CN115133035A

CN115133035A - Positive electrode slurry, method for producing same, secondary battery, battery module, battery pack, and electric device

Info

Publication number: CN115133035A
Application number: CN202211052015.3A
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: 2022-09-30
Anticipated expiration: 2042-08-30
Also published as: CN115133035B; CN117638069A; WO2024045505A1

Abstract

The application provides a positive electrode slurry, a preparation method thereof, a secondary battery, a battery module, a battery pack and an electric device. The positive electrode slurry comprises a positive electrode active substance, a conductive agent, a dispersing agent and a binder, wherein the dispersing agent comprises a first polymer with the weight-average molecular weight of 0.5-15 ten thousand, the binder comprises a second polymer with the weight-average molecular weight of 70-110 ten thousand and a third polymer with the weight-average molecular weight of 130-300 ten thousand, and the first polymer, the second polymer and the third polymer are all polymers containing structural units shown in a formula I. The dispersing agent improves the dispersibility and the processability of the positive electrode slurry, obviously reduces the resistance of a pole piece film layer and improves the cycle performance of the battery; the adhesive can ensure that the pole piece has good adhesive force under the condition of relatively small addition amount, and is beneficial to improving the loading capacity of active substances of the positive pole piece and the energy density of a battery; the crystallinity of the adhesive is improved, and the flexibility of the pole piece is improved.

Description

Positive electrode slurry, method for producing same, secondary battery, battery module, battery pack, and electric device

Technical Field

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

Background

In recent years, secondary batteries are widely used in energy storage power 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, and aerospace. As the application range of the secondary battery becomes wider, higher requirements are also made on the energy density, cycle performance, safety performance, and the like of the secondary battery.

The traditional binder usually needs higher content to meet the requirement of pole piece binding power, so that the promotion of active substance loading in the pole piece is limited, and the promotion of battery energy density is not facilitated. Moreover, the high binder dosage can cause the brittleness problem of the pole piece while improving the compaction density of the pole piece, and the safety and the cycle performance of the battery are reduced. The problem of how to reduce the dosage of the adhesive in the pole piece and improve the brittleness of the pole piece becomes a problem which needs to be solved urgently at present.

Disclosure of Invention

The present invention has been made in view of the above problems, and an object of the present invention is to provide a positive electrode slurry including a positive electrode active material, a conductive agent, a dispersing agent, and a binder, wherein the dispersing agent includes a first polymer having a weight average molecular weight of 0.5 to 15 ten thousand, the binder includes a second polymer having a weight average molecular weight of 70 to 110 ten thousand, and a third polymer having a weight average molecular weight of 130 to 300 ten thousand, and each of the first polymer, the second polymer, and the third polymer is a polymer containing a structural unit represented by formula I,

formula I

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

The first polymer with the weight-average molecular weight of 0.5-15 ten thousand is used as the dispersing agent in the anode slurry, so that the dispersibility and the processability of the anode slurry are improved, the resistance of a pole piece film layer is obviously reduced, and the cycle performance of a battery is improved. The second polymer and the third polymer with different weight average molecular weights are used as the binding agent in the positive pole slurry, and compared with the common binding agent in the prior art, the binding agent can enable the pole piece to have good binding power under the condition of relatively small addition amount, is beneficial to reducing the using amount of the binding agent in the pole piece, and improves the positive pole active material loading amount of the pole piece and the energy density of a battery; in addition, the combination of the second polymer and the third polymer also improves the crystallinity of the adhesive and improves the flexibility of the pole piece.

In any embodiment, the first polymer, the second polymer, and the third polymer are each a halogenated hydrocarbon polymer, each independently selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene.

The polymer has stable chemical properties, excellent electrical properties and good mechanical properties, and is beneficial to preparing a film layer with appropriate flexibility and hardness.

In any embodiment, the particles of the first polymer have a median particle diameter Dv50 of 0.5 μm to 5 μm.

The particle size of the particles of the first polymer is in a proper range, so that the particles are favorable for dissolving in a positive electrode slurry solvent, such as N-methyl pyrrolidone, and the processing difficulty of a glue solution is reduced.

In any embodiment, the first polymer is dissolved in N-methyl pyrrolidone to prepare a first glue solution, and when the mass content of the first polymer is 7% based on the total mass of the first glue solution, the viscosity of the first glue solution is 20-180 mPa · s.

The viscosity of the first polymer is in a proper range, so that the wetting and depolymerization of the positive active substance powder particles are facilitated, the phenomena of agglomeration of the positive active substance, filter screen blockage and the like are reduced, the dispersion performance of the positive slurry is improved, the solid content of the positive slurry and the coating uniformity of the pole piece are improved, and the energy density of the battery is further improved.

In any embodiment, the particles of the second polymer have a median particle diameter Dv50 of 15 to 25 μm.

The particle size of the second polymer is in a proper range, which is beneficial to the dissolution of the second polymer in a positive electrode slurry solvent, such as N-methyl pyrrolidone, reduces the processing difficulty of glue solution, and improves the processing efficiency.

In any embodiment, the second polymer is dissolved in N-methyl pyrrolidone to prepare a second glue solution, and the viscosity of the second glue solution is 2500-4000 mPa · s when the mass content of the second polymer is 7% based on the total mass of the second glue solution.

The viscosity of the second polymer in a suitable range can reduce the order of arrangement of the third polymer molecules and reduce the crystallinity of the third polymer molecules.

In any embodiment, the particles of the third polymer have a median particle diameter Dv50 of 30 μm to 100 μm.

The particle size of the third polymer particles is in a proper range, so that the particles can be dissolved in the positive slurry solvent, the processing difficulty of the glue solution is reduced, and the processing efficiency of the pole piece is improved.

In any embodiment, the third polymer is dissolved in N-methyl pyrrolidone to prepare a third glue solution, and when the mass content of the third polymer is 4% based on the total mass of the third glue solution, the viscosity of the third glue solution is 1500-5000 mPa · s.

The viscosity of the third polymer is in a proper range, the adhesive has good adhesive property, and the pole piece can have excellent adhesive force when the addition amount is low, so that the loading amount of the positive active material and the energy density of the battery can be improved.

In any embodiment, the polydispersity of the third polymer is 2 to 2.3.

In any embodiment, the third polymer has a polydispersity of 2.1 to 2.2.

The polydispersity of the third polymer in the above range helps to maintain the viscosity of the third polymer stable, thereby improving the stability of the production of the pole piece.

In any embodiment, the dispersant is present in an amount of 0.05% to 1% by mass, based on the total mass of solid matter of the positive electrode slurry.

The mass content of the dispersing agent in a proper range can improve the dispersibility of the positive slurry, and has no influence or little influence on the bonding performance of the binder.

In any embodiment, the mass content of the binder is 0.6% to 1.2% based on the total mass of solid matter of the positive electrode slurry.

The mass content of the binder is in a proper range, so that the pole piece has good binding power, the direct contact between the positive active material and the electrolyte is avoided, the occurrence of side reactions is reduced, and the potential safety hazard of the secondary battery is reduced; meanwhile, the addition amount of the binder is relatively low, which is beneficial to reducing the resistance of a pole piece film layer, improving the loading capacity of the positive active material and improving the energy density of the battery.

In any embodiment, the mass ratio of the second polymer to the third polymer in the binder is 1:9 to 8: 2.

When the mass ratio of the second polymer to the third polymer is within the above range, the pole piece can be ensured to have improved flexibility on the premise of having good adhesion.

In any embodiment, the binder has a crystallinity of 25% to 44%.

The crystallinity of the adhesive is in a proper range, so that the flexibility of the pole piece can be improved, the processing of the secondary battery is facilitated, and the potential safety hazard of the secondary battery is reduced.

In any embodiment, the binder has a melting enthalpy of 25J/g to 45J/g.

The melting enthalpy of the binder is in a proper range, so that the crystallinity of the binder is moderate, and the pole piece has excellent flexibility and binding power.

In any embodiment, the positive 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-coated modified material, a conductive metal-coated modified material, or a conductive polymer-coated modified material thereof.

A second aspect of the present application provides a method for preparing a positive electrode slurry, the method comprising the steps of:

step 1: uniformly mixing the positive active substance, the conductive agent and the binder; the binder comprises a second polymer with the weight-average molecular weight of 70-110 ten thousand and a third polymer with the weight-average molecular weight of 130-300 ten thousand,

and 2, step: adding a dispersing agent into the mixture and stirring the mixture to obtain positive electrode slurry, wherein the dispersing agent comprises a first polymer with the weight-average molecular weight of 0.5-15 ten thousand, and the first polymer, the second polymer and the third polymer are all prepared by polymerizing at least one monomer shown in a formula II under a polymerizable condition,

formula II

Wherein R is ₁ 、R ₂ Each independently selected from one or more of hydrogen, fluorine, chlorine and trifluoromethyl.

The method of adding the binder and then adding the dispersant is favorable for realizing the full mixing, adhesion/coating of the positive active material, the conductive agent and the high molecular weight binder, and the dispersant is added afterwards, so that the sedimentation of the positive active material and the high molecular weight binder can be effectively avoided, and the dispersibility and the stability of the positive slurry can be improved.

In any embodiment, the method of making the first polymer comprises the steps of:

providing at least one monomer shown as a formula II, a first initiator and a first solvent, carrying out polymerization reaction for 2-8 hours at the reaction temperature of 55-80 ℃ under normal pressure, stopping the reaction, carrying out solid-liquid separation, and retaining a solid phase to obtain the first polymer.

