Positive pole piece, secondary battery and electronic device

Abstract

The invention provides a positive electrode plate, a secondary battery and an electronic device, the positive electrode plate 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 active material layer comprises a positive electrode conductive agent, the positive electrode conductive agent comprises carbon black particles and a first carbon nanotube with a cluster structure, the first carbon nano tube is formed by arranging a plurality of carbon nano tube units in a bundle shape; wherein the particle size of the carbon black particles is R1, and R1 is more than or equal to 3nm and less than or equal to 40nm; the tube diameter of the first carbon nano tube is 0.5-2 microns, the diameter of a single carbon nano tube unit in the first carbon nano tube is R2, and R2 is more than or equal to 5 nm and less than or equal to 25 nm. Through the arrangement, the secondary battery has a relatively low thickness swelling rate after being stored at a high temperature on the basis of relatively low internal resistance.

Description

Technical Field

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

[0002] Lithium-ion secondary batteries have the advantages of high energy storage density, high open circuit voltage, low self-discharge rate, long cycle life, good safety, etc. They are widely used in various fields such as power storage, mobile electronic devices, electric vehicles, and aerospace equipment. As mobile electronic devices and electric vehicles enter a stage of rapid development, the market has been pursuing lithium-ion secondary batteries with lower internal resistance.

[0003] The existing technology often reduces the internal resistance of lithium-ion batteries by increasing the positive electrode conductive agent content in the positive electrode sheet or reducing the binder content in the positive electrode sheet. However, the reduction in the binder content will reduce the cohesion of the positive electrode active material layer itself, or reduce the bonding force between the positive electrode active material layer and the positive electrode current collector, resulting in the problem of thickness expansion of lithium-ion batteries at high temperatures (≥80°C). Therefore, how to make lithium-ion batteries have good high-temperature storage performance without affecting the internal resistance has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0004] The purpose of the present application is to provide a positive electrode plate, a secondary battery and an electronic device, so that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

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

[0006] The first aspect of the present application provides a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer includes a positive electrode conductive agent, the positive electrode conductive agent includes carbon black particles and a first carbon nanotube with a cluster structure, the first carbon nanotube is composed of a plurality of carbon nanotube units arranged in a bundle; wherein the particle size of the carbon black particles is R 1 , 3nm≤R 1 ≤40nm; the diameter of the first carbon nanotube is 0.5μm to 2μm, and the diameter of a single carbon nanotube unit is R 2 , 5nm≤R 2≤25nm. The positive electrode sheet provided in the first aspect of the present application is provided by adding carbon black particles and first carbon nanotubes with a cluster structure as positive electrode conductive agents in the positive electrode active material layer, and the particle size of the carbon black particles, the tube diameter of the first carbon nanotubes and the diameter of a single carbon nanotube unit are regulated within the scope of the present application. The first carbon nanotubes with a cluster structure are distributed between the particles of the positive electrode active material, and have a long-range conductive effect. The binder adsorbed on its surface will also be brought into the gaps between the particles of the positive electrode active material, resulting in a reduction in the content of the first carbon nanotubes on the surface of the positive electrode active material layer, and the binder content on the surface of the positive electrode active material layer is also reduced, affecting the bonding force between the positive electrode active material layer and the positive electrode current collector; the carbon black particles have a short-range conductive effect, and they have a high specific surface area. After the carbon black particles adsorb the binder on the surface, they are easily distributed on the surface of the positive electrode active material layer, which can make up for the disadvantage of insufficient bonding force brought by the aforementioned first carbon nanotubes. In this way, the first carbon nanotubes and carbon black particles of the present application work synergistically, so that the particles of the positive electrode active material have a high bonding force, and the surface of the positive electrode active material layer also has a high bonding force, which can make the positive electrode active material layer and the positive electrode current collector have a high bonding force, and the positive electrode plate has a high cohesive force. As a result, the secondary battery using the positive electrode plate has a small thickness expansion rate after high-temperature storage, that is, it has good high-temperature storage performance. The combination of the first carbon nanotubes and carbon black particles can also improve the conductivity of the positive electrode plate, so that the secondary battery using the positive electrode plate has a lower internal resistance.

[0007] In one embodiment of the present application, R 1 and R 2 Satisfaction: R 1 ≤2R 2 The particle size R of carbon black particles 1 and the diameter R of a single carbon nanotube unit in the first carbon nanotube 2 When the above relationship is satisfied, the cohesion of the positive electrode sheet can be improved, the positive electrode sheet also has a lower resistance, and the secondary battery has a smaller thickness expansion rate under high-temperature storage.

[0008] In one embodiment of the present application, the positive electrode sheet satisfies at least one of the following characteristics: (1) 8nm≤R 1 ≤15nm; (2) the diameter of the first carbon nanotube is 0.8μm to 1.2μm; (3) 8nm≤R 2 ≤20nm. This is conducive to making the positive electrode sheet have higher cohesion and lower resistance, and the secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0009] In one embodiment of the present application, the mass percentage of carbon black particles is W based on the mass of the positive electrode active material layer. 1 , the mass percentage of the first carbon nanotube is W2 , 0.2% ≤ W 1 ≤1.0%, 0.2%≤W 2 ≤0.8%. When the mass percentage of the first carbon nanotube and the carbon black particles is controlled within the above range, the positive electrode sheet has a higher cohesive force and a lower resistivity, and a secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0010] In one embodiment of the present application, W 1 , W 2 , R 1 and R 2 Satisfy: 0.2≤(W 1 / R 1 ) / (W 2 / R 2 )≤1.3. 1 / R 1 ) / (W 2 / R 2 ) is controlled within the above range, which is beneficial to improving the cohesion of the positive electrode sheet and reducing the resistivity of the positive electrode sheet. The secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0011] In one embodiment of the present application, the length of the first carbon nanotube is 5 μm to 10 μm. By adjusting the length of the first carbon nanotube within the above range, the positive electrode sheet can have a higher cohesive force and a lower resistivity, and the secondary battery has a lower thickness expansion rate after high-temperature storage, and the secondary battery has good processing performance.

[0012] In one embodiment of the present application, the specific surface area of ​​the carbon black particles is 500 m 2 / g to 1600m 2 / g. Controlling the specific surface area of ​​carbon black particles within the above range is conducive to improving the cohesion of the positive electrode sheet while having a lower resistance. The secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0013] In one embodiment of the present application, the specific surface area of ​​the carbon black particles is 800 m 2 / g to 1400m 2 / g. Regulating the specific surface area of ​​carbon black particles within the above range is conducive to further improving the cohesion of the positive electrode sheet, thereby further reducing the thickness expansion rate of the secondary battery after high-temperature storage. The positive electrode sheet also has a high conductivity efficiency.

[0014] In one embodiment of the present application, the positive electrode conductive agent further includes a single distributed second carbon nanotube, the diameter of the second carbon nanotube is 3nm to 20nm, and the length of the second carbon nanotube is 1μm to 3μm. Further introducing a single distributed second carbon nanotube with a diameter and a length within the above range into the positive electrode active material layer as a positive electrode conductive agent, the cohesion of the positive electrode sheet is further improved, the resistance is further reduced, and the thickness expansion rate of the secondary battery after high temperature storage is further reduced.

[0015] In one embodiment of the present application, based on the mass of the positive electrode active material layer, the mass percentage of the second carbon nanotubes is W 3 , 0%<W 3 ≤0.5%. When the mass percentage of the second carbon nanotubes is controlled within the above range, the positive electrode sheet has a higher cohesive force and a lower resistivity, and the secondary battery has a smaller thickness expansion rate after high temperature storage. In one embodiment of the present application, the positive electrode active material layer further includes a binder; based on the positive electrode active material layer, the mass percentage of the positive electrode conductive agent is C 1 , the mass percentage of the binder is C 2 , 0.5≤C 1 / C 2 ≤1.5,0.8%≤C 2 ≤3%. The mass percentage of the binder in the positive electrode active material layer and its ratio to the positive electrode conductive agent C 1 / C 2 The value is regulated within the above range, so that the positive electrode plate has higher cohesion and lower resistivity. The secondary battery using the positive electrode plate has a smaller thickness expansion rate after high-temperature storage and also has a higher energy density.

