Positive electrode sheet, secondary battery, and electronic device

Abstract

The application provides a positive electrode sheet, a secondary battery and an electronic device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector. The positive electrode current collector has a first edge along the width direction of the positive electrode sheet. An edge coating layer is arranged on the surface of the positive electrode current collector between the positive electrode active material layer and the first edge. The edge coating layer comprises an insulating material and a first binder. The first binder comprises a modified polyimide, or the first binder comprises the modified polyimide and polyvinylidene fluoride. The modified polyimide is obtained by modifying polyimide with an acrylate monomer. The main chain of the polyimide is connected with a cyano group and a carboxyl group. The molar ratio of the cyano group, the carboxyl group and the acrylate monomer is (1-10):(10-25):(10-25). The molecular weight of the modified polyimide is 50000 g / mol-500000 g / mol. Through the above arrangement, the capacity and safety performance of the secondary battery are improved.

Description

Technical Field

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

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

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

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

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

[0006] The first aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Along the width direction of the positive electrode sheet, the positive current collector has a first edge. An edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge. The edge coating includes an insulating material and a first adhesive. The first adhesive includes a modified polyimide, or the first adhesive includes a modified polyimide and polyvinylidene fluoride. The modified polyimide is obtained by modifying the polyimide with acrylate monomers. A cyano group and a carboxyl group are attached to the main chain of the polyimide. The molar ratio of cyano group, carboxyl group, and acrylate monomer is (1 to 10):(10 to 25):(10 to 25). The molecular weight of the modified polyimide is 50,000 g / mol to 500,000 g / mol. The positive electrode sheet of this application incorporates a first binder into the edge coating and controls the molar ratio of cyano, carboxyl, and acrylate monomers in the modified polyimide, as well as the molecular weight of the modified polyimide, within the range specified in this application. This gives the modified polyimide suitable rigidity, reducing the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shortening the overlap distance between the edge coating slurry and the positive electrode active material layer, and improving the creep phenomenon in the positive electrode sheet. The modified polyimide also exhibits high adhesion and stability, providing good anchoring strength with the positive electrode current collector. Furthermore, the modified polyimide maintains good stability after immersion in the electrolyte. Applying the modified polyimide to the edge coating results in high interfacial adhesion between the edge coating and the positive electrode current collector. After immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is also improved. Therefore, through the above-mentioned design, the creep phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte can be improved. The improved slope reduction reduces the unproductive active area of ​​the positive electrode active material layer, allowing for greater utilization of the positive electrode active material's capacity. The improved adhesion of the edge coating after electrolyte immersion reduces the probability of it detaching from the positive electrode current collector surface. The edge coating also possesses good insulation and hardness, reducing the likelihood of burrs on the negative electrode puncturing the separator and contacting the positive electrode, thus lowering the risk of short circuits in the secondary battery. Consequently, the capacity and safety performance of the secondary battery will be improved.

[0007] In some embodiments of this application, the mass percentage of the first binder is 10% to 30% based on the quality of the edge coating. Controlling the mass percentage of the first binder within this range is beneficial for ensuring good adhesion and stability of the edge coating, resulting in higher capacity and better safety performance for the secondary battery using the positive electrode sheet.

[0008] In some embodiments of this application, the first binder comprises modified polyimide and polyvinylidene fluoride, with a mass ratio of modified polyimide to polyvinylidene fluoride of 1:(0 to 5). Controlling the mass ratio of modified polyimide to polyvinylidene fluoride in the first binder within the aforementioned range can improve the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion after the edge coating is soaked in electrolyte, thereby improving the capacity and safety performance of the secondary battery.

[0009] In some embodiments of this application, the positive electrode sheet satisfies at least one of the following characteristics: (1) the molecular weight of the modified polyimide is from 200,000 g / mol to 300,000 g / mol; (2) the molar ratio of cyano, carboxyl, and acrylate monomers is (1 to 5):(15 to 25):(15 to 25); (3) the mass percentage of the first binder is 12% to 20% based on the mass of the edge coating; (4) the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(1 to 3). This is beneficial for improving the capacity and safety performance of the secondary battery.

[0010] In some embodiments of this application, the polyimide is formed by polycondensation of a diamine monomer and a dianhydride monomer, with a molar ratio of 1:1. The diamine monomer includes at least one of 4,4'-diaminodiphenyl ether, p-phenylenediamine, or biphenylenediamine. The dianhydride monomer includes a cyano-containing dianhydride monomer and a carboxyl-containing dianhydride monomer, with a molar ratio of 1:(1 to 3). Polyimides prepared by selecting the above-mentioned types of diamine and dianhydride monomers and controlling the molar ratio of the diamine and dianhydride monomers within the above-mentioned range can improve the climbing phenomenon in the positive electrode and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte, thereby improving the capacity and safety performance of the secondary battery.

[0011] In some embodiments of this application, the cyano-containing dianhydride monomers include at least one selected from 2,2'-bis(3-cyano-4-hydroxyphenyl)hexafluoropropane dianhydride, 4,4'-(hexafluoroisopropene)phthalic acid dianhydride, or 3,3'-dicyano-4,4'-oxophthalic acid dianhydride; the carboxyl-containing dianhydride monomers include at least one selected from 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, 4,4'-(hexafluoroisopropene)phthalic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, or biphenyltetracarboxylic acid dianhydride. Using the above-mentioned types of cyano-containing dianhydride monomers and carboxyl-containing dianhydride monomers is beneficial for the preparation of the modified polyimide of this application.

[0012] In some embodiments of this application, the alkyl chain in the acrylate monomer has 4 to 8 carbon atoms. Controlling the number of carbon atoms in the alkyl chain in the acrylate monomer within the above range is beneficial to improving the mobility of the chain segments in the modified polyimide, reducing the rigidity of the imide ring in the polyimide, and enabling the modified polyimide to form a "rigid-flexible block" structure.

[0013] In some embodiments of this application, the acrylate monomer includes at least one of butyl acrylate, isobutyl acrylate, heptyl acrylate, or cyclohexyl acrylate. Using the above-mentioned types of acrylate monomers can enhance the mobility of the chain segments in the modified polyimide, reduce the rigidity of the imide ring in the polyimide, and enable the modified polyimide to form a "rigid-flexible block" structure.

[0014] In some embodiments of this application, the insulating material includes at least one of alumina, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide; the mass percentage of the insulating material is 70% to 90% based on the quality of the edge coating. By selecting the above-mentioned types of insulating materials and controlling the mass percentage of the insulating material in the edge coating within the above range, the safety performance of the secondary battery can be improved while increasing its capacity.

[0015] In some embodiments of this application, the particle size Dv50 of the insulating material is from 0.4 μm to 1.8 μm. Controlling the particle size Dv50 of the insulating material within this range is beneficial for ensuring uniform distribution of the insulating material in the edge coating.

[0016] In some embodiments of this application, the thickness of the edge coating is between 10 μm and 50 μm. Controlling the thickness of the edge coating within this range allows the secondary battery to improve its safety performance while simultaneously increasing its capacity.

[0017] In some embodiments of this application, the width of the edge coating is 2 mm to 5 mm. Controlling the width of the edge coating within this range allows it to function effectively without reducing the width of the positive electrode active material layer, thereby improving the capacity and safety performance of the secondary battery.

[0018] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, which may be at least one of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, or lithium titanate. Selecting the above-mentioned types of positive electrode active materials helps to reduce the penetration depth of the edge coating slurry and shortens the diffusion distance of the edge coating slurry into the positive electrode active material layer slurry, thereby enabling the secondary battery to have high capacity while maintaining good safety performance.

[0019] In some embodiments of this application, the width of the mixed layer between the edge coating and the positive electrode active material layer along the width direction of the positive electrode sheet in its unfolded state is d mm, where 0 ≤ d ≤ 0.5. This indicates that the climbing shadow in the positive electrode sheet is relatively small.

[0020] A second aspect of this application provides a secondary battery comprising the positive electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has high capacity and good safety performance.

