Secondary battery and electric device

Abstract

The invention discloses a secondary battery and electric equipment. The secondary battery comprises an electrode assembly and a plurality of tabs, the electrode assembly comprises a plurality of pole pieces and an isolating membrane, and each pole piece is connected with one tab. And the plurality of pole pieces are composed of two outer-layer pole pieces and a plurality of inner-layer pole pieces. One of the two outer-layer pole pieces is a first pole piece, and the pole lug connected with the first pole piece is a first pole lug; the plurality of inner-layer pole pieces comprise a second pole piece, the pole lug connected with the second pole piece is a second pole lug, and the polarity of the first pole piece is the same as that of the second pole piece; and the resistance value of the second tab is greater than or equal to that of any other tab connected with the inner-layer pole piece. The first tab comprises a first part and a second part, the first part is connected with the first pole piece and the second part, the resistivity of the first part is rho1, the resistivity of the second part is rho2, rho1 is smaller than rho2, the resistance value of the first tab is R1, the resistance value of the second tab is R2, and 1.01 R2 is smaller than or equal to R1. According to the secondary battery, the current density of the first pole piece can be reduced.

Description

Technical Field

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

[0002] The outermost electrode sheet of the stacked electrode assembly in a secondary battery is usually coated with an active material layer only on one side of the current collector, while the inner electrode sheet is coated with an active material layer on both sides of the current collector to fully utilize the active material layer, reduce material costs, and increase the energy density of the battery. Summary of the invention

[0003] For the secondary battery in the prior art, the inventors found that because the outermost electrode sheet is coated with a single layer, while the inner electrode sheet is coated on both sides, and the ratio of various materials in the active material layer of the inner and outer electrode sheets is generally the same, the conductivity of the outermost electrode sheet of the electrode assembly is greater than that of the inner electrode sheet, and the current density of the outermost electrode sheet is significantly higher than that of the inner electrode sheet. During the cycle of the secondary battery, the electrolyte at the outermost electrode sheet is consumed faster, and lithium deposition is easy to occur. Specifically, when the outermost electrode sheet is a positive electrode sheet, lithium deposition is easy to occur on the negative electrode sheet adjacent to it. When the outermost electrode sheet is a negative electrode sheet, lithium deposition is easy to occur on the outermost electrode sheet. Lithium deposition will lead to a decrease in the cycle performance of the secondary battery and a decrease in safety performance.

[0004] In view of the above situation, it is necessary to provide a secondary battery that can improve the uniformity of current distribution between pole pieces of the same polarity in an electrode assembly to reduce the possibility of black spots and lithium deposition.

[0005] The first aspect of an embodiment of the present application provides a secondary battery, which includes an electrode assembly and a plurality of pole tabs. The electrode assembly includes a plurality of pole sheets and a separator, wherein the plurality of pole sheets are stacked along a first direction, the polarities of two adjacent pole sheets are opposite, and a separator is provided between two adjacent pole sheets; the first direction is the thickness direction of the pole sheets, and each pole sheet is connected to at least one pole tab. The plurality of pole sheets are composed of two outer pole sheets and a plurality of inner pole sheets, wherein the outer pole sheets are the pole sheets located at the outermost layer of the electrode assembly, and the inner pole sheets are located at the inner layer of the electrode assembly; the pole sheets include a current collector and an active material layer, wherein the active material layer of the outer pole sheet is provided on a surface of the current collector along the first direction, and the active material layer faces the interior of the electrode assembly; the active material layer of the inner pole sheet is provided on both surfaces of the current collector along the thickness direction. One of the two outer electrode sheets is a first electrode sheet, and the electrode ear connected to the first electrode sheet is the first electrode ear; the plurality of inner electrode sheets include a second electrode sheet, and the electrode ear connected to the second electrode sheet is the second electrode ear, and the polarity of the first electrode sheet and the second electrode sheet is the same; the resistance value of the second electrode ear is greater than or equal to the resistance value of any other electrode ear connected to the inner electrode sheet. The first electrode ear includes a first part and a second part, and the first part connects the first electrode sheet and the second part. The resistivity of the first part is ρ1, and the resistivity of the second part is ρ2, ρ1<ρ2, the resistance value of the first electrode ear is R1, and the resistance value of the second electrode ear is R2, 1.01R2≤R1.

[0006] In this secondary battery, the first pole ear includes a first part and a second part, and the resistivity of the second part is greater than the resistivity of the first part. Compared with a pole ear of the same specification made of the same material as the first part, the resistance value of the first pole ear can be increased, thereby increasing the total resistance value of the first pole ear and the first pole sheet; on this basis, 1.01R2≤R1 is set so that the resistance value of the first pole ear is higher than the resistance value of the second pole ear, which is beneficial to reducing the current density of the first pole sheet, reducing the electrolyte consumption rate at the first pole sheet, and reducing lithium plating.

[0007] In an optional embodiment of the present application, R1≤5R2. When R1≤5R2 is set, the resistance of the first pole piece is not too large, which is beneficial to increase the capacity that can be released by the first pole piece, improve the energy density of the secondary battery, and reduce the possibility of excessive local temperature in the secondary battery, thereby improving the safety of the secondary battery.

[0008] In an optional embodiment of the present application, 1.2R2≤R1≤3R2. Setting R1≥1.2R2 is conducive to further reducing the current density of the first pole piece, reducing the electrolyte consumption rate at the first pole piece, and reducing lithium precipitation; setting R1≤3R2 is conducive to further increasing the capacity that can be released by the first pole piece and improving the energy density of the secondary battery.

[0009] In an optional embodiment of the present application, 0.1mΩ≤R1≤1mΩ. When R1≥0.1mΩ is set, the resistance value of the first pole ear is not too small, which is conducive to increasing the total resistance value of the first pole piece and the first pole ear, so as to reduce the current density on the first pole piece, reduce the possibility of black spots and lithium precipitation, and help to reduce the difficulty of material selection for the first part and the second part; when R1≤1mΩ is set, the resistance value of the first pole ear is not too large, which is conducive to improving the current carrying capacity of the first pole ear, so as to increase the capacity released by the secondary battery, and also help to reduce the possibility of local overtemperature in the secondary battery, and improve the safety of the secondary battery.

