Negative pole piece and electrochemical device and electronic device comprising same

Abstract

The invention discloses a negative pole piece and an electrochemical device and an electronic device comprising the negative pole piece, and belongs to the technical field of energy storage. According to the negative pole piece, a first negative pole active material layer, a second negative pole active material layer and a third negative pole active material layer are sequentially arranged on at least one surface of a negative pole current collector; according to the present invention, the polymer binder containing the hydroxyl and the C-F bond and the polyurethane resin are adopted as the binder, a3 is controlled to be more than a2 and less than a1, and the first negative electrode active material layer has the relatively high binder content, such that the volume expansion of the negative electrode active material is effectively alleviated, and the cycle life of the electrochemical device is effectively prolonged.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and more specifically, to negative electrode plates and electrochemical and electronic devices comprising them. Background Technology

[0002] Electrochemical devices (such as lithium-ion batteries) have become the mainstream choice in the current electronics market due to their high energy density, excellent cycle performance, wide operating temperature range, environmental friendliness, and portability.

[0003] In recent years, as people's demands for thinner and lighter electronic products, faster charging capabilities, higher energy density, and longer lifespan have continued to increase, the performance indicators of electrochemical devices have also faced greater challenges.

[0004] Therefore, this application is submitted. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a negative electrode sheet and an electrochemical device and electronic device containing the same, wherein the negative electrode sheet can effectively improve the cycle life of the electrochemical device.

[0006] To achieve the above objectives, a first aspect of this application provides a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer sequentially disposed on at least one surface of the negative electrode current collector. Each of the first negative electrode active material layer, the second negative electrode active material layer, and the third negative electrode active material layer independently includes a negative electrode active material and a binder, wherein the binder includes a polymer binder containing hydroxyl and CF bonds and a polyurethane resin; The mass percentage of the binder in the first negative electrode active material layer is a1, the mass percentage of the binder in the second negative electrode active material layer is a2, and the mass percentage of the binder in the third negative electrode active material layer is a3, satisfying the condition: a2 < a3 < a1.

[0007] In some embodiments, the hydroxyl- and CF-bonded polymeric binder comprises at least one of hydroxyl-containing polyvinylidene fluoride, hydroxyl-containing polytetrafluoroethylene, and hydroxyl-containing fluorinated polyvinylidene fluoride; and / or The hydroxyl content in the polymer binder containing hydroxyl groups and CF bonds is 0.8~1.5wt%.

[0008] In some of these implementations, at least one of the following (1) to (3) is satisfied: (1) 20% ≤ a1 ≤ 25%; (2) 8% ≤ a2 ≤ 12%; (3) 10%≤a3≤15%.

[0009] In some of these implementations, at least one of the following (4) to (9) is satisfied: (4) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 15-18% in the first negative electrode active layer; (5) The polyurethane resin has a mass percentage content of 5-7% in the first negative electrode active layer; (6) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 6-8% in the second negative electrode active layer; (7) The mass percentage of the polyurethane resin in the second negative electrode active layer is 2-4%; (8) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 6-9% in the third negative electrode active layer; (9) The polyurethane resin in the third negative electrode active layer has a mass percentage content of 4-6%; In some embodiments, the negative electrode active material includes graphite and a metal oxide layer disposed on the outer surface of the graphite, wherein the metal oxide includes at least one of aluminum oxide, zirconium oxide, copper oxide, magnesium oxide, manganese oxide, and nickel oxide. The thickness of the metal oxide layer is 2~10 nm.

[0010] In some embodiments, the graphite has surface pores with an average pore size of 100-200 nm and a porosity of 10-20%.

[0011] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include a toughening agent, the toughening agent having a core-shell structure, the core being polybutadiene and the shell being polymethyl methacrylate; The toughening agent has a mass percentage content of 3-5% in the second negative electrode active layer; The toughening agent has a mass percentage content of 2-3% in the third negative electrode active layer.

[0012] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include graphene quantum dots, wherein the average particle size of the graphene quantum dots is 5~10nm.

[0013] In some embodiments, the graphene quantum dots constitute 0.1% to 0.3% of the mass percentage in the second negative electrode active layer; and / or The graphene quantum dots in the third negative electrode active layer have a mass percentage content of 0.1~0.2%.

[0014] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include a covalent crosslinking agent, wherein the covalent crosslinking agent includes dimethyl cysteine. The covalent crosslinking agent has a mass percentage content of 0.5% to 1% in the second negative electrode active layer; The covalent crosslinking agent has a mass percentage content of 0.3-0.8% in the third negative electrode active layer.

[0015] In some embodiments, the current collector has multiple grooves on its surface near the first negative electrode active material layer; the grooves have a depth of 0.5~1μm, a width of 0.5~1μm, and a spacing of 1~3μm between adjacent grooves.

[0016] In some of these implementations, at least one of the following (10) to (13) is satisfied: (10) The thickness of the current collector is 6~12μm; (11) The thickness of the first negative electrode active material layer is 5~15μm; (12) The thickness of the second negative electrode active material layer is 20~40μm; (13) The thickness of the third negative electrode active layer is 10~25μm.

[0017] A second aspect of this application provides an electrochemical device comprising the aforementioned negative electrode plate.

[0018] The third aspect of this application is an electronic device, including the electrochemical device described above.

[0019] The beneficial effects of this application are as follows: The negative electrode sheet described in this application has a first negative electrode active material layer, a second negative electrode active material layer, and a third negative electrode active material layer sequentially disposed on at least one surface of the negative electrode current collector. This application uses a polymer binder containing hydroxyl and CF bonds and polyurethane resin as binders, and controls a2 < a3 < a1. The first negative electrode active material layer has a relatively high binder content and has excellent interfacial bonding force with the negative electrode current collector. The second negative electrode active material layer has a relatively low binder content, which can effectively improve the energy density of the negative electrode sheet. The third negative electrode active material layer has a moderate binder content, which can effectively form a "micro-buffer unit". Under the stress of winding hot pressing, it can deform and quickly rebound to buffer the surface stress during hot pressing. In addition, the polymer binder containing hydroxyl and CF bonds and polyurethane resin as binders can also effectively improve the adhesion between the negative electrode active material and the binder, while improving the flexibility of the binder, effectively alleviating the volume expansion of the negative electrode active material, and effectively improving the cycle life of the electrochemical device. Attached Figure Description

[0020] The embodiments described in this application are not limited to the accompanying drawings, which are only some of the embodiments described herein. Those skilled in the art can obtain drawings of other embodiments based on the content of this application.

