Secondary battery and electric device

Abstract

The invention discloses a secondary battery and electric equipment, and the secondary battery comprises an electrode assembly with a winding structure, and the electrode assembly comprises a first pole piece. The first pole piece comprises a first current collector and a first active material layer; the first current collector comprises a first surface and a second surface; the first current collector is provided with a first section and a second section, the first surface and the second surface of the first section are provided with the first active material layer, the second surface of the second section is provided with the first active material layer, and the first surface of the second section is not provided with the active material layer. The junction position of the first section and the second section is defined as a junction line, the first pole piece corresponding to the first section comprises a first area, the first area is an area extending L1 from the junction line in the direction from the second section to the first section, and L1 is larger than or equal to 0.5 mm and smaller than or equal to 5mm. The first current collector in the first region is a first sub-section, the first active material layer located on the first surface of the first sub-section is a first part, and at least one concave part is arranged on the first part.

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to a secondary battery and an electrical device. 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] Secondary batteries include electrode assemblies. In related technologies, for reasons of energy density and safety, negative electrode active material slurry is typically applied to one side of the negative electrode sheet of the electrode assembly, typically in a single-sided coating area, or to both sides of the negative electrode sheet, typically in a double-sided coating area. However, such electrode assemblies are prone to lithium deposition during cycling, reducing 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 and electrical equipment to reduce the risk of lithium plating and improve the safety and cycle life of the secondary battery.

[0005] Embodiments of the present application provide a secondary battery comprising an electrode assembly in a wound structure. The electrode assembly includes a first electrode sheet. The first electrode sheet includes a first current collector and a first active material layer disposed on the first current collector. Along a first direction, the first current collector includes a first surface and a second surface, with the first direction being the thickness direction of the first electrode sheet. The first current collector comprises a first segment and a second segment, arranged in sequence. The first surface of the first segment and the second surface of the first segment are both provided with the first active material layer, the second surface of the second segment is provided with the first active material layer, and the first surface of the second segment is not provided with the active material layer. The intersection of the first segment and the second segment is defined as a boundary line. The first electrode sheet corresponding to the first segment includes a first region, which is an area extending L1 from the boundary line in a direction from the second segment to the first segment, with 0.5 mm ≤ L1 ≤ 5 mm. The first current collector in the first region is a first subsegment. The first active material layer located on the first surface of the first subsegment is a first portion. The first portion is provided with at least one recess. The recess in the first portion is formed by the first portion being recessed away from the surface of the first subsegment and toward the first subsegment.

[0006] The starting point for coating the first active material layer slurry on the first surface of the first current collector is the intersection of the single-sided coating area and the double-sided coating area. This location is prone to slurry accumulation, resulting in the thickness of the first part being greater than the thickness of the single-layer first active material layer in the second area. When the first electrode is cold-pressed, the location of the first part is overpressed, resulting in the compaction density of the first part being greater than the compaction density of the first active material layer in the second area, which in turn leads to a high lithium ion transmission impedance at this location and a high risk of lithium plating. By providing a recess on the first part, it is beneficial to the infiltration and storage of the electrolyte, reducing the lithium ion transmission impedance of the first area, shortening the lithium ion transmission distance, reducing the risk of lithium plating on the surface of the first part, and improving the safety and service life of the battery. And by ensuring that L1 ≥ 0.5mm, the area where the recess is provided in the first part is conducive to reducing the risk of lithium plating. By ensuring that L1 ≤ 5mm, while reducing the risk of lithium plating, the impact of excessive removal of the first active material layer due to the formation of the recess on the energy density of the secondary battery is reduced.

[0007] In one or more of the above optional embodiments, the first active material layer located on the second surface of the first sub-segment constitutes the second portion, and the second portion is provided with at least one recessed portion, wherein the recessed portion of the second portion is formed by the surface of the second portion facing away from the first sub-segment and recessed toward the first sub-segment. Providing the recessed portion on the second portion facilitates the infiltration and storage of the electrolyte, further reduces the lithium ion transmission impedance in the first region, shortens the lithium ion transmission distance, reduces the risk of lithium deposition on the second surface of the first sub-segment, and improves the safety and service life of the battery.

[0008] In one or more optional embodiments above, the first electrode corresponding to the first section also includes a second area, and along the winding direction of the electrode assembly, the second area includes a first edge and a second edge; along the winding direction of the electrode assembly, the distance between the first edge and the boundary line is 20 mm, and the distance between the second edge and the tail edge of the electrode assembly is 20 mm; the first area satisfies at least one of the following conditions: (a). 0.5 mm ≤ L1 ≤ 3 mm, which is beneficial to reducing the risk of lithium plating while ensuring that the area where the recess is set in the first part is conducive to further reducing the impact of excessive removal of the first active material layer due to the formation of the recess on the energy density of the secondary battery; (b). Along the first direction, the thickness of the first part is H1, and the thickness of the single-layer first active material layer in the second area is H3, 1.05 ≤ H1 / H3 ≤ 1.2; (c). The compaction density of the first part is ρ1, and the compaction density of the first active material layer in the second area is ρ3, 1.05 ≤ ρ1 / ρ3 ≤ 1.2.

[0009] In one or more of the above optional embodiments, the first electrode sheet corresponding to the second segment includes a third region, which is a region extending L2 from the boundary line along the first segment toward the second segment, with 1mm≤L2≤7mm. The first active material layer in the third segment is a third portion, and the third portion is provided with at least one recessed portion. The recessed portion in the third portion is formed by the surface of the third portion facing away from the second segment and concave toward the second segment. Because the thickness of the first portion is greater than the thickness of the single-layer first active material layer in the second segment, the thickness difference prevents the first active material layer in the third segment from being pressed by the roller, resulting in a thickness greater than that of the first active material layer in the second segment that is normally cold-pressed. The compaction density of the third portion is less than the compaction density of the first active material layer in the second segment, resulting in a long lithium ion migration distance and a high risk of lithium plating. The provision of the recessed portion in the third segment facilitates electrolyte infiltration and storage, reduces the lithium ion transfer impedance in the third segment, shortens the lithium ion transfer distance, reduces the risk of lithium plating on the surface of the third segment, and improves battery safety and service life. And by setting L2≥1mm, the area where the recess is set in the third part is ensured, which is beneficial to reducing the risk of lithium plating. By setting L2≤7mm, on the basis of reducing the risk of lithium plating, the influence on the energy density of the secondary battery caused by excessive removal of the first active material layer due to the formation of the recess is reduced.

