Hard carbon material, preparation method thereof, hard carbon negative electrode sheet and sodium ion battery

Abstract

The application discloses a kind of hard carbon materials and preparation method thereof, hard carbon negative pole piece and sodium ion battery, it is related to battery technical field;The preparation method of hard carbon material includes the following steps: to hemp pole is sequentially screened, broken, carbonized, secondary broken, ground and impurity-removed, and hard carbon precursor is obtained;The hard carbon precursor is sequentially ground, primary burned, secondary burned, washed and dried, and hard carbon material is obtained.The preparation operation is relatively simple, and the price is relatively low, which can reduce the cost of hard carbon material, and at the same time, it can be obtained from hemp manufacturer in large scale, which is convenient for large-scale collection.In addition, the hard carbon material with hemp pole as raw material is applied to the negative pole piece of sodium ion battery, and the gram capacity of hard carbon material is high, which is beneficial to improve the energy density of sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a hard carbon material and its preparation method, a hard carbon negative electrode sheet, and a sodium-ion battery. Background Technology

[0002] The development of sodium-ion batteries is of great significance for alleviating lithium resource shortages, reducing dependence on foreign resources, promoting technological progress, and lowering energy costs. With technological advancements and cost reductions, the application prospects of sodium-ion batteries are becoming increasingly broad, and they will have a profound impact on multiple sectors, including the electric vehicle market, energy storage market, and the power industry. Hard carbon anode materials are crucial for sodium-ion batteries, possessing characteristics such as high capacity, long cycle life, low cost, and high safety. However, currently, hard carbon raw materials are expensive, resulting in high costs. Summary of the Invention

[0003] The main objective of this invention is to provide a hard carbon material and its preparation method, a hard carbon negative electrode sheet, and a sodium-ion battery, with the aim of reducing the cost of hard carbon materials.

[0004] To achieve the objective of this invention, this invention provides a method for preparing hard carbon materials, the method comprising the following steps:

[0005] The hemp stalks were subjected to primary screening, crushing, carbonization, secondary crushing, grinding and impurity removal in sequence to obtain hard carbon precursor;

[0006] The hard carbon precursor was subjected to secondary grinding, initial calcination, secondary calcination, washing, and drying to obtain hard carbon material.

[0007] In one embodiment, the carbonization includes the following steps:

[0008] The crushed hemp stalks are carbonized in air at a temperature of 400-700℃, and then cooled to room temperature.

[0009] In one embodiment, the carbonization heating rate is 4-6°C / min, and the holding time is 3-6h.

[0010] In one embodiment, the initial firing includes the following steps:

[0011] The hard carbon precursor after secondary grinding was initially calcined in an inert gas atmosphere at a temperature of 1000-1400℃.

[0012] In one embodiment, the initial heating rate is 5-10°C / min, and the holding time is 3-6h.

[0013] In one embodiment, the second firing includes the following steps:

[0014] The product after the initial firing is then subjected to a second firing at a temperature of 600-1000℃.

[0015] In one embodiment, the heating rate of the second furnace is 5-10°C / min, and the holding time is 3-6h.

[0016] The present invention also provides a hard carbon material, wherein the hard carbon is prepared by the hard carbon material preparation method described above.

[0017] The present invention also provides a hard carbon negative electrode sheet, wherein the hard carbon negative electrode sheet comprises the hard carbon material as described above.

[0018] The present invention also provides a sodium-ion battery, the sodium-ion battery comprising a hard carbon negative electrode, a positive electrode and a separator as described above, wherein the separator is disposed between the hard carbon negative electrode and the positive electrode.

[0019] The present invention provides a method for preparing hard carbon material using hemp stalks as raw material. First, the hemp stalks are subjected to initial screening, crushing, carbonization, secondary crushing, grinding, and impurity removal to obtain a hard carbon precursor. Then, the hard carbon precursor is subjected to secondary grinding, initial calcination, secondary calcination, washing, and drying to obtain the hard carbon material. Therefore, the preparation process is relatively simple, and hemp stalks, being a waste product from burlap processing, are inexpensive, reducing the cost of hard carbon material. Furthermore, they can be obtained in large quantities from burlap manufacturers, facilitating large-scale collection. When the hard carbon material made from hemp stalks is applied to the negative electrode of a sodium-ion battery, the specific capacity of the resulting hard carbon material is high due to the strong yet porous nature of hemp stalks, which is beneficial for improving the energy density of sodium-ion batteries. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram comparing the charge and discharge curves of Embodiments 1, 2 and 3 of the present invention.

