A carbon-coated iron-based / vanadium-based phosphate composite material, a preparation method and application thereof

Abstract

The application discloses a carbon-coated iron-based / vanadium-based phosphate composite material and a preparation method and application thereof. The composite material is composed of iron-based / vanadium-based phosphate and a carbon layer coated on the surface of the iron-based / vanadium-based phosphate in situ; the chemical formula of the iron-based / vanadium-based phosphate is Na4Fe3(PO4)2P2O7 / Na x V y M 2‑y (PO4)3 or Na3Fe2(PO4)P2O7 / Na x V y M 2‑y (PO4)3, wherein 2<=x<=4 and 1<=y<=2; in the composite material, the molar ratio of the iron-based phosphate and the vanadium-based phosphate is 1:0.2-5. The composite material is prepared by mixing raw materials uniformly and then calcining in one step, and the preparation method has the advantages of simple process and low cost. The sodium ion battery prepared based on the composite material provided by the application has excellent rate performance and cycle stability. Tests show that the initial discharge specific capacity is 125 mAh g ‑1 under 500 mA g ‑1 -1, the capacity retention rate is as high as 96% after 300 cycles, and the capacity can be maintained at 97 mAh g ‑1 under 5 A g ‑1 -1.

Description

Technical Field

[0001] This invention relates to a phosphate composite material, specifically to a carbon-coated iron-based / vanadium-based phosphate composite material, its preparation method, and its application, belonging to the field of sodium-ion battery technology. Background Technology

[0002] Electrochemical energy storage has seen large-scale development due to its high efficiency and low cost. Lithium-ion batteries have been applied in power batteries, portable devices, and other fields, while sodium-ion batteries, with their low cost, abundant raw materials, and high safety, have become a research hotspot in the field of large-scale energy storage. However, sodium ions have a larger radius than lithium ions, which hinders the repeated insertion and extraction of sodium ions in traditional electrode materials, necessitating the development of novel sodium-ion battery electrode materials.

[0003] After years of research, sodium-ion cathode materials have evolved into layered oxide materials, Prussian blue materials, organometallic compound materials, and polyanionic materials. Among these, NASICON-type polyanionic cathode materials have attracted widespread attention due to their structural stability and wide ion migration channels. Iron-based polyanionic materials, such as Na4Fe3(PO4)2P2O7 and Na3Fe2(PO4)P2O7, possess a stable open framework composed of phosphate and pyrophosphate ions, exhibiting high cycle stability, good rate performance, and free ion migration. However, their low voltage plateau and low intrinsic ionic conductivity make it difficult to achieve the theoretical capacity during charge and discharge as sodium-ion cathode materials. Vanadium-based polyanionic materials, such as Na2VTi(PO4)3 or Na4VMn(PO4)3, have a high voltage plateau, high capacity, and high energy density. However, the highly toxic vanadium raw materials and high sintering temperatures during preparation hinder large-scale production. Therefore, how to combine the advantages of both to obtain sodium battery materials with excellent comprehensive performance such as low cost, energy saving and environmental protection, and sodium storage performance has become an urgent technical problem to be solved. Summary of the Invention

[0004] To address the problems existing in the prior art, the first objective of this invention is to provide a carbon-coated iron-based / vanadium-based phosphate composite material. This carbon-coated iron-based / vanadium-based phosphate composite material has a more stable structural framework than traditional vanadium-based materials, which is beneficial for improving the material's cycle stability. Compared with traditional iron-based polyanionic materials, it improves the average voltage and ionic conductivity, thereby increasing the material's energy density. Simultaneously, the in-situ coating of an amorphous carbon layer on the material surface further enhances the material's ionic and electronic conductivity, suppressing side reactions between the electrolyte and the material.

[0005] The second objective of this invention is to provide a method for preparing carbon-coated iron-based / vanadium-based phosphate composite materials. This method involves a one-step process to uniformly mix raw materials and then calcining them to prepare the material. A carbon source is introduced in situ during the raw material mixing process, and the materials are sintered under a specific atmosphere to obtain in-situ carbon-coated nanoparticles. This method is simple, and compared with traditional vanadium-based materials, the sintering temperature is reduced from 800℃ to below 600℃, and the proportion of vanadium is reduced. The preparation process is energy-saving and environmentally friendly, and suitable for large-scale industrial production.

