Secondary battery, method for manufacturing same, and

Abstract

The invention relates to the technical field of secondary batteries, and provides a secondary battery, a manufacturing method thereof and electric equipment, which are at least beneficial to improving the performance of the secondary battery. The secondary battery comprises a positive plate, a negative plate and a diaphragm, the diaphragm is located between the positive plate and the negative plate, and the positive plate comprises a positive current collector and a first conductive layer and a second conductive layer which are sequentially stacked on the surface of the positive current collector; wherein the first conductive layer is made of a first nano metal wire, first graphene and first carbon-coated copper powder, and the second conductive layer is made of a second nano metal wire, second graphene and second carbon-coated copper powder; the diameter of the first nano metal wire is smaller than that of the second nano metal wire, and / or the length-diameter ratio of the first nano metal wire is smaller than that of the second nano metal wire; and the number of sheet layers of the first graphene is smaller than that of sheet layers of the second graphene, and / or the diameter of the sheet layers of the first graphene is smaller than that of the sheet layers of the second graphene.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and particularly relates to a secondary battery, a manufacturing method thereof, and an electrical device. Background Art

[0002] With the rapid development of fields such as new energy vehicles and electronic products, lithium batteries are widely used due to their advantages such as relatively high energy density, long cycle life, and environmental friendliness.

[0003] In a secondary battery, the electrical conductivity of the positive current collector directly affects the internal resistance, energy density, and cycle life of the secondary battery. Usually, a conductive material is provided on the positive current collector to improve the electrical conductivity of the positive current collector. The conductive material uses a carbon-based material (such as conductive carbon black, graphite, etc.). The carbon-based material has problems such as insufficient electrical conductivity, poor interfacial adhesion, low corrosion resistance, and complex processes, which limit the application scenarios of secondary batteries.

[0004] Therefore, improving the positive current collector to make it have high electrical conductivity, strong adhesion, corrosion resistance, and excellent stability is an important means to improve the performance of secondary batteries. Summary of the Invention

[0005] Embodiments of the present application provide a secondary battery, a manufacturing method thereof, and an electrical device, which are at least beneficial to improving the performance of the secondary battery.

[0006] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a secondary battery, including: a positive electrode sheet, a negative electrode sheet, and a separator. The separator is located between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet includes: a positive current collector; a first conductive layer located on the surface of the positive current collector; a second conductive layer located on the surface of the first conductive layer away from the positive current collector. Wherein, the material of the first conductive layer includes a first nano metal wire, a first graphene, and a first copper powder coated with carbon; the material of the second conductive layer includes a second nano metal wire, a second graphene, and a second copper powder coated with carbon; the diameter of the first nano metal wire is smaller than the diameter of the second nano metal wire, and / or, the aspect ratio of the first nano metal wire is smaller than the aspect ratio of the second nano metal wire; and, the number of layers of the first graphene is smaller than the number of layers of the second graphene, and / or, the layer diameter of the first graphene is smaller than the layer diameter of the second graphene.

[0007] In some embodiments, the diameter of the first nano metal wire is greater than or equal to 10 nm and less than or equal to 20 nm; the diameter of the second nano metal wire is greater than 20 nm and less than or equal to 50 nm.

[0008] In some embodiments, the aspect ratio of the first nano metal wire is greater than or equal to 1000 and less than or equal to 1500; the aspect ratio of the second nano metal wire is greater than 1500 and less than or equal to 2000.

[0009] In some embodiments, the number of layers of the first graphene is 3 to 5 layers; the number of layers of the second graphene is 4 to 8 layers.

[0010] In some embodiments, the layer diameter of the first graphene is 5 μm to 15 μm; the layer diameter of the second graphene is 10 μm to 20 μm.

[0011] In some embodiments, the average particle size of the first carbon-coated copper powder is smaller than that of the second carbon-coated copper powder.

[0012] In some embodiments, in the first conductive layer, the total mass ratio of the first graphene and the first nano metal wire to the first conductive layer is greater than the mass ratio of the first carbon-coated copper powder to the first conductive layer; in the second conductive layer, the total mass ratio of the second graphene and the second nano metal wire to the second conductive layer is greater than the mass ratio of the second carbon-coated copper powder to the second conductive layer.

[0013] In some embodiments, in the first conductive layer, the mass ratio of the first graphene to the total mass of the first graphene and the first nano metal wire is the first ratio; in the second conductive layer, the mass ratio of the second graphene to the total mass of the second graphene and the second nano metal wire is the second ratio, and the first ratio is greater than the second ratio.

[0014] In some embodiments, the positive electrode sheet further includes: a third conductive layer, the third conductive layer is located on the surface of the second conductive layer away from the first conductive layer, and the material of the third conductive layer includes a third nano metal wire, a third graphene, a third carbon-coated copper powder and silica, the diameter of the third nano metal wire is smaller than that of the first nano metal wire, and / or, the aspect ratio of the third nano metal wire is smaller than that of the first nano metal wire.

[0015] In some embodiments, the number of layers of the third graphene is smaller than that of the second graphene and greater than that of the first graphene; and / or, the layer diameter of the third graphene is smaller than that of the second graphene and greater than that of the first graphene.

[0016] In some embodiments, the average particle size of the third carbon-coated copper powder is smaller than that of the second carbon-coated copper powder and greater than that of the first carbon-coated copper powder.

