A starch-based hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery

Abstract

The present invention provides a starch-based hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery, belonging to the technical field of battery materials. The starch-based hard carbon negative electrode material includes single crystal particles, and the single crystal particles include polygonal particles with an angular morphology, and the edge angle is 90°-130°. The single crystal particles in the starch-based hard carbon negative electrode material provided by the present invention have a regular morphology and an angular morphology with a specific angle, which can avoid the phenomenon of particle agglomeration and fusion, and the physical and chemical properties of the starch-based hard carbon negative electrode material are significantly improved, the particle size distribution is uniform and the compaction is improved, and the sodium storage sites are also increased, which is beneficial to the improvement of sodium storage capacity and first effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a starch-based hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery. Background Art

[0002] Due to the shortage of lithium resources and the pressure of rising prices, sodium-ion batteries have regained public attention and shown great application potential in areas such as grid energy storage and low-speed vehicles. For sodium-ion battery anode materials, hard carbon is the most promising material for commercial application.

[0003] The advantage of industrial starch as a raw material for hard carbon precursors lies in its high-purity industrial product, ensuring product consistency and stability. Furthermore, the raw material is abundantly available, and the market supply is sufficient and stable, reducing the risk of a single supplier. However, its bottleneck lies in its high processing difficulty and tendency to foam and expand during high-temperature carbonization, which limits commercial production and affects performance. Starch is a polysaccharide, a high-molecular carbohydrate formed by the dehydration and polymerization of glucose molecules. Glucose molecules are connected by α-1,4 glycosidic bonds or α-1,6 glycosidic bonds. Direct high-temperature carbonization of ordinary starch causes foaming and fusion of starch granules. This is because during the carbonization stage, the glycosidic bonds break, forming large amounts of H2O, CO2, and CO small molecules, which are released in large quantities. Although starch can be stabilized by pre-stabilization at low temperatures for a long time before carbonization, this requires a time-consuming stabilization process (more than 72 hours), which is not conducive to commercial mass production. Moreover, after mass production, due to the problems of uneven temperature and insufficient reaction, materials that have been pre-stabilized for a long time or modified by other technical means still have the risk of foaming and expansion at the high-temperature carbonization end.

[0004] In addition, the prior art also has the following problems:

[0005] 1) The unmodified starch-based hard carbon particles exhibited serious agglomeration and fusion, which directly led to problems such as uneven particle size distribution, large D50, and low powder compaction, resulting in substandard physical and chemical properties.

[0006] 2) The properties of the hard carbon precursor largely determine the sodium storage capacity of the hard carbon material after high-temperature carbonization. It is difficult to achieve a high-capacity breakthrough in starch-based hard carbon by simply changing the high-temperature carbonization process.

[0007] 3) Low-solvent molecules in carbonate electrolyte systems (e.g., EC: ethylene carbonate) can enter the interior of the carbon layer of incompletely grown hard carbon, leading to irreversible capacity loss. This is because when the diameter of the solvent molecules is smaller than the diameter of the pore opening, the solvent molecules will penetrate into the bulk of the material. This makes the actual surface area of the starch-based hard carbon material (also known as the "electrolyte-accessible surface area") much larger than the surface area of the material, resulting in excessive capacity loss and low initial efficiency.

[0008] Therefore, how to avoid the agglomeration and fusion of starch-based hard carbon particles, improve the physical and chemical properties of starch-based hard carbon negative electrode materials, and increase the sodium storage capacity and initial efficiency is a technical problem that needs to be solved urgently. Summary of the Invention

[0009] In response to the shortcomings of the prior art, the present invention aims to provide a starch-based hard carbon anode material, a preparation method thereof, and a sodium-ion battery. The starch-based hard carbon anode material provided by the present invention has regular single-crystal particles with angular shapes at specific angles, which prevents particle agglomeration and fusion. Furthermore, the starch-based hard carbon anode material exhibits significantly improved physical and chemical properties, with uniform particle size distribution and enhanced compaction. Furthermore, the material also has increased sodium storage sites, which contributes to improved sodium storage capacity and initial efficiency.

[0010] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material comprises single crystal particles, wherein the single crystal particles have an angular morphology with an edge angle of 90°-130°.

[0012] It should be noted that particle angularity is an important metric used to characterize particle shape. Angularity refers to the sharpness of a particle's edges and corners. Particle angularity can be quantitatively or qualitatively described using a variety of methods. For example, angularity can be assessed by measuring the angle at the particle's corners or observing the particle's outline under a microscope.

[0013] The single crystal particles in the starch-based hard carbon negative electrode material provided by the present invention have regular morphology and angular morphology with specific angles, which can avoid the phenomenon of particle agglomeration and fusion. In addition, the physical and chemical properties of the starch-based hard carbon negative electrode material are significantly improved, the particle size distribution is uniform and the compaction is improved. At the same time, the sodium storage sites are also increased, which is beneficial to the improvement of sodium storage capacity and first efficiency.

[0014] In the present invention, the single-crystal particles have an angular, polygonal morphology with angles ranging from 90° to 130°, for example, 90°, 100°, 110°, 120°, or 130°. This morphology contributes to the packing density and stability of the hard carbon single-crystal particles in the electrode material, thereby improving the energy density and cycling performance of the battery. Furthermore, this angular morphology effectively reduces particle agglomeration during charge and discharge.

