A hard carbon material prepared by atmosphere-adjusted hard carbon closed pores and pseudo-graphite domains and a preparation method and application thereof

Abstract

The application belongs to the technical field of energy storage materials, and particularly relates to a hard carbon material prepared by atmosphere modulation of hard carbon closed pores and pseudo-graphite domains, and a preparation method and application thereof. The application takes biomass walnut shells as raw materials, adopts argon atmosphere or air pre-oxidation combined with high-temperature treatment of argon atmosphere or hydrogen-containing mixed gas, and modulates the closed pores and pseudo-graphite domains of the hard carbon by changing the atmosphere. The raw materials used in the application are low in cost, and the method is simple and easy to scale. The hard carbon material obtained by the application has good electrical conductivity and fast ion transmission characteristics, significantly improves the initial coulomb efficiency, and the rich closed pore structure of the hard carbon material is beneficial to realizing high-platform sodium storage capacity.

Description

Technical Field

[0001] This invention belongs to the field of energy storage materials technology, specifically relating to a hard carbon material prepared by atmosphere-modulated hard carbon closed pores and pseudographite domains, its preparation method, and its application. Background Technology

[0002] The rapid depletion of fossil fuels, severe global climate problems, and the growing demand for electric or hybrid vehicles have led to an increasing need for grid-scale energy storage systems with higher power / energy density and operability under extreme conditions. Lithium-ion batteries, with their high energy density and cycle stability, have been widely used in electronic devices and new energy vehicles. However, the scarcity (0.0017 wt.%) and uneven distribution of lithium resources pose significant obstacles to the widespread application of lithium-ion batteries in large-scale energy storage systems. Therefore, developing new, cost-effective, high-performance, and resource-saving rechargeable batteries is an inevitable trend. Sodium resources are abundant (2.3 wt.%) and inexpensive, making them an ideal alternative to lithium-ion batteries. This is because the radius of sodium ions is much larger than that of lithium ions, and the interlayer spacing of graphite (0.335 nm) is smaller than that of sodium ions. + The required interlayer spacing (0.370 nm) makes graphite unsuitable for storing sodium ions as a negative electrode in lithium-ion batteries, thus necessitating the development of a suitable negative electrode for sodium ion storage.

[0003] Hard carbon, composed of tortuous graphite domains and numerous nanoscale pores, is considered a preferred anode material for sodium-ion batteries. The twisted structure enhances the repulsion and interaction between carbon nanosheets, resulting in a larger interlayer spacing (0.3-0.42 nm), which can benefit Na... + Rapid sodium transport is achieved. The interlayer spacing of pseudographite domains, between 0.37 and 0.40 nm, is optimal for sodium storage. Therefore, controlling the interlayer spacing of hard carbon and the structure of pseudographite domains is key to achieving high-capacity sodium storage.

[0004] The charge-discharge curves of hard carbon for sodium storage exhibit a ramp region (>0.1 V) and a plateau region (<0.1 V), corresponding to sodium ion adsorption at defects, interlayer embedding, and closed-cell filling, respectively. However, improving the plateau capacity of the anode material is key to increasing the energy density of sodium-ion batteries. High plateau capacity requires the coordinated action of pseudographite domains and nanoscale closed-cell structures. Both low and high carbonization temperatures affect the formation of pseudographite domains. Lower temperatures lead to excessive disorder in hard carbon, resulting in too many defects and irreversible sodium ion adsorption, while higher temperatures increase the degree of graphitization, reducing interlayer spacing and hindering effective sodium ion storage, thus decreasing the plateau capacity. The literature "Regulating oxygen functionalities of cellulose derived hard carbon toward superior sodium storage (J. Mater. Chem. A, 2024, 12, 5834.)" modulates the formation of pseudographite domains. Researchers induce molecular chain crosslinking by cleaving cellulose glycosidic bonds at different pre-oxidation temperatures. The oxidative cross-linking process of cellulose generates more C=O while retaining some CO. During pyrolysis, the graphite microcrystal structure becomes finer and longer, forming larger closed pores. Related literature indicates that pre-oxidation can introduce more C=O, hindering graphitization formation during high-temperature carbonization. Furthermore, the literature "A porous biomass-derived anode for high-performance sodium-ion batteries (Carbon, 2018, 129, 695-701.)" shows that reduction treatment can remove some oxygen defects, thereby improving the initial coulombic efficiency of hard carbon anode materials. However, significant challenges remain in the coordinated development of closed pores and pseudographite domains.

