Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. A secondary battery includes a wound electrode assembly. The negative pole piece comprises a negative current collector and a first negative active material layer arranged on the negative current collector. The electrode assembly includes a corner region. The negative pole piece comprises a plurality of first negative pole piece layers located in the corner region, at least one of the plurality of first negative pole piece layers is a first layer, a negative current collector in the first layer is a first conductive region, and a first negative active material layer in the first layer is a first region. The negative pole piece further comprises a second negative active material layer arranged on at least one surface of the first area. The first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material. The cycle size expansion rate of the second negative electrode active material is smaller than that of the first negative electrode active material. The lithium separation risk of the electrode assembly in the corner area can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of energy storage, and in particular, to a secondary battery and an electronic device having the secondary battery. Background Art

[0002] With the popularization of consumer electronic products such as laptop computers, mobile phones, handheld game consoles, tablet computers, mobile power supplies, and drones, people's requirements for the safety performance and cycle life of secondary batteries are becoming increasingly strict.

[0003] A secondary battery generally includes a housing and an electrode assembly disposed within the housing. In related technologies, when the electrode assembly is a wound structure, due to the large stress in the corner area of the electrode assembly and the expansion and deformation of the negative electrode tab during the charge and discharge cycle, the corner area is easily squeezed, and the electrolyte in the corner area is extruded, resulting in a discontinuous distribution of the electrolyte in the corner area and the blockage of the ion channel. Therefore, lithium deposition is likely to occur in the corner area of the electrode assembly, reducing the safety performance and cycle life of the secondary battery. Summary of the Invention

[0004] In view of this, it is necessary to provide a secondary battery that can improve the lithium deposition situation in the corner area of the electrode assembly.

[0005] In addition, it is also necessary to provide an electronic device having the secondary battery.

[0006] A first aspect of the present application provides a secondary battery, including an electrode assembly. The electrode assembly includes a positive electrode tab, a separator, and a negative electrode tab. The positive electrode tab, the separator, and the negative electrode tab are sequentially stacked and wound. The negative electrode tab includes a negative electrode current collector and a first negative electrode active material layer disposed on the negative electrode current collector. The electrode assembly includes a corner area. The negative electrode tab includes a plurality of first negative electrode tab layers located in the corner area, and the plurality of first negative electrode tab layers are spaced apart along the winding direction of the negative electrode tab. At least one of the plurality of first negative electrode tab layers is a first layer. The negative electrode current collector in the first layer is a first conductive region, and the first negative electrode active material layer in the first layer is a first region. The negative electrode tab further includes a second negative electrode active material layer disposed on at least one surface of the first region. The first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material. The cyclic dimensional expansion rate of the second negative electrode active material is less than that of the first negative electrode active material.

[0007] The negative electrode sheet of the present application further includes a second negative electrode active material layer provided on the first region, and the second negative electrode active material in the second negative electrode active material layer has a smaller cyclic size expansion rate compared to the first negative electrode active material in the first negative electrode active material layer. This is beneficial to reducing the volume expansion in the corner region during charge and discharge, thereby improving the risk of lithium precipitation in the negative electrode sheet in the corner region caused by the extrusion of the electrolyte in the corner region when the corner region is squeezed, and improving the safety and cycle life of the secondary battery. In addition, compared with the technical solution in which the first negative electrode active material layer and the second negative electrode active material layer are alternately arranged on the same surface of the negative electrode current collector, the present application is beneficial to reducing the risk of an increase in the thickness of the negative electrode sheet caused by the lamination of the first negative electrode active material layer and the second negative electrode active material layer at the junction, which can also reduce the volume expansion in the corner region during charge and discharge, thereby further reducing the risk of lithium precipitation in the negative electrode sheet in the corner region. Moreover, compared with the technical solution of entirely coating the second negative electrode active material layer on the surface of the negative electrode current collector, the volume energy density of the secondary battery of the present application is greater. It should be noted that the cyclic size expansion rate of the negative electrode active material refers to the change rate of the average size of the negative electrode active material particles when the secondary battery is fully charged compared to the average size of the negative electrode active material particles when the secondary battery is fully discharged.

[0008] Based on the first aspect, in some possible implementation manners, the specific capacity of the second negative electrode active material layer is greater than that of the first negative electrode active material layer. The second negative electrode active material layer has a higher specific capacity compared to the first negative electrode active material layer, which is beneficial to enabling the negative electrode active material located in the corner region to receive more lithium ions and reducing the risk of lithium precipitation in the corner region.

[0009] Based on the first aspect, in some possible implementation manners, the second negative electrode active material layer is located on the surface of the first region facing away from the first conductive region. Therefore, the lithium ions released from the positive electrode active material layer disposed opposite to the first region will preferentially embed in the second negative electrode active material layer. Especially when the first negative electrode active material includes silicon material, the risk of volume expansion of the silicon material due to the embedding of more lithium ions can be reduced, and the risk of lithium precipitation in the negative electrode sheet in the corner region can be further reduced. Moreover, during the production of the negative electrode sheet, after the rolling step, some of the second negative electrode active material may protrude from the surface of the first negative electrode active material layer, and this part of the second negative electrode active material is beneficial to increasing the space for electrolyte storage and circulation, facilitating electrolyte infiltration, thereby further reducing the risk of lithium precipitation in the negative electrode sheet in the corner region and enhancing the safety and cycle life of the secondary battery.

[0010] Based on the first aspect, in some possible implementation manners, the first negative electrode active material includes at least one of graphite and silicon material, and the second negative electrode active material includes at least one of hard carbon, soft carbon, and lithium titanate. Setting the above materials can reduce the risk of lithium precipitation in the negative electrode sheet in the corner region, thereby improving the safety and cycle life of the secondary battery.

[0011] Based on the first aspect, in some possible implementation manners, the silicon material includes at least one of silicon oxide, silicon carbide compound, silicon alloy, and elemental silicon.

[0012] Based on the first aspect, in some possible implementation manners, the ratio of the coating weight per unit area of the second negative electrode active material layer to the coating weight per unit area of the first region is R1, and 2% ≤ R1 ≤ 20%. Therefore, a certain amount of the second negative electrode active material can be made to exist in the corner region, reducing the volume expansion of the corner region during charge and discharge, thereby reducing the risk of lithium plating on the negative electrode sheet in the corner region and improving the safety and cycle life of the secondary battery.

[0013] Based on the first aspect, in some possible implementation manners, 3% ≤ R1 ≤ 15%. Therefore, the risk of a large migration impedance of lithium ions in the second negative electrode active material layer can be reduced, that is, lithium ions can enter the interior of the second negative electrode active material more easily, improving the kinetic performance of the secondary battery, and thereby further reducing the risk of lithium plating on the negative electrode sheet in the corner region.

