Phosphate composite cathode material, preparation and application thereof in sodium ion battery

Abstract

This invention belongs to the field of battery materials, specifically relating to a Na a Fe b (PO4) c X d The preparation method of @C composite material includes a carbon source and synthetic Na a Fe b (PO4) c X d The precursor materials are mixed to obtain a mixture; the mixture is preheated to temperature T in a flowing atmosphere, and then held at that temperature under pressure P to obtain the Na. a Fe b (PO4) c X d @C composite material; the temperature T is 480-620℃, and the pressure P is greater than 0.1MPa and less than or equal to 0.5MPa; the Na a Fe b (PO4) c X d In the above, X is F ‑ P2O7 4‑ One or more of the following: a = 0 < a < 6; b = 0 < b < 5; c = 0 < c < 4; d = 0 < c < 4. This invention also includes materials prepared by the described method and their applications. The process of this invention can effectively improve the interfacial bonding effect of the prepared materials, and can further improve their capacity, long-range cycling performance, and low-temperature stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery materials, and particularly relates to the technical field of iron-based phosphate positive electrode active materials of sodium ion batteries. BACKGROUND

[0002] In modern society, lithium ion batteries are widely used in the 3C field, new energy vehicles and energy storage fields. However, although the existing commercial secondary batteries are mainly lithium secondary batteries, sodium ion batteries similar in principle have not been widely commercially used. The main difficulty lies in that the radius difference between lithium ions and sodium ions is large, which makes it difficult to directly apply the technology suitable for lithium ion batteries to sodium ion batteries. Among them, the positive electrode material plays a leading role.

[0003] Among the many positive electrode systems of sodium ion batteries, iron-based phosphates have a stable open three-dimensional framework, a suitable working voltage, and are rich in resources and environmentally friendly for large-scale energy storage, and are the best choice for commercial sodium ion battery positive electrode materials.

[0004] However, iron-based polyanion materials, especially iron phosphate materials combined with F and pyrophosphate anions, often have lower electrical conductivity, and the preparation process is prone to the problem of impurity phases caused by the combined anions. Therefore, it is necessary to sinter at a lower temperature. However, sintering at a lower temperature makes it difficult to carbonize the carbon source to form high-conductivity carbon, so that it cannot meet the requirements of low impurity phase and high conductivity for preparation, which limits the electrochemical performance of the material.

[0005] In view of the existing literature of the existing process, the prior art also provides some solutions, for example, the Chinese patent document with publication number CN114906832A discloses a preparation method of sodium iron fluorophosphate positive electrode material, which uses sodium ion battery material precursor as raw material and adopts solid phase synthesis method to perform heat treatment under inert atmosphere protection. Again, the Chinese patent document with publication number CN114242968A discloses a carbon-coated sodium iron fluorophosphate material prepared by calcining a mixture precursor after secondary ball milling and tabletting.

[0006] Although the prior art provides some improvement ideas, it is still difficult to effectively balance the problems of impurity phase, crystal interface combination and low carbon conductivity. The electrochemical performance of the material, especially the capacity, long cycle and stability such as low temperature stability, still has a large space for improvement. SUMMARY

[0007] In view of the deficiencies of the prior art, the present application provides a phosphate composite positive electrode material (the present application mainly refers to Na a Fe b (PO4) c X dThe application relates to a preparation method of Na

[0008] The second object of the application is to provide the Na a Fe b (PO4) c X d C composite material and application thereof in a sodium ion battery.

[0009] The third object of the application is to provide a sodium ion battery comprising the Na a Fe b (PO4) c X d C composite material, a positive electrode and a positive electrode material thereof.

[0010] The fourth object of the application is to provide a sodium ion battery comprising the Na a Fe b (PO4) c X d C composite material.

