Modified hard carbon material, and preparation method and application thereof

Abstract

This invention discloses a modified hard carbon material, its preparation method, and its application. The preparation method includes the following steps: S1, heating and holding biomass hard carbon in an aerobic environment to obtain oxygen-doped biomass hard carbon, wherein the heating temperature is 250~350℃, the heating rate is 2~5℃ / min, and the holding time is 2~5h; S2, introducing a protective atmosphere before the end of S1, heating and holding to obtain stabilized biomass hard carbon, wherein the heating temperature is 400℃~800℃, the heating rate is 2~10℃ / min, and the holding time is 0.5~2.5h; S3, heating and holding the biomass hard carbon treated in S2 under a protective atmosphere. This invention combines oxygen doping treatment and stabilization process of biomass hard carbon to obtain hard carbon material through carbon layer rearrangement at high temperature, which can improve carbon yield, eliminate impurities, and has the advantages of high initial charge capacity and high initial efficiency.

Description

Technical Field

[0001] This invention relates to the field of battery anode material technology, specifically to a modified hard carbon material, its preparation method, and its application. Background Technology

[0002] With the recent surge in research on sodium-ion batteries, traditional graphite materials have proven inadequate in forming stable intercalation compounds with sodium ions; the maximum stoichiometry of sodium in graphite is only that of NaCl. 86 or NaC 64 With a capacity of only 12 mAh / g or 36 mAh / g, it is far from meeting the development needs of sodium-ion batteries. Although the capacity and first-efficiency can be improved by using ether electrolytes, the high cost of ether electrolytes and the specific capacity of less than 150 mAh / g make this a drop in the ocean for the development of sodium batteries.

[0003] Hard carbon, due to its large interlayer spacing and complex, varied structure, possesses a large number of sodium ion storage active sites, making it the preferred anode material for the future commercial application of sodium batteries. Hard carbon has a wide range of sources, originating from various aspects of the surrounding environment, and can be broadly categorized into three types: resin, asphalt, and biomass. Limited by cost and development process difficulty, most commercially available hard carbon currently uses biomass hard carbon. Biomass hard carbon has the advantages of high capacity and low cost, making it the mainstream preparation route for hard carbon. However, the carbon yield of biomass hard carbon from precursor to finished product is less than 20%. How to improve the carbon yield of biomass hard carbon is a problem that the industry needs to solve. Existing technologies use bamboo charcoal or anthracite as the hard carbon substrate material and introduce soft carbon materials, which can greatly improve the carbon yield. However, this method not only significantly reduces the electrochemical performance of biomass carbon, such as its initial efficiency or capacity, but also introduces impurities contained in the soft carbon into the product.

[0004] Therefore, how to improve the carbon yield of biomass hard carbon while enhancing its electrochemical performance without introducing impurities is a key issue that has long been faced in the field of biomass hard carbon preparation. Summary of the Invention

[0005] This invention addresses the low carbon yield of traditional biomass hard char. While coating biomass hard char with soft carbon can improve the carbon yield to some extent, it leads to low initial efficiency and low capacity. The inventors have creatively proposed a modified hard carbon material, its preparation method, and its application without introducing soft carbon materials. This invention combines oxygen doping and stabilization processes with biomass hard char, and performs carbon layer rearrangement at high temperature to obtain a hard carbon material that can improve the carbon yield, is free of impurities, and has the advantages of high capacity and high initial efficiency.

[0006] Specifically, to achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a modified hard carbon material, comprising the following steps:

[0008] S1. The biomass hard char is heated and kept warm in an aerobic environment to obtain oxygen-treated biomass hard char, wherein the heating temperature is 250~350℃, the heating rate is 2~5℃ / min, and the holding time is 2~5h.

[0009] S2. Before the end of S1, a protective atmosphere is introduced in advance, and the mixture is heated and kept at a certain temperature to obtain stabilized bio-hard carbon. The heating temperature is 400℃~800℃, the heating rate is 2~10℃ / min, and the holding time is 0.5~2.5 h.

[0010] S3. Biomass hard char treated with S2 is heated under a protective atmosphere and then kept at that temperature.

