Positive electrode material and method thereof, sodium ion battery positive plate and sodium ion battery

Abstract

The invention relates to a positive electrode material and a preparation method thereof, a sodium-ion battery positive plate and a sodium-ion battery. The positive electrode material comprises a mixed sodium iron phosphate matrix and a double-carbon heterostructure penetrating through the inside and the outside of the mixed sodium iron phosphate matrix, the general formula of the mixed sodium iron phosphate matrix is Na4Fe3 (PO4) 2P2O7, and the double-carbon heterostructure comprises a conductive carbon material and an organic carbon source precursor pyrolysis product, the mixed sodium iron phosphate matrix is foamed by gas generated by pyrolysis of the organic carbon source precursor to form a three-dimensional porous structure, and the surfaces of particles of the mixed sodium iron phosphate matrix are coated with pyrolysis products of the organic carbon source precursor; the conductive carbon material penetrates through the three-dimensional porous structure and is connected with the mixed sodium ferric phosphate matrix particles coated with the organic carbon source precursor pyrolysis product, so that a three-dimensional porous interconnected conductive network is formed. According to the positive electrode material, the sodium storage specific capacity can be ensured, the electronic conductivity of the NFPP material is effectively improved, and the rate capability and the cycling stability of the NFPP material are improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode material and its preparation method, a sodium-ion battery positive electrode sheet, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are considered an ideal choice for large-scale energy storage due to their abundant resources and low cost. Among many cathode materials, mixed polyanionic compounds, especially mixed sodium iron phosphate (Na4Fe3(PO4)2P2O7, NFPP), show promising application prospects because they have stable three-dimensional sodium ion diffusion channels, high theoretical specific capacity (approximately 129 mAh / g), and moderate average discharge voltage (approximately 3.2 V), which is higher than that of Na2FeP2O7 and Na2FePO4F. Furthermore, the iron-based raw materials are low-cost and environmentally friendly.

[0003] However, the low electronic conductivity of NFPP materials severely limits their rate performance and cycling stability, thus hindering their practical applications. Common modification methods such as carbon coating can improve conductivity to some extent, but they are usually difficult to construct efficient ion and electron transport networks simultaneously, and traditional single carbon source coating often fails to effectively suppress particle aggregation and optimize material structure.

[0004] Therefore, how to effectively improve the electronic conductivity of NFPP materials and enhance their rate performance and cycle stability has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] Therefore, it is necessary to provide a cathode material and its preparation method, a sodium-ion battery cathode sheet, and a sodium-ion battery to address the issues of how to effectively improve the electronic conductivity of NFPP materials and enhance their rate performance and cycle stability.

[0006] A cathode material comprising a mixed sodium iron phosphate matrix and a dual-carbon heterostructure penetrating the interior and exterior of the mixed sodium iron phosphate matrix, wherein the general formula of the mixed sodium iron phosphate matrix is ​​Na4Fe3(PO4)2P2O7, and the dual-carbon heterostructure comprises a conductive carbon material and pyrolysis products of an organic carbon source precursor, wherein the gas generated by the pyrolysis of the organic carbon source precursor foams the mixed sodium iron phosphate matrix to form a three-dimensional porous structure, the pyrolysis products of the organic carbon source precursor coat the surface of the mixed sodium iron phosphate matrix particles, and the conductive carbon material penetrates the three-dimensional porous structure and connects the mixed sodium iron phosphate matrix particles coated by the pyrolysis products of the organic carbon source precursor, thereby forming a three-dimensional porous interconnected conductive network.

[0007] In the cathode material of the present invention, the dual-carbon heterostructure synergistically modifies the mixed sodium iron phosphate matrix to form a three-dimensional porous interconnected conductive network. This effectively improves the electronic conductivity of the NFPP material, as well as its rate performance and cycle stability, while ensuring the sodium storage specific capacity. The sodium storage electrochemical performance is significantly better than that of pure phase Na4Fe3(PO4)2P2O7 material.

[0008] In one embodiment, the conductive carbon material includes one or a mixture of multiple of carbon nanotubes, surface-treated carbon nanotubes, Ketjen black, acetylene black, graphene, and activated carbon. The surface-treated carbon nanotubes include one or a mixture of more than one of acidified carbon nanotubes, surface-functionalized carbon nanotubes, and alkalized carbon nanotubes.

[0009] In one embodiment, the organic carbon source precursor includes one or a mixture of more of glucose, sucrose, citric acid, ascorbic acid, and dopamine.