[0021] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0022] The beneficial effects of the embodiments of this application are as follows:

[0023] This application provides a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet incorporates modified polyimide in its edge coating, and the molar ratio of cyano, carboxyl, and acrylate monomers, as well as the molecular weight of the modified polyimide, are controlled within the range specified in this application. This allows the modified polyimide to possess suitable rigidity, reducing the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shortening the overlap distance between the edge coating slurry and the positive electrode active material layer, and improving the creeping phenomenon in the positive electrode sheet. The modified polyimide also exhibits high adhesion and stability, providing good anchoring strength with the positive electrode current collector. Furthermore, the modified polyimide maintains good stability after immersion in the electrolyte. Applying the modified polyimide to the edge coating results in high interfacial adhesion between the edge coating and the positive electrode current collector. After immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is also improved. Therefore, through the above-mentioned design, the creeping phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte can be improved. The improved slope reduction reduces the unproductive active area of ​​the positive electrode active material layer, allowing for greater utilization of the positive electrode active material's capacity. The improved adhesion of the edge coating after electrolyte immersion reduces the probability of it detaching from the positive electrode current collector surface. The edge coating also possesses good insulation and hardness, reducing the likelihood of burrs on the negative electrode puncturing the separator and contacting the positive electrode, thus lowering the risk of short circuits in the secondary battery. Consequently, the capacity and safety performance of the secondary battery will be improved.

[0024] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0026] Figure 1 This is a schematic diagram of the positive electrode sheet along its width and length directions in some embodiments of this application;

[0027] Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure along the AA direction;

[0028] Figure 3 This is a schematic diagram of the positive electrode sheet along its width and length directions in another embodiment of this application;

[0029] Figure 4 The infrared spectrum of the modified polyimide in Example 1-1 of this application;

[0030] Figure 5 These are microscope images of the positive electrode plates in Examples 1-4 of this application;

[0031] Figure 6 This is a microscope image of the positive electrode in Comparative Example 2 of this application.

[0032] Figure label:

[0033] 10-Positive electrode sheet; 20-Positive current collector; 21-Positive electrode tab; 30-Positive active material layer; 40-First edge; 50-Edge coating; 60-Mixed layer. Detailed Implementation

[0034] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0035] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0036] The first aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Along the width direction of the positive electrode sheet, the positive current collector has a first edge, and an edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge. The aforementioned "first edge" refers to one edge from which at least one positive electrode tab extends from one of the two opposing edges along the width direction of the positive current collector. The edge coating comprises an insulating material and a first adhesive. The first adhesive comprises modified polyimide, or the first adhesive comprises modified polyimide and polyvinylidene fluoride. The modified polyimide is obtained by modifying the polyimide with acrylate monomers, and the main chain of the polyimide is linked with cyano groups and carboxyl groups. The molar ratio of cyano groups, carboxyl groups, and acrylate monomers is (1 to 10):(10 to 25):(10 to 25). The molecular weight of the modified polyimide is from 50,000 g / mol to 500,000 g / mol.

[0037] For ease of understanding, in this application, a three-dimensional Cartesian coordinate system is established with the length direction of the positive electrode sheet in its unfolded state as X, the width direction as Y, and the thickness direction as Z. It should be understood that the above definitions of directions are for the purpose of conveniently describing this application, and the directions defined in this application can be understood based on the relative positions of the accompanying drawings and actual product elements. It is understood that the length direction, width direction, and thickness direction of the positive electrode current collector, the positive electrode active material layer, and the edge coating are the same as those of the positive electrode sheet. The aforementioned "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 or both surfaces of the positive electrode current collector, and the aforementioned surfaces can be a partial or complete area of ​​the surface of the positive electrode current collector. In some embodiments, such as... Figure 1 and Figure 2 As shown, the positive electrode sheet 10 includes a positive current collector 20 and a positive active material layer 30. The positive active material layer 30 is disposed on two surfaces of the positive current collector 20. Along the width direction Y of the positive electrode sheet 10, the positive current collector 20 has a first edge 40. Multiple positive electrode tabs 21 extend from one side of the first edge 40. The positive electrode tabs 21 can be directly die-cut from the empty foil area of ​​the positive current collector 20. An edge coating 50 is disposed on the surface of the positive current collector 20 between the positive active material layer 30 and the first edge 40. It should be noted that, due to Figure 1 This is a schematic diagram of the structure of the positive electrode 10 along its length direction X and width direction Y. Therefore, only the positive electrode active material layer 30 and the edge coating 50 on one surface of the positive electrode current collector 20 are shown. Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure along the AA direction. Figure 2 The diagram shows a schematic representation of the positive electrode 10 along its length direction X and thickness direction Z. Figure 2The image shows the edge coating 50 disposed on the two surfaces of the positive electrode current collector 20, therefore, it can be understood that... Figure 1 The positive electrode current collector 20 in the positive electrode sheet 10 is also provided with a positive electrode active material layer 30 and an edge coating 50 on another surface.

[0038] For example, the molar ratios of cyano, carboxyl, and acrylate monomers are 1:10:10, 1:10:15, 1:10:25, 1:15:15, 1:15:25, 1:25:25, 5:10:15, 5:15:15, 5:25:25, 10:15:15, 10:15:25, and 10:25:25. The cyano group reacts with cations (such as Al) on the surface of the positive electrode current collector. 3+The modified polyimide forms strong coordination bonds, and the carboxyl groups enhance interfacial wettability through a hydrogen bond network. The cyano and carboxyl groups synergistically construct a dual-mode interface of "chemical anchoring + physical adhesion," resulting in high adhesion between the edge coating and the positive electrode current collector. When the positive electrode is immersed in the electrolyte, the carboxyl groups in the modified polyimide preferentially react with HF in the electrolyte to form a dense layer (such as AlF3) to prevent HF from continuously corroding the positive electrode current collector (such as aluminum foil). The coordination bonds formed by the cyano groups enhance stability through intramolecular charge transfer in the swelling environment of the electrolyte, increasing the coordination bond dissociation energy (120 kJ / mol for unmodified polyimide, better than 180 kJ / mol for modified polyimide). Therefore, after immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is improved. The main chain of the modified polyimide consists of repeating imide rings (-CO-NR-CO-), and its five-membered ring structure has high planar rigidity and a high energy barrier for molecular chain segment movement (glass transition temperature > 300℃), forming a dense physical barrier. In addition, the imide rings are tightly packed together through strong dipole-dipole interactions, constructing a nano-confined network with a pore size of less than 5nm, which blocks the diffusion path of the slurry solvent. Therefore, it can reduce the diffusion of the edge coating slurry towards the positive electrode active material layer, shorten the overlap distance between the edge coating slurry and the positive electrode active material layer, and improve the climbing phenomenon in the positive electrode sheet. However, the imide rings in the polyimide backbone give it a high modulus, which increases the brittleness of the edge coating and makes the positive electrode sheet less flexible in processes such as winding and bending. Therefore, this application introduces aliphatic long chains into the modified polyimide by copolymerizing or grafting acrylate monomers to form a "rigid-flexible block" structure. The COC and -CH2- chains of the acrylate monomers have low free rotation energy barriers (about 5kJ / mol to 10kJ / mol), which can improve the mobility of the chain segments in the modified polyimide, thereby reducing the impact of the high modulus caused by rigidity on the flexibility of the positive electrode sheet. By adjusting the molar ratio of cyano, carboxyl, and acrylate monomers within the range specified in this application, the modified polyimide exhibits good anchoring strength with the positive electrode current collector. Furthermore, the modified polyimide maintains good stability in the electrolyte. When applied to the edge coating, the edge coating exhibits high interfacial adhesion to the positive electrode current collector. After immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is further enhanced. Moreover, the modified polyimide improves the diffusion of the edge coating slurry towards the positive electrode active material layer, shortens the overlap distance between the edge coating slurry and the positive electrode active material layer, and mitigates the climbing phenomenon in the positive electrode.