[0006] The third objective of this invention is to provide an application of a carbon-coated iron-based / vanadium-based phosphate composite material as a cathode material for sodium-ion batteries, enabling the preparation of sodium-ion battery cathodes. The sodium-ion battery cathode material prepared based on the composite material provided by this invention exhibits a high voltage plateau and a stable crystal structure, maintaining good cycle stability over a wide voltage range.

[0007] To achieve the above technical objectives, the present invention provides a carbon-coated iron-based / vanadium-based phosphate composite material, which is composed of iron-based / vanadium-based phosphate and an in-situ carbon coating layer on its surface;

[0008] The chemical formula of the iron-based / vanadium-based phosphate is Na4Fe3(PO4)2P2O7 / Na x V y M 2-y (PO4)3 or Na3Fe2(PO4)P2O7 / Na x V y M 2-y (PO4)3, wherein 2≤x≤4, 1≤y≤2; in the composite material, the molar ratio of iron-based phosphate to vanadium-based phosphate is 1:0.2~5.

[0009] The composite material provided by this invention is based on the synergistic effect of iron-based polyanionic materials and vanadium-based polyanionic materials. By strictly limiting the molar ratio of the two, the advantages of the two are organically combined, and the amount of vanadium element is reduced while ensuring the excellent performance of the composite material. The composite material not only has excellent cycle stability, but also has the advantages of high voltage platform and high energy density.

[0010] As a preferred embodiment, M is at least one of Ti, Fe, Mn, Cr, Al, Mo, Co, Mg, Ni, Zr, and Sc.

[0011] As a preferred embodiment, the mass of the in-situ encapsulated carbon layer is 3 to 15% of the mass of the iron-based / vanadium-based phosphate.

[0012] As a preferred embodiment, the particle size of the composite material is 0.5-20 μm.

[0013] The present invention also provides a method for preparing carbon-coated iron-based / vanadium-based phosphate composite material, wherein raw materials including sodium source, iron source, vanadium source, phosphorus source, carbon source and M source are mixed evenly to obtain a precursor; the precursor is sintered under a protective atmosphere with programmed temperature rise to obtain the final product.

[0014] As a preferred embodiment, the preparation process of the precursor is as follows: the raw material is dispersed in a solvent, sphericalized at 300-500 r / min for 2-6 h, and then the solvent is evaporated to obtain the precursor.

[0015] As a preferred embodiment, the solvent is at least one selected from deionized water, methanol, ethanol, propanol, acetone, and diethyl ether.

[0016] As a preferred embodiment, the solvent evaporation method is one of vacuum drying, spray drying, forced air drying, and freeze drying.

[0017] As a preferred embodiment, the sodium source is at least one selected from sodium acetate, sodium carbonate, sodium bicarbonate, sodium oxalate, and sodium hydroxide.

[0018] As a preferred embodiment, the vanadium source is at least one selected from vanadium pentoxide, ammonium metavanadate, sodium metavanadate, and vanadium oxysulfate.

[0019] As a preferred embodiment, the phosphorus source is at least one selected from phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

[0020] As a preferred embodiment, the carbon source is at least one of citric acid, glucose, and sucrose.

[0021] As a preferred embodiment, the M source is at least one of an inorganic salt, an organic salt, a hydroxide, and an oxide containing the element M.

[0022] As a preferred option, the raw materials also include a reducing agent.

[0023] As a preferred embodiment, the reducing agent is at least one of citric acid, oxalic acid, ascorbic acid, glucose, and hydroxylamine hydrochloride.

[0024] As a preferred embodiment, the molar ratio of the reducing agent to the transition metal element in the raw material is 0 to 5:1.

[0025] As a preferred embodiment, the programmed heating conditions are as follows: heating to 500-650°C at a rate of 1-5°C / min, holding at that temperature for 2-12 hours, and then cooling to room temperature with the furnace.

