Positive electrode sheet, electrochemical device, and electronic device

Abstract

A positive electrode tab includes a positive electrode current collector, an undercoat layer, and a positive electrode active material layer. The undercoat layer is provided between the positive electrode current collector and the positive electrode active material layer. The positive electrode active material layer includes positive electrode active material particles, and the positive electrode active material particles include first particles that penetrate through the undercoat layer. The number of the first particles is n1, and the total number of the positive electrode active material particles connected to the undercoat layer in the positive electrode active material layer is n2, and the following is satisfied: 5% ≤ n1 / n2 ≤ 40%. The present application can reduce ion polarization, improve the rate performance and cycle performance of an electrochemical device. The present application also provides an electrochemical device using the positive electrode tab and an electronic device using the electrochemical device.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and more particularly to a positive electrode, an electrochemical device having the positive electrode, and an electronic device having the electrochemical device. Background Technology

[0002] Electrochemical devices (such as lithium-ion batteries) are widely used in electronic mobile devices, power tools, and electric vehicles, and the demand for energy density in these devices is constantly increasing. Increasing the coating density of the electrodes can reduce the use of current collectors and separators, thereby improving the energy density of electrochemical devices. However, as the coating density of the electrodes increases, the ion transport channels lengthen, transport resistance increases, and ion polarization increases, resulting in a decrease in charging speed and a deterioration in rate performance and long-cycle capability. Summary of the Invention

[0003] Therefore, this application proposes a positive electrode that can improve the rate performance and cycle performance of an electrochemical device. Furthermore, this application also provides an electrochemical device having the positive electrode and an electronic device having the electrochemical device.

[0004] This application provides a positive electrode sheet, including a positive current collector, a base coating, and a positive active material layer. The base coating is disposed between the positive current collector and the positive active material layer. The positive active material layer includes positive active material particles, among which a first particle penetrates the base coating. The number of the first particles is n1, and the total number of positive active material particles in the positive active material layer connected to the base coating is n2, satisfying: 5% ≤ n1 / n2 ≤ 40%.

[0005] This application, by incorporating a first particle penetrating the undercoat layer and controlling the number of this first particle to satisfy n1 / n2≥5%, increases the bonding sites between the positive electrode active material particles and the undercoat layer, thereby improving the connection reliability between the positive electrode active material layer and the undercoat layer. Furthermore, it facilitates contact between the positive electrode active material particles, the undercoat layer, and the current collector, thus improving the conductive network around the positive electrode active material particles. Therefore, it helps reduce polarization, suppress the increase in impedance during cycling, increase charging speed, and improve the rate performance and cycle life of the electrochemical device. Simultaneously, by controlling n1 / n2≤40%, it prevents the undercoat layer or the positive electrode current collector from being damaged by the positive electrode active material particles, thus preventing the positive electrode active material layer from easily detaching during cycling and reducing the risk of decreased cycle performance of the electrochemical device.

[0006] In some implementations, 8% ≤ n1 / n2 ≤ 30%. This can further enhance the bonding effect between the positive electrode active material particles and the base coating, while further reducing the risk of reduced cycle performance due to the shedding of the positive electrode active material layer during cycling.

[0007] In some implementations, the Dv50 and Dv99 of the positive electrode active material particles satisfy the following condition: 0.25 ≤ Dv50 / Dv99 ≤ 0.5. By controlling the Dv50 / Dv99 ratio of the positive electrode active material particles within this range, the risk of a sharp increase in the number of first particles (n1) penetrating the undercoating layer after electrode rolling is reduced when the Dv50 / Dv99 ratio is too small. This could lead to damage to the undercoating layer or the positive electrode current collector by the positive electrode active material particles, making the positive electrode active material layer prone to detachment during cycling and resulting in reduced cycle performance. Simultaneously, it also reduces the risk of a sharp decrease in the number of first particles (n1) penetrating the undercoating layer after electrode rolling when the Dv50 / Dv99 ratio is too large, leading to insufficient reliability of the connection between the positive electrode active material layer and the undercoating layer. Furthermore, keeping the Dv50 / Dv99 ratio within this range also reduces the risk of reduced compaction density of the positive electrode due to inappropriate particle size distribution.

[0008] In some implementations, 0.3 ≤ Dv50 / Dv99 ≤ 0.45. This can further improve the connection reliability between the positive electrode active material layer and the undercoat layer, as well as the positive electrode current collector, thereby improving the cycle performance of the electrochemical device.

[0009] In some implementations, the undercoat includes a conductive agent and a binder. Based on the quality of the undercoat, the mass percentage of the conductive agent in the undercoat is W1, where 30wt% ≤ W1 ≤ 95wt%. By controlling W1 within this range, the overall conductivity of the electrode is improved, thereby further enhancing the rate performance and cycle performance of the electrochemical device. In some implementations, 70wt% ≤ W1 < 90wt%.

[0010] In some implementations, based on the quality of the primer coating, the mass percentage of binder in the primer coating is W2, where 5wt% ≤ W2 ≤ 70wt%. By controlling W2 within this range, the adhesion performance of the primer coating is improved, thereby further enhancing the connection reliability between the positive electrode active material layer and the primer coating, as well as between the primer coating and the positive electrode current collector. In some implementations, 10wt% ≤ W2 ≤ 30wt%.

[0011] In some implementations, the conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, or graphene, which gives the conductive agent good conductivity, thereby improving the conductive network.