[0011] Preferably, the bio-hard charcoal in S1 is cellulose-based biochar or nut shells. Specifically, the cellulose is selected from one or more of microcrystalline cellulose, food cellulose, polymeric cellulose, lignocellulose, cellulose ether, methylcellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; the nut shells are selected from one or more of pecan shells, walnut shells, almond shells, hickory shells, hazelnut shells, peanut shells, acorn shells, apricot shells, macadamia nut shells, and pistachio shells.

[0012] In some embodiments of the present invention, the cellulose is microcrystalline cellulose or food cellulose, but this is not intended to limit the present invention. The preparation method of the present invention can be applied not only to microcrystalline cellulose hard carbon, but also to biomass materials containing cellulose-based precursors, and even most biomass materials can achieve the same effect. In some embodiments of the present invention, the biomass hard carbon in S1 is pecan shell, but this is not intended to limit the present invention; other nut shells described in the present invention can also achieve the same effect.

[0013] Preferably, the heating and heat preservation conditions in S1 are: heating temperature 280℃, heating rate 5℃ / min, and heat preservation time 4~5h.

[0014] Preferably, in step S2, introducing the protective atmosphere before the end of step S1 specifically means introducing the protective atmosphere 10 minutes before the end of step S1.

[0015] Preferably, the protective atmosphere described in S2 and S3 is selected from nitrogen, argon, helium, neon, krypton, xenon, and radon; more preferably, it is argon, with a gas flow rate of 50~200 ml / min. This gas flow rate range fully satisfies the calcination protection requirements of the hard carbon material of the present invention, but does not alter the structural and electrochemical properties of the hard carbon.

[0016] Preferably, the heating and holding conditions in S2 are: heating temperature 400℃, heating rate 5℃ / min, and holding time 1 h.

[0017] Preferably, the stabilized biomass hard carbon obtained in S2 is ball-milled and screened to obtain screened material, which is then subjected to S3 treatment.

[0018] More preferably, the ball milling speed is 800~1000 r / min and the time is 20~30 min.

[0019] More preferably, the screened material is D. 50 Materials with a particle size of 3~20μm.

[0020] Preferably, the biomass hard char in S1 is nut shells. The stabilized biomass hard char obtained in S2 is ball-milled and screened. After obtaining the screened material, it is soaked in hydrochloric acid solution, then washed to neutral, dried, and then subjected to the treatment in S3.

[0021] More preferably, the solid-liquid mass ratio during the hydrochloric acid solution soaking process is 1:30~100.

[0022] Preferably, the conditions for holding the temperature after heating in S3 are: heating temperature of 1300~1400℃, heating rate of 2~5℃ / min, and holding time of 3h.

[0023] Secondly, the present invention provides a modified hard carbon material obtained by the above preparation method.

[0024] Thirdly, the present invention provides the application of the modified hard carbon material in the preparation of sodium-ion battery anode materials.

[0025] Preferably, the application specifically involves: making an electrode sheet from the modified hard carbon material: mixing the modified hard carbon material, PVDF and SP, diluting with an organic solvent, transferring the mixture to an aluminum foil for coating after uniform mixing, and then drying to obtain a negative electrode sheet.

[0026] The beneficial effects of this invention are:

[0027] Precursors in the prior art are often divided into soft carbon and hard carbon. This invention is aimed at biomass hard carbon. Existing technical solutions for improving the carbon production rate of biomass hard carbon often use soft carbon or hard carbon coated with soft carbon as the carbon source. Although the carbon production rate can be improved to a certain extent, the electrochemical performance of the obtained biochar, such as the first efficiency or capacity, is poor. Therefore, it is often difficult to improve the carbon production rate and capacity at the same time. In the preparation method of the modified hard carbon material of this invention, the biomass hard carbon is first subjected to oxygen doping and stabilization treatments, both of which are indispensable. Oxygen doping treatment provides strong active sites through oxygen-containing groups, improving specific capacity and cycle stability through strong chemisorption. Furthermore, the oxygen-containing groups act in the graphite domain, disrupting the inherent crystallinity of the biomass hard carbon, causing it to exhibit more amorphous structures during subsequent pyrolysis, resulting in a higher degree of disorder and increased structural diversity. The subsequent stabilization treatment allows sufficient time for dehydration, decarboxylation, and decarbonization of the biomass hard carbon at low temperatures, further stabilizing the structure. High-temperature pyrolysis ensures sufficient time for carbon atoms to rearrange and recombine, improving the electrical performance of the hard carbon structure. This invention can improve the carbon yield and electrochemical performance of biomass without altering its inherent structure or introducing new impurities, achieving a coulombic efficiency of over 76.70% and a reversible capacity of 285 mAh·g for the first time. -1 In the above, at a current density of 1C, the reversible capacity retention rate can reach up to 98.9% after 100 cycles.

