Positive electrode sheet, secondary battery, and electronic device

Abstract

The application provides a positive electrode sheet, a secondary battery and an electronic device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector. The positive electrode current collector has a first edge in the width direction of the positive electrode sheet. A first edge coating layer is arranged on the surface of the positive electrode current collector between the positive electrode active material layer and the first edge. The first edge coating layer comprises a first insulating material and a first binder. The first binder comprises modified polyimide and polyvinylidene fluoride. The modified polyimide is obtained by modifying polyimide with an acrylate monomer. The modified polyimide contains a polar group. The polar group comprises at least one of a cyano group, a carboxyl group, an amino group, a sulfonic acid group or a phosphate group. The molar ratio of the main chain of the polyimide to the polar group is 1:(0.1-0.6). The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.1-5). Through the above arrangement, the capacity and safety performance of the secondary battery can be improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a positive electrode, a secondary battery, and an electronic device. Background Technology

[0002] Secondary batteries (such as lithium-ion batteries), as efficient and environmentally friendly energy storage devices, have been widely used in consumer electronics, electric vehicles, energy storage power stations, and other fields. During the manufacturing process of secondary batteries, burrs are easily generated on the edges of the positive electrode during cutting. These burrs may pierce the separator and come into contact with the negative electrode, thereby causing a short circuit in the secondary battery and seriously threatening its safety performance.

[0003] To address the aforementioned issues, current market solutions involve coating the edge of the positive electrode sheet with an insulating coating, known as an edge coating. Due to the smooth surface of boehmite, existing edge coatings typically use boehmite as the primary material to ensure a smooth, burr-free surface on the cut positive electrode sheet, thereby improving the safety performance and yield of the secondary battery. However, existing edge coatings are prone to interfacial penetration with the positive electrode active material layer during coating, leading to severe creep phenomenon and affecting the capacity of the secondary battery. Furthermore, the adhesion of existing edge coatings is weak, especially after immersion in electrolyte at high temperatures (≥85℃), further impacting the safety performance of the secondary battery. Therefore, those skilled in the art need to develop a new technical solution to improve the creep phenomenon in the positive electrode sheet and the insufficient adhesion of the edge coating after immersion in electrolyte. Summary of the Invention

[0004] The purpose of this application is to provide a positive electrode sheet, a secondary battery, and an electronic device to improve the problems of ramping phenomenon in the positive electrode sheet and insufficient adhesion of the first edge coating after being soaked in electrolyte, thereby improving the capacity and safety performance of the secondary battery.

[0005] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0006] The first aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Along the width direction of the positive electrode sheet, the positive current collector has a first edge. A first edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge. The first edge coating includes a first insulating material and a first adhesive. The first adhesive includes modified polyimide and polyvinylidene fluoride. The modified polyimide is obtained by modifying the polyimide with acrylate monomers. The modified polyimide contains polar groups, including at least one of cyano, carboxyl, amino, sulfonic acid, or phosphate groups. The molar ratio of the polyimide backbone to the polar groups is 1:(0.1 to 0.6). The mass ratio of the modified polyimide to polyvinylidene fluoride is 1:(0.1 to 5). The positive electrode sheet of this application incorporates a first edge coating containing modified polyimide and polyvinylidene fluoride (PVDF). The molar ratio of the polyimide main chain to polar groups and the mass ratio of modified polyimide to PVDF are controlled within the range specified in this application. PVDF serves as a toughening phase; its flexible CC segments enhance the flexibility of the first edge coating, mitigating the impact of the rigidity of the modified polyimide on the brittleness of the first edge coating. The modified polyimide, through its rigid segments, inhibits interfacial penetration between the first edge coating and the positive electrode active material layer, thereby improving the climbing phenomenon in the positive electrode sheet. The improvement in slope allows for greater utilization of the capacity of the positive electrode active material in the positive electrode active material layer. Modified polyimide possesses excellent adhesion, enhancing the interfacial bonding between the first insulating coating and the positive electrode current collector. This addresses the issue of insufficient adhesion of the first edge coating after immersion in electrolyte, reducing the probability of the first edge coating detaching from the surface of the positive electrode current collector. Furthermore, the first edge coating also exhibits good insulation and hardness, reducing the probability of burrs on the negative electrode sheet puncturing the separator and contacting the positive electrode sheet, thereby mitigating the risk of short circuits in the secondary battery. Ultimately, this improves the capacity and safety performance of the secondary battery.

[0007] In some embodiments of this application, the mass percentage of the first adhesive is 10% to 20% based on the mass of the first edge coating. By controlling the mass percentage of the first adhesive within the above range, it is beneficial to make the first edge coating have good adhesion, thus achieving good interfacial bonding between the first edge coating and the positive current collector.

[0008] In some embodiments of this application, the positive electrode sheet satisfies at least one of the following characteristics: (1) the mass percentage of the first binder is 10% to 15% based on the mass of the first edge coating; (2) the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(0.5 to 3). This is beneficial for enabling the secondary battery to have higher capacity and better safety performance.

[0009] In some embodiments of this application, the molecular weight of the modified polyimide is from 50,000 g / mol to 500,000 g / mol. Controlling the molecular weight of the modified polyimide within this range is beneficial for improving the capacity and safety performance of the secondary battery.

[0010] In some embodiments of this application, the thickness of the first edge coating is between 10 μm and 50 μm. Adjusting the thickness of the first edge coating within this range enables the secondary battery to improve its safety performance while simultaneously increasing its capacity.

[0011] In some embodiments of this application, the width of the first edge coating is 2 mm to 5 mm along the width direction of the positive electrode sheet. Adjusting the width of the first edge coating within this range helps to maximize its function while reducing the width of the positive electrode active material layer, thereby improving the capacity and safety performance of the secondary battery.

[0012] In some embodiments of this application, the areal density of the first edge coating is 0.02 mg / cm³. 2 Up to 0.06 mg / cm 2 Controlling the areal density of the first edge coating within the aforementioned range helps the first edge coating to perform its function, enabling the secondary battery to have higher capacity and safety performance.

[0013] In some embodiments of this application, the areal density of the positive electrode active material layer is 0.1 mg / cm³. 2 Up to 0.3 mg / cm 2 Controlling the areal density of the positive electrode active material layer within the above-mentioned range is beneficial to having more positive electrode active material in the positive electrode active material layer, thereby enabling the positive electrode active material layer to have a higher capacity.

[0014] In some embodiments of this application, the first insulating material includes at least one selected from alumina, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide; the mass percentage of the first insulating material is 80% to 90% based on the mass of the first edge coating. By selecting the above-mentioned types of first insulating materials and controlling the mass percentage of the first insulating material in the first edge coating within the above range, the safety performance of the secondary battery can be improved while increasing its capacity.

[0015] In some embodiments of this application, the particle size Dv50 of the first insulating material is from 0.4 μm to 1.8 μm. Controlling the particle size Dv50 of the first insulating material within this range is beneficial for improving the safety performance of the secondary battery while maintaining its high capacity.

[0016] In some embodiments of this application, a mixed layer is present between the positive electrode active material layer and the first edge coating along the width direction of the positive electrode sheet, and the width of the mixed layer is less than or equal to 0.5 mm. This indicates that the climbing shadow in the positive electrode sheet is smaller.

[0017] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, a second binder, and a conductive agent. Based on the mass of the positive electrode active material layer, the mass percentage content of the second binder is 1.5% to 3.0%. Controlling the mass percentage content of the second binder within the above range is beneficial to achieving good interfacial bonding between the positive electrode active material layer and the positive electrode current collector without affecting the content of the positive electrode active material.

[0018] In some embodiments of this application, the positive electrode current collector has a second edge opposite to the first edge along the width direction of the positive electrode sheet. A second edge coating is provided on the surface of the positive electrode current collector between the positive electrode active material layer and the second edge. The second edge coating includes a second insulating material and a third binder. The third binder includes modified polyimide and polyvinylidene fluoride. The modified polyimide contains polar groups, including at least one of cyano, carboxyl, amino, sulfonic acid, or phosphate groups. The mass ratio of modified polyimide to polyvinylidene fluoride is 1:(0.1 to 5). By providing a second edge coating in the positive electrode sheet and adding the aforementioned type of third binder to the second edge coating, the climbing phenomenon in the positive electrode sheet is improved, and the interfacial bonding force between the second insulating coating and the positive electrode current collector is enhanced, resulting in a secondary battery with higher capacity and better safety performance.