The preparation method of the first polymer has low raw material cost and mild reaction conditions, and is beneficial to the mass production of the dispersing agent.

In any embodiment, the method of making the third polymer comprises the steps of:

providing at least one monomer shown as a formula II, a second initiator and a second solvent, and when the monomer shown as the formula II enables the reaction pressure to reach 6-8 MPa, raising the temperature to 35-60 ℃ to carry out polymerization reaction for 6-10 hours;

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

In the preparation method of the third polymer, the raw materials are easy to obtain, the reaction conditions are safe and controllable, and the method is favorable for the expanded production of the third polymer.

The third aspect of this application provides a secondary battery, including positive pole piece, barrier film, negative pole piece and electrolyte, positive pole piece includes the anodal mass flow body and sets up the anodal rete on the at least one surface of the anodal mass flow body, anodal rete by the preparation of the anodal thick liquids of any one of the first aspect of this application.

A fourth aspect of the present application provides a battery module including the secondary battery of the third aspect of the present application.

A fifth aspect of the present application provides a battery pack including the battery module of the fourth aspect of the present application.

A sixth aspect of the present application provides an electric device including at least one selected from the secondary battery of the third aspect of the present application, the battery module of the fourth aspect of the present application, or the battery pack of the fifth aspect of the present application.

Drawings

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

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

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

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

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

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

Description of reference numerals:

1, a battery pack; 2, putting the box body on the box body; 3, discharging the box body; 4 a battery module; 5 a secondary battery; 51 a housing; 52 an electrode assembly; 53 cover plate.

Detailed Description

Hereinafter, embodiments of the positive electrode active material, the method for producing the same, the positive electrode sheet, the secondary battery, the battery module, the battery pack, and the electrical device according to the present application will be specifically disclosed in detail with reference to the drawings as appropriate. But a detailed description thereof will be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually the same structures may be omitted. This is to avoid unnecessarily obscuring the following description, and to facilitate understanding by those skilled in the art. The drawings and the following description are provided for those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

The "ranges" disclosed herein are defined in terms of lower limits and upper limits, with a given range being defined by a selection of one lower limit and one upper limit that define the boundaries of the particular range. Ranges defined in this manner may or may not include the stated limits and may be arbitrarily combined, i.e., any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for a particular parameter, 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 stated, 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, a numerical range of "0 to 5" means that all real numbers between "0 to 5" have been listed herein, and "0 to 5" is simply a shorthand representation of the combination of these values. In addition, when a parameter is an integer of 2 or more, it is equivalent to disclose 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, if not specifically stated.

All technical and optional features of the present application may be combined with each other to form new solutions, if not otherwise specified.

All steps of the present application may be performed sequentially or randomly, preferably sequentially, if not specifically stated. For example, the method comprises steps (a) and (b), meaning that the method may comprise steps (a) and (b) performed sequentially, and may also comprise steps (b) and (a) performed sequentially. For example, reference to the process further comprising step (c) means that step (c) may be added to the process in any order, for example, the process may comprise steps (a), (b) and (c), may also comprise steps (a), (c) and (b), may also comprise steps (c), (a) and (b), etc.

The terms "comprises" and "comprising" as used herein mean either open or closed unless otherwise specified. For example, the terms "comprising" and "comprises" may mean that other components not listed may also be included or included, or that only listed components may be included or included.

In this application, the term "or" is inclusive, if not 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 not present); a is false (or not present) and B is true (or present); or both a and B are true (or present).

Traditional binders such as PVDF often need higher content (more than 2%) to meet the demand of pole piece binding power, which limits the promotion of active material loading in pole pieces and is not beneficial to the promotion of battery energy density. In addition, when the battery is processed and hot-pressed and shaped, the high binder dosage easily causes the film layer at the corner of the cathode at the innermost circle to be broken due to insufficient tensile force, so that the light transmission phenomenon of the pole piece is caused, the brittleness (or brittle fracture) problem of the pole piece is caused, and the safety and the cycle performance of the battery are reduced. Moreover, the compatibility of the traditional binder and a new generation of positive active material is poor, the dispersibility and stability of the prepared slurry are poor, the positive active material in the pole piece is not uniformly distributed, and the improvement of the battery performance is limited.

[ Positive electrode slurry ]

Based on the above, the present application provides a positive electrode slurry, which comprises a positive electrode active material, a conductive agent, a dispersing agent and a binder, wherein the dispersing agent comprises a first polymer with a weight average molecular weight of 0.5 to 15 ten thousand, the binder comprises a second polymer with a weight average molecular weight of 70 to 110 ten thousand and a third polymer with a weight average molecular weight of 130 to 300 ten thousand, and the first polymer, the second polymer and the third polymer are all polymers containing structural units shown in formula I,

formula I

The term "positive electrode" herein also refers to the "cathode" in the battery. The term "negative electrode" also refers to the "anode" in the battery.

As used herein, the term "dispersant" refers to a chemical compound, polymer, or mixture that facilitates uniform dispersion of particles of a material in a colloidal solution or dispersion.

As used herein, the term "binder" refers to a chemical compound, polymer or mixture that holds a solid object in intimate association with another solid object. The term "binder" also refers to a chemical compound, polymer or mixture that tightly binds together the solid matter in the battery paste during the fabrication of the pole piece.

As used herein, the term "weight average molecular weight" refers to the sum of the product of the weight fraction of molecules of different molecular weight in a polymer and their corresponding molecular weight.

In this context, the term "polymer" encompasses on the one hand a collection of chemically uniform 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 reactions, i.e. compounds or mixtures which can be obtained by reactions, e.g. additions or substitutions, of functional groups in the above-mentioned macromolecules and which can be chemically homogeneous or chemically heterogeneous.

In some embodiments, the dispersant is used in a battery positive electrode slurry to improve dispersibility of the positive electrode slurry. In some embodiments, the dispersant may also be used in battery negative electrode slurry to improve dispersibility of the negative electrode slurry.

Without being bound by any theory, when the weight average molecular weight of the first polymer is 0.5-15 ten thousand, the intermolecular force is small, the adhesion force and the wetting performance are good, the positive active substances in the positive slurry can be well adhered, and the aggregation among the positive active substances is prevented/reduced; the first polymer is dispersed or suspended in a solvent (or a dispersion medium) of the positive electrode slurry through electrostatic repulsion or steric hindrance, so that the dispersibility of the positive electrode slurry is remarkably improved, the positive electrode slurry does not settle after being placed for a certain time, the stability of the positive electrode slurry is improved, and the solid content of the slurry and the coating rate of a pole piece are improved; the positive active substance is uniformly distributed in the pole piece, which is beneficial to improving the electron conduction efficiency of the pole piece, reducing the film resistance of the battery pole piece and improving the cycle performance of the battery.

In the present application, the binder includes a second polymer having a weight average molecular weight of 70 to 110 ten thousand and a third polymer having a weight average molecular weight of 130 to 300 ten thousand. The third polymer has high viscosity, and when the third polymer is used as an adhesive, a small amount of the third polymer is added, so that the pole piece has good adhesive force. However, because the third polymer has high crystallinity, when the third polymer is used alone as a binder, the outer film layer of the prepared pole piece is easy to crack and break due to insufficient plastic deformation stress of the binder during the hot-pressing treatment of the battery. The crack fracture means that in the pole piece processing process, solid matters in the slurry are processed into film layers to be attached to the current collection, cracks are easily generated at the innermost 1-2 circles of cathode corners of the pole piece (or a bare cell) during hot-pressing shaping, and the pole piece is caused to have a light transmission phenomenon. The crack and fracture of the pole piece lead the pole piece to expose fresh aluminum foil while dusting. Along with the circulation, the electrolyte can be decomposed 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 electrically 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.

When the weight average molecular weight of the third polymer is greater than 300 ten thousand, although the addition amount of the binder is further reduced due to the increase of the viscosity of the binder, the problem of uneven dispersion of the positive active material is also aggravated, the dispersibility and stability of the slurry are affected, and finally, the resistance of a pole piece film layer is increased and the cycle performance of the battery is reduced. When the weight average molecular weight of the third polymer is less than 130 ten thousand, the viscosity of the binder is reduced, and although the dispersibility of the slurry is improved, the binding power of the pole piece is reduced. In order to improve the binding power of the pole piece, the dosage of the third polymer and/or the second polymer needs to be increased, and the increase of the dosage of the binding agent can reduce the loading of the positive active material in the pole piece, thereby influencing the energy density of the battery.

Without being bound by any theory, in the application, the second polymer can be inserted into a regular chain segment of a third polymer molecule, so that the order of the molecular structure of the third polymer is reduced, and the crystallinity of the adhesive is reduced, thereby improving the plastic deformation stress of a film layer, ensuring that no crack or fracture is generated during the pole piece hot-pressing treatment, improving the flexibility of the pole piece, and being beneficial to improving the processability of the pole piece and reducing the potential safety hazard of a battery caused by the fracture (or brittle fracture) caused by the pole piece crack.

