Secondary battery and electronic device

Abstract

This application relates to a secondary battery and an electronic device, including 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. A separator is disposed between adjacent positive 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 disposed between two adjacent negative electrode sheets. The first positive electrode sheet includes a first current collector and a first positive electrode coating. Along the first direction, the first current collector includes a first surface and a second surface disposed opposite to each other, with the second surface facing the negative electrode sheet. The first positive electrode coating is disposed on the second surface. The first surface has a plurality of grooves. This reduces the risk of lithium plating in the secondary battery and improves the fast-charging performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and more particularly to a secondary battery and electronic device. Background Technology

[0002] The electrode components of lithium-ion batteries (secondary batteries) are usually made using a stacking or winding process. Lithium-ion battery products made using the stacking process have the characteristics of high energy density, more stable internal structure, and high safety.

[0003] Lithium-ion batteries typically consist of an outermost single-sided positive electrode with a single-sided coating and an innermost double-sided positive electrode with a double-sided coating. However, the delithiation rate of the single-sided positive electrode and the lithium insertion rate of the corresponding negative electrode may not match. Some lithium ions may fail to be inserted into the corresponding negative electrode in time, easily leading to lithium plating. Summary of the Invention

[0004] The embodiments of this application aim to provide a secondary battery and electronic device that can reduce the technical problem of lithium plating in secondary batteries.

[0005] In order to solve its technical problems, the embodiments of this application adopt the following technical solutions:

[0006] In a first aspect, embodiments of this application propose a secondary battery, including 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, with a separator disposed between adjacent positive and negative electrode sheets. The positive electrode sheets include a first positive electrode sheet and a second positive electrode sheet, and a second positive electrode sheet is disposed between two adjacent negative electrode sheets. Along the first direction, the first positive electrode sheet is the outermost electrode sheet of the electrode assembly. The second positive electrode sheet is disposed between two adjacent negative electrode sheets. The first positive electrode sheet includes a first current collector and a first positive electrode coating. Along the first direction, the first current collector includes a first surface and a second surface disposed opposite to each other, with the second surface facing the negative electrode sheet, and the first positive electrode coating disposed on the second surface. The first surface has a plurality of grooves.

[0007] In the above scheme, by setting grooves on the first current collector, the current-carrying area of ​​the first current collector can be reduced, thus increasing the resistance of the first positive electrode. During charging and discharging of the secondary battery, the current flowing through the first positive electrode can be reduced, thereby reducing the current density of the first positive electrode coating and decreasing lithium-ion insertion / extraction reactions. This ensures that the corresponding negative electrode has sufficient capacity to insert lithium ions extracted from the first positive electrode coating per unit time, further reducing lithium plating. This improves the fast-charging performance of the secondary battery while reducing the risk of lithium plating.

[0008] In some embodiments, along the first direction, the thickness of the first current collector is D1, the depth of the groove is D2, and 10% ≤ D2 / D1 ≤ 70%. This can effectively reduce the risk of outer layer lithium plating, and the first current collector still has a certain structural strength, reducing the occurrence of curling. Preferably, 33.33% ≤ D2 / D1 ≤ 50%. To further reduce the risk of outer layer lithium plating, improve the anti-curling ability and anti-tearing ability of the first current collector, and enhance the structural strength of the first current collector.

[0009] In some embodiments, the groove is arranged along the second direction. Along the third direction, the width of the first current collector is W1, and the width of the groove is W2, 0.5% ≤ W2 / W1 ≤ 4%. Among them, the first direction, the second direction, and the third direction are perpendicular to each other in pairs. This can not only effectively reduce the current-carrying area of the first current collector, but also reduce the phenomenon that the first current collector is prone to curling. Preferably, 0.5% ≤ W2 / W1 ≤ 2%. To further optimize the width of the groove, thereby improving the phenomenon that the first current collector is prone to curling.

[0010] In some embodiments, along the third direction, the sum of the widths of all the grooves is W3, 15% ≤ W3 / W1 ≤ 80%. This can reduce the risk of outer layer lithium plating, shorten the difference between the risk of outer layer lithium plating and the risk of inner layer lithium plating, and enable the first current collector to still maintain a high structural strength, reducing the phenomenon of curling. Preferably, 50% ≤ W3 / W1 ≤ 70%. To further reduce the risk of outer layer lithium plating and improve the structural strength of the first current collector.

[0011] In some embodiments, along the third direction, several grooves are arranged in sequence, and there is a first interval L1 between adjacent two grooves, 50% ≤ L1 / W2 ≤ 200%. The spacing between the grooves is moderate, the stress is more evenly distributed, and the curling of the first current collector is reduced. Preferably, 100% ≤ L1 / W2 ≤ 150%. While reducing the deformation of the first current collector, the short-circuit risk of the secondary battery is reduced.

[0012] In some embodiments, along the second direction, the first current collector includes a first end face and a second end face arranged opposite to each other, and the groove penetrates through the first end face and the second end face, which can reduce the current-carrying area of the first current collector, and then reduce the current density of the outer first positive electrode sheet, and can reduce the risk of outer layer lithium plating.

[0013] In some embodiments, the second positive electrode sheet includes a second current collector and second positive electrode coatings provided 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, and along the first direction, the thickness of the second conductive layer is T2, 67% ≤ T1 / T2 ≤ 150%. Preferably, 78% ≤ T1 / T2 ≤ 120%.

[0014] When the thickness T1 of the first conductive layer is less than or equal to the thickness T2 of the second conductive layer, the resistance of the first positive electrode is greater than or equal to the resistance of the second positive electrode. During charging and discharging of the secondary battery, this reduces the current flowing through the first positive electrode, thereby reducing the current density of the first positive electrode coating and decreasing lithium-ion insertion / extraction reactions. This ensures that the corresponding negative electrode has sufficient capacity to insert lithium ions extracted from the first positive electrode coating per unit time, further reducing lithium plating. This allows the secondary battery to adapt to higher charge / discharge rates, improving its fast-charging performance while reducing the risk of lithium plating.

[0015] When the thickness T1 of the first conductive layer is greater than the thickness T2 of the second conductive layer, since the groove is provided on the first current collector in this application, the current carrying area of ​​the first current collector can be reduced. Therefore, appropriately increasing the thickness of the first conductive layer can enhance the anti-curling ability of the first current collector, which is beneficial to reduce the deformation of the first positive electrode sheet while reducing lithium plating.

[0016] Secondly, embodiments of this application provide an electronic device including a secondary battery as described in any of the embodiments of the first aspect above. Attached Figure Description

[0017] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0018] Figure 1 These are schematic diagrams of the structures of some secondary batteries provided in the embodiments of this application;

[0019] Figure 2 These are schematic diagrams of the structures of some electrode components provided in the embodiments of this application;

[0020] Figure 3 These are schematic diagrams of the structures of some electrode components provided in the embodiments of this application;

[0021] Figure 4 These are schematic diagrams of the structure of some first positive electrode plates provided in the embodiments of this application;

[0022] Figure 5 These are schematic diagrams of the structures of some second positive electrode plates provided in the embodiments of this application;

[0023] Figure 6 These are schematic diagrams of the structures of some grooves provided in the embodiments of this application;

[0024] Figure 7 These are schematic diagrams of the structures of some grooves provided in the embodiments of this application;

[0025] Figure 8 These are schematic diagrams of structures where the first electrode tab is a regular electrode tab, as provided in the embodiments of this application;

[0026] Figure 9 These are schematic diagrams of structures where the first electrode tab is an irregular electrode tab, as provided in the embodiments of this application;

[0027] Figure 10 These are schematic diagrams of the structures of some first adapters provided in the embodiments of this application;

[0028] Figure 11 These are schematic diagrams of structures where the second electrode tab is a regular electrode tab, as provided in the embodiments of this application;

[0029] Figure 12 This is a schematic diagram of some irregularly shaped second electrodes provided in the embodiments of this application;

[0030] Figure 13 These are schematic diagrams of the structures of some first current collectors provided in the embodiments of this application;

[0031] Figure 14 These are schematic diagrams of the structures of some second current collectors provided in the embodiments of this application;

[0032] Figure 15 These are schematic diagrams of the structure of some first negative electrode plates provided in the embodiments of this application;

[0033] Figure 16 This is a schematic diagram of the structure of some first negative electrode plates provided in the embodiments of this application.

