A modified graphite material and a method for producing the same

Abstract

The present application relates to a kind of modified graphite material and its preparation method, its preparation method includes the following steps, first graphite is added to certain concentration of oxidant solution, filter and wash after stirring for a period of time quickly, then oxidized graphite and benzoxazine monomer are mixed uniformly and melt coating, finally heat treatment in inert gas can obtain modified graphite material.Benzoxazine resin low shrinkage when solidification forming and pyrolysis high carbon residue, can effectively reduce graphite surface defect and specific surface area, increase the structural compactness of surface coating layer, finally improve the first efficiency and cycle performance of graphite negative electrode material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to a modified graphite material and its preparation method. Background Technology

[0002] Graphite has been used as a negative electrode material for lithium-ion batteries for over two decades and has become the most widely used negative electrode material. Graphite negative electrode materials have unparalleled performance balance, including relatively low cost, high abundance, high energy density, and good structural stability. However, they also have some issues that need improvement, such as more surface defects, high selectivity to electrolytes, high irreversible capacity, and slightly lower initial efficiency.

[0003] Currently, the commonly used method is to coat the surface of graphite materials with a layer of carbon material, including solid-phase coating, liquid-phase coating, and in-situ polymerization coating. Each method has its advantages and disadvantages. For example, solid-phase coating is simple in process, but it also suffers from problems such as poor coating uniformity and repeatability. Liquid-phase coating has the advantage of good coating uniformity, but it causes more pollution and has higher production costs. In-situ polymerization coating involves initiating monomer polymerization to form a polymer that coats the surface of graphite particles. Finally, the graphite is dried, carbonized, and pulverized to obtain carbon-coated graphite material, which is a relatively good coating method.

[0004] CN 103072974 A discloses a method for coating the surface of artificial graphite carbon anode material. This method employs in-situ polymerization coating, first oxidizing the graphite surface, filtering and washing it, then adding it to a phenol solution. After metered addition of formaldehyde solution, hydrochloric acid solution is added to control the pH at 1.9-2.3. Finally, hexamethylenetetramine is added to cure the phenolic resin. The sample is washed, separated, dried, and carbonized to obtain carbon-coated graphite material. CN 104538598A and CN 104966839A disclose a preparation method for coating graphite and / or silicon using dopamine in-situ polymerization. Graphite is mixed with a pre-prepared buffer solution of dopamine hydrochloride (or an alkaline substance) for a certain period, dried, and carbonized under an inert atmosphere to obtain amorphous carbon-coated graphite. CN 115986091 A discloses a carbon-coated graphite anode material, its preparation method, and a battery. The preparation method of the carbon-coated graphite anode material includes the following steps: oxidizing and polymerizing a mixture of artificial graphite, coating agent monomer, organic acid and oxidant to obtain a solid, and then carbonizing the solid to obtain the carbon-coated graphite anode material.

[0005] The aforementioned in-situ polymerization coating patents suffer from complex preparation processes, high costs, and the need for strong acid / base catalysis and a large amount of waste liquid during in-situ polymerization coating. Furthermore, the coating resin exhibits problems such as high volatile matter content, high specific surface area, and low char residue during pyrolysis. These issues affect the coating uniformity of the resin-coated graphite material, adversely impacting the initial efficiency improvement of lithium-ion batteries using coated graphite as a negative electrode material.

[0006] Therefore, there is a need for a coating material with a simple coating process and excellent performance. The modified graphite material formed by coating the graphite surface can serve as the negative electrode of lithium-ion batteries and has better electrochemical performance. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a modified graphite material and its preparation method. By using a benzoxazine resin that has excellent affinity with graphite and exhibits low shrinkage and high pyrolysis residue during curing, surface defects and specific surface area of ​​graphite can be effectively reduced, ultimately improving the initial efficiency and cycle performance of the graphite anode material.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for preparing a modified graphite material, the method comprising the following steps:

[0010] 1) Add graphite matrix to oxidant solution, react, filter and wash to obtain graphite oxide; 2) Mix graphite oxide and benzoxazine monomer evenly and perform melt coating; 3) Heat treatment in inert gas to obtain modified graphite material.

[0011] In some examples, the graphite matrix is ​​selected from natural graphite or synthetic graphite; the particle size of the graphite matrix satisfies: D min ≥1.0μm, particle size D 50 It is 12-20μm.

