Positive electrode sheet, preparation method therefor, secondary battery, and electronic apparatus

Abstract

The present application provides a positive electrode sheet, a preparation method therefor, a secondary battery, and an electronic apparatus. The positive electrode sheet comprises a positive electrode current collector, a positive electrode material layer, and an insulating layer, wherein the positive electrode sheet comprises an uncoated foil region and a coated region, the positive electrode material layer and the insulating layer are located within the coated region, the insulating layer and the positive electrode material layer are connected in a length direction of the positive electrode current collector, and the insulating layer is closer to the uncoated foil region than the positive electrode material layer; in a direction from the positive electrode material layer to the insulating layer, the positive electrode material layer comprises a first edge and a second edge opposite to each other, and the first edge is close to the uncoated foil region. When the positive electrode sheet undergoes a tensile strength test in the aforementioned direction, a fracture location of the positive electrode sheet is between the first edge and the second edge, and a distance between the fracture location and the first edge is x mm, wherein 3 < x ≤ 20. Using the positive electrode sheet provided in the present application in secondary batteries allows the secondary battery to have good safety performance.

Description

A positive electrode sheet and its preparation method, a secondary battery and an electronic device

[0001] This application claims priority to Chinese Patent Application No. 202510111774.X, filed on January 23, 2025, entitled "A Positive Electrode Sheet and its Preparation Method, a Secondary Battery and an Electronic Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of electrochemical technology, and in particular to a positive electrode sheet and its preparation method, a secondary battery, and an electronic device. Background Technology

[0003] With technological advancements, the performance of rechargeable batteries is continuously improving, and their application areas are constantly expanding. Multi-tab rechargeable batteries represent an effective technological improvement to meet the demands of fast charging; the multi-tab design significantly enhances the charging efficiency of rechargeable batteries.

[0004] In the manufacturing process of multi-tab secondary batteries, an insulating layer is typically coated at the edge of the positive electrode material layer to prevent short circuits caused by contact between the positive electrode tab and the negative electrode. However, during the cold pressing process, stress concentration occurs at the boundary between the area where the positive electrode material layer is located and the empty foil area in the positive electrode sheet. This causes wrinkling and deformation of the empty foil area near the boundary, reducing the tensile strength of the empty foil area and affecting the subsequent coating of the insulating layer, thus lowering the overall reliability of the positive electrode sheet. Therefore, during the use of multi-tab secondary batteries, the positive electrode sheet is prone to breakage at the edge of the positive electrode material layer, significantly affecting the safety performance of the secondary battery. Summary of the Invention

[0005] The purpose of this application is to provide a positive electrode sheet and its preparation method, a secondary battery, and an electronic device to improve the overall tensile strength of the positive electrode sheet of the secondary battery, thereby enhancing the safety performance of the secondary battery. The specific technical solution is as follows:

[0006] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0007] The first aspect of this application provides a positive electrode sheet, which includes a positive current collector, a positive electrode material layer, and an insulating layer. The positive electrode sheet includes an empty foil region and a coated region. The positive electrode material layer and the insulating layer are located in the coated region. The insulating layer and the positive electrode material layer are connected along the length direction of the positive current collector, and the insulating layer is closer to the empty foil region than the positive electrode material layer. Along the direction from the positive electrode material layer to the insulating layer, the positive electrode material layer includes a first edge and a second edge, with the first edge closer to the empty foil region. A tensile strength test is performed on the positive electrode sheet, and the fracture location of the positive electrode sheet is located between the first edge and the second edge, with a distance of x mm between the fracture location and the first edge, where 3 < x ≤ 20. In some embodiments of this application, 5 < x ≤ 20. This configuration not only improves the tensile strength near the positive electrode tab and reduces the risk of positive electrode tab breakage, but also enhances the safety performance of the secondary battery.

[0008] In some embodiments of this application, the overall tensile strength of the empty foil area and the area where the insulating layer is disposed in the positive electrode sheet is R1 MPa, and the tensile strength of the area where the positive electrode material layer is disposed in the positive electrode sheet is R2 MPa, where 220 ≤ 1.2R2 < R1 ≤ 280. By keeping R1 and R2 within the above ranges, the tensile strength of the positive electrode tab is improved, which helps to reduce the risk of positive electrode tab breakage, thereby improving the safety performance of the secondary battery.

[0009] In some embodiments of this application, the difference between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 1 μm. In some embodiments of this application, the difference between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 0.8 μm. These settings are beneficial for improving the energy density of the secondary battery. Simultaneously, the distance between the fracture location and the first edge is within the aforementioned range, which helps to increase the tensile strength near the positive electrode tab, reduce the risk of positive electrode tab breakage, and improve the safety performance of the secondary battery.

[0010] In some embodiments of this application, the roughness of the empty foil region is Rzμm, where 2≤Rz≤5. By adjusting Rz within the above range, it is not only beneficial to reduce the risk of positive electrode tab breakage, but the positive electrode current collector with the above roughness also facilitates the subsequent insulation layer to adhere firmly to the positive electrode current collector, making it less prone to falling off, thereby improving the safety performance of the secondary battery.

[0011] In some embodiments of this application, the insulating layer includes an adhesive and a ceramic material, and the insulating layer satisfies at least one of the following characteristics: (1) the mass percentage W1 of the adhesive, based on the mass of the insulating layer, is 5% to 20%; (2) the mass percentage W2 of the ceramic material, based on the mass of the insulating layer, is 80% to 95%. By adjusting the mass percentage of the adhesive and ceramic material in the insulating layer to satisfy at least one of the above, the insulation performance of the positive electrode tab can be improved, thereby improving the safety performance of the secondary battery.

[0012] In some embodiments of this application, the insulating layer includes an adhesive and a ceramic material, and the insulating layer satisfies at least one of the following characteristics: (1) the mass percentage of the adhesive, W1, is 70% to 90% based on the mass of the insulating layer; (2) the mass percentage of the ceramic material, W2, is 10% to 30% based on the mass of the insulating layer. By adjusting the mass percentage of the adhesive and ceramic material in the insulating layer to satisfy at least one of the above, it is beneficial to promote the insulating layer to have both good flexibility and insulation performance, which is beneficial to improve the pass rate of the secondary battery in drop tests and improve the safety performance of the secondary battery.

[0013] In some embodiments of this application, the insulating layer satisfies at least one of the following characteristics: (1) the binder includes at least one of polyurethane, polyacrylic acid, or polyacrylonitrile; (2) the ceramic material includes at least one of boehmite or alumina. By selecting the above-mentioned binder and / or ceramic material, the insulating layer has good insulation properties and is not easily detached during the operation of the secondary battery, which is beneficial to improving the cycle stability and safety performance of the secondary battery.

[0014] In some embodiments of this application, the maximum thickness of the insulating layer is d μm and the width is b mm, where 10 ≤ d ≤ 40 and 1.5 ≤ b ≤ 3.5. In some embodiments of this application, 10 ≤ d ≤ 25; and / or 2.0 ≤ b ≤ 3.0. By adjusting d and b within the above ranges, the energy density and safety performance of the secondary battery can be balanced, enabling the secondary battery to have higher energy density while maintaining high safety performance.

[0015] In some embodiments of this application, the edge of the insulating layer near the positive electrode material layer is designated as the third edge, which coincides with the first edge; alternatively, the third edge is located between the first and second edges, and the distance between the third edge and the first edge is s mm, where 0 ≤ s ≤ 0.5. In some embodiments of this application, 0 ≤ s ≤ 0.3. By adjusting the distance s between the third edge and the first edge within the scope of this application, it is beneficial to promote higher safety performance and higher energy density in the secondary battery.

[0016] In some embodiments of this application, the positive electrode material layer and the insulating layer are disposed on two surfaces of the coating area. Along the direction from the positive electrode material layer to the insulating layer, the distance between the third edges of the insulating layers on both sides of the positive electrode current collector is Δs mm, where 0 ≤ Δs ≤ 0.3. In some embodiments of this application, 0 ≤ Δs ≤ 0.1. By adjusting the distance Δs between the third edges of the insulating layers on both sides of the positive electrode current collector within the above range, it is beneficial to improve the tensile strength near the positive electrode tab, reduce the risk of positive electrode tab breakage, and increase the proportion of the positive electrode material layer by improving the setting accuracy of the insulating layer, thereby improving the energy density of the secondary battery.

[0017] In some embodiments of this application, the positive electrode material layer and the insulating layer are disposed on two surfaces of the coating area, and the maximum thickness difference of the insulating layer on both sides of the positive electrode current collector is Δd μm, where 0 ≤ Δd ≤ 15. In some embodiments of this application, 0 ≤ Δd ≤ 10. By adjusting the value of Δd within the above range, it is beneficial to improve the insulation performance near the positive electrode tab, reduce the risk of breakage of the positive electrode tab, and improve the safety performance of the secondary battery.

[0018] In some embodiments of this application, the positive electrode material layer and the insulating layer are disposed on two surfaces of the coating area, and the difference in width between the insulating layers on both sides of the positive electrode current collector is Δb mm, where 0 ≤ Δb ≤ 0.3. By adjusting Δb within the above range, it is not only beneficial to improve the tensile strength of the positive electrode tab and reduce the risk of positive electrode tab breakage, but also to increase the proportion of the positive electrode material layer by improving precision, thereby increasing the energy density.

