Positive pole piece, secondary battery and electronic device

Abstract

The invention provides a positive pole piece, a secondary battery and an electronic device, the positive pole piece comprises a positive current collector and a positive active material layer arranged on at least one surface of the positive current collector, the positive current collector is provided with a first edge along the width direction of the positive pole piece, a first edge coating is arranged on the surface of the positive electrode current collector between the positive electrode active material layer and the first edge, and the first edge coating 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 through an acrylate monomer, the modified polyimide contains a polar group, and 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 arrangement, the capacity and the safety performance of the secondary battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and particularly relates to a positive electrode sheet, a secondary battery and an electronic device. BACKGROUND

[0002] As a kind of efficient and environmentally friendly energy storage device, secondary batteries (such as lithium ion batteries) have been widely used in consumer electronics, electric vehicles, energy storage power stations and other fields. During the manufacturing process of the secondary battery, burrs are prone to occur on the edge of the positive electrode sheet during the cutting process. These burrs may pierce the separator and come into contact with the negative electrode sheet, thereby causing a short circuit of the secondary battery, which seriously threatens the safety performance of the secondary battery.

[0003] In order to solve the above problems, the current market solution is to coat a layer of insulating coating, i.e. edge coating, on the edge of the positive electrode sheet. Since boehmite has a smooth surface, the existing edge coating usually uses boehmite as the main material to make the surface of the cut positive electrode sheet smooth and burr-free, so as to improve the safety performance and yield of the secondary battery. However, the existing edge coating is prone to interface penetration between the positive active material layer during the coating process, resulting in serious climbing phenomenon, which affects the capacity of the secondary battery. At the same time, the adhesion of the existing edge coating is weak, especially after soaking in electrolyte at high temperature (≥85℃), which affects the safety performance of the secondary battery. Based on this, the person skilled in the art needs to develop a new technical solution to improve the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after soaking in electrolyte. SUMMARY

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

[0005] It should be noted that the present application uses lithium ion batteries as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a positive electrode tab, the positive electrode tab comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode current collector having a first edge along a width direction of the positive electrode tab, a first edge coating layer being disposed 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 comprising a first insulating material and a first binder; the first binder comprising a modified polyimide and a polyvinylidene fluoride, the modified polyimide being obtained by modifying a polyimide with an acrylate monomer, the modified polyimide containing a polar group, the polar group comprising at least one of a cyano group, a carboxyl group, an amino group, a sulfonic acid group or a phosphate group, a molar ratio of a main chain of the polyimide to the polar group being 1:(0.1 to 0.6); a mass ratio of the modified polyimide to the polyvinylidene fluoride being 1:(0.1 to 5). The positive electrode tab of the present application sets the first edge coating layer containing the modified polyimide and the polyvinylidene fluoride, and controls the molar ratio of the main chain of the polyimide to the polar group and the mass ratio of the modified polyimide to the polyvinylidene fluoride within the range of the present application. The polyvinylidene fluoride acts as a toughening phase, and its flexible C-C segment can improve the flexibility of the first edge coating layer to alleviate the influence of the rigidity of the modified polyimide on the brittleness of the first edge coating layer. The modified polyimide inhibits the interfacial penetration between the first edge coating layer and the positive electrode active material layer through its rigid segment to improve the climbing phenomenon in the positive electrode tab. The improvement of the climbing phenomenon can make the capacity of the positive electrode active material in the positive electrode active material layer be more fully utilized. The modified polyimide has good adhesion, which can improve the interfacial bonding force between the first insulating coating layer and the positive electrode current collector. Thus, the problem of insufficient adhesion of the first edge coating layer after soaking in electrolyte can be improved, and the probability of the first edge coating layer falling off from the surface of the positive electrode current collector will be reduced. The first edge coating layer can also have good insulating properties and hardness to reduce the probability of the positive electrode tab burr piercing the separator and contacting the positive electrode tab, thereby reducing the risk of short circuit of the secondary battery. Therefore, the capacity and safety performance of the secondary battery can be improved.

[0007] In some embodiments of the present application, the mass percentage content of the first binder is 10% to 20% based on the mass of the first edge coating layer. By controlling the mass percentage content of the first binder within the above range, the first edge coating layer has good adhesion, and thus the first edge coating layer and the positive electrode current collector have good interfacial bonding force.

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

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

[0010] In some embodiments of the present application, the first edge coating layer has a thickness of 10 μm to 50 μm. Controlling the thickness of the first edge coating layer within the above range is advantageous for improving the safety performance of the secondary battery on the basis of improved capacity.

[0011] In some embodiments of the present application, the first edge coating layer has a width of 2 mm to 5 mm in the width direction of the positive electrode tab. Controlling the width of the first edge coating layer within the above range is advantageous for exerting the function of the first edge coating layer on the basis of reduced width of the positive electrode active material layer, thereby improving the capacity and safety performance of the secondary battery.

[0012] In some embodiments of the present application, the first edge coating layer has an area density of 0.02 mg / cm 2 to 0.06 mg / cm 2 . Controlling the area density of the first edge coating layer within the above range is advantageous for exerting the function of the first edge coating layer, thereby enabling the secondary battery to have high capacity and safety performance.

[0013] In some embodiments of the present application, the positive electrode active material layer has an area density of 0.1 mg / cm 2 to 0.3 mg / cm 2 . Controlling the area density of the positive electrode active material layer within the above range is advantageous for having a large amount of positive electrode active material in the positive electrode active material layer, thereby enabling the positive electrode active material layer to have high capacity.

[0014] In some embodiments of the present application, the first insulating material includes at least one of alumina, boehmite, silica, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide; and the mass percentage content of the first insulating material is 80% to 90% based on the mass of the first edge coating layer. Selecting the first insulating material from the above types and controlling the mass percentage content of the first insulating material within the above range is advantageous for improving the safety performance of the secondary battery on the basis of improved capacity.

[0015] In some embodiments of the present application, the first insulating material has a particle size Dv50 of 0.4 μm to 1.8 μm. Controlling the particle size Dv50 of the first insulating material within the above range is advantageous for improving the safety performance of the secondary battery on the basis of high capacity.

[0016] In some embodiments of the present application, along the width direction of the positive electrode tab, the positive electrode active material layer and the first edge coating layer have a mixed layer, and the width of the mixed layer is less than or equal to 0.5 mm. It is indicated that the climbing shadow in the positive electrode tab is small.

[0017] In some embodiments of the present application, the positive electrode active material layer comprises the positive electrode active material, the second binder and the conductive agent, and the mass percentage of the second binder is 1.5% to 3.0% based on the mass of the positive electrode active material layer. By adjusting the mass percentage of the second binder within the above range, it is beneficial to make the positive electrode active material layer and the positive current collector have good interfacial bonding force without affecting the content of the positive electrode active material.

[0018] In some embodiments of the present application, along the width direction of the positive electrode tab, the positive current collector has a second edge opposite to the first edge, and a second edge coating layer is arranged on the surface of the positive current collector between the positive electrode active material layer and the second edge, the second edge coating layer comprises a second insulating material and a third binder, the third binder comprises a modified polyimide and a polyvinylidene fluoride, 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; and the mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0.1 to 5). By arranging the second edge coating layer in the positive electrode tab and adding the third binder of the above type in the second edge coating layer, the climbing phenomenon in the positive electrode tab is improved, the interfacial bonding force between the second insulating coating layer and the positive current collector is improved, and the secondary battery has higher capacity and good safety performance.

