Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery includes a wound electrode assembly. A positive current collector of a positive electrode tab includes a first surface facing a winding central axis and a second surface opposite to the first surface, and a positive active material layer is disposed on the first surface and the second surface. A negative current collector of a negative electrode tab includes a third surface facing the winding central axis and a fourth surface opposite to the third surface, and a first negative active material layer is disposed on the third surface and the fourth surface. The positive active material layer includes a first region, which is disposed on the first surface and located in a corner region of the electrode assembly. The first negative active material layer includes a second region, which is disposed on the fourth surface and located in the corner region. The first region is located outside the second region and is disposed opposite to the second region. The negative electrode tab further includes a second negative active material layer disposed on the second region, and the second negative active material layer has a greater gravimetric capacity than the first negative active material layer. The present application can improve the lithium precipitation of the electrode assembly in the corner region.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a secondary battery and an electronic device having the secondary battery. Background Technology

[0002] With the increasing popularity of consumer electronics products such as laptops, mobile phones, handheld game consoles, tablets, power banks, and drones, people are becoming more and more demanding in terms of the safety performance and cycle life of rechargeable batteries.

[0003] Secondary batteries typically consist of a casing and an electrode assembly housed within the casing. In related technologies, when the electrode assembly has a wound structure, lithium deposition is prone to occur at the corners, reducing the safety and cycle life of the secondary battery. Summary of the Invention

[0004] In view of this, it is necessary to propose a secondary battery that can improve the lithium plating situation of electrode components in the corner region.

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

[0006] This application provides a secondary battery, including an electrode assembly. The electrode assembly includes a positive electrode, a separator, and a negative electrode, which are sequentially stacked and wound. The positive electrode includes a positive current collector and a positive active material layer. The positive current collector includes a first surface facing the winding center axis and a second surface facing away from the first surface, and the positive active material layer is disposed on the first surface and the second surface respectively. The negative electrode includes a negative current collector and a first negative active material layer. The negative current collector includes a third surface facing the winding center axis and a fourth surface facing away from the third surface, and the first negative active material layer is disposed on the third surface and the fourth surface respectively. The electrode assembly includes a corner region. The positive active material layer includes a first region disposed on the first surface and located in the corner region. The first negative active material layer includes a second region disposed on the fourth surface and located in the corner region. The first region is located outside the second region facing away from the winding center axis and is disposed opposite to the second region. The negative electrode also includes a second negative active material layer disposed on the second region. The specific capacity of the second negative electrode active material layer is greater than that of the first negative electrode active material layer.

[0007] This application further includes a second negative electrode active material layer disposed on the second region, and the specific capacity of the second negative electrode active material layer is higher. This is beneficial to improving the unit area capacity (i.e., lithium intercalation capability) of the negative electrode active material opposite to the first region, that is, increasing the ratio of the unit area capacity of the negative electrode active material layer in the corner region to the positive electrode active material layer opposite to it, i.e., the CB value. This allows lithium ions extracted from the first region to be fully intercalated into the negative electrode active material, thereby improving the problem of insufficient lithium intercalation space caused by the small winding radius and specific surface area of ​​the second region when the first region is located outside the second region. This reduces the risk of lithium plating on the negative electrode in the corner region and improves the safety and cycle life of the secondary battery. In addition, compared with the technical solution of alternating arrangement of the first and second negative electrode active material layers on the fourth surface of the negative electrode current collector, this application is beneficial to reducing the risk of increased negative electrode thickness caused by the stacking of the first and second negative electrode active material layers at the interface, thereby reducing the volume expansion of the negative electrode during charging and discharging, and further improving the cycle life of the secondary battery.

[0008] Based on the first aspect, in some possible implementations, the electrode assembly further includes a flat region, and the corner region is connected to the flat region along the winding direction of the electrode assembly; the length of the overlap area between the second negative electrode active material layer and the flat region is 0 mm to 3 mm.

[0009] Based on the first aspect, in some possible implementations, the second negative electrode active material layer is only disposed in the second region. Compared with the technical solution of coating the entire fourth surface with the second negative electrode active material layer, this application has less impact on the positive electrode potential when the secondary battery is fully charged, thereby reducing safety risks.

[0010] Based on the first aspect, in some possible implementations, the second negative electrode active material layer is located on the surface of the second region facing the winding central axis. Therefore, lithium ions extracted from the first region will preferentially embed into the second region. Especially when the second negative electrode active material includes silicon, the risk of volume expansion of the silicon material due to the embedding of more lithium ions can be reduced, further improving the cycle life of the secondary battery.

[0011] Based on the first aspect, in some possible implementations, the second negative electrode active material layer is located on the surface of the second region facing away from the winding center axis. Compared to the technical solution of setting the second negative electrode active material layer on the surface of the second region facing the winding center axis, it is possible to increase the unit area capacity of the negative electrode active material opposite to the first region while having the same thickness of negative electrode sheet. Moreover, after rolling, a portion of the second negative electrode active material may protrude from the surface of the first negative electrode active material layer. Therefore, this portion of the second negative electrode active material can increase the space for electrolyte storage and circulation, which is beneficial for electrolyte wetting, thereby further reducing the risk of lithium plating on the negative electrode sheet in the corner area and further improving the safety and cycle life of the secondary battery.

[0012] Based on the first aspect, in some possible implementations, the specific capacity ratio of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is 1.1 to 1.5. Therefore, on the one hand, it can increase the unit area capacity of the negative electrode active material opposite to the first region; on the other hand, it can reduce the risk of excessively small interlayer spacing of the second negative electrode active material, leading to high migration resistance of lithium ions within the material. This makes it easier for lithium ions to enter the interior of the second negative electrode active material, improving the kinetic performance of the secondary battery and further reducing the risk of lithium plating on the negative electrode in the corner region.

[0013] Based on the first aspect, in some possible implementations, the specific capacity ratio of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is 1.25 to 1.35. This further reduces the risk of lithium plating on the negative electrode in the corner region.

[0014] Based on the first aspect, in some possible implementations, the first negative electrode active material layer includes a first negative electrode active material, the second negative electrode active material layer includes a second negative electrode active material, the first negative electrode active material includes graphite, and the second negative electrode active material includes silicon. Using the above materials can increase the unit area capacity of the negative electrode active material opposite to the first region, reduce the risk of lithium plating on the negative electrode sheet in the corner region, and improve the safety and cycle life of the secondary battery.

[0015] Based on the first aspect, in some possible implementations, the silicon material is selected from at least one of silicon oxides, silicon carbide compounds, silicon alloys, and elemental silicon.

[0016] Based on the first aspect, in some possible implementations, the second anode active material also includes graphite. By adjusting the content ratio of silicon and graphite in the second anode active material, the specific capacity of the second anode active material layer can be controlled.

[0017] Based on the first aspect, in some possible implementations, the coating weight per unit area of ​​the second negative electrode active material layer is 1.0 mg / cm². 2 Up to 3.0 mg / cm 2 Therefore, on the one hand, it can increase the content of the second negative electrode active material, and on the other hand, it can reduce the risk of ion channel blockage caused by the close contact between the particles of the second negative electrode active material, thereby improving the dynamic performance of the secondary battery. It also facilitates electrolyte wetting, thereby further reducing the risk of lithium plating on the negative electrode sheet in the corner region.

[0018] Based on the first aspect, in some possible implementations, the coating weight per unit area of ​​the second negative electrode active material layer is 1.5 mg / cm³. 2 Up to 3.0 mg / cm 2This is to further increase the content of the second negative electrode active material and further reduce the risk of lithium plating on the negative electrode sheet in the corner region.