[0019] In some embodiments of this application, the thickness of the positive current collector is h μm, where 8 ≤ h ≤ 10. By selecting a positive current collector within the above range, the strength of the positive electrode sheet during cold pressing and processing can be guaranteed, reducing the risk of strip breakage and subsequent tab breakage during electrode processing. At the same time, the use of a relatively thin positive current collector is beneficial to improving energy density, thus achieving the goal of balancing processing stability, safety, and energy density.

[0020] The second aspect of this application provides a method for preparing a positive electrode sheet according to any of the foregoing embodiments, comprising the following steps: obtaining a positive electrode material layer slurry and an insulating layer slurry; placing adhesive tape in a first region and a second region in a positive electrode current collector, then placing the positive electrode material layer slurry in a third region of the positive electrode current collector such that at least a portion of the surface of the adhesive tape is covered by the positive electrode material layer slurry, then drying and cold pressing, then peeling off the adhesive tape, placing the insulating layer slurry in the second region, drying, and obtaining a positive electrode sheet; wherein, the first region is an empty foil region, the second region is used to place the insulating layer, the third region is used to place the positive electrode material layer, and the first region, the second region, and the third region are connected sequentially.

[0021] A third aspect of this application provides a secondary battery comprising a positive electrode sheet as described in any of the foregoing embodiments or a positive electrode sheet obtained according to the foregoing preparation method.

[0022] A fourth aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments.

[0023] The beneficial effects of this application are:

[0024] This application provides a positive electrode sheet, its preparation method, a secondary battery, and an electronic device. The positive electrode sheet includes a positive current collector, a positive electrode material layer, and an insulating layer. The positive electrode sheet includes an empty foil region and a coated region. The positive electrode material layer and the insulating layer are located in the coated region, and the insulating layer and the positive electrode material layer are sequentially connected. The insulating layer is closer to the empty foil region than the positive electrode material layer. Along the direction from the positive electrode material layer to the insulating layer, the positive electrode material layer includes opposing first and second edges, with the first edge closer to the empty foil region. A tensile strength test is performed using a 5mm × 30mm positive electrode sheet. The fracture location of the positive electrode sheet is located between the first and second edges, and the distance between the fracture location and the first edge is x mm, where 20 ≥ x > 3. By controlling x within the above range, the area near the first edge of the positive electrode material layer has higher strength, which helps reduce the possibility of fracture near the first edge of the positive electrode material layer, thereby reducing the risk of positive electrode tab breakage, improving the safety performance of the secondary battery, and balancing energy density.

[0025] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0026] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0027] Figure 1 is a schematic cross-sectional view of the positive electrode sheet along its thickness direction in one embodiment of this application;

[0028] Figure 2 is a schematic cross-sectional view of the positive electrode sheet along its thickness direction in another embodiment of this application;

[0029] Figure 3 is a schematic cross-sectional view of the positive electrode sheet along its thickness direction in another embodiment of this application;

[0030] Figure 4 is a schematic cross-sectional view of the positive electrode sheet along the thickness direction in another embodiment of this application;

[0031] Figure 5 is a schematic diagram of a lithium-ion battery structure according to one embodiment of this application;

[0032] Figure 6a is a photograph of the positive electrode sample in Comparative Example 1 after the tensile strength test;

[0033] Figure 6b is a photograph of the test samples of the positive electrode sheet in Examples 1-5 after tensile strength testing;

[0034] Figure 7a shows the tensile test curve of the positive electrode sample in Comparative Example 1.

[0035] Figure 7b shows the tensile test curves of the positive electrode samples in Examples 1-5;

[0036] Figure 8a is an electron microscope image of the sample obtained by cold pressing in Comparative Example 1, where only a layer of positive electrode material is set on the surface of the positive electrode current collector.

[0037] Figure 8b is an electron microscope image of the samples obtained in Examples 1-5, where only a positive electrode material layer was deposited on the surface of the positive electrode current collector, after cold pressing and removal of the adhesive paper. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0039] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0040] Currently, to meet the demand for fast charging, secondary batteries typically employ a multi-tab design. The tabs are usually the extension of the current collector, acting as conductors. Each tab is the current output point of its corresponding positive or negative electrode. By stacking multiple positive tabs and then welding them together, the positive tabs of the secondary battery are obtained, and by stacking multiple negative tabs and then welding them together, the negative tabs of the secondary battery are obtained, achieving a parallel connection. This allows the secondary battery to be charged and discharged at high rates, such as 5C, 10C, or even higher, to meet the demand for fast charging.

[0041] The positive electrode sheet is typically prepared using a "zebra coating" process. Due to surface tension in the positive electrode material slurry during coating, the thickness of the edge region of the resulting positive electrode material layer is less than that of the central region, resulting in a "thinning zone" at the edge. During the preparation of the positive electrode sheet for secondary batteries, the positive electrode material layer needs to be cold-pressed. During this process, stress concentration usually occurs at the boundary between the positive electrode material layer and the empty foil area, causing the empty foil area near the boundary to easily wrinkle and deform. This boundary becomes a "weak area" of the positive electrode sheet, prone to breakage. Since the positive electrode tab is usually located at the edge of the positive electrode sheet, near the aforementioned boundary, this increases the risk of tab breakage, thus affecting the safety performance of the lithium-ion battery. Furthermore, for multi-tab secondary batteries, an insulating layer is typically placed at the edge of the positive electrode material layer to reduce the risk of short circuits when the portion of the positive current collector acting as the positive electrode tab comes into contact with the negative electrode, thus improving the safety performance of the secondary battery. However, during cold pressing, stress concentration at the edge of the positive electrode material layer can easily cause wrinkling and deformation of the positive current collector, which can also affect the placement of the insulating layer. Especially for positive electrode sheets without a "thinning zone," they often face greater stress concentration during cold pressing, thereby affecting the safety performance of the secondary battery.

[0042] The solution presented in this application can be used in the design of positive electrode material layers with or without thinning regions, reducing the risk of positive electrode tab breakage and thus improving the safety performance of the secondary battery. Based on the above problems, this application provides a secondary battery and electronic device that improves the problem of wrinkling and deformation easily occurring in the empty foil area near the interface, reducing the risk of positive electrode tab breakage and thus improving the safety performance of the secondary battery.

[0043] The first aspect of this application provides a positive electrode sheet, as shown in FIG1. ​​The positive electrode sheet 01 includes a positive current collector 10, a positive electrode material layer 20, and an insulating layer 30. The positive electrode sheet 01 includes an empty foil region 012 and a coated region 011. The positive electrode material layer 20 and the insulating layer 30 are located in the coated region 011. Along the length direction of the positive current collector 10, the insulating layer 30 and the positive electrode material layer 20 are connected and disposed. The "length direction" refers to the direction indicated by the rightmost arrow in FIG5 of this application, or the direction from the positive electrode material layer 20 to the insulating layer 30 in FIG1. ​​The insulating layer 30 is closer to the empty foil region 012 than the positive electrode material layer 20. Along the direction from the positive electrode material layer 20 to the insulating layer 30, that is, the direction indicated by the arrow in FIG1, the positive electrode material layer 20 includes a first edge 21 and a second edge 22 opposite to each other. The first edge 21 is closer to the empty foil region 012.

[0044] Tensile strength tests were conducted using a 5mm × 30mm positive electrode sheet. The fracture location of the positive electrode sheet was between the first and second edges, and the distance between the fracture location and the first edge was x mm, where 3 < x ≤ 20. In some embodiments of this application, 5 < x ≤ 20. For example, x can be 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, 9, 10, 12, 15, 17, 19, 20, or any two values ​​in between. When x ≤ 3, it indicates that the positive electrode sheet tends to fracture near the first edge during the tensile strength test. This is usually because stress is concentrated near the first edge during the cold pressing process, causing wrinkling and deformation of the empty foil area near the first edge, reducing the tensile strength of the empty foil area. Therefore, during the use of the secondary battery, the tabs at the edge of the positive electrode sheet are prone to fracture, reducing the safety performance of the secondary battery. In this application, 20≥x>3 indicates that during the tensile strength test, the fracture location of the positive electrode sheet is located between the first and second edges of the positive electrode material layer, and the distance between the fracture location and the first edge is 20≥x>3. Since 20≥x>3 is within this range, it means that the area near the first edge of the positive electrode material layer is no longer a "weak zone," thereby reducing the risk of positive electrode tab breakage and improving the safety performance of the secondary battery.

[0045] Therefore, the problem of wrinkling and deformation near the first edge of the positive electrode sheet in the secondary battery provided by this application is improved, which can reduce the risk of positive electrode tab breakage and thus improve the safety performance of the secondary battery.

[0046] In this application, a 5mm × 30mm positive electrode sample is used for tensile strength testing, where the size of the region corresponding to the positive electrode material layer is 5mm × 25mm. During testing, tension is applied along the direction from the positive electrode material layer to the insulating layer. Since the size of secondary batteries varies in actual applications, the size of the positive electrode sample used for tensile strength testing can be determined according to the actual size of the secondary battery. Its fracture location and tensile strength will also change accordingly with the change in the size of the region corresponding to the positive electrode material layer in the positive electrode sample. For example, if the size of the region corresponding to the positive electrode material layer in the positive electrode sample along the direction from the positive electrode material layer to the insulating layer is 'a', then 3 < x ≤ a - 5.

