Positive pole piece, secondary battery and electronic device

Abstract

The invention provides a positive pole piece, a secondary battery and an electronic device, the positive pole piece comprises a positive current collector and a positive active material layer arranged on at least one surface of the positive current collector, and the positive active material layer comprises a first carbon material; the first carbon material comprises carbon fibers and single carbon black particles arranged on the surfaces of the carbon fibers and / or chain structures formed by the carbon black particles; the tap density of the first carbon material is 0.005 g / cm < 3 > to 0.30 g / cm < 3 >; the first carbon material has an oil absorption value of 100 ml / 100 g to 1000 ml / 100 g. Through the arrangement, the secondary battery has 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] Secondary batteries (such as lithium-ion batteries) have the advantages of high energy storage density, high open circuit voltage, low self-discharge rate, long cycle life, good safety, etc., and 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 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 of the positive electrode 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 to reduce the internal resistance of the secondary battery.

[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 plate, the positive electrode plate comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a first carbon material; the first carbon material comprising carbon fibers and single carbon black particles and / or a chain structure formed by carbon black particles disposed on the surface of the carbon fibers; the tap density of the first carbon material is 0.005 g / cm 3 Up to 0.30g / cm 3; The oil absorption value of the first carbon material is from 100 ml / 100 g to 1000 ml / 100 g. In the positive electrode sheet provided by the first aspect of the present application, the first carbon material can connect each particle of the positive electrode active material by arranging single carbon black particles and / or chain-like structures formed by carbon black particles on the surface of the carbon fiber, enabling the first carbon material to play both a short-range and a long-range conductive role. Compared with the carbon black material with a conventional particulate distribution in the prior art, the first carbon material can play a more efficient conductive role. The tapped density of the first carbon material is regulated within the scope of the present application, making the first carbon material have a relatively small tapped density and a relatively loose interior, which can better fill the pores between each particle of the positive electrode active material and enhance the continuity of conductivity. The oil absorption value of the first carbon material is regulated within the scope of the present application. The interstitial structure of the first carbon material can adsorb more electrolyte, which is beneficial to improving the electrolyte retention capacity of the positive electrode sheet, enabling the positive electrode sheet to also have good ionic conductivity in the later stage of the secondary battery cycle. Applying the positive electrode sheet to the secondary battery is beneficial to improving the cycle performance of the secondary battery in the later stage of charge and discharge. By selecting the first carbon material and regulating the tapped density and oil absorption value of the first carbon material within the scope of the present application, the positive electrode sheet of the present application is beneficial to constructing a stable and continuous conductive network with good conductive performance in the positive electrode active material layer, and is also beneficial to improving the electrolyte retention capacity of the positive electrode sheet, increasing the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to the secondary battery can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0007] In some embodiments of the present application, the average particle size of the carbon black particles is d0 μm, where 0.005 ≤ d0 ≤ 0.1. Regulating the average particle size of the carbon black particles within the above range is beneficial to constructing a stable and continuous conductive network with good conductive performance in the positive electrode active material layer, and is also beneficial to improving the electrolyte retention capacity of the positive electrode sheet, increasing the ionic conductivity and electronic conductivity of the positive electrode sheet. It can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0008] In some embodiments of the present application, the diameter of the carbon fiber is Φ μm, where 0.01 ≤ Φ ≤ 1.00. Regulating the diameter of the carbon fiber within the above range is beneficial to improving the conductive efficiency of the first carbon material to reduce the internal resistance of the secondary battery.

[0009] In some embodiments of the present application, 1.5 ≤ Φ / d0 ≤ 8. Regulating the value of the ratio Φ / d0 of the carbon fiber diameter to the average particle size of the carbon black particles within the above range can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0010] In some embodiments of the present application, the positive electrode sheet satisfies at least one of the following characteristics: (a) 0.01 ≤ d0 ≤ 0.05; (b) 0.05 ≤ Φ ≤ 0.1. This is beneficial to reducing the internal resistance of the secondary battery.

[0011] In some embodiments of the present application, the length of the first carbon material is L1 μm, where 1 ≤ L1 ≤ 50. Controlling the length of the first carbon material within the above range is beneficial to enabling the first carbon material to play a short-range conductive role and further play its long-range conductive role, which can further reduce the internal resistance of the secondary battery and improve the cycling performance of the secondary battery.

[0012] In some embodiments of the present application, the diameter of the first carbon material is D1 μm, where 0.2 ≤ D1 ≤ 5. Controlling the diameter of the first carbon material within the above range, the positive electrode sheet has a high ionic conductivity and electronic conductivity, thereby reducing the internal resistance of the secondary battery.

[0013] In some embodiments of the present application, the first carbon material satisfies at least one of the following characteristics: (1) the tapped density of the first carbon material is 0.01 g / cm 3 to 0.06 g / cm 3 ; (2) the oil absorption value of the first carbon material is 400 ml / 100 g to 700 ml / 100 g; (3) the length of the first carbon material is L1 μm, where 2 ≤ L1 ≤ 15; (4) the diameter of the first carbon material is D1 μm, where 0.5 ≤ D1 ≤ 2. This is beneficial to reducing the internal resistance of the secondary battery.

[0014] In some embodiments of the present application, based on the mass of the positive electrode active material layer, the mass percentage content of the first carbon material is W1%, where 0.1 ≤ W1 ≤ 2.0. In some embodiments of the present application, based on the mass of the positive electrode active material layer, the mass percentage content of the first carbon material is W1%, where 0.3 ≤ W1 ≤ 1.0. Controlling the mass percentage content of the first carbon material within the above range is beneficial to the uniform distribution of the first carbon material in the positive electrode slurry, playing the good short-range and long-range conductive roles of the first carbon material, enabling the positive electrode sheet to have good ionic conductivity and electronic conductivity, and reducing the internal resistance of the secondary battery.

[0015] In some embodiments of the present application, the positive electrode active material layer includes a second carbon material, the second carbon material includes carbon nanotubes, the diameter of the carbon nanotubes is D2 nm, 5 ≤ D2 ≤ 15, the length of the carbon nanotubes is L2 μm, 1 ≤ L2 ≤ 5; based on the mass of the positive electrode active material layer, the mass percentage content of the carbon nanotubes is W2%, 0.2 ≤ W2 ≤ 0.8. Selecting carbon nanotubes with diameters and lengths within the scope of the present application and controlling the mass percentage content of the carbon nanotubes in the positive electrode active material layer within the above range is beneficial to constructing a more stable conductive network in the positive electrode active material layer, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, thereby reducing the internal resistance of the secondary battery.

[0016] In some embodiments of the present application, the positive electrode active material layer includes a positive electrode binder, the positive electrode binder includes at least one of polyvinylidene fluoride, vinylidene fluoride - hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyamide, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene - butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber or carboxymethyl cellulose salt; based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode binder is W3%, 0.8 ≤ W2 ≤ 3.0. The positive electrode binders of the above types have good adhesion performance. Selecting the positive electrode binders of the above types for use in the positive electrode active material layer and controlling the mass percentage content of the positive electrode binder in the positive electrode active material layer within the above range is beneficial to enabling the secondary battery to have a low probability of demolding on the basis of having a low internal resistance, and the positive electrode sheet has a low thickness expansion rate, so that the secondary battery has good safety performance and cycle performance.

