Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. A secondary battery includes a wound electrode assembly. A positive current collector of the positive pole piece comprises a first surface facing a winding center shaft and a second surface opposite to the first surface, and a positive active material layer is arranged on the first surface and the second surface. A negative current collector of the negative pole piece comprises a third surface facing the winding center shaft and a fourth surface opposite to the third surface, and the first negative active material layer is arranged on the third surface and the fourth surface. The positive electrode active material layer comprises a first area, and the first area is arranged on the first surface and located in the corner area of the electrode assembly. The first negative electrode active material layer comprises a second area which is arranged on the fourth surface and located in the corner area. The first area is located on the outer side of the second area and is opposite to the second area. The negative pole piece further comprises a second negative active material layer arranged in the second area, and the gram capacity of the second negative active material layer is larger than that of the first negative active material layer. The lithium precipitation condition of the electrode assembly in the corner area can be improved.

Description

Technical Field

[0001] The present 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 Art

[0002] With the popularity of consumer electronic products such as laptops, mobile phones, handheld game consoles, tablets, mobile power supplies and drones, people have increasingly stringent requirements on the safety performance and cycle life of secondary batteries.

[0003] A secondary battery generally includes a housing and an electrode assembly disposed in the housing. In the related art, when the electrode assembly is a winding structure, lithium deposition is easy to occur in the corner area of ​​the electrode assembly, which reduces the safety 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 of the electrode assembly in the corner area.

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

[0006] The first aspect of the present application provides a secondary battery, including an electrode assembly. The electrode assembly includes a positive electrode sheet, a separator and a negative electrode sheet, which are stacked and wound in sequence. The positive electrode sheet 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 away from the first surface, and the positive active material layer is respectively arranged on the first surface and the second surface. The negative electrode sheet 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 away from the third surface, and the first negative active material layer is respectively arranged on the third surface and the fourth surface. The electrode assembly includes a corner area. The positive active material layer includes a first area, which is arranged on the first surface and located in the corner area. The first negative active material layer includes a second area, which is arranged on the fourth surface and located in the corner area. The first area is located outside the second area away from the winding center axis and is arranged opposite to the second area. The negative electrode sheet also includes a second negative active material layer arranged on the second area. 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.

[0007] The negative electrode sheet of the present application also includes a second negative electrode active material layer disposed on the second region, and the gram capacity of the second negative electrode active material layer is higher, which is conducive to improving the unit area capacity (i.e., lithium insertion capacity) of the negative electrode active material relative to the first region, that is, improving 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 relative thereto, i.e., the CB value, so that the lithium ions released from the first region can be fully embedded in the negative electrode active material, thereby improving the problem of insufficient lithium insertion space due to the small winding radius and specific surface area of ​​the second region when the first region is located outside the second region, reducing the risk of lithium precipitation of the negative electrode sheet in the corner region, 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 fourth surface of the negative electrode current collector, the present application is conducive to reducing the risk of increasing the thickness of the negative electrode sheet due to the stacking of the first negative electrode active material layer and the second negative electrode active material layer at the junction, thereby helping to reduce the volume expansion of the negative electrode sheet during the charging and discharging process, 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 straight area, and along the winding direction of the electrode assembly, the corner area and the straight area are connected; the length of the overlapping area between the second negative electrode active material layer and the straight area 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 second negative electrode active material layer on the entire fourth surface, the present 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 center axis. Therefore, the lithium ions released from the first region will be preferentially embedded in the second region, especially when the second negative electrode active material includes silicon material, the risk of volume expansion of the silicon material due to embedding 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 away from the winding center axis. Compared with the technical solution of arranging the second negative electrode active material layer on the surface of the second region facing the winding center axis, the unit area capacity of the negative electrode active material relative to the first region can be increased under the premise that the negative electrode sheet has the same thickness, and part of the second negative electrode active material may protrude from the surface of the first negative electrode active material layer after rolling, so this part of the second negative electrode active material can increase the space for electrolyte storage and circulation, which is conducive to electrolyte infiltration, thereby further reducing the risk of lithium deposition of 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 ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is 1.1 to 1.5. Therefore, on the one hand, the unit area capacity of the negative electrode active material relative to the first region can be increased, and on the other hand, the risk of too small spacing between the second negative electrode active material layers, resulting in a large migration impedance of lithium ions inside the material, can be reduced, so that lithium ions can more easily enter the second negative electrode active material, improve the kinetic performance of the secondary battery, and further reduce the risk of lithium deposition in the negative electrode sheet in the corner area.

[0013] Based on the first aspect, in some possible implementations, the ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is 1.25 to 1.35, so as to further reduce the risk of lithium deposition on the negative electrode sheet in the corner area.

[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 a silicon material. Providing the above materials can increase the unit area capacity of the negative electrode active material relative to the first area, reduce the risk of lithium deposition of the negative electrode sheet in the corner area, 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 oxide, silicon carbon compound, silicon alloy and silicon element.

[0016] Based on the first aspect, in some possible implementations, the second negative electrode active material further includes graphite. By adjusting the content ratio of the silicon material and the graphite material in the second negative electrode active material, the gram capacity of the second negative electrode active material layer can be adjusted.

[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 / cm 2 Up to 3.0mg / cm 2 Therefore, on the one hand, the content of the second negative electrode active material can be increased, and on the other hand, the risk of the second negative electrode active material particles being in close contact and causing the ion channel to be blocked can be reduced, thereby improving the kinetic performance of the secondary battery and facilitating electrolyte infiltration, thereby further reducing the risk of lithium deposition of the negative electrode sheet in the corner area.

[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 / cm 2 Up to 3.0mg / cm 2. To further increase the content of the second negative electrode active material and further reduce the risk of lithium plating of the negative electrode sheet in the corner area.

