Sodium ferric sulfate positive electrode material and preparation method thereof

Abstract

The invention discloses a sodium ferric sulfate positive electrode material with high electronic conductivity and a preparation method thereof. The nano-carbon material coated and modified sodium ferric sulfate positive electrode material is prepared by adopting a sol-gel method, so that the agglomeration of the nano-carbon material is reduced, the components are uniformly distributed, and the problem of poor conductivity of sodium ferric sulfate is solved. The nano carbon material with good conductivity is introduced to modify the sodium ferric sulfate, so that the ionic conductivity of the sodium ferric sulfate is improved, and the transmission of sodium ions is facilitated; and in combination with a good conductive network of the nano carbon material, the conductivity and air stability of the nano carbon material coated and modified sodium ferric sulfate can be obviously improved. The preparation method of the nano carbon material coated and modified sodium ferric sulfate provided by the invention is simple, convenient and low in cost, and has relatively strong market competitiveness in the field of large-scale energy storage application.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium-ion batteries, and particularly relates to a sodium ferric sulfate positive electrode material with high electronic conductivity and a preparation method thereof. BACKGROUND

[0002] With the increasing scarcity of global lithium resources, commercial lithium-ion batteries face severe challenges in supporting electric vehicles and large-scale energy storage. Under this background, sodium-ion batteries have become a promising complementary or alternative energy storage technology due to their abundant resources (the content of sodium in the earth's crust is more than 1000 times that of lithium), low cost, and environmental friendliness. They are expected to be widely used in large-scale power storage, such as wind or solar power plants, home energy storage systems, and low-speed transportation tools.

[0003] However, the performance of current sodium-ion batteries still cannot fully meet the stringent requirements of energy storage and electric vehicles, especially in terms of energy density, power density, and working voltage. Therefore, developing sodium-ion battery materials with high voltage, high power density, and high energy density has become a research hotspot. In the sodium-ion battery system, the positive electrode material is a key component that determines the performance of the battery, and its selection is particularly critical. Compared with layered metal oxides, sodium vanadium phosphate, Prussian blue, and organic positive electrode materials, sodium ferric sulfate (Na2Fe(SO4)2) has attracted much attention due to its simple synthesis process, abundant raw materials, obvious cost advantage, environmental friendliness, and relatively high working voltage (~3.8 V vs. Na + / Na). However, this material still has inherent defects such as poor environmental stability and low intrinsic electronic conductivity, which severely restrict its practical application. SUMMARY

[0004] To address the above problems in the prior art, the present application provides a method for preparing a sodium ferric sulfate positive electrode material with high electronic conductivity. The present application uses a sol-gel method to uniformly coat and modify Na2Fe(SO4)2 with high-conductivity carbon materials (such as carbon nanotubes and graphene), to prepare a Na2Fe(SO4)2 / carbon material positive electrode material. Through systematic material characterization and electrochemical testing, it is found that the electronic conductivity and discharge specific capacity of the positive electrode material are significantly improved, and the cycle stability is improved, thus exhibiting good electrochemical performance. The sol-gel method for preparing the sodium ferric sulfate positive electrode material reduces the agglomeration of sodium ferric sulfate and carbon material particles, improves the uniformity of component distribution, not only confirms the effectiveness of the carbon nanotube coating strategy, but also provides a new research idea and technical path for developing high-performance sodium-ion battery positive electrode materials.

[0005] Specifically, one aspect of the present application provides a method for preparing a sodium ferric sulfate positive electrode material, comprising the following steps:

[0006] (1) preparing a nanocarbon material solution: dispersing a nanocarbon material in water to form a nanocarbon material solution, the nanocarbon material being carbon nanotubes or graphene;

[0007] (2) preparing a sodium ferric sulfate precursor solution: dissolving ferrous sulfate heptahydrate, sodium sulfate and an antioxidant in water to form a sodium ferric sulfate precursor solution;

[0008] (3) adding the sodium ferric sulfate precursor solution to the nanocarbon material solution to obtain a mixed solution, heating and stirring, removing water, and obtaining a powder material;

[0009] (4) drying the powder material under vacuum to obtain a precursor of a sodium ferric sulfate positive electrode material;

[0010] (5) sintering the precursor of the sodium ferric sulfate positive electrode material under a protective gas flow to obtain a sodium ferric sulfate positive electrode material.

[0011] In one or more embodiments, in step (1), the concentration of the nanocarbon material in the nanocarbon material solution is 1000-3000 mg / L.

[0012] In one or more embodiments, in step (1), the dispersing includes ultrasonic dispersion, and the time of ultrasonic dispersion is 0.5-2 h.

[0013] In one or more embodiments, in step (1), the average diameter of the carbon nanotubes is 5-50 nm, and the average length is 1-20 μm.

[0014] In one or more embodiments, in step (2), the concentration of the ferrous sulfate heptahydrate in the sodium ferric sulfate precursor solution is 80-100 g / L.

[0015] In one or more embodiments, in step (2), the concentration of the sodium sulfate in the sodium ferric sulfate precursor solution is 40-60 g / L.

[0016] In one or more embodiments, in step (2), the molar ratio of the ferrous sulfate heptahydrate to the sodium sulfate in the sodium ferric sulfate precursor solution is 1:1.

