CN117638072A

CN117638072A - Fluoropolymer, method for producing the same, use of the same, binder composition, secondary battery, battery module, battery pack, and electric device

Info

Publication number: CN117638072A
Application number: CN202310272259.0A
Authority: CN
Inventors: 李�诚; 曾子鹏; 刘会会; 王景明
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2022-08-30
Filing date: 2022-08-30
Publication date: 2024-03-01
Also published as: WO2024045644A1; CN115124638A

Abstract

Provided are a fluoropolymer, a method of preparing the same, a use of the same, a binder composition, a secondary battery, a battery module, a battery pack, and an electric device. The fluorine-containing polymer is a polymer containing structural units shown in a formula I, and the weight average molecular weight of the polymer is 2-15 ten thousand, wherein R ₁ 、R ₂ Each independently selected from hydrogen, fluorine, chlorine or trifluoromethyl. The fluorine-containing polymerThe compound can reduce the crystallinity and melting enthalpy of the binder, so that the flexibility of the pole piece is improved, the fluorine-containing polymer can also improve the dispersibility of battery slurry, the uniform distribution of the positive electrode active material in the pole piece is facilitated, the film resistance of the pole piece is reduced, and the conductivity of the pole piece and the cycle performance of the battery are improved.

Description

Fluoropolymer, method for producing the same, use of the same, binder composition, secondary battery, battery module, battery pack, and electric device

Technical Field

The application relates to the technical field of secondary batteries, in particular to a fluorine-containing polymer, a preparation method and application thereof, an adhesive composition, a secondary battery, a battery module, a battery pack and an electric device.

Background

In recent years, as the application range of secondary batteries is becoming wider, secondary batteries are widely used in energy storage power 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 vehicles, military equipment, aerospace and the like.

Along with the continuous improvement of the requirements of the market on the battery endurance, the loading capacity of the positive electrode active material in the battery manufacturing process is continuously improved, and the surface density and the compaction density are continuously improved so as to meet the requirements on the battery energy density. However, the problem of pole piece brittleness is caused by the improvement of the compaction density of the battery pole piece, and how to improve the brittleness of the pole piece on the basis of ensuring the compaction density of the battery is a problem to be solved at present.

Disclosure of Invention

The present application has been made in view of the above problems, and a first aspect of the present application is to provide a fluoropolymer which is a polymer containing a structural unit represented by formula I and has a weight average molecular weight of 2 to 15 tens of thousands,

wherein R is ₁ 、R ₂ Each independently selected from hydrogen, fluorine, chlorine or trifluoromethyl.

The fluorine-containing polymer provided by the application uses the polymer with the weight average molecular weight of 2-15 ten thousand and containing the structural unit of the formula I, and the fluorine-containing polymer can reduce the crystallinity and the melting enthalpy of the binder, so that the flexibility of the pole piece is improved; the fluorine-containing polymer can also improve the dispersibility of battery slurry, is beneficial to the uniform distribution of active substances in the pole piece, further reduces the film resistance of the pole piece, and improves the conductivity of the pole piece and the cycle performance of the battery.

In any embodiment, the polymer is selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene.

Among the polymers containing structural units shown in the formula I, the polymers with the weight average molecular weight of 2-15 ten thousand have better adhesion effect with the positive electrode active material, are beneficial to improving the uniformity of the dispersion of the positive electrode active material in the pole piece, and enable the positive electrode active material to be uniformly distributed on the surface of the pole piece; meanwhile, the polymer with the molecular weight can be inserted into a regular chain segment of the binder molecules, so that the ordering of the binder molecules is disturbed, the crystallinity is reduced, and the flexibility of the pole piece is improved.

In any embodiment, the particles of the fluoropolymer have a median particle diameter Dv50 of 1 to 4 μm. The polymer particles in the particle size range are beneficial to the dissolution of the polymer in the solvent of the positive electrode slurry, such as N-methyl pyrrolidone, and reduce the processing difficulty of the positive electrode slurry.

In any embodiment, when the fluoropolymer is dissolved in N-methylpyrrolidone to prepare a dope having a mass% of 7%, the viscosity of the prepared dope is 50 to 200 mPas.

In any embodiment, when the fluoropolymer is dissolved in N-methylpyrrolidone to prepare a dope having a mass% of 7%, the viscosity of the dope is 50 to 180 mPas.

Fluoropolymers in this viscosity range are easily mixed with the binder and short chains of fluoropolymer molecules intercalate into long chains of the binder molecules to form an irregular mixture, reducing the crystalline regularity of the fluoropolymer and the binder; and the method is also beneficial to the full adhesion of the fluorine-containing polymer and the positive electrode active material, reduces the phenomena of agglomeration of the positive electrode active material, blockage of a filter screen and the like, improves the dispersion performance of the slurry and is beneficial to improving the solid content of the slurry.

In a second aspect the present application provides a method of preparing a fluoropolymer,

at least one monomer of the formula II is provided,

wherein R is ₁ 、R ₂ Each independently selected from hydrogen, fluorine, chlorine or trifluoromethyl;

polymerizing the monomer under polymerization conditions to prepare a polymer, wherein the weight average molecular weight of the polymer is 2-15 ten thousand.

In the preparation method provided by the application, short chains of prepared fluorine-containing polymer molecules can be inserted into long chains of binder molecules to form a random mixture, so that the crystallinity of the binder is reduced; meanwhile, the fluorine-containing polymer has good adhesion with the positive electrode active material, and aggregation among particles of the positive electrode active material, such as lithium iron phosphate (LFP) powder or lithium nickel cobalt manganese oxide (NCM), is avoided through steric hindrance of the polymer, so that the stability of the slurry is increased.

In any embodiment, the method of making further comprises the steps of:

at least one monomer shown in the formula II is polymerized for 2-8 hours in a non-reactive gas atmosphere at normal pressure and a reaction temperature of 55-75 ℃, the reaction is stopped, solid-liquid separation is carried out, and a solid phase is reserved.

In any embodiment, the method of making further comprises the steps of:

adding a solvent and a dispersing agent into a container, and filling the container with a non-reactive gas; adding an initiator and a pH regulator into the container, regulating the pH value to 6.5-7, adding a monomer shown in a formula II, stirring for 0.5-1 hour, and heating to 55-75 ℃ to perform polymerization reaction.

In the above preparation method, the polymer containing the structural unit shown in the formula I provided in the first aspect of the application can be obtained under the selected conditions. The preparation method has low cost of raw materials and relatively mild reaction conditions, and is beneficial to mass production of fluorine-containing polymers.