[0019] A second aspect of this application provides a secondary battery comprising the positive electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has high capacity and good safety performance.

[0020] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0021] The beneficial effects of the embodiments of this application are as follows:

[0022] This application provides a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet incorporates a first edge coating containing modified polyimide and polyvinylidene fluoride (PVDF). The molar ratio of the polyimide main chain to polar groups and the mass ratio of modified polyimide to PVDF are controlled within the range specified in this application. PVDF serves as a toughening phase; its flexible CC segments enhance the flexibility of the first edge coating, mitigating the impact of the rigidity of the modified polyimide on the brittleness of the first edge coating. The modified polyimide, through its rigid segments, inhibits interfacial penetration between the first edge coating and the positive electrode active material layer, thereby improving the positive electrode's performance. The improvement of the slope phenomenon in the electrode sheet allows for greater utilization of the capacity of the positive active material in the positive electrode active material layer. Modified polyimide possesses excellent adhesion, enhancing the interfacial bonding between the first insulating coating and the positive current collector. This addresses the issue of insufficient adhesion of the first edge coating after electrolyte immersion, reducing the probability of the first edge coating detaching from the positive current collector surface. Furthermore, the first edge coating also exhibits good insulation and hardness, reducing the probability of burrs on the negative electrode sheet puncturing the separator and contacting the positive electrode sheet, thus mitigating the risk of short circuits in the secondary battery. Consequently, the capacity and safety performance of the secondary battery can be improved.

[0023] 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

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0025] Figure 1 This is a schematic diagram of the positive electrode sheet along its width and length directions in some embodiments of this application;

[0026] Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure along the AA direction;

[0027] Figure 3 This is a schematic diagram of the positive electrode sheet along its width and length directions in some other embodiments of this application;

[0028] Figure 4 This is a schematic diagram of the positive electrode sheet along its width and length directions, representing another embodiment of this application.

[0029] Figure label:

[0030] 10-Positive electrode sheet; 20-Positive current collector; 21-Positive electrode tab; 30-Positive active material layer; 40-First edge; 50-First edge coating; 60-Mixed layer; 70-Second edge; 80-Second edge coating. Detailed Implementation

[0031] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only 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.

[0032] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0033] A first aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Along the width direction of the positive electrode sheet, the positive current collector has a first edge, and a first edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge. The first edge coating comprises a first insulating material and a first adhesive. The first adhesive comprises modified polyimide and polyvinylidene fluoride (PVDF). The modified polyimide is obtained by modifying the polyimide with an acrylate monomer. The modified polyimide contains polar groups, including at least one selected from cyano, carboxyl, amino, sulfonic acid, or phosphate groups. Further, the polar groups include at least one selected from cyano or carboxyl groups. The molar ratio of the polyimide backbone to the polar groups is 1:(0.1 to 0.6). The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.1 to 5).

[0034] For ease of understanding, in this application, a three-dimensional Cartesian coordinate system is established with the length direction of the positive electrode sheet in its unfolded state as X, the width direction as Y, and the thickness direction as Z. It should be understood that the above definitions of directions are for the purpose of conveniently describing this application, and the directions defined in this application can be understood based on the relative positions of the elements in the accompanying drawings and the actual product. It is understood that the length direction, width direction, and thickness direction of the positive current collector, the positive active material layer, the first edge coating, and the second edge coating are the same as those of the positive electrode sheet. The aforementioned "positive active material layer disposed on at least one surface of the positive current collector" means that the positive active material layer can be disposed on one or both surfaces of the positive current collector, and the aforementioned surfaces can be a partial or complete area of ​​the surface of the positive current collector. Figures 1 to 3As shown, the positive electrode sheet 10 includes a positive current collector 20 and a positive active material layer 30. The positive active material layer 30 is disposed on two surfaces of the positive current collector 20. The positive current collector 20 has a first edge 40, and a plurality of positive electrode tabs 21 extend from one side of the first edge 40. The positive electrode tabs 21 can be directly die-cut from the empty foil area of ​​the positive current collector 20. A first edge coating 50 is disposed on the surface of the positive current collector 20 between the positive active material layer 30 and the first edge 40. It should be noted that, due to Figure 1 This is a schematic diagram of the structure of the positive electrode 10 along its length direction X and width direction Y. Therefore, only the positive electrode active material layer 30 and the first edge coating 50 on one surface of the positive electrode current collector 20 are shown. Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure along the AA direction. Figure 2 The diagram shows a schematic representation of the positive electrode 10 along its length direction X and thickness direction Z. Figure 2 The image shows a first edge coating 50 disposed on both surfaces of the positive electrode current collector 20, therefore, it can be understood that... Figure 1 The positive electrode current collector 20 in the positive electrode sheet 10 is also provided with a positive electrode active material layer 30 on another surface.

[0035] For example, the molar ratio of the polyimide backbone to the polar groups is 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, or any ratio within any two of the above ranges. For example, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:0.1, 1:0.3, 1:0.5, 1:1.0, 1:1.2, 1:1.5, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3.0, 1:3.3, 1:3.5, 1:4.0, 1:4.1, 1:4.3, 1:4.7, 1:5.0, or any ratio within any two of the above ranges. The PVDF backbone is composed of carbon-carbon (CC) single bonds. This structure has a high degree of spatial freedom, allowing PVDF molecular segments to move and diffuse relatively freely. Especially under high-temperature drying conditions, the chain segment movement intensifies, and PVDF molecular segments migrate more easily into the positive electrode active material layer. This leads to a blurred interface between the PVDF-containing edge coating and the positive electrode active material layer, causing a creeping phenomenon. Polyimide (PI) backbone contains a rigid imide ring structure. This ring structure restricts the degree of freedom of movement of PI molecular segments, allowing PI to maintain a stable morphology during coating and drying. This helps prevent PI from penetrating into the positive electrode active material layer, thus preventing creeping. Furthermore, the rigid imide ring structure of PI provides high adhesion, but its high modulus can increase the brittleness of the first edge coating. Although PVDF molecules contain fluorine atoms (F), their electronegativity is high, and they only rely on van der Waals forces to bind with the positive electrode current collector. Introducing polar groups into the polyimide backbone can improve the interfacial adhesion between the modified polyimide and the positive electrode current collector (such as aluminum foil). For example, when the polar group is a carboxyl group, the carboxyl group can form a stable covalent bond with the hydroxyl groups on the surface of the positive electrode current collector through esterification; when the polar group is a cyano group, the cyano group can bind with cations (such as Al) on the surface of the positive electrode current collector. 3+Coordination occurs, forming a stable metal-ligand complex. The bonding strength is significantly higher than the van der Waals forces between PVDF and the positive electrode current collector. Therefore, the use of modified polyimide can improve the interfacial adhesion between the first edge coating and the positive electrode current collector. When the molar ratio of the polyimide backbone to polar groups is greater than 1:0.1, the molar amount of polar groups is too small, the effect of the polar groups is not significant, and the improvement in interfacial adhesion between the modified polyimide and the positive electrode current collector is not obvious. When the molar ratio of the polyimide backbone to polar groups is less than 1:0.6, the content of polar groups is too high, which leads to poor structural stability of the first binder, deterioration of the bonding performance of the first binder, and an increase in electrochemical side reactions in the secondary battery, affecting the performance of the secondary battery. If the mass ratio of modified polyimide to polyvinylidene fluoride is greater than 1:0.1, the content of modified polyimide is too high. The higher modulus of modified polyimide can easily lead to increased brittleness of the first edge coating, affecting the flexibility of the positive electrode sheet in processes such as winding and bending. If the mass ratio of modified polyimide to polyvinylidene fluoride is less than 1:5.0, the content of polyvinylidene fluoride is too high. The first edge coating slurry diffuses more into the positive electrode active material layer, blurring the interface between the first edge coating and the positive electrode active material layer and causing a severe ramping phenomenon. The capacity of the positive electrode active material in the part of the positive electrode active material layer where the ramping phenomenon occurs cannot be utilized, which will reduce the capacity of the secondary battery.