[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 / cm 2 Up to 2.5 mg / cm 2 .

[0020] Based on the first aspect, in some possible implementations, the positive electrode plate includes a first corner segment located in the corner area, and the first corner segment includes a first area. The negative electrode plate includes a second corner segment located in the corner area, and the second corner segment includes a second area. The first corner segment is located outside the second corner segment away from the winding center axis and is arranged 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 the protrusions is conducive to improving the risk of poor electrolyte infiltration in the corner area due to the increase in local volume expansion after the second negative electrode active material layer (e.g., a second negative electrode active material layer containing silicon material) is provided on the negative electrode plate, thereby improving the ability of the corner area to store electrolyte, thereby further improving the cycle life of the secondary battery.

[0021] Based on the first aspect, in some possible implementations, the first corner section and the second corner section are respectively provided with a plurality of convex portions, so that the electrolyte storage capacity of the corner area can be further improved, 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 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 multiple protrusions of the first corner segment overlaps at least partially 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, while improving the ability of the corner area to store electrolyte, since the protrusion of the second corner segment can be at least partially embedded in the depression that appears on the other side of the first corner segment, the risk of misalignment between the positive electrode sheet and the negative electrode sheet after winding can also be reduced, so that the first area and the second negative electrode active material layer can fully correspond to each other, thereby achieving the effect of reducing the risk of lithium deposition 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 when viewed from the third direction, the multiple protrusions of the first corner section do not overlap with the multiple protrusions of the second corner section, and the first direction, the second direction and the third direction are perpendicular to each other. Therefore, it is beneficial to further improve the ability of the corner area to store electrolyte, 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 connected in sequence in the winding direction. The third surface located in the first conductive region is not provided with a first negative electrode active material layer, and the fourth surface located in the first conductive region is provided with a first negative electrode active material layer. The third surface and the fourth surface located in the second conductive region are both provided with a first negative electrode active material layer. The second negative electrode active material layer is provided on the second conductive region. Considering that the material on both sides of the single-sided coating area is not uniform, resulting in a certain tension in the single-sided coating area after winding, compared with the technical solution of providing the second negative electrode active material layer on the first conductive region, by providing the second negative electrode active material layer on the second conductive region, the risk of the first conductive region being more easily wrinkled under tension can be reduced, thereby 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 from the mth fold to the nth fold of the negative electrode sheet along the winding direction, where m is the first fold of the second conductive region in the negative electrode sheet, and N / 3≤n≤N / 2. Considering that the closer the first negative electrode active material layer is to the winding center axis in the corner area, the higher the risk of lithium plating, by setting the second negative electrode active material layer to be located from the mth fold to the nth fold of the negative electrode sheet, the risk of lithium plating of the first negative electrode active material layer in this part can be improved, further improving the safety and cycle life of the secondary battery. If n>N / 2, the electrode sheet located on the outer circle of the winding squeezes the electrode sheet located on the inner circle of the winding, which can also easily lead to a smaller spacing between the electrode sheets on the inner circle of the winding, and the electrolyte is squeezed out and the electrolyte is not easy to infiltrate, increasing the risk of lithium plating.

[0026] The second aspect of the present application provides an electronic device, comprising the secondary battery. The electronic device is powered by the secondary battery, and the lithium deposition of the electrode assembly in the corner area is improved, so the secondary battery has higher safety and cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic cross-sectional view of a secondary battery provided in accordance with one embodiment of the present application.

[0028] Figure 2 for Figure 1 An enlarged view of the secondary battery shown at frame II.

[0029] Figure 3 for Figure 1 An expanded view of the positive electrode sheet of the secondary battery shown.

[0030] Figure 4 for Figure 1 The negative electrode sheet of the secondary battery shown is an expanded view in some embodiments.

[0031] Figure 5 for Figure 1 The negative electrode plate of the secondary battery shown is an expanded view in some other embodiments.

[0032] Figure 6 for Figure 1 The negative electrode plate of the secondary battery shown is an expanded view in some other embodiments.

[0033] Figure 7 for Figure 1 Enlarged views of secondary batteries in other embodiments are shown.

[0034] Figure 8 for Figure 1 Enlarged views of secondary batteries in other embodiments are shown.

[0035] Fig. 9 for Figure 1 Enlarged views of secondary batteries in other embodiments are shown.

[0036] Fig.10 for Figure 1 Enlarged views of secondary batteries in other embodiments are shown.

[0037] Fig.11 A schematic diagram of the overall structure of an electronic device provided in one embodiment of the present application.

[0038] 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 electrode active material layer 211

[0042] First corner section 21A Negative electrode current collector 220

[0043] Negative electrode sheet 22 first region 2111

[0044] Second corner section 22A groove 2200

[0045] Isolation film 23 First conductive region 2201

[0046] Protrusion 24 Second conductive region 2202

[0047] Secondary battery 100 Second region 2211

[0048] Battery compartment 101 winding center plane P

[0049] The first straight area 201 is wound around the central axis C

[0050] First corner area 202 Winding direction D

[0051] Second straight area 203 First direction X

[0052] Second corner area 204 Second direction Y

[0053] Positive electrode current collector 210 Third direction Z

[0054] First surface 210A Fourth direction Y'

[0055] Second surface 210B Fifth direction Z'

[0056] The third surface 220A

[0057] The following specific implementation methods will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0058] The technical scheme in the embodiment of the present application is described clearly and in detail below. Obviously, the described embodiment is only a part of the embodiment of the present application, rather than all the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0059] 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 being limited to the exemplary embodiments illustrated herein. Rather, these exemplary embodiments are provided so that the present application is thoroughly and in detail communicated to those skilled in the art.

