CN117625088A

CN117625088A - Binder, preparation method, positive electrode slurry, secondary battery and electric device

Info

Publication number: CN117625088A
Application number: CN202310172771.8A
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: 2024-03-01
Also published as: WO2024045506A1; CN115133034A; CN115133034B

Abstract

The application provides a binder, a preparation method, positive electrode slurry, a secondary battery and an electric device. The binder is a polymer containing structural units shown in the formulas I and II, wherein R is ₁ One or more selected from fluorine, chlorine and trifluoromethyl, and the weight average molecular weight of the polymer is 180-500 ten thousand. The binder is added at low levelsThe pole piece can be guaranteed to have enough cohesive force, meanwhile, the flexibility of the pole piece can be further improved, the probability of brittle failure of the pole piece is reduced, and therefore the safety and the cycle performance of the battery are improved.

Description

Binder, preparation method, positive electrode slurry, secondary battery and electric device

The present application is a divisional application based on the application of 202211046282.X, application day 2022, month 08 and 30, and the invention name of "binder, preparation method, positive electrode sheet, secondary battery and electric device".

Technical Field

The application relates to the technical field of secondary batteries, in particular to a binder, a preparation method, positive electrode slurry, a secondary battery 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 structural units represented by the formula I, formula II,

wherein R is ₁ One or more selected from fluorine, chlorine and trifluoromethyl, and the weight average molecular weight of the polymer is 180-500 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% to 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 can enable the pole piece to have excellent flexibility and good adhesive force at a low addition amount, so that the battery can keep good capacity performance in a circulating process.

In any embodiment, the polymer has a polydispersity of 2 to 2.3. Alternatively, the polymer has a polydispersity of 2.1 to 2.2.

The polydispersity coefficient of the polymer is controlled in a proper range, so that the flexibility of the pole piece can be improved, and the pole piece has good adhesive force.

In any embodiment, the polymer has a Dv50 particle size of 50 μm to 160 μm, alternatively the polymer has a Dv50 particle size of 50 μm to 100 μm.

The Dv50 particle size of the polymer is controlled within a proper range, so that the flexibility of the pole piece can be improved, and the pole piece has good adhesive force.

In any embodiment, the crystallinity of the polymer is 34% to 42%, alternatively, the crystallinity of the polymer is 35% to 40%.

The crystallinity of the polymer is controlled within a proper range, so that the flexibility of the pole piece can be improved, and the pole piece has good adhesive force.

In any embodiment, the viscosity of the gum solution containing 4% by mass of the polymer obtained by dissolving the polymer in N-methylpyrrolidone is 2400 mPas to 5000 mPas. Alternatively, the viscosity of a dope containing 4% by mass of the polymer prepared by dissolving the polymer in N-methylpyrrolidone is 2500 mPas to 4000 mPas.

The viscosity of the glue solution of the polymer is controlled within a proper range, so that the pole piece can be ensured to have good bonding performance when the using amount of the bonding agent is in a lower level.

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:

under the polymerizable condition, polymerizing a monomer shown in a formula III and a monomer shown in a formula IV to prepare a polymer;

wherein R is ₂ One or more selected from fluorine, chlorine and trifluoromethyl, and the weight average molecular weight of the polymer is 180-500 ten thousand.

The preparation method of the adhesive is simple, is environment-friendly and is beneficial to industrial production. Meanwhile, the adhesive prepared by the method enables the pole piece to have excellent flexibility and good adhesive force, and the battery has high-level cycle retention rate.

In any embodiment, the mass content of the monomer of formula iv is 0.5% to 15% based on the total mass of the monomers of formulae III and 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 monomers shown in the formula III and the formula IV are reacted for 6 to 12 hours in a non-reactive gas atmosphere at a reaction pressure of 6 to 8MPa and a reaction temperature of 45 to 60 ℃; adding a chain transfer agent, stopping the reaction when the pressure in the reaction system is reduced to 2-2.5 MPa, and carrying out solid-liquid separation to keep a solid phase.

In any embodiment, the method of making further comprises the steps of: adding a solvent and a dispersing agent into a container, vacuumizing the container, and filling non-reactive gas; 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 6-8 MPa; stirring for 30-60 min, heating to 45-60 deg.C, and polymerizing.

The reaction pressure, reaction pressure and reaction temperature of the polymerization reaction are controlled within proper ranges, and the weight average molecular weight of the polymer can be controlled, so that the pole piece has excellent flexibility and good binding force, and the battery has high cycle capacity retention rate.

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

In any embodiment, the positive electrode active material is at least one of lithium iron phosphate and its modified material, lithium nickel cobalt manganese oxide and its modified material, and the modified material is prepared by one or more modification modes of doping, conductive carbon coating, conductive metal coating and conductive polymer coating.

