Negative pole piece, preparation method thereof and battery

Abstract

The invention discloses a negative pole piece, a preparation method thereof and a battery, and the method for preparing the negative pole piece comprises the following steps: at least one surface of a negative current collector is coated with negative slurry to form a negative slurry layer, the negative slurry comprises graphite and an orientation material, and the orientation material comprises at least one of graphene and graphene oxide; performing drying treatment, and removing the solvent in the negative electrode slurry layer to form a negative electrode active material layer so as to obtain a negative electrode plate; and applying an electric field perpendicular to the direction of the negative electrode current collector in the processes of coating the negative electrode slurry and drying, so that the directional material drives the graphite to carry out directional arrangement. The directional material is of a lamellar structure and can drive graphite to be directionally arranged under the action of an electric field, so that more graphite lamellar layers are arranged perpendicular to the negative electrode current collector, the order degree of a graphite particle material is increased, the OI value of graphite is reduced, the fast charging capacity of a graphite negative electrode is improved, graphite particles are arranged in order, the tortuosity of a negative electrode piece can be effectively reduced, and the service life of the negative electrode piece is prolonged. Therefore, pole piece polarization is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet and a preparation method thereof, and a battery. Background Art

[0002] Lithium-ion batteries are widely used in all aspects of human life, such as food, clothing, housing and transportation, due to their high specific energy, long cycle life, no memory and low self-discharge rate. In particular, their application in the field of power batteries has been strongly supported by national policies. With the continuous increase in the number of new energy vehicles, many users hope that new energy vehicles can replenish energy as quickly as traditional oil vehicles, that is, fast charging. Since the overall energy density of lithium-ion power batteries cannot be infinitely improved, it is necessary to improve the charging speed to alleviate the above problems, and the demand for fast-charging graphite negative electrode materials has also shown explosive growth. In the process of fast charging of lithium-ion batteries, the most important limiting factor is that lithium ions quickly move to the negative electrode after being released from the positive electrode, but the embedding speed of lithium ions in the negative electrode is less than the migration speed of lithium ions from the positive electrode. Lithium ions are enriched on the surface of the negative electrode, resulting in the precipitation of metallic lithium on the surface of the negative electrode, which may cause lithium precipitation, thermal runaway, a sharp rise in temperature, large battery temperature difference, poor cycle life and other failure anomalies. Therefore, it is necessary to continuously improve the fast charging technology of lithium-ion batteries to solve the energy replenishment anxiety of new energy power batteries.

[0003] Therefore, the current negative electrode sheet, preparation method thereof and battery still need to be improved. Summary of the invention

[0004] The present invention is based on the inventor's discovery and understanding of the following facts and problems:

[0005] At present, the negative electrode materials of lithium-ion batteries include graphite, silicon and silicon oxide, tin-based materials, lithium metal, lithium titanate and other materials. Among them, graphite materials have become the most widely commercialized negative electrode materials for lithium-ion batteries due to their advantages such as low lithium insertion potential, stable properties, high capacity, large operating temperature window, and low cost. The crystal structure of graphite materials is a layered crystal structure. Graphite crystals are anisotropic. During the charging process, lithium ions cannot enter the interior of the graphite crystal through the ordered sheet surface of graphite, that is, lithium ions cannot be embedded in graphite from the ordered surface of graphite, but need to be embedded in the graphite crystal from the end face of the graphite crystal structure and diffuse in the middle of the graphite sheet to complete the graphite lithium insertion process, such as Figure 1 As shown. Therefore, when the graphite sheets are parallel to the surface of the negative electrode current collector, lithium ions need to diffuse to the end faces of the graphite crystals before they can be embedded in the graphite, thereby increasing the polarization of lithium ion embedding. When the graphite sheets are perpendicular to the negative electrode current collector and the end faces are exposed to the surface of the electrode, there are more embedding sites on the surface of the graphite negative electrode. At the same time, the distance that lithium ions need to diffuse becomes shorter, which is more conducive to the rapid deintercalation of lithium ions, thereby reducing the polarization of the negative electrode side during charging and improving the fast charging capability of the battery system. Figure 2 shown.

