Negative plate, preparation method thereof and battery

Abstract

The invention relates to a negative plate, a preparation method thereof and a battery, the negative plate comprises a current collector, and a first active material layer, a middle layer and a second active material layer which are sequentially stacked on at least one surface of the current collector in the thickness direction of the current collector, and the first active material layer is located between the current collector and the middle layer; the first active substance layer comprises a first binder, the second active substance layer comprises a second binder, and both the first binder and the second binder comprise hydroxyl groups; the middle layer comprises a modified carbon nano tube, and the modified carbon nano tube comprises carboxyl. According to the negative plate in the embodiment of the invention, through the three-layer coating design of the first active material layer, the middle layer and the second active material layer, the energy density of the battery is improved, and meanwhile, the fast charging performance and the cycle performance of the battery can be effectively improved, so that the comprehensive improvement of the multi-aspect performance of the lithium ion battery is realized.

Description

Technical Field

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

[0002] In the current development of lithium-ion battery technology, improving energy density, accelerating charging speed and extending cycle life are the three key directions to break through the bottleneck of electric vehicle endurance.

[0003] However, there is a mutual constraint relationship between the above three performance indicators, and it is difficult to achieve simultaneous optimization. Taking the improvement of battery energy density as an example, the preparation of ultra-thick pole pieces with a high coating amount is a direct and effective strategy. By increasing the coating amount of the pole piece, the proportion of inactive materials such as the current collector, the tab, and the battery shell in the overall battery can be effectively reduced, thereby increasing the specific energy of the battery. However, while this strategy brings about an increase in energy density, it also causes a series of new problems. On the one hand, under high-rate charge and discharge conditions, the diffusion kinetics of lithium ions in the electrode material are limited, which will lead to a decrease in the rate performance of the battery, thereby affecting the charge and discharge efficiency of the battery; on the other hand, after the pole piece with a high coating amount is rolled, it is difficult for the electrolyte to easily infiltrate the pole piece, which will lead to a decrease in the battery's liquid retention capacity, thereby affecting the battery's cycle performance.

[0004] Therefore, how to achieve comprehensive improvement in the multi-faceted performance of lithium-ion batteries is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] In view of this, embodiments of the present application provide a negative electrode sheet, a preparation method thereof, and a battery to solve at least one problem existing in the background technology.

[0006] In a first aspect, an embodiment of the present application provides a negative electrode sheet, comprising: a current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in a thickness direction of the current collector, wherein the first active material layer is located between the current collector and the intermediate layer;

[0007] The first active material layer includes a first binder, the second active material layer includes a second binder, and both the first binder and the second binder include hydroxyl groups;

[0008] The intermediate layer includes modified carbon nanotubes, and the modified carbon nanotubes include carboxyl groups.

[0009] In conjunction with the first aspect of the present application, in an optional embodiment, the negative electrode sheet satisfies at least one of the following characteristics:

[0010] (1) The first binder and the second binder each independently include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin;

[0011] (2) The intermediate layer further comprises a third binder, and the mass ratio of the modified carbon nanotubes to the third binder is (95-99): (1-5);

[0012] (3) The thickness of the first active material layer is 40 μm to 160 μm;

[0013] (4) The thickness of the second active material layer is 40 μm to 160 μm;

[0014] (5) The thickness of the intermediate layer is 10 μm to 30 μm;

[0015] (6) The second active material layer has a porous structure;

[0016] (7) The compaction density of the first active material layer is greater than the compaction density of the second active material layer;

[0017] (8) The compacted density of the first active material layer is 1.5 g / cc to 1.7 g / cc;

[0018] (9) The compacted density of the second active material layer is 1.3 g / cc to 1.6 g / cc;

[0019] (10) The compacted density of the intermediate layer is 1.3 g / cc to 1.7 g / cc.

[0020] In combination with the first aspect of the present application, in an optional embodiment, the first active material layer further includes a first active material, the second active material layer further includes a second active material, and the particle size of the first active material is smaller than the particle size of the second active material; optionally, the particle size D50 of the first active material is 5.5μm~10μm; optionally, the particle size D50 of the second active material is 10μm~18μm.

[0021] In a second aspect, an embodiment of the present application provides a method for preparing a negative electrode sheet, the method comprising the following steps:

[0022] Adding a first active material, a first binder and a first conductive agent into a first solvent and mixing them to obtain a first slurry; the first binder includes a hydroxyl group;

[0023] adding a second active material, a second binder, and a second conductive agent into a second solvent and mixing them to obtain a second slurry; wherein the second binder includes a hydroxyl group;

[0024] adding modified carbon nanotubes and a third binder into a third solvent and mixing them to obtain a third slurry; wherein the modified carbon nanotubes include carboxyl groups;

[0025] The first slurry, the second slurry and the third slurry are stacked and coated on at least one side of the current collector in a manner such that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry, and dried to obtain the negative electrode sheet.

[0026] In combination with the second aspect of the present application, in an optional embodiment, in the step of preparing the second slurry, a pore-forming agent is also added to the second solvent; optionally, the mass ratio of the second active material, the second binder, the second conductive agent and the pore-forming agent is (95.4~95.5):2:2:(0.5~1.5); optionally, the pore-forming agent includes at least one of ammonium bicarbonate, ammonium carbonate, and ammonium chloride.

[0027] In conjunction with the second aspect of the present application, in an optional embodiment, the steps of preparing the first slurry and the second slurry meet at least one of the following characteristics:

[0028] (1) The first binder and the second binder each independently include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin;

[0029] (2) The first active material and the second active material each independently include at least one of graphite, soft carbon, hard carbon, and silicon-based materials;

[0030] (3) the first solvent and the second solvent each independently comprise deionized water;

[0031] (4) The particle size of the first active material is smaller than the particle size of the second active material;

[0032] (5) The particle size D50 of the first active material is 5.5 μm to 10 μm;

[0033] (6) The particle size D50 of the second active material is 10 μm to 18 μm;

[0034] (7) The viscosity of the first slurry is 2000 mPa·s to 5000 mPa·s;

[0035] (8) The viscosity of the second slurry is 2000 mPa·s to 5000 mPa·s.

[0036] In conjunction with the second aspect of the present application, in an optional embodiment, the step of preparing the third slurry satisfies at least one of the following characteristics:

[0037] (1) The mass ratio of the modified carbon nanotubes to the third binder is (95-99): (1-5);

[0038] (2) the third solvent comprises deionized water;

[0039] (3) The viscosity of the third slurry is 300 mPa·s to 1000 mPa·s.

[0040] In conjunction with the second aspect of the present application, in an optional embodiment, the prepared negative electrode sheet includes the current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in a thickness direction of the current collector; the first active material layer is located between the current collector and the intermediate layer; and the method satisfies at least one of the following characteristics:

[0041] (1) The thickness of the first active material layer is 40 μm to 160 μm;

[0042] (2) The thickness of the second active material layer is 40 μm to 160 μm;

[0043] (3) The thickness of the intermediate layer is 10 μm to 30 μm.

