Composite graphite negative electrode material and preparation method thereof, negative electrode plate and lithium ion battery

Abstract

The invention provides a composite graphite negative electrode material and a preparation method thereof, a negative electrode plate and a lithium ion battery. The preparation method of the composite graphite negative electrode material comprises the following steps: preparing graphite oxide; carrying out first calcination treatment on the graphite oxide to obtain graphite particles; mixing graphite particles, phenolic organic matters and a solvent to obtain a mixed solution; mixing the mixed solution, a catalyst, a nitrogen source and an aldehyde organic matter, and reacting to obtain a precursor; and carrying out second calcination treatment on the precursor to obtain the composite graphite negative electrode material. A uniform nitrogen-doped carbon coating layer is formed on the surface of the expanded graphite, so that the electron conductivity of the material is enhanced, the formation and stability of an SEI film are promoted, the first-week coulombic efficiency is remarkably improved, meanwhile, the change of the material volume in the lithium ion intercalation and deintercalation processes is effectively relieved, and the cycling stability is enhanced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically, to a composite graphite anode material and its preparation method, an anode sheet, and a lithium-ion battery. Background Technology

[0002] With the increasing prevalence of portable electronic devices such as mobile phones and laptops, and even electric vehicles, higher demands are being placed on lithium-ion batteries in terms of capacity, high charge-discharge rate performance, cycle stability, safety, and cost. Graphite has long held a dominant position in the field of lithium-ion battery anode materials due to its excellent electrochemical performance and low cost. However, two issues still limit its further development: volume changes during lithium-ion intercalation and deintercalation lead to poor cycle stability, and its microstructure determines a relatively low theoretical capacity (372 mAh·g). -1 ).

[0003] Graphite, as one of the most common anode materials for lithium-ion batteries, occupies a major market share due to its stable crystal structure, high lithium-ion insertion / extraction potential, and low cost. However, graphite also has significant limitations, mainly in two aspects: first, its theoretical specific capacity is much lower than that of many new anode materials; second, the insertion and extraction of lithium ions between graphite layers leads to the expansion and contraction of the graphite lattice, which may cause graphite powder cracking under long-term cycling, thereby reducing the cycle stability and lifespan of the battery. To address these issues, researchers have proposed various modification strategies, including graphite expansion treatment and surface modification.

[0004] Expanded graphite refers to graphite intercalation compounds (GICs) formed under certain conditions by inserting chemicals such as acids, alkalis, and metal salts into the interlayer of graphite and combining with carbon atoms to form a new chemical phase. GICs decompose at certain temperatures, producing gas that causes graphite exfoliation and increases the interlayer spacing, resulting in expanded graphite. Due to the larger interlayer spacing, expanded graphite theoretically has a higher specific capacity and a faster lithium-ion migration rate, thus exhibiting higher energy density and faster charging capability than traditional graphite. However, the larger interlayer spacing and more defects lead to poorer cycle stability and a lower first-cycle coulombic efficiency in expanded graphite.

[0005] Therefore, how to design the microstructure and preparation process of expanded graphite to significantly improve its performance, that is, to prepare a composite graphite anode material with excellent electrochemical performance and cycle stability, and thus improve the various performances of the lithium-ion battery in which it is located, is one of the important technical problems to be solved in this field. Summary of the Invention

[0006] The main objective of this invention is to provide a composite graphite anode material and its preparation method, anode sheet, and lithium-ion battery, so as to solve the problems of poor electrochemical performance and cycle stability of graphite anode materials in the prior art.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a composite graphite anode material, comprising: step S1, preparing graphite oxide; step S2, subjecting the graphite oxide to a first calcination treatment to obtain graphite particles; step S3, mixing the graphite particles, phenolic organic compounds, and a solvent to obtain a mixture; mixing the mixture, a catalyst, a nitrogen source, and aldehyde organic compounds, and reacting to obtain a precursor; the precursor comprising graphite particles and a nitrogen-containing phenolic resin layer coated on the surface of the graphite particles; and step S4, subjecting the precursor to a second calcination treatment to obtain the composite graphite anode material.

[0008] This invention effectively controls the interlayer spacing of expanded graphite by using graphite oxide, avoiding excessive expansion that leads to increased specific surface area and structural damage, thus providing a larger intercalation space and a more stable intercalation environment for lithium ions. Subsequently, by utilizing the carbonization of phenolic resin and nitrogen doping, a uniform nitrogen-doped carbon coating layer is formed on the surface of the expanded graphite. This not only enhances the electronic conductivity of the material but also promotes the formation and stabilization of the SEI film, significantly improving the first-cycle coulombic efficiency. Simultaneously, it effectively mitigates the volume change of the material during lithium ion intercalation and deintercalation, enhancing cycle stability. Specifically, in the above preparation process, graphite oxide is first prepared to form graphite intercalation compounds (GICs). This process increases the interlayer spacing of graphite, creating more lithium ion intercalation sites and theoretically improving the lithium storage capacity of the material. The subsequent first calcination treatment, i.e., heat treatment of graphite oxide at high temperature, aims to remove the intercalating oxidants, restore the layered structure of graphite, and control its expansion degree. This process not only removes foreign substances from the GIC structure but also ensures the formation of graphite particles, preparing for subsequent surface coating. The size and structure of the obtained graphite particles were optimized, forming a core-shell structure with a high specific surface area and stable structure, which provides an ideal platform for subsequent surface modification. In the surface modification stage of step S3, a mixed solution of phenolic organics, catalyst, nitrogen source, and aldehyde organics reacted with the graphite particles to form a nitrogen-containing phenolic resin layer. The introduction of the nitrogen source and the presence of the coating layer improved the surface properties of the graphite core and promoted the formation and stabilization of the SEI film. The nitrogen-containing phenolic resin layer was transformed into a nitrogen-doped carbon coating layer under the second calcination treatment. During this transformation, the electronic conductivity was significantly improved, and the nitrogen-doped sites served as active sites for lithium-ion insertion, thereby improving the lithium storage capacity of the obtained anode material.

