Hard carbon materials, their preparation methods and applications

Abstract

This invention discloses a hard carbon material, its preparation method, and its applications. The hard carbon material comprises a hollow structure, a first hard carbon layer forming the peripheral wall of the hollow structure, and a second hard carbon layer encapsulating the first hard carbon layer. The first hard carbon layer is made of nitrogen-doped hard carbon material, and the second hard carbon layer is made of cellulose sintered hard carbon material. The hard carbon material of this invention exhibits superior charge-discharge performance, and is particularly suitable for sodium-ion battery systems. In the preparation method of the hard carbon material of this invention, a cavity structure is formed in the middle of the material by etching a composite of nickel foam and cellulose gel. A three-dimensional network structure composed of a conductive polymer film and cellulose is formed on the surface of the nickel foam through electrochemical polymerization. The hard carbon material formed after carbonization exhibits superior charge-discharge performance.

Description

Technical Field

[0001] This invention relates to the field of materials preparation technology, and in particular to hard carbon materials, their preparation methods, and applications. Background Technology

[0002] Compared to graphite, hard carbon materials have a larger interlayer spacing and more micropores, resulting in more active sites for lithium-ion insertion and extraction, and thus a higher specific capacity. Furthermore, hard carbon materials exhibit good compatibility with carbonate electrolytes, making them suitable for low-temperature operation. Additionally, hard carbon also boasts excellent high-rate charge-discharge performance and a long cycle life. As an anode material, it can be applied in various technological fields, including lithium-ion batteries, sodium-ion batteries, and supercapacitors. The unique porous structure of hard carbon materials makes them particularly suitable for use as an anode material in sodium-ion batteries.

[0003] With the advancement of new energy development, primarily based on lithium batteries, the current shortage of lithium-ion battery raw materials has become a major factor restricting global automakers' sales. Sodium, belonging to the same group as lithium, possesses similar properties, and its abundant reserves make sodium-ion batteries increasingly advantageous. In the development of sodium-ion battery anode materials, among carbon-based, titanium-based, and alloy anode materials, hard carbon materials currently outperform other types due to their unique structural advantages. Therefore, constructing hard carbon materials with unique structures is beneficial for the development of sodium-ion battery technology.

[0004] Chinese patent CN112225194A discloses a method for preparing hard carbon materials. This method involves adding conductive additives to a hard carbon precursor to improve the material's conductivity, and combining this with variable-speed heating to introduce a large number of oxygen-containing functional groups, increasing the cross-linking degree of the material's framework and improving its strength. This significantly enhances the sodium storage specific capacity, first-cycle coulombic efficiency, and rate performance. However, the conductive material added to the hard carbon precursor undergoes structural changes during several stages of variable-speed heating, affecting its cycle stability. Furthermore, the additives do not substantially improve sodium storage, resulting in a relatively low specific capacity. Therefore, this method does not significantly promote the development of high-energy-density sodium-ion batteries. Summary of the Invention

[0005] Therefore, one of the objectives of this invention is to provide a hard carbon material with a hollow structure and nitrogen doping, which can provide a high-quality and efficient ion channel for ion transport and has better charge and discharge performance.

[0006] The second objective of this invention is to provide a method for preparing hard carbon materials, which involves forming a cavity structure in the middle of the material by etching nickel foam and cellulose gel together, and forming a three-dimensional network structure composed of conductive polymer film and cellulose on the surface of nickel foam by electrochemical polymerization. The hard carbon material formed after carbonization has better charge and discharge performance.

[0007] The third objective of this invention is to provide the application of hard carbon materials, which, when used as negative electrode active materials, exhibit better charge and discharge performance.

[0008] To achieve the aforementioned first objective, a first aspect of the present invention provides a hard carbon material. The hard carbon material includes a hollow structure, a first hard carbon layer forming the peripheral wall of the hollow structure, and a second hard carbon layer enclosing the first hard carbon layer. The first hard carbon layer is made of nitrogen-doped hard carbon material, and the second hard carbon layer is made of hard carbon material formed by sintering cellulose.

[0009] The hollow structure of the hard carbon material of this invention has high porosity, which is beneficial for ion transport. The first hard carbon layer forming the peripheral wall of the hollow structure constitutes the framework structure of the hard carbon material, and nitrogen doping can generate defective active sites for energy storage, thereby improving specific capacity. Cellulose and nitrogen-doped raw materials can form a three-dimensional network structure before sintering. After sintering, nitrogen atoms are bonded within the complex network structure, which can improve the conductivity of the material, thus resulting in better initial efficiency. In summary, the hard carbon material of this invention has excellent charge-discharge performance, and is particularly suitable for sodium-ion battery systems.

[0010] In some embodiments, the hollow structure is tubular.

