Secondary battery and electronic device

Abstract

The application relates to a secondary battery and an electronic device. The secondary battery comprises an electrode assembly, the electrode assembly comprises a plurality of positive electrode sheets and a plurality of negative electrode sheets which are alternately and sequentially arranged along a first direction, and a separator is arranged between adjacent positive electrode sheets and negative electrode sheets. The positive electrode sheet comprises a first positive electrode sheet and a second positive electrode sheet, along the first direction, the first positive electrode sheet is the outermost sheet of the electrode assembly, and the second positive electrode sheet is arranged between two adjacent negative electrode sheets. The first positive electrode sheet comprises a first current collector and a first positive electrode coating arranged on the surface of the first current collector facing the negative electrode sheet. The second positive electrode sheet comprises a second current collector and a second positive electrode coating arranged on the two surfaces of the second current collector. The first current collector comprises a first conductive layer, along the first direction, the thickness of the first conductive layer is T1, the second current collector comprises a second conductive layer, the thickness of the second conductive layer is T2, and T1 <= T2. The lithium precipitation risk of the secondary battery can be reduced, and the fast-charging performance of the secondary battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to a secondary battery and an electronic device. BACKGROUND

[0002] The electrode assembly of a lithium ion battery (secondary battery) is usually made by a stacking process or a winding process. The lithium ion battery product made by the stacking process has the characteristics of high energy density, more stable internal structure and higher safety.

[0003] The lithium ion battery usually includes single-sided positive electrode sheets with the outermost single-sided coating and double-sided positive electrode sheets with the inner double-sided coating. However, the delithiation rate of the single-sided positive electrode sheet and the lithium intercalation rate of the corresponding negative electrode sheet may not match, and part of the lithium ions may be difficult to intercalate into the corresponding negative electrode sheet of the single-sided positive electrode sheet in time, which is prone to cause lithium precipitation. SUMMARY

[0004] Embodiments of the present application aim to provide a secondary battery and an electronic device, which can reduce the technical problem that lithium precipitation is prone to occur in the secondary battery.

[0005] In order to solve the technical problem, the embodiments of the present application adopt the following technical solutions:

[0006] In a first aspect, the embodiments of the present application provide a secondary battery, which includes an electrode assembly, the electrode assembly includes a plurality of positive electrode sheets and a plurality of negative electrode sheets which are alternately and layerwisely arranged along a first direction, an isolation film is arranged between adjacent positive electrode sheets and negative electrode sheets, the positive electrode sheet includes a first positive electrode sheet and a second positive electrode sheet, along the first direction, the first positive electrode sheet is the outermost sheet of the electrode assembly, and the second positive electrode sheet is arranged between two adjacent negative electrode sheets; the first positive electrode sheet includes a first current collector and a first positive electrode coating, the first positive electrode coating is arranged on the surface of the first current collector facing the negative electrode sheet; the second positive electrode sheet includes a second current collector and a second positive electrode coating arranged on both surfaces of the second current collector; the first current collector includes a first conductive layer, along the first direction, the thickness of the first conductive layer is T1; the second current collector includes a second conductive layer, along the first direction, the thickness of the second conductive layer is T2, and T1≤T2.

[0007] In the technical solution, the thickness T1 of the first conductive layer is less than or equal to the thickness T2 of the second conductive layer, so that the current-carrying area of the first conductive layer is less than or equal to the current-carrying area of the second conductive layer, and the resistance of the first positive electrode plate is greater than or equal to the resistance of the second positive electrode plate. When the secondary battery is charging and discharging, the current flowing through the first positive electrode plate can be reduced, and the current density of the first positive electrode coating can be reduced, the deintercalation reaction of lithium ions is reduced, the corresponding negative electrode plate of the first positive electrode plate has sufficient excess lithium ions to be inserted into the first positive electrode coating in unit time, and the lithium precipitation is reduced. While improving the fast charging performance of the secondary battery, the risk of lithium precipitation of the secondary battery is reduced.

[0008] In some embodiments, the ratio of the thickness T1 of the first conductive layer to the thickness T2 of the second conductive layer is 44%≤T1 / T2≤100%, and 1μm≤T2≤30μm. Not only the risk of lithium precipitation of the outer layer is reduced, but also the fast charging performance of the secondary battery is improved, and the first positive electrode plate also has high structural strength, which can reduce the phenomenon of curling of the first positive electrode plate.

[0009] In some embodiments, the ratio of the thickness T1 of the first conductive layer to the thickness T2 of the second conductive layer is 44%≤T1 / T2≤78%. Preferably, 44%≤T1 / T2≤67%, which can further reduce lithium precipitation and shorten the difference between the outer layer lithium precipitation and the inner layer lithium precipitation. Further, 4μm≤T1≤10μm.

[0010] In some embodiments, the first positive electrode plate further comprises an insulating layer, and the insulating layer is arranged on the surface of the first current collector away from the first positive electrode coating. By arranging the insulating layer on the first current collector, the stress distribution of the two sides of the first positive electrode plate can be more uniform, the structural strength of the first positive electrode plate can be enhanced, and the curling of the first positive electrode plate can be reduced.

[0011] In some embodiments, along the first direction, the thickness of the insulating layer is T3, and 10μm≤T3≤20μm. The curling phenomenon of the first positive electrode plate is effectively improved, and the space occupied by the insulating layer can be reduced, and the energy density of the secondary battery is improved.

[0012] In some embodiments, the insulating layer comprises at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer or ethylene-methyl acrylate copolymer.

[0013] In some embodiments, the surface of the first current collector facing the first positive electrode coating has a first reinforcing portion; and / or, the surface of the first current collector away from the first positive electrode coating has a second reinforcing portion. The first reinforcing portion and the second reinforcing portion can enhance the structural strength of the first positive electrode plate and reduce the curling phenomenon of the first positive electrode plate.

[0014] In some embodiments, along the first direction, the thickness of the first positive electrode coating layer is T4, and the thickness of the second positive electrode coating layer is T5, T4 < T5. The thickness of the first positive electrode coating layer being less than the thickness of the second positive electrode coating layer is conducive to improving the curling phenomenon of the first positive electrode tab, and the thinning of the thickness of the first positive electrode coating layer can reduce the amount of lithium extraction of the first positive electrode coating layer, thereby reducing the risk of external lithium precipitation. Wherein, 50%≤T4 / T5≤90%.

