Biomass-based hard carbon material, and preparation method therefor and use thereof

Abstract

Provided in the present application are a biomass-based hard carbon material, and a preparation method therefor and the use thereof. The preparation method comprises: sequentially performing anaerobic baking, impurity removal, oxidative modification and high-temperature carbonization on a biomass raw material to obtain a biomass-based hard carbon material. In the present application, by means of first sequentially performing anaerobic baking and impurity removal, lignin, cellulose, etc., in the biomass raw material are destroyed, and pores and defects are left inside the biomass raw material, such that the material is in a metastable-state structure to expose impurities; thus, impurity removal can be selected at a normal temperature, and an impurity removal effect is good; and the impurity content of the obtained biomass-based hard carbon is low, the ash content thereof can be reduced to 0.5 wt% or below, and the obtained biomass-based hard carbon has a disordered interlayer structure, thereby facilitating the intercalation / deintercalation of sodium ions, such that the material can show relatively high reversible capacity and initial efficiency performance.

Description

A biomass-based hard carbon material and its preparation method and use Technical Field

[0001] The embodiments of the present application relate to the field of battery technology, for example, a biomass-based hard carbon material and its preparation method and use. Background Art

[0002] As a secondary battery, the electrochemical mechanism of sodium-ion batteries is similar to that of lithium batteries. However, compared with lithium-ion batteries, sodium-ion batteries have the following advantages: abundant reserves of raw materials and low prices; sodium-ion batteries are allowed to discharge to 0V; the battery energy density is greater than 100Wh / kg, which is comparable to lithium iron phosphate batteries. It can be seen that its cost advantage is obvious.

[0003] As a negative electrode material for sodium-ion batteries, hard carbon has more outstanding capacity and stability than graphite commonly used in lithium batteries. This is because the radius of sodium ions (0.103nm) is much larger than that of lithium ions (0.071nm), and the small interlayer spacing of graphite is not conducive to the intercalation and deintercalation of sodium ions. The long-range disorder of hard carbon structure, with many defects and micropores, facilitates the adsorption and storage of sodium ions.

[0004] Hard carbon raw materials are mainly divided into biomass, resin, and coal-based. Resin-based hard carbon has good performance but is relatively expensive. Coal-based hard carbon has difficulty increasing its capacity and relies on coal mining for raw materials. In comparison, biomass-based hard carbon has advantages in development and application due to its wide raw material source, easy structure control, and low cost.

[0005] CN115259136A discloses a method for preparing biomass-based hard carbon materials in large quantities using waste biomass. The method comprises using a specific pretreatment solution to destroy the biomass to extract lignin, pre-carbonizing and high-temperature carbonizing the obtained lignin, acid washing at 40-80°C, and drying to obtain the target hard carbon material.

[0006] CN113044827A discloses a method for preparing a nano-carbon composite biomass hard carbon electrode material. The biomass raw material is cleaned and crushed, then heated with an alkaline solution. After cleaning and drying, it is first heated and stirred in a 1-5 mol / L alkaline solution at 90-120°C, then put into a 1-5 mol / L acid solution and heated and stirred at 90-120°C, and then subjected to microwave hydrothermal reaction, pre-carbonization and high-temperature treatment to obtain the target hard carbon material.

[0007] Currently, the production of hard carbon from biomass raw materials is complex and often results in excessive impurities. Consequently, the common industrial method for removing impurities is high-temperature alkaline and acid washing, as described above. However, this method is costly and requires high-quality equipment. Therefore, a new method for producing biomass-based hard carbon materials is needed.

[0008] Summary of the Invention

[0009] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0010] The embodiment of the present application provides a biomass-based hard carbon material and its preparation method and use. The preparation method sequentially subjects the biomass raw material to anaerobic baking, impurity removal, oxidative modification and high-temperature carbonization to obtain a biomass-based hard carbon material. The present application first performs anaerobic baking and impurity removal in sequence, so that the lignin and cellulose in the biomass raw material are destroyed, leaving pores and defects inside, so that the material is in a metastable structure to expose impurities, and then it is possible to choose to remove impurities at room temperature, and the impurity removal effect is excellent. The obtained biomass-based hard carbon has a low impurity content, the ash content can be reduced to below 0.5wt%, and has a disordered interlayer structure, which is conducive to the insertion / extraction of sodium ions, and can exhibit a higher reversible capacity and first-effect performance.

[0011] In a first aspect, the present invention provides a method for preparing a biomass-based hard carbon material, the method comprising the following steps:

[0012] The biomass raw materials are subjected to oxygen-free baking, impurity removal, oxidation modification and high-temperature carbonization in sequence to obtain biomass-based hard carbon materials.

[0013] The present application first performs anaerobic baking and impurity removal in sequence, so that the lignin and cellulose in the biomass raw materials are destroyed, leaving pores and defects inside, so that the material is in a metastable structure to expose impurities, and then impurities can be removed at room temperature with excellent impurity removal effect. The obtained biomass-based hard carbon has a low impurity content, the ash content can be reduced to below 0.5wt%, and has a disordered interlayer structure, which is conducive to the insertion / extraction of sodium ions, and can exhibit higher reversible capacity and first-effect performance.

