Graphite negative electrode material and preparation method and application thereof

Abstract

The invention provides a graphite negative electrode material and a preparation method and application thereof. The graphite negative electrode material comprises graphite, a hard carbon layer coated on the surface of the graphite and a metal oxide layer coated on the surface of the hard carbon layer, wherein the graphite negative electrode material meets the relational expression that # imgabs0 # d002 represents the carbon layer spacing of the graphite negative electrode material, rho represents the compaction density of the graphite negative electrode material, SSA represents the specific surface area of the graphite negative electrode material, Thard represents the average thickness of the hard carbon layer, and Tmetal represents the average thickness of the metal oxide layer. In the application, the hard carbon layer is a fiber-based hard carbon layer, is obtained by carbonizing the organic fibers, is uniformly combined with the graphite, has good strength and high interface stability, and can reduce the specific surface area of the graphite. The metal oxide layer can improve the electronic conductivity and the first coulombic efficiency of the material. And a method. The hard carbon layer and the metal oxide layer can achieve a synergistic effect, so that the rate capability and the specific capacity of the graphite negative electrode material can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and in particular relates to a graphite negative electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries, as an important secondary battery, are widely used in new energy vehicles, portable electronic devices, and other fields. As market requirements for lithium-ion battery energy density and rate performance continue to increase, traditional graphite anode materials have been unable to meet market demand due to their low theoretical specific capacity (372mAh / g) and poor rate performance.

[0003] At present, the main method to improve the energy density and rate performance of graphite negative electrode materials is surface coating, such as coating with soft carbon or hard carbon. However, traditional carbon coatings have problems such as uneven coating and poor interface stability, which cause the composite material to react with the electrolyte, thereby affecting the initial efficiency and rate performance of the battery. For example, the hard carbon coating itself has poor conductivity, resulting in increased lithium ion diffusion impedance and unable to meet the needs of fast charging. Soft carbon coatings often require a higher coating amount and have poor coating effects, and have no added value to the capacity of graphite.

[0004] Therefore, there is an urgent need to develop a new type of composite coating material that can solve the problems of existing coating materials while ensuring high energy density and rate performance.

[0005] The contents of the background technology section are merely the technologies known to the inventors and do not necessarily represent the existing technologies in this field. Summary of the Invention

[0006] In order to solve at least one of the above-mentioned problems in the prior art, the present application provides a graphite negative electrode material and a preparation method thereof.

[0007] The graphite negative electrode material provided in the present application comprises graphite, a hard carbon layer coated on the surface of the graphite, and a metal oxide layer coated on the surface of the hard carbon layer;

[0008] Wherein, the graphite negative electrode material satisfies the relationship: d 002 represents the carbon interlayer spacing of the graphite negative electrode material, ρ represents the compaction density of the graphite negative electrode material, SSA represents the specific surface area of the graphite negative electrode material, T hard represents the average thickness of the hard carbon layer, T metal represents the average thickness of the metal oxide layer.

[0009] In this application, the hard carbon layer is a fiber-based hard carbon layer, obtained by carbonizing organic fibers. It bonds evenly with graphite, exhibits good strength, and exhibits high interfacial stability, while also reducing the specific surface area of the graphite. The metal oxide layer can enhance the material's electronic conductivity and initial coulombic efficiency. Furthermore, the hard carbon layer and metal oxide layer work synergistically to effectively enhance the rate performance and specific capacity of the graphite anode material.

[0010] In some embodiments of the present application, the graphite negative electrode material satisfies at least one of the following conditions:

[0011] a) the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1;

[0012] b) the average thickness of the hard carbon layer is 60 to 90 nm;

[0013] c) the average thickness of the metal oxide layer is 25 to 45 nm;

[0014] d) The hard carbon layer is formed by carbonizing high-performance fibers, wherein the high-performance fibers are high-performance fibers polarized with polydopamine. Optionally, the high-performance fibers include at least one or more of polyethylene fibers, polystyrene fibers, and polyimide fibers.

[0015] Under the above conditions, the hard carbon layer has better bonding strength with graphite, higher interface stability, more uniform interface, and can better reduce the specific surface area of graphite; moreover, the synergistic effect of the hard carbon layer and the metal oxide layer is the best, which can better improve the electronic conductivity and first coulombic efficiency of the material.

[0016] In some embodiments of the present application, the graphite negative electrode material satisfies at least one of the following conditions:

[0017] e) the carbon interlayer spacing d of the graphite negative electrode material 002 0.355~0.361nm;

[0018] f) The specific surface area SSA of the graphite negative electrode material is ≤ 2.5 m 2 / g;

[0019] g) The compaction density of the graphite negative electrode material is ρ≥1.55g / cm 3 ;

[0020] h) the average width L of the graphite crystals in the graphite negative electrode material along the a-axis direction a 75~100nm;

[0021] i) The average height L of the graphite crystals in the graphite negative electrode material along the c-axis direction c It is 25 to 35 nm.

[0022] Under the above conditions, the performance of graphite negative electrode material is better.

