Binder composition, secondary battery, battery module, battery pack and electrical device

Abstract

The binder composition, electrode, battery and electric device include a fluorine-containing polymer A and a copolymer B, wherein the copolymer B includes a structural unit derived from a monomer containing a cyano group and a structural unit derived from a monomer containing an ester group.

Description

Technical Field

[0001] The present application relates to the technical field of lithium batteries, and in particular to an adhesive composition, a secondary battery, a battery module, a battery pack, and an electrical device. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the widespread use of lithium-ion batteries, higher requirements have been placed on their performance and cost.

[0003] Binders are a common material in lithium-ion batteries and are in high demand for electrode plates, separators, and packaging. However, existing binders are expensive and have poor adhesion, increasing battery costs and reducing cycle performance. Therefore, existing binders still need improvement. Summary of the Invention

[0004] The present application has been made in view of the above-mentioned problems, and an object of the present application is to provide an adhesive composition that can reduce adhesive costs and has excellent adhesive performance.

[0005] A first aspect of the present application provides an adhesive composition comprising a fluorine-containing polymer A and a copolymer B, wherein the copolymer B comprises a structural unit derived from a monomer containing a cyano group and a structural unit derived from a monomer containing an ester group.

[0006] Therefore, the present application uses a fluoropolymer A and a copolymer B comprising structural units derived from a monomer containing a cyano group and structural units derived from a monomer containing an ester group as a binder, which can further improve the bonding performance of the binder and the cycle performance of the battery compared to using only the fluoropolymer A or the copolymer B as the binder.

[0007] In any embodiment, the weight average molecular weight of fluoropolymer A is 600,000-900,000, and the weight average molecular weight of copolymer B is 400,000-700,000. Controlling the weight average molecular weight of the polymer can take into account the cohesiveness and processability of the binder. If the weight average molecular weight of the polymer is too low, the brittleness is too great, resulting in insufficient cohesive force of the binder. If the weight average molecular weight of the polymer is too high, it is difficult to disperse the electrode active material. Moreover, the rational combination of polymers with different molecular weights can improve the dispersibility of the electrode active material, thereby improving battery performance.

[0008] In any embodiment, the mass ratio of fluoropolymer A to copolymer B is 1:4-4:1. Reasonable combination of fluoropolymer A and copolymer B within a certain mass range can further improve the bonding performance of the binder and the cycle performance of the battery.

[0009] In any embodiment, the fluorine-containing polymer A is selected from one or more of polyvinylidene fluoride, and copolymers thereof with tetrafluoroethylene, hexafluoropropylene, and trichloroethylene.

[0010] In any embodiment, the monomer containing a cyano group is selected from one or more of acrylonitrile, methacrylonitrile, halogenated acrylonitrile, and methoxyacrylonitrile.

[0011] In any embodiment, the ester-containing monomer is selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isoamyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate. Compared to fluorinated monomers, the aforementioned cyano- and ester-containing monomers are low-cost, not subject to regulatory restrictions, and can be mass-produced, significantly reducing the cost of the adhesive.

[0012] In any embodiment, the mass ratio of structural units derived from monomers containing cyano groups to structural units derived from monomers containing ester groups in copolymer B is 8:1 to 12:1. The cyano-containing monomers can improve the mechanical strength and bonding properties of copolymer B, further enhancing the battery's cycling performance. A small amount of ester-containing monomers can enhance the flexibility of copolymer B, preventing brittle fracture of the electrode. Furthermore, the ester group has a certain ability to absorb and retain electrolyte, improving the poor ionic conductivity of fluoropolymer A.

[0013] In any embodiment, copolymer B further comprises structural units derived from monomers containing groups represented by formula I,

[0014]

[0015] wherein n is selected from 0, 1, 2 or 3.

[0016] The oxygen element in the group represented by Formula I has a greater electronegativity than the nitrogen element in the cyano group. Compared to the cyano group in copolymer B, it more easily forms hydrogen bonds with the electrode active material and the conductive agent, and the bond energy is stronger. This can significantly improve the dispersibility of the slurry, making the solid matter in the slurry less likely to precipitate, and increasing the solid content of the slurry. Furthermore, the addition of the group represented by Formula I can further improve the bonding strength of the battery electrode and the battery's cycle resistance.

[0017] In any embodiment, the monomer containing the group represented by formula I is selected from one or more of N-vinyl pyrrolidone and N-allyl-2-pyrrolidone. The above monomers are low in cost, good in stability, and easy to process and synthesize.

[0018] In any embodiment, the mass content of the structural unit derived from the monomer containing the group represented by formula I is 0.1% to 2%, based on the total mass of copolymer B. The inclusion of an appropriate amount of the group represented by formula I in copolymer B can improve the dispersion properties of copolymer B, making the prepared slurry less likely to precipitate, helping to increase the solid content of the slurry, and thus increasing the loading capacity of the electrode.

[0019] A third aspect of the present application provides a secondary battery comprising an electrode assembly and an electrolyte, wherein the electrode assembly comprises a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material and the binder of the first or second aspect of the present application. The battery has improved cycle performance.

[0020] In any embodiment, the positive electrode active material is a lithium-containing transition metal oxide, which can be selected from lithium iron phosphate, or their doped modified materials, or their conductive carbon coated modified materials, conductive metal coated modified materials or conductive polymer coated modified materials.

[0021] A fourth aspect of the present application provides a battery module including the secondary battery according to the third aspect of the present application.

[0022] The fifth aspect of the present application provides a battery pack comprising the battery module of the fourth aspect of the present application.

[0023] A sixth aspect of the present application provides an electrical device comprising at least one of the secondary battery of the third aspect of the present application, the battery module of the fourth aspect of the present application, and the battery pack of the fifth aspect of the present application.

[0024] The battery module of the fourth aspect and the battery pack of the fifth aspect of the present application include the secondary battery of the third aspect and thus have the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Graphs showing the bonding performance of the adhesives prepared in Example 1 and Comparative Example 1;

[0026] Figure 2 1 is a cycle test curve of the batteries prepared in Example 1 and Comparative Example 1;

[0027] Figure 3 Schematic diagram of a secondary battery according to one embodiment of the present application.

[0028] Figure 4 yes Figure 3 FIG. 1 is an exploded view of a secondary battery according to an embodiment of the present application.

[0029] Figure 5 Schematic diagram of a battery module according to one embodiment of the present application.