[0010] In one or more optional embodiments above, the first electrode corresponding to the first section also includes a second region, and along the winding direction of the electrode assembly, the second region includes a first edge and a second edge; along the winding direction of the electrode assembly, the first edge is 20 mm away from the boundary line, and the second edge is 20 mm away from the tail edge of the electrode assembly; the third region satisfies at least one of the following conditions: (a). Along the first direction, the thickness of the third part is H2, and the thickness of the single-layer first active material layer in the second region is H3, 1.1≤H2 / H3≤1.5; (b). The compaction density of the third part is ρ2, and the compaction density of the first active material layer in the second region is ρ3, 0.65≤ρ2 / ρ3≤0.95.

[0011] In one or more of the above optional embodiments, 1mm 2 The total area of ​​the concave portion in the first section is S1, 0.01 mm 2 ≤S1≤0.15mm 2 . Pass S1 ≥ 0.01mm 2 , ensuring the area of ​​the first recess is conducive to reducing the risk of lithium deposition, through S1≤0.15mm 2 , thereby reducing the influence on the energy density of the secondary battery 100 caused by the excessive removal of the first active material layer due to the formation of the recess.

[0012] In one or more of the above optional embodiments, 0.04 mm2 ≤S1≤0.1mm 2 Further ensuring the area of ​​the first recess is conducive to reducing the risk of lithium plating and further affecting the energy density of the secondary battery.

[0013] In one or more of the above optional embodiments, 1mm 2 The total area of ​​the concave portion in the third part is S2, 0.01mm 2 ≤S2≤0.15mm 2 . Pass S2 ≥ 0.01mm 2 , ensuring the area of ​​the concave portion of the third part is conducive to reducing the risk of lithium deposition, through S2≤0.15mm 2 , reducing the impact of excessive removal of the first active material layer due to the formation of the recess on the energy density of the secondary battery.

[0014] In one or more of the above optional embodiments, 0.04 mm 2 ≤S2≤0.1mm 2 Further ensuring the area of ​​the recessed portion in the third part is beneficial to reducing the risk of lithium plating and further affecting the energy density of the secondary battery.

[0015] In one or more of the above optional embodiments, along the first direction, the thickness of the first portion is H1, the depth of the recess in the first portion is h1, and 1 / 6 ≤ h1 / H1 ≤ 5 / 6. By ensuring h1 / H1 ≥ 1 / 6, the depth of the recess in the first portion is guaranteed, which helps reduce the risk of lithium plating. By ensuring h1 / H1 ≤ 5 / 6, the effect of excessive depth on the energy density of the secondary battery is reduced.

[0016] In one or more optional embodiments above, 1 / 3≤h1 / H1≤2 / 3. Further ensuring the depth of the recess in the first portion is beneficial to reducing the risk of lithium plating and further reducing the impact of excessive depth on the energy density of the secondary battery.

[0017] In one or more of the above optional embodiments, along the first direction, the thickness of the third portion is H2, the depth of the recess in the third portion is h2, and 1 / 6 ≤ h2 / H2 ≤ 5 / 6. By ensuring h2 / H2 ≥ 1 / 6, the depth of the recess in the third portion is guaranteed, which helps reduce the risk of lithium plating. By ensuring h2 / H2 ≤ 5 / 6, the effect of excessive depth on the energy density of the secondary battery is reduced.

[0018] In one or more of the above optional embodiments, 1 / 3≤h2 / H2≤2 / 3. Further ensuring the depth of the concave portion in the third portion is beneficial to reducing the risk of lithium plating and further reducing the impact of excessive depth on the energy density of the secondary battery.

[0019] In one or more optional embodiments above, the first electrode sheet corresponding to the first section further includes a second region, and along the winding direction of the electrode assembly, the second region includes a first edge and a second edge; along the winding direction of the electrode assembly, the distance between the first edge and the boundary line is 20 mm, and the distance between the second edge and the trailing edge of the electrode assembly is 20 mm; the first active material layer in the second region is also provided with at least one recess, 1 mm 2 The total area of ​​the recesses in the first active material layer in the second region is S3,1 mm 2 The total area of ​​the concave portion in the first section is S1, 1 mm 2 The total area of ​​the concave portion in the third portion is S2, the depth of the concave portion in the first portion is h1, the depth of the concave portion in the third portion is h2, along the first direction, the thickness of the single-layer first active material layer in the second region is H3, the depth of the concave portion in the second region is h3, S1 or S2 or h1 or h2 satisfies at least one of the following conditions: (a). 0.5≤S1 / S3≤0.95; (b). 0.6≤S1 / S3≤0.85; (c). 5≤S2 / S3≤0.95; (d). 0.6≤S2 / S3≤0.85; (e). 1.05≤h1 / h3≤1.2; (f). 1.1≤h1 / h3≤1.15; (g). 1.1≤h2 / h3≤1.5;

[0020] (h). 1.2≤h2 / h3≤1.4. This is beneficial to reducing the risk of lithium plating in the second area.

[0021] In one or more of the above optional embodiments, the width of the first electrode sheet after unfolding is defined as the second direction, and the length of the first electrode sheet after unfolding is defined as the third direction, with the first, second, and third directions being perpendicular to each other. The recess extends along the second direction, and when viewed from the first direction, the width of a single recess along the third direction is W1, with 20μm ≤ W1 ≤ 300μm; or, when the recess extends along the third direction, the width of a single recess along the second direction is W1, with 20μm ≤ W1 ≤ 300μm when viewed from the first direction. This helps reduce the risk of lithium plating.

[0022] In one or more optional embodiments above, the width of the first active material layer along the second direction is W, and when the recess extends along the second direction, the length of a single recess along the second direction is W2, and 0.6≤W2 / W≤1. This is beneficial for reducing the risk of lithium plating.

[0023] In one or more optional embodiments above, when viewed along the first direction, a single concave portion is substantially circular, and a diameter of the single concave portion is D1, where 0.5 mm ≤ D1 ≤ 3 mm, which is beneficial for reducing the risk of lithium plating.

[0024] In one or more of the above optional embodiments, the plurality of recesses are spaced apart, with a spacing D2 between adjacent recesses, 0.5 mm ≤ D2 ≤ 3 mm, which is beneficial for reducing the risk of lithium plating.

[0025] In one or more optional embodiments above, the first electrode is a negative electrode.

[0026] An embodiment of the present application provides an electrical device, comprising the secondary battery in any one of the above embodiments.

[0027] The above-mentioned electrical equipment is powered by the above-mentioned secondary battery, and the lithium deposition of the electrode assembly is improved, so the secondary battery has higher safety and cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic cross-sectional views of secondary batteries in some embodiments are shown.

[0029] Figure 2 A schematic structural diagram of the first pole piece in the unfolded state in some embodiments is shown.

[0030] Figure 3 A schematic structural diagram of the first pole piece in an unfolded state from another perspective in some embodiments is shown.

[0031] Figure 4 A structural schematic diagram showing the unfolded state of the first pole piece from another perspective in some embodiments is shown.

[0032] Figure 5 Schematic diagrams of the structure of the first pole piece in the expanded state in other embodiments are shown.

