Positive plate, preparation method thereof and battery

Abstract

The invention relates to a positive plate and a preparation method thereof and a battery, the positive plate comprises a current collector and a safety protection layer and an active material layer arranged on at least one surface of the current collector, and the safety protection layer is located between the current collector and the active material layer; the safety protection layer comprises an electro-deformation material and a conductive material; the conductive material comprises a carbon nanotube and conductive carbon; the mass ratio of the electro-deformation material in the safety protection layer is 80%-90%; under the condition that the voltage applied to the safety protection layer exceeds the deformation voltage threshold value of the electro-deformation material, the volume of the safety protection layer can be expanded. Therefore, the risk of thermal runaway of the battery under high voltage can be well reduced, and the safety performance of the battery is effectively improved; meanwhile, when the voltage applied to the safety protection layer is reduced to be lower than a deformation voltage threshold value of the electro-deformation material, the volume of the safety protection layer can be recovered, and the positive plate can be recovered to be used.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a positive electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] Secondary batteries are currently widely used in a wide range of industries. As the complexity of battery applications increases, the risk of battery misuse often rises. This misuse threatens battery safety. Furthermore, as battery energy density increases, the battery's heat dissipation efficiency decreases. Under the same misuse conditions, the heat accumulated within the battery increases, increasing the risk of thermal runaway.

[0003] The safety of secondary batteries has become one of the obstacles to further expanding their application in areas such as electric vehicles. Therefore, how to reduce the risk of thermal runaway and improve the safety of secondary batteries has become a technical problem that needs to be solved urgently. Summary of the Invention

[0004] In view of this, embodiments of the present application provide a positive electrode sheet, a preparation method thereof, and a battery to solve at least one problem existing in the background technology.

[0005] In a first aspect, an embodiment of the present application provides a positive electrode sheet, comprising: a current collector and a safety protection layer and an active material layer disposed on at least one surface of the current collector, wherein the safety protection layer is located between the current collector and the active material layer;

[0006] The safety protection layer includes an electrodeformable material and a conductive material; the conductive material includes carbon nanotubes and conductive carbon; the electrodeformable material accounts for 80% to 90% of the mass of the safety protection layer;

[0007] When the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand.

[0008] In conjunction with the first aspect of the present application, in an optional embodiment, the security protection layer satisfies at least one of the following characteristics:

[0009] (1) The conductive material accounts for 10% to 20% by mass in the safety protection layer;

[0010] (2) The electrostrictive material includes a relaxor ferroelectric material; optionally, the relaxor ferroelectric material includes a multi-component copolymer of vinylidene fluoride and a monomer and / or polyvinylidene fluoride, wherein the monomer includes at least one of chlorotrifluoroethylene, trifluoroethylene, and hexafluoropropylene;

[0011] (3) The conductive material includes carbon nanotubes and conductive carbon; optionally, the mass ratio of the carbon nanotubes to the conductive carbon is 1:20 to 1:1.5;

[0012] (4) The surface density of the safety protection layer is 25 mg / m 2 ~35 mg / m 2 .

[0013] In conjunction with the first aspect of the present application, in an optional embodiment, the positive electrode sheet satisfies at least one of the following characteristics:

[0014] (1) The active material layer includes a high nickel ternary material; optionally, the chemical formula of the high nickel ternary material is LiNi x Co y M z O2, wherein M is at least one of Mn, Al, Mg, Zr, Ti, W, and B, x ≥ 0.8, and x + y + z = 1;

[0015] (2) The current collector includes at least one of aluminum foil, porous aluminum foil, and carbon-coated aluminum foil.

[0016] In conjunction with the first aspect of the present application, in an optional embodiment, the positive electrode sheet satisfies at least one of the following characteristics:

[0017] (1) The ratio of the sum of the thickness of the safety protection layer and the active material layer to the thickness of the safety protection layer is greater than or equal to 10;

[0018] (2) The thickness of the safety protection layer is 3 μm to 10 μm;

[0019] (3) The thickness of the active material layer is 110 μm to 115 μm.

[0020] In combination with the first aspect of the present application, in an optional embodiment, the active material layer includes a first sub-active layer and a second sub-active layer stacked in sequence in the thickness direction of the current collector, and the first sub-active layer is located between the safety protection layer and the second sub-active layer; the first sub-active layer includes a single crystal material, and the second sub-active layer includes a polycrystalline material.

[0021] In combination with the first aspect of the present application, in an optional embodiment, the active material layer includes a first sub-active layer and a second sub-active layer stacked in sequence in the thickness direction of the current collector, and the first sub-active layer is located between the safety protection layer and the second sub-active layer; the first sub-active layer includes a polycrystalline material, and the second sub-active layer includes a single crystal material.

[0022] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode sheet, the method comprising the following steps:

[0023] Adding an electrodeformable material to a first solvent, stirring until the electrodeformable material is completely dissolved, then adding a conductive material, mixing uniformly, to obtain a mixed slurry; the conductive material includes carbon nanotubes and conductive carbon;

[0024] The mixed slurry is coated on at least one surface of a current collector, and subjected to a first drying process to obtain a safety protection layer; the electrodeformable material accounts for 80% to 90% by weight of the safety protection layer;

[0025] Adding the positive electrode active material, the conductive agent and the binder into the second solvent and mixing them uniformly to obtain a positive electrode slurry;

[0026] The positive electrode slurry is coated on the surface of the safety protection layer away from the current collector, and subjected to a second drying treatment to obtain an active material layer; when the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand.

[0027] In conjunction with the second aspect of the present application, in an optional embodiment, the step of preparing the mixed slurry satisfies at least one of the following characteristics:

[0028] (1) The electrostrictive material includes a relaxor ferroelectric material; optionally, the relaxor ferroelectric material includes a multi-component copolymer of vinylidene fluoride and a monomer and / or polyvinylidene fluoride, wherein the monomer includes at least one of chlorotrifluoroethylene, trifluoroethylene, and hexafluoropropylene;

[0029] (2) the first solvent includes dimethylformamide;

[0030] (3) The mass ratio of the carbon nanotubes to the conductive carbon is 1:20 to 1:1.5;

[0031] (4) In the mixed slurry, the solid-liquid ratio of the electrodeformable material to the first solvent is (1-1.8): (5-12) g / mL;

[0032] (5) The conductive material accounts for 10% to 20% by mass in the safety protection layer;

[0033] (6) Before the stirring step, a heating treatment is also included; optionally, the temperature of the heating treatment is 60°C to 80°C, and the time is 0.5h to 1h.

