Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device. The secondary battery comprises a positive pole piece, an isolating membrane and a negative pole piece, the positive pole piece comprises a positive current collector and a positive material layer arranged on at least one surface of the positive current collector, and the negative pole piece comprises a negative current collector and a negative material layer arranged on at least one surface of the negative current collector; the thickness of the positive electrode material layer is 22-110 [mu] m, the positive electrode material layer comprises a positive electrode conductive agent and a positive electrode binder, the positive electrode conductive agent comprises a carbon nanotube and at least one of a carbon fiber tube or flake graphene, the diameter of the carbon nanotube is D1 nm, 4 < = D1 < = 23, the outer diameter of the carbon fiber tube is D2 nm, 25 < = D2 < = 150. Through the arrangement, the secondary battery has high energy density and good high-temperature charge-discharge performance.

Description

Technical Field

[0001] The present application relates to the field of electrochemistry technology, and particularly to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in digital electronic products, energy storage, drones, power tools, electric vehicles and other products due to their characteristics such as high energy density, long cycle life, high safety, and fast charging ability. With the increasing demand for thinner and lighter products, there is a need for secondary batteries to have higher and higher energy density.

[0003] Conventional methods for increasing energy density include improving the specific capacity of the positive and negative electrode materials, and raising the charging voltage platform, etc. Making the electrode sheet thicker and increasing the surface coating density of the electrode sheet is one of the means in the industry to increase energy density. However, after the thickness of the electrode sheet increases, it means that the lithium-ion transmission path increases, and the ionic conductivity and electronic conductivity will be significantly reduced, resulting in a decrease in the rate discharge capacity of the secondary battery. For secondary batteries of the fast charging (actual charging rate > 1C) type, the thickening of the electrode sheet will lead to a significant deterioration of the charging time. Therefore, how to improve the ionic conductivity and electronic conductivity of the thick electrode sheet and improve the high-temperature charge and discharge performance of the secondary battery with the thick electrode sheet will be the most urgent problem to be solved for the thick electrode sheet to increase the energy density. Summary of the Invention

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to improve the high-temperature charge and discharge performance of the secondary battery.

[0005] It should be noted that in the summary of the invention of the present application, the lithium-ion battery is taken as an example of the secondary battery to explain the present application, but the secondary battery of the present application is not limited to the lithium-ion battery. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a secondary battery, which includes a positive electrode plate, a separator, and a negative electrode plate. The positive electrode plate includes a positive current collector and a positive electrode material layer provided on at least one surface of the positive current collector. The negative electrode plate includes a negative current collector and a negative electrode material layer provided on at least one surface of the negative current collector. The thickness of the positive electrode material layer is from 22 μm to 110 μm. The positive electrode material layer includes a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent includes carbon nanotubes and at least one of carbon fiber tubes or flaky graphene. The diameter of the carbon nanotubes is D1 nm, where 4 ≤ D1 ≤ 23, and the outer diameter of the carbon fiber tubes is D2 nm, where 25 ≤ D2 ≤ 150. By adjusting the thickness of the positive electrode material layer in the secondary battery within the scope of the present application, selecting carbon nanotubes and at least one of carbon fiber tubes or flaky graphene as the positive electrode conductive agent, and adjusting the values of D1 and D2 within the scope of the present application, the use of a thick positive electrode plate can provide a secondary battery with a high capacity, enabling the secondary battery to have a high energy density. Using at least one of carbon nanotubes and carbon fiber tubes or flaky graphene in combination can increase the contact sites between the positive electrode conductive agent and the positive electrode active material particles, form a conductive network between the positive electrode active material particles, which is beneficial to constructing a stable conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus improving the high-temperature charge and discharge performance of the secondary battery.

[0007] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the coating surface density of the positive electrode material layer is from 10 mg / cm 2 to 55 mg / cm 2 ; (2) the thickness of the positive electrode material layer is from 45 μm to 110 μm; (3) the coating surface density of the negative electrode material layer is from 2.5 mg / cm 2 to 35 mg / cm 2 ; (4) the thickness of the negative electrode material layer is from 20 μm to 70 μm. By satisfying the above characteristics, on the basis of having good high-temperature charge and discharge performance, the secondary battery also has a high energy density.

[0008] In some embodiments of the present application, the carbon nanotubes satisfy at least one of the following characteristics: (1) the length of the carbon nanotubes is L1 nm, where 100 ≤ L1 ≤ 6000; (2) the aspect ratio L1 / D1 of the carbon nanotubes satisfies: 100 ≤ L1 / D1 ≤ 300; (3) 8 ≤ D1 ≤ 18. By satisfying the above characteristics, it is beneficial for the carbon nanotubes to construct a stable medium- and short-range conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode plate, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0009] In some embodiments of the present application, the carbon fiber tube satisfies at least one of the following characteristics: (1) the length of the carbon fiber tube is L2 nm, where 2000 ≤ L ≤ 50000; (2) the inner diameter of the carbon fiber tube is D2' nm, where 1.5 ≤ D2' ≤ 35; (3) the aspect ratio L2 / D2 of the carbon fiber tube satisfies: 50 ≤ L2 / D2 ≤ 400; (4) 30 ≤ D2 ≤ 80; (5) the resistivity of the carbon fiber tube is ρ mΩ·cm, where 5 ≤ ρ ≤ 27. Meeting the above characteristics is beneficial to constructing a stable long-range three-dimensional network conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0010] In some embodiments of the present application, the sheet diameter of the flaky graphene is from 5 μm to 15 μm. By adjusting the sheet diameter of the flaky graphene within the above range, face contact is achieved between the flaky graphene and each particle of the positive electrode active material, which is beneficial to further improving the high-temperature charge and discharge performance of the secondary battery.

[0011] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode conductive agent is from 0.2% to 2%. In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode conductive agent is from 0.5% to 1%. By adjusting the mass percentage content of the positive electrode conductive agent within the above range, it is beneficial to prepare a positive electrode material layer with good conductive performance and high capacity, so that the secondary battery can have good high-temperature charge and discharge performance and energy density, and also has a low production cost.

[0012] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) based on the mass of the positive electrode material layer, the mass percentage content of the carbon nanotube is Y1%, where 0.1 ≤ Y1 ≤ 1; (2) based on the mass of the positive electrode material layer, the mass percentage content of the carbon fiber tube is Y2%, where 0.1 ≤ Y2 ≤ 0.5; (3) based on the mass of the positive electrode material layer, the mass percentage content of the flaky graphene is Y3%, where 0.1 ≤ Y3 ≤ 0.5. Meeting the above characteristics, the positive electrode sheet has high ionic conductivity and electronic conductivity, thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0013] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride, and the positive electrode binder satisfies at least one of the following characteristics: (1) in the infrared spectrum of the positive electrode binder, there is a peak located at 1710 cm -1 to 1725 cm -1 、2500 cm -1 to 3300 cm -1Infrared characteristic peaks; (2) The melting point of the positive electrode binder is 157°C to 167°C; (3) The peak decomposition temperature of the thermogravimetric curve of the positive electrode binder is in the range of 470°C to 490°C; (4) Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is 0.5% to 3.0%; (5) The positive electrode binder contains carboxyl groups, and based on the mass of the positive electrode binder, the mass percentage of carboxyl groups is 0.5% to 3.5%; (6) The weight-average molecular weight of the positive electrode binder is Mw, 800,000 ≤ Mw ≤ 1,200,000, the number-average molecular weight of the positive electrode binder is Mn, and the molecular weight distribution Mw / Mn of the positive electrode binder satisfies: 1.5 ≤ Mw / Mn ≤ 3. When the positive electrode binder satisfies the above characteristics, the positive electrode material layer has good adhesion performance, which is beneficial to improving the stability of the conductive network in the positive electrode sheet, thereby further improving the high-temperature charge and discharge performance of the secondary battery.

[0014] In some embodiments of the present application, the positive electrode binder satisfies at least one of the following characteristics: (1) The melting point of the positive electrode binder is 159°C to 165°C; (2) The peak decomposition temperature of the thermogravimetric curve of the positive electrode binder is in the range of 475°C to 481°C; (3) Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is 0.6% to 1.6%; (4) The positive electrode binder contains carboxyl groups, and based on the mass of the positive electrode binder, the mass percentage of carboxyl groups is 0.5% to 1.5%; (5) The molecular weight distribution Mw / Mn of the positive electrode binder satisfies: 2 ≤ Mw / Mn ≤ 2.5. When the positive electrode binder satisfies the above characteristics, the positive electrode material layer has better adhesion performance, which is beneficial to further improving the stability of the conductive network in the positive electrode material layer, thereby better improving the high-temperature charge and discharge performance of the secondary battery.