[0015] In some embodiments, the first conductive layer and the second conductive layer each comprise at least one of aluminum, an aluminum alloy, nickel, a nickel alloy, or stainless steel.

[0016] In some embodiments, the first conductive layer comprises a first sub-conductive layer and a second sub-conductive layer, and the first current collector further comprises a first insulating polymer layer, which is arranged between the first sub-conductive layer and the second sub-conductive layer along the first direction. The arrangement of the first insulating polymer layer causes the first conductive layer to be thinned, the weight of the first positive electrode tab to be reduced, and the weight energy density of the secondary battery to be improved. When the thickness of the first conductive layer is relatively thin, the overall strength of the first current collector is enhanced by increasing the first insulating polymer layer, and the curling phenomenon of the first positive electrode tab is improved.

[0017] In some embodiments, the first insulating polymer layer comprises at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

[0018] In some embodiments, the negative electrode tab comprises a first negative electrode tab, and the first negative electrode tab is arranged between two adjacent positive electrode tabs; the first negative electrode tab comprises a third current collector and a first negative electrode coating layer arranged on both surfaces of the third current collector; and along the first direction, the projection of the first positive electrode coating layer falls within the projection range of the first negative electrode coating layer. This allows the first negative electrode tab to have sufficient excess capacity to embed lithium ions extracted from the first positive electrode tab, thereby reducing the phenomenon of lithium precipitation in the first negative electrode tab.

[0019] In a second aspect, the embodiments of the present application provide an electronic device comprising the secondary battery of any one of the embodiments of the first aspect described above BRIEF DESCRIPTION OF DRAWINGS

[0020] One or more embodiments are exemplified by the accompanying drawings corresponding thereto, which do not constitute a limitation on the embodiments, and elements with the same reference numerals in the drawings represent similar elements, unless otherwise specified. The drawings in the accompanying drawings do not constitute a proportional limit.

[0021] Figure 1 is a structural schematic diagram of some secondary batteries provided by the embodiments of the present application;

[0022] Figure 2is a structural schematic diagram of some electrode assemblies provided by embodiments of the present application;

[0023] Figure 3 is a structural schematic diagram of some electrode assemblies provided by embodiments of the present application;

[0024] Figure 4 is a structural schematic diagram of some first positive electrode tabs provided by embodiments of the present application;

[0025] Figure 5 is a structural schematic diagram of some second positive electrode tabs provided by embodiments of the present application;

[0026] Figure 6 is a structural schematic diagram of some first current collectors provided by embodiments of the present application;

[0027] Figure 7 is a structural schematic diagram of some second current collectors provided by embodiments of the present application;

[0028] Figure 8 is a structural schematic diagram of some insulating layers provided by embodiments of the present application;

[0029] Figure 9 is a structural schematic diagram of some reinforcing portions provided by embodiments of the present application;

[0030] Figure 10 is a structural schematic diagram of some first negative electrode tabs provided by embodiments of the present application;

[0031] Figure 11 is a structural schematic diagram of some first negative electrode tabs provided by embodiments of the present application.

[0032] Legend of reference signs:

[0033] 1000, secondary battery;

[0034] 100, electrode assembly;

[0035] 10, positive electrode tab; 11, first positive electrode tab; 111, first current collector; 1111, first conductive layer; 111a, first sub-conductive layer; 111b, second sub-conductive layer; 1112, first insulating polymer layer; 112, first positive electrode coating layer; 113, insulating layer; 114, first reinforcing portion; 115, second reinforcing portion; 12, second positive electrode tab; 121, second current collector; 1211, first surface; 1212, second surface; 1213, second conductive layer; 121a, third sub-conductive layer; 121b, fourth sub-conductive layer; 1214, second insulating polymer layer; 122, second positive electrode coating layer;

[0036] 20, negative electrode tab; 21, first negative electrode tab; 211, third current collector; 2111, third surface; 2112, fourth surface; 212, first negative electrode coating;

[0037] 30, separator film;

[0038] 200, housing;

[0039] X, first direction; Y, second direction; Z, third direction. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0041] In the description of the embodiments of the present application, the technical terms "first", "second", and the like 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 "several" and "a plurality of" is two or more, unless otherwise explicitly specified.

[0042] In the description of the embodiments of the present application, the term "and / or" is only used to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents a "or" relationship between the front and rear associated objects.

[0043] In this paper, the reference to "embodiments" means that the specific features, structures or characteristics described in combination with the embodiments can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment to other embodiments. In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0044] For the laminated battery, especially the soft package laminated battery, the inside thereof usually includes the outermost positive electrode tab coated with a single-sided coating (hereinafter referred to as a single-sided positive electrode tab) and the innermost positive electrode tab coated with a double-sided coating (hereinafter referred to as a double-sided positive electrode tab). The outermost side of the single-sided positive electrode tab can be provided with a blank foil (without coating), which can reduce the space occupied by the single-sided positive electrode tab and improve the energy density of the battery.

[0045] However, the single-face positive electrode sheet is located at the outermost layer of the electrode assembly, and the restraint force is small, and the current collector in the single-face positive electrode sheet is only coated on one side, which may cause uneven stress distribution on both sides of the current collector, and the current collector is easy to curl towards the side away from the coating, and the curling of the current collector is easy to cause the coating to fall off, affecting the energy density of the secondary battery. In order to reduce the curling problem of the single-face positive electrode sheet, the thickness of the current collector in the single-face positive electrode sheet is usually increased to improve the anti-deformation ability of the single-face positive electrode sheet.

[0046] The inventors of the present application have found that, during the charging and discharging process of the secondary battery, the risk of lithium precipitation (hereinafter referred to as outer layer lithium precipitation) of the corresponding negative electrode sheet of the single-face positive electrode sheet is relatively high. When the secondary battery is charged and discharged, the maximum charging and discharging rate that the double-face positive electrode sheet can withstand is greater than the maximum charging and discharging rate that the single-face positive electrode sheet can withstand without lithium precipitation. In order to improve the rate performance of the secondary battery, the maximum charging and discharging rate of the double-face positive electrode sheet is used as a reference, but this will exceed the bearing range of the single-face positive electrode sheet, so that the risk of outer layer lithium precipitation is higher than that of inner layer lithium precipitation, which is easy to cause safety problems.

