A nitrogen-doped porous hard carbon for sodium-ion battery negative electrode and a preparation method and application thereof

Abstract

The present application relates to the technical field of sodium ion batteries, and particularly relates to a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode, a preparation method and application thereof. The preparation method is to perform a co-pyrolysis reaction of carbon nitride as a self-sacrificial template agent and starch after cross-linking and pre-carbonization, and the carbon nitride has a pore-forming and nitrogen-doping dual function as the self-sacrificial template agent and the dopant. On one hand, the carbon nitride can form pores in the hard carbon and make the hard carbon have a rich pore structure, and on the other hand, the carbon nitride can dope nitrogen elements into the body of the hard carbon. The two functions can greatly increase the sodium storage active sites, store a large amount of sodium ions, and further improve the specific capacity of the sodium ion battery, so that the hard carbon is suitable for being used as the negative electrode of the sodium ion battery. Moreover, the co-pyrolysis of the carbon nitride and the starch solves the problem of expansion, melting and foaming of pure starch pyrolysis. In addition, the preparation method has the characteristics of simple process, simple operation, low production cost and suitability for large-scale production.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode and a preparation method and application thereof. BACKGROUND

[0002] With the development of global society, the use of fossil fuels dominated by coal, oil and natural gas is more and more, and the pollution and destruction to the environment are more and more serious, so that the demand for renewable clean energy is becoming larger and larger. However, renewable clean energy such as solar energy, water energy and wind energy has the problems of regionalism, climaticity and randomness, resulting in unstable power generation and uncertain power generation time, so the development of energy storage technology is urgent. In recent years, with the development of new energy, the demand for energy storage batteries is increasing. In particular, lithium ion batteries are widely concerned due to their high energy density and long cycle life. However, in the face of the serious situation of lithium resource shortage, uneven distribution and high cost, sodium ion batteries have become a new research hotspot in large-scale energy storage technology due to their low cost and abundant natural resources.

[0003] In thermodynamics, the binding energy of sodium ions and carbon is low, and NaC6 structure cannot be formed like LiC6 intercalation compound formed by lithium ions and graphite. Therefore, sodium ions cannot be well inserted into the graphite layer. Therefore, the industry personnel give up the application of graphite to the negative electrode of sodium ion battery, and instead research the feasibility of applying hard carbon to the negative electrode of sodium ion battery. Hard carbon can be prepared by using resin, coal tar pitch, biomass and the like as precursors through a series of chemical changes. From the perspective of market economy, stable supply of precursors in batches and low production process cost are the core factors for the maturity of hard carbon technology. However, the existing hard carbon materials have the following defects: resin-based hard carbon has good performance but the production process is complicated and the raw material price is high; coal tar pitch has a high hydrogen content, but high-temperature pyrolysis can only obtain low-performance soft carbon; biomass has a wide source, but the element content is unstable and is affected by problems such as region, climate and season. Therefore, it is still a difficult problem to find a widely available and low-cost biomass precursor to prepare and synthesize high-performance hard carbon.

[0004] In the prior art, a Chinese patent application with application number CN118538913A discloses that doping boron elements into starch-derived hard carbon can effectively improve ion diffusion rate, and both high first circle coulomb efficiency and capacity are taken into account. A patent application with application number CN118439585A discloses that using sodium carbonate as a template agent enables the starch-derived hard carbon to have a nano-microporous structure, and can effectively inhibit the adverse reaction of starch melting, foaming and swelling during the process of starch pyrolysis. However, the above prior art has the following shortcomings: (1) although starch is a renewable precursor for preparing hard carbon, the microstructure of the hard carbon obtained by directly pyrolyzing starch is a smooth lamellar structure, which is attributed to the fact that starch will melt, swell and foam during high-temperature pyrolysis. When the starch-derived hard carbon that melts, swells and foams is used as a negative electrode of a sodium ion battery and is subjected to cycle test, it is found that the specific capacity is very low, and it is not suitable for use as a negative electrode material of a sodium ion battery. (2) The elements contained in starch are carbon, hydrogen and oxygen, so the hard carbon obtained by pyrolyzing starch has weak conductivity and poor charge transfer capacity, and thus has very low sodium storage capacity and almost no sodium storage performance. (3) The two modification methods of the hard carbon, namely, pore making and doping, need to be completed in two independent steps in sequence, and the template agent and the doping agent used are often not the same chemical. This results in the defects of complicated process flow and high production cost. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the first object of the present application is to provide a preparation method of nitrogen-doped porous hard carbon for a negative electrode of a sodium ion battery, which has simple process, low production cost, solves the problem of pure starch pyrolysis, swelling and melting and foaming, is suitable for use as a negative electrode material of a sodium ion battery, and improves the specific capacity of the battery.

[0006] In order to overcome the shortcomings of the prior art, the second object of the present application is to provide nitrogen-doped porous hard carbon for a negative electrode of a sodium ion battery, which is suitable for use as a negative electrode material of a sodium ion battery, can increase sodium storage active sites, and improve the specific capacity of the battery.

