Preparation method and application of biomass-based modified functional group carbon quantum dot coupled closed-pore structure hard carbon material

Abstract

The invention discloses a preparation method and application of a biomass-based modified functional group carbon quantum dot coupled closed-pore structure hard carbon material, and belongs to the field of battery materials. The preparation method comprises the following steps: firstly, preparing biomass-based carbon quantum dots, modifying functional groups of the carbon quantum dots, putting biomass activated carbon powder and the modified functional group carbon quantum dots into a hydrothermal kettle to react to obtain an intermediate, and calcining to obtain the carbon quantum dot composite hard carbon material. On one hand, the carbon quantum dots of the modified functional groups can be combined with a large number of micropores of the activated carbon to form closed pores and can contain a large number of sodium ions, and on the other hand, the surface modified functional groups and defect sites can provide rich active sites for the sodium ions and provide a large number of sodium storage sites and faster reaction kinetics for sodium ion adsorption; the hard carbon material with a long slope area and a long low platform area is constructed through the dual coupling effect of the carbon dots, the hard carbon material has the characteristics of high capacity, high magnification and high first efficiency, and the performance of the sodium ion battery is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of battery materials, and particularly relates to a preparation method and application of a biomass-based modified functional group carbon quantum dot coupled with a closed pore structure hard carbon material. Background Art

[0002] In recent years, lithium-ion batteries have developed rapidly and been widely used in life. However, resources such as lithium and cobalt in lithium-ion batteries are limited and unevenly distributed in the earth's crust. As a new type of battery with low temperature resistance, safety, and high rate performance, sodium-ion batteries are becoming a research hotspot for new energy storage technologies. Limited by the relatively small interlayer spacing of graphite (0.34 nm) and the weak interaction between sodium ions and graphite layers, it is difficult for sodium ions to enter the graphite layers. Hard carbon materials are regarded as the most commercially promising battery anode materials and have received extensive attention and research.

[0003] However, the specific capacity, initial Coulomb efficiency, and rate performance of current hard carbon cannot meet the commercial requirements of SIBs. At present, many strategies have been proposed to improve the electrochemical performance of hard carbon. For example, increasing the interlayer spacing to enhance the insertion ability of sodium ions, regulating the pore structure to accommodate more sodium metal clusters, changing the graphitization degree and defect concentration to obtain higher sodium affinity, doping heteroatoms to increase conductivity and provide more active sites. Different structures of hard carbon materials have different sodium storage mechanisms. The slope region and the plateau region can generally be regarded as the adsorption sites provided by the carbon material surface and the closed pore structure between the carbon layers. How to regulate the microstructure to obtain high-rate and high-capacity hard carbon materials still needs in-depth research. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems in the related art to some extent. For this purpose, the present invention provides a preparation method and application of a biomass-based modified functional group carbon quantum dot coupled with a closed pore structure hard carbon material. The hard carbon material prepared by the present invention has high rate performance, relatively high reversible specific capacity, and first Coulomb efficiency.

[0005] The preparation method of the biomass-based modified functional group carbon quantum dot coupled with a closed pore structure hard carbon material of the present invention includes the following steps:

[0006] Step 1: Pretreat the biomass powder to remove most of the volatile components in the biomass to obtain a carbonized material;

[0007] Step 2: Prepare biomass carbon quantum dots with modified functional groups;

[0008] Step 3: Mix the carbonized material obtained in Step 1 with the biomass carbon quantum dots obtained in Step 2 and carry out a hydrothermal reaction to obtain an intermediate;

[0009] Step 4: Calcinate the intermediate obtained in Step 3 to obtain a hard carbon material.

[0010] In Step 1, the biomass powder is obtained by crushing one or more of coconut shell, sugarcane, poplar, cotton, wheat, corn straw, reed, bamboo, seaweed, oak, etc., and the size is 10 - 50um. Thus, it is beneficial to obtain a hard carbon material with a higher reversible specific capacity and first Coulombic efficiency.

[0011] In Step 1, the pretreatment is a sintering treatment carried out in an inert atmosphere.

[0012] Furthermore, the inert atmosphere includes at least one of argon atmosphere and nitrogen atmosphere. By selecting the above-mentioned inert atmosphere, it can effectively prevent the biomass powder from reacting with oxygen in the air during the heating process, which is beneficial to obtain a hard carbon material with a higher reversible specific capacity and first Coulombic efficiency.

