WO2024045553A1

WO2024045553A1 - Binder, preparation method, positive electrode sheet, secondary battery and electric device

Info

Publication number: WO2024045553A1
Application number: PCT/CN2023/081606
Authority: WO
Inventors: 段连威; 孙成栋; 刘会会
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-08-30
Filing date: 2023-03-15
Publication date: 2024-03-07
Also published as: CN115133033A; CN117638070A; CN115133033B

Abstract

Provided in the present application are a binder, a preparation method, a positive electrode sheet, a secondary battery and an electric device. The binder comprises first polyvinylidene fluoride and second polyvinylidene fluoride, wherein the weight-average molecular weight of the first polyvinylidene fluoride is 5,000,000-9,000,000, and the weight-average molecular weight of the second polyvinylidene fluoride is smaller than that of the first polyvinylidene fluoride. The binder enables an electrode sheet to have a high bonding force at a low addition amount, and can improve the cycle performance of a battery.

Description

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

cross reference

This application cites Chinese patent application No. 202211045483.8 titled "Binder, preparation method, positive electrode sheet, secondary battery and electrical device" submitted on August 30, 2022, which is fully incorporated by reference. Apply.

Technical field

The present application relates to the technical field of secondary batteries, and in particular to a binder, a preparation method, a positive electrode sheet, a secondary battery 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

This application was made in view of the above-mentioned problems, and its purpose is to provide a binder that can exert excellent bonding force at a low addition amount, so that the pole piece has sufficient bonding strength, and Can improve battery cycle performance.

In order to achieve the above purpose, this application provides a binder. The binder includes a first polyvinylidene fluoride and a second polyvinylidene fluoride. The weight average molecular weight of the first polyvinylidene fluoride is 5 million to 900. W, the weight average molecular weight of the second polyvinylidene fluoride is smaller than the weight average molecular weight of the first polyvinylidene fluoride.

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 polydispersity coefficient of the first polyvinylidene fluoride is 1.8-2.5.

The polydispersity coefficient of the first polyvinylidene fluoride is within an appropriate range, the weight average molecular weight distribution of the first polyvinylidene fluoride is uniform, the performance variance is small, and the stability is high, which can ensure that the first polyvinylidene fluoride and the third polyvinylidene fluoride are included. The binder of dipolyvinylidene fluoride enables the pole piece to have sufficient adhesive force at a low addition amount, further improving the capacity retention rate of the battery during cycling.

In any embodiment, the Dv50 particle size of the first polyvinylidene fluoride is 100 μm to 200 μm.

The Dv50 particle size of the first polyvinylidene fluoride is controlled to be within an appropriate range, and the first polyvinylidene fluoride has good processing properties, so that the first polyvinylidene fluoride and the second polyvinylidene fluoride are bonded together. The agent is easy to process and can ensure the production efficiency of pole pieces and batteries.

In any embodiment, the first polyvinylidene fluoride has a crystallinity of 40% to 45%.

The crystallinity of the first polyvinylidene fluoride is controlled within an appropriate range, so that the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can meet the pole piece bonding force and Based on the battery cycle performance, it will not have an excessive impact on the flexibility of the pole pieces and can meet the use needs of the pole pieces.

In any embodiment, the viscosity of the glue obtained by dissolving the first polyvinylidene fluoride in N-methylpyrrolidone is 2000 mPa·s to 5000 mPa·s, and the mass content of the first polyvinylidene fluoride is 2%. , based on the total mass of glue.

Controlling the viscosity of the first polyvinylidene fluoride glue within an appropriate range and adding a low amount of a binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can ensure that the pole piece has excellent Adhesion.

In any embodiment, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1˜4:1.

Controlling the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride within an appropriate range allows the pole piece to have both good processing performance and adhesion, and the capacity retention rate of the battery during cycling can be further improved. . In addition, the mass ratio of the first polyvinylidene fluoride and the second polyvinylidene fluoride is controlled within an appropriate range, so that the pole piece has sufficient adhesion, Reduce the usage of the first polyvinylidene fluoride, save the cost of binder, and facilitate industrial production.

In any embodiment, the second polyvinylidene fluoride has a weight average molecular weight of 600,000 to 1.1 million.

Controlling the weight average molecular weight of the second polyvinylidene fluoride within an appropriate range and adding a low amount of a binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride can ensure that the pole piece has excellent adhesion. The capacity retention rate of the battery during cycling can be further improved.

A second aspect of the application also provides a method for preparing an adhesive, including the following steps:

Preparing the first polyvinylidene fluoride: providing vinylidene fluoride monomer and solvent, performing a first-stage polymerization reaction to obtain a first product; performing a second-stage polymerization reaction on the first product in a water-insoluble gas atmosphere; adding Chain transfer agent is used to perform the third stage polymerization reaction to obtain the first polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million; blending: co-exist the first polyvinylidene fluoride and the second polyvinylidene fluoride. The adhesive is prepared by mixing, wherein the weight average molecular weight of the second polyvinylidene fluoride is smaller than that of the first polyvinylidene fluoride.

The preparation method of the binder can prepare the first polyvinylidene fluoride with ultra-high molecular weight through segmented polymerization, so that the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can be produced at low The addition amount can meet the demand for the adhesive force of the electrode piece, which helps to increase the loading capacity of the positive active material in the electrode piece and improves the capacity retention rate of the battery during the cycle. In addition, the binder is prepared by blending the ultra-high molecular weight first polyvinylidene fluoride with the relatively low molecular weight second polyvinylidene fluoride, thereby reducing the use of the ultra-high molecular weight first polyvinylidene fluoride. quantity, reducing the cost of the binder, which is conducive to industrial production.

