Secondary battery, electronic equipment and preparation method of secondary battery

Abstract

The invention discloses a secondary battery, electronic equipment and a preparation method of the secondary battery, the secondary battery comprises a shell, a tab and an electrode assembly accommodated in the shell, and the electrode assembly is flat. The electrode assembly comprises a bending section, a first end face and a second end face, the first end face and the second end face are oppositely arranged in the first direction, the second end face is far away from the electrode lug, the electrode assembly comprises an outermost ring electrode piece, and the outermost ring electrode piece comprises a first narrowed edge and a second narrowed edge; the first narrowed edge and the second narrowed edge are oppositely arranged, and the second narrowed edge is far away from the tab. In the first direction, on the bent section, the distance D1 between the first narrowed edge and the first end face is larger than or equal to 0.1 mm, and / or the distance D2 between the second narrowed edge and the second end face is larger than or equal to 0.1 mm. Therefore, the risk that the shell is punctured and electrocorrosion is caused by extension of the outermost ring of pole piece after cyclic charging and discharging is reduced, and the safety performance is improved.

Description

Technical Field

[0001] Embodiments of the present application relate to the field of batteries, and in particular to a secondary battery, an electronic device, and a method for manufacturing a secondary battery. Background Art

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

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

[0004] To solve the above technical problems, in the first aspect of the present application: A secondary battery is provided, including a housing, a tab, and an electrode assembly housed in the housing. The electrode assembly is flat, the electrode assembly includes a bent section, the tab is connected to the electrode assembly and extends out of the housing, and the electrode assembly includes a first electrode tab, a separator, and a second electrode tab that are stacked and wound. Taking the direction of the central axis of the winding of the electrode assembly as the first direction, the electrode assembly includes a first end face and a second end face. Along the first direction, the first end face and the second end face are oppositely arranged, the second end face is far from the tab, the electrode assembly includes an outermost electrode tab, and the outermost electrode tab includes a first narrowed edge and a second narrowed edge. Along the first direction, the first narrowed edge and the second narrowed edge are oppositely arranged, and the second narrowed edge is far from the tab. Wherein, along the first direction, on the bent section, the first narrowed edge is at a distance D1 from the first end face, D1≥0.1mm, and / or the second narrowed edge is at a distance D2 from the second end face, D2≥0.1mm.

[0005] After the secondary battery undergoes multiple cycles of charge and discharge, the electrode tabs of the electrode assembly in the secondary battery extend along the first direction, that is, the dimensions in the first direction increase. The distance between some electrode tabs near the end corners of the housing and the end corners of the housing is relatively close. When the electrode tabs extend, it is easy to pierce the end corners of the housing and thus cause the risk of electrocorrosion. For the secondary battery adopting the above scheme, at least the part near the end corners of the housing, including the outermost electrode tabs, is pre-narrowed, increasing the distance between the electrode tabs at the bent section and the housing, reducing the risk of electrocorrosion caused by the outermost electrode tabs extending in the first direction and piercing the end corners of the housing after the secondary battery is cycled for charge and discharge, which is beneficial to improving the safety performance of the secondary battery.

[0006] In some embodiments, the first pole piece includes a first main body portion and a first narrowing portion connected along the winding direction. The first narrowing portion includes a first narrowing edge and a second narrowing edge. Both the first narrowing edge and the second narrowing edge are connected to the first main body portion. Along the first direction, the distance between the first narrowing edge and the second narrowing edge is M1, and the width dimension of the first main body portion is B2, satisfying B2 > M1. For the first pole piece with this structure, compared with the way of uniformly narrowing the width of the entire first pole piece, the first narrowing portion of the first pole piece with this structure is narrowed in width relative to the first main body portion. The first narrowing portion is at least partially in the outermost circle of the first pole piece in the wound state, and the width of the first main body portion of the first pole piece remains unchanged in the first direction, which is beneficial to reducing the risk of electrical corrosion caused by the extension of the first pole piece, while taking into account the energy density of the secondary battery and reducing the risk of excessive loss of energy density due to width narrowing of the first pole piece.

[0007] In some embodiments, when observed along the first direction, the first narrowing portion is wound around at least one turn. In this way, it is beneficial to reserve an extension space between the outermost circle of the first pole piece wound around and the multiple end corners of the housing, so as to reduce the risk of the first pole piece piercing the housing due to extension in the first direction after the secondary battery is cycled for charging and discharging, which is beneficial to improving the safety performance of the secondary battery, and at the same time, it can also reduce the risk of excessive reduction of the energy density of the secondary battery due to the too long length of the first narrowing portion of the first pole piece.

[0008] In some embodiments, the first narrowing portion is wound around at least two turns. In this way, it is more beneficial to reduce the risk of the first pole piece piercing the housing due to extension in the first direction, and further improves the safety performance of the secondary battery.

[0009] In some embodiments, the first main body portion includes a first winding end edge located in the first end face, a second winding end edge located in the second end face, a first winding starting edge, and a first connecting edge. The first winding end edge and the second winding end edge are oppositely arranged along the first direction, and the first connecting edge and the first winding starting edge are oppositely arranged along the winding direction of the first pole piece;

[0010] The first narrowing portion includes a first winding end edge and a first narrowing starting edge oppositely arranged along the winding direction of the first pole piece. The first narrowing starting edge coincides with the first connecting edge. The first narrowing starting edge includes a first end and a second end oppositely arranged along the first direction. The first end is connected to the first narrowing edge, and the second end is connected to the second narrowing edge.

[0011] Wherein, when the first end of the first narrowing starting edge is connected to the first winding end edge, the second end of the first narrowing starting edge is connected to the second winding end edge, and both the first narrowing edge and the second narrowing edge are arranged at an acute angle with the first narrowing starting edge, the first narrowing portion is arranged in a trapezoidal shape at this time, which is beneficial for the first pole piece to narrow in width in the first direction to reduce the risk of the first pole piece extending and piercing the housing to cause electrical corrosion, and at the same time, it also takes into account reducing the impact on the energy density of the secondary battery due to the width narrowing of the first pole piece.

[0012] Wherein, the first end of the first narrowing starting edge is connected to the first narrowing edge, the first end of the first narrowing starting edge has a first distance from the first winding end edge, the second end of the first narrowing starting edge is connected to the second narrowing edge, the second end of the first narrowing starting edge has a second distance from the second winding end edge, and both the first narrowing edge and the second narrowing edge are arranged at an acute angle with the first narrowing starting edge. In this way, the first narrowing portion is arranged in a trapezoidal shape, which is beneficial for the first pole piece to narrow in width in the first direction to reduce the risk of the first pole piece extending and piercing the housing to cause electrical corrosion, and at the same time, it also takes into account reducing the impact on the energy density of the secondary battery due to the width narrowing of the first pole piece.

[0013] Wherein, when the first narrowing edge and the second narrowing edge are arranged in parallel, and the first end of the first narrowing starting edge has a third distance from the first winding end edge, and the second end of the first narrowing starting edge has a fourth distance from the second winding end edge, the first narrowing portion is arranged in a rectangular shape, which is beneficial for the first pole piece to narrow in width on the relatively two sides in the first direction, and is beneficial for further reducing the risk of the first pole piece extending and piercing the housing to cause electrical corrosion.

[0014] In some embodiments, the first narrowing portion further includes a first winding end edge arranged opposite to the first winding starting edge along the winding direction of the first pole piece. The dimension of the first winding end edge in the first direction is B1, and it satisfies 0.3mm ≤ B2 - B1 ≤ 1mm. In this way, the dimension of the first winding end edge of the first narrowing portion relative to the housing in the first direction is more appropriate. At this time, the extension space reserved by the first narrowing portion of the first pole piece relative to the housing in the first direction is sufficient, and at the same time, the reduction of the active material on the first pole piece is not too large, which is beneficial for taking into account the energy density of the secondary battery while improving the safety performance of the secondary battery.

[0015] In some embodiments, 0.5mm ≤ B2 - B1 ≤ 0.8mm. In this way, the extension space reserved by the first narrowing portion of the first pole piece relative to the housing in the first direction is more appropriate, and it is more beneficial for taking into account the energy density of the secondary battery while improving the safety performance of the secondary battery.

[0016] In some embodiments, along the winding direction of the first pole piece, the first narrowing portion includes a first winding end edge disposed opposite to the first narrowing start edge along the winding direction of the first pole piece. One ends of the first narrowing start edge and the first winding end edge are respectively connected to two ends of the first narrowing edge, and the other ends of the first narrowing start edge and the first winding end edge are respectively connected to two ends of the second narrowing edge. The first main body portion includes a first winding start edge disposed opposite to the first winding end edge in the winding direction of the first pole piece. Wherein, along the winding direction of the first pole piece, the dimension between the first narrowing start edge and the first winding end edge is l1, and the dimension between the first winding start edge and the first winding end edge of the first pole piece is L1, satisfying 5% ≤ l1 / L1 ≤ 40%.

[0017] When l1 / L1 < 5%, the size of the first narrowing portion is small, and the number of winding folds occupied by the first narrowing portion during the winding of the first pole piece is small. Correspondingly, the extension space reserved between the first narrowing portion of the first pole piece and the housing in the first direction is limited, and it only provides partial end corner extension space between the first pole piece and the housing in the first direction, which is not conducive to reserving extension space at multiple end corners of the housing, and is relatively limited in improving the safety performance of the secondary battery 1. When l1 / L1 > 40%, the length of the first narrowing portion of the first pole piece in the winding direction of the first pole piece is too long, which is not conducive to improving the energy density of the secondary battery. When 5% ≤ l1 / L1 ≤ 40%, at this time, in the winding direction of the first pole piece, the length of the first narrowing portion is appropriate, and the number of winding folds that the first pole piece can wind is suitable, which is conducive to reserving more extension space for the first pole piece relative to the housing in the first direction, and is more conducive to reducing the risk of electrical corrosion caused by the first pole piece piercing the housing and short-circuiting due to extension in the first direction. At the same time, it can also take into account the energy density of the secondary battery and reduce the excessive reduction of the energy density of the secondary battery due to the too long length of the first narrowing portion.

[0018] In some embodiments, the secondary battery satisfies 20% ≤ l1 / L2 ≤ 30%, so that it is beneficial to further increase the extension space reserved by the first pole piece relative to the housing in the first direction, further reduce the risk of electrical corrosion caused by the first pole piece piercing the housing and short-circuiting due to extension in the first direction, and is more conducive to improving the safety performance of the secondary battery.

[0019] In some embodiments, the first main body portion includes a first winding end edge located within the first end face, a second winding end edge located within the second end face, a first winding starting edge, and a first connecting edge. The first winding end edge and the second winding end edge are arranged opposite to each other in the first direction, and the first connecting edge and the first winding starting edge are arranged opposite to each other along the winding direction of the first pole piece. The first narrowing portion includes a first winding ending edge and a first narrowing starting edge that are arranged opposite to each other along the winding direction of the first pole piece. The first narrowing starting edge coincides with the first connecting edge. The first narrowing starting edge includes a first end and a second end that are arranged opposite to each other in the first direction. The first end is connected to the first narrowing edge, and the second end is connected to the second narrowing edge. In this way, it is beneficial to reduce the risk of the first pole piece extending and piercing the housing to cause electrical corrosion.

[0020] In some embodiments, the first end of the first narrowing starting edge is connected to the first narrowing edge. The first end of the first narrowing starting edge has a fifth distance from the first winding end edge. The second end of the first narrowing starting edge is connected to the second narrowing edge. The second end of the first narrowing starting edge has a sixth distance from the second winding end edge, and the first narrowing edge and the second narrowing edge are arranged intersectingly. In this way, the first narrowing portion is arranged in a triangular shape, which is beneficial to reduce the risk of the first pole piece extending and piercing the housing to cause electrical corrosion.

[0021] In some embodiments, the electrode assembly includes a sub-outermost pole piece. The sub-outermost pole piece is adjacent to the outermost pole piece in a direction perpendicular to the first direction. The sub-outermost pole piece includes a third narrowing edge and a fourth narrowing edge that are arranged opposite to each other in the first direction. The fourth narrowing edge is far from the pole ear. Wherein, along the first direction, on the bending section, the third narrowing edge has a distance D3 from the first end face, and the fourth narrowing edge has a distance D4 from the second end face, satisfying D3≥0.1mm and / or D4≥0.1mm.

[0022] In this way, for the secondary battery adopting the above scheme, at least the parts of the outermost and sub-outermost poles near the end corners of the housing are narrowed, increasing the distance between the pole piece and the housing at the bending section, reducing the risk of the outermost and sub-outermost pole pieces piercing the end corners of the housing due to extension in the first direction after the secondary battery is charged and discharged cyclically, and thus causing electrical corrosion, which is beneficial to improving the safety performance of the secondary battery.

[0023] In some embodiments, the secondary battery satisfies B2 - B4≥0.1mm, which is beneficial to reducing the risk of lithium plating. And / or, along the winding direction, the dimension of the part where the first winding ending edge exceeds the second winding ending edge is L c , satisfying L c ≥0.1mm. In this way, it is beneficial to further reduce the risk of lithium plating.