[0012] In some implementations, the binder includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylic acid, polyacrylate, acrylate polymers, or polyimide, which gives the binder good adhesion properties and improves the connection reliability between the positive electrode active material layer and the base layer, as well as between the base layer and the positive electrode current collector.

[0013] In some implementations, the first particle includes a second particle embedded in the positive electrode current collector. The thickness of the undercoating layer is d1, and the embedding depth of the second particle in the positive electrode current collector is H, satisfying: 0.18 ≤ H / d1 ≤ 3. By controlling H / d1 ≥ 0.18, the bonding effect between the second particle and the undercoating layer and the positive electrode current collector can be improved, thereby further improving the connection reliability between the positive electrode active material layer and the undercoating layer and the positive electrode current collector. Simultaneously, by controlling H / d1 ≤ 3, the risk of cracking due to gaps between the second particle, the current collector, and the undercoating layer during the winding and bending of the positive electrode sheet can be reduced, thereby suppressing the increase in resistance and improving the rate performance and cycle performance of the electrochemical device.

[0014] In some implementations, 0.45 ≤ H / d1 ≤ 2. This further enhances the bonding effect between the second particle and the base coating and the positive electrode current collector, improves its stability during positive electrode winding and bending, suppresses the increase in resistance, and thus improves the rate performance and cycle performance of the electrochemical device.

[0015] In some implementations, 1μm ≤ d1 ≤ 5μm, which effectively improves the bonding reliability between the positive electrode active material layer and the positive electrode current collector, and reduces the impact on the energy density of the electrochemical device. In other implementations, 1μm ≤ d1 ≤ 3μm.

[0016] In some implementations, 0.5μm ≤ H ≤ 4.5μm is used to reduce the risk of cracking due to gaps between the second particle and the current collector and the undercoating layer when the positive electrode is wound and bent. This suppresses the increase in resistance and improves the rate performance and cycle performance of the electrochemical device. In some implementations, 0.9μm ≤ H ≤ 4μm is used.

[0017] In some implementations, the compaction density of the positive electrode sheet is ρ, 3.5 g / cm³. 3 ≤ρ≤4.4g / cm 3 Higher compaction density is beneficial for increasing the energy density of electrochemical devices. Furthermore, it can reduce the porosity and effective specific surface area of ​​the positive electrode active material layer, thereby reducing electrolyte loss and improving the initial coulombic efficiency and cycle life of the electrochemical device.

[0018] In some implementations, the positive electrode active material particles are made of at least one of lithium transition metal composite oxides or lithium transition metal phosphate compounds.

[0019] A second aspect of this application provides an electrochemical device including an electrode assembly. The electrode assembly includes a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the aforementioned positive electrode.

[0020] A third aspect of this application provides an electronic device including the aforementioned electrochemical device. The electronic device is powered by the electrochemical device, and the electrochemical device has high rate performance and cycle performance. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of an electrochemical device provided in one embodiment of this application.

[0022] Figure 2 for Figure 1 Cross-sectional view of the electrode assembly of the electrochemical device shown in some embodiments.

[0023] Figure 3 for Figure 1 Cross-sectional view of the electrode assembly of the electrochemical device shown in some other embodiments.

[0024] Figure 4 for Figure 2 or Figure 3 A cross-sectional view of the positive electrode of the electrode assembly shown.

[0025] Figure 5 for Figure 2 or Figure 3 A cross-sectional view of the negative electrode of the electrode assembly shown.

[0026] Figure 6 This is a schematic diagram of the overall structure of an electronic device provided in one embodiment of this application.

[0027] Figure 7 The image shows a scanning electron microscope image of the cross-section of the positive electrode sheet prepared in Example 1.

[0028] The following detailed description, in conjunction with the accompanying drawings, will further illustrate this application. Detailed Implementation

[0029] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0030] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to provide a thorough and detailed understanding of this application to those skilled in the art.

[0031] Additionally, for brevity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same numerical values ​​refer to the same elements. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more of the associated enumerated items. Furthermore, it should be understood that when element A is referred to as "connecting" element B, element A may be directly connected to element B, or there may be an intermediate element C and element A and element B may be indirectly connected to each other.

[0032] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," as used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0033] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figures. It should be understood that, in addition to the directions depicted in the figures, spatial terms are intended to include different orientations of the device or apparatus during use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations. It should be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Therefore, a first element, component, region, layer, or portion discussed below may be referred to as a second element, component, region, layer, or portion without departing from the teachings of the exemplary embodiments.

[0034] Please see Figure 1 and Figure 2One embodiment of this application provides an electrochemical device 100, including a housing 10, an electrode assembly 20, an electrolyte, a first tab 30, and a second tab 40. The electrode assembly 20 and the electrolyte are located inside the housing 10. The first tab 30 and the second tab 40 are both electrically connected to the electrode assembly 20 and extend from the inside of the housing 10 to connect to external components (not shown).

[0035] like Figure 2 As shown, the electrode assembly 20 can be a wound structure. The electrode assembly 20 includes a positive electrode 21, a negative electrode 22, and a separator 23, with the separator 23 disposed between the positive electrode 21 and the negative electrode 22. The positive electrode 21, the separator 23, and the negative electrode 22 are sequentially stacked and wound to form the electrode assembly 20. Figure 3 As shown, in some other embodiments, the electrode assembly 20 may also be a stacked structure, with the positive electrode 21, the separator 23 and the negative electrode 22 stacked in sequence to form the electrode assembly 20.