[0013] Furthermore, the temperature of the sintering treatment is 400°C - 1200°C, the time is 2 - 24h, and the heating rate is 0.5°C / min - 20°C / min. Thus, it can effectively promote the pyrolysis and carbonization reactions of the organic components in the biomass powder, and can form a precursor with a high carbon content and a stable structure, which is beneficial to obtain a hard carbon material with a higher reversible specific capacity and first Coulombic efficiency.

[0014] In Step 1, the specific surface area of the carbonized material is 300 - 800m 2 / g. The carbonized material with rich pores is beneficial to the subsequent formation of a closed pore structure, provides a large storage place for sodium ions, and improves the specific capacity.

[0015] In Step 2, the carbon quantum dots are obtained by the following method:

[0016] Add the precursor powder from biomass sources into distilled water, and then add reagents such as nitric acid or perchloric acid, and carry out a hydrothermal reaction in a reaction kettle to obtain carbon quantum dots.

[0017] The precursor powder is selected from bamboo powder or coconut shell powder.

[0018] The mass ratio of the precursor powder to nitric acid or perchloric acid is 1:2 - 4.

[0019] The specific process includes: Add 5g of the precursor powder from biomass sources into 20ml of distilled water, then add 10 - 20ml of reagents such as nitric acid or perchloric acid, and carry out a hydrothermal reaction in a reaction kettle at 180°C for 3 - 10 hours to obtain carbon quantum dots.

[0020] In Step 2, the size of the carbon quantum dots is 1-10 nm, and they have abundant functional groups including carboxyl, hydroxyl, amino, etc. Thus, it is beneficial to obtain a hard carbon material with a high reversible specific capacity and a first Coulombic efficiency.

[0021] In Step 3, based on the total mass of the carbon quantum dots and the carbonized material, the mass ratio of the carbon quantum dots is 1%-10%.

[0022] In Step 3, the temperature of the hydrothermal reaction is 100°C - 200°C, and the reaction time is 3-10 h. Thus, it can effectively promote the coupling of the carbon quantum dots and the carbonized material, and effectively fill the carbon quantum dots into the micropores. Thus, it is beneficial to obtain a hard carbon material with a high reversible specific capacity and high rate performance. If the hydrothermal reaction time is too short, the reaction will be uneven and the composite effect will be poor; if the reaction time is too long, excessive carbon quantum dots will deposit on the carbon surface, consuming too many carbon quantum dots and resulting in a low yield. By limiting the hydrothermal reaction time within the above range, it can not only ensure sufficient reaction but also form a rich, uniform and appropriately sized pore structure.

[0023] In Step 4, the temperature of the calcination is 400°C - 1200°C, and the time is 0.5 h - 24 h. In this step, by mixing and calcining the obtained intermediate, during the mixed calcination process, the functional groups on the surface of the carbon quantum dots will change and melt and carbonize in combination with the micropores on the surface of the carbonized material. An enclosed pore structure can be obtained by internal curing in the pores. The enclosed pore structure can provide a large storage place for sodium ions, improving the reversible specific capacity of the hard carbon material. At the same time, the surface-modified functional groups and defect sites of the carbon quantum dots can provide abundant active sites for sodium ions, providing a large number of sodium storage sites and faster reaction kinetics for sodium ion adsorption. The dual coupling effect of the carbon dots constructs a hard carbon material with both a long sloping region and a long low plateau region. It has the characteristics of high capacity, high rate performance and high initial efficiency, greatly improving the performance of sodium ion batteries. By limiting the calcination temperature within the above range, it can better allow the carbon quantum dots to undergo carbonization combination, forming more and appropriately sized enclosed pore structures and appropriate organic functional groups. Thus, it is beneficial to obtain a hard carbon material with a high reversible specific capacity and a first Coulombic efficiency.

[0024] In Step 4, the D50 particle size of the hard carbon material is 5μm - 15μm, and the specific surface area ≤ 10m 2 / g.

[0025] Application of the biomass-based modified functional group carbon quantum dot coupled enclosed pore structure hard carbon material of the present invention in the preparation of sodium ion batteries.

[0026] Specifically, using the biomass-based modified functional group carbon quantum dot coupled enclosed pore structure hard carbon material to prepare a negative electrode material, using metallic sodium as the counter electrode, and using a NaPF6 solution as the electrolyte.