[0034] In conjunction with the second aspect of the present application, in an optional embodiment, the step of preparing the positive electrode slurry satisfies at least one of the following characteristics:

[0035] (1) The positive electrode active material includes a high nickel ternary material; optionally, the chemical formula of the high nickel ternary material is LiNi x Co y M z O2, wherein M is one or more of Mn, Al, Mg, Zr, Ti, W, and B, x ≥ 0.8, and x + y + z = 1;

[0036] (2) The conductive agent includes at least one of conductive carbon black, carbon fiber, graphene, and carbon nanotubes;

[0037] (3) The binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose;

[0038] (4) In the positive electrode slurry, the mass proportion of the positive electrode active material is 95% to 97%, the mass proportion of the conductive agent is 1% to 1.5%, and the mass proportion of the binder is 2% to 3.5%.

[0039] In a third aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet described in any one of the first aspects or a positive electrode sheet prepared by the preparation method of the positive electrode sheet described in any one of the second aspects.

[0040] Compared with the prior art, the embodiments of the present application have the following beneficial effects:

[0041] The embodiments of the present application provide a positive electrode sheet, a preparation method thereof, and a battery, wherein the positive electrode sheet includes a current collector and a safety protection layer and an active material layer arranged on at least one surface of the current collector, wherein the safety protection layer is located between the current collector and the active material layer; the safety protection layer includes an electrodeformable material and a conductive material; the conductive material includes carbon nanotubes and conductive carbon; the mass proportion of the electrodeformable material in the safety protection layer is 80% to 90%; when the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand. In an embodiment of the present application, a safety protection layer is provided between the current collector and the active material layer, and the safety protection layer includes an electrodeformable material and a conductive material. The carbon nanotubes in the conductive material can form a complete conductive network, and the conductive carbon can play a point-to-point conductive connection role. Through the synergistic effect of the carbon nanotubes and the conductive carbon, the uniform distribution of the current in the safety protection layer can be guaranteed, so that the electrodeformable material can exhibit excellent electrodeformable performance at a relatively low voltage. When the battery is overcharged or in other abnormal use states, causing the voltage applied to the safety protection layer to exceed the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand, so that the spacing between the conductive material particles in the safety protection layer is reduced. Increased, the conductive network between the active material layer and the current collector is partially or even completely blocked. By controlling the mass proportion of the electrodeformable material in the safety protection layer to 80% to 90%, on the one hand, it can ensure that the volume of the safety protection layer is sufficiently expanded under high voltage, thereby effectively reducing the risk of thermal runaway of the battery under high voltage and effectively improving the safety performance of the battery; on the other hand, it can avoid the problem of large loss of battery capacity caused by excessive expansion of the safety protection layer. When the voltage applied to the safety protection layer is reduced to below the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can be restored, so that the conductive connection between the active material layer and the current collector is restored, and the positive electrode sheet can be restored to use.

[0042] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0044] Figure 1 A schematic cross-sectional view of a positive electrode sheet provided in an embodiment of the present application;

[0045] Figure 2 A schematic cross-sectional view of another positive electrode sheet provided in an embodiment of the present application;

[0046] Figure 3 A schematic flow chart of a method for preparing a positive electrode sheet provided in an embodiment of the present application. DETAILED DESCRIPTION

[0047] In order to make the technical solutions and beneficial effects of the present invention more clearly understood, the following is a detailed description of the invention by referring to the accompanying drawings and listing specific embodiments. It should be understood that these embodiments are intended only to illustrate the present invention and are not intended to limit the scope of the invention. The experimental methods in the following examples where specific conditions are not specified are generally based on conventional experimental conditions. Unless otherwise specified, the reagents and raw materials used in the present invention are commercially available.

[0048] In the following description, numerous specific details are provided to provide a more thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced without one or more of these details. In other instances, certain technical features known in the art are not described to avoid confusion with the present application; that is, all features of actual embodiments are not described herein, nor are well-known functions and steps described in detail.

[0049] The purpose of the terms used herein is only to describe specific embodiments and is not intended to limit the present application. When used herein, the singular forms "a", "an", and "the" are intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "comprising" and / or "including", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0050] In order to fully understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below. However, in addition to these detailed descriptions, the present application may also have other implementation methods.

[0051] Unless otherwise defined, technical and scientific terms used in this application have the same meanings as those in the technical field to which this application belongs.

[0052] In the following examples, if no specific techniques or conditions are specified, the methods are generally carried out according to conventional techniques or conditions described in the literature in the field, or according to the product instructions and the conditions recommended by the manufacturer. The numerical ranges in the following examples are all inclusive.

[0053] At lower temperatures and normal voltages, the heat generation rate and total heat of the side reactions inside the battery are relatively low, the battery has sufficient heat dissipation time, and the risk of thermal runaway is relatively low. However, in some abnormal usage conditions, such as when the battery is overcharged, high voltage will cause the electrolyte to decompose rapidly, or the battery will short-circuit, which will often generate a large amount of Joule heat, which can easily lead to thermal runaway. In related technologies, the following three methods are usually used to prevent thermal runaway of batteries. The first method is to optimize the selection and ratio of battery materials, for example, the selection of positive electrode materials, negative electrode materials and electrolytes with stable performance; the second method is to conduct regular inspections and maintenance of the battery; the third method is to install a thermal management system. These methods cost a lot of manpower and material resources, and the protection effect is limited.

[0054] Based on this, the embodiment of the present application provides a positive electrode sheet, such as Figure 1 As shown, the positive electrode sheet includes a current collector 100 and a safety protection layer 200 and an active material layer 300 arranged on at least one surface of the current collector 100, and the safety protection layer 200 is located between the current collector 100 and the active material layer 300; the safety protection layer 200 includes an electrodeformable material and a conductive material; the conductive material includes carbon nanotubes (CNT) and conductive carbon (Super-P); the mass proportion of the electrodeformable material in the safety protection layer 200 is 80% to 90%, for example, it can be 80%, 85%, 90% or any value between any two of the above numerical ranges; when the voltage applied to the safety protection layer 200 exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer 200 can expand.

[0055] In an embodiment of the present application, a safety protection layer 200 is provided between the current collector 100 and the active material layer 300. The safety protection layer 200 includes an electrodeformable material and a conductive material. The carbon nanotubes in the conductive material can form a complete conductive network, and the conductive carbon can play a point-to-point conductive connection role. Through the synergistic effect of the carbon nanotubes and the conductive carbon, the uniform distribution of the current in the safety protection layer 200 can be guaranteed, so that the electrodeformable material can exhibit excellent electrodeformable performance at a relatively low voltage. When the battery is overcharged or in other abnormal use states, causing the voltage applied to the safety protection layer 200 to exceed the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer 200 can expand, so that the spacing between the conductive material particles in the safety protection layer 200 is reduced. Increased, the conductive network between the active material layer 300 and the current collector 100 is partially or even completely blocked. By controlling the mass proportion of the electrodeformable material in the safety protection layer 200 to 80% to 90%, on the one hand, it can ensure that the volume of the safety protection layer 200 is sufficiently expanded under high voltage, thereby effectively reducing the risk of thermal runaway of the battery under high voltage and effectively improving the safety performance of the battery; on the other hand, it can avoid the problem of large loss of battery capacity caused by excessive expansion of the safety protection layer. When the voltage applied to the safety protection layer 200 is reduced to below the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer 200 can be restored, so that the conductive connection between the active material layer 300 and the current collector 100 is restored, and the positive electrode sheet can be restored to use.

