Nmfpp@nvpo f composites, preparation and use thereof

Abstract

This invention belongs to the field of cathode materials, specifically relating to an NMFPP@NVPOF composite material and its preparation and application. The preparation method involves: precipitating VOPO4, a sodium source, and a fluorine source on the surface of an NMFPP active material to coat the NMFPP surface with NVPOF, followed by thermal modification treatment to obtain the NMFPP@NVPOF composite material; the chemical formula of the NMFPP is Na. y Mn x Fe 3‑x P2O7(PO4)2, 0≤x<2.5, 3.8≤y<4.1; the expression for NVPOF is Na3V2(PO4)2O 3‑z F z ,1≤z≤3. Unlike conventional hydrothermal processes, this invention innovatively uses VOPO4, sodium source, and F source to undergo a non-solventothermal low-temperature precipitation reaction induced on the surface of NMFPP. This can form a uniform precipitation interface, and further combined with subsequent thermal modification treatment, the fusion interface can be reconstructed, improving the gradient outward permeation effect of the components. This can significantly improve the air stability and fast-charging performance of the prepared material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to the field of cathode materials for sodium-ion batteries. Technical Background

[0002] With the development of science and technology and the improvement of people's living standards, the demand for energy is growing rapidly, and environmental problems are becoming increasingly prominent, especially the consumption of fossil fuels and greenhouse gas emissions. Against this backdrop, the research and development of environmentally friendly clean energy and energy storage systems such as batteries are particularly important. In recent years, the success of lithium-ion batteries has led to a significant increase in lithium resource consumption. However, the fluctuating price of lithium resources, uneven resource distribution, and poor low-temperature adaptability are detrimental to their further development. Against this backdrop, sodium-ion batteries (SIBs), due to their low cost, abundant resources, and superior low-temperature performance, have gradually become a research hotspot and a potential candidate for next-generation battery technology.

[0003] In recent years, researchers have studied many sodium-ion battery cathode materials, such as layered oxides, polyanionic cathodes, and Prussian blue cathodes. Among them, polyanionic cathode materials are particularly suitable for energy storage due to their stable three-dimensional structure, wide voltage plateau, and high thermal stability. The superior structural stability and rate performance of polyanionic cathode materials have attracted considerable attention from researchers, with sodium pyrophosphate (manganese iron phosphate) cathode material being a prime example. y Mn x Fe 3-x P2O7(PO4)2 / C, 0≤x<2.5, 3.8≤y<4.1 (NMFPP) not only possesses the advantages of polyanionic materials, but its low cost also makes it a very promising cathode material for sodium-ion batteries.

[0004] However, NMFPP suffers from weak air stability, low electrical conductivity, and low energy density at high rates (see Li X, Dai S, Chen Q, et al. Understanding Air Sensitivity and Deterioration Mechanism of Na4Fe3(PO4)2P2O7 Cathode for Na-Ion Battery[J]. Journal of Materials Chemistry A, 2024, 12(43): 29726-29734. and Yang Y, Wang Z, Du C, et al. Decoupling the air sensitivity of Na-layered oxides[J]. Science, 2024, 385(6710): 744-752.), which limits its further development.

[0005] Furthermore, Chinese patent document CN 114068938A discloses a method for preparing a sodium vanadium fluorophosphate-coated sodium iron phosphate pyrophosphate composite material. This method involves obtaining a sodium iron phosphate pyrophosphate precursor through a first-stage hydrothermal reaction, followed by a second-stage hydrothermal encapsulation with a second sodium source, a vanadium source, a second phosphorus source, a fluorine source, and a second complexing agent, and then heat treatment. This process requires hydrothermal treatment, making it difficult to reconstruct new fusion interfaces between the bulk phase and the interfaces of the prepared material. The material's air resistance and high-rate long-cycle performance require further improvement. Summary of the Invention

[0006] To address the challenges of constructing uniform surfaces and fusion interfaces in NMFPP@NVPOF composite materials, as well as the unsatisfactory air stability and high-rate performance of the materials, the primary objective of this invention is to provide a method for preparing NMFPP@NVPOF composite materials. This method aims to prepare composite materials with suitable reconstructed interfaces (coherent interfaces) while also ensuring excellent air stability and fast-charging performance.

[0007] The second objective of this invention is to provide the NMFPP@NVPOF composite material prepared by the aforementioned method and its application in sodium-ion batteries.

[0008] A third objective of this invention is to provide a sodium-ion battery comprising the NMFPP@NVPOF composite material, and its positive electrode and positive electrode material.

[0009] A method for preparing an NMFPP@NVPOF composite material involves precipitating VOPO4, sodium source, and fluorine source on the surface of an NMFPP active material to coat the NMFPP surface with NVPOF, followed by thermal modification treatment to obtain the NMFPP@NVPOF composite material.

