Electrode sheet and preparation method therefor, electrode assembly and secondary battery

Abstract

The present application relates to an electrode sheet and a preparation method therefor, an electrode assembly and a secondary battery. The electrode sheet comprises a current collector, an active material layer and a tab, wherein the current collector has a first surface, and the active material layer is arranged on the first surface; the first surface has a first bare foil region, and the tab is arranged in the first bare foil region; and the electrode sheet comprises a first adhesive layer. The tab has a first portion and a second portion which are connected to each other, the first adhesive layer being bonded between the first portion and the first bare foil region, and the second portion being configured to externally connect to an electronic device. The surface of the first portion facing the first bare foil region has a protruding region, several protruding parts protruding from the protruding region; in the direction of thickness of the tab, the protruding parts protrude relative to the second portion; and the protruding parts are at least partially embedded in the first adhesive layer. The risk of a separator being punctured can be reduced, and there is no need to provide a tab adhesive on the tab or the first bare foil region, thereby improving the energy density of the secondary battery while reducing the cost.

Description

Electrode sheets and their preparation methods, electrode assemblies and secondary batteries

[0001] Cross-reference of related applications

[0002] This application claims priority to Chinese Patent Application No. 202410706651.6, filed on May 31, 2024, entitled “Electrode and its preparation method, electrode assembly and secondary battery”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, and in particular to an electrode sheet and its preparation method, an electrode assembly, and a secondary battery. Background Technology

[0004] The tabs, which serve as a bridge connecting the secondary battery to the external circuit, are usually made of metal. The welding of the tabs to the current collector inside the secondary battery allows the tabs to conduct electricity with the electrode components inside the secondary battery, thereby achieving the purpose of transmitting current.

[0005] To facilitate the welding of the tabs, a double-sided empty foil area is usually required on the current collector, with the welding head and welding seat contacting the current collector on both sides of the empty foil area for welding. However, the welding of the tabs to the empty foil area of ​​the current collector usually results in some welding burrs. These burrs can easily pierce the separator and cause short circuit failure of the positive and negative electrodes inside the secondary battery. Summary of the Invention

[0006] The purpose of this application is to provide an electrode sheet and its preparation method, an electrode assembly and a secondary battery, which aims to improve the technical problems of welding burrs and easy puncture of the separator in the current collector.

[0007] According to a first aspect of this application, an electrode sheet is provided, comprising a current collector, a first active material layer, and a tab. The current collector has a first surface, the first active material layer is disposed on the first surface, the first surface has a first empty foil region, the tab is disposed in the first empty foil region, and the electrode sheet includes a first adhesive layer. The tab has a first portion and a second portion connected together. Along the thickness direction, the first adhesive layer is bonded between the first portion and the first empty foil region. The first portion is electrically connected to the first empty foil region, and the second portion is used for connecting to an external electronic device. The surface of the first portion facing the first empty foil region has a protruding area, and the protruding area has a plurality of protrusions. Along the thickness direction of the tab, the protrusions protrude relative to the second portion, and the protrusions are at least partially embedded in the first adhesive layer.

[0008] In the above technical solution, the tab is directly bonded to the current collector via the first adhesive layer. This not only facilitates operation but also eliminates welding burrs, reducing the risk of the separator being punctured. Furthermore, it eliminates the need for tab adhesive in the tab or the first empty foil area, saving space and increasing the energy density of the secondary battery while reducing costs. Additionally, the surface of the tab facing the first empty foil area has several protrusions. When the first adhesive layer is bonded between the tab and the first empty foil area, the protrusions are at least partially embedded within the first adhesive layer. The protrusions increase the bonding area between the first adhesive layer and the tab, resulting in a stronger bond and improved connection strength between the tab and the current collector. Moreover, the protrusions increase the current-carrying area of ​​the tab, improving its current-carrying capacity and facilitating high-rate charging and discharging of the secondary battery. Simultaneously, the protrusions act as reinforcing ribs, increasing the tab's strength and reducing the risk of tearing.

[0009] In some preferred embodiments, the current collector further has a second surface, and the first surface and the second surface are disposed opposite each other along the thickness direction of the current collector. The electrode also has a second active material layer disposed on the second surface, and when viewed along the thickness direction of the current collector, the projection of the first empty foil area lies on the projection of the second active material layer. Since a solder pad and solder head are not required, the empty foil area can be provided only on the first surface. The single-sided empty foil area can fully utilize the internal space of the secondary battery and improve the energy density of the secondary battery.

[0010] In some preferred embodiments, when viewed along the thickness direction of the current collector, the contact area between all the protrusions and the first empty foil area is S1, and the projected area of ​​the tab on the first empty foil area is S2, where 20% ≤ S1 / S2 ≤ 80%. This facilitates the embedding of each protrusion into the first adhesive layer, improving the bonding strength between the tab and the first adhesive layer, while simultaneously reducing the resistance between the tab and the current collector. If the area occupied by the protrusions is too large, the gap between two adjacent protrusions will become smaller, making it difficult for the adhesive layer to be embedded. This not only leads to poor bonding strength but may also make it difficult for the tab to directly contact the current collector, resulting in higher resistance.

[0011] In some preferred embodiments, at least part of the protrusion contacts the first empty foil area. When the first adhesive layer is bonded between the tab and the current collector, the protrusion can directly contact the current collector, thereby directly realizing the electrical connection between the tab and the current collector.

[0012] In some preferred embodiments, the first adhesive layer comprises at least one of epoxy resin, polyolefin, polystyrene, polymethyl methacrylate, phenolic resin, or styrene-butadiene rubber. These materials possess excellent adhesive properties and chemical resistance, making them suitable for various electrochemical reactions within the secondary battery.

[0013] In some preferred embodiments, the first adhesive layer includes at least one of thermosetting adhesive, pressure-sensitive adhesive, and hot melt adhesive.

[0014] In some preferred embodiments, the length of the first empty foil area is L1 along the width direction of the current collector, and the length of the first adhesive layer is L2, where 1mm ≤ L1 - L2 ≤ 5mm. This facilitates the tab being positioned in the first empty foil area along the width direction of the current collector, while allowing a gap of 1mm to 5mm to be reserved in the width direction of the current collector to prevent direct adhesion between the first adhesive layer and the first active material layer, thereby reducing the impact of the first adhesive layer on the battery energy density.

[0015] Optionally, along the length of the current collector, the width of the first empty foil area is W1, and the width of the first adhesive layer is W2, where 1mm ≤ W1 - W2 ≤ 5mm. This facilitates the placement of the tab in the first empty foil area along the length of the current collector, while allowing a gap of 1mm to 5mm to be reserved along the length of the current collector to prevent direct adhesion between the first adhesive layer and the first active material layer, reducing the impact of the first adhesive layer on the energy density of the secondary battery. This not only facilitates the placement of the tab in the first empty foil area but also facilitates the adhesion and fixation of the tab to the current collector through the first adhesive layer.

[0016] In some preferred embodiments, along the width direction of the current collector, the bonding length between the first adhesive layer and the first empty foil area is L3, where 4mm ≤ L3 ≤ 24mm. Along the length direction of the current collector, the bonding width between the first adhesive layer and the first empty foil area is W3, where 4mm ≤ W3 ≤ 14mm. Sufficient bonding length and width increase the bonding area between the first adhesive layer and the first empty foil area, improving the connection strength, while simultaneously reducing the impact of the first adhesive layer on the energy density of the secondary battery.

