CN118435393A

CN118435393A - Adhesive composition, positive electrode sheet, secondary battery and electric device

Info

Publication number: CN118435393A
Application number: CN202380015763.0A
Authority: CN
Inventors: 李�诚; 曾子鹏; 刘会会; 王景明
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2022-08-30
Filing date: 2023-04-14
Publication date: 2024-08-02

Abstract

The application provides an adhesive composition comprising a first fluoropolymer comprising polyvinylidene fluoride having a weight average molecular weight of 500 to 900 tens of thousands and a second fluoropolymer having a weight average molecular weight of not more than 60 tens of thousands. The adhesive composition has good processability, can enable the pole piece to have high adhesive force under the condition of low addition amount, and can improve the cycle performance of the battery.

Description

Adhesive composition, positive electrode sheet, secondary battery and electric device

Cross reference

The present application refers to a fluoropolymer, its preparation method and use, CN patent application No. 202211044631.4 of positive electrode slurry, secondary battery, battery module, battery pack and electric device, filed on 30 th month 2022, a fluoropolymer, its preparation method and use, CN patent application No. 202211052014.9 of positive electrode slurry, secondary battery, battery module, battery pack and electric device, CN patent application No. 202211044756.7 of 2022, CN patent application No. 202211044756.7 of positive electrode sheet, secondary battery and electric device, CN patent application No. 202211043966.4 of 2022, filed on 30 th month 2022, and CN patent application No. 202211043966.4 of "binder, preparation method, positive electrode sheet, secondary battery and electric device", which are all incorporated herein by reference.

Technical Field

The application relates to the technical field of secondary batteries, in particular to an adhesive, a preparation method, a positive pole piece, a secondary battery, a battery module, a battery pack and an electric device.

Background

In recent years, secondary batteries are widely used in energy storage power supply systems such as hydraulic power, thermal power, wind power and solar power stations, and in various fields such as electric tools, electric bicycles, electric motorcycles, electric automobiles, military equipment, aerospace, and the like. With the popularization of secondary battery applications, higher demands are also being made on its cycle performance, service life, etc.

The binder is a common material in secondary batteries, and there is a great demand for pole pieces, separator films, packaging parts, and the like of the batteries. However, the existing adhesive is poor in adhesion, and a large amount of adhesive is often required to be added to meet the requirement of the adhesive force of the pole piece, so that the improvement of the energy density of the battery is limited. Thus, the existing adhesives remain to be improved.

Disclosure of Invention

The present application has been made in view of the above problems, and an object thereof is to provide an adhesive that can exhibit excellent adhesion at a low addition amount and has good workability.

In order to achieve the above object, the present application provides an adhesive composition comprising a first fluoropolymer having a weight average molecular weight of 500 to 900 ten thousand and a second fluoropolymer having a weight average molecular weight of not more than 60 ten thousand.

The adhesive can ensure that the pole piece has enough adhesive force under the condition of low addition amount, and improves the cycle performance of the battery.

In any embodiment, the second fluoropolymer is present in an amount of 0.25% to 15% by mass based on the total mass of the binder composition.

The mass ratio of the first fluorine-containing polymer to the second fluorine-containing polymer is controlled within a proper range, and the adhesive enables the pole piece to have excellent adhesive force under the condition of low addition amount, so that the capacity retention rate of the battery in the circulating process can be improved. The crystallinity and processability of the binder can be effectively improved by adding a small amount of the second fluoropolymer to the first fluoropolymer, and the cycle performance of the battery can be further improved while reducing the manufacturing cost.

In any embodiment, the binder composition has a crystallinity of no more than 45%, alternatively 20% to 45%.

The second fluorine-containing polymer is added into the first fluorine-containing polymer, so that the crystallinity of the binder can be effectively reduced, and the flexibility of the pole piece is improved.

In any embodiment, the first fluoropolymer comprises structural units derived from vinylidene fluoride.

In any embodiment, the first fluoropolymer further comprises structural units represented by formula I,

Wherein R ₁ comprises one or more of hydrogen, fluorine, chlorine, C _1-3 alkyl containing at least one fluorine atom.

In any embodiment, the first fluoropolymer has a polydispersity of 1.8 to 2.5.

The polydispersity of the first fluoropolymer is in a suitable range for the pole piece to have excellent adhesion and to improve the capacity retention of the battery during cycling. In addition, the proper polydispersion coefficient of the first fluorine-containing polymer can effectively improve the solid content of the slurry and reduce the production cost.

In any embodiment, the crystallinity of the first fluoropolymer is 30% to 46%, alternatively 40% to 46%.

In any embodiment, the viscosity of the dope containing the first fluoropolymer with a mass content of 2% obtained by dissolving the first fluoropolymer in N-methylpyrrolidone is 2000mpa·s to 5000mpa·s.

When the positive electrode slurry is prepared, the binder needs to have certain viscosity to prevent the sedimentation of additives such as positive electrode active materials, conductive agents and the like, so that the slurry can be placed more stably. In the prior art, the glue solution viscosity of 2000 MPa.s-5000 MPa.s is realized by at least containing 7% of binder by mass, and the first fluorine-containing polymer can realize the expected viscosity of the glue solution at the dosage of 2% based on the mass of the glue solution, thereby providing a basis for reducing the content of the binder in the positive electrode film layer.

In any embodiment, the first fluoropolymer comprises one or more of polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer.

In any embodiment, the second fluoropolymer comprises a polymer of structural units of formula II,

Wherein each R ₂、R₃ independently includes at least one of hydrogen, halogen, or C _1-3 alkyl containing at least one fluorine atom.

In any embodiment, each R ₂、R₃ independently comprises at least one of hydrogen, fluorine, chlorine, trifluoromethyl.

In any embodiment, the end groups of the second fluoropolymer contain hydroxyl or ester groups.

The second fluoropolymer has a small weight average molecular weight and a high mass content of end groups therein, and therefore the end groups have a large influence on the properties of the second fluoropolymer. The end group of the second fluorine-containing polymer contains hydroxyl or ester, so that the cohesiveness of the second fluorine-containing polymer can be effectively improved, and the reduction of the cohesiveness of the polar plate caused by the addition of the second fluorine-containing polymer can be reduced.

In any embodiment, the second fluoropolymer has a weight average molecular weight of 0.5 to 60, optionally 0.5 to 15, tens of thousands.

In any embodiment, the second fluoropolymer comprises one or more of polytetrafluoroethylene, polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) copolymer, poly (vinylidene fluoride-tetrafluoroethylene) copolymer, poly (vinylidene fluoride-hexafluoropropylene) copolymer.

In a second aspect of the application, there is provided a method of preparing a binder composition, for preparing a first fluoropolymer: under the polymerizable condition, carrying out a first polymerization reaction on a raw material containing vinylidene fluoride monomers to prepare a first fluorine-containing polymer, wherein the weight average molecular weight of the first fluorine-containing polymer is 500-900 ten thousand; preparing a second fluoropolymer: under the polymerizable condition, carrying out a second polymerization reaction on the fluorine-containing monomer to prepare a second fluorine-containing polymer, wherein the weight average molecular weight of the second fluorine-containing polymer is not more than 60 ten thousand; blending: the first fluoropolymer is blended with the second fluoropolymer to prepare the adhesive composition.

In any embodiment, the preparation method of the first fluoropolymer specifically includes: providing a raw material containing vinylidene fluoride monomers and a reaction solvent, and performing a first-stage polymerization reaction to obtain a first product; carrying out second-stage polymerization reaction on the first product under the water-insoluble gas atmosphere; adding chain transfer agent to perform the third polymerization reaction to obtain polyvinylidene fluoride with weight average molecular weight of 500-900 ten thousand.

In any embodiment, the reaction temperature of the first stage polymerization reaction is 45-60 ℃, the reaction time is 4-10 h, and the initial polymerization pressure is 4-6 MPa.

In any embodiment, the reaction temperature of the second stage polymerization reaction is 60-80 ℃, the reaction time is 2-4 h, and the reaction pressure is 6-8 MPa.

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

In any embodiment, the preparation method of the second fluoropolymer specifically includes:

Carrying out a second polymerization reaction on at least one monomer shown in a formula III in a non-reactive gas atmosphere at a reaction temperature of between 0.1 and 5MPa and between 60 and 90 ℃ for 0.5 to 8 hours, stopping the reaction, carrying out solid-liquid separation, and retaining a solid phase to obtain a second fluorine-containing polymer

Wherein each R ₄、R₅ is independently selected from hydrogen, halogen, or C _1-3 alkyl containing at least one fluorine atom.

In any embodiment, the second polymerization reaction further comprises the steps of:

adding a solvent and a dispersing agent into a container, and filling the container with a non-reactive gas;

And (3) adding a monomer shown in a formula III, heating to 60-90 ℃, and then adding a second initiator and a chain transfer agent to perform a second polymerization reaction.

In any embodiment, the second initiator comprises an inorganic peroxide, which may be selected from potassium persulfate or ammonium persulfate.

The nuclear magnetic test results show that the end groups in the second fluoropolymer include-CF ₂-CH₂ OH or-CF ₂-CH₂OOCCH₃ when inorganic peroxide is used as an initiator. The existence of the hydroxyl and the ester groups in the terminal group can effectively improve the cohesiveness of the second fluorine-containing polymer and reduce the decrease of the cohesiveness of the second fluorine-containing polymer to the adhesive composition.

