Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises a negative electrode sheet (10) and a negative electrode tab (20), wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer (13); the negative electrode current collector (11) comprises a substrate layer (11) and a conductive coating (12) arranged on a surface of the substrate layer (11); the negative electrode material layer (13) is arranged on a surface of the conductive coating (12); the negative electrode tab (20) and the negative electrode sheet (11) are electrically connected by means of bonding via a conductive adhesive (30), and the bonding length of the negative electrode tab (20) and the negative electrode sheet (11) is 6 mm to 10 mm; the conductive coating (12) comprises a conductive agent and a binder; and the material of the substrate layer (11) comprises at least one of polypropylene, polyethylene, polyethylene glycol terephthalate, cellulose, polyimide, polyamide, spandex or aramid fibers. The secondary battery provided by the present application has relatively low impedance and good safety, reliability and discharge rate capability.

Description

Secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on March 28, 2024, with application number 202410369883.7 and invention name “A Secondary Battery and Electronic Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of electrochemical technology, and in particular to a secondary battery and an electronic device. Background Art

[0003] 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 and good safety. They are widely used in various fields such as portable energy storage, electronic equipment, and electric vehicles.

[0004] Currently, most soft-pack lithium-ion batteries on the market use copper current collectors for their negative electrode plates. This heavier copper current collectors contribute to the overall mass of the lithium-ion battery, significantly increasing the risk of internal short circuits in the event of an accidental drop or impact with a nail, impacting the battery's safety and reliability. Using polymers as substrate layers to construct current collectors, replacing existing negative current collectors (such as copper foil), can improve the safety of lithium-ion batteries. However, achieving electrical connectivity between the upper and lower surfaces of the polymer substrate layer while ensuring good discharge rate performance (2C discharge capacity / 0.5C discharge capacity) has become a key issue in the development of lithium-ion batteries. Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and an electronic device to improve the safety, reliability and discharge rate performance of the secondary battery. The specific technical solution is as follows:

[0006] In a first aspect, the present application provides a secondary battery comprising a negative electrode plate and a negative electrode tab. The negative electrode plate 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 disposed on a surface of the substrate layer. The conductive coating layer is provided with the negative electrode material layer. The negative electrode tab and the negative electrode plate are bonded together by a conductive adhesive to form an electrical connection. The bond length between the negative electrode tab and the negative electrode plate is 6 mm to 10 mm. The conductive coating layer comprises a conductive agent and a binder. The substrate layer is made of at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex, or aramid. The negative electrode current collector comprises the substrate layer and the conductive coating. The negative electrode tab and the negative electrode plate are bonded together by the conductive adhesive to form an electrical connection. The bond length can be controlled. Within the scope of the present application, leveraging the advantages of the substrate layer can reduce the mass of the negative electrode current collector, making the secondary battery lighter and improving its safety and reliability. Furthermore, the negative electrode plate can have a lower resistance, 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 impedance of the negative electrode plate is 7Ω to 40Ω, indicating that the negative electrode plate has a lower impedance, which is beneficial to 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 conductive adhesive includes at least one of acrylic conductive adhesive, epoxy conductive adhesive or silicone conductive adhesive; the resistivity of the conductive adhesive is 10 -6 Ω·cm to 10 -4 Ω·cm. Selecting a conductive adhesive within the above range and regulating its resistivity within the above range facilitates the bonding of the negative electrode tab and the negative electrode plate to form a good electrical connection, thereby making the secondary battery lighter and improving its safety and reliability. At the same time, it enables the negative electrode plate to have a lower resistance, reduces the overall impedance of the secondary battery, and thus improves the discharge rate performance of the secondary battery.

[0009] 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 this range facilitates the bonding of the negative electrode tab and the negative electrode plate to form a good electrical connection. It also improves the flatness of the negative electrode plate and reduces its impedance, thereby reducing the weight of the secondary battery. This results in a secondary battery with low impedance, high energy density, and good safety, reliability, and discharge rate performance.

[0010] In some embodiments of the present application, the negative electrode tab is bonded to the surface of the negative electrode material layer. Selecting this method to connect the negative electrode tab and the negative electrode material layer can leverage the advantages of the substrate layer, reduce the mass of the negative electrode current collector, make the secondary battery lighter, and improve its safety and reliability. It can also lower the resistance of the negative electrode plate, reduce the overall impedance of the secondary battery, and thus improve the discharge rate performance of the secondary battery.

[0011] 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 disposed between the negative electrode material layer and the conductive coating. Selecting such a method to connect the negative electrode tab and the negative electrode material layer can achieve a higher flatness of the negative electrode plate, reduce the thickness of the secondary battery, and thus increase the energy density of the secondary battery. It also provides a fast path for electrical transmission, resulting in a lower resistance of the negative electrode plate, reducing the overall impedance of the secondary battery, and improving the discharge rate performance of the secondary battery.

[0012] In some embodiments of the present application, the thickness of the negative electrode tab is 60 μm to 100 μm, and 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. Regulating the thickness of the negative electrode tab within the above range and adjusting the thickness of the negative electrode tab to be less than or equal to the sum of the thicknesses of the conductive coating and the negative electrode material layer can maximize the connection function of the negative electrode tab without affecting the energy density of the secondary battery, thereby improving the flatness of the negative electrode tab, making the secondary battery lighter, and improving its safety and reliability.

[0013] In some embodiments of the present application, the substrate layer includes a first surface and a second surface, the first surface being provided with a first conductive coating, the surface of which is provided with a first negative electrode material layer; the second surface being provided with a second conductive coating, the surface of which is provided with a second negative electrode material layer; the negative electrode tab includes a first negative electrode tab and a second negative electrode tab, the first negative electrode material layer and the first negative electrode tab being electrically connected by bonding with a first conductive adhesive, and the second negative electrode material layer and the second negative electrode tab being electrically connected by bonding with a second conductive adhesive. These features are advantageous in reducing the impedance between the negative electrode tab and the negative electrode material layer, thereby reducing the impedance of the secondary battery and improving the discharge rate performance of the secondary battery.

[0014] In some embodiments of the present application, the thickness of the substrate layer is 5 μm to 20 μm. By regulating the thickness of the substrate layer within the above range, the substrate layer can have good mechanical properties, which is beneficial to improving the safety and reliability of the secondary battery. 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 MPa to 200 MPa. Generally, the longitudinal tensile strength of the substrate layer is much higher than the transverse tensile strength. Therefore, regulating 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 current collector ductility and deformation, and help improve the safety and reliability of the secondary battery.

