Secondary battery and electronic device

Abstract

The invention provides a secondary battery and electronic equipment, an electrode assembly comprises a first pole piece, an isolating membrane and a second pole piece which are stacked and wound, the first pole piece comprises a first starting end in the length direction of the first pole piece, and the first pole piece comprises a first folding part and a first main body part in the length direction of the first pole piece. In the thickness direction of the first pole piece, the first folding part and the first main body part are stacked, and the first main body part extends in the winding direction. In the length direction of the first pole piece, the first folding part comprises a first section and a second section, and the first starting end is located on the first section. The first main body part, the first section and the second section are sequentially stacked in the thickness direction of the first pole piece. In the thickness direction of the first pole piece, the projection of the second section and the projection of the first main body part jointly form a first projection, and the projection of the first starting end is located in the first projection. Short circuit can be reduced, and the service life of the secondary battery can be prolonged.

Description

Technical Field

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

[0002] With the rapid development of modern science and technology, the demand for high-performance energy storage devices in the fields of portable electronic devices, electric vehicles, etc. is growing. As an efficient and environmentally friendly energy storage device, lithium-ion batteries have been widely used in many fields due to their advantages such as high energy density, long cycle life, and low self-discharge rate. In the continuous development of lithium-ion batteries, the safety performance requirements of batteries are getting higher and higher. Summary of the invention

[0003] The present application aims to provide a secondary battery and an electronic device, aiming to improve the safety performance of the battery.

[0004] In the first aspect, the present application proposes a secondary battery, including an electrode assembly, the electrode assembly including a first electrode sheet, a separator and a second electrode sheet stacked and wound, and along the winding direction, the first electrode sheet includes a winding starting end. Along the thickness direction of the first electrode sheet, the electrode sheet adjacent to the winding starting end is the second electrode sheet; it is characterized in that along the length direction of the first electrode sheet, the first electrode sheet includes a first folded portion and a first main body. Along the thickness direction of the first electrode sheet, the first folded portion and the first main body are stacked, and the first main body extends along the winding direction. Along the length direction of the first electrode sheet, the first electrode sheet includes a first starting end, the first folded portion includes a first section and a second section, and the first starting end is located in the first section; along the thickness direction of the first electrode sheet, the first main body, the first section and the second section are stacked in sequence. Along the thickness direction of the first electrode sheet, the projection of the second section and the projection of the first main body together form a first projection, and the projection of the first starting end is located within the first projection.

[0005] In the above technical solution, the projection of the first starting end is located within the first projection, which allows the first main body and / or the second section to wrap and cover the first starting end, which can reduce the cutting burrs of the first starting end from piercing the isolation membrane, thereby reducing the short circuit caused by direct contact between the first starting end and the second electrode, thereby improving the safety performance of the secondary battery. Compared with covering the first starting end by gluing, in the embodiment of the present application, the first starting end is folded to cover the burrs, and there is no risk of burrs being exposed due to aging, damage or puncture of the glue layer, and the first folded portion formed by folding has a more stable structure and has a higher tolerance to temperature and electrolyte environment, which is conducive to extending the service life of the secondary battery.

[0006] In some embodiments, along the thickness direction of the first pole piece, the projection of the first starting end is located within the projection of the second section, and the projection of the first starting end is located within the projection of the first main body. This is to make the second section cover the first starting end on one side of the thickness direction of the first pole piece, and make the first main body cover the first starting end on the other side of the thickness direction of the first pole piece. The first starting end is completely isolated from the isolation membranes on both sides, which can fully reduce the risk of the burrs of the first starting end piercing the isolation membrane, and further improve the safety performance of the secondary battery.

[0007] In some embodiments, the winding tail end and the first tail end are located at the sub-outer pole piece of the electrode assembly. Since there are pole pieces of opposite polarity on both sides, folding the sub-outer pole piece is more effective in reducing the puncture of the isolation membrane.

[0008] In some embodiments, the secondary battery is cylindrical. In the present application, the first starting end is wrapped and covered by folding, which can reduce the possibility of the first starting end piercing the isolation membrane due to slippage, thereby reducing the occurrence of short circuits, which is particularly suitable for cylindrical secondary batteries.

[0009] In some embodiments, the first main body includes a first surface facing the winding center and a second surface away from the winding center, and the first folded portion is superimposed on the first surface of the first main body. Due to the outward tension of the first pole piece, it is easier for the first starting end to pierce the isolation membrane away from the winding center. The first folded portion is superimposed on the first surface of the first main body, and the second surface of the first main body is a flat surface, which has a smoother contact with the isolation membrane, which can further reduce the risk of the isolation membrane being pierced and improve the safety performance of the secondary battery.

[0010] In some embodiments, along the thickness direction of the first main body, the surface of the second section away from the first main body is at least partially in direct contact with the isolation film. No need to perform glue sticking operation on the first starting end, which can simplify the production process and reduce the cost of raw materials, such as reducing the purchase, storage and transportation costs of the glue layer, which is beneficial to reducing costs and increasing efficiency in the production process of secondary batteries.

[0011] In some embodiments, the first pole piece also includes a first current collector and a first active material layer stacked on the first current collector, the first current collector includes a first folded portion, and along the thickness direction of the first pole piece, in the winding turn where the first folded portion is located, the first folded portion does not overlap with the first active material layer.

[0012] In some embodiments, along the width direction of the first pole piece, the width of the first main body is W, the width of the first starting end is W0, and the width of the second section is W2. W0≤W2, the width of the first starting end is smaller, and it is easier to control during the folding operation, which is conducive to the first starting end being completely covered by the second section. In addition, the narrower first starting end generates less stress during folding, and the risk of damage such as tearing and deformation to the first pole piece is lower. In addition, since the width of the first starting end is smaller, the possibility of the first starting end being exposed is lower, which can effectively reduce the exposure of the first starting end due to external forces such as vibration and impact on the secondary battery.

[0013] In some embodiments, along the width direction of the first pole piece, the width of the first main body is W, W0≤W, which is conducive to the first starting end being completely covered by the first main body, reducing the exposure of the first starting end due to external forces such as vibration and impact on the secondary battery.

[0014] In some embodiments, the first section includes a first angle position and a second angle position. Along the length direction of the first pole piece, the first angle position and the second angle position are both located at one end of the first section away from the second section; and, along the width direction of the first pole piece, the first angle position and the second angle position are respectively located at both ends of the first starting end. Observed along the thickness direction of the first section, the first angle position is arc-shaped, and the arc-shaped angle position has no sharp edges. Even when it contacts the isolation membrane, its pressure distribution is more uniform, which can reduce the risk of puncture and damage to the isolation membrane. In addition, the setting of the arc-shaped angle position can make the width of the first starting end smaller, which is conducive to better wrapping the first starting end inside, thereby reducing the occurrence of short circuits caused by burrs piercing the isolation membrane.