[0007] The third object of the present application is to provide an application of the nitrogen-doped porous hard carbon in a negative electrode material of a sodium ion battery.

[0008] The fourth object of the present application is to provide an application of the nitrogen-doped porous hard carbon in a sodium ion battery.

[0009] To achieve the first object of the present application, the technical solution adopted by the present application is as follows:

[0010] The present application provides a preparation method of nitrogen-doped porous hard carbon for a negative electrode of a sodium ion battery, comprising the following steps:

[0011] S1, mixing carbon nitride and starch: carbon nitride, starch and anhydrous ethanol are placed in a container, and then mixed to obtain a mixture;

[0012] S2, solid-liquid separation and drying: the mixture obtained in step S1 is separated into anhydrous ethanol and mixed powder, and the mixed powder is dried;

[0013] S3, cross-linking pre-carbonization: the mixed powder after drying in step S2 is heated for cross-linking pre-carbonization under the protection of inert gas;

[0014] S4, pyrolysis reaction: after the cross-linking pre-carbonization of step S3 is completed, pyrolysis reaction is continued by heating to obtain the sodium ion battery negative electrode nitrogen-doped porous hard carbon.

[0015] Further, in the step S1, the preparation method of the carbon nitride is: under the protection of inert gas, urea is subjected to thermal decomposition reaction to obtain the carbon nitride.

[0016] Further, the preparation method of the carbon nitride is: urea is placed in a porcelain boat, then put into a tube furnace, heated to 500-600℃ at a heating rate of 3-7℃ / min under the protection of nitrogen for 2-4h, and then naturally cooled to room temperature to obtain carbon nitride powder; the flow rate of nitrogen is 15-25mL / min.

[0017] Further, in the step S1, the mass ratio of the carbon nitride and the starch is 1:(1-5); the mass volume ratio of the total mass of the carbon nitride and the starch to the anhydrous ethanol is 3g:(100-300mL); and / or

[0018] In the step S1, the mixing is: the container containing carbon nitride, starch and anhydrous ethanol is placed in an ultrasonic instrument for ultrasonic dispersion for 20-40min, and then stirred with a magnetic stirrer for 5-7h.

[0019] In the preparation method of the sodium ion battery negative electrode nitrogen-doped porous hard carbon, anhydrous ethanol is used as the liquid medium for mixing carbon nitride and starch, and then ultrasonic resonance of an ultrasonic instrument and stirring treatment of a magnetic stirrer are used, so that starch particles and carbon nitride particles are fully dispersed, and carbon nitride and starch are fully mixed, which has the advantages of good mixing effect, and is beneficial to the pore forming of hard carbon by carbon nitride in the subsequent cross-linking pre-carbonization and pyrolysis reaction, and the pore structure is more uniform, and the sodium storage active sites formed by nitrogen entering the hard carbon are also more uniform.

[0020] Further, in the step S2, the mixture obtained in the step S1 is separated into anhydrous ethanol and mixed powder by using a suction filtration device, and the mixed powder is dried in a blast drying oven at 50-70 DEG C for 10-14 h.

[0021] Further, in the step S3, the dried mixed powder is placed in a porcelain boat and sent into a tube furnace, and heated to 180-220 DEG C at a heating rate of 3-7 DEG C / min under the protection of nitrogen and kept for 3-5 h for cross-linking pre-carbonization.

[0022] Further, in the step S4, after the cross-linking pre-carbonization in the step S3, the temperature is continuously increased to 800-1600 DEG C at a heating rate of 3-7 DEG C / min for pyrolysis reaction for 1-3 h, thereby obtaining the nitrogen-doped porous hard carbon for negative electrode of sodium ion battery.

[0023] To achieve the second object of the present application, the technical scheme adopted by the present application is as follows:

[0024] The present application provides a kind of nitrogen-doped porous hard carbon for negative electrode of sodium ion battery, which is prepared by the preparation method of the nitrogen-doped porous hard carbon for negative electrode of sodium ion battery.

[0025] To achieve the third object of the present application, the technical scheme adopted by the present application is as follows:

[0026] The present application provides a kind of sodium ion battery negative electrode, which is prepared from the nitrogen-doped porous hard carbon.

[0027] To achieve the fourth object of the present application, the technical scheme adopted by the present application is as follows:

[0028] The present application provides a kind of sodium ion battery, which is prepared by using the nitrogen-doped porous hard carbon as negative electrode material.

[0029] The template agent has the effect of directional change of the microstructure of the target product (hard carbon), and different template agents have different effects on the microstructure of the target product (hard carbon). Some template agents have the function of pore forming for hard carbon to form a pore structure, and some template agents have the function of changing hard carbon into a specific shape. The template agent in the prior art does not have the property of storing sodium, so it does not contribute to the specific capacity, and the specific capacity is only provided by the hard carbon. The hard carbon with residual template agent is assembled into a button cell. Since the template agent occupies part of the mass of the negative electrode and does not have the property of storing sodium, the specific capacity of the button cell will be reduced, so the template agent needs to be removed in the prior art. If the template agent is not removed, the hard carbon assembled into the button cell will contact the electrolyte inside the battery, and the template agent may have a side reaction with part of the electrolyte, consume the electrolyte, and the impurities generated by the side reaction hinder the contact between the remaining electrolyte and the negative electrode, resulting in less electrolyte participating in the charge and discharge cycle between the positive and negative electrodes, and thus reducing the specific capacity.