[0010] The chemical formula of the NMFPP is Na. y Mn x Fe 3-x P2O7(PO4)2,0≤x<2.5,3.8≤y<4.1;

[0011] The expression for NVPOF is Na3V2(PO4)2O 3-z F z ,1≤z≤3.

[0012] Unlike conventional hydrothermal encapsulation processes, this invention innovatively provides a non-hydrothermal low-temperature precipitation process for forming NVPOF. It innovatively uses NMFPP active material as a substrate, leveraging its surface residual alkali and sodium properties and the good compatibility with NVPOF to induce surface precipitation of VOPO4, sodium sources, and fluorine sources. Further combined with subsequent thermal modification treatment, this triggers interfacial reconstruction and induces gradient exudation of metals and non-metals. Research in this invention shows that the described preparation method can significantly improve the air stability of the material and also enhance its fast-charging performance.

[0013] The research of this invention shows that by directly using VOPO4 as the source of phosphorus and vanadium, and combining it with the deposition-inducing mechanism of the physicochemical characteristics of the NMFPP active material surface described in this invention, it is possible to unexpectedly induce uniform deposition of NVPOF on the NMFPP surface. In addition, it is beneficial to the subsequent reconstruction of the fusion interface, as well as the air stability and fast charging performance of the material.

[0014] In this invention, the VOPO4 can be selected from commercial products or prepared using conventional methods. For example, it can be obtained by pre-reaction of a phosphorus source, a vanadium source, and a reducing agent.

[0015] In this invention, the sodium source is at least one selected from sodium fluoride, sodium carbonate, sodium bicarbonate, sodium hydroxide, and sodium citrate.

[0016] The fluorine source is at least one of sodium fluoride, ammonium fluoride, and polyvinylidene fluoride.

[0017] This invention demonstrates that directly using NMFPP as a deposition substrate unexpectedly induces NVPOF surface precipitation due to its surface physicochemical properties, thereby facilitating the construction of a uniform, stable composite material that promotes the formation of a reconstructed interface. In this invention, the NMFPP can be a commercially available active material or can be prepared using existing methods.

[0018] In this invention, NVPOF is 1 to 10% of the weight of NMFPP; more specifically, it can be 3 to 8%, and even more specifically, it can be 4 to 6%.

[0019] In this invention, the solvent for the precipitation reaction is at least one selected from water, ethanol, isopropanol, and ethylene glycol; preferably, it is a mixed solvent containing water; for example, it can be a water-alcohol mixed solvent, wherein the alcohol includes at least one selected from ethanol, isopropanol, and ethylene glycol. In the mixed solvent, the volume ratio of water to alcohol is, for example, 1 to 5:1, and more preferably 2 to 3:1. Studies have shown that carrying out the precipitation reaction under these preferred conditions helps to further optimize the homogenization and coating of the surface layer based on the core-induced effect, and further facilitates the preparation of composite materials with high stability of the coherent interface, excellent fast charging, and air resistance.

[0020] In this invention, under the aforementioned non-hydrothermal low-temperature precipitation process, further thermal modification treatment is carried out, which can reconstruct the dissolution interface and help improve the air stability and fast charging performance of the material.

[0021] In this invention, the heat treatment atmosphere is a protective atmosphere.

[0022] In this invention, the temperature of the thermal modification treatment is 300-700℃, and can be further 450-550℃;

[0023] Preferably, the heat modification treatment time is 1 to 12 hours, and more preferably 3 to 5 hours.

[0024] The present invention also provides an NMFPP@NVPOF composite material prepared by the above preparation method, which has a core-shell structure, wherein the core includes NMFPP, the shell is NVPOF, and there is a reconstructed fusion interface between the core and the shell.

[0025] The present invention also provides an application of the aforementioned NMFPP@NVPOF composite material, using it as a positive electrode active material for the preparation of sodium-ion batteries.

[0026] The present invention also provides a sodium-ion battery cathode, comprising a current collector and a cathode material composite thereon, wherein the cathode material comprises a cathode active material, and the cathode active material comprises the NMFPP@NVPOF composite material obtained by the present invention.

[0027] The present invention also provides a sodium-ion battery positive electrode, comprising the positive electrode described herein.

[0028] In this invention, the NMFPP@NVPOF composite material described herein can be used as the positive electrode active material based on known raw materials and methods to prepare the desired sodium-ion battery. The sodium-ion battery, apart from containing the NMFPP@NVPOF composite material described herein, can have conventional components, parts, and structural relationships.