[0006] Under high-rate fast charging conditions, the lithium ion migration and diffusion distance of the graphite negative electrode of the lithium-ion power battery is long, and the negative electrode polarization is large, which limits the improvement of the fast charging ability of the battery system. In order to improve the charging capacity of the battery, it is necessary to increase the number of graphite sheets in the pole piece that are perpendicular to the current collector, that is, to reduce the graphite OI value of the graphite negative electrode pole piece. At present, the main means to improve the fast charging performance of the graphite negative electrode and reduce the OI value is to improve the material isotropy by modifying the graphite particles by secondary granulation, surface coating and other material modifications, and increase the proportion of graphite sheets perpendicular to the current collector. The inventors found that in order to optimize the orientation of graphite particles in the graphite negative electrode pole piece, graphene or graphene oxide can be added to the graphite negative electrode slurry, and the orientation of the graphite particles can be oriented by applying electric fields, magnetic fields, etc. during the coating and drying process, and the tortuosity of the graphite negative electrode can be improved to reduce polarization during fast charging and improve the fast charging ability of the battery system.

[0007] The present invention aims to alleviate or solve at least one of the above-mentioned problems to at least some extent.

[0008] In one aspect of the present invention, the present invention proposes a method for preparing a negative electrode sheet. In some embodiments of the present invention, the method for preparing a negative electrode sheet includes: coating a negative electrode slurry on at least one surface of a negative electrode current collector to form a negative electrode slurry layer, wherein the negative electrode slurry includes graphite and a directional material, wherein the directional material includes at least one of graphene and graphene oxide; drying treatment to remove the solvent in the negative electrode slurry layer to form a negative electrode active material layer to obtain a negative electrode sheet; in the process of coating the negative electrode slurry and drying treatment, applying an electric field perpendicular to the direction of the negative electrode current collector so that the directional material drives the graphite to be oriented. Thus, the directional material is a sheet structure, which can drive the graphite to be oriented under the action of the electric field, so that more graphite sheets are arranged perpendicular to the negative electrode current collector, increase the order of the graphite particle material, and reduce the OI value of the graphite to improve the fast charging ability of the graphite negative electrode. The orderly arrangement of graphite particles can effectively reduce the pore tortuosity of the graphite negative electrode sheet, thereby reducing the polarization of the sheet and further improving the fast charging ability of the graphite negative electrode.

[0009] In some embodiments of the present invention, the negative electrode sheet satisfies at least one of the following conditions: in the negative electrode sheet, the OI value of the graphite is ≤10; the tortuosity is ≤3.6. As a result, the graphite particles in the negative electrode sheet are arranged in an orderly manner, and more graphite sheets are perpendicular to the negative electrode current collector, which is conducive to further improving the fast charging capability of the negative electrode sheet.

[0010] In some embodiments of the present invention, the intensity of the electric field is 10V / m-4000V / m, and the frequency of the electric field is 10Hz-100kHz. Under the action of the above electric field, graphene or graphene oxide can better drive the graphite to be oriented, so that more graphite sheets are arranged perpendicular to the negative electrode current collector, thereby further improving the fast charging capability of the negative electrode.

[0011] In some embodiments of the present invention, based on the total mass of the negative electrode active material layer, the mass content of the oriented material is 1%-30%; and / or the number of layers of the oriented material is ≤5.

[0012] In some embodiments of the present invention, the temperature of the drying treatment is 80° C.-120° C. Drying under the above conditions can appropriately shorten the drying time of the electrode sheet and effectively avoid cracking of the negative electrode active material layer.

[0013] In some embodiments of the present invention, after drying, the negative electrode sheet is subjected to roller pressing, and the compaction density of the negative electrode sheet after roller pressing is 1.3 g / cm 3 -2.2g / cm 3 The compaction density within the above range is beneficial to improving the overall performance of the negative electrode sheet.

[0014] In some embodiments of the present invention, the negative electrode active material layer includes a negative electrode active material, the oriented material, a conductive agent and a binder, the negative electrode active material includes graphite, and based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material is 50%-97%, the mass content of the oriented material is 1%-30%, the mass content of the conductive agent is 1%-10%, and the mass content of the binder is 1%-10%.

[0015] In some embodiments of the present invention, the preparation of the negative electrode slurry includes the following steps: adding the binder to the dispersant, stirring, and obtaining a glue solution; mixing the negative electrode active material, the oriented material, and the conductive agent with the glue solution, and homogenizing to obtain the negative electrode slurry. The negative electrode slurry prepared by the above steps has good uniformity, and can be coated on the current collector to obtain a negative electrode slurry layer with good thickness consistency and uniform composition.

[0016] In some embodiments of the present invention, the dispersant includes at least one of water, N,N-dimethylacetamide, tetrahydrofuran, butanone, ethylene glycol, N-methylpyrrolidone, acetone, dimethyl sulfoxide, pyridine, dioxane, N,N-dimethylformamide, and diethylene glycol.

