WO2024045631A1

WO2024045631A1 - Binder composition, positive electrode sheet, secondary battery and electric device

Info

Publication number: WO2024045631A1
Application number: PCT/CN2023/088498
Authority: WO
Inventors: 李�诚; 曾子鹏; 刘会会; 王景明
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-08-30
Filing date: 2023-04-14
Publication date: 2024-03-07

Abstract

Provided in the present application is a binder composition, which comprises a first fluorine-containing polymer and a second fluorine-containing polymer, wherein the first fluorine-containing polymer comprises polyvinylidene fluoride with a weight-average molecular weight of 5,000,000-9,000,000, and the second fluorine-containing polymer has a weight-average molecular weight of no more than 600,000. The binder composition has good processability, enables an electrode sheet to have a high adhesion force at a low addition amount, and can improve the cycle performance of a battery.

Description

Binder composition, positive electrode sheet, secondary battery and electrical device

cross reference

This application cites CN Patent No. 202211044631.4 titled "Fluoropolymer, its preparation method and use, positive electrode slurry, secondary battery, battery module, battery pack and electrical device" submitted on August 30, 2022 Application, CN patent application No. 202211052014.9 titled "Fluoropolymers, their preparation methods and uses, positive electrode slurries, secondary batteries, battery modules, battery packs and electrical devices" submitted on August 30, 2022, The CN patent application No. 202211044756.7 submitted on August 30, 2022, titled "Binder, preparation method, positive electrode sheet, secondary battery and electrical device", and the title "Adhesive" submitted on August 30, 2022 CN Patent Application No. 202211043966.4 "Junction Agent, Preparation Method, Positive Electrode Plate, Secondary Battery and Electric Device", which is fully incorporated into this application by reference.

Technical field

The present application relates to the technical field of secondary batteries, and in particular to an adhesive, a preparation method, a positive electrode sheet, a secondary battery, a battery module, a battery pack and an electrical device.

Background technique

In recent years, secondary batteries have been widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, as well as in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. With the popularization of secondary battery applications, higher requirements have been placed on its cycle performance and service life.

Binders are commonly used materials in secondary batteries and are in great demand for battery pole pieces, separators, packaging, etc. However, the existing binders have poor adhesion and often require a large amount of addition to meet the adhesive strength requirements of the pole pieces, which will limit the improvement of battery energy density. Therefore, existing adhesives still need to be improved.

Contents of the invention

The present application was made in view of the above-mentioned problems, and its purpose is to provide an adhesive that can exhibit excellent adhesive force at a low addition amount and has good processability.

In order to achieve the above object, the present application provides a binder composition. The binder composition includes a first fluoropolymer and a second fluoropolymer. The weight average molecular weight of the first fluoropolymer is 5 million to 9 million, and the weight average molecular weight of the second fluoropolymer does not exceed 600,000.

This binder can ensure sufficient adhesion of the pole pieces at a low addition amount and improve the cycle performance of the battery.

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

Controlling the mass ratio of the first fluoropolymer to the second fluoropolymer within an appropriate range, the binder can be added in a low amount so that the electrode piece has excellent bonding force, which can improve the capacity retention of the battery during cycling. Rate. By adding a small amount of the second fluoropolymer to the first fluoropolymer, the crystallinity and processability of the binder can be effectively improved, thereby reducing the manufacturing cost and further improving the cycle performance of the battery.

In any embodiment, the crystallinity of the binder composition is no higher than 45%, optionally between 20% and 45%.

The addition of the second fluoropolymer to the first fluoropolymer can effectively reduce the crystallinity of the binder and improve the flexibility of the pole piece.

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

In any embodiment, the first fluoropolymer further includes a structural unit represented by Formula I,

Among them, R ₁ includes one or more of hydrogen, fluorine, chlorine, and a C _1-3 alkyl group containing at least one fluorine atom.

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

The polydispersity coefficient of the first fluoropolymer is within an appropriate range, so that the electrode piece has excellent adhesion and improves the capacity retention rate of the battery during cycling. In addition, a suitable polydispersity coefficient of the first fluoropolymer can effectively increase the solid content of the slurry and reduce production costs.

In any embodiment, the first fluoropolymer has a crystallinity of 30% to 46%, optionally 40% to 46%.

In any embodiment, the viscosity of the glue containing 2% mass content of the first fluoropolymer prepared by dissolving the first fluoropolymer in N-methylpyrrolidone is 2000MPa·s～5000MPa. ·s.

When preparing positive electrode slurry, the binder needs to have a certain viscosity to prevent the positive electrode active material and conductive agent and other additives from settling, so that the slurry can be placed more stably. In the existing technology, to achieve a glue viscosity of 2000MPa·s to 5000MPa·s, a binder with a mass content of at least 7% is required. Based on the mass of the glue, the first fluorine-containing polymer of the present application The expected viscosity of the glue can be achieved at a dosage of 2%, which provides a basis for reducing the content of the binder in the positive electrode film layer.

In any embodiment, the first fluoropolymer includes polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer - One or more of 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 represented by Formula II,

Wherein, R ₂ and R ₃ each independently include at least one of hydrogen, halogen or C _1-3 alkyl group containing at least one fluorine atom.

In any embodiment, R ₂ and R ₃ each independently include at least one of hydrogen, fluorine, chlorine, and trifluoromethyl.

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

The weight average molecular weight of the second fluorine-containing polymer is small, and the mass content of the end groups in it is high. Therefore, the end groups have a great influence on the performance of the second fluorine-containing polymer. The end group of the second fluoropolymer contains a hydroxyl group or an ester group, which can effectively improve the adhesion of the second fluoropolymer and reduce the reduction in the adhesion performance of the pole piece caused by the addition of the second fluoropolymer.

In any embodiment, the weight average molecular weight of the second fluoropolymer ranges from 0,500 to 600,000, optionally from 0,500 to 150,000.

In any embodiment, the second fluoropolymer includes polytetrafluoroethylene, polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) ) copolymer, poly(vinylidene fluoride-chlorovinylidene fluoride) copolymer, poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene) copolymer.

A second aspect of the application provides a method for preparing a binder composition to prepare a first fluoropolymer: under polymerizable conditions, a raw material containing vinylidene fluoride monomer is subjected to a first polymerization reaction to prepare a first fluorine-containing polymer. Fluorine-containing polymer, the weight average molecular weight of the first fluorine-containing polymer is 5 million to 9 million; preparing the second fluorine-containing polymer: subjecting the fluorine-containing monomer to a second polymerization reaction under polymerizable conditions to prepare a second fluorine-containing polymer. Fluoropolymer, the weight average molecular weight of the second fluoropolymer does not exceed 600,000; blending: blending the first fluoropolymer and the second fluoropolymer to prepare a binder composition.

In any embodiment, the preparation method of the first fluoropolymer specifically includes: providing a raw material containing vinylidene fluoride monomer and a reaction solvent, performing a first-stage polymerization reaction to obtain a first product; and converting the first The product undergoes the second-stage polymerization reaction in a water-insoluble gas atmosphere; a chain transfer agent is added to perform the third-stage polymerization reaction to obtain polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.

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

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

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

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

At least one monomer represented by formula III is subjected to a second polymerization reaction in a non-reactive gas atmosphere, 0.1-5MPa, and a reaction temperature of 60°C to 90°C for 0.5 to 8 hours, the reaction is stopped, and the solid and liquid are separated, leaving the solid phase to obtain the second fluoropolymer

Among them, R ₄ and R ₅ are each independently selected from hydrogen, halogen or C _1-3 alkyl group containing at least one fluorine atom.

In any embodiment, the second polymerization further includes the following steps:

Add solvent and dispersant to the container, and fill the container with non-reactive gas;

Add the monomer represented by formula III, raise the temperature to 60°C to 90°C, then add the second initiator and chain transfer agent to perform the second polymerization reaction.

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

Nuclear magnetic test results show that when inorganic peroxide is used as the initiator, the end groups in the second fluoropolymer include -CF ₂ -CH ₂ OH or -CF ₂ -CH ₂ OOCCH ₃ . The presence of hydroxyl groups and ester groups in the end groups can effectively improve the adhesiveness of the second fluoropolymer and reduce the decrease in adhesive performance of the adhesive composition caused by its addition.

In any embodiment, the mass content of the initiator is 3%-12% based on the total mass of the monomer represented by Formula III.

In any embodiment, the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.

A third aspect of the present application provides a positive electrode sheet, including 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 includes a positive electrode active material, a conductive agent, and a binder in any embodiment. The composition or the adhesive composition prepared by the preparation method in any embodiment.

The positive electrode sheet has excellent adhesive force when the binder composition is added in a low amount.

