Fluorine-containing polymer, preparation method thereof, positive pole piece, secondary battery and electric device

Abstract

The invention provides a fluorine-containing polymer, a preparation method thereof, a positive pole piece, a secondary battery and an electric device. The fluorine-containing polymer comprises a structural unit derived from vinylidene fluoride and a structural unit derived from an unsaturated carboxylic acid monomer, and the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and can be 5 million to 8 million optionally.

Description

Fluorine-containing polymer, preparation method thereof, positive electrode sheet, secondary battery and electrical device Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a fluorine-containing polymer, a preparation method of the fluorine-containing polymer, a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0002] 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 electrochemical performance and cycle stability.

[0003] Binders are commonly used in secondary batteries and are in high demand in battery electrodes, separators, and packaging. However, the use of binders in existing technologies increases the rigidity of the electrode film layer, reduces its flexibility, and is detrimental to improving the battery's electrochemical performance. Therefore, existing binders still need to be improved.

[0004] Summary of the Invention

[0005] The present application is made in view of the above-mentioned problems, and its purpose is to provide a fluorine-containing polymer that can improve the flexibility and ultimate compaction density of the electrode, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0006] In order to achieve the above-mentioned purpose, the present application provides a fluorine-containing polymer, which comprises structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers, and the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and can be optionally 5 million to 8 million.

[0007] Compared with the polymer binders in the prior art, this fluorine-containing polymer can maintain the bonding strength of the pole piece at a low addition amount while reducing the stiffness of the pole piece film layer, improving the flexibility of the pole piece, and reducing the battery impedance, thereby comprehensively improving the battery's ultimate volume energy density and cycle capacity retention rate.

[0008] The fluorinated polymer with a weight-average molecular weight of 5 million to 8 million can more effectively reduce battery impedance while taking into account the battery's ultimate volume energy density and cycle capacity retention rate.

[0009] In any embodiment, the molar content of the structural unit derived from the unsaturated carboxylic acid monomer is 0.5% to 2.5%, optionally 0.7% to 2.0%, based on the total moles of the structural units in the fluorine-containing polymer.

[0010] Fluoropolymers within the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0011] Based on the total molar number of structural units in the fluoropolymer, a fluoropolymer having a molar content of structural units derived from unsaturated carboxylic acid monomers of 0.7%-2.0% can further reduce battery impedance and more effectively balance the battery's ultimate volume energy density and cycle capacity retention rate.

[0012] In any embodiment, the polydispersity coefficient of the fluoropolymer is from 1.5 to 3.5, optionally from 1.9 to 2.3.

[0013] Fluoropolymers with a polydispersity coefficient of 1.8 to 3.5 can improve the flexibility and ultimate compaction density of the electrode at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0014] Fluoropolymers with a polydispersity coefficient of 1.9 to 2.3 can further increase bonding strength, reduce battery impedance, and improve battery cycle stability.

[0015] In any embodiment, the fluoropolymer has a crystallinity of 40% to 55%, optionally 48% to 55%.

[0016] Fluoropolymers with a crystallinity within the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0017] In any embodiment, the Dv50 particle size of the fluoropolymer is 20 μm to 100 μm.

[0018] Fluoropolymers with Dv50 in the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0019] In any embodiment, the unsaturated carboxylic acid monomer is as shown in Formula I,

[0020] Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

[0021] In any embodiment, R4 comprises an ester group.

[0022] R4 including an ester group can further improve the flexibility and bonding strength of the electrode, and more effectively improve the cycle stability and ultimate volume energy density of the battery.

[0023] In any embodiment, the unsaturated carboxylic acid monomer includes at least one of acrylic acid, methacrylic acid, β-acryloxypropionic acid, and maleic acid.

[0024] Unsaturated carboxylic acid monomers including β-acryloyloxypropionic acid can further improve the flexibility and bonding strength of the electrode compared to other unsaturated carboxylic acid monomers, and more effectively improve the cycle stability and ultimate volume energy density of the battery.

[0025] In any embodiment, the viscosity of the fluorine-containing polymer glue with a mass content of 2% obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone is 1000 mPa·s to 6000 mPa·s.

[0026] By controlling the viscosity of the fluoropolymer glue within an appropriate range, a low amount of fluoropolymer added can enable the slurry to be formed into uniform pole pieces with a certain bonding strength, thereby improving the cycle stability of the battery.

[0027] In any embodiment, the fluorine-containing polymer is at least one of poly(vinylidene fluoride-acrylic acid), poly(vinylidene fluoride-methacrylic acid), poly(vinylidene fluoride-β-acryloyloxypropionic acid), and poly(vinylidene fluoride-maleic acid).

[0028] The second aspect of the present application also provides a method for preparing a fluorine-containing polymer, comprising the following steps:

[0029] Providing a vinylidene fluoride monomer, an unsaturated carboxylic acid monomer, and a solvent, and performing a first stage polymerization reaction to obtain a first product;

[0030] subjecting the first product to a second-stage polymerization reaction under a water-insoluble gas atmosphere;

[0031] A chain transfer agent is added to carry out a third stage polymerization reaction to obtain a fluorine-containing polymer with a weight average molecular weight of 5 million to 9 million.

[0032] The staged polymerization method can produce high-molecular-weight polymers with low polydispersity. The first stage of polymerization forms the first product, the second stage forms molecular segments with the target molecular weight, and the third stage regulates the polymer's molecular weight, reducing the randomness of the polymer's weight-average molecular weight and improving its uniformity. Furthermore, staged polymerization not only improves reactor utilization during polymer production but also saves time by reducing the polymer's residence time in the reactor. The coordinated operation of the first, second, and third stages further enhances polymer production efficiency.

[0033] In any embodiment, the unsaturated carboxylic acid monomer is as shown in Formula I,

[0034] Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

[0035] In any embodiment, the reaction temperature of the first stage polymerization reaction is 45° C. to 60° C., the reaction time is 2 h to 8 h, and the initial condition is that the pressure of the vinylidene fluoride monomer is 4 MPa to 6 MPa.

[0036] In any embodiment, the reaction temperature of the second stage polymerization reaction is 60° C. to 80° C., the reaction time is 2 h to 4 h, and the reaction pressure is 6 MPa to 8 MPa.

[0037] In any embodiment, the reaction time of the third stage polymerization reaction is 1 hour to 2 hours.

[0038] By controlling the reaction pressure, reaction time and reaction temperature of the polymerization reaction at each stage within an appropriate range, the uniformity of the weight-average molecular weight of the polymerization product can be controlled while increasing the weight-average molecular weight of the fluorinated polymer, thereby ensuring that the product has a low polydispersity coefficient and improving the uniformity of product performance. The prepared polymer enables the electrode to have excellent flexibility and adhesion at a low addition amount, and the cycle capacity retention rate of the battery can be further improved.

[0039] In any embodiment, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0040] In any embodiment, the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.