[0028] The modified hard carbon material obtained by this invention has a small specific surface area and an irregular blocky distribution of overall structural layers. Microscopically, it is formed by the stacking of irregular graphene layers, and the surface and interior contain layered porous morphology, ensuring the stability and reliability of the hard carbon structure. Attached Figure Description

[0029] Figure 1 The graph shows the carbon production rate and electrochemical data detection results of the embodiments and comparative examples of the present invention.

[0030] Figure 2 SEM image (2000×) of the modified hard carbon material obtained in Example 7 of the present invention.

[0031] Figure 3 SEM image (5000×) of the modified hard carbon material obtained in Example 7 of this invention;

[0032] Figure 4 The image shows the XRD pattern of the modified hard carbon material obtained in Example 7 of this invention.

[0033] Figure 5 This is the first charge-discharge curve of Embodiment 7 of the present invention;

[0034] Figure 6This is a cycle performance curve diagram of Embodiment 7 of the present invention;

[0035] Figure 7 This is the first charge-discharge curve of Comparative Example 4 of the present invention;

[0036] Figure 8 The XRD patterns are of the modified hard carbon obtained in Comparative Examples 9 and 10. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0038] The preparation method of the modified hard carbon material in this embodiment includes the following steps:

[0039] S1. Weigh 50g of microcrystalline cellulose and place it in a corundum boat. Transfer it to a tube furnace and heat it to 280℃ at a heating rate of 5℃ / min in air atmosphere and hold it for 2 hours to carry out oxygen doping treatment.

[0040] S2. Ten minutes before the S1 program is about to end, nitrogen gas is introduced at a flow rate of 90 ml / min. Then, the temperature is increased to 400℃ at a rate of 5℃ / min and held for 1 hour. After cooling with the furnace, the stabilized biochar can be obtained.

[0041] S3. The stabilized biochar obtained is placed in a high-energy ball mill jar and stirred and dispersed at 800 r / min for 30 min. Powder materials with D50 particle size of 3~20μm are screened out.

[0042] S4. Transfer the screened material to a tube furnace and heat it to 1300℃ for 3 hours under a nitrogen atmosphere. The heating rate is 5℃ / min and the nitrogen flow rate is 90 ml / min. After cooling to room temperature, the modified hard carbon material can be obtained.

[0043] The modified hard carbon material is used to prepare a sodium-ion battery anode material:

[0044] The modified hard carbon material was used to make an electrode sheet. The ratio of modified hard carbon material, PVDF, and SP was 90:5:5. The mixture was diluted with the organic solvent N-methylpyrrolidone and, after being thoroughly mixed, transferred to aluminum foil for coating. The coating was then dried at 110°C and cut by roll forming to obtain the negative electrode sheet, with a compaction density of 0.8~0.9 mg / cm³. -3 ;

[0045] In the coin cell system, the counter electrode is a Na sheet, the separator is a glass fiber membrane, and the electrolyte composition is: 1 mol LiPF6 as solute, DC and DEC as solvents in a 1:1 volume ratio; the additive is FEC at a content of 5%; the 2032 coin cell is assembled in a glove box with a protective atmosphere. Example 2

[0046] Same as Example 1, except that:

[0047] S1 processing conditions: Heating to 280℃ at a heating rate of 5℃ / min in air atmosphere and holding at that temperature for 4 hours. Example 3

[0048] Same as Example 1, except that:

[0049] S1 processing conditions: Heating to 280℃ at a heating rate of 5℃ / min and holding for 5h in air atmosphere;

[0050] S3 processing conditions: Stir and disperse at 1000 r / min for 20 min. Example 4

[0051] Same as Example 3, except that:

[0052] S2 processing conditions: nitrogen gas flow rate of 90 ml / min, followed by heating at 10℃ / min to 400℃ and holding for 1 hour. Example 5

[0053] Same as Example 3, except that step S3 is omitted. Example 6

[0054] Same as Example 3, except that:

[0055] S1 processing conditions: Heating to 280℃ at a heating rate of 3℃ / min and holding for 5h in air atmosphere;

[0056] S2 processing conditions: nitrogen gas flow rate of 90 ml / min, followed by heating at 5℃ / min to 400℃ and holding for 1 hour. Example 7

[0057] Same as Example 3, except that:

[0058] S4 processing conditions: heat to 1300℃ and hold for 3 hours, heating rate 2℃ / min, nitrogen flow rate 90 ml / min. Example 8

[0059] Same as Example 3, except that:

[0060] S4 processing conditions: heat to 1400℃ and hold for 3 hours, heating rate 5℃ / min, nitrogen flow rate 90 ml / min. Example 9

[0061] Same as Example 3, except that:

[0062] The biochar in S1 is food cellulose;

[0063] S4 processing conditions: heat to 1300℃ and hold for 3 hours, heating rate 2℃ / min, nitrogen flow rate 90 ml / min. Example 10

[0064] Same as Example 3, except that:

[0065] The biochar in S1 is pecan shells;

[0066] After S3 treatment, the mixture was soaked overnight with 1 mol / L hydrochloric acid solution and stirred. The solid-liquid mass ratio was 1:40. It was then washed with deionized water to neutralize the solution and dried in a drying oven at 100°C for 12 hours for later use. Example 11

[0067] Same as Example 1, except that:

[0068] The processing conditions for S2 were as follows: nitrogen gas flow rate of 90 ml / min, followed by a further increase in temperature to 400℃ at 2℃ / min and holding at that temperature for 2.5 h. Example 12

[0069] Same as Example 1, except that:

[0070] The processing conditions for S2 were as follows: nitrogen gas flow rate of 90 ml / min, followed by heating at 10℃ / min to 800℃ and holding at that temperature for 0.5 h. Example 13

[0071] Same as Example 1, except that:

[0072] S1 processing conditions: Heating to 250℃ at a heating rate of 2℃ / min and holding for 5h in air atmosphere. Example 14

[0073] Same as Example 1, except that:

[0074] S1 processing conditions: Heating to 350℃ at a heating rate of 5℃ / min and holding for 2 hours in air atmosphere. Comparative Example 1

[0075] Same as Example 1, except that:

[0076] S1 processing conditions: Heating to 200℃ at a heating rate of 5℃ / min and holding for 3h in air atmosphere. Comparative Example 2

[0077] Same as Example 1, except that:

[0078] S1 processing conditions: Heating to 400℃ at a heating rate of 3℃ / min and holding for 2 hours in air atmosphere. Comparative Example 3

[0079] Same as Example 1, except that:

[0080] S2 processing conditions: nitrogen gas flow rate of 90 ml / min, followed by heating at 5℃ / min to 1000℃ and holding for 1 hour. Comparative Example 4

[0081] Weigh 50g of microcrystalline cellulose and place it in a corundum boat. Transfer the vessel to a tube furnace and heat it to 1300℃ for 3 hours under a nitrogen atmosphere. The heating rate is 5℃ / min and the nitrogen flow rate is 90 ml / min. After cooling to room temperature, the hard carbon anode material can be obtained. Comparative Example 5

[0082] Same as Example 3, except that there is no S2 stabilization step. Comparative Example 6

[0083] Same as Example 3, except that:

[0084] S4 processing conditions: heat to 1300℃ and hold for 5 hours, heating rate 5℃ / min, nitrogen flow rate 90 ml / min. Comparative Example 7

[0085] Same as Example 3, except that:

[0086] S4 processing conditions: heat to 1200℃ and hold for 4 hours, heating rate 5℃ / min, nitrogen flow rate 90 ml / min. Comparative Example 8

[0087] Same as Example 3, except that:

[0088] The processing conditions for S4 were: heating to 1600℃ and holding for 2.5 hours, heating rate of 5℃ / min, and nitrogen flow rate of 90 ml / min. Comparative Example 9

[0089] Same as Example 3, except that:

[0090] The carbon source for S1 is petroleum pitch (soft carbon material) at 250°C.

