Secondary battery and electronic device

Abstract

A secondary battery (100) and an electronic device (1000). The secondary battery (100) comprises an electrode assembly (20) and a first tab (30). The electrode assembly (20) comprises a first electrode sheet (21), a second electrode sheet (22) and a separator (23) arranged between the first electrode sheet (21) and the second electrode sheet (22), wherein the first electrode sheet (21), the second electrode sheet (22) and the separator (23) form a wound structure. The first electrode sheet (21) comprises a first current collector (211) and a first active material layer (212) which are stacked, the first current collector (211) is provided with a first connection region (2111), and the first connection region (2111) is not provided with the first active material layer (212). The first tab (30) is welded to the first connection region (2111); the first connection region (2111) comprises a plurality of welding regions (211a); the plurality of welding regions (211a) are arranged in the winding direction of the electrode assembly (20); and at least one welding mark (203) formed by welding the first tab (30) and the first connection region (2111) is provided in the welding regions (211a). In the winding direction of the electrode assembly (20), the welding mark (203) in the welding region (211a) among the plurality of welding regions (211a) closest to the winding center of the electrode assembly (20) has a minimum welding strength. It is conducive to improving the tensile capacity of the first connection region (2111), and reducing the risk of damage to the first electrode sheet (21).

Description

Secondary battery and electronic device Technical Field

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

[0002] The tabs of a secondary battery are welded to the current collector of the electrode assembly, serving as the positive and negative leads. In related art, during the cycling of a secondary battery, the thickness of the electrode assembly's pole pieces repeatedly expands, causing the wound electrode assembly to repeatedly contract and expand. During this process, the weld marks formed by the tabs and current collectors are subjected to stress, which can easily damage or break the current collector.

[0003] Summary of the Invention

[0004] In view of this, the present application provides a secondary battery and an electronic device, which are beneficial to improving the tensile strength of the welding part between the tab and the pole piece, and are beneficial to reducing the risk of damage to the pole piece.

[0005] In a first aspect of the present application, a secondary battery is provided. The secondary battery includes an electrode assembly and a first electrode tab. The electrode assembly includes a first electrode sheet, a second electrode sheet, and a separator disposed between the first electrode sheet and the second electrode sheet. The first electrode sheet, the second electrode sheet, and the separator are stacked and wound to form a wound structure. The first electrode sheet includes a first current collector and a first active material layer disposed on the first current collector. The first current collector is provided with a first connection region, and the first connection region is not provided with the first active material layer. The first electrode tab is welded to the first connection region. The first connection region includes a plurality of welding regions, and the plurality of welding regions are arranged along the winding direction of the electrode assembly. The welding region is provided with at least one weld mark formed by welding the first electrode tab to the first connection region. Along the winding direction of the electrode assembly, the weld mark in the welding region closest to the winding center of the electrode assembly has the smallest welding strength.

[0006] In the above embodiment, among the multiple welding areas, the welding strength of the weld mark in the welding area closest to the winding center of the electrode assembly along the winding direction of the electrode assembly is the smallest, which reduces the damage of the welding to the first current collector in the welding area closest to the winding center of the electrode assembly, thereby helping to improve the tensile strength of the weld mark in the welding area closest to the winding center of the electrode assembly, and helping to resist the pulling force on the welding part of the first electrode ear and the first current collector from the outside to the inside along the winding structure when the electrode assembly expands, and helping to reduce the risk of damage to the electrode sheet.

[0007] In one or more of the above embodiments, along the winding direction of the electrode assembly from outside to inside, the welding strength of the weld marks in different welding areas gradually decreases.

[0008] In the above embodiment, the weld strength decreases as the weld mark approaches the center of the electrode assembly in the winding direction, and the degree of damage to the structural strength of the first current collector caused by the weld is also reduced. This improves the tensile strength of the weld mark on the first electrode sheet, helps resist the pulling force on the weld between the first tab and the first current collector from the outside to the inside of the winding structure when the electrode assembly expands, and helps reduce the risk of damage to the first electrode sheet.

[0009] In one or more of the above embodiments, there is welding tension between the first electrode tab and the first current collector at each weld mark, and the welding tension at the weld marks in different welding areas gradually decreases along the winding direction of the electrode assembly from outside to inside.

[0010] In the above embodiment, the closer the weld is to the center of the winding of the electrode assembly, the less damage it causes to the structural strength of the first current collector. This improves the tensile strength of the weld mark on the first electrode sheet, helps resist the pulling force on the weld between the first tab and the first current collector from the outside to the inside along the winding structure when the electrode assembly expands, and helps reduce the risk of damage to the first electrode sheet.

[0011] In one or more of the above embodiments, the welding tension between the first electrode tab and the first current collector at each weld mark is F, which satisfies 1N≤F≤80N.

[0012] In the above embodiment, when 1N≤F≤80N is satisfied, it is beneficial to reduce the risk of a cold weld due to too low a welding force at the weld mark with the smallest welding force, and it is also beneficial to reduce the risk of damage to the first current collector due to too high a welding force at the weld mark with the largest welding force.

[0013] In one or more of the above embodiments, the number of welding zones is N. Along the winding direction of the electrode assembly, the i-th welding zone is closer to the winding center of the electrode assembly than the i+1-th welding zone. The welding tension of the weld mark of the i-th welding zone is F. i , the welding tension of the weld mark in the i+1th welding zone is F i+1 , satisfying 1N≤F i+1 -F i ≤5N. N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N.

[0014] In the above embodiment, the difference in welding strength between the weld marks in two adjacent welding areas is between 1N and 5N, so that the difference in structural strength between the two adjacent welding areas will not be too large, which is beneficial to reducing the risk of poor welding due to insufficient welding tension, and also helps to reduce the risk of damage to the first current collector due to excessive welding tension.

[0015] In one or more of the above embodiments, the number of weld marks is S, and the average value of the welding tension of the S weld marks is K, which satisfies 5N≤K≤9N. S is a positive integer greater than or equal to 2.

[0016] In the above embodiment, when K satisfies the range of 5N≤K≤9N, it is beneficial to reduce the risk of the first pole ear falling off due to the overall welding tension between the first pole ear and the first pole sheet being too small, and the welding strength of some weld marks is allowed to be reduced to improve the structural strength of the first pole sheet, which is beneficial to achieve the welding tension at the weld marks in different welding areas gradually decreasing along the winding direction of the electrode assembly from the outside to the inside, thereby reducing the risk of damage to the pole sheet.

