Fixed sulfur-doped biomass hard carbon, method of preparation, and sodium-ion battery

Abstract

The application discloses a kind of fixed sulfur doped biomass hard carbon, preparation method and sodium ion battery, preparation method includes the following steps: biomass raw material is pretreated, after being pretreated, the biomass raw material is mixed with (NH4) 2Mg (SO4) 2·6H2O, and obtain mixed material;The mixture is pre-carbonized at first preset temperature, and obtain pre-carbonized material, wherein (NH4) 2Mg (SO4) 2·6H2O is decomposed to generate MgSO4 at the first preset temperature, and the pre-carbonized material is heated from room temperature to second preset temperature to carbonize and obtain sulfur-containing biomass hard carbon, in the heating process, MgSO4 is decomposed into magnesium oxide and sulfur-containing gas, magnesium oxide and carbon matrix are reduced to generate magnesium-containing vapor, and sulfur-containing gas and carbon matrix are reacted to form sulfur-containing biomass hard carbon.The preparation method of the application is a kind of low-cost, simple sulfur-doped hard carbon preparation method based on magnesium sulfur fixation strategy.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a fixed sulfur-doped biomass hard carbon, its preparation method, and a sodium-ion battery. Background Technology

[0002] Currently, sulfur doping of hard carbon is mainly achieved by constructing sulfur-rich polymer precursors or introducing external sulfur sources and mixing them with carbon sources in a solid or liquid phase, followed by co-heating. For example, sulfur doping is achieved by mixing thiourea, sulfates, concentrated sulfuric acid, and thiosulfates with carbon sources in a solid or liquid phase, followed by co-heating.

[0003] Some existing technologies involve expensive raw materials in the preparation of sulfur-containing precursors, resulting in high costs; in some existing technologies, the sulfur source is easily decomposed at high temperatures and the carbon-sulfur bond is unstable, resulting in poor sulfur introduction effect; and some existing technologies use exogenous sulfur-fixing reagents containing calcium and magnesium, which increases reagent costs.

[0004] Therefore, there is an urgent need to develop a sulfur-doped hard carbon material with readily available raw materials and low production costs. Summary of the Invention

[0005] The present invention aims to at least solve the technical problem of high cost of sulfur doping synthesis in the prior art. To this end, one object of the present invention is to propose a method for preparing fixed sulfur doped biomass hard carbon.

[0006] The present invention also provides a fixed sulfur-doped biomass hard carbon prepared by the above-described method for preparing fixed sulfur-doped biomass hard carbon, and a sodium-ion battery comprising the above-described fixed sulfur-doped biomass hard carbon.

[0007] According to a first aspect of the present invention, a method for preparing fixed sulfur-doped biomass hard carbon includes the following steps: pretreating biomass raw materials; mixing the pretreated biomass raw materials with (NH4)2Mg(SO4)2·6H2O to obtain a mixture; pre-carbonizing the mixture at 500°C to obtain a pre-carbonized material; and carbonizing the pre-carbonized material by heating it from room temperature to 1300°C-1500°C to obtain sulfur-containing biomass hard carbon.

[0008] Optionally, the pretreatment step includes: crushing the biomass raw material, passing it through a 50-mesh sieve to obtain undersize material, and mixing the undersize material with (NH4)2Mg(SO4)2·6H2O to obtain a mixture.

[0009] Optionally, in the mixture, the biomass raw material and (NH4)2Mg(SO4)2·6H2O are mixed in a mass ratio of 10:1 to 4:1.

[0010] Optionally, the process of carbonizing the pre-carbonized material includes: cooling the pre-carbonized material to room temperature and then crushing it; and heating the crushed pre-carbonized material from room temperature to 1300℃-1500℃ for carbonization.

[0011] Optionally, the mixture is heated to 500°C at a rate of 3°C / min-5°C / min, held at that temperature for 2 hours, and then cooled to room temperature; and / or, the biomass raw material is mixed with (NH4)2Mg(SO4)2·6H2O by ball milling in a vibrating mill.

[0012] Optionally, the method for preparing fixed sulfur-doped biomass hard carbon further includes the following step: purifying the sulfur-containing biomass hard carbon by acid washing with acid solution.

[0013] Optionally, the acid solution is one or more selected from sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid; and / or, The acid solution has H + The concentration is 1 mol / L-5 mol / L, and the mass ratio of the acid solution to the sulfur-containing biomass hard carbon is 3:1.

