Positive electrode sheet and preparation method therefor, binder, battery, and electric device

Abstract

The present application relates to the technical field of batteries, and provides a positive electrode sheet and a preparation method therefor, a binder, a battery, and an electric device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector, the positive electrode film layer comprises a binder, and the binder comprises a copolymer.

Description

Positive electrode sheet and preparation method thereof, binder, battery and electrical device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority benefit of Chinese patent application No. 202410044131.3 filed on January 11, 2024, and incorporates the entirety of the application into this document. Technical Field

[0003] The present application belongs to the field of battery technology, and specifically relates to a positive electrode plate and a preparation method thereof, a binder, a battery, and an electrical device. Background Art

[0004] As the application of secondary batteries expands, the requirements for their electrochemical performance and safety are also becoming increasingly stringent. Currently, to improve the energy density and conductivity of secondary batteries, a large amount of positive electrode active material and a small amount of additives such as binders are typically added to the film layer of the positive electrode sheet. However, current positive electrode films can react with each other, affecting battery performance. Other issues include poor toughness, easy breakage, and weak adhesion. These factors can make the positive electrode sheet susceptible to breakage or film detachment during winding processes, posing significant safety risks.

[0005] Therefore, it is still necessary to further improve the positive electrode.

[0006] Summary of the Invention

[0007] In response to the above-mentioned problems, the present application aims to provide a positive electrode sheet and a preparation method thereof, a battery, and an electrical device. The positive electrode sheet has high conductivity and high toughness and adhesion.

[0008] In a first aspect, the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises a binder, and the binder comprises a copolymer; the copolymer comprises a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3:

[0009] in, represents a connecting bond, R1, R3 and R4 are the same or different and are independently hydrogen or an alkyl group having 1 to 4 carbon atoms; R2 is an alkyl group having 1 to 8 carbon atoms, and p is any integer from 1 to 8.

[0010] In the positive electrode sheet of the present application, the copolymer is introduced as a binder to improve the flexibility of the positive electrode sheet and the adhesion between the film layer and the current collector. The ester group (-COOR2) introduced in the first structural unit can improve the flexibility of the binder, the amide group (-CONH-) in the second structural unit provides the binder with high adhesion and flexibility, the isocyanate group (-NCO) has high reactivity, which can provide high adhesion between the binder and the positive electrode current collector and between the components in the positive electrode film layer. The cyano group (-CN) in the third structural unit can improve the bonding strength and the solubility of the binder in the positive electrode slurry. In addition, the binder can also maintain a low resistance of the electrode sheet. As a result, the positive electrode sheet using this binder has high toughness and adhesion, as well as high conductivity.

[0011] In some embodiments of the present application, the copolymer is a random copolymer.

[0012] In some embodiments of the present application, the copolymer has a core-shell structure, wherein the core layer includes a polymer segment formed by the first structural unit and the second structural unit, and the shell layer includes a polymer segment formed by the third structural unit. The third structural unit has a polar group cyano (-CN), and the polymer segment formed by it as the shell layer can improve the wettability of the electrolyte to the electrode and the solubility of the copolymer in the positive electrode slurry. The core structure formed by the first structural unit and the second structural unit can reduce the glass transition temperature (Tg) of the copolymer and improve its flexibility.

[0013] In some embodiments of the present application, the glass transition temperature of the copolymer is 5° C. to 15° C. This is beneficial for improving the toughness of the positive electrode sheet.

[0014] In some embodiments of the present application, the number average molecular weight of the copolymer is 50,000 to 80,000. This can provide the positive electrode film with high toughness while improving adhesion to the current collector.

[0015] In some embodiments of the present application, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.2-0.5):(0.4-0.9), thereby improving the solubility of the copolymer in the positive electrode slurry.

[0016] Furthermore, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.3-0.5):(0.5-0.8), thereby further improving the brittleness of the positive electrode sheet and increasing the toughness of the positive electrode sheet.

[0017] In some embodiments of the present application, R1, R3 and R4 are all hydrogen, and R2 is an alkyl group having 2 to 6 carbon atoms.

[0018] In some embodiments of the present application, p is any integer from 2 to 6.

[0019] In some embodiments of the present application, the structure of the first structural unit is as shown in Formula 1-1, the structure of the second structural unit is as shown in Formula 2-1, and the structure of the third structural unit is as shown in Formula 3-1:

[0020] Thus, the overall performance of the positive electrode sheet can be improved.

[0021] In some embodiments of the present application, the copolymer is prepared by a method comprising the following steps:

[0022] In the presence of a first solvent and an initiator, a first monomer having a structure as represented by Formula A is subjected to a first polymerization reaction to obtain a first mixed solution;

[0023] Mixing the first mixed solution with a second monomer represented by formula B and performing a second polymerization reaction to obtain a second mixed solution;

[0024] mixing the second mixed liquid with a third monomer represented by formula C and performing a third polymerization reaction to obtain a third mixed liquid containing the copolymer;

[0025] separating and drying the third mixed liquid to obtain the copolymer in powder form;

[0026] Optionally, the temperature of the first polymerization reaction is 70° C. to 90° C., and the reaction time is 2 h to 5 h.

[0027] Optionally, the temperature of the second polymerization reaction is 70° C. to 90° C., and the reaction time is 2 h to 5 h.

[0028] Optionally, the temperature of the third polymerization reaction is 80° C. to 200° C., and the reaction time is 3 h to 5 h.

[0029] Optionally, the initiator comprises at least one of benzoyl peroxide and diisopropyl peroxide.

[0030] Optionally, based on the weight of the first monomer, the amount of the initiator is 0.1 wt% to 15 wt%.

[0031] Optionally, the first solvent comprises N-methylpyrrolidone.

[0032] In some embodiments of the present application, the binder has a mass content of 0.5% to 2% based on the mass of the positive electrode film layer, thereby improving the bonding effect while minimizing the hardness of the positive electrode sheet.

[0033] In some embodiments of the present application, the positive electrode film layer further comprises a flexibilizer, wherein the flexibilizer comprises at least one of nitrile rubber, polyether, and polystyrene, thereby further improving the toughness of the positive electrode sheet.

[0034] Optionally, the flexibilizer includes nitrile rubber. Thus, the flexibilizer has a high compatibility with the adhesive, which can further improve the brittleness of the electrode and enhance its toughness.

[0035] In some embodiments of the present application, the mass content of the softener is 0.1% to 0.5% based on the total weight of the positive electrode film layer, thereby improving the brittleness of the electrode while also making the positive electrode sheet have higher conductivity.

[0036] In some embodiments of the present application, the positive electrode film layer further comprises a positive electrode active material and a conductive additive. Based on the total weight of the positive electrode film layer, the mass content of the positive electrode active material is 96% to 99%, and the mass content of the conductive additive is 0.2% to 1%. This can improve the conductivity of the battery.

[0037] In some embodiments of the present application, the positive electrode active material includes at least one of lithium iron phosphate and lithium nickel cobalt manganese oxide.

