Starch-based hard carbon as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to starch-based hard carbon as well as a preparation method and application thereof. The starch-based hard carbon comprises a core-shell structure composed of a core and a shell layer, the core comprises hard carbon; the shell layer comprises a nitrogen-doped nano carbon material. Due to the composition and the structure, the starch-based hard carbon disclosed by the invention is good in conductivity, high in capacity and high in cycle performance.

Description

Technical Field

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

[0002] Research on the application of large-capacity lithium-ion batteries is increasing rapidly, and they are seen as the primary choice for large-scale energy storage batteries in future applications such as electric vehicles and energy storage power stations. However, limited lithium reserves and high material costs pose significant obstacles to their widespread application. Developing advanced battery systems with abundant resources and low costs is the inevitable solution to future large-scale energy storage applications. Sodium and lithium are in the same main group, with similar chemical properties and electrode potentials. Furthermore, sodium is abundant and its extraction cost is lower than that of lithium. If sodium were used to replace lithium and a high-performance sodium-ion battery were developed, it would have a significant competitive advantage over lithium-ion batteries.

[0003] Therefore, the search for sodium electrode materials with high capacity and excellent cycle performance has become a research hotspot in the current battery field.

[0004] Hard carbon, as a commonly used negative electrode material for sodium-ion batteries, directly affects the battery's capacity and cycle performance.

[0005] Therefore, it is urgent for those skilled in the art to obtain hard carbon materials with high capacity and good cycling performance. Summary of the Invention

[0006] The present invention aims to address at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a starch-based hard carbon, a preparation method thereof, and applications thereof. The starch-based hard carbon of the present invention has high capacity performance and good cycle performance.

[0007] A first aspect of the present invention provides a starch-based hard carbon.

[0008] Specifically, a starch-based hard carbon comprises a core-shell structure consisting of a core and a shell;

[0009] The core comprises hard carbon;

[0010] The shell layer includes nitrogen-doped nanocarbon material.

[0011] A second aspect of the present invention provides a method for preparing starch-based hard carbon, comprising the following steps:

[0012] Mixing starch with an esterifying agent to react to obtain esterified starch;

[0013] The esterified starch is mixed with the melamine solution to form a suspension, and then the cyanuric acid solution is added to the suspension, mixed, and solid-liquid separation is performed to obtain a solid to obtain a mixture;

[0014] The mixture is subjected to pre-carbonization and carbonization treatment under a protective gas atmosphere to obtain the starch-based hard carbon.

[0015] A third aspect of the present invention provides an application of starch-based hard carbon, specifically a sodium ion battery comprising the starch-based hard carbon.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The starch-based hard carbon of the present invention comprises a core-shell structure consisting of a core and a shell; the core comprises hard carbon; the shell comprises nitrogen-doped nanocarbon material. Such a composition and structure enable the starch-based hard carbon of the present invention to have good electrical conductivity, high capacity and cycle performance.

[0018] (2) The preparation method of the present invention is to mix the esterified starch with the melamine solution and then add the melamine solution in this order so that the melamine is first sufficiently grafted on the surface of the esterified starch, and then utilize the path of forming numerous hydrogen bonds between the melamine and cyanuric acid molecules to form supramolecules coated with the esterified starch. Further, through pre-carbonization and carbonization treatment, the supramolecules form nanocarbon materials, thereby improving the conductivity of the starch-based hard carbon. At the same time, nitrogen elements are introduced on the surface to achieve surface nitrogen doping, thereby improving the conductivity and capacity of the starch-based hard carbon, as well as the cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the charge and discharge curve of the button battery assembled with starch-based hard carbon in Example 1. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application and are not used to limit the present application.

[0021] In the description of this application, the term "including" means "including but not limited to". Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present invention; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0022] In this application, "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural.

[0023] In the present application, “at least one”, “at least one” and other similar descriptions specifically refer to one or more, one or more; “multiple”, “multiple” and other similar descriptions specifically refer to two or more, two or more. “At least one”, “at least one of the following” or similar expressions refer to any combination of these items, including any combination of single items or plural items. For example, “at least one of a, b, or c”, or “at least one of a, b, and c” can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0024] A first aspect of the present invention provides a starch-based hard carbon.

[0025] Specifically, a starch-based hard carbon comprises a core-shell structure consisting of a core and a shell;

[0026] The core comprises hard carbon;

[0027] The shell layer includes nitrogen-doped nanocarbon material.