[0021] Figure 1 This is a schematic diagram of the negative electrode sheet according to one embodiment of this application.

[0022] Figure 2 This is a schematic diagram of the negative electrode sheet according to another embodiment of this application. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0025] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0026] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0027] Unless otherwise specified, all components, raw materials, or instruments used in the embodiments and comparative examples of this application are commercially available, and the components and raw materials used in each parallel experiment are the same.

[0028] In the following description, all figures disclosed herein are approximate values, regardless of whether the terms "about" or "approximately" are used in conjunction. They may vary by 1%, 2%, 5%, or sometimes 10% to 20%. Whenever a range of values ​​with a lower limit RL and an upper limit RU is disclosed, any values ​​falling within that range are specifically disclosed. Specifically, the following values ​​within this range are specifically disclosed: R = RL + k * (RU - RL), where k is a variable with a 1% increment from 1% to 100%, i.e., k is 1%, 2%, 3%, 4%, 5%, ..., 50%, 51%, 52%, ..., 95%, 96%, 97%, 98%, 99%, or 100%. Furthermore, any range of values ​​defined by the two R values ​​as defined above are also specifically disclosed.

[0029] General definition Positive electrode active material: As used herein, the term "positive electrode active material" (also known as positive electrode active material) is defined as a material that is electrochemically active in the positive electrode or cathode. Active material should be understood as a material that can capture and release Li and / or Na ions when subjected to voltage changes over a predetermined time period.

[0030] Negative electrode active material: As used herein, the term "negative electrode active material" (also known as negative electrode active material) is defined as a material that is electrochemically active in a negative electrode or anode. Active material should be understood as a material that can capture and release Li and / or Na ions when subjected to voltage changes over a predetermined time period.

[0031] I. Negative electrode plate Studies have shown that the problem of powder shedding from the negative electrode of lithium-ion batteries has always been a key bottleneck restricting the electrochemical performance and production yield of lithium-ion batteries.

[0032] In the existing technology, the improvement of powder shedding is mostly focused on a single dimension: such as optimizing the type and ratio of binder, or adjusting the hot pressing process parameters, or performing simple surface treatment on the active material. However, these solutions have obvious limitations, such as: (1) structural uniformity defects: the traditional negative electrode sheet adopts a uniform structure of current collector-single active layer. The stress distribution in the active material layer cannot adapt to the gradient stress field during winding and hot pressing, resulting in an imbalance of the bonding force between the corner and the planar area. (2) single interface effect: the existing active material modification mostly relies on a single coating or functional group modification, which makes it difficult to simultaneously achieve chemical bonding with binder, physical anchoring with current collector and improvement of its own structural stability.

[0033] To solve the above technical problems, such as Figure 1 As shown, this application embodiment provides a negative electrode sheet, which includes a negative electrode current collector 1 and a first negative electrode active material layer 2, a second negative electrode active material layer 3 and a third negative electrode active material layer 4 sequentially disposed on at least one surface of the negative electrode current collector. Each of the first negative electrode active material layer, the second negative electrode active material layer, and the third negative electrode active material layer independently includes a negative electrode active material and a binder, wherein the binder includes a polymer binder containing hydroxyl and CF bonds and a polyurethane resin; The mass percentage of the binder in the first negative electrode active material layer is a1, the mass percentage of the binder in the second negative electrode active material layer is a2, and the mass percentage of the binder in the third negative electrode active material layer is a3, satisfying the condition: a2 < a3 < a1.

[0034] The negative electrode sheet described in this application has a first negative electrode active material layer, a second negative electrode active material layer, and a third negative electrode active material layer sequentially disposed on at least one surface of the negative electrode current collector. This application uses a polymer binder containing hydroxyl and CF bonds and polyurethane resin as binders, and controls a2 < a3 < a1. The first negative electrode active material layer has a relatively high binder content, resulting in excellent interfacial bonding with the negative electrode current collector. The second negative electrode active material layer has a relatively low binder content, effectively improving the energy density of the negative electrode sheet. The third negative electrode active material layer has a moderate binder content, effectively forming a "micro-buffer unit" that can deform and quickly rebound under the stress of hot pressing, buffering the surface stress during hot pressing. Furthermore, the polymer binder containing hydroxyl and CF bonds and polyurethane resin as binders effectively improve the adhesion between the binder and the negative electrode active material, while also increasing the binder's flexibility, effectively mitigating the volume expansion of the negative electrode active material, effectively reducing powder shedding, and effectively improving the cycle life of the electrochemical device.

[0035] In some embodiments, the hydroxyl- and CF-bonded polymeric binder comprises at least one of hydroxyl-containing polyvinylidene fluoride, hydroxyl-containing polytetrafluoroethylene, and hydroxyl-containing fluorinated polyvinylidene fluoride; and / or The hydroxyl content in the polymer binder containing hydroxyl groups and CF bonds is 0.8~1.5wt%, for example, it can be 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, or any two of these values.

[0036] In particular, when the polymer binder containing hydroxyl groups and CF bonds is used and the hydroxyl content is in the range of 0.8~1.5wt%, the adhesion can be further improved, the affinity with the negative electrode active material can be increased, the volume expansion of the negative electrode active material can be effectively alleviated, and the cycle life of the electrochemical device can be improved.

[0037] In some embodiments, 20% ≤ a1 ≤ 25%, for example, it can be a range consisting of 20%, 21%, 22%, 23%, 24%, 25%, or any two of these values. In particular, when a1 is within this range, the first negative electrode active material layer and the negative electrode current collector have excellent adhesion, effectively improving the structural stability between the first negative electrode active material layer and the negative electrode current collector.

[0038] In some embodiments, 8%≤a2≤12%, for example, can be a range consisting of 8%, 9%, 10%, 11%, 12% or any two of these values. By controlling a2 within this range, the second negative electrode active material has sufficient negative electrode active material, which can effectively improve the energy density of the electrochemical device, while also improving the adhesion between the second negative electrode active material layer and the first and third negative electrode active material layers, thereby improving the structural stability of the negative electrode sheet.