[0015] In some embodiments of the present application, the thickness of the conductive coating is 0.2 μm to 5 μm. By regulating the thickness of the conductive coating within the above range, the negative electrode plate can have a lower resistance, making the secondary battery lighter, and improving the safety and reliability of the secondary battery while also allowing the secondary battery to have a higher energy density.

[0016] 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; the weight percentage of the conductive agent is 5% to 95% based on the weight of the conductive coating. Selecting the conductive agent and regulating its weight percentage within the above range facilitates the construction of a highly conductive conductive network on the surface of the substrate layer, resulting in a lower resistance of the negative electrode current collector, thereby reducing the weight of the secondary battery while improving the safety, reliability, and discharge rate performance of the secondary battery.

[0017] In some embodiments of the present application, the secondary battery meets 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 is 0.5 μm to 2 μm; (3) the mass percentage of the conductive agent based on the mass of the conductive coating is 30% to 50%; (4) the thickness of the negative electrode material layer is 60 μm to 150 μm. Meeting at least one of the above characteristics is conducive to further improving the safety, reliability and discharge rate performance of the secondary battery.

[0018] The second aspect of the present application provides an electronic device comprising the secondary battery provided in the first aspect of the present application. The secondary battery provided in the present application has good safety, reliability and discharge rate performance, so that the electronic device of the present application has good performance and a long service life.

[0019] The present application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode plate and a negative electrode tab. The negative electrode plate includes a negative electrode current collector and a negative electrode material layer. The negative electrode current collector includes a substrate layer and a conductive coating disposed on the surface of the substrate layer. The surface of the conductive coating is provided with a negative electrode material layer. The negative electrode tab and the negative electrode plate are bonded together by a conductive adhesive to form an electrical connection. The bonding length between the negative electrode tab and the negative electrode plate is 6 mm to 10 mm. The conductive coating includes a conductive agent and a binder. The substrate layer is made of at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex, or aramid. The negative electrode current collector includes a substrate layer and a conductive coating. The negative electrode tab and the negative electrode sheet are bonded together by a conductive glue 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 this application, the advantages of the substrate layer can be utilized to reduce the mass of the negative electrode current collector, making the secondary battery lightweight and improving its safety and reliability. At the same time, it can also make the negative electrode sheet have a lower resistance, reduce the overall impedance of the secondary battery, and thus improve the discharge rate performance of the secondary battery.

[0020] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0022] FIG1 is a schematic structural diagram of an electrode assembly according to an embodiment of the present application;

[0023] FIG2 is a schematic structural diagram of an electrode assembly according to another embodiment of the present application;

[0024] FIG3 is a schematic diagram of a connection method between a negative electrode tab and a negative electrode sheet according to an embodiment of the present application;

[0025] FIG4 is a schematic diagram of a connection method between a negative electrode tab and a negative electrode sheet according to another embodiment of the present application.

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

[0027] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0028] It should be noted that in the following description, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0029] The first aspect of the present application provides a secondary battery comprising a negative electrode plate and a negative electrode tab, wherein the negative electrode plate 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 disposed on the surface of the substrate layer, the surface of the conductive coating being provided with a negative electrode material layer, the negative electrode tab and the negative electrode plate being bonded together by a conductive adhesive to form an electrical connection, and the bonding length between the negative electrode tab and the negative electrode plate is 6 mm to 10 mm, for example, the bonding length between the negative electrode tab and the negative electrode plate is 6 mm, 6.5 mm, 7 mm, 7.5 mm, 8 mm, 8.5 mm, 9 mm, 9.5 mm, 10 mm, or a range consisting of any two of these values. The conductive coating 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 discovered that replacing the commonly used copper foil with the aforementioned negative electrode current collector can reduce the risk of short-circuit heating during secondary battery drops and nailing, thereby improving the safety and reliability of the secondary battery. The negative electrode current collector comprises a substrate layer and a conductive coating. The negative electrode tab and the negative electrode plate are bonded together by a conductive adhesive to form an electrical connection. Leveraging the advantages of the substrate layer can reduce the mass of the negative electrode current collector, making the secondary battery lighter and improving its safety and reliability. Furthermore, it can also provide the negative electrode plate with lower resistance, reducing the overall impedance of the secondary battery and thus improving the discharge rate performance of the secondary battery. If the bond length between the negative electrode tab and the negative electrode plate is too short, for example, less than 6 mm, poor bonding between the negative electrode tab and the negative electrode plate can result, preventing a good electrical connection between the negative electrode tab and the negative electrode plate, affecting the discharge rate performance of the secondary battery. In severe cases, severe localized heating can occur within the secondary battery, affecting the safety and reliability of the secondary battery. If the bond length between the negative electrode tab and the negative electrode plate is too long, for example, greater than 10 mm, capacity loss can occur in the secondary battery. Therefore, by regulating the bonding length between the negative electrode tab and the negative electrode sheet within the scope of the present application, the secondary battery can have a high energy density while improving its discharge rate performance.

[0031] In some embodiments, as shown in FIG1 , an electrode assembly 100 includes a negative electrode sheet 10 and a negative electrode tab 20. The negative electrode sheet 10 includes a negative electrode current collector, which includes a substrate layer 11 and a conductive coating 12 disposed on one surface of the substrate layer 11. A negative electrode material layer 13 is disposed on the surface of the conductive coating 12. The negative electrode tab 20 and the negative electrode sheet 10 are bonded together to form an electrical connection via a conductive adhesive 30. These features are beneficial for reducing the impedance between the negative electrode tab and the negative electrode material layer, thereby reducing the overall impedance of the secondary battery and improving the safety, reliability, and discharge rate performance of the secondary battery.

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

[0033] In some embodiments of the present application, the impedance of the negative electrode plate is 7Ω to 40Ω. For example, the impedance of the negative electrode plate can be 7Ω, 10Ω, 13Ω, 16Ω, 20Ω, 23Ω, 25Ω, 28Ω, 30Ω, 33Ω, 35Ω, 37Ω, 40Ω, or a range consisting of any two of these values. This indicates that the negative electrode plate has a low impedance, which is beneficial for reducing 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 acrylic conductive adhesive, epoxy conductive adhesive or silicone conductive adhesive; the resistivity of the conductive adhesive is 10 -6 Ω·cm to 10 -4 Ω·cm, for example, the resistivity of conductive glue 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 consisting of any two of these values. Selecting a conductive adhesive within the above range and regulating its resistivity within the above range facilitates bonding the negative electrode tab and the negative electrode sheet to form a good electrical connection, thereby reducing the weight of the secondary battery and improving its safety and reliability. It also allows the negative electrode sheet to have a lower resistance, reducing the overall impedance of the secondary battery, thereby improving the discharge rate performance of the secondary battery.