When the weight average molecular weight of the second polymer is more than 110 ten thousand, the resistance of the pole piece film layer is increased and the cycle performance of the battery is reduced due to the increase of the viscosity of the adhesive. In addition, the difference between the length of the second polymer molecule and the length of the third polymer molecule is reduced due to the increase of the length of the second polymer molecule, and the spatial structure of the third polymer molecule is not sufficient to accommodate the long chain of the second polymer molecule, so that the spatial structure of the third polymer molecule cannot be orderly reduced or the crystallinity of the binder cannot be reduced after the two are mixed. When the weight average molecular weight of the second polymer is less than 70 ten thousand, the viscosity of the binder is reduced, so that the pole piece has good binding power, the dosage of the second polymer and/or the third polymer needs to be increased, and the increase of the loading capacity of the positive active material in the pole piece is not facilitated; in addition, because the difference between the molecular chain length of the second polymer and the molecular chain length of the third polymer is too large, the molecular chain of the second polymer cannot form effective physical winding/crosslinking with the molecular chain of the third polymer, the influence on the crystallinity of the third polymer is reduced, and the plastic strain force of the film layer and the flexibility of the pole piece cannot be improved.

In the application, through the combined use of the second polymer and the third polymer with specific weight average molecular weights, especially the combined use of the second polymer with the weight average molecular weight of 70-110 ten thousand and the third polymer with the weight average molecular weight of 130-300 ten thousand, the using amount of the binder in the prior art can be reduced, the pole piece can have good binding power when the additive amount of the binder is low, and the loading amount of a positive electrode active substance in the pole piece and the energy density of a battery can be improved. Meanwhile, the combination of the second polymer and the third polymer also improves the crystallinity of the binder and improves the flexibility of the pole piece.

In some embodiments, the first polymer, the second polymer, and/or the third polymer are capable of dissolving in an oily solvent. In some embodiments, the first polymer, the second polymer, and/or the third polymer are 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 first polymer, the second polymer, and the third polymer are each a halogenated hydrocarbon polymer, each independently selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene.

As used herein, the term "halogenated hydrocarbon polymer" refers to a polymer of a halogen-substituted unsaturated hydrocarbon. The term "halogen" refers to a halogen element including fluorine, chlorine, bromine, iodine.

The polymer has the characteristics of stable chemical property and excellent electrical property, and is generally less or rarely swelled in the electrolyte of the battery. Meanwhile, the polymer has good mechanical properties, and is beneficial to preparing a film layer with appropriate flexibility and hardness.

In some embodiments, the particles of the first polymer have a median particle diameter Dv50 of 0.5 μm to 5 μm. In some embodiments, the particles of the first polymer have a median particle diameter Dv50 of 0.5 to 4.5 μm, 0.5 to 4 μm, 0.5 to 3.5 μm, 0.5 to 3 μm, 0.5 to 2.5 μm, 0.8 to 5 μm, 1 to 4 μm, 2 to 5 μm, or 2 to 4 μm.

The particle size of the first polymer is in a proper range, so that the first polymer can be dissolved in a positive electrode slurry solvent, such as N-methyl pyrrolidone, and the processing difficulty of a glue solution is reduced.

As used herein, the term "median particle diameter Dv 50" refers to the particle diameter corresponding to the percentage of the cumulative particle size distribution of a measured sample reaching 50%, and its physical meaning is that particles having a particle diameter less than (or greater than) 50% of its particle diameter.

In some embodiments, the first polymer is dissolved in N-methylpyrrolidone to prepare a first dope, and the viscosity of the first dope is 20 to 180 mPas when the mass content of the first polymer is 7% based on the total mass of the first dope.

In some embodiments, the first polymer is dissolved in N-methyl pyrrolidone to produce a first dope, and the viscosity of the first dope is 30 to 180mPa · s, 50 to 180mPa · s, 20 to 160mPa · s, 20 to 150mPa · s, 30 to 120mPa · s, or 20 to 50mPa · s when the mass content of the first polymer is 7% based on the total mass of the first dope.

When the viscosity of the first polymer is higher than 180mPa · s (mass content is 7%), wettability with the positive electrode active material is reduced, which is not favorable for dispersing the slurry; the first polymer with the viscosity within a proper range is beneficial to wetting and depolymerization of positive active substance powder particles, reduces the phenomena of agglomeration of positive active substances, filter screen blockage and the like, improves the dispersion performance of positive slurry, improves the solid content of the positive slurry and the coating uniformity of a pole piece, and further improves the energy density of the battery.

In some embodiments, the particles of the second polymer have a median particle diameter Dv50 of 15 to 25 μm. In some embodiments, the particles of the second polymer have a median particle diameter Dv50 of 15 to 23 μm, 15 to 20 μm, 18 to 25 μm, or 20 to 25 μm.

The particle size of the second polymer is in a proper range, which is beneficial to dissolving the second polymer in a positive electrode slurry solvent, such as N-methyl pyrrolidone, reduces the processing difficulty of the glue solution, and improves the processing efficiency.

In some embodiments, the second polymer is dissolved in N-methyl pyrrolidone to prepare a second glue solution, and when the mass content of the second polymer is 7% based on the total mass of the second glue solution, the viscosity of the second glue solution is 2500 to 4000 mPa-s.

In some embodiments, the second polymer is dissolved in N-methyl pyrrolidone to form a second glue solution, and when the second polymer is present in an amount of 7% by mass, based on the total mass of the second glue solution, the second glue solution has a viscosity of 2800 to 4000mPa · s, 3000 to 4000mPa · s, 3200 to 4000mPa · s, 3500 to 4000mPa · s, 2800 to 3800mPa · s, or 2800 to 3500mPa · s.

When the viscosity of the second polymer is higher than 4000mPa · s (the mass content is 7%), the intramolecular or intermolecular force is increased, which is not beneficial to the free swing of the second polymer molecular chain in the positive electrode slurry solvent, and further influences the physical crosslinking with the third polymer molecular chain; when the viscosity of the second polymer is lower than 2800mPa · s, the adhesive property of the adhesive is reduced, and the addition amount of the adhesive needs to be increased to ensure that the pole piece has good adhesive force, so that the resistance of a pole piece film layer is increased, the cycle performance of the battery is damaged, and the increase of the loading amount of the positive active material in the pole piece is not facilitated. When the viscosity of the second polymer is in a proper range, the molecular chain of the second polymer can overcome the intra-molecular or intermolecular acting force to physically crosslink/wind with the molecular chain of the third polymer, so that the order and the crystallization regularity of the molecules of the third polymer are reduced, and the flexibility of the binder is improved.

In some embodiments, the particles of the third polymer have a median particle diameter Dv50 of 30 μm to 100 μm. In some embodiments, the particles of the third polymer have a median particle diameter Dv50 of 30 μm to 80 μm, 30 μm to 60 μm, 40 μm to 80 μm, 50 μm to 80 μm, or 60 μm to 80 μm.

The third polymer has higher viscosity, the dissolving rate in a positive electrode slurry solvent such as N-methyl pyrrolidone is low, and the particle size of the third polymer is in a proper range, so that the processing difficulty of glue solution is reduced, and the processing efficiency of a pole piece is improved.

In some embodiments, the third polymer is dissolved in N-methyl pyrrolidone to form a third glue solution, and the viscosity of the third glue solution is 1500-5000 mPa-s when the mass content of the third polymer is 4% based on the total mass of the third glue solution.

In some embodiments, the third polymer is dissolved in N-methyl pyrrolidone to form a third glue solution, and when the third polymer is present in an amount of 4% by mass, based on the total mass of the third glue solution, the third glue solution has a viscosity of 1700 to 4800mPa · s, 1700 to 4500mPa · s, 1700 to 4300mPa · s, 1700 to 4000mPa · s, 1700 to 3600mPa · s, 1700 to 3500mPa · s, 2000 to 4800mPa · s, 2500 to 4800mPa · s, or 3600 to 4800mPa · s.

When the viscosity of the third polymer is higher than 4800mPa · s (mass content of 4%), although the adhesive property is improved, the dispersibility of the positive electrode slurry is further deteriorated, and the non-uniform distribution of the positive electrode active material in the slurry not only affects the processability of the electrode sheet but also causes defects such as cracks, particle scratches, pinholes, and the like on the surface of the electrode sheet. When the viscosity of the third polymer is lower than 1700 mPas, the adhesive property of the adhesive is obviously reduced, the cohesive force between solid matters in slurry is insufficient, or the adhesive force with a current collector is insufficient, the defect of cracking or demolding is easy to occur, and the safety of a battery using the pole piece is seriously damaged. The viscosity of the third polymer is in a proper range, the adhesive has good adhesive property, and the pole piece can have excellent adhesive force when the addition amount is low, so that the loading amount of the positive active material and the energy density of the battery can be improved.

In some embodiments, the polydispersity of the third polymer is 2 to 2.3.

In some embodiments, the polydispersity of the third polymer is 2.1 to 2.2.

In some embodiments, the dispersant is present in an amount of 0.05% to 1% by mass, based on the total mass of the solid matter of the positive electrode slurry.

In some embodiments, the dispersant is present in an amount of 0.1% to 1%, 0.2% to 1%, 0.3% to 1%, 0.4% to 1%, 0.6% to 1%, 0.05% to 0.8%, 0.05% to 0.6%, or 0.1% to 0.6% by mass based on the total mass of the solid matter of the positive electrode slurry.