[0023] The method for preparing the above-mentioned graphite negative electrode material provided in this application comprises the following steps:

[0024] S1: reacting dopamine with a high-performance fiber, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material;

[0025] S2: mixing the intermediate material with graphite so that the graphite is coated with the intermediate material, and then pyrolyzing the mixture to obtain a graphite precursor;

[0026] S3: Immersing the graphite precursor in a metal salt solution, drying, and then carbonizing at a low temperature to obtain the graphite negative electrode material.

[0027] In this application, dopamine is introduced to form a large number of amino and hydroxyl functional groups on the surface of high-performance fibers, which are then coated with graphite. The groups are then reacted with metal salt solutions and finally carbonized to form a hard carbon layer and a metal oxide layer coated on the graphite surface. The two layers have a synergistic effect and can effectively improve the rate performance and specific capacity of the prepared graphite material.

[0028] In some embodiments of the present application, step S1 includes:

[0029] Dissolving the dopamine and the high-performance fiber in water, stirring at a temperature of 60 to 80° C. for 2 to 5 hours, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material;

[0030] Optionally, the high-performance fiber includes at least one or more of polyethylene fiber, polystyrene fiber and polyimide fiber;

[0031] Optionally, the molecular weight of the high performance fiber is ≥3×10 6 g / moL;

[0032] Optionally, the mass ratio of the dopamine to the high-performance fiber is 1:(1-3).

[0033] Dopamine contains a large number of amino and hydroxyl functional groups. Step S1 uses the polymerized dopamine to polarize the high-performance fiber, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the fiber surface to enhance surface activity.

[0034] In some embodiments of the present application, step S2 includes:

[0035] dispersing the graphite in a solution with a pH of 2.0 to 3.0 to obtain a graphite dispersion solution; and

[0036] The graphite dispersion solution and the intermediate material are stirred and mixed so that the graphite is coated with the intermediate material, and then dried and pyrolyzed in an inert atmosphere at 300-500° C. for 2-4 hours to obtain the graphite precursor;

[0037] Optionally, the average particle size of the graphite is 8 to 20 μm;

[0038] Optionally, the solution is one or more of hydrochloric acid, nitric acid, sulfuric acid and HF;

[0039] Optionally, the stirring and mixing time is 30 to 90 minutes;

[0040] Optionally, the concentration of the graphite dispersion solution is 0.2 to 0.4 kg / L;

[0041] Optionally, the inert atmosphere is one or more of helium, nitrogen, and argon.

[0042] In step S2, graphite is coated with a high-performance fiber with an adhesive film formed on its surface, followed by pyrolysis. Because the fiber surface contains a large number of amino and hydroxyl functional groups, they bind to the graphite through electrostatic interactions and covalent bonding, enhancing the bonding strength between the coating and the graphite and improving interfacial stability. During the pyrolysis process, the fiber shrinks due to heat, partially filling the pores of the graphite and remaining coated on the surface, forming a uniform, stable hard carbon coating, thereby reducing the specific surface area of the graphite.

[0043] In some embodiments of the present application, step S3 includes:

[0044] The graphite precursor is immersed in the metal salt solution, and after drying, carbonized in an inert atmosphere at a temperature of 500 to 700° C. for 1 to 2 hours to obtain the graphite negative electrode material;

[0045] Optionally, the metal salt solution is one or more of aluminum nitrate, ferric nitrate, ferric chloride, and nickel nitrate solution;

[0046] Optionally, the inert atmosphere is one or more of helium, nitrogen, and argon.

[0047] In step S3, the graphite precursor with hydroxyl functional groups can combine with metal salt ions to form a stable structure, thereby coating the metal salt on the surface of the graphite precursor. After drying, low-temperature carbonization is performed to completely carbonize the fibers and metal salt coated on the graphite surface, thereby forming a hard carbon layer and a metal oxide layer. The present application also provides a negative electrode sheet comprising any of the above-described graphite negative electrode materials, or a graphite negative electrode material prepared by any of the above-described preparation methods.

[0048] The present application also provides a negative electrode sheet, which includes any of the graphite negative electrode materials described above, or a graphite negative electrode material prepared by any of the preparation methods described above.

[0049] The present application further provides a lithium-ion battery comprising the above-mentioned negative electrode sheet.

[0050] In some embodiments of the present application, the lithium-ion battery has a reversible discharge specific capacity of ≥375 mAh / g, an initial coulombic efficiency of ≥95%, a capacity retention rate of ≥90% after 200 cycles at 1C, and a capacity retention rate of ≥75% at 5C. The lithium-ion battery provided herein has excellent rate performance and specific capacity.

[0051] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The accompanying drawings, which constitute a part of the present disclosure, are used to provide a further understanding of the present disclosure. The exemplary embodiments of the present disclosure and their descriptions are used to explain the present disclosure and do not constitute an improper limitation to the present disclosure.

[0053] Figure 1 This is a process flow chart for preparing graphite negative electrode materials provided in one embodiment of the present application.

[0054] Figure 2 This is a SEM image of the graphite negative electrode material prepared in one embodiment of the present application, with a magnification of 3000×.

[0055] Figure 3 yes Figure 2 A local enlarged view with a magnification of 10000×.

[0056] Figure 4 This is a SEM image of a graphite negative electrode material prepared in another embodiment of the present application, with a magnification of 3000×.