[0017] In some embodiments of the present invention, the binder includes at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl pyrrolidone, polyethylene oxide, and polytetrafluoroethylene. The above binder has good bonding performance and stability, which is conducive to further improving the overall performance of the negative electrode sheet.

[0018] In some embodiments of the present invention, the conductive agent includes at least one of conductive carbon black and carbon nanotubes, thereby improving the conductivity of the negative electrode sheet and thus improving the charge and discharge performance of the electrode sheet.

[0019] In another aspect of the present invention, the present invention provides a negative electrode sheet. In some embodiments of the present invention, the negative electrode sheet is prepared by the method described above. As a result, the graphite in the negative electrode sheet has a high degree of order, more graphite sheets are arranged perpendicular to the negative electrode current collector, and the tortuosity of the negative electrode sheet is small, which is conducive to reducing the diffusion distance of lithium ions and reducing the polarization of the negative electrode, thereby facilitating improving the fast charging capability of the battery.

[0020] In another aspect of the present invention, the present invention provides a battery. In some embodiments of the present invention, the battery includes the negative electrode sheet described above. Therefore, the battery has all the characteristics and advantages of the negative electrode sheet described above, which will not be repeated here. In general, the battery has good fast charging performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0022] Figure 1 A schematic diagram showing the lithium insertion pathway of graphite sheets;

[0023] Figure 2 A schematic diagram showing how graphene orientation improves the orientation of graphite particles;

[0024] Figure 3 The graphite gram capacity of the negative electrode sheets of various embodiments and comparative examples at different rates is shown. DETAILED DESCRIPTION

[0025] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0026] In one aspect of the present invention, the present invention provides a method for preparing a negative electrode sheet. In some embodiments of the present invention, the method for preparing a negative electrode sheet may include the following steps:

[0027] S100: coating a negative electrode slurry on at least one surface of a negative electrode current collector to form a negative electrode slurry layer.

[0028] In some embodiments of the present invention, the negative electrode slurry may include graphite and an oriented material, and the oriented material may include at least one of graphene and graphene oxide.

[0029] S200: Drying treatment to remove the solvent in the negative electrode slurry layer, form a negative electrode active material layer, and obtain a negative electrode sheet.

[0030] During the coating of the negative electrode slurry and the drying process, an electric field perpendicular to the negative electrode current collector is applied, so that the oriented material drives the graphite to be oriented. Graphene and graphene oxide are sheet structures with obvious anisotropy and better sensitivity to electric and magnetic fields. Adding oriented materials such as graphene and / or graphene oxide to the negative electrode slurry and applying an electric field perpendicular to the plane of the negative electrode current collector (such as copper foil) during the coating of the slurry and the drying process can make the oriented material drive the graphite particles to be oriented, so that more graphite sheets are arranged perpendicular to the negative electrode current collector (such as Figure 2 As shown), the order of graphite particles is improved, the OI value of graphite is reduced, and the fast charging ability of graphite negative electrode materials is improved; graphite particles are arranged in an orderly manner along with graphene, graphene oxide and other sheets, which can effectively reduce the pore tortuosity in the graphite negative electrode sheet to reduce the polarization of the negative electrode sheet and further improve the fast charging ability of the graphite negative electrode. It should be noted that in the present invention, due to reasons such as equipment accuracy, the direction of the electric field can be substantially perpendicular to the plane where the negative electrode current collector is located. For example, the angle formed by the direction of the electric field and the plane where the negative electrode current collector is located is 90±2 degrees.

[0031] In some embodiments of the present invention, during the coating of the negative electrode slurry and the drying process, the intensity of the electric field may be 10V / m-4000V / m, for example, the intensity of the electric field may be 10V / m, 50V / m, 100V / m, 500V / m, 1000V / m, 2000V / m, 4000V / m, etc., and the frequency of the electric field may be 10Hz-100kHz, for example, the frequency of the electric field may be 10Hz, 30Hz, 50Hz, 80Hz, 100kHz, etc. The intensity and frequency of the electric field within the above range are conducive to the directional material driving the graphite particles to be oriented, so that more graphite sheets are arranged perpendicular to the negative electrode current collector, so that the graphite can exert a greater specific capacity, improve the fast charging performance of the electrode sheet, and will not significantly increase the preparation cost of the negative electrode sheet.

[0032] In some embodiments of the present invention, during the coating of the negative electrode slurry and the drying process, an alternating electric field can be applied to the electrode sheet. The intensity of the alternating electric field can be 10V / m-4000V / m, and the frequency of the alternating electric field can be 10Hz-100kHz.