[0005] Therefore, there is an urgent need for a simple, efficient and easily scalable method to synergistically prepare hard carbon anode materials rich in closed-pore and pseudo-graphite domains. Summary of the Invention

[0006] To address existing problems, this invention aims to provide a hard carbon material prepared by atmosphere modulation of closed-cell hard carbon and pseudographite domains, along with its preparation method and applications. This invention uses biomass walnut shells as raw material and employs argon atmosphere or air pre-oxidation combined with high-temperature treatment in an argon atmosphere or a hydrogen-containing mixed gas to modulate the closed-cell and pseudographite domain structures of the hard carbon. In the first stage, argon atmosphere pre-carbonization cleaves C-C bonds, effectively destroying long side chains on individual aromatic rings of lignin; or air pre-oxidation introduces oxygen to promote the formation of CO-C bonds and oxygen-containing covalent bonds, promoting cross-linking during thermal decomposition to form a three-dimensional network structure. In the second stage, during high-temperature processing, argon atmosphere promotes the formation of closed cells, or the reduction in a hydrogen-containing mixed gas facilitates the removal of oxygen-containing covalent bonds and dangling bonds, thereby effectively modulating the pseudographite domains and closed cells of the hard carbon. This significantly improves the plateau capacity and first coulombic efficiency, thus achieving excellent sodium storage performance.

[0007] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains includes the following steps:

[0008] (1) Weigh the walnut shells, wash them with deionized water, and then dry them in a forced-air drying oven.

[0009] (2) Place the dried walnut shells in a crusher for crushing and sieving;

[0010] (3) Wash the sieved walnut shell powder with hydrochloric acid to remove impurities and inorganic components from the walnut shell, then wash it with deionized water until neutral, and place it in a forced-air drying oven for drying.

[0011] (4) Place the acid-washed walnut shell powder in a muffle furnace for pre-oxidation or in a tube furnace for pre-carbonization. After cooling to room temperature, remove the powder to obtain pre-oxidized or pre-carbonized material.

[0012] (5) Place the pre-oxidized or pre-carbonized material obtained in step (4) into a high-temperature tube furnace for high-temperature reaction, cool it to room temperature and take it out to obtain hard carbon material.

[0013] Preferably, the particle size of the walnut shell powder after sieving in step (2) is about 40-75 µm, so as to ensure that the prepared hard carbon anode material rich in pseudographite domains has excellent sodium storage performance and high tap density.

[0014] Preferably, the hydrochloric acid in step (3) is 0.5-2 mol / L, more preferably 1 mol / L, to ensure that the appropriate hydrochloric acid concentration can remove inorganic salts and other impurities.

[0015] Preferably, the pre-carbonization or pre-oxidation temperature in step (4) is 200-400 ℃, more preferably 300 ℃ for pre-oxidation or pre-carbonization, for 1-3 h, and the heating rate is 5 ℃ / min.

[0016] Preferably, in step (4), when the material is placed in a muffle furnace for pre-oxidation, the pre-oxidation atmosphere is air; when the material is placed in a tube furnace for pre-carbonization, the pre-carbonization atmosphere is argon.

[0017] Preferably, the high-temperature reaction temperature in step (5) is 1200-1500 ℃, more preferably 1400 ℃, the reaction time is 1-3 h, and the heating rate is 3 ℃ / min.

[0018] Preferably, the atmosphere for the high-temperature reaction of the pre-oxidized material obtained in step (4) in step (5) is argon; the atmosphere for the high-temperature reaction of the pre-carbonized material obtained in step (4) is a hydrogen-containing mixed gas, and more preferably, the concentration of the hydrogen-containing mixed gas is 5%-10%.

[0019] The present invention also provides the application of the above-mentioned hard carbon material in sodium-ion batteries.

[0020] Beneficial effects

[0021] (1) The raw material used in this invention is walnut shell, which is inexpensive, simple and easy to scale up.