[0016] In one embodiment of the present application, the binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyamide, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber or carboxymethyl cellulose salt. The above-mentioned types of binders are used in the positive electrode active material layer, which is conducive to making the positive electrode plate have a higher cohesive force and making the secondary battery using the positive electrode plate have a smaller thickness expansion rate after high temperature storage.

[0017] In one embodiment of the present application, the positive electrode active material layer further includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium-rich manganese-based materials, or lithium titanate; based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 95.4% to 98.5%. The above types of positive electrode active materials are selected and the content of the positive electrode active materials is regulated within the above range, so that the secondary battery has a higher energy density on the basis of a lower thickness expansion rate after high temperature storage.

[0018] In one embodiment of the present application, the volume density of the positive electrode active material layer is 3.9 g / cm 3 Up to 4.15g / cm 3 Controlling the volume density of the positive electrode active material layer within the above range is beneficial for making the positive electrode sheet have a smaller resistance and also for making the secondary battery have a smaller thickness expansion rate after high-temperature storage.

[0019] In one embodiment of the present application, the cohesive force of the positive electrode sheet is 30 N / m to 85 N / m, indicating that the positive electrode sheet has a relatively high cohesive force.

[0020] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet described in any of the above embodiments. Therefore, the secondary battery has a smaller thickness expansion rate after high-temperature storage, indicating that the secondary battery has good storage performance.

[0021] The third aspect of the present application provides an electronic device, which comprises the secondary battery described in any one of the above embodiments. Therefore, the electronic device has good performance.

[0022] Beneficial effects of this application:

[0023] The present application provides a positive electrode sheet, a secondary battery and an electronic device, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode conductive agent, wherein the positive electrode conductive agent comprises carbon black particles and a first carbon nanotube having a cluster structure, wherein the first carbon nanotube is composed of a plurality of carbon nanotube units arranged in a bundle; wherein the particle size of the carbon black particles is R 1 , 3nm≤R 1 ≤40nm; the diameter of the first carbon nanotube is 0.5μm to 2μm, and the diameter of a single carbon nanotube unit is R 2 , 5nm≤R 2≤25nm. Through the above arrangement, the first carbon nanotubes and the carbon black particles work synergistically, so that the particles of the positive electrode active material have a higher bonding force, the surface of the positive electrode active material layer also has a higher bonding force, which can make the positive electrode active material layer and the positive electrode current collector have a higher bonding force, and the positive electrode plate has a higher cohesive force. As a result, the secondary battery using the positive electrode plate has a smaller thickness expansion rate after high-temperature storage. The combination of the first carbon nanotubes and the carbon black particles can also improve the conductivity of the positive electrode plate, so that the secondary battery using the positive electrode plate has a lower internal resistance.

[0024] Of course, implementing any product or method of the present application does not necessarily require achieving all of the advantages described above at the same time. DETAILED DESCRIPTION

[0025] The following will be combined with the embodiments of the present application to clearly and completely describe the technical solutions in the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0026] It should be noted that in the specific implementation manner of the present application, the present application is explained by taking a lithium-ion battery as an example of a secondary battery, but the secondary battery of the present application is not limited to a lithium-ion battery.

[0027] The first aspect of the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode active material layer disposed on at least one surface of the positive electrode collector. The above-mentioned "positive electrode active material layer disposed on at least one surface of the positive electrode collector" means that the positive electrode active material layer can be disposed on one surface or two surfaces of the positive electrode collector along its own thickness direction, and the above-mentioned "surface" can be a partial surface of the positive electrode collector or the entire surface of the positive electrode collector. The positive electrode active material layer includes a positive electrode conductive agent, and the positive electrode conductive agent includes carbon black particles and a first carbon nanotube with a clustered structure, and the first carbon nanotube is composed of a plurality of carbon nanotube units arranged in a bundle; wherein the particle size of the carbon black particles is R 1 , 3nm≤R 1 ≤40nm; the diameter of the first carbon nanotube is 0.5μm to 2μm, and the diameter of a single carbon nanotube unit is R 2 , 5nm≤R 2 ≤25nm.

[0028] For example, the diameter of the first carbon nanotube is 0.5 μm, 0.7 μm, 0.8 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.5 μm, 1.7 μm, 2 μm or any value between any two of the above ranges. The diameter of the first carbon nanotube is less than 0.5 μm, the first carbon nanotube is too thin, the first carbon nanotube is easy to agglomerate, the first carbon nanotube is unevenly distributed between the particles of the positive electrode active material, and the content of the first carbon nanotube on the surface of the positive electrode active material layer is reduced, and the content of the binder adsorbed on the surface of the first carbon nanotube is also reduced on the surface of the positive electrode active material layer, resulting in the poor bonding between the particles of the positive electrode active material in the positive electrode sheet, or the poor bonding between the positive electrode active material layer and the positive electrode current collector, the cohesive force of the positive electrode sheet is reduced, and the thickness expansion rate of the secondary battery after high temperature storage will increase; the diameter of the first carbon nanotube is greater than 2 μm, the first carbon nanotube is too thick, and at the same first carbon nanotube content, the number of the first carbon nanotubes will be too small, which will reduce the number of bridges between the particles of the positive electrode active material, so that there is no bridge between some particles of the positive electrode active material, which will affect the conductive efficiency and worsen the impedance of the secondary battery. "High temperature" in this application refers to a temperature ≥ 80 ° C.

[0029] For example, the diameter R of a single carbon nanotube unit in the first carbon nanotube is 2 The diameter R of a single carbon nanotube unit in the first carbon nanotube is 5 nm, 7 nm, 8 nm, 10 nm, 12 nm, 13 nm, 16 nm, 17 nm, 18 nm, 20 nm, 21 nm, 22 nm, 25 nm or any value between any two of the above ranges. 2 The diameter of a single carbon nanotube unit is less than 5nm, which makes it easy to make the diameter of the first carbon nanotube too small, resulting in agglomeration of the first carbon nanotubes, uneven distribution of the first carbon nanotubes between the particles of the positive electrode active material, and a decrease in the content of the first carbon nanotubes on the surface of the positive electrode active material layer. The content of the binder adsorbed on the surface of the first carbon nanotubes is also reduced on the surface of the positive electrode active material layer, resulting in a weak bond between the particles of the positive electrode active material in the positive electrode sheet, or a weak bond between the positive electrode active material layer and the positive electrode current collector, a decrease in the cohesive force of the positive electrode sheet, and an increase in the thickness expansion rate of the secondary battery after high-temperature storage; the diameter R of a single carbon nanotube unit in the first carbon nanotube is less than 5nm, which makes the diameter of a single carbon nanotube unit in the first carbon nanotube is less than 5nm, which makes it easy to make the diameter of the first carbon nanotube unit be too small, resulting in a weak bond between the particles of the positive electrode active material in the positive electrode sheet, or a weak bond between the positive electrode active material layer and the positive electrode current collector, a decrease in the cohesive force of the positive electrode sheet, and an increase in the thickness expansion rate of the secondary battery after high-temperature storage; the diameter R of a single carbon nanotube unit in the first carbon nanotube is less than 5nm, which makes it easy to make the diameter of the first carbon nanotube unit be less than 5nm ... 2 If the diameter is larger than 25 nm, the diameter of a single carbon nanotube unit is too large, which will affect the conductivity and increase the resistance of the secondary battery.

[0030] For example, the particle size R of carbon black particles 1The carbon black particles are easily distributed on the surface of the positive electrode active material layer after adsorbing the binder to improve the distribution of the binder in the positive electrode active material layer. When the particle size R of the carbon black particles is 1 If the carbon black particle size is less than 3nm, the carbon black particle size is too small, the carbon black particles are easy to agglomerate in the positive electrode slurry, the probability of uniform distribution in the positive electrode active material layer is small, the probability of improving the distribution uniformity of the binder in the positive electrode active material layer is small, the bonding force between the positive electrode active material layer and the positive electrode current collector is small, resulting in a small cohesive force in the positive electrode sheet, and a large thickness expansion rate of the secondary battery after high temperature storage; the particle size R of the carbon black particles 1 When the carbon black particles are larger than 40nm, the specific surface area is smaller and the content of surface binder is reduced, which will also reduce the bonding force between the particles of the positive electrode active material and the bonding force on the surface of the positive electrode active material layer, thereby reducing the cohesive force of the positive electrode sheet and increasing the thickness expansion of the secondary battery after high-temperature storage.