[0038] In some embodiments of the present application, the adhesion between the positive electrode film layer and the positive electrode current collector is not less than 40 N / m.

[0039] In a second aspect, the present application provides a binder comprising a copolymer, wherein the copolymer comprises a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3:

[0040] in, represents a connecting bond, R1, R3 and R4 are the same or different and are independently hydrogen or an alkyl group having 1 to 4 carbon atoms; R2 is an alkyl group having 1 to 8 carbon atoms, and p is any integer from 1 to 8.

[0041] In the binder of the present application, the copolymer used can improve the flexibility of the positive electrode sheet and improve the bonding ability between the film layer and the current collector. The ester group (-COOR2) introduced in the first structural unit can improve the flexibility of the binder, the amide group (-CONH-) in the second structural unit provides the binder with high bonding and flexibility, and the isocyanate group (-NCO) has high reactivity, which can provide high bonding between the binder and the positive electrode collector and between the components in the positive electrode film layer. The cyano group (-CN) in the third structural unit can improve the bonding strength and the solubility of the binder in the positive electrode slurry. In addition, the binder can also maintain a low resistance of the electrode sheet. As a result, the positive electrode sheet using this binder has high toughness and bonding strength, as well as high conductivity.

[0042] In some embodiments of the present application, the copolymer is a random copolymer.

[0043] In some embodiments of the present application, the copolymer has a core-shell structure, wherein the core layer includes a polymer segment formed by the first structural unit and the second structural unit, and the shell layer includes a polymer segment formed by the third structural unit. The third structural unit has a polar group cyano (-CN), and the polymer segment formed by it as the shell layer can improve the wettability of the electrolyte to the electrode and the solubility of the copolymer in the positive electrode slurry. The core structure formed by the first structural unit and the second structural unit can reduce the glass transition temperature (Tg) of the copolymer and improve its flexibility.

[0044] In some embodiments of the present application, the number average molecular weight of the copolymer is 50,000 to 80,000. This can provide the positive electrode film with high toughness while improving adhesion to the current collector.

[0045] In some embodiments of the present application, the glass transition temperature Tg of the copolymer is 5° C. to 15° C. This is beneficial for improving the toughness of the positive electrode sheet.

[0046] In some embodiments of the present application, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.2-0.5):(0.4-0.9), thereby improving the solubility of the copolymer in the positive electrode slurry.

[0047] Optionally, in the acrylic copolymer, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.3-0.5):(0.5-0.8). This can further improve the brittleness of the positive electrode sheet and enhance the toughness of the positive electrode sheet.

[0048] In some embodiments of the present application, R1, R3 and R4 are all hydrogen, and R2 is an alkyl group having 2 to 6 carbon atoms.

[0049] In some embodiments of the present application, p is any integer from 2 to 6.

[0050] In some embodiments of the present application, the structure of the first structural unit is as shown in Formula 1-1, the structure of the second structural unit is as shown in Formula 2-1, and the structure of the third structural unit is as shown in Formula 3-1:

[0051] Thus, the overall performance of the positive electrode sheet can be improved.

[0052] In a third aspect, the present application provides a method for preparing the positive electrode sheet described in the first aspect of the present application, comprising: applying a positive electrode slurry containing the binder on the positive electrode current collector to form a positive electrode film layer.

[0053] In some embodiments of the present application, the method includes:

[0054] mixing the positive electrode active material, the conductive additive, the binder and the optional flexibilizer with a second solvent to obtain a positive electrode slurry;

[0055] The positive electrode slurry is coated on the surface of the positive electrode current collector, dried, and cold pressed to form a positive electrode film layer.

[0056] In some embodiments of the present application, the mass content of the second solvent in the positive electrode slurry is 10% to 20%, thereby making the positive electrode slurry have an appropriate viscosity and improving coating uniformity.

[0057] In a fourth aspect, the present application provides a battery, comprising the positive electrode sheet described in the first aspect of the present application or the positive electrode sheet prepared by the method described in the third aspect of the present application.

[0058] In a fifth aspect, the present application provides an electrical device comprising the battery described in the fourth aspect of the present application.

[0059] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become obvious from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Various other advantages and benefits will become apparent to those skilled in the art by reading the detailed description of the preferred embodiment below. The accompanying drawings are only for the purpose of illustrating the preferred embodiment and are not to be considered as limiting the present application. In the accompanying drawings:

[0061] FIG1 is a transmission electron micrograph of the acrylamide copolymer prepared in Preparation Example 1;

[0062] FIG2 is a schematic diagram of a battery cell according to an embodiment of the present application;

[0063] FIG3 is an exploded view of a battery cell according to an embodiment of the present application;

[0064] FIG4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0065] FIG5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0066] FIG6 is an exploded view of the battery pack shown in FIG5 according to an embodiment of the present application;

[0067] FIG7 is a schematic diagram of an electrical device using a battery as a power source according to an embodiment of the present application.

[0068] Description of reference numerals:

[0069] 1: Battery pack; 2: Upper case; 3: Lower case; 4: Battery module; 5: Battery cell; 51: Housing; 52: Electrode assembly; 53: Top cover assembly. DETAILED DESCRIPTION

[0070] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0071] In this application, references to "embodiments" mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor do they represent independent or alternative embodiments that are mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0072] " Scope " disclosed in the present application is limited in the form of lower limit and / or upper limit, and given range is limited by selecting a lower limit and / or an upper limit, and the selected lower limit and / or the rear 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 be combined with any upper limit to form the scope of not clearly recording, and any lower limit can be combined with other lower limits to form the scope of not clearly recording, and any upper limit can be combined with any other upper limit to form the scope of not clearly recording. In addition, each separately disclosed point or single numerical value itself can be used as lower limit or upper limit and any other point or single numerical value combination or with other lower limit or upper limit combination to form the scope of not clearly recording.

[0073] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0074] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of this application.

[0075] In addition, in this application, the terms "plurality," "multiple," and "at least one" refer to more than two. "Above" and "below" are inclusive of the number itself. For example, "two or more" includes two itself, such as two, three, four, or more.

[0076] Unless otherwise specified, the term "and / or" in this application simply describes an association between related objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Furthermore, the character " / " in this document generally indicates that the related objects are in an "or" relationship.

[0077] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and any variations thereof are intended to cover non-exclusive inclusions. Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0078] Currently, the binders used for battery electrodes include fluorine-containing binders and fluorine-free binders. Fluorine-containing binders include PVDF, which has high bonding strength. However, the preparation conditions of fluorine-containing binders are strict. The F atoms in their monomer molecular structure are highly electronegative and easily react with positively charged substances (such as metal ions), thereby introducing metal impurities. In addition, during the preparation of the positive electrode slurry, the slurry often needs to be sheared and dispersed at high speed in metal equipment, which also increases the risk of introducing metal impurities into the positive electrode. When metal impurities are introduced into the positive electrode, they will undergo an oxidation reaction to form ions and migrate to the negative electrode to undergo a reduction reaction, resulting in an excessively large direct current resistance (DCR) (>450mΩ), reducing the battery's usability. Fluorine-free binders include polyacrylates, polystyrene, and other binders. Although these binders are stable in nature, their application to positive electrode sheets will cause the sheets to be too brittle, which is not conducive to the subsequent use of the sheets (such as brittle fracture during winding), and often fails to meet the use requirements of the positive electrode sheets.