[0036] Overall, the positive electrode of this application incorporates a first edge coating containing modified polyimide and polyvinylidene fluoride (PVDF). The molar ratio of the polyimide main chain to polar groups and the mass ratio of modified polyimide to PVDF are controlled within the range specified in this application. PVDF, as a toughening phase, uses its flexible CC segments to enhance the flexibility of the first edge coating, thus mitigating the impact of the rigidity of the modified polyimide on the brittleness of the first edge coating. The modified polyimide, through its rigid segments, inhibits interfacial penetration between the first edge coating and the positive electrode active material layer, thereby improving the climbing behavior in the positive electrode. For example, the improved climbing effect allows for greater utilization of the capacity of the positive electrode active material in the positive electrode active material layer; the modified polyimide has good adhesion, which improves the interfacial bonding force between the first insulating coating and the positive electrode current collector. This addresses the issue of insufficient adhesion of the first edge coating after immersion in electrolyte, reducing the probability of the first edge coating falling off the surface of the positive electrode current collector; the first edge coating also possesses good insulation and hardness, reducing the probability of burrs on the negative electrode sheet puncturing the separator and contacting the positive electrode sheet, thus reducing the risk of short circuits in the secondary battery. Therefore, the capacity and safety performance of the secondary battery can be improved.

[0037] This application does not impose any particular restrictions on the method of controlling the molar ratio of the polyimide backbone to the polar groups, as long as the purpose of this application can be achieved. For example, this can be achieved by controlling the amount of reactants containing polar groups added during the preparation of the modified polyimide. This application also does not impose any particular restrictions on the method of controlling the mass ratio of the modified polyimide to polyvinylidene fluoride, as long as the purpose of this application can be achieved. For example, this can be achieved by controlling the mass of the modified polyimide or polyvinylidene fluoride. The type of polar groups in the modified polyimide in this application can be achieved by controlling the type of compounds containing polar groups during the preparation of the modified polyimide. The modified polyimide and polyvinylidene fluoride in this application can also be obtained by purchasing commercially available products that meet the requirements of this application, as long as the purpose of this application can be achieved.

[0038] This application does not impose any particular limitation on the type of acrylate monomer, as long as it can achieve the purpose of this application. For example, the acrylate monomer includes, but is not limited to, at least one of butyl acrylate, isobutyl acrylate, or cyclohexyl acrylate.

[0039] In some embodiments of this application, the mass percentage of the first binder is 10% to 20% based on the mass of the first edge coating. For example, the mass percentage of the first binder is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value between any two of the above ranges. By controlling the mass percentage of the first binder within the above range, it is beneficial to ensure that the first edge coating has good adhesion. This results in good interfacial bonding between the first edge coating and the positive current collector, improving the problem of insufficient adhesion after the first edge coating is soaked in electrolyte. The probability of the first edge coating falling off the surface of the positive current collector is reduced, and the first edge also has better flexibility. Therefore, the secondary battery using the positive electrode sheet has good safety performance while maintaining a high capacity.

[0040] In some embodiments of this application, the mass percentage of the first adhesive is 10% to 15% based on the mass of the first edge coating. For example, the mass percentage of the first adhesive is 10%, 11%, 12%, 13%, 14%, 15%, or any value between any two of the above ranges. By controlling the mass percentage of the first adhesive within the above range, the content of the first insulating material in the first edge coating is further increased. This is beneficial for the first edge coating to have good adhesion while also having lower stiffness, and it can also improve the hardness and insulation performance of the first edge coating, thereby reducing the probability of the negative electrode burrs piercing the separator and contacting the positive electrode, thus reducing the risk of short circuit in the secondary battery. This results in a secondary battery with higher capacity and better safety performance.

[0041] In some embodiments of this application, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(0.5 to 3). For example, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:0.5, 1:1.0, 1:1.2, 1:1.5, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3.0, or any ratio within any two of the above ranges. By controlling the mass ratio of modified polyimide to polyvinylidene fluoride within the above ranges, the rigidity of the modified polyimide and the flexibility of the polyvinylidene fluoride can be better matched, thereby further reducing the ramp-up phenomenon in the positive electrode sheet and allowing the capacity of the positive electrode active material layer to be fully utilized. This results in a further increase in the capacity of the secondary battery while maintaining good safety performance.

[0042] In some embodiments of this application, the molecular weight of the modified polyimide is from 50,000 g / mol to 500,000 g / mol. For example, the molecular weight of the modified polyimide is 50,000 g / mol, 70,000 g / mol, 100,000 g / mol, 150,000 g / mol, 180,000 g / mol, 210,000 g / mol, 250,000 g / mol, 300,000 g / mol, 340,000 g / mol, 370,000 g / mol, 400,000 g / mol, 420,000 g / mol, 460,000 g / mol, 500,000 g / mol, or any value between any two of the above ranges. By controlling the molecular weight of the modified polyimide within the aforementioned range, the modified polyimide can provide suitable rigidity and adhesion to suppress interfacial penetration between the first edge coating and the positive electrode active material layer, reduce the ramping phenomenon in the positive electrode sheet, and also enable good interfacial bonding between the first edge coating and the positive electrode current collector, thus improving the problem of insufficient adhesion of the first edge coating after soaking in electrolyte. This is beneficial for improving the capacity and safety performance of the secondary battery.

[0043] In this application, the molecular weight of the modified polyimide refers to the weight-average molecular weight.

[0044] This application does not impose any particular restrictions on the method of controlling the molecular weight of the modified polyimide, as long as it achieves the purpose of this application. For example, it can be achieved by adjusting the addition ratio of monomers that provide flexible segments, such as acrylate monomers, during the preparation of the modified polyimide, or by purchasing commercially available modified polyimides with molecular weights that meet the requirements of this application.

[0045] This application does not impose any particular limitation on the molecular weight of polyvinylidene fluoride (PVDF). Conventional PVDF in the art can be used, as long as it achieves the purpose of this application. For example, the weight-average molecular weight of PVDF is 50W to 80W.

[0046] In some embodiments of this application, the thickness of the first edge coating is 10 μm to 50 μm. For example... Figure 2 As shown, the thickness of the first edge coating 50 is T 50 As shown. For example, the thickness of the first edge coating is 10μm, 12μm, 16μm, 20μm, 25μm, 27μm, 30μm, 35μm, 40μm, 42μm, 46μm, 50μm, or any value between any two of the above ranges. By controlling the thickness of the first edge coating within the above range, the probability of stress concentration on the positive electrode current collector caused by the pressure on the first insulating material during the cold pressing of the positive electrode sheet, leading to the breakage of the positive electrode sheet, is reduced. It also enables the positive electrode sheet to have a continuous and dense insulating barrier, reducing the probability of microcracks generated during the cutting or winding of the positive electrode sheet. It also reduces the risk of local short circuits or capacity decay caused by excessive electrolyte penetration leading to dissolution of the positive electrode active material and aggravation of interfacial side reactions (such as excessive gas production, thickening of the solid electrolyte interfacial film, etc.). It also reduces the risk of burrs being exposed on the positive electrode current collector, thereby reducing the risk of short circuits in the secondary battery caused by direct contact between the positive electrode sheet and the negative electrode sheet. This allows secondary batteries to improve their safety performance while increasing their capacity.

[0047] In some embodiments of this application, the width of the first edge coating is 2 mm to 5 mm along the width direction of the positive electrode sheet. For example... Figure 1 , Figure 3 and Figure 4 As shown, along the width direction Y of the positive electrode 10, the width of the first edge coating 50 is W... 50 As shown. For example, the width of the first edge coating is 2mm, 2.3mm, 3mm, 3.5mm, 4mm, 4.2mm, 4.6mm, 5mm, or any value between any two of the above ranges. Controlling the width of the first edge coating within the above range helps to maximize its function while reducing the width of the positive electrode active material layer. This improves the climbing phenomenon in the positive electrode sheet and addresses the problem of insufficient adhesion of the first edge coating after immersion in electrolyte, thereby improving the capacity and safety performance of the secondary battery.