[0024] Second aspect of the present application: There is also provided an electronic device, including the secondary battery described above.

[0025] Third aspect of the present application: There is also provided a method for preparing a secondary battery, which is applied to prepare the secondary battery described above. The method for preparing the secondary battery includes: providing a first electrode sheet, a separator, a second electrode sheet, and a housing; cutting and narrowing at least one of the first electrode sheet and the second electrode sheet to obtain at least one narrowed electrode sheet; providing electrode tabs and welding the electrode tabs to the first electrode sheet and the second electrode sheet; stacking and winding the first electrode sheet, the separator, and the second electrode sheet to form a flat electrode assembly. The electrode assembly includes a bent section, and a first end face and a second end face oppositely arranged along the winding central axis of the electrode assembly. Among them, at least a part of the narrowed electrode sheet is wound on the outermost circle of the electrode assembly. The electrode sheet on the outermost circle of the electrode assembly includes a first narrowed edge and a second narrowed edge. In the bent section, there is a distance between the first narrowed edge and the first end face, and / or there is a distance between the second narrowed edge and the second end face; placing the electrode assembly in the housing and performing a packaging process and a liquid injection process to obtain a battery cell; performing a forming process on the battery cell; evacuating and edge-sealing the formed battery cell to obtain a secondary battery.

[0026] In some embodiments, the step of performing a forming process on the battery cell includes: providing a special-shaped forming elastic pad, the special-shaped forming elastic pad including a main body part and a convex part protruding relative to the main body part; setting the battery cell between two adjacent special-shaped forming elastic pads, so that the main body part abuts against the outermost circle electrode sheet, and in the direction of the winding central axis of the electrode assembly, the convex part abuts against the area between the first narrowed edge and the first end face, and the area between the second narrowed edge and the second end face; applying a preset pressure to the special-shaped forming elastic pad; providing a forming environment and performing a forming process on the battery cell under the preset pressure.

[0027] The beneficial effects of the embodiments of the present application are as follows: The secondary battery provided by the embodiments of the present application includes a housing, a tab, and an electrode assembly accommodated in the housing. The electrode assembly is flat and includes a bent section. The tab is connected to the electrode assembly and extends out of the housing. The electrode assembly includes a first electrode tab, a separator, and a second electrode tab that are stacked and wound. Taking the direction of the central axis of the winding of the electrode assembly as the first direction, the electrode assembly includes a first end face and a second end face. Along the first direction, the first end face and the second end face are oppositely arranged, and the second end face is far from the tab. The electrode assembly includes an outermost electrode tab, and the outermost electrode tab includes a first narrowed edge and a second narrowed edge. Along the first direction, the first narrowed edge and the second narrowed edge are oppositely arranged, and the second narrowed edge is far from the tab. Wherein, along the first direction, on the bent section, there is a distance D1 between the first narrowed edge and the first end face, D1≥0.1mm, and / or there is a distance D2 between the second narrowed edge and the second end face, D2≥0.1mm. In this way, the risk that the outermost electrode tab pierces into the end corner of the housing due to the extension of the outermost electrode tab in the first direction after the secondary battery is cycled for charging and discharging, thereby causing electrocorrosion, is reduced, which is beneficial to improving the safety performance of the secondary battery. Description of the Drawings

[0028] One or more embodiments are illustrated by way of example in the accompanying drawings, which do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements, unless otherwise stated, and the drawings in the figures do not constitute a scale limitation.

[0029] Figure 1 is a schematic structural diagram of a secondary battery according to one embodiment of the present application;

[0030] Figure 2 is Figure 1 a schematic diagram after sectioning along the section line NN in

[0031] Figure 3 is Figure 1 a schematic diagram after sectioning along the section line MM in

[0032] Figure 4a is a front view of an electrode assembly according to one embodiment;

[0033] Figure 4b is Figure 4a a rear view of

[0034] Figure 4c is Figure 2 a front view of the first electrode tab in when it is in an unfolded state;

[0035] Figure 4d is Figure 2 a top view of the first electrode tab in when it is in an unfolded state;

[0036] Figure 4e is Figure 2 The front view of the first pole piece of another embodiment in Figure 2 in the unfolded state;

[0037] Figure 4f is Figure 2 The front view of the first pole piece of yet another embodiment in Figure 2 in the unfolded state;

[0038] Figure 4g The front view of the electrode assembly of another embodiment;

[0039] Figure 4h is Figure 4g The rear view of Figure 4g ;

[0040] Figure 4i is Figure 2 The front view of the first pole piece of yet another embodiment in Figure 2 in the unfolded state;

[0041] Figure 4j The front view of the electrode assembly of yet another embodiment;

[0042] Figure 4k is Figure 4j The rear view of Figure 4j ;

[0043] Figure 4l is Figure 2 The front view of the first pole piece of still another embodiment in Figure 2 in the unfolded state;

[0044] Figure 5a is Figure 2 The front view of the second pole piece in Figure 2 in the unfolded state;

[0045] Figure 5b is Figure 2 The top view of the second pole piece in Figure 2 in the unfolded state;

[0046] Figure 5c The front view of the second pole piece of another embodiment in the unfolded state;

[0047] Figure 5d The front view of the second pole piece of yet another embodiment in the unfolded state;

[0048] Figure 5e The front view of the second pole piece of still another embodiment in the unfolded state;

[0049] Figure 5f The front view of the second pole piece of yet another embodiment in the unfolded state;

[0050] Figure 6 The step diagram of the preparation method of the secondary battery of another embodiment of the present application;

[0051] Figure 7 isFigure 6 A further refined step diagram of step S206 in

[0052] Figure 8 It is a schematic diagram when the battery cell is clamped between two special-shaped forming elastic pads;

[0053] In the figure: 1. Secondary battery; 2. Housing; 3. Electrode assembly; 4. Tab;

[0054] 21. End corner; 22. Housing main body; 32. First pole piece; 34. Second pole piece; 36. Separator; 41. First tab; 42. Second tab;

[0055] 301. Outermost pole piece; 302. Second outermost pole piece; 32a. First current collector; 32b. First active material layer; 34a. Second current collector; 34b. Second active material layer; 3a. Bent section; 3b. Straight section; 3c. First end face; 3d. Second end face; 3011. First narrowing edge; 3012. Second narrowing edge; 3021. Third narrowing edge; 3022. Fourth narrowing edge;

[0056] 322. First main body part; 324. First narrowing part;

[0057] 3222. First winding start edge; 3224. First connection edge; 3226. First winding end edge; 3228. Second winding end edge; 3221. First end edge; 3223. Second end edge;

[0058] 3242. First winding end edge; 3244. First narrowing start edge; 3246. First narrowing edge; 3248. Second narrowing edge; 32441. First end; 32442. Second end;

[0059] 342. Second main body part; 344. Second narrowing part;

[0060] 3422. Second winding start edge; 3424. Second connection edge; 3426. Third winding end edge; 3428. Fourth winding end edge; 3421. Third end edge; 3423. Fourth end edge;

[0061] 3442. Second winding end edge; 3444. Second narrowing start edge; 3446. Third narrowing edge; 3448. Fourth narrowing edge; 34441. Third end; 34442. Fourth end. Detailed implementation manner

[0062] For ease of understanding of the present invention, the present invention will be described in more detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that when an element is expressed as "fixed to" another element, it can be directly on the other element, or there can be one or more intermediate elements therebetween. When an element is expressed as "connected to" another element, it can be directly connected to the other element, or there can be one or more intermediate elements therebetween. The terms "upper", "lower", "inner", "outer", "vertical", "horizontal", etc. used in this specification indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0063] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not used to limit the present invention. The term "and / or" used in this specification includes any and all combinations of one or more of the related listed items.

[0064] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0065] As Figures 1-3 shown, a secondary battery 1 provided in one embodiment of the present application includes a housing 2, an electrode assembly 3, and a tab 4. The electrode assembly 3 is received in the housing 2, the tab 4 is connected to the electrode assembly 3 and extends out of the housing 2, and the tab 4 is used to connect to an external electronic device.

[0066] The housing 2 can be made of an aluminum plastic film or other materials, and can be specifically set according to needs as long as it can receive the electrode assembly 3 and be insulated from the electrode assembly 3. In some embodiments, the housing 2 is made of an aluminum plastic film, and at this time the secondary battery 1 is a soft-pack battery cell. Among them, as Figure 3 shown, the housing 2 includes end corners 21 and a housing body 22 connected to the end corners 21.

[0067] Please refer to Figure 2 , Figure 2 is along Figure 1The electrode assembly 3 is obtained by cutting along the cutting line NN in the figure. At this time, the electrode assembly 3 is flat, including a bending segment 3a and a straight segment 3b, and there are at least two bending segments 3a and straight segments 3b. Both ends of each straight segment 3b are connected to the bending segment 3a, and the portion between the straight line OM and the straight line ON is the straight segment 3b, and the portion on one side of the straight line OM and the straight line ON is the bending segment 3a.

[0068] The electrode assembly 3 includes a first pole piece 32, a separator 36 and a second pole piece 34. The separator 36 is disposed between the first pole piece 32 and the second pole piece 34 so that the first pole piece 32 and the second pole piece 34 are insulated from each other. The polarity between the first pole piece 32 and the second pole piece 34 is opposite. For example, when the first pole piece 32 is a positive pole piece, the second pole piece 34 is a negative pole piece. Conversely, when the first pole piece 32 is a negative pole piece, the second pole piece 34 is a positive pole piece. In this embodiment, the first pole piece 32, the separator 36 and the second pole piece 34 are stacked and wound in sequence to form the electrode assembly 3. The direction of the winding center axis of the electrode assembly 3 is defined as the first direction X.

[0069] Among them, a straight section 3b of the same pole piece (such as the first pole piece 32 or the second pole piece 34) represents a fold of the pole piece. For example, when the first pole piece 32 has 20 straight sections 3b, the first pole piece 32 has 20 folds. For another example, when the first pole piece 32 has 8 straight sections 3b, the first pole piece 32 has 8 folds.

[0070] like Figure 2 As shown, the first pole piece 32 includes a first current collector 32a and a first active material layer 32b. Along the thickness direction of the first current collector 32a, the first active material layer 32b is superimposed on at least one surface of the first current collector 32a. The first current collector 32a can be made of aluminum foil, copper foil, titanium foil or nickel foil, etc., which has high strength and high conductivity, and can reduce the tearing of the first current collector 32a. The second pole piece 34 includes a second current collector 34a and a second active material layer 34b. Along the thickness direction of the second current collector 34a, the second active material layer 34b is superimposed on at least one surface of the second current collector 34a. The second current collector 34a can be made of aluminum foil, copper foil, titanium foil or nickel foil, etc.

[0071] When the first electrode sheet 32 ​​is a positive electrode sheet, the second electrode sheet 34 is a negative electrode sheet, and the first active material layer 32b includes a positive electrode active material, a conductive agent, and an adhesive, etc., wherein the positive electrode active material includes 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 manganese iron phosphate.

[0072] In some other embodiments, when the first electrode tab 32 is a negative electrode tab, the second electrode tab 34 is a positive electrode tab. The first active material layer 32b includes a negative electrode active material, a conductive agent, a binder, etc. Among them, the negative electrode active material includes one or more of graphite, soft carbon, hard carbon, elemental silicon, silicon oxide compounds, silicon alloys, etc.

[0073] For the above-mentioned electrode tab 4, as Figure 1 shown, it includes a first electrode tab 41 and a second electrode tab 42. Both the first electrode tab 41 and the second electrode tab 42 are connected to the electrode assembly 3. The polarities of the first electrode tab 41 and the second electrode tab 42 are different, specifically depending on the polarities of the connected electrode tabs. In this embodiment, the first electrode tab 41 is connected to the first electrode tab 32, the second electrode tab 42 is connected to the second electrode tab 34, and both the first electrode tab 41 and the second electrode tab 42 extend out of the housing 2 along the first direction X. When the first electrode tab 32 is a negative electrode tab and the second electrode tab 34 is a positive electrode tab, then the first electrode tab 41 is a negative electrode tab and the second electrode tab 42 is a positive electrode tab.

[0074] To improve the energy density of the secondary battery 1, as Figure 3 shown, during manufacturing, the housing 2 is usually closely attached to the electrode assembly 3. The housing 2 forms an end corner 21 at the end of the wound electrode assembly 3. In the first direction X, the distance between a part of the electrode tab near the end corner 21 of the housing 2 and the end corner 21 is less than the distance between a part of the electrode tab far from the end corner 21 and the housing 2. The inventors of the present application have found through research that after the secondary battery 1 undergoes multiple cycles of charge and discharge, the electrode tabs (including the first electrode tab 32 and the second electrode tab 34) of the electrode assembly 3 will extend in the first direction X, that is, the size of the electrode tab in the width direction will slightly increase, which will cause the electrode tab near the end corner 21 to pierce the housing 2 due to extension and short-circuit with the housing 2, thereby forming a risk of corner corrosion at this position and posing a safety hazard. For this reason, the inventors of the present application have designed the electrode tab (including the first electrode tab 32 and / or the second electrode tab 34), and pre-narrowed the electrode tab to reduce the risk that the electrode tab pierces the housing 2 after the secondary battery 1 undergoes charge and discharge cycles.

