Composite hard carbon negative electrode material, preparation method thereof and sodium ion battery

Abstract

The invention provides a composite hard carbon negative electrode material, a preparation method thereof and a sodium ion battery. The preparation method comprises the following steps: preparing a porous hard carbon material from a biomass raw material; the biomass raw materials comprise cellulose and lignin; mixing the resin precursor solution with a porous hard carbon material, homogenizing and drying to obtain a composite material; curing and cross-linking the composite material in a functional atmosphere to obtain a cross-linked cured body; and carbonizing and post-processing the cross-linked solidified body to obtain the composite hard carbon negative electrode material. According to the invention, biomass raw materials with oxygen-containing functional groups are selected to prepare porous hard carbon, and the porous hard carbon and a resin precursor containing functional groups such as hydroxyl and / or amino can be chemically bonded in a curing and crosslinking process, so that a compact and firmly-attached carbon coating layer is finally formed, and side reaction and volume expansion can be inhibited; and finally, the sodium ion battery has high first efficiency, high capacity and long cycle stability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery materials, and particularly relates to a composite hard carbon negative electrode material, a preparation method thereof and a sodium ion battery. BACKGROUND

[0002] Hard carbon has become the mainstream choice of sodium ion battery negative electrode materials due to its low cost, high sodium storage capacity and other advantages. However, hard carbon has significant volume expansion during sodium intercalation / deintercalation, which affects the structural stability and cycle life. To alleviate the expansion, the method of constructing a porous structure is generally used at present, which buffers the volume change through internal pores. However, the porous structure also increases the contact area of the material and the electrolyte, resulting in the consumption of more sodium ions to form a solid electrolyte interface film (SEI) during the first charge and discharge process, causing the initial efficiency to decrease; at the same time, the continuous side reactions exacerbate the decomposition of the electrolyte and the loss of active sodium, thereby deteriorating the cycle performance of the battery.

[0003] Therefore, the prior art often performs surface coating treatment on the porous hard carbon in order to isolate the electrolyte, reduce side reactions and inhibit volume expansion. However, the conventional coating process often fails to achieve uniform, complete and firm coating of the porous structure, and problems such as non-dense coating layer, incomplete local coverage or peeling off due to stress change during the cycle process are prone to occur, which limits the full play of the protection effect.

[0004] Therefore, how to construct a coating layer on the surface of the porous hard carbon, which is dense in structure, firmly attached and has good electrochemical stability, so as to effectively improve the electrochemical performance of the sodium ion battery, is a technical problem to be solved. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application aims to provide a composite hard carbon negative electrode material, a preparation method thereof and a sodium ion battery. The composite hard carbon negative electrode material prepared based on the method exhibits excellent initial coulombic efficiency, high specific capacity and long cycle stability when used in a sodium ion battery.

[0006] To achieve the purpose of the present application, the following technical solutions are adopted:

[0007] In a first aspect, the present application provides a preparation method of a composite hard carbon negative electrode material, which comprises the following steps:

[0008] The porous hard carbon material is prepared from biomass raw materials; wherein the biomass raw materials include cellulose and lignin.

[0009] The resin precursor solution and the porous hard carbon material are mixed, and after homogenization treatment, drying is performed to obtain a composite material; the resin precursor in the resin precursor solution contains hydroxyl and / or amino groups.

[0010] The composite material is cured and cross-linked in a functional atmosphere to obtain a cross-linked and cured body.

[0011] The cross-linked and cured body is carbonized and post-treated to obtain a composite hard carbon negative electrode material.

[0012] In some embodiments, the biomass raw material is fruit shell, and a mass ratio of cellulose to lignin in the fruit shell is (1-3):1.

[0013] In some embodiments, the fruit shell includes any one or a combination of at least two of peanut shell, walnut shell, almond shell, or coconut shell.

[0014] In some embodiments, the resin precursor in the resin precursor solution includes any one or a combination of at least two of phenolic resin, epoxy resin, or melamine formaldehyde resin.

[0015] In some embodiments, the step of preparing the porous hard carbon material from the biomass raw material includes:

[0016] The biomass raw material is pre-carbonized to obtain the porous hard carbon material.

[0017] The pre-carbonization temperature is 600-900°C, and the pre-carbonization holding time is 1-4 hours.

[0018] In some embodiments, at least one of the following conditions is met:

[0019] (1) The mass concentration of the resin precursor solution is 5-20%.

[0020] (2) A mass ratio of the resin precursor in the resin precursor solution to the porous hard carbon material is 1:(5-20).

[0021] (3) The homogenization treatment includes vacuum impregnation and ultrasonic dispersion performed in sequence.

[0022] In some embodiments, the functional atmosphere includes any one of an air atmosphere, an inert atmosphere containing ammonia, or an inert atmosphere containing hydrogen.

[0023] And / or, the curing and cross-linking temperature is 150-250°C, and the time is 1-3 hours.

[0024] In some embodiments, at least one of the following conditions is met:

[0025] (1) The carbonization atmosphere is an inert atmosphere.

[0026] (2) During the carbonization process, the absolute pressure is 1-5 MPa.

[0027] (3) the carbonization process comprises sequentially performing primary carbonization and secondary carbonization, the temperature of the primary carbonization is 300 DEG C ~ 500 DEG C, and the temperature of the secondary carbonization is 1000 DEG C ~ 1300 DEG C.

[0028] In some embodiments, the post-treatment comprises sequentially performing acid washing, water washing and drying.

[0029] In the second aspect, the application provides a composite hard carbon negative electrode material, which is prepared by the preparation method in the first aspect.

[0030] The composite hard carbon negative electrode material comprises a porous hard carbon inner core and a carbon coating layer coated on the surface of the porous hard carbon inner core.

[0031] In some embodiments, at least one of the following conditions is met:

[0032] (1) the specific surface area of the porous hard carbon inner core is 30 m 2 / g ~ 50 m 2 / g.

[0033] (2) the content of surface oxygen atoms of the porous hard carbon inner core is 5 at% ~ 11.5 at%.

