Fluorine-containing polymer and preparation method therefor, positive electrode sheet, secondary battery and electric device

Abstract

Provided in the present application are a fluorine-containing polymer and a preparation method therefor, a positive electrode sheet, a secondary battery and an electric device. The fluorine-containing polymer comprises a structural unit derived from a monomer as represented by formula I and a structural unit derived from a monomer as represented by formula II, wherein R1, R2 and R3 are each independently selected from hydrogen, fluorine, chlorine or a C1-3 alkyl containing at least one fluorine atom; R4, R5 and R6 are each independently selected from hydrogen and a substituted or unsubstituted C1-5 alkyl; and R7 comprises a substituted or unsubstituted C1-9 alkyl. The molar content of an α crystal form of the fluorine-containing polymer is larger than or equal to 80%, based on the total molar contents of the α crystal form, a β crystal form and a γ crystal form in the fluorine-containing polymer; and the crystallinity of the fluorine-containing polymer is 40-45%. When the fluorine-containing polymer is used as a binder, the flexibility of an electrode sheet can be improved, the slippage among positive electrode active material particles can be enhanced, and the compaction density of the positive electrode sheet can be advantageously increased.

Description

Fluoropolymer, preparation method, positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202310972762.7 filed on August 3, 2023, entitled "Fluoropolymer, preparation method, positive electrode sheet, secondary battery and electrical device", which is incorporated into this application in its entirety by reference. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a fluorine-containing polymer, a preparation method, a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the widespread use of secondary batteries, higher requirements have been placed on their energy density and other performance characteristics.

[0005] Binders are commonly used in secondary batteries and are in high demand in battery electrodes, separators, and packaging. To increase the energy density of secondary batteries, thick coatings of positive and negative electrode active materials are typically required, requiring the use of a large amount of binder. However, conventional binders lack sufficient flexibility during cycling, making the electrodes susceptible to brittle fracture and potentially causing safety issues. Therefore, the flexibility of binders still needs to be improved.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a fluorine-containing polymer. By adding the fluorine-containing polymer as a binder to the positive electrode slurry, the electrode can be thickly coated, and the electrode has high flexibility and is not prone to brittle fracture.

[0008] In order to achieve the above object, the present application provides a fluorine-containing polymer, which comprises a structural unit derived from a monomer represented by formula I and a structural unit derived from a monomer represented by formula II.

[0009] wherein R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl,

[0010] The molar content of the α crystal form of the fluoropolymer is greater than or equal to 80%, and the crystallinity of the fluoropolymer is 40%-45% based on the total molar content of the α crystal form, the β crystal form and the γ crystal form of the fluoropolymer.

[0011] Fluoropolymers contain ester groups, which have a softening effect. In addition, when the molar content of α crystal in the fluoropolymer is greater than or equal to 80% and the crystallinity is 40%-45%, using it as a binder can improve the flexibility of the electrode and will not cause brittle fracture when thickly coated.

[0012] In any embodiment, the molar content of the α-crystalline form of the fluoropolymer is 80% to 92%, based on the total molar content of the α-crystalline form, the β-crystalline form, and the γ-crystalline form of the fluoropolymer.

[0013] Controlling the molar content of the α-crystal within the above range can improve the bonding performance of the fluorinated polymer and improve the cycle performance of the pole piece without affecting the flexibility of the pole piece.

[0014] In any embodiment, R1 in Formula I is fluorine, R2 and R3 are each independently selected from one or more of hydrogen, fluorine, chlorine, and trifluoromethyl, and R5 and R6 are each independently selected from one or two of hydrogen and methyl.

[0015] The above-mentioned limitation on the types of substituents can improve the binding force of the positive electrode active material on the current collector.

[0016] In any embodiment, the molar content of the structural unit derived from the monomer represented by formula II is 5% to 15%, based on the total moles of the structural units of the fluorine-containing polymer.

[0017] In any embodiment, the weight average molecular weight of the fluorinated polymer is 800,000 to 1,000,000.

[0018] The weight-average molecular weight is in a moderate range, the viscosity rebound is small, it can better adhere to the active substance and improve the stability of the slurry.

[0019] In any embodiment, the monomer represented by formula I comprises one or more of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and hexafluoropropylene.

[0020] In any embodiment, the monomer represented by Formula II comprises one or more of methyl acrylate, ethyl acrylate, butyl acrylate, isopentyl acrylate, isooctyl acrylate, methyl methacrylate, and ethyl methacrylate.

[0021] In a second aspect of the present application, a method for preparing a fluorinated polymer is provided, comprising the following steps:

[0022] Polymerization: Under polymerizable conditions, the monomer represented by formula I and the monomer represented by formula II are polymerized to obtain an initial fluorine-containing polymer.

[0023] wherein R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl;

[0024] Recrystallization: Recrystallizing the initial fluoropolymer to obtain a fluoropolymer;

[0025] The molar content of the α crystal form of the fluoropolymer is greater than or equal to 80%. Based on the total molar content of the α crystal form, the β crystal form and the γ crystal form of the fluoropolymer, the crystallinity of the fluoropolymer is 40%-45%.

[0026] The fluorine-containing polymer prepared by the suspension method and recrystallization method in the present application has a high molar content of α crystal form. Using the fluorine-containing polymer prepared by the above method as a binder can significantly improve the flexibility of the pole piece and avoid brittle fracture of the pole piece during processing.