[0022] (2) The present invention modulates the closed pores and pseudo-graphite domains of hard carbon by changing the atmosphere. The pre-carbonization of argon atmosphere or pre-oxidation of air shears the C-C bonds at a certain temperature. Pre-oxidation can also introduce oxygen elements to form oxygen-containing covalent bonds such as COC bonds, which promotes the formation of a stable three-dimensional network structure during pyrolysis. At high temperature, more pseudo-graphite domains and closed nanopores are formed by argon atmosphere or hydrogen mixed atmosphere.

[0023] (3) The pseudo-graphite domains modulated by changing the atmosphere have good conductivity and fast ion transport characteristics; the modulated rich closed-pore structure is conducive to achieving a high sodium storage capacity. Attached Figure Description

[0024] Figure 1 This is a TEM image of the hard carbon prepared in Example 1 of the present invention;

[0025] Figure 2 The XRD patterns are of the hard carbon prepared in Examples 1-3 of this invention;

[0026] Figure 3 This is a second charge-discharge curve of the hard carbon prepared in Examples 1-3 of this invention;

[0027] Figure 4The rate performance diagrams are for the hard carbon prepared in Examples 1-3 of this invention;

[0028] Figure 5 The XRD patterns are of the hard carbon prepared in Examples 1 and 4-8 of this invention.

[0029] Figure 6 The second charge-discharge curves of the hard carbon prepared in Examples 1 and 4-8 of this invention are shown.

[0030] Figure 7 The rate performance diagrams are for the hard carbon prepared in Examples 1 and 4-8 of this invention. Detailed Implementation

[0031] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer with the description. However, the embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0032] Example 1

[0033] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0034] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0035] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water and crush them. Sieve them through a 200-mesh sieve, ensuring that the particle size is below 74 µm, so as to ensure better pre-oxidation and promote the destruction of C-C bonds and the formation of oxygen-containing covalent bonds during the pyrolysis process.

[0036] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0037] (4) The acid-washed walnut shell powder was placed in a muffle furnace for pre-oxidation treatment. The pre-oxidation temperature was 300℃, the pre-oxidation time was 2 h, and the heating rate was 5℃ / min. After the temperature was cooled to room temperature, it was taken out to obtain the pre-oxidized material. The purpose of pre-oxidation at 300℃ for 2 h is to ensure the decomposition of cellulose and hemicellulose and to effectively destroy the long side chains on the single aromatic ring of lignin, and to promote the cross-linking of C and O atoms during the thermal decomposition process to form a three-dimensional network structure.

[0038] (5) The pre-oxidized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400°C for 2 h under a hydrogen-argon mixed atmosphere. The heating rate is 3 °C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material. The hard carbon material has abundant pseudo-graphite domains and closed-pore structure. During the high-temperature process, the hydrogen-argon mixed atmosphere is used to regulate the oxygen content and remove some oxygen defects, which is conducive to the formation of more pseudo-graphite domains and closed pores. The pseudo-graphite domains provide suitable interlayer spacing for the rapid transport of sodium ions, while the abundant closed pores provide more sites for the storage of sodium ions.

[0039] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0040] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0041] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0042] Figure 1 The image shows a TEM image of the hard carbon prepared in Example 1 of this invention. As can be seen from the image, there are abundant pseudo-graphite microcrystalline structures and abundant closed-pore structures, which are beneficial to improving the sodium storage capacity of the plateau region.

[0043] The hard carbon anode material prepared under the above conditions exhibits a current density of 354.5 mAh g⁻¹ at 25 mA / g. -1 Reversible capacity.

[0044] Example 2

[0045] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0046] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0047] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water and crush them. Sieve them through a 200-mesh sieve, ensuring that the particle size is below 74 µm, so as to ensure better pre-oxidation and promote the destruction of C-C bonds and the formation of oxygen-containing covalent bonds during the pyrolysis process.

[0048] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0049] (4) The acid-washed walnut shell powder was placed in a muffle furnace for pre-oxidation treatment. The pre-oxidation temperature was 300℃, the pre-oxidation time was 1 h, and the heating rate was 5℃ / min. After the temperature cooled to room temperature, it was taken out to obtain the pre-oxidized material.