[0047] In some embodiments of this application, the difference ΔT between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 1 μm. In some embodiments of this application, the difference ΔT between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 0.8 μm. For example, the difference between the maximum and minimum thickness can be 0.2 μm, 0.3 μm, 0.5 μm, 0.6 μm, 0.8 μm, or 1 μm. When the difference between the maximum and minimum thickness of the positive electrode material layer 20 is within the range of this application, it indicates that there is no "thinning zone" at the edge of the positive electrode material layer, and the resulting secondary battery has a high energy density. At the same time, the distance between the fracture location and the first edge is within the above range, which is also beneficial to improving the cycle stability and safety performance of the secondary battery. Typically, the outer contour of the cross-section of the positive electrode material layer without a "thinning zone" along its own thickness direction is close to a rectangle. The above-mentioned difference between the maximum and minimum thickness refers to the difference between the maximum and minimum thickness of the positive electrode material in the region extending from the first edge along the direction from the insulating layer to the positive electrode material layer within the above direction.

[0048] The second aspect of this application provides a method for preparing a secondary battery according to any of the foregoing embodiments, comprising the following steps: obtaining a positive electrode material layer slurry and an insulating layer slurry; placing adhesive tape in a first region and a second region in a positive electrode current collector, then placing the positive electrode material layer slurry in a third region of the positive electrode current collector such that at least a portion of the surface of the adhesive tape is covered by the positive electrode material layer slurry, then drying and cold pressing, then peeling off the adhesive tape, placing the insulating layer slurry in the second region, drying, and obtaining a positive electrode sheet; wherein, the first region is an empty foil region, the second region is used to place the insulating layer, the third region is used to place the positive electrode material layer, and the first region, the second region, and the third region are connected sequentially.

[0049] Specifically, the positive current collector includes the first region, the second region, and the third region, wherein the first region corresponds to the empty foil region, and the second and third regions correspond to the coating region.

[0050] In this application, when setting the positive electrode material layer slurry, the size of the adhesive tape covering the slurry can be changed by altering the starting position of the slurry. Generally, the larger the size of the adhesive tape covering the positive electrode material layer slurry, the smaller the ΔT of the positive electrode material layer obtained after peeling off the tape and cold pressing, and vice versa. The aforementioned "size of the adhesive tape covering the positive electrode material layer slurry" refers to the size of the adhesive tape covering the positive electrode material layer slurry along the direction from the insulating layer to the positive electrode material layer.

[0051] This application does not impose any particular limitation on the cold pressing pressure, as long as the purpose of this application can be achieved. For example, the cold pressing pressure can be from 30t to 120t. In some embodiments of this application, the adhesive strength of the adhesive tape is from 0.5N / m to 5N / m. This application does not impose any particular limitation on the thickness of the adhesive tape, as long as the purpose of this application can be achieved. For example, the thickness of the adhesive tape is from 5μm to 20μm.

[0052] Thanks to the aforementioned method for preparing the positive electrode sheet, before applying the positive electrode material layer slurry to the positive electrode current collector and performing cold pressing, an additional step is added: applying adhesive paper to the areas corresponding to the empty foil area and the insulating layer in the positive electrode current collector. Specifically, adhesive paper is applied to the first and second areas, ensuring that at least a portion of the adhesive paper's surface is covered by the positive electrode material layer slurry. The resulting positive electrode sheet is then dried. During cold pressing, the area near the first edge is covered by the adhesive paper and the positive electrode material layer, forming a unified structure. Cold pressing is then performed, and the adhesive paper and the positive electrode material layer on its surface are removed. This achieves the goal of protecting the empty foil area and the area corresponding to the insulating layer near the first edge of the positive electrode material layer during cold pressing. Compared to the current method of directly cold pressing after applying the positive electrode material layer or simultaneously applying both the positive electrode material layer and the insulating layer, this method improves the stress concentration problem at the interface between the coated area and the empty foil area, i.e., the stress concentration problem near the first edge. This alleviates the wrinkling phenomenon near the first edge after cold pressing, increases the tensile strength of the first edge, reduces the risk of positive electrode tab breakage, and ultimately improves the safety performance of the secondary battery.

[0053] In some embodiments of this application, the overall tensile strength of the empty foil area and the area where the insulating layer is disposed in the positive electrode sheet is R1 MPa, and the tensile strength of the coated area is R2 MPa, where 220 ≤ 1.2R2 < R1 ≤ 280. In some embodiments of this application, 220 ≤ R1 ≤ 280; for example, R1 can be 220, 225, 230, 235, 240, 245, 250, 255, 260, 270, 280, or a range of any two of these values. In some embodiments of this application, 190 ≤ R2 ≤ 230; R2 can be 190, 192, 195, 200, 205, 207, 210, 212, 215, 220, 225, 228, 230, or a range of any two of these values. If R1 and R2 are within the aforementioned range, it indicates that both the empty foil region and the coated region have good tensile strength. Simultaneously, 1.2R2 < R1 indicates that the area near the first edge of the positive electrode material layer is no longer a "weak area" of the positive electrode sheet. The increased tensile strength near the first edge helps reduce the risk of positive electrode tab breakage and improves the safety performance of the secondary battery. In this application, the tensile strength of the coated region refers to the tensile strength of the coated region corresponding to the positive electrode material layer.

[0054] In this application, adhesive tape is first applied to the first and second regions of the positive electrode current collector. Then, the adhesive tape and the positive electrode material layer on its surface are removed. After the adhesive tape is removed, the roughness of the areas corresponding to the empty foil region and the insulating layer of the positive electrode current collector in the first and second regions increases compared to the surface roughness of the positive electrode current collector without adhesive tape. Furthermore, the magnitude of the cold pressing pressure during the preparation of the positive electrode sheet also affects the roughness of the empty foil region. Specifically, in some embodiments of this application, the roughness of the empty foil region of the positive electrode sheet is Rz μm, where 2 ≤ Rz ≤ 5. For example, Rz can be 2, 2.5, 3, 3.5, 4, 4.5, 5, or a range consisting of any two values ​​in between. Within the aforementioned range, Rz can improve the tensile strength and flatness of the empty foil area, thereby enhancing the tensile strength near the positive electrode tab and reducing the risk of positive electrode tab breakage. Furthermore, the resulting rougher empty foil area is also beneficial for the adhesion of the subsequent insulating layer coating, improving the bonding force between the insulating layer and the positive electrode current collector, reducing the risk of insulating layer detachment, and thus improving the safety performance of the secondary battery.

[0055] In some embodiments of this application, the insulating layer includes an insulating layer binder and a ceramic material. Based on the mass of the insulating layer, the mass percentage W1 of the insulating layer binder is 5% to 20%, for example, it can be 5%, 7%, 10%, 12%, 15%, 18%, 20%, or any two values ​​within this range. By adjusting the mass percentage of the insulating layer binder within the above range, the adhesion between the positive electrode current collector and the insulating layer can be improved, thereby reducing the occurrence of short circuits between the positive electrode tab and the negative electrode sheet, and thus improving the safety performance of the secondary battery.

[0056] In some embodiments of this application, the insulating layer includes an insulating layer binder and a ceramic material. Based on the mass of the insulating layer, the mass percentage W2 of the ceramic material is 80% to 95%, for example, it can be 80%, 82%, 85%, 88%, 90%, 93%, 95%, or a range of any two values ​​in between. By adjusting the mass percentage of the ceramic material in the insulating layer within the above range, the insulation performance between the positive electrode tab and the negative electrode sheet can be improved, thereby improving the safety performance of the secondary battery.

[0057] In some embodiments of this application, the insulating layer includes an insulating layer binder and a ceramic material. Based on the mass of the insulating layer, the mass percentage W1 of the insulating layer binder is 5% to 20%, and the mass percentage W2 of the ceramic material is 80% to 95%. By adjusting the mass percentages of the binder and ceramic material in the insulating layer to the above ranges, the insulating layer has good insulation performance and good adhesion to the positive electrode current collector, which helps to reduce the risk of short circuit between the positive electrode tab and the negative electrode sheet, and improves the safety performance of the secondary battery.

[0058] In some embodiments of this application, the adhesive for the insulating layer includes at least one of polyurethane, polyacrylic acid, or polyacrylonitrile. This adhesive not only allows the insulating layer to adhere better to the surface of the positive electrode current collector, but also possesses good chemical stability and heat resistance, making it better suited to the operating environment of the positive electrode in the secondary battery. Therefore, by selecting the aforementioned adhesive, the resulting insulating layer exhibits good adhesion to the positive electrode current collector, preventing it from easily detaching during secondary battery operation and improving the cycle stability and safety performance of the secondary battery.

[0059] In some embodiments of this application, the ceramic material includes at least one of boehmite or alumina. This ceramic material not only exhibits good insulation properties at room temperature but also demonstrates good insulation properties at high temperatures. Therefore, even in the event of thermal runaway in the secondary battery, it helps maintain good insulation between the positive electrode tab and the negative electrode sheet, thereby improving the safety performance of the secondary battery.

[0060] In some embodiments of this application, the binder of the insulating layer includes at least one of polyurethane, polyacrylic acid, or polyacrylonitrile, and the ceramic material includes at least one of boehmite or alumina. When the above-mentioned binder is used in conjunction with the ceramic material, it is beneficial to further improve the insulation performance of the insulating layer, better reduce the risk of short circuits caused by contact between the positive electrode tab and the negative electrode, and thus improve the safety performance of the secondary battery.