[0017] In some preferred embodiments, the thickness of the first adhesive layer is T1 along the thickness direction of the current collector, where 2μm ≤ T1 ≤ 50μm. This can improve the connection strength while reducing the impact of the first adhesive layer on the energy density of the secondary battery. Furthermore, 15μm ≤ T1 ≤ 25μm can reduce the detachment of the tab protrusion from the current collector.

[0018] In some preferred embodiments, the length of the first adhesive layer is L2 along the width direction of the current collector, and the length of the protruding area is L4, with 0mm≤L2-L4≤5mm; this facilitates that each protrusion can be embedded in the first adhesive layer.

[0019] Optionally, along the length of the current collector, the width of the first adhesive layer is W2, the width of the protruding area is W4, and the width of the tab located on the first empty foil area is W5, 0mm≤W2-W4≤5mm, 0≤W2-W5≤5mm, which can facilitate the first adhesive layer to bond all the tabs and improve the bonding strength.

[0020] In some preferred embodiments, the width of a single protrusion is W6 along the length direction of the current collector, where 0.05mm≤W6≤1mm, to facilitate the protrusion being embedded in the first adhesive layer.

[0021] Optionally, along the thickness direction of the current collector, the protrusion height of a single protrusion is H, 5μm≤H≤60μm, which facilitates the protrusion to be embedded in the first adhesive layer. Sufficient embedding depth improves the connection strength between the first adhesive layer and the tab, and makes it easy for the protrusion to contact the current collector, thus facilitating the electrical connection between the tab and the current collector.

[0022] Secondly, this application also proposes an electrode assembly, including a separator and at least two electrodes as described in any of the embodiments of the first aspect above, wherein the at least two electrodes are a positive electrode and a negative electrode, and the positive electrode and the separator are stacked and wound around the negative electrode.

[0023] Thirdly, this application also proposes a secondary battery including an electrode assembly as described in any of the embodiments of the second aspect above.

[0024] Fourthly, this application also proposes a method for preparing an electrode sheet as described in any embodiment of the first aspect above, comprising: providing a current collector having a first surface, coating the first surface with a first active material layer, and reserving a first empty foil area on the first surface; providing a first adhesive layer, bonding the first adhesive layer to the first empty foil area; providing an electrode tab having a plurality of protrusions, the protrusions of the electrode tab facing the first adhesive layer, bonding the electrode tab to the first adhesive layer, and pressing the electrode tab to the first adhesive layer such that the protrusions are embedded in the first adhesive layer.

[0025] Additional aspects and advantages of the embodiments of this application will be described, shown, or illustrated in part by way of implementation of the embodiments of this application in the following description. Attached Figure Description

[0026] 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 dimensions in the drawings do not constitute a limitation on scale.

[0027] Figure 1 is a schematic diagram of the structure of an electrode sheet according to an embodiment of this application;

[0028] Figure 2 is a magnified view of part A in Figure 1;

[0029] Figure 3 is a schematic diagram of the bonding of the tab, the first adhesive layer and the current collector according to an embodiment of this application;

[0030] Figure 4 is a schematic diagram of the bonding of the tab, the first adhesive layer and the current collector according to an embodiment of this application;

[0031] Figure 5 is a schematic diagram of the bonding of the tab, the first adhesive layer and the current collector according to an embodiment of this application;

[0032] Figure 6 is a schematic diagram of the bonding of the tab, the first adhesive layer and the current collector according to an embodiment of this application (viewed along the third direction Z);

[0033] Figure 7 is a schematic diagram of the structure of the electrode tab according to an embodiment of this application;

[0034] Figure 8 is a schematic diagram of the structure of the first adhesive layer according to an embodiment of this application;

[0035] Figure 9 is a schematic diagram of the structure of an electrode assembly according to an embodiment of this application;

[0036] Figure 10 is a schematic diagram of the structure of an electrode assembly in the prior art.

[0037] Explanation of reference numerals in the attached drawings: 100, electrode sheet; 10, current collector; 11, first surface; 111, first empty foil area; 12, second surface; 13, embedding groove; 20, first active material layer; 30, tab; 31, first portion; 32, second portion; 311, protruding area; 312, protrusion; 3121, top; 3122, bottom; 33, flat portion; 40, first adhesive layer; 50, second active material layer; 1000, electrode assembly; 100a, positive electrode sheet; 10a, positive current collector; 11a, first positive electrode surface; 111a, first positive electrode empty foil area; 12a, second positive electrode surface; 20a, first positive electrode active material layer; 50a, second positive electrode active material layer; 30a, positive electrode tab; 100b, negative electrode sheet; 10b, negative electrode current collector; 11b, first negative electrode surface; 111b, first negative electrode empty foil area; 12b, second negative electrode surface; 20b, first negative electrode active material layer; 50b, second negative electrode active material layer; 30b, negative electrode tab; 200, separator; 300, electrode sheet adhesive; 400, tab adhesive; X, first direction; Y, second direction; Z, third direction. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0039] In this application, the reference to "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 separate or alternative embodiment that is mutually exclusive with other embodiments.

[0040] 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, "multiple" means two or more, unless otherwise explicitly defined.

[0041] 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.

[0042] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0043] The tab 30 serves as a bridge connecting the secondary battery to the external circuit. It is usually made of metal. The tab 30 is welded to the current collector 10 inside the secondary battery, which allows the tab 30 to conduct with the electrode assembly 1000 inside the secondary battery, thereby achieving the purpose of transmitting current.

[0044] Referring to Figure 10, to facilitate the welding of the positive electrode tab 30a, a double-sided empty foil area (the empty foil area is the area on the current collector 10 where no active material layer is provided) is usually required on the positive electrode current collector 10a. The welding socket contacts the positive electrode current collector 10a in one empty foil area, and the welding head welds the positive electrode tab 30a to the positive electrode current collector 10a in the other empty foil area. The negative electrode sheet 100b is usually configured similarly.

[0045] However, the welding of the positive electrode tab 30a to the positive electrode current collector 10a and the welding of the negative electrode tab 30b to the negative electrode current collector 10b usually results in some welding burrs. These burrs can easily pierce the separator 200, which can easily lead to short circuit failure of the positive electrode 100a and the negative electrode 100b inside the secondary battery.

[0046] To reduce the risk of the separator 200 being punctured, as shown in Figure 10, a double layer of tab adhesive 400 is typically provided at the positive electrode tab 30a position. That is, a layer of tab adhesive 400 is provided on one side of the empty foil of the positive electrode current collector 10a, and a layer of tab adhesive 400 is also provided on the other side. The negative electrode sheet 100b is treated similarly, with the double layer of tab adhesive 400 preventing burrs from puncturing the separator 200.

[0047] Furthermore, two layers of adhesive are required on the positive electrode 100a to reduce the impact of burrs at the negative electrode tab 30b on the positive electrode active material layer. Similarly, two layers of adhesive 300 are required on the negative electrode 100b to reduce the impact of burrs at the positive electrode tab 30a on the negative electrode active material layer. However, the multiple layers of adhesive affect the energy density of the secondary battery.