[0057] Figure 5 yes Figure 4 A local enlarged view with a magnification of 10000×. DETAILED DESCRIPTION

[0058] Hereinafter, only certain exemplary embodiments are briefly described. As will be appreciated by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit or scope of the present invention. Therefore, the drawings and description are to be considered as illustrative in nature and not restrictive.

[0059] The disclosure below provides many different embodiments or examples for implementing the present invention. In order to simplify the disclosure of the present invention, the components and settings of specific examples are described below. Of course, they are merely examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numbers and / or reference letters in different examples. Such repetition is for the purpose of simplicity and clarity and does not in itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the present invention provides examples of various specific processes and materials, but those of ordinary skill in the art will recognize the application of other processes and / or the use of other materials.

[0060] In addition, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. It will also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology and the present invention, and will not be interpreted in an idealized or overly formal sense unless expressly defined as such in this article.

[0061] As used herein, "about" or "approximately" is inclusive of the stated value and means within an acceptable range of deviation from the particular value as determined by one skilled in the art, taking into account the measurement in question and the errors associated with the measurement of the particular quantity (i.e., the limitations of the measurement system). For example, "about" can mean within one or more standard deviations, or within ±30%, ±20%, ±10%, or ±5% of the stated value.

[0062] The following description of the embodiments of the present invention is provided in more detail with reference to the accompanying drawings and examples to provide a better understanding of the present invention and its advantages in various aspects. However, the embodiments and examples described below are for illustrative purposes only and are not intended to limit the present invention.

[0063] The graphite negative electrode material provided in the present application is a core-shell structure, in which graphite is the core and the surface of the core is covered with two layers of shells, the inner shell is a hard carbon layer, and the outer shell is a metal oxide layer. In the present application, the hard carbon layer is a fiber-based hard carbon layer, which is obtained by carbonizing high-performance organic fibers. It is evenly combined with graphite, has good strength, high interface stability, and can reduce the specific surface area of ​​graphite. The metal oxide layer can improve the electronic conductivity and first coulomb efficiency of the material. In the present application, the hard carbon layer and the metal oxide layer can work synergistically to effectively improve the rate performance and specific capacity of the graphite negative electrode material.

[0064] In this application, the graphite negative electrode material satisfies the relationship: Among them, d 002represents the carbon layer spacing of the graphite negative electrode material, ρ represents the compaction density of the graphite negative electrode material, SSA represents the specific surface area of the graphite negative electrode material, T hard The average thickness of the hard carbon layer, T metal Indicates the average thickness of the metal oxide layer. Within the above parameter range, it is shown that the graphite negative electrode material has very good rate performance and specific capacity. In some embodiments of the present application, The value of can be 0.5, 0.7, 0.9, 1.0, 1.1, 1.3 or 1.5.

[0065] Optionally, the graphite includes natural graphite and / or artificial graphite.

[0066] Optionally, the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1. Within this thickness ratio range, the synergistic effect of the hard carbon layer and the metal oxide layer is optimal. In some embodiments of the present application, the thickness ratio may be 1.3:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.3:1, or 3.6:1.

[0067] Optionally, the average thickness of the hard carbon layer is 60 to 90 nm. When the hard carbon layer is within this range, the bonding strength between the hard carbon layer and the graphite is improved, the interfacial stability is enhanced, the interface is more uniform, and the specific surface area of the graphite is further reduced. In some embodiments of the present application, the average thickness of the hard carbon layer may be 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, or 90 nm.

[0068] Optionally, the average thickness of the metal oxide layer is 25 to 45 nm. When the thickness of the metal oxide layer is within this range, it can better improve the electronic conductivity and first coulombic efficiency of the material. In some embodiments of the present application, the average thickness of the metal oxide layer may be 25 nm, 30 nm, 35 nm, 40 nm, or 45 nm.

[0069] Optionally, the hard carbon layer is formed by carbonizing high-performance fibers, which are fibers polarized with polydopamine. In the present application, high-performance fibers refer to fibers with a strength greater than 17.6 cN / dtex and an elastic modulus of more than 440 cN / dtex. Such fibers have high strength and a large elastic modulus, and the hard carbon properties formed after carbonization are also very good, which can more effectively improve the energy density and rate performance of the material. Optionally, the high-performance fibers include one or more of polyethylene fibers, polystyrene fibers, and polyimide fibers. The high-performance fibers polarized with polydopamine can form a large number of amino and hydroxyl functional groups on the fiber surface. In the subsequent mixing process with graphite, under the action of electrostatic interaction and covalent bonding, they are tightly combined with graphite, which can improve the bonding strength between the hard carbon coating and graphite, increase the thickness of the hard carbon coating, and improve the interface stability.

[0070] Optionally, the carbon interlayer spacing d of the graphite negative electrode material 002 The carbon interlayer spacing is large, and there are many lithium storage sites. In some embodiments of the present application, the carbon interlayer spacing d 002 It may be 0.355 nm, 0.356 nm, 0.357 nm, 0.358 nm, 0.359 nm, 0.360 nm or 0.361 nm.

