A hard carbon precursor, hard carbon and preparation method thereof, and sodium battery

Abstract

The application discloses a hard carbon precursor, a hard carbon and a preparation method thereof and a sodium battery, belongs to the technical field of electrochemistry, and overcomes the defects of low compaction density, excessively large specific surface area and low coulomb efficiency of a biomass hard carbon in the prior art. The hard carbon precursor comprises a biomass precursor and a coating layer precursor; raw materials of the biomass precursor are biomasses containing aldehyde groups on surfaces; and the coating layer precursor comprises a polymer. The coating layer precursor reacts with the aldehyde groups on the surface of the raw materials of the biomass precursor, so that firm chemical bonds are formed between the biomass precursor and the coating layer precursor, the compact and uniform coating of the coating layer precursor is realized, the coating layer will not be curled due to the compatibility problem in the temperature rising process of preparing the hard carbon, the coating layer is uniform, the compaction density and the coulomb efficiency of the prepared hard carbon are improved, and the specific surface area is reduced.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical technology, specifically relating to a hard carbon precursor, hard carbon and its preparation method, and sodium batteries. Background Technology

[0002] As one of the alternatives to lithium batteries, sodium batteries have a slightly lower energy density than lithium batteries, but they have good low-temperature performance and extremely low cost of positive electrode materials. If combined with low-cost and readily available negative electrode materials, sodium batteries can replace lithium batteries in fields such as A0-class cars, power tools, and energy storage power stations, reducing the excessive demand for lithium ore and thus alleviating industrial and living costs.

[0003] Graphite and soft carbon materials suffer from poor sodium storage performance due to their lack of closed-cell structure and excessively small carbon layer spacing, with their anode specific capacity often falling below 200 mAh / g, failing to meet ideal requirements. In contrast, during the carbonization process of hard carbon material precursors, the irregular arrangement of carbon layers hinders the growth of graphite crystals, resulting in numerous closed pores and defects, exhibiting a long-range disordered structure. These closed pores and defects provide sodium storage sites, making hard carbon materials an excellent anode material for sodium-ion batteries.

[0004] Compared to soft carbon and graphite, hard carbon has a wider variety of precursors, such as thermosetting resins with more O groups, thermoplastic resins that form more cross-linked structures through carbonization, and biomass. One of the important prerequisites for hard carbon precursor materials is low cost. Because lithium batteries hold an irreplaceable position in terms of performance (such as energy density and cycle performance), sodium batteries, as their alternative, must have a cost advantage. Biomass, such as sugars, bio-lignin, wood scraps, and crop straw, are widely available and inexpensive, making them promising candidates for hard carbon precursor materials.

[0005] For conventional biomass hard carbon, carbon precursors in organisms do not pack as tightly as polymers. For example, cellulose is mainly found in the cell walls and Golgi apparatus of plant cells, which results in low compaction density and excessively large specific surface area of ​​the prepared carbon materials, thus reducing the coulombic efficiency of the anode material.

[0006] If resin-based hard carbon precursors are directly used to coat biomass precursors to improve compaction density, reduce specific surface area, and improve coulombic efficiency, the coating effect will be poor due to the poor wettability of the two. Increasing the coating amount will also increase the material cost, which is not conducive to commercialization. Summary of the Invention

[0007] Therefore, the technical problem to be solved by the present invention is to overcome the defects of low compaction density, excessive specific surface area and low coulombic efficiency of biomass hard carbon in the prior art, thereby providing a hard carbon precursor, hard carbon and its preparation method, and sodium battery.

[0008] To this end, the present invention provides the following technical solution.

[0009] In a first aspect, the present invention provides a hard carbon precursor, comprising a biomass precursor and a coating layer precursor;

[0010] The raw material for the biomass precursor is biomass containing aldehyde groups on its surface;

[0011] The coating precursor includes a polymer.

[0012] Furthermore, the mass ratio of the biomass precursor to the coating layer precursor is (85-98):(15-2).

[0013] Secondly, the present invention provides a method for preparing a hard carbon precursor, comprising the following steps:

[0014] Step 1: Obtain biomass with aldehyde groups on its surface;

[0015] Step 2: Mix biomass containing aldehyde groups on the surface with the coating layer precursor raw material, heat and react to obtain hard carbon precursor.