When the mass content of the dispersing agent is less than 0.05%, the dispersing agent is too low in content, so that the dispersing agent cannot fully coat or attach to the positive active material in the positive slurry, the dispersion of the positive active material is not facilitated, the phenomena of agglomeration, filter screen blockage and the like are easily generated in the powder of the positive slurry, the stability of the positive slurry and the processability of a pole piece are influenced, and the film resistance is increased. When the mass content of the dispersing agent is higher than 1%, the content of the dispersing agent is too high, so that the bonding performance of the binder is reduced, and the cohesive force of the conductive agent and the positive active material is reduced, namely, the bonding force of the pole piece is small, so that the demoulding phenomenon is easy to occur in the processing process or the positive active material diffuses into the negative electrode in the long-term recycling process of the battery, and great potential safety hazard is caused. The mass content of the dispersing agent in a proper range can improve the dispersibility of the positive slurry, and has no influence or little influence on the bonding performance of the binder.

In some embodiments, the binder is present in an amount of 0.6% to 1.2% by mass, based on the total mass of the solid matter of the positive electrode slurry.

In some embodiments, the binder is present in an amount of 0.7% to 1.2%, 0.8% to 1.2%, 0.9% to 1.2%, 1% to 1.2%, 0.6% to 1.1%, 0.6% to 1.0%, 0.6% to 0.9%, or 0.6% to 0.8% by mass based on the total mass of solid matter of the positive electrode slurry.

When the mass content of the binder is higher than 1.2%, although the binding power of the pole piece can be obviously improved, the viscosity of the anode slurry is too high, so that the distribution of the anode active substance in the slurry is not uniform, the dispersibility of the anode slurry and the quality of the pole piece are influenced, the film resistance of the pole piece is improved, and the cycle performance of the battery is reduced. When the mass content of the binder is less than 0.6%, the binding performance of the binder is reduced, so that the binding power of the pole piece is insufficient, a demoulding phenomenon is easy to occur in the processing process, or the positive active substance diffuses into the negative electrode in the long-term recycling process of the battery, so that great potential safety hazard is caused. The mass content of the binder in a proper range can ensure that the pole piece has good binding power, avoid the direct contact of positive active substances and electrolyte, reduce the occurrence of side reactions and reduce the potential safety hazard of the secondary battery; meanwhile, the addition amount of the binder is relatively low, which is beneficial to reducing the resistance of a pole piece film, improving the load capacity of the positive active material and improving the energy density of the battery.

In some embodiments, the mass ratio of the second polymer to the third polymer in the binder is 1:9 to 8:2, or 0.1 to 4:1.

In some embodiments, the mass ratio of the second polymer to the third polymer in the binder is 0.1 to 4:1, 0.25 to 4:1, 0.5 to 4:1, 1 to 4:1, 1.5 to 4:1, 2 to 4:1, 2.5 to 4:1, or 3 to 4:1.

When the mass ratio of the second polymer to the third polymer is within the above range, the pole piece can be ensured to have improved flexibility under the premise of good adhesion.

In some embodiments, the binder has a crystallinity of 25% to 44%.

In some embodiments, the crystallinity of the binder is from 25% to 42%, from 25% to 40%, from 25% to 38%, from 25% to 35%, from 25% to 32%, from 25% to 30%, from 28% to 42%, from 30% to 42%, or from 35% to 42%.

When the crystallinity of the binder is lower than 25%, the cohesive force between solid matters in the pole piece film layers or the adhesive force of the film layers is insufficient, so that the demoulding phenomenon is easy to occur in the battery processing process, or the positive active substances of the battery are diffused to the negative electrode in the long-term recycling process, so that great potential safety hazards are caused. When the crystallinity of the adhesive is higher than 44%, the plastic strain force of the film layer during the hot-pressing treatment of the bare cell is insufficient, and the crack and the fracture are easy to generate. The crystallinity of the binder is in a proper range, so that the flexibility of the pole piece is improved, and the pole piece has proper hardness, and is beneficial to processing the secondary battery and reducing the potential safety hazard of the secondary battery.

In some embodiments, the binder has a melting enthalpy of 25J/g to 45J/g.

In some embodiments, the binder has a melting enthalpy of 28J/g to 45J/g, 30J/g to 45J/g, 32J/g to 45J/g, 35J/g to 45J/g, 28J/g to 43J/g, 28J/g to 40J/g, 28J/g to 38J/g, 28J/g to 35J/g, or 28J/g to 33J/g.

The melting enthalpy of the adhesive is in a proper range, so that the crystallinity of the adhesive is moderate, and the pole piece has excellent flexibility and adhesive force.

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

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

The application also provides a preparation method of the positive electrode slurry, which comprises the following steps:

step 1: uniformly mixing a positive electrode active substance, a conductive agent and a binder; the binder comprises a second polymer with the weight-average molecular weight of 70-110 ten thousand and a third polymer with the weight-average molecular weight of 130-300 ten thousand,

step 2: adding a dispersing agent into the mixture and stirring the mixture to obtain positive electrode slurry, wherein the dispersing agent comprises a first polymer with the weight-average molecular weight of 0.5-15 ten thousand, and the first polymer, the second polymer and the third polymer are all prepared by polymerizing at least one monomer shown in a formula II under a polymerizable condition,

formula II

The method of adding the binder and then adding the dispersant is favorable for realizing the full mixing, adhesion/coating among the positive active material, the conductive agent and the high molecular weight binder, and the dispersant is added later, so that the sedimentation of the positive active material and the high molecular weight binder can be effectively avoided, and the dispersibility and the stability of the positive slurry can be improved.

In some embodiments, the R is ₁ 、R ₂ Are all hydrogen. In some embodiments, the R is ₁ 、R ₂ Are both fluorine. In some embodiments, R ₁ Is hydrogen, R ₂ Is trifluoromethyl. In some embodiments, R ₁ Is trifluoromethyl, R ₂ Is hydrogen.

In some embodiments, the method of preparing the first polymer comprises the steps of:

providing at least one monomer shown as a formula II, a first initiator and a first solvent, carrying out polymerization reaction for 2-8 hours at the reaction temperature of 55-80 ℃ under normal pressure, stopping the reaction, carrying out solid-liquid separation, and retaining a solid phase to obtain a first polymer.

In some embodiments, the method of making the first polymer comprises the steps of:

providing at least one monomer shown as a formula II, a first initiator and a first solvent, carrying out polymerization reaction for 2-8 hours at the reaction temperature of 55-80 ℃ under the non-reactive gas atmosphere and normal pressure, stopping the reaction, carrying out solid-liquid separation, and retaining a solid phase to obtain a first polymer.

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

In some embodiments, in the preparation method of the first polymer, the reaction temperature is 60-80 ℃, 65-80 ℃, 70-80 ℃, or 66-80 ℃, 68-80 ℃, 73-80 ℃, 64-75 ℃ or 55-73 ℃.

In some embodiments, in the method of preparing the first polymer, the reaction time is 2 hours to 4 hours, 2 hours to 3 hours, 2 hours to 6 hours, 3 hours to 8 hours, 3 hours to 6 hours, 4 hours to 8 hours, 4 hours to 6 hours, 6 hours to 8 hours.

In some embodiments, the method of preparing the first polymer further comprises the steps of:

adding a first solvent and a first dispersing aid into a container, and filling a non-reactive gas into the container;

and adding a first initiator and a first pH regulator into the container, regulating the pH value, adding a monomer shown in the formula II, stirring for 0.5-1 hour, heating to 55-80 ℃, and carrying out polymerization reaction for 2-8 hours.

The term "initiator" refers to a substance that, in a polymerization reaction, initiates the polymerization of a monomer. Exemplary initiators are 2-ethyl peroxydicarbonate, t-butyl peroxypivalate, t-amyl peroxypivalate.

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

The term "dispersing aid" refers to a substance capable of promoting uniform dispersion of a monomer in a medium during a synthesis reaction. Exemplary dispersing aids include carboxyethyl cellulose ethers and methyl cellulose ethers.

In some embodiments, in the method for preparing the first polymer, the first solvent is water, which is beneficial to reducing the harm to the environment.

In some embodiments, the first polymer is prepared by a process that adjusts the pH to a value between 6.5 and 7, such as 6.5, 6.8, or 7.

In some embodiments, the first polymer is prepared by a method wherein the stirring time is from 30 minutes to 55 minutes, from 30 minutes to 50 minutes, from 30 minutes to 45 minutes, from 35 minutes to 60 minutes, from 40 minutes to 60 minutes, or from 45 minutes to 60 minutes.

In the preparation method of the first polymer, the prepared first polymer (or dispersing agent) has lower weight average molecular weight and viscosity, has good adhesion with a positive active material, and obviously improves the dispersibility and stability of slurry. The preparation method of the first polymer has low raw material cost and mild reaction conditions, and is beneficial to the mass production of the dispersing agent.

In some embodiments, the method of making the third polymer comprises the steps of:

providing at least one monomer shown as a formula II, a second initiator and a second solvent, raising the temperature to 35-60 ℃ when the reaction pressure of the monomer shown as the formula II reaches 6-8 MPa, and carrying out polymerization reaction for 6-10 hours;

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

In some embodiments, the method of preparing the third polymer is performed under a non-reactive gas atmosphere.

In the third polymer preparation process, the initiator has the same meaning as the initiator in the first polymer preparation process. In some embodiments, the first initiator and the second initiator are the same chemical species. In some embodiments, the first initiator and the second initiator are different chemical species.

In some embodiments, the second initiator is t-butyl peroxypivalate.

The term "chain transfer agent" refers to a chemical species capable of generating a free radical which, upon interaction with a starting molecule, produces a product and another free radical, enabling the reaction to continue. Chain transfer agents may be used to control the chain length of the polymer, i.e., to control the degree of polymerization of the polymer, or the viscosity of the polymer. Exemplary chain transfer agents are cyclohexane.