[0033] Figure 6 Schematic diagrams of the structure of the first pole piece in the expanded state in some other embodiments are shown.

[0034] Figure 7 Shown are schematic structural diagrams of electrical equipment in some embodiments.

[0035] Description of main component symbols:

[0036] Secondary battery 100

[0037] Housing 10

[0038] Electrode assembly 20

[0039] First pole piece 21

[0040] Second pole piece 22

[0041] Isolation film 23

[0042] First current collector 211

[0043] First surface 211A

[0044] Second surface 211B

[0045] First section 2111

[0046] Second section 2112

[0047] First sub-segment 2111A

[0048] First active material layer 212

[0049] Part 1 212A

[0050] Part 2 212B

[0051] Part 3 212C

[0052] First Area 213

[0053] Second area 214

[0054] First Edge 2141

[0055] Second Edge 2142

[0056] Third Area 215

[0057] Junction 101

[0058] Recess 102

[0059] Electrical equipment 200

[0060] First direction X

[0061] Second direction Y

[0062] The third direction Z

[0063] Winding direction S

[0064] The following specific embodiments will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0065] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0066] When a component is referred to as being “disposed on” another component, it can be directly disposed on the other component or there may be a component intervening therebetween. When a component is referred to as being “connected to” another component, it can be directly connected to the other component or there may be a component intervening therebetween.

[0067] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "or / and" as used herein includes any and all combinations of one or more of the associated listed items.

[0068] It can be understood that the term "perpendicular" is used to describe the ideal state between two components. In actual production or use, there may be a state between the two components that is approximately perpendicular or equal. For example, combined with numerical descriptions, perpendicular can refer to the angle between two straight lines being in the range of 90°±10°, perpendicular can also refer to the dihedral angle between two planes being in the range of 90°±10°, and perpendicular can also refer to the angle between a straight line and a plane being in the range of 90°±10°. The two components described as "perpendicular" may not be absolute straight lines or planes, but may be roughly straight lines or planes. From a macroscopic perspective, a component can be considered a "straight line" or a "plane" if the overall extension direction is a straight line or a plane.

[0069] Unless otherwise defined, the term "plurality" herein, when used to describe the number of components, specifically means that the components are two or more.

[0070] The following describes some embodiments of the present application in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features of the embodiments may be combined with each other.

[0071] See also Figure 1 An embodiment of the present application provides a secondary battery 100 , including a housing 10 , an electrode assembly 20 , an electrolyte (not shown), and tabs (not numbered). The electrode assembly 20 and the electrolyte are located within the housing 10 .

[0072] The electrode assembly 20 includes a first electrode sheet 21, a second electrode sheet 22, and a separator 23. The separator 23 is disposed between the first electrode sheet 21 and the second electrode sheet 22. The electrode assembly 20 has a wound structure, that is, the first electrode sheet 21, the separator 23, and the second electrode sheet 22 are stacked and wound. The electrode tabs include a first electrode tab connected to the first electrode sheet 21 and a second electrode tab connected to the second electrode sheet 22. Optionally, the first electrode sheet 21 is a negative electrode sheet, and the second electrode sheet 22 is a positive electrode sheet.

[0073] See also Figures 2 to 4 The first electrode sheet 21 includes a first current collector 211 and a first active material layer 212 disposed on at least a portion of the first current collector 211. Taking the thickness direction of the first electrode sheet 21 as a first direction X, the first current collector 211 includes a first surface 211A and a second surface 211B disposed opposite to each other along the first direction X.

[0074] Along the winding direction S of the electrode assembly 20, the first current collector 211 has a first section 2111 and a second section 2112. The first active material layer 212 is provided on both the first surface 211A and the second surface 211B of the first section 2111. The first active material layer 212 is provided on the second surface 211B of the second section 2112, while the first surface 211A of the second section 2112 is not provided with the first active material layer 212. In other words, the first section 2111 is a double-sided coated area, while the second section 2112 is a single-sided coated area.

[0075] In some embodiments, the intersection of the first segment 2111 and the second segment 2112 is defined as the intersection line 101, and the first pole piece 21 corresponding to the first segment 2111 includes a first area 213, and the first area 213 is an area extending L1 from the intersection line 101 along the direction of the second segment 2112 to the first segment 2111, 0.5mm≤L1≤5mm.

[0076] The first current collector 211 in the first region 213 is a first sub-segment 2111A, and the first active material layer 212 located on the first surface 211A of the first sub-segment 2111A is a first portion 212A. At least one recess 102 is provided on the first portion 212A.

[0077] In some embodiments, the first electrode piece 21 corresponding to the first section 2111 also includes a second area 214. Along the winding direction S of the electrode assembly 20, the second area 214 includes a first edge 2141 and a second edge 2142. The first edge 2141 is 20 mm away from the boundary line 101, and the second edge 2142 is 20 mm away from the tail edge of the electrode assembly 20.

[0078] In some embodiments, along the first direction X, the thickness of the first portion 212A is H1, the thickness of the single first active material layer 212 in the second region 214 is H3, and 1.05≤H1 / H3≤1.2.

[0079] In some embodiments, the compaction density of the first portion 212A is ρ1, the compaction density of the first active material layer 212 in the second region 214 is ρ3, and 1.05≤ρ1 / ρ3≤1.2.

[0080] The starting position of coating the slurry of the first active material layer 212 on the first surface 211A of the first current collector 211 is the junction of the single-sided coating area and the double-sided coating area. This position is prone to slurry accumulation, resulting in the thickness H1 of the first part 212A being greater than the thickness H3 of the single-layer first active material layer 212 in the second area 214. When the first electrode 21 is cold pressed, the position where the first part 212A is located is over-pressurized, resulting in the compaction density of the first part 212A being greater than the compaction density of the first active material layer 212 in the second area 214, which in turn leads to a large lithium ion transmission impedance at this position and a high risk of lithium plating. The present application provides a recess 102 on the first part 212A, which is beneficial to the infiltration and storage of the electrolyte, reduces the lithium ion transmission impedance of the first area 213, shortens the lithium ion transmission distance, reduces the risk of lithium plating on the surface of the first part 212A, and improves the safety and service life of the battery. And by setting L1≥0.5mm, the area where the recess 102 is set in the first part 212A is ensured, which is beneficial to reducing the risk of lithium plating. By setting L1≤5mm, on the basis of reducing the risk of lithium plating, the influence of excessive removal of the first active material layer 212 due to the formation of the recess 102 on the energy density of the secondary battery 100 is reduced.

[0081] Optionally, L1 can be any one of 0.5mm, 0.8mm, 1.0mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm, 2mm, 2.2mm, 2.4mm, 2.6mm, 2.8mm, 3mm, 3.2mm, 3.4mm, 3.6mm, 3.8mm, 4mm, 4.2mm, 4.4mm, 4.6mm, 4.8mm, and 5mm, or a range consisting of any two of them.

