Hard carbon material and preparation method therefor, negative electrode sheet and sodium-ion battery

Abstract

The present application relates to the field of electrochemical negative electrode materials. Disclosed are a hard carbon material and a preparation method therefor, a negative electrode sheet and a sodium-ion battery. The preparation method comprises: pretreatment: taking a bamboo raw material, and heating and distilling same, so as to obtain a pretreated bamboo material from which bamboo vinegar has been removed; reaction: providing a mixed solution comprising an alkali metal hydroxide and a sulfur-containing substance, treating the pretreated bamboo material with the mixed solution, and reacting the pretreated bamboo material with the alkali metal hydroxide and the sulfur-containing substance; and carbonization: subjecting the reacted pretreated bamboo material to acid pickling, and then subjecting same to carbonization, so as to obtain a hard carbon material.

Description

Hard carbon material and preparation method thereof, negative electrode sheet, and sodium ion battery

[0001] Related cross-references

[0002] This disclosure claims priority to the Chinese patent application filed with the Patent Office of China on August 25, 2023, with application number 2023110798229 and application name “Hard carbon materials and preparation methods thereof, negative electrode sheets, and sodium ion batteries”, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present application relates to the field of electrochemical negative electrode materials, and in particular to a hard carbon material and a preparation method thereof, a negative electrode sheet, and a sodium ion battery. Background Art

[0004] Hard carbon materials have become a research hotspot for sodium ion battery anode materials due to their rich sodium storage sites, long charge and discharge cycle life, good safety performance, and large specific surface area.

[0005] However, compared with negative electrode materials such as graphite used in lithium-ion batteries, the hard carbon materials currently used in sodium-ion batteries still have problems such as high production cost, low first-cycle coulombic efficiency, poor rate performance and poor cycle stability, which limits their development in the industrial application of sodium-ion batteries.

[0006] Summary of the Invention

[0007] The embodiments of the present application disclose a hard carbon material and a preparation method thereof, a negative electrode plate, and a sodium ion battery to solve the problems of high production cost, low first-cycle coulombic efficiency, poor rate performance, and poor cycle stability of hard carbon materials in the prior art.

[0008] In a first aspect, an embodiment of the present application provides a method for preparing a hard carbon material, the method comprising the following steps:

[0009] Pretreatment: taking bamboo raw materials and performing heating and distillation to obtain pretreated bamboo materials from which bamboo vinegar is removed;

[0010] Reaction: providing a mixed solution comprising an alkali metal hydroxide and a sulfur-containing substance, and treating the pretreated bamboo material with the mixed solution to allow the pretreated bamboo material to react with the alkali metal hydroxide and the sulfur-containing substance;

[0011] Carbonization: The pretreated bamboo material after the reaction is acid-washed and then carbonized to obtain the hard carbon material.

[0012] In a second aspect, an embodiment of the present application provides a hard carbon material, which is obtained by the preparation method of the hard carbon material described in the first aspect.

[0013] In a third aspect, an embodiment of the present application provides a negative electrode plate, which includes a current collector and an active material layer provided on at least one side surface of the current collector, and the active material layer includes a hard carbon material prepared by the preparation method of the hard carbon material described in the first aspect.

[0014] In a fourth aspect, an embodiment of the present application provides a sodium ion battery, wherein the sodium ion battery includes the negative electrode sheet described in the third aspect.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The hard carbon material prepared by the preparation method in this application example is a bamboo charcoal-based hard carbon material with a rich pore structure. When used as the negative electrode active material in sodium-ion batteries, this hard carbon material exhibits excellent electrolyte wettability, high first-cycle coulombic efficiency, high rate capability, and good cycling stability. Furthermore, since this hard carbon material uses bamboo as a precursor, it is resource-rich and relatively low in production cost, making it particularly suitable for industrial applications in sodium-ion batteries.

[0017] In this embodiment, bamboo vinegar is first removed from the raw bamboo material by heating and distilling, leaving the pretreated bamboo material primarily composed of lignin, cellulose, and hemicellulose, which are rich in oxygen-containing functional groups. This pretreated bamboo material is then reacted with a specific mixed solution. This reaction, combined with a subsequent carbonization step, creates abundant pores in the pretreated bamboo material, yielding a hard carbon material with a rich pore structure.

[0018] Among them, the alkali metal hydroxide in the mixed solution can, on the one hand, increase the solubility of sulfur-containing substances in the mixed solution, and promote the sulfur-containing substances to smoothly enter the defects and pores of the pretreated bamboo material structure to react; on the other hand, it can penetrate into the crystalline area of ​​cellulose and hemicellulose of the pretreated bamboo material and react with oxygen bridges such as hydroxyl groups, phenol groups and ether bonds. The OH in the alkali metal hydroxide - It can weaken the hydrogen bonding force between cellulose and hemicellulose and the saponification of ester bonds between internal molecules. Therefore, as the ester bonds decrease and the porosity increases, the cellulose and hemicellulose are dissolved internally, which reduces the degree of polymerization of the macromolecular sugars of cellulose and hemicellulose. In the subsequent carbonization step, a large number of pore structures appear due to aromatization and carbonization reactions.

[0019] Furthermore, the sulfur-containing substances in the mixed solution, under the synergistic effect of the alkali metal hydroxide, can more smoothly move through the solution into the defects and vacancies of the lignin, undergoing sulfonation reactions with hydrogen and hydroxyl groups on the lignin benzene rings or side chains to form lignin sulfonates. The resulting lignin sulfonates are anionic surfactants that are highly soluble in water. During the subsequent washing and solid-liquid separation of the pretreated bamboo material after the reaction, these lignin sulfonates can be discharged with the washing liquid, leaving a rich pore structure in the lignin, which is also retained during the subsequent carbonization process.