[0036] like Figure 4 As shown, the positive electrode 21 includes a positive current collector 210, a base coating 212, and a positive active material layer 211. The base coating 212 is disposed between the positive current collector 210 and the positive active material layer 211. In some embodiments, the base coating 212 includes a conductive agent and a binder. In some embodiments, the number of base coatings 212 and positive active material layers 211 may each be two. The positive current collector 210 includes a first surface 210a and a second surface 210b disposed opposite to each other. Two base coatings 212 are respectively disposed on the first surface 210a and the second surface 210b. Two positive active material layers 211 are respectively disposed on the two base coatings 212. Figure 5 As shown, the negative electrode 22 includes a negative electrode current collector 220 and a negative electrode active material layer 221.

[0037] The positive electrode current collector 210 can be made of aluminum foil or nickel foil. The positive electrode active material layer 211 comprises positive electrode active material particles, which include compounds capable of reversibly intercalating and deintercalating active ions. In some embodiments, the positive electrode active material particles may comprise at least one of lithium transition metal composite oxides or lithium-containing transition metal phosphate compounds. In some embodiments, the lithium transition metal composite oxide is selected from at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium manganese oxide, or lithium nickel manganese oxide. The lithium-containing transition metal phosphate compound is selected from at least one of lithium iron phosphate or lithium manganese iron phosphate.

[0038] The negative electrode current collector 220 can be at least one of copper foil, nickel foil, or carbon-based current collector. The negative electrode active material layer 221 contains negative electrode active material particles, which can be negative electrode active materials known in the art capable of reversible intercalation and deintercalation of active ions, and this application is not limited thereto. For example, it can be one or more combinations of graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium. Among them, graphite can be selected from one or more combinations of artificial graphite, natural graphite, and modified graphite; silicon-based materials can be selected from one or more combinations of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; tin-based materials can be selected from one or more combinations of elemental tin, tin oxide compounds, and tin alloys.

[0039] The separator 23 comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene, in particular, have a good effect on improving short circuits and can improve the safety of the electrochemical device 100 through a turn-off effect.

[0040] The electrolyte can be in one or more of the following states: gel, solid, and liquid. Liquid electrolytes may include lithium salts and non-aqueous solvents. The lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is chosen as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The non-aqueous solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or combinations thereof. Examples of carbonate compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), and combinations thereof.

[0041] In some embodiments, the conductive agent of the base coating 212 may include at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, or graphene. The binder of the base coating 212 may include at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylic acid, polyacrylate, acrylate polymers, or polyimide.

[0042] In some embodiments, the compaction density of the positive electrode 21 is ρ, 3.5 g / cm³.3 ≤ρ≤4.4g / cm 3 The higher compaction density of the positive electrode 21 is beneficial for improving the energy density of the electrochemical device 100. Furthermore, the higher compaction density can reduce the porosity and effective specific surface area of ​​the positive electrode active material layer 211, thereby reducing electrolyte loss and improving the initial coulombic efficiency and cycle life of the electrochemical device 100.

[0043] The method for measuring the compaction density ρ of the positive electrode 21 may include the following steps: Discharge the electrochemical device 100 to 0 SOC% (State of Charge), disassemble the electrochemical device 100, and clean and dry it. Then, weigh the positive electrode 21 of a certain area A using a balance, and record the weight as W1. Measure the thickness d of the positive electrode 21 using a micrometer. Wash away the positive active material layer 211 and the base coating layer 212 using a solvent, dry it, measure the weight of the positive current collector 210, record it as W2, and measure the thickness d2 of the positive current collector 210 using a micrometer. Then, calculate the compaction density ρ of the positive electrode 21 using the following formula: ρ=(W1-W2) / [(d-d2)×A].

[0044] like Figure 4As shown, the positive electrode active material particles in the positive electrode active material layer 211 include first particles 2111 penetrating the base coating layer 212. The base coating layer 212 includes a third surface 212a connected to the positive electrode current collector 210 and a fourth surface 212b connected to the positive electrode active material layer 211. The first particles 2111 penetrate both the third surface 212a and the fourth surface 212b of the base coating layer 212. Because the first particles 2111 penetrate the base coating layer 212, they provide a riveting effect, thereby improving the connection reliability between the positive electrode active material layer 211 and the base coating layer 212. At the same time, the contact between the first particles 2111 and the positive electrode current collector 210 helps to increase the contact sites between the positive electrode active material particles and the base coating layer 212 and the positive electrode current collector 210, thereby improving the conductive network around the positive electrode active material particles. Typically, the electrode preparation process includes slurry preparation, electrode coating, electrode rolling, and electrode slitting. The purpose of rolling is to make the active materials bond more tightly together and between the active material layer and the current collector, and to make the active material layer thickness more uniform, thereby increasing the compaction density of the electrode. The first particle 2111 in the positive electrode active material layer 211 can be embedded in the base coating layer 212 during the rolling process. The number of the first particle 2111 is n1, and the total number of positive electrode active material particles connected to the base coating layer 212 in the positive electrode active material layer 211 (i.e., positive electrode active material particles that are in contact with the base coating layer 212, which can contact the fourth surface 212b of the base coating layer 212, be embedded in the base coating layer 212, or penetrate the base coating layer 212) is n2, satisfying: 5% ≤ n1 / n2 ≤ 40%. By controlling the number of the first particles 2111 to satisfy n1 / n2≥5%, on the one hand, the number of bonding sites between the positive electrode active material particles and the base coating 212 is increased, thereby improving the connection reliability between the positive electrode active material layer 211 and the base coating 212. On the other hand, it facilitates the contact between the positive electrode active material particles, the base coating 212, and the positive electrode current collector 210, thereby improving the conductive network around the positive electrode active material particles. Therefore, it is beneficial to reduce polarization, suppress the increase in impedance during cycling, increase the charging speed, and improve the rate performance and cycle life of the electrochemical device 100. At the same time, by controlling n1 / n2≤40%, it is possible to prevent the base coating 212 or the positive electrode current collector 210 from being damaged by the positive electrode active material particles (especially the positive electrode active material particles with larger particle sizes), which would make the positive electrode active material layer 211 prone to detachment during cycling, thereby reducing the risk of reduced cycle performance of the electrochemical device 100.

