Soft and hard carbon composite porous negative electrode material prepared through molten salt coupling calcination method regulation and control, and preparation method and application of soft and hard carbon composite porous negative electrode material

Abstract

The invention discloses a soft and hard carbon composite porous negative electrode material prepared through molten salt coupling calcination method regulation and control, and a preparation method and application thereof, and the preparation method comprises the following steps: (1) uniformly mixing polyvinyl chloride particles, alkaline lignin and anhydrous salt to prepare a mixed material; (2) placing the mixture in an inert gas atmosphere for fused salt calcination treatment; and (3) carrying out desalting, suction filtration and drying on the product, then carrying out secondary high-temperature calcination, and finally filtering out through a screen to obtain the soft and hard carbon composite porous negative electrode material. Wherein the mass ratio of the polyvinyl chloride particles to the alkaline lignin is (2-6): 1; the anhydrous salt is composed of one or more salts, and the mass of the anhydrous salt accounts for 5-20% of the total mass of the polyvinyl chloride and the alkaline lignin. The negative electrode material prepared by the technical scheme has rich micro / mesoporous structure and ordered graphite domain structure characteristics, the sodium ion storage capacity is effectively improved, and effective regulation and control on the pore structure of the carbon material are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy storage carbon materials, and in particular to a soft and hard carbon composite porous negative electrode material prepared by a molten salt coupled calcination method, and a preparation method and application thereof. Background Art

[0002] In recent years, with the rapid development of industries such as electric vehicles, smart grids, and mobile devices, the production scale of lithium-ion batteries has expanded dramatically, resulting in an increase in the consumption of lithium ore resources. Sodium-ion batteries with high specific energy and long life, which have cost advantages, have become the new favorite and important development direction of battery research. However, the development and widespread application of high-performance sodium-ion batteries are much slower than those of lithium-ion batteries, mainly because Na + The radius of the ion is Li + ions are large, resulting in Na + Migration is slow, so designing and developing suitable sodium-ion battery electrode materials, especially suitable sodium-ion battery anode materials, is more challenging than lithium-ion battery electrode materials. In short, developing anode materials with good cycle performance and rate performance is crucial for the practical application of sodium-ion batteries.

[0003] Carbon-based materials have become the preferred target for the study of sodium storage anode materials because of their wide sources, rich resources, diverse structures and long life. Among them, graphite materials, as anode materials for sodium ion batteries, are difficult to form sodium-inserted graphite compounds. + can be stored in graphite, resulting in a battery of only about 30 mA hg -1 , the reversible capacity of the battery is suppressed. Compared with graphite, non-graphitizable hard carbon and graphitizable soft carbon have higher capacity as negative electrode materials for sodium ion batteries, which has attracted widespread attention from researchers. Among them, hard carbon materials have the advantages of high sodium storage capacity, low sodium storage voltage, high mechanical strength, and stable structure, but their cost is high and the carbon yield of precursors is low. The precursor cost of soft carbon is low, and it has a good pore structure, that is, a more ordered structure, fewer defects and shorter interlayer spacing. It also has a higher specific surface area, but its sodium storage capacity is low and its mechanical strength is low. In view of the respective advantages of hard carbon and soft carbon, the two are combined. By adjusting the ratio, structure and distribution of hard and soft carbon, the performance of the material can be precisely controlled, which provides a good strategy for the development of low-cost, high-performance carbon-based negative electrode materials. The pore structure of carbon materials, such as pore size and distribution, has a very important influence on sodium storage behavior. It is of great significance to achieve further precise control of the pore structure of carbon materials and construct hard carbon materials with reasonable pore size distribution, moderate specific surface area and stable interface. Summary of the invention

[0004] In order to solve the above-mentioned technical problems, the purpose of the present invention is to provide a soft and hard carbon composite porous negative electrode material prepared by a molten salt coupled calcination method and its preparation method and application, the negative electrode material has rich micro / mesoporous structure and ordered graphite-like domain structure characteristics, effectively improves the sodium ion storage capacity, and realizes effective regulation of the pore structure of the carbon material.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0006] A method for preparing a soft and hard carbon composite porous negative electrode material by regulating and preparing the material through a molten salt coupled calcination method, comprising the following steps:

[0007] (1) uniformly mixing polyvinyl chloride particles, alkaline lignin and anhydrous salt to prepare a mixture;

[0008] (2) placing the above-mentioned mixture in an inert gas atmosphere for molten salt calcination treatment;

[0009] (3) desalting, filtering, drying, and calcining the product at a high temperature for a second time, and finally filtering the product through a sieve to obtain a soft and hard carbon composite porous negative electrode material;

[0010] The mass ratio of polyvinyl chloride particles to alkaline lignin is 2 to 6:1; the anhydrous salt is composed of one or more salts, and its mass accounts for 5% to 20% of the total mass of polyvinyl chloride and alkaline lignin.

[0011] Preferably, in the anhydrous salt, the cation is one or more of an alkali metal element, an alkaline earth metal element and Ni, and the anion is a halogen, NO3 - PO3 3- and SO4 2- One or more of .

[0012] Preferably, the polyvinyl chloride particles are white particles with a degree of polymerization of 68-78 obtained after processing of waste materials.

[0013] Preferably, in step (2), the heating rate is 5 to 10 °C min -1 The calcination temperature is in the range of 500-700°C and the holding time is 1-3h.

[0014] Preferably, in step (3), the material after preliminary carbonization is taken out, washed with deionized water to remove salt, filtered and placed in a drying oven to be completely dried; the dried material is placed in a magnetic boat, and then placed in a tubular furnace again, and subjected to secondary high-temperature calcination under the protection of an argon atmosphere, and then cooled to room temperature with the furnace and taken out; the above material is ball-milled into powder, and filtered out with a 50-200 mesh sieve to obtain a soft and hard carbon composite porous negative electrode material.

[0015] Preferably, in step (3), deionized water in an amount of 10-20 times the volume of the material after preliminary carbonization is added to precipitate anhydrous salt.

[0016] Preferably, in step (3), the heating rate of the secondary high temperature calcination is 5 to 10°C min -1 The calcination temperature is in the range of 1300-1700°C, and the holding time is 1-3h.

[0017] Preferably, in step (3), the ball milling time is 4 to 6 hours and the rotation speed is 500 rpm. -1 .

[0018] A soft and hard carbon composite porous negative electrode material is prepared by regulating the molten salt coupled calcination method, and is prepared by the preparation method as described above.

[0019] A method for preparing soft and hard carbon composite porous negative electrode materials by molten salt coupled calcination method as above and applying the same in negative electrode materials for sodium ion batteries.

[0020] The invention adopts the above technical solution, and has the following beneficial effects:

[0021] (1) The soft and hard carbon composite porous negative electrode material prepared by the present invention has rich micro / mesoporous structure and ordered graphite-like domain structure characteristics. The sodium ion button battery assembled therefrom exhibits a high initial discharge capacity, and the first discharge capacity can reach 421.34 mAh g -1 After 700 cycles, its reversible capacity can still reach 122.32 mAh g -1 ;

[0022] (2) In the present invention, the pore size of the soft and hard carbon composite porous negative electrode material is regulated at the nanometer level by mixing waste plastic polyvinyl chloride, biomass alkaline lignin and anhydrous salt and controlling the ratio of the three, the types of anhydrous salt and the ratio between the anhydrous salts;

[0023] (3) In the present invention, waste plastic polyvinyl chloride is reused. The preparation method of the soft and hard carbon composite porous negative electrode material provided by this scheme is simple and feasible, the raw materials can be selected from a wide range of sources, the raw material cost is low, and it is easy to realize industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart for preparing the soft and hard carbon composite porous negative electrode material in Example 1 of the present invention;

[0025] Figure 2 The scanning electron microscope (SEM) images of the material samples of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention are shown;

[0026] Figure 3 It is a transmission electron microscope (TEM) image of the material samples of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention;

[0027] Figure 4 The charge and discharge curve diagram of the sodium ion battery assembled with the material sample in Example 1 of the present invention;

[0028] Figure 5 This is a graph of cycle life and coulombic efficiency of a sodium ion battery assembled from the material samples in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further elaborated in detail below. The parts not described and disclosed in detail in the following embodiments of the present invention should be understood as the prior art known or should be known to those skilled in the art.