[0017] According to some embodiments of the present application, on the other hand, an embodiment of the present application further provides a method for manufacturing a secondary battery, including: preparing a positive electrode sheet, and the preparation steps include: providing a positive electrode current collector; preparing a first slurry, dispersing a first nano metal wire, a first graphene, and a first copper powder coated with carbon in a solvent, adding a binder and stirring evenly; coating the first slurry on the surface of the positive electrode current collector by a doctor blade coating method, and drying by a thermal curing method to form a first conductive layer; preparing a second slurry, dispersing a second nano metal wire, a second graphene, and a second copper powder coated with carbon in a solvent, adding a binder and stirring evenly; wherein, the diameter of the first nano metal wire is smaller than that of the second nano metal wire, and / or, the aspect ratio of the first nano metal wire is smaller than that of the second nano metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and / or, the layer diameter of the first graphene is smaller than that of the second graphene; coating the second slurry on the surface of the first conductive layer away from the positive electrode current collector by a doctor blade coating method, and drying by a thermal curing method to form a second conductive layer; providing a negative electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet, and then placing them in a housing, and injecting an electrolyte to form a secondary battery.

[0018] In some embodiments, the first slurry includes the following components in parts by mass: 10% - 30% of the first nano metal wire, 5% - 15% of the first graphene, 5% - 10% of the first copper powder coated with carbon, 20% - 40% of the binder, and the balance of deionized water; the second slurry includes the following components in parts by mass: 10% - 30% of the second nano metal wire, 5% - 15% of the second graphene, 5% - 10% of the second copper powder coated with carbon, 20% - 40% of the binder, and the balance of deionized water. The proportion of the above components is the mass percentage of each component in the first slurry.

[0019] In some embodiments, the process parameters of the doctor blade coating method include: the blade gap is 50μm - 200μm, and the moving speed is 1m / min - 10m / min; the process parameters of the thermal curing method include: the temperature is 120°C - 250°C, and the time is 10min - 60min.

[0020] According to some embodiments of the present application, on yet another aspect, an embodiment of the present application further provides an electrical device, which includes the secondary battery in the above embodiments, or the secondary battery prepared by the method for manufacturing a secondary battery in the above embodiments; or, the electrical device includes an energy storage system, and the energy storage system includes the secondary battery in the above embodiments, or the secondary battery prepared by the method for manufacturing a secondary battery in the above embodiments.

[0021] The technical solutions provided by the embodiments of the present application have at least the following advantages: In the secondary battery provided by the embodiment of the present application, the positive electrode sheet includes a positive electrode current collector, and a first conductive layer and a second conductive layer stacked on the positive electrode current collector in sequence. The materials of the first conductive layer and the second conductive layer both include graphene, nano metal wires, and carbon-coated copper powder. The nano metal wires are intertwined with the graphene sheets to form an intertwined conductive network, enabling the carbon-coated copper powder to better connect with the nano metal wires and graphene. Good electrical contact can also be maintained between the loose carbon-coated copper powder particles. Moreover, the carbon-coated copper powder can also reduce the porosity. The three-dimensional conductive network composed of "point-line-plane" is conducive to the rapid conduction of electrons, reduces battery polarization, and lowers the internal resistance of the secondary battery, thereby being beneficial to improving the charge and discharge efficiency and cycle stability of the secondary lithium battery. In addition, the first conductive layer close to the positive electrode current collector serves as the inner layer, and the second conductive layer far from the positive electrode current collector serves as the outer layer. The inner layer uses first nano metal wires with a smaller diameter (and / or a lower aspect ratio) compared to the outer layer to form a high-density conductive network, reducing the interfacial resistance between the positive electrode current collector and the active material and ensuring the rapid transmission of electrons. The outer layer uses second nano metal wires with a larger diameter (and / or a higher aspect ratio) compared to the inner layer to form a long-range continuous conductive network, enhancing the overall electron conduction ability of the positive electrode sheet and reducing the distortion phenomenon during charge and discharge. The gradient conductive network realizes the improvement of the overall conductive performance of the positive electrode sheet. The inner layer uses first graphene with fewer sheets (and / or a smaller diameter) compared to the outer layer, which is beneficial for exposing more active edge sites, shortening the lithium ion diffusion path, and enhancing the reaction kinetics. The outer layer uses second graphene with more sheets (and / or a larger diameter) compared to the inner layer to form a porous structure, promoting the penetration of the electrolyte into the interior of the positive electrode sheet and avoiding the problem of lithium ion transmission being blocked due to excessive densification, optimizing the diffusion of lithium ions and the infiltration effect of the electrolyte. The second nano metal wires with a larger diameter and / or aspect ratio and the second graphene with a larger number of sheets and / or sheet diameter in the outer layer can also provide mechanical support, relieve the volume expansion of the active material during charge and discharge, reduce the pulverization and cracking problems of the positive electrode sheet, enhance the mechanical stability of the positive electrode sheet, and thus be beneficial to improving the cycle performance of the secondary battery. Description of the Drawings

[0022] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplary illustrations do not limit the embodiments unless otherwise stated. The figures in the drawings do not constitute a proportional limitation. To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0023] Figure 1Schematic diagram of the structure of a positive electrode sheet provided by an embodiment of the present application; Figure 2 Schematic diagram of the structure of a winding of graphene and nano metal wires provided by an embodiment of the present application.

[0024] Explanation of reference numerals: 101, positive electrode current collector; 102, first conductive layer; 103, second conductive layer; 201, graphene; 202, nano metal wire. Detailed implementation manners

[0025] As can be seen from the background art, improving the positive electrode current collector to make it have high conductivity, strong adhesion, corrosion resistance and excellent stability is an important means to improve the performance of secondary batteries.

[0026] The embodiments of the present application provide a secondary battery, its manufacturing method and an electrical device. On the positive electrode current collector of the secondary battery, through the gradient structure design of the conductive layer, the conductive network, ion channels and mechanical properties are precisely regulated at the microscopic scale, realizing the coordinated improvement of the high conductivity and long cycle performance of the secondary battery.

[0027] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features.