[0017] In one or more embodiments, in step (2), the concentration of the antioxidant in the sodium ferric sulfate precursor solution is 1-10 g / L.

[0018] In one or more embodiments, in step (2), the antioxidant is selected from one or more of L-ascorbic acid, erythorbic acid and sodium erythorbate.

[0019] In one or more embodiments, in step (3), the mass ratio of sodium ferric sulfate to the nanocarbon material in the mixed solution is 1:(0.01-0.1).

[0020] In one or more embodiments, in step (3), the temperature for heating and stirring is 80-120°C.

[0021] In one or more embodiments, in step (4), the temperature for vacuum drying is 100-200°C, and the drying time is 6-8 hours.

[0022] In one or more embodiments, in step (5), the flow rate of the protective gas is 100-200 mL / min, and the protective gas is selected from one or both of argon and nitrogen.

[0023] In one or more embodiments, in step (5), the sintering temperature is 300-450°C, and the sintering time is 6-8 hours.

[0024] Another aspect of the present application provides a sodium ferric sulfate cathode material prepared according to any one of the embodiments herein.

[0025] Another aspect of the present application provides a cathode electrode sheet, wherein the cathode material layer of the cathode electrode sheet contains the sodium ferric sulfate cathode material according to any one of the embodiments herein.

[0026] Another aspect of the present application provides a sodium ion battery, wherein the sodium ion battery contains the cathode electrode sheet according to any one of the embodiments herein. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 X-ray diffraction pattern of the Na2Fe(SO4)2 cathode material of Comparative Example 1.

[0028] Figure 2 X-ray diffraction pattern of the Na2Fe(SO4)2 / CNTs cathode material of Example 1.

[0029] Figure 3 Cyclic voltammogram of the battery prepared using the Na2Fe(SO4)2 cathode material of Comparative Example 1.

[0030] Figure 4 Cyclic voltammogram of the battery prepared using the Na2Fe(SO4)2 / CNTs cathode material of Example 1.

[0031] Figure 5 SEM image of the Na2Fe(SO4)2 cathode material of Comparative Example 1.

[0032] Figure 6SEM image of Na2Fe(S04)2 / CNTs cathode material of Example 1.

[0033] Figure 7 Resistivity test histogram of Na2Fe(S04)2 / CNTs cathode material of Example 1.

[0034] Figure 8 Constant current charge-discharge curve of battery prepared using Na2Fe(S04)2 cathode material of Comparative Example 1.

[0035] Figure 9 Constant current charge-discharge curve of battery prepared using Na2Fe(S04)2 / CNTs cathode material of Example 1. DETAILED DESCRIPTION

[0036] To enable persons skilled in the art to understand the features and effects of the present application, the following is a general description and definition of the terms and phrases mentioned in this text. Unless otherwise specified, all technical and scientific words used in this text are the usual meanings understood by those skilled in the art for the present application, and in the event of a conflict, the definition in this specification shall prevail.

[0037] Theories or mechanisms described and disclosed herein, whether correct or not, should not be considered limiting the scope of the present application, i.e., the present application can be practiced without relying on any particular theory or mechanism.

[0038] In this text, "comprising", "including", "containing", and similar phrases cover the meanings of "consisting essentially of" and "consisting of", for example, when this text discloses "A comprising B and C", "A consisting essentially of B and C" and "A consisting of B and C" should be considered to have been disclosed herein.

[0039] In this text, all features defined in the form of numerical ranges or percentage ranges, such as values, quantities, contents and concentrations, are for the sake of brevity and convenience. Therefore, the description of numerical ranges or percentage ranges should be considered to have covered and specifically disclosed all possible sub-ranges and individual values within the range (including integers and fractions).

[0040] In this text, unless otherwise specified, percentages refer to mass percentages and ratios refer to mass ratios.

[0041] In this text, the sum of the percentage contents of each component of the composition is 100%.

[0042] In the description herein, it is to be understood that the embodiments or examples are not intended to limit the present application to such embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein, which have the same intended scope of the application, are to be included within the scope of the present application.

[0043] In the description herein, all possible combinations of the technical features in each embodiment or example are not described in order to make the description simple. Therefore, as long as the combinations of the technical features do not contradict each other, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as the scope of the present application.

[0044] The present application is to improve the conductivity and stability of the sodium ion battery cathode material. The sodium iron sulfate is uniformly coated and modified by high-conductive carbon material (such as carbon nanotube CNTs) through sol-gel method to prepare high-conductive sodium iron sulfate cathode material. The sol-gel method reduces the agglomeration of sodium iron sulfate and carbon material particles, and improves the uniformity of component distribution. After the sodium iron sulfate is coated by the carbon material, the conductivity and discharge specific capacity of the sodium iron sulfate cathode material are significantly improved, and the cycle stability is improved, which exhibits good electrochemical performance.