[0015] In some embodiments of the present application, the thickness of the positive electrode material layer is 70 μm to 100 μm, which can further balance the high-temperature charge and discharge performance, energy density, and capacity retention rate.

[0016] In some embodiments of the present application, the adhesion force between the positive electrode material layer and the positive electrode current collector is F1 N / m, 10 ≤ F1 ≤ 70, indicating that there is a high adhesion force between the positive electrode material layer and the positive electrode current collector in the present application, which is beneficial to improving the contact between the positive electrode material layer and the positive electrode current collector, maintaining a good conductive network, thereby being beneficial to improving the stability of the conductive network in the positive electrode sheet, and further improving the high-temperature charge and discharge performance of the secondary battery. The adhesion force between the positive electrode sheet and the separator is F2 N / m, 8 ≤ F2 ≤ 30. It shows that there is a high adhesion force between the positive electrode sheet and the separator in the present application, enabling good contact between the positive electrode sheet and the separator, which is beneficial to ion transport inside the secondary battery, thereby improving the internal ion conductivity of the secondary battery and further improving the high-temperature charge and discharge performance of the secondary battery.

[0017] In some embodiments of the present application, 30.3 ≤ F1 ≤ 47.7 and 17.8 ≤ F2 ≤ 25.1. This indicates that there is a higher adhesive force between the positive electrode material layer and the positive electrode current collector, as well as between the positive electrode sheet and the separator in the present application, which is more conducive to improving the high-temperature charge and discharge performance of the secondary battery.

[0018] The second 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.

[0019] Advantages of the present application:

[0020] The present application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The thickness of the positive electrode material layer is 22 μm to 110 μm. The positive electrode material layer includes a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent includes carbon nanotubes and at least one of carbon fiber tubes or flaky graphene. The diameter of the carbon nanotubes is D1 nm, where 4 ≤ D1 ≤ 23, and the outer diameter of the carbon fiber tubes is D2 nm, where 25 ≤ D2 ≤ 150. In the thick positive electrode sheet, the combination of the positive electrode conductive agents with the above characteristics can increase the contact sites between the positive electrode conductive agent and the positive electrode active material particles, form a conductive network between the positive electrode active material particles, which is beneficial to constructing a stable conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus improving the high-temperature charge and discharge performance of the secondary battery.

[0021] Of course, it is not necessary to achieve all the above-mentioned advantages simultaneously when implementing any product or method of the present application. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application, and those of ordinary skill in the art can also obtain other embodiments based on these drawings.

[0023] Figure 1 It is the infrared spectrum diagram of the positive electrode binder of Example 1-1 of the present application;

[0024] Figure 2 It is the thermogravimetric curve of the positive electrode binder of Example 1-1 of the present application. Detailed Embodiments

[0025] Next, the technical solutions in the present application will be clearly and completely described 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 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 embodiments of the present application, a lithium-ion battery is taken as an example of a secondary battery to explain the present application. However, the secondary battery of the present application is not limited to lithium-ion batteries.

[0027] The inventors found that by increasing the thickness of the positive electrode sheet to increase the areal density of the coating on the surface of the positive electrode sheet, if the conductive network of the positive electrode material layer is not adjusted, when the lithium-ion battery is charged at a high rate (actual charging rate > 1C, that is, fast charging), the lithium ions released from the positive electrode will easily accumulate on the surface of the negative electrode sheet due to excessive polarization, forming lithium dendrites. As the charge and discharge cycles of the lithium-ion battery proceed, the lithium dendrites continue to grow, increasing the risk of piercing the separator membrane and increasing the risk of safety problems in the lithium-ion battery. When the lithium-ion battery is discharged at a high rate (actual discharge rate > 1C), the discharge rate will also deteriorate significantly due to increased polarization, thereby affecting the high-temperature charge and discharge performance of the secondary battery.

[0028] In view of this, a first aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The thickness of the positive electrode material layer is 22 μm to 110 μm. Preferably, the thickness of the positive electrode material layer is 45 μm to 110 μm. More preferably, the thickness of the positive electrode material layer is 70 μm to 100 μm. For example, the thickness of the positive electrode material layer can be 22 μm, 30 μm, 40 μm, 45 μm, 50 μm, 60 μm, 80 μm, 90 μm, 100 μm, 110 μm, or a range composed of any two of these values. The positive electrode material layer includes a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent includes carbon nanotubes and at least one of carbon fiber tubes or flaky graphene. The diameter of the carbon nanotubes is D1 nm, where 4 ≤ D1 ≤ 23. Preferably, 8 ≤ D1 ≤ 18. For example, the value of D1 can be 4, 5, 8, 10, 12, 15, 18, 20, 23, or a range composed of any two of these values. The outer diameter of the carbon fiber tubes is D2 nm, where 25 ≤ D2 ≤ 150. Preferably, 30 ≤ D2 ≤ 80. For example, the value of D2 can be 25, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, 100, 114, 120, 135, 140, 150, or a range composed of any two of these values.

[0029] When the thickness of the positive electrode material layer is too small, for example, less than 22, the capacity provided by the positive electrode material layer for the secondary battery is too small, and the energy density of the secondary battery is too small; when the thickness of the positive electrode material layer is too large, for example, greater than 110, the increase in the lithium ion transmission path causes an increase in polarization, the ionic conductivity and electronic conductivity of the positive electrode plate are too low, the charging speed of the secondary battery is reduced, the rate performance becomes poor, the cycle capacity decays severely, and the volume of the secondary battery will also increase, and the increase in the volume of the secondary battery will result in a loss of energy density. When the value of D1 is too small, for example, less than 4, the diameter of the carbon nanotube is too small, the carbon nanotube is not easily dispersed, and it is easy to agglomerate in the form of a tube bundle and cannot be evenly dispersed in the positive electrode material layer, affecting the ionic conductivity and electronic conductivity of the positive electrode plate and unable to improve the high-temperature charge and discharge performance of the secondary battery; when the value of D1 is too large, for example, greater than 23, the diameter of the carbon nanotube is too large, and in the case of the same dosage, the number of carbon tubes is too small, and the contact area of the carbon nanotube is insufficient, which is not conducive to improving the ionic conductivity and electronic conductivity of the positive electrode plate and cannot improve the high-temperature charge and discharge performance of the secondary battery. When the value of D2 is too small, for example, less than 25, the diameter of the carbon fiber tube is too small, which is not conducive to the dispersion of the carbon fiber tube in the positive electrode material layer, and the agglomeration of the carbon fiber tube affects the ionic conductivity and electronic conductivity of the positive electrode plate and cannot improve the high-temperature charge and discharge performance of the secondary battery; when the value of D2 is too large, for example, greater than 150, the diameter of the carbon fiber tube is too large, and in the case of the same dosage, the number of carbon tubes is too small, and the contact area of the carbon fiber tube is insufficient, thus being not conducive to improving the ionic conductivity and electronic conductivity of the positive electrode plate and unable to improve the high-temperature charge and discharge performance of the secondary battery.

[0030] Overall, controlling the thickness of the positive electrode material layer in the secondary battery within the scope of this application, selecting carbon nanotubes as the positive electrode conductive agent, and at least one of carbon fiber tubes or flaky graphene, and controlling the values of D1 and D2 within the scope of this application can enable the secondary battery to have a high energy density while improving the high-temperature charge and discharge performance of the secondary battery. The use of one-dimensional carbon nanotubes is conducive to connecting the positive active material particles in the positive electrode material layer of the thick electrode plate in series, maintaining the network conductive network between the particles, and reducing the probability of damage to the conductive network during the charge and discharge cycle of the secondary battery. Compared with carbon nanotubes, one-dimensional carbon fiber tubes can connect more positive active material particles to form a long-range, three-dimensional network conductive network, further reducing the probability of damage to the conductive network during the charge and discharge cycle of the secondary battery, making the three-dimensional conductive network in the positive electrode material layer have better stability. At the same time, the carbon fiber tube has a certain stiffness and can stretch and distribute in the positive electrode material layer, reducing the probability of agglomeration of the conductive agent, so that the thick positive electrode plate has excellent electronic conductivity. The use of two-dimensional flaky graphene can achieve surface contact with the positive active material particles, increase the contact sites, improve the conductivity, and the flaky graphene can provide slippage between the positive active material particles, reducing the probability of brittle fracture of the thick positive electrode plate. In this application, the use of a thick positive electrode plate can provide a higher capacity for the secondary battery, enabling the secondary battery to have a higher energy density. Combining at least one of carbon nanotubes, carbon fiber tubes or flaky graphene can increase the contact sites between the positive electrode conductive agent and the positive active material particles, form a conductive network between the positive active material particles, facilitate the construction of a stable conductive network in the positive electrode material layer, improve the ionic conductivity and electronic conductivity of the positive electrode plate, and reduce the polarization impedance during the charge and discharge process of the secondary battery, thereby improving the high-temperature charge and discharge performance of the secondary battery.