[0082] In some embodiments, 0.5 mm ≤ L1 ≤ 3 mm. This ensures that the area where the recess 102 is set in the first portion 212A is conducive to reducing the risk of lithium plating, and further reduces the impact of excessive removal of the first active material layer 212 due to the formation of the recess 102 on the energy density of the secondary battery 100.

[0083] In some embodiments, 1 mm 2 The total area of ​​the recess 102 in the first portion 212A is S1, 0.01 mm 2 ≤S1≤0.15mm 2 . Pass S1 ≥ 0.01mm 2 , ensuring the area of ​​the first portion 212A where the recess 102 is set is beneficial to reducing the risk of lithium deposition, by S1≤0.15mm 2 , thereby reducing the influence on the energy density of the secondary battery 100 caused by the excessive removal of the first active material layer 212 due to the formation of the recess 102 .

[0084] Optionally, S1 can be 0.01 mm 2 , 0.02mm 2 , 0.03mm 2 , 0.04mm 2 , 0.05mm 2 , 0.06mm 2 , 0.07mm 2 , 0.08mm 2 , 0.09mm 2 , 0.1mm 2 , 0.11mm 2 , 0.12mm 2 , 0.13mm 2 , 0.14mm 2 , 0.15mm 2 A range consisting of any one or any two of .

[0085] In some embodiments, 0.04 mm 2 ≤S1≤0.1mm 2 , further ensuring the area of ​​the first portion 212A for setting the recess 102 is beneficial to reducing the risk of lithium plating and further affecting the energy density of the secondary battery 100.

[0086] In some embodiments, along the first direction X, the thickness of the first portion 212A is H1, and the depth of the recess 102 in the first portion 212A is h1, where 1 / 6 ≤ h1 / H1 ≤ 5 / 6. By ensuring h1 / H1 ≥ 1 / 6, the depth of the recess 102 in the first portion 212A is guaranteed, which helps reduce the risk of lithium plating. By ensuring h1 / H1 ≤ 5 / 6, the effect of excessive depth on the energy density of the secondary battery 100 is reduced.

[0087] In some embodiments, 1 / 3≤h1 / H1≤2 / 3, which further ensures the depth of the recess 102 in the first portion 212A, helps reduce the risk of lithium plating, and further reduces the impact of excessive depth on the energy density of the secondary battery 100.

[0088] Optionally, the ratio range of h1 / H1 can be any one of 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6, or a range consisting of any two of them.

[0089] See also Figure 2In some embodiments, the first active material layer 212 located on the second surface 211B of the first sub-segment 2111A is the second portion 212B, and at least one recess 102 is provided on the second portion 212B. Because the thickness of the first portion 212A is greater than the thickness H3 of the single layer of the first active material layer 212 in the second region 214, the overall thickness of the first region 213 of the first electrode 21 increases. When overvoltage occurs at the location of the first portion 212A, overvoltage also occurs at the location of the second portion 212B. Providing the recess 102 on the second portion 212B facilitates the infiltration and storage of the electrolyte, further reducing the lithium ion transmission impedance in the first region 213 and shortening the lithium ion transmission distance. This reduces the risk of lithium deposition on the second surface 211B of the first sub-segment 2111A, thereby improving the safety and service life of the battery.

[0090] In some embodiments, the area range of the recess 102 set on the second part 212B is the same as the area range of the recess 102 set on the first part 212A, and the ratio range of the thickness of the second part 212B to the depth of the recess 102 is the same as the ratio range of the thickness of the first part 212A to the depth of the recess 102.

[0091] See also Figures 2 to 4 In some embodiments, the first electrode 21 corresponding to the second section 2112 includes a third region 215. The third region 215 extends from the boundary line 101 in a direction L2 from the first section 2111 to the second section 2112, with 1 mm ≤ L2 ≤ 7 mm. The first active material layer 212 in the third region 215 is a third portion 212C, and at least one recess 102 is provided in the third portion 212C.

[0092] In some embodiments, along the first direction X, the thickness of the third portion 212C is H2 , the thickness of the single first active material layer 212 in the second region 214 is H3 , and 1.1≤H2 / H3≤1.5.

[0093] In some embodiments, the compaction density of the third portion 212C is ρ2, the compaction density of the first active material layer in the second region 214 is ρ3, and 0.65≤ρ2 / ρ3≤0.95.

[0094] Because the thickness of first portion 212A is greater than the thickness H3 of the single-layer first active material layer 212 in second region 214, the thickness difference prevents the first active material layer 212 in third portion 212C from being pressed by the roller, resulting in the thickness of third portion 212C being greater than the thickness H3 of the first active material layer 212 in second region 214, which is normally cold-pressed. The compaction density of third portion 212C is less than the compaction density of first active material layer 212 in second region 214, resulting in a long lithium ion migration distance and a high risk of lithium plating. Providing recess 102 in third portion 212C facilitates electrolyte infiltration and storage, reduces the lithium ion transfer impedance in third region 215, shortens the lithium ion transfer distance, reduces the risk of lithium plating on the surface of third portion 212C, and improves battery safety and service life. And by setting L2≥1mm, the area where the recess 102 is set in the third part 212C is ensured, which is beneficial to reducing the risk of lithium plating. By setting L2≤7mm, on the basis of reducing the risk of lithium plating, the influence of excessive removal of the first active material layer 212 due to the formation of the recess 102 on the energy density of the secondary battery 100 is reduced.

[0095] In some embodiments, 1 mm 2 The total area of ​​the recess 102 in the third portion 212C is S2, 0.01 mm 2 ≤S2≤0.15mm 2 . Pass S2 ≥ 0.01mm 2 , ensuring the area of ​​the recess 102 in the third portion 212C is provided, which is beneficial to reducing the risk of lithium deposition, and by S2≤0.15mm 2 , thereby reducing the influence on the energy density of the secondary battery 100 caused by the excessive removal of the first active material layer 212 due to the formation of the recess 102 .

[0096] Optionally, S2 can be 0.01 mm 2 , 0.02mm 2 , 0.03mm 2 , 0.04mm 2 , 0.05mm 2 , 0.06mm 2 , 0.07mm 2 , 0.08mm 2 , 0.09mm 2 , 0.1mm 2 , 0.11mm 2 , 0.12mm 2 , 0.13mm 2 , 0.14mm 2 , 0.15mm 2 A range consisting of any one or any two of .

[0097] In some embodiments, 0.04 mm 2 ≤S2≤0.1mm 2 , further ensuring the area of ​​the third portion 212C for setting the recess 102 is beneficial to reducing the risk of lithium plating and further affecting the energy density of the secondary battery 100.

