Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and electronic equipment, 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 tab connected with the first current collector, and the first tab extends out of the first current collector in the second direction. The second positive pole piece comprises a second current collector and a second tab connected with the second current collector, and the second tab extends out of the second current collector. The minimum sectional area of the first tab in the first direction is S1, and the minimum sectional area of the second tab in the first direction is S2, S1lt; and S2. 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] This application relates to the field of electrochemical technologies, and particularly 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 stacking process or a winding process. The lithium-ion battery products made by the stacking process have the characteristics of high energy density, more stable internal structure, and high safety.

[0003] Inside a lithium-ion battery, it usually includes a single-sided positive electrode plate with a coating on the outermost layer and a double-sided positive electrode plate with coatings on both inner layers. However, the lithium deintercalation rate of the single-sided positive electrode plate may not match the lithium intercalation rate of the corresponding negative electrode plate. Some lithium ions may be difficult to embed into the corresponding negative electrode plate of the single-sided positive electrode plate in time, which easily leads to lithium deposition. Summary of the Invention

[0004] The embodiments of this application aim to provide a secondary battery and an electronic device, which can reduce the technical problem that lithium deposition easily occurs on the positive electrode plate outside the secondary battery.

[0005] The embodiments of this application solve their technical problems by adopting the following technical solutions:

[0006] In a first aspect, the embodiments of this application propose a secondary battery. The secondary battery includes an electrode assembly. The electrode assembly includes a plurality of positive electrode plates and a plurality of negative electrode plates that are alternately stacked along a first direction. A separator is provided between adjacent positive electrode plates and negative electrode plates. It is characterized in that the positive electrode plates include a first positive electrode plate and a second positive electrode plate. Along the first direction, the first positive electrode plate is the outermost electrode plate of the electrode assembly, and the second positive electrode plate is arranged between two adjacent negative electrode plates. The first positive electrode plate includes a first current collector and a first tab. The first tab is connected to the first current collector. Along a second direction, the first tab extends out of the first current collector. The second positive electrode plate includes a second current collector and a second tab. The second tab is connected to the second current collector. Along the second direction, the second tab extends out of the second current collector. The minimum cross-sectional area of the first tab in the first direction is S1, and the minimum cross-sectional area of the second tab in the first direction is S2, and S1 < S2. Wherein, the first direction is perpendicular to the second direction.

[0007] In the above solution, the minimum cross-sectional area S1 of the first tab is smaller than the minimum cross-sectional area S2 of the second tab, so the current-carrying area of the first tab is smaller than that of the second tab, making the resistance of the first positive electrode tab greater than that of the second positive electrode tab. When the secondary battery is charged and discharged, the current flowing through the first positive electrode tab can be reduced, thereby reducing the current density of the first positive electrode coating, reducing the lithium-ion insertion and extraction reaction, enabling the negative electrode tab corresponding to the first positive electrode tab to have sufficient margin to insert the lithium ions extracted from the first positive electrode coating per unit time, and further reducing the occurrence of lithium deposition. This enables the secondary battery to adapt to a larger charge-discharge rate, and while improving the fast charging performance of the secondary battery, the risk of lithium deposition in the secondary battery can be reduced.

[0008] In some embodiments, along the first direction, the thickness of the first tab is smaller than the thickness of the second tab, and along the third direction, the minimum width of the first tab is smaller than the minimum width of the second tab. Or, along the first direction, the thickness of the first tab is equal to the thickness of the second tab, and along the third direction, the minimum width of the first tab is smaller than the minimum width of the second tab. Or, along the third direction, the minimum width of the first tab is equal to the minimum width of the second tab, and along the first direction, the thickness of the first tab is smaller than the thickness of the second tab. In the above solutions, the minimum cross-sectional area S1 of the first tab can be made smaller than the minimum cross-sectional area S2 of the second tab.

[0009] In some embodiments, along the first direction, the thickness of the first tab is equal to the thickness of the second tab, along the third direction, the minimum width of the first tab is W1, the minimum width of the second tab is W2, and W1 < W2. Among them, the first direction, the second direction, and the third direction are perpendicular to each other in pairs. The minimum width W2 of the second tab is smaller than the minimum width W1 of the first tab, further making the current-carrying area of the first tab smaller than that of the second tab, and the resistance of the first positive electrode tab greater than that of the second positive electrode tab. When the secondary battery is charged and discharged, the current flowing through the first positive electrode tab is reduced, thereby reducing the current density of the first positive electrode coating, reducing the lithium-ion insertion and extraction reaction, and reducing the occurrence of lithium deposition on the first positive electrode tab.

[0010] In some embodiments, 40% ≤ W1 / W2 ≤ 90%, which can reduce the occurrence of lithium deposition while reducing the impact on the strength of the first tab and reducing the impact on the current-carrying capacity of the first tab. Further, 50% ≤ W1 / W2 ≤ 70%, which can further reduce the occurrence of lithium deposition and enable the first tab to have higher strength and higher current-carrying capacity.

[0011] In some embodiments, 1.2 mm ≤ W1 ≤ 9 mm; preferably, 1.5 mm ≤ W1 ≤ 7 mm, which can reduce the occurrence of lithium deposition while enabling the first tab to have higher current-carrying capacity, and is beneficial to improving the charge-discharge rate performance of the secondary battery.

[0012] In some embodiments, the first current collector includes a first conductive layer, the second current collector includes a second conductive layer; the first pole ear is integrally arranged with the first conductive layer. The integral arrangement can reduce the contact resistance at the connection between the first pole ear and the first conductive layer, so that the current transmission is smoother and the conductivity of the single-sided positive electrode sheet is improved. And / or, the second pole ear is integrally arranged with the second conductive layer. The integral arrangement can reduce the contact resistance at the connection between the second pole ear and the second conductive layer, so that the current transmission is smoother and the conductivity of the double-sided positive electrode sheet is improved. At the same time, the integral arrangement can reduce the production process of the electrode sheet, reduce material and processing costs, and improve production efficiency.

