A positive electrode sheet, a secondary battery including the same, and an electronic device

Abstract

The application provides a positive electrode sheet, a secondary battery comprising the positive electrode sheet and an electronic device, the positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a positive electrode active material and a positive electrode binder, the positive electrode binder being obtained by copolymerization of acrylonitrile, acrylic acid, end monovinyl polydimethylsiloxane, 2-acrylamide-2-methylpropane sulfonic acid and a small-molecule ether monomer; the small-molecule ether monomer comprises at least one of ethylene glycol vinyl ether and diethylene glycol monovinyl ether. Through the above arrangement, the secondary battery has high energy density and good cycle performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a positive electrode, a secondary battery comprising the positive electrode, and an electronic device. Background Technology

[0002] Secondary batteries (lithium-ion batteries) are widely used in smartphones, wearable devices, consumer drones, and electric vehicles due to their advantages such as high energy density, long cycle life, and no memory effect. With the widespread application of lithium-ion batteries in these fields, the market demands increasingly higher energy density and cycle performance from them.

[0003] Currently, the cathode binders available on the market include polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polyimide (PI). However, these binders have problems such as weak adhesion and poor flexibility, which affect the energy density and cycle performance of lithium-ion batteries. Summary of the Invention

[0004] The purpose of this application is to provide a positive electrode, a secondary battery comprising the positive electrode, and an electronic device to improve the cycle performance and energy density of the secondary battery. The specific technical solution is as follows:

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] The first aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material and a positive electrode binder. The positive electrode binder is obtained by copolymerization of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamide-2-methylpropanesulfonic acid, and small molecule ether monomers. The small molecule ether monomers include at least one of ethylene glycol vinyl ether and diethylene glycol monovinyl ether. In this application, the terminal monovinyl polydimethylsiloxane and small molecule ether monomers in the positive electrode binder can reduce the regularity of the -CN and -COOH arrangement, improve the flexibility of the polymer chain in the positive electrode binder, and lower the glass transition temperature. After cold pressing, this is beneficial for obtaining a positive electrode sheet with a high initial compaction density, thereby improving the energy density of the secondary battery. The strongly hygroscopic monomer 2-acrylamido-2-methylpropanesulfonic acid in the positive electrode binder can absorb moisture from the air, making the positive electrode sheet soft and reducing damage to the positive current collector during cold pressing. The strongly polar cyano groups in the side chain of the positive electrode binder are beneficial for lithium-ion transport and improve the adhesion between the positive electrode material layer and the positive current collector. The hydroxyl and carboxyl groups on the molecular chain of the positive electrode binder can form intermolecular hydrogen bonds, increasing the cohesive force of the positive electrode material layer and further improving the energy density of the secondary battery. In summary, the secondary battery using this positive electrode binder has good cycle performance and high energy density.

[0007] In one embodiment of this application, the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers is (3 to 4.7):2:(0.5 to 2.5):(0.8 to 1.2):(1 to 2). Controlling the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers within the above range is beneficial for improving the adhesion between the positive electrode material layer and the positive electrode current collector. This results in higher cohesive forces between the positive electrode material layers, thereby improving the energy density and cycle performance of the secondary battery.

[0008] In one embodiment of this application, the glass transition temperature of the positive electrode binder is -30°C to 30°C, preferably -20°C to 0°C. When the glass transition temperature of the positive electrode binder is within the above range, the positive electrode binder has good bonding properties, which is beneficial to improving the processing performance and initial compaction density of the positive electrode sheet, thereby increasing the energy density of the secondary battery.

[0009] In one embodiment of this application, the weight-average molecular weight of the positive electrode binder is between 850,000 and 1,000,000. When the weight-average molecular weight of the positive electrode binder is within the above range, the positive electrode binder has good bonding properties, and the positive electrode sheet has good structural stability, thereby improving the cycle performance of the secondary battery.

[0010] In one embodiment of this application, the mass percentage of the positive electrode binder is 1% to 3% based on the mass of the positive electrode material layer. When the mass percentage of the positive electrode binder is within the above range, the positive electrode sheet has good bonding performance, which improves the cycle performance of the secondary battery.

[0011] In one embodiment of this application, the adhesion force between the positive electrode material layer and the positive electrode current collector is 40 N / m to 50 N / m. When the adhesion force between the positive electrode material layer and the positive electrode current collector is within the above range, the positive electrode sheet has good structural stability, and the secondary battery has good cycle performance.

[0012] In one embodiment of this application, the cohesive force of the positive electrode material layer is 80 N / m to 90 N / m. When the cohesive force of the positive electrode material layer is within the above range, the positive electrode sheet has good structural stability, and the secondary battery has high energy density.