[0014] Specifically, in terms of raw material composition, the main components that contribute to hard carbon in biomass raw materials are lignin and part of cellulose. Biomass raw materials, especially fibrous biomass raw materials, are composed of hemicellulose, cellulose, lignin and various impurities. The impurities are all present in the structure of hemicellulose, cellulose and lignin. To remove these impurities, it is necessary to destroy the stable connection between hemicellulose, cellulose and lignin. Therefore, this application first performs anaerobic baking to allow hemicellulose, cellulose and lignin to decompose under certain temperature conditions, and the impurities can be exposed; therefore, subsequent impurity removal can be carried out under room temperature conditions, and further elemental analysis of the main impurities in the biomass can be performed to make the subsequent impurity removal targeted.

[0015] Since biomass raw materials are mainly composed of three major components: hemicellulose, cellulose, and lignin, and usually use cellulose as the skeleton, lignin and hemicellulose are tightly wrapped around the skeleton, it is difficult to break the steady-state structure of biomass raw materials by directly removing impurities and oxidative modification without anaerobic baking; since the decomposition temperatures of hemicellulose, cellulose, and lignin increase in sequence, hemicellulose and part of cellulose can be decomposed at a relatively low temperature. Therefore, anaerobic baking can provide natural pore sites for organic carbon sources on the one hand, and on the other hand, it can destroy the original steady-state structure of biomass, which is beneficial to subsequent impurity removal and oxidative modification;

[0016] After the oxygen-free baking is completed, for the impurity removal process and oxidation modification process at this time, since the material is in a metastable structure, some impurities are exposed, and impurities can be removed at room temperature. The hard carbon precursor after impurity removal is mainly composed of a chain structure of C, H, and O, and is still in an organic state. In this state, oxygen-containing functional groups are directly introduced through oxidation modification as connecting bonds between organic segments. The strength is higher than that of hydrogen bonds, which is conducive to the subsequent structural aromatic ring rearrangement.

[0017] In summary, the preparation method described herein can be used to produce normalized hard carbon materials from different biomass feedstocks. Normalization refers to the process of treating essentially different materials to produce materials with similar structures and properties. As mentioned above, the components of biomass feedstocks are hemicellulose, cellulose, lignin, and impurities. Different biomass feedstocks differ in their impurity composition and the content of these three elements. The biomass-based hard carbons obtained through the preparation method described herein have similar structures and similar properties.

[0018] The following are preferred technical solutions of this application, but are not intended to limit the technical solutions provided in this application. Through the following technical solutions, the technical objectives and beneficial effects of this application can be better achieved and realized.

[0019] As a preferred technical solution of the present application, the biomass raw material includes any one or a combination of at least two of poplar sawdust, pine sawdust, lychee sawdust, coconut shell, bamboo sawdust, cotton or wheat straw.

[0020] As a preferred technical solution of the present application, the anaerobic baking is carried out in an inert atmosphere with an oxygen concentration of <1000 ppm, for example, 990 ppm, 900 ppm, 800 ppm, 700 ppm, 600 ppm, 500 ppm, 400 ppm, 300 ppm, 200 ppm, 100 ppm, 50 ppm, 10 ppm or 0 ppm, where 0 ppm means an oxygen-free atmosphere, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0021] Preferably, the inert atmosphere includes any one of nitrogen, argon or helium, or a combination of at least two of them. Typical but non-limiting examples of the combination include a combination of nitrogen and argon, a combination of nitrogen and helium, or a combination of argon and helium.

[0022] Preferably, the temperature of the oxygen-free baking is 200-1000°C, for example, 200°C, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 900°C or 1000°C, and the time is 15-48h, for example, 15h, 18h, 21h, 24h, 27h, 30h, 33h, 36h, 39h, 42h, 45h or 48h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0023] Preferably, after the oxygen-free baking and before the impurity removal, when the material is cooled to less than 50° C., the material is discharged and subjected to a first crushing to obtain first precursor particles, and the impurity removal is performed on the first precursor particles.

[0024] Preferably, the median particle size of the first precursor particles is 0.5 to 1 mm, for example, 0.5 mm, 0.55 mm, 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.85 mm, 0.9 mm, 0.95 mm or 1 mm, etc., but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0025] As a preferred technical solution of the present application, the impurity removal is liquid phase impurity removal, and the biomass raw material after oxygen-free baking is treated in a solution containing an impurity remover.

[0026] Preferably, the impurity remover comprises any one or a combination of at least two of sodium hydroxide, potassium hydroxide, sulfuric acid, hydrochloric acid, phosphoric acid or hydrofluoric acid. Typical but non-limiting examples of the combination include a combination of sodium hydroxide and potassium hydroxide, a combination of sulfuric acid and hydrochloric acid, a combination of hydrochloric acid and phosphoric acid, a combination of hydrochloric acid and hydrofluoric acid, a combination of phosphoric acid and hydrofluoric acid or a combination of sulfuric acid and hydrofluoric acid.

[0027] Preferably, the solution containing the impurity remover also contains additives.

