Ferrous sodium sulfate positive electrode material and method for producing the same, positive electrode sheet, and secondary battery

Abstract

This application provides a sodium ferrous sulfate cathode material, a method for producing the same, a cathode sheet, and a secondary battery, and belongs to the technical field of secondary batteries. The sodium ferrous sulfate cathode material comprises a core layer and a shell layer, the core layer material comprises sodium ferrous sulfate, and the shell layer material is a carbon material, where the thickness of the shell layer is 2 nm to 10 nm, and the Raman spectrum of the sodium ferrous sulfate cathode material has D peaks and G peaks. D / I G The value is (0.8~1):1. The ferrous sulfate sodium cathode material provided in this application has good conductivity, a relatively high charge-discharge ratio, and good electrochemical performance.

Description

[Technical Field]

[0001] This application claims priority to the Chinese patent application No. 202411055731.6, titled "Sodium ferrous sulfate cathode material and method for producing the same, cathode sheet, and secondary battery," filed with the China National Intellectual Property Administration on 1 August 2024, and all of its contents are incorporated into this application by reference.

[0002] This application relates to the technical field of secondary batteries, and more specifically to a sodium ferrous sulfate cathode material, a method for producing the same, a cathode sheet, and a secondary battery. [Background technology]

[0003] Lithium-ion batteries, with their superior electrochemical performance, are already widely used in fields such as mobile phones, laptops, and new energy vehicles, making them an ideal energy storage technology. However, globally, lithium resources are highly unbalanced in distribution, with approximately 70% located in South America, resulting in high production costs. Therefore, there is a need to provide new energy storage battery technologies to mitigate the negative impacts of lithium resource shortages.

[0004] Current research indicates that sodium-ion batteries also possess a certain capacity for storing and releasing electrical energy. Compared to lithium salts, sodium salts have a more abundant storage capacity and lower cost. Among these, sodium ferrous sulfate is relatively inexpensive and has a high voltage. However, sodium ferrous sulfate has low conductivity. Conventional processing methods improve its conductivity by carbon coating, but this still fails to effectively improve the charge ratio and discharge ratio of sodium ferrous sulfate cathode materials, further limiting the application of sodium ferrous sulfate cathode materials. [Overview of the project] [Problems that the invention aims to solve]

[0005] In view of the technical problems existing in the background art, the present application provides a ferrous sulfate sodium positive electrode material, a manufacturing method thereof, a positive electrode sheet, and a secondary battery, and aims to solve the technical problem that the conductivity of the ferrous sulfate sodium positive electrode material is low.

Means for Solving the Problems

[0006] In a first aspect, an embodiment of the present application is a ferrous sulfate sodium positive electrode material, which includes a core layer and a shell layer. The material of the core layer contains ferrous sulfate sodium, the material of the shell layer is a carbon material, the thickness of the shell layer is 2 nm to 10 nm, and the Raman spectrum of the ferrous sulfate sodium positive electrode material has a D peak and a G peak, and the value of I D / I G is (0.8 to 1): 1. A ferrous sulfate sodium positive electrode material is provided.

[0007] In the technical solution of the embodiment of the present application, by designing the structure of the ferrous sulfate sodium positive electrode material and coating a shell layer of a carbon material with a relatively high degree of graphitization on the surface of the ferrous sulfate sodium in the core layer, it helps to improve the conductivity of the ferrous sulfate sodium positive electrode material.

[0008] In some embodiments, the thickness of the shell layer is 2 nm to 5 nm.

[0009] In this embodiment, by further optimizing the thickness of the shell layer, it becomes easier for sodium ions to be released from the positive electrode material, which is beneficial to improving the charge specific capacity and discharge specific capacity of the battery using the ferrous sulfate sodium positive electrode material.

[0010] In some embodiments, in the Raman spectrum of the ferrous sulfate sodium positive electrode material, the value of I D / I G is (0.9 to 1): 1.

[0011] In this embodiment, further control of the D and G peak values ​​and increasing the amorphous carbon content helps to increase the specific surface area of ​​the sodium ferrous sulfate cathode material, thereby improving the wettability of the sodium ferrous sulfate cathode material to the electrolyte when used in a battery.

[0012] In some examples, the sodium ferrous sulfate cathode material has a D50 particle size of 0.5 μm to 5 μm and a BET specific surface area of ​​10 m². 2 / g~25m 2 It is / g.

[0013] In this embodiment, by controlling the D50 particle size and BET specific surface area of ​​the sodium ferrous sulfate cathode material, its processing performance can be improved, making it easier to produce a uniform slurry and improving the overall electrochemical performance of the battery.

[0014] In some examples, the sodium ferrous sulfate cathode material has a ratio of sodium to iron in terms of the amount of each substance.

[0015] In this embodiment, the capacity of a battery using a sodium ferrous sulfate cathode material can be further improved by controlling the ratio of the amounts of sodium and iron elements.

[0016] In a second embodiment, the embodiment of the present application is as follows: A step of obtaining a carbon-coated material by performing a first calcination on a sodium sulfate organic substance or a mixture of a sodium sulfate organic substance and a carbon source, wherein the sodium sulfate organic substance has an alkyl group. The steps include: mixing the above carbon-coated material with a ferrous salt to obtain a mixed material; The process includes the step of performing a second calcination on the above mixed material to obtain a sodium ferrous sulfate cathode material, Here, the carbon source is selected from a carbon material different from sodium sulfate organic matter. The ferrous sodium sulfate positive electrode material includes a core layer and a shell layer, the material of the core layer contains ferrous sodium sulfate, the material of the shell layer is a carbon material, the thickness of the shell layer is 2 nm to 10 nm, and the Raman spectrum of the ferrous sodium sulfate positive electrode material has a D peak and a G peak, and I D / I G The value of is (0.8 to 1):1, and a method for manufacturing a ferrous sodium sulfate positive electrode material is provided.