[0015] In some embodiments, the second corner is arc-shaped, which can reduce the risk of the isolation membrane being punctured and damaged, and is conducive to better wrapping the first starting end inside, thereby reducing the occurrence of short circuits caused by burrs piercing the isolation membrane.

[0016] In some embodiments, the first section includes a first part, a first sub-folding portion, a second sub-folding portion, a first angle position and a second angle position. Along the length direction of the first pole piece, the first angle position and the second angle position are both located at one end of the first section away from the second section. And, along the width direction of the first pole piece, the first angle position and the second angle position are respectively located at both ends of the first starting end. The first angle position is located in the first sub-folding portion, and along the thickness direction of the first pole piece, the first sub-folding portion is folded relative to the first part. There is no need to cut the first angle position, and it is beneficial to better wrap the first angle position and reduce the occurrence of short circuits caused by burrs piercing the isolation membrane. In addition, folding again increases the thickness of the first pole piece at the first sub-folding portion, and the first sub-folding portion is subjected to greater pressure, which is beneficial to fix the position of the first angle position, reduce the exposure of the first angle position due to vibration and impact of the secondary battery, and can effectively improve the stability of the first folding portion structure.

[0017] In some embodiments, the second corner is located at the second sub-folded portion, and along the thickness direction of the first pole piece, the second sub-folded portion is folded relative to the first portion. There is no need to cut the second corner, and it is beneficial to better wrap the second corner, reducing the occurrence of short circuits caused by burrs piercing the isolation film.

[0018] In some embodiments, along the width direction of the first pole piece, the maximum width of the first portion is W1, and the maximum width of the first sub-folding portion is W 11 , the maximum width of the second sub-folding part is W 12 .W1 / 4≤W 11 ≤W1 / 2, and / or, W1 / 4≤W 12 ≤W1 / 2. This is beneficial to the folding of the first sub-folding portion and the second sub-folding portion, improves the structural stability of the first sub-folding portion and the second sub-folding portion, and reduces the impact on the thickness of the first pole piece.

[0019] In some embodiments, along the length direction of the first pole piece, there is a first folding position between the first section and the second section, and there is a second folding position between the second section and the first main body. Along the length direction of the first pole piece, the length from the first folding position to the first starting end is L1, and the length from the second folding position to the first folding position is L2, and L2 ≥ L1. The second section has a larger length, which is conducive to wrapping the first section, reducing the folding of the first section when folding the second section, thereby reducing the impact on the thickness of the first pole piece.

[0020] In some embodiments, when observed along the thickness direction of the first pole piece, along the length direction of the first pole piece, the first section includes a first edge disposed away from the second section, and the first starting end is located at the first edge. The first edge is arc-shaped, and along the length direction of the first pole piece, the first edge protrudes in a direction away from the first section. This is conducive to the second section and the first main body wrapping the first starting end, and during the winding process and the subsequent use of the secondary battery, when contacting the isolation membrane, no sharp corners are generated to cause pressure concentration on the isolation membrane, which can effectively reduce the risk of puncturing the isolation membrane.

[0021] In some embodiments, along the length direction of the first pole piece, the maximum length of the first edge is D, D / 2≤L1≤D, so that the first section is located within the first edge, which is beneficial to the folding of the first section and can easily wrap and cover the first starting end.

[0022] In some embodiments, 1mm≤L1≤L2≤10mm is conducive to the folding of the first section and the second section, reducing the folding of the first section when folding the second section, and reducing the impact on the thickness of the first pole piece. Preferably, 4mm≤L1≤L2≤8mm can reduce the occurrence of short circuits.

[0023] In some embodiments, along the winding direction, the first pole piece includes a winding tail end; along the thickness direction of the first pole piece, the pole piece adjacent to the winding tail end is the second pole piece. Along the length direction of the first pole piece, the first pole piece also includes a second main body and a second folded portion. Along the thickness direction of the first pole piece, the second folded portion is stacked with the second main body, and the second main body extends along the winding direction. Along the length direction of the first pole piece, the first pole piece includes a first tail end, the second folded portion includes a third section and a fourth section, and the first tail end is located in the fourth section; along the thickness direction of the first pole piece, the second main body, the fourth section and the third section are stacked in sequence. Along the thickness direction of the first pole piece, the projection of the third section and the projection of the second main body together form a second projection, and the projection of the first tail end is located within the second projection.

[0024] In the above technical solution, the projection of the first tail end is located within the second projection, so that the third section and / or the second main body can wrap and cover the first tail end, which can reduce the cutting burrs of the first tail end from piercing the isolation membrane, thereby reducing the short circuit caused by direct contact between the first tail end and the second electrode, thereby improving the safety performance of the secondary battery. Compared with covering the first tail end by gluing, in the embodiment of the present application, the burrs are covered by folding the first tail end, and there is no risk of burrs being exposed due to aging, damage or puncture of the glue layer, and the second folded part formed by folding has a more stable structure and has a higher tolerance to temperature and electrolyte environment, which can effectively extend the service life of the secondary battery.

[0025] In some embodiments, the winding tail end and the first tail end are located at the secondary outer ring pole piece of the electrode assembly, and the outermost pole piece of the electrode assembly is part of the second pole piece. Since there are pole pieces of opposite polarity on both sides, folding the secondary outer ring pole piece has a more significant effect on reducing the puncture of the isolation membrane.

[0026] In a second aspect, the present application further proposes an electronic device, comprising a secondary battery as in any embodiment of the first aspect above.

[0027] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of the structure of a secondary battery in some embodiments of the present application;

[0029] Figure 2 An exploded schematic diagram of a secondary battery according to some embodiments of the present application;

[0030] Figure 3 A schematic diagram of the winding structure of an electrode assembly in some embodiments of the present application;

[0031] Figure 4A schematic diagram of the structure of a first pole piece in some embodiments of the present application;

[0032] Figure 5 This is a partial structural schematic diagram of an electrode assembly located at a first starting end in some embodiments of the present application;

[0033] Figure 6 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0034] Figure 7 A partial structural schematic diagram of a first pole piece (first starting end) in some embodiments of the present application;

[0035] Figure 8 This is a schematic structural diagram of the first folding portion of some embodiments of the present application;

[0036] Fig. 9 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0037] Fig.10 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0038] Fig.11 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0039] Fig.12 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0040] Fig.13 A schematic diagram of the structure of a first pole piece in some embodiments of the present application;

[0041] Fig.14 A schematic diagram of folding a first folding portion of some embodiments of the present application;

[0042] Fig.15 This is a schematic diagram of a portion of the structure of an electrode assembly located at a first tail end in some embodiments of the present application;

[0043] Fig.16 A partial structural schematic diagram of a first pole piece (first tail end) in some embodiments of the present application;

[0044] Fig.17 A schematic diagram of folding the second folding portion of some embodiments of the present application;

[0045] Fig.18 This is a schematic structural diagram of the second folding portion of some embodiments of the present application.