[0017] In some embodiments of the present application, the saturated liquid retention rate of the positive electrode sheet is 5% to 20%. It shows that the positive electrode sheet has a high saturated liquid retention rate.

[0018] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet described in any one of the foregoing embodiments. Therefore, the secondary battery has a low internal resistance.

[0019] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any one of the foregoing embodiments. Therefore, the electronic device has good performance in use.

[0020] Advantages of the present application:

[0021] The present application provides a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first carbon material; the first carbon material includes carbon fibers and single carbon black particles and / or chain-like structures formed by carbon black particles provided on the surface of the carbon fibers; the tapped density of the first carbon material is 0.005 g / cm 3 to 0.30 g / cm 3 ; the oil absorption value of the first carbon material is 100 ml / 100 g to 1000 ml / 100 g. In the positive electrode sheet, the first carbon material can connect the particles of the positive electrode active material by providing single carbon black particles and / or chain-like structures formed by carbon black particles on the surface of the carbon fibers, enabling the first carbon material to play both a short-range and a long-range conductive role. Compared with the conventional granular carbon black material in the prior art, the first carbon material can play a more efficient conductive role. By regulating the tapped density of the first carbon material within the scope of this application, the first carbon material has a smaller tapped density, and the interior of the first carbon material is relatively loose, which can better fill the pores between the particles of the positive electrode active material and enhance the conductivity coherence. By regulating the oil absorption value of the first carbon material within the scope of this application, the interstitial structure of the first carbon material can adsorb more electrolyte, which is beneficial to improving the liquid retention ability of the positive electrode sheet for the electrolyte, enabling the positive electrode sheet to also have good ionic conductivity in the later stage of the secondary battery cycle. Applying the positive electrode sheet to the secondary battery is beneficial to improving the cycle performance of the secondary battery in the later stage of charge and discharge. The positive electrode sheet of this application, by selecting the first carbon material and regulating the tapped density and oil absorption value of the first carbon material within the scope of this application, is beneficial to constructing a stable, coherent and well-conductive conductive network in the positive electrode active material layer, and is also beneficial to improving the liquid retention ability of the positive electrode sheet for the electrolyte, and improving the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to the secondary battery can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0022] Of course, it is not necessary for any product or method implementing this application to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of this application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of this application, and those of ordinary skill in the art can also obtain other embodiments based on these drawings.

[0024] Figure 1 It is a schematic diagram of the microscopic structure of the first carbon material in some implementation schemes of this application.

[0025] Reference Signs:

[0026] 10 - First carbon material, 11 - Carbon fiber, 12 - Carbon black particles. Detailed implementation manners

[0027] The technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of 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.

[0028] The first aspect of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The above "positive electrode active material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode active material layer can be provided on one surface or two surfaces of the positive electrode current collector along its own thickness direction. The above "surface" can be a partial surface or the entire surface of the positive electrode current collector. The positive electrode active material layer includes a first carbon material, and the first carbon material includes carbon fibers and single carbon black particles and / or chain-like structures formed by carbon black particles provided on the surface of the carbon fibers. The tapped density of the first carbon material is 0.005 g / cm 3 to 0.30 g / cm 3 ; the oil absorption value of the first carbon material is 100 ml / 100 g to 1000 ml / 100 g.

[0029] It can be understood that in some embodiments, the first carbon material includes carbon fibers and single carbon black particles provided on the surface of the carbon fibers. In some other embodiments, the first carbon material includes carbon fibers and chain-like structures formed by carbon black particles provided on the surface of the carbon fibers. In still some other embodiments, the first carbon material includes carbon fibers and also includes carbon black particles and chain-like structures formed by carbon black particles provided on the surface of the carbon fibers. It should be noted that the above "chain-like structure" refers to a structure obtained by connecting two or more carbon black particles. As Figure 1 shown, the first carbon material 10 includes carbon fibers 11 and carbon black particles 12. Single carbon black particles 12 are provided on the surface of the carbon fibers 11, and chain-like structures formed by two or more carbon black particles 12 are also provided.

[0030] For example, the tapped density of the first carbon material is 0.005 g / cm 3 , 0.01 g / cm 3 , 0.02 g / cm 3 , 0.03 g / cm 3 , 0.04 g / cm 3 , 0.05 g / cm 3 , 0.06 g / cm 3 , 0.09 g / cm3 , 0.10 g / cm 3 , 0.13 g / cm 3 , 0.15 g / cm 3 , 0.20 g / cm 3 , 0.22 g / cm 3 , 0.25 g / cm 3 , 0.27 g / cm 3 , 0.30 g / cm 3 or any value between any two of the above numerical ranges. The tapped density of the first carbon material is less than 0.005 g / cm 3 . The tapped density of the first carbon material is too small, during the preparation of the positive electrode paste, the viscosity of the positive electrode paste becomes too large, resulting in difficult processing, and the first carbon material itself is not easily dispersed, leading to agglomeration, affecting the performance of its electrical conductivity, thereby affecting the liquid retention rate of the positive electrode sheet and the internal resistance of the secondary battery; the tapped density of the first carbon material is greater than 0.30 g / cm 3 . The tapped density of the first carbon material is too large, and the volume of the first carbon material is too small under the same weight. The first carbon material will easily fill the gaps between the positive electrode active materials, affecting the performance of the long-range electrical conductivity of the first carbon material, the electrical conductivity of the first carbon material becomes poor, and there are too few pores in the first carbon material, the liquid retention rate of the positive electrode sheet will decrease, the ionic conductivity of the positive electrode sheet becomes poor, and the subsequent cycle performance of the secondary battery decreases.

[0031] For example, the oil absorption value of the first carbon material is 100 ml / 100 g, 200 ml / 100 g, 300 ml / 100 g, 400 ml / 100 g, 500 ml / 100 g, 600 ml / 100 g, 700 ml / 100 g, 800 ml / 100 g, 900 ml / 100 g, 1000 ml / 100 g or any value between any two of the above numerical ranges. The oil absorption value of the first carbon material is less than 100 ml / 100 g, the oil absorption value of the first carbon material is too small, the amount of electrolyte that can be stored in the interstitial structure of the first carbon material is too small, the positive electrode sheet stores too little electrolyte, the ionic conductivity decreases, and the charge and discharge performance in the later stage of the secondary battery cycle becomes poor; the oil absorption value of the first carbon material is greater than 1000 ml / 100 g, the oil absorption value of the first carbon material is too large, during the preparation of the positive electrode paste, the viscosity of the positive electrode paste becomes too large, resulting in difficult processing, and the first carbon material itself is not easily dispersed, leading to agglomeration, affecting the performance of its electrical conductivity, thereby affecting the liquid retention rate of the positive electrode sheet and the internal resistance of the secondary battery.

