A graphite / silicon composite material, its preparation method and application

Abstract

This application provides a graphite / silicon composite material, its preparation method, and its applications. The graphite / silicon composite material comprises: expanded graphite; silicon nanoparticles located between the layers of the expanded graphite; and a coating layer covering the surface of the expanded graphite; wherein the coating layer includes a conductive agent and a binder. This application combines the high capacity of silicon with the excellent conductivity of graphite, resulting in advantages such as high energy density, excellent rate performance, and high cycle efficiency, while also being environmentally friendly and sustainable.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of lithium ion batteries, in particular, relates to a graphite / silicon composite material and a preparation method and application thereof. BACKGROUND

[0002] In recent years, lithium ion batteries (LIBs) have been widely applied in portable devices, electric vehicles and energy storage grids. The demand for high energy density applications encourages researchers to explore high-capacity, environmentally friendly and low-cost electrode materials. Among the materials that form intermetallic phases with lithium (Li), silicon (Si) is considered to be one of the most promising anode materials to replace the most advanced graphite material. It has a high theoretical specific capacity of 4200 mAh g -1 , and exhibits a lower delithiation potential (~0.4 V vs Li / Li + ), a relatively low voltage hysteresis, while its precursor material (such as silicon dioxide) is very abundant and low-cost. However, the huge volume expansion of silicon (ranging from 300% to 400%) leads to the pulverization of silicon particles and the repeated formation of an unstable solid-electrolyte interphase (SEI), and silicon has low electrical conductivity.

[0003] Graphite anodes have the advantages of structural stability and excellent electrical conductivity, but its theoretical specific capacity is 372 mAh / g, and the energy density is limited to improve. And when the high-power charge and discharge, Li + diffusion inside the particles does not match the rate of lithium deintercalation on the particle surface, and various polarizations, including ohmic polarization, concentration overpotential and charge transfer overpotential, are inevitable. Therefore, from the perspectives of cost and performance, building a reasonable graphite and nano-silicon composite material is considered to be the most reasonable direction to explore new anode materials.

[0004] The prior art discloses a preparation method of a carbon-silicon sandwich material and its application in lithium ion batteries. It comprises the following steps: (1) mixing nano-silicon, graphite material and non-graphite conductive carbon layer material; (2) extruding or rolling the mixture to separate and surround the nano-silicon with the graphite material to form a tight multi-layer sandwich structure; (3) first sintering at 200-1200°C in an inert atmosphere to obtain a carbon-silicon sandwich material precursor; (4) uniformly mixing the carbon-silicon sandwich material precursor with non-graphite conductive carbon layer material, and second sintering at 800-1200°C in an inert atmosphere to obtain carbon-silicon sandwich material particles. The composite material prepared by this method has been sintered twice, however, silicon will melt again to form large particles in a high temperature environment, and although the sandwich layer limits a certain expansion, the expansion inhibition is not obvious due to the volume increase of the silicon aggregation again at high temperature.

[0005] The contents of the background section merely represent the knowledge of the inventors and do not necessarily constitute the prior art. SUMMARY

[0006] To solve at least one problem in the prior art, the first aspect of the present application provides a graphite / silicon composite material. The graphite / silicon composite material comprises:

[0007] expanded graphite;

[0008] silicon nanoparticles, located in the interlayer of the expanded graphite; and

[0009] a coating layer, coated on the surface of the expanded graphite;

[0010] wherein the coating layer comprises a conductive agent and a binder.

[0011] The present application combines the high capacity of silicon and the excellent electrical conductivity of graphite, and has the advantages of high energy density, excellent rate performance, high cycle efficiency, environmental friendliness, and sustainable development. The present application uses expanded graphite as a skeleton, and the expanded graphite with a layered structure can enhance the Li + diffusion rate in the graphite, accelerating the kinetics of the graphite. The silicon nanoparticles are only located in the interlayer of the expanded graphite, and not on the surface, which can effectively inhibit the volume expansion of silicon, and at the same time improve the low electrical conductivity of silicon. The surface coating layer can further reduce the volume expansion of silicon, facilitate the rapid transmission of ions and electrons, and weaken the contact between the material and the electrolyte, significantly improving the cycle performance.

[0012] In some embodiments of the present application, the graphite / silicon composite material satisfies at least one of the following conditions:

[0013] a) the content of silicon in the graphite / silicon composite material is 0.2-30wt%, and the content of carbon is 70-99.8wt%;

[0014] b) the median particle size D50 of the graphite / silicon composite material is 5-15μm; 50

[0015] c) the interlayer spacing of the expanded graphite is 0.4-1.2nm;

[0016] d) the median particle size D50 of the silicon nanoparticles is 0.5-1.2nm; 50

[0017] e) the thickness of the coating layer is 0.01-100nm;

[0018] f) the conductive agent is selected from one or more of carbon black, conductive carbon fiber, graphene, and carbon nanotube;

[0019] ​​g) the binder is a polyacrylic series binder;

[0020] h) the mass ratio of the conductive agent and the binder in the coating layer is 1:(20-30).

[0021] The graphite / silicon composite material meeting the above conditions has better performance.

[0022] The second aspect of the present application provides a preparation method of a graphite / silicon composite material, comprising the following steps:

[0023] S1: dispersing silicon nanoparticles in an organic solvent to obtain an impregnating agent;

[0024] S2: placing expanded graphite into the impregnating agent for vacuum impregnation, so that the silicon nanoparticles are deposited between the layers and on the surface of the expanded graphite, to obtain a first intermediate product;

[0025] S3: performing ultrasonic treatment on the first intermediate product, so that the silicon nanoparticles deposited between the layers of the expanded graphite are uniformly dispersed, and the silicon nanoparticles deposited on the surface of the expanded graphite are removed, to obtain a second intermediate product;

[0026] S4: filtering the second intermediate product, and then performing freeze-drying on the filter residue, to obtain a third intermediate product; and

[0027] S5: performing surface coating on the third intermediate product by using a polymer precursor, to obtain the graphite / silicon composite material; wherein the polymer precursor comprises a conductive agent and a binder.

[0028] The method provided in the present application embeds silicon nanoparticles between graphite layers by vacuum impregnation, and then performs ultrasonic treatment, so that the graphite particles collide with each other, the silicon nanoparticles deposited on the outer surface of the graphite are shaken off and rubbed off, and the silicon nanoparticles deposited between the layers of the graphite are more uniformly dispersed. Next, the organic solvent is removed by filtering and freeze-drying, which can effectively prevent the silicon nanoparticles from being aggregated. Finally, the prepared material is surface-coated, which can further reduce the volume expansion of silicon, weaken the contact between the negative electrode material and the electrolyte, and significantly improve the cycle performance.