[0048] In some embodiments of this application, the areal density of the first edge coating is 0.02 mg / cm³. 2 Up to 0.06 mg / cm 2 For example, the areal density of the first edge coating is 0.02 mg / cm³. 2 0.03 mg / cm 2 0.04 mg / cm 2 0.05 mg / cm 2 0.06 mg / cm 2Or any value between any two of the above-mentioned ranges. Controlling the areal density of the first edge coating within the above range is beneficial for forming a continuous and dense first edge coating on the surface of the positive electrode current collector, allowing the first edge coating to function effectively, improving the climbing phenomenon in the positive electrode sheet, increasing the interfacial bonding force between the first edge coating and the positive electrode current collector, giving the positive electrode sheet good insulation, and reducing the probability of microcracks during cutting or winding. This increases the content of positive active material in the positive active material layer that can contribute to the capacity, improves the adhesion of the first edge coating after soaking in the electrolyte, and reduces the risk of short circuits in the secondary battery due to direct contact between the positive and negative electrode sheets. Therefore, the secondary battery can have higher capacity and safety performance.

[0049] This application does not impose any particular restrictions on the method of controlling the areal density of the first edge coating, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the amount of the first edge coating slurry applied.

[0050] In some embodiments of this application, the areal density of the positive electrode active material layer is 0.1 mg / cm³. 2 Up to 0.3 mg / cm 2 For example, the areal density of the positive electrode active material layer is 0.1 mg / cm³. 2 0.13 mg / cm 2 0.17 mg / cm 2 0.2 mg / cm 2 0.22 mg / cm 2 0.26 mg / cm 2 0.3 mg / cm 2 Or any value between any two of the above ranges. Controlling the areal density of the positive electrode active material layer within the above range is beneficial for having a greater amount of positive electrode active material in the layer, thus resulting in a higher capacity. Therefore, the secondary battery can achieve a higher capacity while maintaining good safety performance.

[0051] This application does not impose any particular restrictions on the method of controlling the areal density of the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling the coating amount of the positive electrode active material layer slurry.

[0052] In some embodiments of this application, the first insulating material includes at least one selected from alumina, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide. Based on the quality of the first edge coating, the mass percentage of the first insulating material is 80% to 90%. For example, the mass percentage of the first insulating material is 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, or any value between any two of the above ranges. Selecting the above-mentioned types of first insulating materials and controlling the mass percentage of the first insulating material in the first edge coating within the above ranges is beneficial for the first insulating coating to have good insulation performance and hardness. This reduces the probability of microcracks forming on the positive electrode sheet during cutting or winding, and also reduces the risk of electrolyte penetration or burr exposure on the positive electrode current collector, thereby reducing the risk of short circuits in the secondary battery due to direct contact between the positive and negative electrode sheets. Therefore, the safety performance of the secondary battery can be improved while increasing its capacity.

[0053] In some embodiments of this application, the particle size Dv50 of the first insulating material is from 0.4 μm to 1.8 μm. For example, the particle size Dv50 of the first insulating material is 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, or any value between any two of the above ranges. Controlling the particle size Dv50 of the first insulating material within the above range is beneficial for ensuring that the first insulating material is uniformly distributed in the first edge coating, thereby improving the interfacial bonding force between the first edge coating and the positive electrode current collector, and mitigating the problem of insufficient adhesion of the first edge coating after immersion in electrolyte. This is beneficial for improving the safety performance of the secondary battery while maintaining a high capacity.

[0054] In this application, "the particle size Dv50 of the first insulating material" refers to the particle size that, starting from the smallest particle size, reaches 50% of the total volume in the particle size distribution of the first insulating material based on volume.

[0055] This application does not impose any particular restrictions on the method of controlling the particle size Dv50 of the first insulating material, as long as the purpose of this application can be achieved. For example, it can be controlled by crushing or sieving.

[0056] In some embodiments of this application, a mixed layer is formed between the positive electrode active material layer and the first edge coating along the width direction of the positive electrode sheet. The width of the mixed layer is less than or equal to 0.5 mm, preferably less than or equal to 0.2 mm. Figure 3As shown, along the width direction Y of the positive electrode 10, a mixing layer 60 is present between the positive electrode active material layer 30 and the first edge coating 50, and the width of the mixing layer 60 is W. 60 As shown. It should be noted that during the preparation of the positive electrode sheet 10, the slurry of the first edge coating 50 diffuses towards the positive active material layer 30, causing the first edge coating slurry to penetrate into the positive active material layer slurry, forming a region containing both the first edge coating slurry and the positive active material layer slurry. This region, after drying, becomes the mixed layer 60. The diffusion process of the first edge coating slurry towards the positive active material layer 30 is also called the "climbing phenomenon," and the region containing both the first edge coating slurry and the positive active material layer slurry, i.e., the mixed layer, can be understood as a "climbing shadow." For example, the width of the mixed layer can be 0 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, or any value between any two of the above ranges. A width within the above range indicates a smaller climbing shadow in the positive electrode sheet, which can improve the capacity of the secondary battery.

[0057] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, a second binder, and a conductive agent. Based on the mass of the positive electrode active material layer, the mass percentage of the second binder is between 1.5% and 3.0%. For example, the mass percentage of the second binder is 1.5%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.3%, 2.5%, 2.6%, 2.8%, 3.0%, or any value between any two of the above ranges. Controlling the mass percentage of the second binder within the above range is beneficial for achieving good interfacial bonding between the positive electrode active material layer and the positive electrode current collector without affecting the content of the positive electrode active material, thereby reducing the probability of the positive electrode active material layer detaching from the surface of the positive electrode current collector. This results in a secondary battery with higher capacity and better safety performance.

[0058] In this application, there are no particular limitations on the content of the aforementioned positive electrode active material and conductive agent, as long as the purpose of this application can be achieved. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode active material is 92% to 97.9%, and the mass percentage content of the conductive agent is 0.6% to 2.2%.

[0059] In some embodiments of this application, the positive electrode active material layer further includes a surfactant. This application does not impose any particular limitation on the type and content of the surfactant, as long as it achieves the purpose of this application. For example, the types of surfactant include, but are not limited to, at least one of sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), or sulfonates. Based on the mass of the positive electrode active material layer, the mass percentage of the surfactant is from 0% to 3.2%.

[0060] In some embodiments of this application, along the width direction of the positive electrode sheet, the positive current collector has a second edge opposite to the first edge, and a second edge coating is provided on the surface of the positive current collector between the positive electrode active material layer and the second edge. Further, at least one positive electrode tab extends from the second edge. Figure 4 As shown, the positive electrode sheet 10 includes a positive current collector 20 and a positive active material layer 30 disposed on the surface of the positive current collector 20. Along the width direction of the positive electrode sheet 10, the positive current collector 20 has a first edge 40 and a second edge 70 opposite to the first edge 40. A plurality of positive electrode tabs 21 extend from one side of the first edge 40, and a plurality of positive electrode tabs 21 also extend from one side of the second edge 70. The positive electrode tabs 21 can be directly die-cut from the empty foil area of ​​the positive current collector 20. A first edge coating 50 is disposed on the surface of the positive current collector 20 between the positive active material layer 30 and the first edge 40, and a second edge coating 80 is disposed on the surface of the positive current collector 20 between the positive active material layer 30 and the second edge 70. The second edge coating comprises a second insulating material and a third adhesive, the third adhesive comprising modified polyimide and polyvinylidene fluoride, the modified polyimide containing polar groups, including at least one selected from cyano, carboxyl, amino, sulfonic acid, or phosphate groups; the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(0.1 to 5). For example, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:0.1, 1:0.5, 1:1.0, 1:1.2, 1:1.5, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3.0, 1:3.3, 1:3.5, 1:3.7, 1:4.0, 1:4.2, 1:4.5, 1:4.8, 1:5.0, or any ratio within any two of the above ranges. A second edge coating is formed in the positive electrode sheet, and the aforementioned type of third binder is added to the second edge coating. The modified polyimide, through its rigid chain segments, inhibits interfacial penetration between the first edge coating and the positive electrode active material layer, thereby improving the ramp-up phenomenon in the positive electrode sheet. This improved ramp-up phenomenon allows the capacity of the positive electrode active material in the positive electrode active material layer to be fully utilized. The modified polyimide also possesses good adhesion, enhancing the interfacial bonding force between the second insulating coating and the positive electrode current collector. This improves the problem of insufficient adhesion after the second edge coating has been soaked in electrolyte, reducing the probability of the second edge coating falling off the surface of the positive electrode current collector. Furthermore, the second edge coating also exhibits good insulation and hardness, reducing the probability of burrs on the negative electrode sheet puncturing the separator and contacting the positive electrode sheet, thus reducing the risk of short circuits in the secondary battery. Therefore, the secondary battery can possess high capacity and good safety performance.