[0041] In any embodiment, the amount of the chain transfer agent used is 1.5% to 4% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0042] The amount of the chain transfer agent is controlled within a suitable range, so that the polymer chain length can be controlled, thereby obtaining a polymer with a suitable molecular weight range.

[0043] In any embodiment, the first stage polymerization reaction includes the following steps: adding a water solvent and a dispersant to a container to remove oxygen in the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH value to 6.5 to 7, and then adding a vinylidene fluoride monomer to make the pressure in the container reach 4MPa to 6MPa; after stirring for 30 minutes to 60 minutes, raising the temperature to 45°C to 60°C, and adding an unsaturated carboxylic acid monomer to carry out the first stage polymerization reaction.

[0044] In any embodiment, the amount of the solvent used is 2 to 8 times the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0045] In any embodiment, the dispersant includes at least one of cellulose, cellulose ether and polyvinyl alcohol; alternatively, the cellulose includes hydroxypropyl methylcellulose, and the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

[0046] In any embodiment, the amount of the dispersant used is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0047] In any embodiment, the initiator includes at least one of t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, t-butyl peroxypivalate, and diisopropyl peroxydicarbonate.

[0048] In any embodiment, the amount of the pH adjuster is 0.05% to 0.2% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0049] In any embodiment, the unsaturated carboxylic acid monomer is added in multiple portions during the first stage polymerization reaction.

[0050] The polymerization reaction of unsaturated carboxylic acid monomer and vinylidene fluoride is exothermic. The heat causes the solvent inside the reactor to volatilize and the pressure to rise rapidly. By adding the solvent multiple times to maintain the reaction pressure stable, the fluorine-containing polymer can be uniformly polymerized with a low polydispersity coefficient.

[0051] A third aspect of the present application provides a positive electrode plate, comprising a positive electrode film layer, wherein the positive electrode film layer comprises a fluorine-containing polymer in any embodiment or a fluorine-containing polymer prepared by the preparation method in any embodiment.

[0052] In any embodiment, the mass fraction of the fluorine-containing polymer is 0.3% to 1.1%, optionally 0.4% to 0.8%, based on the total mass of the positive electrode film layer.

[0053] The fluorinated polymer within the above range can improve the flexibility and ultimate compaction density of the pole piece while maintaining the bonding strength of the pole piece, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0054] In any embodiment, the positive electrode sheet is 3.5g / cm 3- 3.7g / cm 3 The number of bends at the compaction density is not less than 2 times, and can be selected as 2-4 times.

[0055] In any embodiment, the ultimate compaction density of the positive electrode sheet is not less than 3.6 g / cm 3 , optional 3.6g / cm 3 -3.7g / cm 3 .

[0056] In a fourth aspect of the present application, a secondary battery is provided, comprising the positive electrode sheet according to the third aspect of the present application.

[0057] 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

[0058] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0059] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 1 .

[0060] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0061] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0062] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present application.

[0063] FIG6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0064] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0065] Below, the embodiments of the fluorinated polymer and its manufacturing method, positive electrode sheet, secondary battery and electrical device of the present application are described in detail with appropriate reference to the 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 structure 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.

[0066] " 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.

[0067] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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).

[0072] Polyvinylidene fluoride (PVDF) polymers are one of the most widely used binders in secondary batteries. However, the addition of PVDF polymers to the electrode film often degrades electrode flexibility, increases battery impedance, and hinders further increases in electrode density, limiting improvements in battery volumetric energy density and cycling stability.

[0073] [Fluoropolymer]

[0074] Based on this, the present application proposes a fluorine-containing polymer, which comprises structural units derived from vinylidene fluoride and structural units derived from unsaturated carboxylic acid monomers, and the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and can be optionally 5 million to 8 million.

[0075] As used herein, the term "polymer" includes, on the one hand, a collection of chemically homogeneous macromolecules prepared by polymerization, but differing in degree of polymerization, molar mass, and chain length. The term also includes derivatives of such collections of macromolecules formed by polymerization, i.e., compounds that can be obtained by reaction, for example, addition or substitution, of functional groups in the aforementioned macromolecules and that can be chemically homogeneous or chemically heterogeneous.

[0076] As used herein, the term "fluoropolymer" refers to a polymer in which at least one hydrogen atom is replaced by a fluorine atom.

[0077] 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.

[0078] As used herein, the term "unsaturated carboxylic acid monomer" refers to an unsaturated monomer containing a carboxyl functional group.

[0079] In the present application, the weight average molecular weight of the fluoropolymer can be tested by methods known in the art, such as gel chromatography, such as using a Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141). In some embodiments, the testing method is to use a polystyrene solution sample with a mass fraction of 3.0% as a reference and select a matching chromatographic column (oily: Styragel HT5DMF7.8*300mm+Styragel HT4). Use purified N-methylpyrrolidone (NMP) solvent to prepare a 3.0% fluoropolymer glue, and let the prepared solution stand for one day for use. During the test, first use a syringe to draw tetrahydrofuran, rinse, and repeat several times. Then draw 5 ml of the experimental solution, expel the air in the syringe, and wipe the needle tip dry. Finally, slowly inject the sample solution into the injection port. After the reading stabilizes, obtain the data and read the weight average molecular weight.

[0080] In some embodiments, the fluoropolymer has a weight average molecular weight of 5 million, 5.5 million, 6 million, 6.5 million, 7 million, 7.5 million, 8 million, 8.5 million, 9 million, or any value therebetween.

[0081] In some embodiments, fluoropolymers act as binders.

[0082] As used herein, the term "binder" refers to a chemical compound, polymer, or mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium.

[0083] In some embodiments, the binder dispersion medium is an oily solvent. Examples of such oily solvents include, but are not limited to, dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dimethyl carbonate, ethyl cellulose, and polycarbonate. That is, the binder is dissolved in the oily solvent.

[0084] In some embodiments, a binder is used to fix the electrode active material and / or the conductive agent in place and adhere them to the conductive metal component to form an electrode.

[0085] In some embodiments, the binder serves as a positive electrode binder, and is used to bind the positive electrode active material and / or the conductive agent to form an electrode.

[0086] In some embodiments, the binder serves as a negative electrode binder, and is used to bind the negative electrode active material and / or the conductive agent to form an electrode.

[0087] Compared with the polymer binders in the prior art, this fluorine-containing polymer can maintain the bonding strength of the pole piece at a low addition amount while reducing the stiffness of the pole piece film layer, improving the flexibility of the pole piece, and reducing the battery impedance, thereby comprehensively improving the battery's ultimate volume energy density and cycle capacity retention rate.

[0088] The fluorinated polymer with a weight-average molecular weight of 5 million to 8 million can more effectively reduce battery impedance while taking into account the battery's ultimate volume energy density and cycle capacity retention rate.

[0089] In some embodiments, the molar content of the structural units derived from the unsaturated carboxylic acid monomer is 0.5% to 2.5%, optionally 0.7% to 2.0%, based on the total moles of the structural units in the fluorine-containing polymer.