[0022] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0023] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially. Furthermore, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, or solution B, or a solution where both A and B are satisfied simultaneously. In addition, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention. 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.

[0025] To address the current technical problem of expensive raw materials and high costs in hard carbon materials, this invention provides a method for preparing hard carbon materials using hemp stalks as raw materials, aiming to reduce the cost of hard carbon materials.

[0026] Hemp stalks are the peeled stems of hemp and other bast fibers such as hemp and flax. The stalks are slender and sturdy, with abundant pores in carbon products. Hemp stalks are a waste product from hemp cloth processing, with low value, and are easy to collect on a large scale. The sturdy yet porous nature of hemp stalks makes them an ideal material for producing hard carbon, offering both a high hard carbon yield and the advantage of controlled pore structure in later stages, increasing sodium storage sites.

[0027] The method for preparing hard carbon material provided by this invention includes the following steps:

[0028] Step S1 involves sequentially screening, crushing, carbonizing, secondary crushing, grinding, and removing impurities from the hemp stalks to obtain a hard carbon precursor.

[0029] The initial screening process involves placing the hemp stalks on a vibrating screen conveyor for preliminary screening to remove other substances mixed in with the hemp stalks, such as larger particles of impurities like mud and stones adhering to the surface of the hemp stalks.

[0030] The crushing process involves the following steps: After initial screening, the hemp stalks are conveyed to a wood crusher via a vibrating conveyor for crushing. The crushed hemp stalks are small particles or powder. The particle size can be adjusted according to specific experimental or production needs. Smaller particles result in a more complete reaction during subsequent carbonization. Optionally, the particle size of the crushed hemp stalks can be 0.1-1mm (e.g., 0.1mm, 0.3mm, 0.5mm, 0.8mm, 1mm, or any range between the two endpoints). This particle size is beneficial for uniform heating and the carbonization reaction.

[0031] The carbonization process involves carbonizing the crushed hemp stalks in air, followed by natural cooling to room temperature. The heating rate during carbonization is 4-6℃ / min (e.g., 4℃ / min, 5℃ / min, 6℃ / min, or any range between two endpoints), the carbonization temperature is 400-700℃ (e.g., 400℃, 500℃, 600℃, 700℃, or any range between two endpoints), and the holding time is 3-6 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours, or any range between two endpoints). These carbonization conditions allow for the complete decomposition and condensation of the organic components in the hemp stalks, yielding the carbonized product.

[0032] The secondary crushing and grinding process specifically involves the secondary crushing and grinding of the cooled carbonized products, resulting in particles with a particle size of 50-100 mesh (e.g., 50 mesh, 60 mesh, 70 mesh, 80 mesh, 90 mesh, 100 mesh, and any range between two endpoints).

[0033] The impurity removal process specifically involves washing the carbonized products with water to remove dust or small particles that are not fully carbonized from the surface of the carbonized products.

[0034] After impurity removal, a high-purity hard carbon precursor can be obtained.

[0035] Step S2 involves sequentially grinding, initial calcination, secondary calcination, washing, and drying the hard carbon precursor obtained in step S1 to obtain the hard carbon material.

[0036] The secondary grinding process specifically involves grinding the hard carbon precursor twice, resulting in a particle size D50 of 3-9 μm (e.g., 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and any range between two endpoints).

[0037] The purpose of initial firing is to form the basic structure of hard carbon. During initial firing, the cellulose, hemicellulose, lignin and other organic components in the hard carbon precursor further decompose, releasing small molecule gases and leaving the basic carbon framework.

[0038] The initial calcination process involves placing the hard carbon precursor, after secondary grinding, in a furnace purged with inert gas for initial calcination. The heating rate during initial calcination is 5-10℃ / min (e.g., 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any range between two endpoints), the initial calcination temperature is 1000-1400℃ (e.g., 1000℃, 1100℃, 1200℃, 1300℃, 1400℃, or any range between two endpoints), and the holding time is 3-6 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours, or any range between two endpoints). These initial calcination conditions ensure sufficient decomposition of the organic components in the hard carbon precursor without causing structural collapse or excessive graphitization due to excessively high temperatures.