[0046] Description of reference numerals:

[0047] 100. Secondary battery;

[0048] 10. Shell; 11. First wall; 12. Second wall; 13. Main body; 14. Opening;

[0049] 20. Electrode assembly;

[0050] 21. The first pole piece;

[0051] 21a, first folding portion; 21a1, first section; 21a11, first angle; 21a12, second angle; 21a13, first sub-folding portion; 21a14, second sub-folding portion; 21a15, first part; 21a2, second section;

[0052] 21b, second folding portion; 21b1, third section; 21b2, fourth section;

[0053] 211, first current collector; 2111, first starting end; 2112, first ending end; 2113, first main body; 2113a, first surface; 2113b, second surface; 2114, second main body; 2115, first edge;

[0054] 21c, first folding position; 21d, second folding position;

[0055] 212, first active material layer;

[0056] 213, winding starting end; 214, winding ending end;

[0057] 22. second pole piece; 23. isolation film;

[0058] 30, first projection; 40, second projection;

[0059] Z, first direction; X, second direction; Y, third direction;

[0060] S, winding direction; G, winding center; K, axial direction. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly 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, not all of the embodiments.

[0062] Reference to "embodiment" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0063] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0064] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0065] In the first aspect, the present application proposes a secondary battery 100, please refer to Figure 1 and Figure 2 The secondary battery 100 includes a shell 10 and an electrode assembly 20 and an electrolyte (not shown) contained in the shell 10. The electrolyte infiltrates the electrode assembly 20 in the shell 10, thereby generating an electrochemical reaction.

[0066] For the above-mentioned housing 10, the housing 10 may be square, trapezoidal or cylindrical, etc. In the embodiment of the present application, the cylindrical housing 10 is taken as an example, please refer to Figure 1 and Figure 2 The housing 10 includes a first wall portion 11, a second wall portion 12 and a main body portion 13. Along the axial direction K of the secondary battery 100, the first wall portion 11 and the second wall portion 12 can be connected to both ends of the main body portion 13 respectively. The second wall portion 12 can be integrally provided with the main body portion 13 or separately provided. An opening 14 is provided at one end of the main body portion 13 opposite to the second wall portion 12. The electrode assembly 20 is placed in the housing 10 through the opening 14. The housing 10 can be sealed by covering the opening 14 with the first wall portion 11.

[0067] The housing 10 may be made of conductive metal materials such as aluminum, aluminum alloy, steel, stainless steel, nickel, copper or magnesium alloy, so that the housing 10 can lead to a certain polarity of the secondary battery 100, for example, the housing 10 itself is used as the positive electrode or negative electrode of the secondary battery 100. In some other embodiments, the housing 10 may also be made of soft packaging materials, such as aluminum plastic film or copper plastic film.

[0068] Regarding the electrode assembly 20, the electrode assembly 20 is accommodated in the housing 10. Please refer to Figure 3 and Figure 4The electrode assembly 20 includes a first pole piece 21, a second pole piece 22 and a separator 23. The first pole piece 21, the separator 23 and the second pole piece 22 are stacked and wound, for example, stacked along the thickness direction (first direction Z) of the first pole piece 21 and wound along its length direction (second direction X), and the separator 23 is arranged between the first pole piece 21 and the second pole piece 22 for insulating and separating the two.

[0069] The first pole piece 21 and the second pole piece 22 have opposite polarities, for example, the first pole piece 21 is a positive pole piece, and the second pole piece 22 is a negative pole piece. Alternatively, the first pole piece 21 is a negative pole piece, and the second pole piece 22 is a positive pole piece. For example, when a cylindrical shell is used, the first pole piece 21 can be electrically connected to the first wall portion 11, so that the first wall portion 11 leads to one polarity of the secondary battery 100, and the second pole piece 22 can be electrically connected to the second wall portion 12, so that the second wall portion 12 leads to the other polarity of the secondary battery 100. By insulating the first wall portion 11 from the second wall portion 12, the occurrence of short circuit can be reduced.

[0070] Please refer to Figure 4 The first electrode sheet 21 includes a first current collector 211 and a first active material layer 212. Along the thickness direction (first direction Z) of the first current collector 211, the first active material layer 212 is stacked on at least one surface of the first current collector 211. The first current collector 211 can be made of aluminum foil, copper foil, titanium foil or nickel foil, etc., which has high strength and high conductivity, can reduce the tearing of the first current collector 211, and is conducive to improving the charge and discharge rate of the secondary battery 100.

[0071] When the first electrode plate 21 is a positive electrode plate, the first active material layer 212 includes positive electrode active materials, conductive agents, adhesives, etc., wherein the positive electrode active materials include one or more of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide or lithium iron manganese phosphate.

[0072] In some other embodiments, when the first electrode plate 21 is a negative electrode plate, the first active material layer 212 includes a negative electrode active material, a conductive agent, and an adhesive, etc., wherein the negative electrode active material includes one or more of graphite, soft carbon, hard carbon, carbon fiber, elemental silicon, silicon oxides, silicon alloys, etc.

[0073] Please refer to Figure 3 and Figure 4, along the length direction (second direction X) of the first pole piece 21, the first pole piece 21 includes a first starting end 2111 and a first ending end 2112. In the preparation process of the first current collector 211, the current collector roll needs to be cut to form a plurality of first current collectors 211. The first starting end 2111 is the cutting position of the first current collector 211, and the first ending end 2112 is another cutting position of the first current collector 211. The first starting end 2111 is located at the second inner pole piece of the electrode assembly 20, and the innermost pole piece of the electrode assembly 20 is part of the second pole piece 22.

[0074] The cutting burrs of the first starting end 2111 have the risk of piercing the isolation film 23, which may cause the first starting end 2111 to directly contact the second pole piece 22 and cause a short circuit. Before cutting out the first current collector 211, the first current collector 211 needs to be glued, and then the glue layer and the first current collector 211 are cut together, so that the glue layer can cover the first starting end 2111, thereby reducing the burrs of the first starting end 2111 from piercing the isolation film 23 and causing a short circuit.