[0047] The inventors of the present application have further found that lithium precipitation is related to the current density of the coating, and since the single-face positive electrode sheet is provided with a coating on only one side, the current is concentrated on the single-layer coating, and the lithium ion extraction reaction is intense, which makes it difficult for the extracted lithium ions to be inserted into the corresponding negative electrode sheet in time, thereby causing lithium precipitation to occur. Moreover, increasing the thickness of the current collector of the outer layer single-face positive electrode sheet means that the resistance of the current collector of the outer layer single-face positive electrode sheet is smaller, the current density is greater, and the lithium ion extraction reaction is more intense, thereby further increasing the risk of lithium precipitation.

[0048] In order to reduce the above problems, in a first aspect, the present application provides a secondary battery 1000, which is a soft package battery that can be charged and discharged, and can realize the storage and release of electric energy through reversible chemical reactions. The secondary battery 1000 can be a lithium ion battery, a sodium ion battery, or a lithium polymer battery, etc.

[0049] Please refer to Figure 1 The secondary battery 1000 comprises an electrode assembly 100 and a shell 200, and the electrode assembly 100 is accommodated in the shell 200. In combination with Figure 2 , Figure 2 The layer structure of the electrode assembly 100 is shown, and the electrode assembly 100 comprises a positive electrode sheet 10, a negative electrode sheet 20 and a separator 30. Along the first direction X (the thickness direction of the positive electrode sheet 10 and / or the negative electrode sheet 20), a plurality of positive electrode sheets 10 and a plurality of negative electrode sheets 20 are alternately and laminatedly arranged, and a separator 30 is arranged between adjacent positive electrode sheets 10 and negative electrode sheets 20, and the separator 30 is used to insulate and separate the positive electrode sheet 10 and the negative electrode sheet 20.

[0050] The positive electrode tab 10 includes a first positive electrode tab 11 and a second positive electrode tab 12. In the first direction X, the first positive electrode tab 11 is the outermost positive electrode tab 10 of the electrode assembly 100. The second positive electrode tab 12 is the inner positive electrode tab 10 of the electrode assembly 100, that is, the second positive electrode tab 12 is arranged between two adjacent negative electrode tabs 20.

[0051] For the first positive electrode tab 11 described above, refer to Figure 3 and Figure 4 The first positive electrode tab 11 includes a first current collector 111 and a first positive electrode coating 112. The first current collector 111 serves as a conductive substrate, which can be an aluminum foil with a flat and strip-shaped structure as a whole. In other embodiments, the material of the first current collector 111 can also be at least one of aluminum alloy, nickel, nickel alloy, or stainless steel.

[0052] In the embodiments of the present application, the first positive electrode tab 11 adopts a single-sided coating structure, that is, in the first direction X, the first positive electrode coating 112 is only arranged on the surface of the first current collector 111 facing the negative electrode tab 20, and the surface of the first current collector 11 away from the negative electrode tab 20 is arranged with an empty foil (without coating), which can reduce the space occupied by the first positive electrode tab 11 and improve the volume energy density of the secondary battery 1000.

[0053] The first positive electrode coating 112 includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode adhesive, and the like. After the above-mentioned materials are mixed and stirred uniformly, they are coated on the surface of the first current collector 111 facing the negative electrode tab 20, thereby obtaining the first positive electrode coating 112. The positive electrode active material can be selected from at least one of lithium nickel cobalt manganese oxide, lithium cobaltate, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganate, lithium manganese iron phosphate, or cobalt-free material. In the embodiments of the present application, the positive electrode conductive agent and the positive electrode adhesive are not particularly limited as long as they can achieve the purpose of the present application. For example, the positive electrode conductive agent can be selected from at least one of conductive carbon black, carbon nanotube, carbon fiber, flaky graphite, graphene, metal material, or conductive polymer. The positive electrode adhesive can be selected from at least one of polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose, or potassium carboxymethyl cellulose.

[0054] For the second positive electrode tab 12 described above, refer to Figure 3 and Figure 5The second positive electrode tab 12 includes a second current collector 121 and a second coating layer 122 arranged on two surfaces (two surfaces arranged opposite to each other in the thickness direction of the second current collector 121) of the second current collector 121. The second current collector 121 is similar in structure to the first current collector 111 described above, and can be an aluminum foil that is flat and in the form of a strip as a whole. Alternatively, in some other embodiments, the material of the first current collector 111 can also be at least one of an aluminum alloy, nickel, a nickel alloy, or stainless steel.

[0055] In the embodiments of the present application, the second positive electrode tab 12 adopts a double-sided coating structure. Specifically, along the first direction X, the second current collector 121 has a first surface 1211 and a second surface 1212 arranged opposite to each other, and both the first surface 1211 and the second surface 1212 are provided with the second positive electrode coating layer 122. The second positive electrode coating layer 122 is arranged on the inner layer, which can make full use of the space and thus improve the energy density of the secondary battery 1000.

[0056] For the above-mentioned separation film 30, please refer to Figure 2 and Figure 3 The separation film 30 is arranged between the positive electrode tab 10 and the negative electrode tab 20, and is used to insulate and separate the positive electrode tab 10 and the negative electrode tab 20. The separation film 30 can be a PE separation film 30 or a PP separation film 30 containing ceramic, etc. In some embodiments, the surfaces of the separation film 30 facing the positive electrode tab 10 and the negative electrode tab 20 can be provided with an adhesive coating layer. When the positive electrode tab 10, the separation film 30, and the negative electrode tab 20 are stacked, the separation film 30 can be directly bonded between the positive electrode tab 10 and the negative electrode tab 20, which can improve the bonding strength between the positive electrode tab 10, the separation film 30, and the negative electrode tab 20, and thus improve the integrity of the electrode assembly 100.

[0057] Please refer to Figure 4 The first current collector 111 includes a first conductive layer 1111, which is a layer of material capable of conducting electric charges, and the electric charges can flow smoothly in the first conductive layer 1111. The first conductive layer 1111 includes at least one of an electrically conductive metal such as aluminum, an aluminum alloy, nickel, a nickel alloy, or stainless steel.

[0058] When the first current collector 111 is a metal foil current collector (the first current collector 111 is entirely made of metal), the first conductive layer 1111 is the first current collector 111 itself.