[0009] Further, step S1 also includes: step S1-1, mixing graphite raw material with an oxidant solution and obtaining a reaction solution through an oxidation reaction; step S1-2, separating the reaction solution into solid and liquid phases to obtain oxidized graphite; the graphite raw material is selected from one or more of artificial graphite and natural graphite; and / or, the oxidant solution is selected from one or more of perchloric acid solution, concentrated sulfuric acid solution, concentrated nitric acid solution, sodium nitrate solution, potassium nitrate solution, and hydrogen peroxide solution, preferably perchloric acid solution; and / or, the weight ratio of graphite raw material to oxidant in the oxidant solution is 1:(5-15); and / or, the oxidation reaction is carried out at 100℃-200℃. The above preferred scheme can promote the formation of a more stable GIC structure between graphite layers, and while fully oxidizing the graphite raw material, reduce its structural damage and the occurrence of side reactions, thereby further improving the microstructure and structural stability of the obtained composite graphite anode material, and ultimately significantly optimizing the electrochemical performance of the obtained anode material.

[0010] Furthermore, steps S1-2 also include: washing the reaction solution with water until the pH value is 4-7; performing solid-liquid separation on the washed reaction solution to obtain a first solid product; and drying the first solid product to obtain a graphite core material. In this preferred embodiment, washing with water to a neutral or weakly acidic pH range removes residual acidic substances and effectively reduces additional chemical reactions caused by excessively high pH, ​​thereby further improving the purity and structural stability of the graphite oxide product, and ultimately significantly optimizing the various properties of the obtained composite graphite anode material.

[0011] Furthermore, in step S2, the holding temperature for the first calcination treatment is 150℃~350℃, and the holding time is 5h~24h. This preferred method can more effectively protect the structure of the obtained graphite particles while thoroughly removing the oxidant.

[0012] Furthermore, the first calcination treatment is carried out in a first protective atmosphere, and the gas flow rate of the first protective atmosphere is 1000±50 mL / min. This preferred scheme can more effectively eliminate volatile byproducts during the heat treatment process and improve the purity and structural stability of the obtained graphite particles.

[0013] Further, in step S3, the weight ratio of phenolic organics, catalyst, nitrogen source, and aldehyde organics is 10:(1-6):(3-10):(1-4); and / or, the total weight ratio of phenolic organics, catalyst, nitrogen source, and aldehyde organics to the weight of graphite particles is (0.5-4):1; and / or, the phenolic organics are selected from one or more of phenol, aminophenol, nitrophenol, catechol, and resorcinol; and / or, the catalyst is selected from one or more of hydrochloric acid, sulfuric acid, and oxalic acid; and / or, the nitrogen source is selected from one or more of 2-methylimidazole, p-nitroaniline, and polyvinylpyrrolidone; and / or, the aldehyde organics are selected from one or more of formaldehyde, acetaldehyde, propionaldehyde, and benzaldehyde; and / or, the solvent is water and / or methanol. This preferred embodiment facilitates more efficient polymerization of phenolic resins and promotes more uniform incorporation of nitrogen, forming a more structurally stable nitrogen-doped phenolic resin carbon source layer.

[0014] Furthermore, before mixing the mixture, catalyst, nitrogen source, and aldehyde organic compound, step S3 further includes heating the mixture to a first temperature and reacting at the first temperature; the first temperature is 60℃ to 150℃. This preferred embodiment can more effectively activate the chemical reactivity of the phenolic organic compound, catalyst, nitrogen source, and aldehyde organic compound, promote the polymerization reaction, and achieve more uniform incorporation of nitrogen, forming a more uniform and dense nitrogen-containing phenolic resin-coated carbon source layer, thereby more effectively improving the electrochemical performance of the final anode material.

[0015] Furthermore, in step S4, the holding temperature for the second calcination treatment is 700℃~900℃, and the holding time is 2h~6h. This preferred scheme can achieve more thorough carbonization of phenolic resin, and also promote further incorporation of nitrogen elements to form a denser carbon layer with more uniform nitrogen doping, thereby improving the conductivity and structural stability of the obtained composite graphite anode material.

[0016] Furthermore, the second calcination treatment is carried out in a second protective atmosphere, and the gas flow rate of the second protective atmosphere is 50±2 mL / min. This preferred embodiment can further reduce the introduction of impurities, thereby more effectively improving the purity and structural stability of the obtained composite graphite anode material.

[0017] A second aspect of the present invention provides a composite graphite anode material, which is prepared by the method described above for preparing composite graphite anode materials. The resulting anode material has a core-shell structure, wherein the outer layer is nitrogen-doped amorphous carbon obtained by carbonizing nitrogen-containing phenolic resin, and the inner layer is low-expansion graphite obtained by oxidation. In this structure:

[0018] Firstly, the expanded graphite structure, which forms the core, is controllable, preventing excessive peeling that would result in an excessively large specific surface area of ​​graphite, thus enhancing structural stability and long-cycle performance.

[0019] Secondly, the amorphous carbon in the outer layer can effectively mitigate the volume deformation of expanded graphite during lithium-ion insertion and extraction. Furthermore, because expanded graphite has more defect sites, its first-cycle coulombic efficiency is relatively low; however, the doping of heteroatoms with nitrogen provides more active sites, resulting in a higher first-cycle coulombic efficiency.