[0017] In one or more of the above embodiments, along the winding direction of the electrode assembly from outside to inside, the heights of the weld marks in different welding areas in the thickness direction of the first electrode sheet gradually decrease.

[0018] In the above embodiment, the height of the weld marks in different weld zones gradually decreases along the winding direction of the electrode assembly from the outside to the inside. This results in welds closer to the winding center of the electrode assembly causing less damage to the structural strength of the first current collector. This improves the tensile strength of the weld marks on the first electrode sheet, helps resist the pulling force on the weld between the first tab and the first current collector from the outside to the inside of the winding structure when the electrode assembly expands, and reduces the risk of damage to the first electrode sheet.

[0019] In one or more of the above embodiments, the height of the weld mark in the thickness direction of the first pole piece is H, which satisfies 1 um≤H≤100 um.

[0020] In the above embodiment, when 1um≤H≤100um is satisfied, it is beneficial to reduce the risk of poor welding due to too low welding tension at the weld mark with the smallest welding tension, and it is also beneficial to reduce the risk of damage to the first current collector due to too high welding tension at the weld mark with the largest welding tension.

[0021] In one or more of the above embodiments, the number of welding zones is N. Along the winding direction of the electrode assembly, the i-th welding zone is closer to the winding center of the electrode assembly than the i+1-th welding zone. The height of the weld mark of the i-th welding zone in the thickness direction of the first electrode sheet is H. i The height of the weld mark of the i+1th welding area in the thickness direction of the first electrode is H i+1 , satisfying 1um≤H i+1 -H i ≤10um. N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N.

[0022] In the above embodiment, the difference in the height of the weld marks in two adjacent welding areas is between 1um and 10um, so that the difference in the structural strength of the two adjacent welding areas will not be too large, which is beneficial to reducing the risk of poor welding due to insufficient welding tension, and also helps to reduce the risk of damage to the first current collector due to excessive welding tension.

[0023] In one or more of the above embodiments, the number of weld marks is S, the average height of the S weld marks is L, and 5 μm≤L≤15 μm is satisfied. S is a positive integer greater than or equal to 2.

[0024] In the above embodiment, when L satisfies the range of 5um≤L≤15um, it is beneficial to reduce the risk of the first pole ear falling off due to the overall welding tension between the first pole ear and the first pole sheet being too small, and the welding strength of some weld marks is allowed to be reduced to improve the structural strength of the first pole sheet, which is beneficial to achieve the welding tension at the weld marks in different welding areas gradually decreasing along the winding direction of the electrode assembly from the outside to the inside, thereby helping to reduce the risk of damage to the pole sheet.

[0025] In one or more of the above embodiments, the edge of the first electrode tab close to the winding center of the electrode assembly in the winding direction from the outside to the inside of the electrode assembly is defined as the first edge, the minimum distance between the weld marks in the multiple welding areas and the first edge is A, the total width of the multiple weld marks is W, and the maximum distance between the multiple weld marks and the first edge is B=A+W.

[0026] In one or more of the above embodiments, 0.1 mm ≤ A ≤ 1 mm is satisfied.

[0027] In the above embodiment, the distance between the weld mark closest to the first edge among the multiple weld marks and the first edge is between 0.1 mm and 1 mm, so that the weld mark will not be too close to the first edge, which is beneficial to the welding operation, and the weld mark will not be too far from the first edge, which is beneficial to increase the area of ​​the welding area and improve the utilization rate of the first connection area.

[0028] In one or more of the above embodiments, 1 mm ≤ W ≤ 3 mm is satisfied.

[0029] In the above embodiment, when 1mm≤W≤3mm is satisfied, the area occupied by the multiple weld marks is not too small, which is beneficial to increasing the total area of ​​the multiple welding areas, and is beneficial to improving the connection strength between the first tab and the first pole piece. The welding strength of some weld marks is allowed to be reduced to improve the structural strength of the first pole piece, which is beneficial to achieving a gradual decrease in the welding tension at the weld marks in different welding areas along the winding direction of the electrode assembly from the outside to the inside, thereby helping to reduce the risk of damage to the pole piece.

[0030] In one or more of the above embodiments, 1.1 mm ≤ B ≤ 4 mm is satisfied.

[0031] In one or more of the above embodiments, along the winding direction of the electrode assembly from the outside to the inside, the first current collector includes an empty foil segment and a coated segment arranged in sequence, the coated segment is provided with a first active material layer, the empty foil segment is not provided with the first active material layer, at least part of the empty foil segment is located at the outermost circle of the first pole piece, and the first connection area is located in the empty foil segment and at the outermost circle of the first pole piece.

[0032] In the above embodiment, among the multiple welding areas, the welding strength of the weld mark in the welding area closest to the winding center of the electrode assembly along the winding direction of the electrode assembly is the smallest, which reduces the damage caused by welding to the first current collector in the welding area closest to the winding center of the electrode assembly, thereby helping to improve the tensile strength of the weld mark in the welding area closest to the winding center of the electrode assembly, and helping to reduce the risk of damage to the electrode sheet.

[0033] In one or more of the above embodiments, the shape of the weld mark includes at least one of a diamond, a square, and a circle.

[0034] In a second aspect of the present application, an electronic device is further provided, which includes a secondary battery as described in any of the above embodiments.

[0035] In the above embodiment, the probability of damage to the electrode of the secondary battery is reduced, which is beneficial to improving the reliability of the electronic device.

[0036] The secondary battery in the present application includes an electrode assembly and a tab. The electrode assembly includes a first electrode sheet and a second electrode sheet that are stacked and wound, and a separator disposed between the first electrode sheet and the second electrode sheet. The first electrode sheet includes a first current collector and a first active material layer that are stacked. The first current collector is provided with a first connection region in which the first active material layer is not provided. The tab is welded to the first connection region. The first connection region includes a plurality of welding regions arranged along the winding direction of the electrode assembly, and at least one weld mark is provided in the welding region. Along the winding direction of the electrode assembly, the weld mark in the welding region closest to the winding center of the electrode assembly among the plurality of welding regions has the smallest welding strength, which is beneficial to reducing damage to the first current collector caused by welding, thereby improving the tensile strength of the weld mark in the welding region closest to the winding center of the electrode assembly, and is beneficial to resisting the pulling force on the weld portion between the first tab and the first current collector from the outside to the inside along the winding structure when the electrode assembly expands, thereby reducing the risk of damage to the first electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG1 is a schematic structural diagram of a secondary battery provided in one embodiment of the present application.