[0017] In this embodiment, sodium sulfate salts of organic substances or a mixture of sodium sulfate salts of organic substances and a carbon source and a ferrous salt are used as raw materials, and through two firings, the thickness of the shell layer is 2 nm to 10 nm, and I D / I G The value of is (0.8 to 1):1, and a ferrous sodium sulfate positive electrode material with improved conductivity can be obtained.

[0018] In some embodiments, the method for manufacturing a ferrous sodium sulfate positive electrode material is used to manufacture the ferrous sodium sulfate positive electrode material according to any one of the above.

[0019] In some embodiments, the sodium sulfate salts of organic substances are sodium alkyl sulfates having 10 to 20 carbon atoms.

[0020] <{ By selecting sodium alkyl sulfate having 10 to 20 carbon atoms as the sodium sulfate salts of organic substances, a carbon-coated sodium sulfate material can be formed in-situ after the first firing, and its carbon coating effect is relatively high, which is advantageous for improving the wettability of the ferrous sodium sulfate positive electrode material to the electrolyte, and further improves the charging performance and discharging performance of the battery manufactured by the positive electrode material.

[0021] In some embodiments, the temperature of the first firing is 600 °C to 700 °C, the heat preservation time of the first firing is 2 h to 4 h, the temperature of the second firing is 300 °C to 350 °C, and the heat preservation time of the second firing is 20 h to 30 h.

[0022] In this embodiment, controlling the conditions of the first and second firings is advantageous in allowing ferrous ions to enter the core layer and react with the sodium sulfate, which is the core layer material formed in the first firing, at a low temperature to produce sodium ferrous sulfate.

[0023] In some embodiments, the heating rate for the first firing was 4°C / min to 6°C / min, and the heating rate for the second firing was 1°C / min to 2°C / min. Both the first and second firings were carried out in an inert atmosphere.

[0024] In this embodiment, controlling the heating rate during the first and second firing processes is advantageous in making the material heating more uniform, ensuring a more complete reaction, and reducing thermal stress within the material.

[0025] In some examples, the ambient humidity during the first firing was 5% to 10%, and the ambient humidity during the second firing was within 5%.

[0026] In this embodiment, controlling the ambient humidity is advantageous in reducing the amount of carbon material lost.

[0027] In some examples, the mixed material was pulverized before the second firing, and the particle size of the mixed material after pulverization was 1 μm to 3 μm.

[0028] In this embodiment, controlling the particle size of the mixed material is advantageous in shortening the diffusion pathway of ferrous ions and improving the yield of the ferrous sulfate sodium cathode material.

[0029] In some examples, the alkylsulfate sodium having 10 to 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.

[0030] Using the above-mentioned alkyl sodium sulfate with 10 to 20 carbon atoms as a raw material makes it easy to manufacture a ferrous sodium sulfate cathode material with an appropriate carbon shell layer thickness. The above carbon source is a common material for manufacturing carbon coating layers and is readily available. Selecting ferrous acetate and ferrous oxalate as ferrous salts avoids the introduction of other impurities.

[0031] In a third embodiment, the embodiment of the present application provides a positive electrode sheet comprising a current collector, a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material is the positive electrode active material of the first embodiment of the present application or a positive electrode active material manufactured by the manufacturing method of the second embodiment of the present application, and thus has a higher charge-discharge ratio capacity.

[0032] In a fourth embodiment, the embodiment of the present application includes a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet according to the third embodiment of the present application, thereby providing a secondary battery having good electrochemical performance. [Effects of the Invention]

[0033] The above description is merely an outline of the technical solution of the present application. To better understand the technical means of the present application, it can be implemented according to the contents of the specification. In order to make the above and other objectives, features, and advantages of the present application clearer and easier to understand, specific embodiments of the present application are shown below. [Brief explanation of the drawing]

[0034] To more clearly explain the technical solution of this application, the drawings used in this application are briefly introduced below. Naturally, the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these without any creative effort. [Figure 1] This is a surface SEM image of the ferrous sulfate sodium cathode material in Example 1. [Figure 2] This is a surface SEM image of the ferrous sulfate sodium cathode material in Example 2. [Figure 3] This is a surface SEM image of the ferrous sulfate sodium cathode material in Example 3. [Figure 4] This is an XRD graph of the ferrous sulfate sodium cathode material in Examples 1 and 2. [Figure 5] This is a TEM diagram of the sodium ferrous sulfate cathode material in Example 1. [Figure 6] This is a TEM diagram of the sodium ferrous sulfate cathode material in Example 2. [Figure 7] This is a Raman spectrum diagram of the sodium ferrous sulfate cathode material in Example 1. [Figure 8] This is a Raman spectrum diagram of the sodium ferrous sulfate cathode material in Example 2. [Figure 9] This figure shows the test results of the charge ratio capacity and discharge ratio capacity at 0.1C and 0.5C rates for batteries manufactured using the ferrous sulfate sodium cathode material in Example 1. [Figure 10] This figure shows the test results of the charge ratio capacity and discharge ratio capacity at a 0.1C rate for batteries manufactured using ferrous sulfate sodium cathode material in Examples 2 and 3. [Modes for carrying out the invention]

[0035] The following descriptions will detail embodiments of the technical solution of the present application with reference to the drawings. The following embodiments are provided solely to clarify the technical solution of the present application and are therefore for illustrative purposes only, and do not limit the scope of protection of the present application.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art in the field relating to this application. The terms used herein are intended solely to describe specific embodiments and are not intended to limit this application. The terms “including,” “having,” and any variations thereof in the description of this application, claims, and drawings are intended to cover non-exclusive inclusion.