[0015] Preferably, the particle size D50 of the single crystal particles is 5-15 μm, for example, it can be 5 μm, 7 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm, and 80% of the single crystal particles are distributed in the range of 3-30 μm, for example, it can be 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm, etc.

[0016] In this invention, the starch-based hard carbon anode material has a uniform particle size distribution of single crystal particles, which facilitates the insertion and deinsertion of sodium ions, shortens the sodium ion diffusion path, reduces diffusion resistance, and thus improves the battery's charge and discharge performance. Furthermore, the uniform particle size distribution of the anode material helps improve the viscosity and stability of the slurry, reduces the difficulty of coating, and thus optimizes the battery production process.

[0017] It should be noted that 80% of the single crystal particles are distributed in the range of 3-30 μm, which means that the particle size range corresponding to the cumulative distribution percentage from small to large is from 10% to 90%, that is, the interval range of D90~D10 is 3-30 μm.

[0018] Preferably, the single crystal dispersion rate of the single crystal particles is ≥90%, for example, it can be 90%, 92%, 94%, 96%, 98% or 99%.

[0019] It should be noted that the single crystal dispersion rate refers to the ratio of the number of individually dispersed single crystal particles to the total number of particles in a given system. It reflects the degree to which the single crystal particles avoid agglomeration and are evenly distributed in the system.

[0020] In the present invention, a single crystal dispersion rate of the single crystal particles of ≥90% helps to reduce the agglomeration phenomenon between the particles, making the electrode material more uniform and stable, thereby reducing polarization and improving the overall performance of the battery.

[0021] Preferably, the starch-based hard carbon negative electrode material has a graphite-like microcrystalline region, and the average carbon layer spacing in the graphite-like microcrystalline region is 0.38-0.4 nm, for example, 0.38 nm, 0.39 nm or 0.4 nm.

[0022] It should be noted that the preparation of starch-based hard carbon materials usually undergoes a high-temperature carbonization step. When the temperature rises to a certain level, the carbon chains in the starch molecules begin to undergo violent pyrolysis, rearrangement, and polycondensation reactions. Although the final hard carbon material has a disordered overall structure, it still contains some locally oriented, graphite-like small-scale stacks of hexagonal carbon atoms. These stacks are usually only a few nanometers thick and have a limited number of stacked layers, forming graphite-like microcrystalline regions. The graphite-like microcrystalline region can be characterized by XRD or high-resolution transmission electron microscopy. For example, the d(002) value in XRD detection can represent the average carbon interlayer spacing of the graphite-like microcrystalline region. Its value in the range of 0.38-0.40 is conducive to the deintercalation and extraction of sodium ions, thereby improving the kinetic properties of the hard carbon material.

[0023] Preferably, the BET of the starch-based hard carbon negative electrode material is 0.1-5m 2 / g, for example, it can be 0.1m 2 / g, 0.5m 2 / g、1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g or 5m 2 / g, etc.

[0024] In the present invention, the BET of starch-based hard carbon negative electrode material is 0.1-5m 2 / g, the BET value is small, which proves that closed pores are effectively generated, which helps to improve the first efficiency, corresponding to the high first efficiency of ≥90%.

[0025] Preferably, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is greater than 0.9g / cm 3 , for example, it can be 0.91 g / cm 3 , 0.93g / cm 3 , 0.95g / cm 3 , 0.97g / cm 3 , 0.99g / cm 3 or 1.1 g / cm 3 etc., preferably 0.95-1.1 g / cm 3 .

[0026] In the present invention, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is greater than 0.9g / cm 3 , which helps to improve the energy density of the battery.

[0027] Preferably, the starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.06-0.2 cm 3 / g, for example, it can be 0.06cm 3 / g, 0.10cm 3 / g, 0.14cm 3 / g, 0.18cm 3 / g or 0.2cm 3 / g, etc. The pore diameter of the closed pore is <0.4nm, for example, it can be 0.3nm, 0.2nm or 0.1nm, etc.

[0028] It should be noted that the volume of closed pores can be calculated according to the formula: closed pore volume = 1 / true density - 1 / 2.26; the true density is directly tested using a true density meter, which refers to the actual mass of solid per unit volume when the material is in an absolutely dense state. It can be used to analyze parameters such as the presence of closed pores in the material. The BSD-TD-K fully automatic true density and porosity analyzer from Best Instrument Technology Co., Ltd. is used. During the test, the powder can be directly loaded into the sample test cavity.

[0029] It should be noted that the pore diameter of the closed pore is smaller than the solvent molecules and solvated Na + The size of the pores is smaller than 0.4 nm, that is, the pore diameter is less than 0.4 nm, which is conducive to the storage of sodium ions therein and hinders the entry of solvent molecules. This protects the active sites in the pores that can store sodium to a certain extent, avoids excessive capacity loss, thereby improving the first efficiency and being suitable for low-molecular solvent electrolytes. It is worth noting that the nitrogen adsorption value in the table test can characterize the size of the pore diameter. When the pore diameter of the material is smaller than the diameter of the nitrogen molecule (about 0.364 nm), the nitrogen molecules cannot enter the pores, the adsorption amount is close to 0, and the BET value is also close to 0, that is, the smaller the BET value, the smaller the pore diameter, and the closer it is to the diameter of the nitrogen molecule. Therefore, based on the starch-based hard carbon negative electrode material of this application, BET <1m 2 / g, the first efficiency is greater than 90%, indicating that the pore diameter can effectively prevent nitrogen molecules from entering and solvent molecules from entering the pores. Therefore, BET < 1m 2 / g and the first efficiency is greater than 90%, and the pore diameter is less than 0.4nm.