[0098] In some embodiments, along the first direction X, the thickness of the third portion 212C is H2, and the depth of the recess 102 in the third portion 212C is h2, where 1 / 6 ≤ h2 / H2 ≤ 5 / 6. By ensuring h2 / H2 ≥ 1 / 6, the depth of the recess 102 in the third portion 212C is guaranteed, which helps reduce the risk of lithium plating. By ensuring h2 / H2 ≤ 5 / 6, the effect of excessive depth on the energy density of the secondary battery 100 is reduced.

[0099] In some embodiments, 1 / 3≤h2 / H2≤2 / 3, which further ensures the depth of the recess 102 in the third portion 212C, helps reduce the risk of lithium plating, and further reduces the impact of excessive depth on the energy density of the secondary battery 100.

[0100] Optionally, the ratio range of h2 / H2 can be any one of 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6, or a range consisting of any two of them.

[0101] In some embodiments, the first active material layer 212 in the second region 214 is also provided with at least one recess 102, 1 mm 2 The total area of ​​the recesses 102 in the first active material layer 212 in the second region 214 is S3, 1 mm 2 The total area of ​​the recesses 102 in the first portion 212A is S1, and 0.5≤S1 / S3≤0.95. By ensuring that S1 / S3 is ≥0.5, the area of ​​the recesses 102 in the second region 214 is guaranteed, which helps reduce the risk of lithium deposition in the second region 214. By ensuring that S1 / S3 is ≤0.95, the effect of excessive removal of the first active material layer 212 due to the formation of the recesses 102 on the energy density of the secondary battery 100 is reduced.

[0102] Optionally, the ratio of S1 / S3 may be any one of 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two of the above.

[0103] In some embodiments, 1 mm 2The total area of ​​the recess 102 in the third portion 212C is S2, 0.6≤S2 / S3≤0.85, which further ensures the area of ​​the recess 102 in the second region 214, is beneficial to reducing the risk of lithium deposition in the second region 214, and further reduces the influence on the energy density of the secondary battery 100 caused by excessive removal of the first active material layer 212 due to the formation of the recess 102.

[0104] In some embodiments, 0.5≤S2 / S3≤0.95. When S2 / S3≥0.5, the area of ​​the recess 102 in the second region 214 is ensured, which helps reduce the risk of lithium plating. When S2 / S3≤0.95, the effect of excessive removal of the first active material layer 212 due to the formation of the recess 102 on the energy density of the secondary battery 100 is reduced.

[0105] Optionally, the ratio of S2 / S3 may be any one of 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two of the above.

[0106] In some embodiments, 0.6≤S2 / S3≤0.85, further ensuring the area of ​​the recess 102 in the second region 214 , which is beneficial to reducing the risk of lithium plating and further reducing the impact on the energy density of the secondary battery 100 caused by excessive removal of the first active material layer 212 due to the formation of the recess 102 .

[0107] In some embodiments, the depth of the recess 102 in the first portion 212A is h1, and the depth of the recess 102 in the second region 214 is h3, where 1.05 ≤ h1 / h3 ≤ 1.2. By ensuring h1 / h3 ≥ 1.05, the depth of the recess 102 in the second region 214 is guaranteed, which helps reduce the risk of lithium plating. By ensuring h1 / h3 ≤ 1.2, the effect of an excessively large depth of the recess 102 in the second region 214 on the energy density of the secondary battery 100 is reduced.

[0108] Optionally, the ratio of h1 / h3 can be any one of 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, or a range consisting of any two of them.

[0109] In some embodiments, 1.1≤h1 / h3≤1.15, ensuring the depth of the recess 102 in the second region 214 further helps reduce the risk of lithium plating and further reduces the impact of the excessive depth of the recess 102 in the second region 214 on the energy density of the secondary battery 100.

[0110] In some embodiments, the depth of the recess 102 in the third portion 212C is h2, and 1.1≤h2 / h3≤1.5. By ensuring h2 / h3≥1.1, the depth of the recess 102 in the second region 214 is guaranteed, which helps reduce the risk of lithium plating. By ensuring h2 / h3≤1.5, the impact of an excessively large depth of the recess 102 in the second region 214 on the energy density of the secondary battery 100 is reduced.

[0111] Optionally, the ratio of h2 / h3 may be any one of 1.1, 1.2, 1.3, 1.4, 1.5, or a range consisting of any two of them.

[0112] In some embodiments, 1.2≤h2 / h3≤1.4, ensuring the depth of the recess 102 in the second region 214 further helps reduce the risk of lithium plating and further reduces the impact of the excessive depth of the recess 102 in the second region 214 on the energy density of the secondary battery 100.

[0113] In some embodiments, when the first electrode 21 is a negative electrode, the first active material layer 212 is a negative electrode active material. The negative electrode active material is a negative electrode active material known in the art that can reversibly deintercalate active ions, and this application does not limit this. 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 microbeads, silicon-based 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-based 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, and tin alloys.

[0114] In some embodiments, when the negative electrode active material is a silicon-based system, the sum of the thicknesses of the first segment 2111 and the first surface 211A and the second surface 211B of the first segment 2111 is H4, and 0.03 mm ≤ H4 ≤ 0.15 mm.

[0115] Optionally, H4 may be any one of 0.03 mm, 0.04 mm, 0.05 mm, 0.06 mm, 0.07 mm, 0.08 mm, 0.09 mm, 0.1 mm, 0.11 mm, 0.12 mm, 0.13 mm, 0.14 mm, and 0.15 mm, or a range consisting of any two of the ranges.

[0116] In some embodiments, when the negative electrode active material is a graphite system, the sum of the thicknesses of the first segment 2111 and the first surface 211A and the second surface 211B of the first segment 2111 is H5, and 0.06 mm≤H5≤0.15 mm.

[0117] Optionally, H5 may be any one of 0.06 mm, 0.07 mm, 0.08 mm, 0.09 mm, 0.1 mm, 0.11 mm, 0.12 mm, 0.13 mm, 0.14 mm, and 0.15 mm, or a range consisting of any two of the ranges.

[0118] See also Figures 3 to 5 In some embodiments, the width direction along the unfolded first electrode sheet 21 is defined as the second direction Y, and the length direction along the unfolded first electrode sheet 21 is defined as the third direction Z. The first direction X, the second direction Y, and the third direction Z are perpendicular to each other, and the winding direction S of the electrode assembly 20 is consistent with the third direction Z. The recess 102 extends along the second direction Y. When viewed along the first direction X, the width of a single recess 102 along the third direction Z is W1, with 20μm≤W1≤300μm. When W1 ≥ 20μm, electrolyte infiltration and storage are facilitated, reducing the risk of lithium plating. When W1 ≤ 300μm, the impact on the energy density of the secondary battery 100 is reduced.

