Positive electrode sheet, secondary battery and electronic device

Abstract

The present application provides a positive electrode plate, a secondary battery and an electronic device. The positive electrode plate includes a positive electrode current collector and an active layer arranged on at least one surface of the positive electrode current collector. The active layer includes a first positive electrode active material layer and a second positive electrode active material layer. The first positive electrode active material layer is arranged between the positive electrode current collector and the second positive electrode active material layer. The first positive electrode active material layer includes a first positive electrode active material. The particle sphericity of the first positive electrode active material is B, and B is ≥ 0.7. The second positive electrode active material layer includes a second positive electrode active material. The compaction density of the second positive electrode active material layer is A2, 3.2 g / cm 3 ≤A2≤3.8g / cm 3 Through the above arrangement, the positive electrode sheet has a higher elongation at break while having a higher compaction density. The secondary battery using the positive electrode sheet of the present application can have a high energy density while taking into account its safety performance.

Description

Technical Field

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

[0002] In order to solve the severe problems of global energy crisis, environmental pollution, climate change, low-carbon economy, etc., the research and development and application of electric vehicles, large-scale power sources and power sources in the field of energy storage have become inevitable. Secondary batteries (such as lithium-ion batteries) have the advantages of high energy storage density, high open circuit voltage, low self-discharge rate, long cycle life, good safety, etc., and are widely used in various fields such as power storage, mobile electronic devices, electric vehicles and aerospace. As mobile electronic devices and electric vehicles enter a stage of rapid development, the market has put forward higher and higher requirements on the energy density and safety performance of secondary batteries.

[0003] Existing secondary batteries often increase their energy density by increasing the compaction density of the positive electrode sheet, but high compaction density will reduce the elongation at break of the positive electrode sheet. The positive electrode sheet will continue to shrink and expand during the process of lithium insertion and extraction. In the later stage of the secondary battery charge and discharge cycle, the positive electrode sheet is prone to breakage, affecting the safety performance of the secondary battery. Based on this, how to balance the safety performance and energy density of secondary batteries has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0004] The purpose of the present application is to provide a positive electrode plate, a secondary battery and an electronic device, so that the secondary battery has a high energy density while taking into account its safety performance.

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

[0006] The first aspect of the present application provides a positive electrode plate, which includes a positive electrode collector and an active layer arranged on at least one surface of the positive electrode collector, the active layer includes a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer is arranged between the positive electrode collector and the second positive electrode active material layer; the first positive electrode active material layer includes a first positive electrode active material, the particle sphericity of the first positive electrode active material is B, B≥0.7; the second positive electrode active material layer includes a second positive electrode active material, the compaction density of the second positive electrode active material layer is A2, 3.2g / cm 3 ≤A2≤3.8g / cm 3. The positive electrode plate provided in the first aspect of the present application, by regulating the particle sphericity B of the first positive electrode active material in the first positive electrode active material layer close to the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer on the surface of the positive electrode plate within the scope of the present application, the second positive electrode active material layer has a higher compaction density, so that the positive electrode plate as a whole can have a higher compaction density, thereby providing a higher energy density for the secondary battery; the higher particle sphericity of the first positive electrode active material makes the probability of the first positive electrode active material particles causing damage to the positive electrode collector during the cold pressing process of the positive electrode plate smaller, so that the positive electrode plate has a higher elongation at break. In this way, the positive electrode plate has a higher elongation at break while having a higher compaction density. The secondary battery using the positive electrode plate provided in the first aspect of the present application can have a high energy density while taking into account its safety performance.

[0007] In some embodiments of the present application, the positive electrode sheet satisfies at least one of the following characteristics: (1) 0.85≤B≤0.95; (2) 3.5 g / cm 3 ≤A2≤3.7g / cm 3 . It is beneficial to further improve the energy density and safety performance of secondary batteries.

[0008] In some embodiments of the present application, the compaction density of the first positive electrode active material layer is A1, and A1 and A2 satisfy: 0.9≤A1 / A2≤0.98. By regulating the ratio A1 / A2 between the compaction density A1 of the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer within the above range, the first positive electrode active material layer is smaller than the compaction density of the second positive electrode active material layer when it has a suitable compaction density, which is conducive to making the positive electrode sheet as a whole have a higher compaction density, and the positive electrode sheet also has a higher elongation at break.

[0009] In some embodiments of the present application, 3.2 g / cm 3 ≤A1≤3.7g / cm 3 The positive electrode sheet still has a high elongation at break after cold pressing. The secondary battery using the positive electrode sheet has a high safety performance while having a high energy density.