A third aspect of the present application provides a binder composition comprising a binder and a fluoropolymer as described in the first aspect of the present application.

Binders, such as PVDF binders, are polymers with regular short chain branches, with ordered arrangement of molecules, resulting in high crystallinity. The chain length of the fluorine-containing polymer is shorter than that of the adhesive, and small molecules of the fluorine-containing polymer are inserted into regular chain segments of macromolecules of the adhesive in the adhesive composition, so that the order of the fluorine-containing polymer is disturbed, the crystallinity of the adhesive is reduced, and the flexibility of the pole piece is improved; meanwhile, the fluorine-containing polymer also improves the adhesive force to the positive electrode active material, improves the dispersibility, stability and processability of the positive electrode slurry, and is beneficial to preparing the positive electrode plate with high pressure density and high surface density.

In any embodiment, in the binder composition, the binder is a crystalline polymer, optionally polyvinylidene fluoride having a weight average molecular weight of 70 to 110 tens of thousands.

The weight average molecular weight of the binder is controlled, so that the stability and the processability of the positive electrode slurry and the binding force of the positive electrode plate are improved, and the cycle internal resistance increase rate of the battery is further reduced.

In any embodiment, the mass ratio of the fluoropolymer to the binder is from 0.05:1 to 5:1.

In any embodiment, the mass ratio of the fluoropolymer to the binder is from 0.2:1 to 4:1.

In any embodiment, the mass ratio of the fluoropolymer to the binder is from 0.5:1 to 1:1.

The mass ratio of the fluorine-containing polymer to the binder is in a proper range, which is favorable for fully mixing the fluorine-containing polymer and the binder, and is favorable for inserting fluorine-containing polymer molecules into the binder molecules to form a mixture with random arrangement and reduced structural order of the binder molecules, thereby reducing the brittleness of the pole piece and improving the flexibility of the pole piece. In addition, the mass ratio is in a proper range, so that the adhesion and the bonding between solid matters in the positive electrode slurry are facilitated, the positive electrode active material and the conductive agent are stably connected, the pole piece has good adhesive force, the direct contact between the positive electrode active material and the electrolyte can be avoided, and the occurrence of side reaction is reduced.

In any embodiment, the binder composition has a crystallinity of 10% to 45%, alternatively 10% to 40%.

The crystallinity of the adhesive composition is in a proper range, so that the pole piece has improved flexibility on the premise of ensuring certain adhesive force, and the processing process of the pole piece is facilitated.

In any embodiment, the binder composition has a melting enthalpy of 10 to 50J/g, alternatively 13 to 45J/g.

The fusion enthalpy of the adhesive composition is in a proper range, so that moderate crystallinity of the adhesive in the composition can be ensured, and the prepared pole piece has excellent flexibility and stability.

A fourth aspect of the present application provides the use of a fluoropolymer according to the first aspect or a binder composition according to the third aspect in a secondary battery. In the adhesive composition, the fluorine-containing polymer can disturb the order of adhesive molecules so as to reduce the crystallinity of the adhesive, and the flexibility of the pole piece is improved; meanwhile, the fluorine-containing polymer also improves the adhesive force to the positive electrode active material, improves the dispersibility, stability and processability of the positive electrode slurry, and is beneficial to preparing the positive electrode plate with high pressure density and high surface density.

In any embodiment, the fluoropolymer or the fluoropolymer in the binder composition is for use as a battery paste flexibilizer.

A fifth aspect of the present application provides a secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, the positive electrode sheet comprising a positive electrode active material, a conductive agent, and the binder composition of the third aspect of the present application.

In any embodiment, in the positive electrode sheet of the secondary battery, the mass ratio of the binder composition to the positive electrode active material is 1:100 to 3.6:100, optionally 1.6:100 to 2.4:100.

In any embodiment, the secondary battery, the positive electrode active material is a lithium-containing transition metal oxide, and may be selected from lithium iron phosphate or lithium nickel cobalt manganese oxide, or a doping modified material thereof, or at least one of a conductive carbon coating modified material, a conductive metal coating modified material, or a conductive polymer coating modified material thereof.

A sixth aspect of the present application provides a battery module comprising the secondary battery according to the fifth aspect of the present application.

A seventh aspect of the present application provides a battery pack comprising the battery module of the sixth aspect of the present application.

An eighth aspect of the present application provides an electric device including at least one selected from the secondary battery according to the fifth aspect of the present application, the battery module according to the sixth aspect of the present application, or the battery pack according to the seventh aspect of the present application.

Drawings

FIG. 1 is a schematic diagram of a fracture location of a battery pole piece when a crack fracture occurs;

FIG. 2 is a schematic diagram of a theory of crack fracture of a battery pole piece;

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; 61 film layers; 62 current collector.

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).

The energy density of the secondary battery can be improved by improving the compaction density of the battery pole piece, however, after the compaction density of the battery pole piece is improved, when the battery is processed, hot-pressed and shaped, the film layer at the corner of the cathode of the innermost ring is easily stretched out due to insufficient tension, so that the pole piece is light-transmitting, and the problem of brittleness (or brittle failure) of the pole piece is caused.

[ fluoropolymer ]

Based on this, the present application provides a fluorine-containing polymer which is a polymer containing a structural unit represented by the formula I and has a weight average molecular weight of 2 to 15 tens of thousands,

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 or mixtures which can be obtained by reaction, for example addition or substitution, of functional groups in the macromolecules mentioned above and which can be chemically homogeneous or chemically inhomogeneous.

In some embodiments, the fluoropolymer is used in battery paste as a polymer with a softening effect to enhance the flexibility of the pole piece. In some embodiments, the fluoropolymer is used in a battery positive electrode slurry to increase the flexibility of the positive electrode sheet. In some embodiments, the fluoropolymer is used in a battery negative electrode slurry to increase the flexibility of the negative electrode tab.

Herein, the term "positive electrode" also refers to the "cathode" in the battery. The term "negative electrode" also refers to the "anode" in a battery.

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 the pole piece processing process, the slurry is formed into a film layer 61 to be attached to a current collector 62 after the processing steps including cold pressing, as shown in fig. 1, the pole piece (or a bare cell) is easy to generate cracks at the corners of the innermost 1-2 circles of cathodes during hot pressing and shaping, so that the pole piece is easy to generate a light transmission phenomenon, the inner film layer is compressed during cell shaping, positive electrode active material particles on the inner film layer are subjected to extrusion deformation in a constraint space to generate extrusion force on the outer film layer, and the outer film layer is stretched due to insufficient plastic deformation stress or insufficient tension resistance, and is called crack fracture.