[0038] FIG2 is an exploded schematic diagram of a secondary battery provided in one embodiment of the present application.

[0039] FIG3 is a side view of an electrode assembly provided in one embodiment of the present application.

[0040] FIG4 is a partial top view of a first pole piece and a first pole tab provided in one embodiment of the present application.

[0041] FIG5 is a partial side view of a first pole piece and a first pole tab provided in one embodiment of the present application.

[0042] FIG6 is a partial top view of a first pole piece and a first pole tab provided in another embodiment of the present application.

[0043] FIG7 is a partial top view of a first pole piece and a first pole tab provided in yet another embodiment of the present application.

[0044] FIG8 is a schematic diagram of an electronic device according to an embodiment of the present application.

[0045] Description of Main Component Symbols Secondary Battery 100 Housing 10 Electrode Assembly 20 First Pole Sheet 21 First Current Collector 211 First Connection Area 2111 Welding Area 211a Coated Section 2112 Empty Foil Section 2113 First Active Material Layer 212 Second Pole Sheet 22 Second Current Collector 221 Second Active Material Layer 222 Separator 23 First Tab 30 Weld Mark 203 Second Tab 40 Winding Direction X of Electrode Assembly Winding Direction X' of Electrode Assembly from Outside to Inside Length Direction X" of First Pole Sheet Width Direction Y of First Pole Sheet Thickness Direction Z of First Pole Sheet Device Body 200 Electronic Device 1000 DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0047] It should be noted that in this application, the center of the pole piece refers to the center of gravity of the pole piece of the layered structure. It can be understood that the center of gravity of the layered structure can be determined by the suspension method. The layered structure is suspended with a thin wire, and a straight line is made in the vertical direction with the starting point of the thin wire. The layered structure is suspended again with an endpoint different from the first time, and another straight line is made according to the previous method. The intersection of the two straight lines is the center of gravity of the plane shape.

[0048] It should be noted that, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connected," and "fixed" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections. When a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be an intervening component. When a component is considered to be "located on" another component, it can be directly located on the other component or there may be an intervening component.

[0049] Unless otherwise specified, the term "plurality" as used herein means two or more than two.

[0050] The terms "first", "second", etc. are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implying the quantity, specific order or primary and secondary relationship of the technical features indicated.

[0051] It should be noted that when a parameter is greater than, equal to, or less than a certain endpoint value, it should be understood that the endpoint value allows a tolerance of ±5%.

[0052] It should be understood that the dimensions of layers, regions, columns, or protrusions shown in the drawings are provided for better understanding and more convenient description, and the present application is not limited to the dimensions shown in the drawings. In order to make the present invention clear, elements not relevant to the description are omitted from the details of this specification.

[0053] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0054] In related technologies, during the cycle of secondary batteries, the thickness of the electrode sheets of the electrode assembly repeatedly expands, causing the wound electrode assembly to repeatedly shrink and expand. During this process, the weld edges formed by the welding of the tabs and the current collector are subjected to stress, which can easily cause damage and breakage of the current collector.

[0055] The present application discloses a secondary battery, comprising an electrode assembly and a tab. The electrode assembly comprises a first pole piece, a second pole piece, and a separator disposed between the first pole piece and the second pole piece. The first pole piece, the second pole piece, and the separator are stacked and wound to form a wound structure. The first pole piece comprises a first current collector and a first active material layer disposed on the first current collector. The first current collector is provided with a first connection region, and the first connection region is not provided with the first active material layer. The tab is welded to the first connection region. The first connection region comprises a plurality of welding regions, the plurality of welding regions being arranged along the winding direction of the electrode assembly. The welding region is provided with at least one weld mark formed by welding the tab to the first connection region. Along the winding direction of the electrode assembly, the weld mark in the welding region closest to the winding center of the electrode assembly among the plurality of welding regions has the smallest welding strength.

[0056] Among the above-mentioned multiple welding areas, the welding strength of the weld mark in the welding area closest to the winding center of the electrode assembly along the winding direction of the electrode assembly is the smallest. When the welding strength is relatively small, it is beneficial to reduce the damage to the first current collector caused by welding, thereby helping to improve the tensile strength of the weld mark in the welding area closest to the winding center of the electrode assembly, and helping to resist the pulling force on the welding part of the first electrode ear and the first current collector from the outside to the inside along the winding structure when the electrode assembly expands, and helping to reduce the risk of damage to the first electrode sheet.

[0057] The following will describe some embodiments of the present application in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0058] Referring to Figures 1 to 3, an embodiment of the present application provides a secondary battery 100, which includes an electrode assembly 20. The electrode assembly 20 includes a first electrode plate 21, a second electrode plate 22, and an isolation membrane 23 arranged between the first electrode plate 21 and the second electrode plate 22. The polarities of the first electrode plate 21 and the second electrode plate 22 are opposite. The isolation membrane 23 is used to isolate the first electrode plate 21 and the second electrode plate 22. The first electrode plate 21, the second electrode plate 22, and the isolation membrane 23 are stacked and wound to form a winding structure.

[0059] In some embodiments, referring to FIG. 3 , the first electrode 21 includes a first current collector 211 and a first active material layer 212 disposed on the first current collector 211 . The first active material layer 212 and the first current collector 211 are stacked.

[0060] In some embodiments, referring to FIG. 3 , the second electrode 22 includes a second current collector 221 and a second active material layer 222 disposed on the second current collector 221 . The second active material layer 222 and the second current collector 221 are stacked.

[0061] In some embodiments, the first electrode 21 is an anode electrode, and the second electrode 22 is a cathode electrode. In other embodiments, the first electrode 21 is a cathode electrode, and the second electrode 22 is an anode electrode.

[0062] In some embodiments, the cathode electrode sheet includes a cathode current collector and a cathode active material layer that are stacked together, and the anode electrode sheet includes an anode current collector and an anode active material layer that are stacked together.

[0063] In some embodiments, the cathode current collector may be a metal layer including at least one of aluminum, nickel, tantalum, and titanium, such as aluminum foil. The anode current collector may be a metal layer including at least one of copper, nickel, tantalum, and titanium, such as copper foil.

[0064] In some embodiments, the cathode active material layer includes a cathode active material, which may include at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganese iron phosphate, or lithium manganese oxide. The anode active material layer includes an anode active material, which may include at least one of graphite, hard carbon, soft carbon, silicon, a silicon-oxygen material, and a silicon-carbon material.