In any embodiment, the mass content of the initiator is 3% to 12% based on the total mass of the monomers of formula III.

In any embodiment, the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

A third aspect of the present application provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, a conductive agent, and the binder composition in any embodiment or the binder composition prepared by the preparation method in any embodiment.

The positive electrode plate has excellent adhesive force under the condition of low addition amount of the adhesive composition.

In any embodiment, the mass fraction of the binder composition is no more than 1% based on the total mass of the positive electrode film layer.

The mass fraction of the binder is controlled within a proper range, so that the loading amount of active substances in the battery pole piece can be improved under the condition that the pole piece has enough binding force, and the power performance of the battery can be further improved.

In any embodiment, the positive electrode active material is a lithium-containing transition metal oxide.

In any embodiment, the lithium-containing transition metal oxide is lithium iron phosphate or lithium nickel cobalt manganese oxide, or a doping modification thereof, or at least one of a conductive carbon coating modification, a conductive metal coating modification, or a conductive polymer coating modification thereof.

In a fourth aspect of the present application, there is provided a secondary battery comprising an electrode assembly including a separator, a negative electrode tab, and a positive electrode tab of the third aspect of the present application, and an electrolyte.

In a fourth aspect of the present application, there is provided an electric device comprising the secondary battery of the third aspect of the present application.

Drawings

Fig. 1 is a schematic view of a secondary battery according to an embodiment of the present application.

Fig. 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in fig. 1.

Fig. 3 is a schematic view of a battery module according to an embodiment of the present application.

Fig. 4 is a schematic view of a battery pack according to an embodiment of the present application.

Fig. 5 is an exploded view of the battery pack of the embodiment of the present application shown in fig. 4.

Fig. 6 is a schematic view of an electric device in which a secondary battery according to an embodiment of the present application is used as a power source.

Reference numerals illustrate: 1, a battery pack; 2, upper box body; 3, lower box body; 4, a battery module; 5a secondary battery; 51 a housing; 52 electrode assembly; 53 cover plates.

Detailed Description

Hereinafter, embodiments of the positive electrode active material, the method for manufacturing the same, the positive electrode tab, the secondary battery, the battery module, the battery pack, and the electrical device according to the present application are specifically disclosed with reference to the accompanying drawings as appropriate. However, unnecessary detailed description may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of the actual same structure may be omitted. This is to avoid that the following description becomes unnecessarily lengthy, facilitating the understanding of those skilled in the art. Furthermore, the drawings and the following description are provided for a full understanding of the present application by those skilled in the art, and are not intended to limit the subject matter recited in the claims.

The "range" disclosed herein is defined in terms of lower and upper limits, with the given range being defined by the selection of a lower and an upper limit, the selected lower and upper limits defining the boundaries of the particular range. Ranges that are defined in this way can be inclusive or exclusive of the endpoints, and any combination can be made, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3,4 and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise indicated, the numerical range "a-b" represents a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed throughout, and "0-5" is simply a shorthand representation of a combination of these values. When a certain parameter is expressed as an integer of 2 or more, it is disclosed that the parameter is, for example, an integer of 2,3,4,5, 6, 7, 8, 9, 10,11, 12 or the like.

All embodiments of the application and alternative embodiments may be combined with each other to form new solutions, unless otherwise specified.

All technical features and optional technical features of the application may be combined with each other to form new technical solutions, unless specified otherwise.

All the steps of the present application may be performed sequentially or randomly, preferably sequentially, unless otherwise specified. For example, the method comprises steps (a) and (b), meaning that the method may comprise steps (a) and (b) performed sequentially, or may comprise steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), may include steps (a), (c) and (b), may include steps (c), (a) and (b), and the like.

The terms "comprising" and "including" as used herein mean open ended or closed ended, unless otherwise noted. For example, the terms "comprising" and "comprises" may mean that other components not listed may be included or included, or that only listed components may be included or included.

The term "or" is inclusive in this application, unless otherwise specified. For example, the phrase "a or B" means "a, B, or both a and B. More specifically, either of the following conditions satisfies the condition "a or B": a is true (or present) and B is false (or absent); a is false (or absent) and B is true (or present); or both A and B are true (or present).

Fluoropolymers are one of the most widely used types of binders in secondary batteries today. However, conventional fluoropolymers have low viscosities and often require extensive additions to ensure effective bonding of the active material, thereby allowing the pole pieces to achieve effective bonding. However, the increase of the dosage of the traditional fluorine-containing polymer can reduce the load capacity of the active material in the pole piece, affect the improvement of the power performance of the battery, and hardly meet the requirement on the cycle performance of the battery.

[ Adhesive ]

The application provides an adhesive composition comprising a first fluoropolymer and a second fluoropolymer, the first fluoropolymer having a weight average molecular weight of 500 to 900 tens of thousands and the second fluoropolymer having a weight average molecular weight of not more than 60 tens of thousands.

Herein, the term "binder composition" refers to a mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium.

As used herein, the term "fluoropolymer" refers to a polymer having fluoromonomers as the primary synthetic monomer, which on the one hand includes an aggregate of chemically uniform macromolecules prepared by polymerization, but differing in terms of degree of polymerization, molar mass and chain length. The term on the other hand also includes derivatives of such macromolecular assemblies formed by polymerization, i.e. compounds which can be obtained by reaction of functional groups in the macromolecules described above, for example addition or substitution, and which can be chemically homogeneous or chemically inhomogeneous. Fluoropolymers herein include both homopolymers and copolymers.

In this context, the term "weight average molecular weight" refers to the sum of the weight fractions of the polymer occupied by molecules of different molecular weights multiplied by their corresponding molecular weights.

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

In some embodiments, the binder composition is used to fix the electrode active material and/or the conductive agent in place and adhere them to the conductive metal part to form an electrode.

In some embodiments, the binder composition acts as a positive electrode binder for binding a positive electrode active material and/or a conductive agent to form an electrode.

In some embodiments, the binder composition acts as a negative electrode binder for binding a negative electrode active material and/or a conductive agent to form an electrode.

In some embodiments, the first fluoropolymer has a weight average molecular weight of 500 to 900 ten thousand. In some embodiments, the first fluoropolymer has a weight average molecular weight of 500 tens of thousands, 550 tens of thousands, 600 tens of thousands, 650 tens of thousands, 700 tens of thousands, 750 tens of thousands, 800 tens of thousands, 850 tens of thousands, 900 tens of thousands, or any number thereof.

In some embodiments, the second fluoropolymer has a weight average molecular weight of no more than 60 ten thousand. In some embodiments, the weight average molecular weight of the second fluoropolymer may be selected from 0.5, 1,2,3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 or any number therein.

In the present application, the fluoropolymer may be tested for its weight average molecular weight by methods known in the art, such as gel chromatography, e.g., using a Waters 2695 Isocratic HPLC-type gel chromatograph (differential refractive detector 2141). In some embodiments, the test method is to select a matched chromatographic column (oiliness: styragel HT5DMF7.8 x 300mm+Styragel HT4) with a 3.0% mass fraction of polystyrene solution sample as reference. Preparing 3.0% binder glue solution with purified N-methyl pyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. During the test, tetrahydrofuran is firstly sucked by a syringe, and then the syringe is washed, and the test is repeated for several times. Then, 5ml of the test solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And after the indication is stable, acquiring data, and reading the weight average molecular weight.

The fluorine element contained in the first fluorine-containing polymer and the second fluorine-containing polymer and the hydroxyl or/and carboxyl on the surface of the active material and the surface of the current collector form hydrogen bond action, so that the adhesive force of the pole piece can be improved. The first fluorine-containing polymer with the weight average molecular weight of 500-900 ten thousand can improve the adhesive force of the pole piece under the condition of low-level addition amount, and improve the capacity retention rate of the battery in the circulating process. The addition of the second fluorine-containing polymer in the binder can reduce the crystallinity of the binder, improve the flexibility of the pole piece, and also can improve the processing performance of slurry, so that the pole piece cannot be layered in the drying process of preparing the pole piece, the uniformity of dispersing the active material in the pole piece is improved, the sheet resistance is reduced, and the cycle performance of the battery is further optimized. The adhesive composition can ensure that the pole piece has enough adhesive force and flexibility under the condition of low addition amount, and is beneficial to improving the energy density of the battery and the cycle performance of the battery.

In some embodiments, the mass content of the second fluoropolymer is 0.25% to 15% based on the total mass of the binder composition.

In some embodiments, the mass content of the second fluoropolymer may be selected to be 0.25%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 7.5%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any number therein, based on the total mass of the binder composition.

The mass ratio of the first fluorine-containing polymer to the second fluorine-containing polymer is controlled within a proper range, and the adhesive enables the pole piece to have excellent adhesive force under the condition of low addition amount, so that the capacity retention rate of the battery in the circulating process can be improved. The crystallinity of the binder, the flexibility and the processability of the pole piece can be effectively improved by adding a small amount of the second fluorine-containing polymer into the first fluorine-containing polymer, and the cycle performance of the battery can be further improved while the manufacturing cost is reduced.

In some embodiments, the binder composition has a crystallinity of no greater than 45%, alternatively 20% -45%.

In some embodiments, the crystallinity of the binder composition may be selected to be 45%, 40%, 35%, 30%, 25%, or 20%.

In this context, the term "crystallinity" refers to the proportion of crystalline regions in a polymer, and there are regions in the microstructure having a stable ordered arrangement of molecules, the regions in which the molecules are ordered in close proximity being referred to as crystalline regions.