[0013] In some embodiments, the thickness of the first conductive layer is T1, the thickness of the second conductive layer is T2, 47%≤T1 / T2≤100%, so that 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 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, so that the negative electrode plate corresponding to the first positive electrode plate has 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.

[0014] In some embodiments, the ratio of the thickness T1 of the first conductive layer to the thickness T2 of the second conductive layer is 47%≤T1 / T2≤78%. Preferably, 47%≤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.

[0015] In some embodiments, the first conductive layer and the second conductive layer both include at least one of aluminum, aluminum alloy, nickel, nickel alloy or stainless steel. Each material has high conductivity, which is beneficial to improving the charge and discharge rate of the secondary battery, and has high mechanical strength, which can reduce curling of the first conductive layer and the second conductive layer.

[0016] In some embodiments, along the first direction, the projection of the first pole ear and the projection of the second pole ear at least partially overlap. The secondary battery also includes a first adapter, the first pole ear and the second pole ear are stacked along the first direction, and the first adapter is connected to the first pole ear and the second pole ear. It is convenient to stack the first pole ear and the second pole ear into a whole, and connect the first adapter to the whole, which is conducive to simplifying the polarity extraction operation.

[0017] In some embodiments, along the second direction, the width of the first pole ear in the third direction is reduced. The smaller the width, the weaker the current carrying capacity of the first pole ear, thereby increasing the resistance of the first pole ear, which can reduce the current distributed to the first positive electrode plate and reduce the occurrence of lithium plating.

[0018] In a second aspect, an embodiment of the present application provides an electronic device, comprising a secondary battery as described in any embodiment of the first aspect above. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and the figures in the drawings do not constitute proportional limitations unless otherwise stated.

[0020] Figure 1 is a schematic diagram of the structure of some secondary batteries provided in the embodiments of the present application;

[0021] Figure 2 is a schematic diagram of the structure of some electrode assemblies provided in the embodiments of the present application;

[0022] Figure 3 is a schematic diagram of the structure of some electrode assemblies provided in the embodiments of the present application;

[0023] Figure 4 is a schematic diagram of a structure in which some first pole tabs provided in the embodiments of the present application are regular pole tabs;

[0024] Figure 5 is a schematic diagram of a structure in which some first pole tabs provided in the embodiments of the present application are irregular pole tabs;

[0025] Figure 6 It is a schematic diagram of the structure of some first positive electrode sheets provided in the embodiments of the present application;

[0026] Figure 7 It is a schematic diagram of the structure of some second positive electrode sheets provided in the embodiments of the present application;

[0027] Figure 8 is a schematic diagram of the structure of some first adapters provided in the embodiments of the present application;

[0028] Fig. 9 It is a schematic diagram of the structure of some second pole ears provided in the embodiments of the present application as regular pole ears;

[0029] Fig.10 is a schematic diagram of a structure in which some second pole tabs provided in the embodiments of the present application are irregular pole tabs;

[0030] Fig.11 is a schematic diagram of the structure of some first current collectors provided in the embodiments of the present application;

[0031] Fig.12 is a schematic diagram of the structure of some second current collectors provided in the embodiments of the present application;

[0032] Fig.13It is a schematic diagram of the structure of some insulating layers provided in the embodiments of the present application;

[0033] Fig.14 It is a schematic diagram of the structure of some negative electrode sheets provided in the embodiments of the present application;

[0034] Fig.15 is a schematic diagram of the structure of some third pole ears provided in the embodiments of the present application;

[0035] Fig.16 It is a schematic diagram of the structure of some second adapters provided in the embodiments of the present application;

[0036] Fig.17 It is a schematic diagram of the structure of some negative electrode plates provided in the embodiments of the present application.

[0037] Description of reference numerals:

[0038] 1000. Secondary battery;

[0039] 100. Electrode assembly;

[0040] 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 pole ear; 113. first positive electrode coating; 114. insulating layer;

[0041] 12, second positive electrode plate; 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 pole ear; 123, second positive electrode coating;

[0042] 20. negative electrode plate; 21. third current collector; 211. third surface; 212. fourth surface; 22. third pole ear; 23. first negative electrode coating;

[0043] 30. Isolation film;

[0044] 40. A first adapter;

[0045] 50. Second adapter;

[0046] 200, housing;

[0047] X, first direction; Y, second direction; Z, third direction. DETAILED DESCRIPTION

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 secondary battery.

[0053] 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.

[0054] 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) on the negative electrode sheet corresponding to the single-sided positive electrode sheet is higher. When charging and discharging the secondary battery, without 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 greater than the risk of internal lithium deposition, which is easy to cause safety problems.

[0055] The inventor of this application further discovered that lithium precipitation is related to the current density of the coating. Since the coating is only set 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, thus causing lithium precipitation. In addition, the thickness of the current collector of the outer single-sided positive electrode sheet increases, which 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, resulting in a further increase in the risk of lithium precipitation.

[0056] 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.

[0057] Please refer to Figure 1 The secondary battery 1000 includes an electrode assembly 100 and a housing 200, wherein the electrode assembly 100 is accommodated 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.

[0058] 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.

[0059] For the first positive electrode plate 11, please refer to Figure 3, the first positive electrode sheet 11 includes a first current collector 111 and a first pole tab 112. The first pole tab 112 is connected to the first current collector 111, and along the second direction Y (the length direction of the positive electrode sheet 10 and / or the negative electrode sheet 20), the first pole tab 112 extends out of the first current collector 111. The minimum cross-sectional area of ​​the first pole tab 112 in the first direction is S1, wherein the minimum cross-sectional area of ​​the first pole tab 112 is the minimum cross-sectional area when current flows into or out of the first pole tab 112.