[0050] (5) The pre-oxidized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400℃ for 2 h in a hydrogen-argon mixed atmosphere. The heating rate is 3℃ / min. After cooling to room temperature, the material is taken out to obtain hard carbon material. Due to the short oxidation time and long reduction time, more graphite-like domains are formed in the hard carbon, which is not conducive to the storage of sodium ions.

[0051] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0052] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0053] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0054] The hard carbon anode material prepared under the above conditions has a yield of 25 mA g. -1 It has a current density of 306.2 mAhg -1 Reversible capacity.

[0055] Example 3

[0056] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0057] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0058] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water and crush them. Sieve them through a 200-mesh sieve, ensuring that the particle size is below 74 µm, so as to ensure better pre-oxidation and promote the destruction of C-C bonds and the formation of oxygen-containing covalent bonds during the pyrolysis process.

[0059] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0060] (4) The acid-washed walnut shell powder was placed in a muffle furnace for pre-oxidation treatment. The pre-oxidation temperature was 300℃, the pre-oxidation time was 3 h, the heating rate was 5℃ / min, and the powder was taken out after cooling to room temperature to obtain the pre-oxidized material.

[0061] (5) The pre-oxidized material obtained in step (4) was placed in a high-temperature tube furnace and reduced at 1400°C for 2 hours under a hydrogen-argon mixed atmosphere. The heating rate was 3°C / min. After cooling to room temperature, the material was removed to obtain hard carbon material. However, due to the long oxidation time and insufficient reduction time, most of the amorphous carbon material in the hard carbon material did not transform into pseudographite domains.

[0062] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0063] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0064] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0065] The hard carbon anode material prepared under the above conditions has a yield of 25 mA g.-1 It has a current density of 324.5 mAhg -1 Reversible capacity.

[0066] Figure 2 The XRD patterns of Examples 1-3 show that, after appropriate pre-oxidation treatment followed by reduction with an argon-hydrogen mixture, abundant pseudographite domains and closed-cell structures were obtained. A shorter pre-oxidation time results in the formation of more graphite-like domains, which is detrimental to sodium ion storage. An excessively long pre-oxidation time prevents the amorphous carbon material from converting into pseudographite domains.

[0067] Figure 3 The figures show the charge-discharge curves for the second cycle of Examples 1-3. As can be seen from the figures, after appropriate pre-oxidation treatment combined with reduction by an argon-hydrogen mixture, the system exhibits high plateau capacity and specific capacity, with the plateau region capacity reaching as high as 246.7 mAh g⁻¹. -1 .

[0068] Figure 4 The figures show the rate performance of Examples 1-3. As can be seen from the figures, the appropriate pre-oxidation treatment yielded excellent rate performance.

[0069] Example 4

[0070] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0071] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0072] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water and crush them. Sieve them through a 200-mesh sieve, ensuring that the particle size is below 74 µm, so as to ensure better pre-oxidation and promote the destruction of C-C bonds and the formation of oxygen-containing covalent bonds during the pyrolysis process.

[0073] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0074] (4) The acid-washed walnut shell powder was placed in a muffle furnace for pre-oxidation treatment. The pre-oxidation temperature was 300℃, the pre-oxidation time was 2 h, and the heating rate was 5 ℃ / min. After cooling to room temperature, the powder was removed to obtain the pre-oxidized material. The purpose of pre-oxidation at 300℃ for 2 h is to ensure the decomposition of cellulose and hemicellulose and to effectively destroy the long side chains on the single aromatic rings of lignin, thereby promoting the cross-linking of C and O atoms during the thermal decomposition process to form a three-dimensional network structure.

[0075] (5) The pre-oxidized material obtained in step (4) was placed in a high-temperature tube furnace and reduced at 1400 °C for 2 h under an argon atmosphere at a heating rate of 3 °C / min. After cooling to room temperature, the material was removed to obtain hard carbon material. This was to verify that the most suitable system was the oxidation-reduction system. High temperature is conducive to the formation of closed pores, but the introduction of more oxygen-containing functional groups at high temperature hinders the formation of pseudographite domains, resulting in a large number of amorphous structures in the carbonized material.

[0076] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0077] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0078] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0079] The hard carbon anode material prepared under the above conditions has a yield of 25 mA g. -1 It has a current density of 338.2 mAhg -1 Reversible capacity.

[0080] Example 5

[0081] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0082] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0083] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water, crush them, and sieve them through a 200-mesh screen, where the particle size is below 74 µm.