[0031] In one embodiment, the positive electrode conductive agent includes carbon nanotubes and carbon fiber tubes. The carbon nanotubes connect the positive active material particles in series, maintaining the medium-short range network conductive network between the particles. The carbon fiber tubes stretch and distribute in the positive electrode material layer, can connect more positive active material particles, and maintain the long-range three-dimensional conductive network between the particles. Therefore, the combined use of carbon nanotubes and carbon fiber tubes can construct a stable and effective conductive network in the positive electrode material layer, which is conducive to improving the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and improving the high-temperature charge and discharge performance of the secondary battery.

[0032] In another embodiment, the positive electrode conductive agent includes carbon nanotubes and flaky graphene. The carbon nanotubes connect the particles of the positive electrode active material in series, maintaining a short-range network conductive network among the particles. The use of two-dimensional flaky graphene enables surface contact with the particles of the positive electrode active material, increasing the contact sites and improving the conductivity. Moreover, the flaky graphene can provide slippage between the particles of the positive electrode active material, reducing the probability of brittle fracture of the thick positive electrode sheet. Therefore, the combined use of carbon nanotubes and flaky graphene can construct a stable and effective conductive network in the positive electrode material layer, which is beneficial to improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and improving the high-temperature charge and discharge performance of the secondary battery.

[0033] In yet another embodiment, the positive electrode conductive agent includes carbon nanotubes, carbon fiber tubes, and flaky graphene. The combined use of the above three substances is beneficial to further improving the conductive network of the positive electrode material layer, forming a good and stable conductive network among the particles of the positive electrode active material, which is beneficial to improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery. In addition, on the basis of maintaining the conductive performance, the proportion of the positive electrode conductive agent and the positive electrode binder can be reduced, making the proportion of the positive electrode active material higher, and providing a higher energy density for the secondary battery on the basis of having good high-temperature charge and discharge performance.

[0034] In some embodiments of the present application, the coating areal density of the positive electrode material layer is 10 mg / cm 2 to 55 mg / cm 2 . For example, the coating areal density of the positive electrode material layer can be 10 mg / cm 2 , 12 mg / cm 2 , 15 mg / cm 2 , 17 mg / cm 2 , 20 mg / cm 2 , 25 mg / cm 2 , 30 mg / cm 2 , 35 mg / cm 2 , 40 mg / cm 2 , 44 mg / cm 2 , 50 mg / cm 2 , 55 mg / cm 2 or a range composed of any two of these values. Controlling the coating areal density of the positive electrode material layer within the above range is beneficial to enabling the positive electrode material layer to contain more positive electrode active material without affecting the lithium-ion transmission channels in the positive electrode material layer and having a good transmission rate, and can provide a higher capacity for the secondary battery. In this way, the secondary battery has a higher energy density on the basis of having good high-temperature charge and discharge performance.

[0035] In some embodiments of the present application, the coating areal density of the negative electrode material layer is 2.5 mg / cm 2 to 35 mg / cm 2 . For example, the coating areal density of the negative electrode material layer can be 2.5 mg / cm 2 , 5 mg / cm 2 , 7 mg / cm 2 , 9 mg / cm 2 , 12 mg / cm 2 , 15 mg / cm 2 , 17 mg / cm 2 , 20 mg / cm 2 , 25 mg / cm 2 , 30 mg / cm 2 , 35 mg / cm 2 or a range composed of any two of these values. Controlling the coating areal density of the negative electrode material layer within the above range is beneficial to enabling the negative electrode material layer to contain more negative electrode active materials without affecting the lithium ion transport channels in the negative electrode material layer and having a good transport rate, providing more lithium ion insertion and extraction sites for lithium ions, and improving the capacity of the secondary battery. In this way, on the basis of having good high-temperature charge and discharge performance, the secondary battery also has a high energy density.

[0036] In some embodiments of the present application, the thickness of the negative electrode material layer is 20 μm to 70 μm. For example, the thickness of the negative electrode material layer can be 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm or a range composed of any two of these values. Controlling the thickness of the negative electrode material layer within the above range, there are more negative electrode active materials in the negative electrode material layer, which can provide more lithium ion insertion and extraction sites for lithium ions and provide a higher capacity for the secondary battery. In this way, on the basis of having good high-temperature charge and discharge performance, the secondary battery also has a high energy density.

[0037] In some embodiments of the present application, the length of the carbon nanotubes is L1 nm, where 100 ≤ L1 ≤ 6000. For example, the value of L1 can be 100, 300, 500, 800, 1000, 2000, 3500, 4400, 5000, 6000, or a range composed of any two of these values. Controlling the length of the carbon nanotubes within the above range is conducive to the carbon nanotubes connecting the particles of the positive electrode active material in series, maintaining a short-range network conductive network between the particles, and reducing the probability of damage to the conductive network during the charge and discharge cycles of the secondary battery. In this way, the use of carbon nanotubes in the positive electrode sheet is more conducive to constructing a stable conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0038] In some embodiments of the present application, the length-to-diameter ratio L1 / D1 of the carbon nanotubes satisfies: 100 ≤ L1 / D1 ≤ 300. For example, the value of L1 / D1 can be 100, 130, 150, 180, 200, 220, 250, 275, 300, or a range composed of any two of these values. Controlling the length-to-diameter ratio of the carbon nanotubes within the above range is more conducive to the uniform dispersion of the carbon nanotubes in the positive electrode material layer, connecting the particles of the positive electrode active material in series, constructing a uniform and stable conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0039] In the present application, the features in the above embodiments, such as the length of the carbon nanotubes and the length-to-diameter ratio of the carbon nanotubes, can be combined arbitrarily, and the carbon nanotubes satisfy at least one of the above features.

[0040] The present application has no particular limitation on the control method of 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 purchasing commercially available carbon nanotubes and selecting the carbon nanotubes with the required length and diameter in the present application, or it can be achieved by using sanding and screening methods after purchasing commercially available carbon nanotubes.

[0041] In some embodiments of the present application, the length of the carbon fiber tube is L2 nm, where 2000 ≤ L ≤ 50000. For example, the value of L2 can be 2000, 5000, 10000, 20000, 30000, 40000, 50000, or a range composed of any two of these numerical values. Adjusting the length of the carbon fiber tube within the above range can make the carbon fiber tube evenly distributed in the positive electrode material layer, connect more positive electrode active material particles, and at the same time facilitate the long-range distribution of the carbon fiber tube in the positive electrode material layer, forming a long-range, three-dimensional network conductive network, further improving the stability of the three-dimensional conductive network in the positive electrode material layer, increasing the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0042] In some embodiments of the present application, the inner diameter of the carbon fiber tube is D2’ nm, where 1.5 ≤ D2’ ≤ 35. For example, the value of D2’ can be 1.5, 5, 8, 10, 14, 17, 20, 23, 26, 30, 33, 35, or a range composed of any two of these numerical values. Adjusting the inner diameter of the carbon fiber tube within the above range can make the carbon fiber tube have an appropriate pore size, which is beneficial for providing sufficient ion channels. At the same time, the carbon material of the carbon fiber tube wall can provide sufficient electron transport channels, which is beneficial for increasing the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0043] In some embodiments of the present application, the aspect ratio L2 / D2 of the carbon fiber tube satisfies: 50 ≤ L2 / D2 ≤ 400. For example, the value of L2 / D2 can be 50, 80, 100, 150, 180, 200, 250, 300, 320, 360, 400, or a range composed of any two of these numerical values. Adjusting the aspect ratio of the carbon fiber tube within the above range can make the carbon fiber tube evenly distributed in the positive electrode material layer, connect more positive electrode active material particles, and at the same time facilitate the long-range distribution of the carbon fiber tube in the positive electrode material layer, forming a long-range, three-dimensional network conductive network, further improving the stability of the three-dimensional conductive network in the positive electrode material layer, increasing the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0044] In some embodiments of the present application, the resistivity of the carbon fiber tube is ρ mΩ·cm, where 5 ≤ ρ ≤ 27. For example, the value of ρ can be 5, 8, 10, 12, 15, 18, 20, 24, 25, 27, or a range composed of any two of these numerical values. When the resistivity of the carbon fiber tube is within the above range, it indicates that the carbon fiber tube has good electrical conductivity, which is beneficial to improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0045] In the present application, the features in the above embodiments: the length of the carbon fiber tube, the inner diameter of the carbon fiber tube, the aspect ratio of the carbon fiber tube, the resistivity of the carbon fiber tube, etc. can be combined arbitrarily, and the carbon fiber tube satisfies at least one of the above features.