[0060] Recombination Figure 4 , along the third direction Z (the width direction of the positive electrode sheet 10 and / or the negative electrode sheet 20), the minimum width of the first pole ear 112 is W1. The first direction X, the second direction Y and the third direction Z are perpendicular to each other. Among them, the minimum width of the first pole ear 112 refers to the width of the position where the current carrying area is the smallest when current flows into or out of the first pole ear 112. The first pole ear 112 can be a regular shape, such as a rectangle or a square, and the minimum width of the first pole ear 112 is the width of the first pole ear 112 itself. The first pole ear 112 can also be an irregular shape, such as a wave shape, a vase shape or a sawtooth shape. In some embodiments, please refer to Figure 5 Along the second direction Y, the width of the first pole ear 112 in the third direction Z is reduced, and the minimum width of the first pole ear 112 is the width of the position where the intercepting area is the smallest when current flows in or out.

[0061] The first current collector 111 is a conductive substrate, which can be an aluminum foil that is flat and strip-shaped. In some other embodiments, the first current collector 111 can also be made of at least one of aluminum alloy, nickel, nickel alloy or stainless steel.

[0062] Please refer to Figure 8 One end of the first pole tab 112 can be connected to the first current collector 111, and the other end can be connected to the first adapter 40, and the first adapter 40 directly extends out of the housing 200 to lead out the polarity. The material of the first pole tab 112 can be the same as that of the first current collector 111. For example, the first pole tab 112 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0063] When the first current collector 111 is a metal foil current collector (the first current collector 111 is made of metal as a whole), the first pole tab 112 can be die-cut on the first current collector 111. For example, the first pole tab 112 is connected to the first current collector 111 by welding or conductive adhesive bonding. When the first current collector 111 is a composite current collector, the first pole tab 112 can also be die-cut on the first current collector 111.

[0064] In the present application, please refer to Figure 3 and Figure 6The first positive electrode plate 11 adopts a single-sided coating structure, that is, along the first direction X, the first positive electrode coating 113 is only provided on the surface of the first current collector 111 facing the negative electrode plate 20. The surface of the first current collector 111 facing away from the negative electrode plate 20 can be provided with a blank foil (without coating), which can reduce the space occupied by the first positive electrode plate 11 and improve the volume energy density of the secondary battery 1000.

[0065] The first positive electrode coating 113 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 113. 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. In the embodiment of the present application, there is no particular restriction on the positive electrode conductor and the positive electrode adhesive, 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.

[0066] For the second positive electrode sheet 12, please refer to Figure 3 and Figure 7 The second positive electrode plate 12 includes a second current collector 121 and a second pole tab 122. The second pole tab 122 is connected to the second current collector 121, and the second pole tab 122 extends out of the second current collector 121 along the second direction Y. The minimum cross-sectional area of ​​the second pole tab 122 in the first direction is S2, wherein the minimum cross-sectional area of ​​the second pole tab 122 is the minimum cross-sectional area when current flows into or out of the second pole tab 122.

[0067] Recombination Fig. 9 , along the third direction Z, the minimum width of the second pole ear 122 is W2. The minimum width of the second pole ear 122 refers to the width of the position where the current carrying area is the smallest when the current flows into or out of the second pole ear 122. The second pole ear 122 can be a regular shape, such as a rectangle or a square, and the minimum width of the second pole ear 122 is the width of the first pole ear 122 itself. The second pole ear 122 can also be an irregular shape, such as a wave shape, a vase shape, or a sawtooth shape. In some embodiments, please refer to Fig.10 Along the second direction Y, the width of the second pole ear 122 in the third direction Z is reduced, and the minimum width of the second pole ear 122 is the width of the position where the intercepting area is the smallest when current flows in or out.

[0068] The second current collector 121 is a conductive substrate, which can be an aluminum foil that is flat and strip-shaped. In some other embodiments, the first current collector 111 can also be made of at least one of aluminum alloy, nickel, nickel alloy or stainless steel.

[0069] Please refer to Figure 8 One end of the second pole tab 122 can be connected to the second current collector 121, and the other end can be connected to the first adapter 40, and the first adapter 40 directly extends out of the housing 200 to lead out the polarity. The material of the second pole tab 122 can be the same as that of the second current collector 121. For example, the second pole tab 122 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0070] When the second current collector 121 is a metal foil current collector (the second current collector 121 is made of metal as a whole), the second pole tab 122 can be die-cut on the second current collector 121. The second pole tab 122 is connected to the second current collector 121 by welding or conductive adhesive bonding. When the second current collector 121 is a composite current collector, the second pole tab 122 can also be die-cut on the second current collector 121.

[0071] In the present application, please refer to Figure 3 and Figure 7 , 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 first surface 1212 that are arranged opposite to each other, and the first surface 1211 and the first surface 1212 are both provided with a second positive electrode coating 123. The second positive electrode coating 123 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.

[0072] For the above-mentioned 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.

[0073] In the embodiment of the present application, the minimum cross-sectional area S1 of the first pole ear 112 is smaller than the minimum cross-sectional area S2 of the second pole ear 122, and the current-carrying area of ​​the first pole ear 112 is smaller than the current-carrying area of ​​the second pole ear 122, so that the resistance of the first positive electrode plate 11 is greater than 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 113, reducing the deintercalation reaction of lithium ions, so that the negative electrode plate 20 corresponding to the first positive electrode plate 11 has sufficient margin per unit time to embed the lithium ions released from the first positive electrode coating 113, thereby reducing the occurrence of lithium plating. While improving the fast charging performance of the secondary battery 1000, the risk of lithium plating of the secondary battery 1000 can be reduced.

[0074] For the minimum cross-sectional areas of the first pole lug 112 and the second pole lug 122, in some embodiments, along the first direction X, the thickness of the first pole lug 112 is less than the thickness of the second pole lug 122, and along the third direction Z, the minimum width of the first pole lug 112 is less than the minimum width of the second pole lug 122. Alternatively, along the first direction X, the thickness of the first pole lug 112 is equal to the thickness of the second pole lug 122, and along the third direction Z, the minimum width of the first pole lug 112 is less than the minimum width of the second pole lug 122. Alternatively, along the third direction Z, the minimum width of the first pole lug 112 is equal to the minimum width of the second pole lug 122, and along the first direction X, the thickness of the first pole lug 112 is less than the thickness of the second pole lug 122. It is sufficient that the minimum cross-sectional area S1 of the first pole lug 112 is less than the minimum cross-sectional area S2 of the second pole lug 122.