[0039] In some embodiments, the inert gas includes one of nitrogen, argon, and a hydrogen-argon mixture, wherein the volume ratio of hydrogen to argon in the hydrogen-argon mixture is 5:95. Initial calcination is carried out in an inert gas atmosphere, which effectively prevents the hard carbon precursor from undergoing oxidation with oxygen, thus avoiding any impact on the yield and purity of the hard carbon material.

[0040] The purpose of secondary sintering is to further optimize the structure of hard carbon. Through secondary sintering, the pore structure of hard carbon can be adjusted, crystallinity can be improved, and impurities and defects can be reduced. After secondary sintering, the density of hard carbon increases, and its hardness and strength also improve accordingly. When hard carbon material with secondary sintering is used as the negative electrode sheet of sodium-ion batteries, it is beneficial for the insertion and extraction of sodium ions, thereby improving the electrochemical performance of sodium-ion batteries. At the same time, the structure of hard carbon with secondary sintering is more stable and can better maintain its structural integrity during charge-discharge cycles, thus significantly improving the cycle performance of sodium-ion batteries.

[0041] The second firing process involves subjecting the product after the initial firing to a second firing at a temperature of 600-1000℃ (e.g., 600℃, 700℃, 800℃, 900℃, 1000℃, or any range between two endpoints). The heating rate during the second firing is 5-10℃ / min (e.g., 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any range between two endpoints), and the holding time is 3-6 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours, or any range between two endpoints). These second firing conditions allow for better control of the pore structure of hard carbon, resulting in a more stable hard carbon structure.

[0042] The washing process specifically involves acid washing of the product after secondary calcination to remove impurities (such as metallic impurities). Simultaneously, acid washing also etches the surface of the hard carbon material to some extent, increasing its specific surface area and porosity, which is beneficial for the subsequent transport and storage of sodium ions. The acid washing solution used can be hydrochloric acid or sulfuric acid with a mass concentration of 10%-20%.

[0043] The purpose of drying is to remove residual acid and moisture after pickling, preventing them from affecting the performance of hard carbon materials. The drying temperature can be selected from 80-120℃ (e.g., 80℃, 90℃, 100℃, 110℃, 120℃, or any range between two extremes), and the drying time can be selected from 8-12h (e.g., 8h, 9h, 10h, 11h, 12h, or any range between two extremes).

[0044] The present invention provides a method for preparing hard carbon material using hemp stalks as raw material. First, the hemp stalks are subjected to initial screening, crushing, carbonization, secondary crushing, grinding, and impurity removal to obtain a hard carbon precursor. Then, the hard carbon precursor is subjected to secondary grinding, initial calcination, secondary calcination, washing, and drying to obtain the hard carbon material. Therefore, the preparation process is relatively simple, and hemp stalks, being a waste product from burlap processing, are inexpensive, reducing the cost of hard carbon material. Furthermore, they can be obtained in large quantities from burlap manufacturers, facilitating large-scale collection. When the hard carbon material made from hemp stalks is applied to the negative electrode of a sodium-ion battery, the specific capacity of the resulting hard carbon material is high due to the strong yet porous nature of hemp stalks, which is beneficial for improving the energy density of sodium-ion batteries.

[0045] The present invention also provides a hard carbon material, which is prepared by the hard carbon material preparation method described in the above embodiments.

[0046] The present invention also provides a hard carbon negative electrode sheet, which includes the hard carbon material described above.

[0047] The hard carbon negative electrode sheet of the present invention can be prepared by the following steps: (a) preparing a hard carbon negative electrode slurry by mixing hard carbon material, conductive agent, and binder in a mass ratio of (70-90):(3-10):(3-7) to obtain a dry powder mixture, then adding solvent water and stirring to obtain a negative electrode slurry. The solid content of the negative electrode slurry is 40wt%-70wt% (e.g., 40wt%, 50wt%, 60wt%, 70wt%, and any range between two endpoints). (b) uniformly coating the negative electrode slurry onto the surface of aluminum foil, vacuum drying, cold pressing, and cutting to obtain a hard carbon negative electrode sheet. The shape of the hard carbon negative electrode sheet can be circular, square, or other reasonable shapes.

[0048] In some embodiments, the conductive agent includes, but is not limited to, carbon black (such as Super P, Ketjen black, etc.), conductive graphite, carbon nanotubes, and graphene.