[0020] It should be noted that, due to the special nature of materials science and the limitations of existing testing and characterization methods, it is difficult to comprehensively and quantitatively characterize the detailed microstructure of the composite graphite anode material obtained in this invention. However, performance test results have shown that the composite graphite anode material obtained in this invention possesses superior electrochemical performance, especially cycle stability.

[0021] A third aspect of the present invention provides a negative electrode sheet comprising the aforementioned composite graphite negative electrode material, or comprising a composite graphite negative electrode material prepared by the aforementioned method for preparing the composite graphite negative electrode material. Because the aforementioned composite graphite negative electrode material comprises a low-expansion graphite core and a high-performance nitrogen-doped amorphous carbon coating layer, it exhibits superior electrochemical performance, and the negative electrode sheet containing it correspondingly possesses higher energy density and longer cycle life.

[0022] A fourth aspect of the present invention provides a lithium-ion battery comprising the aforementioned negative electrode sheet. Thanks to the composite graphite negative electrode material in the negative electrode sheet, which exhibits excellent electrochemical performance and structural stability, the presence of the coating layer effectively mitigates volume changes during lithium-ion insertion and extraction, maintaining the structural stability and integrity of the material. Furthermore, the incorporation of nitrogen significantly enhances the material's lithium storage capacity and improves electronic conductivity, which is beneficial for kinetic response during rapid charge and discharge. Based on this, the resulting lithium-ion battery possesses higher charge-discharge specific capacity, higher initial efficiency, and a longer service life.

[0023] By applying the technical solution of this invention, the interlayer spacing of expanded graphite is effectively controlled through the preparation of expanded graphite and the removal of intercalation materials. This avoids the increase in specific surface area and structural damage caused by excessive expansion, providing a larger intercalation space and a more stable intercalation environment for lithium ions. Subsequently, a uniform nitrogen-doped carbon coating layer is formed on the surface of the expanded graphite by carbonization of phenolic resin and doping with nitrogen. This not only enhances the electronic conductivity of the material but also promotes the formation and stabilization of the SEI film, significantly improving the first-cycle coulombic efficiency. Simultaneously, it effectively mitigates the volume change of the material during lithium ion intercalation and deintercalation, enhancing cycle stability. Detailed Implementation

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0025] As described in the background section, existing graphite anode materials suffer from poor electrochemical performance and cycle stability. To address these issues, a first aspect of this invention provides a method for preparing a composite graphite anode material, comprising: step S1, preparing graphite oxide; step S2, subjecting the graphite oxide to a first calcination treatment to obtain graphite particles; step S3, mixing the graphite particles, phenolic organic compounds, and a solvent to obtain a mixture; mixing the mixture, a catalyst, a nitrogen source, and aldehyde organic compounds, and reacting to obtain a precursor; the precursor comprising graphite particles and a nitrogen-containing phenolic resin layer coated on the surface of the graphite particles; and step S4, subjecting the precursor to a second calcination treatment to obtain the composite graphite anode material.

[0026] This invention effectively controls the interlayer spacing of expanded graphite by using graphite oxide, avoiding excessive expansion that leads to increased specific surface area and structural damage, thus providing a larger intercalation space and a more stable intercalation environment for lithium ions. Subsequently, by utilizing the carbonization of phenolic resin and nitrogen doping, a uniform nitrogen-doped carbon coating layer is formed on the surface of the expanded graphite. This not only enhances the electronic conductivity of the material but also promotes the formation and stabilization of the SEI film, significantly improving the first-cycle coulombic efficiency. Simultaneously, it effectively mitigates the volume change of the material during lithium ion intercalation and deintercalation, enhancing cycle stability. Specifically, in the above preparation process, graphite oxide is first prepared to form graphite intercalation compounds (GICs). This process increases the interlayer spacing of graphite, creating more lithium ion intercalation sites and theoretically improving the lithium storage capacity of the material. The subsequent first calcination treatment, i.e., heat treatment of graphite oxide at high temperature, aims to remove the intercalating oxidants, restore the layered structure of graphite, and control its expansion degree. This process not only removes foreign substances from the GIC structure but also ensures the formation of graphite particles, preparing for subsequent surface coating. The size and structure of the obtained graphite particles were optimized, forming a core-shell structure with a high specific surface area and stable structure, which provides an ideal platform for subsequent surface modification. In the surface modification stage of step S3, a mixed solution of phenolic organics, catalyst, nitrogen source, and aldehyde organics reacted with the graphite particles to form a nitrogen-containing phenolic resin layer. The introduction of the nitrogen source and the presence of the coating layer improved the surface properties of the graphite core and promoted the formation and stabilization of the SEI film. The nitrogen-containing phenolic resin layer was transformed into a nitrogen-doped carbon coating layer under the second calcination treatment. During this transformation, the electronic conductivity was significantly improved, and the nitrogen-doped sites served as active sites for lithium-ion insertion, thereby improving the lithium storage capacity of the obtained anode material.

[0027] In summary, through the aforementioned series of precise process controls, the technical solution of this invention can accurately adjust the microstructure of graphite, including interlayer spacing, specific surface area, and surface functionalization degree, thereby significantly improving the electrochemical performance of the resulting anode material. Specifically, the structural stability of the material is enhanced, effectively alleviating the volume expansion problem during lithium-ion insertion and extraction, while simultaneously improving the first-pass coulombic efficiency.

