Sodium ferrous sulfate positive electrode material and preparation method therefor, positive electrode sheet and secondary battery

Abstract

The present invention relates to the technical field of secondary batteries. Provided in the present application are a sodium ferrous sulfate positive electrode material and a preparation method therefor, a positive electrode sheet and a secondary battery. The sodium ferrous sulfate positive electrode material comprises a core layer and a shell layer. The material of the core layer comprises sodium ferrous sulfate, and the material of the shell layer is a carbon material, wherein the thickness of the shell layer is 2 nm-10 nm. In addition, the Raman spectrum of the sodium ferrous sulfate positive electrode material exhibits a D peak and a G peak, the value of ID / IG being (0.8-1):1. The sodium ferrous sulfate positive electrode material provided by the present application has good electrical conductivity, higher charge-discharge specific capacity, and good electrochemical performance.

Description

A sodium ferrous sulfate cathode material, its preparation method, cathode sheet, and secondary battery.

[0001] This application claims priority to Chinese Patent Application No. 2024110557316, filed on August 1, 2024, entitled "A sodium ferrous sulfate cathode material and its preparation method, cathode sheet and secondary battery", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of secondary battery technology, specifically to a sodium ferrous sulfate cathode material and its preparation method, cathode sheet, and secondary battery. Background Technology

[0003] Lithium-ion batteries, with their excellent electrochemical performance, have been widely used in mobile phones, laptops, and new energy vehicles, making them an ideal energy storage technology. However, globally, lithium resources are extremely unevenly distributed, with approximately 70% located in South America, leading to high production costs. Therefore, there is an urgent need to provide a new energy storage battery technology to mitigate the adverse effects of lithium resource shortages.

[0004] Current research indicates that sodium-ion batteries also have the function of storing and releasing electrical energy. Compared with lithium salts, sodium salts are more abundant and cheaper. Among them, sodium ferrous sulfate is relatively inexpensive and has a high voltage. However, sodium ferrous sulfate has poor conductivity. The current conventional treatment method is to improve its conductivity through carbon coating, but it still cannot effectively improve the charging specific capacity and discharging specific capacity of sodium ferrous sulfate cathode materials, thus limiting the application of sodium ferrous sulfate cathode materials.

[0005] Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a sodium ferrous sulfate cathode material and its preparation method, cathode sheet and secondary battery, aiming to solve the technical problem of poor conductivity of sodium ferrous sulfate cathode material.

[0007] In a first aspect, embodiments of this application provide a sodium ferrous sulfate cathode material, comprising a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The thickness of the shell layer is 2 nm-10 nm. The Raman spectrum of this sodium ferrous sulfate cathode material exhibits D peaks and G peaks, I... D / I G The value is (0.8-1):1.

[0008] In the technical solution of this application embodiment, by designing the structure of the sodium ferrous sulfate cathode material, a carbon material shell with a high degree of graphitization is coated on the surface of the sodium ferrous sulfate core layer, which helps to improve the conductivity of the sodium ferrous sulfate cathode material.

[0009] In some embodiments, the thickness of the shell is 2nm-5nm.

[0010] In this embodiment, by further optimizing the thickness of the shell layer, sodium ions are more easily extracted from the cathode material, which is beneficial to improving the charge specific capacity and discharge specific capacity of the battery using the sodium ferrous sulfate cathode material.

[0011] In some embodiments, the Raman spectrum of sodium ferrous sulfate cathode material shows that I D / I G The value is (0.9-1):1.

[0012] In this embodiment, the values ​​of the D peak and the G peak are further controlled. The increased content of amorphous carbon helps to increase the specific surface area of ​​the sodium ferrous sulfate cathode material and improve the electrolyte wettability of the sodium ferrous sulfate cathode material when it is applied to the battery.

[0013] In some embodiments, the D50 particle size of the sodium ferrous sulfate cathode material is 0.5 μm-5 μm, and the BET specific surface area is 10 m². 2 / g-25m 2 / g.

[0014] In this embodiment, controlling the D50 particle size and BET specific surface area of ​​the sodium ferrous sulfate cathode material can improve its processing performance, make it easier to prepare a uniform slurry, and improve the overall electrochemical performance of the battery.

[0015] In some embodiments, the molar ratio of sodium to iron in the sodium ferrous sulfate cathode material is (2-2.5):1.

[0016] In this embodiment, by controlling the molar ratio of sodium and iron, the capacity of the battery using sodium ferrous sulfate cathode material can be further improved.

[0017] Secondly, embodiments of this application provide a method for preparing sodium ferrous sulfate cathode material, comprising the following steps:

[0018] A carbon-coated material is obtained by first calcining a sodium sulfate-based organic compound or a mixture of sodium sulfate-based organic compound and a carbon source; wherein the sodium sulfate-based organic compound has alkyl groups.

[0019] The above-mentioned carbon-coated material and ferrous salt are mixed to obtain a mixture.

[0020] The above mixture was calcined a second time to obtain sodium ferrous sulfate cathode material;

[0021] The carbon source mentioned above is selected from carbon materials that are different from sodium sulfate organic compounds;

[0022] The sodium ferrous sulfate cathode material comprises a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The thickness of the shell layer is 2 nm-10 nm. The Raman spectrum of the sodium ferrous sulfate cathode material exhibits D peaks and G peaks. D / I G The value is (0.8-1):1.