[0019] The second aspect of the present application provides a secondary battery, which comprises the positive electrode tab according to any one of the preceding embodiments. Therefore, the secondary battery has higher capacity and good safety performance.

[0020] The third aspect of the present application provides an electronic device, which comprises the secondary battery according to any one of the preceding embodiments. Therefore, the electronic device has good use performance.

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

[0022] The positive electrode tab provided by the embodiment of the present application, the secondary battery and the electronic device, the positive electrode tab is provided with a first edge coating containing modified polyimide and polyvinylidene fluoride, and the molar ratio of the main chain of the polyimide to the polar group and the mass ratio of the modified polyimide to the polyvinylidene fluoride are controlled within the range of the present application. The polyvinylidene fluoride serves as a toughening phase, and its flexible C-C segment can improve the flexibility of the first edge coating to alleviate the influence of the rigidity of the modified polyimide on the brittleness of the first edge coating. The modified polyimide suppresses the interfacial penetration between the first edge coating and the positive active material layer through its rigid segment to improve the climbing phenomenon in the positive electrode tab. The improvement of the climbing phenomenon can make the capacity of the positive active material in the positive active material layer be more fully utilized. The modified polyimide has good adhesion, which can improve the interfacial bonding force between the first insulating coating and the positive current collector. In this way, the problem of insufficient adhesion of the first edge coating after the electrolyte is saturated can be improved, and the probability of the first edge coating falling off from the surface of the positive current collector will be reduced. The first edge coating can also have good insulating performance and hardness to reduce the probability of the negative electrode tab burr piercing the separator and contacting the positive electrode tab, thereby reducing the risk of short circuit of the secondary battery. Therefore, the capacity and safety performance of the secondary battery can be improved.

[0023] Of course, implementing any of the products or methods of the present application does not necessarily require all the advantages described above to be achieved at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0025] Figure 1 Structure schematic diagram of the positive electrode tab of some embodiments of the present application along the width direction and the length direction of itself;

[0026] Figure 2 Structure schematic diagram of the positive electrode tab of some embodiments of the present application along the width direction and the length direction of itself; Figure 1 Structure schematic diagram of the positive electrode tab of some embodiments of the present application along the width direction and the length direction of itself;

[0027] Figure 3 Structure schematic diagram of the positive electrode tab of some embodiments of the present application along the width direction and the length direction of itself;

[0028] Figure 4 Structure schematic diagram of the positive electrode tab of some embodiments of the present application along the width direction and the length direction of itself.

[0029] Reference signs:

[0030] 10 - positive electrode tab; 20 - positive electrode current collector; 21 - positive electrode tab; 30 - positive electrode active material layer; 40 - first edge; 50 - first edge coating; 60 - mixed layer; 70 - second edge; 80 - second edge coating. DETAILED DESCRIPTION

[0031] The technical solutions in the present application will be clearly and completely described below in combination with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. All other embodiments obtained by those skilled in the art based on the present application are within the scope of protection of the present application.

[0032] It should be noted that in the specific embodiments of the present application, the present application is explained by taking lithium ion batteries as examples of secondary batteries, but the secondary batteries of the present application are not limited to lithium ion batteries. The specific technical solutions are as follows:

[0033] The first aspect of the present application provides a positive electrode tab, the positive electrode tab comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode current collector having a first edge along a width direction of the positive electrode tab, and a first edge coating disposed on the surface of the positive electrode current collector between the positive electrode active material layer and the first edge. The first edge coating comprises a first insulating material and a first binder. The first binder comprises a modified polyimide and polyvinylidene fluoride (PVDF), the modified polyimide being obtained by modifying polyimide with an acrylate monomer, the modified polyimide containing a polar group, the polar group comprising at least one of a cyano group, a carboxyl group, an amino group, a sulfonic acid group or a phosphate group. Further, the polar group comprises at least one of a cyano group or a carboxyl group. The molar ratio of the main chain of the polyimide to the polar group 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 the convenience of understanding, in the present application, a three-dimensional rectangular coordinate system is established with the length direction of the positive electrode tab in the 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 facilitating the description of the present application, and the directions defined in the present application can be understood according to the relative positions of the elements of the actual product and the accompanying drawings. It can be understood that the length direction, width direction and thickness direction of each of the positive electrode current collector, positive electrode active material layer, first edge coating and second edge coating are the same as those of the positive electrode tab. The above "positive electrode active material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode active material layer can be disposed on one surface or two surfaces of the positive electrode current collector, and the above surface can be a partial region or the entire region of the surface of the positive electrode current collector. For example, Figures 1 to 3As shown, the positive electrode tab 10 includes a positive electrode current collector 20 and a positive electrode active material layer 30, the positive electrode active material layer 30 is arranged on both surfaces of the positive electrode current collector 20, the positive electrode current collector 20 has a first edge 40, the first edge 40 extends out a plurality of positive electrode tabs 21 on one side, the positive electrode tabs 21 can be directly die-cut from the blanking area of the positive electrode current collector 20, and a first edge coating 50 is arranged on the surface of the positive electrode current collector 20 between the positive electrode active material layer 30 and the first edge 40. It should be noted that, since Figure 1 The positive electrode tab 10 is a structural schematic view along the length direction X and the width direction Y of the positive electrode tab 10, 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 The positive electrode tab 10 is a structural schematic view along the length direction X and the width direction Y of the positive electrode tab 10, 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 1 The positive electrode tab 10 is a structural schematic view along the length direction X and the width direction Y of the positive electrode tab 10, 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 The positive electrode tab 10 is a structural schematic view along the length direction X and the thickness direction Z of the positive electrode tab 10, Figure 2 The positive electrode tab 10 is a structural schematic view along the length direction X and the thickness direction Z of the positive electrode tab 10, Figure 1 The positive electrode tab 10 is a structural schematic view along the length direction X and the thickness direction Z of the positive electrode tab 10,

[0035] For example, the molar ratio of the polyimide backbone to the polar group is 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, or any ratio between any two of the above ranges. For example, the mass ratio of the modified polyimide and the 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 between any two of the above ranges. The backbone of PVDF is composed of carbon-carbon (C-C) single bonds, which has a high degree of spatial freedom, allowing the PVDF molecular segments to move and diffuse relatively freely, especially under high-temperature drying conditions, the chain segment movement intensifies, and the PVDF molecular segments are more likely to migrate to the positive active material layer, resulting in a blurred interface between the edge coating containing only PVDF and the positive active material layer, triggering the climbing phenomenon. The backbone of polyimide (PI) contains a rigid imide ring structure, which can limit the movement freedom of PI molecular segments, allowing PI to maintain a stable morphology during coating and drying, which is beneficial to prevent PI from penetrating into the positive active material layer to achieve the purpose of preventing climbing. In addition, the rigid imide ring structure of PI can provide high adhesion, but its high modulus can easily increase the brittleness of the first edge coating. Although there are fluorine atoms (F) in the PVDF molecule, the electronegativity is high, and it only relies on van der Waals force to combine with the positive current collector, and by introducing polar groups into the backbone of polyimide, it is beneficial to improve the interfacial adhesion between the modified polyimide and the positive 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 group on the surface of the positive current collector through esterification; when the polar group is a cyano group, the cyano group can form a stable covalent bond with the cation (such as Al 3+) occurs, forming a stable metal-ligand complex; the above bonding strength is much higher than the van der Waals force between PVDF and the positive current collector, so the use of modified polyimide can improve the interfacial bonding force between the first edge coating and the positive current collector. When the molar ratio of the polyimide main chain to the polar group is greater than 1:0.1, the molar amount of the polar group is too small, and the effect of the polar group is not obvious, the interfacial adhesion between the modified polyimide and the positive current collector is not obviously improved; when the molar ratio of the polyimide main chain to the polar group is less than 1:0.6, the content of the polar group is too high, which will cause the poor stability of the first binder structure and the deterioration of the bonding performance of the first binder, easily increasing the electrochemical side reaction in the secondary battery, affecting the performance of the secondary battery. The mass ratio of the modified polyimide and the polyvinylidene fluoride is greater than 1:0.1, the content of the modified polyimide is too high, and the higher modulus of the modified polyimide is easy to cause the increase of the brittleness of the first edge coating, affecting the flexibility of the positive electrode sheet in the process of winding, bending and the like; the mass ratio of the modified polyimide and the polyvinylidene fluoride is less than 1:5.0; the content of the polyvinylidene fluoride is too high, the diffusion of the first edge coating slurry to the positive active material layer is more, which causes the interface between the first edge coating and the positive active material layer to be blurred and causes serious climbing phenomenon, and the capacity of the positive active material in the part of the positive active material layer where the climbing phenomenon occurs cannot be developed, which will reduce the capacity of the secondary battery.