[0049] In some embodiments, the binder includes, but is not limited to, sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA).

[0050] The present invention also provides a sodium-ion battery, which includes a hard carbon negative electrode, a positive electrode, and a separator as described above. The separator is located between the hard carbon negative electrode and the positive electrode. The positive electrode includes a positive current collector (such as aluminum foil) and a positive active material coated on the surface of the positive current collector. The positive active material includes layered transition metal oxides, polyanionic compounds, and Prussian blue compounds. The separator includes, but is not limited to, PP, PE, PP / PE, PP / PE / PP separators, and ceramic separators.

[0051] Here, the sodium-ion battery also includes an electrolyte, which is composed of a solvent, a sodium salt, and additives. The sodium salt includes, but is not limited to, sodium perchlorate, sodium trifluoromethanesulfonate, and sodium hexafluorophosphate. The solvent includes, but is not limited to, one or more of dimethyl carbonate, propylene carbonate, ethylene carbonate, and ethylene glycol dimethyl ether.

[0052] The following detailed description of the hard carbon material, its preparation method, the hard carbon negative electrode sheet, and the sodium-ion battery of the present invention is provided through specific embodiments.

[0053] Example 1

[0054] This embodiment provides a method for preparing hard carbon material, including the following steps:

[0055] (1) Place the hemp stalk raw material on a vibrating screen conveyor for preliminary screening to remove other substances mixed in with the hemp stalk.

[0056] (2) The hemp stalks after preliminary screening are fed into a wood crusher for crushing. The particle size of the crushed hemp stalks is 0.5 mm.

[0057] (3) The broken hemp stalks are carbonized in air atmosphere. The heating rate of the carbonization process is 5℃ / min, the carbonization temperature is 500℃, the holding time is 3h, and the carbonization is naturally cooled to room temperature after carbonization.

[0058] (4) The cooled carbonized product is subjected to secondary crushing and grinding. The particle size of the ground particles is 80 mesh. Then, the product is washed with water to remove impurities and obtain hard carbon precursor.

[0059] (5) The hard carbon precursor was subjected to secondary grinding, and the particle size D50 after secondary grinding was 6μm.

[0060] (6) The hard carbon precursor after secondary grinding is placed in a furnace with inert gas and subjected to initial calcination. The heating rate during the initial calcination process is 8℃ / min, the initial calcination temperature is 1200℃, and the holding time is 4h.

[0061] (7) The product after the initial firing is subjected to a second firing at a temperature of 800℃, a heating rate of 6℃ / min, and a holding time of 5h.

[0062] (8) After acid washing and drying, the product after the second firing is obtained as hard carbon material.

[0063] This embodiment also provides a method for assembling a sodium-ion battery, specifically including the following steps:

[0064] 1. Preparation of hard carbon negative electrode sheet: First, the hard carbon material, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber prepared above are mixed in a mass ratio of 90:5:2:3. Then, deionized water is added and stirred to obtain a negative electrode slurry with a solid content of 40wt%. Then, the negative electrode slurry is uniformly coated on the surface of aluminum foil, vacuum dried, cold pressed and cut to obtain a circular hard carbon negative electrode sheet with a diameter of 12mm.

[0065] 2. Preparation of positive electrode sheet: Polyanionic compound material is used as the material for the positive electrode sheet.

[0066] 3. Diaphragm: PP diaphragm is used.

[0067] 4. Electrolyte: Sodium hexafluorophosphate electrolyte is used.

[0068] 5. Assemble the battery: Assemble the hard carbon negative electrode, positive electrode, and separator prepared above into a 2025 coin cell.

[0069] It should be noted that the specific preparation of the positive electrode sheet, the specific composition of the electrolyte, and the specific assembly operation of the 2025 coin cell can be referred to the existing technology, and will not be elaborated here.

[0070] The 2025 coin cell battery assembled in this embodiment was subjected to charge-discharge tests, and its charge-discharge curve is shown below. Figure 1 As shown, its capacity data is shown in Table 1.

[0071] Example 2

[0072] The difference between this embodiment and Embodiment 1 is that the initial firing temperature in step (6) is 1100℃, while all other operations are the same as in Embodiment 1. The charge-discharge curves of the 2025 coin cell assembled in Embodiment 2 are also as shown. Figure 1 As shown, its capacity data is shown in Table 1.

