Secondary battery and electronic device

Abstract

Provided are a secondary battery and an electronic device. The secondary battery comprises an electrode assembly. The electrode assembly comprises a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and negative electrode active material layers arranged on surfaces of the negative electrode current collector. The negative electrode current collector comprises a first area, and the negative electrode active material layers are arranged on two opposite surfaces of the first area. The total thickness of the negative electrode active material layers on the first area is T, wherein 0.07 mm≤T≤0.15 mm. A plurality of recesses are formed on the negative electrode active material layers. The compaction density of the negative electrode active material layers is D (g / cm3), wherein 3T+1≤D≤8T+1.

Description

Secondary batteries and electronic devices 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] Secondary batteries (such as lithium-ion secondary batteries) are widely used in electronic products such as electronic mobile devices, power tools and electric vehicles, and people have increasingly higher requirements for the performance of secondary batteries.

[0003] Related technologies aim to increase the energy density of secondary batteries by increasing the thickness of the active material layer in the electrode. However, thick electrode designs lengthen the lithium ion migration path and increase migration impedance. This makes it difficult for lithium ions to quickly reach the active material layer adjacent to the current collector during cycling, increasing concentration polarization and leading to problems such as reduced kinetic performance and cycle capacity decay. Therefore, for secondary batteries with thick electrode sheets, achieving both high energy density and superior kinetic and cycle performance is a pressing issue.

[0004] Summary of the Invention

[0005] The present application provides a secondary battery that can achieve both high energy density and excellent kinetic and cycle performance, and an electronic device having the secondary battery.

[0006] In a first aspect, the present application provides a secondary battery comprising an electrode assembly. The electrode assembly comprises a negative electrode plate. The negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The negative electrode current collector comprises a first region, and negative electrode active material layers are disposed on two opposite surfaces of the first region. The total thickness of the negative electrode active material layer on the first region is T, 0.07 mm ≤ T ≤ 0.15 mm. The negative electrode active material layer is provided with a plurality of recesses, and the compaction density of the negative electrode active material layer is D, in units of g / cm 3 , 3T+1≤D≤8T+1.

[0007] The present application is directed to a negative electrode sheet with a thicker negative electrode active material layer. By providing a recess on the negative electrode active layer, the degree of electrolyte infiltration into the negative electrode sheet is improved, the ion migration impedance is reduced, and the transmission and embedding of lithium ions in the negative electrode active material layer is facilitated, thereby improving the problems of poor kinetic performance and low cycle capacity attenuation caused by the increase in the thickness of the negative electrode active material layer. At the same time, by setting the relationship between the compaction density and the total thickness, on the one hand, the secondary battery can maintain a higher energy density, and on the other hand, the negative electrode active particles are in close contact to provide an efficient ion transmission path, reduce impedance, and further improve the kinetic performance of the secondary battery. At the same time, more pores can be constructed in the negative electrode active material layer to further improve the degree of electrolyte infiltration, thereby reducing the risk of partial loss of negative electrode active material and lithium precipitation in the recess due to the opening of the recess in the negative electrode active material layer, further improving the cycle performance of the secondary battery. Therefore, the present application sets a recess in the thick electrode sheet and further sets the relationship between the compaction density and the total thickness, which can achieve a balance between the energy density, dynamic performance and cycle performance of the secondary battery, so that the secondary battery can have both high energy density and better dynamic performance and cycle performance. In addition, by setting the relationship between the compaction density and the total thickness, the risk of the negative electrode active material falling off (collapse of the side wall of the recess) when the recess is set can be reduced, and the risk of the particles of the negative electrode active material generating more fresh surface and consuming electrolyte when the recess is set can be reduced, thereby reducing the risk of increased side reactions at the side wall of the recess and worsening the problem of lithium precipitation in the recess.

[0008] Based on the first aspect, in some possible implementations, 3.5T+1≤D≤6.5T+1. This can further improve the energy density of the secondary battery while maintaining close contact between the negative electrode active particles and creating more pores in the negative electrode active material layer to further improve the electrolyte infiltration, thereby further improving the kinetic performance and cycle performance of the secondary battery.

[0009] Based on the first aspect, in some possible implementations, 4.5T+1≤D≤5.5T+1. This can further improve the energy density of the secondary battery while maintaining close contact between the negative electrode active particles and creating more pores in the negative electrode active material layer to further improve the electrolyte infiltration, thereby further improving the kinetic performance and cycle performance of the secondary battery.

[0010] Based on the first aspect, in some possible implementations, 1.2 g / cm 3 ≤D≤2.2g / cm 3 . It can make the secondary battery have both higher energy density and better kinetic performance and cycle performance.

[0011] Based on the first aspect, in some possible implementations, 1.3 g / cm 3≤D≤1.9g / cm 3 . It can make the secondary battery have higher energy density and better kinetic performance and cycle performance.

[0012] Based on the first aspect, in some possible implementations, the electrode assembly is a wound structure, and the electrode assembly includes a straight section and a curved section connected along the winding direction. The negative electrode active material layer includes a first active material region located in the straight section and a second active material region located in the curved section. The compaction density of the first active material region is D1, and the compaction density of the second active material region is D2, 1.3 g / cm 3 ≤D1≤2.0g / cm3,1.2g / cm 3 ≤D2≤1.5g / cm3. This can improve the problem of the second active material area in the curved section being squeezed and the high stress in the electrode piece, which leads to easy lithium deposition. At the same time, the first active material area in the straight section has a higher compaction density. Without significantly deteriorating the dynamic performance and lithium deposition problem in the first active material area, the first active material area can play a greater capacity role, thereby further improving the energy density of the secondary battery.