[0036] Overall, the positive electrode sheet of the present application sets the first edge coating containing modified polyimide and polyvinylidene fluoride, and controls the molar ratio of the polyimide main chain to the polar group and the mass ratio of the modified polyimide and the polyvinylidene fluoride within the range of the present application. As a toughening phase, the flexible C-C segment of polyvinylidene fluoride can improve the flexibility of the first edge coating to alleviate the influence of the rigidity of the modified polyimide on the brittleness of the first edge coating. The rigid segment of the modified polyimide inhibits the interfacial penetration between the first edge coating and the positive active material layer to improve the climbing phenomenon in the positive electrode sheet, and the improvement of the climbing phenomenon can make the capacity of the positive active material in the positive active material layer be more developed. The modified polyimide has good adhesion, which can improve the interfacial bonding force between the first insulating coating and the positive current collector. In this way, the problem of insufficient adhesion of the first edge coating after the electrolyte is filled can be improved, and the probability of the first edge coating falling from the surface of the positive current collector will be reduced. The first edge coating can also have good insulating performance and hardness to reduce the probability of the negative electrode sheet burr piercing the separator and contacting the positive electrode sheet, thereby reducing the risk of short circuit of the secondary battery. Thus, the capacity and safety performance of the secondary battery can be improved.

[0037] The application does not have special restrictions on the regulation method of the molar ratio of the polyimide main chain to the polar group, as long as the purpose of the application can be achieved. For example, it can be achieved by regulating the addition amount of the polar group-containing reactant in the preparation of the modified polyimide. The application does not have special restrictions on the regulation method of the mass ratio of the modified polyimide and the polyvinylidene fluoride, as long as the purpose of the application can be achieved. For example, it can be achieved by regulating the mass of the modified polyimide or the polyvinylidene fluoride. The type of polar group in the modified polyimide in the application can be achieved by regulating the type of polar group-containing compound in the preparation of the modified polyimide. The modified polyimide and the polyvinylidene fluoride of the application can also be obtained by purchasing commercially available products that meet the needs of the application, as long as the purpose of the application can be achieved.

[0038] The application does not have special restrictions on the type of acrylate monomer, as long as the purpose of the application can be achieved. 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 the application, the mass percentage content of the first binder is 10% to 20% based on the mass of the first edge coating. For example, the mass percentage content of the first binder is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value within the range between any two of the above-mentioned values. By regulating the mass percentage content of the first binder within the above-mentioned range, the first edge coating has good adhesion, so that the first edge coating has good interfacial bonding force with the positive current collector, the problem of insufficient adhesion of the first edge coating after soaking in electrolyte can be improved, the probability of the first edge coating falling off from the surface of the positive current collector is reduced, and the first edge also has good flexibility. Therefore, the secondary battery using the positive electrode sheet has high capacity and good safety performance.

[0040] In some embodiments of the application, the mass percentage content of the first binder is 10% to 15% based on the mass of the first edge coating. For example, the mass percentage content of the first binder is 10%, 11%, 12%, 13%, 14%, 15%, or any value within the range between any two of the above-mentioned values. By regulating the mass percentage content of the first binder within the above-mentioned range, the content of the first insulating material in the first edge coating is further increased, which is conducive to making the first edge coating have lower stiffness on the basis of good adhesion, and also can improve the hardness and insulation performance of the first edge coating, so as to reduce the probability of the negative electrode sheet burr piercing the separator and contacting the positive electrode sheet, and reduce the risk of short circuit of the secondary battery. Thus, the secondary battery has high capacity and good safety performance.

[0041] In some embodiments of the present application, the mass ratio of the modified polyimide and the polyvinylidene fluoride is 1:(0.5 to 3). For example, the mass ratio of the modified polyimide and the 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 between any two of the above ranges. By adjusting the mass ratio of the modified polyimide and the polyvinylidene fluoride within the above range, the rigidity of the modified polyimide and the flexibility of the polyvinylidene fluoride can be better matched to further reduce the climbing phenomenon in the positive electrode sheet, so that the capacity of the positive active material in the positive active material layer can be better utilized. Thus, the capacity of the secondary battery can be further improved on the basis of good safety performance.

[0042] In some embodiments of the present application, the molecular weight of the modified polyimide is 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 adjusting the molecular weight of the modified polyimide within the above range, the modified polyimide can provide appropriate rigidity and adhesion to inhibit the interfacial penetration between the first edge coating and the positive active material layer, reduce the climbing phenomenon in the positive electrode sheet, and also enable the first edge coating to have good interfacial bonding with the positive current collector, thereby improving the problem of insufficient adhesion of the first edge coating after soaking in electrolyte. Thus, the capacity and safety performance of the secondary battery can be improved.

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

[0044] The present application does not have a particular limitation on the way of adjusting the molecular weight of the modified polyimide, as long as the purpose of the present application can be achieved. For example, it can be achieved by adjusting the addition ratio of the monomer providing flexible segments, such as the addition ratio of the acrylate monomer, during the preparation of the modified polyimide, or it can be achieved by purchasing commercially available modified polyimide with a molecular weight meeting the requirements of the present application.

[0045] The present application does not have a particular limitation on the molecular weight of the polyvinylidene fluoride, and a conventional polyvinylidene fluoride in the art can be selected, as long as the purpose of the present application can be achieved. For example, the weight average molecular weight of the polyvinylidene fluoride is 50W to 80W.

[0046] In some embodiments of the present application, the thickness of the first edge coating is 10 μm to 50 μm. As shown in Figure 2 the thickness of the first edge coating 50 is shown as T 50 . For example, the thickness of the first edge coating is 10 μm, 12 μm, 16 μm, 20 μm, 25 μm, 27 μm, 30 μm, 35 μm, 40 μm, 42 μm, 46 μm, 50 μm, or any value between any two of the above-mentioned numerical ranges. By regulating the thickness of the first edge coating within the above-mentioned range, the probability of stress concentration on the positive current collector caused by the first insulating material subjected to pressure during the cold pressing of the positive electrode sheet leading to the fracture of the positive electrode sheet is small. It can also enable the positive electrode sheet to have a continuous and dense insulating barrier, reducing the probability of micro-cracks occurring in the positive electrode sheet during cutting or winding, reducing the risk of excessive electrolyte penetration leading to the dissolution of the positive active material, the intensification of the interface side reaction (such as excessive gas production, thickening of the solid-state electrolyte interface film, etc.), and the risk of local short circuit or capacity decay, and reducing the risk of burr exposure on the positive current collector, thereby reducing the risk of direct contact between the positive electrode sheet and the negative electrode sheet leading to short circuit of the secondary battery. Thus, it can enable the secondary battery to improve its safety performance on the basis of improving its capacity.

