Lithium ion secondary battery, preparation method thereof and electric device

Abstract

The invention belongs to the technical field of batteries, and particularly discloses a lithium ion secondary battery, a preparation method thereof and a power utilization device. The lithium ion secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, a carbon nanotube and a first fluorine-containing polymer, and the first fluorine-containing polymer comprises a structural unit derived from vinylidene fluoride and a structural unit derived from an unsaturated carboxylic acid monomer. According to the design mode provided by the invention, the dispersibility of the carbon nanotubes in the positive electrode film layer is improved, so that the carbon nanotubes exert good conductivity, and the direct-current internal resistance of the battery is reduced by improving the conductivity of the positive electrode plate.

Description

Technical Field

[0001] This application relates to the field of batteries, specifically to a lithium-ion secondary battery, its preparation method, and an electrical device thereof. Background Technology

[0002] Lithium-ion rechargeable batteries are widely used in various consumer electronics and electric vehicles, such as portable electronic devices and electric vehicles, due to their outstanding characteristics such as light weight, no pollution, and no memory effect.

[0003] As the application scope of lithium-ion rechargeable batteries becomes wider and wider, the requirements for the performance of lithium-ion rechargeable batteries are also becoming higher and higher. Summary of the Invention

[0004] In view of the above problems, this application provides a lithium-ion secondary battery, a method for preparing the same, and an electrical device thereof, which reduces the DC internal resistance of the battery by improving the conductivity of the positive electrode.

[0005] In a first aspect, this application provides a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, and a separator.

[0006] The aforementioned positive electrode includes a positive current collector and a positive electrode film layer located on at least one side surface of the aforementioned positive current collector;

[0007] The aforementioned positive electrode film includes a positive electrode active material, carbon nanotubes, and a first fluoropolymer;

[0008] The aforementioned first fluoropolymer includes structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers;

[0009] The above-mentioned unsaturated carboxylic acid monomers include the following chemical structural formulas:

[0010]

[0011] In the above chemical structural formula, R1 and R2 each independently include hydrogen and any one of C1-C3 alkyl groups; R3 includes any one or more of carboxyl groups and ester groups.

[0012] In this application, carbon nanotubes are used as a conductive agent in the positive electrode film. Utilizing the excellent conductivity of carbon nanotubes, this application can achieve its conductive function with a relatively small amount. However, carbon nanotubes are prone to aggregation, which affects their conductivity. This application selects a first fluoropolymer as a dispersant. This first fluoropolymer contains structural units of vinylidene fluoride and unsaturated carboxylic acid monomers. Both of these structural units possess a certain polarity and easily generate intermolecular forces with carbon nanotubes. By reducing the forces between adjacent carbon nanotubes, a steric hindrance effect is created between them, increasing the dispersion of carbon nanotubes and reducing their aggregation. The carbon nanotubes then exhibit excellent conductivity, effectively reducing the contact resistance between the positive electrode active materials, improving the conductivity of the positive electrode sheet, and thus helping to reduce the DC resistance of the battery.

[0013] In some implementations, R1 and R2 each independently include hydrogen.

[0014] In these embodiments, this application discloses that the substituent R1 in the first fluoropolymer includes hydrogen or the substituent R2 includes hydrogen or both include hydrogen. This design facilitates the chemical reaction of the monomers to generate the first fluoropolymer and also helps the first fluoropolymer to exhibit good dispersibility.

[0015] In some embodiments, R3 includes either a C1-C4 carboxyl group or a C1-C4 ester group.

[0016] Within a certain range, this application demonstrates that increasing the number of carbon atoms in the carboxyl groups and / or the aforementioned ester groups is beneficial for increasing the molecular weight of the first fluoropolymer. Furthermore, the suspension effect of a first fluoropolymer with a certain molecular weight in carbon nanotube slurry varies due to its own gravity and buoyancy, and these different suspension effects result in different dispersions of carbon nanotubes. Carboxyl groups and / or ester groups with the required number of carbon atoms, as specified in this application, facilitate the suspension of the first fluoropolymer and promote the dispersion of carbon nanotubes.

[0017] In some embodiments, the unsaturated carboxylic acid monomers mentioned above include either β-acryloyloxypropionic acid or succinic acid mono-2-(2-acryloyloxy)hydroxyethanol.

[0018] Compared to other types, the unsaturated carboxylic acid monomers listed above can better disperse carbon nanotubes.

[0019] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million; and / or,

[0020] Based on the total molar number of structural units in the first fluoropolymer mentioned above, the molar content of structural units derived from unsaturated carboxylic acid monomers is 0.7% to 1.5%; and / or,

[0021] The polydispersity index of the first fluoropolymer mentioned above is 1.5 to 3.5.

[0022] This application selects to add a first fluoropolymer with a weight-average molecular weight of 1.5 million to 9 million to the positive electrode film layer. The molecular weight of this type of fluoropolymer is relatively large, and the fluoropolymer with a relatively large molecular weight has good suspension in carbon nanotube slurry. On the one hand, it can effectively improve the dispersibility of carbon nanotubes and thus increase the carbon nanotube content in the carbon nanotube slurry. On the other hand, this type of fluoropolymer also has relatively higher adhesion, which is beneficial to increase the cohesive force between the positive electrode active materials.

[0023] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000; and / or,

[0024] Based on the total molar number of structural units in the first fluoropolymer mentioned above, the molar content of structural units derived from unsaturated carboxylic acid monomers is greater than 1.5% and less than 3.0%; and / or,

[0025] The polydispersity index of the first fluoropolymer mentioned above is 1.5 to 3.5.

[0026] In some embodiments of this application, a first fluoropolymer with a weight-average molecular weight of 50,000 to 200,000 is selected. This type of fluoropolymer has a relatively small molecular weight and is prone to generating strong intermolecular forces. These intermolecular forces enhance the bonding between carbon nanotubes and fluoropolymers, thereby increasing the stability of carbon nanotubes in carbon nanotube slurry and facilitating the carbon nanotubes to exert their conductivity.

[0027] In some embodiments, the mass percentage content of the aforementioned carbon nanotubes in the positive electrode film is 0.02% to 0.4%; and / or,

[0028] The mass percentage content of the first fluoropolymer in the positive electrode film layer is 0.02% to 2%.

[0029] As mentioned above, carbon nanotubes act as conductive agents in the positive electrode film. The first fluoropolymer can increase the dispersion of carbon nanotubes to increase the carbon nanotube content in the carbon nanotube slurry or increase the carbon nanotube stability in the carbon nanotube slurry. Therefore, the content of both in the positive electrode film affects the conductivity of the positive electrode sheet.

[0030] In some embodiments, the average diameter of the carbon nanotubes is ≤10 nm.

[0031] In some embodiments, the average diameter of the carbon nanotubes is 1 nm to 5 nm.

[0032] The carbon nanotubes in this application are further selected as single-walled carbon nanotubes, which have better electron mobility compared to other types of carbon nanotubes.

[0033] In some embodiments, the positive electrode film layer includes carbon black;

[0034] The mass percentage content of the carbon black in the above-mentioned positive electrode film layer is 0-2.5%.

[0035] In some embodiments, the compaction density of the above-mentioned positive electrode film is 3.4 g / cm³. 2 ~3.6g / cm 2 ; and / or,

[0036] The film resistance of the above positive electrode is <1Ω.

[0037] In some embodiments, the film resistance of the positive electrode is greater than 0.02Ω and less than 1Ω.

[0038] The positive electrode of this application has good conductivity.

[0039] In some embodiments, the positive electrode film layer includes a binder, and the binder has a mass percentage content of ≤1.5% in the positive electrode film layer;

[0040] The aforementioned adhesives include any one or more of polyvinylidene fluoride and its modified compounds, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, and hydrogenated nitrile butadiene rubber.

[0041] In some embodiments, the aforementioned vinylidene fluoride and its modified compounds include a second fluoropolymer, the second fluoropolymer comprising structural units derived from vinylidene fluoride and structural units derived from acrylic acid monomers; and / or,

[0042] The weight-average molecular weight of the second fluoropolymer is 1,000,000 to 1,200,000; and / or,

[0043] Based on the total number of moles of structural units in the second fluoropolymer, the molar content of structural units derived from acrylic monomers is 1.4% to 1.6%.

[0044] The second fluoropolymer provided in this application has different performance parameters than the first fluoropolymer described above. The second fluoropolymer is mainly added to the electrode slurry and primarily functions as an adhesive in the positive electrode film. As mentioned above, since the first fluoropolymer added to the carbon nanotubes also possesses a certain degree of adhesiveness, this application can further select to add a relatively smaller amount of the second fluoropolymer to the electrode slurry.

[0045] In some embodiments, the aforementioned positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure.

[0046] In some embodiments, the above-mentioned positive electrode active material includes one or more of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide;

[0047] The Dv50 of the above positive electrode active material is 2μm to 8μm.

[0048] The second aspect of this application is to provide a method for preparing the lithium-ion secondary battery described in the first aspect, comprising the following steps:

[0049] Provide the first fluoropolymer adhesive;

[0050] Preparation of carbon nanotube slurry: Disperse carbon nanotube powder and a first fluoropolymer liquid in a first organic solvent;

[0051] Preparation of positive electrode slurry: The positive electrode active material and carbon nanotube slurry are dispersed in a second organic solvent;

[0052] Preparation of positive electrode sheet: A positive electrode slurry is coated on at least one side surface of the above-mentioned positive electrode current collector to form a positive electrode film layer;

[0053] Preparation of lithium-ion batteries: Assemble the above-mentioned positive electrode, negative electrode and separator together.

[0054] In some embodiments, the mass ratio of the carbon nanotube powder to the first fluoropolymer in the carbon nanotube slurry is 1:(0.4-5).

[0055] In some embodiments, the aspect ratio of the carbon nanotubes is 1,000 to 1,000,000.

[0056] In some embodiments, the preparation of the positive electrode slurry further includes adding carbon black to the second organic solvent; and / or,

[0057] The resistivity of the above-mentioned carbon black powder is <0.03 Ω·cm; and / or,

[0058] The median particle size Dv50 of the above carbon black is ≤30μm.

[0059] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million; and / or,

[0060] The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 2% by mass solution with a viscosity of 1000 mPa·s to 6000 mPa·s.

[0061] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000; and / or,

[0062] The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 7% by mass solution, the viscosity of which is greater than zero and less than or equal to 1000 mPa·s.

[0063] A third aspect of this application is to provide an electrical device comprising the lithium-ion secondary battery described in the first aspect or the lithium-ion secondary battery prepared by the preparation method described in the second aspect.

[0064] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0065] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0066] Figure 1 This is a schematic diagram of the battery structure of some embodiments of this application;

[0067] Figure 2 This is an exploded structural diagram of a battery according to some embodiments of this application;

[0068] Figure 3 This is a schematic diagram of the vehicle structure according to some embodiments of this application;

[0069] Figure 4 This is a schematic diagram of the battery pack structure according to some embodiments of this application;

[0070] Figure 5 This is a structural schematic diagram showing the positional relationship between the positive electrode, the separator, and the negative electrode in some embodiments of this application;

[0071] Figure 6 This is a schematic diagram of the structure of the positive electrode sheet in some embodiments of this application.

[0072] The reference numerals in the detailed embodiments are as follows:

[0073] 10000, vehicles;

[0074] 1000, Battery; 2000, Controller; 3000, Motor;

[0075] 100. Battery cell;

[0076] 200. Box body; 210. First part; 220. Second part;

[0077] 10. Lithium-ion secondary batteries;

[0078] 101. Housing; 102. Electrode assembly; 103. Cover plate;

[0079] 1. Negative electrode plate;

[0080] 2. Positive electrode plate; 21. Positive current collector; 22. Positive electrode film;

[0081] 3. Isolation components;

[0082] The x-axis direction of the coordinate axis: the stacking direction or the thickness direction of the separator;

[0083] The y-axis direction: the length or width direction of the separator. Detailed Implementation

[0084] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the lithium-ion secondary battery, its preparation method, and its power application. However, unnecessary details may be omitted. For example, detailed descriptions of well-known facts and repetitive descriptions of identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0085] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is also expected that ranges of 60–110 and 80–120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0086] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0087] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0088] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0089] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0090] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0091] Unless otherwise specified, in this application, the terms "first," "second," etc., are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features.