[0071] Optionally, the specific surface area SSA of the graphite negative electrode material is ≤ 2.5 m 2 / g. Small specific surface area, fewer surface defects, and better material performance. In some specific embodiments, the specific surface area SSA of the graphite negative electrode material can be 2.5 cm 2 / g, 2.3cm 2 / g, 2.0cm 2 / g, 1.8cm 2 / g, 1.6cm 2 / g, 1.4cm 2 / g or 1.2cm 2 / g.

[0072] Optionally, the compaction density of the graphite negative electrode material is ρ≥1.55g / cm 3 The greater the compaction density, the higher the energy density of the lithium-ion battery. In some embodiments of the present application, the compaction density ρ of the graphite negative electrode material can be 1.55 g / cm 3 , 1.57g / cm 3 , 1.59g / cm 3 , 1.61g / cm 3 , 1.63g / cm 3 , 1.65g / cm 3 , 1.67g / cm 3 or 1.69 g / cm 3 .

[0073] Optionally, the average width L of the graphite crystals along the a-axis in the graphite negative electrode material is a 75~100nm. a The larger the value, the more lithium is inserted and the larger the lithium storage capacity. a The value can be 75nm, 80nm, 85nm, 90nm, 95nm or 100nm.

[0074] Optionally, the average height L of the graphite crystals along the c-axis in the graphite negative electrode material is c 25~35nm. cThe longitudinal stacking of the graphite structure is better when the value is within the above range, and it does not have a negative impact on the lithium ion diffusion and insertion efficiency. c The value may be 25 nm, 27 nm, 29 nm, 31 nm, 33 nm or 35 nm.

[0075] Figure 1 A method for preparing a graphite negative electrode material provided in an embodiment of the present application is shown, including the following steps S1 to S3.

[0076] S1: reacting dopamine with high-performance fibers so that dopamine is polymerized on the surface of the high-performance fibers and forms an adhesive film, thereby obtaining an intermediate material.

[0077] Dopamine contains a large number of amino and hydroxyl functional groups. In this step, the polymerized dopamine is used to polarize the high-performance fiber, so that the dopamine is polymerized on the surface of the high-performance fiber and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the fiber surface and enhancing the surface activity.

[0078] In some embodiments of the present application, step S1 may be specifically as follows: dissolving dopamine and high-performance fiber in water, stirring at a temperature of 60 to 80° C. for 2 to 5 hours, so that dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material.

[0079] Gentle stirring at 60-80°C ensures the oxidative self-polymerization of dopamine, thereby forming an adhesive film on the surface of the high-performance fiber, while preventing other reactions on the fiber itself. In some embodiments of the present application, the reaction temperature can be 60°C, 65°C, 70°C, 75°C, or 80°C.

[0080] As mentioned above, the high performance fiber includes one or more of polyethylene fiber, polystyrene fiber and polyimide fiber. Optionally, the molecular weight of the high performance fiber is ≥3×10 6 g / moL. High-performance fibers with relatively large molecular weight, such as polyethylene fibers and polyimide fibers, can be better coated on the surface of graphite.

[0081] Optionally, the mass ratio of dopamine to high-performance fiber is 1:(1-3). After the oxidative autopolymerization reaction of dopamine, it polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the fiber surface. Within the above mass ratio range, dopamine is not wasted and a large number of amino and hydroxyl functional groups can be formed on the fiber surface. In some embodiments of the present application, the mass ratio of dopamine to high-performance fiber can be 1:1, 2:3, 1:2, 2:5 or 1:3.

[0082] S2: mixing the intermediate material with graphite so that the graphite is coated with the intermediate material, and then pyrolyzing the mixture to obtain a graphite precursor.

[0083] This step uses high-performance fibers (i.e., intermediate materials) with an adhesive film formed on the surface to coat the graphite, which is then pyrolyzed. Because the fiber surface carries a large number of amino and hydroxyl functional groups, they bind to the graphite under electrostatic interactions and covalent bonding, which can increase the bonding strength between the coating and the graphite and improve interfacial stability. During the pyrolysis process, the fiber shrinks due to heat, partially filling the pores of the graphite, and the rest is coated on the surface of the graphite, forming a uniform and stable hard carbon coating, thereby reducing the specific surface area of the graphite.

[0084] In some embodiments of the present application, step S2 may specifically be: dispersing graphite in a solution with a pH of 2.0 to 3.0 to obtain a graphite dispersion solution; and stirring and mixing the graphite dispersion solution with the intermediate material so that the graphite is coated with the intermediate material, and then drying and pyrolyzing at 300 to 500°C in an inert atmosphere for 2 to 4 hours to obtain a graphite precursor.

[0085] Graphite is first acidified in an acidic solution with a pH of 2.0 to 3.0, allowing it to better bond with the high-performance fiber, which has a large number of amino and hydroxyl functional groups on its surface. Optionally, the solution may be one or more of hydrochloric acid, nitric acid, sulfuric acid, or HF. In some embodiments of the present application, the pH of the acidic solution may be 2.0, 2.2, 2.4, 2.6, 2.8, or 3.0.

[0086] Optionally, the average particle size of the graphite is 8 to 20 μm. The average particle size of the graphite directly affects the average particle size of the graphite negative electrode material. Within the above particle size range, the performance of the graphite negative electrode material obtained is better. In some embodiments of the present application, the average particle size of the graphite may be 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm.