[0035] In some embodiments of the present application, the thickness of the conductive adhesive is 3 μm to 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 consisting of any two of these values. Regulating the thickness of the conductive adhesive within the above range is conducive to the adhesion of the negative electrode tab and the negative electrode plate to form a good electrical connection. At the same time, it makes the flatness of the negative electrode plate higher and the negative electrode plate has a lower impedance, which can make the secondary battery lighter and improve its safety and reliability. Thus, the secondary battery has a lower impedance, a higher energy density, and good safety, reliability, and discharge rate performance.

[0036] In some embodiments of the present application, as shown in FIG1 , the negative electrode tab 20 is bonded to the surface of the negative electrode material layer 13. Selecting the above-mentioned method to connect the negative electrode tab and the negative electrode material layer can give full play to the advantages of the substrate layer, reduce the mass of the negative electrode current collector, make the secondary battery lighter and improve its safety and reliability. At the same time, it can also make the negative electrode sheet have a lower resistance, reduce the overall impedance of the secondary battery, and thus improve the discharge rate performance of the secondary battery.

[0037] In some embodiments of the present application, as shown in FIG3 , the negative electrode tab 20 is embedded in the negative electrode material layer 13. Selecting the above method to connect the negative electrode tab and the negative electrode material layer can make the negative electrode sheet have a higher flatness, reduce the thickness of the secondary battery, and thus improve the energy density of the secondary battery. However, embedding the tab in the negative electrode material layer is more difficult to process, and the adhesion is slightly worse than the arrangement of the negative electrode tab adhering to the surface of the negative electrode material layer and the negative electrode tab being arranged between the negative electrode material layer and the conductive coating. Therefore, the discharge rate performance of the secondary battery can be improved, but the improvement effect is slightly worse than the above two arrangements.

[0038] In some embodiments of the present application, as shown in FIG4 , the negative electrode tab 20 is disposed between the negative electrode material layer 13 and the conductive coating 12. Selecting the above-mentioned method to connect the negative electrode tab and the negative electrode material layer can make the negative electrode plate have a higher flatness, reduce the thickness of the secondary battery, and thus improve the energy density of the secondary battery. In addition, this connection method is simple to process and provides a fast channel for electrical transmission, so that the negative electrode plate has a lower resistance, reduces the overall impedance of the secondary battery, and thus can further improve the discharge rate performance of the secondary battery.

[0039] In some embodiments of the present application, the thickness of the negative electrode tab is 60 μm to 100 μm, and 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. For example, the thickness of the negative electrode tab can be 60 μm, 70 μm, 75 μm, 80 μm, 90 μm, 100 μm, or a range consisting of any two values ​​therein. By regulating the thickness of the negative electrode tab within the above range and adjusting the thickness of the negative electrode tab to be less than or equal to the sum of the thicknesses of the conductive coating and the negative electrode material layer, the connecting function of the negative electrode tab can be brought into play without affecting the energy density of the secondary battery, thereby making the flatness of the negative electrode sheet higher, and making the secondary battery lighter and improving its safety and reliability.

[0040] In some embodiments of the present application, the thickness of the substrate layer is 5 μm to 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 consisting of any two of these values. By regulating the thickness of the substrate layer within the above range, the substrate layer can have good mechanical properties, play a supporting role of the substrate layer, and is conducive to improving the safety and reliability of the secondary battery. At the same time, the secondary battery can have a higher energy density.

[0041] In some embodiments of the present application, the transverse tensile strength of the substrate layer is 130MPa to 200MPa, preferably 150MPa to 200MPa. For example, the transverse tensile strength of the substrate layer can be 130MPa, 140MPa, 146MPa, 150MPa, 158MPa, 160MPa, 164MPa, 170MPa, 175MPa, 180MPa, 190MPa, 200MPa or a range consisting of any two of the numerical values. The longitudinal tensile strength of the general substrate layer is much higher than the transverse tensile strength, and the longitudinal tensile strength can meet the process and product requirements. Therefore, by regulating the transverse tensile strength of the substrate layer within the above range, the current collector can have good mechanical properties, reduce the risk of current collector ductility and deformation, and help improve the safety and reliability of the secondary battery.

[0042] In some embodiments of the present application, the thickness of the substrate layer is 5 μm to 20 μm, and the transverse tensile strength of the substrate layer is 130 MPa to 200 MPa, preferably 150 MPa to 200 MPa. Regulating the thickness and transverse tensile strength of the substrate layer within the above ranges can provide the substrate layer with good mechanical properties, which is beneficial for improving the safety and reliability of the secondary battery while also enabling the secondary battery to have a higher 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 consisting of any two of these values. By regulating the thickness of the conductive coating within the above range, the negative electrode plate can have a lower resistance, making the secondary battery lightweight, and being able to improve the safety and reliability of the secondary battery while making the secondary battery have a higher energy density.

[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 fiber; based on the mass of the conductive coating, the mass percentage of the conductive agent is 5% to 95%, preferably 30% to 50%. 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 consisting of any two values ​​therein. Selecting the above-mentioned conductive agent and regulating its mass percentage within the above-mentioned range is conducive to constructing a conductive network with good conductivity on the surface of the substrate layer, so that the negative electrode current collector has a lower resistance, making the secondary battery lightweight while improving the safety, reliability and discharge rate performance of the secondary battery.

[0045] The present application has no particular restrictions 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 may include but is not limited to at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The present application has no particular restrictions on the mass percentage of the binder. For example, based on the mass of the conductive coating, the mass percentage of the binder can be 5% to 95%.

[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 consisting of any two values ​​therein. The above-mentioned "thickness of the negative electrode material layer" refers to the thickness of the single-sided negative electrode material layer. By regulating the thickness of the negative electrode material layer within the above-mentioned range, the negative electrode plate can have better flatness, so that the energy density of the secondary battery is higher, and at the same time, the lithium ions have good ion transport kinetics, so that the secondary battery can have a higher energy density while improving its discharge rate performance.