[0060] In addition, for simplicity and clarity, in the accompanying drawings, the size or thickness of various components, layers may be amplified. Throughout the entire text, the same numerical value refers to the same element. As used herein, the term "and / or", "and / or" includes any and all combinations of one or more related enumerated items. In addition, 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] Further, the use of “may” when describing embodiments of the present application means “one or more embodiments of the present application”.

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

[0063] Spatial related terms, such as "on" and the like can be used herein for convenient description, to describe the relationship between an element or feature and another element (multiple elements) or feature (multiple features) as illustrated in the figure. It should be understood that, in addition to the directions described in the figure, spatial related terms are intended to include different directions of equipment or devices in use or operation. For example, if the equipment in the figure is turned over, the elements described as "above" or "on" other elements or features will be oriented "below" or "below" other elements or features. Therefore, the exemplary term "on" may include the direction above and below. It should be understood that although the terms first, second, third, etc. can be used herein to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are used to distinguish an element, component, region, layer or part from another element, component, region, layer or part. Therefore, the first element, component, region, layer or part discussed below may be referred to as the second element, component, region, layer or part without departing from the teaching of the exemplary embodiment.

[0064] See also Figure 1 In one embodiment of the present application, a secondary battery 100 is provided, comprising a housing 10 , an electrode assembly 20 and an electrolyte (not shown). The electrode assembly 20 and the electrolyte are located in the housing 10 .

[0065] The electrode assembly 20 includes a positive electrode sheet 21, a negative electrode sheet 22 and a separator 23, wherein the separator 23 is disposed between the positive electrode sheet 21 and the negative electrode sheet 22. The positive electrode sheet 21, the separator 23 and the negative electrode sheet 22 are stacked and wound. The positive electrode sheet 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 away from the first surface 210A, and the positive active material layer 211 is disposed on the first surface 210A and the second surface 210B, respectively. The negative electrode sheet 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 winding center axis C and a fourth surface 220B away from the third surface 220A, and the first negative electrode active material layer 221 is respectively disposed on the third surface 220A and the fourth surface 220B. In some embodiments, a negative electrode bottom coating may be further 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, and the negative electrode bottom coating may 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 being peeled off from the negative electrode current collector 220. The material of the negative electrode bottom coating includes a binder and a conductive agent. The binder includes at least one of 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 of carbon black, carbon nanotubes and graphene. In some embodiments, a positive electrode bottom coating may be provided between the positive current collector 210 and the positive active material layer 211, and the positive electrode bottom coating may be used to improve the bonding force between the positive current collector 210 and the positive active material layer 211, and reduce the risk of the positive active material layer 211 being peeled off from the positive current collector 210. The material of the positive electrode bottom coating includes a binder and a conductive agent. The binder includes at least one of 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 of carbon black, carbon nanotubes and graphene. In some embodiments, the positive electrode bottom coating may also be used to improve the safety performance of the secondary battery, and the material of the positive electrode bottom coating includes at least one of alumina, boehmite, silicon dioxide and metal oxide. Among them, a three-dimensional coordinate system is defined by a first direction X, a second direction Y and a third direction Z that are perpendicular to each other. The extension direction of the winding central 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.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 opposite to each other in the second direction Y, and the first corner area 202 and the second corner area 204 are arranged opposite to each other in the third direction Z. The corner area is a bending portion of the electrode assembly 20, and the corner area is a concept opposite 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.

[0066] Among them, 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, 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 and nickel. In some embodiments, the positive electrode active material is selected from lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese ternary material (NCM), lithium nickel cobalt aluminum ternary material (NCA), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4) or at least one of 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 that can reversibly deintercalate active ions. For example, it can include but is not limited to a combination of one or more 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 a combination of one or more of artificial graphite, natural graphite and modified graphite; silicon materials can be selected from a combination of one or more of elemental silicon, silicon oxide compounds, silicon carbon composites, and silicon alloys; tin-based materials can be selected from a combination of one or more of elemental tin, tin oxide compounds, tin alloys, etc.

[0068] 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.

[0069] See also Figure 1 and Figure 2, the positive electrode active material layer 211 includes a first region 2111, and the first region 2111 is disposed on the first surface 210A and is located in the corner area. The first negative electrode active material layer 221 includes a second region 2211, and the second region 2211 is disposed on the fourth surface 220B and is located in the corner area. The first region 2111 is located outside the second region 2211 away from the winding center axis C and is arranged opposite to the second region 2211. In the embodiment of the present application, the following description is given by taking the first region 2111 and the second region 2211 both being located in the first corner area 202 as an example, but it can be understood that the first region 2111 and the second region 2211 can also be located in the second corner area 204. In the embodiment of the present application, the first region 2111 is located outside the second region 2211 away from the winding center axis C and is arranged opposite to the second region 2211, which means that the first region 2111 and the second region 2211 are located on two adjacent circles of pole pieces of the electrode assembly 20, the first region 2111 is located on the outer pole piece of the two circles of pole pieces, the second region 2211 is located on the inner pole piece of the two circles of pole pieces, and the first region 2111 and the second region 2211 are arranged opposite to each other through the isolation film 23. Therefore, when observed from the third direction Z, the first region 2111 and the second region 2211 at least partially overlap.

