High-entropy doped composite polyanionic sodium iron sulfate cathode material, preparation method thereof and sodium ion battery

Abstract

The application provides a high-entropy doped composite polyanionic sodium ferric sulfate positive electrode material, a preparation method thereof and a sodium ion battery. 2+δ Fe 2‑ε M ε (SO4) 3‑μ‑2τ (PO4) μ (P2O7) τ / C, M is at least three metal elements in Ni, Co, Mn, Ca, Mg, Zn and Cu; 0 <= delta <= 0.5, 0

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, and particularly to a high-entropy doped composite polyanionic sodium iron sulfate cathode material, its preparation method, and a sodium-ion battery thereof. Background Technology

[0002] With the rapid development of sodium-ion battery technology, the development of high-performance, low-cost cathode materials has become a research hotspot. Sodium iron sulfate (Na2Fe2(SO4)3), as a cathode material for sodium-ion batteries, has attracted much attention due to its advantages such as high operating voltage (approximately 3.8V), abundant resources, and environmental friendliness. However, the practical application of this material faces multiple challenges: First, its intrinsic electronic conductivity is extremely low, severely limiting its rate performance; second, the specific capacity is relatively low, and its capacity decays significantly during high-temperature cycling; furthermore, ferrous iron is easily oxidized during synthesis, making it difficult to obtain a pure-phase product.

[0003] Currently, the main methods for improving the performance of sodium ferric sulfate have many limitations: traditional solid-state sintering methods are difficult to achieve atomic-level uniform mixing, affecting material consistency; conventional carbon material addition (such as carbon nanotubes, graphene, etc.) can partially improve conductivity, but these carbon materials are prone to agglomeration in electrode slurry, usually requiring the addition of dispersants and sand milling, which increases process complexity and cost; and single cation doping has limited effect on improving material performance.

[0004] Therefore, developing a synthesis method that enables precise control of components, effective structural stability, and a well-constructed conductive network is crucial for promoting the commercial application of sodium ferric sulfate. Summary of the Invention

[0005] The purpose of this invention is to provide a high-entropy doped composite polyanionic sodium ferric sulfate cathode material, its preparation method, and a sodium-ion battery.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing high-entropy doped composite polyanionic sodium ferric sulfate cathode material includes:

[0008] Step 1: Prepare the precursor solution:

[0009] Iron, sodium, sulfur, and phosphorus sources are selected according to the general chemical formula Na. 2+δ Fe 2-ε M ε (SO4) 3-μ-2τ (PO4) μ (P2O7) τThe stoichiometric ratio of the metals is dissolved in deionized water, and a water-soluble organic carbon source and additives are added to prepare a mixed metal salt solution with a total metal ion concentration of 1-4 mol / L; wherein, 0≤δ≤0.5, 0<ε≤0.4, 0≤μ≤0.3, 0≤τ≤0.15, and μ and τ are not simultaneously 0; M is selected from at least three metal elements from Ni, Co, Mn, Ca, Mg, Zn, and Cu;

[0010] Step 2: Low-temperature spray pyrolysis:

[0011] The precursor solution obtained in step one was subjected to spray pyrolysis at a temperature of 350-550℃ to obtain composite sodium ferric sulfate precursor powder.

[0012] Step 3: Mixing

[0013] The precursor powder obtained in step two is mixed with a solid carbon source to obtain a mixture.

[0014] Step 4: Simultaneous Crystallization and Coating:

[0015] The mixture obtained in step three is placed in a rotary kiln and heat-treated under a protective atmosphere at a temperature of 350-550℃ for 3-10 hours. During the heat treatment holding stage, carbon-containing gaseous hydrocarbons are introduced to perform chemical vapor deposition carbon coating. After cooling, a carbon-coated high-entropy doped composite polyanionic iron sulfate sodium cathode material is obtained.

[0016] In a further technical solution, in step one, the iron source is ferrous sulfate; the phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium phosphate, pyrophosphate, and sodium pyrophosphate; the sodium source is at least one of sodium sulfate, sodium pyrophosphate, and sodium phosphate, and the phosphorus source and sodium source may be the same or different; the dopant element M source is a soluble sulfate, nitrate, or chloride corresponding to M; the water-soluble organic carbon source is at least one of glucose, sucrose, citric acid, and ascorbic acid; the additive is at least one of polyacrylic acid, polyvinylpyrrolidone, carboxymethyl chitosan, and polyethylene glycol, and the amount of additive added is 0.5-3% of the total mass of the metal salt.