[0048] To address the aforementioned problems, in a first aspect, this application proposes an electrode 100, which is an important component of a secondary battery. The electrode 100 is typically divided into a positive electrode 100a and a negative electrode 100b, which are usually insulated and separated by a separator 200. Both the positive electrode 100a and the negative electrode 100b contain substances capable of electrochemical reactions. These substances work together with the electrolyte to achieve energy storage and release in the secondary battery. Referring to Figure 1, which illustrates the structure of an electrode 100 according to some embodiments of this application, the electrode 100 includes a current collector 10, a first active material layer 20, tabs 30, and a first adhesive layer 40. The first active material layer 20 is disposed on the current collector 10, and the tabs 30 are bonded to the current collector 10 via the first adhesive layer 40.

[0049] For the aforementioned current collector 10, which serves as the conductive substrate of the electrode 100, it can be made of aluminum foil or copper foil, which are flat and have a strip-like structure. For example, when the current collector 10 is the positive electrode current collector, aluminum foil can be used; when the current collector 10 is the negative electrode current collector, copper foil can be used. Aluminum foil and copper foil have high conductivity, which can effectively reduce the internal resistance of the secondary battery and facilitate the improvement of the energy density and power density of the secondary battery. In addition, aluminum foil and copper foil also have good mechanical strength, which can withstand the expansion and contraction of the secondary battery during charging and discharging, ensuring the stability and safety of the secondary battery.

[0050] Along the thickness direction (third direction Z) of the current collector 10, the current collector 10 has a first surface 11 and a second surface 12 disposed opposite to each other. The first active material layer 20 may be disposed on the first surface 11. In some other embodiments, the electrode 100 further includes a second active material layer 50 disposed on the second surface 12.

[0051] The first active material layer 20 comprises an active material, a conductive agent, and a binder. These components are mixed and stirred until homogeneous, then coated onto the first surface 11 of the current collector 10 to obtain the first active material layer 20. When the electrode 100 is a positive electrode, the active material can be selected from one or more of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, and cobalt-free materials. When the electrode 100 is a negative electrode, the active material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, elemental silicon, silicon oxides, and silicon alloys.

[0052] The current collector 10 has areas without an active material layer. For example, referring to Figures 1 to 3, the first surface 11 has a first empty foil area 111, which is the area on the current collector 10 without an active material layer. The second surface 12 can also have a second empty foil area (not shown in the figures). This double-sided empty foil area facilitates the welding of the tab 30 to the current collector 10. For example, a welding socket receives the current collector 10 in the second empty foil area, and a welding head welds the tab 30 to the current collector 10 in the first empty foil area 111. Alternatively, the second surface 12 can be coated with more of the second active material layer 50 to improve the energy density of the secondary battery. The tab 30 can be electrically connected to the current collector 10 in the first empty foil area 111.

[0053] The aforementioned tab 30 serves as a bridge connecting the secondary battery to an external circuit, allowing the current flowing through the current collector 10 to be directed to the external circuit. Referring to Figures 1 to 3, the tab 30 can have a flat structure, comprising a first portion 31 and a second portion 32 connected to each other. The first portion 31 is disposed in the first empty foil area 111 and is electrically connected to the current collector 10. The second portion 32 of the tab 30 extends beyond the current collector 10 and is used to connect to external electronic devices. In embodiments of this application, the tab 30 can be fixed to the first empty foil area 111 using adhesive.

[0054] Referring to Figure 1, the first adhesive layer 40 is bonded between the first empty foil area 111 and the first portion 31 of the tab 30, fixing the tab 30 to the current collector 10, and electrically connecting the first portion 31 to the first empty foil area 111. Since the tab 30 is directly bonded to the current collector 10 via the first adhesive layer 40, it is not only convenient to operate but also eliminates welding burrs, reducing the risk of puncture to the separator 200. Furthermore, it eliminates the need for tab adhesive on the tab 30 or the first empty foil area 111, saving space and increasing the energy density of the secondary battery while reducing costs. Moreover, conventional welding methods often fail to fully weld the overlapping portion of the tab 30 and the current collector 10, resulting in lower welding strength. The first adhesive layer 40 of this application, however, can fully bond the overlapping portion of the tab 30 and the current collector 10 and fill the gap between them, resulting in a larger connection area and higher connection strength.

[0055] Among them, the surface of the tab 30 facing away from the first adhesive layer 40 can directly contact the separator 200, thereby improving space utilization and increasing the energy density of the secondary battery.

[0056] Referring to Figures 1 to 3, in some embodiments, the second active material layer 50 is disposed on the second surface 12. Viewed along the thickness direction (third direction Z) of the current collector 10, the projection of the first empty foil area 111 lies on the projection of the second active material layer 50. Since the tab 30 and the first empty foil area 111 of the current collector 10 are bonded and fixed by the first adhesive layer 40, welding is not required using a soldering socket and soldering head. Therefore, more second active material layers 50 can be disposed on the second surface 12. For example, an empty foil area can be disposed only on the first surface 11. The single-sided empty foil area can fully utilize the internal space of the secondary battery and improve the energy density of the secondary battery. Simultaneously, no welding burrs are generated on either the first surface 11 or the second surface 12, further reducing the risk of the separator 200 being punctured. Furthermore, neither the first surface 11 nor the second surface 12 requires tab adhesive, which not only fully utilizes space and further improves energy density but also reduces costs.

[0057] In some embodiments, referring to Figures 1 to 3, the surface of the first portion 31 facing the first empty foil area 111 has a protruding area 311, and the protruding area 311 is provided with a plurality of protrusions 312. Along the thickness direction (third direction Z) of the tab 30, the protrusions 312 protrude from the second portion 32, that is, the protrusions 312 protrude relative to the second portion 32 in the thickness direction. The protrusion 312 can be formed on the tab 30 by laser processing or extrusion molding. When the first adhesive layer 40 is bonded between the tab 30 and the first empty foil area 111, the protrusion 312 is at least partially embedded in the first adhesive layer 40. The protrusion 312 can increase the bonding area between the first adhesive layer 40 and the tab 30, making the first adhesive layer 40 and the tab 30 bond more firmly and improving the connection strength between the tab 30 and the current collector 10. In addition, the protrusion 312 makes the current carrying area of ​​the tab 30 larger, improving the current carrying capacity of the tab 30 and making it easier to adapt to the high-rate charging and discharging of the secondary battery. At the same time, the protrusion 312 acts like a reinforcing rib, which can improve the strength of the tab 30, reduce the risk of the tab 30 tearing, and improve the impact resistance of the secondary battery.

[0058] In some embodiments, when viewed along the thickness direction of the current collector 10, the contact area between all the protrusions 312 and the first empty foil area 111 is S1, and the projected area of ​​the tab 30 on the first empty foil area 111 is S2, where 20% ≤ S1 / S2 ≤ 80%. The protrusions 312 increase the bonding area between the first adhesive layer 40 and the tab 30. If the area occupied by the protrusions 312 is too small, the effect on improving the connection strength is weak. If the area occupied by the protrusions 312 is too large, the distance between two adjacent protrusions 312 will easily decrease, which is not conducive to all protruding and embedding into the first adhesive layer 40. Within the above-mentioned area range, the bonding strength between the tab 30 and the first adhesive layer 40 can be improved while facilitating the embedding of each protrusion 312 into the first adhesive layer 40.

[0059] In other embodiments, referring further to FIG4, a plurality of embedding grooves 13 may be provided on the first empty foil area 111 of the current collector 10. The embedding grooves 13 may be formed on the current collector 10 by means of laser processing or extrusion forming. When the first adhesive layer 40 is bonded between the current collector 10 and the tab 30, the first adhesive layer 40 can be embedded into the embedding grooves 13 on the current collector 10, which can increase the bonding area between the first adhesive layer 40 and the current collector 10, and can improve the friction between the protrusion 312 and the first adhesive layer 40, thereby improving the connection strength between the tab 30 and the current collector 10.

