WO2024066504A1

WO2024066504A1 - Binder, preparation method, positive electrode plate, secondary battery, and electric device

Info

Publication number: WO2024066504A1
Application number: PCT/CN2023/101416
Authority: WO
Inventors: 曾子鹏; 孙成栋; 李�诚; 刘会会; 王景明
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-09-30
Filing date: 2023-06-20
Publication date: 2024-04-04
Also published as: CN115286728A

Abstract

The present application provides a binder, a preparation method, a positive electrode plate, a secondary battery, and an electric device. The binder comprises a polymer containing a structural unit derived from a monomer represented by formula I, a structural unit derived from a monomer represented by formula II, and a structural unit derived from a monomer represented by formula III, wherein R1 and R2 are each independently selected from hydrogen or fluorine or chlorine, R3 and R4 are selected from hydrogen or substituted or unsubstituted C1-5 alkyl, and the molar content of the structural unit derived from the monomer represented by formula I is 40%-60% on the basis of the number of total moles of all the structural units in the polymer. According to the binder, the proportion of a fluorine-containing structural unit in the polymer is greatly reduced, and the gel phenomenon of a slurry can be remarkably reduced by adding non-fluorine monomers represented by formula II and formula III, such that the stability of the slurry is improved, the flexibility of the electrode plate is improved, and the dispersibility of a positive electrode active material is improved, thereby reducing the cycle internal resistance growth rate of a film resistor and a battery.

Description

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

cross reference

This application refers to Chinese Patent Application No. 202211207124.8 filed on September 30, 2022, entitled “Binder, Preparation Method, Positive Electrode Plate, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety.

Technical Field

The present application relates to the technical field of secondary batteries, and in particular to a binder, a preparation method, a positive electrode sheet, a secondary battery, a battery module, a battery pack and an electrical device.

Background technique

In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

Binders are commonly used materials in secondary batteries and are widely used in battery pole pieces, separators, packaging, etc. However, traditional binders have high production costs, insufficient production capacity, and great environmental hazards. In addition, gelation is easy to occur during the slurry preparation process, resulting in poor slurry stability and high processing costs. The pole pieces prepared with them have poor conductivity, high resistance, low yield, and unstable battery performance, which makes it difficult to meet the market's requirements for battery cost and performance. Therefore, existing binders still need to be improved.

Summary of the invention

The present application is made in view of the above-mentioned problems, and its purpose is to provide a binder that can significantly reduce the gelation phenomenon of the slurry and improve the stability of the slurry.

The first aspect of the present application provides a binder, the binder comprising a polymer containing a structural unit derived from a monomer represented by formula I, a structural unit derived from a monomer represented by formula II, and a structural unit derived from a monomer represented by formula III.

Wherein, _R1 and _R2 are each independently selected from hydrogen or fluorine or chlorine, _R3 and _R4 are selected from hydrogen or substituted or unsubstituted _C1-5 alkyl, and the molar content of the structural unit derived from the monomer represented by formula I is 40% to 60%, based on the total molar number of all structural units in the polymer.

The binder significantly reduces the proportion of fluorine-containing structural units in the polymer, and the addition of non-fluorine monomers shown in formula II and formula III can significantly reduce the gelation phenomenon of the slurry, improve the stability of the slurry, improve the flexibility of the pole piece, and improve the dispersibility of the positive electrode active material, thereby reducing the membrane resistance and the cycle internal resistance growth rate of the battery.

In any embodiment, the molar content of the structural unit derived from the monomer represented by formula II is 15% to 55%, and the molar content of the structural unit derived from the monomer represented by formula III is 5% to 25%, based on the total molar number of all structural units in the polymer.

By controlling the molar content of the structural units derived from the monomers represented by Formula II and Formula III in the polymer within an appropriate range, not only the stability of the slurry is improved and the adhesion of the electrode can be guaranteed, but also the battery has both excellent first charge efficiency and good cycle capacity retention rate, and the overall performance of the battery is improved.

In any embodiment, the weight average molecular weight of the polymer is 500,000 to 1.2 million.

By controlling the weight-average molecular weight of the polymer within an appropriate range, the electrode has excellent flexibility and good adhesion, lower membrane resistance and internal resistance growth rate, thereby improving the battery's cycle capacity retention rate.

In any embodiment, the monomer represented by formula I is selected from one or more of vinylidene fluoride, tetrafluoroethylene, and chlorotrifluoroethylene.

In any embodiment, the monomer represented by formula II is selected from one or two of propylene and 2-butene.

In any embodiment, the monomer represented by formula III is selected from one or both of acrylic acid and methacrylic acid.

The second aspect of the present application also provides a method for preparing a binder, comprising the following steps:

Under polymerizable conditions, polymerizing the monomer represented by formula I, the monomer represented by formula II, and the monomer represented by formula III to prepare a polymer;

Wherein, R ₁ and R ₂ are selected from hydrogen or fluorine or chlorine, R ₃ and R ₄ are selected from hydrogen or substituted or unsubstituted C _1-5 alkyl, wherein the molar content of the monomer represented by formula I is 40% to 60%, based on the total molar number of all monomers.

The preparation method can significantly reduce the amount of fluorinated monomers to reduce costs, reduce environmental pollution, and is conducive to the increase of binder production. At the same time, the binder prepared by this method can significantly slow down the gelation of the slurry, improve the stability of the slurry, improve the flexibility of the pole piece, and reduce the membrane resistance and the cycle internal resistance growth rate of the battery through the effective dispersion of the positive electrode active material.

In any embodiment, the polymerization reaction comprises the following steps:

Dispersing an initiator, a catalyst, a monomer represented by formula I, a monomer represented by formula II, and a monomer represented by formula III in a solvent, and carrying out a polymerization reaction for 1 h to 2 h in an inert gas atmosphere, a reaction pressure of 3.2 MPa to 4.0 MPa, and a reaction temperature of 40° C. to 60° C.;

When the pressure in the reaction system drops to 2.5 MPa-3 MPa, the reaction is stopped, the solid and liquid are separated, and the solid phase is retained.

By controlling the reaction pressure, reaction temperature and reaction time of the polymerization reaction within an appropriate range, the weight-average molecular weight of the polymer can be controlled, so that the electrode has excellent flexibility, good adhesion, lower membrane resistance and internal resistance growth rate, which is beneficial to improving the battery's cycle capacity retention rate.

In any embodiment, the polymerization reaction further comprises the following steps:

Add solvent and emulsifier into the container, evacuate the container and fill it with inert gas. body;

The system is heated to 40° C. to 60° C., an initiator and a catalyst are added to the container, and then the monomers represented by formula I, II and III are added, and an inert gas is continuously introduced to make the pressure in the container reach 3.2 MPa to 4.0 MPa to carry out a polymerization reaction.