[0023] In this embodiment, using sodium sulfate organic compounds or a mixture of sodium sulfate organic compounds and a carbon source, as well as ferrous salts, as raw materials, a shell with a thickness of 2nm-10nm can be obtained through two calcinations. D / I G The value is (0.8-1):1, which improves the conductivity of sodium ferrous sulfate cathode material.

[0024] In some embodiments, the method for preparing sodium ferrous sulfate cathode material is used to prepare sodium ferrous sulfate cathode material as described in any of the preceding claims.

[0025] In some embodiments, the sodium sulfate salt organic compound is an alkyl sodium sulfate with 10-20 carbon atoms.

[0026] Using alkyl sodium sulfate with 10-20 carbon atoms as an organic sodium sulfate salt allows for in-situ carbon coating of sodium sulfate material after the first calcination. This carbon coating effect is good and helps improve the electrolyte wettability of the sodium ferrous sulfate cathode material, thereby improving the charging and discharging performance of the battery made using this cathode material.

[0027] In some embodiments, the temperature of the first calcination is 600℃-700℃, and the holding time of the first calcination is 2h-4h; the temperature of the second calcination is 300℃-350℃, and the holding time of the second calcination is 20h-30h.

[0028] In this embodiment, by controlling the conditions of the first and second calcinations, it is beneficial for ferrous ions to enter the core layer and react with sodium sulfate, the core layer material formed in the first calcination, at low temperature to generate sodium ferrous sulfate.

[0029] In some embodiments, the heating rate for the first calcination is 4°C / min-6°C / min, and the heating rate for the second calcination is 1°C / min-2°C / min. Both the first and second calcinations are carried out in an inert atmosphere.

[0030] In this embodiment, controlling the heating rate during the first and second calcination processes helps to make the material heat more evenly, react more fully, and reduce the internal thermal stress of the material.

[0031] In some embodiments, the ambient humidity for the first calcination is 5%-10%, and the ambient humidity for the second calcination is less than 5%.

[0032] In this embodiment, controlling the ambient humidity helps reduce the loss of carbon materials.

[0033] In some embodiments, before the second calcination, the mixture is pulverized, and the particle size of the mixture after pulverization is 1μm-3μm.

[0034] In this embodiment, controlling the particle size of the mixture helps to shorten the diffusion path of ferrous ions and improve the yield of sodium ferrous sulfate cathode material.

[0035] In some embodiments, the alkyl sulfate having 10-20 carbon atoms is at least one of sodium dodecyl sulfate, sodium hexadecyl sulfate, sodium octadecyl sulfate, and sodium eicosyl sulfate; and / or, the carbon source is at least one of glucose, sucrose, water-soluble starch, and fructose; and the ferrous salt is at least one of ferrous acetate and ferrous oxalate.

[0036] Using sodium alkyl sulfate with 10-20 carbon atoms as raw materials, sodium ferrous sulfate cathode materials with a suitable carbon shell thickness can be easily prepared. The aforementioned carbon sources are commonly used materials for preparing carbon coating layers and are readily available. Using ferrous acetate and ferrous oxalate as ferrous salts does not introduce other impurities.

[0037] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector, a positive active material, a conductive agent, and a binder. The positive active material is the positive active material of the first aspect of this application, or the positive active material prepared by the preparation method of the second aspect of this application, and therefore has a high charge-discharge specific capacity.

[0038] Fourthly, embodiments of this application provide a secondary battery, including a positive electrode, a separator, an electrolyte, and a negative electrode as described in the third aspect of this application, thus exhibiting good electrochemical performance.

[0039] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0041] Figure 1 is a surface SEM image of the sodium ferrous sulfate cathode material in Example 1;

[0042] Figure 2 is a surface SEM image of the sodium ferrous sulfate cathode material in Example 2;

[0043] Figure 3 is a surface SEM image of the sodium ferrous sulfate cathode material in Example 3;

[0044] Figure 4 shows the XRD patterns of sodium ferrous sulfate cathode materials in Examples 1-2;

[0045] Figure 5 is a TEM image of the sodium ferrous sulfate cathode material in Example 1;

[0046] Figure 6 is a TEM image of the sodium ferrous sulfate cathode material in Example 2;

[0047] Figure 7 shows the Raman spectrum of the sodium ferrous sulfate cathode material in Example 1;

[0048] Figure 8 shows the Raman spectrum of the sodium ferrous sulfate cathode material in Example 2;

[0049] Figure 9 shows the test results of the charging specific capacity and discharging specific capacity of the battery prepared from the sodium ferrous sulfate cathode material in Example 1 at 0.1C and 0.5C rates;

[0050] Figure 10 shows the test results of the charging specific capacity and discharging specific capacity of the batteries prepared from the sodium ferrous sulfate cathode material in Examples 2-3 at a rate of 0.1C. Detailed Implementation

[0051] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0053] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0054] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0055] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0056] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0057] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0058] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0059] Compared to lithium resources, sodium resources are more abundant and lower in cost, making them promising candidates for energy storage. Sodium ferrous sulfate is a commonly used cathode material in sodium-ion batteries, but its conductivity is poor. Currently, carbon coating is typically used to improve the conductivity of sodium ferrous sulfate, but the improvement is limited and cannot significantly enhance the charge and discharge specific capacities of batteries using sodium ferrous sulfate as the cathode material.