[0027] In any embodiment, the recrystallization comprises the steps of:

[0028] Heat treatment: heating and melting the initial fluoropolymer;

[0029] Crystallization: cooling the mixture to 130-150°C at a cooling rate of 40-60°C / min, maintaining the temperature for at least 2 hours, and cooling to room temperature to obtain a fluoropolymer.

[0030] By controlling the recrystallization conditions, the content of α crystal in the fluoropolymer can be effectively increased, making the electrode have good flexibility.

[0031] In any embodiment, the heat treatment process is specifically:

[0032] The initial fluoropolymer is heated to above 200°C at a rate of no more than 20°C / min and maintained at this temperature for at least 1 hour. Optionally, the temperature is 200°C-350°C, within which the fluoropolymer will not decompose.

[0033] Controlling the heat treatment conditions within the above range can completely melt the fluorine-containing polymer without affecting the subsequent crystallization process.

[0034] In the third aspect of the present application, a positive electrode plate is provided, comprising a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, a conductive agent and a binder, the binder comprising a fluoropolymer according to the first aspect of the present application or a fluoropolymer prepared by the preparation method of the second aspect.

[0035] In any embodiment, the positive electrode active material is one or more of lithium-containing phosphates and their modified materials. The modified material is prepared by one or more modification methods of doping, conductive carbon coating, conductive metal coating, and conductive polymer coating, and can be selected from one or more of lithium iron phosphate and its modified materials.

[0036] In any embodiment, the compacted density of the positive electrode film is 2.6 g / cm 3 -2.8g / cm 3 .

[0037] The positive electrode film layer prepared using the fluorine-containing polymer of the present application can obtain the above-mentioned compaction density, which helps to improve the energy density of the positive electrode plate.

[0038] In any embodiment, the bonding force per unit length between the positive electrode film layer and the positive electrode current collector is not less than 20 N / m.

[0039] In any embodiment, after the positive electrode plate undergoes a bending test of not less than 3 times, the positive electrode plate becomes light-transmissive.

[0040] In a fourth aspect of the present application, a secondary battery is provided, comprising a separator, a negative electrode plate, an electrolyte, and the positive electrode plate of the third aspect of the present application.

[0041] In a fifth aspect of the present application, an electrical device is provided, comprising the secondary battery according to the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0043] FIG2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

[0044] 3 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application;

[0045] FIG4 is a graph showing the cycle performance test of Examples 1 and 14 of the present application;

[0046] Reference numerals:

[0047] 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0048] Below, the embodiments of the fluorinated polymer, preparation method, positive electrode sheet, secondary battery and electric device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0049] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0051] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0052] Unless otherwise specified, all steps of the present 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 may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0053] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0054] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0055] Cathode materials such as lithium iron phosphate and lithium manganese oxide have been widely used in secondary batteries due to their low cost and good safety. However, with the widespread use of secondary batteries, higher requirements have been placed on their energy density. Currently, a common method for increasing the energy density of secondary batteries is to thicken the positive electrode coating, but this requires the use of more binder. One of the most widely used binders in secondary batteries is polyvinylidene fluoride. Increasing the amount of polyvinylidene fluoride increases the brittleness of the electrode, resulting in poor processing performance and prone to cracking and brittle fracture during cycling.

[0056] [Fluoropolymer]

[0057] Based on this, the present application proposes a fluorine-containing polymer, which comprises a structural unit derived from a monomer represented by formula I and a structural unit derived from a monomer represented by formula II.

[0058] wherein R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl,

[0059] The molar content of the α crystal form of the fluoropolymer is greater than or equal to 80%. Based on the total molar content of the α crystal form, the β crystal form and the γ crystal form of the fluoropolymer, the crystallinity of the fluoropolymer is 40%-45%.

[0060] In the α-crystal form, F atoms are arranged helically around the molecular chain, facilitating the slippage of the positive electrode active material particles and enhancing flexibility. This increases coating thickness and compaction density, thereby improving the energy density of the positive electrode. Fluoropolymers with a crystallinity above 45% exhibit poor electrode flexibility. Fluoropolymers with a crystallinity below 40% readily absorb electrolyte fluid, swell, and even dissolve, impairing the coating's adhesion. Controlling the fluoropolymer's crystallinity within the above range can achieve a balance between electrode flexibility and coating adhesion.

[0061] In some embodiments, the molar content of the α crystal form of the fluoropolymer is 80%-100%, based on the total molar content of the α crystal form, the β crystal form and the γ crystal form in the fluoropolymer; in some embodiments, the molar content of the α crystal form of the fluoropolymer is 80%-85%, 85%-90%, 90%-92%, or 92%-100%, based on the total molar content of the α crystal form, the β crystal form and the γ crystal form in the fluoropolymer.

[0062] In this context, the term "polymer" includes, on the one hand, chemically uniform macromolecular aggregates prepared by polymerization reactions but differing in degree of polymerization, molar mass and chain length, and, on the other hand, derivatives of such macromolecular aggregates formed by polymerization reactions, i.e. compounds that can be obtained by reactions of functional groups in the above-mentioned macromolecules, such as addition or substitution, and which can be chemically uniform or chemically heterogeneous.