[0075] However, the inventors of the present application have found that during the long-term charge and discharge cycle of the secondary battery 100, the glue layer is at risk of aging, damage or puncture, resulting in the exposure of burrs. For example, during long-term use, the performance of the glue layer may degrade due to factors such as the chemical environment and temperature changes inside the secondary battery 100. In addition, the first pole piece 21 will expand, which may cause the first starting end 2111 to slip along the winding direction S, which may easily cause the first starting end 2111 to be misaligned with the glue layer, which may cause the first starting end 2111 to be exposed, and then easily cause the burrs of the first starting end 2111 to pierce the isolation membrane 23 and cause a short circuit.

[0076] To reduce the above problems, in the embodiments of this application, please refer to Figures 5 to 7 , along the length direction (second direction X) of the first pole piece 21, the first pole piece 21 includes a first folded portion 21a and a first main body portion 2113, along the thickness direction (first direction Z) of the first pole piece 21, the first folded portion 21a and the first main body portion 2113 are stacked, and the first main body portion 2113 extends along the winding direction S. Along the length direction (second direction X) of the first pole piece 21, the first folded portion 21a includes a first section 21a1 and a second section 21a2, and the first starting end 2111 is located at the first section 21a1. Along the thickness direction (first direction Z) of the first pole piece 21, the first main body portion 2113, the first section 21a1 and the second section 21a2 are stacked in sequence.

[0077] For example, in the portion of the first pole piece 21 near the first starting end 2111, the first section 21a1 is folded relative to the second section 21a2, and the first section 21a1 and the second section 21a2 are stacked along the thickness direction (first direction Z) of the first pole piece 21, thereby forming the first folded portion 21a. The first folded portion 21a is then folded relative to the first main body 2113 as a whole, and the first folded portion 21a and the first main body 2113 are stacked along the thickness direction (first direction Z) of the first pole piece 21, so that the first section 21a1 is stacked between the second section 21a2 and the first main body 2113. In which, along the winding direction S, the winding starting end 213 of the first pole piece 21 is located at the connection between the first folded portion 21a and the first main body 2113, and along the thickness direction (first direction Z) of the first pole piece 21, the pole piece adjacent to the winding starting end 213 is the second pole piece 22.

[0078] In the embodiments of this application, please refer to Figure 8 , along the thickness direction (first direction Z) of the first pole piece 21, the projection of the second segment 21a2 and the projection of the first main body 2113 together form a first projection 30 ( Figure 8 The projection of the first starting end 2111 is located within the first projection 30, which allows the first main body 2113 and / or the second section 21a2 to wrap and cover the first starting end 2111, which can reduce the cutting burrs of the first starting end 2111 from piercing the isolation film 23, thereby reducing the short circuit caused by direct contact between the first starting end 2111 and the second pole piece 22, thereby improving the safety performance of the secondary battery 100. Compared with covering the first starting end 2111 by gluing, in the embodiment of the present application, the first starting end 2111 is folded to cover the burrs, and there is no risk of burrs being exposed after the glue layer is aged, damaged or pierced, and the first folded portion 21a formed by folding has a more stable structure and has a higher tolerance to temperature and electrolyte environment, which is conducive to extending the service life of the secondary battery 100.

[0079] The inventors of the present application have also found that for the sub-inner circle pole piece of the electrode assembly 20, there are pole pieces of opposite polarity (second pole piece 22) on both sides in the thickness direction, and the stress on the inner circle is relatively large, and the risk of burrs piercing the isolation membrane 23 is relatively high. In the embodiment of the present application, the winding start end 213 and the first start end 2111 are located at the sub-inner circle pole piece of the electrode assembly 20, and the innermost circle pole piece of the electrode assembly 20 is part of the second pole piece 22. Folding the sub-inner circle pole piece has a more significant effect on reducing the piercing of the isolation membrane 23.

[0080] For the first main body 2113, please refer to Figure 6 and Figure 7After the first section 21a1 and the second section 21a2 are folded, a first folding position 21c is formed between the first section 21a1 and the second section 21a2, that is, the first section 21a1 is folded relative to the second section 21a2 along the first folding position 21c. After the second section 21a2 and the first main body 2113 are folded, a second folding position 21d is formed between the second section 21a2 and the first main body 2113, that is, the second section 21a2 is folded relative to the first main body 2113 along the second folding position 21d. The winding start end 213 is located at the second folding position 21d. The first folding portion 21a has a Q point, and along the winding direction S, the Q point is the farthest point of the first folding portion 21a from the second folding position 21d. Along the thickness direction (first direction Z) of the first pole piece 21, the projection of point Q on the first main body 2113 is P, and a tangent is made along the width direction (third direction Y) of the first pole piece 21 and through point P, and the portion of the first pole piece 21 located from the tangent to the second folding position 21d is the first main body 2113. That is, along the winding direction S, the maximum length of the first folding portion 21a is consistent with the length of the first main body 2113.

[0081] It is understandable that the first folding portion 21a includes but is not limited to a two-layer folding structure. In some other embodiments, the first folding portion 21a may be a multi-layer stacking structure, such as three layers, four layers or five layers, that is, the first folding portion 21a is folded multiple times. Considering the complexity of the process and the impact on the thickness of the first pole piece 21, in the embodiment of the present application, it is preferred that the first folding portion is a two-layer folding structure, that is, including a stacked first section 21a1 and a second section 21a2.

[0082] In some embodiments, please refer to Fig. 9 , along the thickness direction (first direction Z) of the first pole piece 21, the projection of the first starting end 2111 is located within the projection of the second section 21a2, and the projection of the first starting end 2111 is located within the projection of the first main body 2113. That is, the second section 21a2 covers the first starting end 2111 on one side of the thickness direction (first direction Z) of the first pole piece 21, and the first main body 2113 covers the first starting end 2111 on the other side of the thickness direction (first direction Z) of the first pole piece 21, and the first starting end 2111 is completely isolated from the isolation membrane 23 on both sides, which can fully reduce the risk of the burr of the first starting end 2111 piercing the isolation membrane 23, and further improve the safety performance of the secondary battery 100.

[0083] In the embodiment of the present application, the secondary battery 100 is cylindrical. During the winding process of the first pole piece 21 and the second pole piece 22, the first pole piece 21 needs to withstand a certain bending stress. Compared with square batteries or batteries of other structures, the first pole piece 21 of the cylindrical battery forms a relatively tight cylindrical shape after winding, which may cause the first pole piece 21 to form a tension similar to the centripetal force, which is more likely to cause the first pole piece 21 to be partially loose, and then cause the first starting end 2111 to slip. In the embodiment of the present application, the first starting end 2111 of the first pole piece 21 is folded to form a first folded portion 21a, and the first starting end 2111 can be wrapped and covered, which can reduce the burrs of the first starting end 2111 from piercing the isolation membrane 23, thereby reducing the first starting end 2111 from slipping and short circuiting, which is particularly suitable for cylindrical secondary batteries 100.