[0017] In a further technical solution, M is a combination of Ni, Co, and Mn, or a combination of Ni, Co, and Mg, or a combination of Ni, Co, and Zn; in the general chemical formula, δ is 0.05-0.15, ε is 0.04-0.06, μ is 0.05-0.1, and τ is 0-0.05.

[0018] In a further technical solution, in step two, the atomization method of the spray pyrolysis is pressure spray, centrifugal spray or ultrasonic spray, and the pyrolysis temperature is 450-500℃; in step one, the total metal ion concentration of the mixed metal salt solution is 1.5-3.2 mol / L.

[0019] In a further technical solution, in step four, the protective atmosphere is at least one of nitrogen and argon, and the heating rate of the heat treatment is 3-5℃ / min; the carbon-containing gaseous hydrocarbon is at least one of acetylene, ethylene, and methane, preferably acetylene, and its volume ratio with the protective gas is 1:99 to 15:85; the deposition time of chemical vapor deposition is the same as the heat treatment holding stage time, and is controlled within 2-8 hours.

[0020] In a further technical solution, the water-soluble organic carbon source is a liquid-phase carbon source, the solid carbon source is a solid-phase carbon source, and the carbon-containing gaseous hydrocarbon is a gaseous carbon source; through the synergistic effect of the liquid-phase carbon source, the solid-phase carbon source, and the gaseous carbon source, a multiple carbon coating layer is formed.

[0021] In step one, the water-soluble organic carbon source is at least one of glucose, sucrose, citric acid, and ascorbic acid; in step three, the solid carbon source is at least one of glucose, sucrose, and citric acid, and its addition amount is combined with the addition amount of water-soluble organic carbon source in step one and the amount of carbon deposited in step four, according to the carbon content of the final cathode material being 1.3-2.0%; in step four, at least one of acetylene, ethylene, and methane is used as a carbon-containing gaseous hydrocarbon for chemical vapor deposition carbon coating; the carbon in the carbon coating layer is composed of the above-mentioned water-soluble organic carbon source, solid carbon source, and carbon deposited in the gas phase.

[0022] Furthermore, this invention also discloses a high-entropy doped composite polyanionic sodium ferric sulfate cathode material, whose general chemical formula is Na. 2+δ Fe 2-ε M ε (SO4) 3-μ-2τ (PO4) μ (P2O7) τ / C, where M is at least three metallic elements selected from Ni, Co, Mn, Ca, Mg, Zn, and Cu; δ satisfies 0 ≤ δ ≤ 0.5, ε satisfies 0 < ε ≤ 0.4, μ satisfies 0 ≤ μ ≤ 0.3, and τ satisfies 0 ≤ τ ≤ 0.15, and μ and τ are not simultaneously 0; the carbon content of the material is 1.3-2.0%, and it is a carbon-coated high-entropy doped composite polyanionic material, wherein the carbon coating layer is a dense, uniform, multi-layer carbon coating layer formed by combining liquid-phase carbon source, solid-phase carbon source, and gas-phase carbon source.

[0023] In a further technical solution, M is a combination of Ni, Co, and Mn, or a combination of Ni, Co, and Mg, or a combination of Ni, Co, and Zn; in the general chemical formula, δ is 0.05-0.15, ε is 0.04-0.06, μ is 0.05-0.1, and τ is 0-0.05.

[0024] A further technical solution is that, within a voltage window of 2.0~4.3V and a current density of 0.1C, the initial discharge specific capacity of the material is ≥96.8mAh / g (1C=120mAh / g); the rate performance is ≥95.0% (capacity retention rate at 1C / 0.1C); and the capacity retention rate after 100 cycles is ≥96.5% within a voltage window of 2.0~4.3V and a current density of 1C.

[0025] In a further technical solution, the carbon content of the material is 1.5%.

[0026] Furthermore, the present invention also discloses a sodium-ion battery, wherein the positive electrode of the battery comprises the aforementioned sodium iron sulfate positive electrode material.