[0030] The preparation method of the nitrogen-doped porous hard carbon negative electrode for sodium ion batteries of the present application is as follows: the mixed powder of carbon nitride and starch is heated at 180-220 DEG C, and the carbon nitride is continuously cross-linked to wrap the starch. The carbon nitride and starch particles are cross-linked to wrap individual starch particles, blocking the contact and melting between the starch particles. Then, when the temperature is further increased to the pyrolysis temperature, the starch is carbonized into hard carbon, and the carbon nitride is gasified and decomposed in situ to leave pores of different sizes (self-sacrifice); the carbon nitride that is not gasified and decomposed is cross-linked with the hard carbon in situ and becomes part of the hard carbon phase. The co-pyrolysis of carbon nitride and starch changes the microstructure of the starch hard carbon, making the hard carbon with smooth lamellar structure change into hard carbon with rich pore structure, greatly increasing the specific surface area and greatly enriching the storage sites of sodium ions; and a certain amount of nitrogen element is doped into the hard carbon phase, further increasing the storage active sites of sodium ions, so it has the advantages of high first-cycle coulombic efficiency and large specific capacity.

[0031] The preparation method of the nitrogen-doped porous hard carbon for a sodium ion battery negative electrode of the present application is as follows: carbon nitride is mixed with starch and pyrolyzed together. By taking advantage of the characteristics of carbon nitride, i.e. melting into the hard carbon phase derived from starch and self-sacrifice pore-forming, a part of the carbon nitride is melted into the hard carbon phase as a dopant, and the other part of the carbon nitride is decomposed, gasified and volatilized at high temperature, leaving pores of different sizes in situ. Moreover, the nitrogen element is doped without the need for additional process steps to remove excess carbon nitride, because a part of the carbon nitride that does not melt into the hard carbon phase derived from starch can be decomposed, gasified and volatilized at high temperature. In addition, the pore-forming of the hard carbon derived from starch does not require additional process steps to remove the template agent, and the remaining carbon nitride is melted into the hard carbon phase derived from starch. Furthermore, the pyrolysis of carbon nitride and starch together can effectively inhibit the problem of melting, foaming and swelling of starch during high-temperature pyrolysis. Therefore, the process flow of the present application is simple, the cost is low, and the hard carbon has the dual functions of doping and pore-forming.

[0032] Compared with the prior art, the present application has the following advantages:

[0033] (1) The preparation method of the nitrogen-doped porous hard carbon for a sodium ion battery negative electrode of the present application uses carbon nitride as a self-sacrifice template agent and pyrolyzes together with starch after cross-linking and pre-carbonization. The carbon nitride acts as a self-sacrifice template agent and a dopant, and has the dual functions of pore-forming and nitrogen doping. On the one hand, the carbon nitride can form pores in the hard carbon and make the hard carbon have a rich pore structure. On the other hand, the carbon nitride can dope nitrogen elements into the hard carbon phase, and the two work together to greatly increase the sodium storage active sites, store a large amount of sodium ions, and thus improve the specific capacity of the sodium ion battery. In addition, the pyrolysis of carbon nitride and starch together solves the problem of swelling, foaming and melting of pure starch during pyrolysis.

[0034] (2) The preparation method of the nitrogen-doped porous hard carbon for a sodium ion battery negative electrode of the present application can prepare nitrogen-doped porous hard carbon after mixing, separating and drying carbon nitride and starch, cross-linking and pre-carbonizing, and pyrolyzing. This preparation method has the characteristics of simple process, simple operation, low production cost and suitability for large-scale production.

[0035] (3) The nitrogen-doped porous hard carbon for a sodium ion battery negative electrode of the present application has a rich pore structure and dopes nitrogen elements in the hard carbon, which can increase the sodium storage active sites and is suitable for use as a negative electrode material for a sodium ion battery to improve the specific capacity of the battery.

[0036] (4) The sodium ion battery negative electrode of the present application is prepared from the nitrogen-doped porous hard carbon of the present application, and thus has a rich sodium storage active site, can store a large amount of sodium ions, and thus can improve the specific capacity of the sodium ion battery.

[0037] (5) The application of the nitrogen-doped porous hard carbon in the sodium ion battery, the prepared sodium ion battery has rich sodium storage active sites, can store a large amount of sodium ions, and can further improve the specific capacity of the sodium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0039] Figure 1 is an SEM image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 1 of the present application.

[0040] Figure 2 is an SEM image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 2 of the present application.

[0041] Figure 3 is an SEM image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 3 of the present application.