[0028] The starch-based hard carbon with a core-shell structure formed by nitrogen-doped nanocarbon materials coating hard carbon provides the starch-based hard carbon with good conductivity, high capacity and cycle performance.

[0029] In some embodiments of the present invention, the nitrogen-doped nanocarbon material comprises nitrogen-doped carbon nanofibers, which provide starch-based hard carbon with good electrical conductivity, high capacity and cycling performance.

[0030] A second aspect of the present invention provides a method for preparing starch-based hard carbon, comprising the following steps:

[0031] Mixing starch with an esterifying agent to react to obtain esterified starch;

[0032] The esterified starch is mixed with the melamine solution to form a suspension, and then the cyanuric acid solution is added to the suspension, mixed, and solid-liquid separation is performed to obtain a solid to obtain a mixture;

[0033] The mixture is subjected to pre-carbonization and carbonization treatment under a protective gas atmosphere to obtain the starch-based hard carbon.

[0034] Specifically, the present invention provides a method for preparing starch-based hard carbon, comprising the following steps:

[0035] Step S11: mixing starch with an esterifying agent to react to obtain esterified starch;

[0036] Step S12: mixing the esterified starch with the melamine solution to form a suspension, then adding the cyanuric acid solution to the suspension, mixing, separating the solid and the liquid, and collecting the solid to obtain a mixture;

[0037] Step S13: Under a protective gas atmosphere, the mixture is subjected to pre-carbonization and carbonization treatment to obtain the starch-based hard carbon.

[0038] The preparation method of the present invention comprises the following steps: mixing esterified starch with a melamine solution and then adding the melamine solution in this order, so that the melamine is first sufficiently grafted onto the surface of the esterified starch; then, numerous hydrogen bonds are formed between melamine and cyanuric acid molecules to form supramolecules coating the esterified starch; and further, through pre-carbonization and carbonization treatment, the supramolecules form nanocarbon materials, thereby improving the conductivity of the starch-based hard carbon. At the same time, nitrogen is introduced onto the surface to achieve surface nitrogen doping, thereby improving the conductivity, capacity, and cycle performance of the starch-based hard carbon.

[0039] In step S11:

[0040] In some embodiments of the present invention, the starch is conventional starch, such as corn starch, wheat starch, tapioca starch, soybean starch and other conventional starches.

[0041] In some embodiments of the present invention, the D 50 Less than or equal to 10 μm. For example, less than or equal to 6 μm, specifically 1 μm, 5 μm, 6 μm, 9 μm. Controlling the particle size of starch helps to obtain uniformly esterified starch.

[0042] In some embodiments, the esterifying agent includes at least one of an organic acid and an organic anhydride, wherein the organic acid includes at least one of tartaric acid, oxalic acid, malic acid, citric acid, and ascorbic acid; and the organic anhydride includes at least one of maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, and octenylsuccinic anhydride.

[0043] In some embodiments, the mass ratio of the esterifying agent to the starch is (10-20):100, further (12-18):100. Specifically, for example, it is 10:100, 15:100, or 20:100.

[0044] In some embodiments, the temperature for the mixed reaction of the starch and the esterification agent is 85-100°C, further 90-100°C.

[0045] In some embodiments, the starch and the esterification agent are mixed and reacted for 2-6 hours, or further for 4-5 hours.

[0046] It can be understood that the mixing method may include known mixing methods such as stirring or ultrasound.

[0047] It is understandable that the mixing reaction of starch and esterification agent can be carried out in air atmosphere or in a protective atmosphere such as nitrogen, argon, etc.

[0048] In step S12:

[0049] In some embodiments of the present invention, the melamine solution is obtained by dissolving melamine in water at 50-90°C. Furthermore, the water is at a temperature of 80-85°C. Water at this temperature facilitates the dissolution of melamine. The water may be deionized water.

[0050] In some embodiments of the present invention, in the melamine solution, the mass volume ratio of melamine to water is 1 g: (150-250) mL, for example, 1 g: 150 mL, 1 g: 200 mL, or 1 g: 250 mL.

[0051] In some embodiments of the present invention, the mass ratio of melamine to esterified starch in the melamine solution is (0.2-0.6):1, further (0.3-0.5):1, for example, 0.3:1, 0.4:1, or 0.5:1.

[0052] In some embodiments of the present invention, the molar ratio of melamine in the melamine solution to cyanuric acid in the cyanuric acid solution is (0.83-1.2):1, further (0.9-1.1):1, for example, 0.83:1, 0.9:1, or 1.2:1.

