase Method and Their Electrochemical Properties

[0110] This embodiment relates to a sodium iron sulfate material for sodium ion batteries, and a preparation method thereof comprises the following steps:

[0111] Step 1: Dissolve ferrous sulfate heptahydrate and anhydrous sodium sulfate in water at a molar ratio of 2:1. Add a small amount of ascorbic acid (1.5 wt% by weight of the ferrous sulfate heptahydrate) to prevent oxidation of the divalent iron ions. Finally, add a small amount of carbon nanotube aqueous slurry as a conductive agent (1.3% by weight of the total solid content in the solution). Stir thoroughly to form a uniform black slurry.

[0112] Step 2: spray drying the slurry, with the outlet air temperature controlled at ≥100°C, to obtain a uniform precursor powder;

[0113] Step 3: Under nitrogen protection, the precursor powder is calcined at 340°C to promote material melting and crystal growth, and the Na2Fe2(SO4)3 / CNT composite material is obtained by natural cooling.

[0114] Figure 1This is the SEM of the Na2Fe2(SO4)3 / CNT composite material, which is round particles with a particle size of about 10-20um. The particle size distribution is wide and the larger size will increase the diffusion distance of ions inside the particles to a certain extent. The results in Table 1 show that the internal resistance of the material powder is as high as 22~25Ω, which is much higher than the material in the application example. This shows that the Na2Fe2(SO4)3 / CNT composite material is larger in size after one-step synthesis and has low internal uniformity of the particles. There may be ion segregation, resulting in a decrease in electronic conductivity. In addition, XRD refinement can be used to identify that the phase purity of the Na2Fe2(SO4)3 / CNT composite material is only 83.1%, indicating that the phase purity of the material is low and the content of internal defects and dislocations in the structure is high. This is related to the large Coulomb repulsion in the material structure under high temperature conditions in the one-step synthesis method.

[0115] The Na2Fe2(SO4)3 / CNT composite, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. After grinding, the black slurry was coated on aluminum foil using a 150μm four-sided preparation device. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used as the separator. CR2016 button cells were assembled in a glove box.

[0116] A constant current charge and discharge test was carried out on the button battery mentioned above, and the current density was set to 0.1C (where 1C=120mAh / g). It can be seen from the results in Table 1 that in the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is only 90.5mAh / g. This capacity is lower than the material performance in the application example. The main reason is that the phase purity of the material is low, and the effective active material components per unit mass are correspondingly reduced, which reduces the number of charges participating in the redox reaction, and ultimately affects the normal performance of the material capacity. In addition, in terms of rate performance testing, at a rate of 10C, the capacity retention rate of the material synthesized by the solid phase method is only 80.3% relative to that at 0.1C. This result is consistent with the lower ionic conductivity of the material in Table 1 (10 -13S / m) confirm each other, and are far from the performance of the pure phase material in the application example. It can be seen that the presence of impurities or the reduction of the crystallinity of the material will hinder the migration of sodium ions at high current density, thereby causing the rate performance of the material to decline significantly. Finally, the data in Table 1 also show that in the voltage range of 2.0-4.3V, the capacity retention rate of the electrode after 500 cycles at a rate of 1C is only 83.2%, which is significantly lower than the material performance in the application example. The poor cyclic stability of this material is rooted in its low phase purity. Low phase purity will lead to uneven local volume expansion of the material during the sodium ion deintercalation process, which can easily cause problems such as lattice rupture, and ultimately lead to a decrease in the cyclic stability of the material.

[0117] Comparative Example 2 Synthesis of Na2Fe2(SO4)3 / graphene materials by solid phase method and their electrochemical properties

[0118] This embodiment relates to a sodium iron sulfate material for sodium ion batteries, and a preparation method thereof comprises the following steps:

[0119] Step 1: Add anhydrous ferrous sulfate and anhydrous sodium sulfate into a ball mill in a molar ratio of 2:1, and add some graphene as a conductive agent (the weight ratio is 2.5% of the total solid content). The ball-to-material ratio is controlled at 20:1, and the grinding time is greater than 8 hours; Step 2: Under nitrogen protection conditions, the precursor powder is calcined at 350°C to promote material melting and crystal growth, and the Na2Fe2(SO4)3 / graphene material is obtained by natural cooling.

[0120] The results in Table 1 show that the internal resistance of the Na2Fe2(SO4)3 / graphene composite material powder is as high as 39~42Ω, which is higher than the materials in other application examples. This is related to the poor uniformity of the material prepared by the ball milling method. It is difficult for graphene to be evenly dispersed to the material surface through the ball milling process, resulting in a blocked electron transport network, increased internal resistance, and a poor performance of its rate performance. In addition, through XRD curve refinement, it was found that the phase purity of the material was only 74.2%, with low crystallinity, low crystal integrity, and high impurity content. This is related to the uneven distribution of ions during the solid-phase grinding process and the large Coulomb repulsion between Fe-Fe in the crystal structure during the one-step synthesis during the material sintering process. The combination of these two factors ultimately led to a significant decrease in the phase purity of the material.

[0121] Na2Fe2(SO4)3 / graphene material, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. After grinding, the black slurry was coated on aluminum foil using a 150μm four-sided preparation device. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a disc with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used as the separator. CR2016 button cells were assembled in a glove box.