[0039] In some embodiments, 10%≤a3≤15%, for example, can be a range consisting of 10%, 11%, 12%, 13%, 14%, 15% or any two of these values. By controlling a3 within this range, the third negative electrode active material layer can deform under the stress of hot pressing and quickly rebound to buffer the surface stress during hot pressing.

[0040] In some embodiments, the polymer binder containing hydroxyl groups and CF bonds has a mass percentage of 15-18% in the first negative electrode active layer, for example, it can be 15%, 16%, 17%, 18% or any two of these values.

[0041] In some embodiments, the polyurethane resin has a mass percentage content of 5-7% in the first negative electrode active layer, for example, it can be 5%, 5.5%, 6%, 6.5%, 7% or any two of these values.

[0042] By controlling the mass percentage of the polymer binder containing hydroxyl and CF bonds and the polyurethane resin in the first negative electrode active material layer within this range, the first negative electrode active material layer has excellent adhesion and flexibility, which can effectively improve the powder shedding phenomenon and the volume expansion of the negative electrode active material.

[0043] In some embodiments, the polymer binder containing hydroxyl groups and CF bonds has a mass percentage of 6-8% in the second negative electrode active layer, for example, it can be 6%, 6.5%, 7%, 7.5%, 8% or any two of these values.

[0044] In some embodiments, this application does not limit the preparation method of polymer binders containing hydroxyl groups and CF bonds, as long as the hydroxyl content meets the requirements of this application and contains CF bonds.

[0045] For example, by treating a polymer binder containing CF bonds with a hydroxylation agent, a polymer binder with the corresponding hydroxyl content can be obtained.

[0046] For example, the preparation method of the polymer binder containing hydroxyl and CF bonds is as follows: The CF bond-containing polymer binder is dissolved in a solvent to obtain a mixture. A hydroxyl reagent and an initiator are added, the mixture is reacted, washed, and filtered to obtain a polymer binder containing hydroxyl and CF bonds.

[0047] Among them, the CF bond-containing polymer binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride.

[0048] The mass percentage of the CF bond polymer binder in the mixture is 10-15%, for example, it can be 10%, 11%, 12%, 13%, 14%, 15% or any two of these values.

[0049] The hydroxyl reagent includes at least one of hydroxyethyl acrylate, citric acid, lactic acid, malic acid, and tartaric acid.

[0050] In some embodiments, the hydroxyl reagent is 5 to 10% of the total mass of the CF bond-containing polymer binder, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values.

[0051] In some embodiments, the initiator includes at least one selected from azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, tert-butyl hydroperoxide, and ammonium persulfate.

[0052] The hydroxyl reagent is 5 to 10% of the mass of the CF bond polymer binder, for example, it can be 5%, 6%, 7%, 8%, 9%, 10% or any two of these values.

[0053] In some embodiments, the initiator is 0.5 to 1% of the mass of the CF bond-containing polymer binder, for example, it can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any two of these values.

[0054] In some embodiments, the reaction temperature is 80-90°C, for example, it can be 80°C, 82°C, 84°C, 85°C, 86°C, 88°C, 90°C or any two of these values.

[0055] In some embodiments, the reaction time is 0.5 to 5 hours, for example, it can be 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours or any two of these values.

[0056] In some embodiments, the polyurethane resin has a mass percentage content of 2-4% in the second negative electrode active layer, for example, it can be 2%, 2.5%, 3%, 3.5%, 4% or any two of these values.

[0057] By controlling the mass percentage of hydroxyl and CF bond-containing polymer binder and polyurethane resin in the second negative electrode active material layer within this range, the negative electrode active material content in the second negative electrode active material layer is relatively high, which can effectively improve the energy density of the electrochemical device. At the same time, it can improve the adhesion between the binder and the negative electrode active material, effectively alleviating the powder shedding phenomenon.

[0058] In some embodiments, the polymer binder containing hydroxyl groups and CF bonds has a mass percentage of 6-9% in the third negative electrode active layer, for example, it can be 6%, 7%, 8%, 9% or any two of these values.

[0059] In some embodiments, the polyurethane resin in the third negative electrode active layer has a mass percentage of 4 to 6%, for example, it can be 4%, 4.5%, 5%, 5.5%, 6% or any two of these values.

[0060] In some embodiments, the negative electrode active material includes graphite and a metal oxide layer disposed on the outer surface of the graphite, wherein the metal oxide includes at least one of aluminum oxide, zirconium oxide, copper oxide, magnesium oxide, manganese oxide, and nickel oxide. The thickness of the metal oxide layer is 2 to 10 nm, for example, it can be 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm or any two of these values.

[0061] In some embodiments, the number-average molecular weight of the polyurethane resin is 20,000 to 50,000 g / mol, for example, it can be 20,000 g / mol, 25,000 g / mol, 30,000 g / mol, 35,000 g / mol, 40,000 g / mol, 45,000 g / mol, 50,000 g / mol or any two of these values. Controlling the number-average molecular weight of the polyurethane resin within this range can improve the bonding strength and flexibility, and improve the bonding strength with the negative electrode current collector.

[0062] In some embodiments, the glass transition temperature of the polyurethane resin is -40 to -20°C, for example, it can be a range of -40°C, -35°C, -30°C, -25°C, -20°C, or any two of these values. By controlling the glass transition temperature of the polyurethane resin within this range, it can undergo reversible deformation with the volume expansion of the graphite lithium intercalation / de-lithiation anode active material, thus preventing the active material particles from falling off.

[0063] This application provides a metal oxide layer on the outer surface of the graphite. The metal oxide layer can effectively improve the structural stability of the negative electrode active material, prevent graphite from breaking during the rolling process, increase its surface polarity, improve the adhesion with the binder, alleviate the volume expansion of the negative electrode active material, and effectively improve the cycle life of the electrochemical device.

[0064] The metal oxide layer can be deposited on the graphite surface using atomic deposition.

[0065] In some embodiments, the graphite has surface pores with an average pore diameter of 100-200 nm, for example, 100 nm, 120 nm, 140 nm, 150 nm, 160 nm, 180 nm, 200 nm, or any two of these values; and the graphite has a porosity of 10-20%, for example, 10%, 12%, 14%, 15%, 16%, 18%, 20%, or any two of these values.

[0066] The graphite surface has surface pores, which can improve the structural stability between the metal oxide layer and the graphite, enhance the interfacial bonding force, and promote the penetration of binding molecules into the graphite surface, effectively alleviating the volume expansion of graphite and improving the cycle life of the electrochemical device.