[0019] Based on the first aspect, in some possible implementations, the coating weight per unit area of ​​the second negative electrode active material layer is 2.0 mg / cm². 2 Up to 2.5 mg / cm 2 .

[0020] Based on the first aspect, in some possible implementations, the positive electrode includes a first corner segment located in the corner region, the first corner segment comprising a first region. The negative electrode includes a second corner segment located in the corner region, the second corner segment comprising a second region. The first corner segment is located on the outer side of the second corner segment away from the winding center axis and is disposed opposite to the second corner segment. At least one of the first corner segment and the second corner segment includes a plurality of protrusions. The provision of protrusions is beneficial to mitigate the risk of poor electrolyte wetting in the corner region due to increased local volume expansion after the provision of a second negative electrode active material layer (e.g., a second negative electrode active material layer comprising silicon) on the negative electrode, thereby improving the ability of the corner region to store electrolyte and further improving the cycle life of the secondary battery.

[0021] Based on the first aspect, in some possible implementations, the first corner segment and the second corner segment are each provided with multiple protrusions. This further enhances the electrolyte storage capacity of the corner area, thereby further improving the cycle life of the secondary battery.

[0022] Based on the first aspect, in some possible implementations, the extension direction of the winding central axis is the first direction, the thickness direction of the electrode assembly is the second direction, and viewed from the third direction, at least one of the protrusions of the first corner segment at least partially overlaps with at least one of the protrusions of the second corner segment, with the first direction, the second direction, and the third direction being perpendicular to each other. Therefore, while improving the electrolyte storage capacity of the corner area, since the protrusion of the second corner segment can be at least partially embedded in the depression appearing on the other side of the first corner segment, the risk of misalignment between the positive and negative electrode sheets after winding can also be reduced, allowing the first region and the second negative electrode active material layer to fully correspond, thereby reducing the risk of lithium plating on the negative electrode sheet in the corner area.

[0023] Based on the first aspect, in some possible implementations, the extension direction of the winding central axis is the first direction, the thickness direction of the electrode assembly is the second direction, and viewed from the third direction, the multiple protrusions of the first corner segment do not overlap with the multiple protrusions of the second corner segment, and the first direction, the second direction, and the third direction are perpendicular to each other. Therefore, it is beneficial to further improve the electrolyte storage capacity of the corner area, thereby further improving the cycle life of the secondary battery.

[0024] Based on the first aspect, in some possible implementations, the negative electrode current collector includes a first conductive region and a second conductive region sequentially connected in the winding direction. The third surface of the first conductive region is not provided with the first negative electrode active material layer, while the fourth surface of the first conductive region is provided with the first negative electrode active material layer. Both the third and fourth surfaces of the second conductive region are provided with the first negative electrode active material layer. The second negative electrode active material layer is disposed on the second conductive region. Considering the uneven material distribution on both sides of the single-sided coating area, which may result in tension in the single-sided coating area after winding, compared to the technical solution of placing the second negative electrode active material layer on the first conductive region, placing the second negative electrode active material layer on the second conductive region can reduce the risk of the first conductive region wrinkling under tension, further improving the cycle life of the secondary battery.

[0025] Based on the first aspect, in some possible implementations, the negative electrode sheet has N folds along the winding direction, where N is a positive integer greater than 3. The second negative electrode active material layer is located along the winding direction at the m-th to n-th folds of the negative electrode sheet, where m is the first fold of the second conductive region of the negative electrode sheet, and N / 3≤n≤N / 2. Considering that the risk of lithium plating is higher in the first negative electrode active material layer closer to the winding center axis in the corner area, by setting the second negative electrode active material layer at the m-th to n-th folds of the negative electrode sheet, the risk of lithium plating in this part of the first negative electrode active material layer can be improved, further enhancing the safety and cycle life of the secondary battery. If n>N / 2, the electrode sheet located on the outer winding ring will squeeze the electrode sheet located on the inner winding ring, which will also easily lead to a smaller spacing between the electrode sheets on the inner winding ring, causing the electrolyte to be squeezed out and making it difficult for the electrolyte to wet, increasing the risk of lithium plating.

[0026] A second aspect of this application provides an electronic device including the aforementioned secondary battery. The electronic device is powered by the secondary battery, and lithium plating in the corner region of the electrode assembly is improved; therefore, the secondary battery has higher safety and cycle life. Attached Figure Description

[0027] Figure 1 This is a cross-sectional schematic diagram of a secondary battery provided in one embodiment of this application.

[0028] Figure 2 for Figure 1 The enlarged view of the secondary battery shown in box II.

[0029] Figure 3 for Figure 1 The diagram shows the unfolded positive electrode of a secondary battery.

[0030] Figure 4 for Figure 1 The diagram shows the unfolded view of the negative electrode of the secondary battery in some embodiments.

[0031] Figure 5 for Figure 1 The diagram shows the unfolded view of the negative electrode of the secondary battery in some other embodiments.

[0032] Figure 6 for Figure 1 The diagram shows the unfolded view of the negative electrode of the secondary battery in some other embodiments.

[0033] Figure 7 for Figure 1 Enlarged view of the secondary battery in some other embodiments.

[0034] Figure 8 for Figure 1 Enlarged view of the secondary battery in some other embodiments.

[0035] Figure 9 for Figure 1 Enlarged view of the secondary battery in some other embodiments.

[0036] Figure 10 for Figure 1 Enlarged view of the secondary battery in some other embodiments.

[0037] Figure 11 This is a schematic diagram of the overall structure of an electronic device provided in one embodiment of this application.

[0038] Explanation of main component symbols Electronic device 1, fourth surface 220B

[0039] Shell 10 First negative electrode active material layer 221

[0040] Electrode assembly 20 Second negative electrode active material layer 222

[0041] Positive electrode sheet 21; Positive active material layer 211

[0042] First corner segment 21A Negative current collector 220

[0043] Negative electrode plate 22, first region 2111

[0044] Second corner segment 22A, groove 2200

[0045] 23 Isolation membrane 23 First conductive region 2201

[0046] Protrusion 24 Second conductive region 2202

[0047] Secondary battery 100 Secondary area 2211

[0048] Battery compartment 101 winding center surface P

[0049] First straight zone 201, winding center axis C

[0050] First corner zone 202, winding direction D

[0051] Second Straight Zone 203, First Direction X

[0052] Second corner area 204 Second direction Y

[0053] Positive current collector 210 Third direction Z

[0054] First surface 210A, fourth direction Y'

[0055] Second surface 210B, fifth direction Z'

[0056] Third surface 220A

[0057] The following detailed description, in conjunction with the accompanying drawings, will further illustrate this application. Detailed Implementation

[0058] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0059] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to thereby convey this application thoroughly and in detail to those skilled in the art.

[0060] Additionally, for brevity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same numerical values ​​refer to the same elements. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more of the associated enumerated items. Furthermore, it should be understood that when element A is referred to as "connecting" element B, element A may be directly connected to element B, or there may be an intermediate element C and element A and element B may be indirectly connected to each other.

[0061] Furthermore, when describing the implementation of this application, the word "may" refers to "one or more implementations of this application".

[0062] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," as used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0063] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figures. It should be understood that, in addition to the directions depicted in the figures, spatial terms are intended to include different orientations of the device or apparatus during use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations. It should be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Therefore, a first element, component, region, layer, or portion discussed below may be referred to as a second element, component, region, layer, or portion without departing from the teachings of the exemplary embodiments.