[0059] When the first current collector 111 is a composite current collector, the first conductive layer 1111 is a part of the first current collector 111. For example, please refer to Figure 6The first current collector 111 includes a first conductive layer 1111 and a first insulating polymer layer 1112. Specifically, the first conductive layer 1111 includes a first sub-conductive layer 111a and a second sub-conductive layer 111b, and the first insulating polymer layer 1112 is disposed between the first sub-conductive layer 111a and the second sub-conductive layer 111b.

[0060] The first insulating polymer layer 1112 can thin the first conductive layer 1111, reduce the weight of the first positive electrode tab 11, and improve the weight energy density of the secondary battery 1000. In particular, when the first conductive layer 1111 is thin, the first insulating polymer layer 1112 can be added to improve the structural strength of the first current collector 111 and reduce the curling phenomenon of the first positive electrode tab 11. Since the first insulating polymer layer 1112 has electrical insulation, it is difficult for charges to pass through the first insulating polymer layer 1112. Therefore, the first insulating polymer layer 1112 does not change the thickness and conductivity of the conductive part of the first current collector 111. In some embodiments, the first insulating polymer layer 1112 includes at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

[0061] Please refer to Figure 5 The second current collector 121 includes a second conductive layer 1213. Similar to the first conductive layer 1111, the second conductive layer 1213 can conduct charges, allowing the charges to flow smoothly in the second conductive layer 1213. The second conductive layer 1213 includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, or stainless steel.

[0062] When the second current collector 121 is a metal foil current collector (the second current collector 121 is entirely made of metal), the second conductive layer 1213 is the second current collector 121 itself.

[0063] When the second current collector 121 is a composite current collector, the second conductive layer 1213 is part of the second current collector 121. For example, please refer to Figure 7 The second current collector 121 includes a second conductive layer 1213 and a second insulating polymer layer 1214. Specifically, the second conductive layer 1213 includes a third sub-conductive layer 121a and a fourth sub-conductive layer 121b, and the second insulating polymer layer 1214 is disposed between the third sub-conductive layer 121a and the fourth sub-conductive layer 121b.

[0064] The second insulating polymer layer 1214 can thin the second conductive layer 1213, reduce the weight of the second positive electrode tab 12, and improve the weight energy density of the secondary battery 1000. In particular, when the second conductive layer 1213 is thin, the second insulating polymer layer 1214 can be added to improve the structural strength of the second current collector 121 and reduce the curling of the second positive electrode tab 12. Since the second insulating polymer layer 1214 has electrical insulation, it is difficult for charges to pass through the second insulating polymer layer 1214. Therefore, the second insulating polymer layer 1214 does not change the thickness and conductivity of the conductive part of the second current collector 121. In some embodiments, the second insulating polymer layer 1214 includes at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

[0065] In the embodiments of the present application, the thickness of the first conductive layer 1111 is T1 and the thickness of the second conductive layer 1213 is T2 along the first direction X. The thickness T1 of the first conductive layer 1111 is less than or equal to the thickness T2 of the second conductive layer 1213. Therefore, the current-carrying area of the first conductive layer 1111 is less than or equal to the current-carrying area of the second conductive layer 1213. In turn, the resistance of the first positive electrode tab 11 is greater than or equal to the resistance of the second positive electrode tab 12. When the secondary battery 1000 is charging and discharging, the current flowing through the first positive electrode tab 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112 and reducing the deintercalation reaction of lithium ions. The corresponding negative electrode tab 20 of the first positive electrode tab 11 has sufficient capacity to intercalate the lithium ions extracted from the first positive electrode coating 112 within a unit time, thereby reducing the occurrence of lithium precipitation. The secondary battery 1000 can adapt to a larger charging and discharging rate, thereby improving the fast-charging performance of the secondary battery 1000 while reducing the risk of lithium precipitation of the secondary battery 1000.

[0066] For the thickness T1 of the first conductive layer 1111, in the embodiments of the present application, the thickness of the first conductive layer 1111 is less than or equal to the thickness of the second conductive layer 1213, for example, 44%≤T1 / T2≤100%. This range not only reduces the risk of outer lithium precipitation, but also provides the first positive electrode tab 11 with a certain structural strength to reduce the curling phenomenon. The thickness of the second conductive layer 1213 is usually in the range of 1 μm≤T2≤30 μm. In the embodiments of the present application, the thickness T1 of the first conductive layer 1111 is in the range of 0.44 μm≤T1≤30 μm. The specific thickness of the first conductive layer 1111 can be set according to the corresponding relationship and the thickness of the second conductive layer 1213. For example, T1 can be 3 μm, 5 μm, 7 μm, 9 μm, 15 μm, 20 μm, 30 μm, etc.

[0067] In particular, when the second current collector 121 is a composite current collector, the second conductive layer 1213 is usually provided with a thickness in the range of 1 μm≤T2≤30 μm. According to 44%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 is in the range of 0.44 μm≤T1≤30 μm. However, in actual production, it is difficult to make the thickness of the first conductive layer 1111 to be 0.44 μm. The thickness of the first conductive layer can be 1 μm only when the first current collector 111 is a composite current collector. Preferably, 1 μm≤T1≤30 μm. When the second current collector 121 is entirely made of metal, the thickness of the second conductive layer 1213 is in the range of 4 μm≤T2≤30 μm. According to 44%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 is in the range of 1.76 μm≤T1≤30 μm.

[0068] In some preferred embodiments, the thickness T1 of the first conductive layer 1111 is less than the thickness T2 of the second conductive layer 1213, and the ratio of T1 to T2 is in the range of 44%≤T1 / T2≤78%. In order to further reduce the risk of lithium elution from the outside, the difference between lithium elution from the outside and lithium elution from the inside is shortened.

[0069] In some preferred embodiments, the thickness T2 of the second conductive layer 1213 can be set to 6 μm to 15 μm, and according to the relationship between the thickness of the first conductive layer 1111 and the thickness of the second conductive layer 1213, 44%≤T1 / T2≤67%, the thickness T1 of the first conductive layer 1111 can be in the range of 4 μm≤T1≤10 μm. For example, T1 can be 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0070] The inventors have found that thinning the thickness of the first conductive layer 1111 can weaken the strength of the first current collector 111, causing the first current collector 111 to curl at the edges, which can damage the structure of the first positive electrode coating 112, and even cause the first positive electrode coating 112 to fall off and affect the energy density of the secondary battery 1000.