[0030] Figure 6 Schematic diagram of a battery pack according to one embodiment of the present application.

[0031] Figure 7 yes Figure 6 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0032] Figure 8 FIG. 1 is a schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source.

[0033] Description of reference numerals:

[0034] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0035] Below, the embodiments of the binder, preparation method, electrode, battery, and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be 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 understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0036] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this 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.

[0037] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0038] 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.

[0039] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating 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), indicating 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.

[0040] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0041] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0042] PVDF is a commonly used battery binder, but its high cost and poor adhesion contribute to a decrease in battery cycle performance due to weakened adhesion during battery recycling. To address these technical issues, this application has developed a low-cost binder that provides excellent electrode adhesion, significantly improving battery cycle performance.

[0043] [Binder]

[0044] Based on this, the present application proposes an adhesive composition comprising a fluorine-containing polymer A and a copolymer B, wherein the copolymer B comprises a structural unit derived from a monomer containing a cyano group and a structural unit derived from a monomer containing an ester group.

[0045] As used herein, the term "binder composition" refers to a mixture of chemical compounds or polymers that forms a colloidal solution or colloidal dispersion in a dispersion medium (eg, water).

[0046] In some embodiments, the dispersion medium of the binder is an aqueous solvent, such as water.

[0047] In some embodiments, the dispersion medium of the adhesive is an oily solvent. Examples of the oily solvent include, but are not limited to, dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dimethyl carbonate, ethyl cellulose, and polycarbonate.

[0048] In some embodiments, a binder is used to hold the electrode material and / or conductive agent in place and adhere them to the conductive metal component to form an electrode. In some embodiments, the electrode does not contain any conductive agent.

[0049] In some embodiments, the binder serves as a positive electrode binder, and is used to bind the positive electrode active material and / or the conductive agent to form a positive electrode.

[0050] In some embodiments, the binder serves as a negative electrode binder, and is used to bind the negative electrode active material and / or the conductive agent to form a negative electrode.

[0051] As used herein, the term "polymer" includes, on the one hand, chemically homogeneous aggregates of macromolecules prepared by polymerization (copolymerization, homopolymerization), but differing in degree of polymerization, molar mass, and chain length. The term also includes, on the other hand, derivatives of such aggregates of macromolecules formed by polymerization, i.e., compounds or mixtures that can be obtained by reaction, for example, addition or substitution, of functional groups in the aforementioned macromolecules and that can be chemically homogeneous or chemically heterogeneous.

[0052] As used herein, the term "fluoropolymer" refers to a polymer containing the element fluorine.

[0053] As used herein, the term "copolymer" refers to polymers prepared by polymerizing two or more different types of monomers.

[0054] As used herein, the term "cyano" refers to a -CN group.

[0055] In this context, the term "ester group" refers to a group having a general formula of -COOR9 structural unit, wherein R9 is selected from C 1-5 Examples of alkyl and ester groups include, but are not limited to, methyl, ethyl, propyl, butyl, pentyl, and isooctyl groups.

[0056] In this context, the term "substituted" refers to substitution by a substituent, wherein the substituent is independently selected from the group consisting of: hydroxyl, thiol, amino, cyano, nitro, aldehyde, halogen, C 1-6 Alkyl, C 1-6 Alkoxy.

[0057] In some embodiments, copolymer B is selected from one or more of acrylonitrile-methyl acrylate copolymer, acrylonitrile-2-methyl methacrylate copolymer, acrylonitrile-2-ethyl methacrylate copolymer, acrylonitrile-ethyl acrylate copolymer, acrylonitrile-butyl acrylate copolymer, acrylonitrile-isooctyl acrylate copolymer, acrylonitrile-butyl acrylate-hydroxyethyl acrylate copolymer, acrylonitrile-butyl acrylate-ethyl acrylate copolymer, acrylonitrile-isoamyl acrylate-hydroxypropyl acrylate copolymer, acrylonitrile-butyl acrylate-isooctyl acrylate-methyl methacrylate copolymer, and acrylonitrile-butyl acrylate-isooctyl acrylate-ethyl methacrylate copolymer. In some embodiments, copolymer B is acrylonitrile-isooctyl acrylate copolymer.

[0058] Structural units derived from monomers containing cyano groups effectively complex not only with the metal on the current collector surface but also with the metal elements on the electrode active material, ensuring strong adhesion between the electrode active material and the current collector. Furthermore, structural units derived from monomers containing ester groups enhance the flexibility of the electrode sheet, preventing brittle fracture. Furthermore, hydrogen bonding between the cyano and ester groups on copolymer B and the fluorine in fluoropolymer A creates a stronger bond between the electrode active material particles on the electrode sheet.

[0059] The present application uses a fluoropolymer A and a copolymer B comprising structural units derived from a monomer containing a cyano group and structural units derived from a monomer containing an ester group as a binder. Compared with using only the fluoropolymer A or the copolymer B as a binder, the bonding performance of the binder between the electrode active material and the current collector, the electrode active material and the conductive agent, and the electrode active material and / or the conductive agent and the current collector is significantly improved, thereby significantly improving the cycle performance of the battery.

[0060] In some embodiments, the weight average molecular weight of fluoropolymer A is 6×10 5 -9×10 5 The weight average molecular weight of copolymer B is 4×10 5 -7×10 5 In some embodiments, the weight average molecular weight of the fluoropolymer A is selected from 6×10 5 -8×10 5 , or 6×10 5 -7×10 5 , or 7×10 5 -9×10 5 , or 8×10 5 -9×10 5 In some embodiments, the weight average molecular weight of copolymer B is selected from 4×10 5 -7×10 5 , or 4×10 5-6×10 5 , or 4×10 5 -5×10 5 , or 5×10 5 -7×10 5 , or 6×10 5 -7×10 5 .

[0061] As used herein, the term "weight average molecular weight" refers to the sum of the weight fractions of molecules of different molecular weights in a polymer multiplied by their corresponding molecular weights.

[0062] Controlling the weight-average molecular weight of the polymer can balance the adhesiveness and processability of the binder. A polymer with a weight-average molecular weight that is too low will be too brittle, resulting in insufficient binding strength. A polymer with a weight-average molecular weight that is too high will not disperse the electrode active material. Furthermore, the appropriate combination of polymers with different molecular weights can improve the dispersibility of the electrode active material, thereby enhancing battery performance.