[0034] (3) the porosity of the carbon coating layer is 5% ~ 10%.

[0035] (4) the thickness of the carbon coating layer is 10 nm ~ 50 nm.

[0036] (5) the carbon coating layer contains nitrogen atoms, and the content of the nitrogen atoms is 1 at% ~ 5 at%.

[0037] In the third aspect, the application provides a sodium ion battery, and the negative electrode of the sodium ion battery comprises the composite hard carbon negative electrode material in the second aspect.

[0038] Compared with the prior art, the application has the following beneficial effects:

[0039] The present application prepares porous hard carbon by selecting biomass raw materials containing cellulose and lignin. Oxygen-containing functional groups such as hydroxyl, ether bond, methoxyl, carbonyl, etc. are carried in the molecular structure of the biomass material. The porous hard carbon prepared from the biomass material carries oxygen-containing functional groups on the surface. By selecting a resin precursor containing functional groups such as hydroxyl and / or amino, during the cross-linking and curing process, the oxygen-containing functional groups of the porous hard carbon and the functional groups such as hydroxyl and / or amino on the resin precursor are chemically bonded, thereby achieving firm bonding between the coating layer and the hard carbon matrix and effectively preventing the coating layer from falling off. At the same time, after the resin precursor fully penetrates into the pores and is cured, cross-linked and carbonized, a carbon coating layer with dense structure, firm adhesion and good electrochemical stability is formed, which can significantly inhibit the electrolyte side reaction and volume expansion. The composite hard carbon negative electrode material prepared based on the method exhibits excellent first coulombic efficiency, high specific capacity and long cycle stability when used in sodium ion batteries. DETAILED DESCRIPTION

[0040] The technical solutions of the present application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only to help understand the present application and should not be regarded as specific limitations on the present application.

[0041] The present application adopts the following technical solutions:

[0042] In a first aspect, the present application provides a preparation method of a composite hard carbon negative electrode material, which comprises the following steps:

[0043] The porous hard carbon material is prepared by using biomass raw materials; wherein the biomass raw materials include cellulose and lignin.

[0044] The resin precursor solution and the porous hard carbon material are mixed, homogenized and dried to obtain a composite material; the resin precursor in the resin precursor solution contains hydroxyl and / or amino.

[0045] The composite material is cured and cross-linked in a functional atmosphere to obtain a cross-linked and cured body.

[0046] The cross-linked and cured body is carbonized and post-treated to obtain a composite hard carbon negative electrode material.

[0047] The porous hard carbon is prepared by using a biomass raw material containing cellulose and lignin. The biomass material has oxygen-containing functional groups such as hydroxyl, ether bond, methoxyl, carbonyl and the like in the molecular structure. Therefore, the porous hard carbon prepared from the biomass material has oxygen-containing functional groups on the surface. By selecting a resin precursor containing functional groups such as hydroxyl and / or amino, the oxygen-containing functional groups of the porous hard carbon and the functional groups such as hydroxyl and / or amino on the resin precursor are chemically bonded during the cross-linking and curing process, to form a C-O-C bond or an N-C bond, thereby realizing firm combination between the coating layer and the hard carbon matrix and effectively preventing the coating layer from falling off. Meanwhile, the resin precursor fully penetrates into the pores and forms a carbon coating layer with dense structure, firm adhesion and good electrochemical stability after being cured, cross-linked and carbonized, which can significantly inhibit the electrolyte side reaction and volume expansion. The composite hard carbon negative electrode material prepared based on the method has excellent first coulombic efficiency, high specific capacity and long cycle stability when used in a sodium ion battery.

[0048] In some embodiments, the biomass raw material is fruit shell, and the mass ratio of cellulose to lignin in the fruit shell is (1-3):1; for example, it can be 1:1, 1.5:1, 2:1, 2.5:1 or 3:1, etc.

[0049] In some embodiments, the fruit shell includes any one or a combination of at least two of peanut shell, walnut shell, almond shell or coconut shell.

[0050] It should be noted that the test method of the mass ratio of cellulose to lignin is as follows: the neutral detergent fiber, acid detergent fiber and acid insoluble lignin in the raw material are separated and quantified by a series of solvent treatments, and the content and mass ratio of cellulose and lignin are calculated. Specifically, 1) sample pretreatment: the biomass raw material is crushed through a 40-mesh sieve and dried at 105°C to a constant weight (absolute dry sample); the extractant (oil, pigment, etc.) is removed by Soxhlet extraction with a benzene-ethanol mixture (volume ratio 2:1). 2) acidolysis and component separation: first step acidolysis: 0.3g of the pretreated sample is weighed and 3mL of 72wt% concentrated sulfuric acid is added, and the sample is oscillated at 30°C for 60min to preliminarily hydrolyze the cellulose; second step acidolysis: the sulfuric acid is diluted to 4wt%, and the sample is hydrolyzed at 121°C for 60min to completely degrade the cellulose and hemicellulose into monosaccharides (glucose, xylose, etc.); solid-liquid separation: after filtration, the filtrate contains monosaccharides (corresponding to cellulose+ hemicellulose), and the filter residue is acid insoluble lignin. 3) quantitative calculation: cellulose content: the glucose concentration in the filtrate is determined by high performance liquid chromatography (HPLC), and the cellulose content is calculated according to “cellulose = glucose mass x 0.9” (conversion factor of glucose→cellulose); lignin content: the filter residue is dried and weighed, and the ash content (residue after calcination at 575°C) is subtracted to obtain the mass of acid insoluble lignin.

[0051] In some embodiments, the resin precursor in the resin precursor solution includes any one or a combination of at least two of phenolic resin, epoxy resin, or melamine-formaldehyde resin.

[0052] In some embodiments, the steps for preparing porous hard carbon materials from biomass raw materials include:

[0053] Biomass raw materials are pre-carbonized to obtain porous hard carbon materials.

[0054] The pre-carbonization temperature is 600℃~900℃, for example, it can be 600℃, 700℃, 800℃ or 900℃, etc., and the pre-carbonization holding time is 1h~4h, for example, it can be 1h, 2h, 3h or 4h, etc.