[0071] To reduce the above problems, in the embodiments of the present application, please refer to Figure 8The first positive electrode tab 11 further comprises an insulating layer 113 disposed on the surface of the first current collector 111 away from the first positive electrode coating 112. By disposing the insulating layer 113 on the first current collector 111, the strength of the first current collector 111 is improved, and the edge curling phenomenon of the first current collector 111 can be effectively reduced. At the same time, one surface of the first current collector 111 is provided with the first positive electrode coating 112, and the other surface is provided with the insulating layer 113, so that the stress of the first current collector 111 is more evenly distributed, and the curling phenomenon of the first current collector 111 is reduced. In addition, the insulating layer 113 can directly isolate the first positive electrode tab 11 from the shell 200, which can reduce the direct contact between the first positive electrode tab 11 and the shell 200, reduce the corrosion of the first current collector 111, and reduce the risk of short circuit.

[0072] It should be noted that when the first current collector 111 is a metal foil current collector, the insulating layer 113 can be directly disposed on the surface of the first current collector 111 away from the first positive electrode coating 112. When the first current collector 111 is a composite current collector, the insulating layer 113 can be directly disposed on the surface of the first sub-conductive layer 111a away from the first insulating polymer layer 1112.

[0073] In some embodiments, along the first direction X, the thickness of the insulating layer 113 is T3, and 10 pm≤T3≤20 pm. For example, the thickness of T3 can be 10 pm, 15 pm, 20 pm, etc. If the thickness of the insulating layer 113 is too thin, for example, less than 10 pm, it is difficult to effectively improve the curling phenomenon of the first positive electrode tab 11. If the thickness of the insulating layer 113 is too thick, for example, greater than 20 pm, the insulating layer 113 will occupy more space, not only affecting the volume energy density of the secondary battery 1000, but also possibly causing uneven stress distribution, causing the first positive electrode tab 11 to deform, curl, etc. In the embodiments of the present application, the thickness of the insulating layer 113 is 10 pm≤T3≤20 pm, which can effectively reduce the curling and deformation of the first current collector 111 while reducing the impact on the energy density of the secondary battery 1000.

[0074] For the material of the insulating layer 113 described above, there is no particular limitation in the embodiments of the present application, as long as the purpose of the present application can be achieved. For example, the material of the insulating layer 113 includes at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

[0075] In some embodiments, please refer to Figure 3 and Figure 9The surface of the first current collector 111 facing the first positive electrode coating 112 has a first reinforcing portion 114. The first reinforcing portion 114 can enhance the structural strength of the first positive electrode tab 11 and improve the curling phenomenon of the first positive electrode tab 11. In addition, the surface of the first current collector 111 away from the first positive electrode coating 112 has a second reinforcing portion 115. The second reinforcing portion 115 can also enhance the structural strength of the first positive electrode tab 11 and improve the curling phenomenon of the first positive electrode tab 11.

[0076] For example, the first reinforcing portion 114 can be formed by embossing the surface of the first current collector 111 facing the first positive electrode coating 112, or by setting protruding blocks on the surface of the first current collector 111 to enhance the structural strength of the first current collector 111. Similarly, the surface of the first current collector 111 away from the first positive electrode coating 112 can also be embossed or set with protruding blocks.

[0077] In some embodiments, referring to Figure 4 and Figure 5 The thickness of the first positive electrode coating 112 is T4, and the thickness of the second positive electrode coating 122 set on the first surface 1211 is T5, and T4 < T5. Since the thickness of the first current collector 111 is reduced, and only one surface is set with the first positive electrode coating 112, if the first positive electrode coating 112 is too thick, the first positive electrode tab 11 is prone to stress imbalance and curling. If the first positive electrode coating 112 is too thin, the capacity of the secondary battery 1000 is reduced due to the lack of positive active material. Reducing the thickness of the first positive electrode coating 112 to 50% ≤ T4 / T5 < 100% can balance the stress between the first current collector 111 and the first positive electrode coating 112 in the first positive electrode tab 11, and reduce the curling phenomenon of the first positive electrode tab 11. In addition, reducing the thickness of the first positive electrode coating 112 can reduce the amount of lithium extraction of the first positive electrode coating 112 and reduce the risk of lithium precipitation. Preferably, 50% ≤ T4 / T5 ≤ 90%, which can reduce the risk of short circuit of the secondary battery 1000 while reducing the curling of the first positive electrode tab 11.

[0078] In other embodiments, the compaction density of the first positive electrode coating 112 can also be set to be less than that of the corresponding negative electrode coating, which can reduce the amount of lithium extraction of the first positive electrode coating 112, and thus the negative electrode tab has enough excess capacity to embed lithium ions, which can reduce the occurrence of lithium precipitation.

[0079] In the embodiments of the present application, by adjusting the coating weight of the first positive electrode coating layer 112, the capacity ratio of the corresponding negative electrode sheet 20 to the first positive electrode sheet 11 is controlled to be 1.15 to 2, and the capacity ratio of the corresponding negative electrode sheet 20 to the second positive electrode sheet 12 is controlled to be 1.03 to 1.1. Not only can the curling problem of the first positive electrode sheet 11 be improved, but also the risk of lithium extraction from the outer layer can be reduced.

[0080] In some embodiments, please refer to Figure 5 Within the error tolerance range, the thickness T5 of the second positive electrode coating layer 122 arranged on the first surface 1211 is substantially equal to the thickness T6 of the second positive electrode coating layer 122 arranged on the second surface 1212, so that the performance of the secondary battery is more balanced and stable, and the phenomenon of local performance imbalance caused by too large thickness difference of the second positive electrode coating layer 122 on the two surfaces of the second current collector 121 is reduced.

[0081] In some embodiments, please refer to Figure 2 and Figure 3 The negative electrode sheet 20 includes a first negative electrode sheet 21, and the first negative electrode sheet 21 is arranged between two adjacent positive electrode sheets 10. The first negative electrode sheet 21 includes a third current collector 211. The third current collector 211 can adopt a copper foil, a nickel foil or a titanium foil which is overall flat and in a strip-shaped structure. In other embodiments, the third current collector 211 can also adopt a copper alloy foil, a stainless steel foil, a foamed nickel, a foamed copper or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.).