[0029] In some embodiments of the present application, the preparation method further comprises the following steps:

[0030] S0: preparing the expanded graphite by using artificial graphite;

[0031] Optionally, the interlayer spacing of the expanded graphite is 0.4-1.2 nm;

[0032] Optionally, the median particle size D50 of the artificial graphite is 5-15 μm. 50 ​

[0033] Optionally, the artificial graphite is heated at a rate of 10-30℃ / min and kept for 5-60min at 800-1200℃ to obtain the expanded graphite.

[0034] The method of increasing the interlayer spacing of graphite by high-temperature heat treatment can enhance the diffusion rate in the graphite and accelerate the kinetics of the graphite. + The prepared expanded graphite has high conductivity and excellent kinetics.

[0035] In some embodiments of the present application, the solid content of the impregnant in step S1 is 5-25%.

[0036] Optionally, the median particle size D50 of the silicon nanoparticles is 0.5-1.2nm. 50

[0037] Optionally, the organic solvent is selected from one or more of water, ethanol, isopropanol, n-propanol and acetone.

[0038] Optionally, the silicon nanoparticles are dispersed in the organic solvent by stirring, and further optionally, the stirring time is 0.1-2h.

[0039] The impregnant meeting the above conditions has better performance.

[0040] In some embodiments of the present application, the vacuum impregnation time in step S2 is 0.5-8h.

[0041] Optionally, the mass ratio of the expanded graphite to the impregnant is 1:(1-5).

[0042] The vacuum impregnation can successfully embed the silicon nanoparticles into the interlayer of the expanded graphite while preserving the original morphology of the graphite.

[0043] In some embodiments of the present application, the ultrasonic temperature in step S3 is room temperature, the ultrasonic time is 10-120min, and the ultrasonic power is 60-300kHz.

[0044] Optionally, the freeze-drying of the filter residue in step S4 includes: first, rapidly freezing the filter residue in liquid nitrogen, and then placing it in a freeze dryer for freeze-drying; further optionally, the freeze-drying is carried out in a vacuum environment, and the freeze-drying time is 5-24h.

[0045] The ultrasonic treatment is simple and effective, which can shake off and rub off the silicon nanoparticles deposited on the outer surface of the graphite through collision between the graphites, and the silicon nanoparticles deposited in the interlayer of the graphite are more uniformly dispersed.

[0046] ​The second intermediate product is filtered to remove most of the solution, and then the filter residue is freeze-dried. Freeze-drying can maximize the protection of the original structure and shape of the material, preventing the volume of the silicon nanoparticle aggregates from increasing and causing uncontrolled swelling.

[0047] In some embodiments of the present application, in step S5, the third intermediate product is mixed with the polymer precursor and water to prepare a slurry, and then spray-dried, so that the polymer precursor is coated on the surface of the third intermediate product;

[0048] Optionally, the solid content of the slurry is 20-30%.

[0049] Optionally, the mass ratio of the conductive agent to the binder in the polymer precursor is 1:(20-30).

[0050] Optionally, the mass ratio of the polymer precursor to the third intermediate product is (0.5-1):100.

[0051] In the present application, the polymer precursor includes a conductive agent and a binder, which not only isolates the contact between silicon and electrolyte, but also effectively improves the migration rate of lithium ions, thereby improving the charge and discharge efficiency and cycle performance.

[0052] The third aspect of the present application provides a negative electrode sheet comprising the graphite / silicon composite material described above or prepared by the preparation method described above.

[0053] The fourth aspect of the present application provides a lithium ion battery comprising the negative electrode sheet described above.

[0054] The lithium ion battery provided by the present application has high energy density, excellent rate performance, high cycle efficiency and other excellent performance.

[0055] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0056] The accompanying drawings, which form a part of this disclosure, are included to provide a further understanding of the disclosure, illustrative embodiments of the disclosure, and explanations of the disclosure. They are not intended to limit the disclosure unduly.

[0057] Figure 1 is a structural schematic diagram of the graphite / silicon composite material provided by an embodiment of the present application.

[0058] Figure 2 is a process flow diagram for preparing the graphite / silicon composite material provided by an embodiment of the present application.

[0059] Figure 3 is a process flow diagram for preparing a graphite / silicon composite material provided by another embodiment of the present application. DETAILED DESCRIPTION

[0060] In the following description, certain example embodiments will simply be described. As can be appreciated by one skilled in the art, the described embodiments can be modified in various different ways without departing from the spirit or scope of the application. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive.

[0061] The disclosure that follows provides many different embodiments, or examples, for implementing the present application. For the purpose of simplicity and clarity, the description that follows will describe certain examples in terms of specific components and arrangements. Of course, a person of ordinary skill in the art will comprehend that the application can be practiced with a variety of different components and arrangements without departing from the spirit or scope of the application. In addition, the present application will be described in reference to particular examples, but those of ordinary skill in the art will comprehend that the application is not limited to these particular examples. Further, the application provides examples of various specific processes and materials, but those of ordinary skill in the art will comprehend that other processes and / or other materials can be used.

[0062] In addition, unless otherwise indicated, all terms used herein including technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0063] "about" or "approximately," as used herein, includes the recited value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art. For example, "about" can mean within one or more standard deviations, or within ±30%, ±20%, ±10%, or ±5% of the recited value.

[0064] The specific embodiments of the present application will be described in greater detail below with reference to the drawings and examples. However, the specific embodiments described below are merely illustrative and are not intended to be limiting of the present application.

[0065] Figure 1 A graphite / silicon composite material provided by an embodiment of the present application is shown. As shown in FIG. 1, the graphite / silicon composite material includes a graphite layer 10 and a silicon layer 20. The graphite layer 10 and the silicon layer 20 are bonded together to form the graphite / silicon composite material. Figure 1As shown, the graphite / silicon composite provided by the present application comprises expanded graphite 2, silicon nanoparticles 3 and a coating layer 1. The silicon nanoparticles 3 are located between the layers of the expanded graphite 2, and the coating layer 1 is coated on the surface of the expanded graphite 2.