[0053] In some embodiments of the present invention, the cyanuric acid solution is obtained by dissolving cyanuric acid in water at 50-90° C. Furthermore, the temperature of the water is 80-85° C. Water at this temperature facilitates the dissolution of cyanuric acid.

[0054] In some embodiments of the present invention, the mass volume ratio of cyanuric acid to water in the cyanuric acid solution is 1.04 g: (250-350) mL, for example, 1.04 g: 250 mL, 1.04 g: 300 mL, or 1.04 g: 350 mL.

[0055] In some embodiments of the present invention, the temperature of the mixture of the esterified starch and the melamine solution is 50-90° C., or further 60-90° C. This temperature facilitates sufficient grafting reaction of melamine on the surface of the esterified starch.

[0056] In some embodiments of the present invention, the esterified starch and the melamine solution are mixed for 1-4 hours, or more preferably 1-3 hours. Adequate mixing time facilitates sufficient grafting reaction of melamine on the surface of the esterified starch.

[0057] In some embodiments of the present invention, the temperature of the cyanuric acid solution when added to the suspension and mixed is 50-90° C., further 60-90° C. This temperature is conducive to the formation of supramolecules coated with esterified starch by cyanuric acid and melamine.

[0058] In some embodiments of the present invention, the cyanuric acid solution is added to the suspension and mixed for 2-5 hours, preferably 3-5 hours. Adequate mixing time helps cyanuric acid and melamine form supramolecules coated with esterified starch.

[0059] It can be understood that the mixing method may include known mixing methods such as stirring or ultrasound.

[0060] In some embodiments of the present invention, after the cyanuric acid solution is added to the suspension, the suspension is vigorously stirred for 2-3 hours, for example, 2 hours or 3 hours.

[0061] In some embodiments of the present invention, the solid-liquid separation includes centrifugation, washing, and drying operations. The solid-liquid separation process is a conventional separation operation in the art.

[0062] In some embodiments of the present invention, the melamine solution can be added to the esterified starch in the form of melamine and water, respectively, and then mixed. This mixing method can also result in melamine grafting onto the surface of the esterified starch, but the grafting effect may not be as good as mixing the melamine solution with the esterified starch. This is because melamine is evenly dispersed in a melamine solution, allowing it to be evenly grafted onto the surface of the esterified starch when mixed with the esterified starch. However, when melamine is mixed with water and esterified starch simultaneously, the melamine's dispersion uniformity is relatively poor, resulting in a weakened grafting effect.

[0063] In some embodiments of the present invention, the cyanuric acid solution can also be added to the suspension in the form of cyanuric acid and water, respectively, and mixed. This form of mixing can also allow cyanuric acid and melamine to form supramolecules that coat the esterified starch, but the coating effect may not be as good as mixing the suspension in the form of a cyanuric acid solution. This is because in the cyanuric acid solution, cyanuric acid is evenly dispersed. When mixed with the suspension, cyanuric acid can evenly form supramolecules that coat the esterified starch with melamine. However, when cyanuric acid, water, and the suspension are mixed simultaneously, the dispersion uniformity of cyanuric acid is relatively poor, resulting in a weakened coating effect.

[0064] The reaction equation for the mutual conversion and formation of supramolecules between melamine and cyanuric acid is shown below:

[0065]

[0066] Under the conditions of step S12 of the present invention, the rate of mutual conversion between melamine and cyanuric acid is very slow, while the reaction rate of forming supramolecules between melamine and cyanuric acid is significantly faster than the mutual conversion between melamine and cyanuric acid. This facilitates the formation of supramolecules coated with esterified starch by cyanuric acid and melamine.

[0067] In step S13:

[0068] The protective gas includes nitrogen or a rare gas. The rare gas is, for example, one of argon, helium, and krypton. It is understood that the present invention may refer to the description herein for any use of a protective gas.

[0069] In some embodiments of the present invention, the temperature of the pre-carbonization treatment is 450-900° C., for example, 650-750° C. The time of the pre-carbonization treatment can be 1-5 hours, for example, 2-3 hours.

[0070] In some embodiments of the present invention, after the pre-carbonization treatment is completed, the pre-carbonized material is crushed. For example, crushed to D 50 to 1-10μm, more specifically D 502μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm.

[0071] In some embodiments of the present invention, the carbonization temperature is 1200-1700° C., and the time is 0.5-4 hours. Specifically, the carbonization temperature can be 1200-1300° C., 1300-1500° C., 1500-1700° C., etc. The carbonization time can be specifically 0.5-1 hour, 1-2 hours, 2-3 hours, 3-4 hours, etc.

