A binder, electrode sheet, battery

Abstract

The present application provides a kind of binder, electrode sheet, battery, which includes copolymer, the copolymer includes aromatic unsaturated hydrocarbon structural unit, aliphatic conjugated unsaturated hydrocarbon structural unit, unsaturated carboxylic ester structural unit, vinyl carboxylic acid structural unit, crosslinking structural unit;At normal temperature, the gel rate of the copolymer in toluene is greater than or equal to 93%, the binder has high adhesion, which is beneficial to reduce the rebound of pole piece, improve the energy density and cycle life of battery and other performances, solve the defects existing in prior art.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more particularly to an adhesive, electrode sheet, and battery. Background Technology

[0002] Batteries are currently common electrochemical energy storage devices, and electrode sheets are an important component of batteries. Electrode sheets typically include an electrode active material layer, and binders are one of the important components of the electrode active material layer. However, due to factors such as poor adhesion and high usage of binders (such as styrene-butadiene rubber (SBR)), batteries generally suffer from problems such as large electrode rebound, poor cycle life, and low energy density. Summary of the Invention

[0003] This application provides an adhesive, an electrode sheet, and a battery. The adhesive has high adhesion and can reduce electrode rebound, improve the energy density and cycle life of the battery, thus solving the defects of the prior art.

[0004] One aspect of the present invention provides an adhesive comprising a copolymer, said copolymer comprising aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, and crosslinking structural units; wherein, at room temperature, the copolymer has a gelation rate in toluene greater than or equal to 93%.

[0005] According to one embodiment of the present invention, the glass transition temperature of the adhesive is -73~0°C.

[0006] According to one embodiment of the present invention, the average particle size D of the copolymer 50 The range is 100-500nm.

[0007] According to one embodiment of the present invention, the copolymer further includes functional structural units, which include one or more of (meth)acrylonitrile structural units, vinylpyrrolidone structural units, butenyl nitrile structural units, and vinyl ether structural units.

[0008] According to one embodiment of the present invention, the mass ratio of the aromatic unsaturated hydrocarbon structural unit, the aliphatic conjugated unsaturated hydrocarbon structural unit, the unsaturated carboxylic acid ester structural unit, the vinyl carboxylic acid structural unit, the crosslinked structural unit, and the functional structural unit is (0.1-50):(0.1-100):(0.1-100):(0.1-20):(0.1-30):(10-20).

[0009] Another aspect of the present invention provides an electrode sheet comprising an electrode active material layer, the electrode active material layer comprising the above-mentioned binder.

[0010] According to one embodiment of the present invention, the binder accounts for 0.3-1.5% of the mass percentage of the electrode active material layer; and / or, the electrode sheet is a negative electrode sheet.

[0011] Another aspect of the present invention provides a battery comprising the above-described electrode sheet.

[0012] This invention uses a copolymer comprising aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, and crosslinking structural units as a binder. At room temperature, the copolymer has a gelation rate in toluene greater than or equal to 93%, which is beneficial to improving the adhesion of the binder, reducing electrode rebound, and improving the energy density and cycle life of the battery. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0014] According to the inventors' research, improving the adhesiveness of the binder can reduce the amount of binder used in the battery and increase the amount of electrode active material used in the battery, thereby improving the energy density and dynamic performance of the battery. In addition, the high adhesiveness of the binder is also beneficial to reduce electrode rebound and improve the cycle life and other performance of the battery.

[0015] Based on this, embodiments of the present invention provide an adhesive comprising a copolymer, wherein the copolymer comprises aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, and crosslinking structural units; at room temperature, the copolymer has a gelation rate in toluene greater than or equal to 93%; the copolymer comprises a first copolymer that is soluble in toluene at room temperature, the first copolymer having a weight-average molecular weight less than or equal to 500,000 g / mol, and the content of the first copolymer being less than 7%.

[0016] In the above system, the copolymer possesses excellent binder properties, which helps reduce the amount of binder used in the electrode sheet, thereby reducing the obstruction in the transport process of active ions (e.g., lithium ions in lithium-ion batteries), improving the battery's kinetic performance and energy density, and reducing electrode rebound. Furthermore, the copolymer exhibits a high gelation rate, which is beneficial for the long-term cycle stability of the battery, reducing the generation of side reactions and thus improving the battery's cycle performance. Specifically, aromatic unsaturated hydrocarbon structural units act as hard segment skeleton supports, which helps improve the battery's cycle stability; aliphatic conjugated unsaturated hydrocarbon structural units and unsaturated carboxylic acid ester structural units act as flexible thickeners, which helps improve the flexibility and adhesion of the binder; vinyl carboxylic acid structural units help improve the adhesion of the binder to the substrate (e.g., current collector); and the monomers are cross-linked through cross-linking structural units, which helps improve the binder's adhesion and gelation rate.