In any embodiment, the catalyst is a transition metal-based catalyst, and the transition metal catalyst can be selected from any one of Formula IV, Formula V, Formula VI, and Formula VII.

Among them, Ar is Cy is cyclohexyl, Me is methyl, Ph is phenyl, and TMS is trimethylsilyl.

Transition metal-based catalysts can effectively weaken the difference in polymerization rate caused by the strong polarity of the monomer shown in formula I in the polymerization reaction, avoid the occurrence of liquid phase separation and the inability to complete the polymerization reaction, and obtain copolymers with lower fluorine content, which helps to further reduce the amount of fluorine-containing monomers in the binder, reduce the cost of the binder, and improve the brittleness of the electrode.

In any embodiment, the molar content of the monomer represented by formula II is 15% to 55%, and the molar content of the monomer represented by formula III is 5% to 25%, based on the total molar number of all monomers in the polymer.

By controlling the molar content of the monomers represented by formula II and formula III, the molar content of the non-fluorine structural unit in the polymer can be controlled, which helps to improve the stability of the slurry, ensure the adhesion of the pole piece, and make the battery have both excellent first charge efficiency and good cycle capacity. Retention rate and excellent comprehensive performance.

The third aspect of the present application provides a positive electrode plate, including a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material, a conductive agent and a binder provided in the first aspect of the present application or a binder prepared by the preparation method of the second aspect of the present application.

The electrode has excellent flexibility and good adhesion, and has low membrane resistance.

In any embodiment, the positive electrode active material is at least one of lithium nickel cobalt manganese oxide, a doped modified material of lithium nickel cobalt manganese oxide, or a conductive carbon coated modified material, a conductive metal coated modified material, or a conductive polymer coated modified material thereof.

In any embodiment, the mass fraction of the binder is 0.1% to 3%, optionally 0.2% to 1.2%, based on the mass of the positive electrode active material.

Controlling the mass fraction of the binder within an appropriate range can slow down the gelation of the slurry and improve the stability of the slurry.

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

In any embodiment, the secondary battery is a lithium ion battery or a sodium ion battery.

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

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

FIG2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

FIG3 is a schematic diagram of a battery module according to an embodiment of the present application;

FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application;

FIG5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG4 ;

FIG. 6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

Description of reference numerals:
1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate.

Detailed ways

Hereinafter, the embodiments of the binder, preparation method, electrode, battery and electrical device of the present application are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can be inclusive or exclusive of end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60-120 and 80-110 is listed for a specific parameter, it is understood that the range of 60-110 and 80-120 is also expected. In addition, 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 can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been fully listed herein, and "0-5" is just the abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

Unless otherwise specified, all embodiments and optional embodiments of the present application are The formulas can be combined with each other to form new technical solutions.

Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

In the prior art, polyvinylidene fluoride (PVDF) is often used as a pole piece binder. However, PVDF has many problems in use, such as being sensitive to water content during the production process; in the battery recycling process, a large amount of HF will be generated to pollute the environment, and it cannot be recycled on a large scale due to restrictions on environmental protection policies; in the process of mixing with high-capacity positive electrode materials (such as high-nickel ternary materials) to prepare positive electrode slurry, the strong polar groups on PVDF will activate the residual hydroxyl groups on the positive electrode material, and then react with the metal elements (such as nickel elements) in the positive electrode material to form chemical crosslinks, which eventually leads to slurry gelation, affecting the normal preparation of the slurry and subsequent pole piece processing. At the same time, PVDF is easy to crystallize, which is not conducive to the transmission of electrons in the pole piece, which in turn leads to high resistance of the pole piece and poor electron transmission performance, which is not conducive to the performance of high-capacity positive active materials.

[Binder]

Based on this, the present application proposes a binder, which comprises a polymer containing a structural unit derived from a monomer represented by formula I, a structural unit derived from a monomer represented by formula II, and a structural unit derived from a monomer represented by formula III.

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

In this document, the term "polymer" includes, on the one hand, a collection of chemically uniform macromolecules prepared by polymerization reactions but differing in degree of polymerization, molar mass and chain length; on the other hand, it also includes derivatives of such a collection of macromolecules formed by polymerization reactions, i.e. compounds that can be obtained by reactions of functional groups in the above-mentioned macromolecules, such as addition or substitution, and which can be chemically uniform or chemically heterogeneous.

In some embodiments, the dispersion medium of the binder is an aqueous solvent, such as water, that is, the binder is dissolved in the aqueous solvent.

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

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

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

In some embodiments, the binder is used as a negative electrode binder to bind the negative electrode active materials and/or conductive agents to form electrodes.

Herein, the term "C _1-5 alkyl" refers to a straight or branched hydrocarbon chain group consisting only of carbon and hydrogen atoms, with no unsaturated bonds in the group, having from one to five carbon atoms, and attached to the rest of the molecule by a single bond.

As used herein, the term "substituted" means that at least one hydrogen atom of the compound or chemical moiety is replaced by another chemical moiety with a substituent, wherein the substituent is independently selected from: hydroxyl, thiol, amino, cyano, nitro, aldehyde, halogen atom, alkenyl, alkynyl, aryl, heteroaryl, C _1-6 alkyl, C _1-6 alkoxy.

In some embodiments, the monomer represented by formula I is selected from one or more of vinylidene fluoride, tetrafluoroethylene, and chlorotrifluoroethylene.

In some embodiments, the monomer represented by formula II is selected from one or both of propylene and 2-butene.

In some embodiments, the monomer represented by formula III is selected from one or both of acrylic acid and methacrylic acid.

In some embodiments, the polymer includes but is not limited to vinylidene fluoride-propylene-acrylic acid polymer, tetrafluoroethylene-propylene-acrylic acid polymer, trifluorochloroethylene-propylene-acrylic acid polymer, vinylidene fluoride-propylene-methacrylic acid polymer, vinylidene fluoride-2-butene-acrylic acid polymer, tetrafluoroethylene-propylene-methacrylic acid polymer.

The fluorine element contained in the structural unit derived from the monomer shown in formula I forms hydrogen bonds with the hydroxyl or/and carboxyl groups on the surface of the active material and the current collector, so that the pole piece has good adhesion. Compared with the PVDF binder in the prior art, the molar content of the structural unit derived from the monomer shown in formula I in the polymer provided by this application is 40% to 60%, which can reduce the crystallinity of the polymer and improve the flexibility of the pole piece without significantly reducing the adhesion; at the same time, the content of fluorine element is reduced, which is more environmentally friendly and more in line with environmental protection requirements. If the molar content of the structural unit derived from the monomer shown in formula I is too low, the adhesion of the polymer is insufficient, and the active material is prone to fall off from the pole piece during the processing of the pole piece; if the molar content of the structural unit derived from the monomer shown in formula I is too high, the crystallinity of the polymer is large, the flexibility is reduced, the subsequent processing of the pole piece is too brittle, and the current collector is easily exposed during the bending process of the pole piece, or even broken, leaving safety hazards.