[0090] In some embodiments, the molar content of the structural units derived from the unsaturated carboxylic acid monomer can be 0.5%, 0.7%, 1%, 1.5%, 2%, 2.5%, or any range therebetween.

[0091] Fluoropolymers within the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0092] Based on the total molar number of structural units in the fluoropolymer, a fluoropolymer having a molar content of structural units derived from unsaturated carboxylic acid monomers of 0.7%-2.0% can further reduce battery impedance and more effectively balance the battery's ultimate volume energy density and cycle capacity retention rate.

[0093] In some embodiments, the polydispersity coefficient of the fluoropolymer is from 1.5 to 3.5, optionally from 1.9 to 2.3.

[0094] In some embodiments, the polymer has a polydispersity index 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 range therebetween.

[0095] As used herein, the term "polydispersity index" refers to the ratio of the weight average molecular weight of the fluoropolymer to the number average molecular weight of the fluoropolymer.

[0096] As used herein, the term "number average molecular weight" refers to the sum of the products of the mole fractions of molecules of different molecular weights in the fluorine-containing polymer and their corresponding molecular weights.

[0097] In the present application, the polydispersity coefficient can be tested by methods known in the art, such as gel chromatography, such as using a Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141). In some embodiments, a polystyrene solution sample with a mass fraction of 3.0% is used as a reference, and a matching chromatographic column is selected (oily: Styragel HT5DMF7.8*300mm+Styragel HT4). A 3.0% fluoropolymer glue is prepared with purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day for use. During the test, tetrahydrofuran is first drawn into a syringe and rinsed, and repeated several times. Then 5 ml of the experimental solution is drawn, the air in the syringe is expelled, and the needle tip is wiped dry. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired. The weight average molecular weight a and the number average molecular weight b are read respectively. Polydispersity coefficient = a / b.

[0098] Fluoropolymers with a polydispersity coefficient of 1.8 to 3.5 can improve the flexibility and ultimate compaction density of the electrode at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0099] Fluoropolymers with a polydispersity coefficient of 1.9 to 2.3 can further increase bonding strength, reduce battery impedance, and improve battery cycle stability.

[0100] In some embodiments, the fluoropolymer has a crystallinity of 40% to 55%, optionally 48% to 55%.

[0101] In some embodiments, the fluoropolymer has a crystallinity of 40%, 41%, 42%, 43%, 44%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, or any range therebetween.

[0102] In this article, the term "crystallinity" refers to the proportion of crystalline regions in a polymer. There are some regions with stable and regular arrangement of molecules in the microstructure. The region where the molecules are regularly and closely arranged is called a crystalline region.

[0103] In the present application, the crystallinity test can be performed by methods known in the art, such as differential scanning calorimetry. In some embodiments, 0.5 g of fluoropolymer is placed in an aluminum dry pan, shaken flat, and covered with a crucible lid. Under a nitrogen atmosphere, with a purge gas flow of 50 ml / min and a protective gas flow of 70 mL / min, a heating rate of 8°C / min, and a test temperature range of 30°C to 200°C, a differential scanning calorimeter (DSC) model DSC 200 F30 from NETZSCH, Germany, is used to perform the test and eliminate the thermal history.

[0104] This test will obtain the DSC / (Mw / mg) temperature variation curve of the fluoropolymer and integrate it. The peak area is the melting enthalpy ΔH (J / g) of the fluoropolymer. The crystallinity of the fluoropolymer = ΔH / (ΔHm100%) * 100%, where ΔHm100% is the standard melting enthalpy (crystalline melting heat) of polyvinylidene fluoride, and ΔHm100% = 104.7 J / g.

[0105] Fluoropolymers with a crystallinity within the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0106] In some embodiments, the fluoropolymer has a Dv50 particle size of 20 μm to 100 μm.

[0107] In some embodiments, the fluoropolymer has a Dv50 particle size of 20 μm to 35 μm, 35 μm to 45 μm, 45 μm to 55 μm, 55 μm to 65 μm, 65 μm to 75 μm, 75 μm to 85 μm, 85 μm to 95 μm, 25 μm to 45 μm, 45 μm to 65 μm, 65 μm to 85 μm, 85 μm to 100 μm, or any range therebetween.

[0108] In this article, the term "Dv50 particle size" refers to the particle size corresponding to when the cumulative particle size distribution number of particles reaches 50% in the particle size distribution curve. Its physical meaning is that particles with a particle size smaller than (or larger than) it account for 50%.

[0109] Refer to the particle size distribution laser diffraction method in GB / T 19077-2016. Weigh 0.1g to 0.13g of fluoropolymer powder into a 50ml beaker. Add 5g of anhydrous ethanol to the beaker containing the fluoropolymer powder. Place a stirring bar approximately 2.5mm in length in the beaker, and seal with plastic wrap. Ultrasonicate the sample for 5 minutes, then transfer it to a magnetic stirrer and stir at 500 rpm for at least 20 minutes. Two samples from each batch are tested. Determination is performed using a laser particle size analyzer, such as the Mastersizer 2000E from Malvern Instruments Ltd., UK.

[0110] Fluoropolymers with Dv50 in the above range can improve the flexibility and ultimate compaction density of the pole piece at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0111] In some embodiments, the unsaturated carboxylic acid monomer is as shown in Formula I,

[0112] Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

[0113] As used herein, the term "ester group" refers to a group containing -C(O)O-.

[0114] As used herein, the term "carbonyl" refers to a group containing -C(O)-.

[0115] In this context, the term "alkyl" refers to a group having the general formula C n H 2n+1 A monovalent radical derived from a saturated, unbranched or branched aliphatic hydrocarbon by removing one hydrogen atom, wherein n is an integer. 1-3 It refers to an alkyl group wherein n is an integer between 1 and 3.

[0116] In some embodiments, C 1-3 The alkyl group includes at least one of a methyl group, an ethyl group, a propyl group, and an isopropyl group.

[0117] In some embodiments, R4 includes a single bond, ie, the carboxyl functional group is directly attached to the carbon atom that is attached to the carbon-carbon double bond.

[0118] In some embodiments, R4 comprises an ester group.

[0119] R4 including an ester group can further improve the flexibility and bonding strength of the electrode, and more effectively improve the cycle stability and ultimate volume energy density of the battery.

[0120] In some embodiments, the unsaturated carboxylic acid monomer includes at least one of acrylic acid, methacrylic acid, β-acryloxypropionic acid, and maleic acid.

[0121] In some embodiments, the unsaturated carboxylic acid monomer includes β-acryloxypropionic acid.

[0122] Unsaturated carboxylic acid monomers including β-acryloyloxypropionic acid can further improve the flexibility and bonding strength of the electrode compared to other unsaturated carboxylic acid monomers, and more effectively improve the cycle stability and ultimate volume energy density of the battery.

[0123] In some embodiments, the viscosity of the fluorinated polymer glue with a mass content of 2% prepared by dissolving the fluorinated polymer in N-methylpyrrolidone is 1000 mPa·s to 6000 mPa·s.

