A starch-based hard carbon material, a preparation method and application thereof

Abstract

The application provides a starch-based hard carbon material and a preparation method and application thereof, and the preparation method comprises the following steps: (1) performing two-stage programmed oxidation on enzymatic starch under air condition, and then performing plasma treatment to obtain an oxidation precursor; (2) performing activation on the oxidation precursor under an activation atmosphere, and then performing plasma treatment to obtain an activation precursor; and (3) performing high-temperature carbonization on the activation precursor in an inert atmosphere to obtain the starch-based hard carbon material. The obtained starch-based hard carbon material inhibits the melting and expansion of starch in a pyrolysis process, avoids the destruction of the structure and shape of the material obtained through pyrolysis, and thus improves the performance of the hard carbon material in a sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, and more particularly to a starch-based hard carbon material, its preparation method, and its application. Background Technology

[0002] With the introduction and implementation of various carbon emission policies, sodium-ion batteries are considered one of the most promising candidates for large-scale energy storage due to their abundant and vast resource reserves, as well as similar storage mechanisms and manufacturing technologies. Significant progress has been made in the development of low-cost sodium-ion battery cathode materials, such as layered transition metal oxides and Prussian blue cathodes that do not contain expensive Co and Ni elements. Anode materials for sodium-ion batteries are currently a research hotspot. Hard carbon, due to its advantages such as high reversible specific capacity, low voltage plateau, low cost, and wide availability, is a typical and promising hard carbon anode material. Starch, a biomass precursor, is abundant, environmentally friendly, renewable, and biodegradable, making it an excellent choice for preparing hard carbon anode materials.

[0003] Chinese patent CN114988391A discloses a method for preparing hard carbon anode materials and their applications. Starch is used as the substrate for the hard carbon anode material, mixed with nano-silica, and then directly subjected to heat treatment at 150-240℃ to pyrolyze it and form hard carbon material. However, due to the presence of crystalline regions between starch molecular chains, this method of directly pyrolyzing and carbonizing starch to prepare hard carbon materials results in a loose and porous carbon material with problems such as low starch carbon yield, fragmented carbon structure unfavorable for sodium ion storage, and high irreversible capacity, directly affecting the application effect of starch-based anode materials in sodium-ion batteries. This is because starch is a semi-crystalline polymer, containing crystalline and amorphous regions. From a mechanical perspective, starch exists in three aggregation states: glassy, ​​rubbery, and molten. The lowest temperature required for the transition from the glassy to the rubbery state is called the glass transition temperature (Tg) of starch. During rapid heating, the starch temperature rises continuously. When the temperature reaches the boiling point of water, the water inside the starch vaporizes rapidly, causing changes in the aggregated structure of the surrounding starch molecules. When the temperature exceeds the starch Tg, the starch enters a rubbery state and expands under the drive of steam pressure. With the release of pressure and the evaporation of water, thermal expansion is completed. Continuing to heat will burn the starch.

[0004] Glucoamylase, also known as glucoamylase (EC.3.2.1.3.), can cleave α,1-4 glucosidic bonds to produce glucose and cleave α,1-6 glucosidic bonds to convert to glucose, promoting the conversion of more amylose into amylopectin. It can also hydrolyze dextrin, releasing β-D-glucose from the non-reducing ends of glycogen. Treatment with glucoamylase reduces the swelling properties of starch, mitigating thermal expansion. Chinese patent CN105633380A discloses a method for preparing a starch-based porous hard carbon anode material for lithium-ion batteries. This method involves enzymatically hydrolyzing starch with a composite bio-enzyme to create pores, then pre-carbonizing the starch to preserve the pore morphology, and finally carbonizing to obtain a porous lithium-ion battery anode material. The material prepared by this method has a significantly open macroporous structure, increasing electrolyte consumption and being unfavorable for sodium ion storage.

[0005] Furthermore, although enzymatic starch can reduce the thermal expansion of starch to some extent, it cannot completely inhibit the melting and expansion of starch during pyrolysis. Summary of the Invention

[0006] To address the problems of the prior art, this invention provides a starch-based hard carbon material, its preparation method, and its application. This material inhibits the melting and expansion of starch during pyrolysis, preventing the destruction of the material's structural shape and thus improving the performance of the hard carbon material in sodium-ion batteries.