The structural unit derived from the monomer shown in formula II and the structural unit derived from the monomer shown in formula III The unit can introduce disordered units into the periodically arranged segment crystal region formed by the structural unit derived from the monomer shown in formula I, thereby further reducing the crystallinity of the polymer, increasing the mobility of the segment, and improving the flexibility of the pole piece. At the same time, the structural unit derived from the monomer shown in formula II and the structural unit derived from the monomer shown in formula III can weaken the intermolecular force between the structural units derived from the monomer shown in formula I, improve the flexibility of the pole piece, reduce the risk of brittle fracture of high-load high-voltage dense pole pieces, and improve the safety performance of the battery.

The introduction of structural units derived from the monomers shown in formula II and structural units derived from the monomers shown in formula III can improve the solubility of the binder in the solvent, improve the dispersibility of the binder, help the formation of a conductive network, and reduce the film resistance. The carboxyl functional group contained in the structural unit derived from the monomer shown in formula III can react with the alkaline LiOH that is easily generated by the high-nickel ternary material in humid air, avoid alkaline hydrolysis of the slurry in humid air, and further improve the stability of the slurry. Due to the inclusion of the carboxyl functional group, the binder has excellent wettability, dispersibility and stability in the slurry, helps the formation of a conductive network, and can reduce the film resistance.

In summary, the binder can slow down the gelation of the slurry and improve the stability of the slurry. At the same time, it can improve the flexibility of the binder without significantly reducing the bonding force, thereby improving the safety performance of the battery, and improving the electrochemical performance of the battery by improving the dispersion of the positive electrode active material in the electrode sheet to reduce the electrode membrane resistance.

In some embodiments, the molar content of the structural unit derived from the monomer shown in formula II is 15% to 55%, and the molar content of the structural unit derived from the monomer shown in formula III is 5% to 25%, based on the total moles of all structural units in the polymer. In some embodiments, the molar content of the structural unit derived from the monomer shown in formula III is 5% to 10%, 10% to 15%, 15% to 20%, 20% to 25%, 15% to 25%, 5% to 15%, 10% to 25%.

When the molar content of the structural unit derived from the monomer shown in formula III is too high and the molar content of the structural unit derived from the monomer shown in formula II is too low, a large number of free carboxylic acid groups exist in the binder, and the battery consumes a large amount of lithium ions to neutralize the carboxylate during the initial charge and discharge process, resulting in a decrease in the initial performance of the battery. When the molar content of the structural unit derived from the monomer shown in formula III is too low and the molar content of the structural unit derived from the monomer shown in formula II is too high, the stability of the slurry decreases and the internal resistance of the pole piece increases.

By controlling the molar content of the structural unit derived from the monomer represented by Formula III in the polymer within a suitable range, not only the stability of the slurry is improved, but also the battery has both excellent first charge efficiency and good cycle capacity retention rate.

In some embodiments, the weight average molecular weight of the polymer is 500,000 to 1.2 million. In some embodiments, the weight average molecular weight of the polymer is 500,000 to 700,000, 700,000 to 900,000, 900,000 to 1.2 million, 500,000 to 800,000, 800,000 to 1 million, 1 million to 1.2 million.

As used herein, the term "weight average molecular weight" refers to the sum of the products of the weight fractions of molecules with different molecular weights in a polymer and their corresponding molecular weights.

If the weight average molecular weight of the binder is too small, it is difficult to form a three-dimensional network bonding structure and cannot play an effective bonding role. If the weight average molecular weight of the binder is too large, the binder is difficult to dissolve and is easy to agglomerate with the conductive agent, increasing the internal resistance of the membrane. In addition, if the weight average molecular weight of the binder is too high, the viscosity of the slurry will increase, making it difficult to evenly apply, which is not conducive to subsequent processing and production.

By controlling the weight-average molecular weight of the polymer within an appropriate range, the electrode has excellent adhesion, good flexibility, lower membrane resistance and internal resistance growth rate, which is beneficial to improving the battery's cycle capacity retention rate.

In the present application, the weight average molecular weight of the polymer can be tested by methods known in the art, such as gel chromatography, such as Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141). In some embodiments, the test method is to use a polystyrene solution sample with a mass fraction of 3.0% as a reference and select a matching chromatographic column (oily: Styragel HT5DMF7.8*300mm+Styragel HT4). Use purified N-methylpyrrolidone (NMP) solvent to prepare a 3.0% binder glue solution, and let the prepared solution stand for one day for use. During the test, first use a syringe to absorb tetrahydrofuran, rinse, and repeat several times. Then absorb 5 ml of the experimental solution, remove the air in the syringe, and wipe the needle tip dry. Finally, slowly inject the sample solution into the injection port. After the indication is stable, obtain the data and read the weight average molecular weight.

In one embodiment of the present application, a method for preparing a binder is provided, comprising the following steps:

The preparation method can significantly reduce the amount of fluorine-containing monomers, reduce costs, reduce environmental pollution, and is conducive to the increase of binder production. At the same time, the binder prepared by this method can significantly slow down the gelation of the slurry, improve the stability of the slurry, improve the flexibility of the pole piece, and reduce the membrane resistance and the cycle internal resistance growth rate of the battery through the effective dispersion of the positive electrode active material.

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

In some embodiments, the inert gas is a gas that is not easily reactive at ambient temperature and pressure, including but not limited to noble gases and nitrogen.

In some embodiments, the initiator is an organic peroxide; optionally, the organic peroxide includes one or more of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate and tert-butyl peroxypivalate.

In some embodiments, the amount of the initiator used is 0.5% to 1.5% of the total mass of the reaction monomers.

In some embodiments, the catalyst is a transition metal-based catalyst, and the transition metal-based catalyst can be selected from any one of Formula IV, Formula V, Formula VI, and Formula VII.

Among them, Ar is C is cyclohexyl, Me is methyl, Ph is phenyl, and TMS is trimethylsilyl.

In some embodiments, the amount of the catalyst used is 0.1% to 0.4% of the total mass of the reaction monomers.

Transition metal-based catalysts can effectively weaken the difference in polymerization rate caused by the strong polarity of the monomer shown in Formula I in the polymerization reaction, avoid the liquid phase separation phenomenon and the inability to complete the polymerization reaction, help to further reduce the content of fluorine-containing structural units, and achieve a further reduction in the cost of the binder.

By controlling the reaction pressure, reaction temperature and reaction time of the polymerization reaction within an appropriate range, the weight-average molecular weight of the polymer can be controlled, so that the electrode has excellent adhesion, good flexibility, lower membrane resistance and internal resistance growth rate, which is beneficial to improving the battery's cycle capacity retention rate.