[0087] Optionally, the concentration of the graphite dispersion solution is 0.2 to 0.4 kg / L. Within this concentration range, the acidification effect on graphite is better. In some embodiments of the present application, the concentration of the graphite dispersion solution may be 0.2 kg / L, 0.25 kg / L, 0.3 kg / L, 0.35 kg / L, or 0.4 kg / L.

[0088] Optionally, the stirring and mixing time is 30 to 90 minutes. This stirring time allows sufficient mixing of the graphite and the intermediate material to allow the graphite to be uniformly mixed and thus coated with the intermediate material. In some embodiments of the present application, the stirring and mixing time may be 30 minutes, 40 minutes, 50 minutes, 60 minutes, 70 minutes, 80 minutes, or 90 minutes. The stirring and mixing can be performed at room temperature.

[0089] In this application, the pyrolysis temperature is 300-500°C, and the time is 2-4 hours. At this temperature and time, the high-performance fibers coated on the graphite surface with a large number of amino and hydroxyl functional groups will not be completely carbonized, but will still have some hydroxyl functional groups. During the reaction with the metal salt solution, the hydroxyl functional groups can combine with the metal salt ions to form a stable structure. In some embodiments of this application, the pyrolysis temperature can be 300°C, 350°C, 400°C, 450°C, or 500°C, and the time can be 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours.

[0090] Optionally, the drying method is oven drying, vacuum drying or natural drying, etc. Optionally, the inert atmosphere is one or more of helium, nitrogen and argon.

[0091] S3: Immersing the graphite precursor in a metal salt solution, drying it, and then carbonizing it at a low temperature to obtain a graphite negative electrode material.

[0092] In this step, the graphite precursor with hydroxyl functional groups can combine with metal salt ions to form a stable structure, so that the metal salt is coated on the surface of the graphite precursor. After drying, it is carbonized at low temperature so that the fibers and metal salt coated on the graphite surface are completely carbonized, thereby forming a hard carbon layer and a metal oxide layer.

[0093] In some embodiments of the present application, step S3 may specifically include: immersing the graphite precursor in a metal salt solution, and then carbonizing it in an inert atmosphere at a temperature of 500-700° C. for 1-2 hours after drying, thereby obtaining a graphite negative electrode material.

[0094] A carbonization temperature of 500-700°C and a carbonization time of 1-2 hours ensures complete carbonization of the fibers and the metal salt without decomposing the metal salt, thereby obtaining a hard carbon layer and a metal oxide layer. In some embodiments of the present application, the carbonization temperature may be 500°C, 550°C, 600°C, 650°C, or 700°C, and the carbonization time may be 1 hour, 1.5 hours, or 2 hours.

[0095] Optionally, the metal salt solution is one or more of aluminum nitrate, ferric nitrate, ferric chloride, and nickel nitrate solutions. These metal ions are more conducive to improving the performance of the hard carbon negative electrode material.

[0096] Optionally, the drying method is oven drying, vacuum drying or natural drying, etc. Optionally, the inert atmosphere is one or more of helium, nitrogen and argon.

[0097] In this application, dopamine is introduced to form a large number of amino and hydroxyl functional groups on the surface of high-performance fibers, which are then coated with graphite. The groups are then reacted with metal salt solutions and finally carbonized to form a hard carbon layer and a metal oxide layer coated on the graphite surface. The two layers have a synergistic effect and can effectively improve the rate performance and specific capacity of the prepared graphite material.

[0098] The present application further provides a negative electrode sheet, comprising the above-mentioned graphite negative electrode material or the graphite negative electrode material prepared by the above-mentioned preparation method.

[0099] The negative electrode sheet generally includes a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector, wherein the negative electrode film layer may include the above-mentioned graphite negative electrode material.

[0100] The current collector may be a metal foil, such as aluminum foil or copper foil, preferably copper foil. The negative electrode film layer may further include a binder, a conductive agent, and the like. The binder may be, for example, styrene-butadiene rubber (SBR), polyvinylidene chloride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC-Na), sodium alginate, and the like. The conductive agent may be, for example, graphene, carbon nanotubes, Ketjen black, conductive carbon black (SP), and the like. Optionally, the negative electrode film layer may further include other additives, such as a dispersant (e.g., carboxymethyl cellulose (CMC)).

[0101] The present application also provides a lithium-ion battery comprising the above-mentioned negative electrode sheet. The lithium-ion battery also comprises a positive electrode sheet, a separator and an electrolyte. The lithium-ion battery provided in the present application has very good reversible discharge specific capacity, initial coulombic efficiency and capacity retention rate. Optionally, the reversible discharge specific capacity of the lithium-ion battery is ≥375mAh / g. Optionally, the initial coulombic efficiency of the lithium-ion battery is ≥95%. Optionally, the capacity retention rate of the lithium-ion battery after 200 cycles at 1C is ≥90%. Optionally, the capacity retention rate of the lithium-ion battery at 5C is ≥75%.