[0032] Overall, the first carbon material provided in the first aspect of this application can connect each particle of the positive electrode active material by arranging single carbon black particles and / or chain-like structures formed by carbon black particles on the surface of carbon fibers. The first carbon material can play both a short-range and a long-range conductive role. Compared with the conventional granular carbon black material in the prior art, the first carbon material can play a more efficient conductive role. The tapped density of the first carbon material is regulated within the scope of this application, so that the first carbon material has a relatively small tapped density and is relatively loose inside, which can better fill the pores between each particle of the positive electrode active material and enhance the conductivity coherence. The oil absorption value of the first carbon material is regulated within the scope of this application. The interstitial structure of the first carbon material can adsorb more electrolyte, which is beneficial to improving the electrolyte retention capacity of the positive electrode sheet, enabling the positive electrode sheet to also have good ionic conductivity in the later stage of the secondary battery cycle. Applying the positive electrode sheet to the secondary battery is beneficial to improving the cycle performance of the secondary battery in the later stage of charge and discharge. By selecting the first carbon material and regulating the tapped density and oil absorption value of the first carbon material within the scope of this application, the positive electrode sheet of this application is beneficial to constructing a stable, coherent and well-conductive conductive network in the positive electrode active material layer, and is also beneficial to improving the electrolyte retention capacity of the positive electrode sheet, increasing the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to the secondary battery can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0033] In some embodiments of this application, the tapped density of the first carbon material is 0.01 g / cm 3 to 0.06 g / cm 3 . For example, the tapped density of the first carbon material is 0.01 g / cm 3 , 0.02 g / cm 3 , 0.03 g / cm 3 , 0.04 g / cm 3 , 0.05 g / cm 3 , 0.06 g / cm 3 or any value between any two of the above numerical ranges. By regulating the tapped density of the first carbon material within the above range, the first carbon material has a relatively small tapped density and is relatively loose inside, which can better fill the pores between each particle of the positive electrode active material and enhance the conductivity coherence, is beneficial to constructing a stable, coherent and well-conductive conductive network in the positive electrode active material layer, increasing the ionic conductivity and electronic conductivity of the positive electrode sheet, and thus reducing the internal resistance of the secondary battery.

[0034] In some embodiments of the present application, the oil absorption value of the first carbon material is from 400 ml / 100 g to 700 ml / 100 g. For example, the oil absorption value of the first carbon material is 400 ml / 100 g, 500 ml / 100 g, 600 ml / 100 g, 700 ml / 100 g, or any value between any two of the above numerical ranges. By controlling the oil absorption value of the first carbon material within the above range, the interstitial structure of the first carbon material can adsorb more electrolyte, which is beneficial to improving the electrolyte retention capacity of the positive electrode sheet, enabling the positive electrode sheet to also have good ionic conductivity in the later stage of the secondary battery cycle. When the positive electrode sheet is applied to the secondary battery, it can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0035] The present application has no particular limitation on the method of controlling the tap density of the first carbon material, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the hot melt temperature during the preparation process of the first carbon material. Generally speaking, the longer the chain structure of the first carbon material and the richer the branches, the smaller its tap density.

[0036] The present application has no particular limitation on the method of controlling the oil absorption value of the first carbon material, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the addition ratio of carbon black material and polyacrylonitrile (PAN) during the preparation process of the first carbon material. Generally speaking, the longer the chain structure of the first carbon material and the richer the branches, the larger its oil absorption value.

[0037] In some embodiments of the present application, the average particle size of the carbon black particles is d0 μm, where 0.005 ≤ d0 ≤ 0.1. For example, the average particle size d0 of the carbon black particles is 0.005, 0.01, 0.02, 0.025, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or any value between any two of the above numerical ranges. By controlling the average particle size of the carbon black particles within the above range, the probability of agglomeration of the conductive carbon black is small, which is beneficial to the distribution of the conductive carbon black on the surface of the carbon fiber in the form of single particles or chain structures, and also reduces the probability of an increase in the viscosity of the positive electrode paste caused by the agglomeration of the first carbon material or the limitation of its own conductive performance. Thus, it is beneficial to construct a stable and coherent conductive network with good conductive performance in the positive electrode active material layer, and it is also beneficial to improve the electrolyte retention capacity of the positive electrode sheet, and improve the ionic conductivity and electronic conductivity of the positive electrode sheet. When the positive electrode sheet is applied to the secondary battery, it can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0038] In some embodiments of the present application, 0.01 ≤ d0 ≤ 0.05. For example, the average particle size d0 of the carbon black particles is 0.01, 0.02, 0.025, 0.03, 0.04, 0.05, or any value between any two of the above numerical ranges. Controlling the average particle size of the carbon black particles within the above range is conducive to obtaining a first carbon material with good electrical conductivity and a low agglomeration probability, thereby facilitating the construction of a stable and coherent conductive network with good electrical conductivity in the positive electrode active material layer, and also facilitating the improvement of the liquid retention capacity of the positive electrode sheet for the electrolyte, and improving the ionic conductivity and electronic conductivity of the positive electrode sheet. When the positive electrode sheet is applied to a secondary battery, the internal resistance of the secondary battery can be reduced, and the cycle performance of the secondary battery can be improved.

[0039] In the present application, the "average particle size of the carbon black particles" refers to the average value of the equivalent diameters of the primary particles of the carbon black particles. The equivalent diameter generally refers to the diameter of a sphere with the same volume as an object with an irregular shape. There is no particular limitation on the method of controlling the particle size of the carbon black particles in the present application, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the reaction temperature and reaction time during the preparation process of the first carbon material; or, it can be achieved by directly purchasing carbon black particles with a particle size within the range of the present application; or, it can be achieved by means such as crushing, grinding, or ball milling.

[0040] In some embodiments of the present application, the diameter of the carbon fiber is Φ μm, 0.01 ≤ Φ ≤ 1.00. For example, the diameter Φ of the carbon fiber is 0.01, 0.02, 0.05, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.00, or any value between any two of the above numerical ranges. Controlling the diameter of the carbon fiber within the above range is conducive to obtaining a first carbon material with a longer length, and is also conducive to an appropriate number of carbon black particles adhering to the surface of the carbon fiber, facilitating the formation of branches of the chain-like carbon black particles, extending the conductive path in the width direction of the first carbon material, thereby improving the conductive efficiency of the first carbon material to reduce the internal resistance of the secondary battery.

[0041] In some embodiments of the present application, 0.05 ≤ Φ ≤ 0.1. For example, the diameter Φ of the carbon fiber is 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or any value between any two of the above numerical ranges. Controlling the diameter of the carbon fiber within the above range is conducive to obtaining a first carbon material with a longer length, and is also conducive to an appropriate number of carbon black particles adhering to the surface of the carbon fiber, facilitating the formation of branches of the chain-like carbon black particles, extending the conductive path in the width direction of the first carbon material, thereby further improving the conductive efficiency of the first carbon material to further reduce the internal resistance of the secondary battery.

[0042] The method for regulating the diameter of the carbon fiber in this application is not particularly limited, as long as the purpose of this application can be achieved. For example, it can be achieved by regulating the concentration of PAN during the synthesis process.