In the present application, the crystallinity may be tested by methods known in the art, such as differential scanning thermal analysis. In some embodiments, 0.5g of the fluoropolymer composition is placed in an aluminum dry pot, shaken flat, covered with a crucible lid, purged with 50ml/min, with 70ml/min of shielding gas, at a temperature ramp rate of 10 ℃/min under a nitrogen atmosphere, at a test temperature ranging from-100 ℃ to 400 ℃, using a Differential Scanning Calorimeter (DSC) of american TA instruments model Discovery 250, and with elimination of heat history.

This test will result in a DSC/(Mw/mg) versus temperature profile of the fluoropolymer composition and will be integrated with peak areas as the enthalpy of fusion of the fluoropolymer composition ΔH (J/g), binder crystallinity = ΔH/(ΔHm). Times.100%, where ΔHm is the standard enthalpy of fusion of the fluoropolymer (crystalline heat of fusion), ΔHm = 104.7J/g.

In some embodiments, the first fluoropolymer comprises structural units derived from vinylidene fluoride. In some embodiments, the first fluoropolymer is a vinylidene fluoride homopolymer. In some embodiments, the first fluoropolymer is a vinylidene fluoride copolymer.

In some embodiments, the first fluoropolymer further comprises structural units of formula I,

In some embodiments, R ₁ comprises one or more of hydrogen, fluorine, chlorine, trifluoromethyl.

In some embodiments, the first fluoropolymer comprises one or more of polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer.

In some embodiments, the first fluoropolymer has a polydispersity of 1.8 to 2.5. In some embodiments, the polydispersity of the first fluoropolymer may be selected to be 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, or any number therein.

As used herein, the term "polydispersity" refers to the ratio of the weight average molecular weight of a polymer to the number average molecular weight of the polymer.

As used herein, the term "number average molecular weight" refers to the sum of the mole fractions of the polymer taken up by molecules of different molecular weights multiplied by their corresponding molecular weights.

If the polydispersity of the first fluoropolymer is too large, the polymerization degree of the first fluoropolymer is relatively dispersed, which affects the uniformity of the binder, the binder cannot uniformly adhere the positive electrode active material to the current collector, the cycle performance of the battery is affected, and the solid content of the slurry is reduced, so that the energy density of the battery cannot be further improved; if the polydispersity of the first fluoropolymer is too small, the difficulty of the preparation process is high, and the rate of preference is low, resulting in high production cost.

In the present application, the polydisperse coefficient of the first fluoropolymer may be tested by methods known in the art, such as gel chromatography, e.g., using a Waters 2695 Isocratic HPLC-type gel chromatograph (differential refractive detector 2141). In some embodiments, a matched chromatographic column (oiliness: styragel HT5DMF7.8 x 300mm+Styragel HT4) was selected as a reference with a polystyrene solution sample at 3.0% mass fraction. Preparing 3.0% binder glue solution with purified N-methyl pyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. During the test, tetrahydrofuran is firstly sucked by a syringe, and then the syringe is washed, and the test is repeated for several times. Then, 5ml of the test solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And obtaining data after the indication is stable. The weight average molecular weight a and the number average molecular weight b were read separately. Polydisperse coefficient = a/b.

In some embodiments, the crystallinity of the first fluoropolymer is 30% to 46%. In some embodiments, the crystallinity of the first fluoropolymer is 40% to 46%. In some embodiments, the crystallinity of the first fluoropolymer is 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46% and any value therebetween.

In some embodiments, the first fluoropolymer is dissolved in N-methylpyrrolidone to produce a dope containing 2% by mass of the first fluoropolymer having a viscosity of 2000MPa s to 5000MPa s. In some embodiments, the viscosity of the dope containing the first fluoropolymer in an amount of 2% by mass obtained by dissolving the first fluoropolymer in N-methylpyrrolidone may be selected to be any one of 2000MPa·s～2500MPa·s、2500MPa·s～3000MPa·s、3000MPa·s～3300MPa·s、3300MPa·s～3500MPa·s、3500MPa·s～3800MPa·s、3800MPa·s～4000MPa·s、4000MPa·s～4200MPa·s、4200MPa·s～4600MPa·s、4600MPa·s～5000MPa·s、3100MPa·s～3400MPa·s、3400MPa·s～3800MPa·s、3800MPa·s～4600MPa·s、4600MPa·s～5000MPa·s、3600MPa·s～5000MPa·s.

If the viscosity of the first fluorine-containing polymer is too high, the viscosity of the prepared binder solution is too high, stirring is difficult, and the dispersibility of the binder is reduced, so that the binder is difficult to uniformly adhere the positive electrode active material to the current collector, the cycle performance of the battery is affected, and meanwhile, the viscosity of the binder solution is too high, and the speed of the pulping process is reduced; if the viscosity of the first fluoropolymer is too low, the viscosity of the prepared binder solution may be too low, and it may be difficult for the pole piece to have sufficient adhesion at low addition amounts.

In addition, when preparing the positive electrode slurry, the binder needs to have certain viscosity to prevent the sedimentation of the positive electrode active material, the conductive agent and other auxiliary agents, so that the slurry can be placed more stably. In the prior art, the glue solution viscosity of 2000 MPa.s-5000 MPa.s is realized by at least containing 7% of binder by mass, and the first fluorine-containing polymer can realize the expected viscosity of the glue solution at the dosage of 2% based on the mass of the glue solution, thereby providing a basis for reducing the content of the binder in the positive electrode film layer.

The viscosity of the first fluorine-containing polymer solution is controlled within a proper range, so that the pole piece can have excellent bonding performance under the condition of low addition amount of the bonding agent, and the capacity retention rate of the battery in the circulating process is improved.

In some embodiments, the second fluoropolymer comprises structural units represented by formula II,

In some embodiments, each R ₂、R₃ independently comprises at least one of hydrogen, fluorine, chlorine, trifluoromethyl.

In some embodiments, the second fluoropolymer comprises one of polytetrafluoroethylene, polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) copolymer, poly (vinylidene fluoride-tetrafluoroethylene) copolymer, poly (vinylidene fluoride-hexafluoropropylene) copolymer.

In some embodiments, the end groups of the second fluoropolymer contain hydroxyl or ester groups.

The terminal structure of the polymer can be studied by nuclear magnetic resonance. The end group structure of the polymer can be analyzed by ¹⁹ F-NMR and ¹ H-NMR. As an example, dimethyl sulfoxide is used as a solvent, CFCl ₃ is used as a fluorine spectrum standard, and TMS is used as a hydrogen spectrum standard.

In one embodiment of the present application, there is provided a method for preparing an adhesive composition, comprising the steps of:

Preparing a first fluoropolymer: under the polymerizable condition, carrying out a first polymerization reaction on a raw material containing vinylidene fluoride monomers to prepare a first fluorine-containing polymer, wherein the weight average molecular weight of the first fluorine-containing polymer is 500-900 ten thousand;

Preparing a second fluoropolymer: under the polymerizable condition, carrying out a second polymerization reaction on at least one fluorine-containing monomer to prepare a second fluorine-containing polymer, wherein the weight average molecular weight of the second fluorine-containing polymer is not more than 60 ten thousand;

blending: an adhesive composition is prepared by blending the first fluoropolymer with the second fluoropolymer.

As used herein, the term "blend" refers to a process of forming a macroscopically homogeneous material under conditions of temperature and/or shear stress, etc., from two or more materials.

The preparation method of the adhesive is simple, is environment-friendly, reduces the cost and is beneficial to industrial production. Meanwhile, the adhesive prepared by the method ensures that the pole piece has excellent adhesive force, and can improve the capacity retention rate of the battery in the circulating process under the condition of low addition.

In some embodiments, in the blending step, the mass ratio of the first fluoropolymer to the second fluoropolymer is 99.75:0.25 to 85:15. in some embodiments, the mass ratio of the first fluoropolymer to the second fluoropolymer may be selected to be 99.75:0.25, 99.5:0.5, 99.25:0.75, 99:1, 98:2, 95:5, 92.5:7.5, 90: 10. 85:15 or any number thereof.

In some embodiments, the step of synthesizing the first fluoropolymer comprises: providing a raw material containing vinylidene fluoride monomers and a reaction solvent, and performing a first-stage polymerization reaction to obtain a first product;

carrying out second-stage polymerization reaction on the first product in the water-insoluble gas atmosphere;

Adding chain transfer agent to perform the third polymerization reaction to obtain polyvinylidene fluoride with weight average molecular weight of 500-900 ten thousand.

The first product may be a reaction solution obtained after the first-stage polymerization reaction, or may be a product obtained after purification treatment of the reaction solution after the first-stage polymerization reaction.

In some embodiments, multiple parts of the first product are mixed and the second stage polymerization is carried out under an atmosphere of a water insoluble gas. That is, the second stage polymerization is a self-polymerization of the first product. It is understood that multiple parts of the first product can be prepared simultaneously by multiple reaction kettles, or can be prepared multiple times by one reaction kettle. The uniformity of the polymerization product can be improved by a multi-time and sectional synthesis method.