[0055] The present invention uses the above parameters to achieve pre-carbonization of biomass raw materials, which can effectively control the pore structure and surface properties of the obtained porous hard carbon. The above parameters can help form abundant sodium storage pores while avoiding pore collapse or excessive specific surface area due to excessive carbonization, thereby laying a structural foundation for the uniform penetration and chemical bonding of subsequent resin precursors.

[0056] In some embodiments, the mass concentration of the resin precursor solution is 5% to 20%, for example, it can be 5%, 10%, 15% or 20%, etc.

[0057] In this invention, the resin precursor solution at a suitable mass concentration can ensure that it fully penetrates into the porous hard carbon material, and can form a continuous, dense and appropriately thick carbon coating layer after subsequent curing and carbonization. This achieves effective chemical bonding and side reaction suppression while avoiding pore blockage caused by an overly viscous solution or incomplete coating caused by an overly thin solution.

[0058] In some embodiments, the mass ratio of the resin precursor to the porous hard carbon material in the resin precursor solution is 1:(5~20), for example, it can be 1:5, 1:10, 1:15 or 1:20, etc.

[0059] In this invention, the appropriate mass ratio of resin precursor to porous hard carbon material ensures that the resin precursor is uniformly distributed in the pores and surface of the porous hard carbon material, avoiding pore blockage caused by excessive use. This not only optimizes the chemical bonding effect but also synergistically improves the conductivity and structural stability of the material.

[0060] In some embodiments, the homogenization process includes sequential vacuum impregnation and ultrasonic dispersion.

[0061] In this invention, the homogenization process employs sequential vacuum impregnation and ultrasonic dispersion, which helps to achieve full penetration of the resin precursor solution into the pores of porous hard carbon. The vacuum environment eliminates gas within the pores, creating an advantage for solution penetration, while subsequent ultrasonication further breaks up agglomerates and promotes dispersion, ensuring the formation of a uniformly dispersed resin precursor in the pores and on the surface. This lays the foundation for achieving a strong and dense chemical bond and coating layer in the future.

[0062] In some embodiments, the vacuum degree of vacuum impregnation is 0.06 MPa to 1 MPa, for example, it can be 0.06 MPa, 0.1 MPa, 0.3 MPa, 0.5 MPa, 0.7 MPa or 9 MPa, etc.; the vacuum impregnation time is 1 h to 3 h, for example, it can be 1 h, 2 h or 3 h, etc.

[0063] In some embodiments, the ultrasonic dispersion power is 300W~350W, for example, 300W, 320W, 340W or 350W, and the ultrasonic dispersion time is 0.5h~1h, for example, 0.5h, 0.75h or 1h.

[0064] In some embodiments, the functional atmosphere includes any one of an air atmosphere, an ammonia-containing inert atmosphere, or a hydrogen-containing inert atmosphere.

[0065] In some embodiments, curing and crosslinking are carried out under an inert atmosphere containing ammonia.

[0066] It is understandable that using an inert atmosphere containing ammonia for curing and crosslinking has the following effects: On the one hand, it removes oxygen, preventing resin oxidative degradation and oxidation of the hard carbon matrix, thus ensuring the smooth progress of curing and crosslinking; on the other hand, during the curing and crosslinking process, ammonia molecules react with active groups (such as hydroxyl and epoxy groups) in the resin molecules, introducing nitrogen-containing groups such as amino (-NH2) and imino (-NH-) into the crosslinking network of the resin; during subsequent carbonization, these nitrogen-containing groups are retained in the carbon coating layer, forming a nitrogen-doped carbon structure, creating defect sites, and further improving the capacity of the anode material; at the same time, ammonia molecules can neutralize oxygen-containing functional groups (such as carboxyl-COOH) on the porous hard carbon surface to generate urethane groups, which can form stronger chemical bonds (such as hydrogen bonds and covalent bonds) with resin molecules, further improving the bonding strength between the coating layer and the hard carbon matrix.

[0067] In some embodiments, curing and crosslinking are performed under an inert atmosphere containing hydrogen. Using an inert atmosphere containing hydrogen for curing and crosslinking has the following effects: under heating conditions, hydrogen can slightly etch the surface of porous hard carbon, removing residual impurities from the hard carbon surface and creating more surface active sites. These active sites can form stronger chemical bonds (such as C-C bonds and hydrogen bonds) with the active groups of resin molecules (such as hydroxymethyl groups in phenolic resins and epoxy groups in epoxy resins), significantly improving the bonding strength between the coating layer and the hard carbon matrix and preventing coating layer detachment during subsequent battery cycling. Furthermore, the resin produces small volatile molecules such as formaldehyde and moisture during the curing and crosslinking process, and the hydrogen gas flow can quickly purge these volatiles out of the reaction system, preventing them from accumulating in the hard carbon channels and forming bubbles. Simultaneously, the reducing properties of hydrogen can inhibit the adsorption and residue of volatiles on the carbon surface, ultimately forming a denser, non-porous carbon coating layer.

[0068] In some embodiments, the volume fraction of ammonia in the inert atmosphere containing ammonia is 2% to 8%, for example, it can be 2%, 4%, 6% or 8%.

[0069] In some embodiments, the volume fraction of hydrogen in the hydrogen-containing inert atmosphere is 2% to 8%, for example, it can be 2%, 4%, 6% or 8%.

[0070] In some embodiments, the curing crosslinking temperature is 150℃~250℃, for example, 150℃, 175℃, 200℃, 225℃ or 250℃, and the time is 1h~3h, for example, 1h, 2h or 3h.

[0071] In this invention, curing and crosslinking within the above-mentioned temperature and time ranges helps to achieve complete and controllable crosslinking, which is beneficial to forming a highly crosslinked, three-dimensional network structure and to maintaining its shape during subsequent carbonization, thus forming a stable coating layer structure.

[0072] In some embodiments, the carbonization atmosphere is an inert atmosphere. Examples include nitrogen or argon.

[0073] In some implementations, the absolute pressure during carbonization is 1 MPa to 5 MPa, for example, it can be 1 MPa, 2 MPa, 3 MPa, 4 MPa or 5 MPa.