[0016] Furthermore, step 1 includes: dispersing the biomass in a first acid solution, then adding an oxidant, reacting, and obtaining biomass with aldehyde groups on the surface;

[0017] Preferably, the pH of the first acid solution is 1 to 4;

[0018] Preferably, the temperature of the first acid solution is 30–60°C;

[0019] Preferably, the reaction temperature is 30–60°C and the reaction time is 1–6 h;

[0020] Preferably, the oxidant is periodate; for example, sodium periodate; in an acidic environment, the active ingredient of periodate is periodic acid and its salt;

[0021] Preferably, the molar concentration of the oxidant in the first acid solution is 0.07–0.2 mol / L;

[0022] Preferably, the reaction is carried out in a light-protected, nitrogen-protected environment.

[0023] Furthermore, the first acid solution is one or more of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, citric acid, lactic acid, acetic acid, formic acid, and glycolic acid prepared with pure water.

[0024] Furthermore, in step 2, the coating layer precursor raw materials include phenolic substances and aldehyde substances;

[0025] Preferably, step 2 includes: performing a first-stage reaction of biomass containing aldehyde groups on its surface and phenolic substances in an alkaline aqueous solution, and then adding aldehyde substances to perform a second-stage reaction;

[0026] Furthermore, the solute in the alkaline aqueous solution includes one or more of NaOH, Ba(OH)2, Ca(OH)2, Mg(OH)2, Na2CO3, tertiary amines, and hexamethylenetetramine.

[0027] More preferably, the total mass of the phenolic substances and aldehyde substances is 2% to 15% of the mass of the biomass containing aldehyde groups on its surface;

[0028] More preferably, the molar ratio of the phenolic substance to the aldehyde substance is 1:(1 to 1.5);

[0029] More preferably, the pH of the alkaline aqueous solution is 10 to 13;

[0030] More preferably, the temperature of the first stage reaction is ≤60℃;

[0031] More preferably, the reaction time of the first stage is 2 to 6 hours;

[0032] More preferably, the temperature of the second stage reaction is 60–80°C;

[0033] More preferably, the reaction time of the second stage is 2 to 4 hours;

[0034] More preferably, the phenolic substance is at least one of phenol, hydroquinone, resorcinol, catechol, 2-hydroxymethylphenol, 4-hydroxymethylphenol, 2,4-dihydroxymethylphenol, 2,4,6-trihydroxymethylphenol, o-cresol, m-cresol, p-cresol, 1-hydroxy-2,6-dimethylbenzene, 1-hydroxy-3,5-dimethylbenzene, 1-hydroxy-2,5-dimethylbenzene, 1-hydroxy-2,4-dimethylbenzene, 1-hydroxy-3,4-dimethylbenzene, or p-tert-butylphenol;

[0035] More preferably, the aldehyde is at least one of benzaldehyde, terephthalaldehyde, formaldehyde, trioxymethylene, paraformaldehyde, and furfural.

[0036] Furthermore, in step 2, the coating layer precursor raw materials include coupling agents, epoxy oligomers, and curing agents;

[0037] Preferably, step 2 includes: a third-stage reaction of biomass containing aldehyde groups on its surface and a coupling agent in a solvent, followed by the addition of an epoxy oligomer for a fourth-stage reaction, removal of the solvent, and curing.

[0038] More preferably, the coupling agent is an amine substance with bifunctionality or higher, such as at least one of aliphatic amines, unsaturated amines, cyanamides or aromatic amines;

[0039] More preferably, the curing agent is at least one of the following: amines with difunctionality or higher, phenolic resins, and carboxylic anhydrides.

[0040] More preferably, the reaction temperature of the third stage is 40-80°C;

[0041] More preferably, the reaction time of the third stage is 1.5 to 4 hours;

[0042] More preferably, the reaction temperature of the fourth stage is 50-80°C;

[0043] More preferably, the reaction time of the fourth stage is 2 to 4 hours;

[0044] More preferably, the curing temperature is 80-180℃; even more preferably 80-150℃;

[0045] More preferably, the coupling agent comprises 0.1% to 0.5% of the biomass containing aldehyde groups on its surface; even more preferably, it comprises 0.3% to 0.5%.

[0046] More preferably, the epoxy oligomer is one or more of glycidyl ethers, glycidyl esters, glycidyl amines, alicyclic or aliphatic epoxy resins;

[0047] For example, bisphenol A type epoxy resin or bisphenol F type epoxy resin;

[0048] More preferably, the solvent includes at least one of acetone, N,N'-dimethylformamide, toluene, ethanol, or ethyl acetate.