[0011] In some embodiments, the porosity is 30–82%.

[0012] To achieve the second objective mentioned above, a second aspect of the present invention provides a method for preparing hard carbon materials, comprising the steps of:

[0013] (1) Preparation of the first composite

[0014] A cellulose solution is prepared and then mixed with a crosslinking agent to obtain a mixed solution. Nickel foam is then immersed in the mixed solution, and after gelation, a cellulose gel encapsulating nickel foam is obtained, thus obtaining the first composite.

[0015] (2) Electrochemical polymerization yields the second complex.

[0016] Using a solution of conductive polymer monomers containing nitrogen elements as an electroplating solution, and the first composite as a working electrode, electrochemical polymerization is performed to encapsulate the first composite after the conductive polymer monomers have polymerized, thereby obtaining the second composite.

[0017] (3) First sintering

[0018] The second composite is sintered to obtain the first precursor;

[0019] (4) Etching

[0020] The first precursor is reacted with an iron salt solution to remove nickel foam, thereby obtaining a second precursor with a hollow structure.

[0021] (5) Second sintering

[0022] The second precursor is sintered to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0023] The technical advantages of the preparation method of the hard carbon material of this invention are as follows:

[0024] (1) Using nickel foam as a template, it is subsequently etched away by reacting with iron salt solution to form a hollow structure, thus having a high porosity, which is beneficial for ion transport. Moreover, nickel foam is low in cost, and its appearance, such as shape, density per unit area, and thickness, is easy to control. It is also easy to select a suitable electroplating solution and control it for large-scale production.

[0025] (2) Cellulose is selected to form a gel because it is widely available, inexpensive and easy to obtain. Moreover, the fiber itself has unique structural characteristics, forming a three-dimensional network structure of cellulose hydrogel, which has excellent water absorption and certain air permeability. After carbonization, it can form a network structure. At the same time, the fiber surface is rich in functional groups, which can form a unique pore structure after carbonization, which is conducive to ion transport.

[0026] (3) Due to the conductive properties of nickel foam, during electrochemical polymerization, the conductive polymer monomers containing nitrogen elements will selectively aggregate and polymerize at the interface between the cellulose gel and nickel foam through the pore structure of the cellulose gel and form a conductive polymer film. At the same time, the surface of the cellulose gel is rich in groups, which is conducive to the adsorption of conductive polymer monomers. Therefore, the grown conductive polymer film and cellulose gel can form a good three-dimensional network structure. After carbonization, a complex pore structure is formed, which has better ion channels.

[0027] (4) After the first sintering, the light molecules of cellulose itself are mainly removed. Then, etching is performed to remove the nickel foam. The resulting material is a composite material of cellulose and conductive polymer with a unique hollow structure. The conductive polymer can play a good role in the framework structure. Then, a second sintering is performed to reinforce the structure of cellulose and conductive polymer, resulting in a biomass-based hard carbon material with a hollow structure. Due to its unique structural characteristics, it can provide a high-quality and efficient ion channel for ion (especially sodium ion) transport. The N element doping provided by the conductive polymer can generate defective active sites for energy storage, thereby improving the specific capacity.

[0028] In some embodiments, cellulose is at least one of bacterial cellulose, wood pulp cellulose, and cotton cellulose.

[0029] In some embodiments, the alkaline urea solution is an aqueous solution comprising hydroxide and urea, wherein the hydroxide is LiOH, NaOH or KOH.

[0030] In some embodiments, the alkaline urea solution is an aqueous solution comprising hydroxide and urea, wherein the mass ratio of hydroxide to urea is 1 to 10:15.

[0031] In some embodiments, the alkaline urea solution is pre-cooled to below 15°C before the cellulose is dissolved.

[0032] In some embodiments, the cellulose solution and the crosslinking agent are mixed at a temperature below 5°C to obtain a mixed solution.

[0033] In some embodiments, the weight ratio of cellulose solution to crosslinking agent is 1 to 5:1.

[0034] In some embodiments, the crosslinking agent is epichlorohydrin, 1,2,3,4-tetracarboxybutane, or citric acid.

[0035] In some embodiments, the gel forming conditions are: standing reaction at below 10°C for 10 to 15 hours.

[0036] In some embodiments, the concentration of the nitrogen-containing conductive polymer monomer is from 0.1 mol / L to 1 mol / L.

[0037] In some embodiments, the nitrogen-containing conductive polymer monomer is pyrrole, aniline, or indole.

[0038] In some embodiments, the electroplating solution further includes p-toluenesulfonic acid at a concentration of 0.1 mol / L to 0.6 mol / L.

[0039] In some embodiments, the counter electrode used in electrochemical polymerization is a platinum counter electrode or a carbon counter electrode.