[0075] Among them, for the thickness of the first pole tab 112 being equal to the thickness of the second pole tab 122, within the error range, the absolute value of the difference between the thickness of the first pole tab 112 and the thickness of the second pole tab 122 is in the range of 0 to 10 μm, and the thickness of the first pole tab 112 can be regarded as being equal to the thickness of the second pole tab 122.

[0076] It should be noted that, since the thickness of the first pole lug 112 and the thickness of the second pole lug 122 are usually at the micrometer level in actual production, it is difficult to adjust the thickness of the first pole lug 112 and the thickness of the second pole lug 122 in actual production. Therefore, in some preferred embodiments, the absolute value of the difference between the thickness of the first pole lug 112 and the thickness of the second pole lug 122 is in the range of 0 to 10 μm, and when the thickness of the first pole lug 112 and the second pole lug 122 are considered equal, the minimum cross-sectional area S1 of the first pole lug 112 and the minimum cross-sectional area S2 of the second pole lug 122 are adjusted by adjusting the width of the first pole lug 112 and the width of the second pole lug 122.

[0077] When the minimum width W1 of the first pole ear 112 is less than the minimum width W2 of the second pole ear 122, for example, 40% ≤ W1 / W2 ≤ 90%, the influence on the strength of the first pole ear 112 can be reduced while reducing the external lithium ion, and the influence on the current carrying capacity of the first pole ear 112 can be reduced. The width range of the second pole ear 122 is usually set to 3mm ≤ W2 ≤ 10mm. In the embodiment of the present application, the width W1 of the first pole ear 112 has a value range of 1.2mm ≤ W1 ≤ 9mm. The specific width of the first pole ear 112 can be set according to the corresponding relationship and the width of the second pole ear 122. For example, W1 can be 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm or 9mm, etc.

[0078] In some other embodiments, when the secondary battery is a power battery, the width of the first pole ear 112 may also be selected to be 1mm≤W1≤90mm. In the embodiment of the present application, the secondary battery 1000 is preferably a secondary battery 1000 of a consumer electronic device, and the range thereof may be selected to be 1.2mm≤W1≤9mm.

[0079] In some preferred embodiments, the minimum width W1 of the first pole tab 112 is smaller than the minimum width W2 of the second pole tab 122, and the ratio of W1 to W2 is in the range of 50%≤W1 / W2≤70%, which can further reduce the occurrence of lithium plating and enable the first pole tab 112 to have higher strength and higher current carrying capacity.

[0080] In some preferred embodiments, the minimum width W2 of the second pole ear 122 can be set to 3mm to 10mm. According to the relationship between W1 and W2, 50%≤W1 / W2≤70%, the minimum width W1 of the first pole ear 112 can be in the range of 1.5mm≤W1≤7mm. Not only does it reduce lithium plating, but it also makes the first pole ear 112 have a higher current carrying capacity, which is beneficial to improving the charge and discharge rate performance of the secondary battery 1000. For example, W1 can be 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 7mm, etc.

[0081] In some embodiments, along the second direction Y, the width of the first pole ear 112 in the third direction Z is reduced, and the width is smaller. This weakens the current carrying capacity of the first pole ear 112, thereby increasing the resistance of the first pole ear 112, which can reduce the current of the first positive electrode plate 111, thereby reducing the occurrence of lithium plating.

[0082] In some embodiments, please refer to Figure 2 and Figure 3, along the first direction X, the projection of the first pole tab 112 and the projection of the second pole tab 122 at least partially overlap. The secondary battery 1000 further includes a first adapter 40, the first pole tab 112 and the second pole tab 122 are stacked along the first direction X, and the first adapter 40 is connected to the first pole tab 112 and the second pole tab 122, which can facilitate the first pole tab 112 and the second pole tab 122 to be stacked as a whole, and the first adapter 40 is connected to the whole, which is conducive to simplifying the lead-out operation of the polarity.

[0083] In some embodiments, please refer to Figure 6 and Fig.11 The first current collector 111 includes a first conductive layer 1111, which is a material layer capable of conducting current, and current 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, each of which has high conductivity, which is beneficial to improving the charge and discharge rate of the secondary battery 1000, and has high mechanical strength, which can reduce the curling of the first conductive layer 1111.

[0084] The first pole tab 112 can be integrally provided with the first conductive layer 1111, and the integral provision can reduce the resistance at the connection between the first pole tab 112 and the first conductive layer 1111, making the current transmission smoother and improving the conductivity of the first positive electrode plate 11. At the same time, the integral provision can reduce the production process of the plate, reduce the material and processing costs, and improve the production efficiency.

[0085] When the first current collector 111 is a metal foil current collector (the first current collector 111 is made of metal), the first conductive layer 1111 is the first current collector 111 itself, and the first electrode tab 112 can be integrally provided with the first current collector 111. For example, the first conductive layer 1111 and the first electrode tab 112 are directly obtained by die cutting.

[0086] 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 Fig.11 , the 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. The first pole tab 112 may be integrally disposed with the first current collector 111. For example, the metal layers of the first conductive layer 1111 and the first pole tab 112 are directly obtained by die cutting.

[0087] 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 will not change the thickness and conductive properties of the conductive part in the first current collector 111.