[0042] Figure 4 is an SEM image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 4 of the present application.

[0043] Figure 5 is an SEM image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 5 of the present application.

[0044] Figure 6 is an SEM image of a hard carbon prepared in Comparative Example 1 of the present application.

[0045] Figure 7 is an XPS diffraction nitrogen element detection image of a nitrogen-doped porous hard carbon for a sodium ion battery negative electrode prepared in Example 1 of the present application.

[0046] Figure 8 is a real object image of a sodium ion battery of Example 9 of the present application.

[0047] Figure 9 is a constant current charge-discharge curve diagram of a sodium ion battery of Example 6 of the present application.

[0048] Figure 10 is a constant current charge-discharge curve diagram of a sodium ion battery of Example 7 of the present application.

[0049] Figure 11 is a constant current charge-discharge curve diagram of a sodium ion battery of Example 8 of the present application.

[0050] Figure 12 is a constant current charge-discharge curve diagram of a sodium ion battery according to Example 9 of the present application.

[0051] Figure 13 is a constant current charge-discharge curve diagram of a sodium ion battery according to Example 10 of the present application.

[0052] Figure 14 is a constant current charge-discharge curve diagram of a sodium ion battery according to Comparative Example 2 of the present application.

[0053] Figure 15 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Example 6 of the present application.

[0054] Figure 16 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Example 7 of the present application.

[0055] Figure 17 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Example 8 of the present application.

[0056] Figure 18 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Example 9 of the present application.

[0057] Figure 19 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Example 10 of the present application.

[0058] Figure 20 is a 300-cycle long cycle performance diagram of a sodium ion battery according to Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0059] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects more clearly understood, the present application will be further described in detail below in conjunction with examples. It should be understood that the specific examples described herein are merely intended to explain the present application, and are not intended to limit the present application.

[0060] The terms used in the embodiments of the present application are merely for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0061] In the embodiments of the present application, a preparation method of nitrogen-doped porous hard carbon for a sodium ion battery negative electrode comprises the following steps:

[0062] S1, mixing carbon nitride and starch: placing carbon nitride, starch and anhydrous ethanol in a container, and then mixing to obtain a mixture;

[0063] S2, solid-liquid separation and drying: separating the mixture obtained in step S1 into anhydrous ethanol and mixed powder, and drying the mixed powder;

[0064] S3, cross-linking pre-carbonization: heating the dried mixed powder in step S2 under the protection of inert gas for cross-linking pre-carbonization;

[0065] S4, pyrolysis reaction: after the cross-linking pre-carbonization in step S3 is completed, heating continues to carry out pyrolysis reaction, thereby obtaining the sodium ion battery negative electrode nitrogen-doped porous hard carbon.

[0066] In some embodiments, in step S1, the preparation method of the carbon nitride is: under the protection of inert gas, urea is subjected to thermal decomposition reaction to obtain the carbon nitride.

[0067] In some embodiments, the preparation method of the carbon nitride is: placing urea in a porcelain boat, then putting it into a tube furnace, heating from room temperature to 500-600°C at a heating rate of 3-7°C / min under the protection of nitrogen, and then naturally cooling to room temperature after thermal decomposition for 2-4h, thereby obtaining carbon nitride powder; the flow rate of the nitrogen is 15-25mL / min.

[0068] In some embodiments, in step S1, the mass ratio of the carbon nitride to starch is 1:(1-5); the mass-volume ratio of the total mass of the carbon nitride and starch to anhydrous ethanol is 3g:(100-300mL); and / or

[0069] In step S1, the mixing is: placing a container containing carbon nitride, starch and anhydrous ethanol into an ultrasonic instrument for ultrasonic dispersion for 20-40min, and then stirring with a magnetic stirrer for 5-7h.

[0070] In some embodiments, in step S2, the mixture obtained in step S1 is separated into anhydrous ethanol and mixed powder using a suction filtration device, and the mixed powder is placed in a forced air drying oven and dried at 50-70°C for 10-14h.

[0071] In some embodiments, in step S3, the dried mixed powder is placed in a porcelain boat and sent into a tube furnace, heated to 180-220°C at a heating rate of 3-7°C / min under the protection of nitrogen, and then kept at this temperature for 3-5h for cross-linking pre-carbonization.

[0072] In some embodiments, after the cross-linking pre-carbonization of step S3, the temperature is raised at a rate of 3℃ / min-7℃ / min to 800℃-1600℃ for pyrolysis reaction for 1h-3h in step S4, thereby obtaining the nitrogen-doped porous hard carbon for the negative electrode of the sodium ion battery.

[0073] In an embodiment of the present application, the nitrogen-doped porous hard carbon for the negative electrode of the sodium ion battery is prepared by the method described above.

[0074] In an embodiment of the present application, the negative electrode of the sodium ion battery is prepared by the nitrogen-doped porous hard carbon described above.

[0075] In an embodiment of the present application, the sodium ion battery is prepared by using the nitrogen-doped porous hard carbon described above as the negative electrode material.