[0028] Further, step S1 includes: step S1-1, mixing graphite raw material with an oxidant solution and obtaining a reaction solution through an oxidation reaction; step S1-2, separating the reaction solution into solid and liquid phases to obtain graphite oxide. That is, this invention prepares low-expansion graphite, i.e., graphite oxide, through a chemical oxidation method. The graphite raw material used in this invention is selected from one or more of artificial graphite and natural graphite, and is not limited to specific types. Simultaneously, to provide more suitable oxidation capacity and form a more stable GIC structure between graphite layers, the oxidant solution is preferably selected from one or more of perchloric acid solution, concentrated sulfuric acid solution (mass concentration of 97%–99%), concentrated nitric acid solution (mass concentration of 65%–69%), sodium nitrate solution, potassium nitrate solution, and hydrogen peroxide solution. Perchloric acid solution has higher environmental friendliness and higher oxidation efficiency, allowing for more controllable oxidation reaction conditions, thus helping to prepare more structurally stable low-expansion graphite particles, ultimately improving the long-cycle stability of the obtained composite graphite anode material.

[0029] In step S1-1, the preferred weight ratio of graphite raw material to oxidant in the oxidant solution is 1:(5-15) to ensure sufficient oxidation of the graphite raw material while minimizing structural damage, ultimately leading to more significant optimization of the electrochemical performance of the resulting anode material. Furthermore, preferably, the oxidation reaction is carried out at 100°C to 200°C, which more effectively promotes the chemical reaction between graphite layers, reduces side reactions, and further enhances the microstructure and structural stability of the resulting composite graphite anode material. Based on this, to further optimize the GIC structure while improving oxidation efficiency and reducing solvent consumption, the preferred mass concentration of the oxidant solution is 10% to 100%.

[0030] In several typical embodiments, steps S1-2 further include: washing the reaction solution with water until the pH value is 4-7; performing solid-liquid separation on the washed reaction solution to obtain a first solid product; and drying the first solid product to obtain a graphite core material. In this preferred embodiment, washing with water to a neutral or weakly acidic pH range removes residual acidic substances and effectively reduces additional chemical reactions caused by excessively high pH, ​​thereby further improving the purity and structural stability of the graphite oxide product, and ultimately significantly optimizing the various properties of the obtained composite graphite anode material.

[0031] In step S2, the intercalated material in the graphite oxide is removed by a first calcination treatment to restore the layered structure of the graphite while controlling its expansion. Therefore, to thoroughly remove the oxidant while more effectively protecting the structure of the resulting graphite particles, the holding temperature during the first calcination treatment is preferably 150℃~350℃, more preferably 200℃~350℃, and the holding time is 5h~24h. Furthermore, the heating rate of the first calcination treatment is preferably 2±0.5℃ / min to effectively control the particle size and distribution of the resulting graphite particles, thereby improving the electrochemical performance of the final coated graphite anode.

[0032] In several typical embodiments, to provide an oxygen-free environment and more effectively eliminate volatile byproducts from the heat treatment process, thereby improving the purity and structural stability of the resulting graphite particles, the first calcination treatment is preferably carried out in a first protective atmosphere, with a gas flow rate of 1000±50 mL / min. Furthermore, the first protective atmosphere is preferably nitrogen and / or argon.

[0033] In step S3, preferably, the weight ratio of phenolic organics, catalyst, nitrogen source, and aldehyde organics is 10:(1-6):(3-10):(1-4), more preferably 10:(2-4):(3-5):1; and / or, the weight ratio of the total weight of phenolic organics, catalyst, nitrogen source, and aldehyde organics to the weight of graphite particles is (0.5-4):1 (more preferably (1.5-3.5):1). During the coating process of the phenolic resin carbon source layer, feeding the components according to the above-mentioned proportions facilitates more efficient polymerization of the phenolic resin and promotes more uniform incorporation of nitrogen, forming a more structurally stable nitrogen-doped phenolic resin carbon source layer. Furthermore, the above-mentioned dosage ratio, which was optimized by the inventors through a large number of experiments, means that the carbon source layer obtained in this process and the nitrogen-doped carbon layer obtained by final calcination both have a more suitable thickness. It is neither too thin, which would result in poor protection, nor too thick, which would affect the diffusion of lithium ions and the effective utilization rate of the graphite core. Ultimately, it significantly improves the electrochemical performance of the obtained coated graphite anode, especially its cycle stability.

[0034] In several typical embodiments, the phenolic organic compound is selected from one or more of phenol, aminophenol, nitrophenol, catechol, and resorcinol; and / or, the catalyst is selected from one or more of hydrochloric acid, sulfuric acid, and oxalic acid; and / or, the nitrogen source is selected from one or more of 2-methylimidazole, p-nitroaniline, and polyvinylpyrrolidone; and / or, the aldehyde organic compound is selected from one or more of formaldehyde, acetaldehyde, propionaldehyde, and benzaldehyde; and / or, the solvent is water and / or methanol. Compared with other phenols and aldehydes in the art that can form phenolic resins, and other nitrogen sources that can provide nitrogen doping, the above-mentioned types obtained in this invention can construct a more stable polymer carbon source framework, while forming a more uniform dopant, ultimately resulting in a coating layer with better conductivity and more lithium-ion intercalation sites, significantly improving the various properties of the obtained coated graphite anode.

[0035] Furthermore, before mixing the mixture, catalyst, nitrogen source, and aldehyde organic compound, step S4 further includes heating the mixture to a first temperature and reacting at the first temperature; the first temperature is 60℃ to 150℃. These preferred reaction conditions can more effectively activate the chemical reactivity of the phenolic organic compound, catalyst, nitrogen source, and aldehyde organic compound, promote the polymerization reaction, and achieve more uniform incorporation of nitrogen, forming a more uniform and dense nitrogen-containing phenolic resin-coated carbon source layer, thereby more effectively improving the electrochemical performance of the final anode material.