[0075] In some embodiments, please refer to Figure 2 、 Figure 4a and Figure 4b, the electrode assembly 3 includes a first end face 3c and a second end face 3d that are oppositely arranged along the first direction X, and the second end face 3d is arranged away from the tab 4. Among them, the electrode assembly 3 includes an outermost ring electrode tab 301, and the outermost ring electrode tab 301 includes a first narrowed edge 3011 and a second narrowed edge 3012 that are oppositely arranged along the first direction X, and the second narrowed edge 3012 is arranged away from the tab 4. Among them, along the first direction X, on the bending section 3a, there is a distance D1 between the first narrowed edge 3011 and the first end face 3c, and there is a distance D2 between the second narrowed edge 3012 and the second end face 3d, satisfying D1≥0.1mm and / or D2≥0.1mm. In other words, at least one side of the outermost ring electrode tab 301 at the bending section 3a is narrowed in width along the first direction X, and at any position of the first narrowed edge 3011 on the bending section 3a, there is a spacing of at least 0.1mm from the first end face 3c in the first direction X, and / or, at any position of the second narrowed edge 3012 on the bending section 3a, there is a spacing of at least 0.1mm from the second end face 3d in the first direction X. Thus, when the secondary battery 1 is cycled for charging and discharging, the risk that a part of the outermost ring electrode tab 301 at the bending section 3a pierces the end corner 21 of the housing 2 due to extension is reduced, which is beneficial to improving the safety performance of the secondary battery 1.

[0076] In some embodiments, please combine Figure 2 , Figure 4a with Figure 4b , the electrode assembly 3 further includes a second outermost ring electrode tab 302. The second outermost ring electrode tab 302 is arranged adjacent to the outermost ring electrode tab 301 in a direction perpendicular to the first direction X. The second outermost ring electrode tab 302 includes a third narrowed edge 3021 and a fourth narrowed edge 3022 that are oppositely arranged along the first direction X, and the fourth narrowed edge 3022 is arranged away from the tab 4. Among them, along the first direction X, on the bending section 3a, there is a distance D3 between the third narrowed edge 3021 and the first end face 3c, and there is a distance D4 between the fourth narrowed edge 3022 and the second end face 3d, satisfying D3≥0.1mm and / or D4≥0.1mm. In other words, at least one side of the second outermost ring electrode tab 302 at the bending section 3a is narrowed in width along the first direction X, and at any position of the third narrowed edge 3021 on the bending section 3a, there is a spacing of at least 0.1mm from the first end face 3c in the first direction X, and / or, at any position of the fourth narrowed edge 3022 on the bending section 3a, there is a spacing of at least 0.1mm from the second end face 3d in the first direction X. Thus, when the secondary battery 1 is cycled for charging and discharging, the risk that a part of the second outermost ring electrode tab 302 at the bending section 3a pierces the end corner 21 of the housing 2 due to extension is reduced, which is beneficial to improving the safety performance of the secondary battery 1.

[0077] It should be understood that the outermost pole piece 301 in the electrode assembly 3 refers to the outermost portion of the pole piece that is wound. Taking the first pole piece 32 as an example, the outermost pole piece 301 of the electrode assembly 3 refers to the outermost portion of the first pole piece 32 in the wound state. Figure 2 As shown, in the winding direction of the first pole piece 32, Figure 2 The portion between the straight line Q1 and the straight line Q2 in FIG. 1 is the outermost pole piece 301 .

[0078] The secondary outer ring electrode piece 302 in the electrode assembly 3 refers to a portion of the second electrode piece 34 in the wound state that is adjacent to the outermost ring electrode piece 301. Figure 2 As shown, in the winding direction of the second pole piece 34, Figure 2 The portion of the second pole piece 34 between the straight line Q3 and the straight line Q4 is the secondary outer ring pole piece 302 .

[0079] The following takes the first pole piece 32 as an example to describe the structure of the first pole piece 32 of the present application after the width is narrowed in the first direction X, specifically as follows:

[0080] In some embodiments, please combine Figure 2 , Figure 4a and Figure 4b , Figure 2 yes Figure 1 The schematic diagram after cutting along the cutting line NN, Figure 4a is a front view of an electrode assembly 3 of one embodiment, Figure 4b yes Figure 4aBack view. For the first pole piece 32, it includes a first main body portion 322 and a first narrowing portion 324 connected along the winding direction of the first pole piece 32. The first narrowing portion 324 includes a first narrowing edge 3011 and a second narrowing edge 3012, and both the first narrowing edge 3011 and the second narrowing edge 3012 are connected to the first main body portion 322. Among them, along the first direction X, the distance between the first narrowing edge 3011 and the second narrowing edge 3012 is M1, and the width dimension of the first main body portion 322 is B2, satisfying B2 > M1. In this way, compared with the method of uniformly narrowing the width of the entire first pole piece 32, the first narrowing portion 324 of the first pole piece 32 with this structure is narrowed in width relative to the first main body portion 322. The first narrowing portion 324 is at least partially in the outermost circle of the first pole piece 32 in the wound state, and the width of the first main body portion 322 of the first pole piece 32 remains unchanged in the first direction X, which is beneficial to reducing the risk of electrocorrosion caused by the extension of the first pole piece 32, while taking into account the energy density of the secondary battery 1 and reducing the risk of excessive loss of energy density due to width narrowing of the first pole piece 32. It should be noted here that for the distance M1 between the first narrowing edge 3011 and the second narrowing edge 3012 and the width dimension B2 of the first main body portion 322, they can be directly measured by precision measuring tools (such as calipers, micrometers).

[0081] In some embodiments, as Figure 2 shown, when observing along the first direction X, in the winding direction of the first pole piece 32, the first narrowing portion 324 is wound around at least one turn. In this way, it is beneficial to reserve an extension space between the outermost circle of the first pole piece 32 wound around and the plurality of end corners 21 of the housing 2, so as to reduce the risk of the first pole piece 32 piercing the housing 2 due to extension in the first direction X after the secondary battery 1 is charged and discharged cyclically, which is beneficial to improving the safety performance of the secondary battery 1, and at the same time, it can also reduce the risk of excessive reduction of the energy density of the secondary battery 1 due to the too long length of the first narrowing portion 324 of the first pole piece 32. In some embodiments, the first narrowing portion 324 is wound around at least two turns, so it is more beneficial to reduce the risk of the first pole piece 32 piercing the housing 2 due to extension in the first direction X, and further improve the safety performance of the secondary battery 1.

[0082] In some embodiments, please combine Figure 2 、 Figure 4a with Figure 4b shown,[[]]END]] Figure 4a shows the front view of the electrode assembly 3 of one of the embodiments, Figure 4b shows Figure 4aRear view of the electrode assembly 3. The first main body portion 322 includes a first winding start edge 3222 and a first connection edge 3224 that are oppositely arranged along the winding direction of the first pole piece 32, and a first winding end edge 3226 and a second winding end edge 3228 that are oppositely arranged along the first direction X. The first winding end edge 3226 is located within the first end face 3c, and the second winding end edge 3228 is located within the second end face 3d. Both ends of the first winding start edge 3222 are respectively connected to one end of the first winding end edge 3226 and the second winding end edge 3228, and both ends of the first connection edge 3224 are respectively connected to the other ends of the first winding end edge 3226 and the second winding end edge 3228.

[0083] The first narrowing portion 324 includes a first winding end edge 3242 and a first narrowing start edge 3244 that are oppositely arranged along the winding direction of the first pole piece 32, and the first narrowing start edge 3244 coincides with the first connection edge 3224. The dimension of the first winding end edge 3242 in the first direction X is smaller than the dimension of the first main body portion 322 in the first direction X, and the first narrowing portion 324 is at least partially located at the bending section 3a. That is, in the first direction X, at least part of the first narrowing portion 324 overlaps at least part of the projection of the end corner 21 of the housing 2. In other words, the width of the first pole piece 32 is narrowed at the position corresponding to the end corner 21 of the housing 2, so as to reduce the risk that the first pole piece 32 extends and pierces the housing 2 at the end corner 21 after the secondary battery 1 is cycled for charging and discharging, which is beneficial to improving the stability of the secondary battery 1.

[0084] For the convenience of understanding, the structural design of the first pole piece 32 is introduced with the first pole piece 32 in the unfolded state. As Figure 4c shown in Figure 4d FIG. Figure 4c shows a front view of the first pole piece 32 in the unfolded state of one of the embodiments, Figure 4d shows a top view of the first pole piece 32 in the unfolded state, and the width direction V1 of the first pole piece 32 in the unfolded state is parallel to the above-mentioned first direction X1.

[0085] As Figure 4cAs shown, the first pole piece 32 includes a first main body portion 322 and a first narrowing portion 324 connected along the length direction U1. The first main body portion 322 includes a first winding start edge 3222 and a first connection edge 3224 oppositely arranged along the length direction U1, and a first end edge 3221 and a second end edge 3223 oppositely arranged along the width direction V1. Both ends of the first winding start edge 3222 are respectively connected to one ends of the first end edge 3221 and the second end edge 3223, and both ends of the first connection edge 3224 are respectively connected to the other ends of the first end edge 3221 and the second end edge 3223. The first narrowing portion 324 includes a first winding end edge 3242 and a first narrowing start edge 3244 oppositely arranged along the length direction U1. The first narrowing start edge 3244 is collinear with the first connection edge 3224, and the first narrowing start edge 3244 includes a first end 32441 and a second end 32442 oppositely arranged along the width direction V1 of the first pole piece 32. Wherein, the first narrowing portion 324 further includes a first narrowing edge 3246 and a second narrowing edge 3248. Both ends of the first narrowing edge 3246 and the second narrowing edge 3248 are respectively connected to the first winding end edge 3242 and the first main body portion 322.

[0086] When the first pole piece 32 is wound, the first end edge 3221 is wound into a first winding end edge 3226, the second end edge 3223 is wound into a second winding end edge 3228, the first narrowing edge 3246 is wound to form a first narrowing edge 3011, and the second narrowing edge 3248 is wound to form a second narrowing edge 3012. For the dimension M1 between the first narrowing edge 3011 and the second narrowing edge 3012, as Figure 4c shown, taking the first winding end edge 3242 and the first connection edge 3224 when the first pole piece 32 is unfolded as the boundary, in the width direction V1, the dimension between the first narrowing edge 3246 and the second narrowing edge 3248 is M1, and it does not include the distance between the endpoints where the first narrowing edge 3246 and the second narrowing edge 3248 are connected to the first connection edge 3224. As Figure 4c shown, along the width direction V1 perpendicular to the length direction U1 of the first pole piece 32, the dimension of the first winding end edge 3242 is defined as B1, and the dimension of the first main body portion 322 is defined as B2, satisfying B1 < B2. Along the length direction U1 of the first pole piece 32, the dimension between the first winding start edge 3222 and the first winding end edge 3242 is defined as L1, and the dimension between the first narrowing start edge 3244 and the first winding end edge 3242 is defined as l1.

[0087] Please combine with Figures 4a to 4c, when the first electrode tab 32 is wound, since the first end edge 3221 is wound into the first wound end edge 3226, the second end edge 3223 is wound into the second wound end edge 3228, the first narrowing edge 3246 is wound to form the first narrowing edge 3011, and the second narrowing edge 3248 is wound to form the second narrowing edge 3012. At this time, along the first direction X, the dimension B1 of the first wound trailing edge 3242 and the dimension B2 of the first main body portion 322 remain unchanged, and in the winding direction of the first electrode tab 32, the dimension between the first narrowing starting edge 3244 and the first wound trailing edge 3242 is still l1, and the dimension between the first wound starting edge 3222 and the first wound trailing edge 3242 of the first electrode tab 32 is still L1. That is, when determining the dimension L1 between the first wound starting edge 3222 and the first wound trailing edge 3242, it can be obtained by measuring the dimension after unfolding the first electrode tab 32. Similarly, the dimension l1 between the first narrowing starting edge 3244 and the first wound trailing edge 3242 in the winding direction of the first electrode tab 32 can be obtained by measuring the dimension after unfolding the first electrode tab 32.

[0088] In some embodiments, the first narrowing edge 3246 and the second narrowing edge 3248 of the first narrowing portion 324 may be arranged in parallel or at an angle, and may be specifically arranged according to needs.

[0089] In some embodiments, as Figure 4c shown, when the first narrowing edge 3246 and the second narrowing edge 3248 are arranged at an angle and do not intersect, both the first narrowing edge 3246 and the second narrowing edge 3248 are arranged at an acute angle with the first narrowing starting edge 3244, that is, as Figure 4c shown, the included angles α1, α1 < 90°, and the shape of the first narrowing portion 324 is trapezoidal.