[0010] In an optional embodiment of the present application, 1.1ρ1≤ρ2≤5ρ1. When ρ2≥1.1ρ1, the resistivity ratio of the second part to the first part is not too small, which is beneficial to increase the resistance value of the first pole piece and reduce the current density of the first pole piece; when ρ2≤5ρ1, the resistivity ratio of the second part to the first part is not too large, which is beneficial to reduce the difficulty of material selection for the second part on the one hand, and on the other hand, it is beneficial to improve the current distribution uniformity of the pole piece with the same polarity as the first pole piece, and it is beneficial to improve the current carrying capacity of the first pole piece.

[0011] In an optional embodiment of the present application, 2ρ1≤ρ2≤3ρ1. When ρ2≥2ρ1 is set, the resistivity ratio of the second part to the first part is not too small, which is conducive to further increasing the resistance value of the first pole piece and reducing the current density of the first pole piece; when ρ2≤3ρ1 is set, the resistivity ratio of the second part to the first part is not too large, which is conducive to further reducing the difficulty of material selection for the second part on the one hand, and on the other hand, it is conducive to further improving the current distribution uniformity of the pole piece with the same polarity as the first pole piece, and is conducive to further improving the current carrying capacity of the first pole piece.

[0012] In an optional embodiment of the present application, the first pole ear is located on one side of the first pole piece along the second direction, and the second direction is perpendicular to the first direction. The first part is welded to the second part and forms a first weld mark; the secondary battery includes a transition piece, the transition piece welds the second part and forms a second weld mark; when the first pole ear is flattened, along the second direction, the maximum distance between the first weld mark and the second weld mark is L1, and the maximum distance from the first weld mark to the first pole piece is L2, 0.2≤L1 / L2≤0.6. By setting L1 / L2≥0.2, the length of the part of the second part 2232 for passing current is not too short, which is conducive to increasing the resistance of the first pole ear 223 to reduce the current density of the first pole piece 213; by setting L1 / L2≤0.6, the length of the first part is not too short compared to the second part, which is conducive to reducing the difficulty of welding the first part and the second part.

[0013] In an optional embodiment of the present application, the first pole ear is located on one side of the first pole piece along the second direction, and the second direction is perpendicular to the first direction. The cross-sectional area of ​​the first part along its thickness direction is S1, and the cross-sectional area of ​​the second part along its thickness direction is S2, 0.5S1≤S2≤2S1. Setting 0.5S1≤S2≤2S1, and keeping the ratio of S1 to S2 within an appropriate range is conducive to reducing the difference in current carrying capacity between the first part and the second part, and can also make the power and time required for welding the first part and the second part easier to control.

[0014] In an optional embodiment of the present application, the first pole ear is located on one side of the first pole piece along the second direction, and the second direction is perpendicular to the first direction. The first part is welded to the second part and forms a first weld mark, and the secondary battery includes an adapter, and the adapter welds the second part and forms a second weld mark. When the first pole ear is flattened, the distance from the first weld mark to the second weld mark along the second direction is D, and D ≥ 1mm. By setting D ≥ 1mm, the distance between the first weld mark and the second weld mark is not too small, which is conducive to reducing the difficulty of welding the first part and the second part and the second part and the adapter, and improving processing efficiency; it is also conducive to reducing the degree of accumulation of damage caused by welding at the same position of the first pole ear, improving the mechanical properties of the first pole ear, and reducing the possibility of damage and fracture of the first pole ear.

[0015] In an optional embodiment of the present application, the first electrode plate is a negative electrode plate, the material of the first part includes one of copper, nickel, copper-nickel alloy, nickel-plated copper and silver-plated copper, and the material of the second part includes one of nickel-chromium alloy, nickel-iron alloy, stainless steel, copper-beryllium alloy and copper-tin alloy.

[0016] In an optional embodiment of the present application, the first electrode plate is a positive electrode plate, the material of the first part includes one of aluminum, aluminum alloy, nickel-plated aluminum and silver-plated aluminum, and the material of the second part includes one of copper alloy, nickel-based alloy, stainless steel and titanium-based alloy.

[0017] A second aspect of the embodiments of the present application provides an electrical device, which includes a secondary battery as described in any of the above embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the structure of a secondary battery in one embodiment of the present application.

[0019] Figure 2 yes Figure 1 Schematic diagram of the cross-sectional structure at II-II.

[0020] Figure 3 yes Figure 1 Schematic diagram of the cross-sectional structure at III-III in the figure.

[0021] Figure 4 It is a schematic diagram of the connection structure of the first pole piece, the first pole ear and the adapter in a flattened state in one embodiment of the present application.

[0022] Figure 5 yes Figure 4 Schematic diagram of the cross-sectional structure at VV in the middle.

[0023] Figure 6 It is a schematic diagram of the structure of an electrical device in an embodiment of the present application.

[0024] Main component symbols 100. secondary battery; 10. housing; 20. Electrode assembly; 21. Pole piece; 211. Positive electrode piece; 2111. Positive electrode current collector; 2112. Positive electrode active material layer; 2113. Insulating layer; 212. Negative electrode piece; 2121. Negative electrode current collector; 2122. Negative electrode active material layer; 213. First pole piece; 214. Second pole piece; 22. Pole ear; 221. Positive electrode ear; 222. Negative electrode ear; 223. First pole ear; 2231. First part; 2232. Second part; 224. Second pole ear; 23. Isolating film; 24. Adapter; 241. Positive electrode adapter; 242. Negative electrode adapter; 24a. First weld mark; 24b. Second weld mark; 1000 electrical equipment; X, first direction; Y, second direction; Z, third direction.

[0025] The following specific implementation methods will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0026] 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 of the embodiments.

[0027] It should be noted that when an element is considered to be "connected" to another element, it may be directly connected to the other element or there may be a centrally arranged element at the same time. When an element is considered to be "set on" another element, it may be directly set on the other element or there may be a centrally arranged element at the same time. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0028] 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 belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0029] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0030] In the description of the embodiments of the present application, the term "perpendicular" is used to describe an ideal state between two components. In actual production or use, there may be a state that is approximately perpendicular between the two components. The two components described as "perpendicular" may not be absolutely straight lines or planes, but may be roughly straight lines or planes. From a macroscopic point of view, if the overall extension direction is a straight line or a plane, the components can be considered to be "straight lines" or "planes".