[0088] In some embodiments, the first insulating polymer layer 1112 includes at least one of polyester, polyamide, modified polyolefin, olefin copolymer or unsaturated olefin copolymer. Polyester includes but is not limited to polybutylene terephthalate (PBT), polycarbonate (PC), and polyether polyester. Polyamide includes but is not limited to polyamide 6 (PA6) and polyamide 66 (PA66). Modified polyolefin includes but is not limited to polymaleic anhydride, polymethyl methacrylate, and glycidyl methacrylate grafted polyethylene. Olefin copolymers include but are not limited to polypropylene and ethylene-propylene copolymers. Unsaturated olefin copolymers include but are not limited to [methyl or ethylene-(methyl)] acrylate copolymers. The above materials are selected as the material of the first insulating polymer layer 1112 to ensure that the strength, toughness, insulation, and formability of the first insulating polymer layer 1112 meet the requirements of the composite current collector and provide good support for the first conductive layer 1111.

[0089] Please refer to Figure 7 and Fig.12 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 current, and the current 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, each of which has high conductivity, which is beneficial to improving the charge and discharge rate of the secondary battery 1000, and has high mechanical strength, which can reduce the curling of the second conductive layer 1213. Among them, the materials of the first conductive layer 1111 and the second conductive layer 1213 need to be consistent when they are set. For example, the first conductive layer 1111 and the second conductive layer 1213 are both aluminum foil.

[0090] The second pole tab 122 can be integrated with the second conductive layer 1213, which can reduce the resistance at the connection between the second pole tab 122 and the second conductive layer 1213, making the current transmission smoother and improving the conductivity of the second positive electrode plate 12. At the same time, the integrated arrangement can reduce the production process of the plate, reduce material and processing costs, and improve production efficiency.

[0091] When the second current collector 121 is a metal foil current collector (the second current collector 121 is made of metal), the second conductive layer 1213 is the second current collector 121 itself, and the second electrode tab 122 can be integrally provided with the second current collector 121. For example, the second conductive layer 1213 and the second electrode tab 122 are directly obtained by die cutting.

[0092] 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. Fig.12 , 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. The second electrode tab 122 may be integrally disposed with the second current collector 121. For example, the metal layers of the second conductive layer 1213 and the second electrode tab 122 are directly obtained by die cutting.

[0093] The second insulating polymer layer 1214 can reduce the thickness of the second conductive layer 1213, 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 reduce 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 will not change the thickness and conductive properties of the conductive part in the second current collector 121.

[0094] In some embodiments, the second insulating polymer layer 1214 includes at least one of polyester, polyamide, modified polyolefin, olefin copolymer or unsaturated olefin copolymer. Polyester includes but is not limited to polybutylene terephthalate (PBT), polycarbonate (PC), and polyether polyester. Polyamide includes but is not limited to polyamide 6 (PA6) and polyamide 66 (PA66). Modified polyolefin includes but is not limited to polymaleic anhydride, polymethyl methacrylate, and glycidyl methacrylate grafted polyethylene. Olefin copolymers include but are not limited to polypropylene and ethylene-propylene copolymers. Unsaturated olefin copolymers include but are not limited to [methyl or ethylene-(methyl)] acrylate copolymers. The above materials are selected as the material of the second insulating polymer layer 1214 to ensure that the strength, toughness, insulation, and formability of the second insulating polymer layer 1214 meet the requirements of the composite current collector and provide good support for the second conductive layer 1213.

[0095] In some embodiments, please refer to Figure 6 and Figure 7, 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. The thickness T1 of the first conductive layer 1111 is less than or equal to the thickness T2 of the second conductive layer 1213. For example, 47%≤T1 / T2≤100%. This range not only reduces 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 an embodiment of the present application, the thickness T1 of the first conductive layer 1111 ranges from 0.47μm≤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.

[0096] 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 47%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 ranges from 0.47μm≤T1≤30μm. However, in actual production, it is difficult to make the thickness of the first conductive layer 1111 to be 0.47μm. When and only when the first current collector 111 is a composite current collector, the thickness of the first conductive layer 1111 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 47%≤T1 / T2≤100%, the thickness T1 of the first conductive layer 1111 ranges from 1.76μm≤T1≤30μm.

[0097] The thickness T1 of the first conductive layer 1111 is less than or equal to the thickness T2 of the second conductive layer 1213, so that the resistance of the first positive electrode plate 11 is 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 113, reducing the deintercalation reaction of lithium ions, and making the negative electrode plate 20 corresponding to the first positive electrode plate 11 have enough margin per unit time to embed the lithium ions released from the first positive electrode coating 113, 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.

[0098] 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 47%≤T1 / T2≤78%, thereby further reducing the risk of external lithium deposition and preventing the first conductive layer 1111 from curling.

[0099] 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 47% ≤ 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.

[0100] 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 113 and even cause the first positive electrode coating 113 to shed powder, thereby affecting the energy density of the secondary battery 1000.

[0101] In order to reduce the above problems, in the embodiments of this application, please refer to Fig.13 , the first positive electrode plate 11 further includes an insulating layer 114, which is disposed on the surface of the first current collector 111 away from the first positive electrode coating 113. By arranging the insulating layer 114 on the first current collector 111, the structural strength of the first current collector 111 is improved, and the edge curling 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 113, and the other side is provided with the insulating layer 114, so that the stress dispersion of the first current collector 111 is more uniform, and the curling of the first current collector 111 is reduced. In addition, the provision of the insulating layer 114 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.

[0102] It should be noted that when the first current collector 111 is a metal foil current collector, the insulating layer 114 can be directly disposed on the surface of the first current collector 111 away from the first positive electrode coating 113. When the first current collector 111 is a composite current collector, the insulating layer 114 can be directly disposed on the surface of the first sub-conductive layer 111a away from the first insulating polymer layer 1112.

[0103] In some embodiments, along the first direction X, the thickness of the insulating layer 114 is T3, 10μm≤T3≤20μm. For example, the thickness of T3 can be 10μm, 15μm, 20μm, etc. If the thickness of the insulating layer 114 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 114 is too thick, for example, greater than 20μm, the insulating layer 114 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 114 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.

