A hard carbon derived from gramineous plant straw and a preparation method and application thereof

Abstract

The application discloses a hard carbon derived from gramineous plant straw and a preparation method and application thereof, and relates to the following steps: washing gramineous plant straw, drying the straw, grinding the dried straw to obtain a straw precursor, pyrolyzing the straw precursor in nitrogen or inert gas, and acidizing the pyrolyzed straw precursor in acid to obtain the hard carbon material derived from the gramineous plant straw. The application effectively solves the technical problem that, in the prior art, the preparation of hard carbon with high reversible capacity based on gramineous plant straw needs to rely on complex modification methods such as doping and coating, which leads to complicated process steps, increased preparation cost, and difficulty in adapting to industrialized and large-scale production.

Description

Technical Field

[0001] This invention belongs to the technical field of negative electrode materials in sodium-ion batteries, and relates to a hard carbon derived from gramineous plant straw, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, thanks to their mature technology, have long dominated the consumer energy storage sector. However, the extreme scarcity of global lithium resources has kept their costs high. Sodium-ion batteries (SIBs), with their low cost, hold promise for replacing lithium-ion batteries in large-scale energy storage. The performance of the anode material is one of the core factors determining the electrochemical performance of sodium-ion batteries. Because the radius of sodium ions is much larger than that of lithium ions, graphite anodes, commonly used in lithium-ion batteries, are difficult to effectively store sodium in sodium-ion batteries. Hard carbon materials, with their larger interlayer spacing, richer defects, and microporous structure than graphite, are ideal materials for sodium storage. Biomass-based hard carbon materials offer numerous advantages, including renewable raw materials, low cost, and environmental friendliness, aligning with the concept of sustainable development in today's society, and have become a research hotspot in the field of hard carbon anode materials. Biomass straw, as an agricultural waste, has the characteristics of high carbon content, wide availability, and green environmental protection, making it an ideal carbon source for the preparation of hard carbon materials. Among them, grass straw has a high proportion of cellulose and hemicellulose, and after carbonization, it is easy to form a suitable sodium storage pore structure, making it a preferred raw material for biomass hard carbon. Existing technology has realized the preparation of grass straw-based hard carbon and applied it to the anode of sodium-ion batteries, providing a way for the low-cost development of energy storage materials.

[0003] Currently, achieving high reversible capacity in gramineous straw-based hard carbon generally requires structural manipulation through doping, coating, or other complex modification methods. This not only increases the number of process steps but also raises preparation costs, making it difficult to adapt to industrial-scale production. Therefore, developing a method for preparing gramineous straw-based hard carbon with high reversible capacity without complex modification methods such as doping and carbon coating is of profound significance for optimizing the preparation process of SIBs anode hard carbon materials. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a biomass hard carbon derived from grass straw, its preparation method, and its application. This solves the technical problem that the preparation of high reversible capacity hard carbon based on grass straw in the prior art requires complex modification methods such as doping and coating, resulting in cumbersome process steps, increased preparation costs, and difficulty in adapting to industrial-scale production.

[0005] This invention is achieved through the following technical solution: A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: The straw of grass plants is washed, dried and then ground to obtain the straw precursor; S2: The straw precursor is subjected to pyrolysis treatment in nitrogen or inert gas; S3: The pyrolyzed straw precursor is placed in an acidic solution for acidification treatment to obtain biomass hard carbon derived from the straw of the grass family.

[0006] Preferably, the grass plant is at least one of wheat, millet, corn, and sorghum.

[0007] Preferably, the straw precursor is derived from at least one of the stem and leaves.

[0008] Preferably, the inert gas is argon or helium.

[0009] Preferably, during the pyrolysis treatment, the heating rate is 5 °C / min, the pyrolysis temperature is 1200~1500 °C, and the time is 2~4 h.

[0010] Preferably, the acid solution is at least one selected from hydrofluoric acid, sulfuric acid, nitric acid, and hydrochloric acid; the concentration of the acid solution is not greater than 5 mol / L.

[0011] Preferably, the acidification treatment specifically involves placing the pyrolyzed hard carbon material in an acid solution and stirring it at 20-70°C for 2-4 hours to complete the acidification process.

[0012] Preferably, the mass ratio of the pyrolyzed straw precursor to the volume ratio of the acid solution is (10~30) g:(1~2) mL.

[0013] A type of biomass hard carbon derived from grass straw is prepared by the above method.

[0014] The above-mentioned application of hard carbon derived from grass straw in sodium-ion battery anode materials.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a method for preparing biomass hard carbon materials derived from grass straw. This method, by adjusting the process sequence, particularly placing the acidification step after the pyrolysis step, provides a novel approach for preparing high-performance hard carbon materials. Traditional methods often require additional doping or coating processes to enhance the material's reversible capacity, leading to cumbersome processes, high costs, and difficulty in meeting the demands of large-scale production. The core innovation of this invention lies in first directly constructing the basic framework of hard carbon through pyrolysis, followed by one-step activation and purification using acidification. This process selectively removes impurities remaining from pyrolysis and etches the material surface, thereby constructing a well-developed and abundant microporous and mesoporous structure in situ. These newly generated pore structures directly provide numerous active sites for sodium ion adsorption, significantly improving the material's sodium storage capacity and reversibility, with effects equivalent to or even superior to traditional complex modification methods. Therefore, this method eliminates the need for additional modification steps; effective control of the material structure can be achieved through simple acidification, greatly simplifying the process and reducing production costs. Meanwhile, the process is simple, the conditions are mild, and the raw materials used are inexpensive and readily available gramineous straw. The whole process is environmentally friendly and very easy to scale up and produce continuously on existing industrial equipment, providing a practical and feasible technical path for the low-cost and large-scale preparation of high reversible capacity hard carbon anode materials.