[0028] In the description of the embodiments of the present application, the meaning of "a plurality" is more than two, unless otherwise specifically defined.

[0029] Referring to "embodiment" in this article means that the specific features, structures or characteristics described in connection with the embodiment may be included in at least one embodiment of the present application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art understand explicitly and implicitly that the embodiments described herein can be combined with other embodiments.

[0030] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may mean: there is A, there is both A and B, and there is B. In addition, the character " / " in this article generally means that the associated objects before and after are in an "or" relationship.

[0031] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise stated, other components are not excluded, and other components may further be included.

[0032] The terms used in the description of the various embodiments herein are for the purpose of describing particular embodiments only and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "component" is also intended to include the plural form unless the context clearly indicates otherwise.

[0033] The embodiments of the present application will be described in detail below with reference to the accompanying drawings. However, those of ordinary skill in the art can understand that in the various embodiments of the present application, many technical details are provided to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0034] Figure 1 Schematic diagram of the structure of a positive electrode sheet provided by an embodiment of the present application; Figure 2 Schematic diagram of the structure of a wrapped graphene and nanometal wire provided by an embodiment of the present application.

[0035] The secondary battery provided by the embodiment of the present application includes: a positive electrode sheet, a negative electrode sheet, and a separator, and the separator is located between the positive electrode sheet and the negative electrode sheet.

[0036] Reference Figure 1 , the positive electrode sheet includes: a positive electrode current collector 101, a first conductive layer 102, and a second conductive layer 103.

[0037] The first conductive layer 102 is located on the surface of the positive electrode current collector 101, and the material of the first conductive layer 102 includes a first nanometal wire, a first graphene, and a first carbon-coated copper powder.

[0038] The second conductive layer 103 is located on the surface of the first conductive layer 102 away from the positive electrode current collector 101, and the material of the second conductive layer 103 includes a second nanometal wire, a second graphene, and a second carbon-coated copper powder.

[0039] Wherein, the diameter of the first nanometal wire is smaller than the diameter of the second nanometal wire, and / or, the aspect ratio of the first nanometal wire is smaller than the aspect ratio of the second nanometal wire; and, the number of layers of the first graphene is smaller than the number of layers of the second graphene, and / or, the layer diameter of the first graphene is smaller than the layer diameter of the second graphene.

[0040] For example, the following situations: (1) The diameter of the first nanometal wire is smaller than the diameter of the second nanometal wire, and the aspect ratio of the first nanometal wire is smaller than the aspect ratio of the second nanometal wire; and, the number of layers of the first graphene is smaller than the number of layers of the second graphene, and the layer diameter of the first graphene is smaller than the layer diameter of the second graphene.

[0041] (2) The diameter of the first nanometer metal wire is smaller than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is smaller than that of the second nanometer metal wire; and, the number of layers of the first graphene is greater than that of the second graphene, and the layer diameter of the first graphene is smaller than that of the second graphene.

[0042] (3) The diameter of the first nanometer metal wire is smaller than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is smaller than that of the second nanometer metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and the layer diameter of the first graphene is greater than that of the second graphene.

[0043] (4) The diameter of the first nanometer metal wire is smaller than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is greater than that of the second nanometer metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and the layer diameter of the first graphene is smaller than that of the second graphene.

[0044] (5) The diameter of the first nanometer metal wire is smaller than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is greater than that of the second nanometer metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and the layer diameter of the first graphene is greater than that of the second graphene.

[0045] (6) The diameter of the first nanometer metal wire is smaller than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is greater than that of the second nanometer metal wire; and, the number of layers of the first graphene is greater than that of the second graphene, and the layer diameter of the first graphene is smaller than that of the second graphene.

[0046] (7) The diameter of the first nanometer metal wire is greater than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is smaller than that of the second nanometer metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and the layer diameter of the first graphene is smaller than that of the second graphene.

[0047] (8) The diameter of the first nanometer metal wire is greater than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is smaller than that of the second nanometer metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and the layer diameter of the first graphene is greater than that of the second graphene.

[0048] (9) The diameter of the first nanometer metal wire is greater than that of the second nanometer metal wire, and the aspect ratio of the first nanometer metal wire is smaller than that of the second nanometer metal wire; and, the number of layers of the first graphene is greater than that of the second graphene, and the layer diameter of the first graphene is smaller than that of the second graphene.

[0049] In the secondary battery provided by the embodiment of the present application, the positive electrode sheet includes a positive electrode current collector 101, a first conductive layer 102 and a second conductive layer 103 that are sequentially stacked on the positive electrode current collector 101. The materials of the first conductive layer 102 and the second conductive layer 103 both include graphene, nano metal wires and carbon-coated copper powder. The first conductive layer 102 and the second conductive layer 103 are used to improve the electrical conductivity of the positive electrode current collector 101. Refer to Figure 2 , the nano metal wires 202 are intertwined with the sheets of graphene 201 to form an intertwined conductive network, enabling the carbon-coated copper powder to better connect with the nano metal wires and graphene. Good electrical contact can also be maintained between the loose carbon-coated copper powder particles. Moreover, the carbon-coated copper powder can also reduce the porosity. The three-dimensional conductive network composed of "point-line-plane" is conducive to the rapid conduction of electrons, reducing battery polarization, lowering the internal resistance of the secondary battery, and thus being beneficial to improving the charge and discharge efficiency and cycle stability of the secondary lithium battery. In addition, the first conductive layer 102 close to the positive electrode current collector 101 serves as the inner layer, and the second conductive layer 103 far from the positive electrode current collector 101 serves as the outer layer. The inner layer uses first nano metal wires with a finer diameter (and / or a lower aspect ratio) compared to the outer layer to form a high-density conductive network, reducing the interfacial resistance between the positive electrode current collector and the active material and ensuring the rapid transmission of electrons; the outer layer uses second nano metal wires with a coarser diameter (and / or a higher aspect ratio) compared to the inner layer to form a long-range continuous conductive network, enhancing the overall electron conduction ability of the positive electrode sheet and reducing the distortion phenomenon during charge and discharge. The gradient conductive network realizes the improvement of the overall electrical conductivity of the positive electrode sheet. The inner layer uses first graphene with fewer sheets (and / or a smaller diameter) compared to the outer layer, which is beneficial to exposing more active edge sites, shortening the lithium ion diffusion path, and enhancing the reaction kinetics; the outer layer uses second graphene with more sheets (and / or a larger diameter) compared to the inner layer to form a porous structure, promoting the penetration of the electrolyte into the interior of the positive electrode sheet and avoiding the problem of lithium ion transmission being blocked due to excessive densification, optimizing the diffusion of lithium ions and the infiltration effect of the electrolyte. The second nano metal wires with a larger diameter and / or aspect ratio and the second graphene with a larger number of sheets and / or sheet diameter in the outer layer can also provide mechanical support, relieve the volume expansion of the active material during charge and discharge, reduce the pulverization and cracking problems of the positive electrode sheet, enhance the mechanical stability of the positive electrode sheet, and thus be beneficial to improving the cycle performance of the secondary battery.