[0061] In some embodiments of this application, the mass percentage W1 of the insulating layer binder is 70% to 90% based on the quality of the insulating layer. For example, it can be 70%, 72%, 75%, 78%, 80%, 82%, 85%, 87%, 90%, or any combination of two values ​​within this range. A mass percentage of the insulating layer binder within the above range indicates that the insulating layer possesses both good flexibility and adhesion. During manufacturing, it can appropriately disperse the stress between the insulating layer and the positive electrode material layer, and can accommodate a certain degree of minor deformation of the positive electrode sheet during use, thereby improving the safety performance of the secondary battery. Furthermore, when the insulating layer binder includes at least one of the above-mentioned insulating layer binders, the insulating layer can possess better flexibility, adhesion, chemical stability, and heat resistance, and can better adapt to the secondary battery processing and operating environment, thereby improving the yield and safety performance of the secondary battery.

[0062] In some embodiments of this application, the mass percentage W2 of the ceramic material, based on the mass of the insulating layer, is 10% to 30%. For example, W2 can be 10%, 13%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or a range of any two values ​​therein. When the mass percentage of the ceramic material in the insulating layer is within the above range, the insulation resistance of the insulating layer can be controlled within the required range, improving the reliability of the insulating layer and thus enhancing the safety performance of the secondary battery. Furthermore, if the ceramic material includes at least one of boehmite or alumina, the insulating layer will have better insulation effect and heat resistance, which is beneficial for further improving the safety performance of the secondary battery.

[0063] In some embodiments of this application, based on the quality of the insulating layer, the mass percentage W1 of the insulating layer binder is 70% to 90%, and the mass percentage W2 of the ceramic material is 10% to 30%. By adjusting the mass percentages of the binder and ceramic material in the insulating layer within the above ranges, it is beneficial to promote the insulating layer to have both good flexibility and insulation performance. This allows the positive electrode tab and the negative electrode sheet to maintain good insulation performance while exhibiting better flexibility to adapt to the deformation of the positive electrode sheet edge. This is beneficial to improving the pass rate of the secondary battery in drop tests and enhancing the safety performance of the secondary battery.

[0064] In some embodiments of this application, as shown in Figure 1, the edge of the insulating layer 30 near the positive electrode material layer 20 is a third edge 31, which coincides with the first edge 21. In some embodiments of this application, as shown in Figure 2, the edge of the insulating layer 30 near the positive electrode material layer 20 is a third edge 31, which is located between the first edge 21 and the second edge 22. The distance between the third edge 31 and the first edge 21 is s mm, where 0 ≤ s ≤ 0.5. In some embodiments of this application, 0 ≤ s ≤ 0.3. For example, the value of s can be 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range of any two values ​​in between. Wherein, the third edge coincides with the first edge, i.e., s = 0; the third edge is located between the first edge and the second edge, i.e., 0 < s ≤ 0.5. In this application, when setting the insulating layer, the insulating layer slurry is first placed on the positive electrode current collector foil at a certain distance from the first edge of the positive electrode material layer. Because the insulating layer slurry is flowable and has surface tension, it spreads and levels towards the positive electrode material layer until the third edge of the insulating layer coincides with the first edge of the positive electrode material layer, i.e., s = 0. Alternatively, the insulating layer slurry spreads along the first edge of the positive electrode material layer to the surface of the positive electrode material layer under the influence of suction, and then penetrates into the interior of the positive electrode material layer under the influence of gravity, forming a new composite material layer. This composite material layer is part of the insulating layer, and thus the third edge of the insulating layer is located between the first and second edges, i.e., 0 < s ≤ 0.5. The surface of the composite material layer will exhibit a color close to that of the insulating layer and can be clearly observed and tested using a scanning electron microscope (SEM). Therefore, by controlling the distance between the initial position of the insulating layer slurry and the first edge while keeping the amount of insulating layer constant, s can be controlled. By adjusting the distance s between the third edge and the first edge within the scope of this application, the risk of lithium plating in the secondary battery can be reduced, thereby improving the safety performance of the secondary battery, while also taking into account the energy density of the secondary battery.

[0065] In some embodiments of this application, as shown in FIG3, the positive electrode sheet 01 includes a positive current collector 10 and a positive electrode material layer 20 and an insulating layer 30 respectively disposed on two surfaces of the positive current collector 10. The positive electrode material layer 20 includes a first positive electrode material layer 201 disposed on one surface of the positive current collector and a second positive electrode material layer 202 disposed on the other surface of the positive current collector. The insulating layer 30 includes a first insulating layer 301 disposed on one surface of the positive current collector and a second insulating layer 302 disposed on the other surface of the positive current collector. The first positive electrode material layer 201 and the first insulating layer 301 are located on the same side of the positive current collector 10, and the second positive electrode material layer 202 and the second insulating layer 302 are located on the same side of the positive current collector 10. Along the direction from the first positive electrode material layer 201 to the first insulating layer 301, i.e., the direction indicated by the arrow in Figure 3, the distance between the third edge 31 of the first insulating layer 301 and the first edge 21 of the first positive electrode material layer 201 is s1, and the distance between the third edge 31 of the second insulating layer 302 and the first edge 21 of the second positive electrode material layer 202 is s2. The difference between the distances s1 and s2 is Δs mm, where 0 ≤ Δs ≤ 0.3. In some embodiments of this application, 0 ≤ Δs ≤ 0.1. For example, Δs can be 0, 0.02, 0.05, 0.07, 0.1, 0.12, 0.15, 0.18, 0.2, 0.23, 0.25, 0.28, 0.3, or a range consisting of any two values ​​in between. By adjusting the distance Δs between the third edges of the insulating layers on both sides of the positive electrode current collector within the aforementioned range, stress concentration near the first edge of the positive electrode material layer is alleviated, and the tensile strength near the first edge of the positive electrode material layer is improved, thereby reducing the risk of positive electrode tab breakage. Simultaneously, lithium plating and black spots are less likely to occur during secondary battery cycling, thus improving the cycle stability and safety performance of the secondary battery. Furthermore, it also helps to maintain the energy density of the secondary battery.

[0066] In some embodiments of this application, the maximum thickness of the insulating layer is d μm, where 10 ≤ d ≤ 40. In other embodiments, 10 ≤ d ≤ 25. For example, it can be 10, 15, 18, 20, 22, 25, 27, 30, 33, 35, 38, 40, or any two values ​​in between. As shown in Figures 1 and 2, due to the surface tension of the insulating layer slurry during the coating process, the thickness of the resulting insulating layer 30 is not uniform. Therefore, the outer contour of the insulating layer 30 is usually irregular. Along the direction from the positive electrode material layer 20 to the insulating layer 30, i.e., the direction indicated by the arrow in Figure 1, the area from a distance of 21a μm from the first edge to the interface between the insulating layer 30 and the surface of the positive electrode current collector 10 is used as the region for evaluating the thickness parameter of the insulating layer 30. Here, a = 0.2, and the maximum thickness of the insulating layer 30 in this region is d. The maximum thickness d of the insulating layer is controlled by adjusting the coating amount of the insulating layer slurry. By controlling the maximum thickness d of the insulating layer within the above range, it is beneficial to reduce the risk of short circuit between the positive electrode tab and the negative electrode sheet, and to reduce the occurrence of positive electrode current collector breakage during processing, thereby improving the safety performance and production process efficiency of the secondary battery.

[0067] In some embodiments of this application, as shown in Figures 1 and 2, the distance between the third edge 31 and the interface between the insulating layer 30 and the surface of the positive electrode current collector 10 along the direction from the positive electrode material layer 20 to the insulating layer 30 is the width b of the insulating layer 30. In some embodiments of this application, the unit of width b is μm, and 1.5 ≤ b ≤ 3.5. In some embodiments of this application, 2.0 ≤ b ≤ 3.0. For example, 1.5, 1.7, 2, 2.3, 2.5, 2.8, 3, 3.2, 3.5, or any two values ​​in between. Along the length of the positive electrode current collector, when the sum of the dimensions of the positive electrode material layer and the insulating layer is constant, the size of b will affect the size of the positive electrode material layer, thereby affecting the energy density of the secondary battery, and also affecting the number of times the secondary battery passes the drop test. Therefore, by controlling the maximum thickness d and the width b of the insulating layer within the above ranges, the energy density and safety performance of the secondary battery can be balanced, so that the secondary battery has both high safety performance and high energy density.

[0068] In some embodiments of this application, as shown in Figures 3 and 4, the positive electrode sheet 01 includes a positive current collector 10 and a positive electrode material layer 20 and an insulating layer 30 respectively disposed on two surfaces of the positive current collector 10. The positive electrode material layer 20 includes a first positive electrode material layer 201 disposed on one surface of the positive current collector and a second positive electrode material layer 202 disposed on the other surface of the positive current collector. The insulating layer 30 includes a first insulating layer 301 disposed on one surface of the positive current collector and a second insulating layer 302 disposed on the other surface of the positive current collector. The first positive electrode material layer 201 and the first insulating layer 301 are located on the same side of the positive current collector 10, and the second positive electrode material layer 202 and the second insulating layer 302 are located on the same side of the positive current collector 10. The maximum thickness of the first insulating layer 301 is d1, and the maximum thickness of the second insulating layer 302 is d2. The absolute value of the difference between d1 and d2 is Δd, with units of μm, and 0 ≤ Δd ≤ 15. In some embodiments of this application, 0 ≤ Δd ≤ 10. In some embodiments of this application, 0 ≤ Δd ≤ 5. For example, Δd can be 0, 2.5, 5, 7.5, 10, 12.5, 15, or any two values ​​in between. By adjusting the value of Δd within the range of this application, the uniformity of the insulating layer coating on both sides of the positive current collector is better. This not only helps to improve the insulation performance between the positive electrode tab and the negative electrode sheet, but also helps to improve the yield rate of the secondary battery manufacturing process, improve the tensile strength near the positive electrode tab, reduce the risk of breakage of the positive electrode tab, and thus improve the safety performance of the secondary battery.