In any embodiment, the mass fraction of the binder composition does not exceed 1%, based on the total mass of the positive electrode film layer.

Controlling the mass fraction of the binder within an appropriate range, while ensuring that the electrode pieces have sufficient adhesion, can increase the loading of active materials in the battery electrode pieces, helping to further improve the power performance of the battery.

In any embodiment, the positive 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 their doping modification materials, or their conductive carbon coating modification materials, conductive metal coating modification materials At least one of the modified materials is coated with a conductive material or a conductive polymer.

A fourth aspect of the present application provides a secondary battery, including an electrode assembly and an electrolyte. The electrode assembly includes a separator, a negative electrode sheet, and the positive electrode sheet of the third aspect of the present application.

In a fourth aspect of the present application, an electrical device is provided, including the secondary battery of the third aspect of the present application.

Description of drawings

FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

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

Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

Figure 4 is a schematic diagram of a battery pack according to an embodiment of the present application.

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

FIG. 6 is a schematic diagram of a power consumption device using a secondary battery as a power source according to an embodiment of the present application.

Explanation of reference symbols:
1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover.

Detailed ways

Hereinafter, embodiments specifically disclosing the positive electrode active material and its manufacturing method, the positive electrode tab, the secondary battery, the battery module, the battery pack, and the electrical device of the present application will be described in detail with appropriate reference to the drawings. However, unnecessary detailed explanations may be omitted. For example, detailed descriptions of well-known matters may be omitted, or descriptions of substantially the same structure may be repeated. This is to prevent the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application, and are not intended to limit the subject matter described in the claims.

"Ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit that define the boundaries of the particular range. Ranges defined in this manner may be inclusive or exclusive of the endpoints, and may be arbitrarily combined, that is, any lower limit may 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, understand that ranges of 60-110 and 80-120 are also expected. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2- 3, 2-4 and 2-5. In this application, unless stated otherwise, the numerical range "a-b" represents an abbreviated 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 in this article, and "0-5" is just an abbreviation of these numerical combinations. In addition, when stating that a certain parameter is an integer ≥ 2, it is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

If there is no special description, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

If there is no special description, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

If there is no special instructions, all steps of the present application can be performed sequentially or randomly, and are preferably performed sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), indicating that the step Step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or May include steps (c), (a) and (b), etc.

If there is no special explanation, the words "include" and "include" mentioned in this application represent open expressions, which may also be closed expressions. For example, "comprising" and "comprising" may mean that other components not listed may also be included or included, or only the listed components may be included or included.

In this application, the term "or" is inclusive unless otherwise stated. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists) ; Or both A and B are true (or exist).

Fluoropolymers are currently one of the most widely used binder types in secondary batteries. However, the viscosity of traditional fluoropolymers is low, and a large amount of addition is often required to ensure effective bonding of active materials, thereby enabling the pole pieces to achieve effective bonding force. However, increasing the dosage of traditional fluoropolymers will reduce the load of active materials in the pole pieces, affecting the improvement of battery power performance and making it difficult to meet the requirements for battery cycle performance.

[Binder]

The present application provides a binder composition. The binder composition includes a first fluoropolymer and a second fluoropolymer. The weight average molecular weight of the first fluoropolymer is 5 million to 5 million. 9 million, and the weight average molecular weight of the second fluoropolymer does not exceed 600,000.

As used herein, the term "binder composition" refers to a mixture forming a colloidal solution or colloidal dispersion in a dispersion medium.

As used herein, the term "fluoropolymer" refers to polymers with fluorine-containing monomers as the main synthetic monomers. Polymers include, on the one hand, chemically homogeneous polymers prepared by polymerization reactions, but with varying degrees of polymerization, molar mass, and A collection of macromolecules that differ in chain length. The term on the other hand also includes derivatives of aggregates of macromolecules formed by polymerization reactions which are obtainable by reaction, for example addition or substitution, of functional groups in said macromolecules and which may be chemically homogeneous or chemically non-uniform compounds. Fluoride in this article Polymers include both homopolymers and copolymers.

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

In some embodiments, the dispersion medium of the adhesive composition is an oily solvent. Examples of the oily solvent include but are not limited to dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, Dimethyl carbonate, ethyl cellulose, polycarbonate. That is, the binder is dissolved in the oily solvent.

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

In some embodiments, the binder composition serves as a positive electrode binder for binding positive electrode active materials and/or conductive agents to form an electrode.

In some embodiments, the binder composition serves as a negative electrode binder for binding negative electrode active materials and/or conductive agents to form an electrode.

In some embodiments, the first fluoropolymer has a weight average molecular weight of 5 million to 9 million. In some embodiments, the first fluoropolymer has a weight average molecular weight of 5 million, 5.5 million, 6 million, 6.5 million, 7 million, 7.5 million, 8 million, 8.5 million, 9 million, or any value therein.

In some embodiments, the second fluoropolymer has a weight average molecular weight of no more than 600,000. In some embodiments, the weight average molecular weight of the second fluoropolymer can be selected from 0.5 thousand, 10 thousand, 20 thousand, 30 thousand, 40 thousand, 50 thousand, 60 thousand, 70 thousand, 80 thousand, 90 thousand, 10 Ten thousand, 110,000, 120,000, 130,000, 140,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000, 550,000, 600,000 or any value in between.

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

The fluorine element contained in the first fluorine-containing polymer and the second fluorine-containing polymer forms hydrogen bonds with the hydroxyl groups or/and carboxyl groups on the surface of the active material and the surface of the current collector, which can improve the adhesive force of the pole piece. The first fluoropolymer with a weight average molecular weight of 5 million to 9 million can improve the adhesion of the pole pieces at a low level of addition and improve the capacity retention rate of the battery during cycling. The addition of the second fluoropolymer in the binder can reduce the crystallinity of the binder, improve the flexibility of the pole piece, and also improve the processing performance of the slurry. During the drying process of preparing the pole piece, the pole piece There will be no delamination phenomenon, which improves the uniformity of the active material dispersion in the pole piece, reduces the diaphragm resistance, and further optimizes the cycle performance of the battery. The above-mentioned binder composition can ensure sufficient bonding force and flexibility of the electrode piece at a low addition amount, which 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, based on the total mass of the binder composition, the mass content of the second fluoropolymer can be selected from 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 value therein.

Controlling the mass ratio of the first fluoropolymer to the second fluoropolymer within an appropriate range, the binder can be added in a low amount so that the electrode piece has excellent bonding force, which can improve the capacity retention of the battery during cycling. Rate. By adding a small amount of the second fluoropolymer to the first fluoropolymer, the crystallinity of the binder, the flexibility and processability of the pole piece can be effectively improved, thereby reducing the manufacturing cost and further improving the cycle performance of the battery.

In some embodiments, the binder composition has a crystallinity of no greater than 45%, optionally between 20% and 45%.

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

In this article, the term "crystallinity" refers to the proportion of crystalline areas in the polymer. There are some areas with stable and regularly arranged molecules in the microstructure. Areas where the molecules are regularly and closely arranged are called crystalline areas.

In this application, the crystallinity can be tested using methods known in the art, such as differential scanning thermal analysis. In some embodiments, 0.5 g of the fluoropolymer composition is placed in an aluminum dry pot, shaken flat, and the crucible lid is covered, under a nitrogen atmosphere, with a purge gas of 50 ml/min and a protection of 70 ml/min. Air, the heating rate is 10℃/min, the test temperature range is -100℃-400℃, and the differential scanning calorimeter (DSC) of the American TA instrument model Discovery 250 is used to test and eliminate the thermal history.

This test will obtain the DSC/(Mw/mg) curve of the fluoropolymer composition as it changes with temperature and integrate it. The peak area is the melting enthalpy ΔH (J/g) of the fluoropolymer composition. The bonding Agent crystallinity = ΔH/(ΔHm) × 100%, where ΔHm is the standard melting enthalpy (crystalline fusion heat) of fluoropolymer, ΔHm = 104.7J/g.

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

In some embodiments, the first fluoropolymer further includes a structural unit represented by Formula I,

In some embodiments, _R1 includes one or more of hydrogen, fluorine, chlorine, and trifluoromethyl.

In some embodiments, the first fluoropolymer includes polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene copolymer. One or more of chlorofluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer.

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

In this document, the term "polydispersity coefficient" refers to the ratio of the weight average molecular weight of the 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 molecules of different molecular weights in the polymer multiplied by their corresponding molecular weights.