[0060] Optionally, the protrusion 312 on the tab 30 is embedded into the embedding groove 13 on the current collector 10. The engagement of the protrusion 312 and the embedding groove 13 can restrict the relative movement between the tab 30 and the current collector 10, thereby improving the connection strength between the tab 30 and the current collector 10. When the protrusion 312 is embedded in the embedding groove 13, a first adhesive layer 40 is present between the protrusion 312 and the embedding groove 13, and the first adhesive layer 40 simultaneously bonds the protrusion 312 and the embedding groove 13; or, at least a portion of the protrusion 312 contacts the first empty foil area 111, thereby achieving an electrical connection between the tab 30 and the current collector 10.

[0061] Similarly, the first empty foil area 111 can also be provided with several protrusions 312. The protrusions 312 of the first empty foil area 111 are embedded into the first adhesive layer 40, improving the bonding strength between the first adhesive layer 40 and the current collector 10. When the protrusions 312 of the first empty foil area 111 are embedded into the first adhesive layer 40, the protrusions 312 of the first empty foil area 111 can also make contact with the protrusions 312 of the electrode 30, realizing the conduction between the electrode 30 and the current collector 10. Alternatively, the surface of the electrode 30 facing the first adhesive layer 40 is provided with an embedding groove 13, and the protrusions 312 on the first empty foil area 111 are directly embedded into the embedding groove 13 on the electrode 30, increasing the bonding area between the first adhesive layer 40 and the electrode 30 and improving the connection strength.

[0062] The first adhesive layer 40 includes at least one of thermosetting adhesive, pressure-sensitive adhesive, or hot melt adhesive.

[0063] For example, the first adhesive layer 40 can be made of hot melt adhesive. The first adhesive layer 40 can be first bonded to the first empty foil area 111, and then the tab 30 can be bonded to the first adhesive layer 40 by hot pressing. Hot pressing melts the first adhesive layer 40 and causes the protrusion 312 to be inserted into the first adhesive layer 40. After the first adhesive layer 40 is cured, the tab 30 can be fixed to the current collector 10.

[0064] The first adhesive layer 40 can also be a thermosetting adhesive. After the first adhesive layer 40 is bonded to the first empty foil area 111, it is cured by hot pressing, so that the tab 30 is fixed to the current collector 10.

[0065] The first adhesive layer 40 can also be made of pressure-sensitive adhesive. When pressing the tab 30, the first adhesive layer 40, and the current collector 10, the protrusion 312 can be embedded in the first adhesive layer 40, and the first adhesive layer 40 can bond the current collector 10 and the tab 30. In some other embodiments, the first adhesive layer 40 can also be made of both hot melt adhesive and pressure-sensitive adhesive.

[0066] The first adhesive layer 40 can be of various types. For example, the first adhesive layer 40 includes at least one of epoxy resin, polyolefin, polystyrene, polymethyl methacrylate, phenolic resin, or styrene-butadiene rubber. For instance, the first adhesive layer 40 includes epoxy resin, which has excellent adhesive properties and chemical resistance, strong adhesion after immersion in liquid, and minimal swelling. It can adapt to various electrochemical reactions inside the secondary battery, and the epoxy resin can be cured by heating or reacting with a curing agent to form a robust first adhesive layer 40.

[0067] In some embodiments, the first adhesive layer 40 is conductive, and when the first adhesive layer 40 is bonded between the tab 30 and the current collector 10, the tab 30 and the current collector 10 are electrically connected. For example, the first adhesive layer 40 includes conductive metal fillers such as silver, copper, and nickel, making the first adhesive layer 40 conductive, and when the first adhesive layer 40 is bonded between the tab 30 and the current collector 10, the tab 30 and the current collector 10 are electrically connected.

[0068] It should be noted that when the protrusion 312 passes through the first adhesive layer 40 and contacts the first empty foil area 111, the material of the first adhesive layer 40 can be either an insulating material or a conductive adhesive. When the protrusion 312 does not pass through the first adhesive layer 40, the first adhesive layer 40 is a conductive adhesive.

[0069] In some embodiments, when viewed along the thickness direction (third direction Z) of the first electrode 100, the overlap area between the tab 30 and the first empty foil area 111 is S3, and the overlap area between the protrusion 312 and the first empty foil area 111 is S4, where 20% ≤ S3 / S4 ≤ 80%. The protrusion 311 and the first empty foil area 111 have sufficient overlap area. When the first adhesive layer 40 is bonded between the tab 30 and the current collector 10, the sufficient area of ​​the protrusion 311 can increase the bonding area between the tab 30 and the first adhesive layer 40, thereby improving the connection strength between the tab 30 and the current collector 10.

[0070] Regarding the size of the first empty foil area 111, if the size of the first empty foil area 111 is too large, it will affect the energy density of the battery; if the size of the first empty foil area 111 is too small, it will be difficult to accommodate the tab 30, which is not conducive to the bonding and fixing of the tab 30 and the current collector 10.

[0071] As for the size of the first adhesive layer 40, if the size of the first adhesive layer 40 is too small, it may lead to unstable adhesion with the current collector 10 or the tab 30. If the size of the first adhesive layer 40 is too large, it occupies a lot of space and is prone to adhesion with the first active material layer 20, affecting the capacity and energy density of the secondary battery.

[0072] In the embodiments of this application, please refer to Figures 5 and 6. Along the width direction (second direction Y) of the current collector 10, the length of the first empty foil area 111 is L1, and the length of the first adhesive layer 40 is L2, 1mm≤L1-L2≤5mm. This facilitates the tab 30 to be disposed in the first empty foil area 111 along the width direction (second direction Y) of the current collector 10. At the same time, a gap space of 1mm to 5mm can be reserved in the width direction (second direction Y) of the current collector 10 to prevent the first adhesive layer 40 from directly bonding with the first active material layer 20 and reduce the impact of the first adhesive layer 40 on the battery energy density.

[0073] Along the length direction (first direction X) of the current collector 10, the width of the first empty foil area 111 is W1, and the width of the first adhesive layer 40 is W2, 1mm≤W1-W2≤5mm. This facilitates the tab 30 to be positioned in the first empty foil area 111 along the length direction (first direction X) of the current collector 10. At the same time, a gap space of 1mm to 5mm can be reserved in the length direction (first direction X) of the current collector 10 to prevent the first adhesive layer 40 from directly bonding with the first active material layer 20, thereby reducing the impact of the first adhesive layer 40 on the energy density of the secondary battery. This not only facilitates the placement of the tab 30 in the first empty foil area 111, but also facilitates the bonding and fixation of the tab 30 to the current collector 10 through the first adhesive layer 40.

[0074] Optionally, along the width direction (second direction Y) of the current collector 10, the bonding length between the first adhesive layer 40 and the first empty foil area 111 is L3, where 4mm ≤ L3 ≤ 24mm. Along the length direction (first direction X) of the current collector 10, the bonding width between the first adhesive layer 40 and the first empty foil area 111 is W3, where 4mm ≤ W3 ≤ 14mm. Sufficient bonding length and width increase the bonding area between the first adhesive layer 40 and the first empty foil area 111, improving the connection strength, while reducing the impact of the first adhesive layer 40 on the energy density of the secondary battery. The first adhesive layer 40 is typically bonded integrally to the first empty foil area 111, and the width W2 of the first adhesive layer 40 is the same as the bonding width W3 between the first adhesive layer 40 and the first empty foil area 111, i.e., W2 = W3.