[0061] This application does not impose any particular limitation on the types of the second insulating material and the third adhesive, as long as the purpose of this application can be achieved. In some embodiments of this application, the second insulating material may be the same as or different from the first insulating material, and the third adhesive may be the same as or different from the first adhesive. For example, the second insulating material may be selected from the first insulating material. This application does not impose any particular limitation on the content of the second insulating material and the third adhesive, as long as the purpose of this application can be achieved. For example, based on the quality of the second edge coating, the mass percentage content of the second insulating material is 80% to 90%, and the mass percentage content of the third adhesive is 10% to 20%.

[0062] This application does not impose any particular restrictions on the types of positive electrode active materials, second binders, and conductive agents, as long as they 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. 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. 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 polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. For example, the second binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0063] 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).

[0064] This application does not impose any particular limitations on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the positive electrode active material layer can be 30 μm to 120 μm.

[0065] This application does not impose any particular restrictions on the preparation method of modified polyimide. It can be prepared by means known to those skilled in the art, as long as the purpose of this application can be achieved. For example, in some embodiments, the preparation method of modified polyimide includes the following steps: (1) under nitrogen protection in anhydrous N-methylpyrrolidone (NMP) solvent, dianhydride monomer and diamine monomer are reacted at 0°C to 5°C for 5.5h to 6.5h to generate polyamic acid (PAA) solution; (2) acrylate monomer and initiator are added to PAA solution to introduce flexible segments, and then a compound containing polar groups and initiator are added, and polar groups are grafted at 55°C to 65°C for 2.5h to 3.5h to obtain a glue solution after the reaction; (3) the glue solution is heated to 50°C to 60°C, and a dehydrating agent and catalyst are slowly added dropwise, and the reaction is stirred for 1.5h to 2.5h to achieve an imidization degree of 70% to 80%; (4) the product is filtered through a 5μm filter screen, and the output is modified PI glue solution, which is the modified polyimide. This application does not have any particular restrictions on the type of dianhydride monomer, as long as it can achieve the purpose of this application. For example, dianhydride monomers include, but are not limited to, pyromellitic dianhydride (PMDA). This application does not impose any particular limitation on the type of diamine monomer, as long as it achieves the purpose of this application. For example, diamine monomers include, but are not limited to, 4,4'-diaminodiphenyl ether. This application does not impose any particular limitation on the type of compound containing a polar group, as long as it achieves the purpose of this application. For example, compounds containing a polar group include, but are not limited to, at least one of acrylonitrile, dimethylaminoethyl acrylate, acrylic acid, or 2-acrylamido-2-methylpropanesulfonic acid. This application does not impose any particular limitation on the type of initiator in step (2) above, as long as it achieves the purpose of this application. For example, initiators include, but are not limited to, azobisisobutyronitrile (AIBN). This application does not impose any particular limitation on the ratio of the adhesive obtained in step (2) above to the dehydrating agent, as long as it achieves the purpose of this application. For example, the molar ratio of the unclosed-ring ammonium acid unit in the adhesive obtained in step (2) to the dehydrating agent in step (3) is 1:(1 to 2). This application does not impose any particular restrictions on the types of dehydrating agent and catalyst used in step (3) above, as long as they can achieve the purpose of this application. For example, dehydrating agents include, but are not limited to, acetic anhydride, and catalysts include, but are not limited to, triethylamine. This application does not impose any particular restrictions on the molar ratio of dehydrating agent and catalyst in step (3), as long as they can achieve the purpose of this application. For example, the molar ratio of dehydrating agent to catalyst is (1.5 to 2.5):1.

[0066] For example, in some embodiments, the modified polyimide has the following structural formula:

[0067]

[0068] R1 and R2 are each independently selected from cyano, carboxyl, amino, sulfonic acid, and phosphate groups; Ar1 ​​and Ar2 are each independently selected from dianhydride subunit and diamine subunit; m is 90 to 300; n is 90 to 300.

[0069] This application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the positive electrode sheet includes the following steps: (1) preparing a first edge coating slurry and a positive active material layer slurry; (2) coating the first edge coating slurry and the positive active material layer slurry on one surface of the positive current collector along the width direction of the positive electrode sheet, wherein the first edge coating slurry is dried, cold-pressed and cut on the side close to the edge of the positive current collector to form a positive electrode sheet with a positive active material layer and a first edge coating on one side; (3) optionally, repeating step (2) on the other surface of the positive current collector to obtain a positive electrode sheet with a positive active material layer and a first edge coating on both sides. In some other embodiments, the preparation method of the positive electrode sheet includes the following steps: (1) preparing a first edge coating slurry, a second edge coating slurry, and a positive active material layer slurry; (2) coating the first edge coating slurry, the positive active material layer slurry, and the second edge coating slurry on one surface of the positive current collector along the width direction of the positive electrode sheet, wherein the positive active material layer slurry is located between the first edge coating slurry and the second edge coating slurry, and then drying, cold pressing, and cutting to form a positive electrode sheet with the first edge coating, the positive active material layer, and the second edge coating on one side; (3) optionally, repeating step (2) on the other surface of the positive current collector to obtain a positive electrode sheet with the first edge coating, the positive active material layer, and the second edge coating on both sides. This application does not impose any particular restrictions on the above-mentioned drying and cold pressing process parameters, which can be selected by those skilled in the art according to actual conditions, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the solid content of the above-mentioned first edge coating slurry, the positive active material layer slurry, and the second edge coating slurry, as long as the purpose of this application can be achieved. For example, the solid content of the first edge coating slurry and the second edge coating slurry is 20 wt% to 40 wt%, and the solid content of the positive electrode active material layer slurry is 50 wt% to 80 wt%.

[0070] A second aspect of this application provides a secondary battery comprising the positive electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has high capacity and good safety performance.

[0071] In this application, the secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode active 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 of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. Exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc. The negative electrode active 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 microcarbon 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. In some embodiments of this application, the negative electrode active material layer may further include a conductive agent and a fourth binder. This application does not particularly limit the types of conductive agents and fourth binders, as long as they can achieve the purpose of this application. For example, the conductive agent may be selected from the aforementioned conductive agents. For example, the fourth binder may be selected from the aforementioned second binder. This application does not particularly limit the mass ratio of negative electrode active material, conductive agent, and fourth binder in the negative electrode active material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. This application does not particularly limit the thickness of the negative electrode active material layer, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode active material layer is 30 μm to 120 μm. This application does not particularly limit the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0072] In this application, the secondary battery also includes an electrolyte, which comprises lithium salts and non-aqueous solvents. 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 lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. 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). Fluorinated carbonate 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. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 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.

[0073] The secondary battery of this application 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 or rigid plastic. This application does not limit the type of metal; metal casings known in the art can be used, 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.

[0074] 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.

[0075] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has excellent performance.

[0076] 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 input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0077] Example

[0078] 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.

[0079] Test methods and equipment:

[0080] Sampling method for positive electrode:

[0081] After discharging the lithium-ion batteries of each embodiment and comparative example to 3.0V at a constant current of 0.2C, the lithium-ion batteries were disassembled and the positive electrode sheet was removed. The positive electrode sheet was immersed in dimethyl carbonate (DMC) for 20 minutes, and then rinsed with dimethyl carbonate and acetone respectively. After that, the positive electrode sheet was placed in an oven and baked at 80°C for 12 hours to obtain the dried positive electrode sheet. Unless otherwise specified, the following tests all used positive electrode sheets obtained by the above method.

[0082] Test of the mass ratio of modified polyimide to polyvinylidene fluoride:

[0083] The positive electrode sheet was cut to obtain the portion coated with the first edge coating. This portion was immersed in NMP solution and ultrasonically treated at 80℃ for 1 hour to dissolve PVDF and modified PI. The first insulating material was separated by centrifugal filtration (0.1μm filter membrane), and the filtrate was collected. Deionized water (volume ratio, NMP:deionized water = 1:4) was slowly added to the filtrate, and the mixture was stirred for 10 minutes. PVDF dissolved in the NMP / water mixed solvent, and modified PI precipitated out. The PI was then vacuum dried at 60℃ for 24 hours to obtain pure modified polyimide powder. The mass of the modified polyimide was measured. The supernatant was rotary evaporated to dryness at 80℃ to obtain solid PVDF. This solid PVDF was then vacuum dried at 60℃ for 24 hours, and the mass of the PVDF was measured. The mass ratio of modified polyimide to PVDF is the mass ratio of the two.