[0067] The surface pores can be formed by soaking graphite in a strong alkaline solution.

[0068] For example, the strong alkaline solution includes at least one of sodium hydroxide solution and potassium hydroxide solution.

[0069] An exemplary method for forming surface pores on the graphite surface is as follows: graphite is added to a strong alkaline solution and reacted at 70~80°C for 1.5~2 hours, followed by spray drying (spray drying temperature is 80~120°C).

[0070] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include a toughening agent, the toughening agent having a core-shell structure, the core being polybutadiene and the shell being polymethyl methacrylate; The toughening agent has a mass percentage content of 3-5% in the second negative electrode active layer, for example, it can be 3%, 3.5%, 4%, 4.5%, 5% or any two of these values.

[0071] The toughening agent has a mass percentage content of 2-3% in the third negative electrode active layer, for example, it can be 2%, 2.2%, 2.5%, 2.8%, 3% or any two of these values.

[0072] By adding the toughening agent to the second and third negative electrode active material layers, and the toughening agent having an elastic core + rigid shell structure, the core material is in a rubbery state at room temperature, exhibiting excellent elasticity and deformation capacity. When the negative electrode sheet is subjected to mechanical stress from winding and bending or pressure from hot pressing, the core can absorb energy through its own deformation, effectively buffering local stress and preventing the negative electrode active material from detaching (shedding powder) due to rigid impact. The shell is made of polymethyl methacrylate, which is a hard solid at room temperature and has a certain rigidity and strength. It provides stable shape support for the core-shell structured microspheres, preventing the elastic core from agglomerating due to excessive deformation, ensuring that the toughening agent is uniformly dispersed in the negative electrode slurry, thereby further improving the cycle life of the electrochemical device.

[0073] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include graphene quantum dots, and the average particle size of the graphene quantum dots is 5~10nm, for example, it can be 5nm, 6nm, 7nm, 8nm, 9nm, 10nm or any two of these values.

[0074] By adding the graphene quantum dots to the second and third negative electrode active material layers, the interlayer friction during winding can be reduced, the powder shedding caused by mechanical wear can be reduced, and the conductivity of the second and third negative electrode active material layers can be improved, effectively increasing the electron transport efficiency.

[0075] In some embodiments, the mass percentage of the graphene quantum dots in the second negative electrode active layer is 0.1-0.3%, for example, it can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any two of these values.

[0076] The graphene quantum dots in the third negative electrode active layer have a mass percentage content of 0.1% to 0.2%, for example, it can be 0.1%, 0.15%, 0.2% or any two of these values.

[0077] In some embodiments, the second negative electrode active material layer and the third negative electrode active material layer each independently include a covalent crosslinking agent, wherein the covalent crosslinking agent includes dimethyl cysteine. The mass percentage of the covalent crosslinking agent in the second negative electrode active layer is 0.5-1%, for example, it can be 0.5%, 0.6%, 0.8%, 0.9%, 1% or any two of these values.

[0078] The covalent crosslinking agent in the third negative electrode active layer has a mass percentage content of 0.3-0.8%, for example, it can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8% or any two of these values.

[0079] By adding the cystine dimethyl ester to the second and third negative electrode active material layers, the cystine dimethyl ester can undergo reversible fracture and recombination during hot pressing, giving the binder self-healing ability. When microcracks occur locally, it can re-crosslink, preventing the negative electrode active material from falling off and improving the structural stability of the negative electrode sheet.

[0080] like Figure 2 As shown, in some embodiments, the current collector has a plurality of grooves 5 formed on the surface near the first negative electrode active material layer; the depth of the groove is 0.5~1μm, the width of the groove is 0.5~1μm, and the spacing between adjacent grooves is 1~3μm, for example, it can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm or any two of these values.

[0081] By controlling the opening of multiple grooves on the surface of the current collector, the interfacial bonding force between it and the first negative electrode active material layer can be further improved. The grooves can form mechanical alignment with the first negative electrode active material layer, effectively improving the structural stability of the negative electrode sheet.

[0082] In some embodiments, the thickness of the current collector is 6 to 12 μm, for example, it can be 6 μm, 7 μm, 8 μm, 10 μm, 12 μm or any two of these values.

[0083] In some embodiments, the thickness of the first negative electrode active material layer is 5~15μm, for example, it can be 5μm, 6μm, 8μm, 10μm, 12μm, 15μm or any two of these values.

[0084] In some embodiments, the thickness of the second negative electrode active material layer is 20~40μm, for example, it can be 20μm, 22μm, 25μm, 28μm, 30μm, 32μm, 35μm, 38μm, 40μm or any two of these values.

[0085] In some embodiments, the thickness of the third negative electrode active layer is 10~25μm, for example, it can be 10μm, 15μm, 20μm, 22μm, 25μm, 28μm, 30μm, 32μm, 35μm, 38μm, 40μm or any two of these values.

[0086] In some embodiments, the mass percentage of the negative electrode active material in the first negative electrode active material layer is 70-85%, for example, it can be 70%, 72%, 75%, 76%, 77%, 78%, 79%, 80%, 82%, 85% or any two of these values.

[0087] In some embodiments, the negative electrode active material has a mass percentage content of 75-80% in the first negative electrode active material layer.

[0088] In some embodiments, the mass percentage of the negative electrode active material in the second negative electrode active material layer is 79.7% to 90.7%, for example, it can be 79.7%, 80%, 82%, 84%, 85%, 86%, 88%, 90%, 90.5%, 90.7% or any two of these values.

[0089] In some embodiments, the mass percentage of the negative electrode active material in the second negative electrode active material layer is 82.4-87.7%.

[0090] In some embodiments, the mass percentage of the negative electrode active material in the third negative electrode active material layer is 78-89%, for example, it can be 78%, 80%, 82%, 85%, 86%, 88%, 89% or any two of these values.

[0091] In some embodiments, the negative electrode active material has a mass percentage content of 81-87.6% in the third negative electrode active material layer.

[0092] II. Electrochemical Device This application provides an electrochemical device comprising the aforementioned negative electrode. In some embodiments, the electrochemical device further includes a positive electrode, a separator, and an electrolyte, wherein the separator is located between the positive and negative electrode.