[0011] The application provides a preparation method of a Na a Fe b (PO4) c X d C composite material, which comprises the following steps: mixing a carbon source and a precursor raw material for synthesizing Na a Fe b (PO4) c X d to obtain a mixture;

[0012] The mixture is preheated to a temperature T in a flowing atmosphere, and then is subjected to a heat preservation treatment under a pressure P to obtain the Na a Fe b (PO4) c X d C composite material.

[0013] The temperature T is 480-620 DEG C, and the pressure P is greater than 0.1 MPa and less than or equal to 0.5 MPa.

[0014] The Na a Fe b (PO4) cX d X is F - , P2O7 4- , or two or more of them; a is in the range of 0 < a < 6; b is in the range of 0 < b < 5; c is in the range of 0 < c < 4; and d is in the range of 0 < c < 4.

[0015] Na a Fe b (PO4) c X d The preparation process of the material faces the problems of impurity phase and unsatisfactory carbon-grain interface composite effect, the application innovatively preheats in a flowing atmosphere, and performs a holding treatment under the combined action of the temperature T and the pressure P, so that the synergy can be unexpectedly realized, Na a Fe b (PO4) c X d impurity phase can be avoided, in addition, the interface composite effect between the crystal and the carbon can be improved, and thus excellent electrochemical performance can be obtained under a low amount of conductive carbon, and better capacity, long cycle and low-temperature cycle stability can be obtained.

[0016] In the application, the precursor raw material includes Na a Fe b (PO4) c X d in a stoichiometric molar ratio of Na source, Fe source, phosphorus source and X source;

[0017] The Na source is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, sodium phosphate, and sodium hydroxide;

[0018] The Fe source is at least one of iron phosphate, iron oxalate, iron nitrate, iron powder, iron oxide, and ferrous sulfate;

[0019] The phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and iron phosphate;

[0020] The X source is a compound capable of providing X, and is preferably at least one of the phosphorus source, ammonium fluoride, sodium fluoride, and sodium pyrophosphate;

[0021] In the application, the Na a Fe b (PO4) c X d In the application, the ratio between the elements can be adjusted according to the chemical charge balance mode. In the application, as an exemplary embodiment, the X is P2O7 4-At this time, the a can be 3-5, the b can be 2-4, the c can be 1-3, the d can be 1-2, and further, the a:b:c:d can be 4:3:2:1. In addition, as another exemplary embodiment, the X is F, the a can be 1-3, the b can be 1-2, the c can be 1-2, the d can be 1-2, and further, the a:b:c:d can be 2:1:1:1.

[0022] In the present application, the carbon source is water-soluble organic matter, and is further preferably at least one of glucose, sucrose, citric acid, starch, ascorbic acid, cyclodextrin, and polyethylene glycol.

[0023] Preferably, the carbon source is at least one of glucose and starch, and is further preferably 1-3:1-3 of glucose and starch.

[0024] Preferably, the weight ratio of the carbon source to the precursor raw material is 1:5-30, and can be further 1:5-20 in consideration of material cost, and is further preferably 1:6-12.

[0025] In the present application, the carbon source and the precursor raw material can be mixed based on the existing conventional process, for example, the mixing method is dry or wet-dry method.

[0026] For example, the dry method is stirring or dry ball milling.

[0027] The wet method in the wet-dry method is wet ball milling-sand milling, and the dry method is spray drying. The slurry obtained by sand milling has a solid content of 30%-50% and a particle size D50 of 0.2-0.4 μm.

[0028] In the present application, the mixture is placed in an atmosphere furnace, heated to a temperature T in a flowing atmosphere, then pressurized to P using the atmosphere, and treated under the conditions of the temperature T and the pressure P. This unexpectedly improves the phase purity and the interface composite effect between the crystal and the carbon, thereby improving the performance. In the process of the present application, the mixture is placed in an atmosphere furnace, and during the heating to the temperature T, the atmosphere is continuously input into the system through the inlet hole and continuously output from the system through the outlet hole, forming a flowing gas stream. When the temperature reaches T, the outlet is closed, and the atmosphere is input through the inlet hole until the pressure reaches P. The system is then sealed and treated under the conditions of P and T.