[0066] The present application combines the high capacity of silicon and the excellent conductivity of graphite, and has the advantages of high energy density, excellent rate performance, high cycle efficiency, environmental friendliness and sustainable development. The expanded graphite used as the framework in the present application has a layered structure, which can enhance the Li + The diffusion rate in the graphite is increased, and the kinetics of the graphite is accelerated. The silicon nanoparticles are only located between the layers of the expanded graphite, and not on the surface, which can effectively inhibit the volume expansion of silicon and improve the low conductivity of silicon. The surface coating layer can further reduce the volume expansion of silicon, facilitate the rapid transmission of ions and electrons, and weaken the contact between the material and the electrolyte, thereby significantly improving the cycle performance.

[0067] In the present application, the coating layer 1 comprises a conductive agent and a binder. The coating layer provided by the present application not only can isolate the contact between silicon and electrolyte, but also can effectively improve the migration rate of lithium ions, thereby improving the charge and discharge efficiency.

[0068] Optionally, the thickness of the coating layer is 0.01-100 nm. In some specific embodiments, the thickness of the coating layer can be 0.01 nm, 1 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm.

[0069] Optionally, the conductive agent is selected from one or more of carbon black, conductive carbon fiber, graphene and carbon nanotube. Optionally, the binder is a polyacrylic acid series binder, such as polyacrylic acid, lithium polyacrylate, polyacrylic acid-butyl acrylate copolymer, etc.

[0070] Optionally, the mass ratio of the conductive agent to the binder in the coating layer is 1:(20-30). Under this mass ratio, the contact between silicon and electrolyte can be more effectively isolated, and the conductive agent can play a role. In some specific embodiments, the mass ratio of the conductive agent to the binder can be 1:20, 1:21, 1:22, 1:23, 1:24, 1:25, 1:26, 1:27, 1:28, 1:29 or 1:30.

[0071] Optionally, the content of silicon in the graphite / silicon composite is 0.2-30wt%, and the content of carbon is 70-99.8wt%. Under the above content, the high capacity of silicon and the excellent conductivity of graphite can be fully played. In some specific embodiments, the content of silicon can be 0.2wt%, 3wt%, 6wt%, 9wt%, 12wt%, 15wt%, 18wt%, 21wt%, 24wt%, 27wt% or 30wt%. In some specific embodiments, the content of carbon can be 70wt%, 73wt%, 76wt%, 79wt%, 82wt%, 85wt%, 88wt%, 91wt%, 94wt%, 97wt% or 99.8wt%.

[0072] Optionally, the interlayer spacing of the expanded graphite is 0.4-1.2nm. The interlayer spacing in this range can effectively buffer the volume change of the silicon nanoparticles during the process of deintercalation of lithium. In some specific embodiments, the interlayer spacing of the expanded graphite can be 0.4nm, 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1nm, 1.1nm or 1.2nm.

[0073] Optionally, the median particle size D 50 of the silicon nanoparticles is 0.5-1.2nm. The smaller particle size and the smaller absolute expansion are more conducive to the cycle performance. In some specific embodiments, the median particle size D 50 of the silicon nanoparticles can be 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1.0nm, 1.1nm or 1.2nm.

[0074] Figure 2 A preparation method of a graphite / silicon composite provided by an embodiment of the present application is shown, which comprises the following steps S1-S5.

[0075] S1: Disperse the silicon nanoparticles in an organic solvent to obtain an impregnant.

[0076] Optionally, the solid content of the impregnant is 5-25%, that is, 5-25% of the silicon nanoparticles are included in the impregnant. In some specific embodiments, the solid content of the impregnant can be 5%, 7%, 9%, 11%, 13%, 15%, 17%, 19%, 21%, 23% or 25%.

[0077] Optionally, the median particle size D 50 of the silicon nanoparticles is 0.5-1.2nm. The smaller particle size and the smaller absolute expansion are more conducive to the cycle performance. In some specific embodiments, the median particle size D 50 of the silicon nanoparticles can be 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1.0nm, 1.1nm or 1.2nm.

[0078] Optionally, the organic solvent is selected from one or more of water, ethanol, isopropanol, n-propanol, and acetone. Optionally, the silicon nanoparticles are dispersed in the organic solvent using stirring. Optionally, the stirring is performed for a time period of 0.1-2 h. In some embodiments, the stirring is performed for a time period of 0.1 h, 0.5 h, 1 h, 1.5 h, or 2 h.

[0079] S2: The expanded graphite is placed in the impregnating agent for vacuum impregnation, so that the silicon nanoparticles are deposited between the layers and on the surface of the expanded graphite, to obtain a first intermediate product.

[0080] The vacuum impregnation can successfully embed the silicon nanoparticles between the layers of the expanded graphite while preserving the original morphology of the graphite.

[0081] Optionally, the vacuum impregnation is performed for a time period of 0.5-8 h. In some embodiments, the vacuum impregnation is performed for a time period of 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, or 8 h.

[0082] Optionally, the mass ratio of the expanded graphite to the impregnating agent is 1:(1-5). Under this range, the desired content of silicon nanoparticles can be obtained. In some embodiments, the mass ratio of the expanded graphite to the impregnating agent can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, or 1:5.

[0083] S3: The first intermediate product is subjected to ultrasonic treatment, so that the silicon nanoparticles deposited between the layers of the expanded graphite are uniformly dispersed, and the silicon nanoparticles deposited on the surface of the expanded graphite are removed, to obtain a second intermediate product.

[0084] The ultrasonic treatment is simple and effective, and the silicon nanoparticles deposited on the outer surface of the graphite are shaken off and rubbed off through collisions between the graphites, and the silicon nanoparticles deposited between the layers of the graphite are more uniformly dispersed.

[0085] Optionally, the ultrasonic temperature is room temperature, the ultrasonic time is 10-120 min, and the ultrasonic power is 60-300 kHz. Under the above conditions, the ultrasonic effect is better. In some embodiments, the ultrasonic time can be 10 min, 30 min, 50 min, 70 min, 90 min, 100 min, 110 min, or 120 min. In some embodiments, the ultrasonic power can be 60 kHz, 100 kHz, 150 kHz, 200 kHz, 250 kHz, or 300 kHz.

[0086] S4: The second intermediate product is filtered, and then the filter residue is freeze-dried, to obtain a third intermediate product.

[0087] The second intermediate product is filtered to remove most of the solution, and then the residue is subjected to freeze-drying. Freeze-drying can maximize the protection of the original structure and shape of the material, preventing the silicon nanoparticle aggregates from expanding uncontrollably due to the increase in volume.