[0013] Based on the first aspect, in some possible implementations, the electrode assembly is a laminate structure. The negative electrode active material layer includes a third active material region and a fourth active material region disposed around the outer periphery of the third active material region. The compaction density of the third active material region is D3, and the compaction density of the fourth active material region is D4, 1.2 g / cm 3 ≤D3≤2.0g / cm3,1.2g / cm 3 ≤D4≤1.7g / cm3. This can improve the problem of lithium deposition in the fourth active material area located at the edge due to its relatively thin thickness and relatively small amount of negative electrode active material. At the same time, the third active material area located in the center has a higher compaction density. Without significantly deteriorating the kinetic performance and lithium deposition problem in the third active material area, the third active material area can play a greater capacity role, thereby further improving the energy density of the secondary battery.

[0014] Based on the first aspect, in some possible implementations, the width of the recess is 70μm to 100μm, the depth is 5μm to 100μm, and the center-to-center distance between two adjacent recesses is 1mm to 2mm. Appropriately sized recesses can reduce the risk of excessive loss of negative electrode active material, which is conducive to maintaining a high energy density and reducing the risk of lithium deposition in the recesses. They can also inhibit the aggregation of lithium ions in the recesses and maintain the structural stability of the negative electrode sheet to reduce deformation of the electrode assembly, thereby further improving the problem of easy lithium deposition in the recesses after the recesses are opened in thick electrode sheets, thereby improving the cycle performance of the secondary battery. Appropriate recess distribution density can improve electrolyte infiltration, reduce ion migration impedance, and improve the problem of easy lithium deposition in the recesses, thereby improving the dynamic performance and cycle performance of the secondary battery.

[0015] Based on the first aspect, in some possible implementations, the recess is circular or square when viewed in the thickness direction of the negative electrode sheet. A circular recess facilitates uniform diffusion of lithium ions in the negative electrode active material layer, improves electrolyte wetting, reduces ion migration impedance, and improves issues such as poor kinetic performance and low cycle capacity decay caused by increased thickness of the negative electrode active material layer. A square recess increases the contact area between the electrolyte and the negative electrode active material layer at the boundary, facilitating increased diffusion rate of lithium ions in the negative electrode active material layer, thereby further improving issues such as poor kinetic performance and low cycle capacity decay caused by increased thickness of the negative electrode active material layer.

[0016] Based on the first aspect, in some possible implementations, the CB value of the secondary battery is 1 to 1.05, thereby further reducing the risk of lithium plating in the negative electrode active material layer and improving the cycle performance of the secondary battery.

[0017] Based on the first aspect, in some possible implementations, the recess is obtained by removing part of the negative electrode active material layer.

[0018] The second aspect of the present application further provides an electronic device comprising a battery compartment and a secondary battery as described above disposed in the battery compartment. The electronic device is powered by the secondary battery, which can achieve both high energy density and excellent dynamic performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0020] FIG1 is a schematic structural diagram of a secondary battery provided in one embodiment of the present application.

[0021] FIG2 is a partial enlarged view of the negative electrode plate of the secondary battery shown in FIG1 at position II.

[0022] FIG3 is a schematic plan view of the negative electrode plate shown in FIG2 in some embodiments.

[0023] FIG. 4 is a schematic plan view of the negative electrode plate shown in FIG. 2 in some other embodiments.

[0024] FIG5 is a schematic structural diagram of a secondary battery provided in another embodiment of the present application.

[0025] FIG6 is a schematic structural diagram of an electronic device according to an embodiment of the present application.

[0026] Explanation of main component symbols Electronic device 1 Electrode assembly 20 Straight section 20A Curved section 20B Negative electrode sheet 21 Positive electrode sheet 22 Separator 23 Secondary battery 100 Battery compartment 101 Positive current collector 220 Positive electrode active material layer 221 Negative current collector 210 Negative electrode active material layer 211 First region 2100 Recess 2110 First active material area 2111 Second active material area 2112 Third active material area 2113 Fourth active material area 2114 Winding center axis C Winding direction W Thickness T0, T1 Width w Depth h Center distance d Thickness direction X The following specific embodiments will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly 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.

[0028] Below, embodiments of the present application will be described in detail. However, the present application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments illustrated herein. Rather, these exemplary embodiments are provided to make the present application thorough and detailed for those skilled in the art.

[0029] In addition, for the sake of brevity and clarity, the size or thickness of various components or layers may be exaggerated in the accompanying drawings. Throughout the text, the same numerical value refers to the same element. As used herein, the terms "and / or" and "and / or" include 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 can be directly connected to element B, or there may be an intermediate element C and element A and element B can be indirectly connected to each other.

[0030] Further, when describing embodiments of the present application, the use of “may” refers to “one or more embodiments of the present application.”

[0031] The technical terms used herein are for the purpose of describing specific embodiments and are not intended to limit this application. As used herein, the singular is intended to include the plural, unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, refers to the presence of the 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.

[0032] Spatial related terms, such as "on" etc., 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" can 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 can be referred to as the second element, component, region, layer or part, without departing from the teachings of the exemplary embodiments.

[0033] In this application, the relationship between parameter values ​​that is greater than, less than, or not equal to the design relationship needs to exclude the reasonable error of the measuring equipment.

[0034] Referring to Figure 1 , one embodiment of the present application provides a secondary battery 100 comprising a housing (not shown), an electrode assembly 20, and an electrolyte (not shown). The electrode assembly 20 and the electrolyte are both located within the housing. The housing may be a packaging bag encapsulated with a packaging film (e.g., an aluminum-plastic film), e.g., if the secondary battery 100 is a soft-pack battery. In other embodiments, the secondary battery 100 may also be a steel-shell battery, an aluminum-shell battery, or the like.