[0043] In some embodiments of this application, 1.5 ≤ Φ / d0 ≤ 8. For example, the value of Φ / d0 is 1.5, 2, 2.3, 2.7, 3, 3.3, 3.6, 4, 4.2, 4.7, 5, 6, 6.5, 7, 7.2, 7.7, 8 or any value between any two of the above numerical ranges. Regulating the value of the ratio Φ / d0 of the carbon fiber diameter to the average particle size of the carbon black particles within the above range is beneficial for the carbon black particles to adhere to the surface of the carbon fiber, form branches of a chain-like structure, extend the conductive path in the width direction of the first carbon material, facilitate the further filling of the first carbon material into the pores between the particles of the positive electrode active material, enhance the conductivity coherence, construct a stable and coherent conductive network with good conductivity in the positive electrode active material layer, and is also beneficial for improving the liquid retention capacity of the positive electrode sheet for the electrolyte, and improving the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to a secondary battery can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0044] In some embodiments of this application, the length of the first carbon material is L1 μm, and 1 ≤ L1 ≤ 50. For example, the length L1 of the first carbon material is 1, 2, 5, 7, 10, 12, 15, 20, 22, 27, 30, 35, 40, 42, 44, 47, 50 or any value between any two of the above numerical ranges. Regulating the length of the first carbon material within the above range is beneficial for the first carbon material to play its long-range conductive role on the basis of playing its short-range conductive role, enabling the first carbon material to construct a stable and coherent conductive network with good conductivity in the positive electrode active material layer, and improving the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to a secondary battery can reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0045] In some embodiments of this application, the length of the first carbon material is L1 μm, and 2 ≤ L1 ≤ 15. For example, the length L1 of the first carbon material is 2, 5, 7, 10, 12, 15 or any value between any two of the above numerical ranges. Regulating the length of the first carbon material within the above range is beneficial for the first carbon material to further play its long-range conductive role on the basis of playing its short-range conductive role, enabling the first carbon material to construct a stable and coherent conductive network with good conductivity in the positive electrode active material layer, and further improving the ionic conductivity and electronic conductivity of the positive electrode sheet. Applying the positive electrode sheet to a secondary battery can further reduce the internal resistance of the secondary battery and improve the cycle performance of the secondary battery.

[0046] There is no particular limitation on the method for regulating the length of the first carbon material in this application, as long as the object of this application can be achieved. For example, it can be achieved by regulating the length of PAN.

[0047] In some embodiments of this application, the diameter of the first carbon material is D1 μm, where 0.2 ≤ D1 ≤ 5. For example, the diameter D1 of the first carbon material is 0.2, 0.3, 0.5, 0.7, 1.0, 1.4, 1.8, 2, 2.3, 2.7, 3, 3.3, 3.5, 3.9, 4.2, 4.5, 5, or any value between any two of the above numerical ranges. By regulating the diameter of the first carbon material within the above range, the first carbon material has a good conductive path in the width direction, which can improve the conductive efficiency of the first carbon material. The first carbon material forms a coherent and well-conductive conductive network in the positive electrode active material layer, and the positive electrode plate has a high ionic conductivity and electronic conductivity, thereby being able to reduce the internal resistance of the secondary battery.

[0048] In some embodiments of this application, the diameter of the first carbon material is D1 μm, where 0.5 ≤ D1 ≤ 2. For example, the diameter D1 of the first carbon material is 0.5, 0.7, 1.0, 1.4, 1.8, 2, or any value between any two of the above numerical ranges. By regulating the diameter of the first carbon material within the above range, the first carbon material has a good conductive path in the width direction, which can further improve the conductive efficiency of the first carbon material. The first carbon material forms a coherent and well-conductive conductive network in the positive electrode active material layer, and the positive electrode plate has a high ionic conductivity and electronic conductivity, thereby being able to further reduce the internal resistance of the secondary battery.

[0049] There is no particular limitation on the method for regulating the diameter of the first carbon material in this application, as long as the object of this application can be achieved. For example, it can be achieved by regulating the addition ratio of carbon black material and PAN.

[0050] In some embodiments of the present application, based on the mass of the positive electrode active material layer, the mass percentage content of the first carbon material is W1%, and 0.1 ≤ W1 ≤ 2.0. For example, the mass percentage content W1% of the first carbon material is 0.1%, 0.3%, 0.5%, 0.7%, 1.0%, 1.2%, 1.5%, 1.7%, 2.0% or any value between any two of the above numerical ranges. By controlling the mass percentage content of the first carbon material within the above range, during the preparation of the positive electrode slurry, the probability of entanglement of the first slurry is relatively low, which is beneficial to the uniform distribution of the first carbon material in the positive electrode slurry, and thus uniformly distributed in the positive electrode active material layer, connecting the particles of the positive electrode active material, forming a coherent conductive network in the positive electrode active material layer, exerting the good short-range and long-range conductive effects of the first carbon material, enabling the positive electrode sheet to have good ionic conductivity and electronic conductivity, and applying the positive electrode sheet to a secondary battery, the secondary battery has a lower internal resistance.

[0051] In some embodiments of the present application, based on the mass of the positive electrode active material layer, the mass percentage content of the first carbon material is W1%, and 0.3 ≤ W1 ≤ 1.0. For example, the mass percentage content W1% of the first carbon material is 0.3%, 0.5%, 0.7%, 0.9%, 1.0% or any value between any two of the above numerical ranges. By controlling the mass percentage content of the first carbon material within the above range, during the preparation of the positive electrode slurry, the probability of entanglement of the first slurry is further reduced, which is beneficial to the uniform distribution of the first carbon material in the positive electrode slurry, and thus uniformly distributed in the positive electrode active material layer, connecting the particles of the positive electrode active material, forming a coherent conductive network in the positive electrode active material layer, exerting the good short-range and long-range conductive effects of the first carbon material, enabling the positive electrode sheet to have good ionic conductivity and electronic conductivity, and applying the positive electrode sheet to a secondary battery, the secondary battery has a lower internal resistance.

[0052] In some embodiments of the present application, the positive electrode active material layer includes a second carbon material, the second carbon material includes carbon nanotubes, the diameter of the carbon nanotubes is D2 nm, where 5 ≤ D2 ≤ 15, and the length of the carbon nanotubes is L2 μm, where 1 ≤ L2 ≤ 5; based on the mass of the positive electrode active material layer, the mass percentage content of the carbon nanotubes is W2%, where 0.2 ≤ W2 ≤ 0.8. For example, the diameter D2 of the carbon nanotubes is 5 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, or any value between any two of the above numerical ranges. For example, the length L2 of the carbon nanotubes is 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or any value between any two of the above numerical ranges. For example, the mass percentage content W2% of the carbon nanotubes is 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. Selecting carbon nanotubes with diameters and lengths within the scope of the present application and controlling the mass percentage content of the carbon nanotubes in the positive electrode active material layer within the above range is beneficial for the carbon nanotubes to connect the particles of the positive electrode active material in series, maintain the network conductive network between the particles, and reduce the probability of damage to the conductive network caused by the expansion of the positive electrode sheet during the charge and discharge cycles of the secondary battery. It is also beneficial to reduce the probability of entanglement between the first carbon material and the carbon nanotubes, and beneficial for the first carbon material and the carbon nanotubes to be evenly distributed in the positive electrode slurry and thus evenly distributed in the positive electrode active material layer to fully exert their conductive performance. In this way, it is beneficial to construct a more stable conductive network in the positive electrode active material layer, improve the ionic conductivity and electronic conductivity of the positive electrode sheet, and thus reduce the internal resistance of the secondary battery.