[0017] Therefore, the above-mentioned binder has good adhesion, which is beneficial to improving the peel strength between the electrode active material layer and the electrode current collector, reducing electrode rebound, and improving the energy density and cycle life of the battery. At the same time, the copolymer also contains a first copolymer that can be dissolved in toluene at room temperature. The first copolymer has a small molecular weight, which makes the binder have better flexibility and adhesion properties, further improving the processing performance of the binder when applied to the electrode sheet.

[0018] Specifically, at room temperature, the gelation rate of the copolymer in toluene can be greater than or equal to 93%, greater than or equal to 94%, greater than or equal to 95%, greater than or equal to 96%, greater than or equal to 97%, greater than or equal to 98%, greater than or equal to 99%, or any combination thereof, and the upper limit of the gelation rate can be 100% or 99.9%.

[0019] Specifically, the ambient temperature can be 25±5℃, such as 20℃, 21℃, 22℃, 23℃, 24℃, 25℃, 26℃, 27℃, 28℃, 29℃, 30℃, or any combination thereof.

[0020] In the embodiments of the present invention, the gelation rate of the copolymer in toluene can be measured by the following method:

[0021] The copolymer was made into a film. A certain amount of the film was taken as a test sample and weighed using an electronic balance to obtain the initial weight W1 of the sample. The test sample was then immersed in toluene and stored in a room temperature oven for 72 hours (3 days). After that, the test sample was taken out and the excess liquid on its surface was wiped off with filter paper and / or a lint-free cloth to obtain the treated test sample. The treated sample was weighed using an electronic balance to obtain the weight W2 of the sample. The gel rate (i.e., the gel rate of the copolymer in toluene at room temperature) was calculated as W2 / W1×100%. The above operation was repeated three times, and the average value was taken as the gel rate of the copolymer in toluene at room temperature.

[0022] In this application, the aromatic unsaturated hydrocarbon structural units are derived from aromatic unsaturated hydrocarbon monomers, the aliphatic conjugated unsaturated hydrocarbon structural units are derived from aliphatic conjugated unsaturated hydrocarbon monomers, the unsaturated carboxylic acid ester structural units are derived from unsaturated carboxylic acid ester monomers, the vinyl carboxylic acid structural units are derived from vinyl carboxylic acid monomers, and the crosslinking structural units are derived from crosslinking agents. The copolymer is copolymerized from monomer raw materials (i.e., copolymer raw materials) including aromatic unsaturated hydrocarbon monomers, aliphatic conjugated unsaturated hydrocarbon monomers, unsaturated carboxylic acid ester monomers, vinyl carboxylic acid monomers, and crosslinking agents. Specifically, the copolymerization reaction is carried out in the presence of a solvent. The resulting binder exists in the form of an emulsion. The emulsion can be dried to form the above-mentioned adhesive film. Then, the gelation rate of the copolymer in toluene at room temperature is measured according to the above process.

[0023] Specifically, the copolymer as a whole has a high gelation rate (gelation rate ≥ 93%). In this invention, the gelation rate of the copolymer in toluene is basically equivalent to the percentage of the copolymer that is insoluble in toluene at room temperature (hereinafter referred to as the second copolymer) in the total mass of the copolymer.

[0024] In this invention, during the preparation of the above-mentioned copolymer, due to the randomness of the polymerization reaction, a small amount of copolymer with a low degree of polymerization that is soluble in toluene at room temperature (hereinafter referred to as the first copolymer) exists. It is understood that, corresponding to a gelation rate ≥93% of the copolymer, the content of the first copolymer in the copolymer is less than 7%, for example, it can be 6%, 5%, 4%, 3%, 2%, 1%, 0.1%, or any combination thereof. That is, in this invention, it can be considered that when the gelation rate of the copolymer is 93%, the content of the first copolymer in the copolymer is 7%.

[0025] In some embodiments, the copolymer includes a first copolymer and a second copolymer. At room temperature, the first copolymer is soluble in toluene, while the second copolymer is insoluble in toluene. This is because the second copolymer is insoluble in toluene at room temperature, while the first copolymer is soluble in toluene at room temperature. In other words, the copolymer contains two types of copolymers. Generally, the larger the molecular weight of a copolymer, the less soluble it is. Therefore, the molecular weight of the second copolymer is greater than the weight-average molecular weight of the first copolymer. This results in the second copolymer exhibiting better mechanical strength and stability, thereby improving the mechanical strength and stability of the binder. This also helps to increase the gelation rate of the copolymer in toluene at room temperature. Copolymers with high gelation rates can exhibit longer cycle stability and other properties during battery cycling. Simultaneously, the copolymer also contains the first copolymer, which is soluble in toluene at room temperature. The smaller molecular weight of the first copolymer gives the binder better flexibility and adhesion, further improving the processing and performance of the binder when applied to electrode sheets. This further improves the peel strength between the electrode active material layer and the electrode current collector, reduces electrode rebound, and increases the battery's energy density and cycle life.