[0012] In some examples, the artificial graphite raw material can be any material conventionally used in the art to prepare artificial graphite, such as petroleum-based pre-calcined coke, post-calcined petroleum coke, raw needle coke, or post-calcined needle coke, preferably petroleum-based pre-calcined coke with a volatile content of 7%-15%; after the raw material is crushed and shaped and graded, the artificial graphite is graphitized at a temperature of 2800-3200℃ for 20-45 hours.

[0013] In some examples, the oxidant in step 1) is one or more of hydrogen peroxide, hypochlorous acid, perchloric acid, potassium permanganate, persulfate (ammonium, sodium, potassium salt), ferric chloride, concentrated sulfuric acid, or nitric acid, preferably hydrogen peroxide; the mass concentration of the hydrogen peroxide solution is 3%-15%.

[0014] In some examples, the reaction temperature in step 1) is 25-60°C; the reaction time is 3-9 hours, and the reaction is preferably carried out under stirring conditions at a stirring rate of 300-700 r / min.

[0015] In some examples, the mass ratio of the graphite matrix to the oxidant solution in step 1) is 1:3 to 1:15.

[0016] In some examples, the structural formula of the benzoxazine monomer in step 2) is: In this embodiment, at least one R substituent is attached to the benzene ring of the benzoxazine monomer, and it can also have two substituents, such as R2 and R3; the R substituent is preferably located at the meta position of the carbon atom of the benzene ring to which the oxygen atom is attached in the oxazine ring; the R substituent is selected from one or two of hydroxyl, alkoxy, amino, or alkylamino groups. The R1 residue on the nitrogen atom is selected from hydrogen, aryl, aryl containing carboxyl or sulfonic acid groups, hydrocarbon groups or alkylhydroxy or alkylsulfonic acid groups with 1-8 carbon atoms, or polyalkoxy alcohols. In one specific embodiment, the benzoxazine is selected from... One or two of them.

[0017] In some examples, the benzoxazine monomer in step 2) can be synthesized with reference to existing techniques, such as (CN113248673B; "Martos, Alba; Sebastián, Rosa M.; Marquet, Jordi. (2018). STUDIES ONTHERING-OPENING POLYMERIZATION OF BENZOXAZINES: UNDERSTANDING THE EFFECT OF THE SUBSTITUENTS. European Polymer The benzoxazine monomer can also be prepared by the following method: (a) Prepare an 8%-40% formaldehyde aqueous solution of paraformaldehyde, add 0.1%-10% sodium hydroxide solution to adjust the pH of the paraformaldehyde solution to 8-10, and add it to the reactor; (b) Heat the reactor containing the paraformaldehyde solution to 25-50℃, and add the pre-prepared substituted phenol solution to a three-necked flask; (c) Then add aniline dropwise, heat to 70-90℃ and react for 3-6 hours; (d) After the reaction is completed, collect the lower precipitate, filter under reduced pressure and wash with ethanol 3-5 times, then redissolve in acetone / toluene in a 5:1-1:2 ratio for purification, filter to remove the insoluble part, and recrystallize to obtain the benzoxazine monomer.

[0018] In this invention, the solution of the substituted phenol in step (b) is selected from toluene or xylene solutions such as 3-methoxyphenol or 3,5-dihydroxyphenol.

[0019] In some examples, the mass ratio of graphite oxide to benzoxazine monomer in step 2) is 100:3-100:15.

[0020] In some examples, the mixing of graphite oxide and benzoxazine monomer in step 2) can be carried out using conventional methods in the art, such as a ribbon mixer, a gravity-free mixer, a V-type mixer, or a conical mixer, preferably in a V-type mixer; the preferred mixing conditions for the V-type mixer are stirring and mixing at a speed of 300-700 r / min for at least 30 minutes.

[0021] In some examples, the coating described in step 2) is carried out in a horizontal coating vessel, and the preferred temperature for melting coating is 100-250°C, with a melting coating time of 2-6 hours; preferably, the coating temperature is increased in stages, first held at 100-150°C for 1-2 hours, and then raised to 150-200°C and held for 1-5 hours.

[0022] In some examples, the molten coated material may optionally be fused in a fusion machine for 10-60 minutes before step 3).

[0023] In some examples, the heat treatment described in step 3) can be performed using methods conventional in the art, specifically selected from roller kilns, pusher kilns, and box furnaces.

[0024] In some examples, the heat treatment temperature in step 3) is 1100-1350℃; the inert gas is nitrogen or argon; and the heat treatment time is 4-10 hours.