[0020] As can be seen, the embodiments of the present application employ a mixed solution containing an alkali metal hydroxide and a sulfur-containing substance. This not only utilizes the alkali metal hydroxide to dissolve cellulose and hemicellulose to create pores and promote the dissolution of the sulfur-containing substance, but also utilizes the sulfonation reaction between the sulfur-containing substance and lignin to generate a soluble salt to form pores. Through the aforementioned synergistic effect of the alkali metal hydroxide and the sulfur-containing substance, coupled with the subsequent carbonization step, a hard carbon material with a rich pore structure is prepared. The rich pore structure in the hard carbon material facilitates the insertion and extraction of sodium ions, improves electrolyte wetting, increases battery capacity, and enhances battery cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, a brief introduction will be given below to the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, a brief introduction will be given below to the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0023] FIG1 is a process flow chart of a method for preparing a hard carbon material according to an embodiment of the present application;

[0024] FIG2 is a charge and discharge curve diagram of a button battery provided in Example 1 of the present application;

[0025] FIG3 is a charge and discharge curve diagram of a button battery provided in Example 2 of the present application;

[0026] FIG4 is a charge and discharge curve diagram of a button battery provided in Example 3 of the present application;

[0027] FIG5 is a charge and discharge curve diagram of a button-type battery provided in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0029] In this application, terms such as "upper," "lower," "left," "right," "front," "back," "top," "bottom," "inside," "outside," "center," "vertical," "horizontal," "transverse," and "longitudinal" indicate positions or locations based on the positions or locations shown in the accompanying drawings. These terms are primarily intended to better describe this application and its embodiments and are not intended to limit the devices, elements, or components indicated to having a specific orientation, or to being constructed or operated in a specific orientation.

[0030] Furthermore, some of the above terms may be used to express other meanings besides indicating a position or location. For example, the term "on" may also be used to indicate a dependency or connection in certain circumstances. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0031] Furthermore, the terms "installed," "disposed," "provided with," "connected," and "connected" should be interpreted broadly. For example, they can refer to fixed connections, removable connections, or integral structures; mechanical connections or electrical connections; direct connections, indirect connections through an intermediary, or internal communication between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0032] Furthermore, the terms "first," "second," etc., are primarily used to distinguish between different devices, elements, or components (which may or may not be of the same type and configuration), and are not intended to indicate or imply the relative importance or quantity of the devices, elements, or components indicated. Unless otherwise specified, "plurality" means two or more.

[0033] The technical solution provided in this application will be further described below with reference to the embodiments and drawings.

[0034] The embodiments of the present application provide a hard carbon material and a preparation method thereof, a negative electrode plate, and a sodium ion battery. Bamboo raw materials are used as a precursor for preparing the hard carbon material, which has the advantages of low price, abundant stock, and high carbon content. The hard carbon material prepared from bamboo raw materials in the embodiments of the present application can reduce production costs while ensuring that the hard carbon material has good electrochemical properties, which is conducive to promoting the industrialization process of such hard carbon materials. Therefore, there is great potential for using the hard carbon material prepared from bamboo raw materials in the embodiments of the present application in sodium ion batteries for energy storage.

[0035] In the first aspect, as shown in FIG1 , FIG1 is a process flow chart of a method for preparing a hard carbon material according to an embodiment of the present application. The present application provides a method for preparing a hard carbon material, comprising the following steps:

[0036] Pretreatment: taking bamboo raw materials and performing heating and distillation to obtain pretreated bamboo materials from which bamboo vinegar is removed;

[0037] Reaction: providing a mixed solution comprising an alkali metal hydroxide and a sulfur-containing substance, and treating the pretreated bamboo material with the mixed solution to allow the pretreated bamboo material to react with the alkali metal hydroxide and the sulfur-containing substance;

[0038] Carbonization: The pretreated bamboo material after the reaction is acid-washed and then carbonized to obtain the hard carbon material.

[0039] In the above preparation method, the pretreatment process removes the volatile bamboo vinegar from the bamboo raw material. After distillation, the pretreated bamboo material obtained includes substances such as cellulose, hemicellulose, and lignin. Lignin contains active groups such as aromatic hydrocarbons, phenolic hydroxyl groups, alcoholic hydroxyl groups, ether bonds, and benzene rings. Cellulose and hemicellulose are large molecular polysaccharides composed of glucose, containing a large number of hydroxyl groups, phenolic groups, and ether bonds. The two are connected by oxygen bridges, forming a state where crystalline and amorphous phases coexist.

[0040] In the above reaction steps, the alkali metal hydroxide in the mixed solution can, on the one hand, increase the solubility of the sulfur-containing substances in the mixed solution, and promote the sulfur-containing substances to smoothly enter the defects and pores of the pretreated bamboo material structure to react; on the other hand, it can penetrate into the crystalline area of ​​cellulose and hemicellulose in the pretreated bamboo material and react with oxygen bridges such as hydroxyl groups, phenol groups and ether bonds. - It can weaken the hydrogen bonding force between cellulose and hemicellulose and the saponification of ester bonds between internal molecules. Therefore, as the ester bonds decrease and the porosity increases, the cellulose and hemicellulose are dissolved internally, which reduces the degree of polymerization of the macromolecular sugars of cellulose and hemicellulose. In the subsequent carbonization step, a large number of pore structures appear due to aromatization and carbonization reactions.

[0041] Furthermore, the sulfur-containing substances in the mixed solution, under the synergistic effect of the alkali metal hydroxide, can more smoothly move through the solution into the defects and vacancies of the lignin, undergoing sulfonation reactions with hydrogen and hydroxyl groups on the lignin benzene rings or side chains to form lignin sulfonates. The resulting lignin sulfonates are anionic surfactants that are highly soluble in water. During the subsequent washing and solid-liquid separation of the pretreated bamboo material after the reaction, these lignin sulfonates can be discharged with the washing liquid, leaving a rich pore structure in the lignin, which is also retained during the subsequent carbonization process.

[0042] During the carbonization step, acid is used to remove the reaction ash, preventing it from reacting with other substances during the subsequent carbonization process and affecting the pore structure of the carbonized hard carbon material. By controlling the carbonization parameters during the carbonization process, not only can the organic components be decomposed, but a hard carbon material with rich pores can also be obtained.

[0043] In addition, the preparation method of the hard carbon material first pre-treats the bamboo material by distillation, then mixes and stirs the bamboo material with the mixed liquid for reaction, and finally removes impurities by acid washing and carbonization. The preparation method of the hard carbon material is simple and suitable for industrial production.