[0076] The following will be described in conjunction with specific examples.

[0077] Example 1

[0078] A method for preparing a nitrogen-doped porous hard carbon for the negative electrode of a sodium ion battery comprises the following steps:

[0079] Preparation of carbon nitride: 17g of urea was placed in a porcelain boat and put into a tube furnace. Under an inert gas atmosphere, the temperature was raised from room temperature to 550℃ at a rate of 5℃ / min, and pyrolysis was carried out at 550℃ for 3 hours, with a nitrogen flow rate of 20ml / min. After the reaction was completed, the temperature was naturally lowered to room temperature, and about 0.5g of yellowish carbon nitride powder was obtained.

[0080] Preparation of nitrogen-doped porous hard carbon: 0.5g of carbon nitride and 2.5g of starch were placed in a beaker, and an appropriate amount of anhydrous ethanol was added to the beaker, followed by ultrasonic dispersion in an ultrasonic instrument for 30 minutes. Then, the carbon nitride and starch were fully mixed by magnetic stirring for 6 hours. Subsequently, the ethanol and the mixed powder were separated by using a suction filtration device. The mixed powder was dried in a forced air drying oven at 60℃ for 12 hours. The dried mixed powder was placed in a porcelain boat and sent into a tube furnace. Under an inert gas (nitrogen) atmosphere, the temperature was raised from room temperature to 200℃ at a rate of 5℃ / min, and the mixed powder was subjected to cross-linking pre-carbonization at 200℃ for 4 hours; after the cross-linking pre-carbonization was completed, the temperature was raised from 200℃ to 800℃ at a rate of 5℃ / min, and pyrolysis reaction was carried out at 800℃ for 2 hours. After the pyrolysis reaction was completed, the temperature was naturally lowered to room temperature. The target product, nitrogen-doped porous hard carbon, was obtained. It was named CNHC-800.

[0081] Example 2

[0082] A method for preparing a nitrogen-doped porous hard carbon for the negative electrode of a sodium ion battery comprises the following steps:

[0083] Preparation of carbon nitride: Take 17 g of urea and place it in a porcelain boat, and put it into a tube furnace. Under an inert gas atmosphere, heat it from room temperature to 550°C at a rate of 5°C / min, and pyrolyze it at 550°C for 3 hours, with a nitrogen flow rate of 20 ml / min. After the reaction is complete, naturally cool it to room temperature, and obtain about 0.5 g of yellowish carbon nitride powder.

[0084] Preparation of nitrogen-doped porous hard carbon: Take 0.5 g of carbon nitride and 2.5 g of starch and place them in a beaker, add an appropriate amount of anhydrous ethanol to the beaker, and then put it into an ultrasonic instrument, and use ultrasonic dispersion for 30 minutes. Then, use a magnetic stirrer to stir for 6 hours to fully mix the carbon nitride and starch. Then, use a suction filtration device to separate the ethanol and the mixed powder. Place the mixed powder into a forced air drying oven and dry it at 60°C for 12 hours. Place the dried mixed powder into a porcelain boat and put it into a tube furnace. Under an inert gas atmosphere, heat it from room temperature to 200°C at a rate of 5°C / min, and cross-link and pre-carbonize the mixed powder at 200°C for 4 hours; after the cross-linking and pre-carbonization is complete, heat it from 200°C to 1000°C at a rate of 5°C / min, and pyrolyze it at 1000°C for 2 hours. After the pyrolysis reaction is complete, naturally cool it to room temperature. The target product, nitrogen-doped porous hard carbon, is obtained. It is named CNHC-1000.

[0085] Example 3

[0086] A method for preparing a sodium-ion battery negative electrode uses nitrogen-doped porous hard carbon, comprising the following steps:

[0087] Preparation of carbon nitride: Take 17 g of urea and place it in a porcelain boat, and put it into a tube furnace. Under an inert gas atmosphere, heat it from room temperature to 550°C at a rate of 5°C / min, and pyrolyze it at 550°C for 3 hours, with a nitrogen flow rate of 20 ml / min. After the reaction is complete, naturally cool it to room temperature, and obtain about 0.5 g of yellowish carbon nitride powder.

[0088] Preparation of nitrogen-doped porous hard carbon: Take 0.5 g of carbon nitride and 2.5 g of starch and place them in a beaker, add an appropriate amount of anhydrous ethanol to the beaker, and then put it into an ultrasonic instrument, and use ultrasonic dispersion for 30 minutes. Then, use a magnetic stirrer to stir for 6 hours to fully mix the carbon nitride and starch. Then, use a suction filtration device to separate the ethanol and the mixed powder. Place the mixed powder into a forced air drying oven and dry it at 60°C for 12 hours. Place the dried mixed powder into a porcelain boat and put it into a tube furnace. Under an inert gas atmosphere, heat it from room temperature to 200°C at a rate of 5°C / min, and cross-link and pre-carbonize the mixed powder at 200°C for 4 hours; after the cross-linking and pre-carbonization is complete, heat it from 200°C to 1000°C at a rate of 5°C / min, and pyrolyze it at 1000°C for 2 hours. After the pyrolysis reaction is complete, naturally cool it to room temperature. The target product, nitrogen-doped porous hard carbon, is obtained. It is named CNHC-1000.