In some embodiments, the third polymer is prepared by charging a reaction vessel with a monomer of formula II to a reaction pressure of 6MPa to 7MPa or 7MPa to 8 MPa.

In some embodiments, the reaction temperature of the third polymer is 37 ℃ to 60 ℃, 40 ℃ to 60 ℃, 43 ℃ to 60 ℃, 45 ℃ to 60 ℃, 50 ℃ to 60 ℃, 35 ℃ to 55 ℃, 35 ℃ to 50 ℃, or 35 ℃ to 45 ℃.

In some embodiments, the reaction time of the third polymer is 6 hours to 9 hours, 6 hours to 8 hours, 6 hours to 7 hours, 7 hours to 10 hours, 8 hours to 10 hours, 9 hours to 10 hours, or 8 hours to 9 hours.

In some embodiments, the preparation of the third polymer comprises the steps of:

adding a second solvent and a second dispersing aid into a container, and filling the container with a non-reactive gas; adding a second initiator and a second pH regulator into the container, regulating the pH value, then adding a monomer shown in the formula II until the reaction pressure is 6 MPa-8 MPa, stirring for 0.5-1 hour, heating to 35-60 ℃, and carrying out polymerization for 6-10 hours;

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

In the third polymer production method, the meaning of the dispersion aid is the same as that of the dispersion aid in the first polymer production method. In some embodiments, the first dispersing aid and the second dispersing aid are the same chemical species. In some embodiments, the first dispersing aid and the second dispersing aid are different chemicals. In some embodiments, the second dispersion aid is a methyl cellulose ether.

In the third polymer production method, the pH adjuster has the same meaning as that of the pH adjuster in the first polymer production method. In some embodiments, the first pH adjusting agent and the second pH adjusting agent are the same chemical. In some embodiments, the first pH adjusting agent and the second pH adjusting agent are different chemicals.

In some embodiments, the third polymer is prepared by a process in which the pH is adjusted to a value of from 6.5 to 7, for example 6.5, 6.8 or 7.

In some embodiments, in the method for preparing the third polymer, the second solvent is water, which is beneficial to reducing the harm to the environment.

In some embodiments, the third polymer is prepared by a method in which the stirring time is 30 to 55 minutes, 30 to 50 minutes, 30 to 45 minutes, 35 to 60 minutes, 40 to 60 minutes, or 45 to 60 minutes.

In the preparation method of the third polymer, the raw materials are easy to obtain, the reaction conditions are safe and controllable, and the expanded production of the third polymer is facilitated. The third polymer prepared by the method has higher molecular weight and viscosity, and the pole piece can have good adhesive force by adding a small amount of the third polymer when preparing battery slurry.

The secondary battery, the battery module, the battery pack, and the electric device according to the present invention 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. In the process of charging and discharging the battery, active ions are embedded and separated back and forth between the positive pole piece and the negative pole piece. The electrolyte plays a role in conducting ions between the positive pole piece and the negative pole piece. The isolating membrane is arranged between the positive pole piece and the negative pole piece, mainly plays a role in preventing the short circuit of the positive pole and the negative pole, and can enable ions to pass through.

[ Positive electrode sheet ]

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

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

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

In some embodiments, the positive active material may be a positive active material for a battery, which is 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 positive electrode active material of a battery may be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of the lithium transition metal oxide 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 (may also be abbreviated as NCM) ₃₃₃ ）、LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (may also be abbreviated as NCM) ₅₂₃ ）、LiNi _0.5 Co _0.25 Mn _0.25 O ₂ (may also be abbreviated as NCM) ₂₁₁ ）、LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (may also be abbreviated as NCM) ₆₂₂ ）、LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (may also be abbreviated as NCM) ₈₁₁ ) Lithium nickel cobalt aluminum oxides (e.g., LiNi) _0.85 Co _0.15 Al _0.05 O ₂ ) And modified compounds thereof, 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 referred to as LFP for short)), a composite material of lithium iron phosphate and carbon, and lithium manganese phosphate (e.g., LiMnPO) ₄ ) At least one of a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

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), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluoroacrylate resin.

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 components for preparing the positive electrode plate, such as the positive active material, the conductive agent, the binder and any other components, in a solvent (such as N-methylpyrrolidone) to form positive electrode slurry; and coating the positive electrode slurry on a positive electrode current collector, and drying, cold pressing and the like to obtain the positive electrode piece.

[ negative electrode sheet ]

The negative pole piece includes the negative pole mass flow body and sets up the negative pole rete on the negative pole mass flow body at least one surface, the negative pole rete includes negative pole active material.

As an example, the negative electrode current collector has two surfaces opposite in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two surfaces opposite to the negative electrode 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 can be used. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer 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 base material of a polymer material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), Polystyrene (PS), Polyethylene (PE), etc.).

In some embodiments, the negative active material may employ a negative active material for a battery 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 can be at least one selected from elemental silicon, silicon-oxygen compounds, silicon-carbon compounds, silicon-nitrogen compounds and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin-oxygen compounds, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery negative active material may also be used. These negative electrode active materials may be used alone or in combination of two or more.

In some embodiments, the anode 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 may be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

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

In some embodiments, the negative electrode sheet can be prepared by: dispersing the components for preparing the negative electrode plate, such as a negative electrode active material, a conductive agent, a binder and any other components, in a solvent (such as deionized water) to form negative electrode slurry; and coating the negative electrode slurry on a negative electrode current collector, and drying, cold pressing and the like to obtain the negative electrode pole piece.

[ electrolyte ]

The electrolyte plays a role in conducting ions between the positive pole piece and the negative pole piece. The kind of the electrolyte is not particularly limited and may be selected as desired. For example, the electrolyte may be liquid, gel, or all solid.

In some embodiments, the electrolyte is an electrolytic solution. 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-fluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalato borate, lithium dioxaoxalato borate, lithium difluorodioxaoxalato phosphate, and lithium tetrafluorooxalato phosphate.

In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl 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, dimethylsulfone, methylethylsulfone, and diethylsulfone.

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

[ isolation film ]

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

In some embodiments, the material of the isolation 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 exterior package. The exterior package may be used to enclose the electrode assembly and electrolyte.

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 outer package of the secondary battery may also be a pouch, such as a pouch-type pouch. The material of the soft bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

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

In some embodiments, referring to fig. 2, the overwrap may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose to form an accommodating cavity. The housing 51 has an opening communicating with the accommodating chamber, and a cover plate 53 can be provided to cover the opening to close the accommodating 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. An electrode assembly 52 is enclosed within the receiving cavity. The electrolyte is impregnated into the electrode assembly 52. The number of the electrode assemblies 52 contained in the secondary battery 5 may be one or more, and those skilled in the art can select them according to specific practical needs.

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

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

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

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

Fig. 4 and 5 are a battery pack 1 as an example. Referring to fig. 4 and 5, a battery pack 1 may include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery box comprises an upper box body 2 and a lower box body 3, wherein 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. A plurality of battery modules 4 may be arranged in any manner in the battery box.

In addition, this application still provides a power consumption device, power consumption device includes at least one in secondary battery, battery module or the battery package that this application provided. The secondary battery, the battery module, or the battery pack may be used as a power source of the electric device, and may also be used as an energy storage unit of the electric device. The powered device may include, but is not limited to, a mobile device (e.g., a mobile phone, a laptop computer, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, and a satellite, an energy storage system, etc.

As the electricity-using device, a secondary battery, a battery module, or a battery pack may be selected according to the use requirement thereof.

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

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

Examples

Hereinafter, examples of the present application will be described. The following description of the embodiments is merely exemplary in nature and is in no way intended to limit the present disclosure. The examples, where specific techniques or conditions are not indicated, are to be construed according to the techniques or conditions described in the literature in the art or according to the product specifications. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available.

Example 1

1) Preparation of the third Polymer

A10L autoclave was charged with 4kg of deionized water and 0.2g of methyl cellulose ether, evacuated and charged with N ₂ By replacement of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, filling 1Kg of vinylidene fluoride monomer to ensure that the pressure reaches 7Mpa, mixing and stirring for 30min, and heating to 45 ℃ to carry out polymerization reaction; after reacting for 8 hours, adding 25g of cyclohexane to continue the reaction, and stopping the reaction 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 polyvinylidene fluoride binder.

2) Preparation of the Binder

The PVDF second polymer having a weight average molecular weight of 70 ten thousand and the third polymer of example 1 were dissolved in N-methylpyrrolidone (NMP) solution, respectively, to prepare 10% dope, and then mixed at a mass ratio of 1: 9. Wherein PVDF having a weight-average molecular weight of 70 ten thousand is HSV900 model of Akema France, Inc.

Examples 2 to 9 the mass ratio of the PVDF binder having a weight average molecular weight of 70 ten thousand to the third polymer prepared in example 1 was adjusted, and specific parameters are shown in table 1.

Example 10

1) Preparation of the third Polymer

4kg of deionized water and 0.2g of methyl cellulose ether were placed in a 10L autoclave, evacuated and charged with N ₂ By substitution of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1Kg of vinylidene fluoride, allowing the monomer pressure to reach 7Mpa, mixing and stirring for 30min, heating to 45 ℃, and carrying out polymerization reaction; after reacting for 6 hours, adding 30g of cyclohexane to continue the reaction, and stopping the reaction 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 polyvinylidene fluoride binder.