[0063] In some embodiments, the mass ratio of fluoropolymer A to copolymer B is 1:4-4:1. In some embodiments, the mass ratio of fluoropolymer A to copolymer B is 1:4-3:1, or 1:4-2:1, or 1:4-1:1, or 1:2-1:4, or 1:2-4:1, or 1:1-4:1, or 2:1-4:1, or 3:1-4:1. Properly combining fluoropolymer A and copolymer B within a certain mass range can further improve electrode adhesion and battery cycle performance.

[0064] In some embodiments, the fluorine-containing polymer A is selected from one or more of polyvinylidene fluoride, and copolymers thereof with tetrafluoroethylene, hexafluoropropylene, and trichloroethylene.

[0065] In some embodiments, the fluorine-containing polymer A is polyvinylidene fluoride, which is synthesized by an emulsion method, has a volume average particle size Dv50 of 5-10 μm, a crystallinity of 35-40%, and a melting point of 160-170°C.

[0066] Compared with the suspension method for synthesizing PVDF, the polyvinylidene fluoride synthesized by the emulsion method has a large single synthesis capacity and lower cost. The appropriate molecular weight enables the slurry containing the binder of the present application to have both excellent suspension and dispersibility, preventing the binder from settling or agglomerating and causing the binder to be unevenly dispersed in the slurry. The appropriate particle size can effectively reduce the dissolution time of polyvinylidene fluoride and thus reduce the time for preparing the slurry. The appropriate crystallinity of polyvinylidene fluoride can ensure good adhesion without causing brittleness problems of the electrode. The higher melting point prevents polyvinylidene fluoride from melting and inactivating during coating and drying.

[0067] In some embodiments, the monomer containing a cyano group is selected from one or more of acrylonitrile, methacrylonitrile, halogenated acrylonitrile, and methoxyacrylonitrile.

[0068] In some embodiments, the monomer containing an ester group is selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isoamyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

[0069] In some embodiments, the monomer containing an ester group is selected from isooctyl acrylate. Isooctyl acrylate enables copolymer B to have a lower glass transition temperature and better flexibility, which is beneficial for the processing and manufacturing of the electrode.

[0070] Compared with fluorine-containing monomers, the above-mentioned monomers containing cyano groups and monomers containing ester groups have low costs, are not subject to policy restrictions, can be mass-produced, and can significantly reduce the cost of the adhesive.

[0071] In some embodiments, the mass ratio of the structural units derived from the monomer containing a cyano group to the structural units derived from the monomer containing an ester group in copolymer B is 8: 1 to 12: 1. In some embodiments, the mass ratio of the structural units derived from the monomer containing a cyano group to the structural units derived from the monomer containing an ester group in copolymer B is 8: 1 to 11: 1, or 8: 1 to 10: 1, or 8: 1 to 9: 1, or 9: 1 to 12: 1, or 10: 1 to 12: 1, or 11: 1 to 12: 1.

[0072] Monomers containing cyano groups can improve the mechanical strength and bonding properties of copolymer B, and can further improve the cycle performance of the battery. A small amount of monomers containing ester groups can improve the flexibility of copolymer B and avoid brittle fracture of the electrode. At the same time, the ester group has a certain ability to absorb electrolyte and retain liquid, which can improve the poor ionic conductivity of fluorinated polymer A and improve the ion conductivity of the binder.

[0073] In some embodiments, copolymer B further comprises structural units derived from monomers containing a group represented by formula I,

[0074]

[0075] wherein n is selected from 0, 1, 2 or 3.

[0076] When n is 0, the group shown in formula I is When n is 1, the group shown in formula I is When n is 2, the group shown in formula I is When n is 3, the group shown in formula I is

[0077] The oxygen element in the group represented by Formula I has a greater electronegativity than the nitrogen element in the cyano group. Compared with the cyano group in copolymer B, it is easier to form hydrogen bonds with the electrode active material and conductive agent, and the bond energy is stronger. This can significantly improve the dispersibility of the slurry and increase the solid content of the slurry. At the same time, the addition of the group represented by Formula I can further enhance the binding force of the binder and the cycle performance of the battery.

[0078] In some embodiments, the monomer containing the group represented by Formula I is selected from one or more of N-vinyl pyrrolidone and N-allyl-2-pyrrolidone. The above monomers are low in cost, good in stability, and easy to process and synthesize.

[0079] In some embodiments, the mass content of the structural unit derived from the monomer containing the group represented by Formula I is 0.1% to 2%, based on the total mass of the copolymer B. In some embodiments, the mass content of the structural unit derived from the monomer containing the group represented by Formula I is 0.5% to 2%, or 0.5% to 1.5%, based on the total mass of the copolymer B. The inclusion of an appropriate amount of the group represented by Formula I in the copolymer B can improve the dispersion properties of the copolymer B, making the prepared slurry less likely to precipitate, helping to increase the solid content of the slurry, and thus increasing the loading capacity of the electrode.

[0080] The second aspect of the present application provides a binder comprising a copolymer C, wherein the copolymer C comprises a structural unit derived from a monomer containing a cyano group, a structural unit derived from a monomer containing an ester group, and a structural unit derived from a monomer containing a group represented by formula I.

[0081]

[0082] wherein n is selected from 0, 1, 2 or 3.

[0083] When n is 0, the group shown in formula I is When n is 1, the group shown in formula I is When n is 2, the group shown in formula I is When n is 3, the group shown in formula I is

[0084] The electronegativity of the oxygen element in the group shown in Formula I is greater than that of the nitrogen element in the cyano group. Compared with the cyano group in the copolymer, it is easier to form hydrogen bonds with the electrode active material and the conductive agent and the bond energy is stronger, which can greatly improve the dispersibility of the slurry and increase the solid content of the slurry.

[0085] Monomers containing cyano groups can effectively complex with the metals on the current collector and electrode active material, ensuring strong adhesion between the electrode active material and the current collector. Monomers containing ester groups can improve the brittleness of the electrode and prevent brittle fracture of the electrode.

[0086] Therefore, the present application uses a copolymer C comprising structural units derived from a monomer containing a cyano group, structural units derived from a monomer containing an ester group, and structural units derived from a monomer containing a group represented by Formula I as a binder, thereby reducing the cost of the binder without reducing the solid content of the slurry, and improving the bonding performance of the binder and the cycle performance of the battery.

[0087] In some embodiments, the monomer containing a cyano group is selected from one or more of acrylonitrile, methacrylonitrile, halogenated acrylonitrile, and methoxyacrylonitrile.