[0090] Correspondingly, as Figure 4a , Figure 4b and Figure 4cAs shown, after the first pole piece 32 is wound, when viewed along the thickness direction of the electrode assembly 3, the projected shape of the first narrowing portion 324 is trapezoidal. At this time, the first end 32441 of the first narrowing starting edge 3244 is connected to the first winding end edge 3226, the second end 32442 of the first narrowing starting edge 3244 is connected to the second winding end edge 3228, the first narrowing edge 3246 is wound to form the first narrowing edge 3011, the second narrowing edge 3248 is wound to form the second narrowing edge 3012, and both the first narrowing edge 3011 and the second narrowing edge 3012 are arranged at an acute angle with the first narrowing starting edge 3244. In this way, setting the first narrowing portion 324 to be trapezoidal is beneficial for the first pole piece 32 to narrow in width in the first direction X to reduce the risk of the first pole piece 32 extending and piercing the housing 2 to cause electrical corrosion, and at the same time, it also takes into account reducing the impact on the energy density of the secondary battery 1 due to the width narrowing of the first pole piece 32.

[0091] In some other embodiments, as Figure 4e shown, when the first narrowing edge 3246 and the second narrowing edge 3248 are arranged at an angle and do not intersect, both the first narrowing edge 3246 and the second narrowing edge 3248 are arranged at an angle with the first narrowing starting edge 3244, that is, as Figure 4e shown, the included angle is α2, α2 < 90°, and the first end 32441 of the first narrowing starting edge 3244 has a first distance d1 from the first end edge 3221, the second end 32442 of the first narrowing starting edge 3244 has a second distance d2 from the second end edge 3223, d1 > 0, d2 > 0, and d1 and d2 can be equal or unequal. At this time, the shape of the first narrowing portion 324 is still trapezoidal.

[0092] Correspondingly, please combine Figure 4a 、 Figure 4b with Figure 4eAs shown, after the first pole piece 32 is wound, when viewed along the thickness direction of the electrode assembly 3, the projected shape of the first narrowing portion 324 is trapezoidal. At this time, the first end edge 3221 is wound into the first wound end edge 3226, the second end edge 3223 is wound into the second wound end edge 3228, the first narrowing edge 3246 is wound to form the first narrowing edge 3011, the second narrowing edge 3248 is wound to form the second narrowing edge 3012. Both the first narrowing edge 3011 and the second narrowing edge 3012 are arranged at an acute angle with the first narrowing starting edge 3244. And the first end 32441 of the first narrowing starting edge 3244 has a first distance d1 from the first wound end edge 3226, and the second end 32442 of the first narrowing starting edge 3244 has a second distance d2 from the second wound end edge 3228. In this way, setting the first narrowing portion 324 to be trapezoidal is beneficial for the first pole piece 32 to narrow in width in the first direction X to reduce the risk of the first pole piece 32 extending and piercing the housing 2 to cause electrical corrosion, and at the same time, it also takes into account reducing the impact on the energy density of the secondary battery 1 due to the width narrowing of the first pole piece 32.

[0093] In some embodiments, as Figure 4f shown, when the first narrowing edge 3246 and the second narrowing edge 3248 are arranged in parallel, the shape of the first narrowing portion 324 is rectangular. The first narrowing starting edge 3244 coincides with the first connecting edge 3224. The first end 32441 of the first narrowing starting edge 3244 has a third distance d3 from the first end edge 3221, and the second end 32442 of the first narrowing starting edge 3244 has a fourth distance d4 from the second end edge 3223. d3>0, d4>0, and the third distance d3 and the fourth distance d4 may be equal or unequal.

[0094] Correspondingly, please combine Figure 4h 、 Figure 4g and Figure 4f shown. After the first pole piece 32 is wound, when viewed along the thickness direction of the electrode assembly 3, the projected shape of the first narrowing portion 324 is rectangular, and the first narrowing edge 3246 is wound to form the first narrowing edge 3011, the second narrowing edge 3248 is wound to form the second narrowing edge 3012. The first narrowing edge 3011 is parallel to the second narrowing edge 3012. The first end 32441 of the first narrowing starting edge 3244 has a third distance d3 from the first wound end edge 3226, and the second end 32442 of the first narrowing starting edge 3244 has a fourth distance d4 from the second wound end edge 3228. In this way, setting the first narrowing portion 324 to be rectangular is beneficial for the first pole piece 32 to narrow in width on the relatively two sides in the first direction X, which is beneficial for further reducing the risk of the first pole piece 32 extending and piercing the housing 2 to cause electrical corrosion.

[0095] In some embodiments, as Figure 4iAs shown, when the first narrowing edge 3246 and the second narrowing edge 3248 intersect at a point, the first winding end edge 3242 is shortened to become the intersection end point of the first narrowing edge 3246 and the second narrowing edge 3248, that is, at this time, there is no first winding end edge 3242, and the first end 32441 of the first narrowing start edge 3244 is connected to the first end edge 3221, and the second end 32442 of the first narrowing start edge 3244 is connected to the second end edge 3223, and the shape of the first narrowing part 324 is triangular.

[0096] Correspondingly, please combine Figure 4i 、 Figure 4k with Figure 4j As shown, after the first pole piece 32 is wound, when observed along the thickness direction of the electrode assembly 3, the projected shape of the first narrowing part 324 is triangular. At this time, the first narrowing edge 3246 is wound to form the first narrowing edge 3011, the second narrowing edge 3248 is wound to form the second narrowing edge 3012, the first end 32441 of the first narrowing start edge 3244 is connected to the first winding end edge 3226, and the second end 32442 of the first narrowing start edge 3244 is connected to the second winding end edge 3228. Setting the first narrowing part 324 in a triangular shape is beneficial to reducing the risk of the first pole piece 32 extending and piercing the housing 2 to cause electrocorrosion.

[0097] In some embodiments, as Figure 4l described, when the first narrowing edge 3246 and the second narrowing edge 3248 intersect at a point, the first winding end edge 3242 is shortened to become the intersection end point of the first narrowing edge 3246 and the second narrowing edge 3248, that is, at this time, there is no first winding end edge 3242, and the first end 32441 of the first narrowing start edge 3244 is connected to the first narrowing edge 3246 and has a fifth distance d5, and the second end 32442 of the first narrowing start edge 3244 is connected to the second narrowing edge 3248 and has a sixth distance d6. At this time, the shape of the first narrowing part 324 is triangular.

[0098] Correspondingly, please combine Figure 4l 、 Figure 4k with Figure 4jAs shown, after the first pole piece 32 is wound, when observed along the thickness direction of the electrode assembly 3, the projected shape of the first narrowing portion 324 is triangular. At this time, the first narrowing side 3246 is wound to form the first narrowing edge 3011, and the second narrowing side 3248 is wound to form the second narrowing edge 3012. The first narrowing edge 3011 intersects with the second narrowing edge 3012. The first end 32441 of the first narrowing starting side 3244 has a fifth distance d5 from the first winding end side 3226, and the second end 32442 of the first narrowing starting side 3244 has a sixth distance d6 from the second winding end side 3228. Setting the first narrowing portion 324 in a triangular shape is beneficial to reducing the risk that the first pole piece 32 extends and pierces the housing 2 to cause electrical corrosion.

[0099] In some embodiments, please refer to Figure 2 、 Figure 4a and Figure 4b As shown, along the first direction X, the first narrowing edge 3011 and the second narrowing edge 3012 are at least partially located in the bending section 3a, and the dimension B1 of the first winding end edge 3242 is smaller than the dimension B2 of the first main body portion 322. In this way, the dimension of the first narrowing portion 324 in the first direction X is pre-reduced, indirectly increasing the distance between the first narrowing portion 324 and the end corner 21 of the housing 2, so as to achieve the purpose of reserving space for the first pole piece 32 to extend at the end corner 21 during the cyclic charge and discharge of the secondary battery 1, thereby reducing the risk that the first pole piece 32 extends and pierces the housing 2.

[0100] The inventor of the present application found that the difference between the dimension B1 of the first winding end edge 3242 and the dimension B2 of the first main body portion 322 will affect the space size between the first narrowing portion 324 of the first pole piece 32 and the housing 2 in the first direction X. When B2 - B1 < 0.3 mm, the dimension by which the first winding end edge 3242 of the first narrowing portion 324 narrows relative to the housing 2 in the first direction X is relatively limited, that is, the reserved extension space for the first narrowing portion 324 of the first pole piece 32 relative to the housing 2 in the first direction X is relatively limited. When B2 - B1 > 1 mm, the dimension by which the first winding end edge 3242 of the first narrowing portion 324 narrows relative to the housing 2 in the first direction X is relatively large, reducing the active material on the first pole piece 32 and affecting the energy density of the secondary battery 1.

[0101] In some embodiments, the dimension B1 of the first winding end edge 3242 and the dimension B2 of the first main body portion 322 satisfy 0.3 mm ≤ B2 - B1 ≤ 1 mm. In this way, the dimension by which the first winding end edge 3242 of the first narrowing portion 324 narrows relative to the housing 2 in the first direction X is appropriate. At this time, the extension space reserved by the first narrowing portion 324 of the first pole piece 32 relative to the housing 2 in the first direction X is sufficient, and at the same time, the amount of active material reduced on the first pole piece 32 is not too large, which is beneficial to taking into account the energy density of the secondary battery 1 while improving the safety performance of the secondary battery 1. Preferably, 0.5 mm ≤ B2 - B1 ≤ 0.8 mm, and the extension space reserved by the first narrowing portion 324 of the first pole piece 32 relative to the housing 2 in the first direction X is more appropriate, which is more beneficial to taking into account the energy density of the secondary battery 1 while improving the safety performance of the secondary battery 1.

[0102] The inventors of the present application also found that the dimension of the first narrowing portion 324 in the winding direction of the first pole piece 32 affects the safety performance of the secondary battery 1. This is because when the first narrowing portion 324 has different lengths, the number of winding turns during the winding to form the electrode assembly 3 is different. Different numbers of turns affect the space reserved between the end of the first pole piece 32 and the housing 2. In the thickness direction of the electrode assembly 3, the winding turn closer to the first winding start edge 3222 of the first pole piece 32 is farther from the end corner 21 of the housing 2, and the distance from the housing 2 in the first direction X is also larger. When the length of the first narrowing portion 324 is longer, the number of winding turns that the first narrowing portion 324 can make is more, and the winding state of the first pole piece 32 has more winding turns that are dimensionally narrowed in the first direction X, which is beneficial to the first pole piece 32 reserving more space for extension relative to the housing 2 in the first direction X, so as to reduce the risk of electrical corrosion caused by the first pole piece 32 extending and piercing the housing 2 in the first direction X and short-circuiting, and is more beneficial to improving the safety performance of the secondary battery 1.

[0103] In some embodiments, such as Figure 4c , Figure 4e , Figure 4f , Figure 4i , Figure 4lAs shown, the dimension l1 between the first narrowing starting edge 3244 and the first winding ending edge 3242, and the dimension L1 between the first winding starting edge 3222 and the first winding ending edge 3242 of the first pole piece 32. When l1 / L1 < 5%, the dimension of the first narrowing portion 324 is small, and the number of winding folds occupied by the first narrowing portion 324 during the winding of the first pole piece 32 is small. Correspondingly, the extension space reserved by the first narrowing portion 324 of the first pole piece 32 and the housing 2 in the first direction X is limited, and it only plays a role in reserving the extension space of the partial end corners 21 of the first pole piece 32 and the housing 2 in the first direction X, which is not conducive to reserving the extension space at multiple end corners 21 of the housing 2, and is relatively limited for improving the safety performance of the secondary battery 1. When l1 / L1 > 40%, the length of the first narrowing portion 324 of the first pole piece 32 in the winding direction of the first pole piece 32 is too long, which is not conducive to improving the energy density of the secondary battery 1.

[0104] When 5% ≤ l1 / L1 ≤ 40% is satisfied, at this time, in the winding direction of the first pole piece 32, the length of the first narrowing portion 324 is more appropriate, and the number of winding folds that the first pole piece 32 can wind is more suitable, which is conducive to the first pole piece 32 reserving more extension space in the first direction X relative to the housing 2, and is more conducive to reducing the risk of electrical corrosion caused by the first pole piece 32 extending in the first direction X and piercing the housing 2 to cause a short circuit. At the same time, the energy density of the secondary battery 1 can also be taken into account, and the energy density of the secondary battery 1 is not reduced too much due to the too long length of the first narrowing portion 324. Preferably, 20% ≤ l1 / L1 ≤ 30%, which is conducive to further increasing the extension space reserved by the first pole piece 32 relative to the housing 2 in the first direction X, further reducing the risk of electrical corrosion caused by the first pole piece 32 extending in the first direction X and piercing the housing 2 to cause a short circuit, and is more conducive to improving the safety performance of the secondary battery 1.

