Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. Comprising an electrode assembly which is of a lamination structure. The electrode assembly comprises a positive plate, the positive plate comprises a positive current collector and a positive material layer, and the positive material layer is arranged on at least one side of the positive current collector. The positive electrode material layer comprises a first surface deviating from the positive electrode current collector, and the first surface is provided with a plurality of grooves arranged at intervals. The first surface comprises a first area, a second area and a third area which are sequentially arranged in the first direction, and in the first direction, the second area is located in the middle of the first surface. The grooves comprise a plurality of first grooves located in the first area, a plurality of second grooves located in the second area and a plurality of third grooves located in the third area, the number of the first grooves in each square centimeter of the first area is C1, and the number of the second grooves in each square centimeter of the second area is C2; the number of the third grooves in the third area per square centimeter is C < 3 >, 1.5 < = C < 2 > / C < = 5.0, and 1.5 < = C < 2 > / C < 3 > < = 5.0.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in electric vehicles and consumer electronic products due to their advantages of high energy density, high output power, long cycle life, etc. During the charge-discharge cycle of a lithium-ion battery, the electrode sheet will expand and contract repeatedly, and it is easy to have a poor electrolyte infiltration phenomenon in the central area of the battery, causing lithium plating and deteriorating the cycle performance. Summary of the Invention

[0003] An object of this application is to provide a secondary battery, which can improve the problem of poor electrolyte infiltration and improve the cycle performance.

[0004] In a first aspect of this application, a secondary battery is provided, including an electrode assembly, and the electrode assembly is a laminated structure. The electrode assembly includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer, and the positive electrode material layer is provided on at least one side of the positive electrode current collector. The positive electrode material layer includes a first surface facing away from the positive electrode current collector, and the first surface has a plurality of grooves arranged at intervals. The first surface includes a first region, a second region, and a third region arranged in sequence along a first direction. In the first direction, the second region is located in the middle of the first surface. The grooves include a plurality of first grooves located in the first region, a plurality of second grooves located in the second region, and a plurality of third grooves located in the third region. The number of first grooves per square centimeter in the first region is C1, the number of second grooves per square centimeter in the second region is C2, and the number of third grooves per square centimeter in the third region is C3, and 1.5 ≤ C2 / C1 ≤ 5.0, 1.5 ≤ C2 / C3 ≤ 5.0.

[0005] In this application, first grooves, second grooves, and third grooves are respectively provided in the first region, the second region, and the third region on the surface of the positive electrode material layer, and the ratio of the distribution number of second grooves per unit area to the distribution number of first grooves / third grooves per unit area is defined as 1.5 - 5.0, which can improve the infiltration of the electrolyte in the middle part of the laminated structure electrode assembly into the positive electrode sheet. At the same time, it also enables the number of grooves in each region in the first direction to reach a balance, and there is relatively less powder loss when forming the grooves, so that the secondary battery has a lower self-discharge rate (that is, a high K-value excellent rate) and a higher cycle capacity retention rate.

[0006] The electrode assembly with a laminated structure referred to in this application means an electrode assembly formed by laminating a positive electrode sheet, a separator, and a negative electrode sheet in sequence as known in the art.

[0007] In some embodiments, 2 ≤ C2 / C1 ≤ 3.5 and 2 ≤ C2 / C3 ≤ 3.5. When C2 / C1 and C2 / C3 are within the above ranges, the cyclic performance is better improved, and a relatively low self-discharge rate can be achieved at the same time.

[0008] In some embodiments, the positive electrode sheet includes a first positive electrode sheet and a second positive electrode sheet arranged along a second direction. In the second direction, the second positive electrode sheet is located in the middle of the electrode assembly. The number of second grooves in the second region per square centimeter in the first positive electrode sheet is C21, and the number of second grooves in the second region per square centimeter in the second positive electrode sheet is C22, where 1.5 ≤ C22 / C21 ≤ 5.0, and the second direction is the thickness direction of the electrode assembly. When C22 / C21 is within the above range, the number of second grooves in the second region of the second positive electrode sheet located in the middle of the electrode assembly in the second direction (i.e., the stacking direction of the laminated electrode assembly) is appropriate, the powder loss is relatively small, and it is beneficial to improve the electrolyte infiltration performance in the middle region of the electrode assembly in its thickness direction, so that the secondary battery has a relatively low self-discharge rate and a high cyclic capacity retention rate.

