Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and an electronic device. The secondary battery comprises an electrode assembly, the electrode assembly comprises a plurality of positive pole pieces and a plurality of negative pole pieces which are alternately stacked along a first direction, and an isolating membrane is arranged between the adjacent positive pole piece and negative pole piece. The positive pole pieces comprise a first positive pole piece and a second positive pole piece, in the first direction, the first positive pole piece is the outermost pole piece of the electrode assembly, and the second positive pole piece is arranged between the two adjacent negative pole pieces. The first positive pole piece comprises a first current collector and a first positive coating arranged on the surface, facing the negative pole piece, of the first current collector. And the second positive pole piece comprises a second current collector and second positive coating layers arranged on two surfaces of the second current collector. The first current collector comprises a first conducting layer, the thickness of the first conducting layer is T1 in the first direction, the second current collector comprises a second conducting layer, the thickness of the second conducting layer is T2, and T1 is smaller than or equal to T2. The lithium precipitation risk of the secondary battery can be reduced, and the fast charging performance of the secondary battery is improved.

Description

Technical Field

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

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

[0003] Lithium-ion batteries usually include a single-sided positive electrode sheet with a coating on the outermost side and a double-sided positive electrode sheet with a coating on the inner side. However, the lithium removal rate of the single-sided positive electrode sheet may not match the lithium insertion rate of the corresponding negative electrode sheet, and some lithium ions may have difficulty in timely embedding into the negative electrode sheet corresponding to the single-sided positive electrode sheet, which may easily lead to lithium deposition. Summary of the invention

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

[0005] In order to solve the technical problem, the embodiment of the present application adopts the following technical solution:

[0006] In the first aspect, an embodiment of the present application proposes 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 alternately stacked along a first direction, an isolation membrane is arranged between adjacent positive electrode sheets and negative electrode sheets, the positive electrode sheets include a first positive electrode sheet and a second positive electrode sheet, along the first direction, the first positive electrode sheet is the outermost electrode 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, and 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, and the thickness of the first conductive layer is T1 along the first direction; the second current collector includes a second conductive layer, and the thickness of the second conductive layer is T2 along the first direction, and T1≤T2.

[0007] In the above technical solution, if the thickness T1 of the first conductive layer is less than or equal to the thickness T2 of the second conductive layer, then the current-carrying area of ​​the first conductive layer is less than or equal to the current-carrying area of ​​the second conductive layer, thereby making the resistance of the first positive electrode plate greater than or equal to the resistance of the second positive electrode plate. When the secondary battery is charged and discharged, the current flowing through the first positive electrode plate can be reduced, thereby reducing the current density of the first positive electrode coating, reducing the deintercalation reaction of lithium ions, and making the negative electrode plate corresponding to the first positive electrode plate have sufficient margin per unit time to embed the lithium ions released from the first positive electrode coating, thereby reducing the occurrence of lithium plating. While improving the fast charging performance of the secondary battery, the risk of lithium plating of the secondary battery can be 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 is the risk of external lithium deposition reduced, the fast charging performance of the secondary battery is improved, but the first positive electrode sheet also has a higher structural strength, which can reduce the curling of the first positive electrode sheet.

[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 the occurrence of lithium precipitation and shorten the difference between the outer lithium precipitation and the inner lithium precipitation. Further, 4μm≤T1≤10μm.

[0010] In some embodiments, the first positive electrode plate further includes an insulating layer, which is disposed on a surface of the first current collector away from the first positive electrode coating. By disposing the insulating layer on the first current collector, the stress distribution on both sides of the first positive electrode plate can be made 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, 10 μm≤T3≤20 μm, which effectively improves the curling phenomenon of the first positive electrode sheet, reduces the space occupied by the insulating layer, and improves the energy density of the secondary battery.

[0012] In some embodiments, the insulating layer includes 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 reinforcement portion; and / or the surface of the first current collector facing away from the first positive electrode coating has a second reinforcement portion. Both the first reinforcement portion and the second reinforcement portion can enhance the structural strength of the first positive electrode sheet and reduce the curling phenomenon of the first positive electrode sheet.