[0056] It should be noted that Figure 1 The safety protection layer 200 and the active material layer 300 being stacked sequentially on one side surface of the current collector 100 in the thickness direction of the current collector 100 is only an example. In some specific embodiments, the safety protection layer 200 and the active material layer 300 can be stacked sequentially on two opposite surfaces of the current collector 100 in the thickness direction of the current collector 100.

[0057] In the embodiments of the present application, there is no particular limitation on the current collector 100 , and any current collector known to those skilled in the art for use in positive electrode sheets can be used. For example, the current collector 100 can include at least one of aluminum foil, porous aluminum foil, and carbon-coated aluminum foil.

[0058] The electrodeformable material in the safety protection layer 200 can be understood as a material that can deform under the influence of an applied voltage. When the applied voltage reaches the deformation voltage threshold of the electrodeformable material, the electrodeformable material deforms, thereby causing the volume of the safety protection layer 200 to expand. Specifically, the electrodeformable material can be selected based on the type of positive electrode and battery so that the deformation voltage threshold of the electrodeformable material matches the voltage upper limit of the positive electrode active material and the safe voltage of the battery. In some specific embodiments, the deformation voltage threshold of the electrodeformable material is less than or equal to the maximum safe voltage of the battery.

[0059] In some embodiments, the electrodeformable material may include a relaxor ferroelectric material. Because relaxor ferroelectric materials have a good response to voltage changes, the dielectric constant of the safety protection layer 200 increases significantly under high voltage, which can enhance the voltage's ability to control the conductive connection between the active material layer 300 and the current collector 100, thereby effectively preventing thermal runaway.

[0060] Furthermore, the relaxor ferroelectric material may include a multi-polymer of vinylidene fluoride (VDF) and a monomer, one of polyvinylidene fluoride, or a mixture of the two, where the monomer includes, for example, at least one of chlorotrifluoroethylene (CTFE), trifluoroethylene (TrFE), and hexafluoropropylene (HFP). This type of material can be called a PVDF-based relaxor ferroelectric material. The PVDF-based relaxor ferroelectric material can produce significant deformation at a relatively low voltage, providing a higher electroresponsiveness for the safety protection layer 200, having sensitive electrodeformable properties, and being able to respond to voltage changes more quickly to prevent thermal runaway of the battery at high voltage. In a specific example, the electrodeformable material may include poly(vinylidene fluoride-co-chlorotrifluoroethylene), i.e., a binary copolymer of vinylidene fluoride and chlorotrifluoroethylene, which can be abbreviated as P(VDF-CTFE). In another specific example, the electrodeformable material may include poly(vinylidene fluoride-co-hexafluoropropylene), which can be abbreviated as P(VDF-HFP).

[0061] The conductive material in the safety protection layer 200 provides the function of electron transmission to ensure the electrodeformation effect of the electrodeformation material. The conductive material includes carbon nanotubes and conductive carbon, which significantly improves the dielectric properties of the safety protection layer 200 and further enhances the electrodeformation effect of the electrodeformation material. Carbon nanotubes are conducive to forming a relatively complete conductive network, and conductive carbon can play a point-to-point conductive connection role, ensuring the uniform distribution of current during the electrodeformation process, thereby improving the overall performance of the safety protection layer 200. Through the synergistic effect of carbon nanotubes and conductive carbon, the electrodeformation material can exhibit excellent electrodeformation performance at a lower voltage, and achieve the purpose of using as little conductive material as possible, thereby better taking into account the thickness and deformation of the safety layer protection layer.

[0062] In some specific embodiments, the mass ratio of carbon nanotubes to conductive carbon can be between 1:20 and 1:1.5, for example, 1:20, 1:15, 1:10, 1:5, 1:1.5, or any value between any two of the aforementioned ranges. Controlling the mass ratio of carbon nanotubes to conductive carbon within the aforementioned range further enhances the electrodeformability of the electrodeformable material, thereby further improving the overall performance of the safety protection layer 200.

[0063] When the mass proportion of the electrodeformable material in the safety protection layer 200 is too large and the mass proportion of the conductive material is too small, it may affect the conduction and uniform distribution of current during the electrodeformation process, thereby affecting the electrodeformation effect of the electrodeformable material; when the mass proportion of the electrodeformable material in the safety protection layer 200 is too small and the mass proportion of the conductive material is too large, it may affect the deformation of the safety layer. Therefore, in some specific embodiments, the mass proportion of the conductive material in the safety protection layer can be 10% to 20%, for example, 10%, 15%, 20%, or any value between any two of the above numerical ranges. In this way, the electrodeformation effect of the electrodeformable material and the deformation of the safety layer can be better guaranteed.

[0064] If the thickness of the safety protection layer 200 is too small, it may limit the deformation of the safety protection layer 200, which in turn affects the effectiveness of the safety protection layer 200 in blocking the conductive network between the active material layer 300 and the current collector 100 under high voltage. If the thickness of the safety protection layer 200 is too thick, it may affect the electrodeformability efficiency of the electrodeformable material and the energy density of the positive electrode sheet. Therefore, in some specific embodiments, the ratio of the sum of the thickness of the safety protection layer 200 and the active material layer 300 to the thickness of the safety protection layer 200 can be greater than or equal to 10. In this way, the safety protection layer 200 can effectively prevent thermal runaway of the battery under high voltage while better balancing the energy density requirements of the positive electrode sheet.

[0065] In some specific embodiments, the thickness of the security protection layer 200 can be 3μm to 10μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or any value between any two of the above numerical ranges.

[0066] In some specific embodiments, the thickness of the active material layer 300 may be 110 μm to 115 μm, for example, 110 μm, 111 μm, 112 μm, 113 μm, 114 μm, 115 μm, or any value between any two of the above value ranges.

[0067] In some embodiments, the surface density of the security protection layer 200 can be 25 mg / m 2~35 mg / m 2 , for example, 25 mg / m 2 , 26mg / m 2 , 27mg / m 2 , 28mg / m 2 , 29mg / m 2 , 30mg / m 2 , 31mg / m 2 , 32mg / m 2 , 33mg / m 2 , 34mg / m 2 , 35mg / m 2 Or any value between any two of the above numerical ranges. In this way, it is beneficial to take into account the thickness and deformation of the safety protection layer 200 and improve the overall performance of the safety protection layer 200.

[0068] In the embodiment of the present application, there is no particular limitation on the active material in the active material layer 300 , and positive electrode active materials well known to those skilled in the art may be used.

[0069] In some embodiments, the active material layer 300 may include a high nickel ternary material. That is, the active material in the active material layer 300 includes a high nickel ternary material. For example, the chemical formula of the high nickel ternary material may be LiNi x Co y M z O2, wherein M is at least one of Mn, Al, Mg, Zr, Ti, W, and B, and x≥0.8, and x+y+z=1.