[0063] As used herein, the term "fluoropolymer" refers to a macromolecular aggregate formed by copolymerization of a fluorine-containing monomer and an ester group-containing monomer, or a derivative thereof.

[0064] In this article, the term “C 1-3 "Alkyl" refers to a straight or branched chain alkane radical consisting solely of carbon and hydrogen atoms, with no unsaturation present, having one, two, or three carbon atoms, and attached to the remainder of the molecule by a single bond. 1-3 Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, and 1-methylethyl (isopropyl).

[0065] As used herein, the term "substituted" means that at least one hydrogen atom of the compound or chemical moiety is replaced by another chemical moiety with a substituent, wherein the substituents are each independently selected from the group consisting of: hydroxyl, thiol, amino, cyano, nitro, aldehyde, halogen, alkenyl, alkynyl, aryl, heteroaryl, C 1-6 Alkyl, C 1-6Alkoxy.

[0066] In this article, the term “C 1-5 "Alkyl" refers to a straight or branched chain alkane radical consisting solely of carbon and hydrogen atoms, with no unsaturation present, having one, two, three, four, or five carbon atoms, and attached to the remainder of the molecule by a single bond. 1-5 Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, 1-methylethyl (isopropyl), n-butyl, dimethylpropyl (isobutyl), n-pentyl, isopentyl (2-methylbutyl).

[0067] In this article, the term “C 1-9 "Alkyl" refers to a straight or branched chain alkane radical consisting solely of carbon and hydrogen atoms, with no unsaturation present, having one, two, three, four, five, six, seven, eight or nine carbon atoms, and attached to the remainder of the molecule by a single bond. 1-9 Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, 1-methylethyl (isopropyl), n-butyl, 2-methylpropyl (isobutyl), n-pentyl, 2-methylbutyl (isopentyl), n-hexyl, 2-methylpentyl (isohexyl), n-heptyl, 2-methylhexyl (isoheptyl), n-octyl, and 2-methylheptyl (isooctyl).

[0068] In this article, the term "α crystal form" refers to a crystal form in which -CH2-CF2- units in the molecular chain are arranged alternately and in antiparallel space, which is a TGTG' conformation, has a net dipole moment of 0, is not externally polar, belongs to the monoclinic crystal system, and has unit cell parameters: a = 0.496nm, b = 0.964nm, c = 0.462nm.

[0069] In this article, the term "β crystal form" refers to a crystal form in which the molecular chain is planar zigzag, which is a TTTT all-trans conformation. It has a large polarity, good piezoelectricity and ferroelectricity, belongs to the orthorhombic crystal system, and the unit cell parameters are: a = 0.858 nm, b = 0.491 nm, c = 0.256 nm.

[0070] In this article, the term "γ crystal form" refers to the TTTGTTTG' conformation, which has a certain polarity and its unit cell parameters are: a=0.496 nm, b=0.967 nm, c=0.920 nm.

[0071] As used herein, the term "crystallinity" refers to the proportion of crystalline regions in a polymer.

[0072] In some embodiments, the molar content of the α-crystalline form of the fluoropolymer is 80%-92%, based on the total molar content of the α-crystalline form, the β-crystalline form, and the γ-crystalline form of the fluoropolymer. In some embodiments, the molar content of the α-crystalline form of the fluoropolymer is 80%-82%, 82%-84%, 84%-86%, 86%-88%, 88%-90%, 80%-85%, or 85%-92%, based on the total molar content of the α-crystalline form, the β-crystalline form, and the γ-crystalline form of the fluoropolymer.

[0073] By controlling the molar content of the α-crystal in the fluorine-containing polymer within the above range, the flexibility of the electrode can be improved while increasing the bonding force between the positive electrode film layer and the current collector, thereby improving the cycle performance of the positive electrode.

[0074] In some embodiments, R1 in Formula I is fluorine, R2 and R3 are each independently selected from one or more of hydrogen, fluorine, chlorine, and trifluoromethyl, and R5 and R6 are each independently selected from one or two of hydrogen and methyl.

[0075] The above-mentioned limitation on the types of substituents can improve the binding force of the positive electrode active material on the current collector.

[0076] In some embodiments, the molar content of the structural unit derived from the monomer represented by formula II is 5%-15%, based on the total moles of the structural units of the fluoropolymer. In some embodiments, the molar content of the structural unit derived from the monomer represented by formula II is 5%-7%, 7%-9%, 9%-11%, 11%-13%, 13%-15%, 5%-10%, or 10%-15%, based on the total moles of the structural units of the fluoropolymer.

[0077] Controlling the molar content of the structural unit derived from the monomer represented by Formula II within the above range can make the fluorine-containing polymer have better adhesion and help improve the flexibility of the pole piece.

[0078] In some embodiments, the weight average molecular weight of the fluoropolymer is 800,000-1,000,000. In some embodiments, the weight average molecular weight of the fluoropolymer is 800,000-850,000, 850,000-900,000, 900,000-950,000, 950,000-1,000,000, 800,000-900,000, or 900,000-1,000,000.

[0079] As used herein, the term "weight average molecular weight" refers to the sum of the weight fractions of molecules of different molecular weights in a polymer multiplied by their corresponding molecular weights.