[0044] Furthermore, the concentration of the alkali metal hydroxide is 0.1mol / L~0.5mol / L, and the concentration of the sulfur-containing substance is 0.5mol / L~1mol / L; the mass ratio of the alkali metal hydroxide, the sulfur-containing substance and the pretreated bamboo material is (0.1~0.2):(0.2~0.3):1.

[0045] When the concentrations of alkali metal hydroxides and sulfur-containing substances and their mass ratio to pretreated bamboo materials are controlled within the above range, it is beneficial to prepare hard carbon materials with rich pore structures. In this way, it can avoid the excessive solubility of cellulose and hemicellulose when there is an excess of alkali metal hydroxide, which destroys the macroscopic structure of bamboo and makes it difficult to form hard carbon. Furthermore, when the concentration of alkali metal hydroxide is high, the solubility of sulfur-containing substances is enhanced, the reaction of sulfur-containing substances with lignin is accelerated, and the hard carbon material with a high macropore content is prepared, which reduces the first-cycle coulombic efficiency of sodium ion batteries. It can also improve the situation when the concentration of alkali metal hydroxide is low, the solubility of sulfur-containing substances is reduced, the reaction with lignin is reduced, and the pore-forming ability of pretreated bamboo materials is limited. In addition, the etching of cellulose and hemicellulose by alkali metal hydroxide is limited, which is not conducive to the construction of pores and active sites suitable for sodium ions, and has a negative impact on the electrochemical performance of sodium ion batteries.

[0046] Illustratively, the concentration of alkali metal hydroxide is 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.45 mol / L or 0.5 mol / L, etc.; the concentration of sulfur-containing substance is 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, etc.; the mass ratio of alkali metal hydroxide, sulfur-containing substance and pretreated bamboo material is 0.1:0.2:1, 0.12:0.25:1, 0.15:0.2:1, 0.15:0.29:1, 0.19:0.25:1, 0.19:0.29:1, 0.2:0.3:1, etc.

[0047] Furthermore, the alkali metal hydroxide includes at least one of sodium hydroxide or potassium hydroxide, and the sulfur-containing substance includes at least one of sulfite or sulfide.

[0048] For example, the alkali metal hydroxide may be sodium hydroxide, potassium hydroxide, or both. The sulfur-containing substance may be only sulfite, only sulfide, or both sulfite and sulfide. The sulfite may be at least one of sodium sulfite, sodium bisulfite, potassium sulfite, or potassium bisulfite, and the sulfide may be at least one of sodium sulfide, potassium sulfide, and sodium persulfide.

[0049] Furthermore, the mass ratio of the mixed solution to the pretreated bamboo material is 3:1 to 8:1; and / or,

[0050] In the reaction step, the mixed liquid and the pretreated bamboo material are mixed and stirred for 4 hours to 8 hours.

[0051] When the mass ratio of the mixed liquid to the pretreated bamboo material and the reaction mixing time are within this range, it is beneficial to construct a pore structure suitable for sodium ion transport. This can reduce the risk that when the concentration of the mixed liquid is excessive, the degree of reaction with the pretreated bamboo material is large, resulting in a hard carbon material with a high content of macropores, which is not conducive to the storage of sodium ions and the construction of active sites; when the concentration of the mixed liquid is too low, the effect on the pretreated bamboo material is limited, which is not conducive to the construction of a rich pore structure. Exemplarily, the mass ratio of the mixed liquid to the pretreated bamboo material is 3:1, 4:1, 6:1, 7:1, 8:1, etc.; the mixing and stirring is carried out for 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc.; the mixing and stirring method includes mechanical stirring or magnetic stirring.

[0052] It is understandable that, in the reaction step, after the pretreated bamboo material is treated with the mixed liquid, solid-liquid separation and washing operations may be performed to wash the pretreated bamboo material to neutrality.

[0053] Furthermore, in the carbonization step, the pickling acid solution includes at least two of nitric acid, sulfuric acid, hydrochloric acid, perchloric acid or hydrofluoric acid, and the concentration of the acid solution is 0.5 mol / L to 5 mol / L, wherein the mass ratio of the acid solution to the pretreated bamboo material after the reaction is 3:1 to 5:1.

[0054] The present application discovered that after solid-liquid separation and washing to neutrality during the reaction step, the resulting pretreated bamboo material product contains oxides or salts of metal elements such as Al, Ba, Fe, Na, Cu, K, and P. A single acid type can only remove a limited amount of ash. However, using at least two acids selected from nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, or hydrofluoric acid can remove most of the ash using different acid radical ions, thereby preventing side reactions during the subsequent carbonization process that could affect the pore structure of the carbonized hard carbon material. For example, the acid wash solution can be selected from nitric acid and perchloric acid, nitric acid, sulfuric acid, or hydrofluoric acid, or nitric acid, sulfuric acid, perchloric acid, or hydrofluoric acid; the acid concentration can be 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2.5 mol / L, 3.5 mol / L, 4.5 mol / L, 4.9 mol / L, or 5.0 mol / L; and the mass ratio of the acid solution to the pretreated bamboo material after the reaction can be 3:1, 3.5:1, 4:1, or 5:1.

[0055] Furthermore, in the carbonization step, the carbonization temperature is 1200°C to 1500°C, the carbonization time is 4h to 8h, and the heating rate is 2°C / min to 5°C / min; and / or, in the carbonization step, the pretreated bamboo material is carbonized in an inert atmosphere, and the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0056] Keeping the carbonization temperature, carbonization time, heating rate, and inert atmosphere within the aforementioned ranges further enriches the pore structure of the hard carbon material and increases its specific surface area. This prevents the structural collapse of the hard carbon material, which can affect the richness of the pore structure, caused by excessively high carbonization temperatures, prolonged carbonization times, or high carbonization rates. It also avoids the problems of structural collapse leading to a reduced specific surface area and number of active sites, thereby preventing adverse effects on the battery's first-cycle coulombic efficiency, active ion transport rate, and active sites. Illustratively, the carbonization temperature is 1200°C, 1300°C, 1350°C, 1400°C, 1450°C, 1500°C, etc.; the carbonization time is 4h, 4.5h, 5h, 6h, 7h, 7.5h, 8h, etc.; the heating rate is 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min, etc.; the inert gas is, for example, only nitrogen, only helium, a mixed gas including nitrogen and helium, or a mixed gas including nitrogen, argon and helium, etc.