[0070] Specifically, the active material layer 300 may include single crystal materials and / or polycrystalline materials. Single crystal materials have the advantages of good cycle performance and good high-temperature storage performance, which are beneficial to improving the cycle performance and high-temperature stability of the positive electrode sheet; while polycrystalline materials have the advantages of good rate performance and high capacity, which are beneficial to improving the energy density and fast charging performance of the positive electrode sheet. Therefore, in some embodiments, the active material layer 300 may include two sub-active layers, and the material of one of the two sub-active layers may include a single crystal material, and the material of the other may include a polycrystalline material. Here, the single crystal material may, for example, include a high-nickel ternary single crystal material, and the polycrystalline material may, for example, include a high-nickel ternary polycrystalline material.

[0071] In the embodiment of the present application, the active material layer 300 adopts a multi-layer design, utilizing the combined advantages of single crystal materials and polycrystalline materials, which can effectively improve the dynamic performance and cycle stability of the battery while maintaining high safety performance.

[0072] In some specific embodiments, please refer to Figure 2The active material layer 300 may include a first sub-active layer 301 and a second sub-active layer 302 stacked in sequence in the thickness direction of the current collector 100, and the first sub-active layer 301 is located between the safety protection layer 200 and the second sub-active layer 302; the first sub-active layer 301 includes a single crystal material, and the second sub-active layer 302 includes a polycrystalline material.

[0073] In an embodiment of the present application, a first sub-active layer 301 comprising a single crystal material is disposed under a second sub-active layer 302 comprising a polycrystalline material. Since the single crystal material has no internal grain boundaries, it has better structural stability and can reduce the generation of microcracks during the charge and discharge process, thereby improving the cycle stability of the positive electrode sheet. The first sub-active layer 301 comprising a single crystal material can provide better thermal stability in the lower layer and reduce the risk of thermal runaway of the battery. The second sub-active layer 302 located in the upper layer includes a polycrystalline material, which can provide better electrochemical activity and can avoid side reactions between the first sub-active layer 301 comprising a single crystal material and the electrolyte, thereby improving the safety and life of the battery.

[0074] Furthermore, the ratio of the thickness of the first sub-active layer 301 to the sum of the thicknesses of the active material layer 300 and the safety protection layer 200 can be greater than 1 / 2, and the ratio of the thickness of the second sub-active layer 302 to the sum of the thicknesses of the active material layer 300 and the safety protection layer 200 can be less than 1 / 2. This helps improve the overall performance of the positive electrode sheet.

[0075] In some specific embodiments, please continue to refer to Figure 2 The active material layer 300 may include a first sub-active layer 301 and a second sub-active layer 302 stacked in sequence in the thickness direction of the current collector 100, and the first sub-active layer 301 is located between the protective layer and the second sub-active layer 302; the first sub-active layer 301 includes a polycrystalline material, and the second sub-active layer 302 includes a single crystal material.

[0076] In an embodiment of the present application, the first sub-active layer 301 comprising polycrystalline material is arranged in the lower layer of the second sub-active layer 302 comprising single crystal material. Polycrystalline material generally has higher electrical conductivity and larger specific surface area. Placing the first sub-active layer 301 comprising polycrystalline material in the lower layer can improve the conductivity of the positive electrode sheet, thereby improving the rate performance of the battery; and placing the first sub-active layer 301 comprising polycrystalline material in the lower layer helps to reduce the polarization of the battery, increase the median voltage of rate discharge, and reduce side reactions during battery storage.

[0077] In some embodiments, the active material layer 300 may include a positive electrode active material, a conductive agent, and a binder. The mass ratio of the positive electrode active material, the conductive agent, and the binder may be (95-97): (1-1.5): (2-3.5). Furthermore, in the active material layer 300, the mass proportion of the positive electrode active material may be 95%-97%, the mass proportion of the conductive agent may be 1%-1.5%, and the mass proportion of the binder may be 2%-3.5%.

[0078] For example, the conductive agent may include at least one of conductive carbon black, carbon fiber, graphene, and carbon nanotubes, and the binder may include at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

[0079] It should be noted that, when the active material layer 300 includes a first sub-active layer 301 and a second sub-active layer 302, the mass ratio of the positive electrode active material (single crystal material or polycrystalline material), the conductive agent and the binder in the first sub-active layer 301 and the second sub-active layer 302 can be (95~97): (1~1.5): (2~3.5).

[0080] The present application also provides a method for preparing a positive electrode sheet. Figure 3 The method for preparing the positive electrode sheet provided in the embodiment of the present application comprises the following steps:

[0081] S1: adding an electrodeformable material to a first solvent, stirring until the electrodeformable material is completely dissolved, adding a conductive material, and mixing uniformly to obtain a mixed slurry; the conductive material includes carbon nanotubes and conductive carbon;

[0082] S2: coating the mixed slurry on at least one surface of the current collector, and performing a first drying process to obtain a safety protection layer; the electrodeformable material accounts for 80% to 90% by weight in the safety protection layer;

[0083] S3: adding the positive electrode active material, the conductive agent and the binder into the second solvent and mixing them uniformly to obtain a positive electrode slurry;

[0084] S4: coating the positive electrode slurry on the surface of the safety protection layer away from the current collector, and performing a second drying treatment to obtain an active material layer; when the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand.

[0085] In the embodiment of the present application, a safety protection layer is formed between the current collector and the active material layer. Since the safety protection layer includes an electrodeformable material and a conductive material, the carbon nanotubes in the conductive material can form a complete conductive network, and the conductive carbon can play a point-to-point conductive connection role. Through the synergistic effect of the carbon nanotubes and the conductive carbon, the uniform distribution of the current in the safety protection layer can be guaranteed, so that the electrodeformable material can exhibit excellent electrodeformable performance at a relatively low voltage. When the battery is overcharged or in other abnormal use states, resulting in the voltage applied to the safety protection layer exceeding the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand, so that the conductive material particles in the safety protection layer are connected to each other. As the spacing increases, the conductive network between the active material layer and the current collector is partially or even completely blocked. By controlling the mass proportion of the electrodeformable material in the safety protection layer to 80% to 90%, on the one hand, it can ensure that the volume of the safety protection layer expands sufficiently under high voltage, thereby effectively reducing the risk of thermal runaway of the battery under high voltage and effectively improving the safety performance of the battery; on the other hand, it can avoid the problem of large loss of battery capacity due to excessive expansion of the safety protection layer. When the voltage applied to the safety protection layer is reduced to below the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can be restored, so that the conductive connection between the active material layer and the current collector is restored, and the positive electrode sheet can be restored to use.

[0086] It should be understood that although the steps in the above flowchart are shown sequentially as indicated by the arrows, these steps are not necessarily performed in the order indicated by the arrows. Furthermore, at least some of the steps in the above flowchart may include multiple steps or multiple stages, and these steps or stages are not necessarily performed at the same time or sequentially.