[0046] The present application does not particularly limit the method of regulating the length and diameter of the carbon fiber tube, as long as the purpose of the present application can be achieved. For example, it can be achieved by purchasing commercially available carbon fiber tubes and selecting the carbon fiber tubes with the required length and diameter in the present application, or it can be achieved by using sanding and screening methods after purchasing commercially available carbon fiber tubes.

[0047] In some embodiments of the present application, the sheet diameter of the flaky graphene is 5 μm to 15 μm. For example, the sheet diameter of the flaky graphene can be 5 μm, 6 μm, 8 μm, 9 μm, 10 μm, 12 μm, 14 μm, 15 μm, or a range composed of any two of these numerical values. By regulating the sheet diameter of the flaky graphene within the above range, face contact is achieved between the flaky graphene and each particle of the positive electrode active material, increasing the contact sites, improving the ionic conductivity and electronic conductivity of the positive electrode sheet, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus better improving the high-temperature charge and discharge performance of the secondary battery.

[0048] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode conductive agent is 0.2% to 2%, preferably 0.5% to 1%. For example, the mass percentage content of the positive electrode conductive agent can be 0.2%, 0.3%, 0.5%, 1%, 1.4%, 1.5%, 1.8%, 2%, or a range composed of any two of these numerical values. By regulating the mass percentage content of the positive electrode conductive agent within the above range, it is beneficial to prepare a positive electrode material layer with good electrical conductivity and high capacity, so that the secondary battery can have good high-temperature charge and discharge performance and energy density, and also has a low production cost.

[0049] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of carbon nanotubes is Y1%, where 0.1 ≤ Y1 ≤ 1. For example, the value of Y1 can be 0.1, 0.2, 0.3, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range composed of any two of these numerical values. Controlling the content of carbon nanotubes within the above range is beneficial for carbon nanotubes to connect the particles of the positive electrode active material in series, maintaining a short-range network conductive network among the particles, and reducing the probability of damage to the conductive network during the charge and discharge cycles of the secondary battery. In this way, it is more conducive to constructing a stable conductive network in the positive electrode material layer, improving the ionic conductivity and electronic conductivity of the positive electrode plate, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0050] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of carbon fiber tubes is Y2%, where 0.1 ≤ Y2 ≤ 0.5. For example, the value of Y2 can be 0.1, 0.15, 0.2, 0.24, 0.3, 0.35, 0.4, 0.44, 0.5, or a range composed of any two of these numerical values. Controlling the mass percentage content of carbon fiber tubes within the above range is beneficial for better exerting the role of carbon fiber tubes, providing a long-range, three-dimensional network conductive network for the positive electrode material layer, enabling the thick positive electrode plate to have excellent ionic conductivity and electronic conductivity, reducing the polarization impedance during the charge and discharge process of the secondary battery, and thus further improving the high-temperature charge and discharge performance of the secondary battery.

[0051] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of flaky graphene is Y3%, where 0.1 ≤ Y3 ≤ 0.5. For example, the value of Y3 can be 0.1, 0.15, 0.2, 0.24, 0.3, 0.35, 0.44, 0.5, or a range composed of any two of these numerical values. Controlling the content of flaky graphene within the above range is beneficial for better exerting the role of flaky graphene, which can achieve surface contact with the particles of the positive electrode active material, enabling the positive electrode material layer to have a stable and well-performing conductive network, and the positive electrode plate to have high ionic conductivity and electronic conductivity, thereby further improving the high-temperature charge and discharge performance of the secondary battery.

[0052] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride (carboxyl-modified PVDF). In the infrared spectrum of the positive electrode binder, there are peaks located at 1710 cm -1 to 1725 cm -1 and 2500 cm -1 to 3300 cm -1The infrared characteristic peaks. The infrared characteristic peaks within the above range are carboxyl peaks. The presence of the above infrared characteristic peaks in the infrared spectrum of the positive electrode binder indicates that the positive electrode binder contains carboxyl groups, which is beneficial to increasing the affinity of the positive electrode binder for the surface of the positive electrode active material particles, making the binder adhere more uniformly and with a higher coverage on the surface of the positive electrode active particles, thereby improving the adhesion between the positive electrode active material particles, improving the high-temperature and high-voltage stability of the positive electrode material layer, further improving the stability of the conductive network in the positive electrode material layer, and further improving the high-temperature charge and discharge performance of the secondary battery. In addition, with the above characteristics, the amount of the positive electrode binder in the positive electrode sheet can be reduced. On the basis of maintaining the bonding performance, the proportion of the positive electrode binder can be reduced, making the proportion of the positive electrode active material higher, and the positive electrode material layer having a higher specific capacity, so as to provide a higher energy density for the secondary battery on the basis of having good high-temperature charge and discharge performance.

[0053] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride, and the melting point of the positive electrode binder is in the range of 157 °C to 167 °C, preferably in the range of 159 °C to 165 °C. For example, the melting point of the positive electrode binder can be 157 °C, 159 °C, 160 °C, 163 °C, 165 °C, 167 °C or a range composed of any two of these values. The melting point of the positive electrode binder within the above range indicates that the positive electrode binder has good crystallinity and molecular chain structure, which can make the above positive electrode binder have good thermal stability, mechanical properties and processing properties, improve the stability of the conductive network in the positive electrode material layer, and further improve the high-temperature charge and discharge performance of the secondary battery.

[0054] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride, and the peak decomposition temperature of the thermogravimetric curve of the positive electrode binder is in the range of 470 °C to 490 °C. Preferably, the peak decomposition temperature of the thermogravimetric curve of the positive electrode binder is in the range of 475 °C to 481 °C. For example, the peak decomposition temperature of the thermogravimetric curve of the positive electrode binder can be 470 °C, 472 °C, 475 °C, 478 °C, 480 °C, 481 °C, 483 °C, 485 °C, 490 °C or a range composed of any two of these values. The peak decomposition temperature of the thermogravimetric curve of the positive electrode binder within the above range indicates that the positive electrode binder has good thermal stability, which is beneficial to better exerting the role of the above positive electrode binder, improving the stability of the conductive network in the positive electrode material layer, and further improving the high-temperature charge and discharge performance of the secondary battery.

[0055] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride. Based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode binder is 0.5% to 3.0%, preferably 0.6% to 1.6%. For example, the mass percentage content of the positive electrode binder can be 0.5%, 0.6%, 0.8%, 1%, 1.4%, 1.6%, 1.8%, 2%, 2.5%, 2.8%, 3.0% or a range composed of any two of these values. Controlling the mass percentage content of the positive electrode binder within the above range is beneficial to better play the role of the binder, is beneficial to increasing the affinity of the binder for the surface of the positive electrode active material particles, making the binder adhere more uniformly and have a higher coverage on the surface of the positive electrode active particles, thereby improving the adhesion between the positive electrode active material particles, improving the high-temperature and high-voltage stability of the positive electrode material layer, further improving the stability of the conductive network in the positive electrode material layer, and further improving the high-temperature charge-discharge performance of the secondary battery. In addition, by controlling the mass percentage content of the positive electrode binder within the above range, on the basis of maintaining the bonding performance, the lower proportion of the positive electrode binder can make the proportion of the positive electrode active material higher, and the positive electrode material layer has a higher specific capacity, thereby providing a higher energy density for the secondary battery on the basis of having good high-temperature charge-discharge performance.

[0056] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride, and the positive electrode binder contains carboxyl. Based on the mass of the positive electrode binder, the mass percentage content of carboxyl is 0.5% to 3.5%, preferably 0.5% to 1.5%. For example, the mass percentage content of carboxyl can be 0.5%, 1%, 1.4%, 1.8%, 2%, 2.5%, 2.8%, 3%, 3.4%, 3.5% or a range composed of any two of these values. Controlling the mass percentage content of carboxyl within the above range can make the binder have a suitable melting point and crystallinity, and is also beneficial to increasing the polarity of the positive electrode binder, improving the self-bonding performance of the positive electrode binder. The increase in the polarity of the positive electrode binder is also beneficial to increasing the affinity of the binder for the surface of the positive electrode active material particles, making the positive electrode binder adhere more uniformly and have a higher coverage on the surface of the positive electrode active particles, thereby improving the adhesion between the positive electrode active material particles, improving the high-temperature and high-voltage stability of the positive electrode material layer, further improving the stability of the conductive network in the positive electrode material layer, and further improving the high-temperature charge-discharge performance of the secondary battery. In addition, it also has a lower production cost.