The method adopts a sectional method to carry out polymerization reaction, and can prepare the polyvinylidene fluoride with ultrahigh molecular weight, so that the adhesive can meet the requirement of the adhesive force of the pole piece under the condition of low addition, thereby being beneficial to improving the load capacity of the positive electrode active material in the pole piece and improving the capacity retention rate of the battery in the circulating process. Meanwhile, a first product is formed in the first-stage polymerization reaction, a molecular chain segment with target molecular weight is formed in the second-stage polymerization reaction, the molecular weight of the polymer is regulated and controlled in the third-stage polymerization reaction, the uniformity of the weight average molecular weight of polyvinylidene fluoride is prevented from being reduced due to the fact that the molecular weight is too high, and the uniformity of the product is improved. And the utilization rate of the reactor in the preparation process of the polyvinylidene fluoride can be improved by the sectional polymerization, so that the time is saved, and the residence time of the polyvinylidene fluoride in the reactor is reduced. The first stage polymerization reaction, the second stage polymerization reaction and the third stage polymerization reaction are matched with each other to further improve the production efficiency of polyvinylidene fluoride.

In some embodiments, the reaction temperature of the first stage polymerization is 45 ℃ to 60 ℃. In some embodiments, the reaction temperature of the first stage polymerization reaction may be selected from 45℃to 50℃to 55℃to 60℃to 45℃to 55 ℃.

In some embodiments, the reaction time for the first stage polymerization is from 4 hours to 10 hours. In some embodiments, the reaction time of the first stage polymerization reaction may be selected from 4 to 5 hours, 5 to 6 hours, 6 to 7 hours, 7 to 8 hours, 8 to 9 hours, 9 to 10 hours, 4 to 6 hours, 6 to 8 hours, 8 to 10 hours, 5 to 10 hours.

In some embodiments, the initial polymerization pressure is from 4MPa to 6MPa. In some embodiments, the initial polymerization pressure may be selected from 4MPa to 5MPa, 5MPa to 6MPa. In some embodiments, the initial polymerization pressure is above the critical pressure of vinylidene fluoride.

In some embodiments, the reaction temperature of the second stage polymerization reaction is from 60 ℃ to 80 ℃. In some embodiments, the reaction temperature of the second stage polymerization reaction may be selected from 60 ℃ to 70 ℃, 70 ℃ to 80 ℃.

In some embodiments, the reaction time for the second stage polymerization is from 2 hours to 4 hours. In some embodiments, the reaction time for the second stage polymerization reaction may be selected from 2 hours to 3 hours, 3 hours to 4 hours.

In some embodiments, the reaction pressure of the second stage polymerization is from 6MPa to 8MPa. In some embodiments, the reaction pressure of the second stage polymerization reaction may be selected from 6MPa to 7MPa, 7MPa to 8MPa.

In some embodiments, the reaction time for the third stage polymerization is from 1h to 2h.

The reaction pressure, reaction time and reaction temperature of polymerization reaction at each stage are controlled within proper ranges, the uniformity of the weight average molecular weight of a polymerization product can be controlled while the weight average molecular weight of the polyvinylidene fluoride is improved, the product is ensured to have lower polydispersity, the uniformity of the performance of the polyvinylidene fluoride is improved, the pole piece has excellent adhesive force under the condition of low addition of the adhesive, and the cycle capacity retention rate of the battery can be further improved.

In some embodiments, the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

The water-insoluble gas means a gas having a gas solubility of less than 0.1L. The gas solubility refers to the volume of gas when dissolved in 1L of water to reach a saturated state at 20℃under a pressure of 1.013X10 ⁵ Pa. In some embodiments, the water insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, methane.

In some embodiments, the chain transfer agent is used in an amount of 1.5% to 3% of the total mass of vinylidene fluoride monomer during the first fluoropolymer production process. The amount of chain transfer agent may also be 2% or 2.5%, for example.

The chain transfer agent is controlled in a proper range, so that the chain length of the polymer can be controlled, and the polymer with proper molecular weight range and uniform distribution can be obtained.

In some embodiments, the first stage polymerization reaction comprises the steps of:

adding a water solvent and a dispersing agent into a container, and removing oxygen in a reaction system;

adding a first initiator and a pH regulator into the container, regulating the pH value to 6.5-7, and then adding vinylidene fluoride monomer to enable the pressure in the container to reach 4-6 MPa;

stirring for 30-60 min, heating to 45-60 deg.c, and first stage polymerization.

Before the temperature is increased to carry out the polymerization reaction, the materials are uniformly mixed, so that the reaction can be more thoroughly carried out, and the weight average molecular weight, the crystallinity and the particle size of the prepared polymer are more uniform.

In some embodiments, the aqueous solvent is used in an amount of 2 to 8 times the total mass of vinylidene fluoride monomer during the first fluoropolymer production process. The amount of solvent may also be, for example, 3,4, 5,6 or 7 times the total mass of vinylidene fluoride monomer. In some embodiments, the aqueous solvent is deionized water.

In some embodiments, the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol during the first fluoropolymer production process.

In some embodiments, the dispersant comprises one or more of a methyl cellulose ether and a carboxyethyl cellulose ether during the first fluoropolymer production process.

In some embodiments, the dispersant is used in an amount of 0.1% to 0.3% of the total mass of vinylidene fluoride monomer during the first fluoropolymer preparation. The amount of dispersant used may also be, for example, 0.2% of the total mass of vinylidene fluoride monomer.

In some embodiments, the first initiator is an organic peroxide.

In some embodiments, the first initiator comprises one or more of t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and t-butyl peroxypivalate.

In some embodiments, the first initiator is used in an amount of 0.15% to 1% of the total mass of vinylidene fluoride monomer during the first fluoropolymer production process. The amount of the first initiator may also be, for example, 0.2%, 0.4%, 0.6% or 0.8% by mass of the vinylidene fluoride monomer.

In some embodiments, the pH adjuster comprises one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

In some embodiments, the pH adjuster is used in an amount of 0.05% to 0.2% of the total mass of vinylidene fluoride monomer. The amount of the pH adjustor can also be, for example, 0.1% or 0.15% of the total mass of the vinylidene fluoride monomer.

In some embodiments, the method of making the second fluoropolymer specifically comprises:

At least one monomer shown in formula III is polymerized for 0.5 to 8 hours in a non-reactive gas atmosphere at a reaction temperature of between 0.1 and 5MPa and between 60 and 90 ℃, the reaction is stopped, solid-liquid separation is carried out, a solid phase is reserved, and a second fluorine-containing polymer is obtained,

In some embodiments, the reaction pressure of the second polymerization reaction is 0.1MPa, 1MPa, 2MPa, 3MPa, 4MPa, 5MPa, or any value therein.

In some embodiments, the reaction temperature of the second polymerization reaction is 60 ℃, 65 ℃, 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃, or any number therein.

The polymerization reaction is carried out at high temperature and high pressure, so that the reaction efficiency can be improved, the reaction time can be reduced, the reaction conversion rate can be improved, and the uniformity and purity of the product can be improved.

In some embodiments, the second polymerization reaction further comprises the steps of:

And (3) adding a monomer shown in a formula III, heating to 55-75 ℃, adding a second initiator and a chain transfer agent, and performing a second polymerization reaction.

In some embodiments, the second initiator comprises an inorganic peroxide, which may be selected from potassium persulfate or ammonium persulfate.

The nuclear magnetic results indicate that the end groups in the second fluoropolymer include-CF ₂-CH₂ OH or-CF ₂-CH₂OOCCH₃ when inorganic peroxide is used as the initiator. The existence of the hydroxyl and the ester groups in the terminal group can effectively improve the cohesiveness of the second fluorine-containing polymer and reduce the decrease of the cohesiveness of the second fluorine-containing polymer to the adhesive composition.

In some embodiments, the initiator is present in an amount of 3% to 12% by mass based on the total mass of the monomers of formula III.

The high-content initiator is beneficial to improving the reaction efficiency, reducing the reaction time, reducing the polydispersity index of the product and improving the uniformity of the product.

[ Positive electrode sheet ]

The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material, a conductive agent, and a binder composition in some embodiments.

In some embodiments, the mass fraction of the binder composition is no more than 1% based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of binder is 0.4% to 1%. In some embodiments, the mass content of the binder composition is 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or any number therein.

The adhesive composition can effectively improve the adhesive property of the pole piece under the condition of low addition amount, and the pole piece has excellent flexibility and processability, so that the battery has high energy density and cycle performance.

In some embodiments, the positive electrode active material is a lithium-containing transition metal oxide.

In some embodiments, the positive electrode active material is at least one of lithium iron phosphate or lithium nickel cobalt manganese oxide, or a doped modified material thereof, or a conductive carbon coated modified material, a conductive metal coated modified material, or a conductive polymer coated modified material thereof.

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

In some embodiments, the positive electrode active material may employ a positive electrode active material for a battery, which is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: olivine structured lithium-containing phosphates, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery positive electrode active material may be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of the olivine-structured lithium-containing phosphate may include, but are not limited to, at least one of lithium iron phosphate (e.g., liFePO ₄ (also simply LFP)), a composite of lithium iron phosphate and carbon, a composite of lithium manganese phosphate (e.g., liMnPO ₄), a composite of lithium manganese phosphate and carbon, a composite of lithium iron phosphate and manganese phosphate, and a composite of lithium manganese iron phosphate and carbon.

In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an 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.

In some embodiments, the positive electrode sheet may be prepared by: dispersing the above components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; and (3) coating the positive electrode slurry on a positive electrode current collector, and obtaining a positive electrode plate after the procedures of drying, cold pressing and the like.