[0045] In some embodiments, 8% ≤ n1 / n2 ≤ 30%. This can further enhance the bonding effect between the positive electrode active material particles and the base coating 212, and further reduce the risk of reduced cycle performance due to the shedding of the positive electrode active material layer 211 during cycling.

[0046] The testing steps for n1 and n2 may include: cutting the positive electrode sheet 21 using plasma cutting, with the cut surface parallel to the thickness direction of the positive electrode sheet 21; acquiring an image of the cut surface of the positive electrode sheet 21 using a scanning electron microscope; then selecting a region with a length of 500 μm along the extension direction of the positive current collector 210 on the image, and counting the number of positive electrode active material particles penetrating the undercoating layer 212 within this region, denoted as n1; then counting the total number of positive electrode active material particles connected to the undercoating layer 212 within this region, denoted as n2.

[0047] In some embodiments, when the electrode assembly 20 is a wound structure, after the winding is completed, the positive active material layer 211 and the base coating layer 212 and the positive current collector 210 at the corner of the positive electrode 21 still maintain a good connection, which reduces the risk of the positive active material layer 211 and the base coating layer 212 cracking at the corner after the positive electrode 21 is wound, resulting in a decrease in conductivity. This helps to suppress the increase in resistance at the corner after winding and improve the rate performance and cycle performance of the electrochemical device 100.

[0048] In some embodiments, the Dv50 and Dv99 of the positive electrode active material particles satisfy: 0.25 ≤ Dv50 / Dv99 ≤ 0.5. By controlling the Dv50 / Dv99 of the positive electrode active material particles within the above range, this application reduces the risk that when the Dv50 / Dv99 ratio is too small (e.g., Dv99 is too large), the number n1 of the first particles 2111 penetrating the base coating 212 after the electrode is rolled increases sharply, thereby causing the base coating 212 or the positive electrode current collector 210 to be destroyed by the positive electrode active material particles. This reduces the adhesion between the base coating 212 and the positive electrode active material layer 211 and the positive electrode current collector 210, thus increasing the risk that the positive electrode active material layer 211 is prone to detachment during cycling and resulting in reduced cycle performance. Simultaneously, it also reduces the risk of insufficient connection reliability between the positive electrode active material layer 2111 and the base coating 212 due to a sharp decrease in the number n1 of the first particles 2111 penetrating the base coating 212 after the electrode sheet is rolled when the Dv50 / Dv99 ratio is too large (e.g., Dv50 is too large and Dv99 is too small). Furthermore, within the aforementioned range, Dv50 / Dv99 also reduces the risk of reduced compaction density of the positive electrode sheet 21 due to an unsuitable particle size distribution. Here, Dv99 represents the particle size that reaches 99% of the total volume of the positive electrode active material particles in the volume-based particle size distribution, i.e., the volume of positive electrode active material particles smaller than this size accounts for 99% of the total volume of the positive electrode active material particles. Dv50, also known as "median particle size", represents the particle size that accounts for 50% of the total volume of positive electrode active material particles in the particle size distribution on a volume basis, starting from the smallest particle size side. In other words, the volume of positive electrode active material particles smaller than this particle size accounts for 50% of the total volume of positive electrode active material particles.

[0049] In some embodiments, 0.3 ≤ Dv50 / Dv99 ≤ 0.45. This can further improve the connection reliability between the positive electrode active material layer 211, the base coating layer 212, and the positive electrode current collector 210. At the same time, it can also further improve the conductive network around the positive electrode active material particles and improve the cycle performance of the electrochemical device 100.

[0050] The measurement process of Dv50 and Dv99 of the positive electrode active material particles may include the following steps: immersing the positive electrode sheet 21 in an organic solvent (e.g., dimethyl carbonate) to peel off the positive electrode active material layer on the surface; drying and burning off the binder and conductive agent to obtain a positive electrode active material particle sample; dispersing the positive electrode active material particle sample in an aqueous solution; and testing it using a laser particle size analyzer (Master Sizer 3000) to obtain the Dv50 and Dv99 values ​​of the positive electrode active material particles.

[0051] In some embodiments, based on the mass of the base coating 212, the mass percentage of the conductive agent in the base coating 212 is W1, where 30wt% ≤ W1 ≤ 95wt%. By controlling W1 within the above range, the overall conductivity of the electrode is improved, thereby further improving the rate performance and cycle performance of the electrochemical device 100. In some embodiments, 70wt% ≤ W1 ≤ 90wt%.