[0026] As a preferred embodiment, the protective atmosphere is high-purity nitrogen and / or high-purity argon.

[0027] This invention also provides an application of a carbon-coated iron-based / vanadium-based phosphate composite material as the positive electrode active material for sodium batteries, used in the preparation of sodium-ion batteries. The sodium-ion battery prepared using the composite material provided by this invention exhibits excellent performance. Tests conducted within a potential window of 1.5–4.3 V showed that the average operating voltage of the composite material is approximately 3.1 V, and the voltage at 500 mAg is [not specified in the original text]. -1 The initial discharge specific capacity is 125 mAh g. -1 After 300 cycles, the capacity retention rate reached 96%. Furthermore, this composite material exhibits excellent rate performance at 5Ag. -1 It can also maintain 97mAh g -1 The capacity.

[0028] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0029] 1) The carbon-coated iron-based / vanadium-based phosphate composite material provided by this invention combines the advantages of iron-based polyanionic materials and vanadium-based polyanionic materials. It has a more stable structural framework than traditional vanadium-based materials, significantly improving the cycling stability of the material. Compared with traditional iron-based polyanionic materials, it improves the average voltage and ionic conductivity, thereby increasing the energy density of the material. At the same time, the surface of the iron-based / vanadium-based phosphate composite material is coated with an amorphous carbon layer, which further improves the ionic conductivity and electronic conductivity of the material and suppresses the side reactions between the electrolyte and the material.

[0030] 2) The preparation method provided by this invention involves preparing material precursors through ball milling, sol-gel method, spray drying, etc., and introducing a carbon source in situ during the raw material mixing process. After sintering under a specific atmosphere, in-situ carbon-coated nanoparticles are obtained. The preparation process is simple, and compared with traditional vanadium-based materials, the sintering temperature is reduced from 800℃ to below 600℃, and the proportion of vanadium is reduced. The preparation process is energy-saving and environmentally friendly, and suitable for large-scale industrial production.

[0031] 3) In the technical solution provided by this invention, the sodium-ion battery prepared using the composite material provided by this invention has excellent performance. Tests conducted within a potential window of 1.5–4.3V showed that the average operating voltage of the composite material is approximately 3.1V, and at 500mA g… -1 The initial discharge specific capacity is 125 mAh g. -1 After 300 cycles, the capacity retention rate reached 96%. Furthermore, this composite material exhibits excellent rate performance at 5A g. -1 It can also maintain 97mAh g -1 The capacity. Attached Figure Description

[0032] Figure 1 The images show the XRD patterns of the cathode materials prepared in Example 1, Comparative Example 2, and Comparative Example 3.

[0033] Figure 2 The image shows the SEM image and elemental distribution diagram of the carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite cathode material prepared in Example 1.

[0034] Figure 3 The cyclic voltammetry curve of the carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite cathode material prepared in Example 1 is shown.

[0035] Figure 4 The cathode materials prepared in Examples 1, 5, 2, and 3 were subjected to a 50 mAg test. -1 The following is a charge / discharge curve diagram;

[0036] Figure 5 Sodium-ion batteries assembled using the cathode materials of Examples 1, 3, 5, and Comparative Example 3 were tested at 500 mA g. -1 The following is a graph showing the cyclic performance.

[0037] Figure 6 Sodium-ion batteries assembled from the cathode materials of Examples 1, 3, 2, and 3 were tested at 500 mA g. -1 The following is a graph showing the rate performance. Detailed Implementation

[0038] The present invention will be further illustrated by the following examples, but not by limiting the invention.

[0039] Example 1

[0040] In this embodiment, a carbon-coated iron-based / vanadium-based phosphate composite material is provided, with the chemical formula Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 and a molar ratio of 1:1. The preparation method of this battery cathode material includes the following steps:

[0041] Vanadyl acetylacetonate, tetrabutyl titanate, ferric nitrate, sodium acetate, and phosphoric acid were added to an ethanol solution in a stoichiometric ratio of Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃. Citric acid was added in a molar ratio of metal element (except Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 600°C in a reducing atmosphere of Ar / H₂ (containing 5% H₂) for 5 hours. Natural cooling yielded a carbon-coated Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃ composite cathode material.

[0042] This embodiment also provides the application of carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite material in sodium-ion batteries. The preparation of the electrode, the assembly of the battery, and the electrochemical performance testing are as follows:

[0043] A carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite cathode material was mixed with binder and conductive agent and slurryed, then coated onto aluminum foil and cured to obtain the sodium-ion battery cathode. CR2016 coin cells were assembled in an argon glove box using a metallic sodium sheet anode, a glass fiber separator, and 1.0M NaClO4 in PC with 5% FEC electrolyte. The cells were tested at 500mA g / L within a voltage range of 1.5–4.3V. -1 Its cycling performance was tested, and its rate performance was tested at different rates.

[0044] Example 2

[0045] Compared with Example 1, the difference lies in changing the ratio of Na4Fe3(PO4)2P2O7 to Na2VTi(PO4)3, as follows:

[0046] Sodium metavanadate, titanium monoxide, ferric oxide, sodium acetate, and phosphoric acid were added to an ethanol solution at a stoichiometric ratio of Na₄Fe₃(PO₄)₂P₂O₇ to Na₂VTi(PO₄)₃ of 1:5. Citric acid was added at a molar ratio of metal element (excluding Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 600°C in a reducing atmosphere of Ar / H₂ (containing 5% H₂) for 5 hours. Natural cooling yielded an in-situ carbon-coated Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃ (1:5) composite cathode material.

[0047] Example 3

[0048] Compared to Example 1, the difference lies in the amount of citric acid added, as follows:

[0049] Vanadium oxysulfate, titanium dioxide, ferric nitrate, ammonium dihydrogen phosphate, and sodium carbonate were added to an ethanol solution in a stoichiometric ratio of Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃. Citric acid was added in a molar ratio of metal element (except Na) to citric acid of 2:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 600°C in an N₂ atmosphere for 5 hours. Natural cooling yielded a carbon-coated Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃ composite cathode material.

[0050] Example 4

[0051] Compared to Example 1, the difference lies in the material composition, which is changed to: Na4Fe3(PO4)P2O7 / Na4VMn(PO4)3, as detailed below:

[0052] Ammonium metavanadate, manganese acetate, ferrous oxalate, sodium bicarbonate, and phosphoric acid were added to an ethanol solution in a stoichiometric ratio of Na3Fe2(PO4)P2O7 / Na4VMn(PO4)3. Citric acid was added in a molar ratio of metal element (except Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 600°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a carbon-coated Na3Fe2(PO4)P2O7 / Na4VMn(PO4)3 composite cathode material.

[0053] Example 5

[0054] Compared to Example 1, the difference lies in the material composition, which is changed to: Na3Fe2(PO4)P2O7 / Na3V2(PO4)3, as detailed below:

[0055] Vanadium pentoxide, iron powder, sodium acetate, and ammonium dihydrogen phosphate were added to an aqueous solution in a stoichiometric ratio of Na3Fe2(PO4)P2O7 / Na3V2(PO4)3. Citric acid was added in a molar ratio of metal elements (excluding Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by freeze-drying and sintered at 550°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a carbon-coated Na3Fe2(PO4)P2O7 / Na3V2(PO4)3 composite cathode material.

[0056] Example 6

[0057] Compared to Example 1, the difference lies in the material composition, which is changed to: Na3Fe2(PO4)P2O7 / Na4VFe(PO4)3, as detailed below:

[0058] Vanadium pentoxide, ferric oxide, sodium sulfate, and ammonium dihydrogen phosphate were added to an aqueous solution in a stoichiometric ratio of Na3Fe2(PO4)P2O7 / Na4VFe(PO4)3. Citric acid was added in a molar ratio of metal element (except Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The temperature was then increased and stirring continued until most of the solvent evaporated. The mixture was then dried in a vacuum oven at 100°C for 8 hours to obtain a precursor powder. This powder was sintered at 500°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a carbon-coated Na3Fe2(PO4)P2O7 / Na4VMn(PO4)3 composite cathode material.