[0044] In conjunction with the second aspect of the present application, in an optional embodiment, the method for preparing the modified carbon nanotubes includes:

[0045] The carbon nanotubes are placed in an acid solution for oxidation treatment, and after washing and drying, the modified carbon nanotubes are obtained; optionally, the molar concentration of the acid solution is 1 mol / L to 7 mol / L; optionally, the acid solution includes concentrated nitric acid and concentrated sulfuric acid, and the volume ratio of the concentrated nitric acid to the concentrated sulfuric acid is 1:(1 to 5); optionally, the oxidation treatment is carried out under the action of ultrasound, and the oxidation treatment time is 0.5h to 2.5h; optionally, the material-liquid ratio of the carbon nanotubes and the acid solution is (50 to 150) g: (100 to 900) mL; optionally, the temperature of the drying treatment is 40°C to 80°C, and the time is 8h to 16h.

[0046] In a third aspect, an embodiment of the present application provides a battery, comprising the negative electrode sheet described in any one of the first aspects or a negative electrode sheet prepared by the preparation method of the negative electrode sheet described in any one of the second aspects.

[0047] Compared with the prior art, the embodiments of the present application have the following beneficial effects:

[0048] The negative electrode sheet, preparation method thereof, and battery provided in the embodiments of the present application include: a current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in the thickness direction of the current collector, the first active material layer being located between the current collector and the intermediate layer; the first active material layer including a first binder, the second active material layer including a second binder, both the first binder and the second binder including hydroxyl groups; the intermediate layer including modified carbon nanotubes, and the modified carbon nanotubes including carboxyl groups. In the embodiment of the present application, the carboxyl groups in the modified carbon nanotubes can react with the hydroxyl groups in the first binder and the second binder to form ester groups, and the hydroxyl groups contained in the carboxyl groups of the modified carbon nanotubes can also form hydrogen bonds with the hydroxyl groups in the first binder and the second binder. Therefore, the intermediate layer containing the modified carbon nanotubes can act as a bridge, realizing the effective connection between the first active material layer and the second active material layer, and promoting the transmission of active ions in the negative electrode sheet; in addition, because the modified carbon nanotubes have strong conductivity and hydrophilicity, and they themselves have a porous structure, the intermediate layer can not only serve as an excellent conductive medium, but also as a medium to promote the transmission of electrolyte, thereby reducing the resistance of the negative electrode sheet and increasing the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery. The negative electrode sheet in the embodiment of the present application is designed with a three-layer coating of the first active material layer, the intermediate layer and the second active material layer. While improving the battery energy density, it can also effectively improve the fast charging performance and cycle performance of the battery, thereby achieving a comprehensive improvement in the various aspects of the performance of the lithium-ion battery.

[0049] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0051] Figure 1 A schematic cross-sectional view of a negative electrode sheet provided in an embodiment of the present application;

[0052] Figure 2 A schematic flow chart of a method for preparing a negative electrode sheet provided in an embodiment of the present application. DETAILED DESCRIPTION

[0053] In order to make the technical solutions and beneficial effects of the present invention more clearly understood, the following is a detailed description of the invention by referring to the accompanying drawings and listing specific embodiments. It should be understood that these embodiments are intended only to illustrate the present invention and are not intended to limit the scope of the invention. The experimental methods in the following examples where specific conditions are not specified are generally based on conventional experimental conditions. Unless otherwise specified, the reagents and raw materials used in the present invention are commercially available.

[0054] In the following description, numerous specific details are provided to provide a more thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced without one or more of these details. In other instances, certain technical features known in the art are not described to avoid confusion with the present application; that is, all features of actual embodiments are not described herein, nor are well-known functions and steps described in detail.

[0055] The purpose of the terms used herein is only to describe specific embodiments and is not intended to limit the present application. When used herein, the singular forms "a", "an", and "the" are intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "comprising" and / or "including", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0056] In order to fully understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below. However, in addition to these detailed descriptions, the present application may also have other implementation methods.

[0057] Unless otherwise defined, technical and scientific terms used in this application have the same meanings as those in the technical field to which this application belongs.

[0058] In the following examples, if no specific techniques or conditions are specified, the methods are generally carried out according to conventional techniques or conditions described in the literature in the field, or according to the product instructions and the conditions recommended by the manufacturer. The numerical ranges in the following examples are all inclusive.

[0059] The embodiment of the present application provides a negative electrode sheet, such as Figure 1As shown, the negative electrode sheet includes: a current collector 100 and a first active material layer 201, an intermediate layer 202 and a second active material layer 203 stacked in sequence on at least one surface of the current collector 100 in the thickness direction of the current collector 100, the first active material layer 201 is located between the current collector 100 and the intermediate layer 202; the first active material layer 201 includes a first binder, the second active material layer 203 includes a second binder, and both the first binder and the second binder include hydroxyl groups; the intermediate layer 202 includes modified carbon nanotubes, and the modified carbon nanotubes include carboxyl groups.

[0060] In the embodiment of the present application, the carboxyl groups in the modified carbon nanotubes can react with the hydroxyl groups in the first binder and the second binder to form ester groups, and the hydroxyl groups contained in the carboxyl groups of the modified carbon nanotubes can also form hydrogen bonds with the hydroxyl groups in the first binder and the second binder. Therefore, the intermediate layer 202 containing the modified carbon nanotubes can act as a bridge to achieve effective connection between the first active material layer 201 and the second active material layer 203, thereby promoting the transmission of active ions in the negative electrode sheet. In addition, since the modified carbon nanotubes have strong conductivity and hydrophilicity (carboxyl groups are hydrophilic groups) and have a porous structure, the intermediate layer 202 can not only serve as an excellent conductive medium, but also as a medium for promoting electrolyte transmission, thereby reducing the resistance of the negative electrode sheet and increasing the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery. The negative electrode sheet in the embodiment of the present application is designed with a three-layer coating of the first active material layer, the intermediate layer and the second active material layer. While improving the battery energy density, it can also effectively improve the fast charging performance and cycle performance of the battery, thereby achieving a comprehensive improvement in the various aspects of the performance of the lithium-ion battery.

[0061] It is understandable that Figure 1 The first active material layer 201, the intermediate layer 202, and the second active material layer 203 are sequentially stacked on both surfaces of the current collector 100 along the thickness direction, which is only an example. In some other embodiments of the present application, the first active material layer 201, the intermediate layer 202, and the second active material layer 203 can be sequentially stacked on one surface of the current collector 100. Specifically, the first active material layer 201, the intermediate layer 202, and the second active material layer 203 can be sequentially stacked on one surface of the current collector 100 along the thickness direction.

[0062] In some embodiments, the first binder and the second binder may each independently include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin. The types of the first binder and the second binder may be the same or different. On this basis, the first binder and the second binder may also include other types of binders to improve the mechanical properties of the electrode. Specifically, the first binder and the second binder may also include styrene-butadiene rubber, which is beneficial to improving the flexibility of the negative electrode sheet, thereby improving the processing performance of the negative electrode sheet and the overall performance of the battery. Furthermore, in the first binder and the second binder, the mass ratio of styrene-butadiene rubber to the binder containing hydroxyl groups can be (8:2) to (3:7).