[0014] In some embodiments, 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, where T4 < T5. The fact that the thickness of the first positive electrode coating is less than that of the second positive electrode coating is beneficial to improving the curling phenomenon of the first positive electrode sheet, and thinning the thickness of the first positive electrode coating can reduce the amount of lithium deintercalation from the first positive electrode coating, thereby reducing the risk of outer layer lithium deposition. Among them, 50% ≤ T4 / T5 ≤ 90%.

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

[0016] In some embodiments, 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. Along the first direction, the first insulating polymer layer is disposed between the first sub-conductive layer and the second sub-conductive layer. The setting of the first insulating polymer layer thins the first conductive layer, reduces the weight of the first positive electrode sheet, and improves the weight energy density of the secondary battery. When the thickness of the first conductive layer is relatively thin, by increasing the first insulating polymer layer, the overall strength of the first current collector is enhanced, and the curling phenomenon of the first positive electrode sheet is improved.

[0017] In some embodiments, the first insulating polymer layer includes 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 sheet includes a first negative electrode sheet, and a first negative electrode sheet is disposed between two adjacent positive electrode sheets; the first negative electrode sheet includes a third current collector and first negative electrode coatings provided on two surfaces of the third current collector; along the first direction, the projection of the first positive electrode coating falls within the projection range of the first negative electrode coating. This enables the first negative electrode sheet to have sufficient margin to embed the lithium ions released from the first positive electrode sheet, reducing the phenomenon of lithium deposition on the first negative electrode sheet.

[0019] In a second aspect, an embodiment of the present application provides an electronic device, including the secondary battery according to any one of the embodiments in the first aspect above. Description of the Drawings

[0020] One or more embodiments are exemplarily illustrated by corresponding drawings. These exemplary illustrations do not limit the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements, unless otherwise stated, and the drawings in the figures do not constitute a proportional limitation.

[0021] Figure 1 are schematic structural diagrams of some secondary batteries provided by embodiments of the present application;

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

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

[0024] Figure 4 It is a schematic diagram of the structure of some first positive electrode sheets provided in the embodiments of the present application;

[0025] Figure 5 It is a schematic diagram of the structure of some second positive electrode sheets provided in the embodiments of the present application;

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

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

[0028] Figure 8 It is a schematic diagram of the structure of some insulating layers provided in the embodiments of the present application;

[0029] Fig. 9 It is a schematic diagram of the structure of some reinforcement parts provided in the embodiments of the present application;

[0030] Fig.10 is a schematic diagram of the structure of some first negative electrode sheets provided in the embodiments of the present application;

[0031] Fig.11 It is a schematic diagram of the structure of some first negative electrode plates provided in the embodiments of the present application.

[0032] Description of reference numerals:

[0033] 1000. Secondary battery;

[0034] 100. Electrode assembly;

[0035] 10. Positive electrode sheet; 11. First positive electrode sheet; 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; 113. Insulating layer; 114. First reinforcement; 115. Second reinforcement; 12. Second positive electrode sheet; 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;

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

[0037] 30. Isolation film;

[0038] 200, housing;

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

[0040] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments 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", 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 "several" and "multiple" is more than two, unless otherwise clearly and specifically defined.

[0042] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0043] 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. In addition, the technical features involved in the different embodiments of the present application described below may be combined with each other as long as they do not conflict with each other.

[0044] For laminated batteries, especially soft-pack laminated batteries, the interior thereof usually includes a positive electrode sheet coated on one side of the outermost layer (hereinafter referred to as a single-sided positive electrode sheet) and a positive electrode sheet coated on both sides of the inner layer (hereinafter referred to as a double-sided positive electrode sheet). The outermost side of the single-sided positive electrode sheet can be provided with a blank foil (without a coating), which can reduce the space occupied by the single-sided positive electrode sheet and improve the energy density of the battery.

[0045] However, the single-sided positive electrode sheet is located at the outermost layer of the electrode assembly, with a small binding force, and the current collector in the single-sided positive electrode sheet is only coated on one side, resulting in uneven stress distribution on both sides of the current collector, which can easily cause the current collector to curl toward the side away from the coating, and the curling of the current collector can easily cause the coating to fall off, affecting the energy density of the secondary battery. In order to reduce the problem of curling of the single-sided positive electrode sheet, the thickness of the current collector in the single-sided positive electrode sheet is usually increased to improve the deformation resistance of the single-sided 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 deposition (hereinafter referred to as external lithium deposition) of the negative electrode sheet corresponding to the single-sided positive electrode sheet is higher. When charging and discharging the secondary battery, in the absence of lithium deposition, the maximum charge and discharge rate that the double-sided positive electrode sheet can withstand is greater than the maximum charge and discharge rate that the single-sided positive electrode sheet can withstand. In order to improve the rate performance of the secondary battery, the maximum charge and discharge rate of the double-sided positive electrode sheet will be used as a benchmark, but this will exceed the tolerance range of the single-sided positive electrode sheet, making the risk of external lithium deposition higher than the risk of internal lithium deposition, which is likely to cause safety problems.