[0036] In step S4, to achieve more thorough carbonization of the phenolic resin and promote further incorporation of nitrogen to form a denser and more uniformly nitrogen-doped carbon layer, thereby improving the conductivity and structural stability of the resulting composite graphite anode material, the holding temperature for the second calcination treatment is preferably 700℃~900℃, more preferably 800℃~900℃, and the holding time is preferably 2h~6h. Furthermore, the heating rate for the second calcination treatment is preferably 2±0.5℃ / min to reduce local overheating during the heating process, thereby more effectively protecting the structure of the resulting anode material and improving its cycle stability.

[0037] In several typical embodiments, to reduce the introduction of impurities and thus more effectively improve the purity and structural stability of the resulting composite graphite anode material, the second calcination treatment is preferably carried out in a second protective atmosphere, and the gas flow rate of the second protective atmosphere is 50±2 mL / min. Furthermore, the second protective atmosphere is preferably nitrogen and / or argon.

[0038] Before the second calcination treatment, step S6 also includes drying the precursor. Similar to the drying treatment in steps S1-2, in practical applications, vacuum drying is preferred, and the vacuum drying temperature is 60℃~120℃, in order to better protect the material structure, while accelerating the drying process and improving production efficiency.

[0039] A second aspect of the present invention provides a composite graphite anode material, which is prepared by the method described above for preparing composite graphite anode materials. The resulting anode material has a core-shell structure, wherein the outer layer is nitrogen-doped amorphous carbon obtained by carbonizing nitrogen-containing phenolic resin, and the inner layer is low-expansion graphite obtained by oxidation. In this structure:

[0040] Firstly, the expanded graphite structure, which forms the core, is controllable, preventing excessive peeling that would result in an excessively large specific surface area of ​​graphite, thus enhancing structural stability and long-cycle performance.

[0041] Secondly, the amorphous carbon in the outer layer can effectively mitigate the volume deformation of expanded graphite during lithium-ion insertion and extraction. Furthermore, because expanded graphite has more defect sites, its first-cycle coulombic efficiency is relatively low; however, the doping of heteroatoms with nitrogen provides more active sites, resulting in a higher first-cycle coulombic efficiency.

[0042] It should be noted that, due to the special nature of materials science and the limitations of existing testing and characterization methods, it is difficult to comprehensively and quantitatively characterize the detailed microstructure of the composite graphite anode material obtained in this invention. However, performance test results have shown that the composite graphite anode material obtained in this invention possesses superior electrochemical performance, especially cycle stability.

[0043] A third aspect of the present invention provides a negative electrode sheet comprising the aforementioned composite graphite negative electrode material, or comprising a composite graphite negative electrode material prepared by the aforementioned method for preparing the composite graphite negative electrode material. Because the aforementioned composite graphite negative electrode material comprises a low-expansion graphite core and a high-performance nitrogen-doped amorphous carbon coating layer, it exhibits superior electrochemical performance, and the negative electrode sheet containing it correspondingly possesses higher energy density and longer cycle life.

[0044] A fourth aspect of the present invention provides a lithium-ion battery comprising the aforementioned negative electrode sheet. Thanks to the composite graphite negative electrode material in the negative electrode sheet, which exhibits excellent electrochemical performance and structural stability, the presence of the coating layer effectively mitigates volume changes during lithium-ion insertion and extraction, maintaining the structural stability and integrity of the material. Furthermore, the incorporation of nitrogen significantly enhances the material's lithium storage capacity and improves electronic conductivity, which is beneficial for kinetic response during rapid charge and discharge. Based on this, the resulting lithium-ion battery possesses higher charge-discharge specific capacity, higher initial efficiency, and a longer service life.

[0045] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0046] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0047] Example 1

[0048] A method for preparing a composite graphite anode material:

[0049] (1-1) 50g of graphite was added to a 70% perchloric acid solution in a three-necked flask and reacted at 120℃ for 0.5h to obtain a reaction solution. The mass ratio of graphite to perchloric acid in the perchloric acid solution was 1:6.

[0050] (1-2) After the reaction is complete, the solvent is repeatedly washed with water until the pH is 4. The intercalated graphite is separated by centrifugation and then dried at 100°C in a vacuum drying oven to obtain graphite oxide.

[0051] (2) The dried powder (i.e. the obtained graphite oxide) was placed in a ceramic boat and heated in a tube furnace at 200°C for 7 hours (i.e. the first calcination treatment). The heating rate was 2°C / min, the protective atmosphere was nitrogen, and the gas flow rate was 100 mL / min to obtain graphite particles, i.e. low-expansion graphite.

[0052] (3) Add 25g of the obtained low-expansion graphite powder to 30mL of phenol solution, heat the solution to 90℃ and keep it at a constant temperature, then add 3g of oxalic acid, 4g of polyvinylpyrrolidone (PvP) and 10mL of formaldehyde solution in sequence (wherein, the weight ratio of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter is 10:3:4:1, and the ratio of the total weight of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter to the weight of low-expansion graphite powder is 1.88:1), and react at 90℃ for 3h. After the reaction is completed, repeatedly wash the solvent with water and centrifuge to separate the solid powder, and dry it under vacuum at 100℃ to obtain nitrogen-containing phenolic resin-coated low-expansion graphite, i.e., the precursor.