[0047] In the present application, the substrate layer is a film of the above-mentioned polypropylene or other polymers, which can be purchased and tested in combination with the test method of "Transverse Tensile Strength Test of Substrate Layer" provided in the present application, and a film with the required transverse tensile strength is selected as the substrate layer. The present application has no special restrictions on its preparation method, as long as the purpose of the present application can be achieved. For example, it can be obtained by the following preparation method: the granules of the substrate layer material are heated and melted, and the substrate layer film is obtained through extrusion, casting, longitudinal stretching, transverse stretching, heat treatment, cooling and molding. In the present application, the above-mentioned melting temperature can be 240°C to 270, the casting temperature can be 25°C to 40°C, the longitudinal stretching temperature can be 125°C to 145°C, the longitudinal stretching ratio can be 4.5 to 5.5, the transverse stretching temperature can be 150°C to 170°C, the transverse stretching ratio can be 7.5 to 10, the heat treatment temperature can be 170°C to 180°C, and the heat treatment time can be 1h to 4h. Optionally, the colloid particles can be mixed with pore-forming agents, additives, etc. and then heated and melted. This application has no particular restrictions on this and can be selected and adjusted according to actual needs, as long as the purpose of this application can be achieved. This application has no particular restrictions on the weight-average molecular weight (Mw) of the substrate layer material, as long as the purpose of this application can be achieved. For example, it can be 300,000 to 600,000. This application has no particular restrictions on the type of pore-forming agent, and can be selected according to actual needs, as long as the purpose of this application can be achieved. For example, the pore-forming agent can be white oil. In this application, the above-mentioned additives can include but are not limited to coupling agents, antioxidants, etc. This application has no particular restrictions on the type of the above-mentioned additives, and can be selected according to actual needs, as long as the purpose of this application can be achieved. This application has no particular restrictions on the equipment used in the extrusion process, and can be selected according to actual needs, as long as the purpose of this application can be achieved. For example, a twin-screw extruder can be used for extrusion. In this application, the above-mentioned "longitudinal stretching" refers to stretching along the extrusion direction, and the above-mentioned "transverse stretching" refers to stretching in a direction perpendicular to the extrusion direction. The stretch 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 modified by changing the weight-average molecular weight and degree of crosslinking of the substrate material. When other conditions remain unchanged, increasing the weight-average molecular weight increases the tensile strength of the substrate layer, while decreasing the weight-average molecular weight decreases the tensile strength. When other conditions remain unchanged, increasing the degree of crosslinking increases the tensile strength of the substrate layer, while decreasing the degree of crosslinking decreases the tensile strength of the substrate layer.

[0049] The present application does not particularly limit the preparation method of the negative electrode current collector, as long as it can achieve the purpose of the present application. For example, it can be prepared by the following method: the conductive agent and binder are mixed, deionized water is added and stirred evenly, to obtain a conductive coating slurry with a solid content of 20wt% to 40wt%. The conductive coating slurry is then evenly coated on one surface of the substrate layer, and after drying, a negative electrode current collector with a single-sided conductive coating is obtained. The above coating steps are repeated on the other surface of the substrate layer, and after drying, a negative electrode plate with a double-sided conductive coating is obtained.

[0050] The present application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant are mixed, deionized water is added and stirred evenly, and a negative electrode slurry with a solid content of 50wt% to 75wt% is obtained. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on one side is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on both sides is obtained. After coating is completed, the negative electrode sheet is obtained by cold pressing and cutting.

[0051] In the present application, the negative electrode material layer of the present application comprises a negative electrode active material. The present application has no particular restrictions on the type of negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may comprise natural graphite, artificial graphite, mesophase microcarbon beads (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 structure lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium. The present application has no particular restrictions on 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, as long as the purpose of the present application can be achieved.

[0052] The negative electrode material layer of the present application may also include a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant. The present application does not particularly limit the negative electrode conductive agent and the negative electrode binder, as long as they can achieve the purpose of the present application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes, graphite, carbon fiber, carbon nanowire, graphene, a metal material, or a conductive polymer. The above-mentioned metal material may include, but is not limited to, metal powder and / or metal fiber. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder may 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 dispersant may include sodium carboxymethyl cellulose.

[0053] The secondary battery of the present application also includes a positive electrode sheet. The present application has no special restrictions on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode collector and a positive electrode material layer provided on at least one surface of the positive electrode collector. In the present application, the positive electrode material layer can be provided on one surface in the thickness direction of the positive electrode collector, or on two surfaces in the thickness direction of the positive electrode collector. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode collector, or it can be a partial area of ​​the surface of the positive electrode collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved.

[0054] The present application has no particular restrictions 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 may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector), etc. The present application has no particular restrictions 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 5μm to 20μm. In the present application, the positive electrode material layer includes a positive electrode active material, and the present application has no particular restrictions on the type of 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 embeds and deintercalates lithium ions. In some embodiments, the positive electrode active material may include at least one of lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide or lithium iron manganese phosphate. Lithium nickel cobalt manganese oxide may 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 At least one of O2 (NCM955). The above-mentioned positive electrode active materials may be subjected to doping treatment. In some embodiments, the elements used for doping may include at least one of K, Na, Ca, Mg, B, Al, Co, Si, V, Ga, Sn or Zr. The present application has no particular restriction on the thickness of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 30μm to 120μm. The positive electrode material layer of the present application may also include a positive electrode conductor and a positive electrode binder. The present application has no particular restriction on the positive electrode conductor and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor may be at least one of the above-mentioned negative electrode conductors, and the positive electrode binder may be at least one of the above-mentioned negative electrode binders.

[0055] In the present application, there is no particular restriction on the method for preparing the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder are mixed, N-methylpyrrolidone (NMP) is added and stirred evenly to obtain a positive electrode slurry with a solid content of 65wt% to 85wt%. The positive electrode slurry is evenly coated on one surface of the positive electrode collector, and after drying, a positive electrode sheet coated with a positive electrode material layer on one side is obtained. Then, the above coating steps are repeated on the other surface of the positive electrode collector, and after drying, a positive electrode sheet coated with a positive electrode material layer on both sides is obtained. After coating is completed, the positive electrode sheet is obtained by cold pressing and cutting.

[0056] The secondary battery of the present application also includes an electrolyte. The electrolyte of the present application may include a lithium salt and an organic solvent. The present application has no particular restrictions on the type of lithium salt, as long as the purpose of the present application can be achieved. For example, the lithium salt may 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(oxalatoborate) (LiBOB) or lithium difluorooxalatoborate (LiDFOB). The present application has no particular restrictions on the type of the above-mentioned organic solvent, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvent. The above-mentioned carbonate compound may 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 may include but is not limited to at least one of dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or methyl ethyl carbonate. Above-mentioned cyclic carbonate compound can include but not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate or vinyl ethylene carbonate. Above-mentioned carboxylate compound can include but not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate, gamma-butyrolactone, decanolactone, valerolactone or caprolactone. Above-mentioned ether compound can include but not limited to at least one of ethylene glycol dimethyl ether, dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned 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-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0057] The secondary battery of the present application also includes a separator. The present application has no particular restrictions on the separator, as long as it can achieve the purpose of the present application. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefin (PO) separators, polyester films (such as polyethylene terephthalate (PET) films), cellulose films, polyimide films (PI), polyamide films (PA), spandex or aramid films, etc. At least one of the types of separators may include but is not limited to woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, rolled membranes or spun membranes, etc. The separator of the present application may have a porous structure, and the porous layer is provided on at least one surface of the separator, and the porous layer includes inorganic particles and a binder. The inorganic particles may 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 may include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The present application does not particularly limit the size of the pores of the porous structure, as long as the purpose of the present application can be achieved. For example, the pore size can be 0.01 μm to 1 μm. In the present application, the thickness of the isolation membrane is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the isolation membrane can be 4 μm to 12 μm.