[0045] Sodium iron sulfate cathode material

[0046] The present application provides a preparation method of sodium iron sulfate cathode material, which comprises the following steps:

[0047] (1) preparing a carbon material solution: dispersing the carbon material in water to form a carbon material solution;

[0048] (2) preparing a sodium iron sulfate precursor solution: dissolving ferrous sulfate heptahydrate, sodium sulfate and an antioxidant in water to form a sodium iron sulfate precursor solution;

[0049] (3) adding the sodium iron sulfate precursor solution into the carbon material solution, heating and stirring, removing water, and obtaining a powder material;

[0050] (4) vacuum drying the powder material to obtain a precursor of sodium iron sulfate cathode material;

[0051] (5) sintering the precursor of sodium iron sulfate cathode material under inert gas flow to obtain sodium iron sulfate cathode material.

[0052] The preparation method of the sodium ferric sulfate positive electrode material is a sol-gel method, which can reduce particle agglomeration and improve component distribution uniformity. Moreover, the carbon material and the sodium ferric sulfate realize microstructure optimization of the positive electrode material through the sol-gel method, so that the particle size of the positive electrode material is significantly reduced, the interface contact resistance is reduced, the sodium ion diffusion dynamics is improved, and therefore the utilization rate of the positive electrode material under high voltage (such as 2.0 V-4.5 V) is improved. The sodium ferric sulfate positive electrode material of the present application realizes higher battery capacity in a narrower voltage range.

[0053] In the present application, the carbon material has good conductivity and environmental stability. Coating and modifying the appropriate amount of carbon material with sodium ferric sulfate can significantly improve the electrical conductivity of the sodium ferric sulfate positive electrode material and has good cycle stability. In order to ensure uniform distribution of carbon material in the sodium ferric sulfate positive electrode material, the carbon material should be easily dispersed in water and have a small size. Therefore, the carbon material in the present application is selected from one or both of carbon nanotubes or graphene.

[0054] In the present application, in order to ensure that the carbon material can be uniformly dispersed in water and avoid agglomeration, the concentration of the carbon material solution is controlled to be 1000-3000 mg / L, such as 1000 mg / L, 1020 mg / L, 1100 mg / L, 1200 mg / L, 1300 mg / L, 1400 mg / L, 1500 mg / L, 1600 mg / L, 1700 mg / L, 1800 mg / L, 1900 mg / L, 2000 mg / L, 2100 mg / L, 2200 mg / L, 2300 mg / L, 2400 mg / L, 2500 mg / L, 2600 mg / L, 2700 mg / L, 2800 mg / L, 2900 mg / L, 3000 mg / L, or a range formed by any two of the above concentrations.

[0055] In step (1) of the preparation method of the present application, the dispersion includes ultrasonic dispersion, which can fully disperse the carbon material in water; the ultrasonic dispersion time is 0.5-2 h, such as 0.5 h, 1 h, 1.5 h, 2 h.

[0056] In the present application, the diameter of the carbon nanotube is 5-50 nm, such as 5 nm, 7 nm, 8 nm, 10 nm, 11 nm, 15 nm, 18 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or a range consisting of any two of the above diameters, such as 7-11 nm; the length of the carbon nanotube is 1-20 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or a range consisting of any two of the above values, such as 3 μm-15 μm. CNTs that are too short form an unstable conductive network that is prone to breakage during electrode preparation (such as coating, rolling) or cycling, resulting in increased electrode resistance. CNTs that are too long greatly increase the viscosity of the slurry, making it less flowable and posing a significant challenge to the coating process.

[0057] In the present application, the graphene is selected from one or more of graphene quantum dots, graphene nanosheets, graphene microsheets, graphene powder, and graphene films.

[0058] In step (2) of the preparation method of the present application, the concentration of ferrous sulfate heptahydrate in water is 80-100 g / L, such as 80 g / L, 81 g / L, 82 g / L, 83 g / L, 84 g / L, 85 g / L, 86 g / L, 87 g / L, 88 g / L, 89 g / L, 90 g / L, 91 g / L, 92 g / L, 93 g / L, 94 g / L, 95 g / L, 96 g / L, 97 g / L, 98 g / L, 98.78 g / L, 99 g / L, 100 g / L.

[0059] In step (2) of the preparation method of the present application, the concentration of sodium sulfate in water is 40-60 g / L, such as 40 g / L, 41 g / L, 42 g / L, 43 g / L, 44 g / L, 45 g / L, 46 g / L, 47 g / L, 48 g / L, 49 g / L, 50 g / L, 50.47 g / L, 51 g / L, 52 g / L, 53 g / L, 54 g / L, 55 g / L, 56 g / L, 57 g / L, 58 g / L, 59 g / L, 60 g / L. In the present sodium ferrite precursor solution, the molar ratio of ferrous sulfate heptahydrate to sodium sulfate in water is 1:1.

[0060] In the present application, the antioxidant functions to inhibit the oxidation of Fe 2+ and complex and stabilize Fe2+ ensure the material chemical stability and electrochemical performance, and also provide a reducing atmosphere in the sintering step to avoid Fe 2+ being oxidized to Fe 3+ In the present application, the antioxidant is selected from one or more of L-ascorbic acid, erythorbic acid and sodium erythorbate.

[0061] In step (2) of the preparation method of the present application, the concentration of the antioxidant in water is 1-10 g / L, such as 1 g / L, 2 g / L, 3 g / L, 4 g / L, 5 g / L, 6 g / L, 6.22 g / L, 7 g / L, 8 g / L, 9 g / L, 10 g / L.