[0064] Please see Figure 1 One embodiment of this application provides a secondary battery 100, including a housing 10, an electrode assembly 20, and an electrolyte (not shown). The electrode assembly 20 and the electrolyte are located inside the housing 10.

[0065] The electrode assembly 20 includes a positive electrode 21, a negative electrode 22, and a separator 23, with the separator 23 disposed between the positive electrode 21 and the negative electrode 22. The positive electrode 21, the separator 23, and the negative electrode 22 are stacked and wound together. The positive electrode 21 includes a positive current collector 210 and a positive active material layer 211 disposed on the positive current collector 210. The positive current collector 210 includes a first surface 210A facing the winding center axis C and a second surface 210B facing away from the first surface 210A, with the positive active material layer 211 disposed on both the first surface 210A and the second surface 210B. The negative electrode 22 includes a negative current collector 220 and a first negative active material layer 221 disposed on the negative current collector 220. The negative electrode current collector 220 includes a third surface 220A facing the central axis C and a fourth surface 220B facing away from the third surface 220A. A first negative electrode active material layer 221 is disposed on the third surface 220A and the fourth surface 220B, respectively. In some embodiments, a negative electrode undercoating layer may also be disposed between the third surface 220A and the first negative electrode active material layer 221, or between the fourth surface 220B and the first negative electrode active material layer 221. The negative electrode undercoating layer can be used to improve the bonding force between the negative electrode current collector 220 and the first negative electrode active material layer 221, and reduce the risk of the first negative electrode active material layer 221 peeling off from the negative electrode current collector 220. The material of the negative electrode undercoating layer includes an adhesive and a conductive agent. The adhesive includes at least one selected from polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and polyimide (PI); the conductive agent includes at least one selected from carbon black, carbon nanotubes, and graphene. In some embodiments, a positive electrode undercoating layer can be disposed between the positive electrode current collector 210 and the positive electrode active material layer 211. This undercoating layer can improve the bonding force between the positive electrode current collector 210 and the positive electrode active material layer 211, reducing the risk of the positive electrode active material layer 211 peeling off from the positive electrode current collector 210. The materials of the positive electrode undercoating layer include a binder and a conductive agent. The binder includes at least one selected from polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and polyimide (PI); the conductive agent includes at least one selected from carbon black, carbon nanotubes, and graphene. In some embodiments, the positive electrode undercoating layer can also be used to improve the safety performance of the secondary battery. The materials of the positive electrode undercoating layer include at least one selected from alumina, boehmite, silicon dioxide, and metal oxides. A three-dimensional coordinate system is defined by a first perpendicular direction X, a second perpendicular direction Y, and a third perpendicular direction Z. The extension direction of the winding central axis C is the first direction X, which is also the width direction of the positive electrode 21 or the negative electrode 22. The thickness direction of the electrode assembly 20 is the second direction Y.The electrode assembly 20 can be divided into a first straight region 201, a first corner region 202, a second straight region 203, and a second corner region 204, which are sequentially connected in the winding direction D. The first straight region 201 and the second straight region 203 are arranged opposite to each other in the second direction Y, and the first corner region 202 and the second corner region 204 are arranged opposite to each other in the third direction Z. The corner region is the bent portion of the electrode assembly 20, and the corner region is a concept opposite to the straight region. When viewed from the first direction X, the first corner region 202 and the second corner region 204 can be arranged in an arc shape.

[0066] The positive electrode current collector 210 can be made of aluminum foil or nickel foil, and the negative electrode current collector 220 can be made of at least one of copper foil, nickel foil, or carbon-based current collector. The positive electrode active material layer 211 contains a positive electrode active material, which includes compounds that reversibly intercalate and deintercalate metal active ions (such as lithium ions, sodium ions, etc., hereinafter taking lithium ions as an example) (i.e., lithiation intercalation compounds). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. This lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material is selected from lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese ternary materials (NCM), lithium nickel cobalt aluminum ternary materials (NCA), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5 At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

[0067] The first negative electrode active material layer 221 includes a first negative electrode active material, which can be a negative electrode active material known in the art capable of reversible intercalation and deintercalation of active ions. For example, it can be one or more combinations of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium. Among them, graphite can be selected from one or more combinations of artificial graphite, natural graphite, and modified graphite; silicon materials can be selected from one or more combinations of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; tin-based materials can be selected from one or more combinations of elemental tin, tin oxide compounds, and tin alloys.

[0068] The separator 23 comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene comprises at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene.

[0069] Please see Figure 1 and Figure 2The positive electrode active material layer 211 includes a first region 2111, which is disposed on the first surface 210A and located in a corner region. The first negative electrode active material layer 221 includes a second region 2211, which is disposed on the fourth surface 220B and located in a corner region. The first region 2111 is located on the outside of the second region 2211, away from the winding center axis C, and is disposed opposite to the second region 2211. In this embodiment, the following description takes the example where both the first region 2111 and the second region 2211 are located in the first corner region 202. However, it can be understood that the first region 2111 and the second region 2211 can also be located in the second corner region 204. In this embodiment, the first region 2111 is located outside the second region 2211, away from the winding center axis C, and is opposite to the second region 2211. This means that the first region 2111 and the second region 2211 are located on two adjacent coils of electrode sheets of the electrode assembly 20. The first region 2111 is located on the outer layer of the two coils of electrode sheets, and the second region 2211 is located on the inner layer of the two coils of electrode sheets. The first region 2111 and the second region 2211 are opposite to each other separated by the separator 23. Therefore, when viewed from a third party in the Z direction, the first region 2111 and the second region 2211 at least partially overlap.

[0070] Among them, such as Figure 2 As shown, the positive electrode active material layer 211 located in the first corner region 202 is multi-layered. Therefore, the positive electrode active material layer 211 may include one first region 2111 or multiple first regions 2111, that is, at least two positive electrode active material layers 211 located in the first corner region 202 are provided with the first region 2111. Figure 3 and Figure 4 As shown, where Figure 3 for Figure 1 The diagram shown is a schematic of the unfolded structure of the positive electrode plate 21. Figure 4 for Figure 1 The diagram shows the unfolded structure of the negative electrode plate 22. Figure 3 and Figure 4 A three-dimensional coordinate system is established using the first direction X, the fourth direction Y', and the fifth direction Z', where the fourth direction Y' represents the thickness direction of the positive electrode 21 or the negative electrode 22, and the fifth direction Z' represents the length direction of the positive electrode 21 or the negative electrode 22. For example... Figure 3 As shown, when the positive electrode active material layer 211 includes a plurality of first regions 2111, the plurality of first regions 2111 are spaced apart along the fifth direction Z'. Figure 4 As shown, correspondingly, the first negative electrode active material layer 221 also has a plurality of second regions 2211, which are spaced apart along the fifth direction Z'. (Refer to reference...) Figure 2 Multiple second regions 2211 are respectively set opposite to multiple first regions 2111.

