Secondary battery and electronic device

Abstract

The application provides a secondary battery and an electronic device. The secondary battery comprises a negative electrode tab and a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer, the negative electrode current collector comprises a substrate layer and a conductive coating layer arranged on the surface of the substrate layer, the surface of the conductive coating layer is provided with the negative electrode material layer, the negative electrode tab and the negative electrode sheet are bonded by conductive adhesive to form an electrical connection, and the bonding length between the negative electrode tab and the negative electrode sheet is 6 mm to 10 mm; the conductive coating layer comprises a conductive agent and a binder, and the material of the substrate layer comprises at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid. The secondary battery provided by the application has low impedance, good safety reliability and discharge rate performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and particularly relates to a secondary battery and an electronic device. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, have the advantages of high energy storage density, high open-circuit voltage, low self-discharge rate, long cycle life, good safety, and the like, and are widely used in various fields such as portable power storage, electronic devices, electric vehicles, and the like.

[0003] At present, most of the soft package lithium ion batteries on the market use copper current collectors for the negative electrode sheets. Due to the relatively large mass of the copper current collector, the overall mass of the lithium ion battery is relatively large, and in the process of accidental falling, impact and penetration of the lithium ion battery, the risk of internal short circuit of the lithium ion battery is greatly increased, affecting the safety and reliability of the lithium ion battery. Using a polymer as a base material layer to replace the existing negative electrode current collector (such as a copper foil) can make the lithium ion battery have good safety performance, but how to realize the conductive connection of the upper and lower surfaces of the polymer base material layer, while making the lithium ion battery have good discharge rate performance (2C discharge capacity / 0.5C discharge capacity), also becomes a key problem in the development of lithium ion batteries. SUMMARY

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to improve the safety and reliability and discharge rate performance of the secondary battery. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a secondary battery, which comprises a negative electrode sheet and a negative electrode tab, the negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer, the negative electrode current collector comprising a base material layer and a conductive coating layer arranged on the surface of the base material layer, the surface of the conductive coating layer being provided with the negative electrode material layer, the negative electrode tab and the negative electrode sheet being bonded by conductive adhesive to form an electrical connection, the bonding length between the negative electrode tab and the negative electrode sheet being 6mm to 10mm; the conductive coating layer comprises a conductive agent and a binder, and the material of the base material layer comprises at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid. The negative electrode current collector comprises a base material layer and a conductive coating layer, the negative electrode tab and the negative electrode sheet are bonded by conductive adhesive to form an electrical connection and the bonding length is controlled within the scope of the present application, the advantages of the base material layer can reduce the mass of the negative electrode current collector, so that the secondary battery is lightweight and its safety and reliability are improved, and at the same time, the negative electrode sheet can have a lower resistance, the overall impedance of the secondary battery is reduced, and thus the discharge rate performance of the secondary battery is improved.

[0006] In some embodiments of the present application, the impedance of the negative electrode tab is 7 Ω to 40 Ω, indicating that the negative electrode tab has a low impedance, which is conducive to reducing the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery.

[0007] In some embodiments of the present application, the conductive adhesive comprises at least one of an acrylic conductive adhesive, an epoxy conductive adhesive or a silicone conductive adhesive; the resistivity of the conductive adhesive is 10 -6 Ω·cm to 10 -4 Ω·cm. Selecting the conductive adhesive within the above range and adjusting the resistivity thereof within the above range is conducive to bonding the negative electrode tab and the negative electrode tab to form a good electrical connection, which can make the secondary battery lightweight and improve its safety and reliability while making the negative electrode tab have a low resistance, reducing the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery.

[0008] In some embodiments of the present application, the thickness of the conductive adhesive is 3 μm to 6 μm. Adjusting the thickness of the conductive adhesive within the above range is conducive to bonding the negative electrode tab and the negative electrode tab to form a good electrical connection, while making the flatness of the negative electrode tab higher and making the negative electrode tab have a low impedance, which can make the secondary battery lightweight, thereby making the secondary battery have a low impedance, a high energy density, and good safety and reliability and discharge rate performance.

[0009] In some embodiments of the present application, the negative electrode tab is bonded to the surface of the negative electrode material layer. Selecting the above-mentioned way to connect the negative electrode tab and the negative electrode material layer can take advantage of the substrate layer, reduce the mass of the negative electrode current collector, make the secondary battery lightweight and improve its safety and reliability, while also making the negative electrode tab have a low resistance, reducing the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery.

[0010] In some embodiments of the present application, the negative electrode tab is embedded in the negative electrode material layer; or the negative electrode tab is arranged between the negative electrode material layer and the conductive coating. Selecting the above-mentioned way to connect the negative electrode tab and the negative electrode material layer can make the negative electrode tab have a high flatness, reduce the thickness of the secondary battery, thereby improving the energy density of the secondary battery, while providing a fast channel for electrical transmission, making the negative electrode tab have a low resistance, reducing the overall impedance of the secondary battery, and also improving the discharge rate performance of the secondary battery.

[0011] In some embodiments of the present application, the thickness of the negative tab is 60-100 μm, and the thickness of the negative tab is less than or equal to the sum of the thicknesses of the conductive coating and the negative material layer. Controlling the thickness of the negative tab within the above range and adjusting the thickness of the negative tab to be less than or equal to the sum of the thicknesses of the conductive coating and the negative material layer can play a connecting role of the negative tab and not affect the energy density of the secondary battery, so that the flatness of the negative tab is higher, which can make the secondary battery lightweight and improve its safety and reliability.

[0012] In some embodiments of the present application, the substrate layer comprises a first surface and a second surface, the first surface is provided with a first conductive coating, and the surface of the first conductive coating is provided with a first negative material layer; the second surface is provided with a second conductive coating, and the surface of the second conductive coating is provided with a second negative material layer; the negative tab comprises a first negative tab and a second negative tab, the first negative material layer is electrically connected to the first negative tab by the first conductive adhesive, and the second negative material layer is electrically connected to the second negative tab by the second conductive adhesive. Having the above characteristics is conducive to reducing the impedance between the negative tab and the negative material layer, thereby reducing the impedance of the secondary battery and improving the discharge rate performance of the secondary battery.

[0013] In some embodiments of the present application, the thickness of the substrate layer is 5-20 μm, and controlling the thickness of the substrate layer within the above range can make the substrate layer have good mechanical properties, which is conducive to improving the safety and reliability of the secondary battery, and at the same time, the secondary battery can have a higher energy density. And / or, the transverse tensile strength of the substrate layer is 130-200 MPa. Generally, the longitudinal tensile strength of the substrate layer is much higher than the transverse tensile strength, so controlling the transverse tensile strength of the substrate layer within the above range can make the current collector have good mechanical properties, reduce the risk of ductile deformation of the current collector, and be conducive to improving the safety and reliability of the secondary battery.

[0014] In some embodiments of the present application, the thickness of the conductive coating is 0.2-5 μm. By controlling the thickness of the conductive coating within the above range, the negative tab can have a lower resistance, the secondary battery can be lightweight, and at the same time, the secondary battery can have a higher energy density while improving the safety and reliability of the secondary battery.

[0015] In some embodiments of the present application, the conductive agent comprises at least one of conductive graphite, conductive carbon black, carbon nanotubes, graphene, or conductive carbon fibers; and the mass percentage of the conductive agent is 5-95% based on the mass of the conductive coating. Selecting the above conductive agent and controlling the mass percentage within the above range can help to build a conductive network with good conductivity on the surface of the substrate layer, so that the negative current collector has a lower resistance, and at the same time, the secondary battery can be lightweight while improving the safety and reliability and discharge rate performance of the secondary battery.