[0052] In some embodiments, based on the mass of the base coating 212, the mass percentage of the binder in the base coating 212 is W2, where 5 wt% ≤ W2 ≤ 70 wt%. By controlling W2 within the above range, the adhesion performance of the base coating 212 is improved, thereby further enhancing the connection reliability between the positive electrode active material layer 211 and the base coating 212, and between the base coating 212 and the positive electrode current collector 210. In some embodiments, 10 wt% ≤ W2 ≤ 30 wt%.

[0053] like Figure 4As shown, in some embodiments, the first particle 2111 includes a second particle 2112 embedded in the positive electrode current collector 210. The thickness of the base coating 212 is d1, and the depth of the second particle 2112 embedded in the positive electrode current collector 210 is H, satisfying: 0.18≤H / d1≤3. By controlling H / d1≥0.18, the bonding effect between the second particle 2112 and the base coating 212 and the positive electrode current collector 210 can be improved, thereby further improving the connection reliability between the positive electrode active material layer 211 and the base coating 212 and the positive electrode current collector 210. At the same time, by controlling H / d1≤3, the risk of cracking due to gaps between the second particle 2112 and the positive electrode current collector 210 and the base coating 212 when the positive electrode sheet 21 is wound and bent can be reduced, thereby suppressing the increase in resistance and improving the rate performance and cycle performance of the electrochemical device 100. In some embodiments, 0.45≤H / d1≤2. This can further improve the riveting effect between the second particle 2112 and the base coating 212 and the positive electrode current collector 210, and improve its connection stability when the positive electrode sheet 21 is wound and bent, suppress the increase of resistance, and thus improve the rate performance and cycle performance of the electrochemical device.

[0054] In some embodiments, 1 μm ≤ d1 ≤ 5 μm, which effectively improves the bonding reliability between the positive electrode active material layer 211 and the positive electrode current collector 210, and reduces the impact on the energy density of the electrochemical device 100. In some embodiments, 0.5 μm ≤ H ≤ 4.5 μm, which reduces the risk of cracking due to gaps between the second particle 2112 and the positive electrode current collector 210 and the undercoating layer 212 when the positive electrode sheet 21 is wound and bent, thereby suppressing the increase in resistance and improving the rate performance and cycle performance of the electrochemical device 100. In some embodiments, 1 μm ≤ d1 ≤ 3 μm. In some embodiments, 0.9 μm ≤ H ≤ 4 μm.

[0055] In some embodiments, the thickness of the positive electrode current collector 210 is d2, where 7 μm ≤ d2 ≤ 20 μm. This improves the structural stability of the positive electrode current collector 210 and reduces the impact of its thickness on the energy density of the electrochemical device 100. Further, in some embodiments, 8 μm ≤ d2 ≤ 12 μm.

[0056] In some embodiments, the thickness of the positive electrode active material layer 211 is d3, where 35 μm ≤ d3 ≤ 70 μm. Further, in some embodiments, 50 μm ≤ d3 ≤ 70 μm is used, thereby increasing the energy density of the electrochemical device 100.

[0057] The measurement process for H, d1, d2, and d3 may include the following steps: The positive electrode sheet 21 is cut using plasma cutting, with the cut surface parallel to the thickness direction of the positive electrode sheet 21. An image of the cut surface of the positive electrode sheet 21 is acquired using a scanning electron microscope. Then, a region with a length of 500 μm is selected on the image along the extension direction of the positive current collector 210. Within this region, 20 second particles 2112 are selected that penetrate the undercoat 212 and embed into the positive current collector 210. The embedding depth of the second particles 2112 into the positive current collector 210 is measured, and the average value is used to obtain H. For the positive electrode 21 with a base coating 212 and a positive active material layer 211 coated on both sides, 10 sample points are selected in the above-mentioned area where the base coating 212 on both sides is not embedded by positive active material particles. The thickness of the positive electrode 21 at the sample points is measured and the average value is recorded as d. The thickness of the positive current collector 210 at the sample points is measured and the average value is recorded as d2. The total thickness of the positive current collector 210 and the base coating 212 on both sides at the sample points is measured and the average value is recorded as d4. The thickness of the base coating 212 is d1 = (d4 - d2) / 2, and the thickness of the positive active material layer 211 is d3 = (d - d4) / 2.

[0058] In some embodiments, when the state of charge of the electrochemical device 100 is 0% SOC, the resistance of the positive electrode 21 is R1, 0.5 mΩ / 154 mm². 2 ≤R1≤5.0mΩ / 154mm 2 In some embodiments, 0.5mΩ / 154mm 2 ≤R1≤2.0mΩ / 154mm 2 In some embodiments, when the state of charge of the electrochemical device 100 is 50% SOC, the resistance of the positive electrode 21 is R2, 0.2 mΩ / 154 mm². 2 ≤R2≤4.0mΩ / 154mm 2 In some embodiments, 0.2mΩ / 154mm 2 ≤R2≤1.5mΩ / 154mm 2 In some embodiments, when the state of charge of the electrochemical device 100 is 100% SOC, the resistance of the positive electrode 21 is R3, 0.05 mΩ / 154 mm². 2 ≤R3≤2.0mΩ / 154mm 2 In some embodiments, 0.05mΩ / 154mm 2 ≤R3≤1.0mΩ / 154mm 2Because the first particle 2111 penetrates the base coating 212, the contact sites between the positive electrode active material layer 211, the base coating 212, and the positive electrode current collector 210 are increased, thereby improving the conductive network in the positive electrode active material layer 211. Therefore, it is beneficial to suppress the increase in resistance during the charging process of the electrochemical device 100.