[0031] Reference to "embodiment" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. The various embodiments in the present application may be combined with each other in the absence of conflict.

[0032] An embodiment of the present application provides a secondary battery, which includes an electrode assembly and a plurality of pole tabs. The electrode assembly includes a plurality of pole sheets and a separator, wherein the plurality of pole sheets are stacked along a first direction, the polarities of two adjacent pole sheets are opposite, and a separator is provided between two adjacent pole sheets; the first direction is the thickness direction of the pole sheets, and each pole sheet is connected to at least one pole tab. The plurality of pole sheets are composed of two outer pole sheets and a plurality of inner pole sheets, wherein the outer pole sheets are the pole sheets located at the outermost layer of the electrode assembly, and the inner pole sheets are located at the inner layer of the electrode assembly; the pole sheets include a current collector and an active material layer, wherein for the outer pole sheets, the active material layer is provided on a surface of the current collector along the first direction, and the active material layer faces the interior of the electrode assembly; and for the inner pole sheets, the active material layer is provided on both surfaces of the current collector along the thickness direction. One of the two outer electrode sheets is a first electrode sheet, and the electrode ear connected to the first electrode sheet is the first electrode ear; the plurality of inner electrode sheets include a second electrode sheet, and the electrode ear connected to the second electrode sheet is the second electrode ear, and the polarity of the first electrode sheet and the second electrode sheet is the same; the resistance value of the second electrode ear is greater than or equal to the resistance value of any other electrode ear connected to the inner electrode sheet. The first electrode ear includes a first part and a second part, and the first part connects the first electrode sheet and the second part. The resistivity of the first part is ρ1, and the resistivity of the second part is ρ2, ρ1<ρ2, the resistance value of the first electrode ear is R1, and the resistance value of the second electrode ear is R2, 1.01R2≤R1.

[0033] In this secondary battery, the first pole ear includes a first part and a second part, and the resistivity of the second part is greater than the resistivity of the first part. Compared with a pole ear of the same specification made of the same material as the first part, the resistance value of the first pole ear can be increased. On this basis, 1.01R2≤R1≤5R2 is set so that the resistance value of the first pole ear is higher than the resistance value of the second pole ear, which is beneficial to reducing the current density of the first pole piece, reducing the electrolyte consumption rate at the first pole piece, and reducing lithium plating.

[0034] The embodiments of the present application are further described below in conjunction with the accompanying drawings. For the convenience of description, hereinafter, the thickness direction of the electrode sheet of the secondary battery is taken as the first direction X, the direction in which the electrode ear extends out of the electrode sheet is taken as the second direction Y, the first direction X is perpendicular to the second direction Y, and the direction perpendicular to the first direction X and the second direction Y is taken as the third direction Z, thereby establishing a rectangular coordinate system.

[0035] like Figure 1 and Figure 2 As shown, an embodiment of the present application provides a secondary battery 100 , which includes a housing 10 and an electrode assembly 20 , wherein the electrode assembly 20 is accommodated in the housing 10 .

[0036] In some embodiments, the housing 10 is a flexible packaging bag, such as an aluminum-plastic film. In other embodiments, the housing 10 is a hard shell, such as a plastic shell, or a metal shell including at least one of a steel alloy, an aluminum alloy, and a copper alloy.

[0037] In some embodiments, Figure 2 As shown, the electrode assembly 20 includes a plurality of pole pieces 21 and a separation film 23, wherein the plurality of pole pieces 21 are stacked along a first direction X, and the polarities of two adjacent pole pieces 21 are opposite, and a separation film 23 is disposed between two adjacent pole pieces 21. Specifically, according to the difference in polarity, the pole piece 21 is composed of a positive pole piece 211 and a negative pole piece 212; according to the relative positional relationship between the pole pieces 21, the plurality of pole pieces 21 are composed of two outer pole pieces and a plurality of inner pole pieces, wherein the outer pole piece is the pole piece located at the outermost layer of the electrode assembly 20, and the inner pole piece is located at the inner layer of the electrode assembly 20.

[0038] It should be noted that the determination of the "outermost layer" and the "inner layer" is determined by the relative positional relationship between the multiple pole pieces 21. In other words, the relative positional relationship between the isolation film 23 and the pole piece 21 is not considered. For example, the outer side of the outer layer pole piece may also be provided with an isolation film 23. The "inner layer" refers to the position between the two outer layer pole pieces.

[0039] In some embodiments, Figure 2 As shown, the electrode sheet 21 includes a current collector and an active material layer. For the outer electrode sheet, the active material layer is disposed on one surface of the current collector along the first direction X, and the active material layer faces the interior of the electrode assembly 20; for the inner electrode sheet, the active material layer is disposed on both surfaces of the current collector along the first direction X. According to the different polarities of the electrode sheet 21, the current collector is divided into a positive electrode current collector 2111 and a negative electrode current collector 2121, and the active material layer is divided into a positive electrode active material layer 2112 and a negative electrode active material layer 2122.

[0040] Specifically, Figure 2As shown, the positive electrode sheet 211 includes a positive electrode current collector 2111 and a positive electrode active material layer 2112, and the positive electrode active material layer 2112 is disposed on one or both sides of the positive electrode current collector 2111 along the first direction X. The negative electrode sheet 212 includes a negative electrode current collector 2121 and a negative electrode active material layer 2122, and the negative electrode active material layer 2122 is disposed on one or both sides of the negative electrode current collector 2121 along the first direction X. Furthermore, for the positive electrode sheet 211 in the inner electrode sheet, the positive electrode active material layer 2112 is arranged on both sides of the positive electrode collector 2111 along the first direction X, and for the negative electrode sheet 212 in the inner electrode sheet, the negative electrode collector 2121 is arranged on both sides of the negative electrode collector 2121 along the first direction X; for the positive electrode sheet 211 in the outer electrode sheet, the positive electrode active material layer 2112 is only arranged on one side of the positive electrode collector 2111 along the first direction X, and for the negative electrode sheet 212 in the outer electrode sheet, the negative electrode active material layer 2122 is only arranged on one side of the negative electrode collector 2121 along the first direction X.