[0063] In some embodiments, the intermediate layer 202 further includes a third binder, and the mass ratio of the modified carbon nanotubes to the third binder can be (95-99): (1-5), for example, 95:1, 95:3, 95:5, 96:2, 97:2, 98:4, 99:1, 99:5, or any value between any two of the above numerical ranges. In this way, while ensuring the bonding performance between the intermediate layer 202 and the first active material layer 201 and the second active material layer 203, it is beneficial to better achieve effective connection between the first active material layer 201 and the second active material layer 203 through the intermediate layer 202, and it is beneficial to form a better conductive network in the intermediate layer 202 and improve the absorption and transmission capacity of the intermediate layer 202 for the electrolyte, thereby further improving the fast charging capability and cycle performance of the battery.

[0064] In the embodiments of the present application, the type of the third binder is not limited. The third binder may be the same as or different from the first binder and the second binder. For example, the third binder may include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, epoxy resin, and styrene-butadiene rubber.

[0065] It is understood that when the mass proportion of the modified carbon nanotubes in the intermediate layer 202 is too low, on the one hand, the number of carboxyl groups provided is limited, and the ester groups and hydrogen bonds formed between the modified carbon nanotubes and the first binder and the second binder are limited, which is not conducive to the effective connection between the first active material layer 201 and the second active material layer 203 through the intermediate layer 202; on the other hand, it is not conducive to the formation of a relatively complete conductive network in the intermediate layer 202, and at the same time affects the absorption and transmission capacity of the intermediate layer 202 for the electrolyte. Therefore, in some specific embodiments, the mass proportion of the modified carbon nanotubes in the intermediate layer 202 can be greater than or equal to 95%. In this way, the effective connection between the first active material layer 201 and the second active material layer 203 can be better achieved through the intermediate layer 202, and the intermediate layer 202 can serve as an excellent conductive medium and a medium for transmitting electrolyte, effectively reducing the resistance of the negative electrode sheet and improving the liquid absorption and retention capacity of the negative electrode sheet, thereby effectively improving the fast charging performance and cycle performance of the battery.

[0066] In some embodiments, the thickness of the first active material layer 201 can be 40 μm to 160 μm, for example, 40 μm, 80 μm, 120 μm, 160 μm, or any value between any two of the above ranges. This helps to improve the fast charging performance and cycle performance of the battery while ensuring the energy density of the battery.

[0067] In some embodiments, the thickness of the second active material layer 203 can be 40 μm to 160 μm, for example, 40 μm, 80 μm, 120 μm, 160 μm, or any value between any two of the above ranges. This helps to improve the fast charging performance and cycle performance of the battery while ensuring the energy density of the battery.

[0068] It can be understood that when the thickness of the intermediate layer 202 is too thin, it is not conducive to forming a good conductive network and promoting the transmission of the electrolyte; when the thickness of the intermediate layer 202 is too thick, the modified carbon nanotubes are prone to agglomeration, which will also affect the conductivity and liquid absorption capacity of the intermediate layer 202. Therefore, in some embodiments, the thickness of the intermediate layer 202 can be 10μm to 30μm, for example, it can be 10μm, 15μm, 20μm, 25μm, 30μm or any value between any two of the above numerical ranges. In this way, the intermediate layer 202 can better reduce the resistance of the negative electrode sheet and improve the liquid absorption rate of the negative electrode sheet, thereby better improving the fast charging performance and cycle performance of the battery.

[0069] In some embodiments, the second active material layer 203 may have a porous structure.

[0070] In the embodiment of the present application, the second active material layer 203 is located on the surface of the negative electrode sheet and is in direct contact with the electrolyte. The second active material layer 203 has a porous structure, which is not only conducive to the electrolyte penetrating into the electrode sheet, thereby improving the cycle performance of the battery; it is also conducive to shortening the migration path of active ions, thereby improving the battery's rate performance and fast charging capability.

[0071] In some embodiments, the compaction density of the first active material layer 201 may be greater than the compaction density of the second active material layer 203 .

[0072] In the embodiment of the present application, the intermediate layer 202 can serve as an excellent conductive medium and a medium for promoting electrolyte transmission, thereby promoting the absorption of electrolyte by the negative electrode sheet, thereby effectively improving the electrolyte's infiltration effect on the first active material layer 201 located between the intermediate layer 202 and the current collector 100. Therefore, the compaction density of the first active material layer 201 can be greater than the compaction density of the second active material layer 203. In this way, the battery's fast charging capability and cycle performance can be improved while further improving the battery's capacity and energy density.

[0073] In some specific embodiments, the compaction density of the first active material layer 201 may be 1.5 g / cc to 1.7 g / cc. Furthermore, the compaction density of the first active material layer 201 is greater than the compaction density of the second active material layer 203 .

[0074] In some specific embodiments, the compaction density of the second active material layer 203 may be 1.3 g / cc to 1.6 g / cc. Furthermore, the compaction density of the first active material layer 201 is greater than the compaction density of the second active material layer 203 .

[0075] In some embodiments, the compaction density of the intermediate layer 202 may be 1.3 g / cc to 1.7 g / cc. This facilitates the formation of a more complete conductive network in the intermediate layer 202 and improves the electrolyte absorption rate of the negative electrode through the intermediate layer 202, thereby improving the fast charging capability and cycle performance of the battery.

[0076] In some embodiments, the first active material layer 201 further includes a first active material, and the second active material layer 203 further includes a second active material. The particle size of the first active material is smaller than that of the second active material.

[0077] In the embodiment of the present application, the particle size of the first active material is smaller than that of the second active material, which is beneficial to improving the compaction density of the first active material layer 201. In this way, the battery's fast charging capability and cycle performance can be improved while further improving the battery's capacity and energy density.

[0078] Further optionally, the particle size D50 of the first active material may be 5.5 μm to 10 μm; the particle size D50 of the second active material may be 10 μm to 18 μm.

[0079] Illustratively, the first active material and the second active material may each independently include at least one of graphite, soft carbon, hard carbon, and silicon-based materials.

[0080] In some embodiments, the first active material layer 201 further includes a first conductive agent, and the second active material layer 203 further includes a second conductive agent.

[0081] For example, the first conductive agent and the second conductive agent can each independently include at least one of carbon black, carbon nanotubes, graphene, and acetylene black. The first conductive agent and the second conductive agent can be the same or different, and the appropriate conductive agent type can be selected according to actual needs.

[0082] The present application also provides a method for preparing a negative electrode sheet. Figure 2 The method for preparing the negative electrode sheet provided in the embodiment of the present application comprises the following steps:

[0083] S1: adding a first active material, a first binder, and a first conductive agent into a first solvent and mixing them to obtain a first slurry; the first binder includes a hydroxyl group;

[0084] S2: adding a second active material, a second binder, and a second conductive agent into a second solvent and mixing them to obtain a second slurry; the second binder includes a hydroxyl group;

[0085] S3: adding the modified carbon nanotubes and the third binder into a third solvent and mixing them to obtain a third slurry; the modified carbon nanotubes include carboxyl groups;

[0086] S4: Layering and coating the first slurry, the second slurry, and the third slurry on at least one side of the current collector in a manner such that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry, and drying to obtain a negative electrode sheet.