FIG. 2 is a schematic diagram showing the theory of crack fracture of a pole piece, wherein the stress point of the pole piece is subjected to a binding force F from the tangential direction of a film layer when the pole piece is bent ₁ And F ₂ ，F ₁ Decomposed into component forces F in the vertical direction ₁₁ And a component force F in the horizontal direction ₁₂ ；F ₂ Decomposed into component forces F in the vertical direction ₂₁ And a component force F in the horizontal direction ₂₂ . Wherein F is ₁₂ And F is equal to ₂₂ The directions of the forces are the same, and jointly form a force F for extruding the positive electrode active material particles of the inner film layer _Extrusion The method comprises the steps of carrying out a first treatment on the surface of the Correspondingly, the outer film layer is also subjected to the pressing force F _Extrusion . When F _Extrusion When the deformation amount exceeds the elongation at break of the film base material, the current collector generates crack fracture. The crack fracture of the pole piece causes the pole piece to be exposed out of fresh aluminum foil while the powder is dropped. With the circulation, the electrolyte can decompose to generate hydrofluoric acid, and the hydrofluoric acid can corrode the aluminum foil, so that the electrochemical performance and the circulation performance of the battery can be reduced.

Herein, the term "current collector" refers to any conductive substrate capable of conducting current to an electrode during discharge or charge of a secondary battery.

The term "film layer" refers to a coating layer formed after the positive electrode or negative electrode slurry is coated and dried.

Without being bound by any theory, the polymer containing the structural unit shown in the formula I has better adhesion between the polymer with the weight average molecular weight of 2-15 ten thousand and the positive electrode active material, prevents/reduces aggregation between the positive electrode active materials, is beneficial to improving the dispersion performance of the positive electrode slurry, and ensures that the positive electrode active material is uniformly distributed on the surface of the pole piece; meanwhile, the polymer with the molecular weight can be inserted into a regular chain segment of the binder molecules, so that the ordering of the binder molecules is disturbed, and the crystallization is reduced, thereby improving the flexibility of the pole piece, being beneficial to improving the processing process of the pole piece and reducing the potential safety hazard of the battery caused by fracture (or brittle failure) caused by the pole piece crack.

The polymer with the weight-average molecular weight of 2-15 ten thousand and containing the structural unit of formula I is used in the fluorine-containing polymer, the addition of the fluorine-containing polymer in the binder can obviously reduce the fusion enthalpy and crystallinity of the binder composition, and further the flexibility of the pole piece is improved, meanwhile, the fluorine-containing polymer with the low molecular weight can also play a role in effective dispersion, the uniformity of the dispersion of the positive electrode active material in the pole piece is improved, the electron conduction efficiency of the pole piece is improved, and the improvement of the flexibility and uniformity of the pole piece is beneficial to the improvement of the safety performance and the cycle performance of the battery.

In some embodiments, the fluoropolymer is a fluorocarbon polymer.

The term "fluorocarbon polymer" refers to a polymer formed by polymerization of a fluoro-substituted unsaturated monomer, which may be a homopolymer or a copolymer.

In some embodiments, the fluoropolymer is selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene.

In some embodiments, the polymer comprising structural units of formula I is capable of dissolving in an oily solvent. In some embodiments, the polymer comprising structural units of formula I is capable of being dissolved in an aqueous solvent. Exemplary oily solvents include dimethylacetamide, N-dimethylformamide, N-methylpyrrolidone, acetone, and dimethyl carbonate. Examples of aqueous solvents include, but are not limited to, water.

In some embodiments, the particles of the polymer have a median particle diameter Dv50 of 1 to 4 μm. The polymer particles within the particle size range are beneficial to the dissolution of the polymer in a positive electrode slurry solvent, such as N-methyl pyrrolidone, so that the processing difficulty of the glue solution is reduced, and the processing efficiency of the pole piece is improved.

In some embodiments, the particles of the polymer have a median particle diameter Dv50 of 1 to 3 μm,1 to 2 μm,2 to 4 μm, or 2 to 3 μm.

The term "median particle diameter Dv50" refers to the particle diameter corresponding to a cumulative particle size distribution of 50% in the particle size distribution curve, and has a physical meaning that the particles have a particle diameter less than (or greater than) 50% of the particle diameter.

In some embodiments, the fluoropolymer is dissolved to produce a dope having a mass percent of 7% and a viscosity of 50 to 200 mPa-s. In some embodiments, the fluoropolymer is dissolved in N-methylpyrrolidone to prepare the gum solution.

In some embodiments, the polymer comprising structural units of formula I is dissolved in a gum solution made of N-methylpyrrolidone, the viscosity of the polymer being 50 to 180 mPas, 50 to 170 mPas, 50 to 160 mPas, 50 to 150 mPas, 50 to 120 mPas, 60 to 180 mPas, 70 to 180 mPas, 80 to 180 mPas, 90 to 180 mPas or 100 to 180 mPas when the mass percentage of the polymer is 7% based on the mass of the gum solution.

After the fluoropolymer within the viscosity range is mixed with the binder, short chains of the fluoropolymer molecules intercalate into long chains of the binder molecules to form an irregular mixture, reducing the crystalline regularity of the binder; the low viscosity is also beneficial to the sufficient adhesion of the fluorine-containing polymer and the positive electrode active material, reduces the phenomena of agglomeration of the positive electrode active material, blockage of a filter screen and the like, improves the dispersibility of the slurry, and is beneficial to improving the solid content of the slurry.

The application also provides a preparation method of the fluorine-containing polymer,

at least one monomer of the formula II is provided,

As used herein, the term "polymerizable conditions" refers to those conditions that include temperature, pressure, reactant concentration, optional solvent/diluent, reactant mixing/addition parameters selected by one of skill in the art, and other conditions that facilitate the reaction of one or more monomers within at least one polymerization reactor.

In some embodiments, the method of making further comprises the steps of:

The term "non-reactive gas" refers to a gas that does not participate in the polymerization reaction, and exemplary non-reactive gases include any or a combination of argon, helium, and nitrogen.

The term "normal pressure" refers to a standard atmospheric pressure, i.e., 101KPa.