[0014] Optionally, the biomass raw material is one or more of the following: jujube shell, apricot shell, walnut shell, and coconut shell.

[0015] The fixed sulfur-doped biomass hard carbon according to a second aspect of the present invention includes sulfur-containing biomass hard carbon obtained according to any of the preparation methods described above.

[0016] According to a third aspect of the present invention, a sodium-ion battery includes a negative electrode active material, said negative electrode active material comprising any of the fixed sulfur-doped biomass hard carbon described above.

[0017] The method for preparing fixed-sulfur-doped biomass hard carbon according to embodiments of the present invention is a low-cost sulfur fixation method. On one hand, it uses inexpensive biomass raw materials as the carbon source and employs low-cost magnesium ammonium sulfate hexahydrate as both the sulfur source and the sulfur-fixing agent. This not only improves the problem of poor sulfur doping effect caused by the introduction of a single sulfur source but also avoids the introduction of expensive sulfur-rich precursors and sulfur-fixing reagents, reducing production costs. It is a low-cost and efficient sulfur doping method with significant practical application value. On the other hand, the preparation method of the present invention does not limit the biomass raw materials; various biomass precursors can be used. This demonstrates that the raw materials and magnesium ammonium sulfate hexahydrate used in the preparation method are inexpensive, and the preparation process is simple. Furthermore, compared to existing technologies that involve complex processes such as multi-step oxidation, gas-phase sulfidation, and in-situ polymerization coating, the steps of low-temperature pre-carbonization and high-temperature carbonization in the embodiments of the present invention have advantages such as simple process and ease of mass production. In addition, compared with the existing technology that first uses H2S vapor phase etching and then uses MgSO4 for sulfur doping, the present invention utilizes (NH4)2Mg(SO4)2·6H2O to complete thermal decomposition during the pre-carbonization process of biomass, releasing the sulfur source in one go to achieve in-situ sulfur doping, and simultaneously completing sulfur fixation during the high-temperature carbonization process, thereby improving the sulfur doping efficiency.

[0018] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a schematic flowchart of a method for preparing fixed sulfur-doped biomass hard carbon according to an embodiment of the present invention; Figure 2 This is a schematic flowchart of a method for preparing fixed sulfur-doped biomass hard carbon according to an embodiment of the present invention; Figure 3 This is a comparison of the XRD patterns of the products of Example 1 and Comparative Example 1. Detailed Implementation

[0020] The following describes in detail the method for preparing fixed sulfur-doped biomass hard carbon according to embodiments of the present invention.

[0021] like Figure 1 and Figure 2 As shown, the method for preparing fixed sulfur-doped biomass hard carbon according to an embodiment of the present invention includes the following steps: The biomass feedstock is pretreated, and then mixed with (NH4)2Mg(SO4)2·6H2O to obtain a mixture. In other words, the biomass feedstock is used as a precursor and mixed with low-cost magnesium sulfate hexahydrate. The magnesium sulfate hexahydrate can simultaneously serve as a sulfur source and a sulfur-fixing agent.

[0022] The mixture is pre-carbonized at a first preset temperature to obtain pre-carbonized material, wherein (NH4)2Mg(SO4)2·6H2O decomposes at the first preset temperature to generate MgSO4. In this step, the pre-carbonization of the biomass raw material is completed, and (NH4)2Mg(SO4)2·6H2O undergoes preliminary decomposition to generate MgSO4. Furthermore, through pre-carbonization, the cellulose, hemicellulose, and lignin in the biomass raw material undergo dehydration, condensation, and aromatic ring cross-linking to construct a primary stable three-dimensional carbon skeleton, preventing structural collapse and particle adhesion deformation during subsequent carbonization. In addition, pre-carbonization can remove a large amount of volatile matter and tar from the biomass raw material, preventing equipment damage and safety hazards caused by large amounts of tar and volatile matter during subsequent high-temperature carbonization.

[0023] The pre-carbonized material is heated from room temperature to a second preset temperature for carbonization to obtain sulfur-containing biomass hard carbon. During the heating process, MgSO4 decomposes into magnesium oxide and sulfur-containing gas. Magnesium oxide is reduced with the carbon matrix to generate magnesium-containing vapor, and the sulfur-containing gas reacts with the carbon matrix to form sulfur-containing biomass hard carbon. It is understood that magnesium ammonium sulfate not only serves as a sulfur source, but also decomposes into magnesium sulfate, ammonia, nitrogen, and SOx under high-temperature conditions. Heating magnesium sulfate further decomposes it into magnesium oxide and sulfur-containing gas. Magnesium oxide is reduced with the carbon matrix to generate magnesium vapor. The volatility and high pressure generated by the magnesium vapor promote the formation of a porous structure and increase ion transport channels. Simultaneously, the sulfur-containing gas reacts with the carbon matrix to form sulfur-containing biomass hard carbon. The specific reaction equations are as follows: .

