CN115117357B

CN115117357B - Adhesive, preparation method, positive electrode plate, secondary battery and power utilization device

Info

Publication number: CN115117357B
Application number: CN202211043966.4A
Authority: CN
Inventors: 段连威; 孙成栋; 刘会会
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2022-08-30
Filing date: 2022-08-30
Publication date: 2023-05-16
Anticipated expiration: 2042-08-30
Also published as: CN117624441A; CN115117357A; WO2024045619A1

Abstract

The application provides an adhesive, a preparation method, a positive electrode plate, a secondary battery and an electric device. The binder is a polymer containing a structural unit shown in a formula I and a structural unit shown in a formula II,

i is a kind of

Formula II wherein R ₁ Selected from fluorine, chlorine, C containing at least one fluorine atom _1‑3 One or more of alkyl groups, and the weight average molecular weight of the polymer is 500-900 ten thousand. The adhesive can ensure that the pole piece has enough adhesive force under the condition of low addition amount, and can further improve the flexibility of the pole piece and reduce the probability of brittle failure of the pole piece, thereby improving the safety and the cycle performance of the battery.

Description

Adhesive, preparation method, positive electrode plate, secondary battery and power utilization device

Technical Field

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

Background

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

The binder is a common material in secondary batteries, and there is a great demand for pole pieces, separator films, packaging parts, and the like of the batteries. However, the existing adhesive needs to be added in a large amount to ensure that the pole piece has enough adhesive force, and meanwhile, the adhesive cannot keep enough flexibility in the circulating process, so that the pole piece is easy to be broken, and the safety problem is further caused. Thus, the existing adhesives remain to be improved.

Disclosure of Invention

The present invention has been made in view of the above problems, and an object thereof is to provide an adhesive that can provide a pole piece with excellent adhesion at a low addition amount, and that can improve flexibility of the pole piece and further improve cycle performance of a battery.

In order to achieve the above object, the present application provides a binder which is a polymer containing a structural unit represented by formula I and a structural unit represented by formula II,

formula I->

II (II)

Wherein R is ₁ Selected from fluorine, chlorine, C containing at least one fluorine atom _1-3 One or more of alkyl groups, and the weight average molecular weight of the polymer is 500-900 ten thousand.

The adhesive can ensure that the pole piece has enough adhesive force under the condition of low addition amount, and can further improve the flexibility of the pole piece and reduce the probability of brittle failure of the pole piece, thereby improving the safety and the cycle performance of the battery.

In any embodiment, the mass fraction of structural units represented by formula II is 0.5% -15% based on the total mass of the polymer.

When the mass fraction of the structural unit shown in the formula II is in a proper range, the adhesive enables the pole piece to have excellent flexibility and good adhesive force, so that the battery can keep high capacity performance in the circulating process.

In any embodiment, the polymer has a polydispersity of 1.7 to 2.3. Optionally, the polymer has a polydispersity of 1.85 to 2.25.

The polymer has a polydisperse coefficient in a proper range, the weight average molecular weight of the polymer is uniformly distributed, the performance is balanced, the binder can be ensured to have excellent flexibility and binding force under the condition of low addition, and the capacity retention rate of the battery in the circulation process is further improved.

In any embodiment, the polymer has a Dv50 particle size of 100 μm to 200 μm, alternatively the polymer has a Dv50 particle size of 105 μm to 185 μm.

The Dv50 particle size of the polymer is controlled within a proper range, and the polymer with ultra-high molecular weight still has good processing performance, so that the production efficiency of the pole piece and the battery can be ensured.

In any embodiment, the crystallinity of the polymer is 30% -40%. Optionally, the crystallinity of the polymer is 31% -39%.

The crystallinity of the polymer is controlled within a proper range, so that the pole piece has excellent flexibility, the risk of fracture or light leakage in the winding and hot pressing processes is reduced, and the safety performance of the battery can be improved.

In any embodiment, the polymer is one or more of 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.

The second aspect of the present application also provides a method for preparing an adhesive, comprising the steps of: providing a monomer shown in a formula III, a monomer shown in a formula IV and a reaction solvent, and performing a first-stage polymerization reaction to obtain a first product;

formula III->

IV

Wherein R is ₂ Selected from fluorine, chlorine, C containing at least one fluorine atom _1-3 One or more of alkyl groups;

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

and adding a chain transfer agent, and performing a third-stage polymerization reaction to obtain the polymer with the weight average molecular weight of 500-900 ten thousand.

The preparation method can prepare the polymer binder with ultra-high molecular weight through segmented polymerization. The adhesive can ensure that the pole piece has enough adhesive force under the condition of low addition amount, and can further improve the flexibility of the pole piece and reduce the probability of brittle failure of the pole piece, thereby improving the safety and the cycle performance of the battery.

In any embodiment, the mass fraction of the monomer represented by formula iv is 0.5% -15% based on the total mass of the monomer represented by formula III and the monomer represented by formula iv.