[0031] The positive electrode plate provided in the first aspect of the present application is achieved by adding carbon black particles and first carbon nanotubes with a clustered structure as positive electrode conductive agents in the positive electrode active material layer, and the particle size of the carbon black particles, the tube diameter of the first carbon nanotubes, and the diameter of a single carbon nanotube unit in the first carbon nanotubes are regulated within the scope of the present application. The first carbon nanotubes with a clustered structure are distributed between the particles of the positive electrode active material, and have a long-range conductive effect. The binder adsorbed on the surface of the first carbon nanotubes will also be brought into the gaps between the particles of the positive electrode active material, resulting in a decrease in the content of the first carbon nanotubes on the surface of the positive electrode active material layer, and the binder content on the surface of the positive electrode active material layer will also decrease accordingly, affecting the bonding force between the positive electrode active material layer and the positive electrode current collector; the carbon black particles have a short-range conductive effect, and they have a high specific surface area. After the binder is adsorbed on the surface of the carbon black particles, they are easily distributed on the surface of the positive electrode active material layer, which can make up for the disadvantage of insufficient bonding force brought by the aforementioned first carbon nanotubes. In this way, the first carbon nanotubes and carbon black particles of the present application work synergistically, so that the particles of the positive electrode active material have a high bonding force, the surface of the positive electrode active material layer also has a high bonding force, which can make the positive electrode active material layer and the positive electrode current collector have a high bonding force, and the positive electrode plate has a high cohesive force. As a result, the secondary battery using the positive electrode plate has a smaller thickness expansion rate after high-temperature storage. The combination of the first carbon nanotubes and carbon black particles can also improve the conductive efficiency of the positive electrode plate, so that the secondary battery using the positive electrode plate has a lower internal resistance.

[0032] In one embodiment of the present application, R 1 and R 2 Satisfaction: R1 ≤2R 2 The particle size R of carbon black particles 1 and the diameter R of a single carbon nanotube unit in the first carbon nanotube 2 When the above relationship is satisfied, the particle size of the carbon black particles is relatively small and the specific surface area is large, and they can carry an appropriate amount of binder on the surface of the positive electrode active material to balance the first carbon nanotubes with a clustered structure carrying the binder into the gaps between the particles of the positive electrode active material, thereby improving the cohesion of the positive electrode sheet. The positive electrode sheet also has a lower resistance, and the secondary battery has a smaller thickness expansion rate under high-temperature storage.

[0033] In one embodiment of the present application, 8nm≤R 1 ≤15nm. For example, the particle size R of carbon black particles 1 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm or any value between any two of the above numerical ranges. When the particle size of the carbon black particles is regulated within the above range, the probability of agglomeration of the carbon black particles is small, and the carbon black particles also have a higher specific surface area. After having more adhesive force adsorbed on the surface, it is easier to distribute on the surface of the positive electrode active material layer, which can improve the uniform distribution of the binder in the positive electrode active material layer, so that the positive electrode active material layer and the positive electrode current collector have good adhesive force, the positive electrode plate has a higher cohesive force, and the secondary battery using the positive electrode plate has a lower thickness expansion rate after high temperature storage. It also makes the positive electrode plate have a higher conductivity efficiency, and the secondary battery using the positive electrode plate has a lower internal resistance.

[0034] In one embodiment of the present application, the diameter of the first carbon nanotube is 0.8 μm to 1.2 μm. For example, the diameter of the first carbon nanotube is 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm or any value between any two of the above ranges. Regulating the diameter of the first carbon nanotube within the above range is conducive to further promoting the mutual cooperation between the first carbon nanotube and the carbon black particles, so that the positive electrode sheet has a higher cohesive force and a lower resistivity, thereby improving the thickness expansion rate of the secondary battery after high temperature storage without affecting its resistance.

[0035] In one embodiment of the present application, 8nm≤R 2 ≤20nm. For example, the diameter R of a single carbon nanotube unit in the first carbon nanotube 2 is 8 nm, 10 nm, 12 nm, 13 nm, 16 nm, 17 nm, 18 nm, 20 nm or any value between any two of the above ranges. 2Controlling within the above range is beneficial to further promote the mutual coordination between the first carbon nanotubes and the carbon black particles, so that the positive electrode sheet has higher cohesion and lower resistivity, thereby improving the thickness expansion rate of the secondary battery after high-temperature storage without affecting its resistance.

[0036] In the present application, "the particle size of carbon black particles" refers to the average value of the equivalent diameter of the primary particles of carbon black particles. The equivalent diameter generally refers to the diameter of a sphere with the same volume as an object of irregular shape. In the present application, the cross section of the positive electrode sheet is obtained, the area of ​​the carbon black particles on the cross section is measured, and then the diameter of the circle with the same area as the area is used as the equivalent diameter of the carbon black particles. The present application does not particularly limit the method for regulating the particle size of carbon black particles, as long as the purpose of the present application can be achieved. For example, it can be achieved by directly purchasing carbon black particles with a particle size within the range of the present application, or by crushing, grinding or ball milling.

[0037] In one embodiment of the present application, the mass percentage of carbon black particles is W based on the mass of the positive electrode active material layer. 1 , the mass percentage of the first carbon nanotube is W 2 , 0.2% ≤ W 1 ≤1.0%, 0.2%≤W 2 ≤0.8%. For example, the mass percentage of carbon black particles W 1 The mass percentage of the first carbon nanotubes is 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or any value between any two of the above ranges. 2 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8% or any value between any two of the above numerical ranges. The mass percentage of the first carbon nanotubes and carbon black particles is regulated within the above range, and the first carbon nanotubes and carbon black particles can form a good long-range and short-range conductive network in the positive electrode active material layer, provide good conductivity for the positive electrode sheet, and can also provide good conductivity efficiency while providing good bonding between the particles of the positive electrode active material and the surface of the positive electrode active material layer. In this way, the positive electrode sheet has a higher cohesive force and a lower resistivity, and the secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0038] In one embodiment of the present application, W 1 , W 2 , R 1 and R 2 Satisfy: 0.2≤(W 1 / R 1 ) / (W 2 / R 2)≤1.5. For example, (W 1 / R 1 ) / (W 2 / R 2 ) is 0.2, 0.4, 0.5, 0.7, 0.8, 1, 1.1, 1.2, 1.3, 1.4, 1.5 or any value between any two of the above numerical ranges, preferably in the range of 0.2 to 1.3. 1 / R 1 ) / (W 2 / R 2 ) is regulated within the above range, the carbon black particles match the first carbon nanotubes, which is beneficial to improving the cohesion of the positive electrode sheet and reducing the resistivity of the positive electrode sheet. The secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage.

[0039] In one embodiment of the present application, the length of the first carbon nanotube is 5 μm to 10 μm. For example, the length of the first carbon nanotube is 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any value between any two of the above numerical ranges. The length of the first carbon nanotube is regulated within the above range, which is conducive to bridging the first carbon nanotubes to form a long-range conductive network without affecting the processing of the positive electrode active material layer slurry, so that the positive electrode sheet can have a higher cohesive force and a lower resistivity. The secondary battery has a smaller thickness expansion rate after high temperature storage, and the secondary battery has good processing performance.

[0040] In one embodiment of the present application, the specific surface area of ​​the carbon black particles is 500 m 2 / g to 1600m 2 / g. For example, the specific surface area of ​​carbon black particles is 500m 2 / g、600m 2 / g, 620m 2 / g、700m 2 / g、768m 2 / g、800m 2 / g、900m 2 / g、1000m 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g、1400m 2 / g、1500m 2 / g、1600m 2 / g or any value between any two numerical ranges mentioned above. The specific surface area of ​​the carbon black particles is regulated within the above range. The carbon black particles have a higher specific surface area and are more likely to adsorb the binder on their surface. After the carbon black particles adsorb the binder, they are easily evenly distributed on the surface of the positive electrode active material layer, so that the positive electrode active material layer and the positive electrode current collector have good bonding force, which is beneficial to improve the cohesion of the positive electrode plate with lower resistance. The secondary battery using the positive electrode plate has a smaller thickness expansion rate after high-temperature storage.