If the polydispersity coefficient of the first fluoropolymer is too large, the polymerization degree of the first fluoropolymer will be relatively dispersed, affecting the uniformity of the binder, and the binder will not be able to uniformly adhere the positive electrode active material to the current collector. will affect the cycle performance of the battery, and also reduce the solid content of the slurry, making it impossible to further improve the energy density of the battery; if the polydispersity coefficient of the first fluoropolymer is too small, the preparation process will be difficult and the yield rate will be low. , resulting in higher production costs.

In this application, the polydispersity coefficient of the first fluoropolymer can be tested using methods known in the art, such as gel chromatography, such as Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index). Detector 2141) for testing. In some embodiments, a polystyrene solution sample with a mass fraction of 3.0% is used as a reference to select a matching chromatographic column (oil: Styragel HT5DMF7.8*300mm+Styragel HT4). Use purified N-methylpyrrolidone (NMP) solvent to prepare 3.0% adhesive glue solution, and let the prepared solution stand for one day for later use. When testing, first draw in tetrahydrofuran with a syringe, rinse, and repeat several times. Then draw 5 ml of the test solution, remove the air from the syringe, and dry the needle tip. Finally, slowly inject the sample solution into the injection port. Obtain the data after the display is stable. Read the weight average molecular weight a and number average respectively. Subquantity b. Polydispersity coefficient=a/b.

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

In some embodiments, the viscosity of the glue containing 2% mass content of the first fluoropolymer prepared by dissolving the first fluoropolymer in N-methylpyrrolidone is 2000MPa·s to 5000MPa·s. In some embodiments, the viscosity of the glue containing 2% mass content of the first fluoropolymer prepared by dissolving the first fluoropolymer in N-methylpyrrolidone can be selected from 2000 MPa·s to 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 Any one of ~5000MPa·s.

If the viscosity of the first fluoropolymer is too high, the viscosity of the prepared binder solution will be too high, making it difficult to stir, reducing the dispersibility of the binder, making it difficult for the binder to uniformly adhere the positive electrode active material to the collection. In terms of fluid, it affects the cycle performance of the battery. At the same time, the viscosity of the binder solution is too high, which reduces the speed of the pulping process; if the viscosity of the first fluoropolymer is too small, the viscosity of the prepared binder solution will be too small, which is extremely harmful. It is difficult for the sheet to have sufficient adhesion at low additive amounts.

In addition, when preparing the positive electrode slurry, the binder needs to have a certain viscosity to prevent the positive electrode active materials and conductive agents and other additives from settling, so that the slurry can be placed more stably. In traditional technology, to achieve a glue viscosity of 2000MPa·s to 5000MPa·s, a binder with a mass content of at least 7% is required to achieve it, based on the mass of the glue. However, the first fluoropolymer of this application The expected viscosity of the glue can be achieved at a dosage of 2%, which provides a basis for reducing the content of the binder in the positive electrode film layer.

Control the viscosity of the first fluoropolymer solution within an appropriate range so that the pole piece is adhered to the With a low addition amount of binder, it can have excellent bonding performance and improve the capacity retention rate of the battery during cycling.

In some embodiments, the second fluoropolymer includes structural units represented by Formula II,

In some embodiments, R ₂ and R ₃ each independently include at least one of hydrogen, fluorine, chlorine, and trifluoromethyl.

In some embodiments, the second fluoropolymer includes polytetrafluoroethylene, polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer Poly(vinylidene fluoride-chlorovinylidene fluoride) copolymer, poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene) copolymer.

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

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

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

Preparation of the first fluorine-containing polymer: Under polymerizable conditions, the raw material containing vinylidene fluoride monomer is subjected to a first polymerization reaction to prepare the first fluorine-containing polymer. The weight average molecular weight of the first fluorine-containing polymer is 5 million. ~9 million;

Preparing the second fluoropolymer: under polymerizable conditions, at least one fluoromonomer Performing a second polymerization reaction to prepare a second fluoropolymer, the weight average molecular weight of the second fluoropolymer not exceeding 600,000;

Blending: The first fluoropolymer and the second fluoropolymer are blended to prepare an adhesive composition.

In this article, the term "blending" refers to the process of making a macroscopically uniform material from two or more substances under certain conditions such as temperature and/or shear stress.

The preparation method of the binder is simple, environmentally friendly, has reduced costs and is conducive to industrial production. At the same time, the binder prepared by this method enables the pole piece to have excellent bonding force, and can improve the capacity retention rate of the battery during cycling at a low addition amount.

In some embodiments, in the blending step, the mass ratio of the first fluoropolymer to the second fluoropolymer is 99.75:0.25˜85:15. In some embodiments, the mass ratio of the first fluoropolymer to the second fluoropolymer may 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 value among them.

In some embodiments, the synthesis step of the first fluoropolymer includes: providing a raw material containing vinylidene fluoride monomer and a reaction solvent, performing a first-stage polymerization reaction, and obtaining a first product;

Subjecting the first product to a second-stage polymerization reaction in a water-insoluble gas atmosphere;

Add a chain transfer agent and perform the third stage polymerization reaction to obtain polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.

The first product may refer to the reaction liquid obtained after the first-stage polymerization reaction, or may be the product after the reaction liquid is purified after the first-stage polymerization reaction.

In some embodiments, multiple parts of the first product are mixed, and the second stage polymerization reaction is performed under a water-insoluble gas atmosphere. That is, the second-stage polymerization reaction is self-polymerization of the first product. It can be understood that multiple portions of the first product can be simultaneously prepared through multiple reaction kettles, or can be prepared multiple times through one reaction kettle. The uniformity of the polyproduct can be improved through multiple, segmented synthesis methods.

Using a segmented method for polymerization, ultra-high molecular weight polyvinylidene fluoride can be prepared, so that the binder can meet the needs of pole piece bonding strength at a low addition amount, which helps It is beneficial to increase the loading capacity of the positive active material in the pole piece and improve the capacity retention rate of the battery during cycling. At the same time, the first product is formed in the first stage of polymerization reaction, and the molecular chain segment of the target molecular weight is formed in the second stage of polymerization reaction. The third stage of polymerization reaction is used to control the molecular weight of the polymer to avoid excessively high molecular weight and reducing the weight of polyvinylidene fluoride. The uniformity of the average molecular weight improves the uniformity of the product. Moreover, staged polymerization can improve the utilization rate of the reactor during the preparation of polyvinylidene fluoride, save time, and reduce the residence time of polyvinylidene fluoride in the reactor. The first-stage polymerization reaction, the second-stage polymerization reaction, and the third-stage polymerization reaction cooperate with each other to further improve the production efficiency of polyvinylidene fluoride.

In some embodiments, the reaction temperature of the first stage polymerization reaction is 45°C to 60°C. In some embodiments, the reaction temperature of the first stage polymerization reaction can be selected from 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, or 45°C to 55°C.

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

In some embodiments, the initial polymerization pressure is 4MPa to 6MPa. In some embodiments, the initial polymerization pressure can be selected from 4MPa to 5MPa, or 5MPa to 6MPa. In some embodiments, the initial polymerization pressure is higher than the critical pressure of vinylidene fluoride.

In some embodiments, the reaction temperature of the second stage polymerization reaction is 60°C to 80°C. In some embodiments, the reaction temperature of the second stage polymerization reaction can be selected from 60°C to 70°C or 70°C to 80°C.

In some embodiments, the reaction time of the second stage polymerization reaction is 2h to 4h. In some embodiments, the reaction time of the second stage polymerization reaction can be selected from 2h to 3h, or 3h to 4h.

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

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

Control the reaction pressure, reaction time, and reaction temperature of each stage of the polymerization reaction. Within an appropriate range, while increasing the weight average molecular weight of polyvinylidene fluoride, the uniformity of the weight average molecular weight of the polymerization product can be controlled, ensuring that the product has a lower polydispersity coefficient, and improving the balance of polyvinylidene fluoride properties, so that The pole piece has excellent adhesion with a low amount of binder added, and the cycle capacity retention rate of the battery can be further improved.

In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.

Water-insoluble gas refers to a gas with a gas solubility less than 0.1L. Gas solubility refers to the volume of gas when it is dissolved in 1L of water and reaches saturation when the pressure of the gas is 1.013×10 ⁵ Pa at 20°C. In some embodiments, the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.

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

Controlling the amount of chain transfer agent within an appropriate range can control the polymer chain length, thereby obtaining a polymer with a suitable molecular weight range and uniform distribution.

In some embodiments, the first stage polymerization reaction includes the following steps:

Add water solvent and dispersant to the container to remove oxygen from the reaction system;

Add the first initiator and pH regulator to the container to adjust the pH value to 6.5-7, then add vinylidene fluoride monomer to make the pressure in the container reach 4MPa-6MPa;

After stirring for 30 to 60 minutes, the temperature is raised to 45°C to 60°C to carry out the first stage of polymerization reaction.