[0035] The electrode assembly 20 includes a negative electrode sheet 21, a positive electrode sheet 22, and a separator 23, wherein the separator 23 is provided between the positive electrode sheet 22 and the negative electrode sheet 21. As shown in FIG1 , the electrode assembly 20 may be a wound structure, which is formed by stacking the positive electrode sheet 22, the separator 23, and the negative electrode sheet 21 in sequence and then winding them. As shown in FIG1 , the electrode assembly 20 has a winding center axis C perpendicular to the paper surface and a winding direction W wound around the winding center axis C. The winding direction W is the direction of counterclockwise rotation around the winding center axis C as shown in FIG2 . In other embodiments, the winding direction W may also be a clockwise rotation direction.

[0036] As shown in FIG. 5 , in some other embodiments, the electrode assembly 20 may also be a laminate structure, which is formed by alternately stacking the positive electrode sheet 22 , the separator 23 , and the negative electrode sheet 21 .

[0037] Positive electrode

[0038] The positive electrode sheet 22 includes a positive electrode current collector 220 and a positive electrode active material layer 221 disposed on the surface of the positive electrode current collector 220. The positive electrode current collector 220 can be made of aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0039] The positive electrode active material layer 221 includes an active material, such as at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium-rich manganese-based material, or lithium nickel cobalt aluminum oxide.

[0040] The positive electrode active material layer 221 may further include a binder to bind the active material particles to facilitate film formation and improve the bonding strength between the positive electrode active material layer 221 and the positive electrode current collector 220. In some embodiments, the binder may include, but is not limited to, at least one of polyimide, polyvinyl alcohol, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, polyacrylonitrile, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0041] The positive electrode active material layer 221 may further include a conductive agent, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0042] Negative electrode

[0043] The negative electrode sheet 21 includes a negative electrode current collector 210 and a negative electrode active material layer 211 disposed on the surface of the negative electrode current collector 210 .

[0044] The negative electrode current collector 210 can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil or a carbon-based current collector, or any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0045] The negative electrode active material layer 211 includes an active material selected from at least one of a graphite material, an alloy material, lithium metal, and alloys thereof. The graphite material may be selected from at least one of artificial graphite and natural graphite; the alloy material may be selected from at least one of silicon, silicon oxide, tin, and titanium sulfide.

[0046] The negative electrode active material layer 211 may further include a conductive agent, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. Metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. The conductive polymer may be a polyphenylene derivative.

[0047] The negative electrode active material layer 211 may further include a binder, which is used to bind the negative electrode active particles to facilitate film formation and improve the bonding strength between the negative electrode active material layer 211 and the negative electrode current collector 210. In some embodiments, the binder may include, but is not limited to, polyimide, polyvinyl alcohol, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, polyacrylonitrile, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0048] The negative electrode active material layer 211 may further include a dispersant to uniformly disperse the conductive agent and binder, thereby improving the film quality. Dispersants include, but are not limited to, at least one of carboxymethyl cellulose salts, polyacrylates, polyethylene glycol, or polyethylene oxide. In some embodiments, the carboxymethyl cellulose salt may include at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0049] Referring to Figure 2 , the negative electrode current collector 210 includes a first region 2100 , with negative electrode active material layers 211 disposed on opposite surfaces of the first region 2100 . Specifically, the first region 2100 is a double-sided coating area of ​​the negative electrode current collector 210 . The total thickness of the negative electrode active material layer 211 on the first region 2100 is T, where 0.07 mm ≤ T ≤ 0.15 mm. The negative electrode active material layer 211 is provided with a plurality of recesses 2110 , which may be obtained by removing portions of the negative electrode active material layer 211 . A method for measuring the total thickness T may include the following steps: discharging the secondary battery 100 to 3.0 V, disassembling the secondary battery 100 , cleaning, and drying the battery; measuring the thickness T0 of the negative electrode tab 21 using a micrometer; washing off the negative electrode active material layer 211 with a solvent, drying the battery, and measuring the thickness T1 of the negative electrode current collector 210 using a micrometer; and then calculating the total thickness T using the following formula: T = T0 - T1.

[0050] The compacted density of the negative electrode active material layer 211 is D, expressed in g / cm 3 , 3T+1≤D≤8T+1. The method for measuring the compacted density D may include the following steps: discharging the secondary battery 100 to 3.0V, disassembling the secondary battery 100, cleaning, and drying; weighing a negative electrode sheet 21 of a certain area A using a balance, with the weight recorded as W0; measuring the thickness T0 of the negative electrode sheet 21 using a micrometer; washing the negative electrode active material layer 211 with a solvent, drying, and measuring the weight of the negative electrode current collector 210, with the weight recorded as W1; measuring the thickness T1 of the negative electrode current collector 210 using a micrometer; and then calculating the compacted density D using the following formula: D = (W0-W1) / [(T0-T1)×A].

[0051] When the compaction density D is lower than 3T+1, not only does it significantly reduce the energy density of the secondary battery 100, but it also leads to poor contact between the particles of the negative electrode active material, increasing impedance, hindering the transmission of lithium ions in the negative electrode active material layer 211, and resulting in reduced kinetic performance. At the same time, the excessive pores in the negative electrode active material layer 211 increase the contact area between the electrolyte and the negative electrode active material, which easily produces side reactions and forms a solid electrolyte membrane (SEI) on the surface of the negative electrode active material, not only increasing impedance but also worsening the lithium precipitation problem, resulting in reduced cycle performance. In addition, because the particles of the negative electrode active material are relatively loose, the negative electrode active material is easily detached when the recess 2110 is set, and the sidewalls of the recess 2110 collapse, further reducing the cycle performance and even causing short circuit problems.