[0030] The present invention discloses a method for preparing a soft and hard carbon composite porous negative electrode material by controlling a molten salt coupled calcination method, comprising the following steps:

[0031] (1) uniformly mixing polyvinyl chloride particles, alkaline lignin and anhydrous salt to prepare a mixture;

[0032] (2) placing the above-mentioned mixture in an inert gas atmosphere for molten salt calcination treatment;

[0033] (3) desalting, filtering, drying, and calcining the product at a high temperature for a second time, and finally filtering the product through a sieve to obtain a soft and hard carbon composite porous negative electrode material;

[0034] The mass ratio of polyvinyl chloride particles to alkaline lignin is 2 to 6:1; the anhydrous salt is composed of one or more salts, and the mass ratio of the salts is 100:5.26 to 25 of the total mass of polyvinyl chloride and alkaline lignin.

[0035] Among them, in the anhydrous salt, the cation is one or more of alkali metal elements, alkaline earth metal elements and Ni, and the anion is halogen, NO3 - PO3 3- and SO4 2- One or more of .

[0036] PVC particles are white particles with a degree of polymerization of 68-78 after processing of waste materials.

[0037] In step (1), the mixing method includes but is not limited to magnetic stirring, manual mixing, etc.;

[0038] In step (2), the heating rate is 5-10°C min -1 The calcination temperature is in the range of 500-700°C and the holding time is 1-3h.

[0039] In step (3), the material after preliminary carbonization is taken out, washed with deionized water to remove salt, filtered and placed in a drying oven to be completely dried; the dried material is placed in a magnetic boat, and then placed in a tubular furnace again, and a second high-temperature calcination is performed under the protection of an argon atmosphere, and then the material is cooled to room temperature with the furnace and taken out; the above material is ball-milled into powder, and filtered out with a 50-200 mesh sieve to obtain a soft and hard carbon composite porous negative electrode material.

[0040] In step (3), deionized water in an amount 10-20 times the volume of the material after preliminary carbonization is added to precipitate anhydrous salt.

[0041] In step (3), the heating rate of the secondary high temperature calcination is 5 to 10 ° C. min -1 The calcination temperature is in the range of 1300-1700°C, and the holding time is 1-3h.

[0042] In step (3), the ball milling time is 4 to 6 hours, and the rotation speed is 500 rpm. -1 .

[0043] The present invention also discloses a soft and hard carbon composite porous negative electrode material prepared by a molten salt coupled calcination method, which is prepared by the preparation method as described above.

[0044] The present invention also discloses an application of the soft and hard carbon composite porous negative electrode material prepared by the molten salt coupled calcination method as described above in the negative electrode material of a sodium ion battery.

[0045] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0046] Embodiment 1:

[0047] like Figure 1 The material sample is prepared according to the soft and hard carbon composite porous negative electrode material preparation process shown in the figure. The specific steps are as follows:

[0048] First, polyvinyl chloride particles, purchased commercial alkaline lignin and anhydrous sodium chloride were fully mixed in a mass ratio of 68:17:15. The fully mixed mixed powder was placed in a magnetic boat and then placed in a tube furnace. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 700°C at a rate of 5°C / min and kept at this temperature for 2 hours. Then, it was allowed to cool to room temperature with the furnace. The preliminary carbonized material was taken out, washed with 10 to 20 times deionized water, and then filtered and placed in an 80°C drying oven to completely dry. The dried material was placed in a new magnetic boat and placed in a tube furnace again. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 1600°C at a rate of 5°C / min and kept at this temperature for 2 hours. Then, it was allowed to cool to room temperature with the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a soft and hard carbon composite porous negative electrode material sample.

[0049] Embodiment 2:

[0050] First, polyvinyl chloride particles, purchased commercial alkaline lignin, anhydrous sodium chloride and sodium carbonate were fully mixed in a mass ratio of 68:17:15, wherein the mass ratio of anhydrous sodium chloride to sodium carbonate was 1:1. The fully mixed mixed powder was placed in a magnetic boat and then in a tube furnace. Under the protection of an argon atmosphere, the mixture was heated from room temperature at 5 °C min -1 The rate was raised to 700℃ and kept at this temperature for 2h, then it was allowed to cool to room temperature with the furnace, the preliminarily carbonized material was taken out, washed with 10-20 times deionized water, filtered and placed in an 80℃ drying oven to make it completely dry. The dried material was placed in a new magnetic boat and put into the tube furnace again, and under the protection of argon atmosphere, it was heated from room temperature at 5℃min -1 The temperature was raised to 1600°C and kept at this temperature for 2 hours. It was then allowed to cool to room temperature along with the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a soft and hard carbon composite porous negative electrode material sample.

