A Na4Fe 3-x M x (PO4)2P2O7 / C composite material, its preparation and application in sodium-ion batteries

Abstract

The application belongs to the field of sodium ion battery cathodes, and particularly relates to a Na4Fe 3‑x M x (PO4)2P2O7 / C composite material, comprising active nanoparticles and a surface coating layer of the active nanoparticles; the active nanoparticles have a chemical formula of Na4Fe 3‑x M x (PO4)2P2O7, wherein M is at least one of Ni, Co, Mn, Cu, Zn, Mg, Ti, Sn, Zr and Ca; x is 0.03-0.3; and the coating layer comprises amorphous carbon and phenolic resin. The application also relates to a preparation method and application of the material. The material can improve the capacity, high rate and long-term cycle performance of the material based on the combination of components and structures.

Description

Technical fields:

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

[0002] The development of sustainable energy has driven rapid progress in the energy storage field, with electrochemical energy storage, represented by lithium-ion batteries, experiencing rapid development. However, the high cost of lithium and the limited distribution of resources restrict its further development. Meanwhile, sodium-ion batteries, with a similar structure, have received extensive research, and their low price and widespread availability have made them a favorite among investors.

[0003] The cathode material, a key component of sodium-ion batteries, determines their energy density and cost. Compared to transition metals commonly used in lithium-ion batteries, such as Co and Ni, Fe-based materials are more affordable. Combining them with sodium salts can further reduce the cost of sodium-ion batteries, meeting the cost requirements of large-scale energy storage. Among iron-based polyanionic materials, Na4Fe3(PO4)2P2O7 exhibits good structural stability and open sodium-ion transport channels; however, its poor electronic conductivity and low redox potential result in low energy density and poor rate performance in practical applications. Summary of the Invention:

[0004] To address the issue of unsatisfactory electrochemical performance of carbon-based composite sodium iron phosphate pyrophosphate, the primary objective of this invention is to provide a Na4Fe 3-x M x The (PO4)2P2O7 / C composite material aims to provide a positive electrode active material with excellent conductivity, tap density, high rate capability and long-range cycling performance.

[0005] To address the problems of impurity phases, uneven carbon coating, and unsatisfactory tap density in carbon-composite sodium iron pyrophosphate composites, the second objective of this invention is to provide a Na4Fe 3-x M x The preparation method of (PO4)2P2O7 / C composite material solves the problems of impurity phase, coating uniformity and unsatisfactory tap density in the preparation process, and improves the high-rate and long-range cycling performance of the prepared material.

[0006] The third objective of this invention is to provide a Na4Fe 3-x M x Application of (PO4)2P2O7 / C composite material in sodium-ion battery cathode material.

[0007] A fourth objective of this invention is to provide a solution comprising the aforementioned Na4Fe 3-x M xSodium-ion batteries and their positive electrode components made of (PO4)2P2O7 / C composite material.

[0008] A Na4Fe 3-x M x (PO4)2P2O7 / C composite material, including active nanoparticles and a carbon coating layer on their surface;

[0009] The chemical formula of the active nanoparticles is Na₄Fe₂O₃. 3-x M x (PO4)2P2O7, wherein M is at least one of Ni, Co, Mn, Cu, Zn, Mg, Ti, Sn, Zr, and Ca; and x = 0.03 to 0.3;

[0010] The coating layer comprises amorphous carbon and phenolic resin.

[0011] This invention provides a novel Na4Fe 3-x M x The (PO4)2P2O7 / C composite material, based on the aforementioned coating material and the doping of the core M and the joint control of the doping content, can achieve synergy and improve the material's capacity, rate performance and long-range cycling stability.

[0012] In this invention, the combined control of M doping and the doping content and coating material is key to synergistically improving the electrochemical performance of the material.

[0013] Preferably, M is two or more of Ni, Co, Mn, Cu, Zn, Mg, Ti, Sn, Zr, and Ca; more preferably, M is four or five of Ni, Co, Mn, Cu, Zn, Mg, Ti, Sn, Zr, and Ca. This invention has found that using the preferred combination of M can further improve synergy, further enhance the synergistic effect of the core and shell, and contribute to further synergistic improvement in high-rate and long-range cycling stability.