[0075] Regarding the thickness of the first adhesive layer 40, if the thickness of the first adhesive layer 40 is too large, it will affect the energy density of the secondary battery; if the thickness of the first adhesive layer 40 is too small, it will easily lead to unstable adhesion between the tab 30 and the current collector 10, and may also make it difficult for the protrusion 312 to be embedded in the first adhesive layer 40. In the embodiments of this application, along the thickness direction of the current collector 10 (third direction Z), the maximum thickness of the first adhesive layer 40 is T1, 2μm≤T1≤50μm. When the current collector 10, the first adhesive layer 40 and the tab 30 are hot-pressed, the protrusion 312 can be embedded in the first adhesive layer 40, which can make the first adhesive layer 40 stably bond the tab 30 and the current collector 10, thereby improving the connection strength and reducing the impact of the first adhesive layer 40 on the energy density of the secondary battery.

[0076] Furthermore, 15μm≤T1≤25μm, where 15μm to 25μm is typically the finished thickness of the first adhesive layer 40. During the secondary battery preparation process, there is a high-temperature environment that causes the first adhesive layer 40 to rebound. 15μm to 25μm is typically the thickness after the rebound, which can reduce the tab 30 from detaching from the current collector 10.

[0077] In some embodiments, please further refer to FIG7. Along the width direction (second direction Y) of the current collector 10, the length of the first adhesive layer 40 is L2, and the length of the protrusion 311 is L4, 0mm≤L2-L4≤5mm. The length of the first adhesive layer 40 can be the same as or greater than the length of the protrusion 311, so that each protrusion 312 can be embedded in the first adhesive layer 40. The length of the protruding area 311 can be the same as or less than the length of the tab 30. When the length is the same as the tab 30, the bonding between the first adhesive layer 40 and the tab 30 is the same as the bonding between the first adhesive layer 40 and the protruding area 311. The tab 30 does not occupy too much space, which is conducive to improving the energy density of the battery. When the length is less than the tab 30, the first adhesive layer 40 can bond the protruding area 311 and part of the flat area around the protruding area (the area without the protrusion 312). When pressing the current collector 10, the first adhesive layer 40 and the tab 30, the bonding of the flat area is more compact, which can reduce the air entering between the first adhesive layer 40 and the tab 30 and reduce the generation of air bubbles. The protruding area 311 has a larger friction and bonding area. The combination of the flat area and the protruding area 311 can further improve the connection strength between the tab 30 and the current collector 10.

[0078] Similarly, along the length direction (first direction X) of the current collector 10, the width of the first adhesive layer 40 is W2, and the width of the protruding area 311 is W4, where 0mm ≤ W2 - W4 ≤ 5mm. This allows each protrusion 312 to be easily embedded in the first adhesive layer 40 along the length direction (first direction X) of the current collector 10. The width of the tab 30 located in the first empty foil area 111 is W5, where 0 ≤ W2 - W5 ≤ 5mm. The width of the first adhesive layer 40 is greater than the width of the tab 30 located in the first empty foil area 111, allowing the first adhesive layer 40 to completely bond the tab 30, thus improving the bonding strength.

[0079] Regarding the shape of the protrusion 312, viewed along the length direction (first direction X) of the current collector 10, the protrusion 312 can be rectangular, trapezoidal, semi-circular, or arc-shaped, or it can be a strip-shaped structure. This strip-shaped structure can be arranged along the length direction of the tab 30, along the width direction of the tab 30, or along other directions. Multiple strip-shaped protrusions can be arranged along the length direction of the tab 30 or along its width direction. For example, the protrusion 312 is rectangular, and the width of a single protrusion 312 is W6, where 0.05mm ≤ W6 ≤ 1mm, to facilitate embedding the protrusion 312 into the first adhesive layer 40. Alternatively, the protrusion 312 can be trapezoidal. Referring to Figure 8, the protrusion 312 includes a top 3121 and a bottom 3122. The top 3121 is located close to the current collector 10, and the bottom 3122 is located away from the current collector 10. Along the width direction (second direction Y) of the current collector 10, the width of the top 3121 is W7, for example, 0mm < W7 ≤ 1mm, and the width of the bottom 3122 is W8, where W7 < W8. The smaller width of the top 3121 allows the protrusion 312 to be easily embedded into the first adhesive layer 40 when hot-pressing the tab 30, the first adhesive layer 40, and the current collector 10.

[0080] In some embodiments, 0.05mm ≤ W7 ≤ 1mm. When the protrusion 312 is in direct contact with the current collector 10, the top 3121 of the protrusion 312 is the part that directly contacts the current collector 10. If the protrusion 3121 is too sharp, it may cause the protrusion 312 to pierce the current collector 10. In the embodiments of this application, it is preferable that 0.05mm ≤ W7 ≤ 1mm, which can reduce the number of times the protrusion 312 pierces the current collector 10. At the same time, the adaptable width of the top 3121 can facilitate the contact and electrical connection between the top 3121 and the current collector 10.

[0081] In some embodiments, the distance between two adjacent protrusions 312 is D, where 0.1mm ≤ D ≤ 1mm, which facilitates the embedding of each protrusion 312 into the first adhesive layer 40. Furthermore, within the aforementioned distance range, it is easier to form a straight portion 33 between two adjacent protrusions 312. The straight portion 33 facilitates the adhesion of the first adhesive layer 40, reduces air entry between the first adhesive layer 40 and the tab 30, and reduces bubble formation. The protrusions 312 increase the bonding area, and the combination of the straight portion 33 and the embedding groove 32 further enhances the connection strength between the first adhesive layer 40 and the tab 30.

[0082] If the depth of the protrusion 312 is too large, a gap may appear between the tab 30 and the first adhesive layer 40, resulting in weak adhesion. If the depth of the protrusion 312 is too small, the area embedded in the first adhesive layer 40 is insufficient, resulting in poor effect on improving the connection strength.

[0083] In the embodiment of the application, along the thickness direction (third direction Z) of the first electrode 100, the protrusion height of the protrusion 312 is H, 5μm≤H≤60μm, which facilitates the protrusion 312 to be embedded in the first adhesive layer 40. Sufficient embedding depth improves the connection strength between the first adhesive layer 40 and the tab 30, and makes it easy for the protrusion 312 to contact the current collector 10, which facilitates the electrical connection between the tab 30 and the current collector 10.

[0084] Secondly, embodiments of this application also provide an electrode assembly 1000, including a separator 200 and at least two electrode sheets 100 as described in any of the embodiments of the first aspect above. Referring to FIG9, the at least two electrode sheets 100 are a positive electrode sheet 100a and a negative electrode sheet 100b, respectively. The positive electrode sheet 100a, the separator 200, and the negative electrode sheet 100b are stacked or stacked and wound along the thickness direction (third direction Z). The separator 200 is disposed between the positive electrode sheet 100a and the negative electrode sheet 100b for insulating and separating the two.

[0085] In some embodiments, the positive electrode 100a includes a positive current collector 10a, a first positive active material layer 20a, a second positive active material layer 50a, and a positive electrode tab 30a. Along the thickness direction (third direction Z) of the positive current collector 10a, the positive current collector 10a has a first positive electrode surface 11a and a second positive electrode surface 12a. The first positive electrode surface 11a may be provided with the first positive active material layer 20a, and the second positive electrode surface 12a may be provided with the second positive active material layer 50a. The first positive electrode surface 11a has a first positive electrode empty foil region 111a, and the positive electrode tab 30a can be bonded to the first positive electrode empty foil region 111a through the aforementioned first adhesive layer 40.

