Fluorine-containing polymer, method for preparing the same, positive electrode sheet, secondary battery, and electrical device

Abstract

This application provides a fluorine-containing polymer, a method for preparing the same, a positive electrode sheet, a secondary battery, and an electrical device. The fluorine-containing polymer 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 selectively 5 million to 8 million.

Description

[Technical Field]

[0001] This application relates to the technical field of secondary batteries, and more particularly to fluorine-containing polymers, methods for preparing fluorine-containing polymers, positive electrode sheets, secondary batteries, and electrical devices. [Background technology]

[0002] In recent years, secondary batteries have been widely applied in various fields, including energy storage and power systems such as hydroelectric, thermal, wind, and solar power plants, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. As the application of secondary batteries becomes more widespread, the demands for their electrochemical performance and cycle stability are also increasing.

[0003] Binding agents are common materials in secondary batteries, and there is a great demand for them in all parts of the battery, including electrode sheets, separators, and sealing sections. However, the use of conventional binding agents increases the rigidity of the film layer of the electrode sheet, reducing its flexibility and thus not contributing to improving the electrochemical performance of the battery. Therefore, there is still room for improvement in existing binding agents. [Overview of the project]

[0004] This application has been made in view of the above-mentioned problems, and its purpose is to provide a fluorine-containing polymer that can improve the flexibility and critical pressure density of electrode sheets, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0005] To achieve the above objective, this application provides a fluorine-containing polymer comprising a constituent unit derived from vinylidene fluoride and a constituent unit derived from an unsaturated carboxylic acid monomer, wherein the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and selectively 5 million to 8 million.

[0006] Compared with the polymer binders of the prior art, the fluorine-containing polymer can reduce the rigidity of the film layer of the electrode sheet while maintaining the binding strength of the electrode sheet with a small addition amount, improve the flexibility of the electrode sheet, reduce the battery impedance, and thereby comprehensively increase the limiting volume energy density and cycle capacity retention rate of the battery.

[0007] The fluorine-containing polymer with a weight average molecular weight of 5,000,000 to 8,000,000 can more effectively reduce the battery impedance while considering the limiting volume energy density and cycle capacity retention rate of the battery.

[0008] In any embodiment, based on the total number of moles of the structural units in the fluorine-containing polymer, the molar content of the structural units derived from the unsaturated carboxylic acid monomer is 0.5% to 2.5%, and optionally 0.7% to 2.0%.

[0009] The fluorine-containing polymer within the above range can improve the flexibility and limiting pressure density of the electrode sheet with a small addition amount, reduce the battery impedance, and thereby comprehensively increase the limiting volume energy density and cycle capacity retention rate of the battery.

[0010] The fluorine-containing polymer in which the molar content of the structural units derived from the unsaturated carboxylic acid monomer is 0.7% to 2.0% based on the total number of moles of the structural units in the fluorine-containing polymer can further reduce the battery impedance and more effectively balance the limiting volume energy density and cycle capacity retention rate of the battery.

[0011] In any embodiment, the polydispersity coefficient of the fluorine-containing polymer is 1.5 to 3.5, and optionally 1.9 to 2.3.

[0012] The fluorine-containing polymer with a polydispersity coefficient of 1.8 to 3.5 can improve the flexibility and limiting pressure density of the electrode sheet with a small addition amount, reduce the battery impedance, and thereby comprehensively increase the limiting volume energy density and cycle capacity retention rate of the battery.

[0013] Fluorine-containing polymers with a polydispersity coefficient of 1.9 to 2.3 can further enhance bonding strength, reduce battery impedance, and improve battery cycle stability.

[0014] In any embodiment, the crystallinity of the fluorine-containing polymer is 40% to 55%, and selectively 48% to 55%.

[0015] Fluorine-containing polymers with a crystallinity within the above range can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0016] In any embodiment, the Dv50 particle size of the fluorine-containing polymer is 20 μm to 100 μm.

[0017] Fluorine-containing polymers with a Dv50 within the above range can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0018] In any embodiment, the unsaturated carboxylic acid monomer is as shown in formula I, [ka] In the formula, R1, R2, and R3 are each independently hydrogen and C. 1~3 It contains at least one alkyl group, and R4 is an ester group, a carbonyl group, or C 1~3 It contains at least one alkyl group and one single bond.

[0019] In any embodiment, R4 includes an ester group.

[0020] R4, which contains ester groups, can further improve the flexibility and bonding strength of the electrode sheet, thereby more effectively improving the battery's cycle stability and critical volumetric energy density.

[0021] In any embodiment, the unsaturated carboxylic acid monomer includes at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid.

[0022] Unsaturated carboxylic acid monomers, including β-acryloyloxypropionic acid, can further enhance the flexibility and bonding strength of electrode sheets compared to other unsaturated carboxylic acid monomers, thereby more effectively improving the battery's cycle stability and critical volumetric energy density.

[0023] In any embodiment, the fluorine-containing polymer obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone has a viscosity of 1000 mPa·s to 6000 mPa·s when the fluorine-containing polymer has a mass content of 2%.

[0024] By controlling the viscosity of the fluorine-containing polymer colloid within an appropriate range, even with the addition of a small amount of fluorine-containing polymer, the slurry can be formed into a uniform electrode sheet with a certain bonding strength, thereby improving the cycle stability of the battery.

[0025] 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).

[0026] A second aspect of this application also provides a method for preparing a fluorine-containing polymer, which preparation method is: The process involves preparing vinylidene fluoride monomer, an unsaturated carboxylic acid monomer, and a solvent, carrying out a first-step polymerization reaction to obtain a first product, and The first product is subjected to a second polymerization reaction under a non-water-soluble gas atmosphere, The process includes the step of adding a chain transfer agent to carry out a third polymerization reaction to obtain a fluorine-containing polymer with a weight-average molecular weight of 5 million to 9 million.

[0027] By carrying out polymerization reactions in a stepwise manner, polymers with low polydispersity coefficients and high molecular weights can be prepared and obtained. Simultaneously, the first polymerization reaction forms the first product, the second polymerization reaction forms a molecular segment with a target molecular weight, and the third polymerization reaction adjusts the molecular weight of the polymer, reducing excessive randomness in the weight-average molecular weight and improving polymer uniformity. Furthermore, stepwise polymerization not only increases the utilization rate of the reactor during the polymer preparation process but also saves time and shortens the residence time of the polymer in the reactor. By coordinating the first, second, and third polymerization reactions, the polymer production efficiency can be further increased.

[0028] In any embodiment, the unsaturated carboxylic acid monomer is as shown in formula I, [ka] In the formula, R1, R2, and R3 are each independently hydrogen and C. 1~3 It contains at least one alkyl group, and R4 is an ester group, a carbonyl group, or C 1~3 It contains at least one alkyl group and one single bond.

[0029] In any embodiment, the reaction temperature for the first polymerization step is 45°C to 60°C, the reaction time is 2 hours to 8 hours, and the initial conditions include a pressure of 4 MPa to 6 MPa for the vinylidene fluoride monomer.

[0030] In any embodiment, the reaction temperature for 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.

[0031] In any embodiment, the reaction time for the third polymerization reaction is 1 to 2 hours.

[0032] By controlling the reaction pressure, reaction time, and reaction temperature of each polymerization step within appropriate ranges, it is possible to control the uniformity of the weight-average molecular weight of the polymerization product while increasing the weight-average molecular weight of the fluorine-containing polymer. This ensures a product with a relatively low polydispersity coefficient and more uniform product properties. Even when only a small amount of the prepared polymer is added, the electrode sheet will have excellent flexibility and bonding strength, further improving the battery's cycle capacity retention rate.

[0033] In any embodiment, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0034] In any embodiment, the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.

[0035] In any embodiment, the dose of the chain transfer agent is 1.5% to 4% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0036] By controlling the dosage of the chain transfer agent within an appropriate range, the chain length of the polymer can be controlled, and a polymer within an appropriate molecular weight range can be obtained.

[0037] In any embodiment, the first step of the polymerization reaction includes the steps of: adding an aqueous solvent and a dispersant to a container to remove oxygen from the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH to 6.5 to 7, then adding vinylidene fluoride monomer to bring the pressure inside the container to 4 MPa to 6 MPa; and stirring for 30 to 60 minutes, then raising the temperature to 45°C to 60°C, and adding an unsaturated carboxylic acid monomer to carry out the first step of the polymerization reaction.

[0038] In any embodiment, the amount of solvent is 2 to 8 times the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0039] In any embodiment, the dispersant comprises at least one of cellulose, cellulose ether, and polyvinyl alcohol. Selectively, the cellulose comprises hydroxypropyl methylcellulose, and the cellulose ether comprises one or more of methylcellulose ether and carboxyethylcellulose ether.