A fourth aspect of the present application provides a method for preparing a positive electrode sheet, including the steps of: the first stage: mixing the positive electrode active material, the conductive agent and the binder in any embodiment or the binder prepared by the preparation method in any embodiment, and performing first stirring; and a second stage: adding a solvent to carry out second stirring; and a third stage: adding a dispersing agent to perform third stirring to obtain slurry, and controlling the viscosity of the slurry to be 8000-15000 mPa.s; fourth stage: and coating slurry on the positive current collector to obtain a positive plate.

The preparation method is simple and is beneficial to industrial production.

In any embodiment, in the first stirring, the stirring revolution speed is 25 rpm and the stirring time is 30 minutes.

In any embodiment, in the second stirring, the stirring revolution speed is 25 rpm, the stirring rotation speed is 800 to 1000 rpm, and the stirring time is 50 to 80 minutes.

In any embodiment, in the third stirring, the stirring rotation speed is 1200 to 1500 rpm and the stirring time is 50 to 70 minutes.

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

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

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

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

Drawings

FIG. 1 is a graph of adhesion versus displacement for example 1 and comparative example 4;

fig. 2 is a graph of battery capacity retention rate versus cycle number for example 1 and comparative example 4;

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

fig. 4 is an exploded view of the secondary battery of an embodiment of the present application shown in fig. 3;

fig. 5 is a schematic view of a battery module according to an embodiment of the present application;

FIG. 6 is a schematic view of a battery pack according to an embodiment of the present application;

FIG. 7 is an exploded view of the battery pack of one embodiment of the present application shown in FIG. 6;

fig. 8 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 conventional polyvinylidene fluoride has low viscosity, and a large amount of addition is often required to ensure effective bonding of active substances, so that the pole piece achieves effective bonding force. However, on one hand, the improvement of the dosage of the traditional polyvinylidene fluoride can reduce the load capacity of the active material in the pole piece, affect the improvement of the power performance of the battery, and on the other hand, the flexibility of the pole piece can be reduced, 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 be met.

[ adhesive ]

Based on this, the application proposes a binder which is a polymer containing structural units represented by the formula I and the formula 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. compounds which can be obtained by reaction of functional groups in the macromolecules described above, for example addition or substitution, and which can be chemically homogeneous or chemically inhomogeneous.

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 "trifluoromethyl" refers to-CF ₃ 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 180-500 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, the crystallization caused by polymerization of the structural unit shown in the formula I is reduced, and the flexibility of the pole piece is further improved.

If the weight average molecular weight of the polymer is more than 500 ten thousand, the excessively high molecular weight can reduce the flexibility of the pole piece; if the weight average molecular weight of the polymer is less than 180 ten thousand, 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 Waters2695 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 300mm+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% to 15% based on the total mass of the polymer. In some embodiments, the mass fraction of structural units represented by 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, and the capacity retention rate of the battery in the circulation process can be improved.

In some embodiments, the polymer has a polydispersity of 2 to 2.3. In some embodiments, the polymer has a polydispersity of any of 2 to 2.1, 2 to 2.2, 2 to 2.3, 2.1 to 2.2, 2.1 to 2.3.

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 higher; 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 polydispersity coefficient of the polymer is in a proper range, so that the flexibility of the pole piece can be improved, and the pole piece has good adhesive force. 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 HT5DMF7.8 x 300mm+Styragel HT4) is selected as a reference with a polystyrene solution sample having a mass fraction of 3.0%. 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 50 μm to 160 μm. In some embodiments, the Dv50 particle size of the polymer may be selected from any of 50 μm to 60 μm, 60 μm to 70 μm, 70 μm to 80 μm, 80 μm to 90 μm, 90 μm to 100 μm, 100 μm to 110 μm, 110 μm to 120 μm, 120 μm to 130 μm, 130 μm to 140 μm, 140 μm to 150 μm, 150 μm to 160 μm, 50 μm to 70 μm, 70 μm to 90 μm, 90 μm to 110 μm, 110 μm to 130 μm, 130 μm to 150 μm, 60 μm to 80 μm, 80 μm to 100 μm, 60 μm to 140 μm, 50 μm to 100 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 enable the dosage of the binder to be controlled at a lower level, and the binding performance is not excessively and negatively affected, 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.

In this application, the Dv50 particle size of the polymer may be tested by methods known in the art, for example, by weighing 0.1g to 0.13g of the polymer powder in a 50ml beaker, adding 5g of absolute ethanol to the beaker containing the polymer powder, placing a stirrer having a length of about 2.5mm, and sealing with a preservative film, by referring to GB/T19077-2016 particle size distribution laser diffraction method. The sample is placed in an ultrasonic machine for ultrasonic treatment for 5 minutes, transferred to a magnetic stirrer and stirred at a speed of 500 revolutions per minute for more than 20 minutes, and measured by a laser particle size analyzer, such as a Mastersizer 2000E laser particle size analyzer from Markov instruments, UK.