[0029] Beneficial effects:

[0030] Unlike conventional hydrothermal encapsulation methods, this invention innovatively utilizes NMFPP active material as a substrate and leverages its surface physicochemical characteristics and the compatibility between these characteristics and NVOPF. It innovatively induces a non-solventothermal low-temperature precipitation reaction of VOPO4, sodium source, and F source on the surface of NMFPP, thus forming a uniform precipitation interface. Further combined with subsequent thermal modification treatment, the fusion interface can be reconstructed, improving the gradient outward permeation effect of the components. This significantly improves the air stability and fast-charging performance of the prepared material.

[0031] The present invention also shows that the conditions of the processing, especially the solvent in the precipitation stage, help to further optimize the homogenization and coating effect of the surface layer, and facilitate the synergistic construction of coherent interfaces, thereby optimizing the material's fast charging and air resistance stability. Attached Figure Description

[0032] Figure 1 The image shows the XRD pattern of the 95% Na4Fe3(PO4)2P2O7 / 5%NVPOF / C-24h cathode material prepared in Example 1.

[0033] Figure 2 The image shows a comparison of FT-IR values ​​for the 95% Na4Fe3(PO4)2P2O7 / 5% NVPOF / C-24h and Na4Fe3(PO4)2P2O7 / C cathode materials prepared in Example 1.

[0034] Figure 3 This is a comparison chart of the electrochemical performance of the 95% Na4Fe3(PO4)2P2O7 / 5% NVPOF / C-24h cathode material and the Na4Fe3(PO4)2P2O7 / C cathode material prepared in Example 1.

[0035] Figure 4 The image shows the SEM-EDS image of the 95% Na4Fe3(PO4)2P2O7 / 5% NVPOF / C-24h cathode material prepared in Example 1.

[0036] Figure 5 The image shows a comparison of SEM images of the 95% Na4Fe3(PO4)2P2O7 / 5% NVPOF / C-7day and NFPP / C-7day cathode materials prepared in Example 1.

[0037] Figure 6 The image shows the XRD pattern of the 92% Na4Mn2Fe(PO4)2P2O7 / 8% NVPOF / C cathode material prepared in Example 2.

[0038] Figure 7The graph shows a comparison of the electrochemical performance of the 92% Na4Mn2Fe(PO4)2P2O7 / 8% NVPOF / C cathode material prepared in Example 2 and the Na4Mn2Fe(PO4)2P2O7 / C cathode material in Comparative Example 2.

[0039] Figure 8 The image shows a SEM image of the 92% Na4Mn2Fe(PO4)2P2O7 / 8% NVPOF / C cathode material prepared in Example 2.

[0040] Figure 9 The image shows the electrochemical performance of the 97% Na4Fe3(PO4)2P2O7 / 3% NVPOF / C cathode material prepared in Example 3.

[0041] Figure 10 The image shows the XRD pattern of the Na4Fe3(PO4)2P2O7C-24h cathode material prepared in Comparative Example 1.

[0042] Figure 11 The image shows a comparison of the FT-IR spectra of the Na4Fe3(PO4)2P2O7C-24h cathode material and the Na4Fe3(PO4)2P2O7 / C cathode material prepared in Comparative Example 1.

[0043] Figure 12 The image shows the electrochemical performance of the Na4Fe3(PO4)2P2O7C-24h cathode material prepared in Comparative Example 1.

[0044] Figure 13 The image shows the SEM image of the Na4Fe3(PO4)2P2O7C-24h cathode material prepared in Comparative Example 1.

[0045] Figure 14 The image shows the SEM image of the Na4Mn2Fe(PO4)2P2O7 / C cathode material prepared in Comparative Example 2.

[0046] Figure 15 The graph shows the electrochemical performance of the 95% NFPP / C + 5% NVPOF mixture prepared in Comparative Example 3.

[0047] Figure 16 The image shows the SEM image of the 95% NFPP / C + 5% NVPOF mixture prepared in Comparative Example 3.

[0048] Figure 17 The graph shows the electrochemical performance of the 92% NMFPP / C + 8% NVPOF mixture prepared in Comparative Example 4.

[0049] Figure 18 The graph shows the electrochemical performance of the cathode material prepared in Comparative Example 5.

[0050] Table 1 compares the electrochemical performance of the cathode material prepared in Comparative Example 6 with that prepared in Examples 1 and 4. Detailed Implementation

[0051] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0052] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0053] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available products or products that can be prepared by known methods.

[0054] This invention utilizes a method of low-temperature liquid-phase precipitation of sodium vanadium fluorophosphate to coat sodium pyrophosphate (manganese iron phosphate) cathode material. A structurally stable fast-ion conductor is uniformly coated on the surface of NMFPP cathode material. By initiating surface reconstruction, a homogeneous heterogeneous composite material with NMFPP / C as the core and NVPOF as the shell is obtained. This method effectively improves the poor air stability of NMFPP cathode material without reducing energy density and greatly enhances the electrochemical performance of the material, giving NMFPP cathode material a broader prospect.