[0053] The present application does not particularly limit the method of controlling the length and diameter of the carbon nanotubes, as long as the purpose of the present application can be achieved. For example, it can be achieved by the rotation speed and time of sanding during the preparation process of the carbon nanotubes. Generally speaking, the greater the rotation speed and the longer the time of sanding, the smaller the diameter and the shorter the length of the carbon nanotubes; the smaller the rotation speed and the shorter the time of sanding, the larger the diameter and the longer the length of the carbon nanotubes. Or, it can be achieved by purchasing commercially available carbon nanotubes with diameters and lengths that meet the requirements of the present application.

[0054] In some embodiments of the present application, the positive electrode active material layer includes a positive electrode binder, and the positive electrode binder includes at least one of polyvinylidene fluoride, vinylidene fluoride - hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyamide, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene - butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber, or carboxymethyl cellulose salt; based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode binder is W3%, and 0.8 ≤ W2 ≤ 3.0. For example, the mass percentage content W3% of the positive electrode binder is 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.3%, 2.5%, 2.8%, 3.0%, or any value between any two of the above numerical ranges. The positive electrode binders of the above types have good adhesion properties. Selecting the positive electrode binders of the above types for the positive electrode active material layer and controlling the mass percentage content of the positive electrode binder in the positive electrode active material layer within the above range is conducive to the uniform dispersion of the first carbon material and / or carbon nanotubes in the positive electrode slurry to exert the good electrical conductivity of the first carbon material and / or carbon nanotubes, and is also conducive to the positive electrode active material layer having good adhesion to the positive electrode current collector or the separator, which is conducive to the secondary battery having a low release probability on the basis of a low internal resistance and the positive electrode sheet having a low thickness expansion rate, so that the secondary battery has good safety performance and cycle performance.

[0055] In some embodiments of the present application, the saturated liquid retention rate of the positive electrode sheet is 5% to 20%. For example, the saturated liquid retention rate of the positive electrode sheet is 5%, 7%, 9%, 11%, 12%, 15%, 17%, 20%, or any value between any two of the above numerical ranges. Controlling the saturated liquid retention rate of the positive electrode sheet within the above range indicates that the positive electrode sheet has a high saturated liquid retention rate and the positive electrode sheet has a high liquid retention capacity for the electrolyte. The secondary battery can still have sufficient electrolyte in the later stage of charge - discharge cycling, enabling the positive electrode sheet to have good ionic and electronic conductivities, which is conducive to the secondary battery having a low internal resistance and good cycle performance.

[0056] The present application has no particular limitation on the method of controlling the saturated liquid retention rate of the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the oil absorption value of the first carbon material. Generally speaking, the larger the oil absorption value of the first carbon material, the larger the saturated liquid retention rate of the positive electrode sheet; the smaller the oil absorption value of the first carbon material, the smaller the saturated liquid retention rate of the positive electrode sheet.

[0057] In some embodiments of the present application, the positive electrode active material layer includes a positive electrode active material. The present application does not particularly limit the type and content of the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material includes, but is not limited to, at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium-rich manganese-based material, or lithium titanate. For example, based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 94.2% to 98.9%.

[0058] In some embodiments of the present application, the positive electrode active material layer includes a first carbon material, a positive electrode binder, and a positive electrode active material. In some other embodiments of the present application, the positive electrode active material layer includes a first carbon material, a second carbon material, a positive electrode binder, and a positive electrode active material.

[0059] The present application does not particularly limit 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 limit 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 5 μm to 20 μm. The thickness of a single-layer positive electrode active material layer is 30 μm to 120 μm.

[0060] The present application does not particularly limit the preparation method of the first carbon material, as long as the purpose of the present application can be achieved. For example, the preparation method of the first carbon material includes, but is not limited to, the following steps: hot-melt blending carbon black material and polyacrylonitrile (PAN) evenly at 180°C to 200°C to completely melt PAN, and the carbon black material is evenly dispersed in the molten PAN; then electrospinning is used to form carbon fibers, and the carbon fibers are pre-oxidized in an inert atmosphere, carbonized in a nitrogen atmosphere, and graphitized in a nitrogen atmosphere to form the first carbon material, wherein the pre-oxidation temperature is 200°C to 300°C and the time is 2 h to 4 h; the carbonization temperature is 1000°C to 1500°C and the time is 1 h to 2 h; the graphitization temperature is 2500°C to 3000°C and the time is 1 h to 2 h. The present application does not particularly limit the mass ratio of the above-mentioned carbon black material to PAN, as long as the purpose of the present application can be achieved. For example, the mass ratio of the carbon black material to PAN is (1 to 8):1. The present application does not particularly limit the weight-average molecular weight of the above-mentioned PAN, as long as the purpose of the present application can be achieved. For example, the weight-average molecular weight Mw of PAN is 80W to 120W. The present application does not particularly limit the above-mentioned inert atmosphere, as long as the purpose of the present application can be achieved. For example, the inert atmosphere is a mixed gas of air and nitrogen, and the mass ratio of air to nitrogen in the mixed gas is (10 to 30):(70 to 90).

[0061] The present application has no particular limitation on the preparation method of the positive electrode sheet, and a preparation method well-known in the art can be adopted as long as the object 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: (1) uniformly mixing a first carbon material, a positive electrode binder, and a positive electrode active material, adding a solvent, and stirring uniformly 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 first carbon material, a positive electrode binder, and a positive electrode active material, adding a solvent, and stirring uniformly 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. In still another embodiment, carbon nanotubes can be further added in the above step (1) to prepare a positive electrode active material layer containing the first carbon material, the second carbon material, the positive electrode binder, and the positive electrode active material. The present application has no particular limitation on the solid content of the positive electrode slurry as long as the object 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 object of the present application can be achieved. For example, the solvent can include but is not limited to N-methylpyrrolidone (NMP) or deionized water.

[0062] 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 lower internal resistance.

[0063] In some embodiments 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 object 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 both surfaces of the negative electrode current collector, and the above "surface" can be a partial surface 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 object of the present application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, 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 object of the present application can be achieved. For example, the negative electrode active material can include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x(0 < x < 2), at least one of Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithium titanate Li4Ti5O with spinel structure 12 、Li-Al alloy or metallic lithium. In the present application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6 μm to 10 μm, and the thickness of the negative electrode active material layer is 30 μm to 130 μm. Optionally, the negative electrode active material layer may further include at least one of a negative electrode conductive agent, a dispersant or a negative electrode binder. The present application has no particular limitation on the types of the negative electrode conductive agent, the dispersant and the negative electrode binder in the negative electrode active material layer, as long as the object of the present application can be achieved. The present application has no particular limitation on the mass ratio of the negative electrode active material, the negative electrode conductive agent, the dispersant and the negative electrode binder in the negative electrode active material layer, as long as the object of the present application can be achieved. For example, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the dispersant and the negative electrode binder in the negative electrode active material layer is (96 - 98):(0.5 - 2):(0 - 1.5):(1.0 - 1.9).

[0064] In some embodiments of the present application, the secondary battery includes a separator disposed between the positive electrode plate and the negative electrode plate to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and not affect the progress of the electrochemical charge and discharge process. The present application has no particular limitation on the separator, as long as the object of the present application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyolefins (PO) based on polyethylene (PE) and polypropylene (PP), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator may include at least one of a woven film, a non-woven film, a microporous film, a composite film, a rolled film or a spun film. In the present application, there is no particular limitation on the thickness of the separator, as long as the object of the present application can be achieved. For example, the thickness of the separator may be 3 μm to 30 μm.