In some embodiments, the reaction pressure is 3.2 MPa to 3.5 MPa, 3.5 MPa to 4.0 MPa.

The polymerization reaction pressure is high, the pressure of monomers entering the reaction liquid is high, and more monomers enter the reaction liquid, which can lead to large-scale polymerization reactions and an increase in the number of polymers generated. As the number of monomers decreases, the polymer lacks the supply of monomers and the molecular weight generated is relatively small. The polymerization reaction pressure is low, the pressure of the monomer entering the reaction liquid is low, the reaction monomer cannot be continuously replenished, which is not conducive to the continuous polymerization. The molecular weight of the polymerization product is too low to provide sufficient adhesion, and the battery cycle performance is also reduced.

In some embodiments, the reaction temperature is 40°C to 45°C, 45°C to 50°C, or 50°C to 60°C.

If the polymerization temperature is too low, the driving force of copolymerization is small, the polymerization reaction is insufficient, and the molecular weight of the prepared polymer is too low, which can easily cause a significant decrease in adhesion and cycle performance. If the polymerization temperature is too high, a large-scale polymerization reaction will occur, which can easily lead to an increase in the number of polymers generated. As the monomers decrease, the molecular weight of the polymer is relatively small, which affects the adhesion of the electrode and the battery cycle capacity retention rate. By controlling the reaction temperature of the polymerization reaction within an appropriate range, the weight average molecular weight of the polymer can be controlled, so that the battery has a better cycle capacity retention rate during the cycle.

In some embodiments, the reaction time is 1 h to 1.5 h, or 1.5 h to 2 h.

If the polymerization time is too short, the polymerization reaction cannot be continued, and the prepared molecular weight is too small, which will also cause the decrease of adhesion and cycle performance. If the polymerization time is too long, with the continuous consumption of monomers and the decrease of pressure, the polymerization conditions can no longer be met. Prolonging the reaction time cannot continue the polymerization reaction, which reduces production efficiency.

In some embodiments, the polymerization reaction further comprises the following steps:

Adding a solvent and an emulsifier into a container, evacuating the container and then filling it with an inert gas;

The system is heated to 45°C to 60°C, an initiator and a catalyst are added to the container, and then the monomers represented by formula I, II and III are added, and an inert gas is continuously introduced to make the pressure in the container reach 3.2MPa to 4.0MPa to carry out a polymerization reaction.

In some embodiments, the emulsifier is an alkali metal perfluorooctanoate.

In some embodiments, the amount of the emulsifier used is 0.1% to 0.2% by weight of the monomer.

Adding initiators and catalysts in advance can fully activate and disperse the initiators and catalysts, increase the polymerization reaction rate, and improve the product yield.

The binder is prepared by emulsion polymerization, which has a fast polymerization speed and a high molecular weight of the product; after the reaction reaches a high conversion rate, the viscosity of the emulsion system is still very low, the dispersion system is stable, and it is easier to control and achieve continuous operation.

In some embodiments, the molar content of the monomer shown in formula III is 5% to 25%, based on the total moles of all monomers in the polymer preparation process. In some embodiments, the molar content of the monomer shown in formula II is 15% to 55%, based on the total moles of all monomers in the polymer preparation process. In some embodiments, the molar content of the structural unit derived from the monomer shown in formula III is 5% to 10%, 10% to 15%, 15% to 20%, 20% to 25%, 15% to 25%, 5% to 15%, 10% to 25%.

If the molar content of the monomer shown in Formula III is too large, a large number of free carboxylic acid groups will be present in the binder. During the initial charge and discharge process of the battery, a large amount of lithium ions will be consumed to neutralize the carboxylate groups, resulting in a decrease in the initial performance of the battery. If the molar content of the monomer shown in Formula III is too low, the stability of the slurry will decrease and the internal resistance of the electrode will increase. Controlling the molar content of the monomer shown in Formula III within a suitable range will not only improve the stability of the slurry, but also enable the battery to have both excellent initial charge efficiency and good cycle capacity retention, which is beneficial to improving the overall performance of the battery.

[Positive electrode]

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

The positive electrode sheet has excellent flexibility and good adhesion, and has low membrane resistance.

In some embodiments, the positive electrode active material may be a positive electrode active material for a battery known in the art. As an example, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxide (such as LiCoO ₂ ), lithium nickel oxide (such as LiNiO ₂ ), lithium manganese oxide (such as LiMnO ₂ , LiMn ₂ O ₄ ), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (also referred to as NCM ₃₃₃ ), LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (also referred to as NCM ₅₂₃ ), LiNi _0.5 Co _0.25 Mn _0.25 _... The phosphate containing lithium ions may be selected from the group consisting of NCM ₂₁₁ ), LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (also referred to as NCM ₆₂₂ ), LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (also referred to as NCM ₈₁₁ ), lithium nickel cobalt aluminum oxide (such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and modified compounds thereof. Examples of lithium phosphate containing an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), 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 active material is a lithium-containing transition metal oxide.

In some embodiments, the positive electrode active material is at least one of lithium nickel cobalt manganese oxide, a doped modified material of lithium nickel cobalt manganese oxide, or a conductive carbon coated modified material, a conductive metal coated modified material, or a conductive polymer coated modified material thereof.

In some embodiments, the mass fraction of the binder is 0.1% to 3%, optionally 0.2% to 1.2%, based on the mass of the positive electrode active material. In some embodiments, the mass fraction of the binder can be 0.1% to 0.5%, 0.5% to 1%, 1% to 1.5%, 1.5% to 2%, 2% to 2.5%, 2.5% to 3%, 0.2% to 1.03%, 1% to 1.03%, 1.03% to 1.2%, 1.2% to 3%.

When the binder content is too low, the binder cannot exert sufficient bonding effect. On the one hand, the binder cannot fully disperse the conductive agent and active material, resulting in an increase in the film resistance of the electrode; on the other hand, the binder cannot be tightly bonded to the surface of the active material, resulting in easy powder removal on the surface of the electrode, causing the battery's cycle performance to decline.

On the contrary, when the binder content is too high, the viscosity of the slurry is too high, which will cause the binder coating on the surface of the positive electrode active material to be too thick, affecting the transmission of electrons and ions during the battery cycle, increasing the internal resistance of the membrane, and causing the internal resistance growth rate of the electrode during the cycle to increase and the capacity retention rate to decrease.

When the mass fraction of the binder is within this range, the stability of the slurry can be improved, so that the electrode has both excellent resistance and bonding properties, and the battery has better comprehensive cycle performance.