[0047] The inventors of this application have further discovered that lithium precipitation is related to the current density of the coating. Since the coating is only provided on one side of the single-sided positive electrode sheet, the current is concentrated in the single-layer coating, and the reaction of lithium ion insertion and extraction 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. In addition, the increase in the thickness of the current collector of the outer single-sided positive electrode sheet means that the resistance of the current collector of the outer single-sided positive electrode sheet is smaller, the current density is greater, and the reaction of lithium ion insertion and extraction is more intense, which further increases the risk of lithium precipitation.

[0048] In order to reduce the above problems, on the first aspect, the present application proposes a secondary battery 1000, which refers to a rechargeable and dischargeable soft-pack battery that can store and release electrical 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 includes an electrode assembly 100 and a housing 200, wherein the electrode assembly 100 is received in the housing 200. Figure 2 , Figure 2 The stacked structure of the electrode assembly 100 is shown, and the electrode assembly 100 includes 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 stacked, 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 sheet 10 includes a first positive electrode sheet 11 and a second positive electrode sheet 12. Along the first direction X, the first positive electrode sheet 11 is the outermost positive electrode sheet 10 of the electrode assembly 100. The second positive electrode sheet 12 is the inner positive electrode sheet 10 of the electrode assembly 100, that is, the second positive electrode sheet 12 is arranged between two adjacent negative electrode sheets 20.

[0051] For the first positive electrode plate 11, please refer to Figure 3 and Figure 4 The first positive electrode plate 11 includes a first current collector 111 and a first positive electrode coating 112. The first current collector 111 is a conductive substrate, which can be an aluminum foil that is flat and has a strip structure. In some 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 embodiment of the present application, the first positive electrode plate 11 adopts a single-sided coating structure, that is, along 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 plate 20, and the surface of the first current collector 11 away from the negative electrode plate 20 is arranged with a blank foil (no coating is set), which can reduce the space occupied by the first positive electrode plate 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 conductor, and a positive electrode adhesive, etc. The above materials are mixed and stirred evenly and applied to the surface of the first current collector 111 facing the negative electrode plate 20, thereby obtaining the first positive electrode coating 112. Among them, the positive electrode active material can be selected from at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron manganese phosphate or cobalt-free materials. There is no particular restriction on the positive electrode conductor and the positive electrode adhesive in the embodiment of the present application, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor can be selected from at least one of conductive carbon black, carbon nanotubes, carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The positive electrode adhesive can be selected from at least one of polyacrylate, polyimide, polyamide, polyamideimide, polyvinylidene fluoride, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose or potassium carboxymethyl cellulose.

[0054] For the second positive electrode sheet 12, please refer to Figure 3 and Figure 5The second positive electrode plate 12 includes a second current collector 121 and a second coating 122 disposed on two surfaces of the second current collector 121 (two surfaces disposed opposite to each other along the thickness direction of the second current collector 121). The second current collector 121 has a similar structure to the first current collector 111, and may be made of an overall flat and strip-shaped aluminum foil. Alternatively, in some other embodiments, the material of the first current collector 111 may also be at least one of aluminum alloy, nickel, nickel alloy or stainless steel.

[0055] In the embodiment of the present application, the second positive electrode plate 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 that are arranged opposite to each other, and the first surface 1211 and the second surface 1212 are both provided with a second positive electrode coating 122. The second positive electrode coating 122 is provided on both sides of the inner layer, which can make full use of the space, thereby improving the energy density of the secondary battery 1000.