[0016] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the transverse tensile strength of the substrate layer is 150 MPa to 200 MPa; (2) the thickness of the conductive coating layer is 0.5 μm to 2 μm; (3) the mass percentage content of the conductive agent is 30% to 50% based on the mass of the conductive coating layer; (4) the thickness of the negative electrode material layer is 60 μm to 150 μm. Satisfying at least one of the above characteristics is conducive to further improving the safety reliability and discharge rate performance of the secondary battery.

[0017] The second aspect of the present application provides an electronic device comprising the secondary battery provided by the first aspect of the present application. The secondary battery provided by the present application has good safety reliability and discharge rate performance, so that the electronic device of the present application has good use performance and a longer service life.

[0018] The present application has the following beneficial effects:

[0019] The present application provides a secondary battery and an electronic device. The secondary battery comprises a negative electrode tab and a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer, the negative electrode current collector comprises a substrate layer and a conductive coating layer arranged on the surface of the substrate layer, the surface of the conductive coating layer is provided with the negative electrode material layer, the negative electrode tab and the negative electrode sheet are bonded by conductive adhesive to form an electrical connection, and the bonding length between the negative electrode tab and the negative electrode sheet is 6 mm to 10 mm; the conductive coating layer comprises a conductive agent and a binder, and the material of the substrate layer comprises at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid. The negative electrode current collector comprises a substrate layer and a conductive coating layer, the negative electrode tab and the negative electrode sheet are bonded by conductive adhesive to form an electrical connection and the bonding length between the negative electrode tab and the negative electrode sheet is regulated within the scope of the present application, the advantages of the substrate layer can reduce the mass of the negative electrode current collector, so that the secondary battery is lightweight and its safety reliability is improved, and at the same time, the negative electrode sheet can have a lower resistance, the overall impedance of the secondary battery is reduced, and thus the discharge rate performance of the secondary battery is improved.

[0020] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above to be achieved at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0022] Figure 1A structural schematic diagram of an electrode assembly of one embodiment of the present application;

[0023] Figure 2 A structural schematic diagram of an electrode assembly of another embodiment of the present application;

[0024] Figure 3 A schematic diagram of a connection mode of a negative tab and a negative tab sheet of one embodiment of the present application;

[0025] Figure 4 A schematic diagram of a connection mode of a negative tab and a negative tab sheet of another embodiment of the present application.

[0026] The reference signs: electrode assembly 100, negative tab sheet 10, negative tab 20, conductive adhesive 30, substrate layer 11, conductive coating layer 12, negative material layer 13, first surface 11a, second surface 11b, first conductive coating layer 121, second conductive coating layer 122, first negative material layer 131, second negative material layer 132, first negative tab 21, second negative tab 22, first conductive adhesive 31, second conductive adhesive 32. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0028] It should be noted that in the following content, the present application is explained by taking a lithium ion battery as an example of a secondary battery, but the secondary battery of the present application is not limited to a lithium ion battery. The specific technical solutions are as follows:

[0029] The first aspect of the present application provides a secondary battery, which comprises a negative tab sheet and a negative tab, the negative tab sheet comprising a negative current collector and a negative material layer. The negative current collector comprises a substrate layer and a conductive coating layer arranged on the surface of the substrate layer, the surface of the conductive coating layer being provided with the negative material layer, the negative tab and the negative tab sheet being bonded to form an electrical connection by a conductive adhesive, the bonding length between the negative tab and the negative tab sheet being 6mm to 10mm, for example, the bonding length between the negative tab and the negative tab sheet being 6mm, 6.5mm, 7mm, 7.5mm, 8mm, 8.5mm, 9mm, 9.5mm, 10mm or a range composed of any two of the above values. The conductive coating layer comprises a conductive agent and a binder, and the material of the substrate layer comprises at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid.

[0030] The inventors find that the above negative current collector can replace the commonly used copper foil to reduce the risk of short-circuit heat generation during the drop and nail penetration of the secondary battery, and improve the safety and reliability of the secondary battery. The negative current collector includes a substrate layer and a conductive coating layer, and the negative tab and the negative tab are bonded by conductive adhesive to form an electrical connection. The advantages of the substrate layer can reduce the mass of the negative current collector, making the secondary battery lightweight and improving its safety and reliability. At the same time, the negative tab can have a lower resistance, reducing the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery. When the bonding length between the negative tab and the negative tab is too small, for example, less than 6 mm, it will cause poor bonding between the negative tab and the negative tab, and the negative tab and the negative tab cannot achieve good electrical connection, which affects the discharge rate performance of the secondary battery. In severe cases, local severe heat generation occurs inside the secondary battery, affecting the safety and reliability of the secondary battery. When the bonding length between the negative tab and the negative tab is too large, for example, greater than 10 mm, it will cause capacity loss of the secondary battery. Therefore, the bonding length between the negative tab and the negative tab is controlled within the scope of the application, which can make the secondary battery have high energy density while improving its discharge rate performance.

[0031] In some embodiments, as shown in FIG. 1A, the electrode assembly 100 includes a negative tab 20 and a negative tab 10, the negative tab 10 includes a negative current collector, the negative current collector includes a substrate layer 11 and a conductive coating layer 12 disposed on one surface of the substrate layer 11, and the surface of the conductive coating layer 12 is provided with a negative material layer 13. The negative tab 20 and the negative tab 10 are bonded by conductive adhesive 30 to form an electrical connection. Figure 1 The above characteristics are beneficial to reduce the impedance between the negative tab and the negative material layer, thereby reducing the overall impedance of the secondary battery, and can improve the safety and reliability and the discharge rate performance of the secondary battery.

[0032] In other embodiments, the conductive coating is disposed on both surfaces of the substrate layer along the thickness direction, as shown in FIG. 1C. Figure 2As shown, the electrode assembly 100 includes a negative electrode tab 10 and a negative electrode lug 20, the negative electrode tab 10 includes a negative electrode current collector, the negative electrode current collector includes a substrate layer 11, a first conductive coating layer 121 and a second conductive coating layer 122, the substrate layer 11 includes a first surface 11a and a second surface 11b, the first surface 11a is provided with the first conductive coating layer 121, the surface of the first conductive coating layer 121 is provided with a first negative electrode material layer 131; the second surface 11b is provided with the second conductive coating layer 122, the surface of the second conductive coating layer 122 is provided with a second negative electrode material layer 132; the negative electrode lug 20 includes a first negative electrode lug 21 and a second negative electrode lug 22, the first negative electrode material layer 121 and the first negative electrode lug 21 are bonded to form an electrical connection by a first conductive adhesive 31, and the second negative electrode material layer 132 and the second negative electrode lug 22 are bonded to form an electrical connection by a second conductive adhesive 32. With the above characteristics, it is beneficial to reduce the impedance between the negative electrode lug and the negative electrode material layer, thereby reducing the impedance of the secondary battery, and the discharge rate performance of the secondary battery can be improved.

[0033] In some embodiments of the present application, the impedance of the negative electrode tab is 7Ω to 40Ω, for example, the impedance of the negative electrode tab can be 7Ω, 10Ω, 13Ω, 16Ω, 20Ω, 23Ω, 25Ω, 28Ω, 30Ω, 33Ω, 35Ω, 37Ω, 40Ω or a range composed of any two of them. It is explained that the negative electrode tab has a lower impedance, which is beneficial to reduce the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery.