[0055] The steps of this method are as follows: A vanadium source and a phosphorus source are pre-reacted with a reducing agent at a certain temperature for a certain time to obtain a blue slurry containing vanadium and phosphorus; a certain amount of Na is added to the solution of the vanadium-phosphorus slurry. y Mn x Fe 3-x P2O7(PO4)2 / C, 0≤x<2.5, 3.8≤y<4.1 (NMFPP / C) cathode material was used. Sodium and fluorine sources were added, and the temperature was adjusted and the mixture was stirred for a period of time. The pH was then adjusted so that the vanadium-phosphorus slurry reacted with Na3V2(PO4)2O 3-z F z 1≤z≤3 (NVPOF) precipitate uniformly coats the surface of NMFPP / C material; after filtration, washing and drying in a vacuum oven, it is heat-treated in an inert atmosphere to induce surface reconstruction, and a coherent interface is formed between NMFPP and NVPOF particles, resulting in a homogeneous heterogeneous composite material with NMFPP / C as the core and NVPOF as the shell, and a structurally stable fast ion conductor is uniformly coated on the surface of NMFPP material.

[0056] As a preferred embodiment, the solvent of the solution is at least one of water, ethanol, isopropanol, and ethylene glycol, which can ensure good dispersion of the NMFPP / C cathode material;

[0057] As a preferred embodiment, the phosphorus source is at least one selected from phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In this invention, the phosphorus source can provide a large amount of phosphate ions to combine with vanadium ions from the vanadium source.

[0058] As a preferred embodiment, the vanadium source is at least one selected from vanadium pentoxide, vanadium dioxide, ammonium metavanadate, and sodium vanadate.

[0059] As a preferred embodiment, the reducing agent is at least one selected from ascorbic acid, oxalic acid, formic acid, and lactic acid.

[0060] As a preferred embodiment, the sodium source is at least one selected from sodium fluoride, sodium carbonate, sodium bicarbonate, sodium hydroxide, and sodium citrate.

[0061] As a preferred embodiment, the fluorine source is at least one of sodium fluoride, ammonium fluoride, and polyvinylidene fluoride.

[0062] As a preferred embodiment, the reduction temperature is 40-90℃ and the reduction time is 0.1-2h.

[0063] The coating reaction temperature is 60-90℃, and the pH is adjusted to 6-9.

[0064] As a preferred embodiment, the molar ratio of sodium, vanadium, phosphorus and fluorine added is (2-5):(1.5-3):(2-4):(1-5), and more preferably (2-4):(2-2.5):(2.5-3.5):(2-4).

[0065] As a preferred embodiment, the molar ratio of vanadium source to reducing agent is (1:4) to (1:4).

[0066] 1), further preferably (1:3) to (1:1.5).

[0067] As a preferred embodiment, the mass ratio of the theoretically generated NVPOF to the added NMFPP / C is (1:99) to (1:3), and more preferably (1:19) to (3:17).

[0068] As a preferred embodiment, the inert atmosphere is at least one of argon, nitrogen, and argon-hydrogen.

[0069] As a preferred embodiment, the heat treatment temperature is 300–700℃, the heating rate is 1–10℃ / min, and the holding time is 1–12h; more preferably, the temperature is 450℃–650℃, the heating rate is 2–5℃, and the holding time is 4–8h. Excessively high sintering temperatures can easily lead to the formation of impurity phases, while excessively low sintering temperatures result in low interfacial interaction between the NVPOF and the matrix material.

[0070] This invention provides a more specific method for modifying sodium vanadium fluorophosphate cathode materials by low-temperature liquid-phase precipitation coating with sodium iron pyrophosphate (manganese phosphate), which specifically includes the following steps:

[0071] 1) Add phosphorus source, vanadium source and reducing agent to a certain amount of deionized water and carry out reduction reaction at a certain temperature for 0.1-2 hours. Maintain the molar ratio of vanadium in the added vanadium source to phosphorus in the phosphorus source in the solution as (1:3) to (1:1), and the molar ratio of vanadium in the added vanadium source to reducing agent as (1:4) to (1:1).

[0072] 2) Add a certain amount of NMFPP / C cathode material, sodium source, and fluorine source to the solution after the reduction reaction in step 1), and adjust the temperature and pH to obtain a composite material with NMFPP / C uniformly coated by NVPOF. Maintain the molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, and fluorine in the fluorine source at (2-5):(1.5-3):(2-4):(1-5), the temperature at 60-90℃, and the pH at 6-9.