[0047] In some embodiments of the present application, the width of the first edge coating is 2 mm to 5 mm along the width direction of the positive electrode sheet. As shown in Figure 1 , Figure 3 and Figure 4 , the width of the first edge coating 50 is shown as W 50 along the width direction Y of the positive electrode sheet 10. For example, the width of the first edge coating is 2 mm, 2.3 mm, 3 mm, 3.5 mm, 4 mm, 4.2 mm, 4.6 mm, 5 mm, or any value between any two of the above-mentioned numerical ranges. By regulating the width of the first edge coating within the above-mentioned range, it is beneficial to play the role of the first edge coating on the basis of reducing the width of the positive active material layer, to improve the problem of insufficient adhesion of the first edge coating after soaking in electrolyte, and to improve the climbing phenomenon in the positive electrode sheet, thereby improving the capacity and safety performance of the secondary battery.

[0048] In some embodiments of the present application, the areal density of the first edge coating is 0.02 mg / cm 2 to 0.06 mg / cm 2 . For example, the areal density of the first edge coating is 0.02 mg / cm 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 numerical ranges. By regulating the areal density of the first edge coating within the above-mentioned range, a continuous and dense first edge coating can be formed on the surface of the positive current collector, the first edge coating can play its own role, the climbing phenomenon in the positive electrode sheet can be improved, the interfacial bonding force between the first edge coating and the positive current collector can be improved, the positive electrode sheet has good insulation, and the probability of micro-cracks generated during cutting or winding can be reduced. In this way, the content of the positive active material capable of playing a capacity in the positive active material layer is increased, the adhesion of the first edge coating after the electrolyte is exhausted is improved, and the risk of short circuit of the secondary battery caused by the direct contact between the positive electrode sheet and the negative electrode sheet is reduced. Thus, the secondary battery can have high capacity and safety performance.

[0049] The application does not have special restrictions on the regulation method of the areal density of the first edge coating, as long as the purpose of the application can be achieved. For example, it can be achieved by regulating the coating amount of the first edge coating slurry.

[0050] In some embodiments of the application, the areal density of the positive active material layer is 0.1 mg / cm 2 to 0.3 mg / cm 2 . For example, the areal density of the positive active material layer is 0.1 mg / cm 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-mentioned numerical ranges. By regulating the areal density of the positive active material layer within the above-mentioned range, the positive active material layer has more positive active material and higher capacity. Thus, the secondary battery can have high capacity on the basis of good safety performance.

[0051] The application does not have special restrictions on the regulation method of the areal density of the positive active material layer, as long as the purpose of the application can be achieved. For example, it can be achieved by regulating the coating amount of the positive active material layer slurry.

[0052] In some embodiments of the present application, the first insulating material includes at least one of aluminum oxide, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide. The mass percentage of the first insulating material in the first edge coating is 80% to 90% based on the mass of the first edge coating. 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 within a range between any two of the above values. The use of the above-mentioned first insulating material and the adjustment of the mass percentage of the first insulating material in the first edge coating within the above range is conducive to the first insulating coating having good insulation and hardness, reducing the probability of micro-cracks in the positive electrode sheet during cutting or winding, and reducing the risk of electrolyte penetration or exposure of burrs on the positive current collector, thereby reducing the risk of direct contact between the positive electrode sheet and the negative electrode sheet leading to secondary battery short circuit. Thus, the safety performance of the secondary battery can be improved on the basis of improving the capacity.

[0053] In some embodiments of the present application, the particle size Dv50 of the first insulating material is 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 within a range between any two of the above values. The adjustment of the particle size Dv50 of the first insulating material within the above range is conducive to the uniform distribution of the first insulating material in the first edge coating, thereby improving the interfacial bonding force between the first edge coating and the positive current collector and improving the problem of insufficient adhesion of the first edge coating after soaking in electrolyte. Thus, the safety performance of the secondary battery can be improved on the basis of having a higher capacity.

[0054] In the present application, the "particle size Dv50 of the first insulating material" refers to the particle size at which the cumulative volume of the first insulating material particles reaches 50% from the small particle size side in the particle size distribution on a volume basis.

[0055] The present application does not have a particular limitation on the adjustment method of the particle size Dv50 of the first insulating material, as long as the purpose of the present application can be achieved. For example, the adjustment can be achieved by crushing and sieving.

[0056] In some embodiments of the present application, there is a mixed layer between the positive 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, preferably less than or equal to 0.2 mm. For example, the width of the mixed layer is 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, or any value within a range between any two of the above values. The use of the above-mentioned mixed layer and the adjustment of the width of the mixed layer within the above range is conducive to the first edge coating having good insulation and hardness, reducing the probability of micro-cracks in the positive electrode sheet during cutting or winding, and reducing the risk of electrolyte penetration or exposure of burrs on the positive current collector, thereby reducing the risk of direct contact between the positive electrode sheet and the negative electrode sheet leading to secondary battery short circuit. Thus, the safety performance of the secondary battery can be improved on the basis of improving the capacity. Figure 3As shown, along the width direction Y of the positive electrode tab 10, the positive electrode active material layer 30 and the first edge coating 50 have a mixed layer 60 therebetween, and the width of the mixed layer 60 is W 60 As shown, along the width direction Y of the positive electrode tab 10, the positive electrode active material layer 30 and the first edge coating 50 have a mixed layer 60 therebetween, and the width of the mixed layer 60 is W

[0057] In some embodiments of the present application, the positive electrode active material layer comprises the positive electrode active material, the second binder and the conductive agent, and the mass percentage of the second binder is 1.5% to 3.0% based on the mass of the positive electrode active material layer. 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 within a range between any two of the above values. Controlling the mass percentage of the second binder within the above range is conducive to achieving good interfacial bonding force 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 falling off from the surface of the positive electrode current collector. Thus, the secondary battery has higher capacity and good safety performance.

[0058] In the present application, the content of the positive electrode active material and the conductive agent is not particularly limited as long as the purpose of the present application can be achieved. For example, the mass percentage of the positive electrode active material is 92% to 97.9% and the mass percentage of the conductive agent is 0.6% to 2.2% based on the mass of the positive electrode active material layer.

[0059] In some embodiments of the present application, the positive electrode active material layer further comprises a surfactant. The present application does not particularly limit the type and content of the surfactant as long as the purpose of the present application can be achieved. For example, the type of the surfactant includes but is not limited to at least one of sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA) or sulfonate. The mass percentage of the surfactant is 0% to 3.2% based on the mass of the positive electrode active material layer.

[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] The kind of the second insulating material and the third binder is not particularly limited in the present application, as long as the object of the present application can be achieved. In some embodiments of the present application, the second insulating material is the same as or different from the first insulating material, and the third binder is the same as or different from the first binder. For example, the second insulating material can be selected from the first insulating material. The content of the second insulating material and the third binder is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the mass percentage content of the second insulating material is 80% to 90%, and the mass percentage content of the third binder is 10% to 20%, based on the mass of the second edge coating.