[0070] Among them, Figure 2 As shown, the positive electrode active material layer 211 located in the first corner region 202 is multi-layered, and therefore, the positive electrode active material layer 211 may include one first region 2111, or may include multiple first regions 2111, that is, at least two layers of the positive electrode active material layer 211 located in the first corner region 202 are provided with the first region 2111. Figure 3 and Figure 4 As shown, Figure 3 for Figure 1 The structure diagram of the positive electrode sheet 21 after unfolding is shown in FIG. Figure 4 for Figure 1 The structure diagram of the negative electrode plate 22 after unfolding is shown. Figure 3 and Figure 4 A three-dimensional coordinate system is established with a first direction X, a fourth direction Y' and a fifth direction Z', wherein the fourth direction Y' is the thickness direction of the positive electrode sheet 21 or the negative electrode sheet 22, and the fifth direction Z' is the length direction of the positive electrode sheet 21 or the negative electrode sheet 22. 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 arranged at intervals along the fifth direction Z'. Figure 4 As shown, correspondingly, the first negative electrode active material layer 221 is also provided with a plurality of second regions 2211, and the plurality of second regions 2211 are arranged at intervals along the fifth direction Z'. Figure 2 , the multiple second areas 2211 are respectively arranged opposite to the multiple first areas 2111.

[0071] like Figures 1 to 4 As shown, the negative electrode sheet 22 also includes a second negative electrode active material layer 222 disposed on the second region 2211. The gram capacity of the second negative electrode active material layer 222 is greater than the gram capacity of the first negative electrode active material layer 221, thereby increasing the unit area capacity (i.e., lithium insertion capacity) of the negative electrode active material relative to the first region 2111. In some embodiments, when the first negative electrode active material includes graphite, the second negative electrode active material includes a silicon material. Further, the second negative electrode active material may also include a mixture of graphite and silicon materials. 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 materials can alleviate the overall volume expansion of the second negative electrode active material layer 222 and improve the cycle life of the secondary battery 100. At the same time, graphite and silicon materials as the second negative electrode active material can also make full use of the advantages of both to achieve better electrochemical performance. In other embodiments, the implementation of the present application is not limited to the negative electrode active materials listed above, and the types of the first negative electrode active material and the second negative electrode active material can also be selected according to known negative electrode active materials and their gram capacities. In addition, the present application can also modify the negative electrode active material to change the specific capacity value, thereby obtaining negative electrode active materials with different gram capacities.

[0072] In the related art, 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 the lithium ions released from the first region, the second region cannot fully embed these lithium ions due to insufficient lithium embedding space, resulting in lithium deposition. The negative electrode sheet 22 of the present application also includes a second negative electrode active material layer 222 disposed on the second region 2211, and the second negative electrode active material layer 222 has a higher gram capacity, which is conducive to increasing the unit area capacity of the negative electrode active material relative to the first region 2111, that is, increasing 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 relative thereto, that is, the CB value, so that the lithium ions released from the first region 2111 can be fully embedded in the negative electrode active material, thereby improving the problem of insufficient lithium insertion space due to 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, reducing the risk of lithium deposition of the negative electrode sheet 22 in the first corner region 202, and improving the safety and cycle life of the secondary battery 100. It can be understood that in order to further reduce the risk of lithium deposition of the negative electrode sheet 22, the gram capacity of the second negative electrode active material layer 222 in the second corner region 204 can also be increased in a similar manner.

[0073] 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 fourth surface 220B of the negative electrode current collector 220, the present application is conducive to reducing the risk of increasing the thickness of the negative electrode plate 22 due to the stacking of the first negative electrode active material layer 221 and the second negative electrode active material layer 222 at the junction, thereby helping to reduce the volume expansion of the negative electrode plate 22 during the charging and discharging process, and further improve the cycle life of the secondary battery 100. Compared with the technical solution in which the second negative electrode active material layer 222 is coated on the entire surface of the fourth surface 220B of the negative electrode current collector 220, the present application has less impact on the positive electrode potential when the secondary battery 100 is fully charged, thereby reducing safety risks.

[0074] In some embodiments, the ratio of the gram capacity of the second negative electrode active material layer 222 to the gram capacity of the first negative electrode active material layer 221 is 1.1 to 1.5. Wherein, when the first negative electrode active material includes graphite and the second negative electrode active material includes a silicon material, the above-mentioned ratio relationship of gram capacity is satisfied. As an example, the ratio of the gram capacity of the two can be 1.1, 1.2, 1.3, 1.4, 1.5 or any value within the range composed of any two of the above values. On the one hand, it can increase the unit area capacity of the negative electrode active material relative to the first region 2111, and on the other hand, it can reduce the risk of the second negative electrode active material layer 222 being too small to cause the migration impedance of lithium ions inside the material to be large, so that lithium ions can enter the second negative electrode active material more easily, improve the kinetic performance of the secondary battery 100, and further reduce the risk of lithium precipitation of the negative electrode sheet 22 in the first corner region 202.