[0087] In the actual preparation process, the specific steps for preparing the mixed slurry in step S1 may be: first, the electrodeformable material and the first solvent are added to a container, stirred (for example, by magnetic stirring and ultrasonic vibration) until the electrodeformable material is completely dissolved, and then the conductive material is added and mixed evenly to obtain a mixed slurry.

[0088] In some embodiments, after the electrodeformable material and the first solvent are added to the container, a heating treatment may be performed before the stirring step. This facilitates the full and rapid dissolution of the electrodeformable material. Specifically, the heating treatment temperature may be 60°C to 80°C, and the heating time may be 0.5 to 1 hour.

[0089] In some embodiments, the electrodeformable material may include a relaxor ferroelectric material. Because relaxor ferroelectric materials are highly responsive to voltage changes, the dielectric constant of the safety protection layer increases significantly at high voltages, enhancing the voltage's ability to control the conductive connection between the active material layer and the current collector, thereby effectively preventing thermal runaway.

[0090] Further, the relaxor ferroelectric material may include a multi-component copolymer of vinylidene fluoride and a monomer, one of polyvinylidene fluoride or a mixture of the two, wherein the monomer here includes, for example, at least one of chlorotrifluoroethylene, trifluoroethylene and hexafluoropropylene. Such materials may be referred to as PVDF-based relaxor ferroelectric materials. PVDF-based relaxor ferroelectric materials can produce significant deformations at relatively low voltages, providing a higher electro-responsiveness for the safety protection layer, having sensitive electro-deformable properties, being able to respond to voltage changes more quickly, and preventing thermal runaway of the battery at high voltages. In a specific example, the electro-deformable material may include poly (vinylidene fluoride-to-chlorotrifluoroethylene). In another specific example, the electro-deformable material may include poly (vinylidene fluoride-to-hexafluoropropylene).

[0091] In step S1, the solid-to-liquid ratio of the electrodeformable material to the first solvent can be (1-1.8): (5-12) g / mL. Controlling the solid-to-liquid ratio of the electrodeformable material to the first solvent within the above range is conducive to the full dissolution of the electrodeformable material and the formation of an adhesive solution with an appropriate mass concentration, thereby facilitating the uniform dispersion of the conductive material in the adhesive solution. Exemplarily, the first solvent can include dimethylformamide (DMF).

[0092] The conductive material in the safety protection layer provides the function of electron transmission to ensure the electrodeformation effect of the electrodeformation material. The conductive material includes carbon nanotubes and conductive carbon, which significantly improves the dielectric properties of the safety protection layer and further enhances the electrodeformation effect of the electrodeformation material. Carbon nanotubes are conducive to forming a relatively complete conductive network, and conductive carbon can play a point-to-point conductive connection role, ensuring the uniform distribution of current during the electrodeformation process, thereby improving the overall performance of the safety protection layer. Through the synergistic effect of carbon nanotubes and conductive carbon, the electrodeformation material can exhibit excellent electrodeformation performance at a lower voltage, and achieve the purpose of using as little conductive material as possible, thereby better taking into account the thickness and deformation of the safety layer protection layer.

[0093] In some specific embodiments, the mass ratio of carbon nanotubes to conductive carbon can be between 1:20 and 1:1.5, for example, 1:20, 1:15, 1:10, 1:5, 1:1.5, or any value between any two of the aforementioned ranges. Controlling the mass ratio of carbon nanotubes to conductive carbon within the aforementioned range further enhances the electrodeformability of the electrodeformable material, thereby further improving the overall performance of the safety protection layer.

[0094] In some specific embodiments, the conductive material may comprise 10% to 20% by weight of the safety protection layer, for example, 10%, 15%, 20%, or any value between the aforementioned two numerical ranges. Thus, in the resulting safety protection layer, the electrodeformable material comprises 80% to 90% by weight, and the conductive material comprises 10% to 20% by weight, thereby effectively ensuring the electrodeformable effect of the electrodeformable material and the deformation of the safety protection layer.

[0095] In step S2, the mixed slurry can be applied to at least one surface of the current collector using a coating process well known to those skilled in the art. The coating process is not limited herein. The current collector can be, for example, at least one of aluminum foil, porous aluminum foil, and carbon-coated aluminum foil. For example, the mixed slurry can be applied to the aluminum foil surface using a scraper, and the scraper thickness can be 10 μm to 20 μm.

[0096] In step S2, the temperature of the first drying process can be 50° C. to 80° C. and the time of the first drying process can be 4 hours to 7 hours. This can effectively remove residual solvent and ensure the quality and performance of the resulting safety protection layer.

[0097] In step S3, in the step of preparing the positive electrode slurry, the positive electrode active material may include a high nickel ternary material. For example, the chemical formula of the high nickel ternary material may be LiNi x Co y M z O2, wherein M is one or more of Mn, Al, Mg, Zr, Ti, W, and B, x ≥ 0.8, and x + y + z = 1. The conductive agent may include at least one of conductive carbon black, carbon fiber, graphene, and carbon nanotubes. The binder may include at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose. The second solvent may include N-methylpyrrolidone (NMP).

[0098] In some embodiments, in the positive electrode slurry, the mass proportion of the positive electrode active material may be 95% to 97%, the mass proportion of the conductive agent may be 1% to 1.5%, and the mass proportion of the binder may be 2% to 3.5%.

[0099] In some embodiments, in step S3, preparing the positive electrode slurry may include separately preparing a first positive electrode slurry and a second positive electrode slurry. Specifically, preparing the first positive electrode slurry may include: adding a first positive electrode active material, a first conductive agent, and a first binder to a solvent, and mixing them uniformly to obtain the first positive electrode slurry. Preparing the second positive electrode slurry may include: adding a second positive electrode active material, a second conductive agent, and a second binder to a solvent, and mixing them uniformly to obtain the second positive electrode slurry.

[0100] The first positive electrode active material includes a single crystal material (specifically, for example, a high-nickel ternary single crystal material); the second positive electrode active material includes a polycrystalline material (specifically, for example, a high-nickel ternary polycrystalline material). The types of the first conductive agent and the first binder, as well as the second conductive agent and the second binder, can be understood with reference to the types of conductive agents and binders in the above embodiments.

[0101] In some embodiments, in the first positive electrode slurry, the mass proportion of the first positive electrode active material may be 95% to 97%, the mass proportion of the first conductive agent may be 1% to 1.5%, and the mass proportion of the first binder may be 2% to 3.5%.

[0102] In some embodiments, in the second positive electrode slurry, the mass proportion of the second positive electrode active material may be 95% to 97%, the mass proportion of the second conductive agent may be 1% to 1.5%, and the mass proportion of the second binder may be 2% to 3.5%.