[0072] In some embodiments of the present invention, after step S13, i.e., after the carbonization treatment, a coating treatment is further performed. The coating treatment comprises mixing the carbonized material with coating asphalt, followed by coating under a protective gas atmosphere to obtain starch-based hard carbon. The coating treatment can reduce surface defects in the carbonized material, thereby improving the electrochemical performance of the starch-based hard carbon (e.g., increasing the specific charge capacity).

[0073] Specifically, the coating asphalt can be petroleum asphalt with a softening point of 220-280°C. The mass of the coating asphalt used is 5%-12% of the mass of the material after carbonization treatment. For example, 5%-10%. Specifically, for example, 5%, 10%, or 12%.

[0074] In some embodiments of the present invention, the coating treatment is performed at a temperature of 1200-1700° C. for 1-3 hours, or at a temperature of 1300-1500° C. for 0.5-1.5 hours.

[0075] In some embodiments of the present invention, the coating process further includes a crushing process, wherein the material after the carbonization process is completed and the coated asphalt is mixed to form a material to be crushed to D 50 2-8 μm, for example, 3-8 μm, more specifically 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, and 8 μm.

[0076] The present invention also provides a starch-based hard carbon, which is prepared by the preparation method of the starch-based hard carbon provided by the present invention.

[0077] A third aspect of the present invention provides an application of starch-based hard carbon, which can be specifically applied to sodium ion batteries, for example, as a negative electrode material of a sodium ion battery.

[0078] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods.

[0079] Example 1

[0080] A starch-based hard carbon comprising a core-shell structure consisting of a core and a shell;

[0081] The core comprises hard carbon;

[0082] The shell layer includes nitrogen-doped carbon nanofibers.

[0083] A method for preparing starch-based hard carbon comprises the following steps:

[0084] Step (1): corn starch (D 50 less than or equal to 10 μm) and maleic anhydride, the mass of maleic anhydride is 15% of the mass of corn starch, and the reaction is carried out at 100° C. for 4 hours to obtain esterified starch;

[0085] Step (2): 1 g of melamine was dissolved in 200 mL of deionized water under heating and stirring at 80° C. to obtain a melamine solution; 1.04 g of cyanuric acid was dissolved in 300 mL of deionized water under heating and stirring at 80° C. to obtain a cyanuric acid solution; 2 g of esterified starch was added to the melamine solution and stirred (stirring was performed at 80° C.) for 1 hour to form a uniformly dispersed suspension; then the cyanuric acid solution was added to the suspension, stirred at 80° C. for 3 hours, centrifuged, washed, and dried at 80° C. to obtain a mixture;

[0086] Step (3): adding the mixed material into a tube furnace, heating it to 650°C under the protection of inert gas (argon) for pre-carbonization treatment for 2 hours, and crushing the cooled block to obtain a pre-carbonized material;

[0087] Step (4): Place the pre-carbonized material into a tubular furnace, heat it to 1500° C. under the protection of an inert gas (argon) and perform carbonization treatment for 2 h to obtain starch-based hard carbon.

[0088] Example 2

[0089] Compared with Example 1, the difference of Example 2 is that after step (4), it further includes step (5). The specific content of step (5) is as follows:

[0090] The material obtained after the carbonization treatment in step (4) is mixed with the coating asphalt (the coating asphalt is petroleum asphalt with a softening point of 230°C, and the mass of the coating asphalt is 11% of the carbon material), and the powder machine is fully crushed and mixed to control the particle size D 50 The mixture is then placed in a tubular furnace, heated to 1300°C under the protection of inert gas (argon) and coated for 1.5 hours to obtain starch-based hard carbon.

[0091] Example 3

[0092] A method for preparing starch-based hard carbon comprises the following steps:

[0093] Step (1): Wheat starch (D 50 less than or equal to 10 μm) and citric acid are uniformly mixed, the mass of maleic anhydride is 20% of the mass of corn starch, and the reaction is carried out at 85° C. for 6 hours to obtain esterified starch;

[0094] Step (2): 0.4 g of melamine was dissolved in 150 mL of deionized water under heating and stirring at 80° C. to obtain a melamine solution; 0.416 g of cyanuric acid was dissolved in 250 mL of deionized water under heating and stirring at 80° C. to obtain a cyanuric acid solution; 2 g of esterified starch was added to the melamine solution and stirred (stirring was carried out at 80° C.) for 1 hour to form a uniformly dispersed suspension; then the cyanuric acid solution was added to the suspension, vigorously stirred (stirring was carried out at 80° C.) for 3 hours, centrifuged, washed, and dried at 80° C. to obtain a mixture;

[0095] Step (3): adding the mixed material into a tube furnace, heating it to 900°C under the protection of inert gas (argon) for pre-carbonization treatment for 2 hours, and crushing the cooled block to obtain a pre-carbonized material;

[0096] Step (4): Place the pre-carbonized material into a tubular furnace, heat it to 1200° C. under the protection of inert gas (argon) and perform carbonization treatment for 2 h to obtain starch-based hard carbon.