[0037] In the description of the embodiments of this application, technical terms such as "first," "second," etc., are merely used to distinguish different subjects and are not meant to indicate or imply relative importance, nor to implicitly show the number of technical features being referred to, a specific order, or a primary relationship. In the description of the embodiments of this application, unless otherwise specified, "plural" means two or more.

[0038] The “Examples” as used herein means that certain features, structures, or properties described in conjunction with the Examples may be included in at least one Example of the Application. The use of “Examples” in various parts of this Specification does not necessarily refer to the same Example, nor does it refer to an independent or alternative Example that is mutually exclusive with the other Examples. It will be explicitly and implicitly understood by those skilled in the art that the Examples described herein can be combined with other Examples.

[0039] In the description of the embodiments of this application, the terms "and / or" are merely used to describe the relationship between related objects, indicating that three relationships are possible. For example, A and / or B can represent three cases: A exists alone, A and B exist simultaneously, and B exists alone. In this specification, the letter " / " generally indicates that the preceding and following related objects are in an "or" relationship.

[0040] In the description of the embodiments of this application, the term "multiple" means two or more (including two), similarly, "multiple groups" means two or more groups (including two groups), and "multiple sheets" means two or more sheets (including two sheets).

[0041] In the description of the embodiments of this application, the directions or positional relationships indicated by technical terms such as "center," "vertical direction," "horizontal direction," "length," "width," "thickness," "top," "bottom," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inside," "outside," "clockwise," "counterclockwise," "axial direction," "radial direction," and "circumferential direction" are directions or positional relationships shown based on the drawings, and are merely for the purpose of easily explaining and simplifying the embodiments of this application. They do not indicate or imply that the shown devices or elements must have a specific direction or be configured and operated in a specific direction, and therefore do not limit the embodiments of this application.

[0042] In the description of the embodiments of this application, unless otherwise explicitly specified and limited, the technical terms "attachment," "connection," "bonding," and "fixing" should be understood in a broad sense. For example, they may be fixedly connected, detachably connected, or integrated; they may be mechanically connected, electrically connected, directly connected, indirectly connected via an intermediate medium, or be internal communication between two elements or an interaction relationship between two elements. Those skilled in the art will be able to understand the specific meaning of the above terms in the embodiments of this application depending on the specific case.

[0043] Compared to lithium resources, sodium resources are more abundant, less expensive, and have relatively high potential for applications in the energy storage field. Sodium ferrous sulfate is a commonly used cathode material in sodium-ion batteries, but its conductivity is low. Conventionally, the conductivity of sodium ferrous sulfate is generally improved using carbon coating methods, but the improvement effect is limited, and it is still not possible to significantly improve the performance of batteries using sodium ferrous sulfate as a cathode material, such as the charge ratio capacity and discharge ratio capacity.

[0044] To address the technical challenge of low conductivity in ferrous sulfate sodium cathode materials, this invention provides a ferrous sulfate sodium cathode material, a method for producing the same, a cathode sheet, and a secondary battery. The ferrous sulfate sodium cathode material comprises a core layer and a shell layer, the core layer is made of ferrous sulfate sodium, 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 ferrous sulfate sodium cathode material, the conductivity of the ferrous sulfate sodium cathode material can be effectively improved, further enhancing the capacity of the cathode sheet and the secondary battery.

[0045] In a first embodiment, the present invention provides a sodium ferrous sulfate cathode material comprising a core layer and a shell layer, wherein the core layer material comprises sodium ferrous sulfate, the shell layer material is a carbon material, and the thickness of the shell layer is 2 nm to 10 nm, specifically, 2.3 nm, 3.3 nm, 3.4 nm, 3.5 nm, 4.9 nm, 5.4 nm, 5.9 nm, 7.9 nm, etc., and is not limited herein, and the Raman spectrum of the sodium ferrous sulfate cathode material has a D peak and a G peak, D / I G The value of is (0.8~1):1, and specifically it may 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 to these values.

[0046] In the description of this application, the term "D peak" refers to the peak at 1350 cm⁻¹ in the Raman spectrum.-1 It is a peak located nearby, for example, 1200cm. -1 ~1400cm -1 These peaks are located within a certain range and are used to characterize defects in the carbon material of the shell layer.

[0047] In the description of this application, the term "G peak" refers to the peak at 1580 cm⁻¹ in the Raman spectrum. -1 It is a peak located nearby, for example, 1500cm. -1 ~1700cm -1 This peak lies within a certain range and is used to characterize the degree of graphitization of the carbon material in the shell layer.

[0048] In the description of this application, the term "I D / I G " is the ratio of the intensities of the D peak and the G peak in the Raman spectrum.

[0049] In the technical solution of the embodiment of the present application, by controlling the thickness of the shell layer, the conductivity of the ferrous sodium sulfate cathode material can be improved by the carbon material, while the sodium ions can have good diffusion performance in the ferrous sodium sulfate cathode material, and I D / I G By controlling the value of , the sodium ferrous sulfate cathode material can be made to have a higher degree of graphitization and thus higher conductivity.

[0050] Furthermore, in some examples, the thickness of the shell layer is 2 nm to 5 nm.

[0051] In the technical solution of the embodiment of the present application, the charge-discharge ratio capacity of a battery equipped with a sodium ferrous sulfate cathode material can be further improved by further controlling the thickness of the shell layer.

[0052] Furthermore, in some examples, the Raman spectrum of the sodium ferrous sulfate cathode material showed I D / I GThe value is (0.9~1):1.

[0053] In the technical solution of the embodiment of the present application, I D / I G By favoring this value, the sodium ferrous sulfate cathode material, when used in batteries, has good wettability to the electrolyte, a relatively large specific surface area, and a high ion conduction velocity, which helps to further improve the charge-discharge ratio capacity of batteries using the sodium ferrous sulfate cathode material.