[0013] A second aspect of this application provides a secondary battery comprising a negative electrode, a separator, an electrolyte, and a positive electrode as described in any of the foregoing embodiments. The secondary battery of this application exhibits high energy density and good cycle performance.

[0014] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery provided by this application has high energy density and good cycle performance; therefore, the electronic device of this application has a long service life.

[0015] The beneficial effects of this application are:

[0016] This application provides a positive electrode sheet, a secondary battery and an electronic device including the positive electrode sheet. The positive electrode sheet includes a positive current collector and a positive active material layer located on at least one surface of the positive current collector. The positive active material layer includes a positive active material and a positive binder. The positive binder is obtained by copolymerization of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamide-2-methylpropanesulfonic acid and small molecule ether monomers. The small molecule ether monomers include at least one of ethylene glycol vinyl ether and diethylene glycol monovinyl ether. In this application, the terminal monovinyl polydimethylsiloxane and small molecule ether monomers in the positive electrode binder can reduce the regularity of the -CN and -COOH arrangement, improve the flexibility of the polymer chain in the positive electrode binder, and lower the glass transition temperature. After cold pressing, this is beneficial for obtaining a positive electrode sheet with a high initial compaction density, thereby improving the energy density of the secondary battery. The strongly hygroscopic monomer 2-acrylamido-2-methylpropanesulfonic acid in the positive electrode binder can absorb moisture from the air, making the positive electrode sheet soft and reducing damage to the positive current collector during cold pressing. The strongly polar cyano groups in the side chain of the positive electrode binder are beneficial for lithium-ion transport and improve the adhesion between the positive electrode material layer and the positive current collector. The hydroxyl and carboxyl groups on the molecular chain of the positive electrode binder can form intermolecular hydrogen bonds, increasing the cohesive force of the positive electrode material layer and further improving the energy density of the secondary battery. In summary, the secondary battery using this positive electrode binder has good cycle performance and high energy density.

[0017] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0018] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0019] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0020] In existing lithium-ion batteries, polyvinylidene fluoride (PVDF), which is relatively expensive, is generally used as the positive electrode binder. However, the bonding between PVDF and the positive electrode active material, which occurs through van der Waals forces and intermolecular forces, is weak. During charge and discharge, the positive electrode sheet is prone to detachment, affecting the performance of the lithium-ion battery. Furthermore, the hydrogen fluoride produced by the high-temperature decomposition of PVDF corrodes the positive electrode current collector, impacting the battery's lifespan. During the preparation of the positive electrode slurry, free amines in the solvent N-methylpyrrolidone (NMP) attack the PVDF molecular chain, undergoing a defluorination elimination reaction. The resulting double bonds form a three-dimensional network structure, leading to gelation of the positive electrode slurry. Simultaneously, PVDF has low mechanical ductility and weak affinity for the positive electrode active material, thus failing to effectively prevent the pulverization and expansion of the positive electrode active material after long-term cycling. To address the aforementioned issues, two series of adhesives have been developed on the market: polyacrylonitrile (PAN) adhesives and polyimide (PI) adhesives. PAN's highly polar cyano groups can form strong hydrogen bonds and dipole-dipole interactions with the hydroxyl groups on the surface of the positive electrode active material. This allows PAN to be more uniformly distributed on the surface of the positive electrode active material, thereby improving the bonding strength between the positive electrode material layer and the current collector. Furthermore, PAN is a highly polar polymer with minimal swelling in the electrolyte. However, PAN homopolymer adhesives have a high glass transition temperature (Tg) (approximately 104℃), resulting in insufficient flexibility and brittleness at room temperature, leading to a low positive electrode compaction density (PD). While copolymerizing acrylate monomers with acrylonitrile improves flexibility to some extent, problems remain due to the similar polarity and solubility parameters of acrylate monomers and carbonate solvents in the electrolyte. This results in significant swelling of the PAN copolymer in the electrolyte and a decrease in PD after the positive electrode rebounds. Based on this, this application provides a positive electrode binder that can improve the bonding force between the positive electrode material layer and the positive electrode current collector in the positive electrode sheet, as well as the cohesive force of the positive electrode material layer, and has a low glass transition temperature, thereby improving the cycle performance and energy density of lithium-ion batteries.