2) Preparation of the Binder

The PVDF second polymer having a weight average molecular weight of 70 ten thousand and the third polymer of example 10 were dissolved in N-methylpyrrolidone (NMP) solutions, respectively, to prepare 10% dope, and then mixed at a mass ratio of 4: 6.

Example 11

1) Preparation of the third Polymer

A10L autoclave was charged with 4kg of deionized water and 0.2g of methyl cellulose ether, evacuated and charged with N ₂ By replacement of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1Kg of vinylidene fluoride, allowing the monomer pressure to reach 7Mpa, mixing and stirring for 30min, heating to 45 ℃, and carrying out polymerization reaction; after the reaction is carried out for 9 hours, 20g of cyclohexane is added for continuous 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 polyvinylidene fluoride binder.

2) Preparation of the Binder

The other steps are the same as in example 10, except that the third polymer of example 10 is replaced with the third polymer prepared in example 11, see in particular table 1.

Example 12

1) Preparation of the third Polymer

4kg of deionized water and 0.2g of methyl cellulose ether were placed in a 10L autoclave, evacuated and charged with N ₂ By replacement of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, filling 1Kg of vinylidene fluoride into the mixture, enabling the monomer pressure to reach 7Mpa, mixing and stirring the mixture for 30min, and heating the mixture to 37 ℃ to perform polymerization reaction; after reacting for 6 hours, adding 30g of cyclohexane to continue the reaction, and stopping the reaction 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 polyvinylidene fluoride binder.

2) Preparation of the Binder

The other steps are the same as in example 10, except that the third polymer of example 10 is replaced with the third polymer prepared in example 12, see in particular table 1.

Example 13

The PVDF second polymer having a weight average molecular weight of 110 ten thousand and the third polymer of example 1 were dissolved in N-methylpyrrolidone (NMP) solution, respectively, to prepare 10% dope, and then mixed at a mass ratio of 4: 6. Among them, PVDF having a weight average molecular weight of 110 ten thousand is 5130 model number of suwei (shanghai) limited.

Example 14

1) Preparation of the dispersant

Adding 0.4Kg of deionized water and 0.2g of carboxyethyl cellulose ether into a 1L four-neck flask, introducing nitrogen to remove oxygen dissolved in the solution, adding 1g of 2-ethyl peroxydicarbonate and 0.1g of sodium bicarbonate again, filling 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 68 ℃, and carrying out a polymerization reaction for 3 h; and distilling, washing, separating, drying and crushing the polymerization solution to obtain the polyvinylidene fluoride dispersing agent.

2) Preparation of positive pole piece

Adding the lithium iron phosphate serving as the positive active material, the conductive agent carbon black, the binder and the dispersing agent into the adhesive solution prepared in the example 10 according to the weight ratio of 94.8:4:0.8:0.4, uniformly mixing to obtain positive slurry, adding N-methylpyrrolidone (NMP), and adjusting the solid content to be 58%. Uniformly coating the positive electrode slurry on two surfaces of an aluminum foil positive electrode current collector, and then drying to obtain a film layer; and then, obtaining the positive pole piece through cold pressing and slitting.

3) Preparation of negative pole piece

Preparing a negative electrode active material of artificial graphite, a conductive agent of carbon black, a binder of Styrene Butadiene Rubber (SBR), and a thickening agent of sodium carboxymethylcellulose (CMC) according to a weight ratio of 96.2: 0.8: 0.8: 1.2 dissolving in solvent deionized water, and preparing into negative electrode slurry after uniformly mixing; and uniformly coating the negative electrode slurry on two surfaces of the copper foil of the negative current collector for multiple times, and drying, cold pressing and slitting to obtain the negative electrode pole piece.

4) Isolation film

Polypropylene film was used as the separator.

5) Preparation of the 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 Ethyl Methyl Carbonate (EMC) according to the volume ratio of 3/7 uniformly, and mixing LiPF ₆ The lithium salt was dissolved in an organic solvent to prepare a 12.5% solution, to obtain an electrolyte.

6) Preparation of the Battery

The positive electrode plate, the separator and the negative electrode plate prepared in example 14 were sequentially stacked to separate the positive electrode plate from the negative electrode plate, and then wound to obtain a bare cell, and a tab was welded to the bare cell, and the bare cell was placed in an aluminum case, and baked at 80 ℃ to remove water, and then an electrolyte was injected and sealed to obtain an uncharged battery. The lithium ion battery product of example 14 was obtained by sequentially performing the steps of standing, hot-cold pressing, formation, shaping, capacity testing, and the like on the uncharged battery.

Examples 15 to 18 the amount of the positive electrode slurry binder was adjusted, and the other steps were the same as in example 14, specifically referring to table 3.

In examples 19 to 22, the weight average molecular weight of the added dispersant was adjusted, and the other steps were the same as in example 16.

The weight average molecular weight of the first polymer in the dispersant of example 19 was 0.5 ten thousand, and was prepared as follows:

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

The weight average molecular weight of the first polymer in the dispersant of example 20 was 2 ten thousand, and the preparation method was as follows:

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, and charging 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 66 ℃, and carrying out polymerization reaction for 4 h; the polymerization solution is obtained by distillation, washing, separation, drying and crushing.

The weight average molecular weight of the first polymer in the dispersant of example 21 was 8 ten thousand, and the preparation method was as follows:

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, and charging 0.1Kg of vinylidene fluoride, mixing and stirring for 30min, heating to 64 ℃, and carrying out polymerization reaction for 6 h; the polymerization solution is obtained by distillation, washing, separation, drying and crushing.

The weight average molecular weight of the first polymer in the dispersant of example 22 was 15 ten thousand, and the preparation method was as follows:

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

In examples 23 to 26, the content of the dispersant was adjusted, and the other methods were the same as in example 21, and the specific parameters are shown in Table 3.

In examples 27 to 30, the ratio of the second polymer to the third polymer in the binder was adjusted, and the other methods were the same as in example 21, and the specific parameters are shown in table 3.

In examples 31 to 35, the weight average molecular weight of the third polymer in the binder was adjusted to 250 ten thousand, and the mass content of the binder was adjusted, in the same manner as in example 14, and the specific parameters are shown in table 3.

In examples 36 to 38, the weight average molecular weight of the third polymer in the binder was adjusted to 300 ten thousand, and the mass content of the binder was adjusted, in the same manner as in example 14, and the specific parameters are shown in table 3.

In example 39, the weight average molecular weight of the third polymer in the binder was adjusted to 130 ten thousand, and the other methods were the same as in example 14, and the specific parameters are shown in Table 3.

The method of manufacturing the secondary battery in example 40 was similar to that of manufacturing the secondary battery in example 14 except that:

2) preparation of positive pole piece

Adding the positive active material lithium nickel cobalt manganese oxide NCM, the conductive agent carbon black, the binder and the dispersing agent into the glue solution of the binder prepared in the embodiment 13 according to the weight ratio of 94.6:4:1.0:0.4, and uniformly mixing to obtain positive slurry, adding N-methylpyrrolidone (NMP), and adjusting the solid content to be 58%. Uniformly coating the positive electrode slurry on two surfaces of an aluminum foil positive electrode current collector, and then drying to obtain a film layer; and then, obtaining the positive pole piece through cold pressing and slitting.

The weight average molecular weight of the dispersant was adjusted in examples 41 to 42, and the other methods were the same as in example 40, and the specific parameters are shown in Table 3.

The types of the third polymers were adjusted in examples 43 to 44, and the other methods were the same as in example 16, and the other parameters are shown in Table 3.

Example 43 the third polymer was polytetrafluoroethylene prepared by the method comprising:

A10L autoclave was charged with 4kg of deionized water and 0.2g of methyl cellulose ether, evacuated and charged with N ₂ By replacement of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 1Kg of tetrafluoroethylene, allowing the monomer to reach 7Mpa, mixing and stirring for 30min, heating to 45 ℃, and carrying out polymerization reaction; after reacting for 6 hours, adding 30g of cyclohexane to continue the reaction, and stopping the reaction 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 catalyst.

The third polymer in example 44 was a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP) prepared by:

A10L autoclave was charged with 4kg of deionized water and 0.2g of methyl cellulose ether, evacuated and charged with N ₂ By substitution of O ₂ Thirdly, adding 5g of tert-butyl peroxypivalate and 2g of sodium bicarbonate again, charging 0.8Kg of vinylidene fluoride and 0.2Kg of hexafluoropropylene into the mixture, enabling the monomer pressure to reach 7MPa, mixing and stirring the mixture for 30min, heating the mixture to 44 ℃, and carrying out polymerization reaction; after reacting for 6 hours, adding 30g of cyclohexane to continue the reaction, and stopping the reaction 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 catalyst.

The binder of comparative example 1 contained only the third polymer having a weight average molecular weight of 250 ten thousand, the binder of comparative example 2 contained only the second polymer having a weight average molecular weight of 70 ten thousand, and the binder of comparative example 3 contained only the second polymer having a weight average molecular weight of 110 ten thousand, and the other methods are the same as in example 1, and specific parameters are shown in table 1.

The binders of comparative examples 4 to 6 comprise only the second polymer, the binders of comparative examples 7 to 10 comprise only the third polymer, and the other methods are the same as those of example 14, and the specific parameters are shown in Table 3.