[0065] In some embodiments, the isolation film 23 is made of insulating film materials such as polyethylene film, polypropylene film, polyester film, or polyimide film.

[0066] In some embodiments, referring to FIG. 3 , the first current collector 211 is provided with a first connection region 2111 , and the first active material layer 212 is not provided in the first connection region 2111 .

[0067] In some embodiments, the first connection region 2111 is formed at the winding end of the first current collector 211 without coating the first active material layer 212 to form an empty foil segment 2113 , and the area on the empty foil segment 2113 used for welding to the first electrode sheet 21 is the first connection region 2111 .

[0068] In some other embodiments, the first connection region 2111 is formed after a portion of the first active material layer 212 is washed away from the first current collector 211 having the first active material layer 212 , and the current collector of the first connection region 2111 is exposed.

[0069] In some embodiments, referring to FIG. 3 and FIG. 4 , the secondary battery 100 further includes a first electrode tab 30 , which is connected to the electrode assembly 20 .

[0070] In some embodiments, the first electrode tab 30 is connected to the first electrode piece 21 . The material of the first electrode tab 30 may be the same as that of the first current collector 211 . The polarity of the first electrode tab 30 is the same as that of the first electrode piece 21 .

[0071] In some embodiments, referring to FIG. 3 , the secondary battery 100 further includes a second electrode tab 40 , which is connected to the electrode assembly 20 .

[0072] In some embodiments, the second electrode tab 40 is connected to the second electrode piece 22 . The material of the second electrode tab 40 can be the same as that of the second current collector 221 . The polarity of the second electrode tab 40 is the same as that of the second electrode piece 22 .

[0073] In some embodiments, referring to Figures 1 and 2, the secondary battery 100 further includes a shell 10, which accommodates an electrode assembly 20. The electrode assembly 20 is connected to a first electrode tab 30 and a second electrode tab 40. Portions of the first electrode tab 30 and the second electrode tab 40 extend out of the shell 10 to lead the polarity of the electrode assembly 20 out of the shell 10.

[0074] In some embodiments, the secondary battery 100 is a soft-pack battery, and the housing 10 is an aluminum-plastic film. In other embodiments, the secondary battery 100 is a hard-shell battery, and the material of the housing 10 includes any one or more of plastic, steel, or aluminum.

[0075] In some embodiments, an electrolyte (not shown) is further disposed within the housing 10. The electrolyte contains a lithium salt and a solvent. The lithium salt may include at least one of LiPF, LiBF, LiClO, LiB(CH), LiCHSO, LiCFSOLiN(SOCF), LiC(SOCF), or LiBOB. The solvent may be a carbonate compound, a carboxylate compound, an ether compound, other organic solvents, or a combination thereof.

[0076] In some embodiments, referring to FIG. 4 , the first electrode tab 30 is welded to the first connection region 2111. The first connection region 2111 includes a plurality of welding regions 211a. The plurality of welding regions 211a are arranged along the winding direction X of the electrode assembly 20 (when the first electrode sheet 21 is unfolded as shown in FIG. 4 , the plurality of welding regions 211a are arranged along the length direction X″ of the first electrode sheet 21). At least one weld mark 203 formed by welding the first electrode tab 30 to the first connection region 2111 is provided in the welding region 211a. Along the winding direction X of the electrode assembly 20, the weld mark 203 in the welding region 211a closest to the winding center of the electrode assembly 20 has the smallest welding strength.

[0077] When the electrode assembly 20 expands, the area where the first electrode tab 30 is welded to the first connection area 2111 is subjected to a pulling force from the outside to the inside along the electrode assembly 20. Among the multiple welding areas 211a, the weld mark 203 within the welding area 211a closest to the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20 is first subjected to the pulling force. In the present application, among the multiple welding areas 211a, the weld mark 203 within the welding area 211a closest to the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20 has the lowest welding strength. This helps reduce welding damage to the first current collector 211 in the welding area 211a closest to the winding center of the electrode assembly 20. This helps improve the tensile strength of the weld mark 203 within the welding area 211a closest to the winding center of the electrode assembly 20, and helps resist the pulling force on the weld between the first electrode tab 30 and the first current collector 211 from the outside to the inside along the winding structure when the electrode assembly 20 expands, thereby reducing the risk of damage to the electrode sheet.

[0078] It should be noted that the higher the welding strength, the greater the welding force between the first electrode tab 30 and the first connection area 2111, and the degree of damage to the structural strength of the first current collector 211 caused by welding. The welding strength can be characterized by the welding tension and the height of the weld mark 203. The higher the welding strength, the greater the welding tension and the higher the weld mark 203.

[0079] In some embodiments, the welding strengths of the weld marks 203 within the same welding area 211 a are substantially the same.

[0080] In some embodiments, referring to FIG. 4 , a plurality of welding marks 203 are disposed in each welding region 211 a , and the plurality of welding marks 203 in each welding region 211 a are arranged along the width Y direction of the first electrode 21 .

[0081] In some embodiments, the number of weld marks 203 in different welding areas 211 a may be the same or different, and no specific limitation is given herein.

[0082] In some embodiments, the multiple weld marks 203 of the first connection area 2111 are discretely arranged, which is beneficial to improving the uniformity of welding in the first connection area 2111, improving the uniformity of force in the welding area 211a between the first pole ear 30 and the first pole piece 21, reducing the risk of the first pole ear 30 falling off, and reducing the risk of damage to the first pole piece 21.

[0083] In some embodiments, the welding method is ultrasonic welding, and the ultrasonic welding device includes multiple welding heads. By setting different depths of the grooves on different welding heads, weld marks 203 with different weld strengths can be produced. Alternatively, by controlling the energy of different welding heads, weld marks 203 with different weld strengths can be produced.

[0084] In some other embodiments, the welding method is laser welding, and weld marks 203 with different welding strengths are produced by adjusting different powers.

[0085] In some embodiments, referring to FIG. 4 , along the winding direction X' of the electrode assembly 20 from outside to inside, the weld strength of the weld marks 203 in different weld regions 211a gradually decreases, such that, along the winding direction X' of the electrode assembly 20, the weld strength of the weld marks 203 decreases as they are closer to the winding center of the electrode assembly 20, and the degree of damage to the structural strength of the first current collector 211 caused by welding is also reduced. This is beneficial for improving the tensile strength of the weld marks 203 of the first electrode sheet 21, and for resisting the pulling force on the weld between the first electrode tab 30 and the first current collector 211 from outside to inside along the winding structure when the electrode assembly 20 expands, thereby reducing the risk of damage to the first electrode sheet 21.