[0091] S1 processing conditions: Heating to 300℃ at a heating rate of 5℃ / min and holding for 2 hours in air atmosphere. Comparative Example 10

[0092] Same as Example 3, except that:

[0093] The carbon source of S1 is a mixture of soft and hard carbon sources, namely a mixture of microcrystalline cellulose and 250°C petroleum asphalt, wherein the mass ratio of microcrystalline cellulose to 250°C petroleum asphalt is 9:1. Comparative Example 11

[0094] Same as Example 3, except that:

[0095] The carbon source for S1 is petroleum pitch (soft carbon material) at 250°C.

[0096] There is no S1 oxygen doping treatment step or S2 stabilization treatment step.

[0097] Example 1

[0098] The carbon yield and electrochemical data of the examples and comparative examples were tested, where carbon yield = final hard carbon product mass / raw material mass before sintering; the electrochemical data were measured using a Xinwei battery tester, and the test procedure was: 8 hours of rest, 0.1C discharge to 0.01V, 5 minutes of rest, and 0.1C charge to 2V. See the detailed results below. Figure 1 .

[0099] Depend on Figure 1 It can be concluded that, by comparing Examples 1, 2, 13, and 14 of the present invention with Comparative Examples 1 and 2, excessively high or low oxygen doping temperatures in S1 will damage the carbon production rate and electrochemical performance. Therefore, the control of oxygen doping temperature in the S1 process has an important impact on the carbon production rate and electrochemical performance of hard carbon materials.

[0100] The oxygen doping and stabilization steps of this invention are indispensable. As can be seen from the comparison between Examples 1-6 and Examples 11-14 of this invention and Comparative Example 4, oxygen doping and stabilization of biochar can not only improve carbon yield, but also further improve electrochemical performance. Specifically, the incorporation of oxygen atoms into hard carbon exists as carbonyl groups. Compared with hard carbon treated by direct carbonization (Comparative Example 4), the carbon yield can be increased by 5-7%. This increase in carbon yield is mainly due to the combination of oxygen elements with carbon elements inside the precursor body, attacking the microcrystalline region of the carbon layer, giving the obtained hard carbon a carbon structure with higher disorder. The disordered structure causes more closed micropores to be generated during the high-temperature carbonization stage, thereby increasing the capacity of the plateau region.

[0101] A comparison of Example 3 and Comparative Example 5 shows that adding stabilization treatment allows the cellulose precursor sufficient time for dehydration, decarboxylation, and decarbonization at low temperatures, further stabilizing the structure. Subsequent high-temperature pyrolysis ensures sufficient time for carbon atoms to rearrange and recombine, improving the electrical performance of the hard carbon structure. Materials without stabilization treatment are more prone to forming pseudostructures, affecting first-efficiency and cycle reversibility. Furthermore, the stabilization temperature also affects the modified hard carbon material. A comparison of Example 1 and Comparative Example 3 shows that low-temperature treatment allows for the full escape of small molecule gases, resulting in a denser closed-pore structure at the downstream carbonization stage. This allows for precise control of the pore size distribution of the hard carbon anode, contributing to improved electrochemical performance. In contrast, high-temperature treatment (Comparative Example 3) results in a denser structure because small molecule gases do not have enough time to escape from the precursor. In step S4, the high-temperature carbonization pyrolysis makes it more difficult for small gas molecules to escape, hindering the formation of closed micropores and contributing little to the plateau capacity.