[0021] The first aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material and a positive electrode binder. The positive electrode binder is obtained by copolymerization of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers. The small molecule ether monomers include at least one of ethylene glycol vinyl ether and diethylene glycol monovinyl ether. In this application, on one hand, the terminal monovinyl polydimethylsiloxane and small molecule ether monomers in the positive electrode binder can reduce the regularity of the -CN and -COOH arrangement, improve the flexibility of the polymer chain in the positive electrode binder, lower the glass transition temperature, solve the problem of hardness and brittleness of the positive electrode binder during use, and facilitate obtaining a positive electrode sheet with high initial compaction density after cold pressing, thereby improving the energy density of the secondary battery. On the other hand, the highly hygroscopic monomer 2-acrylamide-2-methylpropanesulfonic acid in the positive electrode binder of this application can absorb moisture from the air, reducing the brittleness of the positive electrode sheet and making the prepared positive electrode sheet more flexible. This is beneficial to improving the mechanical properties of the positive electrode sheet, and it also facilitates the slippage of positive electrode active material particles during cold pressing, reducing damage to the positive electrode current collector and increasing the initial compaction density of the positive electrode sheet. Furthermore, the highly polar cyano groups in the side chains of the positive electrode binder are beneficial to lithium-ion transport, giving the positive electrode binder excellent oxidation resistance (>4.7V) in high-voltage systems. Moreover, the highly polar cyano and carboxyl groups can form strong hydrogen bonds and dipole-dipole interactions with the hydroxyl groups on the surface of the positive electrode active material, allowing the positive electrode binder to be more uniformly distributed on the surface of the positive electrode active material and improving the adhesion between the positive electrode material layer and the positive electrode current collector. On the other hand, the hydroxyl and carboxyl groups on the positive electrode binder molecular chain can form intermolecular hydrogen bonds, increasing the cohesive strength between polymer chains, enhancing the cohesive force of the positive electrode material layer, reducing positive electrode rebound, and further improving the energy density of the secondary battery. In summary, the various groups in the positive electrode binder work synergistically, resulting in a positive electrode binder with a low glass transition temperature, good flexibility, and good oxidation resistance. This improves the initial compaction density of the positive electrode sheet, giving it excellent adhesion and mechanical properties. Secondary batteries using this positive electrode binder exhibit good cycle performance and high energy density. Furthermore, the carboxyl groups in acrylic acid and the sulfonic acid groups in 2-acrylamido-2-methylpropanesulfonic acid possess anti-gel properties, maintaining the processing performance of the positive electrode slurry and improving the processing performance of the positive electrode sheet.

[0022] In one embodiment of this application, the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers is (3 to 4.7):2:(0.5 to 2.5):(0.8 to 1.2):(1 to 2). For example, the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers is 3:2:2.5:1:1.5, 3:2:1.8:1.2:2, 4:2:1.5:1:1.5, 4:2:1.5:8:1.7, 4:2:1.5:1.2:1.3, 4.7:2:1.5:0.8:1, or a range consisting of any two of these values. When the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers is controlled within the above range, it is beneficial to reduce the glass transition temperature of the positive electrode binder, have higher adhesion between the positive electrode material layer and the positive electrode current collector, and have higher cohesion between the positive electrode material layers. This increases the initial compaction density of the positive electrode sheet, improves the energy density of the secondary battery, and enhances the structural stability of the positive electrode sheet during cycling, thereby improving the cycle performance of the secondary battery.

[0023] In one embodiment of this application, the structure of the positive electrode binder is shown in formula (I):

[0024]

[0025] Where x is selected from 4000 to 10000, y is selected from 2000 to 4000, z is selected from 20 to 300, p is selected from 200 to 800, q is selected from 800 to 3000, n is 10 to 30, and n is a positive integer.

[0026] In another embodiment of this application, the structure of the positive electrode binder is shown in formula (II):

[0027]

[0028] Where x is selected from 4000 to 10000, y is selected from 2000 to 4000, z is selected from 20 to 300, p is selected from 200 to 800, q is selected from 500 to 2000, n is 10 to 30, and n is a positive integer.

[0029] In one embodiment of this application, the glass transition temperature of the positive electrode binder is -30°C to 30°C, preferably -20°C to 0°C. For example, the glass transition temperature of the positive electrode binder is -30°C, -20°C, -10°C, 0°C, 10°C, 20°C, 30°C, or a range consisting of any two of these values. When the glass transition temperature of the positive electrode binder is within the above range, the glass transition temperature of the positive electrode binder is low, and it has good bonding performance. At the same time, the positive electrode sheet prepared using the positive electrode binder of this application has good flexibility, which is beneficial to improving the processing performance and initial compaction density of the positive electrode sheet, thereby improving the energy density of the secondary battery.