[0059] The testing process for R1, R2, and R3 may include the following steps: The electrochemical device 100, with states of charge of 0%, 50%, and 100% respectively, is disassembled to obtain the corresponding positive electrode 21. Then, the resistance of the positive electrode 21 is tested using a film resistance meter (manufacturer: Yuaneng Technology), with the power supply maintained at 220V and the air pressure greater than 0.7MPa. Next, different positive electrode 21 samples are cut into sizes of 60mm × 80mm. The samples are placed flat on a sample stage, and the sample stage is placed in the test chamber of the resistance meter to begin testing. The test area is a circle with a diameter of 14mm. Throughout the test, the test air pressure is set to zero, thereby measuring R1, R2, and R3 respectively.

[0060] The electrochemical device 100 of this application includes all devices capable of undergoing electrochemical reactions. Specifically, the electrochemical device includes all types of primary or secondary batteries. Optionally, the electrochemical device can be a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0061] Please see Figure 6 This application also provides an electronic device 1, which includes an electrochemical device 100. The electronic device 1 is powered by the electrochemical device 100, and the electrochemical device 100 has high rate capability and cycle performance. The electrochemical device 100 of this application is applicable to electronic devices 1 in various fields. In one embodiment, the electronic device 1 of this application may be, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0062] The present application will be described in detail below through specific embodiments and comparative examples. The electrochemical device is illustrated using a lithium-ion battery as an example, and specific testing methods will be used in conjunction with the description.

[0063] Example 1

[0064] (1) Preparation of the positive electrode sheet: Conductive carbon black, binder polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) are mixed in a certain proportion and stirred under vacuum until the system is homogeneous to obtain a base coating slurry with a solid content of 10%. The mass ratio of conductive carbon black to PVDF in the base coating slurry is 85:15. Then, the base coating slurry is coated on both surfaces of a 10 μm thick aluminum foil for the positive electrode current collector. After drying, a positive electrode current collector containing the base coating is obtained.

[0065] Conductive carbon black and binder polyvinylidene fluoride (PVDF) were mixed in a certain proportion, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 7%. Then, lithium cobalt oxide, the positive electrode active material, was added, and the mixture was stirred under vacuum until the system was homogeneous, obtaining a positive electrode active slurry with a solid content of 75%. The mass ratio of positive electrode active material, conductive carbon black, and binder in the positive electrode active slurry was 97:1:2. The positive electrode active slurry was then coated onto a base layer, dried, and cold-pressed to allow the first particles of the positive electrode active material layer to penetrate the base layer. After cutting and welding the tabs, the positive electrode sheet was obtained.

[0066] The thickness d2 of the positive current collector in the positive electrode sheet is 10 μm, the thickness d1 of the single-sided bottom coating is 2 μm, the thickness d3 of the single-sided positive active material layer is 50 μm, and the parameters such as n1 / n2 are specifically recorded in Table 1.

[0067] (2) Preparation of negative electrode sheet: Artificial graphite, styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) were mixed with deionized water in a mass ratio of 96:2:2 and stirred evenly to obtain a negative electrode active slurry. The negative electrode active slurry was coated on both surfaces of a 6 μm thick negative electrode current collector copper foil, dried, cold-pressed, and then cut and welded with tabs to obtain the negative electrode sheet.

[0068] (3) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) are mixed in a mass ratio of EC:PC:DEC = 1:1:1 to obtain an organic solvent. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a LiPF6 mass concentration of 12.5%.

[0069] (4) Preparation of lithium-ion batteries: The positive electrode, separator, and negative electrode are sequentially stacked and wound to obtain an electrode assembly. The separator is a porous polyethylene (PE) membrane. The electrode assembly is placed in an outer packaging aluminum-plastic film, and then the above-mentioned electrolyte is injected. After formation, degassing, packaging, edge trimming, capacity testing and other processes, a lithium-ion battery is obtained.

[0070] Examples 2-8

[0071] The difference from Example 1 is that the parameters such as Dv50 / Dv99 of the positive electrode active material and the compaction density ρ of the positive electrode sheet are adjusted.

[0072] Comparative Example 1

[0073] The difference from Example 1 is that the positive electrode sheet does not have a base coating.

[0074] Comparative Examples 2-4

[0075] The difference from Example 1 is that the parameters such as Dv50 / Dv99 of the positive electrode active material and the compaction density ρ of the positive electrode sheet are adjusted.

[0076] Test methods

[0077] (1) Testing of positive electrode active materials Dv50 / Dv99

[0078] The positive electrode active material was dispersed in an aqueous solution and tested using a laser particle size analyzer (Master Sizer 3000) to obtain the Dv50 and Dv99 values ​​of the positive electrode active material.

[0079] (2) Testing of n1, n2, H, d1, d2 and d3

[0080] The positive electrode sheet was cut using a plasma cutter, with the cut surface parallel to the thickness direction of the positive electrode sheet. Images of the cut surface of the positive electrode sheet were acquired using a scanning electron microscope (SEM). Then, a 500 μm long region was selected on the image along the extension direction of the positive current collector, and the number of positive active material particles penetrating the undercoat within this region was counted, denoted as n1. Then, the total number of positive active material particles connected to the undercoat within this region was counted, denoted as n2.