[0071] like Figures 1 to 4 As shown, the negative electrode 22 also includes a second negative electrode active material layer 222 disposed on the second region 2211. The specific capacity of the second negative electrode active material layer 222 is greater than that of the first negative electrode active material layer 221, thereby improving the unit area capacity (i.e., lithium intercalation capability) of the negative electrode active material opposite to the first region 2111. In some embodiments, when the first negative electrode active material includes graphite, the second negative electrode active material includes silicon. Further, the second negative electrode active material may also include a mixture of graphite and silicon. By introducing two different negative electrode active materials into the second negative electrode active material layer 222, the second negative electrode active material layer 222 can combine the advantages of the two negative electrode active materials. Since graphite has a certain degree of flexibility, its combination with silicon can alleviate the overall volume expansion of the second negative electrode active material layer 222, improve the cycle life of the secondary battery 100, and at the same time, graphite and silicon together as the second negative electrode active material can make full use of their advantages to achieve better electrochemical performance. In other embodiments, the implementation of this application is not limited to the negative electrode active materials listed above. The types of the first and second negative electrode active materials can also be selected based on known negative electrode active materials and their specific capacities. Furthermore, this application can modify the negative electrode active material to change its specific capacity, thereby obtaining negative electrode active materials with different specific capacities.

[0072] In related technologies, since the first region is located outside the second region away from the winding center axis, the second region has a smaller winding radius and specific surface area than the first region. Therefore, for lithium ions that have escaped from the first region, the second region cannot fully embed these lithium ions due to insufficient lithium intercalation space, resulting in lithium plating. This application further includes a second negative electrode active material layer 222 disposed on the second region 2211, and the specific capacity of the second negative electrode active material layer 222 is higher. This is beneficial to improving the unit area capacity of the negative electrode active material opposite to the first region 2111, that is, improving the ratio of the unit area capacity of the negative electrode active material layer in the first corner region 202 to the positive electrode active material layer 211 opposite to it, i.e., the CB value. This allows the lithium ions extracted from the first region 2111 to be fully embedded in the negative electrode active material, thereby improving the problem of insufficient lithium intercalation space caused by the small winding radius and specific surface area of ​​the second region 2211 when the first region 2111 is located outside the second region 2211. This reduces the risk of lithium plating in the negative electrode 22 in the first corner region 202 and improves the safety and cycle life of the secondary battery 100. It is understood that, in order to further reduce the risk of lithium plating in the negative electrode 22, the specific capacity of the second negative electrode active material layer 222 in the second corner region 204 can also be improved in a similar way.

[0073] Furthermore, compared to the technical solution where the first negative electrode active material layer 221 and the second negative electrode active material layer 222 are alternately arranged on the fourth surface 220B of the negative electrode current collector 220, this application is advantageous in reducing the risk of increased thickness of the negative electrode sheet 22 due to the overlapping of the first negative electrode active material layer 221 and the second negative electrode active material layer 222 at the interface. This, in turn, helps to reduce the volume expansion of the negative electrode sheet 22 during charging and discharging, further improving the cycle life of the secondary battery 100. Compared to the technical solution of coating the entire fourth surface 220B of the negative electrode current collector 220 with the second negative electrode active material layer 222, this application has a smaller impact on the positive electrode potential when the secondary battery 100 is fully charged, thereby reducing safety risks.

[0074] In some embodiments, the specific capacity ratio of the second negative electrode active material layer 222 to the specific capacity ratio of the first negative electrode active material layer 221 is 1.1 to 1.5. This specific capacity ratio is satisfied when the first negative electrode active material comprises graphite and the second negative electrode active material comprises silicon. As an example, the specific capacity ratio can be 1.1, 1.2, 1.3, 1.4, 1.5, or any value within the range of any two of the above values. On the one hand, this increases the unit area capacity of the negative electrode active material opposite the first region 2111; on the other hand, it reduces the risk of excessively small spacing between the second negative electrode active material layers 222, leading to high migration resistance of lithium ions within the material. This makes it easier for lithium ions to enter the second negative electrode active material, improving the kinetic performance of the secondary battery 100 and further reducing the risk of lithium plating on the negative electrode sheet 22 in the first corner region 202.

[0075] Alternatively, the coating weight per unit area of ​​the second negative electrode active material layer 222 can be set to 1.0 mg / cm². 2 Up to 3.0 mg / cm 2 As an example, the coating weight per unit area of ​​the second negative electrode active material layer 222 can be 1.0 mg / cm². 2 1.2 mg / cm 2 1.5 mg / cm 2 1.8 mg / cm 2 2.0 mg / cm 2 2.2 mg / cm 2 2.5 mg / cm 2 2.8 mg / cm 2 3.0 mg / cm 2Or any value within the range of any two of the above values. Therefore, on the one hand, it can increase the content of the second negative electrode active material, and on the other hand, it can reduce the risk of ion channel blockage caused by the close contact between the particles of the second negative electrode active material, thereby improving the dynamic performance of the secondary battery 100. It also facilitates electrolyte wetting and further reduces the risk of lithium plating on the negative electrode 22 in the first corner region 202.

[0076] like Figure 1 and Figure 2 As shown, in some embodiments, the second negative electrode active material layer 222 is located on the surface of the second region 2211 facing the winding central axis C. The second negative electrode active material layer 222 is located between the second region 2211 and the negative electrode current collector 220. Since the second negative electrode active material layer 222 is located on the surface of the second region 2211 facing the winding central axis C, during the charging process of the secondary battery 100, lithium ions extracted from the first region 2111 will preferentially insert into the second region 2211. Especially when the first negative electrode active material includes graphite and the second negative electrode active material includes silicon, the risk of volume expansion of the silicon material due to the insertion of more lithium ions can be reduced, further improving the cycle life of the secondary battery 100. When manufacturing the negative electrode sheet 22, multiple spaced second negative electrode active material layers 222 can be formed on the fourth surface 220B of the negative electrode current collector 220 by gravure coating. Then, a first negative electrode active material layer 221 is formed on the fourth surface 220B with the second negative electrode active material layers 222. The first negative electrode active material layer 221 covers the surface of the second negative electrode active material layer 222 away from the winding center axis C and the side surface 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 aforementioned surface of the second negative electrode active material layer 222 is the second region 2211. During coating, the slurry of the second negative electrode active material is in a liquid state and has fluidity, which can fully fill the gap between two adjacent second negative electrode active material layers 222, making the thickness of the negative electrode sheet 22 more uniform.

[0077] like Figure 5 As shown, in some other embodiments, the second region 2211 may also be located between the negative electrode current collector 220 and the second negative electrode active material layer 222. In this case, it can be understood that when... Figure 5After the negative electrode sheet 22 is wound, the second negative electrode active material layer 222 is located on the surface of the second region 2211 facing away from the winding center axis C. When manufacturing the negative electrode sheet 22, a first negative electrode active material layer 221 can be formed on the fourth surface 220B of the negative electrode current collector 220. Then, multiple mutually spaced second negative electrode active material layers 222 are set on the surface of the first negative electrode active material layer 221 facing away from the winding center axis C by gravure coating. Then, roll pressing is performed so that at least part of the second negative electrode active material is embedded in 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 on the surface of the second negative electrode active material layer 222 facing the winding center axis C is the second region 2211. Compared to the technical solution of setting the second negative electrode active material layer 222 on the surface of the second region 2211 facing the winding central axis C, this embodiment can increase the unit area capacity of the negative electrode active material opposite to the first region 2111 while the negative electrode sheet 22 has the same thickness. Moreover, since some of the second negative electrode active material may protrude from the surface of the first negative electrode active material layer 221 after rolling, this part of the second negative electrode active material can increase the space for electrolyte storage and circulation, which is conducive to electrolyte wetting, thereby further reducing the risk of lithium plating in the negative electrode sheet 22 in the first corner region 202.