[0030] In one embodiment of this application, the weight-average molecular weight of the positive electrode binder is between 850,000 and 1,000,000. For example, the weight-average molecular weight of the positive electrode binder is 850,000, 880,000, 900,000, 920,000, 940,000, 960,000, 980,000, 1,000,000, or a range consisting of any two of these values. When the weight-average molecular weight of the positive electrode binder is within the above range, the positive electrode binder has good bonding properties, and the positive electrode sheet has good structural stability, thereby improving the cycle performance of the secondary battery.

[0031] In one embodiment of this application, the mass percentage of the positive electrode binder is 1% to 3% based on the mass of the positive electrode material layer. For example, the mass percentage of the positive electrode binder is 1%, 1.5%, 2%, 2.5%, 3%, or a range consisting of any two of these values. When the mass percentage of the positive electrode binder is within the above range, the initial compaction density of the positive electrode sheet is high, exhibiting good bonding and mechanical properties, thus improving the cycle performance of the secondary battery.

[0032] In one embodiment of this application, the adhesion force between the positive electrode material layer and the positive electrode current collector is 40 N / m to 50 N / m. For example, the adhesion force between the positive electrode material layer and the positive electrode current collector is 40 N / m, 42 N / m, 44 N / m, 46 N / m, 48 N / m, 50 N / m, or a range consisting of any two of these values. When the adhesion force between the positive electrode material layer and the positive electrode current collector is within the above range, it indicates that the positive electrode sheet has good adhesion performance, thereby the positive electrode sheet has good structural stability, and thus improves the cycle performance of the secondary battery.

[0033] In one embodiment of this application, the cohesive force of the positive electrode material layer is between 80 N / m and 90 N / m. For example, the cohesive force of the positive electrode material layer is 80 N / m, 82 N / m, 84 N / m, 86 N / m, 88 N / m, 90 N / m, or a range consisting of any two of these values. When the cohesive force of the positive electrode material layer is within the above range, it indicates that the positive electrode sheet has good bonding properties, good structural stability, and the secondary battery has a high energy density.

[0034] This application does not impose any particular limitation on the preparation method of the positive electrode binder, as long as it achieves the purpose of this application. For example, the preparation method of the positive electrode binder may include, but is not limited to, the following steps: mixing polymer monomers acrylonitrile, acrylic acid, terminal vinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and ethylene glycol vinyl ether in a certain mass ratio, dispersing in deionized water, heating and stirring in an inert gas atmosphere, adding an initiator after uniform mixing, reacting to obtain positive electrode binder precipitate particles, filtering and drying the positive electrode binder particles to obtain the positive electrode binder. This application does not impose any particular limitation on the inert gas, as long as it achieves the purpose of this application; for example, inert gases include, but are not limited to, nitrogen. This application does not impose any particular limitation on the heating time, as long as it achieves the purpose of this application. For example, the heating time may be 10 to 14 hours. This application does not impose any particular limitation on the type of initiator, as long as it achieves the purpose of this application; for example, initiators include, but are not limited to, potassium persulfate, sodium persulfate, or ammonium persulfate. This application does not impose any particular limitation on the content of the initiator, as long as the purpose of this application can be achieved. For example, based on the mass percentage of the following five polymer monomers—acrylonitrile, acrylic acid, terminal vinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and ethylene glycol—the initiator content can be 0.5% to 2%. This application also does not impose any particular limitation on the reaction time and temperature, as long as the purpose of this application can be achieved. For example, the reaction time can be 10 to 14 hours, and the reaction temperature can be 60°C to 80°C.

[0035] In this application, "a positive electrode material layer located on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along its own thickness direction, or it can be located on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that "surface" here can be the entire surface area of ​​the positive electrode current collector, or it can be a partial surface area of ​​the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application is achieved. This application also has no particular limitation on the positive electrode current collector, as long as the purpose of this application is achieved. For example, the positive electrode current collector can include aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors).

[0036] This application does not impose any particular limitation on the type of positive electrode active material in the positive electrode material layer, as long as it achieves the purpose of this application. For example, the positive electrode active material may include at least one of lithium nickel cobalt manganese oxide (NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials (γLi2MnO3·(1-γ)LiMO2, 0<γ<1, M is a transition element cobalt, nickel or iron), lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate or lithium titanate. This application does not impose any particular limitation on the content of the positive electrode active material, as long as it achieves the purpose of this application. For example, based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode active material is 95% to 97%. In this application, there is no particular limitation on the thickness of the positive electrode current collector and the positive electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the single-sided positive electrode material layer ranges from 30 μm to 120 μm.

[0037] In this application, the positive electrode material layer may include a conductive agent in addition to the positive electrode binder. This application does not impose any particular limitation on the type of conductive agent in the positive electrode material layer, as long as it achieves the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metallic materials, or conductive polymers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the content of the conductive agent, as long as it achieves the purpose of this application. For example, based on the mass of the positive electrode material layer, the mass percentage content of the conductive agent is 1.5% to 2.5%.