When the mass fraction of the monomer shown in the formula IV is in a proper range, the pole piece has excellent flexibility and good binding power, so that the battery can keep high circulation capacity in the circulation process.

In any embodiment, the monomer shown in the formula IV is one or more of chlorotrifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

The raw materials are simple and easy to obtain, the production cost can be greatly reduced, and the yield is improved.

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

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

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

The reaction pressure, reaction time and reaction temperature of the polymerization reaction at each stage are controlled within proper ranges, the weight average molecular weight of the polymer is improved, the uniformity of the weight average molecular weight of the polymer can be ensured, the polymer has lower polydispersity coefficient, the uniformity of the polymer performance is improved, meanwhile, the pole piece has excellent flexibility and cohesive force under the condition of low addition of the binder, and the cycle capacity retention rate of the battery can be further improved.

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

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

In any embodiment, the amount of chain transfer agent is 1.5% -3% of the total mass of the monomer of formula III and the monomer of formula IV.

In any embodiment, the first stage polymerization reaction comprises the steps of:

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

adding an initiator and a pH regulator into the container, regulating the pH value to 6.5-7, and then adding a monomer shown in a formula III and a monomer shown in a formula IV to enable the pressure in the container to reach 4-6 MPa;

stirring for 30-60 min, heating to 45-60 ℃ and carrying out first-stage polymerization.

In any embodiment, the water solvent is used in an amount of 2 to 8 times the total mass of the monomer shown in the formula III and the monomer shown in the formula IV.

In any embodiment, the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol.

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

In any embodiment, the amount of the dispersing agent is 0.1% -0.3% of the total mass of the monomer shown in the formula III and the monomer shown in the formula IV.

In any embodiment, the initiator is an organic peroxide.

In any embodiment, the organic peroxide comprises one or more of t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and t-butyl peroxypivalate.

In any embodiment, the initiator is used in an amount of 0.15% -1% of the total mass of the monomer represented by formula III and the monomer represented by formula IV.

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

In any embodiment, the amount of the pH regulator is 0.05% -0.2% of the total mass of the monomer shown in the formula III and the monomer shown in the formula IV.

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 including a positive electrode active material, a conductive agent, and a binder in any embodiment or a binder prepared by a preparation method in any embodiment.

The positive plate has excellent flexibility and good binding force.

In any embodiment, the mass fraction of the binder is 0.8% -1% based on the total mass of the positive electrode film layer.

The mass fraction of the binder is controlled within a proper range, so that the pole piece has excellent flexibility and binding force, and the battery has high cycle capacity retention rate in the cycle process.

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

In a fifth aspect of the present application, there is provided a battery module including the secondary battery of the fourth aspect of the present application.

In a sixth aspect of the present application, there is provided a battery pack comprising the battery module of the fifth aspect of the present application.

In a seventh aspect of the present application, there is provided an electric device including at least one of the secondary battery of the fourth aspect, the battery module of the fifth aspect, or the battery pack of the sixth aspect of the present application.

Drawings

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

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

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

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

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

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

Reference numerals illustrate:

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

Detailed Description

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

The "range" disclosed herein is defined in terms of lower and upper limits, with a given range being defined by the selection of a lower and an upper limit, the selected lower and upper limits defining the boundaries of the particular range. Ranges that are defined in this way can be inclusive or exclusive of the endpoints, and any combination can be made, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the

minimum range values

1 and 2 are listed, and if the

maximum range values

3,4 and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise indicated, the numerical range "a-b" represents a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed throughout, and "0-5" is simply a shorthand representation of a combination of these values. When a certain parameter is expressed as an integer of 2 or more, it is disclosed that the parameter is, for example, an integer of 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12 or the like.

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

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

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

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

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

Polyvinylidene fluoride is one of the most widely used types of binders in secondary batteries at present. However, the adhesive force of the traditional polyvinylidene fluoride is insufficient, and a large amount of the polyvinylidene fluoride is often required to be added to ensure effective adhesion of active substances, so that the pole piece achieves effective adhesive force. However, the increase of the dosage of the polyvinylidene fluoride binder can reduce the load capacity of the active material in the pole piece on one hand, influence the improvement of the power performance of the battery, and reduce the flexibility of the pole piece on the other hand, so that the pole piece is easy to be brittle broken, and the requirements on the cycle performance and the safety performance of the battery are difficult to meet.

[ adhesive ]

Based on this, the present application proposes a binder which is a polymer containing a structural unit represented by formula I and a structural unit represented by formula II,

Formula I->

II (II)

In this context, the term "binder" refers to a chemical compound, polymer or mixture that forms a colloidal solution or colloidal dispersion in a dispersing medium.

In this context, the term "polymer" includes on the one hand the collection of chemically homogeneous macromolecules prepared by polymerization, but differing in terms of degree of polymerization, molar mass and chain length. The term on the other hand also includes derivatives of such macromolecular assemblies formed by polymerization, i.e. products which can be obtained by reaction, e.g. addition or substitution, of functional groups in the macromolecules described above and which can be chemically homogeneous or chemically inhomogeneous.