[0026] In this invention, the first copolymer is identified as follows: the adhesive copolymer is made into a film, a certain amount of the film is taken as a test sample, weighed with an electronic balance to obtain the initial weight W1 of the sample, the test sample is immersed in toluene, and then stored in a room temperature oven for 72 hours (3 days). After that, the test sample is taken out, filtered and the filter residue is washed, the filter residue is placed in a 100°C oven to dry for 24 hours, the weight of the filter residue is weighed as W2, and the filtrate is collected at the same time. The organic polymer copolymer in the filtrate is the first copolymer, and the weight of the first copolymer is W1-W2.

[0027] In some embodiments, the weight-average molecular weight of the first copolymer is less than or equal to 500,000 g / mol, for example, less than or equal to 500,000 g / mol, less than or equal to 400,000 g / mol, less than or equal to 300,000 g / mol, less than or equal to 200,000 g / mol, less than or equal to 100,000 g / mol, less than or equal to 50,000 g / mol, less than or equal to 40,000 g / mol, or any combination thereof.

[0028] In some embodiments, the glass transition temperature of the binder is -73 to 0°C, such as -70°C, -60°C, -50°C, -40°C, -30°C, -20°C, -10°C, 0°C, or any combination thereof. This is beneficial for balancing the flexibility and adhesion of the binder, reducing electrode rebound, and improving battery cycle life and other performance characteristics.

[0029] In some embodiments, the average particle size D of the copolymer 50The range is 100-500nm, such as 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm or any combination thereof, which is beneficial to further improve the adhesion of the binder, reduce electrode rebound, and improve the energy density and cycle life of the battery.

[0030] The average particle size D of the samples in the embodiments of the present invention 50 For Dv 50 This indicates that, in a volume-based particle size distribution, the particle size that reaches 50% of the total volumetric size from the smallest particle size side can be measured by conventional methods in the art, such as using a laser particle size analyzer. Exemplarily, the average particle size D of the copolymer in the embodiments of the present invention... 50 For Dv 50 This indicates that, in the volumetric particle size distribution, the copolymer particles in the binder reach a particle size that constitutes 50% of the total volumetric particle size, starting from the smallest particle size.

[0031] In some embodiments, aromatic unsaturated hydrocarbon monomers include aromatic vinyl monomers, which include one or more of styrene, α-methylstyrene, vinyltoluene, methylvinylnaphthalene, vinylanthracene, and methylvinylanthracene.

[0032] In some embodiments, the aliphatic conjugated unsaturated hydrocarbon monomer includes one or more of 1,3-butadiene, 1,3-pentadiene, 2-methyl-1,3-butadiene, 1,3-cyclopentadiene, and 1,3-hexadiene.

[0033] In some embodiments, the unsaturated carboxylic acid ester monomers include acrylate monomers and / or alkyl propionate monomers, and the alkyl acrylate monomers may specifically include methacrylate monomers.

[0034] In some specific embodiments, the unsaturated carboxylic acid ester monomer may specifically include (meth)acrylate monomers, which may include one or more of methyl (meth)acrylate, ethyl (meth)acrylate, isobutyl acrylate, butyl (meth)acrylate, 2-isooctyl (meth)acrylate, lauryl (meth)acrylate, octadecyl (meth)acrylate, dodecyl (meth)acrylate, caprolactone, and benzyl acrylate. Specifically, the aforementioned (meth)acrylate monomers include acrylate monomers and / or methacrylate monomers; for example, methyl (meth)acrylate includes methyl acrylate and / or methyl methacrylate.

[0035] In some embodiments, the acrylate monomers include one or more of methyl acrylate, ethyl acrylate, isobutyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, octadecyl acrylate, dodecyl acrylate, caprolactone acrylate, and benzyl acrylate.

[0036] In some embodiments, methacrylates include one or more of methyl methacrylate, ethyl methacrylate, isobutyl acrylate, butyl methacrylate, 2-isooctyl methacrylate, lauryl methacrylate, octadecyl methacrylate, and dodecyl methacrylate.

[0037] In some embodiments, vinyl carboxylic acid monomers (or vinyl unsaturated carboxylic acid monomers) include vinyl carboxylic acid monomers and / or anhydride compounds of vinyl carboxylic acid monomers.