[0080] Controlling the molecular weight of the fluorine-containing polymer within the above range can enable the fluorine-containing polymer to have good adhesion and at the same time be uniformly dispersed in the positive electrode slurry without affecting the flexibility of the electrode sheet.

[0081] In some embodiments, the monomer represented by Formula I comprises one or more of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and hexafluoropropylene.

[0082] In some embodiments, the monomer represented by formula II comprises one or more of methyl acrylate, ethyl acrylate, butyl acrylate, isopentyl acrylate, isooctyl acrylate, methyl methacrylate, and ethyl methacrylate.

[0083] In one embodiment of the present application, a method for preparing a fluorine-containing polymer is provided, comprising the following steps:

[0084] Polymerization: Under polymerizable conditions, the monomer represented by formula I and the monomer represented by formula II are polymerized to obtain an initial fluorine-containing polymer.

[0085] wherein R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl;

[0086] Recrystallization: Recrystallizing the initial fluorine-containing polymer to obtain a fluorine-containing polymer;

[0087] The molar content of the α crystal form of the fluoropolymer is greater than or equal to 80%, based on the total molar content of the α crystal form, the β crystal form and the γ crystal form of the fluoropolymer; the crystallinity of the fluoropolymer is 40%-45%.

[0088] A fluorine-containing polymer with a high α-crystal content can be prepared by combining a suspension method with a recrystallization method. The method is simple, uses cheap raw materials, and is low-cost.

[0089] In some embodiments, the recrystallization comprises the following steps:

[0090] Heat treatment: heating and melting the initial fluorine-containing polymer;

[0091] Crystallization: cooling the mixture to 130° C.-150° C. at a cooling rate of 40° C. / min-60° C. / min, maintaining the temperature for at least 2 hours, and cooling the mixture to room temperature to obtain the fluorine-containing polymer.

[0092] The above-mentioned limitation on the crystallization conditions can increase the content of α crystal in the product, which helps to improve the flexibility of the electrode.

[0093] In some embodiments, the heat treatment process is specifically as follows:

[0094] The initial fluoropolymer is heated to above 200° C. at a rate not higher than 20° C. / min and maintained at this temperature for at least 1 hour.

[0095] The heating rate is controlled within the above range to prevent the polymer from decomposing due to excessive local temperature rise. The heat treatment under the above conditions can completely melt the initial fluorine-containing polymer.

[0096] In one embodiment of the present application, the fluorine-containing polymer may be used in a secondary battery. In some embodiments, the secondary battery includes at least one of a lithium ion battery, a sodium ion battery, a magnesium ion battery, and a potassium ion battery.

[0097] [Positive electrode]

[0098] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, a conductive agent and a binder, and the binder includes a fluoropolymer in some embodiments or a fluoropolymer prepared by the preparation method in some embodiments.

[0099] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0100] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0101] In some embodiments, the positive electrode active material may be a positive electrode active material known in the art for secondary batteries. As an example, the positive electrode active material may include one or more lithium-containing phosphates and their modified materials, wherein the modified materials are prepared by one or more modification methods selected from the group consisting of doping, conductive carbon coating, conductive metal coating, and conductive polymer coating. In some embodiments, the positive electrode active material is one or more lithium iron phosphate and its modified materials. Using the fluoropolymer described herein as a binder can promote the slippage of the lithium-containing phosphate and its modified materials during processing, thereby improving the flexibility of the electrode sheet.

[0102] In some embodiments, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0103] In some embodiments, the compaction density of the positive electrode film layer is 2.6 g / cm 3 -2.8g / cm 3 .

[0104] In this article, the "compacted density" refers to the ratio of the coating surface density to the thickness of the positive electrode film layer, and the "coating surface density" refers to the ratio of the weight of the positive electrode film layer after coating, drying and rolling to the coating area.

[0105] In some embodiments, the bonding force per unit length between the positive electrode film layer and the positive electrode current collector is not less than 20 N / m.

[0106] In some embodiments, after the positive electrode plate undergoes no less than three bending tests, the positive electrode plate becomes light-transmissive.

[0107] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode film layer, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, a positive electrode sheet is obtained.

[0108] [Negative electrode]

[0109] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material, a conductive agent and a binder.

[0110] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0111] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0112] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0113] In some embodiments, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0114] In some embodiments, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0115] In some embodiments, the negative electrode film layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0116] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0117] [Electrolytes]

[0118] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0119] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0120] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0121] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0122] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0123] [Isolation film]

[0124] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0125] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven 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.

[0126] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0127] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0128] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0129] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 having a square structure as an example.

[0130] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0131] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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 thereto.

[0132] Figure 3 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0133] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0134] Example

[0135] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0136] 1. Preparation method

[0137] Example 1

[0138] 1) Preparation of fluorinated polymers

[0139] The polymerization process includes: adding 30 kg of deionized water (with a conductivity of less than or equal to 2 μs / cm) and 10 g of hydroxyethyl cellulose to a reactor in sequence, and closing the reactor; evacuating the reactor and filling it with nitrogen, and repeating the operation until the oxygen concentration in the reactor is less than 100 ppm; introducing vinylidene fluoride monomer into the reactor until the pressure in the reactor reaches 5.0 MPa; raising the temperature in the reactor to 50° C., adding 50 g of diisopropyl peroxydicarbonate and 100 g of acetone to start the reaction, and continuously introducing vinylidene fluoride monomer and methyl methacrylate monomer in proportion to maintain the reaction pressure in the reactor constant during the reaction, wherein the mass of the introduced vinylidene fluoride is 4262.4 g and the mass of the introduced methyl methacrylate is 740 g; the time for adding all the monomers is 10 hours; the reaction is continued for 8 hours after all the monomers are added; when the reaction is completed, the pressure in the reactor reaches 0.2 MPa, and the unreacted vinylidene fluoride monomer is recovered; the reaction product is centrifuged, washed, and dried to obtain an initial fluoropolymer.