[0041] In some embodiments, at least one of the positive electrode current collector 2111 and the negative electrode current collector 2121 is a metal layer. As an illustrative example, the positive electrode current collector 2111 may be a metal layer including at least one of aluminum, nickel, tantalum, and titanium, such as aluminum foil. The negative electrode current collector 2121 may be a metal layer including at least one of copper, nickel, tantalum, and titanium, such as copper foil.

[0042] In some embodiments, at least one of the positive electrode current collector 2111 and the negative electrode current collector 2121 is a composite current collector.

[0043] In some embodiments, the positive electrode active material layer 2112 includes a positive electrode active material, which includes 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.

[0044] In some embodiments, the negative electrode active material layer 2122 includes a negative electrode active material, and the negative electrode active material includes at least one of graphite, hard carbon, soft carbon, silicon, silicon-oxygen material, and silicon-carbon material.

[0045] In some embodiments, Figure 2 and Figure 3 As shown, the secondary battery 100 further includes a plurality of tabs 22, and each pole piece 21 is connected to a tab 22. Specifically, according to the polarity of the connected pole piece 21, the tabs 22 are divided into positive tabs 221 and negative tabs 222, wherein the positive tabs 221 are connected to the positive pole piece 211, and the negative tabs 222 are connected to the negative pole piece 212.

[0046] In some embodiments, Figure 2 and Figure 3As shown, the secondary battery 100 also includes an adapter, which includes a positive electrode adapter 241 and a negative electrode adapter 242. All the positive electrode ears 221 are stacked and welded to form a positive electrode ear 221 bundle. The positive electrode adapter 241 connects the positive electrode ear 221 bundle. All the negative electrode ears 222 are stacked and welded to form a negative electrode ear 222 bundle. The negative electrode adapter 242 connects the negative electrode ear 222 bundle.

[0047] In some embodiments, the isolation film 23 is made of insulating film materials such as polyethylene film, polypropylene film, polyester film or polyimide film.

[0048] In some embodiments, the secondary battery 100 includes an electrolyte (not shown), which is contained in the housing 10 .

[0049] In some embodiments, the electrolyte includes an electrolyte salt. The electrolyte salt includes at least one of an organic lithium salt or an inorganic lithium salt.

[0050] In some embodiments, the electrolyte salt includes but is not limited to at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium hexafluorocesium oxide (LiCsF6), lithium perchlorate (LiClO4) or lithium trifluoromethanesulfonate (LiCF3SO3).

[0051] In some embodiments, Figure 2 As shown, the positive electrode sheet 211 also includes an insulating layer 2113, which is arranged at the edge of the positive electrode collector 2111 connected to the positive electrode ear 221 to reduce the possibility that the burrs on the edge of the positive electrode collector 2111 pierce the isolation film and cause a short circuit between the positive electrode sheet 211 and the negative electrode sheet 212.

[0052] In some embodiments, the material of the insulating layer 2113 includes a ceramic material and a binder.

[0053] In some embodiments, the ceramic material includes at least one of aluminum oxide, magnesium oxide, silicon nitride, silicon carbide, and boehmite.

[0054] In some embodiments, the composition of the binder includes polyvinylidene fluoride (PVDF).

[0055] In some embodiments, Figure 2 As shown, one of the two outer pole pieces is a first pole piece 213, the pole ear 22 connected to the first pole piece 213 is a first pole ear 223, the plurality of inner pole pieces include a second pole piece 214, the pole ear 22 connected to the second pole piece 214 is a second pole ear 224, and the first pole piece 213 and the second pole piece 214 have the same polarity. Figure 4 and Figure 5As shown, the first pole ear 223 includes a first part 2231 and a second part 2232, the first part 2231 connects the first pole piece 213 and the second part 2232, the resistivity of the first part 2231 is ρ1, the resistivity of the second part 2232 is ρ2, ρ1<ρ2; the resistance value of the first pole ear 223 is R1, the resistance value of the second pole ear 224 is R2, 1.01R2≤R1.

[0056] In this secondary battery 100, the first pole tab 223 includes a first part 2231 and a second part 2232, and the resistivity of the second part 2232 is greater than the resistivity of the first part 2231. Compared with the pole tab 22 of the same specification made of the same material as the first part 2231, the resistance value of the first pole tab 223 can be increased. On this basis, 1.01R2≤R1 is set so that the resistance value of the first pole tab 223 is higher than the resistance value of the second pole tab 224, which is conducive to reducing the current density of the first pole sheet, reducing the electrolyte consumption rate at the first pole sheet 213, and reducing lithium precipitation. When the first pole sheet 213 is a positive pole sheet, this embodiment is conducive to reducing the lithium precipitation of the negative pole sheet adjacent to the first pole sheet 213, and when the first pole sheet is a negative pole sheet, this embodiment is conducive to reducing the lithium precipitation of the first pole sheet 213.

[0057] In some embodiments, R1≤5R2. As the resistance of the first pole piece 223 increases, the current density of the first pole piece 213 decreases, thereby reducing the released capacity; and, if the resistance of the first pole piece 223 is too large, it may cause the local temperature of the secondary battery 100 to be too high, thereby reducing the safety of the secondary battery 100. Therefore, by setting R1≤5R2, the resistance of the first pole piece 223 is not too large, which is conducive to increasing the capacity that can be released by the first pole piece 213, improving the energy density of the secondary battery 100, and reducing the possibility of the local temperature of the secondary battery 100 being too high, thereby improving the safety of the secondary battery 100.

[0058] In the embodiment of the present application, the resistivity of the first portion 2231 and the second portion 2232 can be measured by the following method: 1) Take a sample of standard size from the first portion 2231 and the second portion 2232 of the first tab 223 respectively. The sample size can be determined as required. For example, take a sample of 5 mm in length, 3 mm in width, and 10 um in thickness; 2) Place the sample in a measuring device (e.g., a multimeter) to measure the resistance value; 3) According to the measured resistance value and the size of the sample, the resistivity of the sample is calculated using a resistance formula, thereby obtaining the resistivity of the first portion 2231 and the second portion 2232. The resistance formula is: ρ=RS / L, where ρ represents the resistivity, R is the measured resistance value, S is the cross-sectional area of ​​the sample, and L is the length of the sample.