[0010] In one embodiment, the dual-carbon heterostructure accounts for 1% to 10% of the mass fraction of the cathode material.

[0011] A method for preparing any of the above-mentioned cathode materials includes the following steps: Sodium source, iron source, phosphorus source, conductive carbon material and organic carbon source precursor are mixed uniformly according to stoichiometric ratio to obtain precursor material; The precursor material is subjected to a first sintering treatment at 300℃~400℃ under a protective atmosphere, and after sufficient reaction, a pre-sintered material is obtained; and The pre-sintered material is subjected to a second sintering treatment at 500℃~800℃ under a protective atmosphere, and the cathode material is obtained after sufficient reaction.

[0012] The method for preparing the cathode material of this invention involves synergistic modification of a mixed sodium iron phosphate matrix with a dual-carbon heterostructure, forming a three-dimensional porous interconnected conductive network. This effectively improves the electronic conductivity of the NFPP material, as well as its rate performance and cycle stability, while ensuring the sodium storage specific capacity. The sodium storage electrochemical performance is significantly better than that of pure phase Na4Fe3(PO4)2P2O7 material.

[0013] In one embodiment, the sodium source is anhydrous sodium pyrophosphate, the iron source is ferrous oxalate dihydrate, and the phosphorus source is diammonium hydrogen phosphate. The molar ratio of sodium in the sodium source, iron in the iron source, and phosphorus in the phosphorus source is (4+2x):(3-x):4, ​​where the parameter x ranges from -1 to 2.

[0014] In one embodiment, the mass ratio of the conductive carbon material to the organic carbon source precursor is 1:3 to 1:5.

[0015] In one embodiment, the first sintering treatment lasts for 2 to 5 hours, and the second sintering treatment lasts for 6 to 12 hours.

[0016] A sodium-ion battery positive electrode sheet, comprising any of the above-mentioned positive electrode materials.

[0017] The sodium-ion battery cathode of the present invention includes the above-mentioned cathode material. By synergistically modifying the mixed sodium iron phosphate matrix through a dual-carbon heterostructure, a three-dimensional porous interconnected conductive network is formed. This can effectively improve the electronic conductivity of the NFPP material and improve its rate performance and cycle stability while ensuring the sodium storage specific capacity.

[0018] A sodium-ion battery includes the sodium-ion battery positive electrode sheet described above.

[0019] The sodium-ion battery of the present invention includes the above-mentioned sodium-ion battery positive electrode sheet, which includes the above-mentioned positive electrode material. By synergistically modifying the mixed sodium iron phosphate matrix through a dual-carbon heterostructure, a three-dimensional porous interconnected conductive network is formed. This can effectively improve the electronic conductivity of the NFPP material and improve its rate performance and cycle stability while ensuring the sodium storage specific capacity. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method for preparing a positive electrode material according to an embodiment of the present invention; Figure 2 X-ray diffraction patterns of the substances prepared in Example 1 and Comparative Example 1; Figure 3(a) is a scanning electron microscope (SEM) image of the NFPP prepared in Comparative Example 1; Figure 3(b) is a scanning electron microscope (SEM) image of NFPP@C / CNFs prepared in Example 1; Figure 4 The image shows the morphological characteristics of the NFPP@C / CNFs obtained in Example 1. Detailed Implementation

[0021] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0022] 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 invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] One embodiment of the cathode material includes a mixed sodium iron phosphate matrix and a dual-carbon heterostructure penetrating the interior and exterior of the mixed sodium iron phosphate matrix. The general formula of the mixed sodium iron phosphate matrix is ​​Na4Fe3(PO4)2P2O7. The dual-carbon heterostructure includes a conductive carbon material and pyrolysis products of an organic carbon source precursor. The gas generated by the pyrolysis of the organic carbon source precursor foams the mixed sodium iron phosphate matrix to form a three-dimensional porous structure. The pyrolysis products of the organic carbon source precursor coat the surface of the mixed sodium iron phosphate matrix particles. The conductive carbon material penetrates the three-dimensional porous structure and connects the mixed sodium iron phosphate matrix particles coated by the pyrolysis products of the organic carbon source precursor, thereby forming a three-dimensional porous interconnected conductive network.

[0024] In the cathode material of the above embodiments, the mixed sodium iron phosphate matrix is ​​an iron-based mixed polyanionic mixture containing phosphate and pyrophosphate ions.