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

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

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

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

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

In this context, the term "fluorine" refers to the-F group.

In this context, the term "chlorine" refers to a-Cl group.

As used herein, the term "C containing at least one fluorine atom _1-3 Alkyl "refers to an alkyl group containing 1 to 3C atoms having at least one H atom replaced by an F atom. In some embodiments, C contains one fluorine atom _1-3 Alkyl is selected from-CF ₃ Group, -C ₂ F ₆ A group.

In some embodiments, the binder is a halogenated hydrocarbon copolymer, and may be selected from one or more of 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.

The fluorine element contained in the polymer forms hydrogen bond action with hydroxyl or/and carboxyl on the surface of the active material and the surface of the current collector, so that the cohesive force of the pole piece can be improved. The polymer with the weight average molecular weight of 500-900 ten thousand has extremely high cohesive force and intermolecular acting force, and can improve the adhesive force of the pole piece under the condition of low addition amount. The structural unit shown in the formula II in the polymer can introduce disordered units into a crystallization area of a chain segment which is formed by the structural units shown in the formula I and is periodically arranged, so that the crystallinity of the polymer is reduced, the mobility of the chain segment is increased, the flexibility of the pole piece is improved, meanwhile, the polymer contains the structural unit shown in the formula II, the content of the structural unit shown in the formula I can be reduced, and the crystallization caused by polymerization of the structural unit shown in the formula I is reduced, so that the flexibility of the pole piece is further improved.

If the weight average molecular weight of the polymer is too large, the too high weight average molecular weight can reduce the flexibility of the pole piece; if the weight average molecular weight of the polymer is too small, the pole piece cannot be ensured to have enough cohesive force under the condition of low addition amount of the binder.

The adhesive can ensure that the pole piece has enough adhesive force under the condition of lower addition amount, and can further improve the flexibility of the pole piece and reduce the probability of brittle failure of the pole piece, thereby improving the safety and the cycle performance of the battery.

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

In some embodiments, the mass fraction of structural units represented by formula II is 0.5% -15% based on the total mass of the polymer. In some embodiments, the mass fraction of the structural unit shown in formula II may be any one of 0.5% -1%, 1% -2%, 3% -4%, 4% -5%, 5% -6%, 6% -7%, 7% -8%, 8% -9%, 9% -10%, 10% -11%, 11% -12%, 12% -13%, 13% -14%, 14% -15%, 0.5% -3%, 3% -6%, 6% -9%, 9% -12%, 12% -15%, 0.5% -5%, 5% -10%, 10% -15%.

If the mass fraction of the structural unit shown in the formula II is too low, the aim of improving the flexibility of the pole piece is not achieved; if the mass fraction of the structural unit shown in the formula II is too high, the adhesive force of the pole piece is reduced, and the cycle performance of the battery is affected.

When the mass fraction of the structural unit shown in the formula II is in a proper range, the pole piece can have excellent flexibility and good binding force under the condition of low addition amount of the binding agent, so that the battery can keep good capacity performance in the circulating process.

In some embodiments, the polymer has a polydispersity of 1.7 to 2.3. In some embodiments, the polydispersity of the polymer may be selected from any of 1.7-1.85, 1.85-1.95, 1.95-2.05, 2.05-2.15, 1.85-2.25.

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

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

If the polydispersity of the polymer is too large, the order of the polymer is lower, the dispersibility of the binder is affected, the flexibility of the pole piece is reduced, the solid content of the slurry is reduced, and the production cost is increased; if the polydispersity of the polymer is too small, the difficulty of the preparation process is high, the yield is low, and the production cost is high.

The polydisperse coefficient of the polymer is in a proper range, the weight average molecular weight of the polymer is uniformly distributed, the performance is balanced, the pole piece can be ensured to have excellent flexibility and bonding force under the condition of low addition amount of the bonding agent, and the capacity retention rate of the battery in the circulation process is further improved. In addition, the proper polymer has a polydispersion coefficient, so that the solid content of the slurry can be effectively improved, and the production cost is reduced.

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

In some embodiments, the Dv50 particle size of the polymer is 100 μm to 200 μm. In some embodiments, the Dv50 particle size of the polymer may be selected from any of 105 μm to 115 μm, 115 μm to 125 μm, 125 μm to 135 μm, 135 μm to 145 μm, 145 μm to 155 μm, 155 μm to 165 μm, 165 μm to 175 μm, 175 μm to 185 μm, 185 μm to 195 μm, 105 μm to 125 μm, 125 μm to 145 μm, 145 μm to 165 μm, 165 μm to 185 μm, 105 μm to 185 μm, 125 μm to 185 μm.

As used herein, the term "Dv50 particle size" refers to the particle size corresponding to a cumulative particle size distribution of 50% in the particle size distribution curve, in the physical sense that the particle size is less than (or greater than) 50% of its particles.