In some embodiments, the crystallinity of the polymer is 34% to 42%.

In some embodiments, the crystallinity of the polymer may be selected from any of 34% to 36%, 35% to 37%, 36% to 38%, 38% to 40%, 40% to 42%, 39% to 40%, 40% to 41%, 41% to 42%, 35% to 40%.

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. Illustratively, 0.5g of polymer is placed in an aluminum dry pan, shaken flat, covered with a crucible lid, purged under a nitrogen atmosphere at 50 ml/min, warmed at a rate of 10 ℃ per minute with a shielding gas of 70 ml/min, tested at a temperature ranging from-100 ℃ to 400 ℃ using a Differential Scanning Calorimeter (DSC) of american TA instruments model Discovery 250 and heat history is eliminated.

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

In some embodiments, the viscosity of a gum solution containing 4% by mass of the polymer prepared by dissolving the polymer in N-methylpyrrolidone is 2400 mPas to 5000 mPas. In some embodiments, the viscosity of a dope containing 4% by mass of the polymer prepared by dissolving the polymer in N-methylpyrrolidone is any one of 2400 mPa-s to 3000 mPa-s, 3000 mPa-s to 3300 mPa-s, 3300 mPa-s to 3500 mPa-s, 3500 mPa-s to 3800 mPa-s, 3800 mPa-s to 4000 mPa-s, 4000 mPa-s to 4200 mPa-s, 4200 mPa-s to 4600 mPa-s, 4600 mPa-s to 4750 mPa-s, 3100 mPa-s to 3400 mPa-s, 3400 mPa-s to 3800 mPa-s, 3800 mPa-s to 4600 mPa-s, 2500 mPa-s to 4000 mPa-s.

If the viscosity of the polymer glue solution is too high, the viscosity of the adhesive containing the polymer is too high, the adhesive is difficult to stir, the dispersibility of the adhesive is reduced, the flexibility of the pole piece is affected, and meanwhile, the speed of the pulping process is reduced due to the too high viscosity of the adhesive; if the viscosity of the polymer dope is too small, the viscosity of the binder containing the polymer is too small, and the binding force of the pole piece is lowered.

In addition, when preparing the positive electrode slurry, the binder needs to have certain viscosity to prevent the sedimentation of the positive electrode active material, the conductive agent and other auxiliary agents, so that the slurry can be placed more stably. In the prior art, in order to achieve the viscosity of the adhesive solution of 2500 mPas-4000 mPas, at least 7% of adhesive is needed, and the use amount of the adhesive can be controlled to be 4% based on the total mass of the adhesive solution, so that support is provided for reducing the content of the adhesive in the positive electrode film layer.

The viscosity of the binder solution is controlled within a proper range, so that the pole piece can have excellent flexibility and good bonding performance under the condition of low addition amount of the binder.

In this application, the viscosity of the binder may be measured using methods known in the art, such as rotational viscometer measurements. As an example, 14g of polymer and 336g N-methylpyrrolidone (NMP) were weighed separately in 500ml beakers, dispersed by stirring with a forced high-speed mill at 800 rpm, and after stirring time of 120 minutes, air bubbles were removed by ultrasonic shaking for 30 minutes. At room temperature, using a force technology NDJ-5S rotary viscometer to test, selecting a No. 3 rotor to insert glue solution, ensuring that a rotor liquid level mark is level with the glue solution, testing viscosity at a rotor rotating speed of 12 revolutions per minute, and reading viscosity data after 6 minutes.

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

The preparation method of the adhesive is simple, is environment-friendly and is beneficial to industrial production. Meanwhile, the adhesive prepared by the method enables the pole piece to have excellent flexibility and good adhesive force, and the battery has good cycle retention rate.

In some embodiments, the mass fraction of the monomer of formula iv is 0.5% to 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 III is too low, the adhesive force of the pole piece is reduced, and the cycle performance of the battery is affected.

When the mass fraction of the monomer shown in the formula IV is in a proper range, the adhesive enables the pole piece to have excellent flexibility and good adhesive force, and the battery can keep high cycle capacity retention rate in the cycle process.

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

In some embodiments, the monomer shown in formula III and the monomer shown in formula IV are reacted for 6 hours to 12 hours in a non-reactive gas atmosphere at a reaction pressure of 6MPa to 8MPa and a reaction temperature of 45 ℃ to 60 ℃;

adding a chain transfer agent, stopping the reaction when the pressure in the reaction system is reduced to 2-2.5 MPa, and carrying out solid-liquid separation to keep a solid phase.