[0007] This invention is achieved through the following technical solution:

[0008] A method for preparing a starch-based hard carbon material includes the following steps:

[0009] (1) Enzymatic starch is oxidized in two stages under air conditions and then subjected to plasma treatment to obtain an oxidized precursor.

[0010] (2) The oxidized precursor is activated in an activating atmosphere and then subjected to plasma treatment to obtain the activated precursor;

[0011] (3) The activated precursor is carbonized at high temperature in an inert atmosphere to obtain starch-based hard carbon material.

[0012] Preferably, the preparation method of the enzymatic starch in step (1) is as follows: dissolve starch and enzyme in water, stir to carry out enzyme treatment, and then filter and dry to obtain enzymatic starch.

[0013] Preferably, the conditions for the two-stage oxidation process in step (1) are as follows: the oxidation temperature of the first stage is 120-180℃ and the oxidation time is 0.5-8 hours; the oxidation temperature of the second stage is 200-250℃ and the oxidation time is 1-8 hours.

[0014] Preferably, the conditions for plasma treatment in step (1) are: temperature of 180-350°C, treatment time of 1-8h, and treatment atmosphere of one or more combinations of N2, CO2, CO, Ar, H2, O2, CH4, NH3 and H2S.

[0015] Preferably, the activation conditions in step (2) are: the activation atmosphere is one or more combinations of water vapor, carbon dioxide, hydrogen sulfide, sulfur dioxide, acetylene and propyne; the activation temperature is 500 to 1100°C; and the activation time is 1 to 20 hours.

[0016] Preferably, the conditions for plasma treatment in step (2) are: temperature of 40-150°C, treatment time of 1-12h, and treatment atmosphere of one or more of N2, CO2, CO, Ar, H2, O2, CH4, NH3 and H2S.

[0017] Starch-based hard carbon material obtained using the preparation method described above.

[0018] An electrode on which the starch-based hard carbon material is loaded.

[0019] A sodium-ion battery, the sodium-ion battery including a negative electrode, the negative electrode being the electrode described above.

[0020] The application of the starch-based hard carbon material or the electrode in sodium-ion batteries.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] This invention involves a two-stage oxidation process of enzymatically modified starch in air followed by plasma treatment to obtain an oxidized precursor. The oxidized precursor is then activated in an active atmosphere and further treated with plasma to obtain an activated precursor. Finally, the activated precursor is carbonized at high temperature in an inert atmosphere to obtain a hard carbon anode material. In the first stage of oxidation, the temperature of the enzymatically modified starch is below the starch's Tg temperature but above the boiling point of water. During this oxidation process, surface and internal moisture evaporates, forming primary open channels. Oxygen-containing functional groups are grafted onto the starch surface and channels. In the second stage of oxidation, numerous oxygen-containing functional groups are grafted onto the starch channels and surface. Dehydration and carbonization of the surface and channels stabilize the channel structure, facilitating the escape of water vapor from within the starch molecules, resulting in deep dehydration and preventing thermal expansion while preserving the morphology of the starch granules. Subsequent plasma treatment breaks the existing chemical bonds (C=C, C=O, C=N, Si-Si, Si-O, etc.) on the sample surface. Free radicals in the plasma combine with these bonds to form a network of cross-linked structures. After thorough plasma treatment, the starch undergoes deep cross-linking. During subsequent activation, the interlayer spacing of the carbon material increases, which is beneficial for sodium storage, and the number of micropores increases. Then, plasma etching is performed, with a large amount of plasma conducting purely physical impacts. This not only removes existing contaminants and impurities from the material surface but also creates an etching effect, making the surface "rough" and forming many micro-pits and grooves, further increasing the number of micropores. Subsequently, during high-temperature carbonization, the micropores collapse and transform into closed pores under thermal action, further improving sodium storage performance. This invention uses a combination of enzymatic starch treatment, two-stage oxidation, and plasma treatment to suppress the melting and expansion of starch during pyrolysis, preventing the destruction of the material's structural shape and thus improving the initial coulombic efficiency. The plasma treatment after activation further increases the material's sodium storage capacity. The plasma treatment and enzymatic starch treatment methods used in this invention are green and scalable, using starch as a widely available raw material. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a scanning electron microscope image of the starch-based hard carbon material prepared in Example 1 of this invention;

[0025] Figure 2 It is a scanning electron microscope image of the raw material, corn starch;