[0016] Furthermore, the grass species mentioned are at least one of wheat, millet, corn, and sorghum. These specific straws serve as precursors, and their naturally porous structure and abundant carbon source provide a good structural basis for the subsequent preparation of hard carbon materials with excellent sodium storage performance.

[0017] Furthermore, the inert gas is argon or helium. Argon, as a protective gas, has stable chemical properties and can effectively isolate oxygen, preventing the straw precursor from being oxidized and burned during high-temperature pyrolysis, thus ensuring the smooth progress of the carbonization reaction.

[0018] Furthermore, during the pyrolysis treatment, the heating rate is 5 °C / min, the pyrolysis temperature is 1200~1500 °C, and the time is 2~4 h. This temperature control program is conducive to the full carbonization of straw organic matter and the orderly arrangement of graphite microcrystals. The specific heating rate can avoid structural cracking or excessive defects caused by excessive heating; while the high temperature of 1200~1500 °C is sufficient to remove volatiles and form a stable hard carbon skeleton. Excessively high pyrolysis temperature will cause the hard carbon to be more graphitized, reduce the interlayer spacing, and reduce the hard carbon capacity. Excessively low pyrolysis temperature will cause the hard carbon to be insufficiently pyrolyzed, resulting in low structural strength and reduced material cycle performance. This temperature range avoids excessive graphitization caused by excessive temperature, thereby preserving the disordered structure and micropores that are conducive to sodium ion insertion, ensuring that the material has high reversible capacity.

[0019] Furthermore, the acid solution is at least one of hydrofluoric acid, sulfuric acid, nitric acid, and hydrochloric acid. These inorganic acids have strong corrosiveness and reactivity, and can effectively dissolve impurities generated during pyrolysis, playing a role in deep purification and chemical etching, thereby constructing a rich pore structure on the surface and inside of the material, increasing the specific surface area and active sites.

[0020] Furthermore, the concentration of the acid solution is no greater than 5 mol / L. This concentration ensures the acidification effect while effectively reducing the corrosive requirements on the equipment and improving operational safety. The lower acid concentration is sufficient to remove impurities and perform moderate etching, avoiding excessive corrosion, material structure collapse, or the generation of excessive harmful byproducts caused by excessively high concentrations (such as concentrated acid), which is beneficial for environmental control and cost savings.

[0021] Furthermore, the acidification treatment specifically involves placing the pyrolyzed hard carbon material in an acid solution and stirring it at 20-70°C for 2-4 hours to complete the acidification process. These mild reaction conditions are sufficient to activate the etching ability of the acid solution, ensuring a uniform reaction and avoiding safety hazards caused by violent reactions. Under these conditions, the acid solution can fully penetrate into the interior of the hard carbon material, gradually dissolving impurities and forming a uniform pore structure, ensuring a consistent and stable acidification effect, which is beneficial for obtaining products with consistent performance.

[0022] Furthermore, the mass ratio of the pyrolyzed straw precursor to the volume ratio of the acid solution is (10~30) g:(1~2) mL. This ratio ensures that the acid solution can fully wet and contact the surface of the hard carbon material, avoiding incomplete reaction or excessive local concentration due to insufficient acid. A suitable solid-liquid ratio is beneficial to improving acidification efficiency, ensuring uniform treatment of the material, and avoiding acid waste, which meets the requirements of raw material utilization rate in green chemistry and industrial production. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic flowchart of a method for preparing a biomass hard carbon material derived from grass straw according to the present invention. Figure 2 XPS images of samples prepared in Examples 1, 2, 3 and Comparative Example 1 of the present invention; Figure 3The images show the SEM images and EDS spectra of the hard carbon materials prepared in Example 2 and Comparative Example 1 at different magnifications. Figure 4 The nitrogen adsorption / desorption curves (a) and pore size distribution diagrams (b) of the hard carbon materials prepared in Examples 1, 2, 3 and Comparative Example 1 of this invention are shown. Figure 5 The charge-discharge curves of the hard carbon materials prepared in Examples 1, 2, 3 and Comparative Example 1 of this invention are shown below. Figure 6 The following are the rate-up curves of the hard carbon materials prepared in Examples 1, 2, 3 and Comparative Example 1 of the present invention; Figure 7 The cycling curves of the hard carbon materials prepared in Examples 1, 2, 3 and Comparative Example 1 of the present invention are shown. Figure 8 This is a comparison chart of the capacity of hard carbon materials prepared in Examples 1, 2, 3 and Comparative Example 1 of the present invention; Figure 9 The thermal decomposition characteristics of the leaves (HAMSC1400-L) and stems (HAMSC1400-S) of straw in this invention are shown, where (a) is the thermogravimetric curve and (b) is the DTG curve. Figure 10 The BET curves and pore size distribution curves of the straw stalks in this invention after being subjected to deep acid treatment (HAMSC1400-S) and mild acid treatment (LAMSC1400-S), respectively. Figure 11 The images show the charge-discharge capacity curves (a) and ICE comparison (b) of straw stalks after deep acid treatment (HAMSC1400-S) and mild acid treatment (LAMSC1400-S) in this invention. Figure 12 The figures show the cyclic curves of straw stalks after deep acid treatment (HAMSC1400-S) and mild acid treatment (LAMSC1400-S) in this invention, where (a) represents 30 cycles and (b) represents 100 cycles. Detailed Implementation

[0025] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0026] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0027] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0028] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0029] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0030] like Figure 1 As shown, this invention provides a method for preparing biomass hard carbon derived from grass straw, comprising the following steps: S1: Pretreatment of precursor materials: The straw of the grass family was cleaned with an ultrasonic cleaner, dried and then ground in a pulverizer to obtain straw precursors.