The term "non-reactive gas" as used herein refers to a gas that does not react with the reactants in the reaction system, and common non-reactive gases are inert gases such as argon and nitrogen.

In some embodiments, the reaction pressure is one of 6MPa to 6.5MPa, 6.5MPa to 7MPa, 7MPa to 7.5MPa, 7.5MPa to 8MPa, 6MPa to 7MPa, 7MPa to 8 MPa.

In some embodiments, the reaction temperature is one of 45 ℃ to 50 ℃, 50 ℃ to 55 ℃, 55 ℃ to 60 ℃, 45 ℃ to 55 ℃, 50 ℃ to 60 ℃.

In some embodiments, the polymerization reaction time is one of 6 hours to 7 hours, 7 hours to 8 hours, 8 hours to 9 hours, 9 hours to 10 hours, 10 hours to 11 hours, 11 hours to 12 hours, 6 hours to 8 hours, 6 hours to 10 hours.

The polymerization reaction pressure is large, the pressure of the monomer entering the reaction solution is large, the monomer enters the reaction solution more, the occurrence of a large-scale polymerization reaction can be caused, the number of generated polymers is increased, the polydispersity coefficient is large, along with the reduction of the monomer, the lack of the supply of the monomer to the polymers leads to relatively smaller weight average molecular weight of the generated polymers, and the cohesive force of the pole pieces and the battery circulation capacity retention rate are influenced.

The polymerization reaction pressure is smaller, the pressure of the monomer entering the reaction solution is smaller, the reaction monomer cannot be continuously supplemented, the continuous polymerization is not facilitated, the weight average molecular weight of the prepared polymer is too low, the requirement on the cohesive force cannot be met, and the battery cycle performance is also reduced.

The polymerization reaction temperature is lower, the promotion force of copolymerization is smaller, the polymerization reaction is insufficient, the weight average molecular weight of the prepared polymer is smaller, the cohesive force is greatly reduced, and the cycle performance is obviously reduced.

The polymerization reaction temperature is higher, so that a large-scale polymerization reaction can be caused, the quantity of generated polymers is increased, and as the monomer is reduced, the polymers lack of supply of the monomer, so that the weight average molecular weight of the generated polymers is relatively smaller, and the adhesive force of the pole piece and the battery cycle capacity retention rate are influenced.

The polymerization reaction time is short, the polymerization reaction cannot be continuously carried out, the weight average molecular weight of the prepared polymer is smaller, and the cohesive force and the cycle performance are also reduced.

The polymerization reaction time is longer, the pressure is reduced along with the continuous consumption of the monomer, the condition that the polymerization reaction can occur cannot be reached, the polymerization reaction can not be continuously carried out after the reaction time is prolonged, and the production efficiency is reduced.

The reaction pressure, reaction temperature and reaction time of the polymerization reaction are controlled within proper ranges, and the weight average molecular weight of the polymer can be controlled, so that the pole piece has excellent binding force, and the battery has better cycle capacity retention rate in the cycle process.

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

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, and the amount of chain transfer agent may also be 2% or 2.5%, for example. The amount of chain transfer agent is controlled within a suitable range, which enables the polymer chain length to be controlled, thereby obtaining a polymer of a suitable weight average molecular weight range.

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

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

adding an initiator and a pH regulator into a container, regulating the pH value to 6.5-7, then adding a monomer shown in a formula III and a monomer shown in a formula IV,

the pressure in the container reaches 6MPa to 8MPa;

stirring for 30-60 min, heating to 45-60 deg.C, and polymerizing.

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 prepared polymer has more proper polydispersity, crystallinity and particle size.

In some embodiments, the solvent is used in an amount of 2 to 8 times the total mass of the monomer of formula III and the monomer of formula IV. The amount of 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 solvent is an aqueous solvent.

In some embodiments, the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol; optionally, 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; alternatively, 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 initiator may also be used in an amount of, for example, 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 pH adjustor is 0.05% to 0.2% of the total mass of the monomer of formula III and the monomer of 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.

[ 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% to 1% based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of binder is 0.8% to 0.85%, 0.85% to 0.9%, 0.9% to 0.95%, 0.95% to 1%, 0.85% to 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.

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.

In some embodiments, the positive electrode active material is a lithium-containing transition metal oxide. In some embodiments, the positive electrode active material is at least one of lithium iron phosphate, lithium nickel cobalt manganese oxide, a doping modification material for lithium iron phosphate, a doping modification material for lithium nickel cobalt manganese oxide, or a conductive carbon coating modification material, a conductive metal coating modification material, a conductive polymer coating modification material thereof.