[0084] Test for first adhesive content:

[0085] The positive electrode sheet is cut to obtain the area coated with the first edge coating. The first edge coating is dissolved in N-methylpyrrolidone. The mixture of the first insulating material and the first adhesive is obtained by centrifugation. The weight loss ratio at 400℃ to 700℃ is obtained by testing with a synchronous thermal analyzer (instrument model STA449F3, test temperature: 25℃ to 700℃, heating rate: 10℃ / min, test atmosphere: nitrogen). The mass percentage content of the first adhesive is obtained by analysis.

[0086] Test for the second adhesive content:

[0087] The positive electrode sheet is cut to obtain the area coated with the positive electrode active material layer. The positive electrode active material layer is dissolved in N-methylpyrrolidone. The mixture of conductive agent and second binder is obtained by centrifugation. The weight loss ratio at 300℃ to 600℃ is obtained by testing with a synchronous thermal analyzer (instrument model STA449F3, test temperature: 25℃ to 600℃, temperature rise: 10℃ / min, test atmosphere: nitrogen). The mass percentage content of the second binder is obtained by analysis.

[0088] Molecular weight testing of modified polyimide:

[0089] The positive electrode sheet was cut to obtain the portion coated with the first edge coating. This portion was immersed in NMP solution and ultrasonically treated at 80℃ for 1 hour to dissolve PVDF and modified PI. The first insulating material was separated by centrifugation and filtration (0.1μm filter membrane). The filtrate was collected, and deionized water (volume ratio, NMP:deionized water = 1:4) was slowly added to the filtrate. After stirring for 10 minutes, PVDF dissolved in the NMP / water mixed solvent, and modified PI precipitated out. The precipitate was then dried under vacuum at 60℃ for 24 hours to obtain pure modified PI powder. Referring to the national standard GB / T21863-2008, gel permeation chromatography (GPC) was used with tetrahydrofuran as the eluent; the molecular weight of the modified polyimide was determined by gel permeation chromatography (instrument model: ACQUITY APC). The molecular weight in this application refers to the weight-average molecular weight.

[0090] Test of the thickness of the first edge coating:

[0091] The area of ​​the positive electrode sheet coated with the first edge coating was argon-ion polished along both the width and thickness directions of the positive electrode sheet to obtain a cross-section of the first edge coating. The morphology of the cross-section of the first edge coating along the thickness direction was observed and scanned electron microscopy (SEM) images were taken using a field emission scanning electron microscope (Philips XL-30). The thickness of the first edge coating was measured using SEM. The above measurements were performed at three random locations on the first edge coating, and the average value was taken as the thickness of the first edge coating.

[0092] Test of the width of the first edge coating:

[0093] Argon ion polishing was performed on the area of ​​the positive electrode sheet coated with the first edge coating along the width and thickness directions of the positive electrode sheet to obtain the cross section of the first edge coating. The morphology of the cross section of the first edge coating along the thickness direction was observed and scanned electron micrographs were taken using a field emission scanning electron microscope (Philips, XL-30). The width of the first edge coating was measured by scanning electron microscopy.

[0094] Testing the width of the blending layer:

[0095] Argon ion polishing was performed on the positive electrode sheet along its width and thickness directions to obtain a cross section. The morphology of the cross section of the mixed layer along the thickness direction of the overlapping area of ​​the first edge coating and the positive electrode active material layer was observed and photographed using a field emission scanning electron microscope (Philips, XL-30). The width of the mixed layer was measured by scanning electron microscopy.

[0096] Areal density test:

[0097] (1) Test of the surface density of the first edge coating:

[0098] After punching and breaking the area of ​​the positive electrode sheet coated with the first edge coating into a small circular piece with radius R, the weight is measured as m1. The first edge coating is scraped off from both surfaces of the small circular piece, and the weight of the positive current collector is measured as m2. Then, the areal density of the first edge coating = (m1-m2) / (2πR) 2 (Unit: mg / cm³) 2 .

[0099] (2) Testing of the layer density of the positive electrode active material:

[0100] After punching and breaking the area of ​​the positive electrode sheet coated with the positive electrode active material layer into a small circular piece with radius R, the weight is measured as m1. The positive electrode active material layer is scraped off from both surfaces of the small circular piece, and the weight of the positive electrode current collector is measured as m2. Therefore, the areal density of the positive electrode active material layer = (m1 - m2) / (2πR) 2 (Unit: mg / cm³) 2 .

[0101] Test of particle size Dv50 of the first insulating material:

[0102] The positive electrode sheet is cut to obtain the part coated with the first edge coating. This part is immersed in NMP solution and ultrasonically treated at 80°C for 1 hour to dissolve PVDF and modified PI. The first insulating material is separated by centrifugal filtration (0.1μm filter membrane), the filter residue is collected, and the filter residue is dried to obtain the first insulating material.

[0103] The particle size distribution was tested using a MasterSizer 2000 laser particle size analyzer, and the volume average particle size of the first insulating material was obtained.

[0104] Test of the adhesion of the first edge coating:

[0105] The adhesion between the first edge coating and the positive current collector was measured by a 180° peel test. For a positive electrode sheet with a first edge coating and a positive active material layer coated on both sides, the first edge coating and the positive active material layer on one surface of the positive current collector were scraped off.

[0106] (1) Test of adhesion force F1 before immersion:

[0107] (a) After cutting the positive electrode sheet into 20mm×80mm samples, place them in a sample container, add electrolyte until the sample is completely submerged, place the sample container in an 85℃ vacuum oven for 24 hours, and then take out the sample and dry it.

[0108] (b) Attach a 20mm × 80mm double-sided adhesive tape (NITTO.NO5000NS) to a steel plate, then attach the sample to the tape with the positive current collector facing down. Connect a 20mm × 90mm paper strip to the sample using the double-sided adhesive tape, adhering it to the surface of the first edge coating away from the positive current collector. Roll the test strip 8 times with a 2kg roller to obtain the test sample. Perform the test using a tensile testing machine. Fix the test sample on the test table, fold the paper strip 180° upwards, and secure it with a clamp. Then, start pulling the paper strip at a speed of 50mm / min until the first edge coating on the double-sided adhesive surface separates from the positive current collector, ending the test and saving the test data. Calculate the adhesion force between the first edge coating and the positive current collector based on the tensile force and displacement during separation, in N / m.

[0109] (2) Test of adhesion force F2 after soaking:

[0110] After cutting the positive electrode sheet into 20mm × 80mm samples, they were placed in a sample container. Electrolyte was added until the samples were completely submerged. The sample container was then placed in a vacuum oven at 85℃ for 24 hours. After removing the samples and drying them, step (b) was performed as described above to obtain the adhesion force between the first edge coating and the positive current collector after immersion, expressed in N / m. The electrolyte used was the electrolyte from Example 1-1.

[0111] Stiffness test:

[0112] After peeling or scraping the first edge coating from the positive current collector, the first insulating coating and NMP are dispersed in a mass ratio of 1:3 using a single-bar disperser for 3 hours until completely dissolved and uniformly dispersed. The resulting slurry is then coated onto a 10μm aluminum foil using a flatbed coater with a 300μm blade. After drying in an oven at 120℃ for 30 minutes, an electrode sheet is formed. Samples with a length × width × thickness of 80mm × 38mm × 0.03mm are cut from the electrode sheet using a sampling machine. A stiffness tester (Guangdong Beidou Precision Instruments Co., Ltd., PT-208) is used, with the sample bending length set to 50mm. The stiffness of the sample is measured when it is bent from 0° to 90°. Five parallel samples are measured in each group, and the average value is taken as the final stiffness value.