[0009] In some embodiments, 2 ≤ C22 / C21 ≤ 3.5. When C22 / C21 is within the above range, the cyclic performance is better improved, and a relatively low self-discharge rate can be achieved at the same time.

[0010] In some embodiments, 4 ≤ C22 ≤ 13. When C22 is within the above range, the number of second grooves in the positive electrode sheets located in the middle and on one side of the electrode assembly in the second direction is appropriate, which can improve the electrolyte infiltration performance in the middle region of the electrode assembly in its thickness direction, and the powder loss during the formation of the second grooves is relatively small, so that the secondary battery has a relatively low self-discharge rate and a high cyclic capacity retention rate.

[0011] In some embodiments, in the second direction, the number of layers of the positive electrode sheet is N, and the number of layers of the second positive electrode sheet is N1, where 1 / 3N ≤ N1 ≤ 2 / 3N. When N1 / N is within the above range, the number of second positive electrode sheets located in the middle of the electrode assembly in the second direction is relatively large, which is beneficial to improve the electrolyte infiltration performance in the middle region of the electrode assembly in its thickness direction, and the number of first positive electrode sheets and the number of second positive electrode sheets can reach a balance, with less powder loss during the formation of the grooves, so that the secondary battery has a relatively low self-discharge rate and a high cyclic capacity retention rate.

[0012] In some embodiments, the area of the groove is 0.7 mm 2 – 7.0 mm 2 . When the area of the groove is within the above range, the area of the groove is appropriate, and the secondary battery can have a relatively low self-discharge rate and a high cyclic capacity retention rate.

[0013] In some embodiments, the grooves are dot-shaped or linear-shaped.

[0014] In some embodiments, the grooves are formed by pressing the positive electrode material layer, which is beneficial to reducing the impact on the energy density.

[0015] The second aspect of the present application provides an electronic device, including any one of the above secondary batteries. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of a secondary battery provided by an embodiment of the present application.

[0017] Figure 2 It is a schematic diagram of an electrode assembly provided by an embodiment of the present application.

[0018] Figure 3 It is a schematic diagram of a positive electrode sheet provided by an embodiment of the present application.

[0019] Figure 4 It is a stacked schematic diagram of a positive electrode sheet and a negative electrode sheet provided by an embodiment of the present application.

[0020] Figure 5 It is a schematic diagram of an electronic device provided by an embodiment of the present application. Detailed Embodiments

[0021] The technical solutions in the embodiments of the present application will be clearly and detailedly described below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application.

[0022] Hereinafter, the embodiments of the present application will be described in detail. However, the present application can be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Instead, these exemplary embodiments are provided so that the present application can be conveyed to those skilled in the art thoroughly and in detail.

[0023] In addition, for the sake of simplicity and clarity, in the drawings, the sizes or thicknesses of various components and layers may be enlarged. Throughout the text, the same numerical values refer to the same elements. As used herein, the terms "and / or", "as well as / or" include any and all combinations of one or more related listed items. In addition, it should be understood that when element A is referred to as "connected" to element B, or element A is referred to as "abutting" element B, element A can be directly connected to element B, or there may be an intermediate element C and elements A and B can be indirectly connected to each other.

[0024] Further, when describing the embodiments of the present application, the use of "may" refers to "one or more embodiments of the present application".

[0025] The technical terms used herein are for the purpose of describing specific embodiments and are not intended to limit the present application. As used herein, the singular form is intended to also include the plural form unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, refers to the presence of the recited features, values, steps, operations, elements, and / or components, but does not preclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0026] 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 parts, these elements, components, regions, layers, and / or parts should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or part from another element, component, region, layer, or part. Thus, the first element, component, region, layer, or part discussed below may be referred to as the second element, component, region, layer, or part without departing from the teachings of the exemplary embodiments.

[0027] Please refer to Figure 1 , an embodiment of the present application provides a secondary battery 100, including a housing 10, an electrode assembly 20, an electrolyte, and electrode terminals 30. The electrode assembly 20 and the electrolyte are accommodated in the housing 10. The electrode terminals 30 are connected to the electrode assembly 20 and extend out from one side of the housing 10 along the first direction X to connect to external components. In this embodiment, the first direction X refers to the length direction of the secondary battery 100. In this embodiment, the number of the electrode terminals 30 is two, which are a positive electrode terminal and a negative electrode terminal respectively, and the two electrode terminals 30 are located on the same side of the secondary battery 100. It is worth noting that in other embodiments of the present application, the first direction X may also be the width direction of the secondary battery 100.