[0024] It can be seen that through the pre-carbonization and subsequent carbonization processes, the N in (NH4)2Mg(SO4)2·6H2O can be basically converted into gas and escape.

[0025] Therefore, the method for preparing fixed-sulfur-doped biomass hard carbon according to embodiments of the present invention is a low-cost sulfur fixation method. On the one hand, it uses inexpensive biomass raw materials as carbon sources and employs low-cost magnesium ammonium sulfate hexahydrate as both a sulfur source and a sulfur-fixing agent. This not only improves the problem of poor sulfur doping effect caused by the introduction of a single sulfur source, but also avoids the introduction of expensive sulfur-rich precursors and sulfur-fixing reagents, reducing production costs. It is a low-cost and efficient sulfur doping method with significant practical application value. On the other hand, the preparation method of the embodiments of the present invention does not impose restrictions on biomass raw materials; various biomass precursors can be used in this scheme. It is evident that the raw materials and magnesium ammonium sulfate hexahydrate used in the preparation method are inexpensive, and the preparation process is simple. Furthermore, compared with the complex processes in the prior art that include multi-step oxidation, gas-phase sulfidation, and in-situ polymerization coating, the steps of low-temperature pre-carbonization and high-temperature carbonization in the embodiments of the present invention have the advantages of simple process and ease of mass production. In addition, compared with the existing technology that first uses H2S vapor phase etching and then uses MgSO4 for sulfur doping, the present invention utilizes (NH4)2Mg(SO4)2·6H2O to complete thermal decomposition during the pre-carbonization process of biomass, releasing the sulfur source in one go to achieve in-situ sulfur doping, and simultaneously completing sulfur fixation during the high-temperature carbonization process, thereby improving the sulfur doping efficiency.

[0026] According to one embodiment of the present invention, the pretreatment step includes: crushing the biomass raw material, passing it through a 50-mesh sieve to obtain undersize material, and mixing the undersize material with (NH4)2Mg(SO4)2·6H2O to obtain a mixture. For example, the biomass raw material is crushed, passed through a 50-mesh sieve to obtain undersize material, and then mixed with (NH4)2Mg(SO4)2·6H2O at a mass ratio of 10:1 to 4:1 to obtain a mixture. In this embodiment, the biomass raw material is first crushed, then passed through a 50-mesh sieve, and the undersize material is used for subsequent processes. In this embodiment, using the undersize material from the 50-mesh sieve can remove some larger particles, improving the completeness of the subsequent mixing and pre-carbonization reaction. In addition, the selection of the 50-mesh sieve also takes into account the yield; if the mesh number is too small, more large particles will remain, and if the mesh number is too large, the yield will be low.

[0027] In some specific embodiments of the present invention, the biomass raw material and (NH4)2Mg(SO4)2·6H2O are mixed in a mass ratio of 10:1 to 4:1. For example, the biomass raw material is crushed and sieved to obtain undersize material, which is then mixed with (NH4)2Mg(SO4)2·6H2O in a mass ratio of 10:1 to 4:1 to obtain the mixture. In this embodiment, when mixing the undersize material with (NH4)2Mg(SO4)2·6H2O, the mass ratio of the undersize material to (NH4)2Mg(SO4)2·6H2O is 10:1 to 4:1, for example, a mass ratio of 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, etc. By adopting the above-mentioned mass ratio range, the sulfur doping content can be kept within an optimal range. This not only ensures that the active sites are within an appropriate range, avoiding accelerated consumption of the SEI film, reducing side reactions, and guaranteeing high first-efficiency and rate performance, but also improves the overall electrode conductivity, avoids a sharp increase in polarization under high current, and prevents local carbon layer distortion and collapse, thus ensuring smooth ion diffusion channels and reducing Na+ doping. + The migration resistance within the pores allows for rapid insertion / extraction even under high current. In this embodiment, by employing the aforementioned mass ratio range, sulfur doping within a preset range can increase surface defects and polar sites, thereby enhancing sodium storage capacity. Simultaneously, it increases the carbon interlayer spacing, strengthens capacitive sodium storage, accelerates surface reactions, and improves the rate capability.