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

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

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

Control the reaction pressure and reaction at each stage of the polymerization reaction in preparing the first polyvinylidene fluoride. When the time and reaction temperature are within a suitable range, while increasing the weight average molecular weight of the first polyvinylidene fluoride, the uniformity of the weight average molecular weight of the first polyvinylidene fluoride can be ensured, ensuring that the product has a lower The polydispersity coefficient improves the uniformity and stability of the performance of the first polyvinylidene fluoride, thereby ensuring consistent performance within and between batches of the binder containing the first polyvinylidene fluoride and the second 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 any embodiment, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

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

In any embodiment, the amount of chain transfer agent used is 1.5% to 3% of the total mass of vinylidene fluoride monomer.

In any embodiment, the first stage polymerization reaction includes the following steps:

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

Add 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.

In any embodiment, the amount of solvent used is 2 to 8 times the total mass of vinylidene fluoride monomer.

In any embodiment, the dispersant includes one or more of cellulose ethers and polyvinyl alcohol.

In any embodiment, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

In any embodiment, the amount of dispersant is 0.1% to 0.3% of the total mass of vinylidene fluoride monomer.

In any embodiment, the initiator is an organic peroxide.

In any embodiment, the organic peroxide includes tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethylperoxydicarbonate, diisopropylperoxydicarbonate One or more of acid esters and tert-butyl peroxypivalate.

In any embodiment, the amount of initiator used is 0.15% to 1% of the total mass of vinylidene fluoride monomer.

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

In any embodiment, the amount of pH adjuster is 0.05% to 0.2% of the total mass of vinylidene fluoride monomer.

In any embodiment, in the blending step, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1˜4:1.

Controlling the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride within an appropriate range can allow the pole piece to have excellent adhesion, and the capacity retention rate of the battery during cycling can be further improved. In addition, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is controlled within an appropriate range, so that the usage amount of the first polyvinylidene fluoride can be reduced when the pole piece has sufficient adhesive force. , save the cost of binder and facilitate industrial production.

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. Or the adhesive prepared by the preparation method in any embodiment.

In any embodiment, the mass fraction of the binder is 0.6% to 0.8%, based on the total mass of the positive electrode film layer.

Controlling the mass fraction of the binder within an appropriate range can help improve the capacity retention rate of the battery during cycling and enable the battery to have high cathode energy density.

In a fourth aspect of the present application, a secondary battery is provided, including an electrode assembly and an electrolyte. The electrode assembly includes the positive electrode sheet, isolation film and negative electrode sheet of the third aspect of the present application. Optionally, the secondary battery is a lithium-ion battery or a sodium-ion battery.

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

Description of drawings

Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

Figure 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in Figure 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;

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

Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application;

Figure 7 is a bonding force-displacement diagram of Example 24 and Comparative Example 2;

Figure 8 is a graph showing the battery capacity retention rate and the number of cycles in Example 24 and Comparative Example 2.

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. Additionally, 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 The ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise stated, the numerical range "ab" 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, mentioning that the method may also include step (c) means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c). , may also include steps (a), (c) and (b), may also 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).

Polyvinylidene fluoride is currently one of the most widely used binder types in secondary batteries. However, the viscosity of traditional polyvinylidene fluoride 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 polyvinylidene fluoride 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]

This application provides a binder. The binder includes a first polyvinylidene fluoride and a second polyvinylidene fluoride. The first polyvinylidene fluoride has a weight average molecular weight of 5 million to 9 million, and the second polyvinylidene fluoride has a weight average molecular weight of 5 million to 9 million. The weight average molecular weight of the polyvinylidene fluoride is less than the weight average molecular weight of the first polyvinylidene fluoride.

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

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

In some embodiments, binders are used to hold electrode active materials and/or conductive agents in place and adhere them to conductive metal components to form electrodes.

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

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

In this article, the term "polyvinylidene fluoride" refers to polymers with vinylidene fluoride as the main synthetic monomer. The polymers include, on the one hand, chemically homogeneous polymers prepared by polymerization reactions, but with varying degrees of polymerization and 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. Polyvinylidene fluoride herein includes 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 structural formula of the first polyvinylidene fluoride is as shown in Formula I, and the structural formula of the second polyvinylidene fluoride is as shown in Formula II,

Among them, m and n are integers, respectively representing the degree of polymerization of the first polyvinylidene fluoride and the second polyvinylidene fluoride. m is greater than n, that is, the degree of polymerization and the weight average molecular weight of the first polyvinylidene fluoride are greater than The degree of polymerization and weight average molecular weight of the second polyvinylidene fluoride.

In some embodiments, the first polyvinylidene fluoride includes polyvinylidene fluoride homopolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride -Hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoroethylene One or more of propylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer.

In some embodiments, the second polyvinylidene fluoride includes polyvinylidene fluoride homopolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene copolymer , Vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer one or more of the things.

In some embodiments, the first polyvinylidene fluoride has a weight average molecular weight of 5 million to 9 million. In some embodiments, the upper limit or lower limit of the weight average molecular weight of the first polyvinylidene fluoride is selected from the group consisting of 5.1 million, 5.5 million, 6 million, 6.5 million, 7 million, 7.5 million, 8 million, 8.5 million, and 9 million. anyone.

The fluorine element contained in the first polyvinylidene fluoride and the second polyvinylidene fluoride 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, so that the pole piece has good adhesion. The first polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million has great cohesion and intermolecular force, which can improve the adhesion of the pole pieces at low levels of addition and improve the performance of the battery during the cycle. capacity retention rate. The addition of the second polyvinylidene fluoride to the binder can greatly reduce the cost of the binder. At the same time, since the first polyvinylidene fluoride and the second polyvinylidene fluoride have the same structural units and excellent compatibility, they can During the drying process of preparing pole pieces, the pole pieces will not delaminate, and high-quality pole pieces can be obtained.

The above-mentioned binder can ensure sufficient adhesion 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 this application, the weight average molecular weight of the first polyvinylidene fluoride can be measured 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, 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 a 3.0% binder 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.

In some embodiments, the first polyvinylidene fluoride has a polydispersity coefficient of 1.8 to 2.5. In some embodiments, the polydispersity coefficient of the first polyvinylidene fluoride can be any one of 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, and 2.5.