[0087] In an embodiment of the present application, the first slurry, the second slurry and the third slurry are stacked and coated on at least one side surface of the current collector in such a manner that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry. After drying, a first active material layer (corresponding to the first slurry), an intermediate layer (corresponding to the third slurry) and a second active material layer (corresponding to the second slurry) stacked in sequence are formed on the current collector to obtain a negative electrode sheet. The carboxyl groups in the modified carbon nanotubes in the intermediate layer can react with the hydroxyl groups in the first binder of the first active material layer and the second binder of the second active material layer to form ester groups, and the hydroxyl groups contained in the carboxyl groups of the modified carbon nanotubes can also form hydrogen bonds with the hydroxyl groups in the first binder and the second binder. Therefore, the intermediate layer containing modified carbon nanotubes can act as a bridge to achieve effective connection between the first active material layer and the second active material layer, and promote the transmission of active ions in the negative electrode sheet. In addition, since the modified carbon nanotubes have strong conductivity and hydrophilicity (carboxyl groups are hydrophilic groups), and they themselves have a porous structure, the intermediate layer can not only serve as an excellent conductive medium, but also as a medium to promote the transmission of electrolyte, thereby reducing the resistance of the negative electrode sheet and improving the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery. The negative electrode sheet in the embodiment of the present application is designed with a three-layer coating of the first active material layer, the intermediate layer and the second active material layer. While improving the battery energy density, it can also effectively improve the fast charging performance and cycle performance of the battery, thereby achieving a comprehensive improvement in the various performance aspects of the lithium-ion battery.

[0088] It should be understood that although the steps in the above flowchart are shown sequentially as indicated by the arrows, these steps are not necessarily performed in the order indicated by the arrows. Furthermore, at least some of the steps in the above flowchart may include multiple steps or multiple stages, and these steps or stages are not necessarily performed at the same time or sequentially.

[0089] In step S1 , a first active material, a first binder, and a first conductive agent are added to a first solvent and mixed to obtain a first slurry; the first binder includes a hydroxyl group.

[0090] For example, the first active material may include at least one of graphite, soft carbon, hard carbon, and a silicon-based material. The first binder may include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin. The first conductive agent may include at least one of carbon black, carbon nanotubes, graphene, and acetylene black. The first solvent may include deionized water.

[0091] In an actual preparation process, the viscosity of the first slurry can be 2000 mPa·s to 5000 mPa·s, which is beneficial for improving the stability of the first slurry and the quality of subsequent slurry coating.

[0092] In step S2, a second active material, a second binder, and a second conductive agent are added to a second solvent and mixed to obtain a second slurry; the second binder includes hydroxyl groups.

[0093] For example, the second active material may include at least one of graphite, soft carbon, hard carbon, and a silicon-based material. The second binder may include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin. The second conductive agent may include at least one of carbon black, carbon nanotubes, graphene, and acetylene black. The second solvent may include deionized water.

[0094] In an actual preparation process, the viscosity of the second slurry can be 2000 mPa·s to 5000 mPa·s, which is beneficial for improving the stability of the second slurry and the quality of subsequent slurry coating.

[0095] In some embodiments, the particle size of the first active material is smaller than that of the second active material. This helps increase the compaction density of the formed first active material layer. Optionally, the particle size D50 of the first active material can be 5.5 μm to 10 μm. The particle size D50 of the second active material can be 10 μm to 18 μm.

[0096] In the embodiments of the present application, the intermediate layer acts as an excellent conductive medium and a medium that facilitates electrolyte transfer, promoting electrolyte absorption by the negative electrode sheet, thereby effectively improving the electrolyte's wetting effect on the first active material layer located between the intermediate layer and the current collector. Therefore, the compaction density of the first active material layer can be greater than the compaction density of the second active material layer. This can further increase the battery's capacity and energy density while improving its fast charging capability and cycle performance.

[0097] In some specific embodiments, the compaction density of the first active material layer may be 1.5 g / cc to 1.7 g / cc. Furthermore, the compaction density of the first active material layer is greater than the compaction density of the second active material layer.

[0098] In some specific embodiments, the compaction density of the second active material layer may be 1.3 g / cc to 1.6 g / cc. Furthermore, the compaction density of the first active material layer is greater than the compaction density of the second active material layer.

[0099] In some embodiments, in step S2 of preparing the second slurry, a pore-forming agent may be added to the second solvent. For example, the pore-forming agent may include at least one of ammonium bicarbonate, ammonium carbonate, and ammonium chloride.

[0100] After the second slurry is coated, it will be dried. During the drying process, the pore-forming agent evaporates, thereby forming a porous structure in the second active material layer formed after drying. Taking the pore-forming agent ammonium bicarbonate as an example, ammonium bicarbonate can be decomposed into ammonia, water and carbon dioxide during the drying process after coating. The generated gas evaporates from the second active material layer, which can increase the porosity of the second active material layer and enhance its liquid absorption capacity. In the embodiment of the present application, the second active material layer is located on the surface of the negative electrode sheet and is in direct contact with the electrolyte. The formation of a porous structure in the second active material layer is not only conducive to the electrolyte penetrating into the electrode sheet, thereby improving the cycle performance of the battery; it is also conducive to shortening the migration path of the active ions, thereby improving the rate performance and fast charging capability of the battery.

[0101] When the amount of pore-forming agent added is too high, it will affect the structural stability of the second active material layer and the energy density of the negative electrode sheet. When the amount of pore-forming agent added is too low, the effect of promoting electrolyte infiltration into the electrode sheet is limited. Therefore, in some specific embodiments, the mass ratio of the second active material, the second binder, the second conductive agent, and the pore-forming agent can be (95.4-95.5):2:2:(0.5-1.5), for example, 95.4:2:2:0.5, 95:2:2:1, 95.5:2:2:1.5, or any other ratio within the above ratio range.

[0102] In step S3, the modified carbon nanotubes and the third binder are added into a third solvent and mixed to obtain a third slurry; the modified carbon nanotubes include carboxyl groups.

[0103] For example, the third binder may include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, epoxy resin, and styrene-butadiene rubber. The third solvent may include deionized water.

[0104] In an actual preparation process, the viscosity of the third slurry may be 300 mPa·s to 1000 mPa·s, which is beneficial for improving the stability of the second slurry and the quality of subsequent slurry coating.

[0105] In some embodiments, the mass ratio of the modified carbon nanotubes to the third binder can be (95-99): (1-5), for example, 95:5, 99:5, 98:4, 97:3, 96:2, 95:1, 99:1, or any value between any two of the above numerical ranges. In this way, while ensuring the bonding performance between the intermediate layer and the first active material layer and the second active material layer, it is beneficial to better achieve effective connection between the first active material layer and the second active material layer through the intermediate layer, and it is beneficial to form a better conductive network in the intermediate layer and improve the absorption and transmission capacity of the intermediate layer for the electrolyte, thereby further improving the fast charging capability and cycle performance of the battery.

[0106] In some embodiments, the method for preparing modified carbon nanotubes may include: placing carbon nanotubes in an acid solution, performing an oxidation treatment, and washing and drying to obtain modified carbon nanotubes.