If the Dv50 particle size of the polymer is too large, the polymer is relatively difficult to dissolve, the dispersibility of the binder is reduced, so that the flexibility of the pole piece is reduced, and meanwhile, the polymer is difficult to dissolve, so that the speed of the pulping process is reduced; if the Dv50 particle size of the polymer is too small, the adhesion of the pole piece is lowered.

The Dv50 particle size of the polymer is controlled within a proper range, so that the solubility of the adhesive can be improved, the flexibility of the pole piece is improved, and the pole piece has better adhesive force. Meanwhile, the Dv50 particle size of the polymer in a proper range can also control the dosage of the binder at a lower level, and can not cause excessive negative influence on the binding performance, thereby effectively improving the condition that the pole piece and the battery performance are damaged due to the high dosage of the binder in the traditional technology.

Referring to a GB/T19077-2016 particle size distribution laser diffraction method, weighing 0.1 g-0.13 g of polymer powder by using a 50 ml beaker, weighing 5g of absolute ethyl alcohol, adding the obtained mixture into the beaker filled with the polymer powder, placing a stirrer with the length of about 2.5 mm, and sealing by using a preservative film. The sample is put into an ultrasonic machine for ultrasonic treatment for 5 minutes, and is transferred to a magnetic stirrer to be stirred for more than 20 minutes at the speed of 500 revolutions per minute, and 2 samples are extracted from each batch of products for testing and averaging. The measurement is performed using a laser particle size analyzer, such as a Mastersizer 2000E laser particle size analyzer from malvern instruments, england.

In some embodiments, the crystallinity of the polymer is 30% -40%. In some embodiments, the crystallinity of the polymer may be selected from any of 30% -32%, 32% -34%, 34% -36%, 36% -38%, 38% -39%, 31% -33%, 33% -35%, 35% -37%, 37% -39%, 31% -39%.

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

If the crystallinity of the polymer is too large, the mobility of the polymer chain segment is reduced, the flexibility of the pole piece is affected, and meanwhile, the polymer is difficult to dissolve, so that the speed of the pulping process is reduced; if the crystallinity of the polymer is too small, the degree of ordered close packing of the polymer molecular chains decreases, affecting the chemical and thermal stability of the binder.

The crystallinity of the polymer is controlled within a proper range, so that when the consumption of the binder is in a lower level, the pole piece can have excellent flexibility and good binding force, thereby being beneficial to improving the loading capacity of active substances and the cycle performance of the battery.

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

This test will give a DSC curve for the polymer, the curve being integrated to give a peak area as the melting enthalpy of the polymer Δh (J/g), the polymer crystallinity = (Δh/Δhm) ×100%, where Δhm is the standard melting enthalpy of polyvinylidene fluoride (crystalline heat of fusion), Δhm=104.7J/g.

In some embodiments, the polymer is one or more of 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.

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

providing a monomer shown in a formula III, a monomer shown in a formula IV and a reaction solvent, and performing a first-stage polymerization reaction to obtain a first product;

formula III->

IV

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

The polymer with ultra-high molecular weight can be prepared by adopting a sectional method for polymerization reaction, so that the adhesive can meet the requirement of the adhesive force of the pole piece under the condition of low addition, and meanwhile, the pole piece has excellent flexibility, thereby being beneficial to improving the capacity retention rate of the battery in the circulating process. Meanwhile, a first product with relatively low weight average molecular weight is formed in the first-stage polymerization reaction, a molecular chain segment with target molecular weight is formed in the second-stage polymerization reaction, the molecular weight of the polymer is regulated and controlled in the third-stage polymerization reaction, the situation that the weight average molecular weight of the polymer is too high in randomness is avoided, and the uniformity of the polymer is improved. And the segmented polymerization not only can improve the utilization rate of the reactor in the preparation process of the polymer, but also can save time and reduce the residence time of the polymer in the reactor. The first-stage polymerization reaction, the second-stage polymerization reaction and the third-stage polymerization reaction are matched with each other, so that the production efficiency of the polymer can be further improved.

It is understood that the first product may be a reaction solution formed by the monomer shown in formula III, the monomer shown in formula iv and the reaction solvent, or a product obtained by processing and purifying the reaction solution.

In some embodiments, the mass fraction of the monomer of formula iv is 0.5% -15% based on the total mass of the monomer of formula III and the monomer of formula iv. In some embodiments, the mass fraction of the monomer represented by formula IV may be any one of 0.5% -1%, 1% -2%, 3% -4%, 4% -5%, 5% -6%, 6% -7%, 7% -8%, 8% -9%, 9% -10%, 10% -11%, 11% -12%, 12% -13%, 13% -14%, 14% -15%, 0.5% -3%, 3% -6%, 6% -9%, 9% -12%, 12% -15%, 0.5% -5%, 5% -10%, 10% -15%.

If the mass fraction of the monomer shown in the formula IV is too low, the purpose of improving the flexibility of the pole piece is not achieved; if the mass fraction of the monomer shown in the formula IV is too high, the adhesive force of the pole piece is reduced, and the cycle performance of the battery is affected.