[0050] The positive electrode sheet further includes a positive electrode coating (not shown in the figure), the positive electrode coating is located on the surface of the second conductive layer 103 away from the first conductive layer 102, and the positive electrode coating includes a positive electrode active material, a conductive agent, and a binder. That is, the positive electrode current collector 101, the first conductive layer 102, and the second conductive layer 103 are used to form the base layer of the positive electrode coating, and the first conductive layer 102 and the second conductive layer 103 are used to improve the conductivity of the positive electrode current collector 101, so as to improve the electron conduction ability between the positive electrode coating and the positive electrode current collector 101, and further improve the cycle performance of the secondary battery.

[0051] The positive electrode active material includes a layered structure LiM x O2 (M = Co, Ni, Mn) positive electrode active material, a spinel structure LiMn2O4 positive electrode active material, or an olivine structure LiFePO4 positive electrode active material. The conductive agent includes at least one of Super P, acetylene black, Super S, KS-6, KS-15, SFG-6, SFG-15, or Ketjen black. The binder includes at least one of polyvinylidene fluoride (PVDF), polyimide (PI), polyacrylic acid (PAA), and carboxymethyl cellulose (CMC).

[0052] When the diameter or aspect ratio of the nano metal wire is too large, it is easy to cause excessive winding, resulting in too large porosity and affecting the mechanical strength of the conductive layer; when the diameter or aspect ratio of the nano metal wire is small, it is easy to cause insufficient density of the network nodes.

[0053] In the first conductive layer 102, the diameter of the first nano metal wire is greater than or equal to 10 nm and less than or equal to 20 nm, for example, it can be specifically 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm.

[0054] In the second conductive layer 103, the diameter of the second nano metal wire is greater than 20 nm and less than or equal to 50 nm, for example, it can be specifically 21 nm, 23 nm, 28 nm, 30 nm, 33 nm, 36 nm, 39 nm, 40 nm, 42 nm, 45 nm, 48 nm, or 50 nm.

[0055] In the first conductive layer 102, the aspect ratio of the first nano metal wire is greater than or equal to 1000 and less than or equal to 1500, for example, it can be specifically 1000, 1111, 1234, 1350, 1480, or 1500, etc.

[0056] In the second conductive layer 103, the aspect ratio of the second nano metal wire is greater than 1500 and less than or equal to 2000, for example, it can be specifically 1540, 1630, 1745, 1852, 1933, or 2000.

[0057] It should be noted that in the first conductive layer 102, the diameters or aspect ratios of different single first nano metal wires may be different. The diameter of the first nano metal wire refers to the average diameter of the first nano metal wire per unit mass, and the aspect ratio of the first nano metal wire refers to the average aspect ratio of the first nano metal wire per unit volume. Similarly, in the second conductive layer 103, the diameter of the second nano metal wire refers to the average diameter of the second nano metal wire per unit mass, and the aspect ratio of the second nano metal wire refers to the average aspect ratio of the second nano metal wire per unit volume.

[0058] When the number of graphene sheets is small, the conductivity is high, but the mechanical strength is poor, and problems such as wrinkles and cracks in the conductive layer are likely to occur; when the number of graphene sheets is too large, it is easy to cause an interlayer contact resistance, which in turn leads to an increase in the resistivity of the positive electrode sheet.

[0059] In the first conductive layer 102, the number of graphene sheets of the first graphene is 3 to 5 layers, for example, it can specifically be 3 layers, 4 layers, or 5 layers.

[0060] In the second conductive layer 103, the number of graphene sheets of the second graphene is 4 to 8 layers, for example, it can specifically be 4 layers, 5 layers, 6 layers, 7 layers, or 8 layers.

[0061] When the sheet diameter of graphene is too large, it is easy to form a continuous conductive surface, but there is interfacial stress concentration; when the sheet diameter of graphene is too small, the number of interfaces can be increased, but the loss of electron transition will increase accordingly.

[0062] In the first conductive layer 102, the sheet diameter of the first graphene is 5 μm to 15 μm, for example, it can specifically be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or 15 μm.

[0063] In the second conductive layer 103, the sheet diameter of the second graphene is 10 μm to 20 μm, for example, it can specifically be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm.