[0062] In step (3) of the preparation method of the present application, the mass ratio of sodium ferric sulfate to carbon material in the mixed solution is 1:(0.01-0.1), such as 1:0.01, 1:0.015, 1:0.018, 1:0.02, 1:0.022, 1:0.025, 1:0.028, 1:0.03, 1:0.032, 1:0.035, 1:0.038, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1. When the mass ratio of the sodium ferric sulfate precursor solution to the carbon material solution is less than 1:0.01, the content of the carbon material in the positive electrode material is too low, which does not significantly improve the electrical conductivity of the positive electrode material, and the performance improvement of the battery is limited. When the mass ratio of the carbon material solution to the sodium ferric sulfate precursor solution exceeds 1:0.1, the content of the carbon material in the positive electrode material is too high, which results in too small mass ratio of sodium ferric sulfate, and the sodium ion battery cannot achieve high battery capacity.

[0063] In step (3) of the preparation method of the present application, the temperature for heating and stirring is 80-120℃, which is to accelerate water evaporation at a suitable temperature without damaging the properties of the positive electrode material precursor. The temperature can be 80℃, 90℃, 100℃, 110℃, 120℃.

[0064] In step (4) of the preparation method of the present application, the temperature for vacuum drying is 100-200℃, and the drying time is 6-8 hours. The temperature can be 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, and the drying time can be 6 h, 6.5 h, 7 h, 7.5 h, 8 h.

[0065] In step (5) of the preparation method, the sintering temperature is 300-450°C, and the sintering time is 6-8 hours. The sintering temperature can be 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C, or 450°C. The sintering time can be 6 hours, 6.5 hours, 7 hours, 7.5 hours, or 8 hours.

[0066] In step (5) of the preparation method, the flow rate of the inert gas stream is 100-200 mL / min, such as 100 mL / min, 120 mL / min, 150 mL / min, 180 mL / min, or 200 mL / min. The inert gas is selected from argon or nitrogen.

[0067] Positive electrode tab

[0068] The positive electrode tab generally includes a positive electrode current collector and a positive electrode material layer formed on the surface of the positive electrode current collector. Therefore, the present application also provides a positive electrode tab, which includes the positive electrode material as described in any of the embodiments herein in the positive electrode material layer. The positive electrode current collector can be a copper foil, an aluminum foil, a titanium foil, a nickel foil, an iron foil, a zinc foil, or the like, and is preferably an aluminum foil. The positive electrode material layer includes a positive electrode active material, a conductive agent, and a binder. The positive electrode material layer is obtained by coating a positive electrode slurry including the positive electrode active material, the conductive agent, the binder, and a solvent onto the positive electrode current collector, and then performing rolling, die cutting, and baking. The solvent can be N-methyl pyrrolidone. In an exemplary embodiment, the positive electrode material, the conductive agent, the binder, and the solvent can be mixed and stirred uniformly to obtain the positive electrode slurry. The conductive agent can be commonly used in the art, and is preferably selected from one or more of conductive carbon black (SP), carbon fiber (CF), acetylene black, Ketjen black, graphene, and carbon nanotube. The binder can be commonly used in the art, and is preferably selected from one or more of fluorine-containing resin, polypropylene resin, fiber-type binder, and polyimide-type binder. In some embodiments, the fluorine-containing resin is polyvinylidene fluoride (PVDF). The mass of the positive electrode active material can be 90 wt%-98 wt%, such as 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, or 98 wt%, based on the total weight of the positive electrode material layer. The content of the conductive agent can be 0.5 wt%-5 wt%, such as 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, or 5 wt%. The content of the binder can be 1.5 wt%-6 wt%, such as 1.5 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or 6 wt%.

[0069] Negative electrode tab

[0070] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on the surface of the negative electrode current collector. The negative electrode current collector can be a material with excellent conductivity commonly used in the art, such as a copper foil. The negative electrode material layer includes a negative electrode active material, a conductive agent, and a binder. The negative electrode material layer is obtained by coating a negative electrode slurry including a negative electrode active material, a conductive agent, a binder, and a solvent onto the negative electrode current collector, followed by rolling, die cutting, and baking. The negative electrode material layer or the negative electrode slurry can further include an optional thickening agent and / or a dispersing agent. The negative electrode active material, the conductive agent, and the binder can be sequentially added to the solvent to prepare the negative electrode slurry, and further to prepare the negative electrode sheet. In exemplary embodiments, the thickening agent is dissolved in the solvent, and then the negative electrode active material, the conductive agent, and the binder, which are uniformly mixed, are added. The solvent can be a solvent commonly used in the art, such as deionized water. The negative electrode active material can be one or more selected from the group consisting of graphite (including artificial graphite, natural graphite), silicon-carbon negative electrode, hard carbon, and sodium titanate, and is preferably hard carbon. The conductive agent is preferably one or more selected from the group consisting of conductive carbon black, carbon fiber, acetylene black, ketjen black, graphene, and carbon nanotube. The binder is preferably one or more selected from the group consisting of fluorine-containing resin, polypropylene resin, fiber-type binder, rubber-type binder, and polyimide-type binder, such as styrene butadiene rubber (SBR). In some embodiments, the negative electrode active material in the negative electrode material layer is graphite, the conductive agent is conductive carbon black, and the binder is styrene butadiene rubber, and the thickening agent is sodium carboxymethyl cellulose (CMC). The mass ratio of each component in the negative electrode material layer can be conventional, for example, the content of the negative electrode active material can be 90 wt% to 98 wt%, preferably 92 wt% to 98 wt%, such as 93 wt%, 95 wt%, 95.7 wt%, 96 wt%, 97 wt%, based on the total weight of the negative electrode material layer, the content of the conductive agent can be 0.5 wt% to 5 wt%, preferably 1.0 wt% to 3 wt%, such as 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, the content of the binder can be 0.5 wt% to 5 wt%, preferably 1.5 wt% to 4 wt%, such as 1.8 wt%, 2.0 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, and the content of the thickening agent can be 0.5 wt% to 5 wt%, such as 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%.