[0030] In a second aspect, the present invention provides a method for preparing the starch-based hard carbon negative electrode material as described in the first aspect, the preparation method comprising the following steps:

[0031] An oxidant and starch are mixed to carry out an oxidation reaction to obtain oxidized modified starch.

[0032] The oxidatively modified starch is subjected to low-temperature pretreatment and high-temperature carbonization treatment in sequence to obtain the starch-based hard carbon negative electrode material.

[0033] In the preparation method provided by the present invention, starch is oxidatively modified, which solves the problem of difficulty in starch processing. Oxygen-containing functional groups are grafted onto the surface of the oxidatively modified starch, and the oxygen-containing functional groups and starch molecules are fully cross-linked with each other, which is beneficial to inhibiting the growth of graphite crystallites during the subsequent high-temperature carbonization process, inhibiting graphitization, generating more disordered carbon structures, and increasing sodium storage sites, thereby facilitating the improvement of sodium storage capacity and the first efficiency, and is suitable for low-molecular solvent electrolytes.

[0034] In the preparation method provided by the present invention, starch is oxidatively modified to release part of the water and small molecules contained in the starch, thereby playing a stabilizing role, greatly reducing the time cost of low-temperature pretreatment, shortening the production cycle, and facilitating commercial production.

[0035] Preferably, the viscosity of the oxidized modified starch is 6-14 mPa·s, for example, 6 mPa·s, 8 mPa·s, 10 mPa·s, 12 mPa·s or 14 mPa·s.

[0036] In the present invention, the viscosity of the oxidized modified starch is used to characterize the oxidation degree of the oxidized modified starch. The oxidation degree of the oxidized starch is significantly negatively correlated with the viscosity. A viscosity of 6-14 mPa·s indicates a decrease in viscosity. After the starch is oxidized, the content of oxygen-containing functional groups increases.

[0037] Preferably, the crystallinity of the oxidized modified starch is 8-20%, for example, 8%, 10%, 12%, 14%, 16%, 18% or 20%.

[0038] In the present invention, the crystallinity of the oxidized modified starch is used to characterize the degree of oxidation of the oxidized modified starch. A crystallinity of 8-20% indicates that the crystallinity is reduced, and the oxidation degree of the starch is increased by the oxidation process.

[0039] Preferably, the oxidatively modified starch contains oxygen-containing functional groups, and the oxygen-containing functional groups include carboxyl groups.

[0040] In the present invention, the oxygen-containing functional groups and starch molecules are more fully cross-linked with each other, thereby hindering the growth of graphite crystallites during the subsequent high-temperature carbonization process, inhibiting graphitization, generating more disordered carbon structures, and increasing sodium storage sites, which is beneficial to the improvement of sodium storage capacity. In addition, the oxygen-containing functional groups can generate volatile components such as carbon monoxide and carbon dioxide. The growth of the carbon layer will cause the pores left by the overflow of gas molecules to be transformed from open pores to nano-closed pore structures during the high-temperature carbonization process, which is beneficial to the storage of sodium ions therein and hindering the entry of solvent molecules. This protects the active sites in the pores that can store sodium to a certain extent, avoids excessive capacity loss, thereby improving the initial efficiency and being suitable for low-molecular solvent electrolytes.

[0041] Preferably, the oxygen-containing functional groups in the oxidatively modified starch further include any one of carbonyl, hydroxyl or quinone groups, or a combination of at least two of them.

[0042] In this invention, carbonyl and carboxyl groups act as stable functional groups during low-temperature pretreatment, synergistically building a stable cross-linked structure and promoting disordering of the carbon structure. Furthermore, small gas molecules (such as CO2 and CO) released by carbonyl and carboxyl groups during high-temperature carbonization further alter the microstructure of the carbon material, thus playing a dual regulatory role.

[0043] Preferably, the mass content of oxygen-containing functional groups in the oxidized modified starch is 0.05-0.8%, for example, it can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%.

[0044] In the present invention, the mass content of oxygen-containing functional groups in the oxidized modified starch is used to characterize the oxidation degree of the oxidized modified starch. A mass content of 0.05-0.8% helps to fully inhibit graphitization, generate more disordered carbon structures, and increase sodium storage sites, thereby facilitating the improvement of sodium storage capacity and first efficiency, and is suitable for low-molecular solvent electrolytes. In addition, it can fully release part of the water and small molecules contained in the starch, play a stabilizing role, and is conducive to commercial production.

[0045] Preferably, the oxidant includes any one of hydrogen peroxide, sodium hypochlorite, potassium permanganate, chlorate or ammonium persulfate, or a combination of at least two thereof. For example, the chlorate may be sodium chlorate or potassium chlorate.

[0046] Preferably, the starch comprises any one of corn starch, wheat starch, rice starch, tapioca starch or sweet potato starch, or a combination of at least two thereof.