[0064] It should be noted that in the first conductive layer, the number of graphene sheets and the sheet diameter corresponding to different first graphene may be different. The number of graphene sheets of the first graphene refers to the average number of graphene sheets per unit mass of the first graphene, and the sheet diameter of the first graphene refers to the average sheet diameter per unit mass of the first graphene. Similarly, in the second conductive layer 103, the number of graphene sheets of the second graphene refers to the average number of graphene sheets per unit mass of the second graphene, and the sheet diameter of the second graphene refers to the average sheet diameter per unit mass of the second graphene.

[0065] In some embodiments, the ratio of the sheet diameter of graphene to the length of the nano metal wire is 0.4 to 0.6, for example, specifically 0.4, 0.45, 0.5, 0.55 or 0.6. That is, in the first conductive layer 102, the ratio of the sheet diameter of the first graphene to the length of the first nano metal wire is 0.4 to 0.6; in the second conductive layer 103, the ratio of the sheet diameter of the second graphene to the length of the second nano metal wire is 0.4 to 0.6. When the ratio of the sheet diameter of graphene to the length of the nano metal wire is within the above range, it is beneficial to keep the winding degree of graphene and the nano metal wire within an appropriate range, avoiding problems such as excessive porosity caused by over-winding or insufficient network node density caused by under-winding.

[0066] In some embodiments, the average particle size of the first carbon-coated copper powder is smaller than that of the second carbon-coated copper powder. On the one hand, the average particle size of the carbon-coated copper powder in the inner layer is smaller than that in the outer layer, which is beneficial for matching the nano metal wires with a finer diameter (and / or a lower aspect ratio) and graphene with fewer sheets (and / or a smaller diameter) in the inner layer compared to the outer layer, so as to form a continuous three-dimensional conductive network structure; on the other hand, the smaller particle size of the carbon-coated copper powder in the inner layer can reduce the proportion of ineffective mass, improve the utilization rate of active materials, allow a higher loading of active materials, and indirectly increase the energy density.

[0067] The average particle size refers to the particle size corresponding to the cumulative particle size distribution percentage of a sample reaching 50%.

[0068] In the first conductive layer 102, the average particle size of the first carbon-coated copper powder is greater than or equal to 1 μm and less than or equal to 2 μm, for example, specifically 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm. The average particle size herein refers to the average particle size corresponding to the first carbon-coated copper powder per unit mass.

[0069] In the second conductive layer 103, the average particle size of the second carbon-coated copper powder is greater than or equal to 1.5 μm and less than or equal to 2.5 μm, for example, specifically 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm or 2.5 μm.

[0070] In some embodiments, in the first conductive layer 102, the total mass ratio of the first graphene and the first nanometal wire to the mass ratio of the first conductive layer 102 is greater than the mass ratio of the first carbon-coated copper powder to the mass ratio of the first conductive layer 102; in the second conductive layer 103, the total mass ratio of the second graphene and the second nanometal wire to the mass ratio of the second conductive layer 103 is greater than the mass ratio of the second carbon-coated copper powder to the mass ratio of the second conductive layer 103. In other words, the conductive layer uses nanometal wires and graphene as the main conductive materials and carbon-coated copper powder as the auxiliary conductive material, which is conducive to forming a more continuous three-dimensional conductive skeleton, reducing the electron transmission resistance inside the positive electrode sheet, and reducing the mass ratio of carbon-coated copper powder can reduce its agglomeration risk in the cycle and avoid local insufficient conductivity.

[0071] In some embodiments, in the first conductive layer 102, the mass ratio of the first graphene to the total mass of the first graphene and the first nanometal wire is a first ratio; in the second conductive layer 103, the mass ratio of the second graphene to the total mass of the second graphene and the second nanometal wire is a second ratio, and the first ratio is greater than the second ratio. In other words, in the first conductive layer 102, the first graphene accounts for a higher proportion in the main conductive material; in the second conductive layer 103, the second nanometal wire accounts for a higher proportion in the main conductive material. Graphene has extremely high intrinsic conductivity and ultra-large specific surface area. A higher proportion of graphene in the first conductive layer 102 can quickly establish a high-density, short-range electron transmission channel, reduce the interface resistance between the positive electrode current collector and the active material, and ensure that electrons are efficiently injected from the positive electrode current collector into the positive electrode sheet; the one-dimensional structure of nanometal wires is easier to form a long-range continuous conductive network. The proportion of nanometal wires in the second conductive layer 103 is higher, and the conductive path can be mainly extended to the surface of the positive electrode sheet with nanometal wires, reducing the overall electron transmission impedance of the positive electrode sheet. At the same time, its rigid structure provides mechanical support for the second conductive layer 103 and suppresses electrode deformation.

[0072] In some embodiments, the thickness of the first conductive layer 102 is 2 μm to 5 μm, for example, 2 μm, 3 μm, 4 μm or 5 μm. The thickness of the second conductive layer 103 is 2 μm to 5 μm, for example, 2 μm, 3 μm, 4 μm or 5 μm.

[0073] In some embodiments, the thickness of the first conductive layer 102 is equal to the thickness of the second conductive layer 103 .

[0074] In some embodiments, the positive electrode sheet may further include: a third conductive layer (not shown), the third conductive layer is located on the surface of the second conductive layer away from the first conductive layer, and the material of the third conductive layer includes third nanometer metal wires, third graphene, third carbon-coated copper powder, and silicon dioxide. The diameter of the third nanometer metal wires is smaller than that of the first nanometer metal wires, and / or the aspect ratio of the third nanometer metal wires is smaller than that of the first nanometer metal wires. When the third conductive layer is used as the outermost layer of the positive electrode sheet, the diameter and / or aspect ratio of the third nanometer metal wires are relatively small to form short-cut fibers, which are used to maintain local conductivity while reducing the surface roughness of the positive electrode sheet. At the same time, silicon dioxide is added to fill the surface pores, improving the corrosion resistance and wear resistance of the positive electrode sheet.