[0049] Furthermore, in step 2, the mass of the epoxy oligomer is 2%-10% of the biomass containing aldehyde groups on its surface;

[0050] or

[0051] In step 2, the mass of the epoxy oligomer is 2%-6% of the biomass containing aldehyde groups on its surface; after the fourth stage reaction is completed, 80%-160% of the mass of the epoxy oligomer is added to phenolic resin.

[0052] Thirdly, the present invention provides a method for preparing hard carbon, wherein the raw materials include the hard carbon precursor described above or the hard carbon precursor prepared according to the above method.

[0053] Preferably, the method for preparing the hard carbon includes the following steps:

[0054] Step 1': Pre-carbonize the hard carbon precursor to obtain a pre-carbonized product;

[0055] Step 2': Crush the pre-carbonized product into powder;

[0056] Step 3': Carbonize the powder to obtain the hard carbon;

[0057] More preferably, in step 1', the pre-carbonization temperature is 500-800℃;

[0058] More preferably, in step 1', the pre-carbonization and heat preservation time is 1 to 4 hours;

[0059] More preferably, in step 2', the powder D50 is 5-10 μm;

[0060] More preferably, in step 3', the carbonization temperature is 1100-1600℃;

[0061] More preferably, in step 3', the carbonization time is 1 to 4 hours.

[0062] Fourthly, the present invention provides a hard carbon prepared according to the method described above.

[0063] Fifthly, the present invention provides a sodium battery in which the negative electrode material includes the aforementioned hard carbon.

[0064] Step 1 includes: dispersing the biomass in a first acid solution, then adding an oxidizing agent, reacting to obtain biomass with aldehyde groups on its surface. The oxidizing agent oxidizes the C2 and C3 hydroxyl groups of the cellulose on the surface of the biomass into aldehyde groups on C2 and C3, and breaks the C-C bonds between C2 and C3. Taking periodic acid / salt as an example, the reaction mechanism is shown in the following formula:

[0065]

[0066] Furthermore, after the reaction in step 1 is completed, pure water is used to wash away any residual impurities on the surface of the biomass containing aldehyde groups.

[0067] In the first possible design, in step 2, the raw materials for the coating layer precursor include phenolic substances and aldehyde substances;

[0068] Preferably, step 2 includes: performing a first-stage reaction of biomass containing aldehyde groups on its surface and phenolic substances in an alkaline aqueous solution, and then adding aldehyde substances to perform a second-stage reaction;

[0069] More preferably, the temperature of the first stage reaction is ≤60℃ to ensure that the phenolic substances react mainly with the aldehyde groups on the biomass surface via addition. After the first and second stage reactions, a biomass precursor-phenolic resin in-situ coating structure is finally formed.

[0070] More preferably, the second reaction gradually polymerizes the unreacted free phenols in the system into a three-dimensional structure.

[0071] In the second possible design, in step 2, the coating layer precursor raw materials include coupling agents, epoxy oligomers, and curing agents;

[0072] Preferably, step 2 includes: a third-stage reaction of biomass containing aldehyde groups on its surface and a coupling agent in a solvent, followed by the addition of an epoxy oligomer for a fourth-stage reaction, removal of the solvent, and a curing reaction.

[0073] A small amount of ethylenediamine is added as a coupling agent. At 50-80℃, the ethylenediamine reacts with the aldehyde groups on the surface of the biomass containing aldehyde groups to form a Schiff base bridging structure. Subsequently, an epoxy oligomer is added to in situ coat the biomass containing aldehyde groups on its surface.

[0074] Finally, ethylenediamine is added to the system as a curing agent, the temperature is raised to 80-150℃ and kept at that temperature for 1-2 hours to complete the curing process, forming a three-dimensional structure that is insoluble and infusible.

[0075] When the curing agent is a diamine, the amount of curing agent depends on the amount of epoxy oligomer and its epoxy equivalent. The mass of diamine curing agent / mass of epoxy oligomer = (molecular weight of diamine / total number of active hydrogen atoms in the amine molecule) / epoxy equivalent.

[0076] In a third possible design, in step 2, the coating layer precursor raw material includes a coupling agent, an epoxy oligomer, and a curing agent; phenolic resin is used as the curing agent. Preferably, the phenolic resin is one or more of phenolic resin, o-cresol phenolic resin, bisphenol A phenolic resin, and bisphenol F phenolic resin.