[0054] Furthermore, in some examples, the D50 particle size of the sodium ferrous sulfate cathode material is 0.5 μm to 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 thereto. The BET specific surface area is 10 m². 2 / g~25m 2 It is / g, specifically 12.4m 2 / g, 15.4m 2 / g, 16.8m 2 / g, 20.4m 2 / g, 21.8m 2 It could also be / g, etc., and is not limited to this.

[0055] In the description of this application, the term "D50 particle size" refers to the particle size corresponding to the point when the cumulative particle size distribution percentage of the sample reaches 50%, meaning that 50% of the particles are larger than that size, and 50% are smaller.

[0056] In the technical solution of the embodiment of the present application, controlling the D50 particle size of the sodium ferrous sulfate cathode material is advantageous in improving the compressive density of the sodium ferrous sulfate cathode material, further improving the energy density of the battery manufactured using the sodium ferrous sulfate cathode material, and controlling its BET specific surface area helps in manufacturing a uniform cathode slurry, thereby greatly contributing to improving the overall electrochemical performance of the cathode sheet and secondary battery.

[0057] Furthermore, in some embodiments, the sodium ferrous sulfate cathode material has a ratio of sodium to iron content of (2-2.5):1, and may be specifically 2.05:1, 2.08:1, 2.1:1, etc., but is not limited thereto.

[0058] Furthermore, in some examples, the core layer material of the sodium ferrous sulfate cathode material is sodium ferrous sulfate, and the general formula of the sodium ferrous sulfate cathode material is Na x Fe y (SO4) z It is @C, and the material of its core layer is Na x Fe y (SO4) z The shell layer material is C, where 2≦x≦2.5, 1≦y≦1.25, and 2≦z≦2.5. For example, it may be Na2Fe(SO4)2.

[0059] In the technical solution of the embodiment of the present application, by controlling the ratio of the amounts of sodium and iron elements, a sodium ferrous sulfate cathode material with fewer defects and higher structural integrity can be manufactured, which helps to improve the charge-discharge ratio capacity. Furthermore, the slurry manufactured from this sodium ferrous sulfate cathode material is more stable and easier to coat.

[0060] In a second embodiment, the embodiment of the present application is as follows: A step of obtaining a carbon-coated material by performing a first calcination on a sodium sulfate organic substance or a mixture of a sodium sulfate organic substance and a carbon source, wherein the sodium sulfate organic substance contains an alkyl group. The steps include: mixing the above carbon-coated material with a ferrous salt to obtain a mixed material; The process includes the step of performing a second calcination on the above mixed material to obtain a sodium ferrous sulfate cathode material, Here, the carbon source is selected from a carbon material different from sodium sulfate organic matter. The above ferrous sodium sulfate cathode material comprises a core layer and a shell layer, wherein the core layer material contains ferrous sodium sulfate, the shell layer material is carbon, the shell layer thickness is 2 nm to 10 nm, and the Raman spectrum of the ferrous sodium sulfate cathode material has D peaks and G peaks. D / I G The present invention provides a method for producing a sodium ferrous sulfate cathode material in which the value of is (0.8~1):1.

[0061] In the technical solution of the embodiment of the present application, by performing a first calcination on a sodium sulfate organic substance, or a mixture of a sodium sulfate organic substance and a carbon source, a shell layer with excellent uniformity and a high degree of graphitization can be formed in situ on the sodium sulfate surface. The second calcination ensures the thickness of the shell layer while allowing sufficient solid-phase reaction between the ferrous salt and the carbon coating material, thereby producing a ferrous sodium sulfate material with a stable core layer and improving the stability of the ferrous sodium sulfate cathode material.

[0062] Furthermore, in some examples, the sodium sulfate organic compounds are alkyl sodium sulfates having 10 to 20 carbon atoms.

[0063] In the technical solution of the embodiment of this application, by selecting alkyl sodium sulfate having 10 to 20 carbon atoms, a carbon-coated sodium sulfate material can be formed in situ during the first sintering process, and it has relatively few impurities and a high coating effect.

[0064] Furthermore, in some embodiments, the temperature of the first firing is 600°C to 700°C, and may specifically be 600°C, 670°C, 700°C, etc., but is not limited thereto; the holding time for the first firing is 2h to 4h, and may specifically be 2h, 3h, 4h, etc., but is not limited thereto; and after the first firing is completed, the carbon-coated material is discharged after its temperature has dropped to ≤120°C.

[0065] In the technical solution of the embodiment of the present application, controlling the conditions of the first firing allows for sufficient decomposition of the alkyl sodium sulfate, forming a carbon coating layer in situ on the sodium sulfate surface, which is advantageous for improving the conductivity of the ferrous sodium sulfate cathode material.

[0066] Furthermore, in some embodiments, the temperature of the second firing is 300°C to 350°C, and may specifically be 300°C, 330°C, 350°C, etc., but is not limited thereto. The holding time for the second firing is 20h to 30h, and may specifically be 20h, 25h, 30h, etc., but is not limited thereto. After the second firing is completed, the sodium ferrous sulfate cathode material is discharged after its temperature has dropped to ≤100°C.

[0067] In the technical solution of the embodiment of the present application, controlling the conditions of the second firing is advantageous in further improving the charge ratio capacity and discharge ratio capacity of a battery using a sodium ferrous sulfate cathode material.

[0068] Furthermore, in some embodiments, the heating rate for the first firing is 4°C / min to 6°C / min, and may specifically be 4°C / min, 5°C / min, 6°C / min, etc., but is not limited thereto; the heating rate for the second firing is 1°C / min to 2°C / min, and may specifically be 1°C / min, 2°C / min, etc., but is not limited thereto; the first and second firings are carried out in an inert atmosphere, the oxygen content in the first firing process is controlled to less than 5 ppm, and the oxygen content in the second firing process is controlled to less than 5 ppm, and the inert atmosphere includes, but is not limited to, nitrogen gas, argon gas, helium gas, or a mixture thereof.