[0089] Example 4

[0090] A method for preparing a nitrogen-doped porous hard carbon for a sodium-ion battery negative electrode, comprising the following steps:

[0091] Preparation of carbon nitride: 17 g of urea was placed in a porcelain boat and put into a tube furnace. Under an inert gas atmosphere, it was heated from room temperature to 550°C at a heating rate of 5°C / min, and pyrolyzed at 550°C for 3 hours, with a nitrogen flow rate of 20 ml / min. After the reaction was completed, it was naturally cooled to room temperature, and about 0.5 g of yellowish carbon nitride powder was obtained.

[0092] Preparation of nitrogen-doped porous hard carbon: 0.5 g of carbon nitride and 2.5 g of starch were placed in a beaker, and an appropriate amount of anhydrous ethanol was added to the beaker, followed by placing it in an ultrasonic instrument for ultrasonic dispersion for 30 minutes. Then, the carbon nitride and starch were fully mixed by magnetic stirring for 6 hours. Then, the ethanol and the mixed powder were separated using a suction filtration device. The mixed powder was placed in a forced air drying oven and dried at 60°C for 12 hours. The dried mixed powder was placed in a porcelain boat and sent to a tube furnace. Under an inert gas atmosphere, it was heated from room temperature to 200°C at a heating rate of 5°C / min, and cross-linked pre-carbonized at 200°C for 4 hours; after cross-linked pre-carbonization was completed, it was heated from 200°C to 1400°C at a rate of 5°C / min, and pyrolyzed at 1400°C for 2 hours. After the pyrolysis reaction was completed, it was naturally cooled to room temperature. The target product, nitrogen-doped porous hard carbon, was obtained. It was named CNHC-1400.

[0093] Example 5

[0094] A method for preparing a nitrogen-doped porous hard carbon for a sodium-ion battery negative electrode, comprising the following steps:

[0095] Preparation of carbon nitride: 17 g of urea was placed in a porcelain boat and put into a tube furnace. Under an inert gas atmosphere, it was heated from room temperature to 550°C at a heating rate of 5°C / min, and pyrolyzed at 550°C for 3 hours, with a nitrogen flow rate of 20 ml / min. After the reaction was completed, it was naturally cooled to room temperature, and about 0.5 g of yellowish carbon nitride powder was obtained.

[0096] Preparation of nitrogen-doped porous hard carbon: 0.5 g of carbon nitride and 2.5 g of starch were placed in a beaker, and an appropriate amount of anhydrous ethanol was added to the beaker, followed by placing in an ultrasonic instrument for ultrasonic dispersion for 30 minutes. Then the carbon nitride and starch were fully mixed by magnetic stirring for 6 hours. Then the ethanol and the mixed powder were separated using a suction filtration device. The mixed powder was placed in a porcelain boat and sent into a tube furnace. Under an inert gas atmosphere, the mixed powder was heated from room temperature to 200°C at a rate of 5°C / min, and then held at 200°C for 4 hours for crosslinking and pre-carbonization. After the crosslinking and pre-carbonization was completed, the mixed powder was heated from 200°C to 1600°C at a rate of 5°C / min, and then pyrolyzed at 1600°C for 2 hours. After the pyrolysis reaction was completed, the temperature was naturally lowered to room temperature. The target product, nitrogen-doped porous hard carbon, was obtained. It was named CNHC-1600.

[0097] Example 6

[0098] A sodium ion battery, a sodium ion battery negative electrode using the nitrogen-doped porous hard carbon prepared in Example 1 as a negative electrode material, was assembled into a button cell.

[0099] Example 7

[0100] A sodium ion battery, a sodium ion battery negative electrode using the nitrogen-doped porous hard carbon prepared in Example 2 as a negative electrode material, was assembled into a button cell.

[0101] Example 8

[0102] A sodium ion battery, a sodium ion battery negative electrode using the nitrogen-doped porous hard carbon prepared in Example 3 as a negative electrode material, was assembled into a button cell.

[0103] Example 9

[0104] A sodium ion battery, a sodium ion battery negative electrode using the nitrogen-doped porous hard carbon prepared in Example 4 as a negative electrode material, was assembled into a button cell.

[0105] In this embodiment, the actual picture of the sodium ion battery prepared is shown in Figure 7 .

[0106] Example 10

[0107] A sodium ion battery, a sodium ion battery negative electrode using the nitrogen-doped porous hard carbon prepared in Example 5 as a negative electrode material, was assembled into a button cell.