[0077] Preferably, the molecular weight of the phenolic resin is 400 to 3000.

[0078] Phenolic resin is a linear or first-order thermosetting phenolic resin.

[0079] Ethylenediamine, a coupling agent, is added. At 50-80℃, ethylenediamine first reacts with the aldehyde groups on the surface of the biomass to form a Schiff base bridging structure. Subsequently, an epoxy oligomer is added to coat the biomass in situ, with the amount added being 2-6% of the mass of the biomass containing aldehyde groups on the surface; finally, phenolic resin is added as a curing agent.

[0080] Sub-scheme 1: Add thermoplastic phenolic resin (80%-150% of the mass of epoxy oligomer) and hexamethylenetetramine (8-10% of the mass of phenolic resin), heat to 80-150℃ and keep warm for 1-5 hours to complete curing, so that the entire coating layer forms a cross-linked interlocking structure, making it insoluble and infusible.

[0081] Sub-scheme 2: Add methyl phenolic resin (thermosetting) (the amount added is 90%-160% of the amount of epoxy oligomer), heat to 80-150℃, and keep at the temperature for 1-5 hours to complete the curing, so that the entire coating layer forms a cross-linked interlocking structure, making it insoluble and infusible.

[0082] In the above system, phenolic resin acts as a curing agent, and during subsequent carbonization, the epoxy resin with low residual carbon can also play a role in regulating the pore structure.

[0083] Furthermore, in step 1, before dispersing the biomass in the first acid solution, the method further includes: using a second acid solution to deash the biomass.

[0084] Preferably, the pH of the second acid solution is <2, and the pickling time is 2-6 hours;

[0085] Preferably, after ash removal, the biomass is washed with pure water;

[0086] Preferably, the second acid solution is one or more of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, citric acid, lactic acid, acetic acid, formic acid, and glycolic acid prepared with pure water.

[0087] (2) In step 1, the biomass is one or more of sawdust, bamboo powder, walnut shell powder, rice husk powder, camellia shell powder, rapeseed oil shell powder, tea seed cake, corn cob, and sugarcane bagasse powder.

[0088] Preferably, the biomass is at the millimeter level.

[0089] Furthermore, the curing process is preferably completed in an oven (with vacuum).

[0090] The technical solution of this invention has the following advantages:

[0091] 1. The hard carbon precursor provided by the present invention includes a biomass precursor and a coating layer precursor; the raw material of the biomass precursor is biomass containing aldehyde groups on its surface; the coating layer precursor includes a polymer.

[0092] The reaction between the aldehyde groups on the surface of the coating precursor and the biomass precursor raw material forms a strong chemical bond between the biomass precursor and the coating precursor, thereby achieving a tight and uniform coating of the coating precursor. This prevents the coating precursor from shrinking due to compatibility issues during the heating process of hard carbon preparation, resulting in a uniform coating layer. Consequently, the compressive density and coulombic efficiency of the hard carbon are improved, while the specific surface area is reduced.

[0093] 2. The method for preparing the hard carbon precursor provided by the present invention, wherein step 1 includes: dispersing the biomass in a first acid solution, then adding an oxidant, reacting, and obtaining biomass with aldehyde groups on its surface. The present invention uses chemically controllable means to introduce active groups onto the surface of biomass.

[0094] 3. In the preparation method of hard carbon precursor provided by the present invention, step 1 is carried out in a light-proof and nitrogen-protected environment to avoid the decomposition of the oxidant itself.

[0095] The pH of the first acid solution is 1 to 4, which can maintain the stability of the oxidant. If the pH is too low, the oxidant is too reactive and easily undergoes a reduction reaction. If the pH is too high, the oxidizing effect of the oxidant decreases.

[0096] 4. The method for preparing hard carbon precursor provided by the present invention, wherein step 2 includes: performing a first-stage reaction of biomass containing aldehyde groups on the surface and phenolic substances in an alkaline aqueous solution, and then adding aldehyde substances to perform a second-stage reaction.

[0097] This invention uses in-situ polymerization coating technology, which makes it easier for monomers and oligomers with lower viscosity to spread on the surface of biomass precursors. The post-crosslinking process can avoid the phenomenon of uneven coating caused by the softening of the coating layer precursor during the subsequent carbonization process.