[0119] Optionally, W1 can be any one of 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 110μm, 120μm, 130μm, 140μm, 150μm, 160μm, 170μm, 180μm, 190μm, 200μm, 210μm, 220μm, 230μm, 240μm, 250μm, 260μm, 270μm, 280μm, 290μm, 300μm, or a range consisting of any two of them.

[0120] In some embodiments, the width of the first active material layer 212 along the second direction Y is W, and the length of a single recess 102 along the second direction Y is W2, where 0.6≤W2 / W≤1. When W2 / W≥0.6, electrolyte infiltration and storage are facilitated, reducing the risk of lithium plating. When W2 / W equals 1, the recess 102 extends through the first active material layer 212 along the second direction Y, further facilitating electrolyte infiltration and storage, reducing the risk of lithium plating.

[0121] Optionally, the ratio of W2 / W may be any one of 0.6, 0.7, 0.8, 0.9, 1, or a range consisting of any two of them.

[0122] See also Figure 5 In some embodiments, the recess 102 extends along the third direction Z. When viewed along the first direction X, the width of a single recess 102 along the second direction Y is W1.

[0123] In some embodiments, along the first direction X, the width of the recess 102 gradually decreases from the first active material layer 212 toward the first current collector 211 , and the width W1 of a single recess 102 refers to the maximum width of the recess 102 .

[0124] See also Figure 6 Observed along the first direction X, a single recess 102 is generally circular, with a diameter D1 of 0.5 mm ≤ D1 ≤ 3 mm, which helps reduce the risk of lithium deposition. The longest straight line connecting two points on the circumference of the recess 102 and passing through the center of the recess 102 is the diameter.

[0125] Optionally, D1 can be any one of 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.1mm, 1.2mm, 1.3mm, 1.4mm, 1.5mm, 1.6mm, 1.7mm, 1.8mm, 1.9mm, 2mm, 2.1mm, 2.2mm, 2.3mm, 2.4mm, 2.5mm, 2.6mm, 2.7mm, 2.8mm, 2.9mm, and 3mm, or a range consisting of any two of them.

[0126] In some embodiments, the plurality of recesses 102 are spaced apart, and the distance between adjacent recesses 102 is D2, 0.5 mm ≤ D2 ≤ 3 mm, which is beneficial for reducing the risk of lithium plating.

[0127] Optionally, D2 can be any one of 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.1mm, 1.2mm, 1.3mm, 1.4mm, 1.5mm, 1.6mm, 1.7mm, 1.8mm, 1.9mm, 2mm, 2.1mm, 2.2mm, 2.3mm, 2.4mm, 2.5mm, 2.6mm, 2.7mm, 2.8mm, 2.9mm, and 3mm, or a range consisting of any two of them.

[0128] See also Figure 7 , one embodiment of the present application further provides an electric device 200, which includes the above-mentioned secondary battery 100. Among them, the secondary battery 100 of the present application is applicable to electric devices 200 in various fields. The electric device 200 is powered by the above-mentioned secondary battery 100, and the lithium plating of the electrode assembly 20 is improved, so the secondary battery 100 has higher safety and cycle life. In one embodiment, the electric device 200 of the present application can be, but is not limited to, electronic equipment, drones, backup power supplies, electric vehicles, electric motorcycles, electric power-assisted bicycles, electric tools, large household battery modules, etc.

[0129] The present application is described in detail below through specific embodiments and comparative examples. The present application is described using a wound lithium-ion secondary battery 100, a first electrode 21 being a negative electrode, and a second electrode 22 being a positive electrode as an example, with specific preparation processes and testing methods. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0130] Example 1

[0131] (1) Preparation of positive electrode sheet: 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 97.6:1:1.4, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 71.5wt%, and stirred evenly. In advance, a foam glue is applied to a portion of the surface of the positive electrode current collector, i.e., aluminum foil, with a thickness of 8μm. The slurry is evenly coated on one side of the aluminum foil, heated to cause the foam glue to fall off so that a portion of the aluminum foil surface is exposed, and then dried at 90°C. The above coating steps are repeated on the other side of the aluminum foil to obtain a double-sided coated positive electrode sheet. The initial positive electrode sheet is cold pressed to obtain a single-layer coating of a positive electrode active material layer with a thickness of 44μm, and then cut and other processes are performed to obtain a positive electrode sheet. A first tab is welded to the exposed aluminum foil. The first tab is made of aluminum.

[0132] (2) Preparation of negative electrode sheet: The negative electrode active materials artificial graphite, conductive carbon black (Super P), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 98:0.8:0.7:0.5, deionized water is added as a solvent, and a slurry with a weight percentage of 50wt% is prepared and stirred evenly. The slurry is evenly coated on the first and second surfaces of the negative electrode current collector, i.e., copper foil, with a thickness of 5μm, to obtain a double-sided coated negative electrode sheet. The initial negative electrode sheet is rolled to obtain a negative electrode active material layer with a coating thickness of 54μm. Then, laser grooves are made in the single-sided and double-sided junction area of ​​the electrode sheet to remove part of the active material. At the same time, a laser is used to clean off part of the active material at a specific position in the middle of the electrode sheet, and a second electrode tab is welded to the exposed copper foil. The second electrode tab is made of nickel.

[0133] (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.

[0134] (4) Preparation of isolation film: A polyethylene (PE) film with a thickness of 5 μm was selected.

[0135] (5) Preparation of secondary battery: The first electrode, the separator and the second electrode are stacked and wound in sequence to obtain Figure 1 The electrode assembly shown is constructed by placing a 150μm thick aluminum-plastic film with a dented surface in an assembly fixture, with the dent facing upward. The electrode assembly is then placed within the dent. Electrolyte is then injected into the dent of the aluminum-plastic film, and the first and second tabs are extended out of the aluminum-plastic film before packaging and formation to produce a secondary battery.

[0136] Comparative Example 1

[0137] The difference from Example 1 is that in Comparative Example 1, no recess is provided in the first portion.

[0138] Comparative Example 2, Comparative Example 3

[0139] The difference from Example 1 is that in Comparative Example 1, the recess is not provided in the first section.

[0140] Examples 2 to 63

[0141] The difference from Example 1 lies in the location of the recess, the values ​​of L1, L2, S1, S2, etc., and the specific differences are listed in Table 1. Except for the differences listed in Table 1, the other parameters are the same as those of Example 1.

[0142] Then, 50 secondary batteries of each embodiment and comparative example were taken for lithium deposition test and volume energy density test, and the test results are recorded in Table 1. The area and depth of the concave portion and the thickness of the first active material were measured using a scanning electron microscope (SEM).