[0108] Comparative Example 1

[0109] A method for preparing a hard carbon, comprising the following steps:

[0110] The starch was cross-linked and pre-carbonized at 200 °C for 4 hours under inert gas atmosphere at a temperature rising rate of 5 °C / min from room temperature to 200 °C; and then pyrolyzed at 1400 °C for 2 hours at a temperature rising rate of 5 °C / min from 200 °C to 1400 °C. After the pyrolysis, the product was cooled to room temperature naturally. A hard carbon was obtained, which was named as HC-1400.

[0111] Comparative Example 2

[0112] A sodium ion battery was assembled by using the hard carbon prepared in Comparative Example 1 as the negative electrode material.

[0113] (I) Morphology characterization by scanning electron microscope

[0114] The nitrogen-doped porous hard carbons prepared in Examples 1 to 5 and the hard carbon prepared in Comparative Example 1 were respectively characterized by scanning electron microscope (SEM), as shown in FIG. 1. Figures 1 to 6

[0115] As can be seen from FIG. 1, the nitrogen-doped porous hard carbon prepared in Example 1 at 800 °C has a pore structure. Figure 1

[0116] As can be seen from FIG. 2, the nitrogen-doped porous hard carbons prepared in Examples 2 to 5 at 1000 °C, 1200 °C, 1400 °C and 1600 °C respectively all have abundant pore structures. Figures 2 to 5

[0117] As can be seen from FIG. 3, the hard carbon prepared in Comparative Example 1 at 1400 °C has a lamellar microstructure and does not have a pore structure, because the hard carbon is not doped with nitrogen. Figure 6 (II) Nitrogen element detection by XPS diffraction

[0118] The nitrogen-doped porous hard carbon prepared in Example 1 was detected for nitrogen element by X-ray photoelectron spectroscopy (XPS), as shown in FIG. 4. As can be seen from FIG. 4, the nitrogen is successfully doped into the hard carbon, and the nitrogen atoms exist in the nitrogen-doped porous hard carbon body in the form of "graphitic nitrogen". Graphitic nitrogen is a type of nitrogen atom existing in carbon materials, one graphitic nitrogen atom is bonded with three carbon atoms, and itself contains a lone pair of electrons, thereby improving the charge transfer efficiency of the hard carbon.

[0119] Figure 7 (III) Morphology characterization of the button cell Figure 7 The photograph of the sodium ion battery prepared in Example 9 is shown in FIG. 5. As can be seen from FIG. 5, the sodium ion battery has a button cell structure.

[0120]

[0121] The photograph of the sodium ion battery prepared in Example 9 is shown in FIG. 5. As can be seen from FIG. 5, the sodium ion battery has a button cell structure. Figure 8 Figure 8 ​​​​​​It can be seen that the button cell prepared by adopting the nitrogen-doped porous hard carbon prepared by the co-pyrolysis reaction of carbon nitride and starch as the negative electrode material does not have the swelling and foaming condition, and the problem of swelling, melting and foaming of pure starch pyrolysis is solved.

[0122] The starch starts to dehydrate and carbonize at 300 DEG C, and is completely carbonized at about 600 DEG C. The decomposition temperature of carbon nitride is above 600 DEG C. Therefore, the starch particles are isolated by carbon nitride before being completely carbonized at 600 DEG C, and cannot be melted and foamed.

[0123] (IV) Constant current charge and discharge test

[0124] The sodium ion batteries prepared in Example 6 to Example 10 and Comparative Example 2 were respectively subjected to the constant current charge and discharge test at a current density of 30 mA / g, and the test results are respectively shown in Table 1 to Table 5. Figures 9 to 14 .

[0125] From Table 1 to Table 5, Figure 9 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Example 6 is 404.5 mAh / g, the discharge specific capacity is 207 mAh / g, and the first cycle coulombic efficiency is 51.17%.

[0126] From Table 1 to Table 5, Figure 10 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Example 7 is 398.2 mAh / g, the discharge specific capacity is 245.3 mAh / g, and the first cycle coulombic efficiency is 61.59%.

[0127] From Table 1 to Table 5, Figure 11 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Example 8 is 430.9 mAh / g, the discharge specific capacity is 277.1 mAh / g, and the first cycle coulombic efficiency is 64.31%.

[0128] From Table 1 to Table 5, Figure 12 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Example 9 is 443.5 mAh / g, the discharge specific capacity is 295.4 mAh / g, and the first cycle coulombic efficiency is 66.61%.

[0129] From Table 1 to Table 5, Figure 13 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Example 10 is 394.2 mAh / g, the discharge specific capacity is 269 mAh / g, and the first cycle coulombic efficiency is 68.24%.

[0130] From Table 1 to Table 5, Figure 14 It can be seen that the first discharge specific capacity of the sodium ion battery prepared in Comparative Example 2 is only 249.9 mAh / g, the discharge specific capacity is only 64.6 mAh / g, and the first cycle coulombic efficiency is only 25.86%.

[0131] It can be seen that the sodium ion battery prepared by Comparative Example 2 has a small initial discharge specific capacity, a small discharge specific capacity, and a very low first cycle coulombic efficiency. The sodium ion battery prepared by the present application has the advantages of large initial discharge specific capacity, large discharge specific capacity, and high first cycle coulombic efficiency.