[0028] According to one embodiment of the present invention, the first preset temperature is 500℃, and the second preset temperature is 1300℃-1500℃. That is, the mixture is first pre-carbonized at 500℃ to obtain pre-carbonized material. In this step, heating at 500℃ completes the pre-carbonization of the biomass raw material on the one hand, and (NH4)2Mg(SO4)2·6H2O undergoes preliminary decomposition to generate MgSO4 on the other hand. The pre-carbonized material is then heated from room temperature to 1300℃-1500℃ for carbonization to obtain sulfur-containing biomass hard carbon. Understandably, magnesium ammonium sulfate not only serves as a sulfur source, but also decomposes into magnesium sulfate, ammonia, nitrogen, and SOx under high-temperature conditions. Magnesium sulfate further decomposes into magnesium oxide and sulfur-containing gases above 900℃. When the temperature exceeds 1300℃, magnesium oxide is reduced to magnesium vapor by the carbon matrix. The volatility and high pressure generated by the magnesium vapor promote the formation of a porous structure and increase ion transport channels. Simultaneously, the sulfur-containing gases react with the carbon matrix to form sulfur-containing biomass hard carbon. Furthermore, the initial preset temperature of 500℃ can prevent the prematurely generated MgO from thermally agglomerating during subsequent high-temperature carbonization, thus improving the uniformity of sulfur doping.

[0029] In some specific embodiments of the present invention, during pre-carbonization, the mixture is heated to 500°C at a rate of 3°C / min-5°C / min, held at that temperature for 2 hours, and then cooled to room temperature. In actual operation, the mixture can be placed in a crucible. For example, the mixture obtained in the aforementioned steps can be placed in a graphite crucible, heated to 500°C at a rate of 3°C / min-5°C / min, held at that temperature for 2 hours, and then cooled to room temperature before being removed.

[0030] Optionally, the carbonization process of the pre-carbonized material includes: cooling the pre-carbonized material to room temperature and then crushing it to obtain powder. For example, the mixture is heated to 500°C and then crushed to a D50 of 6±0.5 μm using an air jet mill. Subsequently, the crushed pre-carbonized material is heated from room temperature to 1300°C-1500°C for carbonization.

[0031] Optionally, when mixing biomass raw materials with (NH4)2Mg(SO4)2·6H2O, ball milling in a vibrator can improve the thoroughness of mixing. For example, mixing the undersized material with (NH4)2Mg(SO4)2·6H2O in a vibrator and ball milling for 30 minutes can achieve thorough mixing with good mixing effect. The frequency of the vibrator is not limited.

[0032] According to one embodiment of the present invention, the preparation method of fixed sulfur-doped biomass hard carbon further includes the following steps: acid washing and purification of the sulfur-containing biomass hard carbon using an acid solution. That is, after high-temperature carbonization to complete sulfur fixation, purification and water washing can be performed after the formation of stable CS bonds to remove impurities and residual MgO. It is understood that placing the biomass hard carbon in an acid solution can remove impurities such as Ca, Na, and Si, as well as residual MgO. In other words, purification with an acid solution removes impurities and residual MgO. The acid purification after high-temperature carbonization removes ash impurities from the biomass hard carbon and also removes residual MgO.

[0033] In some specific embodiments of the present invention, the acid solution is one or more of sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, etc., which can efficiently remove impurities.

[0034] According to one embodiment of the present invention, the H of the acid solution + With a concentration of 1 mol / L to 5 mol / L and a mass ratio of acid solution to sulfur-containing biomass hard carbon of 3:1, it can be both economical and highly efficient in removing impurities.

[0035] In some specific embodiments of the present invention, the biomass raw material is one or more of the following: jujube shell, apricot shell, walnut shell, coconut shell, etc., which is low in cost.

[0036] This invention also discloses a fixed sulfur-doped biomass hard carbon, comprising the sulfur-containing biomass hard carbon obtained by the preparation method described in any of the above embodiments, which has advantages such as improved capacity, first-efficiency, and rate performance. For example, sulfur doping can increase active sites, increase interlayer spacing, and improve rate performance.

[0037] The present invention also discloses a sodium-ion battery, including a negative electrode active material, wherein the negative electrode active material comprises fixed sulfur-doped biomass hard carbon, thereby improving the performance of the sodium-ion battery.

[0038] The preparation method of fixed sulfur-doped biomass hard carbon and its products according to specific embodiments of the present invention will be described in detail below.