[0034] Explanation of reference numerals in the attached figures:

[0035] 1000, secondary battery;

[0036] 100. Electrode assembly;

[0037] 10. Positive electrode sheet; 11. First positive electrode sheet; 111. First current collector; 1111. First surface; 1112. Second surface; 1113. First end face; 1114. First conductive layer; 111a. First sub-conductive layer; 111b. Second sub-conductive layer; 1115. First insulating polymer layer; 1116. Groove; 1117. Second end face; 112. First positive electrode coating; 113. First tab; 12. Second positive electrode sheet; 121. Second current collector; 1211. Third surface; 1212. Fourth 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; 123. Second tab;

[0038] 20. Negative electrode sheet; 21. First negative electrode sheet; 211. Third current collector; 2111. Fifth surface; 2112. Sixth surface; 212. First negative electrode coating;

[0039] 30. Separating membrane;

[0040] 40. First adapter;

[0041] 200. Shell;

[0042] X, first direction;

[0043] Y, the second direction;

[0044] Z, Third-party orientation. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0046] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "several" and "more than" mean two or more, unless otherwise explicitly defined.

[0047] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0048] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0049] A stacked battery typically consists of an outermost positive electrode sheet with a single-sided coating (hereinafter referred to as a single-sided positive electrode sheet) and an innermost positive electrode sheet with a double-sided coating (hereinafter referred to as a double-sided positive electrode sheet). The outermost part of the single-sided positive electrode sheet can be empty foil (without an active material layer), which can reduce the space occupied by the single-sided positive electrode sheet and improve the energy density of the secondary battery.

[0050] However, since the single-sided positive electrode is located on the outermost layer of the electrode assembly, its binding force is relatively small. The current collector is coated only on one side, leading to uneven stress distribution on both sides. This can cause the current collector to curl towards the side away from the coating, which in turn can cause the coating to peel off, affecting the energy density of the secondary battery. To reduce the curling problem of the single-sided positive electrode, the thickness of the current collector in the single-sided positive electrode is usually increased to improve its resistance to deformation.

[0051] The inventors of this application have discovered that during the charging and discharging process of a secondary battery, the negative electrode corresponding to a single-sided positive electrode has a higher risk of lithium plating (hereinafter referred to as outer layer lithium plating). When charging and discharging a secondary battery, without lithium plating, the maximum charge / discharge rate that a double-sided positive electrode can withstand is greater than that of a single-sided positive electrode. To improve the rate performance of the secondary battery, the maximum charge / discharge rate of the double-sided positive electrode is used as a benchmark, but this exceeds the tolerance range of a single-sided positive electrode, making the risk of outer layer lithium plating higher than that of inner layer lithium plating, which can easily lead to safety issues.

[0052] Further research by the inventors of this application revealed that lithium plating is related to the current density of the coating. Because the coating is only applied to one side of the single-sided positive electrode, the current is concentrated in the single-layer coating, resulting in a vigorous lithium-ion insertion / extraction reaction. This makes it difficult for the corresponding negative electrode to insert the extracted lithium ions in time, thus leading to lithium plating. Furthermore, increasing the current collector thickness of the outer single-sided positive electrode means that the current collector has lower resistance and higher current density, resulting in a more vigorous lithium-ion insertion / extraction reaction and further increasing the risk of lithium plating.

[0053] To reduce the aforementioned problems, firstly, this application proposes a secondary battery 1000, which refers to a rechargeable soft-pack battery that can store and release electrical energy through a reversible chemical reaction. The secondary battery 1000 can be a lithium-ion battery, a sodium-ion battery, or a lithium polymer battery, etc.

[0054] Please refer to Figure 1 The secondary battery 1000 includes an electrode assembly 100 and a housing 200, with the electrode assembly 100 housed within the housing 200. Please refer to... Figure 2 , Figure 2The layered structure of electrode assembly 100 is shown. Electrode assembly 100 includes a positive electrode 10, a negative electrode 20, and a separator 30. Along a first direction X (the thickness direction of the positive electrode 10 and / or the negative electrode 20), a plurality of positive electrode 10 and a plurality of negative electrode 20 are alternately stacked. A separator 30 is disposed between adjacent positive electrode 10 and negative electrode 20 to insulatingly separate the positive electrode 10 and the negative electrode 20.

[0055] The positive electrode 10 includes a first positive electrode 11 and a second positive electrode 12. Along the first direction X, the first positive electrode 11 is the outermost positive electrode 10 of the electrode assembly 100. The second positive electrode 12 is the inner positive electrode 10 of the electrode assembly 100, that is, the second positive electrode 12 is disposed between two adjacent negative electrode 20.

[0056] For the first positive electrode plate 11 mentioned above, please refer to... Figure 3 and Figure 4 The first positive electrode 11 includes a first current collector 111 and a first positive electrode coating 112. The first current collector 111 serves as a conductive substrate and can be made of aluminum foil that is flat and has a strip-like structure. 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.

[0057] In this embodiment, the first positive electrode 11 adopts a single-sided coating structure, that is, along the first direction X, the first positive electrode coating 112 is only disposed on the surface of the first current collector 111 facing the negative electrode 20. For details, please refer to... Figure 4 The first current collector 111 includes a first surface 1111 and a second surface 1112 disposed opposite to each other. The second surface 1112 faces the negative electrode 20, and the first positive electrode coating 112 is disposed on the second surface 1112. The first surface 1111 of the first positive electrode 11 has no corresponding negative electrode 20 and coating, which can reduce the space occupied by the first positive electrode 11 and improve the volumetric energy density of the secondary battery 1000.

[0058] The first positive electrode coating 112 includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. These materials are mixed, stirred evenly, and coated onto the surface of the first current collector 111 facing the negative electrode 20, thereby obtaining the first positive electrode coating 112. The positive electrode active material can be selected from at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, or cobalt-free materials. In this application embodiment, there are no particular limitations on the positive electrode conductive agent and the positive electrode binder, as long as they can achieve the purpose of this application. For example, the positive electrode conductive agent can be selected from at least one of conductive carbon black, carbon nanotubes, carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The positive electrode binder can be selected from at least one of polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose, or potassium carboxymethyl cellulose.

[0059] For the second positive electrode plate 12 mentioned above, please refer to... Figure 3 and Figure 5 The second positive electrode 12 includes a second current collector 121 and a second positive electrode coating 122 disposed on two surfaces of the second current collector 121 (two surfaces disposed opposite 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 described above, and can be made of an integrally flat and strip-shaped aluminum foil. Alternatively, in some other embodiments, the material of the second current collector can be at least one of aluminum alloy, nickel, nickel alloy, or stainless steel.