[0014] Based on the first aspect, in some possible implementation manners, the coating weight per unit area of the second negative electrode active material layer is W1, 0.3 mg / cm 2 ≤ W1 ≤ 1.5 mg / cm 2 . On the one hand, a certain amount of the second negative electrode active material can be made to exist in the corner region, reducing the volume expansion of the corner region during charge and discharge, thereby reducing the risk of lithium plating on the negative electrode sheet in the corner region. On the other hand, the risk of a large migration impedance of lithium ions in the second negative electrode active material layer can be reduced, improving the kinetic performance of the secondary battery, and thereby further reducing the risk of lithium plating on the negative electrode sheet in the corner region.

[0015] Based on the first aspect, in some possible implementation manners, the ratio of the area of the second negative electrode active material layer to the area of the first region is R2, and 50% ≤ R2 ≤ 100%. The extending direction of the winding central axis of the electrode assembly is the first direction, the thickness direction of the electrode assembly is the second direction, and the first direction, the second direction, and the third direction are perpendicular to each other pairwise. By defining the area ratio of the second negative electrode active material layer on the first region, a certain amount of the second negative electrode active material can be made to exist in the corner region, reducing the volume expansion of the corner region during charge and discharge, thereby reducing the risk of lithium plating on the negative electrode sheet in the corner region.

[0016] Based on the first aspect, in some possible implementation manners, the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The ratio of the capacity per unit area of the composite coating formed by the first region and the second negative electrode active material layer to the capacity per unit area of the positive electrode active material layer is CB, and 1.04 ≤ CB ≤ 1.15. By defining the value of CB, the lithium ions released from the positive electrode active material layer can be fully embedded in the composite coating formed by the first region and the second negative electrode active material layer, thereby further reducing the risk of lithium metal deposition on the negative electrode tab in the corner region, and at the same time avoiding material waste caused by too large CB value, etc.

[0017] Based on the first aspect, in some possible implementation manners, 1.05 ≤ CB ≤ 1.12, thereby reducing the risk of large migration impedance of lithium ions inside the material, improving the kinetic performance of the secondary battery, and further reducing the risk of lithium metal deposition on the negative electrode tab in the corner region.

[0018] The second aspect of the present application provides an electronic device, including the above-mentioned secondary battery. The electronic device is powered by the above-mentioned secondary battery, and the volume expansion of the electrode assembly in the corner region is improved. Therefore, the risk of squeezing out the electrolyte in the corner region and causing lithium metal deposition on the negative electrode tab in the corner region can be reduced, thereby improving the safety and cycle life of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a cross-sectional schematic diagram of a secondary battery provided by an embodiment of the present application.

[0020] Figure 2 It is Figure 1 an enlarged view of the secondary battery shown at box II.

[0021] Figure 3A It is Figure 1 a developed view of the negative electrode tab of the secondary battery shown.

[0022] Figure 3B It is a developed view of the negative electrode tab of the secondary battery in some other embodiments.

[0023] Figure 4 It is a cross-sectional schematic diagram of a secondary battery provided by another embodiment of the present application.

[0024] Figure 5 It is Figure 4 an enlarged view of the secondary battery shown at box V.

[0025] Figure 6A It is Figure 4 a developed view of the negative electrode tab of the secondary battery shown.

[0026] Figure 6BIt is an unfolded view of the negative electrode tab of a secondary battery in some other embodiments.

[0027] Figure 7 It is a schematic diagram of the overall structure of an electronic device provided by an embodiment of the present application.

[0028] Main component symbol description: Electronic device 1, Fourth surface 220B

[0029] Housing 10, First negative electrode active material 221

[0030] Electrode assembly 20, Second negative electrode active material 222

[0031] Positive electrode tab 21, Positive electrode active material layer 211

[0032] Negative electrode tab 22, Negative electrode current collector 220

[0033] First negative electrode tab layer 22A, First conductive region 2201

[0034] Separator 23, Second conductive region 2202

[0035] Secondary battery 100, First region 2211

[0036] Battery compartment 101, Second region 2212

[0037] First flat region 201, Winding central axis C

[0038] First corner region 202, Winding direction D

[0039] Second flat region 203, First direction X

[0040] Second corner region 204, Second direction Y

[0041] Positive electrode current collector 210, Third direction Z

[0042] First surface 210A, Fourth direction Y’

[0043] Second surface 210B, Fifth direction Z’

[0044] Third surface 220A

[0045] The following specific embodiments will further illustrate the present application in conjunction with the above-mentioned drawings. Specific embodiments

[0046] The technical solutions in the embodiments of the present application will be described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all 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 description of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application.

[0047] 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. Rather, these exemplary embodiments are provided so that the present application will be thorough and detailed and will convey to those skilled in the art.

[0048] In addition, for the sake of brevity and clarity, in the drawings, the dimensions 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 of the associated listed items. In addition, it should be understood that when element A is referred to as being "connected to" element B, element A may be directly connected to element B, or there may be an intermediate element C and elements A and B may be indirectly connected to each other.

[0049] Furthermore, when describing the embodiments of the present application, "may" means "one or more embodiments of the present application".

[0050] The professional 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 forms are also intended to include the plural forms unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, means 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.

[0051] Spatial relative terms, such as "above", etc., may be used in this document for convenience of description to describe the relationship of one element or feature illustrated in the figures with another element(s) or feature(s). It should be understood that, in addition to the directions described in the figures, spatial relative terms are intended to include different directions of the device or apparatus during use or operation. For example, if the device in the figures is turned over, an element described as "above" or "on" another element or feature will be oriented "below" or "beneath" the other element or feature. Thus, the exemplary term "above" can include both upward and downward directions. It should be understood that although terms such as first, second, third, etc. may be used in this document 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, a first element, component, region, layer, or part discussed below may be referred to as a second element, component, region, layer, or part without departing from the teachings of the exemplary embodiments.

[0052] Please refer to Figure 1 , an embodiment of the present application provides a secondary battery 100, including a housing 10, an electrode assembly 20, and an electrolyte (not shown in the figure). The electrode assembly 20 and the electrolyte are located inside the housing 10.