In some embodiments, the monomer of formula iv is one or more of chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene.

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

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 may be selected from any one of 4 hours to 5 hours, 5 hours to 6 hours, 6 hours to 7 hours, 7 hours to 8 hours, 8 hours to 9 hours, 9 hours to 10 hours, 4 hours to 6 hours, 6 hours to 8 hours, 8 hours to 10 hours, and 5 hours to 10 hours.

In some embodiments, the initial pressure of the first stage polymerization reaction is 4mpa to 6mpa. In some embodiments, the initial pressure of the first stage polymerization reaction is 4MPa to 5MPa or 5MPa to 6MPa.

In some embodiments, the reaction temperature of the second stage polymerization reaction is 60 ℃ to 80 ℃. In some embodiments, the reaction temperature of the second stage polymerization reaction is 60 ℃ to 70 ℃ or 70 ℃ to 80 ℃.

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

In some embodiments, the reaction pressure of the second stage polymerization is from 6mpa to 8mpa. In some embodiments, the reaction pressure of the second stage polymerization is 6mpa to 7mpa or 7mpa to 8mpa.

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

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

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

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

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

In some embodiments, the amount of chain transfer agent is 1.5% to 3% of the total mass of the monomer of formula III and the monomer of formula IV. The amount of chain transfer agent used may also be, for example, 2% or 2.5% of the total mass of the monomer of formula III and the monomer of formula IV.

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

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

Before the polymerization reaction is carried out by heating, the materials are uniformly mixed, so that the reaction can be more thoroughly carried out, and the obtained polymer has more uniform polydisperse coefficient, crystallinity and particle size.

In some embodiments, the amount of the aqueous solvent is 2 to 8 times the total mass of the monomer of formula III and the monomer of formula IV. The amount of the aqueous solvent may also be, for example, 3, 4, 5, 6 or 7 times the total mass of the monomer of formula III and the monomer of formula IV. In some embodiments, the aqueous solvent is deionized water.

In some embodiments, the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol.

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

In some embodiments, the dispersant is used in an amount of 0.1% to 0.3% of the total mass of the monomer of formula III and the monomer of formula IV. The amount of the dispersant used may be, for example, 0.2% by mass of the total mass of the monomer represented by formula III and the monomer represented by formula IV.

In some embodiments, the initiator is an organic peroxide.

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

In some embodiments, the initiator is used in an amount of 0.15% to 1% of the total mass of the monomer of formula III and the monomer of formula IV. The amount of initiator used may also be selected, for example, to be 0.2%, 0.4%, 0.6% or 0.8% of the total mass of the monomer of formula III and the monomer of formula IV.

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

In some embodiments, the amount of the pH adjustor is 0.05% to 0.2% of the total mass of the monomer represented by formula III and the monomer represented by formula IV. The amount of the pH adjustor can also be, for example, 0.1% or 0.15% of the total mass of the monomer represented by the formula III and the monomer represented by the formula IV.

The positive plate has excellent flexibility and good binding force.

In some embodiments, the mass fraction of the binder is 0.8% -1% based on the total mass of the positive electrode film layer.

[ Positive electrode sheet ]

The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises a positive electrode active material, a conductive agent, a binder in some embodiments or a binder prepared by a preparation method in some embodiments.

The positive plate has excellent flexibility and good binding force.

In some embodiments, the mass fraction of the binder is 0.8% -1% based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of the binder may be any one of 0.8% -0.85%, 0.85% -0.9%, 0.9% -0.95%, 0.95% -1%, 0.85% -0.95%.

If the mass fraction of the binder is too high, the binder coating layer coated on the surface of the positive electrode active material is too thick, and the pole piece is brittle and has poor toughness. In addition, excessive binder can cause the load capacity of the positive electrode active material in the pole piece to be reduced, so that the energy density of the battery is reduced, and the capacity of the battery is reduced.

If the mass fraction of the binder is too low, a sufficient bonding effect cannot be achieved, on one hand, enough conductive agent and positive electrode active material cannot be bonded together, and the bonding force of the pole piece is small; on the other hand, the adhesive cannot be tightly combined with the surface of the active substance, so that the surface of the pole piece is easy to be destoner, and the cycle performance of the battery is reduced.

The mass fraction of the binder is controlled within a proper range, so that the pole piece has excellent flexibility and good binding force, and the battery has good cycle capacity retention rate in the cycle process.