[0058] The secondary battery of the present application also includes a shell for accommodating a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, as well as other components known in the field of secondary batteries. The present application does not limit the above-mentioned other components. The present application has no special restrictions on the shell, and it can be a shell known in the art, as long as it can achieve the purpose of the present application. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal. The present application does not limit the type of metal, and a metal hard shell known in the art can be used, as long as it can achieve the purpose of the present application. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0059] This application does not specifically limit the type of secondary battery; it may include any device that undergoes an electrochemical reaction. For example, secondary batteries may include, but are not limited to, lithium metal secondary batteries, lithium ion batteries, sodium ion batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries. This application does not specifically limit the shape of the secondary battery, as long as it can achieve the purpose of this application.

[0060] The preparation process of a secondary battery is well known to those skilled in the art and is not particularly limited in this application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, separator, negative electrode sheet and separator in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, 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 sheet, separator, negative electrode sheet and separator in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the packaging bag to prevent pressure rise and overcharge and discharge inside the secondary battery.

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

[0062] The present application does not particularly limit the type of electronic device, and the electronic device may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0063] Example

[0064] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0065] Test methods and equipment:

[0066] Sampling method for negative electrode sheet and negative electrode current collector:

[0067] Take a fully discharged lithium-ion battery (discharge process is to discharge to 3.0V at a current of 0.2C), disassemble it, remove the negative electrode, soak it in dimethyl carbonate (DMC) for 20 minutes, rinse it with DMC and acetone, and then place it in an oven and bake it at 80℃ for 12 hours to obtain the negative electrode sample. Scrape off the negative electrode material layer on the surface of the negative electrode, wash the negative electrode current collector with deionized water, and then bake it at 80℃ for 12 hours to 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 and the like can all be sampled using the above-mentioned sampling method.

[0069] Test of the bonding length between the negative electrode tab and the negative electrode sheet:

[0070] Use a tape measure to measure the total length of the negative electrode tab L1mm, and then measure the vertical length L2mm of the negative electrode tab extending beyond the negative electrode sheet. The bonding length between the negative electrode tab and the negative electrode sheet is L3 = L1-L2, in mm.

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

[0072] Use a micrometer to measure the thickness H1μm of the negative electrode current collector, then scrape off the conductive coating on the surface of the substrate layer and measure the thickness H2μm of the substrate layer. The thickness of the conductive coating is H3=H1-H2, in μm.

[0073] Transverse tensile strength test of substrate layer:

[0074] The transverse tensile strength of the substrate layer was tested by a high-speed rail tensile machine. The conductive coating on the surface of the negative electrode current collector was scraped off to obtain a substrate layer sample. The substrate layer sample was punched out with a 20mm×100mm die, and 10 splines were punched out in each group. A circle of wrinkle glue was wrapped around both ends of each spline (for easy clamping with a clamp) to obtain the spline to be tested. The tensile speed of the high-speed rail tensile machine was set to 50mm / min, and the initial clamp spacing was 40mm. The spline to be tested was placed in the middle of the clamp, and the upper and lower ends were clamped with clamps. Then the high-speed rail tensile machine was started to start the tensile test until the sample was broken, and the tensile curve was recorded. Three splines were measured in each group, and the average value was calculated to obtain the transverse tensile strength of the substrate layer.

[0075] Impedance test of negative electrode:

[0076] The impedance of the negative electrode sheet was tested using a resistance meter (Yuanneng Technology). The negative electrode sheet of each embodiment or comparative example was cut into a rectangular sample measuring 20 mm wide by 60 mm long. A separator measuring 25 mm wide by 70 mm long was stacked with the negative electrode sheet sample, which served as insulation. The sheet was then folded in half along the midpoint of its 60 mm long side (i.e., 30 mm from either end of the long side) so that the two wide sides overlapped. Test probes were placed on the upper and lower surfaces along the thickness direction near a corner of the wide side, with the probes 2 mm from each adjacent edge of the negative electrode sheet. The impedance of the sample was then measured. The test was repeated for five samples per group, and the average value was calculated to obtain the resistance of the negative electrode sheet.

[0077] Impedance testing of lithium-ion batteries:

[0078] An electrochemical workstation (Bio-Logic, France) was used to test electrochemical impedance spectroscopy (EIS) to characterize the impedance of the lithium-ion battery. The specific test method was to measure the ratio of the disturbance signal X and the response signal Y with a frequency of 10 μHz to 1 MHz, and obtain the real part Z', imaginary part Z", modulus |Z| and phase angle φ of the impedance 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 testing:

[0080] The safety and reliability of lithium-ion batteries are evaluated by the impact pass rate. The higher the impact pass rate, the better the safety and reliability of the lithium-ion battery. Under a 25°C environment, the lithium-ion battery of the embodiment or comparative example is first directly charged to 4.45V using 0.2C, and then the 4.45V constant voltage is increased to 0.025C until it is fully charged. The fully charged lithium-ion battery is placed on the test table, and a round rod with a diameter of φ15.8mm and a length of 15.8cm is placed at the center of the surface of the lithium-ion battery parallel to the table, with the longitudinal axis of the round rod perpendicular to the test table. A 9.6kg heavy hammer is used to drop vertically and freely from a height of 610mm from the test table onto the upper end of the round rod, and the bottom end of the round rod collides with the lithium-ion battery. Finally, the surface temperature of the lithium-ion battery is tested. Judgment criteria: no fire or heat is passed. 100 lithium-ion batteries are tested for each embodiment or comparative example.

[0081] Impact pass rate = number of lithium ions passing the test / 100×100%.