[0047] Preferably, the specific steps of mixing include:

[0048] The starch and the solvent are mixed to obtain a starch emulsion solution, and then a pH adjuster is added to adjust the pH, followed by the addition of an oxidizing agent.

[0049] In the present invention, liquid-phase oxidation modification of starch can increase the degree of oxidation of starch and make the oxidation uniform.

[0050] Preferably, the solvent includes water, for example, distilled water.

[0051] Preferably, the mass concentration of the starch milk solution is 20-50%, for example, it can be 20%, 30%, 40% or 50%.

[0052] Preferably, the pH adjuster comprises sodium hydroxide solution.

[0053] In the present invention, the purposes of adjusting pH are: 1) affecting the oxidation rate: regardless of the type of oxidant, its oxidation rate is related to the pH value of the reaction medium; 2) affecting the degree of oxidation: as the pH value changes from low to high, the content of oxygen-containing functional groups gradually increases, affecting the degree of oxidation of starch.

[0054] Preferably, the mass volume ratio concentration of the oxidant is 1-20 g / L, for example, 1 g / L, 5 g / L, 10 g / L, 15 g / L or 20 g / L.

[0055] Preferably, the mass ratio of the starch to the oxidant is 1:(0.002-0.04), for example, it can be 1:0.002, 1:0.005, 1:0.01, 1:0.02, 1:0.03 or 1:0.04, etc.

[0056] In the present invention, a suitable mass ratio of starch and oxidant helps to controllably adjust the degree of oxidation, while too little or too much will affect the material properties.

[0057] Preferably, the temperature of the oxidation reaction is 30-50°C, for example, 30°C, 40°C or 50°C.

[0058] Preferably, the temperature of the low-temperature pretreatment is 150-350°C, for example, 150°C, 200°C, 250°C, 300°C or 350°C.

[0059] Preferably, the low-temperature pretreatment time is 6-48 hours, for example, it can be 6 hours, 12 hours, 18 hours, 24 hours, 30 hours, 36 hours, 42 hours or 48 hours.

[0060] In the present invention, the time cost of low-temperature pretreatment is greatly reduced, which is of great significance.

[0061] Preferably, the temperature of the high-temperature carbonization treatment is 1300-1600°C, for example, 1300°C, 1400°C, 1500°C or 1600°C.

[0062] Preferably, the high-temperature carbonization treatment time is 4-10 hours, for example, 4 hours, 6 hours, 8 hours or 10 hours.

[0063] Preferably, the preparation method comprises the following steps:

[0064] (1) Starch and a solvent are mixed to obtain a starch emulsion solution with a mass concentration of 20-50%, and then a pH adjuster is added to adjust the pH value to 6-9 (for example, 6, 7, 8 or 9, etc.), followed by adding an oxidant with a mass volume concentration of 1-20 g / L, and performing an oxidation reaction under stirring to obtain oxidized modified starch.

[0065] The oxidized modified starch has a viscosity of 6-14 mPa·s, a crystallinity of 8-20%, and a moisture content of ≤11% (for example, 11%, 10%, 8%, 6%, or 4%); the oxidized modified starch contains oxygen-containing functional groups, including carboxyl groups, and the mass content of oxygen-containing functional groups in the oxidized modified starch is 0.05-0.8%; the mass ratio of starch to oxidant is 1:(0.002-0.04); and the oxidation reaction temperature is 30-50° C., and the time is 0.5-3 hours.

[0066] (2) In an air atmosphere, the temperature is raised to 150-350°C at a heating rate of 1-5°C / min (for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min or 5°C / min, etc.), and the oxidized modified starch is subjected to a low-temperature pretreatment for 6-48 hours. Then, in a protective atmosphere (for example, a nitrogen atmosphere), the temperature is raised to 1300-1600°C at a heating rate of 1-10°C / min (for example, 1°C / min, 3°C / min, 5°C / min, 7°C / min or 9°C / min, etc.) and subjected to a high-temperature carbonization treatment for 4-10 hours. After the treatment, the temperature is lowered, and the starch-based hard carbon negative electrode material is crushed and sieved to obtain the starch-based hard carbon negative electrode material.

[0067] In a third aspect, the present invention provides a sodium ion battery, wherein the negative electrode plate of the sodium ion battery includes the starch-based hard carbon negative electrode material as described in the first aspect.

[0068] The numerical range described in the present invention includes not only the point values listed above, but also any point values between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values included in the range.

[0069] Compared with the prior art, the present invention has the following beneficial effects:

[0070] (1) The single crystal particles in the starch-based hard carbon negative electrode material provided by the present invention have regular morphology and angular morphology with specific angles, which can avoid the phenomenon of particle agglomeration and fusion. In addition, the physical and chemical properties of the starch-based hard carbon negative electrode material are significantly improved, the particle size distribution is uniform, and the compaction is improved. At the same time, the sodium storage sites are also increased, which is beneficial to the improvement of sodium storage capacity and first efficiency.

[0071] (2) In the preparation method provided by the present invention, starch is oxidatively modified, which solves the problem of difficulty in starch processing. The surface of the oxidatively modified starch is grafted with oxygen-containing functional groups, and the oxygen-containing functional groups and starch molecules are fully cross-linked with each other, which is beneficial to inhibiting the growth of graphite crystallites in the subsequent high-temperature carbonization process, inhibiting graphitization, generating more disordered carbon structures, and increasing sodium storage sites, thereby facilitating the improvement of sodium storage capacity and first efficiency, and being suitable for low-molecular solvent electrolytes.