[0073] Example 3

[0074] The difference between this embodiment and Embodiment 1 is that the firing temperature in step (7) is 700℃, while all other operations are the same as in Embodiment 1. The charge-discharge curve of the 2025 button cell assembled in Embodiment 2 is also as shown. Figure 1 As shown, its capacity data is shown in Table 1.

[0075] from Figure 1 It can be seen that, compared to Example 1, the charging plateau of the charge-discharge curve of Example 2 is slightly higher after the initial firing temperature is reduced, indicating that the battery polarization is slightly greater. Compared to Example 1, the slope region of the charge-discharge curve of Example 3 is further to the left after the second firing temperature is reduced, indicating slightly poorer kinetics and a relatively lower specific capacity. This shows that the initial firing temperature mainly affects the battery polarization, while the second firing temperature mainly affects the battery specific capacity.

[0076] Example 4

[0077] The difference between this embodiment and Embodiment 1 is that the initial firing temperature in step (6) is 1400℃, while all other operations are the same as in Embodiment 1. The capacity data of the 2025 button cell assembled in Embodiment 4 are shown in Table 1.

[0078] Example 5

[0079] The difference between this embodiment and Embodiment 1 is that the second firing temperature in step (7) is 900℃, while all other operations are the same as in Embodiment 1. The capacity data of the 2025 button cell assembled in Embodiment 5 are shown in Table 1.

[0080] Table 1. Capacity data of 2025 button cells assembled in Examples 1-5

[0081] Serial Number Discharge specific capacity (mAh / g) Charging specific capacity (mAh / g) First-time efficiency (%) Example 1 380.7 344.5 90.49 Example 2 383.6 346.8 90.41 Example 3 375.1 340.5 90.78 Example 4 373.7 337.6 90.35 Example 5 372.5 336.9 90.44

[0082] As can be seen from the data in Table 1, in the preparation of hard carbon materials in Examples 1-5 of the present invention, by reasonably controlling the initial calcination temperature and the second calcination temperature during the preparation process, the hard carbon materials prepared in this way are used to prepare hard carbon negative electrode sheets and assembled into 2025 coin cells. The discharge specific capacity of the obtained batteries can reach more than 372.5 mAh / g, and the charge specific capacity can reach more than 336.9 mAh / g. The specific capacity is high, which is conducive to improving the energy density of the battery. The initial efficiency can reach more than 90%.

[0083] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or system that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or system. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or system that includes that element.

[0084] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent articles or equivalent process changes made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for producing a hard carbon material, characterized by, The preparation method comprises the following steps: The hemp rod is sequentially subjected to primary screening, crushing, carbonization, secondary crushing, grinding and impurity removal to obtain a hard carbon precursor, wherein the hemp rod is the stem of a hemp variety after peeling, and the carbonization comprises: carbonizing the crushed hemp rod in an air atmosphere, and then cooling to room temperature; The hard carbon precursor is sequentially subjected to secondary grinding, primary sintering, secondary sintering, washing and drying to obtain a hard carbon material, wherein the primary sintering comprises: primary sintering the secondary ground hard carbon precursor in an inert gas atmosphere, and the temperature of the primary sintering is 1000-1400℃; The secondary sintering comprises: secondary sintering the product after the primary sintering, and the temperature of the secondary sintering is 600-1000℃.

2. The method for producing a hard carbon material according to claim 1, wherein The carbonization temperature is 400-700℃.

3. The method for producing a hard carbon material according to claim 2, wherein The heating rate of the carbonization is 4-6℃ / min, and the holding time is 3-6h.

4. The method for producing a hard carbon material according to claim 1, wherein The heating rate of the primary sintering is 5-10℃ / min, and the holding time is 3-6h.

5. The method for producing a hard carbon material according to any one of claims 1 to 4, wherein The heating rate of the secondary sintering is 5-10℃ / min, and the holding time is 3-6h.

6. A hard carbon material characterized in that, The hard carbon material is prepared by the preparation method of the hard carbon material according to any one of claims 1 to 5.

7. A hard carbon negative electrode sheet, characterized by, The hard carbon negative electrode sheet comprises the hard carbon material according to claim 6.

8. A sodium-ion battery, characterized in that, The sodium ion battery comprises the hard carbon negative electrode sheet, a positive electrode sheet and a separator according to claim 7, and the separator is arranged between the hard carbon negative electrode sheet and the positive electrode sheet.

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