[0069] In the technical solutions of the embodiments of the present application, the heating rate mainly affects the manufacturing rate and the uniformity of the components of the ferrous sulfate sodium cathode material. By heating under the above conditions, the heat absorption inside and outside the object being fired is made uniform, which helps to produce a ferrous sulfate sodium cathode material with more uniform components and higher overall performance.

[0070] Furthermore, in some examples, the ambient humidity was controlled to 5% to 10% during the first firing process, and controlled to within 5% during the second firing process.

[0071] In the technical solution of the embodiment of the present invention, the amount of carbon loss is reduced by controlling the ambient humidity.

[0072] Furthermore, in some examples, after mixing the carbon-coated material with the ferrous salt, a grinding treatment is performed, and the particle size of the material after grinding is 1 μm to 3 μm, and may specifically be 1 μm, 2.1 μm, 3 μm, etc., and is not limited thereto.

[0073] In the technical solution of the embodiment of the present application, the diffusion path of ferrous ions can be shortened by controlling the particle size of the material, and the ferrous sodium sulfate cathode material, in which the core layer contains ferrous sodium sulfate and the shell layer is made of carbon material, is easier to manufacture and has high conductivity.

[0074] Furthermore, in some embodiments, the grinding process includes, but is not limited to, polishing, ball milling, and air-jet grinding. Specifically, the grinding process can be performed using a grinder, and the grinding gas source is nitrogen gas with a pressure of 0.3 MPa to 0.5 MPa, a dew point of less than -60°C, and an oxygen content of less than 1 ppm.

[0075] Furthermore, in some embodiments, the crushed material is transported into a mixer via a pipe and mixed for a mixing time of 60 min to 120 min to obtain a mixed material with a uniform component distribution.

[0076] Furthermore, in some examples, after the second firing is completed, the material is demagnetized under constant temperature and humidity conditions to obtain a sodium ferrous sulfate cathode material.

[0077] In the technical solution of the embodiment of the present application, the content of impurities in the sodium ferrous sulfate cathode material can be reduced by demagnetization.

[0078] Furthermore, in some examples, the alkylsulfate sodium having 10 to 20 carbon atoms is at least one of dodecyl sulfate sodium, hexadecyl sulfate sodium, octadecyl sulfate sodium, and eicosyl sulfate sodium, more preferably dodecyl sulfate sodium, hexadecyl sulfate sodium, and octadecyl sulfate sodium. The alkylsulfate sodium having 10 to 20 carbon atoms used has a purity of ≥99% and is obtained by purifying industrial-grade alkylsulfate sodium having 10 to 20 carbon atoms, and the purification method is as follows.

[0079] Industrial-grade alkylsulfate with 10 to 20 carbon atoms is dissolved in ethanol at a temperature of 50°C to 70°C, with a mass ratio of 1:(5 to 10) of alkylsulfate with 10 to 20 carbon atoms to ethanol. Demagnetization is then performed until the magnetic substance content in the solution is less than 0.5 ppm, after which demagnetization is stopped. The solution is then cooled, filtered, dried, and purified alkylsulfate with 10 to 20 carbon atoms is obtained. Further demagnetization is performed until the magnetic substance content in the solution is less than 0.05 ppm, after which demagnetization is stopped, and the magnetic substance content in the purified alkylsulfate with 10 to 20 carbon atoms is less than 0.2 ppm.

[0080] Taking the purification of industrial-grade sodium dodecyl sulfate as an example, when industrial-grade sodium dodecyl sulfate is purified using the above method, the magnetic substance content can be reduced from 6.7 ppm to 0.12 ppm.

[0081] In the technical solution of the embodiment of the present application, by selecting the above material and performing a first firing, a carbon-coated sodium sulfate material with an appropriate thickness of carbon shell layer can be produced in situ, and the uniformity of the thickness of the carbon shell layer is higher.

[0082] By adding a carbon source and controlling its usage, it is possible to improve the wettability of the sodium ferrous sulfate cathode material to the electrolyte, thereby increasing the ion conduction rate, reducing erosion of the sodium ferrous sulfate cathode material by the electrolyte, and improving its cycle performance.

[0083] In some examples, the carbon source is at least one of glucose, sucrose, water-soluble starch, and fructose.

[0084] Furthermore, in some examples, the ferrous salt is at least one of ferrous acetate and ferrous oxalate.

[0085] In the technical solution of the embodiment of the present application, when a ferrous sodium sulfate cathode material is manufactured using the above ferrous salt, the acetate group and oxalic acid group form carbon dioxide during the second calcination, without introducing other impurities, and the manufactured ferrous sodium sulfate cathode material has high conductivity.

[0086] In a third embodiment, the embodiment of the present application provides a cathode sheet comprising a sodium ferrous sulfate cathode material according to the first embodiment or a sodium ferrous sulfate cathode material manufactured by the manufacturing method according to the second embodiment.

[0087] In a fourth embodiment, an embodiment of the present application provides a secondary battery including a positive electrode sheet according to the third embodiment.

[0088] The following are some specific examples, which are illustrative and intended only to interpret this application and not to limit it. Where specific techniques or conditions are not specified in the examples, the techniques and conditions described in the relevant art literature or product descriptions shall be followed. Unless otherwise specified, the reagents or equipment used are all commercially available, common products.

[0089] 1. Manufacturing method The C10-C20 alkylsulfate sodium used in the examples and comparative examples is obtained by purifying industrial-grade C10-C20 alkylsulfate sodium, and the purification method is as follows: Industrial-grade C10-C20 alkylsulfate sodium is added to ethanol at a temperature of 50°C to 70°C and dissolved, where the mass ratio of C10-C20 alkylsulfate sodium to ethanol is 1:(5-10). Subsequently, the solution is transported by pump to a permanent magnet type iron remover for circulating demagnetization. When the magnetic substance content in the solution is less than 0.05 ppm, the circulating demagnetization is stopped, and the solution is then cooled to a temperature of 5°C to 15°C. The obtained material is filtered and dried to obtain C10-C20 alkylsulfate sodium with a purity of 99% or higher for use.