[0088] In some embodiments, the mass content of the structural unit derived from the monomer containing a cyano group is 80% to 95%, based on the total mass of the copolymer C. In some embodiments, the mass content of the structural unit derived from the monomer containing a cyano group is 81% to 95%, or 82% to 95%, or 83% to 95%, or 84% to 95%, or 85% to 95%, or 86% to 95%, or 87% to 95%, or 88% to 95%, or 88% to 94%, or 88% to 93%, or 88% to 92%, or 88% to 91%, based on the total mass of the copolymer C. Within this range, the copolymer C can further improve the binding performance of the binder and the cycle performance of the battery.

[0089] In some embodiments, the monomer containing an ester group is selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isoamyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

[0090] In some embodiments, the monomer containing an ester group is selected from isooctyl acrylate. Isooctyl acrylate enables copolymer C to have a lower glass transition temperature and better flexibility, which is beneficial for the processing and manufacturing of the electrode.

[0091] Compared with fluorine-containing monomers, the above-mentioned monomers containing cyano groups and monomers containing ester groups have low costs, are not subject to policy restrictions, can be mass-produced, and can significantly reduce the cost of the adhesive.

[0092] In some embodiments, the mass content of the structural unit derived from the monomer containing an ester group is 8% to 12%, based on the total mass of the copolymer C. In some embodiments, the mass content of the structural unit derived from the monomer containing an ester group is 8% to 11%, or 9% to 11%, based on the total mass of the copolymer C. Within this range, the copolymer C can further improve the binding performance of the binder and the cycle performance of the battery.

[0093] In some embodiments, the mass ratio of structural units derived from monomers containing cyano groups to structural units derived from monomers containing ester groups in copolymer C is 8:1 to 12:1. The cyano-containing monomers can improve the mechanical strength and adhesion of copolymer B, further enhancing the battery's cycling performance. A small amount of ester-containing monomers can enhance the flexibility of copolymer B, preventing brittle fracture of the electrode. Furthermore, the ester group's ability to absorb and retain electrolyte can improve the poor ionic conductivity of fluoropolymer A. Within this range, copolymer C can further enhance electrode adhesion and battery cycling performance.

[0094] In some embodiments, the monomer containing the group represented by formula I is selected from one or more of N-vinyl pyrrolidone and N-allyl-2-pyrrolidone.

[0095] In some embodiments, the mass content of the structural unit derived from the monomer containing the group represented by Formula I is 0.1% to 2%, or 0.5% to 1.5%, based on the total mass of the copolymer C. The inclusion of an appropriate amount of the group represented by Formula I in the copolymer B can improve the dispersion properties of the copolymer B, so that the prepared slurry contains more electrode active material, which helps to increase the solid content of the slurry.

[0096] In some embodiments, the weight average molecular weight of copolymer C is 4×10 5 -7×10 5 In some embodiments, the weight average molecular weight of copolymer C is selected from 4×10 5 -7×10 5 , or 4×10 5 -6×10 5 , or 4×10 5 -5×10 5 , or 5×10 5 -7×10 5 , or 6×10 5 -7×10 5 .

[0097] Controlling the weight-average molecular weight of copolymer C can balance the adhesiveness and processability of the binder. If the weight-average molecular weight of copolymer C is too low, it will be too brittle, resulting in insufficient adhesive strength. If the weight-average molecular weight of copolymer C is too high, it will not disperse the electrode active material. Furthermore, the appropriate combination of copolymers with different molecular weights can improve the dispersibility of the electrode active material, thereby enhancing battery performance.

[0098] In some embodiments, the volume average particle size Dv50 of the copolymer C is 5-20 μm.

[0099] In this article, the term "Dv50" refers to the particle size at which the cumulative particle size distribution percentage reaches 50%. Its physical meaning is that 50% of the particles are larger than this size, and 50% of the particles are smaller than this size. Dv50 is also called the median diameter or median particle size.

[0100] A large average particle size (Dv50) of copolymer C can cause dissolution difficulties. Poor slurry dispersion can lead to agglomeration of the conductive agent or electrode active material with the binder, clogging the filter and impacting production. Agglomerates can also be washed onto the coating head, causing scratches on the coating particles and affecting coating quality. A suitable average particle size (Dv50) helps increase the dissolution rate of copolymer C in the solvent and improve electrode processing efficiency.

[0101] In some embodiments, the intrinsic viscosity of copolymer C is 0.8-1.1 dl / g.

[0102] In this article, the term "intrinsic viscosity" refers to the most commonly used method for expressing the viscosity of polymer solutions. It is defined as the reduced viscosity when the concentration of a polymer solution approaches zero. This refers to the contribution of individual molecules to the solution's viscosity and reflects the polymer's characteristics, with its value being independent of concentration. The intrinsic viscosity herein refers to the intrinsic viscosity measured in N,N-dimethylacetamide at 30°C.

[0103] The following method is used to test intrinsic viscosity in this application: First, weigh a sample m1 (0.15-0.17 g) of the finished powder of copolymer C and place it in a 100 mL conical flask. Add V1 (50-60 mL) of N,N-dimethylacetamide using a pipette, seal the conical flask, and calculate the solution concentration (C0 = m1 / V1). Place the conical flask in a 60°C constant-temperature water bath to dissolve the solution for 2.5 hours. Filter the dissolved sample solution through a sand core filter to prevent particulate impurities from clogging the Ubbelohde viscometer. Next, use a disposable plastic pipette to draw the filtered N,N-dimethylacetamide into a clean Ubbelohde viscometer, rinsing it at least 4-5 times with the solvent. Use a pipette to draw 10 mL of N,N-dimethylacetamide into the Ubbelohde viscometer. Place the Ubbelohde viscometer in a constant-temperature water bath at 30.0°C ± 0.1°C for 15-20 minutes. After that, measure the outflow time and record t0. Finally, use a disposable plastic pipette to absorb the filtered glue prepared in the first step, and rinse the plastic pipette with the glue at least 4-5 times; use a pipette to draw 10mL of glue into the Ubbelohde viscometer; place the Ubbelohde viscometer in a constant temperature water bath at 30.0℃±0.1℃, keep it for 15-20 minutes, then measure the outflow time and record t1. The measured value of the intrinsic viscosity is (t1 / t0) / C0.