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 polyvinylidene fluoride is too large, the degree of polymerization of the first polyvinylidene fluoride is relatively dispersed, thereby affecting the bonding between the first polyvinylidene fluoride and the second polyvinylidene fluoride. Due to the uniformity of the agent, the binder cannot evenly adhere the positive active material to the current collector, which affects the cycle performance of the battery. It also reduces the solid content of the slurry and cannot further improve the energy density of the battery; if the first polymerization bias The polydispersity coefficient of vinyl difluoride is too small, the preparation process is difficult, and the yield rate is low, resulting in high production costs.

The polydispersity coefficient of the first polyvinylidene fluoride is within an appropriate range, the weight average molecular weight distribution of the first polyvinylidene fluoride is uniform, and the performance is uniform, which can ensure that the first polyvinylidene fluoride and the second polyvinylidene fluoride are included. The ethylene binder enables the pole pieces to have sufficient adhesive force at a low addition amount, further improving the capacity retention rate of the battery during cycling. In addition, poly-partial two Vinyl fluoride has a suitable polydispersity coefficient, which can effectively increase the solid content of the slurry and reduce production costs.

In this application, the polydispersity coefficient 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). 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 molecular weight b respectively, and the polydispersity coefficient = a/b.

In some embodiments, the Dv50 particle size of the first polyvinylidene fluoride is 100 μm to 200 μm. In some ways, the Dv50 particle size of the first polyvinylidene fluoride can be selected from any one of 120 μm to 200 μm, 120 μm to 160 μm, 120 μm to 180 μm, and 160 μm to 200 μm.

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

If the Dv50 particle size of the first polyvinylidene fluoride is too large, it will be relatively difficult for the first polyvinylidene fluoride to dissolve, thereby reducing the dispersibility of the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride. , affecting the uniform distribution of the positive active material on the current collector, affecting the cycle performance of the battery. At the same time, it is difficult to dissolve the first polyvinylidene fluoride, which reduces the speed of the pulping process; if the Dv50 particle size of the first polyvinylidene fluoride If it is too small, the adhesive force of the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride will decrease, causing the adhesive force of the pole piece to decrease.

Controlling the Dv50 particle size of the first polyvinylidene fluoride within an appropriate range allows the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride to have good processing properties and ensure the stability of the pole piece and the battery. Productivity. At the same time, the Dv50 particle size of the first polyvinylidene fluoride in a suitable range can also make the first polyvinylidene fluoride and the second polyvinylidene fluoride The amount of binder can be controlled at a low level without excessive negative impact on the bonding performance, thus effectively improving the performance limitations of the pole piece and battery caused by the high amount of binder in traditional technology. .

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 polyvinylidene fluoride powder, then weigh 5g of absolute ethanol, and add it to the first polyvinylidene fluoride container. In a beaker of vinylidene fluoride powder, put a stirrer with a length of about 2.5mm and seal it with plastic wrap. Put the sample into an ultrasonic machine for 5 minutes, transfer to a magnetic stirrer and stir at a speed of 500 rpm for more than 20 minutes. Take 2 samples from each batch of products and test them and take the average. 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.

In some embodiments, the first polyvinylidene fluoride has a crystallinity of 40% to 45%. In some embodiments, the crystallinity of the first polyvinylidene fluoride may be any one of 41%, 42%, 43%, 44%, or 45%.

If the crystallization of the first polyvinylidene fluoride is too small, the degree of regular and dense packing of the molecular chains of the first polyvinylidene fluoride is reduced, which affects the chemical stability and thermal stability of the first polyvinylidene fluoride, thereby affecting the Chemical and thermal stability of a binder comprising first polyvinylidene fluoride and second polyvinylidene fluoride. However, if the crystallinity of the first polyvinylidene fluoride is too large, the crystallinity of the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride will be increased, causing the flexibility of the pole piece to decrease, and at the same time First polyvinylidene fluoride has difficulty dissolving, reducing the speed of the pulping process.

The crystallinity of the first polyvinylidene fluoride of the present application is within a suitable range, and the binder can satisfy the adhesive force of the electrode piece and battery cycle performance at a low addition amount without affecting the flexibility of the electrode piece. Excessive impact.

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 first polyvinylidene fluoride is placed in an aluminum crucible, shaken flat, and the crucible lid is covered. Under a nitrogen atmosphere, a purge gas of 50 ml/min is used, and a purge gas of 70 ml/min is used. minutes of protective gas, a heating rate of 10°C per minute, a test temperature range of -100°C to 400°C, and a differential scanning calorimeter (DSC) of the American TA Instruments model Discovery 250 for testing and elimination of thermal history.

This test will obtain the DSC curve of the first polyvinylidene fluoride. After integrating the curve, the peak area is the melting enthalpy ΔH (J/g) of the polymer. The crystallinity of the first polyvinylidene fluoride = ΔH/ (ΔHm)×100%, where ΔHm is the standard melting enthalpy (crystalline fusion heat) of polyvinylidene fluoride, ΔHm=104.7J/g.

In some embodiments, the viscosity of the glue prepared by dissolving the first polyvinylidene fluoride in N-methylpyrrolidone is 2000 mPa·s to 5000 mPa·s, and the mass content of the first polyvinylidene fluoride is 2%. , based on the total mass of glue. In some embodiments, the viscosity of the glue prepared by dissolving the first polyvinylidene fluoride in N-methylpyrrolidone can be selected from 2100mPa·s～2700mPa·s, 2700mPa·s～3400mPa·s, 3400mPa·s～ Any one of 3800mPa·s, 3800mPa·s～4300mPa·s, 4300mPa·s～4800mPa·s, 2100mPa·s～3400mPa·s, 2100mPa·s～4800mPa·s, 3400mPa·s～4800mPa·s.

