Sodium ferric sulfate positive electrode material, preparation method thereof, positive electrode sheet and sodium ion battery

Abstract

The application discloses a sodium ferric sulfate positive electrode material, a preparation method and a positive electrode sheet and a sodium ion battery, and relates to a sodium ferric sulfate positive electrode material, which comprises Na2Fe2(SO4)3, the surface of the Na2Fe2(SO4)3 is coated with a carbon coating layer, and the carbon coating layer is doped with N elements and F elements. After the N and F anions are doped, defects are formed in the surface carbon coating layer, sodium ion adsorption and storage are facilitated, the conductivity of the positive electrode material is greatly improved, the migration rate of Na + is improved, the rate performance of the material is improved, the F elements help to relieve the interface side reaction between the electrolyte and the electrode surface, and the cycle life of the positive electrode material is improved.

Description

Technical Field

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

[0002] In recent years, with the rapid development of renewable energy sources such as solar, wind, and tidal energy, the demand for high-power, low-cost energy storage facilities has increased significantly. High-power, high-energy-density, and low-cost battery systems have always been a target of in-depth research. Due to the abundance and low cost of sodium resources, rechargeable sodium-ion batteries (SIBs) are among the most promising batteries for large-scale application. As a crucial component of sodium-ion batteries, cathode materials play a decisive role in improving electrochemical performance and reducing the overall cost of the battery. For practical applications, cathode materials need to have high redox potentials and power densities. Polyanionic cathode materials not only have high potential plateaus but also relatively stable structures, ensuring that complex phase transitions do not occur during cycling or voltage decay, and are compatible with PO4. 3- In comparison, SO4 2- Higher electronegativity and stronger inductive effect endow the materials with higher operating voltages. Among them, Na₂Fe₂(SO₄)₃ stands out, exhibiting a high voltage plateau of 3.8V, dependent solely on Fe. 2+ / Fe 3+ As a redox center, and Na + Ionic components occupy Fe 2+ The lattice positions of the ions give the material excellent sodium ion transport channels; in Na + During the ion insertion / extraction process, the volume change of the material is only 1.6%, which gives Na2Fe2(SO4)3 a longer cycle life.

[0003] However, Na2Fe2(SO4)3 has inherently low electronic conductivity, which severely inhibits its electrochemical performance and hinders its application as a cathode material. Currently, carbon coating has been widely used to improve the electrochemical performance of polyanionic materials; however, conventional carbon coating requires in-situ carbonization at high temperatures (500℃~800℃), which is problematic for Na2Fe2(SO4)3 materials, especially SO42-containing materials. 2- Decomposition occurs at temperatures exceeding 450°C during heat treatment, making it difficult to apply the strategy of improving conductivity through in-situ carbon coating via high-temperature carbonization. Summary of the Invention

[0004] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, this invention proposes a sodium iron sulfate cathode material, its preparation method, a cathode sheet, and a sodium-ion battery, to address the difficulty of applying the strategy of in-situ carbon coating through high-temperature carbonization to improve conductivity to Na2Fe2(SO4)3 materials.

[0005] In a first aspect, the present invention provides a sodium iron sulfate cathode material comprising Na2Fe2(SO4)3, wherein the surface of the Na2Fe2(SO4)3 is coated with a carbon coating layer, and the carbon coating layer is doped with N and F elements.

[0006] In a first aspect of the invention, the N and F anion-doped carbon coating layer on the surface greatly improves the conductivity of the cathode material and enhances the Na+ conductivity. + The migration rate of the F element is increased, which improves the rate performance of the material. At the same time, the introduction of F element forms a thin and tough NaF surface layer on the material surface, which greatly enhances the material surface's ability to resist electrolyte erosion and provides stable protection for the cathode material during long-term cycling, thereby improving the cycle life of the cathode material.

[0007] In a second aspect of the present invention, a method for preparing a sodium ferric sulfate cathode material is provided, comprising the following steps:

[0008] (1) FeSO4 and Na2SO4 were ball-milled to obtain the precursor;

[0009] (2) The precursor is sintered under inert gas protection to obtain Na2Fe2(SO4)3;

[0010] (3) Polyethylene glycol, NH4F and Na2Fe2(SO4)3 are uniformly dispersed in methanol, and the methanol is evaporated to obtain a solid mixture;

[0011] (4) The solid mixture is sintered under inert gas protection to obtain a doped carbon-coated sodium iron sulfate cathode material.