[0104] Controlling the intrinsic viscosity of copolymer C within an appropriate range allows copolymer C to have both excellent bonding and processing properties. This avoids ineffective bonding due to too low a viscosity, and also avoids difficulties in slurry stirring, preparation, and coating due to too high a viscosity.

[0105] The third aspect of the present application provides a method for preparing a binder, comprising the following steps:

[0106] Providing monomers containing cyano groups, monomers containing ester groups, and monomers containing groups shown in formula I,

[0107]

[0108] wherein n is selected from 0, 1, 2 or 3;

[0109] A monomer containing a cyano group, a monomer containing an ester group, and a monomer containing a group represented by formula I are polymerized under polymerizable conditions to prepare a copolymer C.

[0110] In some embodiments, the copolymer C is obtained by conventional emulsion polymerization using an anionic emulsifier.

[0111] In some embodiments, polymerizing a monomer containing a cyano group, a monomer containing an ester group, and a monomer containing a group shown in Formula I under polymerizable conditions to prepare a copolymer C includes reacting a first amount of the monomer containing a cyano group, the monomer containing an ester group, and the monomer containing a group shown in Formula I with a first amount of a reaction solvent, a first amount of an emulsifier, a first amount of a pH buffer, and a first amount of an initiator at a first polymerization temperature under polymerization pressure; and after the first-stage reaction, adding a second amount of the monomer containing a cyano group, the monomer containing an ester group, and the monomer containing a group shown in Formula I with a second amount of a reaction solvent, a second amount of an emulsifier, a second amount of a pH buffer, and a second amount of an initiator to the system for a second-stage reaction at a second polymerization temperature.

[0112] In some embodiments, the emulsifier is selected from one or more alkali metal salts or alkyl salts of perfluorooctanoic acid. The alkali metal salt of perfluorooctanoic acid is selected from one or more of sodium perfluorooctanoate and potassium perfluorooctanoate. The alkyl salt is selected from one or more of alkyl sulfates and alkyl sulfonates.

[0113] In some embodiments, the initiator is selected from peroxides, the peroxides are selected from one or more of persulfate inorganic peroxides and peroxide carbonates, the inorganic peroxides are selected from one or more of ammonium persulfate and potassium persulfate, and the peroxide carbonates are selected from diisopropyl peroxydicarbonate.

[0114] In some embodiments, the reaction solvent is deionized water.

[0115] In some embodiments, the pH buffer is selected from one or more of aqueous ammonia, potassium carbonate, and potassium bicarbonate.

[0116] In some embodiments, the first amount of the monomer containing a cyano group, the monomer containing an ester group, and the monomer containing the group represented by Formula I is 75-90% of the total amount of each monomer formula; the first amount of the reaction solvent is 70-80% of the total amount of monomers added in the first stage reaction; the first amount of the emulsifier is 0.2-0.3% of the total amount of monomers added in the first stage reaction; the first amount of the pH buffer is 0.05-0.2% of the total amount of monomers added in the first stage reaction; and the first amount of the initiator is 0.15-1% of the total amount of monomers added in the first stage reaction. The first polymerization temperature is 70-80° C., and the reaction time of the first stage reaction is 2-3 hours.

[0117] In some embodiments, the second amount of the monomer containing a cyano group, the monomer containing an ester group, and the monomer containing the group shown in Formula I is 10-25% of the formula amount of each monomer, the second amount of the reaction solvent is 20-30% of the total amount of monomers added to the second stage reaction, the second amount of the emulsifier is 0.05-0.1% of the total amount of monomers added to the second stage reaction, the second amount of the initiator is 0.05-0.3% of the total amount of monomers added to the second stage reaction, the second polymerization temperature is 85-90°C, and the reaction time of the second stage reaction is 3-4 hours.

[0118] In some embodiments, the mass ratio of the monomer containing a cyano group to the monomer containing an ester group is 8:1 to 12:1, and the mass content of the monomer containing the group represented by Formula I is 0.1% to 2%, based on the total mass of copolymer C.

[0119] In some embodiments, the monomer containing a cyano group is selected from one or more of acrylonitrile, methacrylonitrile, halogenated acrylonitrile, and methoxyacrylonitrile; and / or the monomer containing an ester group is selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isopentyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate; and / or the monomer containing a group represented by formula I is selected from one or more of N-vinyl pyrrolidone and N-propenyl pyrrolidone.

[0120] The method has low cost for preparing monomers and mild reaction conditions, and can reduce the cost of the binder.

[0121] [Positive electrode]

[0122] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0123] 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 either or both of the two opposite surfaces of the positive electrode current collector.

[0124] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. 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.).

[0125] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, 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 may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), 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 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0126] In some embodiments, the positive electrode film layer may further include a conductive agent. For 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.

[0127] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder of the present application 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.

[0128] [Negative electrode]

[0129] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0130] 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 either or both of the two opposite surfaces of the negative electrode current collector.

[0131] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. 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, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0132] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well 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 oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0133] In some embodiments, the negative electrode film layer may further include another 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).

[0134] 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.

[0135] In some embodiments, the negative electrode film layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0136] In some embodiments, the negative electrode sheet can be prepared by the following method: 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 current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0137] [Electrolytes]

[0138] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0139] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0140] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0141] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0142] In some embodiments, the electrolyte may further include additives. For example, the additives may include 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, and additives that improve battery high or low temperature performance.

[0143] [Isolation film]

[0144] 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 and mechanical stability can be selected.

[0145] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0146] [Secondary battery]

[0147] The present application provides a secondary battery comprising an electrode assembly and an electrolyte, wherein the electrode assembly comprises a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material and a binder according to any embodiment of the present application or a binder prepared by a preparation method according to any embodiment of the present application. The secondary battery has improved cycle performance.

[0148] In some embodiments, the positive electrode active material is a lithium-containing transition metal oxide, which can be lithium iron phosphate, or their doped modified materials, or their conductive carbon coated modified materials, conductive metal coated modified materials or conductive polymer coated modified materials.

[0149] In some embodiments, the lithium-containing transition metal oxide can be selected from lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide, lithium titanate, or at least one of their doped modified materials, or their conductive carbon coated modified materials, conductive metal coated modified materials or conductive polymer coated modified materials.

[0150] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0151] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0152] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0153] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0154] The present application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square or any other shape. For example, Figure 3 The secondary battery 5 is a square structure as an example.

[0155] In some embodiments, reference Figure 4 , the outer packaging may include a shell 51 and a cover plate 53. 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 membrane 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.