[0040] In some embodiments, electrochemical polymerization employs a bipolar electrochemical oxidation method.

[0041] In some embodiments, the sintering in step (3) is carried out in a high-temperature carbonization furnace, and the sintering temperature is 300°C to 500°C, and the sintering time is 1h to 5h.

[0042] In some embodiments, the sintering in step (5) is carried out in a high-temperature carbonization furnace, and the sintering temperature is 1000°C to 1600°C, and the sintering time is 1h to 6h.

[0043] In some embodiments, the sintering in step (3) and the sintering in step (5) are both carried out in a high-temperature carbonization furnace, and the gas atmosphere is nitrogen, helium or argon.

[0044] In some embodiments, the iron salt solution is a FeCl3 solution or a Fe(NO3)3 solution.

[0045] In some embodiments, the iron salt solution is an aqueous solution with an iron salt concentration of 0.5 mol / L to 1.5 mol / L.

[0046] In some embodiments, the first precursor is immersed in an iron salt solution for 6 to 10 hours to react and remove the nickel foam.

[0047] To achieve the aforementioned third objective, a third aspect of the present invention provides the application of the aforementioned hard carbon material in negative electrode materials and in sodium-ion batteries or supercapacitors. Sodium-ion batteries using the aforementioned hard carbon material as the negative electrode active material exhibit superior charge-discharge performance. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the hard carbon material of the present invention. Detailed Implementation

[0049] The hard carbon material of this invention has a unique porous structure, with a hollow interior and a mesh structure in the outer hard carbon layer, and is doped with nitrogen. The material has excellent charge and discharge performance and can be used in multiple technical fields such as lithium-ion batteries, sodium-ion batteries, and supercapacitors, and is especially suitable for use as a negative electrode material in sodium-ion batteries.

[0050] Sodium-ion batteries consist of a positive electrode, a negative electrode, a separator, and an electrolyte.

[0051] The positive electrode sheet is obtained by coating a positive electrode slurry containing a positive electrode active material, a binder, and a conductive agent onto a positive electrode current collector, followed by drying, cold pressing, and die cutting. The mass ratio of the positive electrode active material, binder, and conductive agent can be 90–99:0.5–10:0.5–10. The positive electrode active material is a material capable of releasing and accepting sodium ions, and may be, but is not limited to, one or more of sodium cobalt oxide, sodium manganese oxide, sodium-based multi-element transition metal compounds, sodium transition metal phosphates, and sodium transition metal fluorophosphates. The binder may be, but is not limited to, at least one of polyvinyl chloride, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinylpyrrolidone, styrene-butadiene rubber, acrylic styrene-butadiene rubber, and epoxy resin. Conductive agents are used to improve the conductivity of the positive electrode. Examples include, but are not limited to, carbon-containing materials such as carbon black, acetylene black, Ketjen black, and carbon fiber; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof. The positive electrode current collector can be aluminum foil. The solvent for the positive electrode slurry can be N-methylpyrrolidone or N-vinylpyrrolidone.

[0052] The separator can be a conventional insulating porous polymer film or an inorganic porous film, specifically, but not limited to, a single layer or multiple layers of polyethylene and polypropylene, such as a polyethylene / polypropylene double-layer separator, a polyethylene / polypropylene / polypropylene triple-layer separator, or a polypropylene / polypropylene / polypropylene triple-layer separator. An insulating layer that allows ions to pass through but not electrons can also be provided on the separator to prevent short circuits in the secondary battery during thermal shrinkage.

[0053] The electrolyte comprises a non-aqueous organic solvent, a sodium salt, and additives. The non-aqueous organic solvent may include chain carbonates, cyclic carbonates, carboxylic acid esters, or lactones. Chain carbonates may be dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. Cyclic carbonates may be ethylene carbonate, propylene carbonate, or butyl carbonate. Carboxylic acid esters may be methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, or ethyl propionate. Lactones may be γ-butyrolactone, β-decyl lactone, γ-pentyl lactone, or γ-caprolactone. The sodium salt may be one or more of NaPF6, NaClO4, NaAlCl4, NaFeCl4, NaSO3CF3, NaBCl4, NaNO3, NaPOF4, NaSCN, NaCN, NaAsF6, NaCF3CO2, NaSbF6, NaC6H5CO2, Na(CH3)C6H4SO3, NaHSO4, and NaB(C6H5)4, with a concentration of 0.5M to 2.0M. Additives may also be added to the electrolyte to improve battery performance; these additives may account for at least 0.1% to 10% of the electrolyte mass. These additives include, but are not limited to, one or more of vinyl sulfite (GS), fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), 4-methylethylene sulfate, 4-propylethylene sulfate, propylene sulfate, 4-methylpropylsulfite, and 4-propylpropylene sulfate.