In some embodiments, a method for preparing a positive electrode sheet is provided, including the steps of: the first stage: mixing the positive electrode active material, the conductive agent, and the binder in any embodiment or the binder prepared by the preparation method in any embodiment, and performing first stirring; and a second stage: adding a solvent to carry out second stirring; and a third stage: adding a dispersing agent to perform third stirring to obtain slurry, and controlling the viscosity of the slurry to be 8000-15000 mpa.s; fourth stage: and coating slurry on the positive current collector to obtain a positive plate.

The preparation method is simple and is beneficial to industrial production. By the preparation method, sedimentation of the high molecular weight binder in the slurry can be reduced, and slurry quality and uniformity of the pole piece are improved.

In some embodiments, in the first agitation, the agitation revolution speed is 25 revolutions per minute and the agitation time is 30 minutes.

In some embodiments, in the second stirring, the stirring revolution speed is 25 rpm, the stirring rotation speed is 800 to 1000 rpm, and the stirring time is 50 to 80 minutes.

In some embodiments, in the third stirring, the stirring autorotation speed is 1200 to 1500 minutes and the stirring time is 50 to 70 minutes.

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 materialsOther conventional materials that can be used as a battery positive electrode active material can also 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.

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. 3 is a secondary battery 5 of a square structure as one example.

In some embodiments, referring to fig. 4, the outer package may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and a side plate connected to the bottom plate, where the bottom plate and the side plate enclose a receiving chamber. The housing 51 has an opening communicating with the accommodation chamber, and the cover plate 53 can be provided to cover the opening to close the accommodation chamber. The positive electrode tab, the negative electrode tab, and the separator may be formed into the electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is enclosed in the accommodating chamber. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and those skilled in the art may select according to specific practical requirements.

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

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

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

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

Fig. 6 and 7 are battery packs 1 as an example. Referring to fig. 6 and 7, a battery case and a plurality of battery modules 4 disposed in the battery case may be included in the battery pack 1. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be covered on the lower box body 3 and forms a closed space for accommodating the battery module 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

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

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

Fig. 8 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

Into a 10L autoclave were charged 4kg of deionized water and 2g of methyl cellulose ether, evacuated and purged with N ₂ Replacement O ₂ Three times, 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate are added again and charged0.94kg of vinylidene fluoride and 0.06kg of chlorotrifluoroethylene are mixed and stirred for 30min, the temperature is raised to 45 ℃, 30g of cyclohexane is added for continuous reaction after 6h of reaction, and the reaction is stopped when the pressure in the reaction kettle is reduced to 2 MPa. And centrifuging the reaction system, collecting a solid phase, washing and drying to obtain the vinylidene fluoride-chlorotrifluoroethylene polymer.

2) Preparation of positive electrode plate

3990g of lithium iron phosphate, 40.8g of vinylidene fluoride-chlorotrifluoroethylene copolymer binder and 49.4g of acetylene black are placed in a planetary stirring tank, and the revolution speed is 25r/min, and the stirring is carried out for 30min, wherein the mass fraction of the binder is 1% based on the total mass of the positive electrode film layer;

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

adding 12.3g of dispersing agent into a stirring tank, stirring for 60min at revolution speed of 25r/min and rotation speed of 1250 r/min;

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

If the viscosity is higher, NMP solution is added to reduce the viscosity to the above viscosity interval, and then the positive electrode slurry is obtained according to revolution speed of 25r/min and rotation speed of 1250r/min and stirring for 30 min. The prepared positive electrode slurry is scraped on a carbon-coated aluminum foil, and the single side of the scraping weight is 500 mg/(1540 mm) ² ) Baking at 110deg.C for 15min, cold pressing to obtain a pressed density of 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 6

Substantially the same as in example 1, except that the polymerization times were adjusted to 8h, 10h, 11h, 11.5h, 12h, respectively, and the mass of cyclohexane was adjusted to 25g, 20g, 18.5g, 16.5g, 15g, respectively, the specific parameters are shown in Table 1.

Examples 7 to 10

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 11 to 14

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 15 to 16

Substantially the same as in example 1, except that 0.06kg of chlorotrifluoroethylene was replaced with 0.03kg of chlorotrifluoroethylene and 0.03kg of tetrafluoroethylene, 0.02kg of chlorotrifluoroethylene, 0.02kg of tetrafluoroethylene and 0.02kg of hexafluoropropylene, respectively.

Examples 17 to 18

Substantially the same as in example 3, except that 0.06kg of chlorotrifluoroethylene was replaced with 0.03kg of chlorotrifluoroethylene and 0.03kg of tetrafluoroethylene, 0.02kg of chlorotrifluoroethylene, 0.02kg of tetrafluoroethylene and 0.02kg of hexafluoropropylene, respectively.