[0025] In the cathode material of the above embodiments, the pyrolysis product of the organic carbon source precursor (i.e., carbon layer) is coated on the surface of the mixed sodium iron phosphate matrix particles. This coating layer can prevent the material from directly contacting the electrolyte and suppress the side reactions generated in the electrochemical reaction.

[0026] In the cathode material described above, a dual-carbon heterostructure is used as a conductive material and simply composited with Na4Fe3(PO4)2P2O7. This allows the dual-carbon heterostructure to uniformly penetrate the interior and exterior of the mixed sodium iron phosphate matrix, forming a special structure with a three-dimensional porous interconnected network. This effectively improves the electronic conductivity of the NFPP material, as well as its rate performance and cycle stability, while maintaining the sodium storage specific capacity. The sodium storage electrochemical performance is significantly better than that of pure-phase Na4Fe3(PO4)2P2O7 material.

[0027] Based on the aforementioned embodiments, the conductive carbon material includes one or a mixture of multiple selected from carbon nanotubes, surface-treated carbon nanotubes, Ketjen black, acetylene black, graphene, and activated carbon. These types of conductive carbon materials possess good conductivity and can effectively improve the electronic conductivity of NFPP materials.

[0028] Based on the aforementioned embodiments, surface-treated carbon nanotubes include one or a mixture of more than one of acidified carbon nanotubes, surface-functionalized carbon nanotubes, and alkalized carbon nanotubes. These types of surface-treated carbon nanotubes have functionalized groups on their surfaces, such as carboxyl groups (-COOH). The presence of these functional groups increases the surface hydrophilicity of the carbon material, allowing it to mix thoroughly with the raw materials in solution. Furthermore, the functional groups in the carbon material itself lead to localized charge imbalances, which can adsorb polar substances during the reaction process, acting as 3D templates and promoting the formation of the final three-dimensional network structure of the material.

[0029] Based on the aforementioned embodiments, the organic carbon source precursor includes one or more of glucose, sucrose, citric acid, ascorbic acid, and dopamine. These types of organic carbon source precursors can undergo carbonization during high-temperature pyrolysis. The generated gas foams the mixed sodium iron phosphate matrix, forming a three-dimensional porous structure. Residual carbon coats the surface of the mixed sodium iron phosphate matrix, while conductive carbon material acts as a hard template and connecting bridge, linking the product particles together to ultimately obtain the Na4Fe3(PO4)2P2O7@C material.

[0030] Based on the aforementioned embodiments, the dual-carbon heterostructure accounts for 1% to 10% of the mass fraction of the cathode material. This allows for maximizing the electronic conductivity of the material and optimizing the ion transport channels without significantly sacrificing the capacity of the active material. Furthermore, the mass fraction of the dual-carbon heterostructure in the cathode material can be, but is not limited to, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0031] In the cathode material of the present invention, the dual-carbon heterostructure synergistically modifies the mixed sodium iron phosphate matrix to form a three-dimensional porous interconnected conductive network. This effectively improves the electronic conductivity of the NFPP material, as well as its rate performance and cycle stability, while ensuring the sodium storage specific capacity. The sodium storage electrochemical performance is significantly better than that of pure phase Na4Fe3(PO4)2P2O7 material.

[0032] Please see Figure 1 The method for preparing the positive electrode material according to one embodiment of the present invention includes the following steps: S10. Sodium source, iron source, phosphorus source, conductive carbon material and organic carbon source precursor are mixed evenly according to stoichiometric ratio to obtain precursor material.

[0033] The sodium, iron, and phosphorus sources are conventional materials used in the preparation of NFPP materials in this field, and this invention does not limit the sodium, iron, and phosphorus sources. Further, the sodium source can be, for example, anhydrous sodium pyrophosphate, the iron source can be, for example, ferrous oxalate dihydrate, and the phosphorus source can be, for example, diammonium hydrogen phosphate.

[0034] In one embodiment, the molar ratio of sodium in the sodium source, iron in the iron source, and phosphorus in the phosphorus source is (4+2x):(3-x):4, ​​where the parameter x ranges from -1 to 2.

[0035] In one embodiment, the mass ratio of the conductive carbon material to the organic carbon source precursor is 1:3 to 1:5. Further, the mass ratio of the conductive carbon material to the organic carbon source precursor may be, but is not limited to, 1:3, 1:4, or 1:5.