[0027] Carbon quantum dots are zero-dimensional carbon-based nanomaterials with a particle size usually below 10 nm. Carbon quantum dots have a small size (< 10 nm) and a rich distribution of organic functional groups on their surface. In the present invention, carbon quantum dots are first prepared and their functional groups are modified. Due to the extremely strong adsorption ability of the micropores of activated carbon, the carbon quantum dots are compounded into the pores of activated carbon through a hydrothermal reaction. On the one hand, the carbon quantum dots with modified functional groups can combine with a large number of micropores of activated carbon to form closed pores, reducing the specific surface area of the hard carbon material and avoiding direct contact with the electrolyte, which can reduce the side reactions with the electrolyte and reduce the formation of the SEI film, thereby being beneficial to improving the first Coulomb efficiency of the hard carbon material. Moreover, the closed pore structure of the hard carbon material can store a large amount of sodium ions, improving the reversible specific capacity of the hard carbon material. On the other hand, the surface-modified functional groups and defect sites can provide rich active sites for sodium ions, providing a large number of sodium storage sites and faster reaction kinetics for sodium ion adsorption. The dual coupling effect of the carbon dots constructs a hard carbon material with both a long sloping region and a long low plateau region. It has the characteristics of high capacity, high rate, and high first efficiency, greatly improving the performance of sodium-ion batteries. Thus, by using the method provided by the present invention, the prepared hard carbon material has a high reversible specific capacity and first Coulomb efficiency.

[0028] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0030] Figure 1 is a flowchart for the preparation of the hard carbon material according to the embodiment of the present application.

[0031] Figure 2 is a scanning electron microscope image of the hard carbon material according to Example 1 of the present application.

[0032] Figure 3 is a cycling performance graph of the sodium-ion battery according to Example 1 of the present application at a current density of 30 mA / g.

[0033] Figure 4 is a high-resolution transmission electron microscope image of the hard carbon material according to Comparative Example 1 of the present application.

[0034] Figure 5 is a high-resolution transmission electron microscope image of the hard carbon material according to Example 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] Embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0036] In the first aspect of the present invention, the present invention provides a method for preparing a modified functional group carbon quantum dot-coupled hard carbon material. According to an embodiment of the present invention, with reference to Figure 1 , the method includes:

[0037] S1. Pre-carbonize the biomass powder to obtain a carbonized material.

[0038] In this step, by pre-carbonizing the biomass powder, most of the volatile components in the biomass can be removed, which is beneficial to subsequent processing steps.

[0039] According to some specific embodiments of the present invention, the biomass powder includes at least one of coconut shell, sugarcane, poplar, cotton, wheat, corn straw, reed, bamboo, seaweed, and oak. The above-mentioned biomass powder is renewable, which helps to reduce the consumption of fossil fuels and lower carbon emissions, and can greatly reduce costs and improve economic benefits.

[0040] S2. Prepare modified functional group biomass carbon quantum dots.

[0041] S3. Put the carbon quantum dots and the carbonized material into a hydrothermal reactor for reaction to obtain an intermediate.

[0042] S4. Calcinate the intermediate to obtain a hard carbon material.

[0043] Hard carbon materials with different structures have different sodium storage mechanisms. The slope region and the plateau region can generally be regarded as the closed pore structure between the adsorption sites provided by the carbon material surface and the carbon layer. How to regulate the microstructure to obtain hard carbon materials with high rate performance and high capacity is still a challenge. Carbon quantum dots are zero-dimensional carbon-based nanomaterials with a particle size usually below 10 nm. Carbon quantum dots have a small size (< 10 nm) and are rich in organic functional groups on the surface. This preparation method first prepares bio-based carbon quantum dots and modifies their functional groups. The micropores of activated carbon have extremely strong adsorption capacity. Carbon quantum dots are compounded into the pores of activated carbon through a hydrothermal reaction. On the one hand, the carbon quantum dots with modified functional groups can combine with a large number of micropores of activated carbon to form closed pores, reducing the specific surface area of the hard carbon material and avoiding direct contact with the electrolyte, which can reduce the side reactions with the electrolyte and reduce the formation of the SEI film, thus being beneficial to improving the first Coulombic efficiency of the hard carbon material. And the closed pore structure of the hard carbon material can store a large amount of sodium ions, improving the reversible specific capacity of the hard carbon material. On the other hand, the surface-modified functional groups and defect sites can provide abundant active sites for sodium ions, providing a large number of sodium storage sites and faster reaction kinetics for sodium ion adsorption. The dual coupling effect of carbon dots constructs a hard carbon material with both a long slope region and a long low plateau region. It has the characteristics of high capacity, high rate performance, and high first efficiency, greatly improving the performance of sodium-ion batteries. Therefore, by using the method of this application, the prepared hard carbon material has a high reversible specific capacity and first Coulombic efficiency.