[0074] In this invention, the absolute pressure is set to 1MPa~5MPa during the carbonization process, which is beneficial to forming a denser coating structure.

[0075] In some implementations, the carbonization process includes sequential primary carbonization and secondary carbonization.

[0076] In some embodiments, the temperature for primary carbonization is 300°C to 500°C, for example, 300°C, 400°C, or 500°C.

[0077] In some embodiments, the temperature for secondary carbonization is 1000°C to 1300°C, for example, 1000°C, 1100°C, 1200°C or 1300°C.

[0078] This invention employs a multi-stage carbonization process with gradient heating, which helps to form a denser and more uniform carbon layer structure.

[0079] In some embodiments, the post-treatment includes sequential pickling, washing, and drying. Pickling removes inorganic impurities remaining in the carbon material after carbonization; for example, dilute hydrochloric acid or dilute nitric acid can be used for pickling. The purpose of washing is to thoroughly remove residual acid and dissolved ions and neutralize to neutral.

[0080] In some embodiments, the preparation method includes the following steps:

[0081] (1) Preparation of porous hard carbon materials, the steps include:

[0082] (1-1) Provide fruit shells containing cellulose and lignin, and wash and crush the fruit shells to obtain fruit shell powder; wherein the mass ratio of cellulose to lignin is (1~3):1.

[0083] (1-2) In an inert atmosphere (exemplary, such as nitrogen or argon, etc.), the nut shell powder is pre-carbonized at 600℃~900℃ for 1h~4h to obtain porous hard carbon material.

[0084] (2) Dissolve the resin precursor in a solvent to obtain a resin precursor solution with a mass concentration of 5% to 20%.

[0085] The resin precursor solution is mixed with porous hard carbon material, and then vacuum impregnated and ultrasonically dispersed to make the resin precursor uniformly distributed in the pores and surface of the porous hard carbon material. Finally, it is dried to obtain the composite material.

[0086] The mass ratio of the resin precursor to the porous hard carbon material is 1:(5~20); the solvent includes any one of water, ethanol or tetrahydrofuran.

[0087] (3) In an inert atmosphere containing ammonia or an inert atmosphere containing hydrogen, the composite material is cured and crosslinked at 150℃~250℃ for 1h~3h, so that the formed resin forms a chemical bond with the surface functional groups of the porous hard carbon material, and a crosslinked cured body is obtained.

[0088] In the inert atmosphere containing ammonia, the volume fraction of ammonia is 2% to 8%; in the inert atmosphere containing hydrogen, the volume fraction of hydrogen is 2% to 8%.

[0089] (4) In an inert atmosphere with an absolute pressure of 1 MPa to 5 MPa, the cross-linked curing body is first heated to 300℃ to 500℃ at a heating rate of 1℃ / min to 3℃ / min (e.g., 1℃ / min, 2℃ / min, or 3℃ / min, etc.) for 0.5h to 1.5h (e.g., 0.5h, 1h, or 1.5h, etc.), and then heated to 1000℃ to 1300℃ at a heating rate of 4℃ / min to 6℃ / min (e.g., 4℃ / min, 5℃ / min, or 6℃ / min, etc.) for 2h to 5h (e.g., 2h, 3h, 4h, or 5h, etc.) to obtain the carbonized product.

[0090] The carbonization products are washed and dried to obtain composite hard carbon anode material.

[0091] In a second aspect, the present invention provides a composite hard carbon anode material, which is prepared by the preparation method of the first aspect.

[0092] The composite hard carbon anode material includes a porous hard carbon core and a carbon coating layer covering the surface of the porous hard carbon core.

[0093] In some embodiments, the specific surface area of ​​the porous hard carbon core is 30 m². 2 / g~50m 2 / g, for example, could be 30m 2 / g、35m 2 / g、40m 2 / g、45m 2 / g or 50m 2 / g etc.

[0094] In this invention, the aforementioned specific surface area is beneficial to the preparation process. During vacuum impregnation, the resin precursor solution enters the pores of the porous hard carbon core. At the same time, when the composite hard carbon anode material is applied in sodium-ion batteries, the porous structure can increase the specific surface area and active sites, contributing more capacity, and can also suppress the volume change caused by sodium ion insertion.

[0095] It should be noted that the specific surface area testing method includes: using the low-temperature nitrogen adsorption-desorption method, the specific surface area of ​​the sample is calculated using the Brunauer-Emmett-Teller (BET) model within the range of relative pressure (P / P0) of 0.05~0.30, as follows: 0.3g of porous hard carbon sample (i.e., for uncoated porous hard carbon material) is placed in a sample tube and degassed at 120℃ under vacuum for 9h; the pretreated sample tube is placed in a specific surface area analyzer, and nitrogen is introduced at liquid nitrogen temperature (~196℃), and the amount of nitrogen adsorbed at different relative pressures (P / P0, usually 0.05~0.3) is tested to obtain the adsorption isotherm.

[0096] In some embodiments, the surface oxygen atom content of the porous hard carbon core is 5 at% to 11.5 at%, for example, it can be 5 at%, 6 at%, 7 at%, 8 at%, 9 at%, 10 at%, or 11.5 at%. This demonstrates that the porous hard carbon surface has oxygen-containing functional groups, which can chemically bond with hydroxyl or amino groups in the resin precursor.

[0097] It should be noted that the surface oxygen atom content of the porous hard carbon core can be measured by XPS (X-ray photoelectron spectroscopy).

[0098] In some embodiments, the porosity of the carbon coating is 5% to 10%, for example, it can be 5%, 6%, 7%, 8%, 9% or 10%.

[0099] In this invention, the increased density of the carbon coating layer can suppress direct contact between the electrolyte and the porous hard carbon core. The porous material has a large specific surface area, and the electrolyte will form an SEI film on its inner and outer surfaces, causing sodium ion consumption. By reducing the porosity of the carbon coating layer, the irreversible consumption of sodium ions can be effectively reduced, thereby improving the first coulombic efficiency of the battery.