[0012] In a second aspect of the invention, a compound containing sodium and iron sources is subjected to high-speed ball milling to obtain a precursor, which is then sintered to obtain Na2Fe2(SO4)3. Na2Fe2(SO4)3 is placed in methanol, and polyethylene glycol (PEG) and NH4F (nitrogen and fluorine sources) are added and stirred until homogeneous. PEG and NH4F are impregnated and adhered to the surface of Na2Fe2(SO4)3. Subsequently, PEG is carbonized on the surface of Na2Fe2(SO4)3 to form a coating layer, while NH4F is doped with N and F elements in the coating layer after sintering, resulting in a carbon-coated sodium iron sulfate cathode material. Simultaneously, the prepared cathode material exhibits relatively uniform surface coating, good coating effect, and a simple preparation process with low cost, environmental friendliness, and ease of large-scale industrial production.

[0013] According to an embodiment of the present invention, the molar ratio of FeSO4 to Na2SO4 is 1:(0.25~1.5).

[0014] According to an embodiment of the present invention, the ball milling conditions include: a ball-to-material ratio of (15-20):1 and a rotation speed of 300 rpm to 500 rpm.

[0015] According to an embodiment of the present invention, the ball milling time is 5h to 8h.

[0016] According to an embodiment of the present invention, the sintering conditions are 200°C to 400°C;

[0017] According to an embodiment of the present invention, the sintering time is 10h to 15h.

[0018] According to an embodiment of the present invention, the evaporation temperature is 80°C to 100°C.

[0019] According to an embodiment of the present invention, the mass ratio of the polyethylene glycol to the Na2Fe2(SO4)3 is (1-5):100.

[0020] According to an embodiment of the present invention, the mass ratio of NH4F to Na2Fe2(SO4)3 is (0.1-0.5):100.

[0021] According to an embodiment of the present invention, the sintering conditions are 200°C to 400°C;

[0022] According to an embodiment of the present invention, the sintering time is 5h to 15h.

[0023] In a third aspect of the invention, a positive electrode sheet is provided, comprising sodium iron sulfate positive electrode material prepared by the preparation method described in the second aspect. This improves the rate performance and cycle stability of the battery.

[0024] In a fourth aspect of the invention, a sodium-ion battery is provided, comprising the positive electrode sheet described in the third aspect. This results in high rate performance and cycle stability.

[0025] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0026] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0027] Figure 1 SEM image of the cathode material prepared in Example 1;

[0028] Figure 2 The XRD diffraction pattern of the cathode material prepared in Example 1;

[0029] Figure 3 This is a graph showing the first charge-discharge curve of the battery made from the positive electrode material in Example 1. Detailed Implementation

[0030] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0031] In a first aspect of the invention, a sodium iron sulfate cathode material is provided, comprising Na2Fe2(SO4)3, wherein the surface of the Na2Fe2(SO4)3 is coated with a carbon coating layer, wherein the carbon coating layer is doped with N and F elements.

[0032] In a first aspect of the invention, the N and F anion-doped carbon coating layer on the surface greatly improves the conductivity of the cathode material and enhances the Na+ conductivity. + The migration rate of the F element is increased, which improves the rate performance of the material. At the same time, the introduction of F element forms a thin and tough NaF surface layer on the material surface, which greatly enhances the material surface's ability to resist electrolyte erosion and provides stable protection for the cathode material during long-term cycling, thereby improving the cycle life of the cathode material.

[0033] In a second aspect of the present invention, a method for preparing a sodium ferric sulfate cathode material is provided. In some embodiments, the preparation method includes the following steps:

[0034] (1) FeSO4 and Na2SO4 are ball-milled to obtain a precursor, thereby achieving mechanical alloying. Optionally, the molar ratio of FeSO4 to Na2SO4 is 1:(0.25~1.5), for example, 1:0.25, 1:1, 1:1.5, etc. Optionally, the ball-to-material ratio is (15~20):1, for example, 15:1, 18:1, 20:1, etc.; the rotation speed is 300rpm~500rpm, for example, 300rpm, 400rpm, 500rpm, etc. This results in more uniform mixing. Further, the ball-milling time is 5h~8h, for example, 5h, 6h, 7h, 8h, etc.

[0035] (2) The precursor is sintered under an inert gas atmosphere to obtain Na2Fe2(SO4)3; optionally, the sintering conditions are 200℃~400℃, for example, 200℃, 250℃, 300℃, 350℃, 400℃, etc., thereby ensuring that the solid-phase reaction proceeds completely while avoiding excessively high temperatures that could lead to SO42-. 2- Decomposition. Optionally, the sintering time is 10h to 15h.

[0036] (3) Polyethylene glycol, NH4F, and Na2Fe2(SO4)3 are dispersed in methanol, and the methanol is evaporated to obtain a solid mixture. Thus, polyethylene glycol and NH4F are attached to Na2Fe2(SO4)3. In some specific embodiments, the evaporation temperature is 80°C to 100°C, for example, 80°C, 90°C, 95°C, 100°C, etc. This completes the evaporation of methanol.