[0052] When the compaction density D is higher than 8T+1, although the energy density of the secondary battery 100 is improved, the contact between the particles of the negative electrode active material is too close, and the ion channel is blocked, which reduces the kinetic performance of the secondary battery 100. In addition, the lack of pores for the electrolyte to enter in the negative electrode active material layer 211 leads to poor electrolyte wettability, thereby worsening the lithium plating problem and reducing the cycle performance of the secondary battery 100. In addition, because the contact between the particles of the negative electrode active material is too close, when the recess 2110 is set, the particles of the negative electrode active material will produce more fresh surfaces and consume electrolyte, resulting in an increase in side reactions at the side walls of the recess 2110, thereby worsening the lithium plating problem in the recess 2110 and reducing the cycle performance of the secondary battery 100.

[0053] In the present application, for a negative electrode sheet 21 having a thicker negative electrode active material layer 211, a recess 2110 is provided on the negative electrode active layer. The recess 2110 can accommodate the electrolyte, thereby improving the degree of electrolyte infiltration into the negative electrode sheet 21, reducing the ion migration impedance, and facilitating the transmission and embedding of lithium ions in the entire negative electrode active material layer 211, thereby improving the problems of poor kinetic performance and low cycle capacity attenuation caused by the increase in the thickness of the negative electrode active material layer 211. At the same time, by setting the relationship between the compaction density D of the negative electrode active material layer 211 and the total thickness T of the negative electrode active material layer 211, on the one hand, the secondary battery 100 can maintain a high energy density, and on the other hand, the negative electrode active particles are in close contact to provide an efficient ion transmission path, reducing impedance, thereby further improving the kinetic performance of the secondary battery 100. At the same time, more pores can be created in the negative electrode active material layer 211 to further improve the degree of electrolyte infiltration, thereby reducing the risk of partial loss of negative electrode active material due to the provision of recesses 2110 in the negative electrode active material layer 211 and causing lithium deposition within the recesses 2110, further improving the cycle performance of the secondary battery 100. Therefore, the present application provides recesses 2110 in the negative electrode active material layer 211 of the thick electrode sheet and further sets the relationship between the compaction density D and the total thickness T of the negative electrode active material layer 211, thereby achieving a balance between the energy density, kinetic performance, and cycle performance of the secondary battery 100, so that the secondary battery 100 can achieve both high energy density and excellent kinetic performance and cycle performance. In addition, setting the compaction density D can also reduce the risk of the negative electrode active material falling off (the side wall of the recess 2110 collapses) when the recess 2110 is set, and at the same time reduce the risk of the particles of the negative electrode active material producing more fresh surfaces and consuming the electrolyte when the recess 2110 is set. Therefore, the risk of increased side reactions at the side wall of the recess 2110 leading to worsening lithium deposition in the recess 2110 can be reduced.

[0054] Furthermore, 3.5T+1≤D≤6.5T+1 can be set, which can further improve the energy density of the secondary battery 100 while maintaining close contact between the negative electrode active particles and creating more pores in the negative electrode active material layer 211 to further improve the electrolyte infiltration degree, thereby further improving the kinetic performance and cycle performance of the secondary battery 100. Furthermore, 4.5T+1≤D≤5.5T+1 can be set, thereby further improving the energy density, kinetic performance, and cycle performance of the secondary battery 100.

[0055] In some embodiments, 1.2 g / cm 3 ≤D≤2.2g / cm 3 By further setting a specific range of compaction density D, the secondary battery 100 can have both a higher energy density and better dynamic performance and cycle performance. 3≤D≤1.9g / cm 3 , which can make the secondary battery 100 have both higher energy density and better dynamic performance and cycle performance. As an example, the compaction density can be 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 Or any value within the range formed by any two of the above values.

[0056] As shown in FIG1 , when the electrode assembly 20 is a wound structure, the electrode assembly 20 may include a straight section 20A and a curved section 20B connected along a winding direction W. The negative electrode active material layer 211 includes a first active material region 2111 located in the straight section 20A and a second active material region 2112 located in the curved section 20B. The compaction density of the first active material region 2111 may be set to D1, and the compaction density of the second active material region 2112 may be set to D2, 1.3 g / cm 3 ≤D1≤2.0g / cm3,1.2g / cm 3 ≤D2≤1.5g / cm3. By further setting specific ranges for the compaction density of the first active material region 2111 and the second active material region 2112, the problem of the second active material region 2112 being squeezed in the curved section 20B and the high stress in the electrode, which leads to easy lithium deposition, can be improved. At the same time, the compaction density of the first active material region 2111 in the straight section 20A is further increased. Without significantly deteriorating the dynamic performance and lithium deposition problem in the first active material region 2111, the first active material region 2111 can play a greater capacity role, thereby further improving the energy density of the secondary battery 100.