[0057] Furthermore, in the pretreatment step, the bamboo raw material is subjected to two-stage heating distillation, the first stage of distillation is at a temperature of 100°C to 150°C and a time of 10h to 20h, and the second stage of distillation is at a temperature of 200°C to 250°C and a time of 6h to 8h.

[0058] In the pretreatment step, a two-stage heating distillation is set up. The temperature of the first distillation is 100°C to 150°C and the time is 10h to 20h, and the free water in the bamboo raw material is removed within this temperature and time range; the temperature of the second distillation is 200°C to 250°C and the time is 6h to 8h, and the adsorbed water and part of the fiber saturated water in the bamboo raw material are removed within this temperature and time range to obtain a pretreated bamboo material containing lignin, cellulose and hemicellulose, which is then blended with subsequent processes to obtain a hard carbon material with a rich pore structure. Illustratively, the temperature of the first distillation stage is 100°C, 110°C, 120°C, 130°C, 140°C, 145°C, 150°C, etc.; the time of the first distillation stage is 10h, 12h, 15h, 17h, 19h, 20h, etc.; the temperature of the second distillation stage is 200°C, 210°C, 220°C, 230°C, 240°C, 245°C, 250°C, etc.; the time of the first distillation stage is 6h, 6.5h, 7h, 7.5h, 8h, etc.

[0059] Furthermore, after the pretreatment step and before the reaction step, the method for preparing the hard carbon material further comprises crushing: crushing and sieving the pretreated bamboo material to make the particle size of the pretreated bamboo material 5 μm≤Dv50≤10 μm.

[0060] Controlling the particle size of the pretreated bamboo material within the above range is beneficial to the subsequent battery pulping process, increasing volumetric energy density, and reducing irreversible capacity loss. If the particle size is too high, not only will coating become more difficult, but the particles will also be too large, limiting the amount of small particles that can fill the gaps between large particles, reducing the compaction density of the electrode and lowering the volumetric energy density of the battery. If the particle size is too low, the specific surface area in contact with the electrolyte increases, increasing the amount of charge consumed by the SEI film formed during the initial charge and discharge, and increasing irreversible capacity loss. For example, the particle size of the pretreated bamboo material is 5μm≤Dv50≤7μm, 6μm≤Dv50≤9μm, 8μm≤Dv50≤9.6μm, 9.6μm≤Dv50≤10μm, etc.

[0061] Furthermore, the moisture content of the pretreated bamboo material is 20% to 40%.

[0062] Controlling the moisture content of the pretreated bamboo material within the range of 20% to 40% can produce a hard carbon material with a rich pore structure. This prevents excessively high moisture content in the pretreated bamboo material from reacting with sulfur-containing substances, affecting the reaction between the sulfur-containing substances and lignin, and limiting the pore structure of the prepared hard carbon material. It also prevents excessively low moisture content, which can cause the pretreated bamboo material to crack during pulverization, resulting in a powder that cannot maintain its intact shape and hinders the formation of a pore structure. Exemplary moisture contents of the pretreated bamboo material include 20%, 25%, 30%, 35%, 40%, and so on.

[0063] Furthermore, the growth period of the bamboo raw material is 3 to 5 years, and the diameter of the bamboo raw material is 7 cm to 12 cm.

[0064] Bamboo raw materials of the above growth period and diameter have the characteristics of abundant resources, easy acquisition and processing, mature and stable cell structure, and moderate water content in bamboo, and are more suitable for the preparation of the hard carbon material in the embodiment of this application. When the growth period is too long and the diameter is too large, on the one hand, the difficulty of obtaining bamboo raw materials increases, and on the other hand, the water content of bamboo raw materials decreases, which is not conducive to the construction of pore structure in the subsequent reaction process; if the growth period is too short, the amount of bamboo raw materials available is limited, and the moisture content of bamboo raw materials is rich, which increases the difficulty of controlling the moisture content of bamboo materials. For example, the growth period of bamboo raw materials is 3 years, 4 years or 5 years, etc.; the diameter is 7 cm, 8 cm, 10 cm, 11 cm or 12 cm, etc.

[0065] In a second aspect, an embodiment of the present application provides a hard carbon material, which is obtained by the preparation method of the hard carbon material described in the first aspect.

[0066] Furthermore, the specific surface area of ​​the hard carbon material is 400m 2 / g~500m 2 / g, true density is 2.15g / cm 3 ≤ρ≤2.5g / cm 3 , pore volume is 0.08 cm 3 / g≤V≤0.12cm 3 / g.

[0067] The embodiment of the present application controls the specific surface area, true density and pore volume of the hard carbon material within the above ranges, which is beneficial to improving the electrochemical performance of the hard carbon material. Among them, the true density is used to measure the total pore characteristics of the hard carbon material, and the pore volume is tested by the CO2 adsorption and desorption method, which is used to measure the micropore characteristics of the hard carbon material. The hard carbon material within this range has a high specific surface area, a high total pore content, and a rich microporous structure under the premise of a high total pore content, which is beneficial to the construction of rich active sites, ion migration, and electrolyte infiltration, thereby improving the electrochemical performance of the battery. In this way, it is possible to avoid the problems that when the specific surface area is too small and the pore structure is limited, the number of active sites of the hard carbon material is reduced, which is not conducive to the migration of sodium ions and the infiltration of the electrolyte; it is also possible to avoid the problems that when the pore volume is small, the micropore content in the material is small, the storage of sodium ions is small, and the electrochemical performance of the hard carbon material is adversely affected.