[0029] In the present application, the atmosphere during the heating and the temperature maintaining process is a protective atmosphere, and is preferably at least one of nitrogen and inert gas.

[0030] In the present application, the heating rate of the heating stage is 1-5℃ / min, preferably 2-3℃ / min.

[0031] In the present application, the atmosphere flow rate of the heating stage is 1000-2000ml / min.

[0032] In the present application, the temperature T is preferably 500-600℃, further preferably 540-560℃.

[0033] In the present application, the pressure P is preferably 0.15-0.45MPa.

[0034] In the present application, the holding treatment time at temperature T and P can be 5-20h, and considering the treatment efficiency, it can be further 6-12h.

[0035] In the present application, the holding process includes two-stage gradient pressure holding processes, wherein the pressure P1 of the first-stage holding process is 0.15-0.25MPa; the P2 of the second-stage holding process is 1.2-3 times of P1, and is specifically preferably 0.3-0.45MPa. The time of the first-stage holding process can be, for example, 2-5h, and can be further 3-4h; the time of the second-stage holding process can be, for example, 3-10h, and can be further 5-6h.

[0036] In the present application, by controlling the pressure values of the two stages, the crystal density and the carbon layer uniformity are unexpectedly synergistically affected, and the uniform and dense distribution of the carbon layer on the material surface can be achieved.

[0037] Preferably, after the holding treatment, airflow breaking treatment is carried out, and the D50 of the broken particles is controlled to be 3-6μm.

[0038] The present application also provides a Na a Fe b (PO4) c X d @C composite material prepared by the preparation method.

[0039] In the present application, due to the preparation method, the prepared material can be endowed with special micro characteristics, and the material prepared by the preparation method has excellent performance, which can have low conductive carbon content, excellent capacity, long cycle and low temperature stability.

[0040] The present application also provides a Na a Fe b (PO4) c X d @C composite material prepared by the preparation method.

[0041] In the present application, the Na a Fe b (PO4) c X d @C composite material can be prepared based on known processes.

[0042] The present application also provides a positive electrode material of a sodium ion battery, comprising a positive electrode active material, wherein the positive electrode material comprises the Na a Fe b (PO4) c X d @C composite material prepared by the preparation method of the present application.

[0043] In the present application, the content of the Na a Fe b (PO4) c X d @C composite material in the positive electrode active material is more than 10 wt.%, further more than 50 wt.%, and further more than 90 wt.%.

[0044] In the present application, the positive electrode material can further comprise a binder.

[0045] In the present application, the binder is at least one of PVDF and CMC.

[0046] In the present application, the positive electrode material can further comprise a conductive agent, and the conductive agent is preferably at least one of acetylene black, ketjen black and Super P.

[0047] In the present application, the content of the binder in the positive electrode material is 1-15 wt.%, the content of the conductive agent is less than 15 wt.%, further less than 5 wt.%, and more preferably 2-4 wt.%. In the present application, the material prepared by the preparation method has excellent performance, and can exhibit excellent capacity, long cycle and low temperature stability at a low content of conductive carbon.

[0048] The present application also provides a positive electrode of a sodium ion battery, comprising a current collector and a positive electrode material coated on the surface of the current collector.

[0049] The present application also provides a sodium ion battery, wherein the positive electrode of the sodium ion battery comprises the Na a Fe b (PO4) c X d @C composite material.

[0050] The sodium ion battery described in the application has the Na a Fe b (PO4) c X d In addition to the Na

[0051] Beneficial effects

[0052] The application innovatively preheats in a circulating atmosphere, and performs a holding treatment under the combined action of the temperature T and the pressure P, so that synergy can be unexpectedly achieved, and the Na a Fe b (PO4) c X d Heterogeneous phase, in addition, the interface composite effect between the crystal and the carbon can be improved, and thus excellent electrochemical performance can be obtained at a low content of the conductive carbon, and better capacity, long cycle and low-temperature cycle stability can be obtained.