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

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

In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[Negative electrode]

The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

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

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

In some embodiments, the negative electrode active material may be a negative electrode active material for a battery known in the art. As an example, the negative electrode 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, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to In addition to these materials, other conventional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of 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 may further include a conductive agent, which 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 optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[Electrolytes]

The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

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

In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

In some embodiments, the solvent may be selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, At least one of 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, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[Isolation film]

In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[Secondary battery]

In one embodiment of the present application, a secondary battery is provided, including an electrode assembly and an electrolyte, wherein the electrode assembly includes a separator, a negative electrode sheet, and a positive electrode sheet of any embodiment.

In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

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

In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. For example, FIG. 1 is a square structure as an example. Secondary battery 5. Optionally, the secondary battery is a lithium ion battery or a sodium ion battery.

In some embodiments, referring to FIG. 2 , the outer package may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[Battery module]

In some embodiments, secondary batteries may be assembled into a battery module. The number of 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.

FIG3 is a battery module 4 as an example. Referring to FIG3 , in the battery module 4, a plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[Battery Pack]

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

FIG4 and FIG5 are battery packs 1 as an example. Referring to FIG4 and FIG5, the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be covered on the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[Electrical devices]

In one embodiment of the present application, there is provided an electrical device, including any implementation At least one of the secondary battery of any embodiment, the battery module of any embodiment, or the battery pack of any embodiment.

The electrical device includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

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

Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

Example

Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used that do not indicate the manufacturer are all conventional products that can be obtained commercially.

1. Preparation method

Example 1

1) Preparation of binder

In a stainless steel autoclave equipped with a stirrer, 8 times the weight of the monomer was deionized water and 0.15% of the weight of the monomer was alkali metal sodium perfluorooctanoate. The autoclave was sealed, vacuumed, and filled with nitrogen. After three times of this process, the temperature was raised to 50°C with stirring. 1% of the weight of the monomer was diisopropyl peroxydicarbonate and 0.2% of the weight of the monomer was added into the autoclave through the material port. Among them, the structural formula of the nickel-based catalyst is shown in the figure below.

Among them, Ar is Me is methyl and Ph is phenyl.

Vinylidene fluoride, propylene and acrylic acid are slowly and continuously added in a molar ratio of 8:11:1, and the nitrogen is pressurized to 3.5MPa until the monomers are added. Stirring is continued for about 1 to 2 hours, and the pressure is reduced to 2.8MPa. Stirring is stopped and the polymerization reaction is completed. The polymerization product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer, which is used as a binder.

2) Preparation of positive electrode

LiNi _0.8 Co _0.1 Mn _0.1 O ₂ lithium nickel cobalt manganese (NCM) material, conductive agent carbon black, binder of Example 1, and N-methylpyrrolidone (NMP) were stirred and mixed in a weight ratio of 96.9:2:1:21 to obtain a positive electrode slurry; the positive electrode slurry was then evenly coated on the positive electrode collector, and then dried, cold pressed, and cut to obtain a positive electrode sheet.

3) Preparation of negative electrode sheet

The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium hydroxymethyl cellulose (CMC) are dissolved in the solvent deionized water in a weight ratio of 96.2:0.8:0.8:1.2, and the negative electrode slurry is prepared after being evenly mixed; the negative electrode slurry is evenly coated on the negative electrode collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

4) Preparation of electrolyte

In an argon atmosphere glove box (H ₂ O<0.1ppm, O ₂ <0.1ppm), organic solvents ethylene carbonate (EC)/ethyl methyl carbonate (EMC) were mixed uniformly in a volume ratio of 3/7, 12.5% LiPF ₆ lithium salt was added and dissolved in the organic solvent, and stirred uniformly to obtain an electrolyte.

5) Isolation film

Polypropylene film is used as the isolation film.

6) Preparation of batteries

The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is between the positive and negative electrodes to play an isolating role, and then wound to obtain a bare cell, the tabs are welded to the bare cell, and the bare cell is placed in an aluminum shell and baked at 80°C to remove water, and then the electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery then goes through the processes of static, hot and cold pressing, formation, shaping, capacity testing, etc. to obtain a battery product.

The preparation methods of the batteries of Examples 2 to 16 and 28 are similar to those of the battery of Example 1, but the molar ratios of vinylidene fluoride, propylene and acrylic acid monomers are adjusted. The specific parameters are shown in Table 1.

The preparation methods of the batteries of Examples 17 to 21 are similar to those of the battery of Example 11, but the molecular weight of the binder is adjusted. The specific parameters are shown in Table 1. The preparation methods are as follows:

The weight average molecular weight of the binder in Example 17 is 400,000, and the preparation method is:

In a stainless steel autoclave equipped with a stirrer, deionized water 8 times the weight of the monomer and 0.2% of the weight of alkali metal sodium perfluorooctanoate as an emulsifier are added, the reactor is sealed, vacuumed, and filled with nitrogen. After repeating this process three times, the temperature is raised to 60° C. by stirring, and diisopropyl peroxydicarbonate 1.4% of the weight of the monomer and a nickel-based catalyst 0.3% of the weight of the monomer are added from the material port, while vinylidene fluoride, propylene and acrylic acid are slowly and continuously added in a molar ratio of 8:11:1. At the same time, the nitrogen is pressurized to 3.2 MPa until the monomers are added. Stirring is continued for about 1 to 2 hours, and the pressure is reduced to 2.5 MPa, and stirring is stopped, and the polymerization reaction is terminated; the polymerization product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer.

The weight average molecular weight of the binder in Example 18 is 500,000, and the preparation method is:

In a stainless steel autoclave equipped with a stirrer, 8 times the weight of the monomer of deionized water and 0.2% of the weight of the monomer of alkali metal sodium perfluorooctanoate were added, the reactor was sealed, vacuumed, and filled with nitrogen. After repeating this three times, the temperature was raised to 55°C with stirring, and 1.2% of the weight of the monomer of diisopropyl peroxydicarbonate and 0.25% of the weight of the monomer of a nickel-based catalyst were added from the material port, and vinylidene fluoride, propylene and acrylic acid were slowly and continuously added in a molar ratio of 8:11:1. At the same time, the nitrogen was pressurized to 3.3MPa until the monomer was added. Stirring was continued for about 1 to 2h, and the pressure was reduced to 2.65MPa, and stirring was stopped, and the polymerization reaction was terminated; the polymerization The product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer.

The weight average molecular weight of the binder in Example 19 is 1 million, and the preparation method is:

In a stainless steel autoclave equipped with a stirrer, deionized water 8 times the weight of the monomer and 0.1% of the weight of alkali metal sodium perfluorooctanoate are added, the reactor is sealed, vacuumed, and filled with nitrogen. After repeating this process three times, the temperature is raised to 48° C. by stirring, and diisopropyl peroxydicarbonate 1.0% of the weight of the monomer and a nickel-based catalyst 0.20% of the weight of the monomer are added from the material port, while vinylidene fluoride, propylene and acrylic acid are slowly and continuously added in a molar ratio of 8:11:1. At the same time, the nitrogen is pressurized to 3.6 MPa until the monomers are added. Stirring is continued for about 1 to 2 hours, and the pressure is reduced to 2.9 MPa, and stirring is stopped, and the polymerization reaction is terminated; the polymerization product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer.