[0053] (4) Place the low-expansion graphite powder coated with nitrogen-containing phenolic resin in a ceramic boat and heat it in a tube furnace at 900°C for 4 hours (i.e., the second calcination treatment). The heating rate is 2°C / min, the protective atmosphere is nitrogen, and the gas flow rate is 50mL / min. Nitrogen-doped carbon-coated low-expansion graphite composite material, i.e. composite graphite anode material, is obtained.

[0054] Example 2

[0055] A method for preparing a composite graphite anode material:

[0056] The only difference between this embodiment and embodiment 1 is that in step (2), the holding temperature of the first calcination treatment is changed to 100°C.

[0057] Example 3

[0058] A method for preparing a composite graphite anode material:

[0059] The only difference between this embodiment and embodiment 1 is that in step (4), the holding temperature of the second calcination treatment is changed to 700°C.

[0060] Example 4

[0061] A method for preparing a composite graphite anode material:

[0062] (1-1) Add 50g of graphite (same type as in Example 1) to a concentrated sulfuric acid solution (98% by mass) in a three-necked flask and react at 200°C for 0.5h to obtain a reaction solution. The mass ratio of graphite to sulfuric acid in the concentrated sulfuric acid solution is 1:5.

[0063] (1-2) After the reaction is complete, the solvent is repeatedly washed with water until the pH is 4. The intercalated graphite is separated by centrifugation and then dried at 100°C in a vacuum drying oven to obtain graphite oxide.

[0064] (2) The dried powder (i.e. the obtained graphite oxide) was placed in a ceramic boat and heated in a tube furnace at 300°C for 7 hours. The heating rate was 2°C / min, the protective atmosphere was nitrogen, and the gas flow rate was 100mL / min. Graphite particles, i.e. low-expansion graphite, were obtained.

[0065] Steps (3) and (4) are consistent with those in Example 1.

[0066] Example 5

[0067] A method for preparing a composite graphite anode material:

[0068] The difference between this embodiment and Embodiment 1 lies only in step (1-1), specifically:

[0069] (1-1) Add 50g of graphite (same type as in Example 1) to a hydrogen peroxide solution (30% by mass) in a three-necked flask and react at 220°C for 2 hours. The mass ratio of graphite to hydrogen peroxide in the hydrogen peroxide solution is 1:10.

[0070] Example 6

[0071] A method for preparing a composite graphite anode material:

[0072] The difference between this embodiment and embodiment 1 lies only in step (3), specifically:

[0073] (3) Add 25g of the obtained low-expansion graphite powder to 50mL of phenol solution, heat the solution to 90℃ and keep it at a constant temperature, then add 3g of oxalic acid, 5g of PvP and 20mL of formaldehyde solution in sequence, and react at 90℃ for 3h. After the reaction is complete, repeatedly wash the solvent with water and centrifuge to separate the solid powder, and dry it under vacuum at 100℃ to obtain nitrogen-containing phenolic resin-coated low-expansion graphite, i.e., the precursor.

[0074] In this embodiment, the weight ratio of phenolic organics, catalyst, nitrogen source and aldehyde organics is 10:6:10:4, and the weight ratio of the total weight of phenolic organics, catalyst, nitrogen source and aldehyde organics to the weight of low-expansion graphite powder is 3.12:1.

[0075] Example 7

[0076] A method for preparing a composite graphite anode material:

[0077] The only difference between this embodiment and Embodiment 1 is that the reaction temperature in step (1-1) is changed to 80°C.

[0078] Example 8

[0079] A method for preparing a composite graphite anode material:

[0080] The only difference between this embodiment and Embodiment 1 is that the reaction temperature in step (1-1) is changed to 240°C.

[0081] Example 9

[0082] A method for preparing a composite graphite anode material:

[0083] The only difference between this embodiment and Embodiment 1 is that the holding temperature of the first calcination treatment in step (2) is changed to 400°C and the holding time is changed to 3h.

[0084] Example 10

[0085] A method for preparing a composite graphite anode material:

[0086] The difference between this embodiment and embodiment 1 is that the amount of each raw material is changed in step (3) so that the weight ratio of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter is changed to 10:0.5:12:0.8, and the weight ratio of the total weight of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter to the weight of low-expansion graphite powder is changed to 0.2:1.

[0087] Example 11

[0088] A method for preparing a composite graphite anode material:

[0089] The difference between this embodiment and embodiment 1 is that the amount of each raw material is changed in step (3) so that the weight ratio of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter is changed to 10:8:2:5, and the weight ratio of the total weight of phenolic organic matter, catalyst, nitrogen source and aldehyde organic matter to the weight of low-expansion graphite powder is changed to 5:1.

[0090] Example 12

[0091] A method for preparing a composite graphite anode material:

[0092] The only difference between this embodiment and Embodiment 1 is that the heating temperature and reaction temperature in step (3) are both changed to 50°C.

[0093] Example 13

[0094] A method for preparing a composite graphite anode material:

[0095] The only difference between this embodiment and Embodiment 1 is that the heating temperature and reaction temperature in step (3) are both changed to 180°C.

[0096] Example 14

[0097] A method for preparing a composite graphite anode material:

[0098] The only difference between this embodiment and Embodiment 1 is that the holding temperature of the second calcination treatment in step (4) is changed to 1000℃ and the holding time is changed to 1h.

[0099] Comparative Example 1

[0100] A method for preparing a negative electrode material:

[0101] The only difference between this comparative example and Example 1 is that no nitrogen source was added in step (3).