[0156] In the fourth aspect of the present application, an electrode is provided, comprising an electrode active material and a binder composition or binder in any embodiment, wherein the electrode active material may be a positive electrode active material, and the positive electrode active material may be a transition metal oxide containing lithium. In some embodiments, the transition metal oxide containing lithium may be selected from lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide, lithium titanate, or at least one of their doped modified materials, or their conductive carbon coated modified materials, conductive metal coated modified materials, or conductive polymer coated modified materials. In some embodiments, the transition metal oxide containing lithium may be selected from lithium iron phosphate, or their doped modified materials, or their conductive carbon coated modified materials, conductive metal coated modified materials, or conductive polymer coated modified materials.

[0157] The electrode has higher adhesion, which makes the battery have better cycle performance.

[0158] [Battery Module]

[0159] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0160] Figure 5 4 is an example of a battery module. Figure 5 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of secondary batteries 5 may further be fixed by fasteners.

[0161] 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.

[0162] [Battery Pack]

[0163] 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 based on the application and capacity of the battery pack.

[0164] Figure 6 and Figure 7 The battery pack 1 is used as an example. Figure 6 and Figure 7The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0165] [Electrical devices]

[0166] A sixth aspect of the present application provides an electric device comprising a secondary battery in any embodiment, a battery module in any embodiment, or a battery pack in any embodiment. The electric device has a longer battery life.

[0167] The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc.

[0168] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0169] Figure 8 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0170] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0171] Example

[0172] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0173] Example 1

[0174] 1) Preparation of a binder containing copolymer C

[0175] Copolymer C was prepared as follows: A 10-liter stainless steel autoclave was used for polymerization, rotating at 100 rpm. The system was first checked for tightness, then the autoclave was evacuated and filled with nitrogen to remove oxygen, repeated three times. 2000 g of deionized water, 1.6 g of aqueous ammonia, and 3.2 g of sodium alkyl sulfate were added, followed by 1408 g of acrylonitrile monomer, 176 g of isooctyl acrylate monomer, and 16 g of N-vinyl pyrrolidone monomer. The polymerization pressure was evacuated to 4.2 MPa, the temperature was raised to 55°C, and the mixture was allowed to stand for 0.8 h. After that, 8 g of ammonium persulfate was added, the temperature was raised to 75°C, and the polymerization reaction was stirred for 2-3 h. To the autoclave, 352 g of acrylonitrile monomer, 44 g of isooctyl acrylate monomer, 4 g of N-vinyl pyrrolidone monomer, 400 g of deionized water, 1.6 g of sodium alkyl sulfate, and 0.8 g of ammonium persulfate were continuously added, the temperature was raised to 90°C, and the reaction was maintained for 4 h. The polymerized article is obtained by flash evaporation and then washed with deionized water until the conductivity of the washing liquid is less than 1*10 -8 s / cm, and after vacuum drying, N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was obtained. The prepared copolymer had a weight average molecular weight of 700,000, a particle size Dv50 of 15 μm, and an intrinsic viscosity of 1.1 dl / g.

[0176] The binder was prepared as follows: 4g of polyvinylidene fluoride (fluoropolymer A) and 4g of N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer were added to 250g of N-methyl pyrrolidone solution, stirred at 500 rpm for 90 minutes in a dispersing machine, and then defoamed in an ultrasonic cleaner for 30 minutes. The polyvinylidene fluoride was 601A, produced by East Sunshine Co., Ltd., synthesized by an emulsion method, with a weight-average molecular weight of 900,000, a particle size Dv50 of 20 μm, a crystallinity of 40%, and a melting point of 170°C.

[0177] 2) Preparation of button cells

[0178] 398g of lithium iron phosphate and 2.8g of conductive carbon black were added to an agate mortar and dry-mixed for 15 minutes. The dry-mixed product was added to the binder and stirred at 1200 rpm on a stirring disperser for 90 minutes to prepare a lithium battery positive electrode slurry.

[0179] The above slurry was scraped onto carbon-coated aluminum foil, baked at 110°C for 15 minutes, cold-pressed and cut into discs with a diameter of 15 mm, and then made into button batteries with metal lithium sheets, isolation membranes, and electrolytes.

[0180] 3) Isolation film

[0181] Polypropylene film is used as the isolation film.

[0182] 4) Preparation of electrolyte

[0183] In an argon atmosphere glove box (H2O <0.1ppm, O2 <0.1ppm), the organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3 / 7, LiPF6 lithium salt was added and dissolved in the organic solvent, and stirred evenly to prepare a 1M LiPF6EC / EMC solution to obtain an electrolyte.

[0184] The batteries of Examples 2 to 25 and the button battery of Comparative Example 1 were prepared in a similar manner to the button battery of Example 1, except that the raw materials and ratios for preparing the copolymer C or the ratios of the components in the binder were adjusted. Specific parameters are shown in Table 1.

[0185] In Examples 2 to 7, the mass ratio of polyvinylidene fluoride to copolymer C was adjusted, and other parameters remained consistent with those in Example 1. Specific parameters are shown in Table 1.

[0186] In Examples 8 to 11, the monomer polymerization ratio of acrylonitrile and isooctyl acrylate in the N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was adjusted, and other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0187] In Examples 12 to 13, the monomer polymerization ratio of acrylonitrile and isooctyl acrylate in the N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was adjusted, and the mass ratio of polyvinylidene fluoride and N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was set to 2:1. Other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0188] In Examples 14 to 15, the monomer polymerization ratio of acrylonitrile and isooctyl acrylate in the N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was adjusted, and the mass ratio of polyvinylidene fluoride and N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was set to 3:1. Other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0189] In Examples 16 to 17, the monomer polymerization ratio of acrylonitrile and isooctyl acrylate in the N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was adjusted, and the mass ratio of polyvinylidene fluoride and N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was set to 4:1. Other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0190] In Examples 18 to 20, the mass of N-vinyl pyrrolidone added to polymer B was adjusted, and the molar ratio of acrylonitrile to isooctyl acrylate monomers in the N-vinyl pyrrolidone-acrylonitrile-isooctyl acrylate copolymer was set to 10:1. Other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0191] During the preparation of the copolymers in Examples 21 to 22, after the secondary rehydration temperature was raised to 90°C, the reaction time was adjusted from 4 h to 3 h and 2.5 h, respectively, and the molar ratio of acrylonitrile and isooctyl acrylate monomers in the N-vinyl pyrrolidone-modified acrylonitrile-isooctyl acrylate copolymer was set to 10:1. Other parameters and steps were consistent with Example 1. Specific parameters are shown in Table 1.