[0124] In some embodiments, the viscosity of the fluorinated polymer glue with a mass content of 2% prepared by dissolving the fluorinated polymer in N-methylpyrrolidone is 1000mPa·s, 1200mPa·s, 1600mPa·s, 2000mPa·s, 2100mPa·s, 2500mPa·s, 2650mPa·s, 2700mPa·s, 2800mPa·s, 3000mPa·s, 3500mPa·s, 4000mPa·s, 4500mPa·s, 5000mPa·s, 6000mPa·s or any numerical range therebetween.

[0125] In this application, the viscosity of the fluoropolymer solution can be tested using methods known in the art, such as the rotational viscometer test method. As an example, 14g of fluoropolymer and 686g of N-methylpyrrolidone (NMP) are weighed in a 500mL beaker to prepare a 2% glue solution by mass. The solution is then dispersed using a Lichen high-speed grinder at a speed of 800r / min. After stirring for 120min, ultrasonic vibration is performed for 30min to remove bubbles. A Lichen Technology NDJ-5S rotational viscometer is used, with rotor No. 3 inserted into the glue solution until it does not pass the scale line. The viscosity is tested at 12r / min and the viscosity data is read after 6min.

[0126] By controlling the viscosity of the fluoropolymer glue within an appropriate range, a low amount of fluoropolymer added can enable the slurry to be formed into uniform pole pieces with a certain bonding strength, thereby improving the cycle stability of the battery.

[0127] In some embodiments, the fluorine-containing polymer is at least one of poly(vinylidene fluoride-acrylic acid), poly(vinylidene fluoride-methacrylic acid), poly(vinylidene fluoride-β-acryloyloxypropionic acid), and poly(vinylidene fluoride-maleic acid).

[0128] In one embodiment of the present application, a method for preparing a fluorine-containing polymer is provided, comprising the following steps: providing a vinylidene fluoride monomer, an unsaturated carboxylic acid monomer and a solvent, carrying out a first stage polymerization reaction to obtain a first product; subjecting the first product to a second stage polymerization reaction in a water-insoluble gas atmosphere; adding a chain transfer agent, carrying out a third stage polymerization reaction to obtain a fluorine-containing polymer having a weight-average molecular weight of 5 million to 9 million.

[0129] The staged polymerization method can produce high-molecular-weight polymers with low polydispersity. The first stage of polymerization forms the first product, the second stage forms molecular segments with the target molecular weight, and the third stage regulates the polymer's molecular weight, reducing the randomness of the polymer's weight-average molecular weight and improving its uniformity. Furthermore, staged polymerization not only improves reactor utilization during polymer production but also saves time by reducing the polymer's residence time in the reactor. The coordinated operation of the first, second, and third stages further enhances polymer production efficiency.

[0130] It can be understood that the first product can be either a reaction solution formed by vinylidene fluoride monomer and solvent, or a product obtained by processing and purifying the reaction solution.

[0131] In some embodiments, the unsaturated carboxylic acid monomer is as shown in Formula I,

[0132] Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

[0133] In some embodiments, the reaction temperature of the first stage polymerization reaction is 45°C to 60°C. In some embodiments, the reaction temperature of the first stage polymerization reaction is 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, or 45°C to 55°C.

[0134] In some embodiments, the reaction time of the first stage polymerization reaction is 2 hours to 8 hours. In some embodiments, the reaction time of the first stage polymerization reaction is 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours or any range therebetween.

[0135] In some embodiments, the polymerization pressure of the first stage polymerization reaction is 4 MPa to 6 MPa.

[0136] In some embodiments, the polymerization pressure of the first stage polymerization reaction is 4 MPa to 5 MPa, or 5 MPa to 6 MPa.

[0137] In some embodiments, the reaction temperature of the second stage polymerization reaction is 60° C. to 80° C. In some embodiments, the reaction temperature of the second stage polymerization reaction is 60° C. to 70° C. or 70° C. to 80° C.

[0138] In some embodiments, the reaction time of the second stage polymerization reaction is 2 hours to 4 hours. In some embodiments, the reaction time of the second stage polymerization reaction is 2 hours to 3 hours, or 3 hours to 4 hours.

[0139] In some embodiments, the reaction pressure of the second stage polymerization reaction is 6 MPa to 8 MPa. In some embodiments, the reaction pressure of the second stage polymerization reaction is 6 MPa to 7 MPa, or 7 MPa to 8 MPa.

[0140] In some embodiments, the reaction time of the third stage polymerization reaction is 1 h to 2 h.

[0141] By controlling the reaction pressure, reaction time and reaction temperature of the polymerization reaction at each stage within an appropriate range, the uniformity of the weight-average molecular weight of the polymerization product can be controlled while achieving an increase in the weight-average molecular weight of the polymer, ensuring that the product has a low polydispersity coefficient and improving the uniformity of product performance. The prepared polymer enables the electrode to have excellent flexibility and adhesion at a low addition amount, and the battery's cycle capacity retention rate can be further improved.

[0142] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0143] A water-insoluble gas is one with a solubility of less than 0.1 L. The solubility of a gas is the volume of the gas when it reaches saturation in 1 L of water at 20°C and a pressure of 1.013 × 105 Pa.

[0144] In some embodiments, the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.

[0145] In some embodiments, the amount of the chain transfer agent is 1.5% to 4% of the total mass of the vinylidene fluoride monomer and the monomer of Formula I. The amount of the chain transfer agent can also be, for example, 2%, 2.5%, 3%, 3.5%, or any range therebetween.

[0146] The amount of the chain transfer agent is controlled within a suitable range, so that the polymer chain length can be controlled, thereby obtaining a polymer with a suitable molecular weight range.

[0147] In some embodiments, the first stage polymerization reaction includes the following steps: adding a water solvent and a dispersant to a container to remove oxygen in the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH value to 6.5-7, and then adding a vinylidene fluoride monomer to make the pressure in the container reach 4MPa-6MPa; after stirring for 30 minutes to 60 minutes, raising the temperature to 45°C to 60°C, and adding an unsaturated carboxylic acid monomer to carry out the first stage polymerization reaction.

[0148] In some embodiments, the amount of solvent used is 2 to 8 times the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid. For example, the amount of solvent used can also be 3, 4, 5, 6, or 7 times the total mass of the vinylidene fluoride monomer and the monomer represented by Formula I.

[0149] In some embodiments, the solvent is water.

[0150] In some embodiments, the dispersant includes at least one of cellulose, cellulose ether, and polyvinyl alcohol; alternatively, the cellulose includes hydroxypropyl methylcellulose, and the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

[0151] In any embodiment, the amount of the dispersant used is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0152] In some embodiments, the initiator includes at least one of t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, t-butyl peroxypivalate, and diisopropyl peroxydicarbonate.