[0028] Viewed along the second direction Z perpendicular to the first direction X, the secondary battery 100 may have a regular shape, such as a rectangle, a circle, etc., or an irregular shape, such as a T shape, an L shape, etc. In this embodiment, viewed along the second direction Z, the secondary battery 100 is rectangular. In the present application, the second direction Z refers to the thickness direction of the secondary battery 100.

[0029] The housing 10 may be a packaging bag encapsulated with a packaging film (such as an aluminum-plastic film or a steel-plastic film), that is, the secondary battery 100 is a soft-pack battery. Specifically, the housing 10 includes a main body portion 11 and a packaging portion 12. The main body portion 11 is provided with a receiving cavity for receiving the electrode assembly 20, and the packaging portion 12 extends from the edge of the main body portion 11 and is used to seal the main body portion 11. The electrode terminal 30 passes through the packaging portion 12 and extends outward. In other embodiments, the housing 10 is a metal housing, such as a steel shell or an aluminum shell, etc.; in some other embodiments, the housing 10 may also be made of a polymer plastic material.

[0030] Please refer to Figure 2 , the electrode assembly 20 includes a positive electrode sheet 21, a negative electrode sheet 22, and a separator 23 disposed between the positive electrode sheet 21 and the negative electrode sheet 22. The electrode assembly 20 is a stacked structure. The positive electrode sheet 21, the separator 23, and the negative electrode sheet 22 are sequentially stacked along the second direction Z to form a stacked structure.

[0031] The positive electrode sheet 21 includes a positive electrode current collector 211 and a positive electrode material layer 212 provided on at least one surface of the positive electrode current collector 211, that is, the positive electrode material layer 212 is provided on at least one surface of the positive electrode current collector 211. The positive electrode current collector 211 includes at least one of Ni, Ti, Cu, Ag, Au, Pt, Fe, Al, and their combinations. The positive electrode material layer 212 includes a positive electrode active material, and the positive electrode active material may include at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium-rich manganese-based materials, lithium nickel cobalt aluminate, and their combinations. In some embodiments, the positive electrode material layer 212 further includes a binder and optionally includes a conductive agent. The binder can improve the binding between the active material particles and can also improve the binding between the active material and the current collector. The binder includes but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, etc. The conductive agent can improve the conductivity of the electrode sheet. The conductive agent includes but is not limited to graphite, carbon nanotubes, graphene, carbon black, acetylene black, metal powders, etc.

[0032] The negative electrode sheet 22 includes a negative electrode current collector 221 and a negative electrode material layer 222 provided on at least one surface of the negative electrode current collector 221. The negative electrode current collector 221 includes at least one of Ni, Ti, Cu, Ag, Au, Pt, Fe, Al, and their combinations. The negative electrode material layer 222 includes a negative electrode active material, and the negative electrode active material includes one or more of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon-carbon composite, lithium titanate, and metals capable of forming alloys with lithium. In some embodiments, the negative electrode material layer 222 further includes a binder and optionally includes a conductive agent. The binder can improve the binding between active material particles and can also improve the binding between the active material and the current collector. The binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, etc. The conductive agent can improve the conductivity of the electrode sheet. The conductive agent includes, but is not limited to, graphite, carbon black, acetylene black, carbon nanotubes, graphene, metal powder, etc.

[0033] Please refer to Figure 3 As shown, the positive electrode material layer 212 includes a first surface 212A facing away from the positive electrode current collector 211, and the first surface 212A has a plurality of grooves 24 arranged at intervals. Along the first direction X, the first surface 212A includes a first region 212A1, a second region 212A2, and a third region 212A3 arranged in sequence along the first direction X. In the first direction X, the second region 212A2 is located in the middle of the first surface 212A. The grooves 24 include a plurality of first grooves 241 located in the first region 212A1, a plurality of second grooves 242 located in the second region 212A2, and third grooves 243 located in the third region 212A3. The region of the first surface 212A where the plurality of first grooves 241 are provided is the first region 212A1, and the edges of the outermost first grooves 241 enclose the first region 212A1. The region of the first surface 212A where the plurality of second grooves 242 are provided is the second region 212A2, and the edges of the outermost second grooves 242 enclose the second region 212A2. The region of the first surface 212A where the plurality of third grooves 243 are provided is the third region 212A3, and the edges of the outermost third grooves 243 enclose the third region 212A3. In some embodiments, the center line of the second region 212A2 coincides with the center line of the positive electrode sheet 21, and the first region 212A1 and the third region 212A3 are symmetrically arranged with respect to the center line of the positive electrode sheet 21.