[0037] In some specific embodiments, the mass ratio of polyethylene glycol to Na₂Fe₂(SO₄)₃ is (1-5):100, for example, 1:100, 2:100, 3:100, 4:100, 5:100, etc. Therefore, the amount of polyethylene glycol doping is beneficial for obtaining a coating layer of ideal thickness, which significantly improves conductivity while avoiding the problem of excessively thick coating layers causing longer sodium ion migration paths. In some specific embodiments, the mass ratio of NH₄F to Na₂Fe₂(SO₄)₃ is (0.1-0.5):100, for example, 0.1:100, 0.15:100, 0.2:100, 0.3:100, 0.4:100, 0.5:100, etc. Therefore, after F and N element doping, defects will form in the surface carbon layer, which is beneficial for sodium ion adsorption and storage, thereby increasing the sodium ion diffusion rate and improving the conductivity of the cathode material. If the doping amount is too low, the defects will not be obvious and the effect will be minimal; if the doping amount is too high, it will damage the surface carbon layer structure, aggravate surface side reactions, and cause performance deterioration.

[0038] (4) The solid mixture is sintered under an inert gas atmosphere to obtain a carbon-coated sodium iron sulfate cathode material. This carbonizes the PEG attached to Na₂Fe₂(SO₄)₃, and NH₄F is used to dope the carbon layer with N and F. Optionally, sintering is performed at 200°C to 400°C under an argon atmosphere, for example, 200°C, 250°C, 300°C, 350°C, 400°C, etc., to obtain a carbon-coated Na₂Fe₂(SO₄)₃; this facilitates the formation of a uniformly coated carbon layer and avoids the decomposition of SO₄. Optionally, the sintering time is 5 h to 15 h.

[0039] In a second aspect of the present invention, a compound containing sodium and iron sources is subjected to high-speed ball milling to obtain a precursor, which is then sintered to obtain Na2Fe2(SO4)3. The obtained Na2Fe2(SO4)3 is placed in methanol, and carbon sources PEG and NH4F are added. After thorough stirring, the mixture is evaporated at a high temperature of 80°C to 100°C to obtain a carbon-coated sodium iron sulfate cathode material. At the same time, the prepared cathode material has a relatively uniform surface coating, a good coating effect, and a simple preparation process with low cost, environmental friendliness, and ease of large-scale industrial production.

[0040] In a third aspect of the invention, a positive electrode sheet is provided, comprising sodium iron sulfate positive electrode material prepared by the preparation method described in the second aspect. This improves the rate performance and cycle stability of the battery.

[0041] In a fourth aspect of the invention, a sodium-ion battery is provided, comprising the positive electrode sheet described in the third aspect. This results in high rate performance and cycle stability.

[0042] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0043] Example 1

[0044] (1) FeSO4·7H2O and Na2SO4 were placed in a box furnace and heated to 200℃ at a heating rate of 2℃ / min under an argon atmosphere. The temperature was held for 15h to obtain FeSO4 and Na2SO4.

[0045] (2) FeSO4 and Na2SO4 were placed in a ball mill jar at a molar ratio of 2:3 and a ball-to-material ratio of 20:1. The mixture was ball-milled at 300 rpm for 8 hours to obtain the precursor.

[0046] (3) The precursor was heated to 350°C in a box furnace under argon protection at a heating rate of 2°C / min, and then sintered at 350°C for 15h to obtain Na2Fe2(SO4)3.

[0047] (4) The Na2Fe2(SO4)3 cathode material, PEG, and NH4F are uniformly dispersed in methanol solvent and stirred. After being thoroughly stirred, the mixture is evaporated at 80°C. The mass ratio of PEG to cathode material is 2%, and the mass ratio of NH4F to cathode material is 0.15%.

[0048] (5) The cathode material was heated to 300°C in a box furnace under argon protection at a heating rate of 2°C / min, and then sintered at 300°C for 8 hours to obtain doped carbon-coated Na2Fe2(SO4)3.

[0049] Example 2

[0050] Everything else is the same as in Example 1, except that in step (3), Na2Fe2(SO4)3 is obtained by sintering at 400°C for 15 hours.

[0051] Example 3

[0052] Everything else is the same as in Example 1, except that in step (3), Na2Fe2(SO4)3 is obtained by sintering at 300°C for 15 hours.

[0053] Comparative Example 1

[0054] Everything else is the same as in Example 1, except that step (4) is omitted.

[0055] Comparative Example 2

[0056] Everything else is the same as in Example 1, except that NH4F is not added in step (4).

[0057] Comparative Example 3

[0058] Everything else is the same as in Example 1, except that PEG is not added in step (4).

[0059] I. The product of Example 1 was subjected to SEM and XRD tests, and the results are shown in [the table below]. Figure 1 and Figure 2 .