[0081] Within this length region, 20 second particles that penetrate the undercoat and are embedded in the positive current collector are selected. The embedding depth of the second particles in the positive current collector is measured, and the average value is taken to obtain H. Within this length region, 10 sample points where the positive active material particles are not embedded in the undercoat on both sides are selected. The thickness of the positive electrode sheet at the sample point is measured, and the average value is taken as d. The thickness of the positive current collector at the sample point is measured, and the average value is taken as d2. The total thickness of the positive current collector and the undercoat on both sides at the sample point is measured, and the average value is taken as d4. The thickness of the undercoat is d1 = (d4 - d2) / 2, and the thickness of the positive active material layer is d3 = (d - d4) / 2.

[0082] Among them, the cross-sectional scanning electron microscope image of the positive electrode sheet in Example 1 is as follows: Figure 7 As shown. From Figure 7It can be seen that the first particle penetrates the base coating, and the second particle in the first particle is embedded in the positive current collector (aluminum foil).

[0083] (3) Testing of the compaction density ρ of the positive electrode sheet

[0084] The lithium-ion battery was discharged to 0% SOC, and the positive electrode sheet was obtained by disassembly. It was then cleaned with dimethyl carbonate (DMC) and dried. A positive electrode sheet sample of a certain area A was then punched out and weighed using a balance; the weight is recorded as W1. The thickness of the positive electrode sheet sample was measured using a micrometer and recorded as d. The positive active material layer and the base coating on the surface of the positive current collector were removed using the solvent NMP, and the sample was dried. The weight of the positive current collector was recorded as W2, and its thickness was measured using a micrometer and recorded as d2. The compaction density ρ of the positive electrode sheet was then calculated using the following formula: ρ=(W1-W2) / [(d-d2)×A].

[0085] (4) Resistance test of the positive electrode at 0% SOC, 50% SOC, and 100% SOC states.

[0086] In each embodiment and comparative example, three lithium-ion batteries were used as parallel samples for charge-discharge testing. At (25±3)℃, the lithium-ion batteries were charged at a constant current of 0.5C to a voltage of 4.5V, and then charged at a constant voltage of 4.5V until the cutoff current reached 0.05C. This state of charge (SOC) was recorded as 100%. The batteries were then discharged at a constant current of 0.2C to 3.0V, and this state of charge (SOC) was recorded as 0%. Based on this, the above charge-discharge treatment was performed on the three lithium-ion batteries to obtain batteries with states of charge of 0%, 50%, and 100%, respectively. These batteries were then disassembled to obtain the corresponding positive electrode plates, and the resistance of each positive electrode plate was measured. The resistances R1, R2, and R3 of the positive electrode plates at 0%, 50%, and 100% SOC were obtained.

[0087] The electrode resistance test method is as follows: A film resistance meter (manufacturer: Yuaneng Technology) is used, with the power supply of the resistance meter maintained at 220V and the air pressure greater than 0.7Mpa. The positive electrode is cut into samples of 60mm×80mm size, and the samples are placed flat in the sample stage. Then, the sample stage is placed in the test chamber of the resistance meter and the test begins. The test area is a circle with a diameter of 14mm. Throughout the test, the test air pressure is set to zero.

[0088] (5) Test of resistance growth rate r1 at the corner after the positive electrode is wound

[0089] Take the positive electrode sheet before winding and test its resistance according to the above electrode resistance test method. Record it as R4. After winding the positive electrode sheet into an electrode assembly, disassemble it and take the positive electrode sheet in the corner area. Test its resistance according to the above electrode resistance test method. Record it as R5. Then the resistance growth rate at the corner of the positive electrode sheet after winding is r1=(R5-R4) / R4×100%.

[0090] (6) Test of the resistance growth rate r2 of the positive electrode after battery cycling

[0091] At (25±3)℃, the lithium-ion battery is charged at a constant current of 0.5C until the voltage reaches 4.5V; then charged at a constant voltage of 4.5V until the cutoff current is 0.05C, and then discharged at a constant current of 0.2C until 3.0V, thus completing one cycle. This cycle is repeated 800 times. Then, the battery is disassembled, and the positive electrode is removed. The resistance of the positive electrode in the flat area is tested according to the electrode resistance test method described above, and recorded as R6. Therefore, r2 = (R6 - R4) / R4 × 100%.

[0092] The parameters and corresponding performance test results of the positive electrode are recorded in Table 1.

[0093] Table 1

[0094]

[0095] Please refer to Table 1. Compared with Comparative Examples 1-4, the positive electrode active material particles in Examples 1-8 satisfy 0.25≤Dv50 / Dv99≤0.50, and the compaction density ρ of the positive electrode sheet satisfies 3.5 g / cm³. 3 ≤ρ≤4.4g / cm 3 The positive electrode sheets in Examples 1-8 satisfy 5% ≤ n1 / n2 ≤ 40% and n1 / n2 ≥ 5%. On the one hand, this increases the bonding sites between the positive active material particles and the base coating, thereby improving the connection reliability between the positive active material layer and the base coating. On the other hand, it facilitates the contact between the positive active material particles, the base coating, and the current collector, thereby improving the conductive network around the positive active material particles. Simultaneously, n1 / n2 ≤ 40% prevents the base coating or the positive current collector from being damaged by the positive active material particles, thus preventing the positive active material layer from easily detaching during electrode winding or cycling. Therefore, the positive electrode sheets in Examples 1-8 maintain a good connection between the positive active material layer, the base coating, and the positive current collector after winding and cycling, and the resistance growth rates r1 and r2 of the positive electrode sheets are both low. The positive electrode sheets in Examples 1-5 and Example 8 satisfy 8% ≤ n1 / n2 ≤ 30%, further reducing the growth rate r1 after winding and the growth rate r2 after cycling.