The performance test results of the binders prepared in examples 1 to 13 and comparative examples 1 to 3 are shown in table 1, the performance test results of the polymers in examples 1 to 44 and comparative examples 1 to 10 are shown in table 2, the performance test results of the pole pieces and batteries prepared in examples 1 to 44 and comparative examples 1 to 10 are shown in table 3, and the test methods are as follows:

measurement of Performance

1. Method for testing weight average molecular weight and polydispersity

A Waters 2695 Isocratic HPLC type gel chromatograph (differential refractometer 2141) was used. A3.0% by weight sample of polystyrene solution was used as a reference and a matching column (oily: Styragel HT5 DMF7.8 x 300mm + Styragel HT 4) was selected. Preparing 3.0% of adhesive glue solution by using purified N-methylpyrrolidone (NMP) solvent, and standing the prepared solution for one day for later use. In the test, tetrahydrofuran is firstly sucked up by a syringe, and the test is carried out by washing and repeating for several times. Then 5ml of the test solution was aspirated, the air in the syringe was removed and the tip of the needle was wiped dry. And finally, slowly injecting the sample solution into the sample inlet. And acquiring the detection data of the weight average molecular weight and the polydispersity coefficient after the readout is stable.

2. Method for testing median particle diameter Dv50

The particle size distribution was determined by the laser diffraction method of GB/T19077-2016 using a Malvern Mastersizer 2000E laser particle size analyzer.

3. Viscosity measurement

Respectively dissolving a first polymer, a second polymer and a third polymer in an N-methylpyrrolidone (NMP) solvent, wherein the first polymer and the second polymer are prepared into glue solution with the solid content of 7%, and the third polymer is prepared into glue solution with the solid content of 4%. Select suitable rotor, fix the viscometer rotor, place the glue solution in viscometer rotor below, the scale mark that the rotor was just submerged to thick liquids, instrument model: shanghai squareness NDJ-5S, rotor: 61# (0-500 mPas), 62# (500-2500 mPas), 63# (2500-10000 mPas), 64# (10000-50000 mPas), rotation speed: 12r/min, test temperature: and (4) at 25 ℃, the testing time is 5min, and the data is stably read after the data is displayed.

4. Method for testing crystallinity and fusion enthalpy

The first polymer and the second polymer in examples 1-14 are respectively dissolved in an N-methyl pyrrolidone (NMP) solution to prepare a 10% glue solution, the glue solutions of the first polymer and the second polymer in the binder of examples 1-14 are weighed according to the mass ratio and mixed, then the uniformly stirred and dispersed mixed solution is placed in a glue film preparation container and dried for 2 days at 100 ℃, then the glue film is cut into small pieces of 2 x 2cm and placed in an aluminum dry pot, the pieces are leveled, a crucible cover is covered, a blowing gas of 50mL/min and a protective gas of 70mL/min are used under a nitrogen atmosphere, the temperature rising rate is 10 ℃/min, the testing temperature range is-100 ℃ to 400 ℃, and the testing is carried out by using a Differential Scanning Calorimeter (DSC) of a U.S. TA instrument model number Discovery 250, and the thermal history is eliminated.

The DSC/(Mw/mg) of the adhesive film along with the temperature change curve is obtained through the test, and the integral is carried out, so that the peak area is the melting enthalpy delta H (J/g) of the adhesive film, and the calculation is carried out according to the following formula:

wherein Δ Hm100% is the standard enthalpy of fusion (crystalline heat of fusion) of PVDF, and Δ Hm100% = 104.7J/g.

5. Measurement of the resistance of the positive electrode film layer:

and cutting the dried anode slurry (film layer) at the left, middle and right parts of the anode piece into small round pieces with the diameter of 3 mm. And starting the power supply of the pole piece resistance instrument of the meta-energy science and technology, placing the power supply in a proper position of a probe of the pole piece resistance instrument, clicking a start button, and reading when the reading is stable. And testing two positions of each small wafer, and finally calculating the average value of six measurements to obtain the film resistance of the electrode sheet.

6. Method for testing brittleness of pole piece

The prepared positive pole piece is cut into a test sample with the size of 20 multiplied by 100mm for standby. Bending and folding the pole piece, fixing, rolling once by using a 2 kg-weight cylindrical roller, and checking whether the folded part of the pole piece is transparent or not and leaks metal; if no light-transmitting metal leakage exists, the pole piece is reversely folded and fixed, and is rolled once again to check whether the folded position of the pole piece is light-transmitting metal leakage or not, and the steps are repeated until the folded position of the pole piece is light-transmitting metal leakage and the light-transmitting rolling times are recorded. Three samples were taken for testing and the average was taken.

7. Method for testing adhesive force

Referring to the national standard GBT 2790-:

a pole piece sample with the width of 30mm and the length of 100-160mm is cut by a blade, and the special double-sided adhesive tape is attached to the steel plate, wherein the width of the adhesive tape is 20mm, and the length of the adhesive tape is 90-150 mm. And (3) sticking the pole piece sample intercepted in the front on a double-faced adhesive tape, enabling the test surface to face downwards, and rolling for three times in the same direction by using a press roller.

And inserting a paper tape with the width equal to that of the pole piece sample and the length of 250mm below a pole piece current collector, and fixing the paper tape by using wrinkle glue.

And (3) opening a power supply (the sensitivity is 1N) of the three-wire tensile machine, lighting the indicating lamp, adjusting the limiting block to a proper position, and fixing one end of the steel plate, which is not attached to the pole piece sample, by using the lower clamp. And turning the paper tape upwards, fixing the paper tape by using an upper clamp, adjusting the position of the upper clamp by using an 'up' button and a 'down' button on a manual controller attached to a tensile machine, and then testing and reading a numerical value. And dividing the force of the pole piece when the stress of the pole piece is balanced by the width of the adhesive tape to be used as the adhesive force of the pole piece with unit length so as to represent the adhesive force strength between the positive pole film layer and the current collector.

8. Method for testing capacity retention rate of battery

A lithium iron phosphate system: taking example 14 as an example, the battery capacity retention rate test procedure is as follows: the cell corresponding to example 14 was charged at 25 ℃ to 3.65V at a constant current of 1/3C, further charged at a constant voltage of 3.65V to a current of 0.05C, left for 5min, and further discharged at 1/3C to 2.5V, and the resulting capacity was designated as initial capacity C0. Repeating the steps on 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:

during the test, the first cycle corresponds to n =1, the second cycle corresponds to n =2, and … … the 100 th cycle corresponds to n = 100. The data of the battery capacity retention rate corresponding to example 14 in table 3 is the data measured after 500 cycles under the above-mentioned test conditions, i.e., the value of P500. The test procedure of comparative example 4 and other examples was the same as above;

lithium nickel cobalt manganese oxide NCM system: taking example 40 as an example, at 25 ℃, a battery corresponding to example 40 is charged to 4.4V at a constant current of 1/3C, then charged to a current of 0.05C at a constant voltage of 4.4V, left for 5min, and then discharged to 2.8V at 1/3C, and the obtained capacity is marked as initial capacity C0. Other testing steps are the same as those of the lithium iron phosphate system.

The parameters and test results of examples 1 to 44 and comparative examples 1 to 10 are shown in tables 1, 2 and 3 below.

From the above results, it is understood that in examples 3 to 8 and 10 to 12, the binder is prepared using the second polymer having a weight average molecular weight of 70 ten thousand, a median particle diameter Dv50 of 15 μm, and a viscosity of 4000mPa · s, and the third polymer having a weight average molecular weight of 130 ten thousand to 300 ten thousand and a polydispersity number of 2.05 to 2.2, and the enthalpy of fusion and the crystallinity of examples 3 to 8 and 10 to 12 are decreased compared to comparative example 2 using only the second polymer having a weight average molecular weight of 70 ten thousand as the binder, indicating that the flexibility of the binder is improved by mixing the second polymer and the third polymer.

In examples 1 to 9, the binder was prepared using the second polymer having a weight average molecular weight of 70 ten thousand and the third polymer having a weight average molecular weight of 250 ten thousand in a mass ratio of 1:9 to 9:1, and the addition of the second polymer reduced the enthalpy of fusion and the crystallinity of the binder and improved the flexibility of the binder, as compared to comparative example 1 in which only the third polymer having a weight average molecular weight of 250 ten thousand was used as the binder.

In example 13, a binder was prepared using a second polymer having a weight average molecular weight of 110 ten thousand, a median particle diameter Dv50 of 25 μm, and a viscosity of 2500mPa · s, and a third polymer having a weight average molecular weight of 250 ten thousand at a mass ratio of 4:6, which shows that the enthalpy of fusion and the crystallinity of the binder are reduced and the flexibility of the binder is improved by mixing the second polymer and the third polymer, compared to comparative example 3 in which only the second polymer having a weight average molecular weight of 110 ten thousand is used as the binder.