[0044] According to some specific embodiments of the present invention, by sintering the biomass powder obtained in step S1, the organic components in the biomass powder undergo pyrolysis and carbonization reactions, and a precursor with a high carbon content and a stable structure can be formed.

[0045] According to some specific embodiments of the present invention, the inert atmosphere includes at least one of an argon atmosphere and a nitrogen atmosphere. By selecting the above-mentioned inert atmosphere, it can effectively prevent the biomass powder from reacting with oxygen in the air during the heating process, which is beneficial to obtaining a hard carbon material with a high reversible specific capacity and first Coulombic efficiency.

[0046] According to some specific embodiments of the present invention, the specific surface area of the carbonized material is 300 - 800 m 2 / g. The carbonized material with rich pores is beneficial to the subsequent formation of a closed pore structure, providing a large storage place for sodium ions and increasing the specific capacity.

[0047] According to some specific embodiments of the present invention, the carbon quantum dots are characterized in that the size of the carbon quantum dots is about 1-10 nm, and they have rich functional groups including carboxyl groups, hydroxyl groups, amino groups, etc. The carbon quantum dots with appropriate size need to match the size of the rich micropores of the carbonized material itself. If they are too small, the micropores of the carbonized material will be blocked; if they are too large, a closed pore structure cannot be formed.

[0048] According to some specific embodiments of the present invention, the sintering temperature is 400°C - 1200°C. For example, the sintering temperature can be 400°C, 800°C, 1000°C, 1200°C, etc. Thus, it can effectively promote the pyrolysis and carbonization reactions of the organic components in the biomass powder, and can form a precursor with a high carbon content and a stable structure, which is beneficial to obtaining a hard carbon material with a high reversible specific capacity and a first Coulomb efficiency.

[0049] According to some specific embodiments of the present invention, the heating rate of sintering is 0.5°C / min - 20°C / min. For example, the heating rate of sintering can be 0.5°C / min, 1°C / min, 5°C / min, 10°C / min, 15°C / min, 20°C / min, etc. Thus, it can effectively promote the pyrolysis and carbonization reactions of the organic components in the biomass powder, and can form a precursor with a high carbon content and a stable structure, which is beneficial to obtaining a hard carbon material with a high reversible specific capacity and a first Coulomb efficiency.

[0050] According to some specific embodiments of the present invention, the temperature of the hydrothermal reaction is 100°C - 200°C, and the time is 0.5 h - 8 h. For example, the reaction temperature can be 100°C, 150°C, 200°C, etc., and the reaction time can be 0.5 h, 1 h, 2 h, 4 h, 6 h, 8 h, etc. Thus, it can effectively promote the coupling of carbon quantum dots and carbonized material, and effectively fill the carbon quantum dots into the micropores. Therefore, it is beneficial to obtain a hard carbon material with a high reversible specific capacity and a high rate performance.

[0051] If the hydrothermal reaction time is too short, the reaction will be uneven and the composite effect will be poor; if the reaction time is too long, excessive carbon quantum dots will deposit on the carbon surface, consuming too many carbon quantum dots and resulting in a low yield. By limiting the hydrothermal reaction time within the above range, it can not only ensure sufficient reaction but also form a rich, uniform and appropriately sized pore structure. Therefore, it is beneficial to obtain a hard carbon material with a high reversible specific capacity and a high rate performance.

[0052] According to some specific embodiments of the present invention, the sintering time is 2h - 24h. For example, the sintering time can be 2h, 5h, 10h, 15h, 20h, 24h, etc. Thereby, it can effectively promote the pyrolysis and carbonization reactions of the organic components in the biomass powder, and can form a precursor with a high carbon content and a stable structure, which is beneficial to obtaining a hard carbon material with a high reversible specific capacity and a first Coulomb efficiency.

[0053] According to some specific embodiments of the present invention, based on the total mass of the carbon quantum dots and the intermediate, the mass ratio of the carbon quantum dots is 1% - 10%. For example, it can be 1%, 3%, 5%, 10%, etc. By limiting the mass ratio of the carbon quantum dots within the above range, the pores can be effectively blocked, avoiding incomplete pore blocking due to insufficient pore filler, and preventing the complete filling of the pore structure and the blockage of the closed pore structure due to excessive carbon quantum dots. Thereby, a hard carbon material with a high reversible specific capacity can be obtained.