[0039] 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. The modified polyimide has a molecular weight less than 50,000 g / mol. If the molecular weight is too small, the imide ring segments in the modified polyimide are too short, resulting in insufficient rigidity to block the diffusion of the slurry solvent, thus the improvement of the slope phenomenon in the positive electrode is not significant. If the molecular weight of the modified polyimide is greater than 500,000 g / mol, the molecular weight is too large, resulting in excessively long imide ring segments and an excessively high modulus, leading to excessive brittleness of the edge coating and poor flexibility of the positive electrode during winding, bending, and other processes. It should be noted that the molecular weight of the modified polyimide in this application refers to the weight-average molecular weight.

[0040] In some embodiments of this application, the first binder comprises modified polyimide. In other embodiments of this application, the first binder comprises modified polyimide and polyvinylidene fluoride (PVDF). PVDF has the characteristic of increasing toughness; when used in combination with modified polyimide, its flexible CC segments can enhance the flexibility of the edge coating, thereby mitigating the effect of the rigidity of the modified polyimide on the brittleness of the edge coating and improving the flexibility of the positive electrode sheet in processes such as winding and bending.

[0041] Overall, the positive electrode sheet of this application, by adding a first binder to the edge coating and controlling the molar ratio of cyano, carboxyl, and acrylate monomers in the modified polyimide, as well as the molecular weight of the modified polyimide, within the range specified in this application, enables the modified polyimide to possess suitable rigidity. This reduces the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shortens the overlap distance between the edge coating slurry and the positive electrode active material layer, and improves the climbing phenomenon in the positive electrode sheet. The modified polyimide also exhibits high adhesion and stability, providing good anchoring strength with the positive electrode current collector. Furthermore, the modified polyimide retains good stability after immersion in the electrolyte. Applying the modified polyimide to the edge coating results in high interfacial adhesion between the edge coating and the positive electrode current collector. After immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is also improved. Therefore, through the above-mentioned design, the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte can be improved. The improved slope reduction reduces the unproductive active area of ​​the positive electrode active material layer, allowing for greater utilization of the positive electrode active material's capacity. The improved adhesion of the edge coating after electrolyte immersion reduces the probability of it detaching from the positive electrode current collector surface. The edge coating also possesses good insulation and hardness, reducing the likelihood of burrs on the negative electrode puncturing the separator and contacting the positive electrode, thus lowering the risk of short circuits in the secondary battery. Consequently, the capacity and safety performance of the secondary battery will be improved.

[0042] In some embodiments of this application, the molar ratio of cyano, carboxyl, and acrylate monomers is (1 to 5):(15 to 25):(15 to 25). For example, the molar ratio of cyano, carboxyl, and acrylate monomers is 1:15:15, 1:15:25, 1:25:25, 5:15:15, or 5:25:25. Controlling the molar ratio of cyano, carboxyl, and acrylate monomers within the aforementioned range facilitates good anchoring strength between the modified polyimide and the positive electrode current collector. Furthermore, the modified polyimide maintains good stability in the electrolyte. Applying the modified polyimide to the edge coating results in high interfacial adhesion between the edge coating and the positive electrode current collector. After immersion in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is further enhanced. Moreover, the modified polyimide improves the diffusion of the edge coating slurry towards the positive electrode active material layer, shortening the overlap distance between the edge coating slurry and the positive electrode active material layer, and mitigating the creeping phenomenon in the positive electrode. Therefore, by improving the insufficient adhesion of the edge coating after immersion in the electrolyte, the creeping phenomenon in the positive electrode is better mitigated, resulting in higher capacity and safety performance of the secondary battery.

[0043] This application does not impose any particular restrictions on the method of controlling the molar ratio of cyano, carboxyl, and acrylate monomers, as long as the purpose of this application can be achieved. For example, this can be achieved by controlling the amount of diamine monomer, dianhydride monomer, and acrylate monomer added during the preparation of the modified polyimide. The modified polyimide and polyvinylidene fluoride of this application can also be obtained by purchasing commercially available products that meet the requirements of this application, as long as the purpose of this application can be achieved.

[0044] In some embodiments of this application, the molecular weight of the modified polyimide is between 200,000 g / mol and 300,000 g / mol. For example, the molecular weight of the modified polyimide is 200,000 g / mol, 210,000 g / mol, 220,000 g / mol, 230,000 g / mol, 240,000 g / mol, 250,000 g / mol, 260,000 g / mol, 270,000 g / mol, 280,000 g / mol, 290,000 g / mol, 300,000 g / mol, or any value between any two of the above ranges. Controlling the molecular weight of the modified polyimide within the above range is beneficial for giving the modified polyimide suitable rigidity. The modified polyimide can block the diffusion of the slurry solvent, improve the climbing phenomenon in the positive electrode sheet, reduce the brittleness of the edge coating, and give the positive electrode sheet good flexibility in processes such as winding and bending.

[0045] This application does not impose any particular restrictions on the method of controlling the molecular weight of the modified polyimide, as long as it achieves the purpose of this application. For example, it can be achieved by adjusting the proportion of flexible monomers, such as acrylate monomers, during the preparation of the modified polyimide, or it can be achieved by purchasing commercially available modified polyimide with a molecular weight that meets the requirements of this application.

[0046] This application does not impose any particular limitation on the molecular weight of polyvinylidene fluoride (PVDF). Conventional PVDF in the art can be used, as long as it achieves the purpose of this application. For example, the weight-average molecular weight of PVDF is 50W to 80W.

[0047] In some embodiments of this application, the mass percentage of the first binder is between 10% and 30%, depending on the quality of the edge coating. For example, the mass percentage of the first binder is 10%, 12%, 15%, 17%, 20%, 23%, 24%, 26%, 30%, or any value between any two of the above ranges. Controlling the mass percentage of the first binder within the above range is beneficial for the edge coating to have good adhesion and stability, i.e., good interfacial adhesion between the edge coating and the positive current collector, and high adhesion even after the edge coating is immersed in the electrolyte, thus reducing the probability of the edge coating falling off the surface of the positive current collector. It also reduces the diffusion of the edge coating slurry towards the positive active material layer, reducing the mid-climbing phenomenon of the positive electrode sheet. Therefore, the secondary battery using the positive electrode sheet has higher capacity and better safety performance.

[0048] In some embodiments of this application, the mass percentage of the first binder is 12% to 20% based on the quality of the edge coating. For example, the mass percentage of the first binder is 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value between any two of the above ranges. Controlling the mass percentage of the first binder within the above range is more conducive to the matching of the first binder and the insulating material in the edge coating. The edge coating, while possessing good adhesion and stability, also exhibits high hardness and insulation properties, reducing the probability of burrs on the negative electrode sheet puncturing the separator and contacting the positive electrode sheet, thereby reducing the risk of short circuits in the secondary battery. This further improves the safety performance of the secondary battery while maintaining a high capacity.

[0049] In some embodiments of this application, the first binder comprises modified polyimide and polyvinylidene fluoride, wherein the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(0 to 5). For example, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:0, 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 between any two of the above ranges. By controlling the mass ratio of modified polyimide and polyvinylidene fluoride in the first binder within the aforementioned range, the resulting first binder exhibits good adhesion, good stability in the electrolyte, and suitable rigidity. Therefore, applying the first binder to the edge coating can reduce the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shorten the overlap distance between the edge coating slurry and the positive electrode active material layer, and improve the creeping phenomenon in the positive electrode sheet. The first binder also exhibits good anchoring strength with the positive electrode current collector, and the edge coating has high interfacial adhesion to the positive electrode current collector. After the positive electrode sheet is immersed in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is also improved. Therefore, the creeping phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte can be improved, thereby enhancing the capacity and safety performance of the secondary battery.

[0050] In some embodiments of this application, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:(1 to 3). For example, the mass ratio of modified polyimide to polyvinylidene fluoride is 1:1.0, 1:1.2, 1:1.5, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3.0, or any ratio within any two of the above ranges. By controlling the mass ratio of modified polyimide and polyvinylidene fluoride in the first binder within the aforementioned range, the resulting first binder exhibits better adhesion, better stability in the electrolyte, and more suitable rigidity. Therefore, applying the first binder to the edge coating can further reduce the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shortening the overlap distance between the edge coating slurry and the positive electrode active material layer, thereby further improving the climbing phenomenon in the positive electrode sheet. The first binder also provides better anchoring strength with the positive electrode current collector, and the edge coating exhibits higher interfacial adhesion between the edge coating and the positive electrode current collector. After the positive electrode sheet is immersed in the electrolyte, the adhesion between the edge coating and the positive electrode current collector can also be further improved. Therefore, the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte can be further improved, and the capacity and safety performance of the secondary battery will be further enhanced.