[0096] Examples 9-14

[0097] The difference from Example 1 is that the contents of conductive agent and binder W1 and W2 in the base coating are adjusted.

[0098] Table 2

[0099]

[0100] Please refer to Table 2. In Examples 1 and 9-14, the conductive agent satisfies 30wt% ≤ W1 ≤ 95wt%, and the binder satisfies 5wt% ≤ W2 ≤ 70wt%. This is beneficial for improving the conductivity and connection performance between the positive electrode active material layer and the positive electrode current collector, thus suppressing the resistance increase of the positive electrode sheet during winding and cycling. Among them, in Examples 1 and 9-11, when 70wt% ≤ W1 ≤ 90wt% and 10wt% ≤ W2 ≤ 30wt%, the resistance growth rates r1 and r2 of the positive electrode sheet after winding and cycling are further reduced.

[0101] Examples 15-17

[0102] The difference between Examples 15-17 and Example 1 is that the thickness d1 of the base coating was adjusted.

[0103] Table 3

[0104]

[0105] Please refer to Table 3. Comparing Examples 1 and 15-17, it can be seen that as the thickness d1 of the undercoat decreases, n1 / n2, H, and H / d1 all increase. The bonding effect between the second particle and the positive current collector is enhanced, thereby further improving the connection reliability between the positive active material layer, the undercoat, and the positive current collector. Therefore, the resistance growth rates r1 and r2 of the positive electrode sheet after winding and cycling are reduced. Combining Tables 1 and 3, it can be seen that for 0.9μm≤H≤4μm and 0.45≤H / d1≤2, the resistance growth rates r1 and r2 of the positive electrode sheet after winding and cycling are further reduced.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application.

Claims

1. A positive electrode sheet, wherein, The positive electrode tab comprises a positive electrode current collector, a base coating layer and a positive electrode active material layer, the base coating layer is arranged between the positive electrode current collector and the positive electrode active material layer, the positive electrode active material layer comprises positive electrode active material particles, the positive electrode active material particles comprise first particles penetrating through the base coating layer, the number of the first particles is n1, the total number of the positive electrode active material particles connected to the base coating layer in the positive electrode active material layer is n2, and the following condition is met: 5%≤n1 / n2≤40%.

2. The cathode sheet of claim 1, wherein, 8%≤n1 / n2≤30%.

3. The cathode sheet of claim 1, wherein, The Dv50 and Dv99 of the positive electrode active material particles meet the following condition: 0.25≤Dv50 / Dv99≤0.

5.

4. The cathode sheet of claim 3, wherein, 0.3≤Dv50 / Dv99≤0.

45.

5. The cathode sheet of claim 1, wherein, The base coating layer comprises a conductive agent and a binder, and at least one of the following conditions is met: (1) the mass percentage content of the conductive agent in the base coating layer is W1 based on the mass of the base coating layer, and 30wt%≤W1≤95wt%; (2) the mass percentage content of the binder in the base coating layer is W2 based on the mass of the base coating layer, and 5wt%≤W2≤70wt%; (3) the conductive agent comprises at least one of conductive carbon black, acetylene black, ketjen black, carbon nanotubes or graphene; (4) the binder comprises at least one of polyvinylidene fluoride, vinylidene-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylic acid, polyacrylate, acrylate polymer or polyimide.

6. The cathode sheet of claim 5, wherein, The positive electrode tab meets at least one of the following conditions: (1) 70wt%≤W1≤90wt%; (2) 10wt%≤W2≤30wt%.

7. The cathode sheet of claim 1, wherein, The first particles comprise second particles embedded in the positive electrode current collector, the thickness of the base coating layer is d1, and the depth of the second particles embedded in the positive electrode current collector is H, and at least one of the following conditions is met: (1) 0.18≤H / d1≤3; (2) 1μm≤d1≤5μm; (3) 0.5μm≤H≤4.5μm.

8. The cathode sheet of claim 7, wherein, The positive electrode tab meets at least one of the following conditions: (1) 0.45≤H / d1≤2; (2) 1μm≤d1≤3μm; (3) 0.9μm≤H≤4μm.

9. The cathode sheet of claim 1, wherein, The positive electrode tab meets at least one of the following conditions: (1) the compacted density of the positive electrode tab is p, 3.5 g / cm 3 ≤ p ≤ 4.4 g / cm 3 ; (2) the material of the positive electrode active material particles comprises at least one of lithium transition metal composite oxide or lithium-containing transition metal phosphate compound.

10. An electrochemical device comprising an electrode assembly, the electrode assembly comprising a positive electrode sheet, a separator film, and a negative electrode sheet, wherein, The positive electrode tab is the positive electrode tab according to any one of claims 1 to 9.

11. An electronic device, wherein, The electrochemical device comprises the positive electrode tab according to claim 10.

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