[0054] According to some specific embodiments of the present invention, the calcination temperature is 400°C - 1200°C. In this step, by mixing and calcining the obtained intermediate, during the mixing and calcining process, the functional groups on the surface of the carbon quantum dots will change, melt and carbonize in combination with the micropores on the surface of the carbonized material, and a closed pore structure can be obtained by internal curing in the pores. The closed pore structure can provide a large storage space for sodium ions, improving the reversible specific capacity of the hard carbon material. At the same time, the surface-modified functional groups and defect sites of the carbon quantum dots can provide abundant active sites for sodium ions, providing a large number of sodium storage sites and faster reaction kinetics for sodium ion adsorption. The dual coupling effect of the carbon dots constructs a hard carbon material with both a long sloping region and a long low plateau region. It has the characteristics of high capacity, high rate, and high first efficiency, greatly improving the performance of sodium-ion batteries. By limiting the calcination temperature within the above range, it is better for the carbon quantum dots to undergo carbonization combination, forming more closed pore structures with appropriate sizes and appropriate organic functional groups. Thereby, it is beneficial to obtaining a hard carbon material with a high reversible specific capacity and a first Coulomb efficiency.

[0055] According to some specific embodiments of the present invention, the heating rate of the calcination is 0.5°C / min - 20°C / min. For example, it can be 0.5°C / min, 1°C / min, 15°C / min, 20°C / min, etc. By limiting the heating rate of the calcination within the above range, it can be avoided that the coupling effect is uneven due to too slow or too fast heating. Thereby, it is beneficial to obtaining a hard carbon material with a high reversible specific capacity and a first Coulomb efficiency.

[0056] According to some specific embodiments of the present invention, the calcination time is 0.5h - 24h. For example, it can be 0.5h, 1h, 5h, 10h, 15h, 20h, 24h, etc. By limiting the calcination time within the above range, the material can be sintered more fully. A shorter time will result in insufficient sintering, and a longer time will lead to greater energy consumption. Thus, it is beneficial to obtain hard carbon materials with higher reversible specific capacity and initial Coulombic efficiency.

[0057] According to some specific embodiments of the present invention, the preparation method further includes: grinding the hard carbon material and sieving it. Through grinding, the particles of the hard carbon material can be broken into smaller sizes, and precise screening can be carried out through the sieve mesh, removing large particles and impurities in the material, making the particle distribution of the hard carbon material more uniform, helping to improve the performance stability of the material, and reducing performance fluctuations caused by uneven particle sizes.

[0058] According to some specific embodiments of the present invention, the D50 particle size of the hard carbon material is 5μm - 15μm. For example, it can be 5μm, 10μm, 15μm, etc. By limiting the D50 particle size of the hard carbon material within the above range, it is beneficial for the rapid insertion and extraction of sodium ions in the hard carbon material, thus supporting a high-rate charge-discharge process.

[0059] Thus, by using the method of the present application, a hard carbon material with closed-pore porosity can be obtained, which has rich organic functional groups and a large interlayer spacing, being beneficial for the insertion and extraction and adsorption of sodium ions. Moreover, the preparation process is simple, the production cost is low, and large-scale production can be carried out. And the battery prepared from the hard carbon material obtained by the present application can reach a first charging capacity of more than 400 mAh / g and a first Coulombic efficiency as high as more than 90% at 0.1C.

[0060] According to some specific embodiments of the present invention, the specific surface area of the hard carbon material is not more than 10 m 2 / g. For example, it can be 1 m 2 / g, 5 m 2 / g, 8 m 2 / g, 10 m 2 / g, etc. A larger specific surface area will result in a larger contact area between the electrolyte and the electrode material in the first cycle of the battery. This large contact area will cause the electrolyte to decompose to form a large SEI film, leading to a significant reduction in the first Coulombic efficiency. By controlling the specific surface area within 10 m 2 / g through hydrothermal process treatment, the first Coulombic efficiency can be greatly improved.

[0061] According to an embodiment of the present invention, the battery includes the above-mentioned hard carbon material or the hard carbon material prepared by the above-mentioned method. Thus, the battery has high specific capacity, initial Coulombic efficiency, and rate performance.