[0073] 3) The Na3V2(PO4)2O synthesized in the liquid phase obtained in step 2) 3-z F z ,1≤z≤3(NVPOF) covering Na y Mn x Fe 3- x The P2O7(PO4)2 / C,0≤x<2.5,3.8≤y<4.1 (NMFPP) material was filtered, washed, and then dried in a vacuum oven to obtain the powder to be sintered.

[0074] 4) The powder from step 3) is subjected to heat treatment to obtain the NMFPP / NVPOF / C composite cathode material. The sintering temperature is 300-700℃, and the holding time is 1-12h.

[0075] Example 1

[0076] 1) Dilute 0.065 mol of 85% phosphoric acid with deionized water in a flask, and then add 0.026 mol of vanadium pentoxide and 0.078 mol of oxalic acid dihydrate while stirring. The molar ratio of phosphorus to vanadium in the slurry is 1.25:1, and the molar ratio of vanadium to oxalic acid dihydrate is 2:3. At the same time, heat the slurry to 70℃ and stir for 1 hour. Finally, the slurry turns dark blue.

[0077] 2) Add 204.516g of Na4Fe3(PO4)2P2O7 / C (carbon content between 1.5% and 3%) (NFPP / C) to the vanadium-phosphorus slurry obtained in step 1) so that the mass ratio of NVPOF to NFPP / C is 5%:95%. After stirring for a period of time, add 0.104mol of NaF, heat to 90℃ and stir for 20min. Then adjust the pH to 6 by adding 0.5mol / L NaOH and maintain the temperature while stirring for 20min to obtain a composite material in which NVPOF is uniformly coated on the surface of NFPP / C.

[0078] 3) The slurry in step 2) is filtered and washed three times. The precipitate after washing is placed in an 80°C vacuum oven and dried to obtain the composite material powder to be sintered.

[0079] 4) The powder from step 3) is sintered under an argon atmosphere and held at 480℃ for 4 hours. The heating rate is 3℃ / min. After sintering, the powder is cooled to room temperature to obtain a 95% NFPP / 5% NVPOF / C cathode material.

[0080] 5) Expose the powder from step 4) to air with a humidity of 30% for 24 hours to obtain...

[0081] 95% NFPP / 5% NVPOF / C-24h cathode material.

[0082] like Figure 1 XRD pattern of 95% NFPP / 5% NVPOF / C-24h.

[0083] like Figure 2 The FT-IR spectra of 95% NFPP / 5% NVPOF / C and 95% NFPP / 5% NVPOF / C-24h were obtained, but the results did not show up at 1480 cm⁻¹. -1 CO3 detected nearby 2- No Na2CO3 was generated on the material surface, and it exhibited better air stability compared to the FT-IR of Comparative Example 2.

[0084] The 95% NFPP / 5% NVPOF / C and 95% NFPP / 5% NVPOF / C-24h positive electrode materials prepared by the method in Example 1 were mixed uniformly with acetylene black and PVDF binder at a mass ratio of 7:2:1 using NMP as a solvent in a homogenizer to obtain a uniformly mixed slurry. The obtained slurry was coated onto aluminum foil and dried in a vacuum oven at 120°C for 12 hours. It was then punched into a disc-shaped electrode with a diameter of 12 mm and assembled into a CR2025 coin cell in a pure argon glove box. At 25°C, with charge / discharge cutoff voltages set to 1.7–4.3V, 0.1C constant current charge / discharge tests, 1C long-cycle tests, and tests at different charge / discharge rates were performed. Figure 3 As shown, the prepared 95%NFPP / 5%NVPOF / C has high capacity, with an NVPOF charge-discharge platform around 3.45V providing a certain capacity. It also exhibits good rate performance and cycle stability. Furthermore, 95%NFPP / 5%NVPOF / C-24h demonstrates good air stability while maintaining electrochemical performance. In addition, the performance of 95%NFPP / 5%NVPOF / C and 95%NFPP / 5%NVPOF / C-24h is greatly improved at different rates, especially at high rates. This is all due to the formation of a coherent interface, which enhances the electrochemical activity of the material.

[0085] like Figure 4 The image shows the SEM-EDS image of 95% NFPP / 5% NVPOF / C-24h. The image reveals a grooved surface, with no sodium carbonate or similar film coating on the secondary spheres. V and F elements are uniformly distributed on the surface of the secondary spheres, and the material surface exhibits significant roughness, indicating surface reconstruction during the liquid-phase coating process. This is consistent with the SEM image of Comparative Example 1. Figure 13 SEM of Comparative Example 3 Figure 16 To create a contrast.