[0192] In Example 23, N-allyl-2-pyrrolidone was used to modify acrylonitrile-isooctyl acrylate copolymer, and other parameters were consistent with those in Example 1. The specific parameters are shown in Table 1.

[0193] In Example 24, N-vinyl pyrrolidone was not added during the preparation of the copolymer, and the molar ratio of acrylonitrile to isooctyl acrylate monomers in the acrylonitrile-isooctyl acrylate copolymer was set to 10:1. Other parameters remained consistent with Example 1. Specific parameters are shown in Table 1.

[0194] In Example 25, polyvinylidene fluoride 401A produced by East Sunshine Co., Ltd. was used, and its weight-average molecular weight was 600,000. Other steps were consistent with those in Example 1. Specific parameters are shown in Table 1.

[0195] In Comparative Example 1, polyvinylidene fluoride alone was used as a binder; in Comparative Example 2, the N-vinyl pyrrolidone-modified acrylonitrile-isooctyl acrylate copolymer prepared in Example 9 was used as a binder; and in Comparative Example 3, the acrylonitrile-isooctyl acrylate copolymer prepared in Example 24 was used as a binder. The other steps were the same as in Example 1. The specific parameters are shown in Table 1.

[0196] The relevant parameters of the adhesives of Examples 1 to 25 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0197] In addition, the electrode sheets and batteries obtained in Examples 1 to 25 and Comparative Examples 1 to 3 were subjected to performance tests. The test methods are as follows, and the test results are shown in Table 1.

[0198] 1. Test of structural unit types of adhesives

[0199] The sample was pressed into a KBr pellet using the pellet transmission method. The KBr background blank was subtracted by the transmission method to obtain the sample test spectrum. The instrument model was Nicolet 5700 (Thermo Nicolet, USA). The standard linearity was better than 0.07% and the resolution was 0.09 cm. -1 , wave number range: 400~4000cm -1 , sensitivity <9.65*10 -5 Abls. Used to examine the structure and chemical bonds of molecules.

[0200] 2. Molecular weight test

[0201] A Waters 2695 Isocratic HPLC gel chromatography instrument (differential refractive index detector 2141) was used. A 3.0% polystyrene solution sample was used as a reference, and a matching chromatographic column was selected (oil-based: Styragel HT5 DMF 7.8*300mm + Styragel HT4). A 3.0% adhesive solution was prepared using purified N-methylpyrrolidone (NMP) solvent. The prepared solution was allowed to stand for one day before use. During testing, tetrahydrofuran was first drawn into a syringe and rinsed several times. Then, 5 ml of the experimental solution was drawn, the air in the syringe was expelled, and the needle tip was wiped dry. Finally, the sample solution was slowly injected into the injection port. Data was acquired after the reading stabilized.

[0202] 3. Adhesion test (between the active material layer of the positive electrode sheet and the positive electrode current collector)

[0203] With reference to the national standard GBT 2790-1995 "Test method for 180° peel strength of adhesives", the adhesion test process of the examples and comparative examples of the present application is as follows:

[0204] Use a blade to cut a sample with a width of 30mm and a length of 100-160mm. Apply special double-sided tape to the steel plate with a width of 20mm and a length of 90-150mm. Attach the electrode sample cut earlier to the double-sided tape with the test surface facing down. Finally, use a roller to roll it three times in the same direction.

[0205] Insert a paper tape with a width equal to the electrode and a length 80-200 mm greater than the length of the sample under the electrode and secure it with wrinkle glue.

[0206] Turn on the power of the Sansi tensile testing machine (sensitivity is 1N), the indicator light will light up, adjust the limit block to the appropriate position, and fix the end of the steel plate without the electrode with the lower clamp. Fold the paper tape upwards and fix it with the upper clamp. Use the "up" and "down" buttons on the manual controller attached to the tensile testing machine to adjust the position of the upper clamp. Then perform the test and read the value. Figure 1 The bonding strength comparison data of Example 1 and Comparative Example 1 shown

[0207] 4. Battery capacity retention test

[0208] Taking Example 1 as an example, the battery capacity retention rate test process is as follows: at 25°C, the battery corresponding to Example 1 is charged to 3.65V at a constant current of 1 / 3C, then charged to a current of 0.05C at a constant voltage of 3.65V, left for 5 minutes, and then discharged to 2.5V at a constant voltage of 1 / 3C. The obtained capacity is recorded as the initial capacity C0. Repeat the above steps for the same battery and record the discharge capacity Cn of the battery after the nth cycle. The battery capacity retention rate after each cycle is Pn = Cn / C0*100%, with the 100 point values P1, P2...P100 as the vertical coordinates and the corresponding number of cycles as the horizontal coordinates, as shown in the attached figure. Figure 2 The graphs of battery capacity retention and cycle number of Example 1 and Comparative Example 1 are shown.

[0209] 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 data corresponding to Example 1 in Table 1 is the data measured after 500 cycles under the above test conditions, i.e., the P500 value. The testing procedures for Comparative Example 1 and the other examples are the same as above.

[0210] 5. Slurry solid content test

[0211] Solids content test method: Prepare a glass Petri dish and record the weight m1. Place a portion of the prepared cathode slurry into the glass Petri dish and record the total weight m2. Heat the Petri dish containing the cathode slurry in a drying oven at 120°C for 1 hour. Weigh the dried Petri dish and record the weight m3. Solids content = (m3 - m1) / (m2 - m1) * 100%.

[0212]

[0213]

[0214]

[0215]

[0216] like Figure 1 and Figure 2 As shown, compared with Comparative Example 1, in Example 1, the bonding performance of the binder and the capacity retention rate of the battery are improved due to the addition of the acrylonitrile-isooctyl acrylate copolymer modified by the monomer having a pyrrolidone group.

[0217] As shown in Table 1, Examples 1-25 all provide a binder composition comprising polyvinylidene fluoride and an acrylonitrile-isooctyl acrylate copolymer comprising structural units derived from acrylonitrile and isooctyl acrylate. Compared to Comparative Examples 1-3, these compositions all achieve superior results, improving the adhesive properties and battery capacity retention.