[0079] To this end, a first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises a binder, wherein the binder comprises a copolymer, and the copolymer comprises a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3:

[0080] in, represents a connecting bond, R1, R3 and R4 are the same or different and are independently hydrogen or an alkyl group having 1 to 4 carbon atoms; R2 is an alkyl group having 1 to 8 carbon atoms, and p is any integer from 1 to 8.

[0081] In the positive electrode sheet of the present application, the copolymer used is an acrylic ester copolymer. Introducing this copolymer as a binder can improve the flexibility of the positive electrode sheet and improve the bonding ability between the film layer and the current collector. The ester group (-COOR2) in the first structural unit can improve the flexibility of the binder, the amide group (-CONH-) in the second structural unit provides the binder with high bonding and flexibility, and the isocyanate group (-NCO) has high reactivity, which can make the binder have high bonding strength with the positive electrode collector and between the components in the positive electrode film layer; the cyano group (CN) in the third structural unit can improve the bonding strength and the solubility of the binder in the positive electrode slurry; and the binder can also maintain a low resistance of the electrode sheet. As a result, the positive electrode sheet has high toughness and bonding strength, and is also highly conductive.

[0082] In some embodiments, R1, R3 and R4 can each independently be hydrogen, methyl, ethyl, n-propyl, isopropyl, etc. Alternatively, R1, R3 and R4 are all hydrogen.

[0083] In Formula 1, R2 may include a linear alkyl group having 1 to 8 carbon atoms or a branched alkyl group having 3 to 8 carbon atoms; the number of carbon atoms in the alkyl group may be, for example, 1, 2, 3, 4, 5, 6, 7, or 8. Specific examples of R2 include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-pentyl, and n-hexyl. Alternatively, R2 is an alkyl group having 2 to 6 carbon atoms.

[0084] In Formula 2, p can be 1, 2, 3, 4, 5, 6, 7, or 8. Optionally, p is any integer from 2 to 6.

[0085] In some embodiments, the copolymer is a random copolymer.

[0086] In some embodiments, the copolymer has a core-shell structure, wherein the core layer includes a polymer segment formed by the first structural unit and the second structural unit, and the shell layer includes a polymer segment formed by the third structural unit. The third structural unit has a polar group cyano (-CN), and the polymer segment formed by it as the shell layer can improve the wettability of the electrolyte to the electrode and the solubility of the copolymer in the positive electrode slurry. The core structure formed by the first structural unit and the second structural unit can reduce the glass transition temperature (Tg) of the copolymer and improve its flexibility.

[0087] In some embodiments, the copolymer has a glass transition temperature (Tg) of 5°C to 15°C, for example, 5°C, 6°C, 8°C, 10°C, 11°C, 12°C, 13°C, 14°C, or 15°C. This improves the toughness of the positive electrode. The glass transition temperature can be measured using a dilatometer method, as described in the Examples below.

[0088] In some embodiments, the copolymer has a number average molecular weight of 50,000 to 80,000, for example, 50,000, 60,000, 70,000, or 80,000. This allows the positive electrode film to have high toughness while also improving adhesion to the current collector. The number average molecular weight can be measured using gel permeation chromatography (GPC), as described in the Examples below.

[0089] In some embodiments, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.2-0.5):(0.4-0.9), for example, 1:0.2:0.5, 1:0.5:0.5, 1:0.3:0.7, 1:0.3:0.8, 1:0.2:0.9, etc. This can improve the solubility of the copolymer in the positive electrode slurry. The molar amounts of the three structural units can be calculated based on the monomer feed amounts.

[0090] Optionally, the molar ratio of the first structural unit, the second structural unit, and the third structural unit is 1:(0.3-0.5):(0.5-0.8), thereby making the positive electrode sheet have higher toughness.

[0091] In some embodiments, the structure of the first structural unit is as shown in Formula 1-1, the structure of the second structural unit is as shown in Formula 2-1, and the structure of the third structural unit is as shown in Formula 3-1:

[0092] Thus, the overall performance of the positive electrode sheet can be improved.

[0093] In the present application, the copolymer can be formed, for example, by a free radical-initiated polymerization reaction of a first monomer having a structure as shown in Formula A, a second monomer having a structure as shown in Formula B, and a third monomer having a structure as shown in Formula C:

[0094] In Formulas A to C, the definitions of R1, R2, R3, R4 and p are as described above and will not be repeated here.

[0095] In some embodiments, the copolymer is prepared by a method comprising the following steps:

[0096] In the presence of a first solvent and an initiator, the first monomer (acrylic ester monomer) is subjected to a first polymerization reaction to obtain a first mixed solution;

[0097] Mixing the first mixed solution with the second monomer (acrylamide monomer) and performing a second polymerization reaction to obtain a second mixed solution;

[0098] mixing the second mixed liquid with the third monomer (acrylonitrile monomer) and performing a third polymerization reaction to obtain a third mixed liquid containing the copolymer;

[0099] The third mixed liquid is separated and dried to obtain a powdery copolymer.

[0100] In the preparation method provided in this embodiment, the mixing and polymerization reactions can be carried out under the protection of an inert atmosphere (eg, nitrogen).

[0101] In this embodiment, the first polymerization reaction can form an acrylic acid ester segment serving as the core layer. The first solvent can include an organic solvent, such as an oil-soluble organic solvent; and the initiator can include an oil-soluble initiator. Alternatively, the first solvent includes N-methylpyrrolidone (NMP).

[0102] Optionally, the temperature of the first polymerization reaction is 70°C to 90°C, such as 70°C, 75°C, 80°C, or 90°C; and the reaction time is 2h, 3h, 4h, or 5h.

[0103] Optionally, the initiator includes at least one of benzoyl peroxide (BPO) and diisopropyl peroxide (AIBN).

[0104] Optionally, based on the weight of the first monomer, the amount of the initiator used is 0.1wt% to 15wt%, for example, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 7wt%, 10wt%, 12wt%, 15wt%, etc.

[0105] In this embodiment, in order to control the reaction rate and improve product uniformity, the second monomer can be mixed with the first mixed solution by slowly adding the monomer in drops. During the addition process, the second monomer can be swelled, allowing the core layer to continue to grow.

[0106] Optionally, the temperature of the second polymerization reaction can be 70°C to 90°C, such as 70°C, 75°C, 80°C, or 90°C; and the reaction time can be 2h to 5h, such as 2h, 3h, 4h, or 5h.

[0107] In this embodiment, the third monomer is an oil-soluble monomer. During mixing and reaction with the second mixed liquid, it may have a higher affinity for the selected first solvent (e.g., NMP), tending to polymerize on the surface of the core layer to form a shell layer. Furthermore, to control the reaction rate and improve product uniformity, the third monomer may be slowly added dropwise to the second mixed liquid.