[0086] The negative electrode sheet 100b includes a negative electrode current collector 10b, a first negative electrode active material layer 20b, a second negative electrode active material layer 50b, and a negative electrode tab 30b. The negative electrode current collector 10b has a first negative electrode surface 11b and a second negative electrode surface 12b. The first negative electrode active material layer 20b is disposed on the first negative electrode surface 11b, and the second negative electrode active material layer 50b is disposed on the second negative electrode surface 12b. The first positive electrode active material layer 20a is disposed facing the second negative electrode active material layer 50b. A first negative electrode empty foil region 111b is disposed on the second negative electrode surface 12b of the negative electrode current collector 10b. The negative electrode tab 30b can be bonded to the first negative electrode empty foil region 111b through a first adhesive layer 40.

[0087] Referring to Figure 9, since the positive electrode tab 30a is bonded to the first positive electrode empty foil region 111a through the first adhesive layer 40, no tab adhesive is needed at the positive electrode tab 30a and / or the first positive electrode empty foil region 111a. Furthermore, along the third direction Z, no electrode adhesive is needed at the position of the second positive electrode active material layer 50a corresponding to the first positive electrode empty foil region 111a. Similarly, no tab adhesive is needed at the negative electrode tab 30b and / or the first negative electrode empty foil region 111b. Along the third direction Z, no electrode adhesive is needed at the position of the first negative electrode active material layer 20b corresponding to the first negative electrode empty foil region 111b. Reducing the amount of tab adhesive and the two layers of electrode adhesive saves space and significantly improves the energy density of the secondary battery.

[0088] Optionally, along the third direction Z, an electrode adhesive 300 is provided at the position of the first positive electrode active material layer 20a corresponding to the position of the first negative electrode empty foil region 111b. This can alleviate lithium-ion insertion / extraction at this location, ensuring that the negative electrode 100b has sufficient margin to insert lithium ions extracted from the positive electrode 100a, thus mitigating lithium plating in the secondary battery. A receiving space can also be formed on the first positive electrode active material layer 20a, and the electrode adhesive 300 can be disposed within this space, reducing the impact of the electrode adhesive 300 on the electrode thickness and improving the energy density of the secondary battery.

[0089] Thirdly, this application also proposes a secondary battery including an electrode assembly 1000 as described in any embodiment of the second aspect above.

[0090] Fourthly, this application also proposes a method for preparing an electrode 100 as described in any embodiment of the first aspect above, comprising: providing a current collector 10 having a first surface 11, coating the first surface 11 with a first active material layer 20, and reserving a first empty foil area 111 on the first surface 11; providing a first adhesive layer 40, bonding the first adhesive layer 40 to the first empty foil area 111; providing an electrode tab 30 having a plurality of protrusions 312, the protrusions 312 of the electrode tab 30 facing the first adhesive layer 40, bonding the electrode tab 30 to the first adhesive layer 40, and pressing the electrode tab 30 to the first adhesive layer 40 such that the protrusions 312 are embedded in the first adhesive layer 40.

[0091] The tab 30 is directly bonded to the current collector 10 through the first adhesive layer 40, which is not only convenient to operate, but also eliminates welding burrs, reducing the risk of the separator 200 being punctured. It also eliminates the need to set tab adhesive on the tab 30 or the first empty foil area 111, saving the space occupied by the tab adhesive. This can improve the energy density of the secondary battery while reducing costs. In addition, the surface of the tab 30 facing the first empty foil area 111 has several protrusions 312. When the first adhesive layer 40 is bonded between the tab 30 and the first empty foil area 111, the protrusions 312 are at least partially embedded in the first adhesive layer 40. The protrusions 312 can increase the bonding area between the first adhesive layer 40 and the tab 30, making the first adhesive layer 40 and the tab 30 bond more firmly and improving the connection strength between the tab 30 and the current collector 10. Furthermore, the protrusions 312 make the current-carrying area of ​​the tab 30 larger, improving the current-carrying capacity of the tab 30 and making it easier to adapt to the high-rate charging and discharging of the secondary battery. At the same time, the protrusions 312 act like reinforcing ribs, which can improve the strength of the tab 30 and reduce the risk of the tab 30 tearing.

[0092] Experiment 1: [Drop Test of Lithium-ion Batteries]

[0093] Example 1: Preparation of Lithium-ion Batteries

[0094] <Preparation of the positive electrode>:

[0095] The positive electrode active material is lithium iron phosphate, the positive electrode conductive agent is acetylene black, and the positive electrode binder is polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10⁻⁶). 5 The materials were mixed at a mass ratio of 94:3:3, with N-methylpyrrolidone (NMP) added as a solvent. The mixture was stirred under vacuum until a homogeneous positive electrode slurry with a solid content of 75 wt% was obtained. The positive electrode slurry was then uniformly coated onto one surface of a 6 μm thick aluminum foil used as a positive electrode current collector, and dried at 90°C to obtain a positive electrode sheet with a single-sided coating of positive active material (80 μm thick). The above steps were then repeated on the other surface of the aluminum foil, with a first empty foil area of ​​10 mm width and 22 mm length pre-reserved on one surface, resulting in a positive electrode sheet with a double-sided coating of positive active material.

[0096] An epoxy resin first adhesive layer is bonded to the first empty foil area. The bonding width of the first adhesive layer in the first empty foil area is 8 mm, with a 1 mm gap reserved on each side from the positive electrode active material layer. The bonding length is 21 mm, with a 1 mm gap reserved in the length direction from the positive electrode active material layer. The thickness of the first adhesive layer is 20 μm.

[0097] An aluminum sheet is used as the positive electrode tab. A protrusion is formed on the positive electrode tab by mechanical pressing. These protrusions enclose a raised area with a width of 4 mm. The width of the positive electrode tab is 6 mm, and the length of the raised area on the first empty foil area is 18 mm, consistent with the length of the positive electrode tab on the first empty foil area. The length of the raised area is less than the length of the first adhesive layer, with a difference of 3 mm. The positive electrode tab and the current collector with the first adhesive layer are fixed by hot pressing. The protrusion of the positive electrode tab faces the first adhesive layer, and the height of the protrusion is 20 μm. During hot pressing, the protrusion is embedded in the first adhesive layer.

[0098] <Preparation of Negative Electrode Sheets>

[0099] Graphite powder (negative electrode active material), conductive carbon black (Super P) (conductive agent), and styrene-butadiene rubber (SBR) (binder) were mixed in a weight ratio of 97.5:1:1.5. Deionized water was then added as a solvent to prepare a slurry with a solid content of 50 wt%, and the mixture was stirred thoroughly. The slurry was then uniformly coated onto one surface of a 5 μm thick copper foil negative electrode current collector and dried at 110 °C to obtain a negative electrode active material with a weight of 9.1 mg / cm³. 2 Single-sided negative electrode sheet. After completing the above steps, the single-sided coating of the negative electrode sheet is complete. Then, repeat the above steps on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a double-sided coating of the negative electrode active material layer. A second empty foil area is reserved on the negative electrode sheet, with a width of 10 mm and a length of 22 mm. A first epoxy resin adhesive layer is bonded to the first empty foil area. The bonding width of the first adhesive layer in the first empty foil area is 8 mm, with a 1 mm gap between it and the negative electrode active material layer on both sides. The bonding length is 21 mm, with a 1 mm gap between it and the negative electrode active material layer along the length direction. The thickness of the first adhesive layer is 20 μm.