[0053] The electrode assembly 20 includes a positive electrode plate 21, a negative electrode plate 22, and a separator 23. The separator 23 is disposed between the positive electrode plate 21 and the negative electrode plate 22. The positive electrode plate 21, the separator 23, and the negative electrode plate 22 are stacked and wound. The positive electrode plate 21 includes a positive electrode current collector 210 and a positive electrode active material layer 211 disposed on the positive electrode current collector 210. The positive electrode current collector 210 includes a first surface 210A facing the winding central axis C and a second surface 210B facing away from the first surface 210A. The positive electrode active material layer 211 is disposed on the first surface 210A and the second surface 210B respectively. The negative electrode plate 22 includes a negative electrode current collector 220 and a first negative electrode active material layer 221 disposed on the negative electrode current collector 220. The negative electrode current collector 220 includes a third surface 220A facing the winding central axis C and a fourth surface 220B facing away from the third surface 220A. The first negative electrode active material layer 221 is disposed on the third surface 220A and the fourth surface 220B respectively.

[0054] Among them, a three-dimensional coordinate system is defined by a first direction X, a second direction Y and a third direction Z which are perpendicular to each other. The extension direction of the winding center axis C is the first direction X, which is also the width direction of the positive electrode sheet 21 or the negative electrode sheet 22. The thickness direction of the electrode assembly 20 is the second direction Y. Among them, the electrode assembly 20 can be divided into a first straight area 201, a first corner area 202, a second straight area 203 and a second corner area 204 which are sequentially connected in the winding direction D. The first straight area 201 and the second straight area 203 are arranged oppositely in the second direction Y, and the first corner area 202 and the second corner area 204 are arranged oppositely in the third direction Z. Among them, the corner area is a bending portion of the electrode assembly 20, and the corner area is a concept relative to the straight area. When viewed from the first direction X, the first corner area 202 and the second corner area 204 can be arranged in an arc shape.

[0055] Among them, the positive electrode current collector 210 can be made of aluminum foil or nickel foil. The positive electrode active material layer 211 contains a positive electrode active material, and the positive electrode active material includes a compound (i.e., a lithiated intercalation compound) that can reversibly embed and de-embed metal active ions (such as lithium ions, sodium ions, etc., and lithium ions are taken as an example below). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, nickel and iron. In some embodiments, the positive electrode active material is selected from lithium cobalt oxide (LiCoO 2 ), lithium nickel cobalt manganese ternary material (NCM), lithium nickel cobalt aluminum ternary material (NCA), lithium manganese oxide (LiMn 2 O 4 ), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ) or lithium iron phosphate (LiFePO 4 ) at least one of.

[0056] The negative electrode current collector 220 may be made of at least one of copper foil, nickel foil or carbon-based current collector. The first negative electrode active material layer 221 includes a first negative electrode active material, and the first negative electrode active material may be a negative electrode active material known in the art that can reversibly deintercalate active ions. For example, it may include but is not limited to a combination of one or more of graphite and silicon materials. Among them, graphite may be selected from a combination of one or more of artificial graphite, natural graphite and modified graphite; silicon material may be selected from a combination of one or more of silicon element, silicon oxide, silicon carbon compound, and silicon alloy.

[0057] The isolation film 23 includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene or ultra-high molecular weight polyethylene.

[0058] Please refer to Figures 1 to 3A , the negative electrode tab 22 includes a plurality of first negative electrode tab layers 22A located in the corner region (marked in Figure 3A ), the plurality of first negative electrode tab layers 22A are arranged at intervals along the winding direction D, and at least one of the plurality of first negative electrode tab layers 22A is the first layer. The negative electrode current collector 220 in the first layer is a conductive region 2201, and the negative electrode current collector 220 located in the flat region is the second conductive region 2202. In the following description of the embodiments of the present application, taking the first conductive region 2201 as the region where the negative electrode current collector 220 is located in the first corner region 202 and the second conductive region 2202 as the region where the negative electrode current collector 220 is located in the first flat region 201 as an example for illustration. However, it can be understood that the first conductive region 2201 can also refer to the region where the negative electrode current collector 220 is located in the second corner region 204. Similarly, the second conductive region 2202 can also refer to the region where the negative electrode current collector 220 is located in the second flat region 203. The first negative electrode active material layer 221 in the first layer is the first region 2211 provided on the first conductive region 2201, and the first negative electrode active material layer 221 provided on the second conductive region 2202 is the second region 2212. The first region 2211 and the second region 2212 are connected along the winding direction D. Wherein when the first conductive region 220 is a double-sided coating region, the first region 2211 is the first negative electrode active material layer 221 on one surface of the first conductive region 220.

[0059] Figure 3A is Figure 1 the schematic structural diagram of the unfolded negative electrode tab 22 shown in Figure 3A , a three-dimensional coordinate system is established with the first direction X, the fourth direction Y', and the fifth direction Z'. Wherein the fourth direction Y' is the thickness direction of the negative electrode tab 22, and the fifth direction Z' is the length direction of the negative electrode tab 22. As Figure 3A shown, when the negative electrode tab 22 is unfolded, the plurality of first negative electrode tab layers 22A are arranged at intervals along the fifth direction Z'. At least one of the plurality of first negative electrode tab layers 22A is the layer located in the first corner region 202 after the electrode assembly 20 is wound, that is, the first layer. The negative electrode current collector 220 in the first layer is the first conductive region 2201, and the first negative electrode active material layer 221 in the first layer is the first region 2211. Figure 3A It is shown that in some embodiments, there are multiple first layers among the plurality of first negative electrode tab layers 22A. Therefore, when the negative electrode tab 22 is unfolded, there are multiple first conductive regions 2201, and the first region 2211 is provided on at least one surface of each first conductive region 2201.

[0060] Such as Figure 3BAs shown, in some other embodiments, a first layer may also exist among the multiple first negative electrode sheet layers 22A. Therefore, when the negative electrode sheet 22 is unfolded, it has a first conductive region 2201, and a first region 2211 is provided on at least one surface of the first conductive region 2201.

[0061] As Figures 1 to 3B shown, the negative electrode sheet 22 further includes a second negative electrode active material layer 222 provided on at least one surface of the first region 2211, but the second negative electrode active material layer 222 is not provided on the second region 2212. The second negative electrode active material layer 222 includes a second negative electrode active material. Among them, the cyclic size expansion rate of the second negative electrode active material is smaller than that of the first negative electrode active material. By introducing the second negative electrode active material with a relatively small cyclic size expansion rate, it is beneficial to reduce the volume expansion of the first corner region 202 during charge and discharge. Among them, the cyclic size expansion rate of the first negative electrode active material or the second negative electrode active material can be characterized by the diameter change rate of the negative electrode active material particles of the secondary battery 100 in the fully charged and fully discharged states respectively.

[0062] Moreover, the specific capacity of the second negative electrode active material layer can be set to be greater than that of the first negative electrode active material layer. Since the specific capacity of the second negative electrode active material layer is relatively larger and the number of lithium ions that can be embedded is more, when the lithium ions are released from the positive electrode active material layer 211 opposite to the first region 2211, the degree of lithium intercalation of the second negative electrode active material layer 222 is reduced, which is also beneficial to reducing the risk of lithium precipitation in the first corner region 202 during charge and discharge.