[0108] Optionally, to increase the degree of reaction, the temperature of the third polymerization reaction can be 80° C. to 200° C., for example, 80° C., 100° C., 120° C., 140° C., 150° C., 170° C., 180° C., 200° C., etc.; the reaction time can be 3 h to 5 h, for example, 3 h, 4 h, 5 h, etc. In addition, to control the degree of reaction and system stability, the temperature can be increased by heating in stages, and the temperature can be controlled within the temperature range of the third polymerization reaction.

[0109] As a specific example, the first monomer is n-butyl acrylate (BA), the second monomer structure is shown in the following formula B-1 (this monomer can be purchased from Yunnan Enjie New Materials Co., Ltd., brand ADY-5130), and the third monomer is acrylonitrile (AN):

[0110] In some specific embodiments, the copolymer has a structure as shown in Formula 4:

[0111] Wherein, m, n, and o represent the degree of polymerization, respectively, m:n:o=1:(0.2-0.5):(0.4-0.9), and may alternatively be 1:(0.3-0.5):(0.5-0.8). It should be understood that Formula 4 and m, n, and o only represent the type and number of structural units, and do not represent the connection method of each structural unit.

[0112] In some embodiments, the binder content is 0.5% to 2% by mass based on the mass of the positive electrode film layer, for example, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2%, etc. This improves the bonding effect while minimizing the hardness of the positive electrode sheet.

[0113] In some embodiments, the positive electrode film layer further comprises a flexibilizer, wherein the flexibilizer comprises at least one of nitrile butadiene rubber (NBR), polyethers (e.g., polyoxypropylene glycol), and polystyrene, thereby further improving the toughness of the positive electrode sheet.

[0114] Furthermore, the flexibilizer includes nitrile rubber. Thus, the flexibilizer has a high compatibility with the adhesive, which can further improve the brittleness of the electrode and enhance the toughness of the material.

[0115] In some embodiments, the mass content of the softener is 0.1% to 0.5% based on the total weight of the positive electrode film layer, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc. Thus, while improving the brittleness of the electrode sheet, the positive electrode sheet also has higher conductivity.

[0116] In some embodiments, the positive electrode film further comprises a positive electrode active material and a conductive additive. Based on the total weight of the positive electrode film, the mass content of the positive electrode active material is 96% to 99%, and the mass content of the conductive additive is 0.2% to 1%. This can improve the conductivity of the battery.

[0117] Generally, the positive electrode active material can be selected from positive electrode materials of secondary batteries, for example, it can be a positive electrode material of a lithium secondary battery or a positive electrode material of a sodium secondary battery.

[0118] In some embodiments, the positive electrode active material may be a positive electrode active material of a lithium secondary battery. As an example, the positive electrode active material may include one or more of a lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and their respective modified compounds. 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 known as NCM333); LiNi 0.5 Co 0.2 Mn 0.3 O2, also known as NCM523; LiNi 0.8 Co 0.1 Mn 0.1 O2, also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi 0.8 Co 015 Al 0.05at least one of O2) and its modified compounds, etc. Examples of the lithium-containing phosphate with olivine structure may include but are not limited to lithium iron phosphate (such as LiFePO4, also known 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 manganese iron phosphate, and at least one of a composite material of lithium manganese iron phosphate and carbon.

[0119] Optionally, the positive electrode active material includes at least one of lithium iron phosphate (LFP) and lithium nickel cobalt manganese oxide (NCM).

[0120] In some embodiments, the positive electrode active material can adopt the positive electrode active material of a sodium secondary battery. As an example, the positive electrode active material may include at least one of Prussian blue (PBA)-type, oxide-type, and polyanion compound-type sodium battery positive electrode materials. Specific examples of the Prussian blue (PBA)-type sodium battery positive electrode materials include but are not limited to Na4Fe2(CN)6, Na4Fe(CN)6, Na 1.72 MnFe2(CN)6, NaNiFe(CN)6. The oxide-type sodium battery positive electrode material (Na x MO2, 0 < x ≤ 1, M is a transition metal element) is composed of a transition metal oxide, and the variable-valence transition metals involved mainly include vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Specific examples thereof include but are not limited to NaCrO2, NaMnO2, NaMnO2, Na 0.61 Ti 0.48 Mn 0.52 O2, Na[Fe 0.5 Co 0.5 O2, NaMnO2, Na[Ni 0.25 Fe 0.5 Mn 0.25 O2. The polyanion compound-type sodium battery positive electrode material is composed of sodium, a transition metal, and an anion. Among them, the transition metals mainly include iron, vanadium, cobalt, etc., and the anions mainly include phosphate, pyrophosphate, fluorophosphate, and sulfate. Specific examples thereof include but are not limited to NaMnFe2(PO4)6, Na2MnP2O7, Na3V2(PO4)3, Na2Fe2(SO4)3, NaFePO4, Na3V2(PO4)2F3, Na4Co3(PO4)2(P2O7).

[0121] In some embodiments, the conductive additive may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0122] In some embodiments, the positive electrode current collector includes a metal foil or a composite current collector. Specific examples of the metal foil include aluminum foil and aluminum alloy 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.). As some examples, specific examples of the composite current collector may include composite aluminum foil, composite aluminum alloy foil, etc.

[0123] Optionally, the positive electrode current collector includes at least one of aluminum foil, aluminum alloy foil, composite aluminum foil or composite aluminum alloy foil.

[0124] In some embodiments, the thickness of the positive electrode current collector is 10 μm to 60 μm, for example, 10 μm, 12 μm, 13 μm, 14 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, etc.

[0125] In some embodiments, the thickness of the positive electrode film layer is 50 μm to 200 μm, for example, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 95 μm, 120 μm, 130 μm, 150 μm, 172 μm, 180 μm, 190 μm, 200 μm, etc.

[0126] In some embodiments, the adhesion between the positive electrode film layer and the positive electrode current collector is not less than 40 N / m. The adhesion can be tested according to GB2792-1998, and the specific test method is shown in the examples below.

[0127] A second aspect of the present application provides a binder including a copolymer, wherein the copolymer includes a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3:

[0128] in, represents a connecting bond, R1, R3 and R4 are the same or different and are independently hydrogen or an alkyl group having 1 to 4 carbon atoms; R2 is an alkyl group having 1 to 8 carbon atoms, and p is any integer from 1 to 8.

[0129] In some embodiments, the binder is a positive electrode binder.

[0130] In the binder of the present application, the copolymer used can improve the flexibility of the positive electrode sheet and improve the bonding ability between the film layer and the current collector. The ester group (-COOR2) introduced by the first structural unit can improve the flexibility of the binder, the amide group (-CONH-) of the second structural unit provides the binder with high bonding and flexibility, and the isocyanate group (-NCO) has high reactivity, which can provide high bonding between the binder and the positive electrode collector and between the components in the positive electrode film layer. The cyano group (-CN) of the third structural unit can improve the bonding strength and the solubility of the binder in the positive electrode slurry. In addition, the binder can also maintain a low resistance of the electrode sheet. As a result, the positive electrode sheet using this binder has high toughness and bonding strength, as well as high conductivity.