[0057] As shown in FIG5 , when the electrode assembly 20 of some other embodiments is a laminated structure, the negative electrode active material layer 211 includes a third active material region 2113 and a fourth active material region 2114 arranged around the outer periphery of the third active material region 2113. Since the negative electrode sheet 21 needs to undergo a pressing process during production, the fourth active material region 2114 can extend in a direction away from the third active material region 2113 during pressing, and the third active material region 2113 is surrounded by the fourth active material region 2114, so that the movable space is limited. Therefore, the thickness of the third active material region 2113 can be greater than the thickness of the fourth active material region 2114. Therefore, the compaction density of the third active material region 2113 can be set to D3, and the compaction density of the fourth active material region 2114 can be set to D4, 1.3 g / cm 3 ≤D3≤2.2g / cm3,1.2g / cm 3≤D4≤1.7g / cm3. By further setting the specific range of the compaction density of the third active material area 2113 and the fourth active material area 2114, the problem of easy lithium deposition due to the relatively thin thickness and relatively less negative electrode active material in the fourth active material area 2114 located at the edge can be improved. At the same time, the compaction density of the third active material area 2113 located in the center is further improved. Without significantly deteriorating the kinetic performance and lithium deposition problem in the third active material area 2113, the third active material area 2113 can play a greater capacity role, thereby further improving the energy density of the secondary battery 100. Among them, the width of the fourth active material area 2114 can be 2mm to 10mm, for example, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, or any specific value between any two adjacent values ​​mentioned above. The width is the distance between any position of the fourth active material region 2114 away from the outer periphery of the third active material region 2113 and the inner periphery of the fourth active material region 2114 adjacent to the outer periphery.

[0058] In some embodiments, the CB value of the secondary battery 100 is 1 to 1.05, thereby further reducing the risk of lithium plating in the negative electrode active material layer 211. The CB value is the ratio of the capacity per unit area of ​​the negative electrode active material layer 211 to the capacity per unit area of ​​the positive electrode active material layer 221. The CB value measurement method may include the following steps: 1) allowing the secondary battery 100 to stand at a test temperature of 25°C for 30 minutes; 2) charging to 4.5V at a constant current of 0.2C, and then discharging to 0.02C at a constant voltage; 3) allowing to stand for 5 minutes; 4) discharging to 3V at a constant voltage of 0.2C and recording the discharge capacity Q1 at 0.2C; 5) allowing to stand for five minutes; 6) charging to 4.5V at a constant current of 0.2C, and then discharging to a capacity cutoff of 0.02C at a constant voltage; 7) allowing to stand for 5 minutes; 8) discharging to 3V at a constant voltage of 0.2C; 9) drawing a dv / dq curve for the charging capacity in step 6, drawing a horizontal tangent at the cutoff voltage and reading the corresponding abscissa, which is recorded as the capacity value Q, and then calculating the CB value using the following formula: CB = Q / Q1.

[0059] As shown in Figures 3 and 4, in some embodiments, the width w of the recess 2110 is 70 μm to 100 μm, the depth h is 5 μm to 100 μm, and the center-to-center spacing d between two adjacent recesses 2110 is 1 mm to 2 mm. Appropriately sized recesses 2110 can reduce the risk of excessive loss of negative electrode active material, thereby maintaining a high energy density and reducing the risk of lithium deposition within the recesses 2110. They can also inhibit lithium ion aggregation within the recesses 2110 and maintain the structural stability of the negative electrode sheet 21, thereby reducing deformation of the electrode assembly 20. This further improves the problem of lithium deposition within the recesses 2110 in thick electrode sheets, thereby enhancing the cycling performance of the secondary battery 100. Furthermore, an appropriate distribution density of the recesses 2110 can improve electrolyte wetting, reduce ion migration resistance, and improve the problem of lithium deposition within the recesses 2110 in thick electrode sheets, thereby enhancing the cycling performance and kinetic performance of the secondary battery 100. The uniform distribution between the recesses 2110 can also reduce the risk of excessively high local temperatures of the negative electrode sheet 21. When gas is generated inside the secondary battery 100, it is also beneficial to quickly discharge the gas from the electrode assembly 20 to reduce the risk of pressure accumulation inside the electrode assembly 20. This can also improve the problem of lithium precipitation in the recesses 2110 and improve the cycle performance of the secondary battery 100. In addition, by setting the relationship between the compaction density D and the total thickness T, the present application can reduce the risk of collapse of the side walls of the recesses 2110 when the recesses 2110 are set, even when the recesses 2110 are small in size or the distribution density is small. Among them, the recesses 2110 can be formed by laser drilling. Laser drilling has high precision and is conducive to obtaining recesses 2110 that are small in size and uniformly distributed.

[0060] Referring to FIG3 , in some embodiments, when viewed in the thickness direction X of the negative electrode sheet 21 (i.e., the direction perpendicular to the paper in FIG3 ), the recess 2110 is square. The square recess 2110 can increase the contact area between the electrolyte and the negative electrode active material layer 211 at the boundary, which helps to increase the diffusion rate of lithium ions in the negative electrode active material layer 211, thereby further improving the problems of poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer 211. Referring to FIG4 , when viewed in the thickness direction X of the negative electrode sheet 21, the recess 2110 can also be circular. The circular recess 2110 can facilitate the uniform diffusion of lithium ions in the negative electrode active material layer 211, improve electrolyte infiltration, and reduce ion migration resistance, thereby further improving the problems of poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer 211.

[0061] Isolation film

[0062] The separator 23 comprises a porous membrane layer, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator 23 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0063] electrolyte

[0064] The state of the electrolyte can be one or more of a gel state, a solid state, and a liquid state. In some embodiments, the liquid electrolyte comprises a lithium salt and an organic solvent. The lithium salt can be selected from, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, tris(trifluoromethylsulfonyl)methyl lithium (LiC(SO2CF3)3), lithium bis(oxalatoborate) (LiBOB), and lithium difluorophosphate (LiPO2F 2) one or more. For example, the lithium salt is selected from LiPF6 because it can give high ionic conductivity and improve cycle characteristics. The organic solvent can be a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, other organic solvents or a combination thereof. Examples of carbonate compounds include but are not limited to diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or a combination thereof.