[0034] In some embodiments of the present application, the conductive adhesive includes at least one of an acrylic conductive adhesive, an epoxy conductive adhesive or a silicone conductive adhesive; the resistivity of the conductive adhesive is 10 -6 Ω·cm to 10 -4 Ω·cm, for example, the resistivity of the conductive adhesive can be 10 -6 Ω·cm, 0.5×10 -5 Ω·cm, 10 -5 Ω·cm, 0.5×10 -4 Ω·cm, 10 -4 Ω·cm or a range composed of any two of them. The conductive adhesive in the above range is selected and its resistivity is controlled in the above range, which is beneficial to the bonding of the negative electrode lug and the negative electrode tab to form a good electrical connection, can make the secondary battery lightweight and improve its safety and reliability, at the same time, the negative electrode tab has a lower resistance, reduces the overall impedance of the secondary battery, and improves the discharge rate performance of the secondary battery.

[0035] In some embodiments of the present application, the thickness of the conductive adhesive is 3-6 μm, for example, the thickness of the conductive adhesive can be 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm or a range between any two of the above values. Controlling the thickness of the conductive adhesive within the above range is conducive to the bonding of the negative tab and the negative tab sheet to form a good electrical connection, while at the same time making the flatness of the negative tab sheet higher and the impedance of the negative tab sheet lower, which can make the secondary battery lightweight and improve its safety and reliability, so that the secondary battery has lower impedance, higher energy density, and good safety and reliability and discharge rate performance at the same time.

[0036] In some embodiments of the present application, as shown in FIG. 1, the negative tab 20 is bonded to the surface of the negative material layer 13. Selecting the above-mentioned way to connect the negative tab and the negative material layer can take advantage of the substrate layer, reduce the mass of the negative current collector, make the secondary battery lightweight and improve its safety and reliability, and at the same time, the negative tab sheet can have lower resistance, which reduces the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery. Figure 1

[0037] In some embodiments of the present application, as shown in FIG. 2, the negative tab 20 is embedded in the negative material layer 13. Selecting the above-mentioned way to connect the negative tab and the negative material layer can make the negative tab sheet have higher flatness, reduce the thickness of the secondary battery, thereby improving the energy density of the secondary battery, but embedding the tab in the negative material layer is more difficult in process, and the adhesion is slightly worse than the setting mode of bonding the negative tab to the surface of the negative material layer and setting the negative tab between the negative material layer and the conductive coating, thereby improving the discharge rate performance of the secondary battery but the improvement effect is slightly worse than the above two setting modes. Figure 3

[0038] In some embodiments of the present application, as shown in FIG. 3, the negative tab 20 is disposed between the negative material layer 13 and the conductive coating 12. Selecting the above-mentioned way to connect the negative tab and the negative material layer can make the negative tab sheet have higher flatness, reduce the thickness of the secondary battery, thereby improving the energy density of the secondary battery, and this connection mode is simple in process, at the same time provides a fast channel for electrical transmission, makes the negative tab sheet have lower resistance, reduces the overall impedance of the secondary battery, thereby further improving the discharge rate performance of the secondary battery. Figure 4

[0039] ​​​In some embodiments of the present application, the thickness of the negative tab is 60-100 μm, and the thickness of the negative tab is less than or equal to the sum of the thicknesses of the conductive coating and the negative material layer. For example, the thickness of the negative tab can be 60 μm, 70 μm, 75 μm, 80 μm, 90 μm, 100 μm, or a range defined by any two of the above values. Controlling the thickness of the negative tab within the above range and adjusting the thickness of the negative tab to be less than or equal to the sum of the thicknesses of the conductive coating and the negative material layer can play a connecting role of the negative tab and not affect the energy density of the secondary battery, so that the flatness of the negative tab is high, the secondary battery can be lightened, and the safety and reliability of the secondary battery can be improved.

[0040] In some embodiments of the present application, the thickness of the substrate layer is 5-20 μm, for example, the thickness of the substrate layer can be 5 μm, 6 μm, 8 μm, 12 μm, 14 μm, 15 μm, 16 μm, 17 μm, 19 μm, 20 μm, or a range defined by any two of the above values. Controlling the thickness of the substrate layer within the above range can make the substrate layer have good mechanical properties, play a supporting role of the substrate layer, and be conducive to improving the safety and reliability of the secondary battery, and at the same time, the secondary battery can have a high energy density.

[0041] In some embodiments of the present application, the transverse tensile strength of the substrate layer is 130-200 MPa, preferably 150-200 MPa. For example, the transverse tensile strength of the substrate layer can be 130 MPa, 140 MPa, 146 MPa, 150 MPa, 158 MPa, 160 MPa, 164 MPa, 170 MPa, 175 MPa, 180 MPa, 190 MPa, 200 MPa, or a range defined by any two of the above values. Generally, the longitudinal tensile strength of the substrate layer is much higher than the transverse tensile strength, and the longitudinal tensile strength can meet the process and product requirements, so controlling the transverse tensile strength of the substrate layer within the above range can make the current collector have good mechanical properties, reduce the risk of extension deformation of the current collector, and be conducive to improving the safety and reliability of the secondary battery.

[0042] In some embodiments of the present application, the thickness of the substrate layer is 5-20 μm, and the transverse tensile strength of the substrate layer is 130-200 MPa, preferably 150-200 MPa. Controlling the thickness and the transverse tensile strength of the substrate layer within the above range can make the substrate layer have good mechanical properties and mechanical properties, be conducive to improving the safety and reliability of the secondary battery, and at the same time, the secondary battery can have a high energy density.

[0043] In some embodiments of the present application, the thickness of the conductive coating is 0.2 μm to 5 μm, preferably 0.5 μm to 2 μm. For example, the thickness of the conductive coating is 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 5 μm, or a range defined by any two of the above values. By adjusting the thickness of the conductive coating within the above range, the negative electrode sheet can have a lower resistance, the secondary battery can be lightweight, and the secondary battery can have a higher energy density while improving the safety and reliability of the secondary battery.

[0044] In some embodiments of the present application, the conductive agent includes at least one of conductive graphite, conductive carbon black, carbon nanotubes, graphene, or conductive carbon fibers; and the mass percentage of the conductive agent is 5% to 95%, preferably 30% to 50%, based on the mass of the conductive coating. For example, the mass percentage of the conductive agent can be 5%, 10%, 14%, 20%, 23%, 25%, 30%, 34%, 35%, 38%, 40%, 43%, 45%, 48%, 50%, 60%, 70%, 80%, 90%, 95%, or a range defined by any two of the above values. By selecting the conductive agent described above and adjusting the mass percentage of the conductive agent within the above range, a conductive network with good conductivity can be constructed on the surface of the substrate layer, the negative electrode current collector can have a lower resistance, and the secondary battery can be lightweight while improving the safety and reliability and the discharge rate performance of the secondary battery.

[0045] The present application does not have a particular limitation on the type of binder in the conductive coating as long as the purpose of the present application can be achieved. For example, the binder can include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, polyacrylic acid, polyacrylic acid salt, carboxymethyl cellulose sodium, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The present application does not have a particular limitation on the mass percentage of the binder. For example, the mass percentage of the binder can be 5% to 95%, based on the mass of the conductive coating

[0046] In some embodiments of the present application, the thickness of the negative electrode material layer is 60 μm to 150 μm. For example, the thickness of the negative electrode material layer can be 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, or a range defined by any two of the above values. The "thickness of the negative electrode material layer" refers to the thickness of the single-sided negative electrode material layer. Controlling the thickness of the negative electrode material layer within the above range can result in a negative electrode sheet with good flatness, a secondary battery with high energy density, and good ion transport kinetics of lithium ions, thereby improving the discharge rate performance of the secondary battery while maintaining high energy density.