[0040] In any embodiment, the dose of the dispersant is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0041] In any embodiment, the initiator includes at least one of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, tert-butyl peroxypivalate, and diisopropyl peroxydicarbonate.

[0042] In any embodiment, the dose 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.

[0043] In any embodiment, the unsaturated carboxylic acid monomer is added in multiple steps during the first polymerization reaction.

[0044] The polymerization reaction between unsaturated carboxylic acid monomers and vinylidene fluoride is an exothermic reaction. The heat causes the solvent in the reaction vessel to volatilize, leading to a rapid increase in pressure. By adding the solvent multiple times to stabilize and maintain the reaction pressure, fluorine-containing polymers can be uniformly polymerized with a relatively low polydispersity coefficient.

[0045] A third aspect of this application provides a positive electrode sheet comprising a positive electrode film layer, wherein the positive electrode film layer comprises a fluorine-containing polymer according to any embodiment, or a fluorine-containing polymer prepared by a preparation method according to any embodiment.

[0046] In any embodiment, the mass fraction of the fluorine-containing polymer is 0.3% to 1.1%, and optionally 0.4% to 0.8%, based on the total mass of the positive electrode film layer.

[0047] The fluorine-containing polymer within the above range can improve the flexibility and limiting pressure density of the electrode sheet while maintaining the binding strength of the electrode sheet, reduce the battery impedance, and thereby comprehensively increase the limiting volume energy density and cycle capacity retention rate of the battery.

[0048] In any embodiment, the number of bendable times of the positive electrode sheet at a pressure density of 3.5 g / cm 3 ~3.7 g / cm 3 is 2 or more, and optionally 2 to 4.

[0049] In any embodiment, the limiting pressure density of the positive electrode sheet is 3.6 g / cm 3 or more, and optionally 3.6 g / cm 3 ~3.7 g / cm 3 is.

[0050] In the fourth aspect of the present application, a secondary battery including the positive electrode sheet according to the third aspect of the present application is provided.

[0051] In the fifth aspect of the present application, an electric device including the secondary battery according to the fourth aspect of the present application is provided.

Brief Description of the Drawings

[0052] [Figure 1] It is a schematic diagram of a secondary battery according to an embodiment of the present application. [Figure 2] It is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1. [Figure 3] It is a schematic diagram of a battery module according to an embodiment of the present application. [Figure 4] It is a schematic diagram of a battery pack according to an embodiment of the present application. [Figure 5] It is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4. [Figure 6] This is a schematic diagram of an electrical device in which a secondary battery is used as a power source according to one embodiment of this application. [Modes for carrying out the invention]

[0053] The following describes in detail the fluorine-containing polymer and its manufacturing method, positive electrode sheet, secondary battery, and embodiment of the electrical device specifically disclosed in this application, with appropriate reference to the accompanying drawings. However, unnecessary detailed explanations may be omitted. For example, detailed explanations of well-known matters and redundant explanations of identical structures may be omitted. This is to avoid unnecessarily verbose explanations, making it easier for those skilled in the art to understand. Furthermore, the drawings and the following explanation are provided to enable those skilled in the art to fully understand this application and are not intended to limit the intent of the claims.

[0054] The “range” disclosed in this application is limited by a lower and upper limit, and a given range is limited by selecting one lower limit and one upper limit, and the boundaries of the special range are limited by the selected lower and upper limits. Such limited ranges may or may not include endpoint values ​​and can be combined arbitrarily, that is, any lower limit and any upper limit can be combined to form a single range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, the ranges 60-110 and 80-120 are also understood to be predictable. Similarly, if the minimum range values ​​are listed as 1 and 2, and the maximum range values ​​are listed as 3, 4 and 5, then the ranges 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5 are all predictable. In this application, unless otherwise specified, the numerical range “a-b” represents an abbreviated expression for any combination of real numbers between a and b, where both a and b are real numbers. For example, the numerical range "0 to 5" indicates that all real numbers between "0 to 5" are listed in this specification, and "0 to 5" is merely an abbreviated representation of combinations of these numbers. Furthermore, when a parameter is described as being an integer of 2 or more, it is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12.

[0055] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0056] Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0057] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, but preferably sequentially. For example, when the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, when the method further includes step (c), it means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or steps (a), (c), and (b), or steps (c), (a), and (b), or otherwise.

[0058] Unless otherwise specified, the terms "includes" and "incorporates" in this application are open, but may also be closed. For example, the terms "includes" and "incorporates" may further include or incorporate other components not listed, or may include or incorporate only the listed components.

[0059] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following conditions satisfy the "A or B" condition: 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).

[0060] Polyvinylidene fluoride polymer is currently one of the most widely used binders in secondary batteries. However, adding polyvinylidene fluoride polymer to the film layer of an electrode sheet can degrade the flexibility of the electrode sheet and increase battery impedance, making it difficult to further increase the pressure density of the electrode sheet and limiting further improvements in the battery's volumetric energy density and cycle stability.

[0061] [Fluorine-containing polymer] Based on this, the present application provides a fluorine-containing polymer comprising constituent units derived from vinylidene fluoride and constituent units derived from unsaturated carboxylic acid monomers, wherein the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and selectively 5 million to 8 million.

[0062] In this specification, the term “polymer” includes, on the one hand, a collection of polymers that are chemically homogeneous but differ in degree of polymerization, molar mass and chain length, prepared by polymerization reactions. On the other hand, the term also includes derivatives of such polymer collections formed by polymerization reactions, i.e., compounds obtained by the reaction of functional groups in the polymers, e.g., addition or substitution, which may be chemically homogeneous or heterogeneous.

[0063] In this specification, the term "fluorine-containing polymer" refers to a polymer in which at least one hydrogen atom is replaced by a fluorine atom.

[0064] In this specification, the term "weight-average molecular weight" refers to the sum of the products of the weight fractions of molecules with different molecular weights and their corresponding molecular weights in a polymer.

[0065] In this specification, the term "unsaturated carboxylic acid monomer" refers to an unsaturated monomer containing a carboxyl functional group.

[0066] In this application, the weight-average molecular weight of fluorine-containing polymers can be measured using methods known in the art, such as gel chromatography, or by employing a Waters 2695 Isocratic HPLC-type gel chromatograph (differential refractive index detector 2141). In some embodiments, the measurement method involves using a 3.0% polystyrene solution sample as a reference and selecting a matched chromatography column (oil-based: Styragel HT5DMF 7.8*300mm + Styragel HT4). A 3.0% fluorine-containing polymer colloid is prepared in purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day to prepare it for use. During measurement, tetrahydrofuran is first drawn into the syringe, washed, and repeated several times. Next, 5 ml of the experimental solution is drawn into the syringe, air is removed, and the tip of the needle is wiped dry. Finally, the sample solution is gradually injected into the inlet. After the displayed value stabilizes, the data is acquired and the weight-average molecular weight is read.

[0067] In some embodiments, the weight-average molecular weight of the fluorine-containing polymer is 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 between any two of these values.

[0068] In some embodiments, a fluorine-containing polymer is used as a binder.

[0069] In this specification, the term "binding agent" refers to a chemical compound, polymer, or mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium.

[0070] In some embodiments, the dispersion medium for the binder is an oily solvent, and examples of oily solvents include, but are not limited to, dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dimethyl carbonate, ethylcellulose, and polycarbonate. That is, the binder is dissolved in the oily solvent.

[0071] In some embodiments, a binder is used to fix electrode active materials and / or conductive materials in appropriate positions and to attach them to conductive metal components to form electrodes.

[0072] In some embodiments, the binder is used as a positive electrode binder to adhere the positive electrode active material and / or conductive material to form an electrode.

[0073] In some embodiments, a binder is used as a negative electrode binder to adhere the negative electrode active material and / or conductive material to form an electrode.

[0074] Compared to conventional polymer binders, this fluorine-containing polymer maintains the bonding strength of the electrode sheet with a smaller amount of additive, while reducing the rigidity of the film layer of the electrode sheet, improving the flexibility of the electrode sheet, and reducing battery impedance. As a result, it can comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0075] The fluorine-containing polymer, with a weight-average molecular weight of 5 million to 8 million, can more effectively reduce battery impedance while considering the battery's critical volumetric energy density and cycle capacity retention rate.

[0076] In some embodiments, the molar content of constituent units derived from unsaturated carboxylic acid monomers in the fluorine-containing polymer is 0.5% to 2.5%, and selectively 0.7% to 2.0%, based on the total number of moles of constituent units.