[0071] Sodium-ion battery

[0072] Sodium-ion batteries generally include five core components, a positive electrode, a negative electrode, an electrolyte, a separator, and a current collector. The positive electrode, upon charging, releases sodium ions from the positive electrode; upon discharging, the sodium ions return to the positive electrode. The negative electrode, upon charging, intercalates sodium ions into the negative electrode; upon discharging, the sodium ions are released from the negative electrode. The electrolyte provides a pathway for the transport of sodium ions between the positive and negative electrodes. The separator physically separates the positive and negative electrodes to prevent short-circuiting while allowing the free passage of sodium ions. The separator can be a polymeric separator, a ceramic separator, or a polymeric / ceramic composite separator. The polymeric separator includes a single-layer polymeric separator and a multi-layer polymeric separator. The single-layer polymeric separator includes a polyethylene (PE) separator and a polypropylene (PP) separator.

[0073] The five parts are assembled in a certain order, and after packaging, standing, formation, and shaping, a sodium-ion battery can be obtained. The form of the sodium-ion battery of the present application is not particularly limited and can be a cylindrical sodium-ion battery, a soft-packaged sodium-ion battery, or an aluminum-cased sodium-ion battery.

[0074] In some embodiments, the present application also provides a sodium-ion battery comprising the positive electrode material and / or the positive electrode tab as described in any of the embodiments herein.

[0075] The present application has the following beneficial effects:

[0076] 1. The preparation method of the sodium iron sulfate positive electrode material is simple and easy to operate, can make the carbon material and sodium iron sulfate uniformly distributed, reduce particle agglomeration, and reduce the particle size of the positive electrode material.

[0077] 2. The sodium iron sulfate positive electrode material has a smaller and more uniform particle size, so the interface contact resistance of the positive electrode material is low, the sodium ion diffusion dynamics is improved, and thus the utilization rate of the positive electrode material at high voltage is improved, enabling higher battery capacity within a narrower voltage range.

[0078] 3. The sodium iron sulfate positive electrode material has good cycle stability.

[0079] The present application will be described below in the manner of specific examples. It should be understood that these examples are merely illustrative and are not intended to limit the scope of the present application. The methods, reagents, and materials used in the examples and comparative examples are conventional in the art unless otherwise specified. The raw material compounds in the examples and comparative examples can be purchased through commercial channels.

[0080] In the examples and comparative examples, Na2SO4 was purchased from Adamas-beta, with a purity of >99.0%; FeSO4·7H2O was purchased from Macklin, with a purity of >99.0%; L-ascorbic acid was purchased from Macklin, with a purity of >99.0%; carbon nanotubes were purchased from Tiannai Technology, with a brand of LB270-44, an average diameter of 7-11 nm, and an average length of 3 μm-15 μm.

[0081] Example 1

[0082] In this example, a sodium iron sulfate / CNTs positive electrode material was prepared by a sol-gel method, and the specific steps were as follows:

[0083] (1) Preparation of carbon nanotube solution: CNTs were ultrasonically dispersed in water for 1 h to form a CNTs solution with a concentration of 1020 mg / L;

[0084] (2) Preparation of sodium iron sulfate precursor solution: ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid were dissolved in water to form a sodium iron sulfate precursor solution, wherein the concentrations of ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid were 98.78 g / L, 50.47 g / L and 6.22 g / L, respectively;

[0085] (3) The sodium iron sulfate precursor solution was added to the CNTs solution, so that the mass ratio of sodium iron sulfate to CNTs in the mixed solution was 1:0.02, heated to 100°C and stirred, and the water was removed to obtain a powder material;

[0086] (4) The powder material was dried at 120°C under vacuum for 6 h to obtain a precursor of the sodium iron sulfate positive electrode material;

[0087] (5) The precursor of the sodium iron sulfate positive electrode material was sintered at a temperature of 350°C under an argon gas flow (120 mL / min) for 6 h to obtain a sodium iron sulfate / CNTs positive electrode material.