[0027] In a further technical solution, the positive electrode is made by mixing sodium iron sulfate positive electrode material, conductive agent, and binder in a mass ratio of 90:5:5; the binder is polyvinylidene fluoride, and the conductive agent is at least one of carbon black, carbon nanotubes, and graphene; the sodium-ion battery is a coin cell, a pouch cell, or a cylindrical cell, preferably a CR2032 type coin cell.

[0028] The terms “include,” “including,” and “have” used in this article are all open-ended, meaning they include but are not limited to.

[0029] Unless otherwise specified, the terms used herein generally have their ordinary meaning in the context of the art, the subject matter, and the specific context. Certain terms used to describe this case will be discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing this case.

[0030] The working principle and advantages of this invention are as follows:

[0031] This invention achieves innovative improvements in four aspects—material structure, conductivity, preparation efficiency, and industrial application—through an integrated and innovative design combining high-entropy doping, composite polyanion, multiple carbon coating, and an integrated process. The technical performance is significantly superior to existing solutions, as detailed below:

[0032] I. This invention constructs a high-entropy doping system by introducing at least three metal elements (Ni, Co, Mn, etc.) and combining them with a composite polyanionic framework of sulfate, phosphate, and pyrophosphate to achieve a dual effect of "cation disorder stability + anion co-regulation": On the one hand, high-entropy doping fills crystal defects and suppresses structural distortions through the lattice occupancy and electronic complementarity of multi-metal ions, significantly improving the structural stability of the material during sodium ion insertion / extraction; on the other hand, the electronegativity difference and spatial configuration complementarity of the composite polyanions broaden the sodium ion migration channels, reduce the ion diffusion energy barrier, and optimize the electronic structure of the material, making the intrinsic electronic conductivity more than an order of magnitude higher than that of traditional sodium iron sulfate, fundamentally solving the technical limitations of limited performance improvement of single doping or single polyanionic systems.

[0033] II. This invention employs a multi-layer carbon coating strategy of "in-situ coating of liquid-phase organic carbon source + dense layer deposition by gas-phase CVD" to form a continuous and uniform conductive network: the liquid-phase carbon source (glucose, sucrose, etc.) is atomically mixed with the precursor during the spray pyrolysis stage, providing a basic conductive pathway within the material; the gas-phase CVD deposition (acetylene, ethylene, etc.) forms a dense outer carbon film during crystallization, which not only enhances interfacial electronic conduction but also suppresses side reactions between the material and the electrolyte. This design completely overcomes the shortcomings of traditional externally added carbon materials (carbon nanotubes, graphene, etc.) in poor dispersibility in slurries and the need for additional sand milling, improving the electronic conduction efficiency of the material by more than 50%, significantly optimizing rate performance, and achieving a capacity retention rate of over 95% at a high current of 1C.

[0034] Third, this invention employs an integrated process of "spray pyrolysis + rotary kiln simultaneous crystallization and coating": spray pyrolysis technology ensures atomic-level uniform mixing of iron, sodium, dopant elements, and carbon sources, avoiding the component segregation problem of traditional solid-state sintering methods; the rotary kiln dynamic heat treatment enables simultaneous crystallization and CVD carbon coating, simplifying the production process and shortening the process cycle, improving production efficiency by more than 30% compared to step-by-step processes. Simultaneously, precise control of process parameters (pyrolysis temperature 350-550℃, holding time 3-10 hours, etc.) results in uniform grain size and high crystallinity of the material, ultimately achieving a simultaneous improvement in specific capacity, cycle stability, and rate performance—the initial discharge specific capacity at 0.1C reaches 96.8-98.2 mAh / g, and the capacity retention rate after 100 cycles at 1C exceeds 96.5%, with overall performance significantly superior to the comparative samples.

[0035] IV. This invention combines high performance with low cost, and has broad prospects for industrial application: At the material level, the iron and sodium sources, as well as the doping elements used, are all abundant and low-cost raw materials, with no dependence on rare metals, meeting the cost requirements of large-scale energy storage; at the process level, spray pyrolysis and rotary kilns are both mature mass-production equipment, requiring no special customization, and the process is simple and can be continuously produced, suitable for large-scale scaling; at the performance level, the material's high capacity, long cycle life, and high rate capability can meet the application needs of sodium-ion batteries in new energy storage, low-speed electric vehicles, and other fields. This solution provides a practical and feasible technical path for the commercial application of sodium iron sulfate cathode materials, and is expected to promote the popularization of sodium-ion batteries in the field of large-scale energy storage, possessing significant economic and social value. Attached Figure Description