[0051] Embodiment 3:

[0052] First, polyvinyl chloride particles, purchased commercial alkaline lignin, anhydrous sodium chloride and sodium carbonate were fully mixed in a mass ratio of 72:18:10, wherein the mass ratio of anhydrous sodium chloride and sodium carbonate was 1:1. The fully mixed mixed powder was placed in a magnetic boat, and then placed in a tube furnace. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 700°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then allowed to cool to room temperature with the furnace. The preliminary carbonized material was taken out, washed with 10 to 20 times deionized water, and then filtered and placed in an 80°C drying oven to completely dry. The dried material was placed in a new magnetic boat, and placed in a tube furnace again. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 1600°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then allowed to cool to room temperature with the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a soft and hard carbon composite porous negative electrode material sample.

[0053] Embodiment 4:

[0054] First, polyvinyl chloride particles, purchased commercial alkaline lignin, anhydrous sodium chloride and sodium carbonate were fully mixed in a mass ratio of 68:17:15, wherein the mass ratio of anhydrous sodium chloride and sodium carbonate was 1:2. The fully mixed mixed powder was placed in a magnetic boat, and then placed in a tube furnace. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 700°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then allowed to cool to room temperature with the furnace. The preliminary carbonized material was taken out, washed with 10 to 20 times deionized water, and then filtered and placed in an 80°C drying oven to completely dry. The dried material was placed in a new magnetic boat, and placed in a tube furnace again. Under the protection of an argon atmosphere, the temperature was raised from room temperature to 1600°C at a rate of 5°C / min, and kept at this temperature for 2 hours, and then allowed to cool to room temperature with the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a soft and hard carbon composite porous negative electrode material sample.

[0055] Comparative Example 1:

[0056] First, polyvinyl chloride particles and purchased commercial alkaline lignin were fully mixed at a mass ratio of 4:1. The mixed powder was placed in a magnetic boat and then in a tube furnace. Under the protection of argon atmosphere, the mixture was heated from room temperature at 5℃ min -1 The rate was raised to 700 °C and kept at this temperature for 2 h. Then, it was allowed to cool to room temperature with the furnace. The preliminarily carbonized material was taken out and put into the tube furnace again. Under the protection of argon atmosphere, the temperature was increased from room temperature to 5 °C min -1 The temperature was raised to 1600°C and kept at this temperature for 2 hours. The material was then cooled to room temperature in the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a material sample.

[0057] Comparative Example 2:

[0058] First, polyvinyl chloride particles were placed in a magnetic boat and then in a tube furnace. Under the protection of an argon atmosphere, the temperature was increased from room temperature to 5 °C min -1 The temperature was raised to 700°C and kept at this temperature for 2 hours. The material was then cooled to room temperature in the furnace. The carbonized material was taken out and ball-milled into fine powder to obtain a material sample.

[0059] The properties of the soft and hard carbon composite porous negative electrode material samples obtained in the above-mentioned embodiments and the material samples obtained in the comparative examples are analyzed below.

[0060] Scanning electron microscope (SEM) inspection:

[0061] like Figure 2As shown, there are scanning electron microscope (SEM) images of the material samples of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention, wherein Figure (a) and Figure (b) are SEM images of the material sample of Comparative Example 1, Figure (c), Figure (d) and Figure (g) are SEM images of the material sample of Comparative Example 2, and Figure (e) and Figure (f) are SEM images of the material sample of Example 1.