[0041] In one embodiment of the present application, the specific surface area of ​​the carbon black particles is 800 m 2 / g to 1400m 2 / g. For example, the specific surface area of ​​carbon black particles is 800m 2 / g、900m 2 / g、1000m 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g、1400m 2 / g or any value between any two numerical ranges mentioned above. Regulating the specific surface area of ​​carbon black particles within the above range is conducive to further improving the cohesion of the positive electrode sheet, thereby further reducing the thickness expansion rate of the secondary battery after high-temperature storage. The positive electrode sheet also has a higher conductivity efficiency.

[0042] The present application has no particular restrictions on the method for regulating the particle size and specific surface area of ​​the carbon black particles, as long as the purpose of the present application can be achieved. For example, it can be achieved by mechanical crushing (such as ball milling). Generally, when other preparation conditions remain unchanged, the particle size of the carbon black particles decreases and the specific surface area of ​​the carbon black particles increases when the ball milling time is extended; the particle size of the carbon black particles increases and the specific surface area of ​​the carbon black particles decreases when the ball milling time is shortened.

[0043] In one embodiment of the present application, the positive electrode conductive agent further comprises a single distributed second carbon nanotube, the diameter of the second carbon nanotube is 3nm to 20nm, and the length of the second carbon nanotube is 1μm to 3μm. For example, the diameter of the second carbon nanotube is 3nm, 5nm, 7nm, 9nm, 10nm, 12nm, 14nm, 16nm, 18nm, 19nm, 20nm or any value between any two of the above numerical ranges. For example, the length of the second carbon nanotube is 1μm, 1.2μm, 1.5μm, 1.7μm, 1.8μm, 2μm, 2.2μm, 2.5μm, 2.7μm, 2.8μm, 3μm or any value between any two of the above numerical ranges. A second carbon nanotube with a single distribution and a diameter and a length within the above range is further introduced into the positive electrode active material layer as a positive electrode conductive agent. The second carbon nanotube has a medium-range conductive effect and can cooperate with the first carbon nanotube and carbon black particles to establish a short-range, medium-range and long-range conductive network in the positive electrode active material layer, so that the positive electrode sheet has a better conductive effect, and the positive electrode active material particles in the positive electrode sheet and the surface of the positive electrode active material layer have better bonding. In this way, the cohesion of the positive electrode sheet is further improved, the resistance is further reduced, and the thickness expansion rate of the secondary battery after high-temperature storage is further reduced.

[0044] In one embodiment of the present application, based on the mass of the positive electrode active material layer, the mass percentage of the second carbon nanotubes is W 3 , 0%<W 3 ≤0.5%. For example, the mass percentage of the second carbon nanotube is 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or any value between any two of the above numerical ranges. Regulating the mass percentage of the second carbon nanotube within the above range is conducive to forming a good mid-range conductive network in the positive electrode active material layer, which cooperates with the first carbon nanotubes and carbon black particles to provide good conductivity for the positive electrode active material layer. The positive electrode active material particles in the positive electrode plate and the surface of the positive electrode active material layer have better bonding. As a result, the positive electrode plate has higher cohesion and lower resistivity, and the secondary battery has a smaller thickness expansion rate after high temperature storage.

[0045] In one embodiment of the present application, the positive electrode active material layer further includes a binder; based on the positive electrode active material layer, the mass percentage of the positive electrode conductive agent is C 1 , the mass percentage of the binder is C 2 , 0.5≤C 1 / C 2 ≤1.5,0.8%≤C 2 ≤3%. For example, C 1 / C 2The value of is 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5 or any value between any two of the above ranges. 2 0.8%, 0.9%, 1.0%, 1.2%, 1.5%, 1.7%, 1.9%, 2.0%, 2.1%, 2.3%, 2.5%, 2.6%, 2.8%, 3% or any value between any two of the above ranges. The mass percentage of the binder in the positive electrode active material layer and its ratio to the positive electrode conductive agent C 1 / C 2 The value of is regulated within the above range, which is conducive to matching the content of the positive electrode conductive agent and the binder, the positive electrode sheet has a good conductive effect, the positive electrode active material particles have good bonding force, the positive electrode active material layer and the positive electrode current collector have good bonding force, and more positive electrode active materials can be added to the positive electrode active material layer as much as possible to provide more capacity for the positive electrode sheet. As a result, the positive electrode sheet has a higher cohesive force and a lower resistivity, and the secondary battery using the positive electrode sheet has a smaller thickness expansion rate after high-temperature storage and also has a higher energy density.

[0046] The mass percentage of the positive electrode conductive agent C 1 There is no special restriction, as long as the purpose of this application can be achieved. For example, the mass percentage of the positive electrode conductive agent C 1 It is between 0.4% and 2%.

[0047] In one embodiment of the present application, the binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyamide, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber or carboxymethyl cellulose salt. The above-mentioned types of binders have good bonding properties. The above-mentioned types of binders are used in the positive electrode active material layer, which is conducive to making the positive electrode plate have a higher cohesive force, so that the secondary battery using the positive electrode has a smaller thickness expansion rate after high temperature storage.

[0048] In one embodiment of the present application, the positive electrode active material layer also includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium nickel cobalt aluminum, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium-rich manganese-based materials or lithium titanate. Based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 95.4% to 98.5%. For example, the mass percentage of the positive electrode active material is 95.4%, 95.7%, 96.0%, 96.3%, 96.5%, 97.0%, 97.5%, 98.0%, 98.1%, 98.5% or any value between any two of the above numerical ranges. Selecting the above-mentioned types of positive electrode active materials and regulating the content of the positive electrode active materials within the above range is conducive to providing a larger capacity for the positive electrode active material layer, so that the secondary battery has a higher energy density on the basis of having a lower thickness expansion rate after high temperature storage.

[0049] In one embodiment of the present application, the volume density of the positive electrode active material layer is 3.9 g / cm 3 Up to 4.15g / cm 3 For example, the volume density of the positive electrode active material layer is 3.9 g / cm 3 、3.95g / cm 3 、3.97g / cm 3 4.02g / cm 3 4.05g / cm 3 , 4.07g / cm 3 , 4.09g / cm 3 , 4.11g / cm 3 4.13g / cm 3 4.15g / cm 3 Or any value between any two of the above numerical ranges. Adjusting the volume density of the positive electrode active material layer within the above range is beneficial to making the positive electrode sheet have a smaller resistance and also beneficial to making the secondary battery have a smaller thickness expansion rate after high temperature storage.

[0050] The present application has no particular restrictions on the method for regulating the volume density of the positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, it can be achieved by regulating the particle size of the positive electrode active material, the cold pressing pressure and time during the preparation of the positive electrode sheet, etc.

[0051] In one embodiment of the present application, the cohesion of the positive electrode sheet is 30N / m to 85N / m. For example, the cohesion of the positive electrode sheet is 30N / m, 40N / m, 42N / m, 50N / m, 55N / m, 60N / m, 62N / m, 65N / m, 70N / m, 75N / m, 80N / m, 85N / m or any value between any two of the above numerical ranges. It shows that the positive electrode sheet has a high cohesion. The secondary battery using the positive electrode sheet has a small thickness expansion rate after high temperature storage.

[0052] The present application has no particular restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil or aluminum alloy foil, etc. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm. The thickness of the single-layer positive electrode active material layer is 30μm to 120μm.

[0053] The present application has no particular restrictions on the method for preparing carbon black particles, as long as the purpose of the present application can be achieved. For example, the method for preparing carbon black particles includes but is not limited to the following steps: placing the raw materials in a reactor to undergo a cracking reaction at a high temperature of 1000°C to 1500°C to generate carbon black particles, blowing the carbon black particles out of the furnace tube, cooling and collecting them. The present application has no particular restrictions on the types of the above raw materials, as long as the purpose of the present application can be achieved. For example, the raw materials may include but are not limited to acetylene or tar.