[0036] In one embodiment, the process of uniformly mixing the sodium source, iron source, phosphorus source, conductive carbon material, and organic carbon source precursor in a stoichiometric ratio involves: mixing and grinding the sodium source, iron source, phosphorus source, conductive carbon material, and organic carbon source precursor in a stoichiometric ratio; then adding the ground material to a ball mill jar, adding ethanol and zirconium beads, and ball milling under a protective atmosphere. Further, the ball-milled material can be dried at low temperature in a vacuum oven.

[0037] Furthermore, the protective atmosphere can be any inert atmosphere such as argon or other atmospheres that do not participate in the reaction.

[0038] S20. The precursor material obtained in step S10 is subjected to a first sintering treatment at 300℃~400℃ under a protective atmosphere. After sufficient reaction, the pre-sintered material is obtained.

[0039] In step S20, pre-sintering can partially carbonize the organic matter in the precursor material and remove volatile components from the material.

[0040] The temperature of the first sintering treatment may be, but is not limited to, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃ or 400℃.

[0041] In one embodiment, the first sintering treatment takes 2 to 5 hours. Further, the first sintering treatment may take, but is not limited to, 2 hours, 3 hours, 4 hours, or 5 hours.

[0042] Furthermore, the protective atmosphere can be any inert atmosphere such as argon or other atmospheres that do not participate in the reaction.

[0043] S30. The pre-sintered material obtained in step S20 is subjected to a second sintering treatment at 500℃~800℃ under a protective atmosphere, and the cathode material is obtained after full reaction.

[0044] After a second sintering process, an electrode material with a special three-dimensional porous interconnected structure was prepared. This special morphology has interconnected pores inside. During the material synthesis process, the organic carbon source precursor forms a three-dimensional porous structure. The conductive carbon material can connect the materials and act as a hard film to effectively prevent the aggregation between material particles. This porous structure allows the prepared positive electrode material to fully contact the electrolyte when used as the positive electrode of the battery, increasing the active sites of electrochemical reactions, reducing the migration path required for ions during charging and discharging, and fully utilizing the sodium storage performance of the material.

[0045] The temperature of the second sintering treatment can be, but is not limited to, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃.

[0046] In one embodiment, the second sintering process takes 6 to 12 hours. Further, the second sintering process may take, but is not limited to, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours.

[0047] The method for preparing the cathode material of this invention involves synergistic modification of a mixed sodium iron phosphate matrix with a dual-carbon heterostructure, forming a three-dimensional porous interconnected conductive network. This effectively improves the electronic conductivity of the NFPP material, as well as its rate performance and cycle stability, while ensuring the sodium storage specific capacity. The sodium storage electrochemical performance is significantly better than that of pure phase Na4Fe3(PO4)2P2O7 material.

[0048] Furthermore, the method for preparing the cathode material according to the present invention is low in cost, generates no harmful waste or waste liquid during the preparation process, and has a simple production process, making it suitable for large-scale industrial production in actual production.

[0049] One embodiment of the sodium-ion battery positive electrode sheet includes any of the above-described positive electrode materials.

[0050] The sodium-ion battery cathode of the present invention includes the above-mentioned cathode material. By synergistically modifying the mixed sodium iron phosphate matrix through a dual-carbon heterostructure, a three-dimensional porous interconnected conductive network is formed. This can effectively improve the electronic conductivity of the NFPP material and improve its rate performance and cycle stability while ensuring the sodium storage specific capacity.

[0051] One embodiment of the sodium-ion battery includes the sodium-ion battery positive electrode sheet described above.

[0052] The sodium-ion battery of the present invention includes the above-mentioned sodium-ion battery positive electrode sheet, which includes the above-mentioned positive electrode material. By synergistically modifying the mixed sodium iron phosphate matrix through a dual-carbon heterostructure, a three-dimensional porous interconnected conductive network is formed. This can effectively improve the electronic conductivity of the NFPP material and improve its rate performance and cycle stability while ensuring the sodium storage specific capacity.

[0053] Referring to the above embodiments, in order to make the technical solution of the present invention more specific, clear and easy to understand, examples of the technical solution of the present invention are given below. However, it should be noted that the content to be protected by the present invention is not limited to the following embodiments.