[0031] The grass species mentioned are at least one of wheat, millet, corn, and sorghum.

[0032] The grass species mentioned is millet.

[0033] The straw precursor is derived from at least one of the stems and leaves of grass straw; preferably, the straw precursor is derived from the stems of grass straw.

[0034] The straw of grass plants was cleaned using an ultrasonic cleaner. Specifically, the precursor was cleaned alternately with purified water and ethanol for 60-90 minutes each time, until the washing solution no longer had a noticeable color.

[0035] After cleaning, the drying temperature is 50~60 ℃ and the time is 18~24 h.

[0036] S2: Pyrolysis of precursor materials: Pyrolysis is carried out in a tube furnace under the protection of an inert gas atmosphere. After pyrolysis, the ash within the material migrates to the material surface. The formation process may involve silicon elements in the precursor transforming into a molten state under high-temperature pyrolysis, and then gradually migrating to the material surface under the drive of surface tension, accumulating in large quantities on the material surface. As the pyrolysis process ends and the system gradually cools, the molten ash in the molten state agglomerates into spherical shapes driven by the minimization of surface energy. During the pyrolysis treatment, the heating rate is 5 °C / min, the pyrolysis temperature is 1200~1500 °C, and the time is 2~4 h; preferably, the pyrolysis temperature is 1400 °C, at which the hard carbon material prepared has the optimal interlayer spacing.

[0037] The inert gas is argon or helium; S3: Acidification of the precursor material: The material was acidified with acid to remove surface ash, achieving simultaneous hard carbon impurity removal and surface structure regulation. After washing to pH 7, the material was dried to obtain the biomass hard carbon material derived from the straw of the gramineous plant. Acidification of the material removed surface ash; during the acidification process, spherical silica-containing ash on the material surface was removed, leaving numerous nanoscale pore structures in the positions originally occupied by spherical nanoparticles. This achieved simultaneous hard carbon impurity removal and surface structure regulation.

[0038] The acid solution is at least one selected from hydrofluoric acid, sulfuric acid, nitric acid, and hydrochloric acid. Hydrofluoric acid and hydrochloric acid can remove metal oxides and silicate impurities from hard carbon and can etch the material surface.

[0039] The acidification process specifically involves placing the pyrolyzed hard carbon material in an acid solution and treating it under heating and stirring conditions at 20-70 °C for 2-4 hours to complete the acidification process. The mass ratio of the pyrolyzed straw precursor to the volume ratio of the acid solution is (10~30) g:(1~2) mL.

[0040] The concentration of the acid solution is no greater than 5 mol / L.

[0041] Grasses are widely cultivated in many countries and regions around the world. Every year, a large amount of straw is generated as agricultural waste. Traditional methods of straw disposal, such as burning or direct disposal, cause serious environmental pollution. Grass straw possesses a unique biological structure, with its natural multi-level pores and composition containing cellulose, hemicellulose, and lignin, providing a good foundation for preparing hard carbon materials with rich pore structure and suitable interlayer spacing. This invention uses grass straw as a precursor to prepare biomass hard carbon through high-temperature pyrolysis, and then removes impurities and improves its performance through a one-step acidification process. This avoids complex modification methods such as doping or carbon coating, achieving simultaneous impurity removal and surface structure regulation. Specifically, this invention discloses a grass straw-derived biomass hard carbon and its preparation method. This method utilizes the ash in the straw precursor to migrate to the material surface and agglomerate into spheres during high-temperature pyrolysis, which is then removed through a one-step acidification process, leaving pores in the occupied locations on the material surface. The acidification treatment effectively removes impurities from the material and improves its electrochemical performance. The porous structure created while removing ash can further enhance the material's capacity.

[0042] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0043] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0044] Example 1 A method for preparing biomass hard carbon derived from grass straw includes the following steps: (1) Precursor treatment: First, a certain amount of straw was cut into small pieces less than 3 cm. It was then cleaned alternately with purified water and ethanol in an ultrasonic cleaner until the washing liquid no longer had obvious color. Each cleaning time was 90 min. The cleaned straw pieces were placed in an electric heating drying oven and dried at 60 ℃ for 24 h. Then, they were ground in a pulverizer to obtain millet straw precursor.

[0045] (2) Pyrolysis of precursor: 3 g of millet straw precursor was weighed using an analytical balance. The weighed precursor was placed in an alumina ceramic boat and carbonized in a tube furnace filled with argon gas at a heating rate of 5 °C / min. The carbonization temperature was 1400 °C and the pyrolysis holding time was 2 h.

[0046] (3) Acidification of hard carbon: The pyrolyzed hard carbon material was acidified for 4 h with 5 mol / L hydrofluoric acid under heating and stirring at 65 ℃. After acidification, the material was ultrasonically washed with ultrapure water until the pH value was 7, and then centrifuged multiple times. After each centrifugation, the supernatant was taken out and tested for chloride ions with silver nitrate solution acidified with 0.1 M dilute nitric acid. Centrifugation was stopped when no obvious white precipitate was formed. The corresponding hard carbon material was labeled FMSC1400 after drying.