Examples 19 to 20

Substantially the same as in example 12, except that 0.06kg of chlorotrifluoroethylene was replaced with 0.03kg of chlorotrifluoroethylene and 0.03kg of tetrafluoroethylene, 0.02kg of chlorotrifluoroethylene, 0.02kg of tetrafluoroethylene and 0.02kg of hexafluoropropylene, respectively.

Comparative example 1

Substantially the same as in example 1, the reaction pressure of the polymerization reaction was adjusted to 5MPa, the reaction temperature was adjusted to 38℃and the reaction time was adjusted to 3 hours, and the mass of cyclohexane was adjusted to 42g, and the specific parameters are shown in Table 1.

Comparative example 2

Substantially the same as in example 1, except that the polymerization temperature was adjusted to 35℃and the mass of cyclohexane was adjusted to 40g, only 1kg of vinylidene fluoride monomer was polymerized, and the specific parameters are shown in Table 1.

Comparative example 3

Substantially the same as in example 1, except that the reaction time of the polymerization reaction was adjusted to 5 hours, the mass of cyclohexane was adjusted to 36g, and only 1kg of vinylidene fluoride was polymerized as a monomer, the specific parameters are shown in Table 1.

Comparative example 4

Substantially the same as in example 1, the binder was polyvinylidene fluoride, purchased from the east yang optical company under the model number 701A, 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.

Comparative example 5

Substantially the same as in example 1, except that only 1kg of vinylidene fluoride was polymerized, the specific parameters are shown in Table 1.

Comparative example 6

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 7

Substantially the same as in example 12, the polymerization monomer was only 1kg of vinylidene fluoride, and the specific parameters are shown in Table 1.

2. Performance testing

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 300mm+Styragel HT4) was selected. Preparing 3.0% binder glue solution with purified N-methyl pyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. During the test, tetrahydrofuran is firstly sucked by a syringe, and then the syringe is washed, and the test is repeated for several times. Then, 5ml of the test solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And after the indication is stable, acquiring data, and reading the weight average molecular weight.

2) Polydisperse coefficient testing

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

3) Dv50 test

With reference to GB/T19077-2016 particle size distribution laser diffraction method, weighing 0.1 g-0.13 g of binder powder with a 50ml beaker, adding 5g of absolute ethyl alcohol into the beaker filled with the binder powder, placing a stirrer with the length of about 2.5mm, and sealing with a preservative film. And (3) putting the sample into an ultrasonic machine for ultrasonic treatment for 5min, transferring the sample into a magnetic stirrer, stirring the sample for more than 20min at a speed of 500r/min, and taking 2 samples from each batch of products for testing and averaging. The measurement is performed using a laser particle size analyzer, such as a Mastersizer2000E laser particle size analyzer from malvern instruments, england.

4) Crystallinity test

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

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

5) Adhesive viscosity test

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

2. Pole piece performance test

1) Average roll count test

The cold-pressed pole piece is sampled along the transverse direction, and can be 4cm in transverse width, 25cm in longitudinal length and 100cm in area ² The method comprises the steps of (1) doubling up samples in a longitudinal direction of 25cm, pre-doubling up the samples, placing the pre-doubled up samples on a plane of an experiment table, rolling for 1 time by using a 2kg cylinder roller, checking whether light leakage points exist or not by using light, recording rolling times without the light leakage points, reversely folding the experimental samples along the longitudinal crease, observing the crease pair until the light leakage points appear and recording rolling times n1. Repeating the above operation to obtain the light transmission rolling times n2 and n3 of the second strip and the third strip, and calculating the average rolling times= (n1+n2+n3)/3.

2) 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 pole piece 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.

A paper tape with the width equal to the width of the pole piece sample 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 sample, 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 force-displacement diagram of example 1 and comparative example 4 shown in fig. 1 was obtained by dividing the force of the pole piece when the force of the pole piece was balanced by the width of the adhesive tape as the adhesive force of the pole piece per unit length to characterize the adhesive strength between the positive electrode film layer and the current collector.

3. Battery performance test

1) Battery capacity retention test

The battery capacity retention test procedure was as follows: at 25 ℃, the button cell was charged to 3.65V at a constant current of 1/3C, then charged to 0.05C at a constant voltage of 3.65V, left for 5min, then discharged to 2.5V at 1/3C, and the resulting capacity was recorded as initial capacity C0. Repeating the above steps for the same battery, and recording the discharge capacity Cn of the battery after the nth cycle, wherein the battery capacity retention ratio Pn= (Cn/C0) ×100% after each cycle takes 500 point values of P1 and P2 … … P500 as ordinate and the corresponding cycle times as abscissa, to obtain the graph of the battery capacity retention ratio and the cycle times. The battery capacity retention rate versus cycle number graphs for example 1 and comparative example 4 shown in fig. 2.