[0092] Unless otherwise specified, in this application, the term "multiple" means two or more (including two), similarly, "multiple sets" means two or more (including two sets), and "multiple pieces" means two or more (including two pieces).

[0093] Unless otherwise specified, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0094] Lithium-ion rechargeable batteries have been widely used in various products due to their advantages such as high energy density, long cycle life, and safety and reliability. In recent years, with the significant increase in demand for lithium-ion rechargeable batteries as an energy source, higher requirements have been placed on their performance, such as high conductivity.

[0095] If the conductivity of the conductive additive can be improved without affecting the battery's cycle charge and discharge performance, it will help improve the battery's high conductivity.

[0096] Based on the above considerations, in order to improve the conductivity of the battery, a lithium-ion secondary battery, its preparation method, and its power device were obtained by conducting relevant experimental research according to the above design concept.

[0097] First, this application discloses a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, and a separator; wherein the positive electrode, separator, and negative electrode are sequentially stacked to form a wound cell or a stacked cell. Meanwhile, the positive electrode includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector. The positive electrode film layer includes a positive active material, carbon nanotubes, and a first fluorinated polymer. The first fluorinated polymer includes structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers; the unsaturated carboxylic acid monomers have the following chemical structural formula:

[0098]

[0099] In the above chemical structural formula, R1 and R2 each independently include hydrogen and any one of C1-C3 alkyl groups; R3 includes any one or more of carboxyl groups and ester groups.

[0100] In this application, carbon nanotubes in the positive electrode film serve as a conductive agent. Due to their excellent conductivity, they can exert their conductive function with a relatively small amount. However, carbon nanotubes are prone to aggregation, which affects their conductivity. This application selects a first fluoropolymer as a dispersant to increase the dispersion of carbon nanotubes and reduce their aggregation. This allows the carbon nanotubes to exert their excellent conductivity, effectively reducing the contact resistance between the positive electrode active materials, improving the conductivity of the positive electrode sheet, and thus helping to reduce the DC resistance of the battery.

[0101] Therefore, the design method provided in this application is beneficial to improving the conductivity of the entire positive electrode to reduce the DC resistance of the battery.

[0102] Meanwhile, the first fluoropolymer provided in this application has an adhesive effect. The first fluoropolymer is dispersed on the surface of the positive electrode active material and between the positive electrode active materials to complete the binding of the positive electrode active material, which is beneficial to reduce the volume change rate of the positive electrode active material.

[0103] Therefore, the lithium-ion secondary battery provided in this application is beneficial to improving the user experience.

[0104] The lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte composed of the aforementioned lithium-ion secondary battery. The outer packaging of the lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the lithium-ion secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, including but not limited to polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0105] This application does not impose any particular limitation on the shape of the lithium-ion secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1This is an example of a square-structured lithium-ion secondary battery 10.

[0106] According to some embodiments of this application, reference is made to Figure 2 The outer packaging may include a housing 101 and a cover plate 103. The housing 101 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 101 has an opening communicating with the receiving cavity, and the cover plate 103 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 102 through a winding process or a stacking process. The electrode assembly 102 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 102. The lithium-ion secondary battery 10 may contain one or more electrode assemblies 102, which can be selected by those skilled in the art according to specific practical needs.

[0107] The electrode assembly 102 provided in this application is beneficial to improving the performance of lithium-ion secondary batteries when applied in lithium-ion secondary batteries. The lithium-ion secondary battery can be used as a power source for an electrical device or as an energy storage unit for an electrical device. The electrical device is used in the power field, such as mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited to the above fields.

[0108] For ease of explanation, some embodiments of this application are illustrated using a vehicle as an example of an electrical device.

[0109] Please refer to Figure 3 , Figure 3 This is a schematic diagram of the structure of a vehicle 10000 provided in some embodiments of this application. The vehicle 10000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. A battery 1000 is disposed inside the vehicle 10000, and the battery 10000 can be located at the bottom, front, or rear of the vehicle 10000. The battery 10000 can be used to power the vehicle 10000; for example, the battery 10000 can serve as the operating power source for the vehicle 10000. The vehicle 10000 may also include a controller 2000 and a motor 3000. The controller 2000 is used to control the battery 10000 to supply power to the motor 3000, for example, to meet the power needs of the vehicle 10000 during startup, navigation, and driving.

[0110] In some embodiments of this application, the battery 1000 can not only serve as the operating power source for the vehicle 10000, but also as the driving power source for the vehicle 10000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 10000.

[0111] Please refer to Figure 4 , Figure 4 This is an exploded view of a battery 1000 provided in some embodiments of this application. The battery 1000 includes a housing 200 and a battery cell 100. A conventional battery cell includes a primary battery or a secondary battery. This application specifically protects a secondary battery, namely a lithium-ion secondary battery 10. The battery cell 100 is housed within the housing 200. The housing 200 provides space for the battery cell 100, and the housing 200 can adopt various structures.

[0112] In some embodiments, the housing 200 may include a first portion 210 and a second portion 220, which overlap each other, and together define a receiving space for accommodating the battery cell 100. The second portion 220 may be a hollow structure with one open end, and the first portion 210 may be a plate-like structure, with the first portion 210 covering the open side of the second portion 220 so that the first portion 210 and the second portion 220 together define the receiving space; alternatively, the first portion 210 and the second portion 220 may both be hollow structures with one open side, with the open side of the first portion 210 covering the open side of the second portion 220. Of course, the housing 200 formed by the first portion 210 and the second portion 220 may be of various shapes, such as a cylinder, a cuboid, etc.

[0113] In battery 1000, there can be multiple battery cells 100, which can be connected in series, parallel, or in a mixed manner. A mixed connection means that multiple battery cells 100 are connected in both series and parallel configurations. Multiple battery cells 100 can be directly connected in series, parallel, or in a mixed manner, and then the entire assembly of the multiple battery cells 100 is housed within housing 200. Alternatively, battery 1000 can also be composed of multiple battery cells 100 first connected in series, parallel, or in a mixed manner to form battery modules, and then these modules are connected in series, parallel, or in a mixed manner to form a whole, which is also housed within housing 200. Battery 1000 may also include other structures; for example, it may include a busbar component for electrical connection between the multiple battery cells 100.

[0114] Lithium-ion secondary batteries

[0115] This application discloses a lithium-ion secondary battery in some embodiments, comprising a positive electrode, a negative electrode, and a separator; wherein the positive electrode, separator, and negative electrode are sequentially stacked to form a wound cell or a cell. Meanwhile, the positive electrode includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector. The positive electrode film layer includes a positive active material, carbon nanotubes, and a first fluorinated polymer. The first fluorinated polymer includes structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers; the unsaturated carboxylic acid monomers have the following chemical structural formula:

[0116]

[0117] In the above chemical structural formula, R1 and R2 each independently include hydrogen and any one of C1-C3 alkyl groups; R3 includes any one or more of carboxyl groups and ester groups.

[0118] The positive electrode, separator, and negative electrode of this application can be formed into a secondary battery using winding or stacking processes. Specifically, this application... Figure 5 The diagram illustrates a lithium-ion secondary battery 10 formed using a winding method, combined with... Figure 5 It is known that a negative electrode 1 or a positive electrode 2 is placed between two adjacent separators 3, and the negative electrode 1 and the positive electrode 2 are alternately arranged along the stacking direction (coordinate axis x direction). The number and size of the negative electrode 1 and / or the positive electrode 2 can be selected according to the actual situation, and will not be elaborated in this application. Furthermore, this application... Figure 5 Only one winding method is shown in the illustration. Other stacking or winding methods are within the scope of protection of this application.

[0119] like Figure 6 The diagram illustrates that the positive electrode 2 includes a positive current collector 21 and a positive electrode film 22 located on at least one side surface of the positive current collector 21. Figure 6 The diagram illustrates a positive electrode film 22 disposed on either side of the surface of the positive electrode current collector 21. The positive electrode film 22 may also be located on both sides of the positive electrode current collector 21. The formation of the positive electrode film 22 on the surface of the positive electrode current collector 21 can be any method conventional in the art, such as coating, deposition, etc.

[0120] In this application, "polymer" refers to an aggregate of chemically homogeneous macromolecules prepared by polymerization reactions, but differing in degree of polymerization, molar mass, and chain length. The term also includes, on the other hand, derivatives of such aggregates of macromolecules formed by polymerization reactions, i.e., compounds that can be obtained through reactions of the functional groups in the aforementioned macromolecules, such as addition or substitution, and which may be chemically homogeneous or chemically heterogeneous.

[0121] The fluoropolymers in this application refer to polymers in which at least one hydrogen atom is replaced by a fluorine atom.

[0122] The unsaturated carboxylic acid monomers in this application refer to unsaturated monomers containing carboxyl functional groups.

[0123] The substituents R1 and R2 in this application each independently include hydrogen or any one of C1-C3 alkyl groups, and the C1-C3 alkyl groups may include methyl, ethyl, propyl and their isomers.

[0124] R3 in this application includes one or more of carboxyl and ester groups. An ester group refers to a group containing -C(O)O-. A carboxyl group refers to a group containing C(O)OH. These types of substituents are beneficial for further enhancing the dispersibility of carbon nanotubes by fluoropolymers.

[0125] The carbon nanotubes in this application refer to carbon allotropes comprising cylindrical layers of one or more carbon atoms, wherein the carbon atoms are covalently bonded to form a hexagonal tile pattern (i.e., graphene sheets), which forms a hollow tube structure with a diameter of up to several hundred nanometers. Here, graphene refers to nanotubes made of sp... 2 A planar sheet, one atom thick, composed of bonded carbon atoms, where the carbon atoms are tightly stacked in a honeycomb lattice.

[0126] The carbon nanotubes of this application have good electron mobility and mainly function as conductive agents in the positive electrode film.

[0127] The carbon nanotubes in this application include any one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0128] The single-walled carbon nanotubes of this application refer to a single cylindrical layer of carbon atoms, while the multi-walled carbon nanotubes of this application refer to two or more layers of carbon atoms connected by intermolecular forces, or a single layer of carbon atoms that is rolled around a hollow cylindrical core several times.

[0129] The carbon nanotubes of this application may be substituted with functional groups or other defects, depending on their production and purification methods, particularly at the ends of the tubes. For example, carbon nanotubes may include oxygen, sulfur, nitrogen, fluorine, or other substituent atoms, and may include, for example, carbonyl, hydroxyl, thiol, amine, and / or amide functional groups. Amorphous carbon and residual catalysts, such as iron or nickel, may also be present, among other impurities. Exemplary carbon nanotube synthesis processes include arc discharge, laser ablation, chemical vapor deposition (CVD), and high-pressure carbon monoxide dismutation (HiPCO). Some common post-synthesis treatments or modifications of carbon nanotubes include ozone treatment, ozone and hydrogen peroxide treatment, hydrochloric acid treatment, sodium hydroxide / potassium hydroxide treatment, and / or thermal treatment.

[0130] The carbon nanotubes of this application can be characterized using various techniques available in the art. For example, X-ray photoelectron spectroscopy (XPS) can be used to measure the content of nitrogen, oxygen, sulfur, or fluorine / halogen, and can indicate the level of impurities and functionalization. Raman spectroscopy can be used to indicate the purity level of the graphene sheets that make up the carbon nanotubes, i.e., their cleanliness. BET measurements can be used to measure the surface area of ​​the carbon nanotubes, and these measurements are affected by the properties of the carbon nanotube structure (e.g., single-walled versus multi-walled nanotubes) and the functionalization and defects of the nanotube structure, which may modify the measured values ​​relative to theoretical values. Finally, scanning electron microscopy can also be used to analyze the surface of the carbon nanotubes, as well as the shape and size of the particles.