[0051] In some embodiments of this application, the polyimide is formed by polycondensation of a diamine monomer and a dianhydride monomer, wherein the molar ratio of the diamine monomer to the dianhydride monomer is 1:1; the diamine monomer includes at least one selected from 4,4'-diaminodiphenyl ether, p-phenylenediamine, or biphenyl diamine; the dianhydride monomer includes a cyano-containing dianhydride monomer and a carboxyl-containing dianhydride monomer, wherein the molar ratio of the cyano-containing dianhydride monomer to the carboxyl-containing dianhydride monomer is 1:(1 to 3). For example, the molar ratio of the cyano-containing dianhydride monomer to the carboxyl-containing dianhydride monomer is 1:1, 1:1.3, 1:1.5, 1:1.7, 1:2, 1:2.2, 1:2.5, 1:2.7, 1:3, or any ratio within any two of the above ranges. The polyimide prepared by selecting the aforementioned diamine monomers and dianhydride monomers and controlling their molar ratio within the specified range has cyano and carboxyl groups attached to its main chain. This modified polyimide exhibits good adhesion, stability, and suitable rigidity. When applied to the edge coating of the positive electrode sheet, it reduces the diffusion of the edge coating slurry into the positive electrode active material layer slurry, shortens the overlap distance between the edge coating slurry and the positive electrode active material layer, and improves the creeping phenomenon in the positive electrode sheet. The first binder has good anchoring strength with the positive electrode current collector, and the edge coating has high interfacial adhesion to the positive electrode current collector. After the positive electrode sheet is immersed in the electrolyte, the adhesion between the edge coating and the positive electrode current collector is also improved. Therefore, it can improve the creeping phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in the electrolyte, thereby improving the capacity and safety performance of the secondary battery.

[0052] In some embodiments of this application, the cyano-containing dianhydride monomers include at least one selected from 2,2'-bis(3-cyano-4-hydroxyphenyl)hexafluoropropane dianhydride, 4,4'-(hexafluoroisopropene)phthalic acid dianhydride, or 3,3'-dicyano-4,4'-oxophthalic acid dianhydride; the carboxyl-containing dianhydride monomers include at least one selected from 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, 4,4'-(hexafluoroisopropene)phthalic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, or biphenyltetracarboxylic acid dianhydride. Using the above-mentioned types of cyano-containing dianhydride monomers and carboxyl-containing dianhydride monomers is beneficial for the preparation of the modified polyimide of this application.

[0053] In some embodiments of this application, the alkyl chain in the acrylate monomer has 4 to 8 carbon atoms. For example, the alkyl chain in the acrylate monomer has 4, 5, 6, 7, or 8 carbon atoms. Controlling the number of carbon atoms in the alkyl chain of the acrylate monomer within the above range is beneficial for introducing longer aliphatic chains into the polyimide during modification, thereby improving the mobility of chain segments in the modified polyimide, reducing the rigidity of the imide rings in the polyimide, and enabling the modified polyimide to form a "rigid-flexible block" structure. Applying this modified polyimide to the positive electrode sheet enables the positive electrode sheet to exhibit good flexibility in processes such as winding and folding.

[0054] In some embodiments of this application, the acrylate monomer includes at least one selected from butyl acrylate, isobutyl acrylate, heptyl acrylate, or cyclohexyl acrylate. Using the above-mentioned types of acrylate monomers to modify the polyimide can improve the mobility of the chain segments in the modified polyimide, reduce the rigidity of the imide rings in the polyimide, and enable the modified polyimide to form a "rigid-flexible block" structure. Applying this modified polyimide to the positive electrode sheet enables the positive electrode sheet to exhibit good flexibility in processes such as winding and folding.

[0055] In some embodiments of this application, the insulating material includes at least one selected from alumina, boehmite, silicon dioxide, barium sulfate, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide, or barium oxide; the mass percentage of the insulating material is 70% to 90% based on the quality of the edge coating. For example, the mass percentage of the insulating material is 70%, 73%, 75%, 77%, 80%, 82%, 84%, 87%, 90%, or any value between any two of the above ranges. Selecting the above-mentioned types of insulating materials and controlling the mass percentage of the insulating material in the edge coating within the above ranges is beneficial for the edge coating to have good insulation performance and hardness, reducing the probability of microcracks forming on the positive electrode sheet during cutting or winding, and also reducing the risk of electrolyte penetration or burr exposure on the positive current collector, thereby reducing the risk of short circuit in the secondary battery due to direct contact between the positive and negative electrode sheets. Thus, the safety performance of the secondary battery can be improved while increasing its capacity.

[0056] In some embodiments of this application, the particle size Dv50 of the insulating material is from 0.4 μm to 1.8 μm. For example, the particle size Dv50 of the insulating material is 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, or any value between any two of the above ranges. Controlling the particle size Dv50 of the insulating material within the above range is beneficial for ensuring uniform distribution of the insulating material in the edge coating, thereby improving the interfacial bonding force between the edge coating and the positive electrode current collector, and mitigating the problem of insufficient adhesion of the edge coating after immersion in electrolyte. This, in turn, helps to improve the safety performance of the secondary battery while maintaining a high capacity.

[0057] In this application, "particle size Dv50 of insulating material" refers to the particle size that, in the volume-based particle size distribution of insulating material particles, reaches 50% of the cumulative volume from the smallest particle size side.

[0058] This application does not impose any particular restrictions on the method of controlling the particle size Dv50 of the insulating material, as long as the purpose of this application can be achieved. For example, it can be controlled by crushing or screening.

[0059] In some embodiments of this application, such as Figure 2 As shown, the thickness T of the edge coating 50 50 The thickness ranges from 10 μm to 50 μm. For example, the thickness of the edge coating can be 10 μm, 15 μm, 20 μm, 23 μm, 27 μm, 30 μm, 33 μm, 35 μm, 40 μm, 42 μm, 44 μm, 46 μm, 50 μm, or any value between any two of the above ranges. By controlling the thickness of the edge coating within the above range, the probability of stress concentration on the positive electrode current collector due to pressure on the insulating material during the cold pressing of the positive electrode sheet, leading to breakage of the positive electrode sheet, is reduced. It also enables the positive electrode sheet to have a continuous and dense insulating barrier, reducing the probability of microcracks forming during the cutting or winding process of the positive electrode sheet, and reducing the risk of electrolyte penetration or burr exposure on the positive electrode current collector, thereby reducing the risk of short circuit in the secondary battery due to direct contact between the positive and negative electrode sheets. Thus, the safety performance of the secondary battery can be improved while increasing its capacity.

[0060] In some embodiments of this application, such as Figure 1 and Figure 3 As shown, along the width direction Y of the positive electrode 10, the width W of the edge coating 50 is... 50The width is 2mm to 5mm. For example, the width of the edge coating can be 2mm, 2.4mm, 2.7mm, 3.2mm, 3.5mm, 4mm, 4.2mm, 4.6mm, 5mm, or any value between any two of the above ranges. Controlling the width of the edge coating within this range helps to maximize its function without reducing the width of the positive electrode active material layer, thereby improving the climbing phenomenon in the positive electrode sheet and addressing the problem of insufficient adhesion of the edge coating after soaking in electrolyte.

[0061] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, which may be at least one of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, or lithium titanate. Selecting the above-mentioned positive electrode active materials facilitates the formation of a loose positive electrode structure, reduces the capillary force of the positive electrode active material layer slurry, decreases the penetration depth of the edge coating slurry, and shortens the diffusion distance of the edge coating slurry into the positive electrode active material layer slurry. This improves the climbing phenomenon in the positive electrode sheet, enabling the secondary battery to have high capacity while maintaining good safety performance.