[0131] The other features of the copolymer described in the second aspect of the present application and its preparation method are as described in the first aspect of the present application and will not be repeated here.

[0132] The third aspect of the present application provides a method for preparing the positive electrode sheet described in the first aspect of the present application, comprising: applying a positive electrode slurry containing the binder on the positive electrode current collector to form a positive electrode film layer.

[0133] In some specific embodiments, the method comprises:

[0134] mixing the positive electrode active material, the conductive additive, the binder and the optional flexibilizer with a second solvent to obtain a positive electrode slurry;

[0135] The positive electrode slurry is coated on the surface of the positive electrode current collector, dried, and cold pressed to form a positive electrode film layer.

[0136] Optionally, the mass content of the second solvent in the positive electrode slurry is 10% to 20%, for example, 10%, 15%, 20%, etc. Thus, the positive electrode slurry can have an appropriate viscosity, thereby improving coating uniformity.

[0137] As an example, the second solvent includes N-methylpyrrolidone (NMP).

[0138] In some embodiments, the viscosity of the positive electrode slurry at 25° C. may be 3000 mPa·s to 5000 mPa·s.

[0139] In some embodiments, the drying temperature may be 55°C to 90°C.

[0140] The fourth aspect of the present application provides a battery, comprising the positive electrode sheet described in the first aspect of the present application or the positive electrode sheet prepared by the preparation method described in the third aspect of the present application.

[0141] In the present application, the battery may be a secondary battery. It is understood that the secondary battery may include a lithium secondary battery or a sodium secondary battery.

[0142] In some embodiments, the secondary battery may further include a negative electrode, a separator, and an electrolyte. During the battery's charge and discharge processes, active ions are intercalated and released between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0143] [Negative electrode]

[0144] In some embodiments, the negative electrode sheet may include a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode material.

[0145] In some embodiments, the negative electrode current collector may include a metal foil or a composite current collector. The metal foil is, for example, copper foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one side of the polymer base layer. The material of the metal layer includes, but is not limited to, copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys, and the polymer material of the polymer base layer includes, but is not limited to, polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0146] In the present application, the negative electrode material may include negative electrode active materials for secondary batteries known in the art. For 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, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may include at least one of elemental tin, tin oxides, and tin alloys.

[0147] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may include, for example, 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), carboxymethyl cellulose (CMC), and carboxymethyl chitosan (CMCS).

[0148] In some embodiments, the negative electrode film layer may further optionally include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0149] In some embodiments, the negative electrode film layer may optionally contain other additives, such as a thickener. Specific examples of thickeners include, but are not limited to, sodium carboxymethyl cellulose (CMC-Na).

[0150] The present application does not particularly limit the preparation method of the negative electrode sheet, and the negative electrode sheet can be prepared by referring to existing methods. For example, the negative electrode components, such as the negative electrode material, conductive agent, and binder, are dispersed in a solvent (e.g., deionized water) to form a negative electrode slurry; the negative electrode slurry is then coated on a negative electrode current collector, and the negative electrode sheet is obtained by drying and cold pressing.

[0151] [Electrolytes]

[0152] 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.

[0153] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0154] In some embodiments, the electrolyte salt may include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0155] In some embodiments, the solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0156] In some embodiments, the electrolyte may optionally include additives. Such additives may include, for example, negative electrode film-forming additives, positive electrode film-forming additives, and additives capable of improving certain battery properties, such as additives that improve battery overcharge performance, or additives that improve battery high or low temperature performance.

[0157] [Isolation film]

[0158] The separator is placed between the positive and negative electrodes to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. This application does not impose any particular restrictions on the type of separator, and any porous separator known in the art may be used.

[0159] In some embodiments, the material of the separator can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. Furthermore, the separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0160] In some embodiments, a ceramic coating and / or a metal oxide coating is further provided on the isolation membrane.

[0161] 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.

[0162] In some embodiments, the battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0163] In some embodiments, the outer packaging may include a housing and a cover plate. The housing may include a bottom plate and side plates connected to the bottom plate, with the bottom plate and side plates enclosing a receiving cavity. The housing may have an opening communicating with the receiving cavity, and the cover plate may be positioned over the opening to seal the receiving cavity. The electrode assembly may be enclosed within the receiving cavity.

[0164] In some embodiments, the outer packaging of the battery is a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.

[0165] In other embodiments, the outer packaging of the battery is a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0166] The battery of the present application may include a battery cell form, a battery module form, and a battery pack form. The battery cell, battery module, and battery pack of the present application will be described below with reference to the accompanying drawings as appropriate.

[0167] The present application has no particular limitation on the shape of the battery, which can be cylindrical, square, or any other shape. For example, FIG2 shows a square-structured battery cell 5 as an example.

[0168] In some embodiments, referring to Figure 3, the outer packaging may include a shell 51 and a top cover assembly 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 top cover assembly 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 be formed into 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 battery cell can be one or more, and those skilled in the art can select according to specific actual needs.

[0169] In some embodiments, batteries may be assembled into a battery module. The number of batteries contained in the battery module may be one or more. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0170] Figure 4 shows an example battery module 5. Referring to Figure 4 , within the battery module 4, multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 5 may be secured together using fasteners.

[0171] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0172] 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.

[0173] Figures 5 and 6 illustrate an example battery pack 1. Referring to Figures 5 and 6 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0174] The fifth aspect of the present application provides an electrical device comprising the battery described in the fourth aspect of the present application.

[0175] Batteries, battery modules, and battery packs can be used as power sources or energy storage units for electrical devices. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks), electric trains, ships, satellites, and energy storage systems.

[0176] As an electrical device, a battery, battery module or battery pack can be selected according to its usage requirements.

[0177] Figure 7 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the battery, a battery pack or battery module can be used.

[0178] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be light and thin, and may use a battery as a power source.

[0179] 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.

[0180] Preparation Example 1

[0181] This preparation example is used to illustrate the preparation method of the acrylic ester copolymer with a core-shell structure of the present application.

[0182] Under nitrogen protection, solvent NMP, initiator BPO and first monomer butyl acrylate (BA) were added to the reaction bottle and stirred evenly, and the temperature was raised to 80° C. and stirred for reaction for 3 h to obtain a first mixed solution;

[0183] Slowly add the second monomer, acrylamide monomer (structure shown in Formula B-1, the same below) dropwise into the first mixed solution over half an hour, and continue stirring and reacting at 80°C for 3 hours to obtain a second mixed solution;

[0184] Slowly dropwise add the third monomer, acrylonitrile monomer (AN), into the second mixed solution over half an hour, then raise the temperature to 20°C for stirring and react, raising the temperature by 20°C every 45 minutes for a total of 3 hours to obtain a third mixed solution;

[0185] The third mixed liquid was separated to remove the solvent, and the separated product was vacuum dried to obtain a powdery acrylic copolymer, which was designated as FCB-1.