[0069] In some embodiments of this application, as shown in Figures 3 and 4, the positive electrode sheet 01 includes a positive current collector 10 and a positive electrode material layer 20 and an insulating layer 30 respectively disposed on two surfaces of the positive current collector 10. The insulating layer 30 includes a first insulating layer 301 disposed on one surface of the positive current collector and a second insulating layer 302 disposed on the other surface of the positive current collector. The width of the first insulating layer 301 is b1, and the width of the second insulating layer 302 is b2. The absolute value of the difference between b1 and b2 is Δb mm, where 0 ≤ Δb ≤ 0.3. In some embodiments of this application, 0 ≤ Δb ≤ 0.1. For example, Δb can be 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, or any two values ​​in between. By adjusting Δb within the above range, it is beneficial to the tensile strength near the first edge of the positive electrode material layer, which helps to reduce the risk of positive electrode tab breakage and improve the safety performance of the secondary battery. Furthermore, Δb within the aforementioned range is also beneficial for the transport of active ions, such as lithium ions, between the positive and negative electrodes in the secondary battery, thereby improving the energy density of the secondary battery.

[0070] To mitigate the problems of wrinkling and breakage of the positive electrode current collector caused by stress concentration at the edge of the positive electrode material layer during the manufacturing process of secondary batteries, a thicker positive electrode current collector, such as 12 μm, is typically used. However, using such a thick current collector significantly reduces the energy density of the secondary battery. In some embodiments of this application, the thickness of the positive electrode current collector is h μm, where 8 ≤ h ≤ 10. For example, h can be 8, 9, 10, or any two values ​​within this range. By selecting a current collector within the above range, wrinkling of the empty foil area of ​​the positive electrode sheet during cold pressing can be mitigated, reducing the risk of positive electrode tab breakage and improving the safety performance of the secondary battery. Furthermore, it is beneficial to increase the energy density of the secondary battery.

[0071] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0072] In this application, positive current collectors of different thicknesses can be purchased and selected by measuring with a micrometer.

[0073] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0074] The positive electrode material layer may further include a conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The positive electrode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and positive electrode binder in the positive electrode material layer; those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0075] This application does not impose any particular limitation on the thickness of the cathode material layer, as long as it achieves the purpose of this application. For example, the thickness of the cathode material layer can be from 30 μm to 120 μm.

[0076] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a positive electrode binder. This application does not impose any particular limitation on the conductive agent and the positive electrode binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and positive electrode binders.

[0077] A third aspect of this application provides a secondary battery comprising a positive electrode sheet as described in any of the foregoing embodiments or a positive electrode sheet obtained according to the foregoing preparation method.

[0078] In this application, the secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion thereof; this application does not have any particular limitation, as long as the purpose of this application is achieved.

[0079] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0080] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0081] In some embodiments of this application, the negative electrode material layer may further include a conductive agent and a negative electrode binder. This application does not particularly limit the types of conductive agents and negative electrode binders, as long as they achieve the purpose of this application. For example, they may be at least one of the aforementioned conductive agents and positive electrode binders. This application does not particularly limit the mass ratio of the negative electrode active material, conductive agent, and negative electrode binder in the negative electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of this application is achieved.

[0082] This application does not impose any particular limitation on the thickness of the negative electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode material layer can be from 30 μm to 120 μm. This application also does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector can be from 4 μm to 15 μm.

[0083] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a negative electrode binder. This application does not impose any particular limitation on the conductive agent and the negative electrode binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agent and negative electrode binder.

[0084] In this application, the secondary battery also includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0085] In some embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0086] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0087] In some embodiments of this application, the inorganic layer comprises inorganic particles and a membrane binder. This application does not impose any particular limitation on the inorganic particles; for example, the inorganic particles may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not impose any particular limitation on the membrane binder; for example, the membrane binder may be at least one of the above-mentioned positive electrode binders. In some embodiments of this application, the polymer layer comprises a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0088] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 30 μm.

[0089] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0090] This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0091] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0092] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0093] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0094] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0095] A fourth aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance in use.

[0096] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is 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, household large-capacity batteries, and lithium-ion capacitors.

[0097] Example

[0098] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0099] Test methods and equipment:

[0100] Drop performance test:

[0101] The lithium-ion battery under test was placed in an environment of 25°C and charged at a constant current of 0.5C to a voltage of 4.5V. Then it was charged at a constant voltage of 4.5V to a cutoff current of 0.05C and left to stand for 5 minutes to reach a fully charged state.

[0102] Place the lithium-ion battery in the fixed fixture, check its appearance and take photos before and after the test;

[0103] The metal drop test was conducted from a height of 1.8m on the metal drop floor, with one drop from each of the two sides (head and tail) and one drop from each of the four corners. A total of 7 rounds of tests were carried out, with 6 drops per round. The drop order was: head -> tail -> head right corner -> tail right corner -> head left corner -> tail left corner (angle: 45±15 degrees).

[0104] The voltage and internal resistance of the lithium-ion battery were measured after the drop test and 24 hours later. The criteria for passing the drop test were: no fire, no explosion, no smoke, no leakage, and voltage drop <50mV. Ten lithium-ion batteries were tested for each example or comparative example, and the number of batteries that passed was recorded as: number of batteries that passed / number of tests. For example, 9 / 10 means that out of 10 lithium-ion batteries tested, 9 lithium-ion batteries passed the test.

[0105] Fracture location test, tensile strength test:

[0106] Under conditions of 25℃ ambient temperature and less than 2% ambient humidity, the lithium-ion battery was disassembled, the positive electrode sheet was removed, and residual electrolyte on the surface of the positive electrode sheet was absorbed with lint-free paper. A portion of the positive electrode sheet, including the positive electrode tab and the coated area, was cut along the direction from the positive electrode material layer to the insulating layer as a test sample. The sample was 30mm long and 5mm wide, and the corresponding area of ​​the positive electrode material layer was 25mm long and 5mm wide. The edges of the cut positive electrode sheet were free of obvious burrs. Samples were taken from each lithium-ion battery for testing. The samples were fixed between the clamping terminals of a tensile testing machine (LJ2005), the limit blocks of the tensile testing machine were adjusted, and the test was started at a speed of 10mm / min until the sample broke. The breakage location was recorded, and subsequent tests were performed. The tensile strength measured by the tensile testing machine was also recorded.

[0107] x: Place the positive electrode material layer of the positive electrode sheet face up under a VHX5000 microscope. During observation, adjust the microscope magnification between 10 and 50x until a photograph can be taken that simultaneously shows the first edge of the positive electrode material layer and the fracture location of the positive electrode sheet. Measure x using the microscope software. If the edge of the fracture location is irregular, use the distance between it and the closest first edge. If the positive electrode material layer and the insulating layer are located on two surfaces of the coated area, and Δs > 0, measure the distance between the first edge of each surface and the fracture location, and record the smaller distance. Record the four test results as x1, x2, x3, and x4, respectively. Finally, calculate the average of x1, x2, x3, and x4 to obtain x.

[0108] (1) If the fracture location is between the first and second edges of the positive electrode material layer and x > 3, the tensile strength at this time is R2. Take another positive electrode sample, which only includes the area where the insulating layer is set and the area where the positive electrode tab is set, and repeat the above steps to perform tensile strength test. The tensile strength when the sample breaks is R1.

[0109] (2) If the fracture location is between the first edge and the empty foil area, or if the fracture location is between the first edge and the second edge and x≤3, the tensile strength at this time is R1. Take another sample, the positive electrode sample only includes the area where the positive electrode material layer is set, and repeat the above steps to perform tensile strength testing. The tensile strength when the sample breaks is R2.

[0110] Both R1 and R2 were tested 4 times, and the average tensile strength of each was taken as the final test result.

[0111] Roughness test:

[0112] At an ambient temperature of 25℃, the lithium-ion battery was disassembled, and the positive electrode sheet was removed and soaked in dimethyl carbonate (DMC) for 20 minutes. Then, the positive electrode sheet was placed in an oven and dried at 80℃ for 12 hours. The empty foil area was placed flat on a stage with one surface facing upwards and fixed. The stage was then placed flat directly below the objective lens of a microscope (Keyence VXH-6000). The microscope was adjusted, and the roughness test mode was selected. At 300x magnification, the roughness of the empty foil area near the junction of the positive electrode tab and the insulating layer was measured. The test was repeated in three fields of view of the empty foil area, and the average of the three tests was taken as the roughness of the empty foil area of ​​the positive electrode sheet.