[0078] Please see Figure 6 In some embodiments, when the second region 2211 is located between the negative electrode current collector 220 and the second negative electrode active material layer 222, at least one groove 2220 can also be formed on the second negative electrode active material layer 222. The groove 2220 can be obtained by laser removal of a portion of the second negative electrode active material layer 222. The groove 2220 also increases the space for electrolyte storage and flow, facilitating electrolyte wetting and further reducing the risk of lithium plating on the negative electrode sheet 22 within the first corner region 202.

[0079] like Figure 1 As shown, in some embodiments, the positive electrode 21 includes a first corner segment 21A located in the first corner region 202, the first corner segment 21A including the aforementioned first region 2111. The negative electrode 22 includes a second corner segment 22A located in the first corner region 202, the second corner segment 22A including the aforementioned second region 2211. The first corner segment 21A is located outside the second corner segment 22A, away from the winding center axis C, and is disposed opposite to the second corner segment 22A. (Refer to reference...) Figure 7The first corner segment 21A includes a plurality of protrusions 24. These protrusions 24 are integrally disposed on the first corner segment 21A and protrude from the first corner segment 21A in a direction toward or away from the winding center axis C. By providing protrusions 24 on the first corner segment 21A, it is beneficial to mitigate the risk of poor electrolyte wetting in the first corner region 202 due to local volume expansion caused by the provision of a second negative electrode active material layer 222 (e.g., a second negative electrode active material layer 222 containing silicon material) on the negative electrode sheet 22, thereby improving the electrolyte storage capacity of the first corner region 202 and further improving the cycle life of the secondary battery 100. When manufacturing the positive electrode 21, the positive electrode 21 can be embossed by a pressure roller, so that the first corner section 21A of the positive electrode 21 forms multiple protrusions 24. Through the embossing process, multiple protrusions 24 can be formed on one side of the first corner section 21A, while multiple depressions appear on the other side of the first corner section 21A.

[0080] like Figure 8 As shown, in some embodiments, the second corner segment 22A includes a plurality of protrusions 24. These protrusions 24 are integrally disposed on the second corner segment 22A and protrude from the second corner segment 22A in a direction toward or away from the winding center axis C. By providing protrusions 24 on the first corner segment 21A, it is also beneficial to mitigate the risk of poor electrolyte wetting in the first corner region 202 due to the increased local volume expansion caused by the provision of the second negative electrode active material layer 222 (e.g., the provision of a second negative electrode active material layer 222 containing silicon material) on the negative electrode sheet 22, thereby improving the electrolyte storage capacity of the first corner region 202 and further improving the cycle life of the secondary battery 100.

[0081] like Figure 9 As shown, in other embodiments, the first corner segment 21A and the second corner segment 22A are respectively provided with a plurality of protrusions 24. In this way, the ability of the first corner region 202 to store electrolyte can be further improved, thereby further improving the cycle life of the secondary battery 100.

[0082] Viewed from a third-party perspective (Z), at least one of the plurality of protrusions 24 of the first corner segment 21A at least partially overlaps with at least one of the plurality of protrusions 24 of the second corner segment 22A. For example, the plurality of protrusions 24 of the first corner segment 21A may be arranged to overlap with the plurality of protrusions 24 of the second corner segment 22A. Furthermore, the overlapping protrusions 24 are arranged to protrude in the same direction. For example, when the protrusions 24 of the first corner segment 21A protrude toward the winding center axis C, the protrusions 24 of the second corner segment 22A also protrude toward the winding center axis C, so that the protrusions 24 of the second corner segment 22A can be at least partially embedded in the recess appearing on the other side of the first corner segment 21A. When manufacturing the electrode assembly 20, the positive electrode 21, the separator 23, and the negative electrode 22 can be stacked and then embossed, so that the protrusions 24 of the first corner segment 21A and the protrusions 24 of the second corner segment 22A can fully overlap. Therefore, while improving the electrolyte storage capacity of the first corner region 202, the risk of misalignment between the positive electrode 21 and the negative electrode 22 can be reduced, so that the first region 2111 and the second negative electrode active material layer 222 can be fully corresponded after winding, thereby reducing the risk of lithium plating in the first corner region 202.

[0083] like Figure 10 As shown, the multiple protrusions 24 of the first corner segment 21A can also be configured not to overlap with the multiple protrusions 24 of the second corner segment 22A. This is beneficial to further improve the electrolyte storage capacity of the first corner area 202, thereby further improving the cycle life of the secondary battery 100.

[0084] like Figure 1 and Figure 4 As shown, the negative electrode current collector 220 may include a first conductive region 2201 and a second conductive region 2202 sequentially connected in the winding direction D. The third surface 220A of the first conductive region 2201 does not have the first negative electrode active material layer 221, while the fourth surface 220B of the first conductive region 2201 has the first negative electrode active material layer 221, meaning the first conductive region 2201 is a single-sided coated area. Both the third surface 220A and the fourth surface 220B of the second conductive region 2202 have the first negative electrode active material layer 221, meaning the second conductive region 2202 is a double-sided coated area. In some embodiments, the second negative electrode active material layer 222 is disposed on the second conductive region 2202. Considering the uneven material on both sides of the single-sided coating area, which may cause a certain tension in the single-sided coating area after winding, compared with the technical solution of setting the second negative electrode active material layer 222 on the first conductive area 2201, setting the second negative electrode active material layer 222 on the second conductive area 2202 can reduce the risk that the first conductive area 2201 is more likely to wrinkle under tension, and further improve the cycle life of the secondary battery 100.

[0085] Furthermore, the negative electrode sheet 22 is defined to have N folds along the winding direction D, where N is a positive integer greater than 3. The second negative electrode active material layer 222 is located along the winding direction D at the m-th to n-th folds of the negative electrode sheet 22, where m is the first fold of the second conductive region 2202 in the negative electrode sheet 22, and N / 3≤n≤N / 2. The number of second negative electrode active material layers 222 located at the m-th to n-th folds of the negative electrode sheet 22 can be one or more. Since the second negative electrode active material layer 222 is located at the m-th to n-th folds of the negative electrode sheet 22, it is located on the second conductive region 2202 closer to the winding center axis C along the winding direction D. Considering that the risk of lithium plating is higher in the first negative electrode active material layer 221 closer to the winding center axis C in the first corner region 202 (e.g., the electrolyte wettability may be insufficient in the part of the first corner region 202 closer to the winding center axis C, or the difference in capacity per unit area between the positive and negative electrode sheets is greater in the first corner region 202 closer to the winding center axis C), the risk of lithium plating in this part of the first negative electrode active material layer 221 can be improved by setting the second negative electrode active material layer 222 to be located in the m-th to n-th fold of the negative electrode sheet 22, thereby further improving the safety and cycle life of the secondary battery 100. In this embodiment of the application, a virtual plane passing through the winding center axis C and perpendicular to the second direction Y is defined as the winding center plane P. The winding center plane P divides the electrode assembly 20 into two parts with approximately the same thickness in the second direction Y. Each turn of the winding center plane and the electrode assembly 20 has two intersections in the first corner area 202 and the second corner area 204, respectively. Each fold of the winding refers to the intersection of the winding center plane P and one corner area as the starting edge of the fold, and the extension along the winding direction D to the intersection of the winding center plane P and another corner area as the ending edge of the fold.