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

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

In some embodiments, the positive electrode active material may employ a positive electrode active material for a battery, which is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: olivine structured lithium-containing phosphates, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery positive electrode active material may be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (e.g., liCoO) ₂ ) Lithium nickel oxide (e.g. LiNiO) ₂ ) Lithium manganese oxide (e.g. LiMnO ₂ 、LiMn ₂ O ₄ ) Lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., 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 (also referred to as NCM) ₅₂₃ ）、LiNi _0.5 Co _0.25 Mn _0.25 O ₂ (also referred to as NCM) ₂₁₁ ）、LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (also referred to as NCM) ₆₂₂ ）、LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (also referred to as NCM) ₈₁₁ ) Lithium nickel cobalt aluminum oxide (e.g. LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) And at least one of its modified compounds and the like. Examples of olivine structured lithium-containing phosphates may include, but are not limited to, lithium iron phosphate (e.g., liFePO ₄ (also abbreviated as LFP)), composite material of lithium iron phosphate and carbon, and manganese lithium phosphate (such as LiMnPO) ₄ ) At least one of a composite material of lithium manganese phosphate and carbon, and a composite material of lithium manganese phosphate and carbon.

In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

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

[ negative electrode sheet ]

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

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

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

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

In some embodiments, the negative electrode film layer further optionally includes a binder. The binder may be at least one selected from Styrene Butadiene Rubber (SBR), polyacrylic acid (PAA), sodium Polyacrylate (PAAs), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium Alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

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

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

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

[ electrolyte ]

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

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

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

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

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

[ isolation Membrane ]

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

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

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

Secondary battery

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

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

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

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

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

[ Battery Module ]

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

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

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

[ Battery pack ]

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

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

[ electric device ]

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

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

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

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

Examples

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

Example 1

1) Preparation of the adhesive

First stage polymerization: into an autoclave of No. 1 and No. 2 and 10L were charged 4kg of deionized water and 2.5g of methyl cellulose ether, and vacuum was applied and N was used ₂ Replacement O ₂ Adding 5g of tert-butyl pivalate peroxide and 2g of sodium bicarbonate again for three times, charging 0.94kg of vinylidene fluoride and 0.06kg of chlorotrifluoroethylene, enabling the pressure to reach 5MPa, mixing and stirring for 30min, heating to 45 ℃ and reacting for 4h;

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

third stage polymerization: after 38g of cyclohexane was added, the reaction was continued for 1 hour, and the reaction was stopped. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the vinylidene fluoride-chlorotrifluoroethylene copolymer binder.

2) Preparation of positive electrode plate

Putting 3961.8 g lithium iron phosphate, 32.8g of polyvinylidene fluoride binder and 57.4g of acetylene black into a planetary stirring tank, and stirring for 20-30 min at revolution speed of 25r/min, wherein the mass fraction of the binder is 0.6% based on the total mass of the positive electrode film layer;

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

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

And after the stirring is finished, testing the viscosity of the slurry, and controlling the viscosity to 8000-15000 mPa.s.

If the viscosity is higher, NMP solution is added to reduce the viscosity to the above viscosity interval, and then the mixture is stirred for 30min according to revolution speed of 25 r/min and rotation speed of 1250r/min, so as to obtain the positive electrode slurry. The prepared positive electrode slurry is scraped on a carbon-coated aluminum foil, and the single side of the scraping weight is 550 mg/(1540 mm) ² ) Baking at 110deg.CCold pressing for 15min to compact to 2.7g/cm ³ Cutting into a wafer with the diameter of 15mm to obtain the positive electrode plate.

3) Negative pole piece

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

4) Isolation film

A polypropylene film was used as a separator.

5) Preparation of electrolyte

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

6) Preparation of a Battery

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

Examples 2 to 5

Substantially the same as in example 1, except that the reaction times in the first polymerization stage were adjusted to 5h, 6h, 7h and 8h, respectively, and the chain transfer agent cyclohexane in the third polymerization stage was adjusted to 33g, 28g, 23g and 18g, respectively, the specific parameters are shown in Table 1.

Examples 6 to 9

Substantially the same as in example 1, except that the total amount of vinylidene fluoride and chlorotrifluoroethylene monomer to be added was kept unchanged, the mass fraction of chlorotrifluoroethylene was adjusted, based on the total mass of vinylidene fluoride and chlorotrifluoroethylene monomer, and specific parameters are shown in Table 1.

Examples 10 to 13

Substantially the same as in example 1, except that the mass fraction of the vinylidene fluoride-chlorotrifluoroethylene copolymer binder was adjusted, the specific parameters are shown in table 1 based on the total mass of the positive electrode film layer.

Examples 14 to 15

Substantially the same as in example 1, except that 0.06kg of chlorotrifluoroethylene was replaced with 0.06kg of tetrafluoroethylene and 0.06kg of hexafluoropropylene, respectively. The specific parameters are shown in table 1.

Comparative example 1

Substantially the same as in example 1, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 2

Substantially the same as in example 2, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 3

Substantially the same as in example 3, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 4

Substantially the same as in example 4, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 5

Substantially the same as in example 5, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

Comparative example 6

Substantially the same as in example 1, the binder was polyvinylidene fluoride having a weight average molecular weight of 80 ten thousand, purchased from the Huaan corporation under the model number 605, and the mass fraction of the binder was adjusted to 2.5% based on the total mass of the positive electrode film layer, and specific parameters are shown in table 1.