[0054] The negative electrode sheet is obtained by coating a negative electrode slurry containing a negative electrode active material, a binder, and a conductive agent onto a negative electrode current collector, followed by drying, cold pressing, and die cutting. The negative electrode active material can be the hard carbon material of this invention alone, or it can be mixed with other negative electrode active materials (such as silicon-based negative electrode active materials, natural graphite, artificial graphite, etc.). The binder is used to improve the adhesion between negative electrode active material particles and between the negative electrode active material particles and the negative electrode current collector. It is selected from at least one of polyvinyl chloride, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, styrene-butadiene rubber, and acrylic styrene-butadiene rubber. The hard carbon material of this invention itself has good conductivity, so a conductive agent can be added or omitted depending on the actual use. The solvent for the negative electrode slurry can be N-methylpyrrolidone or N-vinylpyrrolidone. The negative electrode current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metals, etc.

[0055] The structure of the hard carbon material 100 of the present invention is as follows: Figure 1 As shown, the structure includes a hollow structure 10, a first hard carbon layer 30 forming the peripheral wall of the hollow structure 10, and a second hard carbon layer 50 enclosing the first hard carbon layer 30. The first hard carbon layer 30 is made of nitrogen-doped hard carbon material, and the second hard carbon layer 50 is made of hard carbon material sintered from cellulose. The hollow structure 10 is tubular, therefore the outline of the nickel foam is also tubular. The porosity of the hard carbon material 100 is 30-82%, and examples, but not limited to, 30%, 33%, 37%, 42%, 46%, 50%, 55%, 60%, 65%, 70%, 74%, 78%, and 82%.

[0056] The present invention relates to a method for preparing hard carbon materials, comprising the following steps:

[0057] (1) Preparation of the first composite

[0058] A cellulose solution is prepared and then mixed with a crosslinking agent to obtain a mixed solution. Nickel foam is then immersed in the mixed solution, and after gelation, a cellulose gel encapsulating nickel foam is obtained, thus obtaining the first composite.

[0059] (2) Electrochemical polymerization yields the second complex.

[0060] Using a solution of conductive polymer monomers containing nitrogen elements as the electroplating solution, and the first composite as the working electrode, electrochemical polymerization is carried out to encapsulate the first composite after the conductive polymer monomers are polymerized, thereby obtaining the second composite.

[0061] (3) First sintering

[0062] The second composite is sintered to obtain the first precursor;

[0063] (4) Etching

[0064] The first precursor is reacted with an iron salt solution to remove nickel foam, thereby obtaining a second precursor with a hollow structure.

[0065] (5) Second sintering

[0066] The second precursor is sintered to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0067] In step (1), preparing the cellulose solution involves dissolving cellulose in an alkaline urea solution, with the cellulose concentration ranging from 3% to 10 wt%. The specific concentration of cellulose may be, but is not limited to, 3%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%. Since cellulose is a macromolecular polysaccharide, it is insoluble in water, but dissolving it in an alkaline urea solution can produce a cellulose gel with better uniformity. Cellulose is at least one of bacterial cellulose, wood pulp cellulose, and cotton cellulose, and its sources are abundant. The alkaline urea solution is an aqueous solution comprising hydroxide and urea, wherein the hydroxide is LiOH, NaOH, or KOH. The mass ratio of hydroxide to urea is 1–10:15, and the specific mass ratio can be, but is not limited to, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, or 1:15. The alkaline urea solution is pre-cooled to below 15°C before cellulose dissolution to control the rate at which cellulose gels form.

[0068] A mixed solution is obtained by mixing the cellulose solution and the crosslinking agent at a temperature below 5°C to prevent the cellulose from gelling before the nickel foam is immersed in the mixed solution. The weight ratio of the cellulose solution to the crosslinking agent is 1 to 5:1, and the weight ratio can be, but is not limited to, 1:1, 2:1, 3:1, 4:1, or 5:1. The crosslinking agent is epichlorohydrin, 1,2,3,4-tetracarboxybutane, or citric acid. As an example, epichlorohydrin is used as the crosslinking agent because it can make the hydrogen bonds in the cellulose more densely crosslinked, resulting in a smaller pore size and better performance of the material after carbonization. The gelation conditions are: static reaction at a temperature below 10°C for 10 to 15 hours. In actual operation, the gelation temperature can be 10°C, 9°C, 8°C, 7°C, 6°C, 5°C, 4°C, 3°C, 2°C, or 1°C. The gelation time can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, or 15 hours. As an example, the gel forming temperature was 5°C and the time was 12 hours.