[0038] Specifically, the vinyl carboxylic acid monomers include one or more of acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, sorbic acid, and cinnamic acid.

[0039] In some embodiments, the crosslinking agent includes a multifunctional crosslinking agent, which includes a multifunctional crosslinking monomer and / or a polymer of the multifunctional crosslinking monomer.

[0040] Specifically, the multifunctional crosslinking monomers include one or more of the following: trimethylolpropane triacrylate, 2(ethoxy)trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, methacryloxypropyltrimethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, allyltrimethoxysilane, methylallyltrimethoxysilane, and aminopropyltrimethoxysilane.

[0041] In some embodiments, the copolymer further includes functional structural units, which include one or more of (meth)acrylonitrile structural units, vinylpyrrolidone structural units, butenyl nitrile structural units, and vinyl ether structural units. These functional structural units are beneficial for improving the affinity between the binder and the electrode active material, further improving the adhesion of the binder, reducing electrode rebound, and improving the energy density and cycle performance of the battery.

[0042] Specifically, the functional structural units are derived from the functional monomers in the copolymer raw materials, including one or more of (meth)acrylonitrile monomers, vinylpyrrolidone monomers, butenyl nitrile monomers, and vinyl ether monomers.

[0043] In some embodiments, the mass ratio of aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, cross-linked structural units, and functional structural units is (0.1-50):(0.1-100):(0.1-100):(0.1-20):(0.1-30):(10-20), which is more conducive to improving the adhesion of the binder, further improving the peel strength between the electrode active material layer and the electrode current collector, reducing electrode rebound, and improving the energy density and cycle life of the battery.

[0044] Specifically, the mass ratio of aromatic unsaturated hydrocarbon monomers, aliphatic conjugated unsaturated hydrocarbon monomers, unsaturated carboxylic acid ester monomers, vinyl carboxylic acid monomers, crosslinking agents, and functional monomers in the copolymer raw materials is basically equal to the mass ratio of aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, crosslinking structural units, and functional structural units.

[0045] Specifically, the mass ratio of aromatic unsaturated hydrocarbon monomers to aliphatic conjugated unsaturated hydrocarbon monomers is (0.1:100) to (50:0.1), for example, (0.1:100), (1:90), (5:80), (10:70), (20:60), (20:50), (30:30), (40:20), (45:10), (50:0.1), or any combination thereof.

[0046] Specifically, the mass ratio of aromatic unsaturated hydrocarbon monomers to unsaturated carboxylic acid ester monomers is (0.1:100) to (50:0.1), for example, (0.1:100), (1:90), (5:80), (10:70), (20:60), (20:50), (30:30), (40:20), (45:10), (50:0.1) or any combination thereof.

[0047] Specifically, the mass ratio of aromatic unsaturated hydrocarbon monomers to vinyl carboxylic acid monomers is (0.1:20) to (50:0.1), for example (0.1:20), (1:15), (10:10), (20:5), (40:4), (50:0.1) or any combination thereof.

[0048] Specifically, the mass ratio of the crosslinking agent to the aromatic unsaturated hydrocarbon monomer is (0.1:50) to (30:0.1), for example, (0.1:50), (1:40), (10:30), (15:20), (20:10), (25:5), (30:0.1) or any combination thereof.

[0049] The present invention also provides a method for preparing the above-mentioned adhesive, comprising the following steps: mixing the raw materials of the copolymer and then performing a copolymerization reaction under the action of a catalyst to obtain the adhesive.

[0050] Specifically, the raw materials of the copolymer can be mixed in a solvent, including water. Specifically, the catalyst (or auxiliary agent) includes a persulfide, specifically sodium persulfide and / or potassium persulfide, and the mass ratio of the catalyst added to the total amount of the copolymer raw materials is (0.1-3):(20.5-320), for example, it can be a range of (0.1:320), (0.5:250), (1:150), (2:75), (3:20.5), or any combination thereof.

[0051] In practice, the raw materials of the copolymer are mixed and reacted at 50℃-80℃ (referred to as the first temperature) for 5-10 hours (referred to as the first time) under the action of a catalyst to obtain an intermediate product. Then, the temperature is raised to 80℃-92℃ (referred to as the second temperature) and reacted for 5-10 hours (referred to as the second time) to obtain the binder.

[0052] Specifically, the first temperature can be a range of 50℃, 60℃, 70℃, 80℃ or any two of them, the first time can be a range of 5h, 6h, 7h, 8h, 9h, 10h or any two of them, the second temperature can be a range of 80℃, 83℃, 85℃, 88℃, 90℃, 92℃ or any two of them, and the second time can be a range of 5h, 6h, 7h, 8h, 9h, 10h or any two of them.