[0102] The lithium-ion battery of the present application is a secondary battery, which refers to a battery that can be recharged to activate the active material after the battery is discharged and continue to be used. Generally, a secondary battery includes a positive electrode, a negative electrode, a separator and an electrolyte. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode and the negative electrode. The separator is arranged between the positive electrode and the negative electrode to act as an isolation. The electrolyte acts as a conductor of ions between the positive electrode and the negative electrode.

[0103] The present invention will be described below with reference to specific embodiments. The process condition values taken in the following examples and comparative examples are exemplary, and their acceptable numerical ranges are as shown in the aforementioned summary of the invention. For process parameters not particularly noted, conventional techniques can be used. Unless otherwise specified, reagents and instruments used in the technical scheme provided by the present invention can be purchased from conventional channels or the market. It should be noted that, in the absence of conflict, the features in the embodiments in this application and the embodiments can be combined with each other.

[0104] Example 1

[0105] This embodiment prepares a graphite negative electrode material, and the specific steps are as follows:

[0106] 1) Dopamine and a molecular weight of 3×10 6 g / moL polyethylene fibers were mixed and dissolved in 200 mL of deionized water at a mass ratio of 1:2, and stirred at 60°C for 4 h to obtain an intermediate material;

[0107] 2) 50 g of natural flake graphite was dispersed in 200 mL of a hydrochloric acid solution with a pH of 2.0, and then mixed with the intermediate material in a ratio of 1:1. After vacuum drying, the mixture was heated to 300°C at a rate of 3°C / min in a nitrogen atmosphere and pyrolyzed for 4 h to form a hard carbon layer and obtain a graphite precursor.

[0108] 3) Immerse the graphite precursor in a 0.5M mixed solution of ferric nitrate (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) (Fe:Co molar ratio 1:1) and stir at 60°C for 2 hours. After drying, heat the mixture to 700°C at a rate of 3°C / min under argon protection and carbonize it for 1 hour to form a metal oxide layer to obtain a graphite negative electrode material.

[0109] Example 2

[0110] This embodiment prepares a graphite negative electrode material, and the specific steps are as follows:

[0111] 1) Dopamine and a molecular weight of 3×10 6 g / moL polyethylene fibers were mixed and dissolved in 200 mL of deionized water at a mass ratio of 1:3, and stirred at 60°C for 4 h to obtain an intermediate material;

[0112] 2) 80 g of artificial graphite was dispersed in 200 mL of hydrochloric acid solution with a pH of 3.0, and then mixed with the intermediate material in a ratio of 1:1. After vacuum drying, the mixture was heated to 500°C at a rate of 3°C / min in a nitrogen atmosphere and pyrolyzed for 2 h to form a hard carbon layer to obtain a graphite precursor.

[0113] 3) Immerse the graphite precursor in a 0.6 M mixed solution of nickel nitrate (Ni(NO3)2) and ferric nitrate (Fe(NO3)3) (Fe:Ni molar ratio 1:2) and stir at 60°C for 2 hours. After drying, heat the mixture to 700°C at a rate of 3°C / min under argon protection and carbonize for 1.5 hours to form a metal oxide layer to obtain a graphite negative electrode material.

[0114] Example 3

[0115] This embodiment prepares a graphite negative electrode material, and the specific steps are as follows:

[0116] 1) Dopamine and a molecular weight of 3×10 6 g / moL polyethylene fibers were mixed and dissolved in 200 mL of deionized water at a mass ratio of 2:3, and stirred at 70°C for 4 h to obtain an intermediate material;

[0117] 2) 50 g of natural flake graphite was dispersed in 200 mL of hydrochloric acid solution with a pH of 2.5, and then mixed with the intermediate material in a ratio of 1:1. After vacuum drying, the mixture was heated to 400°C at a rate of 3°C / min in a nitrogen atmosphere and pyrolyzed for 2 h to form a hard carbon layer to obtain a graphite precursor;

[0118] 3) Immerse the graphite precursor in a 0.6 M mixed solution of nickel nitrate (Ni(NO3)2) and ferric nitrate (Fe(NO3)3) (Fe:Ni molar ratio 1:2) and stir at 60°C for 2 hours. After drying, heat the mixture to 700°C at a rate of 3°C / min under argon protection and carbonize for 1.5 hours to form a metal oxide layer to obtain a graphite negative electrode material.

[0119] Example 4

[0120] This embodiment prepares a graphite negative electrode material, and the specific steps are as follows:

[0121] 1) Dopamine and a molecular weight of 3×10 7 g / moL polystyrene fibers were mixed and dissolved in 200 mL of deionized water at a mass ratio of 1:2, and stirred at 70°C for 4 h to obtain an intermediate material;

[0122] 2) 50 g of natural flake graphite was dispersed in 200 mL of hydrochloric acid solution with a pH of 2.5, and then mixed with the intermediate material in a ratio of 1:1. After vacuum drying, the mixture was heated to 500°C at a rate of 3°C / min in a nitrogen atmosphere and pyrolyzed for 4 h to form a hard carbon layer to obtain a graphite precursor;

[0123] 3) Immersing the graphite precursor in a 0.4 M ferric chloride (FeCl3) solution and stirring at 60°C for 2 h; after drying, heating to 700°C at 3°C / min under argon protection and carbonizing for 1.5 h to form a metal oxide layer to obtain a graphite negative electrode material.