[0068] For example, the specific surface area of ​​the hard carbon material is 400 m 2 / g, 420m 2 / g, 450m 2 / g, 470m 2 / g, 490m 2 / g、500m 2 / g, etc., and the true density is 2.15g / cm 3 , 2.20g / cm 3 , 2.25g / cm 3 , 2.30g / cm 3 , 2.45g / cm 3 , 2.5g / cm 3 etc., the pore volume is 0.08 cm 3 / g, 0.09cm 3 / g, 0.10cm 3 / g, 0.11cm 3 / g, 0.12cm 3 / g, etc.

[0069] In a third aspect, the present application provides a negative electrode plate, which includes a current collector and an active material layer provided on at least one side surface of the current collector, wherein the active material layer includes a hard carbon material prepared by the preparation method of the hard carbon material as described in the first aspect.

[0070] It should be emphasized that the negative electrode current collector is coated with a single layer of hard carbon material or both sides of the negative electrode current collector are coated with hard carbon material.

[0071] In a fourth aspect, an embodiment of the present application provides a sodium ion battery, wherein the sodium ion battery includes the negative electrode sheet described in the third aspect.

[0072] The structure of the battery includes a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. A hard carbon material is coated on one side or both sides of the negative electrode plate, and the battery is assembled.

[0073] The technical solution of the present application will be further explained below in conjunction with more specific embodiments and experimental test results.

[0074] Example 1

[0075] This embodiment provides a hard carbon material, and a method for preparing the hard carbon material includes the following steps:

[0076] Pretreatment: Three-year-old bamboo with a diameter of 7 cm was selected and cut into strips. After cleaning, the strips were used as bamboo raw materials for distillation. Two heating temperatures were set: the first temperature was 150°C and the heat preservation time was 15 hours; the second temperature was 300°C and the heat preservation time was 6 hours. The pretreated bamboo material with bamboo vinegar removed and a moisture content of 20% was obtained;

[0077] Reaction: The pretreated bamboo material is coarsely crushed with a roller crusher, pulverized with a jet mill, and then sieved to a particle size of 5 μm ≤ Dv50 ≤ 7 μm. The material is then added to a mixture containing sodium hydroxide, sodium sulfite, and sodium sulfide, with the mass ratio of the mixture to the pretreated bamboo material being 3:1; wherein the concentration of sodium hydroxide in the mixture is 0.5 mol / L, the concentration of sodium sulfite is 0.5 mol / L, and the concentration of sodium sulfide is 0.5 mol / L. The mixture and the pretreated bamboo material are heated and stirred in boiling water for 8 hours, followed by solid-liquid separation, and washed with deionized water until neutral;

[0078] Carbonization: The pretreated bamboo material after the reaction is added to an acid solution for pickling to remove ash. The mass ratio of the acid solution to the pretreated bamboo material after the reaction is 3:1, wherein the acid solution is a mixed acid solution formed by 2 mol / L nitric acid and 1 mol / L hydrogen fluoride. After stirring for 6 hours, the solid-liquid separation and washing are carried out until neutral, and then carbonization is carried out. The carbonization conditions are a carbonization temperature of 1200°C, a carbonization time of 8 hours, and nitrogen as the protective gas. After screening, a hard carbon material can be obtained.

[0079] The specific surface area of ​​the hard carbon material is 435 m 2 / g, true density is 2.23g / cm 3 , pore volume is 0.10 cm 3 / g.

[0080] Example 2

[0081] The difference between this embodiment and the first embodiment is that:

[0082] Pretreatment: Four-year-old bamboo with a diameter of 10 cm was cut into strips, cleaned, and used as bamboo raw material for distillation. Two heating temperatures were set: the first temperature was 120°C for 20 hours; the second temperature was 400°C for 6 hours. The pretreated bamboo material was obtained after the bamboo vinegar liquid was removed and the moisture content was 40%.

[0083] Reaction: The pretreated bamboo material is coarsely crushed with a roller crusher, pulverized with a jet mill, and then sieved to a particle size of 6 μm ≤ Dv50 ≤ 8 μm. The material is then added to a mixture containing sodium hydroxide, sodium sulfite, and sodium sulfide, with the mass ratio of the mixture to the pretreated bamboo material being 5:1. The concentration of potassium hydroxide in the mixture is 0.2 mol / L, the concentration of sodium bisulfite is 0.8 mol / L, and the concentration of sodium persulfide is 0.8 mol / L. The mixture and the pretreated bamboo material are heated and stirred in boiling water for 8 hours, followed by solid-liquid separation, and washed with deionized water until neutral.

[0084] Carbonization: The pretreated bamboo material after the reaction is added to an acid solution for pickling to remove ash. The mass ratio of the acid solution to the pretreated bamboo material after the reaction is 5:1, wherein the hydrofluoric acid is 1 mol / L and the sulfuric acid is 1 mol / L. Stir for 6 hours, separate the solid and liquid and wash until neutral, and then carbonize. The carbonization conditions are a carbonization temperature of 1300°C, a carbonization time of 6 hours, and argon as the protective gas. After screening, a hard carbon material can be obtained.

[0085] The specific surface area of ​​the hard carbon material is 460 m 2 / g, true density is 2.29g / cm 3 , pore volume is 0.12 cm 3 / g.

[0086] Example 3

[0087] The difference between this embodiment and the first embodiment is that:

[0088] Pretreatment: Five-year-old bamboo with a diameter of 12 cm was cut into strips, cleaned, and then distilled as a bamboo raw material. Two heating temperatures were set: the first temperature was 150°C and the heat was kept for 15 hours; the second temperature was 350°C and the heat was kept for 8 hours. The pretreated bamboo material was obtained after the bamboo vinegar liquid was removed and the moisture content was 30%.

[0089] Reaction: The pretreated bamboo material was coarsely crushed with a roller crusher, pulverized with a jet mill, and then sieved to a particle size of 5 μm ≤ Dv50 ≤ 7 μm. The mass ratio of the mixed solution to the pretreated bamboo material was 8:1. The concentration of sodium hydroxide in the mixed solution was 0.5 mol / L, the concentration of potassium sulfite was 1 mol / L, and the concentration of sodium persulfide was 1 mol / L. The stirring time was 6 hours.