[0077] In some embodiments, the molar content of constituent units derived from unsaturated carboxylic acid monomers is selectively within a numerical range of 0.5%, 0.7%, 1%, 1.5%, 2%, 2.5%, or any of these binary values.

[0078] Fluorine-containing polymers within the above range can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0079] A fluorine-containing polymer having a molar content of unsaturated carboxylic acid monomers at 0.7% to 2.0% of the total number of moles of constituent units in the fluorine-containing polymer can further reduce battery impedance and more effectively achieve both the limiting volumetric energy density and cycle capacity retention rate of the battery.

[0080] In some embodiments, the polydispersity coefficient of the fluorine-containing polymer is 1.5 to 3.5, and selectively 1.9 to 2.3.

[0081] In some embodiments, the polydispersity coefficient of the polymer is within the range 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 of these binary ranges.

[0082] In this specification, the term "polydispersion coefficient" refers to the ratio of the weight-average molecular weight of a fluorine-containing polymer to the number-average molecular weight of a fluorine-containing polymer.

[0083] In this specification, the term "number-average molecular weight" refers to the sum of the products of the mole fractions of molecules with different molecular weights and their corresponding molecular weights in a fluorine-containing polymer.

[0084] In this application, the polydispersity coefficient can be measured using a method known in the art, for example, by employing gel chromatography, and for example, by employing a Waters 2695 Isocratic HPLC-type gel chromatograph (differential refractive index detector 2141). In some embodiments, a 3.0% polystyrene solution sample is used as a reference, and a matched chromatography column (oil-based: Styragel HT5DMF 7.8*300mm + Styragel HT4) is selected. A 3.0% fluorine-containing polymer colloid is prepared in purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for 1 day to prepare it for use. At the time of measurement, first, tetrahydrofuran is drawn into the syringe, washed, and repeated several times. Next, 5 ml of the experimental solution is drawn into the syringe, the air in the syringe is removed, and the tip of the needle is wiped dry. Finally, the sample solution is gradually injected into the inlet. Data is acquired after the displayed value has stabilized. The weight-average molecular weight a and the number-average molecular weight b are read, respectively. The coefficient of multivariance is a / b.

[0085] Fluorine-containing polymers with polydispersity coefficients of 1.8 to 3.5 can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0086] Fluorine-containing polymers with a polydispersity coefficient of 1.9 to 2.3 can further enhance bonding strength, reduce battery impedance, and improve battery cycle stability.

[0087] In some embodiments, the crystallinity of the fluorine-containing polymer is 40% to 55%, and selectively 48% to 55%.

[0088] In some embodiments, the crystallinity of the fluorine-containing polymer is within a numerical range of 40%, 41%, 42%, 43%, 44%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, or any of these binary values.

[0089] In this specification, the term "crystallinity" refers to the proportion of the polymer occupied by crystalline regions. Within the microstructure, there are regions where molecules are stably and regularly arranged, and regions where molecules are regularly and closely arranged are called crystalline regions.

[0090] In this application, the degree of crystallinity can be measured using methods known in the art, such as differential scanning thermal analysis. In some embodiments, 0.5 g of fluorine-containing polymer is placed in an aluminum dry pan, shaken to flatten it, and covered with a crucible lid. Under a nitrogen atmosphere, with a purge gas of 50 ml / min, a protective gas of 70 mL / min, a heating rate of 8 °C / min, and a measurement temperature range of 30 °C to 200 °C, measurements are taken using a differential scanning calorimeter (DSC) model DSC 200 F30 manufactured by NETZSCH GmbH, Germany, to erase the thermal history.

[0091] In this measurement, the temperature fluctuation curve of DSC / (Mw / mg) of the fluorine-containing polymer is obtained and integrated. The peak area represents the enthalpy of fusion ΔH (J / g) of the fluorine-containing polymer. The degree of crystallinity of the fluorine-containing polymer = ΔH / (ΔHm100%)*100%, where ΔHm100% is the reference enthalpy of fusion (heat of fusion) of polyvinylidene fluoride, and ΔHm100% = 104.7 J / g.

[0092] Fluorine-containing polymers with a crystallinity within the above range can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and comprehensively enhance the critical volumetric energy density and cycle capacity retention rate of the battery.

[0093] In some embodiments, the Dv50 particle size of the fluorine-containing polymer is 20 μm to 100 μm.

[0094] In some embodiments, the Dv50 particle size of the fluorine-containing polymer is within the ranges of 20μm~35μm, 35μm~45μm, 45μm~55μm, 55μm~65μm, 65μm~75μm, 75μm~85μm, 85μm~95μm, 25μm~45μm, 45μm~65μm, 65μm~85μm, 85μm~100μm, or any numerical range between these two values.

[0095] In this specification, the term "Dv50 particle size" refers to the particle size at which the cumulative particle size distribution ratio of particles reaches 50% in the particle size distribution curve. Its physical meaning is that particles smaller (or larger) than that size account for 50% of the total particle size.

[0096] Referring to the particle size distribution laser diffraction method GB / T19077-2016, weigh 0.1g to 0.13g of fluorine-containing polymer powder into a 50ml beaker, then weigh 5g of anhydrous ethanol and add it to the beaker containing the fluorine-containing polymer powder. Insert a stirring bar approximately 2.5mm in length and seal with plastic wrap. Place the sample in an ultrasonic device for 5 minutes, then transfer to a magnetic stirrer and stir at a speed of 500 r / min for at least 20 minutes. Take two samples from each batch of product for measurement. The measurement is performed using a laser particle size analyzer such as the Mastersizer 2000E manufactured by Malvern, UK.

[0097] Fluorine-containing polymers with a Dv50 within the above range can improve the flexibility and critical pressure density of electrode sheets with small amounts of additive, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0098] In some embodiments, the unsaturated carboxylic acid monomer is as shown in formula I, [ka] In the formula, R1, R2, and R3 are each independently hydrogen and C. 1~3 It contains at least one alkyl group, and R4 is an ester group, a carbonyl group, or C1~3 It contains at least one alkyl group and one single bond.

[0099] In this specification, the term "ester group" refers to a group containing -C(O)O-.

[0100] In this specification, the term "carbonyl group" refers to a group containing -C(O)-.

[0101] In this specification, the term “alkyl group” means a saturated, unbranched, or branched aliphatic hydrocarbon from which one hydrogen atom has been removed, with the general formula C n H 2n+1 This refers to a monovalent base having the following properties, where n is an integer. 1~3 This refers to an alkyl group where n is an integer from 1 to 3.

[0102] In some embodiments, C 1~3 The alkyl group contains at least one of the following groups: methyl, ethyl, propyl, or isopropyl.

[0103] In some embodiments, R4 contains a single bond, i.e., the carboxyl functional group is directly bonded to a carbon atom that is bonded to a carbon-carbon double bond.

[0104] In some embodiments, R4 includes an ester group.

[0105] R4, which contains ester groups, can further improve the flexibility and bonding strength of the electrode sheet, thereby more effectively improving the battery's cycle stability and critical volumetric energy density.

[0106] In some embodiments, the unsaturated carboxylic acid monomer includes at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid.

[0107] In some embodiments, the unsaturated carboxylic acid monomer includes β-acryloyloxypropionic acid.

[0108] Unsaturated carboxylic acid monomers, including β-acryloyloxypropionic acid, can further enhance the flexibility and bonding strength of electrode sheets compared to other unsaturated carboxylic acid monomers, thereby more effectively improving the battery's cycle stability and critical volumetric energy density.

[0109] In some embodiments, the fluorine-containing polymer obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone has a viscosity of 1000 mPa·s to 6000 mPa·s when the fluorine-containing polymer has a mass content of 2%.

[0110] In some embodiments, the viscosity of a fluorine-containing polymer colloid with a mass content of 2%, obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone, is within the range of 1000 mPa·s, 1200 mPa·s, 1600 mPa·s, 2000 mPa·s, 2100 mPa·s, 2500 mPa·s, 2650 mPa·s, 2700 mPa·s, 2800 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, 6000 mPa·s, or any of these binary ranges.

[0111] In this application, the viscosity of a fluorine-containing polymer solution may be measured using methods known in the art, such as rotational viscometer measurement. For example, 14 g of fluorine-containing polymer and 686 g of N-methylpyrrolidone (NMP) are weighed into a 500 mL beaker to prepare a colloid with a mass fraction of 2%, which is then dispersed by stirring at a rotational speed of 800 r / min using a high-speed grinder manufactured by LICHEN, stirred for 120 minutes, and then bubbles are removed by ultrasonic shaking for 30 minutes. Using a rotational viscometer NDJ-5S manufactured by LICHEN, rotor No. 3 is selected and inserted into the colloid below the scale line, and the viscosity is measured at 12 r / min, with the viscosity data read after 6 minutes.