[0060] In this embodiment, the second positive electrode 12 adopts a double-sided coating structure. Specifically, along the first direction X, the second current collector 121 has a third surface 1211 and a fourth surface 1212 disposed opposite to each other, and both the third surface 1211 and the fourth surface 1212 are provided with a second positive electrode coating 122. The inner layer is provided with a second positive electrode coating 122 on both sides, which can make full use of space and thus improve the energy density of the secondary battery 1000.

[0061] For the aforementioned separator membrane 30, please refer to... Figure 2 and Figure 3 A separator 30 is disposed between the positive electrode 10 and the negative electrode 20 to provide insulation between them. The separator 30 may be a ceramic-containing PE separator 30 or a PP separator 30, etc. In some embodiments, an adhesive coating may be provided on the surfaces of the separator 30 facing the positive electrode 10 and the negative electrode 20. When the positive electrode 10, the separator 30, and the negative electrode 20 are stacked, the separator 30 can be directly bonded between the positive electrode 10 and the negative electrode 20 to improve the bonding strength between them, thereby enhancing the overall integrity of the electrode assembly 100.

[0062] Please refer to Figure 4 and Figure 6 The first surface 1111 is provided with a plurality of grooves 1116. The position of the grooves 1116 on the first surface 1111 is not specifically limited. For example, the grooves 1116 may be arranged along the second direction Y on the first surface 1111. The grooves 1116 may also be arranged along the third direction Z on the first surface 1111, or staggered along the second direction Y and the third direction Z on the first surface 1111. The spacing between adjacent grooves 1116 is not specifically limited; the spacing may be equal or unequal. Wherein, the second direction Y is the length direction of the first current collector 111, the third direction Z is the width direction of the first current collector 111, and the first direction X, the second direction Y, and the third direction Z are all perpendicular to each other.

[0063] In this embodiment, the first surface 1111 is provided with a plurality of grooves 1116, which reduces the current-carrying area of ​​the first current collector 111, thereby increasing the resistance of the first positive electrode 11. During charging and discharging of the secondary battery 1000, the current flowing through the first positive electrode 11 is reduced, thereby reducing the current density of the first positive electrode coating 112 and decreasing lithium-ion insertion / extraction reactions. This ensures that the negative electrode 20 corresponding to the first positive electrode 11 has sufficient capacity to insert lithium ions extracted from the first positive electrode coating 112 per unit time, thus reducing lithium plating. This improves the fast-charging performance of the secondary battery 1000 while reducing the risk of lithium plating.

[0064] For the groove 1116 mentioned above, please refer to... Figure 6 and Figure 7 Along the first direction X, the thickness of the first current collector 111 is D1, and the depth of the groove 1116 is D2, with 10% ≤ D2 / D1 ≤ 70%. This range not only reduces the risk of outer-layer lithium plating, but also provides the first current collector 111 with a certain structural strength, reducing the likelihood of curling. In some embodiments, the thickness of the first current collector 111 ranges from 5 μm ≤ D1 ≤ 20 μm. For example, when the thickness D1 of the first current collector 111 is 10 μm, the depth D2 of the groove 1116 is 1 μm to 7 μm. When the thickness D1 of the first current collector 111 is 5 μm, the depth D2 of the groove 1116 is 0.5 μm to 3.5 μm. It should be noted that when the depth of the groove 1116 is 3.5 μm, the thickness at the connection between the grooves 1116 is only 1.5 μm, which is too thin and prone to breakage. Therefore, in actual production, the thickness at the connection between grooves 1116 should be as great as possible to 3μm to reduce the risk of breakage at the connection between grooves 1116 and grooves 1116.

[0065] In some preferred embodiments, the ratio of the depth D2 of the groove 1116 to the thickness D1 of the first current collector 111 is in the range of 33.33% ≤ D2 / D1 ≤ 50%. This further reduces the risk of outer layer lithium plating, improves the anti-curling and anti-tear ability of the first current collector 111, and enhances the structural strength of the first current collector 111.

[0066] In some embodiments, please refer to Figure 6 The groove 1116 is arranged along the second direction Y and along the third direction Z. The width of the first current collector 111 is W1, and the width of a single groove 1116 is W2, with 0.5% ≤ W2 / W1 ≤ 4%. In this embodiment, if a single groove 1116 is too wide, the structural strength inside the groove 1116 is weak, making the first current collector 111 prone to curling. If a single groove 1116 is too narrow, the reduction in the current-carrying area of ​​the first current collector 111 is not significant. Controlling the width of a single groove 1116 to 0.5% ≤ W2 / W1 ≤ 4% not only effectively reduces the current-carrying area of ​​the first current collector 111 but also reduces the tendency for the first current collector 111 to curl.

[0067] Furthermore, the ratio of the width W2 of a single groove 1116 to the width W1 of the first current collector 111 is 0.5% ≤ W2 / W1 ≤ 2%. This is to further optimize the width of the single groove 1116, thereby improving the tendency of the first current collector 111 to curl.

[0068] In some embodiments, a plurality of grooves 1116 are arranged sequentially along a third direction Z. The grooves 1116 may have the same width or different widths. The sum of the widths of all grooves 1116 is W3, and 15% ≤ W3 / W1 ≤ 80%. If the total width of all grooves 1116 is too large a proportion of the width of the first current collector 111, the structural strength of the first current collector 111 will be weakened and prone to curling. If the total width of all grooves 1116 is too small a proportion of the width of the first current collector 111, the reduction in the current-carrying area of ​​the first current collector 111 is not significant, resulting in an insignificant reduction in the risk of outer layer lithium plating. When the sum of the widths W3 of all grooves 1116 is within the range of 15% ≤ W3 / W1 ≤ 80%, the risk of outer layer lithium plating can be reduced, the difference between the risks of outer layer lithium plating and inner layer lithium plating can be shortened, and the first current collector 111 can have higher structural strength, reducing the phenomenon of curling.

[0069] In some preferred embodiments, the ratio of the sum of the widths W3 of all the grooves 1116 to the width W1 of the first current collector 111 is in the range of 50% ≤ W3 / W1 ≤ 70%. This is to further reduce the risk of outer layer lithium plating and improve the structural strength of the first current collector 111.

[0070] In some embodiments, please refer to Figure 7Along the third direction Z, a plurality of grooves 1116 are arranged sequentially, with a first interval L1 between adjacent grooves 1116, where 50%≤L1 / W2≤200%, making the spacing between grooves 1116 moderate, the stress distribution more uniform, and reducing the curling of the first current collector 111. Preferably, 100%≤L1 / W2≤150%, which can reduce the deformation of the first current collector 111 while reducing the short-circuit risk of the secondary battery 1000.

[0071] In some embodiments, please refer to Figure 6 Along the second direction Y, the first current collector 111 includes a first end face 1113 and a second end face 1117 disposed opposite to each other, and a groove 1116 penetrates the first end face 1113 and the second end face 1117. The groove 1116 may completely penetrate the first end face 1113 and the second end face 1117, or the groove 1116 may partially penetrate the first end face 1113 and the second end face 1117. This reduces the current-carrying area of ​​the first current collector 111, thereby reducing the current density of the outer first positive electrode 11 and lowering the risk of outer layer lithium plating. Specifically, when the groove 1116 completely penetrates the first end face 1113 and the second end face 1117, the current-carrying area of ​​the first current collector 111 is smaller, the resistance of the first positive electrode 11 is larger, and the risk of outer layer lithium plating is reduced more significantly.