[ Negative electrode sheet ]

The negative electrode plate comprises a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a negative electrode active material.

As an example, the anode current collector has two surfaces opposing in its own thickness direction, and the anode film layer is provided on either one or both of the two surfaces opposing the anode current collector.

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

In some embodiments, the anode active material may employ an anode active material for a battery, which is well known in the art. As an example, the anode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, and the like. 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 oxide, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery anode active material may be used. These negative electrode active materials may be used alone or in combination of two or more.

In some embodiments, the negative electrode film layer further optionally includes 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).

In some embodiments, the negative electrode film layer further optionally includes a conductive agent. The conductive agent is at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

In some embodiments, the negative electrode film layer may optionally further include other adjuvants, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

In some embodiments, the negative electrode sheet may be prepared by: dispersing the above components for preparing the negative electrode sheet, such as a negative electrode active material, a conductive agent, a binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; and coating the negative electrode slurry on a negative electrode current collector, and obtaining a negative electrode plate after the procedures of drying, cold pressing and the like.

[ Electrolyte ]

The electrolyte plays a role in ion conduction between the positive electrode plate and the negative electrode plate. The application is not particularly limited in the kind of electrolyte, and may be selected according to the need. For example, the electrolyte may be liquid, gel, or all solid.

In some embodiments, the electrolyte is an electrolyte. The electrolyte includes an electrolyte salt and a solvent.

In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis-fluorosulfonyl imide, lithium bis-trifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalato borate, lithium difluorodioxaato phosphate, and lithium tetrafluorooxalato phosphate.

In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methylethyl 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, methyl sulfone, and diethyl sulfone.

In some embodiments, the electrolyte further optionally includes an additive. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives capable of improving certain properties of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high or low temperature performance of the battery, and the like.

[ Isolation Membrane ]

In some embodiments, a separator is further included in the secondary battery. The type of the separator is not particularly limited, and any known porous separator having good chemical stability and mechanical stability can be used.

In some embodiments, the material of the isolating film may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene and fluorine-containing polymer. The separator may be a single-layer film or a multilayer composite film, and is not particularly limited. When the separator is a multilayer composite film, the materials of the respective layers may be the same or different, and are not particularly limited.

In some embodiments, the positive electrode tab, the negative electrode tab, and the separator may be manufactured into an electrode assembly through a winding process or a lamination process.

In some embodiments, the secondary battery may include an outer package. The outer package may be used to encapsulate the electrode assembly and electrolyte described above.

In some embodiments, the outer package of the secondary battery may be a hard case, such as a hard plastic case, an aluminum case, a steel case, or the like. The exterior package of the secondary battery may also be a pouch type pouch, for example. The material of the flexible bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

The shape of the secondary battery is not particularly limited in the present application, and may be cylindrical, square, or any other shape. For example, fig. 1 is a secondary battery 5 of a square structure as one example.

In some embodiments, referring to fig. 2, the outer package may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and a side plate connected to the bottom plate, where the bottom plate and the side plate enclose a receiving chamber. The housing 51 has an opening communicating with the accommodation chamber, and the cover plate 53 can be provided to cover the opening to close the accommodation chamber. The positive electrode tab, the negative electrode tab, and the separator may be formed into the electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is enclosed in the accommodating chamber. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and those skilled in the art may select according to specific practical requirements.

In some embodiments, the secondary batteries may be assembled into a battery module, and the number of secondary batteries included in the battery module may be one or more, and the specific number may be selected by one skilled in the art according to the application and capacity of the battery module.

Fig. 3 is a battery module 4 as an example. Referring to fig. 3, in the battery module 4, a plurality of secondary batteries 5 may be sequentially arranged in the longitudinal direction of the battery module 4. Of course, the arrangement may be performed in any other way. The plurality of secondary batteries 5 may be further fixed by fasteners.

Alternatively, the battery module 4 may further include a case having an accommodating space in which the plurality of secondary batteries 5 are accommodated.

In some embodiments, the above battery modules may be further assembled into a battery pack, and the number of battery modules included in the battery pack may be one or more, and a specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

Fig. 4 and 5 are battery packs 1 as an example. Referring to fig. 4 and 5, a battery case and a plurality of battery modules 4 disposed in the battery case may be included in the battery pack 1. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be covered on the lower box body 3 and forms a closed space for accommodating the battery module 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

In addition, the application also provides an electric device which comprises at least one of the secondary battery, the battery module or the battery pack. The secondary battery, the battery module, or the battery pack may be used as a power source of the power consumption device, and may also be used as an energy storage unit of the power consumption device. The power utilization device may include mobile devices (e.g., cell phones, notebook computers, etc.), electric vehicles (e.g., electric-only vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

As the electricity consumption device, a secondary battery, a battery module, or a battery pack may be selected according to the use requirements thereof.

Fig. 6 is an electrical device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or the like. In order to meet the high power and high energy density requirements of the secondary battery by the power consumption device, a battery pack or a battery module may be employed.

As another example, the device may be a cell phone, tablet computer, notebook computer, or the like. The device is generally required to be light and thin, and a secondary battery can be used as a power source.

Examples

Hereinafter, embodiments of the present application are described. The following examples are illustrative only and are not to be construed as limiting the application. The examples are not to be construed as limiting the specific techniques or conditions described in the literature in this field or as per the specifications of the product. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.

Example 1

1) Preparation of the adhesive composition

Preparing a first fluoropolymer: first stage polymerization: adding 4kg of deionized water and 2g of methyl cellulose ether into an autoclave of No. 1 and No. 2 and 10L, vacuumizing and replacing O ₂ with N ₂ for three times, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1kg of vinylidene fluoride monomer to enable the pressure to reach 5MPa, mixing and stirring for 30min, heating to 45 ℃, and reacting for 7.5h;

second stage polymerization: transferring the reaction liquid in the reaction kettles 1 and 2 into a reaction kettle 3, charging nitrogen to the pressure of 7MPa, heating to 70 ℃, and stirring for reaction for 3 hours;

Third stage polymerization: after 22.5g of cyclohexane was added, the reaction was continued for 1 hour, and the reaction was stopped. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the polyvinylidene fluoride binder.

The polydispersion coefficient of the polyvinylidene fluoride binder with the weight average molecular weight of 750 ten thousand is 2.2, the crystallinity is 45 percent, and the Dv50 particle diameter is 100 mu m; the solution was dissolved in N-methylpyrrolidone solution to prepare a solution having a mass percent concentration of 4wt%, and the solution viscosity was found to be 4900 MPa.s.

Preparing a second fluoropolymer:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen several times to remove oxygen, followed by adding 120g of vinylidene fluoride monomer gas. After the monomers were added, the reaction temperature was controlled at 85℃and after the addition of a water-soluble ammonium persulfate initiator and 0.14g of isopropyl alcohol chain transfer agent, the reaction was started, the addition amount of the initiator was about 8% of the total amount of the added monomers, the polymerization time was 1 hour, and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the polyvinylidene fluoride polymer.

Blending a first fluoropolymer with a second fluoropolymer, the mass ratio of the first fluoropolymer to the second fluoropolymer being 99.75:0.25 to obtain an adhesive composition comprising a first fluoropolymer and a second vinylidene fluoride.

2) Preparation of positive electrode plate

Mixing lithium iron phosphate, a binder composition and acetylene black in a planetary stirring tank at revolution speed of 25r/min for 30min, wherein the mass fraction of the binder composition is 1.2% based on the total mass of the positive electrode film layer;

2.4kg of N-methylpyrrolidone (NMP) solution is added into a stirring tank, the revolution speed is 25r/min, the rotation speed is 900r/min, and stirring is carried out for 70min;

Adding 12.3g of polyvinylpyrrolidone dispersing agent into a stirring tank, stirring for 60min at revolution speed of 25r/min and rotation speed of 1300 r/min;

After the stirring is finished, the viscosity of the slurry is tested, and the viscosity is controlled to be 8000-15000 MPa.s.

If the viscosity is higher, adding N-methyl pyrrolidone (NMP) solution to reduce the viscosity to the range, adding NMP solution, and stirring for 30min according to revolution speed of 25r/min and rotation speed of 1200-1500 r/min to obtain anode slurry; and (3) scraping the prepared positive electrode slurry on a carbon-coated aluminum foil, baking for 15min at 110 ℃, cold pressing, and cutting into wafers with the diameter of 15mm to obtain the positive electrode plate.

3) Negative pole piece

And taking the metal lithium sheet as a negative electrode sheet.

4) Isolation film

A polypropylene film was used as a separator.

5) Preparation of electrolyte

In an argon atmosphere glove box (H ₂O<0.1ppm,O₂ <0.1 ppm), uniformly mixing an organic solvent of Ethylene Carbonate (EC)/ethylmethyl carbonate (EMC) according to a volume ratio of 3/7, adding LiPF ₆ lithium salt to dissolve in the organic solvent, uniformly stirring, and preparing a 1M LiPF ₆ EC/EMC solution to obtain an electrolyte.

6) Preparation of a Battery

The positive electrode tab, the negative electrode tab, the separator and the electrolyte in example 1 were assembled into a button cell in a button cell box.

Examples 2 to 9

The mass ratio of the first fluoropolymer to the second fluoropolymer in the binder composition was adjusted and other preparation methods were the same as in example 1, with specific parameters shown in table 1.