[0014] Preferably, M comprises Ni, Co, Mn, Cu, and Mg, or comprises calcium, zinc, tin, titanium, and zirconium. Preferably, when a combination of multiple elements is used, the molar content of each element in M ​​is 10–30 mol.%.

[0015] Preferably, M is 0.05 to 0.15; more preferably, it is 0.05 to 0.1.

[0016] Preferably, the coating layer is a carbon-organic composite material layer consisting of a composite carbon source with a molecular weight less than or equal to 200, polyethylene glycol, and phenolic resin, which is carbonized at 450–600°C to form a composite carbon source containing amorphous carbon and residual organic carbon source.

[0017] Preferably, the Na4Fe 3-x M x In the (PO4)2P2O7 / C composite material, the weight of active nanoparticles is 90-98%;

[0018] Preferably, the thickness of the carbon coating layer is 1–10 nm;

[0019] Preferably, the size of the active nanoparticles is 100–600 nm.

[0020] Preferably, the tap density is 1.1 to 1.6 g / cc.

[0021] A second objective of this invention is to provide the aforementioned Na4Fe 3-x M x A method for preparing (PO4)2P2O7 / C composite materials has been developed, but early in the technology development process, it was discovered that the phase stability of sodium iron pyrophosphate is lower than that of maricite-NaFePO4, and maricite-NaFePO4 impurities are easily formed during the preparation process. Furthermore, metal doping of sodium iron pyrophosphate further exacerbates the presence of maricite-NaFePO4 impurities. In addition, nanostructuring and carbon coating of sodium iron pyrophosphate help improve its conductivity, but also present new problems such as uneven carbon coating uniformity and unsatisfactory tap density, thus affecting the electrochemical performance of the prepared material. To address the problems of impurities, uneven carbon coating uniformity, unsatisfactory tap density, and unsatisfactory electrochemical performance that need to be overcome in the preparation process of the material described in this invention, the following solutions are provided:

[0022] A Na4Fe 3-x M x The preparation method of (PO4)2P2O7 / C composite material involves reducing and calcining a mixture containing sodium source, iron source, metal M source, phosphorus source, and synergistic carbon source at a temperature of 450–600°C to obtain the Na4Fe composite material. 3- x M x (PO4)2P2O7 / C composite material;

[0023] The synergistic carbon sources include carbon source A, carbon source B, and carbon source C, wherein the molecular weight of carbon source A is less than or equal to 200; carbon source B is polyethylene glycol; and carbon source C is phenolic resin.

[0024] In this invention, a novel combination of carbon source A, carbon source B, and carbon source C is used as the carbon source. This is further combined with the doping of the aforementioned metal M and the joint control of the reduction calcination method and temperature. This allows for the synergistic promotion of Na₄Fe₂O₃ by utilizing the different thermal behaviors of the carbon source components during the reduction calcination stage.3-x M x The synthesis of (PO4)2P2O7 improves phase purity and reduces thermodynamically stable impurities. In addition, it can improve the uniformity of carbon coating structure and the composition characteristics of carbon-organic composite layer, improve tap density, and improve the electrochemical performance of the prepared material, especially significantly improving high-rate and long-range cycling stability.

[0025] Preferably, the sodium source is at least one of sodium carbon source, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, disodium monohydrogen phosphate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium pyrophosphate.

[0026] Preferably, the iron source is at least one of ferric nitrate, ferrous oxalate, ferrous acetate, ferrous sulfate, ferric oxide, ferrous oxide, and ferric oxide.