[0113] Capacity testing:

[0114] The lithium-ion batteries of each embodiment and comparative example were charged at 25°C with a constant current of 0.3C to 3.6V, and then charged with a constant voltage of 3.6V to 0.05C; left to stand for 10 minutes; discharged at a constant current of 0.3C to 2.5V; the charge-discharge cycle was repeated 3 times according to the above procedure, and left to stand for 30 minutes; during discharge, the equipment (name: lithium battery charge-discharge test equipment, manufacturer: Shenzhen Xinwei New Energy Technology Co., Ltd.) calculated the capacity of the lithium-ion battery by monitoring the discharge current and time.

[0115] Safety performance testing:

[0116] The lithium-ion batteries in each embodiment and comparative example were subjected to charge-discharge cycle tests in a 60°C constant temperature chamber. The charge-discharge voltage range was 2.5V to 3.6V. The batteries were charged at a constant current of 0.5C to 3.6V, then charged at a constant voltage of 3.6V to 0.05C and allowed to rest for 5 minutes. Finally, they were discharged at a constant current of 0.5C to 2.5V. This charge-discharge cycle was repeated 200 times. The lithium-ion batteries were disassembled, and the positive electrode was removed to check for peeling of the first edge coating. If the coating did not peel off, the battery passed; if it peeled off completely or partially, the battery failed. 100 lithium-ion batteries were tested for each embodiment or comparative example. The pass rate (%) was calculated as: (Number of passing batteries / 100) × 100%.

[0117] Example 1-1

[0118] <Preparation of the positive electrode>

[0119] The positive electrode active material is lithium iron phosphate, the conductive agent is Super P, and the second binder is PVDF (weight average molecular weight is 7×10). 6 The mixture of sodium polyacrylate and surfactant in a mass ratio of 97:0.9:1.9:0.2 was dispersed in N-methylpyrrolidone (NMP) and stirred under vacuum to obtain a positive electrode slurry with a solid content of 75wt% and a homogeneous system.

[0120] Preparation of modified polyimide: (1) In anhydrous NMP solvent, under nitrogen protection, dianhydride monomer PMDA and diamine monomer ODA were reacted at 5°C for 6h to generate polyamic acid (PAA) solution, wherein the molar ratio of dianhydride monomer to diamine monomer was 1:1; (2) Butyl acrylate and initiator AIBN were added to PAA solution, followed by cyano-containing acrylonitrile, carboxyl-containing acrylic acid and secondary AIBN, and the reaction was carried out at 60°C for 3h to graft polar groups, and the reaction was carried out to obtain a glue solution; (3) The glue solution was heated to 60°C, and the dehydrating agent acetic anhydride and the catalyst triethylamine were slowly added dropwise (molar ratio, acetic anhydride:triethylamine = 2:1, acetic anhydride:unclosed amic acid unit in glue solution = 1.5:1), and the reaction was stirred for 2h; (4) The modified polyimide was obtained by filtering through a 5μm filter. The modified polyimide described above contains polar cyano and carboxyl groups, with a molar ratio of cyano to carboxyl groups of 1:1. The molar ratio of the polyimide backbone to the polar groups in the modified polyimide is 1:0.3. The molecular weight of the modified polyimide is 300,000 g / mol.

[0121] After uniformly mixing the first insulating material and the first binder, the mixture was dispersed in NMP and stirred until homogeneous to obtain a first edge coating slurry with a solid content of 30 wt% and a viscosity of 10158.7 mPa. Based on the mass of the first edge coating, the mass percentage of the first insulating material W1 = 88%, and the mass percentage of the first binder W2 = 12%. The first insulating material was selected from boehmite with a particle size Dv50 = 1.1 μm, and the first binder consisted of modified polyimide and polyvinylidene fluoride (weight average molecular weight 65W), with a mass ratio of modified polyimide to polyvinylidene fluoride of 1:3.

[0122] The positive electrode slurry and the first edge coating slurry are uniformly coated on one surface of a 10 μm thick aluminum foil used as a positive electrode current collector, and then dried at 120°C to obtain a semi-finished positive electrode sheet with a single-sided coating of positive active material layer and first edge coating. The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive active material layer and first edge coating (see structure). Figure 1 But not with Figure 1 (For limitations). After cold pressing, cutting, and welding of tabs, a positive electrode sheet with dimensions of 74mm × 867mm is obtained for use. The tabs are welded to the side with the first edge coating, the thickness of which is T. 50 =20μm, width W 50 =3.5mm. The areal density of the first edge coating is 0.04mg / cm³. 2 The areal density of the positive electrode active material layer is 0.2 mg / cm³. 2 .

[0123] <Preparation of Negative Electrode Sheets>

[0124] Artificial graphite (negative electrode active material), styrene-butadiene rubber (fourth binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97.4:1.4:1.2. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120 °C to obtain a negative electrode sheet with a single-sided coating of negative electrode active material layer. The coating weight of the negative electrode active material layer was 142 mg / 1540 mm². 2 Then, repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode active material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with a size of 78mm×875mm for later use.

[0125] <Preparation of Electrolyte>

[0126] In an environment with a water content of less than 10 ppm, ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a mass ratio of 40:40:20 to obtain an organic solvent. Then, lithium salt LiPF6 was added to the organic solvent and mixed evenly to obtain an electrolyte with a lithium salt concentration of 1.17 mol / L.

[0127] <Septum>

[0128] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0129] <Preparation of Lithium-ion Batteries>

[0130] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0131] Examples 1-2 to Examples 1-13

[0132] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0133] In Examples 1-1 to 1-5, the change in the mass ratio of modified polyimide to polyvinylidene fluoride was achieved by adjusting the mass of polyvinylidene fluoride.

[0134] Examples 2-1 to 2-6

[0135] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0136] Examples 3-1 to 3-14

[0137] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Examples 1-1.

[0138] In Examples 3-11 to 3-14, when the mass percentage of the second binder changes, the mass percentage of the positive electrode active material changes accordingly, while the mass percentages of the conductive agent and surfactant remain unchanged. The sum of the mass percentages of the positive electrode active material, conductive agent, second binder, and surfactant is 100%.

[0139] Comparative Example 1

[0140] Except for the first binder in <Preparation of Positive Electrode Sheet> being modified polyimide, the rest is the same as in Example 1-1.

[0141] Comparative Example 2

[0142] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Examples 1-1. The change in the mass ratio of modified polyimide to polyvinylidene fluoride is achieved by controlling the mass of polyvinylidene fluoride.

[0143] Comparative Example 3

[0144] Except for the fact that the first binder in the <Preparation of Positive Electrode Sheet> is polyvinylidene fluoride, the rest is the same as in Example 1-1.

[0145] Comparative Example 4

[0146] Except for the first binder being polyimide in the <Preparation of Positive Electrode Sheet>, the rest is the same as in Example 1-1.

[0147] Comparative Example 5

[0148] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Examples 1-1. The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 3.

[0149] Table 1

[0150]

[0151]

[0152] Note: In Table 1, "\" indicates that there is no corresponding parameter.

[0153] As can be seen from Examples 1-1 to 1-7 and Comparative Examples 1 to 5, in the secondary battery of this application, the positive electrode sheet is provided with a first edge coating containing modified polyimide and polyvinylidene fluoride, and the molar ratio of the main chain of polyimide to polar groups and the mass ratio of modified polyimide to polyvinylidene fluoride are controlled within the range of this application. This enables the portion of the positive electrode sheet with the first edge coating to have a smaller stiffness, the mixed layer in the positive electrode sheet to have a smaller width, and the first edge coating to have a higher adhesion force F2 between the positive electrode current collector and the positive electrode after soaking. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after soaking in electrolyte have been improved, and the capacity and safety performance of the secondary battery have been improved. In contrast, the secondary battery in the comparison case does not contain modified polyimide or polyvinylidene fluoride in the first edge coating of its positive electrode, or the mass ratio of modified polyimide to polyvinylidene fluoride is not within the scope of this application, or the molar ratio of the main chain of polyimide to polar groups is not within the scope of this application. The portion of the positive electrode in which the first edge coating is provided has greater stiffness or the mixed layer in the positive electrode has a greater width. After immersion, the first edge coating has a lower adhesion force F2 between it and the positive current collector. The secondary battery has a lower capacity and / or throughput, indicating that the climbing phenomenon in the positive electrode and the problem of insufficient adhesion force of the first edge coating after immersion in electrolyte have not been improved.