Examples 10 to 16

The weight average molecular weight of the second fluoropolymer in the binder composition was adjusted and the specific parameters are shown in table 1, as described in example 4.

In example 10, polyvinylidene fluoride had a weight average molecular weight of 0.5 ten thousand and was produced by the following method:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen several times to remove oxygen, followed by adding 120g of vinylidene fluoride monomer gas. After the addition of the monomers, the reaction temperature was controlled at 87℃and after the addition of the water-soluble ammonium persulfate initiator and 0.15g of isopropyl alcohol chain transfer agent, the reaction was started up with an initiator addition amount of about 8% of the total amount of the added monomers. The polymerization time was 0.8h and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the polyvinylidene fluoride polymer.

In example 11, polyvinylidene fluoride has a weight average molecular weight of 2 ten thousand and is prepared by the following steps:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen several times to remove oxygen, followed by adding 120g of vinylidene fluoride monomer gas. After the addition of the monomers, the reaction temperature was controlled at 83℃and after the addition of the water-soluble ammonium persulfate initiator and 0.14g of isopropyl alcohol chain transfer agent, the reaction was started up with an initiator addition amount of about 8% of the total amount of the added monomers. The polymerization time was 1h and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the polyvinylidene fluoride polymer.

In example 12, polyvinylidene fluoride has a weight average molecular weight of 8 ten thousand and is prepared by the following steps:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen several times to remove oxygen, followed by adding 120g of vinylidene fluoride monomer gas. After the monomer was added, the reaction temperature was controlled at 82℃and after the addition of the water-soluble ammonium persulfate initiator and 0.12g of isopropyl alcohol chain transfer agent, the reaction was started up with the addition of the initiator being about 8% of the total amount of the added monomer. The polymerization time was 1.5h and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the polyvinylidene fluoride polymer.

In example 13, polyvinylidene fluoride had a weight average molecular weight of 15 ten thousand and was produced by the following method:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen several times to remove oxygen, followed by adding 120g of vinylidene fluoride monomer gas. After the monomers were added, the reaction temperature was controlled at 79℃and after the addition of a water-soluble ammonium persulfate initiator and 0.12g of isopropyl alcohol chain transfer agent, the reaction was started, the addition amount of the initiator was about 7.0% of the total amount of the added monomers, the polymerization time was 1.5 hours, and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the polyvinylidene fluoride polymer.

In examples 14, 15 and 16, polyvinylidene fluoride was commercially available.

Example 17 is substantially the same as example 4 except that the first fluoropolymer has a weight average molecular weight of 600 ten thousand; example 18 is essentially the same as example 12 except that the first fluoropolymer has a weight average molecular weight of 600 ten thousand; example 19 is substantially the same as example 15 except that the first fluoropolymer has a weight average molecular weight of 600 ten thousand; the preparation method of polyvinylidene fluoride with weight average molecular weight of 600 ten thousand comprises the following steps:

first stage polymerization: adding 4kg of deionized water and 2g of methyl cellulose ether into an autoclave of No. 1 and No. 2 and 10L, vacuumizing and replacing O ₂ with N ₂ for three times, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1kg of vinylidene fluoride monomer to enable the pressure to reach 5MPa, mixing and stirring for 30min, heating to 45 ℃ and reacting for 5h;

Third stage polymerization: after 35g of cyclohexane was added, the reaction was continued for 1 hour, and the reaction was stopped. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the first fluorine-containing polymer.

The polydispersion coefficient of the polyvinylidene fluoride is 2, the crystallinity is 42%, and the Dv50 particle size is 140 mu m; the solution was dissolved in N-methylpyrrolidone solution to prepare a solution having a mass percent concentration of 2wt%, and the solution viscosity was found to be 2700 MPa.s.

Examples 20-22 were prepared in substantially the same manner as examples 17-19 except that the first fluoropolymer had a weight average molecular weight of 900 ten thousand, and the specific preparation method was:

first stage polymerization: adding 4kg of deionized water and 2g of methyl cellulose ether into an autoclave of No. 1 and No. 2 and 10L, vacuumizing and replacing O ₂ with N ₂ for three times, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1kg of vinylidene fluoride monomer to enable the pressure to reach 5MPa, mixing and stirring for 30min, heating to 45 ℃, and reacting for 8h;

third stage polymerization: after 20g of cyclohexane was added, the reaction was continued for 1 hour, and the reaction was stopped. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the catalyst.

The polydispersion coefficient of the polyvinylidene fluoride is 2.3, the crystallinity is 46%, and the Dv50 particle size is 200 mu m; the solution was dissolved in N-methylpyrrolidone solution to prepare a solution having a mass percent concentration of 2wt%, and the solution viscosity was measured to be 4300 MPa.s.

The preparation methods of examples 23 to 27 were substantially the same as example 4 except that the kind of the second fluoropolymer was changed, and in example 23, the second fluoropolymer was polytetrafluoroethylene having a weight average molecular weight of 1 ten thousand, and the preparation method was:

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 1.0g of tert-amyl peroxypivalate and 0.1g of potassium carbonate again, adding 0.1Kg of tetrafluoroethylene, mixing and stirring for 30min, heating to 68 ℃ and carrying out polymerization reaction for 3h; and distilling, washing, separating, drying and crushing the polymerization solution to obtain the polytetrafluoroethylene.

In example 24, a second fluoropolymer was prepared from a vinylidene fluoride-hexafluoropropylene copolymer having a weight average molecular weight of 1 ten thousand by:

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 1.0g of tert-amyl peroxypivalate and 0.1g of potassium carbonate again, adding 0.8Kg of vinylidene fluoride and 0.2Kg of hexafluoropropylene, mixing and stirring for 30min, heating to 68 ℃ and carrying out polymerization reaction for 4h; the polymerization solution is distilled, washed, separated, dried and crushed to obtain the polyvinylidene fluoride-hexafluoropropylene.

In example 25, a second fluoropolymer having a weight average molecular weight of 1 ten thousand polyvinylidene fluoride was prepared by:

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 1.0g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 68 ℃, and carrying out polymerization reaction for 3h; the polymerization solution is distilled, washed, separated, dried and crushed to obtain polyvinylidene fluoride with the weight average molecular weight of 1 ten thousand, namely the second fluorine-containing polymer.

In example 26, the second fluoropolymer was polytetrafluoroethylene having a weight average molecular weight of 1 ten thousand prepared by:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, nitrogen was added to replace oxygen several times to remove oxygen, then 120g of tetrafluoroethylene monomer gas was added, the reaction temperature was controlled at 50 ℃, ammonium persulfate initiator was added, and the reaction was started, the addition amount of the initiator was about 15% of the total amount of the monomers. Controlling the reaction pressure to 0.7MPa for 1h, discharging polytetrafluoroethylene from the bottom of the kettle after polymerization, and obtaining powdery materials through filtering, washing, drying and grinding.

In example 27, a second fluoropolymer was prepared from a vinylidene fluoride-hexafluoropropylene copolymer having a weight average molecular weight of 1 ten thousand by:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen gas several times to remove oxygen, followed by addition of 100g of vinylidene fluoride monomer gas and 11.9g of hexafluoropropylene. After the addition of the monomers, the reaction temperature was controlled at 85 ℃. After the addition of the water-soluble ammonium persulfate initiator and 0.14g of isopropyl alcohol chain transfer agent, the reaction was started, the addition amount of the initiator was about 8% of the total amount of the monomers, the polymerization time was 1 hour, and the pressure was maintained at 4.4MPa. And washing and drying the reaction product to obtain the second fluorine-containing polymer.

The types of second fluoropolymers in examples 28-32 were identical to examples 23-27, respectively, except that the second fluoropolymer had a weight average molecular weight of 8 ten thousand.

In example 28, the second fluoropolymer was polytetrafluoroethylene having a weight average molecular weight of 8 ten thousand prepared by:

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.9g of tert-amyl peroxypivalate and 0.1g of potassium carbonate again, adding 0.1Kg of tetrafluoroethylene, mixing and stirring for 30min, heating to 64 ℃ and carrying out polymerization reaction for 6h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

In example 29, the second fluoropolymer was a vinylidene fluoride-hexafluoropropylene copolymer having a weight average molecular weight of 8 ten thousand prepared by:

Adding 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.9g of tert-amyl peroxypivalate and 0.1g of potassium carbonate again, charging 0.8kg of vinylidene fluoride and 0.2kg of hexafluoropropylene, mixing and stirring for 30min, heating to 64 ℃, and carrying out polymerization reaction for 7h; the polymerization solution is distilled, washed, separated, dried and crushed to obtain the polyvinylidene fluoride-hexafluoropropylene.

In example 30, the second fluoropolymer was polyvinylidene fluoride having a weight average molecular weight of 8 ten thousand prepared by:

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.9g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 64 ℃, and carrying out polymerization reaction for 6h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

In example 31, the second fluoropolymer was polytetrafluoroethylene having a weight average molecular weight of 8 ten thousand prepared by:

In example 32, a second fluoropolymer having a weight average molecular weight of 8 ten thousand was prepared as a vinylidene fluoride-hexafluoropropylene copolymer by:

In a 0.5L autoclave, 219g of deionized water and 0.1g of hydroxypropyl methylcellulose were added, and replaced with nitrogen gas several times to remove oxygen, followed by addition of 100g of vinylidene fluoride monomer gas and 11.9g of hexafluoropropylene. After the addition of the monomers, the reaction temperature was controlled at 85 ℃. After the addition of the water-soluble ammonium persulfate initiator and 0.12g of isopropyl alcohol chain transfer agent, the reaction was started, and the initiator was added in an amount of about 8% of the total amount of the monomers. The polymerization time was 1h and the pressure was maintained at 4.6MPa. And washing and drying the reaction product to obtain the second fluorine-containing polymer.