[0057] In some embodiments of the present application, the positive electrode binder includes carboxyl-modified polyvinylidene fluoride. The weight-average molecular weight of the positive electrode binder is Mw, where 800,000 ≤ Mw ≤ 1,200,000. The number-average molecular weight of the positive electrode binder is Mn. The molecular weight distribution Mw / Mn of the positive electrode binder satisfies: 1.5 ≤ Mw / Mn ≤ 3. Preferably, 2 ≤ Mw / Mn ≤ 2.5. For example, Mw can be 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000 or a range composed of any two of these values. Mw / Mn can be 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3 or a range composed of any two of these values. By regulating the weight-average molecular weight and its molecular weight distribution of the positive electrode binder within the above ranges, the molecular weight distribution of the positive electrode binder is relatively concentrated, the probability of agglomeration of more small-molecule binders is low, and the probability of difficulty in melting of more large-molecule binders during heating is also low, which is more conducive to better exerting the role of the binder. Therefore, it is not only beneficial to improve the dispersibility of the binder, reduce the amount of the binder used, increase the bonding force between the positive electrode active material particles, and further improve the stability of the conductive network in the positive electrode material layer; but also enables the positive electrode active material particles to have appropriate acting forces, achieve effective slip during the cold pressing process, improve the risk of damage to the positive electrode current collector, reduce the probability of brittle fracture of the positive electrode sheet, improve the manufacturing yield of the positive electrode sheet, and enable the secondary battery to have good high-temperature charge and discharge performance while having a low production cost.

[0058] The present application does not particularly limit the number-average molecular weight Mn of the positive electrode binder, as long as the object of the present application can be achieved. For example, 300,000 ≤ Mn ≤ 800,000.

[0059] In some embodiments of the present application, the bonding force between the positive electrode material layer and the positive electrode current collector is F1 N / m, where 10 ≤ F1 ≤ 70. Preferably, 30.3 ≤ F1 ≤ 47.7. For example, the value of F1 can be 10, 12, 15, 17, 20.6, 30.3, 40, 47.7, 50, 60, 70 or a range composed of any two of these values. When the value of F1 is within the above range, it shows that there is a high bonding force between the positive electrode material layer and the positive electrode current collector of the present application, which is beneficial to improving the contact between the positive electrode material layer and the positive electrode current collector, maintaining a good conductive network, and thus is beneficial to improving the stability of the conductive network in the positive electrode sheet, and further improving the high-temperature charge and discharge performance of the secondary battery.

[0060] In some embodiments of the present application, the adhesion force between the positive electrode sheet and the separator is F2 N / m, where 8 ≤ F2 ≤ 30, and preferably 17.8 ≤ F2 ≤ 25.1. For example, the value of F2 can be 8, 10, 14, 17.8, 20.2, 23.5, 25.1, 26, 30, or a range composed of any two of these values. When the value of F2 is within the above range, it indicates that there is a high adhesion force between the positive electrode sheet and the separator of the present application, enabling good contact between the positive electrode sheet and the separator, which is beneficial to ion transport inside the secondary battery, thereby improving the ion conductivity inside the secondary battery and further enhancing the high-temperature charge and discharge performance of the secondary battery.

[0061] In the present application, the positive electrode material layer further includes a positive electrode active material. The present application does not particularly limit the type 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 cobalt oxide (LCO), 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. In some embodiments, based on the mass of the positive electrode material layer, the mass percentage content of the positive electrode active material is 96.5% to 99.1%. For example, the mass percentage content of the positive electrode active material can be 96.5%, 96.7%, 96.9%, 97.1%, 97.5%, 97.7%, 98.0%, 98.2%, 98.5%, 98.7%, 99.0%, 99.1%, or a range composed of any two of these values.

[0062] 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 can 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, 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.

[0063] 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 purpose of the present application can be achieved. For example, in one embodiment, the preparation method of the positive electrode sheet includes but is not limited to the following steps: (1) uniformly mixing a positive electrode conductive agent, a positive electrode 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) uniformly 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 material layer provided 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 positive electrode 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 uniformly coated on one surface of the positive electrode current collector to form a positive electrode material layer; (3) repeating step (2) on the other surface of the positive electrode current collector, and obtaining a positive electrode sheet with positive electrode material layers provided on both sides through cold pressing and slitting. 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.

[0064] 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 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, SnO2, spinel-structured lithium titanate Li4Ti5O 12, at least one of Li-Al alloy or metallic lithium. In the present application, there is no particular limitation on the thickness of the negative electrode current collector, 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. Optionally, the negative electrode material layer may further include at least one of a negative electrode conductive agent, a negative electrode dispersant, or a negative electrode binder. In the present application, there is no particular limitation on the types of the negative electrode conductive agent, the negative electrode dispersant, and the negative electrode binder in the negative electrode material layer, as long as the object of the present application can be achieved. In the present application, there is no particular limitation on the mass ratio of the negative electrode active material, the negative electrode conductive agent, the negative electrode dispersant, and the negative electrode binder in the negative electrode 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 negative electrode dispersant, and the negative electrode binder in the negative electrode material layer is (96 to 98):(0.5 to 2):(0 to 1.5):(1 to 1.9).

[0065] In the present application, there is 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) mainly composed of 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 calendared 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.

[0066] The secondary battery of the present application further includes an electrolyte, which may include a lithium salt and an organic solvent. The present application places no particular limitation on the type of the lithium salt, as long as the object of the present application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), or lithium difluoro(oxalato)borate (LiDFOB). The present application places no particular limitation on the type of the above-mentioned organic solvent, as long as the object of the present application can be achieved. For example, it may include, but is not limited to, at least one of carbonate compounds, carboxylate compounds, ether compounds, or other organic solvents. The above-mentioned carbonate compounds may include, but are not limited to, at least one of linear carbonate compounds or cyclic carbonate compounds. The above-mentioned linear carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The above-mentioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinylene carbonate (VEC). The above-mentioned carboxylate compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone. The above-mentioned ether compounds may include, but are not limited to, at least one of ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0067] The secondary battery further includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, as well as other components known in the field of secondary batteries. The present application places no limitation on the above-mentioned other components. The present application places no particular limitation on the housing, and it may be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing may be a hard shell housing or a flexible housing. The material of the hard shell housing may be a metal. The present application places no limitation on the type of the metal, and a metal hard shell housing known in the art may be adopted, as long as the object of the present application can be achieved. The flexible housing may be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0068] The secondary battery of the present application is not particularly limited, and it 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.

[0069] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no special limitation in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. Alternatively, stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fix the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, place the electrode assembly into the housing, inject an electrolyte into the housing and seal it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. may be placed in the housing as needed to prevent the pressure inside the secondary battery from rising and overcharging / discharging.

[0070] The second 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.

[0071] The electronic device of the present application is not particularly limited, and it may be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a moped, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium ion capacitor.

[0072] Examples

[0073] 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. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0074] Test methods and equipment:

[0075] Sampling method for positive electrode sheet and negative electrode sheet:

[0076] After discharging the lithium-ion batteries of each example and comparative example at a constant current of 0.2C to 3.0V, disassemble the lithium-ion batteries to take out the positive electrode sheet and the negative electrode sheet. Immerse the positive electrode sheet in dimethyl carbonate (DMC) for 20 min, and then rinse it with dimethyl carbonate and acetone in sequence. Clean the negative electrode sheet with deionized water, and then place the two in an oven and bake at 80°C for 12 h to obtain the dried positive electrode sheet and negative electrode sheet.

[0077] Thickness test of the positive electrode material layer and the negative electrode material layer:

[0078] For the positive electrode sheet and the negative electrode sheet, the thickness Hz of the positive electrode material layer and the thickness Hf of the negative electrode material layer are measured by observing with a scanning electron microscope (SEM). Cut the positive electrode sheet and the negative electrode sheet along their own thickness directions by plasma cutting to obtain cross-sections for SEM observation. Randomly measure three positions of the above samples and take the average value to obtain the thickness Hz of the positive electrode material layer and the thickness Hf of the negative electrode material layer. Hz is the thickness of the single-sided positive electrode material layer, and Hf is the thickness of the single-sided positive electrode material layer.