[0053] It is understood that those skilled in the art can adjust the degree of polymerization of the prepared copolymer by adjusting the content of each component in the raw materials and the catalyst, as well as the reaction temperature and time, thereby adjusting the copolymer gel rate.

[0054] The present invention also provides an electrode sheet, including an electrode active material layer, wherein the electrode active material layer includes the above-mentioned binder or a binder prepared according to the above-mentioned binder preparation method, and has properties corresponding to the above-mentioned binder, which will not be described in detail here.

[0055] Specifically, the electrode sheet includes an electrode current collector and an electrode active material layer located on at least one side of the electrode current collector. The electrode active material layer includes the above-mentioned binder or a binder prepared according to the above-mentioned binder preparation method.

[0056] In some embodiments, the binder accounts for 0.3-1.5% of the mass of the electrode active material layer, for example, 0.3%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, or any combination thereof. Since the binder of this application has good bonding properties, in practical applications, only a small amount needs to be added to obtain a good bonding effect, which is beneficial to improving the energy density and other electrochemical performance of the battery.

[0057] In some embodiments, the electrode sheet is a negative electrode sheet. The negative electrode sheet has a large rebound during the charging and discharging process of the battery, which leads to the degradation of the battery's cycle life and other performance characteristics. Adding the above-mentioned binder to the negative electrode sheet can help reduce the rebound of the negative electrode sheet and improve the battery's cycle life and other performance characteristics.

[0058] Specifically, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector. The negative electrode active material layer includes the above-mentioned binder or a binder prepared according to the above-mentioned binder preparation method.

[0059] In practice, the negative electrode current collector includes copper foil, and the negative electrode active material layer also includes negative electrode active material, negative electrode conductive agent, and thickener.

[0060] In the above system, the binder, through its interaction with the negative electrode active material and the negative electrode current collector (or foil), helps to improve the peel strength (or peel force of the negative electrode sheet) between the negative electrode active material layer and the negative electrode current collector, as well as the cohesive force of the negative electrode sheet. This helps to reduce electrode rebound and improve the electrochemical performance of the battery, such as cycle performance.

[0061] In some embodiments, the average particle size D of the negative electrode active material 50 The particle size is 2-20 μm, for example, 2 μm, 4 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, or any combination thereof. According to the inventors' research, in the process of preparing the negative electrode sheet, it is necessary to disperse the negative electrode active material, binder, and other components in the negative electrode solvent to form a negative electrode slurry. Increasing the amount of binder will lead to an increase in the viscosity of the negative electrode slurry, which is not conducive to the dispersion of the negative electrode active material, especially the negative electrode active material with a small particle size. The binder in the embodiments of this invention has high adhesion, which can reduce the amount of binder used, thereby reducing the viscosity of the negative electrode slurry, which is beneficial to the dispersion of the negative electrode active material, improving the peel strength between the negative electrode active material layer and the negative electrode current collector, reducing electrode rebound, and improving the cycle life and other performance characteristics of the battery.

[0062] In some embodiments, the negative electrode active material includes graphite. Due to its surface tension and other properties, graphite has poor dispersibility in the negative electrode slurry. The binder in the embodiments of the present invention has high adhesion, which can reduce the amount of binder used, thereby reducing the viscosity of the negative electrode slurry, which is beneficial to the dispersion of graphite, improving the peel strength between the negative electrode active material layer and the negative electrode current collector, reducing electrode rebound, and improving the cycle life and other performance of the battery.

[0063] Specifically, the negative electrode conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNTs), acetylene black, graphene, Ketjen black, and carbon fiber.

[0064] In some embodiments, the thickener may comprise a water-soluble polymer, including one or more of carboxymethyl cellulose salts, polyacrylic acid, and modified forms of the aforementioned polymers. Carboxymethyl cellulose salts include lithium carboxymethyl cellulose and / or sodium carboxymethyl cellulose (CMC).

[0065] In this embodiment of the invention, the negative electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, components used to form the negative electrode active material layer, such as negative electrode active material, conductive agent, thickener, and binder, can be dispersed in a negative electrode solvent, such as water, to prepare a negative electrode slurry. This slurry is then coated onto the surface of the negative electrode current collector, and after drying, rolling, and other processes, the negative electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing negative electrode sheets using the coating method, and are not particularly limited thereto.

[0066] This invention also provides a battery comprising the above-described electrode sheet, having properties corresponding to the above-described electrode sheet, which will not be described in detail here.

[0067] In some embodiments, the battery described above may be a lithium-ion battery.

[0068] Generally, a battery includes a cell, an electrolyte, and a casing that encapsulates the cell. The electrolyte is injected into the cell inside the casing. The cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The cell can be a stacked cell, meaning it is composed of alternating layers of positive electrode, separator, and negative electrode.