[0053] In the application, the type of the carbon source is controlled by the optimized holding treatment mechanism, so that synergy can be further achieved, and the Na a Fe b (PO4) c X d The interface adaptation effect of the crystal grain, the crystal and the carbon can further exhibit excellent capacity, long cycle and low-temperature cycle stability at a low composite ratio of the conductive carbon. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 The first circle charge-discharge curve of the sodium iron pyrophosphate phosphate prepared in Example 1; DETAILED DESCRIPTION

[0055] Since the poly-anion positive electrode material type involved in the application is more, in order to effectively illustrate the technical effects of the application, the Na4Fe3(PO4)2P2O7 is selected for comparison. However, the selection of the material type in this embodiment is not limited to the iron-based phosphate material, and the above-mentioned sodium ion battery positive electrode material or the positive electrode material meeting the general formula requirement also has similar performance, and also belongs to the protection scope of the application.

[0056] A preparation method of the application, for example, includes the following steps:

[0057] (1) Mixing: the sodium source, the iron source, the phosphorus source and the carbon source are weighed and mixed according to the stoichiometric ratio Na a Fe b (PO4) c X d The raw materials are weighed and uniformly mixed;

[0058] (2) sand grinding: adding an appropriate amount of solvent to the initial mixture obtained in step (1) to control the solid content of the slurry to be 30%-50%, and then sand grinding to a slurry particle size D50 of 0.2-0.4 μm;

[0059] (3) spray drying: spray drying the slurry obtained in step (2) to obtain a precursor;

[0060] (4) sintering: heating the precursor obtained in step (3) to T in a flowing protective atmosphere, then closing the gas outlet, and controlling the pressure of the system to be P, and then heat treatment under the combined action of the temperature T and the pressure P;

[0061] (5) crushing: air flow crushing the material obtained in step (4) to control D50 to be 3-6 μm;

[0062] Further, the sodium source is at least one of sodium carbonate, sodium bicarbonate, an organic acid salt of sodium, sodium phosphate, and sodium hydroxide;

[0063] Further, the iron source is at least one of ferric phosphate, ferrous oxalate, ferric nitrate, ferrous sulfate, iron powder, and iron oxide;

[0064] Further, the phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and ferric phosphate;

[0065] Further, the carbon source is at least one of water-soluble organic matter, and further preferably glucose, citric acid, starch, ascorbic acid, cyclodextrin, and polyethylene glycol;

[0066] Further, in step (1), the mixing is assisted by ball milling or sand milling, and the particle size is controlled to be 1-2 μm;

[0067] Further, in step (2), the sand grinding speed is controlled to be 1500-2000 rpm, the time is controlled to be 0.5-3 h, and the particle size is controlled to be 0.2-0.3 μm;

[0068] Further, in step (3), the spray drying temperature is controlled to be greater than 70°C at the outlet;

[0069] The following are specific embodiments:

[0070] Example 1

[0071] The specific material of this example is Na4Fe3(PO4)2P2O7, and the preparation process is as follows:

[0072] S1: The precursor raw materials of iron phosphate (10.2 g), sodium acetate, and ammonium dihydrogen phosphate were weighed according to the stoichiometric molar ratio, and were dispersed in water (liquid-solid ratio, for example, 5 ml / g) together with glucose (organic carbon source, organic carbon source: precursor raw material, mixed in a weight ratio of 1:8) to obtain a suspension A by stirring;

[0073] S2: The suspension A was transferred to a sand mill, and the particle size was controlled to D50 < 300 nm after sand milling at a speed of 2000 rpm for 1 h;

[0074] S3: The precursor solution of S2 was subjected to spray drying, the outlet temperature was controlled to 100°C, and the flow rate was 35 ml / min; and spherical precursors were obtained;