[0140] Recrystallization: 50 g of the initial fluoropolymer powder was added to a 250 ml corundum crucible; the crucible was placed in a muffle furnace, the gas in the furnace was replaced with nitrogen, and the furnace was kept closed; the temperature was increased to 230 ° C at a rate of 5 ° C / min, and maintained for 1 hour to eliminate the thermal history; the temperature was decreased to 140 ° C at a rate of 45 ° C / min, and the temperature was kept constant for 3 hours; the product was removed from the muffle furnace and cooled to room temperature to obtain a fluoropolymer.

[0141] 2) Preparation of positive electrode sheet

[0142] Lithium iron phosphate (LiFePO4), conductive carbon black, the fluorinated polymer of Example 1, polyvinyl pyrrolidone dispersant, and N-methyl pyrrolidone (NMP) were stirred in a weight ratio of 97:0.7:2:0.3:38, and the solid content of the positive electrode slurry was 62%. The positive electrode slurry was then evenly coated on the positive electrode current collector, and then dried, cold pressed, and cut to obtain a positive electrode sheet, wherein the coating surface density was 21 mg / cm 2 , compacted density is 2.45g / cm 3 .

[0143] 3) Preparation of negative electrode sheet

[0144] The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water in a weight ratio of 96.2:0.8:0.8:1.2, and mixed evenly to prepare a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0145] 4) Isolation film

[0146] Polypropylene film is used as the isolation film.

[0147] 5) Preparation of electrolyte

[0148] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3 / 7, LiPF6 lithium salt was added and dissolved in the organic solvent, the mass content of LiPF6 in the solution was 12.5%, and stirred evenly to obtain the electrolyte of Example 1.

[0149] 6) Preparation of batteries

[0150] The positive electrode sheet, separator, and negative electrode sheet of Example 1 were stacked in order, with the separator positioned between the positive and negative electrode sheets to provide isolation. The cells were then wound to obtain a bare cell. The tabs were welded to the bare cell and placed in an aluminum casing. The cells were then baked at 80°C to remove moisture. The electrolyte was then injected and sealed to obtain an uncharged battery. The uncharged battery then underwent a series of steps, including resting, hot and cold pressing, formation, shaping, and capacity testing, to obtain the lithium-ion battery product of Example 1.

[0151] In Examples 2-4, the cooling rate during the recrystallization process was adjusted to thereby adjust the α-crystal molar content and crystallinity of the fluoropolymer. Other parameters remained the same as in Example 1. Specific parameters are shown in Table 1.

[0152] In Examples 5-8, the molar content of the structural unit derived from methyl methacrylate in the fluoropolymer was adjusted, and the molar content of the structural unit derived from vinylidene fluoride in the fluoropolymer was changed accordingly. Other parameters remained the same as in Example 1. Specific parameters are shown in Table 1.

[0153] In Examples 9-12, the polymerization temperature and the mass of the initiator diisopropyl peroxydicarbonate were adjusted to prepare the fluoropolymers, so that the fluoropolymers had different weight-average molecular weights. The parameters are as follows; other parameters were consistent with those in Example 1. Specific parameters are shown in Table 1.

[0154] In Example 9, the polymerization reaction temperature was adjusted to 55° C., and the mass of diisopropyl peroxydicarbonate was adjusted to 55 g;

[0155] In Example 10, the polymerization reaction temperature was adjusted to 52.5° C., and the mass of diisopropyl peroxydicarbonate was adjusted to 52.5 g;

[0156] In Example 11, the polymerization reaction temperature was adjusted to 47.5° C., and the mass of diisopropyl peroxydicarbonate was adjusted to 47.5 g;

[0157] In Example 12, the polymerization reaction temperature was adjusted to 45° C., and the mass of diisopropyl peroxydicarbonate was adjusted to 45 g.

[0158] In Example 13, methyl methacrylate was replaced with ethyl acrylate, and other parameters remained the same as in Example 1. Specific parameters are shown in Tables 1 and 2.

[0159] In Example 14, the positive electrode active material lithium iron phosphate is replaced by LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), other parameters are consistent with those in Example 1, and specific parameters are shown in Tables 1 and 2.

[0160] In Comparative Example 1, polyvinylidene fluoride was used as the fluorine-containing polymer, and other parameters were the same as those in Example 1. For specific parameters, see Table 1.

[0161] In Comparative Example 2, the cooling rate, crystallization temperature, and crystallization time during the recrystallization process were adjusted to thereby adjust the α-crystal molar content and crystallinity of the fluoropolymer. Other parameters were the same as those in Example 1. For specific parameters, see Table 1.