[0069] The embodiments of the present invention can be used to prepare batteries by conventional methods in the art. For example, positive electrode, separator and negative electrode can be stacked alternately to obtain a stacked cell; then the cell is placed in a casing and subjected to conventional processes such as electrolyte injection, encapsulation, settling, formation and capacity testing to obtain the battery.

[0070] The positive electrode sheet in the embodiments of the present invention can be a conventional positive electrode sheet in the art. Generally, the positive electrode sheet includes a positive current collector and a positive active material layer located on at least one side surface of the positive current collector. Specifically, the positive active material layer can be provided on one side surface of the positive current collector, or positive active material layers can be provided on both sides of the positive current collector in the thickness direction.

[0071] Specifically, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a positive electrode binder, all of which can be conventional materials in the art. For example, the positive electrode active material may include one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials. The ternary cathode materials may include nickel-cobalt-manganese ternary materials and / or nickel-cobalt-aluminum ternary materials. The conductive agent may include one or more of conductive carbon black (Super. P), conductive graphite, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber. The positive electrode binder may include one or more of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, etc.

[0072] The embodiments of the present invention may employ conventional positive current collectors in the art, for example, positive current collectors may include aluminum foil.

[0073] In this embodiment of the invention, the positive electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the components used to form the positive electrode active material layer, such as the positive electrode conductive agent and the positive electrode binder, can be dispersed in a positive electrode solvent, such as N-methylpyrrolidone (NMP), to prepare a positive electrode slurry. This slurry is then coated onto the surface of the positive electrode current collector, and after drying, rolling, and other processes, the positive electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing positive electrode sheets using the coating method, and are not particularly limited thereto.

[0074] In this embodiment of the invention, the separator is used to separate the positive electrode and the negative electrode to prevent the positive electrode and the negative electrode from short-circuiting due to contact. The separator in this embodiment of the invention can be a conventional separator in the art, such as polypropylene membrane (PP membrane) or polyethylene (PE membrane), but is not limited thereto.

[0075] In the embodiments of the present invention, the electrolyte can be a non-aqueous electrolyte. The non-aqueous electrolyte generally includes lithium salt, carbonate solvent and additives. The carbonate solvent may include one or more of cyclic carbonate solvent and chain carbonate solvent, specifically including ethylene carbonate (EC), including one or more of dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). The lithium salt may include one or more of lithium hexafluorophosphate (LiPF6) and lithium perchlorate. The additives may include one or more of fluoroethylene carbonate, difluoroethylene carbonate, fluorocyclotriphosphazene, and hexafluorocyclotriphosphazene.

[0076] In this embodiment of the invention, the battery cell can be packaged using conventional housing materials in the art. The housing may include, for example, soft packaging materials such as aluminum-plastic film (in which case the battery is a soft-pack battery), but is not limited to this.

[0077] The present invention will be further described below through specific embodiments.

[0078] Example 1

[0079] 1. Preparation of adhesive

[0080] Two parts by mass of α-methylstyrene, 40 parts by mass of 1,3-butadiene, two parts by mass of itaconic acid, five parts by mass of lauryl methacrylate, and two parts by mass of pentaerythritol triacrylate were mixed in water. 0.5 parts by mass of sodium persulfate were added, and the mixture was heated to 60°C and stirred for 6 hours to obtain an intermediate product. Then, the mixture was heated to 80°C and aged for 10 hours to obtain the binder.

[0081] 2. Preparation of negative electrode sheet

[0082] Graphite (D) 50 Mix the binder prepared in step 1 with SP, CMC, and 12nm by mass ratio of 96:1:1:1.2, add deionized water, stir evenly, and prepare a negative electrode slurry.

[0083] The negative electrode slurry is coated on both the front and back surfaces of a copper foil. After drying and rolling, a negative electrode coating is formed on both the front and back surfaces of the copper foil, thus producing a negative electrode sheet.

[0084] 3. Preparation of positive electrode sheet

[0085] Lithium iron phosphate, SP, and PVDF were mixed in a mass ratio of 97.5:1:1.5, N-methylpyrrolidone was added, and the mixture was stirred evenly to prepare a positive electrode slurry. The positive electrode slurry was then coated on both sides of an aluminum foil, and the positive electrode sheet was obtained after drying and rolling.

[0086] 4. Battery assembly

[0087] The positive electrode, separator (PP film) and negative electrode are alternately stacked to assemble a stacked battery cell;

[0088] The stacked cells are placed in an aluminum-plastic film, with the electrolyte injection port remaining. Electrolyte (EC:EMC:DEC=5:2:3) is injected into the film through the injection port, and then the injection port is sealed to obtain the battery precursor. After aging, formation and aging treatments, and capacity testing, a soft-pack battery is obtained.