[0059] The resistance value of the first electrode tab 223 can be measured by the following method: According to the flow direction of the current on the first pole ear 223 when the secondary battery 100 is working, one end of the first pole ear 223 is electrically connected to an electrode of a measuring device (such as a multimeter), and the other end of the first pole ear 223 is connected to another electrode of the measuring device, and the resistance value of the first pole ear 223 is obtained according to the measurement value given by the measuring device.

[0060] In the case where the first pole lug 223 is welded to the adapter to form the second weld mark 24b, since the first pole lug 223 is electrically connected to other pole lugs 22 of the same polarity, the first pole lug 223 needs to be separated from the other pole lugs 22 of the same polarity. At this time, the first pole lug 223 can be cut off from the connection position between the first pole lug 223 and the first pole piece 213 to separate the first pole lug 223 from the first pole piece 213, and then the first pole lug 223 can be cut off from the edge of the second weld mark 24b to separate the first pole lug 223 from the other pole lugs 22 of the same polarity, and the first pole lug 223 is electrically connected to a measuring device (such as a multimeter), so as to measure the resistance value of the first pole lug 223.

[0061] The resistance value of the second electrode tab 224 can be obtained in the following manner: According to the flow direction of the current on the second pole ear 224 when the secondary battery 100 is working, one end of the second pole ear 224 is electrically connected to an electrode of a measuring device (such as a multimeter), and the other end of the second pole ear 224 is connected to another electrode of the measuring device, and the resistance value of the first pole ear 223 is obtained according to the measurement value given by the measuring device.

[0062] In the case where the second pole lug 224 is welded to the adapter to form the second weld mark 24b, since the second pole lug 224 is electrically connected to other pole lugs 22 of the same polarity, the second pole lug 224 needs to be separated from the other pole lugs 22 of the same polarity. At this time, the second pole lug 224 can be cut off from the connection position between the second pole lug 224 and the second pole piece 214 to separate the second pole lug 224 from the second pole piece 214, and then the second pole lug 224 can be cut off from the edge of the second weld mark 24b to separate the second pole lug 224 from the other pole lugs 22 of the same polarity, and the second pole lug 224 is electrically connected to a measuring device (such as a multimeter) to measure the resistance value of the second pole lug 224.

[0063] It should be added that, under normal circumstances, the thickness of the pole ear 22 connected to the inner pole piece along the first direction X and the width along the third direction are basically consistent (ignoring the tolerance during manufacturing). At this time, in order to simplify the testing process, the pole ear 22 connected to the inner pole piece can be flattened, and the length of the pole ear 22 connected to the inner pole piece can be compared, and the longest one can be determined as the second pole ear 224, and the resistance value can be further measured.

[0064] In some embodiments, the first portion 2231 is welded to the second portion 2232. Welding methods include, but are not limited to, ultrasonic welding and laser welding.

[0065] In some embodiments, the first portion 2231 and the second portion 2232 are bonded using conductive adhesive.

[0066] In some embodiments, 1.2R2≤R1≤3R2. Setting R1≥1.2R2 is conducive to further reducing the current density of the first pole piece 213, reducing the electrolyte consumption rate at the first pole piece 213, and reducing lithium deposition; setting R1≤3R2 is conducive to further increasing the capacity that can be released by the first pole piece 213 and improving the energy density of the secondary battery 100.

[0067] In some embodiments, 0.1mΩ≤R1≤1mΩ. When R1≥0.1mΩ is set, the resistance value of the first pole ear 223 is not too small, which is conducive to increasing the total resistance value of the first pole piece 213 and the first pole ear 223, so as to reduce the current density on the first pole piece 213, reduce the possibility of black spots and lithium precipitation, and help to reduce the difficulty of selecting materials for the first part 2231 and the second part 2232; when R1≤1mΩ is set, the resistance value of the first pole ear 223 is not too large, which is conducive to improving the current carrying capacity of the first pole ear 223, so as to increase the capacity released by the secondary battery 100, and also help to reduce the possibility of excessive local temperature of the secondary battery 100, and improve the safety of the secondary battery 100.

[0068] In some embodiments, 1.1ρ1≤ρ2≤5ρ1. When ρ2≥1.1ρ1, the resistivity ratio of the second part 2232 to the first part 2231 is not too small, which is beneficial to increase the resistance value of the first pole piece 223 and reduce the current density of the first pole piece 213; when ρ2≤5ρ1, the resistivity ratio of the second part 2232 to the first part 2231 is not too large, which is beneficial to reduce the difficulty of material selection for the second part 2232 on the one hand, and is beneficial to improve the current distribution uniformity of the pole piece 21 with the same polarity as the first pole piece 213 on the other hand, and is beneficial to improve the current carrying capacity of the first pole piece 223.

[0069] In some embodiments, 2ρ1≤ρ2≤3ρ1. When ρ2≥2ρ1 is set, the resistivity ratio of the second part 2232 to the first part 2231 is not too small, which is conducive to further improving the resistance value of the first pole piece 223 and reducing the current density of the first pole piece 213; when ρ2≤3ρ1 is set, the resistivity ratio of the second part 2232 to the first part 2231 is not too large, which is conducive to further reducing the difficulty of material selection for the second part 2232 on the one hand, and on the other hand, it is conducive to further improving the current distribution uniformity of the pole piece 21 with the same polarity as the first pole piece 213, and is conducive to further improving the current carrying capacity of the first pole piece 223.

[0070] In some embodiments, Figure 4 and Figure 5 As shown, the first pole ear 223 is located on one side of the first pole piece 213 along the second direction Y; the first portion 2231 is welded to the second portion 2232 to form a first weld mark 24a; the secondary battery 100 includes a transition piece 24, and the transition piece 24 is welded to the second portion 2232 to form a second weld mark 24b. In the embodiment of the present application, the weld mark (the first weld mark 24a and the second weld mark 24b) refers to the area where the weld point formed during the welding process is located. Figure 4 The dotted frame that contains the solder joint represents the solder mark.