[0069] In step (2), the nitrogen-containing conductive polymer monomer is pyrrole, aniline, or indole. The concentration of the nitrogen-containing conductive polymer monomer is 0.1 mol / L to 1 mol / L. For example, the concentration of the conductive polymer monomer may be, but is not limited to, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, or 1 mol / L. The electroplating solution also includes p-toluenesulfonic acid with a concentration of 0.1 mol / L to 0.6 mol / L. The concentration of p-toluenesulfonic acid may be, but is not limited to, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, or 0.6 mol / L. Adding p-toluenesulfonic acid to the electroplating solution can increase its conductivity and improve the electroplating effect. Furthermore, during the electrochemical polymerization of conductive polymer monomers, sulfonate groups can be polymerized into the molecular chain, enhancing the electrochemical performance of the conductive polymer. The electrochemical polymerization uses a platinum or carbon counter electrode and employs a bipolar electrochemical oxidation method.

[0070] In step (3), sintering is carried out in a high-temperature carbonization furnace under a nitrogen, helium, or argon atmosphere. The sintering temperature is 300℃ to 500℃, and the sintering time is 1h to 5h. Specifically, the sintering temperature may be, but is not limited to, 300℃, 330℃, 350℃, 380℃, 400℃, 420℃, 450℃, 480℃, or 500℃. The sintering time may be, but is not limited to, 1h, 2h, 3h, 4h, or 5h.

[0071] In step (4), the iron salt solution is either FeCl3 solution or Fe(NO3)3 solution. As an example, FeCl3 solution is used as the etching solution, which helps control costs and overcomes pollution problems during preparation. Furthermore, the FeCl3 recovery process is simple. The iron salt solution is an aqueous solution with an iron salt concentration of 0.5 mol / L to 1.5 mol / L. The specific iron salt concentration can be, but is not limited to, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, or 1.5 mol / L. The first precursor can react and remove the nickel foam by soaking in the iron salt solution for 6 to 10 hours. The soaking time can be, but is not limited to, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours.

[0072] In step (5), sintering is carried out in a high-temperature carbonization furnace under a nitrogen, helium, or argon atmosphere. The sintering temperature is 1000℃ to 1600℃, and the sintering time is 1h to 6h. Specifically, the sintering temperature may be, but is not limited to, 1000℃, 1100℃, 1200℃, 1300℃, 1400℃, 1500℃, or 1600℃. The sintering time may be, but is not limited to, 1h, 2h, 3h, 4h, 5h, or 6h.

[0073] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0074] Example 1

[0075] This embodiment uses a hard carbon material, and its preparation method includes:

[0076] (1) Preparation of the first composite

[0077] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0078] (2) Electrochemical polymerization yields the second complex.

[0079] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0080] (3) First sintering

[0081] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0082] (4) Etching

[0083] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0084] (5) Second sintering

[0085] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0086] The porosity of the hard carbon material was determined to be 54% using a porosity meter.

[0087] Example 2

[0088] This embodiment uses a hard carbon material, and its preparation method includes:

[0089] (1) Preparation of the first composite

[0090] Cotton cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% cotton cellulose solution, with a NaOH to urea mass ratio of 1:15. Then, 1,2,3,4-tetracarboxybutane and the cotton cellulose solution were dissolved at a mass ratio of 1:5 and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cotton cellulose gel encapsulating nickel foam, thus completing the first composite.

[0091] (2) Electrochemical polymerization yields the second complex.

[0092] A 0.5 mol / L aniline monomer solution was prepared, and then a 0.3 mol / L p-toluenesulfonic acid solution was added to the aniline monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0093] (3) First sintering

[0094] The second composite was placed in a high-temperature carbonization furnace with an argon atmosphere, and sintered at 400°C for 3 hours to obtain the first precursor.

[0095] (4) Etching

[0096] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0097] (5) Second sintering

[0098] The second precursor was placed in a high-temperature carbonization furnace with an argon atmosphere and sintered at 1100℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0099] The porosity of the hard carbon material was measured to be 62% using a porosity meter.

[0100] Example 3

[0101] This embodiment uses a hard carbon material, and its preparation method includes:

[0102] (1) Preparation of the first composite

[0103] Wood pulp cellulose was dissolved in a pre-cooled KOH / urea solution at 5°C to prepare a 3wt% wood pulp cellulose solution, with a KOH to urea mass ratio of 6:15. Epichlorohydrin and the wood pulp cellulose solution were then dissolved in the solution at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a wood pulp cellulose gel encapsulating nickel foam, thus completing the first composite.

[0104] (2) Electrochemical polymerization yields the second complex.

[0105] A 1 mol / L indole monomer solution was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the indole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0106] (3) First sintering

[0107] The second composite was placed in a high-temperature carbonization furnace with a helium atmosphere, and sintered at 500°C for 3 hours to obtain the first precursor.