[0153] In some embodiments, the amount of the pH adjuster is 0.05% to 0.2% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0154] In some embodiments, the unsaturated carboxylic acid monomer is added in multiple portions during the first stage polymerization reaction.

[0155] The polymerization reaction of unsaturated carboxylic acid monomer and vinylidene fluoride is exothermic. The heat causes the solvent inside the reactor to volatilize and the pressure to rise rapidly. By adding the solvent multiple times to maintain the reaction pressure stable, the fluorine-containing polymer can be uniformly polymerized with a low polydispersity coefficient.

[0156] [Positive electrode]

[0157] The positive electrode sheet includes a positive electrode film layer, and the positive electrode film layer includes a fluorine-containing polymer according to any embodiment or a fluorine-containing polymer prepared by a preparation method according to any embodiment.

[0158] In some embodiments, the mass fraction of the fluorine-containing polymer is 0.3% to 1.1%, optionally 0.4% to 0.8%, based on the total mass of the positive electrode slurry.

[0159] In some embodiments, the mass fraction of the fluoropolymer is 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, 1.05%, 1.1% or any range therebetween.

[0160] The fluorinated polymer within the above range can improve the flexibility and ultimate compaction density of the pole piece while maintaining the bonding strength of the pole piece, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0161] In some embodiments, the positive electrode sheet is 3.5 g / cm 3 -3.7g / cm 3 The average number of bends at the compaction density is not less than 2 times, and can be selected as 2-4 times.

[0162] In some embodiments, the positive electrode sheet is 3.5 g / cm 3 -3.7g / cm 3 The average number of bends at the compaction density can be selected as 2 times, 3 times, 4 times or any range of values ​​therebetween.

[0163] The average number of times the positive electrode can be bent can be measured by any known method. The compaction density is calculated by dividing the mass of the positive electrode film layer on one side by the volume of the positive electrode film layer. The compaction density is 3.6g / cm 3 The positive electrode sheet is cut into 20×100mm 2 The size of the test sample is folded in half forward, flattened with a 2kg roller, and unfolded to check whether there is light transmittance through the gap. If no light transmittance occurs, fold it in half in the opposite direction, flatten it with a 2kg roller, and check again against the light. Repeat this process until light transmittance occurs through the gap, and record the number of folds. Repeat the test ten times and take the average value as the average number of bends of the positive electrode sheet to characterize the flexibility of the positive electrode sheet.

[0164] In some embodiments, the ultimate compaction density of the positive electrode sheet is not less than 3.6 g / cm 3 , optional 3.6g / cm 3 -3.7g / cm 3 .

[0165] In some embodiments, the ultimate compaction density of the positive electrode sheet can be 3.6 g / cm 3 、3.65g / cm3 、3.7g / cm 3 or any range of values ​​between them.

[0166] The ultimate compaction density refers to the maximum compaction density when the pole piece's elongation is no less than 7‰ and the pole piece's flexible folding times are no less than 2 times. Increasing the ultimate compaction density of the pole piece is beneficial to increasing the volume energy density of the battery.

[0167] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector.

[0168] The positive electrode sheet has good flexibility.

[0169] 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.

[0170] 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 and a metal layer formed on at least one surface of the polymer material base. 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 a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0171] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0172] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, 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.

[0173] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, 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 current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0174] [Negative electrode]

[0175] 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.

[0176] 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.

[0177] 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 a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0178] 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.

[0179] In some embodiments, the negative electrode film layer may further include a binder. 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).

[0180] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0181] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0182] 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.

[0183] [Electrolytes]

[0184] 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.

[0185] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] [Isolation film]

[0190] 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.

[0191] 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.

[0192] 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.

[0193] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0194] 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.

[0195] 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.

[0196] 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.

[0197] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0198] Figure 3 shows an example battery module 4. Referring to Figure 3 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0199] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0200] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0201] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0202] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric 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.

[0203] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0204] Figure 6 shows an example of an electric device. This device is 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.

[0205] 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.

[0206] Example

[0207] 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.

[0208] 1. Preparation method

[0209] Example 1

[0210] 1) Preparation of fluorinated polymers

[0211] First stage polymerization reaction: 4 kg of deionized water and 2 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2, and the atmosphere was evacuated and O2 was replaced with N2 three times. 5 g of tert-butyl peroxypivalate and 2 g of sodium bicarbonate were added again, and 1 kg of vinylidene fluoride monomer was added to achieve a pressure of 5 MPa. The mixture was mixed and stirred for 30 min, and the temperature was raised to 45°C. At the same time, an aqueous solution containing 22.5 g of β-acryloyloxypropionic acid was added in multiple portions, and the reaction was allowed to proceed for 3.0 h.

[0212] Second stage polymerization reaction: transfer the reaction liquid in reactors 1 and 2 to reactor 3, fill with nitrogen to a pressure of 7 MPa, raise the temperature to 70°C, and stir the reaction for 3 hours;

[0213] The third stage of polymerization reaction: After adding 40 g of cyclohexane, the reaction was continued for 1 hour, and then the reaction was stopped. The reaction system was centrifuged, and the solid phase was collected, washed, and dried to obtain a polyvinylidene fluoride copolymer.

[0214] 2) Preparation of positive electrode sheet

[0215] 4935g of NCM (nickel-cobalt-manganese ternary material) and 30g of fluoropolymer were stirred in a planetary stirring tank at an orbital speed of 25r / min for 20min, wherein the mass fraction of the fluoropolymer was 0.6% based on the total mass of the positive electrode film layer;

[0216] Add 1.67 kg of N-methylpyrrolidone (NMP) solution into the stirring tank, rotate at a speed of 25 r / min and a speed of 800-1000 / min, and stir for 50 minutes.

[0217] Add 5 g of dispersant hydrogenated nitrile rubber (HNBR) and 30 g of single-walled carbon nanotubes (CNT) into a stirring tank and stir at a revolution speed of 25 r / min and a rotation speed of 1200-1500 r / min for 50 min;

[0218] After stirring, the slurry viscosity was tested and the viscosity was controlled at 8000-15000 MPa·s.

[0219] If the viscosity is too high, add N-methylpyrrolidone (NMP) solution to reduce the viscosity to the above range. After adding the NMP solution, stir at an orbital speed of 25 r / min and an autorotation speed of 1250 r / min for 30 minutes to obtain the positive electrode slurry. After adding the NMP solution, stir at an orbital speed of 25 r / min and an autorotation speed of 1200-1500 r / min for 30 minutes to obtain the positive electrode slurry. Apply the prepared positive electrode slurry to carbon-coated aluminum foil, bake at 110°C for 15 minutes, cold press, and cut into 15mm diameter discs to obtain the positive electrode sheets.

[0220] 3) Negative electrode

[0221] The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium hydroxymethyl cellulose (CMC-Na) are dissolved in the solvent deionized water in a mass 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 current collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0222] 4) Isolation film

[0223] Polypropylene film is used as the isolation film.