[0027] Preferably, the M source is at least one of oxides, sulfates, nitrates, acetates, and oxalates containing M;

[0028] Preferably, the M source is two or more of Ni, Co, Mn, Cu, Zn, Mg, Ti, Sn, Zr, and Ca, and more preferably four or five of them. More preferably, the metal M source is Ni, Co, Mn, and Cu, and also includes at least one of Zn, Mg, Ti, Sn, Zr, and Ca; most preferably, it includes Ni, Co, Mn, Cu, and Mg, or includes calcium, zinc, tin, titanium, and zirconium. This invention has found that using the preferred combination of M can further improve synergy, further improve the synergistic effect of the core and shell, and contribute to further synergistic improvement in rate capability and long-range stability.

[0029] Preferably, the phosphorus source is at least one selected from ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, and diammonium hydrogen phosphate.

[0030] Preferably, the sodium source, iron source, M source, and phosphorus source are weighed according to the molar ratio of Na, Fe, M, and P elements of 4:3 to x:x:4.

[0031] In this invention, the synergistic carbon source, metal M source doping, x content control, and combined control of reduction calcination and temperature are key to synergistically improving the phase purity of the target active material, reducing thermodynamically stable impurities, improving the carbon coating structure and tap density, and improving electrochemical performance.

[0032] Preferably, the carbon source A is a polyhydroxy sugar with a molecular weight of less than or equal to 200, preferably at least one of sucrose and glucose;

[0033] Preferably, the carbon source B is polyethylene glycol with a molecular weight of 2500-20000, more preferably 3000-8000, and even more preferably 5000-6500;

[0034] Preferably, the carbon source C is a phenolic resin. The phenolic resin can be a polymer formed by the condensation polymerization of phenol and aldehyde in any form, for example, it can be a phenol-formaldehyde polymer or a substituted phenol-formaldehyde polymer, such as 2123, 2123F, and 2130.

[0035] Preferably, in the synergistic carbon source, the ratio of carbon source A, carbon source B and carbon source C is 5-7:2-4:0.5-3; more preferably 6:3:0.5-2; and even more preferably 6:3:0.7-1.

[0036] Preferably, the total mass of the synergistic carbon source is the theoretically synthesized Na4Fe 3-x M x The content of (PO4)2P2O7 is 10-30% by mass, more preferably 10-20%, even more preferably 12-18%, and most preferably 12-15%.

[0037] In this invention, the components can be mixed using existing methods to form the mixture. Preferably, the sodium source, iron source, metal M source, phosphorus source, and synergistic carbon source are subjected to wet ball milling, sand milling, and spray drying to obtain the mixture.

[0038] Preferably, the medium in the wet ball milling stage is an organic solvent; the organic solvent is a C1-C3 alcohol, acetone, etc.

[0039] Preferably, there are no special requirements for the solid content in the ball milling stage; for example, it can be 30-60%, and the ball-to-material ratio can be 5-20:1.

[0040] Preferably, the ball milling speed is not particularly important; for example, it can be 100-1000 rpm, preferably 500-800 rpm.

[0041] Preferably, the ball milling time can be adjusted according to the preparation needs, for example, 20 to 60 minutes;

[0042] Preferably, the particle size requirement in the ball-milled slurry is D50≤2μm and D100≤4μm.

[0043] Preferably, the rotation speed during the sanding stage is not particularly important; for example, it can be 1200-2500 rpm, preferably 1600-2300 rpm.

[0044] Preferably, the grinding time can be adjusted according to the preparation needs, for example, between 5 and 40 minutes;

[0045] Preferably, the particle size requirement in the slurry after sand milling is D50≤600nm and D100≤1.5μm.

[0046] In this invention, the inlet temperature of the spray drying stage is 180-270℃, the spray rate is 20-50L / h, and the carrier gas is nitrogen or argon.

[0047] Preferably, the D50 of the spray-dried particles is 10–20 μm.

[0048] In this invention, the combination of M doping, doping x control, and synergistic carbon source, along with the combined control of reduction calcination method and temperature, helps to further reduce the difficulty of synthesizing the target phase caused by doping, reduce thermodynamically stable impurities, and improve the uniformity of the coating layer and the tap density of the particles, thereby synergistically improving the electrochemical performance of the prepared material, especially its capacity and rate performance.