[0108] (4) Etching

[0109] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0110] (5) Second sintering

[0111] The second precursor was placed in a high-temperature carbonization furnace with a helium atmosphere, and sintered at 1200℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0112] The porosity of the hard carbon material was measured to be 74% using a porosity meter.

[0113] Example 4

[0114] This embodiment uses a hard carbon material, and its preparation method includes:

[0115] (1) Preparation of the first composite

[0116] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 6wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Citric acid and the bacterial cellulose solution were then dissolved in the solution at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in this mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0117] (2) Electrochemical polymerization yields the second complex.

[0118] A pyrrole monomer solution with a concentration of 0.2 mol / L was prepared, and then a 0.5 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0119] (3) First sintering

[0120] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 500°C for 3 hours to obtain the first precursor.

[0121] (4) Etching

[0122] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0123] (5) Second sintering

[0124] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1600℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0125] The porosity of the hard carbon material was measured to be 68% using a porosity meter.

[0126] Example 5

[0127] This embodiment uses a hard carbon material, and its preparation method includes:

[0128] (1) Preparation of the first composite

[0129] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 4wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 9:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0130] (2) Electrochemical polymerization yields the second complex.

[0131] A pyrrole monomer solution with a concentration of 0.2 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0132] (3) First sintering

[0133] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 400°C for 3 hours to obtain the first precursor.

[0134] (4) Etching

[0135] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0136] (5) Second sintering

[0137] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1400℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0138] The porosity of the hard carbon material was measured to be 78% using a porosity meter.

[0139] Example 6

[0140] This embodiment uses a hard carbon material, and its preparation method includes:

[0141] (1) Preparation of the first composite

[0142] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 1 wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 9:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0143] (2) Electrochemical polymerization yields the second complex.

[0144] A pyrrole monomer solution with a concentration of 0.4 mol / L was prepared, and then a 0.4 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0145] (3) First sintering

[0146] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0147] (4) Etching

[0148] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0149] (5) Second sintering

[0150] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1500℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0151] The porosity of the hard carbon material was measured to be 82% using a porosity meter.

[0152] Example 7

[0153] This embodiment uses a hard carbon material, and its preparation method includes:

[0154] (1) Preparation of the first composite

[0155] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 10wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0156] (2) Electrochemical polymerization yields the second complex.

[0157] A pyrrole monomer solution with a concentration of 1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0158] (3) First sintering

[0159] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0160] (4) Etching

[0161] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0162] (5) Second sintering

[0163] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1600℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0164] The porosity of the hard carbon material was measured to be 63% using a porosity meter.

[0165] Example 8

[0166] This embodiment uses a hard carbon material, and its preparation method includes:

[0167] (1) Preparation of the first composite

[0168] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0169] (2) Electrochemical polymerization yields the second complex.

[0170] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0171] (3) First sintering

[0172] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0173] (4) Etching

[0174] The first precursor was soaked in a 1.5 mol / L Fe(NO3)3 solution for 10 h, then removed and washed. The first precursor was then reacted with FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0175] (5) Second sintering

[0176] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0177] The porosity of the hard carbon material was measured to be 52% using a porosity meter.

[0178] Example 9

[0179] This embodiment uses a hard carbon material, and its preparation method includes:

[0180] (1) Preparation of the first composite

[0181] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 2°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a mass ratio of 1:3 and stirred at 2°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 8°C for 10 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0182] (2) Electrochemical polymerization yields the second complex.

[0183] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0184] (3) First sintering

[0185] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0186] (4) Etching

[0187] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0188] (5) Second sintering

[0189] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0190] The porosity of the hard carbon material was measured to be 62% using a porosity meter.

[0191] Example 10

[0192] This embodiment uses a hard carbon material, and its preparation method includes:

[0193] (1) Preparation of the first composite

[0194] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, 1,2,3,4-tetracarboxybutane and the bacterial cellulose solution were dissolved in a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0195] (2) Electrochemical polymerization yields the second complex.

[0196] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0197] (3) First sintering

[0198] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0199] (4) Etching

[0200] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0201] (5) Second sintering

[0202] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0203] The porosity of the hard carbon material was determined to be 32% using a porosity meter.

[0204] Example 11

[0205] This embodiment uses a hard carbon material, and its preparation method includes:

[0206] (1) Preparation of the first composite

[0207] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Citric acid and the bacterial cellulose solution were then dissolved in the solution at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in this mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0208] (2) Electrochemical polymerization yields the second complex.

[0209] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared, and then a 0.6 mol / L p-toluenesulfonic acid solution was added to the pyrrole monomer solution to prepare an electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0210] (3) First sintering

[0211] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0212] (4) Etching

[0213] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0214] (5) Second sintering

[0215] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0216] The porosity of the hard carbon material was found to be 50% using a porosity meter.