[0033] In some embodiments of the present invention, based on the total mass of the negative electrode active material layer, the mass content of the oriented material can be 1%-30%, for example, the mass content of the oriented material can be 1%, 2%, 5%, 10%, 12%, 15%, 20%, 30%, etc. When the mass content of the oriented material is within the above range, more graphite particles can be driven to be oriented under the action of the electric field, so that the graphite particles are arranged in an orderly manner, the order of the graphite particle material is increased, the OI value of the graphite is reduced, and the tortuosity of the negative electrode sheet is reduced, thereby reducing the polarization of the sheet and improving the fast charging ability of the graphite negative electrode material.

[0034] In some embodiments of the present invention, the oriented material may be graphene, and the mass content of the graphene may be 1%-30% based on the total mass of the negative electrode active material layer.

[0035] In some other embodiments of the present invention, the oriented material may be graphene oxide, and the mass content of the graphene oxide may be 1%-30% based on the total mass of the negative electrode active material layer.

[0036] In other embodiments of the present invention, the oriented material may be a mixture of graphene and graphene oxide, and the sum of the mass contents of graphene and graphene oxide may be 1%-30% based on the total mass of the negative electrode active material layer. The mass ratio of graphene and graphene oxide is not specifically limited in the present invention, and those skilled in the art may set and adjust it according to actual needs.

[0037] In some embodiments of the present invention, the number of layers of the oriented material may be less than or equal to 5. For example, the oriented material is a sheet structure, and the number of layers of the oriented material may be 1, 2, 3, 4 or 5. When the number of layers of the oriented material is ≤ 5, the anisotropy of the oriented material is more obvious, and under the action of the electric field, the graphite particles can be better driven to be oriented.

[0038] In some embodiments of the present invention, the oriented material may include graphene and / or graphene oxide. The graphene and / or graphene oxide may be a single-layer structure or a sheet structure with no more than 5 layers.

[0039] In some embodiments of the present invention, in the negative electrode sheet prepared by the method proposed in the present invention, the OI value of graphite is ≤10, for example, the OI value of graphite can be 3, 4, 5, 8, 10, etc. The graphite orientation (OI value) is a parameter that describes the degree of order of graphite sheet arrangement in graphite materials. The (110) crystal plane diffraction signal collected by XRD comes from the graphite with a layer structure perpendicular to the electrode sheet, and the (002) and (004) crystal plane diffraction signals come from the graphite with a layer structure parallel to the electrode sheet. Therefore, the orientation of the graphite electrode can be described by the ratio of the (002) or (004) diffraction peak intensity (or integrated area) to the (110) diffraction peak intensity (or integrated area): OI = I (002) / I (110) or OI = I (004) / I (110). In the present invention, the OI value of graphite is I (002) / I (110).

[0040] In the negative electrode sheet, the OI value of graphite is ≤10, the graphite has a higher degree of order, the number of graphite flakes perpendicular to the negative electrode current collector is greater, more end faces are exposed to the surface of the negative electrode sheet, and the surface of the negative electrode sheet has more embedding sites, which is beneficial to shorten the diffusion distance of lithium ions, thereby helping to improve the fast charging capability of the graphite negative electrode.

[0041] In some embodiments of the present invention, the tortuosity of the negative electrode sheet prepared by the method proposed in the present invention is ≤3.6, for example, the tortuosity may be 2.2, 2.5, 3, 3.2, 3.6, etc. Thus, the tortuosity is small, the distance that lithium ions need to diffuse becomes shorter, which is more conducive to the rapid deintercalation of lithium ions, so as to reduce the polarization of the negative electrode side during charging, thereby improving the fast charging capability of the graphite negative electrode.

[0042] In some embodiments of the present invention, the negative electrode active material layer may include a negative electrode active material, an oriented material, a conductive agent and a binder, wherein the negative electrode active material may include graphite, the oriented material may include graphene and / or graphene oxide, and based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material may be 50%-97%, the mass content of the oriented material may be 1%-30%, the mass content of the conductive agent may be 1%-10%, and the mass content of the binder may be 1%-10%. For example, the mass content of the negative electrode active material may be 50%, 60%, 70%, 80%, 90%, 97%, etc., the mass content of the oriented material may be 1%, 2%, 5%, 10%, 15%, 30%, etc., the mass content of the conductive agent may be 1%, 3%, 5%, 8%, 10%, etc., and the mass content of the binder may be 1%, 2%, 5%, 7%, 10%, etc.