[0072] (3) In the preparation method provided by the present invention, the starch is oxidatively modified, which releases part of the water and small molecules contained in the starch, plays a stabilizing role, greatly reduces the time cost of low-temperature pretreatment, shortens the production cycle, and is conducive to commercial production.

[0073] (4) The starch-based hard carbon negative electrode material provided by the present invention has a reversible capacity of up to 335 mAh / g and a first-cycle coulombic efficiency of up to 91.2%. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 This is an SEM image of the starch-based hard carbon negative electrode material provided in Example 7 of the present invention at a magnification of 1K.

[0075] Figure 2 This is an SEM image of the starch-based hard carbon negative electrode material provided in Example 7 of the present invention at a magnification of 5K.

[0076] Figure 3 This is an SEM image of the starch-based hard carbon negative electrode material provided in Example 7 of the present invention at a magnification of 2K.

[0077] Figure 4 This is the charge and discharge curve of the starch-based hard carbon negative electrode material provided in Example 7 of the present invention.

[0078] Figure 5 Particle size distribution curve of the starch-based hard carbon negative electrode material provided in Example 7 of the present invention.

[0079] Figure 6 This is an SEM image of the starch-based hard carbon negative electrode material provided in Comparative Example 2 of the present invention at a magnification of 2K.

[0080] Figure 7 This is the charge and discharge curve of the starch-based hard carbon negative electrode material provided in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0081] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0082] Example 1

[0083] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 110°.

[0084] The single-crystal particles had a particle size (D50) of 10 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 99%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.39 nm.

[0085] The BET of the starch-based hard carbon negative electrode material is 2.3m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.94g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.068 cm 3 / g.

[0086] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0087] (1) Wheat starch and distilled water were mixed to obtain a starch emulsion solution with a mass concentration of 30%, and then a 3 wt % sodium hydroxide solution was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3 g / L was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0088] The viscosity of the oxidized modified starch is 6 mPa·s, the crystallinity is 10%, and the moisture content is 5%; the oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.52%; the mass ratio of the wheat starch to sodium hypochlorite is 1:0.02; the temperature of the oxidation reaction is 35° C., and the time is 3 hours.

[0089] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1300 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0090] Example 2

[0091] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 100°.

[0092] The single-crystal particles had a particle size (D50) of 12 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 93%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.393 nm.

[0093] The BET of the starch-based hard carbon negative electrode material is 3.2m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.95g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.062 cm 3 / g.

[0094] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0095] (1) Cassava starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0096] The viscosity of the oxidized modified starch is 10 mPa·s, the crystallinity is 12%, and the moisture content is 8%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.37%. The mass ratio of the cassava starch to sodium hypochlorite is 1:0.01. The temperature of the oxidation reaction is 35° C., and the time is 3 hours.

[0097] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1300 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0098] Example 3

[0099] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 110°.

[0100] The single-crystal particles had a particle size (D50) of 12 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 95%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.392 nm.

[0101] The BET of the starch-based hard carbon negative electrode material is 3.3m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.95g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.07 cm 3 / g.

[0102] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0103] (1) Corn starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0104] The viscosity of the oxidized modified starch is 7 mPa·s, the crystallinity is 11%, and the moisture content is 8%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.49%. The mass ratio of the corn starch to sodium hypochlorite is 1:0.03. The temperature of the oxidation reaction is 35° C., and the time is 3 hours.

[0105] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1300 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0106] Example 4

[0107] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 90°.

[0108] The single-crystal particles had a particle size (D50) of 10 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 96%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.398 nm.

[0109] The BET of the starch-based hard carbon negative electrode material is 2.0 m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.96g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.0935 cm 3 / g.

[0110] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0111] (1) Corn starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0112] The viscosity of the oxidized modified starch is 11 mPa·s, the crystallinity is 15%, and the moisture content is 2%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.38%. The mass ratio of the corn starch to sodium hypochlorite is 1:0.04. The temperature of the oxidation reaction is 35° C., and the time is 2 hours.

[0113] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1300 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0114] Example 5

[0115] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 130°.

[0116] The single-crystal particles had a particle size (D50) of 13 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 98%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.389 nm.

[0117] The BET of the starch-based hard carbon negative electrode material is 2.3m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.93g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.0712 cm 3 / g.

[0118] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0119] (1) Corn starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0120] The viscosity of the oxidized modified starch is 14 mPa·s, the crystallinity is 18%, and the moisture content is 5%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.21%. The mass ratio of the corn starch to sodium hypochlorite is 1:0.04. The temperature of the oxidation reaction is 35° C., and the time is 1 hour.

[0121] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1300 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0122] Example 6

[0123] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 120°.

[0124] The single-crystal particles had a particle size (D50) of 9 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 90%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.387 nm.

[0125] The BET of the starch-based hard carbon negative electrode material is 0.8m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.97g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.1304 cm 3 / g, the pore diameter of closed pores is less than 0.4nm.

[0126] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0127] (1) Corn starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0128] The viscosity of the oxidized modified starch is 11 mPa·s, the crystallinity is 15%, and the moisture content is 5%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.38%. The mass ratio of the corn starch to sodium hypochlorite is 1:0.02. The temperature of the oxidation reaction is 35° C., and the time is 2 hours.