[0132] (V) Constant current charge and discharge test

[0133] The sodium ion batteries prepared by Example 6 to Example 10 and Comparative Example 2 were respectively subjected to 300 cycle long cycle performance tests at a current density of 1000 mAh / g, and the test results are respectively shown in Table 1. Figures 15 to 20 .

[0134] From Figure 15 It can be seen that the sodium ion battery prepared by Example 6 has an initial discharge specific capacity of 168.4 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is 126.9 mAh / g, the capacity retention rate is 75.3%, and the cycle stability is good.

[0135] From Figure 16 It can be seen that the sodium ion battery prepared by Example 7 has an initial discharge specific capacity of 205.8 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is 154.5 mAh / g, the capacity retention rate is 75.0%, and the cycle stability is good.

[0136] From Figure 17 It can be seen that the sodium ion battery prepared by Example 8 has an initial discharge specific capacity of 236.3 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is 196.9 mAh / g, the capacity retention rate is 83.3%, and the cycle stability is good.

[0137] From Figure 18 It can be seen that the sodium ion battery prepared by Example 9 has an initial discharge specific capacity of 293.4 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is 222.8 mAh / g, the capacity retention rate is 75.9%, and the cycle stability is good.

[0138] From Figure 19 It can be seen that the sodium ion battery prepared by Example 10 has an initial discharge specific capacity of 190.9 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is 159.3 mAh / g, the capacity retention rate is 83.4%, and the cycle stability is good.

[0139] From Figure 20It can be seen that the initial discharge specific capacity of the sodium ion battery prepared in Comparative Example 2 is only 35.5 mAh / g at a current density of 1000 mAh / g. After 300 cycles of charge and discharge, the discharge specific capacity is only 11.2 mAh / g.

[0140] It can be seen that the specific capacity of the sodium ion battery prepared in Comparative Example 2 is very low and has almost no sodium storage performance. The specific capacity of the sodium ion battery prepared in the present application is large and has excellent sodium storage performance.

[0141] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a nitrogen-doped porous hard carbon for a sodium-ion battery anode, characterized by, The method comprises the following steps: S1, mixing carbon nitride and starch: placing carbon nitride, starch and anhydrous ethanol in a container, then placing the container containing carbon nitride, starch and anhydrous ethanol into an ultrasonic instrument for ultrasonic dispersion for 20-40 min, and then stirring for 5-7 h by using a magnetic stirrer to obtain a mixture; the mass ratio of carbon nitride to starch is 1:(1-5); the mass-volume ratio of the total mass of carbon nitride and starch to anhydrous ethanol is 3 g:(100-300 mL); S2, solid-liquid separation and drying: separating the mixture obtained in step S1 by using a suction filtration device to separate anhydrous ethanol and a mixed powder, and placing the mixed powder in a blast drying oven for drying at 50-70 ℃ for 10-14 h; S3, cross-linking pre-carbonization: placing the dried mixed powder in a porcelain boat and sending it into a tube furnace, heating at a rate of 3-7 ℃ / min to 180-220 ℃ under the protection of an inert gas, and keeping the temperature for 3-5 h for cross-linking pre-carbonization; S4, pyrolysis reaction: after the cross-linking pre-carbonization in step S3 is completed, heating and pyrolysis reaction is continued, thereby obtaining the nitrogen-doped porous hard carbon for sodium ion battery negative electrode.

2. The method for preparing nitrogen-doped porous hard carbon for sodium-ion battery negative electrode as described in claim 1, characterized in that, In step S1, the preparation method of the carbon nitride is: under the protection of an inert gas, subjecting urea to a thermal decomposition reaction to obtain the carbon nitride.

3. The method of claim 2, wherein the method is characterized by: The preparation method of the carbon nitride is: placing urea in a porcelain boat, then placing it into a tube furnace, heating at a rate of 3-7 ℃ / min to 500-600 ℃ under the protection of nitrogen gas for 2-4 h, and then naturally cooling to room temperature to obtain carbon nitride powder; the flow rate of the nitrogen gas is 15-25 mL / min.

4. The method for preparing nitrogen-doped porous hard carbon for sodium-ion battery negative electrode as described in claim 1, characterized in that, In step S4, after the cross-linking pre-carbonization in step S3 is completed, heating at a rate of 3-7 ℃ / min to 800-1600 ℃ for 1-3 h for pyrolysis reaction, thereby obtaining the nitrogen-doped porous hard carbon for sodium ion battery negative electrode.

5. A nitrogen-doped porous hard carbon for a sodium-ion battery anode, characterized by, A method for preparing a nitrogen-doped porous hard carbon for sodium ion battery negative electrode according to any one of claims 1-4.

6. A sodium-ion battery anode, characterized in that, The nitrogen-doped porous hard carbon according to claim 5 is prepared.

7. A sodium-ion battery, characterized in that, The nitrogen-doped porous hard carbon according to claim 5 is used as a negative electrode material.

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