[0025] In another aspect, the present invention provides modified graphite materials prepared by the above method and their application in lithium battery anodes.

[0026] The beneficial effects of this invention are as follows:

[0027] The benzoxazine resin monomer selected in this invention contains electron-donating groups, which makes the ring-opening polymerization of the oxazine ring easier and reduces the curing temperature of the resin. After surface oxidation treatment, graphite materials will generate phenolic hydroxyl, epoxy, or carbonyl compounds, which catalyze the curing of benzoxazine and can also reduce the curing temperature of benzoxazine resin. At the same time, it enhances the affinity with benzoxazine resin and improves the adhesion of benzoxazine resin to the graphite surface. The low shrinkage and high pyrolysis char rate of benzoxazine resin during curing can effectively reduce graphite surface defects and specific surface area, increase the structural density of the surface coating layer, and ultimately improve the first-pass efficiency and cycle performance of graphite anode materials. Detailed Implementation

[0028] To further illustrate the present invention, preferred embodiments are described below in conjunction with specific examples. However, it should be noted and understood that these specific examples are only for further illustrating the features and methods of the present invention, and are not intended to limit the present invention.

[0029] All percentages in this invention are by weight, unless otherwise stated. The raw materials used in the examples are as follows:

[0030] Aniline (chemically pure, Beijing Bailingwei Technology Co., Ltd.);

[0031] Phenol (chemically pure, Beijing Bailingwei Technology Co., Ltd.);

[0032] Paraformaldehyde (chemically pure, Shanghai Aladdin Biochemical Technology Co., Ltd.);

[0033] 3-Methoxyphenol (chemically pure, Shanghai Mairui Biochemical Technology Co., Ltd.);

[0034] Phloroglucinol (chemically pure, Wuhan Xinyang Ruihe Chemical Technology Co., Ltd.);

[0035] Hydrogen peroxide solution (30%, Sinopharm Chemical Reagent Co., Ltd.);

[0036] Toluene (analytical grade, Sinopharm Chemical Reagent Co., Ltd.);

[0037] Natural graphite (spherical, Qingdao Jinhui Graphite Co., Ltd.)

[0038] Example 1

[0039] 1) Preparation of benzoxazine monomer: Using 3-methoxyphenol, aniline, and paraformaldehyde as starting materials and toluene as the reaction solvent, the monomer was synthesized by a solution method well known in the art: (a) 60.0 g of paraformaldehyde was prepared into a 37 wt% formaldehyde aqueous solution, and then 4% sodium hydroxide aqueous solution was added to adjust the pH of the paraformaldehyde solution to 9.5, which was then added to a three-necked flask; (b) The flask containing the paraformaldehyde solution was heated to 50 °C, and 248.0 g of a pre-prepared toluene solution of 3-methoxyphenol (mass concentration of 50%) was added to the three-necked flask; (c) Then 93.0 g of aniline was added dropwise, and the temperature was raised to 80 °C for 4 hours; (d) After the reaction was completed, the lower precipitate was collected, filtered under reduced pressure, washed three times with ethanol, and then redissolved in a 3 / 1 acetone / toluene solution for purification. The insoluble portion was removed by filtration, and the benzoxazine monomer was obtained by recrystallization, with the structure as follows:

[0040] 2) Preparation of graphite oxide: 10 kg of natural graphite was added to 60 kg of hydrogen peroxide aqueous solution with a mass concentration of 4%, and stirred at 300 rpm for 9 hours at 50°C until the system turned bright yellow. After oxidation was completed, the system was filtered under reduced pressure and washed 5 times with deionized water to obtain oxidized graphite.

[0041] 3) Curing and coating of benzoxazine resin: The graphite oxide obtained in step 2) and benzoxazine monomer (the mass ratio of graphite oxide to benzoxazine monomer is 100:3) are mixed in a V-type mixer at 700 rpm for 30 minutes. Then, the mixture is added to a horizontal coating reactor and kept at 120°C at 300 rpm for 1 hour. The temperature is then raised to 200°C and maintained at 700 rpm for 5 hours.

[0042] 4) Preparation of modified graphite material: The benzoxazine resin that was cured and coated on the surface of graphite oxide in step 3) was pyrolyzed and carbonized at 1200℃ for 9 hours in a nitrogen atmosphere to obtain the modified graphite material.

[0043] Example 2

[0044] The difference from Example 1 is that the mass concentration of hydrogen peroxide was 10% when preparing graphene oxide, and the mixture was stirred at 700 rpm for 3 hours at 25°C.