[0086] The secondary battery 100 of this application can be a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0087] Please see Figure 11This application also provides an electronic device 1, which includes a battery compartment 101 and a secondary battery 100 disposed within the battery compartment 101. The secondary battery 100 of this application is applicable to electronic devices 1 in various fields. The electronic device 1 is powered by the secondary battery 100, and the lithium plating situation of the electrode assembly 20 at the first corner region 202 is improved, thus the secondary battery 100 has high safety and cycle life. In one embodiment, the electronic device 1 of this application may be, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable C-type devices, mini CD-ROMs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0088] The present application will be described in detail below through specific embodiments and comparative examples. The secondary battery 100 is a lithium-ion secondary battery, and the specific preparation process and testing methods are used to illustrate the present application. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0089] Example 1

[0090] (1) Preparation of positive electrode 21: Lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 96.5:1.5:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%, and the mixture was stirred evenly. Foaming adhesive was first applied to a portion of the surface of the positive current collector 210 (aluminum foil) with a thickness of 9 μm. The slurry was then uniformly coated onto the first surface 210A of the aluminum foil. The slurry was heated to remove the foaming adhesive and expose a portion of the aluminum foil surface. The foil was then dried at 90°C. The coating process was repeated on the second surface 210B of the aluminum foil to obtain a double-sided coated initial positive electrode. The initial positive electrode was cold-pressed to obtain a positive active material layer 211 with a thickness of 77 μm coated on one side of the aluminum foil. After cutting and other processes, the positive electrode 21 was obtained. Then, the positive electrode tab is welded onto the exposed aluminum foil. The positive electrode tab is made of aluminum.

[0091] (2) Preparation of negative electrode 22: The second negative electrode active material, silicon carbide, artificial graphite, conductive carbon black, and polyacrylic acid binder are mixed in a weight ratio of 8:89:1.5:1.5. Deionized water is added as a solvent to prepare a slurry with a weight percentage of 55 wt%. The slurry is stirred evenly and then coated intermittently on the fourth surface 220B of the negative electrode current collector 220 (i.e., copper foil) with a thickness of 5 μm by gravure coating. The slurry is then dried at 90 °C. Then, the first negative electrode active material, artificial graphite, conductive carbon black, and polyacrylic acid binder (PAA) are mixed in a weight ratio of 97:1.5:1.5. Deionized water is added as a solvent to prepare another slurry with a weight percentage of 55 wt%, and the slurry is stirred evenly. The slurry is uniformly coated onto the third surface 220A of the copper foil and then dried at 90°C. The above coating steps are repeated on the fourth surface 220B of the copper foil. A second negative electrode active material slurry is coated onto a portion of the first negative electrode active material layer and dried at 90°C to obtain a double-sided coated initial negative electrode sheet. Then, the initial negative electrode sheet is rolled to obtain a first negative electrode active material layer 221 with a single coating thickness of 60 μm and a second negative electrode active material layer 222 with a single coating thickness of 40 μm. Then, a portion of the first negative electrode active material layer is etched away using a laser to expose the copper foil. A negative electrode tab is soldered onto the exposed copper foil. The negative electrode tab is made of nickel.

[0092] (3) Preparation of electrolyte: In a dry argon atmosphere, the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are first mixed in a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0093] (4) Preparation of the isolation membrane 23: A polyethylene (PE) membrane with a thickness of 9 μm was selected.

[0094] (5) Preparation of secondary battery 100: The positive electrode 21, the separator 23 and the negative electrode 22 are sequentially stacked and wound to obtain Figure 1 The electrode assembly 20 shown has a second negative electrode active material layer 222 located on the surface of the second region 2211 facing the winding center axis C after winding. Then, electrolyte is injected into the pit of the aluminum-plastic film, and the positive and negative electrode tabs are led out of the aluminum-plastic film and then encapsulated to obtain the secondary battery 100.

[0095] Example 2

[0096] The difference from Example 1 lies in the preparation of the negative electrode sheet 22. Specifically, a first negative electrode active material is first coated on the surface of the negative electrode current collector 220, and then a second negative electrode active material is coated at intervals, so that the wound second region 2211 is located between the negative electrode current collector 220 and the second negative electrode active material layer 222 (that is, the second negative electrode active material layer 222 is located on the surface of the second region 2211 away from the winding center axis C).

[0097] Comparative Example 1

[0098] The difference from Example 1 is that the second negative electrode active material layer is omitted in the negative electrode sheet.

[0099] Comparative Example 2

[0100] The difference from Example 1 is that the first negative electrode active material and the second negative electrode active material are alternately coated on the fourth surface of the negative electrode current collector, and the second negative electrode active material is located in the first corner area after winding, that is, only the second negative electrode active material is located in the first corner area.

[0101] Then, lithium plating tests and cycle capacity retention tests were performed on the secondary batteries of each embodiment and comparative example. The test results are recorded in Table 1.

[0102] The test steps for the cycling interface are as follows: 1) At a test temperature of 25℃, the secondary battery 100 is left to stand for 30 minutes, and then charged in stages according to the following charging steps: (a) 2.0C constant current charging to 4.23V, constant voltage charging to 1.8C; (b) 1.8C constant current charging to 4.3V, constant voltage charging to 1.4C; (c) 1.4C constant current charging to 4.4V, constant voltage charging to 1.0C; (d) 1.0C constant current charging to 4.5V, constant voltage charging to 0.05C; after standing for 10 minutes, discharge according to the following steps: 1C DC discharge to 3V. The above charge and discharge process constitutes one cycle, and is repeated 1000 times. 2) Disassemble the electrode assembly 20 and inspect the surface of the negative electrode 22 in the first corner area 202. If there is a gray area, it indicates lithium plating; if there is no gray area, there is no lithium plating. The degree of lithium plating is divided into slight lithium plating, moderate lithium plating, and severe lithium plating. Slight lithium plating is defined as lithium plating area less than 0.5% of the total area of ​​the negative electrode 22; moderate lithium plating is defined as lithium plating area of ​​0.5% to 5% of the total area of ​​the negative electrode 22; and severe lithium plating is defined as lithium plating area greater than 5% of the total area of ​​the negative electrode 22.

[0103] The cycle capacity retention test steps are as follows: 1) Perform charge and discharge cycles according to the above steps, and record the discharge capacity of the first cycle and the discharge capacity of the 1000th cycle; 2) Calculate the cycle capacity retention rate = (discharge capacity of the secondary battery after 1000 cycles / discharge capacity of the first cycle) × 100%.

[0104] Table 1

[0105]

[0106] As can be seen from the data in Table 1, compared with Comparative Examples 1 to 2, Examples 1 and 2 improved the risk of lithium plating on the negative electrode sheet in the first corner region by setting the second negative electrode active material silicon material on the second region, and also improved the cycle capacity retention rate of the secondary battery. Compared to Example 2, in Example 1, the second region is located on the surface of the first region facing the winding center axis, i.e., the second region is located between the first region and the negative electrode current collector. During the lithium-ion intercalation / deintercalation process, lithium ions preferentially intercalate into the first negative electrode active material layer. The second negative electrode active material layer has a lower degree of lithium intercalation and a smaller expansion rate. During long-term cycling, the second negative electrode active material is less damaged and less pulverized, which is beneficial to extending the cycle life of the second negative electrode active material. Moreover, the first negative electrode active material layer located on top can suppress the expansion of the second negative electrode active material layer to a certain extent, reducing the risk of electrolyte shortage caused by the small spacing between the electrodes in the first corner area. This is beneficial to increasing the space for electrolyte storage and circulation and is conducive to electrolyte wetting. Therefore, the risk of lithium plating on the negative electrode in the first corner area in Example 1 is further reduced, and the cycle capacity retention rate of the secondary battery is higher.