In example 33, a first fluoropolymer was prepared as a vinylidene fluoride-chlorotrifluoroethylene copolymer having a weight average molecular weight of 750 ten thousand, in substantially the same manner as in example 3, except that:

First stage polymerization: adding 4kg of deionized water and 2.5g of methyl cellulose ether into an autoclave of No. 1 and No. 2 and 10L, vacuumizing and replacing O ₂ with N ₂ for three times, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 0.94kg of vinylidene fluoride and 0.06kg of chlorotrifluoroethylene, enabling the pressure to reach 5MPa, mixing and stirring for 30min, heating to 45 ℃ and reacting for 6.5h;

Third stage polymerization: after 25.5g of cyclohexane was added, the reaction was continued for 1 hour, and the reaction was stopped. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the vinylidene fluoride-chlorotrifluoroethylene copolymer binder.

Comparative examples 1-4 are substantially identical to examples 1, 17, 20, 33, except that the second fluoropolymer is not included in the binder, only the first fluoropolymer.

The binder in comparative example 5 was polyvinylidene fluoride having a weight average molecular weight of 80 ten thousand, which was grade 701A polyvinylidene fluoride produced by eastern yang optical company.

2. Battery performance test

1. Adhesive composition property testing

1) Weight average molecular weight test

A Waters 2695 Isocratic HPLC gel chromatograph (differential refractive detector 2141) was used. A sample of 3.0% by mass polystyrene solution was used as a reference and a matched column (oiliness: styragel HT5DMF7.8 x 300mm+Styragel HT4) was selected. Preparing 3.0% binder glue solution with purified N-methyl pyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. During the test, tetrahydrofuran is firstly sucked by a syringe, and then the syringe is washed, and the test is repeated for several times. Then, 5ml of the test solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And after the indication is stable, acquiring data, and reading the weight average molecular weight.

2) Polydisperse coefficient testing

A Waters 2695 Isocratic HPLC gel chromatograph (differential refractive detector 2141) was used. A sample of 3.0% by mass polystyrene solution was used as a reference and a matched column (oiliness: styragel HT5DMF7.8 x 300mm+Styragel HT4) was selected. Preparing 3.0% binder glue solution with purified N-methyl pyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. During the test, tetrahydrofuran is firstly sucked by a syringe, and then the syringe is washed, and the test is repeated for several times. Then, 5ml of the test solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And obtaining data after the indication is stable. The weight average molecular weight a and the number average molecular weight b were read separately. Polydisperse coefficient = a/b.

3) Dv50 test

With reference to GB/T19077-2016 particle size distribution laser diffraction method, weighing 0.1 g-0.13 g of first fluoropolymer powder by using a 50ml beaker, weighing 5g of absolute ethyl alcohol, adding into the beaker filled with the first fluoropolymer powder, placing a stirrer with the length of about 2.5mm, and sealing by using a preservative film. And (3) putting the sample into an ultrasonic machine for ultrasonic treatment for 5min, transferring the sample into a magnetic stirrer, stirring for more than 20min by using 500r/min, and taking 2 samples from each batch of products for testing and averaging. The measurement is performed using a laser particle size analyzer, such as a Mastersizer 2000E laser particle size analyzer from malvern instruments, england.

4) Crystallinity test

Placing 0.5g of the first fluoropolymer in an aluminum dry pot, shaking, covering a crucible cover, blowing 50ml/min of a purge gas under a nitrogen atmosphere, and heating at a temperature rising rate of 10 ℃/min with a protective gas of 70ml/min, testing at a temperature ranging from-100 ℃ to 400 ℃, and testing and eliminating heat history by using a Differential Scanning Calorimeter (DSC) with a model of Discovery 250 of the TA Instrument in the United states.

This test will result in a DSC/(Mw/mg) versus temperature profile for the first fluoropolymer and integrate the peak area, i.e. the melting enthalpy of the first fluoropolymer Δh (J/g), binder crystallinity = Δh/(Δhm100%). 100%, where Δhm100% is the standard melting enthalpy of the fluoropolymer (crystalline heat of fusion), Δhm100% = 104.7J/g.

5) Glue viscosity test

14G of a first fluorine-containing polymer and 336g N-methylpyrrolidone (NMP) are weighed by a 500ml beaker respectively to prepare a glue solution with the mass fraction of 4%, a high-speed grinding machine is used for stirring and dispersing, the rotating speed is 800r/min, and after the stirring time is 120min, air bubbles are removed by ultrasonic vibration for 30min. At room temperature, using a force technology NDJ-5S rotary viscometer to test, selecting a No. 3 rotor to insert glue solution, ensuring that a rotor liquid level mark is level with the glue solution, testing viscosity at a rotor rotating speed of 12r/min, and reading viscosity data after 6 min.

2. Slurry performance test

1) Slurry flowability test

And taking a proper amount of positive electrode slurry by using a medicine spoon, and observing whether the natural downflow of the positive electrode slurry is smooth. If the natural downflow is smooth, judging that the natural downflow is OK; if the fluidity is poor, the slurry is jelly-like and is agglomerated, which indicates that gel appears, and is judged to be NG.

2) Slurry filtration performance test

Placing a 500ml beaker at the lower end of a 200-mesh filter screen bracket, taking 500ml of slurry, placing the slurry in a filter screen for filtering, recording the time when the volume of the slurry in the beaker reaches 300ml, and judging the filtering performance of the slurry at the time, wherein the filtering time is lower than 120s, and the filtering performance of the slurry is OK; if the slurry cannot pass through the filter screen, the slurry is poor in filtering performance, and the judgment is "NG".

3. Pole piece performance test

1) Adhesion test

Referring to GB-T2790-1995 national standard "180 DEG peel Strength test method of adhesive", the adhesion test procedure of the examples and comparative examples of the application is as follows:

cutting a sample with the width of 30mm and the length of 100-160mm by a blade, and sticking a special double-sided adhesive tape on a steel plate, wherein the width of the adhesive tape is 20mm and the length of the adhesive tape is 90-150mm. The positive electrode film layer surface of the pole piece sample intercepted in the front is stuck on a double-sided adhesive tape, and then is rolled three times along the same direction by a 2kg press roller.

Paper tape with the width equal to the width of the pole piece and the length of 250mm is fixed on the pole piece current collector and is fixed by using crepe adhesive.

And (3) turning on think carefully a power supply (sensitivity is 1N) of the tension machine, turning on an indicator lamp, adjusting a limiting block to a proper position, and fixing one end of the steel plate, which is not attached with the pole piece, by using a lower clamp. The paper tape is turned upwards and fixed by an upper clamp, and the position of the upper clamp is adjusted by using an 'up' button and a 'down' button on a manual controller attached to a pulling machine. Then testing is performed and the values are read. The adhesive strength between the positive electrode film layer and the current collector is represented by dividing the force of the pole piece when the pole piece is stressed and balanced by the width of the adhesive tape as the adhesive force of the pole piece in unit length.

2) Measurement of positive electrode film resistance:

Cutting the dried positive electrode slurry (film layer) at the left, middle and right parts of the positive electrode plate into small wafers with the diameter of 3mm. And (3) starting a power supply of the element energy science and technology pole piece resistance meter, placing the power supply at a proper position of a probe of the pole piece resistance meter, clicking a start button, and reading after the indication is stable. And testing two positions of each small wafer, and finally calculating the average value of six measurements, namely the film resistance of the pole piece.

3) Pole piece brittleness test

The positive electrode sheet in the examples was cut into test specimens of 20X 100mm size for use. The pole piece is bent, folded and fixed, a rolling roller with the weight of 2kg is used for rolling once, and whether light is transmitted and metal leaks at the folded position of the pole piece are checked; if no light-transmitting metal leakage exists, the pole piece is reversely folded and fixed, a rolling roller of 2kg is used for rolling once, whether the light-transmitting metal leakage exists at the folded position of the pole piece is checked, the steps are repeated until the light-transmitting metal leakage exists at the folded position of the pole piece, and the folded light-transmitting times of the positive pole piece are recorded. The average value was calculated by measuring 3 groups in parallel.

3. Battery performance test

1) Battery capacity retention test

The battery capacity retention test procedure was as follows: at 25 ℃, the button cell was charged to 3.65V at a constant current of 1/3C, then charged to 0.05C at a constant voltage of 3.65V, left for 5min, then discharged to 2.5V at 1/3C, and the resulting capacity was recorded as initial capacity C0. Repeating the above steps for the same battery, and recording the discharge capacity Cn of the battery after the nth cycle, wherein the battery capacity retention ratio Pn=Cn/C0 after each cycle is 100%, the 500 point values of P1 and P2 … … P500 are taken as ordinate, and the corresponding cycle times are taken as abscissa, so as to obtain a graph of the battery capacity retention ratio and the cycle times.

In this test procedure, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and … … th cycle corresponds to n=500. The battery capacity retention rate data corresponding to examples 1 to 41 or comparative examples 1 to 3 in table 1 are data measured after 500 cycles under the above test conditions, i.e., P500 values.