[0063] In some embodiments, the first negative electrode active material includes at least one of graphite and silicon material, and the second negative electrode active material includes at least one of hard carbon, soft carbon and lithium titanate. For example, when the first negative electrode active material includes graphite, the second negative electrode active material includes hard carbon, and hard carbon has a smaller cyclic size expansion rate and a higher specific capacity compared with graphite. Another example is that when the first negative electrode active material includes silicon material, the second negative electrode active material includes hard carbon, and hard carbon has a smaller cyclic size expansion rate compared with silicon material. Another example is that when the first negative electrode active material includes graphite, the second negative electrode active material includes lithium titanate or soft carbon, and lithium titanate or soft carbon has a smaller cyclic size expansion rate compared with graphite.

[0064] The negative electrode sheet 22 of the present application further includes a second negative electrode active material layer 222 provided on the first region 2211, and the cyclic dimensional expansion rate of the second negative electrode active material in the second negative electrode active material layer 222 is smaller, which is beneficial to reducing the volume expansion of the first corner region 202 during charge and discharge, thereby improving the risk of lithium deposition of the negative electrode sheet 22 in the first corner region 202 after being extruded, and enhancing the safety and cycle life of the secondary battery 100. Optionally, in order to further reduce the risk of lithium deposition of the negative electrode sheet 22 in the first corner region 202, a plurality of first negative electrode sheet layers 22A may be provided with the second negative electrode active material layer 222. It can be understood that in order to reduce the risk of lithium deposition of the negative electrode sheet 22 in the second corner region 204, a second negative electrode active material with a smaller cyclic volume expansion can also be provided in the second corner region 204 in a similar manner.

[0065] In addition, compared with the technical solution in which the first negative electrode active material layer 221 and the second negative electrode active material layer 222 are alternately arranged on the surface of the negative electrode current collector 220, the present application is beneficial to reducing the risk of the thickness increase of the negative electrode sheet 22 caused by the lamination at the junction of the first negative electrode active material layer 221 and the second negative electrode active material layer 222, which can also reduce the volume expansion in the first corner region 202 during charge and discharge, thereby further reducing the risk of lithium deposition of the negative electrode sheet 22 in the first corner region 202. Moreover, compared with the technical solution of entirely coating the second negative electrode active material layer 222 on the surface of the negative electrode current collector 220, the secondary battery 100 of the present application has a larger volume energy density.

[0066] Such as Figure 1 and Figure 2As shown, in some embodiments, the second negative electrode active material layer 222 is located on the surface of the first region 2211 facing away from the first conductive region 2201. That is, the first region 2211 is located between the corresponding second negative electrode active material layer 222 and the first conductive region 2201. Due to the position of the second negative electrode active material layer 222, during the charging process, the lithium ions released from the positive electrode active material layer 211 disposed opposite to the first region 2211 will preferentially embed into the second negative electrode active material layer 222. Especially when the first negative electrode active material includes silicon material, the risk of volume expansion of the silicon material due to the embedding of a large amount of lithium ions can be reduced, further reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202, and improving the safety and cycle life of the secondary battery 100. When manufacturing the negative electrode sheet 22, the first negative electrode active material layer 221 can be formed on the third surface 220A and the fourth surface 220B of the negative electrode current collector 220 respectively, and then one or more spaced-apart second negative electrode active material layers 222 are disposed on the surface of the first negative electrode active material layer 221 facing away from the negative electrode current collector 220 by intaglio coating, and then roll-pressed so that at least part of the second negative electrode active material embeds into the first negative electrode active material layer 221 to obtain the negative electrode sheet 22. At this time, the first negative electrode active material layer 221 located between the second negative electrode active material layer 222 and the negative electrode current collector 220 is the first region 2211. Since part of the second negative electrode active material may protrude from the surface of the first negative electrode active material layer 221 after roll-pressing, this part of the second negative electrode active material can increase the space for electrolyte storage and circulation, facilitate electrolyte infiltration, further reduce the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202, and improve the safety and cycle life of the secondary battery 100.

[0067] As Figures 4 to 6A shown, in some other embodiments, the second negative electrode active material layer 222 can also be located between the first region 2211 and the first conductive region 2201. Specifically, Figure 6A shows that there are multiple first layers in the multiple first negative electrode sheet layers 22A. Therefore, when the negative electrode sheet 22 is unfolded, there are multiple first conductive regions 2201. On at least one surface of each first conductive region 2201, there is a first region 2211, and the second negative electrode active material layer 222 is located between the first region 2211 and the first conductive region 2201. As Figure 6B shown, there can also be one first layer in the multiple first negative electrode sheet layers 22A. Therefore, when the negative electrode sheet 22 is unfolded, there is one first conductive region 2201. On at least one surface of this first conductive region 2201, there is a first region 2211, and the second negative electrode active material layer 222 is located between this first region 2211 and the first conductive region 2201.

[0068] When manufacturing the negative electrode sheet 22, one or more spaced-apart second negative electrode active material layers 222 can be formed on the third surface 220A and the fourth surface 220B of the negative electrode current collector 220 by gravure coating. Then, a first negative electrode active material layer 221 is provided on the third surface 220A and the fourth surface 220B of the negative electrode current collector 220 respectively. The first negative electrode active material layer 221 covers the surface of the second negative electrode active material layer 222 facing away from the negative electrode current collector 220 and the side surfaces of the second negative electrode active material layer 222. After rolling, the negative electrode sheet 22 is obtained. At this time, the first negative electrode active material layer 221 located on the surface of the second negative electrode active material layer 222 facing away from the negative electrode current collector 220 is the first region 2211. When coating, the slurry of the second negative electrode active material is in a liquid state and has fluidity, and can fully fill the gaps between two adjacent second negative electrode active material layers 222, so that the thickness of the negative electrode sheet 22 is relatively uniform.

[0069] In some embodiments, the coating weight per unit area of the second negative electrode active material layer 222 is W1, and the coating weight per unit area of the first region 2211 is W2. Then, the ratio R1 of W1 to W2 satisfies: 2% ≤ R1 < 20%. As an example, R1 can be 2%, 4%, 8%, 12%, 16%, 20% or any value within the range composed of any two of the above values. By setting the ratio of the coating weights per unit area of the two, a certain amount of the second negative electrode active material can be provided in the first corner region 202, reducing the volume expansion of the first corner region 202 during charge and discharge, thereby reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202. Optionally, 3% ≤ R1 ≤ 15% can be set. By setting the upper limit of the ratio of the coating weights per unit area of the two, the risk that the migration impedance of lithium ions in the second negative electrode active material layer 222 is relatively large can be reduced, that is, lithium ions can enter the second negative electrode active material more easily, improving the kinetic performance of the secondary battery 100, and further reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202.