[0086] In some embodiments, welding tension exists between the first electrode tab 30 and the first current collector 211 at each weld mark 203 , and the welding tension at the weld marks 203 in different weld regions 211 a gradually decreases along the winding direction X′ of the electrode assembly 20 from outside to inside.

[0087] The welding tension at the weld marks 203 in different welding regions 211a gradually decreases along the winding direction X' of the electrode assembly 20 from the outside to the inside. This results in a weld closer to the winding center of the electrode assembly 20 in the winding direction X of the electrode assembly 20 causing less damage to the structural strength of the first current collector 211. This helps improve the tensile strength of the weld marks 203 of the first electrode sheet 21, helps resist the pulling force on the weld between the first electrode tab 30 and the first current collector 211 from the outside to the inside along the winding structure when the electrode assembly 20 expands, and helps reduce the risk of damage to the first electrode sheet 21.

[0088] Welding tensile testing method: After fully discharging the finished secondary battery 100, disassemble it to obtain the first electrode sheet 21 to be measured. Cut the welding pieces of the first electrode sheet 30 and the first connection area 2111 of the first electrode sheet 21 along the width direction Y of the first electrode sheet 21 to obtain the welding pieces of the first electrode sheet 30 and the first current collector 211 corresponding to different welding areas 211a. Each welding piece of the first electrode sheet 30 and the first current collector 211 in each welding area 211a is placed on a high-speed rail tensile testing machine and subjected to a tensile test at a speed of 1 mm / s along the width direction Y of the first electrode sheet 21. The maximum tensile force is recorded as the welding tensile force at the weld mark 203 corresponding to the welding area 211a.

[0089] In an embodiment in which the welding tension at the weld mark 203 in different welding areas 211a gradually decreases along the winding direction X' of the electrode assembly 20 from the outside to the inside, the welding tension between the first electrode tab 30 and the first current collector 211 at each weld mark 203 is F, satisfying 1N≤F≤80N. This is beneficial to reducing the risk of a cold weld due to the welding tension at the weld mark 203 with the smallest welding tension being too small, and is also beneficial to reducing the risk of the first current collector 211 being easily broken due to the welding tension at the weld mark 203 with the largest welding tension being too large.

[0090] As an example, the number of welding areas 211a is N, the Nth welding area 211a is farthest from the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20, and the first welding area 211a is closest to the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20. The welding tension at the weld mark 203 in different welding areas 211a gradually decreases along the winding direction X' from the outside to the inside of the electrode assembly 20, so that the welding tension at the weld mark 203 in the i-th welding area 211a is F i , and F i <F i+1 , where N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N. Then it satisfies: F N ≤80N, F1≥1N.

[0091] For example, F can specifically be 80N, 78N, 75N, 72N, 70N, 68N, 65N, 62N, 60N, 58N, 55N, 52N, 50N, 48N, 45N, 42N, 40N, 38N, 35N, 32N, 30N, 28N, 25N, 22N, 20N, 18N, 15N, 12N, 10N, 8N, 5N, 4N, 3N, 2N or 1N.

[0092] In some embodiments, the number of welding regions 211a is N. Along the winding direction X of the electrode assembly 20, the i-th welding region 211a is closer to the winding center of the electrode assembly 20 than the i+1-th welding region 211a. The welding tension of the weld mark 203 of the i-th welding region 211a is F. i The welding tension of the weld mark 203 of the i+1 welding area 211a is F i+1 , satisfying 1N≤F i+1 -F i ≤5N, so that the difference in welding strength of the weld marks 203 in two adjacent welding areas 211a is between 1N-5N, so that the difference in structural strength of the two adjacent welding areas 211a will not be too large, which is beneficial to reducing the risk of cold welding and the risk of damage to the first pole piece 21 due to excessive welding tension.

[0093] As an illustrative example, F i+1 -Fi Specifically, it can be any one of 5N, 4N, 3N, 2N or 1N.

[0094] In some embodiments, the number of weld marks 203 is S, and the average welding tension of the S weld marks 203 is K, satisfying 5N≤K≤9N, where S is a positive integer greater than or equal to 2. When K satisfies the range of 5N≤K≤9N, it is beneficial to reduce the risk of the first electrode tab 30 detaching due to the overall welding tension between the first electrode tab 30 and the first electrode sheet 21 being too low, allowing the welding strength of some weld marks 203 to be reduced to improve the structural strength of the first electrode sheet 21. This is beneficial to gradually reduce the welding tension at the weld marks 203 in different welding areas 211a along the winding direction X' from the outside to the inside of the electrode assembly 20, thereby reducing the risk of damage to the electrode sheet.

[0095] As an illustrative example, K can be any one of 9N, 8N, 7N, 6N, 5N, 4N, 3N, 2N or 1N.

[0096] In some embodiments, referring to FIG. 5 , along the winding direction X′ of the electrode assembly 20 from outside to inside, the heights of the weld marks 203 in different welding regions 211 a in the thickness direction Z of the first electrode sheet 21 gradually decrease.

[0097] The height of the weld marks 203 in different weld areas 211a gradually decreases along the winding direction X' of the electrode assembly 20 from the outside to the inside. This reduces the damage to the structural strength of the first current collector 211 as the welds are closer to the winding center of the electrode assembly 20 in the winding direction X. This helps improve the tensile strength of the weld marks 203 of the first electrode sheet 21, helps resist the pulling force on the weld between the first electrode tab 30 and the first current collector 211 from the outside to the inside along the winding structure when the electrode assembly 20 expands, and helps reduce the risk of damage to the first electrode sheet 21.

[0098] In some embodiments, referring to FIG5 , the height of the weld mark 203 in the thickness direction Z of the first pole piece 21 is H, satisfying 1um≤H≤100um, which is beneficial to reducing the risk of a cold weld due to too small a welding tension at the weld mark 203 where the welding tension is the smallest, and is also beneficial to reducing the risk of damage to the first current collector 211 due to too large a welding tension at the weld mark 203 where the welding tension is the largest.

[0099] As an example, the number of welding areas 211a is N, the Nth welding area 211a is farthest from the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20, and the first welding area 211a is closest to the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20. The welding tension at the weld marks 203 in different welding areas 211a gradually decreases along the winding direction X' from the outside to the inside of the electrode assembly 20, so that the height of the weld mark 203 of the i-th welding area 211a in the thickness direction Z of the first electrode sheet 21 is H i , and H i <H i+1 , where N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N. Then it satisfies: H N ≤100um, -H1≥1um.