[0086] like Figure 5 The images show SEM images of 95% NFPP / 5% NVPOF / C-7day and NFPP / C-7day. The comparison between 95% NFPP / 5% NVPOF / C-7day and NFPP / C-7day shows that the former still maintains the same morphology as the material exposed to air, while the latter has a large number of needle-like substances due to side reactions with air. Combined with the comparison of electrochemical performance in Comparative Example 1, the air stability of the material is greatly improved.

[0087] Example 2

[0088] 1) Dilute 0.065 mol of 85% phosphoric acid with ethanol in a flask, and then add 0.026 mol of vanadium pentoxide and 0.078 mol of oxalic acid dihydrate while stirring. The molar ratio of phosphorus to vanadium in the slurry is 1.25:1, and the molar ratio of vanadium to oxalic acid dihydrate is 2:3. At the same time, heat the slurry to 70℃ and stir for 1 hour. Finally, the slurry turns dark blue.

[0089] 2) Add 123.786g of Na4Mn2Fe(PO4)2P2O7 / C (NM2FPP / C) to the vanadium-phosphorus slurry obtained in step 1) so that the mass ratio of NVPOF:NMFPP / C to be generated is 8%:92%. After stirring for a period of time, add 0.104mol of NaF and stir at 90℃ for 20min. Then adjust the pH to 6.5 by adding 0.5mol / L NaOH and stir at the same temperature for 20min to obtain a composite material in which NVPOF is uniformly coated on the surface of NMFPP / C.

[0090] 3) The slurry in step 2) is filtered and washed three times. The precipitate after washing is placed in a vacuum oven at 70°C and dried to obtain the composite material powder to be sintered.

[0091] 4) The powder from step 3) was pre-calcined in an argon atmosphere and held at 500℃ for 4 hours with a heating rate of 3℃ / min. After sintering, it was cooled to room temperature to obtain a 92% NM2FPP / 8% NVPOF / C cathode material.

[0092] like Figure 6 The XRD pattern is for 92% NM2FPP / 8% NVPOF / C.

[0093] The 92% NM2FPP / 8% NVPOF / C cathode material prepared by the method in Example 2, acetylene black, and PVDF binder were mixed uniformly at a mass ratio of 7:2:1. NMP was used as a solvent, and the mixture was homogenized in a homogenizer to obtain a uniform slurry. The obtained slurry was coated onto aluminum foil and dried in a vacuum oven at 120°C for 12 hours. It was then punched into a disc-shaped electrode with a diameter of 12 mm. This electrode was then assembled into a CR2025 coin cell in a pure argon glove box. At 25°C, with charge / discharge cutoff voltages set to 1.7–4.3V, 0.1C constant current charge / discharge tests, 1C long-cycle tests, and tests at different charge / discharge rates were performed. Figure 7 As shown, it can be clearly seen that the 92% NM2FPP / 8% NVPOF / C cathode has a charge-discharge plateau at around 3.4V, and its electrochemical performance is significantly improved in terms of capacity, cycle performance and rate performance compared to the unmodified NM2FPP / C.

[0094] like Figure 8The image shows a SEM image of 92% NM2FPP / 8% NVPOF / C. The image is compared with... Figure 14 Compared with the SEM image of Comparative Example 2, it can be seen that the surface of the material after liquid phase coating has grooves, i.e. pores, and is not as smooth as that of Comparative Example 2. This indicates that there is a surface reconstruction phenomenon during the liquid phase coating process, which is in line with the expected results.

[0095] Example 3

[0096] 1) Dilute 0.065 mol of 85% phosphoric acid with a 1:1 mixture of deionized water and anhydrous ethanol in a flask, and then add 0.026 mol of vanadium pentoxide and 0.052 mol of oxalic acid dihydrate while stirring. The molar ratio of phosphorus to vanadium in the slurry is 1.25:1, and the molar ratio of vanadium to oxalic acid dihydrate is 1:1. At the same time, heat the slurry to 70°C and stir for 1 hour. Finally, the slurry turns dark blue.

[0097] 2) Add 348.04g of Na4Fe3(PO4)2P2O7C (carbon content between 1.5% and 3%) to the vanadium-phosphorus slurry obtained in step 1) so that the mass ratio of NVPOF to NFPP / C is 3%:97% according to theory. After stirring for a period of time, add 0.078mol of NaF and stir at 70℃ for 20 minutes to obtain a composite material in which NVPOF is uniformly coated on the surface of NFPP / C.

[0098] 3) The slurry in step 2) is filtered and washed three times. The washed precipitate is placed in an 80°C vacuum oven and dried to obtain the composite material powder to be sintered.

[0099] 4) The powder from step 3) is sintered under an argon atmosphere and held at 500℃ for 4 hours. The heating rate is 3℃ / min. After sintering, the powder is cooled to room temperature to obtain a 97% NFPP / 3% NVPOF / C cathode material.