[0102] As can be seen from the comparison between Example 3 and Comparative Examples 6-8, the high-temperature pyrolysis conditions in step S4 also have a significant impact on carbon yield and electrochemical performance. The parameters for S4 specified in this invention are designed to form a closed microporous structure, which helps to improve the capacity of the plateau region. On the other hand, it can produce a hard carbon product with lower defects, effectively avoiding the consumption of sodium ions during SEI film formation on the hard carbon surface, thus improving the first-efficiency. However, if the pyrolysis temperature is insufficient (as in Comparative Example 7), the number of closed micropores formed will be insufficient, and the degree of surface defects will be high, resulting in poor electrochemical performance. On the other hand, if the pyrolysis temperature is too high (Comparative Example 8) or the holding time is prolonged (Comparative Example 6), the pore walls of the formed micropores will be thicker, and sodium ions will enter and form sodium clusters, making it more difficult for them to escape from the pores, resulting in a rapid decline in capacity and first-efficiency.

[0103] A comparison of Example 3 and Comparative Examples 9, 10, and 11 reveals a significant difference in the effects of oxygen doping on both soft and hard carbon precursor materials. In Example 3, the oxygen doping primarily aims to allow oxygen atoms from the air to exist as functional groups within the hard carbon. The increase in carbon yield mainly stems from the addition of carbon atoms. Furthermore, the capacity improvement is achieved through the chemisorption of oxygen-containing functional groups, without altering the original structural characteristics of the hard carbon. In contrast, the soft carbon in Comparative Example 9 exhibits a high degree of order in its carbon layers and a narrow interlayer spacing (3.4-3.5 nm), severely hindering sodium ion insertion and resulting in a very low capacity. The oxygen doping treatment primarily works by using oxygen to regulate the disorder of the soft carbon structure, increasing the interlayer insertion capacity. The difference in electrochemical performance between Comparative Example 9 and Comparative Example 11 is achieved through the increased interlayer spacing. However, the increase in interlayer spacing (3.56 nm) in the hard carbon obtained through oxygen doping is not significant, resulting in minimal improvement in the low-voltage plateau capacity. The carbon source in Comparative Example 10 was a combination of soft carbon and hard carbon. This combination is a physical bond. During the carbonization process of asphalt, there is a three-phase transformation from solid to liquid to solid, while the hard carbon from cellulose undergoes a solid-phase transformation throughout. The resulting hard carbon consists of asphalt coated on the surface of cellulose. Since the capacity contribution of the composite material mainly comes from cellulose, the asphalt coating slightly reduces the interlayer spacing, hindering the insertion of sodium ions, thus exhibiting lower capacity and first-efficiency. In summary, oxygen doping treatment for hard carbon precursor materials achieves higher carbon yield and better electrochemical performance through the chemical adsorption of oxygen-containing groups; for soft carbon, it improves disorder and increases interlayer spacing to improve low-voltage plateau capacity. The two treatment principles are different, and oxygen doping treatment is more effective for hard carbon precursor materials.

[0104] This invention, through a combination of oxygen doping and stabilization processes, ensures a closed micropore distribution within the carbon layer. This, combined with the surface pore distribution, forms a hierarchical pore structure, optimizing the electrode / electrolyte surface. This approach achieves an initial coulombic efficiency of 83.38% and a reversible capacity of 311 mAh.g. -1 At a current density of 1C, the reversible capacity retention rate is 98.9% after 100 cycles.

[0105] Example 2

[0106] Figure 2 The image shows a SEM image (2000×) of the modified hard carbon material obtained in Example 7 of this invention. Figure 3 The image shows a SEM image (5000×) of the modified hard carbon material obtained in Example 7 of this invention. It can be seen that the obtained hard carbon material is distributed in an irregular block shape. The magnified image shows that the graphene layers are distributed in an irregular sheet-like disordered manner. The graphene layers in different directions are wrapped together to obtain hard carbon, which has a high degree of disorder. Oxygen doping can make the surface of hard carbon have more sodium ion active sites, which can contribute more capacity.