[0059] Example 7

[0060] Compared to Example 1, the difference lies in the material composition, which is changed to: Na3Fe2(PO4)P2O7 / Na2VTi(PO4)3, as detailed below:

[0061] Vanadium pentoxide, titanium dioxide, iron tetroxide, sodium carbonate, and phosphoric acid were added to an aqueous solution in a stoichiometric ratio of Na3Fe2(PO4)P2O7 / Na2VTi(PO4)3. Citric acid was added in a molar ratio of metal elements (except Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The temperature was then increased and stirring continued until most of the solvent evaporated. The mixture was then dried in a vacuum oven at 100°C for 8 hours to obtain a precursor powder. This powder was sintered at 600°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a carbon-coated Na3Fe2(PO4)P2O7 / Na2VTi(PO4)3 composite cathode material.

[0062] Example 8

[0063] Compared with Example 1, the difference lies in that the material composition is changed to: Na4Fe3(PO4)P2O7 / Na3V 1.5 Al 0.5 (PO4)3, as detailed below:

[0064] Ammonium metavanadate, aluminum nitrate, ferric nitrate, ammonium dihydrogen phosphate, and ammonium sodium acetate were prepared according to the formula Na4Fe3(PO4)P2O7 / Na3V. 1.5 Al 0.5(PO4)3 was added to an aqueous solution in a stoichiometric ratio, and citric acid was added in a molar ratio of 1:3 for the metal elements (except Na). The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The temperature was then increased and stirring continued until most of the solvent evaporated. The mixture was then placed in a vacuum oven and dried at 100°C for 8 hours to obtain a precursor powder. This powder was then sintered at 600°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a carbon-coated Na4Fe3(PO4)P2O7 / Na3V 1.5 Al 0.5 (PO4)3 composite cathode material.

[0065] Comparative Example 1

[0066] This comparative example provides a method for preparing carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite materials.

[0067] The preparation method of the carbon-coated Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite material in this comparative example is similar to that in the examples, except for the calcination conditions:

[0068] Vanadyl acetylacetonate, tetrabutyl titanate, ferric nitrate, sodium acetate, and phosphoric acid were added to an ethanol solution in a stoichiometric ratio of Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃. Citric acid was added in a molar ratio of metal element (except Na) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 800°C in a reducing atmosphere of Ar / H₂ (containing 5% H₂) for 5 hours. Natural cooling yielded a carbon-coated Na₄Fe₃(PO₄)₂P₂O₇ / Na₂VTi(PO₄)₃ composite cathode material.

[0069] The electrode preparation, battery assembly, and performance testing steps are the same as in Example 1.

[0070] Comparative Example 2

[0071] This comparative example provides a method for preparing Na4Fe3(PO4)2P2O7 cathode material.

[0072] The preparation method of the Na4Fe3(PO4)2P2O7 cathode material in this comparative example is similar to that in the examples, except for the material composition:

[0073] Ferric nitrate, sodium acetate, and phosphoric acid were added to an ethanol solution in the stoichiometric ratio of Na4Fe3(PO4)2P2O7. Citric acid was added in a molar ratio of Fe to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 600°C in a reducing atmosphere of Ar / H2 (containing 5% H2) for 5 hours. Natural cooling yielded a Na4Fe3(PO4)2P2O7 cathode material.

[0074] Comparative Example 3

[0075] This comparative example provides a method for preparing Na2VTi(PO4)3 cathode material.

[0076] The preparation method of the Na2VTi(PO4)3 cathode material in this comparative example is similar to that in the examples, except for the material composition:

[0077] Vanadium acetylacetonate, tetrabutyl titanate, and phosphoric acid were added to an ethanol solution in the stoichiometric ratio of Na₂VTi(PO₄)₃. Citric acid was added in a molar ratio of metal elements (V and Ti) to citric acid of 1:3. The mixture was heated and stirred at 80°C for 3 hours until uniformly dispersed. The precursor powder was obtained by spray drying and sintered at 800°C in a reducing atmosphere of Ar / H₂ (containing 5% H₂) for 5 hours. Natural cooling yielded a Na₂VTi(PO₄)₃ cathode material.