[0100] For example, H can specifically be 100um, 98um, 95um, 92um, 90um, 88um, 85um, 82um, 80um, 78um, 75um, 72um, 70um, 68um, 65um, 62um, 60um, 58um, 55um, 52um, 50um, 48um, 45um, 42um, 40um, 38um, 35um, 32um, 30um, 28um, 25um, 22um, 20um, 18um, 15um, 12um, 10um, 8um, 5um, 4um, 3um, 2um or 1um.

[0101] In some embodiments, the number of welding areas 211a is N. Along the winding direction X of the electrode assembly 20, the i-th welding area 211a is closer to the winding center of the electrode assembly 20 than the i+1-th welding area 211a. The height of the weld mark 203 of the i-th welding area 211a in the thickness direction Z of the first electrode sheet 21 is H. i The height of the weld mark 203 of the i+1th welding area 211a in the thickness direction Z of the first electrode 21 is H i+1 , satisfying 1um≤H i+1 -H i ≤10um, so that the height difference of the weld marks 203 in two adjacent welding areas 211a is between 1um and 10um, so that the difference in the structural strength of the two adjacent welding areas 211a will not be too large, which is beneficial to reducing the risk of cold welding and the risk of damage to the first electrode 21 due to excessive welding tension. As an example, H i+1 -H i Specifically, it can be any one of 10N, 9N, 8N, 7N, 6N, 5N, 4N, 3N, 2N or 1N.

[0102] In some embodiments, the number of weld marks 203 is S, and the average height of the S weld marks 203 is L, satisfying 5 μm ≤ L ≤ 15 μm, where S is a positive integer greater than or equal to 2. When L satisfies the range of 5 μm ≤ L ≤ 15 μm, this helps reduce the risk of the first electrode tab 30 detaching due to excessively low overall welding tension between the first electrode tab 30 and the first electrode sheet 21. This allows for reduced welding strength of some weld marks 203 to improve the structural strength of the first electrode sheet 21. This helps achieve a gradual decrease in welding tension at the weld marks 203 in different weld areas 211 a along the winding direction X' from the outside to the inside of the electrode assembly 20, thereby reducing the risk of electrode sheet damage.

[0103] As an illustrative example, L can be any one of 15um, 14um, 13um, 12um, 11um, 10um, 9um, 8um, 7um, 6um or 5um.

[0104] In order to verify the influence of the welding tension at the weld mark 203 and the height of the weld mark 203 on the cold welding and electrode damage, the following tests were conducted:

[0105] Cold soldering test:

[0106] After fully discharging the finished secondary battery 100, disassemble it to obtain the first electrode sheet 21 to be measured. Cut along the width direction Y of the first electrode sheet 21 at the first connection area 2111 between the first electrode tab 30 and the first electrode sheet 21 to obtain welding pieces corresponding to different welding areas 211a of the first electrode tab 30 and the first current collector 211. The welding pieces of the first electrode tab 30 and the first current collector 211 in different welding areas 211a are each placed on a high-speed rail tensile tester and subjected to a tensile test at a speed of 1 mm / s along the width direction Y of the first electrode sheet 21. After the test, observe whether there is any residue of the first current collector 211 at the weld mark 203 of the first electrode tab 30. If no residue is observed, the weld mark 203 is a cold joint; otherwise, there is no cold joint.

[0107] Pole damage test:

[0108] Perform a battery cycle test on the secondary battery 100: Place the secondary battery 100 in a 25°C environment and let it rest for 30 minutes. Then, perform the following charge and discharge steps: Charge at a constant current of 2.5C to 4.2V, then charge at a constant voltage of 0.5C; then charge at a constant current of 0.5C to 4.45V, then charge at a constant voltage of 0.02C; let it rest for 5 minutes, then discharge at a constant current of 1C to 3V, and let it rest for 5 minutes. This constitutes one cycle. Repeat these steps for 500 cycles, then disassemble the secondary battery 100 to inspect the first electrode 21 for damage.

[0109] In the above test, 20 batteries were tested for each embodiment or comparative example. If there was no cold solder joint on the first electrode 21 and the first electrode 21 was not damaged, the battery passed the test, otherwise it failed. Pass rate = (pass number / 20) × 100%.

[0110] The specific implementation of the secondary battery 100 in the examples and comparative examples will be described below.

[0111] Example:

[0112] A secondary battery 100 is assembled as follows:

[0113] (1) Preparation of the anode electrode: The anode active material artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 96:1.5:2.5, deionized water is added as a solvent, and a slurry with a weight percentage of 70 wt% is prepared and stirred evenly. The slurry is evenly coated on one surface of an anode current collector copper foil with a thickness of 10 μm, leaving an empty foil area at the edge of the copper foil, and dried at 110°C to obtain an anode electrode sheet with a coating thickness of 150 μm and coated on one side with an anode active material layer. The above steps are repeated on the other surface of the anode electrode sheet to obtain an anode electrode sheet with a double-sided coating of an anode active material layer. Then, the winding end of the anode electrode sheet is not coated with the anode active material layer, and the empty foil section 2113 not coated with the anode active material layer has a first connection area 2111. A copper foil is welded to the first connection area 2111 to serve as an anode tab. The first connection region 2111 includes multiple welding regions 211a, which are arranged along the winding direction X of the electrode assembly 20. Multiple weld marks 203 formed by welding the anode tab to the first connection region 2111 are provided within the welding regions 211a. The weld marks 203 within each welding region 211a are arranged along the width direction Y of the anode electrode sheet. Along the winding direction X' of the electrode assembly 20 from outside to inside, the weld strength of the weld marks 203 in different welding regions 211a gradually decreases.

[0114] (2) Preparation of cathode electrode: The cathode active material lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:1.0:1.5, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 75wt%, and stirred evenly. The slurry is evenly coated on one surface of a cathode current collector aluminum foil with a thickness of 12μm, leaving an empty foil area at the edge of the aluminum foil, and then dried at 90°C to obtain a cathode electrode with a cathode active material layer thickness of 100μm. When preparing another first electrode 21 coated on both sides, repeat the above coating steps on the other surface of the aluminum foil. Then, a portion of the cathode electrode is not provided with a cathode active material layer and the current collector is exposed, and the current collector is welded to an aluminum foil to serve as a cathode tab.