[0060] To address the technical problem of poor conductivity in sodium ferrous sulfate cathode materials, this application provides a sodium ferrous sulfate cathode material, its preparation method, a cathode electrode, and a secondary battery. The sodium ferrous sulfate cathode material comprises a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. By controlling the thickness of the shell layer and the intensity of the D and G peaks in the Raman spectrum of the sodium ferrous sulfate cathode material, the conductivity of the sodium ferrous sulfate cathode material can be effectively improved; consequently, the capacity of the cathode electrode and the secondary battery is also increased.

[0061] In one aspect, embodiments of this application provide a sodium ferrous sulfate cathode material, comprising a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The thickness of the shell layer is 2nm-10nm, specifically 2.3nm, 3.3nm, 3.4nm, 3.5nm, 4.9nm, 5.4nm, 5.9nm, 7.9nm, etc., and is not limited thereto. The Raman spectrum of this sodium ferrous sulfate cathode material exhibits D peaks and G peaks, I... D / I G The value is (0.8-1):1, which can be 0.953:1, 0.961:1, 0.962:1, 0.968:1, 0.978:1, 0.987:1, 0.991:1, etc., and is not limited here.

[0062] In the description of this application, the term "D peak" refers to the peak located at 1350 cm⁻¹ in the Raman spectrum. -1 Nearby peaks, for example, located at 1200cm -1 -1400cm -1 Peaks within a certain range are used to characterize defects in the carbon material of the shell.

[0063] In the description of this application, the term "G peak" refers to the peak located at 1580 cm⁻¹ in the Raman spectrum. -1 Nearby peaks, for example, located at 1500cm -1 -1700cm -1 The peaks within the range are used to characterize the degree of graphitization of the carbon material in the shell.

[0064] In the description of this application, the term "I" D / I G"This refers to the ratio of the intensity of the D peak to the G peak in the Raman spectrum."

[0065] In the technical solution of this application embodiment, by controlling the thickness of the shell, on the one hand, the conductivity of the sodium ferrous sulfate cathode material can be improved by using carbon materials, and on the other hand, sodium ions can have good diffusion performance in the sodium ferrous sulfate cathode material; in addition, by controlling the thickness of I D / I G Controlling the value can give the sodium ferrous sulfate cathode material a higher degree of graphitization, resulting in better conductivity.

[0066] Furthermore, in some embodiments, the shell thickness is 2nm-5nm.

[0067] In the technical solution of this application embodiment, by further controlling the thickness of the shell layer, the charge-discharge specific capacity of the battery with sodium ferrous sulfate cathode material can be further improved.

[0068] Furthermore, in some embodiments, the Raman spectrum of the sodium ferrous sulfate cathode material shows that I D / I G The value is (0.9-1):1.

[0069] In the technical solution of this application embodiment, by modifying I D / I G The optimal value of sodium ferrous sulfate cathode material is selected. When applied to batteries, sodium ferrous sulfate cathode material has good wettability with electrolyte, and its specific surface area is relatively large and its ion transport rate is high, which helps to further improve the charge and discharge specific capacity of batteries using sodium ferrous sulfate cathode material.

[0070] Furthermore, in some embodiments, the D50 particle size of the sodium ferrous sulfate cathode material is 0.5 μm-5 μm, specifically 1.27 μm, 2.39 μm, 2.42 μm, 2.49 μm, 2.87 μm, 3.61 μm, etc., and is not limited here; the BET specific surface area is 10 m². 2 / g-25m 2 / g, specifically 12.4m 2 / g, 15.4m 2 / g, 16.8m 2 / g, 20.4m 2 / g, 21.8m 2 / g, etc., are not limited here.

[0071] In the description of this application, the term "D50 particle size" refers to the particle size corresponding to a cumulative particle size distribution percentage of 50% for a sample, that is, the proportion of particles larger than D50 is 50%, and the proportion of particles smaller than D50 is also 50%.

[0072] In the technical solution of this application embodiment, controlling the D50 particle size of the sodium ferrous sulfate cathode material is beneficial to improving the compaction density of the sodium ferrous sulfate cathode material, thereby improving the energy density of the battery prepared from the sodium ferrous sulfate cathode material; in addition, controlling its BET specific surface area helps to prepare a uniform cathode slurry, which is of great help to improve the overall electrochemical performance of the cathode sheet and the secondary battery.

[0073] Furthermore, in some embodiments, the molar ratio of sodium to iron in the sodium ferrous sulfate cathode material is (2-2.5):1, specifically 2.05:1, 2.08:1, 2.1:1, etc., which are not limited here.

[0074] Furthermore, in some embodiments, the core layer of the sodium ferrous sulfate cathode material is made of sodium ferrous sulfate, and the general chemical formula of the sodium ferrous sulfate cathode material is Na. x Fe y (SO4) z @C, whose core material is Na x Fe y (SO4) z The shell material is C, where 2≤x≤2.5, 1≤y≤1.25, and 2≤z≤2.5. For example, it can be Na2Fe(SO4)2, etc.