[0107] Example 3

[0108] Figure 4 , Figure 8 The images show the XRD patterns of the hard carbon obtained in Examples 7, 9, and 10 of this invention, respectively. In Examples 7 and 10, the two broad peaks near 24° and 43° correspond to the (002) and (110) planes of the graphite crystals, respectively, indicating that the obtained materials have a good amorphous carbon structure. The 23° position corresponds to graphite sheet information. Calculations using the Bragg equation show that the interlayer spacing of the graphite sheets in both examples is 3.9 nm and 3.78 nm, respectively, which meets the sodium ion insertion size (>3.7 nm), corresponding to a higher plateau capacity. While Comparative Example 9 improved its disorder to a certain extent through oxygen doping, it still had a higher Sp... 3 The graphite has a hybrid carbon structure, so both characteristic peaks are relatively sharp, exhibiting a distinct soft carbon peak structure. The interlayer spacing of the graphite is 3.53 nm, which hinders the interlayer sodium storage capacity.

[0109] Example of effect 4

[0110] Figure 5 This is the first charge-discharge curve of Embodiment 7 of the present invention; Figure 6 The cycling performance curve of Embodiment 7 of the present invention; Figure 7 This is the initial charge-discharge curve for Comparative Example 4 of this invention. It can be seen that the hard carbon material obtained through oxygen doping and stabilization processes of this invention achieves an initial coulombic efficiency of 83.38% and a reversible capacity of 311 mAh·g. -1 The material exhibits excellent electrochemical performance with a reversible capacity retention of 98.9% after 100 cycles at a current density of 1C, while the untreated hard carbon material (Comparative Example 4) has an initial coulombic efficiency of only 74.83% and a reversible capacity of 214 mAh.g. -1 The reason is that the oxygen doping treatment converts oxygen in the air into carbonyl groups inside the hard carbon and fixes sodium ions through strong chemical adsorption. The stabilization process promotes the escape of small molecule gases from the material at low temperatures to form a carbon layer framework structure. The high-temperature pyrolysis process makes it easier to obtain a better carbon layer structure. Therefore, the hard carbon obtained by this invention can improve the carbon production rate while having high initial charge capacity and high initial efficiency.

[0111] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a modified hard carbon material, characterized in that, Includes the following steps: S1. The biomass hard char is heated and kept warm in an aerobic environment to obtain oxygen-treated biomass hard char, wherein the heating temperature is 250~350℃, the heating rate is 2~5℃ / min, and the holding time is 2~5h. S2. Before the end of S1, a protective atmosphere is introduced, and the mixture is heated and kept at a certain temperature to obtain stabilized biomass hard char. The heating temperature is 400℃~800℃, the heating rate is 2~10℃ / min, and the holding time is 0.5~2.5 h. S3. The biomass hard char treated in S2 is heated and then kept at a temperature under a protective atmosphere. The conditions for holding the temperature after heating are: heating temperature of 1300~1400℃, heating rate of 2~5℃ / min, and holding time of 3h.

2. The preparation method according to claim 1, characterized in that, The biomass hard char in S1 is cellulose-based biomass hard char or nut shells.

3. The preparation method according to claim 1, characterized in that, The heating and heat preservation conditions in S1 are: heating temperature 280℃, heating rate 5℃ / min, and heat preservation time 4~5h.

4. The preparation method according to claim 1, characterized in that, The heating and holding conditions in S2 are: heating temperature 400℃, heating rate 5℃ / min, and holding time 1 h.

5. The preparation method according to claim 1, characterized in that, The stabilized biomass hard carbon obtained from S2 is ball-milled and screened, and the screened material is then subjected to S3 treatment.

6. The preparation method according to claim 1, characterized in that, The biomass hard char in S1 is nut shells. The stabilized biomass hard char obtained in S2 is ball-milled and screened. After the screened material is obtained, it is soaked in hydrochloric acid solution, then washed to neutral, dried and then subjected to the treatment in S3.

7. The preparation method according to claim 1, characterized in that, The protective atmosphere mentioned in S2 and S3 is selected from one of nitrogen, argon, helium, neon, krypton, xenon, and radon.

8. A modified hard carbon material obtained by the preparation method according to any one of claims 1 to 7.

9. The application of the modified hard carbon material according to claim 8 in the preparation of sodium-ion battery anode materials.

Images