[0088] Optionally, the freeze-drying of the residue includes: first, subjecting the residue to quick freezing in liquid nitrogen, and then placing the residue in a freeze-drying machine for freeze-drying. Optionally, the freeze-drying is performed in a vacuum environment, which can maximize the protection of the original structure and shape of the material. Optionally, the freeze-drying time is 5-24 hours. In some specific embodiments, the freeze-drying time can be 5 hours, 7 hours, 9 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, or 24 hours.

[0089] S5: coating the third intermediate product with a polymer precursor to obtain a graphite / silicon composite material.

[0090] In this application, the polymer precursor includes a conductive agent and a binder, which not only can isolate the silicon from the electrolyte, but also can effectively improve the migration rate of lithium ions, thereby improving the charge and discharge efficiency and the cycle performance.

[0091] Optionally, the step can be specifically as follows: preparing a slurry by mixing the third intermediate product, the polymer precursor, and water, and then spray-drying to coat the polymer precursor on the surface of the third intermediate product.

[0092] Optionally, the solid content of the slurry is 20-30%. In some specific embodiments, the solid content of the slurry can be 20%, 22%, 24%, 26%, 28%, or 30%.

[0093] Optionally, the mass ratio of the conductive agent to the binder in the polymer precursor is 1:(20-30). In some specific embodiments, the mass ratio of the conductive agent to the binder can be 1:20, 1:21, 1:22, 1:23, 1:24, 1:25, 1:26, 1:27, 1:28, 1:29, or 1:30.

[0094] Optionally, the mass ratio of the polymer precursor to the third intermediate product is (0.5-1):100. Under this range, the formed coating layer will not be too thick or too thin, which is more conducive to improving the performance of the material. In some specific embodiments, the mass ratio of the polymer precursor to the third intermediate product can be 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, or 1:100.

[0095] Optionally, as shown in Figure 3 the above method can further include a step S0: preparing expanded graphite from artificial graphite.

[0096] Optionally, the artificial graphite is heated at a temperature of 800-1200℃, and then is kept at the temperature, so as to obtain the expanded graphite. At the temperature, the van der Waals force between the graphite layers can be broken, so that the graphite expands in volume and the interlayer spacing is increased.

[0097] The expanded graphite prepared by the method of increasing the interlayer spacing of graphite through high-temperature heat treatment can enhance the Li + The diffusion rate in the graphite is increased, and the kinetics of the graphite is accelerated. The prepared expanded graphite has high conductivity and excellent kinetics.

[0098] Optionally, the heating rate is 10-30℃ / min, and the holding time is 5-60min. In some specific embodiments, the heating rate can be 10℃ / min, 12℃ / min, 14℃ / min, 16℃ / min, 18℃ / min, 20℃ / min, 22℃ / min, 24℃ / min, 26℃ / min, 28℃ / min or 30℃ / min. In some specific embodiments, the holding time can be 5min, 10min, 20min, 30min, 40min, 50min or 60min.

[0099] Optionally, the interlayer spacing of the expanded graphite is 0.4-1.2nm. In some specific embodiments, the interlayer spacing of the expanded graphite can be 0.4nm, 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1nm, 1.1nm or 1.2nm.

[0100] Optionally, the median particle size D 50 of the artificial graphite is 5-15μm. In some specific embodiments, the median particle size D 50 of the artificial graphite can be 5μm, 7μm, 9μm, 11μm, 13μm or 15μm.

[0101] The method provided in the application embeds the silicon nanoparticles between the graphite layers through vacuum impregnation, and then performs ultrasonic treatment, so that the graphite particles collide with each other, the silicon nanoparticles deposited on the outer surface of the graphite are shaken off and rubbed off, and the silicon nanoparticles deposited between the graphite layers are more uniformly dispersed. Next, the organic solvent is removed by filtration and freeze-drying, which can effectively prevent the silicon nanoparticles from being aggregated. Finally, the prepared material is coated on the surface, which can further reduce the volume expansion of silicon and weaken the contact between the negative electrode material and the electrolyte, and significantly improve the cycle performance.

[0102] The application combines the high capacity of silicon and the excellent conductivity of graphite, has the advantages of high energy density, excellent rate performance, high cycle efficiency, etc., is environmentally friendly, and has sustainable development. The expanded graphite used in the application as a skeleton has a layered structure, which can enhance the Li+ The diffusion rate inside the graphite accelerates the kinetics of the graphite. The silicon nanoparticles are only located in the interlayer of the expanded graphite, and not on the surface, which can effectively inhibit the volume expansion of silicon, and at the same time improve the low conductivity of silicon. The surface coating layer can further reduce the volume expansion of silicon, facilitate the rapid transmission of ions and electrons, and weaken the contact between the material and the electrolyte, thereby significantly improving the cycle performance.

[0103] The application further provides a negative electrode sheet comprising the graphite / silicon composite material or the graphite / silicon composite material prepared by the above preparation method.

[0104] The negative electrode sheet generally comprises a negative electrode current collector and a negative electrode film layer arranged on the negative electrode current collector, wherein the negative electrode film layer can comprise the graphite / silicon composite material.

[0105] The current collector can be a metal foil, such as an aluminum foil, a copper foil, etc., and is preferably a copper foil. The negative electrode film layer can further comprise a binder, a conductive agent, etc. The binder can be, for example, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC-Na), sodium alginate, etc. The conductive agent can be, for example, graphene, carbon nanotubes, Ketjen black, conductive carbon black (SP), etc. Optionally, the negative electrode film layer can further comprise other auxiliary agents, such as a dispersing agent (e.g., carboxymethyl cellulose (CMC)), etc.

[0106] The application further provides a lithium ion battery comprising the above negative electrode sheet. The lithium ion battery further comprises a positive electrode sheet, a separator, and an electrolyte.

[0107] The lithium ion battery of the application is a secondary battery, which refers to a battery that can continue to be used by activating the active material through charging after discharging. Generally, a secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet and serves as a barrier. The electrolyte serves as a conductor of ions between the positive electrode sheet and the negative electrode sheet.

[0108] The positive electrode sheet can comprise a current collector and a positive electrode sheet film layer. The current collector can be a metal foil, such as an aluminum foil, a copper foil, etc. The specific type of active material of the positive electrode sheet film layer is not limited and can be selected according to actual needs. The application does not limit the types of separator and electrolyte, which can be selected according to actual needs.

[0109] The lithium ion battery provided by the application has high energy density, excellent rate performance, high cycle efficiency, and other excellent properties.