[0036] Appendix Figure 1 This is a scanning electron microscope image of the spray pyrolysis precursor of Embodiment 1 of the present invention;

[0037] Appendix Figure 2 This is a scanning electron microscope image of sodium ferric sulfate material from Example 1 of the present invention;

[0038] Appendix Figure 3 The first charge-discharge curve of the coin cell assembled with the positive electrode material prepared in Example 1 of the present invention under 0.1C rate cycling. Detailed Implementation

[0039] The present invention will be clearly described below with illustrations and detailed description. Any person skilled in the art who understands the embodiments of the present invention can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0040] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of this case.

[0041] Example 1

[0042] This embodiment provides a method for preparing a high-entropy doped composite polyanionic sodium ferric sulfate cathode material, the specific steps of which are as follows:

[0043] S1. Preparation of precursor solution:

[0044] According to the general chemical formula Na 2.10 Fe 1.95 Ni 0.02 Co 0.02 Mn 0.01 (SO4) 2.9 (PO4) 0.1According to the stoichiometric ratio, 0.390 mol of ferrous sulfate (FeSO4·7H2O), 0.180 mol of sodium sulfate (Na2SO4), 0.020 mol of sodium phosphate (Na3PO4·12H2O), 0.004 mol of nickel sulfate (NiSO4·6H2O), 0.004 mol of cobalt sulfate (CoSO4·7H2O), and 0.002 mol of manganese sulfate (MnSO4·H2O) were weighed and dissolved in 547 mL of deionized water. Then, 2% glucose (as a water-soluble organic carbon source) and 1.5% polyacrylic acid (as an additive, based on the total mass of the metal salts) were added. The mixture was magnetically stirred at 500 rpm for 2 hours at 30°C until the solution was completely clear and transparent, thus preparing a precursor solution with a total metal ion concentration of 1.5 mol / L.

[0045] S2, Low-temperature spray pyrolysis:

[0046] The precursor solution was atomized using a pressure spray device, with the atomization pressure set at 0.3 MPa and the spray rate at 5 mL / min. The temperature inside the pyrolysis furnace was controlled at 450°C, and the pyrolysis atmosphere was air. After atomization, the precursor solution entered the pyrolysis furnace, where it was instantly dried and underwent a preliminary reaction. The powder falling from the bottom of the furnace was collected to obtain composite sodium ferric sulfate precursor powder.

[0047] S3, Mixing:

[0048] Based on the design value of 1.4% carbon content of the final product, 100g of the above precursor powder and 2.35g of sucrose (solid carbon source) were weighed and placed in a ball mill. Agate balls were used as the grinding medium, the ball-to-material ratio was 20:1, the rotation speed was 300 rpm, and the mixture was ground for 2 hours to obtain a uniform mixture.

[0049] S4. Simultaneous crystallization and coating:

[0050] The mixture was fed into a rotary kiln, and nitrogen was introduced as a protective atmosphere at a flow rate of 0.5 L / min. The temperature was raised to 500 °C at a rate of 3 °C / min and held for 6 hours. During this holding period, a mixture of acetylene and nitrogen (volume ratio C2H2:N2 = 8:92) was simultaneously introduced for chemical vapor deposition carbon coating at a total flow rate of 0.3 L / min. After the holding period, the material was allowed to cool naturally to room temperature with the kiln, and then removed to obtain a carbon-coated high-entropy doped composite polyanionic sodium iron sulfate cathode material. Carbon and sulfur analysis showed that the carbon content of the material was 1.38%.

[0051] The performance of the cathode material prepared in this embodiment was tested: the initial discharge specific capacity was 97.1 mAh / g (1C=120 mAh / g) under a voltage window of 2.0~4.3V and a current density of 0.1C; the capacity retention rate at a current density of 1C was 95.5% (relative to 0.1C); and the capacity retention rate was 96.7% after 100 cycles under a voltage window of 2.0~4.3V and a current density of 1C.