If the viscosity of the first polyvinylidene fluoride glue is too high, the viscosity of the prepared binder solution including the first polyvinylidene fluoride and the second polyvinylidene fluoride will be too high, making it difficult to stir and reducing the viscosity. The dispersion of the binder makes it difficult for the binder to evenly adhere the positive active material to the current collector, affecting the cycle performance of the battery. At the same time, if the preparation contains the first polyvinylidene fluoride and the second polyvinylidene fluoride The viscosity of the binder solution is too large, which reduces the speed of the pulping process; if the viscosity of the first polyvinylidene fluoride glue is too small, the prepared solution containing the first polyvinylidene fluoride and the second polyvinylidene fluoride The viscosity of the ethylene binder solution will be too small, and it will be difficult for the pole piece to have sufficient bonding force at low addition amounts.

In addition, when preparing the positive electrode slurry, the binder solution needs to have a certain viscosity to prevent the positive electrode active material and conductive agent from settling and enable the slurry to be placed more stably. In traditional technology, to achieve a glue viscosity of 2500 mPa·s to 5000 mPa·s, at least the mass fraction of the binder in the glue must reach 7%. However, the first polyvinylidene fluoride in this application has a viscosity of 2%. The expected viscosity of the glue can be achieved at the dosage, which provides a basis for reducing the content of the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride in the positive electrode film layer.

In this application, the viscosity of the binder solution can be tested using methods known in the art, such as the rotational viscometer test method. As an example, use a 500ml beaker to weigh 7g of the first polyvinylidene fluoride and 343g of N-methylpyrrolidone (NMP) respectively, configure it into a glue solution with a mass fraction of 2%, and use a Lichen high-speed grinder to stir and disperse at a speed of 800 rpm. /min, stir for 120 minutes and then ultrasonic vibrate 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 12 rpm, 6 Read the viscosity data after a few minutes.

In some embodiments, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1˜4:1. In some embodiments, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride can be any one of 1:1, 2:1, 3:1, and 4:1.

If the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is too large, that is, the quality of the first polyvinylidene fluoride is too high and the purpose of cost reduction cannot be achieved; if the first polyvinylidene fluoride The mass ratio to the second polyvinylidene fluoride is too small, that is, the mass of the first polyvinylidene fluoride is too low, which causes the adhesive force of the pole piece to decrease and affects the cycle performance of the battery.

Controlling the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride within an appropriate range, the binder can be added in a low amount so that the pole piece has excellent bonding force, which can improve the performance of the battery during the cycle. Capacity retention rate. In addition, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is controlled within an appropriate range, so that the pole piece can reduce the usage amount of the first polyvinylidene fluoride while having sufficient adhesive force. Save the cost of binder and facilitate industrial production.

In some embodiments, the second polyvinylidene fluoride has a weight average molecular weight of 600,000 to 1.1 million. In some embodiments, the weight average molecular weight of the second polyvinylidene fluoride can be any one of 600,000, 700,000, 800,000, 900,000, 1 million, and 1.1 million.

If the weight average molecular weight of the second polyvinylidene fluoride is too large, the purpose of cost reduction will not be achieved; if the weight average molecular weight of the second polyvinylidene fluoride is too small, the first polyvinylidene fluoride and the second polyvinylidene fluoride will not be included. The adhesive force of the polyvinylidene fluoride binder decreases, which in turn causes the adhesive force of the pole piece to decrease.

Control the weight average molecular weight of the second polyvinylidene fluoride within an appropriate range, and add a low amount of The binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can ensure that the pole piece has excellent bonding force, and the capacity retention rate of the battery during cycling can be further improved.

In one embodiment of the present application, a method for preparing a binder is provided, including the following steps: preparing a first polyvinylidene fluoride: providing a vinylidene fluoride monomer and a solvent, and performing a first-stage polymerization reaction to obtain a first-stage polyvinylidene fluoride monomer. One product; the first product is subjected to a second-stage polymerization reaction in a water-insoluble gas atmosphere; a chain transfer agent is added to perform a third-stage polymerization reaction to obtain a first polyylidene fluoride with a weight average molecular weight of 5 million to 9 million Ethylene; blending: the first polyvinylidene fluoride and the second polyvinylidene fluoride are blended to prepare a binder, wherein the weight average molecular weight of the second polyvinylidene fluoride is smaller than the first polyvinylidene fluoride.

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.

It can be understood that the first product may refer to the reaction liquid obtained after the first-stage polymerization reaction of vinylidene fluoride monomer and solvent, or may refer to the polymer obtained 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. 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.

The preparation method of the binder can prepare the first polyvinylidene fluoride with ultra-high molecular weight through segmented polymerization, so that the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can be produced at low The addition amount can meet the demand for the adhesive force of the electrode piece, which helps to increase the loading capacity of the positive active material in the electrode piece and improves the capacity retention rate of the battery during the cycle. At the same time, the first product is formed in the first-stage polymerization reaction, the second-stage polymerization reaction forms the molecular chain segment with the target molecular weight, and the third-stage polymerization reaction is used to control the molecular weight of the first polyvinylidene fluoride to avoid excessive reduction in molecular weight. The uniformity of the weight average molecular weight of the first polyvinylidene fluoride improves the uniformity of the product. Moreover, staged polymerization can improve the utilization rate of the reactor during the preparation process of the first polyvinylidene fluoride, save time, and reduce the residence time of the first 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 the first polyvinylidene fluoride.

In addition, the binder is prepared by blending the ultra-high molecular weight first polyvinylidene fluoride with the relatively low molecular weight second polyvinylidene fluoride, thereby reducing the use of the ultra-high molecular weight first polyvinylidene fluoride. quantity, reducing the cost of the binder, which is conducive to industrial production.

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 any one of 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, and 45°C to 55°C.

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

In some embodiments, the initial pressure of the first stage polymerization reaction is 4MPa˜6MPa. In some embodiments, the initial pressure of the first stage polymerization reaction is 4MPa˜5MPa or 5MPa˜6MPa. In some embodiments, the initial pressure of the first stage polymerization 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 is 60°C to 70°C or 70°C to 80°C.