[0060] Figure 1 The image shows a SEM image of the cathode material prepared in Example 1. Figure 1 As can be seen, the synthesized sodium ferric sulfate cathode material is distributed in blocks, with relatively rough particle surfaces and a single particle size of approximately 3 μm.

[0061] Figure 2 The XRD diffraction pattern of the cathode material prepared in Example 1 is shown below. Figure 2 As can be seen, the characteristic peaks of the synthesized sodium ferric sulfate cathode material correspond well, and there are also many peaks with lower intensity, which may be due to sintering.

[0062] II. Button Battery Preparation: The positive electrode materials prepared in the examples and comparative examples were applied to button batteries for electrochemical performance testing. The battery assembly process is as follows: The positive electrode material, polyvinylidene fluoride (PVDF), and conductive carbon black were mixed in a mass ratio of 8:1:1 and dissolved in N-methylpyrrolidone (NMP) solvent to prepare a positive electrode slurry. The slurry was stirred until there were no particles inside. The positive electrode slurry was then uniformly coated onto aluminum foil on a coating machine to form an electrode sheet. The electrode sheet was placed in a vacuum drying oven at 120°C and vacuum dried for 12 hours. The electrode sheet was then removed and rolled on a roller press for later use. The electrode sheets were cut into 12mm diameter circles using a cutting machine, dried in a vacuum drying oven at 60℃ for 12 hours, and then weighed using an electronic balance. Finally, the positive electrode sheet, sodium sheet, spring sheet, gasket, positive electrode shell, negative electrode shell, separator, and electrolyte were assembled into a CR2025 coin cell in an argon-filled glove box. The electrolyte was a 1M NaPF6 solution dissolved in a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) (by volume, EC:DMC = 1:1). The sodium sheet served as the counter electrode. The charge / discharge voltage range was 2.0V–4.5V, the initial charge / discharge rate was 0.1C, and the nominal specific capacity was 1C = 120mAh / g. Figure 3 This is a graph showing the first charge-discharge curve of the battery made with the positive electrode material in Example 1. Figure 3 It can be seen that the synthesized sodium ferric sulfate cathode material exhibits a discharge specific capacity as high as 92 mAh / g within the voltage range of 2V to 4.5V, which is far higher than the capacity of the cathode material in the comparative example. This is attributed to the defects formed in the carbon coating layer on the surface after N and F anion doping, which is beneficial for sodium ion adsorption and storage. The test data for cycle performance testing at a charge-discharge rate of 1C for 50 cycles are shown in Table 1.

[0063] Table 1

[0064]

[0065] As shown in Table 1, based on the test results of the examples and comparative examples, the carbon-coated sodium ferric sulfate cathode materials doped with N and F elements in Examples 1-3 exhibit superior electrochemical performance, higher discharge specific capacity, better rate performance, and more stable cycle performance. This is attributed to the uniform coating layer formed on the surface, which helps to improve the conductivity of the material and enhance the Na+ content. + It also improves the diffusion rate, alleviates the interfacial side reactions between the cathode material and the electrolyte, and enhances cycle life.

[0066] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0067] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a sodium ferric sulfate cathode material, characterized in that, Includes the following steps: (1) FeSO4 and Na2SO4 were ball-milled to obtain the precursor; (2) The precursor is sintered under inert gas protection to obtain Na2Fe2(SO4)3; (3) Disperse polyethylene glycol, NH4F and Na2Fe2(SO4)3 in methanol, and evaporate the methanol to obtain a solid mixture; (4) The solid mixture is sintered under inert gas protection to obtain a doped carbon-coated sodium iron sulfate cathode material; In step (3), the mass ratio of polyethylene glycol to Na2Fe2(SO4)3 is (1~5):100; In step (3), the mass ratio of NH4F to Na2Fe2(SO4)3 is (0.1~0.5):100; The sodium ferric sulfate cathode material includes Na2Fe2(SO4)3, the surface of which is coated with a carbon coating layer, and the carbon coating layer is doped with N and F elements.

2. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, In step (1), the molar ratio of FeSO4 to Na2SO4 is 1:(0.25~1.5).

3. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, In step (1), the ball milling conditions include: The ball-to-material ratio is (15-20):1, and the rotation speed is 300 rpm to 500 rpm.

4. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, The ball milling time is 5 h to 8 h.

5. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, In step (2), the sintering conditions are 200℃~400℃.

6. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, The sintering time is 10 h to 15 h.

7. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, In step (3), after the reaction is complete, evaporation is carried out at 80℃~100℃.

8. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, In step (4), the sintering conditions are 200℃~400℃.

9. The method for preparing sodium ferric sulfate cathode material according to claim 1, characterized in that, The sintering time is 5 h to 15 h.

10. A positive electrode plate, characterized in that, The sodium ferric sulfate cathode material prepared by any of the preparation methods described in claims 1 to 9.

11. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 10.

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