[0054] The present application has no particular restrictions on the preparation method of the first carbon nanotube, as long as the purpose of the present application can be achieved. For example, the preparation method of the first carbon nanotube includes but is not limited to the following steps: selecting a cobalt-based or iron-based catalyst, depositing the catalyst on a silicon substrate, and placing it in a reactor; introducing an inert protective gas into the reactor, and heating the reactor to 700°C to 1000°C; introducing a reaction gas into the reactor, and the carbon formed by the cracking of the reaction gas in the reactor is deposited on the catalyst to form a carbon nanotube bundle with a cluster structure; and obtaining the first carbon nanotube within the length and diameter range of the present application by low-speed sand grinding and dispersion. The present application has no particular restrictions on the rotation speed of the above-mentioned low-speed sand grinding and dispersion, as long as the purpose of the present application can be achieved. For example, the rotation speed is 900r / min to 1300r / min.

[0055] The present application has no particular restrictions on the preparation method of the second carbon nanotube, as long as the purpose of the present application can be achieved. For example, the preparation method of the second carbon nanotube includes but is not limited to the following steps: select a cobalt-based or iron-based catalyst, deposit the catalyst on a silicon substrate, and place it in a reactor; introduce an inert protective gas into the reactor, and heat the reactor to 600°C to 900°C; introduce a reaction gas into the reactor, and the carbon formed by the cracking of the reaction gas in the reactor is deposited on the catalyst to form a carbon nanotube bundle with a cluster structure; and obtain a single dispersed second carbon nanotube within the length and diameter range of the present application through high-speed sand grinding and dispersion. The present application has no particular restrictions on the rotation speed of the above-mentioned high-speed sand grinding and dispersion, as long as the purpose of the present application can be achieved. For example, the rotation speed is 1700r / min to 2300r / min.

[0056] The present application does not particularly limit the type of the above-mentioned inert gas, as long as the purpose of the present application can be achieved. For example, the inert gas includes but is not limited to any one of argon or nitrogen. The present application does not particularly limit the type of the above-mentioned reaction gas, as long as the purpose of the present application can be achieved. For example, the reaction gas includes but is not limited to acetylene. The present application does not particularly limit the type of the above-mentioned cobalt-based catalyst, and those skilled in the art can choose as needed, as long as the purpose of the present application can be achieved. For example, cobalt-based catalysts include but are not limited to cobalt-nickel alloys. The present application does not particularly limit the type of the above-mentioned iron-based catalyst, and those skilled in the art can choose as needed, as long as the purpose of the present application can be achieved. For example, iron-based catalysts include but are not limited to iron-magnesium alloys.

[0057] Generally speaking, the diameter and length of the first carbon nanotube can be regulated by the rotation speed and time during the above-mentioned low-speed sand milling process. Generally speaking, the greater the speed of the sand milling and the longer the time, the smaller the diameter of the first carbon nanotube and the shorter the length; the smaller the speed of the sand milling and the shorter the time, the larger the diameter of the first carbon nanotube and the longer the length. The present application has no particular restrictions on the method of regulating the diameter of a single carbon nanotube unit in the first carbon nanotube, as long as the purpose of the present application can be achieved. For example, it can be achieved by regulating at least one of the temperature, the type of catalyst, the size of the catalyst or the shape of the catalyst during the synthesis process. The diameter of the second carbon nanotube can be achieved by regulating at least one of the temperature, the type of catalyst, the size of the catalyst or the shape of the catalyst during the synthesis process. The length of the second carbon nanotube can be achieved by regulating the rotation speed and time during the above-mentioned high-speed sand milling process. Generally speaking, the greater the speed of the sand milling and the longer the time, the shorter the length of the second carbon nanotube; the smaller the speed of the sand milling and the shorter the time, the longer the length of the second carbon nanotube.

[0058] The present application has no particular limitation on the preparation method of the positive electrode sheet, and the preparation methods well-known in the art can be adopted as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode sheet includes but is not limited to the following steps: (2) uniformly mixing a positive electrode conductive agent, a binder, and a positive electrode active material in the contents of the present application, adding a solvent, and stirring evenly to obtain a positive electrode slurry; (2) coating the positive electrode slurry on one surface of the positive electrode current collector, drying and cold pressing to form a positive electrode sheet with a positive electrode active material layer disposed on one side. In another embodiment, the preparation method of the positive electrode sheet includes the following steps: (1) uniformly mixing a positive electrode conductive agent, a binder, and a positive electrode active material in the contents of the present application, adding a solvent, and stirring evenly to obtain a positive electrode slurry; (2) drying the positive electrode slurry coated on one surface of the positive electrode current collector to form a positive electrode active material layer; (3) repeating step (2) on the other surface of the positive electrode current collector, and cold pressing and slitting to obtain a positive electrode sheet with positive electrode active material layers disposed on both sides. The present application has no particular limitation on the solid content of the positive electrode slurry as long as the purpose of the present application can be achieved. For example, the solid content of the positive electrode slurry is 50 wt% to 80 wt%. The present application has no particular limitation on the type of the above solvent as long as the purpose of the present application can be achieved. For example, the solvent may include but is not limited to N-methylpyrrolidone (NMP) or deionized water.

[0059] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has a small thickness expansion rate after high-temperature storage, indicating that the secondary battery has good storage performance.

[0060] In one embodiment of the present application, the secondary battery includes a negative electrode sheet. The present application has no particular limitation on the negative electrode sheet as long as the purpose of the present application can be achieved. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode active material layer is disposed on one surface or two surfaces of the negative electrode current collector. The above "surface" may be a partial surface of the negative electrode current collector or the entire surface of the negative electrode current collector. The present application has no particular limitation on the negative electrode current collector as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, etc. The negative electrode active material layer of the present application contains a negative electrode active material. The present application has no particular limitation on the type of the negative electrode active material as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO 2 、spinel-structured lithium titanate Li 4 Ti 5O 12 , Li-Al alloy or lithium metal. In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6μm to 10μm, and the thickness of the negative electrode active material layer is 30μm to 130μm. Optionally, the negative electrode active material layer may also include at least one of a negative electrode conductor, a dispersant or a negative electrode binder. The present application does not particularly limit the types of negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer, as long as the purpose of the present application can be achieved. The present application does not particularly limit the mass ratio of negative electrode active materials, negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the mass ratio of negative electrode active materials, negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer is (96-98): (0.5-2): (0-1.5): (1.0-1.9).

[0061] In one embodiment of the present application, the secondary battery includes a diaphragm, which is arranged between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet, prevent the internal short circuit of the secondary battery, allow the electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spinning membrane. The present application has no particular restrictions on the thickness of the diaphragm, as long as the purpose of the present application can be achieved.

[0062] In one embodiment of the present application, the secondary battery includes an electrolyte. The present application does not particularly limit the type of the electrolyte, and those skilled in the art can select an electrolyte known in the art according to actual needs, as long as the purpose of the present application can be achieved.

[0063] In one embodiment of the present application, the secondary battery includes a packaging bag, which is used to contain a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The present application does not particularly limit the type of packaging bag, and those skilled in the art can select packaging bags known in the art according to actual needs, as long as the purpose of the present application can be achieved.

[0064] The secondary battery of the present application is not particularly limited, and may include any device that undergoes an electrochemical reaction. For example, the secondary battery may include, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0065] The present application does not particularly limit the preparation method of the secondary battery, and the preparation method known in the art can be selected as long as the purpose of the present application can be achieved. For example, the preparation method of the secondary battery includes but is not limited to the following steps: stacking the diaphragm, the positive electrode sheet, the diaphragm and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly of a winding structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the diaphragm, the positive electrode sheet, the diaphragm and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure to obtain an electrode assembly of a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery.

[0066] The third aspect of the present application provides an electronic device, which comprises the secondary battery described in any one of the above embodiments. Therefore, the electronic device has good performance.