[0131] In this application, carbon nanotubes in the positive electrode film serve as a conductive agent. A conductive agent is a substance with electrical conductivity that collects microcurrents between positive electrode active materials and between the positive electrode active materials and the positive electrode current collector, thereby reducing the contact resistance of the electrode and accelerating the electron mobility. This application leverages the excellent conductivity of carbon nanotubes to achieve their conductive function with a relatively small amount. However, carbon nanotubes are prone to aggregation, which affects their conductivity. Therefore, this application selects a first fluoropolymer as a dispersant. This first fluoropolymer contains structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers. Both types of structural units possess a certain polarity and readily generate intermolecular forces with carbon nanotubes. By reducing the forces between adjacent carbon nanotubes, a steric hindrance effect is created, increasing the dispersion of carbon nanotubes and reducing their aggregation. This allows the carbon nanotubes to exhibit good conductivity, effectively reducing the contact resistance between the positive electrode active materials, improving the conductivity of the positive electrode sheet, and ultimately reducing the DC resistance of the battery. Meanwhile, the structural units of vinylidene fluoride and unsaturated carboxylic acid monomers contained in the first fluoropolymer provided in this application also possess adhesive properties, and therefore can also be used as binders. Binders refer to chemical compounds, polymers, or mixtures that form colloidal solutions or colloidal slurries in a dispersion medium. The first fluoropolymer of this application is dispersed on the surface of and between the positive electrode active material to bind the positive electrode active material, which helps to reduce the volume change rate of the positive electrode active material.

[0132] When the carbon nanotubes of this application contain functional groups of oxygen, sulfur, nitrogen, fluorine or other substituent atoms, the carbon nanotubes readily generate intermolecular forces or hydrogen bonds with the first fluorinated polymer. On the one hand, this promotes the dispersion of the carbon nanotubes, and on the other hand, the carbon nanotubes and the first fluorinated polymer can easily form a binding network to better bind the positive electrode active material.

[0133] In summary, the design provided in this application is beneficial for improving the conductivity of the entire positive electrode to reduce the DC resistance of the battery, and can also reduce the volume change rate of the positive electrode active material to improve the cycle life of the battery.

[0134] In some embodiments, R1 and R2 each independently include hydrogen.

[0135] In these embodiments, this application discloses that the substituent R1 in the first fluoropolymer includes hydrogen or the substituent R2 includes hydrogen or both include hydrogen. This design facilitates the chemical reaction of the monomers to generate the first fluoropolymer and also helps the first fluoropolymer to exert its adhesive properties.

[0136] In some embodiments, R3 includes either a C1-C4 carboxyl group or a C1-C4 ester group.

[0137] In these embodiments, this application discloses that the carboxyl group can be a single carboxyl group (containing at least one carbon atom) or a carboxyl group containing 2 to 4 carbon atoms of an alkyl group. Similarly, the ester group can be a single ester group (containing at least one carbon atom) or an ester group containing 2 to 4 carbon atoms of an alkyl group. Within a certain range, increasing the number of carbon atoms is beneficial for increasing the molecular weight of the first fluoropolymer. Furthermore, the suspension effect of a first fluoropolymer of a certain molecular weight in carbon nanotube slurry varies due to its own gravity and buoyancy, and these different suspension effects result in different dispersions of carbon nanotubes. Carboxyl groups and / or ester groups that meet the carbon atom number requirements of this application facilitate the suspension of the first fluoropolymer and are beneficial for the dispersion of carbon nanotubes.

[0138] In some embodiments, the unsaturated carboxylic acid monomers mentioned above include either β-acryloyloxypropionic acid or succinic acid mono-2-(2-acryloyloxy)hydroxyethanol.

[0139] The β-acryloyloxypropionic acid of this application has the following structural formula:

[0140]

[0141] The succinic acid mono-2-(2-acryloyloxy)hydroxyethanol of this application has the following structural formula:

[0142]

[0143] Compared to other types, the unsaturated carboxylic acid monomers listed above in this application can better disperse carbon nanotubes and better exert their binding properties.

[0144] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million.

[0145] Based on the total number of moles of structural units in the first fluoropolymer mentioned above, the molar content of structural units derived from unsaturated carboxylic acid monomers is 0.7% to 1.5%.

[0146] The polydispersity index of the first fluoropolymer mentioned above is 1.5 to 3.5.

[0147] The weight-average molecular weight in this application refers to the sum of the products of the weight fraction of molecules with different molecular weights in the polymer and their corresponding molecular weights.

[0148] The weight-average molecular weight of the first fluoropolymer in this application can be determined using methods known in the art, such as gel permeation chromatography (GPC), specifically a Waters 2695 Isocratic HPLC gel permeation chromatograph (differential refractive index detector 2141). In some embodiments, the test method involves using a polystyrene solution sample of a certain mass fraction as a reference and selecting a matching chromatographic column (oil-based: Styragel HT5DMF7.8*300mm + StyragelHT4). A fluoropolymer gel solution of a certain mass fraction is prepared using purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day for later use. During testing, tetrahydrofuran is first drawn into a syringe for rinsing, and this is repeated several times. Then, 5 ml of the experimental solution is drawn, air is expelled from the syringe, and the needle tip is dried. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired, and the weight-average molecular weight is read.

[0149] This application selects to add a first fluoropolymer with a weight-average molecular weight of 1.5 million to 9 million to the positive electrode film layer. The molecular weight of this type of fluoropolymer is relatively large, and the fluoropolymer with a relatively large molecular weight has good suspension in carbon nanotube slurry. On the one hand, it can effectively improve the dispersibility of carbon nanotubes and thus increase the carbon nanotube content in the carbon nanotube slurry. On the other hand, this type of fluoropolymer also has relatively higher adhesion, which is beneficial to increase the cohesive force between the positive electrode active materials.

[0150] In these embodiments, this application provides that the weight-average molecular weight of the first fluoropolymer is any one of 1.5 million, 2 million, 2.5 million, 3 million, 3.5 million, 4 million, 4.5 million, 5 million, 5.5 million, 6 million, 6.5 million, 7 million, 7.5 million, 8 million, 8.5 million, or 9 million, or any one of the ranges of both above.

[0151] In this application, the molar content of structural units derived from unsaturated carboxylic acid monomers in the first fluoropolymer is 0.7% to 1.5%, which facilitates the dispersion of carbon nanotubes by the first fluoropolymer.

[0152] In these embodiments, this application provides that the molar content of the structural units derived from unsaturated carboxylic acid monomers is any one of 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any one of the ranges of both above.

[0153] The polydispersity factor in this application refers to the ratio of the weight-average molecular weight of the fluoropolymer to its number-average molecular weight. The number-average molecular weight is the sum of the products of the mole fractions of molecules with different molecular weights in the fluoropolymer and their corresponding molecular weights.

[0154] In this application, the polydispersity index (PDI) can be tested using methods known in the art, such as gel permeation chromatography (GPC), specifically a Waters 2695 Isocratic HPLC gel permeation chromatograph (differential refractive index detector 2141). In some embodiments, a polystyrene solution sample of a certain mass fraction is used as a reference, and a matching chromatographic column (oil-based: Styragel HT5DMF7.8*300mm + Styragel HT4) is selected. A fluoropolymer gel solution of a certain mass fraction is prepared using purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day for later use. During testing, tetrahydrofuran is first drawn into a syringe for rinsing, and this is repeated several times. Then, 5 ml of the experimental solution is drawn, air is expelled from the syringe, and the needle tip is dried. Finally, the sample solution is slowly injected into the injection port. Data is obtained after the reading stabilizes. The weight-average molecular weight a and the number-average molecular weight b are read separately. Polydispersity index = a / b.

[0155] The first fluoropolymer in this application has a weight-average molecular weight of 1.5 million to 9 million and also satisfies a polydispersity index of 1.5 to 3.5, which is beneficial for better dispersibility of carbon nanotubes.

[0156] In these embodiments, the polydispersity factor of the first fluoropolymer is provided to be any one of 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5 or any one of the above two ranges.

[0157] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000.

[0158] Based on the total number of moles of structural units in the first fluoropolymer, the molar content of structural units derived from unsaturated carboxylic acid monomers is greater than 1.5% and less than 3.0%.

[0159] The polydispersity index of the first fluoropolymer mentioned above is 1.5 to 3.5.

[0160] The definitions and measurement methods of weight-average molecular weight, polydispersity index, etc., described herein are the same as those described above.

[0161] In some embodiments of this application, a first fluoropolymer with a weight-average molecular weight of 50,000 to 200,000 is selected. This type of fluoropolymer has a relatively small molecular weight and is prone to generating strong intermolecular forces. These intermolecular forces enhance the bonding between carbon nanotubes and fluoropolymers, thereby increasing the stability of carbon nanotubes in carbon nanotube slurry and facilitating the carbon nanotubes to exert their conductivity.

[0162] In some embodiments, the mass percentage content of the above-mentioned carbon nanotubes in the positive electrode film layer is 0.02% to 0.4%;

[0163] The mass percentage content of the first fluoropolymer in the positive electrode film layer is 0.02% to 2%.

[0164] As mentioned above, carbon nanotubes act as conductive agents in the positive electrode film. The first fluoropolymer can increase the dispersion of carbon nanotubes to increase the carbon nanotube content in the carbon nanotube slurry or increase the carbon nanotube stability in the carbon nanotube slurry. Therefore, the content of both in the positive electrode film affects the conductivity of the positive electrode sheet.

[0165] In these embodiments, this application discloses that the mass percentage content of carbon nanotubes in the positive electrode film is any one of 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, 0.25%, 0.3%, 0.35%, and 0.4%, or any one of the ranges of both above.

[0166] In these embodiments, this application discloses that the mass percentage content of the first fluoropolymer in the positive electrode film layer is any one of 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2.0%, or any one of the ranges of both above.

[0167] In some embodiments, the average diameter of the carbon nanotubes is ≤10 nm.

[0168] In some embodiments, the average diameter of the carbon nanotubes is 1 nm to 10 nm.

[0169] In some embodiments, the average diameter of the carbon nanotubes is 1 nm to 5 nm.

[0170] In some embodiments, the average diameter of the carbon nanotubes is 1 nm to 3 nm.

[0171] The average diameter of the carbon nanotubes in this application refers to the average outer diameter of the carbon nanotubes.

[0172] The method for determining the average diameter of carbon nanotubes in this application includes any conventional method in the art, such as using a high-power electron microscope. Specifically, the average diameter testing method is as follows: 1) Instrument model: TEM transmission electron microscope; 2) Adjust the magnification of the equipment: 100K; 3) Select 100 carbon nanotubes in the field of view for statistical analysis and take the average value L1 (nm).

[0173] In these embodiments, this application discloses that the average diameter of the carbon nanotubes is any one of 10nm, 9nm, 8nm, 7nm, 6nm, 5nm, 4nm, 3nm, 2nm, 1nm or any one of the above ranges.

[0174] The carbon nanotubes in this application are further selected as single-walled carbon nanotubes, which have better electron mobility compared to other types of carbon nanotubes.

[0175] In these embodiments, the average diameter of the carbon nanotubes is provided as any one of 1 nm, 2 nm, 3 nm, or any one of the ranges of both above.

[0176] In some embodiments, the positive electrode film layer includes carbon black;

[0177] The carbon black content in the positive electrode film layer is 0-2.5% by mass.

[0178] As described above, carbon nanotubes serve as a conductive agent in the positive electrode film layer. The carbon nanotubes are one-dimensional tubular structures. To further increase the conductivity of the conductive agent, this application also chooses to use dotted conductive agent carbon black.

[0179] The carbon black content disclosed in these embodiments refers to the actual amount added during the preparation process, ignoring any changes in content. To reduce the impact of carbon black usage on the amount of positive electrode active material, this application selects a carbon black usage amount that meets the above-mentioned range.

[0180] In these embodiments, this application discloses that the mass percentage content of carbon black in the above-mentioned positive electrode film layer is any one of 0, 0.1%, 0.5%, 1%, 2%, 2.5%, or any one of the numerical ranges between any two of the above.

[0181] The carbon black in this application is an industrial product, whose main component is elemental carbon, and contains small amounts of oxygen, hydrogen and sulfur.

[0182] In some embodiments, the compaction density of the above-mentioned positive electrode film is 3.4 g / cm³. 2 ~3.6g / cm 2 ;

[0183] The film resistance of the above positive electrode is <1Ω.

[0184] The compaction density of the positive electrode film layer in this application can be used to characterize the energy density of the material. The compaction density of the positive electrode film layer = the areal density of the positive electrode film layer / the thickness of the positive electrode film layer. The thickness of the positive electrode film layer includes the distance between the two end faces of the positive electrode film layer along the thickness direction. The areal density of the positive electrode film layer = the weight of a single-sided positive electrode film layer / the area of ​​a single-sided positive electrode film layer. The weight of a single-sided positive electrode film layer can be obtained by weighing, and the area of ​​a single-sided positive electrode film layer can be obtained by using the area calculation formula according to the shape of the film layer. The test method for the compaction density of the positive electrode film layer is given in the test method of the following specific embodiments of this application, and will not be repeated here.