[0047] In the present application, the substrate layer is a film of the above-mentioned polymer such as polypropylene, which can be purchased and tested according to the "test method for transverse tensile strength of the substrate layer" provided in the present application, and a film with the desired transverse tensile strength is selected as the substrate layer. The method for preparing the substrate layer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the substrate layer film can be prepared by heating and melting the glue particles of the substrate layer material, and then performing extrusion, casting, longitudinal stretching, transverse stretching, heat treatment, cooling and molding, and the like. In the present application, the temperature for melting can be 240°C to 270°C, the temperature for casting can be 25°C to 40°C, the temperature for longitudinal stretching can be 125°C to 145°C, the longitudinal stretching ratio can be 4.5 to 5.5, the temperature for transverse stretching can be 150°C to 170°C, the transverse stretching ratio can be 7.5 to 10, and the temperature for heat treatment can be 170°C to 180°C, and the heat treatment time can be 1 h to 4 h. Alternatively, the glue particles can be mixed with pore-forming agents, additives, and the like before being heated and melted, and the present application does not particularly limit this, which can be selected and adjusted according to actual needs, as long as the purpose of the present application can be achieved. The weight average molecular weight (Mw) of the substrate layer material is not particularly limited in the present application, as long as the purpose of the present application can be achieved, which can be, for example, 300,000 to 600,000. The type of pore-forming agent is not particularly limited in the present application, which can be selected according to actual needs, as long as the purpose of the present application can be achieved. For example, the pore-forming agent can be white oil. In the present application, the above-mentioned additives can include, but are not limited to, coupling agents, antioxidants, and the like, and the type of the above-mentioned additives is not particularly limited in the present application, which can be selected according to actual needs, as long as the purpose of the present application can be achieved. The equipment used in the extrusion process is not particularly limited in the present application, which can be selected according to actual needs, as long as the purpose of the present application can be achieved. For example, a twin-screw extruder can be used for extrusion. In the present application, the "longitudinal stretching" refers to stretching in the extrusion direction, and the "transverse stretching" refers to stretching in a direction perpendicular to the extrusion direction. The stretching ratio refers to the ratio of the size of the film after stretching to the size before stretching.

[0048] Generally, the tensile strength of the substrate layer can be changed by changing the weight average molecular weight and the crosslinking degree of the substrate layer material. When other conditions remain unchanged, the tensile strength of the substrate layer increases as the weight average molecular weight increases, and the tensile strength of the substrate layer decreases as the weight average molecular weight decreases. When other conditions remain unchanged, the tensile strength of the substrate layer increases as the crosslinking degree increases, and the tensile strength of the substrate layer decreases as the crosslinking degree decreases.

[0049] The preparation method of the negative electrode current collector is not particularly limited in the present application, as long as the purpose of the present application can be achieved, for example, the following method can be used: the above-mentioned conductive agent and binder are mixed, deionized water is added and stirred uniformly to obtain a conductive coating slurry with a solid content of 20wt% to 40wt%. Then the conductive coating slurry is uniformly coated on one surface of the substrate layer, and after drying, a single-sided conductive coating negative electrode current collector is obtained. Repeat the above coating steps on the other surface of the substrate layer, and after drying, a double-sided conductive coating negative electrode current collector is obtained.

[0050] The preparation method of the negative electrode current collector is not particularly limited in the present application, as long as the purpose of the present application can be achieved, for example, the following method can be used: the above-mentioned conductive agent and binder are mixed, deionized water is added and stirred uniformly to obtain a conductive coating slurry with a solid content of 20wt% to 40wt%. Then the conductive coating slurry is uniformly coated on one surface of the substrate layer, and after drying, a single-sided conductive coating negative electrode current collector is obtained. Repeat the above coating steps on the other surface of the substrate layer, and after drying, a double-sided conductive coating negative electrode current collector is obtained.

[0051] In the present application, the negative electrode material layer of the present application comprises a negative electrode active material. The type of negative electrode active material is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the negative electrode active material can comprise at least one of natural graphite, artificial graphite, mesocarbon 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, lithium titanate Li4Ti5O 12 with spinel structure, Li-Al alloy or metallic lithium. The mass ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder in the negative electrode material layer is not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0052] The negative electrode material layer of the present application can further include a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersing agent. The present application does not have a particular limitation on the negative electrode conductive agent and the negative electrode binder, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can include, but is not limited to, at least one of conductive carbon black, carbon nanotube, graphite, carbon fiber, carbon nanowire, graphene, a metallic material, or a conductive polymer. The above-mentioned metallic material can include, but is not limited to, a metal powder and / or a metal fiber, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer can include, but is not limited to, at least one of a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder can include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyamide-imide, styrene butadiene rubber, or polyvinylidene fluoride. The negative electrode dispersing agent can include sodium carboxymethyl cellulose.

[0053] The secondary battery of the present application further includes a positive electrode tab. The present application does not have a particular limitation on the positive electrode tab, as long as the purpose of the present application can be achieved. For example, the positive electrode tab includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. In the present application, the positive electrode material layer can be disposed on one surface in the thickness direction of the positive electrode current collector, or can be disposed on both surfaces in the thickness direction of the positive electrode current collector. It is noted that the "surface" herein can be the entire area of the surface of the positive electrode current collector, or can be a partial area of the surface of the positive electrode current collector, and the present application does not have a particular limitation, as long as the purpose of the present application can be achieved.

[0054] The present application does not have a particular limitation on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector), etc. The present application does not have a particular limitation on the thickness of the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 µm to 20 µm. In the present application, the positive electrode material layer includes a positive electrode active material, and the present application does not have a particular limitation on the type of the positive electrode active material, as long as the purpose of the present application can be achieved. The positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions. In some embodiments, the positive electrode active material can include at least one of nickel cobalt manganese acid lithium (NCM), nickel cobalt aluminum acid lithium, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium iron manganese phosphate, etc. The nickel cobalt manganese acid lithium can include, but is not limited to, LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn 0.2O2(NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), or LiNi 0.9 Co 0.05 Mn 0.05 O2(NCM955). The above positive electrode active materials can be subjected to a doping treatment. In some embodiments, the element used for doping can include at least one of K, Na, Ca, Mg, B, Al, Co, Si, V, Ga, Sn, or Zr. The thickness of the positive electrode material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 30 to 120 pm. The positive electrode material layer of the present application can further include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent and the positive electrode binder are not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the positive electrode conductive agent can be at least one of the above-mentioned negative electrode conductive agents, and the positive electrode binder can be at least one of the above-mentioned negative electrode binders.

[0055] In the present application, the method for preparing the positive electrode tab is not particularly limited, as long as the object of the present application can be achieved, for example, it can be prepared by the following method: mixing the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder, adding N-methyl pyrrolidone (NMP) and stirring uniformly to obtain a positive electrode slurry with a solid content of 65 to 85 wt%. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector, and after drying, a single-sided positive electrode material layer coated positive electrode tab is obtained. Then repeat the above coating step on the other surface of the positive electrode current collector, and after drying, a double-sided positive electrode material layer coated positive electrode tab is obtained. After the coating is completed, the positive electrode tab is obtained by cold pressing and cutting.