[0047] Example 2 A method for preparing biomass hard carbon derived from grass straw includes the following steps: (1) Precursor treatment: First, a certain amount of straw was cut into small pieces less than 3 cm. It was then cleaned alternately with purified water and ethanol in an ultrasonic cleaner until the washing liquid no longer had obvious color. Each cleaning time was 90 min. The cleaned straw pieces were placed in an electric heating drying oven and dried at 60 ℃ for 24 h. Then, they were ground in a pulverizer to obtain millet straw precursor.

[0048] (2) Pyrolysis of precursor: 3 g of millet straw precursor was weighed using an analytical balance. The weighed precursor was placed in an alumina ceramic boat and carbonized in a tube furnace filled with argon gas at a heating rate of 5 °C / min. The carbonization temperature was 1400 °C and the pyrolysis holding time was 2 h.

[0049] (3) Acidification of hard carbon: The pyrolyzed hard carbon material was acidified for 3 h under heating and stirring at 65 ℃ with 2.5 mol / L hydrofluoric acid and 0.33 mol / L hydrochloric acid. After acidification, the material was ultrasonically washed with ultrapure water until the pH value was 7, and then centrifuged multiple times. After each centrifugation, the supernatant was taken out and tested for chloride ions with silver nitrate solution acidified with 0.1 M dilute nitric acid. Centrifugation was stopped when no obvious white precipitate was formed. The corresponding hard carbon material was labeled AMSC1400 after drying.

[0050] Example 3 A method for preparing biomass hard carbon derived from grass straw includes the following steps: (1) Precursor treatment: First, the stalks were separated from a certain amount of straw and cut into small segments less than 3 cm. They were then washed alternately with purified water and ethanol in an ultrasonic cleaner until the washing liquid no longer had a noticeable color. Each washing session lasted 90 min. The washed straw segments were then placed in an electric heating drying oven and dried at 60 ℃ for 24 h. After that, they were ground in a pulverizer to obtain millet straw precursor.

[0051] (2) Pyrolysis of precursor: 3 g of millet straw precursor was weighed using an analytical balance. The weighed precursor was placed in an alumina ceramic boat and carbonized in a tube furnace filled with argon gas at a heating rate of 5 °C / min. The carbonization temperature was 1400 °C and the pyrolysis holding time was 2 h.

[0052] (3) Acidification of hard carbon: The pyrolyzed hard carbon material was acidified by stirring at 20°C for 3 h with 1 mol / L hydrofluoric acid and 0.1 mol / L hydrochloric acid. After acidification, the material was ultrasonically washed with ultrapure water until the pH value was 7, and then centrifuged multiple times. After each centrifugation, the supernatant was taken out and tested for chloride ions with silver nitrate solution acidified with 0.1 M dilute nitric acid. Centrifugation was stopped when no obvious white precipitate was formed. The corresponding hard carbon material was labeled as LAMSC1400-S after drying.

[0053] Comparative Example 1 The difference from Example 1 is that the hard carbon in this comparative example is not treated with acidification after pyrolysis, and the resulting hard carbon is denoted as MSC1400.

[0054] Example 4 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: Clean the straw fragments alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has obvious color. Each cleaning time is 60 minutes. Place the cleaned straw fragments into an electric heating drying oven and dry them at 50 ℃ for 24 h. Then put them into a pulverizer for grinding to obtain the straw precursor.

[0055] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1200 ℃ for 4 h. S3: The pyrolyzed hard carbon material was placed in 5 mol / L sulfuric acid and treated with heating and stirring at 20 °C for 4 h to complete the acidification process, remove surface ash, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the Gramineae family. The mass ratio of the pyrolyzed straw precursor to the volume of the acid solution was 10 g: 1 mL.

[0056] Example 5 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: Clean the straw fragments alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has obvious color. Each cleaning time is 90 minutes. Place the cleaned straw fragments into an electric heating drying oven and dry them at 60 ℃ for 18 h. Then put them into a pulverizer for grinding to obtain the straw precursor.

[0057] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1500 ℃ for 2 h. S3: The pyrolyzed hard carbon material was placed in 4 mol / L nitric acid and treated with heating and stirring at 70 °C for 2 h to complete the acidification process, remove surface ash, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the Gramineae family. The mass ratio of the pyrolyzed straw precursor to the volume of nitric acid was 30 g: 2 mL.

[0058] Example 6 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: Clean the straw fragments alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has obvious color. Each cleaning time is 80 minutes. Place the cleaned straw fragments into an electric heating drying oven and dry them at 55 ℃ for 20 h. Then put them into a pulverizer for grinding to obtain the straw precursor.

[0059] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1300 ℃ for 3 h. S3: The pyrolyzed hard carbon material was placed in 5 mol / L hydrochloric acid and treated at 50 °C with stirring for 3 h to complete the acidification process, remove surface ash, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the Gramineae family. The mass ratio of the pyrolyzed straw precursor to the volume ratio of hydrochloric acid was 20 g: 1.5 mL.

[0060] Example 7 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: First, the stalks are separated from a certain amount of straw and cut into small segments less than 3 cm. They are then cleaned alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has a noticeable color. Each cleaning session lasts for 80 minutes. The cleaned straw segments are then placed in an electric heating drying oven and dried at 55 ℃ for 20 h. Subsequently, they are ground in a pulverizer to obtain the straw precursor.