3. Analysis of test results for examples and comparative examples

Batteries of each example and comparative example were prepared according to the above-described methods, and each performance parameter was measured, and the results are shown in tables 1 and 2 below.

TABLE 1

Table 1, below

TABLE 2

As can be seen from a comparison of examples 1 to 33 with comparative example 5, the adhesive composition includes a first fluoropolymer having a weight average molecular weight of 500 to 900 ten thousand and a second fluoropolymer having a weight average molecular weight of not more than 60 ten thousand. Compared with the polyvinylidene fluoride binder with the weight average molecular weight of 80 ten thousand used in the prior art, the binder composition provided by the application effectively reduces the binder dosage, is beneficial to further improving the loading capacity of the pole piece active material and improves the energy density of the battery.

As can be seen from examples 1-33, the first fluoropolymer comprises structural units derived from vinylidene fluoride, which may be a homopolymer or a copolymer; the second fluoropolymer comprises at least one of polytetrafluoroethylene, polyvinylidene fluoride, and poly (vinylidene fluoride-hexafluoropropylene) copolymer.

As can be seen from the comparison of examples 1 to 9 with comparative example 1, the adhesive composition having the mass content of the second fluoropolymer of 0.25% to 15% based on the total mass of the adhesive composition can optimize the processability of the adhesive while maintaining high adhesive properties, improve the brittleness of the electrode sheet, improve the uniformity of the electrode sheet, and thus improve the cycle performance of the battery, compared to the use of the macromolecular polyvinylidene fluoride adhesive alone. With the increase of the use content of the second fluorine-containing polymer in the adhesive composition, the adhesive force of the adhesive composition is gradually reduced, the crystallinity is firstly reduced and then increased, the flexibility of the pole piece is firstly increased and then reduced,

As can be seen from the comparison of examples 10 to 22 with comparative example 1, the weight average molecular weight of the second fluoropolymer was 40 to 60 ten thousand, and the addition amount of the second fluoropolymer to the binder composition was 1.5% to substantially achieve the technical effect achieved by the addition amount of the second fluoropolymer being 0.5 to 15 ten thousand, 1%.

It can be seen from examples 23 to 33 that the second fluoropolymer prepared using ammonium persulfate as the initiator can further improve the retention of the adhesive property of the pole piece without significantly degrading the adhesive property of the pole piece while improving the processability of the slurry.

Nuclear magnetic resonance results show that the existence of hydroxyl and ester groups in the end groups of the second fluoropolymer prepared by taking ammonium persulfate as an initiator can effectively improve the adhesive property of the second fluoropolymer compared with the fluoropolymer prepared by taking 2-ethyl peroxydicarbonate as the initiator, wherein the end groups of the second fluoropolymer comprise-CF ₂-CH₂OH、-CF₂-CH₂OOCCH₃.

The present application is not limited to the above embodiment. The above embodiments are merely examples, and embodiments having substantially the same configuration and the same effects as those of the technical idea within the scope of the present application are included in the technical scope of the present application. Further, various modifications that can be made to the embodiments and other modes of combining some of the constituent elements in the embodiments, which are conceivable to those skilled in the art, are also included in the scope of the present application within the scope not departing from the gist of the present application.

Claims

An adhesive composition comprising a first fluoropolymer having a weight average molecular weight of 500 to 900 tens of thousands and a second fluoropolymer having a weight average molecular weight of not more than 60 tens of thousands.
The adhesive composition according to claim 1, wherein the mass content of the second fluoropolymer is 0.25-15% based on the total mass of the adhesive composition.
The adhesive composition according to claim 1 or 2, wherein the crystallinity of the adhesive composition is not higher than 45%, optionally 20% -45%.
A binder composition according to any one of claims 1 to 3, characterized in that the first fluoropolymer comprises structural units derived from vinylidene fluoride.
The adhesive composition according to any one of claims 1 to 4, wherein the first fluoropolymer further comprises structural units of formula I,

Wherein R ₁ comprises one or more of fluorine, chlorine, C _1-3 alkyl containing at least one fluorine atom.
The binder composition of any one of claims 1-5 wherein the first fluoropolymer has a polydispersity of 1.8 to 2.5.
The adhesive composition according to any one of claims 1 to 6, wherein the crystallinity of the first fluoropolymer is 30-46%, optionally 40-46%.
The adhesive composition according to any one of claims 1 to 7, wherein a viscosity of a dope containing 2% by mass of the first fluoropolymer obtained by dissolving the first fluoropolymer in N-methylpyrrolidone is 2000 MPa-s to 5000 MPa-s.
The adhesive composition of any one of claims 1 to 8, wherein the first fluoropolymer comprises one or more of polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer.
The adhesive composition according to any one of claims 1 to 9, wherein the second fluoropolymer comprises structural units of formula II,

Wherein each R ₂、R₃ independently includes at least one of hydrogen, halogen, C _1-3 alkyl containing at least one fluorine atom.
The adhesive composition according to any one of claims 1 to 10, wherein each R ₂、R₃ independently comprises at least one of hydrogen, fluorine, chlorine, trifluoromethyl.
The adhesive composition according to any one of claims 1 to 11, wherein the end groups of the second fluoropolymer contain hydroxyl or ester groups.
The adhesive composition according to any one of claims 1 to 12, wherein the second fluoropolymer has a weight average molecular weight of 0.5 to 60 tens of thousands.
The adhesive composition of any one of claims 1 to 13, wherein the second fluoropolymer comprises one of polytetrafluoroethylene, polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) copolymer, poly (vinylidene fluoride-tetrafluoroethylene) copolymer, poly (vinylidene fluoride-vinylidene chloride) copolymer, poly (vinylidene fluoride-hexafluoropropylene) copolymer.
A method for preparing an adhesive composition is characterized in that,

Preparing a first fluoropolymer: under the polymerizable condition, carrying out a first polymerization reaction on a raw material containing vinylidene fluoride monomers to prepare a first fluorine-containing polymer, wherein the weight average molecular weight of the first fluorine-containing polymer is 500-900 ten thousand;

Preparing a second fluoropolymer: carrying out a second polymerization reaction on the fluorine-containing monomer under the polymerizable condition to prepare a second fluorine-containing polymer, wherein the weight average molecular weight of the second fluorine-containing polymer is not more than 60 ten thousand;

blending: an adhesive composition is prepared by blending the first fluoropolymer with the second fluoropolymer.
The method of preparing a binder composition according to claim 15, wherein the method of preparing a first fluoropolymer specifically comprises:

providing a raw material containing vinylidene fluoride monomers and a reaction solvent, and performing a first-stage polymerization reaction to obtain a first product;

Carrying out second-stage polymerization reaction on the first product under the water-insoluble gas atmosphere;

Adding chain transfer agent to perform the third polymerization reaction to obtain polyvinylidene fluoride with weight average molecular weight of 500-900 ten thousand.
The method according to claim 16, wherein the reaction temperature of the first stage polymerization is 45 to 60 ℃, the reaction time is 4 to 10 hours, and the initial polymerization pressure is 4 to 6MPa.
The process according to claim 16 or 17, wherein the second polymerization stage is carried out at a reaction temperature of 60 to 80 ℃ for a reaction time of 2 to 4 hours and a reaction pressure of 6 to 8MPa.
The method according to any one of claims 16 to 18, wherein the reaction time of the third stage polymerization is 1 to 2 hours.
The method according to any one of claims 15 to 19, characterized in that the method for preparing the second fluoropolymer specifically comprises:

Carrying out a second polymerization reaction on at least one monomer shown in a formula III in a non-reactive gas atmosphere at a reaction temperature of between 0.1 and 5MPa and between 60 and 90 ℃ for 0.5 to 8 hours, stopping the reaction, carrying out solid-liquid separation, and retaining a solid phase to obtain a second fluorine-containing polymer

Wherein each R ₄、R₅ independently includes at least one of hydrogen, halogen, C _1-3 alkyl containing at least one fluorine atom.
The method of any one of claims 15 to 20, wherein the second polymerization reaction further comprises the steps of:

adding a solvent and a dispersing agent into a container, and filling the container with a non-reactive gas;

And (3) adding a monomer shown in a formula III, heating to 60-90 ℃, and then adding a second initiator and a chain transfer agent to perform a second polymerization reaction.
The method of claim 21, wherein the second initiator comprises an inorganic peroxide selected from the group consisting of potassium persulfate and ammonium persulfate.
The production method according to claim 21 or 22, wherein the mass content of the second initiator is 3% to 12% based on the total mass of the monomer represented by formula III.
The method of preparing a binder composition according to any one of claims 16 to 23, wherein the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.
A positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, a conductive agent, and the binder composition according to any one of claims 1 to 14 and the binder composition prepared by the preparation method according to any one of claims 15 to 24.
The positive electrode sheet according to claim 25, wherein the mass fraction of the binder composition is not more than 1% based on the total mass of the positive electrode film layer.
The positive electrode sheet according to claim 25 or 26, wherein the positive electrode active material is a lithium-containing transition metal oxide, optionally lithium iron phosphate or lithium nickel cobalt manganese oxide, or a doping modified material thereof, or at least one of a conductive carbon coating modified material, a conductive metal coating modified material, or a conductive polymer coating modified material thereof.
A secondary battery comprising an electrode assembly and an electrolyte, the electrode assembly comprising a separator, a negative electrode tab, and the positive electrode tab of any one of claims 25-27.
An electric device comprising the secondary battery according to claim 28.