[0100] The 97% NFPP / 3% NVPOF / C cathode material, prepared by the method in Example 3, acetylene black, and PVDF binder were mixed uniformly at a mass ratio of 7:2:1. Using NMP as a solvent, the mixture was homogenized in a homogenizer to obtain a uniform slurry. The obtained slurry was coated onto aluminum foil and dried in a vacuum oven at 120°C for 12 hours. It was then punched into a disc-shaped electrode with a diameter of 12 mm and assembled into a CR2025 coin cell in a pure argon glove box. At 25°C, with charge / discharge cutoff voltages set to 1.7–4.3V, 0.1C constant current charge / discharge tests, 1C long-cycle tests, and tests at different charge / discharge rates were performed. Figure 9As shown, it can be clearly seen that the overall curve of the 97% NFPP / 3% NVPOF / C cathode is almost identical to that of the example, exhibiting good rate performance, capacity, and the best cycle performance. This is because the amount of NVPOF coating is moderate, and NFPP / C can be better dispersed in anhydrous ethanol than in water, which is more conducive to uniform NVPOF coating, thereby facilitating interface reconstruction, effectively activating sodium intercalation sites, and the appropriate amount of coating promotes cycle stability, making it the preferred solution.

[0101] Example 4

[0102] Compared to Example 1, the only difference is that ethanol is added to the precipitation reaction system in step 2, wherein the volume ratio of water to ethanol in the system is 2:1; the total amount of solvent and other operations and parameters are the same as in Example 1.

[0103] Comparative Example 1

[0104] The only difference between this comparative example and Example 1 is that Na4Fe3(PO4)2P2O7 / C (NFPP / C) was not modified with NVPOF coating, and NFPP / C was exposed to air with a humidity of 30% for 24 hours to obtain NFPP / C-24h.

[0105] The XRD peak intensity of NFPP / C-24h was significantly reduced, and the (004) and (222) peaks split. This is because the poor air stability of NFPP / C leads to the exposure of Na3 sites in the material. + Changes caused by extraction from the crystal lattice ( Figure 10 The FT-IR results for NFPP / C-24h are as follows: Figure 11 As shown, at 1480cm -1 CO3 detected nearby 2- The characteristic peaks indicate the formation of Na2CO3 on the material surface. The electrochemical performance of the material is therefore significantly affected; the first-cycle efficiency at 0.1C is 107.58%, and the charging plateau near 2.7V, where Na3 sites undergo Na+ ion removal, disappears, resulting in a significant decrease in both cycle performance and discharge specific capacity (see...). Figure 12 ).

[0106] The SEM image of the NFPP / C-24h cathode material prepared in Comparative Example 1 is as follows: Figure 13 As shown in the figure, the surface of the secondary spherical particles is very smooth, which is due to the presence of a Na2CO3 film on the surface.

[0107] Comparative Example 2

[0108] The only difference between this comparative example and Example 2 is that NVPOF was not used for coating, and Na4Mn2Fe(PO4)2P2O7 / C (NM2FPP / C) was prepared.

[0109] The electrochemical data of the Na4Mn2Fe(PO4)2P2O7 / C (NM2FPP / C) cathode material prepared in Comparative Example 2 are compared with those prepared in Example 2. Figure 6 As shown, the SEM image is as follows: Figure 14 As shown.

[0110] The results show that the lack of NVPOF coating treatment for NM2FPP / C leads to a decrease in the electrochemical performance of the cathode material.

[0111] Comparative Example 3

[0112] The only difference between this comparative example and Example 1 is that Na4Fe3(PO4)2P2O7 / C (NFPP / C) was not added to the pre-reacted vanadium oxide slurry for NVPOF coating modification. Instead, Na4Fe3(PO4)2P2O7 / C (NFPP / C) and NVPOF / C were physically mixed and sintered under the same conditions to obtain a 95% NFPP / C + 5% NVPOF mixture.

[0113] The electrochemical data of the 95% NFPP / C + 5% NVPOF mixture prepared in Comparative Example 3 are compared with the electrochemical data of the bare NFPP / C material. Figure 15 As shown; SEM image of 95% NFPP / C + 5% NVPOF mixture is shown below. Figure 16 As shown.

[0114] It can be seen that there is a clear charge-discharge plateau of NVPOF / C at around 3.4V, indicating that simple mixing and sintering of NFPP / C and NVPOF / C cannot achieve the excellent effect of different components in the same system.