[0043] In some embodiments of the present invention, the negative electrode active material in the negative electrode plate may be graphite, and the mass content of the graphite may be 50%-97% based on the total mass of the negative electrode active material layer.

[0044] In some embodiments of the present invention, the binder may include at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl pyrrolidone, polyethylene oxide, and polytetrafluoroethylene. The above materials have good bonding properties, which are beneficial to improving the bonding strength between the negative electrode active material layer and the negative electrode current collector, so that the electrode sheet maintains good stability during the charge and discharge process.

[0045] In some embodiments of the present invention, the conductive agent may include at least one of conductive carbon black and carbon nanotubes. Adding the above materials is beneficial to improving the conductivity of the negative electrode active material layer.

[0046] In some embodiments of the present invention, the negative electrode slurry can be applied on one surface of the negative electrode current collector, and the negative electrode sheet obtained after drying has a negative electrode active material layer. In other embodiments of the present invention, the negative electrode slurry can be applied on both surfaces of the negative electrode current collector (such as copper foil), and after drying, a negative electrode active material layer is provided on each of the two surfaces of the negative electrode current collector. The specific method of applying the negative electrode slurry is not particularly limited in the present invention, and those skilled in the art can select it according to actual conditions and needs. In some embodiments of the present invention, the negative electrode slurry can be applied on the surface of the negative electrode current collector by a doctor blade method.

[0047] In some embodiments of the present invention, preparing the negative electrode slurry comprises the following steps: adding a binder to a dispersant, stirring, and obtaining a glue solution; mixing the negative electrode active material, the oriented material, and the conductive agent with the glue solution, and homogenizing to obtain the negative electrode slurry. In some embodiments of the present invention, the graphite particle material, graphene, the conductive agent, and the binder in the glue solution can be mixed and homogenized in a ratio of (50-97): (1-30): (1-10): (1-10) by a slurry mixing technology.

[0048] In some other embodiments of the present invention, the binder, the negative electrode active material, the directional material and the conductive agent may be added to the dispersant and stirred evenly to obtain the negative electrode slurry.

[0049] In some embodiments of the present invention, the dispersant may include at least one of water, N,N-dimethylacetamide, tetrahydrofuran, butanone, ethylene glycol, N-methylpyrrolidone, acetone, dimethyl sulfoxide, pyridine, dioxane, N,N-dimethylformamide, and diethylene glycol.

[0050] After coating the negative electrode slurry on the surface of the negative electrode current collector to form a negative electrode slurry layer, a drying treatment is performed. In some embodiments of the present invention, the drying treatment temperature may be 80°C-120°C, for example, the drying treatment temperature may be 80°C, 90°C, 100°C, 110°C, 120°C, etc. Drying the electrode sheet at the above temperature can shorten the drying treatment time, and the negative electrode active material layer is not easy to crack.

[0051] In some specific embodiments of the present invention, the dispersant in the negative electrode slurry may be water, and the temperature of the drying treatment may be 80°C-120°C.

[0052] In some specific embodiments of the present invention, after the drying process, the negative electrode sheet can be subjected to a roller pressing process, and the compaction density of the negative electrode sheet after the roller pressing process can be 1.3 g / cm 3 -2.2g / cm 3 For example, the compaction density of the negative electrode sheet can be 1.3g / cm 3 , 1.5g / cm 3 , 1.8g / cm 3 , 2.0g / cm 3 , 2.2g / cm 3 wait.

[0053] In general, the method proposed in the present invention has the following beneficial effects in preparing negative electrode sheets: (1) Graphene and / or graphene oxide and other oriented materials are added during the homogenization of graphite negative electrode slurry, so that the oriented materials and graphite particle materials are evenly mixed, and the plane surface of graphene and / or graphene oxide can effectively fit with the graphite layer surface of graphite particles, increase the conductivity of graphite particles, and improve the tortuosity of the electrode sheet. (2) By increasing the effect of the electric field in the coating process and the drying process, the oriented material drives the graphite particles to be arranged in an orderly manner during the slurry coating stage and the drying stage, thereby improving the order of the graphite particle material, reducing the OI value of graphite, and making more graphite sheets arranged perpendicular to the negative electrode current collector, so as to achieve the purpose of improving the fast charging capacity of the graphite negative electrode material. (3) The graphite particles are arranged in an orderly manner with oriented materials such as graphene sheets and graphene oxide sheets, which can effectively reduce the pore tortuosity in the graphite negative electrode sheet, thereby reducing the polarization of the electrode sheet and further improving the fast charging capacity of the graphite negative electrode.