[0026] Figure 3 This is a charge-discharge curve of the starch-based hard carbon material prepared in Example 1 of this invention;

[0027] Figure 4 This is a scanning electron microscope image of the starch-based hard carbon material prepared in Comparative Example 1 of this invention;

[0028] Figure 5 This is a scanning electron microscope image of the starch-based hard carbon material prepared in Comparative Example 2 of this invention;

[0029] Figure 6 This is a scanning electron microscope image of the starch-based hard carbon material prepared in Comparative Example 7 of this invention. Detailed Implementation

[0030] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0031] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0032] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying the method steps, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0033] The preparation method of the starch-based hard carbon material of the present invention includes the following steps:

[0034] (1) Enzymatic starch is oxidized in two stages under air conditions and then subjected to plasma treatment to obtain an oxidized precursor.

[0035] (2) The oxidized precursor is activated in an activating atmosphere and then subjected to plasma treatment to obtain the activated precursor;

[0036] (3) The activated precursor is carbonized at high temperature in an inert atmosphere to obtain starch-based hard carbon material.

[0037] In some specific embodiments of the present invention, the preparation method of the enzymatic starch in step (1) is as follows: starch and enzyme are dissolved in water, stirred for enzyme treatment, and then filtered and dried to obtain enzymatic starch; the type of enzyme is preferably a saccharifying enzyme with an enzyme activity of 80,000 u / g to 200,000 u / g, an enzyme treatment time of 4 to 12 hours, and an enzyme treatment temperature of 25 to 65°C; the starch is one or a combination of sweet potato starch, potato starch, rice starch, wheat starch, and corn starch.

[0038] In some specific embodiments of the present invention, the two-stage oxidation process in step (1) includes: the first stage oxidation temperature is 120-180℃, the heating rate is 0.5-3℃ / min, and the oxidation time is 0.5-8 hours; the second stage oxidation temperature is 200-250℃, the heating rate is 0.5-2℃ / min, and the oxidation time is 1-8 hours.

[0039] In some specific embodiments of the present invention, the plasma treatment conditions in step (1) are as follows: temperature is 180-350℃, treatment time is 1-8h, vacuum degree is 50-500pa, treatment power is 50-1000W, treatment atmosphere is one or more combinations of N2, CO2, CO, Ar, H2, O2, CH4, NH3, and H2S, and treatment atmosphere flow rate is 5-300ml / min.

[0040] In some specific embodiments of the present invention, the activation conditions in step (2) are as follows: the activation atmosphere is one or more combinations of water vapor, carbon dioxide, hydrogen sulfide, sulfur dioxide, acetylene and propyne; the activation temperature is 500 to 1100°C, the heating rate is 1 to 6°C / min, and the activation time is 1 to 20 hours.

[0041] In some specific embodiments of the present invention, the plasma treatment conditions in step (2) are as follows: temperature is 40-150℃, treatment time is 1-12h, vacuum degree is 50-500pa, treatment power is 50-1000W, treatment atmosphere is one or more of N2, CO2, CO, Ar, H2, O2, CH4, NH3 and H2S, and treatment atmosphere flow rate is 5-300ml / min.

[0042] In some specific embodiments of the present invention, the conditions for high-temperature carbonization in step (3) are: carbonization temperature of 1000-1700℃, heating rate of 2-8℃ / min, and carbonization time of 0.5-5 hours; the inert atmosphere is one or more combinations of nitrogen, argon, helium and neon.

[0043] In the following embodiments, the half-cell testing method is as follows: The negative electrode material samples prepared in each embodiment are mixed evenly in an aqueous solution with conductive agent Super P, binder CMC (sodium carboxymethyl cellulose), and binder SBR (styrene-butadiene rubber) at a mass ratio of 90:5:2.5:2.5. Then, the mixture is pressed onto copper foil to obtain a negative electrode sheet. The negative electrode sheet is then placed in a vacuum drying oven at 110°C and vacuum dried for 4 hours for later use. Using a sodium metal sheet as the negative electrode, a solution of 1 mol / L sodium hexafluorophosphate in ethylene carbonate and dimethyl carbonate (the ratio of ethylene carbonate to dimethyl carbonate is 1:1 vol%) is used as the electrolyte, and a Whatman GF / D glass fiber membrane is used as the separator. A CR2032 button sodium-ion battery is assembled and charged and discharged. The charging and discharging method is constant current 0.1C discharge to 2mV, rest for 10 minutes, then discharge at 0.01C to 1mV, and finally charge at 0.1C to 3V.