[0186] In this preparation example, the molar ratio of the first monomer BA, the second monomer acrylamide monomer, and the third monomer AN was 1:0.5:0.5, the mass ratio of the three monomers to NMP was 50:100, and the mass ratio of BPO to BA was 0.5:5. GPC analysis showed that the number average molecular weight of the copolymer was 70,000 and Tg = 8°C. Figure 1 is a transmission electron microscopy (TEM) image of the copolymer, showing that the copolymer has a core-shell structure.

[0187] Comparative Preparation Example 1

[0188] An acrylic acid ester copolymer was prepared according to the method of Preparation Example 1, except that the third monomer AN was not used. The second mixed liquid was directly separated to remove the solvent. The separated product was vacuum dried to obtain a powdered acrylic acid ester copolymer (a copolymer formed by BA and acrylamide monomers), which was recorded as FCB-D1.

[0189] Preparation Example 2

[0190] A core-shell acrylic copolymer was prepared according to the method of Preparation Example 1, except that the monomer amounts were adjusted so that the molar ratio of the first monomer BA, the second monomer acrylamide monomer, and the third monomer AN was 1:0.3:0.8. The acrylic copolymer obtained in this Preparation Example was designated FCB-2. GPC analysis revealed a number average molecular weight of 80,000 and a Tg of 13°C.

[0191] Preparation Example 3

[0192] A core-shell acrylic copolymer was prepared according to the method of Preparation Example 1, except that the monomer amounts were adjusted so that the molar ratio of the first monomer BA, the second monomer acrylamide monomer, and the third monomer AN was 1:0.2:0.9. The acrylic copolymer obtained in this Preparation Example was designated FCB-3. GPC analysis revealed a number average molecular weight of 60,000 and a Tg of 15°C.

[0193] The following Examples 1-17 are used to illustrate the positive electrode sheet and the preparation method thereof of the present application.

[0194] NBR was purchased from Hengshui Ruien Rubber & Plastic Technology Co., Ltd. with the brand name L-NBR01.

[0195] The fluorine-free linear adhesive is polymethyl methacrylate (PMMA), purchased from Hubei Kediya Technology Co., Ltd., with the brand name ADY-701B.

[0196] Example 1

[0197] By mass fraction, 98.8% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 0.8% binder FCB-1 and solvent NMP were stirred and dispersed uniformly to obtain a positive electrode slurry with a solid mass content of 85%;

[0198] The positive electrode slurry was coated on one surface of aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0199] Comparative Example 1

[0200] The positive electrode sheet was prepared according to the method of Example 1, except that the binder FCB-1 was replaced by a fluorine-free linear binder PMMA of equal mass.

[0201] Example 2

[0202] By mass fraction, 97.6% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 2.0% binder FCB-1 and solvent NMP were stirred and dispersed uniformly to obtain a positive electrode slurry with a solid mass content of 85%;

[0203] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0204] Comparative Example 2

[0205] The positive electrode sheet was prepared according to the method of Example 2, except that the binder FCB-1 was replaced by a fluorine-free linear binder PMMA.

[0206] Example 3

[0207] By mass fraction, 98.4% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 1.2% binder FCB-1 and solvent NMP were stirred and dispersed uniformly to obtain a positive electrode slurry with a solid mass content of 85%;

[0208] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0209] Example 4

[0210] By mass fraction, 98.6% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 0.8% binder FCB-1, and 0.2% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0211] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0212] Comparative Example 3

[0213] The positive electrode sheet was prepared according to the method of Example 4, except that the binder FCB-1 was replaced by a fluorine-free linear binder PMMA.

[0214] Example 5

[0215] By mass fraction, 97.4% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 2.0% binder FCB-1, and 0.2% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0216] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0217] Comparative Example 4

[0218] The positive electrode sheet was prepared according to the method of Example 5, except that the binder FCB-1 was replaced by a fluorine-free linear binder PMMA.

[0219] Example 6

[0220] By mass fraction, 98.3% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 0.8% binder FCB-1, and 0.5% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0221] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0222] Example 7

[0223] By mass fraction, 98.1% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 1.2% binder FCB-1, and 0.3% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0224] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0225] Example 8

[0226] By mass fraction, 98.8% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 0.5% binder FCB-1, and 0.3% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0227] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0228] Example 9

[0229] By mass fraction, 98.7% lithium iron phosphate (LFP), 0.4% conductive carbon black (SP), 0.8% binder FCB-1, and 0.1% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0230] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0231] Example 10

[0232] A positive electrode sheet was prepared according to the method of Example 4, except that the binder FCB-1 was replaced by FCB-2.

[0233] Example 11

[0234] A positive electrode sheet was prepared according to the method of Example 5, except that the binder FCB-1 was replaced by FCB-2.

[0235] Example 12

[0236] A positive electrode sheet was prepared according to the method of Example 4, except that the binder FCB-1 was replaced by FCB-3.

[0237] Example 13

[0238] A positive electrode sheet was prepared according to the method of Example 5, except that the binder FCB-1 was replaced by FCB-3.

[0239] Example 14

[0240] By mass fraction, 98.2% lithium iron phosphate (LFP), 0.8% conductive carbon black (SP), 0.8% binder FCB-1, and 0.2% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0241] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0242] Example 15

[0243] By mass fraction, 98.8% lithium iron phosphate (LFP), 0.2% conductive carbon black (SP), 0.8% binder FCB-1, and 0.2% softener NBR were stirred and dispersed uniformly with the solvent NMP to obtain a positive electrode slurry with a solid mass content of 85%;

[0244] The positive electrode slurry was coated on one surface of an aluminum foil (thickness of 14 μm), heated to 85°C and maintained for 10 minutes, and then cold pressed into a sheet to form a positive electrode sheet with a positive electrode film layer thickness of 95 μm.

[0245] Example 16

[0246] A positive electrode plate was prepared according to the method of Example 4, except that NBR was replaced with polyoxypropylene glycol (PPG, purchased from Suzhou Senfida Chemical Co., Ltd., brand 25322-69-4).

[0247] Example 17

[0248] A positive electrode sheet was prepared according to the method of Example 4, except that NBR was replaced by polystyrene (PS, purchased from Dongguan Xinhe Plastics Co., Ltd., brand HIPS GH-660).

[0249] Comparative Example 5

[0250] The positive electrode sheet was prepared according to the method of Example 1, except that the binder FCB-1 was replaced by the fluorine-containing binder PVDF.

[0251] Comparative Example 6

[0252] A positive electrode sheet was prepared according to the method of Example 1, except that the binder FCB-1 was replaced by FCB-D1.

[0253] In the above embodiments and comparative examples, the composition of the positive electrode film layer is shown in Table 1.

[0254] Test section

[0255] 1. Binder

[0256] 1) Molecular weight test

[0257] A Waters 2695 Isocratic HPLC gel chromatography instrument (2141 differential refractive index detector) was used. A 3.0% polystyrene solution was used as a reference, and a matching chromatographic column (oil-based: Styragel HT5 DMF 7.8*300mm + Styragel HT4) was selected. A 3.0% binder solution was prepared in purified NMP solvent and tested at 25°C.