[0067] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device can include, but is not limited to: a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0068] Example

[0069] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

[0070] Test methods and equipment:

[0071] The particle size R of carbon black particles 1 The test:

[0072] a) discharging the lithium-ion battery at a constant current of 0.2C to 3.0V, and after reaching a full discharge state, disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0073] b) Soaking the positive electrode in dimethyl carbonate (DMC) at room temperature for 60 minutes, taking it out, and drying it at room temperature;

[0074] c) taking the positive electrode sheet and obtaining a cross section of the positive electrode active material layer on the positive electrode sheet by liquid nitrogen brittle fracture;

[0075] d) The cross section obtained in c) was observed under a scanning electron microscope (SEM), and the diameter of the carbon black particles was tested at 10 different positions, and the average value was taken as the particle size, wherein the sample size of the carbon black particles was 50 particles.

[0076] The first carbon nanotube diameter and length test:

[0077] a) discharging the lithium-ion battery at a constant current of 0.2C to 3.0V, and after reaching a full discharge state, disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0078] b) Soak the positive electrode in DMC at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0079] c) taking the positive electrode sheet and obtaining a cross section of the positive electrode active material layer on the positive electrode sheet by liquid nitrogen brittle fracture;

[0080] d) Observe the cross section obtained in c) under SEM, test 10 different positions, test the diameter / length of the first carbon nanotube, take the average as the target value, and test 30 first carbon nanotubes. The first carbon nanotubes are carbon nanotubes with a cluster structure in the cross section.

[0081] The diameter R of a single carbon nanotube unit in the first carbon nanotube 2 The test:

[0082] a) discharging the lithium-ion battery at a constant current of 0.2C to 3.0V, and after reaching a full discharge state, disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0083] b) Soak the positive electrode in DMC at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0084] c) taking the positive electrode sheet and obtaining a cross section of the positive electrode active material layer on the positive electrode sheet by liquid nitrogen brittle fracture;

[0085] d) Observe the cross section obtained in c) under SEM, test 10 different positions, test the diameter of a single carbon nanotube unit in the first carbon nanotube, take the average as the target value, and test 30 first carbon nanotubes. The first carbon nanotube is a carbon nanotube with a cluster structure in the cross section.

[0086] Test of carbon black particle specific surface area:

[0087] The specific surface area of ​​the carbon black particles in each embodiment and comparative example was tested by nitrogen adsorption using a specific surface area analyzer (TristarⅡ3020M, Micromeritics Instruments, USA). The specific test was conducted in accordance with the national standard GB / T19587-2017 "Determination of the specific surface area of ​​solid substances by gas adsorption BET method".

[0088] Second, the test of carbon nanotube diameter and length:

[0089] a) discharging the lithium-ion battery at a constant current of 0.2C to 3.0V, and after reaching a full discharge state, disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0090] b) Soak the positive electrode in DMC at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0091] c) taking a positive electrode plate and obtaining a cross section of the positive electrode active material layer on the plate by liquid nitrogen brittle fracture;

[0092] d) Observe the cross section obtained in c) under SEM, test 10 different positions, test the diameter / length of the second carbon nanotube, take the average as the target value, and test the number of second carbon nanotubes to be 30. Among them, the second carbon nanotube is a single carbon nanotube distributed in the cross section.

[0093] Test of the volume density of the positive electrode active material layer:

[0094] a) The lithium-ion battery is charged at a constant current rate of 0.2C to 3.0V to reach full discharge state;

[0095] b) disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0096] c) soaking the positive electrode obtained in b) in DMC at room temperature for 30 minutes, and then taking it out to dry;

[0097] d) Take the positive electrode sheet in c), obtain the cross section of the positive electrode active material layer on the positive electrode sheet by plasma cutting, observe and test the thickness of the positive electrode current collector under SEM, test 10 locations, and take the average value h (cm);

[0098] e) Take the positive electrode from c) and use a fixed area S (cm 2 ) cutter to obtain the membrane, and the weight of the membrane is W. 1 (g) Use a micrometer to measure the thickness of the diaphragm at 5 points and take the average value as H 1 (cm), then its volume density is W 1 / (H 1 -h) / S; The body density of 6 films was tested in each embodiment and comparative example, and the average value was taken as the final value of the body density.

[0099] Test of cohesion of positive electrode:

[0100] a) discharging the lithium-ion battery at a constant current of 0.2C to 3.0V, and after reaching a full discharge state, disassembling the lithium-ion battery to obtain a positive electrode sheet;

[0101] b) Soak the positive electrode in DMC at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0102] c) Take the positive electrode sheet and use the high-speed rail tensile machine commonly used in the lithium battery industry and the 90° angle method to test the cohesion of the positive electrode active material layer:

[0103] The positive electrode sheet in b) is made into a strip with a width of 30mm and a length of 10cm, with the positive electrode active material layer on one side (referred to as side A) facing upward along the length direction, and the other side is adhered to the steel plate with double-sided tape; then one end of a tape with a width of 20mm is attached to the center position of side A along the width direction, leaving a blank end of about 5cm long. The steel plate is fixed to the corresponding position of the high-speed rail tensile machine, and the blank end of the tape is clamped in the chuck. When the tension of the clamp is greater than 0kgf and less than 0.02kgf, the high-speed rail tensile machine can be used for testing. The average tension in the stable area is finally measured and recorded as the bonding force between the positive electrode active material layer and the positive electrode current collector. In particular, it is required that the ratio of the standard deviation of the bonding force data in this stable area to the average value does not exceed 10%.

[0104] Thickness expansion test:

[0105] After the lithium-ion battery is charged at a constant current of 0.5C to 4.5V and reaches a fully charged state, its thickness is measured and recorded as the initial thickness.

[0106] The fully charged lithium-ion battery obtained above was placed in a high and low temperature box at 80°C for storage, and the thickness of the lithium-ion battery after storage for 7 hours was recorded as the final thickness. The thickness of the lithium-ion battery was tested using a PPG thickness gauge with a thickness measuring pressure of 300g.

[0107] Thickness expansion rate of lithium-ion battery (%) = (final thickness - initial thickness) / initial thickness × 100%.

[0108] Resistance test:

[0109] 1) Discharge the lithium-ion battery at a constant current of 0.2C to 3.0V, reaching the full discharge state;

[0110] 2) Disassembling the lithium-ion battery to obtain the positive electrode sheet;

[0111] 3) Soak the positive electrode obtained in 2) in DMC at room temperature for 30 minutes, then take it out and dry it;

[0112] 4) Use BER1200 film resistance tester to test the positive electrode sheet resistance obtained in 3). The adjacent test points are 2mm to 3mm apart. Test 15 different points. The average resistance of all test points is the full wet film resistance of the positive electrode sheet. The test parameters are: pressure head area 153.94mm 2 , pressure 3.5t, holding time 50s.

[0113] The resistance of the positive electrode plate is used to characterize the internal resistance of the lithium-ion battery. The smaller the resistance, the smaller the internal resistance of the lithium-ion battery.

[0114] Example 1-1

[0115] <Preparation of positive electrode sheet>

[0116] Preparation of carbon black particles: acetylene is used as the raw material, and the raw material is placed in a reaction furnace for cracking at 1500°C to generate carbon black particles, which are blown out of the furnace tube, cooled and collected. After crushing and screening, the particle size R shown in Table 1 is obtained. 1 of carbon black particles.

[0117] Preparation of the first carbon nanotube: an iron-magnesium alloy catalyst is selected, the catalyst is deposited on a silicon substrate, and placed in a reactor; argon gas is introduced into the reactor, and the temperature of the reactor is adjusted to 800°C; acetylene is introduced into the reactor, and carbon formed by the cracking of acetylene in the reactor is deposited and grown on the catalyst to form carbon nanotubes; and the first carbon nanotube with a cluster structure is obtained by low-speed (1100 r / min) sand grinding and dispersion.