[0044] Example 1

[0045] 10g of corn starch was dissolved in 20g of water, and then 0.2g of 100,000 active saccharifying enzyme was added. The mixture was stirred at 55℃ and 400r / min for 6.5h, followed by filtration and drying to obtain enzymatically modified starch. The enzymatically modified starch was then placed in a temperature-controlled oven for oxidation, with the temperature increased to 170℃ at a rate of 2℃ / min and held for 2h, followed by increasing the temperature to 240℃ at a rate of 1℃ / min and holding for 5h. Immediately afterwards, it was treated with Ar and O2 plasma for 2h at a flow rate of 30ml / min for both Ar and O2 plasmas at 250℃. With a vacuum of 250 Pa and a processing power of 500 W, an oxidized precursor was obtained. The oxidized precursor was heated to 600 °C at a rate of 5 °C / min under a carbon dioxide atmosphere and held for 1.5 h. Immediately afterwards, it was treated in Ar plasma for 2 h at an Ar flow rate of 60 ml / min, a temperature of 60 °C, a vacuum of 250 Pa, and a processing power of 500 W to obtain an activated precursor. Finally, the activated precursor was heated to 1400 °C at a rate of 5 °C / min under a nitrogen atmosphere and carbonized for 2 h to obtain a hard carbon anode material.

[0046] The obtained hard carbon anode material was tested in a half-cell configuration. The first discharge specific capacity was 425 mAh / g, the reversible specific capacity was 404 mAh / g, and the initial efficiency was 95%. Its charge-discharge curve is shown below. Figure 3 As shown.

[0047] Figure 1 The image shows a scanning electron microscope image of the obtained starch-based hard carbon material. Figure 1 As can be seen from this, compared with starch raw materials ( Figure 2 The near-spherical shape of the starch granules obtained after pyrolysis is not significantly changed; the only change is the size of the spheres, with the particle size becoming smaller after pyrolysis.

[0048] Example 2

[0049] 10g of corn starch was dissolved in 20g of water, and then 0.2g of 100,000 active saccharifying enzyme was added. The mixture was stirred at 400 rpm for 8 hours at 50℃, then filtered and dried to obtain enzymatically modified starch. The enzymatically modified starch was then placed in a temperature-controlled oven for oxidation. The temperature was increased to 140℃ at a rate of 2℃ / min and held for 3 hours, followed by an increase to 220℃ at a rate of 1℃ / min and held for 1.5 hours. Immediately afterwards, the mixture was treated with Ar and NH3 plasma for 2 hours at a flow rate of 30 ml / min for both Ar and NH3 plasmas at 220℃. The oxidation precursor was obtained by heating the precursor to 700°C at a vacuum of 250 Pa and a processing power of 500 W under a hydrogen sulfide atmosphere. The precursor was then heated to 700°C at a rate of 5°C / min and held for 2 hours. Immediately afterwards, it was treated in Ar plasma for 2 hours at an Ar flow rate of 60 ml / min, a temperature of 60°C, a vacuum of 250 Pa, and a processing power of 500 W to obtain the activated precursor. Finally, the activated precursor was heated to 1050°C at a rate of 5°C / min and carbonized for 2 hours under a nitrogen atmosphere to obtain the hard carbon anode material.

[0050] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 415 mAh / g, the reversible specific capacity was 353 mAh / g, and the first efficiency was 85%.

[0051] Example 3

[0052] 10g of corn starch was dissolved in 20g of water, and then 0.2g of 150,000 active saccharifying enzyme was added. The mixture was stirred at 400 rpm for 4.5 hours at 60℃, then filtered and dried to obtain enzymatically modified starch. The enzymatically modified starch was then placed in a temperature-controlled oven for oxidation. The temperature was increased to 130℃ at a rate of 2.5℃ / min and held for 3 hours. Then, the temperature was increased to 230℃ at a rate of 0.5℃ / min and held for 2 hours. Immediately afterwards, the mixture was treated with Ar and CO2 plasma for 2 hours at a flow rate of 30 ml / min for both Ar and CO2 plasmas at 26℃. An oxidized precursor was obtained at 0℃, a vacuum of 250Pa, and a processing power of 500W. The oxidized precursor was then heated to 800℃ at a rate of 5℃ / min under a sulfur dioxide atmosphere and held for 2 hours. Immediately afterwards, it was treated in Ar plasma for 2 hours at an Ar flow rate of 60ml / min, a temperature of 60℃, a vacuum of 250Pa, and a processing power of 500W to obtain an activated precursor. Finally, the activated precursor was heated to 1300℃ at a rate of 5℃ / min under a nitrogen atmosphere and carbonized for 2 hours to obtain a hard carbon anode material.