[0082] Discharge rate performance test:

[0083] The discharge rate performance of the lithium-ion battery is evaluated by the 2C / 0.5C discharge rate. The higher the 2C / 0.5C discharge rate, the better the discharge rate performance of the lithium-ion battery. Under a 25°C environment, the lithium-ion battery of the embodiment or the comparative example is charged to 4.45V with a 0.5C constant current, and then charged to 0.025C at a constant voltage of 4.45V to a fully charged state, and then discharged to 3.0V at a current of 0.5C. The above charging and discharging process is repeated 3 times, and the average capacity of the three discharges is calculated as the 0.5C discharge capacity. Then, it is charged to 4.45V with a 0.5C DC charge, and then charged to 0.025C at a constant voltage of 4.45V to a fully charged state, and then discharged to 3.0V at a current of 2C. The above charging and discharging process is repeated 3 times, and the average capacity of the three discharges is calculated as the 2C discharge capacity.

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

[0085] Example 1-1

[0086] <Preparation of Negative Electrode Current Collector>

[0087] Using a melt process, polypropylene pellets (weight-average molecular weight Mw = 400,000, purchased from BASF, Germany) were heated to 250°C for melting, extruded using a twin-screw extruder, and cast onto a steel rod at 35°C. The film was first stretched longitudinally at 130°C using a longitudinal traction force with a draw ratio of 5, then stretched transversely at 160°C with a draw ratio of 8, and then heat-treated at 175°C for 3 hours. Finally, the film was cooled and formed to obtain a substrate film. Carbon nanotubes (CNTs), a conductive agent, and polymethyl acrylate (Mw = 20,000) as a binder were mixed in a 50:50 mass ratio, deionized water was added, and stirred to obtain a conductive coating slurry with a solid content of 25 wt%. The conductive coating slurry was evenly applied to one surface of the substrate layer prepared above and dried at 85°C for 4 hours to obtain a negative electrode current collector coated on one side with a conductive coating. The above steps were repeated on the other surface of the substrate layer to obtain a negative electrode current collector coated on both sides with a conductive coating. After drying at 85°C under vacuum for 4 hours, the negative electrode current collector was obtained by cold pressing and slitting. 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] Graphite (negative electrode active material), conductive carbon black (negative electrode conductive agent), carboxymethyl cellulose (negative electrode thickener), and styrene-butadiene rubber (negative electrode binder) were mixed in a mass ratio of 94:2:2:2, and deionized water was added and stirred to obtain a negative electrode slurry with a solid content of 54 wt%. The negative electrode slurry was evenly applied to one surface of the negative electrode current collector and dried at 85°C for 4 hours to obtain a negative electrode sheet coated on one side with a 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 coated on both sides with a negative electrode material layer. After drying at 85°C under vacuum for 4 hours, the negative electrode sheet was cold pressed, cut, and slit to obtain a negative electrode sheet with a size of 78 mm x 875 mm. Finally, a conductive silicone 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 via the conductive adhesive 30, as shown in Figure 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 / cm 3 .

[0090] <Preparation of positive electrode sheet>

[0091] The positive electrode active material LiCoO2, the positive electrode conductive agent conductive carbon black, and the positive electrode binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95.2:2.2:2.6, and N-methylpyrrolidone is added and stirred evenly to obtain a positive electrode slurry with a solid content of 72wt%. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and the aluminum foil is dried at 85°C for 4h to obtain a positive electrode sheet coated with a positive electrode material layer on one side. The above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. Then, after drying at 85°C under vacuum conditions for 4h, the positive electrode sheet with a specification of 74mm×867mm is obtained by cold pressing, cutting, and slitting. Finally, the positive electrode aluminum tab is welded on the positive electrode sheet. Among them, the compaction density of the positive electrode material layer is 4.15g / cm 3 The thickness of the single-sided positive electrode material layer is 90μm.

[0092] <Isolation Film>

[0093] PVDF and alumina ceramics were mixed in a mass ratio of 1:2, NMP was added as a solvent, and a ceramic layer slurry with a solid content of 12wt% was prepared. The slurry was evenly coated on both surfaces of a polyethylene (PE) substrate with a thickness of 5μm. After drying, an isolation membrane with a double-sided coating of 2μm alumina ceramic layer was obtained. PVDF was added to the NMP solvent and stirred evenly to prepare a PVDF slurry with a solid content of 25wt%, and then 2.5mg / 1540.25mm was coated on the surface of the alumina ceramic layer. 2The PVDF was dried at 85° C. for 4 hours to obtain an isolation membrane coated with an alumina ceramic layer and a PVDF bonding 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, and then lithium salt lithium hexafluorophosphate (LiPF6) was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1 mol / L.

[0096] <Preparation of lithium-ion batteries>

[0097] The prepared positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order, with the separator placed between the positive and negative electrode sheets to act as an insulator, and then wound to form an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried in an 85°C vacuum oven for 12 hours to remove moisture, and then injected with the prepared electrolyte. After vacuum packaging, standing, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), shaping, capacity testing, and secondary packaging, a lithium-ion battery is obtained.

[0098] Example 1-2 to Example 1-28

[0099] The parameters were adjusted according to Table 1, and the rest were the same as Example 1-1. In Examples 1-15 to 1-17, the weight average molecular weight and crosslinking degree of the polypropylene particles were adjusted to change the transverse tensile strength of the substrate layer.

[0100] Example 2-1

[0101] Except that the negative electrode sheet was prepared according to the following <Preparation of Negative Electrode Sheet> process, the rest was 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 thickener carboxymethyl cellulose, and the negative electrode binder styrene-butadiene rubber were mixed in a mass ratio of 94:2:2:2, and deionized water was added and stirred evenly to obtain a negative electrode slurry with a solid content of 54 wt%. The negative electrode slurry was evenly coated on one surface of the above-mentioned negative electrode current collector, and the negative electrode tab coated with a conductive adhesive (organic silicone conductive adhesive) was embedded in the above-mentioned negative electrode slurry. The negative electrode tab was dried at 85°C for 4 hours to obtain a negative electrode sheet coated with a negative electrode material layer on one side. As shown in Figure 3, the negative electrode tab 20 was embedded in the middle of the negative electrode material layer 13. The above steps were repeated on the other surface of the negative electrode current collector to obtain a negative electrode sheet coated with a negative electrode material layer on both sides. After drying at 85°C under vacuum conditions for 4 hours, the negative electrode sheet with a specification of 78 mm × 875 mm was obtained through cold pressing, cutting, and slitting. The compaction density of the negative electrode material layer is 1.75 g / cm 3 .

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

[0105] Example 2-2

[0106] Except that the negative electrode sheet was prepared according to the following <Preparation of Negative Electrode Sheet> process, the rest was the same as Example 1-1.