[0062] Depend on Figure 2 It can be seen that the sample of comparative example 1 is in the form of lamellar blocks and particles agglomerated and attached to the lamellar carbon, and the sample of comparative example 2 is composed of carbon nanolayers and nanoparticles. By comparing comparative examples 1 and 2, it can be seen that the composite of soft and hard carbon makes the gaps, surfaces and surroundings of lamellar hard carbon dispersed with soft carbon particles, which will help sodium ions to increase the reaction activity by utilizing the defect sites of hard carbon when they are deintercalated in soft carbon. In Example 1, after the composite of soft and hard carbon and the pores are formed by molten salt, certain pores are formed on the surface and middle part of the material sample. These pores can alleviate the growth of sodium dendrites when the material reacts on a low potential platform to a certain extent, avoid sodium dendrites from destroying the material structure, causing structural collapse, reducing safety hazards, and improving cycle stability.

[0063] Transmission electron microscopy (TEM) detection: Figure 3 Shown are projection electron microscope (TEM) images of the material samples of Example 1, Comparative Example 1 and Comparative Example 2, wherein Figure (a) is the TEM image of the sample of Comparative Example 1, Figure (b) is the TEM image of the sample of Comparative Example 2, and Figure (c) is the TEM image of the sample of Example 1.

[0064] Depend on Figure 3 It can be seen that the material samples of Comparative Examples 1 and 2 show disordered structures, while the material sample of Example 1 shows abundant micro / mesoporous and ordered graphite-like domain structures. It can be seen that the pore-forming treatment of anhydrous salt NaCl effectively increases the content of ordered graphite-like domains, improves its ion transport capacity, conductivity and sodium storage active sites, and thus improves its capacity.

[0065] Mesopore size detection:

[0066] The pore sizes of the micro / mesopores in the material samples in Examples 1 to 4, Comparative Examples 1 and 2 were analyzed by BET pore size analysis, and the data obtained are as follows:

[0067] Table 1: Mesopore diameter data in the material samples in Examples 1 to 4, Comparative Examples 1 and 2.

[0068] Group Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Pore ​​size (nm) 3.9nm 3.5nm 3.8nm 2.9nm No mesopores No mesopores

[0069] As can be seen from Table 1 above, according to the comparison between Example 1 and Example 2, under the condition that the total proportion of anhydrous salt is certain, the type of salt is changed, and the sodium chloride that does not decompose during pyrolysis is mixed with the sodium carbonate that produces carbon dioxide during pyrolysis in a ratio of 1:1, and the mesopore diameter of the material becomes smaller; according to the comparison between Example 2 and Example 3, under the condition that the type and ratio of anhydrous salt are not changed, the ratio of anhydrous salt to polyvinyl chloride particles and purchased commercial alkaline lignin is reduced, and the mesopore of the material becomes larger; according to the comparison between Example 2 and Example 4, under the condition that the total proportion of anhydrous salt is certain and the type of anhydrous salt is unchanged, as the proportion of sodium carbonate in the pyrolysis process increases, the mesopore diameter of the material becomes smaller. It can be seen from this that the mesopore diameter of the material can be effectively and controllably adjusted by adjusting the total proportion of anhydrous salt, the type of anhydrous salt, and the ratio between anhydrous salts.

[0070] Electrical performance testing:

[0071] The soft and hard carbon composite porous negative electrode material samples obtained in the above embodiments and the material samples obtained in the comparative examples are used to prepare a sodium ion battery. The specific preparation method is as follows:

[0072] According to the mass ratio of material sample: conductive agent: binder of 8:1:1, the corresponding materials are weighed respectively, and then a certain amount of N-methylpyrrolidone (NMP) is added as a solvent, and the above things are mixed and configured into a slurry. After the slurry is configured, the slurry is scraped on the aluminum foil by the scraping method, and the scraping thickness is selected to be 100μm. The scraped pole piece is placed in an 80℃ blast oven for drying for 3h, and the dried pole piece is transferred into a 120℃ vacuum oven for 24h. After taking it out, it is punched into a pole piece with a diameter of 12mm, and then weighed to calculate the pole piece load. The average load of the negative pole piece is 1.24mg. Finally, it is assembled into a battery, and the battery shell adopts a CR2032 type battery shell. Assemble the button battery in the order of positive shell, negative plate, diaphragm, electrolyte, sodium sheet, spring gasket, and negative shell. After adding the electrolyte, let it stand overnight, and then the battery is charged and discharged. The charge and discharge current density is 0.5C and the voltage range is 0.01~2.5V.