[0103] Step S4 may include coating the first positive electrode slurry and the second positive electrode slurry on the surface of the safety protection layer away from the current collector, and performing a second drying process to obtain an active material layer.

[0104] In the embodiment of the present application, the active material layer adopts a multi-layer design, utilizing the combined advantages of single crystal materials and polycrystalline materials, which can effectively improve the battery's kinetic performance and cycle stability while maintaining high safety performance.

[0105] In one embodiment, the first positive electrode slurry and the second positive electrode slurry can be stacked and coated on the surface of the safety protection layer away from the current collector in a manner such that the first positive electrode slurry is closer to the safety protection layer than the second positive electrode slurry. After a second drying treatment, a first sub-active layer and a second sub-active layer are obtained, and the first sub-active layer is located between the safety protection layer and the second sub-active layer.

[0106] In an embodiment of the present application, a first sub-active layer comprising a single crystal material is formed on the lower layer of a second sub-active layer comprising a polycrystalline material. Since the single crystal material has no internal grain boundaries, it has better structural stability and can reduce the generation of microcracks during the charge and discharge process, thereby improving the cycle stability of the positive electrode sheet. The first sub-active layer comprising a single crystal material can provide better thermal stability in the lower layer and reduce the risk of thermal runaway of the battery. The second sub-active layer located in the upper layer includes a polycrystalline material, which can provide better electrochemical activity and can avoid side reactions between the first sub-active layer comprising a single crystal material and the electrolyte, thereby improving the safety and life of the battery.

[0107] Furthermore, the ratio of the thickness of the first sub-active layer to the sum of the thicknesses of the active material layer and the safety protection layer can be greater than 1 / 2, and the ratio of the thickness of the second sub-active layer to the sum of the thicknesses of the active material layer and the safety protection layer can be less than 1 / 2. This helps improve the overall performance of the positive electrode sheet.

[0108] In another embodiment, the first positive electrode slurry and the second positive electrode slurry can be stacked and coated on the surface of the safety protection layer away from the current collector in a manner such that the second positive electrode slurry is closer to the safety protection layer than the first positive electrode slurry. After a second drying treatment, a first sub-active layer and a second sub-active layer are obtained, and the first sub-active layer is located between the safety protection layer and the second sub-active layer.

[0109] In an embodiment of the present application, a first sub-active layer comprising polycrystalline material is formed on the lower layer of a second sub-active layer comprising single crystal material. Polycrystalline material generally has higher electrical conductivity and a larger specific surface area. Forming the first sub-active layer comprising polycrystalline material on the lower layer can improve the conductivity of the positive electrode sheet, thereby improving the rate performance of the battery; and forming the first sub-active layer comprising polycrystalline material on the lower layer helps to reduce the polarization of the battery, increase the median voltage of rate discharge, and reduce side reactions during battery storage.

[0110] In step S4, the positive electrode slurry can be coated on the surface of the safety protection layer away from the current collector using a coating process well known to those skilled in the art. The coating process is not limited here.

[0111] In step S4, the temperature of the second drying treatment may be 50° C. to 80° C. This can effectively remove the residual solvent and ensure the quality and performance of the obtained safety protection layer and active material layer.

[0112] Furthermore, in the final cathode sheet, the ratio of the sum of the thicknesses of the safety protection layer and the active material layer to the thickness of the safety protection layer can be greater than or equal to 10. In this way, the safety protection layer can effectively prevent thermal runaway of the battery at high voltage while also better meeting the energy density requirements of the cathode sheet.

[0113] In some specific embodiments, the thickness of the security protection layer can be 3μm to 10μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or any value between any two of the above numerical ranges.

[0114] In some specific embodiments, the thickness of the active material layer may be 110 μm to 115 μm, for example, 110 μm, 111 μm, 112 μm, 113 μm, 114 μm, 115 μm, or any value between any two of the above value ranges.

[0115] In some embodiments, the surface density of the safety protection layer can be 25 mg / m 2 ~35 mg / m 2 , for example, 25 mg / m 2 , 26mg / m 2 , 27mg / m 2 , 28mg / m 2 , 29mg / m 2 , 30mg / m 2 , 31mg / m 2 , 32mg / m 2 , 33mg / m 2 , 34mg / m 2 , 35mg / m 2 Or any value between any two of the above numerical ranges. In this way, it is beneficial to take into account the thickness and deformation of the safety protection layer and improve the overall performance of the safety protection layer.

[0116] An embodiment of the present application further provides a battery, which includes the positive electrode sheet described in any of the above embodiments or a positive electrode sheet prepared by the method for preparing the positive electrode sheet described in any of the above embodiments.

[0117] The battery in this application is a secondary battery. Further, the secondary battery may include a lithium-ion battery.

[0118] It should be understood that since the battery in the embodiments of the present application includes the positive electrode sheet described in any of the above embodiments or includes a positive electrode sheet prepared by the method for preparing the positive electrode sheet described in any of the above embodiments, the beneficial effects of the positive electrode sheet in the above embodiments are applicable to the battery. The battery in the embodiments of the present application has high safety performance.

[0119] The technical solution of the present application is further described below with reference to a number of embodiments and comparative examples.

[0120] Example 1

[0121] The preparation of the positive electrode sheet in this embodiment includes the following steps:

[0122] Step S101: 1.5 g of P(VDF-CTFE) particles (electrodeformable material) were added to 8 mL of DMF (first solvent), heated at 70°C for 1 hour, and then magnetically stirred and ultrasonically vibrated for more than 30 minutes until the P(VDF-CTFE) particles were completely dissolved. 0.357 g of conductive carbon and 0.018 g of carbon nanotubes (conductive material) were then added and mixed uniformly to obtain a mixed slurry.

[0123] Step S102: coating the mixed slurry prepared in step S101 on one surface of the aluminum foil (current collector) along the thickness direction with a scraper thickness of 10 μm; then drying in a 70° C. oven (first drying process) to obtain a safety protection layer;

[0124] Step S103: adding high nickel ternary single crystal material (first positive electrode active material), conductive carbon and oily carbon nanotubes (first conductive agent), and PVDF (first binder) to NMP in a mass ratio of 97:2:1, and mixing them evenly to obtain a first positive electrode slurry;

[0125] Step S104: adding high nickel ternary polycrystalline material (second positive electrode active material), conductive carbon and oily carbon nanotubes (second conductive agent) and PVDF (second binder) to NMP in a mass ratio of 97:2:1, and mixing them evenly to obtain a second positive electrode slurry;

[0126] Step S105: Using a double-layer coating die head, the first positive electrode slurry and the second positive electrode slurry are simultaneously coated on the surface of the safety protection layer away from the aluminum foil in a manner that the first positive electrode slurry is closer to the safety protection layer than the second positive electrode slurry, and dried in a 70°C oven (second drying treatment) to obtain a first sub-active layer and a second sub-active layer (active material layer). The first sub-active layer is located between the safety protection layer and the second sub-active layer, and the positive electrode sheet is obtained after cold pressing. The coating surface density of the first positive electrode slurry and the second positive electrode slurry is 155g / m 2 The compaction density of the positive electrode is 3.45g / cm 3 .