[0185] In these embodiments, the compaction density of the positive electrode film is given as 3.4 g / cm³. 2 3.5g / cm 2 3.6g / cm 2 It can be any one of the above or any one of the numerical ranges between any two of the above.

[0186] The film resistance of the positive electrode in this application refers to the resistance value of the positive electrode measured using conventional methods in the art, and is mainly used to reflect the conductivity of the positive electrode. The methods for measuring the film resistance of the positive electrode in this application include conventional methods in the art, such as the four-probe method, which are described in detail in the following specific embodiments.

[0187] The film resistance of the positive electrode in this application is <1Ω, which indicates that the positive electrode has good conductivity.

[0188] In some embodiments of this application, the film resistance of the positive electrode is provided to be greater than 0.02Ω and less than 1Ω.

[0189] In some embodiments of this application, the film resistance of the positive electrode is given as 0.02Ω to 0.8Ω.

[0190] In some embodiments, the positive electrode film layer includes a binder, wherein the binder comprises ≤1.5% by mass in the positive electrode film layer;

[0191] The aforementioned adhesives include any one or more of polyvinylidene fluoride and its modified compounds, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, and hydrogenated nitrile butadiene rubber.

[0192] The meaning of the binder in this application is as described above. The first fluoropolymer in this application can be used as a binder, thus reducing the amount of other binders used in the positive electrode film layer. This application selects the binder to have a mass percentage content of ≤1.5% in the above-mentioned positive electrode film layer. The other binders are any types conventional in the art, and this application only provides examples of a few.

[0193] In some embodiments, the above-mentioned positive electrode film layer includes an adhesive, which includes a second fluoropolymer, the second fluoropolymer including structural units derived from vinylidene fluoride and structural units derived from acrylic acid monomers;

[0194] The weight-average molecular weight of the aforementioned second fluoropolymer is 1 million to 1.2 million.

[0195] Based on the total number of moles of structural units in the second fluoropolymer, the molar content of structural units derived from acrylic monomers is 1.4% to 1.6%.

[0196] In some embodiments of this application, a second fluoropolymer is further selected as the binder, and the meaning and determination method of the weight-average molecular weight of the second fluoropolymer are the same as those described above.

[0197] The second fluoropolymer selected in this application, together with the first fluoropolymer, is beneficial for achieving synergistic bonding properties.

[0198] In these embodiments, this application provides that the weight-average molecular weight of the second fluoropolymer is any one of 1,000,000, 1,100,000, or 1,200,000, or any one of the numerical ranges between any two of the above.

[0199] In these embodiments, this application provides that the molar content of structural units derived from acrylic monomers in the second fluoropolymer is any one of 1.4%, 1.5%, 1.6%, or any value within a range between any two of the above.

[0200] In some embodiments, the above-mentioned positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure.

[0201] The types of positive electrode active materials in this application are not specifically limited; the above are just examples of a few.

[0202] In some embodiments, the above-mentioned positive electrode active material includes one or more of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide;

[0203] The Dv50 of the above positive electrode active material is 2μm to 8μm.

[0204] The positive electrode active material of this application includes one or more of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide, and carbon nanotubes are dispersed around the positive electrode active material, which is beneficial to improving the conductivity of the positive electrode sheet.

[0205] The Dv50 of this application includes a particle size distribution where 50% of the volume is larger than the particle size and 50% of the volume is smaller than the particle size; it is also known as the median diameter and is commonly used to represent the average particle size. In these embodiments, this application selects to determine the particle size distribution using laser diffraction particle size analysis, specifically referring to standard GB / T19077-2016 to obtain the particle size distribution map, and then calculating the result.

[0206] This application selects positive electrode active materials that meet the above-mentioned range values, so as to facilitate the dispersion of carbon nanotubes and fluoropolymers on the surface of each positive electrode active material.

[0207] In these embodiments, this application discloses that the Dv50 of the positive electrode active material is any one of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or any one of the numerical ranges between any two of the above.

[0208] [Positive electrode plate]

[0209] According to some embodiments of this application, as described above, the positive electrode sheet includes a positive current collector and a positive electrode film layer located on at least one side surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, a conductive agent, a binder, etc., and each component is as described above.

[0210] The positive current collector in this application can be a metal foil or a composite current collector. The metal foil can be an aluminum foil, and the composite current collector can include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector can be formed by forming a metal material, such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy, on a polymer substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE).

[0211] [Preparation method of positive electrode sheet]

[0212] According to some embodiments of this application, this application provides a method for preparing a positive electrode sheet, comprising the following preparation process:

[0213] S1. Provide a first fluoropolymer adhesive solution;

[0214] S2. Preparation of carbon nanotube slurry: Disperse carbon nanotube powder and a first fluoropolymer liquid in a first organic solvent;

[0215] S3. Preparation of positive electrode slurry: Disperse the positive electrode active material and carbon nanotube slurry in a second organic solvent;

[0216] S4. Preparation of positive electrode sheet: The positive electrode slurry is coated on at least one side surface of the positive electrode current collector to form a positive electrode film layer.

[0217] In some embodiments, the method for preparing the first fluoropolymer includes: providing vinylidene fluoride monomer, unsaturated carboxylic acid monomer and organic solvent, performing a first-stage polymerization reaction to obtain a first product; performing a second-stage polymerization reaction on the first product under a non-water-soluble gas atmosphere; and adding a chain transfer agent to perform a third-stage polymerization reaction.

[0218] This application employs a segmented polymerization method to facilitate the preparation of the first fluoropolymer. The first product is formed in the first stage of polymerization, the molecular chain segments with the target molecular weight are formed in the second stage, and the third stage is used to control the polymer's molecular weight, reducing excessive randomness in the weight-average molecular weight and improving polymer uniformity. Furthermore, segmented polymerization not only improves reactor utilization during polymer preparation but also saves time by reducing the polymer's residence time in the reactor. The coordinated action of the first, second, and third stages of polymerization further enhances polymer production efficiency.

[0219] The general structural formula of the unsaturated carboxylic acid monomers in this application is as described above.

[0220] The structural formula of the vinylidene fluoride monomer in this application is as follows:

[0221]

[0222] In some embodiments, the reaction temperature of the first polymerization reaction is 45°C to 60°C, and the reaction time is 2h to 8h.

[0223] In these embodiments, the application provides that the reaction temperature of the first polymerization reaction is any one of 45°C, 46°C, 47°C, 48°C, 49°C, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C, or 60°C, or any value within a range between any two of the above.

[0224] In these embodiments, the first polymerization reaction time is provided as any one of 2h, 3h, 4h, 5h, 6h, 7h, 8h or any one of the numerical ranges between any two of the above.

[0225] In some embodiments, the polymerization pressure of the first polymerization reaction is 4 MPa to 6 MPa.

[0226] In some embodiments, the reaction temperature of the second polymerization reaction is 60°C to 80°C, and the reaction time is 2h to 4h.

[0227] In these embodiments, this application provides that the reaction temperature for the second-stage polymerization reaction is any one of 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, or 80°C, or any value within a range between any two of the above.

[0228] In these embodiments, the second polymerization reaction time is given as any one of 2h, 3h, 4h or any one of the numerical ranges between any two of the above.

[0229] In some embodiments, the polymerization pressure of the second polymerization reaction is 6 MPa to 8 MPa.

[0230] In some embodiments, the reaction time of the third polymerization reaction is 1 h to 2 h.

[0231] This application controls the reaction pressure, reaction time, and reaction temperature of each stage of the polymerization reaction within a suitable range, which is beneficial to controlling the uniformity of the weight-average molecular weight of the polymerization product.

[0232] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0233] In some embodiments, a non-water-soluble gas refers to a gas with a solubility of less than 0.1 L. Gas solubility is defined as the gas pressure at 20°C being 1.013 × 10⁻⁶. 5 Pa is the volume of gas that dissolves in 1 L of water to reach saturation.

[0234] In some embodiments, the non-water-soluble gas includes one or more of nitrogen, oxygen, hydrogen, and methane.

[0235] In some embodiments, the amount of chain transfer agent used is 1.5% to 4% of the total mass of vinylidene fluoride monomer and unsaturated carboxylic acid monomer. Controlling the amount of chain transfer agent within a suitable range allows for controllable polymer chain length, thereby obtaining polymers with a suitable molecular weight range.

[0236] In some embodiments, the method for preparing the first fluoropolymer liquid includes dispersing the first fluoropolymer in N-methylpyrrolidone to form a liquid having a certain mass percentage content and a certain viscosity.

[0237] In some embodiments, the preparation process of the carbon nanotube slurry includes:

[0238] Kneading: The first fluoropolymer is dispersed in an organic solvent to form a glue solution with a certain mass fraction. Carbon nanotube powder is added to the glue solution and kneaded thoroughly to obtain blend 1.

[0239] Pre-stirring: Continue to add organic solvent and the above-mentioned first fluoropolymer solution to blend 1, mix well until the whole system becomes homogeneous;

[0240] High-pressure homogenization: Dispersion is performed using a high-pressure homogenizer.

[0241] This application discloses, in some embodiments, organic solvents including N-methylpyrrolidone.

[0242] In some embodiments, the mass ratio of carbon nanotube powder to the first fluoropolymer in the carbon nanotube slurry is 1:(0.4-5).

[0243] As described above in this application, the first fluoropolymer has different weight-average molecular weight ranges, and the carbon nanotubes have different diameters. That is, different carbon nanotubes require different amounts of the first fluoropolymer, but all meet the above-mentioned range. At this amount, the first fluoropolymer can effectively disperse the carbon nanotubes.

[0244] In these embodiments, this application discloses that the mass ratio between carbon nanotube powder and the first fluoropolymer described above is any one of 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:2, 1:3, 1:4, 1:5 or any one of the numerical ranges between any two of the above.

[0245] In some embodiments, the aspect ratio of the carbon nanotubes is 1000 to 1000000.

[0246] The carbon nanotubes of this application possess a one-dimensional tubular structure, thus having a specific length and diameter. The length includes the length of the tubular structure corresponding to the longer dimension (the third dimension) compared to the other two dimensions, and the distance between one end of the tubular structure. This one-dimensional tubular length can be obtained by photographing the carbon nanotubes, for example, using a scanning electron microscope (SEM) or a scanning transmission electron microscope (STEM). Similarly, the diameter of the carbon nanotube includes the dimension of one of the two smaller dimensions. This can be obtained by photographing the cross-section of the one-dimensional tubular structure and calculating the distance between one end of the cross-section. This application describes the average length and average diameter, specifically measured using the following method: Average diameter testing method: 1) Instrument type: TEM transmission electron microscope; 2) Magnification adjustment: 100K; 3) Measure the diameter of 100 carbon nanotubes within the field of view and statistically analyze the results, taking the average value L1 (nm). Average tube length test method: 1) Instrument model: SEM scanning electron microscope; 2) Adjust the magnification of the equipment: 30K / 40K; 4) Select 100 carbon tubes and measure their lengths for statistical analysis, and take the average value L2 (um). Then the length-to-diameter ratio is L2 / L1.

[0247] The aspect ratio of the carbon nanotubes in this application is 1,000 to 1,000,000. This is the aspect ratio of the raw material. In reality, the aspect ratio decreases in carbon nanotube slurry or positive electrode film. However, due to the dispersing effect of the first fluoropolymer in this application, the aspect ratio of the carbon nanotubes does not decrease too much, which is beneficial to the good conductivity of the carbon nanotubes.

[0248] In some embodiments, the preparation of the positive electrode slurry described above further includes adding carbon black to the second organic solvent;

[0249] The resistivity of the carbon black powder is <0.03Ω·cm;

[0250] The median particle size Dv50 of the above carbon black is ≤30μm.

[0251] The Dv50 of this application includes a particle size distribution where 50% of the volume is larger than the particle size and 50% of the volume is smaller than the particle size; it is also known as the median diameter and is commonly used to represent the average particle size. In these embodiments, this application selects to determine the particle size distribution using laser diffraction particle size analysis, specifically referring to standard GB / T19077-2016 to obtain the particle size distribution map, and then calculating the result.