[0217] Example 12

[0218] This embodiment uses a hard carbon material, and its preparation method includes:

[0219] (1) Preparation of the first composite

[0220] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the first composite.

[0221] (2) Electrochemical polymerization yields the second complex.

[0222] A pyrrole monomer solution with a concentration of 0.1 mol / L was prepared as the electroplating solution. The first composite was used as the working electrode, and a platinum sheet was used as the counter electrode. The composite was placed in the electroplating solution, and electrochemical polymerization was carried out using a bipolar electrochemical oxidation method to obtain the second composite.

[0223] (3) First sintering

[0224] The second composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0225] (4) Etching

[0226] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0227] (5) Second sintering

[0228] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0229] The porosity of the hard carbon material was determined to be 59% using a porosity meter.

[0230] Comparative Example 1

[0231] This embodiment uses a hard carbon material, and its preparation method includes:

[0232] (1) Preparation of composite

[0233] Bacterial cellulose was dissolved in a pre-cooled LiOH / urea solution at 5°C to prepare a 3wt% bacterial cellulose solution, with a LiOH to urea mass ratio of 3:15. Then, epichlorohydrin and the bacterial cellulose solution were dissolved at a 1:1 mass ratio and stirred at 0°C to obtain a mixed solution. Nickel foam was then immersed in the mixed solution and allowed to stand at 5°C for 12 hours until gelation occurred, yielding a cellulose gel encapsulating the nickel foam, thus completing the composite.

[0234] (2) First sintering

[0235] The composite was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 300°C for 3 hours to obtain the first precursor.

[0236] (3) Etching

[0237] The first precursor was soaked in a 1 mol / L FeCl3 solution for 8 hours, then removed and washed. The first precursor was reacted with the FeCl3 solution to remove the nickel foam, thus obtaining a second precursor with a hollow structure.

[0238] (4) Second sintering

[0239] The second precursor was placed in a high-temperature carbonization furnace with a nitrogen atmosphere, and sintered at 1000℃ for 3 hours to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

[0240] The porosity of the hard carbon material was measured to be 28% using a porosity meter.

[0241] The hard carbon anodes obtained in Examples 1 to 12 and Comparative Example 1 were used as the anodes of coin cells, with sodium metal sheets as the cathodes and 1 mol / L NaPF6 / EC+DEC (wt:wt, 1:1) as the electrolyte, and were assembled into CR2032 coin cells. Battery testing was conducted using the Blue Battery Testing System, employing a constant current charge-discharge method. The test steps were: ① resting for 10 min; ② discharging at a rate of 0.1C until the voltage ≤ 0V; ③ resting for 10 min; ④ charging at a rate of 0.1C until the voltage ≥ 2.5V. The test results are shown in Table 1.

[0242] Table 1. Electrochemical performance of hard carbon materials obtained from each group of examples.

[0243] Example Porosity (%) Reversible specific capacity (mAh / g) First-time coulomb efficiency (%) Example 1 54 349 92 Example 2 62 365 90 Example 3 74 390 93 Example 4 68 370 95 Example 5 78 421 92 Example 6 82 404 90 Example 7 63 415 89 Example 8 52 367 89 Example 9 62 359 88 Example 10 32 326 84 Example 11 50 311 85 Example 12 59 303 82 Comparative Example 1 28 280 75

[0244] As shown in Table 1, compared to Comparative Example 1, the hard carbon materials formed by etching and electrochemical polymerization in Examples 1-12 have higher porosity, reversible specific capacity, and initial coulombic effect. This is due to the conductive properties of nickel foam. During electrochemical polymerization, nitrogen-containing conductive polymer monomers selectively aggregate and polymerize at the interface between the cellulose gel and nickel foam through the porous structure of the cellulose gel, forming a conductive polymer film. Simultaneously, the cellulose gel surface is rich in functional groups, which facilitates the adsorption of conductive polymer monomers. Therefore, the grown conductive polymer film and cellulose gel can form a good three-dimensional network structure. After carbonization, a complex mesh structure is formed, providing excellent ion channels. After etching away the nickel foam, the resulting material is a composite material of cellulose and conductive polymer with a unique hollow structure. The conductive polymer effectively acts as a skeletal structure. A second sintering process further reinforces the structure of the cellulose and conductive polymer, resulting in a biomass-based hard carbon material with a hollow structure. Due to its unique structural characteristics, it provides high-quality and efficient ion channels for ion transport, while the nitrogen doping provided by the conductive polymer generates defective active sites for energy storage, thereby improving the specific capacity.

[0245] Comparing Examples 1 and 10 and 11, it can be seen that the material has better performance when the crosslinking agent is epichlorohydrin.