[0127] Example 2

[0128] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, with the main differences being:

[0129] In step S101 , the amount of conductive carbon added is 0.2 g, and the amount of carbon nanotubes added is 0.1 g.

[0130] Example 3

[0131] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, with the main differences being:

[0132] In step S101 , the amount of conductive carbon added is 0.16 g, and the amount of carbon nanotubes added is 0.07 g.

[0133] Example 4

[0134] In step S101 , the amount of conductive carbon added is 0.13 g, and the amount of carbon nanotubes added is 0.04 g.

[0135] Example 5

[0136] In step S101 , the amount of conductive carbon added is 0.1 g, and the amount of carbon nanotubes added is 0.066 g.

[0137] Comparative Example 1

[0138] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Example 1, with the main differences being:

[0139] In step S101, no conductive carbon is added, and only 0.1 g of carbon nanotubes are added as a conductive material.

[0140] Comparative Example 2

[0141] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Example 1, with the main differences being:

[0142] In step S101, no carbon nanotubes are added, and only 0.2 g of conductive carbon is added as a conductive material.

[0143] Comparative Example 3

[0144] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Example 1, with the main differences being:

[0145] In step S101 , the amount of P(VDF-CTFE) particles added is 3 g.

[0146] The performance of the positive electrode sheets prepared in the above embodiments and comparative examples was tested.

[0147] (1) Electrodeformation test of the positive electrode sheet: The positive electrode sheet prepared as described above was clamped between glass plates and placed on a CCD (Charge-Coupled Device) detection device. First, the thickness of the positive electrode sheet was measured with a ruler, which was recorded as D1. Then, a constant current of 4.4V was passed through both ends of the positive electrode sheet, and the thickness of the positive electrode sheet when the current was applied was measured with a ruler, which was recorded as D2. Electrodeformation = ((D2-D1) / D1)*100%.

[0148] The positive electrode sheets prepared in the above embodiments and comparative examples were prepared into batteries to test the battery performance. The battery preparation steps are as follows:

[0149] Preparation of negative electrode sheet: Graphite, conductive carbon and carbon nanotubes, polyacrylic acid (PAA) and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 97:1:2, added to deionized water, and stirred to form a uniform and stable negative electrode slurry. The negative electrode slurry was evenly coated on the copper foil with a coating surface density of 110g / m 2 After drying and cold pressing, the negative electrode sheet was obtained, and the compaction density of the negative electrode sheet was 1.60g / cm 3 ;

[0150] Preparation of electrolyte: LiPF6 was dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1 to prepare an electrolyte with a concentration of 1.2 mol / L;

[0151] Assemble the battery: stack the positive electrode sheet, separator, negative electrode sheet, and separator in sequence, and wind them to obtain the battery cell; the separator uses a PP film with a thickness of 13μm; inject the electrolyte into the dry battery cell, soak it for 10 hours, and then charge it at 45°C. The formation process is: charge it at 0.05C to 3.4V, and then charge it at 0.2C to 3.75V; after aging at room temperature for 24 hours, perform the second sealing, and finally divide the capacity to complete the battery production.

[0152] Battery discharge current test: The battery with a designed capacity of 4.4Ah prepared above was tested using a charge and discharge instrument. The battery was charged to 50% SOC and an electric field of 3.8kV / mm~3.95kV / mm, i.e. 4.5V voltage, was applied under constant voltage charging. The current value after the electric field was applied and the capacity displayed after charging for 30 minutes were observed on the charge and discharge instrument.

[0153] The test results of the above tests are shown in Table 1.

[0154] Table 1

[0155]

[0156]

[0157] From the data in Table 1, it can be seen that, compared with Comparative Examples 1 and 2, the positive electrode sheets prepared in Examples 1 to 5 all underwent larger electrodeformation when an electric field was applied, and the current values ​​of the corresponding batteries after the electric field was applied dropped to a lower value, indicating that the conductive network between the active material layer and the current collector in the positive electrode sheet was effectively blocked. In Comparative Examples 1 and 2, the conductive material only contained one of carbon nanotubes and conductive carbon. Without the synergistic effect of carbon nanotubes and conductive carbon, it was difficult to ensure the uniform distribution of current in the safety protection layer, resulting in a decrease in the electrodeformability of the electrodeformable material. At the same voltage, the electrodeformation of the positive electrode sheet decreased relative to that of Examples 1 to 5, resulting in a significant increase in the current values ​​of the corresponding batteries after the electric field was applied relative to that of Examples 1 to 5. This indicates that the conductive network between the active material layer and the current collector was not effectively blocked, and therefore could not effectively reduce the risk of thermal runaway of the battery at high voltage. In Comparative Example 3, since the amount of electrodeformable material added in the safety protection layer is too high, the mass proportion of the electrodeformable material in the safety protection layer is greater than 90%. It can be seen from the data in Table 1 that under the same voltage, the electrodeformation of the positive electrode sheet of Comparative Example 3 is significantly increased relative to that of Examples 1 to 5. Although the current value of the corresponding battery after the application of the electric field is reduced to a very low value, the risk of thermal runaway of the battery under high voltage can be effectively reduced. However, the 30-min charging capacity of the battery after the electric field is applied at 50% SOC in Comparative Example 3 is significantly reduced relative to that of Examples 1 to 5. This shows that the volume expansion of the positive electrode sheet is too large, which will cause a large loss of battery capacity. After the voltage drops, it is difficult to ensure that the battery can be restored to use.From the above, it can be seen that in the present application, a safety protection layer is set between the current collector and the active material layer. Since the safety protection layer includes electrodeformable material and conductive material, the carbon nanotubes in the conductive material can form a complete conductive network, and the conductive carbon can play a point-to-point conductive connection role. Through the synergistic effect of carbon nanotubes and conductive carbon, the uniform distribution of current in the safety protection layer can be guaranteed, so that the electrodeformable material can exhibit excellent electrodeformable performance at a lower voltage. When the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand, so that the spacing between the conductive material particles in the safety protection layer increases, and the active material layer and the current collector are closer. The conductive network between them is partially or even completely blocked. By controlling the mass proportion of the electrodeformable material in the safety protection layer to 80% to 90%, on the one hand, it can ensure that the volume of the safety protection layer expands sufficiently under high voltage, thereby effectively reducing the risk of thermal runaway of the battery under high voltage and effectively improving the safety performance of the battery; on the other hand, it can avoid the problem of large loss of battery capacity caused by excessive expansion of the safety protection layer. When the voltage applied to the safety protection layer is reduced to below the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can be restored, so that the conductive connection between the active material layer and the current collector is restored, and the positive electrode sheet can be restored to use, that is, the battery can be restored to use.