[0252] In some embodiments, the Dv50 of the carbon black is 2 μm to 30 μm. Carbon black that meets this range is easily dispersed in the positive electrode film and forms a good conductive network together with other conductive agents.

[0253] In these embodiments, this application discloses that the carbon black has a Dv50 of any one of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, and 30μm, or any one of the ranges of both above.

[0254] In some embodiments, the resistivity of the carbon black powder is <0.03 Ω·cm.

[0255] The powder resistivity in this application is also the volume resistivity, which refers to the resistance of a powder material per unit length and per unit area at room temperature (10℃~30℃). The method for measuring the powder resistivity is any conventional method in the art. This application selects: using a powder resistivity measuring device (manufactured by Mitsubishi Chemical Analysis Technology Co., Ltd.: Lorester GP Powder Resistivity Measuring System MCP-PD-51), with a sample mass of 1.2g, a powder probe unit (four-probe probe unit) using a ring electrode, an electrode spacing of 5.0mm, an electrode radius of 1.0mm, a sample radius of 12.5mm, and measuring the volume resistivity [Ω·cm] of the conductive powder under a certain pressure (3t) with a 90V voltage limiter applied.

[0256] In some embodiments, the resistivity of the carbon black powder is 0.001 Ω·cm to 0.02 Ω·cm.

[0257] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million.

[0258] The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 2% by mass solution with a viscosity of 1000 mPa·s to 6000 mPa·s.

[0259] This application controls the viscosity of the first fluoropolymer within a certain range, which helps to reduce its impact on the electrode slurry. The viscosity measurement method of this application includes any method conventional in the art, such as a rotational viscometer. Specifically, a Brook DV2T viscometer is used. At 25°C, 200 ml of the adhesive solution is placed in a 250 ml beaker. A 64# rotor is inserted into the slurry, the device rotates at 100 r / min, the start test button is pressed, and the viscosity data is recorded after 5 minutes.

[0260] In some embodiments, the weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000.

[0261] The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 7% by mass solution, the viscosity of which is greater than zero and less than or equal to 1000 mPa·s.

[0262] In some embodiments, the first organic solvent and the second organic solvent each independently include one or more of N-methylpyrrolidone, dimethylacetamide, N,N-dimethylformamide, acetone, dimethyl carbonate, and polycarbonate.

[0263] [Negative electrode plate]

[0264] The negative electrode sheet of this application includes a negative current collector and a negative electrode film layer located on one or both surfaces of the negative current collector. Generally, the negative electrode film layer is located on both surfaces of the negative current collector, and is formed by methods such as coating or deposition. In this application, both sides will be used as examples.

[0265] In some embodiments, this application discloses that the compaction density of the negative electrode film layer is ≥1.65 g / cm³. 3 .

[0266] In this application, the compaction density of the negative electrode film can be used to characterize the energy density of the material; however, the compaction density of the negative electrode film is used to evaluate the overall compaction density of the negative electrode sheet. The compaction density of the negative electrode film = areal density of the negative electrode film / thickness of the negative electrode film. The thickness of the negative electrode film includes the distance between the two end faces of the negative electrode film along the thickness direction. The areal density of the negative electrode film = weight of a single positive electrode layer / area of ​​a single positive electrode layer. The weight of a single positive electrode layer can be obtained by weighing, and the area of ​​a single negative electrode layer can be obtained using the area calculation formula based on the shape of the film. In these embodiments, this application lists a compaction density of the negative electrode film ≥ 1.65 g / cm³. 3 .

[0267] This application discloses in some embodiments that the negative electrode film layer includes a negative electrode active material, and the active material includes one or more of carbonaceous materials, silicon-based materials, silicon-carbon composite materials, tin-based materials and their alloys. The carbonaceous materials in this application include one or more combinations of artificial graphite, natural graphite, soft carbon, and hard carbon. Among them, artificial graphite, natural graphite, soft carbon, and hard carbon include any form of material conventional in the art, and include any manufacturer and model conventional in the art. The silicon-based materials in this application include one or two of silicon-oxygen materials or silicon-carbon materials, or silicon-carbon composites. The tin-based materials and their alloys in this application include, but are not limited to, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, etc. Furthermore, this application is not limited to these materials; other conventional materials that can be used as negative electrode active materials for lithium-ion batteries or sodium-ion batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0268] In some embodiments, this application discloses that the conductive agent includes one or more of dot-shaped conductive agents, linear conductive agents, and planar conductive agents. The dot-shaped conductive agent includes one or more of conductive carbon black (Super P or Super S), acetylene black, conductive graphite (KS-6 or KS-15 or SFG-6 or SFG-15), and Ketjen black. The linear conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon nanofibers. The planar conductive agent includes, but is not limited to, graphene.

[0269] This application discloses in some embodiments that the negative electrode film layer includes a binder and a dispersant. The binder includes, but is not limited to, polyvinyl alcohol, polyethylene glycol, sodium carboxymethyl cellulose, polyethylene oxide, polyacrylic acid, polyacrylamide, sodium alginate, styrene-butadiene rubber (SBR), etc. The dispersant also includes any type conventional in the art, such as cellulose and its salts, specifically including, but not limited to, methylcellulose, carboxymethyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, etc.

[0270] The method of forming the negative electrode film layer in this application includes mixing the above-mentioned raw materials with a solvent (such as deionized water) in a certain mass ratio to form a negative electrode slurry, defoaming the negative electrode slurry, uniformly coating the negative electrode slurry on both sides of the negative electrode current collector; controlling the coating weight on one side; drying, and compacting it to a certain compaction density using a cold press to obtain a negative electrode sheet containing a negative electrode film layer.

[0271] [Isolation membrane]

[0272] Some embodiments of this application disclose a separator membrane. This application does not have any particular limitation on the type of separator membrane, and any well-known porous structure separator membrane with good chemical and mechanical stability can be selected.

[0273] In some embodiments, the separator includes a substrate material layer and a coating disposed on the surface of the substrate material layer; the substrate material of the substrate material layer includes one or more of polyethylene, polypropylene, poly(p-phenylene terephthalamide), polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, and polyamide; the coating includes a ceramic coating and / or a polymer coating. The substrate material layer has good lithium ion permeability, which is beneficial to lithium ion migration; the coating disposed on the surface of the substrate material layer can further improve the mechanical properties of the separator. Optionally, the ceramic particles in the ceramic coating include one or more of SiO2, Al2O3, AlOOH, CaO, TiO2, MgO, ZnO, ZrO2, Mg(OH)2, and BaSO4. Optionally, the polymer material of the polymer coating includes one or more of polyethylene (PE), polypropylene (PP), poly(p-phenylene terephthalamide) (PPTA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), and polyamide (PA). The polymer coating can be made of the same or different material as the substrate material layer, and the thicknesses of the polymer coating and the substrate material layer can be different. Optionally, the thickness of the polymer coating is less than the thickness of the substrate material layer.

[0274] In other embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0275] Electrolyte

[0276] Some embodiments of this application disclose an electrolyte, which can be liquid, solid, or gel-like. Solid state refers to a solid electrolyte, liquid state to a liquid electrolyte, and gel-like state to a gel electrolyte. The secondary battery of this application uses a liquid electrolyte, i.e., an electrolyte. This electrolyte contains an electrolyte salt and an organic solvent. The electrolyte salt can be any type conventional in the art, including, but not limited to, inorganic metal salts such as RClO4, RAsF6, RPF6, RBF4, RSbF6, RSO3F, RN(FSO2)2, etc.; fluorinated organometallic salts such as RCF3SO3, RN(FSO2)(CF3SO2), RN(CF3SO2)2, RN(C2F5SO2)2, cyclic 1,3-hexafluoropropane disulfonylimide lithium, cyclic 1,2-tetrafluoroethane disulfonylimide lithium, RN(CF3SO2)(C4F9S O2), RC(CF3SO2)3, RPF4(CF3)2, RPF4(C2F5)2, RPF4(CF3SO2)2, RPF4(C2F5SO2)2, RBF2(CF3)2, RBF2(C2F5)2, RBF2(CF3SO2)2, RBF2(C2F5SO2)2, etc.; and metal salts containing dicarboxylic acid complexes, such as lithium bis(oxalate)borate, lithium difluorooxalate borate, lithium tri(oxalate)phosphate, lithium difluorobis(oxalate)phosphate, lithium tetrafluoro(oxalate)phosphate, etc. Here, both the metal and R contain lithium ions.

[0277] According to some embodiments of this application, the concentration of the electrolyte salt in the electrolyte is 0.1 mol / L to 4 mol / L. In these embodiments, this application discloses that the concentration of the electrolyte salt is any one of 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, 2.8 mol / L, 3 mol / L, 3.2 mol / L, 3.5 mol / L, 3.8 mol / L, and 4 mol / L, or any one of the above ranges.

[0278] As described above, the organic solvent comprises one or more of carboxylic acid esters, carbonates, and ethers. Specifically, the carboxylic acid esters comprise one or more of ethyl acetate (EA), methyl acetate (MA), ethyl propionate (EP), propyl acetate (PA), methyl propionate (MP), methyl butyrate (MB), ethyl butyrate (EB), and 1,4-butyrolactone (GBL); the carbonates comprise one or more of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), and fluoroethylene carbonate (FEC). The ethers comprise at least one of tetrahydrofuran, dimethyl tetrahydrofuran, tetrahydropyran, dimethyl tetrahydropyran, 1,2-dimethoxyethane, dipropylene glycol dimethyl ether, or dimethyl phthalate. The organic solvents of this application further include one or two of nitrile solvents and sulfone solvents. The nitrile solvents include one or more of acetonitrile (AN), glutaronitrile (GLN), and adiponitrile (ADN). The sulfone solvents include at least one or a combination of two of sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0279] According to some embodiments of this application, the electrolyte further comprises a film-forming stabilizer, which includes a positive electrode film-forming stabilizer and a negative electrode film-forming stabilizer. The positive electrode film-forming stabilizer comprises carbonate additives and / or sulfate additives. The carbonate additives include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC). The sulfate additives include cyclic sulfonate additives and / or sulfated hydrocarbon ester additives; further, the cyclic sulfonate additives include one or more of 1,3-propanesulfonate lactone (PS), propenesulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (FPS); the sulfated hydrocarbon ester additives include one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS). The negative electrode film-forming stabilizer includes one or more of boron lithium salts, phosphorus-containing lithium salts, and sulfur-containing lithium salts; boron-containing lithium salts include one or more of lithium tetrafluoroborate (LiBF4), lithium bis(oxalate)borate (LiBOB), and lithium bis(oxalate)borate (LiDFOB); phosphorus-containing lithium salts include one or more of lithium difluorophosphate (LiPO2F2), lithium fluorophosphate (Li2PO3F), and lithium phosphate (Li3PO4). Sulfur-containing lithium salts include one or more of lithium fluorosulfonate (LiFSO3), lithium sulfate (Li2SO4), and lithium aminosulfonate (LiSO3NH2).

[0280] The secondary battery of this application will be described in detail below with reference to specific embodiments.

[0281] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0282] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0283] This application may employ conventional inorganic chemistry techniques within the art. In the following examples, efforts have been made to ensure the accuracy of the figures used (including quantities, temperatures, reaction times, etc.), but some experimental errors and biases should be considered. Temperatures (in degrees Celsius) used in the following examples are expressed in °C, and pressures are at or near atmospheric pressure. All reagents were purchased from AR-grade suppliers, and all reactions were carried out under argon protection. Unless otherwise stated, all reagents were obtained commercially.

[0284] Experimental materials:

[0285] Lithium nickel cobalt manganese oxide (LiNi) 0.8 CO 0.1 Mn 0.1 O2): Commercially available;

[0286] Graphite: Commercially available;

[0287] Carbon nanotubes: commercially available;

[0288] First fluoropolymer: Preparation;

[0289] Second fluoropolymer: Commercially available;

[0290] Carbon black: Commercially available;

[0291] Hydrogenated nitrile butadiene rubber: Commercially available.

[0292] Preparation Example 1

[0293] A method for preparing a first fluoropolymer (weight-average molecular weight of 350W) is provided, comprising the following preparation process:

[0294] First stage polymerization reaction: 3 kg of deionized water and 1.6 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. 4 g of tert-butyl peroxypentanoate and 2 g of sodium bicarbonate were added again, and 1 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, heated to 45 °C, and an aqueous solution containing 15.2 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 5.0 h.