[0105] The second pole piece 34

[0106] In some embodiments, please refer to Figure 2 、 Figure 4a and Figure 4b , Figure 4a which shows the front view of the electrode assembly 3. Figure 4bThe back view of the electrode assembly 3 is shown. For the second pole piece 34, it includes a second main body portion 342 and a second narrowing portion 344 connected along the winding direction of the second pole piece 34. The second narrowing portion 344 includes a third narrowing edge 3021 and a fourth narrowing edge 3022, and both the third narrowing edge 3021 and the fourth narrowing edge 3022 are connected to the second main body portion 342. Among them, along the first direction X, the distance between the third narrowing edge 3021 and the fourth narrowing edge 3022 is M2, and the width dimension of the second main body portion 342 is B4, satisfying B4 > M2. In this way, compared with the way of uniformly narrowing the width of the entire second pole piece 34, the second narrowing portion 344 of the second pole piece 34 with this structure is width-narrowed relative to the second main body portion 342. The second narrowing portion 344 is at least partially in the outermost circle of the second pole piece 34 in the wound state, and the width of the second main body portion 342 of the second pole piece 34 remains unchanged in the first direction X, which is beneficial to reducing the risk of electrical corrosion caused by the extension of the second pole piece 34. At the same time, it takes into account the energy density of the secondary battery 1 and reduces the risk of excessive loss of energy density due to width narrowing of the second pole piece 34. It should be noted here that for the distance M2 between the third narrowing edge 3021 and the fourth narrowing edge 3022 and the width dimension B4 of the second main body portion 342, they can be directly measured by precise measuring tools (such as calipers, micrometers).

[0107] In some embodiments, please refer to Figure 2 、 Figure 4a and Figure 4b , Figure 4a which shows the front view of the wound electrode assembly, Figure 4b and the back view of the wound electrode assembly is shown. The second main body portion 342 includes a second winding start edge 3422 and a second connection edge 3424 that are oppositely arranged along the winding direction of the second pole piece 34, and a third winding end edge 3426 and a fourth winding end edge 3428 that are oppositely arranged along the first direction X. Both ends of the second winding start edge 3422 are respectively connected to one end of the third winding end edge 3426 and the fourth winding end edge 3428, and both ends of the second connection edge 3424 are respectively connected to the other ends of the third winding end edge 3426 and the fourth winding end edge 3428.

[0108] The second narrowing portion 344 includes a second winding end edge 3442 and a second narrowing starting edge 3444 which are oppositely arranged, and a third narrowing edge 3021 and a fourth narrowing edge 3022 which are oppositely arranged along the first direction X. The fourth narrowing edge 3022 is arranged away from the tab 4. The second narrowing starting edge 3444 coincides with the second connecting edge 3424. The dimension of the second winding end edge 3442 in the first direction X is smaller than the dimension of the second main body portion 342 in the first direction X, and the second narrowing portion 344 is at least partially located at the bending section 3a. That is, in the first direction X, at least part of the second narrowing portion 344 overlaps at least part of the projection of the end corner 21 of the housing 2. In other words, the width of the second pole piece 34 is narrowed at the corresponding end corner 21 of the housing 2 to reduce the risk that the second pole piece 34 extends and pierces into the housing 2 at the end corner 21 after the secondary battery 1 is cycled for charging and discharging, which is beneficial to improving the stability of the secondary battery 1. In this embodiment, the second main body portion 342 overlaps with the first main body portion 322, and the second narrowing portion 344 overlaps with the first narrowing portion 324.

[0109] For the convenience of understanding, the structural design of the second pole piece 34 is introduced with the second pole piece 34 in the unfolded state. As Figure 5a shown in Figure 5b and Figure 5a which show the front view of the second pole piece 34 in the unfolded state, Figure 5b and the top view of the second pole piece 34 in the unfolded state, and the width direction V2 of the second pole piece 34 in the unfolded state is parallel to the above-mentioned first direction X.

[0110] As Figure 5a shown, the second pole piece 34 includes a second main body portion 342 and a second narrowing portion 344 which are connected along the length direction U2. The second main body portion 342 includes a second winding starting edge 3422 and a second connecting edge 3424 which are oppositely arranged along the length direction U2, and a third end edge 3421 and a fourth end edge 3423 which are oppositely arranged along the width direction V2. The two ends of the second winding starting edge 3422 are respectively connected to one ends of the third end edge 3421 and the fourth end edge 3423, and the two ends of the second connecting edge 3424 are respectively connected to the other ends of the third end edge 3421 and the fourth end edge 3423. The second narrowing portion 344 includes a second winding end edge 3442 and a second narrowing starting edge 3444 which are oppositely arranged along the length direction U2. The second narrowing starting edge 3444 is collinear with the second connecting edge 3424. The second narrowing starting edge 3444 includes a third end 34441 and a fourth end 34442 which are oppositely arranged along the width direction V2 of the second pole piece 34. Among them, the second narrowing portion 344 further includes a third narrowing edge 3446 and a fourth narrowing edge 3448. The two ends of the third narrowing edge 3446 and the fourth narrowing edge 3448 are respectively connected to the second winding end edge 3442 and the second main body portion 342.

[0111] When the second pole piece 34 is wound, the third end edge 3421 is wound into the third wound end edge 3426, the fourth end edge 3423 is wound into the fourth wound end edge 3428, the third narrowing edge 3446 is wound to form the third narrowing edge 3021, and the fourth narrowing edge 3448 is wound to form the fourth narrowing edge 3022. For the dimension M2 between the third narrowing edge 3021 and the fourth narrowing edge 3022, as Figure 5a shown, with the second wound finishing edge 3442 and the second connecting edge 3424 when the second pole piece 34 is unfolded as the boundary, in the width direction V2, the dimension between the third narrowing edge 3446 and the fourth narrowing edge 3448 is M2, and it does not include the distance between the endpoints where the third narrowing edge 3446 and the fourth narrowing edge 3448 are connected to the second connecting edge 3424.

[0112] As Figure 5a shown, along the width direction V2 perpendicular to the length direction U2 of the second pole piece 34, the dimension of the second wound finishing edge 3442 is defined as B3, and the dimension of the second main body portion 342 is defined as B4, satisfying B3 < B4. Along the length direction U2 of the second pole piece 34, the dimension between the second wound starting edge 3422 and the second wound finishing edge 3442 is defined as L2, and the dimension between the second narrowing starting edge 3444 and the second wound finishing edge 3442 is defined as l2.

[0113] Please refer to again Figure 4a and Figure 4b , when the second pole piece 34 is wound, the third end edge 3421 is wound into the third wound end edge 3426, the fourth end edge 3423 is wound into the fourth wound end edge 3428, the third narrowing edge 3446 is wound to form the third narrowing edge 3021, and the fourth narrowing edge 3448 is wound to form the fourth narrowing edge 3022. The dimension B3 of the second wound finishing edge 3442 and the dimension B4 of the second main body portion 342 remain unchanged. And in the winding direction of the second pole piece 34, the dimension between the second narrowing starting edge 3444 and the second wound finishing edge 3442 is still l2, and the dimension between the second wound starting edge 3422 and the second wound finishing edge 3442 of the second pole piece 34 is still L2. That is to say, when determining the dimension between the second wound starting edge 3422 and the second wound finishing edge 3442, it can be obtained by measuring the dimension after unfolding the second pole piece 34. Similarly, the dimension l2 between the second narrowing starting edge 3444 and the second wound finishing edge 3442 in the winding direction of the second pole piece 34 can be obtained by measuring the dimension after unfolding the second pole piece 34.

[0114] Thus, when the above-mentioned second pole piece 34, first pole piece 32 and separator 36 are wound together to form the electrode assembly 3, since the second pole piece 34 is also dimensionally narrowed in the first direction X, it is beneficial to further reduce the risk of the second pole piece 34 piercing into the end corner 21 of the housing 2 due to the extension of the second pole piece 34 in the first direction X after the secondary battery 1 is cycled for charging and discharging, which is conducive to further improving the safety performance of the secondary battery 1.

[0115] In some embodiments, the third narrowing edge 3446 and the fourth narrowing edge 3448 of the second narrowing portion 344 may be arranged in parallel or at an angle, and may be specifically arranged according to needs.

[0116] In some embodiments, as Figure 5a shown, when the third narrowing edge 3446 and the fourth narrowing edge 3448 are arranged at an angle and do not intersect, both the third narrowing edge 3446 and the fourth narrowing edge 3448 are arranged at an acute angle with the second narrowing starting edge 3444, that is, as Figure 5a shown by the angle β1, β1 < 90°, and the shape of the second narrowing portion 344 is trapezoidal.

[0117] Correspondingly, as Figure 4a 、 Figure 4b and Figure 5a shown, after the second pole piece 34 is wound, when viewed along the thickness direction of the electrode assembly 3, the projected shape of the second narrowing portion 344 is trapezoidal. At this time, the third end 34441 of the second narrowing starting edge 3444 is connected to the third winding end edge 3426, the fourth end 34442 of the second narrowing starting edge 3444 is connected to the fourth winding end edge 3428, the third narrowing edge 3446 is wound to form the third narrowing edge 3021, the fourth narrowing edge 3448 is wound to form the fourth narrowing edge 3022, and both the third narrowing edge 3021 and the fourth narrowing edge 3022 are arranged at an acute angle with the second narrowing starting edge 3444. Thus, setting the second narrowing portion 344 to a trapezoidal shape is beneficial for the second pole piece 34 to narrow in width in the first direction X to reduce the risk of the second pole piece 34 extending and piercing the housing 2 to cause electrical corrosion, while also taking into account reducing the impact on the energy density of the secondary battery 1 due to the width narrowing of the second pole piece 34.

[0118] In some embodiments, as Figure 5c shown, when the third narrowing edge 3446 and the fourth narrowing edge 3448 are arranged at an angle and do not intersect, both the third narrowing edge 3446 and the fourth narrowing edge 3448 are arranged at an angle with the second narrowing starting edge 3444, that is, as Figure 5cThe included angle β2 shown is such that β2 < 90°, and the third end 34441 of the second narrowing starting side 3444 has a seventh distance d7 from the third end side 3421, and the fourth end 34442 of the second narrowing starting side 3444 has an eighth distance d8 from the fourth end side 3423. d7 > 0, d8 > 0, and d7 and d8 may be equal or unequal. At this time, the shape of the second narrowing portion 344 is still trapezoidal.

[0119] Correspondingly, as Figure 4a shown in Figure 4b and Figure 5c After the second pole piece 34 is wound, when viewed along the thickness direction of the electrode assembly 3, the projected shape of the second narrowing portion 344 is trapezoidal. At this time, the third end side 3421 is wound into a third wound end side 3426, the fourth end side 3423 is wound into a fourth wound end side 3428, the third narrowing side 3446 is wound to form a third narrowing edge 3021, and the fourth narrowing side 3448 is wound to form a fourth narrowing edge 3022. Both the third narrowing edge 3021 and the fourth narrowing edge 3022 are arranged at an acute angle with the first narrowing starting side 3244. And the third end 34441 of the second narrowing starting side 3444 has a seventh distance d7 from the third wound end side 3426, and the fourth end 34442 of the second narrowing starting side 3444 has an eighth distance d8 from the fourth wound end side 3428. In this way, setting the second narrowing portion 344 to be trapezoidal is beneficial for the second pole piece 34 to narrow in width in the first direction X to reduce the risk of the second pole piece 34 extending and piercing the housing 2 to cause electrical corrosion, and at the same time, it also takes into account reducing the impact on the energy density of the secondary battery 1 due to the width narrowing of the second pole piece 34.

[0120] In some embodiments, as Figure 5d shown, when the third narrowing side 3446 and the fourth narrowing side 3448 are arranged in parallel, the shape of the second narrowing portion 344 is rectangular. The second narrowing starting side 3444 coincides with the second connecting side 3424. The third end 34441 of the second narrowing starting side 3444 has a ninth distance d9 from the third end side 3421, and the fourth end 34442 of the second narrowing starting side 3444 has a tenth distance d 10 , d9 > 0, d 10 > 0, and the ninth distance d9 and the tenth distance d 10 may be equal or unequal.

[0121] Correspondingly, as Figure 4h , Figure 4g and Figure 5dAs shown, after the second pole piece 34 is wound, when observed along the thickness direction of the electrode assembly 3, the projected shape of the second narrowing portion 344 is rectangular, and the third narrowing side 3446 is wound to form a third narrowing edge 3021, and the fourth narrowing side 3448 is wound to form a fourth narrowing edge 3022. The third narrowing edge 3021 is parallel to the fourth narrowing edge 3022. The third end 34441 of the second narrowing starting side 3444 has a ninth distance d9 from the third winding end side 3426, and the fourth end 34442 of the second narrowing starting side 3444 has a tenth distance d from the fourth winding end side 3428. 10 In this way, setting the second narrowing portion 344 to be rectangular is beneficial to the width narrowing of the two opposite sides of the second pole piece 34 in the first direction X, and is beneficial to further reducing the risk that the second pole piece 34 extends and pierces the housing 2 to cause electrical corrosion.

[0122] In some embodiments, as Figure 5e shown, when the third narrowing side 3446 and the fourth narrowing side 3448 intersect at a point, the second winding end side 3442 is shortened to become the intersection point of the third narrowing side 3446 and the fourth narrowing side 3448, that is, there is no second winding end side 3442 at this time, and the third end 34441 of the second narrowing starting side 3444 is connected to the third end side 3421, and the fourth end 34442 of the second narrowing starting side 3444 is connected to the fourth end side 3423, and the shape of the second narrowing portion 344 is triangular.