[0056] For the isolation film 30, please refer to Figure 2 and Figure 3 , the separator 30 is disposed between the positive electrode sheet 10 and the negative electrode sheet 20, and is used to insulate and separate the positive electrode sheet 10 and the negative electrode sheet 20. The separator 30 may be a PE separator 30 or a PP separator 30 containing ceramics. In some embodiments, the separator 30 facing the positive electrode sheet 10 and the surface facing the negative electrode sheet 20 may be provided with an adhesive coating. When the positive electrode sheet 10, the separator 30 and the negative electrode sheet 20 are stacked, the separator 30 may be directly bonded between the positive electrode sheet 10 and the negative electrode sheet 20, and is used to improve the bonding strength between the positive electrode sheet 10, the separator 30 and the negative electrode sheet 20, thereby improving 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 material layer capable of conducting electric charges, and electric charges can flow smoothly in the first conductive layer 1111. The first conductive layer 1111 includes at least one of conductive metals such as aluminum, aluminum alloy, nickel, 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 provision of the first insulating polymer layer 1112 can make the first conductive layer 1111 thinner, reduce the weight of the first positive electrode plate 11, and improve the weight energy density of the secondary battery 1000. In particular, when the thickness of the first conductive layer 1111 is relatively thin, the first insulating polymer layer 1112 is added to improve the structural strength of the first current collector 111 and reduce the curling phenomenon of the first positive electrode plate 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 conductive properties of the conductive part in 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 so that the charges can 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 a part of the second current collector 121. 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 provision of the second insulating polymer layer 1214 can make the second conductive layer 1213 thinner, reduce the weight of the second positive electrode plate 12, and improve the weight energy density of the secondary battery 1000. In particular, when the thickness of the second conductive layer 1213 is relatively thin, the second insulating polymer layer 1214 is added to improve the structural strength of the second current collector 121 and improve the curling phenomenon of the second positive electrode plate 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 conductive properties of the conductive part in 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 embodiment of the present application, along the first direction X, the thickness of the first conductive layer 1111 is T1, and the thickness of the second conductive layer 1213 is T2. If the thickness T1 of the first conductive layer 1111 is less than or equal to the thickness T2 of the second conductive layer 1213, then 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, thereby making the resistance of the first positive electrode plate 11 greater than or equal to the resistance of the second positive electrode plate 12. When the secondary battery 1000 is charged and discharged, the current flowing through the first positive electrode plate 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112, reducing the deintercalation reaction of lithium ions, and making the negative electrode plate 20 corresponding to the first positive electrode plate 11 have sufficient margin per unit time to embed the lithium ions released from the first positive electrode coating 112, thereby reducing the occurrence of lithium plating. The secondary battery 1000 can be adapted to a larger charge and discharge rate, and the risk of lithium plating of the secondary battery 1000 can be reduced while improving the fast charging performance of the secondary battery 1000.

[0066] For the thickness T1 of the first conductive layer 1111, in the embodiment 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 can not only reduce the risk of external lithium plating, but also the first positive electrode plate 11 has a certain structural strength, reducing the phenomenon of curling. The thickness range of the second conductive layer 1213 is usually set to 1μm ≤ T2 ≤ 30μm. In the embodiment of the present application, the thickness T1 of the first conductive layer 1111 ranges from 0.44μm ≤ T1 ≤ 30μm. 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 3μm, 5μm, 7μm, 9μm, 15μm, 20μm, 30μm, etc.

[0067] In particular, when the second current collector 121 is a composite current collector, the thickness range of the second conductive layer 1213 is often set to 1μm≤T2≤30μm. According to 44%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 ranges from 0.44μm≤T1≤30μm. However, in actual production, it is difficult to make the thickness of the first conductive layer 1111 to be 0.44μm. When and only when the first current collector 111 is a composite current collector, the thickness of the first conductive layer may be 1μm. Preferably, 1μm≤T1≤30μm. When the second current collector 121 is made of metal as a whole, the thickness range of the second conductive layer 1213 is 4μm≤T2≤30μm. According to 44%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 ranges from 1.76μm≤T1≤30μm.

[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%, so as to further reduce the risk of external lithium chromatography and shorten the difference between external lithium chromatography and internal lithium chromatography.

[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 of the present application have found that reducing the thickness of the first conductive layer 1111 may weaken the strength of the first current collector 111, resulting in edge curling of the first current collector 111, which may easily damage the structure of the first positive electrode coating 112 and even cause the first positive electrode coating 112 to shed powder, thereby affecting the energy density of the secondary battery 1000.