[0028] Preferably, the additive comprises any one or a combination of at least two of thiourea, urea, disodium edetate, citric acid, aqueous ammonia or sodium gluconate, typical but non-limiting examples of the combination include a combination of thiourea and urea, a combination of thiourea and disodium edetate, a combination of citric acid and disodium edetate, a combination of aqueous ammonia and urea, or a combination of sodium gluconate and citric acid.

[0029] The present application preferably uses a combination of an impurity remover and an additive to produce a synergistic effect, so that the pretreated precursor is further decomposed at room temperature, leaving abundant pores and defects inside the material after further decomposition. At this time, the material is in a metastable structure, and a replacement reaction occurs between the impurities and the impurity remover. The impurities are dissolved in the aqueous solution in a free state, thereby further improving the effect of impurity removal at room temperature.

[0030] It should be noted that the role of the additive is to open the segment structure of the hard carbon precursor and promote the impurity removal effect of the impurity remover; the impurity remover is determined according to the type of impurity elements in the material. The impurity elements can be divided into alkaline metal / non-metal elements and acidic metal / non-metal elements. After testing, the types of impurity metal / non-metal elements with higher content are obtained, so that the impurity remover can be used in a targeted manner.

[0031] Preferably, the liquid phase impurity removal is carried out at a rotation speed of 100 to 500 rpm, for example, 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, 450 rpm or 500 rpm, etc., but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0032] Preferably, the treatment time of the liquid phase impurity removal is 3 to 24 hours, for example, 3 hours, 6 hours, 9 hours, 12 hours, 15 hours, 18 hours, 21 hours or 24 hours, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0033] Preferably, after the impurity removal and before the oxidative modification, washing is first performed until the pH is 7, and then the second crushing is performed after drying.

[0034] Preferably, the drying temperature is 80-120°C, for example, 80°C, 90°C, 100°C, 110°C or 120°C, and the drying time is 8-12h, for example, 8h, 9h, 10h, 11h or 12h, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0035] Preferably, the second crushing obtains second precursor particles, and the second precursor particles are oxidatively modified. The median particle size of the second precursor particles is 4 to 10 μm, for example, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0036] The present application preferably performs coarse crushing (first crushing) after anaerobic baking and fine crushing (second crushing) after impurity removal. By reducing the particle size and coordinating with structural solidification (oxidative modification), the pores and defects can be exposed to the greatest extent. Through oxidative modification, oxygen-containing functional groups are introduced into the material structure as connecting bonds between chain segments. The strength is higher than that of hydrogen bonds, which is conducive to the subsequent structural aromatic ring rearrangement; and in the subsequent high-temperature carbonization process, as the temperature slowly rises, due to the existence of tension stress in the material itself, the material continues to shrink, achieving a "self-repairing" effect, "repairing" the pores on the surface, thereby forming a closed-pore structure.

[0037] As a preferred technical solution of the present application, the oxidation modification method includes oxygen-containing sintering and / or liquid-phase oxidation.

[0038] One of the purposes of the oxidative modification carried out in this application is to introduce oxygen-containing functional groups. When the reaction is sufficient, a portion of the oxygen molecules react with carbon to form oxygen-containing functional groups as active sites, while another portion of the oxygen reacts with part of the carbon to generate CO and / or CO2, thereby further forming pores on the surface and inside of the material. These pores contribute to the storage of sodium ions, thereby improving the electrochemical performance of the material.

[0039] Preferably, the oxygen concentration of the oxygen-containing sintering is 10% to 15%, for example, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5% or 15%, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0040] Preferably, the temperature of the oxygen-containing sintering is 400-600°C, for example, 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, 520°C, 540°C, 560°C, 580°C or 600°C, and the time is 1-5h, for example, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h or 5h, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0041] Preferably, the liquid phase oxidation processes the biomass raw material after impurities removal in a solution containing an oxidant;

[0042] Preferably, the treatment time of the liquid-phase oxidation is 6 to 12 h, for example, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h or 12 h, and the treatment temperature is -3 to 5 ° C, for example, -3 ° C, -2 ° C, -1 ° C, 0 ° C, 1 ° C, 2 ° C, 3 ° C, 4 ° C or 5 ° C, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0043] As a preferred technical solution of the present application, the temperature of the high-temperature carbonization is 1600-1800°C, for example, 1600°C, 1620°C, 1640°C, 1660°C, 1680°C, 1700°C, 1720°C, 1740°C, 1760°C, 1780°C or 1800°C, and the time is 3-20h, for example, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h or 20h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0044] Preferably, the heating rate of the high-temperature carbonization is 0.5 to 3°C / min, for example, 0.5°C / min, 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min or 3°C / min, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0045] Preferably, the high-temperature carbonization is carried out under an inert atmosphere.