[0158] The present invention proposes a new type of voltage-sensitive positive electrode sheet, which utilizes the characteristics of electrodeformable materials that can produce obvious electrodeformation at relatively low voltages. A safety protection layer containing electrodeformable materials and conductive materials is provided between the current collector and the active material layer. The safety protection layer has sensitive electrodeformable performance at relatively low voltages. For example, under the action of a 4.4kV / mm electric field, it will produce a large deformation, so that the electrical contact between the material area (active material layer) and the current collector is disconnected, thereby preventing the battery from causing thermal runaway when the voltage exceeds the upper limit of use, providing overvoltage protection for the battery, and enhancing the safety performance of the battery. After reducing the battery voltage, the battery cell can be restored to use. The safety protection layer of the present invention achieves excellent dielectric properties and electrodeformation efficiency, has a high-voltage self-blocking function, and is particularly suitable for preventing scenarios where thermal failure is caused by overcharging.

[0159] It should be noted that the positive electrode sheet embodiments, positive electrode sheet preparation method embodiments and battery embodiments provided in this application belong to the same concept; the technical features in the technical solutions recorded in each embodiment can be arbitrarily combined without conflict.

[0160] It should be understood that the above embodiments are exemplary and are not intended to encompass all possible implementations of the present application. Various modifications and variations may be made based on the above embodiments without departing from the scope of the present disclosure. Similarly, the various technical features of the above embodiments may be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments merely illustrate several implementations of the present invention and do not limit the scope of protection of the patent of the present invention.

Claims

1. A positive electrode sheet, characterized in that: include: a current collector and a safety protection layer and an active material layer disposed on at least one surface of the current collector, wherein the safety protection layer is located between the current collector and the active material layer; The safety protection layer includes an electrodeformable material and a conductive material; the conductive material includes carbon nanotubes and conductive carbon; the electrodeformable material accounts for 80% to 90% of the mass of the safety protection layer; When the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand.

2. The positive electrode sheet according to claim 1, characterized in that The security protection layer satisfies at least one of the following characteristics: (1) The conductive material accounts for 10% to 20% by mass in the safety protection layer; (2) The electrostrictive material includes a relaxor ferroelectric material; optionally, the relaxor ferroelectric material includes a multi-component copolymer of vinylidene fluoride and a monomer and / or polyvinylidene fluoride, wherein the monomer includes at least one of chlorotrifluoroethylene, trifluoroethylene, and hexafluoropropylene; (3) The conductive material includes carbon nanotubes and conductive carbon; optionally, the mass ratio of the carbon nanotubes to the conductive carbon is 1:20 to 1:1.5; (4) The surface density of the safety protection layer is 25 mg / m 2 ~35 mg / m 2 .

3. The positive electrode sheet according to claim 1, characterized in that The positive electrode sheet satisfies at least one of the following characteristics: (1) The active material layer includes a high nickel ternary material; optionally, the chemical formula of the high nickel ternary material is LiNi x Co y M z O2, wherein M is at least one of Mn, Al, Mg, Zr, Ti, W, and B, x ≥ 0.8, and x + y + z = 1; (2) The current collector includes at least one of aluminum foil, porous aluminum foil, and carbon-coated aluminum foil.

4. The positive electrode sheet according to claim 1, characterized in that The positive electrode sheet satisfies at least one of the following characteristics: (1) The ratio of the sum of the thickness of the safety protection layer and the active material layer to the thickness of the safety protection layer is greater than or equal to 10; (2) The thickness of the safety protection layer is 3 μm to 10 μm; (3) The thickness of the active material layer is 110 μm to 115 μm.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The active material layer includes a first sub-active layer and a second sub-active layer stacked in sequence in the thickness direction of the current collector, and the first sub-active layer is located between the safety protection layer and the second sub-active layer; the first sub-active layer includes a single crystal material, and the second sub-active layer includes a polycrystalline material.

6. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The active material layer includes a first sub-active layer and a second sub-active layer stacked in sequence in the thickness direction of the current collector, and the first sub-active layer is located between the safety protection layer and the second sub-active layer; the first sub-active layer includes a polycrystalline material, and the second sub-active layer includes a single crystal material.

7. A method for preparing a positive electrode sheet, characterized in that: The method comprises the following steps: Adding an electrodeformable material to a first solvent, stirring until the electrodeformable material is completely dissolved, adding a conductive material, and mixing uniformly to obtain a mixed slurry; the conductive material includes carbon nanotubes and conductive carbon; The mixed slurry is coated on at least one surface of a current collector, and subjected to a first drying process to obtain a safety protection layer; the electrodeformable material accounts for 80% to 90% by weight of the safety protection layer; Adding the positive electrode active material, the conductive agent and the binder into the second solvent and mixing them uniformly to obtain a positive electrode slurry; The positive electrode slurry is coated on the surface of the safety protection layer away from the current collector, and subjected to a second drying treatment to obtain an active material layer; when the voltage applied to the safety protection layer exceeds the deformation voltage threshold of the electrodeformable material, the volume of the safety protection layer can expand.

8. The method for preparing a positive electrode sheet according to claim 7, wherein: The step of preparing the mixed slurry satisfies at least one of the following characteristics: (1) The electrostrictive material includes a relaxor ferroelectric material; optionally, the relaxor ferroelectric material includes a multi-component copolymer of vinylidene fluoride and a monomer and / or polyvinylidene fluoride, wherein the monomer includes at least one of chlorotrifluoroethylene, trifluoroethylene, and hexafluoropropylene; (2) the first solvent includes dimethylformamide; (3) The mass ratio of the carbon nanotubes to the conductive carbon is 1:20 to 1:1.5; (4) In the mixed slurry, the solid-liquid ratio of the electrodeformable material to the first solvent is (1-1.8): (5-12) g / mL; (5) The conductive material accounts for 10% to 20% by mass in the safety protection layer; (6) Before the stirring step, a heating treatment is also included; optionally, the temperature of the heating treatment is 60°C to 80°C, and the time is 0.5h to 1h.

9. The method for preparing a positive electrode sheet according to claim 7, wherein: The step of preparing the positive electrode slurry satisfies at least one of the following characteristics: (1) The positive electrode active material includes a high nickel ternary material; optionally, the chemical formula of the high nickel ternary material is LiNi x Co y M z O2, wherein M is one or more of Mn, Al, Mg, Zr, Ti, W, and B, x ≥ 0.8, and x + y + z = 1; (2) The conductive agent includes at least one of conductive carbon black, carbon fiber, graphene, and carbon nanotubes; (3) The binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose; (4) In the positive electrode slurry, the mass proportion of the positive electrode active material is 95% to 97%, the mass proportion of the conductive agent is 1% to 1.5%, and the mass proportion of the binder is 2% to 3.5%.

10. A battery, characterized in that: A positive electrode sheet comprising the positive electrode sheet according to any one of claims 1 to 6 or a positive electrode sheet prepared by the preparation method of the positive electrode sheet according to any one of claims 7 to 9.

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