[0075] In addition, the coating weight per unit area of ​​the second negative electrode active material layer 222 may be set to 1.0 mg / cm 2 Up to 3.0mg / cm 2 As an example, the coating weight per unit area of ​​the second negative electrode active material layer 222 may be 1.0 mg / cm 2 , 1.2mg / cm 2 , 1.5mg / cm 2 、1.8mg / cm 2 , 2.0mg / cm 2 , 2.2mg / cm 2 , 2.5mg / cm 2 , 2.8mg / cm 2 、3.0mg / cm 2Or any value within the range formed by any two of the above values. Therefore, on the one hand, the content of the second negative electrode active material can be increased, and on the other hand, the risk of the second negative electrode active material particles being in close contact with each other and causing the ion channel to be blocked can be reduced, thereby improving the kinetic performance of the secondary battery 100, and also facilitating electrolyte infiltration, further reducing the risk of lithium deposition of the negative electrode sheet 22 in the first corner area 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, the lithium ions released from the first region 2111 will be preferentially embedded in the second region 2211, especially when the first negative electrode active material includes graphite and the second negative electrode active material includes silicon material, the risk of volume expansion of the silicon material due to embedding more lithium ions can be reduced, and the cycle life of the secondary battery 100 can be further improved. When manufacturing the negative electrode sheet 22, a plurality of mutually 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, and then a first negative electrode active material layer 221 is arranged on the fourth surface 220B having the second negative electrode active material layer 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 of the second negative electrode active material layer 222, and the negative electrode sheet 22 is obtained after rolling, and the first negative electrode active material layer 221 located on the above 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, and can fully fill the gap between two adjacent second negative electrode active material layers 222, so that the thickness of the negative electrode sheet 22 is relatively 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 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 collector 220, and then a plurality of mutually spaced second negative electrode active material layers 222 are arranged on the surface of the first negative electrode active material layer 221 away from the winding center axis C by gravure coating, and then rolled 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 which 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 with the technical solution of setting the second negative electrode active material layer 222 on the surface of the second region 2211 facing the winding center axis C, this embodiment can increase the unit area capacity of the negative electrode active material relative to the first region 2111 under the premise that the negative electrode plate 22 has the same thickness, and because part 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 beneficial to electrolyte infiltration, thereby further reducing the risk of lithium deposition of the negative electrode plate 22 in the first corner area 202.

[0078] See also 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 may be provided on the second negative electrode active material layer 222. The groove 2220 may be obtained by laser removal of a portion of the second negative electrode active material layer 222. The provision of the groove 2220 is also conducive to increasing the space for electrolyte storage and circulation, and is conducive to electrolyte infiltration, thereby further reducing the risk of lithium deposition of the negative electrode sheet 22 in the first corner region 202.

[0079] like Figure 1 As shown, in some embodiments, the positive electrode sheet 21 includes a first corner section 21A located in the first corner area 202, and the first corner section 21A includes the above-mentioned first area 2111. The negative electrode sheet 22 includes a second corner section 22A located in the first corner area 202, and the second corner section 22A includes the above-mentioned second area 2211. The first corner section 21A is located outside the second corner section 22A away from the winding center axis C and is arranged opposite to the second corner section 22A. Figure 7, the first corner section 21A includes a plurality of convex portions 24. The convex portions 24 are integrally arranged on the first corner section 21A, and are arranged to protrude from the first corner section 21A in a direction toward the winding center axis C or away from the winding center axis C. By providing the convex portions 24 on the first corner section 21A, it is helpful to improve the risk of poor electrolyte infiltration in the first corner area 202 due to the increase in local volume expansion after the second negative electrode active material layer 222 (e.g., the second negative electrode active material layer 222 containing silicon material) is provided on the negative electrode sheet 22, thereby improving the ability of the first corner area 202 to store electrolyte, thereby further improving the cycle life of the secondary battery 100. When manufacturing the positive electrode sheet 21, the positive electrode sheet 21 can be embossed by a pressing roller so that the first corner section 21A of the positive electrode sheet 21 forms multiple protrusions 24, and 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 other embodiments, the second corner segment 22A includes a plurality of protrusions 24. The protrusions 24 are integrally arranged on the second corner segment 22A, and are protruded from the second corner segment 22A in a direction toward the winding center axis C or away from the winding center axis C. By providing the protrusions 24 on the first corner segment 21A, it is also helpful to improve the risk of poor electrolyte infiltration in the first corner area 202 due to the increase in local volume expansion after the second negative electrode active material layer 222 (e.g., the second negative electrode active material layer 222 containing silicon material) is provided on the negative electrode sheet 22, thereby improving the ability of the first corner area 202 to store electrolyte, thereby further improving the cycle life of the secondary battery 100.

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

[0082] Among them, when viewed from the third direction Z, at least one of the multiple protrusions 24 of the first corner segment 21A overlaps at least partially with the multiple protrusions 24 of the second corner segment 22A. For example, the multiple protrusions 24 of the first corner segment 21A can be arranged to overlap with the multiple protrusions 24 of the second corner segment 22A. Further, the overlapping protrusions 24 are arranged to protrude in the same direction. For example, when the protrusions 24 of the first corner segment 21A are arranged to protrude toward the winding center axis C, the protrusions 24 of the second corner segment 22A are also arranged to 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 depression appearing on the other side of the first corner segment 21A. When manufacturing the electrode assembly 20, the positive electrode sheet 21, the separator 23 and the negative electrode sheet 22 can be stacked and then embossed, so that the protrusions 24 of the first corner segment 21A can fully overlap with the protrusions 24 of the second corner segment 22A. Therefore, the ability of the first corner area 202 to store electrolyte can be improved while the risk of misalignment between the positive electrode sheet 21 and the negative electrode sheet 22 can be reduced, so that the first area 2111 and the second negative electrode active material layer 222 can fully correspond to each other after winding, thereby reducing the risk of lithium deposition in the first corner area 202.

[0083] like Fig.10 As shown, the plurality of protrusions 24 of the first corner section 21A and the plurality of protrusions 24 of the second corner section 22A may also be arranged not to overlap, which is beneficial to further improve the ability of the first corner area 202 to store electrolyte, 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 connected in sequence in the winding direction D. The third surface 220A located in the first conductive region 2201 is not provided with the first negative electrode active material layer 221, and the fourth surface 220B located in the first conductive region 2201 is provided with the first negative electrode active material layer 221, that is, the first conductive region 2201 is a single-sided coating region. The third surface 220A and the fourth surface 220B located in the second conductive region 2202 are both provided with the first negative electrode active material layer 221, that is, the second conductive region 2202 is a double-sided coating region. In some embodiments, the second negative electrode active material layer 222 is provided on the second conductive region 2202. Taking into account 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 disposing the second negative electrode active material layer 222 on the first conductive area 2201, by disposing the second negative electrode active material layer 222 on the second conductive area 2202, the risk of the first conductive area 2201 being more easily wrinkled under the action of tension can be reduced, thereby further improving the cycle life of the secondary battery 100.