[0046] As a preferred technical solution of this application, the preparation method comprises the following steps:

[0047] (1) placing the biomass raw material in an inert atmosphere with no oxygen or an oxygen concentration of less than 1000 ppm in the system, performing oxygen-free baking at 200-1000° C. for 15-48 hours, cooling to less than 50° C., discharging the material and performing a first crushing to obtain millimeter-sized first precursor particles with a median particle size of 0.5-1 mm;

[0048] (2) mixing the first precursor particles with an aqueous solution containing an impurity remover and an additive, stirring at 100-500 rpm for 3-24 hours for liquid phase impurity removal, washing with water until the pH is 7, drying at 80-120° C. for 8-12 hours, and performing a second crushing to obtain micron-sized second precursor particles with a median particle size of 4-10 μm;

[0049] (3) placing the second precursor particles in an inert atmosphere with an oxygen concentration of 10% to 15% in the system, performing oxygen-containing sintering at 400 to 600° C. for 15 to 48 hours, and cooling to room temperature to obtain third precursor particles;

[0050] Alternatively, the second precursor particles are mixed with an aqueous solution containing an oxidant, stirred at -3 to 5° C. for 6 to 12 hours, and dried to obtain third precursor particles;

[0051] (4) Under the protection of inert gas, the temperature is increased at a rate of 0.5 to 3°C / min, and the third precursor particles are subjected to high-temperature carbonization treatment at 1600 to 1800°C for 3 to 20 hours. After cooling to room temperature, a biomass-based hard carbon material is obtained.

[0052] In a second aspect, an embodiment of the present application provides a biomass-based hard carbon material obtained using the preparation method described in the first aspect.

[0053] Preferably, the ash content of the hard carbon material is less than 0.5wt%, for example, 0.45wt%, 0.40wt%, 0.35wt%, 0.30wt%, 0.25wt%, 0.20wt%, 0.15wt%, 0.10wt%, 0.05wt% or 0.01wt%, etc., but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0054] Preferably, the pore size of the hard carbon material is 0.5 to 20 nm, for example, 0.5 nm, 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm or 20 nm, etc., but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0055] Preferably, the true density of the hard carbon material is 1.3 to 2.0 g / cm 3 , for example 1.3 g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 , 1.9g / cm 3 or 2.0g / cm 3 etc., but are not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0056] Preferably, the specific surface area of ​​the hard carbon material is less than 5 g / m 2 , for example 4.8g / m 2 4.5g / m 2 , 4g / m 2 3.5g / m 2 , 3g / m 2 , 2.5g / m 2 , 2g / m 2 , 1.5g / m 2 , 1g / m 2 , 0.5g / m 2 or 0.1g / m 2etc., but are not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0057] In a third aspect, an embodiment of the present application provides a negative electrode plate containing the biomass-based hard carbon material described in the second aspect.

[0058] In a fourth aspect, an embodiment of the present application provides a battery comprising the negative electrode sheet described in the third aspect.

[0059] Compared with the related technical solutions, the embodiments of the present application have at least the following beneficial effects:

[0060] (1) The embodiment of the present application first performs anaerobic baking and impurity removal in sequence, so that the lignin and cellulose in the biomass raw material are destroyed, leaving pores and defects inside, so that the material is in a metastable structure to expose impurities, and then impurities can be removed at room temperature, and the impurity removal effect is excellent. The obtained biomass-based hard carbon has a low impurity content, and the ash content can be reduced to below 0.5wt%. It has a disordered interlayer structure, which is conducive to the insertion / extraction of sodium ions and can exhibit a high reversible capacity and first-effect performance;

[0061] (2) The embodiment of the present application uses a synergistic effect of the impurity remover and the additive, so that the pretreated precursor is further decomposed at room temperature. After further decomposition, abundant pores and defects are left inside the material. At this time, the material is in a metastable structure, and a replacement reaction occurs between the impurities and the impurity remover. The impurities are free and dissolved in the aqueous solution, thereby further improving the effect of room temperature impurity removal;

[0062] (3) In the embodiment of the present application, coarse crushing (first crushing) is performed after anaerobic baking, and fine crushing (second crushing) is performed after impurities are removed. By reducing the particle size and performing structural solidification, the pores and defects can be exposed to the greatest extent. In the subsequent high-temperature carbonization process, as the temperature slowly rises, the material shrinks continuously due to the tension stress of the material itself, achieving a "self-repairing" effect, "repairing" the pores on the surface, thereby forming a closed-pore structure;

[0063] (4) The embodiments of the present application introduce oxygen-containing functional groups through oxidation modification. When the reaction is sufficient, a portion of the oxygen molecules react with carbon to form oxygen-containing functional groups as active sites, while another portion of the oxygen reacts with part of the carbon to generate CO and / or CO2, thereby further forming pores on the surface and inside of the material. The pores contribute to the storage of sodium ions, thereby improving the electrochemical performance of the material.

[0064] (5) The preparation method described in the examples of this application is widely applicable to biomass raw materials and has low cost, which has practical significance for the industrial application of hard carbon negative electrode materials.

[0065] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0067] FIG1 is a SEM image of the first precursor particles obtained in Example 1;

[0068] FIG2 is a SEM image of the biomass-based hard carbon material obtained in Example 1;

[0069] FIG3 is a charge and discharge curve diagram of the biomass-based hard carbon material obtained in Example 1;

[0070] FIG4 is an XRD pattern of the biomass-based hard carbon material obtained in Example 1. DETAILED DESCRIPTION

[0071] The technical solution of this application is further explained below through specific implementation methods.