The weight average molecular weight of the binder in Example 20 is 1.2 million, and the preparation method is:

In a stainless steel autoclave equipped with a stirrer, deionized water 8 times the weight of the monomer and 0.1% of the weight of perfluorooctanoic acid alkali metal salt are added, the reactor is sealed, vacuumed, and filled with nitrogen. After repeating this process three times, the temperature is raised to 46° C. by stirring, and 0.9% of the weight of diisopropyl peroxydicarbonate and 0.18% of the weight of a nickel-based catalyst are added from a material port, while vinylidene fluoride, propylene and acrylic acid are slowly and continuously added in a molar ratio of 8:11:1. At the same time, the nitrogen is pressurized to 3.7 MPa until the monomers are added. Stirring is continued for about 1 to 2 hours, and the pressure is reduced to 2.9 MPa, and stirring is stopped, and the polymerization reaction is terminated. The polymerization product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer.

The weight average molecular weight of the binder in Example 21 is 1.4 million, and the preparation method is:

In a stainless steel autoclave equipped with a stirrer, deionized water 8 times the weight of the monomer and 0.1% of the weight of perfluorooctanoic acid alkali metal salt are added, the reactor is sealed, vacuumed, and filled with nitrogen. After repeating this process three times, the temperature is raised to 44° C. by stirring, and 0.8% of the weight of diisopropyl peroxydicarbonate and 0.18% of the weight of a nickel-based catalyst are added from a material port, while vinylidene fluoride, propylene and acrylic acid are slowly and continuously added in a molar ratio of 8:11:1. At the same time, the nitrogen is pressurized to 4.0 MPa until the monomers are added. Stirring is continued for about 1 to 2 hours, and the pressure is reduced to 3 MPa, and stirring is stopped, and the polymerization reaction is terminated. The polymerization product is centrifuged, washed, and dried to obtain a vinylidene fluoride-propylene-acrylic acid copolymer.

The preparation methods of the batteries of Examples 22 to 25 are similar to those of the battery of Example 1, but the mass fraction of the binder is adjusted. The specific parameters are shown in Table 1.

The preparation method of the battery of Example 26 is similar to that of the battery of Example 11, but the propylene monomer is replaced by 2-butene monomer, and the molar ratio remains unchanged. The specific parameters are shown in Table 1.

The preparation method of the battery of Example 27 is similar to that of the battery of Example 11, but the acrylic acid monomer is replaced by a methacrylic acid monomer, and the molar ratio remains unchanged. The specific parameters are shown in Table 1.

The battery of Example 29 is similar to the battery of Example 1 in the preparation method of the battery of Example 1, but the polymerization monomers are adjusted to vinylidene fluoride, tetrafluoroethylene, propylene and acrylic acid monomers, wherein the molar ratio of vinylidene fluoride, tetrafluoroethylene, propylene and acrylic acid monomers is 7.5:0.5:11:1, and the specific parameters are shown in Table 1.

The battery of Example 30 is similar to the battery of Example 1 in the preparation method of the battery of Example 1, but the polymerization monomers are adjusted to tetrafluoroethylene, propylene and acrylic acid monomers, wherein the molar ratio of tetrafluoroethylene, propylene and acrylic acid monomers is 8:11:1. The specific parameters are shown in Table 1.

The battery of Example 31 is similar to the battery of Example 1 in preparation method, but the polymerization monomers are adjusted to vinylidene fluoride, chlorotrifluoroethylene, propylene and acrylic acid monomers, wherein the molar ratio of vinylidene fluoride, chlorotrifluoroethylene, propylene and acrylic acid monomers is 7.5:0.5:11:1, and the specific parameters are shown in Table 1.

The preparation method of the battery of Comparative Example 1 is similar to that of the battery of Example 1, but the polymerization monomer is only vinylidene fluoride monomer. The specific parameters are shown in Table 1.

The preparation method of the battery of Comparative Example 2 is similar to that of the battery of Example 1, but the polymerization monomers are vinylidene fluoride monomer and propylene monomer, the molar content of the vinylidene fluoride monomer is 60%, and the molar content of propylene is 40%. The specific parameters are shown in Table 1.

The preparation method of the battery of Comparative Example 3 is similar to that of the battery of Example 1, but the polymerization monomers are vinylidene fluoride monomer and acrylic acid monomer, the molar content of vinylidene fluoride monomer is 60%, and the molar content of acrylic acid is 40%. The specific parameters are shown in Table 1.

The preparation methods of the batteries of Comparative Examples 4 to 6 are similar to those of the battery of Example 1, but the molar ratios of vinylidene fluoride, propylene and acrylic acid monomers are adjusted. The specific parameters are shown in Table 1.

2. Test Method

1. Test of adhesive properties

1) Weight average molecular weight test

A Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141) was used. A polystyrene solution sample with a mass fraction of 3.0% was used as a reference, and a matching chromatographic column (oily: Styragel HT5 DMF7.8*300mm+Styragel HT4) was selected. A 3.0% polymer gel solution was prepared with purified N-methylpyrrolidone (NMP) solvent, and the prepared solution was allowed to stand for one day for use. During the test, tetrahydrofuran was first drawn with a syringe and rinsed, and repeated several times. Then 5 ml of the experimental solution was drawn, the air in the syringe was removed, and the needle tip was wiped dry. Finally, the sample solution was slowly injected into the injection port. After the test, the outflow curve, molecular weight distribution curve and molecular weight statistical results were output.

2. Slurry property test

1) Slurry viscosity test

After the slurry is shipped, take 500 ml of the slurry and place it in a beaker. Use a rotational viscometer, select the rotor, set the speed to 12 rpm, and set the rotation time to 5 minutes. After the value stabilizes, read and record the viscosity value.

2) Slurry stability test

After re-stirring the slurry for 30 minutes, take a certain amount of slurry and pour it into the sample bottle of the stability instrument. After putting it into the sample bottle, close the test tower cover, open the test tower cover, and a scanning curve will begin to appear on the test interface, and the sample stability test will begin. The test will be completed after more than 48 hours of continuous testing.

3. Performance test of pole piece

1) Test of the membrane resistance of the positive electrode

Cut small discs with a diameter of 10mm from the left, middle and right sides of the electrode. Turn on the indicator light of Yuanneng Technology's electrode resistor meter, place the probe in the appropriate position of the membrane resistor meter, click the "start" button, and wait for the reading to stabilize before reading. Test two positions of each small disc, and finally calculate the average value of six measurements, which is the membrane resistance of the electrode.