[0085] Further, it is defined that the negative electrode sheet 22 has 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 from the mth fold to the nth fold of the negative electrode sheet 22 along the winding direction D, 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 the second negative electrode active material layers 222 located from the mth fold to the nth fold of the negative electrode sheet 22 can be one or more. Since the second negative electrode active material layer 222 is located from the mth fold to the nth fold of the negative electrode sheet 22, the second negative electrode active material layer 222 is located on the second conductive region 2202 closer to the winding center axis C along the winding direction D. Taking into account that the lithium plating risk of the first negative electrode active material layer 221 is higher as it is closer to the winding center axis C in the first corner area 202 (for example, the electrolyte wettability of the part closer to the winding center axis C in the first corner area 202 may be insufficient, and for another example, the difference in unit area capacity of the positive and negative electrode sheets is greater as they are closer to the winding center axis C in the first corner area 202), the second negative electrode active material layer 222 is arranged to be located from the mth fold to the nth fold of the negative electrode sheet 22, so as to improve the lithium plating risk of this part of the first negative electrode active material layer 221, thereby further improving the safety and cycle life of the secondary battery 100. In the embodiment of the present 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. The winding center plane and each circle of the electrode assembly 20 have 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 is the starting edge of the fold, and the intersection along the winding direction D to the winding center plane P and another corner area is the ending edge of the fold.

[0086] The secondary battery 100 of the present application may 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.

[0087] See also Fig.11, an embodiment of the present application also provides an electronic device 1, 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 suitable for electronic devices 1 in various fields. The electronic device 1 is powered by the above-mentioned secondary battery 100, and the lithium precipitation of the electrode assembly 20 in the first corner area 202 is improved, so the secondary battery 100 has higher safety and cycle life. In one embodiment, the electronic device 1 of the present application can 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, an LCD TV, a portable cleaner, a portable C machine, a mini CD, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium ion capacitor, etc.

[0088] The present application is described in detail below through specific embodiments and comparative examples. Among them, the secondary battery 100 is a lithium-ion secondary battery as an example and the present application is described in combination with a specific preparation process and a testing method. Those skilled in the art should understand that the preparation method described in the present application is only an example, and any other suitable preparation method is within the scope of the present application.

[0089] Example 1

[0090] (1) Preparation of positive electrode sheet 21: The positive electrode active material lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 96.5:1.5:2, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 75wt%, and stirred evenly. Preliminarily, a foam glue is attached to a part of the surface of the positive electrode current collector 210, i.e., the aluminum foil with a thickness of 9μm, and the slurry is evenly coated on the first surface 210A of the aluminum foil, heated to make the foam glue fall off so that part of the surface of the aluminum foil is exposed, and then dried at 90°C, and the above coating steps are repeated on the second surface 210B of the aluminum foil to obtain an initial positive electrode sheet coated on both sides. The initial positive electrode sheet is cold pressed to obtain a positive electrode active material layer 211 with a thickness of 77μm coated on one side of the aluminum foil, and then cut and other processes are performed to obtain a positive electrode sheet 21. Then, the positive electrode tab is welded on the exposed aluminum foil, and the material of the positive electrode tab is aluminum.

[0091] (2) Preparation of negative electrode sheet 22: The second negative electrode active material silicon carbon compound, 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, and a slurry with a weight percentage of 55wt% is prepared, and the mixture is stirred evenly. The slurry is coated on the fourth surface 220B of the negative electrode current collector 220, i.e., the copper foil, with a thickness of 5μm, by gravure coating, and 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, and another slurry with a weight percentage of 55wt% is prepared, and the mixture is stirred evenly. The slurry is uniformly coated on 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, and the second negative electrode active material slurry is coated on part of the first negative electrode active material layer, and dried at 90°C to obtain an initial negative electrode sheet coated on both sides. Then, the initial negative electrode sheet is rolled to obtain a first negative electrode active material layer 221 with a single coating thickness of 60μm and a second negative electrode active material layer 222 with a single coating thickness of 40μm. Then, a laser is used to etch away part of the first negative electrode active material layer and expose the copper foil, and a negative electrode tab is welded on the exposed copper foil, and the material of the negative electrode tab is nickel.

[0092] (3) Preparation of 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, then add lithium salt lithium hexafluorophosphate (LiPF6) 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 isolation film 23: A polyethylene (PE) film with a thickness of 9 μm was selected.

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

[0095] Example 2

[0096] The difference from Example 1 lies in the preparation of the negative electrode sheet 22, specifically, first coating the first negative electrode active material on the surface of the negative electrode collector 220, and then coating the second negative electrode active material at intervals, so that the second region 2211 after winding is located between the negative electrode 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 plate.

[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 collector, and after winding, the second negative electrode active material is located in the first corner area, that is, only the second negative electrode active material is located in the first corner area.

[0101] Then, the secondary batteries of each embodiment and comparative example were subjected to a lithium deposition test and a cycle capacity retention rate test, respectively. The test results are recorded in Table 1.

[0102] Among them, the test steps of the cycle interface are as follows: 1) At a test temperature of 25°C, the secondary battery 100 is allowed to stand for 30 minutes, and step-charged 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 charging and discharging process is one cycle, which is repeated 1000 times. 2) Disassemble the electrode assembly 20 and check the surface of the negative electrode plate 22 of the first corner area 202. 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 slight lithium deposition, moderate lithium deposition and severe lithium deposition. Mild lithium deposition means that the lithium deposition area is less than 0.5% of the entire area of ​​the negative electrode plate 22 , moderate lithium deposition means that the lithium deposition area is 0.5% to 5% of the entire area of ​​the negative electrode plate 22 , and severe lithium deposition means that the lithium deposition area is greater than 5% of the entire area of ​​the negative electrode plate 22 .