[0154] The molecular weight of modified polyimide affects the capacity and safety performance of secondary batteries. As can be seen from Examples 1-1, 1-8 to 1-11, secondary batteries using modified polyimide with a molecular weight within the range of this application have a smaller stiffness in the portion of the positive electrode sheet where the first edge coating is provided, a smaller width in the mixed layer of the positive electrode sheet, and a higher adhesion force F2 between the first edge coating and the positive current collector after immersion. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after immersion in electrolyte have been improved. The secondary battery has a higher capacity and better safety performance.

[0155] The type of polar groups in modified polyimide affects the capacity and safety performance of secondary batteries. As can be seen from Examples 1-1, 1-12, and 1-13, secondary batteries using modified polyimide with polar groups within the scope of this application have a smaller stiffness in the portion of the positive electrode sheet where the first edge coating is provided, a smaller width in the mixed layer of the positive electrode sheet, and a higher adhesion force F2 between the first edge coating and the positive current collector after immersion. The secondary batteries have higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after immersion in electrolyte have been improved, and the secondary batteries have higher capacity and better safety performance.

[0156] Table 2

[0157]

[0158] The areal density of the first edge coating affects the capacity and safety performance of the secondary battery. As seen in Examples 1-1, 2-1 to 2-4, secondary batteries with an areal density of the first edge coating within the range of this application exhibit a smaller width of the mixed layer in the positive electrode sheet, higher adhesion F2 between the first edge coating and the positive current collector after immersion, and higher capacity and throughput. This indicates that the climbing phenomenon in the positive electrode sheet and the insufficient adhesion of the first edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance. Compared to Examples 1-1, 2-2, and 2-3, although Example 2-1 has higher capacity and throughput, its smaller areal density of the first edge coating may lead to insufficient coverage of the positive current collector, causing metal leakage and posing a short-circuit safety hazard. Therefore, it is not the preferred option.

[0159] The areal density of the positive electrode active material layer affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 2-5 and 2-6, the secondary battery with the areal density of the positive electrode active material layer within the range of this application has a smaller width of the mixed layer in the positive electrode sheet, and a higher adhesion force F2 between the first edge coating and the positive electrode current collector after immersion. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after immersion in electrolyte have been improved. The secondary battery has a higher capacity and better safety performance.

[0160] Table 3

[0161]

[0162]

[0163] The mass percentage W1 of the first binder and the mass percentage W2 of the first insulating material in the first edge coating affect the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4, 3-1 to 3-5, when the mass percentage W1 of the first binder and the mass percentage W2 of the first insulating material in the first edge coating are within the scope of this application, the portion of the positive electrode sheet in which the first edge coating is provided has a smaller stiffness, the mixed layer in the positive electrode sheet has a smaller width, and the first edge coating and the positive current collector have a higher adhesion force F2 after soaking. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after soaking in electrolyte have been improved, and the secondary battery has a higher capacity and better safety performance. Among them, compared with Examples 1-4 and Examples 3-1 to 3-3, although Example 3-4 has a higher capacity and throughput, the part of the positive electrode sheet with the first edge coating is more rigid, the positive electrode sheet has slightly poorer flexibility in winding, bending and other processes, and the part of the positive electrode sheet with the first edge coating is more prone to brittle fracture, so it is not preferred.

[0164] The type of first insulating material affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4 and Examples 3-6, when the type of first insulating material selected is within the scope of this application, the portion of the positive electrode sheet with the first edge coating has a smaller stiffness, the mixed layer in the positive electrode sheet has a smaller width, and the first edge coating and the positive current collector have a higher adhesion force F2 after soaking. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after soaking in electrolyte have been improved. The secondary battery has a higher capacity and better safety performance.

[0165] The particle size Dv50 of the first insulating material affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4, 3-7 to 3-10, the secondary battery with a particle size Dv50 of the first insulating material within the scope of this application has a smaller stiffness in the portion of the positive electrode sheet where the first edge coating is provided, a smaller width in the mixed layer in the positive electrode sheet, and a higher adhesion force F2 between the first edge coating and the positive current collector after immersion. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion force of the first edge coating after immersion in electrolyte have been improved. The secondary battery has a higher capacity and better safety performance.

[0166] The mass percentage W3 of the second binder in the positive electrode active material layer affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4 and 3-11 to 3-14, secondary batteries using a second binder with a mass percentage W3 within the scope of this application exhibit lower stiffness in the portion of the positive electrode sheet where the first edge coating is located, a smaller width in the mixed layer of the positive electrode sheet, and higher adhesion F2 between the first edge coating and the positive electrode current collector after immersion. This results in higher capacity and throughput for the secondary battery, indicating that the climbing phenomenon in the positive electrode sheet and the insufficient adhesion of the first edge coating after immersion in electrolyte have been improved. The secondary battery exhibits higher capacity and better safety performance. Compared to Examples 1-4, 3-12, and 3-13, although Example 3-11 has higher capacity and throughput, it is not preferred because the mass percentage of the second binder in the positive electrode active material layer is lower, resulting in poorer adhesion between the positive electrode active material layer and the positive electrode current collector, posing a risk of demolding during use.

[0167] 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.

[0168] 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.

[0169] 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 and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive current collector has a first edge along the width direction of the positive electrode sheet, and a first edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge, the first edge coating comprising a first insulating material and a first adhesive; The first insulating material includes at least one of alumina, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide; The first adhesive comprises modified polyimide and polyvinylidene fluoride, wherein the modified polyimide is obtained by modifying the polyimide with acrylate monomers, and the modified polyimide contains polar groups, wherein the polar groups include at least one selected from cyano, carboxyl, amino, sulfonic acid, or phosphate groups, and the molar ratio of the main chain of the polyimide to the polar groups is 1:(0.1 to 0.6). The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.1 to 5).

2. The positive electrode sheet according to claim 1, wherein, Based on the quality of the first edge coating, the mass percentage of the first adhesive is 10% to 20%.

3. The positive electrode sheet according to claim 2, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1) Based on the quality of the first edge coating, the mass percentage of the first adhesive is 10% to 15%; (2) The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.5 to 3).

4. The positive electrode sheet according to claim 1, wherein, The modified polyimide has a molecular weight of 50,000 g / mol to 500,000 g / mol.

5. The positive electrode sheet according to claim 1, wherein, The thickness of the first edge coating is 10 μm to 50 μm.

6. The positive electrode sheet according to claim 1, wherein, Along the width direction of the positive electrode sheet, the width of the first edge coating is 2 mm to 5 mm.

7. The positive electrode sheet according to claim 1, wherein, The areal density of the first edge coating is 0.02 mg / cm³. 2 Up to 0.06 mg / cm 2 .

8. The positive electrode sheet according to claim 1, wherein, The areal density of the positive electrode active material layer is 0.1 mg / cm³. 2 Up to 0.3 mg / cm 2 .

9. The positive electrode sheet according to claim 1, wherein, Based on the quality of the first edge coating, the mass percentage of the first insulating material is 80% to 90%.

10. The positive electrode sheet according to claim 1, wherein, The particle size Dv50 of the first insulating material is 0.4 μm to 1.8 μm.

11. The positive electrode sheet according to any one of claims 1 to 9, wherein, Along the width direction of the positive electrode sheet, there is a mixed layer between the positive active material layer and the first edge coating, and the width of the mixed layer is less than or equal to 0.5 mm.

12. The positive electrode sheet according to any one of claims 1 to 10, wherein, The positive electrode active material layer includes a positive electrode active material, a second binder, and a conductive agent. Based on the mass of the positive electrode active material layer, the mass percentage of the second binder is 1.5% to 3.0%.

13. The positive electrode sheet according to claim 12, wherein, Along the width direction of the positive electrode sheet, the positive current collector has a second edge opposite to the first edge. A second edge coating is provided on the surface of the positive current collector between the positive active material layer and the second edge. The second edge coating includes a second insulating material and a third adhesive. The third adhesive includes modified polyimide and polyvinylidene fluoride. The modified polyimide contains polar groups, and the polar groups include at least one of cyano, carboxyl, amino, sulfonic acid, or phosphate groups. The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.1 to 5).

14. A secondary battery, wherein, The secondary battery includes the positive electrode sheet according to any one of claims 1 to 13.

15. An electronic device, wherein, The electronic device includes the secondary battery as described in claim 14.

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