[0054] Example 1 Weigh out 2.66 g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4 g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64 g of diammonium hydrogen phosphate (((NH4)2HPO4), 0.45 g of sucrose, and 0.1 g of acidified carbon nanofibers. The preparation steps for the acidified carbon nanofibers are as follows: Weigh 100 mg of commercially available carbon nanofibers, place them in a 100 ml hydrothermal reactor liner, add 60 ml of nitric acid solution, and hydrothermally heat in a 140°C forced-air drying oven for 6 h; after cooling, remove the reactants, centrifuge, and freeze-dry in an oven; finally, collect and label the material as CNFs.

[0055] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0056] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C / CNFs.

[0057] Example 2 Weigh out 2.66g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64g of diammonium hydrogen phosphate (((NH4)2HPO4), 0.46g of glucose, and 0.1g of acidified carbon fiber.

[0058] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0059] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C / CNFs-A.

[0060] Example 3 Weigh out 2.66g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64g of diammonium hydrogen phosphate (((NH4)2HPO4), 0.49g of citric acid, and 0.1g of acidified carbon fiber.

[0061] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0062] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C / CNFs-B.

[0063] Example 4 Weigh out 2.66 g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4 g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64 g of diammonium hydrogen phosphate (((NH4)2HPO4), 0.45 g of sucrose, and 0.1 g of commercial carbon fiber. Place the weighed materials in a mortar and grind them. After grinding, transfer the materials to a 50 ml zirconia ball mill jar, add 5 ml of ethanol and an appropriate amount of zirconia balls. Ball mill the materials under an argon atmosphere for 4 hours, then remove them and dry them in a vacuum drying oven for 12 hours (at 80℃) to obtain the precursor material.

[0064] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C / CNFs-C.

[0065] Example 5 Weigh out 2.66 g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4 g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64 g of diammonium hydrogen phosphate (((NH4)2HPO4), 0.45 g of sucrose, and 0.1 g of acetylene black. Place the weighed materials in a mortar and grind them. After grinding, transfer the materials to a 50 ml zirconia ball mill jar, add 5 ml of ethanol and an appropriate amount of zirconia balls. Ball mill the materials under an argon atmosphere for 4 hours, then remove them and dry them in a vacuum drying oven for 12 hours (at 80℃) to obtain the precursor material.

[0066] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C / CNFs-D.

[0067] Comparative Example 1 Weigh out 2.66g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4g of ferrous oxalate dihydrate (FeC2O4·2H2O), and 2.64g of diammonium hydrogen phosphate (((NH4)2HPO4), i.e., the molar ratio of Na:Fe:P is 4:3:4.

[0068] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0069] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed to obtain a grayish-green powder, sodium iron pyrophosphate, denoted as NFPP.

[0070] Comparative Example 2 Weigh out 2.66g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64g of diammonium hydrogen phosphate (((NH4)2HPO4) and 0.1g of acidified carbon fiber.

[0071] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0072] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP / CNFs.

[0073] Comparative Example 3 Weigh out 2.66g of anhydrous sodium pyrophosphate (Na4P2O7), 5.4g of ferrous oxalate dihydrate (FeC2O4·2H2O), 2.64g of diammonium hydrogen phosphate (((NH4)2HPO4) and 0.45g of sucrose.

[0074] The weighed material was placed in a mortar and ground. After grinding, it was added to a 50ml zirconia ball mill jar, along with 5ml of ethanol and an appropriate amount of zirconia balls. The material was ball-milled in an argon atmosphere for 4 hours, then removed and dried in a vacuum drying oven at 80℃ for 12 hours to obtain the precursor material.

[0075] The precursor material was placed in a tube furnace and calcined at 350°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 600°C and held for 6 hours. After cooling, the material was removed, yielding a black cathode material, denoted as NFPP@C.

[0076] Performance testing: (1) XRD: X-ray diffraction was performed on the materials of Example 1 and Comparative Example 1, and the results are as follows: Figure 2 As shown. By Figure 2 It can be seen that the substances prepared in Example 1 and Comparative Example 1 have the same NFPP diffraction peaks as those in the standard spectrum, indicating that the synthesized substances are NFPP@C / CNFs and NFPP.