[0075] In some embodiments, the length of the third nanometer metal wires is less than 5 μm.

[0076] Furthermore, the number of layers of the third graphene is less than that of the second graphene and greater than that of the first graphene; and / or, the layer diameter of the third graphene is less than that of the second graphene and greater than that of the first graphene. That is, the number of layers and / or layer diameter of the graphene are used to match the size of the third nanometer metal wires, which is conducive to the construction of a local conductive network.

[0077] Furthermore, the average particle size of the third carbon-coated copper powder is less than that of the second carbon-coated copper powder and greater than that of the first carbon-coated copper powder. The size of the carbon-coated copper powder needs to cooperate with the size of the third nanometer metal wires and the third graphene to construct a good three-dimensional conductive network structure.

[0078] In some embodiments, the thickness of the third conductive layer is 2 μm to 5 μm, for example, specifically it can be 2 μm, 3 μm, 4 μm or 5 μm.

[0079] Correspondingly, another embodiment of the present application further provides a manufacturing method of a secondary battery, which can be used to manufacture the secondary battery provided in the above embodiments. For the same or corresponding parts as the previous embodiment, reference can be made to the corresponding description of the previous embodiment, and details will not be elaborated below.

[0080] On the other hand, a method for manufacturing a secondary battery provided by an embodiment of the present application includes: preparing a positive electrode sheet, and the preparation steps include: providing a positive electrode current collector; preparing a first slurry, dispersing a first nano metal wire, a first graphene, and a first copper-coated carbon powder in a solvent, adding a binder and stirring evenly; coating the first slurry on the surface of the positive electrode current collector by a doctor blade coating method and drying by a thermal curing method to form a first conductive layer; preparing a second slurry, dispersing a second nano metal wire, a second graphene, and a second copper-coated carbon powder in a solvent, adding a binder and stirring evenly; wherein, the diameter of the first nano metal wire is smaller than that of the second nano metal wire, and / or, the aspect ratio of the first nano metal wire is smaller than that of the second nano metal wire; and, the number of layers of the first graphene is smaller than that of the second graphene, and / or, the layer diameter of the first graphene is smaller than that of the second graphene; coating the second slurry on the surface of the first conductive layer away from the positive electrode current collector by a doctor blade coating method and drying by a thermal curing method to form a second conductive layer; providing a negative electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then placing them into a housing, and injecting an electrolyte to form a secondary battery.

[0081] Before preparing the first slurry or the second slurry, the first graphene or the second graphene can be surface-modified, for example, surface-treated with an amino silane coupling agent to improve the dispersibility and interfacial bonding force of the first graphene or the second graphene.

[0082] When preparing the first slurry or the second slurry, the viscosity of the first slurry or the second slurry can be adjusted to 1000 cp - 1400 cp to facilitate the coating of the slurry and avoid the inability to form a film due to too low viscosity or difficult coating due to too high viscosity.

[0083] In some embodiments, the first slurry includes the following components in parts by mass: 10% - 30% of the first nano metal wire, 5% - 15% of the first graphene, 5% - 10% of the first copper-coated carbon powder, 20% - 40% of the binder, and the balance of deionized water; the second slurry includes the following components in parts by mass: 10% - 30% of the second nano metal wire, 5% - 15% of the second graphene, 5% - 10% of the second copper-coated carbon powder, 20% - 40% of the binder, and the balance of deionized water.

[0084] In some embodiments, the first slurry or the second slurry may further include functional additives, such as boron nitride nanosheets (1% - 3%) for improving thermal conductivity and mechanical strength; phosphoric acid high corrosion inhibitor (0.5% - 2%) for reacting with the positive electrode current collector to form a passivation film protection layer; polyether modified defoamer (0.1% - 0.5%) for removing air bubbles in the slurry; and leveling agent (0.2% - 1%) for improving the fluidity of the slurry.

[0085] It should be noted that the "%" in the above components represents the mass percentage of the substance in the first slurry or the second slurry.

[0086] In some embodiments, the process parameters of the blade coating method include: the blade gap is 50 μm to 200 μm, and the moving speed is 1 m / min to 10 m / min; the process parameters of the thermal curing method include: the temperature is 120 °C to 250 °C, and the time is 10 min to 60 min. The process method combining the blade coating method and the thermal curing method is beneficial to balancing performance and cost-effectiveness.

[0087] In the method for preparing a secondary battery provided by the embodiments of the present application, when preparing the positive electrode sheet, a first conductive layer and a second conductive layer are sequentially formed on the positive electrode current collector. The materials of the first conductive layer and the second conductive layer both include graphene, nano metal wires, and carbon-coated copper powder. The nano metal wires are intertwined with the graphene sheets to form an intertwined conductive network, enabling the carbon-coated copper powder to better connect with the nano metal wires and graphene. Good electrical contact can also be maintained between the loose carbon-coated copper powder particles. Moreover, the carbon-coated copper powder can also reduce the porosity. The three-dimensional conductive network composed of "point-line-plane" is conducive to the rapid conduction of electrons, reducing battery polarization, and lowering the internal resistance of the secondary battery, thereby being beneficial to improving the charge and discharge efficiency and cycle stability of the secondary lithium battery. In addition, the first conductive layer close to the positive electrode current collector is used as the inner layer, and the second conductive layer far from the positive electrode current collector is used as the outer layer. The inner layer uses first nano metal wires with a smaller diameter (and / or a lower aspect ratio) compared to the outer layer to form a high-density conductive network, reducing the interfacial resistance between the positive electrode current collector and the active material and ensuring the rapid transmission of electrons; the outer layer uses second nano metal wires with a larger diameter (and / or a higher aspect ratio) compared to the inner layer to form a long-range continuous conductive network, enhancing the overall electron conduction ability of the positive electrode sheet and reducing the distortion phenomenon during the charge and discharge process. The gradient conductive network realizes the improvement of the overall conductive performance of the positive electrode sheet. The inner layer uses first graphene with fewer sheets (and / or a smaller diameter) compared to the outer layer, which is beneficial to exposing more active edge sites, shortening the lithium ion diffusion path, and enhancing the reaction kinetics; the outer layer uses second graphene with more sheets (and / or a larger diameter) compared to the inner layer to form a porous structure, promoting the penetration of the electrolyte into the interior of the positive electrode sheet and avoiding the problem of lithium ion transmission being blocked due to excessive densification, optimizing the diffusion of lithium ions and the wetting effect of the electrolyte. The second nano metal wires with a larger diameter and / or aspect ratio and the second graphene with a larger number of sheets and / or sheet diameter in the outer layer can also provide mechanical support, relieve the volume expansion of the active material during the charge and discharge process, reduce the pulverization and cracking problems of the positive electrode sheet, enhance the mechanical stability of the positive electrode sheet, and thereby be beneficial to improving the cycle performance of the secondary battery.