[0090] (Example 1) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is as follows.

[0091] (1) Nitrogen gas is introduced to maintain an oxygen content of 30±10 ppm, and the first calcination is performed on sodium dodecyl sulfate (with a purity of 99.3%) under a nitrogen atmosphere, with a heating rate of 5°C / min, a first calcination temperature of 670°C, and a holding time of 3 hours. The product from the first calcination is cooled to ≤120°C and then discharged to obtain the product from the first calcination (i.e., carbon-coated material), and exhaust gas is extracted during the holding process, ensuring a humidity of 8%~8.5%.

[0092] (2) The product of the first calcination is mixed with ferrous acetate and pulverized by a jet mill until the particle size is 2.1 μm. The pulverizing gas source is nitrogen gas with a pressure of 0.41 MPa, a dew point of less than -60°C, and an oxygen content of less than 1 ppm. After pulverization, the mixture is transported through a pipe into a ribbon mixer and mixed for 90 mins to obtain the mixed material. The ratio of the amount of sodium in the product of the first calcination to the amount of iron in the ferrous acetate is 2.08:1.

[0093] (3) Nitrogen gas is introduced to reduce the oxygen content to less than 5 ppm, and the mixed material is subjected to a second firing at a heating rate of 1.5°C / min, with a second firing temperature of 330°C, and a holding time of 25 hours for the second firing. The product from the second firing is cooled to a temperature of ≤100°C before being discharged to obtain the second firing product, and exhaust gas is extracted during the holding process, ensuring a humidity of 5% or less.

[0094] (4) The product from the second firing was sieved using a 100-mesh ultrasonic vibrating sieve, and demagnetized in a constant temperature and humidity environment (temperature 25±1℃ and humidity ≤10%) to obtain a sodium ferrous sulfate cathode material.

[0095] (Example 2) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is as follows.

[0096] (1) Nitrogen gas is introduced to adjust the oxygen content to 20±10 ppm, and the first calcination is performed on sodium dodecyl sulfate (purity 99.1%), with a heating rate of 6°C / min, a first calcination temperature of 700°C, and a holding time of 2 hours for the first calcination. The product from the first calcination is cooled to ≤120°C and then discharged to obtain the product from the first calcination, and the exhaust gas is extracted during the holding process, ensuring a humidity of 6±1%.

[0097] (2) The product of the first calcination is mixed with ferrous oxalate and pulverized by a jet mill until the particle size is 1 μm. The pulverizing gas source is nitrogen gas with a pressure of 0.3 MPa, a dew point of less than -60°C, and an oxygen content of less than 1 ppm. After pulverization, the mixture is transported through a pipe into a double cone mixer and mixed for 60 min to obtain the mixed material. The ratio of the amount of sodium in the product of the first calcination to the amount of iron in ferrous acetate is 2.05:1.

[0098] (3) Nitrogen gas is introduced to maintain an oxygen content of less than 5 ppm, and the mixed material is subjected to a second firing at a heating rate of 1°C / min, with a second firing temperature of 300°C, a holding time of 30 hours, the product from the second firing is cooled to ≤100°C and then discharged to obtain the product from the second firing, the exhaust gas is extracted during the holding process, and the humidity is guaranteed to be within 5%.

[0099] (4) The product from the second firing was sieved using an 80-mesh ultrasonic vibrating sieve, and demagnetized in a constant temperature and humidity environment (temperature 25±1℃ and humidity ≤10%) to obtain a sodium ferrous sulfate cathode material.

[0100] (Example 3) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is as follows.

[0101] (1) Nitrogen gas is introduced to adjust the oxygen content to 40±10 ppm, and the first calcination is performed on sodium dodecyl sulfate (purity 99.4%), with a heating rate of 4°C / min, a first calcination temperature of 600°C, and a holding time of 4 hours for the first calcination. The product from the first calcination is cooled to ≤120°C and then discharged to obtain the product from the first calcination, and the exhaust gas is extracted during the holding process, ensuring a humidity of 9%~10%.

[0102] (2) The product of the first calcination is mixed with ferrous acetate and pulverized by a jet mill until the particle size is 3 μm. The pulverizing gas source is nitrogen gas with a pressure of 0.5 MPa, a dew point of less than -60°C, and an oxygen content of less than 1 ppm. After pulverization, the mixture is transported through a pipe into an inclined-axis mixer and mixed for 120 min to obtain the mixed material. The ratio of the amount of sodium in the product of the first calcination to the amount of iron in the ferrous acetate is 2.10:1.

[0103] (3) Perform a second firing of the mixed material under a nitrogen atmosphere, with a heating rate of 2°C / min, a second firing temperature of 350°C, a holding time of 20 hours, and discharge the product from the second firing after it has cooled to ≤100°C to obtain the second firing product. Extract the exhaust gas during the holding process and ensure the humidity is within 5%.

[0104] (4) The product from the second firing was sieved using a 150-mesh ultrasonic vibrating sieve, and demagnetized in a constant temperature and humidity environment (temperature 25±1℃ and humidity ≤10%) to obtain a sodium ferrous sulfate cathode material.

[0105] (Example 4) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is the same as in Example 1, except that sodium dodecyl sulfate is replaced with sodium hexadecyl sulfate (with a purity of 99.4%).

[0106] (Example 5) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is the same as in Example 1, except that sodium dodecyl sulfate is replaced with sodium octadecyl sulfate (with a purity of 99.3%).