[0061] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1400 ℃ for 3 h. S3: The pyrolyzed hard carbon material was placed in a solution of 5 mol / L hydrofluoric acid and 0.5 mol / L hydrochloric acid, and treated at 65 ℃ with stirring for 6 h to complete the acidification process, remove ash from the material surface, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the gramineous plant. The mass ratio of the pyrolyzed straw precursor to the volume ratio of hydrochloric acid was 20 g: 1.5 mL. The corresponding hard carbon material obtained after drying was labeled HAMSC1400-S.

[0062] Example 8 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: First, the leaf parts are sorted out from a certain amount of straw and cut into small pieces less than 3 cm. They are then cleaned alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has obvious color. Each cleaning time is 80 minutes. The cleaned straw pieces are then placed in an electric heating drying oven and dried at 55 ℃ for 20 h. After that, they are put into a pulverizer for grinding to obtain the straw precursor.

[0063] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1400 ℃ for 3 h. S3: The pyrolyzed hard carbon material was placed in a solution of 5 mol / L hydrofluoric acid and 0.5 mol / L hydrochloric acid, and treated at 65 ℃ with stirring for 6 h to complete the acidification process, remove surface ash, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the Gramineae family. The mass ratio of the pyrolyzed straw precursor to the volume ratio of hydrochloric acid was 20 g: 1.5 mL. The corresponding hard carbon material obtained after drying was labeled HAMSC1400-L.

[0064] Example 9 A method for preparing biomass hard carbon derived from grass straw includes the following steps: S1: Clean the straw fragments alternately with pure water and ethanol in an ultrasonic cleaner until the washing liquid no longer has obvious color. Each cleaning time is 60 minutes. Place the cleaned straw fragments into an electric heating drying oven and dry them at 50 ℃ for 24 h. Then put them into a pulverizer for grinding to obtain the straw precursor.

[0065] S2: Place the straw precursor in a nitrogen or inert atmosphere and pyrolyze it at 1000 ℃ for 4 h. S3: The pyrolyzed hard carbon material was placed in 5 mol / L hydrochloric acid and treated at 65 °C with stirring for 4 h to complete the acidification process, remove surface ash, and achieve simultaneous treatment of hard carbon impurity removal and surface structure regulation. After washing to pH 7, it was dried to obtain the biomass hard carbon material derived from the straw of the Gramineae family. The mass ratio of the pyrolyzed straw precursor to the volume of the acid solution was 10 g: 1 mL.

[0066] To verify the performance of the straw-derived biomass hard carbon in this invention, untreated hard carbon and acidified hard carbon from this invention were assembled into batteries, and their performance was tested as follows: The manufacturing process of the electrode sheet in this invention is as follows: The prepared hard carbon material, conductive agent Ketjen black (KB), and binder polyvinylidene fluoride (PVDF) were thoroughly mixed and ground in a mass ratio of 8:1:1. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added dropwise, and the slurry was coated onto aluminum foil using a 20 μm thick doctor blade. The foil was dried in a vacuum oven at 55 °C for 12 h. Finally, the aluminum foil was cut using a tablet press to form an electrode sheet with a diameter of 14 mm. The battery was then assembled in a glove box with a water oxygen value of less than 0.1 ppm for subsequent testing.

[0067] The battery assembly process in this invention is as follows: The battery is assembled in the following order: negative electrode shell, spring, gasket, sodium plate, separator, electrolyte, electrode plates, and positive electrode shell. The separator is made of glass fiber. The electrolyte formula is dimethyl carbonate (DMC) and ethylene carbonate (EC) in a volume ratio of 1:1. Finally, the battery is pressed tightly with a battery packaging machine at a pressure of 1.25. The battery is then left to stand for 12 hours before constant current charge and discharge testing.

[0068] Test 1: Structural Morphology Test of Hard Carbon Materials (1) XPS test of hard carbon materials XPS testing can characterize the elemental composition of a sample. XPS testing was performed on Examples 1, 2, 3, and Comparative Example 1, and the test results are shown below. Figure 2 As shown in the figure, the characteristic Ca 2p peak located near the binding energy of approximately 347 eV can be clearly observed in the spectrum of the unacidified MSC1400 sample, indicating the presence of calcium impurities in the raw material. However, in the spectra of FMSC1400, AMSC1400, and LAMSC1400-S samples treated with different acidification methods, the Ca 2p peak was significantly weakened or even completely disappeared, which strongly proves that acidification treatment can effectively dissolve and remove calcium impurities.

[0069] (2) SEM testing of hard carbon materials The surface morphology of hard carbon materials can directly reflect the effect of acidification on the material structure. SEM tests were performed on the hard carbon materials prepared in Example 2 and Comparative Example 1, and the test results are shown below. Figure 3 As shown in the figure, the surface of the un-acidified MSC1400 sample is relatively dense, with tightly packed particles and indistinct pore structure. The silicon element distribution in the elemental distribution map is relatively uniform but locally enriched, indicating the presence of numerous aggregated phases in the original material. However, the AMSC1400 sample treated with acid in Example 2 exhibits a significant change in surface morphology: clearer particle boundaries, increased surface roughness, and the appearance of numerous uniformly distributed micropores and mesopores with a richer and more controllable pore size distribution. This demonstrates that the acid treatment effectively etched away some impurity phases on the material surface, thereby exposing more pore structures and increasing the specific surface area and porosity of the material. Simultaneously, the silicon elemental distribution map of AMSC1400 shows a more diffuse distribution, indicating that the acid treatment removed some silicon-containing impurities or altered the silicon's occurrence state, further verifying the dual role of acid treatment in impurity removal and structure regulation. Therefore, by adjusting the acidification conditions, the pore size distribution, surface roughness, and elemental distribution of hard carbon materials can be effectively controlled, thereby optimizing the ion transport channels and active site exposure when used as electrode materials, providing a structural basis for improving electrochemical performance.