Before raising the temperature to carry out the polymerization reaction, mix the materials evenly first, so that the reaction can proceed more thoroughly, and the weight average molecular weight, crystallinity and particle size of the prepared polymer will be more uniform.

In some embodiments, during the preparation process of the first fluorine-containing polymer, the amount of water solvent used is 2 to 8 times the total mass of vinylidene fluoride monomer. The amount of solvent used may also be, for example, 3, 4, 5, 6 or 7 times the total mass of vinylidene fluoride monomer. In some embodiments, the water solvent is deionized water.

In some embodiments, during the preparation of the first fluoropolymer, the dispersant includes one or more of cellulose ether and polyvinyl alcohol.

In some embodiments, during the preparation of the first fluoropolymer, the dispersant includes one or more of methylcellulose ether and carboxyethylcellulose ether.

In some embodiments, during the preparation process of the first fluoropolymer, the amount of dispersant is 0.1% to 0.3% of the total mass of vinylidene fluoride monomer. 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 includes t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and One or more types of tert-butyl peroxypivalate.

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

In some embodiments, the pH adjusting agent includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and ammonia.

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

In some embodiments, the preparation method of the second fluoropolymer specifically includes:

Polymerize at least one monomer represented by formula III in a non-reactive gas atmosphere, 0.1-5MPa, and a reaction temperature of 60°C to 90°C for 0.5 to 8 hours, stop the reaction, separate the solid and liquid, and retain the solid phase, Obtain a second fluoropolymer,

Wherein, R ₄ and R ₅ are each independently selected from hydrogen, halogen or C _1-3 alkyl group containing at least one fluorine atom.

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°C, 65°C, 70℃, 75℃, 80℃, 85℃, 90℃ or any value among them.

Carrying out polymerization reactions under high temperature and high pressure can improve reaction efficiency, reduce reaction time, increase reaction conversion rate, and improve product uniformity and purity.

In some embodiments, the second polymerization reaction further includes the following steps:

Add the monomer represented by Formula III, raise the temperature to 55°C to 75°C, add a second initiator and a chain transfer agent, and perform the second polymerization reaction.

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

NMR results show that when inorganic peroxide is used as the initiator, the end groups in the second fluoropolymer include -CF ₂ -CH ₂ OH or -CF ₂ -CH ₂ OOCCH ₃ . The presence of hydroxyl groups and ester groups in the end groups can effectively improve the adhesiveness of the second fluoropolymer and reduce the decrease in adhesive performance of the adhesive composition caused by its addition.

In some embodiments, the mass content of the initiator is 3%-12% based on the total mass of the monomer represented by Formula III.

High content of initiator helps to improve reaction efficiency, reduce reaction time, reduce the polydispersity coefficient of the product, and improve the uniformity of the product.

[Positive pole piece]

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 includes 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 does not exceed 1%, based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of the binder is 0.4% to 1%. In some embodiments, the mass content of the adhesive composition is 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or any value therein.

The adhesive composition can effectively improve the bonding performance of the pole piece at a low addition amount. The pole piece has excellent flexibility and processing properties, allowing the battery to have high energy density and cycle performance.

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

In some embodiments, the positive active material is lithium iron phosphate or lithium nickel cobalt manganese oxide, or their doped modified materials, or their conductive carbon coating modified materials, conductive metal coating modified materials, or conductive The polymer coats at least one of the modified materials.

In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming metal materials (aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on polymer material substrates (such as polypropylene (PP), polyterephthalate It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

In some embodiments, the cathode active material may be a cathode active material known in the art for batteries. As an example, the cathode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials of batteries can also be used. Only one type of these positive electrode active materials may be used alone, or two or more types may be used in combination. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO ₂ ), lithium nickel oxides (such as LiNiO ₂ ), lithium manganese oxides (such as LiMnO ₂ , LiMn ₂ O ₄ ), lithium Nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (also referred to as NCM ₃₃₃ ), LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (can also be abbreviated to NCM ₅₂₃ ), LiNi _0.5 Co _0.25 Mn _0.25 O ₂ (can also be abbreviated to NCM ₂₁₁ ), LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (can also be abbreviated to NCM ₆₂₂ ), LiNi At least one of _0.8 Co _0.1 Mn _0.1 O ₂ (also referred to as NCM ₈₁₁ ), lithium nickel cobalt aluminum oxide (such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and its modified compounds. The olivine structure contains Examples of lithium phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), composites of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), At least one of a composite material of lithium manganese phosphate and carbon, a composite material of lithium manganese iron phosphate, or a composite material of lithium manganese iron phosphate and carbon.

In some embodiments, the positive electrode film layer optionally further 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 can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components in a solvent (such as N -methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode piece can be obtained.

[Negative pole piece]

The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, where the negative electrode film layer includes a negative electrode active material.

As an example, the negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

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

In some embodiments, the negative active material may be a negative active material known in the art for batteries. As an example, the negative 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 selected from at least one of elemental silicon, silicon oxide compounds, silicon carbon composites, silicon nitrogen composites and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as battery negative electrode active materials can also be used. material. Only one type of these negative electrode active materials may be used alone, or two or more types may be used in combination.

In some embodiments, the negative electrode film layer optionally further includes a binder. The binder can be selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polysodium acrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), poly At least one of methacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

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

In some embodiments, the negative electrode film layer optionally includes other auxiliaries, such as thickeners (such as sodium carboxymethylcellulose (CMC-Na)) and the like.

In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative active materials, conductive agents, binders and any other components in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode piece can be obtained.

[electrolyte]

The electrolyte plays a role in conducting ions between the positive and negative electrodes. There is no specific restriction on the type of electrolyte in this application, and it can be selected according to needs. For example, the electrolyte can be liquid, gel, or completely solid.

In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes electrolyte salts and solvents.

In some embodiments, the electrolyte salt may be selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bistrifluoromethanesulfonimide, trifluoromethane At least one of lithium sulfonate, lithium difluorophosphate, lithium difluoroborate, lithium dioxaloborate, lithium difluorodioxalate phosphate and lithium tetrafluoroxalate phosphate.

In some embodiments, the solvent may be selected from the group consisting of ethylene carbonate, propylene carbonate, methylethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, Ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate At least one of ester, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

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

[Isolation film]

In some embodiments, the secondary battery further includes a separator film. There is no particular restriction on the type of isolation membrane in this application. Any well-known porous structure isolation membrane with good chemical stability and mechanical stability can be used.

In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and fluoropolymer. The isolation film can be a single-layer film or a multi-layer composite film, with no special restrictions. When the isolation film is a multi-layer composite film, the materials of each layer can be the same or different, and there is no particular limitation.

In some embodiments, the positive electrode piece, the negative electrode piece and the separator film can be made into an electrode assembly through a winding process or a lamination process.

In some embodiments, the secondary battery may include an outer packaging. The outer packaging can be used to package the above-mentioned electrode assembly and electrolyte.

In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft bag, such as a bag-type soft bag. The material of the soft bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

This application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square or any other shape. For example, FIG. 1 shows a square-structured secondary battery 5 as an example.

In some embodiments, referring to FIG. 2 , the outer package may include a housing 51 and a cover 53 . Among them, the housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose Form a receiving cavity. The housing 51 has an opening communicating with the accommodation cavity, and the cover plate 53 can cover the opening to close the accommodation cavity. The positive electrode piece, the negative electrode piece and the isolation film can be formed into the electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is packaged in the containing cavity. The electrolyte soaks into the electrode assembly 52 . The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

In some embodiments, secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in the battery module can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery module.

Figure 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 arranged in sequence along the length direction of the battery module 4 . Of course, it can also be arranged in any other way. Furthermore, the plurality of secondary batteries 5 can be fixed by fasteners.

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

In some embodiments, the above-mentioned battery modules can also be assembled into a battery pack. The number of battery modules contained in the battery pack can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery pack.

Figures 4 and 5 show the battery pack 1 as an example. Referring to FIGS. 4 and 5 , the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box 2 and a lower box 3 . The upper box 2 can be covered with the lower box 3 and form a closed space for accommodating the battery module 4 . Multiple battery modules 4 can be arranged in the battery box in any manner.

In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided by the present application. The secondary battery, battery module, or battery pack may be used as a power source for the electrical device, or may be used as an energy storage unit for the electrical device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric Golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.

As the power-consuming device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

Figure 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, etc. In order to meet the high power and high energy density requirements of the secondary battery for the electrical device, a battery pack or battery module can be used.