Examples 14 to 18, 31 to 35, 36 to 38, 39 and 43 to 44 secondary batteries were prepared using 0.40% by mass of a dispersant having a weight average molecular weight of 1 ten thousand and 0.80% to 1.2%, 0.60% to 0.8%, 0.80% to 1.2%, 0.80% to 0.8%, 1.00% by mass of a binder, respectively, using a second polymer having a weight average molecular weight of 70 ten thousand and a comparative example 130 (median diameter Dv50 of 60 μm, viscosity of 3600mPa · s), 250 ten thousand (median diameter Dv50 of 80 μm, viscosity of 4300mPa · s), 300 ten thousand (median diameter Dv50 of 100 μm, viscosity of 4800mPa · s), 130 (comparative example 130 (median diameter Dv50 of 30 μm, viscosity of 1700mPa · s), 180 (PTFE/PVDF-HFP) using a third polymer having a weight average molecular weight of 180 to 10 comparative example 7, the film resistance is reduced, and the capacity retention rate of the pole piece is obviously improved in average rolling times and 500 circulation times; compared with comparative example 5 in which a secondary battery is prepared by only using a second polymer with the weight average molecular weight of 70 ten thousand, the film resistance of the electrode sheets in examples 14-18 is reduced, and the average rolling times, the adhesive force and the capacity retention rate after 500 cycles of the electrode sheet are obviously improved. The data in table 1 show that the second polymer and the third polymer are mixed in a set proportion range, so that the crystallinity of the adhesive can be reduced, the flexibility of the adhesive can be improved, and the flexibility of the pole piece can be improved; meanwhile, the addition of the dispersing agent can further improve the dispersibility of the slurry, the processability of the pole piece is obviously improved, the uniformity of slurry coating is improved, the resistance of the pole piece is reduced, and the cycle performance of the battery is improved. The combined use of the second polymer and the third polymer can ensure that the pole piece film layer still keeps excellent cohesive force on the basis of having good processability, thereby ensuring the cycle safety of the secondary battery.

Examples 19 to 22, and 40 to 42 secondary batteries were prepared using a dispersant and a binder having a weight average molecular weight of 0.5 ten thousand (median particle diameter Dv50 of 0.5 μm, viscosity of 20mPa · s), 1 ten thousand (median particle diameter Dv50 of 0.8 μm, viscosity of 30mPa · s), 2 ten thousand (median particle diameter Dv50 of 1 μm, viscosity of 50mPa · s), 8 ten thousand (median particle diameter Dv50 of 2 μm, viscosity of 120mPa · s), 15 ten thousand (median particle diameter Dv50 of 4 μm, viscosity of 180mPa · s), respectively, and the binder was prepared using a second polymer having a weight average molecular weight of 70 ten thousand and a third polymer having a weight average molecular weight of 180 ten thousand, median particle diameter Dv50 of 60 μm, viscosity of 3600mPa · s, the second polymer having a weight average molecular weight of 110, and the third polymer having a weight average molecular weight of 250 ten thousand, respectively. Compared with comparative examples 5 to 6 and 8 to 9, the film resistance of the secondary batteries in examples 19 to 22 and 40 to 42 is reduced, and the average rolling frequency and the capacity retention rate of 500 cycles of the pole piece are obviously improved. The data in the table 1 show that the crystallinity of the adhesive can be reduced and the flexibility of the adhesive can be improved after the second polymer and the third polymer are mixed, so that the flexibility of the pole piece can be improved; the dispersing agent with the weight-average molecular weight of less than 15 ten thousand is helpful for improving the dispersibility of the slurry, so that the slurry is uniformly coated, the resistance of the pole piece is reduced, and the cycle performance of the battery is improved.

In examples 23 to 26, secondary batteries were manufactured using 0.05 to 1.00% by mass of a dispersant having a weight average molecular weight of 8 ten thousand and a binder, which was manufactured using a second polymer having a weight average molecular weight of 70 ten thousand and a third polymer having a weight average molecular weight of 180 ten thousand, respectively. Compared with the comparative examples 5 and 8, the resistance of the pole piece film layer of the secondary battery in the examples 23-26 is obviously reduced, and the adhesive force is not greatly reduced, which shows that the dispersant in the range is beneficial to improving the stability and the processability of the positive pole slurry, improving the uniformity of slurry distribution and reducing the resistance of the pole piece, and meanwhile, the adhesive force of the pole piece is not greatly reduced.

In examples 27 to 30, secondary batteries were manufactured using a dispersant having a weight average molecular weight of 8 ten thousand and a binder, which were manufactured using a second polymer having a weight average molecular weight of 70 ten thousand and a third polymer having a weight average molecular weight of 180 ten thousand in a mass ratio of 1:9 to 8:2, respectively. Compared with the comparative examples 5 and 8, the sheet resistance of the pole piece of the secondary battery in the examples 27-30 is obviously reduced, the average rolling times of the pole piece are also obviously improved, and good adhesive force and capacity retention rate of 500 times of circulation are still kept. The combination of the second polymer and the third polymer in the range is beneficial to improving the crystallinity and the flexibility of the adhesive, so that the flexibility of the pole piece is improved, and the pole piece is ensured to have good adhesive force; and the addition of the dispersant improves the dispersibility and stability of the slurry, and improves the film resistance and the battery cycle performance.

In example 26, a secondary battery is prepared by using a dispersant with the mass content of 1% and a binder with the mass content of 1%, and in comparative example 4, a secondary battery is prepared by using a second polymer with the mass content of 2.5%, although the total amount of the additives added in example 26 is smaller than that in comparative example 4, the resistance of a pole piece film layer of the secondary battery in example 26 is obviously reduced, the average rolling frequency and the capacity retention rate of the pole piece after 500 times of circulation are also obviously improved, and meanwhile, good binding power is still kept. The method is beneficial to reducing the impedance increasing multiplying power of the battery, improving the cycle performance of the battery, and improving the compaction density of the pole piece and the energy density of the battery.

The present application is not limited to the above embodiments. The above embodiments are merely examples, and embodiments having substantially the same configuration as the technical idea and exhibiting the same operation and effect within the technical scope of the present application are all included in the technical scope of the present application. In addition, various modifications that can be conceived by those skilled in the art are applied to the embodiments and other embodiments are also included in the scope of the present application, in which some of the constituent elements in the embodiments are combined and constructed, without departing from the scope of the present application.

Claims

1. A positive electrode slurry is characterized by comprising a positive electrode active material, a conductive agent, a dispersing agent and a binder, wherein the dispersing agent comprises a first polymer with the weight-average molecular weight of 0.5-15 ten thousand, the binder comprises a second polymer with the weight-average molecular weight of 70-110 ten thousand and a third polymer with the weight-average molecular weight of 130-300 ten thousand, and the first polymer, the second polymer and the third polymer are all polymers containing structural units shown in a formula I,

formula I

2. The cathode slurry according to claim 1, wherein the first polymer, the second polymer, and the third polymer are each independently selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, a copolymer of vinylidene fluoride, and hexafluoropropylene.

3. The positive electrode slurry according to claim 1, wherein the median diameter Dv50 of the particles of the first polymer is 0.5 μm to 5 μm.

4. The positive electrode slurry according to any one of claims 1 to 3, wherein the first polymer is dissolved in N-methylpyrrolidone to prepare a first slurry, and the viscosity of the first slurry is 20 to 180 mPa-s when the mass content of the first polymer is 7% based on the total mass of the first slurry.

5. The positive electrode slurry according to claim 1, wherein the particles of the second polymer have a median diameter Dv50 of 15 to 25 μm.

6. The positive electrode slurry according to claim 1 or 5, wherein the second polymer is dissolved in N-methylpyrrolidone to prepare a second glue solution, and the viscosity of the second glue solution is 2500-4000 mPa-s when the mass content of the second polymer is 7% based on the total mass of the second glue solution.

7. The positive electrode slurry according to claim 1, wherein the median diameter Dv50 of the particles of the third polymer is 30 μm to 100 μm.

8. The cathode slurry according to claim 1 or 7, wherein the third polymer is dissolved in N-methyl pyrrolidone to prepare a third glue solution, and based on the total mass of the third glue solution, when the mass content of the third polymer is 4%, the viscosity of the third glue solution is 1500-5000 mPa-s.

9. The positive electrode slurry according to claim 1 or 7, wherein the polydispersity index of the third polymer is 2 to 2.3.

10. The cathode slurry according to claim 1 or 2, wherein the dispersant is contained in an amount of 0.05% to 1% by mass based on the total mass of the solid matter of the cathode slurry.

11. The positive electrode slurry according to claim 1 or 2, wherein the binder is contained in an amount of 0.6% to 1.2% by mass based on the total mass of the solid matter of the positive electrode slurry.

12. The positive electrode slurry according to claim 1 or 2, wherein the mass ratio of the second polymer to the third polymer in the binder is 1:9 to 8: 2.

13. The positive electrode slurry according to claim 1 or 2, wherein the binder has a crystallinity of 25% to 44%.

14. The positive electrode slurry according to claim 1 or 2, wherein the binder has a melting enthalpy of 25 to 45J/g.

15. The positive electrode slurry according to claim 1, wherein the positive electrode active material is a lithium-containing transition metal oxide.

16. The positive electrode slurry according to claim 15, wherein the lithium-containing transition metal oxide is lithium iron phosphate or lithium nickel cobalt manganese oxide, or a doping modification material thereof, or at least one of a conductive carbon coating modification material, a conductive metal coating modification material, or a conductive polymer coating modification material thereof.

17. A preparation method of positive electrode slurry is characterized by comprising the following steps:

formula II

18. The method of claim 17, wherein the first polymer is prepared by a method comprising the steps of:

19. The method of claim 17, wherein the third polymer is prepared by a method comprising the steps of:

20. A secondary battery, comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer is made from the positive electrode slurry of any one of claims 1 to 16 or the positive electrode slurry prepared by the method of any one of claims 17 to 19.

21. A battery module characterized by comprising the secondary battery according to claim 20.

22. A battery pack comprising the battery module according to claim 21.

23. An electric device comprising at least one selected from the secondary battery according to claim 20, the battery module according to claim 21, and the battery pack according to claim 22.