[0093] The electrochemical device of this application includes any device in which an electrochemical reaction occurs, and specific examples include all kinds of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0094] Positive electrode sheet The electrochemical device of this application includes a positive electrode, wherein the positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector.

[0095] In some of these embodiments, the type of positive current collector is not particularly limited, and it may be any material known to be suitable for use as a positive current collector.

[0096] In some embodiments, the positive current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0097] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of carbon plate, carbon film, carbon cylinder, etc.

[0098] In some embodiments, the positive electrode active layer includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent.

[0099] In some embodiments, the positive electrode active material is selected from LiCoO2, LiNiO2, and LiNi. x Mn y O2, Li 1+z Ni x Mn y Co 1-x-y O2, LiNi x Co y Al z The group consisting of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, Li2MnO3, LiFeO2, LiFePO4, LiMnPO4 and combinations thereof, wherein each x is independently 0.2 to 0.9; each y is independently 0.1 to 0.45; and each z is independently 0 to 0.2.

[0100] In some embodiments, the positive electrode active material is Li 1+x Ni a Mn b Co c Al (1-a-b-c) O2; where -0.2≤x≤0.2, 0≤a<1, 0≤b<1, 0≤c<1 and a+b+c≤1.

[0101] In some embodiments, the positive electrode active material has the general formula Li 1+x Ni a Mn b Co c Al (1-a-b-c) O2, where 0.33≤a≤0.92, 0.33≤a≤0.9, 0.33≤a≤0.8, 0.5≤a≤0.92, 0.5≤a≤0.9, 0.5≤a≤0.8, 0.6≤a≤0.92 or 0.6≤a≤0.9; 0≤b≤0.5, 0≤b≤0.3, 0.1≤b≤0.5, 0.1≤b≤0.4, 0.1≤b≤0.3, 0.1≤b≤0.2 or 0.2≤b≤0.5; 0≤c≤0.5, 0≤c≤0.3, 0.1≤c≤0.5, 0.1≤c≤0.4, 0.1≤c≤0.3, 0.1≤c≤0.2 or 0.2≤c≤0.5.

[0102] In some embodiments, the positive electrode active material is doped with a dopant selected from the group consisting of Fe, Ni, Mn, Al, Mg, Zn, Ti, La, Ce, Sn, Zr, Ru, Si, Ge, and combinations thereof.

[0103] In some embodiments, the positive electrode active material includes LiNi. 0.33 Mn 0.33 Co 0.33 O2(NMC333), LiNi 0.4 Mn 0.4 Co0.2 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2(NMC532), LiNi 0.6 Mn 0.2 Co 0.2 O2(NMC622), LiNi 0.7 Mn 0.15 Co 0.15 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2(NMC811), LiNi 0.92 Mn 0.04 Co 0.04 O2, LiNi 0.8 Co 0.15 Al 0.05 At least one of O2 (NCA) and LiNiO2 (LNO).

[0104] In some embodiments, the positive electrode binder includes binder materials comprising at least one of the following: polyvinylidene fluoride (PVDF), poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, polyacrylic acid, polyacrylonitrile, polyimide, polyurethane, polyvinyl butyral, polyvinylpyrrolidone (PVP), acrylic acid-acrylonitrile-acrylamide copolymer, and acrylic acid-acrylonitrile-acrylate copolymer. This application is not limited to the above materials and also includes other materials that can be used as battery positive electrode binders.

[0105] In some embodiments, the positive electrode conductive agent includes at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon tubes, carbon nanotubes, activated carbon, and mesoporous carbon. This application is not limited to the above materials, but also includes other materials that can be used as positive electrode conductive agents in batteries.

[0106] diaphragm The separator separates the negative and positive electrodes and provides a pathway for lithium-ion migration. The use of the separator is not particularly limited, as long as it is a separator commonly used in lithium-ion secondary batteries. In particular, separators with low resistance to electrolyte ion movement and excellent electrolyte permeability are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin-based polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc.) or laminated structures with two or more layers. Alternatively, nonwoven fabrics formed from conventional porous nonwoven fabrics (e.g., glass fibers with high melting points, polyethylene terephthalate fibers, etc.) can be used. Furthermore, coated separators containing ceramic components or polymer materials to ensure heat resistance or mechanical strength can be used, and can optionally be used as single-layer or multi-layer structures.

[0107] Generally, a diaphragm includes a substrate and a coating applied to the surface of the substrate.

[0108] Substrate In some embodiments, the porous substrate is, but is not limited to, at least one of polyolefins, polyesters, polyacetals, polyamides, polyethylene terephthalate, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyacrylonitrile, polyvinylidene fluoride, polyoxymethylene, polyoxymethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, polysulfone, and polymethyl methacrylate. Some non-limiting examples of polyolefins include at least one of polyethylene (PE), ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polypropylene (PP), polyethylene-polypropylene copolymer (PE-PP), and polyethylene-polypropylene-polyethylene copolymer.

[0109] Inorganic particles In some embodiments, the coating is disposed on one side of the substrate. In some embodiments, the coating is disposed on both sides of the substrate. The inorganic filler comprises Al2O3, SiO2, TiO2, ZrO2, Mg(OH)2, MgO, SnO2, CaCO3, BaSO4, TiN, AlN, Na2O.mTiO2, K2O.nTiO2, BaOx, MTiO3, and combinations thereof, wherein m is 3 or 6, n is 1, 2, 4, 6, or 8, x is 1 or 2, and M is Ba, Sr, or Ca.

[0110] In some embodiments, the inorganic filler includes one or more of alumina, hydrated alumina, boehmite, magnesium hydroxide, magnesium oxide, titanium dioxide, zirconium oxide, and barium sulfate.

[0111] Coating adhesive In some embodiments, the binder is a water-soluble polymer. In some embodiments, the water-soluble polymer is a homopolymer or copolymer.

[0112] In some embodiments, the water-soluble binder includes at least one of polyamide, polycarboxylate, polycarboxylic acid, polyacrylic acid, polyacrylate, polymethacrylic acid, polymethacrylate, polyvinyl alcohol, polyvinyl acetate, polyacrylamide, cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, carboxymethyl cellulose, cyanoethyl cellulose, nitrile rubber (NBR), styrene-butadiene rubber (SBR), and latex.