[0053] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 380 mAh / g, the reversible specific capacity was 323 mAh / g, and the first efficiency was 85%.

[0054] Example 4

[0055] 10g of corn starch was dissolved in 20g of water, and then 0.2g of 150,000 active saccharifying enzyme was added. The mixture was stirred at 400 rpm for 11 hours at 45℃, then filtered and dried to obtain enzymatically modified starch. The enzymatically modified starch was then placed in a temperature-controlled oven for oxidation. The temperature was increased to 160℃ at a rate of 1℃ / min and held for 3 hours, followed by increasing the temperature to 230℃ at a rate of 1℃ / min and holding for 2 hours. Immediately afterwards, it was treated with Ar and H2 plasma for 2 hours at a flow rate of 30 ml / min for both Ar and H2 plasmas at 260℃. An oxidized precursor was obtained under a vacuum of 250 Pa and a processing power of 500 W. The oxidized precursor was then heated to 700 °C at a rate of 5 °C / min under an acetylene atmosphere and held for 2 h. Immediately afterwards, it was treated in Ar plasma for 2 h at an Ar flow rate of 60 ml / min, a temperature of 60 °C, a vacuum of 250 Pa, and a processing power of 500 W to obtain an activated precursor. Finally, the activated precursor was heated to 1350 °C at a rate of 5 °C / min under a nitrogen atmosphere and carbonized for 2 h to obtain a hard carbon anode material.

[0056] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 453 mAh / g, the reversible specific capacity was 421 mAh / g, and the first efficiency was 93%.

[0057] Example 5

[0058] Dissolve 10g of cornstarch in 20g of water, then add 0.2g of... 150,000 active saccharifying enzymes were added, and the mixture was stirred at 400 r / min for 5 h at 50 °C. After filtration and drying, enzymatic starch was obtained. The enzymatic starch was then placed in a temperature-controlled oven for oxidation, with the temperature increased to 140 °C at a rate of 1 °C / min and held for 3 h. The temperature was then increased to 210 °C at a rate of 1 °C / min and held for 3 h. Following this, the mixture was treated in Ar and CO plasma for 2 h at a flow rate of 30 ml / min for both Ar and CO plasmas, a temperature of 260 °C, a vacuum of 250 Pa, and a processing power of 500 W to obtain an oxidation precursor. This precursor was then heated to 800 °C at a rate of 5 °C / min and held for 2 h in a propyne atmosphere to obtain a pre-carbonized precursor. Finally, the pre-carbonized precursor was carbonized to 1400 °C at a rate of 5 °C / min in a nitrogen atmosphere for 2 h to obtain a hard carbon anode material.

[0059] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 418 mAh / g, the reversible specific capacity was 376 mAh / g, and the first efficiency was 90%.

[0060] Example 6

[0061] It is basically the same as Example 1, except that the starch is sweet potato starch.

[0062] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 405 mAh / g, the reversible specific capacity was 365 mAh / g, and the first efficiency was 90%.

[0063] Example 7

[0064] It is basically the same as Example 1, except that the starch is potato starch.

[0065] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 399 mAh / g, the reversible specific capacity was 355 mAh / g, and the first efficiency was 89%.

[0066] Example 8

[0067] It is basically the same as Example 1, except that the starch is rice starch.

[0068] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 403 mAh / g, the reversible specific capacity was 367 mAh / g, and the first efficiency was 91%.

[0069] Example 9

[0070] It is basically the same as Example 1, except that the starch is wheat starch.

[0071] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 424 mAh / g, the reversible specific capacity was 373 mAh / g, and the first efficiency was 88%.

[0072] Example 10

[0073] It is basically the same as Example 1, except that the activation atmosphere is water vapor.

[0074] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 446 mAh / g, the reversible specific capacity was 415 mAh / g, and the first efficiency was 93%.

[0075] Comparative Example 1

[0076] It is basically the same as Example 1, except that the starch is oxidized directly without enzyme treatment.