[0075] In the technical solution of this application embodiment, controlling the molar ratio of sodium and iron elements can prepare a sodium ferrous sulfate cathode material with fewer defects and better structural integrity, which helps to improve the charge-discharge specific capacity. In addition, the slurry prepared with this sodium ferrous sulfate cathode material has higher stability and is easier to coat and process.

[0076] Secondly, embodiments of this application provide a method for preparing sodium ferrous sulfate cathode material, comprising the following steps:

[0077] A carbon-coated material is obtained by first calcining a sodium sulfate-based organic compound or a mixture of sodium sulfate-based organic compound and a carbon source; wherein the sodium sulfate-based organic compound contains alkyl groups.

[0078] The above-mentioned carbon-coated material and ferrous salt are mixed to obtain a mixture.

[0079] The above mixture was calcined a second time to obtain sodium ferrous sulfate cathode material;

[0080] The carbon source mentioned above is selected from carbon materials that are different from sodium sulfate organic compounds;

[0081] The aforementioned sodium ferrous sulfate cathode material comprises a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The shell layer has a thickness of 2 nm-10 nm. The Raman spectrum of the sodium ferrous sulfate cathode material exhibits D and G peaks, and I... D / I G The value is (0.8-1):1.

[0082] In the technical solution of this application embodiment, by performing a first calcination with sodium sulfate salt organic matter, or a mixture of sodium sulfate salt organic matter and carbon source, a shell layer with good uniformity and high degree of graphitization can be formed in situ on the surface of sodium sulfate. The second calcination can ensure the thickness of the shell layer on the one hand, and allow the ferrous salt to fully react with the above-mentioned carbon coating material to generate sodium ferrous sulfate material with a stable core layer, thereby improving the stability of sodium ferrous sulfate cathode material.

[0083] Furthermore, in some embodiments, the sodium sulfate salt organic compound is an alkyl sodium sulfate with 10-20 carbon atoms.

[0084] In the technical solution of this application embodiment, alkyl sodium sulfate with 10-20 carbon atoms is selected so that carbon-coated sodium sulfate material can be formed in situ during the first sintering process. It has fewer impurities and better coating effect.

[0085] Furthermore, in some embodiments, the temperature of the first calcination is 600℃-700℃, specifically 600℃, 670℃, 700℃, etc., which is not limited here. The holding time of the first calcination is 2h-4h, specifically 2h, 3h, 4h, etc., which is not limited here. After the first calcination is completed, the material is discharged after the temperature of the carbon-coated material drops to ≤120℃.

[0086] In the technical solution of this application embodiment, controlling the conditions of the first calcination is beneficial to fully decompose the sodium alkyl sulfate, form a carbon coating layer in situ on the surface of the sodium sulfate, and improve the conductivity of the sodium ferrous sulfate cathode material.

[0087] Furthermore, in some embodiments, the temperature of the second calcination is 300℃-350℃, specifically 300℃, 330℃, 350℃, etc., which is not limited here. The holding time of the second calcination is 20h-30h, specifically 20h, 25h, 30h, etc., which is not limited here. After the second calcination is completed, the material is discharged after the temperature of the sodium ferrous sulfate cathode material drops to ≤100℃.

[0088] In the technical solution of this application embodiment, controlling the conditions of the second calcination is beneficial to further improve the charging specific capacity and discharging specific capacity of the battery using sodium ferrous sulfate cathode material.

[0089] Furthermore, in some embodiments, the heating rate of the first calcination is 4℃ / min-6℃ / min, specifically 4℃ / min, 5℃ / min, 6℃ / min, etc., which is not limited here; the heating rate of the second calcination is 1℃ / min-2℃ / min, specifically 1℃ / min, 2℃ / min, etc., which is not limited here; the first and second calcinations are carried out in an inert atmosphere, and the oxygen content is controlled to be less than 50ppm during the first calcination and less than 5ppm during the second calcination; the inert atmosphere includes, but is not limited to, nitrogen, argon, helium or a mixture thereof.

[0090] In the technical solution of this application embodiment, the heating rate mainly affects the preparation rate and composition uniformity of sodium ferrous sulfate cathode material. Heating under the above conditions can make the internal and external heating of the calcined object uniform, which helps to prepare sodium ferrous sulfate cathode material with more uniform composition and better comprehensive performance.

[0091] Furthermore, in some embodiments, the ambient humidity is controlled at 5%-10% during the first calcination process and at less than 5% during the second calcination process.

[0092] In the technical solution of this application embodiment, the amount of carbon loss is reduced by controlling the ambient humidity.

[0093] Furthermore, in some embodiments, the carbon-coated material and ferrous salt are mixed and then pulverized. The particle size of the pulverized material is 1μm-3μm, specifically 1μm, 2.1μm, 3μm, etc., which are not limited here.

[0094] In the technical solution of this application embodiment, by controlling the particle size of the material, the diffusion path of ferrous ions can be shortened, making it easier to prepare a sodium ferrous sulfate cathode material with a core layer of sodium ferrous sulfate and a shell of carbon material, which has high conductivity.