[0088] Comparative Example 1

[0089] In this comparative example, a sodium iron sulfate positive electrode material was prepared by a sol-gel method, and the specific steps were as follows:

[0090] (1) Preparation of sodium iron sulfate precursor solution: ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid were dissolved in water to form a sodium iron sulfate precursor solution, wherein the concentrations of ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid were 98.78 g / L, 50.47 g / L and 6.22 g / L, respectively;

[0091] (2) The sodium iron sulfate precursor solution was heated to 100°C and stirred, and the water was removed to obtain a powder material;

[0092] (3) The powder material is dried under vacuum at 120°C for 6 h to obtain a precursor of the sodium ferric sulfate cathode material;

[0093] (4) The precursor of the sodium ferric sulfate cathode material is sintered under an argon gas stream (120 mL / min) at 350°C for 6 h to obtain the sodium ferric sulfate cathode material.

[0094] Comparative Example 2

[0095] In this comparative example, a sodium ferric sulfate / CNTs cathode material is prepared by a spray drying method.

[0096] (1) A carbon nanotube solution is prepared according to Example 1: CNTs are ultrasonically dispersed in water for 1 h to form a CNTs solution with a concentration of 1020 mg / L;

[0097] (2) A sodium ferric sulfate precursor solution is prepared according to Example 1: ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid are dissolved in water to form a sodium ferric sulfate precursor solution, wherein the concentrations of the ferrous sulfate heptahydrate, sodium sulfate and L-ascorbic acid are 98.78 g / L, 50.47 g / L and 6.22 g / L, respectively;

[0098] (3) The sodium ferric sulfate precursor solution is added to the CNTs solution so that the mass ratio of sodium ferric sulfate to CNTs in the mixed solution is 1:0.02, and a mixed solution is obtained;

[0099] (4) A sodium ferric sulfate / CNTs cathode material is prepared by a spray drying method, and the drying steps are as follows: the mixed solution is spray dried, and the inlet temperature is set to 280°C and the outlet temperature is set to 80°C. The solution is atomized into tiny droplets, which are rapidly dried in hot air to obtain a precursor powder.

[0100] (5) Heat treatment (sintering): The precursor powder obtained by spray drying is heat treated in a tube furnace. Under an inert atmosphere, it is sintered at 350°C for 6 hours to finally obtain a Na2Fe(SO4)2 / CNTs powder sample.

[0101] Application Example

[0102] The cathode materials of Example 1 and Comparative Example 1 are made into batteries for cyclic voltammetry curve testing and constant current charge and discharge testing. The preparation method of the battery is as follows:

[0103] 1. Preparation of the positive electrode sheet

[0104] (1) Preparation of the slurry:

[0105] Ingredients: positive active material, carbon black, binder (PVDF) are mixed in a certain proportion to obtain a solid mixture, wherein the mass percentage of the positive active material is 90 wt%, the mass percentage of the carbon black is 5 wt%, and the mass percentage of the binder is 5 wt%.

[0106] Stirring: in a volumetric flask, the solid mixture is mixed with an organic solvent (N-methyl pyrrolidone) to form a uniform slurry with appropriate viscosity.

[0107] (2) Coating: coating is performed using a coating machine, and an aluminum foil is placed. Vacuum is turned on to make the aluminum foil adsorbed on the surface of the coating machine. The slurry is poured onto the aluminum foil and evenly distributed, and then coated. After coating, cover and heat at 120°C for 20 minutes. Dry the electrode sheet. Transfer to a drying oven and vacuumize, dry at 80°C for 12h.

[0108] 2. Preparation of electrolyte and separator

[0109] (1) Preparation of electrolyte

[0110] Sodium salt: sodium hexafluorophosphate. Organic solvent: ethylene carbonate (EC), diethyl carbonate (DEC).

[0111] Preparation process:

[0112] In a highly dry and water-free glove box, sodium hexafluorophosphate is dissolved in a mixed solution of EC:DEC (1:1 v / v), and the concentration of sodium hexafluorophosphate in the electrolyte is 1 mol / L. After strict purification, filtration and quality detection, the water and impurity contents are extremely low (ppm level).

[0113] (2) Preparation of separator

[0114] The separator is a microporous membrane for isolating the positive and negative electrodes to prevent short circuiting, while allowing sodium ions to pass through. The present application uses polypropylene (PP) as the separator.

[0115] 3. Negative electrode sheet

[0116] The negative electrode sheet used in the battery of the present application is a sodium sheet.

[0117] 4. Assembly of the battery

[0118] The assembly process of the battery is carried out in a glove box filled with high-purity inert gas (such as argon), ensuring that the water and oxygen contents in the glove box are stably below 0.1 ppm. The assembly sequence is positive shell-positive electrode sheet-separator-electrolyte-sodium sheet-gasket-spring-negative shell. The assembled battery is packaged in a button cell sealing machine and placed in a box in sequence.

[0119] Test example

[0120] (1) X-ray diffraction (XRD) characterization: The testing instrument is an X-ray diffractometer (model: D8 ADVANCE). Test 50-200 mg of powder sample, use copper target material (Cu Kα), set the test voltage to 40-50 kV, the scanning range is 2θ=5°-90°, and the scanning speed is 5° / min.