[0107] In the embodiment of the present application, the carbon nanotubes are oxidized by an acid solution, so that the surface defects of the carbon nanotubes are fully oxidized by the acid, thereby forming a large number of carboxyl groups on the surface of the carbon nanotubes.

[0108] In the actual preparation process, the molar concentration of the acid solution can be 1 mol / L to 7 mol / L. The acid solution can include concentrated nitric acid and concentrated sulfuric acid, and the volume ratio of concentrated nitric acid to concentrated sulfuric acid can be 1: (1 to 5). The oxidation treatment can be carried out under the action of ultrasound, and the oxidation treatment time can be 0.5h to 2.5h. This can improve the effect and efficiency of the oxidation treatment. After the oxidation treatment, the oxidized carbon nanotubes can be thoroughly washed with anhydrous ethanol, and then vacuum dried at 40°C to 80°C for 8h to 16h.

[0109] In some specific embodiments, the material-liquid ratio of carbon nanotubes to acid solution can be (50-150) g: (100-900) mL. This helps the carbon nanotubes to be fully dispersed in the acid solution, exposing more surface areas, thereby enabling the formation of a large number of carboxyl groups on the carbon nanotube surfaces.

[0110] In step S4, the first slurry, the second slurry and the third slurry are stacked and coated on at least one side of the current collector in a manner such that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry. After drying, a negative electrode sheet is obtained.

[0111] In multi-layer coating, although the upper and lower slurries are compatible, there is still a small gap between the two, so that there is a gap between the active material layers formed, which will hinder the transmission of active ions and affect the conductivity of the pole piece. In the embodiment of the present application, after drying in the coating oven, the carboxyl groups on the surface of the modified carbon nanotubes in the third slurry can form hydrogen bonds and / or ester groups with the hydroxyl groups of the first binder in the first slurry and the second binder in the second slurry, thereby effectively connecting the first active material layer and the second active material layer through the intermediate layer to promote the transmission of active ions. Since the modified carbon nanotubes have strong conductivity and hydrophilicity (carboxyl groups are hydrophilic groups), and they themselves have a porous structure, the intermediate layer can not only serve as an excellent conductive medium, but also as a medium to promote the transmission of electrolyte, thereby reducing the resistance of the negative electrode sheet and improving the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery.

[0112] In actual production, a three-layer coating die can be used to simultaneously apply the first, second, and third slurries to the current collector in a predetermined stacking pattern, followed by drying to produce the negative electrode sheet. This facilitates control of the coating amount and uniformity of each slurry layer, improving overall coating quality, and significantly increasing production efficiency and reducing costs.

[0113] Of course, the present embodiments do not exclude the use of a single-layer coating die to coat the first, second, and third slurries on at least one side of the current collector in a predetermined stacking manner. Specifically, for example, the first slurry is first coated on at least one side of the current collector and dried to form a first active material layer. Next, the third slurry is coated on the first active material layer and dried to form an intermediate layer. Next, the second slurry is coated on the intermediate layer and dried to form a second active material layer.

[0114] The negative electrode sheet prepared in the embodiment of the present application includes a current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in the thickness direction of the current collector; the first active material layer is located between the current collector and the intermediate layer.

[0115] In some embodiments, the thickness of the first active material layer may be 40 μm to 160 μm, for example, 40 μm, 80 μm, 120 μm, 160 μm, or any value between any two of the aforementioned ranges. This helps improve the battery's fast charging performance and cycle performance while ensuring the battery's energy density.

[0116] In some embodiments, the thickness of the second active material layer may be 40 μm to 160 μm, for example, 40 μm, 80 μm, 120 μm, 160 μm, or any value between any two of the aforementioned ranges. This helps improve the battery's fast charging performance and cycle performance while ensuring the battery's energy density.

[0117] It can be understood that when the thickness of the intermediate layer is too thin, it is not conducive to forming a good conductive network and promoting the transmission of the electrolyte; when the thickness of the intermediate layer is too thick, the modified carbon nanotubes are prone to agglomeration, which will also affect the conductivity and liquid absorption capacity of the intermediate layer. Therefore, in some embodiments, the thickness of the intermediate layer can be 10μm to 30μm, for example, it can be 10μm, 15μm, 20μm, 25μm, 30μm or any value between any two of the above numerical ranges. In this way, the intermediate layer can better reduce the resistance of the negative electrode sheet and improve the liquid absorption rate of the negative electrode sheet, thereby better improving the fast charging performance and cycle performance of the battery.

[0118] An embodiment of the present application further provides a battery, which includes the negative electrode sheet described in any of the above embodiments or the negative electrode sheet prepared by the method for preparing the negative electrode sheet described in any of the above embodiments.

[0119] It should be understood that since the battery in the embodiments of the present application includes the negative electrode sheet described in any of the above embodiments or the negative electrode sheet prepared by the preparation method of the negative electrode sheet described in any of the above embodiments, the beneficial effects of the negative electrode sheet in the above embodiments are all applicable to the battery.

[0120] In some embodiments, the battery may be a secondary battery, which may be, for example, a lithium-ion battery or a sodium-ion battery. Typically, a secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are intercalated and released back and forth between the positive electrode and the negative electrode. The electrolyte conducts active ions between the positive electrode and the negative electrode. The separator is disposed between the positive electrode and the negative electrode, primarily to prevent a short circuit between the positive and negative electrodes while allowing active ions to pass through.

[0121] In the actual preparation process of the battery, first, the positive electrode sheet, separator, negative electrode sheet, and separator can be stacked in sequence, and after assembly, a bare cell is obtained. The assembly method can be, for example, a stacking method or a winding method; then, the electrolyte is injected into the dried bare cell, and after formation and aging, a secondary battery is obtained. The negative electrode sheet is the negative electrode sheet described in any of the above embodiments or the negative electrode sheet prepared by the preparation method of the negative electrode sheet described in any of the above embodiments. The positive electrode sheet includes a positive electrode current collector (specifically, for example, aluminum foil) and a positive electrode active material layer coated on the positive electrode current collector. Specifically, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material may include, for example, at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt manganese aluminum oxide; the conductive agent may include, for example, at least one of conductive carbon black, carbon nanotubes, and acetylene black; and the binder may include, for example, polyvinylidene fluoride (PVDF). The separator may be, for example, a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, or a non-woven fabric. The electrolyte may include, for example, a lithium salt and a solvent or a sodium salt and a solvent. The types of the lithium salt, sodium salt, and solvent are not specifically limited and may be selected according to actual needs.

[0122] The technical solution of the present application is further described below with reference to a number of embodiments and comparative examples.