[0071] To reduce the above problems, in the embodiments of this application, please refer to Figure 8The first positive electrode plate 11 also includes an insulating layer 113, which is disposed on the surface of the first current collector 111 away from the first positive electrode coating 112. By arranging 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, the two surfaces of the first current collector 111, one side is provided with the first positive electrode coating 112, and the other side is provided with the insulating layer 113, so that the stress dispersion of the first current collector 111 is more uniform, and the curling phenomenon of the first current collector 111 is reduced. In addition, the provision of the insulating layer 113 can directly isolate the first positive electrode plate 11 from the shell 200, which can reduce the direct contact between the first positive electrode plate 11 and the shell 200, reduce the corrosion of the first current collector 111, and reduce the possible short circuit risk.

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

[0074] There is no particular limitation on the material of the insulating layer 113 in the present embodiment, 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 Fig. 9, the surface of the first current collector 111 facing the first positive electrode coating 112 has a first strengthening portion 114, and the first strengthening portion 114 can enhance the structural strength of the first positive electrode sheet 11 and improve the curling phenomenon of the first positive electrode sheet 11. And / or, the surface of the first current collector 111 facing away from the first positive electrode coating 112 has a second strengthening portion 115. The second strengthening portion 115 can also enhance the structural strength of the first positive electrode sheet 11 and improve the curling phenomenon of the first positive electrode sheet 11.

[0076] For example, the first strengthening portion 114 can be formed by embossing the surface of the first current collector 111 facing the first positive electrode coating 112, or by providing a protruding block. The protruding block can be provided at a specific part 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 facing away from the first positive electrode coating 112 can also be embossed or provided with a protruding block.

[0077] In some embodiments, please refer 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 provided on the first surface 1211 is T5, and T4 < T5. Since the thickness of the first current collector 111 is thinned and the first positive electrode coating 112 is provided on only one surface, if the first positive electrode coating 112 is too thick, the first positive electrode sheet 11 is prone to stress imbalance and is likely to cause curling. If the first positive electrode coating 112 is too thin, there is less positive active material and the capacity of the secondary battery 1000 is reduced. Reducing the thickness of the first positive electrode coating 112 to 50% ≤ T4 / T5 < 100% makes the stress balance between the first current collector 111 and the first positive electrode coating 112 in the first positive electrode sheet 11 and reduces the curling phenomenon of the first positive electrode sheet 11. And reducing the thickness of the first positive electrode coating 112 can reduce the amount of lithium deintercalation of the first positive electrode coating 112 and reduce the risk of outer layer lithium deposition. 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 sheet 11.

[0078] In some other embodiments, the compaction density of the first positive electrode coating 112 can also be set to be less than the compaction density of the corresponding negative electrode coating, which can reduce the amount of lithium deintercalation of the first positive electrode coating 112, and then enable the negative electrode sheet to have enough margin to embed lithium ions, and reduce the occurrence of lithium deposition.

[0079] In the embodiment of the present application, by adjusting the coating weight of the first positive electrode coating 112, the capacity ratio of the negative electrode sheet 20 corresponding to the first positive electrode sheet 11 to the capacity ratio of the first positive electrode sheet 11 is controlled to be 1.15 to 2, and the capacity ratio of the negative electrode sheet 20 corresponding to the second positive electrode sheet 12 to the capacity ratio of the second positive electrode sheet 12 is 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 external lithium chromatography can be reduced.

[0080] In some embodiments, please refer to Figure 5 Within the allowable error range, the thickness T5 of the second positive electrode coating 122 set on the first surface 1211 is approximately equal to the thickness T6 of the second positive electrode coating 122 set on the second surface 1212, so that the performance of the secondary battery is more balanced and stable, and the local performance imbalance caused by the excessive difference in thickness of the second positive electrode coating 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 be made of copper foil, nickel foil or titanium foil that is flat and has a strip structure. In some other embodiments, the third current collector 211 can also be made of copper alloy foil, stainless steel foil, foamed nickel, 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 embodiment of the present application, the first negative electrode plate 21 adopts a double-sided coating structure. Fig.10 The first negative electrode sheet 21 further includes a first negative electrode coating 212 disposed on both 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 disposed opposite to each other, and the first negative electrode coating 212 is disposed on both the third surface 2111 and the fourth surface 2112.