[0071] In some embodiments, Figure 4 As shown, in the state where the first pole ear 223 is flattened, along the second direction Y, the maximum distance between the first weld mark 24a and the second weld mark 24b is L1, and the maximum distance from the first weld mark 24a to the first pole piece 213 is L2, 0.2≤L1 / L2≤0.6. When L1 / L2≥0.2, the length of the portion of the second part 2232 for passing current is not too short, which is beneficial to increase the resistance value of the first pole ear 223 to reduce the current density of the first pole piece 213; when L1 / L2≤0.6, the length of the first part 2231 is not too short compared to the second part 2232, which is beneficial to reduce the difficulty of welding the first part 2231 and the second part 2232. Among them, the maximum distance between the first weld mark 24a and the second weld mark 24b is the maximum distance between the welding points in the first weld mark 24a and the second weld mark 24b. When measuring the distance between two welding points, make tangents of the two welding points along the third direction Z respectively, and both welding points are located between the two tangents, and then measure the distance between the two welding points. The maximum distance from the first welding mark 24a to the first pole piece 213 is the maximum distance from the welding point in the first welding mark 24a to the first pole piece 213. When measuring, select the welding point farthest from the first pole piece 213 in the first welding mark 24a, make a tangent of this welding point along the third direction Z, and this welding point is closer to the first pole piece 213 than the tangent, and then measure the distance from the tangent to the first pole piece 213.

[0072] In some embodiments, Figure 4As shown, when the first pole ear 223 is flattened, the distance from the first weld mark 24a to the second weld mark 24b along the second direction Y is D, and D≥1mm. By setting D≥1mm, the distance between the first weld mark 24a and the second weld mark 24b is not too small, which is conducive to reducing the difficulty of welding the first part 2231 and the second part 2232 and the second part 2232 and the adapter, and improving the processing efficiency; it is also conducive to reducing the degree of accumulation of damage caused by welding at the same position of the first pole ear 223, improving the mechanical properties of the first pole ear 223, and reducing the possibility of damage and fracture of the first pole ear 223. Among them, the adapter 24 refers to one of the positive adapter 241 and the negative adapter 242. When the first pole piece 213 is a positive pole piece, the adapter 24 is the positive pole adapter 241, and when the first pole piece 213 is a negative pole piece, the adapter 24 is the negative pole adapter 242. The distance between the first weld mark 24 a and the second weld mark 24 b is the distance along the second direction Y between two closest weld points in the first weld mark 24 a and the second weld mark 24 b.

[0073] In some embodiments, the first pole ear 223 is located on one side of the first pole piece 213 along the second direction Y. The cross-sectional area of ​​the first portion 2231 along its thickness direction is S1, and the cross-sectional area of ​​the second portion 2232 along its thickness direction is S2, and 0.5S1≤S2≤2S1. Setting 0.5S1≤S2≤2S1 and keeping the ratio of S1 to S2 within an appropriate range is conducive to reducing the difference in current carrying capacity between the first portion 2231 and the second portion 2232, and can also make the power and time required for welding the first portion 2231 and the second portion 2232 easier to control.

[0074] In some embodiments, the first electrode plate 213 is a negative electrode plate 212, the material of the first part 2231 includes but is not limited to one of copper, nickel, copper-nickel alloy, nickel-plated copper and silver-plated copper, and the material of the second part 2232 includes but is not limited to one of nickel-chromium alloy, nickel-iron alloy, stainless steel, copper-beryllium alloy and copper-tin alloy.

[0075] In some embodiments, the first electrode plate 213 is the positive electrode plate 211, the material of the first part 2231 includes but is not limited to one of aluminum, aluminum alloy, nickel-plated aluminum and silver-plated aluminum, and the material of the second part 2232 includes but is not limited to one of copper alloy, nickel-based alloy, stainless steel and titanium-based alloy.

[0076] like Figure 6 As shown, an embodiment of the present application further provides an electric device 1000 , which includes the secondary battery 100 involved in any of the aforementioned embodiments.

[0077] In order to verify the influence of the scheme provided by the present application on the cycle performance of the secondary battery 100, the inventor of the present application conducted the following experiment, which includes 3 groups of comparative examples and 11 groups of embodiments, each group of comparative examples and each group of embodiments includes 21 secondary batteries 100, and the first pole piece 213 of the secondary battery 100 used in the comparative examples and the embodiments is a positive pole piece 211. It should be noted that in order to simplify the experimental process and improve the welding effect of the first part 2231 and the second part 2232 of the first pole ear 223, in this experiment, the flow area of ​​the first part 2231 and the second part 2232 of the first pole ear 223 is set to be the same, that is, the width and thickness of the first part 2231 and the second part 2232 are set to be the same, so that by adjusting the length of the first part 2231 and the second part 2232 and the material of the second part 2232, the resistance value of the first pole ear 223 can be adjusted.

[0078] The preparation process of the secondary battery 100 in Example 1 includes the following steps: (1) Preparation of positive electrode sheet 211: Active materials lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), CNT (carbon nanotube), polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97.5:0.5:0.5:1.5, N-methylpyrrolidone (NMP) is added as a solvent, and a positive electrode active material with a solid content of 75wt% is prepared, and stirred evenly for use. A 0.1mm thick aluminum foil is used as the positive electrode current collector 2111. The above active material is evenly coated on a surface of the positive electrode current collector 2111 along the first direction X using a slit coater, and then dried at 90°C to obtain a positive electrode sheet 211 coated with a positive electrode active material layer 2112 on one side. Two such positive electrode sheets 211 are prepared as the two outermost layers of the electrode assembly 20. Repeat the above coating steps on the other side of the positive electrode current collector 2111 along the first direction X to obtain a positive electrode sheet 211 coated with a positive electrode active material layer 2112 on both sides. Then the coated positive electrode sheet 211 is cold pressed. The area of ​​the positive electrode current collector 2111 not covered by the positive electrode active material layer 2112 is an empty foil area, and the empty foil area is die-cut to obtain a positive electrode ear 221. For the positive electrode sheet 211 coated with a positive electrode active material layer 2112 on one side, a part of the positive electrode ear 221 is cut off, and the remaining part is the first part 2231 described above. A copper alloy foil is welded on the first part 2231 as the second part 2232, thereby obtaining the first electrode ear 223 described above. The length of the positive electrode ear 221 after welding the copper alloy foil is equal to the length of the electrode ear 22 before cutting. The welding point of the first part 2231 and the second part 2232 forms a first weld mark 24a. One of the two positive electrode sheets 211 coated with a positive electrode active material layer 2112 on one side is determined as the first electrode sheet 213, and the positive electrode ear 221 connected thereto is the first electrode ear 223. The two electrodes of the multimeter are electrically connected to the first electrode ear 223, one of the electrodes of the multimeter is connected to the connection between the first electrode ear 223 and the positive electrode current collector 2111, and the other electrode of the multimeter is moved at different positions of the first electrode ear 223, and the moving direction is the length direction of the first electrode ear 223, so that different lengths of the first electrode ear 223 are electrically connected to the multimeter. When the reading of the multimeter is the same as the resistance value required to be controlled for the first electrode ear 223, the position of the multimeter is marked as a reference point. When welding the positive electrode adapter 241 and the first electrode ear 223, the edge of the second weld mark 24b coincides with the reference point, so that the resistance value provided by the first electrode ear 223 when the secondary battery 100 is used is the value required to be controlled in the experiment. The positive electrode adapter 241 is welded to the positive electrode ear 221 in the subsequent steps.