[0102] Comparative Example 2

[0103] A method for preparing a negative electrode material:

[0104] The only difference between this comparative example and Example 1 is that the graphite particles (i.e., low-expansion graphite) obtained in step (2) of Example 1 are directly used as the final negative electrode material sample.

[0105] Comparative Example 3

[0106] A method for preparing a negative electrode material:

[0107] This comparative example directly uses the untreated graphite raw material used in Example 1 as the final negative electrode material sample.

[0108] Comparative Example 4

[0109] A method for preparing a negative electrode material:

[0110] The only difference between this comparative example and Example 1 is that the graphite raw material was not processed in steps (1) to (2), but 25g of graphite raw material was directly added to 30mL of phenol solution, and steps (3) and (4) were performed.

[0111] Comparative Example 5

[0112] A method for preparing a negative electrode material:

[0113] The only difference between this comparative example and Example 1 is that in step (3), the low-expansion graphite powder, phenol solution, oxalic acid, PvP and formaldehyde solution are directly mixed simultaneously, and subsequent steps are carried out according to the conditions in Example 1.

[0114] Battery sample preparation:

[0115] 1) Dissolve 80 wt% of the negative electrode material sample, 10 wt% of Super P, 5 wt% of CMC, and 5 wt% of SBR in a certain amount of deionized water, resulting in a solid content of 20 wt%. Stir continuously to form a uniform electrode slurry. Coat the electrode slurry onto the surface of a copper foil and vacuum dry at 100°C for 12 hours. The final electrode sheet has an active material loading of approximately 1.5 mg / cm³. 2 .

[0116] 2) Using the electrode sheet as the working electrode, lithium metal as the reference electrode and counter electrode, a 25μm PP / PE / PP composite membrane as the separator, diethyl carbonate and ethylene carbonate in a 1:1 volume ratio as the electrolyte, and 1mol / L lithium hexafluorophosphate as the electrolyte solution, a 2032-type button cell was assembled in a glove box under an argon atmosphere.

[0117] The negative electrode material samples obtained from the above embodiments and comparative examples were used to prepare corresponding battery samples.

[0118] Electrical performance testing methods for battery samples:

[0119] (1) Initial discharge capacity, initial charge capacity and first-cycle coulombic efficiency: tested at a current density of 0.1C, with a voltage window of 0.005V to 3.0V.

[0120] (2) Specific capacitance of 4C: obtained by testing within a voltage window of 0.005V to 3.0V.

[0121] (3) Specific capacity after 5000 cycles: tested at a current density of 1C, with a voltage window of 0.005V to 3.0V.

[0122] The battery samples corresponding to each embodiment and comparative example were subjected to the above tests, and the results are shown in Table 1.

[0123] Table 1

[0124]

[0125]

[0126] As can be seen from the above description, compared with the comparative examples, the nitrogen-doped carbon-coated low-expansion graphite prepared in the above embodiments of the present invention has the characteristics of high capacity, fast charging and high stability, and is a high-performance lithium-ion battery anode material.

[0127] Specifically, in each embodiment:

[0128] Comparing Examples 2 and 9 with Example 1, it can be seen that in the preparation of low-expansion graphite, a lower calcination temperature leads to intercalation residue, and the residual Cl will damage the lithium-ion active sites, reducing the first-cycle coulombic efficiency and rate performance of the electrode material; while a higher temperature may cause structural damage. Therefore, by gradually optimizing the temperature of the first calcination treatment, the structure of the obtained graphite particles can be more effectively protected while completely removing the oxidant.

[0129] Comparing Examples 3 and 14 with Example 1, especially in Example 3, if the carbonization temperature of the phenolic resin is lowered, the conductivity of the outer carbon coating is insufficient, leading to a decrease in battery rate performance. Therefore, by optimizing the temperature of the second calcination treatment, more thorough carbonization of the phenolic resin can be achieved, while also promoting further incorporation of nitrogen elements, forming a denser carbon layer with more uniform nitrogen doping, further improving the conductivity and structural stability of the obtained composite graphite anode material.

[0130] Example 4 uses concentrated sulfuric acid as an oxidant to prepare low-expansion graphite. The performance of the prepared negative electrode material is similar to that of Example 1, indicating that the two have similar structures. However, it is difficult to ignore that the preparation process of Example 4 is more dangerous, and the gas produced when concentrated sulfuric acid is heated and decomposed is more toxic, resulting in a decrease in environmental friendliness and an increase in the cost of exhaust gas treatment.

[0131] In Example 5, hydrogen peroxide was used as an oxidant to prepare low-expansion graphite. The capacity exhibited was slightly lower than that in Example 1. This is because hydrogen peroxide has a weaker oxidizing power, resulting in a smaller interlayer spacing of the prepared low-expansion graphite.

[0132] Comparing Examples 6, 10, and 11 with Example 1, it can be seen that the cycle stability of Example 6 is almost the same, but the specific capacity is relatively reduced; while the overall performance of Examples 10 and 11 is reduced. That is to say, by optimizing the weight ratio of phenolic organics, catalysts, nitrogen sources, and aldehyde organics, as well as the ratio of the total weight of phenolic organics, catalysts, nitrogen sources, and aldehyde organics to the weight of low-expansion graphite powder, the thickness of the outer carbon coating can be optimized accordingly. This ensures that it is neither too thin, resulting in poor protection, nor too thick, affecting the diffusion of lithium ions and the effective utilization rate of the graphite core, ultimately significantly improving the electrochemical performance, especially the cycle stability, of the obtained coated graphite anode.

[0133] Comparing Examples 7 and 8 with Example 1, it can be seen that by optimizing the reaction temperature of the oxidation reaction between the graphite raw material and the oxidant solution, the chemical reaction between the graphite layers can be promoted more effectively, the occurrence of side reactions can be reduced, and the microstructure and structural stability of the obtained composite graphite anode material can be further improved.