[0295] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 7 MPa, heat to 65°C, and stir for 3 hours.

[0296] The third polymerization reaction: After adding 30g of cyclohexane, the reaction continued for 1 hour. The reaction was then stopped, and the reaction system was centrifuged to collect the solid phase. After washing and drying, the first fluoropolymer was obtained.

[0297] Preparation Example 2

[0298] A method for preparing a first fluoropolymer (weight-average molecular weight of 20W) is provided, comprising the following preparation process:

[0299] First stage polymerization reaction: 0.5 kg of deionized water and 0.2 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. Then, 0.45 g of tert-butyl peroxypentanoate and 0.18 g of sodium bicarbonate were added, and 0.13 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, heated to 45 °C, and an aqueous solution containing 2.5 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 5.0 h.

[0300] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 6 MPa, heat to 60°C, and stir for 3 hours.

[0301] The third polymerization reaction: After adding 10g of cyclohexane, the reaction continued for 1 hour. The reaction was then stopped, and the reaction system was centrifuged to collect the solid phase. After washing and drying, the first fluoropolymer was obtained.

[0302] [Testing the performance of the first fluoropolymer]

[0303] ①Test weight-average molecular weight:

[0304] A Waters 2695 Isocratic HPLC gel electrophoresis system (differential refractive index detector 2141) was used. A polystyrene solution with a mass fraction of 2.0%–7.0% was used as a reference, and a matched column (oil-based: Styragel HT5DMF7.8*300mm + Styragel HT4) was selected. Fluoropolymer gel solutions with a mass fraction of 2.0%–7.0% were prepared using purified N-methylpyrrolidone (NMP) solvent and allowed to stand for one day. For testing, tetrahydrofuran was first used to flush the sample, repeated several times. Then, 5 ml of the experimental solution was drawn, air was expelled from the syringe, and the needle tip was dried. Finally, the sample solution was slowly injected into the injection port. Data was acquired after the reading stabilized, and the weight-average molecular weight was recorded.

[0305] ② Test the polydispersity index:

[0306] A Waters 2695 Isocratic HPLC gel electrophoresis system (differential refractive index detector 2141) was used. A polystyrene solution with a mass fraction of 2.0%–7.0% was used as a reference, and a matched column (oil-based: Styragel HT5DMF7.8*300mm + Styragel HT4) was selected. Fluoropolymer gel solutions with a mass fraction of 2.0%–7.0% were prepared using purified N-methylpyrrolidone (NMP) solvent and allowed to stand for one day. For testing, tetrahydrofuran was first used to flush the solution, repeated several times. Then, 5 ml of the experimental solution was drawn, air was expelled from the syringe, and the needle tip was dried. Finally, the sample solution was slowly injected into the injection port. Data was acquired after the readings stabilized. The weight-average molecular weight *a* and the number-average molecular weight *b* were read. The polydispersity index was calculated as a / b.

[0307] ③ Test the viscosity of the adhesive:

[0308] Weigh 14g of the first fluoropolymer and a certain mass of N-methylpyrrolidone (NMP) into a 500ml beaker and prepare a glue solution with a certain mass fraction. In Preparation Example 1, the first fluoropolymer was prepared into a glue solution with a mass fraction of 2%, and in Preparation Example 2, the first fluoropolymer was prepared into a glue solution with a mass fraction of 7%.

[0309] The mixture was dispersed using a Lichen high-speed grinder at 800 rpm for 120 minutes, followed by ultrasonic vibration for 30 minutes to remove air bubbles. A Lichen NDJ-5S rotational viscometer was then used, with rotor #3 inserted to the mark above the graduations. The viscosity was measured at 12 rpm for 6 minutes, and the data was read afterward.

[0310] The performance parameters of the first fluoropolymers prepared in Preparation Example 1 and Preparation Example 2 according to the above test methods are listed in Table 1-1 below:

[0311] Table 1-1 List of performance parameters of the first fluoropolymer

[0312]

[0313] Example 1-1

[0314] A method for preparing a lithium-ion battery is provided, comprising the following steps:

[0315] Preparation of positive electrode sheet:

[0316] Preparation of carbon nanotube slurry: 10 parts by mass of the first fluoropolymer liquid (2% by mass) prepared in Preparation Example 1 were dispersed in N-methylpyrrolidone, and 1 part by mass of carbon nanotube powder (average diameter 1.5 nm, aspect ratio 200000) was added. The rotation speed was set to 25 rpm, the revolution speed to 800 rpm, and the stirring time was 30 minutes. The mixture was kneaded to obtain blend 1. N-methylpyrrolidone was added to blend 1 for dilution. The remaining 40 parts by mass of the first fluoropolymer liquid were added while stirring. The stirring speed was 1000 rpm, and the stirring time was 60 minutes. Cooling water circulation was turned on until the mixture became uniform. The mixture was then dispersed using a high-pressure homogenizer to obtain carbon nanotube slurry. The carbon nanotube content in the carbon nanotube slurry is shown in Table 1-2.

[0317] Preparation of positive electrode slurry: The positive electrode active material LiNi... 0.8 CO 0.1 Mn 0.1 O2 (median particle size Dv50 is 6.5 μm) and the above carbon nanotube slurry are dispersed in N-methylpyrrolidone, and then conductive agent carbon black (powder resistivity is 0.01 Ω·cm, median particle size Dv50 is 18 μm) and binder hydrogenated nitrile rubber are added and thoroughly mixed to form positive electrode slurry;

[0318] Preparation of the positive electrode sheet: A positive electrode slurry is coated onto both sides of the positive electrode current collector to form a positive electrode film layer; the film layer is dried and cold-pressed to achieve a compaction density of 3.5 g / cm³. 2 The positive electrode sheet.

[0319] The types and contents of each component in the positive electrode film are shown in Table 1-3.

[0320] Preparation of negative electrode sheet:

[0321] Graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) were dissolved in deionized water at a weight ratio of 96.2:1.8:0.8:1.2. The mixture was stirred under vacuum until homogeneous to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of the copper foil of the negative electrode current collector. The foil was dried in a nine-section oven with the following temperatures set sequentially: 100℃ / 100℃ / 95℃ / 85℃ / 85℃ / 80℃ / 80℃ / 80℃ / 60℃. The resulting material was then compacted using a cold press to obtain the negative electrode sheet.

[0322] Preparation of electrolyte:

[0323] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate, diethyl carbonate, and dimethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain a solvent. Lithium hexafluorophosphate is added to the solvent to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L.

[0324] Provide isolation membrane

[0325] A porous polyethylene (PE) membrane with a thickness of 13 μm was used as the separator.

[0326] Preparation of lithium-ion secondary batteries

[0327] The positive electrode, separator, and negative electrode are wound in sequence, with the separator acting as a separator between the positive and negative electrodes, and the corresponding components are assembled to form a wound bare cell. The wound bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion secondary battery is obtained.

[0328] Examples 1-2

[0329] A method for preparing a lithium-ion secondary battery is provided. The method differs from that in Example 1-1 in that the carbon nanotube slurry uses the first fluorinated polymer obtained in Preparation Example 2, and the first fluorinated polymer with a mass percentage of 7% is used when preparing the carbon nanotube slurry. The mass ratio of carbon nanotubes to the first fluorinated polymer in the carbon nanotube slurry is 1:2. All other aspects are the same as in Example 1-1.

[0330] Comparative Example 1

[0331] A method for preparing a lithium-ion secondary battery is provided. This method differs from Examples 1-1 in that a commercially available fluoropolymer is used in the carbon nanotube slurry. This fluoropolymer comprises structural units derived from vinylidene fluoride and structural units derived from acrylic acid monomers. Based on the total molar number of structural units in the fluoropolymer, the molar content of the structural units derived from acrylic acid monomers is 1.5%, and the weight-average molecular weight of the fluoropolymer is 1.1 million. Furthermore, the mass ratio of carbon nanotubes to the first fluoropolymer in the carbon nanotube slurry is 1:5; all other aspects remain the same as in Examples 1-1.

[0332] Comparative Example 2

[0333] A method for preparing a lithium-ion secondary battery is provided. This method differs from Example 1-1 in that a commercially available fluoropolymer is used in the carbon nanotube slurry. The fluoropolymer includes structural units derived from vinylidene fluoride and has a weight-average molecular weight of 1.1 million. The mass ratio of carbon nanotubes to the first fluoropolymer in the carbon nanotube slurry is 1:5. All other aspects remain the same as in Example 1-1.

[0334] Based on the actual amount of each raw material component added and the stable state of the carbon nanotube slurry formed, this application calculates the mass percentage content of carbon nanotubes in the carbon nanotube slurries of Examples 1-1, Examples 1-2, and Comparative Examples 1 and 2, as shown in Table 1-2.

[0335] Table 1-2 Mass percentage content of carbon nanotubes and first fluoropolymer in carbon nanotube slurry

[0336]

[0337] As shown in Table 1-2, the fluoropolymer designed in this application, compared with the fluoropolymers of the prior art, is also beneficial to increase the carbon nanotube content in the carbon nanotube slurry while controlling the amount used relatively less.

[0338] In this application, the carbon nanotube slurries prepared in the above embodiments and comparative examples are used to prepare positive electrode slurries and further form positive electrode films. The amount of each carbon nanotube slurry used is adjusted according to the actual situation to control the mass percentage content of carbon nanotubes in the positive electrode films of each embodiment and comparative example to be 0.10%. The mass percentage is calculated based on the amount of each component added in the positive electrode film, as shown in Tables 1-3.

[0339] Table 1-3 Mass percentage content of each component in the positive electrode film

[0340]

[0341] The remaining components in Table 1-3 contain positive electrode active materials, and the sum of all components is 100%.

[0342] [Testing the adhesion and cohesion of the positive electrode sheet]

[0343] ④ Test adhesion:

[0344] Referring to GB / T2790-1995 national standard "Test Method for 180° Peel Strength of Adhesives", the bonding force test process of the positive electrode sheet in the embodiments and comparative examples of this application is as follows: A sample with a width of 30mm and a length of 100-160mm is cut with a blade. Special double-sided adhesive tape is applied to a steel plate, with a tape width of 20mm and a length of 90-150mm. The positive electrode film layer of the previously cut electrode sheet sample is then attached to the double-sided adhesive tape and rolled three times in the same direction using a 2kg pressure roller. A paper strip with a width equal to the electrode sheet and a length of 250mm is fixed to the current collector of the electrode sheet and secured with wrinkle adhesive. The power supply of the Sansi tensile testing machine (sensitivity 1N) is turned on; the indicator light illuminates. The limit block is adjusted to a suitable position, and the end of the steel plate without the electrode sheet attached is fixed with the lower clamp. The paper strip is folded upwards and fixed with the upper clamp. The position of the upper clamp is adjusted using the "up" and "down" buttons on the manual controller attached to the tensile testing machine. Then, the test is performed and the values ​​are read. The force when the electrode is in equilibrium is divided by the width of the tape to obtain the bonding force per unit length of the electrode, measured in N / m, to characterize the bonding strength between the positive electrode film and the current collector.

[0345] ⑤ Test cohesion:

[0346] Equipment used: Instron 336; the specific testing procedure is as follows:

[0347] Cut the positive electrode sheet into a 100mm long and 10mm wide sample. Take a 25mm wide stainless steel plate and attach the sample to it using 11mm wide 3M double-sided tape, ensuring the current collector is bonded to the tape. Roll the sample surface back and forth three times (300mm / min) with a 2000g roller. Then, attach a 10mm wide and 50μm thick tape (model NITTO.NO5000NS) to the surface of the positive electrode film and roll it back and forth three times (300mm / min) with a 2000g roller. Bend the tape 180 degrees and manually peel it 25mm away from the positive electrode film. Fix the sample on an Instron 336 tensile testing machine, ensuring the peeled surface is aligned with the machine's force line (i.e., perform a 180° peel). Pre-stretch at a peeling speed of 100mm / min for 10mm, then allow the tensile testing machine to return to its original position. Finally, peel continuously at 300mm / min to obtain the cohesive force curve. Take the average value of the stable segment as the peeling force F0. Calculate the cohesive force F1 of the tested electrode using the following formula: F1 = F0 / width of the sample. The unit of measurement for F1 is N / m.