[0115] (3) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were first mixed in a mass ratio of EC:EMC:DEC = 30:50:20 to form a basic organic solvent. Then, lithium salt lithium hexafluorophosphate (LiPF6) was added to the basic organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0116] (4) Preparation of the isolation film 23: The isolation film 23 is a three-layer structure, which includes a first adhesive layer, a first base material layer, and a first adhesive layer. The first base material layer is made of polyethylene (PE), and the first adhesive layer contains a first adhesive. The first adhesive layer also contains inorganic ceramic particles Al2O3.

[0117] (5) Preparation of electrode assembly 20: The cathode electrode sheet, the separator 23 and the anode electrode sheet are stacked and the stacked structure is hot-pressed for 10 seconds at a temperature of 80° C. and a pressure of 1.5 MPa to form an electrode assembly 20 for standby use.

[0118] (6) Assembly of the electrode assembly 20: Place the aluminum-plastic film with the cavities formed in it into an assembly fixture, with the cavities facing upward. Place the electrode assembly 20 in the cavities and apply external force to tighten. Then, place another aluminum-plastic film with the cavities formed in it, with the cavities facing downward, over the electrode assembly 20. Heat-seal the two aluminum-plastic films around their edges using a hot press to obtain the assembled electrode assembly 20.

[0119] (7) Liquid injection and packaging: The electrolyte is injected into the assembled electrode assembly 20, and the secondary battery 100 is manufactured through processes such as vacuum packaging, static standing, hot pressing, and shaping.

[0120] Comparative Example:

[0121] The difference from the embodiment is that the welding strength of each weld mark 203 formed by welding the anode electrode sheet and the anode tab in comparative example 1 is basically the same, while the welding strength of multiple weld marks 203 formed by welding the anode electrode sheet and the anode tab in comparative example 2 is gradually increased along the winding direction X' from the outside to the inside of the electrode assembly 20, and the welding strength of the weld marks 203 in different welding areas 211a increases gradually.

[0122] The main parameter control and test results of each embodiment and comparative example are shown in Table 1:

[0123] Table 1

[0124] According to Table 1 above, relative to Comparative Examples 1 and 2, Examples 1-12 satisfy the following requirements: along the winding direction X of the electrode assembly 20, the welding strength of the weld mark 203 in the welding area 211a closest to the winding center of the electrode assembly 20 among the multiple welding areas 211a is the smallest, which is beneficial to reducing the damage of welding to the first current collector 211 in the welding area 211a closest to the winding center of the electrode assembly 20, thereby facilitating the improvement of the tensile strength of the weld mark 203 in the welding area 211a closest to the winding center of the electrode assembly 20, and reducing the risk of damage to the electrode sheet.

[0125] According to Table 1 above, compared with Examples 1 and 12, Examples 2-11 satisfy the following conditions: 1N≤F i+1 -F i ≤5N, so that the difference in structural strength between two adjacent welding areas 211a will not be too large, which is beneficial to reducing the risk of cold welding and the risk of damage to the first pole piece 21 due to excessive welding tension.

[0126] According to Table 1 above, compared with Examples 1 and 12, Examples 2-11 satisfy the following conditions: 1um≤H i+1 -H i ≤10um, so that the difference in structural strength between two adjacent welding areas 211a will not be too large, which is beneficial to reducing the risk of cold welding and the risk of damage to the first pole piece 21 due to excessive welding tension.

[0127] According to Table 1 above, relative to Examples 13 and 18, Example 6, and Examples 14-17 satisfy the following requirement: 5N≤K≤9N, which is beneficial to reducing the risk of the first pole tab 30 detaching due to the overall welding tension between the first pole tab 30 and the first pole piece 21 being too small, and is beneficial to reducing the possibility of cold welding. It also allows the welding strength of some weld marks 203 to be reduced to improve the structural strength of the first pole piece 21, and is beneficial to achieving a gradual decrease in the welding tension at the weld marks 203 in different welding areas 211a along the winding direction X' from the outside to the inside of the electrode assembly 20, thereby helping to reduce the risk of pole piece damage.

[0128] In some embodiments, referring to FIG. 4 , the edge of the first electrode tab 30 located near the winding center of the electrode assembly 20 in the winding direction X' from the outside to the inside of the electrode assembly 20 is defined as the first edge. The minimum distance A between the weld marks 203 in the multiple welding regions 211a and the first edge satisfies 0.1 mm ≤ A ≤ 1 mm. This ensures that the distance between the weld mark 203 closest to the first edge among the multiple weld marks 203 and the first edge is between 0.1 mm and 1 mm. This prevents the weld mark 203 from being too close to the first edge, thereby facilitating welding operations. It also prevents the weld mark 203 from being too far from the first edge, thereby increasing the area of ​​the welding region 211a and improving the utilization rate of the first connection region 2111.

[0129] As an illustrative example, A can be any one of 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm or 1 mm.

[0130] In some embodiments, please refer to Figure 4. Along the winding direction X of the electrode assembly 20 (the length direction Y of the first electrode sheet 21 when the first electrode sheet 21 is unfolded), the total width of the multiple weld marks 203 is W, satisfying 1mm≤W≤3mm, so that the area occupied by the multiple weld marks 203 is not too small, which is beneficial to increase the total area of ​​the multiple welding areas 211a, and is beneficial to improve the connection strength between the first electrode tab 30 and the first electrode sheet 21. The welding strength of some weld marks 203 is allowed to be reduced to improve the structural strength of the first electrode sheet 21, which is beneficial to achieve that the welding tension at the weld marks 203 in different welding areas 211a gradually decreases along the winding direction X' from the outside to the inside of the electrode assembly 20, thereby helping to reduce the risk of damage to the electrode sheet.

[0131] In some embodiments, referring to FIG. 4 , the maximum distance between the plurality of weld marks 203 and the first edge is B=A+W.

[0132] In one or more of the above embodiments, please refer to Figures 3 and 4. Along the winding direction X' of the electrode assembly 20 from the outside to the inside, the first current collector 211 includes an empty foil segment 2113 and a coated segment 2112 arranged in sequence, the coated segment 2112 is provided with a first active material layer 212, the empty foil segment 2113 is not provided with the first active material layer 212, at least a portion of the empty foil segment 2113 is located at the outermost circle of the first electrode sheet 21, and the first connection area 2111 is located at the empty foil segment 2113 and at the outermost circle of the first electrode sheet 21.