[0112] By controlling the viscosity of the fluorine-containing polymer colloid within an appropriate range, even with the addition of a small amount of fluorine-containing polymer, the slurry can be formed into a uniform electrode sheet with a certain bonding strength, thereby improving the cycle stability of the battery.

[0113] 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).

[0114] In one embodiment of this application, a method for preparing a fluorine-containing polymer is provided, which includes the steps of: preparing 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 under a non-water-soluble gas atmosphere; and adding a chain transfer agent to carry out a third-stage polymerization reaction to obtain a fluorine-containing polymer having a weight-average molecular weight of 5 million to 9 million.

[0115] By carrying out polymerization reactions in a stepwise manner, polymers with low polydispersity coefficients and high molecular weights can be prepared and obtained. Simultaneously, the first polymerization reaction forms the first product, the second polymerization reaction forms a molecular segment with a target molecular weight, and the third polymerization reaction adjusts the molecular weight of the polymer, reducing excessive randomness in the weight-average molecular weight and improving polymer uniformity. Furthermore, stepwise polymerization not only increases the utilization rate of the reactor during the polymer preparation process but also saves time and shortens the residence time of the polymer in the reactor. By coordinating the first, second, and third polymerization reactions, the polymer production efficiency can be further increased.

[0116] It is understood that the first product may be a reaction solution formed from vinylidene fluoride monomer and a solvent, or it may be a product obtained by processing and purifying the above reaction solution.

[0117] In some embodiments, the unsaturated carboxylic acid monomer is as shown in formula I, [ka] In the formula, R1, R2, and R3 are each independently hydrogen and C. 1~3 It contains at least one alkyl group, and R4 is an ester group, a carbonyl group, or C 1~3 It contains at least one alkyl group and one single bond.

[0118] In some embodiments, the reaction temperature of the first polymerization step is 45°C to 60°C. In some embodiments, the reaction temperature of the first polymerization step is 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, or 45°C to 55°C.

[0119] In some embodiments, the reaction time for the first polymerization step is 2 to 8 hours. In some embodiments, the reaction time for the first polymerization step is within the range of 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, or any binary range thereof.

[0120] In some embodiments, the polymerization pressure for the first stage polymerization reaction is 4 MPa to 6 MPa.

[0121] In some embodiments, the polymerization pressure for the first stage polymerization reaction is 4 MPa to 5 MPa, or 5 MPa to 6 MPa.

[0122] In some embodiments, the reaction temperature of the second polymerization step is 60°C to 80°C. In some embodiments, the reaction temperature of the second polymerization step is 60°C to 70°C or 70°C to 80°C.

[0123] In some embodiments, the reaction time for the second polymerization step is 2 to 4 hours. In some embodiments, the reaction time for the second polymerization step is 2 to 3 hours or 3 to 4 hours.

[0124] In some embodiments, the reaction pressure of the second polymerization reaction is 6 MPa to 8 MPa. In some embodiments, the reaction pressure of the second polymerization reaction is 6 MPa to 7 MPa or 7 MPa to 8 MPa.

[0125] In some embodiments, the reaction time for the third polymerization step is 1 to 2 hours.

[0126] By controlling the reaction pressure, reaction time, and reaction temperature of each polymerization step within appropriate ranges, it is possible to control the uniformity of the weight-average molecular weight of the polymerization product while achieving an increase in the weight-average molecular weight of the polymer. This ensures a product with a relatively low polydispersity coefficient and more uniform product properties. Even when only a small amount of the prepared polymer is added, the electrode sheet will have excellent flexibility and bonding strength, further improving the battery's cycle capacity retention rate.

[0127] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0128] Non-water-soluble gases refer to gases with a gas solubility of less than 0.1 L. Gas solubility refers to the volume of gas when dissolved in 1 L of water to a saturated state at 20°C and a gas pressure of 1.013 × 10⁵ Pa.

[0129] In some embodiments, the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.

[0130] In some embodiments, the dose of the chain transfer agent is 1.5% to 4% of the total mass of vinylidene fluoride monomer and the monomer represented by formula I. The dose of the chain transfer agent may be, for example, 2%, 2.5%, 3%, 3.5%, or within any binary range thereof.

[0131] By controlling the dosage of the chain transfer agent within an appropriate range, the chain length of the polymer can be controlled, and a polymer within an appropriate molecular weight range can be obtained.

[0132] In some embodiments, the first step of the polymerization reaction includes the steps of: adding an aqueous solvent and a dispersant to a container to remove oxygen from the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH to 6.5-7, then adding vinylidene fluoride monomer to bring the pressure inside the container to 4MPa-6MPa; and stirring for 30-60 minutes, then raising the temperature to 45°C-60°C, and adding an unsaturated carboxylic acid monomer to carry out the first step of the polymerization reaction.

[0133] In some embodiments, the amount of solvent is 2 to 8 times the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer. The amount of solvent may be, for example, 3, 4, 5, 6, or 7 times the total mass of the vinylidene fluoride monomer and the monomer represented by formula I.

[0134] In some embodiments, the solvent is water.

[0135] In some embodiments, the dispersant comprises at least one of cellulose, cellulose ether, and polyvinyl alcohol. Selectively, the cellulose comprises hydroxypropyl methylcellulose, and the cellulose ether comprises one or more of methylcellulose ether and carboxyethylcellulose ether.

[0136] In any embodiment, the dose of the dispersant is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

[0137] In some embodiments, the initiator includes at least one of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, tert-butyl peroxypivalate, and diisopropyl peroxydicarbonate.

[0138] In some embodiments, the dose 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.

[0139] In some embodiments, the unsaturated carboxylic acid monomer is added in multiple steps during the first polymerization reaction.

[0140] The polymerization reaction between unsaturated carboxylic acid monomers and vinylidene fluoride is an exothermic reaction. The heat causes the solvent in the reaction vessel to volatilize, leading to a rapid increase in pressure. By adding the solvent multiple times to stabilize and maintain the reaction pressure, fluorine-containing polymers can be uniformly polymerized with a relatively low polydispersity coefficient.

[0141] [Positive electrode sheet] The positive electrode sheet includes a positive electrode film layer containing a fluorine-containing polymer in any embodiment, or a fluorine-containing polymer prepared by a preparation method in any embodiment.

[0142] In some embodiments, the mass fraction of the fluorine-containing polymer is 0.3% to 1.1% of the total mass of the positive electrode slurry, and selectively 0.4% to 0.8%.

[0143] In some embodiments, the mass fraction of the fluorine-containing polymer is within a numerical range of 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 of these binary values.

[0144] Fluorine-containing polymers within the above range can improve the flexibility and critical pressure density of the electrode sheet while maintaining the bonding strength of the electrode sheet, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0145] In some embodiments, the positive electrode sheet is 3.5 g / cm². 3 ~3.7g / cm 3 The average number of bends possible at the given compression density is 2 or more, selectively 2 to 4 times.

[0146] In some embodiments, the positive electrode sheet is 3.5 g / cm². 3 ~3.7g / cm 3 The average number of bends possible at a given pressure density is selectively within a numerical range of 2, 3, 4, or any binary value between these.

[0147] The average number of bends the positive electrode sheet can withstand can be measured by any known method. The pressure density is calculated by dividing the mass of one positive electrode film layer by the volume of the positive electrode film layer. Pressure density: 3.6 g / cm³ 3 The positive electrode sheet is 20 x 100 mm 2 The sample is cut to a size suitable for measurement, folded in the forward direction, flattened with a 2kg roller, and then unfolded to check if light can pass through the gaps. If light does not pass through, it is folded in the reverse direction, flattened with a 2kg roller, and then checked again by exposing it to light. This process is repeated until light can pass through the gaps, and the number of folds is recorded. The measurement is repeated 10 times, and the average value is used to characterize the flexibility of the positive electrode sheet as the average number of bends the positive electrode sheet can withstand.

[0148] In some embodiments, the critical pressure density of the positive electrode sheet is 3.6 g / cm³. 3 The above is the result, and selectively 3.6 g / cm³ 3 ~3.7g / cm 3 That is the case.

[0149] In some embodiments, the critical pressure density of the positive electrode sheet is selectively 3.6 g / cm³. 3 3.65 g / cm³ 3 3.7 g / cm³ 3 , or within the numerical range between any two of these values.