[0065] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.5 mol / L to 1.5 mol / L. The lithium salt dissolves in the organic solvent and ionizes, partially forming solvated lithium ions and corresponding anion clusters, which provide ion conductivity. Therefore, increasing the concentration of the lithium salt in the electrolyte can help improve the conductivity of the electrolyte, enhance the kinetic performance of the secondary battery 100, and further reduce the possibility of lithium plating. The concentration of the lithium salt in the electrolyte can be measured using an ion chromatograph.

[0066] Referring to FIG6 , one embodiment of the present application further provides an electronic device 1 including a battery compartment 101 and the aforementioned secondary battery 100. The electronic device 1 may include, but is not limited to, a laptop computer, a pen-type 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 television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, lighting fixtures, toys, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0067] The present application is described below by way of specific examples and comparative examples. It should be understood by those skilled in the art that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0068] Example 1

[0069] (1) Preparation of the positive electrode plate 22: 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. Pre-apply foam glue to a portion of the surface of the positive electrode current collector 220, i.e., aluminum foil, with a thickness of 12μm. The slurry is evenly coated on one surface of the aluminum foil, heated to cause the foam glue to fall off so that a portion of the surface of the aluminum foil is exposed, and then dried at 90°C to obtain a positive electrode active material layer 221 with a coating thickness of 74μm. Repeat the above coating steps on the other surface of the aluminum foil to obtain a double-sided coated positive electrode plate 22. Then, the positive electrode plate 22 is cold pressed, and a positive electrode tab is welded to the exposed aluminum foil. The positive electrode tab is made of aluminum.

[0070] (2) Preparation of the negative electrode plate 21: The negative electrode active materials artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 96:1.5:2.5, and deionized water is added as a solvent to prepare a slurry with a weight percentage of 70 wt%, and stirred evenly. Pre-apply foam glue to the surface of the 8 μm negative electrode current collector 210, i.e., the copper foil. The slurry is evenly coated on one surface of the copper foil, heated to cause the foam glue to fall off so that the surface of the aluminum foil is exposed, and then dried at 110°C to obtain a negative electrode active material layer 211 with a coating thickness of 50 μm. Repeat the above coating steps on the other surface of the copper foil to obtain a double-sided coated negative electrode plate 21, i.e., the total thickness T of the two negative electrode active material layers 211 is 100 μm. Then, the negative electrode active material layer 211 is cold pressed. Laser drilling is then used to create multiple recesses 2110 in each negative electrode active material layer 211. The recesses 2110 are 70 μm wide and 10 μm deep, with a 1 mm center-to-center spacing between adjacent recesses 2110. The exposed copper foil is then welded to the negative electrode tab, which is made of copper.

[0071] (3) Preparation of electrolyte: In a dry argon atmosphere, organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were first mixed at a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0072] (4) Preparation of the isolation film 23: A polyethylene (PE) film with a thickness of 9 μm was selected.

[0073] (5) Assembly of the secondary battery 100: The positive electrode sheet 22, separator 23, and negative electrode sheet 21 are stacked and wound in sequence to form the electrode assembly 20. A 150 μm thick aluminum-plastic film with cavities punched and formed is placed in an assembly fixture with the cavities facing upward, and the electrode assembly 20 is placed in the cavities. Electrolyte is injected into the cavities of the aluminum-plastic film, and the positive and negative electrode tabs are led out of the aluminum-plastic film. Then, the battery is formed and packaged to obtain the secondary battery 100 shown in FIG1 .

[0074] Examples 2-13 and Comparative Examples 1-2

[0075] The difference from Example 1 lies in the relationship between D and T.

[0076] Comparative Example 3

[0077] The difference from Example 1 is that no recess 2110 is provided on the negative electrode active material layer 211 .

[0078] Example 14

[0079] (1) Preparation of the positive electrode plate 22: 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. The slurry is evenly coated on one surface of the positive electrode current collector 220, i.e., an aluminum foil with a thickness of 12μm, leaving an empty foil area at the edge of the aluminum foil, and then dried at 90°C to obtain a positive electrode active material layer 221 with a coating thickness of 74μm. The above coating steps are repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode plate 22. Then, the excess empty foil area is removed by laser die-cutting to obtain a positive electrode tab.

[0080] (2) Preparation of the negative electrode plate 21: The negative electrode active materials artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 96:1.5:2.5, deionized water is added as a solvent, and a slurry with a weight percentage of 70 wt% is prepared and stirred evenly. The slurry is evenly coated on one surface of the 8 μm negative electrode current collector 210, i.e., a copper foil, leaving an empty foil area at the edge of the copper foil, and then dried at 110°C to obtain a negative electrode active material layer 211 with a coating thickness of 65 μm. The above coating steps are repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode plate 21. Then, the total thickness T of the two layers of negative electrode active material layers 211 is 130 μm. Then, the negative electrode active material layer 211 is cold pressed. Laser drilling is then used to create multiple recesses 2110 on each negative electrode active material layer 211. The recesses 2110 are 70 μm wide and 10 μm deep, with a 1 mm center-to-center spacing between adjacent recesses 2110. Laser die-cutting is then used to remove the excess foil area, yielding the negative electrode tabs.

[0081] (3) Preparation of electrolyte: In a dry argon atmosphere, organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were first mixed at a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0082] (4) Preparation of the isolation film 23: A polyethylene (PE) film with a thickness of 9 μm was selected.

[0083] (5) Assembly of the secondary battery 100: The positive electrode sheet 22, separator 23, and negative electrode sheet 21 are stacked in sequence to form a laminated electrode assembly 20. An aluminum-plastic film (150 μm thick) with cavities formed therein is placed in an assembly fixture with the cavities facing upward, and the electrode assembly 20 is placed in the cavities. Electrolyte is injected into the cavities of the aluminum-plastic film, and the positive and negative electrode tabs are led out of the aluminum-plastic film. Chemical formation and packaging are then performed to obtain the secondary battery 100 shown in FIG5 .