2) Adhesion test of positive electrode

Cut the positive electrode sheet into test specimens of 20* ^100mm2 size for later use; use double-sided tape to adhere the side of the specimen to be tested, and use a roller to compact it so that the double-sided tape and the specimen are completely fitted; the other side of the double-sided tape of the specimen is adhered to the stainless steel surface, and one end of the specimen is bent in the opposite direction with a bending angle of 180°; use a high-speed rail tensile testing machine to test, fix one end of the stainless steel to the fixture below the tensile testing machine, and fix the bent end of the specimen to the fixture above, adjust the angle of the specimen to ensure that the upper and lower ends are in a vertical position, and then stretch the specimen at a speed of 50mm/min. Until the sample is completely peeled off from the substrate, the displacement and force in the process are recorded. It is generally believed that the force when the force is balanced is the bonding force of the electrode.

3) Pole brittleness test

The positive electrode slurry is coated on the surface of the current collector (such as), and the electrode is made into a pole piece after drying and cold pressing. The prepared pole piece is cut into a test sample of 20* ^100mm2 size for standby use; first bend the pole piece and fold it in half and fix it, and use a 2kg rolling roller to roll it once to check whether the folded part of the pole piece is light-transmitting and leaking metal; if there is no light-transmitting and metal-leaking, fold the pole piece in reverse and fix it, and use a 2kg rolling roller to roll it once to check whether the folded part of the pole piece is light-transmitting and leaking metal, and repeat the above steps until the folded part of the pole piece is light-transmitting and leaking metal. Take three samples for testing and take the average value.

4. Battery performance test

1) Battery capacity retention test

The battery capacity retention rate test process is as follows: at 25°C, the prepared battery is charged to 4.3V at a constant current of 1/3C, then charged to a current of 0.05C at a constant voltage of 4.3V, left for 5 minutes, and then discharged to 2.8V at 1/3C. The obtained capacity is recorded as the initial capacity C0. Repeat the above steps for the same battery mentioned above, and record the discharge capacity Cn of the battery after the nth cycle at the same time. Then, after each cycle, the battery capacity retention rate Pn=Cn/C0*100%, with the 100 point values of P1, P2...P100 as the ordinate, and the corresponding number of cycles as the abscissa, to obtain a curve chart of the battery capacity retention rate and the number of cycles. During this test, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and...the 100th cycle corresponds to n=100. The battery capacity retention rate data corresponding to the embodiment or comparative example in Table 2 is the data measured after 500 cycles under the above test conditions, that is, the value of P500. The test process of the comparative example and other embodiments is the same as above.

2) Battery DC impedance test

The battery DC impedance test process is as follows: at 25°C, charge the prepared battery at a constant current of 1/3C to 4.3V, then charge at a constant voltage of 4.3V to a current of 0.05C, leave it for 5 minutes, and record the voltage V1. Then discharge it at 1/3C for 30s, record the voltage V2, and then (V2-V1)/(1/3C) to obtain the internal resistance DCR1 of the battery after the first cycle. Repeat the above steps for the same battery, and record the internal resistance DCRn (n=1, 2, 3...100) of the battery after the nth cycle. The 100 point values of DCR3...DCR100 are used as the vertical coordinates, and the corresponding number of cycles is used as the horizontal coordinate, so as to obtain a curve graph of battery discharge DCR and cycle number.

In this test process, the first cycle corresponds to n=1, the second cycle corresponds to n=2, ... the 100th cycle corresponds to n=100. In Table 2, the battery internal resistance increase ratio = (DCRn-DCR1)/DCR1*100%, and the test process of the comparative example and other embodiments is the same as above. The data in Table 2 are measured after 100 cycles under the above test conditions.

3) Power-off initial test

At 2.8-4.3V, charge the button cell at 0.1C to 4.3V, then charge at 4.3V at constant voltage until the current is ≤0.05mA, let stand for 2min, the charge capacity at this time is recorded as C0, then discharge at 0.1C to 2.8V, the discharge capacity at this time is the initial gram capacity recorded as D0. The first efficiency is calculated as D0/C0*100%.

3. Test Results

The property test results of the binders obtained in the above Examples 1 to 31 and Comparative Examples 1 to 6 are shown in Table 1, and the performance test results of the slurry, pole piece and battery are shown in Table 2.

Table 1

Table 2

According to the above results, the binders in Examples 1 to 31 all contain polymers containing structural units derived from vinylidene fluoride, tetrafluoroethylene or trifluorochloroethylene, structural units derived from propylene or 2-butene, and structural units derived from acrylic acid or methacrylic acid, and the molar content of structural units derived from vinylidene fluoride in the polymer is 40% to 60%. From the comparison of Examples 1 to 16, Examples 26 to 31 and Comparative Example 1, it can be seen that the vinylidene fluoride-propylene-acrylic acid copolymer binder, vinylidene fluoride-propylene-methacrylic acid copolymer binder, vinylidene fluoride-2-butene-acrylic acid copolymer binder, vinylidene fluoride-tetrafluoroethylene-propylene-acrylic acid copolymer binder, tetrafluoroethylene-propylene-acrylic acid copolymer or vinylidene fluoride-chlorotrifluoroethylene-propylene-acrylic acid copolymer with vinylidene fluoride content within the above range can significantly slow down the gelation phenomenon of the slurry, improve the flexibility of the pole piece, and reduce the membrane resistance and the cycle internal resistance growth rate of the battery.

From the comparison between Examples 13 to 15 and Comparative Example 2, it can be seen that the polymer containing structural units derived from acrylic acid can not only significantly slow down the slurry gel phenomenon, reduce the internal resistance of the diaphragm, improve the adhesion of the diaphragm, reduce the cycle internal resistance growth rate of the battery, improve the cycle capacity retention rate of the battery, and optimize the comprehensive performance of the battery during the cycle; but also improve the flexibility of the electrode, reduce the probability of brittle fracture of the electrode, and improve the safety performance of the battery.

From the comparison between Examples 13 to 15 and Comparative Example 3, it can be seen that the structural units derived from propylene in the polymer can reduce the viscosity of the slurry, improve the dispersibility of the slurry, reduce the cycle internal resistance growth rate of the battery, and improve the first charge efficiency of the battery.

From the comparison of Examples 1 to 16 and Comparative Examples 4 to 6, it can be seen that the molar content of the structural units derived from vinylidene fluoride in the polymer is 40% to 60%. Based on the total molar number of all structural units in the polymer, the electrode has both excellent flexibility and good adhesion, and the battery has both excellent cycle capacity retention and good first charge efficiency.