[0034] The number of the first grooves 241 in the first region 212A1 per square centimeter is C1, the number of the second grooves 242 in the second region 212A2 per square centimeter is C2, and the number of the third grooves 243 in the third region 212A3 per square centimeter is C3, where 1.5 ≤ C2 / C1 ≤ 5.0 and 1.5 ≤ C2 / C3 ≤ 5.0. For example, C2 / C1 can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or a range composed of any two of these values, and C2 / C3 can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or a range composed of any two of these values. In the electrode assembly 20 of the laminated structure, in the first direction X, in the electrode region in the middle of the electrode plate, the problem of poor electrolyte infiltration is likely to occur, resulting in poor cycle performance of the secondary battery. The grooves 24 provided on the positive electrode plate 21 can serve as electrolyte channels to guide the electrolyte located at the edge of the electrode plate into the interior of the electrode plate and store the electrolyte, improving the electrolyte infiltration effect and thus improving the cycle performance. By making C2 / C1 and C2 / C3 within the above ranges, the density of the grooves 24 in the second region 212A2 in the middle of the positive electrode plate 21 is greater, which can further improve the problem of poor electrolyte infiltration in the middle region of the positive electrode plate 21. At the same time, the number of the grooves 24 provided on the first region 212A1, the second region 212A2, and the third region 212A3 of the positive electrode plate 21 in the first direction X can reach a balance, and relatively few particles (such as Li-Co particles) fall off when forming the grooves 24, so that the secondary battery 100 has a lower self-discharge rate (i.e., a high K value excellent rate) and a higher cycle capacity retention rate. When C2 / C1 < 1.5 (or C2 / C3 < 1.5), the number of the grooves 24 in the second region 212A1 in the middle of the positive electrode plate 21 is relatively small, and the improvement of the electrolyte infiltration and cycle performance is limited. When C2 / C1 > 5.0 and C2 / C1 > 5.0, the number of the grooves 24 in the second region 212A1 is too large, and relatively many positive electrode particles fall off when forming the grooves 24, resulting in the diaphragm being easily punctured and causing an internal short circuit, with a lower K value excellent rate and a higher self-discharge rate.

[0035] In some embodiments, 2 ≤ C2 / C1 ≤ 3.5 and 2 ≤ C2 / C3 ≤ 3.5. When C2 / C1 and C2 / C3 are within the above ranges, the effect of improving the cycle performance is better, and a lower self-discharge rate can be taken into account.

[0036] Please refer to Figure 3 and Figure 4, in some embodiments, the positive electrode sheet 21 includes a first positive electrode sheet 21A and a second positive electrode sheet 21B arranged along the second direction Z. In the second direction Z, the second positive electrode sheet 21B is located in the middle of the electrode assembly 20. In this embodiment, the second direction Z is the thickness direction of the electrode assembly 20, that is, the direction in which the positive electrode sheet, negative electrode sheet, and separator in the electrode assembly 20 are stacked. In some embodiments, in the second direction Z, the first positive electrode sheet 21A may be located on one or both sides of the electrode assembly 20. In this application, the fact that the second positive electrode sheet 21B is located in the middle of the electrode assembly 20 means that in the thickness direction of the electrode assembly, the second positive electrode sheet 21B is not the positive electrode sheet closest to the upper and lower end faces of the electrode assembly, but is stacked in the middle area of the electrode assembly. In some embodiments, the first positive electrode sheet 21A is located on opposite sides of the second positive electrode sheet 21B in the second direction Z. It can be understood that in the second direction Z, a separator and a negative electrode sheet are provided between the first positive electrode sheet 21A and the second positive electrode sheet 21B. In the first positive electrode sheet 21A, the number of the first grooves 241 in the first region 212A1 per square centimeter is C11, the number of the second grooves 242 in the second region 212A2 per square centimeter is C21, and the number of the third grooves 243 in the third region 212A3 per square centimeter is C31, where 1.5 ≤ C21 / C11 ≤ 5.0 and 1.5 ≤ C21 / C31 ≤ 5.0. In the second positive electrode sheet 21B, the number of the first grooves 241 in the first region 212A1 per square centimeter is C12, the number of the second grooves 242 in the second region 212A2 per square centimeter is C22, and the number of the third grooves 243 in the third region 212A3 per square centimeter is C32, where 1.5 ≤ C22 / C12 ≤ 5.0 and 1.5 ≤ C22 / C32 ≤ 5.0. In some embodiments, 1.5 ≤ C22 / C21 ≤ 5.0. For example, C22 / C21 may be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or a range composed of any two of these values. In the second direction Z, the second region 212A2 of the second positive electrode sheet 21B located in the middle of the electrode assembly 20 is more likely to have a problem of poor infiltration. When C22 / C21 is within the above range, the density of the grooves 24 in the second region 212A2 of the second positive electrode sheet 21B located in the middle area of the electrode assembly 20 in the second direction Z is greater, which can improve the problem of poor electrolyte infiltration in the middle area in the thickness direction of the electrode assembly 20, and the number of the second grooves 242 on the positive electrode sheet 21 located in the middle area and one side of the electrode assembly 20 in the thickness direction can reach a balance, and the powder loss is relatively small when forming the second grooves 242, so that the secondary battery 100 has a lower self-discharge rate and a higher cycle capacity retention rate.