[0065] In one embodiment of the present application, the secondary battery includes an electrolyte. The present application has no particular limitation on the type of the electrolyte, and those skilled in the art can select a well-known electrolyte in the art according to actual needs, as long as the object of the present application can be achieved.

[0066] In one embodiment of the present application, the secondary battery includes a packaging bag for accommodating the positive electrode plate, the negative electrode plate, the separator and the electrolyte. The present application has no particular limitation on the type of the packaging bag, and those skilled in the art can select a well-known packaging bag in the art according to actual needs, as long as the object of the present application can be achieved.

[0067] 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: lithium metal secondary batteries, lithium ion secondary batteries (lithium ion batteries), lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

[0068] The present application has no particular limitation on the preparation method of the secondary battery, and a preparation method well-known in the art can be selected as long as the object 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 a separator, a positive electrode sheet, a separator, and a negative electrode sheet in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking a separator, a positive electrode sheet, a separator, and a negative electrode sheet in sequence, then fixing the four corners of the entire laminated structure to obtain a laminated electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain a secondary battery.

[0069] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good performance in use.

[0070] The electronic device of the present application is not particularly limited and may be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household storage batteries, and lithium ion capacitors.

[0071] Examples

[0072] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods.

[0073] Testing method and device:

[0074] Sampling method for the positive electrode sheet:

[0075] At 25 °C, the lithium ion battery is discharged at a constant current of 0.2C to 3.0V until it reaches a fully discharged state. After reaching the fully discharged state, the lithium ion battery is disassembled, and the positive electrode sheet is soaked in dimethyl carbonate (DMC) at room temperature for 60 min, taken out and dried at room temperature to obtain the positive electrode sheet.

[0076] Unless otherwise specified, the positive electrode sheets in the following test methods are all obtained by the above method.

[0077] Testing of tapped density:

[0078] (1) Take the positive electrode sheet and soak it in the solvent N-methylpyrrolidone (NMP) to remove the film, dissolve the film layer of the positive electrode active material layer in the solvent, and use a disperser to disperse it evenly to obtain the first slurry;

[0079] (2) Take the above first slurry, and use centrifugation to separate the positive electrode binder in the first slurry to obtain a second slurry containing the positive electrode active material and the first carbon material;

[0080] (3) Use hydrochloric acid to digest the positive electrode active material (lithium cobaltate or lithium nickel cobalt manganate) in the above second slurry. For the positive electrode active material of lithium iron phosphate type, use hydrochloric acid-hydrogen peroxide to digest it, and then dry it to obtain the first carbon material;

[0081] (4) Take 2 g to 10 g of the first carbon material in (3) and put it into a tap density tester. The model of the tap density tester can be VTD-100. Record its weight as M (g). Set the vibration frequency of the vibrator to 250 times / min. After vibrating 5000 times in total, record its volume as V (cm 3 )), then the tapped density of the first carbon material (g / cm 3 ) = M / V.

[0082] Testing of oil absorption value of the first carbon material:

[0083] (1) Take the positive electrode sheet and soak it in the solvent N-methylpyrrolidone (NMP) to remove the film, dissolve the film layer of the positive electrode active material layer in the solvent, and use a disperser to disperse it evenly to obtain the first slurry;

[0084] (2) Take the above first slurry, and use centrifugation to separate the positive electrode binder in the first slurry to obtain a second slurry containing the positive electrode active material and the first carbon material;

[0085] (3) Use hydrochloric acid to digest the positive electrode active material (lithium cobaltate or lithium nickel cobalt manganate) in the above second slurry. For the positive electrode active material of lithium iron phosphate type, use hydrochloric acid-hydrogen peroxide to digest it, and then dry it to obtain the first carbon material;

[0086] (4) Use the paraffin oil + torque method to test the oil absorption value of the first carbon material:

[0087] Select an oil absorption meter of model DABS-H. Add the first carbon material (sample) into the mixing tank of the oil absorption meter, and add paraffin oil to the sample at a constant dropping rate of 4 ml / min using a constant dropper. As the oil absorption value of the sample increases, the mixture changes from a free-flowing state to a semi-plastic agglomerate, and the viscosity of the mixture continuously increases. This viscosity is transmitted to the torque sensing system of the oil absorption meter. When the viscosity of the mixture reaches a torque value of 2 WN·m, the oil absorption meter and the burette automatically close simultaneously. Read directly from the reading burette the volume of the added oil. The volume of oil absorbed per unit mass of the sample is the oil absorption value of the sample.

[0088] Testing the average particle size of carbon black particles:

[0089] (1) Take the positive electrode plate and obtain the cross-section of the positive active material layer on the positive electrode plate by brittle fracture with liquid nitrogen.

[0090] (2) Observe the cross-section obtained in (1) under a scanning electron microscope (SEM). Test 10 different positions, measure the equivalent diameter of carbon black particles, and take the average value as the average particle size, where the sample size of carbon black particles is 50.

[0091] Testing the diameter of carbon fiber:

[0092] (1) Take the positive electrode plate and obtain the cross-section of the positive active material layer on the positive electrode plate by brittle fracture with liquid nitrogen.

[0093] (2) Observe the cross-section obtained in (1) under a scanning electron microscope (SEM). Test 10 different positions, measure the diameter of carbon fiber, and take the average value as the final value, where the total sample size is 30.

[0094] Testing the length and diameter of the first carbon material:

[0095] a) Take the positive electrode plate and obtain the cross-section of the positive active material layer on the positive electrode plate by brittle fracture with liquid nitrogen.

[0096] b) Observe the cross-section obtained in a) under a scanning electron microscope (SEM). Test 10 different positions. For each first carbon material, take the average value of the diameters at the 3 positions with the largest diameters as its final diameter. Test at least 30, and then take the average value as the final value.

[0097] c) Observe the cross-section obtained in a) under a scanning electron microscope (SEM). Test 10 different positions, measure the length of the middle carbon fiber, and take the average value as the length of the first carbon material, where the total sample size is not less than 30.

[0098] Testing the diameter and length of carbon nanotubes:

[0099] a) Take the positive electrode plate and obtain the cross-section of the positive active material layer on the positive electrode plate by brittle fracture with liquid nitrogen;

[0100] b) Observe the cross-section obtained in a) under SEM, test 10 different positions, measure the diameter and length of the carbon nanotubes, and take their average value as the target value. The number of carbon nanotubes to be measured is 30.

[0101] Test for the liquid retention rate of the positive electrode plate:

[0102] a) Take the positive electrode plate and measure its mass as M1. Immerse it in the electrolyte at 25 °C for 24 h, then take it out and air-dry it for 10 min in an environment with a temperature of 25 °C ± 2 °C and a humidity of 10% to 15%. Weigh it and record the mass as M2. Then the liquid retention rate (%) = (M2 - M1) / M1 × 100%.