[0113] Energy density test:

[0114] (1) Test of discharge capacity: At an ambient temperature of 25°C, the lithium-ion battery was charged at a constant current of 1C to a voltage of 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 0.2C to a voltage of 3.0V, left to stand for 5 minutes. The discharge capacity at this time was recorded as C, and the unit was Wh.

[0115] (2) Lithium-ion battery size test: As shown in Figure 5, along the extension direction of the positive electrode tab 70, i.e., the direction of the arrow in the figure, the lithium-ion battery 02 includes a fifth edge 401 and a sixth edge 601. The edge of the positive electrode tab 70 is designated as the fifth edge 401. The region extending 3mm from the fifth edge 401 in the opposite direction to the extension direction of the positive electrode tab 70 is designated as the head region 40. The region extending 3mm from the sixth edge 601 in the extension direction of the positive electrode tab 70 is designated as the tail region 60. The remaining region between the head region 40 and the tail region 60 is designated as the middle region 50. The distance between the fifth edge 401 and the sixth edge 601 is the length L of the lithium-ion battery 02, and the length of the fifth edge 401 is the width M of the lithium-ion battery 02. The dimension of the lithium-ion battery 02 in the stacking direction is its thickness D. The length L and width M of the lithium-ion battery are measured using a laser measuring instrument. The thickness of the head region (40), the middle region (50), and the tail region (60) were measured using a micrometer. Each region was measured three times, and the average of the three measurements was taken as the thickness of the corresponding region. The thickness D of the lithium-ion battery was calculated using the following formula: D = 1 / 3 × (Da + Db + Dc). Where D, L, and M are in dm.

[0116] (3) Calculation of energy density X of lithium-ion battery: X=C / (D×L×M);

[0117] In this study, the energy density X0 of Comparative Example 1 is designated as "low," with X0 = 712.7 Wh / L. Each 0.5% increase represents a gradient in energy density, and the resulting lithium-ion batteries are sorted from low to high as follows: High > Upper-Medium > Medium > Lower-Medium > Low. Specifically, energy densities greater than X0 and less than or equal to 1.005X0 are designated as "lower-Medium," greater than 1.005X0 and less than or equal to 1.010X0 as "medium," greater than 1.010X0 and less than or equal to 1.015X0 as "upper-Medium," and greater than 1.015X0 are designated as "high."

[0118] Dimension measurement:

[0119] d, b, s: The cross-section of the positive electrode sheet along its thickness direction was ion-polished and then observed and measured under a VHX5000 microscope at 500x magnification. For each dimension, 10 points were measured, and the average value was taken as the final result.

[0120] Δd, Δb, Δs: Measure d1, d2, b1, b2, s1, and s2 of the two surfaces respectively, and calculate Δd, Δb, and Δs. Δd = |d1 - d2|, Δb = |b1 - b2|, Δs = |s1 - s2|.

[0121] h: Use a micrometer to measure 10 points and take the average as the final result.

[0122] Example 1-1

[0123] <Preparation of Insulating Layer>

[0124] The insulating layer adhesive polyvinylidene fluoride and the ceramic material boehmite were mixed evenly in N-methylpyrrolidone (NMP) solvent at a mass ratio of 8:2 to obtain an insulating layer slurry with a solid content of 35 wt%.

[0125] <Preparation of the positive electrode>

[0126] Conductive carbon black (a conductive agent) and polyvinylidene fluoride (PVDF) (a positive electrode binder) were mixed, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a conductive adhesive solution with a solid content of 7%. Lithium cobalt oxide (the positive electrode active material) was then added to the conductive adhesive solution, and the mixture was stirred under vacuum until homogeneous, resulting in a positive electrode material slurry with a solid content of 75%. The mass ratio of the positive electrode active material, conductive agent, and positive electrode binder was 97:1:2.

[0127] A 9μm thick aluminum foil was selected as the positive electrode current collector. On one surface of the current collector, a 10μm thick adhesive tape was attached to the area corresponding to the empty foil region and the insulating layer. Then, a positive electrode material layer slurry was applied to the third region, with the slurry covering the adhesive tape by 8mm along the direction from the insulating layer to the positive electrode material layer. The foil was dried at 105℃ for 15 minutes to obtain a positive electrode sheet with a single-sided coating of 75μm thick positive electrode material layer. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material layer. A cold pressing process was then performed at a pressure of 90t. The adhesive tape was then peeled off, and an insulating layer slurry was applied to the first edge of the positive electrode material layer on both sides of the current collector. The coating amount of the insulating layer was 48mg / 1540.25mm. 2 Then, the positive electrode sheet is obtained by drying. The positive electrode sheet is cut into fixed sizes, and the positive electrode tab is obtained by cutting from the corresponding position of the empty foil area. Then, it is vacuum dried at 85°C for 4 hours to obtain a positive electrode sheet with a specification of 58mm×65mm for use. The thickness of the single-sided positive electrode material layer is 46μm, and the difference between the maximum and minimum thickness ΔT is 1μm. Specific parameters are shown in Tables 1-1, 1-2, and 2. The thickness of the single-sided positive electrode material layer mentioned above refers to the area of ​​the positive electrode material layer excluding the edge area. The edge area is the area where the first edge extends 10mm along the direction from the insulating layer to the positive electrode material layer, and the area where the second edge extends 10mm along the direction from the positive electrode material layer to the insulating layer. The following examples and comparative examples are understood in the same way.

[0128] <Preparation of Negative Electrode Sheets>

[0129] Artificial graphite (anode material), styrene-butadiene rubber (binder), and sodium carboxymethyl cellulose (binder) were mixed in a mass ratio of 96:2:2. Deionized water was added as a solvent, and the mixture was stirred until homogeneous to obtain a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 85°C to obtain a negative electrode sheet with a single-sided coating of 100 μm thick negative electrode material. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing and cutting, negative electrode tabs were cut from the corresponding positions in the empty foil area and vacuum dried at 120°C for 12 hours to obtain a negative electrode sheet with a size of 60 mm × 67 mm for later use.

[0130] <Preparation of Electrolyte>

[0131] In an environment with a water content of less than 10 ppm, ethylene carbonate, propylene carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the electrolyte. The molar concentration of the electrolyte salt was 1.15 mol / L based on the mass of the electrolyte.

[0132] <Preparation of the diaphragm>

[0133] A porous polyethylene film with a thickness of 7μm (supplied by Celgard) is used.

[0134] <Preparation of Lithium-ion Batteries>

[0135] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator acting as a separator between the positive and negative electrodes. The four corners of the stacked structure are then secured with tape to obtain the stacked electrode assembly. Multiple positive electrode tabs of the multiple positive electrode sheets are then integrated and welded to form the positive electrode tab of the lithium-ion battery, and multiple negative electrode tabs of the multiple negative electrode sheets are integrated and welded to form the negative electrode tab of the lithium-ion battery. The electrode assembly is placed in an aluminum-plastic film outer packaging foil and dehydrated at 80°C. The prepared electrolyte is then injected, followed by vacuum sealing, settling, formation, shaping, and capacity testing to obtain a soft-pack lithium-ion battery. The formation temperature is 80°C, and the settling time is 2 hours.

[0136] Examples 1-2 to Examples 1-6

[0137] Except for adjusting the relevant preparation parameters according to Table 1-1, the rest is the same as in Example 1-1.

[0138] Examples 1-7 to Examples 1-9

[0139] Except for adjusting the relevant preparation parameters according to Table 1-1, the rest is the same as in Examples 1-5. Among them, in the preparation process of the positive electrode sheet, after the adhesive tape is applied to the positive current collector, the difference between the maximum and minimum thickness of the positive electrode material layer is shown in Table 1 by changing the starting coating position of the positive electrode material layer slurry.

[0140] Examples 1-10 to Examples 1-27

[0141] Except for adjusting the relevant preparation parameters according to Table 1-1, the rest is the same as in Examples 1-5. Among them, in Examples 1-16 to 1-21, along the direction from the insulating layer to the positive electrode material layer, the sum of the dimensions of the positive electrode material layer and the insulating layer is constant. When b1 and b2 increase or decrease compared to Examples 1-5, the dimension of the positive electrode material layer decreases or increases accordingly.

[0142] Examples 1-28 to Examples 1-39

[0143] Except for adjusting the relevant preparation parameters according to Table 1-2, the rest is the same as in Examples 1-5. Among them, in Examples 1-36 to 1-38, along the direction from the insulating layer to the positive electrode material layer, the sum of the dimensions of the positive electrode material layer and the insulating layer is constant. When b2 decreases compared to Examples 1-5, the dimension of the positive electrode material layer increases accordingly.

[0144] Examples 2-1 to 2-5

[0145] Except for adjusting the relevant preparation parameters according to Table 2, the other examples are the same as those in Examples 1-5. In Examples 2-1 to 2-3, the roughness of the empty foil region and the compaction density of the positive electrode material layer are adjusted by changing the cold pressing pressure, as shown in Table 2.

[0146] Comparative Example 1

[0147] Except for the preparation of the positive electrode sheet according to the following steps, the rest is the same as in Examples 1-5:

[0148] <Preparation of the positive electrode>

[0149] The slurry for the positive electrode material layer is the same as in Example 1-1.