[0056] The secondary battery of the present application further includes an electrolyte. The electrolyte of the present application can include a lithium salt and an organic solvent. The kind of the lithium salt of the present application is not particularly limited as long as the object of the present application can be achieved, for example, the lithium salt can include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), or lithium difluoro(oxalato)borate (LiDFOB). The kind of the above-mentioned organic solvent of the present application is not particularly limited as long as the object of the present application can be achieved, for example, can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The above-mentioned carbonate compound can include, but is not limited to, at least one of a chain carbonate compound or a cyclic carbonate compound. The above-mentioned chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, or methyl ethyl carbonate. The above-mentioned cyclic carbonate compound can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The above-mentioned carboxylate compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or hexanolactone. The above-mentioned ether compound can include, but is not limited to, at least one of dimethyl ether of ethylene glycol, 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 above-mentioned other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0057] The secondary battery of the present application further includes a separator. The separator of the present application is not particularly limited as long as the object of the present application can be achieved, for example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), polyolefin (PO) separator mainly including polytetrafluoroethylene, polyester film (for example, polyethylene terephthalate (PET) film), cellulose film, polyimide film (PI), polyamide film (PA), spandex or aramid film, etc. The type of the separator can include, but is not limited to, at least one of woven film, non-woven film (non-woven fabric), microporous film, composite film, calendered film or spunlaced film, etc. The separator of the present application can have a porous structure, a porous layer is provided on at least one surface of the separator, the porous layer includes inorganic particles and a binder, the inorganic particles can include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The binder can include at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, carboxymethyl cellulose na, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The size of the pore diameter of the porous structure is not particularly limited as long as the object of the present application can be achieved, for example, the size of the pore diameter can be 0.01 μm to 1 μm. In the present application, the thickness of the separator is not particularly limited as long as the object of the present application can be achieved, for example, the thickness of the separator can be 4 μm to 12 μm.

[0058] The secondary battery of the present application further includes a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte, and other components known in the art of secondary batteries, which are not limited by the present application. The case of the present application is not particularly limited and can be a case known in the art as long as the object of the present application can be achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be metal, which is not limited by the present application, and a metal hard case known in the art can be used as long as the object of the present application can be achieved. The flexible case can be a metal plastic film, for example, an aluminum plastic film, a steel plastic film, etc.

[0059] The type of the secondary battery of the present application is not particularly limited and can include any device in which an electrochemical reaction occurs. For example, the secondary battery can include, but is not limited to, a lithium metal secondary battery, a lithium ion battery, a sodium ion battery, a lithium polymer secondary battery, a lithium ion polymer secondary battery. The shape of the secondary battery of the present application is not particularly limited as long as the object of the present application can be achieved.

[0060] The preparation process of the secondary battery is well known to those skilled in the art, and the present application is not particularly limited, for example, can include but is not limited to the following steps: stacking the positive electrode sheet, the separator, the negative electrode sheet and the separator in order, and winding, folding and the like according to the needs to obtain the electrode assembly of the winding structure, placing the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing to obtain the secondary battery; or stacking the positive electrode sheet, the separator, the negative electrode sheet and the separator in order, then fixing the four corners of the entire stack structure with adhesive tape to obtain the electrode assembly of the stack structure, placing the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing to obtain the secondary battery. In addition, the overcurrent prevention element, guide plate and the like can also be placed in the packaging bag as needed, so as to prevent the pressure rise in the secondary battery and overcharge and discharge.

[0061] The second aspect of the present application provides an electronic device comprising the secondary battery provided by the first aspect of the present application. The secondary battery provided by the present application has good safety reliability and discharge rate performance, so that the electronic device of the present application has good use performance and longer service life.

[0062] The present application does not particularly limit the type of electronic device, which can be any electronic device known in the prior art. In some embodiments, the electronic device can include but is not limited to notebook computers, pen input computers, mobile computers, electronic book players, portable telephones, portable facsimile machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic organizers, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, power-assisted bicycles, bicycles, lighting appliances, toys, game machines, watches, power tools, flashlights, cameras, household large storage batteries and lithium ion capacitors, etc.

[0063] Embodiments

[0064] Hereinafter, embodiments and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations were carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0065] Test methods and equipment:

[0066] Negative electrode sheet and negative electrode current collector sampling method:

[0067] Take the lithium ion battery after full discharge (discharge process is discharged to 3.0V at 0.2C current), disassemble and take out the negative electrode sheet, soak in dimethyl carbonate (DMC) for 20min, then rinse with DMC and acetone in turn, then place it in an oven, bake at 80℃ for 12h, obtain the negative electrode sheet sample. Scrape off the negative electrode material layer on the surface of the negative electrode sheet, wash the negative electrode current collector with deionized water, then bake at 80℃ for 12h, obtain the negative electrode current collector sample.

[0068] Hereinafter, the thickness of the substrate layer and the thickness of the conductive coating, the transverse tensile strength of the substrate layer, the impedance of the negative electrode sheet, etc. can be sampled by the above sampling method.

[0069] The bonding length test between the negative electrode tab and the negative electrode sheet:

[0070] Measure the total length L1 mm of the negative electrode tab with a tape measure, then measure the vertical length L2 mm of the negative electrode tab beyond the negative electrode sheet, then the bonding length L3 between the negative electrode tab and the negative electrode sheet = L1-L2, unit: mm.

[0071] Thickness test of the substrate layer and the conductive coating:

[0072] Measure the thickness H1 μm of the negative electrode current collector with a micrometer, then scrape off the conductive coating on the surface of the substrate layer, measure the thickness H2 μm of the substrate layer, then the thickness H3 of the conductive coating = H1-H2, unit: μm.

[0073] Transverse tensile strength test of the substrate layer:

[0074] Test the transverse tensile strength of the substrate layer by a high-iron tensile testing machine. Scrape off the conductive coating on the surface of the negative electrode current collector to obtain a substrate layer sample. Cut the substrate layer sample into 10 strips with a 20mm×100mm knife die, wrap a piece of crepe tape around the two ends of each strip (for easy clamping by the clamp), and obtain the test strips. Set the tensile speed of the high-iron tensile testing machine to 50mm / min, the initial clamp distance to 40mm, place the test strips in the middle of the clamps, clamp the upper and lower ends with the clamps, then start the high-iron tensile testing machine to begin the tensile test until the sample is broken, and record the tensile curve. Measure three strips in each group, calculate the average value to obtain the transverse tensile strength of the substrate layer.

[0075] Impedance test of the negative electrode sheet:

[0076] The resistance of the negative electrode sheet was tested using a resistance meter (Yuan Energy Technology). The negative electrode sheet of each example or comparative example was cut into a rectangular sample with a width of 20 mm and a length of 60 mm. A separator film with a width of 25 mm and a length of 70 mm was stacked with the negative electrode sheet sample, and the separator film served as an insulator. Then, the rectangular sample was folded in half along the midpoint of the length of 60 mm (i.e., 30 mm from each end of the length), so that the two widths overlapped. Test probes were placed on the upper and lower surfaces in the thickness direction at a corner near the width, and the distance between the probes and the adjacent edges of the negative electrode sheet was 2 mm. The resistance of the sample was then tested, and the average value was calculated based on 5 samples in each group to obtain the resistance of the negative electrode sheet.

[0077] Impedance test of lithium ion battery:

[0078] The electrochemical impedance spectroscopy (EIS) was tested using an electrochemical workstation (Bio-Logic, France) to characterize the impedance of the lithium ion battery. The specific test method was to determine the ratio of the perturbation signal X and the response signal Y at a frequency of 10 μHz to 1 MHz to obtain the real part Z', the imaginary part Z", the modulus |Z| and the phase angle φ at different frequencies. Then, the EIS impedance spectrum of the lithium ion battery was obtained. The EIS impedance spectrum was curve-fitted using EIS analysis software to obtain the impedance of the lithium ion battery.