As another example, the device may be a mobile phone, a tablet, a laptop, etc. The device is usually required to be thin and light, and a secondary battery can be used as a power source.

Example

Hereinafter, examples of the present application will be described. The embodiments described below are illustrative and are only used to explain the present application and are not to be construed as limitations of the present application. If specific techniques or conditions are not specified in the examples, the techniques or conditions described in literature in the field or product instructions will be followed. If the manufacturer of the reagents or instruments used is not indicated, they are all conventional products that can be purchased commercially.

Example 1

1) Preparation of adhesive composition

Preparation of the first fluorine-containing polymer: First-stage polymerization reaction: Add 4 kg of deionized water and 2 g of methylcellulose ether to No. 1 and No. 2 10L autoclaves, evacuate and replace O ₂ with N ₂ three times, Add 5g of tert-butylperoxypivalate and 2g of sodium bicarbonate again, and fill in 1kg of vinylidene fluoride monomer to bring the pressure to 5MPa. Mix and stir for 30min, raise the temperature to 45°C, and react for 7.5h;

The second stage of polymerization reaction: transfer the reaction liquid in No. 1 and No. 2 reactors to No. 3 reactor, fill with nitrogen to a pressure of 7MPa, raise the temperature to 70°C, and stir for 3 hours;

The third stage of polymerization reaction: add 22.5g of cyclohexane and continue the reaction for 1 hour, then stop the reaction. The reaction system is centrifuged and the solid phase is collected, washed and dried to obtain the polyvinylidene fluoride binder.

The polyvinylidene fluoride binder with a weight average molecular weight of 7.5 million has a polydispersity coefficient of 2.2, a crystallinity of 45%, and a Dv50 particle size of 100 μm; it is dissolved in an N-methylpyrrolidone solution to produce a mass concentration of 4wt% solution, the viscosity of the solution was measured to be 4900MPa·s.

Preparation of the second fluoropolymer:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace it with nitrogen several times to remove oxygen, and then add 120g of vinylidene fluoride monomer gas. After adding the monomer, control the reaction temperature at 85°C. After adding the water-soluble ammonium persulfate initiator and 0.14g isopropyl alcohol chain transfer agent, the reaction starts. The amount of initiator added is about 8% of the total amount of monomer added. , the polymerization reaction time is 1h, and the pressure is maintained at 4.4MPa. After the reaction product is washed with water and dried, a polyvinylidene fluoride polymer is obtained.

The first fluoropolymer and the second fluoropolymer are blended, and the mass ratio of the first fluoropolymer to the second fluoropolymer is 99.75:0.25 to obtain a mixture containing the first fluoropolymer and the second partial fluoropolymer. Vinylidene fluoride adhesive composition.

2) Preparation of positive electrode pieces

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

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

Add 12.3g of polyvinylpyrrolidone dispersant into the mixing tank, stir for 60 minutes at a revolution speed of 25r/min and a rotation speed of 1300r/min;

After stirring is completed, test the viscosity of the slurry and control the viscosity between 8000 and 15000 MPa·s.

If the viscosity is high, add N-methylpyrrolidone (NMP) solution to reduce the viscosity to the above range. After adding the NMP solution, stir for 30 minutes at a revolution speed of 25 r/min and a rotation speed of 1200 to 1500 r/min to obtain the positive electrode slurry; The prepared positive electrode slurry is scraped onto the carbon-coated aluminum foil, baked at 110°C for 15 minutes, and then cold-pressed and cut into discs with a diameter of 15 mm to obtain the positive electrode sheet.

3) Negative pole piece

Metal lithium sheets are used as negative electrode sheets.

4) Isolation film

Use polypropylene film as the isolation film.

5) Preparation of electrolyte

In an argon atmosphere glove box (H ₂ O <0.1ppm, O ₂ <0.1ppm), mix the organic solvent ethylene carbonate (EC)/ethyl methyl carbonate (EMC) evenly according to the volume ratio of 3/7, and add LiPF Dissolve _{the 6} lithium salt in the organic solvent, stir evenly, and prepare a 1M LiPF ₆ EC/EMC solution to obtain an electrolyte.

6) Preparation of battery

The positive electrode sheet, negative electrode sheet, separator and electrolyte in Example 1 were assembled into a button battery in a buck box.

Example 2-9

The mass ratio of the first fluoropolymer and the second fluoropolymer in the adhesive composition is adjusted. Other preparation methods are the same as in Example 1. The specific parameters are as shown in Table 1.

Examples 10-16

The weight average molecular weight of the second fluoropolymer in the adhesive composition is adjusted. Other methods are the same as in Example 4. The specific parameters are as shown in Table 1.

In Example 10, the weight average molecular weight of polyvinylidene fluoride is 0.5 million, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace it with nitrogen several times to remove oxygen, and then add 120g of vinylidene fluoride monomer gas. After adding the monomer, control the reaction temperature at 87°C. After adding the water-soluble ammonium persulfate initiator and 0.15g isopropyl alcohol chain transfer agent, the reaction starts. The amount of initiator added is about 8% of the total amount of monomer added. . The polymerization reaction time was 0.8h, and the pressure was maintained at 4.4MPa. After the reaction product is washed with water and dried, a polyvinylidene fluoride polymer is obtained.

In Example 11, the weight average molecular weight of polyvinylidene fluoride is 20,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace it with nitrogen several times to remove oxygen, and then add 120g of vinylidene fluoride monomer gas. After adding the monomer, control the reaction temperature at 83°C. After adding the water-soluble ammonium persulfate initiator and 0.14g isopropyl alcohol chain transfer agent, the reaction starts. The amount of initiator added is about the total amount of monomer added. 8% of the amount. The polymerization reaction time was 1 h, and the pressure was maintained at 4.4MPa. After the reaction product is washed with water and dried, a polyvinylidene fluoride polymer is obtained.

In Example 12, the weight average molecular weight of polyvinylidene fluoride is 80,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace it with nitrogen several times to remove oxygen, and then add 120g of vinylidene fluoride monomer gas. After adding the monomer, control the reaction temperature at 82°C. After adding the water-soluble ammonium persulfate initiator and 0.12g isopropyl alcohol chain transfer agent, the reaction starts. The amount of initiator added is about 8% of the total amount of monomer added. . The polymerization reaction time was 1.5h, and the pressure was maintained at 4.4MPa. After the reaction product is washed with water and dried, a polyvinylidene fluoride polymer is obtained.

In Example 13, the weight average molecular weight of polyvinylidene fluoride is 150,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace it with nitrogen several times to remove oxygen, and then add 120g of vinylidene fluoride monomer gas. After adding the monomer, control the reaction temperature at 79°C. After adding the water-soluble ammonium persulfate initiator and 0.12g isopropyl alcohol chain transfer agent, the reaction starts. The amount of initiator added is approximately 7.0% of the total amount of monomer added. The polymerization reaction time was 1.5h, and the pressure was maintained at 4.4MPa. After the reaction product is washed with water and dried, a polyvinylidene fluoride polymer is obtained.

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

Embodiment 17 is basically the same as Embodiment 4, except that the weight average molecular weight of the first fluoropolymer is 6 million; Embodiment 18 is basically the same as Embodiment 12, except that the weight average molecular weight of the first fluoropolymer is 600 10,000; Example 19 is basically the same as Example 15, except that the weight average molecular weight of the first fluoropolymer is 6 million; the preparation method of polyvinylidene fluoride with a weight average molecular weight of 6 million is:

The first stage of polymerization reaction: Add 4kg of deionized water and 2g of methylcellulose ether to the 10L autoclave of No.1 and No.2, vacuum and replace _O2 with _N2 three times, and add 5g of tert-butyl peroxide again Pivalate and 2g of sodium bicarbonate, and filled with 1kg of vinylidene fluoride monomer to bring the pressure to 5MPa, mix and stir for 30 minutes, raise the temperature to 45°C, and react for 5 hours;

The second stage of polymerization reaction: transfer the reaction liquid in reactors No. 1 and 2 to reactor No. 3. Fill the kettle with nitrogen to a pressure of 7MPa, raise the temperature to 70°C, and stir for 3 hours;

The third stage of polymerization reaction: add 35g of cyclohexane and continue the reaction for 1 hour, then stop the reaction. The reaction system is centrifuged and the solid phase is collected, washed and dried to obtain the first fluoropolymer.

The polyvinylidene fluoride has a polydispersity coefficient of 2, a crystallinity of 42%, and a Dv50 particle size of 140 μm. Dissolve it in N-methylpyrrolidone solution to make a solution with a mass concentration of 2wt%. The measured The solution viscosity is 2700MPa·s.