[0054] In another aspect of the present invention, the present invention proposes a negative electrode sheet. In some embodiments of the present invention, the negative electrode sheet can be prepared using the method described above. As a result, the negative electrode active material graphite in the negative electrode sheet has a higher degree of order and a lower OI value, and more graphite sheets are arranged perpendicular to the negative electrode current collector, which is conducive to exerting the capacity of the negative electrode active material and improving the fast charging ability of the sheet; and the pore tortuosity of the negative electrode sheet is low, which can reduce the polarization of the sheet, thereby further improving the fast charging ability of the sheet.

[0055] In another aspect of the present invention, the present invention provides a battery. In some embodiments of the present invention, the battery may include the negative electrode sheet described above. Thus, the battery has all the features and advantages of the negative electrode sheet described above, which will not be repeated here. In general, the battery has good fast charging performance and can be used in electric vehicles, hybrid vehicles, electric bicycles and other electrical devices.

[0056] In some embodiments of the present invention, the battery may be a lithium-ion battery. In some specific embodiments of the present invention, the lithium-ion battery may include a positive electrode sheet, a separator, and the negative electrode sheet described above.

[0057] In some embodiments of the present invention, the positive electrode sheet may include a positive current collector and a positive active material layer located on at least one surface of the positive current collector, and the positive active material layer may include a positive active material, a positive conductive agent, a positive binder, etc. In some embodiments of the present invention, the positive active material may include at least one of lithium iron phosphate, lithium manganate, lithium cobaltate, ternary material nickel cobalt manganate, etc., the positive conductive agent may include at least one of conductive carbon black and carbon nanotubes, and the positive binder may include PVDF, etc.

[0058] The present invention will be described below by specific examples, and it will be appreciated by those skilled in the art that the following specific examples are only for illustrative purposes, and do not limit the scope of the present invention in any way. In addition, in the following examples, unless otherwise specified, the materials and equipment used are all commercially available. If in the following examples, specific processing conditions and processing methods are not clearly described, then conditions and methods known in the art can be used to process.

[0059] Example 1

[0060] In Example 1, natural graphite is selected as graphite, graphene is selected as graphene sheet (single layer), and the negative electrode conductive agent includes conductive carbon black and carbon nanotubes. Based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material graphite is 90%, the mass content of the graphene is 5%, the mass content of the conductive agent (the mass ratio of conductive carbon black to carbon nanotubes is 5:1) is 2%, and the mass content of the binder (the mass ratio of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyvinyl pyrrolidone is 2:2:1) is 3%.

[0061] (1) Preparation of graphite negative electrode sheet

[0062] Step 1: First, water, sodium carboxymethyl cellulose, styrene-butadiene rubber and polyvinyl pyrrolidone are mixed at a mass ratio of 90:4:4:2 by stirring at a low speed to form a glue solution.

[0063] Step 2: Mix the graphite particle material, graphene, conductive agent, and the binder in the glue in the first step in a mass ratio of 90:5:2:3 through slurry mixing technology to form a graphite negative electrode slurry.

[0064] Step 3: After preparing the slurry according to the above ratio, the graphite negative electrode slurry is coated on the surface of the copper foil current collector by scraping. During the coating process, an external electric field device is used to align the pole piece in an electric field orientation. The frequency of the alternating electric field used is 500 Hz, the intensity is 500 V / m, and the direction of the electric field is perpendicular to the plane direction of the current collector.

[0065] Step 4: Place the copper foil current collector coated with graphite negative electrode slurry in a drying device and slowly dry it at 90°C. During the drying process, use an external electric field device to align the pole pieces. The frequency of the alternating electric field is 500Hz and the intensity is 500V / m. After drying, roll the pole pieces to obtain graphite negative electrode pieces. The compaction density of the pole pieces is 1.7g / cm 3 .

[0066] (2) Assembling lithium-ion half-cells (graphite / metal lithium half-cells) and symmetrical batteries

[0067] The graphite negative electrode sheet serves as the positive electrode of the half-cell, and the lithium sheet serves as the negative electrode, forming a lithium-ion half-cell with the diaphragm and electrolyte.

[0068] Take a graphite negative electrode sheet as the positive electrode, and another graphite negative electrode sheet as the negative electrode, and form a symmetrical battery with a diaphragm and an electrolyte.