[0129] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1400 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0130] Example 7

[0131] This embodiment provides a starch-based hard carbon negative electrode material, wherein the starch-based hard carbon negative electrode material includes single crystal particles. The single crystal particles have an angular morphology with an edge angle of 120°.

[0132] The single-crystal particles had a particle size (D50) of 8 μm, with 80% of the single-crystal particles distributed within the range of 3-30 μm. The single-crystal dispersion of the single-crystal particles was 97%. The starch-based hard carbon anode material had a graphite-like microcrystalline region with an average carbon interlayer spacing of 0.381 nm.

[0133] The BET of the starch-based hard carbon negative electrode material is 0.4m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.99g / cm 3 The starch-based hard carbon negative electrode material has closed pores with a closed pore volume of 0.181 cm 3 / g, the pore diameter of closed pores is less than 0.4nm.

[0134] This embodiment also provides a method for preparing the starch-based hard carbon negative electrode material, the preparation method comprising the following steps:

[0135] (1) Corn starch and distilled water were mixed to obtain a starch emulsion solution with a concentration of 30%, and then a sodium hydroxide solution with a concentration of 3 wt% was added to adjust the pH value to 9. Subsequently, sodium hypochlorite with a mass volume ratio of 3% was added and an oxidation reaction was carried out under stirring to obtain oxidized modified starch, which was then centrifuged and washed.

[0136] The viscosity of the oxidized modified starch is 11 mPa·s, the crystallinity is 15%, and the moisture content is 4%. The oxidized modified starch contains carboxyl groups, and the mass content of the carboxyl groups in the oxidized modified starch is 0.38%. The mass ratio of the corn starch to sodium hypochlorite is 1:0.035. The temperature of the oxidation reaction is 35° C., and the time is 2 hours.

[0137] (2) The oxidized modified starch was pretreated at low temperature for 10 h in a medium-temperature furnace in an air atmosphere at a heating rate of 3 °C / min to 200 °C, and then cooled to below 50 °C. A sample was taken to obtain a low-temperature pretreated material, which was then transferred to a high-temperature furnace in a nitrogen atmosphere and heated to 1500 °C at a heating rate of 2 °C / min for a high-temperature carbonization treatment for 4 h. After the treatment, the sample was cooled to below 50 °C and then pulverized and sieved to obtain the starch-based hard carbon negative electrode material.

[0138] Figure 1 、 Figure 2 and Figure 3 The SEM images of the starch-based hard carbon negative electrode material provided in this embodiment at different magnifications are shown respectively. It can be seen from the figure that the starch-based hard carbon negative electrode material has no obvious particle agglomeration, high particle dispersion, and single crystal particles have an angular morphology with an edge angle of 120° and a D50 of 8 μm.

[0139] Figure 4The charge and discharge curves of the starch-based hard carbon negative electrode material provided in this embodiment are shown. As can be seen from the figure, at a current density of 0.1C, the reversible specific capacity of the starch-based hard carbon negative electrode material can reach 335.8mAh / g, and the first-cycle coulombic efficiency is 91.2%, which shows a significant performance improvement.

[0140] Figure 5 The particle size distribution curve of the starch-based hard carbon negative electrode material provided in this embodiment is shown. As can be seen from the figure, the particle size distribution in the starch-based hard carbon negative electrode material is uniform, and there are no agglomerated large particles, which helps to improve compaction.

[0141] Example 8

[0142] The difference between this embodiment and embodiment 1 is that the sodium hypochlorite in step (1) is replaced by a combination of hydrogen peroxide and ammonium persulfate, so that the oxidized modified starch contains carboxyl and carbonyl groups, and the mass content of carboxyl and carbonyl groups is 0.45%.

[0143] The rest of the preparation methods and parameters remained the same as in Example 1.

[0144] Example 9

[0145] The difference between this embodiment and embodiment 1 is that the temperature of the high-temperature carbonization treatment in step (2) is reduced so that the pore diameter of the closed pores in the starch-based hard carbon negative electrode material is greater than 0.4 nm.

[0146] The rest of the preparation methods and parameters remained the same as in Example 1.

[0147] Example 10

[0148] The difference between this embodiment and embodiment 1 is that the mass ratio of the wheat starch and sodium hypochlorite in step (1) is adjusted so that the mass content of the carboxyl group in the oxidized modified starch is 0.02%.

[0149] The rest of the preparation methods and parameters remained the same as in Example 1.

[0150] Example 11

[0151] The difference between this embodiment and embodiment 1 is that the mass ratio of the wheat starch and sodium hypochlorite in step (1) is adjusted so that the mass content of the carboxyl group in the oxidized modified starch is 1%.

[0152] The rest of the preparation methods and parameters remained the same as in Example 1.

[0153] Example 12

[0154] The difference between this embodiment and embodiment 1 is that no sodium hydroxide solution is added for pH adjustment in step (1).

[0155] The rest of the preparation methods and parameters remained the same as in Example 1.

[0156] Example 13

[0157] The difference between this embodiment and embodiment 1 is that the temperature of the oxidation reaction in step (1) is 25°C.

[0158] The rest of the preparation methods and parameters remained the same as in Example 1.