[0224] 5) Preparation of electrolyte

[0225] 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, and stirred evenly to prepare a 1M LiPF6EC / EMC solution to obtain an electrolyte.

[0226] 6) Preparation of batteries

[0227] 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.

[0228] Examples 2 to 5

[0229] The results are basically the same as Example 1, except that the reaction time in the first stage polymerization reaction is adjusted to 5 h, 6 h, 7 h, and 8 h, respectively, and the cyclohexane in the third stage polymerization reaction is adjusted to 35 g, 30 g, 25 g, and 20 g, respectively. The specific parameters are shown in Table 1.

[0230] Examples 6 to 9

[0231] The method is basically the same as Example 2, except that the molar content of β-acryloxypropionic acid is adjusted. The specific parameters are shown in Table 1 based on the total molar number of structural units in the fluorine-containing polymer.

[0232] Examples 10 to 13

[0233] This is essentially the same as Example 2, except that the mass fraction of the vinylidene fluoride-β-acryloxypropionic acid copolymer was adjusted. While maintaining the total mass of the electrode film layer unchanged, the mass content of the positive electrode active material was adjusted accordingly when adjusting the content of the fluorinated copolymer. The specific parameters are shown in Table 1.

[0234] Examples 14 to 16

[0235] The method is basically the same as Example 2, except that 1 mol% of β-acryloxypropionic acid is replaced by 1 mol% of acrylic acid, 1 mol% of methacrylic acid, and 1 mol% of maleic acid, respectively. The specific parameters are shown in Table 1.

[0236] Example 17

[0237] The process is basically the same as Example 2, except that the aqueous solution of β-acryloxypropionic acid is added once in the first polymerization reaction. The specific parameters are shown in Table 1.

[0238] Comparative Example 1

[0239] Basically the same as Example 1, the positive electrode plate used a polyvinylidene fluoride-acrylic acid copolymer with a weight-average molecular weight of 1.1 million, purchased from Solvay in the United States. The added amount of the positive electrode binder was 2.0%, and the mass content of the positive electrode active material was adjusted accordingly. The specific parameters are shown in Table 1.

[0240] Comparative Example 2

[0241] The process is basically the same as Example 2, except that the polymerization monomer is only vinylidene fluoride. The specific parameters are shown in Table 1.

[0242] 2. Battery performance test

[0243] 1. Fluoropolymer property test

[0244] 1) Weight average molecular weight test

[0245] A Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141) was used. A 3.0% polystyrene solution sample was used as a reference, and a matching chromatographic column was selected (oil-based: Styragel HT5DMF7.8*300mm+Styragel HT4). A 3.0% fluoropolymer glue 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 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 reading stabilized, the data was acquired and the weight-average molecular weight was read.

[0246] 2) Polydispersity coefficient test

[0247] A Waters 2695 Isocratic HPLC gel chromatography instrument (differential refractive index detector 2141) was used. A 3.0% polystyrene solution sample was used as a reference, and a matching chromatographic column was selected (oil-based: Styragel HT5DMF7.8*300mm+Styragel HT4). A 3.0% fluoropolymer glue 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 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. The data was acquired after the reading stabilized. The weight-average molecular weight a and number-average molecular weight b were read separately. Polydispersity coefficient = a / b.

[0248] 3) Dv50 test

[0249] Refer to the particle size distribution laser diffraction method in GB / T 19077-2016. Weigh 0.1g to 0.13g of fluoropolymer powder into a 50ml beaker. Add 5g of anhydrous ethanol to the beaker containing the fluoropolymer powder. Place a stirring bar approximately 2.5mm in length in the beaker and seal with plastic wrap. Ultrasonicate the sample for 5 minutes, then transfer it to a magnetic stirrer and stir at 500 rpm for at least 20 minutes. Test two samples from each batch. Laser particle size analyzers, such as the Mastersizer 2000E from Malvern Instruments Ltd., UK, are conveniently used for measurement.

[0250] 4) Crystallinity test

[0251] 0.5 g of fluoropolymer was placed in an aluminum dry pan, shaken flat, and covered with a crucible lid. In a nitrogen atmosphere, with a purge gas of 50 mL / min and a protective gas of 70 ml / min, the heating rate was 8°C / min, and the test temperature range was 30°C to 200°C. A differential scanning calorimeter (DSC) model DSC 200F30 from NETZSCH, Germany, was used for testing and eliminating the thermal history.

[0252] This test will obtain the DSC / (Mw / mg) temperature variation curve of the fluoropolymer and integrate it. The peak area is the melting enthalpy ΔH (J / g) of the fluoropolymer. The crystallinity of the fluoropolymer = ΔH / (ΔHm100%) * 100%, where ΔHm100% is the standard melting enthalpy (crystalline melting heat) of polyvinylidene fluoride, and ΔHm100% = 104.7 J / g.

[0253] 5) Glue viscosity test

[0254] In a 500ml beaker, weigh 14g of fluoropolymer and 686g of N-methylpyrrolidone (NMP) to create a 2% (mass fraction) adhesive solution. Stir and disperse using a Lichen high-speed grinder at 800 rpm for 120 minutes, then ultrasonically vibrate for 30 minutes to remove air bubbles. Using a Lichen Technology NDJ-5S rotational viscometer, insert the No. 3 rotor into the adhesive solution until it submerges the scale line. Measure the viscosity at 12 rpm and read the viscosity after 6 minutes.

[0255] 2. Bond strength test

[0256] With reference to GB-T2790-1995, "Test Method for 180° Peel Strength of Adhesives," the adhesion test procedure for the Examples and Comparative Examples of this application is as follows: Use a blade to cut a sample 30 mm wide and 100-160 mm long. Apply a special double-sided tape with a width of 20 mm and a length of 90-150 mm to a steel plate. Apply the positive electrode film surface of the previously cut electrode sample to the double-sided tape, then roll it three times in the same direction with a 2 kg roller. Secure a paper tape 250 mm long and the same width as the electrode to the electrode current collector and secure it with crepe adhesive. Turn on the Sansi tensile testing machine (sensitivity 1 N), illuminate the indicator light, adjust the stop block to the appropriate position, and secure the end of the steel plate not attached to 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 when the electrode is under force balance divided by the width of the tape is used as the bonding force of the electrode per unit length to characterize the bonding strength between the positive electrode film layer and the current collector.

[0257] 3. Extreme compaction density test of pole piece

[0258] The double-sided coated electrode is cold-pressed by a roller press to test the elongation of the electrode after cold pressing, and the flexibility of the electrode after cold pressing is also evaluated.

[0259] By increasing the pressure of the roller press, pole pieces with different compaction densities will be obtained. As the pressure increases, the compaction density of the pole piece increases, the elongation of the pole piece increases, and the flexibility of the pole piece decreases. A pole piece with too high an elongation can easily cause the pole piece to warp, while a pole piece with too low flexibility can easily lead to brittle fracture. Therefore, the maximum compaction density corresponding to the pole piece when the elongation of the pole piece is not less than 7‰ and the number of pole piece flexibility folds is not less than 2 is defined as the limit compaction density.