In some embodiments, the reaction temperature is 55 ℃ to 73 ℃,55 ℃ to 70 ℃,55 ℃ to 66 ℃,55 ℃ to 64 ℃,55 ℃ to 62 ℃,58 ℃ to 75 ℃,60 ℃ to 75 ℃,62 ℃ to 75 ℃, or 65 ℃ to 75 ℃.

In some embodiments, the reaction time is from 2 hours to 7 hours, from 2 hours to 6 hours, from 2 hours to 4 hours, from 4 hours to 8 hours, from 6 hours to 8 hours, or from 7 hours to 8 hours.

In some embodiments, the method of making further comprises the steps of:

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

adding an initiator and a pH regulator into the container, regulating the pH value to 6.5-7, adding a monomer shown in a formula II, stirring for 0.5-1 hour, and heating to 55-75 ℃ to perform polymerization reaction.

The term "initiator" refers to a substance that initiates polymerization of a monomer during polymerization. Exemplary initiators are, for example, 2-ethyl peroxydicarbonate, t-butyl peroxypivalate and t-amyl peroxypivalate.

The term "pH adjuster" refers to a substance that can change the pH of a solution or dispersion medium, including increasing acidity or increasing alkalinity. Exemplary pH adjusting agents are sodium bicarbonate, sodium carbonate and sodium hydroxide.

The term "dispersant" refers to a substance capable of promoting uniform dispersion of material particles in a medium to form a stable suspension. Exemplary dispersants include carboxyethyl cellulose ether.

In some embodiments, the reaction solvent is water, which is beneficial for reducing environmental hazards.

In some embodiments, the pH is adjusted to 6.5,6.8 or 7.

In some embodiments, the agitation time may be 30 minutes to 55 minutes, 30 minutes to 50 minutes, 30 minutes to 45 minutes, 35 minutes to 60 minutes, 40 minutes to 60 minutes, or 45 minutes to 60 minutes.

In the preparation method provided by the application, the short chains of the prepared fluorine-containing polymer molecules can be inserted into the long chains of the binder molecules to form a random mixture, so that the crystallinity of the binder is reduced, the brittleness of the pole piece is reduced, and the flexibility of the pole piece is improved; meanwhile, the fluorine-containing polymer has good adhesion effect with the positive electrode active material, and the aggregation among the positive electrode active material, such as lithium iron phosphate (LFP) powder or ternary positive electrode material (NCM) particles, is avoided through the steric hindrance of the polymer, so that the stability of the slurry is increased, the preparation of the positive electrode plate with the positive electrode material uniformly distributed is facilitated, the positive electrode film resistance is reduced, the capacity retention rate of the battery after 500 circles is improved, and the battery performance is improved.

In addition, in the preparation method, the polymer containing the structural unit shown in the formula I provided by the application with the effect can be obtained under the selected condition. The preparation method has the advantages of low cost of raw materials, relatively mild reaction conditions and small environmental hazard, and is beneficial to the industrial production of the fluorine-containing polymer.

[ adhesive composition ]

The application provides a binder composition comprising a binder and the fluoropolymer described above.

The binder, for example PVDF binder, is a polymer with regular short branched chains, and molecules are orderly arranged, so that the crystallinity of the polymer is higher, and the pole piece prepared by using the binder is strained due to stress deformation at the bent part of the pole piece in the bending treatment step of battery processing, and the film layer on the outer side of the pole piece is strained due to insufficient plastic deformation stress of the binder. Without being bound by any theory, the chain length of the fluoropolymer provided by the application is shorter than that of the adhesive, and in the adhesive composition, molecules of the fluoropolymer can be inserted into the regular chain segments of the adhesive molecules, so that the ordering of the molecules is disturbed, the crystallinity of the adhesive is reduced, the flexibility of the adhesive is improved, the plastic deformation stress of the film layer can be improved, and the film layer can resist extrusion force enough to not be stretched when the pole piece is processed and bent, namely, the flexibility of the pole piece is improved. Meanwhile, the fluorine-containing polymer also improves the adhesive force to the positive electrode active material, improves the dispersibility, stability and processability of the positive electrode slurry, and is beneficial to preparing the positive electrode plate with high pressure density and high surface density.

In some embodiments, in the binder composition, the binder is a crystalline polymer and the binder is polyvinylidene fluoride having a weight average molecular weight of 70 to 110 tens of thousands.

In some embodiments, in the binder composition, the binder is polyvinylidene fluoride having a weight average molecular weight of 70 to 100 tens of thousands, 70 to 90 tens of thousands, 80 to 110 tens of thousands, or 90 to 100 tens of thousands.

The polyvinylidene fluoride binder has the characteristics of stable chemical property and excellent electrical property, and is usually less or hardly swelled in the electrolyte of the battery. When the weight average molecular weight of the binder is less than 70 ten thousand, the viscosity and the binding force of the binder are reduced, so that the binding force between the film layer and the current collector is insufficient and the film layer and the current collector fall off from the pole piece. When the weight average molecular weight of the binder is higher than 110 ten thousand, the problem of uneven dispersion of the positive electrode active material is further aggravated due to the increase of the viscosity of the binder, so that the stability of the slurry and the processability of the pole piece are affected, and the increase of the resistance of the pole piece film layer and the reduction of the battery performance are finally caused. The weight average molecular weight of the binder can further reduce the cycle internal resistance increase rate of the battery while improving the stability and the processability of the positive electrode slurry and the binding force of the positive electrode plate in a proper range; and when the adhesive is mixed with the fluorine-containing polymer, the adhesive with the weight average molecular weight in a proper range has a proper space structure, and can well form a staggered random space structure with molecules of the fluorine-containing polymer, so that the order of the space structure of the molecules of the adhesive is disturbed or reduced, and the crystallinity of the adhesive is further reduced.

In some embodiments, the mass ratio of the fluoropolymer to the binder is from 0.05:1 to 5:1.

In some embodiments, the mass ratio of the fluoropolymer to the binder is from 0.2:1 to 4:1.

In some embodiments, the mass ratio of the fluoropolymer to the binder is from 0.5:1 to 1:1.

In some embodiments, the mass ratio of the fluoropolymer to the binder is 0.1:1 to 5:1,0.2:1 to 5:1,0.5:1 to 5:1,1:1 to 5:1,2:1 to 5:1,3:1 to 5:1, or 4:1 to 5:1.