[0115] Comparative Example 4

[0116] The only difference between this comparative example and Example 2 is that Na4Mn2Fe(PO4)2P2O7 / C (NM2FPP / C) was not added to the pre-reacted vanadium oxide slurry for NVPOF coating modification. Instead, NVPOF / C was first prepared using the process conditions described above. Then, Na4Mn2Fe(PO4)2P2O7 / C (NM2FPP / C) and NVPOF / C were physically mixed and sintered under the same conditions to obtain a 92% NM2FPP / 8% NVPOF / C mixture.

[0117] The electrochemical data of the 92% NM2FPP / 8% NVPOF / C mixture prepared in Comparative Example 4 are compared with the electrochemical data of the bare NFPP / C material. Figure 17 As shown.

[0118] It can be seen that there is a clear charge-discharge plateau of NVPOF / C at around 3.4V, indicating that simple mixing and sintering of NM2FPP / C and NVPOF / C cannot achieve the excellent effect of different components in the same system.

[0119] Comparative Example 5

[0120] Compared with Example 2, the only difference is that the treatment in step 1 was not performed. Instead, the phosphorus source and vanadium source from step 1 were directly added to the precipitation system in step 2 for co-precipitation. All other operations and parameters were the same as in Example 1.

[0121] Its electrochemical curve is as follows Figure 18 As shown, the significant difference compared to Example 2 is that there is no obvious electrochemical plateau at around 2.5V. This is due to uneven coating and poor surface reconstruction effect after subsequent heat treatment.

[0122] Comparative Example 6

[0123] Compared with Example 1, the only difference is that in step 2, no co-precipitation treatment is performed. Instead, the mixed system is subjected to hydrothermal reaction at a temperature of 130-140°C to obtain the composite material. All other operations and parameters are the same as in Example 1.

[0124] The electrochemical performance of each case is shown in Table 1.

[0125] Table 1

[0126]

[0127] As demonstrated in Example 1, the low-temperature precipitation process described in this invention yields superior electrochemical performance and air stability. In particular, it provides excellent fast-charging performance. Furthermore, research indicates that performing the low-temperature precipitation treatment in a water-alcohol system further optimizes the fast-charging and air stability of the prepared materials.

Claims

1. A method for preparing an NMFPP@NVPOF composite material, characterized in that, VOPO4, sodium source and fluorine source were precipitated on the surface of NMFPP active material to coat NVPOF on the surface of NMFPP, and then thermally modified to obtain the NMFPP@NVPOF composite material. The chemical formula of the NMFPP is Na. y Mn x Fe 3-x P2O7(PO4)2, 0≤x<2.5,3.8≤y<4.1; The expression for NVPOF is Na3V2(PO4)2O 3-z F z ,1≤z≤3.

2. The preparation method of the NMFPP@NVPOF composite material as described in claim 1, characterized in that, The VOPO4 is obtained through a pre-reaction of a phosphorus source, a vanadium source, and a reducing agent.

3. The preparation method of the NMFPP@NVPOF composite material as described in claim 1, characterized in that, The sodium source is at least one of sodium fluoride, sodium carbonate, sodium bicarbonate, sodium hydroxide, and sodium citrate; The fluorine source is at least one of sodium fluoride, ammonium fluoride, and polyvinylidene fluoride.

4. The method for preparing the NMFPP@NVPOF composite material as described in claim 1, characterized in that, The NVPOF is 1 to 10% of the weight of NMFPP.

5. The method for preparing the NMFPP@NVPOF composite material as described in claim 1, characterized in that, The solvent for the precipitation reaction is at least one of water, ethanol, isopropanol, and ethylene glycol.

6. The method for preparing the NMFPP@NVPOF composite material as described in claim 5, characterized in that, The solvent for the precipitation reaction is a mixed solvent containing water.

7. The method for preparing the NMFPP@NVPOF composite material as described in claim 1, characterized in that, The temperature for heat modification is 300~700℃.

8. The method for preparing the NMFPP@NVPOF composite material as described in claim 7, characterized in that, The heat modification treatment time is 1~12 hours.

9. An NMFPP@NVPOF composite material prepared by the preparation method according to any one of claims 1 to 8, characterized in that, It has a core-shell structure, wherein the core includes NMFPP, the shell is NVPOF, and there is a reconstructed fusion interface between the core and the shell.

10. An application of the NMFPP@NVPOF composite material according to claim 9, characterized in that, It is used as a positive electrode active material in the preparation of sodium-ion batteries.

11. A positive electrode for a sodium-ion battery, comprising a current collector and a positive electrode material composited thereon, said positive electrode material comprising a positive electrode active material, wherein, The positive electrode active material comprises the NMFPP@NVPOF composite material prepared by the preparation method according to any one of claims 1 to 8.

12. A sodium-ion battery, characterized in that, It includes the positive electrode as described in claim 11.

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