[0083] The first negative electrode coating 212 includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode adhesive, etc. The above-mentioned material components are mixed and stirred evenly and applied to the third surface 2111 and the fourth surface 2112 of the third current collector 211, thereby obtaining the first negative electrode coating 212. Among them, the negative electrode active material can be selected from at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Sn, SnO, SnO2, Li-Al alloy, metallic lithium, etc. In the embodiment of the present application, there is no particular restriction on the negative electrode conductive agent and the negative electrode adhesive, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can include at least one of the above-mentioned positive electrode conductive agents, and the negative electrode adhesive can include at least one of the above-mentioned positive electrode adhesives.

[0084] In some embodiments, please refer to Fig.11 The thickness T7 of the first negative electrode coating 212 set on the third surface 2111 is approximately equal to the thickness T8 of the first negative electrode coating 212 set on the fourth surface 2112, so that the performance of the secondary battery is more balanced and stable, and the local performance imbalance caused by the excessive difference in thickness of the first negative electrode coating 212 on the two surfaces of the third current collector 211 is reduced.

[0085] In some embodiments, please refer 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 plate 21 has sufficient margin to embed the lithium ions released from the first positive electrode plate 11, which can reduce the lithium deposition phenomenon of the first negative electrode plate 21.

[0086] For further information, please refer to Figure 3 , Figure 4 and Fig.11 , along the length direction of the first positive electrode sheet 11 (the second direction Y), the length of the first positive electrode coating 112 is L1. Along the length direction of the first negative electrode sheet 21 (the second direction Y), the length of the first negative electrode coating 212 is L2, then 0.6mm≤L2-L1≤4mm. Along the width direction of the first positive electrode sheet 11 (the third direction Z), the width of the first positive electrode coating 112 is W1. Along the width direction of the first negative electrode sheet 21 (the third direction Z), the width of the first negative electrode coating 212 is W2, then 0.6mm≤W2-W1≤4mm. The negative electrode sheet has enough margin to accommodate the lithium ions released from the positive electrode sheet, which can further reduce the phenomenon of lithium precipitation in the first negative electrode sheet 21, and limit 0.6mm≤L2-L1≤4mm and 0.6mm≤W2-W1≤4mm, so that the secondary battery has a higher energy density.

[0087] In the second aspect, the embodiment of the present application also proposes an electronic device, including a secondary battery 1000 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it can be used for any electronic device known in the prior art. For example, the electronic device includes but is not limited to a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0088] In the embodiments of the present application, a lithium ion secondary battery is taken as an example to perform a lithium deposition test on it.

[0089] The test is as follows:

[0090] During the battery assembly preparation stage, a reference electrode 1 is placed between the negative electrode sheet and the separator corresponding to the first positive electrode sheet, and a reference electrode 2 is placed between the center negative electrode sheet and the separator (the center negative electrode sheet refers to the negative electrode sheet in the middle or near the middle of the total negative electrode sheets in the secondary battery. For example, if the total number of negative electrode sheets is 14, the center negative electrode sheet is the 7th layer of negative electrode sheets, and both sides can be used). The production 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. Soak one end of the copper wire (about 30 mm) in concentrated sulfuric acid for 2 h to remove the surface insulating paint. Then use alcohol ultrasonic cleaning for 10 to 15 minutes, repeat 2 to 3 times, and finally dry it in an oven for use.

[0092] (2) Insert copper wire into the battery cell: In the designated insertion layer, place the processed end of the copper wire at the center of the separator corresponding to the negative electrode surface, and fix the copper wire with adhesive tape at the edge of the separator. Cover the copper wire with a separator about 10 mm wide and the same length as the width of the negative electrode sheet to separate the copper wire from the negative electrode sheet. Finally, continue the electrode assembly and liquid injection packaging process according to the normal stacking assembly sequence. The copper wire is led out from the side aluminum-plastic film packaging edge. Because the copper wire is thin enough, no additional packaging treatment is required.

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

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

[0095] The test process is as follows:

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

[0097] (2) Charging: Charge the battery according to the normal charging process of the battery. The present embodiment and the comparative example both adopt the following process: first apply 3C constant current charging, charge to 4.5V and then switch to constant voltage charging mode, and stop charging after the current drops to 0.05C. Obtain V changing with time t1 , extract the minimum value as V1, and obtain the V that changes with time t2 , extract the minimum value as V2.