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

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 is 6MPa˜7MPa or 7MPa˜8MPa.

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

By controlling the reaction pressure, reaction time, and reaction temperature of the polymerization reaction at each stage within a suitable range, the weight average molecular weight of the first polyvinylidene fluoride can be controlled while increasing the weight average molecular weight of the first polyvinylidene fluoride. Uniformity, ensuring that the product has a lower polydispersity coefficient, improving the consistency of the performance of the first polyvinylidene fluoride, and thereby ensuring the performance of the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride. stability, making the pole pieces It has excellent adhesion with a low addition amount of binder, 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 any one of nitrogen, oxygen, hydrogen, and methane.

In some embodiments, 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 used may also be, for example, 2% or 2.5% of the total mass of vinylidene fluoride monomer.

Controlling the dosage of the chain transfer agent within a suitable range can control the polymer chain length, thereby obtaining the first polyvinylidene fluoride with a suitable molecular weight range and uniform distribution.

In some embodiments, the first-stage polymerization reaction includes the following steps: adding a solvent and a dispersant to the container to remove oxygen from the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH value to 6.5-7, Then add vinylidene fluoride monomer to bring the pressure in the container to 4MPa to 6MPa; stir for 30 to 60 minutes, then raise the temperature to 45°C to 60°C to perform the first stage of polymerization.

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

In some embodiments, the amount of 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 solvent is deionized water.

In some embodiments, the dispersant includes one or more of cellulose ethers and polyvinyl alcohol.

In some embodiments, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

In some embodiments, the amount of dispersant is 0.5% of the total mass of vinylidene fluoride monomer. 0.1%～0.3%. The amount of dispersant used may also be, for example, 0.2% of the total mass of vinylidene fluoride monomer.

In some embodiments, the initiator is an organic peroxide.

In some embodiments, organic peroxides include t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethylperoxydicarbonate, diisopropylperoxydicarbonate, and One or more types of tert-butyl peroxypivalate.

In some embodiments, the amount of initiator is 0.15% to 1% of the total mass of vinylidene fluoride monomer. The amount of initiator used may also be, for example, 0.2%, 0.4%, 0.6% or 0.8% of the total mass of 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 any embodiment, the amount of pH adjuster is 0.05% to 0.2% of the total mass of vinylidene fluoride monomer. The amount of pH adjuster used may also be, for example, 0.1% or 0.15% of the total mass of vinylidene fluoride monomer.

In some embodiments, in the blending step, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1 to 4:1. In some embodiments, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride can be any one of 1:1, 2:1, 3:1, and 4:1.

Controlling the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride within an appropriate range allows the pole piece to have excellent adhesion, and the capacity retention rate of the battery during cycling can be further improved. In addition, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is controlled within an appropriate range, so that the pole piece can reduce the usage amount of the first polyvinylidene fluoride while having sufficient adhesive force. Save the cost of binder and facilitate industrial production.

[Positive pole piece]

The positive electrode sheet includes a positive electrode current collector and is 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 some embodiments or a binder prepared by a preparation method in some embodiments.

The positive electrode sheet has excellent bonding force with a low additive amount of binder.

In some embodiments, the mass fraction of the binder is 0.6% to 0.8%, based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of the binder in the total mass of the positive electrode film layer is 0.6% to 0.7% or 0.7% to 0.8%.

If the mass fraction of the binder is too high, too much binder will cause the loading capacity of the positive active material in the pole piece to decrease, resulting in a reduction in the energy density of the battery and a reduction in battery capacity.

If the mass fraction of the binder is too low, sufficient bonding effect cannot be achieved. On the one hand, a sufficient amount of conductive agent and positive active material cannot be bonded together, and the bonding force of the electrode piece is small; on the other hand, the bonding force The agent cannot be tightly combined with the surface of the active material, causing the surface of the electrode piece to easily come off, resulting in a decrease in the cycle performance of the battery.

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

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

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, this application is not limited to these materials, and other materials that can be used as electrical Traditional materials for battery cathode active materials. 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 ), lithium manganese phosphate and carbon. At least one of composite materials, lithium iron manganese phosphate, and composite materials of lithium iron manganese 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. composite set The fluid 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 base material (such as polypropylene (PP), polyethylene terephthalate It is formed on base materials such as alcohol 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. 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.

[electrolytes]

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, ethylpropyl carbonate, Butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate At least one of ester, 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 polyvinylidene fluoride. 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 . 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 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, and 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.

1. Preparation method

Example 1

1) Preparation of adhesive

Preparation of the first polyvinylidene fluoride: In the first stage of polymerization reaction, add 4 kg of deionized water and 2 g of methylcellulose ether to the 10L autoclave No. 1 and No. 2, evacuate and replace O ₂ with N ₂ three times, Add 5g of tert-butyl peroxypivalate 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 4h; second stage For 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 Stir and react for 3 hours; in the third stage of polymerization reaction, add 40g of cyclohexane and continue the reaction for 1 hour to stop the reaction. The polymer is centrifuged, washed, and dried to obtain the first polyvinylidene fluoride.

The second polyvinylidene fluoride: purchased from Shandong Deyi New Materials Co., Ltd., model DY-5, weight average molecular weight 800,000, polydispersity coefficient 1.85, Dv50 15μm, crystallinity 40%, dissolved in N -The viscosity of the glue with a mass fraction of 7% after methylpyrrolidone is 2300 mpa·s.

The first polyvinylidene fluoride and the second polyvinylidene fluoride are blended, and the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1 to obtain the first polyvinylidene fluoride. Binder for ethylene and a second polyvinylidene fluoride.

2) Preparation of positive electrode pieces

Put 3961.8g lithium iron phosphate, 24.6g binder, and 57.4g acetylene black in a planetary stirring tank at a revolution speed of 25 r/min and stir for 25 minutes. The mass fraction of the binder is 0.6%, based on the positive electrode film layer. total mass;

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 dispersant into the mixing tank and stir for 60 minutes at a revolution speed of 25r/min and a rotation speed of 1250r/min;

After stirring is completed, test the viscosity of the slurry and control the viscosity at 8000 to 15000 mPa·s.