[0062] In some embodiments of this application, the positive electrode active material layer further includes a positive electrode active material, a second binder, a conductive agent, and a surfactant. This application does not impose any particular limitation on the content of the positive electrode active material, the second binder, the conductive agent, and the surfactant, as long as the purpose of this application is achieved. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode active material is 94% to 98.5%, the mass percentage content of the second binder is 0.9% to 3.0%, the mass percentage content of the conductive agent is 0.5% to 2.2%, and the mass percentage content of the surfactant is 0.1% to 0.9%. This application does not impose any particular limitation on the types of the second binder, the conductive agent, and the surfactant, as long as the purpose of this application is achieved. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers; the conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. For example, the second binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. For example, the surfactant may include, but is not limited to, at least one of sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), polyacrylic acid, or sulfonates.

[0063] In some embodiments of this application, along the width direction of the positive electrode sheet in its unfolded state, the width of the mixed layer between the edge coating and the positive electrode active material layer is d mm, 0 ≤ d ≤ 0.5, preferably 0 ≤ d ≤ 0.2. For example... Figure 3As shown, along the width direction Y of the positive electrode sheet 10 in its unfolded state, there is a mixed layer 60 between the positive active material layer 30 and the edge coating 50, the width of which is denoted by d. It should be noted that during the preparation of the positive electrode sheet 10, the slurry of the edge coating 50 diffuses towards the positive active material layer 30, causing the edge coating slurry to penetrate into the positive active material layer slurry, forming a region containing both the edge coating slurry and the positive active material layer slurry. After drying, this region becomes the mixed layer 60. The diffusion of the edge coating slurry towards the positive active material layer 30 is also called the "climbing phenomenon," and the region containing both the edge coating slurry and the positive active material layer slurry, i.e., the mixed layer, can be understood as a "climbing shadow." For example, d can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, or any value between any two of the above ranges. A width within the above range indicates a smaller climbing shadow in the positive electrode sheet, which will improve the capacity of the secondary battery.

[0064] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0065] This application does not impose any particular limitations on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the positive electrode active material layer can be 30 μm to 120 μm.

[0066] This application does not impose any particular limitation on the preparation method of modified polyimide, as long as it can achieve the purpose of this application. For example, the preparation method of modified polyimide includes the following steps: (1) mixing diamine monomer and dianhydride monomer in a molar ratio of 1:1 and then carrying out a polycondensation reaction to generate a reactant; (2) adding acrylate monomer and initiator to the reactant to modify the polyimide to obtain modified polyimide, wherein the molar ratio of acrylate monomer to reactant is (0.1 to 0.25):1. In some embodiments, a compound containing a polar group may also be added in step (2) above to introduce a polar group. This application does not impose any particular limitation on the type of compound containing a polar group, as long as it can achieve the purpose of this application. For example, compounds containing a polar group include, but are not limited to, at least one of acrylonitrile, dimethylaminoethyl acrylate, acrylic acid, or 2-acrylamido-2-methylpropanesulfonic acid. This application does not impose any particular limitation on the type of initiator in step (2) above, as long as it can achieve the purpose of this application. For example, initiators include, but are not limited to, azobisisobutyronitrile (AIBN).

[0067] For example, in some embodiments, the modified polyimide has the following structural formula:

[0068]

[0069] R1 and R2 are each independently selected from cyano, carboxyl, amino, sulfonic acid, and phosphate groups; Ar1 ​​and Ar2 are each independently selected from dianhydride subunit and diamine subunit; m is 90 to 300; n is 90 to 300.

[0070] This application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the positive electrode sheet includes the following steps: (1) preparing an edge coating slurry and a positive active material layer slurry; (2) along the width direction of the positive electrode sheet, coating the edge coating slurry and the positive active material layer slurry on one surface of the positive current collector, wherein the edge coating slurry is disposed on the side close to the edge of the positive current collector, and after drying, cold pressing and cutting, a positive electrode sheet with a positive active material layer and an edge coating on one side is formed; (3) optionally, step (2) is repeated on the other surface of the positive current collector to obtain a positive electrode sheet with a positive active material layer and an edge coating on both sides. In some other embodiments, the preparation method of the positive electrode sheet includes the following steps: (1) preparing an edge coating slurry and a positive active material layer slurry; (2) along the width direction of the positive electrode sheet, sequentially coating the edge coating slurry, the positive active material layer slurry, and the edge coating slurry on one surface of the positive current collector, wherein the edge coating slurry is respectively disposed on both sides of the positive active material layer slurry, and then drying, cold pressing, and cutting to form a positive electrode sheet with a positive active material layer and two edge coatings on one side; (3) optionally, repeating step (2) on the other surface of the positive current collector to obtain a positive electrode sheet with a positive active material layer and edge coatings on both sides. This application does not impose any particular restrictions on the above-mentioned drying and cold pressing process parameters, which can be selected by those skilled in the art according to actual conditions, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the solid content of the above-mentioned edge coating slurry and positive active material layer slurry, as long as the purpose of this application can be achieved. For example, the solid content of the edge coating slurry is 18 wt% to 26 wt%, and the solid content of the positive electrode active material layer slurry is 50 wt% to 80 wt%. By adjusting the edge coating slurry within the above range, the positive electrode sheet exhibits better processing performance.

[0071] A second aspect of this application provides a secondary battery comprising the positive electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has high capacity and good safety performance.

[0072] 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 third binder. This application does not particularly limit the types of conductive agents and third 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 third 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 third binder in the negative electrode active material layer. Those skilled in the art can choose 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.

[0073] In this application, the secondary battery also includes an electrolyte, which comprises lithium salts and non-aqueous solvents. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0074] The secondary battery of this application also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal or rigid plastic. This application does not limit the type of metal; metal casings known in the art can be used, as long as they achieve the purpose of this application. The flexible casing can be a metal-plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0075] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery.

[0076] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has excellent performance.

[0077] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0078] Example

[0079] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0080] Test methods and equipment:

[0081] Sampling method for positive electrode:

[0082] After discharging the lithium-ion batteries of each embodiment and comparative example to 3.0V at a constant current of 0.2C, the lithium-ion batteries were disassembled and the positive electrode sheet was removed. The positive electrode sheet was immersed in dimethyl carbonate (DMC) for 20 minutes, and then rinsed with dimethyl carbonate and acetone respectively. After that, the positive electrode sheet was placed in an oven and baked at 80°C for 12 hours to obtain the dried positive electrode sheet. Unless otherwise specified, the following tests all used positive electrode sheets obtained by the above method.

[0083] Testing of the infrared spectrum of modified polyimide:

[0084] The infrared spectrum of the modified polyimide was measured using a Nicoleti S50 Fourier transform infrared spectrometer.

[0085] Test of the molar amounts of cyano, carboxyl, and acrylate monomers:

[0086] Cut the positive electrode sheet to obtain the portion with the edge coating. Immerse this portion in NMP solution and sonicate at 80℃ for 1 hour to dissolve PVDF and modified PI. Separate the insulating material by centrifugation and filtration (0.1μm filter membrane). Collect the filtrate and slowly add deionized water (volume ratio, NMP:deionized water = 1:4). Stir for 10 minutes. PVDF dissolves in the NMP / water mixed solvent, and modified PI precipitates out. Dry under vacuum at 60℃ for 24 hours to obtain pure modified PI powder, which is designated as the sample. Acrylic ester segments preferentially decompose at 200℃ to 300℃. The molar amount of acrylate is calculated using thermogravimetric analysis (TGA). The molar amount of CN is calculated by measuring the mass percentage (%N) of nitrogen in the sample using an elemental analyzer (such as a CHNS elemental analyzer). Since the carboxyl group is acidic, its molar amount can be measured by titration with an alkaline standard solution.