[0070] In some embodiments, 0.3 mg / cm 2 ≤ W1 ≤ 1.5 mg / cm 2 。As an example, W1 can be 0.3 mg / cm 2 、0.5 mg / cm 2 、0.8 mg / cm 2 、1.2 mg / cm 2 、1.5 mg / cm 2Or any value within the range formed by any two of the above values. By setting the lower limit of the coating weight per unit area of the second negative electrode active material layer 222, a certain amount of the second negative electrode active material can be provided in the first corner region 202, reducing the volume expansion of the first corner region 202 during charge and discharge, thereby reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202. By setting the upper limit of the coating weight per unit area of the second negative electrode active material layer 222, the risk of a large migration impedance of lithium ions in the second negative electrode active material layer 222 can be reduced, that is, lithium ions can enter the interior of the second negative electrode active material more easily, improving the kinetic performance of the secondary battery 100, and thereby further reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202.

[0071] In some embodiments, the ratio of the area of the second negative electrode active material layer 222 to the area of the first region 2211 is R2, and 50% ≤ R2 ≤ 100%. As an example, R2 can be 50%, 60%, 70%, 80%, 90%, 100% or any value within the range formed by any two of the above values. By defining the area ratio of the second negative electrode active material layer 222 on the first region 2211, a certain amount of the second negative electrode active material can be provided in the first corner region 202, reducing the volume expansion of the first corner region 202 during charge and discharge, thereby reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202.

[0072] In some embodiments, the ratio of the capacity per unit area of the composite coating formed by the first region 2211 and the second negative electrode active material layer 222 to the capacity per unit area of the positive electrode active material layer 211 is CB, and 1.04 ≤ CB ≤ 1.15. Among them, the capacity per unit area can reflect the lithium intercalation ability of the negative electrode active material or the lithium deintercalation ability of the positive electrode active material. As an example, the CB value can be 1.04, 1.06, 1.08, 1.10, 1.12, 1.14, 1.15 or any value within the range formed by any two of the above values. By defining the CB value, the lithium ions deintercalated from the positive electrode active material layer 211 can be fully intercalated into the composite coating formed by the first region 2211 and the second negative electrode active material layer 222, thereby further reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202. Optionally, 1.05 ≤ CB ≤ 1.12 can be set, thereby reducing the risk of a large migration impedance of lithium ions inside the material, improving the kinetic performance of the secondary battery 100, and thereby further reducing the risk of lithium deposition on the negative electrode sheet 22 in the first corner region 202.

[0073] Among them, the secondary battery 100 of the present application can be a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

[0074] Please refer to Figure 7 In an embodiment of the present application, an electronic device 1 is further provided. The electronic device 1 includes a battery compartment 101 and the above-mentioned secondary battery 100 disposed in the battery compartment 101. Among them, the secondary battery 100 of the present application is applicable to electronic devices 1 in various fields. The electronic device 1 is powered by the above-mentioned secondary battery 100, and the lithium deposition condition of the electrode assembly 20 in the corner area is improved. Therefore, the secondary battery 100 has high safety and cycle life. In an embodiment, the electronic device 1 of the present application may be, but is not limited to, a laptop 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 headset, a video recorder, a liquid crystal TV, a portable cleaner, a portable C machine, 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 moped, a 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.

[0075] The present application will be described in detail below through specific examples and comparative examples. Among them, taking the secondary battery 100 as a lithium-ion secondary battery as an example and combining 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.

[0076] Example 1

[0077] (1) Preparation of the positive electrode tab 21: Mix the positive electrode active material lithium cobaltate (LiCoO 2 ), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) according to a weight ratio of 98:1:1, add N-methylpyrrolidone (NMP) as a solvent, and formulate a slurry with a solid content of 75 wt%. Stir evenly. Coat the slurry evenly on the first surface 210A of the positive electrode current collector 210 with a thickness of 9 μm, that is, aluminum foil, and then dry it at 90 °C. Repeat the above coating steps on the second surface 210B of the aluminum foil to obtain an initially double-sided coated positive electrode tab. Cold press the initially positive electrode tab to obtain a positive electrode active material layer 211 with a single-sided coating thickness of 45 μm on the aluminum foil, and then obtain the positive electrode tab 21 through processes such as cutting. Then, weld a positive electrode tab on the empty aluminum foil at one end in the length direction of the positive electrode current collector 210. The material of the positive electrode tab is aluminum.

[0078] (2) Preparation of the negative electrode sheet 22: Mix the first negative electrode active material artificial graphite, sodium carboxymethyl cellulose dispersant (CMC), and styrene-butadiene rubber binder (SBR) in a weight ratio of 97:1.8:1.2, add deionized water as a solvent, and formulate a slurry with a weight percentage of 55 wt%. Stir evenly. Coat the slurry evenly on the third surface 220A of the 5-μm negative electrode current collector 220, i.e., the copper foil, and then dry it at 90°C. Repeat the above coating steps on the fourth surface 220B of the copper foil to obtain an initially double-sided coated negative electrode sheet. Then, mix the second negative electrode active material hard carbon, sodium carboxymethyl cellulose dispersant (CMC), and styrene-butadiene rubber binder (SBR) in a weight ratio of 97:1.8:1.2, add deionized water as a solvent, and formulate another slurry with a weight percentage of 55 wt%. Stir evenly, and then coat this slurry at intervals on the surface of the first region 2211 facing away from the negative electrode current collector 220 by means of gravure coating, and then dry it at 90°C. Roll press the initially obtained negative electrode sheet to obtain a first negative electrode active material layer 221 with a single coating thickness of 45 μm and a second negative electrode active material layer 222 with a single coating thickness of 10 μm. The number of layers of the wound negative electrode sheet 22 is 24 layers, among which the number of layers of the negative electrode sheet 22 in the first corner region 202 and the second corner region 204 is 12 layers. The number of the first layer in Example 1 is 6, and all the first layers are located in the first corner region 202. Then, weld a negative electrode tab on the empty copper foil at one end in the length direction of the negative electrode current collector 220. The material of the negative electrode tab is nickel.

[0079] (3) Preparation of the electrolyte: In a dry argon atmosphere, first mix the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of EC:EMC:DEC = 30:50:20, and then add lithium salt lithium hexafluorophosphate (LiPF 6 ) to the organic solvents, dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0080] (4) Preparation of the separator 23: Select a polyethylene (PE) film with a thickness of 5 μm.