[0159] Example 14

[0160] The difference between this embodiment and embodiment 1 is that the temperature of the oxidation reaction in step (1) is 60°C.

[0161] The rest of the preparation methods and parameters remained the same as in Example 1.

[0162] Comparative Example 1

[0163] The difference between this comparative example and Example 1 is that step (1) is not performed, and the time of low-temperature pretreatment in step (2) is 72 hours.

[0164] The starch-based hard carbon negative electrode material provided in this comparative example does not contain single crystal particles.

[0165] The BET of the starch-based hard carbon negative electrode material is 8.9m 2 / g, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.86g / cm 3 The closed pore volume of the starch-based hard carbon negative electrode material is small, which is 0.021cm 3 / g, and the pore diameter of the closed pore is 0.5nm.

[0166] The rest of the preparation methods and parameters remained the same as in Example 1.

[0167] Comparative Example 2

[0168] The difference between this comparative example and Example 7 is that step (1) is not performed, and the time of low-temperature pretreatment in step (2) is 72 hours.

[0169] The starch-based hard carbon negative electrode material provided in this comparative example has serious particle agglomeration and fusion, a very low proportion of single crystal particles, and a particle size D50 of 16.8 μm.

[0170] The BET of the starch-based hard carbon negative electrode material is 2.89m 2 / g, and the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is 0.88 / cm 3 The closed pore volume of the starch-based hard carbon negative electrode material is small, which is 0.043 cm 3 / g, and the pore diameter of the closed pore is 0.43nm.

[0171] The rest of the preparation methods and parameters remained the same as in Example 7.

[0172] Figure 6 The SEM image of the starch-based hard carbon negative electrode material provided in this comparative example at 2K magnification is shown. It can be seen from the image that the particles in the starch-based hard carbon negative electrode material are severely agglomerated.

[0173] Figure 7 The charge and discharge curve of the starch-based hard carbon negative electrode material provided in this comparative example is shown. It can be seen from the figure that at a current density of 0.1C, the reversible capacity of the starch-based hard carbon negative electrode material is only 296mAh / g, and the first-cycle coulombic efficiency is 87.5%.

[0174] Performance Testing

[0175] The starch-based hard carbon negative electrode materials provided in the above examples and comparative examples are prepared into sodium ion batteries, and the specific steps include:

[0176] The above-mentioned starch-based hard carbon negative electrode material is combined with conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone, mixed and ground in a mass ratio of 8:1:1, and then coated on copper foil. After drying, roller pressing and punching, the negative electrode sheet is obtained; in an argon glove box, a sodium sheet is used as the counter electrode, a glass fiber membrane is used as the diaphragm, and a 1.5 mol / L electrolyte (the solvent is NaPF6, and the solvent includes ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 1:2:2) is assembled into a standard CR2032 button battery.

[0177] The reversible specific capacity and first efficiency tests of the above sodium ion battery were carried out under the following test conditions: test voltage range 0-2V, test at 0.1C current density.

[0178] The test results are shown in Table 1.

[0179] Table 1

[0180]

[0181] analyze:

[0182] It can be seen from the above table that the measured reversible capacity of the starch-based hard carbon negative electrode materials prepared in Comparative Examples 1 and 2 is difficult to exceed 300 mAh / g, the particles are severely agglomerated, the particle size is too large, and the powder compaction is also low. There are problems such as difficulty in improving the capacity, low initial efficiency, and substandard physical and chemical properties. Examples 1-7 that have undergone an oxidation process not only reduce the time cost of low-temperature pretreatment, but also effectively improve the sodium storage capacity, initial efficiency, and physical and chemical properties.

[0183] From the comparative analysis of Examples 1, 3-5 and 10-11, it can be seen that the higher the degree of oxidation, the better the battery performance. As the oxidation time increases, the carboxyl content does increase, which promotes sufficient cross-linking between carboxyl groups and starch molecules, helps to increase sodium storage capacity and promote the formation of closed-pore structure. However, when the degree of oxidation is too high, the situation becomes complicated. On the one hand, the degree of breakage of the starch molecular chain is aggravated, resulting in a decrease in molecular weight; on the other hand, the oxidant can penetrate into the crystalline area of the starch and react. This process destroys the originally dense crystalline structure, making it loose, thereby reducing the relative crystallinity. This structural change, in turn, reduces the degree of cross-linking between molecular chains, which has an adverse effect on battery performance. Therefore, in order to maximize battery performance, the degree of oxidation must be precisely controlled during the oxidative modification process of starch.

[0184] It can be seen from Examples 1 and 8 that if carboxyl groups and carbonyl groups are grafted onto starch for modification, it is beneficial to construct a stable cross-linked structure and promote the disordering of the carbon structure.

[0185] It can be seen from Examples 1 and 9 that if the pore diameter of the closed pores in the starch-based hard carbon negative electrode material is greater than 0.4 nm, it is not conducive to hindering the entry of solvent molecules, resulting in excessive capacity loss, thereby reducing the first efficiency and being unsuitable for low-molecular solvent electrolytes.

[0186] It can be seen from Examples 1 and 12 that if sodium hydroxide solution is not added in step (1) for pH adjustment, it is not conducive to the oxidation reaction and affects the oxidation efficiency.