The preparation methods of Examples 20-22 and Examples 17-19 are basically the same. The difference is that the weight average molecular weight of the first fluoropolymer is 9 million. The specific preparation method is:

The first stage of polymerization reaction: Add 4kg of deionized water and 2g of methylcellulose ether to the 10L autoclave of No.1 and No.2, vacuum and replace _O2 with _N2 three times, and add 5g of tert-butyl peroxide again Pivalate and 2g of sodium bicarbonate, and filled with 1kg of vinylidene fluoride monomer to bring the pressure to 5MPa, mix and stir for 30 minutes, raise the temperature to 45°C, and react for 8 hours;

The third stage of polymerization reaction: add 20g of cyclohexane and continue the reaction for 1 hour, then stop the reaction. Centrifuge the reaction system and collect the solid phase, wash and dry it.

The polyvinylidene fluoride has a polydispersity coefficient of 2.3, a crystallinity of 46%, and a Dv50 particle size of 200 μm. Dissolve it in N-methylpyrrolidone solution to make a solution with a mass percentage concentration of 2wt%. The measured The solution viscosity is 4300MPa·s.

The preparation methods of Examples 23-27 are basically the same as Example 4, except that the type of the second fluoropolymer is changed. In Example 23, the second fluoropolymer is polytetrafluoroethylene with a weight average molecular weight of 10,000. Ethylene, preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, add nitrogen gas to remove dissolved oxygen in the solution, and then add 1.0g of tert-amyl peroxide pivalate and 0.1 g of potassium carbonate, add 0.1Kg of tetrafluoroethylene, mix and stir for 30 minutes, raise the temperature to 68°C, and perform a polymerization reaction for 3 hours; the polymerization solution is distilled, washed, separated, dried, and pulverized to obtain polytetrafluoroethylene.

In Example 24, the second fluoropolymer is a vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 10,000, and the preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, add nitrogen gas to remove dissolved oxygen in the solution, and then add 1.0g of tert-amyl peroxide pivalate and 0.1 g of potassium carbonate, add 0.8Kg of vinylidene fluoride and 0.2Kg of hexafluoropropylene, mix and stir for 30 minutes, raise the temperature to 68°C, and perform a polymerization reaction for 4 hours; the polymerization solution is distilled, washed, separated, dried, and pulverized to obtain polyvinylidene fluoride. Fluoroethylene-hexafluoropropylene.

In Example 25, the second fluoropolymer is polyvinylidene fluoride with a weight average molecular weight of 10,000, and the preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, add nitrogen gas to remove the dissolved oxygen in the solution, and then add 1.0g of 2-ethylperoxydicarbonate and 0.1g of sodium bicarbonate, and add 0.1Kg of vinylidene fluoride, mix and stir for 30 minutes, raise the temperature to 68°C, and perform a polymerization reaction for 3 hours; the polymerization solution is distilled, washed, separated, dried, and pulverized to obtain a polymer with a weight average molecular weight of 10,000 Vinylidene fluoride, the second fluoropolymer.

In Example 26, the second fluoropolymer is polytetrafluoroethylene with a weight average molecular weight of 10,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, add nitrogen to replace the oxygen several times to remove oxygen, then add 120g of tetrafluoroethylene monomer gas, and control the reaction temperature at 50 ℃, add ammonium persulfate initiator, the reaction starts, the amount of initiator added is about 15% of the total amount of monomers. The reaction pressure is controlled to 0.7MPa and the time is 1 hour. After the polymerization is completed, the polytetrafluoroethylene is discharged from the bottom of the kettle and is filtered, washed, dried, and ground to obtain powdery material.

In Example 27, the second fluoropolymer is a vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 10,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace with nitrogen several times to remove oxygen, and then add 100g of vinylidene fluoride monomer gas and 11.9g of hexafluoropropylene. After adding the monomer, the reaction temperature was controlled at 85°C. After adding water-soluble ammonium persulfate initiator and 0.14g isopropyl alcohol chain transfer agent, the reaction started, and the initiator The added amount is about 8% of the total amount of monomers, the polymerization reaction time is 1 hour, and the pressure is maintained at 4.4MPa. After the reaction product is washed with water and dried, the second fluoropolymer is obtained.

The type of the second fluoropolymer in Examples 28-32 is the same as that in Examples 23-27, respectively, except that the weight average molecular weight of the second fluoropolymer is 80,000.

In Example 28, the second fluoropolymer is polytetrafluoroethylene with a weight average molecular weight of 80,000, and the preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, add nitrogen gas to remove the dissolved oxygen in the solution, and then add 0.9g of tert-amyl peroxide pivalate and 0.1g. of potassium carbonate, and filled with 0.1Kg of tetrafluoroethylene, mixed and stirred for 30 minutes, heated to 64°C, and polymerized for 6 hours; the polymerization solution was obtained by distilling, washing, separating, drying, and pulverizing.

In Example 29, the second fluoropolymer is a vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 80,000, and the preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, add nitrogen gas to remove the dissolved oxygen in the solution, and then add 0.9g of tert-amyl peroxide pivalate and 0.1g. of potassium carbonate, add 0.8kg of vinylidene fluoride and 0.2kg of hexafluoropropylene, mix and stir for 30 minutes, raise the temperature to 64°C, and perform a polymerization reaction for 7 hours; the polymerization solution is distilled, washed, separated, dried, and pulverized to obtain polyvinylidene fluoride Ethylene-hexafluoropropylene.

In Example 30, the second fluoropolymer is polyvinylidene fluoride with a weight average molecular weight of 80,000, and the preparation method is:

Add 0.4kg of deionized water and 0.2g of carboxyethyl cellulose ether to a 1L four-necked flask, pass in nitrogen to remove the dissolved oxygen in the solution, and then add 0.9g of 2-ethylperoxydicarbonate and 0.1g of sodium bicarbonate, and add 0.1Kg of vinylidene fluoride, mix and stir for 30 minutes, raise the temperature to 64°C, and perform a polymerization reaction for 6 hours; the polymerization solution is obtained by distilling, washing, separating, drying, and pulverizing.

In Example 31, the second fluoropolymer is polytetrafluoroethylene with a weight average molecular weight of 80,000, and the preparation method is:

In a 0.5L autoclave, add 219g deionized water and 0.1g hydroxypropyl methylcellulose, add nitrogen to replace the oxygen several times to remove oxygen, and then add tetrafluoroethylene monomer gas 120g, control the reaction temperature at 50°C, add ammonium persulfate initiator, and start the reaction. The amount of initiator added is about 15% of the total amount of monomers. The reaction pressure is controlled to 0.7MPa and the time is 1 hour. After the polymerization is completed, the polytetrafluoroethylene is discharged from the bottom of the kettle and is filtered, washed, dried, and ground to obtain powdery material.

In Example 32, the second fluoropolymer is a vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 80,000, and the preparation method is:

In a 0.5L autoclave, add 219g of deionized water and 0.1g of hydroxypropyl methylcellulose, replace with nitrogen several times to remove oxygen, and then add 100g of vinylidene fluoride monomer gas and 11.9g of hexafluoropropylene. After adding the monomer, the reaction temperature was controlled at 85°C. After adding water-soluble ammonium persulfate initiator and 0.12g isopropyl alcohol chain transfer agent, the reaction started. The amount of initiator added was about 8% of the total amount of monomers. The polymerization reaction time was 1 h, and the pressure was maintained at 4.6 MPa. After the reaction product is washed with water and dried, the second fluoropolymer is obtained.

In Example 33, the first fluorine-containing polymer is a vinylidene fluoride-chlorotrifluoroethylene copolymer with a weight average molecular weight of 7.5 million. Others are basically the same as Example 3. The preparation method of the first fluorine-containing polymer is:

The first stage of polymerization reaction: Add 4kg of deionized water and 2.5g of methylcellulose ether to the 10L autoclave of No.1 and No.2, evacuate and replace _O2 with _N2 three times, and then add 5g of tert-butyl hydrochloride Oxidize pivalate and 2g of sodium bicarbonate, and fill in 0.94kg of vinylidene fluoride and 0.06kg of chlorotrifluoroethylene to bring the pressure to 5MPa. Mix and stir for 30 minutes, raise the temperature to 45°C, and react for 6.5 hours;

The third stage of polymerization reaction: add 25.5g of cyclohexane and continue the reaction for 1 hour, then stop the reaction. The reaction system is centrifuged and the solid phase is collected, washed and dried to obtain a vinylidene fluoride-chlorotrifluoroethylene copolymer binder.

Comparative Examples 1-4 are basically the same as Examples 1, 17, 20, and 33, except that the binder does not contain the second fluoropolymer, but only the first fluoropolymer.