[0072] Those skilled in the art should understand that the embodiments are only intended to help understand the present application and should not be regarded as specific limitations of the present application.

[0073] Example 1

[0074] This embodiment provides a biomass-based hard carbon material, which is prepared by the following method:

[0075] (1) 1 kg of bamboo chips were placed in a nitrogen atmosphere with an oxygen concentration of less than 200 ppm, heated to 400°C, treated for 12 hours, cooled to below 50°C, and crushed to a median particle size of 1 mm using a jaw crusher to obtain first precursor particles;

[0076] (2) The first precursor particles obtained in step (1) were placed in a stirred tank, and sodium hydroxide, sodium gluconate, and water were added in a mass ratio of 22:4:74, stirred at room temperature for 3 hours, washed until neutral, dried, and crushed by air jet mill to a median particle size of 5 μm to obtain second precursor particles;

[0077] (3) The second precursor particles of step (2) are placed in a nitrogen atmosphere with the oxygen concentration controlled at 8%, and the temperature is rapidly raised to 800°C within 5°C / min, and the treatment is carried out for 6 hours to introduce oxygen-containing functional groups to obtain third precursor particles. (4) Under argon conditions, the third precursor particles obtained in step (3) are subjected to high-temperature closed-pore treatment, the temperature is raised to 1700°C at 2°C / min, kept warm for 8 hours, and cooled to room temperature to obtain a biomass-based hard carbon material with an internal porous structure.

[0078] Figures 1 and 2 are SEM images of the first precursor particles obtained in Example 1 and the biomass-based hard carbon material obtained after sintering in the high-temperature stage, respectively. It can be seen from the figure that the surface of the first precursor particles contains a large amount of particulate impurities. The surface of the biomass-based hard carbon material obtained after de-impurity carbonization is smooth, and the particle size is significantly reduced after carbonization. Figure 3 is a charge and discharge curve of the biomass-based hard carbon material obtained in Example 1. It can be seen from the figure that the electrochemical properties of the biomass-based hard carbon are: 334.43mAh / g for the first charge and 88.65% for the first charge. Figure 4 is an XRD diagram of the biomass-based hard carbon material obtained in Example 1. It can be seen from the figure that the hard carbon material has no impurities, and the lattice spacing is about 3.8nm by calculation.

[0079] Example 2

[0080] This embodiment provides a biomass-based hard carbon material, which is prepared by the following method:

[0081] (1) 1 kg of pine sawdust was placed in a nitrogen atmosphere with an oxygen concentration of less than 200 ppm, heated to 250°C, treated for 48 hours, cooled to below 50°C, and crushed to a median particle size of 1 mm using a jaw crusher to obtain first precursor particles;

[0082] (2) The first precursor particles obtained in step (1) were placed in a stirred tank, and sodium hydroxide, thiourea, and water were added in a mass ratio of 12:7:81 in sequence, stirred at room temperature for 3 h, washed until neutral, dried, and crushed by air jet mill to a median particle size of 5 μm to obtain second precursor particles;

[0083] (3) The second precursor particles of step (2) are placed in a nitrogen atmosphere with the oxygen concentration controlled at 8%, and the temperature is rapidly raised to 800°C within 5°C / min, and treated for 6 hours to introduce oxygen-containing functional groups to obtain third precursor particles. (4) Under argon conditions, the third precursor particles obtained in step (3) are subjected to high-temperature closed-pore treatment, the temperature is raised to 1700°C at 2°C / min, kept warm for 8 hours, and cooled to room temperature to obtain a biomass-based hard carbon material with an internal porous structure.

[0084] Example 3

[0085] This embodiment provides a biomass-based hard carbon material, which is prepared by the following method:

[0086] (1) 1 kg of poplar sawdust was placed in a nitrogen atmosphere with an oxygen concentration of less than 200 ppm, heated to 600°C, treated for 28 hours, cooled to below 50°C, and crushed to a median particle size of 1 mm using a jaw crusher to obtain first precursor particles;

[0087] (2) placing the first precursor particles obtained in step (1) in a stirred tank containing 20% ​​HF, stirring at room temperature for 3 h, washing until neutral, drying, and crushing by jet milling to a median particle size of 5 μm to obtain second precursor particles;

[0088] (3) The second precursor particles of step (2) are placed in 500 mL of a mixed solution of phosphoric acid and sulfuric acid with a molar ratio of 3:1, stirred at a temperature of 0-3°C with a stirrer, and 3 g of potassium permanganate is added every two minutes, a total of 60 g is added in batches, and stirred for 6 hours. Finally, 100 mL of hydrogen peroxide is added and stirred for 12 hours. The particles are washed until neutral, oxygen-containing functional groups are introduced, and dried to obtain third precursor particles.

[0089] (4) Under argon conditions, the third precursor particles obtained in step (3) are subjected to high-temperature closed-pore treatment, the temperature is raised to 1600°C at 2°C / min, kept at this temperature for 8 hours, and then cooled to room temperature to obtain a biomass-based hard carbon material with an internal porous structure.