[0107] <Preparation of negative electrode sheet>

[0108] The negative electrode active material graphite, the negative electrode conductive agent conductive carbon black, the negative electrode thickener carboxymethyl cellulose, and the negative electrode binder styrene-butadiene rubber were mixed in a mass ratio of 94:2:2:2, and deionized water was added and stirred evenly to obtain a negative electrode slurry with a solid content of 54 wt%. A conductive adhesive (organic silicone adhesive) was applied to the surface of the conductive coating, and the negative electrode tab 20 was bonded to the surface of the conductive coating 12 via the conductive adhesive 30, as shown in Figure 4. The negative electrode slurry was then evenly applied to one surface of the above-mentioned negative electrode current collector and dried at 85°C for 4 hours to obtain a negative electrode sheet coated on one side with a 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 coated on both sides with a negative electrode material layer. After drying under vacuum conditions at 85°C for 4 hours, the negative electrode sheet was cold pressed, cut, and slit to obtain a negative electrode sheet with a size of 78 mm × 875 mm. The compaction density of the negative electrode material layer was 1.75 g / cm 3 .

[0109] The bonding length between the negative electrode tab and the negative electrode sheet and the thickness of the conductive adhesive are the same as those in Example 1-1. The thickness of the negative electrode tab and the thickness of the single-sided negative electrode material layer (the thickness of the single-sided negative electrode material layer in the area where the projection of the negative electrode material layer and the negative electrode tab in the thickness direction does not overlap) are shown in Table 2.

[0110] Example 2-3 to Example 2-7

[0111] Except for adjusting the parameters according to Table 2, the rest is the same as Example 2-1.

[0112] Comparative Example 1

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

[0114] Comparative Example 2

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

[0116] Comparative Example 3 to Comparative Example 4

[0117] Except that a copper metal layer is evaporated on the surface of the substrate layer to replace the conductive coating, and the negative electrode tab is connected as shown in Table 1, the rest is the same as Example 1-1.

[0118] Comparative Example 5 to Comparative Example 6

[0119] Except for adjusting the bonding length according to Table 1, the rest is the same as Example 1-1.

[0120] The relevant parameters and performances of each embodiment and each comparative example are shown in Table 1 and Table 2.

[0121] Table 1 Note: In Table 1, " / " indicates that the corresponding substance or parameter does not exist. "Adhesion" means that the negative electrode tab and the negative electrode sheet are bonded together by conductive adhesive to form an electrical connection. "Welding" means that the negative electrode tab and the negative electrode sheet are connected by welding to form an electrical connection.

[0122] It can be seen from Examples 1-1 to 1-28 and Comparative Examples 1 to 6 that the negative electrode current collector includes a substrate layer and a conductive coating, and the negative electrode tab and the negative electrode plate are bonded to form an electrical connection through a conductive adhesive and the bonding length is regulated within the scope of the present application. This can make the negative electrode plate have a lower impedance, and the lithium-ion battery also has a lower impedance, a higher impact pass rate and a discharge rate, indicating that the lithium-ion battery has a lower impedance, good safety and reliability, and a discharge rate performance. In Comparative Example 1, the negative electrode current collector is copper foil, and the negative electrode tab is connected to the negative electrode sheet by welding. In Comparative Example 2, the negative electrode current collector is copper foil and no conductive coating is provided, and the negative electrode tab is connected to the negative electrode sheet by welding. In Comparative Example 3, the conductive coating is replaced by a copper metal layer, and the negative electrode tab is connected to the negative electrode sheet by welding. In Comparative Example 4, the conductive coating is replaced by a copper metal layer, and the negative electrode tab is connected to the negative electrode sheet by bonding. The bonding lengths of Comparative Examples 5 and 6 are not within the scope of this application. The lithium-ion batteries of Comparative Examples 1 to 4 and 6 have a lower impact pass rate, and the lithium-ion battery of Comparative Example 5 has a lower discharge rate, indicating that it is difficult to take into account both the safety performance and discharge rate performance of lithium-ion batteries.

[0123] It can be seen from Examples 1-1 to 1-6 that by selecting the material of the substrate layer within the scope of this application, the resistance of the negative electrode plate is low, and the lithium-ion battery has a low impedance, a high impact pass rate and a discharge rate, indicating that the lithium-ion battery has a low impedance, good safety and reliability, and a discharge rate performance.

[0124] It can be seen from Example 1-1, Example 1-7 to Example 1-8, Comparative Example 5 to Comparative Example 6 that when the bonding length between the negative electrode tab and the negative electrode sheet is too small, such as in Comparative Example 5, the impedance of the negative electrode sheet is high, and the lithium-ion battery has a high impedance and a low discharge rate. When the bonding length between the negative electrode tab and the negative electrode sheet is too large, such as in Comparative Example 6, the lithium-ion battery has a low impact pass rate. This shows that it is difficult to take into account both the safety performance and the discharge rate performance of the lithium-ion battery. Therefore, by regulating the bonding length between the negative electrode tab and the negative electrode sheet within the scope of this application, the resistance of the negative electrode sheet can be lowered, and the lithium-ion battery has a lower impedance, a higher impact pass rate and a discharge rate, which means that the lithium-ion battery has low impedance, good safety and reliability, and discharge rate performance.

[0125] The type and resistivity of the conductive adhesive generally affect the impedance, safety, reliability, and discharge rate performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-9, and 1-10, selecting a conductive adhesive within the scope of this application and adjusting its resistivity within this scope results in lower resistance of the negative electrode sheet, lower impedance of the lithium-ion battery, higher impact pass rate, and higher discharge rate, demonstrating that the lithium-ion battery has low impedance, good safety, reliability, and discharge rate performance.

[0126] The thickness of the conductive adhesive generally affects the impedance, safety, reliability, and discharge rate performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-11, and 1-12, adjusting the thickness of the conductive adhesive within the scope of this application results in lower resistance of the negative electrode sheet, lower impedance, higher impact pass rate, and higher discharge rate for the lithium-ion battery, demonstrating that the lithium-ion battery has low impedance, good safety, reliability, and discharge rate performance.

[0127] The thickness of the substrate layer generally affects the safety and reliability of lithium-ion batteries. As can be seen from Examples 1-1, 1-13, and 1-14, adjusting the thickness of the substrate layer within the range of this application does not significantly change the resistance of the negative electrode sheet or the impedance of the lithium-ion battery, and the lithium-ion battery has a high impact pass rate, indicating that the lithium-ion battery has good safety and reliability.

[0128] The transverse tensile strength of the substrate layer generally affects the safety and reliability of lithium-ion batteries. As can be seen from Examples 1-1, 1-15, and 1-17, adjusting the transverse tensile strength of the substrate layer within the scope of this application does not significantly change the resistance of the negative electrode sheet or the impedance of the lithium-ion battery, and the lithium-ion battery has a high impact pass rate, indicating that the lithium-ion battery has good safety and reliability.