From the comparison of Examples 1 to 3, Example 16 and Example 28, it can be seen that the molar content of the structural unit derived from acrylic acid is 5% to 25%. Based on the total molar number of all structural units in the polymer, the battery has both excellent first charge efficiency and good cycle capacity retention rate.

From the comparison between Example 11, Examples 18 to 20 and Example 17 and Example 21, it can be seen that when the weight average molecular weight of the vinylidene fluoride-propylene-acrylic acid copolymer is 500,000 to 1.2 million, the binder enables the electrode to have excellent flexibility, good adhesion, and low membrane resistance and internal resistance growth rate, which is beneficial to improving the battery's cycle capacity retention rate.

From the comparison of Example 1, Examples 22 to 25 and Comparative Example 1, it can be seen that the mass fraction of the vinylidene fluoride-propylene-acrylic acid copolymer binder is 0.1% to 3%. Based on the mass of the positive electrode active material, the binder can slow down the gelation of the slurry and improve the stability of the slurry. From the comparison of Example 1, Examples 23 to 24 and Examples 22 and Example 25, it can be seen that when the mass fraction of the binder is 0.2% to 1.2%, the slurry has further improved stability, reduced membrane internal resistance and battery internal resistance growth rate, thereby making the battery have a high cycle capacity retention rate.

From the comparison of Examples 1 to 3, 4 to 6, 7 to 9, 10 to 12 and 13 to 15, it can be seen that the reduction of the vinylidene fluoride content helps to improve the flexibility of the pole piece. When the vinylidene fluoride content is constant, increasing the acrylic acid content within the range of 5% to 25% helps to improve the bonding force of the pole piece, improve the dispersibility of the positive electrode active material, reduce the membrane resistance and reduce the internal resistance growth rate during the cycle.

It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having substantially the same structure and the same effects as the technical idea within the technical solution of the present application are all included in the technical scope of the present application. Furthermore, other embodiments in which various modifications that can be conceived by those skilled in the art are added to the embodiments and some components of the embodiments are combined to form other embodiments are also included in the scope of the present application without departing from the gist of the present application.

Claims

A binder, characterized in that the binder comprises a polymer containing a structural unit derived from a monomer represented by formula I, a structural unit derived from a monomer represented by formula II, and a structural unit derived from a monomer represented by formula III

Wherein, R 1 and R 2 are each independently selected from hydrogen or fluorine or chlorine, R 3 and R 4 are selected from hydrogen or substituted or unsubstituted C 1-5 alkyl, and the molar content of the structural unit derived from the monomer represented by formula I is 40% to 60%, based on the total molar number of all structural units in the polymer.
The binder according to claim 1 is characterized in that the molar content of the structural unit derived from the monomer represented by formula II is 15% to 55%, and the molar content of the structural unit derived from the monomer represented by formula III is 5% to 25%, based on the total molar number of all structural units in the polymer.
The binder according to claim 1 or 2, characterized in that the weight average molecular weight of the polymer is 500,000 to 1.2 million.
The binder according to any one of claims 1 to 3, characterized in that the monomer represented by formula I is selected from one or more of vinylidene fluoride, tetrafluoroethylene, and chlorotrifluoroethylene.
The binder according to any one of claims 1 to 4, characterized in that the monomer represented by formula II is selected from one or two of propylene and 2-butene.
The binder according to any one of claims 1 to 5, characterized in that the monomer represented by formula III is selected from one or both of acrylic acid and methacrylic acid.
A method for preparing a binder, characterized in that it comprises the following steps:

Under polymerizable conditions, polymerizing the monomer represented by formula I, the monomer represented by formula II, and the monomer represented by formula III to prepare a polymer;

Wherein, R 1 and R 2 are selected from hydrogen or fluorine or chlorine, R 3 and R 4 are selected from hydrogen or substituted or unsubstituted C 1-5 alkyl, wherein the molar content of the monomer represented by formula I is 40% to 60%, based on the total molar number of all monomers.
The preparation method according to claim 7, characterized in that the polymerization reaction comprises the following steps:

Dispersing an initiator, a catalyst, a monomer represented by formula I, a monomer represented by formula II, and a monomer represented by formula III in a solvent, and carrying out a polymerization reaction for 1 h to 2 h in an inert gas atmosphere, a reaction pressure of 3.2 MPa to 4.0 MPa, and a reaction temperature of 40° C. to 60° C.;

When the pressure in the reaction system drops to 2.5 MPa-3.0 MPa, the reaction is stopped, the solid and liquid are separated, and the solid phase is retained.
The preparation method according to claim 7 or 8, characterized in that the polymerization reaction comprises the following steps:

Adding a solvent and an emulsifier into a container, evacuating the container and then filling it with an inert gas;

The system is heated to 40°C to 60°C, an initiator and a catalyst are added to the container, and then the monomers represented by formula I, II and III are added, and the mixture is continuously introduced into the container. The inert gas makes the pressure in the container reach 3.2MPa-4.0MPa to carry out the polymerization reaction.
The preparation method according to claim 8 or 9, characterized in that the catalyst is a transition metal-based catalyst, and the transition metal-based catalyst is selected from any one of the catalysts represented by formula IV, formula V, formula VI, and formula VII,

Among them, Ar is Cy is cyclohexyl, Me is methyl, Ph is phenyl, and TMS is trimethylsilyl.
The preparation method according to any one of claims 7 to 10, characterized in that the molar content of the monomer represented by formula II is 15% to 55%, and the molar content of the monomer represented by formula III is 5% to 25%, based on the total molar number of all monomers in the polymer preparation process.
A positive electrode plate, characterized in that it includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material, a conductive agent and a binder as described in any one of claims 1 to 6 or a binder prepared by the preparation method as described in any one of claims 7 to 11.
The positive electrode sheet according to claim 12, characterized in that the positive electrode The active material is a transition metal oxide containing lithium.
The positive electrode plate according to claim 12 is characterized in that the positive electrode active material is at least one of lithium nickel cobalt manganese oxide, a doped modified material of lithium nickel cobalt manganese oxide, or a conductive carbon coated modified material, a conductive metal coated modified material, and a conductive polymer coated modified material of lithium nickel cobalt manganese oxide.
The positive electrode sheet according to any one of claims 12 to 14, characterized in that the mass fraction of the binder is 0.1% to 3%, based on the mass of the positive electrode active material.
The positive electrode sheet according to any one of claims 12 to 14, characterized in that the mass fraction of the binder is 0.2% to 1.2%, based on the mass of the positive electrode active material.
A secondary battery, characterized in that it comprises an electrode assembly and an electrolyte, wherein the electrode assembly comprises a separator, a negative electrode sheet and the positive electrode sheet according to any one of claims 12 to 16.
The secondary battery according to claim 17 is characterized in that the secondary battery is a lithium ion battery or a sodium ion battery.
An electrical device, characterized by comprising the secondary battery according to claim 17 or 18.