[0077] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 379 mAh / g, the reversible specific capacity was 330 mAh / g, and the first efficiency was 87%.

[0078] Figure 4The image shows a scanning electron microscope image of the hard carbon anode material obtained in this comparative example. It can be seen from the image that the material is in an amorphous state, that is, the starch without enzyme treatment breaks into an amorphous shape during the subsequent carbonization. However, as mentioned above, the starch after enzyme treatment and two stages of oxidation does not change its shape during the subsequent carbonization and remains spherical. This indicates that the starch can maintain its original shape and not be destroyed after enzyme treatment and subsequent oxidation.

[0079] Comparative Example 2

[0080] It is basically the same as Example 1, except that the enzymatic starch does not undergo two-stage oxidation treatment.

[0081] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 377 mAh / g, the reversible specific capacity was 298 mAh / g, and the first efficiency was 79%.

[0082] Figure 5 The image shows a scanning electron microscope image of the hard carbon anode material in this comparative example. It can be seen from the image that the material is in an amorphous state. The starch that has not undergone two-stage oxidation treatment breaks into an amorphous shape during subsequent carbonization. In Example 1, the starch that has undergone two-stage oxidation treatment has not had its shape destroyed after carbonization, indicating that two-stage oxidation is beneficial to maintaining the morphology of the material.

[0083] Comparative Example 3

[0084] It is basically the same as Example 1, except that it does not undergo carbon dioxide activation treatment.

[0085] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 335 mAh / g, the reversible specific capacity was 288 mAh / g, and the first efficiency was 86%.

[0086] Comparative Example 4

[0087] It is basically the same as Example 1, except that it does not undergo high-temperature carbonization treatment.

[0088] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 274 mAh / g, the reversible specific capacity was 208 mAh / g, and the first efficiency was 76%.

[0089] Comparative Example 5

[0090] It is basically the same as Example 1, except that no plasma treatment is performed after the two-stage oxidation.

[0091] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 385 mAh / g, the reversible specific capacity was 312 mAh / g, and the first efficiency was 81%.

[0092] Comparative Example 6

[0093] It is basically the same as Example 1, except that no plasma treatment is performed after activation.

[0094] The obtained hard carbon anode material was tested in a half-cell, and the first discharge specific capacity was 352 mAh / g, the reversible specific capacity was 292 mAh / g, and the first efficiency was 83%.

[0095] Comparative Example 7

[0096] It is basically the same as Example 1, except that the two-stage oxidation is changed to direct oxidation, that is, oxidation at 240°C for 7 hours in air.

[0097] The obtained hard carbon anode material was tested in a half-cell test. The first discharge specific capacity was 340 mAh / g, the reversible specific capacity was 248 mAh / g, and the first efficiency was 73%.

[0098] Figure 6 The image shows a scanning electron microscope image of the hard carbon anode material obtained in this comparative example. It can be seen from the image that the material is in an amorphous state. The starch treated by direct oxidation also breaks into an amorphous shape during subsequent carbonization. Combined with Example 1 and Comparative Example 2, it can be shown that direct oxidation cannot overcome the breakage of the material during carbonization, while two-stage oxidation can effectively maintain the morphology of the material. This is because after the starch is directly oxidized into a rubbery state, the internal water vapor cannot be released, causing expansion.

[0099] To more intuitively demonstrate the performance of the materials prepared by this invention, Table 1 below is a comparison table of the main parameters of some specific embodiment samples.

[0100] Table 1 Comparison of main parameters of samples in specific embodiments

[0101]

[0102]