[0037] In some embodiments, 2 ≤ C22 / C21 ≤ 3.5. When C22 / C21 is within the above range, the cyclic performance is better improved, and a relatively low self-discharge rate can be achieved at the same time.

[0038] In some embodiments, 4 ≤ C22 ≤ 13. For example, the number of the second grooves 242 in the second region 212A2 per square centimeter of the second positive electrode sheet 21B can be 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13. When C22 is within the above range, the number of the second grooves 242 in the second region 212A2 of the second positive electrode sheet 21B located in the middle of the electrode assembly 20 in the second direction Z is appropriate, which can improve the problem of poor electrolyte infiltration in the middle region in the thickness direction of the electrode assembly 20, and less powder is dropped during the formation of the second grooves 242, so that the secondary battery 100 has a relatively low self-discharge rate and a relatively high cyclic capacity retention rate.

[0039] In some embodiments, in the second direction Z, the number of layers of the positive electrode sheet 21 is N, and the number of layers of the second positive electrode sheet 21B is N1, where 1 / 3N ≤ N1 ≤ 2 / 3N. When N1 / N is within the above range, the proportion of the second positive electrode sheets 21B with a higher density of the second grooves 24 in the positive electrode sheet 21 is relatively large, which is beneficial to improving the problem of poor electrolyte infiltration in the middle region in the thickness direction of the electrode assembly 20, and the number of the first positive electrode sheets 21A and the number of the second positive electrode sheets 21B in the positive electrode sheet 21 can reach a balance, and relatively less powder is dropped during the formation of the grooves 24, so that the secondary battery 100 has a relatively low self-discharge rate and a relatively high cyclic capacity retention rate.

[0040] In some embodiments, the area of the groove 24 is 0.7 mm 2 –7.0 mm 2 . For example, the area of the groove 24 can be 0.7 mm 2 、1 mm 2 、1.5 mm 2 、2 mm 2 、2.5 mm 2 、3 mm 2 、3.5 mm 2 、4 mm 2 、4.5 mm 2 、5 mm 2 、5.5 mm 2 、6 mm 2 、6.5 mm 2 、7 mm 2Or a range formed by any two of these numerical values. When the area of the groove 24 is within the above range, the area of the groove 24 is appropriate, and relatively less powder drops off during the formation of the groove 24, and the groove 24 can store an appropriate amount of electrolyte to improve the wetting performance, so that the secondary battery 100 has a lower self-discharge rate and a higher cycle capacity retention rate.

[0041] In some embodiments, the groove 24 is in a dot shape or a line shape. The dot-shaped groove 24 can be circular, rectangular, polygonal, etc., and the dot-shaped grooves 24 can be arranged orderly or disorderly. The line-shaped groove 24 can be straight, broken line-shaped, curved, etc.

[0042] In some embodiments, the groove 24 is formed by pressing the positive electrode material layer 212 with an embossing roller having bumps under a certain pressure. The size of the groove 24 and the distribution number of the grooves 24 per unit area can be adjusted by the distribution of the bumps on the surface of the embossing roller. By forming the groove 24 in the way of pressing the positive electrode material layer 212, the capacity of the positive electrode material layer 212 can be kept unchanged, so as to reduce the influence on the energy density.