[0260] The compacted density is calculated by dividing the mass of the single-sided positive electrode film layer by the volume of the positive electrode film layer.

[0261] The test method for elongation is as follows:

[0262] Lay the electrode flat on a horizontal table and cut it into sections, with each electrode about 100 cm long; remove the copper foil at the edge of the electrode substrate, and be careful to keep the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the cold pressing roller), ensuring that the electrode part is completely covered by the coating, and use a steel ruler to measure the length between the marking points at the same position of the length and width at the head and tail of the electrode, estimate it to 0.1mm, and record the length before cold pressing; after cold pressing by the cold press, record the length between the corresponding marking points after cold pressing, and use (length after cold pressing - length before cold pressing) / length before cold pressing as the elongation of the electrode.

[0263] The test method for the number of flexibility folds is shown below.

[0264] 4. The electrode is 3.6g / cm 3 Flexibility and folding times test at compacted density

[0265] The compacted density is 3.6 g / cm 3 The positive electrode sheet is cut into 20×100mm 2 Fold the test specimen in the forward direction, flatten it with a 2kg roller, unfold it and check against the light to see if there is any light transmittance. If there is no light transmittance, fold it in the reverse direction, flatten it with a 2kg roller, and check against the light again. Repeat this process until there is light transmittance. Record the number of folds. Repeat the test ten times and take the average value as the reference data for the flexibility of the electrode.

[0266] 5. Battery performance test

[0267] 1) DC impedance test

[0268] At 25°C, the secondary battery was charged at a constant current rate of 1 / 3C to 4.25V. It was then charged at a constant voltage rate of 4.25°C to a current of 0.05C and allowed to rest for 5 minutes. The battery was then discharged at a rate of 1 / 3C for 90 minutes, adjusting the electrode assembly to 50% SOC. The battery was allowed to rest for 60 minutes, and then discharged at a rate of 4C for 30 seconds. The 50% SOC discharge DCR was calculated based on the test data. The 50% SOC discharge DCR from the second cycle is used as the test result for this example.

[0269] 2) Battery capacity retention test

[0270] The battery capacity retention test procedure is as follows: At 25°C, charge a button cell at a constant current of 1 / 3C to 3.65V, then charge at a constant voltage of 3.65V to a current of 0.05C, let stand for 5 minutes, and then discharge at a constant current of 1 / 3C to 2.5V. The resulting capacity is recorded as the initial capacity C0. Repeat these steps for the same battery, and simultaneously record the battery's discharge capacity Cn after the nth cycle. The battery capacity retention rate after each cycle is Pn = Cn / C0 * 100%. The battery cycle capacity retention rate is calculated as Pn = Cn / C0 * 100%, with the 100 points P1, P2, ..., P100 as the vertical axis and the corresponding number of cycles as the horizontal axis.

[0271] During this test, the first cycle corresponds to n = 1, the second cycle corresponds to n = 2, ..., and the 100th cycle corresponds to n = 100. The battery capacity retention rate data corresponding to Examples 1 to 17 or Comparative Examples 1 to 6 in Table 1 are the data measured after 500 cycles under the above test conditions, i.e., the P500 value.

[0272] 3) Battery volume energy density test at ultimate compaction density

[0273] The battery at the ultimate compaction density was left at rest at 25°C for 2 hours to ensure that the battery temperature was 25°C. After charging the battery at 0.33C at 25°C to the charge cut-off voltage, constant voltage charging was continued at the charge cut-off voltage until the current reached 0.05C and the charge was cut off (where C0 represents the rated capacity of the battery). After the battery was left at rest at 25°C for 1 hour, it was discharged at 0.33C at 25°C to the discharge cut-off voltage, and the total discharge energy of the battery was recorded as E0;

[0274] Measure the battery volume as V0;

[0275] Battery volume energy density = battery discharge energy E0 / battery volume V0.

[0276] 3. Analysis of test results of various embodiments and comparative examples

[0277] Batteries of various embodiments and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Tables 1, 2, and 3 below.

[0278] Table 1

[0279] Table 2

[0280] Table 3

[0281] According to the above results, the fluoropolymers of Examples 1 to 17 contain structural units derived from vinylidene fluoride and at least one structural unit derived from acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid, and the weight average molecular weight of the fluoropolymers is 5 million to 9 million.

[0282] From the comparison of Examples 1 to 17 and Comparative Example 1, it can be seen that the vinylidene fluoride-β-acryloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binders with a weight-average molecular weight of 5 million to 9 million can improve the flexibility and ultimate compaction density of the electrode at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery compared to the general binders in the prior art.

[0283] As can be seen from Table 3, the fluorine-containing polymer provided by the present disclosure can also improve the flexibility and ultimate compaction density of the electrode compared to the vinylidene fluoride homopolymer with the same weight-average molecular weight and the same addition content, thereby comprehensively improving the ultimate volume energy density and cycle capacity retention rate of the battery.

[0284] It can be seen from Examples 1-5 that the fluorine-containing polymer with a weight average molecular weight of 5 million to 8 million can more effectively reduce the battery impedance while taking into account the battery's ultimate volume energy density and cycle capacity retention rate.

[0285] It can be seen from Examples 1 to 17 that, based on the total molar number of structural units in the fluoropolymer, the molar content of structural units derived from unsaturated carboxylic acid monomers is 0.5%-2.5%, and based on the total molar number of structural units in the fluoropolymer, vinylidene fluoride-β-acryloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binders can improve the flexibility and ultimate compaction density of the electrode at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0286] As can be seen from Examples 2 and 6-9, based on the total molar number of structural units in the fluoropolymer, the molar content of the structural units derived from the unsaturated carboxylic acid monomer is 0.7%-2.0%. The fluoropolymer can further reduce the battery impedance and more effectively take into account the battery's ultimate volume energy density and cycle capacity retention rate.

[0287] It can be seen from Examples 1 to 17 that the vinylidene fluoride-β-acryloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binders with a polydispersity index of 1.8 to 3.5 can improve the flexibility and ultimate compaction density of the electrode at a low addition amount, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery compared to the general binders in the prior art.

[0288] As can be seen from the examples, the fluorine-containing polymer with a polydispersity index of 1.9 to 2.3 can further improve the bonding strength, reduce the battery impedance, and improve the cycle stability of the battery.

[0289] It can be seen from Examples 2 and 14-16 that the fluorinated polymer whose monomer of Formula I includes β-acryloxypropionic acid can more effectively reduce the battery impedance, improve the flexibility of the electrode, and further improve the ultimate volume energy density and cycle stability of the battery.

[0290] It can be seen from Examples 1 to 17 that when the mass fraction of the binder of vinylidene fluoride-β-acryloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer is 0.3% to 1.1% based on the total mass of the positive electrode film layer, the fluorine-containing polymer can improve the flexibility and ultimate compaction density of the electrode while maintaining the bonding strength of the electrode, reduce the battery impedance, and thereby comprehensively improve the ultimate volume energy density and cycle capacity retention rate of the battery.