[0100] It should be noted that the methods for testing the porosity of the carbon coating include: 1) Using the BET method: testing the N2 adsorption-desorption curve, calculating the specific surface area using the Brunauer-Emmett-Teller formula, and then analyzing the pore volume using the Barrett-Joyner-Halenda (BJH) model to derive the pore volume of micropores (<2nm) + mesopores (2~50nm). 2) Using the mercury intrusion porosimetry: utilizing the surface tension of mercury, testing the intrusion volume of mercury under different pressures to characterize the pore volume of macropores (>50nm). 3) Adding the pore volumes measured by the BET method and the mercury intrusion porosimetry, and then dividing by the true density of hard carbon, the total porosity is approximately obtained.

[0101] In some embodiments, the thickness of the carbon coating layer is 10~50nm, for example, it can be 10nm, 20nm, 30nm, 40nm or 50nm.

[0102] In this invention, the carbon coating layer of the above-mentioned thickness can effectively block the direct contact between the electrolyte and the active material, thereby significantly reducing side reactions and suppressing volume expansion. At the same time, this thickness avoids a significant increase in ion / electron transport resistance and a decrease in overall energy density caused by excessive coating thickness, ensuring that the anode material has both high initial efficiency, high rate capability and long cycle life.

[0103] It should be noted that the thickness of the carbon coating can be obtained by observing the TEM and then calculating the average value at several locations.

[0104] In some embodiments, the carbon coating contains nitrogen atoms, with the nitrogen atom content ranging from 1 at% to 5 at%, for example, it can be 1 at%, 2 at%, 3 at%, 4 at%, or 5 at%.

[0105] This invention involves doping nitrogen atoms into the carbon coating layer, which can create defect sites and further improve the capacity performance of the battery.

[0106] It should be noted that the nitrogen atom content can be characterized using the XPS method.

[0107] Thirdly, the present invention provides a sodium-ion battery, wherein the negative electrode of the sodium-ion battery includes a composite hard carbon negative electrode material as described in the second aspect.

[0108] The numerical range of this invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this invention will not exhaustively list the specific point values ​​included in the range.

[0109] Example 1

[0110] This embodiment provides a method for preparing a composite hard carbon anode material, the method comprising the following steps:

[0111] (1) Preparation of porous hard carbon materials, the steps include:

[0112] (a) Provide almond shells containing cellulose and lignin, and wash and crush the almond shells and pass them through a 200-mesh sieve to obtain almond shell powder; wherein the mass ratio of cellulose to lignin is 1.5:1.

[0113] (b) The almond shell powder was pre-carbonized at 800°C for 2 hours in an argon atmosphere to obtain a porous hard carbon material.

[0114] (2) Dissolve phenolic resin in ethanol to obtain a resin precursor solution with a mass concentration of 10%.

[0115] The resin precursor solution is mixed with the porous hard carbon material, and then subjected to vacuum impregnation for 2 hours and ultrasonic dispersion for 0.5 hours to ensure that the resin precursor is uniformly distributed in the pores and surface of the porous hard carbon material. The solvent is then removed by rotary evaporation to obtain the composite material.

[0116] The mass ratio of the resin precursor to the porous hard carbon material is 1:10; the vacuum degree of the vacuum impregnation is 0.08 MPa; and the ultrasonic power of the ultrasonic dispersion is 300 W.

[0117] (3) The composite material is cured and crosslinked at 200°C for 2 hours in an air atmosphere, so that the formed resin forms chemical bonds with the surface functional groups of the porous hard carbon material to obtain a crosslinked cured body.

[0118] (4) In an argon atmosphere, the cross-linked solidified body is first heated to 400°C at a heating rate of 2°C / min for 1 hour for primary carbonization, and then heated to 1200°C at a heating rate of 5°C / min for 3 hours for secondary carbonization to obtain the carbonized product.

[0119] The carbonization product was acid-washed with 1 mol / L HCl, then washed with water until neutral, and finally vacuum-dried at 80°C for 12 h to obtain the composite hard carbon anode material.

[0120] This embodiment also provides a composite hard carbon anode material, which is prepared by the preparation method described above.

[0121] The composite hard carbon anode material comprises a porous hard carbon core and a carbon coating layer covering the surface of the porous hard carbon core; the specific surface area of ​​the porous hard carbon core is 42 m². 2 / g; the surface oxygen atom content of the porous hard carbon core is 9.3 at%; the porosity of the carbon coating layer is 9.5%; the thickness of the carbon coating layer is 25 nm.

[0122] Example 2

[0123] This embodiment provides a method for preparing a composite hard carbon anode material, the method comprising the following steps:

[0124] (1) Preparation of porous hard carbon materials, the steps include:

[0125] (a) Provide walnut shells containing cellulose and lignin, and wash and crush the walnut shells, and pass them through a 200-mesh sieve to obtain walnut shell powder; wherein the mass ratio of cellulose to lignin is 1:1.

[0126] (b) The walnut shell powder was pre-carbonized at 600°C for 4 hours in an argon atmosphere to obtain a porous hard carbon material.

[0127] (2) Dissolve epoxy resin in ethanol to obtain a resin precursor solution with a mass concentration of 5%.

[0128] The resin precursor solution is mixed with the porous hard carbon material, and then subjected to vacuum impregnation for 2 hours and ultrasonic dispersion for 0.5 hours to ensure that the resin precursor is uniformly distributed in the pores and surface of the porous hard carbon material. The solvent is then removed by rotary evaporation to obtain the composite material.

[0129] The mass ratio of the resin precursor to the porous hard carbon material is 1:5; the vacuum degree of the vacuum impregnation is 0.08 MPa; and the ultrasonic power of the ultrasonic dispersion is 300 W.

[0130] (3) The composite material is cured and crosslinked at 150°C for 3 hours in a nitrogen atmosphere containing ammonia, so that the resin formed forms a chemical bond with the surface functional groups of the porous hard carbon material, and a crosslinked cured body is obtained.

[0131] In the nitrogen atmosphere containing ammonia, the volume fraction of ammonia is 2%.