[0095] Furthermore, in some embodiments, the pulverization process includes, but is not limited to, grinding, ball milling, and air jet milling; specifically, a pulverizer can be used for pulverization, and the pulverizing gas source is nitrogen with a pressure of 0.3MPa-0.5MPa, a dew point below -60℃, and an oxygen content below 1ppm.

[0096] Furthermore, in some embodiments, the pulverized material is transported through a pipeline to a mixer for mixing, and the mixing time is 60-120 minutes to obtain a mixture with uniform component distribution.

[0097] Furthermore, in some embodiments, after the second calcination, the material is demagnetized under constant temperature and humidity conditions to obtain sodium ferrous sulfate cathode material.

[0098] In the technical solution of this application embodiment, the content of impurities in the sodium ferrous sulfate cathode material can be reduced by demagnetizing.

[0099] Furthermore, in some embodiments, the alkyl sulfate with 10-20 carbon atoms is at least one selected from sodium dodecyl sulfate, sodium hexadecyl sulfate, sodium octadecyl sulfate, and sodium eicosyl sulfate, more preferably sodium dodecyl sulfate, sodium hexadecyl sulfate, or sodium octadecyl sulfate; the alkyl sulfate with 10-20 carbon atoms used has a purity ≥99%, obtained by purifying industrial-grade alkyl sulfate with 10-20 carbon atoms; the purification method is as follows:

[0100] Industrial-grade alkyl sulfate with 10-20 carbon atoms was dissolved in ethanol at 50℃-70℃, with a mass ratio of sodium sulfate with 10-20 carbon atoms to ethanol of 1:(5-10). The solution was then demagnetized until the content of magnetic material in the solution was below 0.5 ppm. The solution was then cooled, filtered, and dried to obtain purified alkyl sulfate with 10-20 carbon atoms. Further, demagnetization was stopped until the content of magnetic material in the solution was below 0.05 ppm, resulting in a purified alkyl sulfate with 10-20 carbon atoms containing less than 0.2 ppm of magnetic material.

[0101] Taking the purification of industrial-grade sodium dodecyl sulfate as an example, the magnetic material content of industrial-grade sodium dodecyl sulfate can be reduced from 6.7 ppm to 0.12 ppm by using the above method.

[0102] In the technical solution of this application embodiment, the above-mentioned materials are selected for the first calcination, which can generate carbon-coated sodium sulfate material with a moderate carbon shell thickness in situ, and the carbon shell thickness has good uniformity.

[0103] Adding a carbon source and controlling its dosage can help improve the electrolyte wettability of sodium ferrous sulfate cathode material, increase the ion transport rate, reduce the erosion of sodium ferrous sulfate cathode material by the electrolyte, and improve its cycle performance.

[0104] In some embodiments, the carbon source is at least one of glucose, sucrose, water-soluble starch, and fructose.

[0105] Furthermore, in some embodiments, the ferrous salt is at least one of ferrous acetate and ferrous oxalate.

[0106] In the technical solution of this application embodiment, when preparing sodium ferrous sulfate cathode material with the above-mentioned ferrous salt, acetate and oxalate will form carbon dioxide during the second calcination, without introducing other impurities, and the obtained sodium ferrous sulfate cathode material has good conductivity.

[0107] Thirdly, embodiments of this application provide a positive electrode sheet, including the sodium ferrous sulfate positive electrode material of the first aspect or the sodium ferrous sulfate positive electrode material prepared by the preparation method of the second aspect.

[0108] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet of the third aspect.

[0109] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0110] I. Preparation Method

[0111] The alkyl sulfate with 10-20 carbon atoms used in the examples and comparative examples was obtained by purifying industrial-grade alkyl sulfate with 10-20 carbon atoms. The purification method was as follows: industrial-grade alkyl sulfate with 10-20 carbon atoms was dissolved in ethanol at a temperature of 50℃-70℃, wherein the mass ratio of alkyl sulfate with 10-20 carbon atoms to ethanol was 1:(5-10). Then, it was pumped to a permanent magnet separator for circulating demagnetization until the content of magnetic material in the solution was lower than 0.05ppm, at which point the circulating demagnetization was stopped. Then, the solution was cooled to a temperature of 5℃-15℃. The obtained material was filtered and dried to obtain alkyl sulfate with 10-20 carbon atoms with a purity greater than or equal to 99%.

[0112] Example 1

[0113] One embodiment of the sodium ferrous sulfate cathode material of this application is prepared by the following method:

[0114] (1) Nitrogen gas is introduced to make the oxygen content 30±10ppm. Sodium dodecyl sulfate (purity 99.3%) is calcined for the first time under nitrogen atmosphere. The heating rate is 5℃ / min, the temperature of the first calcination is 670℃, and the holding time of the first calcination is 3h. After the product of the first calcination is cooled to ≤120℃, it is discharged to obtain the product of the first calcination (i.e., carbon-coated material). During the holding process, the waste gas is removed and the humidity is kept at 8%-8.5%.