[0079] (2) Preparation of negative electrode sheet 212: Active materials artificial graphite, conductive carbon black (Super P), styrene-butadiene rubber (SBR), and CMC (sodium carboxymethyl cellulose) are mixed in a weight ratio of 97:0.5:1.3:1.2, and deionized water is added as a solvent to prepare a negative electrode active material material with a weight percentage of 50wt%, and stirred evenly for use. A copper foil with a thickness of 0.1mm is used as a negative electrode current collector 2121. The above negative electrode active material is evenly coated on one surface of the negative electrode current collector 2121 along the first direction X using a slit coater, and then dried at 110°C to obtain a negative electrode sheet 212 coated with a negative electrode active material layer 2122 on one side. The above steps are repeated on the other surface of the negative electrode current collector 2121 along the thickness direction to obtain a negative electrode sheet 212 coated with a negative electrode active material layer 2122 on both sides. Then the coated negative electrode sheet 212 is cold pressed. The area of ​​the negative electrode current collector 2121 that is not covered by the negative electrode active material layer 2122 is an empty foil area, and the empty foil area is die-cut to obtain the negative electrode ear 222 .

[0080] (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 well to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0081] (4) Preparation of isolation membrane 23: A polyethylene porous polymer film is used as the isolation membrane 23.

[0082] (5) Preparation of electrode assembly 20: The positive electrode sheet 211, the separator 23 and the negative electrode sheet 212 are stacked along the first direction X to obtain the electrode assembly 20. The two outermost electrode sheets 21 of the electrode assembly 20 are positive electrode sheets 211 coated with a positive electrode active material layer 2112 on one side. The positive electrode ears 221 are stacked to form a positive electrode ear bundle, and the positive electrode adapter 241 is welded. The welding point between the positive electrode ear bundle and the positive electrode adapter 241 forms a second weld mark 24b; the negative electrode ears 222 are stacked to form a negative electrode ear bundle, and the negative electrode adapter 242 is connected.

[0083] (6) Assembly of secondary battery 100: Place the aluminum-plastic film with a hole formed in the assembly fixture, with the hole facing upward, place the electrode assembly 20 in the hole, and apply external force to press it. Then, cover the electrode assembly 20 with another aluminum-plastic film with a hole formed in the hole facing downward, and heat-seal the three edges of the two aluminum-plastic films by hot pressing. The unsealed edge is the side where the positive electrode adapter 241 and the negative electrode adapter 242 extend out of the shell 10. Then, inject electrolyte through the unsealed edge, and after vacuum packaging, standing, hot pressing, shaping and other processes, the secondary battery 100 is obtained. The obtained secondary battery has a length of 80 mm, a width of 50 mm, and a thickness of 5 mm.

[0084] The preparation method of the secondary battery 100 in Comparative Examples 1 to 3 is basically the same as that in Example 1, except that the positive electrode ear 221 of the outermost positive electrode sheet 211 of the secondary battery 100 in Comparative Examples 1 to 3 is not processed, that is, the positive electrode ear 221 in Comparative Examples 1 to 3 is made of the same single material as the positive electrode current collector 2111. In Comparative Examples 1 to 3, the lengths of the outermost positive electrode ear 221 of different experimental groups are different, so the resistance values ​​are also different, which are represented by R1 in Table 1.

[0085] The preparation method of the secondary battery 100 in Examples 2 to 11 is substantially the same as that in Example 1, and the difference is that the resistance value of the first pole tab 223 in Examples 2 to 11 is different from that in Example 1. It is understandable that, for those skilled in the art, the resistance value of the first pole tab 223 can be assisted in adjusting by replacing the material of the second portion 2232 and changing the length of the second portion 2232, so that the resistance value of the first pole tab 223 reaches the target value, that is, the value listed in the R1 column in Table 1.

[0086] After the secondary batteries 100 in the comparative example and the embodiment are prepared, for each experimental group, 20 secondary batteries 100 are randomly selected to perform a long cycle test, and the remaining one secondary battery 100 is tested for capacity retention rate.

[0087] (I) The specific process of the long cycle test is as follows: 1) Maintain the test temperature at 25°C; 2) The secondary battery 100 was left to stand for 30 min; 3) 5C constant current charging to 4.25V, then constant voltage charging to 3C; 4) 3C constant current charging to 4.35V, then constant voltage charging to 1.5C; 5) 1.5C constant current charge to 4.45V, then constant voltage charge to 0.05C; 6) Let stand for 5 minutes; 7) 0.7C constant current discharge to 3V; 8) Let stand for 5 minutes; 9) Repeat steps 3 to 8 800 times; 10) The secondary batteries 100 are disassembled, lithium deposition is observed, and the number of secondary batteries 100 with lithium deposition on the electrode sheet 212 adjacent to the first electrode sheet 213 is counted.

[0088] (II) The specific process of capacity retention rate test is as follows: First, the initial capacity of the secondary battery in comparative example 3 is measured as a1, and then the initial capacity of the secondary batteries in other experimental groups is measured as a2. The capacity retention rate is (a1-a2) / a1. The results are expressed as percentages and recorded in Table 1.

[0089] The initial capacity test steps are as follows: 1) Maintain the test temperature at 25°C; 2) The secondary battery 100 was left to stand for 60 min; 3) 0.5C constant current discharge to 3V; 4) Let stand for 60 minutes; 5) 0.7C constant current charging to 4.50V, constant voltage charging to 0.025C; 6) Let stand for 10 minutes to obtain the initial capacity.