[0150] The critical pressure density refers to the maximum pressure density corresponding to the case where the electrode sheet has an elongation rate of 7‰ or more and its flexibility allows it to be folded two or more times. Increasing the critical pressure density of the electrode sheet helps to improve the volumetric energy density of the battery.

[0151] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector.

[0152] The positive electrode sheet exhibits excellent flexibility.

[0153] For example, a positive electrode current collector has two opposing surfaces in its own thickness direction, and the positive electrode film layer is provided on one or both of the two opposing surfaces of the positive electrode current collector.

[0154] In some embodiments, a metal foil or a composite current collector can be used as the positive electrode current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate layer and a metal layer formed on at least one surface of the polymer material substrate layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer material substrate (for example, a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0155] In some embodiments, the positive electrode active material may be a positive electrode active material known in the art for batteries. For example, the positive electrode active material may include at least one of olivine-structured lithium-containing phosphates, lithium transition metal oxides, and modified compounds of each thereof. However, this application is not limited to these materials, and other conventional materials usable as positive electrode active materials for batteries may be used. These positive electrode active materials may be used individually or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxide (LiCoO2, etc.), lithium nickel oxide (LiNiO2, etc.), lithium manganese oxide (LiMnO2, LiMn2O4, etc.), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM 333 (Also abbreviated as LiNi) 0.5 Co 0.2 Mn 0.3 O2(NCM 523 (Also abbreviated as LiNi) 0.5 Co 0.25 Mn 0.25 O2(NCM 211 (Also abbreviated as LiNi) 0.6 Co 0.2 Mn 0.2 O2(NCM 622 (Also abbreviated as LiNi) 0.8 Co 0.1 Mn 0.1 O2(NCM 811 (Also abbreviated as LiNi) Lithium nickel cobalt aluminum oxide (LiNi 0.85 Co 0.15 Al 0.05Examples include, but are not limited to, lithium iron phosphate (LiFePO4 (also abbreviated as LFP)), lithium iron phosphate and carbon composite materials, lithium manganese phosphate (LiMnPO4), lithium manganese phosphate and carbon composite materials, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composite materials.

[0156] In some embodiments, the cathode film layer selectively further comprises 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.

[0157] In some embodiments, the positive electrode sheet can be manufactured as follows: The above components for manufacturing the positive electrode sheet, such as a positive electrode active material, a conductive agent, a binder, and any other components, are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, the positive electrode slurry is coated onto a positive electrode current collector, and the positive electrode sheet can be obtained through processes such as baking and cold pressing.

[0158] [Negative electrode sheet] 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, the negative electrode film layer containing a negative electrode active material.

[0159] For example, the negative electrode current collector has two opposing surfaces in its own thickness direction, and the negative electrode film layer is provided on one or both of the two opposing surfaces of the negative electrode current collector.

[0160] In some embodiments, a metal foil or a composite current collector can be used as the negative electrode current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys) on a polymer material substrate (for example, a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0161] In some embodiments, the negative electrode active material can be any negative electrode active material known in the art for batteries. For example, the negative electrode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be at least one selected from elemental silicon, silicon oxygen compounds, silicon carbon composites, silicon nitrogen composites, and silicon alloys. The tin-based material may be at least one selected from elemental tin, tin oxygen compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials usable as negative electrode active materials for batteries may be used. These negative electrode active materials may be used individually or in combination of two or more types.

[0162] In some embodiments, the negative electrode film layer further selectively comprises a binder. The binder may be at least one selected from 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).

[0163] In some embodiments, the negative electrode film layer further selectively comprises a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0164] In some embodiments, the negative electrode film layer further comprises other additives, such as a selective thickener (e.g., sodium carboxymethylcellulose (CMC-Na)).

[0165] In some embodiments, the negative electrode sheet can be manufactured as follows: Components for manufacturing the negative electrode sheet, such as a negative electrode active material, a conductive agent, a binder, and any other components, are dispersed in a solvent (e.g., deionized water) to form a negative electrode slurry; the negative electrode slurry is applied to a negative electrode current collector; and the negative electrode sheet can be obtained through processes such as oven drying and cold pressing.

[0166] [Electrolyte] The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. In this application, the type of electrolyte is not specifically limited and can be selected as needed. For example, the electrolyte may be liquid, gel-like, or all-solid.

[0167] In some embodiments, the electrolyte is an electrolyte solution, which comprises an electrolyte salt and a solvent.

[0168] In some embodiments, the electrolyte salt may be at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0169] In some embodiments, the solvent may be at least one selected from ethylene carbonate, propylene carbonate, ethylmethyl 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, sulfolane, dimethyl sulfone, ethylmethyl sulfone, and diethyl sulfone.

[0170] In some embodiments, the electrolyte further selectively includes additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may further include additives that can improve certain aspects of the battery's performance, such as additives that can improve the battery's overcharge performance, or additives that can improve the battery's high-temperature or low-temperature performance.

[0171] [Separator] In some embodiments, the secondary battery further includes a separator. In this application, the type of separator is not particularly limited, and any known porous structure separator having good chemical and mechanical stability can be selected.

[0172] In some embodiments, the material of the separator may be at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multilayer composite film, and is not particularly limited. If the separator is a multilayer composite film, the materials of each layer may be the same or different, and are not particularly limited.

[0173] In some embodiments, the positive electrode sheet, negative electrode sheet, and separator can be assembled into an electrode assembly by a winding process or a lamination process.

[0174] In some embodiments, the secondary battery may include an casing. This casing can be used to enclose the electrode assembly and electrolyte.

[0175] In some embodiments, the casing of the secondary battery may be a rigid case, such as a hard plastic case, an aluminum case, or a steel case. The casing of the secondary battery may also be a soft pack, such as a pouch-type soft pack. The material of the soft pack may be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0176] In this application, the shape of the secondary battery is not particularly limited and may be cylindrical, rectangular, or any other shape. For example, Figure 1 shows a rectangular secondary battery 5 as an example.

[0177] In some embodiments, referring to Figure 2, the exterior may include a case 51 and a cover plate 53. Here, the case 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and side plates surround the case 51 to form a housing cavity. The case 51 has an opening that communicates with the housing cavity, and the cover plate 53 can cover the opening to seal the housing cavity. The positive electrode sheet, negative electrode sheet and separator can be formed into an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is sealed within the housing cavity. The electrolyte is impregnated into the electrode assembly 52. ​​The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and a person skilled in the art can select according to the specific actual needs.

[0178] In some embodiments, the secondary battery may be assembled as a battery module, and the number of secondary batteries included in the battery module may be one or more, the specific number of which can be selected by those skilled in the art depending on the application and capacity of the battery module.

[0179] Figure 3 shows an example of a battery module 4. Referring to Figure 3, in the battery module 4, the multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, they may be arranged in any other manner. Furthermore, the multiple secondary batteries 5 may be fixed by fastening members.

[0180] Selectively, the battery module 4 may further include a housing having a housing space, in which a plurality of secondary batteries 5 are housed.

[0181] In some embodiments, the battery modules may be assembled as a battery pack, and the number of battery modules included in the battery pack may be one or more, the specific number of which can be selected by those skilled in the art depending on the application and capacity of the battery pack.

[0182] Figures 4 and 5 show an example of a battery pack 1. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided within the battery box. The battery box includes an upper box 2 and a lower box 3, the upper box 2 covering the lower box 3 and forming a sealed space for housing the battery modules 4. The plurality of battery modules 4 may be arranged within the battery box in any manner.

[0183] Furthermore, this application provides an electrical device comprising at least one of the secondary battery, battery module, or battery pack provided herein. The secondary battery, battery module, or battery pack may be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), trains, ships and satellites, energy storage systems, etc.

[0184] The aforementioned electrical device can be selected as a secondary battery, battery module, or battery pack, depending on its intended use.

[0185] Figure 6 shows an example of an electrical device. This electrical device may be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the requirements for high power output and high energy density of secondary batteries, this electrical device may use a battery pack or battery module.

[0186] Other examples of devices may include mobile phones, tablet computers, and laptop computers. Such devices are typically required to be lightweight and thin, and may use rechargeable batteries as a power source.

[0187] Examples Examples of the present application are described below. The examples described below are illustrative and are for interpretive purposes only, and should not be understood as limiting this application. Unless otherwise specified in the examples, specific techniques or conditions are followed in accordance with the techniques or conditions or product specifications described in the literature in the art. Unless otherwise specified, the reagents or equipment used are common commercially available products.