[0038] The present application does not particularly limit the preparation method of the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: (1) mixing a positive electrode active material, a positive electrode binder, and a conductive agent in a certain mass ratio, adding a solvent and stirring evenly to obtain a positive electrode slurry; (2) coating the positive electrode slurry on one surface of the positive electrode current collector, and after drying, forming a positive electrode material layer on one surface of the positive electrode current collector; (4) coating the positive electrode slurry on the other surface of the positive electrode current collector, and after drying, forming a positive electrode material layer on each of the two surfaces of the positive electrode current collector; (5) obtaining the positive electrode sheet through cold pressing and slicing. The present application does not particularly limit the solvent in the positive electrode slurry, as long as the purpose of the present application can be achieved. For example, it can be N-methylpyrrolidone.

[0039] The second aspect of the present application provides a secondary battery, which includes a negative electrode sheet, a separator, an electrolyte, and the positive electrode sheet described in any of the foregoing embodiments. The positive electrode sheet in the present application has a low glass transition temperature and a high initial compaction density, has a high adhesion force between the positive electrode material layer and the positive electrode current collector, and has a high cohesive force between the positive electrode material layers. Therefore, the secondary battery of the present application has a high energy density and good cycling performance.

[0040] The present application does not particularly limit the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer provided on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be provided on one surface of the negative electrode current collector along its own thickness direction, or can be provided on both surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be a partial area of the surface of the negative electrode current collector. The present application does not particularly limit it, as long as the purpose of the present application can be achieved. The present application does not particularly limit the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.). The negative electrode material layer of the present application includes a negative electrode active material. The present application does not particularly limit the type of the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material can include natural graphite, artificial graphite, mesophase microcarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12The negative electrode material layer contains at least one of the following: Li-Al alloy or metallic lithium. In this application, there are no particular limitations on the thickness of the negative electrode current collector or the negative electrode material layer, as long as the purpose of this application is achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm, and the thickness of the negative electrode material layer is 30 μm to 130 μm. Optionally, the negative electrode material layer may further include a conductive agent and a negative electrode binder. This application does not particularly limit the type of conductive agent in the negative electrode material layer, as long as the purpose of this application is achieved. For example, the conductive agent may be the same type as the conductive agent in the aforementioned positive electrode material layer. This application does not particularly limit the type of negative electrode binder in the negative electrode material layer, as long as the purpose of this application is achieved. For example, the negative electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. This application does not impose any particular restrictions on the mass ratio of negative electrode active material, conductive agent and negative electrode binder in the negative electrode material layer, as long as the purpose of this application can be achieved.

[0041] This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. The diaphragm of this application may have a porous structure, and this application does not impose any particular limitation on the size of the pores in the porous structure of the diaphragm, as long as it achieves the purpose of this application. For example, the pore size may be from 0.01 μm to 1 μm. This application does not impose any particular limitation on the thickness of the diaphragm, as long as it achieves the purpose of this application. For example, the thickness of the diaphragm may be from 5 μm to 10 μm.

[0042] The electrolyte in the secondary battery of this application includes lithium salts and non-aqueous solvents. The lithium salt may include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluoroborate. This application does not limit the content of lithium salts in the electrolyte, as long as the purpose of this application is achieved. This application does not particularly limit the non-aqueous solvent, as long as the purpose of this application is achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate, or vinylene carbonate. Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1,2-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0043] The secondary battery of this application also includes a packaging bag for containing the positive electrode, negative electrode, separator, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0044] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of this application, the secondary battery may include, but is not limited to, a lithium-ion secondary battery, a lithium polymer secondary battery, or a lithium-ion polymer secondary battery.

[0045] This application does not impose any particular limitation on the shape of the secondary battery, as long as it can achieve the purpose of this application. For example, the shape of the secondary battery may include, but is not limited to: square, cylindrical, irregular shape (such as L-shaped, H-shaped, etc.).

[0046] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery.

[0047] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery provided by this application has high energy density and good cycle performance; therefore, the electronic device of this application has a long service life.