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

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

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

[0101] In the embodiments of the present application, taking the current collector of the positive electrode sheet in the lithium-ion secondary battery as a metal foil current collector as an example, the preparation of each component of the lithium-ion secondary battery is as follows:

[0102] Example 1

[0103] (1) Preparation of positive electrode sheet: Lithium cobalt oxide, a positive electrode active material, acetylene black, a positive electrode conductive agent, and polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5 ) were mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added as a solvent, a positive electrode slurry with a solid content of 75 wt% was prepared, and the slurry was stirred evenly under a vacuum mixer.

[0104] Aluminum foil is selected as the first current collector (first conductive layer), and the first current collector and the first conductive layer have 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 4μm. The slurry is evenly coated on one surface of the first current collector and dried to form a first positive electrode coating with a thickness of 34μm on the surface of the first current collector, thereby obtaining a first positive electrode sheet with a single-sided coating of positive electrode active material. The length of the first positive electrode sheet is 69mm and the width is 69mm.

[0105] Aluminum foil is selected as the second current collector (second conductive layer), and the second current collector and the second conductive layer have 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 9μm. The slurry is evenly coated on the first surface and the second surface of the second current collector, and dried to form a second positive electrode coating with a thickness of 34μm on both the first surface and the second surface, to obtain a second positive electrode sheet coated with positive electrode active material on both sides. The length of the second positive electrode sheet is 69mm and the width is 69mm.

[0106] (2) Preparation of negative electrode sheet: The negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (SuperP), and binder styrene acrylic rubber (SD-3) were mixed in a weight ratio of 89.5:8:1:1.5, and then deionized water was added as a solvent to prepare a negative electrode slurry with a solid content of 50wt%, and stirred evenly.

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

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

[0109] (4) Preparation of isolation membrane: A polyethylene porous membrane is used as a substrate layer, and a ceramic layer containing alumina ceramic and polyvinylidene fluoride binder is coated on one surface of the substrate layer as an isolation membrane, wherein the mass percentage of alumina ceramic in the ceramic layer is 95%, the thickness of the substrate layer is 5 μm, and the thickness of the ceramic layer is 1.8 μm.

[0110] (5) Preparation of electrode assembly: nickel sheets and aluminum sheets with specifications of 12 mm × 6 mm are selected as tabs, the aluminum tabs are welded to the positive electrode sheets (aluminum foil of the positive electrode sheets), and the nickel tabs are welded to the negative electrode sheets. The positive electrode sheets, separators and negative electrode sheets are stacked to form an electrode assembly for use. Among them, the first positive electrode sheet has 2 layers, the second positive electrode sheet has 11 layers, the first negative electrode sheet has 12 layers, and the separator has 24 layers.

[0111] (6) Electrode assembly assembly: Place the aluminum-plastic film formed by punching holes in an assembly fixture, with the pit side facing upward, place the electrode assembly in the pit, and set seals at the two tabs, and apply external force to press tightly. Then, cover the electrode assembly with another aluminum-plastic film formed by punching holes with the pit side facing downward, and heat-seal the four sides of the two aluminum-plastic films by hot pressing to obtain an assembled electrode assembly.

[0112] (7) Liquid injection packaging: The assembled electrode assembly is injected with electrolyte, and after vacuum packaging, static standing, hot pressing, shaping and other processes, a lithium-ion secondary battery is produced.

[0113] Different from Example 1, the relevant parameters of Examples 2 to 12 can be referred to in Table 1 below.

[0114] In Example 8, an insulating layer made of polyamide material is disposed on the surface of the first current collector away from the first positive electrode coating, wherein the thickness of the insulating layer is 15 μm.

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

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

[0117] Comparative Examples 1 to 3 are different from Examples 1 to 12 in that 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, it can be seen that when T1 / T2≤100%, that is, when the thickness of the first conductive layer is less than the thickness of the second conductive layer, Δ is large, that is, the absolute value of Δ is small, indicating that the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography is small. Therefore, in the embodiments of the present application, by limiting T1≤T2, the risk of lithium chromatography can be effectively reduced.