[0134] Comparing Examples 12 and 13 with Example 1, it can be seen that by optimizing the heating temperature and reaction temperature of the mixture, the chemical reactivity of phenolic organic compounds, catalysts, nitrogen sources and aldehyde organic compounds can be activated more effectively, promoting the polymerization reaction and more uniform incorporation of nitrogen elements, forming a more uniform and dense nitrogen-containing phenolic resin coating carbon source layer, thereby more effectively improving the electrochemical performance of the final negative electrode material.

[0135] As for the proportions of each pair:

[0136] In Comparative Example 1, the carbon coating on the outer layer was not doped with nitrogen, so its first-cycle coulombic efficiency was relatively low. Although the discharge specific capacity did not decay in the first cycle, the increased number of carbon defects led to an increase in irreversible active sites, resulting in significantly worse cycle performance.

[0137] Comparative Example 2, lacking an outer carbon coating, exhibits higher specific capacity and fast-charging capability compared to Comparative Example 3, consistent with the advantages of expanded graphite. However, the increased carbon defects and larger interlayer spacing result in lower first-cycle coulombic efficiency and poorer cycling stability.

[0138] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0139] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a composite graphite anode material, characterized in that, include: Step S1: Prepare graphite oxide; Step S2, the graphite oxide undergoes a first calcination treatment to obtain graphite particles; Step S3: The graphite particles, phenolic organic compounds, and solvent are mixed to obtain a mixture; the mixture, catalyst, nitrogen source, and aldehyde organic compounds are mixed and reacted to obtain a precursor; the precursor includes the graphite particles and a nitrogen-containing phenolic resin layer coating the surface of the graphite particles. Step S4: The precursor undergoes a second calcination treatment to obtain the composite graphite anode material.

2. The method for preparing the composite graphite anode material according to claim 1, characterized in that, Step S1 further includes: Step S1-1: Mix graphite raw material with oxidant solution, and obtain reaction solution through oxidation reaction; In step S1-2, the reaction solution is subjected to solid-liquid separation to obtain the graphite oxide; The graphite raw material is selected from one or more of artificial graphite and natural graphite; and / or, The oxidant solution is selected from one or more of perchloric acid solution, concentrated sulfuric acid solution, concentrated nitric acid solution, sodium nitrate solution, potassium nitrate solution, and hydrogen peroxide solution, preferably perchloric acid solution; and / or, The weight ratio of the graphite raw material to the oxidant in the oxidant solution is 1:(5-15); and / or, The oxidation reaction is carried out at 100℃ to 200℃.

3. The method for preparing the composite graphite anode material according to claim 2, characterized in that, Step S1-2 further includes: The reaction solution was washed with water until the pH value was 4-7; The reaction solution after washing with water is subjected to solid-liquid separation to obtain a first solid product; The first solid product is dried to obtain the graphite core material.

4. The method for preparing the composite graphite anode material according to any one of claims 1 to 3, characterized in that, In step S2, the holding temperature of the first calcination treatment is 150℃~350℃, and the holding time is 5h~24h.

5. The method for preparing the composite graphite anode material according to claim 4, characterized in that, The first calcination treatment is carried out in a first protective atmosphere, and the gas flow rate of the first protective atmosphere is 1000±50mL / min.

6. The method for preparing the composite graphite anode material according to any one of claims 1 to 3, characterized in that, In step S3 The weight ratio of the phenolic organic compound, the catalyst, the nitrogen source, and the aldehyde organic compound is 10:(1-6): (3-10): (1-4); and / or, The total weight ratio of the phenolic organic compounds, the catalyst, the nitrogen source, and the aldehyde organic compounds to the weight of the graphite particles is (0.5–4):1; and / or, The phenolic organic compound is selected from one or more of phenol, aminophenol, nitrophenol, catechol, and resorcinol; and / or, The catalyst is selected from one or more of hydrochloric acid, sulfuric acid, and oxalic acid; and / or, The nitrogen source is selected from one or more of 2-methylimidazolium, p-nitroaniline, and polyvinylpyrrolidone; and / or, The aldehyde organic compound is selected from one or more of formaldehyde, acetaldehyde, propionaldehyde, and benzaldehyde; and / or the solvent is water and / or methanol.

7. The method for preparing the composite graphite anode material according to claim 6, characterized in that, Before mixing the mixture, the catalyst, the nitrogen source, and the aldehyde organic compound, step S3 further includes heating the mixture to a first temperature and carrying out the reaction at the first temperature; the first temperature is 60°C to 150°C.

8. The method for preparing the composite graphite anode material according to any one of claims 1 to 3, characterized in that, In step S4, the holding temperature of the second calcination treatment is 700℃~900℃, and the holding time is 2h~6h.

9. The method for preparing the composite graphite anode material according to claim 8, characterized in that, The second calcination treatment is carried out in a second protective atmosphere, and the gas flow rate of the second protective atmosphere is 50±2 mL / min.

10. A composite graphite anode material, characterized in that, The composite graphite anode material is prepared by the method described in any one of claims 1 to 9.

11. A negative electrode sheet, characterized in that, The negative electrode sheet includes the composite graphite negative electrode material as described in claim 10, or the negative electrode sheet includes the composite graphite negative electrode material prepared by the method for preparing the composite graphite negative electrode material according to any one of claims 1 to 9.

12. A lithium-ion battery, characterized in that, The lithium-ion battery includes the negative electrode sheet as described in claim 11.

Images