[0088] According to some embodiments of the present application, on the other hand, the present application embodiments further provide an electrical device, the electrical device includes a secondary battery as in the above embodiments, or a secondary battery prepared by the manufacturing method of the secondary battery as in the above embodiments; alternatively, the electrical device includes an energy storage system, and the energy storage system includes a secondary battery as in the above embodiments, or a secondary battery prepared by the manufacturing method of the secondary battery as in the above embodiments.

[0089] The following are specific embodiments of the present application.

[0090] The secondary batteries corresponding to Embodiments 1 to 10 are prepared by the following method. The differences between Embodiments 1 to 10 lie in the different parameters of the first nano metal wire, the first graphene, the second nano metal wire, the second graphene, the first copper-coated carbon powder, and the second copper-coated carbon powder. Table 1 shows the parameters of the first nano metal wire, the first graphene, the second nano metal wire, the second graphene, the first copper-coated carbon powder, and the second copper-coated carbon powder in various embodiments provided by the embodiments of the present application. For details, see Table 1.

[0091] (1) Provide aluminum foil (positive current collector).

[0092] (2) Prepare the first slurry. Disperse 20% of the first nano metal wire, 10% of the first graphene, and 8% of the first copper-coated carbon powder in deionized water, and add 30% of the waterborne polyurethane-acrylic copolymer (binder) and stir evenly; coat the first slurry on the surface of the positive current collector by the doctor blade coating method and dry it by the thermal curing method to form the first conductive layer.

[0093] (3) Prepare the second slurry. Disperse 20% of the second nano metal wire, 10% of the second graphene, and 8% of the second copper-coated carbon powder in deionized water, and add 30% of the waterborne polyurethane-acrylic copolymer (binder) and stir evenly; coat the second slurry on the surface of the first conductive layer away from the positive current collector by the doctor blade coating method and dry it by the thermal curing method to form the second conductive layer.

[0094] (4) Prepare the positive electrode slurry. Prepare a positive electrode slurry with a solid content of 60% by mixing 57% of LiFePO4, 1.8% of Super P, 1.2% of PVDF, and deionized water. Coat the positive electrode slurry on the surface of the second conductive layer away from the first conductive layer and dry it to form a positive electrode coating. The positive current collector, the first conductive layer, the second conductive layer, and the positive electrode coating constitute the positive electrode sheet.

[0095] (5) Provide a negative electrode sheet and a separator. Wind or stack the positive electrode sheet, the separator, and the negative electrode sheet, then put them into a housing, and inject electrolyte to form a secondary battery.

[0096] Comparative Examples 1 to 3 The difference between Comparative Example 1 and Example 1 is that only the first conductive layer is fabricated on the positive current collector in Comparative Example 1, and the second conductive layer is not fabricated.

[0097] The difference between Comparative Example 2 and Example 1 is that the second conductive layer is directly fabricated on the positive current collector in Comparative Example 2, and the first conductive layer is not fabricated.

[0098] The difference between Comparative Example 3 and Example 1 is that the positions of the first conductive layer and the second conductive layer on the positive current collector in Comparative Example 3 are replaced.

[0099] Table 1

[0100] Continued Table 1

[0101] The internal resistance, charge-discharge efficiency, capacity retention rate of the secondary batteries corresponding to Examples 1 to 10 and Comparative Examples 1 to 3 are tested, and the resistivity and contact angle of the positive electrode sheets corresponding to Examples 1 to 10 and Comparative Examples 1 to 3 are tested.

[0102] Charge-discharge test process: Place the secondary battery to be tested in an environment of 25°C, charge it at a current of 0.5C to 3.65V, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a current of 0.5C to 2.5V. Repeat the above steps 100 times to test the capacity retention rate and charge-discharge efficiency of the secondary battery.

[0103] The test results are shown in Table 2.

[0104] Table 2

[0105] By comparing the results of Examples 1 to 10 and Comparative Examples 1 to 3, it can be found that for the secondary battery provided in the embodiments of the present application, the conductive layer on the positive electrode sheet adopts a gradient structure design, which is beneficial to reducing the resistivity of the positive electrode sheet, improving the wettability of the positive electrode sheet, enhancing the charge-discharge efficiency and cycle performance of the secondary battery, and reducing the internal resistance of the secondary battery.

[0106] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present application. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.