[0121] When 44% ≤ T1 / T2 ≤ 100% is satisfied, as the ratio of T1 to T2 decreases, the value of Δ gradually increases, and the absolute value of Δ gradually decreases, indicating that the difference between the outer layer lithium plating risk and the inner layer lithium plating risk gradually decreases, improving the charge and discharge rate at which the outer layer does not undergo lithium plating under the same working conditions, and thus enhancing the fast charging performance of the secondary battery. In particular, when the ratio of T1 to T2 is 44%, the outer layer lithium plating risk is close to the inner layer lithium plating risk, indicating that the discharge rate at which the outer layer does not undergo lithium plating is basically equal to the discharge rate at which the inner layer does not undergo lithium plating under the same working conditions, and the fast charging performance effect of the secondary battery is optimal.

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

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

[0124] Combining Example 1, Example 8, and Example 9, in Example 1, the first positive electrode sheet is not provided with an insulating layer and a reinforcing portion, and it can be observed that the first positive electrode sheet shows slight curling. In Example 8, the first positive electrode sheet is provided with an insulating layer, and it can be observed that the first positive electrode sheet has no curling, indicating that the insulating layer can improve the structural strength of the first positive electrode sheet and reduce the edge curling phenomenon of the first positive electrode sheet. In Example 9, the first positive electrode sheet is provided with a reinforcing portion, and it can be observed that the first positive electrode sheet has no curling, indicating that the reinforcing portion can enhance the structural strength of the first positive electrode sheet and improve the edge curling phenomenon of the first positive electrode sheet.

[0125] In combination with Example 2, Example 10 to Example 12, the thickness T4 of the first positive electrode coating and the thickness T5 of the second positive electrode coating in Example 2 are equal, the first positive electrode plate is slightly curled, and the voltage difference is -1.54mV. In Example 10 to Example 12, 50%≤T4 / T5≤90%, the voltage difference is greater than -1.54mV, 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 desorption of the first positive electrode coating and reduce the risk of external lithium chromatography. Further, when 50%≤T4 / T5≤70%, in Example 10 and Example 11, the first positive electrode plate did not curl, indicating that the thickness of the first coating is reduced, which can reduce the uneven stress on both sides of the first positive electrode plate, thereby reducing the curling of the first positive electrode plate. And the absolute values ​​of Δ are all less than 1, the risk of external lithium chromatography is close to the risk of internal lithium chromatography, 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, rather than to limit them. Under the concept of the present application, the technical features in the above embodiments or different embodiments may also be combined, the steps may be implemented in any order, and there are many other changes in different aspects of the present application as described above, which are not provided in detail for the sake of simplicity. Although the present application has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly comprises a plurality of positive electrode sheets and a plurality of negative electrode sheets alternately stacked along a first direction, and a separator is disposed between adjacent positive electrode sheets and negative electrode sheets, characterized in that: 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 electrode sheet of the electrode assembly, and the second positive electrode sheet is arranged between two adjacent negative electrode sheets. The first positive electrode plate comprises a first current collector and a first positive electrode coating, wherein the first positive electrode coating is disposed on a surface of the first current collector facing the negative electrode plate; The second positive electrode plate includes a second current collector and a second positive electrode coating disposed on two 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. Along the first direction, the thickness of the second conductive layer is T2, where T1≤T2.

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

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

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

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

6. The secondary battery according to claim 1, characterized in that: The first positive electrode plate further includes an insulating layer, and the insulating layer is disposed on a surface of the first current collector facing away from the first positive electrode coating.

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

8. The secondary battery according to claim 6, characterized in that: 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 in that: The surface of the first current collector facing the first positive electrode coating has a first reinforcement portion; and / or the surface of the first current collector facing away from the first positive electrode coating has a second reinforcement portion.

10. The secondary battery according to claim 1, characterized in that: Along the first direction, the thickness of the first positive electrode coating is T4, the thickness of the second positive electrode coating is T5, and the thickness of T4 is <T5。 11. The secondary battery according to claim 10, characterized in that: 50%≤T4 / T5≤90%.

12. The secondary battery according to claim 1, characterized in that: 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 in that: 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. Along the first direction, the first insulating polymer layer is disposed between the first sub-conductive layer and the second sub-conductive layer.

14. The secondary battery according to claim 13, characterized in that: 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, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 14.

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