[0124] Example 5

[0125] This embodiment prepares a graphite negative electrode material, and the specific steps are as follows:

[0126] 1) Dopamine and a molecular weight of 3×10 6 g / moL polyimide fibers were mixed and dissolved in 200 mL of deionized water at a mass ratio of 1:2, and stirred at 60°C for 4 h to obtain an intermediate material;

[0127] 2) 50 g of natural flake graphite was dispersed in 200 mL of hydrochloric acid solution with a pH of 2.5, and then mixed with the intermediate material in a ratio of 1:1. After vacuum drying, the mixture was heated to 300°C at 3°C / min in a nitrogen atmosphere and pyrolyzed for 2 h to form a hard carbon layer to obtain a graphite precursor;

[0128] 3) Immerse the graphite precursor in a 0.5M mixed solution of iron oxide (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) (Fe:Co molar ratio 1:1) and stir at 60°C for 2 hours; after drying, heat to 600°C at 3°C / min under argon protection and carbonize for 2 hours to form a metal oxide layer to obtain a graphite negative electrode material.

[0129] Comparative Example 1

[0130] The difference between this comparative example and Example 1 is that the dopamine in step 1) is replaced by phenolic resin.

[0131] Comparative Example 2

[0132] The difference between this comparative example and Example 1 is that step 2) is omitted, and step 3) is directly carried out using the intermediate material.

[0133] Comparative Example 3

[0134] The difference between this comparative example and Example 1 is that step 3) is performed first and then step 2), that is, step 3) is performed using the intermediate material first to form a metal oxide layer, and then step 2) is performed to form a hard carbon layer on the surface of the metal oxide layer.

[0135] Comparative Example 4

[0136] The difference between this comparative example and Example 1 is that the pyrolysis temperature in step 2) is 600° C. and the pyrolysis time is 2 h.

[0137] Comparative Example 5

[0138] The difference between this comparative example and Example 1 is that the pyrolysis temperature in step 2) is 200° C. and the pyrolysis time is 2 h.

[0139] Comparative Example 6

[0140] The difference between this comparative example and Example 1 is that there is no step 3), that is, the graphite precursor material in step 2) is the final product.

[0141] Comparative Example 7

[0142] The difference between this comparative example and Example 1 is that in step 1), polyethylene fiber is not used, and dopamine is directly dissolved in deionized water and stirred at 60° C. for 4 h to obtain an intermediate material.

[0143] Test Example 1

[0144] The graphite negative electrode materials of Examples 1 to 5 and Comparative Examples 1 to 7 were subjected to N2 adsorption and desorption to detect the specific surface area. The specific surface area was determined by gas adsorption method, and the interlayer spacing was calculated by XRD. The incident light wavelength was The TEM cross section was used to measure the coating thickness, as shown in Table 1.

[0145] The SEM image of the graphite negative electrode material prepared in Example 1 is shown in Figure 2 and Figure 3 The SEM image of the graphite negative electrode material prepared in Example 2 is shown in Figure 4 and Figure 5 .in Figure 2 The magnification is 3000×, Figure 3 for Figure 2 The local enlarged picture, with a magnification of 10000×, Figure 4 The magnification is 3000×, Figure 5 for Figure 4 A local enlarged view with a magnification of 10000×.

[0146] Table 1

[0147]

[0148] As shown in Examples 1-5 and Comparative Examples 1-7 in Table 1, the hard carbon / metal oxide coating significantly reduces the specific surface area of the resulting graphite anode material by covering the pores on the graphite surface. Furthermore, the hard carbon / metal oxide fills the interstices between graphite particles, increasing the compaction density of the resulting graphite anode material. Furthermore, the disordered nature of the surface-coated hard carbon increases the interlayer spacing of the resulting graphite anode material. After high-temperature carbonization, the microcrystalline structure of the surface coating grows, effectively improving rate performance.

[0149] Test Example 2

[0150] The graphite negative electrode materials obtained in Examples 1 to 5 and Comparative Examples 1 to 7 were used as negative electrode materials to prepare lithium-ion batteries according to the following methods. The preparation method of a button-type lithium-ion battery comprises the following steps:

[0151] According to the ratio of active material: SP: CMC: SBR = 94.5:1.5:1.5:2.5, the negative electrode material, SP, CMC, and SBR were weighed respectively and evenly mixed in deionized water to form a slurry; the evenly mixed slurry was coated on the aluminum foil current collector, baked in an 80°C oven for 1 hour, and then taken out and cooled to room temperature.

[0152] Adjust the rolling gap and roll the electrode. Cut the rolled electrode into small discs with a diameter of 14 mm and weigh them as m1. Similarly, cut the aluminum foil current collector into small discs with a diameter of 14 mm and weigh them as m2. Where (m1 - m2) * 0.945 is the mass of the active material, recorded as m3. The weighed discs are then placed in an 80°C oven under vacuum for 12 hours.