[0082] In the embodiments of the present application, the first negative electrode sheet 21 adopts a double-sided coating structure. Specifically, please further refer to Figure 10 The first negative electrode sheet 21 further includes a first negative electrode coating layer 212 arranged on the two surfaces of the third current collector 211. Along the first direction X, the third current collector 211 has a third surface 2111 and a fourth surface 2112 arranged oppositely, and the third surface 2111 and the fourth surface 2112 are both provided with the first negative electrode coating layer 212.

[0083] The first negative electrode coating 212 comprises a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder, etc. The above-mentioned material components are uniformly mixed and coated on the third surface 2111 and the fourth surface 2112 of the third current collector 211, so as to obtain the first negative electrode coating 212. The negative electrode active material can be selected from at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Sn, SnO, SnO2, Li-Al alloy, lithium metal, etc. In the embodiments of the present application, the negative electrode conductive agent and the negative electrode binder are not particularly limited, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can comprise at least one of the above-mentioned positive electrode conductive agents, and the negative electrode binder can comprise at least one of the above-mentioned positive electrode binders.

[0084] In some embodiments, referring to Figure 11 , the thickness T7 of the first negative electrode coating 212 arranged on the third surface 2111 is substantially equal to the thickness T8 of the first negative electrode coating 212 arranged on the fourth surface 2112, so that the performance of the secondary battery is more balanced and stable, and the phenomenon of local performance imbalance caused by too large thickness difference of the first negative electrode coating 212 on the two surfaces of the third current collector 211 is reduced.

[0085] In some embodiments, referring to Figure 3 , along the first direction X, the projection of the first positive electrode coating 112 falls within the projection range of the first negative electrode coating 212, so that the first negative electrode tab 21 has sufficient excess to embed the lithium ions extracted by the first positive electrode tab 11, and the phenomenon of lithium precipitation of the first negative electrode tab 21 can be reduced.

[0086] Further, referring to Figure 3 , Figure 4 and Figure 11 , along the length direction (second direction Y) of the first positive electrode tab 11, the length of the first positive electrode coating 112 is L1. Along the length direction (second direction Y) of the first negative electrode tab 21, the length of the first negative electrode coating 212 is L2, then 0.6mm≤L2-L1≤4mm. Along the width direction (third direction Z) of the first positive electrode tab 11, the width of the first positive electrode coating 112 is W1. Along the width direction (third direction Z) of the first negative electrode tab 21, the width of the first negative electrode coating 212 is W2, then 0.6mm≤W2-W1≤4mm. So that the negative electrode tab has sufficient excess to embed the lithium ions extracted by the positive electrode tab, the phenomenon of lithium precipitation of the first negative electrode tab 21 can be further reduced, and limiting 0.6mm≤L2-L1≤4mm and 0.6mm≤W2-W1≤4mm can make the secondary battery have a higher energy density.

[0087] In a second aspect, the embodiments of the present application also provide an electronic device comprising the secondary battery 1000 of any of the embodiments of the first aspect. The electronic device of the embodiments of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device includes, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0088] In the embodiments of the present application, lithium ion secondary batteries are taken as examples for lithium extraction tests.

[0089] The tests are as follows:

[0090] In the battery assembly preparation stage, the reference electrode 1 is placed in the middle layer of the negative electrode sheet corresponding to the first positive electrode sheet and the separator, and the reference electrode 2 is placed in the middle layer of the separator between the center negative electrode sheet and the separator. (The center negative electrode sheet refers to the negative electrode sheet at the middle position or near the middle position of the total negative electrode sheet in the secondary battery. For example, if the total number of negative electrode sheets is 14, the center negative electrode sheet is the 7th negative electrode sheet, and the positions on both sides can be used.) The manufacturing process of the reference electrode is as follows:

[0091] (1) Copper wire pretreatment: select a copper wire with a diameter of 30 μm and a length of 100 mm, immerse one end (about 30 mm) of the copper wire in concentrated sulfuric acid for 2 h to remove the surface insulating paint, then ultrasonically clean with alcohol for 10 to 15 minutes, repeat 2 to 3 times, and finally dry in an oven for standby use.

[0092] (2) Copper wire insertion into the battery: at the specified insertion layer, place the treated end of the copper wire on the negative electrode surface at the center position of the separator, and fix the copper wire at the edge of the separator with adhesive tape. Cover a piece of separator with a width of about 10 mm and a length equal to the width of the negative electrode sheet on the copper wire to separate the copper wire from the negative electrode sheet. Finally, continue the electrode assembly assembly and liquid injection packaging process in the normal stacking sequence. The copper wire is drawn out from the side of the aluminum plastic film packaging edge. Since the copper wire is thin enough, no additional packaging treatment is required.

[0093] (3) Copper wire adapter welding: weld the copper wire to the nickel tab and fix it to the aluminum plastic film shell with adhesive tape. Use the positive and negative poles of a multimeter to clamp the reference electrode and the positive or negative electrode of the battery. If there is voltage, it indicates that the three-electrode battery is made normally.

[0094] (4) Lithium plating: using a charge-discharge instrument, the positive electrode is clamped to the positive electrode of the battery, and the negative electrode is clamped to the reference electrode. Charge at 20 μA for 10 h; then the positive electrode is clamped to the negative electrode of the battery, and the negative electrode is clamped to the reference electrode. Charge at 20 μA for 10 h.

[0095] The test procedure is as follows:

[0096] (1) Preparation: An electrochemical workstation is used as the test equipment to provide the charging current and monitor the battery voltage, including: the voltage between the positive and negative electrodes of the battery, the voltage V t1 between the negative tab and reference electrode 1, and the voltage V t2 between the negative tab and reference electrode 2.

[0097] (2) Charging: The battery is charged according to the normal charging process. The examples and comparative examples of the present application use the following process: first, apply a constant current of 3C to charge, then switch to constant voltage charging mode when the voltage reaches 4.5V, and stop when the current drops to 0.05C. Obtain V t1 as a function of time, and extract the minimum value as V1. Obtain V t2 as a function of time, and extract the minimum value as V2.

[0098] (3) Calculate the voltage difference Δ = V1 - V2.

[0099] Evaluation principle: The lower the negative potential, the higher the risk of lithium precipitation.