[0188] 1. Preparation method Example 1 1) Preparation of fluorine-containing polymers Polymerization step 1: 4 kg of deionized water and 2 g of methylcellulose ether were added to 10 L autoclaves 1 and 2, and the mixture 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 packed in to a pressure of 5 MPa. The mixture was stirred for 30 minutes, the temperature was raised to 45°C, and an aqueous solution containing 22.5 g of β-acryloyloxypropionic acid was added in multiple batches, and the reaction was allowed to proceed for 3.0 hours.

[0189] Second stage polymerization reaction: The reaction solutions in reaction vessels 1 and 2 were transferred to reaction vessel 3, nitrogen was added until the pressure reached 7 MPa, the temperature was raised to 70°C, and the reaction was carried out for 3 hours with stirring.

[0190] Third stage polymerization reaction: 40 g of cyclohexane was added, and the reaction was continued for 1 hour before being stopped. After centrifuging the reaction system, the solid phase was recovered, washed, and dried to obtain polyvinylidene fluoride copolymer.

[0191] 2) Preparation of the positive electrode sheet 4935g of NCM (nickel-cobalt-manganese ternary material) and 30g of fluorine-containing polymer were placed in a planetary stirring tank and stirred for 20 minutes at a rotational speed of 25 r / min. Here, the mass fraction of the fluorine-containing polymer was 0.6% based on the total mass of the positive electrode film layer.

[0192] 1.67 kg of N-methylpyrrolidone (NMP) solution was placed in a stirring tank and stirred for 50 minutes at an orbital rotation speed of 25 r / min and a rotational rotation speed of 800-1000 rpm.

[0193] 5g of hydrogenated nitrile rubber (HNBR) and 30g of single-walled carbon nanotubes (CNTs) were placed in a stirring tank as dispersants, and the tank was stirred for 50 minutes at an orbital rotation speed of 25 r / min and a rotational rotation speed of 1200-1500 r / min.

[0194] After stirring was complete, the slurry viscosity was measured and controlled to 8000-15000 MPa·s.

[0195] If the viscosity was too high, an N-methylpyrrolidone (NMP) solution was added to reduce the viscosity to within the above range. After adding the NMP solution, the mixture was stirred for 30 minutes at an orbital speed of 25 r / min and a rotational speed of 1250 r / min to obtain a positive electrode slurry. After adding the NMP solution, the mixture was stirred for 30 minutes at an orbital speed of 25 r / min and a rotational speed of 1200-1500 r / min to obtain a positive electrode slurry. The obtained positive electrode slurry was knife-coated onto the top surface of carbon-coated aluminum foil, baked at 110°C for 15 minutes, cold-pressed, and then cut into 15 mm diameter discs to obtain a positive electrode sheet.

[0196] 3) Negative electrode sheet A negative electrode slurry was prepared by dissolving artificial graphite as the active material, carbon black as the conductive agent, styrene-butadiene rubber (SBR) as the binder, and sodium hydroxymethylcellulose (CMC-Na) as the thickener in deionized water as the solvent in a mass ratio of 96.2:0.8:0.8:1.2 and mixing them uniformly. The negative electrode slurry was then uniformly applied to the copper foil of the negative electrode current collector one or more times, and a negative electrode sheet was obtained by oven drying, cold pressing, and cutting.

[0197] 4) Separator A polypropylene film was used as the separator.

[0198] 5) Preparation of the electrolyte In an argon-atmosphered glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC), both organic solvents, were uniformly mixed in a 3 / 7 volume ratio. LiPF6 lithium salt was added and dissolved in the organic solvent, and the mixture was uniformly stirred to prepare a 1 M LiPF6EC / EMC solution, thereby obtaining the electrolyte.

[0199] 6) Battery preparation The positive electrode sheet, separator, and negative electrode sheet of Example 1 were stacked in this order, with the separator acting to separate them from the positive electrode sheet. These were then wound to obtain a bare cell, tabs were welded to the bare cell, the bare cell was placed in an aluminum case, and immediately after baking at 80°C to remove water, the electrolyte was injected and sealed to obtain an uncharged battery. The uncharged battery was then subjected to sequential processes such as standing, hot pressing, cold pressing, chemical conversion, molding, and capacity measurement to obtain the lithium-ion battery product of Example 1.

[0200] Examples 2-5 The procedure is basically the same as in Example 1, except that the reaction times in the first polymerization step were adjusted to 5h, 6h, 7h, and 8h, respectively, and the amount of cyclohexane in the third polymerization step was adjusted to 35g, 30g, 25g, and 20g, respectively. The specific parameters are shown in Table 1.

[0201] Examples 6-9 This example is essentially the same as Example 2, except that the molar content of β-acryloyloxypropionic acid was adjusted based on the total number of moles of constituent units in the fluorine-containing polymer. The specific parameters are shown in Table 1.

[0202] Examples 10-13 This example is basically the same as Example 2, except that the mass fraction of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer was adjusted, the total mass of the electrode sheet film layer was kept the same, and the mass content of the positive electrode active material was appropriately adjusted while adjusting the content of the fluorinated copolymer. The specific parameters are shown in Table 1.

[0203] Examples 14-16 This is essentially the same as Example 2, except that 1% molar β-acryloyloxypropionic acid was replaced with 1% molar acrylic acid, 1% molar methacrylic acid, and 1% molar maleic acid, respectively. The specific parameters are shown in Table 1.

[0204] Example 17 The procedure is essentially the same as in Example 2, except that an aqueous solution of β-acryloyloxypropionic acid was added once to the first step of the polymerization reaction. The specific parameters are shown in Table 1.

[0205] Comparative Example 1 The cathode sheet used was a polyvinylidene fluoride-acrylic acid copolymer with a weight-average molecular weight of 1.1 million, purchased from Solvay, Inc. in the United States. The amount of cathode binder added was 2.0%, and the mass content of the cathode active material was adjusted as appropriate. Except for these differences, the procedure was basically the same as in Example 1. The specific parameters are shown in Table 1.

[0206] Comparative Example 2 This is essentially the same as Example 2, except that the polymerization monomer is solely vinylidene fluoride. The specific parameters are shown in Table 1.

[0207] 2. Battery performance testing 1. Measurement of the properties of fluorine-containing polymers 1) Measurement of weight-average molecular weight A Waters 2695 isocratic HPLC-type gel chromatograph (differential refractive index detector 2141) was used. A 3.0% polystyrene solution sample was used as a reference, and a matching chromatography column (oil-based: Styragel HT5DMF 7.8*300mm + Styragel HT4) was selected. A 3.0% fluorine-containing polymer colloid was prepared in purified N-methylpyrrolidone (NMP) solvent, and the prepared solution was allowed to stand for 1 day to prepare for use. For measurement, tetrahydrofuran was first aspirated using a syringe, washed, and repeated several times. Next, 5 ml of the experimental solution was aspirated, air was removed from the syringe, and the tip of the needle was wiped dry. Finally, the sample solution was gradually injected into the inlet. After the displayed value stabilized, data was acquired, and the weight-average molecular weight was read.

[0208] 2) Measurement of the multivariance coefficient A Waters 2695 isocratic HPLC-type gel chromatograph (differential refractive index detector 2141) was used. A 3.0% polystyrene solution sample was used as a reference, and a matching chromatography column (oil-based: Styragel HT5DMF 7.8*300mm + Styragel HT4) was selected. A 3.0% fluorine-containing polymer colloid was prepared in purified N-methylpyrrolidone (NMP) solvent, and the prepared solution was allowed to stand for 1 day to prepare for use. For measurement, tetrahydrofuran was first aspirated using a syringe, washed, and repeated several times. Next, 5 ml of the experimental solution was aspirated, air was removed from the syringe, and the tip of the needle was wiped dry. Finally, the sample solution was gradually injected into the inlet. Data was acquired after the displayed value stabilized. The weight-average molecular weight a and number-average molecular weight b were read, respectively. The polydispersity coefficient was a / b.

[0209] 3) Measurement of Dv50 Referring to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method, 0.1g to 0.13g of fluorine-containing polymer powder was weighed into a 50ml beaker, 5g of anhydrous ethanol was weighed, and these were added to the beaker containing the fluorine-containing polymer powder. A stirring bar approximately 2.5mm in length was placed inside, and the beaker was sealed with plastic wrap. The sample was placed in an ultrasonic device for 5 minutes, then transferred to a magnetic stirrer and stirred at a speed of 500 r / min for more than 20 minutes. Two samples were extracted from each batch of product and measured. The measurements were conveniently performed using laser particle size analyzers such as the Mastersizer 2000E laser particle size analyzer manufactured by Malvern, UK.