[0084] Comparative Examples 4-6

[0085] The difference from Example 14 lies in the relationship between D and T.

[0086] Ten secondary batteries of each comparative example and embodiment were taken for energy density, kinetic performance and cycle performance tests. The test results are recorded in Table 1.

[0087] The energy density test steps for secondary batteries are as follows: 1) The secondary battery is allowed to rest at a test temperature of 25°C for 30 minutes, then charged at a constant current of 0.7C to 4.50V. Then, it is charged at a constant voltage of 4.50V to 0.05C, allowed to rest for 5 minutes, and discharged at a constant current of 0.2C to 3.0V. The discharge capacity C of the secondary battery is then calculated. 2) The secondary battery is charged at a constant current of 0.7C to 3.95V, then charged at a constant voltage of 3.95V to 0.05C. The length (L), width (W), and height (H) of the secondary battery are then measured using a laser thickness gauge. Energy density (ED) = C / (L × W × H), expressed in Wh / L. The energy density of 10 samples is averaged.

[0088] The dynamic performance of secondary batteries can be characterized by low-temperature discharge performance. The low-temperature discharge performance test steps are as follows: 1) The secondary battery is allowed to stand at a test temperature of 25°C for 5 minutes, then discharged at a constant current of 0.2C to 3.0V, and allowed to stand for 5 minutes. This discharge capacity is the initial discharge capacity; 2) Charge at a constant current of 0.2C to 4.50V, then charge at a constant voltage of 4.50V to 0.02C, and allow to stand for 5 minutes; 3) Adjust the furnace temperature to -20°C, stand for 60 minutes, then discharge at a constant current of 0.2C to 3.0V, and then allow to stand for 5 minutes. The ratio of the secondary battery's discharge capacity after low-temperature discharge to the initial capacity multiplied by 100% is the low-temperature capacity retention rate. The low-temperature capacity retention rates of 10 samples are averaged.

[0089] The test steps for the cyclic capacity retention rate of secondary batteries are as follows: 1) The secondary battery is allowed to rest at a test temperature of 25°C for 30 minutes. Then, it is charged to 4.43V at a constant current of 1C. Then, it is charged to 0.05C at a constant voltage. After resting for 5 minutes, it is discharged to 3.0V at a rate of 0.7C. This discharge capacity is the initial discharge capacity and is calculated as 100%. 2) The above charge and discharge steps are repeated for 1000 cycles. The ratio of the discharge capacity after the cycle to the initial capacity of the secondary battery is multiplied by 100% to obtain the capacity retention rate. The capacity retention rates of the 10 samples are averaged.

[0090] The lithium plating test steps of the secondary battery are as follows: 1) Charge the secondary battery after the cycle test to 4.43V at a constant current of 1C, and then charge it to 0.05C at a constant voltage; 2) Disassemble the above secondary battery and check whether lithium plating occurs in the concave area of ​​the negative electrode. If the lithium plating area is greater than or equal to 2mm 2 , it is determined to be lithium deposition and the test fails. The proportion of samples that pass the lithium deposition test among the 10 samples is the lithium deposition test pass rate. The results are recorded in Table 1.

[0091] Table 1

[0092] As can be seen from the data in Table 1, for the electrode assembly of the wound structure, compared to the thick electrode sheet with a recess in Comparative Example 1-2, Example 1-11 sets the compaction density D and the total thickness T to meet 3T+1≤D≤8T+1 while setting the recess, so that the secondary battery can maintain a higher energy density while improving the ion transmission path so that the low-temperature capacity retention rate of the secondary battery is improved, and also improves the degree of electrolyte infiltration, thereby improving the lithium plating test pass rate and the cycle capacity retention rate of the secondary battery. Therefore, the secondary battery of Example 1-11 can take into account both higher energy density and better low-temperature capacity retention rate and cycle capacity retention rate. Although the compaction density D and total thickness T of Comparative Example 3 meet the above relationship, no recess is set, so the ion transmission path and electrolyte infiltration are poor, and the low-temperature capacity retention rate and cycle capacity retention rate of the secondary battery are low. Further, Example 2-8 meets 3.5T+1≤D≤6.5T+1, which can further improve the energy density, low-temperature capacity retention rate and cycle capacity retention rate of the secondary battery. Furthermore, Examples 4-6 satisfy 4.5T+1≤D≤5.5T+1, which can further improve the energy density, low-temperature capacity retention rate, and cycle capacity retention rate of the secondary battery.

[0093] Similarly, for the laminated electrode assembly, compared to the thick electrode sheets with recesses in Comparative Examples 4-5, Example 14 achieves both a higher energy density and excellent low-temperature and cycle capacity retention by providing recesses while also setting the compaction density D and total thickness T to meet specific conditions. Comparative Example 6, while satisfying the aforementioned relationship in terms of compaction density D and total thickness T, lacks recesses, resulting in poor ion transport and electrolyte wettability, and low low-temperature and cycle capacity retention.

[0094] Moreover, the compacted density of Examples 1-14 satisfies 1.2 g / cm 3 ≤D≤2.2g / cm 3 When the secondary battery is charged, it can take into account higher energy density and better low-temperature capacity retention rate and cycle capacity retention rate.

[0095] Examples 15-18

[0096] The difference from Example 1 is that in the wound electrode assembly 20 , the compaction density D1 of the first active material region 2111 located in the straight section 20A is different from the compaction density D2 of the second active material region 2112 located in the curved section 20B.