[0110] The application will be illustrated below with reference to specific examples. The values of the process conditions taken in the following examples and comparative examples are exemplary, and the values of the ranges thereof are as indicated in the foregoing summary of the application. For the process parameters not specifically mentioned, refer to the conventional techniques. Unless otherwise specified, the reagents and instruments used in the technical solutions provided by the application can be purchased from the conventional channels or the market. It should be noted that the examples in the present application and the features in the examples can be combined with each other without conflict.

[0111] Example 1

[0112] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0113] (1) The artificial graphite with a median particle size D 50 is 10.0-11.0 μm is subjected to rapid heating at 1000°C, the heating rate is 15°C / min, and the constant temperature time is 30 min, to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0114] (2) The nanosilicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain an impregnating agent with uniform dispersion and a solid content of 8%.

[0115] (3) The impregnating agent and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0116] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time is 60 min, and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0117] (5) The second intermediate product is filtered to remove most of the solution. Then, it is rapidly frozen in liquid nitrogen and subjected to freeze-drying in a freeze-drying machine for 12 h to obtain a third intermediate product. The freeze-drying machine is operated in a vacuum environment.

[0118] (6) The third intermediate product, a polymer precursor, and pure water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain a final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0119] Example 2

[0120] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0121] (1) The artificial graphite with a median particle size D 50The artificial graphite with a median particle size D50 of 7.0-8.0 μm is rapidly heated at 900 ℃, the heating rate is 10 ℃ / min, and the constant temperature time is 20 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0122] (2) The median particle size D50 of the expanded graphite is 7.0-8.0 μm. 50 The nanosilicon with a median particle size D50 of 0.5-0.8 nm is dispersed in an isopropyl alcohol solution, and stirred for 0.3 h to obtain the impregnant with a uniform dispersion and a solid content of 6%.

[0123] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 2 h to obtain the first intermediate product.

[0124] (4) The first intermediate product is ultrasonically treated at room temperature, the ultrasonic time is 30 min, and the ultrasonic power is 50 kHz, to obtain the second intermediate product.

[0125] (5) The second intermediate product is filtered to remove most of the solution, and then rapidly frozen in liquid nitrogen and freeze-dried in a freeze dryer for 8 h to obtain the third intermediate product, wherein the freeze dryer is operated in a vacuum environment.

[0126] (6) The third intermediate product, the polymer precursor and pure water are uniformly mixed to prepare a slurry. The slurry is spray-dried to obtain the final product, wherein the solid content of the slurry is 20%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:20, and the mass ratio of the polymer precursor to the third intermediate product is 0.5:100.

[0127] Example 3

[0128] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0129] (1) The median particle size D50 of the expanded graphite is 7.0-8.0 μm. 50 The artificial graphite with a median particle size D50 of 13.0-14.0 μm is rapidly heated at 1100 ℃, the heating rate is 20 ℃ / min, and the constant temperature time is 40 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0130] (2) The median particle size D50 of the expanded graphite is 7.0-8.0 μm. 50 The nanosilicon with a median particle size D50 of 0.5-0.8 nm is dispersed in an isopropyl alcohol solution, and stirred for 0.8 h to obtain the impregnant with a uniform dispersion and a solid content of 10%.

[0131] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 6 h to obtain the first intermediate product.

[0132] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time is 90 min and the ultrasonic power is 160 kHz, to obtain a second intermediate product.

[0133] (5) The second intermediate product is filtered to remove most of the solution. Then, the second intermediate product is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 16 h to obtain a third intermediate product. The freeze-drier is operated in a vacuum environment.

[0134] (6) The third intermediate product, a polymer precursor and pure water are mixed to obtain a slurry. The slurry is spray-dried to obtain a final product. The solid content of the slurry is 30%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:30, and the mass ratio of the polymer precursor to the third intermediate product is 1:100.

[0135] Example 4

[0136] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0137] (1) The artificial graphite with a median particle size D 50 of 5.0-6.0 μm is subjected to rapid heating at 1000°C, the heating rate is 15°C / min, and the constant temperature time is 30 min, to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0138] (2) The nanosilicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain an impregnating agent with a uniform dispersion and a solid content of 8%.

[0139] (3) The impregnating agent and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0140] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time is 60 min and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0141] (5) The second intermediate product is filtered to remove most of the solution. Then, the second intermediate product is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 12 h to obtain a third intermediate product. The freeze-drier is operated in a vacuum environment.

[0142] (6) The third intermediate product, polymer precursor and purified water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0143] Example 5

[0144] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0145] (1) The artificial graphite with a median particle size D 50 of 10.0-11.0 μm is subjected to rapid heating at 1000°C, the heating rate is 15°C / min, and the constant temperature time is 30 min to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0146] (2) The nanosilicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain an impregnating agent with uniform dispersion and a solid content of 25%.

[0147] (3) The impregnating agent and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0148] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, the ultrasonic treatment time is 60 min, and the ultrasonic power is 120 kHz to obtain a second intermediate product.

[0149] (5) The second intermediate product is filtered to remove most of the solution. Then, it is rapidly frozen in liquid nitrogen and subjected to freeze-drying in a freeze-drying machine for 12 h to obtain a third intermediate product. The freeze-drying machine operates in a vacuum environment.

[0150] (6) The third intermediate product, polymer precursor and purified water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0151] Example 6

[0152] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0153] (1) The artificial graphite with a median particle size D 50The artificial graphite with a size of 10.0-11.0 μm is rapidly heated at 1000 ℃, the heating rate is 15 ℃ / min, and the constant temperature time is 30 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0154] (2) The median particle size D 50 The nanosilicon with a size of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain the impregnant with a uniform dispersion and a solid content of 8%.

[0155] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 1:1 for 4 h to obtain a first intermediate product.

[0156] (4) The first intermediate product is ultrasonically treated at room temperature, the ultrasonic time is 60 min, and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0157] (5) The second intermediate product is filtered to remove most of the solution, and then rapidly frozen in liquid nitrogen and freeze-dried in a freeze dryer for 12 h to obtain a third intermediate product, wherein the freeze dryer is operated in a vacuum environment.

[0158] (6) The third intermediate product, a polymer precursor and pure water are uniformly mixed to prepare a slurry. The slurry is spray-dried to obtain a final product, wherein the solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0159] Example 7

[0160] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0161] (1) The median particle size D 50 The artificial graphite with a size of 10.0-11.0 μm is rapidly heated at 1000 ℃, the heating rate is 15 ℃ / min, and the constant temperature time is 30 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0162] (2) The median particle size D 50 The nanosilicon with a size of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain the impregnant with a uniform dispersion and a solid content of 8%.