[0062] The kind of the positive electrode active material, the second binder and the conductive agent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the positive electrode active material can include, but is not limited to, at least one of lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate or lithium titanate. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotube (CNT), carbon fiber, flake graphite, graphene, metal material or conductive polymer, and the conductive carbon black can include, but is not limited to, at least one of acetylene black or ketjen black. The above-mentioned carbon nanotube can include, but is not limited to, single-walled carbon nanotube and / or multi-walled carbon nanotube. The above-mentioned carbon fiber can include, but is not limited to, vapor grown carbon fiber (VGCF) and / or nanocarbon fiber. The above-mentioned metal material can include, but is not limited to, metal powder and / or metal fiber, and in particular, the metal can include, but is not limited to, at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymer can include, but is not limited to, at least one of polyphenylene derivative, polyaniline, polythiophene, polyacetylene or polypyrrole. For example, the second binder can 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, butadiene styrene rubber or polyvinylidene fluoride.

[0063] The positive electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved, and for example, can include aluminum foil, aluminum alloy foil or composite current collector (e.g., aluminum-carbon composite current collector) and the like.

[0064] The thickness of the positive electrode current collector and the positive electrode active material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the positive electrode active material layer is 30 μm to 120 μm.

[0065] The preparation method of the modified polyimide is not particularly limited in the present application, and can be prepared in a manner known to those skilled in the art, as long as the purpose of the present application can be achieved. For example, in some embodiments, the preparation method of the modified polyimide comprises the following steps: (1) reacting a dianhydride monomer and a diamine monomer in anhydrous N-methyl pyrrolidone (NMP) solvent under nitrogen protection at 0°C to 5°C for 5.5 h to 6.5 h to generate a polyamic acid (PAA) solution; (2) adding an acrylate monomer and an initiator to the PAA solution to introduce a flexible segment, and then adding a compound containing a polar group and an initiator, and reacting at 55°C to 65°C for 2.5 h to 3.5 h to graft the polar group, and obtaining a glue solution after reaction; (3) slowly adding a dehydrating agent and a catalyst to the glue solution, and stirring at 50°C to 60°C for 1.5 h to 2.5 h to make the imidization degree reach 70% to 80%; (4) filtering through a 5 μm filter screen, and the output is a modified PI glue solution, i.e. the modified polyimide is obtained. The type of the dianhydride monomer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the dianhydride monomer includes but is not limited to pyromellitic dianhydride (PMDA). The type of the diamine monomer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the diamine monomer includes but is not limited to 4,4'-oxydianiline. The type of the compound containing a polar group is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the compound containing a polar group includes but is not limited to at least one of acrylonitrile, dimethylaminoethyl acrylate, acrylic acid, or 2-acrylamide-2-methylpropanesulfonic acid. The type of the initiator in the above step (2) is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the initiator includes but is not limited to azobisisobutyronitrile (AIBN). The addition ratio of the glue solution obtained in the above step (2) to the dehydrating agent is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the molar ratio of the un-closed ring amic acid unit in the glue solution obtained in step (2) to the dehydrating agent in step (3) is 1:(1 to 2). The type of the dehydrating agent and the catalyst in the above step (3) is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the dehydrating agent includes but is not limited to acetic anhydride, and the catalyst includes but is not limited to triethylamine. The molar ratio of the dehydrating agent to the catalyst in step (3) is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the molar ratio of the dehydrating agent to the catalyst is (1.5 to 2.5):1.

[0066] For example, in some embodiments, the structural formula of the modified polyimide is as follows:

[0067]

[0068] wherein R1and R2are each independently selected from cyano, carboxyl, amino, sulfonic acid group, phosphate group; Ar1and Ar2are each independently selected from dianhydride radical, diamine radical; m is 90 to 300; n is 90 to 300.

[0069] The preparation method of the positive electrode tab is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, in some embodiments, the preparation method of the positive electrode tab comprises 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 in the width direction of the positive electrode tab, wherein the first edge coating slurry is on one side close to the edge of the positive current collector, and the positive electrode tab with the positive active material layer and the first edge coating arranged on one side is formed by drying, cold pressing and cutting; (3) optionally, repeating step (2) on the other surface of the positive current collector to obtain the positive electrode tab with the positive active material layer and the first edge coating arranged on both sides. In other embodiments, the preparation method of the positive electrode tab comprises 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 in the width direction of the positive electrode tab, wherein the positive active material layer slurry is located between the first edge coating slurry and the second edge coating slurry, and the positive electrode tab with the first edge coating, the positive active material layer and the second edge coating arranged on one side is formed by drying, cold pressing and cutting; (3) optionally, repeating step (2) on the other surface of the positive current collector to obtain the positive electrode tab with the first edge coating, the positive active material layer and the second edge coating arranged on both sides. The process parameters of the above drying and cold pressing are not particularly limited in the present application, and those skilled in the art can select them according to the actual situation, as long as the purpose of the present application can be achieved. The solid content of the above first edge coating slurry, positive active material layer slurry and second edge coating slurry is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the solid content of the first edge coating slurry and the second edge coating slurry is 20wt% to 40wt%, and the solid content of the positive active material layer slurry is 50wt% to 80wt%.

[0070] The second aspect of the present application provides a secondary battery, which comprises the positive electrode tab according to any one of the preceding embodiments. Therefore, the secondary battery has higher 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 the present application, the secondary battery further includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited in the present application as long as the object of the present application is achieved. For example, the lithium salt can 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. The content of the lithium salt in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved. The non-aqueous solvent is not particularly limited in the present application as long as the object of the present application is achieved, for example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluoro-carbonate compound. The chain carbonate compound can include, but is 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 cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The fluoro-carbonate compound can include, but is 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-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, 1-ethoxy-1-methoxy ethane, 2-methyl tetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the non-aqueous solvent in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved.

[0073] The secondary battery of the present application further comprises a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte solution, and other components known in the art of secondary batteries, and the present application does not limit the above-mentioned other components. The case is not particularly limited in the present application, and can be a case known in the art as long as the purpose of the present application can be achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be metal or hard plastic, and the type of metal is not limited in the present application, and a metal case known in the art can be used as long as the purpose of the present application can be achieved. The flexible case can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, and the like.

[0074] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited, for example, the preparation process of the secondary battery can include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and the like according to the need to obtain the electrode assembly of the winding structure, placing the electrode assembly into the case, injecting the electrolyte into the case and sealing to obtain the secondary battery. Alternatively, the positive electrode sheet, the separator and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain the electrode assembly of the stack structure, the electrode assembly is placed into the case, the electrolyte is injected into the case and sealed to obtain the secondary battery.

[0075] The third aspect of the present application provides an electronic device comprising the secondary battery of any one of the preceding embodiments. Thus, the electronic device provided by the present application has good use performance.

[0076] The type of electronic device is not particularly limited in the present application, and it can be any electronic device known in the art. In some embodiments of the present application, the electronic device can include but is not limited to a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large battery, and a lithium ion capacitor, and the like.

[0077] Examples

[0078] Hereinafter, examples and comparative examples are given to more specifically explain the embodiments of the present application. Various tests and evaluations were carried out according to the following methods.

[0079] Test methods and apparatus:

[0080] Sampling method of the positive electrode sheet:

[0081] After the lithium ion battery of each example and comparative example was discharged at 0.2C constant current to 3.0V, the lithium ion battery was disassembled to take out the positive electrode sheet, the positive electrode sheet was soaked in dimethyl carbonate (DMC) for 20 min, then was rinsed with dimethyl carbonate and acetone in turn, and then the positive electrode sheet was placed in an oven and baked at 80°C for 12h to obtain a dried positive electrode sheet. The following tests were performed on the positive electrode sheet obtained by the above method unless otherwise specified.