[0062] The solution of the present disclosure will be explained below in conjunction with embodiments. Those skilled in the art will understand that the following embodiments are only used to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. For those without specific techniques or conditions noted in the embodiments, they shall be carried out according to the techniques or conditions described in the literature in this field or according to the product specifications.

[0063] The technical solution of the present invention will be further analyzed and explained below through specific embodiments.

[0064] Embodiment 1:

[0065] 1. Put 10 g of bamboo powder into a nitrogen atmosphere and sinter it at a heating rate of 5 °C / min to a set temperature of 800 °C for 1 h to obtain a carbonized material.

[0066] 2. Prepare 4 g of coconut shell carbon as a carbon source and place it in 60.00 ml of distilled water. Add 10 ml of concentrated nitric acid, and magnetically stir for 10 min at a rotation speed of 500 r / min. Then put the suspension system into a hydrothermal reaction kettle with a polytetrafluoroethylene liner. Then place the reaction kettle in an electrothermal constant temperature forced air drying oven and react at a constant temperature of 200 °C for 10 h. After the reaction solution is cooled to room temperature, centrifuge for 10 min at 1000 r / min. Take the centrifuged solution and ultrafiltrate and separate it with a 1KD ultrafiltration membrane, and dialyze for 24 h to obtain carbon quantum dots.

[0067] 3. After mixing the carbon quantum dots and the carbonized material, where the mass ratio of the carbon quantum dots is 6%, add them to the reaction kettle and react at 150 °C for 2 h.

[0068] 4. Subsequently, heat the obtained intermediate to 1000 °C at a rate of 10 °C / min and hold for 4 hours, and then cool it to room temperature to obtain a hard carbon material.

[0069] 5. Preparation method of a sodium ion battery: Mix the above-mentioned hard carbon material with carbon black and polyvinylidene fluoride in a mass ratio of 90:5:5 to form a slurry. Then evenly scrape the slurry onto the current collector copper foil, dry it at a temperature of 100 °C for 30 min, and cut it into electrode sheets as the negative electrode. The battery is assembled in a glove box under an Ar atmosphere, using metallic sodium as the counter electrode and a 1 mol / L NaPF6 (volume ratio of ethylene carbonate to diethyl carbonate is 1:1) solution as the electrolyte to assemble a button cell. Test it in the voltage range of 0 - 3 V, with a current of 30 mA / g.

[0070] For the hard carbon materials of Embodiments 2 - 8 and Comparative Examples 1 - 4, except for the different partial experimental parameters listed in the table, the rest are the same as those in Embodiment 1.

[0071] Partial experimental parameters of the hard carbon materials of Embodiments 1 - 8 and Comparative Examples 1 - 4 are shown in Table 1.

[0072]

[0073] Wherein:

[0074] Comparative Example 1 is to directly perform the hydrothermal reaction in Step 3 on the carbonized material prepared in Step 1 without adding carbon quantum dots.

[0075] Comparative Example 2 is a carbonized material without performing the hydrothermal reaction and without adding carbon quantum dots.

[0076] Comparative Example 3 is to omit the calcination process in Step 4 to investigate the influence of calcination on the material.

[0077] Comparative Example 4 is to directly mix and calcine the carbonized material obtained in Step 1 and the carbon quantum dots obtained in Step 2, omitting the hydrothermal reaction process in Step 3.

[0078] Testing and Analysis

[0079] Under the same conditions, the reversible specific capacity test and the first Coulombic efficiency test were respectively carried out on the hard carbon materials obtained in the above Examples 1-8 and Comparative Examples 1-4. The specific test methods are as follows:

[0080] Reversible specific capacity test: The battery needs to be fully static for 6 h before performing the electrochemical test. Constant current charge and discharge are adopted, the current magnitude is set to 30 mA / g, and the voltage magnitude is set to 0-3 V. Among them, the discharge process is: the current magnitude is 30 mA / g, and it is discharged at a constant current of 0.1C to 0 V, static for 10 min, and then discharged at a constant current of 0.02C to 0 V; the charging process is: the current magnitude is 30 mA / g, and it is charged at a constant current of 0.1C to 3 V.

[0081] Reversible specific capacity = button battery charging capacity / electrode active material.