If the viscosity is high, add NMP solution to reduce it to the above viscosity range, then stir for 30 minutes at a revolution speed of 25 r/min and a rotation speed of 1250 r/min to obtain the positive electrode slurry. The prepared positive electrode slurry is spread on 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) at a volume ratio of 3/7, and add Dissolve the LiPF 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.

Examples 2-3

Basically the same as Example 1, the difference is that the reaction time in the first stage polymerization reaction of the first polyvinylidene fluoride is adjusted to 6h and 8h respectively, and the cyclohexane in the third stage polymerization reaction is adjusted to 30g, 20g, the specific parameters are shown in Table 1.

Examples 4 to 7

It is basically the same as Example 1, except that the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride during the blending process is adjusted. The specific parameters are as shown in Table 1.

Examples 8 to 11

It is basically the same as Example 1, except that the mass fraction of the binder is adjusted, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

Example 12

It is basically the same as Example 1, except that the second polyvinylidene fluoride is 605 purchased from Huaan Company, with a weight average molecular weight of 600,000, a polydispersity coefficient of 2.05, a Dv50 of 13.4 μm, a crystallinity of 42%, and a dissolved The viscosity of the glue formulated with a mass fraction of 7% after N-methylpyrrolidone is 3000 mPa·s, and the specific parameters are shown in Table 1.

Example 13

It is basically the same as Example 1, except that the second polyvinylidene fluoride is 202E purchased from Shenzhou Company, with a weight average molecular weight of 1.1 million, a polydispersity coefficient of 2.0, a Dv50 of 11.5 μm, a crystallinity of 42%, and a dissolved The viscosity of the glue formulated with a mass fraction of 7% after N-methylpyrrolidone is 4100 mPa·s.

Examples 14 to 16

Basically the same as Example 1, the difference is that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.7%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Examples 17-19

Basically the same as Example 1, the difference is that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.8%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Examples 20 to 22

Basically the same as Example 2, except that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.6%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Example 23

It is basically the same as Example 2, except that the mass fraction of the binder is adjusted to 0.7%, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

Examples 24 to 26

Basically the same as Example 2, except that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.7%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Example 27

It is basically the same as Example 2, except that the mass fraction of the binder is adjusted to 0.8%, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

Examples 28-30

Basically the same as Example 2, except that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.8%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Examples 31 to 33

It is basically the same as Example 3, except that the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride during the blending process is adjusted. The specific parameters are as shown in Table 1.

Example 34

It is basically the same as Example 3, except that the mass fraction of the binder is adjusted to 0.7%, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

Examples 35 to 37

Basically the same as Example 3, the difference is that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.7%, based on the positive electrode The total mass of the film layer is measured, and the specific parameters are shown in Table 1.

Example 38

It is basically the same as Example 3. The difference is that during the blending process, the mass fraction of the binder is adjusted to 0.8%, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

Examples 39-41

Basically the same as Example 3, the difference is that during the blending process, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride was adjusted, and the mass fraction of the binder was adjusted to 0.8%, based on the positive electrode film The total mass of the layer is measured, and the specific parameters are shown in Table 1.

Example 42

Basically the same as Example 2, the difference is that the polymerized monomers are adjusted to 0.94kg of vinylidene fluoride and 0.06kg of chlorotrifluoroethylene to prepare a vinylidene fluoride-chlorotrifluoroethylene copolymer. The specific parameters are as shown in the table 1 shown.

Example 43

Basically the same as Example 2, the difference is that the polymerized monomers are adjusted to 0.94kg of vinylidene fluoride and 0.06kg of tetrafluoroethylene to prepare a vinylidene fluoride-tetrafluoroethylene copolymer. The specific parameters are as shown in Table 1 Show.

Example 44

It is basically the same as Example 2, except that the polymerized monomers are adjusted to 0.94kg of vinylidene fluoride and 0.06kg of hexafluoropropylene to prepare a vinylidene fluoride-hexafluoropropylene copolymer. The specific parameters are as shown in Table 1. Show.

Example 45

It is basically the same as Example 2, except that the second polyvinylidene fluoride is replaced with a vinylidene fluoride-chlorotrifluoroethylene copolymer with a weight average molecular weight of 800,000, purchased from Huaxia Shenzhou New Materials Co., Ltd., model number 202D, the specific parameters are shown in Table 1.

Comparative example 1

Basically the same as Example 1, the binder only contains the second polyvinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 2

Basically the same as Comparative Example 1, the mass fraction of the binder was adjusted to 2.5%, based on the total mass of the positive electrode film layer. The specific parameters are as shown in Table 1.

2. Performance test

1. Binder property testing

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 a 3.0% binder 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 a 3.0% binder 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 polyvinylidene fluoride powder, then weigh 5g of absolute ethanol, and add it to the first polyvinylidene fluoride container. In a beaker containing vinylidene fluoride powder, put a stirrer with a length of about 2.5 mm. Mix and seal 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 from Malvern Instruments Co., Ltd. in the United Kingdom.

4) Crystallinity test

Place 0.5g of the first polyvinylidene fluoride in an aluminum crucible, 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 at 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 curve of the first polyvinylidene fluoride, and integrate the curve. The peak area is the melting enthalpy ΔH (J/g) of the first polyvinylidene fluoride. The crystallinity = (ΔH/ΔHm) × 100%, where ΔHm is the standard melting enthalpy (crystalline fusion heat) of polyvinylidene fluoride, ΔHm = 104.7J/g.