[0072] In some embodiments, please refer to Figure 3 The first positive electrode 11 also includes a first tab 113. The first tab 113 is connected to the first current collector 111, and the first tab 113 extends out of the first current collector 111 along the second direction Y. Then combined with... Figure 8 Along the third direction Z, the minimum width of the first tab 113 is W4. The minimum width of the first tab 113 refers to the width at the position where the current-carrying area is smallest when current flows into or out of the first tab 113. The first tab 113 can be a regular shape, such as a rectangle or a square, and the minimum width of the first tab 113 is the width of the first tab 113 itself. The first tab 113 can also be an irregular shape, such as a wavy, vase-shaped, or sawtooth shape. In some embodiments, please refer to... Figure 9 Along the second direction Y, the width of the first electrode 113 is reduced in the third direction Z. The minimum width of the first electrode 113 is the width at the position where the current-cutting area is the smallest when the current flows in or out.

[0073] Please refer to Figure 10 One end of the first electrode tab 113 can be connected to the first current collector 111, and the other end can be connected to the first adapter 40. The first adapter 40 extends directly out of the housing 200 to lead out the polarity. The material of the first electrode tab 113 can be the same as that of the first current collector 111. For example, the first electrode tab 113 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0074] When the first current collector 111 is a metal foil current collector (the first current collector 111 is entirely made of metal), the first tab 113 can be die-cut into the first current collector 111. Alternatively, the first tab 113 can be connected to the first current collector 111 by welding or bonding with conductive adhesive. When the first current collector 111 is a composite current collector, the first tab 113 can also be die-cut into the first current collector 111.

[0075] Please refer to Figure 3 The second positive electrode 12 includes a second tab 123, which is connected to the second current collector 121, and extends out of the second current collector 121 along the second direction Y. Figure 11 Along the third direction Z, the minimum width of the second tab 123 is W5. The minimum width of the second tab 123 refers to the width of the position where the current-carrying area is smallest when current flows into or out of the second tab 123. The second tab 123 can be a regular shape, such as a rectangle or a square, in which case the minimum width of the second tab 123 is the width of the second tab 123 itself. The second tab 123 can also be an irregular shape, such as a wavy, vase-shaped, or sawtooth shape. In some embodiments, please refer to... Figure 12 Along the second direction Y, the width of the second electrode 123 is reduced in the third direction Z. The minimum width of the second electrode 123 is the width at the position where the current-cutting area is the smallest when the current flows in or out.

[0076] Please refer to Figure 10 One end of the second tab 123 can be connected to the second current collector 121, and the other end can be connected to the first adapter 40. The first adapter 40 extends directly out of the housing 200 to lead out the polarity. The material of the second tab 123 can be the same as that of the second current collector 121. For example, the second tab 123 can be made of at least one of aluminum alloy, nickel, nickel alloy, stainless steel, etc.

[0077] When the second current collector 121 is a metal foil current collector (the second current collector 121 is entirely made of metal), the second tab 123 can be die-cut from the second current collector 121. Alternatively, the second tab 123 can be connected to the second current collector 121 by welding or bonding with conductive adhesive. When the second current collector 121 is a composite current collector, the second tab 123 can also be die-cut from the second current collector 121.

[0078] In this embodiment, along the third direction Z, the minimum width W4 of the first tab 113 is less than the minimum width W5 of the second tab 123. Therefore, the current-carrying area of ​​the first tab 113 is smaller than that of the second tab 123, resulting in a greater resistance of the first positive electrode 11 than the second positive electrode 12. During charging and discharging of the secondary battery 1000, the current flowing through the first positive electrode 11 can be reduced, thereby reducing the current density of the first positive electrode coating 112 and decreasing the lithium-ion insertion / extraction reaction. This ensures that the negative electrode 20 corresponding to the first positive electrode 11 has sufficient capacity to insert lithium ions extracted from the first positive electrode coating 112 per unit time, thus reducing lithium plating. This allows the secondary battery 1000 to adapt to higher charge / discharge rates, improving its fast-charging performance while reducing the risk of lithium plating.

[0079] In some embodiments, 40% ≤ W4 / W5 ≤ 90% can reduce the impact on the strength of the first tab 113 and the current carrying capacity of the first tab 113 while reducing the outer layer lithium plating. The width of the second tab 123 is in the range of 3mm ≤ W5 ≤ 10mm. In this embodiment, the width W4 of the first tab 113 is in the range of 1.2mm ≤ W4 ≤ 9mm. The specific width of the first tab 113 can be set according to the corresponding relationship and the width of the second tab 123. For example, W4 can be 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm or 9mm, etc.

[0080] In some other embodiments, when the secondary battery 1000 is a power battery, the width of the first tab 113 can also be selected as 1mm≤W4≤90mm. In the embodiments of this application, the secondary battery 1000 is preferably a secondary battery 1000 of a consumer electronic device, and its range can be selected as 1.2mm≤W4≤9mm.

[0081] In some preferred embodiments, the ratio of W4 to W5 ranges from 50% ≤ W4 / W5 ≤ 70%. The minimum width W5 of the second tab 123 can be set from 3 mm to 10 mm. Based on the above correspondence, the minimum width W4 of the first tab 113 can range from 1.5 mm ≤ W1 ≤ 7 mm. This can further reduce lithium plating and enable the first tab 113 to have higher strength and higher current carrying capacity.

[0082] In some embodiments, please refer to Figure 3 Along the first direction X, the projection of the first electrode 113 at least partially overlaps with the projection of the second electrode 123. Combined with... Figure 10The secondary battery 1000 also includes a first adapter 40, a first tab 113 and a second tab 123 stacked along a first direction X, the first adapter 40 is connected to the first tab 113 and the second tab 123, so that the first tab 113 and the second tab 123 can be stacked into a whole, and the first adapter 40 is used for the connection of this whole, which helps to simplify the polarity lead-out operation.

[0083] In some embodiments, please refer to Figure 4 and Figure 12 The first current collector 111 includes a first conductive layer 1114, which is a metal layer capable of conducting current, allowing current to flow smoothly. The first conductive layer 1114 includes at least one conductive metal such as aluminum, aluminum alloy, nickel, nickel alloy, or stainless steel. Each material possesses high conductivity, which is beneficial for improving the charge / discharge rate of the secondary battery 1000, and also has high mechanical strength, reducing the risk of curling of the first conductive layer 1114.

[0084] When the first current collector 111 is a metal foil current collector (the first current collector 111 is made entirely of metal), the first conductive layer 1114 is the first current collector 111 itself.

[0085] When the first current collector 111 is a composite current collector, the first conductive layer 1114 is a part of the first current collector 111. For example, please refer to... Figure 13 The first current collector 111 includes a first conductive layer 1114 and a first insulating polymer layer 1115. Specifically, the first conductive layer 1114 includes a first sub-conductive layer 111a and a second sub-conductive layer 111b, and the first insulating polymer layer 1115 is disposed between the first sub-conductive layer 111a and the second sub-conductive layer 111b.