[0258] 2) Morphology test

[0259] Transmission electron microscopy (TEM, model JEM-2100Plus) was used to observe the microstructure of the adhesive. Test conditions: The adhesive was completely dispersed with alcohol before TEM testing.

[0260] 3) Glass transition temperature (Tg) test

[0261] The Tg test using the dilatometer method involves loading an appropriate amount of binder into a dilatometer, then pumping water into the dilatometer under negative pressure by evacuating the chamber. The dilatometer is then heated in an oil bath at a specific heating rate, and the change in the height of the water column with temperature is recorded to produce a water column height-temperature curve. Due to the sudden change in the binder's volume before and after its glass transition temperature, the water column height-temperature curve has an inflection point, corresponding to the temperature at this inflection point, which is the glass transition temperature.

[0262] 2. Pole performance test

[0263] 1) Brittleness test

[0264] The prepared positive electrode sheet was cut into test specimens of 20 mm × 100 mm size for later use.

[0265] Bend the electrode in half and secure it, then use a 5kg roller to roll it. Record whether the sample breaks after two folds (if it does not break after one fold, fold the electrode in half in the opposite direction at the same position and roll it a second time). Take three samples from each group for testing and calculate the average value.

[0266] 2) Pole tensile test

[0267] The prepared positive electrode sheet was cut into test specimens of 20 mm × 100 mm size for later use.

[0268] A universal material testing machine (INSTRON 5969) was used to stretch the positive electrode sheet along the length direction until it broke, with a stretching rate of 50 mm / min.

[0269] The cross-sectional shrinkage is calculated according to the following formula:

[0270] Area reduction (%) = [(A0-A) / A0] × 100%;

[0271] A0 represents the cross-sectional area of ​​the sample before stretching, and A represents the cross-sectional area of ​​the necked portion after stretching and breaking.

[0272] Three samples were taken from each group for testing and the average value was taken.

[0273] 3) Pole piece adhesion

[0274] The test was conducted in accordance with the national standard GB2792-1998. A universal material testing machine (INSTRON 5969) was used to test the 180° peel strength of the electrode at a tensile rate of 50 mm / min.

[0275] The procedure is as follows: Take a sample 30mm wide and 100mm to 160mm long. Apply a special double-sided tape 20mm wide and 90mm to 150mm long to a steel plate. Place the positive electrode film layer of the previously cut electrode sample on the double-sided tape and then roll it three times in the same direction with a 2kg roller. Secure a 250mm long paper tape, the same width as the electrode, beneath the electrode current collector and secure it with corrugated tape. Turn on the equipment until the indicator light illuminates. Adjust the stop block to the appropriate position. Secure the end of the steel plate not attached to the electrode with the lower clamp. Fold the paper tape upward and secure it with the upper clamp. Use the up and down buttons on the manual controller included with the tensile testing machine to adjust the clamp position. Then perform the test and read the value. The force at equilibrium on the electrode divided by the width of the tape is used as the electrode adhesion force per unit length, which represents the bond strength between the positive electrode film layer and the current collector.

[0276] Three samples were taken from each group for testing and the average value was taken.

[0277] 4. Battery performance test

[0278] 1) Battery preparation

[0279] [Positive electrode]

[0280] The positive electrode sheets prepared in the embodiment and the comparative example were respectively used.

[0281] [Isolation film]

[0282] Polypropylene film is used as the isolation film.

[0283] [Electrolyte]

[0284] LiPF6 with a concentration of 1 mol / L was used as the electrolyte (the solvents were EC and EMC, with a volume ratio of 3:7).

[0285] [Negative electrode]

[0286] The negative electrode material hard carbon, conductive agent carbon black Super P, binder CMC and solvent water are mixed and stirred in a mass ratio of 8:1:1:10 to obtain a negative electrode slurry; the negative electrode slurry is then evenly coated on one surface of the negative electrode collector (copper foil); and then dried, cold pressed and cut to obtain a negative electrode sheet.

[0287] The positive electrode sheet, separator (porous polyethylene film), and negative electrode sheet obtained above are cut into round pieces and arranged in sequence, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role. The separator is soaked in the above electrolyte and compacted to obtain a button-type lithium-ion battery.

[0288] 2) DC resistance (DCR) test

[0289] The DCR test was performed using a DCR meter. Before the HPPC test, the cell was discharged at a 1C rate at 25°C. The cell was then fully charged using a constant current and constant voltage method and left for 1 hour to restore electrochemical and thermal equilibrium. The OCV value at 100% SOC was recorded. An HPPC test was performed at the recommended pulse rate to obtain the charge and discharge DCR at 100% SOC. The test results are shown in Table 2.

[0290] Table 1

[0291] In Table 1, "BA / acrylamide / AN" refers to the molar ratio of three monomers: BA, the acrylamide monomer represented by Formula B-1, and AN.

[0292] Table 2

[0293] Combining Table 1 and Table 2, it can be seen that compared with Comparative Examples 1-4 using conventional fluorine-free binders, the positive electrode film layer of Examples 1-17 introducing the acrylic copolymer as a binder can effectively improve the brittleness of the electrode, so that the electrode has a higher cross-sectional expansion rate and higher toughness. At the same time, it also makes the electrode have both higher peel strength and lower DC internal resistance (DCR < 440mΩ), which can meet the subsequent application requirements of the electrode.

[0294] Furthermore, a comparison of Examples 4-5 and 7 with Examples 1-3 shows that the introduction of an NBR softener into the positive electrode film further improves electrode brittleness and enhances electrode toughness. Correspondingly, a comparison of Comparative Examples 3-4 with Comparative Examples 1-2 shows that, when using a conventional fluorine-free linear binder, even the addition of an NBR softener into the positive electrode film has little effect on improving electrode brittleness, and the cross-sectional shrinkage is also poor. This indicates that the introduction of a softener into the positive electrode film is compatible with the acrylic copolymer employed in this application, enabling both to work together to improve brittleness and enhance toughness.

[0295] Comparing Examples 4-5 and 10-11 with Examples 12-13, it can be seen that when the molar ratio of the three structural units of BA / acrylamide / AN is controlled within the range of 1:(0.3-0.5):(0.6-0.8), the toughness of the positive electrode sheet can be further improved.

[0296] Comparing Example 4 with Examples 16-17, it can be seen that using NBR as a softener can further improve the overall performance of the positive electrode.

[0297] Comparing Example 1 with Comparative Examples 1 and 5, it can be seen that, compared with a conventional fluorine-free linear binder (Comparative Example 1), while the use of PVDF (Comparative Example 5) as a binder can also improve the electrode brittleness, PVDF results in excessively high DC resistance of the electrode, while the positive electrode of Example 1 has both high toughness and low resistance. This shows that the use of an acrylic copolymer as a binder in this application not only gives the positive electrode sheet containing it comparable toughness to that of a positive electrode sheet using a fluorine-containing binder, but also improves the conductivity of the electrode sheet.