[0039] Example 1 S1. Take the raw material of jujube shell and crush it through a small crusher until it passes through a 50-mesh sieve. Take 500g of the material that passes through the sieve and 83.33g of (NH4)2Mg(SO4)2·6H2O (mass ratio of 6:1) and put them into a vibrating ball mill and ball mill for 30 minutes to complete the mixing. S2. Place the mixture in a graphite crucible and heat it to 500℃ in a box furnace at 5℃ / min. After holding at this temperature for 2 hours, cool it to room temperature and remove it. Then, use an air jet mill to pulverize it to a D50 of 6±0.5um. S3. Take 100g of the material from step S2 and place it in a graphite crucible. Heat it to 1400℃ in a tube furnace at 5℃ / min and keep it at that temperature for 2 hours. After it naturally cools to room temperature, take it out to obtain sulfur-containing biomass hard carbon. S4. Take 50g of sulfur-containing biomass hard carbon and add it to 150g of 3mol / L hydrochloric acid solution for purification. After purification, wash repeatedly with deionized water until the filtrate is neutral. Then dry it until the moisture content is below 2% to obtain the final sulfur-containing biomass hard carbon.

[0040] Example 2 The difference between Example 2 and Example 1 is that the mass ratio of the raw material under the 50-mesh sieve to magnesium ammonium sulfate hexahydrate is 7:1, while the other steps are the same.

[0041] Example 3 The difference between Example 3 and Example 1 is that the mass ratio of the raw material under the 50-mesh sieve to magnesium ammonium sulfate hexahydrate is 8:1, while the other steps are the same.

[0042] Example 4 The difference between Example 4 and Example 1 is that the mass ratio of the raw material under the 50-mesh sieve to magnesium ammonium sulfate hexahydrate is 10:1, while the other steps are the same.

[0043] Example 5 The difference between Example 5 and Example 1 is that the mass ratio of the raw material under the 50-mesh sieve to magnesium ammonium sulfate hexahydrate is 4:1, while the other steps are the same.

[0044] Example 6 The difference between Example 6 and Example 1 is that the high-temperature carbonization temperature is 1300℃, while the other steps are the same.

[0045] Example 7 The difference between Example 7 and Example 1 is that the high-temperature carbonization temperature is 1500℃, while the other steps are the same.

[0046] Example 8 The difference between Example 8 and Example 1 is that the raw material is changed to walnut shells, while the other steps are the same.

[0047] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that (NH4)2Mg(SO4)2·6H2O was not added in step S1, and step S1 only contained 500g of undersize material, while the other steps were the same.

[0048] The performance of the products from the above embodiments and comparative examples will now be tested.

[0049] First, the battery is manufactured.

[0050] The negative electrode materials provided in the above examples and comparative examples were dispersed in an N-methylpyrrolidone (NMP) solution with conductive agent SP and binder PVDF at a mass ratio of 90:5:5. The mixture was thoroughly stirred to obtain a slurry, which was then coated, dried, and formed into an electrode sheet with a diameter of 12 mm. A sodium metal sheet was used as the counter electrode, glass fiber as the separator, and 1M NaPF6 was used as the electrolyte. A button cell (model CR2032) was assembled in the following order: negative electrode casing → sodium sheet → separator → negative electrode material sheet → gasket → spring sheet → positive electrode casing.

[0051] Subsequently, the prepared button cells were subjected to the following performance tests: (1) Battery test.

[0052] The nominal specific capacity is 200mAh / g.

[0053] (2) First-efficiency and specific capacity tests.

[0054] The system was subjected to a series of processes, including 2 hours of rest, 0.1C, 0V discharge, 10 minutes of rest, 0.02C, 0V discharge, 30 seconds of rest, and 0.1C, 2.0V charging. The initial reversible specific capacity and initial coulombic efficiency were then recorded.

[0055] (3) Ratio performance test.

[0056] The device was subjected to a series of processes, including 2 hours of rest, 0.1C, 0V discharge, 10 minutes of rest, 0.02C, 0V discharge, 30 seconds of rest, and 0.1C, 2.0V charging. It was then charged and discharged at 0.2C, 0.5C, and 1C, with a voltage window of 0V-2V.

[0057] The test results for each embodiment and comparative example are shown in Table 1 below.