[0218] The weight-average molecular weight of the polyvinylidene fluoride in Examples 1 to 25 is 600,000 to 900,000, and the weight-average molecular weight of the acrylonitrile-isooctyl acrylate copolymer is 400,000 to 700,000. Compared with Comparative Examples 1 to 3, these examples all achieved good results, improving the adhesive properties of the binder and the battery capacity retention rate.

[0219] In Examples 1 to 25, the mass ratio of polyvinylidene fluoride to acrylonitrile-isooctyl acrylate copolymer in the binder composition is 1:4 to 4:1. Compared with Comparative Examples 1 to 3, they all achieved good results, improving the bonding performance of the binder and the battery capacity retention rate.

[0220] The mass ratio of the structural units derived from acrylonitrile to the structural units derived from isooctyl acrylate in the acrylonitrile-isooctyl acrylate copolymers in Examples 1 to 25 was 8:1 to 12:1, which all achieved good results, improving the bonding performance of the binder and the battery capacity retention rate. When the mass ratio of the structural units derived from acrylonitrile to the structural units derived from isooctyl acrylate in the acrylonitrile-isooctyl acrylate copolymers was 8:1 to 10:1, the bonding performance of the binder and the battery capacity retention rate were further improved.

[0221] The acrylonitrile-2-ethylhexyl acrylate copolymers in Examples 1-23 and 25 were modified with monomers bearing pyrrolidone groups, resulting in the copolymers containing structural units derived from monomers bearing pyrrolidone groups. Compared to the unmodified acrylonitrile-2-ethylhexyl acrylate copolymers, the modified acrylonitrile-2-ethylhexyl acrylate copolymers exhibited stronger adhesive properties and battery capacity retention, while also further increasing the solids content of the slurry.

[0222] The mass content of the structural units derived from the monomer containing a pyrrolidone group in Examples 1 to 23 and 25 is 0.1% to 2% based on the total mass of the acrylonitrile-isooctyl acrylate copolymer. Within this range, the copolymer improves the adhesive properties of the binder, the battery capacity retention rate, and the solid content of the slurry. The mass content of the structural units derived from the monomer containing a pyrrolidone group in Examples 1 to 23 and 25 is either 0.5% to 1.5% or 0.5% to 1.0%. Within this range, the solid content of the slurry is further improved.

[0223] The acrylonitrile-2-ethylhexyl acrylate copolymers in Examples 1-23 and 25 were modified with a monomer containing a pyrrolidone group, with the weight content of structural units derived from acrylonitrile ranging from 80% to 95% based on the total weight of the copolymer. Within this range, the copolymers improved the adhesive properties of the binder, the battery capacity retention, and the solids content of the slurry.

[0224] The acrylonitrile-2-octyl acrylate copolymers in Examples 1-23 and 25 were modified with a monomer containing a pyrrolidone group, with the weight content of structural units derived from 2-octyl acrylate ranging from 8% to 12% based on the total weight of the copolymer. Within this range, the copolymers improved the adhesive properties of the binder, battery capacity retention, and the solids content of the slurry.

[0225] Comparison of Comparative Example 2 and Comparative Example 3 shows that modifying the binder with a monomer having a group represented by Formula I can increase the solid content in the positive electrode slurry. Comparison of Example 9 and Example 24 shows that, under the same conditions, modifying the binder with a monomer having a group represented by Formula I can increase the solid content in the positive electrode slurry, significantly improve the bonding force between the corresponding current collector and the negative electrode material layer, and significantly improve the capacity retention rate of the battery.

[0226] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that: The invention comprises an electrode assembly and an electrolyte, wherein the electrode assembly comprises a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material and a binder, wherein the binder comprises a fluoropolymer A and a copolymer B, wherein the mass ratio of the fluoropolymer A to the copolymer B is 1:4 to 4:1; wherein the copolymer B comprises a structural unit derived from a monomer containing a cyano group and a structural unit derived from a monomer containing an ester group, and wherein the copolymer B further comprises a structural unit derived from a monomer containing a group represented by formula I. Formula I, wherein n is selected from 0, 1, 2 or 3; The mass content of the structural unit derived from the monomer containing the group represented by formula I is 0.1% to 2%, based on the total mass of the copolymer B; The mass ratio of the structural units derived from the monomer containing a cyano group to the structural units derived from the monomer containing an ester group in the copolymer B is 8:1 to 12:

1.

2. The secondary battery according to claim 1, wherein The weight average molecular weight of the fluorine-containing polymer A is 600,000 to 900,000, and the weight average molecular weight of the copolymer B is 400,000 to 700,000.

3. The secondary battery according to claim 1, wherein The fluorine-containing polymer A is selected from one or more of polyvinylidene fluoride, and copolymers thereof with tetrafluoroethylene, hexafluoropropylene, and trichloroethylene.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The monomer containing a cyano group is selected from one or more of acrylonitrile, methacrylonitrile, halogenated acrylonitrile and methoxyacrylonitrile.

5. The secondary battery according to any one of claims 1 to 3, characterized in that: The monomer containing an ester group is selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isoamyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

6. The secondary battery according to any one of claims 1 to 3, characterized in that: The mass ratio of the structural units derived from the monomer containing a cyano group to the structural units derived from the monomer containing an ester group in the copolymer B is 8:1 to 10:

1.

7. The secondary battery according to any one of claims 1 to 3, characterized in that: The monomer containing the group represented by formula I is selected from one or more of N-vinyl pyrrolidone and N-allyl-2-pyrrolidone.

8. The secondary battery according to any one of claims 1 to 3, characterized in that: The mass content of the structural unit derived from the monomer containing the group represented by formula I is 0.5% to 1.5%, based on the total mass of the copolymer B.

9. The secondary battery according to any one of claims 1 to 3, characterized in that: The mass content of the structural unit derived from the monomer containing the group represented by formula I is 0.5% to 1.0%, based on the total mass of the copolymer B.

10. The secondary battery according to any one of claims 1 to 3, characterized in that: The positive electrode active material is a lithium-containing transition metal oxide.

11. The secondary battery according to any one of claims 1 to 3, characterized in that: The positive electrode active material is lithium iron phosphate, or doped modified materials thereof, or at least one of conductive carbon coated modified materials, conductive metal coated modified materials or conductive polymer coated modified materials thereof.

12. A battery module, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 11.

13. A battery pack, characterized in that: A battery module comprising the battery module according to claim 12.

14. An electrical device, characterized in that: The battery comprises at least one selected from the secondary battery according to any one of claims 1 to 11, the battery module according to claim 12, or the battery pack according to claim 13.

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