[0103] As shown in Table 1, the hard carbon anode material prepared from corn starch in Example 1 exhibits superior performance compared to the sweet potato starch, potato starch, rice starch, and wheat starch used in Examples 6-9. When the activation atmosphere is water vapor, both the initial discharge specific capacity and reversible specific capacity are improved compared to carbon dioxide in Example 1, while the initial efficiency is essentially the same. Compared to the direct oxidation of starch in Comparative Example 1 without enzyme treatment, Example 1, which uses enzyme-treated starch, shows improved performance in terms of initial discharge specific capacity, reversible specific capacity, and initial efficiency. This indicates that enzyme treatment of starch can improve the electrochemical performance of the final hard carbon material, because untreated starch contains more amylose, resulting in poor cross-linking. Compared to Comparative Example 3 without activation treatment, Example 1 shows improved performance in terms of initial discharge specific capacity, reversible specific capacity, and initial efficiency. This indicates that activation treatment can improve the electrochemical performance of the final hard carbon material, because activation increases the interlayer spacing of the carbon layers, leading to an increase in the number of micropores and an increase in sodium storage capacity after carbonization. Compared to Comparative Example 4, which did not undergo high-temperature carbonization, Example 1 showed improved performance in terms of first-cycle discharge specific capacity, reversible specific capacity, and first-time efficiency. This indicates that high-temperature carbonization can improve the electrochemical performance of the final hard carbon material, because unclosed micropores are detrimental to sodium storage. Compared to Comparative Example 5, which did not undergo plasma treatment after oxidation, Example 1 showed improved performance in terms of first-cycle discharge specific capacity, reversible specific capacity, and first-time efficiency. This indicates that plasma treatment after oxidation can improve the electrochemical performance of the final hard carbon material, because the starch crosslinking effect is poor without plasma treatment. Compared to Comparative Example 6, which did not undergo plasma treatment after activation, Example 1 showed improved performance in terms of first-cycle discharge specific capacity, reversible specific capacity, and first-time efficiency. This indicates that plasma treatment after activation can improve the electrochemical performance of the final hard carbon material, because the number of micropores increases after plasma etching, and the sodium storage capacity increases after carbonization. Compared to Comparative Example 2, which underwent oxidation treatment, Comparative Example 7 showed improved performance in terms of initial discharge specific capacity, reversible specific capacity, and initial effect. This indicates that oxidation treatment of starch can improve the electrochemical performance of the final hard carbon material. This is because the oxidation process increases the oxygen-containing functional groups on the surface of the starch particles, and the closed pores formed after carbonization are beneficial for sodium storage. Compared to Comparative Example 7, which underwent direct oxidation, Example 1 showed significantly improved performance in terms of initial discharge specific capacity, reversible specific capacity, and initial effect. This indicates that two-stage oxidation can improve the electrochemical performance of the final hard carbon material compared to direct oxidation. This is because the morphology and structure of the starch particles are preserved after two-stage oxidation.

[0104] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0105] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a starch-based hard carbon material, characterized in that, Includes the following steps: (1) Enzymatic starch is oxidized in two stages under air conditions and then subjected to plasma treatment to obtain an oxidized precursor; the preparation method of the enzymatic starch is as follows: starch and enzyme are dissolved in water, stirred for enzyme treatment, and then filtered and dried to obtain enzymatic starch; the type of enzyme is preferably a saccharifying enzyme; the conditions of the two-stage oxidation are: the first stage oxidation temperature is 120~180℃, and the oxidation time is 0.5~8 hours; the second stage oxidation temperature is 200~250℃, and the oxidation time is 1~8 hours; the conditions of the plasma treatment are: the temperature is 180~350℃, the treatment time is 1~8h, and the treatment atmosphere is one or more combinations of N2, CO2, CO, Ar, H2, O2, CH4, NH3 and H2S; (2) The oxidized precursor is activated in an activating atmosphere and then subjected to plasma treatment to obtain the activated precursor; (3) The activated precursor is carbonized at high temperature in an inert atmosphere to obtain starch-based hard carbon material.

2. The method for preparing starch-based hard carbon material according to claim 1, characterized in that, The activation conditions in step (2) are as follows: the activation atmosphere is one or more combinations of water vapor, carbon dioxide, hydrogen sulfide, sulfur dioxide, acetylene and propyne; the activation temperature is 500~1100℃; and the activation time is 1~20 hours.

3. The method for preparing starch-based hard carbon material according to claim 1, characterized in that, The conditions for plasma treatment in step (2) are: temperature of 40~150℃, treatment time of 1~12h, and treatment atmosphere of one or more of N2, CO2, CO, Ar, H2, O2, CH4, NH3 and H2S.

4. Starch-based hard carbon material obtained by the preparation method according to any one of claims 1 to 3.

5. An electrode, characterized in that, The electrode is loaded with the starch-based hard carbon material as described in claim 4.

6. A sodium-ion battery, characterized in that, The sodium-ion battery includes a negative electrode, which is the electrode according to claim 5.

7. The application of the starch-based hard carbon material of claim 4 or the electrode of claim 5 in a sodium-ion battery.

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