[0098] 5. The method for preparing hard carbon precursor provided by the present invention, wherein the raw material of the coating layer precursor in step 2 includes a coupling agent, an epoxy oligomer, and a curing agent; step 2 includes: carrying out a third-stage reaction of biomass containing aldehyde groups on the surface and the coupling agent in a solvent, then adding the epoxy oligomer to carry out a fourth-stage reaction, and adding the curing agent to cure.

[0099] This invention utilizes a cross-linking reaction between biomass containing aldehyde groups on its surface and a coupling agent and a polymer to form a covalent bond between the coating layer precursor and the biomass precursor. This prevents the coating layer precursor from shrinking due to compatibility issues during the heating process, resulting in a uniform coating layer and improved electrochemical performance. Detailed Implementation

[0100] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0101] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0102] Example 1

[0103] This embodiment discloses a method for preparing hard carbon, including the following steps:

[0104] (1) Ash removal treatment: The biomass was washed with an acid solution of pH=1 for 4 hours, followed by washing with pure water to remove residual ash. In this embodiment, the acid solution was an aqueous solution of hydrochloric acid, the biomass was sawdust, and the median particle size of the biomass was 250 μm.

[0105] (2) Introduction of aldehyde groups on the surface of biomass: In a light-protected, nitrogen-protected environment, biomass was dispersed in a hydrochloric acid aqueous solution at pH 3 and 50°C with a solid-liquid ratio of 50%. After uniform dispersion, sodium periodate was added within 30 minutes to achieve a total molar concentration of periodic acid + sodium periodate of 0.07 mol / L. After all the sodium periodate was added, the reaction was continued at 50°C for 6 hours. After the reaction was completed, residual impurities on the surface of the biomass were washed away with pure water.

[0106] (3) In the kneading equipment, the product obtained in (2) is mixed with 10% of its mass of phenol in a NaOH aqueous solution with pH 10 and reacted at 40°C for 4 hours; then formaldehyde with a mass of 1.3 times that of phenol is added, the temperature is raised to 80°C, and the reaction is carried out again for 3 hours.

[0107] (4) The product obtained in (3) is fed into a carbonization device and pre-carbonized at 600℃ for 2 hours to obtain a pre-carbonized product.

[0108] (5) Use a crushing-sieving device to crush the product of step (4) into powder with a D50 of 7 μm.

[0109] (6) The product of (5) is sent to a high-temperature carbonization device and kept at 1250°C for 4 hours to obtain hard carbon.

[0110] Example 2

[0111] This embodiment discloses a method for preparing hard carbon, including the following steps:

[0112] (1) Ash removal treatment: The biomass was washed with an acid solution of pH=1.3 for 6 hours, and then washed with pure water to remove residual ash. In this embodiment, the acid solution was an aqueous solution of hydrochloric acid, the biomass was bamboo powder, and the particle size of the biomass was 200μm.

[0113] (2) Introduction of aldehyde groups on the surface of biomass: In a light-proof, nitrogen-protected environment, biomass was dispersed in a hydrochloric acid aqueous solution at pH 4 and temperature 45℃ with a solid-liquid ratio of 30%. After uniform dispersion, sodium periodate was added within 40 minutes to make the total molar concentration of periodic acid + sodium periodate in the solution 0.08 mol / L. After all the sodium periodate was added, the reaction continued for 5 hours. After the reaction was completed, residual impurities on the surface of the biomass were washed away with pure water.

[0114] (3) In a kneading device, the product obtained in (2) and 0.5% of its mass of ethylenediamine are mixed in acetone and reacted at 60°C for 2 hours; then 10% of the mass of bisphenol A glycidyl ether epoxy resin (molecular weight 300) containing aldehyde groups on its surface is added, the temperature is raised to 80°C, and the reaction is carried out for 3 hours, while the solvent is gradually evaporated. Then, the curing agent ethylenediamine is added, and the temperature is raised to 140°C for curing for 2 hours.

[0115] (4) The product obtained in (3) is fed into a carbonization device and carbonized at 650°C for 3 hours to obtain a pre-carbonized product.

[0116] (5) Use a crushing-sieving device to crush the product of step (4) into powder with a D50 of 6 μm.

[0117] (6) The product of (5) is sent to a high-temperature carbonization device, heated to 1300°C for 3 hours to obtain hard carbon.

[0118] Example 3

[0119] This embodiment discloses a method for preparing hard carbon, including the following steps:

[0120] (1) Ash removal treatment: The biomass was washed with an acid solution of pH=0.7 for 3 hours, and then washed with pure water to remove residual ash. In this embodiment, the acid solution was an aqueous solution of hydrochloric acid, the biomass was walnut shell powder, and the particle size of the biomass was 300μm.