[0078] Table 1 Summary of the electrochemical properties of the materials obtained in Examples 1-8

[0079]

[0080] In the composite material provided in this embodiment of the invention, the iron-based polyanionic material and the vanadium-based polyanionic material are not simply physically mixed during the preparation process, but rather form a two-phase composite material, producing a synergistic effect, thereby improving the electrochemical performance of the material as a cathode material for sodium-ion batteries. For example... Figure 1 As shown, the novel Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite material prepared in Example 1 contains all the diffraction peaks of the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 2 and the Na2VTi(PO4)3 material prepared in Comparative Example 3, indicating that the material contains a two-phase structure of Na4Fe3(PO4)2P2O7 and Na2VTi(PO4)3. Figure 2 It can be seen that the spray drying method produces materials with spherical morphology and uniform distribution of elements. Therefore, this method can be used to synthesize pure-phase and homogeneous iron-based / vanadium-based phosphate composite materials.

[0081] Depend on Figure 3It can be seen that the composite material obtained in Example 1 of the present invention has multiple sets of redox peaks in the range of 1.5 to 4.3 V, including redox peaks of Ti, Fe and V elements. The peak intensity is relatively high and the peak width is relatively narrow, indicating that the material has good conductivity. Moreover, the offset between the redox peaks in the same set is small, indicating that the polarization of the material is small. Furthermore, for this composite material, the two constituent structures take turns to carry out electrochemical reactions, which mutually restricts the volume expansion of the two, which is beneficial to improving cycle stability.

[0082] Furthermore, to investigate the application of the materials in sodium-ion batteries, the materials included in Examples 1 and 5, and Comparative Examples 2 and 3 were used as positive electrode active materials in half-cells, and their electrochemical performance was studied. The above materials were tested at 50 mA g in the range of 1.5–4.3 V. -1 Charge-discharge curves at current density Figure 4 (c) The Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 2 contains Fe 3+ / Fe 2+ Electrochemical reaction platform Figure 4 (d) The Na2VTi(PO4)3 prepared in Comparative Example 3 contains V 4+ / V 3+ Ti 4+ / Ti 3+ V 3+ / V 2+ Electrochemical reaction platform Figure 4 (a) The charge-discharge curve of the Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite material prepared in Example 1 shows that includes multiple electrochemical reaction platforms of Ti, Fe, and V elements, and... Figure 3 Corresponding to the CV diagram shown, this composite material combines the advantages of both materials, exhibiting a high theoretical specific capacity and a high average voltage. Similarly, the Na3Fe2(PO4)P2O7 / Na3V2(PO4)3 composite material prepared in Example 5 also contains multiple electrochemical reaction platforms of Fe and V elements, demonstrating a high average voltage and specific capacity.

[0083] Furthermore, the present invention also subjected the above materials to a 500mA g test. -1 The results of the 100-cycle test are as follows: Figure 5 As shown, the composite material prepared in Example 1 exhibits high cycling stability, with a capacity retention rate of 92.8% after 500 cycles. Although the material prepared in Comparative Example 2 has a higher capacity retention rate, its specific capacity is too low, less than 60 mA g. -1Comparing Example 1 and Example 3 reveals that reducing the carbon content has little impact on the capacity retention of the material before 200 cycles, but after 200 cycles, the capacity curve decreases significantly, and the capacity retention at 500 mA g⁻¹ is reduced. -1 The capacity at current density indicates that an appropriate amount of carbon coating can improve the electrochemical cycling stability of the material.