[0103] Test for the 360 s DC internal resistance (DCR) of the lithium-ion battery at 20% SOC (state of charge):

[0104] Take the lithium-ion battery in the example or comparative example and conduct the following tests at 25 ± 2 °C:

[0105] (1) Stand still for 2 h;

[0106] (2) Then charge at a constant current of 1.0C to 4.5V, and charge at a constant voltage of 4.5V to 0.025C; stand still for 2 h;

[0107] (3) Then discharge at a constant current of 0.2C to 3.0V, and take the discharge capacity of this step as the variable C1. Stand still for 5 h;

[0108] (4) Then charge at a constant current of 1.0C1 to 4.5V, and charge at a constant voltage of 4.5V to 0.025C1; stand still for 10 min;

[0109] (5) Then discharge at a constant current of 0.1C1 to 0.2C1, stand still for 15 min, record the voltage of the lithium-ion battery at this time as V1, and then discharge at a rate of 1C1 for 360 s and record the voltage as V2. Then the 360 s DCR (mΩ) at 25 °C and 20% SOC = (V1 - V2) / 1C1.

[0110] Example 1-1

[0111] <Preparation of the positive electrode plate>

[0112] Preparation of the first carbon material: The carbon black material and polyacrylonitrile (PAN, weight-average molecular weight Mw = 1.1 million) are melt-blended uniformly at 190 °C until PAN is completely melted and the carbon black is uniformly dispersed in the molten PAN; then electrospinning is used to form carbon fibers, and the carbon fibers are pre-oxidized at 280 °C for 3 h in a mixed atmosphere of air and nitrogen (mass ratio of air to nitrogen is 20:80), carbonized at 1350 °C for 1.5 h in a nitrogen atmosphere, and graphitized at 2700 °C for 1.5 h in a nitrogen atmosphere to form the first carbon material. Among them, the mass ratio of the carbon black material to PAN is 4:1.

[0113] The cathode active material lithium cobaltate, the first carbon material, and the cathode binder polyvinylidene fluoride (PVDF, weight-average molecular weight is 0.8 million) are mixed, and N-methylpyrrolidone (NMP) is added as a solvent, and stirred in a vacuum mixer until a cathode slurry with a solid content of 75 wt% and a uniform system is obtained. The cathode slurry is uniformly coated on one surface of a cathode current collector aluminum foil with a thickness of 6 μm and dried at 90 °C to obtain a cathode electrode sheet with a single-sided coated cathode active material layer. Then, the above steps are repeated on the other surface of the aluminum foil to obtain a cathode electrode sheet with a double-sided coated cathode active material layer. After cold pressing and slitting, a cathode electrode sheet with a specification of 74 mm × 851 mm is obtained for use. Among them, the coating weight of the cathode active material layer is 280 mm / 1540.25 mm 2 。

[0114] Among them, based on the mass of the cathode active material layer, the mass percentage content of the cathode active material is 98%, the mass percentage content W1 of the first carbon material is 0.8%, and the mass percentage content of the cathode binder is 1.2%.

[0115] <Preparation of the anode electrode sheet>

[0116] The anode active material artificial graphite, the anode conductive agent acetylene black, the anode binder styrene-butadiene rubber (SBR, weight-average molecular weight is 5×10 6 ) and the dispersant carboxymethyl cellulose (CMC) are mixed according to a mass ratio of 96:2:1:1, and then deionized water is added as a solvent, and stirred in a vacuum mixer until an anode slurry with a solid content of 50 wt% and a uniform system is obtained. The anode slurry is uniformly coated on one surface of an anode current collector copper foil with a thickness of 8 μm and dried at 90 °C to obtain an anode electrode sheet with a single-sided coated anode active material layer (thickness 130 μm). Then, the above steps are repeated on the other surface of the copper foil to obtain an anode electrode sheet with a double-sided coated anode active material layer. After cold pressing and slitting, an anode electrode sheet with a specification of 76 mm × 856 mm is obtained for use.

[0117] <Preparation of the separator>

[0118] Use a polyethylene (PE) porous film with a thickness of 8 μm as the separator.

[0119] <Preparation of electrolyte>

[0120] In a dry argon atmosphere, mix the organic solvents ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 30:50:20 to obtain a basic electrolyte. Then, add lithium hexafluorophosphate to the basic electrolyte and dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0121] <Preparation of lithium-ion battery>

[0122] Stack the separator, positive electrode sheet, separator, and negative electrode sheet in sequence and wind them to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film packaging bag, inject the electrolyte after drying, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, degassing, and trimming.

[0123] Examples 1-2 to 1-31

[0124] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Example 1-1.

[0125] Example 2-1

[0126] Except for further introducing carbon nanotubes in the <Preparation of positive electrode sheet> and adjusting the mass percentage contents of the first carbon material, carbon nanotubes, and positive electrode binder according to Table 2, with the mass percentage content of the positive electrode active material changing accordingly, the rest are the same as Example 1-1.

[0127] Among them, the sum of the mass percentage contents of the first carbon material, carbon nanotubes, positive electrode binder, and positive electrode active material is 100%.

[0128] Examples 2-2 to 2-4

[0129] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as Example 1-1.

[0130] Among them, when the mass percentage content of the first carbon material changes, the mass percentage content of the positive electrode active material changes accordingly, and the mass percentage content of the positive electrode binder remains unchanged.

[0131] Example 2-5

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

[0133] Among them, when the mass percentage content of the first carbon material changes, the mass percentage content of the positive electrode active material changes accordingly, and the mass percentage contents of the positive electrode binder and carbon nanotubes remain unchanged.

[0134] Examples 2-6

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

[0136] Among them, when the mass percentage content of the first carbon material changes, the mass percentage content of the positive electrode active material changes accordingly, and the mass percentage content of the positive electrode binder remains unchanged.

[0137] Examples 2-7 to Examples 2-19

[0138] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as in Example 2-1.

[0139] Among them, when the mass percentage content of the first carbon material and / or carbon nanotubes changes, the mass percentage content of the positive electrode active material changes accordingly, and the mass percentage content of other components in the positive electrode active material layer remains unchanged.

[0140] Examples 3-1 to Examples 3-6

[0141] Except for adjusting the relevant preparation parameters according to Table 3, the rest are the same as in Example 2-12.

[0142] Among them, when the mass percentage content of the positive electrode binder changes, the mass percentage content of the positive electrode active material changes accordingly, and the mass percentage content of other components in the positive electrode active material layer remains unchanged.

[0143] Comparative Examples 1 and Comparative Example 6

[0144] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Examples 1-1.

[0145] The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 3.

[0146] Table 1

[0147]

[0148]

[0149] As can be seen from Examples 1-1 to 1-31 and Comparative Examples 1 to 6, for the secondary batteries of the embodiments of the present application, by adding the first carbon material with single carbon black particles and / or chain-like structures formed by carbon black particles provided on the surface of carbon fibers to the positive electrode active material layer, and regulating the tapped density and oil absorption value of the first carbon material within the scope of the present application, the 360s DC internal resistance of the secondary battery at 20% SOC is lower, indicating that the secondary battery has a lower internal resistance. While for the secondary batteries of the comparative examples, the tapped density and oil absorption value of the first carbon material are not within the scope of the present application, and the 360s DC internal resistance of the secondary batteries of the comparative examples at 20% SOC is higher.