[0052] Example 2

[0053] This embodiment provides a method for preparing a high-entropy doped composite polyanionic sodium ferric sulfate cathode material, the specific steps of which are as follows:

[0054] S1. Preparation of precursor solution:

[0055] According to the general chemical formula Na 2.00 Fe 1.96 Ni 0.015 Co 0.015 Mg 0.01 (SO4) 2.9 (P2O7) 0.05 According to the stoichiometric ratio, 0.397 mol of ferrous sulfate (FeSO4·7H2O), 0.182 mol of sodium sulfate (Na2SO4), 0.010 mol of sodium pyrophosphate (Na4P2O7·10H2O), 0.003 mol of nickel sulfate (NiSO4·6H2O), 0.003 mol of cobalt sulfate (CoSO4·7H2O), and 0.002 mol of magnesium sulfate (MgSO4·7H2O) were weighed and dissolved in 405 mL of deionized water. Citric acid (a water-soluble organic carbon source) at 3% of the total metal ion mass and polyvinylpyrrolidone (an additive, based on the total metal salt mass) were added. The mixture was magnetically stirred at 600 rpm for 1.5 hours at 40 °C to prepare a precursor solution with a total metal ion concentration of 2.0 mol / L.

[0056] S2, Low-temperature spray pyrolysis:

[0057] Atomization was performed using a centrifugal spray device, with the rotary table speed set at 15000 rpm and the feed rate at 8 mL / min. The pyrolysis temperature was controlled at 500℃, and the pyrolysis atmosphere was air. The pyrolysis products were collected to obtain precursor powder.

[0058] S3, Mixing:

[0059] Based on a final product carbon content of 1.5%, weigh 100g of precursor powder and 2.6g of glucose, put them into a high-speed mixer, and mix them at 1000rpm for 1 hour to obtain a mixture.

[0060] S4. Simultaneous crystallization and coating:

[0061] The mixture was fed into a rotary kiln, and argon gas was introduced as a protective atmosphere at a flow rate of 0.4 L / min. The temperature was raised to 450 °C at a rate of 5 °C / min and held for 8 hours. During the holding period, a mixture of ethylene and nitrogen (volume ratio C2H4:N2 = 5:95) was introduced at a total flow rate of 0.25 L / min for CVD coating. After cooling, the material was removed to obtain the target cathode material. Carbon and sulfur analysis showed that the carbon content of the material was 1.52%.

[0062] Performance test results: 0.1C initial discharge specific capacity is 96.8mAh / g; 1C / 0.1C rate performance is 95.1%; 1C, 100-cycle capacity retention is 96.9%.

[0063] Example 3

[0064] This embodiment provides a method for preparing a high-entropy doped composite polyanionic sodium ferric sulfate cathode material, the specific steps of which are as follows:

[0065] S1. Preparation of precursor solution:

[0066] According to the general chemical formula Na 2.1 Fe 1.95 Ni 0.01 Co 0.02 Zn 0.02 (SO4) 2.8 (PO4) 0.1 (P2O7) 0.05 According to the stoichiometric ratio, 0.390 mol of ferrous sulfate (FeSO4·7H2O), 0.160 mol of sodium sulfate (Na2SO4), 0.020 mol of sodium phosphate (Na3PO4·12H2O), 0.010 mol of sodium pyrophosphate (Na4P2O7·10H2O), 0.002 mol of nickel sulfate (NiSO4·6H2O), 0.004 mol of cobalt sulfate (CoSO4·7H2O), and 0.004 mol of zinc sulfate (ZnSO4·7H2O) were weighed and dissolved in 256 mL of deionized water. 2.5% sucrose (a water-soluble organic carbon source) and 0.8% carboxymethyl chitosan (an additive, based on the total mass of the metal salts) were added, and the mixture was magnetically stirred at 550 rpm for 2.5 hours at 35°C to prepare a precursor solution with a total metal ion concentration of 3.2 mol / L.

[0067] S2, Low-temperature spray pyrolysis:

[0068] An ultrasonic spray device was used for atomization, with the ultrasonic frequency set at 1.7 MHz and the feed rate at 3 mL / min. The pyrolysis temperature was 450 ℃, and the pyrolysis atmosphere was air. The precursor powder was collected.

[0069] S3, Mixing:

[0070] Based on a final product carbon content of 1.5%, weigh 100g of precursor powder and 2.45g of citric acid, put them into a high-speed mixer, and mix at 1200rpm for 3 hours to obtain a mixture.