When the mass ratio of the fluorine-containing polymer to the binder is lower than 0.05:1, short chain molecules of the fluorine-containing polymer are inserted into the binder molecules, the crystallinity of the binder molecules is slightly affected because the content of the fluorine-containing polymer is too low to damage or reduce the order of the spatial structure of the binder molecules; when the mass ratio of the fluorine-containing polymer to the binder is higher than 5:1, the content of the fluorine-containing polymer is too high, so that the viscosity and/or the binding force of the binder are obviously reduced, and the defect that the slurry is easy to crack or demould due to insufficient cohesive force between solid substances in the slurry or insufficient binding force with a current collector in the pole piece processing process is overcome. The mass ratio of the fluorine-containing polymer to the binder is in a proper range, which is favorable for fully mixing the fluorine-containing polymer and the binder, and is favorable for inserting fluorine-containing polymer molecules into the binder molecules to form a mixture with random arrangement and reduced structural order of the binder molecules, thereby reducing the brittleness of the pole piece and improving the flexibility of the pole piece. In addition, the mass ratio is in a proper range, so that the adhesion and the bonding between solid matters in the positive electrode slurry are facilitated, the positive electrode active material and the conductive agent are stably connected, the pole piece has good bonding force, the direct contact between the positive electrode active material and the electrolyte can be avoided, and the occurrence of side reaction is reduced; the internal resistance of the film layer of the battery is reduced, the battery performance is improved, for example, the resistance of the positive electrode film layer can be obviously reduced, and the capacity retention rate of the battery after 500 circles is obviously improved.

In some embodiments, the binder composition has a crystallinity of 10% to 45%.

In some embodiments, the binder composition has a crystallinity of 10% to 40%.

In some embodiments, the binder composition has a crystallinity of 10% to 35%,10% to 30%,10% to 25%,15% to 40%,20% to 40%, or 25% to 40%.

When the crystallinity of the binder composition is lower than 10%, after the slurry is processed into a film layer of a pole piece, the cohesion between solid matters in the film layer is insufficient, or the adhesion of the film layer is insufficient, so that a demolding phenomenon is easy to occur in the processing process, or the anode active material of the battery is easily diffused to the anode in the long-term recycling process, so that great potential safety hazards are caused. When the crystallinity of the adhesive composition is higher than 45%, the plastic strain of the film layer is insufficient when the bare cell is subjected to hot pressing treatment, and crack fracture is easy to occur. The crystallinity of the adhesive composition is in a proper range, so that the pole piece has improved flexibility on the premise of ensuring certain adhesive force, the folding light transmission times of the positive pole piece can be obviously improved, the processing process of the pole piece is facilitated, and the potential safety hazard of a battery is reduced.

In some embodiments, the binder composition has a melting enthalpy of 10 to 50J/g.

In some embodiments, the binder composition has a melting enthalpy of 13 to 45J/g.

In some embodiments, the binder composition has a melting enthalpy of 13 to 45J/g,15 to 45J/g,18 to 45J/g,20 to 45J/g,25 to 45J/g,13 to 43J/g,13 to 40J/g,13 to 38J/g,13 to 35J/g, or 13 to 30J/g.

The melting enthalpy of the binder composition is in a suitable range, which ensures reduced crystallization performance and improved flexibility of the binder in the composition, and increased flexibility of the resulting pole piece.

The application also provides application of the fluorine-containing polymer or the binder composition containing the fluorine-containing polymer in a secondary battery.

In the fluorine-containing polymer or the adhesive composition, the fluorine-containing polymer can disturb/reduce the order of the adhesive molecules so as to reduce the crystallinity of the adhesive, improve the flexibility of the adhesive molecules, further improve the flexibility of the pole piece and be beneficial to the processing process of the battery; meanwhile, the fluorine-containing polymer also improves the adhesion to the positive electrode active material, reduces the phenomena of agglomeration of the positive electrode material, blockage of a filter screen and the like, improves the dispersibility, stability and processability of the positive electrode slurry, and is beneficial to improving the solid content of the pole piece and preparing the positive electrode pole piece with high pressure density and high surface density.

In some embodiments, the use includes use of the fluoropolymer or a binder composition containing the fluoropolymer to improve the flexibility of a battery pole piece.

In some embodiments, the use includes the use of the fluoropolymer or a binder composition containing the fluoropolymer to improve the flexibility of a battery positive electrode sheet. In some embodiments, the use includes the use of the fluoropolymer or a binder composition containing the fluoropolymer to improve the flexibility of a battery negative electrode sheet.

In some embodiments, the fluoropolymer in the fluoropolymer or binder composition is used as a battery paste flexibilizer.

As used herein, the term "flexibilizing agent" refers to a chemical compound, polymer or mixture that is capable of increasing the flexibility of an adhesive/binder film layer. The secondary battery, the battery module, the battery pack, and the electric device of the present application will be described below with reference to the drawings as appropriate.

In one embodiment of the present application, a secondary battery is provided.

In general, a secondary battery includes a positive electrode tab, a negative electrode tab, an electrolyte, and a separator. During the charge and discharge of the battery, active ions are inserted and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in ion conduction between the positive electrode plate and the negative electrode plate. The isolating film is arranged between the positive pole piece and the negative pole piece, and mainly plays a role in preventing the positive pole piece and the negative pole piece from being short-circuited, and meanwhile ions can pass through the isolating film.

[ Positive electrode sheet ]

The positive pole piece comprises a positive current collector and a positive film layer arranged on at least one surface of the positive current collector, wherein the positive film layer comprises a positive active material.

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 ) Or their doping modified materials, or at least one of their conductive carbon coating modified materials, conductive metal coating modified materials or conductive polymer coating modified materials. 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, a doping modified material of lithium manganese phosphate and carbon, or a conductive carbon coating modified material, a conductive metal coating modified material or a conductive polymer coating modified material of the composite material.

In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluoroacrylate resin.

In some embodiments, the adhesive further comprises the adhesive composition of the third aspect of the present application.

In some embodiments, the mass ratio of the binder composition to the positive electrode active material in the positive electrode sheet is 1:100 to 3.6:100.

In some embodiments, the mass ratio of the binder composition to the positive electrode active material in the positive electrode sheet is 1.6:100 to 2.4:100.

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 softening agent fluorine-containing polymer

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

2) Preparation of adhesive composition glue

The adhesive composition comprises the softening agent prepared in the example 1 and an adhesive with a weight average molecular weight of 110 ten thousand, wherein the softening agent and the adhesive are dissolved in N-methyl pyrrolidone according to a mass ratio of 0.05:1 to prepare a glue solution, and the mass percentage of the polymer in the glue solution is 7%. Wherein the binder is PVDF having a weight average molecular weight of 110 ten thousand, available from Suwei (Shanghai) Inc.