[0048] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0049] Example

[0050] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0051] Test methods and equipment:

[0052] Adhesion test between the positive electrode material layer and the positive electrode current collector:

[0053] Take the positive electrode sheet obtained according to the <Preparation of Positive Electrode Sheet> steps, and punch it using a mold to obtain a test strip with a length of 100 mm and a width of 20 mm. Clean the surface of the steel plate with alcohol, and attach a 70 mm long and 20 mm wide double-sided adhesive tape (NITTO, NO5000NS) to the steel plate, ensuring no air bubbles are generated during the application process. Place the test strip centered on the double-sided adhesive tape, with the test side facing down. Use crepe tape (high-tack masking tape) to connect and fix a 75 mm long and 20 mm wide paper strip to one end of the test strip. Manually push a 2 kg rubber roller back and forth on the test strip 4 times to obtain the test sample. Test the sample using a tensile testing machine (Instron 3365). The test sample is fixed on the test stage, then the paper tape is folded upwards at 90° and secured with a clamp. The tensile testing machine is then used to slowly pull the paper tape at a speed of 10 mm / min until the positive electrode material layer on the double-sided adhesive surface separates from the positive electrode current collector, ending the test. The average tensile force in the stable region is recorded as the adhesive force between the positive electrode material layer and the positive electrode current collector, expressed in N / m.

[0054] Cohesive strength test of cathode material layer:

[0055] Take the positive electrode sheet obtained according to the <Preparation of Positive Electrode Sheet> steps, and punch it using a mold to obtain a test strip with a length of 80 mm and a width of 20 mm. Clean the surface of the steel plate with alcohol, and attach a 60 mm long and 20 mm wide double-sided adhesive tape (NITTO, NO5000NS) to the steel plate, ensuring no air bubbles are generated during the application process. Center the test strip on the double-sided adhesive tape, with the test side facing upwards. Center the green adhesive tape (Topi adhesive paper, 20 mm wide and 80 mm long) on ​​the test strip. Cut a 60 mm long and 20 mm wide paper strip and insert it into the gap between the test strip and the green adhesive tape, with an overlap of 15 mm. Manually push a 2 kg rubber roller back and forth on the test strip 4 times to obtain the test sample. Test the test sample using a tensile testing machine (Instron 3365). The test sample is fixed on the test stage, then the paper tape is folded upwards 180° and secured with a clamp. The tensile testing machine then slowly pulls the paper tape at a speed of 10 mm / min until the green adhesive separates from the positive electrode material layer on the positive electrode surface, at which point the test ends. The average tensile force in the stable region is recorded as the cohesive force of the positive electrode material layer, expressed in N / m.

[0056] Initial compaction density test of positive electrode sheet:

[0057] Remove the lithium-ion battery after capacity testing, take out the positive electrode sheet to be tested, and use a stamping die to cut 12 small round pieces (1540.25 mm² in area). 2 After zeroing the electronic scale, use tweezers to place the cut small round pieces onto the weighing platform and record the mass of each small round piece: m1, m2, m3, m4, m5, m6, m7, m8, m9, m 10 m 11 m 12 Use a micrometer to measure the thickness h at the four positions (top, bottom, left, and right) of each small circular piece. 11 h 12 h 13 h 14 h 21 h 22 h 23 h 24 h 31 h 32 h 33 h 34 h 41 h 42 h 43 h 44 h 51 h 52 h 53 h 54 h 61 h 62 h63 h 64 h 71 h 72 h 73 h 74 h 81 h 82 h 83 h 84 h 91 h 92 h 93 h 94 h 101 h 102 h 103 h 104 h 111 h 112 h 113 h 114 h 121 h 122 h 123 h 124 The thickness of each small disc is the average of four thicknesses, namely h1, h2, h3, h4, h5, h6, h7, h8, h9, and h1. 10 h 11 h 12 Then the initial compaction density (P.D1) of the first small disc is equal to (m1 - m). 圆片正极集流体 ) / [1540.25(h1-h 圆片正极集流体 The unit is g / cc. Replacing m1 and h1 of the first small disc with the corresponding values ​​of the remaining 11 small discs yields the PD of the other 11 small discs. The average PD of the 12 small discs is the PD of the tested positive electrode. Where m... 圆片正极集流体 The area is 1540.25 mm². 2 The mass of the disc positive current collector, h 圆片正极集流体 This indicates the thickness of the positive electrode current collector.

[0058] Glass transition temperature test:

[0059] The glass transition temperature (Tg) of the positive electrode binder was measured by differential scanning calorimetry (DSC). 5 mg of positive electrode binder sample was weighed and heated from -150 °C to 50 °C at a heating rate of 5 °C / min. The Tg of the positive electrode binder was obtained by DSC curve, with the unit being °C.

[0060] Weight-average molecular weight test:

[0061] The weight-average molecular weight of the positive electrode binder was determined using an Agilent 1200 series liquid chromatograph. The binder was dissolved in N-methylpyrrolidone to prepare a 0.5% (w / w) solution. After filtration through an oil-based filter membrane, the solution was injected into the Agilent 1200 series liquid chromatograph for analysis to obtain the weight-average molecular weight of the positive electrode binder.