[0087] Molecular weight testing of modified polyimide:

[0088] The positive electrode sheet was cut to obtain the portion with the edge coating. This portion was immersed in NMP solution and ultrasonically treated at 80℃ for 1 hour to dissolve PVDF and modified PI. The first insulating material was separated by centrifugation and filtration (0.1μm filter membrane). The filtrate was collected, and deionized water (volume ratio, NMP:deionized water = 1:4) was slowly added to the filtrate. The mixture was stirred for 10 minutes, and PVDF dissolved in the NMP / water mixed solvent. Modified PI precipitated out and was dried under vacuum at 60℃ for 24 hours to obtain pure modified PI powder. Referring to the national standard GB / T 21863-2008, gel permeation chromatography (GPC) was used with tetrahydrofuran as the eluent; the weight-average molecular weight of the modified polyimide was determined by gel permeation chromatography (instrument model: ACQUITYAPC).

[0089] Tests for the content of the first adhesive and insulating material:

[0090] (1) Test of the content of the first adhesive agent:

[0091] The positive electrode sheet is cut to obtain the area to be coated with the edge coating. The edge coating is dissolved in N-methylpyrrolidone. The mixture of insulating material and first adhesive is obtained by centrifugation. The weight loss ratio at 400℃ to 700℃ is obtained by testing with a synchronous thermal analyzer (instrument model STA449F3, test temperature: 25℃ to 700℃, temperature rise: 10℃ / min, test atmosphere: nitrogen). The mass percentage content of the first adhesive is obtained by analysis.

[0092] (2) Testing of insulation material content:

[0093] The mass percentage of insulating material (%) = 100% - the mass percentage of the first adhesive.

[0094] Test of the mass ratio of modified polyimide to polyvinylidene fluoride:

[0095] Cut the positive electrode sheet to obtain the portion with the edge coating. Immerse this portion in NMP solution and sonicate at 80℃ for 1 hour to dissolve PVDF and modified PI. Separate the insulating material by centrifugal filtration (0.1μm filter membrane), collect the filtrate, and slowly add deionized water (volume ratio, NMP:deionized water = 1:4). Stir for 10 minutes. PVDF dissolves in the NMP / water mixed solvent, and modified PI precipitates out. Dry under vacuum at 60℃ for 24 hours to obtain pure modified polyimide powder. Weigh the modified polyimide to obtain its mass. Evaporate the supernatant to dryness at 80℃ to obtain solid PVDF. Dry under vacuum at 60℃ for 24 hours and weigh the PVDF to obtain its mass. The mass ratio of modified polyimide to PVDF is the same as the mass ratio of the two.

[0096] Testing of particle size Dv50 of insulating materials:

[0097] Cut the positive electrode sheet to obtain the part with the edge coating. Immerse this part in NMP solution and sonicate at 80°C for 1 hour to dissolve PVDF and modified PI. Separate the insulating material by centrifugal filtration (0.1μm filter membrane), collect the filter residue, and dry the filter residue to obtain the insulating material.

[0098] The particle size distribution was tested using a MasterSizer 2000 laser particle size analyzer, and the volume average particle size of the insulating material was obtained.

[0099] Testing the thickness of the edge coating:

[0100] The area of ​​the positive electrode sheet with the edge coating was argon-ion polished along both the width and thickness directions to obtain a cross-section of the edge coating. The morphology of the cross-section along the thickness direction of the edge coating was observed and scanned electron microscopy (SEM) was performed using a field emission scanning electron microscope (Philips XL-30). The thickness of the edge coating was then measured using SEM. The above measurements were performed at three random locations on the edge coating, and the average value was taken as the thickness of the edge coating.

[0101] Testing the width of the edge coating:

[0102] Argon ion polishing was performed on the area of ​​the positive electrode sheet with the edge coating along the width and thickness directions of the positive electrode sheet to obtain the cross section of the edge coating. The morphology of the cross section of the edge coating along the thickness direction was observed and scanned electron micrographs were taken using a field emission scanning electron microscope (Philips, XL-30). The width of the edge coating was measured by scanning electron microscopy.

[0103] Testing the width of the blending layer:

[0104] Argon ion polishing was performed on the positive electrode sheet along its width and thickness directions to obtain a cross section. The morphology of the cross section of the mixed layer along the thickness direction, i.e., the area where the edge coating and the positive electrode active material layer overlapped, was observed and photographed using a field emission scanning electron microscope (Philips, XL-30). The width of the mixed layer was measured by scanning electron microscopy.

[0105] Testing the adhesion of the edge coating:

[0106] The adhesion between the edge coating and the positive current collector was measured by a 180° peel test. For positive electrode sheets with edge coating and positive active material layer coated on both sides, the edge coating and positive active material layer on one surface of the positive current collector were scraped off.

[0107] (1) Test of adhesion force F1 before immersion:

[0108] (a) After cutting the positive electrode sheet into 20mm×80mm samples, place them in a sample container, add electrolyte until the sample is completely submerged, place the sample container in an 85℃ vacuum oven for 24 hours, and then take out the sample and dry it.

[0109] (b) Attach a 20mm × 80mm double-sided adhesive tape (NITTO.NO5000NS) to a steel plate, then attach the sample to the tape with the positive current collector facing down. Connect a 20mm × 90mm paper strip to the sample using the double-sided adhesive tape, adhering it to the surface of the edge coating away from the positive current collector. Roll the test strip 8 times with a 2kg roller to obtain the test sample. Perform the test using a tensile testing machine. Fix the test sample on the test table, fold the paper strip 180° upwards, and secure it with a clamp. Then, start pulling the paper strip at a speed of 50mm / min until the edge coating on the double-sided adhesive surface separates from the positive current collector, ending the test and saving the test data. Calculate the adhesion force between the edge coating and the positive current collector based on the tensile force and displacement during separation, in N / m.

[0110] (2) Test of adhesion force F2 after soaking:

[0111] After cutting the positive electrode sheet into 20mm × 80mm samples, they were placed in a sample container. Electrolyte was added until the samples were completely submerged. The sample container was then placed in a vacuum oven at 85℃ for 24 hours. After removing the samples and drying them, step (b) above was performed to obtain the adhesion force between the edge coating and the positive current collector after immersion, expressed in N / m. The electrolyte used was the electrolyte from Example 1-1.

[0112] Stiffness test:

[0113] After peeling or scraping the edge coating from the positive current collector, the first insulating coating and NMP are dispersed using a single-bar disperser for 3 hours until completely dissolved and uniformly dispersed. The resulting slurry is then coated onto a 10μm aluminum foil using a flatbed coater with a 300μm blade. After drying in an oven at 120℃ for 30 minutes, an electrode sheet is formed. Samples with a length × width × thickness of 80mm × 38mm × 0.03mm are cut from the electrode sheet using a sampler. A stiffness tester (Guangdong Beidou Precision Instruments Co., Ltd., PT-208) is used, with the sample bending length set to 50mm. The stiffness of the sample is measured when it is bent from 0° to 90°. Five parallel samples are measured in each group, and the average value is taken as the final stiffness value.

[0114] Capacity testing:

[0115] (1) When lithium iron phosphate is selected as the positive electrode active material, the test method is as follows:

[0116] The lithium-ion battery was charged at 25°C with a constant current of 0.3C to 3.6V, and then charged at 3.6V with a constant voltage to 0.05C; left to stand for 10 minutes; and discharged at a constant current of 0.3C to 2.5V. This charge-discharge cycle was repeated 3 times, followed by a 30-minute stand. During discharge, the equipment (name: lithium battery charge-discharge test equipment, manufacturer: Shenzhen Xinwei New Energy Technology Co., Ltd.) calculated the capacity of the lithium-ion battery by monitoring the discharge current and time.

[0117] (2) When lithium nickel cobalt manganese oxide is selected as the positive electrode active material, the test method is as follows:

[0118] The lithium-ion battery was charged at 25°C with a constant current of 0.5C to 4.2V, and then charged at 4.2V with a constant voltage to 0.05C; left to stand for 10 minutes; and discharged at a constant current of 0.2C to 2.8V. This charge-discharge cycle was repeated 3 times, followed by a 30-minute stand. During discharge, the equipment (name: lithium battery charge-discharge test equipment, manufacturer: Shenzhen Xinwei New Energy Technology Co., Ltd.) calculated the capacity of the lithium-ion battery by monitoring the discharge current and time.