[0086] The provision of the first insulating polymer layer 1115 allows for a thinner first conductive layer 1114, reducing the weight of the first positive electrode 11 and increasing the gravimetric energy density of the secondary battery 1000. Specifically, when the first conductive layer 1114 is thin, increasing the thickness of the first insulating polymer layer 1115 enhances the structural strength of the first current collector 111 and reduces the curling phenomenon of the first positive electrode 11. Because the first insulating polymer layer 1115 is electrically insulating, charges have difficulty passing through it; therefore, the first insulating polymer layer 1115 does not change the thickness or conductivity of the conductive portion in the first current collector 111.

[0087] In some embodiments, the first insulating polymer layer 1115 comprises 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 selection of the above materials as the material for the first insulating polymer layer 1115 ensures that the strength, toughness, insulation, and moldability of the first insulating polymer layer 1115 meet the requirements of the composite current collector, providing good support for the first conductive layer 1114.

[0088] In some embodiments, when the first current collector 111 is a composite current collector, a groove 1116 may also be provided in the first conductive layer 1114. For example, a groove 1116 may be provided on the surface of the first sub-conductive layer 111a away from the first insulating polymer layer 1115 to reduce the risk of lithium plating on the outer layer.

[0089] In some embodiments, the second current collector 121 includes a second conductive layer 1213. Similar to the first conductive layer 1114, the second conductive layer 1213 can conduct current, allowing the current to flow smoothly within it. The second conductive layer 1213 comprises at least one of aluminum, aluminum alloy, nickel, nickel alloy, or stainless steel. Each material has high conductivity, which is beneficial for improving the charge / discharge rate of the secondary battery 1000, and also has high mechanical strength, which can reduce the curling of the second conductive layer 1213. The materials of the first conductive layer 1114 and the second conductive layer 1213 must be consistent during installation; for example, both the first conductive layer 1114 and the second conductive layer 1213 can be aluminum foil.

[0090] When the second current collector 121 is a metal foil current collector (the second current collector 121 is made entirely of metal), the second conductive layer 1213 is the second current collector 121 itself.

[0091] 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. For example, please refer to... Figure 14 The second current collector 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.

[0092] The provision of the second insulating polymer layer 1214 allows for a thinner second conductive layer 1213, reducing the weight of the second positive electrode and increasing the gravimetric energy density of the secondary battery 1000. Specifically, when the second conductive layer 1213 is thin, increasing the thickness of the second insulating polymer layer 1214 enhances the structural strength of the second current collector 121, mitigating the curling phenomenon of the second positive electrode 12. Because the second insulating polymer layer 1214 is electrically insulating, charge has difficulty passing through it; therefore, the second insulating polymer layer 1214 does not change the thickness or conductivity of the conductive portion in the second current collector 121. In this embodiment, the second insulating polymer layer 1214 includes at least one material from the first insulating polymer layer 1115 described above.

[0093] In some embodiments, please refer to Figure 4 and Figure 5 Along the first direction X, the thickness of the first conductive layer 1114 is T1. The thickness of the second conductive layer 1213 is T2. The ratio of the thickness T1 of the first conductive layer 1114 to the thickness T2 of the second conductive layer 1213 is 67% ≤ T1 / T2 ≤ 150%. The thickness range of the second conductive layer 1213 is 1 μm ≤ T2 ≤ 30 μm. In the embodiments of this application, the thickness T1 of the first conductive layer 1114 ranges from 0.67 μm ≤ T1 ≤ 45 μm. It should be noted that in the embodiments of this application, the thickness of the first conductive layer 1114 is the thickness of the portion of the first current collector 111 without grooves. When the first current collector 111 is entirely made of metal, the thickness T1 of the first conductive layer 1114 is equal to the thickness D1 of the first current collector 111.

[0094] When the thickness T1 of the first conductive layer 1114 is less than or equal to the thickness T2 of the second conductive layer 1213, the resistance of the first positive electrode 11 is greater than or equal to the resistance of the second positive electrode 12. During charging and discharging of the secondary battery 1000, the current flowing through the first positive electrode 11 is reduced, thereby reducing the current density of the first positive electrode coating 112 and decreasing lithium-ion insertion / extraction reactions. This allows the corresponding negative electrode 20 to have sufficient capacity to insert lithium ions extracted from the first positive electrode coating 112 per unit time, further reducing lithium plating. This enables the secondary battery 1000 to adapt to higher charge / discharge rates, improving its fast-charging performance while reducing the risk of lithium plating.

[0095] When the thickness T1 of the first conductive layer 1114 is greater than the thickness T2 of the second conductive layer 1213, in this application, a groove 1116 is provided on the first current collector 111, which can reduce the current carrying area of ​​the first current collector 111. Therefore, appropriately increasing the thickness of the first conductive layer 1114 can enhance the anti-curling ability of the first current collector 111, which is beneficial to reduce the deformation of the first positive electrode 11 while reducing lithium plating.

[0096] In some preferred embodiments, the thickness T1 of the first conductive layer 1114 is less than the thickness T2 of the second conductive layer 1213, and the ratio of T1 to T2 is in the range of 78% ≤ T1 / T2 ≤ 120%. This is to further reduce the risk of outer layer lithium plating and reduce the curling of the first conductive layer 1114.

[0097] In some preferred embodiments, the thickness T2 of the second conductive layer 1213 can be set to 6 μm to 15 μm. Based on the relationship between the thickness of the first conductive layer 1114 and the thickness of the second conductive layer 1213 (78 ≤ T1 / T2 ≤ 120%), the thickness T1 of the first conductive layer 1114 can range from 4.68 μm ≤ T1 ≤ 18 μm. For example, T1 can be 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, etc.

[0098] In some embodiments, please refer to Figure 2 The negative electrode 20 includes a first negative electrode 21, which is disposed between two adjacent positive electrode 10s. Figure 3 The first negative electrode 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-like structure. In some other embodiments, the third current collector 211 can also be made of copper alloy foil, stainless steel foil, nickel foam, copper foam, or composite current collectors (e.g., lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.).

[0099] In this embodiment, the first negative electrode 21 adopts a double-sided coated structure. Further refer to... Figure 15 The first negative electrode 21 also includes a first negative electrode coating 212 disposed on the two surfaces of the third current collector 211. Along the first direction X, the third current collector 211 has a fifth surface 2111 and a sixth surface 2112 disposed opposite to each other, and both the fifth surface 2111 and the sixth surface 2112 are provided with the first negative electrode coating 212.

[0100] The first negative electrode coating 212 includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. These material components are mixed and stirred evenly and then coated onto the fifth surface 2111 and the sixth surface 2112 of the third current collector 211, thereby obtaining the first negative electrode coating 212 and the second negative electrode coating 213. The negative electrode active material can be selected from at least one of the following: natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composite, tin, stannous oxide, tin oxide, lithium-aluminum alloy, and metallic lithium. In this application embodiment, there are no particular limitations on the negative electrode conductive agent and the negative electrode binder, as long as the purpose of this 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 binder may include at least one of the above-mentioned positive electrode binders.

[0101] In some embodiments, please refer to Figure 4 and Figure 5 The thickness of the first positive electrode coating 112 is T3, and the thickness of the second positive electrode coating 122 is T4, with 50% ≤ T3 / T4 < 100%. Because the thickness of the first current collector 111 is reduced, and the first positive electrode coating 112 is only applied to one surface, if the first positive electrode coating 112 is too thick, the first positive electrode sheet 11 is prone to stress imbalance, easily causing curling. If the first positive electrode coating 112 is too thin, there is less positive electrode active material, reducing the capacity of the secondary battery 1000. Reducing the thickness of the first positive electrode coating 112 to 50% ≤ T3 / T4 < 100% achieves stress balance between the first current collector 111 and the first positive electrode coating 112 in the first positive electrode sheet 11, reducing the curling phenomenon of the first positive electrode sheet 11. Furthermore, reducing the thickness of the first positive electrode coating 112 can reduce the amount of lithium delithiation from the first positive electrode coating 112, lowering the risk of outer layer lithium plating. Preferably, 50%≤T3 / T4<90% can reduce the curling of the first positive electrode 11 while reducing the short-circuit risk of the secondary battery 1000.