[0163] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 0.5 h to obtain a first intermediate product.

[0164] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time is 60 min and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0165] (5) The second intermediate product is filtered to remove most of the solution. Then, the second intermediate product is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 12 h to obtain a third intermediate product. The freeze-drier is operated in a vacuum environment.

[0166] (6) The third intermediate product, a polymer precursor and pure water are mixed to obtain a slurry. The slurry is spray-dried to obtain a final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0167] Example 8

[0168] In this example, a graphite / silicon composite material is prepared, and the specific steps are as follows:

[0169] (1) Artificial graphite with a median particle size D 50 of 10.0-11.0 μm is subjected to rapid heating at 1000°C, wherein the heating rate is 15°C / min and the constant temperature time is 30 min, to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0170] (2) Nanosilicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain an impregnating agent with a uniform dispersion and a solid content of 8%.

[0171] (3) The impregnating agent and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0172] (4) The first intermediate product is subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time is 60 min and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0173] (5) The second intermediate product is filtered to remove most of the solution. Then, the second intermediate product is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 5 h to obtain a third intermediate product. The freeze-drier is operated in a vacuum environment.

[0174] (6) The third intermediate product, the polymer precursor, and purified water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0175] Comparative Example 1

[0176] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0177] (1) The nanosilicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, stirred for 0.5 h, and a uniformly dispersed impregnant with a solid content of 8% is obtained.

[0178] (2) The impregnant and artificial graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product. The median particle size D 50 of the artificial graphite is 10.0-11.0 μm.

[0179] (3) The first intermediate product is subjected to ultrasonic treatment at room temperature, the ultrasonic treatment time is 60 min, and the ultrasonic power is 120 kHz to obtain a second intermediate product.

[0180] (4) The second intermediate product is filtered to remove most of the solution. Then it is rapidly frozen in liquid nitrogen and subjected to freeze-drying in a freeze-drying machine for 12 h to obtain a third intermediate product. The freeze-drying machine operates in a vacuum environment.

[0181] (5) The third intermediate product, the polymer precursor, and purified water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0182] Comparative Example 2

[0183] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0184] (1) The artificial graphite with a median particle size D 50 of 10.0-11.0 μm is rapidly heated at 1000°C, the heating rate is 5°C / min, and the constant temperature time is 30 min.

[0185] (2) The artificial graphite with a median particle size D 50Nano-sized silicon (0.5–0.8 nm) was dispersed in an ethanol solution and stirred for 0.5 h to obtain a uniformly dispersed impregnating agent with a solid content of 8%.

[0186] (3) The impregnating agent and the graphite treated in step (1) are vacuum impregnated at a mass ratio of 3:1 for 4 hours to obtain the first intermediate product.

[0187] (4) The first intermediate product is subjected to ultrasound at room temperature for 60 min and the ultrasound power is 120 kHz to obtain the second intermediate product.

[0188] (5) The second intermediate product was filtered to remove most of the solution. Then it was rapidly frozen in liquid nitrogen and freeze-dried in a freeze dryer for 12 hours to obtain the third intermediate product. The freeze dryer was operated in a vacuum environment.

[0189] (6) The third intermediate product, polymer precursor, and purified water are mixed evenly to obtain a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%. The polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, with a mass ratio of single-walled carbon nanotubes to lithium polyacrylate of 1:25. The mass ratio of polymer precursor to third intermediate product is 0.8:100.

[0190] Comparative Example 3

[0191] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0192] (1) The median particle size D 50 Expanded graphite with an interlayer spacing of 0.8–1.2 nm was obtained by rapidly heating artificial graphite with a thickness of 10.0–11.0 μm at 1000 °C at a heating rate of 15 °C / min and holding at the temperature for 30 min.

[0193] (2) The median particle size D 50 Nano-sized silicon (0.5–0.8 nm) was dispersed in an ethanol solution and stirred for 0.5 h to obtain a uniformly dispersed impregnating agent with a solid content of 8%.

[0194] (3) Impregnating agent and expanded graphite at a mass ratio of 3:1 under stirring for 4 hours to obtain the first intermediate product.

[0195] (4) The first intermediate product is subjected to ultrasound at room temperature for 60 min and the ultrasound power is 120 kHz to obtain the second intermediate product.

[0196] (5) The second intermediate product is filtered to remove most of the solution. Then it is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 12 hours to obtain a third intermediate product. The freeze-drier is operated in a vacuum environment.

[0197] (6) The third intermediate product, the polymer precursor and pure water are mixed to obtain a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0198] Comparative Example 4

[0199] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0200] (1) The artificial graphite with a median particle size D 50 of 10.0-11.0 μm is rapidly heated at 1000°C, the heating rate is 15°C / min, and the constant temperature time is 30 min to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0201] (2) The nano-silicon with a median particle size D 50 of 0.5-0.8 nm is dispersed in an ethanol solution, stirred for 0.5 h to obtain an impregnating agent with uniform dispersion and a solid content of 8%.

[0202] (3) The impregnating agent and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0203] (4) The first intermediate product is filtered to remove most of the solution. Then it is rapidly frozen in liquid nitrogen and freeze-dried in a freeze-drier for 12 hours to obtain a second intermediate product. The freeze-drier is operated in a vacuum environment.

[0204] (6) The second intermediate product, the polymer precursor and pure water are mixed to obtain a slurry. The slurry is spray-dried to obtain the final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0205] Comparative Example 5

[0206] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0207] (1) The artificial graphite with a median particle size D 50The artificial graphite with a size of 10.0-11.0 μm is rapidly heated at 1000 ℃, the heating rate is 15 ℃ / min, and the constant temperature time is 30 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0208] (2) The median particle size D 50 The nanosilicon with a size of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain the impregnant with a uniform dispersion and a solid content of 8%.

[0209] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0210] (4) The first intermediate product is ultrasonically treated at room temperature, the ultrasonic time is 60 min, and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0211] (5) The second intermediate product is filtered to remove most of the solution, and then dried in a vacuum drying box for 12 h to obtain a third intermediate product.

[0212] (6) The third intermediate product, a polymer precursor, and pure water are uniformly mixed to prepare a slurry. The slurry is spray-dried to obtain a final product. The solid content of the slurry is 25%, the polymer precursor is composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0213] Comparative Example 6

[0214] The graphite / silicon composite material is prepared according to the following specific steps:

[0215] (1) The median particle size D 50 The artificial graphite with a size of 10.0-11.0 μm is rapidly heated at 1000 ℃, the heating rate is 15 ℃ / min, and the constant temperature time is 30 min, to obtain the expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0216] (2) The median particle size D 50 The nanosilicon with a size of 0.5-0.8 nm is dispersed in an ethanol solution, and stirred for 0.5 h to obtain the impregnant with a uniform dispersion and a solid content of 8%.