[0122] The button cell was subjected to constant current charge and discharge tests, with the current density set at 0.1C (1C=120mAh / g). According to the results in Table 1, within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode was 72.4mAh / g, which is much lower than the level in the application examples. This shows that the actual capacity of the material prepared by the solid-phase method is significantly reduced due to low phase purity, poor crystallinity and low content of active substances. In addition, the rate performance test results show that the capacity retention rate of the material at a rate of 10C is only 69.5% of that at 0.1C, indicating that its rate performance is poor. This is consistent with the lower ionic conductivity (10 -14 S / m), indicating that the presence of the impurity phase increases the energy barrier for ion migration, seriously hindering the rapid migration of ions, thereby affecting the rapid charge and discharge capabilities of the material. Finally, the data in Table 1 also show that in the voltage range of 2.0-4.3V, the capacity retention rate of the electrode after 500 cycles at a rate of 1C is only 72.1%, and the stability is significantly reduced compared with the application example. This may be due to the presence of impurities in the material, which leads to uneven structural expansion and contraction during the process of sodium insertion and deintercalation, causing crystal cracking. In addition, the larger specific surface area of the material may also accelerate the side reactions between the material interface and the electrolyte, further worsening the cycle stability.

[0123] Table 1 Performance test results

[0124] Example performance Application Example 1 Application Example 2 Comparative Example 1 Comparative Example 2 Phase purity (%) 99.3 99.5 83.1 74.2 Internal resistance (Ω) 3~5 4~7 22~25 39~42 Ionic conductivity (S / m) <![CDATA[10 -9 ]]> <![CDATA[10 -10 ]]> <![CDATA[10 -13 ]]> <![CDATA[10 -14 ]]> 0.1C specific capacity mAh / g (2.0-4.5V) 116.8 114.3 90.5 72.4 Rate performance (10C / 0.1C) 96.5 94.1 80.3 69.5 Cycle stability (%) / 500 cycles at 1C (2.0-4.3V) 98.2 96.5 83.2 72.1

[0125] The above embodiments are merely specific embodiments of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the scope of the present invention, and such obvious alternatives are all within the scope of protection of the present invention.

Claims

1. A method for preparing sodium iron sulfate material for sodium ion battery with high stability structure, characterized in that The following steps are involved: S1. Preparation of precursor solution: Sodium source, iron source, sulfuric acid and antioxidant are mixed and dissolved by wet process under controlled temperature to form a precursor solution; S2. Preparation of amorphous Na2Fe2(SO4)3 precursor: In a protective atmosphere, the precursor solution is evaporated at low temperature to obtain an amorphous Na2Fe2(SO4)3 precursor; S3. Preparation of partially crystalline Na2Fe2(SO4)3 crystals: calcining an amorphous Na2Fe2(SO4)3 precursor at low temperature in a protective atmosphere to form partially crystalline Na2Fe2(SO4)3 crystals; Preparation of S4 and Na2Fe2(SO4)3 / C slurry: In a protective atmosphere, partially crystallized Na2Fe2(SO4)3 crystals and a conductive agent are dispersed in an organic solvent and wet-ground to form a Na2Fe2(SO4)3 / C slurry; S5. In a protective atmosphere, the Na2Fe2(SO4)3 / C slurry is temperature-controlled and evaporated to form a Na2Fe2(SO4)3 / C composite material with a highly stable structure.

2. The method for preparing the sodium ferric sulfate material for sodium ion battery with high stability structure according to claim 1, characterized in that: In step S1, the temperature is controlled at 25-100°C.

3. The method for preparing the sodium ferric sulfate material for sodium ion battery with high stability structure according to claim 1, characterized in that: In step S2, the temperature of the low-temperature evaporation is <0°C.

4. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: In step S3, the low-temperature calcination temperature is 80-250° C., and the time is >0.01 h.

5. The method for preparing the sodium ferric sulfate material for sodium ion battery with high stability structure according to claim 1, characterized in that: In step S5, the evaporation temperature is controlled to be 60-200°C.

6. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: In step S2, the moisture content in the amorphous Na2Fe2(SO4)3 precursor is less than 0.2 wt%.

7. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: In step S4 , the grinding method is one or more of sand milling, ball milling, and high-speed dispersion.

8. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: The protective atmosphere is one or more of the non-oxygen atmospheres of argon, helium, nitrogen, carbon dioxide, and hydrogen.

9. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium peroxide; The iron source is one or more of iron powder, ferrous oxide, ferric oxide, ferric oxide, ferric hydroxide, ferrous hydroxyhydroxide, ferrous sulfate, and ferric sulfate; The antioxidant is one or more of ascorbic acid, riboflavin, cysteine, glutathione, phytic acid, anthocyanin, and tea polyphenols.

10. The method for preparing the sodium ferric sulfate material for sodium ion batteries with a high stability structure according to claim 1, wherein: 。 11. The conductive agent is one or more of carbon black and its derivatives, carbon nanotubes and their slurry, carbon fiber, graphene and its slurry, and flake graphite; The organic solvent is one or more of ether compounds, ketone compounds, and benzene compounds; the ether compounds are one or more of tetrahydrofuran, dimethoxyethane, trihydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethyl ether, and 2-methyl-tetrahydrofuran; the ketone compounds are one or more of 9-fluorenone b, benzophenone, acetone, and cyclohexanone; and the benzene compounds are one or more of 4-methylbiphenyl, toluene, xylene, and phenol.

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