[0143] Lithium plating test method: (1) Maintain the test temperature at 25°C and let the secondary battery 100 stand for 30 minutes; (2) Discharge at a constant current of 0.5C to 3V; (3) Let it stand for 10 minutes; (4) Charge at a constant current of 1.2C to 4.5V, and then charge at a constant voltage of 0.05C; (5) Let it stand for 10 minutes; (8) Repeat steps 2 to 5 100 times; (9) Disassemble the secondary battery 100 and observe whether lithium plating occurs on the negative electrode sheet. Count the number of secondary batteries 100 with lithium plating on the negative electrode sheet in each group. The number of secondary batteries 100 with lithium plating on the negative electrode sheet in this group of experiments is N. The lithium plating rate of the secondary batteries 100 in this group of experiments is N / 50.

[0144] Volume energy density test method: Under 25°C environmental conditions, let the secondary battery stand for 30 minutes, discharge it at a constant current of 0.2C to 3.0V, and let it stand for 10 minutes; charge it at a constant current of 0.2C to 4.5V, then charge it at a constant voltage to 0.05C, and let it stand for 10 minutes; then discharge it at a constant current of 0.2C to 3V, let it stand for 10 minutes, and record the discharge capacity C0; 2) 1400gPPG is used to measure the thickness of the battery cell, and CCD is used to measure the length and width of the battery cell; 3) Volume energy density is calculated using the following formula: platform voltage × C0 / volume of secondary battery

[0145] Table 1

[0146]

[0147]

[0148]

[0149]

[0150] It can be seen from Comparative Examples 1 and 2, Comparative Example 3 and Examples 1 to 63 in Table 1 that providing a recess in the first portion is beneficial to reducing the risk of lithium deposition in the interface area between the single-sided coating area and the double-sided coating area.

[0151] From Comparative Examples 1 to 3 and Examples 1 to 4 in Table 1, it can be seen that the first region is in the range of 0.5 mm ≤ L1 ≤ 5 mm, which is beneficial for reducing the risk of lithium deposition in the interface between the single-sided and double-sided coated regions while also achieving a high volumetric energy density. Furthermore, 0.5 mm ≤ L1 ≤ 3 mm is preferred.

[0152] From Example 1, Example 5 and Example 9 in Table 1, it can be seen that compared with Example 1, in Example 5, recesses are provided in the first part and the second part, and the volume energy density is slightly reduced, which further reduces the risk of lithium deposition in the boundary area between the single-sided coating area and the double-sided coating area. Compared with Example 5, in Example 9, recesses are provided in the first part, the second part and the third part, and the volume energy density is slightly reduced, which further reduces the risk of lithium deposition in the boundary area between the single-sided coating area and the double-sided coating area.

[0153] From Examples 9 to 15 in Table 1, it can be seen that the third region is in the range of 1 mm ≤ L2 ≤ 7 mm, which is beneficial to reducing the risk of lithium deposition in the interface area between the single-sided coating area and the double-sided coating area while taking into account a higher volume energy density.

[0154] From Example 1 and Example 16 to Example 21 in Table 1, it can be seen that 1 mm 2 The total area of ​​the concave portion in the first part is 0.01 mm 2≤S1≤0.15mm 2 The range is beneficial to reducing the risk of lithium plating in the interface area between the single-sided coating area and the double-sided coating area, while taking into account a higher volume energy density.

[0155] From Example 9 and Example 22 to Example 27 in Table 1, it can be seen that 1 mm 2 The total area of ​​the concave portion in the third part is 0.01mm 2 ≤S2≤0.15mm 2 The range is beneficial to reducing the risk of lithium plating in the interface area between the single-sided coating area and the double-sided coating area, while taking into account a higher volume energy density.

[0156] From Examples 1 and 28 to 33 in Table 1, it can be seen that the ratio of the thickness of the first portion to the depth of the recessed portion of the first portion satisfies 1 / 6 ≤ h1 / H1 ≤ 5 / 6, which helps reduce the risk of lithium deposition in the interface between the single-sided coating area and the double-sided coating area while also achieving a high volume energy density. Furthermore, preferably, 1 / 3 ≤ h1 / H1 ≤ 2 / 3.

[0157] From Examples 9 and 34 to 39 in Table 1, it can be seen that the ratio of the thickness of the third portion to the depth of the recessed portion of the third portion satisfies 1 / 6 ≤ h2 / H2 ≤ 5 / 6, which helps reduce the risk of lithium deposition in the interface region between the single-sided coating area and the double-sided coating area while also achieving a high volume energy density. Furthermore, preferably, 1 / 3 ≤ h2 / H2 ≤ 2 / 3.

[0158] From Example 1 and Example 40 to Example 45 in Table 1, it can be seen that 1 mm 2 The total area of ​​the concave portion in the first part is 1 mm 2 The ratio of the total area of ​​the recessed portions in the first active material layer in the second region satisfies 0.5≤S1 / S3≤0.95, which is beneficial to reducing the risk of lithium deposition in the second region while taking into account a higher volume energy density. When S1 / S3 < 0.5, the kinetics of the second region is poor and the risk of lithium deposition in the second region is high; when S1 / S3 > 0.95, the second region does not need such a large area of ​​grooves, which loses energy density. Furthermore, 0.6≤S1 / S3≤0.85 is preferred.

[0159] From Example 9 and Example 46 to Example 51 in Table 1, it can be seen that 1 mm 2 The total area of ​​the concave portion in the third part is 1 mm 2The ratio of the total area of ​​the recessed portions in the first active material layer in the second region satisfies 0.5≤S2 / S3≤0.95, which is beneficial to reducing the risk of lithium deposition in the second region while taking into account a higher volume energy density. When S2 / S3 < 0.5, the kinetics of the second region is poor and the risk of lithium deposition in the second region is high; when S2 / S3 > 0.95, the second region does not need such a large area of ​​grooves, which loses energy density. Furthermore, 0.6≤S2 / S3≤0.85 is preferred.

[0160] From Examples 1 and 52 to 57 in Table 1, it can be seen that the ratio of the depth of the recess in the first portion to the depth of the recess in the second region satisfies 1.05 ≤ h1 / h3 ≤ 1.2, which helps reduce the risk of lithium deposition in the second region while also achieving a high volumetric energy density. Furthermore, preferably, 1.1 ≤ h1 / h3 ≤ 1.15.

[0161] From Examples 9 and 58 to 63 in Table 1, it can be seen that the ratio of the depth of the recess in the third portion to the depth of the recess in the second region satisfies 1.1 ≤ h2 / h3 ≤ 1.5, which helps reduce the risk of lithium deposition in the second region while also achieving a high volumetric energy density. Furthermore, preferably, 1.2 ≤ h2 / h3 ≤ 1.4.

[0162] Those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present application and are not intended to limit the present application. As long as they are within the spirit of the present application, appropriate changes and modifications to the above embodiments fall within the scope disclosed in the present application.