[0115] (2) The first calcination product was mixed with ferrous acetate and pulverized to a particle size of 2.1 μm using an air jet mill. The pulverizing gas source was nitrogen with a pressure of 0.41 MPa, a dew point below -60℃, and an oxygen content below 1 ppm. After pulverization, the mixture was transported through a pipeline to a ribbon mixer for mixing. The mixing time was 90 min to obtain a mixed material. The molar ratio of sodium in the first calcination product to iron in ferrous acetate was 2.08:1.

[0116] (3) Introduce nitrogen to make the oxygen content less than 5 ppm, and calcine the mixture for the second time. The heating rate is 1.5℃ / min, the temperature of the second calcination is 330℃, and the holding time of the second calcination is 25h. After the product of the second calcination is cooled to ≤100℃, it is discharged to obtain the product of the second calcination. During the holding process, the waste gas is removed and the humidity is kept below 5%.

[0117] (4) The second calcination product was screened using a 100-mesh ultrasonic vibrating screen and demagnetized in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%) to obtain sodium ferrous sulfate cathode material.

[0118] Example 2

[0119] One embodiment of the sodium ferrous sulfate cathode material of this application is prepared by the following method:

[0120] (1) Nitrogen gas is introduced to make the oxygen content 20±10ppm. Sodium dodecyl sulfate (purity 99.1%) is calcined for the first time. The heating rate is 6℃ / min, the temperature of the first calcination is 700℃, and the holding time of the first calcination is 2h. After the product of the first calcination is cooled to ≤120℃, it is discharged to obtain the product of the first calcination. During the holding process, the waste gas is removed and the humidity is kept at 6±1%.

[0121] (2) The first calcination product was mixed with ferrous oxalate and pulverized to a particle size of 1 μm using an air jet mill. The pulverizing gas source was nitrogen with a pressure of 0.3 MPa, a dew point below -60℃, and an oxygen content below 1 ppm. After pulverization, the mixture was transported through a pipeline to a double cone mixer for mixing. The mixing time was 60 min to obtain a mixed material. The molar ratio of sodium in the first calcination product to iron in ferrous acetate was 2.05:1.

[0122] (3) Introduce nitrogen to make the oxygen content less than 5 ppm, and calcine the mixture for the second time. The heating rate is 1℃ / min, the temperature of the second calcination is 300℃, and the holding time of the second calcination is 30h. After the product of the second calcination is cooled to ≤100℃, it is discharged to obtain the product of the second calcination. During the holding process, the waste gas is removed and the humidity is kept below 5%.

[0123] (4) The second calcination product was screened using an 80-mesh ultrasonic vibrating screen and demagnetized in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%) to obtain sodium ferrous sulfate cathode material.

[0124] Example 3

[0125] One embodiment of the sodium ferrous sulfate cathode material of this application is prepared by the following method:

[0126] (1) Nitrogen gas is introduced to make the oxygen content 40±10ppm. Sodium dodecyl sulfate (purity 99.4%) is calcined for the first time. The heating rate is 4℃ / min, the temperature of the first calcination is 600℃, and the holding time of the first calcination is 4h. After the product of the first calcination is cooled to ≤120℃, it is discharged to obtain the product of the first calcination. During the holding process, the waste gas is removed and the humidity is kept at 9%-10%.

[0127] (2) The first calcination product was mixed with ferrous acetate and pulverized to a particle size of 3 μm using an air jet mill. The pulverizing gas source was nitrogen with a pressure of 0.5 MPa, a dew point below -60℃, and an oxygen content below 1 ppm. After pulverization, the mixture was transported through a pipeline to an inclined mixer for mixing. The mixing time was 120 min to obtain a mixed material. The molar ratio of sodium in the first calcination product to iron in ferrous acetate was 2.10:1.

[0128] (3) The mixture is calcined for the second time under a nitrogen atmosphere. The heating rate is 2℃ / min, the temperature of the second calcination is 350℃, and the holding time of the second calcination is 20h. The product of the second calcination is discharged after the temperature drops to ≤100℃. The waste gas is removed during the holding process, and the humidity is kept below 5%.

[0129] (4) The second calcination product was screened using a 150-mesh ultrasonic vibrating screen and demagnetized in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%) to obtain sodium ferrous sulfate cathode material.

[0130] Example 4

[0131] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that sodium hexadecyl sulfate (purity 99.4%) is used instead of sodium dodecyl sulfate.

[0132] Example 5

[0133] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that sodium octadecyl sulfate (purity 99.3%) is used instead of sodium dodecyl sulfate.

[0134] Example 6

[0135] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that sodium eicosyl sulfate (purity 99.1%) is used instead of sodium dodecyl sulfate.

[0136] Example 7

[0137] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that the temperature of the first calcination is 700°C.

[0138] Example 8

[0139] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that, in step (2), ferrous acetate is added at a molar ratio of sodium in the first calcination product to iron in ferrous acetate of 2.5:1.

[0140] Example 9

[0141] One embodiment of the sodium ferrous sulfate cathode material of this application. The difference between the preparation method of the sodium ferrous sulfate cathode material of this embodiment and that of Example 1 is that, in step (2), ferrous acetate is added at a molar ratio of sodium element in the first calcination product to iron element in ferrous acetate of 1.5:1.

[0142] Example 10

[0143] One embodiment of the sodium ferrous sulfate cathode material of this application differs from that of Example 1 only in that, in step (2), ferrous acetate is added at a molar ratio of sodium in the first calcination product to iron in ferrous acetate of 3:1.