[0102] In some embodiments, please refer to Figure 5 The thickness T4 of the second positive electrode coating 122 on the third surface 1211 and the thickness T5 of the second positive electrode coating 122 on the second surface 1112 are approximately equal within the error range of -0.5μm≤T5-T4≤0.5μm, which makes the performance of the secondary battery 1000 more balanced and stable, and reduces the phenomenon of local performance imbalance caused by excessive thickness difference of the second positive electrode coating 122 on the two surfaces of the second current collector 121.

[0103] In some embodiments, please refer to Figure 16The thickness T6 of the first negative electrode coating 212 on the fifth surface 2111 and the thickness T7 of the first negative electrode coating 212 on the sixth surface 2112 are approximately equal within the error range of -0.5μm≤T7-T6≤0.5μm, which makes the performance of the secondary battery 1000 more balanced and stable, and reduces the phenomenon of local performance imbalance caused by excessive thickness difference of the first negative electrode coating 212 on the two surfaces of the third current collector 211.

[0104] 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 21 has enough margin to embed the lithium ions extracted from the first positive electrode 11, which can reduce the phenomenon of lithium plating on the first negative electrode 21.

[0105] For further details, please refer to Figure 4 and Figure 16 Along the length direction (second direction Y) of the first positive electrode 11, the length of the first positive electrode coating 112 is L2. Along the length direction (second direction Y) of the second positive electrode 12, the length of the first negative electrode coating 212 is L3, so 0.6mm ≤ L3 - L2 ≤ 4mm. Along the width direction (third direction Z) of the first positive electrode 11, the width of the first positive electrode coating 112 is W6. Along the width direction (third direction Z) of the second positive electrode 12, the width of the first negative electrode coating 212 is W7, so 0.6mm ≤ W7 - W6 ≤ 4mm. This allows the negative electrode 20 to have sufficient margin to accommodate lithium ions extracted from the positive electrode 10, further reducing lithium plating on the first negative electrode 21. Furthermore, limiting 0.6mm ≤ L3 - L2 ≤ 4mm and 0.6mm ≤ W7 - W6 ≤ 4mm allows the secondary battery 1000 to have a higher energy density.

[0106] Secondly, embodiments of this application also provide an electronic device including a secondary battery 1000 as described in any of the embodiments of the first aspect above. The electronic device in the embodiments of this application is not particularly limited and can be any electronic device known in the prior art. For example, electronic devices include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0107] In the embodiments of this application, a lithium-ion secondary battery is used as an example to perform a lithium plating test.

[0108] The test is as follows:

[0109] During the battery module fabrication stage, reference electrode 1 is placed between the negative electrode corresponding to the first positive electrode and the separator layer, and reference electrode 2 is placed between the central negative electrode and the separator layer (the central negative electrode refers to the negative electrode located in the middle or near the middle of the total number of negative electrodes in the secondary battery. For example, if the total number of negative electrodes is 14, the central negative electrode is the 7th negative electrode layer, and it can be located on either side). The fabrication process of the reference electrode is as follows:

[0110] (1) Copper wire pretreatment: Select copper wire with a diameter of 30μm and a length of 100mm. Immerse one end of the copper wire (about 30mm) in concentrated sulfuric acid for 2 hours to remove the surface insulating varnish. Then, use alcohol to ultrasonically clean for 10 to 15 minutes, repeat 2 to 3 times, and finally dry in an oven for later use.

[0111] (2) Copper wire insertion into the battery: At the designated insertion layer, place one end of the processed copper wire at the center of the corresponding separator on 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 approximately 10mm wide and the same length as the negative electrode sheet, thus separating the copper wire from the negative electrode sheet. Finally, continue the electrode assembly and electrolyte injection encapsulation process according to the normal stacking assembly sequence. The copper wire is led out from the side aluminum-plastic film encapsulation edge. Since the copper wire is thin enough, no additional encapsulation treatment is required.

[0112] (3) Copper wire transfer welding: Weld the copper wire to the nickel tab and fix it to the aluminum-plastic film shell with adhesive tape. Use the positive and negative terminals of a multimeter to clamp the reference electrode and the positive or negative terminals of the battery. If there is voltage, it means that the three-electrode battery is made normally.

[0113] (4) Lithium plating: A charge-discharge apparatus is used, with the positive electrode clamped to the positive electrode of the battery and the negative electrode clamped to the reference electrode, and the battery is charged with a current of 20μA for 10h; then the positive electrode clamped to the negative electrode of the battery and the negative electrode clamped to the reference electrode, and the battery is charged with a current of 20μA for 10h.

[0114] The testing process is as follows:

[0115] (1) Preparation: An electrochemical workstation was used as the testing equipment to provide charging current and monitor battery voltage, including the voltage between the positive and negative electrodes and the voltage V between the negative electrode tab and reference electrode 1. t1 The voltage V between the negative electrode tab and the reference electrode 2 t2 .

[0116] (2) Charging: The battery is charged according to the normal charging procedure. Both the embodiments and comparative examples in this application adopt the following procedure: first, a constant current of 3C is applied for charging; after charging to 4.5V, the charging mode is switched to constant voltage charging; the charging stops when the current drops to 0.05C. The V value over time is obtained. t1 Extract the minimum value as V1 to obtain V as a function of time. t2 The minimum value is V2.

[0117] (3) Calculate the voltage difference Δ = V1 - V2.

[0118] Evaluation principle: The lower the negative electrode potential, the higher the risk of lithium plating.

[0119] Evaluation criteria: Δ>0 indicates a higher risk of lithium plating in the inner layer than in the outer layer; Δ<0 indicates a higher risk of lithium plating in the outer layer than in the inner layer; Δ=0 indicates that the risk of lithium plating in the outer layer is equal to that in the inner layer. Furthermore, the larger the absolute value of Δ, the greater the difference in lithium plating risk between the inner and outer layers.

[0120] Example 1

[0121] Preparation of lithium-ion secondary batteries

[0122] (1) Preparation of the first positive electrode: The positive active material lithium cobalt oxide, the positive conductive agent acetylene black, and the positive binder polyvinylidene fluoride (PVDF, weight average molecular weight of 5×10⁻⁶) are prepared. 5 The mixture was prepared by mixing the materials at a mass ratio of 97:1.5:1.5, adding N-methylpyrrolidone (NMP) as a solvent, and preparing a positive electrode slurry with a solid content of 75wt%. The mixture was then stirred evenly under a vacuum mixer.

[0123] Aluminum foil is selected as the first current collector (first conductive layer). The first current collector and the first conductive layer have the same structure. The aluminum foil has a room temperature conductivity of 3.774e7 S / m. The thickness D1 of the first current collector is 15 μm, the width W1 is 69 mm, and the length is 69 mm. Positive electrode slurry is uniformly coated onto the second surface of the first current collector and dried to form a first positive electrode coating with a thickness of 34 μm on the second surface, resulting in a first positive electrode sheet with a single-sided coating of positive electrode active material.