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

[0083] The positive electrode sheet was cut to obtain a part coated with the first edge coating, the part was immersed in an NMP solution and ultrasonically treated at 80°C for 1h to dissolve the PVDF and the modified PI, the first insulating material was separated by centrifugal filtration (0.1 pm filter membrane), the filtrate was collected, deionized water was slowly added to the filtrate (volume ratio, NMP: deionized water = 1:4), stirred for 10 min, the PVDF was dissolved in the NMP / water mixed solvent, and the modified PI was precipitated and separated out, and then vacuum dried at 60°C for 24h to obtain a pure modified polyimide powder, and the mass of the modified polyimide was obtained by weighing. The supernatant was rotary evaporated at 80°C until dryness to obtain PVDF solid, which was vacuum dried at 60°C for 24h, and the mass of the PVDF was obtained by weighing. The mass ratio of the modified polyimide to the PVDF is the mass ratio of the two.

[0084] Test of content of the first binder:

[0085] The positive electrode sheet was cut to obtain a region coated with the first edge coating, the first edge coating was dissolved in N-methyl pyrrolidone, and a mixture of the first insulating material and the first binder was obtained by centrifugation, and the weight loss ratio at 400°C to 700°C was obtained by testing with a simultaneous thermal analyzer (instrument model STA449F3), test temperature: 25°C to 700°C, temperature rise: 10°C / min, test atmosphere: nitrogen), and the mass percentage content of the first binder was analyzed.

[0086] Test of content of the second binder:

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

[0088] Test of molecular weight of the modified polyimide:

[0089] Cut the positive electrode sheet to obtain the part coated with the first edge coating, immerse the part in NMP solution, and ultrasonically treat at 80°C for 1 h to dissolve the PVDF and the modified PI. The first insulating material is separated by centrifugal filtration (0.1 μm filter membrane), the filtrate is collected, deionized water is slowly added to the filtrate (volume ratio, NMP: deionized water = 1:4), and stirring is performed for 10 min. The PVDF is dissolved in the NMP / water mixed solvent, and the modified PI is precipitated and separated out. Vacuum drying is performed at 60°C for 24 h to obtain a pure modified PI powder. The molecular weight of the modified polyimide is tested by gel permeation chromatography (GPC) according to the national standard GB / T 21863-2008, using tetrahydrofuran as the eluent. The molecular weight in this application refers to the weight average molecular weight.

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

[0091] The cross section of the first edge coating is obtained by argon ion polishing of the area of the positive electrode sheet coated with the first edge coating along the width direction and the thickness direction of the positive electrode sheet. The morphology of the cross section of the first edge coating along the thickness direction is observed by field emission scanning electron microscopy (Philips, XL-30 type), and a scanning electron microscope photograph is taken. The thickness of the first edge coating is measured by scanning electron microscopy. The above measurement is randomly taken at three positions of the first edge coating, and the average value is the thickness of the first edge coating.

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

[0093] The cross section of the first edge coating is obtained by argon ion polishing of the area of the positive electrode sheet coated with the first edge coating along the width direction and the thickness direction of the positive electrode sheet. The morphology of the cross section of the first edge coating along the thickness direction is observed by field emission scanning electron microscopy (Philips, XL-30 type), and a scanning electron microscope photograph is taken. The width of the first edge coating is measured by scanning electron microscopy.

[0094] Test of width of the mixed layer:

[0095] The cross section is obtained by argon ion polishing of the positive electrode sheet along the width direction and the thickness direction of the positive electrode sheet. The morphology of the cross section of the mixed layer along the thickness direction is observed by field emission scanning electron microscopy (Philips, XL-30 type) at the area where the first edge coating and the positive active material layer overlap, i.e., the mixed layer, and a scanning electron microscope photograph is taken. The width of the mixed layer is measured by scanning electron microscopy.

[0096] Test of areal density:

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

[0098] The area of the positive electrode tab coated with the first edge coating is punched into a small disc with a radius of R, and then weighed as m1. The first edge coating on both surfaces of the small disc is scraped off, and the weight of the positive electrode current collector is weighed as m2. The areal density of the first edge coating is (m1-m2) / (2πR), with units of mg / cm2. 2 2 .

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

[0100] The area of the positive electrode tab coated with the positive electrode active material layer is punched into a small disc with a radius of R, and then weighed as m1. The positive electrode active material layer on both surfaces of the small disc is scraped off, and the weight of the positive electrode current collector is weighed as m2. The areal density of the positive electrode active material layer is (m1-m2) / (2πR), with units of mg / cm2. 2 2 .

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

[0102] A positive electrode tab is cut to obtain a portion coated with the first edge coating. The portion is immersed in an NMP solution and ultrasonically treated at 80°C for 1 h to dissolve the PVDF and modified PI. The first insulating material is separated by centrifugal filtration (0.1 μm filter membrane), and the filter residue is collected. After drying, the first insulating material is obtained.

[0103] The particle size distribution of the first insulating material is tested by a laser particle size analyzer MasterSizer 2000 to obtain the volume average particle size of the first insulating material.

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

[0105] The adhesion between the first edge coating and the positive electrode current collector is tested by 180° peeling. If the positive electrode tab is coated with the first edge coating and the positive electrode active material layer on both sides, the first edge coating and the positive electrode active material layer on one surface of the positive electrode current collector are scraped off.

[0106] (1) Test of the adhesion F1 before soaking:

[0107] (a) The positive electrode tab is cut into a sample of 20 mm x 80 mm, which is then placed in a sample jar. Electrolyte is added until the sample is completely immersed. The sample jar is placed in a vacuum oven at 85°C for 24 h, and then the sample is taken out and dried.

[0108] ​​(b) A double-sided tape (NITTO NO 5000NS) of 20 mm x 80 mm is attached to a steel plate, and then the sample is attached to the double-sided tape with the positive current collector facing down. A paper tape of 20 mm x 90 mm is attached to the sample through the double-sided tape, and is attached to the surface of the first edge coating away from the positive current collector. A small stick of 2 kg in mass is pushed by hand to roll on the test sample for 8 times to obtain a test sample. The test is performed using a tensile testing machine. The test sample is fixed on the test table, the paper tape is folded upward by 180°, and is fixed by a clamp. Then the tensile testing machine starts to pull the paper tape at a speed of 50 mm / min until the test is ended after the first edge coating on the surface of the double-sided tape is separated from the positive current collector, and the test data is saved. The adhesion between the first edge coating and the positive current collector is calculated according to the tensile force and the displacement of stretching when the first edge coating is separated from the positive current collector, and is expressed by N / m.

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

[0110] After the positive electrode sheet is cut into a sample of 20 mm x 80 mm, the sample is placed in a sample tank, and an electrolyte is added until the sample is completely immersed. The sample tank is placed in a vacuum oven at 85°C for 24 h, and then the sample is taken out and dried. After drying, step (b) is performed to obtain the adhesion between the first edge coating and the positive current collector after soaking, which is expressed by N / m. The electrolyte is the electrolyte in Example 1-1.