[0123] Example 1

[0124] The method for preparing the negative electrode sheet in this embodiment includes the following steps:

[0125] Step S101: adding graphite (a first active material), sodium carboxymethyl cellulose and styrene-butadiene rubber (a first binder) mixed in a mass ratio of 5.5:4.5, and conductive carbon SP (a first conductive agent) in a mass ratio of 96:2:2 to deionized water, and stirring and mixing thoroughly to obtain a first slurry with a viscosity of 3500 mPa·s;

[0126] Step S102: adding graphite (second active material), sodium carboxymethyl cellulose and styrene-butadiene rubber (second binder) mixed in a mass ratio of 5.5:4.5, conductive carbon SP (second conductive agent), and ammonium bicarbonate (pore-forming agent) in a mass ratio of 95:2:2:1 to deionized water, and stirring thoroughly to obtain a second slurry with a viscosity of 3500 mPa·s;

[0127] Step S103: adding the modified carbon nanotubes and the third binder into deionized water at a mass ratio of 97:3, and stirring and mixing them thoroughly to obtain a third slurry with a viscosity of 650 mPa·s;

[0128] Step S104: Using a three-layer coating die head, the first slurry, the second slurry and the third slurry prepared in the above steps S101 to S103 are stacked and coated on the negative electrode current collector copper foil in a manner that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry. After drying, a first active material layer, an intermediate layer and a second active material layer stacked in sequence are formed on the current collector. After tableting, a negative electrode sheet is obtained; wherein the mass ratio of the first slurry, the second slurry and the third slurry is 1:1:0.2, the thickness of the first active material layer is 100 μm, the thickness of the intermediate layer is 20 μm, and the thickness of the second active material layer is 100 μm.

[0129] In addition, the preparation method of the modified carbon nanotubes used in the above step S103 is as follows: first, 100 g of carbon nanotubes (CNTs) are weighed and placed in 500 ml of 4 mol / L mixed acid (concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 1:3); next, ultrasonic treatment is performed for 1.5 hours under the action of ultrasound to allow the surface defects of the CNTs to be fully oxidized by concentrated nitric acid and concentrated sulfuric acid to form a large number of carboxyl groups; next, the oxidized CNTs are fully washed with anhydrous ethanol and vacuum dried at 60°C for 12 hours to obtain a black powder, i.e., modified carbon nanotubes, which can be named CNT-COOH.

[0130] Example 2

[0131] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0132] In step S102 , the mass ratio of graphite, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon SP, and ammonium bicarbonate is 95.5:2:2:0.5.

[0133] Example 3

[0134] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0135] In step S102 , the mass ratio of graphite, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon SP, and ammonium bicarbonate is 95.4:2:2:1.5.

[0136] Example 4

[0137] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0138] In step S103 , the mass ratio of the modified carbon nanotubes to the third binder is 95:4.

[0139] Example 5

[0140] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0141] In step S103 , the mass ratio of the modified carbon nanotubes to the third binder is 99:1.

[0142] Example 6

[0143] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0144] The thickness of the intermediate layer formed in step S104 is 10 μm.

[0145] Example 7

[0146] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0147] The thickness of the intermediate layer formed in step S104 is 30 μm.

[0148] Example 8

[0149] The preparation method of the negative electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0150] In step S102 , the pore-forming agent ammonium bicarbonate is not added, and the mass ratio of graphite, sodium carboxymethyl cellulose, styrene-butadiene rubber, and conductive carbon SP is adjusted to 95:3:2.

[0151] Comparative Example 1

[0152] The preparation method of the negative electrode sheet in this comparative example includes the following steps:

[0153] Step S201: adding graphite (a first active material), sodium carboxymethyl cellulose and styrene-butadiene rubber (a first binder) mixed in a mass ratio of 5.5:4.5, and conductive carbon SP (a first conductive agent) in a mass ratio of 96:2:2 to deionized water, and stirring thoroughly to obtain a first slurry with a viscosity of 3500 mPa·s;

[0154] Step S202: adding graphite (second active material), sodium carboxymethyl cellulose and styrene-butadiene rubber (second binder) mixed in a mass ratio of 5.5:4.5, conductive carbon SP (second conductive agent), and ammonium bicarbonate (pore-forming agent) in a mass ratio of 95:2:2:1 to deionized water, and stirring thoroughly to obtain a second slurry with a viscosity of 3500 mPa·s;

[0155] Step S203: Using a double-layer coating die head, the first slurry and the second slurry prepared in the above steps S201 and S202 are stacked and coated on the negative electrode current collector copper foil in a manner such that the first slurry is closer to the current collector than the second slurry. After drying, a first active material layer and a second active material layer stacked in sequence are formed on the current collector, and a negative electrode sheet is obtained after pressing; wherein the mass ratio of the first slurry to the second slurry is 1:1, the thickness of the first active material layer is 100 μm, and the thickness of the second active material layer is 100 μm.

[0156] That is, in Comparative Example 1, compared to Example 1, no intermediate layer is provided between the first active material layer and the second active material layer.

[0157] Comparative Example 2

[0158] The preparation method of the negative electrode sheet in this comparative example includes the following steps:

[0159] Step S301: adding graphite (first active material), sodium carboxymethyl cellulose and styrene-butadiene rubber (first binder) mixed in a mass ratio of 5.5:4.5, and conductive carbon SP (first conductive agent) in a mass ratio of 96:2:2 to deionized water, and stirring and mixing thoroughly to obtain a negative electrode slurry with a viscosity of 3500 mPa·s;

[0160] Step S302: coating the negative electrode slurry prepared in step S301 on the negative electrode current collector copper foil, drying it, forming an active material layer on the current collector, and pressing it to obtain a negative electrode sheet; wherein the thickness of the active material layer is 200 μm.

[0161] Comparative Example 3

[0162] The preparation method of the negative electrode sheet in this comparative example is basically the same as that in Example 1, except that:

[0163] In step S103, the modified carbon nanotubes are replaced with unmodified ordinary carbon nanotubes.

[0164] The performance of the negative electrode sheets prepared in the above embodiments and comparative examples was tested.

[0165] (1) Negative electrode sheet liquid absorption rate test: The negative electrode sheet was cut into a sample with a size of 5 cm × 5 cm, and the weight was recorded as a. The sample was placed in the electrolyte and soaked for 1 hour to allow it to fully absorb the electrolyte. After the absorption was completed, the sample was taken out and hung for 5 minutes to dry the electrolyte attached to the surface of the sample, and the weight was weighed again and recorded as b. The liquid absorption rate of the negative electrode sheet is ((b a) / a) × 100%.

[0166] (2) Negative electrode sheet resistance test: Place the negative electrode sheet in a sheet resistance tester for testing to obtain the sheet resistance of the negative electrode sheet.

[0167] The test results of the above tests are shown in Table 1.

[0168] The negative electrode sheets prepared in the above embodiments and comparative examples were prepared into batteries to test the electrochemical performance of the batteries.

[0169] The preparation steps of the battery are as follows: lithium nickel cobalt manganese oxide powder, conductive carbon and PVDF (binder) are mixed in a mass ratio of 8:1:1 to obtain a mixed material, and the mixed material is fully stirred in an NMP solvent to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil, dried, and pressed to obtain a positive electrode sheet; the negative electrode sheet is the negative electrode sheet prepared in the above embodiment and comparative example; the diaphragm is a polypropylene film; ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1, and then the fully dried lithium salt LiPF6 is dissolved in the above mixed organic solvent at a ratio of 1 mol / L to prepare an electrolyte; the positive electrode sheet, diaphragm, negative electrode sheet, and diaphragm are stacked in sequence, wound to form a battery core, and then dried, electrolyte injected, packaged, formed, and capacity divided to obtain a lithium ion battery.