[0162] In Comparative Example 3, the cooling rate, crystallization temperature and crystallization time during the recrystallization process were adjusted to adjust the α-crystal molar content and crystallinity of the fluoropolymer. Other parameters were the same as those in Example 1. For specific parameters, see Table 1.

[0163] 2. Test Method

[0164] 1. Characterization of Fluoropolymer Properties

[0165] (1) Weight average molecular weight of fluorinated polymer

[0166] A Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141) was used. A polystyrene solution sample with a mass fraction of 3.0% was used as a reference, and a matching chromatographic column was selected (oily: Styragel HT5 DMF 7.8×300 mm + Styragel HT4). A 3.0% fluoropolymer solution was prepared using purified N-methylpyrrolidone (NMP) solvent. The prepared solution was allowed to stand for one day before use. During the test, tetrahydrofuran was first drawn into a syringe and rinsed, and repeated several times. Then 5 mL of the experimental solution was drawn, the air in the syringe was expelled, and the needle tip was wiped dry. Finally, the sample solution was slowly injected into the injection port. After the display stabilized, the data was acquired and the weight-average molecular weight was read.

[0167] (2) Molar content of α-crystal of fluoropolymer

[0168] The molar content of α-crystal was tested by FTIR. Specifically, the FTIR test instrument was a Nieolet 5700 infrared spectrometer, which was scanned 32 times. The wave number range of the spectrum was 700-4000 cm-1 From the obtained spectrum, the characteristic absorption wave number of α crystal form is 795cm - 1 The absorption intensity of the β-crystal is 839 cm -1 The absorption intensity and the wave number 811 cm-1, which is the characteristic absorption of γ-type -1 The absorption intensity of each peak corresponding to the above wavenumber is obtained by connecting the starting and ending points of the formation with a straight line. The intersection of this straight line and the peak wavenumber is set as A, the point where the spectrum intersects the peak wavenumber is set as B, and the point where the transmittance at the peak wavenumber is 0% is set as C. The lengths between AC and BC are calculated, and Log(AC / BC) is used as the absorption intensity at each wavenumber.

[0169] The molar content of α crystal form is as described above, and the wave number 795 cm corresponding to α crystal form, β crystal form and γ crystal form is calculated. -1 、wave number 839cm -1 and wave number 811cm -1 The absorption intensity is calculated by substituting it into the following formula:

[0170] Molar content of α-crystal = 795 cm -1 Absorption intensity / (795cm -1 Absorption intensity +839cm -1 Absorption intensity +811cm -1 absorption intensity)×100%.

[0171] (3) Crystallinity of fluoropolymers

[0172] The crystallinity was measured using a differential scanning calorimeter (DSC) of the American TA Instruments model Discovery 250. Specifically, the fluoropolymer was dissolved in N-methylpyrrolidone solution to prepare a 10% fluoropolymer mass fraction adhesive solution, which was then dried at 100°C for 2 days. The adhesive film was then cut into small pieces of 2 cm*2 cm and placed in an aluminum crucible. The pieces were shaken flat and the crucible lid was closed. In a nitrogen atmosphere, the purge gas was blown at a rate of 50 mL / min, the protective gas flow rate was 70 mL / min, the heating rate was set to 10°C / min, and the test temperature range was -100°C-400°C. This test can obtain the DSC / (Mw / mg) temperature variation curve of the film, and integrate the peak area to calculate the melting enthalpy ΔH (J / g) of the film. The film crystallinity = ΔH / (ΔHm100%) * 100%, where ΔHm100% is the standard melting enthalpy (crystalline melting heat) of PVDF, ΔHm100% = 104.7 J / g.

[0173] 2. Property test of positive electrode

[0174] (1) Flexibility

[0175] The cold-pressed positive electrode sheet is cut into test specimens of 20mm×100mm size; after being folded in half forward, it is flattened with a 2kg roller, and unfolded to check whether there is light transmittance through the gap. If no light transmittance occurs, it is folded in half in the opposite direction, flattened with a 2kg roller, and checked again against the light. The test is repeated three times. If no light transmittance occurs, it indicates that the positive electrode sheet has a certain degree of flexibility.

[0176] (2) Adhesion

[0177] The battery was charged and discharged at 1C for 100 cycles at 25°C. The battery was disassembled at full discharge, and the cathode electrode was removed. The cathode electrode was rinsed three times in dimethyl carbonate and dried. The electrode was then subjected to an adhesion test.

[0178] With reference to the national standard GB / T 2790-1995 "Test method for 180° peel strength of adhesives", the adhesion test process of the examples and comparative examples of the present application is as follows:

[0179] Use a blade to cut a 30mm wide and 100-160mm long electrode sample. Apply a 20mm wide and 90-150mm long double-sided tape to the steel plate. Attach the previously cut electrode sample to the double-sided tape with the test surface facing down. Then use a roller to roll the sample three times in the same direction.

[0180] Insert a paper tape with a width equal to that of the electrode sample and a length of 250 mm under the electrode current collector and fix it with wrinkle glue.