[0090] Example 4

[0091] This embodiment provides a biomass-based hard carbon material. Step (3) of the method for preparing the biomass-based hard carbon material is:

[0092] The second precursor particles of step (2) are placed in 500 mL of a mixed solution of phosphoric acid and sulfuric acid with a molar ratio of 3:1, stirred at 0-3°C with a stirrer, and 3 g of potassium permanganate is added every two minutes, and a total of 60 g is added in batches, stirred for 6 hours, and finally 100 mL of hydrogen peroxide is added, stirred for 12 hours, washed until neutral, and oxygen-containing functional groups are introduced. After drying, third precursor particles are obtained;

[0093] Apart from this, other conditions are exactly the same as those in Example 2.

[0094] Example 5

[0095] This embodiment provides a biomass-based hard carbon material. In the preparation method of the biomass-based hard carbon material, urea is used instead of thiourea in step (2). Apart from this, other conditions are exactly the same as those in Example 2.

[0096] Example 6

[0097] This embodiment provides a biomass-based hard carbon material. In the preparation method of the biomass-based hard carbon material, disodium ethylenediaminetetraacetate is used in step (2) to replace thiourea. Apart from this, other conditions are exactly the same as those in Example 2.

[0098] Example 7

[0099] This embodiment provides a biomass-based hard carbon material. In the preparation method of the biomass-based hard carbon material, ammonia water is used instead of thiourea in step (2). Apart from this, other conditions are exactly the same as those in Example 2.

[0100] Example 8

[0101] This embodiment provides a biomass-based hard carbon material. In the preparation method of the biomass-based hard carbon material, sodium gluconate is used instead of thiourea in step (2). Apart from this, other conditions are exactly the same as those in Example 2.

[0102] Comparative Example 1

[0103] This comparative example provides a biomass-based hard carbon material, which is prepared by the following method:

[0104] (1) Place 1 kg of bamboo chips in an argon atmosphere with an oxygen concentration below 200 ppm, heat to 1600 °C at 5 °C / min, keep warm for 10 h, cool to room temperature, and cool to below 50 °C before discharging;

[0105] (2) The sample obtained in step (1) is subjected to a jet mill to reduce the median particle size to 5 μm, thereby obtaining a biomass-based hard carbon material.

[0106] Comparative Example 2

[0107] This comparative example provides a biomass-based hard carbon material. The preparation method of the biomass-based hard carbon material does not perform step (2), and directly performs step (3) on the first precursor particles obtained in step (1). Apart from this, other conditions are exactly the same as those in Example 2.

[0108] Comparative Example 3

[0109] This comparative example provides a biomass-based hard carbon material. The preparation method of the biomass-based hard carbon material does not perform step (3), and directly performs step (4) on the second precursor particles obtained in step (2). Apart from this, other conditions are exactly the same as those in Example 2.

[0110] Comparative Example 4

[0111] This comparative example provides a biomass-based hard carbon material. The preparation method of the biomass hard carbon material differs from that of Example 2 in that oxygen-free baking, oxidative modification, impurity removal, and high-temperature carbonization are sequentially performed. The preparation method is as follows:

[0112] (1) 1 kg of coconut shell was placed in a nitrogen atmosphere with an oxygen concentration of less than 200 ppm, heated to 250° C., treated for 48 hours, cooled to below 50° C., and crushed to a median particle size of 1 mm using a jaw crusher to obtain first precursor particles;

[0113] (2) placing the first precursor particles obtained in step (1) in a nitrogen atmosphere with an oxygen concentration controlled at 8%, rapidly heating to 800°C within 5°C / min, treating for 6 hours, introducing oxygen-containing functional groups, and crushing by air flow milling to a median particle size of 5 μm to obtain second precursor particles;

[0114] (3) placing the second precursor particles of step (2) in a stirred tank, adding sodium hydroxide, thiourea and water in a mass ratio of 12:7:81 in sequence, stirring at room temperature for 3 hours, washing until neutral, and drying to obtain third precursor particles;

[0115] (4) Under argon conditions, the third precursor particles obtained in step (3) are subjected to high-temperature closed-pore treatment, the temperature is raised to 1700°C at 2°C / min, kept at this temperature for 8 hours, and then cooled to room temperature to obtain a biomass-based hard carbon material.

[0116] Comparative Example 5

[0117] This comparative example provides a biomass-based hard carbon material. The preparation method of the biomass hard carbon material differs from that of Example 2 in that oxygen-free baking, oxidative modification, high-temperature carbonization, and impurity removal are sequentially performed. The preparation method is as follows:

[0118] (1) 1 kg of coconut shell was placed in a nitrogen atmosphere with an oxygen concentration of less than 200 ppm, heated to 250° C., treated for 48 hours, cooled to below 50° C., and crushed to a median particle size of 1 mm using a jaw crusher to obtain first precursor particles;

[0119] (2) placing the first precursor particles obtained in step (1) in a nitrogen atmosphere with an oxygen concentration controlled at 8%, rapidly heating to 800°C within 5°C / min, treating for 6 hours, introducing oxygen-containing functional groups, and crushing by air flow milling to a median particle size of 5 μm to obtain second precursor particles;

[0120] (3) Under argon conditions, the second precursor particles of step (2) were subjected to a high-temperature closed-pore treatment, the temperature was raised to 1700° C. at 2° C. / min, kept at this temperature for 8 h, and then cooled to room temperature to obtain third precursor particles;

[0121] (4) The third precursor particles obtained in step (3) are placed in a stirred tank, and sodium hydroxide, thiourea and water are added in a mass ratio of 12:7:81 in sequence, stirred at room temperature for 3 hours, washed to neutrality, and dried to obtain a biomass-based hard carbon material.