2. Battery performance test

1. Adhesive property test

1) Weight average molecular weight test

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

2) Polydisperse coefficient testing

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

3) Dv50 test

With reference to a GB/T19077-2016 particle size distribution laser diffraction method, weighing 0.1 g-0.13 g of binder powder by using a 50ml beaker, weighing 5g of absolute ethyl alcohol, adding the obtained mixture into a beaker filled with the binder powder, placing a stirrer with the length of about 2.5mm, and sealing by using a preservative film. And (3) putting the sample into an ultrasonic machine for ultrasonic treatment for 5min, and transferring the sample into a magnetic stirrer for stirring at a speed of 500r/min for more than 20 min. Conveniently, the measurement is carried out using a laser particle size analyzer, such as a Mastersizer 2000E laser particle size analyzer from Markov instruments, UK.

4) Crystallinity test

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

This test will give a DSC/(Mw/mg) versus temperature curve for the binder and integrate the peak area, i.e. the melting enthalpy of the binder Δh (J/g), binder crystallinity = (Δh/Δhm) ×100%, where Δhm is the standard melting enthalpy of polyvinylidene fluoride (crystalline heat of fusion), Δhm=104.7J/g.

2. Pole piece performance test

1) Adhesion test

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

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

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

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

3. Battery performance test

1) Battery capacity retention test

The battery capacity retention test procedure was as follows: at 25 ℃, the button cell was charged to 3.65V at a constant current of 1/3C, then charged to 0.05C at a constant voltage of 3.65V, left for 5min, then discharged to 2.5V at 1/3C, and the resulting capacity was recorded as initial capacity C0. Repeating the above steps for the same battery, and recording the discharge capacity Cn of the battery after the nth cycle, wherein the battery capacity retention rate pn=cn/c0 after each cycle is 100%, the 500 point values of P1 and P2 … … P500 are taken as ordinate, and the corresponding cycle times are taken as abscissa, so as to obtain a graph of the battery capacity retention rate and the cycle times.

In this test procedure, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and the 500 th cycle of … … corresponds to n=500. The battery capacity retention rate data corresponding to examples 1 to 15 or comparative examples 1 to 6 in table 1 are data measured after 500 cycles under the above test conditions, i.e., P500 values.

3. Analysis of test results for examples and comparative examples

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

Table 1 parameters and performance test tables of examples 1 to 15 and comparative examples 1 to 6

From the above results, the binders in examples 1 to 15 all comprise polymers comprising structural units derived from vinylidene fluoride and at least one structural unit derived from chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, and the weight average molecular weight of the polymers is 500 to 900 ten thousand. From the comparison of example 1 and comparative example 1, the comparison of example 2, examples 6 to 9 and comparative example 2, the comparison of example 3 and comparative example 3, the comparison of example 4 and comparative example 4, and the comparison of example 5 and comparative example 5, it is apparent that the introduction of a comonomer in the polymer can improve the flexibility of the pole piece, reduce the risk of breakage or light leakage during the winding and hot-pressing process, and improve the safety performance of the battery without significantly reducing the adhesive force of the pole piece.

Compared with comparative example 6, the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-tetrafluoroethylene copolymer or vinylidene fluoride-hexafluoropropylene copolymer binder with the weight average molecular weight of 500-900 ten thousand can ensure that the pole piece has enough binding force under the condition of low addition, and meanwhile, the flexibility of the pole piece can be further improved, so that the capacity retention rate of the battery in the circulation process can be improved, and the condition that the performance of the pole piece and the battery is damaged due to the high-dosage binder in the traditional technology is effectively improved.

As is clear from comparison of examples 1, 6, 8, 7 and 9, when the mass fraction of the chlorotrifluoroethylene in the vinylidene fluoride-chlorotrifluoroethylene copolymer is 0.5% -15%, the binder enables the pole piece to have excellent flexibility and binding force based on the total mass of the vinylidene fluoride-chlorotrifluoroethylene copolymer, so that the battery can maintain good capacity performance in the circulating process.

As is clear from examples 1 to 15, the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-tetrafluoroethylene copolymer or vinylidene fluoride-hexafluoropropylene copolymer binder having a polydispersity of 1.7 to 2.3 gives the pole piece excellent flexibility and adhesion at low addition amounts, so that the battery has a high capacity retention during cycling.

From examples 1 to 15, it was found that the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-tetrafluoroethylene copolymer or vinylidene fluoride-hexafluoropropylene copolymer binder having a Dv50 particle diameter of 100 μm to 200 μm provides the pole piece with excellent flexibility and adhesion even at a low addition amount, so that the battery has a high capacity retention during the cycle.

From examples 1 to 15, it was found that the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-tetrafluoroethylene copolymer or vinylidene fluoride-hexafluoropropylene copolymer binder having a crystallinity of 30% to 40% provides the pole piece with excellent flexibility and adhesion at a low addition amount, so that the battery has a high capacity retention rate during the cycle.