[0049] Preferably, the reduction calcination stage is carried out in an atmosphere containing reducing gas;

[0050] Preferably, the reducing gas is at least one of H2, CO, CH4, and C2H2, and more preferably hydrogen.

[0051] Preferably, the atmosphere during the reduction and calcination stage also includes a carrier gas, which is at least one of nitrogen and an inert gas.

[0052] Preferably, in the atmosphere of the reduction calcination stage, the volume content of reducing gas is greater than or equal to 1%, preferably 5-10%;

[0053] Preferably, the temperature of the reduction calcination stage is 480–530°C; more preferably, it is 500–520°C.

[0054] Preferably, the heating rate during the reduction calcination stage is 0.5–3 °C / min, and more preferably 1–2.5 °C / min;

[0055] Preferably, the reduction calcination time is 6-18 hours, and more preferably 10-14 hours.

[0056] A preferred embodiment of the present invention is Na4Fe 3-x M x The preparation scheme of (PO4)2P2O7 / C composite material includes the following steps: Sodium source, iron source, metal M source, phosphorus source, and synergistic carbon source are mixed uniformly by wet ball milling, followed by a second sand milling process to prepare a nano-scale slurry; subsequently, the slurry is subjected to spray drying-reduction calcination treatment to obtain Na4Fe. 3-x M x (PO4)2P2O7 / C composite material.

[0057] In this invention, the preparation method can endow the prepared material with special microscopic and chemical properties. In addition, the material prepared by the preparation method can take into account excellent phase purity, coating uniformity, encapsulation structure and material characteristics as well as tap density, and can exhibit excellent electrochemical performance, especially helping to improve high-rate and long-range cycling stability.

[0058] The present invention also provides the aforementioned Na4Fe 3-x M x The application of (PO4)2P2O7 / C composite material in the preparation of sodium-ion batteries;

[0059] Preferably, it is used as a positive electrode active material for the preparation of sodium-ion batteries.

[0060] The present invention also provides a sodium-ion battery cathode material, comprising the active material Na4Fe 3-x M x (PO4)2P2O7 / C composite material.

[0061] Preferably, the positive electrode material further comprises a binder and a conductive agent. The binder and conductive agent can be materials known in the industry; for example, the binder can be PVDF, and the conductive agent can be, for example, acetylene black. The content of each component can also be adjusted based on existing methods; for example, the binder content is, for example, 5–15 wt%; the conductive agent content is, for example, 5–15 wt%, with the balance being the active material.

[0062] The present invention also provides a sodium-ion battery cathode, which comprises the Na4Fe ...3Fe4Fe4Fe4Fe4Fe4Fe3Fe4Fe4Fe4Fe3Fe4Fe4Fe4Fe3Fe4Fe4Fe3Fe4Fe4Fe3Fe4Fe4Fe3Fe4Fe4Fe3Fe4Fe4Fe3 3-x M x (PO4)2P2O7 / C composite material for cathode.

[0063] The present invention also includes a sodium-ion battery, comprising a positive electrode current collector and a positive electrode material composited on the surface of the current collector, wherein the active material in the positive electrode material comprises the aforementioned Na4Fe 3-x M x (PO4)2P2O7 / C composite material.

[0064] This invention has the following significant features:

[0065] (1) This invention provides a novel Na4Fe 3-x M x The (PO4)2P2O7 / C composite material, based on the aforementioned coating material and the doping of the core M and the joint control of the doping content, can achieve synergy and improve the electrochemical performance of the material, such as capacity, rate capability, and long-range cycling stability.

[0066] (2) In the M described in this invention, the combination of M, particularly Ni, Co, Mn, Cu and Mg, combined with the control of the x content, can further achieve the integration of the core and coating materials, and obtain high-entropy Na4Fe. 3-x M x (PO4)2P2O7 / C composite material can make full use of the synergistic effect of various elements, improve the crystal structure of the material, diffuse the channels for sodium ion diffusion, accelerate the sodium ion transport rate, achieve high discharge specific capacity at high rate, and improve the stability of the material during long cycle.