[0111] Test of stiffness:

[0112] After the first edge coating is peeled off or scraped off from the positive current collector, the first insulating coating: NMP with a mass ratio of 1:3 is dispersed using a single-rod disperser for 3 h until it is completely dissolved and uniformly dispersed. The obtained slurry is coated on a 10 μm aluminum foil using a flat blade coater with a 300 μm doctor blade. After baking in an oven at 120°C for 30 min, an electrode sheet is prepared. A sample of length x width x thickness = 80 mm x 38 mm x 0.03 mm is cut from the electrode sheet using a sampling machine. A stiffness tester (Guangdong Beidou Precision Instrument Co., Ltd., PT-208) is used to set the bending length of the sample to 50 mm. The stiffness of the sample is measured when the sample 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] Test of capacity:

[0114] The lithium ion batteries in each example and comparative example were charged at 25℃ with a constant current of 0.3C to 3.6V, charged at 3.6V with a constant voltage to 0.05C, rested for 10 min, discharged at 0.3C to 2.5V, and cycled with charge and discharge according to the foregoing procedure for 3 times, rested for 30 min; during discharging, the equipment (name: lithium battery charge-discharge test equipment, manufacturer: Shenzhen Xinwei New Energy Technology Co., Ltd.) monitored the discharge current and time to calculate the capacity of the lithium ion battery.

[0115] Test of safety performance:

[0116] The lithium ion batteries in each example and comparative example were subjected to charge-discharge cycle test in a 60℃ constant temperature box, with a charge-discharge voltage range of 2.5V to 3.6V, charged at 0.5C to 3.6V with a constant current, then charged at 3.6V with a constant voltage to 0.05C and rested for 5 min, discharged at 0.5C to 2.5V with a constant current, and cycled the above charge-discharge process for 200 times. The lithium ion battery was disassembled, and the positive electrode sheet was taken out to see if the coating on the first edge fell off. If not, it was passed; if it completely or partially fell off, it was failed. 100 lithium ion batteries were tested for each example or comparative example, and the pass rate (%) = number of passes / 100 x 100%.

[0117] Example 1-1

[0118] Preparation of positive electrode sheet

[0119] The positive electrode active material lithium iron phosphate, the conductive agent Super P, the second binder PVDF (weight average molecular weight 7 x 10 6 ) and the surfactant sodium polyacrylate were mixed in a mass ratio of 97:0.9:1.9:0.2, then dispersed in N-methyl pyrrolidone (NMP), and stirred under the action of a vacuum stirrer to obtain a positive electrode slurry with a solid content of 75wt% and a uniform 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℃ for 6h to form a polyamide acid (PAA) solution, wherein the molar ratio of dianhydride monomer and diamine monomer was 1:1; (2) butyl acrylate and initiator AIBN were added to the PAA solution, and then acrylonitrile containing cyano group and acrylic acid containing carboxyl group and secondary AIBN were added, and the polar groups were grafted at 60℃ for 3h, and the reaction obtained a glue solution; (3) the glue solution was warmed to 60℃, and dehydrating agent acetic anhydride and catalyst triethylamine (molar ratio, acetic anhydride: triethylamine = 2:1, acetic anhydride: un-closed ring amide acid unit in the glue solution = 1.5:1) were slowly added dropwise, and stirred for 2h; (4) filtered through a 5μm filter screen to obtain the modified polyimide. The above modified polyimide contains polar groups cyano and carboxyl, the molar ratio of cyano and carboxyl in the polar group is 1:1, and the molar ratio of the main chain of the modified polyimide and the polar group is 1:0.3. The molecular weight of the modified polyimide is 300000g / mol.

[0121] After the first insulating material and the first binder were uniformly mixed, they were dispersed in NMP to obtain a first edge coating slurry with a solid content of 30wt% and a viscosity of 10158.7mPa·s. Among them, the mass percentage of the first insulating material W1 = 88% and the mass percentage of the first binder W2 = 12% based on the mass of the first edge coating. The first insulating material is selected from boehmite with a particle size Dv50 = 1.1μm, and the first binder is composed of modified polyimide and polyvinylidene fluoride (weight average molecular weight 65W), and the mass ratio of the modified polyimide and the polyvinylidene fluoride is 1:3.

[0122] The positive electrode slurry and the first edge coating slurry were respectively uniformly coated on one surface of the positive current collector aluminum foil with a thickness of 10μm, and then dried at 120℃ to obtain a semi-finished positive electrode sheet with a single-sided coated positive active material layer and a first edge coating. Then the above steps were repeated on the other surface of the aluminum foil, i.e. a positive electrode sheet with a double-sided coated positive active material layer and a first edge coating was obtained (structure see Figure 1 , but not limited to Figure 1 ). After cold pressing, the sheet was cut and the tabs were welded to obtain a positive electrode sheet with a specification of 74mm×867mm. Among them, the tabs were welded on the side where the first edge coating was arranged, and the thickness T 50 of the first edge coating was 20μm, the width W 50 of the first edge coating was 3.5mm. The area density of the first edge coating was 0.04mg / cm 2 , and the area density of the positive active material layer was 0.2mg / cm 2 .

[0123] <Preparation of negative electrode sheet>

[0124] The negative active material artificial graphite, the fourth binder styrene-butadiene rubber, and the conductive agent acetylene black were mixed in a mass ratio of 97.4:1.4:1.2, deionized water was added as a solvent, and a slurry with a solid content of 45wt% was prepared. After uniform stirring in a vacuum stirrer, a negative electrode slurry was obtained. The negative electrode slurry was uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm, and dried at 120℃ to obtain a negative electrode sheet with a single-side coated negative active material layer. The coated weight of the negative active material layer was 142mg / 1540mm 2 Then, the above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-side coated negative active material layer. After drying at 120℃ and cold pressing, the sheet was cut and the tabs were welded to obtain a negative electrode sheet with a size of 78mm×875mm for use.

[0125] <Preparation of electrolyte>

[0126] In an environment with a water content of less than 10ppm, 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 uniformly to obtain an electrolyte with a lithium salt concentration of 1.17mol / L.

[0127] <Separator>

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

[0129] <Preparation of lithium ion battery>

[0130] The positive electrode sheet, the separator, and the negative electrode sheet prepared above were stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to act as a separator. An electrode assembly was obtained by winding. The electrode assembly was placed in an aluminum-plastic film packaging bag, and the water was removed at 80℃. The electrolyte prepared above was injected, and the lithium ion battery was obtained after vacuum packaging, standing, formation, degassing, and edge cutting. The upper limit voltage of the formation was 4.15V, the formation temperature was 70℃, and the standing time of the formation was 2h.

[0131] Examples 1-2 to 1-13

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

[0133] Among them, the mass ratio of modified polyimide and polyvinylidene fluoride in Examples 1-1 to 1-5 was changed by adjusting the mass of polyvinylidene fluoride.

[0134] Examples 2-1 to 2-6

[0135] The rest is the same as Example 1-1 except that the relevant preparation parameters are adjusted according to Table 2.

[0136] Examples 3-1 to 3-14

[0137] The rest is the same as Example 1-1 except that the relevant preparation parameters are adjusted according to Table 3.

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

[0139] Comparative Example 1

[0140] The rest is the same as Example 1-1 except that the first binder in the preparation of the positive electrode sheet is modified polyimide.

[0141] Comparative Example 2

[0142] The rest is the same as Example 1-1 except that the relevant preparation parameters are adjusted according to Table 1. The mass ratio of modified polyimide and polyvinylidene fluoride is changed by adjusting the mass of polyvinylidene fluoride.

[0143] Comparative Example 3

[0144] The rest is the same as Example 1-1 except that the first binder in the preparation of the positive electrode sheet is polyvinylidene fluoride.

[0145] Comparative Example 4

[0146] The rest is the same as Example 1-1 except that the first binder in the preparation of the positive electrode sheet is polyimide.

[0147] Comparative Example 5

[0148] The rest is the same as Example 1-1 except that the relevant preparation parameters are adjusted according to Table 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 means no corresponding parameter.