[0062] Cyclic performance test:

[0063] Cycle performance was evaluated by the capacity retention rate of lithium-ion batteries. After formation, the lithium-ion batteries were placed in a constant-temperature environment at 25°C and charged at a constant current of 0.6C to 4.5V, then charged at a constant voltage to the cutoff current of 0.05C. After being fully charged and left to stand for 3 minutes, they were discharged at 0.5C to 3.0V, and the discharge capacity was recorded as D0. Cycling tests were then conducted using a 0.6C charge-0.5C discharge cycle for 500 cycles, and the discharge capacity after the 500th cycle was recorded as D1. The capacity retention rate (%) of the lithium-ion battery after 500 cycles at room temperature (25°C) is calculated as D1 / D0 × 100%.

[0064] Example 1-1

[0065] <Preparation of Positive Electrode Binder>

[0066] The polymer monomers acrylonitrile, acrylic acid, terminal vinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and ethylene glycol vinyl ether were mixed in a mass ratio of 4:2:1.5:1:1.5 and dispersed in deionized water in a four-necked flask. The mixture was mechanically stirred at 300 rpm under nitrogen protection and heating conditions. When the temperature was raised from room temperature to 70°C, potassium persulfate (K2S2O8) initiator (1% of the total mass of the above five compounds) was added. After reacting at a constant temperature of 70°C for 12 hours, positive electrode binder precipitates were obtained. The positive electrode binder precipitates were then filtered and dried to obtain the positive electrode binder.

[0067] <Preparation of the positive electrode>

[0068] Lithium cobalt oxide (LiCoO2), the positive electrode active material, the positive electrode binder prepared above, and conductive carbon black were dispersed in N-methylpyrrolidone and stirred until homogeneous to obtain a positive electrode slurry with a solid content of 70 wt%, wherein the mass ratio of LiCoO2, positive electrode binder, and conductive carbon black in the solid components was 96:2:2. The above positive electrode slurry was uniformly coated onto one surface of a positive electrode current collector aluminum foil with a thickness of 8 μm and dried at 120°C to obtain a single-sided positive electrode sheet with a positive electrode material layer thickness of 70 μm. The above operation steps were then repeated on the other surface of the positive electrode current collector aluminum foil to obtain a double-sided positive electrode sheet with a positive electrode active material coating. The coated positive electrode sheet was cold-pressed and then cut into sheets with a specification of 70 mm × 800 mm for later use.

[0069] <Preparation of Negative Electrode Sheets>

[0070] Artificial graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed at a mass ratio of 97.6:1.1:1.3. Deionized water was then added as a solvent, and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 70 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 120°C to obtain a single-sided coated negative electrode sheet with a 120 μm thick negative electrode active material layer. The same process was repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. The coated negative electrode sheet was then cold-pressed and cut into sheets measuring 74 mm × 800 mm for later use.

[0071] <Preparation of Electrolyte>

[0072] In a glove box filled with a dry argon atmosphere, organic solvents propylene carbonate (PC), diethyl carbonate (DEC), and ethylene carbonate (EC) were mixed in a mass ratio of 1:1:1. Then, lithium hexafluorophosphate (LiPF6) was added to the organic solvents to dissolve and mix thoroughly to obtain an electrolyte with a LiPF6 concentration of 1.15 mol / L.

[0073] <Preparation of the diaphragm>

[0074] A porous polyethylene film with a thickness of 5 μm (supplied by Celgard) was used as the separator.

[0075] <Preparation of Lithium-ion Batteries>

[0076] The prepared positive electrode, separator, and negative electrode are stacked sequentially and then wound to obtain an electrode assembly. After the tabs are welded, the electrode assembly is placed in an aluminum-plastic film and dried in a vacuum oven at 80°C for 12 hours to remove moisture. Then, the prepared electrolyte is injected, and the lithium-ion battery is obtained after vacuum sealing, standing, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), capacity testing, and shaping.

[0077] Examples 1-2 to 1-27

[0078] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0079] Examples 2-1 to 2-4

[0080] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0081] Comparative Example 1

[0082] Except for the use of PVDF (model: HSV900, Arkema, France) as the positive electrode binder, the rest is the same as in Example 1-1.

[0083] Comparative Example 2

[0084] Except for the use of polyvinylpyrrolidone (Mw=40000, Sigma-Aldrich Trading Ltd.) as the positive electrode binder, the rest is the same as in Example 1-1.

[0085] Comparative Example 3

[0086] Except for the use of an aqueous dispersion of acrylonitrile multi-element copolymer (model: LA136D, Sichuan Indile Materials Technology Group Co., Ltd.) as the positive electrode binder, the rest is the same as in Examples 1-1.