Examples 2-9 the mass ratio of the flexibilizing agent to the binder was adjusted, the other steps being the same as in example 1, see in particular table 1.

Example 10

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.8g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 62 ℃, and carrying out polymerization reaction for 7h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

The other steps were the same as in example 4.

Example 11

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.7g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 60 ℃, and carrying out polymerization reaction for 8h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

The other steps were the same as in example 4.

Example 12

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.6g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 58 ℃ and carrying out polymerization reaction for 8h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

The other steps were the same as in example 4.

Example 13

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 0.9g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 66 ℃, and carrying out polymerization reaction for 4h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

The other steps were the same as in example 4.

Example 14

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove dissolved oxygen in the solution, adding 1.2g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, adding 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 73 ℃, and carrying out polymerization reaction for 2h; the polymerization solution is obtained through distillation, washing, separation, drying and crushing.

The other steps were the same as in example 4.

Example 15

The flexibilizer prepared in example 1 and PVDF having a weight average molecular weight of 70 ten thousand were used as binder compositions, PVDF being purchased from Amara France Co., ltd, the other steps being identical to those of example 1.

The mass ratio of the softening agent to the binder was adjusted in examples 16 to 23, and the other steps were the same as in example 15, referring specifically to table 1.

Example 24

The same procedure as in example 10 was followed using as binder composition a flexibilizer having a weight average molecular weight of 12 ten thousand and PVDF having a weight average molecular weight of 70 ten thousand prepared in example 10, see in particular table 1.

Example 25

The same procedure as in example 11 was followed using the flexibilizer prepared in example 11 and PVDF having a weight average molecular weight of 70 ten thousand as binder composition, see in particular Table 1.

Example 26

The same procedure as in example 12 was followed using the flexibilizer prepared in example 12 and PVDF having a weight average molecular weight of 70 ten thousand as binder compositions, see in particular Table 1.

Example 27

The same procedure as in example 13 was followed using the flexibilizer prepared in example 13 and PVDF having a weight average molecular weight of 70 ten thousand as binder composition, see in particular Table 1.

Example 28

The same procedure as in example 14 was followed using the flexibilizer prepared in example 14 and PVDF having a weight average molecular weight of 70 ten thousand as binder compositions, see in particular Table 1.

Example 29

1) Preparation of positive electrode plate

Positive electrode active material NCM (lithium nickel cobalt manganese oxide), conductive agent carbon black and binder composition were mixed according to 100:3:2, adding the adhesive composition glue solution prepared in the example 1, and uniformly mixing to obtain the positive electrode slurry. Uniformly coating the anode slurry on two surfaces of an aluminum foil anode current collector, and then drying to obtain a film layer; and then cold pressing and cutting are carried out to obtain the positive pole piece.

2) Preparation of negative electrode plate

Artificial graphite as a cathode active material, carbon black as a conductive agent, styrene-butadiene rubber (SBR) as a binder and sodium carboxymethylcellulose (CMC) as a thickener according to the weight ratio of 96.2:0.8:0.8:1.2, dissolving in deionized water serving as a solvent, and uniformly mixing to prepare negative electrode slurry; and uniformly coating the negative electrode slurry on two surfaces of a negative electrode current collector copper foil for a plurality of times, and drying, cold pressing and cutting to obtain a negative electrode plate.

3) Isolation film

A polypropylene film was used as a separator.

4) Preparation of electrolyte

In an argon atmosphere glove box (H ₂ O<0.1ppm，O ₂ <0.1 ppm), mixing organic solvents of Ethylene Carbonate (EC) and methyl ethyl carbonate (EMC) uniformly according to a volume ratio of 3/7, and mixing LiPF ₆ The lithium salt was dissolved in an organic solvent to prepare a 12.5% solution, to obtain an electrolyte.

5) Preparation of a Battery

The positive electrode sheet, the isolating film and the negative electrode sheet prepared in the embodiment 29 are sequentially stacked, the isolating film is positioned between the positive electrode sheet and the negative electrode sheet to play a role of isolation, then the bare cell is obtained by winding, the tab is welded on the bare cell, the bare cell is arranged in an aluminum shell, baking and dewatering are carried out at 80 ℃, then electrolyte is injected and sealing is carried out, and the uncharged battery is obtained. The uncharged battery was subjected to the processes of standing, hot and cold pressing, formation, shaping, capacity test and the like in order, to obtain the lithium ion battery product of example 29.

Example 30

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 5, with the other steps being the same as in example 29, see in particular table 2.

Example 31

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 7, with the other steps being the same as in example 29, see in particular table 2.

Example 32

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 8, with the other steps being the same as in example 29, see in particular table 2.

Example 33

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 9, with the other steps being the same as in example 29, see in particular table 2.

Example 34

A positive electrode sheet was prepared using the binder composition paste prepared in example 4, wherein the mass ratio of the binder composition to the positive electrode active material was 1.0:100, and the other steps were the same as in example 29, see in particular table 2.

Examples 35-44 the mass ratio of the binder composition to the positive electrode active material was adjusted, and the other steps were the same as in example 29, see in particular table 2.

Example 45

A positive electrode sheet was prepared using the binder composition paste prepared in example 10, wherein the mass ratio of the binder composition to the positive electrode active material was 2.0:100, and the other steps were the same as in example 29, see in particular table 2.

Example 46

A positive electrode sheet was prepared using the binder composition paste prepared in example 11, wherein the mass ratio of the binder composition to the positive electrode active material was 2.0:100, and the other steps were the same as in example 29, see in particular table 2.

Example 47

A positive electrode sheet was prepared using the binder composition paste prepared in example 13, wherein the mass ratio of the binder composition to the positive electrode active material was 2.0:100, and the other steps were the same as in example 29, see in particular table 2.

Example 48

1) Preparation of positive electrode plate

The positive electrode active material lithium iron phosphate, conductive agent carbon black and binder composition were mixed according to 100:4:3, and adding the adhesive composition glue solution prepared in the example 15, and uniformly mixing to obtain the positive electrode slurry. Uniformly coating the anode slurry on two surfaces of an aluminum foil anode current collector, and then drying to obtain a film layer; and then cold pressing and cutting are carried out to obtain the positive pole piece. Other steps are the same as in example 29, see in particular table 2.

Example 49

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 18, with the other steps being the same as in example 48, see in particular table 2.

Example 50

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 22, with the other steps being the same as in example 48, see in particular table 2.