[0075] S4: The precursors were transferred to a box furnace, and were heated from room temperature to 500°C (marked as temperature T) under flowing Ar (flow rate of 1500 ml / min); then the exhaust port was closed, the pressure of the system was controlled to 0.2 MPa (marked as P1) by Ar, and the heat preservation treatment was carried out at this pressure and temperature for 4 h; then the pressure of the system was controlled to 0.3 MPa (marked as P2) by Ar, and the heat preservation treatment was carried out at this pressure and temperature for 6 h;

[0076] S5: The material obtained in S4 was subjected to airflow crushing, and the D50 was controlled to be less than 2 μm.

[0077] Example 2

[0078] Compared with Example 1, the only difference is that in step S4, the temperature T is 580°C, the pressure P1 is 0.15 MPa, and the pressure P2 is 0.45, and the other conditions are not changed;

[0079] Example 3

[0080] Compared with Example 1, the only difference is that in step S4, a single-stage heat preservation process is used, and the experimental groups are as follows:

[0081] Group A: The heat preservation of P2 section is omitted, and only the heat preservation treatment under T / P1 conditions is carried out, and the time t1 is the same as the total time of t1 and t2 in Example 1, that is, the heat preservation time under T / P1 in the single-stage heat preservation process is 10 h.

[0082] Group B: The heat preservation of P1 section is omitted, and only the heat preservation treatment under T / P2 conditions is carried out, and the time t2 is the same as the total time of t1 and t2 in Example 1, that is, the heat preservation time under T / P1 in the single-stage heat preservation process is 10 h.

[0083] Example 4

[0084] The difference between Example 1 and the present example is that the type of carbon source is changed, and the experimental groups are as follows:

[0085] A: the organic carbon source is sucrose

[0086] B: the organic carbon source is ascorbic acid

[0087] C: the organic carbon source is a mixture of glucose and sucrose in a weight ratio of 1:1, and the total weight of the organic carbon source is the same as in Example 1.

[0088] Example 5

[0089] The difference between Example 1 and the present example is that in S4, the flow rate of Ar during the temperature rising stage is 1000 ml / min, and the temperature is raised from room temperature to 550°C (marked as temperature T) at a rate of 2° / min; then the exhaust port is closed, and the pressure of the system is controlled to 0.25 MPa (marked as P1) by Ar, and the system is kept at this pressure and temperature for 3 h; then the pressure of the system is controlled to 0.35 MPa (marked as P2) by Ar, and the system is kept at this pressure and temperature for 5 h.

[0090] Example 6 (X is F)

[0091] The specific material of the present example is Na2FePO4F, and the preparation process is as follows:

[0092] S1: the precursor raw materials of iron phosphate (18 g), sodium fluoride, and sodium bicarbonate are weighed according to the stoichiometric ratio, and then the organic carbon source (glucose, the weight ratio of the precursor raw materials to the organic carbon source is 1:10) is added and dispersed in water (the liquid-solid ratio is, for example, 5 ml / g), and a suspension A is obtained by stirring;

[0093] S2: the suspension A is transferred to a sand mill, and the rotation speed is controlled to 2000 rpm; after sand milling for 1 h, the particle size is controlled to D50<300 nm;

[0094] S3: the precursor solution of S2 is subjected to spray drying, the outlet temperature is controlled to 100°C, and the flow rate is controlled to 35 ml / min; and a spherical precursor is obtained;

[0095] S4: the precursor is transferred to a box furnace, and the temperature is raised from room temperature to 580°C (marked as temperature T) under flowing Ar (the flow rate is 1200 ml / min) at a rate of 3° / min; then the exhaust port is closed, and the pressure of the system is controlled to 0.2 MPa (marked as P) by Ar, and the system is kept at this pressure and temperature for 6 h;

[0096] S5: the material obtained in S4 is subjected to gas flow crushing, and the D50 is controlled to be less than 2 μm.