[0071] S4. Simultaneous crystallization and coating:

[0072] The mixture was fed into a rotary kiln, and nitrogen was introduced as a protective atmosphere at a flow rate of 0.6 L / min. The temperature was raised to 500 °C at a rate of 4 °C / min and held for 4 hours. During the holding period, a mixture of methane and argon (volume ratio CH4:Ar = 10:90) was introduced at a total flow rate of 0.3 L / min for CVD coating. After cooling, the material was removed to obtain the target cathode material. Carbon and sulfur analysis showed that the carbon content of the material was 1.42%.

[0073] Performance test results: 0.1C initial discharge specific capacity is 98.2mAh / g; 1C / 0.1C rate performance is 95.6%; 1C, 100-cycle capacity retention is 96.5%.

[0074] Comparative Example 1

[0075] The preparation method of this comparative example is basically the same as that of Example 1, except that: during the simultaneous crystallization and coating process in S4, only nitrogen gas is introduced as a protective atmosphere, and no carbon-containing gaseous hydrocarbons are introduced, that is, no chemical vapor deposition carbon coating is performed.

[0076] Performance test results: 0.1C initial discharge specific capacity is 87.4mAh / g; 1C / 0.1C rate performance is 82.5%; 1C, 100-cycle capacity retention is 91.9%; actual carbon content is 1.23%.

[0077] Comparative Example 2

[0078] The preparation method of this comparative example is basically the same as that of Example 1, the only difference being that: no doping elements such as Ni, Co, and Mn are added when preparing the precursor solution in S1, and the chemical formula is Na 2.1 Fe2(SO4) 2.9 (PO4) 0.1 The remaining steps are exactly the same as in Example 1. Elemental analysis showed that the actual carbon content was 1.51%.

[0079] Comparative Example 3

[0080] The preparation method of this comparative example is basically the same as that of Example 1, the only difference being that no phosphorus source (sodium phosphate) is added when preparing the precursor solution in S1. The chemical formula is Na 2.1 Fe 1.90 Ni0.02 Co 0.02 Mn 0.01 (SO4)3, the remaining steps are exactly the same as in Example 1. Elemental analysis showed that the actual carbon content was 1.46%.

[0081] Performance testing and results analysis:

[0082] The sodium ferric sulfate cathode material prepared in Examples 1-3 and Comparative Examples 1-3, a conductive agent (such as conductive carbon black SuperP), and a binder PVDF were mixed at a mass ratio of 90:5:5 to prepare an electrode slurry. This slurry was uniformly coated onto an aluminum foil current collector and then vacuum-dried at 80-120°C for 4-8 hours, followed by rolling (pressure 3-5t) to form a cathode sheet. Using a sodium metal sheet as the counter electrode, glass fiber as the separator, and 1 mol / L NaPF6 / EC-DMC (volume ratio 1:1) as the electrolyte, CR2032 coin cells were assembled in an argon-protected glove box. After the cells were allowed to stand for 12 hours, charge-discharge tests were performed. The test voltage window was 2.0-4.3V, and the test ambient temperature was 25°C. The first-cycle discharge specific capacity was tested at a current density of 0.1C (1C=120mAh / g), and the capacity retention rate was tested over 100 cycles at a 1C current density and a voltage window of 2.0-4.3V.

[0083] Table 1 Performance of cathode materials

[0084]

[0085] Test results show that Examples 1-3 of this invention significantly outperform Comparative Example 1 in terms of core electrochemical performance. Specifically, at a rate of 0.1C, the initial discharge specific capacity of the examples reaches 96.8-98.2 mAh / g, which is about 11%-12% higher than Comparative Example 1 (87.4 mAh / g) without CVD coating, about 7%-9% higher than Comparative Example 2 (90.2 mAh / g) without high-entropy doping, and about 8%-10% higher than Comparative Example 3 (89.5 mAh / g) without composite polyanions. This indicates that the synergistic effect of high-entropy doping and composite polyanions effectively improves the reversible capacity of the material. In terms of rate performance, the 1C / 0.1C capacity retention rates of Examples 1-3 are all higher than 95%, while those of Comparative Example 1 are only 82.5%, Comparative Example 2 is 86.8%, and Comparative Example 3 is 85.3%. This demonstrates that the multi-carbon-coated network constructed in this invention significantly improves electronic conduction efficiency. Simultaneously, high-entropy doping and the composite polyanionic framework lower the sodium ion diffusion barrier, enabling the material to maintain excellent capacity output even under high current. Regarding long-term cycling stability, after 100 cycles at a 1C current density, the capacity retention rates of Examples 1-3 all exceeded 96.5%, while Comparative Example 1 showed 91.9%, Comparative Example 2 92.6%, and Comparative Example 3 93.2%. This indicates that the structural design of this invention effectively suppresses structural degradation and interfacial side reactions during repeated sodium ion insertion / extraction processes, endowing the material with excellent long cycle life.