[0124] The first surface of the first current collector is grooved by laser drilling. The groove depth D2 is 1μm, the groove width W2 is 1.04mm, and the sum of the groove widths W3 is 52mm.

[0125] (2) Preparation of the second positive electrode: The positive active material lithium cobalt oxide, the positive conductive agent acetylene black, and the positive binder polyvinylidene fluoride (PVDF, weight average molecular weight of 5×10⁻⁶) are prepared. 5The mixture was prepared by mixing the materials at a mass ratio of 97:1.5:1.5, adding N-methylpyrrolidone (NMP) as a solvent, and preparing a positive electrode slurry with a solid content of 75wt%. The mixture was then stirred evenly under a vacuum mixer.

[0126] Aluminum foil was selected as the second current collector (second conductive layer). The second current collector and the second conductive layer have the same structure. The aluminum foil has a room temperature conductivity of 3.774e7 S / m. The second current collector has a thickness of 9 μm, a width of 69 mm, and a length of 69 mm. Positive electrode slurry was uniformly coated onto the third and fourth surfaces of the second current collector and dried. A second positive electrode coating with a thickness of 34 μm was formed on both the third and fourth surfaces, resulting in a second positive electrode sheet with double-sided coating of positive electrode active material.

[0127] (3) 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) are mixed in a weight ratio of 89.5:8:1:1.5, and then deionized water is added as a solvent to prepare a negative electrode slurry with a solid content of 50wt%, and stirred evenly.

[0128] Copper foil was selected as the third current collector. The copper foil has a room temperature conductivity of 5.998e7 S / m and a thickness of 6 μm. The negative electrode slurry was uniformly 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, resulting in a first negative electrode sheet with a double-sided negative electrode coating. The length of the first negative electrode sheet is 70 mm and the width is 69.7 mm.

[0129] (4) Preparation of electrolyte: In a dry argon atmosphere, ethylene carbonate (EC) is first prepared.

[0130] 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. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the basic organic solvent, dissolved, and mixed evenly to obtain an electrolyte with a LiPF6 mass concentration of 12.5%.

[0131] (5) Preparation of the isolation membrane: A porous polyethylene membrane is used as the substrate layer, and a ceramic layer containing alumina ceramic and polyvinylidene fluoride binder is coated on one side of the substrate layer as the isolation membrane. 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.

[0132] (6) Electrode assembly preparation: The above-mentioned positive electrode, separator, and negative electrode are stacked. Nickel and aluminum sheets with a specification of 12mm×6mm are selected as tabs. The aluminum tabs are welded to the positive electrode (the aluminum foil of the positive electrode), and the nickel tabs are welded to the negative electrode to form an electrode assembly for later use. Among them, the first positive electrode has 2 layers, the second positive electrode has 11 layers, the first negative electrode has 12 layers, and the separator has 24 layers.

[0133] (7) Electrode assembly assembly: Place the punched aluminum-plastic film in the assembly fixture with the punched surface facing up, place the electrode assembly in the punch, and set a seal at the two adapter lugs after overall welding, and apply external force to press it tight. Then cover the electrode assembly with another punched aluminum-plastic film with the punched surface facing down, and heat seal the two aluminum-plastic films around the perimeter by hot pressing to obtain the assembled electrode assembly.

[0134] (8) Liquid injection and encapsulation: Electrolyte is injected into the assembled electrode assembly, and after vacuum encapsulation, standing, hot pressing formation, shaping and other processes, lithium-ion secondary batteries are obtained.

[0135] The relevant parameters for Examples 2 to 16 can be found in Table 1 below.

[0136] Unlike Example 1, in Example 9 the groove depth D2 is 5μm, the ratio of groove depth D2 to the thickness D1 of the first current collector is 33.33%, and the sum of the groove widths W3 is 6.9mm.

[0137] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not have a groove.

[0138] Table 1

[0139]

[0140] According to Table 1 above, and in conjunction with Examples 1 to 8 and Comparative Example 1, when the sum of the widths of the grooves, W3, is a constant, the value of Δ gradually increases and the absolute value of Δ gradually decreases as the groove depth D2 increases. This indicates that setting the groove can reduce the risk of lithium plating on the outer layer, reduce the difference between the risk of lithium plating on the outer layer and the risk of lithium plating on the inner layer, and improve the fast charging performance of the secondary battery. However, as the groove depth increases, the thickness of the first positive electrode sheet decreases, and the first positive electrode sheet is prone to curling and tearing. Therefore, in conjunction with Examples 2 to 7, when the groove depth is 10%≤D2 / D1≤70%, the absolute value of Δ is relatively small, and the first positive electrode sheet does not curl or only curls slightly.

[0141] Compared to Embodiments 5 and 6, the voltage difference Δ is closer, but the groove depth is deeper in Embodiment 6, and the first positive electrode is prone to curling. Therefore, in the embodiments of this application, it is further preferred that 33.33%≤D2 / D1≤50%.

[0142] According to Table 1 above, and in conjunction with Examples 9 to 16 and Comparative Example 1, when the groove depth D2 is a constant, as the sum of the groove widths W3 increases, the value of Δ gradually increases, the absolute value of Δ gradually decreases, the risk of outer-layer lithium plating gradually decreases, and the difference between the risk of outer-layer lithium plating and the risk of inner-layer lithium plating gradually decreases. In Example 9, when W3 / W1 equals 10%, the voltage difference Δ is less than 10mV, indicating that the difference between the risk of outer-layer lithium plating and the risk of inner-layer lithium plating is large, and the reduction in the risk of outer-layer lithium plating is not significant. In Example 16, when W3 / W1 equals 85%, the total width of the groove is larger than the width of the first current collector, and the first current collector is prone to stress imbalance, leading to curling of the first positive electrode sheet. Therefore, in the embodiments of this application, in conjunction with Examples 10 to 15, 15% ≤ W3 / W1 ≤ 80% can be selected. In Examples 12 and 13, when 50% ≤ W3 / W1 ≤ 70%, Δ is relatively large, and the absolute value of Δ is relatively small, indicating that the difference between the outer and inner lithium plating is small. Furthermore, the total width of the groove is moderate relative to the width of the first current collector, reducing the occurrence of curling in the first positive electrode sheet. Preferably, in the embodiments of this application, 50% ≤ W3 / W1 ≤ 70% is used.

[0143] Unlike Example 1, in Example 17, the groove depth D2 is 5 μm, the ratio of groove depth D2 to the thickness D1 of the first current collector is 33.33%, the width W2 of a single groove is 0.21 mm, and the sum of the groove widths W3 is 6.9 mm. The relevant parameters for Examples 18 to 24 can be found in Table 2 below.

[0144] Table 2

[0145]

[0146] According to Table 2 above, combined with Examples 17 to 24 and Comparative Example 1, it can be seen that when the groove depth D2 is a constant value, as the width W2 of a single groove increases, the value of Δ gradually increases and the absolute value of Δ gradually decreases, indicating that the risk of lithium plating in the outer layer gradually decreases, the difference between the risk of lithium plating in the outer layer and the risk of lithium plating in the inner layer gradually decreases, and the fast charging performance of the secondary battery is improved.