As can be seen from the comparison of examples 2, 11-12, 10 and 13, when the mass fraction of the vinylidene fluoride-chlorotrifluoroethylene copolymer binder based on the total mass of the positive electrode film layer is 0.8% -1%, the binder can enable the pole piece to have excellent flexibility and good binding power, so that the battery has high capacity retention rate in the circulating process.

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

Claims

1. The positive electrode plate comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises a positive electrode active material, a conductive agent and a binder, the binder is a polymer containing a structural unit shown in a formula I and a structural unit shown in a formula II,

wherein R is ₁ Selected from fluorine, chlorine, C containing at least one fluorine atom _1-3 And one or more of alkyl, wherein the mass fraction of the structural unit shown in the formula II is 0.5-15%, the weight average molecular weight of the polymer is more than 500 ten thousand and not more than 900 ten thousand based on the total mass of the polymer, the crystallinity of the polymer is 30-40%, the polydispersity index of the polymer is 1.7-2.3, and the mass fraction of the binder is 0.8-1%, based on the total mass of the positive electrode film layer.

2. The positive electrode sheet according to claim 1, wherein the polymer has a Dv50 particle size of 100 μm to 200 μm.

3. The positive electrode sheet according to claim 1 or 2, wherein the polymer is one or more of 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.

4. A method for preparing an adhesive, comprising the steps of:

wherein R is ₂ Selected from fluorine, chlorine, C containing at least one fluorine atom _1-3 One or more of alkyl groups; the mass fraction of the monomer shown in the formula IV is 0.5-15%, based on the total mass of the monomer shown in the formula III and the monomer shown in the formula IV; the first products are polymerized with each other under the atmosphere of water-insoluble gas to carry out second-stage polymerization reaction;

adding a chain transfer agent to perform a third-stage polymerization reaction to obtain a polymer with a weight average molecular weight of more than 500 ten thousand and not more than 900 ten thousand, wherein the crystallinity of the polymer is 30-40%, and the polydispersity of the polymer is 1.7-2.3.

5. The preparation method according to claim 4, wherein the monomer shown in the formula IV is one or more of chlorotrifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

6. The process according to claim 4, wherein the reaction temperature of the first polymerization stage is 45 to 60℃and the reaction time is 4 to 10 hours, and the initial pressure is 4 to 6MPa.

7. The process according to any one of claims 4 to 6, wherein the second polymerization stage has a reaction temperature of 60 to 80 ℃, a reaction time of 2 to 4 hours, and a reaction pressure of 6 to 8MPa.

8. The production method according to any one of claims 4 to 6, wherein the reaction time of the third-stage polymerization reaction is 1 to 2 hours.

9. The production method according to any one of claims 4 to 6, wherein the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

10. The method according to any one of claims 4 to 6, wherein the water-insoluble gas is one or more selected from nitrogen, oxygen, hydrogen, and methane.

11. The production method according to any one of claims 4 to 6, wherein the chain transfer agent is used in an amount of 1.5% to 3% of the total mass of the monomer represented by formula III and the monomer represented by formula IV.

12. The preparation method according to any one of claims 4 to 6, wherein the first stage polymerization reaction comprises the steps of:

Adding an initiator and a pH regulator into the container, regulating the pH value to 6.5-7, and then adding monomers shown in the formulas III and IV to enable the pressure in the container to reach 4-6 MPa;

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

13. The preparation method according to claim 12, wherein the amount of the aqueous solvent is 2 to 8 times the total mass of the monomer represented by formula III and the monomer represented by formula iv.

14. The method of preparation of claim 12, wherein the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol.

15. The method of preparation of claim 14, wherein the cellulose ether comprises one or more of a methyl cellulose ether and a carboxyethyl cellulose ether.

16. The preparation method according to claim 12, wherein the amount of the dispersant is 0.1% to 0.3% of the total mass of the monomer represented by formula III and the monomer represented by formula iv.

17. The method of claim 12, wherein the initiator is an organic peroxide.

18. The method of preparation of claim 17, wherein the organic peroxide comprises one or more of t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and t-butyl peroxypivalate.

19. The preparation method according to claim 12, wherein the amount of the initiator is 0.15% to 1% of the total mass of the monomer represented by formula III and the monomer represented by formula iv.

20. The method of claim 12, wherein the pH adjuster comprises one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

21. The preparation method according to claim 12, wherein the amount of the pH adjustor is 0.05% to 0.2% of the total mass of the monomer represented by formula III and the monomer represented by formula iv.

22. A secondary battery comprising a positive electrode sheet as claimed in any one of claims 1 to 3 or comprising the binder prepared by the preparation method of any one of claims 4 to 21, a separator, a negative electrode sheet, and an electrolyte.

23. The secondary battery according to claim 22, wherein the secondary battery is a lithium ion battery or a sodium ion battery.

24. A battery module comprising the secondary battery according to claim 22 or 23.

25. A battery pack comprising the battery module of claim 24.

26. An electric device comprising at least one selected from the secondary battery according to claim 22 or 23, the battery module according to claim 24, and the battery pack according to claim 25.