[0121] (2) Introduction of aldehyde groups on the surface of biomass: In a light-proof, nitrogen-protected environment, biomass was dispersed in a hydrochloric acid aqueous solution at pH=2 and temperature of 60℃, with a solid-liquid ratio of 40%. After uniform dispersion, sodium periodate was added within 35 minutes to make the total molar concentration of periodic acid + sodium periodate in the solution 0.09 mol / L. After all the sodium periodate was added, the reaction continued for 4 hours. After the reaction was completed, residual impurities on the surface of the biomass were washed away with pure water.

[0122] (3) In a kneading device, the product obtained in (2) and 0.4% of its mass of ethylenediamine were mixed with acetone and reacted at 65°C for 1.5 h. Then, 5% of the biomass mass of bisphenol A glycidyl ether epoxy resin (molecular weight 300) obtained in (2) was added, and the temperature was raised to 75°C for 4 h, while the solvent was gradually evaporated. 8% of the biomass mass of phenolic resin (phenolic resin with a molecular weight of 2000 prepared from phenol and formaldehyde) was added, and the temperature was further raised to 150°C for curing for 5 h.

[0123] (4) The product obtained in (3) is fed into a carbonization device and carbonized at 500°C for 4 hours to obtain a pre-carbonized product.

[0124] (5) Use a crushing-sieving device to crush the product of step (4) into powder with a D50 of 8 μm.

[0125] (6) The product of (5) is sent to a high-temperature carbonization device and kept at 1450°C for 2 hours to obtain hard carbon.

[0126] Comparative Example 1

[0127] This comparative example discloses a method for preparing hard carbon, which is basically the same as that in Example 1, except that it does not include the step of (2) introducing an aldehyde group.

[0128] Comparative Example 2

[0129] This comparative example discloses a method for preparing hard carbon, which is basically the same as that in Example 2, except that it does not include the step of introducing an aldehyde group (2).

[0130] Comparative Example 3

[0131] This comparative example discloses a method for preparing hard carbon, which is basically the same as that in Example 3, except that it does not include the step of (2) introducing an aldehyde group.

[0132] Comparative Example 4

[0133] This comparative example discloses a method for preparing hard carbon, comprising the following steps:

[0134] (1) Ash removal treatment: The biomass was washed with an acid solution of pH=1 for 4 hours, followed by washing with pure water to remove residual ash. In this embodiment, the acid solution was an aqueous solution of hydrochloric acid, the biomass was sawdust, and the median particle size of the biomass was 250 μm.

[0135] (2) The product obtained in (1) is fed into a carbonization device and pre-carbonized at 600℃ for 2 hours to obtain a pre-carbonized product.

[0136] (3) Use a crushing-sieving device to crush the product of step (2) into powder with a D50 of 7 μm.

[0137] (4) The product of (3) is sent to a high-temperature carbonization device and kept at 1250°C for 4 hours to obtain hard carbon.

[0138] Test case

[0139] 1. The compaction density of the hard carbon prepared in Examples 1-3 and Comparative Examples 1-4 was tested. The test process was as follows: weigh the sample, put it into a special mold for compaction density testing, place the assembled mold in the center of the test platform, apply 1T pressure to the mold and record the mold scale at this time, then apply 5T pressure to the mold and record the mold scale at this time. Calculate the compaction density of the item under 1T and 5T pressure based on the mass and volume of the item.

[0140] 2. The specific surface area of ​​the hard carbon prepared in Examples 1-3 and Comparative Examples 1-4 was tested. The test process was as follows: First, the sample and the control sample were placed in the test tube, then heated to 150 degrees Celsius, and then purged with nitrogen for 30 minutes. After purging, the sample and the test tube were cooled, and the mass of the sample and the control sample at this time was recorded. The standard sample was connected in parallel to the test gas path (nitrogen) that was exactly the same as the sample to be tested, and the adsorption and desorption process was carried out together with the sample to be tested. The specific surface area of ​​the sample = mass of the standard sample × desorption peak area of ​​the sample to be tested × specific surface area of ​​the standard sample ÷ mass of the sample to be tested ÷ desorption peak area of ​​the standard sample.