[0084] In some embodiments, please refer to Figure 5 , the first main body 2113 includes a first surface 2113a facing the winding center G and a second surface 2113b away from the winding center G. The inventors of the present application have found that due to the outward tension of the first pole piece 21, it is easier for the first starting end 2111 to pierce the isolation membrane 23 away from the winding center G. In the embodiment of the present application, the first folding portion 21a is superimposed on the first surface 2113a of the first main body 2113, that is, the first folding portion 21a is located on the side of the first main body 2113 facing the winding center G, and the second surface 2113b of the first main body 2113 is a flat surface, which has a smoother contact with the isolation membrane 23, which can further reduce the risk of the isolation membrane 23 being pierced, and improve the safety performance of the secondary battery 100.

[0085] In some embodiments, along the thickness direction (first direction Z) of the first main body 2113, the surface of the second section 21a2 away from the first main body 2113 is at least partially in direct contact with the isolation film 23. In the embodiment of the present application, the first starting end 2111 is wrapped and covered by the second section 21a2 and the first main body 2113, which can reduce the risk of burrs piercing the isolation film 23. Therefore, there is no need to perform a gluing operation on the first starting end 2111, and the first starting end 2111 can be in direct contact with the adjacent isolation film 23. The production process can be simplified, and the cost of raw materials can be reduced, such as reducing the purchase, storage and transportation costs of the glue layer, which is beneficial to reducing costs and increasing efficiency in the production process of the secondary battery 100.

[0086] As for the first folded portion 21a and the first active material layer 212, the inventors of the present application have found that the first folded portion 21a covers the first active material layer 212, which will reduce the contact area between the first active material layer 212 and the electrolyte, and the first active material layer 212 cannot fully participate in the electrochemical reaction, which affects the energy density of the secondary battery 100. In addition, the first folded portion 21a squeezes and restrains the first active material layer 212, which may cause the first active material to bear additional mechanical stress during the charge and discharge process, which may easily weaken the binding force between the active materials, and easily cause powdering and falling off, further reducing the utilization rate of the first active material layer 212.

[0087] In the implementation of this application, please refer to Figure 7 , along the thickness direction (first direction Z) of the first pole piece 21, at the winding circle where the first folded portion 21a is located, that is, Figure 7 In the first winding circle starting from the winding starting end 213, the first folded portion 21a does not overlap with the first active material layer 212, which is beneficial for the first active material layer 212 to fully participate in the electrochemical reaction, improve the energy density of the secondary battery 100, and reduce the pulverization and shedding of the first active material layer 212.

[0088] In some embodiments, please refer to Fig.10 , along the width direction of the first pole piece 21 (third direction Y), the width of the first starting end 2111 is W0, and the width of the second section 21a2 is W2, W0≤W2, the width of the first starting end 2111 is smaller, and it is easier to control during the folding operation, which is conducive to the first starting end 2111 being completely covered by the second section 21a2, and the narrower first starting end 2111 generates less stress during folding, and the risk of damage such as tearing and deformation to the first pole piece 21 is lower, which is not only conducive to improving production efficiency, but also conducive to improving product quality and reducing the defective rate caused by the folding process. In addition, the width of the first starting end 2111 is smaller, and the possibility of the first starting end 2111 being exposed is lower, which can effectively reduce the exposure of the first starting end 2111 due to external forces such as vibration and impact on the secondary battery 100.

[0089] Based on the same inventive concept, along the width direction of the first pole piece 21 (the third direction Y), the width of the first main body 2113 is W, W0≤W, which is conducive to the first starting end 2111 being completely covered by the first main body 2113, thereby reducing the exposure of the first starting end 2111 due to external forces such as vibration and impact on the secondary battery 100.

[0090] In some embodiments, please refer to Fig.11, the first section 21a1 includes a first corner position 21a11 and a second corner position 21a12. Along the length direction of the first pole piece 21 (the second direction X), the first corner position 21a11 and the second corner position 21a12 are both located at one end of the first section 21a1 away from the second section 21a2. And, along the width direction of the first pole piece 21 (the third direction Y), the first corner position 21a11 and the second corner position 21a12 are respectively located at both ends of the first starting end 2111. Observing along the thickness direction (the first direction Z) of the first section 21a1, the first corner position 21a11 is arc-shaped, for example, the corner position of the first starting end 2111 is cut to form an arc-shaped corner position. The arc-shaped corner position has no sharp edges, and even when it contacts the isolation membrane 23, its pressure distribution is more uniform, which can reduce the risk of the isolation membrane 23 being punctured and damaged. Furthermore, the setting of the arc angle can make the width of the first starting end 2111 smaller, which is conducive to better wrapping the first starting end 2111 inside, thereby reducing the occurrence of short circuits caused by burrs piercing the isolation membrane 23.

[0091] Based on the same inventive concept, when observed along the thickness direction (first direction Z) of the first section 21a1, the second corner 21a12 is arc-shaped, which can reduce the risk of the isolation membrane 23 being punctured and damaged, and is conducive to better wrapping the first starting end 2111 inside, thereby reducing the occurrence of short circuits caused by burrs piercing the isolation membrane 23.

[0092] In some embodiments, please refer to Fig.12 , the first angle 21a11 and the second angle 21a12 are both right angles. The first section 21a1 also includes a first part 21a15, a first sub-folding portion 21a13, and a second sub-folding portion 21a14. The first angle 21a11 is located at the first sub-folding portion 21a13, and along the thickness direction (first direction Z) of the first pole piece 21, the first sub-folding portion 21a13 is folded relative to the first part 21a15. There is no need to cut the first angle 21a11, and it is beneficial to better wrap the first angle 21a11, reducing the occurrence of short circuits caused by burrs piercing the isolation membrane 23. In addition, folding again increases the thickness of the first pole piece 21 at the first sub-folding portion 21a13, and the first sub-folding portion 21a13 is subjected to greater pressure, which is beneficial to fix the position of the first angle 21a11, reduce the exposure of the first angle 21a11 due to vibration and impact of the secondary battery 100, and can effectively improve the stability of the structure of the first folding portion 21a.

[0093] Based on the same inventive concept, the second corner 21a12 is located at the second sub-folding portion 21a14, and along the thickness direction (first direction Z) of the first pole piece 21, the second sub-folding portion 21a14 is folded relative to the first portion 21a15. There is no need to cut the second corner 21a12, and it is beneficial to better wrap the second corner 21a12, reducing the occurrence of short circuits caused by burrs piercing the isolation film 23. In addition, the pressure of the first pole piece 21 at the second sub-folding portion 21a14 is greater, which is beneficial to fix the position of the second corner 21a12, reduce the exposure of the second corner 21a12 due to vibration and impact of the secondary battery 100, and effectively improve the stability of the first folding portion 21a structure.