[0118] The positive electrode active material lithium cobalt oxide, the positive electrode conductive agent, and the binder polyvinylidene fluoride (PVDF, weight average molecular weight of 80W) are mixed, and N-methylpyrrolidone (NMP) is added as a solvent. Stir under the action of a vacuum mixer until the solid content is 75wt% and the system is uniform. The positive electrode slurry is evenly coated on one surface of the positive electrode current collector aluminum foil with a thickness of 6μm, and dried at 90°C to obtain a positive electrode sheet coated with a positive electrode active material layer on one side. After that, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing and slitting, a positive electrode sheet with a specification of 74mm×851mm is obtained for standby use. Among them, the coating weight of the positive electrode active material layer is 280mm / 1540.25mm 2 .

[0119] The positive electrode conductive agent includes the carbon black particles and the first carbon nanotubes prepared above. Based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 97.8%, and the mass percentage of the carbon black particles is W 1 =0.3%, mass percentage of the first carbon nanotube W 2=0.5%, mass percentage of binder C 2 =1.4%.

[0120] <Preparation of negative electrode sheet>

[0121] The negative electrode active material artificial graphite, the negative electrode conductive agent acetylene black, and the negative electrode binder styrene butadiene rubber (SBR, weight average molecular weight 5×10 6 ), dispersant carboxymethyl cellulose (CMC) are mixed in a mass ratio of 96:2:1:1, and then deionized water is added as a solvent, and stirred under the action of a vacuum mixer until the solid content is 50wt% and the system is uniform. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 8μm, and dried at 90°C to obtain a negative electrode sheet with a single-sided coating of a negative electrode active material layer (thickness 130μm). After that, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of a negative electrode active material layer. After cold pressing and slitting, a negative electrode sheet with a specification of 76mm×856mm is obtained for standby use.

[0122] <Preparation of Separator>

[0123] A polyethylene (PE) porous film with a thickness of 8 μm was used as the separator.

[0124] <Preparation of Electrolyte>

[0125] In a dry argon atmosphere, organic solvents ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain a basic electrolyte, and then lithium salt lithium hexafluorophosphate was added to the basic electrolyte to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0126] <Preparation of lithium-ion batteries>

[0127] The separator, positive electrode sheet, separator and negative electrode sheet are stacked in order and wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, and after drying, the electrolyte is injected, and a lithium-ion battery is obtained through vacuum packaging, standing, formation, degassing, trimming and other processes.

[0128] Example 1-2 to Example 1-33

[0129] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0130] In the positive electrode active material layer, when the mass percentage of carbon black particles and / or the first carbon nanotubes changes, the mass percentage of the positive electrode active material changes accordingly, and the mass percentage of the binder remains unchanged. The sum of the mass percentages of carbon black particles, the first carbon nanotubes, the positive electrode active material and the binder is 100%.

[0131] Example 2-1 to Example 2-4

[0132] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0133] Example 3-1 to Example 3-6

[0134] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 1-1.

[0135] In the positive electrode active material layer, when the mass percentage of carbon black particles and / or the first carbon nanotubes changes, the mass percentage of the positive electrode active material changes accordingly, and the mass percentage of the binder remains unchanged. The sum of the mass percentages of carbon black particles, the first carbon nanotubes, the positive electrode active material and the binder is 100%.

[0136] Embodiment 3-7

[0137] <Preparation of positive electrode sheet>

[0138] The carbon black particles and the first carbon nanotubes prepared in Example 1-1 were selected.

[0139] Preparation of the second carbon nanotube: an iron-magnesium alloy catalyst is selected, the catalyst is deposited on a silicon substrate, and placed in a reactor; argon gas is introduced into the reactor, and the temperature of the reactor is adjusted to 700°C; acetylene is introduced into the reactor, and the carbon formed by the cracking of acetylene in the reactor is deposited and grown on the catalyst to form carbon nanotubes; and the second carbon nanotubes are dispersed by high-speed (2000r / min) sand grinding to obtain a single distributed second carbon nanotube.

[0140] The positive electrode active material lithium cobalt oxide, the positive electrode conductive agent, and the binder polyvinylidene fluoride (PVDF, weight average molecular weight of 80W) are mixed, and N-methylpyrrolidone (NMP) is added as a solvent. Stir under the action of a vacuum mixer until the solid content is 75wt% and the system is uniform. The positive electrode slurry is evenly coated on one surface of the positive electrode current collector aluminum foil with a thickness of 6μm, and dried at 90°C to obtain a positive electrode sheet coated with a positive electrode active material layer on one side. After that, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing and slitting, a positive electrode sheet with a specification of 74mm×851mm is obtained for standby use. Among them, the coating weight of the positive electrode active material layer is 280mm / 1540.25mm 2 .

[0141] The positive electrode conductive agent includes the carbon black particles, the first carbon nanotubes and the second carbon nanotubes prepared above. The mass percentages of the positive electrode active material, the carbon black particles, the first carbon nanotubes, the second carbon nanotubes and the binder are shown in Table 3.

[0142] Example 3-8 to Example 3-12

[0143] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 3-7.

[0144] Example 3-13

[0145] In addition to adjusting the relevant preparation parameters according to Table 3, the particle size R 1 Adjusted to 40nm, specific surface area adjusted to 500m 2 / g, except that the diameter of the second carbon nanotube is adjusted to 20nm and the length is adjusted to 1μm, the rest is the same as Example 3-7.

[0146] Embodiment 3-14

[0147] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 3-7.

[0148] Example 4-1 to Example 4-7

[0149] Except for adjusting the relevant preparation parameters according to Table 4, the rest is the same as Example 1-1.

[0150] Example 5-1 to Example 5-9

[0151] Except for adjusting the relevant preparation parameters according to Table 5, the rest is the same as Example 3-7.

[0152] Comparative Example 1 to Comparative Example 13

[0153] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. The preparation parameters and performance data of each embodiment and comparative example are shown in Tables 1 to 5.

[0154] Table 1

[0155]

[0156]

[0157]

[0158] Note: “\” in Table 1 means there is no corresponding parameter.

[0159] From Examples 1-1 to 1-33 and Comparative Examples 1 to 13, it can be seen that the secondary battery of the present application embodiment, by adding carbon black particles and the first carbon nanotubes with a clustered structure as the positive electrode conductive agent in the positive electrode active material layer, and regulating the particle size of the carbon black particles and the diameter of the first carbon nanotubes and the diameter of the single carbon nanotube unit in the first carbon nanotubes within the scope of the present application, the positive electrode sheet has a higher cohesive force and a lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance. In the secondary battery of the comparative example, the particle size R of the carbon black particles is 1 , at least one of the diameter of the first carbon nanotube or the diameter of a single carbon nanotube unit in the first carbon nanotube is outside the scope of the present application, the cohesion of the positive electrode plate is lower, the secondary battery has a larger thickness expansion rate after being stored at 80°C for 7 hours, or the positive electrode plate has a larger resistance, indicating that the secondary battery in the comparative example cannot have both lower internal resistance and higher high-temperature storage performance.

[0160] The particle size of carbon black particles usually affects the internal resistance and high-temperature storage performance of secondary batteries. From Examples 1-1 to 1-10, Comparative Examples 1 and 2, and Comparative Examples 9 to 13, it can be seen that the secondary battery using carbon black particles with a particle size within the range of the present application has a higher cohesive force and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0161] The diameter of the first carbon nanotube usually affects the internal resistance and high-temperature storage performance of the secondary battery. From Example 1-1, Example 1-11 to Example 1-16, Comparative Example 3 and Comparative Example 4, Comparative Example 7 to Comparative Example 13, it can be seen that the secondary battery whose diameter of the first carbon nanotube is within the scope of the present application has a higher cohesive force and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0162] The diameter of a single carbon nanotube unit in the first carbon nanotube usually affects the internal resistance and high-temperature storage performance of the secondary battery. It can be seen from Example 1-1, Example 1-17 to Example 1-23, and Comparative Example 5 to Comparative Example 8 that the secondary battery whose diameter of a single carbon nanotube unit in the first carbon nanotube is within the scope of the present application has a positive electrode sheet with a higher cohesion and a lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance. Among them, although the positive electrode sheet of Comparative Example 6 has a higher cohesion and the secondary battery has a relatively small thickness expansion rate after being stored at 80°C for 7 hours, its positive electrode sheet has an excessively large resistance, indicating that the secondary battery of Comparative Example 6 cannot take into account both internal resistance and high-temperature storage performance.