[0090] After the test, the experimental results are recorded in Table 1: Table 1 It can be seen from Table 1 that in Comparative Examples 1 to 3, the resistance value of the first pole ear 223 is less than the resistance value of the second pole ear 224, while in Examples 1 to 11, the resistance value of the first pole ear 223 of the secondary battery 100 is greater than the resistance value of the second pole ear 224. In Example 1, the outermost positive pole ear 221 of the secondary battery 100, compared with Comparative Examples 1 to 3, the number of lithium deposition phenomena on the adjacent pole pieces of the first pole piece 213 of the secondary battery in Examples 1 to 11 is significantly less than that in Comparative Examples 1 to 3. It can be seen that the first pole ear 223 is prepared by two materials, so that R1≥1.01R2, which is conducive to reducing the current density on the first pole piece 213, so as to reduce the possibility of lithium deposition on the adjacent pole piece of the first pole piece 213.

[0091] In Examples 1 to 10, the secondary battery 100 satisfies 1.01R2≤R1≤5R2, and the capacity retention rate of the secondary battery 100 in Examples 1 to 10 is higher than that in Example 11. It can be seen that by setting R1≤5R2, the resistance of the first pole lug 223 is not too large, which is conducive to increasing the capacity that can be released by the first pole piece 213 and improving the energy density of the secondary battery.

[0092] In Examples 2 to 6, the secondary battery satisfies 1.2R2≤R1≤3R2. Compared with Example 1, the number of lithium deposition phenomena on the adjacent pole pieces of the first pole piece 213 of the secondary battery 100 in Examples 2 to 6 is less, and the capacity retention rate of the secondary battery 100 in Examples 2 to 6 is higher than that in Examples 7 to 11. It can be seen that setting 1.2R2≤R1≤3R2 and setting the resistance ratio of the first pole piece 223 and the second pole piece 224 within an appropriate range is conducive to reducing the possibility of lithium deposition on the adjacent pole piece of the first pole piece 213 while taking into account the energy density of the secondary battery 100.

[0093] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present application and are not intended to be limiting of the present application. As long as they are within the spirit and scope of the present application, appropriate changes and modifications to the above embodiments are within the scope of the present application.

Claims

1. A secondary battery, characterized in that: include: An electrode assembly, the electrode assembly comprising a plurality of pole pieces and a separation film, the plurality of pole pieces are stacked along a first direction, two adjacent pole pieces have opposite polarities, and the separation film is disposed between two adjacent pole pieces; the first direction is a thickness direction of the pole pieces; A plurality of pole lugs, each of the pole pieces is connected to at least one of the pole lugs; The plurality of electrode sheets are composed of two outer electrode sheets and a plurality of inner electrode sheets, wherein the outer electrode sheets are the electrode sheets located at the outermost layer of the electrode assembly, and the inner electrode sheets are located at the inner layer of the electrode assembly; the electrode sheets include a current collector and an active material layer, wherein the active material layer of the outer electrode sheet is disposed on a surface of the current collector along the first direction, and the active material layer faces the interior of the electrode assembly; and the active material layer of the inner electrode sheet is disposed on both surfaces of the current collector along the thickness direction; One of the two outer electrode sheets is a first electrode sheet, and the electrode ear connected to the first electrode sheet is a first electrode ear; the plurality of inner electrode sheets include a second electrode sheet, and the electrode ear connected to the second electrode sheet is a second electrode ear, and the first electrode sheet and the second electrode sheet have the same polarity; the resistance value of the second electrode ear is greater than or equal to the resistance value of any other electrode ear connected to the inner electrode sheet; The first pole lug includes a first portion and a second portion, the first portion connects the first pole piece and the second portion, the resistivity of the first portion is ρ1, the resistivity of the second portion is ρ2, ρ1<ρ2; The resistance value of the first electrode tab is R1, the resistance value of the second electrode tab is R2, and 1.01R2≤R1.

2. The secondary battery according to claim 1, wherein: R1≤5R2.

3. The secondary battery according to claim 2, characterized in that: 1.2R2≤R1≤3R2.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: 0.1mΩ≤R1≤1mΩ.

5. The secondary battery according to claim 1, wherein: 1.1ρ1≤ρ2≤5ρ1.

6. The secondary battery according to claim 5, characterized in that: 2ρ1≤ρ2≤3ρ1.

7. The secondary battery according to claim 1, wherein: The first pole ear is located at one side of the first pole piece along a second direction, and the second direction is perpendicular to the first direction; The first part is welded to the second part to form a first weld mark; The secondary battery includes a transfer piece, and the transfer piece is welded to the second part to form a second weld mark; When the first pole tab is flattened, along the second direction, the maximum distance between the first weld mark and the second weld mark is L1, and the maximum distance from the first weld mark to the first pole piece is L2, and 0.2≤L1 / L2≤0.

6.

8. The secondary battery according to claim 1, wherein: The first pole ear is located at one side of the first pole piece along a second direction, and the second direction is perpendicular to the first direction; The cross-sectional area of ​​the first portion along the thickness direction thereof is S1, the cross-sectional area of ​​the second portion along the thickness direction thereof is S2, and 0.5S1≤S2≤2S1.

9. The secondary battery according to claim 1, wherein: The first pole ear is located at one side of the first pole piece along a second direction, and the second direction is perpendicular to the first direction; The first part is welded to the second part to form a first weld mark; The secondary battery includes a transfer piece, and the transfer piece is welded to the second part to form a second weld mark; When the first electrode tab is flattened, a distance from the first weld mark to the second weld mark along the second direction is D, where D≥1 mm.

10. The secondary battery according to claim 1, wherein: The first electrode sheet is a negative electrode sheet, the material of the first part includes one of copper, nickel, copper-nickel alloy, nickel-plated copper and silver-plated copper, and the material of the second part includes one of nickel-chromium alloy, nickel-iron alloy, stainless steel, copper-beryllium alloy and copper-tin alloy; or The first electrode plate is a positive electrode plate, the material of the first part includes one of aluminum, aluminum alloy, nickel-plated aluminum and silver-plated aluminum, and the material of the second part includes one of copper alloy, nickel-based alloy, stainless steel and titanium-based alloy.

11. An electrical device, characterized in that: The invention comprises the secondary battery as claimed in any one of claims 1 to 10.

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