[0081] (5) Preparation of the secondary battery 100: Stack and wind the positive electrode sheet 21, the separator 23, and the negative electrode sheet 22 in sequence to obtain the Figure 1 shown electrode assembly 20. After winding, the second negative electrode active material layer 222 is located on the surface of the corresponding first region 2211 facing away from the first conductive region 2201. Then, inject the electrolyte into the pit of the aluminum-plastic film, lead out the positive electrode tab and the negative electrode tab outside the aluminum-plastic film and then encapsulate them to obtain the secondary battery 100.

[0082] Example 2

[0083] The difference from Example 1 is that the first negative electrode active material is replaced with the same amount of silicon carbide compound.

[0084] Example 3

[0085] The difference from Example 1 is that the second negative electrode active material is replaced with the same amount of lithium titanate.

[0086] Example 4

[0087] The difference from Example 1 is that the second negative electrode active material is replaced with the same amount of soft carbon.

[0088] Example 5

[0089] The difference from Example 1 lies in the preparation of the negative electrode sheet 22. Specifically, the second negative electrode active material is first coated on the surface of the negative electrode current collector 220, and then the first negative electrode active material is coated, so that the second negative electrode active material layer 222 after winding is located between the corresponding first region 2211 and the first conductive region 2201.

[0090] Example 6

[0091] The difference from Example 1 is that the number of the first layer is 1.

[0092] Example 7

[0093] The difference from Example 1 is that the number of the first layer is 3.

[0094] Example 8

[0095] The difference from Example 1 is that the number of the first layer is 9.

[0096] Example 9

[0097] The difference from Example 1 is that the number of the first layer is 12.

[0098] Example 10

[0099] The difference from Example 1 is that all the first layers are arranged in the second corner area 204.

[0100] Example 11

[0101] The difference from Example 1 is that 6 layers of the first layer are arranged in each of the first corner area 202 and the second corner area 204.

[0102] Comparative Example 1

[0103] The difference from Example 1 is that the second negative electrode active material layer is omitted in the negative electrode sheet.

[0104] Comparative Example 2

[0105] The difference from Example 1 is that the first negative electrode active material and the second negative electrode active material are alternately coated on each surface of the negative electrode current collector, and after winding, the second negative electrode active material is located on the first conductive region of the first corner region. Then, the secondary batteries of each example and comparative example were respectively subjected to a cyclic capacity retention rate test, a cyclic interface test, a cyclic size expansion rate test of the negative electrode active material particles, and a specific capacity test of the negative electrode active material. The test results are recorded in Table 1.

[0106] Among them, the test steps for the cyclic capacity retention rate are as follows: 1) At a test temperature of 25 °C, the secondary battery is charged to 4.48 V at a constant current and constant voltage of 1.5C, cut off at 0.05C, left standing for 5 minutes, and then discharged at a constant current of 1C to 3V, left standing for 5 minutes. This process is recorded as 1 cycle, and the discharge capacity of the first cycle is recorded; 2) The charge and discharge are repeated 1000 times according to the above steps, and the discharge capacity of the battery after the 1000th full charge is recorded. Among them, the cyclic capacity retention rate = (the discharge capacity of the battery after 1000 cycles / the discharge capacity of the first cycle) × 100%.

[0107] The test steps for the cyclic interface are as follows: 1) The battery after 1000 cycles is charged at a constant current of 1.5C to 4.48 V, and then charged at a constant voltage of 4.48 V until the current is 0.05C (fully charged, 100% SOC); 2) The electrode assembly 20 is disassembled, and the surface of the negative electrode sheet 22 in the first corner region 202 is inspected. If there is a gray area, it is lithium deposition. If there is no gray area, there is no lithium deposition. Among them, the degree of lithium deposition is divided into no lithium deposition, slight lithium deposition, moderate lithium deposition, and severe lithium deposition. Slight lithium deposition means that the lithium deposition area is less than 0.5% of the overall area of the negative electrode sheet 22, moderate lithium deposition means that the lithium deposition area is 0.5% - 5% of the overall area of the negative electrode sheet 22, and severe lithium deposition means that the lithium deposition area is greater than 5% of the overall area of the negative electrode sheet 22.

[0108] The test steps for the cyclic size expansion rate of the negative electrode active material particles are as follows: 1) At a test temperature of 25 °C, the secondary battery is charged to 100% SOC (State of Charge, charge state); 2) The electrode assembly 20 is disassembled, and a certain area of the negative electrode sheet 22 is punched out from the first flat region 201 or the second flat region 203 as sample 1. The sample 1 is polished in cross-section and observed using a scanning electron microscope (SEM), and the average size of the first negative electrode active material particles in the fully charged state is recorded as L a1 ; The sample 1 is assembled with a lithium sheet into a button battery. After standing for 4 hours, the above button battery is discharged to 0% SOC, and the average size of the first negative electrode active material particles in the fully discharged state is recorded in a similar manner as L b1 ; Calculate the cyclic size expansion rate of the first negative electrode active material particles = (L a1 - L b1) / L b1 × 100%; 3) Punch out a certain area of the negative electrode tab 22 from the first layer of the first corner area 202. Place the obtained negative electrode tab 22 under a microscope to observe its cross-section. It can be observed that there is a first negative electrode active material layer 221 close to the negative electrode current collector 220 and a second negative electrode active material layer 222 disposed on the side of the first negative electrode active material 221 away from the negative electrode current collector 220, and there is an obvious demarcation line between the first negative electrode active material layer 221 and the second negative electrode active material layer 222. Use a scraper to scrape off and collect a part of the second negative electrode active material layer 222 located in the upper layer to obtain Sample 2 (ensure that the scraped part does not exceed the demarcation line between the first negative electrode active material layer 221 and the second negative electrode active material layer 222 observed under the microscope, so that the collected Sample 2 does not contain the first negative electrode active material layer 221); Observe the above Sample 2 using a scanning electron microscope, and record the average size of the second negative electrode active material particles in the fully charged state as L a2 ; Combine the above Sample 2 with a lithium sheet to form a button battery. After standing for 4 hours, discharge the above button battery to 0% SOC, and record the average size of the second negative electrode active material particles in the fully discharged state as L in a similar manner b2 ; Calculate the cyclic size expansion rate of the second negative electrode active material particles = (L a2 - L b2 ) / L b2 × 100%.