[0073] Figure 4 The charge and discharge curve of the sodium ion battery assembled with the material sample in Example 1 of the present invention at a current density of 0.5C and a potential window of 0.01-2.5V, and its first discharge capacity can reach 421.34mAh g -1 , the reversible discharge capacity is 276.39 mAh g -1 , showing good electrochemical performance and high sodium storage capacity.

[0074] Figure 5The cycle life and coulomb efficiency of the sodium ion battery assembled with the material sample in Example 1 of the present invention at a current density of 5C, and the specific capacity at the beginning of the cycle is 140 mAh g -1 After 700 cycles, the specific capacity dropped to 125 mAh g -1 The uniform nanopore structure makes the hard carbon material have good conductivity and electrolyte permeability. Therefore, the coulombic efficiency of the sodium ion battery is always maintained at about 100% during the cycle process, and the coulombic efficiency is high.

[0075] The present invention realizes effective regulation of the pore size of soft and hard carbon composite porous negative electrode materials by adjusting the types and proportions of cations and anions in molten salt through a molten salt coupled calcination method, and applies it to the negative electrode of sodium ion batteries, providing an effective preparation method for the development of negative electrodes for high-energy, high-power, long-life, and fast-charging sodium ion batteries.

[0076] All features described in the description, the attached claims and the drawings are essential features for the invention both individually and in any combination.

[0077] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Ordinary technicians in the field can change, modify, replace, modify, delete some features, add features or re-combine features to form a technical solution in the above embodiments without departing from the principles and purpose of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the innovative principles of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method, characterized in that: The following steps are involved: (1) uniformly mixing polyvinyl chloride particles, alkaline lignin and anhydrous salt to prepare a mixture; (2) placing the above-mentioned mixture in an inert gas atmosphere for molten salt calcination treatment; (3) desalting, filtering, drying, and calcining the product at a high temperature for a second time, and finally filtering the product through a sieve to obtain a soft and hard carbon composite porous negative electrode material; The mass ratio of polyvinyl chloride particles to alkaline lignin is 2 to 6:1; the anhydrous salt is composed of one or more salts, and its mass accounts for 5% to 20% of the total mass of polyvinyl chloride and alkaline lignin.

2. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 1, characterized in that: In anhydrous salts, the cations are one or more of alkali metal elements, alkaline earth metal elements and Ni, and the anions are halogens, NO3 - PO3 3- and SO4 2- One or more of .

3. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 2, characterized in that: PVC particles are white particles with a degree of polymerization of 68-78 after processing of waste materials.

4. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 1, characterized in that: In step (2), the heating rate is 5-10°C min -1 The calcination temperature is in the range of 500-700°C and the holding time is 1-3h.

5. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 1, characterized in that: In step (3), the material after preliminary carbonization is taken out, washed with deionized water to remove salt, filtered and placed in a drying oven to be completely dried; the dried material is placed in a magnetic boat, and then placed in a tubular furnace again, and a second high-temperature calcination is performed under the protection of an argon atmosphere, and then the material is cooled to room temperature with the furnace and taken out; the above material is ball-milled into powder, and filtered out with a 50-200 mesh sieve to obtain a soft and hard carbon composite porous negative electrode material.

6. The method for preparing a soft and hard carbon composite porous negative electrode material by controlling the molten salt coupled calcination method according to claim 5, characterized in that: In step (3), deionized water in an amount 10-20 times the volume of the material after preliminary carbonization is added to precipitate anhydrous salt.

7. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 5, characterized in that: In step (3), the heating rate of the secondary high temperature calcination is 5 to 10 ° C. min -1 The calcination temperature is in the range of 1300-1700°C, and the holding time is 1-3h.

8. The method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method according to claim 5, characterized in that: In step (3), the ball milling time is 4 to 6 hours, and the rotation speed is 500 rpm. -1 .

9. A method for preparing a soft and hard carbon composite porous negative electrode material by molten salt coupled calcination method, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. An application of the soft and hard carbon composite porous negative electrode material prepared by the molten salt coupled calcination method as claimed in claim 9 in the negative electrode material of sodium ion battery.

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