[0079] Safety and reliability test:

[0080] The safety and reliability of the lithium ion battery was evaluated by impact pass rate. The higher the impact pass rate, the better the safety and reliability of the lithium ion battery. The lithium ion battery of the example or comparative example was first charged at 0.2 C to 4.45 V, and then held at 4.45 V to 0.025 C to full charge. The lithium ion battery in full charge was placed on the test table, a round rod with a diameter of φ15.8 mm and a length of 15.8 cm was placed at the center of the surface of the lithium ion battery parallel to the table, and the longitudinal axis of the round rod was perpendicular to the test table. A 9.6 kg weight was dropped vertically from a height of 610 mm above the test table to the upper end of the round rod in a free state, and the bottom end of the round rod impacted the lithium ion battery. Finally, the temperature of the surface of the lithium ion battery was tested. The criteria for passing were no fire and no heat. 100 lithium ion batteries were tested for each example or comparative example.

[0081] Impact pass rate = number of lithium ion batteries passing the test / 100 x 100%.

[0082] Discharge rate performance test:

[0083] The discharge rate performance of the lithium ion battery was evaluated by 2C / 0.5C discharge rate. The higher the 2C / 0.5C discharge rate, the better the discharge rate performance of the lithium ion battery. The lithium ion battery of the example or the comparative example was charged at 0.5C constant current to 4.45V, and then held at 4.45V to 0.025C to full charge state, and then discharged at 0.5C current to 3.0V, and the above charging and discharging process was repeated 3 times, and the average capacity of the three discharges was calculated as the 0.5C discharge capacity. Then, the lithium ion battery was charged at 0.5C direct current to 4.45V, and then held at 4.45V to 0.025C to full charge state, and then discharged at 2C current to 3.0V, and the above charging and discharging process was repeated 3 times, and the average capacity of the three discharges was calculated as the 2C discharge capacity.

[0084] 2C / 0.5C discharge rate = 2C discharge capacity / 0.5C discharge capacity x 100%.

[0085] Example 1-1

[0086] <Preparation of negative electrode current collector>

[0087] The polypropylene colloidal particles (weight average molecular weight Mw = 400,000, purchased from BASF, Germany) were heated to 250°C for melting, and then extruded by a twin-screw extruder, and then cast onto a steel roll at 35°C. First, longitudinal stretching was performed at 130°C by longitudinal traction force, and the stretching ratio was 5. Then, transverse stretching was performed at 160°C by transverse traction force, and the stretching ratio was 8. Then, heat treatment was performed at 175°C for 3h, and finally, the substrate layer film was cooled and shaped. The conductive agent carbon nanotube (CNT) and the binder polymethyl acrylate (Mw = 20,000) were mixed according to the mass ratio of 50:50, and then deionized water was added and stirred uniformly to obtain a conductive coating slurry with a solid content of 25wt%. The conductive coating slurry was uniformly coated on one surface of the substrate layer prepared above, and then dried at 85°C for 4h to obtain a negative electrode current collector coated with a conductive coating on one side. The above steps were repeated on the other surface of the substrate layer to obtain a negative electrode current collector coated with a conductive coating on both sides. Then, the negative electrode current collector was dried at 85°C under vacuum for 4h, and then cold-pressed and cut to obtain the negative electrode current collector. The thickness and transverse tensile strength of the substrate layer, and the thickness of the conductive coating are shown in Table 1.

[0088] <Preparation of negative electrode sheet>

[0089] A negative electrode active material (graphite), a negative electrode conductive agent (conductive carbon black), a negative electrode thickener (carboxymethyl cellulose), and a negative electrode binder (styrene-butadiene rubber) were mixed in a mass ratio of 94:2:2:2. Deionized water was added and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 54 wt%. The negative electrode slurry was uniformly coated onto one surface of the aforementioned negative electrode current collector and dried at 85°C for 4 hours to obtain a negative electrode sheet with a single-sided coating of the negative electrode material layer. The above steps were repeated on the other surface of the negative electrode current collector to obtain a negative electrode sheet with a double-sided coating of the negative electrode material layer. After drying under vacuum at 85°C for 4 hours, the sheet was cold-pressed, cut, and slit to obtain a negative electrode sheet with a size of 78 mm × 875 mm. Finally, conductive silicone conductive adhesive was applied to the surface of the negative electrode material layer, and the negative electrode tab 20 was bonded to the surface of the negative electrode material layer 13 using conductive adhesive 30. Figure 1 As shown in Table 1, the bonding length between the negative electrode tab and the negative electrode sheet, and the thickness of the conductive adhesive are shown in Table 1. The thickness of the negative electrode tab and the thickness of the single-sided negative electrode material layer are shown in Table 2. The compaction density of the negative electrode material layer is 1.75 g / cm³. 3 .

[0090] <Preparation of the positive electrode>

[0091] LiCoO2 (positive electrode active material), conductive carbon black (positive electrode conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 95.2:2.2:2.6. N-methylpyrrolidone was added and stirred until homogeneous, resulting in a positive electrode slurry with a solid content of 72 wt%. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector. The aluminum foil was dried at 85°C for 4 hours to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode material layer. After drying under vacuum at 85°C for 4 hours, the sheet was cold-pressed, cut, and slit to obtain a positive electrode sheet with a size of 74 mm × 867 mm. Finally, positive electrode tabs were welded onto the positive electrode sheet. The compaction density of the positive electrode material layer was 4.15 g / cm³. 3 The thickness of the single-sided positive electrode material layer is 90 μm.

[0092] <Isolation membrane>

[0093] PVDF and alumina ceramic were mixed at a mass ratio of 1:2, and NMP was added as a solvent to prepare a ceramic layer slurry with a solid content of 12 wt%. The slurry was stirred evenly and then uniformly coated onto both surfaces of a 5 μm thick polyethylene (PE) substrate. After drying, a release film with a 2 μm thick alumina ceramic layer on both sides was obtained. PVDF was added to NMP solvent and stirred evenly to prepare a PVDF slurry with a solid content of 25 wt%. Then, a 2.5 mg / 1540.25 mm thick PVDF slurry was coated onto the surface of the alumina ceramic layer.2 PVDF, and dried at 85℃ for 4h to obtain the separator film coated with the alumina ceramic layer and the PVDF adhesive layer on both sides.

[0094] <Preparation of electrolyte>

[0095] In a dry argon atmosphere glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC) and ethyl propionate (EP) were mixed in a mass ratio of EC:PC:DEC:EP = 3:1:3:3, then lithium salt lithium hexafluorophosphate (LiPF6) was added to the organic solvent to dissolve and mix uniformly to obtain an electrolyte with a lithium salt concentration of 1 mol / L.

[0096] <Preparation of lithium ion battery>

[0097] The prepared positive electrode sheet, separator film, negative electrode sheet, and separator film were stacked in order, with the separator film between the positive electrode sheet and the negative electrode sheet to act as a separator, and then wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum plastic film packaging bag and dried in a vacuum oven at 85℃ for 12h to remove water, and the above prepared electrolyte was injected. After vacuum packaging, standing, formation (0.02C constant current charging to 3.5V, and then 0.1C constant current charging to 3.9V), shaping, capacity testing, secondary packaging, and other processes, a lithium ion battery was obtained.

[0098] Examples 1-2 to 1-28

[0099] Except for adjusting the parameters according to Table 1, the rest is the same as Example 1-1. Among them, in Examples 1-15 to 1-17, the transverse tensile strength of the substrate layer was changed by adjusting the weight average molecular weight and crosslinking degree of the polypropylene colloidal particles.

[0100] Example 2-1

[0101] Except for preparing the negative electrode sheet according to the following <Preparation of negative electrode sheet> process, the rest is the same as Example 1-1.