[0291] It can be seen from Examples 2 and 10-13 that, based on the total mass of the positive electrode film layer, when the mass fraction of the fluorine-containing polymer is 0.4%-0.8%, the battery impedance can be further reduced and the ultimate volume energy density and cycle stability of the battery can be improved.

[0292] 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 The fluorine-containing polymer comprises a structural unit derived from vinylidene fluoride and a structural unit derived from an unsaturated carboxylic acid monomer, and the weight average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and can be optionally 5 million to 8 million.

2. The fluorinated polymer according to claim 1, characterized in that The molar content of the structural unit derived from the unsaturated carboxylic acid monomer is 0.5%-2.5%, optionally 0.7%-2.0%, based on the total molar number of the structural units in the fluorine-containing polymer.

3. The fluorinated polymer according to claim 1 or 2, characterized in that The polydispersity coefficient of the fluorine-containing polymer is 1.5-3.5, and can be optionally 1.9-2.

3.

4. The fluorine-containing polymer according to any one of claims 1 to 3, characterized in that The crystallinity of the fluorine-containing polymer is 40% to 55%, and can be optionally 48% to 55%.

5. The fluorine-containing polymer according to any one of claims 1 to 4, characterized in that The Dv50 particle size of the fluorine-containing polymer is 20 um to 100 um.

6. The fluorine-containing polymer according to any one of claims 1 to 5, characterized in that The unsaturated carboxylic acid monomer is as shown in Formula I, Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

7. The fluorine-containing polymer according to any one of claims 1 to 6, characterized in that R4 includes an ester group.

8. The fluorine-containing polymer according to any one of claims 1 to 7, characterized in that The unsaturated carboxylic acid monomer includes at least one of acrylic acid, methacrylic acid, β-acryloxypropionic acid, and maleic acid.

9. The fluorine-containing polymer according to any one of claims 1 to 8, characterized in that The viscosity of the fluorine-containing polymer glue solution with a mass content of 2% obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone is 1000mPa·s to 6000mPa·s.

10. The fluorine-containing polymer according to any one of claims 1 to 9, characterized in that The fluorine-containing polymer includes at least one of poly(vinylidene fluoride-acrylic acid), poly(vinylidene fluoride-methacrylic acid), poly(vinylidene fluoride-β-acryloxy propionic acid), and poly(vinylidene fluoride-maleic acid).

11. A method for preparing a fluorine-containing polymer, characterized in that: The following steps are involved: Providing a vinylidene fluoride monomer, an unsaturated carboxylic acid monomer and a solvent, and performing a first stage polymerization reaction to obtain a first product; subjecting the first product to a second stage polymerization reaction in a water-insoluble gas atmosphere; A chain transfer agent is added to carry out a third stage polymerization reaction to obtain a fluorine-containing polymer with a weight average molecular weight of 5 million to 9 million.

12. The preparation method according to claim 11, characterized in that: The unsaturated carboxylic acid monomer is as shown in Formula I, Wherein, R1, R2, and R3 each independently include hydrogen, C 1-3 At least one of the alkyl groups, R4 includes an ester group, a carbonyl group, a C 1-3 At least one of an alkyl group and a single bond.

13. The preparation method according to claim 11 or 12, characterized in that: The reaction temperature of the first stage polymerization reaction is 45° C. to 60° C., the reaction time is 2 hours to 8 hours, and the polymerization pressure is 4 MPa to 6 MPa.

14. The preparation method according to any one of claims 11 to 13, characterized in that: The reaction temperature of the second stage polymerization reaction is 60° C. to 80° C., the reaction time is 2 hours to 4 hours, and the reaction pressure is 6 MPa to 8 MPa.

15. The preparation method according to any one of claims 11 to 14, characterized in that: The reaction time of the third stage polymerization reaction is 1 hour to 2 hours.

16. The preparation method according to any one of claims 11 to 15, characterized in that: The chain transfer agent includes one or more of cyclohexane, isopropanol, methanol and acetone.

17. The preparation method according to any one of claims 11 to 16, characterized in that: The water-insoluble gas includes one or more of nitrogen, oxygen, hydrogen and methane.

18. The preparation method according to any one of claims 11 to 17, characterized in that: The amount of the chain transfer agent used is 1.5% to 4% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

19. The preparation method according to any one of claims 11 to 18, characterized in that: The first stage polymerization reaction comprises the following steps: Adding a solvent and a dispersant into the container to remove oxygen from the reaction system; Adding an initiator and a pH adjuster into the container, adjusting the pH value to 6.5 to 7, and then adding vinylidene fluoride monomer to make the pressure in the container reach 4 MPa to 6 MPa; After stirring for 30 to 60 minutes, the temperature is raised to 45 to 60° C., and unsaturated carboxylic acid monomer is added to carry out the first stage polymerization reaction.

20. The preparation method according to claim 19, characterized in that: The amount of the solvent used is 2 to 8 times the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

21. The preparation method according to claim 19 or 20, characterized in that: The dispersant includes at least one of cellulose, cellulose ether and polyvinyl alcohol; optionally, the cellulose includes hydroxypropyl methylcellulose, and the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

22. The preparation method according to any one of claims 19 to 21, characterized in that: The amount of the dispersant used is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

23. The preparation method according to any one of claims 19 to 22, characterized in that: The initiator includes at least one of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, tert-butyl peroxypivalate, and diisopropyl peroxydicarbonate.

24. The preparation method according to any one of claims 19 to 23, characterized in that: The amount of the pH regulator used is 0.05% to 0.2% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

25. The preparation method according to any one of claims 19 to 24, characterized in that: In the first stage polymerization reaction, the unsaturated carboxylic acid monomer is added in multiple times.

26. A positive electrode plate, characterized in that: The positive electrode plate comprises a positive electrode film layer, and the positive electrode film layer comprises the fluorine-containing polymer according to any one of claims 1 to 10 or the fluorine-containing polymer prepared by the preparation method according to any one of claims 11 to 25.

27. The positive electrode sheet according to claim 26, characterized in that: The mass fraction of the fluorine-containing polymer is 0.3% to 1.1%, and can be optionally 0.4% to 0.8%, based on the total mass of the positive electrode film layer.

28. The positive electrode sheet according to claim 26 or 27, characterized in that: The positive electrode sheet is 3.5g / cm 3- 3.7g / cm 3 The number of bends at the compaction density is not less than 2 times, and can be selected as 2-4 times.

29. The positive electrode sheet according to any one of claims 26 to 28, characterized in that: The ultimate compaction density of the positive electrode sheet is not less than 3.6 g / cm 3 , optional 3.6g / cm 3 -3.7g / cm 3 .

30. A secondary battery, characterized in that: A positive electrode sheet comprising the positive electrode sheet described in any one of claims 26 to 29.

31. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 30.

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