[0181] Turn on the Sansi tensile testing machine (sensitivity 1N), the indicator light will illuminate, adjust the limit block to the appropriate position, and secure the end of the steel plate specimen without the electrode with the lower clamp. Fold the paper tape upward and secure it with the upper clamp. Use the "up" and "down" buttons on the manual controller included with the tensile testing machine to adjust the position of the upper clamp, then perform the test and read the value. The force at the electrode when the force is balanced is divided by the width of the tape as the bonding force per unit length of the electrode, which represents the bonding strength between the positive electrode film layer and the current collector.

[0182] (3) Maximum compaction density

[0183] Cut the positive electrode into test specimens of 20mm*100mm size for later use. Bend the electrode and fold it in half and fix it, use a 2kg rolling roller to roll it once, check whether the folded part of the electrode is translucent and leaks metal. If not, fold the electrode in half and fix it, use the same method to roll it once, check whether the folded part of the electrode is translucent and leaks metal. If it is not translucent and leaks metal after three times of repeated testing, reduce the thickness of the electrode, increase the compaction density, and retest until the electrode is translucent and leaks metal after being folded and rolled three times. The compaction density of the positive electrode that does not show any translucent or metal leakage is taken as the maximum compaction density. The initial compaction density of the embodiment and the comparative example is the same. The compaction density increases by 0.02g / cm for each test. 3 .

[0184] 3. Battery

[0185] (1) Analysis of cyclic diving phenomenon

[0186] The test procedure for analyzing battery cycle discharge is as follows: At 25°C, the prepared battery is charged at a constant current of 1 / 3C to 4.3V, then charged at a constant voltage of 4.3V to a current of 0.05C, left for 5 minutes, and then discharged at 1 / 3C to 2.8V. The resulting capacity is recorded as the initial capacity C0. These steps are repeated for the same battery, and the battery's discharge capacity Cn after the nth cycle is recorded. The battery capacity retention rate after each cycle is calculated as Pn = Cn / C0 × 100%. A graph of battery capacity retention rate versus cycle number is generated, with capacity retention rate as the vertical axis and the corresponding cycle number as the horizontal axis.

[0187] 3. Analysis of test results of various embodiments and comparative examples

[0188] The fluorine-containing polymers, positive electrodes and batteries of the embodiments and comparative examples were prepared according to the above method, and various properties were measured. The flexibility test result Y represents opacity after three repetitions. The specific results are shown in Tables 1 and 2 below.

[0189] Table 1

[0190] Table 2

[0191] From the above test results, it can be seen that the binders in Examples 1-14 are all fluoropolymers described in this application, and the maximum compaction density of the positive electrode film prepared by the fluoropolymer can reach 2.6 g / cm 3 The above has good flexibility, can meet the requirements of thick coating, and helps to improve the energy density of the positive electrode.

[0192] The binder used in Comparative Example 1 is polyvinylidene fluoride, and the molar content of the α-crystal is too low, which cannot effectively improve the flexibility of the positive electrode film layer. The maximum compaction density is low, and the improvement effect on energy density is weak. The binder used in Comparative Example 2 is vinylidene fluoride-methyl methacrylate copolymer, and its molar content of the α-crystal is also less than 80%, and the maximum compaction density cannot reach 2.5g / cm 3 The above also cannot effectively improve the energy density of the positive electrode. The crystallinity of the fluorine-containing polymer in Comparative Example 3 is not within the range defined in this application, and the bonding force is extremely low.

[0193] In addition, by comparing the test results of Examples 1-4, it can be found that when the molar content of the α crystal is higher than 92%, the bonding force is significantly reduced. This result shows that when the molar content of the α crystal in the fluoropolymer is 80%-92%, the positive electrode sheet has good flexibility, is suitable for thick coating, and improves the compaction density. There is good bonding between the positive electrode film layer and the positive electrode current collector, and the cycle stability is higher.

[0194] By comparing the test results of Example 1 and Examples 5-8, it can be found that the content of Formula II in Example 5 is relatively low, the maximum compaction density is low, and the flexibility of the positive electrode sheet is not as good as that of Example 1 and Examples 6-8. The content of Formula II in Example 8 is higher and the bonding force is relatively low. This result shows that when the content of Formula II is 5%-15%, the positive electrode sheet has both good flexibility and bonding force.

[0195] Comparing the test results of Example 1 and Examples 9-12, it can be found that the molecular weight of the fluoropolymer in Example 9 is relatively low, and its bonding force is lower than 26 N / m, while the molecular weight of the fluoropolymer in Example 12 is relatively high, and its maximum compacted density is 2.7 g / cm 3 The above results show that when the weight average molecular weight of the fluorinated polymer is 800,000-1,000,000, a positive electrode sheet with good flexibility and adhesion can be prepared, and the battery has a high energy density and good cycle performance.

[0196] The formula II of the fluorine-containing polymer described in Example 13 is ethyl acrylate. The bonding strength and maximum compaction density of the prepared positive electrode sheet are slightly lower than those in Example 1, but it still has good bonding performance and is more flexible than conventional positive electrode sheets.