[0086] The significant improvement in the aforementioned performance is attributed to the multiple effects of the material design and preparation method of this invention. First, the high-entropy doping and the composite polyanionic framework work together to effectively stabilize the crystal structure and broaden the sodium ion migration channels. Second, the multi-layer carbon coating network constructed by combining spray pyrolysis and CVD technology significantly improves the electronic conductivity of the material and suppresses interfacial side reactions. The combined effect of these factors fundamentally improves the shortcomings of traditional unmodified sodium ferric sulfate materials, such as poor conductivity and insufficient structural stability, thereby achieving a breakthrough in its comprehensive electrochemical performance.

[0087] like Figure 1 The image shows the precursor obtained from spray pyrolysis. Figure 2 As shown, after simultaneous crystallization and CVD coating, the particle morphology of the material tends to be more regular. The integrated process of this invention ensures the high uniformity of the material composition, enabling the material to maintain ideal specific capacity under high compaction density, and significantly improving the volumetric energy density of sodium-ion batteries.

[0088] Appendix Figure 3 The graph shows the first charge-discharge curves of a coin cell assembled from the cathode material prepared in Example 1 of this invention, under a 0.1C rate cycle. The blue curve represents the discharge curve, and the orange curve represents the charging curve.

[0089] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-entropy doped composite polyanionic sodium ferric sulfate cathode material, characterized in that: include: Step 1: Prepare the precursor solution: Iron, sodium, sulfur, and phosphorus sources are selected according to the general chemical formula Na. 2+δ Fe 2-ε M ε (SO4) 3-μ-2τ (PO4) μ (P2O7) τ The stoichiometric ratio of the metals is dissolved in deionized water, and a water-soluble organic carbon source and additives are added to prepare a mixed metal salt solution with a total metal ion concentration of 1-4 mol / L; wherein, 0≤δ≤0.5, 0<ε≤0.4, 0≤μ≤0.3, 0≤τ≤0.15, and μ and τ are not simultaneously 0; M is selected from at least three metal elements from Ni, Co, Mn, Ca, Mg, Zn, and Cu; Step 2: Low-temperature spray pyrolysis: The precursor solution obtained in step one was subjected to spray pyrolysis at a temperature of 350-550℃ to obtain composite sodium ferric sulfate precursor powder. Step 3: Mixing The precursor powder obtained in step two is mixed with a solid carbon source to obtain a mixture. Step 4: Simultaneous Crystallization and Coating: The mixture obtained in step three is placed in a rotary kiln and heat-treated under a protective atmosphere at a temperature of 350-550℃ for 3-10 hours. During the heat treatment holding stage, carbon-containing gaseous hydrocarbons are introduced to perform chemical vapor deposition carbon coating. After cooling, a carbon-coated high-entropy doped composite polyanionic iron sulfate sodium cathode material is obtained.

2. The preparation method of the high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 1, characterized in that: In step one, the iron source is ferrous sulfate; the phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium phosphate, pyrophosphate, and sodium pyrophosphate; the sodium source is at least one of sodium sulfate, sodium pyrophosphate, and sodium phosphate, and the phosphorus source and sodium source may be the same or different; the dopant element M source is a soluble sulfate, nitrate, or chloride corresponding to M; the water-soluble organic carbon source is at least one of glucose, sucrose, citric acid, and ascorbic acid; the additive is at least one of polyacrylic acid, polyvinylpyrrolidone, carboxymethyl chitosan, and polyethylene glycol, and the amount of additive added is 0.5-3% of the total mass of the metal salt.

3. The preparation method of the high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 1, characterized in that: M is a combination of Ni, Co, and Mn, or a combination of Ni, Co, and Mg, or a combination of Ni, Co, and Zn; in the general chemical formula, δ is 0.05-0.15, ε is 0.04-0.06, μ is 0.05-0.1, and τ is 0-0.