[0107] Examples 3 to 16

[0108] The difference from Example 1 is that the ratio of the specific capacity of the second negative electrode active material layer 222 to the specific capacity of the first negative electrode active material layer 221, or the coating weight per unit area of ​​the second negative electrode active material layer, and the number of folds n of the second negative electrode active material layer along the winding direction of the negative electrode sheet are specifically recorded in Table 2.

[0109] The ratio of the specific capacity of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer 221 is controlled by adjusting the specific capacity of the second negative electrode active material layer 222. In addition to silicon carbide compounds, the second negative electrode active material may also include graphite. The specific capacity of the second negative electrode active material layer 222 is controlled by adjusting the ratio of silicon carbide compounds to graphite in the second negative electrode active material. The compositions of the second negative electrode active materials in Examples 3 to 16 are as follows:

[0110] Example 3: The weight ratio of silicon carbide, artificial graphite, conductive carbon black and polyacrylic acid binder is 3:94:1.5:1.5.

[0111] Example 4: The weight ratio of silicon carbide, artificial graphite, conductive carbon black and polyacrylic acid binder is 3.57:93.43:1.5:1.5.

[0112] Example 5: The weight ratio of silicon carbide, artificial graphite, conductive carbon black and polyacrylic acid binder is 10.74:86.26:1.5:1.5.

[0113] Example 6: The weight ratio of silicon carbide, artificial graphite, conductive carbon black and polyacrylic acid binder is 15.03:81.97:1.5:1.5.

[0114] Example 7: The weight ratio of silicon carbide, artificial graphite, conductive carbon black and polyacrylic acid binder is 17:80:1.5:1.5.

[0115] Examples 8 to 16: The weight ratio of each component in the second negative electrode active material is the same as in Example 1.

[0116] Then, lithium plating tests and cycle capacity retention tests were performed on the secondary batteries of each embodiment and comparative example. The test results are recorded in Table 2.

[0117] The specific capacity of the first negative electrode active material layer 221 can be measured in the following ways: 1) At a test temperature of 25°C, the secondary battery 100 is discharged to 0SOC%, and the negative electrode sheet 22 is disassembled. A certain area of ​​single-sided coated negative electrode sheet 22 is punched out from the negative electrode sheet 22 of the first flat region 201 as a sample. The sample is weighed using a balance and the weight is recorded as W1. Then, a negative electrode current collector 220 of the same area is punched out and weighed using a balance and the weight is recorded as W2; 2) The above sample is combined with a lithium sheet to form a button battery. After standing for 4 hours, it is discharged to 0.005V with a constant current of 20μA, stood for 5 minutes, and then charged to 2.0V with a constant current of 20μA. The capacity Q1 of the button battery is measured; 3) The specific capacity of the first negative electrode active material layer 221 is calculated as Q1 / (W1-W2).

[0118] The specific capacity of the second negative electrode active material layer 222 can be measured in the following way: 1) At a test temperature of 25°C, the secondary battery 100 is discharged to 0% SOC, and the negative electrode sheet 22 is obtained by disassembly. A certain area of ​​the negative electrode sheet 22 is punched out from the negative electrode sheet 22 in the first corner region 202 as a sample. The sample is placed under a microscope to observe the cross-section of the sample. A clear boundary line can be observed between the second negative electrode active material layer 222 near the negative electrode current collector 220 and the first negative electrode active material layer 221 far from the negative electrode current collector 220. A scraper is used to remove all the first negative electrode active material layer 221 on the upper layer and part of the second negative electrode active material layer on the lower layer. 1) Scrape off material layer 222 (ensuring the scraped portion exceeds the boundary between the first negative electrode active material layer 221 and the second negative electrode active material layer 222 observed under a microscope), then continue scraping and collect the lower layer of the second negative electrode active material layer 222. Weigh the collected second negative electrode active material layer 222 using a balance, and record the weight as W3; 2) Combine the above sample with lithium sheets to form a button cell. After standing for 4 hours, discharge at a constant current of 20 μA to 0.005V, stand for 5 minutes, and then charge at a constant current of 20 μA to 2.0V. Measure the capacity Q2 of the button cell; 3) Calculate the specific capacity of the second negative electrode active material layer 222 = Q2 / W3. Then calculate the ratio of the specific capacity of the second negative electrode active material layer 222 to the specific capacity of the first negative electrode active material layer 221.

[0119] The unit area coating weight of the second negative electrode active material layer 222 can be measured in the following ways: 1) Discharge the secondary battery 100 to 0SOC% at a test temperature of 25℃, disassemble to obtain the negative electrode sheet 22, clean it with dimethyl carbonate (DMC) and then dry it; 2) Cut a single-sided coated negative electrode sheet 22 with an area of ​​S1 from the negative electrode sheet 22 of the first flat region 201 as a sample of the first negative electrode active material layer, weigh its mass W4, observe its cross-section under a scanning electron microscope to obtain the thickness d1 of the first negative electrode active material layer, cut a negative electrode current collector 220 with an area of ​​S1 and weigh it with a balance to obtain its mass W5, and calculate the density ρ1 of the first negative electrode active material d1 = (W 4- W5) / (S1×d1); 3) Take the negative electrode 22 with a punched area of ​​S2 from the negative electrode 22 in the first corner region 202 as the sample of the first corner region, weigh it using a balance, and record the weight as W. 总 The thickness d2 of the first negative electrode active material layer 221 was obtained by observing the cross-section of the sample in the first corner area under a scanning electron microscope. The first negative electrode active material layer 221 and the second negative electrode active material layer 222 of the sample were washed away with the solvent N-methylpyrrolidone (NMP), dried, and the weight of the negative electrode current collector 220 was weighed and recorded as W0. The coating weight per unit area of ​​the second negative electrode active material was calculated by the following formula: W=[(W 总-W0)-ρ1×S2×d2] / S2.

[0120] Table 2

[0121]

[0122]

[0123] As can be seen from the data in Examples 1, 3 to 7 in Table 2, when the ratio of the specific capacity of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is less than 1.1, the improvement in the interface condition and cycle capacity retention of the first corner region is not significant. When the ratio of the specific capacity of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is less than or equal to 1.35, as the specific capacity ratio increases, the interface condition of the first corner region gradually improves, and the cycle capacity retention gradually increases. This is because as the specific capacity of the second negative electrode active material layer increases, the first corner region can receive more lithium ions. The increased number of electrodes improves lithium plating in the first corner region, thus enhancing cycle capacity retention. However, when the specific capacity ratio of the second negative electrode active material layer to the first negative electrode active material layer is greater than or equal to 1.5, the interface condition in the first corner region deteriorates, and the cycle capacity retention gradually decreases. This is because when the specific capacity of the second negative electrode active material layer is too large, its expansion rate increases accordingly. This easily leads to a reduction in the spacing between the electrodes in the first corner region, making it easier for the electrolyte to be squeezed out and difficult to wet. This increases the risk of lithium plating in the first corner region due to insufficient electrolyte, resulting in a decrease in cycle capacity retention. Therefore, the specific capacity ratio of the second negative electrode active material layer to the first negative electrode active material layer is selected to be 1.1 to 1.5. Further, the specific capacity ratio is preferably 1.25 to 1.35.