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 20 or comparative examples 1 to 7 in table 1 are data measured after 500 cycles under the above test conditions, i.e., P500 values.

The results of performance tests on the binders, pole pieces and batteries obtained in examples 1 to 20 and comparative examples 1 to 7 described above are shown in table 1.

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 for examples 1 to 20 and comparative examples 1 to 7

/>

Fig. 1 is a graph showing the adhesion force versus displacement of example 1 and comparative example 4, and it can be seen from the graph that the adhesion force of example 1 is significantly higher than that of comparative example 4 at the same displacement, indicating that the adhesive of the vinylidene fluoride-chlorotrifluoroethylene copolymer provides the pole piece with excellent adhesion force at a lower adhesive addition amount. Fig. 2 is a graph of the battery capacity retention rate versus the cycle number of the example 1 and the comparative example 4, and it can be seen from the graph that the cycle capacity retention rate of the example 1 is significantly higher than that of the comparative example 4 after the battery is cycled 500 times, which indicates that the vinylidene fluoride-chlorotrifluoroethylene copolymer binder can improve the cycle capacity retention rate of the battery during the cycling process with a lower binder addition, and effectively improve the pole piece and the battery performance damage caused by the high binder use in the conventional technology.

From the above results, it is understood that the binders in examples 1 to 20 each comprise a polymer comprising a structural unit derived from vinylidene fluoride and further comprising any one of structural units derived from chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, and the weight average molecular weight of the polymer is 180 to 500 tens of thousands. From the comparison of examples 1 to 10, examples 15 to 18 and comparative example 1, and from the comparison of example 12, example 19, example 20 and comparative examples 2 to 3, it is apparent that the use of the polymer having a weight average molecular weight of 180 to 500 ten thousand as a binder can provide a pole piece having excellent adhesion and flexibility at a low addition amount, thereby improving the capacity retention rate of the battery during the cycle.

From the comparison of example 1, examples 6 to 10, examples 15 to 16 and comparative example 5, and from the comparison of example 3, examples 17 to 18 and comparative example 6, it is apparent from the comparison of example 12, examples 19 to 20 and comparative example 7 that the introduction of the comonomer in the high molecular weight binder can improve the flexibility of the pole piece, reduce the risk of breakage or light leakage during winding and hot pressing processes, and improve the safety performance of the battery without significantly reducing the adhesive force of the pole piece.

Examples 1 to 20 compared with comparative example 4, the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer or vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer binder having a weight average molecular weight of 150 to 500 tens of thousands of the binder provided that the pole piece has excellent flexibility and adhesion, thereby improving the capacity retention rate of the battery during the cycle, and effectively improving the damage of the pole piece and battery performance caused by the high-usage of the binder in the conventional art.

As is apparent from the comparison of examples 1, examples 7 to 9 and example 10, when the mass fraction of the chlorotrifluoroethylene in the vinylidene fluoride-chlorotrifluoroethylene copolymer is 0.5% to 15%, the binder gives the pole piece both excellent flexibility and good adhesion, so that the battery maintains good capacity performance during the cycle, based on the total mass timing of the vinylidene fluoride-chlorotrifluoroethylene copolymer.

As is apparent from examples 1 to 20, the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer or vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer binder having a polydispersity of 2 to 2.3 can improve the flexibility of the pole piece and provide the battery with good adhesion. From examples 1 to 6, it is known that the vinylidene fluoride-chlorotrifluoroethylene copolymer binder having a polydispersity of 2.1 to 2.2 can improve the flexibility of the electrode sheet and provide the battery with good adhesion.

From examples 1 to 20, it is known that a vinylidene fluoride-chlorotrifluoroethylene copolymer or a vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer or a vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer or a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer binder having a Dv50 particle diameter of 50 μm to 160 μm can improve the flexibility of the pole piece and provide a battery with good adhesion. As is clear from the comparison of examples 1 to 3 and examples 4 to 6, when the vinylidene fluoride-chlorotrifluoroethylene copolymer has a Dv50 particle diameter of 50 μm to 100. Mu.m, the flexibility of the pole piece can be further improved.

From examples 1 to 20, it is known that the vinylidene fluoride-chlorotrifluoroethylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer or vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer or vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer binder having a crystallinity of 34% to 42% can improve the flexibility of the pole piece and provide the battery with good adhesion. As is apparent from comparison of examples 1, 2 and 3 to 6, and comparison of examples 1, 9 and 7 to 8 and 10, the binder can improve flexibility of the electrode sheet and provide good adhesion to the battery when the crystallinity of the vinylidene fluoride-chlorotrifluoroethylene copolymer is 35 to 40%.