[0090] Carbonization: The pretreated bamboo material after the reaction is added to acid solution for pickling to remove ash. The mass ratio of the acid solution and the pretreated bamboo material after the reaction is 5:1, wherein the acid solution is 1 mol / L hydrochloric acid, 0.8 mol / L hydrofluoric acid, and 2 mol / L nitric acid. Stir for 6 hours, separate the solid and liquid and wash until neutral, and then carbonize. The carbonization conditions are carbonization temperature of 1500℃ and carbonization time of 6 hours; nitrogen is used as the protective gas, and hard carbon material can be obtained after screening.

[0091] The specific surface area of ​​the hard carbon material is 440 m 2 / g, true density is 2.37g / cm 3 , the pore volume is 0.11 cm 3 / g.

[0092] Example 4

[0093] The only difference between this embodiment and the first embodiment is that in the pretreatment step, after two stages of distillation, a pretreated bamboo material with a moisture content of 10% is obtained.

[0094] Example 5

[0095] The only difference between this embodiment and the first embodiment is that, in the pretreatment step, after two stages of distillation, a pretreated bamboo material with a moisture content of 70% is obtained.

[0096] Example 6

[0097] The only difference between this embodiment and the first embodiment is that, in the reaction step, the mass ratio of the mixed liquid to the pretreated bamboo material is 1:1.

[0098] Example 7

[0099] The only difference between this embodiment and the first embodiment is that, in the reaction step, the mass ratio of the mixed liquid to the pretreated bamboo material is 8:1.

[0100] Example 8

[0101] The only difference between this embodiment and the first embodiment is that, in the reaction step, the alkali metal hydroxide in the mixed solution is 0.5 mol / L sodium hydroxide and 0.2 mol / L potassium hydroxide.

[0102] Embodiment 9

[0103] The only difference between this embodiment and the first embodiment is that, in the reaction step, the sulfur-containing substance in the mixed solution is 0.5 mol / L sodium sulfite.

[0104] Example 10

[0105] The only difference between this embodiment and the first embodiment is that, in the reaction step, the sulfur-containing substance in the mixed solution is 0.5 mol / L sodium sulfide.

[0106] Comparative Example 1

[0107] Pretreatment: 3-year-old bamboo with a diameter of 7 cm was cut into strips, cleaned, and used as the raw material for distillation. Two heating temperatures were set: the first temperature was 150°C for 15 hours; the second temperature was 300°C for 6 hours. The pretreated bamboo material was obtained after the bamboo vinegar was removed and the moisture content was 20%.

[0108] Carbonization: The pretreated bamboo material after the reaction is added to an acid solution for pickling to remove ash. The mass ratio of the acid solution to the pretreated bamboo material after the reaction is 3:1, wherein the acid solution is a mixed acid solution formed by 2 mol / L nitric acid and 1 mol / L hydrogen fluoride. After stirring for 6 hours, the solid-liquid separation and washing are carried out until neutral, and then carbonization is carried out. The carbonization conditions are a carbonization temperature of 1200°C, a carbonization time of 8 hours, and nitrogen as the protective gas. After screening, a hard carbon material can be obtained.

[0109] Comparative Example 2

[0110] The only difference between this comparative example and Example 1 is that, in the reaction step, the mixed solution contains only 0.5 mol / L sodium hydroxide.

[0111] Pore ​​structure performance test:

[0112] 1 g to 3 g of the hard carbon materials of Examples 1 to 10 and Comparative Examples 1 and 2 are weighed and placed in a true density tester. Helium is used as the medium to test the true density of the materials.

[0113] 30-500 mg of the hard carbon materials from Examples 1-10 and Comparative Examples 1-2 were weighed and placed in a sample tube. CO adsorption and desorption were performed at 195 K at various pre-set pressures to determine the carbon dioxide adsorption capacity of the samples. Adsorption isotherms were generated. The data were processed by computer, and the specific surface area and desorption pore volume were calculated from the adsorption isotherms. The test results are shown in Table 1, which shows the parameters that distinguish Examples 4-10 and Comparative Examples 1-2 from Example 1, as well as the pore structure test results.

[0114] Table 1 Summary of components, parameters and pore structures of Examples and Comparative Examples

[0115] As shown in Table 1, the true density and specific surface area of ​​Examples 1 to 10 are greater than those of Comparative Example 1. Comparative Example 1 does not use a mixed solution to react with the pretreated bamboo material. Therefore, the hard carbon material produced in Comparative Example 1 has a lower true density and a reduced total pore content, which is not conducive to sodium ion migration and leads to a reduction in battery capacity. In contrast, the hard carbon materials produced in Examples 1 to 10, which use a mixed solution to react with the pretreated bamboo material, have a higher true density and a higher total pore content, which facilitates the construction of pores suitable for sodium ion transport, thereby improving battery performance. Furthermore, the pore volume of Examples 1 to 10 is 4 to 6 times higher than that of Comparative Example 1. For sodium-ion batteries, the microporous structure facilitates the storage of more sodium ions, increasing the battery's first-cycle coulombic efficiency and improving the battery rate and cycling performance.

[0116] The true density, specific surface area, and pore volume of Examples 1 to 10 were all greater than those of Comparative Example 2. In Comparative Example 2, only alkali metal hydroxide was used to react with the pretreated bamboo material, indicating the limited ability of a single alkali metal hydroxide to act on the pretreated bamboo material. The resulting hard carbon material after carbonization had a less porous structure than that of Examples 1 to 10, a microscopic manifestation of the lower vacuum density, specific surface area, and pore volume. Consequently, the capacity of the battery prepared from this hard carbon material was also lower than that of Examples 1 to 10.

[0117] Furthermore, a comparison of Examples 1, 9, and 10 reveals that the pore structure parameters of Example 1 are significantly superior to those of Examples 9 and 10, and that the pore structure of Example 10 is also superior to that of Example 9. This is because the sulfide salts in sulfur-containing substances are readily soluble in water and possess strong reducing power, while sulfites are generally obtained by oxidizing sulfide salts and have poor reducing power. Therefore, the reactivity of sulfites with the lignin functional groups in the pretreated bamboo material is weaker than that of sulfides. Therefore, the pore-forming ability of the selected sulfur-containing substances is ranked from highest to lowest as follows: Example 1 > Example 10 > Example 9, resulting in a larger pore volume in Example 1.