[0069] (3) Test of OI value of graphite in graphite negative electrode and tortuosity of negative electrode

[0070] The peak intensities of the (002) and (110) peaks of graphite in the graphite negative electrode sheet were tested by XRD, and the OI value of graphite was I(002) / I(110).

[0071] The tortuosity of the electrode was tested by testing the electrochemical impedance spectrum of a symmetrical battery (both the positive and negative electrodes were graphite negative electrode sheets).

[0072] The OI value of the graphite in the negative electrode sheet and the tortuosity test results of the negative electrode sheet are recorded in Table 1.

[0073] (4) Electrochemical performance test of lithium-ion half-cell (graphite / metal lithium half-cell)

[0074] First, the lithium-ion half-cell was charged and discharged at a rate of 0.1C in the voltage range of 0.010V-1.5V, and the lithium-ion half-cell was activated for five cycles. Then, the charge and discharge test was carried out at 0.2C, 0.5C, 1C, 1.5C, 2C, and 3C at 25°C to determine the charging capacity of the graphite negative electrode.

[0075] Table 1

[0076] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 OI value 5.2 5 9.7 7.4 13.8 11.7 12.9 Tortuosity 2.16 2.1 3.54 2.54 4.47 3.82 4.25

[0077] Example 2

[0078] The difference from Example 1 is that the frequency of the alternating electric field used in the process of coating the negative electrode slurry and drying is 100 kHz and the intensity is 4000 V / m. The remaining parameters and steps are the same as those in Example 1.

[0079] Depend on Figure 3 It can be seen that when the external electric field is large, the graphite particles are more obviously oriented and arranged, which can significantly improve the fast charging capability of the battery. As shown in Table 1, compared with Comparative Examples 1-3, in Example 2, the OI value of the graphite in the graphite negative electrode sheet and the tortuosity of the graphite negative electrode sheet are significantly reduced, and the improvement effect is obvious.

[0080] Example 3

[0081] Different from Example 1, based on the total mass of the negative electrode active material layer, the mass content of graphite is 94%, the mass content of graphene is 1%, the mass content of the conductive agent is 2%, and the mass content of the binder is 3%. The remaining parameters and steps are the same as those in Example 1.

[0082] Depend on Figure 3 It can be seen that adding a small amount of graphene can also drive the graphite particles to have a certain degree of directional orientation under the action of an external electric field, which can improve the fast charging ability of the battery to a certain extent. As shown in Table 1, compared with Comparative Examples 1-3, in Example 3, the OI value of the graphite in the graphite negative electrode sheet and the tortuosity of the negative electrode sheet are also reduced to a certain extent.

[0083] Example 4

[0084] The difference from Example 1 is that during the coating of the negative electrode slurry and the drying process, the frequency of the alternating electric field used is 10 Hz and the intensity is 10 V / m. The remaining parameters and steps are the same as those in Example 1.

[0085] Depend on Figure 3 It can be seen that under the action of the external electric field of Example 4, graphene can also drive the graphite particles to be oriented to a certain extent, which can improve the fast charging ability of the battery to a certain extent. As shown in Table 1, compared with Comparative Examples 1-3, in Example 4, the OI value of the graphite in the graphite negative electrode sheet and the tortuosity of the negative electrode sheet are also reduced to a certain extent.

[0086] Comparative Example 1

[0087] Different from Example 1, no graphene was added (the mass content of graphite in the negative electrode active material layer was 95%), and no external electric field equipment was used to orient the electrode. The remaining parameters and steps were the same as those in Example 1.

[0088] Comparative Example 2

[0089] The difference from Example 1 is that no external electric field equipment is used to orient the electrode piece. The remaining parameters and steps are the same as those in Example 1.

[0090] Depend on Figure 3 It can be seen that when only graphene is added without the action of an external electric field, the graphite particles and graphene sheets have no driving force for directional arrangement, and directional orientation does not occur. The fast charging capability of the battery is not significantly improved, and the OI value of the graphite in the graphite negative electrode sheet and the tortuosity of the negative electrode sheet do not decrease significantly. However, the presence of graphene also has the effect of improving the fast charging capability of the electrode sheet. Therefore, the fast charging capability of the graphite negative electrode sheet in Comparative Example 2 is slightly higher than that of Comparative Example 1 and Comparative Example 3, but lower than the fast charging capability of the graphite negative electrode of Examples 1-4.

[0091] Comparative Example 3

[0092] The difference from Example 1 is that no graphene is added (the mass content of graphite in the negative electrode active material layer is 95%), but an external electric field device is used to orient the electrode. The remaining parameters and steps are the same as those in Example 1.