[0104] There is no particular restriction on the material of the above-mentioned insulating layer 114 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 114 includes at least one of polyester, polyamide, modified polyolefin, olefin copolymer, unsaturated olefin copolymer, etc. Polyester includes but is not limited to polybutylene terephthalate (PBT), polycarbonate (PC), and polyether polyester. Polyamide includes but is not limited to polyamide 6 (PA6) and polyamide 66 (PA66). Modified polyolefin includes but is not limited to polymaleic anhydride, polymethyl methacrylate, and glycidyl methacrylate grafted polyethylene. Olefin copolymers include but are not limited to polypropylene and ethylene-propylene copolymers. Unsaturated olefin copolymers include but are not limited to [methyl or ethylene-(methyl)] acrylate copolymers.

[0105] In some embodiments, please refer to Fig.14 and Fig.15 The negative electrode sheet 20 includes a third current collector 21 and a third electrode tab 22. The third electrode tab 22 is connected to the third current collector 21, and the third electrode tab 22 extends out of the third current collector 21 along the second direction Y.

[0106] The third current collector 21 is a conductive substrate, which can be made of copper foil, nickel foil or titanium foil that is flat and strip-shaped. In some other embodiments, the third current collector 21 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.).

[0107] Please refer to Fig.16One end of the third pole tab 22 can be connected to the third current collector 21, and the other end can be connected to the second adapter 50. The second adapter 50 directly extends out of the housing 200 to lead out the polarity and facilitate conduction with the external circuit. The material of the third pole tab 22 can be selected to be consistent with the material of the third current collector 21, such as copper foil. In some other embodiments, at least one of nickel foil, titanium foil, etc. can also be used.

[0108] In some embodiments, please refer to Figure 2 Along the first direction X, the third pole tab 22 is staggered with the first pole tab 112, and the third pole tab 22 is staggered with the second pole tab 122 to distinguish the positive pole tab from the negative pole tab, and facilitate the stacking and pressing of the positive pole tabs and the stacking and pressing of the negative pole tabs.

[0109] In the embodiment of the present application, the negative electrode plate 20 adopts a double-sided coating structure. Figure 3 and Fig.14 The negative electrode sheet 20 further includes a first negative electrode coating 23 disposed on two surfaces of the third current collector 21. Along the first direction X, the third current collector 21 has a third surface 211 and a fourth surface 212 disposed opposite to each other, and the third surface 211 and the fourth surface 212 are both provided with the first negative electrode coating 23.

[0110] The first negative electrode coating 23 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 coated on the third surface 211 and the fourth surface 212 of the third current collector 21, thereby obtaining the first negative electrode coating 23. 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 may include at least one of the above-mentioned positive electrode conductive agents, and the negative electrode adhesive may include at least one of the above-mentioned positive electrode adhesives.

[0111] In some embodiments, please refer to Figure 6 and Figure 7, the thickness of the first positive electrode coating 113 is T4, and the thickness of the second positive electrode coating 123 is T5, where T4 < T5. Since the thickness of the first current collector 111 is thinned and the first positive electrode coating 113 is provided on only one surface, if the first positive electrode coating 113 is too thick, stress imbalance is likely to occur in the first positive electrode sheet 11, easily causing curling. If the first positive electrode coating 113 is too thin, there is less positive active material, and the capacity of the secondary battery 1000 decreases. Reducing the thickness of the first positive electrode coating 113 to 50% ≤ T4 / T5 < 100% facilitates stress balance between the first current collector 111 and the first positive electrode coating 113 in the first positive electrode sheet 11, reducing the curling phenomenon of the first positive electrode sheet 11. And reducing the thickness of the first positive electrode coating 113 can reduce the amount of lithium deintercalation in the first positive electrode coating 113 and lower the risk of outer 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.

[0112] In some other embodiments, the compaction density of the first positive electrode coating 113 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 in the first positive electrode coating 113, and then enable the negative electrode sheet 20 to have sufficient margin to embed lithium ions, reducing the occurrence of lithium deposition.

[0113] In some embodiments, please refer to Figure 7 , the thickness T5 of the second positive electrode coating 123 provided on the first surface 1211 and the thickness T6 of the second positive electrode coating 123 provided on the second surface 1212 are approximately equal within an error range of -0.5 μm ≤ T6 - T5 ≤ 0.5 μm, making the performance of the secondary battery 1000 more balanced and stable, and reducing the phenomenon of local performance imbalance caused by too large a thickness difference between the second positive electrode coatings 123 on the two surfaces of the second current collector 121.

[0114] In some embodiments, please refer to Fig.17 , the thickness T7 of the first negative electrode coating 23 provided on the third surface 211 and the thickness T8 of the first negative electrode coating 23 provided on the fourth surface 212 are approximately the same within an error range of -0.5 μm ≤ T8 - T7 ≤ 0.5 μm, making the performance of the secondary battery 1000 more balanced and stable, and reducing the situation of local performance imbalance caused by too large a thickness difference between the first negative electrode coatings 23 on the two surfaces of the third current collector 21.

[0115] In some embodiments, please refer to Figure 3 , along the first direction X, the projection of the first positive electrode coating 113 falls within the projection range of the first negative electrode coating 23, enabling the negative electrode sheet 20 to have sufficient margin to embed the lithium ions released from the first positive electrode sheet 11, and reducing the phenomenon of lithium deposition in the negative electrode sheet 20.

[0116] For further information, please refer to Figure 2 , Figure 6 and Fig.17 , along the length direction of the first positive electrode sheet 11 (the second direction Y), the length of the first positive electrode coating 113 is L1. Along the length direction of the negative electrode sheet 20 (the second direction Y), the length of the first negative electrode coating 23 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 113 is W3. Along the width direction of the negative electrode sheet 20 (the third direction Z), the width of the first negative electrode coating 23 is W4, then 0.6mm≤W4-W3≤4mm, so that the negative electrode sheet 20 has enough margin to accommodate the lithium ions released from the positive electrode sheet 10, which can further reduce the lithium precipitation phenomenon of the negative electrode sheet 20, and limit 0.6mm≤L2-L1≤4mm and 0.6mm≤W4-W3≤4mm, so that the secondary battery 1000 has a higher energy density.