[0094] Regarding the width of the first sub-folding portion 21a13, if the width of the first sub-folding portion 21a13 is too small, it may be difficult to fold, and its structure is unstable after folding, and the fixing effect is poor, which may easily cause the first sub-folding portion 21a13 to unfold, thereby causing the first corner 21a11 to be exposed, and the risk of piercing the isolation membrane 23 is high. If the width is too large, it may overlap with the second sub-folding portion 21a14, which may easily cause the first folding portion 21a to be uneven, affecting the thickness of the first pole piece 21. In the embodiments of this application, please refer to Fig.13 Along the width direction of the first pole piece 21 (the third direction Y), the maximum width of the first part 21a15 is W1, and the maximum width of the first sub-folding portion 21a13 is W11, W1 / 4≤W11≤W1 / 2, which is beneficial to the folding of the first sub-folding portion 21a13, improves the structural stability of the first sub-folding portion 21a13, and reduces the impact on the thickness of the first pole piece 21.

[0095] Based on the same inventive concept, along the width direction (third direction Y) of the first pole piece 21, the maximum width of the second sub-folding portion 21a14 is W 12 , W1 / 4≤W 12 ≤W1 / 2, which is beneficial to the folding of the second sub-folding portion 21a14, improves the structural stability of the second sub-folding portion 21a14, and reduces the impact on the thickness of the first pole piece 21.

[0096] In some embodiments, please refer to Fig.14 , along the length direction (second direction X) of the first pole piece 21, the length from the first folding position 21c to the first starting end 2111 is L1, that is, the length of the first section 21a1 is L1, and the length from the second folding position 21d to the first folding position 21c is L2, that is, the length of the second section 21a2 is L2, and L2≥L1. In the embodiment of the present application, the length of the second section 21a2 is greater, which is conducive to wrapping and covering the first section 21a1, reducing the folding of the first section 21a1 when folding the second section 21a2, thereby reducing the impact on the thickness of the first pole piece 21.

[0097] Among them, 1mm≤L1≤L2≤10mm is conducive to the folding of the first section 21a1 and the second section 21a2, reducing the folding of the first section 21a1 when folding the second section 21a2, and reducing the impact on the thickness of the first pole piece 21. Preferably, 4mm≤L1≤L2≤8mm can reduce the occurrence of short circuits.

[0098] Please refer to Fig.14 , observed along the thickness direction (first direction Z) of the first pole piece 21, along the length direction (second direction X) of the first pole piece 21, the first section 21a1 includes a first edge 2115 set away from the second section 21a2, and the first starting end 2111 is located at the first edge 2115. The first edge 2115 is arc-shaped, and along the length direction (second direction X) of the first pole piece 21, the first edge 2115 protrudes in the direction away from the first section 21a1. In the embodiment of the present application, the first edge 2115 is arc-shaped, which is conducive to the second section 21a2 and the first main body 2113 wrapping the first starting end 2111, and in the winding process and the subsequent use of the secondary battery 100, when contacting with the isolation membrane 23, no sharp angle will be generated to cause pressure concentration on the isolation membrane 23, which can effectively reduce the risk of piercing the isolation membrane 23.

[0099] It should be noted that when the first edge 2115 is in an arc shape, the farthest point of the first edge 2115 from the first folding position 21c is M, and point M is the first starting end 2111. When the first edge 2115 is arranged along the width direction (third direction Y) of the first pole piece 21, the first edge 2115 is the first starting end 2111.

[0100] Please refer to Fig.14 , along the length direction (second direction X) of the first pole piece 21, the maximum length of the first edge 2115 is D, D / 2≤L1≤D, so that the first section 21a1 is located inside the first edge 2115, which is beneficial to the folding of the first section 21a1 and the wrapping and covering of the first starting end 2111. Specifically, the maximum length of the first edge 2115 can be selected to be 2mm≤D≤10mm.

[0101] The inventor of the present application has found that the cutting burrs of the first tail end 2112 also have the risk of piercing the isolation film 23. To reduce this problem, please refer to Figures 15 to 17, along the length direction of the first pole piece 21 (the second direction X), the first pole piece 21 further includes a second main body 2114 and a second folded portion 21b, and the second main body 2114 extends along the winding direction S. Along the thickness direction of the first pole piece 21 (the first direction Z), the second folded portion 21b and the second main body 2114 are stacked. Along the length direction of the first pole piece 21 (the second direction X), the second folded portion 21b includes a third section 21b1 and a fourth section 21b2, and the first tail end 2112 is located at the fourth section 21b2. Along the thickness direction of the first pole piece 21 (the first direction Z), the second main body 2114, the fourth section 21b2 and the third section 21b1 are stacked in sequence.

[0102] For example, in the portion of the first pole piece 21 near the first tail end 2112, the fourth section 21b2 is stacked relative to the third section 21b1, and the third section 21b1 and the fourth section 21b2 are stacked along the thickness direction (first direction Z) of the first pole piece 21, thereby forming the second folded portion 21b. The second folded portion 21b is then folded relative to the second main body 2114 as a whole, and the second folded portion 21b and the second main body 2114 are stacked along the thickness direction (first direction Z) of the first pole piece 21, so that the fourth section 21b2 is stacked between the third section 21b1 and the second main body 2114.

[0103] In the embodiments of this application, please refer to Fig.18 , along the thickness direction (first direction Z) of the first pole piece 21, the projection of the third segment 21b1 and the projection of the second main body 2114 together form a second projection 40 ( Fig.18 The projection of the first tail end 2112 is located within the second projection 40, which makes the third section 21b1 and / or the second main body 2114 wrap and cover the first tail end 2112, which can reduce the cutting burrs of the first tail end 2112 from piercing the isolation film 23, thereby reducing the short circuit caused by direct contact between the first tail end 2112 and the second pole piece 22, thereby improving the safety performance of the secondary battery 100. Compared with covering the first tail end 2112 by gluing, in the embodiment of the present application, the first tail end 2112 is folded to cover the burrs, and there is no risk of burrs being exposed after the glue layer is aged, damaged or pierced, and the second folded portion 21b formed by folding has a more stable structure and has a higher tolerance to temperature and electrolyte environment, which can effectively extend the service life of the secondary battery 100.

[0104] The related structure of the second folding portion 21b can be similarly arranged with reference to the above-mentioned first folding portion 21a, and will not be described one by one in this application. Through the arrangement of the first folding portion 21a and the second folding portion 21b, the burrs of the first starting end 2111 and the first tail end 2112 of the first pole piece 21 can be effectively wrapped and covered, which can further reduce the risk of the burrs piercing the isolation membrane 23 and improve the safety performance of the secondary battery 100. Among them, along the winding direction S, the winding tail end 214 of the first pole piece 21 is located at the connection between the second folding portion 21b and the second main body 2114, and along the thickness direction of the first pole piece 21 (the first direction Z), the pole piece adjacent to the winding tail end 214 is the second pole piece 22.