[0079] Coating surface density test:

[0080] Obtain the positive electrode sheet and the negative electrode sheet according to the above sampling method of the positive electrode sheet and the negative electrode sheet. Perform the test according to the following test steps:

[0081] (1) Punch the positive electrode sheet into small round pieces with a radius of R and weigh the weight as m1. Scrape off the positive electrode material layers on both surfaces of the positive electrode sheet and weigh the weight of the positive electrode current collector as m2. Then the coating surface density Cz of the positive electrode sheet = (m1 - m2) / (2πR 2 ), and the unit is mg / cm 2 .

[0082] (2) Punch the negative electrode sheet into small round pieces with a radius of R and weigh the weight as m3. Scrape off the negative electrode material layers on both surfaces of the negative electrode sheet and weigh the weight of the negative electrode current collector as m4. Then the coating surface density Cf of the negative electrode sheet = (m3 - m4) / (2πR 2 ), and the unit is mg / cm 2 .

[0083] Size parameter test of the positive electrode conductive agent:

[0084] The positive electrode sheet sample is obtained according to the above sampling method for the positive electrode sheet and the negative electrode sheet. The cross-section of the positive electrode active material layer of the positive electrode sheet along its own thickness direction is obtained by brittle fracture with liquid nitrogen. The carbon nanotubes, carbon fiber tubes and flaky graphene in the positive electrode material layer are observed by scanning electron microscopy. The diameter D1 of the carbon nanotubes, the outer diameter D2 and the inner diameter D2' of the carbon fiber tubes are measured at a magnification of 30,000 times; the length L1 of the carbon nanotubes, the length L2 of the carbon fiber tubes and the sheet diameter A of the flaky graphene are measured at a magnification of 30,000 times. The above size parameters are obtained by testing 10 different positions in the positive electrode material layer, measuring 30 values for each parameter, and then calculating the average value. Among them, due to the irregular shape of the flaky graphene, the sheet diameter of the flaky graphene refers to the diameter of the circumscribed circle of the graphene.

[0085] Measurement of the resistivity of carbon fiber tubes:

[0086] The carbon fiber tube powder is pressed into a thin sheet with a thickness of 100 μm to obtain a sample for resistivity measurement. Then, the resistance of the thin sheet is measured by a four-probe tester, and the resistivity f of the carbon fiber tube is obtained by calculation, with the unit of mΩ·cm.

[0087] Measurement of the mass percentage content of the positive electrode conductive agent:

[0088] The positive electrode sheet is dissolved in NMP solution, and after centrifugal separation, a mixture of the conductive agent and the binder is obtained. After drying, a mixed powder of the conductive agent and the binder is obtained. Through thermogravimetric analysis (TG), the substance decomposed at 300 °C to 500 °C is the binder. The content of the conductive agent is obtained by the weight difference method, and the mass percentage content Y% of the positive electrode conductive agent is obtained by calculation.

[0089] Measurement of the mass percentage content of the positive electrode binder:

[0090] The positive electrode sheet is dissolved in NMP solution, and a mixture of the conductive agent and the positive electrode binder is obtained by centrifugal separation. Through a synchronous thermal analyzer (instrument model: STA449F3), test temperature: 25 °C to 600 °C, heating rate: 10 °C / min), the weight loss ratio at 300 °C to 600 °C is measured, and the mass percentage content X% of the positive electrode binder is analyzed.

[0091] Measurement of the mass percentage content of carboxyl groups:

[0092] The positive electrode binder is tested and characterized by a Nicolet iS50 Fourier transform infrared spectrometer and an HLD-NMR-II nuclear magnetic resonance spectrometer, and the mass percentage content Xs% of carboxyl groups in the positive electrode binder is analyzed.

[0093] Infrared spectrum test:

[0094] The infrared spectrum of the cathode binder was tested by a Nicolet iS50 Fourier transform infrared spectrometer.

[0095] Melting point test:

[0096] The melting point of the cathode binder was tested using a differential scanning calorimeter (DSC). An aluminum crucible was used for the test. The weight of the binder sample was 10 mg. The test temperature range was from room temperature to 400 °C, and the heating rate was 10 °C / min for the melting point test. The measured melting point of the cathode binder was denoted as Tm, with the unit of °C.

[0097] Thermogravimetric test:

[0098] Take 0.5 g of the cathode binder sample and add it to a ceramic crucible. A synchronous thermal analyzer (model STA449F3) was used for the test. The heating range was set from 25 °C to 600 °C, and the heating rate was 10 °C / min. The thermogravimetric curve of the cathode binder was measured to obtain the thermal decomposition temperature of the binder, denoted as Td, with the unit of °C.

[0099] Molecular weight test:

[0100] Referring to the national standard: GB / T 21863-2008 Gel Permeation Chromatography (GPC) with tetrahydrofuran as the eluent; the weight-average molecular weight Mw and number-average molecular weight Mn of the cathode binder were tested by Gel Permeation Chromatography (instrument model ACQUITY APC), and the molecular weight distribution Mw / Mn was calculated.

[0101] Adhesion test:

[0102] (1) Adhesion test between the cathode material layer and the cathode current collector

[0103] The adhesion force F1 between the positive electrode material layer and the positive electrode current collector is measured by a 180° peel test. The lithium-ion battery is discharged at a constant current of 0.2C to 3.0V and then disassembled. The separator and the negative electrode plate are peeled off to obtain the positive electrode plate. If the positive electrode plate is coated with the positive electrode material layer on both sides, the positive electrode material layer on one surface of the positive electrode current collector is scraped off. The positive electrode plate is soaked in DMC for 20 min to remove the electrolyte, and then the positive electrode plate is cut into test strips of 15 mm × 54 mm. A double-sided tape (NITTO.NO5000NS) of 15 mm × 55 mm is attached to the steel plate, and then the test strip is attached to the double-sided tape with the positive electrode current collector facing down. A paper tape of 15 mm × 70 mm is connected to the test strip through the double-sided tape and adhered to the surface of the positive electrode material layer away from the positive electrode current collector. A small rod with a mass of 2 kg is pushed by hand to roll on the test strip 8 times to obtain a test sample. A tensile testing machine is used for testing. The test sample is fixed on the test bench, the paper tape is folded 180° upward and fixed by a clamp. Subsequently, the tensile testing machine starts to pull the paper tape at a speed of 50 mm / min until the positive electrode material layer on the surface of the double-sided tape and the positive electrode current collector are separated, and then the test ends. The test data is saved. The adhesion force F1 between the positive electrode material layer and the positive electrode current collector is calculated according to the tensile force and the tensile displacement when they are separated, and the unit is N / m.

[0104] (2) Adhesion force test between the positive electrode plate and the separator

[0105] The adhesion force F2 between the positive electrode plate and the separator is measured by a 180° peel test. The lithium-ion battery is discharged at a constant current of 0.2C to 3.0V and then disassembled. The negative electrode plate is peeled off to obtain the positive electrode plate and the separator. If the positive electrode plate is coated with the positive electrode material layer on both sides, the positive electrode material layer on the surface of the positive electrode current collector away from the separator is scraped off, and then the positive electrode plate and the separator are soaked in DMC for 20 min to remove the electrolyte on their surfaces. Then they are cut into test strips of 15 mm × 54 mm. A double-sided tape (NITTO.NO5000NS) of 15 mm × 55 mm is attached to the steel plate, and then the test strip is attached to the double-sided tape with the positive electrode current collector facing down. A paper tape of 15 mm × 70 mm is connected to the test strip through the double-sided tape and adhered to the surface of the separator. A small rod with a mass of 2 kg is pushed by hand to roll on the test strip 8 times to obtain a test sample. A tensile testing machine is used for testing. The test sample is fixed on the test bench, the paper tape is folded 180° upward and fixed by a clamp. Subsequently, the tensile testing machine starts to pull the paper tape at a speed of 50 mm / min until the positive electrode plate and the separator on the surface of the double-sided tape are separated, and then the test ends. The test data is saved. The adhesion force F2 between the positive electrode plate and the separator is calculated according to the tensile force and the tensile displacement when they are separated, and the unit is N / m.

[0106] Full charge time test:

[0107] In an environment of 35 °C, the lithium-ion battery is charged at a constant current of 1.0 C until 4.5 V, and then changed to constant voltage charging until the cut-off current of 0.05 C, obtaining the state of the lithium-ion battery as 100% SOC (state of charge). The charging time to reach this state is the full charge time of 100% SOC, denoted as T, with the unit of second (s).