[0100] A nickel sheet is used as the negative electrode tab. A protrusion is formed on the negative electrode tab by mechanical pressing. These protrusions enclose a raised area with a width of 4 mm. The width of the negative electrode tab is 6 mm. The length of the raised area on the second empty foil area is 18 mm, consistent with the length of the negative electrode tab on the first empty foil area. The length of the raised area is less than the length of the first adhesive layer, with a difference of 3 mm. The negative electrode tab and the current collector with the first adhesive layer are fixed by hot pressing. The protrusion of the negative electrode tab faces the first adhesive layer, and the height of the protrusion is 20 μm. During hot pressing, the protrusion is embedded in the first adhesive layer.

[0101] <Preparation of the separating membrane>

[0102] A porous polyethylene (PE) film with a thickness of 8 μm was used as the separator.

[0103] <Electrolyte Preparation>

[0104] In a dry argon atmosphere, ethylene carbonate, methyl ethyl carbonate and diethyl carbonate are mixed in a mass ratio of 30:50:20 to obtain an organic solution. Then, lithium hexafluorophosphate is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0105] <Preparation of Lithium-ion Batteries>

[0106] The separator, positive electrode, separator, and negative electrode prepared above are stacked in sequence and wound to obtain an electrode assembly. The electrode assembly is then hot-pressed at a pressure of 5 MPa and a temperature of 65°C for 10 seconds. The electrode assembly is placed in an aluminum-plastic film packaging bag, with both the positive and negative electrode tabs extending from the top seal edge of the packaging bag. After removing moisture at 80°C, electrolyte is injected and the bag is sealed.

[0107] In Comparative Example 1, unlike Example 1, the positive electrode tab is welded to the positive current collector, and the negative electrode tab is welded to the negative current collector.

[0108] Drop test method: The lithium-ion batteries were pretreated at 25℃ and allowed to stand at room temperature for 60 minutes. The voltage of the lithium-ion batteries before the drop test was measured. The lithium-ion batteries were then placed in a fixture and dropped freely from a height of 1.5m using a drop device in the following sequence: head-tail-right head corner-right tail corner-left head corner-left tail corner (angle: 45±15°), repeated 6 times. After the drop test, the batteries were allowed to stand at room temperature for 24 hours, and the voltage of the lithium-ion batteries was measured and recorded. The appearance of the lithium-ion batteries was checked and photographed before and after the test. The criteria for passing the drop test were: no smoke, no leakage, and voltage drop <30mV. 100 lithium-ion batteries were tested, and the number of batteries that passed the test was X, with a pass rate of X / 100. The test results are shown in Table 1 below.

[0109] Table 1

[0110] According to Table 1 above, and in conjunction with Example 1 and Comparative Examples 1 and 2, it can be seen that when the tab and current collector are bonded using a first adhesive layer, and a protrusion is provided on the tab, the drop test pass rate is significantly better than that of Comparative Examples 1 and 2. Comparative Example 1 uses welding, and welding burrs easily puncture the separator. When the lithium-ion battery experiences drops or other impacts, these burrs are more likely to puncture the separator, leading to a short circuit between the positive and negative electrodes and causing the lithium-ion battery to fail. Example 1, however, uses a first adhesive layer, which eliminates welding burrs, effectively reducing the risk of the separator being punctured and thus improving the impact resistance of the lithium-ion battery. Comparative Example 2 uses bonding but does not have a protrusion, therefore its failure rate is relatively higher than the solution with a protrusion.

[0111] Meanwhile, welding is difficult to weld all the overlapping parts of the tab and the current collector, resulting in a small connection area. In contrast, bonding can bond all the overlapping areas of the tab and the current collector. After pressing, the first adhesive layer fills the gap between the tab and the current collector, and the protrusions further increase the connection area, thereby improving the connection strength. Higher connection strength means stronger resistance to deformation and tearing of the tab. After a battery collision, it can reduce the tilting of the tab, thereby reducing contact with the tab or electrode of the other pole and effectively reducing the risk of short circuit.

[0112] Experiment 2: Energy Density Test and Tensile Strength Test

[0113] Taking the lithium-ion batteries of Example 1 and Comparative Example 1 as examples, their energy density was tested. In Comparative Example 1, the positive electrode used two tab adhesives with a thickness of 8 μm and dimensions of 15*30 mm to cover the empty foil areas on both sides of the positive electrode tab, and a positive electrode adhesive sheet with a thickness of 8 μm and dimensions of 15 mm*30 mm to cover the position of the positive electrode active material layer corresponding to the negative electrode tab. The negative electrode used two tab adhesives with a thickness of 8 μm and dimensions of 14 mm*28 mm to cover the empty foil areas on both sides of the negative electrode tab, and a negative electrode adhesive sheet with a thickness of 8 μm and dimensions of 15 mm*30 mm to cover the position of the positive electrode active material layer corresponding to the negative electrode tab.

[0114] The relevant parameters in Examples 2 to 9 are shown in Table 2 below.

[0115] Energy density testing method: A square lithium-ion battery was prepared using the above method. The battery was charged to 4.2V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.2V, and discharged to 2.5V with a constant current of 0.2C. The discharge energy E was recorded. The external dimensions of the square battery were measured, and its volume, V, was calculated. The energy density W = E / V. The test results are shown in Table 2 below.

[0116] Tensile strength test method: Disassemble the battery to obtain the positive electrode and the negative electrode. Taking the positive electrode as an example, use a high-speed rail tensile testing machine to fix the tab to the lower end of the tensile testing machine and fix the current collector to the upper end of the high-speed rail tensile testing machine, keeping both ends on the same vertical plane. Set the speed of the tensile testing machine to 50mm / min, pull the tab and the current collector, and record the tensile force F (in N) when the tab and the current collector are pulled apart.

[0117] Table 2

[0118] Based on Table 2 above, and according to Examples 1 to 9 and Comparative Example 1, it can be seen that when the first adhesive layer is used to bond the tab and the current collector, and a protrusion is provided on the tab, its tensile strength is significantly higher than that of Comparative Example 1. This is because the protrusion can be embedded in the first adhesive layer to restrict the relative movement between the first adhesive layer and the tab, and increase the connection area between the first adhesive layer and the tab, thereby improving the tensile strength between the tab and the current collector.

[0119] Comparative Example 2 uses bonding but does not have protrusions, so its tensile strength is relatively lower than that of the scheme with protrusions.

[0120] In Examples 1 and 3 to 9, the tensile strength is superior to that in Example 2. In Examples 1 and 3 to 9, the width of the first adhesive layer is greater than 4 mm, which increases the connection area between the tab and the current collector, thus improving the connection strength of the tab. In Examples 1 and 3 to 8, the energy density is higher than that in Example 9. This may be because the width of the first adhesive layer is too large, which easily leads to the first adhesive layer directly adhering to the active material layer, affecting the battery capacity. Furthermore, in Examples 8 and 9, the width of the first adhesive layer does not significantly improve the tensile strength. This may be because part of the first adhesive layer adheres to the active material layer, resulting in poor adhesion in that part. Therefore, in this application, the width of the first adhesive layer is selected as 4 mm ≤ W2 ≤ 14 mm, that is, the adhesion width between the first adhesive layer and the first empty foil area is 4 mm ≤ W3 ≤ 14 mm.