[0097] Comparative Example 1

[0098] Compared with Example 1, the difference of Comparative Example 1 is that step (2) is not performed, and the other processes are the same as those of Example 1.

[0099] Comparative Example 2

[0100] Compared with Example 1, the difference of Comparative Example 2 is step (2), and the other steps are the same as those of Example 1. The specific contents of step (2) of Comparative Example 2 are as follows:

[0101] Step (2): 1 g of melamine is dissolved in 200 mL of deionized water under heating and stirring at 80° C. to obtain a melamine solution; 1.04 g of cyanuric acid is dissolved in 300 mL of deionized water under heating and stirring at 80° C. to obtain a cyanuric acid solution; the melamine solution and the cyanuric acid solution are added together to 2 g of esterified starch and stirred (stirring is carried out at 80° C.) for 4 h, centrifuged, washed, and dried at 80° C. to obtain a mixture.

[0102] Comparative Example 3

[0103] Compared with Example 1, the difference of Comparative Example 3 is step (2), and the other steps are the same as those of Example 1. The specific contents of step (2) of Comparative Example 3 are as follows:

[0104] Step (2): 1 g of melamine is dissolved in 200 mL of deionized water under heating and stirring at 80° C. to obtain a melamine solution; 1.04 g of cyanuric acid is dissolved in 300 mL of deionized water under heating and stirring at 80° C. to obtain a cyanuric acid solution; 2 g of esterified starch is added to the cyanuric acid solution and vigorously stirred (stirring is carried out at 80° C.) for 3 h to form a uniformly dispersed mixture, and then the melamine solution is added to the mixture, stirred (stirring is carried out at 80° C.) for 1 hour, centrifuged, washed, and dried at 80° C. to obtain a mixture.

[0105] Product effect testing

[0106] The starch-based hard carbons prepared in Examples 1-3 and Comparative Examples 1-3 were respectively prepared into button batteries.

[0107] The specific steps for preparing the button battery are as follows: starch-based hard carbon is mixed with conductive carbon Super-P and PVDF (polyvinylidene fluoride) in a mass ratio of 92:3:3. Then, N-methylpyrrolidone (NMP) is used as the solvent. After mixing evenly, it is coated on aluminum foil and dried in a vacuum oven at 102°C. After roller pressing, it is cut into circular electrode pieces with a diameter of 8mm. A sodium sheet is used as the negative electrode. The electrolyte is a NaPF6 lipid electrolyte model NP-001. The separator is a glass fiber separator. The button battery is assembled in an argon-filled glove box.

[0108] The charge and discharge performance of the prepared button battery was tested. The battery test was carried out using a Xinwei battery test system. The test method adopted constant current and constant voltage discharge and constant current charging. The charge and discharge test was carried out at a current density of 0.1C. The discharge cut-off voltage was 0V and the charge cut-off voltage was 2.5V. The test results are shown in Table 1 (charge and discharge efficiency = charge specific capacity / discharge specific capacity*100%). The charge and discharge curves of the button battery assembled from starch-based hard carbon in Example 1 are shown in Table 1. Figure 1 .

[0109] Table 1

[0110] Charge specific capacity (mAh / g) Charge and discharge efficiency (%) Example 1 300.47 86.28 Example 2 306.01 88.07 Example 3 295.07 86.75 Comparative Example 1 270.66 84.22 Comparative Example 2 276.41 84.56 Comparative Example 3 275.44 84.39

[0111] As can be seen from Table 1, compared with comparative examples 1-3, the corresponding charge specific capacity and charge-discharge efficiency of Examples 1-3 of the present invention are improved to a certain extent. The charge specific capacity can reach 300 mAh / g, and the charge-discharge efficiency can reach up to 88.07%.