[0103] The test steps for cycle capacity retention rate 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] It can be seen from the data in Table 1 that, compared with Comparative Examples 1 and 2, Example 1 and Example 2 improve the risk of lithium plating of the negative electrode sheet in the first corner area and improve the cycle capacity retention rate of the secondary battery by arranging the second negative electrode active material silicon material on the second area. Compared with Example 2, in Example 1, the second region is arranged on the surface of the first region facing the winding center axis, that is, the second region is located between the first region and the negative electrode current collector. Then, in the process of lithium ion deintercalation, lithium ions are preferentially embedded in the first negative electrode active material layer, and the second negative electrode active material layer has a lower lithium intercalation degree and a smaller expansion rate. During a long cycle, 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. The first negative electrode active material layer located on the upper layer can inhibit the expansion of the second negative electrode active material layer to a certain extent, reducing the risk of electrolyte shortage in the first corner area due to the small spacing between the pole pieces, which is beneficial to increasing the space for electrolyte storage and circulation, and is beneficial to electrolyte infiltration. Therefore, the risk of lithium plating of the negative electrode pole piece in the first corner area in Example 1 is further reduced, and the cycle capacity retention rate of the secondary battery is higher.

[0107] Embodiment 3 to Embodiment 16

[0108] The difference from Example 1 is that the ratio of the gram capacity of the second negative electrode active material layer 222 to the gram 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 fold number n of the negative electrode sheet at which the second negative electrode active material layer is located along the winding direction are specifically recorded in Table 2.

[0109] The ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer 221 is regulated by regulating the gram capacity of the second negative electrode active material layer 222. In addition to the silicon-carbon compound, the second negative electrode active material may also include graphite. The gram capacity of the second negative electrode active material layer 222 is regulated by regulating the ratio of the silicon-carbon compound and graphite in the second negative electrode active material. The compositions of the second negative electrode active materials in Examples 3 to 16 are respectively:

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

[0111] Example 4: The weight ratio of silicon carbon compound, 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 carbon compound, 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 carbon compound, 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 carbon compound, artificial graphite, conductive carbon black and polyacrylic acid binder is 17:80:1.5:1.5.

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

[0116] Then, the secondary batteries of each embodiment and comparative example were subjected to a lithium deposition test and a cycle capacity retention rate test. The test results are recorded in Table 2.

[0117] Among them, the gram 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, the secondary battery 100 is discharged to 0SOC%, and the negative electrode plate 22 is disassembled to obtain the negative electrode plate 22. A certain area of ​​the single-sided coated negative electrode plate 22 is punched out from the negative electrode plate 22 of the first straight area 201 as a sample, and it is weighed using a balance, and the weight is recorded as W1. Then, the negative electrode current collector 220 of the same area is punched out, and it is weighed using a balance, and the weight is recorded as W2; 2) The above sample and the lithium sheet are combined into a button battery. After standing for 4 hours, it is discharged to 0.005V at a constant current of 20μA, stood for 5 minutes, and then charged to 2.0V at a constant current of 20μA, and the capacity of the button battery is measured Q1; 3) The gram capacity of the first negative electrode active material layer 221 is calculated as Q1 / (W1-W2).

[0118] The gram 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 sheet 22, punch out a certain area of ​​the negative electrode sheet 22 from the negative electrode sheet 22 of the first corner area 202 as a sample, place the sample under a microscope to observe the cross-section of the sample, and observe a clear boundary between the second negative electrode active material layer 222 close to the negative electrode current collector 220 and the first negative electrode active material layer 221 away from the negative electrode current collector 220, and use a scraper to scrape the entire first negative electrode active material layer 221 located on the upper layer and part of the second negative electrode active material layer 221 located on the lower layer. 1) scrape off the material layer 222 (make sure the scraped off part exceeds the boundary line between the first negative electrode active material layer 221 and the second negative electrode active material layer 222 observed under the microscope), then continue to scrape off and collect the second negative electrode active material layer 222 in the lower layer, and 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 sheet to form a button battery, let it stand for 4 hours, discharge it at a constant current of 20μA to 0.005V, let it stand for 5 minutes, and then charge it at a constant current of 20μA to 2.0V, and measure the capacity of the button battery Q2; 3) calculate the gram capacity of the second negative electrode active material layer 222 = Q2 / W3. Then calculate the ratio of the gram capacity of the second negative electrode active material layer 222 to the gram capacity of the first negative electrode active material layer 221.

[0119] The coating weight per unit area of ​​the second negative electrode active material layer 222 can be measured by the following method: 1) at a test temperature of 25°C, the secondary battery 100 is discharged to 0 SOC%, and the negative electrode sheet 22 is disassembled to obtain the negative electrode sheet 22, which is washed with dimethyl carbonate (DMC) and then dried; 2) a single-sided coated negative electrode sheet 22 with an area of ​​S1 is punched out from the negative electrode sheet 22 of the first flat region 201 as a first negative electrode active material layer sample, and its mass W4 is weighed, and its cross-section is observed under a scanning electron microscope to obtain the thickness d1 of the first negative electrode active material layer, and a negative electrode current collector 220 with an area of ​​S1 is cut and weighed using a balance to obtain its mass W5, and the density ρ1 of the first negative electrode active material is calculated as (W 4- W5) / (S1×d1); 3) punching out a negative electrode sheet 22 with an area of ​​S2 from the negative electrode sheet 22 of the first corner area 202 as a first corner area sample, and weighing it using a balance, and the weight is recorded as W 总 The cross section of the first corner area sample is observed under a scanning electron microscope to obtain the thickness d2 of the first negative electrode active material layer 221. The first negative electrode active material layer 221 and the second negative electrode active material layer 222 of the sample are washed away with a solvent N-methylpyrrolidone (NMP), and then dried. The weight of the negative electrode current collector 220 is weighed and recorded as W0. The coating weight per unit area of ​​the second negative electrode active material is calculated by the following formula: W = [(W 总-W0)-ρ1×S2×d2] / S2.