[0070] (3) Nitrogen adsorption / desorption test of hard carbon materials The pore structure of hard carbon materials has a decisive influence on their electrochemical performance. While a well-developed pore structure can provide abundant sodium ion adsorption active sites and sodium storage micropores, thereby increasing the material capacity, an excessively high specific surface area often leads to a decrease in the initial coulombic efficiency (ICE), negatively impacting cycle stability. In this invention, the BET specific surface area and pore size distribution of the hard carbon materials prepared in Examples 1-3 and Comparative Example 1 were tested, and the results are as follows: Figure 4 As shown. Figure 4(a) shows the nitrogen adsorption-desorption isotherms. As can be seen from the figure, the adsorption capacity of MSC1400 (unacidified sample) is relatively low, and the curve is flat, indicating that its pore structure is not well-developed. However, the adsorption capacity of samples treated with different acidification methods (FMSC1400, AMSC1400, LAMSC1400-S) increases sharply when the relative pressure P / P0 is close to 1.0, showing typical type IV isotherm characteristics accompanied by type H4 hysteresis loops, indicating that there is a rich mesoporous structure in the material. Among them, AMSC1400 has the highest adsorption capacity, indicating that it has the largest specific surface area and the most developed pores. Figure 4 (b) shows the pore size distribution curves. As can be seen from the figure, MSC1400 only has a weak peak in the 2-5 nm range, while the acidified sample shows a significant main peak in the 3-5 nm range. Furthermore, AMSC1400 exhibits the highest peak intensity and a more concentrated distribution, indicating that the acidification treatment effectively constructs a uniform mesoporous structure. Although FMSC1400 and LAMSC1400-S also exhibit mesoporous characteristics, their peak intensities are weaker, indicating that their pore development is lower than that of AMSC1400. This demonstrates that different acidification methods have varying abilities to regulate pore structure. The acidification conditions in Example 2 (AMSC1400) are most conducive to forming a high specific surface area and a uniform mesoporous distribution. However, considering performance, an excessively high specific surface area may exacerbate the formation of the solid electrolyte interphase (SEI) film, thereby reducing the electrochemical efficiency (ICE). Therefore, by adjusting the acidification process parameters, precise control of the pore structure of hard carbon materials can be achieved, striking a balance between increasing sodium storage capacity and maintaining a high initial coulombic efficiency. This provides an effective way to optimize the overall electrochemical performance of hard carbon anode materials.

[0071] Test 2: Assembly and Performance Testing of Sodium-ion Batteries To evaluate the battery performance of the hard carbon anode materials prepared in Examples 1, 2, 3, and Comparative Example 1, the batteries were assembled in the following order: anode shell, spring sheet, gasket, sodium sheet, separator, electrolyte, electrode sheet, and positive electrode shell. The separator was made of glass fiber, and the electrolyte formulation was DMC:EC = 1:1. Finally, the batteries were pressed together with a battery packaging machine at a pressure of 1.25. After the batteries were left to stand for 12 hours, constant current charge-discharge tests were performed.

[0072] (1) Constant current charge and discharge test The present invention conducted constant current charge-discharge tests on Examples 1, 2, 3 and Comparative Example 1 to test the reversible specific capacity and coulombic efficiency of the materials.

[0073] (2) Ratio performance test The present invention compared Example 1, Example 2, Example 3, and Comparative Example 1 at 20 mA g. -1 50 mA g -1 100mA g -1200 mA g -1 500 mA g -1 Rate testing was conducted at a current density to characterize the rate performance of the material.

[0074] (3) Cyclic performance test The present invention applies to Examples 1, 2, 3, and Comparative Example 1 at 20 mA g. -1 Cyclic tests were conducted at current densities to characterize the cycling performance of the material under different acidification conditions.

[0075] Figure 5 The figures show the charge-discharge curves of the hard carbon materials prepared in Examples 1, 2, 3, and Comparative Example 1 of this invention. As can be seen from the figures, the specific capacity of the un-acidified MSC1400 sample is approximately 170 mAh·g. - ¹ The charging capacity is low, and the initial coulombic efficiency (ICE) is significantly low, indicating a large irreversible capacity loss. This may be due to the presence of numerous surface functional groups or impurities, leading to excessive SEI film formation. In contrast, the acid-treated FMSC1400, AMSC1400, and LAMSC1400-S samples all exhibited significantly improved reversible capacity and ICE. The LAMSC1400-S sample, in particular, achieved a specific capacity of approximately 400 mAh·g. - ¹, and the ICE is also high, indicating that its structural optimization effect is the best. FMSC1400 and AMSC1400 also showed high capacity, but slightly lower than LAMSC1400-S, and the voltage plateaus differed slightly. From the curve shape, all samples showed a long voltage plateau in the 0.1–1.0 V range, corresponding to the adsorption / intercalation behavior of sodium ions at micropores or defect sites. The plateau of LAMSC1400-S was longer and more stable, indicating that it has more sodium storage active sites and better reaction kinetics. In addition, the second and third curves highly overlapped, indicating that the acid-treated material has good cycle stability and the structure remains stable during charge and discharge. In summary, acid treatment effectively removed impurities, optimized the pore structure and surface chemistry, thereby significantly improving the sodium storage capacity and first coulombic efficiency of hard carbon materials.