[0089] Example 2

[0090] The difference from Example 1 is that the mass fraction of α-methylstyrene is 50, the mass fraction of 1,3-butadiene is 100, the mass fraction of itaconic acid is 0.1, and the mass fraction of lauryl methacrylate is 0.1. The remaining steps and conditions are the same as in Example 1.

[0091] Example 3

[0092] The difference from Example 1 is that the mass fraction of pentaerythritol triacrylate is 1, while the remaining steps and conditions are the same as in Example 1.

[0093] Example 4

[0094] The difference from Example 1 is that it also contains 15 parts of a functional monomer, which is acrylonitrile.

[0095] Examples 5 to 12

[0096] The difference from Example 1 is that the types and mass fractions of the binder raw material monomers are different, as shown in Table 1. The remaining steps and conditions are the same as in Example 1.

[0097] Comparative Examples 1 to 5

[0098] The difference from Example 1 is that the types and mass fractions of the binder raw material monomers are different, as shown in Table 1. The remaining steps and conditions are the same as in Example 1.

[0099] The copolymers, binders, negative electrode sheets, and batteries in the examples and comparative examples were tested using the following procedures, and the results are shown in Tables 2 and 3:

[0100] (1) Gel rate test: The adhesive copolymer was made into a film, and a certain amount of the film was taken as a test sample. The initial weight of the sample was obtained by weighing it with an electronic balance. The test sample was immersed in toluene and then stored in a room temperature oven for 72 hours (3 days). The test sample was then taken out, filtered and the filter residue was washed, and the filtrate was collected at the same time. The filter residue was placed in a 100℃ oven for 24 hours to obtain the treated test sample. The weight of the treated sample was obtained by weighing it with an electronic balance. The gel rate (i.e., the gel rate of the copolymer in toluene at room temperature) = W2 / W1×100% was calculated. The above operation was repeated three times, and the average value was taken as the gel rate of the copolymer in toluene at room temperature.

[0101] (2) Particle size test: The particle size of polymer particles was tested using a laser particle size analyzer (Zhuhai Omec Instrument Co., Ltd., model: LS-909E).

[0102] (3) Weight-average molecular weight test of the first copolymer: Detection instrument: Waters e2695-2414; Chromatographic column: Shodex KD-806M; Instrument method: Organic NMP solution was used as the mobile phase, and the mobile phase was in single-pass mode. The UV detector was turned off during the test; Column temperature: 55℃; Injection volume: 50μL; Mobile phase: 0.03 mol / L NMP solution with added LiBr; Elution method: Isocratic elution at a flow rate of 0.7mL / min for 50min to obtain the elution curve of the first copolymer. The weight-average molecular weight of the first copolymer was calculated by the calibration curve and is shown in Table 2; The first copolymer is the organic polymer copolymer in the filtrate during the gelation test.

[0103] (4) Negative electrode rebound test: The initial thickness T0 is the thickness of the negative electrode after rolling. It is charged to 4.35V with constant current and constant voltage at 1C and the cutoff current is 0.05C. It is discharged to 2.75V with constant current at 1C. It is then charged to 4.35V with constant current and constant voltage at 1C and the cutoff current is 0.05C. At this time, the thickness T1 of the negative electrode in the fully charged state is measured. The rebound of the negative electrode when fully charged is calculated as T1 / T0×100%. The results are shown in Table 3.

[0104] (5) Battery energy density (mass energy density) test: The mass of the battery cell is accurately measured using an electronic balance and recorded as m (unit: kg). The battery cell is charged at a constant current of 0.5C to the specified charging cutoff voltage (4.35V) under constant temperature conditions (25°C), and then charging is stopped. Then it is discharged at 0.5C to the cutoff voltage (2.75V). The current I0 (A) and discharge time T0 (h) during the discharge process are recorded. Then the capacity of the battery cell C (unit: ampere-hour) = I0 × T0; the capacity of the battery cell E (watt-hour) = C × V a (V) a(where ρ is the average voltage during the discharge process), then the battery's mass energy density ρ m =E / m (watt-hours / kg), the results are shown in Table 3.

[0105] (6) Battery cycle performance (i.e. cycle life) test: Discharge the cell at 0.5C constant current to 2.75V, charge it at constant current and constant voltage to 4.35V, let it stand, and then discharge it at constant current to 2.75V. Record the discharge capacity of the first cycle as X1. After cycling in this way for 500 cycles, record the discharge capacity of the 500th cycle as X2. Then the cycle performance of the battery = X2 / X1×100%. The results are shown in Table 3.