[0100] Evaluation criteria: Δ > 0, indicating that the risk of outer lithium precipitation is not higher than that of inner lithium precipitation; Δ < 0, indicating that the risk of outer lithium precipitation is higher than that of inner lithium precipitation; Δ = 0, indicating that the risk of outer lithium precipitation is equal to that of inner lithium precipitation. The larger the absolute value of Δ, the greater the difference between the inner and outer lithium precipitation.

[0101] In the examples of the present application, the current collector of the positive electrode tab in the lithium ion secondary battery is taken as a metal foil current collector, and the preparation of each component of the lithium ion secondary battery is as follows:

[0102] Example 1

[0103] (1) Preparation of positive electrode tab: mix the positive electrode active material lithium cobaltate, the positive electrode conductive agent acetylene black, and the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight 5 x 10 5 ) in a mass ratio of 97:1.5:1.5, add N-methyl pyrrolidone (NMP) as a solvent, adjust the positive electrode slurry to a solid content of 75 wt%, and stir uniformly in a vacuum stirrer.

[0104] The aluminum foil is selected as the first current collector (first conductive layer), and the first current collector and the first conductive layer are the same structure. The room temperature conductivity of the aluminum foil is 3.774e7 S / m, and the thickness T1 of the first conductive layer is equal to 4 pm. The slurry is uniformly coated on one surface of the first current collector, dried, and a first positive electrode coating with a thickness of 34 pm is formed on the surface of the first current collector to obtain a first positive electrode sheet with single-sided coating of positive electrode active material. The length of the first positive electrode sheet is 69 mm, and the width is 69 mm.

[0105] The aluminum foil is selected as the second current collector (second conductive layer), and the second current collector and the second conductive layer are the same structure. The room temperature conductivity of the aluminum foil is 3.774e7 S / m, and the thickness T2 of the second conductive layer is 9 pm. The slurry is uniformly coated on the first surface and the second surface of the second current collector, dried, and a second positive electrode coating with a thickness of 34 pm is formed on the first surface and the second surface to obtain a second positive electrode sheet with double-sided coating of positive electrode active material. The length of the second positive electrode sheet is 69 mm, and the width is 69 mm.

[0106] (2) Preparation of the negative electrode sheet: The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (Super P), and binder phenylpropylene rubber (SD-3) are mixed in a weight ratio of 89.5:8:1:1.5, then deionized water is added as a solvent to prepare a negative electrode slurry with a solid content of 50 wt%, and the mixture is stirred uniformly.

[0107] The copper foil is selected as the third current collector. The room temperature conductivity of the copper foil is 5.998e7 S / m, and the thickness is 6 pm. The slurry is uniformly coated on both surfaces of the third current collector, dried, and a first negative electrode coating with a thickness of 42 pm is formed on both surfaces of the third current collector to obtain a first negative electrode sheet with double-sided coating of negative electrode coating. The length of the first negative electrode sheet is 70 mm, and the width is 69.7 mm.

[0108] (3) Preparation of the electrolyte: In a dry argon atmosphere, first, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC: EMC: DEC = 30:50:20 to form a basic organic solvent, then lithium salt lithium hexafluorophosphate (LiPF6) is added to the basic organic solvent to dissolve and mix uniformly to obtain an electrolyte with a LiPF6 mass concentration of 12.5%.

[0109] (4) Preparation of the separator film: A polyethylene porous film is used as a base layer, and an aluminum oxide ceramic and polyvinylidene fluoride binder containing ceramic layer is coated on one side surface of the base layer as a separator film. The mass percentage of aluminum oxide ceramic in the ceramic layer is 95%, the thickness of the base layer is 5 pm, and the thickness of the ceramic layer is 1.8 pm.

[0110] (5) Electrode assembly preparation: Nickel and aluminum sheets with a size of 12 mm x 6 mm were selected as tabs. The aluminum tab was welded to the positive electrode tab (aluminum foil of the positive electrode tab), and the nickel tab was welded to the negative electrode tab. The above-mentioned positive electrode tab, separator, and negative electrode tab were stacked to form an electrode assembly for use. The number of layers of the first positive electrode tab was 2, the number of layers of the second positive electrode tab was 11, the number of layers of the first negative electrode tab was 12, and the number of layers of the separator was 24.

[0111] (6) Electrode assembly assembly: The punched aluminum-plastic film was placed in the assembly jig with the pit surface facing up. The electrode assembly was placed in the pit, and a sealing member was provided at both tabs, and an external force was applied to compress it. Then, another punched aluminum-plastic film was placed on the electrode assembly with the pit surface facing down. The four sides of the two aluminum-plastic films were heat-sealed by hot pressing to obtain an assembled electrode assembly.

[0112] (7) Liquid injection packaging: The assembled electrode assembly was injected with electrolyte, and after vacuum packaging, standing, hot pressing, and shaping, a lithium ion secondary battery was obtained.

[0113] Different from Example 1, the related parameters of Examples 2 to 12 can refer to Table 1 as follows.

[0114] In Example 8, the insulating layer of the polyamide material was arranged on the surface of the first current collector away from the first positive electrode coating, and the thickness of the insulating layer was 15 μm.

[0115] In Example 9, the surface of the first current collector facing the first positive electrode coating was embossed to form a first reinforcing portion.

[0116] In Example 10, the thickness T1 of the first conductive layer was 5 μm, the thickness T4 of the first positive electrode coating was 17 μm, and the thickness of the second positive electrode coating was 34 μm.

[0117] Comparative Examples 1 to 3, different from Examples 1 to 12, T2 < T1.

[0118] Table 1

[0119]

[0120] According to Table 1 above, in combination with Examples 1 to 7 and Comparative Examples 1 to 3, when T1 / T2 ≤ 100%, that is, the thickness of the first conductive layer is less than the thickness of the second conductive layer, Δ is larger, that is, the absolute value of Δ is smaller, indicating that the difference between the outer lithium precipitation risk and the inner lithium precipitation risk is smaller. Therefore, in the embodiments of the present application, by limiting T1 ≤ T2, the lithium precipitation risk can be effectively reduced.