[0122] The ash content of the hard carbon materials obtained in the examples and comparative examples was tested using the national standard method GB / T 17664-1999. The results are recorded in Table 1.

[0123] The hard carbon materials obtained in the Examples and Comparative Examples were dissolved in deionized water with sodium carboxymethyl cellulose, a super P conductive agent, and a polymer binder at a ratio of 95:2:1:2 to form a slurry. The slurry was then coated onto copper foil and dried in a drying oven at 80°C for 4-10 hours to produce a hard carbon negative electrode. Finally, button cells were assembled in an argon-filled glove box using an electrolyte consisting of NaClO₄ dissolved in ethylene carbonate and propylene carbonate at a 1:1 volume ratio. Sodium metal foil served as the counter and reference electrodes. Electrochemical performance was measured using an electrochemical workstation and other equipment, and the results are reported in Table 1.

[0124] Table 1

[0125] From Table 1 we can see that:

[0126] The specific surface area of ​​the biomass-based hard carbon sample prepared in the embodiment is smaller than that of comparative example 1. The specific surface area of ​​the sample after medium-temperature treatment in the embodiment is larger than the specific surface area after high-temperature carbonization. This is only because the temperature is rapidly raised to the medium-temperature stage, which causes the structure in the material to rearrange rapidly. Due to the rapid temperature change, the molecular weight moves faster and faster, causing the connections between the structures to gradually become messy, and a large number of pores and defects to be exposed; with the further slow increase in temperature and the presence of catalytic gas, the impurity elements will be removed. Due to the change in temperature, the tension stress of the material itself exists, causing the material to shrink continuously, achieving a "self-repairing" effect, "repairing" the surface pores without affecting the formation of the internal pore structure, thereby reducing the specific surface area.

[0127] The electrochemical performance of the hard carbon product prepared in the embodiment is better than that of comparative example 1. This is because during the high-temperature catalytic carbonization process, on the one hand, the impurities contained in the material are catalytically volatilized, and on the other hand, the material undergoes structural rearrangement and can repair surface defects by itself, so that the material forms closed pores, the specific surface area is reduced, and the sodium ions forming SEI are reduced, thereby improving the capacity and initial efficiency of the material.

[0128] The electrochemical performance of Example 2 is better than that of Example 4. This is because in Example 4, liquid-phase oxidation is carried out in solution. In addition to the effect of the oxidant on the material, the aqueous solution under the action of a strong oxidant also affects the structure of the material, resulting in an excessively large specific surface area of ​​the material and abundant internal pores, which makes it impossible for the material to shrink and close the pores through its own tension, resulting in performance differences from Example 2.

[0129] The electrochemical performance of Example 2 is better than that of Comparative Examples 2 and 3. Since the material of Comparative Example 2 has not been impurity-removed, it has a great impact on the capacity and initial efficiency of the material. The presence of impurities leads to a low carbon content in the material, and untreated Si and other impurities react with carbon to form SiC, causing the material structure to mutate, making it difficult to achieve excellent energy storage performance. The material of Comparative Example 3 has been impurity-removed and has relatively excellent performance. Since the pores of Comparative Example 3 are all natural pores, the energy storage is limited, and there is a large difference in capacity compared with Example 2.

[0130] The electrochemical performance of Example 2 is better than that of Comparative Examples 4 and 5. The process sequence of the comparative examples is adjusted, so that the material cannot achieve the effect of impurity removal. The presence of impurities leads to a low carbon content of the material. Untreated Si and other impurities react with carbon to form SiC, causing the material structure to mutate, making it difficult to achieve excellent energy storage performance.

[0131] The present application uses the above-mentioned embodiments to illustrate the detailed structural features of the present application. However, the present application is not limited to the above-mentioned detailed structural features, that is, it does not mean that the present application must rely on the above-mentioned detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present application, equivalent replacements of selected components of the present application, addition of auxiliary components, selection of specific methods, etc., all fall within the scope of protection and disclosure of the present application.

[0132] The preferred embodiments of the present application are described in detail above. However, the present application is not limited to the specific details of the above embodiments. Within the technical concept of the present application, various simple modifications can be made to the technical solution of the present application, and these simple modifications all fall within the scope of protection of the present application.

[0133] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, this application will not further describe various possible combinations.

[0134] In addition, the various implementation methods of the present application may be arbitrarily combined, and as long as they do not violate the concept of the present application, they should also be regarded as the contents disclosed in the present application.

Claims

1. A method for preparing a biomass-based hard carbon material, comprising the following steps: The biomass raw materials are subjected to oxygen-free baking, impurity removal, oxidation modification and high-temperature carbonization in sequence to obtain biomass-based hard carbon materials.