[0170] The electrochemical performance tests of the battery are as follows:

[0171] (1) First coulombic efficiency (first effect) test: first, charge the battery to 3.75V at a constant current of 0.1C to obtain the formation capacity C0; then divide the battery into smaller capacities, charge it to 4.25V at a constant current of 0.33C, and then charge it to I≤0.05C at a constant voltage of 4.25V to obtain the capacity C1 of the divided battery; after standing for 5 minutes, discharge the battery to 2.8V at a constant current of 1C to obtain the discharge capacity C2; then discharge it to 2.5V at a constant current of 0.1C to obtain the discharge capacity C3; finally, discharge it to 2.5V at a constant current of 0.01C to obtain the discharge capacity C4; the first effect is calculated according to ((C2+C3+C4) / (C0+C1))*100%.

[0172] (2) Constant current ratio test: The constant current ratio refers to the ratio of the amount of electricity charged by constant current to the total amount of electricity charged by constant current and constant voltage when charging a battery cell. The larger the constant current ratio, the smaller the polarization degree of the battery charge, that is, the better the fast charging capability of the battery. The relevant test process is as follows: At 25°C, first charge the battery to 4.25V at a constant current of 1C. The capacity obtained by this constant current charging step is recorded as A. Then charge the battery at a constant voltage of 4.25V to a current of I≤0.05C. The capacity obtained by the constant voltage step is recorded as B. The constant current ratio is (A / (A+B))*100%.

[0173] (3) Capacity retention test after 100 cycles: At 25°C, the battery was first charged to 4.25V at a constant current of 1C, and then charged at a constant voltage at 4.25V to a current of I≤0.05C. The 1C charging capacity of the battery was recorded as c0. After standing for 15 minutes, the battery was discharged to 2.5V at a constant current of 1C to obtain the discharge capacity c1 after one cycle. According to this cycle process, the 100-cycle discharge capacity c100 was recorded, and the battery capacity retention rate after 100 cycles was obtained as (c100 / c0)*100%.

[0174] The test results of the above tests are shown in Table 2.

[0175] Table 1

[0176] Negative electrode liquid absorption rate / % Negative electrode sheet resistance / mΩ Example 1 132 0.52 Example 2 126 0.57 Example 3 130 0.55 Example 4 127 0.72 Example 5 125 0.68 Example 6 122 0.65 Example 7 121 0.66 Example 8 113 0.67 Comparative Example 1 117 0.79 Comparative Example 2 110 0.85 Comparative Example 3 122 0.65

[0177] Table 2

[0178]

[0179]

[0180] It can be seen from the data in Table 1 that the liquid absorption rate of the negative electrode sheets in Examples 1 to 7 is higher than that of the negative electrode sheets in Comparative Examples 1 and 2, and the membrane resistance of the negative electrode sheet is significantly reduced. From the comparison of the data of Example 1 and Comparative Example 3 in Table 1, it can be seen that in Comparative Example 3, since the middle layer is added with unmodified ordinary carbon nanotubes, the liquid absorption rate of the negative electrode sheet is significantly reduced relative to the liquid absorption rate of the negative electrode sheet in Example 1. It can be seen that in the negative electrode sheet of the present application, the modified carbon nanotubes in the middle layer have strong conductivity and hydrophilicity (carboxyl group is a hydrophilic group), and it itself has a porous structure, so that the middle layer can not only serve as an excellent conductive medium, but also as a medium to promote the transmission of electrolyte, thereby reducing the resistance of the negative electrode sheet and improving the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery.

[0181] A comparison of the data from Examples 1, 6, and 7 in Table 1 shows that the intermediate layer thickness of the negative electrode sheet in Example 6 is relatively thin, while that of the negative electrode sheet in Example 7 is relatively thick. Both results in a decrease in the liquid absorption rate of the negative electrode sheet and an increase in the sheet resistance of the negative electrode sheet. This indicates that in the negative electrode sheet of this application, an intermediate layer thickness of 10 μm to 30 μm is a more preferred technical solution.

[0182] A comparison of the data between Examples 1 and 8 in Table 1 shows that, because no pore-forming agent was added during the preparation of the second slurry for the negative electrode sheet in Example 8, the porosity in the second active material layer was not increased, which in turn affected the liquid absorption rate of the negative electrode sheet. This shows that, in the method for preparing a negative electrode sheet of the present application, adding a pore-forming agent during the preparation of the second slurry to form a porous structure in the second active material layer and increase the porosity in the second active material layer is a more preferred technical solution.

[0183] As can be seen from the data in Table 2, the first effect, constant current ratio, and capacity retention rate after 100 cycles of the batteries containing the negative electrode sheets of Examples 1 to 8 are all improved compared to Comparative Examples 1 and 2. A comparison of the data of Example 1 and Comparative Example 3 in Table 2 shows that in Comparative Example 3, since the intermediate layer contains unmodified ordinary carbon nanotubes, the first effect, constant current ratio, and capacity retention rate after 100 cycles of the corresponding battery are all reduced compared to those of the battery containing the negative electrode sheet of Example 1. It can be seen from this that in the negative electrode sheet of the present application, the carboxyl groups in the modified carbon nanotubes in the intermediate layer can react with the hydroxyl groups in the first binder of the first active material layer and the second binder of the second active material layer to form ester groups, and the hydroxyl groups contained in the carboxyl groups of the modified carbon nanotubes can also form hydrogen bonds with the hydroxyl groups in the first binder and the second binder. Therefore, the intermediate layer containing modified carbon nanotubes can act as a bridge to achieve effective connection between the first active material layer and the second active material layer, and promote the transmission of active ions in the negative electrode sheet; in addition, because the modified carbon nanotubes have strong conductivity and hydrophilicity (carboxyl groups are 0 hydrophilic groups), and they themselves have a porous structure, the intermediate layer can not only serve as an excellent conductive medium, but also as a medium to promote the transmission of electrolyte, thereby reducing the resistance of the negative electrode sheet and improving the liquid absorption rate of the negative electrode sheet, thereby improving the fast charging performance and cycle performance of the battery.

[0184] From the data comparison of Examples 1, 6, and 7 in Table 2, it can be seen that the intermediate layer thickness of the negative electrode sheet in Example 6 is relatively thin, while the intermediate layer thickness of the negative electrode sheet in Example 7 is relatively thick, which leads to a decrease in the first efficiency, constant current ratio, and capacity retention rate after 100 cycles of the corresponding battery. This further shows that in the negative electrode sheet of this application, the intermediate layer thickness of 10μm to 30μm is a more preferred technical solution.

[0185] From the data comparison of Example 1 and Example 8 in Table 2, it can be seen that since no pore-forming agent was added during the preparation of the second slurry for the negative electrode sheet in Example 8, the porosity in the second active material layer was not improved, which affected the liquid absorption rate of the negative electrode sheet, and further led to a decrease in the first efficiency, constant current ratio, and capacity retention rate after 100 cycles of the corresponding battery. This further shows that in the method for preparing the negative electrode sheet of the present application, adding a pore-forming agent during the preparation of the second slurry to form a porous structure in the second active material layer and improve the porosity in the second active material layer is a more preferred technical solution.