[0118] Furthermore, true density reflects the density after removing the internal pores or inter-particle pores of the hard carbon material. A higher true density indicates a greater mass per unit volume of the hard carbon material, which in turn reflects a higher purity of the hard carbon material's components. Specifically, the prepared hard carbon material contains fewer oxygen-containing functional groups, indicating a high degree of reaction between the sulfur-containing substance and the pretreated bamboo material, resulting in a rich pore structure. Since the true density of Example 1 is higher than that of Examples 9 and 10, this indicates that the simultaneous use of sulfite and sulfide salts results in a more complete reaction with the pretreated bamboo material.

[0119] To test the effectiveness of the hard carbon materials described in the above examples and comparative examples for use in sodium-ion battery materials, this application used the hard carbon materials in the positive electrode of a half-cell to prepare a 2430-type button battery and conducted testing and evaluation of their effectiveness. In practical applications, the hard carbon materials were used as the active material for the negative electrode of a sodium-ion battery.

[0120] The button battery preparation method comprises the following steps:

[0121] A hard carbon material, a conductive agent, and a binder in a mass ratio of 94:2:2:2 are dissolved in a solvent and uniformly mixed to obtain a mixed solution, wherein the hard carbon material is the hard carbon material of Examples 1 to 10 and Comparative Examples 1 to 2, the conductive agent is acetylene black, the binder is sodium carboxymethyl cellulose and styrene-butadiene rubber, and the solvent is deionized water;

[0122] The mixed solution was coated on a copper foil and vacuum dried for 10 hours to obtain a positive electrode sheet, which was then rolled and punched.

[0123] NaPF6 was used as the sodium salt electrolyte at a concentration of 1 mol / L. Ethylene carbonate (EC) and diethylene carbonate (DEC) were used as the solvent in a 1:1 volume ratio. Polyethylene (PE) served as the separator, and a sodium sheet served as the negative electrode. The button-type battery was assembled in an argon atmosphere.

[0124] Test data

[0125] The button cells prepared in Examples 1 to 10 and Comparative Examples 1 and 2 were subjected to the following tests:

[0126] Test 1:

[0127] The assembled button cells were charged and discharged using a CT2001A blue battery test system, discharging to 5 mV at current rates of 0.1C (current density 30 mA / g), 0.08C (current density 24 mA / g), 0.05C (current density 15 mA / g), 0.03C (current density 9 mA / g), and 0.005C (current density 1.5 mA / g), and then charging to 2.0 V at a current rate of 0.1C (current density 30 mA / g).

[0128] Table 2 Electrochemical performance test

[0129] As shown in Figure 2 and Table 2, in the electrochemical test of the battery charge, the discharge specific capacity, charge specific capacity, and first-cycle coulombic efficiency of Examples 1 to 10 after mixed solution treatment are all better than those of Comparative Example 1, indicating that mixed solution treatment can produce a hard carbon material with a rich pore structure. The pore structure is conducive to storing more active sites, facilitating the infiltration of the electrode solution, ion and electron transport, and thus improving the electrochemical performance of the hard carbon material. By analyzing Examples 1, 4, and 5, it was found that the electrochemical performance of Examples 4 and 5 was lower than that of Example 1. Among them, the specific surface area, pore volume, and true density of Examples 4 and 5 were smaller, indicating that excessively high or low water content will affect the pore structure of the hard carbon material during the subsequent preparation process, thereby adversely affecting the performance of the battery. Compared with Comparative Examples 1 and 2, it can be seen that Examples 4 to 5 have higher electrochemical performance and rich pore structures, indicating that although the moisture content will affect the battery pore structure and thus affect the battery performance, compared with Comparative Examples 1 and 2, it is found that their performance degradation is weaker than that of Comparative Examples 1 and 2, indicating that although the moisture content of the pretreated bamboo material will affect the performance of the hard carbon material, the use of the mixed liquid to react with the pretreated bamboo material in combination with the subsequent carbonization step can still achieve a good pore-forming effect.

[0130] The discharge specific capacity, charge specific capacity, and first-cycle coulombic efficiency of Examples 1 to 10 are higher than those of Comparative Example 2, indicating that when only alkali metal hydroxide is used to act on the pretreated bamboo material, the true density and pore volume of the obtained hard carbon material are small, and its pore-forming ability is limited. Therefore, the discharge specific capacity, charge specific capacity, and first-cycle coulombic efficiency are low, indicating that the pore-forming ability of the reaction of alkali metal hydroxide and sulfur-containing substances in the mixed solution with the pretreated bamboo material is better than that of using only alkali metal hydroxide.

[0131] Furthermore, current sodium-ion batteries generally suffer from poor initial coulombic efficiency. Compared to lithium-ion batteries using graphite as the negative electrode active material, which can achieve initial coulombic efficiency of 90% or even over 95%, current sodium-ion batteries struggle to achieve initial coulombic efficiency exceeding 85%. The initial coulombic efficiency of several embodiments, such as Examples 1 to 3 of the present application, can reach over 85%, demonstrating that the hard carbon materials of the present embodiments are beneficial in significantly improving indicators such as the initial coulombic efficiency of sodium-ion batteries.

[0132] Test 2:

[0133] Cut the negative electrode sheet of the button cell into 2cm×5cm strips and place them in a glove box filled with argon. Use a pipette to precisely aspirate 100μL of electrolyte and drop it vertically onto the center of the cut sheet 5cm above it. The timer starts when the electrolyte contacts the sheet and ends when the electrolyte completely wets the sheet. The mass m (mg) of each sheet (excluding the copper foil substrate) and the wetting time t (s) are calculated to determine the electrode uptake rate v (mg / s).