[0077] (2) SEM: The NFPP of Comparative Example 1 and the NFPP@C / CNFs of Example 1 were characterized by scanning electron microscopy, and Figures 3(a) and 3(b) were obtained. Figure 3(a) shows the morphology of the NFPP material prepared in Comparative Example 1. As can be seen from Figure 3(a), the NFPP material prepared in Comparative Example 1 consists of irregular large-sized particles. Figure 3(b) shows the morphology of the NFPP@C / CNFs prepared in Example 1. As can be seen from Figure 3(b), the particle size of NFPP@C / CNFs with carbon material introduced is significantly reduced. This is because the sucrose is carbonized during high-temperature calcination, which reduces the particle size. The incorporation of acidified carbon fiber further reduces the agglomeration between particles. Figure 3(b) also shows that the introduction of dual carbon material forms a three-dimensional porous interconnected conductive network structure. It is preliminarily judged that this structure with highly conductive carbon fiber material as the backbone can effectively improve the overall conductivity of the material and reduce the volume expansion caused by sodium ion insertion and extraction during the electrochemical reaction, thereby improving the electrochemical performance of the material, especially the rate performance.

[0078] (3) The NFPP@C / CNFs of Example 1 were characterized by transmission electron microscopy and subjected to Fourier and inverse Fourier transforms to obtain... Figure 4 . Figure 4 Images a and b are TEM images of the material, clearly showing the doped acidified carbon fibers and carbon layers. The carbon nanofibers are uniformly incorporated into the material and connected in series with adjacent particles. The carbon layers (i.e., sucrose pyrolysis products) are uniformly distributed on the material surface, forming a coating layer. This coating layer prevents direct contact between the material and the electrolyte, suppressing side reactions during the electrochemical reaction. The material exhibits an overall honeycomb-like nanoporous structure, consistent with the SEM characterization images. Figure 4 Image c is a high-resolution TEM image of the material. The obvious lattice fringes in the image indicate that the material has a high degree of crystallinity. This can be further verified by Fourier and inverse Fourier transforms (e.g.,...). Figure 4 In sections d and f), we obtained clear alternating light and dark stripes. Measurements showed the stripe spacing to be 0.367 nm, corresponding to the (410) crystal plane of the material, consistent with our XRD-refined structure. These results demonstrate the successful preparation of the mixed sodium iron phosphate modified with a dual-carbon heterostructure.

[0079] (4) The substances prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests. The test methods are as follows, and the test results are shown in Table 1.

[0080] Powder resistivity: Using a powder resistivity tester and a four-terminal test method, a certain mass of material powder is subjected to a high voltage of 30 MPa, and the current flux in the test material is measured to calculate the powder resistivity. The lower the value, the better the conductivity of the material.

[0081] BET: The specific surface area of ​​the material was accurately determined using the nitrogen adsorption BET method. The sample was pretreated by vacuum degassing at 120°C for 4 hours. Nitrogen adsorption-desorption isotherms were collected using a Quantachrome Autosorb-iQ analyzer at a liquid nitrogen temperature of -196°C. Based on BET theory, the specific surface area of ​​the material was calculated within a linear range of relative pressure (P / P0) of 0.05–0.25.

[0082] First-cycle discharge specific capacity: The material under test was fabricated into a coin cell and subjected to cyclic voltammetry (CV) testing at 1.8-3.8V vs. Na / Na. + Within the potential window, the charge / discharge rate was 0.1C (1C = 129 mAh·g⁻¹), and the CV curve was measured at a rate of 0.1 mV / s.

[0083] 100-cycle retention rate: Same as the first-cycle discharge specific capacity test method, but in this test, the charge-discharge program is set to repeat 100 times.

[0084] High-rate first-cycle discharge specific capacity: Same as the first-cycle discharge specific capacity test method, but in this test, the charge / discharge rate is 5C.

[0085] Table 1 As can be seen from Table 1: (1) The powder resistivity of the materials prepared in Examples 1 to 5 of the present invention is low, while the specific surface area, first-cycle discharge specific capacity, cycle retention rate and high-rate first-cycle discharge specific capacity are high. This indicates that the modification of the mixed sodium iron phosphate matrix with a double carbon heterostructure in Examples 1 to 5 of the present invention can effectively improve the electronic conductivity of NFPP materials and improve their rate performance and cycle stability. That is, the double carbon heterostructure plays a significant and effective role in improving the cycle retention rate and high-rate performance of the materials.