Example 51

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 23, with the other steps being the same as in example 48, see in particular table 2.

Example 52

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 19, with the other steps being the same as in example 48, see in particular table 2.

Examples 53-54 the mass ratio of the binder composition to the positive electrode active material was adjusted, and the other steps were the same as in example 48, see in particular table 2.

Example 55

1) Preparation of softening agent

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

2) Preparation of adhesive composition glue

The adhesive composition comprises the flexibilizer prepared in the example 55 and a PVDF adhesive with a weight average molecular weight of 110 ten thousand, wherein the flexibilizer and the adhesive are dissolved in N-methyl pyrrolidone in a mass ratio of 0.5:1 to prepare a glue solution, and the mass percentage of the polymer in the glue solution is 7%.

The positive electrode sheet was prepared using the binder composition paste prepared by the above method, wherein the mass ratio of the binder composition to the positive electrode active material was 2.0:100, and the other steps were the same as in example 29, with specific reference to table 2.

Example 56

1) Preparation of softening agent

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

2) Preparation of adhesive composition glue

The adhesive composition comprises the flexibilizer prepared in the example 56 and a PVDF adhesive with a weight average molecular weight of 110 ten thousand, wherein the flexibilizer and the adhesive are dissolved in N-methyl pyrrolidone in a mass ratio of 0.5:1 to prepare a glue solution, and the mass percentage of the polymer in the glue solution is 7%.

Comparative example 1

Only PVDF binder having a weight average molecular weight of 8 ten thousand was used in the binder composition glue solution, and the other steps were the same as in example 1, see specifically table 1.

Comparative example 2

Only PVDF binder having a weight average molecular weight of 70 ten thousand was used in the binder composition dope, and the other steps were the same as in example 1, with specific reference to table 1.

Comparative example 3

Only PVDF binder having a weight average molecular weight of 110 ten thousand was used in the binder composition glue solution, and the other steps were the same as in example 1, see specifically table 1.

Comparative example 4

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 12, with the other steps being the same as in example 29, see in particular table 2.

Comparative example 5

Positive electrode sheets were prepared using the binder composition glue solution prepared in example 26, with the other steps being the same as in example 48, see in particular table 2.

The parameters related to the binder compositions and positive electrode materials of examples 1 to 56 and comparative examples 1 to 5 are shown in table 1 below. The flexibility-enhancing agents, binder compositions, positive electrode pastes, electrode sheets and batteries obtained in examples 1 to 56 and comparative examples 1 to 5 were subjected to performance tests as follows:

performance measurement

1. Weight average molecular weight test method

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 was selected (oiliness: styragel HT5 DMF 7.8. Times. 300mm+Styragel HT4). Preparing 3.0% of polymer glue solution to be tested by using the 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.

2. Determination of median particle diameter Dv50

With reference to a GB/T19077-2016 particle size distribution laser diffraction method, 0.1 g-0.13 g of polymer sample to be measured is weighed by a 50mL beaker, 5g of absolute ethyl alcohol is added, and after a stirrer with the size of about 2.5mm is placed, the mixture is sealed by a preservative film. After ultrasonic treatment for 5min, the samples are transferred to a magnetic stirrer and stirred for more than 20min at 500 rpm, and 2 samples are extracted for each batch of products for testing. The test was performed using a Mastersizer 2000E laser particle size analyzer, malvern instruments, uk.

3. Crystallinity and melting enthalpy determination

Dissolving the softening agent and the binder of examples 1-28 in N-methylpyrrolidone (NMP) solution respectively to prepare 10% glue solution, weighing and mixing the glue solution according to the mass ratio of the softening agent to the binder in the binders of examples 1-28, placing the uniformly stirred and dispersed mixed solution into a glue film preparation container, drying for 2 days at 100 ℃, cutting the glue film into small pieces with the length of 2X 2cm, placing the small pieces into an aluminum dry pot, shaking, covering a crucible cover, blowing 50mL/min of a protective gas at the temperature rising rate of 10 ℃/min under nitrogen atmosphere, testing at the temperature ranging from-100 ℃ to 400 ℃ by using a Differential Scanning Calorimeter (DSC) with the model of a TA instrument of the United states of Disconnector 250, and eliminating heat history.

The DSC/(Mw/mg) curve of the adhesive film along with the temperature is obtained by the test, and is integrated, wherein the peak area is the melting enthalpy delta H (J/g) of the adhesive film, and the peak area is calculated according to the following formula:

film crystallinity = Δh/(Δhm100%) x 100%

Where Δhm100% is the standard enthalpy of fusion (crystalline heat of fusion) of PVDF, Δhm100% = 104.7J/g.

4. Viscosity test

Dissolving a sample to be tested in an N-methyl pyrrolidone (NMP) solvent, preparing a glue solution with 7% of solid content (mass percent), selecting a proper rotor, fixing a viscometer rotor, placing the glue solution below the viscometer rotor, and just submerging the scale marks of the rotor by the glue solution, wherein the type of the instrument is as follows: shanghai Fang Rui NDJ-5S, rotor: 61# (0-500 mPas), 62# (500-2500 mPas), 63# (2500-10000 mPas, 64# (10000-50000 mPas), rotating speed: 12r/min, testing temperature: 25 ℃, testing time: 5min, and reading data after the reading is stable.

5. Measurement of positive electrode film resistance:

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

6. Pole piece brittleness test

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

7. Cycle capacity retention test

When the positive electrode active material is lithium nickel cobalt manganese oxide NCM, the method for measuring the circulation capacity retention rate is as follows:

taking example 29 as an example, the battery capacity retention test procedure is as follows: the corresponding battery of example 29 was charged to 4.4V at a constant current of 1/3C, then charged to 0.05C at a constant voltage of 4.4V, left to stand for 5 minutes, then discharged to 2.8V at 1/3C, and the resulting capacity was designated as initial capacity C0. Repeating the steps for the same battery, and simultaneously recording the discharge capacity Cn of the battery after the nth cycle, wherein the capacity retention rate of the battery after each cycle is as follows:

Pn＝Cn/C0×100％

In this test procedure, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and. The battery capacity retention rate data corresponding to example 29 in table 2 is data measured after 500 cycles under the above-described test conditions, i.e., the value of P500. Comparative example 3 and other examples were tested as above.