In Comparative Example 5, the binder is polyvinylidene fluoride with a weight average molecular weight of 800,000, which is the 701A brand polyvinylidene fluoride produced by Dongguang Sunshine Company.

2. Battery performance test

1. Testing of adhesive composition properties

1) Weight average molecular weight test

Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141) was used. Use a polystyrene solution sample with a mass fraction of 3.0% as a reference to select a matching chromatographic column (oil: Styragel HT5DMF7.8*300mm+Styragel HT4). Use purified N-methylpyrrolidone (NMP) solvent to prepare 3.0% adhesive glue solution, and let the prepared solution stand for one day for later use. When testing, first draw in tetrahydrofuran with a syringe, rinse, and repeat several times. Then draw 5 ml of the test solution, remove the air from the syringe, and dry the needle tip. Finally, slowly inject the sample solution into the injection port. After the display is stable, obtain the data and read the weight average molecular weight.

2) Polydispersion coefficient test

Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141) was used. Use a polystyrene solution sample with a mass fraction of 3.0% as a reference to select a matching chromatographic column (oil: Styragel HT5DMF7.8*300mm+Styragel HT4). Use purified N-methylpyrrolidone (NMP) solvent to prepare 3.0% adhesive glue solution, and let the prepared solution stand for one day for later use. When testing, first draw in tetrahydrofuran with a syringe, rinse, and repeat several times. Then draw 5 ml of the test solution, remove the air from the syringe, and dry the needle tip. Finally, slowly inject the sample solution into the injection port. Obtain the data after the display is stable. Read the weight average molecular weight a and number average molecular weight b respectively. Polydispersity coefficient=a/b.

3)Dv50 test

Referring to the GB/T 19077-2016 particle size distribution laser diffraction method, use a 50ml beaker to weigh 0.1g~0.13g of the first fluoropolymer powder, then weigh 5g of absolute ethanol, and add it to the container containing the first fluoropolymer. Put a stirrer with a length of about 2.5mm into a beaker containing the powdered material and seal it with plastic wrap. Put the sample into the ultrasonic machine for 5 minutes, transfer to the magnetic stirrer and stir at 500r/min for more than 20 minutes. Take 2 samples from each batch of products for testing and take the average value. Use a laser particle size analyzer for measurement, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Co., Ltd. in the United Kingdom.

4) Crystallinity test

Place 0.5g of the first fluoropolymer in an aluminum dry pot, shake it flat, cover the crucible lid, and use a purge gas of 50ml/min and a protective gas of 70ml/min under a nitrogen atmosphere with a heating rate of 10 ℃/min, test temperature range -100℃~400℃, use American TA Instruments model Discovery 250 differential scanning calorimeter (DSC) to test and eliminate thermal history.

This test will obtain the DSC/(Mw/mg) curve of the first fluoropolymer versus temperature and integrate it. The peak area is the melting enthalpy ΔH (J/g) of the first fluoropolymer. The bonding Agent crystallinity = ΔH/(ΔHm100%)*100%, where ΔHm100% is the standard melting enthalpy (crystalline fusion heat) of fluoropolymer, ΔHm100% = 104.7J/g.

5) Glue viscosity test

Use a 500ml beaker to weigh 14g of the first fluoropolymer and 336g of N-methylpyrrolidone (NMP) respectively, and prepare a glue solution with a mass fraction of 4%. Use a Lichen high-speed grinder to stir and disperse, with a rotation speed of 800r/min and a stirring time of After 120 minutes, ultrasonic vibration was performed for 30 minutes to remove bubbles. At room temperature, use the Lichen Technology NDJ-5S rotational viscometer for testing. Use the No. 3 rotor to insert the glue liquid to ensure that the rotor liquid level mark is level with the glue liquid level. Test the viscosity at a rotor speed of 12r/min. After 6 minutes Just read the viscosity data.

2. Slurry performance test

1) Slurry fluidity test

Use a medicine spoon to take an appropriate amount of positive electrode slurry and observe whether the natural downward flow of the positive electrode slurry is smooth. If it flows naturally and smoothly, it is judged as OK; if the fluidity is poor and the slurry appears jelly-like and lumps, indicating gel formation, it is judged as NG.

2) Slurry filtration performance test

Take a 500ml beaker and place it at the lower end of the 200 mesh filter holder. Take 500ml of the slurry and filter it in the filter. Record the time when the volume of the slurry in the beaker reaches 300ml. This time is used to judge the filtration performance and filtration time of the slurry. Lower than 120s, it indicates that the filtration performance of the slurry is OK; if the slurry cannot penetrate the filter, it indicates that the filtration performance of the slurry is poor and is judged as "NG".

3. Pole piece performance test

1) Adhesion test

Refer to the GB-T2790-1995 national standard "Test Method for 180° Peel Strength of Adhesives". The bonding force testing process of the application examples and comparative examples is as follows:

Use a blade to cut a sample with a width of 30mm and a length of 100-160mm, and stick the special double-sided tape on the steel plate with a width of 20mm and a length of 90-150mm. Paste the positive electrode film layer of the pole piece sample intercepted earlier on the double-sided tape, and then roll it three times in the same direction with a 2kg pressure roller.

Fix a paper tape with the same width as the pole piece and a length of 250mm on the pole piece current collector, and fix it with wrinkle glue.

Turn on the power of the Sansi tensile machine (sensitivity is 1N), the indicator light is on, adjust the limit block to the appropriate position, and fix the end of the steel plate that is not attached to the pole piece with the lower clamp. Fold the paper tape upward and fix it with the upper clamp. Use the "up" and "down" buttons on the manual controller that comes with the tensile machine to adjust the position of the upper clamp. Then perform the test and read the value. Divide the force of the pole piece when the force is balanced by the width of the tape as the bonding force of the pole piece per unit length to characterize the bonding strength between the positive electrode film layer and the current collector.

2) Determination of positive electrode film resistance:

Cut the dried positive electrode slurry (film layer) on the left, middle and right sides of the positive electrode piece into small discs with a diameter of 3mm. Turn on the power of Yuaneng Technology's pole piece resistance meter, place it in the appropriate position of the "probe" of the pole piece resistance meter, click the "Start" button, and wait until the reading is stable, then read it. Each small disc is tested at two positions, and finally the average of the six measurements is calculated, which is the film resistance of the pole piece.

3) Pole piece brittleness test

Cut the positive electrode sheet in the example into a test sample with a size of 20×100 mm for later use. Bend the pole piece in half and fix it. Use a 2kg rolling roller to roll it once. Check whether there is light transmission and metal leakage at the folded part of the pole piece. If there is no light transmission and metal leakage, fold the pole piece in reverse and fix it. Use a 2kg roller. Roll it once with a rolling roller to check whether light is transmitted through the folded part of the pole piece and metal is leaking out. Repeat the above steps until light is transmitted through the folded part and metal leaks out. Record the number of times the positive electrode piece is folded and light is transmitted through. Three groups were measured in parallel and the average value was calculated.

3. Battery performance test

1)Battery capacity retention test

The battery capacity retention rate test process is as follows: at 25°C, press the button battery to 1/3C Charge with constant current to 3.65V, then charge with constant voltage of 3.65V until the current is 0.05C, leave it aside for 5 minutes, and then discharge with 1/3C to 2.5V. The resulting capacity is recorded as the initial capacity C0. Repeat the above steps for the same battery, and at the same time record the discharge capacity Cn of the battery after the nth cycle. Then the battery capacity retention rate after each cycle Pn=Cn/C0*100%, based on P1, P2...P500, the 500 Each point value is the ordinate, and the corresponding number of cycles is the abscissa, and a curve graph of battery capacity retention rate and number of cycles is obtained.

During this test, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and the 500th 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 is the data measured after 500 cycles under the above test conditions, that is, the value of P500.

3. Analysis of test results of each embodiment and comparative example

Batteries of each example and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 and Table 2 below.

Table 1

Continued table 1

Table 2

It can be seen from the comparison between Examples 1-33 and Comparative Example 5 that the adhesive composition includes a first fluoropolymer and a second fluoropolymer, and the weight average molecular weight of the first fluoropolymer is is 5 million to 9 million, and the weight average molecular weight of the second fluoropolymer does not exceed 600,000. Compared with the polyvinylidene fluoride binder with a weight average molecular weight of 800,000 used in the prior art, the binder composition provided by this application effectively reduces the amount of binder and helps to further improve the activity of the pole piece. The material loading capacity increases the energy density of the battery.