[0058] Table 1 Through the Figure 3 Analysis of the XRD patterns of Example 1 and Comparative Example 1 shows that the spectrum of Comparative Example 1 is shifted to the right compared to Example 1, and θ increases. According to Bragg's 2dsinθ=nλ, an increase in θ leads to a decrease in interlayer spacing d. Therefore, after sulfur doping, the interlayer spacing is larger. Table 1 shows that the anode materials in each example have higher specific capacity, first-efficiency, and better rate performance than Comparative Example 1. This is because moderate sulfur doping can increase surface defects and polar sites, improve sodium storage capacity, increase carbon interlayer spacing, enhance capacitive sodium storage, accelerate surface reactions, and improve rate performance.

[0059] The method for preparing fixed sulfur-doped biomass hard carbon according to some embodiments of the present invention includes, but is not limited to, the following beneficial effects: First, it uses biomass raw materials, which have the advantage of low price. It does not require expensive mesophase carbon microspheres (MCMB) or EDOT monomers, and the raw materials are readily available and the production cost is low. Secondly, it eliminates complex processes such as multi-step oxidation, gas-phase sulfidation, and in-situ polymerization coating. For example, some embodiments of the present invention use pretreatment, low-temperature pre-carbonization, high-temperature carbonization, and purification steps, which have the advantages of simple process and good mass production. Third, the thermal decomposition of the double salt (NH4)2Mg(SO4)2·6H2O during the pre-carbonization of biomass is completed, releasing the sulfur source in one go to achieve in-situ sulfur doping, while simultaneously completing sulfur fixation during the high-temperature carbonization process, thus improving the S doping efficiency.

[0060] In summary, the method for preparing fixed sulfur-doped biomass hard carbon according to embodiments of the present invention innovatively introduces magnesium sulfate hexahydrate as both a sulfur source and a sulfur-fixing reagent. By controlling the mass ratio of magnesium sulfate hexahydrate to raw materials and the high-temperature carbonization temperature, sulfur-doped biomass hard carbon with improved capacity, first-efficiency, and rate performance is prepared.

[0061] Other configurations and operations of the sodium-ion battery according to embodiments of the present invention are known to those skilled in the art and will not be described in detail here.

[0062] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example.

[0063] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing fixed sulfur-doped biomass hard carbon, characterized in that, Includes the following steps: The biomass raw material is pretreated, and the pretreated biomass raw material is mixed with (NH4)2Mg(SO4)2·6H2O to obtain a mixture. The mixture is pre-carbonized at a first preset temperature to obtain a pre-carbonized material, wherein (NH4)2Mg(SO4)2·6H2O decomposes at the first preset temperature to generate MgSO4. The pre-carbonized material is heated from room temperature to a second preset temperature for carbonization to obtain sulfur-containing biomass hard carbon. During the heating process, MgSO4 decomposes into magnesium oxide and sulfur-containing gas. Magnesium oxide is reduced with the carbon matrix to generate magnesium-containing vapor, and sulfur-containing gas reacts with the carbon matrix to form sulfur-containing biomass hard carbon.

2. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 1, characterized in that, Preprocessing steps include: The biomass raw material is crushed and passed through a 50-mesh sieve to obtain the undersize material. The undersize material is then mixed with (NH4)2Mg(SO4)2·6H2O to obtain a mixture.

3. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 1 or 2, characterized in that, In the mixture, the biomass raw material and (NH4)2Mg(SO4)2·6H2O are mixed in a mass ratio of 10:1 to 4:

1.

4. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 1, characterized in that, The first preset temperature is 500℃, and the second preset temperature is 1300℃-1500℃.

5. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 4, characterized in that, The mixture is heated to 500°C at a rate of 3°C / min-5°C / min, held at that temperature for 2 hours, and then cooled to room temperature.

6. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 1, characterized in that, It also includes the following steps: The sulfur-containing biomass hard carbon was purified by acid washing with acid solution.

7. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 6, characterized in that, The acid solution is one or more of sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid; and / or, The acid solution has H + The concentration is 1 mol / L-5 mol / L, and the mass ratio of the acid solution to the sulfur-containing biomass hard carbon is 3:

1.

8. The method for preparing fixed sulfur-doped biomass hard carbon according to claim 1, characterized in that, The biomass raw materials are one or more of the following: jujube shells, apricot shells, walnut shells, and coconut shells.

9. A type of fixed sulfur-doped biomass hard carbon, characterized in that, This includes sulfur-containing biomass hard carbon obtained by any of the preparation methods described in claims 1-8.

10. A sodium-ion battery, characterized in that, It includes a negative electrode active material, which comprises the fixed sulfur-doped biomass hard carbon as described in claim 9.

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