[0144] Comparative Example 1

[0145] A sodium ferrous sulfate cathode material, the preparation method of which is as follows:

[0146] Glucose, ferrous acetate, and sodium sulfate were mixed in a molar ratio of 0.18:2.05:1. The mixture was then heated to 700°C at a rate of 6°C / min under a nitrogen atmosphere and held at 700°C for 2 hours, ensuring the humidity was below 5%. The calcined product was then sieved using an 80-mesh ultrasonic vibrating sieve and demagnetized in a constant temperature and humidity chamber (temperature 25±1°C, humidity ≤10%) to obtain sodium ferrous sulfate cathode material.

[0147] II. Testing Methods

[0148] 1. Property testing of sodium ferrous sulfate cathode material

[0149] 1) Surface morphology: The surface morphology of the sodium ferrous sulfate cathode material was observed using SEM and TEM;

[0150] 2) Shell thickness: Observed using TEM;

[0151] 3) Compacted density: Tested using a compaction density meter, with a test pressure of 3T and a compaction time of 30s;

[0152] 4) Tap density: Tested using a tap density meter with 5000 vibrations;

[0153] 5) BET: Tested using a BET analyzer, nitrogen adsorption method;

[0154] 6) D50 particle size: measured using a laser particle size analyzer;

[0155] 7)I D / I G Value: The Raman spectrum of the sample to be tested is determined based on the intensity of peak D. D With the intensity of peak G I G Calculate I D / I G The value;

[0156] 8) Powder resistivity: The four-probe method was used to test the resistivity at a pressure of 10 MPa.

[0157] 9) Iron dissolution amount: Add 1g of the test sample to 100mL of 0.1mol / L hydrogen fluoride-ethanol solution, stir and dissolve at 45℃ for 30min, then filter, and measure the iron content in the filtrate, which is the iron dissolution amount.

[0158] 2. Battery property testing

[0159] Sodium ferrous sulfate positive electrode material, polyvinylidene fluoride, and conductive carbon black were mixed in a mass ratio of 85:7:8 and coated onto aluminum foil. Then, an electrode sheet was prepared with a compaction density of 2.2 g / mL. A sodium sheet was used as the negative electrode, and a 1 mol / L sodium perchlorate solution was used as the electrolyte. The cells were assembled into a coin cell and measured at 25±0.1℃.

[0160] Using a constant current charge and discharge mode, charging and discharging tests were conducted at rates of 0.1C and 0.5C, respectively. The charging cutoff voltage was 4.5V, and the discharging cutoff voltage was 2.0V. The specific capacity of each battery during the first charge and the specific capacity during the first discharge were tested.

[0161] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0162] Figures 1-3 are SEM images of the sodium ferrous sulfate cathode material in Examples 1-3, respectively. As shown in Figures 1-3, the sodium ferrous sulfate cathode material disclosed in this application has a relatively uniform particle size and is cubic in shape. Figure 4 is the XRD pattern of the sodium ferrous sulfate cathode material in Examples 1-2. As shown in this figure, the obtained crystal has good structural integrity and no other impurity phases are generated. Figures 5-6 are TEM images of the sodium ferrous sulfate cathode material in Examples 1-2. As shown in Figures 5 and 6, there is an obvious carbon coating layer, and the carbon coating layer is relatively dense, without flocculent carbon or floating carbon. Figures 7-8 are Raman spectra of the sodium ferrous sulfate cathode material in Examples 1-2. As shown in Figures 7-8, the sodium ferrous sulfate cathode material in Examples 1-2 exhibits... D / I G The values ​​are all less than 1, indicating a relatively high degree of graphitization and good electronic conductivity.

[0163] Table 1 shows the performance test results of sodium ferrous sulfate cathode materials in the examples and comparative examples.

[0164] Table 1

[0165] As shown in Table 1, the sodium ferrous sulfate cathode material prepared by the method in the embodiments of this application has I D / I G The values ​​are all less than 1, indicating a high degree of graphitization, which helps to improve the conductivity of the sodium ferrous sulfate cathode material and enhance its electrochemical performance. Comparing the test results of Example 1 and Example 7, it can be found that as the primary calcination temperature increases, I... D / I G The value decreases, and the degree of graphitization increases.

[0166] In Comparative Example 1, alkyl sodium sulfate with 10-20 carbon atoms was not used, resulting in a relatively poor carbon coating effect. The prepared sodium ferrous sulfate cathode material had a lower I... D / IG The value is relatively high, and the degree of graphitization is low.

[0167] Table 2 shows the test results of the first charge specific capacity and first discharge specific capacity of the batteries made from the sodium ferrous sulfate cathode material in the examples and comparative examples at 0.1C and 0.5C rates.