[0150] A positive electrode current collector aluminum foil with a thickness h of 9 μm was selected. A positive electrode material layer slurry was deposited on one surface of the current collector and dried at 105℃ for 15 min to obtain a positive electrode sheet with a single-sided coating of 75 μm thick positive electrode material layer. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material layer and a "thinning zone". An insulating layer slurry was then coated on the first edge of the positive electrode material layer on both sides of the current collector, with a coating amount of 48 mg / 1540.25 mm. 2Then, it is dried and cold-pressed at a pressure of 90t to obtain the positive electrode sheet. The positive electrode sheet is cut into fixed sizes, and the positive electrode tabs are obtained by cutting from the corresponding positions of the empty foil area. Then, it is vacuum dried at 85℃ for 4 hours to obtain a positive electrode sheet with a specification of 58mm×65mm for use. Specific parameters are shown in Table 1. The thickness of the single-sided positive electrode material layer is 46μm. The above-mentioned thickness of the single-sided positive electrode material layer refers to the thickness of the region from 10mm away from the first edge to the second edge of the positive electrode material layer.

[0151] Comparative Example 2

[0152] Except for the preparation of the positive electrode sheet according to the following steps, the rest is the same as in Examples 1-5:

[0153] <Preparation of the positive electrode>

[0154] The slurry for the positive electrode material layer is the same as in Example 1-1.

[0155] A 9μm thick aluminum foil was selected as the positive electrode current collector. A 10μm thick adhesive tape was applied to the area of ​​the positive electrode current collector corresponding to the empty foil area and the insulating layer. Then, a positive electrode material layer slurry was applied to one surface of the positive electrode current collector, covering the adhesive tape. The slurry was dried at 105℃ for 15 minutes to obtain a positive electrode sheet with a 75μm thick positive electrode material layer on one side. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode material layer. The adhesive tape was then peeled off, and an insulating layer slurry was applied to the first edge of the positive electrode material layer on both sides of the positive electrode current collector. The coating amount of the insulating layer was 48mg / 1540.25mm. 2 Then, after drying, it undergoes cold pressing at a pressure of 90t to obtain the positive electrode sheet. The positive electrode sheet is cut to a fixed size, with the positive electrode tabs obtained by cutting from the corresponding position in the empty foil area. It is then vacuum dried at 85℃ for 4 hours to obtain a positive electrode sheet with dimensions of 58mm × 65mm for later use. Specific parameters are shown in Table 1. The thickness of the single-sided positive electrode material layer is 46μm.

[0156] The preparation parameters and performance tests of each embodiment and comparative example are shown in Tables 1-1, 1-2 and 2.

[0157] Table 1-1 Note: In Comparative Example 1, the energy density is marked as "low", which means that its energy density X0 = 712.7 Wh / L.

[0158] Table 1-2 Note: In Table 1-2, d1 = 20 μm, b1 = 2.5 mm, and s1 = 0.2 mm.

[0159] As can be seen from Examples 1-1 to 1-39 and Comparative Examples 1 to 2-2, in the above examples, during the preparation of the positive electrode sheet, by introducing adhesive paper before cold pressing and peeling off the adhesive paper after cold pressing before setting the insulating layer, the fracture location of the positive electrode sheet obtained in the examples is between the first edge and the second edge of the positive electrode material layer and 20≥x>3, resulting in a higher number of lithium-ion batteries passing the drop test. This indicates that the tensile strength near the first edge of the positive electrode material layer of this application is improved, the risk of positive electrode tab breakage is reduced, and the lithium-ion battery has higher safety performance. However, as can be seen from Comparative Example 1, as shown in Figure 6a, the overall tensile strength of the empty foil area of ​​the positive electrode sheet and the area where the insulating layer is set in the positive electrode sheet is lower. When conducting the tensile strength test, x≤3, that is, the fracture occurs near the first edge of the positive electrode material layer, and there are many wrinkles and severe deformation at the junction of the coated area and the empty foil area. As shown in Figure 6b, the positive electrode sheets of Examples 1-5 fractured between the first and second edges of the positive electrode material layer during tensile strength testing, with 20 ≥ x > 3, indicating a higher number of lithium-ion cells passing the drop test. In Figures 6a and 6b, the different colored areas from top to bottom represent the empty foil area, the area with the insulating layer, and the area with the positive electrode material layer, respectively. This demonstrates that the safety performance of the lithium-ion battery conforming to this application has been improved while maintaining the energy density of the lithium-ion battery.

[0160] The difference between the maximum and minimum thickness of the positive electrode material layer typically affects the energy density and safety performance of a secondary battery. As can be seen from Examples 1-5 and 1-7 to 1-9, when the difference between the maximum and minimum thickness of the positive electrode material layer is within the range of this application, the resulting lithium-ion battery passes the drop test more frequently while maintaining a higher energy density. This demonstrates that when the difference between the maximum and minimum thickness of the positive electrode material layer is within the range of this application, the resulting lithium-ion battery exhibits both high safety performance and high energy density. Figure 7a shows the tensile test curve of the positive electrode sheet test sample in Comparative Example 1, and Figure 7b shows the tensile test curve of the positive electrode sheet test sample in Examples 1-5. The two tensile strength test curves in the tensile test curves represent the tensile strength test curves of the test sample from the same lithium-ion battery. In Figures 7a and 7b, area A represents the empty foil area, the insulating layer, and the corresponding area within 3 mm from the first edge along the direction from the insulating layer to the positive electrode material layer. Area B represents the positive electrode material layer area from 3 mm from the first edge along the direction from the insulating layer to the second edge. As can be seen from the figure, when stretched to the same extent, the tensile strength of the above embodiment is greater, while the comparative embodiment fractures faster under the same tensile stress. This demonstrates that the positive electrode sheet of this application has higher tensile strength, and the tensile strength near the first edge of the positive electrode material layer is improved, thereby reducing the risk of positive electrode tab breakage and improving the safety performance of lithium-ion batteries.

[0161] Furthermore, the positive electrode sheets in Examples 1-1 to 1-39 did not exhibit wrinkles during the cold pressing process, while Comparative Examples 1 and 2 showed obvious wrinkles during the cold pressing process. Specifically, as shown in Figures 8a and 8b, Figure 8a is an electron microscope image of the sample obtained in Comparative Example 1 with only a positive electrode material layer on the surface of the positive electrode current collector after cold pressing, and Figure 8b is an electron microscope image of the sample obtained in Examples 1-5 with only a positive electrode material layer on the surface of the positive electrode current collector after cold pressing and removal of the adhesive tape. As can be seen from the figures, the left side is the area coated with the positive electrode material layer, and the right side is the empty foil area. In Comparative Example 1, no adhesive tape was applied during cold pressing, and the interface between the positive electrode material layer and the aluminum foil had many wrinkles and severe deformation, with several diagonal lines. This indicates that during the cold pressing process, stress concentration occurred at the first edge of the positive electrode material layer, resulting in deformation and causing fewer lithium-ion batteries to pass the drop test. In Examples 1-5, adhesive tape was first applied to the areas corresponding to the empty foil area and the insulating layer in the positive electrode current collector before cold pressing. The empty foil area adjacent to the first edge of the positive electrode material layer was relatively flat and without wrinkles. This demonstrates that using the positive electrode sheet in this application can improve the stress concentration problem near the first edge during cold pressing, thereby reducing the risk of the positive electrode sheet breaking near the first edge of the positive electrode material layer, and thus improving the safety performance of the lithium-ion battery. In addition, referring to Figure 8b, the surface of the empty foil area without the positive electrode material layer has a fish-scale-like morphology, which is mainly caused by the application and removal of adhesive tape.

[0162] In the insulating layer of the positive electrode sheet, the type and content of the insulating layer binder and ceramic material typically affect the results of drop tests. As can be seen from Examples 1-1 to 1-6, when the content of the insulating layer binder and ceramic material is within the range specified in this application, the resulting lithium-ion battery has a higher number of passes in drop tests and a higher energy density. This indicates that when the content of the insulating layer binder and ceramic material is within the range specified in this application, the resulting lithium-ion battery exhibits good safety performance while maintaining good energy density. A comparison of Examples 1-5 and 1-39 shows that when the types of insulating layer binder and ceramic material are within the range specified in this application, the resulting lithium-ion battery has a higher number of passes in drop tests and a higher energy density. This also indicates that when the types of insulating layer binder and ceramic material are within the range specified in this application, the resulting lithium-ion battery exhibits good safety performance while maintaining good energy density.

[0163] The maximum thickness d of the insulating layer typically affects the safety performance and manufacturing process yield of lithium-ion batteries, while the width b typically affects the energy density and safety performance. As seen in Examples 1-5 and 1-10 to 1-15, in this application, d is always less than or equal to the thickness of the positive electrode material layer. Therefore, changes in d have virtually no impact on the volumetric energy density of the secondary battery. However, when d is too large, as in Examples 1-15, there is a risk of strip breakage during mass production, affecting the manufacturing process yield of the positive electrode sheet. When the maximum thickness d of the insulating layer is within the range of this application, a higher number of lithium-ion batteries pass the drop test. Along the length of the positive electrode current collector, the sum of the dimensions of the positive electrode material layer and the insulating layer is constant. Therefore, changes in b affect the dimension of the positive electrode material layer, thus affecting the energy density of the secondary battery. As seen in Examples 1-5 and 1-16 to 1-21, when the width b of the insulating layer is within the range of this application, a higher number of lithium-ion batteries pass the drop test, and the energy density is also higher. This demonstrates that when the values ​​of d and b are within the range of this application, the resulting lithium-ion battery has good safety performance while maintaining adequate energy density.