Claims

1. A secondary battery comprising: An electrode assembly, wherein the electrode assembly is in a wound structure and includes a first electrode sheet; The first pole piece includes a first current collector and a first active material layer provided on the first current collector. Along a first direction, the first current collector includes a first surface and a second surface. The first direction is a thickness direction of the first pole piece. The first current collector has a first section and a second section arranged in sequence, the first surface of the first section and the second surface of the first section are both provided with the first active material layer, the second surface of the second section is provided with the first active material layer, and the first surface of the second section is not provided with an active material layer; characterized in that: The intersection of the first segment and the second segment is defined as a boundary line. The first pole piece corresponding to the first segment includes a first region. The first region is a region extending L1 from the boundary line along the direction from the second segment to the first segment, and 0.5 mm ≤ L1 ≤ 5 mm. The first current collector in the first region is a first sub-segment, the first active material layer located on the first surface of the first sub-segment is a first part, and at least one recess is provided on the first part. The recess in the first part is a recess formed by the first part being recessed toward the first sub-segment from the surface of the first sub-segment.

2. The secondary battery according to claim 1, wherein The first active material layer located on the second surface of the first sub-segment is the second part, and at least one recess is provided on the second part. The recess in the second part is a recess formed by the second part being recessed toward the first sub-segment away from the surface of the first sub-segment.

3. The secondary battery according to claim 1, wherein The first electrode sheet corresponding to the first section further includes a second region, the second region including a first edge and a second edge disposed opposite each other along the winding direction of the electrode assembly; along the winding direction of the electrode assembly, the first edge is 20 mm from the boundary line, and the second edge is 20 mm from the trailing edge of the electrode assembly; the first region satisfies at least one of the following conditions: (a). 0.5mm≤L1≤3mm; (b) along the first direction, the thickness of the first portion is H1, the thickness of the single layer of the first active material layer in the second region is H3, 1.05≤H1 / H3≤1.2; (c) The compaction density of the first portion is ρ1, the compaction density of the first active material layer in the second region is ρ3, and 1.05≤ρ1 / ρ3≤1.

2.

4. The secondary battery according to claim 1 or 2, wherein: The first pole piece corresponding to the second section includes a third area, the third area being an area extending L2 from the boundary line along the first section toward the second section, 1 mm ≤ L2 ≤ 7 mm; The first active material layer in the third region is a third portion. The third portion is provided with at least one recess. The recess in the third portion is formed by the surface of the third portion facing away from the second segment and recessed toward the second segment.

5. The secondary battery according to claim 4, wherein The first electrode sheet corresponding to the first section further includes a second region, and along the winding direction of the electrode assembly, the second region includes a first edge and a second edge; along the winding direction of the electrode assembly, the first edge is 20 mm from the boundary line, and the second edge is 20 mm from the trailing edge of the electrode assembly; the third region satisfies at least one of the following conditions: (a) Along the first direction, the thickness of the third portion is H2, the thickness of the single layer of the first active material layer in the second region is H3, 1.1≤H2 / H3≤1.5; (b) The compaction density of the third portion is ρ2, the compaction density of the first active material layer in the second region is ρ3, and 0.65≤ρ2 / ρ3≤0.

95.

6. The secondary battery according to claim 1, wherein 1mm 2 The total area of ​​the recess in the first part is S1, 0.01mm 2 ≤S1≤0.15mm 2 .

7. The secondary battery according to claim 6, wherein 0.04mm 2 ≤S1≤0.1mm 2 。 8. The secondary battery according to claim 4, wherein 1mm 2 The total area of ​​the recess in the third portion is S2, 0.01 mm 2 ≤S2≤0.15mm 2 .

9. The secondary battery according to claim 8, wherein 0.04mm 2 ≤S2≤0.1mm 2 。 10. The secondary battery according to claim 1, wherein Along the first direction, the thickness of the first portion is H1, the depth of the recess in the first portion is h1, and 1 / 6≤h1 / H1≤5 / 6.

11. The secondary battery according to claim 10, wherein 1 / 3≤h1 / H1≤2 / 3.

12. The secondary battery according to claim 4, wherein Along the first direction, the thickness of the third portion is H2, the depth of the recess in the third portion is h2, and 1 / 6≤h2 / H2≤5 / 6.

13. The secondary battery according to claim 12, wherein 1 / 3≤h2 / H2≤2 / 3.

14. The secondary battery according to claim 4, wherein The first electrode sheet corresponding to the first section also includes a second area, and along the winding direction of the electrode assembly, the second area includes a first edge and a second edge; along the winding direction of the electrode assembly, the first edge is 20 mm away from the boundary line, and the second edge is 20 mm away from the trailing edge of the electrode assembly; the first active material layer in the second area is also provided with at least one recess, 1 mm 2 The total area of ​​the recesses in the first active material layer in the second region is S3,1 mm 2 The total area of ​​the recess in the first part is S1,1mm 2 The total area of ​​the recesses in the third portion is S2, the depth of the recesses in the first portion is h1, the depth of the recesses in the third portion is h2, along the first direction, the thickness of the single-layer first active material layer in the second region is H3, the depth of the recesses in the second region is h3, S1 or S2 or h1 or h2 satisfies at least one of the following conditions: (a).0.5≤S1 / S3≤0.95; (b).0.6≤S1 / S3≤0.85; (c).0.5≤S2 / S3≤0.95; (d).0.6≤S2 / S3≤0.85; (e).1.05≤h1 / h3≤1.2; (f).1.1≤h1 / h3≤1.15; (g).1.1≤h2 / h3≤1.5; (h).1.2≤h2 / h3≤1.

4.

15. The secondary battery according to claim 1, wherein The width direction of the first pole piece after unfolding is taken as the second direction, the length direction of the first pole piece after unfolding is taken as the third direction, and the first direction, the second direction and the third direction are perpendicular to each other; The recess extends along the second direction. When viewed along the first direction, a width of a single recess along the third direction is W1, and 20 μm ≤ W1 ≤ 300 μm. Alternatively, the recessed portion extends along the third direction, and when viewed along the first direction, a width of a single recessed portion along the second direction is W1, where 20 μm ≤ W1 ≤ 300 μm.

16. The secondary battery according to claim 15, wherein The width of the first active material layer along the second direction is W. When the recessed portion extends along the second direction, the length of a single recessed portion along the second direction is W2, and 0.6≤W2 / W≤1.

17. The secondary battery according to claim 1, wherein When viewed along the first direction, each of the recesses is substantially circular, and the diameter of each of the recesses is D1, where 0.5 mm ≤ D1 ≤ 3 mm.

18. The secondary battery according to claim 15 or 17, wherein: The plurality of recesses are arranged at intervals, and the distance between adjacent recesses is D2, 0.5 mm ≤ D2 ≤ 3 mm.

19. The secondary battery according to claim 1, wherein The first electrode is a negative electrode.

20. An electrical device, characterized in that: The secondary battery according to any one of claims 1 to 19 is included.

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