[0187] It can be seen from Examples 1 and 13-14 that if the temperature of the oxidation reaction is too low, the relative movement between molecules in the reaction system is too slow, and the probability of starch molecules contacting and reacting with the oxidant is low, so less carboxyl groups are generated and the oxidation degree is too low; if the temperature of the oxidation reaction is too high, part of the oxidized starch will degrade and dissolve in water, and will be washed away during washing after the reaction is completed, which will also lead to a decrease in the carboxyl content.

[0188] The applicant states that while the above-described embodiments illustrate the process of the present invention, the present invention is not limited to the above-described process steps, nor does it imply that the present invention must rely on the above-described process steps for implementation. Those skilled in the art will appreciate that any improvements to the present invention, equivalent substitutions for the raw materials used, additions of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A starch-based hard carbon negative electrode material, characterized in that: The starch-based hard carbon negative electrode material includes single crystal particles, and the single crystal particles include polygonal particles with an angular morphology, with an edge angle of 90°-130°; The starch-based hard carbon negative electrode material has a graphite-like microcrystalline region, and the average carbon layer spacing in the graphite-like microcrystalline region is 0.38-0.4 nm; The starch-based hard carbon negative electrode material has closed pores with a volume of 0.06-0.2 cm 3 / g, the pore diameter of closed pores is <0.4nm; The single crystal dispersion rate of the single crystal particles is ≥90%.

2. The starch-based hard carbon negative electrode material according to claim 1, characterized in that: The particle size D50 of the single crystal particles is 5-15 μm, and 80% of the single crystal particles are distributed in the range of 3-30 μm.

3. The starch-based hard carbon negative electrode material according to claim 1, characterized in that: The BET of the starch-based hard carbon negative electrode material is 0.1-5m 2 / g; And / or, the compaction density of the starch-based hard carbon negative electrode material under a pressure of 1t is greater than 0.9g / cm 3 .

4. A method for preparing a starch-based hard carbon negative electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: mixing an oxidant and starch to carry out an oxidation reaction to obtain oxidized modified starch; The oxidized modified starch is subjected to a low-temperature pretreatment and a high-temperature carbonization treatment in sequence to obtain the starch-based hard carbon negative electrode material; The temperature of the low-temperature pretreatment is 150-350°C.

5. The preparation method according to claim 4, characterized in that The viscosity of the oxidized modified starch is 6-14 mPa·s; And / or, the crystallinity of the oxidized modified starch is 8-20%.

6. The preparation method according to claim 4, characterized in that The oxidatively modified starch contains oxygen-containing functional groups, and the oxygen-containing functional groups include carboxyl groups; And / or, the oxygen-containing functional groups in the oxidatively modified starch further include any one or a combination of at least two of carbonyl, hydroxyl or quinone groups; And / or, the mass content of oxygen-containing functional groups in the oxidized modified starch is 0.05-0.8%.

7. The preparation method according to claim 4, characterized in that The oxidant includes any one of hydrogen peroxide, sodium hypochlorite, potassium permanganate, chlorate or ammonium persulfate, or a combination of at least two thereof; And / or, the starch comprises any one or a combination of at least two of corn starch, wheat starch, rice starch, tapioca starch or sweet potato starch; And / or, the specific steps of mixing include: Mixing starch and a solvent to obtain a starch emulsion solution, then adding a pH regulator to adjust the pH, and then adding an oxidant; And / or, the mass volume ratio concentration of the oxidant is 1-20 g / L; and / or, the mass ratio of the starch to the oxidant is 1:(0.002-0.04); and / or, the temperature of the oxidation reaction is 30-50° C.; And / or, the low temperature pretreatment time is 6-48h; And / or, the temperature of the high-temperature carbonization treatment is 1300-1600°C; And / or, the high-temperature carbonization treatment time is 4-10 hours.

8. The preparation method according to claim 4, characterized in that The preparation method comprises the following steps: (1) Starch and a solvent are mixed to obtain a starch emulsion solution with a mass concentration of 20-50%, and then a pH adjuster is added to adjust the pH value to 6-9, followed by adding an oxidant with a mass volume ratio concentration of 1-20 g / L, and an oxidation reaction is carried out under stirring to obtain oxidized modified starch; The oxidized modified starch has a viscosity of 6-14 mPa·s, a crystallinity of 8-20%, and a moisture content of ≤11%. The oxidized modified starch contains oxygen-containing functional groups, including carboxyl groups, and the mass content of oxygen-containing functional groups in the oxidized modified starch is 0.05-0.8%. The mass ratio of the starch to the oxidant is 1:(0.002-0.04). The oxidation reaction temperature is 30-50° C., and the reaction time is 0.5-3 hours. (2) The oxidized modified starch is heated to 150-350°C at a heating rate of 1-5°C / min in an air atmosphere, and the oxidized modified starch is subjected to a low-temperature pretreatment for 6-48 hours. The oxidized modified starch is then heated to 1300-1600°C at a heating rate of 1-10°C / min in a protective atmosphere for a high-temperature carbonization treatment for 4-10 hours. After the treatment, the oxidized modified starch is cooled, crushed, and sieved to obtain the starch-based hard carbon negative electrode material.

9. A sodium ion battery, characterized in that: The negative electrode sheet of the sodium ion battery includes the starch-based hard carbon negative electrode material according to any one of claims 1 to 3.

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