[0105] In the embodiment of the present application, the winding tail end 214 and the first tail end 2112 are located at the sub-outer pole piece of the electrode assembly 20, and the outermost pole piece of the electrode assembly 20 is part of the second pole piece 22. Since there are pole pieces of opposite polarity on both sides, folding at the sub-outer pole piece has a more significant effect on reducing the puncture of the isolation membrane 23.

[0106] In the second aspect, the present application also proposes an electronic device, including a secondary battery 100 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, electronic devices include but are not limited to Bluetooth headsets, mobile phones, tablets, laptops, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys, electric airplane toys, and the like, and spacecraft can include airplanes, rockets, space shuttles, and spacecrafts, and the like.

[0107] Embodiment 1:

[0108] <Preparation of positive electrode sheet>

[0109] The positive electrode active material is lithium cobalt oxide, the positive electrode conductive agent is acetylene black, and the positive electrode binder is polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5) are mixed in a mass ratio of 94:3:3, N-methylpyrrolidone (NMP) is added as a solvent, and a positive electrode slurry with a solid content of 75wt% is prepared, and the mixture is stirred evenly under a vacuum mixer. An aluminum foil with a thickness of 12μm, a length of 400mm, and a width of 3.5mm is used as the positive electrode current collector, and the width W0 of the starting end (first starting end) of the positive electrode current collector is 3.5mm. The positive electrode slurry is evenly coated on one surface of the positive electrode current collector aluminum foil, and empty foil areas not coated with the positive electrode slurry are reserved at both ends of the aluminum foil, and dried at 110°C to obtain a positive electrode sheet coated with a single-sided positive electrode active material layer. Afterwards, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a double-sided positive electrode active material layer. The coating weight of the positive electrode sheet is 25mg / cm 2 .

[0110] <Preparation of negative electrode sheet>

[0111] The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (Super P), and binder styrene-butadiene rubber (SBR) are mixed in a weight ratio of 87.5:10:1:1.5, and then deionized water is added as a solvent to prepare a negative electrode slurry with a solid content of 50wt%, and stirred evenly. A copper foil with a thickness of 10μm, a length of 430mm, and a width of 4mm is selected as the negative electrode current collector. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector copper foil, and an empty foil area without negative electrode slurry is reserved at the beginning and end of the copper foil. Dry at 90°C to obtain a single-sided negative electrode sheet. After the above steps are completed, the single-sided coating of the negative electrode sheet has been completed. After that, the above steps are repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a double-sided coating of the negative electrode active material layer. The coating weight of the negative electrode sheet is 6.5mg / cm 2 .

[0112] <Preparation of Separator Film>

[0113] Polyethylene (PE) with a thickness of 7 μm and a 2 μm aluminum oxide (Al2O3) coated porous film were used as isolation membranes.

[0114] <Electrolyte Preparation>

[0115] In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solution, and then lithium hexafluorophosphate 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.

[0116] <Preparation of lithium-ion batteries>

[0117] An aluminum sheet with a width of 3 mm is used as the positive electrode tab, and the positive electrode tab is welded to the empty foil area of ​​the positive electrode sheet. A nickel sheet with a width of 3 mm is used as the negative electrode tab, and the negative electrode tab is welded to the empty foil area of ​​the negative electrode sheet. An empty foil area of ​​the positive electrode collector is folded twice to form a three-layer folded structure of the second section, the first section and the first main body, wherein the starting end (first starting end) of the first current collector is located in the first section, and along the length direction of the first electrode sheet, the length of the first section is L1, which is 0.5 mm, and the length of the second section is L2, which is 0.5 mm. Along the thickness direction of the first current collector, the projection of the second section and the projection of the first main body together form the first projection, and the projection of the first starting end is located within the first projection.

[0118] The separator, positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order and wound to obtain an electrode assembly. The electrode assembly is placed in a cylindrical shell, and the positive electrode tab is welded to the top wall of the shell, and the negative electrode tab is welded to the bottom wall of the shell. After removing moisture at 80°C, the electrolyte is injected, and the secondary battery is obtained through the processes of packaging and liquid injection, formation, capacity testing, voltage internal resistance testing, etc. Among them, the inner diameter D1 of the shell is 10.5mm, and the outer diameter of the electrode assembly is D2=10.35mm.

[0119] The relevant parameters in Examples 2 to 26 and Comparative Example 1 are shown in Table 1 below, wherein in Comparative Example 1, the positive electrode current collector is formed by first pasting adhesive tape and then cutting the current collector roll, and the adhesive layer directly covers the first starting end. In Examples 24 to 26, the first starting end is cut so that the width W0 of the first starting end is reduced.

[0120] Charge and discharge cycle test: The lithium-ion battery prepared above was placed at room temperature 25°C and left to stand for 30 minutes in an environment with a test temperature of 25°C, and then charged to 4.45V according to the following charging steps:

[0121] (1) 2.5C constant current charging to 4.15V, constant voltage charging cut-off current to 1.5C;

[0122] (2) 1.5C constant current charging to 4.18V, constant voltage charging cut-off current to 0.5C;

[0123] (3) 0.5C constant current charging to 4.45V, constant voltage charging cut-off current to 0.02C;

[0124] After standing for 10 minutes, discharge according to the following steps:

[0125] 1C constant current discharge to 3V, this is one cycle.

[0126] The charge and discharge cycle is 1000 times, and the internal short circuit is tested. If it occurs, it is considered as failure. Each group tests 20 samples, the number of failures is N, and the failure rate is N / 20.

[0127] Determination of internal short circuit: constant voltage charging is difficult to cut off, resulting in a high charging capacity (more than 3%). Disassembly and observation show that the isolation membrane at the first starting end of the electrode is melted and damaged.

[0128] Table 1

[0129]

[0130]

[0131] According to the above Table 1, combined with Examples 1 to 26 and Comparative Example 1, it can be seen that when the positive electrode plate is folded near the first starting end, the test failure rate can be effectively reduced. This is because when the projection of the first starting end is located within the first projection, the first main body and / or the second section wraps and covers the first starting end, which can reduce the cutting burrs of the first starting end from piercing the isolation membrane, thereby reducing the short circuit caused by direct contact between the first starting end and the second electrode plate, thereby improving the safety performance of the secondary battery.