[0043] Please refer to Figure 5 , an embodiment of the present application further provides an electronic device 1, including any one of the above secondary batteries 100. The electronic device 1 of the present application can be, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo earphone, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium-ion capacitor, etc.

[0044] The performance of the secondary battery provided by the present application is described below through specific examples and comparative examples. Among them, taking the secondary battery as a soft-pack battery cell as an example and combining with the specific preparation process and test method to describe the present application. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0045] Example 1

[0046] Preparation of the positive electrode sheet: The positive active material (lithium cobaltate), conductive agent (conductive carbon black), and binder (polyvinylidene fluoride) are dissolved in an N-methylpyrrolidone solution in a weight ratio of 97.5:1:1.5 to form a positive electrode slurry with a solid content of 75%. An 8-μm aluminum foil is used as the current collector, and the positive electrode slurry is coated on both surfaces of the positive electrode current collector to obtain a positive electrode material layer. The thickness of the single-sided positive electrode material layer is 50 μm. Subsequently, it undergoes cold pressing and cutting to obtain the positive electrode sheet. The size of the positive electrode sheet is 60 mm × 40 mm, and the positive electrode tab is integrally formed with the aluminum foil current collector by cutting. Then, an embossing roller is used to press the positive electrode material layer along the length direction of the positive electrode sheet to form multiple grooves, thereby forming a first region, a second region, and a third region. The size of the three regions is 20 mm × 40 mm, the grooves are circular, and the area of a single groove in the three regions is 0.7 mm 2 . The parameters such as the groove density, quantity, and distance between grooves of the first positive electrode sheet and the second positive electrode sheet are shown in Table 1 and Table 2. The structure of the positive electrode sheet is as shown in Figure 3 .

[0047] Preparation of the negative electrode sheet: The negative active material (graphite), conductive agent (conductive carbon black), thickening agent (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) are mixed in a mass ratio of 97.5:1:0.5:1, and then deionized water is added as a solvent and stirred evenly to obtain a negative electrode slurry with a solid content of 50 wt%. A 6-μm copper foil is used as the current collector, and the negative electrode slurry is coated on both surfaces of the negative electrode current collector to obtain a negative electrode material layer. The thickness of the single-sided negative electrode material layer is 75 μm. Subsequently, it undergoes cold pressing and cutting to obtain the negative electrode sheet. The size of the negative electrode sheet is 62 mm × 82 mm, and the negative electrode tab is integrally formed with the copper foil current collector by cutting.

[0048] Preparation of the separator: A polyethylene film is selected as the separator.

[0049] Preparation of the electrolyte: Ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) are mixed in a weight ratio of 20:30:20:28:2 to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 and the organic solvent are mixed in a weight ratio of 8:92 to obtain the electrolyte.

[0050] Preparation of the lithium-ion battery: The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed between the positive electrode sheet and the negative electrode sheet to obtain an electrode assembly; the total number of positive electrode sheets is 24, the number N1 of the second positive electrode sheets is 8, and the rest are the first positive electrode sheets. The second positive electrode sheets are located in the middle of the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and hot-pressed and bonded at a preset pressure, and then the lithium-ion battery is obtained after liquid injection and formation.

[0051] Except for the parameter differences shown in Table 1, the other parameters of Examples 1-18 and Comparative Examples 1-3 are the same as those of Example 1, and the specific changed parameters are shown in Table 1 below. Among them, C1 = C3, C11 = C13, C21 = C23, and C21 / C11 = C22 / C12 = C2 / C1.

[0052] The test methods for each parameter of the present application are described below.

[0053] (1) Test of the distribution quantity, area, and spacing of the grooves:

[0054] Take the positive electrode sheet of unit area in different regions, visually count the distribution quantity of the grooves in the unit area of this region, and calculate the distribution quantity of the grooves per square centimeter in this region.

[0055] Take a positive electrode sheet of a certain area, scan the grooves through a three-dimensional optical profiler, obtain the highest point and the lowest point of the grooves, calculate the horizontal distance between the highest point and the lowest point to obtain the radius r, and obtain the area of the grooves through the radius r.