[0133] The inventors have discovered that during cycling of a wound electrode assembly 20, lithium intercalation and deintercalation in the first electrode sheet 21 causes repeated expansion and expansion of the thickness of the first electrode sheet 21, resulting in repeated contraction and expansion of the electrode assembly 20. The outermost first electrode sheet 21 is subject to the greatest tensile force. In this embodiment, among the multiple weld zones 211a, the weld mark 203 in the weld zone 211a closest to the winding center of the electrode assembly 20 along the winding direction X of the electrode assembly 20 has the lowest weld strength. This reduces damage to the first current collector 211 in the weld zone 211a closest to the winding center of the electrode assembly 20. This improves the tensile strength of the weld mark 203 in the weld zone 211a closest to the winding center of the electrode assembly 20, thereby reducing the risk of electrode sheet damage.

[0134] In some embodiments, referring to FIG. 4 , FIG. 6 and FIG. 7 , the shape of the weld mark 203 includes at least one of a diamond, a square and a circle.

[0135] In some embodiments, the connection method between the second pole tab 40 and the second pole piece 22 adopts a connection method that is basically the same as the connection method between the first pole tab 30 and the first pole piece 21. The specific implementation method of the connection between the second pole tab 40 and the second pole piece 22 can refer to the embodiment of the connection between the first pole tab 30 and the first pole piece 21, and will not be repeated here.

[0136] Referring to FIG8 , an embodiment of the present application further provides an electronic device 1000, which includes the secondary battery 100 of any of the aforementioned embodiments. Because the electronic device 1000 employs the technical solution of the secondary battery 100 of any of the aforementioned embodiments, it at least has the beneficial effects brought about by the technical solution of the secondary battery 100 of any of the aforementioned embodiments, which will not be further elaborated here.

[0137] In some embodiments, referring to FIG. 8 , the electronic device 1000 further includes a device body 200 , and the secondary battery 100 is installed in the device body 200 .

[0138] In some embodiments, the electronic device 1000 may be a mobile phone, a tablet computer, an e-reader, AR glasses, VR glasses, etc., which are not listed here one by one.

[0139] In addition, those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present application and are not intended to limit the present application. As long as they are within the substantive scope of the present application, appropriate changes and modifications to the above embodiments are within the scope disclosed in the present application.

Claims

1. A secondary battery, characterized in that: include: An electrode assembly, the electrode assembly comprising a first electrode sheet, a second electrode sheet, and a separator disposed between the first electrode sheet and the second electrode sheet, the first electrode sheet, the second electrode sheet, and the separator being stacked and wound to form a wound structure; the first electrode sheet comprising a first current collector and a first active material layer disposed on the first current collector, the first current collector being provided with a first connection region, the first connection region being free of the first active material layer; A first electrode tab, wherein the first electrode tab is welded to the first connection area, the first connection area includes a plurality of welding areas, the plurality of welding areas are arranged along the winding direction of the electrode assembly, and at least one weld mark formed by welding the first electrode tab to the first connection area is provided in the welding area, and along the winding direction of the electrode assembly, the welding strength of the weld mark in the welding area closest to the winding center of the electrode assembly among the plurality of welding areas is the smallest.

2. The secondary battery according to claim 1, wherein Along the winding direction of the electrode assembly from outside to inside, the welding strength of the weld marks in different welding areas gradually decreases.

3. The secondary battery according to claim 2, wherein There is welding tension between the first electrode tab and the first current collector at each welding mark, and the welding tension at the welding marks in different welding areas gradually decreases along the winding direction of the electrode assembly from outside to inside.

4. The secondary battery according to claim 3, wherein The welding tension between the first electrode tab and the first current collector at each weld mark is F, and satisfies 1N≤F≤80N.

5. The secondary battery according to claim 4, wherein The number of the welding zones is N. Along the winding direction of the electrode assembly, the i-th welding zone is closer to the winding center of the electrode assembly than the i+1-th welding zone. The welding tension of the weld mark of the i-th welding zone is F. i The welding tension of the weld mark of the i+1th welding zone is F i+1 , satisfying 1N≤F i+1 -F i ≤5N; N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N.

6. The secondary battery according to claim 4, characterized in that The number of the weld marks is S, and the average welding tension of the S weld marks is K, which satisfies 5N≤K≤9N; S is a positive integer greater than or equal to 2.

7. The secondary battery according to claim 2, characterized in that Along the winding direction of the electrode assembly from outside to inside, the heights of the weld marks in different welding areas in the thickness direction of the first electrode sheet gradually decrease.

8. The secondary battery according to claim 7, wherein: The height of the weld mark in the thickness direction of the first pole piece is H, which satisfies 1um≤H≤100um.

9. The secondary battery according to claim 8, characterized in that The number of the welding areas is N. Along the winding direction of the electrode assembly, the i-th welding area is closer to the winding center of the electrode assembly than the i+1-th welding area. The height of the weld mark of the i-th welding area in the thickness direction of the first electrode sheet is H. i The height of the weld mark of the i+1th welding area in the thickness direction of the first electrode is H i+1 , satisfying 1um≤H i+1 -H i ≤10um; N is a positive integer greater than or equal to 2, and i is a positive integer greater than or equal to 1 and less than N.

10. The secondary battery according to claim 8, wherein The number of the weld marks is S, the average height of the S weld marks is L, and the value satisfies 5 μm ≤ L ≤ 15 μm; S is a positive integer greater than or equal to 2.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: The edge of the first electrode tab on the side close to the winding center of the electrode assembly in the winding direction from the outside to the inside of the electrode assembly is defined as the first edge, the minimum distance between the weld marks in the multiple welding areas and the first edge is A, the total width of the multiple weld marks is W, and the maximum distance between the multiple weld marks and the first edge is B=A+W, and the secondary battery satisfies at least one of the following conditions (1) to (3): (1) 0.1mm≤A≤1mm; (2) 1mm≤W≤3mm; (3)1.1mm≤B≤4mm.

12. The secondary battery according to any one of claims 1 to 10, characterized in that: Along the winding direction of the electrode assembly from the outside to the inside, the first current collector includes an empty foil segment and a coated segment arranged in sequence, the coated segment is provided with the first active material layer, and the empty foil segment is not provided with the first active material layer. At least part of the empty foil segment is located at the outermost circle of the first pole piece, and the first connection area is located in the empty foil segment and at the outermost circle of the first pole piece.

13. The secondary battery according to any one of claims 1 to 10, characterized in that: The shape of the weld mark includes at least one of a diamond, a square, and a circle.

14. An electronic device, characterized in that: The secondary battery according to any one of claims 1 to 13 is included.

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