[0246] Comparing Examples 1 and 12, it can be seen that the electrochemical performance of the material obtained by adding p-toluenesulfonic acid to the electroplating solution is better.

[0247] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A hard carbon material, characterized in that, It includes a hollow structure, a first hard carbon layer forming the peripheral wall of the hollow structure, and a second hard carbon layer wrapping the first hard carbon layer. The first hard carbon layer is made of nitrogen-doped hard carbon material, and the second hard carbon layer is made of hard carbon material sintered from cellulose. Both the first hard carbon layer and the second hard carbon layer have a mesh structure.

2. The hard carbon material according to claim 1, characterized in that, The hollow structure is tubular.

3. The hard carbon material according to claim 1, characterized in that, The porosity is 30-82%.

4. A method for preparing hard carbon materials, characterized in that, Including the following steps: (1) Preparation of the first composite A cellulose solution is prepared and then mixed with a crosslinking agent to obtain a mixed solution. Nickel foam is then immersed in the mixed solution, and after gelation, a cellulose gel encapsulating nickel foam is obtained, thus obtaining the first composite. (2) Electrochemical polymerization yields the second complex. Using a solution of conductive polymer monomers containing nitrogen elements as an electroplating solution, and the first composite as a working electrode, electrochemical polymerization is performed to encapsulate the first composite after the conductive polymer monomers have polymerized, thereby obtaining the second composite. (3) First sintering The second composite is sintered to obtain the first precursor; (4) Etching The first precursor is reacted with an iron salt solution to remove nickel foam, thereby obtaining a second precursor with a hollow structure. (5) Second sintering The second precursor is sintered to obtain a composite hard carbon material with a hollow structure and nitrogen doping.

5. The method for preparing hard carbon material according to claim 4, characterized in that, The preparation of the cellulose solution involves dissolving cellulose in an alkaline urea solution, wherein the concentration of the cellulose is 3% to 10 wt%.

6. The method for preparing hard carbon material according to claim 5, characterized in that, Includes at least one of the following features (1) to (4): (1) The cellulose is at least one of bacterial cellulose, wood pulp cellulose and cotton cellulose; (2) The alkaline urea solution is an aqueous solution comprising hydroxide and urea, wherein the hydroxide is LiOH, NaOH or KOH; (3) The alkaline urea solution is an aqueous solution comprising hydroxide and urea, wherein the mass ratio of hydroxide to urea is 1 to 10:15; (4) The alkaline urea solution is pre-cooled to below 15°C before the cellulose is dissolved.

7. The method for preparing hard carbon material according to claim 4, characterized in that, Includes at least one of the following features (1) to (15): (1) The cellulose solution and the crosslinking agent are mixed at a temperature below 5°C to obtain the mixed solution; (2) The weight ratio of the cellulose solution to the crosslinking agent is 1 to 5:1; (3) The crosslinking agent is epichlorohydrin, 1,2,3,4-tetracarboxybutane or citric acid; (4) The gel forming conditions are: standing reaction at 10°C or below for 10 to 15 hours; (5) The concentration of the nitrogen-containing conductive polymer monomer is 0.1 mol / L to 1 mol / L; (6) The nitrogen-containing conductive polymer monomer is pyrrole, aniline or indole; (7) The electroplating solution further includes p-toluenesulfonic acid with a concentration of 0.1 mol / L to 0.6 mol / L; (8) The counter electrode used in the electrochemical polymerization is a platinum counter electrode or a carbon counter electrode; (9) The electrochemical polymerization is carried out using a bipolar electrochemical oxidation method; (10) The sintering in step (3) is carried out in a high-temperature carbonization furnace, and the sintering temperature is 300°C to 500°C, and the sintering time is 1h to 5h. (11) The sintering in step (5) is carried out in a high-temperature carbonization furnace, and the sintering temperature is 1000℃ to 1600℃, and the sintering time is 1h to 6h. (12) The sintering in step (3) and the sintering in step (5) are both carried out in a high-temperature carbonization furnace, and the gas atmosphere is nitrogen, helium or argon. (13) The iron salt solution is FeCl3 solution or Fe(NO3)3 solution; (14) The iron salt solution is an aqueous solution with an iron salt concentration of 0.5 mol / L to 1.5 mol / L; (15) The first precursor is soaked in the iron salt solution for 6 to 10 hours to react and remove the foamed nickel.

8. The application of the hard carbon material according to any one of claims 1 to 3, or the hard carbon material prepared by the preparation method according to any one of claims 4 to 7, in anode materials.

9. The application of the hard carbon material prepared by the method of preparing the hard carbon material according to any one of claims 1 to 3, or according to any one of claims 4 to 7, as a negative electrode material in sodium-ion batteries or supercapacitors.

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