[0132] (4) In an argon atmosphere with an absolute pressure of 1 MPa, the cross-linked solidified body is first heated to 300°C at a heating rate of 1°C / min for 1.5 h of primary carbonization, and then heated to 1000°C at a heating rate of 4°C / min for 5 h of secondary carbonization to obtain the carbonized product.

[0133] The carbonization product was acid-washed with 1 mol / L HCl, then washed with water until neutral, and finally vacuum-dried at 80°C for 12 h to obtain the composite hard carbon anode material.

[0134] This embodiment also provides a composite hard carbon anode material, which is prepared by the preparation method described above.

[0135] The composite hard carbon anode material comprises a porous hard carbon core and a carbon coating layer covering the surface of the porous hard carbon core; the specific surface area of ​​the porous hard carbon core is 39 m². 2 / g; the surface oxygen atom content of the porous hard carbon core is 8.8 at%; the porosity of the carbon coating layer is 8.6%; the thickness of the carbon coating layer is 27 nm; the carbon coating layer contains nitrogen atoms, and the nitrogen atom content is 1.8 at%.

[0136] Example 3

[0137] This embodiment provides a method for preparing a composite hard carbon anode material, the method comprising the following steps:

[0138] (1) Preparation of porous hard carbon materials, the steps include:

[0139] (a) Provide coconut shells containing cellulose and lignin, and wash and crush the coconut shells and pass them through a 200-mesh sieve to obtain coconut shell powder; wherein the mass ratio of cellulose to lignin is 3:1.

[0140] (b) The coconut shell powder was pre-carbonized at 900°C for 1 hour in an argon atmosphere to obtain a porous hard carbon material.

[0141] (2) Melamine-formaldehyde resin was dissolved in ethanol to obtain a resin precursor solution with a mass concentration of 20%.

[0142] The resin precursor solution is mixed with the porous hard carbon material, and then subjected to vacuum impregnation for 2 hours and ultrasonic dispersion for 1 hour to ensure that the resin precursor is uniformly distributed in the pores and surface of the porous hard carbon material. The solvent is then removed by rotary evaporation to obtain the composite material.

[0143] The mass ratio of the resin precursor to the porous hard carbon material is 1:20; the vacuum degree of the vacuum impregnation is 0.09 MPa; and the ultrasonic power of the ultrasonic dispersion is 350 W.

[0144] (3) The composite material is cured and crosslinked at 250°C for 1 hour in a nitrogen atmosphere containing hydrogen, so that the resin formed forms a chemical bond with the surface functional groups of the porous hard carbon material to obtain a crosslinked cured body.

[0145] In the hydrogen-containing nitrogen atmosphere, the volume fraction of hydrogen is 8%.

[0146] (4) In an argon atmosphere with an absolute pressure of 5 MPa, the cross-linked solidified body is first heated to 500°C at a heating rate of 3°C / min for 0.5 h of primary carbonization, and then heated to 1300°C at a heating rate of 6°C / min for 2 h of secondary carbonization to obtain the carbonized product.

[0147] The carbonization product was acid-washed with 1 mol / L HCl, then washed with water until neutral, and finally vacuum-dried at 80°C for 12 h to obtain the composite hard carbon anode material.

[0148] This embodiment also provides a composite hard carbon anode material, which is prepared by the preparation method described above.

[0149] The composite hard carbon anode material comprises a porous hard carbon core and a carbon coating layer covering the surface of the porous hard carbon core; the specific surface area of ​​the porous hard carbon core is 43 m². 2 / g; the surface oxygen atom content of the porous hard carbon core is 8.2 at%; the porosity of the carbon coating layer is 9.2%; the thickness of the carbon coating layer is 33 nm; the carbon coating layer contains nitrogen atoms, and the nitrogen atom content is 2.9 at%.

[0150] Examples 4-12

[0151] The differences between Examples 4-12 and Example 1 are summarized in Tables 1 and 2.

[0152] The remaining preparation methods and parameters are consistent with those in Example 1.

[0153] Table 1

[0154] Table 2

[0155] Example 13

[0156] The difference between this embodiment and Embodiment 5 is that the mass ratio of the resin precursor to the porous hard carbon material is 1:25.

[0157] The remaining preparation methods and parameters are consistent with those in Example 5.

[0158] Example 14

[0159] The difference between this embodiment and embodiment 5 is that the mass ratio of the resin precursor to the porous hard carbon material is 1:2.

[0160] The remaining preparation methods and parameters are consistent with those in Example 5.

[0161] Example 15

[0162] The difference between this embodiment and Embodiment 5 is that the volume fraction of ammonia in the nitrogen atmosphere containing ammonia is 1%.

[0163] The remaining preparation methods and parameters are consistent with those in Example 5.

[0164] Example 16

[0165] The difference between this embodiment and Embodiment 5 is that the volume fraction of ammonia in the nitrogen atmosphere containing ammonia is 9%.

[0166] The remaining preparation methods and parameters are consistent with those in Example 5.

[0167] Example 17

[0168] The difference between this embodiment and embodiment 5 is that the pressure in the argon atmosphere in step (4) is 0.5 MPa.

[0169] The remaining preparation methods and parameters are consistent with those in Example 5.

[0170] Example 18

[0171] The difference between this embodiment and embodiment 5 is that the pressure in the argon atmosphere described in step (4) is 6 MPa.

[0172] The remaining preparation methods and parameters are consistent with those in Example 5.

[0173] Comparative Example 1

[0174] The difference between this comparative example and Example 5 is that steps (2), (3), and (4) are omitted. Instead, the peanut shell powder provided in step (1) is directly carbonized at 1200°C to obtain a hard carbon anode material; the specific surface area of ​​the hard carbon anode material is 210 m². 2 / g, surface oxygen content is 8at%, porosity is 25%.

[0175] The remaining preparation methods and parameters are consistent with those in Example 5.

[0176] Comparative Example 2

[0177] The difference between this comparative example and Example 5 is that the melamine-formaldehyde resin is replaced with coal-based needle-shaped coke pitch; the specific surface area of ​​the resulting hard carbon anode material is 85 m². 2 / g, surface oxygen content is 7.5at, coating thickness is 30nm, and coating porosity is 12%.