[0109] The steps for testing the specific capacity of the negative electrode active material are as follows: The specific capacity of the first negative electrode active material layer 221 can be measured by the following method: 1) At a test temperature of 25 °C, discharge the secondary battery 100 to 0 SOC%, disassemble to obtain the negative electrode tab 22, punch out a certain area of the single-sided coated negative electrode tab 22 from the negative electrode tab 22 in the first flat area 201 as Sample 3, and weigh it using a balance, and record the weight as W a1 , then punch out the negative electrode current collector 220 with the same area, and weigh it using a balance, and record the weight as W'; 2) Combine the above sample with a lithium sheet to form a button battery. After standing for 4 hours, discharge it at a constant current of 20 μA to 0.005 V, stand for 5 minutes, and then charge it at a constant current of 20 μA to 2.0 V to measure the capacity Q of the button battery 1 ; 3) Calculate the specific capacity of the first negative electrode active material layer 221 = Q 1 / (W a1 - W').

[0110] The specific capacity of the second negative electrode active material layer 222 can be measured in the following manner: 1) At a test temperature of 25°C, discharge the secondary battery 100 to 0% SOC, disassemble to obtain the negative electrode plate 22, punch a certain area of the negative electrode plate 22 from the negative electrode plate 22 in the first corner area 202, place the punched negative electrode plate 22 under a microscope to observe its cross-section, and it is possible to observe the first negative electrode active material layer 221 close to the negative electrode current collector 220 and the second negative electrode active material layer 222 provided on the side of the first negative electrode active material 221 away from the negative electrode current collector 220. Moreover, there is an obvious demarcation line between the first negative electrode active material layer 221 and the second negative electrode active material layer 222. Use a scraper to scrape off and collect a part of the second negative electrode active material layer 222 located on the upper layer to obtain sample 2 (ensuring that the scraped-off part does not exceed the demarcation line between the first negative electrode active material layer 221 and the second negative electrode active material layer 222 observed under the microscope so that the collected sample 2 does not contain the first negative electrode active material layer 221); weigh the collected second negative electrode active material layer 222 using a balance, and record the weight as W a2 ; 2) Compose a button battery with the above sample 2 and a lithium sheet. After standing for 4 hours, discharge at a constant current of 20 μA to 0.005 V, stand for 5 minutes, and then charge at a constant current of 20 μA to 2.0 V to measure the capacity Q of the button battery 2 ; 3) Calculate the specific capacity of the second negative electrode active material layer 222 = Q 2 / W a2 .

[0111] Table 1

[0112]

[0113]

[0114]

[0115] As can be seen from the data in Table 1, compared with Comparative Examples 1 to 3, in Example 1, by disposing the second negative electrode active material hard carbon on a plurality of first regions, the risk of lithium deposition on the negative electrode sheet in the first corner region is improved, and the cycle capacity retention rate of the secondary battery is increased. In Examples 2 to 4, by changing the type of the first negative electrode active material or the second negative electrode active material, the risk of lithium deposition on the negative electrode sheet in the first corner region can also be improved, and the cycle capacity retention rate of the secondary battery is increased. Among them, the particle size growth rate of the second negative electrode active material in Examples 1 to 4 after full charge and full discharge does not exceed 5%, indicating that the above-mentioned second negative electrode active materials all have a small cycle volume expansion. Compared with Example 5, Example 1 is beneficial to increasing the space for electrolyte storage and circulation, facilitating electrolyte infiltration. Therefore, the risk of lithium deposition on the negative electrode sheet in the first corner region is further reduced, and the cycle capacity retention rate of the secondary battery is higher. From the experimental data of Example 1 and Examples 6 to 9, it can be seen that as the number of the first layers increases, the cycle capacity retention rate increases. From this, it can be known that as the number of the first layers increases, the degree of lithium deposition decreases. Actually, when the number of the first layer in the first corner region increases to 9 layers or more, there is no lithium deposition in the first corner region. The final statistical result is "slight lithium deposition" because there is no first layer in the second corner region in Example 1 and Examples 6 to 9, and there is lithium deposition in the second corner region. From Comparative Examples 1 to 3, Example 1 and Example 10, it can be seen that disposing the same number of first layers in the first corner region or the second corner region can reduce the degree of lithium deposition and increase the cycle capacity retention rate of the secondary battery. From Example 1, Example 10 and Example 11, it can be seen that disposing the second negative electrode active material layer in the first corner region and the second corner region simultaneously is beneficial to further reducing the degree of lithium deposition and increasing the cycle capacity retention rate.

[0116] Examples 12 to 25

[0117] The difference from Example 1 lies in that the values of R1, W1, R2 or CB are specifically recorded in Table 2.

[0118] Then, the lithium deposition test and the cycle capacity retention rate test are respectively carried out on the secondary batteries of each example and comparative example. The test results of each are recorded in Table 2.

[0119] The coating weight per unit area W2 of the first negative electrode active material can be measured by the following method: 1) At a test temperature of 25°C, discharge the secondary battery 100 to 0 SOC%, disassemble to obtain the negative electrode sheet 22, and dry it after cleaning with dimethyl carbonate (DMC); 2) Punch out a negative electrode sheet 22 with an area of S 1 from the negative electrode sheet 22 in the first flat region 201 as a sample, and weigh it using a balance, and the weight is recorded as W 总1; Use the solvent N-methylpyrrolidone (NMP) to wash away the first negative electrode active material layer 221 of the sample, dry it, weigh the negative electrode current collector 220, and record it as W 01 ; Calculate the coating weight per unit area by the following formula: W2 = (W 总1 - W 01 ) / S 1 .

[0120] The coating weight per unit area W1 of the second negative electrode active material can be measured in the following way: 1) At a test temperature of 25 °C, discharge the secondary battery 100 to 0 SOC%, disassemble to obtain the negative electrode plate 22, clean it with dimethyl carbonate (DMC) and then dry it; 2) Punch out a negative electrode plate 22 with an area of S 2 from the negative electrode plate 22 in the first corner area 202 as a sample, weigh it using a balance, and record the weight as W 总2 ; Use the solvent N-methylpyrrolidone (NMP) to wash away the first negative electrode active material layer 221 and the second negative electrode active material layer 222 of the sample, dry it, weigh the negative electrode current collector 220, and record it as W 02 ; Calculate the coating weight per unit area by the following formula: W12 = W1 + W2 = (W 总2 - W 02 ) / S 2 . After calculating W2 by the above method, W1 = W12 - W2 can be calculated by the following formula.

[0121] R1 can be obtained by calculating the ratio of W1 to W2.