[0106] (7) Glass transition temperature test of adhesive: Instrument used: Setalam-Setline, France. The adhesive was kept at 15℃ for 30 min, and then the heating program was set from 15℃ to 180℃, with a heating rate of 10℃ / min and an N2 flow rate of 40 mL / min. The heat flow change of the adhesive during the heating process was measured. When the specific heat capacity C appeared, the glass transition temperature was measured. p The temperature range of abrupt changes (baseline offset) is the glass transition temperature of the adhesive, and the results are shown in Table 3.

[0107] (8) Peel strength test of negative electrode sheet (i.e. peel force of negative electrode sheet); After the negative electrode sheet prepared above is compacted, the peel strength of the negative electrode sheet is measured by a tensile testing machine. The test method refers to the test method of peel strength of adhesive tape in GB / T 2792-2014. The peel strength of the negative electrode sheet is shown in Table 3.

[0108] Table 1 Raw materials for binder preparation

[0109]

[0110] Table 2 Properties of Copolymers and Binders

[0111]

[0112] Table 3 Performance of negative electrode and battery

[0113]

[0114] Compared to Comparative Examples 1-5, it can be found that the copolymer raw materials in Examples 1-12 include aromatic unsaturated hydrocarbon monomers, aliphatic conjugated unsaturated hydrocarbon monomers, unsaturated carboxylic acid ester monomers, vinyl carboxylic acid monomers, and crosslinking agents (i.e., the copolymers in the binder include aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, and crosslinking structural units). At room temperature, the copolymer has a gelation rate of greater than or equal to 93% in toluene. This binder has high adhesion, which is beneficial for reducing electrode rebound, improving battery energy density and cycle life, and solving the defects of the prior art.

[0115] Compared to other embodiments, the copolymer in Example 4 also includes functional structural units, which can help improve the affinity between the binder and the electrode active material, further improve the adhesion of the binder, reduce electrode rebound, and improve the energy density and cycle performance of the battery.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An adhesive, characterized in that, The copolymers include aromatic unsaturated hydrocarbon structural units, aliphatic conjugated unsaturated hydrocarbon structural units, unsaturated carboxylic acid ester structural units, vinyl carboxylic acid structural units, and crosslinking structural units. The mass ratio of the aromatic unsaturated hydrocarbon structural unit, the aliphatic conjugated unsaturated hydrocarbon structural unit, the unsaturated carboxylic acid ester structural unit, the vinyl carboxylic acid structural unit, and the crosslinked structural unit is (2-50):(10-100):(0.1-100):(0.1-20):(1-30). At room temperature, the copolymer has a gelation rate in toluene greater than or equal to 93%; The copolymer includes a first copolymer that is soluble in toluene at room temperature, the weight-average molecular weight of the first copolymer is less than or equal to 500,000 g / mol, and the content of the first copolymer is less than 7%. The glass transition temperature of the adhesive is -73~0℃; The binder is prepared by a process including the following steps: mixing monomer raw materials including aromatic unsaturated hydrocarbon monomers, aliphatic conjugated unsaturated hydrocarbon monomers, unsaturated carboxylic acid ester monomers, vinyl carboxylic acid monomers, and crosslinking agents in water, reacting at 50℃-80℃ for 5-10h under the action of a catalyst, and then heating to 80℃-92℃ for 5-10h to obtain the binder; The crosslinking agent is selected from one or more of the following: trimethylolpropane triacrylate, 2(ethoxy)trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, and dipentaerythritol hexaacrylate.

2. The adhesive according to claim 1, characterized in that, The average particle size D of the copolymer 50 The range is 100-500nm.

3. The adhesive according to claim 1 or 2, characterized in that, The copolymer further includes functional structural units, which include one or more of (meth)acrylonitrile structural units and butenyl nitrile structural units.

4. The adhesive according to claim 3, characterized in that, The mass ratio of the aromatic unsaturated hydrocarbon structural unit, the aliphatic conjugated unsaturated hydrocarbon structural unit, the unsaturated carboxylic acid ester structural unit, the vinyl carboxylic acid structural unit, the crosslinked structural unit, and the functional structural unit is (2-50):(10-100):(0.1-100):(0.1-20):(1-30):(10-20).

5. An electrode sheet, characterized in that, It includes an electrode active material layer, wherein the electrode active material layer includes the binder according to any one of claims 1-4.

6. The electrode sheet according to claim 5, characterized in that, The binder accounts for 0.3-1.5% of the mass percentage of the electrode active material layer; And / or, the electrode sheet is a negative electrode sheet.

7. A battery, characterized in that, Includes the electrode sheet as described in claim 5 or 6.

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