[0197] The positive electrode active material in Example 14 is a ternary positive electrode material, and its maximum compaction density is significantly higher than that of other examples, and its flexibility is excellent, but its adhesion is significantly lower than that of Example 1. Figure 4 is a cycle performance test diagram of Example 1 and Example 14 of the present application. It can be seen from the figure that Example 14 has a diving phenomenon after about 800 cycles. This result shows that the type of positive electrode active material will have a certain impact on the adhesion and flexibility of the positive electrode sheet. For the ternary positive electrode material, the molar content of α crystal form in the fluorine-containing polymer is greater than 80%, which will weaken the adhesion between the positive electrode film layer and the positive electrode current collector. During the cycle, the positive electrode film layer will fall off from the surface of the positive electrode current collector, resulting in a cycle capacity diving phenomenon. However, its flexibility is excellent, and the maximum compaction density can reach 3.5g / cm 3 ,This result shows that the fluorine-containing polymer described in this application can be used to thickly coat the ternary positive electrode material.

[0198] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A fluorine-containing polymer, characterized in that comprising a structural unit derived from a monomer represented by formula I and a structural unit derived from a monomer represented by formula II, Wherein, R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl, The molar content of the α-crystal form of the fluorine-containing polymer is greater than or equal to 80%, based on the total molar content of the α-crystal form, the β-crystal form and the γ-crystal form of the fluorine-containing polymer; the crystallinity of the fluorine-containing polymer is 40%-45%.

2. The fluorinated polymer according to claim 1, characterized in that The molar content of the α-crystal form of the fluorine-containing polymer is 80%-92%, based on the total molar content of the α-crystal form, the β-crystal form and the γ-crystal form of the fluorine-containing polymer.

3. The fluorine-containing polymer according to claim 1 or 2, characterized in that In the formula I, R1 is fluorine, R2 and R3 are each independently selected from one or more of hydrogen, fluorine, chlorine, and trifluoromethyl, and R5 and R6 are each independently selected from one or two of hydrogen and methyl.

4. The fluorine-containing polymer according to claim 3, characterized in that The molar content of the structural unit derived from the monomer represented by formula II is 5% to 15%, based on the total molar number of the structural units of the fluorine-containing polymer.

5. The fluorine-containing polymer according to claim 1 or 2, characterized in that The weight average molecular weight of the fluorine-containing polymer is 800,000-1,000,000.

6. The fluorine-containing polymer according to claim 1 or 2, characterized in that The monomer represented by formula I comprises one or more of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and hexafluoropropylene.

7. The fluorine-containing polymer according to claim 1, characterized in that The monomer represented by formula II comprises one or more of methyl acrylate, ethyl acrylate, butyl acrylate, isopentyl acrylate, isooctyl acrylate, methyl methacrylate and ethyl methacrylate.

8. A method for preparing a fluorine-containing polymer, characterized in that: The following steps are involved: Polymerization: Under polymerizable conditions, the monomers represented by formula I and the monomers represented by formula II are polymerized to obtain an initial fluorine-containing polymer, Wherein, R1, R2, and R3 are each independently selected from hydrogen, fluorine, chlorine, or a C 1-3 Alkyl, R4, R5, R6 are each independently selected from hydrogen, substituted or unsubstituted C 1- 5 alkyl, R7 contains substituted or unsubstituted C 1-9 alkyl; Recrystallization: Recrystallizing the initial fluorine-containing polymer to obtain a fluorine-containing polymer; The molar content of the α-crystal form of the fluorine-containing polymer is greater than or equal to 80%, based on the total molar content of the α-crystal form, the β-crystal form and the γ-crystal form of the fluorine-containing polymer; the crystallinity of the fluorine-containing polymer is 40%-45%.

9. The preparation method according to claim 8, characterized in that: The recrystallization The following steps are involved: Heat treatment: heating and melting the initial fluorine-containing polymer; Crystallization: cooling to 130°C-150°C at a cooling rate of 40°C / min-60°C / min, keeping warm for at least 2 hours, and cooling to room temperature to obtain the fluorine-containing polymer.

10. The preparation method according to claim 9, characterized in that: The heat treatment process is specifically as follows: The initial fluorine-containing polymer is heated to above 200° C. at a rate not higher than 20° C. / min and maintained at this temperature for at least 1 hour.

11. A positive electrode sheet, characterized in that: The invention comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises a positive electrode active material, a conductive agent and a binder, and the binder comprises a fluorine-containing polymer according to any one of claims 1 to 7 or a fluorine-containing polymer prepared by the preparation method according to any one of claims 8 to 10.

12. The positive electrode sheet according to claim 11, characterized in that: The positive electrode active material is one or more of lithium-containing phosphates and modified materials thereof, and the modified materials are prepared by one or more modification methods including doping, conductive carbon coating, conductive metal coating, and conductive polymer coating.

13. The positive electrode sheet according to claim 12, characterized in that: The positive electrode active material is one or more of lithium iron phosphate and its modified materials.

14. The positive electrode sheet according to any one of claims 11 to 13, characterized in that: The compaction density of the positive electrode film layer is 2.6 g / cm 3 -2.8g / cm 3 .

15. The positive electrode sheet according to any one of claims 11 to 13, characterized in that: The bonding force per unit length between the positive electrode film layer and the positive electrode current collector is not less than 20 N / m.

16. The positive electrode sheet according to any one of claims 11 to 13, characterized in that: After the positive electrode plate has been subjected to no less than three bending tests, light is translucent in the positive electrode plate.

17. A secondary battery, characterized in that: The invention comprises a separator, a negative electrode sheet, an electrolyte and a positive electrode sheet as claimed in any one of claims 11 to 16.

18. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 17.

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