[0163] The mass percentage of carbon black particles W 1 It usually affects the internal resistance and high temperature storage performance of the secondary battery. From Examples 1-1, 1-24 to 1-29, it can be seen that the mass percentage of carbon black particles selected is W 1 The secondary battery within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0164] The mass percentage of the first carbon nanotube is W 2 It usually affects the internal resistance and high temperature storage performance of the secondary battery. From Example 1-1, Example 1-30 to Example 1-33, it can be seen that the mass percentage of the first carbon nanotube is W 2 The secondary battery within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0165] (W 1 / R 1 ) / (W 2 / R 2 ) value usually affects the internal resistance and high temperature storage performance of the secondary battery. From Examples 1-1, 1-2 to 1-10, it can be seen that the selection of (W 1 / R 1 ) / (W 2 / R 2 ) value within the scope of the present application, the positive electrode plate has higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7h, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0166] Table 2

[0167]

[0168]

[0169] The length of the first carbon nanotube usually affects the internal resistance of the secondary battery. It can be seen from Examples 1-1, 2-1 to 2-4 that the secondary battery whose length of the first carbon nanotube is within the scope of this application has a higher cohesive force and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance and also has a lower internal resistance. Among them, the secondary battery of Example 2-4, compared with Examples 1-1, 2-1 to 2-3, has a lower resistance, but due to the longer length of the first carbon nanotube, it will affect the processing of the positive electrode slurry, resulting in the uniformity of the distribution of various substances in the positive electrode slurry, and it is also easy to increase the difficulty of coating the positive electrode slurry on the surface of the positive electrode collector, thereby reducing the processing performance of the secondary battery and increasing the production cost.

[0170] Table 3

[0171]

[0172]

[0173] Mass percentage of binder C 2 It usually affects the internal resistance and high temperature storage performance of the secondary battery. From Examples 1-1, 3-1 to 3-6, it can be seen that the mass percentage of the binder selected is C 2 The secondary battery within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0174] A second carbon nanotube is further introduced into the positive electrode active material layer, and the mass percentage of the second carbon nanotube is W 3 It usually affects the internal resistance and high temperature storage performance of the secondary battery. From Examples 1-1, 3-7 to 3-13, it can be seen that the mass percentage of the second carbon nanotubes W 3 The secondary battery within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0175] Mass percentage of positive electrode conductive agent C 1 And the mass percentage of binder C 2 The ratio C 1 / C 2 It usually affects the internal resistance and high temperature storage performance of the secondary battery. From Examples 1-1, 3-1 to 3-6, it can be seen that the mass percentage of the positive electrode conductive agent C 1 And the mass percentage of binder C 2 The ratio C 1 / C 2 The secondary battery within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0176] The mass percentage and type of positive electrode active materials usually affect the internal resistance and high-temperature storage performance of secondary batteries. It can be seen from Examples 1-1, 3-1 to 3-14 that the secondary batteries whose mass percentage and type of positive electrode active materials are within the scope of this application have higher cohesion and lower resistance, and the secondary batteries have a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary batteries have good high-temperature storage performance without affecting the internal resistance. Among them, the content of positive electrode active materials in Example 3-13 is relatively low, which will lead to a decrease in the energy density of the secondary battery.

[0177] Table 4

[0178]

[0179]

[0180] The type of binder usually affects the internal resistance and high-temperature storage performance of the secondary battery. It can be seen from Examples 1-1, 4-1 and 4-3 that the secondary battery using the type of binder within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0181] The bulk density of the positive electrode active material layer usually affects the internal resistance and high-temperature storage performance of the secondary battery. It can be seen from Examples 1-1, 4-4 to 4-7 that the secondary battery with a bulk density of the positive electrode active material layer within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0182] Table 5

[0183]

[0184] Note: “\” in Table 5 means there is no corresponding parameter.

[0185] The diameter of the second carbon nanotube usually affects the internal resistance and high-temperature storage performance of the secondary battery. It can be seen from Examples 1-1, 5-1 and 5-5 that the secondary battery in which the second carbon nanotube is introduced and the diameter of the second carbon nanotube is selected to be within the scope of the present application has a positive electrode sheet with higher cohesion and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0186] The length of the second carbon nanotube usually affects the internal resistance and high-temperature storage performance of the secondary battery. It can be seen from Examples 5-1, 5-6 to 5-9 that the secondary battery with the length of the second carbon nanotube within the scope of the present application has a higher cohesive force and lower resistance, and the secondary battery has a smaller thickness expansion rate after being stored at 80°C for 7 hours, indicating that the secondary battery has good high-temperature storage performance without affecting the internal resistance.

[0187] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0188] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0189] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode conductive agent, wherein the positive electrode conductive agent comprises carbon black particles and a first carbon nanotube having a cluster structure, wherein the first carbon nanotube is composed of a plurality of carbon nanotube units arranged in a bundle; wherein: The particle size of the carbon black particles is R1, 3nm≤R1≤40nm; The diameter of the first carbon nanotube is 0.5 μm to 2 μm, the diameter of a single carbon nanotube unit is R2, and 5 nm ≤ R2 ≤ 25 nm.

2. The positive electrode sheet according to claim 1, wherein: The positive electrode sheet satisfies at least one of the following characteristics: (1) 8nm≤R1≤15nm; (2) The diameter of the first carbon nanotube is 0.8 μm to 1.2 μm; (3)8nm≤R2≤20nm.

3. The positive electrode sheet according to claim 1, wherein: Based on the mass of the positive electrode active material layer, the mass percentage of the carbon black particles is W1, the mass percentage of the first carbon nanotubes is W2, 0.2%≤W1≤1.0%, 0.2%≤W2≤0.8%.

4. The positive electrode sheet according to claim 3, wherein: W1, W2, R1 and R2 satisfy: 0.2≤(W1 / R1) / (W2 / R2)≤1.

3.

5. The positive electrode sheet according to claim 1, wherein: R1 and R2 satisfy: R1≤2R2.

6. The positive electrode sheet according to claim 1, wherein: The length of the first carbon nanotubes is 5 μm to 10 μm.

7. The positive electrode sheet according to claim 1, wherein: The specific surface area of ​​the carbon black particles is 500 m 2 / g to 1600m 2 / g.

8. The positive electrode sheet according to claim 1, wherein: The specific surface area of ​​the carbon black particles is 800 m 2 / g to 1400m 2 / g.

9. The positive electrode sheet according to claim 1, wherein: The positive electrode conductive agent further includes a single distributed second carbon nanotube, the diameter of the second carbon nanotube is 3 nm to 20 nm, and the length of the second carbon nanotube is 1 μm to 3 μm.

10. The positive electrode sheet according to claim 9, wherein: Based on the mass of the positive electrode active material layer, the mass percentage of the second carbon nanotubes is W3, 0%<W3≤0.5%.

11. The positive electrode sheet according to claim 1 or 9, wherein: The positive electrode active material layer further includes a binder; Based on the positive electrode active material layer, the mass percentage of the positive electrode conductor is C1, the mass percentage of the binder is C2, 0.5≤C1 / C2≤1.5, 0.8%≤C2≤3%.

12. The positive electrode sheet according to claim 11, wherein: The binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyamide, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber or carboxymethyl cellulose salt.

13. The positive electrode sheet according to claim 1, wherein: The positive electrode active material layer further includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium-rich manganese-based material or lithium titanate; The mass percentage of the positive electrode active material is 95.4% to 98.5% based on the mass of the positive electrode active material layer.

14. The positive electrode sheet according to claim 1, wherein: The volume density of the positive electrode active material layer is 3.9 g / cm 3 Up to 4.15g / cm 3 .

15. The positive electrode sheet according to claim 1, wherein: The cohesive force of the positive electrode plate is 30 N / m to 85 N / m.

16. A secondary battery, wherein: The secondary battery comprises the positive electrode sheet according to any one of claims 1 to 15.

17. An electronic device, wherein: The electronic device includes the secondary battery according to claim 16.

Images