[0084] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0085] (4) The acid-washed walnut shell powder is placed in a tube furnace and pre-carbonized under an argon atmosphere. The pre-carbonization temperature is 300 ℃, the pre-carbonization time is 2 h, the heating rate is 5 ℃ / min, and the pre-carbonized material is obtained after the temperature is cooled to room temperature.

[0086] (5) The pre-carbonized material obtained in step (4) is placed in a high-temperature tube furnace and reacted at 1400 °C under an argon atmosphere for 2 h with a heating rate of 3 °C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material.

[0087] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0088] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0089] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0090] Because no hydrogen or oxygen is introduced under these conditions, the material after high-temperature carbonization has the most amorphous structures and fewer pseudo-graphite domains.

[0091] The hard carbon anode material prepared under the above conditions has a yield of 25 mA g. -1 It has a current density of 326.8 mAhg -1 Reversible capacity.

[0092] Example 6

[0093] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0094] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0095] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water, crush them, and sieve them through a 200-mesh screen, where the particle size is below 74 µm.

[0096] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0097] (4) The acid-washed walnut shell powder is placed in a tube furnace and pre-carbonized under an argon atmosphere. The pre-carbonization temperature is 300 ℃, the pre-carbonization time is 2 h, the heating rate is 5 ℃ / min, and the pre-carbonized material is obtained after the temperature is cooled to room temperature.

[0098] (5) The pre-carbonized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400°C for 2 hours under a hydrogen-argon mixed atmosphere. The heating rate is 3°C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material.

[0099] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0100] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0101] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0102] Because this invention involves only hydrogen reduction without the introduction of oxygen under these conditions, excessive oxygen removal occurs during high-temperature carbonization. Therefore, the material after high-temperature carbonization exhibits more graphite-like domains, which is detrimental to sodium ion storage.

[0103] The hard carbon anode material prepared under the above conditions has a yield of 25 mA g. -1 It has a current density of 327.9 mAhg -1 Reversible capacity.

[0104] Example 7

[0105] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0106] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0107] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water, crush them, and sieve them through a 200-mesh screen, where the particle size is below 74 µm.

[0108] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0109] (4) The acid-washed walnut shell powder is placed in a tube furnace and pre-carbonized under a hydrogen atmosphere. The pre-carbonization temperature is 300 ℃, the pre-carbonization time is 2 h, the heating rate is 5 ℃ / min, and the pre-carbonized material is obtained after the temperature is cooled to room temperature.

[0110] (5) The pre-carbonized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400 °C for 2 h under an argon atmosphere. The heating rate is 3 °C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material.

[0111] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0112] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0113] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0114] Because the low-temperature hydrogen reduction under these conditions removes a small amount of oxygen, it prevents carbon atoms from connecting, resulting in the carbon material after high-temperature treatment exhibiting more amorphous structures.

[0115] The hard carbon anode material prepared under the above conditions exhibits a current density of 321.6 mAh / g at 25 mA / g. -1 Reversible capacity.

[0116] Example 8

[0117] A method for preparing hard carbon materials by atmosphere modulation of closed-cell hard carbon and pseudographite domains

[0118] (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying.

[0119] (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water, crush them, and sieve them through a 200-mesh screen, where the particle size is below 74 µm.

[0120] (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. Sufficient stirring time ensures that the hydrochloric acid can fully remove inorganic salts and other impurities from the walnut shell.

[0121] (4) The acid-washed walnut shell powder is placed in a tube furnace and pre-carbonized under a hydrogen-argon mixed atmosphere. The pre-carbonization temperature is 300 ℃, the pre-carbonization time is 2 h, the heating rate is 5 ℃ / min, and the pre-carbonized material is obtained after the temperature is cooled to room temperature.

[0122] (5) The pre-carbonized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400°C for 2 hours under a hydrogen-argon mixed atmosphere. The heating rate is 3°C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material.

[0123] (6) Preparation of electrode sheet: Weigh the prepared hard carbon material rich in pseudographite domains, acetylene black and binder PVDF in a mass ratio of 8:1:1 and mix them evenly in a mortar. Add an appropriate amount of N-methylpyrrolidone (NMP) solvent, stir evenly to obtain a slurry and coat it on copper (Cu) foil. Dry it in a vacuum drying oven at 120 ℃ for 12 h and cut it into 12 mm circular negative electrode sheet for later use.