[0120] Table 2

[0121]

[0122]

[0123] From the data of Example 1 and Example 3 to Example 7 in Table 2, it can be seen that when the ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is less than 1.1, the improvement of the interface condition and the cycle capacity retention rate of the first corner area is not significant; when the ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is less than or equal to 1.35, as the gram capacity ratio increases, the interface condition of the first corner area gradually improves, and the cycle capacity retention rate gradually increases. This is because as the gram capacity of the second negative electrode active material layer increases, the first corner area can receive lithium ions. The number of sub-elements increases, thereby improving the lithium deposition in the first corner area and improving the cycle capacity retention rate; when the ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is greater than or equal to 1.5, the interface condition in the first corner area deteriorates instead, and the cycle capacity retention rate gradually decreases. This is because when the gram capacity of the second negative electrode active material layer is too large, its expansion rate increases accordingly, which easily leads to a decrease in the spacing between the pole pieces in the first corner area, and the electrolyte is easily squeezed out and the electrolyte is not easy to infiltrate, increasing the risk of lithium deposition in the first corner area due to lack of electrolyte, and causing the cycle capacity retention rate to decrease. Therefore, the ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is selected to be 1.1 to 1.5. Furthermore, the gram capacity ratio is preferably 1.25 to 1.35.

[0124] It can be seen from the experimental data of Examples 1 and 8 to 13 that when the coating weight per unit area of ​​the second negative electrode active material layer is less than 1.0 mg / cm 2 When the coating weight per unit area of ​​the second negative electrode active material layer is less than or equal to 2.5 mg / cm 2 As the coating weight per unit area increases, the interface condition of the first corner area gradually improves, and the cycle capacity retention rate 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 can be received by the first corner area increases, thereby improving the lithium deposition condition of the first corner area and improving the cycle capacity retention rate. When the coating weight per unit area of ​​the second negative electrode active material layer is greater than or equal to 3.0 mg / cm 2When this occurs, the interface condition in the first corner region 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 in the volume energy density of the secondary battery, but also easily leads to a decrease in the spacing between the electrode sheets in the first corner region. The electrolyte is easily extruded and is not easily infiltrated, increasing the risk of lithium deposition in the first corner region 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] From the experimental data of Example 1 and Examples 14 to 16, it can be seen that when n < N / 3, the improvement in the interface condition and the cycle capacity retention rate in the first corner region is not significant; when n > N / 2, the interface condition in the first corner region also deteriorates and the cycle capacity retention rate decreases. 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 spacing between the electrode sheets in the inner winding circle. The electrolyte is easily extruded and is not easily infiltrated, thereby increasing the risk of lithium deposition in the first corner region 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 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 positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, the positive electrode current collector comprises a first surface facing the winding center axis and a second surface away from the first surface, and the positive electrode active material layer is respectively arranged on the first surface and the second surface; the negative electrode sheet comprises a negative electrode current collector and a first negative electrode active material layer, the negative electrode current collector comprises a third surface facing the winding center axis and a fourth surface away from the third surface, and the first negative electrode active material layer is respectively arranged on the third surface and the fourth surface; 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, 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 plate further includes a second negative electrode active material layer disposed on the second region, 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.

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

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

4. The secondary battery according to claim 1, wherein The second negative electrode active material layer is located on a surface of the second region away from the winding central axis.

5. The secondary battery according to claim 1, wherein A groove is provided on a surface of the second negative electrode active material layer facing away from the second region.

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

7. The secondary battery according to claim 1, wherein The ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is 1.1 to 1.

5.

8. The secondary battery according to claim 7, wherein: The ratio of the gram capacity of the second negative electrode active material layer to the gram capacity of the first negative electrode active material layer is 1.25 to 1.

35.

9. The secondary battery according to claim 1, wherein 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 a silicon material.

10. The secondary battery according to claim 9, wherein The silicon material is selected from at least one of silicon oxide, silicon carbon compound, silicon alloy and silicon element.

11. The secondary battery according to claim 9, wherein The second negative electrode active material further includes graphite.

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

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

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

15. The secondary battery according to claim 1, wherein The positive electrode sheet includes a first corner segment located in the corner area, the first corner segment includes the first area, the negative electrode sheet includes a second corner segment located in the corner area, the second corner segment includes the second area, the first corner segment is located on the outside of the second corner segment away from the winding center axis and is arranged 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 according to claim 15, wherein The first corner section and the second corner section are respectively provided with the plurality of protrusions.

17. The secondary battery according to 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 multiple protrusions of the first corner segment at least partially overlaps with at least one of the multiple protrusions of the second corner segment, and the first direction, the second direction and the third direction are perpendicular to each other.

18. The secondary battery according to 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 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.

19. The secondary battery according to claim 1, wherein The negative electrode current collector includes a first conductive region and a second conductive region connected sequentially in a 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 the third surface and the fourth surface of the second conductive region are both provided with the first negative electrode active material layer; the second negative electrode active material layer is provided on the second conductive region.

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

21. An electronic device, wherein: The electronic device includes the secondary battery according to any one of claims 1 to 20.

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