[0056] Take a positive electrode sheet of a certain area, scan the grooves through a three-dimensional optical profiler, obtain the lowest point of the grooves, and the horizontal distance between the lowest points of two adjacent grooves is the spacing between two adjacent grooves.

[0057] (2) Test of the cycle capacity retention rate:

[0058] At 25°C, charge the lithium-ion battery according to the following charging steps:

[0059] 1) Constant current at 1.65C until 4.10V, then constant voltage until cutoff at 1.55C;

[0060] 2) Constant current at 1.55C until 4.20V, then constant voltage until cutoff at 1.4C;

[0061] 3) Constant current at 1.4C until 4.24V, then constant voltage until cutoff at 1.1C;

[0062] 4) Constant current at 1.1C until 4.27V, then constant voltage until cutoff at 0.7C;

[0063] 5) Constant current at 0.7C until 4.30V, then constant voltage until cutoff at 0.4C;

[0064] 6) Constant current at 0.4C until 4.5V, then constant voltage until cutoff at C / 40.

[0065] The above charge and discharge process is one cycle. Record the discharge capacity after the first cycle as the initial discharge capacity. Cycle 1000 times according to the above charge and discharge process, and record the discharge capacity after 1000 cycles as the discharge capacity after cycling. The capacity retention rate = the discharge capacity after cycling / the initial discharge capacity × 100%.

[0066] (3) K - value test (pressure drop test per unit time):

[0067] At 25 °C, charge the lithium - ion battery at a constant current of 0.7C to 3.85V, then charge it at a constant voltage until 0.05C. Measure the open - circuit voltage of the lithium - ion battery at this time, denoted as OCV 1. After standing for 48 hours, measure the open - circuit voltage of the lithium - ion battery again, denoted as OCV2. The pressure drop per unit time is denoted as K, and K=(OCV 1 - OCV2) / 48. If the K - value is less than 0.07 mV / h, it is determined to pass the K - value test (OK); if the K - value is greater than or equal to 0.07 mV / h, it is determined to fail the K - value test (NG). Take 10,000 lithium - ion batteries for the test and record the passing rate of the K - value test (excellent rate of K - value).

[0068] Table 1

[0069]

[0070] As can be seen from Table 1, compared with the comparative examples, in Examples 1 - 6, grooves are provided, and the distribution numbers of the grooves in the first region, the second region, and the third region per unit area satisfy 1.5 ≤ C 2 / C 1 ≤ 5.0 and 1.5 ≤ C 2 / C 3 ≤ 5, making the distribution numbers of the grooves in each region relatively balanced. Thus, it has both a relatively high excellent rate of K - value and capacity retention rate, and has a better effect on improving self - discharge and cycling. In Comparative Example 2, C2 / C 1 < 1.5, the distribution numbers of the grooves in the first region and the third region are relatively large, and more powder drops off, and the excellent rate of K - value is small, that is, the self - discharge rate is relatively high, and the effect of improving self - discharge is poor; in Comparative Example 3, C 2 / C 1 > 5, the distribution numbers of the grooves in the first region and the third region are relatively small, and the effect of improving the cycling performance is poor. Examples 1, 3, and 4 satisfy 2 ≤ C2 / C1 ≤ 3.5 and 2 ≤ C 2 / C 3 ≤ 3.5, and the distribution numbers of the grooves in each region are more balanced. When having a higher cycling capacity retention rate, they also have a relatively high excellent rate of K - value.

[0071] Examples 3, 8 - 12 satisfy 1.5 ≤ C22 / C21 ≤ 5.0. In the thickness direction of the battery, the distribution quantity of the grooves in the second region of each positive electrode sheet is relatively balanced, so that it has both a high K - value excellent rate and a capacity retention rate. In Example 7, C22 / C21 < 1.5. In the thickness direction of the battery, the distribution quantity of the grooves in the second region of the positive electrode sheet on one side is large, there is more powder falling off, the K - value excellent rate is small, and the effect of improving self - discharge is poor. In Example 13, C2 / C1 > 5. In the thickness direction of the battery, the distribution quantity of the grooves in the second region of the positive electrode sheet on one side is small, and the effect of improving the cycle performance is poor. Examples 3, 9, 10 satisfy 2 ≤ C22 / C21 ≤ 3.5. The distribution quantity of the grooves in the second region of each positive electrode sheet is more balanced. When having a higher cycle capacity retention rate, it has both a high K - value excellent rate.