[0097] Comparative Example 1

[0098] Compared with Example 1, the only difference is that step S4 remains normal sintering, without adjusting the pressure in the furnace, i.e. P1 / P2 is atmospheric pressure, and other steps remain unchanged.

[0099] Comparative Example 2

[0100] Compared with Example 1, the only difference is that step S4 uses negative pressure sintering, i.e. P1 / P2 in the soaking sintering stage is set to 0.01 MPa, and other steps remain unchanged.

[0101] Comparative Example 3

[0102] Compared with Example 1, the only difference is that step S4 uses high pressure sintering, i.e. P1 / P2 in the soaking sintering stage is set to 1 MPa, and other steps remain unchanged.

[0103] Comparative Example 4

[0104] Compared with Example 1, the only difference is that a solid-phase pressing forming process is used, i.e. the mixture in S1 is pressed into a solid under a mechanical pressure of 20 MPa, and then directly subjected to a soaking treatment at T for 10 h, without a pressure P adjustment process in the soaking sintering stage, and other steps remain unchanged.

[0105] Comparative Example 5

[0106] Compared with Example 1, the only difference is that in step S4, the process of heating to T is not treated in a flowing atmosphere, but the outlet of the closed reaction furnace is controlled, and the pressure in the heating process is controlled at P1, and other operations and parameters are the same as in Example 1.

[0107] Comparative Example 6

[0108] Compared with Example 1, the only difference is that in step S4, the temperature T is 680°C, and other operations and parameters are the same as in Example 1.

[0109] The positive electrode materials prepared in each example and comparative example are subjected to electrical performance testing:

[0110] The main steps of the test are:

[0111] (1) Using a 2032 type battery shell, the positive electrode is the prepared positive electrode material tab, the current collector is aluminum foil, the active material (the material finally prepared in each example and comparative example) is: conductive carbon (acetylene black): PVDF = 94:3:3, the negative electrode is sodium metal, a fiber separator (Whatman Grade GF / D) is used, and the electrolyte is 1M NaPF6 (pure PC + 5% FEC) to assemble the battery.

[0112] (2) setting a 3C rate charging program, a voltage interval of 2.0V-4.0V, a cycle temperature of room temperature, and a cycle of 500 times, with a 12h standing time;

[0113] The test results are shown in Table 1:

[0114]

[0115]

[0116] In conclusion, the process of the application can achieve good technical effects, and can improve the long-range cycle and low-temperature performance of the prepared material.

Claims

1. A Na a Fe b (PO4) c X d @C composite material, characterized by, mixing a precursor raw material containing a carbon source and synthetic Na a Fe b (PO4) c X d to obtain a mixture; The mixture is preheated to a temperature T in a flowing atmosphere and then subjected to a pressure treatment at a pressure P to produce the Na a Fe b (PO4) c X d @C composite The temperature T is 480-620℃, the pressure P is greater than 0.1 MPa and less than or equal to 0.5 MPa; Na a Fe b (PO4) c X d wherein X is F - , P2O7 4- , or both; a has a value in the range 0 < a < 6; b has a value in the range 0 < b < 5; c has a value in the range 0 < c < 4; and d has a value in the range 0 < c < 4.

2. The Na of claim 1, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by, The precursor raw materials include Na a Fe b (PO4) c X d The stoichiometric molar ratio of the Na source, the Fe source, the phosphorus source, and the X source is Na:Fe:(PO4):X. The Na source is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, sodium phosphate, and sodium hydroxide; The Fe source is at least one of ferric phosphate, ferric oxalate, ferric nitrate, iron powder, iron oxide, and ferrous sulfate; The P source is at least one of phosphoric acid, sodium phosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and ferric phosphate; The X source is a compound capable of providing X.

3. The Na of claim 2, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by The X source is at least one of the P source, ammonium fluoride, sodium fluoride, and sodium pyrophosphate.

4. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by The carbon source is a water-soluble organic substance.