[0117] 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.

[0118] 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.

[0119] The test is as follows:

[0120] 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:

[0121] (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.

[0122] (2) Insert copper wire into the battery: 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.

[0123] (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.

[0124] (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.

[0125] The test process is as follows:

[0126] (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 .

[0127] (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.

[0128] (3) Calculate the voltage difference Δ=V1-V2.

[0129] Evaluation principle: The lower the negative electrode potential, the higher the risk of lithium deposition.

[0130] Evaluation criteria: Δ>0, the risk of lithium in the inner layer is greater than that of lithium in the outer layer; Δ<0, the risk of lithium in the outer layer is higher than that of lithium in the inner layer; Δ=0, 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.

[0131] Example 1

[0132] Preparation of lithium-ion secondary batteries

[0133] (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.

[0134] 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 current collector is 15μm. 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.

[0135] 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 current collector is 9 μm. 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 34 μm 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 69 mm and the width is 69 mm.

[0136] (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.

[0137] A copper foil was selected as the third current collector. The room temperature conductivity of the copper foil was 5.998e7 S / m and the thickness was 6 μm. The slurry was evenly coated on both surfaces of the third current collector and dried. A first negative electrode coating with a thickness of 42 μm was formed on both surfaces of the third current collector to obtain a negative electrode sheet with a negative electrode coating on both sides. The length of the negative electrode sheet was 70 mm and the width was 69.7 mm.

[0138] (3) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC),

[0139] Ethyl methyl 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, 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%.

[0140] (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.

[0141] (5) Preparation of electrode assembly: The prepared positive electrode sheets, separators and negative electrode sheets are stacked, and an aluminum sheet with a length of 12 mm and a thickness of 70 μm is selected as the first electrode ear, and the first electrode ear is welded to the first current collector, and the minimum width of the first electrode ear is 1.8 mm. An aluminum sheet with a length of 12 mm and a thickness of 70 μm is selected as the second electrode ear, and the second electrode ear is welded to the second current collector, and the minimum width of the second electrode ear is 6 mm. A nickel sheet with a size of 12 mm × 6 mm is selected as the third electrode ear, and the third electrode ear is welded to the third current collector. The electrode ears of the above-mentioned positive electrode sheets are welded as a whole and then led out through the first adapter, and the electrode ears of the above-mentioned negative electrode sheets are welded as a whole and then led out through the second adapter to form an electrode assembly for standby use. Among them, the number of layers of the first positive electrode sheet is 2, the number of layers of the second positive electrode sheet is 11, the number of layers of the negative electrode sheet is 12, and the number of layers of the separator is 24.

[0142] (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 a seal at the two transfer tabs after integral welding, 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.

[0143] (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.

[0144] It should be noted that in Example 1, the thickness of the first pole lug and the second pole lug are consistent, and the minimum cross-sectional area S1 of the first pole lug and the minimum cross-sectional area S2 of the second pole lug are adjusted by adjusting the width of the first pole lug and the second pole lug.

[0145] Different from Example 1, the relevant parameters in Examples 2 to 21 and Comparative Examples 1 to 6 can be referred to in Table 1 below.

[0146] Table 1

[0147]

[0148]

[0149] According to the above Table 1, in combination with Examples 1 to 7 and Comparative Examples 1 to 2, Examples 8 to 14 and Comparative Examples 3 to 4, Examples 15 to 21 and Comparative Examples 5 to 6, it can be known that when the ratio of the minimum width W1 of the first pole ear to the minimum width W2 of the second pole ear is less than 1, that is, the ratio of the minimum cross-sectional area S1 of the first pole ear to the minimum cross-sectional area S2 of the second pole ear is less than 1, as the ratio of W1 to W2 decreases, the value of Δ gradually increases, and the absolute value of Δ gradually decreases, indicating that the risk of external lithium chromatography is reduced, and the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography is gradually reduced, which improves the discharge rate of the outer layer without lithium chromatography, and improves the fast charging performance of the secondary battery.

[0150] When the minimum width W2 of the second pole ear is constant, the smaller the ratio of the minimum width W1 of the first pole ear to the minimum width W2 of the second pole ear, the smaller the minimum width W1 of the first pole ear, the closer Δ is to 0, and the risk of lithium deposition in the outer layer is closer to the risk of lithium deposition in the inner layer. However, in Example 1, Example 8 and Example 15, the overall width of the first pole ear is large, and the minimum width of the first pole ear is small, which may cause the strength at the minimum width of the first pole ear to be weak, and may cause the first pole ear to be damaged and torn. And the minimum width of the first pole ear is small, which may cause the resistance of the first pole ear to be too large, and will also affect the charge and discharge rate performance of the battery. Therefore, in the embodiments of the present application, in combination with Examples 2 to 7, Examples 9 to 14 and Examples 16 to 21, 40% ≤ W1 / W2 ≤ 90% can be selected. Among them, the minimum width W1 of the first pole ear can be selected as 1.2mm ≤ W1 ≤ 9mm.

[0151] Compared with Example 2, in Example 3, the width of the first pole ear is larger, but Δ is closer, indicating that the risk of lithium deposition in Example 2 and Example 3 is closer. However, the first pole ear with a wider width in Example 3 has higher strength and stronger current carrying capacity. It is similar in Example 9 and Example 10, and Example 16 and Example 17. Therefore, in the embodiments of the present application, it can be further preferred that 50%≤W1 / W2≤70%. Among them, the minimum width W1 of the first pole ear can be further preferably 1.5mm≤W1≤7mm.