[0108] Capacity retention rate test:

[0109] In an environment of 45 °C, the lithium-ion battery is charged at a constant current of 0.7 C until the cut-off voltage of 4.53 V, then charged at a constant voltage until 0.025 C, left standing for 30 min, and discharged at a constant current of 0.2 C until 3.0 V. Record the discharge capacity as C0; left standing for 5 min, then charge the lithium-ion battery at a constant current of 0.7 C until the cut-off voltage of 4.5 V again, then charge at a constant voltage until 0.025 C, left standing for 30 min, and discharge at a constant current of 1.0 C until 3.0 V. Record the discharge capacity as C1. Then the 1C discharge capacity retention rate (%) = C1 / C0 × 100%.

[0110] Energy density test:

[0111] First, measure the dimensions of the lithium-ion battery to obtain the thickness T, width W, and length L. Calculate the volume V through the following formula: V = T × W × L. Then, charge and discharge the lithium-ion battery according to the following operation process: charge at a constant current of 2 C until 4.3 V, and then charge at a constant voltage of 4.3 V until 0.05 C; then discharge at a constant current of 0.5 C until 2.5 V to obtain the discharge capacity E. The energy density (ED) of the lithium-ion battery can be calculated through the following formula: ED (Wh / cm 3 ) = E / V.

[0112] Example 1-1

[0113] <Preparation of the positive electrode sheet>

[0114] Mix the positive electrode conductive agents, namely carbon nanotubes, carbon fiber tubes, and flaky graphene, with the positive electrode binder, carboxyl-modified PVDF (Mw = 1000000, Mw / Mn = 2.2). Add N-methylpyrrolidone (NMP) to formulate a conductive adhesive solution with a solid content of 7 wt%. Then add the positive electrode active material, lithium cobaltate, and continue to stir under a vacuum mixer until the system becomes homogeneous to obtain a slurry with a solid content of 75 wt%. Uniformly coat the positive electrode slurry on one surface of a positive electrode current collector aluminum foil with a thickness of 10 μm, and dry it at 120 °C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer. Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. After drying at 120 °C, perform cold pressing, and then cut, weld the tabs to obtain a positive electrode sheet with a specification of 74 mm × 867 mm for standby.

[0115] Based on the mass of the positive electrode material layer, the mass percentage content Z% of the positive electrode active material, the mass percentage content X% of the positive electrode binder, the mass percentage content Y% of the positive electrode conductive agent, the mass percentage content Y1% of the carbon nanotubes, the mass percentage content Y2% of the carbon fiber tubes, and the mass percentage content Y3% of the flaky graphene are shown in Table 2.

[0116] <Preparation of the negative electrode sheet>

[0117] Mix the negative electrode active material, artificial graphite, the negative electrode binder, styrene-butadiene rubber, and the negative electrode conductive agent, acetylene black, according to a mass ratio of 97.4:1.4:1.2. Add deionized water as a solvent to formulate a slurry with a solid content of 45 wt%. After stirring evenly with a vacuum mixer, obtain the negative electrode slurry. Uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 6 μm, and dry it at 120 °C to obtain a negative electrode sheet with a single-sided coated negative electrode material layer. Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. After drying at 120 °C, perform cold pressing, and then cut, weld the tabs to obtain a negative electrode sheet with a specification of 78 mm × 875 mm for standby.

[0118] <Preparation of the electrolyte>

[0119] In an environment with a water content of less than 10 ppm, mix ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) according to a mass ratio of 1:1:1 to obtain an organic solvent. Then add the lithium salt LiPF6 to the organic solvent and mix evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of the lithium salt is 12.5%, and the rest is the organic solvent.

[0120] <Separator>

[0121] A porous polyethylene film with a thickness of 7 μm (provided by Celgard) was used as the separator.

[0122] <Preparation of Lithium-Ion Batteries>

[0123] The above-prepared positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, with the separator placed between the positive electrode sheet and the negative electrode sheet to play a separating role, and then wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum-plastic film packaging bag, dehydrated at 80 °C, and the above-prepared electrolyte was injected. After vacuum packaging, standing, forming, degassing, trimming and other processes, a lithium-ion battery was obtained.

[0124] Examples 1-2 to 1-34

[0125] Except for adjusting the relevant parameters according to Table 1, the rest were the same as Example 1-1.

[0126] Examples 2-1 to 2-11

[0127] Except for adjusting the relevant parameters according to Table 2, the rest were the same as Example 1-1.

[0128] Examples 3-1 to 3-21

[0129] Except for adjusting the relevant parameters according to Table 3, the rest were the same as Example 1-1.

[0130] Example 3-22

[0131] Except for using ordinary PVDF (not carboxyl-modified, Mw = 1000000, Mw / Mn = 2.2), the rest were the same as Example 1-1.

[0132] Comparative Example 1

[0133] Except for using conductive carbon black (Super P, specific surface area of 60 m 2 / g) as the positive electrode conductive agent, the rest were the same as Example 1-1.

[0134] Comparative Examples 2 to 10

[0135] Except for adjusting the relevant parameters according to Table 1, the rest were the same as Example 1-1.

[0136] The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 3.

[0137]

[0138]

[0139] As can be seen from Examples 1-1 to 1-34 and Comparative Examples 1 to 10, when the thickness of the positive electrode material layer in the secondary battery of the present application is within the scope of the present application, at least one of carbon nanotubes and conductive carbon fibers or flaky graphene is selected and used in combination, and the values of D1 and D2 are adjusted within the scope of the present application, the secondary battery can have a shorter charging time and a higher discharge capacity retention rate, indicating that the secondary battery of the example has better high-temperature charge and discharge performance, and the secondary battery has a higher energy density. For the secondary battery of the comparative example, the thickness of the positive electrode material layer is outside the scope of the present application, the value of D1 or D2 is outside the scope of the present application, or at least one of carbon nanotubes and conductive carbon fibers or flaky graphene is not selected and used in combination. The secondary battery of the comparative example has a longer charging time, or a lower discharge capacity retention rate, or a lower energy density.

[0140] The thickness of the positive electrode material layer generally affects the high-temperature charge and discharge performance and energy density of the secondary battery. As can be seen from Examples 1-1 to 1-6, Comparative Example 3 and Comparative Example 4, adjusting the thickness of the positive electrode material layer within the scope of the present application can make the secondary battery have a shorter charging time, a higher discharge capacity retention rate and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and a higher energy density.

[0141] The coating surface density of the positive electrode material layer, the thickness and coating surface density of the negative electrode material layer generally affect the high-temperature charge and discharge performance and energy density of the secondary battery. As can be seen from Examples 1-1 to 1-6, adjusting the coating surface density of the positive electrode material layer, the thickness and coating surface density of the negative electrode material layer within the scope of the present application can make the positive electrode material layer have a higher porosity, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and a higher energy density.

[0142] The diameter, length and aspect ratio of carbon nanotubes generally affect the high-temperature charge and discharge performance and energy density of the secondary battery. As can be seen from Example 1-1 and Examples 1-7 to 1-18, adjusting the diameter, length and aspect ratio of carbon nanotubes within the scope of the present application can make the secondary battery have a shorter charging time, a higher discharge capacity retention rate and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and a higher energy density.

[0143] The outer diameter, inner diameter, length, length-diameter ratio, and resistivity of the carbon fiber tube generally affect the high-temperature charge-discharge performance and energy density of the secondary battery. As can be seen from Examples 1-1, 1-19 to 1-30, adjusting the outer diameter, inner diameter, length, length-diameter ratio, and resistivity of the carbon fiber tube within the scope of this application can enable the secondary battery to have a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge-discharge performance and a high energy density.

[0144] The sheet diameter of the flaky graphene generally affects the high-temperature charge-discharge performance and energy density of the secondary battery. As can be seen from Examples 1-1, 1-31 to 1-34, adjusting the value of the sheet diameter A of the flaky graphene within the scope of this application can enable the secondary battery to have a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge-discharge performance and a high energy density.

[0145] Table 2

[0146]

[0147] Note: " / " in Table 2 indicates the non-existence of the corresponding substance or parameter.

[0148] The content of the positive electrode conductive agent and the mass percentage contents of the carbon nanotubes, carbon fiber tubes, and flaky graphene generally affect the high-temperature charge-discharge performance and energy density of the secondary battery. As can be seen from Examples 1-1, 2-1 to 2-11, adjusting the content of the positive electrode conductive agent and the mass percentage contents of the carbon nanotubes, carbon fiber tubes, and flaky graphene within the scope of this application can result in a higher adhesion force between the positive electrode material layer and the positive electrode current collector and between the positive electrode plate and the separator, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge-discharge performance and a high energy density.