Claims

1. A secondary battery, characterized in that, Comprising: A positive electrode sheet, a negative electrode sheet, and a separator, the separator being located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet comprising: A positive electrode current collector; A first conductive layer located on the surface of the positive electrode current collector; A second conductive layer located on the surface of the first conductive layer away from the positive electrode current collector; Wherein, the material of the first conductive layer includes a first nano metal wire, a first graphene, and a first copper powder coated with carbon, and the material of the second conductive layer includes a second nano metal wire, a second graphene, and a second copper powder coated with carbon; The diameter of the first nano metal wire is less than the diameter of the second nano metal wire, and / or, the aspect ratio of the first nano metal wire is less than the aspect ratio of the second nano metal wire; and, The number of layers of the first graphene is less than the number of layers of the second graphene, and / or, the layer diameter of the first graphene is less than the layer diameter of the second graphene.

2. The secondary battery according to claim 1, characterized in that, The diameter of the first nano metal wire is greater than or equal to 10 nm and less than or equal to 20 nm; the diameter of the second nano metal wire is greater than 20 nm and less than or equal to 50 nm.

3. The secondary battery according to claim 1, wherein The aspect ratio of the first nano metal wire is greater than or equal to 1000 and less than or equal to 1500; the aspect ratio of the second nano metal wire is greater than 1500 and less than or equal to 2000.

4. The secondary battery according to claim 1, characterized in that, The number of layers of the first graphene is 3 to 5 layers; the number of layers of the second graphene is 4 to 8 layers.

5. The secondary battery according to claim 1, characterized in that, The layer diameter of the first graphene is 5 μm to 15 μm; the layer diameter of the second graphene is 10 μm to 20 μm.

6. The secondary battery according to claim 1, characterized in that, The average particle size of the first copper powder coated with carbon is less than the average particle size of the second copper powder coated with carbon.

7. The secondary battery according to claim 1, wherein In the first conductive layer, the total mass of the first graphene and the first nano metal wire accounts for a mass ratio of the first conductive layer greater than the mass ratio of the first copper powder coated with carbon in the first conductive layer; in the second conductive layer, the total mass of the second graphene and the second nano metal wire accounts for a mass ratio of the second conductive layer greater than the mass ratio of the second copper powder coated with carbon in the second conductive layer.

8. The secondary battery according to claim 1, wherein In the first conductive layer, the mass ratio of the first graphene to the total mass of the first graphene and the first nano metal wire is a first ratio; in the second conductive layer, the mass ratio of the second graphene to the total mass of the second graphene and the second nano metal wire is a second ratio, and the first ratio is greater than the second ratio.

9. The secondary battery according to claim 1, characterized in that, The positive electrode sheet further includes: a third conductive layer located on the surface of the second conductive layer away from the first conductive layer, and the material of the third conductive layer includes a third nano metal wire, a third graphene, a third copper powder coated with carbon, and silicon dioxide, and the diameter of the third nano metal wire is less than the diameter of the first nano metal wire, and / or, the aspect ratio of the third nano metal wire is less than the aspect ratio of the first nano metal wire.

10. The secondary battery according to claim 9, characterized in that, The number of layers of the third graphene is less than that of the second graphene and greater than that of the first graphene; and / or, the layer diameter of the third graphene is less than that of the second graphene and greater than that of the first graphene.

11. The secondary battery according to claim 9 or 10, characterized in that, The average particle size of the third carbon-coated copper powder is less than that of the second carbon-coated copper powder and greater than that of the first carbon-coated copper powder.

12. A method for manufacturing a secondary battery, characterized in that, Comprising: Preparing a positive electrode sheet, and the preparation steps include: Providing a positive electrode current collector; Preparing a first slurry, dispersing a first nano metal wire, a first graphene, and a first carbon-coated copper powder in a solvent, adding a binder, and stirring evenly; Coating the first slurry on the surface of the positive electrode current collector by a doctor blade coating method, and drying by a thermal curing method to form a first conductive layer; Preparing a second slurry, dispersing a second nano metal wire, a second graphene, and a second carbon-coated copper powder in a solvent, adding a binder, and stirring evenly; wherein, the diameter of the first nano metal wire is less than that of the second nano metal wire, and / or, the aspect ratio of the first nano metal wire is less than that of the second nano metal wire; and, the number of layers of the first graphene is less than that of the second graphene, and / or, the layer diameter of the first graphene is less than that of the second graphene; Coating the second slurry on the surface of the first conductive layer away from the positive electrode current collector by a doctor blade coating method, and drying by a thermal curing method to form a second conductive layer; Providing a negative electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet, putting them into a housing, and injecting an electrolyte to form a secondary battery.

13. The manufacturing method of the secondary battery according to claim 12, wherein, The first slurry comprises the following components in parts by mass: 10% - 30% of the first nano metal wire, 5% - 15% of the first graphene, 5% - 10% of the first carbon-coated copper powder, 20% - 40% of the binder, and the balance deionized water; the second slurry comprises the following components in parts by mass: 10% - 30% of the second nano metal wire, 5% - 15% of the second graphene, 5% - 10% of the second carbon-coated copper powder, 20% - 40% of the binder, and the balance deionized water.

14. The manufacturing method of the secondary battery according to claim 12, characterized in that, The process parameters of the doctor blade coating method include: the blade gap is 50μm - 200μm, and the moving speed is 1m / min - 10m / min; the process parameters of the thermal curing method include: the temperature is 120°C - 250°C, and the time is 10min - 60min.

15. An electrical device, characterized in that, The electrical equipment includes the secondary battery according to any one of claims 1 - 11, or the secondary battery prepared by the manufacturing method of the secondary battery according to any one of claims 12 - 14; or, the electrical equipment includes an energy storage system, and the energy storage system includes the secondary battery according to any one of claims 1 - 11, or the secondary battery prepared by the manufacturing method of the secondary battery according to any one of claims 12 - 14.

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