[0076] Figure 6 The figures show the rate capability curves of the hard carbon materials prepared in Examples 1, 2, 3, and Comparative Example 1 of this invention. As can be seen from the figures, the un-acidified MSC1400 sample at 20 mA·g... -1 The specific capacity is low at low current densities, and increases further as the current density increases to 500 mA·g. -1The capacity decayed rapidly, exhibiting poor rate performance, indicating limited ion transport kinetics, possibly due to underdeveloped pore structure or high surface impedance. In contrast, samples treated with different acidification methods all showed significantly improved rate performance, with the LAMSC1400-S sample performing best: at 20 mA·g -1 The specific capacity is as high as approximately 400 mAh·g -1 Even at 500 mA·g -1 It can still maintain about 142 mAh·g at high rates -1 The reversible capacity, and when the current density drops back to 20 mA·g -1 The capacity was almost completely recovered, demonstrating excellent structural stability and reversibility. The FMSC1400 and AMSC1400 samples also showed superior rate performance compared to MSC1400, but their overall capacity and high-rate retention were slightly lower than AMSC1400. This performance difference is mainly attributed to the acidification treatment effectively modulating the pore structure of the material. In particular, AMSC1400 may have formed a more uniform mesoporous network and a higher specific surface area, which is beneficial for electrolyte penetration and rapid sodium ion transport. Simultaneously, acidification removed some surface impurities, reduced interfacial impedance, and improved electrode reaction kinetics. Therefore, by optimizing the acidification process, the rate performance of hard carbon materials can be significantly improved.

[0077] Figure 7 The figures show the cycling curves of the hard carbon materials prepared in Examples 1, 2, 3, and Comparative Example 1 of this invention. As can be seen from the figures, the specific capacity of the un-acidified MSC1400 sample continuously decreased during cycling, with an initial capacity of approximately 120 mAh·g. -1 After 100 cycles, the concentration dropped to 106 mAh·g. -1 The following results indicate poor structural stability and severe side reactions, with significant irreversible capacity loss due to continuous SEI film growth. In contrast, the acidified samples all exhibited superior cycling stability. Among them, the LAMSC1400-S sample showed the most outstanding performance, with an initial reversible capacity of approximately 330 mAh·g⁻¹. -1 It remained stable at 320 mAh·g after 100 cycles. -1 The above results demonstrate a capacity retention exceeding 95%, and the coulombic efficiency rapidly increases to over 98% after the second cycle and remains relatively stable, indicating a highly reversible sodium ion insertion / extraction mechanism and a stable interfacial structure. The FMSC1400 and AMSC1400 samples also exhibited good cycling performance, with capacities stabilizing at approximately 200 and 240 mAh·g, respectively. -1The coulombic efficiency remained above 95%, significantly better than MSC1400. This performance difference mainly stems from the fact that acidification treatment effectively removed surface impurities, optimized the pore structure, and enhanced the material's structural compactness, thereby suppressing side reactions and volume expansion. In particular, the acidification process of LAMSC1400-S may have achieved an optimal balance between surface passivation and pore structure, enabling the material to maintain efficient ion transport and structural integrity during long-term cycling. Therefore, acidification treatment is a key means to improve the cycle stability and coulombic efficiency of hard carbon anode materials.

[0078] Figure 8 This is a capacity comparison chart of the hard carbon materials prepared in Examples 1, 2, 3, and Comparative Example 1 of the present invention. As can be seen from the chart, the initial specific capacity of the un-acidified MSC1400 sample is only 160.79 mAh·g. -1 The capacity was significantly lower than that of other acidified samples, indicating insufficient sodium storage active sites or impaired ion transport. After acidification, the capacity of all samples increased significantly, with FMSC1400, AMSC1400, and LAMSC1400-S reaching 268.54, 314.73, and 399.93 mAh·g, respectively. -1 The sodium storage capacity of hard carbon materials gradually increases with the optimization of the acidification process. In particular, the LAMSC1400-S sample exhibits the highest reversible capacity, indicating that its acidification conditions are most favorable for constructing abundant sodium storage micropores and defect structures. Meanwhile, the line graph shows that the initial coulombic efficiency also significantly improves with increasing capacity: MSC1400's efficiency is only 20.26%, indicating severe irreversible side reactions; while the efficiency of the acidified samples increases sequentially, with LAMSC1400-S reaching 42.27%. Although still below the ideal value, this represents a significant improvement, indicating that acidification effectively removes surface functional groups and impurities, reducing excessive SEI film formation. It is noteworthy that although LAMSC1400-S has the highest capacity, its coulombic efficiency remains relatively low, possibly related to its excessively high specific surface area leading to increased interfacial side reactions. In summary, acidification significantly improves the sodium storage capacity and reaction reversibility of hard carbon materials by controlling the pore structure and surface chemical state.