[0164] The distance *s* between the third edge of the insulating layer and the first edge of the positive electrode material layer in the positive electrode sheet typically affects the energy density and safety performance of lithium-ion batteries. As can be seen from Examples 1-5, 1-22 to 1-27, when *s* falls within the scope of this application, the resulting lithium-ion battery not only has a higher energy density but also better safety performance. However, when the value of *s* is too large, as in Examples 1-27, although it is beneficial to improve the safety performance of the lithium-ion battery, it will affect the transport of lithium ions between the positive and negative electrode sheets, thereby affecting the electrochemical performance of the lithium-ion battery.

[0165] As can be seen from Examples 1-5 and Examples 1-28 to Examples 1-31, when the positive electrode material layer and the insulating layer are disposed on the two surfaces of the coating area, and the distance Δs between the third edges of the insulating layer on both sides of the positive electrode current collector is within the range of this application, the resulting lithium-ion battery not only has a higher energy density but also a higher pass rate in drop tests. This indicates that the lithium-ion battery with Δs within the range of this application has higher safety performance while maintaining energy density. Furthermore, with Δs within the range of this application, the resulting lithium-ion battery not only has a higher energy density but also better kinetic performance, and a lower risk of lithium plating on the positive electrode sheet. As in Examples 1-31, when Δs is too large, the resulting lithium-ion battery has poor kinetic performance, and the positive electrode sheet removed after cycling exhibits slight lithium plating and black spots.

[0166] As can be seen from Examples 1-5 and Examples 1-32 to 1-35, when the positive electrode material layer and the insulating layer are disposed on the two surfaces of the coating area, the maximum thickness difference Δd between the insulating layers on both sides of the positive electrode current collector is within the range of this application. The resulting lithium-ion battery has a higher number of successful drop tests and a higher energy density. This indicates that when Δd is within the range of this application, the safety performance of the lithium-ion battery can be improved while maintaining energy density. Furthermore, Δd being within the above range indicates better consistency in the thickness of the coating layer on both sides of the positive electrode sheet, making it less prone to curling during the preparation process and thus less likely to affect the process yield.

[0167] The difference in width Δb between the two sides of the positive current collector in the positive electrode sheet usually affects the safety performance of lithium-ion batteries. As can be seen from Examples 1-36 to 1-38, when Δb is within the range of this application, the resulting lithium-ion battery has a higher number of passes and higher energy density in the drop test, thus demonstrating that the safety performance and energy density of the lithium-ion battery are balanced.

[0168] Table 2

[0169] Examples 1-5 and Examples 2-1 to 2-3 involve controlling the coating amount of the positive electrode material layer to remain constant, and adjusting the cold pressing pressure during the preparation of the positive electrode sheet to change the compaction density and roughness of the empty foil area. As can be seen from the above examples, the roughness of the empty foil area of ​​the positive electrode sheet in the above examples is within the scope of this application. The overall tensile strength R1 of the empty foil area and the area where the insulating layer is set in the positive electrode sheet is better than the tensile strength R2 of the area where the positive electrode material layer is set in the positive electrode sheet. When subjected to tension, the fracture location is located between the first and second edges of the positive electrode material layer, and 20≥x>3. The resulting lithium-ion battery has a higher number of passes in drop tests, indicating that the lithium-ion battery with a roughness of the empty foil area within the scope of this application has a lower risk of positive electrode tab breakage and higher safety performance.

[0170] The thickness of the positive electrode current collector typically affects the tensile strength and energy density of the positive electrode sheet. As seen in Examples 1-5, 2-4, and 2-5, the tensile strengths R1 and R2 vary with the thickness of the positive electrode current collector. Even when the thickness of the positive electrode current collector is controlled within the scope of this application, the tensile strength of the positive electrode material coating area of ​​the obtained positive electrode sheet is still superior to that of the empty foil area and the insulating layer coating area. The fracture locations of the positive electrode sheet are all within the positive electrode material coating area, and 20 ≥ x > 3. This demonstrates that the lithium-ion battery within the scope of this application has both higher safety performance and higher energy density.

[0171] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0172] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0173] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A positive electrode sheet, comprising a positive current collector, a positive electrode material layer, and an insulating layer, wherein the positive electrode sheet further comprises an empty foil region and a coating region, the positive electrode material layer and the insulating layer are located in the coating region, the insulating layer and the positive electrode material layer are connected and disposed, and along the length direction of the positive current collector, the insulating layer is closer to the empty foil region than the positive electrode material layer; Along the direction from the positive electrode material layer to the insulating layer, the positive electrode material layer includes a first edge and a second edge opposite to each other, the first edge being close to the empty foil area; the positive electrode sheet is subjected to a tensile strength test along the direction, the fracture location of the positive electrode sheet is located between the first edge and the second edge, and the distance between the fracture location and the first edge is x mm, 3 < x ≤ 20.

2. The positive electrode sheet according to claim 1, wherein, 5＜x≤20。 3. The positive electrode sheet according to claim 1, wherein, The difference between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 1 μm.

4. The positive electrode sheet according to claim 1, wherein, The overall tensile strength of the empty foil area and the area in the positive electrode sheet where the insulating layer is disposed is R1 MPa, and the tensile strength of the area in the positive electrode sheet where the positive electrode material layer is disposed is R2 MPa, 220≤1.2R2<R1≤280.

5. The positive electrode sheet according to claim 1, wherein, The roughness of the empty foil region is Rzμm, where 2≤Rz≤5.

6. The positive electrode sheet according to claim 1, wherein, The insulating layer comprises an adhesive and a ceramic material, and the insulating layer satisfies at least one of the following characteristics: (1) Based on the mass of the insulating layer, the mass percentage of the adhesive W1 is 5% to 20%; (2) Based on the mass of the insulating layer, the mass percentage W2 of the ceramic material is 80% to 95%.

7. The positive electrode sheet according to claim 1, wherein, The insulating layer comprises an adhesive and a ceramic material, and the insulating layer satisfies at least one of the following characteristics: (1) Based on the mass of the insulating layer, the mass percentage of the adhesive W1 is 70% to 90%; (2) Based on the mass of the insulating layer, the mass percentage of the ceramic material W2 is 10% to 30%.

8. The positive electrode sheet according to claim 6 or 7, wherein the insulating layer satisfies at least one of the following characteristics: (1) The adhesive includes at least one of polyvinylidene fluoride, polyurethane, polyacrylic acid or polyacrylonitrile; (2) The ceramic material includes at least one of boehmite or alumina.

9. The positive electrode sheet according to any one of claims 1 to 7, wherein the maximum thickness of the insulating layer is d μm and the width is b mm, wherein, 10≤d≤40, 1.5≤b≤3.

5.

10. The positive electrode sheet according to claim 9, wherein, 10≤d≤25; and / or, 2.0≤b≤3.

0.

11. The positive electrode sheet according to any one of claims 1 to 7, wherein, The edge of the insulating layer near the positive electrode material layer is the third edge, which coincides with the first edge, or the third edge is located between the first edge and the second edge, and the distance between the third edge and the first edge is s mm, where 0 ≤ s ≤ 0.

5.

12. The positive electrode sheet according to claim 11, wherein, 0≤s≤0.3。 13. The positive electrode sheet according to claim 11, wherein the positive electrode material layer and the insulating layer are disposed on two surfaces of the coating region, and wherein the positive electrode material layer and the insulating layer have at least one of the following characteristics: (1) Along the direction, the distance between the third edges of the insulating layers on both sides of the positive current collector is Δs mm, 0 ≤ Δs ≤ 0.3; (2) The maximum thickness difference of the insulating layer on both sides of the positive current collector is Δdμm, 0≤Δd≤15.

14. The positive electrode sheet according to claim 13, wherein it satisfies at least one of the following characteristics: (1) 0 ≤ Δs ≤ 0.1; (2) 0 ≤ Δd ≤ 10.

15. The positive electrode sheet according to claim 13, wherein, The positive electrode material layer and the insulating layer are disposed on two surfaces of the coating area, and the difference in width of the insulating layer on both sides of the positive electrode current collector is Δb mm, where 0 ≤ Δb ≤ 0.

3.

16. The positive electrode sheet according to claim 1, wherein, The difference between the maximum and minimum thickness of the positive electrode material layer is less than or equal to 0.8 μm.

17. A method for preparing a positive electrode sheet as described in any one of claims 1 to 16, comprising the following steps: Obtain the positive electrode material layer paste and the insulating layer paste; Adhesive tape is placed in the first and second regions of the positive electrode current collector. Then, the positive electrode material layer slurry is placed in the third region of the positive electrode current collector, such that at least a portion of the surface of the adhesive tape is covered by the positive electrode material layer slurry. Then, the tape is dried and cold-pressed. The adhesive tape is then peeled off, and the insulating layer slurry is placed in the second region and dried to obtain the positive electrode sheet. The first region is an empty foil region, the second region is used to set the insulating layer, and the third region is used to set the positive electrode material layer, and the first region, the second region and the third region are connected in sequence.

18. A secondary battery comprising a positive electrode sheet according to any one of claims 1 to 16 or a positive electrode sheet obtained by the preparation method according to claim 17.

19. An electronic device comprising the secondary battery of claim 18.

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