From examples 1 to 20, it was found that when the vinylidene fluoride-chlorotrifluoroethylene copolymer or the vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer or the vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer or the vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer binder was dissolved in N-methylpyrrolidone, the viscosity of the binder solution having a mass content of 4% was 2400 mPas to 5000 mPas. The adhesive has enough viscosity under the condition of low addition amount, so that the pole piece can be ensured to have enough adhesive force under the condition of low addition of the adhesive.

From the comparison of examples 1, 7 to 9 and 10, examples 1 to 4 and examples 5 to 6, it is understood that when the vinylidene fluoride-chlorotrifluoroethylene copolymer is dissolved in N-methylpyrrolidone, the viscosity of the binder solution obtained is 2500 mPas to 4000 mPas, and the mass percentage of the binder in the binder solution is 4%. The adhesive has enough viscosity under the condition of low addition amount, and can ensure the adhesive force of the pole piece under the condition of low addition amount of the adhesive.

As is clear from the comparison between examples 1 to 6 and comparative example 1, when the reaction pressure of the polymerization reaction of vinylidene fluoride and chlorotrifluoroethylene is 6MPa to 8MPa, the reaction temperature is 45 ℃ to 60 ℃ and the reaction time is 6h to 12h, the molecular weight of the prepared vinylidene fluoride-chlorotrifluoroethylene copolymer is 180 ten thousand to 500 ten thousand, and the adhesive can enable the pole piece to have excellent flexibility and adhesive force and can improve the capacity retention rate of the battery in the circulating process.

From the comparison of examples 6, 12 to 13 and 11 and 14, when the mass fraction of the binder of the vinylidene fluoride-chlorotrifluoroethylene copolymer is 0.8% to 1%, the binder can provide the pole piece with excellent flexibility and adhesion based on the total mass of the positive electrode film layer.

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. A binder is characterized in that the binder is a polymer containing structural units shown in the formula I and the formula II,

wherein R is ₁ One or more selected from fluorine, chlorine and trifluoromethyl, wherein the weight average molecular weight of the polymer is 180-500 ten thousand.

2. The binder of claim 1 wherein the structural unit of formula II has a mass fraction of 0.5% to 15% based on the total mass of the polymer.

3. The binder of claim 1 wherein the polymer has a polydispersity of 2 to 2.3.

4. A binder according to any one of claims 1 to 3, wherein the polymer has a Dv50 particle size of 50 μm to 160 μm.

5. A binder according to any one of claims 1 to 3 wherein the crystallinity of the polymer is 34% to 42%.

6. A binder according to any one of claims 1 to 3, wherein the polymer is dissolved in N-methylpyrrolidone to obtain a dope containing 4% by mass of the polymer having a viscosity of 2400 mPa-s to 5000 mPa-s.

7. A binder according to any one of claims 1 to 3 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.

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

wherein R is ₂ One or more selected from fluorine, chlorine and trifluoromethyl, wherein the weight average molecular weight of the polymer is 180-500 ten thousand.

9. The preparation method according to claim 8, wherein the mass content of the monomer represented by formula IV is 0.5% to 15% based on the total mass of the monomers represented by formula III and formula IV.

10. The preparation method of claim 8, wherein the monomer shown in the formula IV is one or more of chlorotrifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

11. The preparation method according to any one of claims 8 to 10, characterized in that the polymerization reaction comprises the steps of:

the monomer shown in the formula III and the formula IV reacts for 6 hours to 12 hours in a non-reactive gas atmosphere at a reaction pressure of 6MPa to 8MPa and a reaction temperature of 45 ℃ to 60 ℃;

12. The production method according to any one of claims 8 to 10, characterized in that the production method further 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 6-8 MPa;

stirring for 30-60 min, heating to 45-60 deg.C, and polymerizing.

13. A positive electrode slurry, characterized in that the positive electrode slurry comprises the binder according to any one of claims 1 to 7 or the binder prepared by the preparation method according to any one of claims 8 to 12.

14. The positive electrode slurry according to claim 13, wherein the mass fraction of the binder is 0.8% to 1% based on the total mass of solid matters in the positive electrode slurry.

15. The positive electrode slurry according to claim 13 or 14, further comprising a positive electrode active material that is a lithium-containing transition metal oxide.

16. The positive electrode slurry according to claim 15, wherein the positive electrode active material is at least one of lithium iron phosphate and its modified material, lithium nickel cobalt manganese oxide and its modified material, and the modified material is prepared by one or more modification modes of doping, conductive carbon coating, conductive metal coating, conductive polymer coating.

17. A secondary battery comprising an electrode assembly and an electrolyte, the electrode assembly comprising a separator, a negative electrode tab, and a positive electrode tab prepared from the positive electrode slurry of any one of claims 13 to 16.

18. An electric device comprising the secondary battery according to claim 17.