[0178] The remaining preparation methods and parameters are consistent with those in Example 5.

[0179] Comparative Example 3

[0180] The difference between this comparative example and Example 5 is that the peanut shells are replaced with corn starch.

[0181] The remaining preparation methods and parameters are consistent with those in Example 5.

[0182] Comparative Example 4

[0183] The difference between this comparative example and Example 5 is that vacuum impregnation and ultrasonic dispersion are not performed, i.e., homogenization is not performed.

[0184] The remaining preparation methods and parameters are consistent with those in Example 5.

[0185] Comparative Example 5

[0186] The difference between this comparative example and Example 5 is that the nitrogen atmosphere containing ammonia is replaced with a pure argon atmosphere.

[0187] The remaining preparation methods and parameters are consistent with those in Example 5.

[0188] Performance testing

[0189] The battery is fabricated based on the composite hard carbon anode material provided in the above embodiments and comparative examples. The specific steps include: mixing the composite hard carbon anode material, binder (sodium-based CMC), and conductive carbon black in a mass ratio of 8:1:1 to form a negative electrode sheet; using a sodium sheet as the counter electrode, the electrolyte is a 1 mol / L sodium hexafluorophosphate solution (the solvent is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1, with 5 vol% of fluoroethylene carbonate (FEC) added as an additive), and the separator is a PE separator; assembling to obtain a CR2032 coin cell.

[0190] 1) Initial Coulombic Efficiency Test: The assembled CR2032 coin cell was left to stand at room temperature for 12 hours to allow the electrolyte to fully wet the electrodes. It was then discharged at a constant current of 0.2C to 0.001V, left to stand for 30 minutes, and then charged at a constant current and constant voltage of 0.2C to 2V, and left to stand for 30 minutes. Initial Coulombic Efficiency = Initial Charge Capacity / Initial Discharge Capacity.

[0191] 2) Cycle performance test: After the initial coulombic efficiency test, the battery was placed in a constant temperature environment of 25℃±2℃ and charged at a constant current rate of 1C until the charging cutoff voltage was 2V. After resting for 10 minutes, it was discharged at a constant current rate of 1C until the discharge cutoff voltage was 0.001V, and then rested for 10 minutes. A total of 500 cycles were performed. Capacity retention rate = (500th cycle discharge capacity / 2nd cycle discharge capacity) × 100% (the capacity of the 2nd cycle was selected as the benchmark to exclude the irreversible capacity effect of the first cycle).

[0192] The test results are shown in Table 3.

[0193] Table 3

[0194] analyze:

[0195] As shown in Table 3, this invention prepares porous hard carbon by selecting biomass raw materials containing cellulose and lignin. The oxygen-containing functional groups naturally present in its molecular structure can chemically bond with the active groups (such as hydroxyl and amino groups) in the resin precursor during the curing and cross-linking process, thereby achieving a strong bond between the coating layer and the hard carbon matrix and effectively preventing the coating layer from falling off. At the same time, after the resin precursor fully penetrates the pores and is cured, cross-linked, and carbonized, a dense, firmly attached carbon coating layer with good electrochemical stability is formed, which can significantly suppress electrolyte side reactions and volume expansion. The composite hard carbon anode material prepared by this method exhibits excellent initial coulombic efficiency, high specific capacity, and long cycle stability when used in sodium-ion batteries, with an initial efficiency of up to 92% and a capacity retention of up to 94% after 500 cycles at 1C rate.

[0196] A comparison of Examples 5 and 13-14 shows that if the mass ratio of the resin precursor to the porous hard carbon material is too small, the carbon coating layer thickness is insufficient and cannot completely cover the pores on the surface of the hard carbon core. The side reactions between the electrolyte and the hard carbon surface are not sufficiently suppressed, resulting in a decrease in the initial coulombic efficiency, accelerated capacity decay during cycling, and low capacity retention. If the mass ratio of the resin precursor to the porous hard carbon material is too large, the carbon coating layer is too thick, which increases the sodium ion transport resistance, leading to a decrease in specific capacity and a deterioration in overall electrochemical performance.

[0197] A comparison of Examples 5 and 15-16 shows that if the volume fraction of ammonia in the nitrogen atmosphere containing ammonia is too small, the chemical bonding efficiency between the resin and the functional groups on the surface of hard carbon will decrease during the curing and crosslinking process. The amount of nitrogen atoms doped in the carbon coating layer will be insufficient, the adhesion of the coating layer will decrease, and local detachment will easily occur during cycling, with the capacity retention rate dropping below 89%. If the volume fraction of ammonia in the nitrogen atmosphere containing ammonia is too large, the corrosiveness of the atmosphere will be enhanced, which will lead to excessive removal of oxygen atoms from the surface of the hard carbon core. The porosity of the carbon coating layer will increase slightly, the interfacial bonding force between the hard carbon matrix and the coating layer will weaken, and the long-term cycling stability will decline slightly.

[0198] As can be seen from the comparison between Example 5 and Examples 17-18, if the pressure in the argon atmosphere in step (4) is too low, the carbon coating layer structure will be insufficiently dense and the porosity will be too high during the carbonization process, which will not effectively suppress the side reactions caused by electrolyte wetting and reduce the capacity retention rate. If the pressure in the argon atmosphere in step (4) is too high, the hard carbon core pores will be excessively compressed, the specific surface area will decrease, the sodium ion storage sites will be reduced, resulting in a slight decrease in specific capacity. Moreover, the high pressure does not bring performance improvement, but increases the process cost instead.

[0199] As can be seen from the comparison between Example 5 and Comparative Example 1, if the peanut shell powder is directly carbonized to obtain the hard carbon anode material, there is no carbon coating layer for protection, and the specific surface area of ​​the hard carbon material is significantly increased (reaching 210m²). 2 / g), the side reactions between the electrolyte and the hard carbon surface are intense, the solid electrolyte interface (SEI) film formed is unstable and too thick, resulting in a significant drop in the initial coulombic efficiency. During the cycling process, the SEI film continues to rupture and regenerate, resulting in severe capacity decay. The capacity retention rate after 500 cycles is low, far lower than that of composite hard carbon materials with carbon coating.