[0067] (3) Regarding the Na4Fe described in this invention 3-x M x The preparation process of (PO4)2P2O7 / C composite materials is more prone to problems such as the difficulty in preparing impurity phases. This invention innovatively uses a combination of carbon source A, carbon source B, and carbon source C as the carbon source, and further combines the doping of the aforementioned metal M with the joint control of reduction calcination method and temperature. In this way, the different thermal behaviors of the carbon source components during the reduction calcination stage can be utilized to synergistically promote the formation of Na4Fe 3-x M x The synthesis of (PO4)2P2O7 improves phase purity, reduces thermodynamically stable impurities, improves the uniformity of carbon coating structure, regulates the encapsulation material and structure, improves tap density, and improves the electrochemical performance of the prepared material.

[0068] The preparation method described in this invention can successfully prepare high-purity, uniformly coated Na₄Fe₄ based on a single high-temperature reduction and calcination step. 3-x M x The (PO4)2P2O7 / C composite material prepared by this process has uniform spherical particles, good processing performance, and does not require pre-sintering, which can reduce energy consumption and further reduce the manufacturing cost of the material. The preparation method of this invention can effectively inhibit the growth of primary particles, achieve particle nano-sizing, shorten the ion diffusion distance, enhance the conductivity of the material, and improve the discharge specific capacity of the material.

[0069] (4) The method described in this invention is simple to operate, has a short process, and is easy to prepare industrially, and has good development prospects in the field of large-scale energy storage. Attached Figure Description

[0070] Figure 1 The image shows the XRD pattern of the material prepared in Example 1 of this invention.

[0071] Figure 2 This is a SEM image of the material prepared in Example 1 of the present invention. Detailed Implementation

[0072] Example 1

[0073] (1) Ferrous oxalate, nickel acetate, manganese acetate, cobalt acetate, copper acetate, magnesium acetate, and sodium dihydrogen phosphate were weighed in a molar ratio of 2.9:0.02:0.02:0.02:0.02:0.02:4, totaling 30 kg. The total weight of the co-existing carbon source was equal to the theoretical product (Na₄Fe₂O₃). 2.9 (NiMnCoCuMg) 0.02 The 12% of (PO4)2P2O7) was used as a co-carbon source. The mass ratio of glucose, polyethylene glycol 6000 and 2123 type phenolic resin was 6:3:0.75. 60 kg of ethanol was added as a solvent and the mixture was ball-milled for 40 min at a ball mill speed of 800 rpm. The particle size was controlled to be D50≤2μm and D100≤4μm.

[0074] (2) The slurry from step (1) was transferred to a sand mill. The sand mill speed was 1800 rpm and the time was 20 min. The particle size was controlled to be D50≤600nm and D100≤1.5μm. Then it was transferred to a spray drying process with an inlet temperature of 270℃ and a spray rate of 25L / h to prepare the precursor powder.

[0075] (3) The precursor powder from step (2) is placed in a box furnace under 5% hydrogen / nitrogen protection for high-temperature calcination at a temperature of 500℃ (T1) for 12 hours at a heating rate of 2.5℃ / min to obtain Na4Fe. 2.9 (NiMnCoCuMg) 0.02 (PO4)2P2O7 / C composite material.

[0076] Example 2

[0077] Compared with Example 1, the only difference is that the proportions of the elements in step (1) are adjusted as follows:

[0078] A: Ferrous oxalate, nickel acetate, manganese acetate, cobalt acetate, copper acetate, magnesium acetate, and sodium dihydrogen phosphate are mixed in a molar ratio of 2.95:0.01; 0.01; 0.01:0.01:0.01:4.

[0079] B: Ferrous oxalate, nickel acetate, manganese acetate, cobalt acetate, copper acetate, magnesium acetate, and sodium dihydrogen phosphate are changed to a molar ratio of 2.7:0.06:0.06:0.06:0.06:0.06:4, while other steps remain unchanged.