[0186] The present invention mainly utilizes a three-layer coating technology to design a gradient gap for the negative electrode sheet. The middle layer of the negative electrode sheet includes modified carbon nanotubes. By forming hydrogen bonds and ester groups with the active groups (carboxyl groups) on the surface of the modified carbon nanotubes and the groups (hydroxyl groups) on the binder in the upper and lower layers (the first active material layer in the lower layer and the second active material layer in the upper layer), a conductive network between the upper and lower layers of the negative electrode sheet can be effectively constructed, and the upper and lower layers can be well combined, thereby accelerating the transmission of active ions (lithium ions) between the upper and lower layers, further reducing the internal resistance of the battery, improving the fast charging performance of the battery, and reducing the charging time; at the same time, the hydrophilic group carboxyl groups on the surface of the modified carbon nanotubes and their own porous properties can further enhance the liquid absorption capacity of the negative electrode sheet, thereby improving the battery cycle performance.

[0187] It should be noted that the negative electrode sheet embodiments, negative electrode sheet preparation method embodiments and battery embodiments provided in this application belong to the same concept; the technical features in the technical solutions recorded in each embodiment can be arbitrarily combined without conflict.

[0188] It should be understood that the above embodiments are exemplary and are not intended to include all possible implementations. Various modifications and changes may be made to the above embodiments without departing from the scope of the present disclosure. Similarly, the various technical features of the above embodiments may be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments merely express several implementations of the present invention and do not limit the scope of protection of the patent of the present invention.

Claims

1. A negative electrode sheet, characterized in that: include: a current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in a thickness direction of the current collector, wherein the first active material layer is located between the current collector and the intermediate layer; The first active material layer includes a first binder, the second active material layer includes a second binder, and both the first binder and the second binder include hydroxyl groups; The intermediate layer includes modified carbon nanotubes, and the modified carbon nanotubes include carboxyl groups.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode sheet satisfies at least one of the following characteristics: (1) The first binder and the second binder each independently include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin; (2) The intermediate layer further comprises a third binder, and the mass ratio of the modified carbon nanotubes to the third binder is (95-99): (1-5); (3) The thickness of the first active material layer is 40 μm to 160 μm; (4) The thickness of the second active material layer is 40 μm to 160 μm; (5) The thickness of the intermediate layer is 10 μm to 30 μm; (6) The second active material layer has a porous structure; (7) The compaction density of the first active material layer is greater than the compaction density of the second active material layer; (8) The compacted density of the first active material layer is 1.5 g / cc to 1.7 g / cc; (9) The compacted density of the second active material layer is 1.3 g / cc to 1.6 g / cc; (10) The compacted density of the intermediate layer is 1.3 g / cc to 1.7 g / cc.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: The first active material layer also includes a first active material, and the second active material layer also includes a second active material. The particle size of the first active material is smaller than that of the second active material. Optionally, the particle size D50 of the first active material is 5.5 μm to 10 μm. Optionally, the particle size D50 of the second active material is 10 μm to 18 μm.

4. A method for preparing a negative electrode sheet, characterized in that: The method comprises the following steps: Adding a first active material, a first binder and a first conductive agent into a first solvent and mixing them to obtain a first slurry; the first binder includes a hydroxyl group; adding a second active material, a second binder, and a second conductive agent into a second solvent and mixing them to obtain a second slurry; wherein the second binder includes a hydroxyl group; adding modified carbon nanotubes and a third binder into a third solvent and mixing them to obtain a third slurry; wherein the modified carbon nanotubes include carboxyl groups; The first slurry, the second slurry and the third slurry are stacked and coated on at least one side of the current collector in a manner such that the first slurry is closer to the current collector than the third slurry and the third slurry is closer to the current collector than the second slurry, and dried to obtain the negative electrode sheet.

5. The method for preparing a negative electrode sheet according to claim 4, wherein: In the step of preparing the second slurry, a pore-forming agent is also added to the second solvent; optionally, the mass ratio of the second active material, the second binder, the second conductive agent and the pore-forming agent is (95.4~95.5):2:2:(0.5~1.5); optionally, the pore-forming agent includes at least one of ammonium bicarbonate, ammonium carbonate and ammonium chloride.

6. The method for preparing a negative electrode sheet according to claim 4, wherein: The steps of preparing the first slurry and the second slurry satisfy at least one of the following characteristics: (1) The first binder and the second binder each independently include at least one of sodium carboxymethyl cellulose, polyvinyl alcohol, chitosan, phenolic resin, and epoxy resin; (2) The first active material and the second active material each independently include at least one of graphite, soft carbon, hard carbon, and silicon-based materials; (3) the first solvent and the second solvent each independently comprise deionized water; (4) The particle size of the first active material is smaller than the particle size of the second active material; (5) The particle size D50 of the first active material is 5.5 μm to 10 μm; (6) The particle size D50 of the second active material is 10 μm to 18 μm; (7) The viscosity of the first slurry is 2000 mPa·s to 5000 mPa·s; (8) The viscosity of the second slurry is 2000 mPa·s to 5000 mPa·s.

7. The method for preparing a negative electrode sheet according to claim 4, wherein: The step of preparing the third slurry satisfies at least one of the following characteristics: (1) The mass ratio of the modified carbon nanotubes to the third binder is (95-99): (1-5); (2) the third solvent comprises deionized water; (3) The viscosity of the third slurry is 300 mPa·s to 1000 mPa·s.

8. The method for preparing a negative electrode sheet according to claim 4, wherein: The prepared negative electrode sheet includes the current collector and a first active material layer, an intermediate layer, and a second active material layer sequentially stacked on at least one surface of the current collector in the thickness direction of the current collector; the first active material layer is located between the current collector and the intermediate layer; and the method satisfies at least one of the following characteristics: (1) The thickness of the first active material layer is 40 μm to 160 μm; (2) The thickness of the second active material layer is 40 μm to 160 μm; (3) The thickness of the intermediate layer is 10 μm to 30 μm.

9. The method for preparing a negative electrode sheet according to any one of claims 4 to 8, characterized in that: The preparation method of the modified carbon nanotubes comprises: The carbon nanotubes are placed in an acid solution for oxidation treatment, and after washing and drying, the modified carbon nanotubes are obtained; optionally, the molar concentration of the acid solution is 1 mol / L to 7 mol / L; optionally, the acid solution includes concentrated nitric acid and concentrated sulfuric acid, and the volume ratio of the concentrated nitric acid to the concentrated sulfuric acid is 1:(1 to 5); optionally, the oxidation treatment is carried out under the action of ultrasound, and the oxidation treatment time is 0.5h to 2.5h; optionally, the material-liquid ratio of the carbon nanotubes and the acid solution is (50 to 150) g: (100 to 900) mL; optionally, the temperature of the drying treatment is 40°C to 80°C, and the time is 8h to 16h.

10. A battery, characterized in that: A negative electrode sheet comprising the negative electrode sheet according to any one of claims 1 to 3 or a negative electrode sheet prepared by the method for preparing the negative electrode sheet according to any one of claims 4 to 9.

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