[0124] (7) The button cell assembly is carried out in a glove box filled with inert gas. The hard carbon material electrode prepared in step (5) is used as the negative electrode, the commercial electrolyte 1M NaPF6DIGLYME=100 Vol% is used as the electrolyte, and the metal Na sheet is used as the counter electrode. The button cell is then assembled.

[0125] (8) The assembled 2025 button battery was charged and discharged on a constant current test system with a voltage range of 0-2 V.

[0126] Under these conditions, the invention removes a small amount of oxygen through low-temperature hydrogen reduction, and further promotes the formation of graphitization by continuing hydrogen reduction during the high-temperature carbon atom rearrangement process.

[0127] The hard carbon anode material prepared under the above conditions exhibits a current density of 326.3 mAh / g at 25 mA / g. -1 Reversible capacity.

[0128] Figure 5 XRD patterns of pre-carbonization or pre-oxidation under different atmospheres and high-temperature treatment under different atmospheres are shown. The figures reveal that the first stage of pre-oxidation or pre-carbonization cleaves C-C bonds, effectively destroying long side chains on individual aromatic rings of lignin; or air pre-oxidation introduces oxygen to promote the formation of CO-C bonds and oxygen-containing covalent bonds, promoting cross-linking during thermal decomposition to form a three-dimensional network structure, further regulating the pseudographite domains and closed-cell structure. It was found that pre-oxidation and high-temperature hydrogen reduction are beneficial for achieving a reasonable combination of pseudographite domains and closed-cell structures.

[0129] Figure 6 The second charge-discharge curves for pre-carbonization or pre-oxidation in different atmospheres and high-temperature treatment in different atmospheres are shown in the figure. As can be seen from the figure, pre-oxidation and high-temperature hydrogen reduction yield the highest specific capacity and the longest plateau capacity.

[0130] Figure 7 The rate performance of pre-carbonization or pre-oxidation under different atmospheres and high-temperature treatment under different atmospheres is shown in the figure. As can be seen from the figure, the best rate performance was obtained by pre-oxidation and high-temperature hydrogen reduction.

[0131] Analysis of the above embodiments shows that both long and short oxidation times are not conducive to the formation of hard carbon closed pores; the structure of pseudo-graphite domains and closed pores can be optimized by adjusting the treatment under different atmospheres.

[0132] The foregoing description illustrates the basic principles, main features, and advantages of this invention. It should be understood by those skilled in the art that this invention is not limited to the specific descriptions of the above embodiments. The embodiments and descriptions provided merely illustrate the principles of this invention. Any variations and modifications without departing from the spirit and scope of this invention should be considered part of this invention and protected. Therefore, the scope of protection of this invention should be determined according to the appended claims and their equivalents.

Claims

1. A method for preparing hard carbon materials by atmosphere-modulated hard carbon closed-cell and pseudographite domains, characterized in that, Includes the following steps: (1) Weigh 500 g of walnut shells and wash them in deionized water. After washing, place them in a drying oven for drying. (2) Weigh 200 g of walnut shells that have been washed and dried with deionized water, crush them, and sieve them through a 200-mesh screen, where the particle size is below 74µm; (3) Weigh 5 g of sieved walnut shell powder and place it in 1 mol / L hydrochloric acid and stir for 12 h. Wash with deionized water until neutral and dry in a forced-air drying oven at 60 ℃. (4) The acid-washed walnut shell powder was placed in a muffle furnace for pre-oxidation treatment. The pre-oxidation temperature was 300 ℃, the pre-oxidation time was 2 h, the heating rate was 5 ℃ / min, and the pre-oxidized material was obtained after the temperature cooled to room temperature. (5) The pre-oxidized material obtained in step (4) is placed in a high-temperature tube furnace and reduced at 1400 °C for 2 h under a hydrogen-argon mixed atmosphere. The heating rate is 3 °C / min. After cooling to room temperature, the material is taken out to obtain hard carbon material.

2. The hard carbon material obtained by the preparation method described in claim 1.

3. The application of the hard carbon material as described in claim 2 in sodium-ion batteries.

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