[0107] (Example 6) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is the same as in Example 1, except that sodium dodecyl sulfate is replaced with sodium eicosyl sulfate (with a purity of 99.1%).

[0108] (Example 7) This is one embodiment of the ferrous sodium sulfate cathode material of the present application, and the method for producing the ferrous sodium sulfate cathode material of this embodiment is the same as in Example 1, except that the temperature of the first firing is 700°C.

[0109] (Example 8) This is an embodiment of the ferrous sulfate sodium cathode material of the present application, and the method for producing the ferrous sulfate sodium cathode material of this embodiment is the same as in Example 1, except that in step (2), ferrous acetate is added such that the ratio of the amount of sodium element in the product of the first calcination to the amount of iron element in the ferrous acetate is 2.5:1.

[0110] (Example 9) This is an embodiment of the ferrous sulfate sodium cathode material of the present application, and the method for producing the ferrous sulfate sodium cathode material of this embodiment is the same as in Example 1, except that in step (2), ferrous acetate is added such that the ratio of the amount of sodium element in the product of the first calcination to the amount of iron element in the ferrous acetate is 1.5:1.

[0111] (Example 10) This is an embodiment of the ferrous sulfate sodium cathode material of the present application, and the method for producing the ferrous sulfate sodium cathode material of this embodiment is the same as in Example 1, except that in step (2), ferrous acetate is added such that the ratio of the amount of sodium element in the product of the first calcination to the amount of iron element in the ferrous acetate is 3:1.

[0112] (Comparative Example 1) The ferrous sulfate sodium cathode material is manufactured as follows.

[0113] Glucose, ferrous acetate, and sodium sulfate were mixed in a ratio of 0.18:2.05:1, and then heated to 700°C at a rate of 6°C / min under a nitrogen atmosphere. The mixture was then maintained at 700°C for 2 hours, ensuring that the humidity was within 5%, and the calcined product was sieved using an 80-mesh ultrasonic vibrating sieve. The mixture was then demagnetized in a constant temperature and humidity environment (temperature 25±1°C and humidity ≤10%) to obtain a sodium ferrous sulfate cathode material.

[0114] 2. Test Method 1. Characteristic testing of ferrous sulfate sodium cathode material 1) Surface morphology: The surface morphology of the ferrous sulfate sodium cathode material was observed using SEM and TEM. 2) Thickness of the shell layer: Observed using TEM, 3) Compression density: Tested using a compression density meter, with a test pressure of 3T and a pressing time of 30s. 4) Tap density: Tested using a tap density meter, with a vibration count of 5000 times. 5) BET: The test was conducted using a BET test machine, and the nitrogen adsorption method was used. 6) D50 particle size: Tested using a laser particle size analyzer. 7) I D / I G Value of: The Raman spectrometer of the test sample was examined, and the intensity of the D peak I was measured. D and the intensity of the G peak I G Based on I D / I G Calculate the value of, 8) Powder resistivity: Tested using the four-probe method under a pressure of 10 MPa. 9) Iron elution amount: 1 g of the test sample was added to 100 mL of a 0.1 mol / L hydrogen fluoride-ethanol solution, dissolved by stirring at 45°C for 30 min, and then filtered. The amount of iron eluted, which is the iron element content in the obtained filtrate, was measured.

[0115] 2. Battery characteristic testing A sodium ferrous sulfate cathode material, polyvinylidene fluoride, and conductive carbon black were mixed in a mass ratio of 85:7:8 and coated onto aluminum foil. Subsequently, an electrode sheet was manufactured with a compressed density of 2.2 g / mL. A sodium sheet was used as the anode, and the electrolyte was a 1 mol / L sodium perchlorate solution. These were assembled into a coin-type battery and measured in an environment of 25 ± 0.1°C.

[0116] Using a constant current charge / discharge mode, charging and discharging tests were performed at rates of 0.1C and 0.5C, respectively. The charge cutoff voltage was set to 4.5V and the discharge cutoff voltage to 2.0V, and the initial charge ratio capacity and initial discharge ratio capacity of each battery were tested.

[0117] III. Analysis of Test Results for Each Example and Comparative Example Figures 1 to 3 are surface SEM images of the ferrous sodium sulfate cathode material in Examples 1 to 3, respectively. As can be seen from Figures 1 to 3, the particles of the ferrous sodium sulfate cathode material disclosed herein are uniform in size and form cubic particles. Figure 4 is an XRD image of the ferrous sodium sulfate cathode material in Examples 1 to 2. As can be seen from this figure, the obtained crystals have relatively high structural integrity and no other impurities are generated. Figures 5 to 6 are TEM images of the ferrous sodium sulfate cathode material in Examples 1 to 2. As can be seen from Figures 5 and 6, a clear carbon coating layer is present, the carbon coating layer is relatively dense, and there is no condensed carbon or floating carbon. Figures 7 to 8 are Raman spectral images of the ferrous sodium sulfate cathode material in Examples 1 to 2. As can be seen from Figures 7 to 8, the I D / I G The values ​​for each parameter are all less than 1, indicating a relatively high degree of graphitization and good electronic conductivity.

[0118] Table 1 shows the performance test results of the ferrous sulfate sodium cathode materials in the examples and comparative examples.

[0119] [Table 1] As can be seen from Table 1, the sodium ferrous sulfate cathode material I produced by the method in the embodiment of this application D / I G The values ​​of all are less than 1, indicating a relatively high degree of graphitization, which helps to improve the conductivity of the sodium ferrous sulfate cathode material and enhance its electrochemical performance. As can be seen from the comparison of the test results of Example 1 and Example 7, when the temperature of the first firing increases, I D / I G The value decreases, and the degree of graphitization increases.