[0086] (2) Compared with Comparative Example 1 (pure phase NFPP), the powder resistivity of Example 1 was reduced by more than 4 orders of magnitude, indicating that the modification of the mixed sodium iron phosphate matrix with a dual-carbon heterostructure in Example 1 of the present invention can effectively improve the electronic conductivity of NFPP material; the specific surface area of ​​Example 1 is higher than that of Comparative Example 1, indicating that the dual-carbon modification successfully constructed a richer pore structure, which is beneficial to electrolyte wetting and ion transport; the first-cycle discharge specific capacity, cycle retention rate and high-rate first-cycle discharge specific capacity of Example 1 are much higher than those of Comparative Example 1, indicating that the modification of the mixed sodium iron phosphate matrix with a dual-carbon heterostructure in Example 1 of the present invention can improve its rate performance and cycle stability.

[0087] (3) Comparing Example 1 with Comparative Example 2 (CNFs only) and Comparative Example 3 (sucrose carbon only), the resistivity of Comparative Example 2 (4088.1 Ω / cm) was much higher than that of Example 1, and its 5C capacity (40.5 mAh / g) was only half that of Example 1. This indicates that although it only had a conductive network, it lacked the coating and particle refinement of amorphous carbon, resulting in limited effectiveness. The resistivity of Comparative Example 3 (189,752.1 Ω / cm) was still very high, and its 5C capacity (12.2 mAh / g) was very poor. This indicates that although it only had coated carbon, it lacked the long-range conductive pathway constructed by CNFs, and thus could not achieve high-rate performance. This shows that conductive carbon (CNFs) and organic carbon source precursor (sucrose) pyrolytic carbon are both indispensable and have a significant synergistic effect. Only by working together can low resistance, high capacity, high rate performance, and long lifetime be achieved.

[0088] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0089] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A positive electrode material, characterized in that, The cathode material includes a mixed sodium iron phosphate matrix and a dual-carbon heterostructure penetrating the interior and exterior of the mixed sodium iron phosphate matrix. The general formula of the mixed sodium iron phosphate matrix is ​​Na4Fe3(PO4)2P2O7. The dual-carbon heterostructure includes a conductive carbon material and pyrolysis products of an organic carbon source precursor. The gas generated by the pyrolysis of the organic carbon source precursor foams the mixed sodium iron phosphate matrix to form a three-dimensional porous structure. The pyrolysis products of the organic carbon source precursor coat the surface of the mixed sodium iron phosphate matrix particles. The conductive carbon material penetrates the three-dimensional porous structure and connects the mixed sodium iron phosphate matrix particles coated by the pyrolysis products of the organic carbon source precursor, thereby forming a three-dimensional porous interconnected conductive network.

2. The cathode material according to claim 1, characterized in that, The conductive carbon material includes one or a mixture of multiple of the following: carbon nanotubes, surface-treated carbon nanotubes, Ketjen black, acetylene black, graphene, and activated carbon. The surface-treated carbon nanotubes include one or a mixture of more than one of acidified carbon nanotubes, surface-functionalized carbon nanotubes, and alkalized carbon nanotubes.

3. The cathode material according to claim 1, characterized in that, The organic carbon source precursor includes one or a mixture of more of glucose, sucrose, citric acid, ascorbic acid, and dopamine.

4. The cathode material according to claim 1, characterized in that, The dual-carbon heterostructure accounts for 1% to 10% of the mass fraction of the cathode material.

5. A method for preparing a cathode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Sodium source, iron source, phosphorus source, conductive carbon material and organic carbon source precursor are mixed uniformly according to stoichiometric ratio to obtain precursor material; The precursor material is subjected to a first sintering treatment at 300℃~400℃ under a protective atmosphere, and after sufficient reaction, the pre-sintered material is obtained. as well as The pre-sintered material is subjected to a second sintering treatment at 500℃~800℃ under a protective atmosphere, and the cathode material is obtained after sufficient reaction.

6. The method for preparing the cathode material according to claim 5, characterized in that, The sodium source is anhydrous sodium pyrophosphate, the iron source is ferrous oxalate dihydrate, and the phosphorus source is diammonium hydrogen phosphate. The molar ratio of sodium in the sodium source, iron in the iron source, and phosphorus in the phosphorus source is (4+2x):(3-x):4, ​​where the parameter x ranges from -1 to 2.

7. The method for preparing the cathode material according to claim 5, characterized in that, The mass ratio of the conductive carbon material to the organic carbon source precursor is 1:3 to 1:

5.

8. The method for preparing the cathode material according to claim 5, characterized in that, The first sintering process takes 2 to 5 hours, and the second sintering process takes 6 to 12 hours.

9. A sodium-ion battery positive electrode, characterized in that, The cathode material includes any one of claims 1 to 4.

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

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