[0152] Different from Example 1, in Example 22, the thickness T1 of the first conductive layer is 4 μm, the thickness T2 of the second conductive layer is 9 μm, and the minimum width of the first pole tab is 5.4 mm, and the minimum width of the second pole tab is 6 mm. The relevant parameters in Examples 23 to 40 can be referred to in Table 2 below.

[0153] Table 2

[0154] <![CDATA[T1(μm)]]> <![CDATA[T2(μm)]]> <![CDATA[T1 / T2(%)]]> Voltage difference Δ(mV) Embodiment 22 4.2 9 47％ 0 Embodiment 23 5 9 56％ -1.45 Embodiment 24 6 9 67％ -3.03 Embodiment 25 7 9 78％ -5.15 Embodiment 26 8 9 89％ -6.51 Embodiment 27 9 9 100％ -7.49 Embodiment 28 10 10 100％ -7.73 Embodiment 29 11 9 122％ -8.67 Embodiment 30 13 9 144％ -9.62 Embodiment 31 15 9 167％ -10.33 Embodiment 32 7.5 15 50％ -1.07 Embodiment 33 10 15 67％ -3.26 Embodiment 34 12 15 80％ -5.55 Embodiment 35 15 15 100％ -7.98 Embodiment 36 2.5 6 41％ 0.06 Embodiment 37 3 6 50％ -0.30 Embodiment 38 4 6 67％ -2.71 Embodiment 39 5 6 83％ -5.21 Embodiment 40 6 6 100％ -7.01

[0155] According to Table 2 above, in combination with Examples 22 to 31, 32 to 35, and 36 to 40, when the ratio of the thickness T1 of the first conductive layer to the thickness T2 of the second conductive layer is less than 1, 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 risk of external lithium chromatography is reduced, and the difference between the risk of external lithium chromatography and the risk of internal lithium chromatography gradually decreases, thereby improving the discharge rate of the outer layer without lithium chromatography and improving the fast charging performance of the secondary battery.

[0156] When the ratio of the thickness T1 of the first conductive layer to the thickness T2 of the second conductive layer is smaller, Δ is closer to 0, and the risk of lithium deposition in the outer layer is closer to the risk of lithium deposition in the inner layer, or even the risk of lithium deposition in the outer layer is lower than the risk of lithium deposition in the inner layer. However, in Example 36, the ratio of T1 to T2 is small, and the thickness of the first conductive layer is thin, which may cause the first positive electrode sheet to curl. Therefore, in the embodiments of the present application, in combination with Examples 22 to 28, and Examples 37 to 40, 47% ≤ T1 / T2 ≤ 100% can be selected.

[0157] In Examples 22 to 25, when 47% ≤ T1 / T2 ≤ 78%, the voltage difference Δ is relatively large, and the absolute value of Δ is relatively small, indicating that the difference between the outer and inner lithium chromatography is small. In Examples 24 and 25, the thickness of the first conductive layer T1 differs by only 1 μm, but the voltage difference of Example 24 is closer to 0, and the risk of outer lithium chromatography is closer to the risk of inner lithium chromatography. Combined with Examples 32 to 33, and Examples 37 to 38, when 47% ≤ T1 / T2 ≤ 67%, it is shown that the risk of outer lithium chromatography is closer to the risk of inner lithium chromatography and the difference is smaller. In the embodiments of the present application, 47% ≤ T1 / T2 ≤ 67% may be preferred. Among them, the thickness T1 of the first conductive layer may be preferably 3μm ≤ T1 ≤ 10μm. However, when the first conductive layer T1 is 3μm, its strength is weak and curling is prone to occur. Therefore, in the embodiments of the present application, 4μm ≤ T1 ≤ 10μm may be further preferred.

[0158] 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 includes a first current collector and a first electrode tab, wherein the first electrode tab is connected to the first current collector, and the first electrode tab extends out of the first current collector along the second direction; The second positive electrode plate comprises a second current collector and a second electrode tab, the second electrode tab is connected to the second current collector, and the second electrode tab extends out of the second current collector along the second direction; The minimum cross-sectional area of ​​the first pole tab in the first direction is S1, and the minimum cross-sectional area of ​​the second pole tab in the first direction is S2, S1 <S2; The first direction is perpendicular to the second direction.

2. The secondary battery according to claim 1, characterized in that: Along the first direction, the thickness of the first pole ear is equal to the thickness of the second pole ear. Along the third direction, the minimum width of the first pole ear is W1, and the minimum width of the second pole ear is W2. <W2; The first direction, the second direction and the third direction are perpendicular to each other.

3. The secondary battery according to claim 2, characterized in that: 40%≤W1 / W2≤90%.

4. The secondary battery according to claim 3, characterized in that: 50%≤W1 / W2≤70%.

5. The secondary battery according to claim 3, characterized in that: 1.2mm≤W1≤9mm.

6. The secondary battery according to claim 5, characterized in that: 1.5mm≤W1≤7mm.

7. The secondary battery according to claim 1, characterized in that: The first current collector includes a first conductive layer, and the second current collector includes a second conductive layer; Along the first direction, the thickness of the first conductive layer is T1, the thickness of the second conductive layer is T2, and T1≤T2. 8 . The secondary battery according to claim 7 , wherein the first electrode tab is integrally provided with the first conductive layer; and / or the second electrode tab is integrally provided with the second conductive layer.

9. The secondary battery according to claim 7, characterized in that: 47%≤T1 / T2≤100%.

10. The secondary battery according to claim 9, characterized in that: 47%≤T1 / T2≤78%.

11. The secondary battery according to claim 9, characterized in that: 4μm≤T1≤10μm.

12. The secondary battery according to claim 7, 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: Along the first direction, a projection of the first pole tab at least partially overlaps a projection of the second pole tab; The secondary battery further includes a first adapter, the first pole tab and the second pole tab are stacked along the first direction, and the first adapter is connected to the first pole tab and the second pole tab.

14. The secondary battery according to claim 2, characterized in that: Along the second direction, the first electrode tab is arranged with a width reduced in the third direction.

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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