[0210] 4) Measurement of crystallinity 0.5 g of fluorine-containing polymer was placed in an aluminum dry pan, shaken to flatten, and covered with a crucible lid. Measurements were taken under a nitrogen atmosphere using a differential scanning calorimeter (DSC) model DSC200F30 manufactured by NETZSCH GmbH, Germany, with a purge gas flow rate of 50 mL / min, a protective gas flow rate of 70 mL / min, a heating rate of 8 °C / min, and a measurement temperature range of 30 °C to 200 °C, and the thermal history was erased.

[0211] In this measurement, the temperature fluctuation curve of DSC / (Mw / mg) for the fluorine-containing polymer was obtained and integrated. The peak area represented the enthalpy of fusion ΔH (J / g) of the fluorine-containing polymer. The degree of crystallinity of the fluorine-containing polymer was given by ΔH / (ΔHm100%)*100%, where ΔHm100% is the reference enthalpy of fusion (heat of fusion) of polyvinylidene fluoride, and ΔHm100% = 104.7 J / g.

[0212] 5) Measurement of colloidal viscosity 14 g of fluorine-containing polymer and 686 g of N-methylpyrrolidone (NMP) were weighed into a 500 ml beaker to prepare a 2% mass colloid. The colloid was dispersed by stirring at a rotation speed of 800 r / min for 120 min using a LICHEN high-speed grinder, and then bubbles were removed by ultrasonic shaking for 30 min. Using a LICHEN NDJ-5S rotational viscometer, rotor No. 3 was selected and inserted into the colloid so as to exceed the scale line, and the viscosity was measured at 12 r / min. The viscosity data was then read after 6 min.

[0213] 2. Measurement of bonding strength Referring to the national standard GB-T2790-1995 "Experimental Method for 180° Peel Strength of Adhesives," the bonding strength measurement process for the examples and comparative examples of this application was as follows: A sample with a width of 30 mm and a length of 100-160 mm was cut with a blade, and a special double-sided tape with a width of 20 mm and a length of 90-150 mm was attached to the steel plate. After attaching the positive electrode film layer of the electrode sheet sample cut earlier to the double-sided tape, it was rotated three times in the same direction using a 2 kg roller. A paper tape with the same width as the electrode sheet and a length of 250 mm was attached to the current collector of the electrode sheet and fixed with crepe adhesive. The power of the Sansi tensioning machine (sensitivity 1 N) was turned on, and when the indicator lamp lit up, the limit block was adjusted to the appropriate position, and the end of the steel plate that did not have the electrode sheet attached was fixed with the lower clamp. The paper tape was folded upward and fixed with the upper clamp, and the position of the upper clamp was adjusted using the "up" and "down" buttons on the manual controller attached to the tensioning machine. Next, the measurement was performed and the value was read. The bonding force of the electrode sheet per unit length was defined as the force at which the electrode sheet reached a state of force equilibrium, divided by the tape width, and this was used to characterize the bonding strength between the positive electrode film layer and the current collector.

[0214] 3. Measurement of the critical pressure density of the electrode sheet. Electrode sheets coated on both sides were cold-pressed using a roller press. The elongation of the electrode sheets after cold pressing was measured, and the flexibility of the electrode sheets after cold pressing was evaluated.

[0215] By increasing the pressure of the roller press, electrode sheets with different pressure densities were obtained. As the pressure increased, the pressure density of the electrode sheet increased, the elongation of the electrode sheet increased, and the flexibility of the electrode sheet decreased. If the elongation of the electrode sheet was too high, the electrode sheet became prone to warping, and if the flexibility of the electrode sheet was too low, the electrode sheet became brittle and prone to fracture. Therefore, the critical pressure density was defined as the corresponding maximum pressure density of the electrode sheet when the elongation of the electrode sheet was 7‰ or higher and the number of folds due to the flexibility of the electrode sheet was 2 or more.

[0216] The pressure density was calculated by dividing the mass of one side of the positive electrode film layer by the volume of the positive electrode film layer.

[0217] The method for measuring the elongation rate was as follows:

[0218] The electrode sheets were laid flat on a horizontal table and cut so that each electrode sheet was approximately 100 cm long. The copper foil of the base material was removed from the edges of the electrode sheets, and care was taken to ensure that the cut edges of the electrode sheets were parallel to the MD direction (perpendicular to the direction of the cold press roller) of the electrode sheet, and that the electrode sheet was completely covered with the coating. The front and back of the electrode sheet were measured using a steel ruler, and the length between the marking points at positions where the width was the same in the length direction was estimated to the nearest 0.1 mm and recorded as the length before cold pressing. After cold pressing with a roller press, the length between the corresponding marking points after cold pressing was recorded, and the elongation rate of the electrode sheet was calculated as (length after cold pressing - length before cold pressing) / length before cold pressing.

[0219] The method for measuring the number of folds based on flexibility was as follows:

[0220] 4. Compression density: 3.6 g / cm³ 3 Measurement of the number of times the electrode sheet can be folded due to its flexibility. Compression density: 3.6 g / cm³ 3 The positive electrode sheet is 20 x 100 mm 2 A sample of the required size was cut, folded in the forward direction, flattened with a 2kg roller, and then unfolded to check if light could pass through the gaps. If light did not pass through, it was folded in the reverse direction, flattened with a 2kg roller, and tested again with light. This process was repeated until light could pass through the gaps, and the number of folds was recorded. The measurement was repeated 10 times, and the average value was used as reference data for the flexibility of the electrode sheet.

[0221] 5. Battery performance testing 1) Measurement of DC impedance The secondary battery was charged at 25°C at a rate of 1 / 3C to 4.25V with a constant current, then charged at a constant voltage of 4.25°C to a current of 0.05C and left for 5 minutes. Next, it was discharged at a rate of 1 / 3C for 90 minutes, the electrode assembly was adjusted to 50% SOC and left standing for 60 minutes, and then discharged at a rate of 4C for 30 seconds. Based on the measurement data, the 50% SOC discharge DCR was obtained. The 50% SOC discharge DCR of the second cycle was used as the measurement result for this example.

[0222] 2) Measurement of battery capacity retention rate The battery capacity retention rate was measured as follows: A button cell was charged to 3.65V with a constant current of 1 / 3C at 25°C, then charged to 0.05C with a constant voltage of 3.65V, left for 5 minutes, and then discharged to 2.5V with 1 / 3C. The obtained capacity was recorded as the initial capacity C0. The above steps were repeated for the same battery, and the discharge capacity Cn of the battery after the nth cycle was recorded. The battery capacity retention rate Pn after each cycle was calculated as Pn = Cn / C0 × 100%, and the values ​​of 100 points P1, P2...P100 were used as the vertical coordinate, and the corresponding number of cycles as the horizontal coordinate to obtain the battery cycle capacity retention rate.

[0223] In this measurement, the first cycle corresponds to n=1, the second cycle to n=2, ... the 100th cycle to n=100. The battery capacity retention rate data corresponding to Examples 1-17 or Comparative Examples 1-6 in Table 1 are data measured after repeating 500 cycles under the above measurement conditions, i.e., the P500 value.

[0224] 3) Measurement of battery volumetric energy density at critical pressure density The battery at its limit pressure density was left standing at 25°C for 2 hours to maintain the battery temperature at 25°C. At 25°C, the battery was charged at 0.33C until it reached the charge cutoff voltage, and then constant voltage charging was continued at the same cutoff voltage until the current reached 0.05C and charging was stopped (where C0 represents the battery's rated capacity). After leaving the battery standing at 25°C for 1 hour, the battery was discharged at 25°C at 0.33C until it reached the discharge cutoff voltage, and the total discharge energy of the battery was recorded as E0.

[0225] The volume of the battery was measured as V0.

[0226] The volumetric energy density of a battery = battery discharge energy E0 / battery volume V0.

[0227] III. Analysis of the test results of each example and comparative example. Batteries for each example and comparative example were prepared according to the method described above, and each performance parameter was measured. The results are shown in Tables 1, 2, and 3 below.

[0228] [Table 1]

[0229] [Table 2]

[0230] [Table 3] From the above results, it can be seen that the fluorine-containing polymers of Examples 1 to 17 contain constituent units derived from vinylidene fluoride and at least one of constituent units derived from acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid, and that the weight-average molecular weight of the fluorine-containing polymers is between 5 million and 9 million.

[0231] A comparison of Examples 1-17 with Comparative Example 1 shows that vinylidene fluoride-β-acryloyloxypropionic 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 improve the flexibility and critical pressure density of electrode sheets with a smaller amount of additive compared to conventional binders, reduce battery impedance, and thereby comprehensively improve the critical volumetric energy density and cycle capacity retention rate of the battery.