[0079] Figure 9This invention illustrates the thermal decomposition characteristics of different parts of the straw. HAMSC1400-L represents the leaves (product of Example 8), and HAMSC1400-S represents the stems (product of Example 7). Figure (a) shows the thermogravimetric curve, and (b) shows the DTG curve. The thermogravimetric curve can be divided into four distinct stages. The white portion corresponds to the low-temperature drying stage, with temperatures ranging from 0 to 240 °C. The mass loss in this stage is primarily due to moisture evaporation. The mass losses of the leaves and stems in this stage are 5.38% and 8.12%, respectively. The stems lose more mass in this stage because the straw structure is more porous than the leaves, making it easier to dehydrate during the pre-treatment drying stage. In contrast, the stem structure is more compact, resulting in more residual moisture after pre-treatment. The second stage, with a temperature range of 240-320 °C (corresponding to the light green portion in the figure), involves the decomposition of hemicellulose at this temperature. The third stage, with a temperature range of 320-385 ℃ (corresponding to the light blue area in the figure), involves significant mass loss due to cellulose decomposition. The final stage, above 385 ℃, primarily results in lignin decomposition due to mass loss. Furthermore, as shown in Figure (b), the rapid weight loss between 240-320 ℃ is related to hemicellulose decomposition. Both materials exhibit a sharp weight loss peak within the 320-385 ℃ range, corresponding to rapid cellulose decomposition. When the pyrolysis temperature exceeds 385 ℃, the precursor decomposition rate slows significantly. This is because lignin's thermal stability is far superior to that of cellulose and hemicellulose, resulting in a slower weight loss rate during pyrolysis. Based on a semi-quantitative estimation of the weight loss rate at each stage, the contents of cellulose, hemicellulose, and lignin in the straw leaves and stalks were 22.1%, 43.8%, and 11.1%, and 19.6%, 33.3%, and 12%, respectively. This indicates that the stalks of millet straw contain more lignin, while the leaves are mainly composed of cellulose and hemicellulose. After pyrolysis, the mass residue rates of the leaves and stalks were 21.7% and 32.2%, respectively. The pyrolysis residue mainly consisted of hard carbon, indicating that the stalk portion had a higher carbon yield.

[0080] Figure 10 The figures (a) and (b) show the BET curves and pore size distribution curves of the straw stalks after deep acid treatment (HAMSC1400-S) and mild acid treatment (LAMSC1400-S) respectively, in this invention. HAMSC1400-S represents the sample after deep acid treatment, corresponding to the product of Example 7, while LAMSC1400-S represents the sample after mild acid treatment, corresponding to the product of Example 3. As shown in Figure (a), the isotherm type of the lightly acid-treated material is between that of Type III and Type II isotherms, and it exhibits an H3 hysteresis loop. This indicates that the surface pore structure of LAMSC1400-S is predominantly mesoporous, but the pore structure may be relatively deep. As shown in Figure (b), the number of pores in the lightly acid-treated material is significantly reduced throughout the entire range. This indicates that light acid treatment did not etch any more micropores onto the material surface, and the pore structure of the treated material remains dominated by mesopores.

[0081] Figure 11 The figures show the charge-discharge capacity curves (a) and ICE comparison (b) of straw stalks treated with deep acid (HAMSC1400-S) and mild acid (LAMSC1400-S), respectively. As can be seen from the figures, the reversible capacity and ICE of LAMSC1400-S are significantly higher than those of HAMSC1400-S. To further investigate the reasons for the increased capacity of LAMSC1400-S, the reversible capacity of the material was divided into a plateau region and a ramp region. In LAMSC1400-S, the ramp region accounts for 32.18% of the total capacity, lower than the 43.39% of HAMSC1400-S. This is because LAMSC1400-S has a smaller specific surface area and limited active sites provided by surface defects. However, its ramp region capacity is still slightly higher than that of HAMSC1400-S, mainly due to the higher ICE of LAMSC1400-S. The increased ICE reduces the amount of sodium ions irreversibly consumed in the first week, indirectly increasing the ramp region capacity.

[0082] Figure 12 The figures show the cyclic curves of straw stalks after deep acid treatment (HAMSC1400-S) and mild acid treatment (LAMSC1400-S) in this invention. (a) represents 30 cycles, and (b) represents 100 cycles. As can be seen from the figures, the LAMSC1400-S material at 500 mA g... -1 It still maintains 142 mAh g at current density -1 The reversible capacity indicates that the material treated with mild acid exhibits higher rate performance. The mildly acid-treated hard carbon material retained 96.4% of its capacity after 100 cycles, slightly higher than the 94.65% of HAMSC1400-S. This is mainly because the mild acidification strategy used in LAMSC1400-S is less likely to disrupt the stability of the carbon skeleton within the material. Furthermore, the inert support provided by silicon carbide also enhances the material's cycling stability.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing hard carbon derived from gramineous plant straw, characterized in that, Includes the following steps: S1: The straw of grass plants is washed, dried and then ground to obtain the straw precursor; S2: The straw precursor is subjected to pyrolysis treatment in nitrogen or inert gas; S3: The pyrolyzed straw precursor is placed in an acidic solution for acidification treatment to obtain biomass hard carbon derived from the straw of the grass family.

2. The method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The grass species mentioned are at least one of wheat, millet, corn, and sorghum.

3. The method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The straw precursor is derived from at least one of the stem and leaves.

4. The method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The inert gas is argon or helium.

5. A method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, During the pyrolysis treatment, the heating rate is 5 °C / min, the pyrolysis temperature is 1200~1500 °C, and the time is 2~4 h.

6. A method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The acid solution is at least one of hydrofluoric acid, sulfuric acid, nitric acid, and hydrochloric acid; the concentration of the acid solution is not greater than 5 mol / L.

7. A method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The acidification treatment specifically involves placing the pyrolyzed hard carbon material in an acid solution and stirring it at 20-70°C for 2-4 hours to complete the acidification process.

8. The method for preparing hard carbon derived from gramineous plant straw according to claim 1, characterized in that, The mass ratio of the pyrolyzed straw precursor to the volume ratio of the acid solution is (10~30)g:(1~2)mL.

9. A type of hard carbon derived from grass straw, characterized in that, It is prepared by the method described in any one of claims 1 to 8.

10. The application of the gramineous plant straw-derived biomass hard carbon as described in claim 9 in the anode material of sodium-ion batteries.

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