[0150] The average particle size d0 of the carbon black particles usually also affects the internal resistance of the secondary battery. As can be seen from Examples 1-1, 1-7 to 1-12, for the secondary batteries with the average particle size d0 of the selected carbon black particles within the scope of the present application, their 360s DC internal resistance at 20% SOC is lower, indicating that the secondary battery has a lower internal resistance.

[0151] The diameter Φ of the carbon fiber usually also affects the internal resistance of the secondary battery. As can be seen from Examples 1-1, 1-13 to 1-19, for the secondary batteries with the diameter Φ of the selected carbon fiber within the scope of the present application, their 360s DC internal resistance at 20% SOC is lower, indicating that the secondary battery has a lower internal resistance.

[0152] The ratio Φ / d0 of the diameter of the carbon fiber to the average particle size of the carbon black particles usually also affects the internal resistance of the secondary battery. As can be seen from Examples 1-1, 1-7 to 1-19, for the secondary batteries with the ratio Φ / d0 of the diameter of the selected carbon fiber to the average particle size of the carbon black particles within the scope of the present application, their 360s DC internal resistance at 20% SOC is lower, indicating that the secondary battery has a lower internal resistance.

[0153] The length L1 and diameter D1 of the first carbon material usually also affect the internal resistance of the secondary battery. As can be seen from Examples 1-1, 1-20 to 1-31, when the length L1 and diameter D1 of the first carbon material change, the tapped density and oil absorption value of the first carbon material change accordingly. For the secondary batteries with the length L1 and diameter D1 of the selected first carbon material within the scope of the present application, their 360s DC internal resistance at 20% SOC is lower, indicating that the secondary battery has a lower internal resistance.

[0154] Table 2

[0155]

[0156] Note: “\” in Table 2 indicates no corresponding parameter.

[0157] The mass percentage content W1 of the first carbon material usually also affects the internal resistance of the secondary battery. It can be seen from Examples 1-1, 2-1 to 2-6 that for the secondary battery with the mass percentage content W1 of the first carbon material within the scope of this application, its 360s DC internal resistance at 20% SOC is relatively low, indicating that the secondary battery has a low internal resistance.

[0158] When carbon nanotubes are further introduced into the positive electrode active material layer, the mass percentage content W2 of the carbon nanotubes usually also affects the internal resistance of the secondary battery. It can be seen from Examples 1-1, 2-7 to 2-10, 2-12 that for the secondary battery with the mass percentage content W2 of the carbon nanotubes within the scope of this application, its 360s DC internal resistance at 20% SOC is relatively low, indicating that the secondary battery has a low internal resistance.

[0159] The diameter D2 of the carbon nanotubes usually also affects the internal resistance of the secondary battery. It can be seen from Examples 2-11 to 2-15 that for the secondary battery with the diameter D2 of the carbon nanotubes within the scope of this application, its 360s DC internal resistance at 20% SOC is relatively low, indicating that the secondary battery has a low internal resistance.

[0160] The length L2 of the carbon nanotubes usually also affects the internal resistance of the secondary battery. It can be seen from Examples 2-12, 2-16 to 2-19 that for the secondary battery with the length L2 of the carbon nanotubes within the scope of this application, its 360s DC internal resistance at 20% SOC is relatively low, indicating that the secondary battery has a low internal resistance.

[0161] Table 3

[0162]

[0163] The type of the positive electrode binder and its mass percentage content W3 usually also affect the internal resistance of the secondary battery. It can be seen from Examples 2-12, 3-1 to 3-6 that for the secondary battery with the type of the positive electrode binder and its mass percentage content W3 within the scope of this application, its 360s DC internal resistance at 20% SOC is relatively low, indicating that the secondary battery has a low internal resistance.

[0164] It should be noted that, in this document, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0165] Each embodiment in this specification is described in a related manner. For the same or similar parts among the embodiments, reference can be made to each other, and the differences between each embodiment and other embodiments are emphasized.

[0166] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A positive electrode plate, comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a first carbon material; The first carbon material comprises carbon fibers and single carbon black particles disposed on the surface of the carbon fibers and / or a chain-like structure formed by the carbon black particles; The tap density of the first carbon material is 0.005 g / cm 3 to 0.30 g / cm 3 ; The oil absorption value of the first carbon material is 100 ml / 100 g to 1000 ml / 100 g.

2. The positive electrode sheet according to claim 1, wherein The average particle size of the carbon black particles is d0 μm, where 0.005 ≤ d0 ≤ 0.

1.

3. The positive electrode sheet according to claim 2, wherein, The diameter of the carbon fibers is Φ μm, where 0.01 ≤ Φ ≤ 1.

00.

4. The positive electrode sheet according to claim 3, wherein 1.5 ≤ Φ / d0 ≤ 8.

5. The positive electrode sheet according to claim 2 or 3, wherein, The positive electrode plate satisfies at least one of the following characteristics: (a) 0.01 ≤ d0 ≤ 0.05; (b) 0.05 ≤ Φ ≤ 0.

1.

6. The positive electrode sheet according to claim 1, wherein, The length of the first carbon material is L1 μm, where 1 ≤ L1 ≤ 50.

7. The positive electrode sheet according to claim 1, wherein, The diameter of the first carbon material is D1 μm, where 0.2 ≤ D1 ≤ 5.

8. The positive electrode sheet according to claim 1, wherein, The first carbon material satisfies at least one of the following characteristics: (1) The tap density of the first carbon material is 0.01 g / cm 3 to 0.06 g / cm 3 ; (2) The oil absorption value of the first carbon material is 400 ml / 100 g to 700 ml / 100 g; (3) The length of the first carbon material is L1 μm, where 2 ≤ L1 ≤ 15; (4) The diameter of the first carbon material is D1 μm, where 0.5 ≤ D1 ≤ 2.

9. The positive electrode sheet according to claim 1, wherein, Based on the mass of the positive active material layer, the mass percentage content of the first carbon material is W1%, where 0.1 ≤ W1 ≤ 2.

0.

10. The positive electrode sheet according to claim 1 or 9, wherein, Based on the mass of the positive active material layer, the mass percentage content of the first carbon material is W1%, where 0.3 ≤ W1 ≤ 1.

0.

11. The positive electrode sheet according to claim 1, wherein, The positive active material layer comprises a second carbon material, the second carbon material comprises carbon nanotubes, the diameter of the carbon nanotubes is D2 nm, where 5 ≤ D2 ≤ 15, and the length of the carbon nanotubes is L2 μm, where 1 ≤ L2 ≤ 5; Based on the mass of the positive active material layer, the mass percentage content of the carbon nanotubes is W2%, where 0.2 ≤ W2 ≤ 0.

8.

12. The positive electrode sheet according to claim 1, wherein, The positive active material layer comprises a positive electrode binder, and the positive electrode binder comprises at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyamide, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, polypropylene, polyethylene, polytetrafluoroethylene, polyetherimide, hydrogenated nitrile rubber or carboxymethyl cellulose salt; Based on the mass of the positive active material layer, the mass percentage content of the positive electrode binder is W3%, where 0.8 ≤ W2 ≤ 3.

0.

13. The positive electrode sheet according to claim 1, wherein, The saturation liquid retention rate of the positive electrode plate is 5% to 20%.

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

15. An electronic device, wherein, The electronic device comprises the secondary battery according to claim 14.

Images