[0123] Correspondingly, as Figure 4k , Figure 4j and Figure 5e shown, after the second pole piece 34 is wound, when observed along the thickness direction of the electrode assembly 3, the projected shape of the second narrowing portion 344 is triangular. At this time, the third narrowing side 3446 is wound to form a third narrowing edge 3021, and the fourth narrowing side 3448 is wound to form a fourth narrowing edge 3022. The third end 34441 of the second narrowing starting side 3444 is connected to the third winding end side 3426, and the fourth end 34442 of the second narrowing starting side 3444 is connected to the fourth winding end side 3428. Setting the second narrowing portion 344 to be triangular is beneficial to reducing the risk that the second pole piece 34 extends and pierces the housing 2 to cause electrical corrosion.

[0124] In some embodiments, as Figure 5f described, when the third narrowing side 3446 and the fourth narrowing side 3448 intersect at a point, the second winding end side 3442 is shortened to become the intersection point of the third narrowing side 3446 and the fourth narrowing side 3448, that is, there is no second winding end side 3442 at this time, and the third end 34441 of the second narrowing starting side 3444 is connected to the third narrowing side 3446 and has an eleventh distance d 11The fourth end 34442 of the second narrowing starting edge 3444 is connected to the fourth narrowing edge 3448 and has a twelfth distance d. 12 At this time, the shape of the second narrowing portion 344 is triangular.

[0125] Correspondingly, after the second pole piece 34 is wound, when observed along the thickness direction of the electrode assembly 3, as Figure 4k 、 Figure 4j and Figure 5f shown, the projected shape of the second narrowing portion 344 is triangular. At this time, the third narrowing edge 3446 is wound to form a third narrowing margin 3021, the fourth narrowing edge 3448 is wound to form a fourth narrowing margin 3022, the third narrowing margin 3021 intersects with the fourth narrowing margin 3022, the third end 34441 of the second narrowing starting edge 3444 has an eleventh distance d from the third winding end edge 3426. 11 The fourth end 34442 of the second narrowing starting edge 3444 has a twelfth distance d from the fourth winding end edge 3428. 12 Setting the second narrowing portion 344 to a triangular shape is beneficial to reducing the risk that the second pole piece 34 extends and pierces the housing 2 to cause electrical corrosion.

[0126] In some embodiments, as Figure 2 、 Figure 4a and Figure 4b shown, along the first direction X, at least a part of the third narrowing margin 3021 and the fourth narrowing margin 3022 is located in the bending section 3a, and the dimension B3 of the second winding end edge 3442 is smaller than the dimension B4 of the second main body portion 342. In this way, the dimension of the second narrowing portion 344 in the first direction X is reduced in advance, indirectly increasing the space between the second narrowing portion 344 and the end corner 21 of the housing 2, so as to achieve the purpose of reserving space for the extension of the second pole piece 34 at the end corner 21 during the cyclic charge and discharge of the secondary battery 1, thereby reducing the risk that the second pole piece 34 extends and pierces the housing 2.

[0127] The inventors of the present application found that the difference between the dimension B3 of the second winding end edge 3442 and the dimension B4 of the second main body portion 342 will affect the size of the space between the second narrowing portion 344 of the second pole piece 34 and the housing 2 in the first direction X. When B4 - B3 < 0.3 mm, the dimension by which the second winding end edge 3442 of the second narrowing portion 344 narrows relative to the housing 2 in the first direction X is relatively limited, that is, the reserved extension space for the second narrowing portion 344 of the second pole piece 34 relative to the housing 2 in the first direction X is relatively limited. When B4 - B3 > 1 mm, the dimension by which the second winding end edge 3442 of the second narrowing portion 344 narrows relative to the housing 2 in the first direction X is relatively large, reducing the active material on the second pole piece 34 and greatly affecting the energy density of the secondary battery 1.

[0128] In some embodiments, the dimension B3 of the second winding end edge 3442 and the dimension B4 of the second main body portion 342 satisfy 0.3 mm ≤ B4 - B3 ≤ 1 mm. In this way, the dimension by which the second winding end edge 3442 of the second narrowing portion 344 narrows relative to the housing 2 in the first direction X is appropriate. At this time, the extension space reserved by the second narrowing portion 344 of the second pole piece 34 relative to the housing 2 in the first direction X is sufficient, and at the same time, the amount of active material reduced on the second pole piece 34 is not too large, which is beneficial to taking into account the energy density of the secondary battery 1 while improving the safety performance of the secondary battery 1. Preferably, 0.5 mm ≤ B4 - B3 ≤ 0.8 mm. In this way, the extension space reserved by the second narrowing portion 344 of the second pole piece 34 relative to the housing 2 in the first direction X is more appropriate, and it is more beneficial to taking into account the energy density of the secondary battery 1 while improving the safety performance of the secondary battery 1.

[0129] The inventors of the present application also found that the dimension of the second narrowing portion 344 in the winding direction of the second pole piece 34 affects the safety performance of the secondary battery 1. This is because when the second narrowing portion 344 has different lengths, the number of winding turns during the winding to form the electrode assembly 3 is different. Different numbers of turns affect the space reserved between the end of the second pole piece 34 and the housing 2. In the thickness direction of the electrode assembly 3, the winding turn closer to the starting edge of the winding of the second pole piece 34 is farther away from the end corner 21 of the housing 2, and the distance from the housing 2 in the first direction X is also larger. When the length of the second narrowing portion 344 is longer, the number of winding turns that the second narrowing portion 344 can make is more. The winding turns of the second pole piece 34 in the winding state have more winding turns that are dimensionally narrowed in the first direction X, which is beneficial to the second pole piece 34 reserving more space for extension relative to the housing 2 in the first direction X, so as to reduce the risk of electrical corrosion caused by the second pole piece 34 extending and piercing the housing 2 in the first direction X and short-circuiting, and is more beneficial to improving the safety performance of the secondary battery 1.

[0130] In some embodiments, such as Figure 5a 、 Figures 5c to 5f, the dimension l2 between the second narrowing starting edge 3444 and the second winding ending edge 3442, and the dimension L2 between the second winding starting edge 3422 and the second winding ending edge 3442 of the second pole piece 34. When l2 / L2 < 5%, the size of the second narrowing portion 344 is small, and the number of winding folds occupied by the second narrowing portion 344 during the winding of the second pole piece 34 is small. Correspondingly, the extension space reserved for the second narrowing portion 344 of the second pole piece 34 and the housing 2 in the first direction X is limited, and it only plays a role in reserving the extension space for some end corners 21 of the second pole piece 34 and the housing 2 in the first direction X, which is not conducive to reserving the extension space at multiple end corners 21 of the housing 2, and is relatively limited for improving the safety performance of the secondary battery 1. When l2 / L2 > 40%, the length of the second narrowing portion 344 of the second pole piece 34 in the winding direction of the second pole piece 34 is too long, which is not conducive to improving the energy density of the secondary battery 1.

[0131] When 5% ≤ l2 / L2 ≤ 40% is satisfied, at this time, in the winding direction of the second pole piece 34, the length of the second narrowing portion 344 is more appropriate, and the number of winding folds that the second pole piece 34 can wind is more suitable, which is conducive to the second pole piece 34 reserving more extension space relative to the housing 2 in the first direction X, and is more conducive to reducing the risk of electrical corrosion caused by the second pole piece 34 extending and piercing the housing 2 in the first direction X to cause a short circuit. At the same time, it can also take into account the energy density of the secondary battery 1 and reduce the excessive reduction of the energy density of the secondary battery 1 due to the too long length of the second narrowing portion 344. Preferably, 20% ≤ l2 / L2 ≤ 30%, which is conducive to further increasing the extension space reserved for the second pole piece 34 relative to the housing 2 in the first direction X, further reducing the risk of electrical corrosion caused by the second pole piece 34 extending and piercing the housing 2 in the first direction X to cause a short circuit, and is more conducive to improving the safety performance of the secondary battery 1.

[0132] In some embodiments, such as Figure 4a and Figure 4bAs shown, the first electrode tab 32 is a negative electrode tab, and the second electrode tab 34 is a positive electrode tab. Along the first direction X, the dimension B2 of the first main body portion 322 of the first electrode tab 32 and the dimension B4 of the second main body portion 342 of the second electrode tab 34 satisfy B2 - B4 ≥ 0.1 mm. That is, along the first direction X, the edge of the first electrode tab 32 extends beyond the edge of the second electrode tab 34, which is beneficial to reducing the risk of lithium plating. When B2 - B4 < 0.1 mm, the space provided by the first electrode tab 32 for the lithium ions of the second electrode tab 34 to be embedded is insufficient, which easily leads to the occurrence of lithium plating. It should be noted here that the distance D1 between the first narrowed edge 3011 and the first end face 3c should be understood as the distance between the first narrowed edge 3011 and the end face where the first winding end edge 3226 is located, and the distance D2 between the second narrowed edge 3012 and the second end face 3d should be understood as the distance between the second narrowed edge 3012 and the end face where the second winding end edge 3228 is located. Similarly, the distance D3 between the third narrowed edge 3021 and the first end face 3c should be understood as the distance between the third narrowed edge 3021 and the end face where the third winding end edge 3426 is located, and the distance D4 between the fourth narrowed edge 3022 and the second end face 3d should be understood as the distance between the fourth narrowed edge 3022 and the end face where the fourth winding end edge 3428 is located.

[0133] In some embodiments, along the winding direction, the dimension of the portion where the first winding end edge 3242 exceeds the second winding end edge 3442 is L c , satisfying L c ≥ 0.1 mm. That is, in the winding direction, the edge of the first electrode tab 32 extends beyond the edge of the second electrode tab 34, which is beneficial to reducing the risk of lithium plating. When L c < 0.1 mm, the space provided by the first electrode tab 32 for the lithium ions of the second electrode tab 34 to be embedded is insufficient, which easily leads to the occurrence of lithium plating.

[0134] It can be understood that the lengths of the first narrowed portion 324 and the second narrowed portion 344 can be set as needed, but the lengths of the first narrowed portion 324 and the second narrowed portion 344 will affect the number of turns of narrowing when the electrode tabs (including the first electrode tab 32 and the second electrode tab 34) are wound. The more the number of turns of narrowing, the lower the risk that the electrode tab extends relative to the housing 2 in the first direction X and pierces the housing 2, resulting in electrocorrosion.

[0135] In some embodiments, such as Figure 2As shown, when observing along the first direction X, in the winding direction of the second pole piece 34, the second narrowing portion 344 is wound around at least one turn. In this way, it is beneficial to reserve an extension space between the outermost circle of the second pole piece 34 wound and arranged and the plurality of end corners 21 of the housing 2, so as to reduce the risk that the second pole piece 34 pierces the housing 2 due to extension in the first direction X after the secondary battery 1 is charged and discharged cyclically, which is beneficial to improving the safety performance of the secondary battery 1. In some embodiments, the second narrowing portion 344 is wound around at least two turns, so that it is more beneficial to reduce the risk that the second pole piece 34 pierces the housing 2 due to extension in the first direction X, and further improves the safety performance of the secondary battery 1.

[0136] The secondary battery 1 provided by the embodiment of the present application includes a housing 2, a tab 4, and an electrode assembly 3 accommodated in the housing 2. The electrode assembly 3 is flat. The electrode assembly 3 includes a bent section 3a. The tab 4 is connected to the electrode assembly 3 and extends out of the housing 2. The electrode assembly 3 includes a first pole piece 32, a separator 36, and a second pole piece 34 that are stacked and wound. Taking the direction of the central axis of the winding of the electrode assembly 3 as the first direction X, the electrode assembly 3 includes a first end face 3c and a second end face 3d. Along the first direction X, the first end face 3c and the second end face 3d are arranged opposite to each other. The second end face 3d is far from the tab 4. The electrode assembly 3 includes an outermost pole piece 301. The outermost pole piece 301 includes a first narrowing edge 3011 and a second narrowing edge 3012. Along the first direction X, the first narrowing edge 3011 and the second narrowing edge 3012 are arranged opposite to each other. The second narrowing edge 3012 is far from the tab 4. Among them, along the first direction X, on the bent section 3a, there is a distance D1 between the first narrowing edge 3011 and the first end face 3c, D1≥0.1 mm, and / or there is a distance D2 between the second narrowing edge 3012 and the second end face 3d, D2≥0.1 mm. In this way, the risk that the outermost pole piece 301 pierces the end corner 21 of the housing 2 due to extension in the first direction after the secondary battery 1 is charged and discharged cyclically and then causes electrocorrosion is reduced, which is beneficial to improving the safety performance of the secondary battery.

[0137] An electronic device provided by another embodiment of the present application includes the secondary battery 1 in the above embodiment. 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, the electronic device includes but is not limited to a Bluetooth headset, a mobile phone, a tablet computer, a laptop computer, an electric toy, an electric tool, a battery car, an electric vehicle, a ship, a spacecraft, and the like. Among them, the electric toy can include a fixed or mobile electric toy. For example, a game console, an electric vehicle toy, an electric ship toy, an electric aircraft toy, and the like. The spacecraft can include an airplane, a rocket, a space shuttle, a spaceship, and the like.