[0082] First Coulombic efficiency test: The test program is set through software, which includes five steps: static, constant current discharge, pause, constant current charge, and cycle. Note that the battery test is carried out under constant temperature conditions, and the battery needs to be fully static for 8 h before performing the electrochemical test. Then place the prepared battery in an environment of 25°C, discharge it at a constant current of 0.1C to a voltage of 0.01V, record the initial discharge specific capacity as Q1, and then charge it at a constant current of 0.1C to 3V, record the charging specific capacity as Q2, and the reversible specific capacity in Table 2 is the charging specific capacity.

[0083] First Coulombic efficiency (%) = Q2 / Q1×100%

[0084] Figure 2 Shows the scanning electron microscope image of the hard carbon material of Example 1 of the present application. It can be seen from the figure that the surface of the material is smooth and the particle size distribution is uniform.

[0085] Figure 3 The cyclic performance graph of the sodium-ion battery according to Embodiment 1 of the present application at a current density of 30 mA / g is shown. It can be seen from the graph that after hydrothermal treatment with carbon quantum dots, the reversible capacity is as high as 412 mAh / g, the initial Coulombic efficiency reaches 91.6%, it has a rich sodium storage pore structure, and the specific capacity in the sloping region reaches 237 mAh / g, thus supporting a high-rate charge-discharge process and showing great potential in the field of sodium-ion batteries.

[0086] The test results are shown in Table 2.

[0087]

[0088] Combined with Table 1, it can be obtained that compared with Comparative Examples 1-4, the hard carbon materials obtained in Examples 1-8 all have higher reversible capacities and higher initial Coulombic efficiencies.

[0089] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", "some implementation manners" or "some examples", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0090] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A preparation method of a biomass-based modified functional group carbon quantum dot-coupled closed pore structure hard carbon material, characterized in that It includes the following steps: Step 1: Pretreat the biomass powder to remove most of the volatile components in the biomass, obtaining a carbonized material with a specific surface area of 300 - 800 m 2 / g; Step 2: Prepare biomass carbon quantum dots with modified functional groups, and the size of the biomass carbon quantum dots with modified functional groups is 1-10 nm; Step 3: Mix the carbonized material obtained in Step 1 with the biomass carbon quantum dots obtained in Step 2 and carry out a hydrothermal reaction to obtain an intermediate; Step 4: Calcinate the intermediate obtained in Step 3 to obtain a hard carbon material.

2. The preparation method according to claim 1, wherein: In Step 1, the biomass powder is obtained by crushing one or several of coconut shell, sugarcane, poplar wood, cotton, wheat, corn straw, reed, bamboo, seaweed, oak, etc., and the size is 10-50 um.

3. The preparation method according to claim 1, wherein: In Step 1, the pretreatment is a sintering treatment carried out in an inert atmosphere, the temperature of the sintering treatment is 400°C - 1200°C, and the time is 2-24 h.

4. The preparation method according to claim 1, wherein: In Step 2, the biomass carbon quantum dots with modified functional groups are obtained by the following method: Add the precursor powder from biomass sources to distilled water, then add nitric acid or perchloric acid, and carry out a hydrothermal reaction in a reaction kettle to obtain biomass carbon quantum dots with modified functional groups; The precursor powder is selected from bamboo powder or coconut shell powder; The mass ratio of the precursor powder to nitric acid or perchloric acid is 1:2-4.

5. The preparation method according to claim 4, wherein: The reaction temperature of the hydrothermal reaction is 180°C, and the reaction time is 3-10 hours.

6. The preparation method according to claim 1, wherein: In Step 3, based on the total mass of the carbon quantum dots and the carbonized material, the mass percentage of the carbon quantum dots is 1%-10%.

7. The preparation method according to claim 1, wherein: In Step 3, the temperature of the hydrothermal reaction is 100°C - 200°C, and the reaction time is 3-10 h.

8. The preparation method according to claim 1, wherein: In Step 4, the temperature of the calcination is 400°C - 1200°C, and the time is 0.5 h-24 h.

9. The preparation method according to claim 1, wherein: In Step 4, the D50 particle size of the hard carbon material is 5 μm - 15 μm, and the specific surface area ≤ 10 m 2 / g.

10. The application of the biomass-based modified functional group carbon quantum dot-coupled closed-pore structure hard carbon material prepared by the preparation method according to any one of claims 1-9 in the preparation of a sodium-ion battery, wherein: Use the biomass-based modified functional group carbon quantum dot-coupled closed-pore structure hard carbon material to prepare a negative electrode material, use metallic sodium as the counter electrode, and use a NaPF6 solution as the electrolyte.

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