[0134] Table 3 Electrode liquid absorption rate test

[0135] As can be seen from Table 3, compared with Comparative Example 1, Examples 1 to 10 have a short infiltration time and a high liquid absorption rate, indicating that Examples 1 to 10 have stronger wetting ability for the electrolyte, and the hard carbon material prepared in the present application has a good wetting effect of the electrolyte. This is because the hard carbon material preparation method of the present application uses a mixed solution containing an alkali metal hydroxide and a sulfur-containing substance to react with the pretreated bamboo material, and the prepared hard carbon material has a rich pore structure, which reduces the ion transmission path, is conducive to the shuttle of sodium ions between the positive and negative electrodes, reduces the battery interface resistance, and thus improves the battery rate and cycle stability.

[0136] The wetting ability of Examples 1 to 10 is higher than that of Comparative Example 2. In Comparative Example 2, there is no sulfur-containing substance involved in the reaction step, only alkali metal hydroxide. This shows that the internal transmission channels of the hard carbon material obtained by the carbonization step of the single alkali metal hydroxide reacting with the pretreated bamboo material become fewer, and the contact area between the hard carbon particles becomes smaller, which affects the diffusion rate of the electrolyte from the outside to the inside of the electrode, thereby illustrating that the sulfur-containing substance has a pore-forming effect. The mixed use of the sulfur-containing substance and the alkali metal hydroxide makes the obtained hard carbon material have a rich pore structure, which is conducive to the wetting of the electrolyte.

[0137] In summary, test data 1 to 3 demonstrate that using bamboo raw materials as precursors, pre-treating the bamboo material to remove the bamboo vinegar solution, and then reacting it with the mixed solution in a reaction step, combined with a subsequent carbonization step, produces a hard carbon material with a rich pore structure. This hard carbon material preparation method offers the advantages of simplicity, easy parameter control, and low cost. The resulting hard carbon material possesses a rich pore structure suitable for sodium ion transport, improving electrolyte wettability, increasing active material utilization, and enhancing ion and electron transport rates, thereby enhancing the electrochemical performance of the battery.

[0138] The above is a detailed introduction to the hard carbon material and preparation method, negative electrode plate, and sodium ion battery disclosed in the embodiments of the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the hard carbon material and preparation method, negative electrode plate, and sodium ion battery: At the same time, for those skilled in the art, according to the ideas of the present application, there will be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present application. 。

Claims

1. A method for preparing a hard carbon material, wherein: The method for preparing the hard carbon material comprises the following steps: Pretreatment: taking bamboo raw materials and performing heating and distillation to obtain pretreated bamboo materials from which bamboo vinegar is removed; Reaction: providing a mixed solution including an alkali metal hydroxide and a sulfur-containing substance, and treating the pretreated bamboo material with the mixed solution to make the pretreated bamboo material react with the alkali metal hydroxide and the sulfur-containing substance; Carbonization: The pretreated bamboo material after the reaction is acid-washed and then carbonized to obtain the hard carbon material.

2. The method for preparing a hard carbon material according to claim 1, wherein: The concentration of the alkali metal hydroxide is 0.1 mol / L to 0.5 mol / L, and the concentration of the sulfur-containing substance is 0.5 mol / L to 1 mol / L; and / or, The mass ratio of the alkali metal hydroxide, the sulfur-containing substance and the pretreated bamboo material is (0.1-0.2):(0.2-0.3):

1.

3. The method for preparing a hard carbon material according to claim 1, wherein: The alkali metal hydroxide includes at least one of sodium hydroxide or potassium hydroxide, and the sulfur-containing substance includes at least one of sulfite or sulfide salt.

4. The method for preparing a hard carbon material according to any one of claims 1 to 3, wherein: In the step of reacting, the mass ratio of the mixed solution to the pretreated bamboo material is 3:1 to 8:1; and / or, In the reaction step, the mixed solution is mixed and stirred with the pretreated bamboo material for 4 hours to 8 hours.

5. The method for preparing a hard carbon material according to claim 1, wherein: The pickling acid solution includes at least two of nitric acid, sulfuric acid, hydrochloric acid, perchloric acid or hydrofluoric acid, and the concentration of the acid solution is 0.5 mol / L to 5 mol / L, wherein the mass ratio of the acid solution to the pretreated bamboo material after the reaction is 3:1 to 5:

1.

6. The method for preparing a hard carbon material according to claim 1, wherein: In the carbonization step, the carbonization temperature is 1200° C. to 1500° C., the carbonization time is 4 h to 8 h, and the heating rate is 2° C. / min to 5° C. / min; and / or, In the carbonization step, the pretreated bamboo material is carbonized in an inert atmosphere, wherein the inert atmosphere includes at least one of nitrogen, argon and helium.

7. The method for preparing a hard carbon material according to claim 1, wherein: In the pretreatment step, the bamboo raw material is subjected to two-stage heating distillation, the first stage of distillation is at a temperature of 100°C to 150°C and a time of 10h to 20h, and the second stage of distillation is at a temperature of 200°C to 250°C and a time of 6h to 8h.

8. The method for preparing a hard carbon material according to claim 1, wherein: The moisture content of the pretreated bamboo material is 20% to 40%; and / or, After the pretreatment step and before the reaction step, the method for preparing the hard carbon material further comprises crushing: crushing and sieving the pretreated bamboo material to make the particle size of the pretreated bamboo material 5μm≤Dv50≤10μm; and / or, The growth period of the bamboo raw material is 3 to 5 years, and the diameter of the bamboo raw material is 7 cm to 12 cm.

9. A hard carbon material, wherein: The hard carbon material is obtained by the method for preparing a hard carbon material according to any one of claims 1 to 8.

10. The hard carbon material according to claim 9, wherein The specific surface area of ​​the hard carbon material is 400 m 2 / g~500m 2 / g, true density is 2.15g / cm 3 ≤ρ≤2.5g / cm 3 , pore volume is 0.08 cm 3 / g≤V≤0.12cm 3 / g.

11. A negative electrode sheet, wherein: The negative electrode plate includes a current collector and an active material layer disposed on at least one side surface of the current collector, and the active material layer includes a hard carbon material prepared by the method for preparing a hard carbon material according to any one of claims 1 to 8.

12. A sodium ion battery, wherein: The sodium ion battery comprises the negative electrode sheet according to claim 11.

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