[0122] The area ratio R2 can be measured in the following way: 1) At a test temperature of 25 °C, discharge the secondary battery 100 to 0 SOC%, observe the secondary battery 100 using CT (Computed Tomography), divide the first corner area 202 according to the morphology of the current collector under CT, then measure the arc length of the first corner area 202, disassemble to obtain the negative electrode plate 22, clean it with dimethyl carbonate (DMC) and then dry it; 2) Flatten the negative electrode plate 22, and the measured arc length is the length of the first corner area 202, recorded as L 1 , use a scale to measure the width of the above negative electrode plate 22, which is the width of the first corner area 202, recorded as L 2 ; 3) Completely punch out the first corner area 202 in the above negative electrode plate 22 as a sample; 4) Observe the cross-section of the sample with a scanning electron microscope (SEM), measure the length of the second negative electrode active material layer 222, recorded as L 3 , measure the width of the second negative electrode active material layer 222, recorded as L 4 ; Calculate the area ratio by the following formula: R2 = (L 1×L 2 ) / (L 3 ×L 4 )。

[0123] The test steps for the CB value are as follows: 1) At a test temperature of 25 °C, charge the secondary battery to 100% SOC (State of Charge); 2) Disassemble the electrode assembly 20, and punch out the positive electrode tab 21 and the negative electrode tab 22 with the same area from the positive electrode tab 21 and the negative electrode tab 22 in the first corner area 202 as the positive electrode sample and the negative electrode sample; 3) Assemble a positive electrode sample and a negative electrode sample into a button cell, discharge it at a constant current of 20 μA until 3.0 V, and measure the discharge capacity Q of the button cell 1 ; 4) Assemble a negative electrode sample and a lithium sheet into a button cell, let it stand for 4 hours, then discharge it at a constant current of 20 μA until 0.005 V, and measure the discharge capacity Q of the button cell 2 , and then calculate the CB value = Q 1 / (Q 1 +Q 2 )×100%. The measurement method for the volume energy density is as follows: 1) Use a caliper to measure the length (L), width (W), and thickness (H) of the secondary battery 100 respectively, and obtain its volume V = L×W×H. 2) Measure the energy of the battery: fully charge the above battery, let it stand for 5 minutes, then discharge it at a constant current of 0.2C until 3V, and record the discharge energy E; calculate the volume energy density ED of the battery through the following formula: ED = E / V

[0124] Table 2

[0125]

[0126]

[0127] As can be seen from the data in Table 2, compared with Example 1, in Examples 12 to 18, by adjusting the coating weight per unit area W1 of the second negative electrode active material layer, the ratio R1 of the coating weight per unit area of the second negative electrode active material layer to the coating weight per unit area of the first region was adjusted. When R1 < 2, since the coating weight per unit area W1 of the second negative electrode active material layer was small, the improvement of the lithium deposition situation in the corner area was small, and the increase in the cycle capacity retention rate was small; when 2 ≤ R1 < 20, as R1 increased, the lithium deposition situation in the corner area was improved, and the cycle capacity retention rate increased; when R1 ≥ 20, since the coating weight per unit area of the second negative electrode active material layer was too large, the compaction density after rolling in the corner area was too large, resulting in poor kinetic performance. As R1 increased, the lithium deposition situation in the corner area deteriorated instead, and the corresponding cycle capacity retention rate also decreased. In addition, due to the large W1, the thickness of the second negative electrode active material layer was large, resulting in a serious loss of the volume energy density of the secondary battery. Considering taking into account the interface situation, cycle capacity retention rate and volume energy density in the corner area, 2% ≤ R1 ≤ 20% was defined. Further, 3% ≤ R1 ≤ 15% was preferably selected. Wherein W1 satisfies 0.3mg / cm 2 ≤ W1 ≤ 1.5mg / cm 2 , 1.04 ≤ CB ≤ 1.15, and further preferably 1.05 ≤ CB ≤ 1.12.

[0128] Compared with Example 1, in Examples 19 to 25, by adjusting the area ratio R2 of the second negative electrode active material layer in the first corner area, the risk of lithium deposition in the first corner area can be further improved, and the cycle capacity retention rate of the secondary battery can be increased. When R2 < 50%, since the area ratio of the second negative electrode active material layer is small, the improvement of the lithium deposition situation in the corner area is small, and the increase in the cycle capacity retention rate is small; when 50% ≤ R2 ≤ 100%, as R2 increases, the lithium deposition area in the corner area decreases, and the cycle capacity retention rate increases; when R2 > 100%, the loss of the volume energy density of the secondary battery is relatively serious. Therefore, in order to take into account the interface situation, cycle capacity retention rate and volume energy density in the corner area, 50% ≤ R2 ≤ 100% is defined. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not restrictive. Although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly comprises a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet, the separator and the negative electrode sheet are sequentially stacked and wound, wherein: The negative electrode sheet comprises a negative electrode current collector and a first negative electrode active material layer disposed on the negative electrode current collector, the electrode assembly comprises a corner region, the negative electrode sheet comprises a plurality of first negative electrode sheet layers located in the corner region, the plurality of first negative electrode sheet layers are arranged at intervals along a winding direction of the negative electrode sheet, at least one of the plurality of first negative electrode sheet layers is a first layer, the negative electrode current collector in the first layer is a first conductive region, and the first negative electrode active material layer in the first layer is a first region; The negative electrode sheet further includes a second negative electrode active material layer disposed on at least one surface of the first region, the first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material; The cycle size expansion rate of the second negative electrode active material is smaller than the cycle size expansion rate of the first negative electrode active material.

2. The secondary battery according to claim 1, wherein The gram capacity of the second negative electrode active material layer is greater than the gram capacity of the first negative electrode active material layer.

3. The secondary battery according to claim 1, wherein The second negative electrode active material layer is located on a surface of the first region away from the first conductive region.

4. The secondary battery according to claim 1, wherein The first negative electrode active material includes at least one of graphite and silicon material, and the second negative electrode active material includes at least one of hard carbon, soft carbon and lithium titanate.

5. The secondary battery according to claim 4, wherein: The silicon material includes at least one of silicon oxide, silicon carbon compound, silicon alloy and silicon element.

6. The secondary battery according to claim 1, wherein The ratio of the coating weight per unit area of ​​the second negative electrode active material layer to the coating weight per unit area of ​​the first region is R1, and 2%≤R1≤20%.

7. The secondary battery according to claim 6, wherein: 3％≤R1≤15％。 8. The secondary battery according to claim 1, wherein The coating weight per unit area of ​​the second negative electrode active material layer is W1,0.3 mg / cm 2 ≤W1≤1.5mg / cm 2 .

9. The secondary battery according to claim 1, wherein A ratio of an area of ​​the second negative electrode active material layer to an area of ​​the first region is R2, and 50%≤R2≤100%.

10. The secondary battery according to claim 1, wherein The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and a ratio of a unit area capacity of a composite coating formed by the first region and the second negative electrode active material layer to a unit area capacity of the positive electrode active material layer is CB, 1.04≤CB≤1.

15.

11. The secondary battery according to claim 10, wherein 1.05≤CB≤1.

12.

12. An electronic device, wherein: The electronic device includes the secondary battery according to any one of claims 1 to 11.

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