[0113] In some embodiments, the monomers constituting the water-soluble polymer include at least one of a monomer containing a carboxylic acid group, a monomer containing an amide group, a monomer containing a cyanide group, and a monomer containing an ester group.

[0114] In some embodiments, the monomers containing a carboxylic acid group include monocarboxylic acids, dicarboxylic acids, anhydrides of dicarboxylic acids, and derivatives thereof. Some non-limiting examples of monocarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, 2-ethylacrylic acid, and isocrotonic acid. Some non-limiting examples of dicarboxylic acids include maleic acid, fumaric acid, itaconic acid, and methylmaleic acid. Some non-limiting examples of anhydrides of dicarboxylic acids include maleic anhydride, acrylic anhydride, methylmaleic anhydride, and dimethylmaleic anhydride.

[0115] In some embodiments, the monomer containing an amide group includes at least one of acrylamide and methacrylamide. In some embodiments, the monomer containing a cyano group includes at least one of acrylonitrile and α-alkylacrylonitrile. In some embodiments, the monomer containing a nitrile group is at least one of methacrylonitrile, α-ethylacrylonitrile, α-isopropylacrylonitrile, α-methoxyacrylonitrile, 3-methoxyacrylonitrile, and 3-ethoxyacrylonitrile.

[0116] In some embodiments, the monomer containing an ester group includes at least one of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, and n-butyl methacrylate.

[0117] In some embodiments, the monomer accounts for 10-90% of the polymer. In some embodiments, the binder is an oil-soluble polymer.

[0118] In some of these embodiments, non-limiting examples of oil-soluble polymers include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyester, polyether, polyvinyl compounds, polyolefins, rubber, polyvinylpyrrolidone, polystyrene, nitrile rubber (NBR), styrene-butadiene rubber (SBR), latex, acrylonitrile-styrene-butadiene copolymer, halogenated polymers, fluorinated polymers, chlorinated polymers, unsaturated polymers, conjugated diene polymers, and combinations thereof.

[0119] polymer coating In some embodiments, the polymer includes at least one of fluoropolymers, polyester polymers, acrylic polymers, polyurethane polymers, polyamide polymers, vinyl polymers, polyether polymers, polycarboxylic acid polymers, cellulose and its derivatives, or rubber / elastomers.

[0120] In some embodiments, the polymer includes at least one of polyvinylidene fluoride (PVDF), a homopolymer or copolymer formed from vinylidene fluoride and another copolymerizable monomer (such as one or more of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, trichloroethylene, and fluoroethylene), polytetrafluoroethylene, fluorinated ethylene propylene copolymer, perfluorosulfonic acid resin, polycarboxylate, polycaprolactone, polyacrylate, polyurethane, polyamide, polyimide, polyacrylonitrile, polyethylene oxide, polyacrylic acid, polymethacrylic acid, polymethacrylate, polyvinyl alcohol, polyvinyl acetate, polyacrylamide, polyphenylene ether, polyvinyl chloride compound, polystyrene, polyvinylpyrrolidone, acrylonitrile-styrene-butadiene copolymer, polyphenylene sulfide, polyetheretherketone, polyarylamide, polypyrrole, polyaniline, polythiophene, polyethylene glycol, polylactic acid, cellulose (which may be cellulose nanofibers), cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, carboxymethyl cellulose, cyanoethyl cellulose, nitrile rubber (NBR), styrene-butadiene rubber (SBR), and latex.

[0121] In some embodiments, the polyvinylidene fluoride resin includes at least one homopolymer of polyvinylidene fluoride (i.e., polyvinylidene fluoride) and a copolymer formed of polyvinylidene fluoride and another copolymerizable monomer (such as at least one of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, trichloroethylene, and vinyl fluoride).

[0122] electrolytes The electrochemical device of this application also includes an electrolyte.

[0123] In some embodiments, the electrolyte includes at least one of a gel electrolyte, a solid electrolyte, and a liquid electrolyte.

[0124] In some embodiments, the liquid electrolyte comprises a non-aqueous solvent and a lithium salt.

[0125] In some embodiments, the lithium salt includes at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, and lithium difluoroborate.

[0126] In some embodiments, the non-aqueous solvent may be at least one of carbonate compounds, carboxylic acid ester compounds, and ether compounds.

[0127] In some embodiments, the carbonate compound includes at least one of chain carbonate compounds, cyclic carbonate compounds, and fluorocarbonate compounds.

[0128] In some embodiments, the chain carbonate compound includes diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), and combinations thereof. Examples of fluorocarbonate compounds are at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, and trifluoromethylethylene carbonate.

[0129] In some embodiments, examples of carboxylic acid ester compounds are at least one of methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonic acid lactone, caprolactone, and methyl formate.

[0130] In some embodiments, examples of ether compounds are dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

[0131] In some embodiments, the non-aqueous solvent also includes at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters.

[0132] III. Electronic Devices This application also provides an electronic device, including the electrochemical device described in this application.

[0133] The application of the electrochemical device in this application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the electrochemical device includes, but is not limited to, mobile phones, smartphones, laptops, tablets, wearable devices, smartwatches, smart bracelets, smart glasses, power banks, televisions, game consoles, game controllers, digital cameras, smart speakers, headphones, keyboards, mice, monitors, drones, audio equipment, home appliances, toys, power tools, automobiles, motorcycles, electric bicycles, bicycles, robots, robot dogs, industrial robots, android robots, etc.

[0134] The following uses a lithium-ion battery as an example and combines specific embodiments to illustrate the preparation of a lithium-ion battery. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0135] Example 1 A method for preparing a lithium-ion battery includes the following steps: (1) Preparation of polyvinylidene fluoride containing hydroxyl groups: Polyvinylidene fluoride (PVDF) was dissolved in the solvent N-methylpyrrolidone to prepare a mixture with a PVDF content of 12 wt%. Hydroxyethyl acrylate and azobisisobutyronitrile (AIO) were added, and the mixture was reacted at 80 °C for 5 h. After washing and filtration, hydroxyl-containing PVDF (OH-PVDF, with a hydroxyl content of 1 wt%) was obtained.