[0200] As can be seen from the comparison between Example 5 and Comparative Example 2, if the melamine-formaldehyde resin is replaced with coal-based needle-shaped tar pitch, the compatibility between the coal-based needle-shaped tar pitch and the functional groups on the surface of hard carbon is poor, and a strong chemical bond cannot be formed. The porosity of the prepared carbon coating layer increases, and the structure is loose and easy to fall off. At the same time, the coal-based needle-shaped tar pitch does not contain nitrogen, so nitrogen doping modification cannot be achieved, resulting in low initial coulombic efficiency, failure of the coating layer's protective effect during cycling, low capacity retention, and electrochemical performance that is significantly inferior to the melamine-formaldehyde resin system.

[0201] As can be seen from the comparison between Example 5 and Comparative Example 3, if peanut shells are replaced with corn starch, which does not contain cellulose and lignin, then there will be a lack of natural oxygen-containing functional groups to form chemical bonds with the resin precursor. The resin can only adhere to the surface of starch-based hard carbon through physical adsorption, and the carbon coating layer formed is easy to fall off. Moreover, the pore structure of starch-based hard carbon is significantly different from that of biomass shell-based hard carbon, resulting in a decrease in sodium ion transport efficiency, leading to an initial coulombic efficiency of less than 85%, a capacity retention rate of less than 88%, and a significant decline in overall performance.

[0202] As can be seen from the comparison between Example 5 and Comparative Example 4, if vacuum impregnation and ultrasonic dispersion are not performed, the resin precursor cannot be uniformly penetrated into the pores of the hard carbon core, but can only adhere to the surface of the hard carbon. The resulting carbon coating layer has an uneven thickness (fluctuating between 15-35 nm), with no coating protection in some areas. The electrolyte is prone to side reactions in these areas, leading to a decrease in the initial coulombic efficiency, accelerated capacity decay during cycling, and low capacity retention.

[0203] As can be seen from the comparison between Example 5 and Comparative Example 5, if the nitrogen atmosphere containing ammonia is replaced with a pure argon atmosphere, the curing and crosslinking process lacks a nitrogen source and an active atmosphere. The number of chemical bonds between the resin and the functional groups on the hard carbon surface is significantly reduced, and the adhesion strength is greatly reduced. During the cycle, the coating layer is easily peeled off from the hard carbon surface, and it is unable to continuously suppress the side reactions and volume expansion of the electrolyte, resulting in a significant decrease in capacity retention and a significant decrease in the first coulombic efficiency. The electrochemical performance is significantly worse than that of the nitrogen atmosphere system containing ammonia.

[0204] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a composite hard carbon anode material, characterized in that, The preparation method includes the following steps: Porous hard carbon materials are prepared using biomass raw materials; wherein, the biomass raw materials include cellulose and lignin; The resin precursor solution is mixed with the porous hard carbon material, homogenized, and then dried to obtain the composite material; the resin precursor in the resin precursor solution contains hydroxyl and / or amino groups. The composite material is cured and crosslinked in a functional atmosphere to obtain a crosslinked cured body; The cross-linked cured body is carbonized and post-treated to obtain the composite hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, The biomass raw material is fruit shell; in the fruit shell, the mass ratio of cellulose to lignin is (1~3):1; The shell includes any one or a combination of at least two of the following: peanut shell, walnut shell, almond shell, or coconut shell. And / or, the resin precursor in the resin precursor solution includes any one or a combination of at least two of phenolic resin, epoxy resin, or melamine-formaldehyde resin.

3. The preparation method according to claim 1, characterized in that, The steps for preparing porous hard carbon materials from biomass raw materials include: The porous hard carbon material is obtained by pre-carbonizing the biomass raw material. The pre-carbonization temperature is 600℃~900℃, and the pre-carbonization holding time is 1h~4h.

4. The preparation method according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The mass concentration of the resin precursor solution is 5%~20%; (2) The mass ratio of the resin precursor in the resin precursor solution to the porous hard carbon material is 1:(5~20); (3) The homogenization process includes vacuum impregnation and ultrasonic dispersion performed sequentially.

5. The preparation method according to claim 1, characterized in that, The functional atmosphere includes any one of an air atmosphere, an inert atmosphere containing ammonia, or an inert atmosphere containing hydrogen. And / or, the curing crosslinking temperature is 150℃~250℃, and the time is 1h~3h.

6. The preparation method according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The carbonization atmosphere is an inert atmosphere; (2) During the carbonization process, the absolute pressure is 1 MPa to 5 MPa; (3) The carbonization process includes a first-stage carbonization and a second-stage carbonization performed sequentially. The temperature of the first-stage carbonization is 300℃~500℃, and the temperature of the second-stage carbonization is 1000℃~1300℃.

7. The preparation method according to claim 1, characterized in that, The post-processing includes pickling, washing, and drying performed sequentially.

8. A composite hard carbon anode material, characterized in that, The composite hard carbon anode material is prepared by the preparation method described in any one of claims 1 to 7; The composite hard carbon anode material includes a porous hard carbon core and a carbon coating layer covering the surface of the porous hard carbon core.

9. The composite hard carbon anode material according to claim 8, characterized in that, At least one of the following conditions must be met: (1) The specific surface area of ​​the porous hard carbon core is 30 m². 2 / g~50m 2 / g; (2) The surface oxygen atom content of the porous hard carbon core is 5 at%~11.5 at%; (3) The porosity of the carbon coating layer is 5%~10%; (4) The thickness of the carbon coating layer is 10 nm to 50 nm; (5) The carbon coating layer contains nitrogen atoms, and the content of nitrogen atoms is 1 at% to 5 at%.

10. A sodium-ion battery, characterized in that, The negative electrode of the sodium-ion battery includes the composite hard carbon negative electrode material as described in claim 8 or 9.

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