[0093] Depend on Figure 3It can be seen that under the action of the external electric field, the graphite particles did not undergo obvious directional orientation, and the effect on improving the fast charging capacity of the battery was not obvious. As shown in Table 1, compared with Comparative Example 1, in Comparative Example 3, the OI value and tortuosity of the graphite negative electrode sheet did not decrease significantly, and the improvement effect was not obvious. In other words, without the addition of graphene, the effect of the external electric field on improving the OI value of graphite and the tortuosity of the negative electrode sheet was not obvious, and the charging capacity was not significantly improved.

[0094] In general, it can be seen from the test results of Examples 1-4 and Comparative Examples 1-3 that by adding anisotropic graphene and / or graphene oxide materials to the negative electrode slurry, combining the graphene sheets and / or graphene oxide sheets with the surface sheets of graphite particle materials by utilizing their similarity, applying external electric field forces during coating the negative electrode slurry and drying the electrode sheets, and utilizing the ordered arrangement characteristics of graphene and / or graphene oxide under the action of an electric field (or electromagnetic field) to improve the orderliness of the crystal structure of the graphite particle material in the graphite negative electrode sheet, the OI value of the graphite in the negative electrode sheet and the tortuosity of the negative electrode sheet can be effectively improved, and the fast charging capability of the graphite negative electrode can be effectively improved.

[0095] In the description of the present invention, the orientations or positional relationships indicated by terms such as "vertical" and "parallel" are based on the orientations or positional relationships shown in the accompanying drawings and are only for the convenience of describing the present invention rather than requiring that the present invention must be constructed and operated in a specific orientation. Therefore, they should not be understood as limitations on the present invention.

[0096] In the description of this specification, the description with reference to the terms "one embodiment", "another embodiment", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment are included in at least one embodiment of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0097] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a negative electrode sheet, characterized in that: include: Applying a negative electrode slurry on at least one surface of a negative electrode current collector to form a negative electrode slurry layer, wherein the negative electrode slurry includes graphite and an oriented material, and the oriented material includes at least one of graphene and graphene oxide; Drying to remove the solvent in the negative electrode slurry layer to form a negative electrode active material layer to obtain a negative electrode sheet; During the coating and drying process of the negative electrode slurry, an electric field perpendicular to the negative electrode current collector is applied so that the oriented material drives the graphite to be oriented.

2. The method according to claim 1, characterized in that: The negative electrode plate meets at least one of the following conditions: In the negative electrode sheet, the OI value of graphite is ≤10; Tortuosity ≤3.

6.

3. The method according to claim 1, characterized in that The intensity of the electric field is 10V / m-4000V / m, and the frequency of the electric field is 10Hz-100kHz.

4. The method according to claim 1, characterized in that: Based on the total mass of the negative electrode active material layer, the mass content of the oriented material is 1%-30%; And / or, the number of layers of the oriented material is ≤5.

5. The method according to claim 1, characterized in that The temperature of the drying treatment is 80℃-120℃.

6. The method according to claim 1, characterized in that After drying, the negative electrode sheet is rolled and the compaction density of the negative electrode sheet after rolling is 1.3g / cm 3 -2.2g / cm 3 .

7. The method according to any one of claims 1 to 6, characterized in that The negative electrode active material layer includes a negative electrode active material, the oriented material, a conductive agent and a binder, wherein the negative electrode active material includes graphite. Based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material is 50%-97%, the mass content of the oriented material is 1%-30%, the mass content of the conductive agent is 1%-10%, and the mass content of the binder is 1%-10%.

8. The method according to claim 7, characterized in that The preparation of the negative electrode slurry comprises the following steps: Adding the binder into the dispersant and stirring to obtain a glue solution; The negative electrode active material, the oriented material and the conductive agent are mixed with the glue solution and homogenized to obtain the negative electrode slurry; Optionally, the dispersant includes at least one of water, N,N-dimethylacetamide, tetrahydrofuran, butanone, ethylene glycol, N-methylpyrrolidone, acetone, dimethyl sulfoxide, pyridine, dioxane, N,N-dimethylformamide, and diethylene glycol; Optionally, the binder includes at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl pyrrolidone, polyethylene oxide, and polytetrafluoroethylene; Optionally, the conductive agent includes at least one of conductive carbon black and carbon nanotubes.

9. A negative electrode plate, characterized in that: The negative electrode plate is prepared by the method according to any one of claims 1 to 8.

10. A battery, characterized in that: Including the negative electrode sheet as described in claim 9.

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