[0153] As can be seen from Examples 1-1 to 1-7, Comparative Examples 1 to 5, the secondary battery of the present application, the positive electrode tab has a smaller stiffness of the part provided with the first edge coating, a smaller width of the mixed layer in the positive electrode tab, a higher adhesive force F2 between the first edge coating and the positive electrode current collector after soaking, a higher capacity and a higher pass rate, indicating that the climbing phenomenon in the positive electrode tab and the problem of insufficient adhesive force of the first edge coating after soaking of the electrolyte are improved, and the capacity and safety performance of the secondary battery are improved. The secondary batteries of the comparative examples do not contain modified polyimide or polyvinylidene fluoride in the first edge coating of the positive electrode tab, or the mass ratio of the modified polyimide and the polyvinylidene fluoride is not within the range of the present application, or the molar ratio of the main chain of the polyimide and the polar group is not within the range of the present application, or the part provided with the first edge coating of the positive electrode tab has a larger stiffness or the mixed layer in the positive electrode tab has a larger width, the first edge coating has a lower adhesive force F2 between the positive electrode current collector after soaking, the secondary battery has a lower capacity and / or pass rate, indicating that the climbing phenomenon in the positive electrode tab and the problem of insufficient adhesive force of the first edge coating after soaking of the electrolyte are not improved.

[0154] The molecular weight of the modified polyimide affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 1-8 to 1-11, the secondary battery with the molecular weight of the modified polyimide within the range of the present application has a smaller stiffness of the part provided with the first edge coating in the positive electrode tab, a smaller width of the mixed layer in the positive electrode tab, a higher adhesive force F2 between the first edge coating and the positive electrode current collector after soaking, a higher capacity and a higher pass rate, indicating that the climbing phenomenon in the positive electrode tab and the problem of insufficient adhesive force of the first edge coating after soaking of the electrolyte are improved, and the secondary battery has a higher capacity and good safety performance.

[0155] The type of polar group in the modified polyimide affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 1-12 and 1-13, the secondary battery with the type of polar group in the modified polyimide within the range of the present application has a smaller stiffness of the part provided with the first edge coating in the positive electrode tab, a smaller width of the mixed layer in the positive electrode tab, a higher adhesive force F2 between the first edge coating and the positive electrode current collector after soaking, a higher capacity and a higher pass rate, indicating that the climbing phenomenon in the positive electrode tab and the problem of insufficient adhesive force of the first edge coating after soaking of the electrolyte are improved, and the secondary battery has a higher capacity and good safety performance.

[0156] Table 2

[0157]

[0158] The face density of the first edge coating layer affects the capacity and safety performance of the secondary battery. As can be seen from Example 1-1, Example 2-1 to Example 2-4, the secondary battery selected with the face density of the first edge coating layer within the range of the present application has a smaller width of the mixed layer in the positive electrode tab, a higher adhesion F2 between the first edge coating layer and the positive current collector after soaking, a higher capacity and a higher passing rate, indicating that the problems of climbing phenomenon in the positive electrode tab and insufficient adhesion of the first edge coating layer after soaking of the electrolyte are improved, and the secondary battery has a higher capacity and good safety performance. Compared with Example 1-1, Example 2-2 and Example 2-3, although Example 2-1 has a higher capacity and a higher passing rate, the face density of the first edge coating layer in Example 2-1 is smaller, which may cause insufficient coverage of the first edge coating layer on the positive current collector, leading to metal leakage, so that the secondary battery has a safety hazard of short circuit, and therefore is not preferred.

[0159] The face density of the positive active material layer affects the capacity and safety performance of the secondary battery. As can be seen from Example 1-1, Example 2-5 and Example 2-6, the secondary battery selected with the face density of the positive active material layer within the range of the present application has a smaller width of the mixed layer in the positive electrode tab, a higher adhesion F2 between the first edge coating layer and the positive current collector after soaking, a higher capacity and a higher passing rate, indicating that the problems of climbing phenomenon in the positive electrode tab and insufficient adhesion of the first edge coating layer after soaking of the electrolyte are improved, and the secondary battery has a higher capacity and good safety performance.

[0160] Table 3

[0161]

[0162]

[0163] The mass percentage content W1 of the first binder and the mass percentage content 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, Examples 3-1 to 3-5, the secondary battery selected with the mass percentage content W1 of the first binder and the mass percentage content W2 of the first insulating material in the first edge coating within the range of the present application has a smaller stiffness of the part of the positive electrode tab provided with the first edge coating, a smaller width of the mixed layer in the positive electrode tab, a higher bonding force F2 between the first edge coating and the positive current collector after soaking, a higher capacity and a higher pass rate, indicating that the problems of the climbing phenomenon in the positive electrode tab and the insufficient bonding force of the first edge coating after soaking of the electrolyte have been improved, and the secondary battery has a higher capacity and good safety performance. Among them, compared with Examples 1-4, Examples 3-1 to 3-3, although Example 3-4 has a higher capacity and a higher pass rate, the stiffness of the part of the positive electrode tab provided with the first edge coating is greater, the flexibility of the positive electrode tab in the process of winding, bending and the like is slightly worse, and the part of the positive electrode tab provided with the first edge coating is also more prone to brittle fracture, and therefore is not preferred.

[0164] The type of the first insulating material affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4, Example 3-6, the secondary battery selected with the type of the first insulating material within the range of the present application has a smaller stiffness of the part of the positive electrode tab provided with the first edge coating, a smaller width of the mixed layer in the positive electrode tab, a higher bonding force F2 between the first edge coating and the positive current collector after soaking, a higher capacity and a higher pass rate, indicating that the problems of the climbing phenomenon in the positive electrode tab and the insufficient bonding force of the first edge coating after soaking of the electrolyte have been improved, and the secondary battery has a higher capacity and good 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, Examples 3-7 to 3-10, the secondary battery selected with the particle size Dv50 of the first insulating material within the range of the present application has a smaller stiffness of the part of the positive electrode tab provided with the first edge coating, a smaller width of the mixed layer in the positive electrode tab, a higher bonding force F2 between the first edge coating and the positive current collector after soaking, a higher capacity and a higher pass rate, indicating that the problems of the climbing phenomenon in the positive electrode tab and the insufficient bonding force of the first edge coating after soaking of the electrolyte have been improved, and the secondary battery has a higher capacity and good safety performance.

[0166] The mass percentage content W3 of the second binder in the positive active material layer affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-4, Examples 3-11 to 3-14, the secondary battery with the mass percentage content W3 of the second binder in the positive active material layer within the range of the present application has a smaller stiffness of the part of the positive electrode tab provided with the first edge coating, a smaller width of the mixed layer in the positive electrode tab, a higher bonding force F2 between the first edge coating and the positive current collector after soaking, a higher capacity and passing rate, indicating that the problems of the climbing phenomenon in the positive electrode tab and the insufficient bonding force after the first edge coating is soaked with electrolyte are improved, and the secondary battery has a higher capacity and good safety performance. Compared with Examples 1-4, Examples 3-12 and 3-13, although Example 3-11 has a higher capacity and passing rate, because the mass percentage content of the second binder in the positive active material layer is lower, the bonding force between the positive active material layer and the positive current collector is poor, and there is a risk of demolding in use, it is not preferred.

[0167] It should be noted that the relational terms herein such as first and second and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any such actual relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0168] Each of the embodiments in the specification is described in a relevant manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the difference from other embodiments.

[0169] The above only describes the preferred embodiments of the present application and does not limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present 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 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, 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; 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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