[0121] (2) Cyclic voltammetry curve (CV) test: the model is PMC-1000-AT. The prepared button cell is tested on an electrochemical workstation, the starting voltage is set to 2 V, the end voltage is set to 4.5 V, and the scanning direction is: the positive electrode material is usually set to positive scanning (scanning from 2.0 V to 4.5 V, corresponding to the charging / desodium process). The scanning rate is generally set to 0.1 mV / s, and the cycle number is set to three times.

[0122] (3) Scanning electron microscope (SEM): The testing instrument is a field emission scanning electron microscope (model: JSM-7900F, JOEL). 1-2 mg of sample is pasted on the surface of conductive glue for SEM characterization, the voltage is set to 5-10 kV, and the magnification is 15000 times (scale 1 μm) and 30000 times (scale 100 nm) respectively.

[0123] (4) Conductivity test: The testing instrument is a four-probe powder resistivity tester (model: ST2742B). When testing, set the initial pressure to 7 MPa and the final pressure to 15 MPa, and the pressure step is 1 MPa. After setting, put the powder material in. In order to maintain consistency, the mass or height put in each time is kept consistent, then click start to test, and get the data of resistivity and pressure.

[0124] (5) Galvanostatic charge-discharge (GCD) test: The testing instrument is an electrochemical workstation (model: LAND) to test the material in half cell at room temperature (25±2℃), the current density of the test is 0.1 C (1 C=91 mA g -1 ), and the test voltage range is 2.0 V-4.5 V.

[0125] Figure 1 and Figure 2The XRD patterns of Na₂Fe(SO₄)₂ prepared in Comparative Example 1 and Na₂Fe(SO₄)₂ / CNTs prepared in Example 1 are shown. The results show that the main diffraction peaks of both Na₂Fe(SO₄)₂ and Na₂Fe(SO₄)₂ / CNTs perfectly match the standard card for Na₂Fe(SO₄)₂ (PDF#21-1360), confirming the successful synthesis of the target phase. Additional diffraction peaks were observed near 2θ = 10°, 20°, and 30°, consistent with the characteristic peaks of Na₆Fe(SO₄)₄ (PDF#29-1218), indicating the presence of a small amount of impurity phase in the samples. This impurity phase needs to be suppressed in subsequent synthesis, but its current content has a controllable impact on performance.

[0126] Figure 3 and Figure 4 Cyclic voltammetry (CV) curves of batteries prepared using Na2Fe(SO4)2 in Comparative Example 1 and Na2Fe(SO4)2 / CNTs in Example 1 are shown, respectively. Figure 3 Na2Fe(SO4)2 only shows Fe 2+ / Fe 3+ The single redox peak and the small area of ​​the CV curve indicate that its intrinsic electrochemical activity is low. Figure 4 In Na2Fe(SO4)2 / CNTs, the main peak (Fe) is modulated by the conductive network and interface of CNTs. 2+ / Fe 3+ The polarization of the CNTs decreased, and the reaction kinetics improved; the new secondary peaks may be due to the additional contribution of CNTs or interface optimization; the area of ​​the CV curve increased, consistent with the higher discharge capacity in the galvanostatic test.

[0127] Figure 5 and Figure 6 SEM images of Na2Fe(SO4)2 prepared in Comparative Example 1 and Na2Fe(SO4)2 / CNTs prepared in Example 1 are shown, respectively, characterizing the microstructure of the two cathode materials. Figure 5 The right figure shows that Na2Fe(SO4)2 exhibits an irregular blocky morphology with a wide particle size distribution (200 nm-2 μm). Some particles have undergone severe agglomeration due to high-temperature sintering, forming larger secondary particles. Figure 5 (Left figure). This non-uniform particle distribution may lead to discontinuous electron conduction paths, increasing interfacial contact resistance; sodium ion diffusion kinetics are restricted, reducing the utilization rate of active materials inside large particles. Figure 6 The right figure shows that the size distribution of Na2Fe(SO4)2 / CNTs is relatively narrow (200 nm-500 nm). Figure 6The left figure shows that the carbon nanotubes in Na2Fe(SO4)2 / CNTs form a three-dimensional interwoven network on the material surface, uniformly coating the Na2Fe(SO4)2 particles, which can significantly improve the conductivity of the material.

[0128] Figure 7 The resistivity histogram for Na2Fe(SO4)2 / CNTs prepared in Example 1 is shown. Experimental tests revealed that the resistivity of the pure-phase Na2Fe(SO4)2 material was too high (exceeding the instrument's measurable range, >10). 6 The resistivity of the material is extremely low (Ω·cm), making accurate measurement impossible and indicating that the material itself has very poor electronic conductivity, which limits its application in electrochemical energy storage and other fields. A Na₂Fe(SO₄)₂ / CNTs cathode material was prepared using a carbon nanotube coating strategy, and the resistivity test results are as follows: Figure 7 The results show that the conductivity of the composite cathode material is significantly improved to 10. -3 The S / cm ratio was improved by at least three orders of magnitude compared to pure-phase Na2Fe(SO4)2. These results indicate that the introduction of CNTs effectively constructed a conductive network, significantly enhancing the material's electron transport capability, thus making it more suitable as an electrode material for applications in batteries or supercapacitors. Furthermore, this optimization strategy not only improved the material's conductivity but may also enhance its structural stability and cycling performance, laying the foundation for subsequent electrochemical performance studies.