[0120] In Comparative Example 1, since alkyl sodium sulfate with 10 to 20 carbon atoms was not used, the carbon coating effect was relatively low, and the manufactured ferrous sodium sulfate cathode material ID / I G The value is relatively high, and the degree of graphitization is low.

[0121] Table 2 shows the test results of the initial charge ratio capacity and initial discharge ratio capacity at 0.1C and 0.5C rates for batteries manufactured with ferrous sulfate sodium cathode material in the examples and comparative examples.

[0122] [Table 2] As can be seen from the comparison of the test results of Examples 1, 4, 5, and 6, when the alkyl sodium sulfate with 10 to 20 carbon atoms is at least one of dodecyl sodium sulfate, hexadecyl sodium sulfate, or octadecyl sodium sulfate, the charge ratio capacity and discharge ratio capacity are higher and the rate performance is superior. This is thought to be because the thickness of the carbon coating layer of the ferrous sodium sulfate cathode material manufactured using the above raw materials is appropriate. Furthermore, as can be seen from the comparison of the test results of Example 1 and Comparative Example 1, the performance of the ferrous sodium sulfate cathode material manufactured by mixing a general sodium source and a carbon source is inferior to the performance of the ferrous sodium sulfate cathode material manufactured by carbon coating alkyl sodium sulfate with 10 to 20 carbon atoms in situ. Its degree of graphitization is low, and its effect on improving electrochemical performance is limited.

[0123] As can be seen from the comparison of the test results of Example 1 and Example 7, a higher degree of graphitization is not necessarily better. D / I G When the value of (0.9~1):1, the charge ratio capacity, discharge ratio capacity, and rate performance of the sodium ferrous sulfate cathode material all improve.

[0124] As can be seen from the comparison of the test results of Example 1 and Examples 8-10, when the ratio of the amount of sodium element in the product of the first calcination to the amount of iron element in the ferrous salt is (2-2.5):1, the charge ratio capacity and discharge ratio capacity are higher, and the electrochemical performance is higher.

[0125] Figure 9 shows the test results of the charge ratio capacity and discharge ratio capacity at 0.1C and 0.5C rates for batteries manufactured with the ferrous sodium sulfate cathode material in Example 1, and Figure 10 shows the test results of the charge ratio capacity and discharge ratio capacity at 0.1C rate for batteries manufactured with the ferrous sodium sulfate cathode material in Examples 2 and 3. The test results are the same as in Table 2, indicating that the charge ratio capacity and discharge ratio capacity of the ferrous sodium sulfate cathode material in the examples of this application are relatively high, and that its rate performance is relatively high.

[0126] Furthermore, this application is not limited to the embodiments described above. The embodiments described above are merely illustrative, and any embodiment that has substantially the same configuration as the technical gist and produces the same effects within the scope of the technical solution of this application is included within the technical scope of this application. In addition, other forms that are constructed by combining some of the components of the embodiments, by implementing various modifications that a person skilled in the art could conceive of the embodiments, without departing from the spirit of this application, are also included within the scope of this application.

Claims

1. A sodium ferrous sulfate cathode material comprising a core layer and a shell layer, wherein the core layer is made of sodium ferrous sulfate, the shell layer is made of carbon, the shell layer has a thickness of 2 nm to 10 nm, and the Raman spectrum of the sodium ferrous sulfate cathode material has a D peak and a G peak, I Ｄ / I Ｇ A sodium ferrous sulfate cathode material characterized by having a value of (0.8 to 1):

1.

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

1.

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

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

1.

5. A step of obtaining a carbon-coated material by performing a first calcination on a sodium sulfate organic substance or a mixture of a sodium sulfate organic substance and a carbon source, wherein the sodium sulfate organic substance has an alkyl group, The steps include mixing the carbon-coated material with a ferrous salt to obtain a mixed material, The step includes performing a second calcination on the mixed material to obtain a sodium ferrous sulfate cathode material, Here, the carbon source is selected from a carbon material different from the sodium sulfate organic matter. The ferrous sodium sulfate cathode material comprises a core layer and a shell layer, the core layer is made of ferrous sodium sulfate, the shell layer is made of carbon, the shell layer has a thickness of 2 nm to 10 nm, and the Raman spectrum of the ferrous sodium sulfate cathode material has a D peak and a G peak. Ｄ / I Ｇ A method for producing a sodium ferrous sulfate cathode material, characterized in that the value of is (0.8 to 1):

1.

6. The method for producing a sodium ferrous sulfate cathode material according to claim 5, characterized in that the aforementioned sodium sulfate organic material is an alkyl sodium sulfate having 10 to 20 carbon atoms.

7. The temperature for the first firing is 600°C to 700°C, the holding time for the first firing is 2 hours to 4 hours, the temperature for the second firing is 300°C to 350°C, the holding time for the second firing is 20 hours to 30 hours, and / or The heating rate for the first firing is 4°C / min to 6°C / min, the heating rate for the second firing is 1°C / min to 2°C / min, and both the first and second firings are performed in an inert atmosphere, and / or The method for producing a sodium ferrous sulfate cathode material according to claim 6, characterized in that the ambient humidity for the first firing is 5% to 10%, and / or, before the second firing, the mixed material is subjected to a grinding treatment, and the particle size of the mixed material after the grinding treatment is 1 μm to 3 μm.

8. A method for producing a sodium ferrous sulfate cathode material according to any one of claims 5 to 7, characterized in that the alkyl sodium sulfate having 10 to 20 carbon atoms is at least one of sodium dodecyl sulfate, sodium hexadecyl sulfate, sodium octadecyl 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 comprising a current collector, a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material comprises a sodium ferrous sulfate positive electrode material described in any one of claims 1 to 4, or a sodium ferrous sulfate positive electrode material manufactured by a method for manufacturing a sodium ferrous sulfate positive electrode material described in any one of claims 5 to 8.

10. A secondary battery comprising a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet described in claim 9.

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