[0149] Table 3

[0150]

[0151]

[0152] The types and mass percentage contents of the positive electrode binder usually affect the high-temperature charge and discharge performance and energy density of the secondary battery. It can be seen from Examples 1-1, 3-1 to 3-6, and 3-22 that by selecting carboxyl-modified PVDF as the positive electrode binder and controlling its mass percentage content within the scope of this application, the adhesion between the positive electrode material layer and the positive electrode current collector and the adhesion between the positive electrode plate and the separator can be relatively high, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and high energy density.

[0153] The mass percentage content of carboxyl in the positive electrode binder usually affects the high-temperature charge and discharge performance and energy density of the secondary battery. It can be seen from Examples 1-1, 3-6 to 3-11 that by controlling the mass percentage content of carboxyl in the positive electrode binder within the scope of this application, the adhesion between the positive electrode material layer and the positive electrode current collector and the adhesion between the positive electrode plate and the separator can be relatively high, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and high energy density.

[0154] The weight-average molecular weight Mw and the molecular weight distribution Mw / Mn of the positive electrode binder usually affect the high-temperature charge and discharge performance and energy density of the secondary battery. It can be seen from Examples 1-1, 3-16 to 3-21 that by controlling the weight-average molecular weight Mw and the molecular weight distribution Mw / Mn of the positive electrode binder within the scope of this application, the adhesion between the positive electrode material layer and the positive electrode current collector and the adhesion between the positive electrode plate and the separator can be relatively high, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and high energy density.

[0155] The melting point and decomposition temperature of the positive electrode binder usually affect the high-temperature charge and discharge performance and energy density of the secondary battery. It can be seen from Examples 1-1, 3-7 to 3-21 that by controlling the melting point and decomposition temperature of the positive electrode binder within the scope of this application, the adhesion between the positive electrode material layer and the positive electrode current collector and the adhesion between the positive electrode plate and the separator can be relatively high, and the secondary battery has a shorter charging time, a higher discharge capacity retention rate, and energy density, indicating that the secondary battery has good high-temperature charge and discharge performance and high energy density.

[0156] In Table 3, in the infrared spectra of the positive electrode binders of Examples 1-1, 3-1 to 3-21, there are all peaks located at 1710 cm -1 to 1725 cm -1 and 2500 cm -1 to 3300 cm-1 IR characteristic peaks. For example, the IR spectrum of the positive electrode binder (carboxyl-modified PVDF) in Example 1-1 is shown in Figure 1. From Figure 1 it can be seen that in the IR spectrum of the positive electrode binder, there are peaks located at 1710 cm -1 to 1725 cm -1 , 2500 cm -1 to 3300 cm -1 . These IR characteristic peaks are the stretching vibration peaks of carboxyl groups, indicating that the positive electrode binder of the present application contains carboxyl groups. In the IR spectrum of the positive electrode binder PVDF in Example 3-22, there are no IR characteristic peaks in the above wavelength bands, indicating that this binder does not contain carboxyl groups.

[0157] Figure 2 Figure 2 shows the thermogravimetric curve of the positive electrode binder in Example 1-1 of the present application. Figure 2 In this figure, the thermogravimetric curve of the positive electrode binder includes a DTG curve and a TG curve. It can be seen that the temperature of the decomposition peak in the DTG curve of the positive electrode binder is 478.5 °C, indicating that the positive electrode binder of the present application has a high thermal decomposition temperature, that is, it has good heat resistance.

[0158] It should be noted that in this article, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0159] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other, and the differences between each embodiment and other embodiments are emphasized.

[0160] 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 secondary battery, comprising a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, and the negative electrode sheet comprises the negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector; The thickness of the positive electrode material layer is 22 μm to 110 μm, and the positive electrode material layer includes a positive electrode conductor and a positive electrode binder. The positive electrode conductor includes carbon nanotubes, and at least one of carbon fiber tubes or flaky graphene. The diameter of the carbon nanotubes is D1 nm, 4≤D1≤23, and the outer diameter of the carbon fiber tube is D2 nm, 25≤D2≤150.

2. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1) The coating surface density of the positive electrode material layer is 10 mg / cm 2 Up to 55mg / cm 2 ; (2) The thickness of the positive electrode material layer is 45 μm to 110 μm; (3) The coating surface density of the negative electrode material layer is 2.5 mg / cm 2 Up to 35mg / cm 2 ; (4) The thickness of the negative electrode material layer is 20 μm to 70 μm.

3. The secondary battery according to claim 1, wherein The carbon nanotubes satisfy at least one of the following characteristics: (1) The length of the carbon nanotube is L1 nm, 100≤L1≤6000; (2) The aspect ratio L1 / D1 of the carbon nanotubes satisfies: 100≤L1 / D1≤300; (3)8≤D1≤18。 4. The secondary battery according to claim 1, wherein The carbon fiber tube satisfies at least one of the following characteristics: (1) The length of the carbon fiber tube is L2 nm, 2000≤L≤50000; (2) The inner diameter of the carbon fiber tube is D2'nm, 1.5≤D2'≤35; (3) The aspect ratio L2 / D2 of the carbon fiber tube satisfies: 50≤L2 / D2≤400; (4)30≤D2≤80； (5) The resistivity of the carbon fiber tube is ρmΩ·cm, 5≤ρ≤27.

5. The secondary battery according to claim 1, wherein The flake graphene has a diameter of 5 μm to 15 μm.

6. The secondary battery according to claim 1, wherein Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode conductor is 0.2% to 2%.

7. The secondary battery according to claim 6, wherein Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode conductor is 0.5% to 1%.

8. The secondary battery according to claim 6, wherein It satisfies at least one of the following characteristics: (1) Based on the mass of the positive electrode material layer, the mass percentage of the carbon nanotubes is Y1%, 0.1≤Y1≤1; (2) Based on the mass of the positive electrode material layer, the mass percentage of the carbon fiber tube is Y2%, 0.1≤Y2≤0.5; (3) Based on the mass of the positive electrode material layer, the mass percentage of the flaky graphene is Y3%, 0.1≤Y3≤0.

5.

9. The secondary battery according to claim 1, wherein The positive electrode binder includes carboxyl-modified polyvinylidene fluoride, and the positive electrode binder satisfies at least one of the following characteristics: (1) In the infrared spectrum of the positive electrode binder, there is a -1 Up to 1725cm -1 、2500cm -1 Up to 3300cm -1 The infrared characteristic peaks of (2) The melting point of the positive electrode binder is 157°C to 167°C; (3) The peak decomposition temperature of the thermal weight loss curve of the positive electrode binder is in the range of 470°C to 490°C; (4) Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is 0.5% to 3.0%; (5) The positive electrode binder contains carboxyl groups, and the mass percentage of the carboxyl groups is 0.5% to 3.5% based on the mass of the positive electrode binder; (6) The weight average molecular weight of the positive electrode binder is Mw, 800000≤Mw≤1200000, the number average molecular weight of the positive electrode binder is Mn, and the molecular weight distribution Mw / Mn of the positive electrode binder satisfies: 1.5≤Mw / Mn≤3.

10. The secondary battery according to claim 9, wherein The positive electrode binder satisfies at least one of the following characteristics: (1) The melting point of the positive electrode binder is 159°C to 165°C; (2) The peak decomposition temperature of the thermal weight loss curve of the positive electrode binder is in the range of 475°C to 481°C; (3) Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is 0.6% to 1.6%; (4) The positive electrode binder contains carboxyl groups, and the mass percentage of the carboxyl groups is 0.5% to 1.5% based on the mass of the positive electrode binder; (5) The molecular weight distribution Mw / Mn of the positive electrode binder satisfies: 2≤Mw / Mn≤2.

5.

11. The secondary battery according to claim 1, wherein The thickness of the positive electrode material layer is 70 μm to 100 μm.

12. The secondary battery according to any one of claims 1 to 11, wherein The bonding force between the positive electrode material layer and the positive electrode current collector is F1 N / m, 10≤F1≤70, and the bonding force between the positive electrode plate and the isolation film is F2 N / m, 8≤F2≤30.

13. The secondary battery according to claim 12, wherein: 30.3≤F1≤47.7, 17.8≤F2≤25.

1. 14 . An electronic device comprising the secondary battery according to claim 1 .

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