[0138] As Figures 6-7As shown in the figure, the preparation method of the secondary battery 1 provided by another embodiment of the present application is applied to prepare the secondary battery 1 in the above embodiment. The steps of the preparation method include:

[0139] Step S201: Provide a first electrode tab 32, a separator 36, a second electrode tab 34, and a housing 2;

[0140] Step S202: Cut and narrow at least one of the first electrode tab 32 and the second electrode tab 34 to obtain at least one narrowed electrode tab;

[0141] The narrowing process means cutting on the preset length of the first electrode tab 32, so that the width dimension of a part of the area of the first electrode tab 32 is smaller than the width dimension of another part of the area, so that the first electrode tab 32 forms a first main body part 322 and a first narrowing part 324, and / or the second electrode tab 34 forms a second main body part 342 and a second narrowing part 344.

[0142] Step S203: Provide a tab 4 and weld the tab 4 to the first electrode tab 32 and the second electrode tab 34;

[0143] The welding method of the tab 4 to the first electrode tab 32 or the second electrode tab 34 includes, but is not limited to, laser welding and ultrasonic welding.

[0144] Step S204: Stack and wind the first electrode tab 32, the separator 36, and the second electrode tab 34 to form a flat electrode assembly 3. The electrode assembly 3 includes a bent section 3a, and a first end face 3c and a second end face 3d oppositely arranged along the winding central axis direction of the electrode assembly 3. Among them, at least part of the narrowed electrode tab is wound on the outermost circle of the electrode assembly 3. The electrode tab on the outermost circle of the electrode assembly 3 includes a first narrowed edge 3011 and a second narrowed edge 3012. In the bent section 3a, there is a distance between the first narrowed edge 3011 and the first end face 3c, and / or there is a distance between the second narrowed edge 3012 and the second end face 3d;

[0145] Step S205: Place the electrode assembly 3 in the housing 2 and perform encapsulation process and liquid injection process to obtain a battery unit;

[0146] Step S206: Perform formation treatment on the battery unit;

[0147] In some embodiments, please combine Figures 6 to 8 , Step S206 includes the following steps:

[0148] Step S2061: Provide a special-shaped formation elastic pad 8. The special-shaped formation elastic pad 8 includes a main body part 82 and a convex part 81 protruding relative to the main body part 82;

[0149] Step S2062: Place the battery cell 9 between two adjacent shaped into elastic pads 8, so that the main body portion 82 abuts against the outermost pole piece 301, and in the direction of the electrode assembly 3 winding around the central axis, the convex portion 81 abuts against the region between the first narrowed edge 3011 and the first end face 3c, and the region between the second narrowed edge 3012 and the second end face 3d;

[0150] As Figure 8 shown, Figure 8 The assembly drawing of two shaped into elastic pads 8 and the battery cell during processing is shown. The two end portions of the battery cell 9 in the first direction X respectively abut against the convex portions 81, and the region between the two end portions abuts against the main body portion 82 of the shaped into elastic pad. In this way, it is beneficial to reduce the risk of uneven pressure caused by the thickness difference due to the narrowing of some regions of the first pole piece 32 and / or the second pole piece 34, and it is beneficial to the uniform pressure of the wound electrode assembly, thereby improving the formation effect.

[0151] Step S2063: Apply a preset pressure to the shaped into elastic pad;

[0152] Step S2064: Provide a formation environment for the battery cell under the preset pressure and perform formation.

[0153] The formation environment refers to the environmental condition requirements such as temperature, air humidity, and air circulation required for the formation of secondary batteries. Formation is a process of activating the internal chemical reaction of secondary batteries through charge and discharge. Its purpose is to form a SEI film. The SEI film is a passivation film generated during the first cycle of secondary batteries, which can ensure the safety and long service life of subsequent charge and discharge cycles.

[0154] Step S207: Evacuate the formed battery cell and perform edge sealing treatment to obtain the secondary battery 1.

[0155] To facilitate the understanding of the technical solution of the present application, the following is a comparative experiment done by the inventor on secondary batteries made of pole pieces with different lengths of narrowed portions and narrowed widths in the width direction (applicable to the first pole piece or the second pole piece).

[0156] Example 1

[0157] (1) Preparation of the negative electrode plate: The negative electrode active material artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed according to a weight ratio of 96:1.5:2.5, deionized water is added, and a slurry with a solid content of 70 wt% is prepared and stirred evenly. The slurry is evenly coated on one surface of the negative electrode current collector copper foil, dried, and a negative electrode plate with a negative electrode active material layer coated on one side is obtained. The above steps are repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode plate with negative electrode active material layers coated on both sides. After cold pressing, the negative electrode plate is cut into the required specifications for use.

[0158] (2) Preparation of the positive electrode plate: The positive electrode active material lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed according to a weight ratio of 97.5:1.0:1.5, N-methylpyrrolidone (NMP) is added, and a slurry with a solid content of 75 wt% is prepared and stirred evenly. The slurry is evenly coated on one surface of the positive electrode current collector aluminum foil, dried, and a positive electrode plate with a positive electrode active material layer coated on one side is obtained. On the other surface of the positive electrode current collector aluminum foil, the above steps are repeated to obtain a positive electrode plate with positive electrode active material layers coated on both sides. After cold pressing, the positive electrode plate is cut into another required specification for use.

[0159] (3) Preparation of the electrolyte: In a dry argon atmosphere, first, the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC = 30:50:20, and then the lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvents and dissolved and mixed evenly to obtain an electrolyte with a LiPF6 concentration of 12.5% based on the mass of the electrolyte.

[0160] (4) Narrowing treatment of the negative electrode plate: The negative electrode plate is cut at a length 3% away from the end edge, so that the negative electrode plate forms an un-narrowed first main part 322 and a narrowed first narrowing part 324. Among them, the length of the first narrowing part 324 accounts for 1% of the total length of the negative electrode plate, the distance between the first narrowing edge 3011 and the first end face 3c is 0.1 mm, and the size of the first winding end edge 3242 of the first narrowing part 324 differs from the size of the width of the first main part 322 by 0.6 mm, and the first narrowing part 324 of the negative electrode plate maintains only 0.5 folds when winding.

[0161] (5) Preparation of the electrode assembly: Stack and wind the separator, the negative electrode sheet, the separator, and the positive electrode sheet in sequence and fix them for standby. Among them, when winding the negative electrode sheet, the first narrowing part maintains only 1 fold. The separator is a polyethylene (PE) film with a thickness of 15 μm. Pole tabs are welded on both the positive electrode sheet and the negative electrode sheet. The positive pole tab connected to the positive electrode sheet is made of aluminum (Al), and the negative pole tab connected to the negative electrode tab is made of nickel (Ni). The two pole tabs are arranged side by side. The thickness h of both the positive and negative pole tabs is 0.2 mm, and the width of the pole tab is 5 mm;

[0162] (6) Assembly of the electrode assembly: Place the aluminum-plastic film (with a thickness of 150 μm) formed with a pit into the assembly fixture with the pit surface facing up. Place the electrode assembly into the pit, and set pole tab glue in the area corresponding to the pole tabs of the electrode assembly at the edge of the aluminum-plastic film. Lead out both the positive and negative pole tabs of the electrode assembly outside the aluminum-plastic film, and perform top sealing and side sealing by means of hot pressing to obtain the assembled electrode assembly.

[0163] (7) Liquid injection and encapsulation: Inject electrolyte into each cavity respectively, and seal it after hot pressing, formation, and degassing.

[0164] Comparative Example 1

[0165] The difference from Example 1 is that the negative electrode sheet is not subjected to narrowing treatment.

[0166] Comparative Example 2

[0167] The difference from Example 1 is that when the negative electrode sheet is subjected to narrowing treatment, the distance between the first narrowing edge and the first end face is 0.05 mm, the length of the first narrowing part 324 accounts for 1% of the total length of the negative electrode sheet, and when winding the negative electrode sheet, the first narrowing part 324 maintains only 0.5 fold.

[0168] Example 2

[0169] The difference from Example 1 is that when the negative electrode sheet is subjected to narrowing treatment, the length of the first narrowing part 324 accounts for 3% of the total length of the negative electrode sheet, and when winding the negative electrode sheet, the first narrowing part 324 maintains only 1 fold.

[0170] Example 3

[0171] The difference from Example 1 is that when the negative electrode sheet is subjected to narrowing treatment, the length of the first narrowing part 324 accounts for 5% of the total length of the negative electrode sheet, and when winding the negative electrode sheet, the first narrowing part 324 maintains only 2 folds.

[0172] Example 4

[0173] The difference from Example 1 is that when the negative electrode tab is narrowed, the length of the first narrowing portion 324 accounts for 10% of the total length of the negative electrode tab, and when the negative electrode tab is wound, the first narrowing portion 324 only has 4 folds.

[0174] Example 5

[0175] The difference from Example 1 is that when the negative electrode tab is narrowed, the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, and when the negative electrode tab is wound, the first narrowing portion 324 only has 8 folds.

[0176] Example 6

[0177] The difference from Example 1 is that when the negative electrode tab is narrowed, the length of the first narrowing portion 324 accounts for 30% of the total length of the negative electrode tab, and when the negative electrode tab is wound, the first narrowing portion 324 only has 12 folds.

[0178] Example 7

[0179] The difference from Example 1 is that when the negative electrode tab is narrowed, the length of the first narrowing portion 324 accounts for 40% of the total length of the negative electrode tab, and when the negative electrode tab is wound, the first narrowing portion 324 only has 16 folds.

[0180] Example 8

[0181] The difference from Example 1 is that when the negative electrode tab is narrowed, ensure that the length of the first narrowing portion 324 accounts for 50% of the total length of the negative electrode tab, and when the negative electrode tab is wound, the first narrowing portion 324 only has 20 folds.

[0182] Example 9

[0183] The difference from Example 1 is that when the negative electrode tab is narrowed, ensure that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, and the dimension of the first winding end edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main body portion 322 by 0.1 mm, and when the negative electrode tab is wound, the first narrowing portion 324 only has 8 folds.

[0184] Example 10

[0185] The difference from Example 1 is that when the negative electrode tab is narrowed, ensure that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, and the dimension of the first winding end edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main body portion 322 by 0.3 mm, and when the negative electrode tab is wound, the first narrowing portion only has 8 folds.

[0186] Example 11

[0187] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, the dimension of the first winding trailing edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main portion 322 by 0.4 mm, and the first narrowing portion 324 maintains only 8 folds when the negative electrode tab is wound.

[0188] Example 12

[0189] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, the dimension of the first winding trailing edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main portion 322 by 0.5 mm, and the first narrowing portion 324 maintains only 8 folds when the negative electrode tab is wound.

[0190] Example 13

[0191] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, the dimension of the first winding trailing edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main portion 322 by 0.6 mm, and the first narrowing portion 324 maintains only 8 folds when the negative electrode tab is wound.

[0192] Example 14

[0193] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, the dimension of the first winding trailing edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main portion 322 by 0.8 mm, and the first narrowing portion 324 maintains only 8 folds when the negative electrode tab is wound.

[0194] Example 15

[0195] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing portion 324 accounts for 20% of the total length of the negative electrode tab, the dimension of the first winding trailing edge 3242 of the first narrowing portion 324 differs from the dimension of the width of the first main portion 322 by 1 mm, and the first narrowing portion 324 maintains only 8 folds when the negative electrode tab is wound.

[0196] Example 16

[0197] The difference from Example 1 is that when the negative electrode tab is narrowed, it is ensured that the length of the first narrowing part 324 accounts for 20% of the total length of the negative electrode tab, and the dimension of the first winding end edge 3242 of the first narrowing part 324 differs from the width dimension of the first main part 322 by 2 mm, and when the negative electrode tab is wound, the first narrowing part 324 maintains only 8 folds.

[0198] Table 1 below shows the results of the phenomena presented by the conventional secondary battery and the secondary battery 1 of the present application after the same power-on duration and the same number of charge and discharge cycles as observed by the inventors of the present application. The observed phenomena mainly include the occurrence of lithium deposition and the occurrence of corrosion corners in the secondary battery 1. For whether lithium deposition occurs in the secondary battery 1, the secondary battery 1 can be disassembled to observe whether the tabs turn black or gray. If so, it is determined that lithium deposition occurs in the secondary battery 1. For whether corrosion corners occur in the secondary battery 1, it can be determined by observing whether the positions of the eight end corners 21 (four on the upper side and four on the lower side) of the housing 2 turn black or gray. If the end corners at this position turn black or gray, it is determined that electrocorrosion occurs at this end corner. At the same time, if several end corners turn black or gray, the number of corrosion corners corresponds to several.