5. The Na of claim 4, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by The carbon source is at least one of glucose, sucrose, citric acid, starch, ascorbic acid, cyclodextrin, and polyethylene glycol.

6. The Na of claim 4 a Fe b (PO4) c X d A method for producing a composite material, characterized by, The weight ratio of the carbon source to the precursor raw material is 1:5-30.

7. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by The mixing method is dry mixing or wet-dry mixing; The dry mixing is stirring or dry ball milling; The wet method in the wet-dry mixing is wet ball milling-sand milling, and the drying method is spray drying; The solid content of the slurry obtained by sand milling is 30%-50%, and the particle size D50 is 0.2-0.4 μm.

8. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by, The atmosphere during the heating and holding processes is a protective atmosphere.

9. The Na of claim 8, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by The atmosphere during the heating and holding processes is at least one of nitrogen and inert gas.

10. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by, The heating rate in the heating stage is 1-5℃ / min.

11. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by, The atmosphere flow rate in the heating stage is 1000-2000 ml / min.

12. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by, The temperature T is 500-600℃.

13. The Na of claim 12, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by The temperature T is 540-560℃.

14. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material of a carbon fiber and a metal, characterized by The pressure P is 0.15-0.45 MPa.

15. The Na of claim 12 a Fe b (PO4) c X d A method for producing a composite material of a carbon fiber-reinforced resin, characterized by The holding process includes two-stage gradient pressure holding processes, wherein the pressure P1 of the first-stage holding process is 0.15-0.25 MPa, and the P2 of the second-stage holding process is 1.2-3 times of P1.

16. The Na of claim 15, wherein the Na is a Na a Fe b (PO4) c X d A method for producing a composite material, characterized by, The P2 of the second-stage holding process is 0.3-0.45 MPa.

17. The Na of claim 1 wherein a Fe b (PO4) c X d A method for producing a composite material, characterized by, After the holding treatment, airflow breaking treatment is performed to control the D50 of the broken particles to be 3-6 μm.

18. Na@C composite prepared by the method of any one of claims 1-17. a Fe b (PO4) c X d @C composite.

19. Na@C composite prepared by the method of any one of claims 1 to 17. a Fe b (PO4) c X d use of Na@C composite, characterized in that It is used as a positive electrode active material for preparing a sodium ion battery.

20. A cathode material for a sodium-ion battery, comprising a cathode active material, characterized in that, The positive electrode material comprises Na a Fe b (PO4) c X d @C composite.

21. The cathode material of the sodium-ion battery of claim 20, wherein, The positive electrode active material, wherein the Na a Fe b (PO4) c X d The content of the @C composite material is more than 10 wt.%.

22. The positive electrode material of the sodium-ion battery of claim 21, wherein, In the positive electrode active material, the Na a Fe b (PO4) c X d @C composite material content is more than 50 wt.%.

23. The positive electrode material of the sodium-ion battery of claim 22, wherein, In the positive electrode active material, the Na a Fe b (PO4) c X d @C composite material content is more than 90 wt.%.

24. The cathode material of the sodium-ion battery of claim 20, wherein, The positive electrode material further contains a binder.

25. The positive electrode material of the sodium-ion battery of claim 24, wherein, The binder is at least one of PVDF and CMC.

26. The cathode material of the sodium-ion battery of claim 24, wherein, The positive electrode material further contains a conductive agent.

27. The positive electrode material of the sodium-ion battery of claim 26, wherein, The conductive agent is at least one of acetylene black, ketjen black, and SP.

28. The positive electrode material of the sodium-ion battery of claim 26, wherein, In the positive electrode material, the content of the binder is 1-15 wt.%, and the content of the conductive agent is less than 15 wt.%.

29. A sodium-ion battery, characterized in that, The sodium ion battery includes the Na a Fe b (PO4) c X d @C composite.

30. The sodium-ion battery of claim 29, wherein, The positive electrode contains the positive electrode material of any one of claims 21-28.

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