5) Glue viscosity test

Use a 500ml beaker to weigh 7g of the first polyvinylidene fluoride and 343g of N-methylpyrrolidone (NMP) respectively, and prepare a glue solution with a mass fraction of 2%. Use a Lichen high-speed grinder to stir and disperse, rotating at 800r/min. 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. Pole piece performance test

1) Adhesion test

Referring to GB-T 2790-1995 National Standard "Test Method for 180° Peel Strength of Adhesives", the bonding force testing process of the examples and comparative examples of this application 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 the paper tape with the same width as the pole piece and a length of 250mm on the pole piece current collector on the body and fixed 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 values. 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, and obtain Example 24 as shown in Figure 7 and the bond force-displacement plot of Comparative Example 2.

3. Battery performance test

1)Battery capacity retention test

The battery capacity retention rate test process is as follows: At 25°C, charge the button battery to 3.65V at a constant current of 1/3C, then charge to a constant voltage of 3.65V until the current is 0.05C, leave it aside for 5 minutes, and then discharge at 1/3C to 2.5V, and 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 graph of the battery capacity retention rate and the number of cycles of Example 24 and Comparative Example 2 is obtained as shown in FIG. 8 .

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 45 or Comparative Examples 1 to 2 in Table 1 is the data measured after 500 cycles under the above test conditions, that is, the value of P500.

The performance test results of the binders, pole pieces and batteries obtained in the above-mentioned Examples 1 to 45 and Comparative Examples 1 to 2 are shown in Table 1.

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 below.

Table 1 Parameters and performance tests of Examples 1 to 45 and Comparative Examples 1 to 2

Figure 7 is a bonding force-displacement diagram between Example 24 and Comparative Example 2. It can be seen from the figure that at the same displacement, the bonding force of Example 24 is significantly higher than that of Comparative Example 2, indicating that When the amount of binder added is low, the binder provided by the present application including the first polyvinylidene fluoride and the second polyvinylidene fluoride enables the pole piece to have excellent bonding force. Figure 8 is a graph showing the battery capacity retention rate and the number of cycles of Example 24 and Comparative Example 2. It can be seen from the figure that after the battery is cycled 500 times, the cycle capacity retention rate of Example 24 is significantly higher than that of Comparative Example 2. , indicating that the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride provided by the present application can improve the cycle capacity retention rate of the battery during cycling when the amount of binder added is low. , effectively improving the performance limitations of pole pieces and batteries caused by high amounts of binders in traditional technologies.

According to the above results, it can be seen that the binders in Examples 1 to 45 all include a first polyvinylidene fluoride and a second polyvinylidene fluoride, and the weight average molecular weight of the first polyvinylidene fluoride is 5 million to 9 million. , the weight average molecular weight of the second polyvinylidene fluoride is smaller than the weight average molecular weight of the first polyvinylidene fluoride.

From the comparison between Examples 1 to 7, Examples 12 to 13, Examples 20 to 22, Examples 31 to 33, and Examples 42 to 45 and Comparative Example 1, it can be seen that the first polyvinylidene fluoride and the second polyvinylidene fluoride are The binder of vinylidene fluoride can give the pole piece excellent adhesion at a low addition amount and improve the capacity retention rate of the battery during cycling.

From the comparison between Examples 1 to 45 and Comparative Example 2, it can be seen that when the amount of binder added is low, the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride makes the pole piece have The excellent bonding force improves the capacity retention rate of the battery during cycling, effectively improving the performance limitations of the pole piece and battery caused by the high amount of binder in traditional technology.

It can be seen from Examples 1 to 45 that the polydispersity coefficient of the first polyvinylidene fluoride in the binder is 1.8 to 2.5, and the low addition amount of the first polyvinylidene fluoride and the second polyvinylidene fluoride is The binder can make the pole piece have excellent adhesion, and the battery has a high capacity retention rate during cycling.

It is known from Examples 1 to 45 that the Dv50 particle size of the first polyvinylidene fluoride in the binder is 100 μm to 200 μm, and the low addition amount includes the first polyvinylidene fluoride and the second polyvinylidene fluoride. The binder can make the pole piece have excellent bonding force, and the battery has a high capacity retention rate during cycling.

It is known from Examples 1 to 45 that the crystallinity of the first polyvinylidene fluoride in the binder is 40% to 45%, and the low addition amount includes the first polyvinylidene fluoride and the second polyvinylidene fluoride. The ethylene binder can make the pole piece have excellent adhesion, and the battery has a high capacity retention rate during cycling.

It is known from Examples 1 to 45 that the viscosity of the first polyvinylidene fluoride glue with a mass content of 2% prepared by dissolving the first polyvinylidene fluoride in N-methylpyrrolidone is 2000mPa·s～5000mPa. ·s, so that the binder containing the first polyvinylidene fluoride and the second polyvinylidene fluoride can ensure sufficient bonding force of the pole piece at a low addition amount.

From the comparison of Example 1, Examples 5 to 7 and Example 4, it can be seen that when the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride in the binder is 1:1 to 4:1, the low The added amount of the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride enables the pole piece to have excellent adhesive force, and the capacity retention rate of the battery during cycling can be further improved.

It is known from Example 1 and Examples 12 to 13 that the weight average molecular weight of the second polyvinylidene fluoride in the binder is 600,000 to 1.1 million, including the first polyvinylidene fluoride and the second polyvinylidene fluoride. The vinyl fluoride binder can make the pole piece have excellent bonding force at a low addition amount, and the electric The pool's capacity retention during cycling is improved.

From the comparison of Example 1, Examples 9-10 and Example 8, it can be seen that when the mass fraction of the binder is 0.6%-0.8%, based on the total mass of the positive electrode film layer, the first polyvinylidene fluoride and the third polyvinylidene fluoride are included. The binder of dipolyvinylidene fluoride can ensure that the pole pieces have sufficient adhesion, and the capacity retention rate of the battery during cycling is further improved. From the comparison of Example 1, Examples 9-10 and Example 11, it can be seen that when the mass fraction of the binder is 0.9%, the battery cycle performance is not significantly improved, but is not conducive to the improvement of the battery energy density.