It can be seen from Examples 1-33 that the first fluorine-containing polymer includes structural units derived from vinylidene fluoride, which can be a homopolymer or a copolymer; the second fluorine-containing polymer includes polytetrafluoroethylene, polyvinylidene fluoride At least one of ethylene and poly(vinylidene fluoride-hexafluoropropylene) copolymer.

It can be seen from the comparison between Examples 1-9 and Comparative Example 1 that compared to using a macromolecular polyvinylidene fluoride binder alone, based on the total mass of the binder composition, the second fluoropolymer has A binder composition with a mass content of 0.25% to 15% can optimize the processing performance of the binder while maintaining high bonding performance, improve the brittleness of the pole piece, and improve the uniformity of the pole piece, thereby improving the cycle performance of the battery. As the content of the second fluoropolymer in the binder composition increases, the adhesive force of the binder composition gradually decreases, the crystallinity first decreases and then increases, and the pole piece flexibility first increases and then decreases.

From the comparison between Examples 10-22 and Comparative Example 1, it can be seen that when the weight average molecular weight of the second fluoropolymer is 400,000-600,000, the addition amount of the second fluoropolymer in the adhesive composition is 1.5%. When the second fluoropolymer is 0.5-150,000, the technical effect achieved by adding 1% can be basically achieved.

It can be seen from Examples 23-33 that the second fluoropolymer prepared using ammonium persulfate as the initiator can improve the processing performance of the slurry and further improve the retention rate of the bonding performance of the pole piece without causing adhesion to the pole piece. The junction performance will be significantly reduced.

Nuclear magnetic resonance results show that the end groups of the second fluoropolymer prepared with ammonium persulfate as the initiator include -CF ₂ -CH ₂ OH and -CF ₂ -CH ₂ OOCCH ₃ . Compared with the end groups of the second fluoropolymer prepared with 2-ethyl peroxidation Dicarbonate is a fluoropolymer prepared as an initiator. The presence of hydroxyl groups and ester groups in the end groups of the fluoropolymer can effectively improve the bonding performance of the second fluoropolymer.

It should be noted that the present application is not limited to the above-described embodiment. The above-mentioned embodiments are only examples. Within the scope of the technical solution of the present application, embodiments that have substantially the same structure as the technical idea and exert the same functions and effects are included in the technical scope of the present application. In addition, technical skills in the art may be applied to the embodiments without departing from the gist of the present application. Various modifications that a person can think of and other forms of construction by combining some of the constituent elements in the embodiments are also included in the scope of this application.

Claims

A binder composition, characterized in that the binder composition includes a first fluoropolymer and a second fluoropolymer, and the first fluoropolymer has a weight average molecular weight of 5 million to 9 million, and the weight average molecular weight of the second fluoropolymer does not exceed 600,000.
The adhesive composition according to claim 1, wherein the mass content of the second fluoropolymer is 0.25% to 15% based on the total mass of the adhesive composition.
The adhesive composition according to claim 1 or 2, characterized in that the crystallinity of the adhesive composition is not higher than 45%, optionally 20%-45%.
The adhesive composition according to any one of claims 1 to 3, wherein the first fluoropolymer contains structural units derived from vinylidene fluoride.
The adhesive composition according to any one of claims 1 to 4, characterized in that the first fluoropolymer further contains a structural unit represented by formula I,

Among them, R 1 includes one or more of fluorine, chlorine, and a C 1-3 alkyl group containing at least one fluorine atom.
The adhesive composition according to any one of claims 1 to 5, wherein the first fluoropolymer has a polydispersity coefficient of 1.8 to 2.5.
The adhesive composition according to any one of claims 1 to 6, characterized in that the crystallinity of the first fluoropolymer is 30% to 46%, optionally 40% to 46%.
The adhesive composition according to any one of claims 1 to 7, characterized by The glue containing the first fluoropolymer with a mass content of 2%, prepared by dissolving the first fluoropolymer in N-methylpyrrolidone, has a viscosity of 2000MPa·s to 5000MPa·s.
The adhesive composition according to any one of claims 1 to 8, wherein the first fluoropolymer includes polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, Vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene - One or more hexafluoropropylene copolymers.
The adhesive composition according to any one of claims 1 to 9, wherein the second fluoropolymer contains a structural unit represented by formula II,

Wherein, R 2 and R 3 each independently include at least one of hydrogen, halogen, and C 1-3 alkyl group containing at least one fluorine atom.
The adhesive composition according to any one of claims 1 to 10, wherein R 2 and R 3 each independently include at least one of hydrogen, fluorine, chlorine, and trifluoromethyl.
The adhesive composition according to any one of claims 1 to 11, wherein the end group of the second fluoropolymer contains a hydroxyl group or an ester group.
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 million to 600.000.
The adhesive composition according to any one of claims 1 to 13, wherein the second fluoropolymer includes polytetrafluoroethylene, polyvinylidene fluoride, poly(vinylidene fluoride- Hexafluoropropylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, One of poly(vinylidene fluoride-chlorovinylidene fluoride) copolymer and poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene) copolymer.
A method for preparing an adhesive composition, characterized in that:

Preparing the first fluorine-containing polymer: Under polymerizable conditions, the raw material containing vinylidene fluoride monomer is subjected to a first polymerization reaction to prepare the first fluorine-containing polymer, the weight average molecular weight of the first fluorine-containing polymer is 5 million to 9 million;

Preparing the second fluorine-containing polymer: Under polymerizable conditions, the fluorine-containing monomer is subjected to a second polymerization reaction to prepare a second fluorine-containing polymer, and the weight average molecular weight of the second fluorine-containing polymer does not exceed 600,000;

Blending: The first fluoropolymer and the second fluoropolymer are blended to prepare an adhesive composition.
The preparation method of the adhesive composition according to claim 15, characterized in that the preparation method of the first fluoropolymer specifically includes:

Provide raw materials and reaction solvents containing vinylidene fluoride monomer, perform a first-stage polymerization reaction, and obtain a first product;

The first product is subjected to a second stage polymerization reaction in a water-insoluble gas atmosphere;

Add a chain transfer agent and perform the third stage polymerization reaction to obtain polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.
The preparation method according to claim 16, characterized in that the reaction temperature of the first stage polymerization reaction is 45°C to 60°C, the reaction time is 4h to 10h, and the initial polymerization pressure is 4MPa to 6MPa.
The preparation method according to claim 16 or 17, characterized in that the reaction temperature of the second stage polymerization reaction is 60°C to 80°C, the reaction time is 2h to 4h, and the reaction pressure is 6MPa to 8MPa.
The preparation method according to any one of claims 16 to 18, characterized in that The reaction time of the third stage polymerization reaction is 1h to 2h.
The preparation method according to any one of claims 15 to 19, characterized in that the preparation method of the second fluoropolymer specifically includes:

At least one monomer represented by formula III is subjected to a second polymerization reaction in a non-reactive gas atmosphere, 0.1-5MPa, and a reaction temperature of 60°C to 90°C for 0.5 to 8 hours, the reaction is stopped, and the solid and liquid are separated, leaving the solid phase to obtain the second fluoropolymer

Among them, R 4 and R 5 each independently include at least one of hydrogen, halogen, and a C 1-3 alkyl group containing at least one fluorine atom.
The preparation method according to any one of claims 15 to 20, characterized in that the second polymerization reaction further includes the following steps:

Add solvent and dispersant to the container, and fill the container with non-reactive gas;

Add the monomer represented by formula III, raise the temperature to 60°C to 90°C, then add the second initiator and chain transfer agent to perform the second polymerization reaction.
The preparation method according to claim 21, characterized in that the second initiator contains an inorganic peroxide, which can be selected from potassium persulfate or ammonium persulfate.
The preparation method according to claim 21 or 22, characterized in that, based on the total mass of the monomer represented by formula III, the mass content of the second initiator is 3%-12%.
The preparation method of the adhesive composition according to any one of claims 16 to 23, wherein the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol and acetone. .
A positive electrode sheet, including 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 includes a positive electrode active material, a conductive agent and the adhesive according to any one of claims 1 to 14. The adhesive composition and the adhesive 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 does not exceed 1%, based on the total mass of the positive electrode film layer.
The cathode plate according to claim 25 or 26, characterized in that the cathode active material is a lithium-containing transition metal oxide, which may be lithium iron phosphate or lithium nickel cobalt manganese oxide, or doping thereof. Modified materials, or at least one of their conductive carbon-coated modified materials, conductive metal-coated modified materials, or conductive polymer-coated modified materials.
A secondary battery, characterized in that it includes an electrode assembly and an electrolyte. The electrode assembly includes a separator, a negative electrode piece, and the positive electrode piece according to any one of claims 25 to 27.
An electrical device, characterized by comprising the secondary battery according to claim 28.