[0136] (2) Preparation of negative electrode active material (graphite negative electrode): Graphite with a particle size of 8 μm (Dv50) was added to a 1 mol / L sodium hydroxide solution and reacted at 70 °C for 2 h. After spray drying, graphite particles were obtained. The graphite particles were then placed in a plasma treatment device. Using trimethylaluminum as the aluminum source, under plasma excitation, trimethylaluminum and oxygen formed alumina coating on the graphite surface. The plasma power was controlled at 150 W for 8 min. Subsequently, the particles were annealed at 350 °C for 5 min in the oxygen range to form an alumina coating layer with a thickness of 7.5 μm on the surface of the graphite particles, thus obtaining a graphite anode. The graphite particles have surface pores with an average pore size of 100 nm and a porosity of 10%.

[0137] (3) Preparation of negative electrode sheet: S1. Using a copper foil with a thickness of 10μm as the negative electrode current collector, multiple mutually spaced grooves are formed on one surface of the negative electrode current collector by laser etching. The groove spacing is 3μm, the groove width is 1μm, and the groove depth is 1μm. S2. The graphite anode, binder (OH-PVDF), and polyurethane resin (weight-average molecular weight of 35000 g / mol, glass transition temperature of -30℃) are mixed evenly at a mass ratio of 75:18:7. The solvent NMP (N-methylpyrrolidone) is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain the first anode active material layer slurry with a solid content of 48%. The first anode active material layer slurry is coated on the surface of the current collector and dried to form a first anode active material layer with a thickness of 8μm. S3. Graphite anode, OH-PVDF, polyurethane resin, toughening agent (core is polybutadiene, shell is polymethyl methacrylate, EXL-2314), graphene quantum dots, and dimethyl cysteine ​​are mixed evenly in a mass ratio of 87.7:6:2:3:0.3:1. Solvent NMP (N-methylpyrrolidone) is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain a first anode active material layer slurry with a solid content of 45%. The second anode active material layer slurry is then coated onto the surface of the first anode active material layer and dried to form a second anode active material layer with a thickness of 30 μm. S3. Graphite anode, OH-PVDF, polyurethane resin, toughening agent, graphene quantum dots, and dimethyl cysteine ​​are mixed evenly in a mass ratio of 87.6:6:2:3:0.1:0.3. NMP (N-methylpyrrolidone) solvent is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain the first anode active material layer slurry with a solid content of 46%. The third anode active material layer slurry is then coated onto the surface of the second anode active material layer and dried to form a third anode active material layer with a thickness of 16 μm.

[0138] (4) Preparation of the positive electrode sheet: The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are thoroughly mixed in an N-methylpyrrolidone solvent system at a mass ratio of 95:2.5:2.5. The mixture is then coated onto the positive electrode current collector Al foil, dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0139] (5) A PE membrane with a thickness of 7 μm was used as the separator; (6) Preparation of electrolyte: At room temperature, in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): ethylene carbonate (VC) were mixed evenly in a mass ratio of 20:30:20:28:2 to obtain a mixed organic solvent. Lithium salt LiPF6 was added to the mixed organic solvent and stirred evenly to obtain the electrolyte.

[0140] The mass ratio of the lithium salt LiPF6 to the mixed organic solvent is 12:88.

[0141] (7) Assembly of lithium-ion batteries: The positive electrode, separator and negative electrode are stacked in sequence, so that the separator is between the positive and negative electrode to play a role in isolation. After being wound into a square bare cell, it is put into the outer packaging, then baked to remove water, injected with the corresponding electrolyte, sealed, and after standing, hot and cold pressing, formation and capacity testing, the lithium-ion battery is obtained.

[0142] The parameters of the negative electrode are shown in Tables 1, 2, and 3.

[0143] Examples 2-4 The difference between Examples 2-4 and Example 1 is that the composition of the first negative electrode active material layer is changed, as shown in Table 1.

[0144] Examples 5-7 The difference between Examples 5-7 and Example 1 is that the composition of the second negative electrode active material layer is changed, as shown in Table 2.

[0145] Examples 8-10 The difference between Examples 8-10 and Example 1 is that the composition of the third negative electrode active material layer is changed, as shown in Table 3.

[0146] Examples 11-13 Examples 11-13 differ from Example 1 in that the composition of the second and third negative electrode active material layers is changed, as shown in Tables 2 and 3.

[0147] Example 14 The difference between Example 14 and Example 1 is that the negative electrode active material is different.

[0148] In Example 14, the negative electrode active material is Dv50 graphite with a particle size of 12 μm.

[0149] Example 15 The difference between Example 15 and Example 1 is that the negative electrode current collector in Example 15 does not have a groove.

[0150] Example 16 The difference between Example 16 and Example 1 is that the parameters of the groove are changed.

[0151] In Example 16, the groove has a depth of 0.5 μm, a width of 0.5 μm, and a spacing of 1 μm.

[0152] Example 17 The difference between Example 17 and Example 1 is that the preparation of the negative electrode active material (graphite negative electrode) is different.

[0153] Preparation of negative electrode active material (graphite negative electrode): Graphite was added to a 2.2 mol / L sodium hydroxide solution and reacted at 80°C for 2 hours. The mixture was then spray-dried at an inlet air temperature of 110°C and an outlet air temperature of 65°C to obtain graphite particles. These particles were then placed in a plasma treatment device. Using trimethylaluminum as the aluminum source, under plasma excitation, trimethylaluminum and oxygen formed alumina coating on the graphite surface. The plasma power was controlled at 250W for 12 minutes. Subsequently, the particles were annealed at 350°C for 5 minutes in an oxygen concentration of 30%, forming an alumina coating layer with a thickness of 8.5 μm on the surface of the graphite particles, thus obtaining a graphite anode. The graphite particles have surface pores with an average pore size of 200 nm and a porosity of 20%.

[0154] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not contain a first negative electrode active material layer.

[0155] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not contain a second negative electrode active material layer.

[0156] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that Comparative Example 3 does not contain a third negative electrode active material layer.

[0157] Comparative Examples 4-6 The difference between Comparative Examples 4-6 and Example 1 is that the composition of the first negative electrode active material layer, the second negative electrode active material layer and the third negative electrode active material layer are changed, as shown in Tables 1-3.