[0121] When 44%≤T1 / T2≤100%, as the ratio of T1 and T2 decreases, the value of Δ gradually increases, and the absolute value of Δ gradually decreases, indicating that the difference between the outer lithium precipitation risk and the inner lithium precipitation risk gradually decreases, improving the charge-discharge rate of the outer layer without lithium precipitation under the same working condition, thereby improving the fast charging performance of the secondary battery. In particular, when the ratio of T1 and T2 is 44%, the outer lithium precipitation risk and the inner lithium precipitation risk are close to consistent, indicating that the discharge rate of the outer layer without lithium precipitation and the discharge rate of the inner layer without lithium precipitation are basically equal under the same working condition, and the fast charging performance of the secondary battery is optimal.

[0122] In embodiments 1 to 4, the absolute value of Δ is less than 6, indicating that the outer lithium precipitation risk is further reduced, and the difference between the outer lithium precipitation and the inner lithium precipitation is small. In the embodiments of the present application, 44%≤T1 / T2≤78% can be selected. Further, in embodiments 1 to 3, the absolute value of Δ is less than 4, indicating that the outer lithium precipitation risk and the inner lithium precipitation risk are closer and the difference is smaller, and 44%≤T1 / T2≤67% is preferred.

[0123] In Comparative Examples 1 to 3, as the first conductive layer thickness T1 continues to increase, the ratio of the first conductive layer thickness T1 to the second conductive layer thickness T2 increases, and the value of Δ becomes smaller and smaller, indicating that the outer lithium precipitation risk gradually increases relative to the inner lithium precipitation risk. When 100%<T1 / T2, the absolute value of Δ becomes larger and larger, indicating that the difference between the inner lithium precipitation risk and the outer lithium precipitation risk becomes larger, causing the fast charging performance of the secondary battery to decrease, increasing the safety risk during use of the secondary battery, and affecting the service life of the battery.

[0124] In combination with embodiments 1, 8 and 9, the first positive electrode tab in embodiment 1 is not provided with an insulating layer and a reinforcing part, and it can be observed that the first positive electrode tab is slightly curled. The first positive electrode tab in embodiment 8 is provided with an insulating layer, and it can be observed that the first positive electrode tab is not curled, indicating that the insulating layer can improve the structural strength of the first positive electrode tab and reduce the edge curling phenomenon of the first positive electrode tab. The first positive electrode tab in embodiment 9 is provided with a reinforcing part, and it can be observed that the first positive electrode tab is not curled, indicating that the reinforcing part can enhance the structural strength of the first positive electrode tab and improve the edge curling phenomenon of the first positive electrode tab.

[0125] In combination with Embodiment 2, Embodiments 10-12, the thickness T4 of the first positive electrode coating and the thickness T5 of the second positive electrode coating in Embodiment 2 are equal, and the first positive electrode tab is slightly curled, and the voltage difference is -1.54 mV. In Embodiments 10-12, 50%≤T4 / T5≤90%, and the voltage difference is greater than -1.54 mV, indicating that the thickness of the first positive electrode coating is less than the thickness of the second positive electrode coating, which is beneficial to reduce the amount of lithium stripping of the first positive electrode coating and reduce the risk of external lithium stripping. Further, when 50%≤T4 / T5≤70%, in Embodiments 10 and 11, the first positive electrode tab does not appear to be curled, indicating that the thickness of the first coating is reduced, which can reduce the phenomenon of uneven stress on both sides of the first positive electrode tab, and thus reduce the curling of the first positive electrode tab. And the absolute value is less than 1, the risk of external lithium stripping is close to the risk of internal lithium stripping, which improves the fast charging performance of the secondary battery.

[0126] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; under the idea of the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above, which are not provided in details for simplicity; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery comprising an electrode assembly including a plurality of positive electrode tabs and a plurality of negative electrode tabs alternately stacked in a first direction, with a separator film provided between adjacent ones of the positive electrode tabs and the negative electrode tabs, characterized by, The positive electrode tab includes a first positive electrode tab and a second positive electrode tab, along the first direction, the first positive electrode tab is the outermost tab of the electrode assembly, and the second positive electrode tab is arranged between two adjacent negative electrode tabs; The first positive electrode tab includes a first current collector and a first positive electrode coating, and the first positive electrode coating is arranged on the surface of the first current collector facing the negative electrode tab; The second positive electrode tab includes a second current collector and a second positive electrode coating arranged on both surfaces of the second current collector; The first current collector includes a first conductive layer, and along the first direction, the thickness of the first conductive layer is T1; The second current collector includes a second conductive layer, and along the first direction, the thickness of the second conductive layer is T2, and T1≤T2.

2. The secondary battery according to claim 1, characterized by 1μm≤T2≤30μm, 44%≤T1 / T2≤100%.

3. The secondary battery according to claim 2, characterized by 44%≤T1 / T2≤78%.

4. The secondary battery according to claim 3, characterized by 44%≤T1 / T2≤67%.

5. The secondary battery according to claim 3 or 4, characterized by 4μm≤T1≤10μm.

6. The secondary battery according to claim 1, characterized by The first positive electrode tab further includes an insulating layer arranged on the surface of the first current collector away from the first positive electrode coating.

7. The secondary battery according to claim 6, characterized by Along the first direction, the thickness of the insulating layer is T3, and 10μm≤T3≤20μm.

8. The secondary battery according to claim 6, characterized by The insulating layer includes at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

9. The secondary battery according to claim 1, characterized by The surface of the first current collector facing the first positive electrode coating has a first reinforcing portion; and / or, the surface of the first current collector away from the first positive electrode coating has a second reinforcing portion.

10. The secondary battery according to claim 1, characterized by Along the first direction, the thickness of the first positive electrode coating is T4, and the thickness of the second positive electrode coating is T5, and T4<T5.

11. The secondary battery according to claim 10, characterized by 50%≤T4 / T5≤90%.

12. The secondary battery according to claim 1, characterized by The first conductive layer and the second conductive layer each include at least one of aluminum, aluminum alloy, nickel, nickel alloy, or stainless steel.

13. The secondary battery according to claim 1, characterized by The first conductive layer includes a first sub-conductive layer and a second sub-conductive layer, and the first current collector further includes a first insulating polymer layer arranged between the first sub-conductive layer and the second sub-conductive layer along the first direction.

14. The secondary battery according to claim 13, characterized by The first insulating polymer layer includes at least one of polyethylene terephthalate, polycarbonate, polyamide, polyethylene, polypropylene, vinyl acetate copolymer, or ethylene-methyl acrylate copolymer.

15. An electronic device, comprising: The secondary battery includes any one of claims 1-14.

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