[0045] Example 3

[0046] The difference from Example 2 is that in the curing and coating of benzoxazine resin, the mass ratio of graphite oxide to benzoxazine monomer is 100:9. After mixing in a V-type mixer at 500 rpm for 45 minutes, it is added to a horizontal coating kettle, first kept at 150°C at 700 rpm for 1 hour, and then heated to 200°C and maintained at 700 rpm for 3 hours.

[0047] Example 4

[0048] The difference from Example 2 is that in the curing and coating of benzoxazine resin, the mass ratio of graphite oxide to benzoxazine monomer is 100:15. After mixing for 1 hour in a V-type mixer at 300 rpm, it is added to a horizontal coating kettle, first kept at 150°C at 700 rpm for 1 hour, and then heated to 200°C and maintained at 700 rpm for 3 hours.

[0049] Example 5

[0050] The difference from Example 1 is that the starting materials for the synthesis of the benzoxazine monomer are resorcinol, aniline, and paraformaldehyde, reacted according to a functional group (hydroxyl:amino:aldehyde) ratio of 3:1:2. The resulting benzoxazine monomer has the following structure: Graphite oxide and benzoxazine monomer were mixed in a V-type mixer at 500 rpm for 30 minutes, and then added to a horizontal coating reactor. The mixture was first kept at 100°C at 500 rpm for 1 hour, and then heated to 150°C and maintained at 500 rpm for 5 hours.

[0051] Example 6

[0052] The difference from Example 2 is that the graphite used is artificial graphite.

[0053] Preparation of graphite oxide: 10 kg of artificial graphite (D... 50 It has a diameter of 15.1 μm and a specific surface area of ​​1.43 m². 2 / g,) was added to 150 kg of 12% hydrogen peroxide aqueous solution, and stirred at 25°C and 500 rpm for 6 hours. After oxidation was completed, the oxidized graphite was obtained by vacuum filtration using a water pump connected to a vacuum filtration flask and washing the filter cake with deionized water 5 times. Curing and coating of benzoxazine resin: The oxidized graphite obtained in step 1) and benzoxazine monomer (the mass ratio of graphite oxide to benzoxazine monomer is 100:3) were mixed in a V-type mixer at 500 rpm for 30 minutes. Then, the mixture was added to a horizontal coating kettle and kept at 120°C and 300 rpm for 2 hours. The temperature was then raised to 175°C and maintained at 700 rpm for 4 hours.

[0054] Preparation of modified graphite material: The benzoxazine resin that was cured and coated on the surface of graphite oxide in step 2) was pyrolyzed and carbonized at 1350℃ for 4 hours in a nitrogen atmosphere to obtain the modified graphite material.

[0055] Comparative Example 1

[0056] Unoxidized and uncoated natural graphite, D 50 It has a diameter of 16.1 μm and a specific surface area of ​​7.91 m². 2 / g, tap density 0.91g / cm³ 3 .

[0057] Comparative Example 2

[0058] The difference from Example 1 is that the natural graphite was not oxidized and was cured and coated with benzoxazine resin.

[0059] Comparative Example 3

[0060] The difference from Example 1 is that the benzoxazine monomer is 3,4-dihydro-3-phenyl-2H-1,3-benzoxazine.

[0061] Performance and tethering tests

[0062] The testing methods for relevant parameters of the modified graphite material in this invention are as follows (unless otherwise specified, conventional testing methods in the art are used):

[0063] Average particle size: The average particle size was measured using a Malvern-Mastersizer 3000 laser particle size analyzer.

[0064] Specific surface area: The specific surface area was measured using a BSD-BET 400 specific surface area meter.

[0065] Coin cell half-cell test: (1) The (modified) graphite / carbon black / styrene-butadiene rubber / hydroxymethyl cellulose in the examples and comparative examples were fed in a mass ratio of 95.5 / 1.5 / 1.5 / 1.5 and stirred evenly in water to make a negative electrode slurry. The slurry was then coated on a battery-grade copper foil and dried in a vacuum oven at 110°C for 4 hours. The carbon-coated graphite negative electrode sheet was then cut by rolling. (2) The counter electrode was a lithium metal sheet, the separator was polyethylene, and the electrolyte included LiPF6 with a molar concentration of 1 mol / L and a solvent (the solvent included ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 1 / 1 / 1) to prepare a coin cell half-cell.