[0119] Safety performance testing:

[0120] (1) When lithium iron phosphate is selected as the positive electrode active material, the test method is as follows:

[0121] The lithium-ion batteries in each embodiment and comparative example were subjected to charge-discharge cycle tests in a 25°C constant temperature chamber. The charge-discharge voltage range was 2.5V to 3.6V. The batteries were charged at a constant current of 0.5C to 3.6V, then charged at a constant voltage of 3.6V to 0.05C and allowed to rest for 5 minutes. Finally, they were discharged at a constant current of 0.5C to 2.5V. This charge-discharge cycle was repeated 100 times. The lithium-ion batteries were disassembled, and the positive electrode was removed to check for edge coating peeling. If the coating did not peel off, the battery passed; if it peeled off completely or partially, the battery failed. 100 lithium-ion batteries were tested for each embodiment or comparative example. The pass rate (%) was calculated as: (Number of passing batteries / 100) × 100%.

[0122] (2) When lithium nickel cobalt manganese oxide is selected as the positive electrode active material, the test method is as follows:

[0123] The lithium-ion batteries in each embodiment and comparative example were subjected to charge-discharge cycle tests in a 25°C constant temperature chamber. The charge-discharge voltage range was 2.8V to 4.2V. The batteries were charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C and allowed to rest for 5 minutes. Finally, they were discharged at a constant current of 0.5C to 2.8V. This charge-discharge cycle was repeated 100 times. The lithium-ion batteries were disassembled, and the positive electrode was removed to check for edge coating peeling. If the coating did not peel off, the battery passed; if it peeled off completely or partially, the battery failed. 100 lithium-ion batteries were tested for each embodiment or comparative example. The pass rate (%) was calculated as: (Number of passing batteries / 100) × 100%.

[0124] Example 1-1

[0125] <Preparation of the positive electrode>

[0126] The positive electrode active material is lithium iron phosphate, the conductive agent is Super P, and the second binder is PVDF (weight average molecular weight is 7×10). 6 The mixture of sodium polyacrylate and surfactant in a mass ratio of 97:0.9:1.9:0.2 was dispersed in N-methylpyrrolidone (NMP) and stirred under vacuum to obtain a positive electrode slurry with a solid content of 75wt% and a homogeneous system.

[0127] Preparation of modified polyimide: Under nitrogen protection in anhydrous NMP solvent, diamine monomer and dianhydride monomer were mixed at a molar ratio of 1:1 and subjected to polycondensation to generate a reactant solution. Butyl acrylate, cyano-containing acrylonitrile, carboxyl-containing acrylic acid, and the initiator azobisisobutyronitrile (AIBN) were added to the reactant solution, and the mixture was reacted at 60°C for 3 hours to generate a gel. Acetic anhydride as a dehydrating agent and triethylamine as a catalyst were added to the gel, and the mixture was stirred for 2 hours. The modified polyimide was obtained by filtration. The diamine monomer was 4,4'-diaminodiphenyl ether; the dianhydride monomers included cyano-containing dianhydride monomers and carboxyl-containing dianhydride monomers, with a molar ratio of cyano-containing dianhydride monomers to carboxyl-containing dianhydride monomers of 1:2; the cyano-containing dianhydride monomer was 2,2'-bis(3-cyano-4-hydroxyphenyl)hexafluoropropane dianhydride, and the carboxyl-containing dianhydride monomer was 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride. The acrylate monomer is butyl acrylate. The molecular weight of the modified polyimide is 250,000 g / mol. The molar ratio of cyano groups, carboxyl groups, and acrylate monomers in the modified polyimide is 1:25:15.

[0128] After the insulating material and the first binder were mixed evenly, they were dispersed in NMP and stirred evenly to obtain an edge coating slurry with a solid content of 25 wt% and a viscosity of 6273.7 mPa·s. Based on the mass of the edge coating, the mass percentage of the insulating material W1 = 88%, and the mass percentage of the first binder W2 = 12%. The insulating material was selected from boehmite with a particle size Dv50 = 1.1 μm. The first binder consisted of modified polyimide and polyvinylidene fluoride (weight average molecular weight 60 W), with a mass ratio of modified polyimide to polyvinylidene fluoride of 1:1.

[0129] The positive electrode slurry and edge coating slurry are uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector, and then dried at 120°C to obtain a semi-finished positive electrode sheet with a single-sided coating of positive active material and an edge coating. The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive active material and an edge coating (see structure). Figure 1 But not with Figure 1 (For limitations). After cold pressing, cutting, and welding of tabs, a positive electrode sheet with dimensions of 74mm × 867mm is obtained for use. The thickness T of the edge coating is specified. 50 =20μm, width W 50 =3.5mm.

[0130] <Preparation of Negative Electrode Sheets>

[0131] Artificial graphite (negative electrode active material), styrene-butadiene rubber (fourth binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97.4:1.4:1.2. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120 °C to obtain a negative electrode sheet with a single-sided coating of negative electrode active material layer. The coating weight of the negative electrode active material layer was 142 mg / 1540 mm². 2 Then, repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode active material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with a size of 78mm×875mm for later use.

[0132] <Preparation of Electrolyte>

[0133] In an environment with a water content of less than 10 ppm, ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a mass ratio of 40:40:20 to obtain an organic solvent. Then, lithium salt LiPF6 was added to the organic solvent and mixed evenly to obtain an electrolyte with a lithium salt concentration of 1.17 mol / L.

[0134] <Septum>

[0135] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0136] <Preparation of Lithium-ion Batteries>

[0137] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0138] Examples 1-2 to Examples 1-14

[0139] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0140] Examples 2-1 to 2-7

[0141] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0142] Examples 3-1 to 3-3

[0143] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Examples 1-1.

[0144] Examples 4-1 to 4-11

[0145] Except for adjusting the relevant preparation parameters according to Table 4, the rest is the same as in Examples 1-1.

[0146] Example 5-1

[0147] Except for adjusting the relevant preparation parameters according to Table 5, the rest is the same as in Examples 1-1.

[0148] Example 5-2

[0149] Except for adjusting the relevant preparation parameters according to Table 5, the rest is the same as in Examples 1-3.

[0150] Comparative Example 1

[0151] Except for replacing the modified polyimide in <Preparation of Positive Electrode Sheet> with polyimide, the rest is the same as in Example 1-1.

[0152] Comparative Example 2

[0153] Except that the first binder in the <Preparation of Positive Electrode Sheet> is polyvinylidene fluoride and does not include modified polyimide, the rest is the same as in Example 1-1.

[0154] Comparative Examples 3 to 6

[0155] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0156] The preparation parameters and performance data of each embodiment and comparative example are shown in Tables 1 to 5.

[0157] Table 1

[0158]

[0159] Note: In Table 1, "\" indicates that there is no corresponding parameter.

[0160] As can be seen from Examples 1-1 to 1-10 and Comparative Examples 1 to 6, the secondary battery of this application has a positive electrode sheet with an edge coating containing modified polyimide and polyvinylidene fluoride. By controlling the molar ratio of cyano, carboxyl and acrylate monomers in the modified polyimide and the molecular weight of the modified polyimide within the range of this application, the part of the positive electrode sheet with the edge coating has a smaller stiffness, the mixed layer in the positive electrode sheet has a smaller width, and the edge coating and the positive current collector have a higher adhesion after immersion. The secondary battery has a higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the problem of insufficient adhesion of the edge coating after immersion in electrolyte have been improved, and the capacity and safety performance of the secondary battery have been improved. In contrast, the secondary battery in the comparison does not contain modified polyimide in the edge coating of its positive electrode, or the molar ratio of cyano, carboxyl and acrylate monomers is not within the range of this application, or the molecular weight of the modified polyimide is not within the range of this application. The mixed layer in its positive electrode has a large width, and the edge coating has low adhesion to the positive current collector after immersion. The secondary battery has low capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the problem of insufficient adhesion of the edge coating after immersion in electrolyte have not been improved.