[0080] Example 3

[0081] Compared to Example 1, the only difference is that the raw material in step (1) is replaced with,

[0082] A: Ferrous oxalate, nickel acetate, and sodium dihydrogen phosphate are mixed in a molar ratio of 2.9:0.1:4;

[0083] B: Ferrous oxalate, cobalt acetate, and sodium dihydrogen phosphate are in a molar ratio of 2.9:0.1:4;

[0084] C: Ferrous oxalate, manganese acetate, and sodium dihydrogen phosphate are in a molar ratio of 2.9:0.1:4;

[0085] D: Ferrous oxalate, copper acetate, and sodium dihydrogen phosphate are mixed in a molar ratio of 2.9:0.1:4;

[0086] E: Ferrous oxalate, magnesium acetate, and sodium dihydrogen phosphate are mixed in a molar ratio of 2.9:0.1:4, with all other steps remaining unchanged.

[0087] Example 4

[0088] Compared with Example 1, the only difference is that the raw materials in step (1) are replaced with ferrous oxalate, calcium acetate, zinc acetate, tin oxalate, titanium oxalate, zirconium acetate and sodium dihydrogen phosphate in a molar ratio of 2.9:0.02:0.02:0.02:0.02:0.02:4, while the others remain unchanged.

[0089] Example 5

[0090] Compared to Example 1, the only difference is that the roasting temperature in step (3) is adjusted to:

[0091] A: 480℃;

[0092] B: 530℃;

[0093] C: The mass ratio of glucose, polyethylene glycol 6000, and type 2123 phenolic resin in the synergistic carbon source is 6:3:1; the total weight of the synergistic carbon source is the theoretical product (Na4Fe). 2.9 (NiMnCoCuMg) 0.02 The atmosphere in step 3 is 10% methane, which is 18% of (PO4)2P2O7.

[0094] The other steps remain unchanged.

[0095] Comparative Example 1

[0096] Compared with Example 1, the only difference is that no doped metal is added in step (1), and ferrous oxalate and sodium dihydrogen phosphate are mixed in a molar ratio of 3:4, while the other steps remain unchanged.

[0097] Comparative Example 2

[0098] Compared with Example 1, the only difference is that in step (1), the molar ratio of ferrous oxalate, nickel acetate, manganese acetate, cobalt acetate, copper acetate, magnesium acetate, and sodium dihydrogen phosphate is changed to 2.5:0.1:0.1:0.1:0.1:0.1:4, while the other steps remain unchanged.

[0099] Comparative Example 3

[0100] Compared with Example 1, the only difference is that the synergistic carbon source in step (1) is adjusted to glucose, polyethylene glycol and phenolic resin are missing, and the total amount is the same as the composite carbon source in Example 1, while the other steps remain unchanged.

[0101] Comparative Example 4

[0102] Compared with Example 1, the only difference is that the synergistic carbon source in step (1) is adjusted to glucose and polyethylene glycol, phenolic resin is missing, and the ratio and total amount of the two are the same as the composite carbon source in Example 1, while the other steps remain unchanged.

[0103] Comparative Example 5

[0104] Compared with Example 1, the only difference is that the synergistic carbon source in step (1) is adjusted to glucose, polyethylene glycol and polyvinyl chloride resin, and the proportion and total amount are the same as the composite carbon source in Example 1, while the other steps remain unchanged.

[0105] Comparative Example 6

[0106] Compared with Example 1, the only difference is that the atmosphere in step (3) is adjusted to pure argon, while the other steps remain unchanged.

[0107] Comparative Example 7

[0108] Compared with Example 1, the only difference is that the temperature in step (3) is adjusted to 650°C, while the other steps remain unchanged.