[0087] Comparative Example 4

[0088] Except for the use of polyvinyl imidazole (Mw=400000, Beijing Innocare Technology Co., Ltd.) as the positive electrode binder, the rest is the same as in Example 1-1.

[0089] Comparative Example 5

[0090] Except for the use of polyimide (Mw=200000, Shenzhen Yanyi New Materials Co., Ltd.) as the positive electrode binder, the rest is the same as in Example 1-1.

[0091] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0092] Table 1

[0093]

[0094]

[0095] Note: " / " in Table 1 indicates that there are no relevant preparation parameters.

[0096] As can be seen from Examples 1-1 to 1-27 and Comparative Examples 1 to 5, the positive electrode of the lithium-ion battery in this application includes the positive electrode binder provided in this application. The positive electrode binder has a lower glass transition temperature, resulting in greater adhesion between the positive electrode material layer and the positive electrode current collector, greater cohesion of the positive electrode material layer, higher initial compaction density of the positive electrode after capacity testing, and higher cycle capacity retention of the lithium-ion battery. This indicates that the lithium-ion battery in this application has higher energy density and better cycle performance. As can be seen from Comparative Examples 1 to 5, when the positive electrode binder provided in this application is not used in the positive electrode, the adhesion between the positive electrode material layer and the positive electrode current collector is smaller, the cohesion of the positive electrode material layer is smaller, the initial compaction density of the positive electrode after capacity testing is lower, and the cycle capacity retention of the lithium-ion battery is lower. This indicates that the lithium-ion battery in the comparative examples of this application has lower energy density and poorer cycle performance.

[0097] The mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-10 and Examples 1-11 to 1-15, when the mass ratio of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamido-2-methylpropanesulfonic acid, and small molecule ether monomers is within the range of this application, the glass transition temperature of the positive electrode binder is lower, the adhesion between the positive electrode material layer and the positive electrode current collector is greater, the cohesion of the positive electrode material layer is greater, the initial compaction density of the positive electrode sheet after capacity testing is higher, and the cycle capacity retention rate of the lithium-ion battery is higher. This indicates that the lithium-ion batteries in the embodiments of this application have high energy density and good cycle performance.

[0098] Table 2

[0099]

[0100] The mass percentage of the positive electrode binder typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-4, when the mass percentage of the positive electrode binder is within the range specified in this application, the glass transition temperature of the positive electrode binder is lower, the adhesion between the positive electrode material layer and the positive electrode current collector is greater, the cohesion of the positive electrode material layer is greater, the initial compaction density of the positive electrode sheet after capacity testing is higher, and the cycle capacity retention rate of the lithium-ion battery is higher. This indicates that the lithium-ion batteries in the embodiments of this application have high energy density and good cycle performance.

[0101] The terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed or inherent to such process, method, or article.

[0102] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0103] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector, the positive electrode material layer comprising a positive electrode active material and a positive electrode binder, the positive electrode binder being obtained by copolymerization of acrylonitrile, acrylic acid, terminal monovinyl polydimethylsiloxane, 2-acrylamide-2-methylpropanesulfonic acid, and small molecule ether monomers; the small molecule ether monomers comprising at least one of ethylene glycol vinyl ether and diethylene glycol monovinyl ether; The mass ratio of the acrylonitrile, the acrylic acid, the terminal monovinyl polydimethylsiloxane, the 2-acrylamide-2-methylpropanesulfonic acid and the small molecule ether monomer is (3 to 4.7): 2: (0.5 to 2.5): (0.8 to 1.2): (1 to 2).

2. The positive electrode sheet according to claim 1, wherein, The glass transition temperature of the positive electrode binder is -30°C to 30°C.

3. The positive electrode sheet according to claim 1, wherein, The glass transition temperature of the positive electrode binder is -20°C to 0°C.

4. The positive electrode sheet according to claim 1, wherein, The weight-average molecular weight of the positive electrode binder is between 850,000 and 1,000,000.

5. The positive electrode sheet according to claim 1, wherein, Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is 1% to 3%.

6. The positive electrode sheet according to claim 1, wherein, The adhesion force between the positive electrode material layer and the positive electrode current collector is 40 N / m to 50 N / m.

7. The positive electrode sheet according to claim 1, wherein, The cohesive force of the positive electrode material layer is 80 N / m to 90 N / m.

8. A secondary battery comprising a negative electrode, a separator, an electrolyte, and a positive electrode as described in any one of claims 1 to 7.

9. An electronic device comprising the secondary battery of claim 8.

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