[0217] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0218] (4) The first intermediate product is ultrasonically treated at room temperature, the ultrasonic time is 60 min, and the ultrasonic power is 120 kHz, to obtain a second intermediate product.

[0219] (5) The second intermediate product is filtered to remove most of the solution. Then it is quickly frozen in liquid nitrogen and placed in a freeze dryer for 12 hours to obtain the final product. The freeze dryer is operated in a vacuum environment.

[0220] Comparative Example 7

[0221] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0222] (1) The artificial graphite with a median particle size D 50 of 10.0-11.0 μm is rapidly heated at 1000°C with a heating rate of 15°C / min and a constant temperature time of 30 min to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0223] (2) The nanosilicon with a median particle size D 50 of 1.3-2.0 nm is dispersed in an ethanol solution and stirred for 0.5 h to obtain an impregnant with uniform dispersion and a solid content of 8%.

[0224] (3) The impregnant and the expanded graphite are vacuum-impregnated at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0225] (4) The first intermediate product is ultrasonically treated at room temperature, with an ultrasonic treatment time of 60 min and an ultrasonic power of 120 kHz, to obtain a second intermediate product.

[0226] (5) The second intermediate product is filtered to remove most of the solution. Then it is quickly frozen in liquid nitrogen and placed in a freeze dryer for 12 hours to obtain a third intermediate product. The freeze dryer is operated in a vacuum environment.

[0227] (6) The third intermediate product, a polymer precursor, and pure water are mixed uniformly to prepare a slurry. The slurry is spray-dried to obtain a final product. The solid content of the slurry is 25%, the polymer precursor consists of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of single-walled carbon nanotubes to lithium polyacrylate is 1:25, and the mass ratio of the polymer precursor to the third intermediate product is 0.8:100.

[0228] Comparative Example 8

[0229] This comparative example prepares a graphite / silicon composite material, and the specific steps are as follows:

[0230] (1) The artificial graphite with a median particle size D 50 of 10.0-11.0 μm is rapidly heated at 1000°C with a heating rate of 15°C / min and a constant temperature time of 30 min to obtain expanded graphite with an interlayer spacing of 0.8-1.2 nm.

[0231] (2) The median particle size D 50 The nanosilicon with a particle size of 0.5-0.8 nm was dispersed in water and stirred for 0.5 h to obtain an impregnant with a uniform dispersion and a solid content of 8%.

[0232] (3) The impregnant was vacuum-impregnated with expanded graphite at a mass ratio of 3:1 for 4 h to obtain a first intermediate product.

[0233] (4) The first intermediate product was subjected to ultrasonic treatment at room temperature, wherein the ultrasonic treatment time was 60 min and the ultrasonic power was 120 kHz, to obtain a second intermediate product.

[0234] (5) The second intermediate product was filtered to remove most of the solution. Then, the product was rapidly frozen in liquid nitrogen and subjected to freeze-drying in a freeze-drying machine for 12 h to obtain a third intermediate product. The freeze-drying machine was operated in a vacuum environment.

[0235] (6) The third intermediate product, a polymer precursor and pure water were uniformly mixed to prepare a slurry. The slurry was spray-dried to obtain a final product. The solid content of the slurry was 25%, the polymer precursor was composed of single-walled carbon nanotubes and lithium polyacrylate, the mass ratio of the single-walled carbon nanotubes to the lithium polyacrylate was 1:25, and the mass ratio of the polymer precursor to the third intermediate product was 0.8:100.

[0236] Test Example 1

[0237] Test samples: graphite / silicon composites provided in Examples 1-8 and composites provided in Comparative Examples 1-8.

[0238] Carbon content testing instrument: resistance muffle furnace SX-8-10.

[0239] Silicon content testing method: magnetic material digestion method.

[0240] Particle size (D 50 ) testing instrument: Malvern, Master Size 3000.

[0241] The specific test results are shown in Table 1 below:

[0242] Table 1

[0243]

[0244] Test Example 2

[0245] Test samples: graphite / silicon composites provided in Examples 1-8 and composites provided in Comparative Examples 1-8.

[0246] Test method: The prepared graphite negative electrode material was prepared into an electrode sheet, specifically as follows:

[0247] Each of the above samples is mixed with conductive agent carbon black (Surper P) and CMC (carboxymethyl cellulose) binder in a mass ratio of 97:2:1 to obtain a slurry, which is then coated on a copper foil to form a negative electrode sheet. The prepared negative electrode sheet is assembled with a lithium metal electrode sheet to form a lithium ion button cell. LiPF6 is dissolved in an electrolyte of ethylene carbonate / diethyl carbonate / methyl ethyl carbonate = 2:3:1 at a concentration of 1 mol / L. After the button cell is assembled, capacity, rate performance, and cycle performance tests are performed, as shown below:

[0248] (a) 0.1C discharge specific capacity test steps: ① stand for 5h; ② 0.05C discharge to 0.005V; ③ 0.05mA constant current discharge to 0.005V; ④ stand for 5min; ⑤ 0.01mA constant current discharge to 0.005V; ⑥ stand for 5min; ⑦ 0.1C charge to 2V; ⑧ stand for 5min;

[0249] (b) 2C discharge specific capacity test steps: ① stand for 5h; ② 0.05C discharge to 0.005V; ③ 0.05mA constant current discharge to 0.005V; ④ stand for 5min; ⑤ 0.01mA constant current discharge to 0.005V; ⑥ stand for 5min; ⑦ 0.1C charge to 2V; ⑧ stand for 5min; ⑨ 2C discharge to 0.005V, ⑩ stand for 30min; 0.1C discharge to 0.005V, stand for 30min; 0.05C discharge to 0.005V stand for 30min; 2C charge to 2V

[0250] (c) 5C discharge specific capacity test steps: ① stand for 5h; ② 0.05C discharge to 0.005V; ③ 0.05mA constant current discharge to 0.005V; ④ stand for 5min; ⑤ 0.01mA constant current discharge to 0.005V; ⑥ stand for 5min; ⑦ 0.1C charge to 2V; ⑧ stand for 5min; ⑨ 5C discharge to 0.005V, ⑩ stand for 30min; 0.1C discharge to 0.005V, stand for 30min; 0.05C discharge to 0.005V stand for 30min; 5C charge to 2V

[0251] (d) 0.1C 200 cycle capacity retention rate test steps: same as the 0.1C discharge specific capacity test steps, set the program to cycle for 200 times, then stop, and compare the capacity retention rate.