[0298] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A positive electrode sheet, which includes a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector, the positive electrode film layer contains a binder, the binder includes a copolymer, and the copolymer includes a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3: Among them, Denotes a linking bond, R1, R3 and R4 are the same or different and each independently is hydrogen or an alkyl group having 1 to 4 carbon atoms; R2 is an alkyl group having 1 to 8 carbon atoms, and p is any integer from 1 to 8.

2. The positive electrode sheet according to claim 1, wherein, The copolymer is a random copolymer.

3. The positive electrode sheet according to claim 1 or 2, wherein The copolymer has a core-shell structure, and the core layer includes a polymerized chain segment formed by the first structural unit and the second structural unit, and the shell layer includes a polymerized chain segment formed by the third structural unit.

4. The positive electrode sheet according to any one of claims 1-3, wherein, The binder satisfies one or more of the following characteristics: (a) The number average molecular weight of the copolymer is 50,000 to 80,000; (b) In the copolymer, the molar ratio of the first structural unit, the second structural unit and the third structural unit is 1∶(0.2 - 0.5)∶(0.4 - 0.9); (c) The glass transition temperature of the copolymer is 5°C to 15°C.

5. The positive electrode sheet according to any one of claims 1-4, wherein, In the copolymer, the molar ratio of the first structural unit, the second structural unit and the third structural unit is 1∶(0.3 - 0.5)∶(0.5 - 0.8).

6. The positive electrode sheet according to any one of claims 1-5, wherein, The copolymer satisfies one or more of the following characteristics: (a) R1, R3 and R4 are all hydrogen, and R2 is an alkyl group having 2 to 6 carbon atoms; (b) p is any integer from 2 to 6.

7. The positive electrode sheet according to any one of claims 1-6, wherein, The structure of the first structural unit is shown in Formula 1-1, the structure of the second structural unit is shown in Formula 2-1, and the structure of the third structural unit is shown in Formula 3-1:

8. The positive electrode sheet according to any one of claims 1-7, wherein, The copolymer is prepared by a method including the following steps: In the presence of a first solvent and an initiator, a first monomer having a structure as shown in formula A is subjected to a first polymerization reaction to obtain a first mixed solution; The first mixed solution is mixed with a second monomer shown in formula B and subjected to a second polymerization reaction to obtain a second mixed solution; The second mixed solution is mixed with a third monomer shown in formula C and subjected to a third polymerization reaction to obtain a third mixed solution containing the copolymer; Separate and dry the third mixture to obtain the copolymer in powder form; 9. The positive electrode sheet according to claim 8, wherein The method for preparing the copolymer satisfies one or more of the following conditions: (a) The temperature of the first polymerization reaction is 70°C to 90°C, and the reaction time is 2 h to 5 h; (b) The temperature of the second polymerization reaction is 70°C to 90°C, and the reaction time is 2 h to 5 h; (c) The temperature of the third polymerization reaction is 80°C to 200°C, and the reaction time is 3 h to 5 h; (d) The initiator includes at least one of benzoyl peroxide and diisopropyl peroxide; (e) Based on the weight of the first monomer, the dosage of the initiator is 0.1 wt% to 15 wt%; (f) The first solvent includes N-methylpyrrolidone.

10. The positive electrode sheet according to any one of claims 1-9, wherein, Based on the mass of the positive electrode film layer, the mass content of the binder is 0.5% to 2%.

11. The positive electrode sheet according to any one of claims 1-10, wherein, The positive electrode film layer further contains a softening agent, and the softening agent includes at least one of nitrile rubber, polyethers and polystyrene.

12. The positive electrode sheet according to claim 11, wherein, The softening agent satisfies one or more of the following characteristics: (a) Based on the total weight of the positive electrode film layer, the mass content of the softening agent is 0.1% to 0.5%; (b) The softening agent includes nitrile rubber.

13. The positive electrode sheet according to any one of claims 1 to 12, wherein, The positive electrode film layer further includes a positive electrode active material and a conductive additive. Based on the total weight of the positive electrode film layer, the mass content of the positive electrode active material is 96% to 99%, and the mass content of the conductive additive is 0.2% to 1%.

14. The positive electrode sheet according to claim 13, wherein, The positive electrode active material includes at least one of lithium iron phosphate and lithium nickel cobalt manganese oxide.

15. The positive electrode sheet according to any one of claims 1-14, wherein, The adhesion between the positive electrode film layer and the positive electrode current collector is not less than 40 N / m.

16. An adhesive, wherein, Comprising a copolymer, and the copolymer comprises a first structural unit represented by Formula 1, a second structural unit represented by Formula 2, and a third structural unit represented by Formula 3: Among them, Represents a connecting bond, R1, R3 and R4 are the same or different, and each independently is hydrogen or an alkyl group with 1 to 4 carbon atoms; R2 is an alkyl group with 1 to 8 carbon atoms, and p is any integer from 1 to 8.

17. The binder according to claim 16, wherein, The copolymer is a random copolymer.

18. The binder according to claim 16 or 17, wherein, The copolymer has a core-shell structure, and the core layer includes a polymer chain segment formed by the first structural unit and the second structural unit, and the shell layer includes a polymer chain segment formed by the third structural unit.

19. The binder according to any one of claims 16 - 18, wherein, The copolymer satisfies one or more of the following characteristics: (a) The number average molecular weight of the copolymer is 50,000 to 80,000; (b) In the copolymer, the molar ratio of the first structural unit, the second structural unit and the third structural unit is 1:(0.2 - 0.5):(0.4 - 0.9); (c) The glass transition temperature of the copolymer is 5°C to 15°C.

20. The binder according to any one of claims 16-19, wherein, In the copolymer, the molar ratio of the first structural unit, the second structural unit and the third structural unit is 1:(0.3 - 0.5):(0.5 - 0.8).

21. The binder according to any one of claims 16 - 20, wherein, The copolymer satisfies one or more of the following characteristics: (a) R1, R3 and R4 are all hydrogen, and R2 is an alkyl group with 2 to 6 carbon atoms; (b) p is any integer from 2 to 6.

22. A method for preparing the positive electrode sheet according to any one of claims 1-15, wherein, Includes: Applying the positive electrode paste containing the binder on the positive electrode current collector to form a positive electrode film layer.

23. The method according to claim 22, wherein, Includes: Mixing the positive electrode active material, the conductive additive, the binder and an optional softening agent with a second solvent to obtain a positive electrode paste; Coating the positive electrode paste on the surface of the positive electrode current collector, followed by drying and cold pressing to form a positive electrode film layer.

24. The method according to claim 22 or 23, wherein In the positive electrode paste, the mass content of the second solvent is 10% - 20%.

25. A battery, wherein, Includes the positive electrode plate according to any one of claims 1 - 15 or the positive electrode plate prepared by the method according to any one of claims 22 - 24.

26. An electrical device, wherein, Includes the battery according to claim 25.

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