[0097] Examples 19-22

[0098] The difference from Example 14 is that the compaction density D3 of the third active material region 2113 located in the central region is different from the compaction density D4 of the fourth active material region 2114 located in the edge region.

[0099] Table 2

[0100] As shown in Table 2, for wound electrode assemblies, compared to Example 1, Examples 15-17 had higher compaction densities for the first active material region compared to the second active material region. While the secondary battery's lithium plating pass rate, low-temperature capacity retention, and cycle capacity retention rates did not significantly decrease, the energy density was significantly improved. Compared to Example 18, Examples 15-17 reduced the impact of the compaction density of the first active material region on the lithium plating pass rate, low-temperature capacity retention, and cycle capacity retention rates by limiting the upper limit of the compaction density.

[0101] Compared to Example 14, for electrode assemblies with a laminated structure, the third active material region of Examples 19-21 had a higher compaction density than the fourth active material region. The lithium plating pass rate, low-temperature capacity retention rate, and cycle capacity retention rate of the secondary batteries did not significantly decrease, but the energy density was significantly improved. Compared to Example 22, Examples 19-21 reduced the impact of the third active material region's compaction density on the lithium plating pass rate, low-temperature capacity retention rate, and cycle capacity retention rate by limiting its upper limit.

[0102] Examples 23-37

[0103] The difference from Example 1 lies in the relevant dimensions of the recess 2110 .

[0104] Table 3

[0105] As can be seen from the data in Table 3, compared to Examples 26 and 32, the size of the recess in Examples 1, 23-25, and 28-31 is appropriately increased, which can improve the degree of electrolyte infiltration in the entire negative electrode active material layer and reduce the ion migration impedance. Therefore, the lithium plating pass rate of the secondary battery is improved, and the cycle capacity retention rate and low-temperature cycle retention rate of the secondary battery are also improved. Compared to Examples 27 and 33, the upper limit of the recess size in Examples 1, 23-25, and 28-31 is limited, avoiding the loss of more negative electrode active material, and reducing the risk of side reactions caused by the increase in contact area between the electrolyte and the negative electrode active material, thereby reducing impedance. Therefore, the secondary battery can maintain a higher energy density, and the lithium plating pass rate and cycle capacity retention rate are also improved.

[0106] Compared to Example 36, the distribution density of the recesses in Examples 1 and 34-35 is appropriately increased, thereby avoiding the loss of more negative electrode active materials and reducing the risk of side reactions caused by the increased contact area between the electrolyte and the negative electrode active materials, thereby reducing impedance. Therefore, the secondary battery can maintain a high energy density, and the lithium plating pass rate and cycle capacity retention rate are also improved. Compared to Example 37, the upper limit of the recess distribution density is limited in Examples 1 and 34-35, thereby increasing the degree of electrolyte infiltration in the entire negative electrode active material layer and reducing ion migration impedance. Therefore, the lithium plating pass rate of the secondary battery is improved, and the cycle capacity retention rate and low-temperature cycle retention rate of the secondary battery are also improved.

[0107] The above disclosure is only a preferred embodiment of the present application, and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A secondary battery comprising an electrode assembly, wherein the electrode assembly comprises a negative electrode sheet, wherein: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on a surface of the negative electrode current collector. The negative electrode current collector includes a first region. The negative electrode active material layer is provided on two opposite surfaces of the first region. The total thickness of the negative electrode active material layer on the first region is T, and 0.07 mm ≤ T ≤ 0.15 mm. The negative electrode active material layer is provided with a plurality of recesses, and the compaction density of the negative electrode active material layer is D, in units of g / cm 3 , 3T+1≤D≤8T+1.

2. The secondary battery according to claim 1, wherein 3.5T+1≤D≤6.5T+1.

3. The secondary battery according to claim 2, wherein 4.5T+1≤D≤5.5T+1.

4. The secondary battery according to any one of claims 1 to 3, wherein 1.2g / cm 3 ≤D≤2.2g / cm 3 。 5. The secondary battery according to claim 4, wherein 1.3g / cm 3 ≤D≤1.9g / cm 3 。 6. The secondary battery according to any one of claims 1 to 3, wherein The electrode assembly is a wound structure, comprising a straight section and a curved section connected along the winding direction. The negative electrode active material layer comprises a first active material region located in the straight section and a second active material region located in the curved section. The compaction density of the first active material region is D1, and the compaction density of the second active material region is D2, 1.3 g / cm 3 ≤D1≤2.0g / cm 3 , 1.2g / cm 3 ≤D2≤1.5g / cm 3 .

7. The secondary battery according to any one of claims 1 to 3, wherein The electrode assembly is a laminated structure. The negative electrode active material layer includes a third active material region and a fourth active material region arranged around the outer periphery of the third active material region. The compaction density of the third active material region is D3, and the compaction density of the fourth active material region is D4, 1.3 g / cm 3 ≤D3≤2.2g / cm 3 , 1.2g / cm 3 ≤D4≤1.7g / cm 3 .

8. The secondary battery according to any one of claims 1 to 7, wherein The width of the concave portion is 70 μm to 100 μm, the depth is 5 μm to 100 μm, and the center distance between two adjacent concave portions is 1 mm to 2 mm.

9. The secondary battery according to any one of claims 1 to 8, wherein When viewed from the thickness direction of the negative electrode sheet, the recess is circular or square.

10. The secondary battery according to any one of claims 1 to 9, wherein The secondary battery has a CB value of 1 to 1.

05.

11. The secondary battery according to any one of claims 1 to 9, wherein The concave portion is obtained by removing a portion of the negative electrode active material layer.

12. An electronic device, wherein: The invention comprises a battery compartment and a secondary battery according to any one of claims 1 to 11 arranged in the battery compartment.

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