[0158] Table 1 Table 2 Table 3 Performance testing I. Follower Loss Rate: 1) Sample preparation: The negative electrode sheets of each embodiment and comparative example were cut into 100mm×100mm square samples, and three samples were tested in parallel for each group. The samples were fixed in a vacuum drying oven and dried at 120℃ for 2 hours. After cooling to room temperature, the initial mass (m0) was weighed using an electronic balance with an accuracy of 0.1mg. 2) Test calculation: A reciprocating friction tester (friction head pressure 5N, friction speed 100mm / min, friction times 50 times) was used to conduct a friction test on the surface of the electrode (third negative electrode active material layer). All the powder that was rubbed off was collected. The collected powder was dried and weighed (m1). The powder loss rate = (m1 / m0) × 100%. The average value of 3 samples was taken as the final result.

[0159] II. Cyclic capacity retention rate: 1) Test conditions: Cyclic testing was conducted using a battery tester under a constant temperature environment of 25℃; Charging: Charge at a constant current of 0.5C to 4.45V, then switch to constant voltage charging until the current is ≤0.05C; Discharge: 0.5C constant current discharge to 3.0V; 2) Calculation: Record the discharge capacity (C1) of the first cycle and the discharge capacity (C2) of the 1000th cycle. 1000 Cyclic capacity retention rate = (C) 1000 (C1) × 100%, take the average of 5 batteries as the final result.

[0160] III. Ratio Capacity Retention Rate: 1) Test conditions: A constant temperature environment (25℃) with the same cycle capacity retention rate was used for charge-discharge tests at different rates. Base capacity: First, charge and discharge at 0.2C 3 times, and record the capacity of the 3rd discharge as the baseline capacity (C0).

[0161] Rate testing: Discharge was performed sequentially at a constant current of 5C (charging was 0.5C), and the discharge capacity (C) at each rate was recorded. n ) 2) Calculation: Capacity retention rate at a certain multiplier = (C n ( / C0)×100%, take the average of 5 batteries, and use the 5C rate capacity retention rate as the core evaluation indicator in the table (reflecting the high rate charge and discharge capability).

[0162] Table 3 As can be seen from Table 4, by using a polymer binder containing hydroxyl and CF bonds and polyurethane resin as binders, and controlling a2 < a3 < a1, the first negative electrode active material layer has a relatively high binder content, which effectively alleviates the volume expansion of the negative electrode active material and effectively improves the cycle life of the electrochemical device.

[0163] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a first negative electrode active material layer, a second negative electrode active material layer and a third negative electrode active material layer sequentially disposed on at least one surface of the negative electrode current collector; Each of the first negative electrode active material layer, the second negative electrode active material layer, and the third negative electrode active material layer independently includes a negative electrode active material and a binder, wherein the binder includes a polymer binder containing hydroxyl and CF bonds and a polyurethane resin; The mass percentage of the binder in the first negative electrode active material layer is a1, the mass percentage of the binder in the second negative electrode active material layer is a2, and the mass percentage of the binder in the third negative electrode active material layer is a3, satisfying the condition: a2 < a3 < a1.

2. The negative electrode sheet according to claim 1, characterized in that, The hydroxyl- and CF-bonded polymer binder includes at least one of hydroxyl-containing polyvinylidene fluoride, hydroxyl-containing polytetrafluoroethylene, and hydroxyl-containing fluorinated polyvinylidene fluoride; and / or The hydroxyl content in the polymer binder containing hydroxyl groups and CF bonds is 0.8~1.5wt%.

3. The negative electrode sheet according to claim 1, characterized in that, Satisfy at least one of the following (1) to (3): （1）20%≤a1≤25%； （2）8%≤a2≤12%； （3）10%≤a3≤15%。 4. The negative electrode sheet according to claim 1, characterized in that, Satisfy at least one of the following (4) to (9): (4) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 15-18% in the first negative electrode active layer; (5) The polyurethane resin has a mass percentage content of 5-7% in the first negative electrode active layer; (6) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 6-8% in the second negative electrode active layer; (7) The mass percentage of the polyurethane resin in the second negative electrode active layer is 2-4%; (8) The polymer binder containing hydroxyl groups and CF bonds has a mass percentage content of 6-9% in the third negative electrode active layer; (9) The mass percentage of the polyurethane resin in the third negative electrode active layer is 4~6%.

5. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material includes graphite and a metal oxide layer disposed on the outer surface of the graphite, wherein the metal oxide includes at least one of aluminum oxide, zirconium oxide, copper oxide, magnesium oxide, manganese oxide, and nickel oxide.

6. The negative electrode sheet according to claim 5, characterized in that, The thickness of the metal oxide layer is 2~10 nm; and / or The graphite has surface pores with an average pore diameter of 100-200 nm and a porosity of 10-20%.

7. The negative electrode sheet according to claim 1, characterized in that, The second and third negative electrode active material layers each independently include a toughening agent, which has a core-shell structure, with the core being polybutadiene and the shell being polymethyl methacrylate. The toughening agent has a mass percentage content of 3-5% in the second negative electrode active layer; The toughening agent has a mass percentage content of 2-3% in the third negative electrode active layer.

8. The negative electrode sheet according to claim 1, characterized in that, The second and third negative electrode active material layers each independently include graphene quantum dots, and the average particle size of the graphene quantum dots is 5~10nm.

9. The negative electrode sheet according to claim 8, characterized in that, The graphene quantum dots in the second negative electrode active layer have a mass percentage of 0.1-0.3%; and / or The graphene quantum dots in the third negative electrode active layer have a mass percentage content of 0.1~0.2%.

10. The negative electrode sheet according to claim 1, characterized in that, The second and third negative electrode active material layers each independently include a covalent crosslinking agent, and the covalent crosslinking agent includes dimethyl cysteine. The covalent crosslinking agent has a mass percentage content of 0.5% to 1% in the second negative electrode active layer; The covalent crosslinking agent has a mass percentage content of 0.3-0.8% in the third negative electrode active layer.

11. The negative electrode sheet according to claim 1, characterized in that, The current collector has multiple grooves on its surface near the first negative electrode active material layer; the grooves have a depth of 0.5~1μm, a width of 0.5~1μm, and a spacing of 1~3μm between adjacent grooves.

12. The negative electrode sheet according to claim 1, characterized in that, Satisfy at least one of the following (10) to (13): (10) The thickness of the current collector is 6~12μm; (11) The thickness of the first negative electrode active material layer is 5~15μm; (12) The thickness of the second negative electrode active material layer is 20~40μm; (13) The thickness of the third negative electrode active layer is 10~25μm.

13. An electrochemical device, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 12.

14. An electronic device, characterized in that, Includes the electrochemical device as described in claim 13.

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