[0129] The type of conductive agent generally affects the impedance, safety, reliability, and discharge rate performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-18, and 1-19, using conductive agents within the scope of this application results in lower negative electrode sheet resistance, lower impedance, higher impact pass rate, and higher discharge rate for lithium-ion batteries, demonstrating that lithium-ion batteries have lower impedance, better safety, reliability, and better discharge rate performance.

[0130] The weight percentage of the conductive agent generally affects the impedance, safety, reliability, and discharge rate performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-20, and 1-24, by adjusting the weight percentage of the conductive agent within the range of this application, the resistance of the negative electrode sheet is low, and the lithium-ion battery has low impedance, high impact pass rate, and discharge rate, indicating that the lithium-ion battery has low impedance, good safety, reliability, and discharge rate performance.

[0131] The thickness of the conductive coating usually affects the impedance, safety, reliability and discharge rate performance of the lithium-ion battery. It can be seen from Examples 1-1, 1-25 to 1-28 that by regulating the thickness of the conductive coating within the scope of this application, the resistance of the negative electrode plate is low, and the lithium-ion battery has a low impedance, a high impact pass rate and a discharge rate, indicating that the lithium-ion battery has a 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 its energy density will also decrease due to the increase in the thickness of the conductive coating. Therefore, when the thickness of the conductive coating is in the range of 0.2μm to 2μm, the lithium-ion battery can have better overall performance.

[0132] Table 2

[0133] The connection method of the negative electrode tab generally affects the impedance, safety, reliability, and discharge rate performance of a lithium-ion battery. As can be seen from Examples 1-1, 2-1, and 2-2, using the negative electrode tab connection method within the scope of this application results in lower negative electrode tab resistance, lower impedance, higher impact pass rate, and higher discharge rate for the lithium-ion battery, demonstrating that the lithium-ion battery has low impedance, good safety, reliability, and discharge rate performance.

[0134] The thickness of the negative electrode tab usually affects the impedance, safety, reliability and discharge rate performance of the lithium-ion battery. From Example 2-1, Example 2-3 to Example 2-4, Example 2-7, it can be seen that when the thickness of the negative electrode tab is regulated within the scope of this application, the resistance of the negative electrode sheet is low, and the lithium-ion battery has a low impedance, a high impact pass rate and a discharge rate, indicating that the lithium-ion battery has a low impedance, good safety, reliability and discharge rate performance. From Example 2-1 and Example 2-7, it can be seen that when the thickness of the negative electrode tab is greater than the sum of the thickness of the conductive coating and the negative electrode material layer, the flatness of the negative electrode sheet will be poor, and the impact pass rate and discharge rate of the lithium-ion battery will be slightly lower. That is, when the thickness of the negative electrode tab is less than or equal to the sum of the thickness of the conductive coating and the negative electrode material layer, the lithium-ion battery has better safety, reliability and discharge rate performance.

[0135] The thickness of the negative electrode tab usually affects the impedance, safety, reliability and discharge rate performance of the lithium-ion battery. It can be seen from Example 2-1, Example 2-5 to Example 2-6 that by regulating the thickness of the negative electrode tab within the scope of this application, the resistance of the negative electrode sheet is low, and the lithium-ion battery has a low impedance, a high impact pass rate and a discharge rate, indicating that the lithium-ion battery has a low impedance, good safety, reliability and discharge rate performance. In addition, it can be seen that if the negative electrode tab is close to the negative electrode material layer (such as Example 2-5, the two have the same thickness), the process requirements are high, and the negative electrode tab may slightly protrude from the surface of the negative electrode material layer, resulting in an uneven surface of the negative electrode material layer, affecting the impact pass rate and discharge rate of the lithium-ion battery.

[0136] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0137] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0138] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles 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 sheet and a negative electrode tab, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer, wherein the negative electrode current collector comprises a substrate layer and a conductive coating disposed on a surface of the substrate layer, wherein the surface of the conductive coating is provided with the negative electrode material layer. Bonding the negative electrode tab and the negative electrode sheet to form an electrical connection through a conductive adhesive, wherein the bonding length between the negative electrode tab and the negative electrode sheet is 6 mm to 10 mm; The conductive coating layer includes a conductive agent and a binder, and the material of the substrate layer includes at least one of polypropylene, polyethylene, polyethylene terephthalate, cellulose, polyimide, polyamide, spandex or aramid.

2. The secondary battery according to claim 1, wherein The impedance of the negative electrode plate is 7Ω to 40Ω.

3. The secondary battery according to claim 1, wherein The conductive adhesive includes at least one of acrylic conductive adhesive, epoxy resin conductive adhesive or silicone conductive adhesive; the resistivity of the conductive adhesive is 10 -6 Ω·cm to 10 -4 Ω·cm.

4. The secondary battery according to claim 1, wherein The thickness of the conductive adhesive is 3 μm to 6 μm.

5. The secondary battery according to claim 1, wherein The negative electrode tab is adhered to the surface of the negative electrode material layer.

6. The secondary battery according to claim 1, wherein 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.

7. The secondary battery according to claim 6, wherein The thickness of the negative electrode tab is 60 μm to 100 μm, and 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.

8. The secondary battery according to claim 1, wherein The substrate layer includes a first surface and a second surface, the first surface is provided with a first conductive coating, the surface of the first conductive coating is provided with a first negative electrode material layer; the second surface is provided with a second conductive coating, the surface of the second conductive coating is provided with a second negative electrode material layer; The negative electrode tab includes a first negative electrode tab and a second negative electrode tab, the first negative electrode material layer and the first negative electrode tab are electrically connected by bonding with a first conductive adhesive, and the second negative electrode material layer and the second negative electrode tab are electrically connected by bonding with a second conductive adhesive.

9. The secondary battery according to any one of claims 1 to 7, 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.

10. The secondary battery according to any one of claims 1 to 7, wherein The conductive coating has a thickness of 0.2 μm to 5 μm.

11. The secondary battery according to any one of claims 1 to 7, wherein The conductive agent includes at least one of conductive graphite, conductive carbon black, carbon nanotubes, graphene or conductive carbon fiber; Based on the mass of the conductive coating, the mass percentage of the conductive agent is 5% to 95%.

12. The secondary battery according to any one of claims 1 to 7, which 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 is 0.5 μm to 2 μm; (3) Based on the mass of the conductive coating, the mass percentage of the conductive agent is 30% to 50%; (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 claim 1 .

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