2. The preparation method according to claim 1, wherein The biomass raw material includes any one of poplar sawdust, pine sawdust, litchi sawdust, coconut shell, bamboo sawdust, cotton or wheat straw, or a combination of at least two of them.

3. The preparation method according to claim 1 or 2, wherein The oxygen-free baking is carried out in an inert atmosphere with an oxygen concentration of less than 1000 ppm.

4. The preparation method according to any one of claims 1 to 3, wherein: The inert atmosphere includes any one of nitrogen, argon or helium, or a combination of at least two of them.

5. The preparation method according to any one of claims 1 to 4, wherein: The temperature of the oxygen-free baking is 200 to 1000° C. and the time is 15 to 48 hours; Preferably, after the oxygen-free baking and before the impurity removal, when the temperature is cooled to less than 50° C., the material is discharged and first crushed to obtain first precursor particles, and the first precursor particles are impurity removed; Preferably, the median particle size of the first precursor particles is 0.5-1 mm.

6. The preparation method according to any one of claims 1 to 5, wherein: The impurity removal is liquid phase impurity removal, and the biomass raw material after oxygen-free baking is treated in a solution containing an impurity remover.

7. The preparation method according to claim 6, wherein: The impurity remover includes any one of sodium hydroxide, potassium hydroxide, sulfuric acid, hydrochloric acid, phosphoric acid or hydrofluoric acid, or a combination of at least two thereof; Preferably, the solution containing the impurity remover also contains an additive; Preferably, the additive includes any one of thiourea, urea, disodium ethylenediaminetetraacetate, citric acid, ammonia water or sodium gluconate, or a combination of at least two thereof.

8. The preparation method according to claim 6 or 7, wherein: The liquid phase impurity removal is carried out at a rotation speed of 100 to 500 rpm; Preferably, the treatment time of the liquid phase impurity removal is 3 to 24 hours.

9. The preparation method according to any one of claims 1 to 8, wherein: After the impurity removal and before the oxidation modification, washing is first performed until the pH is 7, and then the second crushing is performed after drying; Preferably, the drying temperature is 80-120°C and the drying time is 8-12h; Preferably, the second crushing obtains second precursor particles, and the second precursor particles are oxidatively modified, and the median particle size of the second precursor particles is 4-10 μm.

10. The preparation method according to any one of claims 1 to 9, wherein: The oxidation modification method includes oxygen-containing sintering and / or liquid phase oxidation; Preferably, the oxygen concentration of the oxygen-containing sintering is 10% to 15%; Preferably, the oxygen-containing sintering temperature is 400-600°C and the time is 1-5h; Preferably, the liquid phase oxidation processes the impurity-removed biomass raw material in a solution containing an oxidant; Preferably, the liquid phase oxidation treatment time is 6 to 12 hours, and the treatment temperature is -3 to 5°C.

11. The preparation method according to any one of claims 1 to 10, wherein: The high temperature carbonization temperature is 1600-1800°C and the time is 3-20h; Preferably, the heating rate of the high temperature carbonization is 0.5-3°C / min; Preferably, the high-temperature carbonization is carried out under an inert atmosphere.

12. The preparation method according to any one of claims 1 to 11, comprising the steps of: (1) placing the biomass raw material in an inert atmosphere, wherein there is no oxygen or the oxygen concentration is less than 1000 ppm in the system, performing oxygen-free baking at 200 to 1000° C. for 15 to 48 hours, and when the temperature is cooled to less than 50° C., discharging the material and performing a first crushing to obtain millimeter-sized first precursor particles with a median particle size of 0.5 to 1 mm; (2) Mixing the first precursor particles with an aqueous solution containing an impurity remover and an additive, stirring at 100 to 500 rpm for 3 to 24 hours for liquid phase impurity removal, washing with water until the pH value is 7, and drying at 80 to 120°C 8 to 12 hours, performing a second crushing to obtain micrometer-sized second precursor particles with a median particle size of 4 to 10 μm; (3) placing the second precursor particles in an inert atmosphere with an oxygen concentration of 10% to 15% in the system, performing oxygen-containing sintering at 400 to 600° C. for 15 to 48 hours, and cooling to room temperature to obtain third precursor particles; Alternatively, the second precursor particles are mixed with an aqueous solution containing an oxidant, stirred at -3 to 5° C. for 6 to 12 hours, and dried to obtain third precursor particles; (4) Under the protection of inert gas, the temperature is increased at a rate of 0.5 to 3°C / min, and the third precursor particles are subjected to high-temperature carbonization treatment at 1600 to 1800°C for 3 to 20 hours. After cooling to room temperature, a biomass-based hard carbon material is obtained.

13. A biomass-based hard carbon material obtained by the preparation method according to any one of claims 1 to 12; Preferably, the ash content of the hard carbon material is less than 0.5wt%; Preferably, the pore size of the hard carbon material is 0.5 to 20 nm; Preferably, the true density of the hard carbon material is 1.3-2.0 g / cm 3 ; Preferably, the specific surface area of ​​the hard carbon material is less than 5 g / m 2 .

14. A negative electrode plate comprising the biomass-based hard carbon material according to claim 13.

15. A battery comprising the negative electrode sheet according to claim 14.

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