[0232] Table 3 shows that the fluorine-containing polymers provided in this disclosure, like vinylidene fluoride homopolymers with the same weight-average molecular weight and the same amount added, improve the flexibility and critical pressure density of electrode sheets, thereby comprehensively increasing the critical volumetric energy density and cycle capacity retention rate of batteries.

[0233] As described in Examples 1 to 5, the above fluorine-containing polymers with a weight-average molecular weight of 5 million to 8 million can more effectively reduce battery impedance while considering the critical volumetric energy density and cycle capacity retention rate of the battery.

[0234] Examples 1 to 17 show that, based on the total number of moles of constituent units in the fluorine-containing polymer, the molar content of constituent units derived from unsaturated carboxylic acid monomers is 0.5% to 2.5%, and based on the total number of moles of constituent units in the fluorine-containing polymer, the vinylidene fluoride-β-acryloyloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binders can improve the flexibility and critical pressure density of the electrode sheet with small amounts of additive, reduce battery impedance, and thereby comprehensively improve the critical volumetric energy density and cycle capacity retention rate of the battery.

[0235] As described in Examples 2, 6-9, a fluorine-containing polymer having a molar content of unsaturated carboxylic acid monomers at 0.7% to 2.0% based on the total number of moles of constituent units in the fluorine-containing polymer can further reduce battery impedance and more effectively achieve both the critical volumetric energy density and cycle capacity retention rate of the battery.

[0236] Examples 1 to 17 show that vinylidene fluoride-β-acryloyloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binders with polydispersity coefficients of 1.8 to 3.5 improve the flexibility and critical pressure density of electrode sheets with smaller additive amounts compared to conventional binders, reduce battery impedance, and thereby comprehensively improve the critical volumetric energy density and cycle capacity retention rate of the battery.

[0237] As described in the examples, fluorine-containing polymers with polydispersity coefficients of 1.9 to 2.3 can further enhance bonding strength, reduce battery impedance, and improve battery cycle stability.

[0238] As described in Examples 2, 14-16, fluorine-containing polymers containing β-acryloyloxypropionic acid as the monomer of formula I can more effectively reduce battery impedance, increase the flexibility of electrode sheets, and further improve the critical volumetric energy density and cycle stability of the battery.

[0239] Examples 1 to 17 show that when the mass fraction of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer, vinylidene fluoride-acrylic acid copolymer, vinylidene fluoride-methacrylic acid copolymer, and vinylidene fluoride-maleic acid copolymer binder 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 critical pressure density of the electrode sheet while maintaining the bonding strength of the electrode sheet, reduce battery impedance, and thereby comprehensively increase the critical volumetric energy density and cycle capacity retention rate of the battery.

[0240] As described in Examples 2, 10-13, when the mass fraction of the fluorine-containing polymer is 0.4% to 0.8% based on the total mass of the positive electrode film layer, it can be seen that the battery impedance can be further reduced and the limiting volumetric energy density and cycle stability of the battery can be improved.

[0241] It should be noted that this application is not limited to the embodiments described above. The embodiments described above are merely illustrative, and all embodiments having substantially the same technical idea and achieving the same function and effect within the scope of the technical solution of this application are included in the technical scope of this application. Furthermore, other forms that are constructed by adding various modifications to the embodiments that a person skilled in the art could conceive of, and by combining some of the components of the embodiments, are also included in the scope of this application, without departing from the gist of this application. [Explanation of symbols]

[0242] 1 Battery pack 2. Top box 3. Lower box 4 Battery Modules 5 Secondary battery 51 cases 52 Electrode assembly 53 Top cover assembly

Claims

1. A fluorine-containing polymer comprising constituent units derived from vinylidene fluoride and constituent units derived from unsaturated carboxylic acid monomers, characterized in that the weight-average molecular weight of the fluorine-containing polymer is 5 million to 9 million, and selectively 5 million to 8 million.

2. The fluorine-containing polymer according to claim 1, characterized in that, based on the total number of moles of constituent units in the fluorine-containing polymer, the molar content of constituent units derived from unsaturated carboxylic acid monomers is 0.5% to 2.5%, and selectively 0.7% to 2.0%.

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

3.

4. The fluorine-containing polymer according to any one of claims 1 to 3, characterized in that the degree of crystallinity of the fluorine-containing polymer is 40% to 55%, and selectively 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 μm to 100 μm.

6. The aforementioned unsaturated carboxylic acid monomer is represented by formula I, 【Chemistry 1】 In the formula, R １ , R ２ , R ３ Each of them independently produces hydrogen and C １～３ It contains at least one alkyl group, R ４ is an ester group, carbonyl group, C １～３ A fluorine-containing polymer according to any one of claims 1 to 5, characterized by containing at least one of an alkyl group and a single bond.

7. R ４ A fluorine-containing polymer according to any one of claims 1 to 6, characterized in that it contains 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 comprises at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid.

9. The fluorine-containing polymer according to any one of claims 1 to 8, characterized in that the fluorine-containing polymer obtained by dissolving the fluorine-containing polymer in N-methylpyrrolidone has a colloidal viscosity of 1,000 mPa·s to 6,000 mPa·s at a mass content of 2%.

10. The fluorine-containing polymer according to any one of claims 1 to 9, characterized in that the fluorine-containing polymer comprises 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).

11. A method for preparing a fluorine-containing polymer, The present invention provides vinylidene fluoride monomer, unsaturated carboxylic acid monomer and solvent, and carries out a first step polymerization reaction to obtain a first product. The first product is subjected to a second polymerization reaction under a non-water-soluble gas atmosphere, A preparation method characterized by comprising the step of adding a chain transfer agent to carry out a third-stage polymerization reaction to obtain a fluorine-containing polymer having a weight-average molecular weight of 5 million to 9 million.

12. The aforementioned unsaturated carboxylic acid monomer is represented by formula I, 【Chemistry 2】 wherein R １ , R ２ , R ３ are each independently hydrogen, C １～３ alkyl group containing at least one of them, and R ４ is an ester group, a carbonyl group, C １～３ alkyl group, and contains at least one of a single bond, and the production method according to claim 11

13. The preparation method according to claim 11 or 12, characterized in that the reaction temperature of the first step 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 step 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 step polymerization reaction is 1 to 2 hours.

16. The preparation method according to any one of claims 11 to 15, characterized in that the chain transfer agent comprises 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 non-water-soluble gas contains 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 dose of the chain transfer agent is 1.5% to 4% of the total mass of the vinylidene fluoride monomer and the unsaturated carboxylic acid monomer.

19. The polymerization reaction in the first step, The steps include placing the solvent and dispersant in a container and removing oxygen from the reaction system, The process involves adding an initiator and a pH adjuster to the container, adjusting the pH to 6.5 to 7, and then adding vinylidene fluoride monomer to bring the pressure inside the container to 4 MPa to 6 MPa. A preparation method according to any one of claims 11 to 18, characterized by comprising the steps of stirring for 30 to 60 minutes, raising the temperature to 45°C to 60°C, and adding an unsaturated carboxylic acid monomer to carry out a first-stage polymerization reaction.

20. The preparation method according to claim 19, characterized in that the amount of the solvent 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 comprises at least one of cellulose, cellulose ether, and polyvinyl alcohol, wherein selectively, the cellulose comprises hydroxypropyl methylcellulose, and the cellulose ether comprises one or more of methylcellulose ether and carboxyethylcellulose ether.

22. The preparation method according to any one of claims 19 to 21, characterized in that the amount of the dispersant 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 comprises 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 dose of the pH adjusting agent 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 the unsaturated carboxylic acid monomer is added in multiple steps during the polymerization reaction in the first step.

26. A positive electrode sheet comprising a positive electrode film layer, wherein the positive electrode film layer comprises a fluorine-containing polymer according to any one of claims 1 to 10, or a 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 selectively 0.4% to 0.8% based on the total mass of the positive electrode film layer.

28. 3.5 g / cm² of the aforementioned positive electrode sheet ３ ~3.7 g / cm ３ The positive electrode sheet according to claim 26 or 27, characterized in that the number of times it can be bent at a given pressure density is two or more, selectively two to four times.

29. The critical pressure density of the aforementioned positive electrode sheet is 3.6 g / cm³. ３ The above is the result, and selectively 3.6 g / cm³ ３ ~3.7 g / cm ３ A positive electrode sheet according to any one of claims 26 to 28, characterized in that it is the positive electrode sheet according to any one of claims 26 to 28.

30. A secondary battery characterized by including a positive electrode sheet as described in any one of claims 26 to 29.

31. An electrical device characterized by including the secondary battery described in claim 30.

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