[0153] The vacuum-dried discs were transferred to a glove box and assembled into lithium-ion button cells using the lithium sheet as the counter and auxiliary electrodes, an electrolyte solution of 1M LiPF6 / EC:DEC (1:1), and a glass fiber separator. The cells were maintained in a glove box with oxygen and water concentrations below 0.01 ppm. The assembled cells were allowed to rest for 12 hours. The electrochemical performance of the rested cells was then tested at constant current on a Wuhan Blue Electric battery testing system. The test results are shown in Table 2.

[0154] Table 2

[0155]

[0156] Table 2 shows that hard carbon / metal oxide coating effectively improves the specific capacity and initial coulombic efficiency of the prepared graphite anode. Comparing the capacity retention during cycling at 1C, surface coating with metal oxides effectively increases ion transport and enhances rate performance. Hard carbon coating reduces the specific surface area of graphite, and pyrolysis and carbonization at a certain temperature form a stable surface-coated hard carbon structure, which facilitates the subsequent attachment of metal salts.

[0157] Obviously, the above embodiments are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all embodiments here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A graphite negative electrode material, characterized in that The invention comprises graphite, a hard carbon layer coated on the surface of the graphite, and a metal oxide layer coated on the surface of the hard carbon layer; Wherein, the graphite negative electrode material satisfies the relationship: d 002 represents the carbon interlayer spacing of the graphite negative electrode material, ρ represents the compaction density of the graphite negative electrode material, SSA represents the specific surface area of the graphite negative electrode material, T hard represents the average thickness of the hard carbon layer, T metal represents the average thickness of the metal oxide layer.

2. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material meets at least one of the following conditions: a) the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1; b) the average thickness of the hard carbon layer is 60 to 90 nm; c) the average thickness of the metal oxide layer is 25 to 45 nm; d) The hard carbon layer is formed by carbonizing high-performance fibers, wherein the high-performance fibers are high-performance fibers polarized with polydopamine. Optionally, the high-performance fibers include at least one or more of polyethylene fibers, polystyrene fibers, and polyimide fibers.

3. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material meets at least one of the following conditions: e) the carbon interlayer spacing d of the graphite negative electrode material 002 0.355~0.361nm; f) The specific surface area SSA of the graphite negative electrode material is ≤ 2.5 m 2 / g; g) The compaction density of the graphite negative electrode material is ρ≥1.55g / cm 3 ; h) the average width L of the graphite crystals in the graphite negative electrode material along the a-axis direction a 75~100nm; i) The average height L of the graphite crystals in the graphite negative electrode material along the c-axis direction c It is 25 to 35 nm.

4. A method for preparing the graphite negative electrode material according to any one of claims 1 to 3, characterized in that: The steps include: S1: reacting dopamine with a high-performance fiber, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material; S2: mixing the intermediate material with graphite so that the graphite is coated with the intermediate material, and then pyrolyzing the mixture to obtain a graphite precursor; S3: Immersing the graphite precursor in a metal salt solution, drying, and then carbonizing at a low temperature to obtain the graphite negative electrode material.

5. The method according to claim 4, characterized in that Step S1 includes: Dissolving the dopamine and the high-performance fiber in water, stirring at a temperature of 60 to 80° C. for 2 to 5 hours, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material; Optionally, the high-performance fiber includes at least one or more of polyethylene fiber, polystyrene fiber and polyimide fiber; Optionally, the molecular weight of the high performance fiber is ≥3×10 6 g / moL; Optionally, the mass ratio of the dopamine to the high-performance fiber is 1:(1-3).

6. The method according to claim 4, characterized in that Step S2 includes: dispersing the graphite in a solution with a pH of 2.0 to 3.0 to obtain a graphite dispersion solution; and The graphite dispersion solution and the intermediate material are stirred and mixed so that the graphite is coated with the intermediate material, and then dried and pyrolyzed in an inert atmosphere at 300-500° C. for 2-4 hours to obtain the graphite precursor; Optionally, the average particle size of the graphite is 8 to 20 μm; Optionally, the solution is one or more of hydrochloric acid, nitric acid, sulfuric acid and HF; Optionally, the stirring and mixing time is 30 to 90 minutes; Optionally, the concentration of the graphite dispersion solution is 0.2 to 0.4 kg / L; Optionally, the inert atmosphere is one or more of helium, nitrogen, and argon.

7. The method according to claim 4, characterized in that Step S3 includes: The graphite precursor is immersed in the metal salt solution, and after drying, carbonized in an inert atmosphere at a temperature of 500 to 700° C. for 1 to 2 hours to obtain the graphite negative electrode material; Optionally, the metal salt solution is one or more of aluminum nitrate, ferric nitrate, ferric chloride, and nickel nitrate solution; Optionally, the inert atmosphere is one or more of helium, nitrogen, and argon.

8. A negative electrode sheet, characterized in that: The invention comprises the graphite negative electrode material according to any one of claims 1 to 3, or the graphite negative electrode material prepared by the method according to any one of claims 4 to 7.

9. A lithium-ion battery, characterized in that: Including the negative electrode sheet according to claim 8.

10. The lithium-ion battery according to claim 9, characterized in that The lithium ion battery has a reversible discharge specific capacity of ≥375 mAh / g, an initial coulombic efficiency of ≥95%, a capacity retention rate of ≥90% after 200 cycles at 1C, and a capacity retention rate of ≥75% at 5C.

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