[0252] Process data: select 4 data with the range less than 3 mAh, remove the maximum and minimum and average.

[0253] The specific test results are shown in Table 2 below:

[0254] Table 2

[0255]

[0256] From the above, it can be seen that the lithium ion battery provided by the present application has high energy density, excellent rate performance, high cycle efficiency and other excellent performances. In Comparative Example 1, when directly using artificial graphite without expansion treatment, the silicon nanoparticles cannot enter between the graphite layers, so the performance of the prepared material is not good. In Comparative Example 2, the heating rate is too slow, the expansion effect of the graphite is poor, and therefore the deposition of the silicon nanoparticles is also less. In Comparative Example 3, vacuum impregnation is not used, and the deposition effect of the silicon nanoparticles is also poor. In Comparative Example 4, ultrasonic is not used, the silicon nanoparticles on the surface of the expanded graphite are not removed, and the silicon nanoparticles between the layers of the expanded graphite are also unevenly distributed. In Comparative Example 5, drying is directly carried out in a drying oven, the silicon nanoparticles are easy to agglomerate together, and then cause the volume of the aggregate to become large and bring about an uncontrollable expansion effect. In Comparative Example 6, the polymer precursor is not coated, and the negative electrode material directly contacts with the electrolyte, so the performance of the material becomes poor. In Comparative Example 7, the silicon nanoparticles used have a large particle size, and cannot be effectively deposited between the layers of the expanded graphite, so the performance of the negative electrode material obtained is poor. In Comparative Example 8, water-dispersed silicon nanoparticles are used, the silicon nanoparticles are easy to agglomerate in water, so the performance of the negative electrode material prepared is also not good.

[0257] Obviously, the above embodiments are only examples for clearly illustrating the present application, and are not intended to limit the implementation modes. Based on the above description, other different forms of changes or variations can also be made by those skilled in the art. Here, all the implementation modes do not need to be exhausted, and the obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A graphite / silicon composite material, characterized by, The graphite / silicon composite material comprises: expanded graphite; silicon nanoparticles, only located in the interlayer of the expanded graphite; and a coating layer coated on the surface of the expanded graphite. The preparation method of the graphite / silicon composite material comprises the following steps: The coating layer comprises a conductive agent and a binder, the content of silicon in the graphite / silicon composite material is 0.2-30 wt%, the content of carbon is 70-99.8 wt%, the interlayer spacing of the expanded graphite is 0.4-1.2 nm, the median particle size D 50 of the silicon nanoparticles is 0.5-1.2 nm. S0: after the artificial graphite is rapidly heated at 800-1200 ℃ and then kept for a certain time, the expanded graphite is prepared; the heating rate is 10-30 ℃ / min, and the keeping time is 5-60 min; S2: the expanded graphite is put into the impregnating agent for vacuum impregnation, so that the silicon nanoparticles are deposited in the interlayer and on the surface of the expanded graphite, and a first intermediate product is obtained; the interlayer spacing of the expanded graphite is 0.4-1.2 nm, and the mass ratio of the expanded graphite to the impregnating agent is 1:(1-5); S1 : dispersing silicon nanoparticles in an organic solvent to obtain an impregnating agent; the median particle size D50 of the silicon nanoparticles is 0.5-1.2 nm; 50 0.5-1.2 nm; S3: the first intermediate product is subjected to ultrasonic treatment, so that the silicon nanoparticles deposited in the interlayer of the expanded graphite are uniformly dispersed, and the silicon nanoparticles deposited on the surface of the expanded graphite are removed, and a second intermediate product is obtained; S4: the second intermediate product is filtered, and then the filter residue is freeze-dried to obtain a third intermediate product; and S5: the third intermediate product is coated with a polymer precursor to obtain the graphite / silicon composite material; wherein the polymer precursor comprises a conductive agent and a binder. The graphite / silicon composite material satisfies at least one of the following conditions:

2. The graphite / silicon composite of claim 1, wherein b) the thickness of the coating layer is 0.01-100 nm; a) the median particle size D50 of the graphite / silicon composite material is in the range of 5 to 15 pm; 50 5 to 15 pm; c) the conductive agent is selected from one or more of carbon black, conductive carbon fiber, graphene and carbon nanotube; d) the binder is a polyacrylic acid series binder; e) the mass ratio of the conductive agent to the binder in the coating layer is 1:(20-30).

3. The graphite / silicon composite material according to claim 1, wherein step S1 satisfies at least one of the following conditions: The median particle size D50 of the artificial graphite is 5-15 μm. 50 is 5-15 μm.

4. The graphite / silicon composite of claim 1, wherein the solid content of the impregnating agent is 5-25%; the organic solvent is selected from one or more of ethanol, isopropanol, n-propanol and acetone; the silicon nanoparticles are dispersed in the organic solvent by stirring, and the stirring time is 0.1-2 h. In step S2, the vacuum impregnation time is 0.5-8 h.

5. The graphite / silicon composite of claim 1, wherein In step S3, the ultrasonic temperature is room temperature, the ultrasonic time is 10-120 min, and the ultrasonic power is 60-300 kHz.

6. The graphite / silicon composite of claim 1, wherein In step S4, the freeze-drying of the filter residue comprises: first, the filter residue is rapidly frozen in liquid nitrogen, and then it is placed in a freeze dryer for freeze-drying; the freeze-drying is carried out in a vacuum environment, and the freeze-drying time is 5-24 h.

7. The graphite / silicon composite of claim 1, wherein In step S5, the third intermediate product is mixed with the polymer precursor and water to prepare a slurry, and then spray drying is carried out, so that the polymer precursor is coated on the surface of the third intermediate product.

8. The graphite / silicon composite of claim 1, wherein, Step S5 satisfies at least one of the following conditions:

9. The graphite / silicon composite of claim 8, wherein, the solid content of the slurry is 20-30%; the mass ratio of the conductive agent to the binder in the polymer precursor is 1:(20-30); the mass ratio of the polymer precursor to the third intermediate product is (0.5-1):

100. ​ 10. A negative electrode sheet characterized by comprising: The graphite / silicon composite material according to any one of claims 1 to 9.

11. A lithium-ion battery, characterized by The negative electrode sheet according to claim 10.

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