[0141] 3. Sodium battery preparation: Hard carbon powder, conductive agent SuperP, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed evenly at a mass ratio of 94.5:1.5:1.5:2.5. A suitable amount of water is added and stirred to form a slurry. The slurry is then evenly coated onto a current collector copper foil. After drying, it is cut into circular electrode sheets with a diameter of 18 mm. Under vacuum conditions, the electrode sheets are dried at 80°C with forced air for about 3 hours, then vacuum dried at 80°C for 6 hours, and finally rolled to a surface density of 4-7 mg / cm³. 2 The cells were then transferred to a glove box for later use. The assembly of the simulated battery was carried out in an Ar atmosphere glove box, using a sodium metal sheet as the counter electrode and 1M NaPF6 dissolved in ethylene carbonate solution as the electrolyte, to assemble a CR2430 button cell.

[0142] 4. The coin cell performance of the sodium-ion battery was tested using a constant current charge-discharge mode at a current density of 0.1C. Cyclic charge-discharge performance was tested under the conditions of a discharge cutoff voltage of 0V and a charge cutoff voltage of 2.0V. The effective capacity (mAh / g) and coulombic efficiency (%) of the first cycle were measured and recorded.

[0143] The test results are shown in Table 1.

[0144] Table 1 Performance parameters of hard carbon and sodium-ion batteries

[0145]

[0146]

[0147] A comparison of Example 1 with Comparative Example 1, Example 2 with Comparative Example 2, and Example 3 with Comparative Example 3 shows that oxidation treatment of biomass precursor materials and introduction of aldehyde groups improves the 1T and 5T compaction density, first-cycle effective capacity, and coulombic efficiency of hard carbon materials.

[0148] A comparison of Examples 1-3 and Comparative Example 4 shows that after using the oxidative treatment-resin modified biomass precursor, the 1T and 5T compaction densities, first-cycle effective capacity, and coulombic efficiency of the material are significantly improved.

[0149] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing hard carbon, characterized in that, Includes the following steps: Step 1': Pre-carbonize the hard carbon precursor to obtain a pre-carbonized product; Step 2': Crush the pre-carbonized product into powder; Step 3': Carbonize the powder to obtain the hard carbon; In step 1', the pre-carbonization temperature is 500-800℃; In step 3', the carbonization temperature is 1100-1600℃; The hard carbon precursor includes a biomass precursor and a coating layer precursor; The raw material for the biomass precursor is biomass containing aldehyde groups on its surface; The coating layer precursor includes a polymer; The mass ratio of the biomass precursor to the coating layer precursor is (85-98):(15-2). The preparation method of the hard carbon precursor includes the following steps: Step 1: Obtain biomass with aldehyde groups on its surface; Step 2: Mix biomass containing aldehyde groups on the surface with the coating layer precursor raw material, heat and react to obtain hard carbon precursor; In step 2, the coating layer precursor raw materials include phenolic substances and aldehyde substances; or in step 2, the coating layer precursor raw materials include coupling agents, epoxy oligomers and curing agents.

2. The method for preparing hard carbon according to claim 1, characterized in that, Step 1 includes: dispersing the biomass in a first acid solution, then adding an oxidant, reacting, and obtaining biomass with aldehyde groups on its surface.

3. The method for preparing hard carbon according to claim 2, characterized in that, The pH of the first acid solution is 1 to 4.

4. The method for preparing hard carbon according to claim 2, characterized in that, The temperature of the first acid solution is 30~60℃.

5. The method for preparing hard carbon according to claim 2, characterized in that, The reaction temperature is 30~60℃, and the reaction time is 1~6h.

6. The method for preparing hard carbon according to claim 2, characterized in that, The oxidant is periodate.

7. The method for preparing hard carbon according to claim 2, characterized in that, The molar concentration of the oxidant in the first acid solution is 0.07~0.2 mol / L.

8. The method for preparing hard carbon according to claim 2, characterized in that, The reaction takes place in a light-protected environment under nitrogen protection.

9. The method for preparing hard carbon according to claim 1, characterized in that, Step 2 includes: performing a first-stage reaction on biomass and phenolic substances containing aldehyde groups on their surfaces in an alkaline aqueous solution, and then adding aldehyde substances to perform a second-stage reaction.

10. The method for preparing hard carbon according to claim 9, characterized in that, The total mass of the phenolic and aldehyde substances is 2% to 15% of the mass of biomass containing aldehyde groups on its surface.