[0084] Furthermore, the present invention also conducted a magnification test on the materials obtained in Examples 1, 3, 2, and 3, and the test results are as follows: Figure 6 As shown, the reduction in carbon content not only slightly decreased the discharge specific capacity of the material, but also severely affected its performance at high-rate currents (3Ag). -1 Only 46.6mAh g was retained. -1 The discharge specific capacity is significantly lower than 98.6 mAh g in Example 1. -1 Furthermore, by comparing Examples 1, 2, and 3, it can be found that the Na4Fe3(PO4)2P2O7 / Na2VTi(PO4)3 composite material combines the advantages of both materials in terms of rate performance, possessing both the high rate performance of Na3Fe2(PO4)P2O7 and the high capacity of Na3V2(PO4)3. Table 1 lists some electrochemical performance data of the materials obtained in Examples 1-8. It can be seen that the iron-based / vanadium-based phosphate composite material has significant advantages in sodium-ion battery applications. It achieves a synergistic effect in structure, combining the advantages of the two materials in the composite material, possessing multiple electrochemical reaction platforms, significantly improving specific capacity, and exhibiting high stability. Appropriate carbon coating can enhance the rate performance of the material, resulting in a high specific capacity at higher current densities.

[0085] In summary, the iron-based / vanadium-based phosphate composite material provided by this invention, which is homogeneously mixed by methods such as ball milling and spray drying, exhibits excellent electrochemical performance as a positive electrode for sodium-ion batteries.

Claims

1. A carbon-coated iron-based / vanadium-based phosphate composite material, characterized in that: It consists of iron-based / vanadium-based phosphate and its in-situ coated carbon layer; The chemical formula of the iron-based / vanadium-based phosphate is Na4Fe3(PO4)2P2O7 / Na x V y M 2-y (PO4)3 or Na3Fe2(PO4)P2O7 / Na x V y M 2-y (PO4)3, Wherein, 2≤x≤4, 1≤y≤2; in the composite material, the molar ratio of iron-based phosphate to vanadium-based phosphate is 1:0.2~5. The carbon-coated iron-based / vanadium-based phosphate composite material is obtained by the following preparation method: raw materials including sodium source, iron source, vanadium source, phosphorus source, carbon source and M source are mixed evenly to obtain a precursor; the precursor is sintered under a protective atmosphere with programmed temperature rise to obtain the final product. The temperature program conditions are as follows: heating to 500-650℃ at a rate of 1-5℃ / min, holding at that temperature for 2-12 hours, and then cooling to room temperature with the furnace; the protective atmosphere is high-purity nitrogen and / or high-purity argon.

2. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 1, characterized in that: M is at least one of Ti, Fe, Mn, Cr, Al, Mo, Co, Mg, Ni, Zr, and Sc.

3. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 1, characterized in that: The mass of the in-situ coated carbon layer is 3-15% of the mass of the iron-based / vanadium-based phosphate; The particle size of the composite material is 0.5~20μm.

4. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 1, characterized in that: The preparation process of the precursor is as follows: the raw material is dispersed in a solvent, ball-milled at 300~500r / min for 2~6h, and then the solvent is evaporated to obtain the precursor.

5. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 4, characterized in that: The solvent is at least one selected from deionized water, methanol, ethanol, propanol, acetone, and diethyl ether. The solvent evaporation method is one of vacuum drying, spray drying, forced air drying, and freeze drying.

6. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 1, characterized in that: The sodium source is at least one of sodium acetate, sodium carbonate, sodium bicarbonate, sodium oxalate, and sodium hydroxide. The vanadium source is at least one of vanadium pentoxide, ammonium metavanadate, sodium metavanadate, and vanadium oxysulfate. The phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate. The carbon source is at least one of citric acid, glucose, and sucrose; The M source is at least one of inorganic salts, organic salts, hydroxides, and oxides containing the M element.

7. The carbon-coated iron-based / vanadium-based phosphate composite material according to claim 1, characterized in that: The raw materials also include a reducing agent; The reducing agent is at least one of citric acid, oxalic acid, ascorbic acid, glucose, and hydroxylamine hydrochloride.

8. The application of the carbon-coated iron-based / vanadium-based phosphate composite material according to any one of claims 1 to 3, characterized in that: Sodium-ion batteries are prepared using sodium as the positive electrode active material.

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