[0102] <Preparation of negative electrode sheet>

[0103] The negative electrode active material graphite, the negative electrode conductive agent conductive carbon black, the negative electrode thickening agent carboxymethyl cellulose, and the negative electrode binder styrene-butadiene rubber were mixed in a mass ratio of 94:2:2:2, deionized water was added and stirred uniformly to obtain a negative electrode slurry with a solid content of 54wt%. The negative electrode slurry was uniformly coated on one surface of the above-mentioned negative electrode current collector, and the negative electrode tab coated with conductive adhesive organic silicone conductive adhesive was embedded between the above-mentioned coated negative electrode slurry, and dried at 85℃ for 4h to obtain a negative electrode sheet coated with a negative electrode material layer on one side, as shown in Figure 3As shown, the negative tab 20 is embedded in the middle of the negative material layer 13. The above steps are repeated on the other surface of the negative current collector, i.e. a negative tab sheet with double-sided negative material layers is obtained. Then after drying at 85°C under vacuum for 4h, the negative tab sheet with a size of 78mm x 875mm is obtained through cold pressing, cutting and slitting. Among them, the compacted density of the negative material layer is 1.75g / cm 3 .

[0104] The bonding length between the negative tab and the negative tab sheet, the thickness of the conductive adhesive, and the thickness of the negative tab are the same as those in Example 1-1. The thickness of the negative tab and the thickness of the single-sided negative material layer are shown in Table 2.

[0105] Example 2-2

[0106] Except that the negative tab sheet is prepared according to the following <Preparation of negative tab sheet> process, the rest is the same as Example 1-1.

[0107] <Preparation of negative tab sheet>

[0108] The negative active material graphite, the negative conductive agent conductive carbon black, the negative thickening agent carboxymethyl cellulose, and the negative binder styrene-butadiene rubber are mixed in a mass ratio of 94:2:2:2, and deionized water is added and stirred uniformly to obtain a negative slurry with a solid content of 54wt%. The conductive adhesive organic silicone conductive adhesive is coated on the surface of the conductive coating, and the negative tab 20 is bonded to the surface of the conductive coating 12 through the conductive adhesive 30, as shown in Figure 4 Then the negative slurry is uniformly coated on one surface of the above-mentioned negative current collector, and is subjected to drying treatment at 85°C for 4h to obtain a negative tab sheet with a single-sided negative material layer. The above steps are repeated on the other surface of the negative current collector, i.e. a negative tab sheet with double-sided negative material layers is obtained. Then after drying at 85°C under vacuum for 4h, the negative tab sheet with a size of 78mm x 875mm is obtained through cold pressing, cutting and slitting. Among them, the compacted density of the negative material layer is 1.75g / cm 3 .

[0109] The bonding length between the negative tab and the negative tab sheet, the thickness of the conductive adhesive, and the thickness of the negative tab are the same as those in Example 1-1. The thickness of the negative tab and the thickness of the single-sided negative material layer (the thickness of the single-sided negative material layer in the non-overlapping area of the thickness direction between the negative material layer and the negative tab) are shown in Table 2.

[0110] Examples 2-3 to 2-7

[0111] Except that the parameters are adjusted according to Table 2, the rest is the same as Example 2-1.

[0112] Comparative Example 1

[0113] The rest is the same as Example 1-1 except that the Cu foil with a thickness of 10 μm is used as the substrate layer, and the negative tab is connected to the negative tab sheet by welding.

[0114] Comparative Example 2

[0115] The rest is the same as Example 1-1 except that the Cu foil with a thickness of 12 μm is used as the negative current collector, and the surface of the copper foil is not coated with a conductive coating, and the negative tab is connected to the negative tab sheet by welding.

[0116] Comparative Examples 3-4

[0117] The rest is the same as Example 1-1 except that the copper metal layer is deposited on the surface of the substrate layer instead of the conductive coating, and the negative tab is connected in the manner shown in Table 1.

[0118] Comparative Examples 5-6

[0119] The rest is the same as Example 1-1 except that the bonding length is adjusted according to Table 1.

[0120] The relevant parameters and properties of each example and each comparative example are shown in Tables 1 and 2.

[0121] Table 1

[0122]

[0123]

[0124] Note: " / " in Table 1 indicates that the corresponding substance or parameter does not exist, and the connection method "bonding" means that the negative tab and the negative tab sheet are bonded to form an electrical connection by conductive adhesive. The connection method "welding" means that the negative tab and the negative tab sheet are connected to form an electrical connection by welding.

[0125] As can be seen from Examples 1-1 to 1-28, Comparative Examples 1 to 6, the negative current collector comprising the substrate layer and the conductive coating, the negative tab and the negative tab are bonded by the conductive adhesive to form an electrical connection and the bonding length is controlled within the scope of the application, which can make the negative tab have a lower impedance, and the lithium ion battery has a lower impedance, a higher impact pass rate and a discharge rate. It is shown that the lithium ion battery has a lower impedance, good safety reliability and discharge rate performance. In Comparative Example 1, the negative current collector is a copper foil, the negative tab is connected to the negative tab by welding, in Comparative Example 2, the negative current collector is a copper foil and no conductive coating is provided, the negative tab is connected to the negative tab by welding, in Comparative Example 3, the conductive coating is replaced by a copper metal layer, and the negative tab is connected to the negative tab by welding. In Comparative Example 4, the conductive coating is replaced by a copper metal layer, and the negative tab is connected to the negative tab by bonding, the bonding length of Comparative Examples 5 and 6 is not within the scope of the application, and the lithium ion batteries of Comparative Examples 1 to 4 and Comparative Example 6 have a lower impact pass rate. The lithium ion battery of Comparative Example 5 has a lower discharge rate, which shows that the safety performance and discharge rate performance of the lithium ion battery are difficult to balance.

[0126] As can be seen from Examples 1-1 to 1-6, the material of the substrate layer within the scope of the application is selected, the resistance of the negative tab is lower, the lithium ion battery has a lower impedance, a higher impact pass rate and a discharge rate, which shows that the lithium ion battery has a lower impedance, good safety reliability and discharge rate performance.

[0127] As can be seen from Examples 1-1, 1-7 to 1-8, Comparative Examples 5 to 6, when the bonding length between the negative tab and the negative tab is too small, for example, Comparative Example 5, the impedance of the negative tab is higher, and the lithium ion battery has a higher impedance and a lower discharge rate. When the bonding length between the negative tab and the negative tab is too large, for example, Comparative Example 6, the lithium ion battery has a lower impact pass rate. It is shown that the safety performance and discharge rate performance of the lithium ion battery are difficult to balance. Therefore, the bonding length between the negative tab and the negative tab is controlled within the scope of the application, which can make the resistance of the negative tab lower, the lithium ion battery has a lower impedance, a higher impact pass rate and a discharge rate. It is shown that the lithium ion battery has a lower impedance, good safety reliability and discharge rate performance.

[0128] The type and resistivity of the conductive adhesive generally affect the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-9 to Example 1-10, the selection of the conductive adhesive within the scope of the present application and the regulation of its resistivity within the scope of the present application, the resistance of the negative electrode sheet is lower, the lithium ion battery has lower impedance, higher impact pass rate and discharge rate, which indicates that the lithium ion battery has lower impedance, good safety reliability and discharge rate performance.

[0129] The thickness of the conductive adhesive generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-11 to Example 1-12, the regulation of the thickness of the conductive adhesive within the scope of the present application, the resistance of the negative electrode sheet is lower, the lithium ion battery has lower impedance, higher impact pass rate and discharge rate, which indicates that the lithium ion battery has lower impedance, good safety reliability and discharge rate performance.

[0130] The thickness of the substrate layer generally affects the safety reliability of the lithium ion battery. As can be seen from Example 1-1, Example 1-13 to Example 1-14, the regulation of the thickness of the substrate layer within the scope of the present application, the resistance of the negative electrode sheet and the impedance of the lithium ion battery have no obvious change, the lithium ion battery has higher impact pass rate, which indicates that the lithium ion battery has good safety reliability.