[0199] Among them, the tabs (including the first tab and the second tab) of the conventional secondary battery used in the experiment in Table 1 below have the same length as the tabs of the secondary battery 1 of the present application, and the number of folds of tab winding is also the same. The difference is that the first tab of the secondary battery of the present application is provided with a first narrowing part. The specific experimental results are as follows:

[0200] Table 1

[0201]

[0202]

[0203] For Comparative Example 1 in Table 1 above, the values of l1 / L1, D1, and B2 - B1 being 0 mean that the tab of the secondary battery 1 is not provided with the first narrowing part 324, that is, the tab is not subjected to the narrowing process. At this time, the average number of corrosion corners of this conventional secondary battery is 8, indicating that electrocorrosion occurs at each end corner due to the extension of the tab. In Comparative Example 2, l1 / L1 being 1% means that the tab is subjected to the narrowing process, and the narrowed part accounts for 1% of the total length of the tab. And D1 being 0.05 mm means that the distance between the first narrowing edge and the first end face at the bending section is only 0.05 mm, and the number of folds of the narrowed part is 0.5 fold.

[0204] In Examples 1-16, l1 / L1 is greater than 0, indicating that the electrode sheets of the secondary battery 1 in Examples 1-16 have all been narrowed. And in Examples 1-8, as the value of l1 / L1 increases, the average number of corrosion angles of the secondary battery 1 gradually decreases, indicating that the longer the length of the first narrowing portion 324, the more extension space there is between the electrode sheet and the housing 2, and the less likely the electrode sheet is to pierce the housing 2 due to extension after cyclic charge and discharge, resulting in electrochemical corrosion, which is more conducive to improving the safety performance of the secondary battery 1. And the probability of lithium plating occurring in the secondary battery 1 is also lower.

[0205] In Examples 9-16, while keeping l1 / L1 unchanged, the value of B2 - B1 is changed to represent that the first narrowing portion 324 maintains the same length but has different narrowing widths. As the narrowing width increases, the average number of corrosion angles of the secondary battery 1 gradually decreases, indicating that there is more extension space between the electrode sheet and the housing 2 in the first direction X, thereby reducing the risk of the electrode sheet piercing the housing 2 due to extension and resulting in electrochemical corrosion, which is also conducive to improving the stability of the secondary battery.

[0206] It can be seen from Examples 3 to 7 that when 5% ≤ l1 / L1 ≤ 40%, the occurrence of corrosion angles in the secondary battery 1 is reduced by at least half, which reflects from the side that the longer the length of the first narrowing portion 324, the lower the risk of the electrode sheet piercing the housing 2 due to extension and causing electrochemical corrosion, which is conducive to improving the safety performance of the secondary battery 1. And the probability of lithium plating occurring in the secondary battery 1 in Examples 3-7 is lower than that in the secondary battery 1 of the comparative example, which is conducive to improving the stability of the secondary battery.

[0207] Furthermore, it can be seen from Examples 5 to 6 that when 20% ≤ l1 / L1 ≤ 30%, the number of the average corrosion angles is basically close to 0, indicating that in the secondary battery 1, the electrode sheet hardly causes the phenomenon of piercing the housing 2 due to extension and resulting in electrochemical corrosion, and the lithium plating situation is also improved, which is beneficial to improving the safety and stability of the secondary battery.

[0208] There are almost no corrosion angle problems in the secondary battery 1 in Examples 7 and 8. However, as l1 / L1 increases, there will be a large energy density loss in the secondary battery 1. Considering from the perspective of energy density, generally, examples such as Examples 7 and 8 are not adopted.

[0209] As can be seen from Embodiment 10 to Embodiment 15, when 0.3 mm ≤ B2 - B1 ≤ 1 mm, the occurrence of corrosion angles in the secondary battery 1 is reduced by at least half. This reflects that when the first narrowing portion 324 has the same length, the smaller the dimension of the first narrowing portion 324 in the width direction, the lower the risk of the electrode tab piercing the housing 2 due to electrode tab extension and causing electrocorrosion, which is beneficial to improving the safety performance of the secondary battery 1 and can also reduce the risk of lithium plating in the secondary battery 1.

[0210] Furthermore, as can be seen from Embodiment 12 to Embodiment 14, when 0.5 mm ≤ B2 - B1 ≤ 0.8 mm, the average number of corrosion angles is no more than 2. Compared with the secondary battery 1 in Embodiment 10, the average number of corrosion angles has decreased by at least half, which reflects that the risk of the secondary battery 1 piercing the housing and causing electrocorrosion due to electrode tab extension is lower within this range, and it is more beneficial to improve the safety performance of the secondary battery 1.

[0211] There is almost no corrosion angle problem in the secondary battery 1 in Embodiment 15 and Embodiment 16. However, as B2 - B1 increases, there will be a large capacity loss in the secondary battery 1. Considering the battery capacity, embodiments such as Embodiment 15 and 16 are generally not adopted.

[0212] The above description is only the implementation mode of the present invention and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied to other related technical fields, shall be included in the patent protection scope of the present invention by the same token.

Claims

1. A secondary battery, comprising a shell, a tab, and an electrode assembly contained in the shell, wherein the electrode assembly is flat, the electrode assembly comprises a bent section, the tab is connected to the electrode assembly and extends out of the shell, the electrode assembly comprises a first pole sheet, a separator, and a second pole sheet stacked and wound, characterized in that: The direction of the winding central axis of the electrode assembly is taken as the first direction, The electrode assembly includes a first end face and a second end face, along the first direction, the first end face is arranged opposite to the second end face, and the second end face is far away from the pole ear, the electrode assembly includes an outermost pole piece, and the outermost pole piece includes a first narrowing edge and a second narrowing edge, along the first direction, the first narrowing edge and the second narrowing edge are arranged opposite to each other, and the second narrowing edge is far away from the pole ear, and at least one of the following conditions is satisfied: (1) Along the first direction, on the bending section, the first narrowed edge is at a distance D1 from the first end surface, and D1 is ≥ 0.1 mm; (2) Along the first direction, on the bending section, the second narrowed edge is at a distance D2 from the second end surface, and D2 ≥ 0.1 mm.

2. The secondary battery according to claim 1, characterized in that: The first pole piece includes a first main body portion and a first narrowed portion connected along a winding direction, the first narrowed portion includes a first narrowed edge and a second narrowed edge, the first narrowed edge and the second narrowed edge are both connected to the first main body portion, Along the first direction, the distance between the first narrowed edge and the second narrowed edge is M1, and the width of the first main body is B2, satisfying B2>M1.

3. The secondary battery according to claim 2, characterized in that: When viewed along the first direction, the first narrowed portion is wound at least once.

4. The secondary battery according to claim 3, characterized in that: The first narrowing portion is wound at least twice.

5. The secondary battery according to any one of claims 2 to 4, characterized in that: The first main body portion includes a first winding end edge located in the first end surface, a second winding end edge located in the second end surface, a first winding starting edge and a first connecting edge, the first winding end edge and the second winding end edge are arranged opposite to each other along the first direction, and the first connecting edge and the first winding starting edge are arranged opposite to each other along the winding direction of the first pole piece; The first narrowing portion includes a first winding tail edge and a first narrowing starting edge that are relatively arranged along the winding direction of the first pole piece, the first narrowing starting edge coincides with the first connecting edge, the first narrowing starting edge includes a first end and a second end that are relatively arranged along the first direction, the first end is connected to the first narrowing edge, and the second end is connected to the second narrowing edge, satisfying any one of the following conditions: (1) The first end of the first narrowed starting edge is connected to the first winding end edge, the second end of the first narrowed starting edge is connected to the second winding end edge, and the first narrowed edge and the second narrowed edge are both arranged at an acute angle with the first narrowed starting edge; (2) the first end of the first narrowed starting edge is connected to the first narrowed edge, the first end of the first narrowed starting edge is at a first distance from the first winding end edge, the second end of the first narrowed starting edge is connected to the second narrowed edge, the second end of the first narrowed starting edge is at a second distance from the second winding end edge, and the first narrowed edge and the second narrowed edge are both arranged at an acute angle with the first narrowed starting edge; (3) The first narrowing edge is arranged in parallel with the second narrowing edge, and the first end of the first narrowing starting edge is at a third distance from the first winding end edge, and the second end of the first narrowing starting edge is at a fourth distance from the second winding end edge.

6. The secondary battery according to claim 5, characterized in that: The first narrowing portion further includes a first winding tail edge arranged opposite to the first winding starting edge along the winding direction of the first pole piece, and a size of the first winding tail edge in the first direction is B1, satisfying 0.3mm≤B2-B1≤1mm.

7. The secondary battery according to claim 6, characterized in that: Satisfies 0.5mm≤B2-B1≤0.8mm.

8. The secondary battery according to any one of claims 2 to 4, characterized in that: Along the winding direction of the first pole piece, the first narrowing portion includes a first winding tail edge arranged opposite to the first narrowing starting edge along the winding direction of the first pole piece, one end of the first narrowing starting edge and the first winding tail edge are respectively connected to the two ends of the first narrowing edge, and the other ends of the first narrowing starting edge and the first winding tail edge are respectively connected to the two ends of the second narrowing edge; The first main body portion includes a first winding starting edge arranged opposite to the first winding ending edge in the winding direction of the first pole piece, Among them, along the winding direction of the first pole piece, the dimension between the first narrowing starting edge and the first winding ending edge is l1, and the dimension between the first winding starting edge and the first winding ending edge of the first pole piece is L1, satisfying 5%≤l1 / L1≤40%.

9. The secondary battery according to claim 8, characterized in that: Satisfies 20%≤l1 / L2≤30%.

10. The secondary battery according to any one of claims 2 to 4, characterized in that: The first main body portion includes a first winding end edge located in the first end surface, a second winding end edge located in the second end surface, a first winding starting edge and a first connecting edge, the first winding end edge and the second winding end edge are arranged opposite to each other along the first direction, and the first connecting edge and the first winding starting edge are arranged opposite to each other along the winding direction of the first pole piece; The first narrowing portion includes a first winding tail edge and a first narrowing starting edge that are relatively arranged along the winding direction of the first pole piece, the first narrowing starting edge coincides with the first connecting edge, the first narrowing starting edge includes a first end and a second end that are relatively arranged along the first direction, the first end is connected to the first narrowing edge, and the second end is connected to the second narrowing edge, satisfying any one of the following conditions: (4) the first end of the first narrowed starting edge is connected to the first winding end edge, the second end of the first narrowed starting edge is connected to the second winding end edge, and the first narrowed edge and the second narrowed edge are arranged to intersect; (5) The first end of the first narrowing starting edge is connected to the first narrowing edge, the first end of the first narrowing starting edge is at a fifth distance from the first winding end edge, the second end of the first narrowing starting edge is connected to the second narrowing edge, the second end of the first narrowing starting edge is at a sixth distance from the second winding end edge, and the first narrowing edge and the second narrowing edge are arranged to intersect.

11. The secondary battery according to claim 1, characterized in that: The electrode assembly includes a secondary outer ring pole piece, which is arranged adjacent to the outermost ring pole piece in a direction perpendicular to the first direction, and the secondary outer ring pole piece includes a third narrowed edge and a fourth narrowed edge arranged opposite to each other along the first direction, and the fourth narrowed edge is arranged away from the pole ear, wherein, along the first direction, on the bending section, the third narrowed edge is at a distance D3 from the first end face, and the fourth narrowed edge is at a distance D4 from the second end face, satisfying D3≥0.1mm and / or D4≥0.1mm.

12. An electronic device, characterized in that: Comprising the secondary battery as claimed in any one of claims 1 to 11.

13. A method for preparing a secondary battery, used for preparing the secondary battery as claimed in any one of claims 1 to 11, characterized in that: The method for preparing the secondary battery comprises: Providing the first pole piece, the isolation membrane, the second pole piece and the housing; Cutting and narrowing at least one of the first pole piece and the second pole piece to obtain at least one narrowed pole piece; Providing the pole lug, and welding the pole lug to the first pole piece and the second pole piece; The first pole piece, the isolation film and the second pole piece are stacked and wound to form the flat electrode assembly, wherein the electrode assembly comprises a bending section, and a first end face and a second end face arranged opposite to each other along the direction of the winding center axis of the electrode assembly, wherein at least a portion of the narrowed pole piece is wound on the outermost circle of the electrode assembly, and the pole piece on the outermost circle of the electrode assembly comprises a first narrowed edge and a second narrowed edge, and in the bending section, the first narrowed edge is at a distance from the first end face, and / or the second narrowed edge is at a distance from the second end face; Placing the electrode assembly in the housing and performing packaging and liquid injection processes to obtain a battery unit; Performing a formation treatment on the battery cell; The formed battery cell is evacuated and edge-sealed to obtain a secondary battery.

14. The preparation method according to claim 13, characterized in that: The step of performing formation treatment on the battery cell comprises: Provide a profiled elastic pad, the profiled elastic pad comprising a main body and a protruding portion protruding relative to the main body; The battery unit is arranged between two adjacent special-shaped elastic pads, so that the main body abuts against the outermost pole piece, and in the direction of the winding center axis of the electrode assembly, the protrusion abuts against the area between the first narrowed edge and the first end face, and the area between the second narrowed edge and the second end face; Applying a preset pressure to the profiled elastic pad; A formation environment is provided to the battery cells at a preset pressure and formation is performed.

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