[0072] It can be seen from Examples 3, 14 - 18 that when 4 ≤ C22 ≤ 13 is satisfied, in the thickness direction of the battery, the distribution quantity of the grooves in the second region of the positive electrode sheet on one side is more appropriate, so that it has both a high K - value excellent rate and a capacity retention rate. In Example 14, C22 < 4. The distribution quantity of the grooves in the second region of the positive electrode sheet on one side is small, and the effect of improving the cycle performance is poor. In Example 18, C22 > 13. The distribution quantity of the grooves in the second region of the positive electrode sheet on one side is large, there is more powder falling off, and the K - value excellent rate is small.

[0073] Except for the parameter differences shown in Table 2, the other parameters of Examples 19 - 27 are the same as those of Example 3.

[0074] Table 2

[0075]

[0076] It can be seen from Table 2 that Examples 3, 20 - 21 satisfy 1 / 3 ≤ N1 / N ≤ 2 / 3. In the thickness direction of the battery, the proportion of the number of positive electrode sheets with more grooves distributed in the second region in the middle region to the total number of positive electrode sheets is more appropriate, so that it has both a high K - value excellent rate and a capacity retention rate. In Example 19, N1 / N < 1 / 4. The number of positive electrode sheets with more grooves distributed in the second region in the middle region is small, and the effect of improving the cycle performance is poor. In Example 22, N1 / N > 2 / 3. The number of positive electrode sheets with more grooves distributed in the second region in the middle region is large, there is more powder falling off, and the K - value excellent rate is small.

[0077] It can be seen from Examples 3, 23 - 27 that the area range of the grooves is 0.7mm 2 to 7mm 2 When, it can have both a high K - value excellent rate and a capacity retention rate. In Example 23, the area of the grooves is less than 0.7mm2 , the effect of improving the cycle performance is poor. In Example 27, the area of the groove is greater than 7 mm 2 , there is more powder falling off, resulting in a smaller excellent rate of K value.

[0078] The above-disclosed is only the preferred embodiment of the present application. Of course, the scope of rights of the present application cannot be limited thereby. Therefore, equivalent changes made according to the claims of the present application still fall within the scope covered by the present application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly is a laminate structure, the electrode assembly comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer, the positive electrode material layer is disposed on at least one side of the positive electrode current collector, characterized in that: The positive electrode material layer includes a first surface facing away from the positive electrode current collector, the first surface has a plurality of grooves arranged at intervals, the first surface includes a first region, a second region and a third region arranged in sequence along a first direction, in the first direction, the second region is located in the middle of the first surface, the grooves include a plurality of first grooves located in the first region, a plurality of second grooves located in the second region and a plurality of third grooves located in the third region, the number of the first grooves in the first region per square centimeter is C1, the number of the second grooves in the second region per square centimeter is C2, the number of the third grooves in the third region per square centimeter is C3, 1.5≤C2 / C1≤5.0, 1.5≤C2 / C3≤5.

0.

2. The secondary battery according to claim 1, wherein: 2≤C 2 / C 1≤3.5, 2≤C 2 / C 3≤3.

5.

3. The secondary battery according to claim 1, wherein: The positive electrode sheet includes a first positive electrode sheet and a second positive electrode sheet arranged along a second direction. In the second direction, the second positive electrode sheet is located in the middle of the electrode assembly. The number of the second grooves in the first positive electrode sheet in the second area per square centimeter is C 21, and the number of the second grooves in the second positive electrode sheet in the second area per square centimeter is C 22, 1.5≤C 22 / C 21≤5.0, and the second direction is the thickness direction of the electrode assembly.

4. The secondary battery according to claim 3, characterized in that: 2≤C 22 / C 21≤3.

5.

5. The secondary battery according to claim 3, characterized in that: 4≤C 22≤13。 6. The secondary battery according to claim 3, characterized in that: In the second direction, the number of layers of the positive electrode sheets is N, the number of layers of the second positive electrode sheets is N1, and 1 / 3N≤N 1≤2 / 3N.

7. The secondary battery according to claim 1, wherein: The area of ​​the groove is 0.70 mm 2 -7.0mm 2 .

8. The secondary battery according to claim 1, wherein: The grooves are in a dot shape or a line shape.

9. The secondary battery according to claim 1, wherein: The groove is formed by pressing the positive electrode material layer.

10. An electronic device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 9.

Images