[0121] Furthermore, in Examples 1 and 3 to 5, the energy density is not only higher than that of Examples 6 to 9, but the tensile strength is also higher than that of Example 2. In Examples 6 to 9, as the width of the first adhesive layer increases, the effect on improving tensile strength becomes less and less significant because the width of the first adhesive layer is greater than the width of the first empty foil area. Part of the first adhesive layer is directly bonded to the active material layer, and the portion bonded to the active material layer has a smaller effect on improving the bonding strength. In Examples 1 and 3 to 5, the width of the first adhesive layer is smaller than the width of the first empty foil area, which reduces direct bonding to the active material layer, and the sufficient width improves the bonding strength while having a smaller impact on energy density. Therefore, in this application, the width difference between the first empty foil area and the first adhesive layer is selected to be 1mm ≤ W1 - W2 ≤ 5mm. 。

[0122] Experiment 3: [Resistance Test of the Tabs]

[0123] The first adhesive layer is bonded between the tab and the current collector. Using conventional resistance testing methods, the voltage and current flowing through the current collector are measured using a voltmeter and an ammeter. The resistance between the tab and the current collector is calculated using the formula R = U / I.

[0124] Example 10 differs from Example 1 in that the projected length and width of the electrode in the first empty foil area are 18mm*5, and the projected area of ​​the electrode in the first empty foil area is S2 = 90mm². 2 The protrusions are trapezoidal in shape, with a height of 20μm. The length and width of the bottom of each protrusion are 0.5mm x 0.5mm, and the projected area of ​​a single protrusion on the first empty foil area is 0.25mm². 2 The number of protrusions is 36.

[0125] The relevant parameters in Examples 11 to 15 are shown in Table 3 below.

[0126] Table 3

[0127] According to Table 3 above, and in conjunction with Examples 10 to 15 and Comparative Example 1, it can be seen that when protrusions are provided on the tab, the resistance between the tab and the current collector can be effectively reduced. This is because the protrusions increase the current-carrying area of ​​the tab, resulting in lower resistance and stronger current-carrying capacity. In Examples 10 and 14, the resistance is lower than that in Example 15. In Example 15, the area occupied by all the protrusions is too large, making it difficult for the adhesive layer to be embedded between adjacent protrusions. This may result in the tab not being able to directly contact the current collector, and only conducting electricity through the first adhesive layer, leading to a larger resistance between the tab and the current collector. Therefore, in the embodiments of this application, 20% ≤ S1 / S2 ≤ 80% is selected to facilitate the embedding of each protrusion into the first adhesive layer, improve the bonding strength between the tab and the first adhesive layer, and at the same time reduce the resistance between the tab and the current collector.

[0128] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. An electrode sheet, comprising a current collector, a first active material layer, and a tab, wherein the current collector has a first surface, the first active material layer is disposed on the first surface, the first surface has a first empty foil region, and the tab is disposed on the first empty foil region, characterized in that, The electrode sheet further includes a first adhesive layer; The electrode has a first part and a second part connected to each other. Along the thickness direction of the electrode, the first adhesive layer is bonded between the first part and the first empty foil area. The first part is electrically connected to the first empty foil area. The second part is used for connecting external electronic devices. The first portion has a protruding area on the surface facing the first empty foil area. The protruding area has a plurality of protrusions. Along the thickness direction of the tab, the protrusions protrude relative to the second portion, and the protrusions are at least partially embedded in the first adhesive layer.

2. The electrode sheet according to claim 1, characterized in that, The current collector also has a second surface, and the first surface and the second surface are disposed opposite to each other along the thickness direction of the current collector; The electrode also has a second active material layer, which is disposed on the second surface. When viewed along the thickness direction of the current collector, the projection of the first empty foil area is located on the projection of the second active material layer.

3. The electrode sheet according to claim 1, characterized in that, Viewed along the thickness direction of the current collector, the contact area between all the protrusions and the first empty foil area is S1, and the projected area of ​​the electrode tab on the first empty foil area is S2, where 20% ≤ S1 / S2 ≤ 80%.

4. The electrode sheet according to claim 1, characterized in that, At least a portion of the protrusion contacts the first empty foil area.

5. The electrode sheet according to claim 4, characterized in that, The first adhesive layer includes at least one of epoxy resin, polyolefin, polystyrene, polymethyl methacrylate, phenolic resin or styrene-butadiene rubber.

6. The electrode sheet according to claim 4, characterized in that, The first adhesive layer includes at least one of thermosetting adhesive, pressure-sensitive adhesive, and hot melt adhesive.

7. The electrode sheet according to claim 1, characterized in that, Along the width direction of the current collector, the length of the first empty foil area is L1, the length of the first adhesive layer is L2, and 1mm ≤ L1 - L2 ≤ 5mm; and / or, Along the length direction of the current collector, the width of the first empty foil area is W1, the width of the first adhesive layer is W2, and 1mm≤W1-W2≤5mm.

8. The electrode sheet according to claim 1, characterized in that, Along the width direction of the current collector, the bonding length between the first adhesive layer and the first empty foil area is L3, 4mm≤L3≤24mm; Along the length direction of the current collector, the bonding width between the first adhesive layer and the first empty foil area is W3, where 4mm ≤ W3 ≤ 14mm.

9. The electrode sheet according to claim 1, characterized in that, Along the thickness direction of the current collector, the maximum thickness of the first adhesive layer is T1, where 2μm≤T1≤50μm.

10. The electrode sheet according to claim 9, characterized in that, 15μm≤T1≤25μm.

11. The electrode sheet according to claim 1, characterized in that, Along the width direction of the current collector, the length of the first adhesive layer is L2, the length of the protruding area is L4, and 0mm ≤ L2 - L4 ≤ 5mm; and / or, Along the length direction of the current collector, the width of the first adhesive layer is W2, the width of the protruding area is W4, and the width of the electrode tab located on the first empty foil area is W5, 0mm≤W2-W4≤5mm, 0≤W2-W5≤5mm.

12. The electrode sheet according to claim 1, characterized in that, Along the length direction of the current collector, the width of a single protrusion is W6, where 0.05mm ≤ W6 ≤ 1mm; Along the thickness direction of the current collector, the protrusion height of a single protrusion is H, where 5μm≤H≤60μm.

13. The electrode sheet according to claim 1, characterized in that, The protrusion includes a top near the current collector, and the width of the top along the width direction of the current collector is W7, where 0.05mm≤W7≤1mm.

14. An electrode assembly, characterized in that, It includes a separator and at least two electrodes as described in any one of claims 1 to 13, wherein the at least two electrodes are a positive electrode and a negative electrode, and the positive electrode and the separator are stacked and wound together with the negative electrode.

15. A secondary battery, characterized in that, Includes the electrode assembly as described in claim 14.

16. A method for preparing an electrode sheet as described in any one of claims 1 to 13, characterized in that, include: A current collector is provided, the current collector having a first surface, a first active material layer being coated on the first surface, and a first empty foil area being reserved on the first surface; A first adhesive layer is provided, and the first adhesive layer is bonded to the first empty foil area; A tab is provided, the tab having a plurality of protrusions facing the first adhesive layer, and the tab is bonded to the first adhesive layer, and the tab is pressed to the first adhesive layer such that the protrusions are embedded in the first adhesive layer.

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