[0129] Figure 8 and Figure 9 The constant current charge-discharge curves (initial three-cycle charge-discharge curves) of the batteries prepared using Na2Fe(SO4)2 in Comparative Example 1 and Na2Fe(SO4)2 / CNTs in Example 1 are shown respectively. The tests were conducted at 0.1 C (1 C = 91 mA g). -1 The discharge was carried out at a current density of ), and the first-cycle discharge specific capacity of pure phase Na2Fe(SO4)2 was determined according to... Figure 8 The calculated value is 67.06 mAh g. -1 This indicates that its intrinsic conductivity is low, resulting in insufficient utilization of the active material. In subsequent cycles, capacity decay is significant, with the discharge capacity dropping to 61.22 mAh g⁻¹ in the third cycle. -1 The capacity retention rate was 91.3%, indicating that Na2Fe(SO4)2 undergoes certain irreversible phase transitions or interfacial side reactions during charge and discharge. The first-cycle discharge specific capacity of the Na2Fe(SO4)2 / CNTs cathode material increased to 72.98 mAh g⁻¹. -1 This indicates that the introduction of carbon nanotubes effectively enhanced the electron conductivity of the electrode and improved the utilization rate of the active material. The discharge capacity remained at 70.11 mAh g⁻¹ in the third cycle. -1, the capacity retention rate is as high as 96.1%, far superior to pure-phase Na2Fe(SO4)2 material, confirming that CNTs coating not only improves the initial capacity, but also significantly improves the cycle stability.

[0130] The spray drying method of Comparative Example 2, although seemingly short in process, has the core problem of unavoidable Fe 2+ oxidation and relatively rough composite effect during the preparation process. These two defects directly lead to the decrease of the electrochemical activity of the final material, the imperfect conductive network, and thus poor performance in specific capacity, rate performance and cycle life, etc. The sol-gel method of Example 1, through a mild and controllable solution chemical process and protection by reducing / inert atmosphere throughout the process, perfectly solves the above problems, and can synthesize high-performance positive electrode materials with pure phase, uniform composition and perfect conductive network.

[0131] Therefore, the preparation method represented by Example 1 is not only more reasonable in science, but also has more advantages in technology and economy, and is the preferred scheme for preparing high-performance Na2Fe(SO4)2 / CNTs composite positive electrode material.

Claims

1. A method for preparing sodium ferric sulfate cathode material, characterized in that, The method includes the following steps: (1) Preparation of nano-carbon material solution: Disperse nano-carbon material in water to form nano-carbon material solution, wherein the nano-carbon material is carbon nanotube or graphene; (2) Preparation of sodium ferric sulfate precursor solution: Dissolve ferrous sulfate heptahydrate, sodium sulfate and antioxidant in water to form sodium ferric sulfate precursor solution; (3) Add the sodium ferric sulfate precursor solution to the nano carbon material solution to obtain a mixed solution, heat and stir to remove water, and obtain powder material; (4) The powder material is dried under vacuum to obtain the precursor of sodium ferric sulfate cathode material; (5) The precursor of sodium ferric sulfate cathode material is sintered under a protective gas flow to obtain sodium ferric sulfate cathode material.

2. The method as described in claim 1, characterized in that, The method has one or more of the following characteristics: In step (1), the concentration of nano-carbon material in the nano-carbon material solution is 1000-3000 mg / L; In step (1), the dispersion includes ultrasonic dispersion, and the ultrasonic dispersion time is 0.5-2 h; In step (1), the carbon nanotubes have an average diameter of 5-50 nm and an average length of 1-20 μm.

3. The method as described in claim 1, characterized in that, The method has one or more of the following characteristics: In step (2), the concentration of ferrous sulfate heptahydrate in the sodium ferric sulfate precursor solution is 80-100 g / L; In step (2), the concentration of sodium sulfate in the sodium ferric sulfate precursor solution is 40-60 g / L; In step (2), the molar ratio of ferrous sulfate heptahydrate to sodium sulfate in the sodium ferric sulfate precursor solution is 1:1; In step (2), the concentration of the antioxidant in the sodium ferric sulfate precursor solution is 1-10 g / L; In step (2), the antioxidant is selected from one or more of L-ascorbic acid, isoascorbic acid and sodium isoascorbate.

4. The method as described in claim 1, characterized in that, In step (3), the mass ratio of sodium ferric sulfate to nano-carbon material in the mixed solution is 1:(0.01-0.1).

5. The method as described in claim 1, characterized in that, In step (3), the heating and stirring temperature is 80℃-120℃.

6. The method as described in claim 1, characterized in that, In step (4), the temperature for vacuum drying is 100℃-200℃ and the drying time is 6-8 hours.

7. The method as described in claim 1, characterized in that, In step (5), the flow rate of the protective gas is 100-200 mL / min, and the protective gas is selected from one or both of argon and nitrogen; the sintering temperature is 300℃-450℃, and the sintering time is 6-8 hours.

8. The sodium ferric sulfate cathode material prepared by any one of claims 1-7.

9. A positive electrode sheet, characterized in that, The positive electrode material layer of the positive electrode sheet contains the sodium ferric sulfate positive electrode material as described in claim 8.

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

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