It is known from Example 1 and Examples 42 to 44 that the first polyvinylidene fluoride in the binder is vinylidene fluoride homopolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride - Tetrafluoroethylene copolymer or vinylidene fluoride-hexafluoropropylene copolymer, low addition amount of the binder including the first polyvinylidene fluoride and the second polyvinylidene fluoride can make the pole piece have excellent Adhesion, the battery has high capacity retention during cycling.

It is known from Example 1 and Example 45 that the second polyvinylidene fluoride in the binder is vinylidene fluoride homopolymer or vinylidene fluoride-chlorotrifluoroethylene copolymer, and the low addition amount includes the second polyvinylidene fluoride. The binder of one polyvinylidene fluoride and the second polyvinylidene fluoride can make the pole piece have excellent bonding force, and the battery has a high capacity retention rate during cycling.

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, within the scope that does not deviate from the gist of the present application, various modifications to the embodiments that can be thought of by those skilled in the art, and other forms constructed by combining some of the constituent elements in the embodiments are also included in the scope of the present application. .

Claims

A binder, characterized in that the binder includes a first polyvinylidene fluoride and a second polyvinylidene fluoride, and the first polyvinylidene fluoride has a weight average molecular weight of 5 million to 900 W, the weight average molecular weight of the second polyvinylidene fluoride is smaller than the weight average molecular weight of the first polyvinylidene fluoride.
The adhesive according to claim 1, wherein the polydispersity coefficient of the first polyvinylidene fluoride is 1.8 to 2.5.
The adhesive according to claim 1 or 2, wherein the Dv50 particle size of the first polyvinylidene fluoride is 100 μm to 200 μm.
The adhesive according to any one of claims 1 to 3, wherein the first polyvinylidene fluoride has a crystallinity of 40% to 45%.
The adhesive according to any one of claims 1 to 4, characterized in that the viscosity of the glue prepared by dissolving the first polyvinylidene fluoride in N-methylpyrrolidone is 2000mPa·s～5000mPa. ·s, the mass content of the first polyvinylidene fluoride in the glue solution is 2%, based on the total mass of the glue solution.
The adhesive according to any one of claims 1 to 5, characterized in that the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1-4: 1.
The adhesive according to any one of claims 1 to 6, wherein the second polyvinylidene fluoride has a weight average molecular weight of 600,000 to 1.1 million.
A method for preparing an adhesive, characterized in that it includes the following steps:

Preparing the first polyvinylidene fluoride: providing vinylidene fluoride monomer and solvent, performing a first-stage polymerization reaction to obtain a first product; subjecting the first product to a second-stage polymerization reaction in a water-insoluble gas atmosphere ; Add a chain transfer agent and perform a third-stage polymerization reaction to obtain the first polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million;

Blending: blending the first polyvinylidene fluoride and the second polyvinylidene fluoride to prepare the adhesive, wherein the weight average molecular weight of the second polyvinylidene fluoride is smaller than that of the first polyvinylidene fluoride. Polyvinylidene fluoride.
The preparation method according to claim 8, characterized in that the reaction temperature of the first stage polymerization reaction is 45°C to 60°C, the reaction time is 4 hours to 10 hours, and the initial pressure is 4MPa to 6MPa.
The preparation method according to claim 8 or 9, characterized in that the reaction temperature of the second stage polymerization reaction is 60°C to 80°C, the reaction time is 2 hours to 4 hours, and the reaction pressure is 6MPa to 8MPa.
The preparation method according to any one of claims 8 to 10, characterized in that the reaction time of the third stage polymerization reaction is 1 hour to 2 hours.
The preparation method according to any one of claims 8 to 11, characterized in that the chain transfer agent is selected from one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.
The preparation method according to any one of claims 8 to 12, characterized in that the water-insoluble gas is selected from any one of nitrogen, oxygen, hydrogen, and methane.
The preparation method according to any one of claims 8 to 13, characterized in that the amount of the chain transfer agent is 1.5% to 3% of the total mass of the vinylidene fluoride monomer.
The preparation method according to any one of claims 8 to 14, characterized in that the first stage polymerization reaction includes the following steps:

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

Add 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, and the first stage of polymerization reaction is carried out. answer.
The preparation method according to claim 15, characterized in that the amount of the solvent is 2 to 8 times the total mass of the vinylidene fluoride monomer.
The preparation method according to claim 15 or 16, characterized in that the dispersant includes one or more of cellulose ether and polyvinyl alcohol.
According to the preparation method of claim 17, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.
The preparation method according to any one of claims 15 to 18, characterized in that the amount of the dispersant is 0.1% to 0.3% of the total mass of the vinylidene fluoride monomer.
The preparation method according to any one of claims 15 to 19, characterized in that the initiator is an organic peroxide.
The preparation method according to claim 20, wherein the organic peroxide includes tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, One or more of diisopropyl peroxydicarbonate and tert-butyl peroxypivalate.
The preparation method according to any one of claims 15 to 21, characterized in that the amount of the initiator is 0.15% to 1% of the total mass of the vinylidene fluoride monomer.
The preparation method according to any one of claims 15 to 22, wherein the pH adjuster includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate and ammonia water.
The preparation method according to any one of claims 15 to 23, characterized in that the amount of the pH adjuster is 0.05% to 0.2% of the total mass of the vinylidene fluoride monomer.
The preparation method according to any one of claims 8 to 24, wherein in the blending step, the mass ratio of the first polyvinylidene fluoride to the second polyvinylidene fluoride is 1:1～4:1.
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 bonding agent according to any one of claims 1 to 7. agent or a binder prepared by the preparation method according to any one of claims 8 to 25.
The positive electrode sheet according to claim 26, wherein the mass fraction of the binder is 0.6% to 0.8%, based on the total mass of the positive electrode film layer.
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 claim 26 or 27.
The secondary battery according to claim 28, wherein the secondary battery is a lithium-ion battery or a sodium-ion battery.
An electrical device, characterized in that it includes the secondary battery according to claim 28 or 29.