[0348] ⑥ Test the film resistance of the positive electrode:

[0349] Cut 3*3mm small circles from the left, center, and right sides of the positive electrode. Turn on the indicator light of the Yuaneng Technology electrode resistance meter, place the probe at the appropriate position on the meter, and click the "Start" button. Once the reading stabilizes, take the reading. Test two positions for each small circle, and finally calculate the average of the six measurements, which is the film resistance of that electrode.

[0350] ⑦ Method for determining the compaction density of the positive electrode film:

[0351] The thickness H of the positive electrode film was obtained using a micrometer.

[0352] Take a positive electrode sheet that has been coated on one side and cold-pressed (if it is a double-sided coated electrode sheet, the electrode film layer on one side can be wiped off first), cut it into a small circular piece with an area of ​​S0, weigh it, and record its weight as M1; then wipe off the electrode film layer of the positive electrode sheet after weighing it, weigh the current collector, and record it as M0. The surface density of the positive electrode film layer is D = (M1 - M0) / S0.

[0353] Then the compaction density of the positive electrode film is equal to the surface density D of the positive electrode film / the thickness H of the positive electrode film.

[0354] The performance parameters of the positive electrode sheets prepared according to the above testing methods are listed in Tables 1-4 below:

[0355] Table 1-4 Performance List of Positive Electrode Films

[0356] Serial Number Adhesion force / N Cohesion / N / m diaphragm resistance / Ω <![CDATA[Compaction density / g / cm 2 > Example 1-1 20.1 134 0.215 3.5 Examples 1-2 18.7 119 0.345 3.5 Comparative Example 1 23.1 120 0.843 3.4 Comparative Example 2 24.5 123 0.915 3.4

[0357] As shown in Tables 1-4, the fluoropolymer provided in this application, under the premise of controlling a relatively smaller amount, exhibits excellent conductivity due to its good dispersion in the positive electrode film layer, thereby improving the conductivity of the positive electrode sheet. Meanwhile, the content of the first fluoropolymer in Example 1-1 is much lower than that in Comparative Example 1, but the cohesive force of the positive electrode film layer is basically the same. The content of the first fluoropolymer in Example 1-2 is also lower than that in Comparative Example 1. Although the adhesive force and cohesive force of Example 1-2 are lower than those of Example 1-1, they are not significant. This indicates that the fluoropolymer of this application can still exhibit good adhesive properties.

[0358] This application further designs the following preparation examples and embodiments to investigate the effects of carbon nanotubes on battery performance at different addition amounts, as well as the effects of fluoropolymers with different properties on battery performance.

[0359] Example 2-1

[0360] A method for preparing a lithium-ion secondary battery is provided, which differs from Example 1-1 in that the binder hydrogenated nitrile rubber in the positive electrode film is replaced with a commercially available second fluoropolymer (weight-average molecular weight of 1.1 million, and based on the total molar number of structural units in the second fluoropolymer, the molar content of structural units derived from acrylic monomers is 1.5%). All other aspects remain the same as in Example 1-1.

[0361] The performance parameters of the positive electrode sheet prepared in Example 2-1 were obtained according to the above testing method, as shown in Table 2-1 below:

[0362] Table 2-1 Performance List of Positive Electrode Films

[0363] Serial Number Adhesion force / N Cohesion / N / m diaphragm resistance / Ω <![CDATA[Compaction density / g / cm 2 > Example 1-1 20.1 134 0.215 3.5 Example 2-1 22.9 136 0.226 3.5

[0364] As shown in Table 2-1, this application further selects to add a second fluoropolymer as a binder to the positive electrode film layer. This binder, together with the first fluoropolymer, facilitates better adhesion of carbon nanotubes and positive electrode active materials in the positive electrode film layer.

[0365] Example 2-2

[0366] A method for preparing a lithium-ion secondary battery is provided, which differs from Examples 1-2 in that the binder hydrogenated nitrile rubber in the positive electrode film is replaced with a commercially available second fluoropolymer (weight-average molecular weight of 1.1 million, and based on the total molar number of structural units in the second fluoropolymer, the molar content of structural units derived from acrylic monomers is 1.5%). All other aspects remain the same as in Examples 1-2.

[0367] The performance parameters of the positive electrode sheet prepared in Example 2-2 were obtained according to the above testing method, as shown in Table 2-2 below:

[0368] Table 2-2 Performance List of Positive Electrode Films

[0369] Serial Number Adhesion force / N Cohesion / N / m diaphragm resistance / Ω <![CDATA[Compaction density / g / cm 2 <!-- 24 -->]]> Examples 1-2 18.7 119 0.345 3.5 Example 2-2 20.1 125 0.362 3.5

[0370] As shown in Table 2-2, this application further selects to add a second fluoropolymer as a binder to the positive electrode film layer. This binder, together with the first fluoropolymer, facilitates better adhesion of carbon nanotubes and positive electrode active materials in the positive electrode film layer.

[0371] Examples 3-1 to 3-5

[0372] A method for preparing a lithium-ion secondary battery is provided. The details are as follows:

[0373] Example 3-1

[0374] A method for preparing a lithium-ion secondary battery is provided. This method differs from that in Example 1-1 in that the amount of carbon nanotube slurry added in the preparation of the positive electrode slurry is changed so that the carbon nanotube content in the positive electrode film is 0.02%, while all other aspects remain the same. That is, the mass ratio of carbon nanotubes to the first fluoropolymer in the carbon nanotube slurry is 1:1, the carbon nanotube content in the positive electrode film is 0.02%, and the content of the first fluoropolymer in the positive electrode film is 0.02%.

[0375] Example 3-2

[0376] A method for preparing a lithium-ion secondary battery is provided. This method differs from that in Example 1-1 in that the amount of carbon nanotube slurry added in the preparation of the positive electrode slurry is changed so that the carbon nanotube content in the positive electrode film is 0.12%, while all other aspects remain the same. That is, the mass ratio of carbon nanotubes to the first fluorinated polymer in the carbon nanotube slurry is 1:1, the carbon nanotube content in the positive electrode film is 0.12%, and the content of the first fluorinated polymer in the positive electrode film is 0.12%.

[0377] Example 3-3

[0378] A method for preparing a lithium-ion secondary battery is provided. This method differs from that in Example 1-1 in that the carbon nanotubes used have a diameter of 6.7 nm and an aspect ratio of 50000. Furthermore, the amount of the first fluoropolymer in the carbon nanotube slurry differs from that in Example 1-1 (the mass ratio of carbon nanotubes to the first fluoropolymer in the carbon nanotube slurry is 2:1), so that the carbon nanotube content in the positive electrode film is 0.2% and the first fluoropolymer content in the positive electrode film is 0.1%.

[0379] Examples 3-4

[0380] A method for preparing a lithium-ion secondary battery is provided. This method differs from that in Example 1-1 in that the carbon nanotubes used have a diameter of 9.8 nm and an aspect ratio of 10000; and the amount of the first fluoropolymer in the carbon nanotube slurry is different from that in Example 1-1 (the mass ratio of carbon nanotubes to the first fluoropolymer in the carbon nanotube slurry is 2:1), so that the carbon nanotube content in the positive electrode film is 0.4% and the first fluoropolymer content in the positive electrode film is 0.2%.

[0381] Examples 3-5

[0382] A method for preparing a lithium-ion secondary battery is provided. The method differs from that in Examples 3-4 in that the carbon black in the positive electrode film is removed, while all other aspects remain the same as in Examples 3-4.

[0383] Examples 3-1 to 3-5 were obtained, and the total mass ratio of the first fluoropolymer and hydrogenated nitrile rubber in each example was 1.5%.

[0384] The mass percentages of each component in the positive electrode film are shown in Table 3-1.

[0385] Table 3-1 Mass percentage of each component in the positive electrode film

[0386]

[0387] The components in Table 3-1 are the same as those in Table 1-3 above. The remaining components include the positive electrode active material, and the sum of all components is 100%.

[0388] The performance of Example 3, measured according to the above testing methods, is listed in Table 3-2.

[0389] Table 3-2 Performance List of Positive Electrode Films

[0390] Serial Number Adhesion force / N Cohesion / N / m diaphragm resistance / Ω <![CDATA[Compaction density / g / cm 2 > Example 1-1 20.1 134 0.215 3.5 Example 3-1 17.8 121 0.682 3.5 Example 3-2 22.3 138 0.208 3.5 Example 3-3 19.2 128 0.287 3.5 Examples 3-4 28.9 145 0.256 3.5 Examples 3-5 27.6 142 0.224 3.5

[0391] As shown in Table 3-2, the fluoropolymer provided in this application has the ability to disperse carbon nanotubes with different diameters. For example, the average diameter of the carbon nanotubes dispersed in Examples 1-1, 3-1, and 3-2 is 1.5 nm, the average diameter of the carbon nanotubes dispersed in Example 3-2 is 6.7 nm, and the average diameter of the carbon nanotubes dispersed in Example 3-3 is 9.8 nm. Furthermore, under the premise of this good dispersion, for carbon nanotubes of the same diameter, the conductivity of the positive electrode is enhanced as the carbon nanotube content increases within a certain range.

[0392] Further analysis of Examples 3-4 and 3-5 reveals that carbon black, together with carbon nanotubes, is beneficial for improving the conductivity of the positive electrode sheet.

[0393] Examples 4-1 to 4-3

[0394] A method for preparing a lithium-ion battery is provided. This method differs from that in Example 1-1 in that different first fluorinated polymers are obtained by adjusting the process parameters and dosages.

[0395] In Example 4-1, the weight-average molecular weight of the first fluoropolymer was 1.5 million; in Example 4-2, the weight-average molecular weight of the first fluoropolymer was 9 million; and in Example 4-3, the weight-average molecular weight of the first fluoropolymer was 50,000.

[0396] Furthermore, the preparation method of the first fluoropolymer in Example 4-1 includes:

[0397] First stage polymerization reaction: 1.5 kg of deionized water and 0.8 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. 2 g of tert-butyl peroxypentanoate and 1 g of sodium bicarbonate were added again, and 0.5 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, heated to 45 °C, and an aqueous solution containing 10 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 5.0 h.

[0398] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 7.5 MPa, heat to 65°C, and stir for 3 hours;

[0399] The third polymerization reaction: After adding 20g of cyclohexane, the reaction continued for 1 hour. The reaction was then stopped, and the solid phase was collected by centrifugation. After washing and drying, the first fluoropolymer was obtained.

[0400] The preparation method of the first fluoropolymer in Example 4-2 includes:

[0401] First stage polymerization reaction: 5 kg of deionized water and 4.5 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. Then, 11.5 g of tert-butyl peroxypentanoate and 5.5 g of sodium bicarbonate were added, and 2.8 kg of vinylidene fluoride monomer was added to bring the pressure to 5.5 MPa. The mixture was stirred for 30 min, and the temperature was raised to 50 °C. At the same time, an aqueous solution containing 43.6 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 5.0 h.

[0402] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 6.5 MPa, heat to 70°C, and stir for 3 hours;

[0403] The third polymerization reaction: After adding 80g of cyclohexane, the reaction continued for 2 hours. The reaction was then stopped, and the reaction system was centrifuged to collect the solid phase. After washing and drying, the first fluoropolymer was obtained.

[0404] The preparation method of the first fluoropolymer in Examples 4-3 includes:

[0405] First stage polymerization reaction: 0.5 kg of deionized water and 0.2 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. Then, 0.15 g of tert-butyl peroxypentanoate and 0.04 g of sodium bicarbonate were added, and 0.05 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, heated to 45 °C, and an aqueous solution containing 0.8 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 3.0 h.

[0406] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 6 MPa, heat to 60°C, and stir for 3 hours.

[0407] The third polymerization reaction: After adding 10g of cyclohexane, the reaction continued for 1 hour. The reaction was then stopped, and the reaction system was centrifuged to collect the solid phase. After washing and drying, the first fluoropolymer was obtained.

[0408] The performance parameters of each of the first fluoropolymers are shown in Table 4-1 below.

[0409] Table 4-1 List of performance parameters of the first fluoropolymer

[0410]

[0411] Based on the actual amount of each raw material component added and the stable state of the carbon nanotube slurry formed, this application calculates the carbon nanotube content in the carbon nanotube slurry of Examples 4-1 to 4-3 above, as shown in Table 4-2.