[0168] Table 2

[0169] Comparing the test results of Examples 1, 4, 5, and 6, it can be found that when the alkyl sodium sulfate with 10-20 carbon atoms is at least one of sodium dodecyl sulfate, sodium hexadecyl sulfate, or sodium octadecyl sulfate, its charge specific capacity and discharge specific capacity are higher, and its rate performance is better. This may be because the carbon coating thickness of the sodium ferrous sulfate cathode material prepared using the above raw materials is moderate. Furthermore, comparing the test results of Example 1 and Comparative Example 1, it can be found that the performance of the sodium ferrous sulfate cathode material prepared by mixing ordinary sodium and carbon sources is not as good as that of the sodium ferrous sulfate cathode material prepared by in-situ carbon coating of alkyl sodium sulfate with 10-20 carbon atoms. Its graphitization degree is low, and its improvement effect on electrochemical performance is limited.

[0170] Comparing the test results of Examples 1 and 7, it can be found that a higher degree of graphitization is not necessarily better. When I D / I G When the value is (0.9-1):1, the charging specific capacity, discharging specific capacity and rate performance of the sodium ferrous sulfate cathode material are all improved.

[0171] Comparing the test results of Example 1 and Examples 8-10, it can be found that when the molar ratio of sodium element in the first calcination product to iron element in the ferrous salt is (2-2.5):1, its charging specific capacity and discharging specific capacity are higher, and it has better electrochemical performance.

[0172] Figure 9 shows the test results of the charging specific capacity and discharging specific capacity of the battery prepared with sodium ferrous sulfate cathode material in Example 1 at 0.1C and 0.5C rates. Figure 10 shows the test results of the charging specific capacity and discharging specific capacity of the battery prepared with sodium ferrous sulfate cathode material in Examples 2-3 at 0.1C rate. The test results are the same as those in Table 2, indicating that the sodium ferrous sulfate cathode material in the examples of this application has a high charging specific capacity and discharging specific capacity, and its rate performance is good.

[0173] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium ferrous sulfate cathode material, characterized in that, It includes a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The thickness of the shell layer is 2 nm-10 nm. The Raman spectrum of the sodium ferrous sulfate cathode material shows D peaks and G peaks. D / I G The value is (0.8-1):

1.

2. The sodium ferrous sulfate cathode material according to claim 1, characterized in that, The thickness of the shell layer is 2nm-5nm; and / or, in the Raman spectrum of the sodium ferrous sulfate cathode material, I D / I G The value is (0.9-1):

1.

3. The sodium ferrous sulfate cathode material according to claim 1 or 2, characterized in that, The sodium ferrous sulfate cathode material has a D50 particle size of 0.5 μm-5 μm and a BET specific surface area of ​​10 m². 2 / g-25m 2 / g.

4. The sodium ferrous sulfate cathode material according to claim 1, characterized in that, In the sodium ferrous sulfate cathode material, the molar ratio of sodium to iron is (2-2.5):

1.

5. A method for preparing a sodium ferrous sulfate cathode material, characterized in that, Includes the following steps: A carbon-coated material is obtained by first calcining a sodium sulfate organic compound or a mixture of sodium sulfate organic compound and a carbon source, wherein the sodium sulfate organic compound has alkyl groups. The carbon-coated material and the ferrous salt are mixed to obtain a mixture. The mixture is calcined a second time to obtain the sodium ferrous sulfate cathode material; The carbon source is selected from carbon materials that are different from the sodium sulfate organic compounds; The sodium ferrous sulfate cathode material comprises a core layer and a shell layer. The core layer is made of sodium ferrous sulfate, and the shell layer is made of carbon. The thickness of the shell layer is 2 nm-10 nm. The Raman spectrum of the sodium ferrous sulfate cathode material exhibits D peaks and G peaks. D / I G The value is (0.8-1):

1.

6. The method for preparing sodium ferrous sulfate cathode material according to claim 5, characterized in that, The sodium sulfate salt organic compound is an alkyl sodium sulfate with 10-20 carbon atoms.

7. The method for preparing sodium ferrous sulfate cathode material according to claim 6, characterized in that, The first calcination temperature is 600℃-700℃, and the holding time for the first calcination is 2h-4h; the second calcination temperature is 300℃-350℃, and the holding time for the second calcination is 20h-30h; and / or The heating rate for the first calcination is 4℃ / min-6℃ / min, and the heating rate for the second calcination is... The rate is 1℃ / min-2℃ / min, and the first and second calcinations are carried out in an inert atmosphere; and / or The ambient humidity during the first calcination is 5%-10%; and / or, before the second calcination, the mixture is pulverized, and the particle size of the mixture after pulverization is 1μm-3μm.

8. The method for preparing the sodium ferrous sulfate cathode material according to any one of claims 5 to 7, characterized in that, The alkyl sulfate with 10-20 carbon atoms is at least one of sodium dodecyl sulfate, sodium hexadecyl sulfate, sodium octadecyl sulfate, and sodium eicosyl sulfate; and / or, the carbon source is at least one of glucose, sucrose, water-soluble starch, and fructose; and / or, the ferrous salt is at least one of ferrous acetate and ferrous oxalate.

9. A positive electrode sheet, characterized in that, It includes a current collector, a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material includes the sodium ferrous sulfate positive electrode material according to any one of claims 1 to 4 or the sodium ferrous sulfate positive electrode material prepared by the preparation method of the sodium ferrous sulfate positive electrode material according to any one of claims 5 to 8.

10. A secondary battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and a negative electrode, wherein the positive electrode is the positive electrode as described in claim 9.

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