[0161] in, Figure 4 The infrared spectrum of the modified polyimide in Example 1-1 is shown below. Figure 4 It can be seen from this that 1776.78cm -1 The corresponding C=O asymmetric stretching in the imide is 723.03 cm. -1 Corresponding to the outward bending of the imide ring, 1716.95 cm -1 The peak at 2926.10 cm⁻¹ is a superposition of the C=O ester group in the acrylate ring, the C=O ester group in the imide ring, and the ester group in the carboxyl group. -1 The peak represents the stretching vibrations of -CH2 and -CH3 in acrylates, at 3306.11 cm⁻¹. -1 The peak represents the stretching vibration of the -OH group in the carboxyl group, at 2232.29 cm⁻¹. -1 The peak represents the stretching vibration of the cyano group, but it is not obvious due to the small amount added. Figure 5 These are microscope images of the positive electrode sheet from Example 1-1. Figure 5 It can be seen that the edge coating 50 does not diffuse into the positive electrode active material layer 30, and there is no mixing layer 60 between the edge coating 50 and the positive electrode active material layer 30. Figure 6 Here are microscope images of the positive electrode in Comparative Example 2, from... Figure 6 It can be seen that the edge coating 50 diffuses into the positive electrode active material layer 30, resulting in a mixed layer 60 between the edge coating 50 and the positive electrode active material layer 30, with a width d = 0.5 mm.

[0162] The mass ratio of modified polyimide to polyvinylidene fluoride in the first binder affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 1-11 to 1-14, secondary batteries using a mass ratio of modified polyimide to polyvinylidene fluoride within the range of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a smaller width in the mixed layer of the positive electrode, and higher adhesion force F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion force of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0163] Table 2

[0164]

[0165] The types of diamine monomers and dianhydride monomers affect the capacity and safety performance of secondary batteries. As can be seen from Examples 1-1, 2-1 to 2-3, secondary batteries using diamine monomers and dianhydride monomers within the scope of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a smaller width in the mixed layer of the positive electrode, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0166] The molar ratio of cyano-dianhydride monomers to carboxyl-dianhydride monomers affects the capacity and safety performance of secondary batteries. As can be seen from Examples 1-1, 2-4 to 2-7, secondary batteries using a molar ratio of cyano-dianhydride monomers to carboxyl-dianhydride monomers within the range of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a smaller width in the mixed layer of the positive electrode, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0167] Table 3

[0168]

[0169] The number of carbon atoms in the alkyl chain of the acrylate monomer and the type of acrylate monomer affect the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 3-1 to 3-3, secondary batteries using acrylate monomers with the number of carbon atoms in the alkyl chain and the type of acrylate monomer within the scope of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a smaller width in the mixed layer of the positive electrode, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0170] Table 4

[0171]

[0172]

[0173] The mass percentage W1 of the first binder and the mass percentage W2 of the insulating material in the edge coating affect the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 4-1 to 4-6, secondary batteries using edge coatings with mass percentages W1 of the first binder and W2 of the insulating material within the scope of this application exhibit lower stiffness in the portion of the positive electrode sheet where the edge coating is located, a smaller width in the mixed layer of the positive electrode sheet, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0174] The type of insulating material affects the capacity and safety performance of a secondary battery. As can be seen from Examples 1-1 and 4-7, secondary batteries using insulating materials within the scope of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a narrower width in the mixed layer of the positive electrode, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0175] The particle size Dv50 of the insulating material affects the capacity and safety performance of the secondary battery. As can be seen from Examples 1-1, 4-8 to 4-11, secondary batteries using insulating materials with a particle size Dv50 within the scope of this application exhibit lower stiffness in the edge coating portion of the positive electrode, a smaller width in the mixed layer of the positive electrode, and higher adhesion F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode and the insufficient adhesion of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0176] Table 5

[0177]

[0178]

[0179] The type of positive electrode active material affects the capacity and safety performance of secondary batteries. As can be seen from Examples 1-1, 1-3, 5-1, and 5-2, secondary batteries using positive electrode active materials within the scope of this application exhibit lower stiffness in the edge coating portion of the positive electrode sheet, a smaller width in the mixed layer of the positive electrode sheet, and higher adhesion force F2 between the edge coating and the positive current collector after immersion. These secondary batteries demonstrate higher capacity and throughput, indicating that the climbing phenomenon in the positive electrode sheet and the insufficient adhesion force of the edge coating after immersion in electrolyte have been improved, resulting in higher capacity and better safety performance.

[0180] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0181] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0182] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive current collector has a first edge along the width direction of the positive electrode sheet, and an edge coating is disposed on the surface of the positive current collector between the positive active material layer and the first edge, the edge coating comprising an insulating material and a first adhesive; The first adhesive comprises a modified polyimide, or the first adhesive comprises the modified polyimide and polyvinylidene fluoride; The modified polyimide is obtained by modifying the polyimide with acrylate monomers, and the main chain of the polyimide is connected with cyano groups and carboxyl groups; the molar ratio of the cyano groups, the carboxyl groups and the acrylate monomers is (1 to 10):(10 to 25):(10 to 25). The modified polyimide has a molecular weight of 50,000 g / mol to 500,000 g / mol.

2. The positive electrode sheet according to claim 1, wherein, Based on the quality of the edge coating, the first adhesive has a mass percentage content of 10% to 30%.

3. The positive electrode sheet according to claim 1, wherein, The first adhesive comprises the modified polyimide and the polyvinylidene fluoride, wherein the mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(0 to 5).

4. The positive electrode sheet according to claim 3, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1) The molecular weight of the modified polyimide is from 200,000 g / mol to 300,000 g / mol; (2) The molar ratio of the cyano group, the carboxyl group and the acrylate monomer is (1 to 5):(15 to 25):(15 to 25); (3) Based on the quality of the edge coating, the mass percentage of the first adhesive is 12% to 20%; (4) The mass ratio of the modified polyimide to the polyvinylidene fluoride is 1:(1 to 3).

5. The positive electrode sheet according to claim 1, wherein, The polyimide is formed by polycondensation of a diamine monomer and a dianhydride monomer, wherein the molar ratio of the diamine monomer to the dianhydride monomer is 1:

1. The diamine monomer includes at least one of 4,4'-diaminodiphenyl ether, p-phenylenediamine, or biphenylenediamine; The dianhydride monomers include cyano-containing dianhydride monomers and carboxyl-containing dianhydride monomers, wherein the molar ratio of the cyano-containing dianhydride monomers to the carboxyl-containing dianhydride monomers is 1:(1 to 3).

6. The positive electrode sheet according to claim 5, wherein, The cyano-containing dianhydride monomer includes at least one of 2,2'-bis(3-cyano-4-hydroxyphenyl)hexafluoropropane dianhydride, 4,4'-(hexafluoroisopropene) diaphthalic acid dianhydride or 3,3'-dicyano-4,4'-oxobisphthalic acid dianhydride. The carboxyl-containing dianhydride monomer includes at least one of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene) phthalic anhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, or biphenyltetracarboxylic dianhydride.

7. The positive electrode sheet according to claim 1, wherein, The alkyl chain in the acrylate monomer has 4 to 8 carbon atoms.

8. The positive electrode sheet according to claim 1, wherein, The acrylate monomer includes at least one of butyl acrylate, isobutyl acrylate, heptyl acrylate, or cyclohexyl acrylate.

9. The positive electrode sheet according to any one of claims 1 to 7, wherein, The 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 edge coating, the mass percentage of the insulating material is 70% to 90%.

10. The positive electrode sheet according to any one of claims 1 to 7, wherein, The particle size Dv50 of the insulating material is from 0.4 μm to 1.8 μm.

11. The positive electrode sheet according to claim 1, wherein, The thickness of the edge coating is 10 μm to 50 μm; and / or the width of the edge coating is 2 mm to 5 mm.

12. The positive electrode sheet according to claim 1, wherein, The positive electrode active material layer includes a positive electrode active material, which includes at least one of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, or lithium titanate.

13. The positive electrode sheet according to claim 1, wherein, Along the width direction of the unfolded state of the positive electrode sheet, the width of the mixed layer between the edge coating and the positive electrode active material layer is d mm, where 0 ≤ d ≤ 0.

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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