[0132] In combination with Examples 1 to 23, in Examples 1 to 2, the length L1 of the first section and the length L2 of the second section are relatively small, which is not only difficult to fold, but also may cause the starting end to be exposed because the second section is too short, posing a risk of short circuit. In Examples 22-23, the length L1 of the first section and the length L2 of the second section are relatively large, which may result in a large space between the first section and the second section and the first main body. Slight shaking or collision of the secondary battery may cause the first section to be misaligned and expose the cutting position, which also poses a risk of short circuit. In Examples 3 to 21, the test failure rate is relatively low. In the embodiments of the present application, 1mm≤L1≤L2≤10mm can be selected. In Examples 10 to 14, the test failure rate is further reduced. In the embodiments of the present application, 4mm≤L1≤L2≤8mm is preferred.

[0133] In Examples 24 to 26, the first starting end is trimmed so that the width of the first starting end is smaller than the overall width of the positive electrode sheet, which can further reduce the test failure rate.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Under the concept of the present application, the technical features in the above embodiments or different embodiments may also be combined, the steps may be implemented in any order, and there are many other changes in different aspects of the present application as described above, which are not provided in detail for the sake of simplicity. Although the present application has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly comprises a first electrode sheet, a separator and a second electrode sheet which are stacked and wound, wherein along the winding direction, the first electrode sheet comprises a winding start end; along the thickness direction of the first electrode sheet, the electrode sheet adjacent to the winding start end is the second electrode sheet; characterized in that: Along the length direction of the first pole piece, the first pole piece includes a first folded portion and a first main body portion connected to each other; Along the thickness direction of the first pole piece, the first folded portion and the first main body portion are stacked, and the first main body portion extends along the winding direction; Along the length direction of the first pole piece, the first pole piece includes a first starting end, the first folded portion includes a first section and a second section, and the first starting end is located in the first section; along the thickness direction of the first pole piece, the first main body, the first section and the second section are stacked in sequence; Along the thickness direction of the first pole piece, the projection of the second segment and the projection of the first main body together form a first projection, and the projection of the first starting end is located within the first projection.

2. The secondary battery according to claim 1, characterized in that: Along the thickness direction of the first pole piece, the projection of the first starting end is located within the projection of the second segment, and / or the projection of the first starting end is located within the projection of the first main body.

3. The secondary battery according to claim 1, characterized in that: The winding starting end is located at the secondary inner circle pole piece of the electrode assembly.

4. The secondary battery according to claim 1, characterized in that: The secondary battery is cylindrical.

5. The secondary battery according to claim 1, characterized in that: The first main body portion includes a first surface facing a winding center and a second surface away from the winding center, and the first folded portion is overlapped on the first surface of the first main body portion.

6. The secondary battery according to claim 1, characterized in that: Along the thickness direction of the first main body, at least a portion of a surface of the second segment facing away from the first main body is in direct contact with the isolation film.

7. The secondary battery according to claim 1, characterized in that: The first pole piece also includes a first current collector and a first active material layer stacked on the first current collector. The first current collector includes the first folded portion. Along the thickness direction of the first pole piece, in the winding turn where the first folded portion is located, the first folded portion does not overlap with the first active material layer.

8. The secondary battery according to any one of claims 1 to 7, characterized in that: Along the width direction of the first pole piece, the width of the first main body is W, the width of the first starting end is W0, and the width of the second section is W2; W0≤W2, and / or, W0≤W.

9. The secondary battery according to any one of claims 1 to 7, characterized in that: The first segment includes a first angular position and a second angular position; Along the length direction of the first pole piece, the first angle position and the second angle position are both located at an end of the first segment away from the second segment; and along the width direction of the first pole piece, the first angle position and the second angle position are respectively located at two ends of the first starting end; Observing along the thickness direction of the first section, the first angle is arc-shaped, and / or the second angle is arc-shaped.

10. The secondary battery according to any one of claims 1 to 7, characterized in that: The first section includes a first portion, a first subfolded portion, a second subfolded portion, a first angle position, and a second angle position; Along the length direction of the first pole piece, the first angle position and the second angle position are both located at an end of the first segment away from the second segment; and along the width direction of the first pole piece, the first angle position and the second angle position are respectively located at two ends of the first starting end; The first corner is located at the first sub-folding portion, and along the thickness direction of the first pole piece, the first sub-folding portion is folded relative to the first portion; The second angle is located at the second sub-folding portion, and along the thickness direction of the first pole piece, the second sub-folding portion is folded relative to the first portion.

11. The secondary battery according to claim 10, characterized in that: Along the width direction of the first pole piece, the maximum width of the first portion is W1, and the maximum width of the first sub-folded portion is W 11 , the maximum width of the second sub-folding portion is W 12 ; W1 / 4≤W 11 ≤W1 / 2, and / or, W1 / 4≤W 12 ≤W1 / 2.

12. The secondary battery according to any one of claims 1 to 7, characterized in that: Along the length direction of the first pole piece, there is a first folding position between the first section and the second section, and there is a second folding position between the second section and the first main body; Along the length direction of the first pole piece, the length from the first folding position to the first starting end is L1, and the length from the second folding position to the first folding position is L2, and L2≥L1.

13. The secondary battery according to claim 12, characterized in that: Observed along the thickness direction of the first pole piece, along the length direction of the first pole piece, the first section includes a first edge disposed away from the second section, and the first starting end is located at the first edge; The first edge is in an arc shape, and along the length direction of the first pole piece, the first edge protrudes in a direction away from the first section.

14. The secondary battery according to claim 13, characterized in that: Along the length direction of the first pole piece, the maximum length of the first edge is D, and D / 2≤L1≤D.

15. The secondary battery according to claim 12, characterized in that: 1mm≤L1≤L2≤10mm.

16. The secondary battery according to claim 15, characterized in that: 4mm≤L1≤L2≤8mm.

17. The secondary battery according to any one of claims 1 to 7, characterized in that: Along the winding direction, the first pole piece includes a winding tail end; along the thickness direction of the first pole piece, the pole piece adjacent to the winding tail end is the second pole piece; Along the length direction of the first pole piece, the first pole piece further includes a second main body portion and a second folding portion; Along the thickness direction of the first pole piece, the second folded portion and the second main body portion are stacked, and the second main body portion extends along the winding direction; Along the length direction of the first pole piece, the first pole piece includes a first tail end, the second folded portion includes a third section and a fourth section, and the first tail end is located in the fourth section; along the thickness direction of the first pole piece, the second main body, the fourth section and the third section are stacked in sequence; Along the thickness direction of the first pole piece, the projection of the third segment and the projection of the second main body together form a second projection, and the projection of the first tail end is located within the second projection.

18. The secondary battery according to claim 17, characterized in that: The winding tail end is located at the secondary outer ring pole piece of the electrode assembly.

19. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 18.

Images