[0412] Table 4-2 Carbon nanotube content in carbon nanotube slurry

[0413]

[0414] In the positive electrode film layers of Examples 4-1 to 4-3 above, the content of each carbon nanotube is 0.1%, the content of hydrogenated nitrile rubber is 1.5%, and the content of carbon black is 2%, as detailed in Table 4-3 below.

[0415] Table 4-3 Mass percentage content of each component in the positive electrode film.

[0416]

[0417] The remaining components in Table 4-3 include the positive electrode active material, and the sum of all components is 100%.

[0418] The performance of the lithium-ion batteries obtained in Examples 4-1 to 4-3 above is illustrated in Table 6 below.

[0419] Example 5-1

[0420] A method for preparing a lithium-ion battery is provided. This method differs from that of Example 1-1 in that different first fluorinated polymers are obtained by adjusting the type of monomer; otherwise, it is the same as that of Example 1. In the first fluorinated polymer of Example 5-1, the unsaturated carboxylic acid monomer is succinic acid mono-2-(2-acryloyloxy)hydroxyethanol.

[0421] Furthermore, the preparation method of the first fluoropolymer in Example 5-1 includes:

[0422] First stage polymerization reaction: 3 kg of deionized water and 1.6 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and the oxygen was replaced with nitrogen three times. Then, 4 g of tert-butyl peroxypentyl ester and 2 g of sodium bicarbonate were added, and 1 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, and the temperature was raised to 45 °C. At the same time, an aqueous solution containing 23.2 g of succinic acid mono-2-(2-acryloyloxy)hydroxyethanol was added in multiple batches. The reaction was carried out for 6.0 h.

[0423] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 7 MPa, heat to 65°C, and stir for 3 hours.

[0424] The third polymerization reaction: After adding 40g of cyclohexane, the reaction continued for 1 hour. The reaction was then stopped, and the reaction system was centrifuged to collect the solid phase. After washing and drying, the first fluoropolymer was obtained.

[0425] The performance parameters of the first fluoropolymer are shown in Table 5-1 below.

[0426] Table 5-1 List of performance parameters of the first fluoropolymer

[0427] Serial Number weight average molecular weight polydispersion coefficient Adhesive viscosity / mPa·s Dimonomer molar content Example 1-1 350W 2.0 1500 1 Example 5-1 350W 2.0 1600 1

[0428] Based on the actual amount of each raw material component added and the stable state of the carbon nanotube slurry formed, this application calculates the carbon nanotube content in the carbon nanotube slurry of Example 5-1 above, as shown in Table 5-2.

[0429] Table 5-2 Carbon nanotube content in carbon nanotube slurry

[0430]

[0431] Table 5-3 Mass percentage content of each component in the positive electrode film layer

[0432]

[0433] The performance of the lithium-ion secondary battery prepared in Example 5-1 is illustrated in Table 6 below.

[0434] [Battery Performance Test]

[0435] ⑧ Cyclic stability, test method for capacity retention after 500 cycles:

[0436] At 25℃, the lithium-ion secondary battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage to a current of 0.05C (corresponding to 100% SOC). After resting for 5 minutes, the secondary battery was discharged at a constant current of 1C to the lower limit of 2.8V (corresponding to 0% SOC), and the discharge capacity at this point was recorded, which is the discharge capacity of the first cycle. The secondary battery was subjected to a cyclic charge-discharge test according to the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 500 cycles at 25℃ = discharge capacity after 500 cycles / discharge capacity of the first cycle × 100%.

[0437] ⑨ Test method for DC resistance (DCR) growth rate after 500 cycles:

[0438] Testing the initial resistance (DCR1) of the battery: At room temperature (25℃), the lithium-ion battery was discharged to 50% SOC using a 1 / 3C1 current, followed by a 4C1 current for 10 seconds. The open-circuit voltage before discharge is recorded as U1, and the voltage at the end of discharge is recorded as U2. U2-U1 is recorded as ΔU1. ΔU1 is related to the current I. 4C1 The ratio is the initial internal resistance of the battery, DCR1, i.e., DCR1 = ΔU1 / I. 4C1 C1 represents the initial capacity of the battery. The testing method includes: at 25°C, charging the individual battery cells to 4.25V with a constant current of 0.33C; further charging with a constant voltage of 4.25V to a current of 0.05C; letting it rest for 30 minutes; and then discharging the lithium-ion battery to 2.8V with a constant current of 0.33C. The discharge capacity is recorded as C1. The specific process for discharging to 50% SOC is as follows: at 25°C, charging the battery to 4.25V with a constant current of 0.33C1; further charging with a constant voltage of 4.25V to a current of 0.05C1; letting it rest for 30 minutes; and then discharging with a constant current of 0.33C1 to 0.5C1 (cutoff).

[0439] Following the above cycle method, charge to 100% SOC after 500 cycles.

[0440] Test of resistance DCR2 after 500 battery cycles: At room temperature (25℃), the lithium-ion battery was discharged to 50% SOC, followed by a 4C2 discharge for 10 seconds. The open-circuit voltage before discharge was recorded as U3, and the final voltage after discharge was recorded as U4. U4-U2 was recorded as ΔU2. ΔU2 is related to the current I. 4C2The ratio is the internal resistance DCR2, i.e., DCR2 = ΔU2 / I4C2. Where C2 is the capacity of the battery after 500 cycles. The test method includes: at 25℃, first charging the lithium-ion battery to 4.25V with a constant current of 0.33C, then further charging it to 0.05C with a constant voltage of 4.25V, letting it rest for 30 minutes, and then discharging it to 2.8V with a constant current of 0.33C. The discharge capacity is recorded as C2. The specific process for discharging to 50% SOC is as follows: at 25℃, first charging the battery to 4.25V with a constant current of 0.33C2, then further charging it to 0.05C2 with a constant voltage of 4.25V, letting it rest for 30 minutes, and then discharging it to 0.5C2 with a constant current of 0.33C2 until it is cut off.

[0441] The DCR growth rate after 500 cycles = (DCR2-DCR1) / DCR1, which can represent the DCR growth during the cycle. The larger the DCR growth, the more the battery DCR deteriorates under the influence of current density gradient, conductive network, battery polarization and other factors during the cycle, affecting the battery dynamic performance and cycle life.

[0442] Table 6 Battery Performance List

[0443]

[0444]

[0445] In summary, the design method provided in this application is beneficial to improving the dispersion of carbon nanotubes in the positive electrode film, enabling the carbon nanotubes to exert good conductivity, thereby reducing the DC internal resistance of the battery by improving the conductivity of the positive electrode sheet, and ultimately improving the cycle stability of the battery.

[0446] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium-ion secondary battery, characterized in that: Includes positive electrode plate, negative electrode plate and separator; The positive electrode includes a positive current collector and a positive film layer located on at least one side surface of the positive current collector; The positive electrode film layer includes a positive electrode active material, carbon nanotubes, and a first fluoropolymer. The first fluoropolymer includes structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers; The unsaturated carboxylic acid monomers include the following chemical structural formulas: In the chemical structural formula, R1 and R2 each independently include hydrogen and any one of C1-C3 alkyl groups; R3 includes any one or more of carboxyl and ester groups.

2. The lithium-ion secondary battery according to claim 1, characterized in that: R1 and R2 each independently include hydrogen.

3. The lithium-ion secondary battery according to any one of claims 1 to 2, characterized in that: The R3 includes any one of C1-C4 carboxyl groups or C1-C4 ester groups.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that: The unsaturated carboxylic acid monomers include either β-acryloyloxypropionic acid or succinic acid mono-2-(2-acryloyloxy)hydroxyethanol.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized in that: The weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million; and / or, Based on the total molar number of structural units in the first fluoropolymer, the molar content of structural units derived from unsaturated carboxylic acid monomers is 0.7% to 1.5%; and / or, The polydispersity index of the first fluoropolymer is 1.5 to 3.

5.

6. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized in that: The weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000; and / or, Based on the total molar number of structural units in the first fluoropolymer, the molar content of structural units derived from unsaturated carboxylic acid monomers is greater than 1.5% and less than 3.0%; and / or, The polydispersity index of the first fluoropolymer is 1.5 to 3.

5.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, characterized in that: The carbon nanotubes constitute 0.02% to 0.4% of the positive electrode film by mass; and / or, The mass percentage content of the first fluoropolymer in the positive electrode film layer is 0.02% to 2%.

8. The lithium-ion secondary battery according to any one of claims 1 to 7, characterized in that: The average diameter of the carbon nanotubes is ≤10nm.

9. The lithium-ion secondary battery according to claim 8, characterized in that: The average diameter of the carbon nanotubes is 1 nm to 5 nm.

10. The lithium-ion secondary battery according to any one of claims 1 to 9, characterized in that: The positive electrode film layer includes carbon black; The carbon black content in the positive electrode film layer is 0-2.5% by mass.

11. The lithium-ion secondary battery according to any one of claims 1 to 10, characterized in that: The compaction density of the positive electrode film is 3.4 g / cm³. 2 ~3.6g / cm 2 ; and / or, The film resistance of the positive electrode is <1Ω.

12. The lithium-ion secondary battery according to claim 11, characterized in that: The film resistance of the positive electrode is greater than 0.02Ω and less than 1Ω.

13. The lithium-ion secondary battery according to any one of claims 1 to 12, characterized in that: The positive electrode film layer includes a binder, and the binder contains ≤1.5% by mass in the positive electrode film layer; The adhesive includes one or more of polyvinylidene fluoride and its modified compounds, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, and hydrogenated nitrile rubber.

14. The lithium-ion secondary battery according to claim 13, characterized in that: The vinylidene fluoride and its modified compounds include a second fluoropolymer, the second fluoropolymer comprising structural units derived from vinylidene fluoride and structural units derived from acrylic acid monomers; and / or, The second fluoropolymer has a weight-average molecular weight of 1,000,000 to 1,200,000; and / or, Based on the total number of moles of structural units in the second fluoropolymer, the molar content of structural units derived from acrylic monomers is 1.4% to 1.6%.

15. The lithium-ion secondary battery according to any one of claims 1 to 14, characterized in that: The positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure.

16. The lithium-ion secondary battery according to any one of claims 1 to 15, characterized in that: The positive electrode active material includes one or more of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide; The Dv50 of the positive electrode active material is 2μm to 8μm.

17. A method for preparing a lithium-ion secondary battery according to claim 1, characterized in that: The process includes the following: Provide the first fluoropolymer adhesive; Preparation of carbon nanotube slurry: Disperse carbon nanotube powder and a first fluoropolymer liquid in a first organic solvent; Preparation of positive electrode slurry: The positive electrode active material and carbon nanotube slurry are dispersed in a second organic solvent; Preparation of positive electrode sheet: A positive electrode slurry is coated on at least one side surface of the positive electrode current collector to form a positive electrode film layer; Preparation of lithium-ion secondary battery: Assemble the positive electrode, negative electrode and separator together.

18. The method for preparing a lithium-ion secondary battery according to claim 17, characterized in that: In the carbon nanotube slurry, the mass ratio of the carbon nanotube powder to the first fluoropolymer is 1:(0.4-5).

19. The method for preparing a lithium-ion battery according to any one of claims 17 to 18, characterized in that: The aspect ratio of the carbon nanotubes is 1000 to 1000000.

20. The method for preparing a lithium-ion secondary battery according to any one of claims 17 to 19, characterized in that: The preparation of the positive electrode slurry further includes adding carbon black to the second organic solvent; and / or, The resistivity of the carbon black powder is <0.03 Ω·cm; and / or, The median particle size Dv50 of the carbon black is ≤30μm.

21. The method for preparing a lithium-ion secondary battery according to any one of claims 17 to 20, characterized in that: The weight-average molecular weight of the first fluoropolymer is 1.5 million to 9 million; and / or, The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 2% by mass solution, the viscosity of which is 1000 mPa·s to 6000 mPa·s.

22. The method for preparing a lithium-ion secondary battery according to any one of claims 17 to 20, characterized in that: The weight-average molecular weight of the first fluoropolymer is 50,000 to 200,000; and / or, The first fluoropolymer is dispersed in N-methylpyrrolidone to form a 7% by mass solution, wherein the viscosity of the soluble solution is greater than zero and less than or equal to 1000 mPa·s.

23. An electrical device, characterized in that: The lithium-ion secondary battery includes any one of claims 1 to 16 or any one of claims 17 to 22.

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