[0112] The above is a detailed introduction to the starch-based hard carbon provided in the examples of the present application, its preparation method and application. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above examples is only used to help understand the method of the present application and its core idea; at the same time, for technical personnel in this field, based on the ideas of the present application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A starch-based hard carbon, characterized in that A core-shell structure consisting of a core and a shell; The core comprises hard carbon; The shell layer includes nitrogen-doped nanocarbon material.

2. A method for preparing starch-based hard carbon according to claim 1, characterized in that: The following steps are involved: Mixing starch with an esterifying agent to react to obtain esterified starch; The esterified starch is mixed with the melamine solution to form a suspension, and then the cyanuric acid solution is added to the suspension, mixed, and solid-liquid separation is performed to obtain a solid to obtain a mixture; The mixture is subjected to pre-carbonization and carbonization treatment under a protective gas atmosphere to obtain the starch-based hard carbon.

3. The preparation method according to claim 2, characterized in that The starch comprises at least one of corn starch, wheat starch, tapioca starch and soybean starch; And / or, the starch D 50 Less than or equal to 10μm; And / or, the esterifying agent comprises at least one of an organic acid and an organic acid anhydride, wherein the organic acid comprises at least one of tartaric acid, oxalic acid, malic acid, citric acid, and ascorbic acid; and the organic acid anhydride comprises at least one of maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, and octenylsuccinic anhydride; And / or, the mass ratio of the esterifying agent to the starch is (10-20):100; and / or, the temperature of the mixed reaction of the starch and the esterifying agent is 85-100° C.; And / or, the starch and the esterifying agent are mixed and reacted for 2-6 hours.

4. The preparation method according to claim 2, characterized in that The melamine solution is obtained by dissolving melamine in water at 50-90°C; And / or, in the melamine solution, the mass volume ratio of melamine to water is 1 g: (150-250) mL; And / or, the mass ratio of melamine to esterified starch in the melamine solution is (0.2-0.6):1; and / or, the molar ratio of melamine in the melamine solution to cyanuric acid in the cyanuric acid solution is (0.83-1.2):1; And / or, the cyanuric acid solution is obtained by dissolving cyanuric acid in water at 50-90° C.; And / or, in the cyanuric acid solution, the mass volume ratio of cyanuric acid to water is 1.04 g: (250-350) mL.

5. The preparation method according to claim 2, characterized in that The temperature when the esterified starch and the melamine solution are mixed is 50-90°C; and / or, the esterified starch and the melamine solution are mixed for 1-4 hours; and / or, the temperature of the cyanuric acid solution when added to the suspension and mixed is 50-90° C.; and / or, the cyanuric acid solution is added to the suspension and mixed for 2-5 hours; and / or, adding the cyanuric acid solution to the suspension and stirring for 2-3 hours; And / or, the solid-liquid separation includes centrifugation, washing, and drying operations.

6. The preparation method according to claim 2, characterized in that The step of mixing the esterified starch with the melamine solution to form a suspension comprises: adding melamine and water to the esterified starch respectively and mixing them to form the suspension; And / or, the adding the cyanuric acid solution into the suspension and mixing includes: adding cyanuric acid and water into the suspension respectively and mixing them.

7. The preparation method according to claim 2, characterized in that The protective gas includes nitrogen or a rare gas, and the rare gas is one of argon, helium, and krypton; And / or, the pre-carbonization temperature is 450-900°C; And / or, the pre-carbonization time may be 1-5 hours; and / or, after the pre-carbonization treatment is completed, the obtained pre-carbonized material is crushed; And / or, the carbonization temperature is 1200-1700° C., and the time is 0.5-4 hours.

8. The preparation method according to claim 2, characterized in that After the carbonization treatment is completed, a coating treatment is also included. The coating treatment process includes: mixing the material after the carbonization treatment with coating asphalt, and then performing the coating treatment under a protective gas atmosphere to obtain the starch-based hard carbon.

9. The preparation method according to claim 8, characterized in that The coating asphalt is a petroleum asphalt with a softening point of 220-280°C; and / or, the mass of the coated asphalt is 5%-12% of the mass of the material after the carbonization treatment; And / or, the coating process also includes a crushing process before the coating process, wherein the material after the carbonization process is completed and the coated asphalt is mixed to form a material to be crushed to D 50 2-8μm; And / or, the coating treatment is carried out at a temperature of 1200-1700° C. and for a time of 1-3 hours.

10. A sodium ion battery, characterized in that: The invention comprises the starch-based hard carbon according to claim 1 or the starch-based hard carbon prepared by the preparation method of the starch-based hard carbon according to any one of claims 2 to 9.

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