[0109] The electrical properties of sodium iron pyrophosphate prepared in each embodiment and comparative example were tested:

[0110] The main steps of the test are as follows:

[0111] (1) Using a 2032 model battery case, the positive electrode is a sodium ferrous fluorophosphate electrode, the current collector is aluminum foil, the active material (materials prepared in the examples and comparative examples) is: conductive carbon (acetylene black) : PVDF = 8:1:1, the negative electrode is sodium metal, and a fiber separator (model Whatman Grade GF / D) is used. The electrolyte is 1M NaPF6 (EC:PC = 1:1, 5% FEC) to assemble the battery.

[0112] (2) The resting time is 8 hours. Set a charge-discharge program with a rate of 50C (1C = 129mA / g) and a voltage range of 1.7V-4.3V. 2C cycle 1000 times.

[0113] (3) The theoretical specific capacity of sodium iron pyrophosphate is 129 mAh / g; the test results are shown in Table 1:

[0114] Table 1

[0115]

[0116] In summary, by using the synergistic carbon source optimization of raw material ratio described in this invention, combined with ball milling-sand milling-spray drying, it is possible to achieve compatibility between particle nano-sizing and high tap density, thereby improving the cycling stability of the material. On this basis, further control of the synergistic carbon source ratio optimization, content, sand milling process, and calcination process parameters can further synergistically improve the electrochemical performance of the material at high rates.

Claims

1. A Na₄Fe 3-x M x (PO4)2P2O7 / C composite material, characterized in that, This includes active nanoparticles and their surface coatings; The chemical formula of the active nanoparticles is Na₄Fe. 3-x M x (PO4)2P2O7, wherein M includes Ni, Co, Mn, Cu and Mg, or includes calcium, zinc, tin, titanium and zirconium; The x value is 0.03 to 0.3; x is the sum of the doping amounts of all elements in M. The coating layer comprises amorphous carbon and phenolic resin; The Na4Fe 3-x M x The preparation method of (PO4)2P2O7 / C composite material is as follows: a mixture containing sodium source, iron source, metal M source, phosphorus source and synergistic carbon source is reduced and calcined at a temperature of 450~600℃ to obtain the Na4Fe 3-x M x (PO4)2P2O7 / C composite material; The synergistic carbon sources include carbon source A, carbon source B, and carbon source C, wherein carbon source A is a polyhydroxy sugar with a molecular weight of less than or equal to 200; carbon source B is polyethylene glycol; and carbon source C is phenolic resin. In the aforementioned synergistic carbon source, the weight ratio of carbon source A, carbon source B, and carbon source C is 5~7:2~4:0.5~3.

2. The Na4Fe as described in claim 1 3-x M x (PO4)2P2O7 / C composite material, characterized in that, In M, the molar content of each element is 10~30 mol.

3. The Na4Fe as described in claim 1 3-x M x (PO4)2P2O7 / C composite material, characterized in that, The Na4Fe 3-x M x In the (PO4)2P2O7 / C composite material, the weight of active nanoparticles is 90-98%; The thickness of the coating layer is 1~10nm; The size of the active nanoparticles is 100~600 nm; The tap density is 1.1~1.6 g / cc.

4. A Na4Fe according to any one of claims 1 to 3 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by: The mixture containing sodium source, iron source, metal M source, phosphorus source and synergistic carbon source is reduced and calcined at a temperature of 450~600℃ to obtain the Na4Fe. 3-x M x (PO4)2P2O7 / C composite material; The synergistic carbon sources include carbon source A, carbon source B, and carbon source C, wherein carbon source A is a polyhydroxy sugar with a molecular weight of less than or equal to 200; carbon source B is polyethylene glycol; and carbon source C is phenolic resin. In the aforementioned synergistic carbon source, the weight ratio of carbon source A, carbon source B, and carbon source C is 5~7:2~4:0.5~3.

5. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by: The carbon source A is at least one of sucrose and glucose; The carbon source B is polyethylene glycol with a molecular weight of 2500-20000; In the aforementioned synergistic carbon source, the weight ratio of carbon source A, carbon source B, and carbon source C is 6:3:0.5~2; The total mass of the synergistic carbon source is the theoretically synthesized Na4Fe 3-x M x 10-30% of the mass of (PO4)2P2O7.

6. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The sodium source is at least one of the following: sodium carbon source, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, disodium monohydrogen phosphate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium pyrophosphate. The iron source is at least one of ferric nitrate, ferrous oxalate, ferrous acetate, ferrous sulfate, ferric oxide, ferrous oxide, and magnetite. The phosphorus source is at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen phosphate, and diammonium hydrogen phosphate. The metal source M is at least one of oxides, sulfates, nitrates, acetates, and oxalates containing M; Sodium source, iron source, M source, and phosphorus source are mixed according to the molar ratio of Na, Fe, M, and P elements of 4:3 to x:x:

4.

7. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The mixture is prepared by wet ball milling, sand milling and spray drying of sodium source, iron source, metal M source, phosphorus source and synergistic carbon source; The medium used in the wet ball milling stage is an organic solvent; The solid content in the ball milling stage is 30-60%, and the ball-to-material ratio is 5-20:1; The ball mill speed is 100~1000 rpm; The ball milling time is 20-60 minutes; The particle size requirement for the ball-milled slurry is D50≤2μm, D100≤4μm; The rotation speed during the sand milling stage is 1200~2500 rpm; The grinding time is 5 to 40 minutes; The particle size requirement for the slurry after sand milling is D50 ≤ 600 nm and D100 ≤ 1.5 μm; The inlet temperature of the spray drying stage is 180~270℃, the spray rate is 20~50L / h, and the carrier gas is nitrogen or argon.

8. The Na4Fe as described in any one of claims 4 to 7 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The reduction calcination stage is carried out in an atmosphere containing reducing gas; The reducing gas is at least one of H2, CO, CH4, and C2H2.

9. The Na4Fe as described in claim 8 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The atmosphere during the reduction and calcination stage also includes a carrier gas, which is at least one of nitrogen and an inert gas. In the atmosphere of the reduction calcination stage, the volume content of reducing gas is greater than or equal to 1%.

10. The Na4Fe as described in claim 9 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, In the atmosphere of the reduction calcination stage, the volume content of reducing gas is 5-10%.

11. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The temperature during the reduction calcination stage is 480~530℃.

12. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The heating rate during the reduction calcination stage is 0.5~3℃ / min.

13. The Na4Fe as described in claim 4 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The reduction calcination time is 6~18h.

14. The Na4Fe as described in claim 13 3-x M x The method for preparing (PO4)2P2O7 / C composite material is characterized by, The reduction calcination time is 10~14h.

15. A Na₄Fe according to any one of claims 1 to 3 3-x M x (PO4)2P2O7 / C composite material or Na4Fe prepared by the preparation method according to any one of claims 4 to 14 3-x M x The application of (PO4)2P2O7 / C composite material is characterized by, Used in the manufacture of sodium-ion batteries.

16. The application as described in claim 15, characterized in that, It is used as a positive electrode active material in the preparation of sodium-ion batteries.

17. The application as described in claim 15, characterized in that, The positive electrode is prepared by using it as the positive electrode active material, mixing it with a conductive agent and a binder to form a slurry, coating it onto the positive electrode current collector, and then drying and curing it.

18. A positive electrode material for a sodium-ion battery, characterized in that, Contains Na4Fe as described in any one of claims 1 to 3 3- x M x (PO4)2P2O7 / C composite material or Na4Fe prepared by the preparation method according to any one of claims 4 to 14 3-x M x (PO4)2P2O7 / C composite material.

19. The positive electrode material of the sodium-ion battery as described in claim 18, characterized in that, It also contains conductive agents and adhesives; The Na4Fe 3-x M x The content of (PO4)2P2O7 / C composite material is greater than or equal to 60wt%.

20. The positive electrode material of the sodium-ion battery as described in claim 19, characterized in that, The Na4Fe 3-x M x The content of (PO4)2P2O7 / C composite material is 70~90wt%.

21. A sodium-ion battery, characterized in that, It includes the cathode material as described in any one of claims 18 to 20.

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