11. The method for preparing hard carbon according to claim 9, characterized in that, The molar ratio of the phenolic substances to the aldehydes is 1:(1~1.5).

12. The method for preparing hard carbon according to claim 9, characterized in that, The pH of the alkaline aqueous solution is 10-13.

13. The method for preparing hard carbon according to claim 9, characterized in that, The temperature of the first stage reaction is ≤60℃.

14. The method for preparing hard carbon according to claim 9, characterized in that, The reaction time for the first stage is 2 to 6 hours.

15. The method for preparing hard carbon according to claim 9, characterized in that, The temperature of the second stage reaction is 60~80℃.

16. The method for preparing hard carbon according to claim 9, characterized in that, The second stage of the reaction takes 2 to 4 hours.

17. The method for preparing hard carbon according to claim 9, characterized in that, The phenolic substance is at least one of phenol, hydroquinone, resorcinol, catechol, 2-hydroxymethylphenol, 4-hydroxymethylphenol, 2,4-dihydroxymethylphenol, 2,4,6-trihydroxymethylphenol, o-cresol, m-cresol, p-cresol, 1-hydroxy-2,6-dimethylbenzene, 1-hydroxy-3,5-dimethylbenzene, 1-hydroxy-2,5-dimethylbenzene, 1-hydroxy-2,4-dimethylbenzene, 1-hydroxy-3,4-dimethylbenzene, or p-tert-butylphenol.

18. The method for preparing hard carbon according to claim 9, characterized in that, The aldehydes are at least one of benzaldehyde, terephthalaldehyde, formaldehyde, trioxymethylene, paraformaldehyde, and furfural.

19. The method for preparing hard carbon according to claim 1, characterized in that, Step 2 includes: a third-stage reaction of biomass containing aldehyde groups on its surface and a coupling agent in a solvent, followed by the addition of an epoxy oligomer for a fourth-stage reaction, removal of the solvent, and curing.

20. The method for preparing hard carbon according to claim 19, characterized in that, The coupling agent is an amine substance with bifunctionality or higher.

21. The method for preparing hard carbon according to claim 20, characterized in that, The coupling agent is at least one of aliphatic amines, unsaturated amines, cyanamides, or aromatic amines.

22. The method for preparing hard carbon according to claim 19, characterized in that, The curing agent is at least one of the following: amines with difunctionality or higher, phenolic resins, and carboxylic anhydrides.

23. The method for preparing hard carbon according to claim 19, characterized in that, The reaction temperature of the third stage is 40-80℃.

24. The method for preparing hard carbon according to claim 19, characterized in that, The reaction time for the third stage is 1.5 to 4 hours.

25. The method for preparing hard carbon according to claim 19, characterized in that, The reaction temperature of the fourth stage is 50-80℃.

26. The method for preparing hard carbon according to claim 19, characterized in that, The reaction time for the fourth stage is 2-4 hours.

27. The method for preparing hard carbon according to claim 19, characterized in that, The curing temperature is 80-180℃.

28. The method for preparing hard carbon according to claim 19, characterized in that, The coupling agent is 0.1% to 0.5% of the biomass containing aldehyde groups on its surface.

29. The method for preparing hard carbon according to claim 19, characterized in that, The epoxy oligomers are one or more of glycidyl ethers, glycidyl esters, glycidyl amines, alicyclic or aliphatic epoxy resins.

30. The method for preparing hard carbon according to claim 19, characterized in that, The solvent includes at least one of acetone, N,N'-dimethylformamide, toluene, ethanol, or ethyl acetate.

31. The method for preparing hard carbon according to claim 19, characterized in that, In step 2, the mass of the epoxy oligomer is 2%-10% of the biomass containing aldehyde groups on its surface; or In step 2, the mass of the epoxy oligomer is 2%-6% of the biomass containing aldehyde groups on its surface; after the fourth stage reaction is completed, 80%-160% of the mass of the epoxy oligomer is added with phenolic resin.

32. The method for preparing hard carbon according to claim 1, characterized in that, In step 1', the pre-carbonization and heat preservation time is 1~4h.

33. The method for preparing hard carbon according to claim 1, characterized in that, In step 2', the powder D50 is 5-10μm.

34. The method for preparing hard carbon according to claim 1, characterized in that, In step 3', the carbonization time is 1~4h.

35. A hard carbon prepared by the method according to any one of claims 1-34.

36. A sodium battery, characterized in that, The negative electrode material includes the hard carbon as described in claim 35.

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