[0066] Table 1. Test results of physicochemical and electrochemical properties of anode materials

[0067]

[0068] The modified graphite in Examples 1-5, after coating treatment, showed a decrease in specific surface area compared to Comparative Examples 1-2 before coating, especially the natural graphite, which showed a significant decrease in specific surface area after coating. Moreover, although the specific surface area of ​​the coated Comparative Examples 2 and 3 was higher than that of the Examples, it was significantly lower than that of the uncoated Comparative Example 1. At the same time, the modified graphite in the Examples also showed higher initial efficiency and cycle performance in the coin cell test compared to the uncoated Comparative Examples.

[0069] This invention cannot list all the embodiments involved in the invention's description; only a few specific implementation examples are given. However, those skilled in the art will readily recognize that the foregoing embodiments are merely specific forms in which the invention can be implemented. The invention is not limited to the aforementioned specific details and can be implemented in other specific forms without departing from its main characteristics. Therefore, the specific embodiments should be considered exemplary rather than restrictive in any way. The scope of the invention has been indicated by the claims rather than the detailed description; any changes made based on this, as long as they fall within the meaning and scope of the equivalents of the claims, should be considered part of the invention.

Claims

1. A method for preparing a modified graphite material, the method comprising the following steps: 1) Add graphite matrix to oxidant solution, react, filter and wash to obtain graphite oxide; 2) Mix graphite oxide and benzoxazine monomer evenly and perform melt coating; 3) Heat treatment in inert gas to obtain modified graphite material.

2. The preparation method according to claim 1, characterized in that, The graphite matrix is ​​selected from natural graphite or artificial graphite; the particle size of the graphite matrix satisfies: D min ≥1.0μm, particle size D 50 It is 12-20μm.

3. The preparation method according to claim 1, characterized in that, The oxidant mentioned in step 1) is one or more of hydrogen peroxide, hypochlorous acid, perchloric acid, potassium permanganate, persulfate, ferric chloride, concentrated sulfuric acid, or nitric acid; the mass concentration of the hydrogen peroxide solution is 3%-15%.

4. The preparation method according to any one of claims 1-3, characterized in that, The reaction temperature in step 1) is 25-60℃; the reaction time is 3-9 hours, the reaction is carried out under stirring conditions, and the stirring rate is 300-700 r / min; and / or, the mass ratio of graphite matrix to oxidant solution in step 1) is 1:3-1:

15.

5. The preparation method according to any one of claims 1-3, characterized in that, The structural formula of the benzoxazine monomer in step 2) is: In this embodiment, at least one R substituent is attached to the benzene ring of the benzoxazine monomer, and the R substituent is located at the meta position of the carbon atom of the benzene ring to which the oxygen atom is attached in the oxazine ring; the R substituent is selected from one or two of hydroxyl, alkoxy, amino or alkylamino; and / or, the R1 residue on the nitrogen atom is selected from hydrogen, aryl, aryl containing carboxyl or sulfonic acid, hydrocarbon or alkylhydroxy or alkylsulfonic acid or polyalkoxy alcohol having 1-8 carbon atoms.

6. The preparation method according to any one of claims 1-3, characterized in that, The benzoxazine is selected from , One or two of them.

7. The preparation method according to any one of claims 1-3, characterized in that, The mass ratio of graphite oxide to benzoxazine monomer in step 2) is 100:3-100:15; and / or, the mixing equipment for graphite oxide and benzoxazine monomer in step 2) is selected from one of a ribbon mixer, a gravity-free mixer, a V-type mixer, or a conical mixer; the preferred mixing conditions for the V-type mixer are stirring and mixing at a speed of 300-700 r / min for at least 30 minutes.

8. The preparation method according to any one of claims 1-3, characterized in that, The coating described in step 2) is carried out in a horizontal coating kettle. The melting coating temperature is 100-250℃ and the melting coating time is 2-6 hours. The coating temperature is increased in stages. First, it is kept at 100-150℃ for 1-2 hours, and then the temperature is increased to 150-200℃ and kept for 1-5 hours.

9. The preparation method according to any one of claims 1-3, characterized in that, The heat treatment described in step 3) is specifically selected from one of the following: roller kiln, pusher kiln, and box furnace; and / or, the heat treatment temperature described in step 3) is 1100-1350℃; the inert gas is nitrogen or argon; and the heat treatment time is 4-10 hours.

10. Modified graphite prepared by the preparation method according to any one of claims 1-9.

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