When the positive electrode active material is lithium iron phosphate, the method for measuring the cyclic capacity retention rate is as follows:

taking example 48 as an example, the battery capacity retention test procedure is as follows: the corresponding cell of example 48 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 to stand for 5min, then discharged to 2.5V at 1/3C, and the resulting capacity was designated as initial capacity CO. Repeating the steps for the same battery, and simultaneously recording the discharge capacity Cn of the battery after the nth cycle, wherein the capacity retention rate of the battery after each cycle is as follows:

Pn＝Cn/C0×100％

in this test procedure, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and. The battery capacity retention rate data corresponding to example 48 in table 2 is data measured after 500 cycles under the above-described test conditions, i.e., the value of P500. Comparative example 2 and other examples were tested as above.

The composition parameters and the detection results of the adhesive composition are shown in Table 1 below. The above battery parameters and the detection results are shown in table 2 below.

TABLE 1 adhesive composition parameters and test results

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From the above results, examples 1 to 14 provided a softening agent having a weight average molecular weight of 0.5 to 20 ten thousand and a PVDF binder composition having a weight average molecular weight of 110 ten thousand, and the binder compositions in examples 1 to 14 of the present application showed significantly reduced melting enthalpy and crystallinity compared to comparative example 3, which contained only a PVDF binder having a weight average molecular weight of 110 ten thousand, indicating improved flexibility of the polymer in the composition.

Examples 15-28 provided softening agents having weight average molecular weights of 0.5-20 ten thousand and PVDF binder compositions having weight average molecular weights of 70 ten thousand, the binder compositions of examples 15-28 of the present application exhibited significantly reduced melting enthalpy and crystallinity, indicating increased flexibility of the polymer in the composition, as compared to comparative example 2 which contained only PVDF binder having a weight average molecular weight of 70 ten thousand.

The binder compositions provided in examples 2-9 and examples 15-23, which included a flexibilizer having a weight average molecular weight of 8 ten thousand, exhibited significantly lower melting enthalpy and crystallinity than comparative example 1, which contained only a flexibilizer having a weight average molecular weight of 8 ten thousand, indicating increased flexibility of the polymer in the composition.

Examples 29 to 47 use a binder composition comprising a flexibilizer having a weight average molecular weight of 2 to 15 ten thousand and a PVDF having a weight average molecular weight of 110 ten thousand to prepare a battery, and the positive electrode film layer resistance prepared in examples 29 to 47 is reduced and the capacity retention after 500 cycles of the battery is improved, compared to comparative example 3 in which a battery was prepared using only a PVDF binder having a weight average molecular weight of 110 ten thousand, indicating that the flexibilizer improves the dispersibility of the positive electrode slurry, thereby reducing the electrode sheet resistance and improving the battery performance; the improvement of the doubling light transmission times of the positive pole piece shows that the softening agent also reduces the crystallinity of the adhesive, thereby improving the flexibility of the pole piece.

Comparative example 4 a battery was prepared using a flexibilizer having a weight average molecular weight of 20 ten thousand and a binder composition having a weight average molecular weight of 110 ten thousand PVDF, and examples 39, 45 to 47, and 55 to 56 compared with comparative example 4, the positive electrode film resistance was significantly reduced and the capacity retention after 500 cycles of the battery was significantly improved, indicating that the conductivity of the electrode sheet and the cycle performance of the battery were improved; meanwhile, the folding light transmission times of the positive pole piece are obviously improved, and the flexibility of the pole piece is improved.

Examples 48 to 54 use a binder composition comprising a flexibilizer having a weight average molecular weight of 8 ten thousand and a PVDF having a weight average molecular weight of 70 ten thousand to prepare a battery, and compared with comparative example 2 in which a battery was prepared using only a PVDF binder having a weight average molecular weight of 70 ten thousand, the positive electrode film prepared in examples 48 to 54 had a reduced resistance, and the capacity retention after 500 cycles of the battery was improved, indicating that the flexibilizer improved the dispersibility of the positive electrode slurry, thereby reducing the resistance of the electrode sheet and improving the battery performance; the doubling light transmission times of the positive pole piece are improved, which indicates that the softening agent also reduces the crystallinity of the binder, thereby improving the flexibility of the pole piece.

Comparative example 5 a battery was prepared using a flexibilizer having a weight average molecular weight of 20 ten thousand and a binder composition having a weight average molecular weight of 70 ten thousand PVDF, and example 49 significantly reduced the resistance of the positive electrode film layer compared to comparative example 4, and significantly improved the capacity retention after 500 cycles of the battery, indicating improved conductivity of the electrode sheet and cycle performance of the battery; meanwhile, the number of times of light transmittance of the positive electrode sheet in the embodiment 49 is also obviously increased, which indicates that the flexibility of the sheet is increased.

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 secondary battery binder composition, comprising: polyvinylidene fluoride with weight average molecular weight of 70-110 ten thousand, and fluorine-containing polymer with weight average molecular weight of 2-8 ten thousand;

The fluorine-containing polymer is selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, copolymer of vinylidene fluoride and hexafluoropropylene;

the melting enthalpy of the adhesive composition is 10-50J/g.

2. The secondary battery binder composition according to claim 1, wherein the binder composition has a melting enthalpy of 13 to 45J/g, optionally 13 to 35J/g.

3. The secondary battery binder composition according to claim 1 or 2, wherein the mass ratio of the fluoropolymer to the binder is 0.05:1 to 5:1, optionally 0.2:1 to 4:1.

4. A secondary battery binder composition according to any one of claims 1 to 3, wherein the binder composition has a crystallinity of 10% to 45%, optionally 10% to 40%.

5. The secondary battery binder composition according to any one of claims 1 to 4, wherein the secondary battery comprises a positive electrode active material comprising lithium nickel cobalt manganese oxide.

6. Use of a binder composition for improving the flexibility of a positive electrode sheet of a secondary battery, the binder composition comprising: polyvinylidene fluoride with weight average molecular weight of 70-110 ten thousand, and fluorine-containing polymer with weight average molecular weight of 2-8 ten thousand;

the melting enthalpy of the adhesive composition is 10-50J/g.

7. A secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, the positive electrode sheet comprising a positive electrode active material, a conductive agent, and the secondary battery binder composition of any one of claims 1 to 5.

8. The secondary battery according to claim 7, wherein the positive electrode active material comprises a lithium-containing transition metal oxide, optionally lithium nickel cobalt manganese oxide.

9. The secondary battery according to claim 7 or 8, wherein the mass ratio of the binder composition to the positive electrode active material in the positive electrode sheet is 1:100 to 3.6:100, optionally 1.6:100 to 2.4:100.

10. An electric device comprising the secondary battery according to any one of claims 7 to 9.