[0147] In Examples 17 and 18, the voltage difference Δ is relatively close, but in Example 18, the width of a single groove is relatively large, which can reduce the accumulation of electrons on the surface of the first current collector, resulting in a more uniform current distribution. In Examples 23 and 24, Δ is relatively close, but in Example 24, the width of a single groove is wider, which reduces the mechanical strength of the first current collector and makes it prone to curling. Therefore, in the embodiments of this application, combining Examples 18 to 23, 0.5% ≤ W2 / W1 ≤ 4% can be selected.

[0148] In Examples 18 to 21, as the width of a single groove gradually increases, Δ increases relatively quickly, significantly reducing the risk of lithium plating on the outer layer. Furthermore, the width of a single groove is a suitable proportion to the width of the first current collector, resulting in better mechanical strength of the first positive electrode. In Examples 21 to 23, Δ increases more slowly, and the width of a single groove is relatively large compared to the width of the first current collector, making the first positive electrode prone to curling. Therefore, in the embodiments of this application, it is preferable that 0.5% ≤ W2 / W1 ≤ 2%.

[0149] Unlike Example 1, in Example 25, the groove depth D2 is 5 μm, the ratio of groove depth D2 to the thickness D1 of the first current collector is 33.33%, the width W2 of a single groove is 1.04 mm, and the first distance L1 between two adjacent grooves is 0.3 mm. The relevant parameters for Examples 26 to 32 can be found in Table 3 below.

[0150] Table 3

[0151]

[0152] According to Table 3 above, and in conjunction with Examples 25 to 32, when the groove depth D2 is a fixed value, as the width of the first distance L1 between two adjacent grooves increases, the phenomenon of curling of the first positive electrode sheet is significantly improved.

[0153] In Example 25, although Δ was -6.1 mV, the first positive electrode showed moderate curling. In Example 32, the absolute value of Δ was relatively large, indicating a significant difference between the risk of lithium plating in the outer and inner layers. In the embodiments of this application, combined with Examples 26 to 31, the first positive electrode did not curl or only slightly curled, and Δ was relatively small, indicating a relatively low risk of lithium plating in the outer layer. Therefore, 50% ≤ L1 / W2 ≤ 200% can be selected.

[0154] In Examples 27 to 31, the absolute value of the voltage difference Δ is relatively small, indicating that the difference between the risk of lithium plating in the outer layer and the inner layer is relatively low. Furthermore, no curling was observed in the first positive electrode, indicating that the first positive electrode structure has good strength. In Examples 29 and 31, Δ differs by only 0.3 mV. However, in Example 31, the width of the first spacing L1 is too large, causing electrons to easily accumulate between adjacent grooves, resulting in excessively high local current density and affecting the performance of the secondary battery. Therefore, in the embodiments of this application, 100% ≤ L1 / W2 ≤ 150% is preferred.

[0155] Unlike Example 1, in Example 33, the thickness T1 of the first conductive layer is 4 μm, that is, the thickness D1 of the first current collector is 4 μm, the thickness T2 of the second conductive layer is 9 μm, and the ratio of the groove depth D2 to the thickness D1 of the first current collector is 33.33%. The relevant parameters in Examples 34 to 42 can be referred to in Table 4 below.

[0156] Table 4

[0157]

[0158] According to Table 4 above, and in conjunction with Examples 33 to 42, when the ratio of the groove depth D2 to the thickness D1 of the first current collector is constant, the value of Δ gradually increases as the ratio of T1 to T2 decreases, indicating that the risk of outer layer lithium plating gradually decreases. When 44% ≤ T1 / T2 < 67%, the risk of outer layer lithium plating is even lower than that of inner layer lithium plating. However, in Examples 33 to 34, because the thickness T1 of the first conductive layer is relatively thin and a groove is provided on the first conductive layer, the first positive electrode sheet is very prone to curling. When 67% ≤ T1 / T2 < 160%, the absolute value of Δ gradually decreases as the ratio of T1 to T2 decreases, indicating that the difference between the risk of outer layer lithium plating and the risk of inner layer lithium plating gradually decreases. However, in Example 42, the value of Δ is relatively small, and the absolute value of Δ is relatively large, indicating that the difference between the risk of outer layer lithium plating and the risk of inner layer lithium plating is large. Therefore, in the embodiments of this application, in conjunction with Examples 35 to 42, 67% ≤ T1 / T2 ≤ 150% can be selected.

[0159] In Examples 35 and 36, Δ is close to 0, indicating a low risk of outer layer lithium plating in both. However, the thickness of the first conductive layer in Example 36 is relatively thick, resulting in better structural strength of the first positive electrode and reduced curling. In Examples 36 to 40, the voltage difference Δ is relatively large, indicating a lower risk of outer layer lithium plating, while the absolute value of Δ is relatively small, indicating a smaller difference between outer and inner layer lithium plating. This improves the discharge rate of the non-lithium-plated outer layer and the fast-charging performance of the secondary battery. Therefore, in the embodiments of this application, 78% ≤ T1 / T2 < 120% is preferably preferred.

[0160] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of this application as described above, which are not provided in detail for the sake of brevity; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A secondary battery comprising an electrode assembly including a plurality of positive electrode tabs and a plurality of negative electrode tabs alternately stacked in a first direction, with a separator film provided between adjacent ones of the positive electrode tabs and the negative electrode tabs, characterized by, The positive electrode tab includes a first positive electrode tab and a second positive electrode tab, along the first direction, the first positive electrode tab is the outermost tab of the electrode assembly, and the second positive electrode tab is arranged between two adjacent negative electrode tabs; The first positive electrode tab includes a first current collector and a first positive electrode coating, along the first direction, the first current collector includes oppositely arranged first and second surfaces, the second surface faces the negative electrode tab, and the first positive electrode coating is arranged on the second surface; The first surface is provided with a groove, along the first direction, the thickness of the first current collector is D1, the depth of the groove is D2, and 10%≤D2 / D1≤70%.

2. The secondary battery according to claim 1, characterized by 33.33%≤D2 / D1≤50%.

3. The secondary battery according to claim 1, characterized by The groove is arranged along the second direction, and along the third direction, the width of the first current collector is W1, and the width of the groove is W2, 0.5%≤W2 / W1≤4%. Wherein, the first direction, the second direction and the third direction are perpendicular to each other.

4. The secondary battery according to claim 3, characterized by 0.5%≤W2 / W1≤2%.

5. The secondary battery according to claim 3, characterized by Along the third direction, the sum of the widths of all the grooves is W3, and 15%≤W3 / W1≤80%.

6. The secondary battery according to claim 5, characterized by 50%≤W3 / W1≤70%.

7. The secondary battery according to claim 3, characterized by Along the third direction, a plurality of grooves are arranged in sequence, and adjacent two grooves have a first interval L1, 50%≤L1 / W2≤200%.

8. The secondary battery according to claim 7, characterized by 100%≤L1 / W2≤150%.

9. The secondary battery according to claim 3, characterized by Along the second direction, the first current collector includes oppositely arranged first and second end surfaces, and the groove penetrates the first and second end surfaces.

10. The secondary battery according to claim 1, characterized by The second positive electrode tab includes a second current collector and a second positive electrode coating arranged on both surfaces of the second current collector; The first current collector includes a first conductive layer, and along the first direction, the thickness of the first conductive layer is T1. The second current collector includes a second conductive layer, and along the first direction, the thickness of the second conductive layer is T2, 67%≤T1 / T2≤150%.

11. The secondary battery according to claim 10, characterized by 78%≤T1 / T2≤120%.

12. An electronic device, comprising: The secondary battery includes any one of claims 1-11.

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