[0124] The experimental data from Examples 1 and 8-13 show that when the coating weight per unit area of ​​the second negative electrode active material layer is less than 1.0 mg / cm³, the coating effect is achieved. 2 When the interface condition and cycle capacity retention rate of the first corner region are not significantly improved; when the coating weight per unit area of ​​the second negative electrode active material layer is less than or equal to 2.5 mg / cm³, the improvement is not significant. 2 As the coating weight per unit area increases, the interface condition of the first corner region gradually improves, and the cycle capacity retention gradually increases. This is because as the coating weight per unit area of ​​the second negative electrode active material layer increases, the number of lithium ions that the first corner region can receive increases, thereby improving the lithium plating situation in the first corner region and improving the cycle capacity retention. When the coating weight per unit area of ​​the second negative electrode active material layer is greater than or equal to 3.0 mg / cm³, the cycle capacity retention is improved. 2At this time, the interface condition of the first corner area deteriorates instead, and the cycle capacity retention rate gradually decreases. This is because when the coating weight per unit area of the second negative electrode active material layer is too large, the thickness of the second negative electrode active material layer increases correspondingly. This not only causes a loss of the volume energy density of the secondary battery, but also easily causes the distance between the electrode sheets in the first corner area to decrease. The electrolyte is easily extruded and the electrolyte is not easily infiltrated, increasing the risk of lithium deposition in the first corner area due to the lack of electrolyte and resulting in a decrease in the cycle capacity retention rate. Therefore, the coating weight per unit area of the second negative electrode active material layer is selected to be 1.0 mg / cm 2 ~3.0 mg / cm 2 . Further, the coating weight per unit area of the second negative electrode active material layer is preferably 1.5 mg / cm 2 ~3.0 mg / cm 2 . Further, the coating weight per unit area of the second negative electrode active material layer is preferably 2.0 mg / cm 2 ~2.5 mg / cm 2 .

[0125] It can be seen from the experimental data of Example 1, Examples 14 to 16 that when n < N / 3, the improvement of the interface condition and the cycle capacity retention rate of the first corner area is not significant; when n > N / 2, the interface condition of the first corner area will also deteriorate and the cycle capacity retention rate will decrease. This is because when the number of winding turns of the second negative electrode active material layer is too large, the electrode sheets located on the outer winding circle squeeze the electrode sheets located on the inner winding circle, resulting in a decrease in the distance between the electrode sheets in the inner winding circle. The electrolyte is easily extruded and the electrolyte is not easily infiltrated, thereby increasing the risk of lithium deposition in the first corner area due to the lack of electrolyte and resulting in a decrease in the cycle capacity retention rate. Therefore, N / 3 ≤ n ≤ N / 2.

[0126] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to be limiting. 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, the electrode assembly including a positive electrode, a separator, and a negative electrode, wherein the positive electrode, the separator, and the negative electrode are sequentially stacked and wound together, wherein... The positive electrode includes a positive current collector and a positive active material layer. The positive current collector includes a first surface facing the winding central axis and a second surface facing away from the first surface. The positive active material layer is disposed on the first surface and the second surface, respectively. The negative electrode includes a negative current collector and a first negative active material layer. The negative current collector includes a third surface facing the winding central axis and a fourth surface facing away from the third surface. The first negative active material layer is disposed on the third surface and the fourth surface, respectively. The electrode assembly includes a corner region, the positive electrode active material layer includes a first region, the first region is disposed on the first surface and located in the corner region, the first negative electrode active material layer includes a second region, the second region is disposed on the fourth surface and located in the corner region, and the first region is located outside the second region away from the winding center axis and is disposed opposite to the second region. The negative electrode sheet further includes a second negative electrode active material layer disposed on the second region, wherein the specific capacity of the second negative electrode active material layer is greater than that of the first negative electrode active material layer.

2. The secondary battery as described in claim 1, wherein, The electrode assembly further includes a flat region, and the corner region and the flat region are connected along the winding direction of the electrode assembly; the length of the overlap area between the second negative electrode active material layer and the flat region is 0mm to 3mm.

3. The secondary battery as described in claim 2, wherein, The second negative electrode active material layer is disposed only in the second region.

4. The secondary battery as described in claim 1, wherein, The second negative electrode active material layer is located on the surface of the second region opposite to the winding center axis.

5. The secondary battery as described in claim 1, wherein, The surface of the second negative electrode active material layer opposite to the second region has a groove.

6. The secondary battery as described in claim 1, wherein, The second negative electrode active material layer is located on the surface of the second region facing the winding central axis.

7. The secondary battery as described in claim 1, wherein, The ratio of the specific capacity of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is 1.1 to 1.

5.

8. The secondary battery as described in claim 7, wherein, The ratio of the specific capacity of the second negative electrode active material layer to the specific capacity of the first negative electrode active material layer is 1.25 to 1.

35.

9. The secondary battery as described in claim 1, wherein, 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 first negative electrode active material includes graphite, and the second negative electrode active material includes silicon.

10. The secondary battery as claimed in claim 9, wherein, The silicon material is selected from at least one of silicon oxides, silicon carbide compounds, silicon alloys, and elemental silicon.

11. The secondary battery as claimed in claim 9, wherein, The second negative electrode active material also includes graphite.

12. The secondary battery as claimed in claim 1, wherein, The coating weight per unit area of ​​the second negative electrode active material layer is 1.0 mg / cm². 2 Up to 3.0 mg / cm 2 .

13. The secondary battery as described in claim 12, wherein, The coating weight per unit area of ​​the second negative electrode active material layer is 1.5 mg / cm². 2 Up to 3.0 mg / cm 2 .

14. The secondary battery as claimed in claim 13, wherein, The coating weight per unit area of ​​the second negative electrode active material layer is 2.0 mg / cm². 2 Up to 2.5 mg / cm 2 .

15. The secondary battery as claimed in claim 1, wherein, The positive electrode includes a first corner segment located in the corner region, the first corner segment containing the first region; the negative electrode includes a second corner segment located in the corner region, the second corner segment containing the second region; the first corner segment is located outside the second corner segment away from the winding center axis and is disposed opposite to the second corner segment; at least one of the first corner segment and the second corner segment includes a plurality of protrusions.

16. The secondary battery as claimed in claim 15, wherein, The first corner segment and the second corner segment are respectively provided with the plurality of protrusions.

17. The secondary battery as claimed in claim 16, wherein, The extension direction of the winding center axis is the first direction, the thickness direction of the electrode assembly is the second direction, and when viewed from the third direction, at least one of the plurality of protrusions of the first corner segment and at least one of the plurality of protrusions of the second corner segment at least partially overlap, and the first direction, the second direction and the third direction are perpendicular to each other.

18. The secondary battery as claimed in claim 16, wherein, The extension direction of the winding center axis is the first direction, the thickness direction of the electrode assembly is the second direction, and when viewed from the third direction, the plurality of protrusions of the first corner segment and the plurality of protrusions of the second corner segment do not overlap, and the first direction, the second direction and the third direction are perpendicular to each other.

19. The secondary battery as claimed in claim 1, wherein, The negative electrode current collector includes a first conductive region and a second conductive region connected sequentially in the winding direction. The third surface of the first conductive region is not provided with the first negative electrode active material layer, the fourth surface of the first conductive region is provided with the first negative electrode active material layer, and both the third and fourth surfaces of the second conductive region are provided with the first negative electrode active material layer. The second negative electrode active material layer is disposed on the second conductive region.

20. The secondary battery as claimed in claim 19, wherein, The negative electrode sheet has N folds along the winding direction, where N is a positive integer greater than 3. The second negative electrode active material layer is located from the mth fold to the nth fold along the winding direction, where m is the first fold of the second conductive region in the negative electrode sheet. N / 3≤n≤N / 2.

21. An electronic device, wherein, The electronic device includes a secondary battery as described in any one of claims 1 to 20.

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