[0131] The transverse tensile strength of the substrate layer generally affects the safety reliability of the lithium ion battery. As can be seen from Example 1-1, Example 1-15 to Example 1-17, the regulation of the transverse tensile strength of the substrate layer within the scope of the present application, the resistance of the negative electrode sheet and the impedance of the lithium ion battery have no obvious change, the lithium ion battery has higher impact pass rate, which indicates that the lithium ion battery has good safety reliability.

[0132] The type of conductive agent generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-18 to Example 1-19, the selection of the conductive agent within the scope of the present application, the resistance of the negative electrode sheet is lower, the lithium ion battery has lower impedance, higher impact pass rate and discharge rate, which indicates that the lithium ion battery has lower impedance, good safety reliability and discharge rate performance.

[0133] The mass percentage content of the conductive agent generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-20 to Example 1-24, the regulation of the mass percentage content of the conductive agent within the scope of the present application, the resistance of the negative electrode sheet is lower, the lithium ion battery has lower impedance, higher impact pass rate and discharge rate, which indicates that the lithium ion battery has lower impedance, good safety reliability and discharge rate performance.

[0134] The thickness of the conductive coating generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-25 to Example 1-28, the thickness of the conductive coating is regulated within the scope of the present application, the resistance of the negative electrode tab is low, the lithium ion battery has low impedance, high impact pass rate and discharge rate, which indicates that the lithium ion battery has low impedance, good safety reliability and discharge rate performance. In Example 1-28, as the thickness of the conductive coating further increases, the impact pass rate and discharge rate of the lithium ion battery cannot be further improved, and the energy density will also decrease due to the increase in the thickness of the conductive coating, so that when the thickness of the conductive coating is in the range of 0.2 μm to 2 μm, the lithium ion battery has better comprehensive performance.

[0135] Table 2

[0136]

[0137]

[0138] The connection mode of the negative electrode tab generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-2, the negative electrode tab is connected in a mode within the scope of the present application, the resistance of the negative electrode tab is low, the lithium ion battery has low impedance, high impact pass rate and discharge rate, which indicates that the lithium ion battery has low impedance, good safety reliability and discharge rate performance.

[0139] The thickness of the negative electrode tab generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 2-1, Example 2-3 to Example 2-4, Example 2-7, the thickness of the negative electrode tab is regulated within the scope of the present application, the resistance of the negative electrode tab is low, the lithium ion battery has low impedance, high impact pass rate and discharge rate, which indicates that the lithium ion battery has low impedance, good safety reliability and discharge rate performance. As can be seen from Example 2-1 and Example 2-7, when the thickness of the negative electrode tab is greater than the sum of the thicknesses of the conductive coating and the negative electrode material layer, the flatness of the negative electrode tab is poor, which makes the impact pass rate and discharge rate of the lithium ion battery slightly lower, that is, when the thickness of the negative electrode tab is less than or equal to the sum of the thicknesses of the conductive coating and the negative electrode material layer, the lithium ion battery has better safety reliability and discharge rate performance.

[0140] The thickness of the negative tab generally affects the impedance, safety reliability and discharge rate performance of the lithium ion battery. As can be seen from Example 2-1, Example 2-5 to Example 2-6, regulating the thickness of the negative tab is within the scope of the present application, the resistance of the negative tab is lower, the lithium ion battery has lower impedance, higher impact pass rate and discharge rate, which indicates that the lithium ion battery has lower impedance, good safety reliability and discharge rate performance. In addition, it can also be seen that if the negative tab is close to the negative material layer (such as Example 2-5, the thicknesses of the two are the same), the process requirement is higher, the negative tab may slightly protrude from the surface of the negative material layer, resulting in uneven surface of the negative material layer, affecting the impact pass rate and discharge rate of the lithium ion battery.

[0141] It should be noted that the relational terms herein such as first and second and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any such actual relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0142] Each of the embodiments in the specification is described in a relevant manner, and the same and similar parts between the embodiments can be referred to each other, and each embodiment focuses on the difference from other embodiments.

[0143] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery, comprising a negative electrode tab and a negative electrode tab, the negative electrode tab comprising a negative electrode current collector and a negative electrode material layer, the negative electrode current collector comprising a substrate layer and an electrically conductive coating layer disposed on a surface of the substrate layer, a surface of the electrically conductive coating layer being provided with the negative electrode material layer, the negative electrode tab and the negative electrode tab being bonded by an electrically conductive adhesive to form an electrical connection, a bonding length between the negative electrode tab and the negative electrode tab being 6 mm to 10 mm; the electrically conductive coating layer comprising an electrically conductive agent and a binder, a material of the substrate layer being selected from at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid; a mass percentage content of the electrically conductive agent being 30% to 95% based on a mass of the electrically conductive coating layer. The impedance of the negative electrode tab is 7 Ω to 40 Ω. The thickness of the electrically conductive adhesive is 3 μm to 6 μm. The negative electrode tab is bonded to a surface of the negative electrode material layer.

2. The secondary battery according to claim 1, wherein The negative electrode tab is embedded in the negative electrode material layer; or 3. The secondary battery according to claim 1, wherein The conductive adhesive includes at least one of an acrylic conductive adhesive, an epoxy conductive adhesive, or a silicone conductive adhesive; the conductive adhesive has a resistivity of 10 -6 Ω·cm to 10 -4 Ω·cm.

4. The secondary battery according to claim 1, wherein The negative electrode tab is disposed between the negative electrode material layer and the electrically conductive coating layer.

5. The secondary battery according to claim 1, wherein The thickness of the negative electrode tab is 60 μm to 100 μm, the thickness of the negative electrode tab being less than or equal to a sum of thicknesses of the electrically conductive coating layer and the negative electrode material layer.

6. The secondary battery according to claim 1, wherein The substrate layer comprises a first surface and a second surface, the first surface being provided with a first electrically conductive coating layer, a surface of the first electrically conductive coating layer being provided with a first negative electrode material layer; the second surface being provided with a second electrically conductive coating layer, a surface of the second electrically conductive coating layer being provided with a second negative electrode material layer; The negative electrode tab comprises a first negative electrode tab and a second negative electrode tab, the first negative electrode material layer and the first negative electrode tab being bonded by a first electrically conductive adhesive to form an electrical connection, the second negative electrode material layer and the second negative electrode tab being bonded by a second electrically conductive adhesive to form an electrical connection.

7. The secondary battery according to claim 6, wherein The thickness of the substrate layer is 5 μm to 20 μm; and / or the transverse tensile strength of the substrate layer is 130 MPa to 200 MPa.

8. The secondary battery according to claim 1, wherein The thickness of the electrically conductive coating layer is 0.2 μm to 5 μm. The electrically conductive agent comprises at least one of electrically conductive graphite, electrically conductive carbon black, carbon nanotubes, graphene or electrically conductive carbon fibers.

9. The secondary battery according to any one of claims 1 to 7, wherein 12.The secondary battery according to any one of claims 1 to 7, satisfying at least one of the following characteristics:

10. The secondary battery according to any one of claims 1 to 7, wherein (1) the transverse tensile strength of the substrate layer is 150 MPa to 200 MPa; 11. The secondary battery according to any one of claims 1 to 7, wherein (2) the thickness of the electrically conductive coating layer is 0.5 μm to 2 μm; (3) the mass percentage content of the electrically conductive agent is 30% to 50% based on the mass of the electrically conductive coating layer; (4) the thickness of the negative electrode material layer is 60 μm to 150 μm. 13.An electronic device comprising the secondary battery according to any one of claims 1 to 12. ​ ​ ​

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