05.

4. The preparation method of the high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 1, characterized in that: In step two, the atomization method of the spray pyrolysis is pressure spray, centrifugal spray or ultrasonic spray, and the pyrolysis temperature is 450-500℃; in step one, the total metal ion concentration of the mixed metal salt solution is 1.5-3.2 mol / L.

5. The method for preparing the high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 1, characterized in that: In step four, the protective atmosphere is at least one of nitrogen and argon, and the heating rate of the heat treatment is 3-5℃ / min; the carbon-containing gaseous hydrocarbon is at least one of acetylene, ethylene, and methane, and its volume ratio with the protective gas is 1:99 to 15:85; the deposition time of chemical vapor deposition is the same as the heat treatment holding stage time, and is controlled within 2-8 hours.

6. The method for preparing the high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 1, characterized in that: The water-soluble organic carbon source is a liquid-phase carbon source, the solid carbon source is a solid-phase carbon source, and the carbon-containing gaseous hydrocarbon is a gaseous carbon source; through the synergistic effect of the liquid-phase carbon source, solid-phase carbon source, and gaseous carbon source, a multiple carbon coating layer is formed. In step one, the water-soluble organic carbon source is at least one of glucose, sucrose, citric acid, and ascorbic acid; in step three, the solid carbon source is at least one of glucose, sucrose, and citric acid, and its addition amount is combined with the addition amount of water-soluble organic carbon source in step one and the amount of carbon deposited in step four, according to the carbon content of the final cathode material being 1.3-2.0%; in step four, at least one of acetylene, ethylene, and methane is used as a carbon-containing gaseous hydrocarbon for chemical vapor deposition carbon coating; the carbon in the carbon coating layer is composed of the above-mentioned water-soluble organic carbon source, solid carbon source, and carbon deposited in the gas phase.

7. A high-entropy doped composite polyanionic sodium ferric sulfate cathode material, characterized in that: Prepared by the preparation method according to any one of claims 1-6, with the general chemical formula Na 2+δ Fe 2-ε M ε (SO4) 3-μ-2τ (PO4) μ (P2O7) τ / C, where M is at least three metallic elements selected from Ni, Co, Mn, Ca, Mg, Zn, and Cu; δ satisfies 0 ≤ δ ≤ 0.5, ε satisfies 0 < ε ≤ 0.4, μ satisfies 0 ≤ μ ≤ 0.3, and τ satisfies 0 ≤ τ ≤ 0.15, and μ and τ are not simultaneously 0; the carbon content of the material is 1.3-2.0%, and it is a carbon-coated high-entropy doped composite polyanionic material, wherein the carbon coating layer is a dense, uniform, multi-layer carbon coating layer formed by combining liquid-phase carbon source, solid-phase carbon source, and gas-phase carbon source.

8. The high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 7, characterized in that: M is a combination of Ni, Co, and Mn, or a combination of Ni, Co, and Mg, or a combination of Ni, Co, and Zn; in the general chemical formula, δ is 0.05-0.15, ε is 0.04-0.06, μ is 0.05-0.1, and τ is 0-0.

05.

9. The high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 7, characterized in that: Within a voltage window of 2.0–4.3 V and a current density of 0.1 C, the material exhibits an initial discharge specific capacity ≥ 96.8 mAh / g (1 C = 120 mAh / g); rate performance ≥ 95.0% (capacity retention at 1 C / 0.1 C); and capacity retention ≥ 96.5% after 100 cycles within a voltage window of 2.0–4.3 V and a current density of 1 C.

10. The high-entropy doped composite polyanionic sodium ferric sulfate cathode material according to claim 7, characterized in that: The carbon content of the material is 1.5%.

11. A sodium-ion battery, characterized in that: The positive electrode of the battery comprises sodium ferric sulfate as the positive electrode material according to any one of claims 7-10.

12. The sodium-ion battery according to claim 11, characterized in that: The positive electrode is made by mixing sodium iron sulfate positive electrode material, conductive agent, and binder in a mass ratio of 90:5:5; the binder is polyvinylidene fluoride, and the conductive agent is at least one of carbon black, carbon nanotubes, and graphene; the sodium-ion battery is a button cell, a pouch cell, or a cylindrical cell.

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