[0010] In some embodiments of the present application, the volume average particle size of the first positive electrode active material is Dv50 -1 , 3μm≤Dv50 -1 ≤8μm; the volume average particle size of the second positive electrode active material is Dv50 -2 , 3μm≤Dv50 -2≤8μm. During the cold pressing process of the positive electrode sheet, the first positive electrode active material particles cause less damage to the positive electrode sheet, so that the positive electrode sheet has a higher elongation at break, and the positive electrode sheet as a whole also has a higher compaction density. As a result, the secondary battery has higher energy density and safety performance.

[0011] In some embodiments of the present application, the total thickness of the active layer is H, the single layer thickness of the first positive electrode active material layer is H1, 0.1≤H1 / H≤0.8, 50μm≤H≤120μm. The value of the ratio of the single layer thickness of the first positive electrode active material layer to the total thickness of the active layer H1 / H and the total thickness H of the active layer are regulated within the above range, so that the secondary battery has a high energy density on the basis of having good safety performance.

[0012] In some embodiments of the present application, 20 μm ≤ H1 ≤ 95 μm, which enables the secondary battery to have good processing performance and low production cost on the basis of good safety performance and high energy density.

[0013] In some embodiments of the present application, a carbon material is disposed on at least part of the surface of the first positive electrode active material, and based on the mass of the first positive electrode active material, the mass percentage of the carbon material is C, 0.5%≤C≤3%; B and C satisfy: 15%≤C / B×10×100%≤30%. The mass percentage C of the carbon material on the surface of the first positive electrode active material and the relationship between C and the particle sphericity B of the first positive electrode active material are regulated within the above range, and the positive electrode sheet has good conductivity while having a higher elongation at break.

[0014] In some embodiments of the present application, the first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide; the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide. Selecting the above-mentioned types of materials as the first positive electrode active material and the second positive electrode active material is conducive to making the secondary battery have good performance, such as good cycle performance and safety performance, high energy density, etc.

[0015] The second aspect of the present application provides a secondary battery, the secondary battery comprising the positive electrode sheet described in any of the above embodiments. Therefore, the secondary battery has high energy density while taking into account its safety performance.

[0016] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any one of the above embodiments. Therefore, the electronic device has good performance.

[0017] Beneficial effects of this application:

[0018] The present invention provides a positive electrode sheet, a secondary battery and an electronic device. The positive electrode sheet is provided with a first positive electrode active material layer near the first positive electrode active material layer and a second positive electrode active material layer on the surface of the positive electrode sheet having a particle sphericity B ≥ 0.7 and a compaction density A2 satisfying 3.2 g / cm 3 ≤A2≤3.8g / cm 3 , so that the second positive electrode active material layer has a higher compaction density, so that the positive electrode plate as a whole can have a higher compaction density, thereby providing a higher energy density for the secondary battery; the higher particle sphericity of the first positive electrode active material makes the probability of the first positive electrode active material particles causing damage to the positive electrode current collector during the cold pressing process of the positive electrode plate smaller, so that the positive electrode plate has a higher elongation at break. In this way, the positive electrode plate has a higher elongation at break while having a higher compaction density. When the positive electrode plate is applied to the secondary battery, the secondary battery can have a high energy density while taking into account its safety performance.

[0019] Of course, implementing any product or method of the present application does not necessarily require achieving all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0021] Figure 1 This is a schematic diagram of the cross-sectional structure of the positive electrode sheet along its thickness direction and length direction in some implementation schemes of the present application.

[0022] Reference numerals: 100 - positive electrode sheet; 10 - positive electrode current collector; 20 - active layer; 11 - first positive electrode active material layer; 12 - second positive electrode active material layer. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present application and the accompanying drawings to clearly and completely describe the technical solutions in the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0024] It should be noted that in the specific implementation manner of the present application, the present application is explained by taking a lithium-ion battery as an example of a secondary battery, but the secondary battery of the present application is not limited to a lithium-ion battery.

[0025] The first aspect of the present application provides a positive electrode plate, which includes a positive electrode collector and an active layer arranged on at least one surface of the positive electrode collector, the active layer includes a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer is arranged between the positive electrode collector and the second positive electrode active material layer; the first positive electrode active material layer includes a first positive electrode active material, the particle sphericity of the first positive electrode active material is B, B≥0.7; the second positive electrode active material layer includes a second positive electrode active material, the compaction density of the second positive electrode active material layer is A2, 3.2g / cm 3 ≤A2≤3.8g / cm 3 The above-mentioned "sphericity" refers to the sphericity known in the art.

[0026] For ease of understanding, in this application, the length direction of the positive electrode plate itself is defined as X, and the thickness direction is defined as Z. It should be understood that the above definition of the direction is for the purpose of convenient description of this application, and the direction defined in this application can be understood based on the relative positions of the drawings and the actual product elements. In addition, the length direction and thickness direction of the positive electrode collector, the first positive electrode active material layer, and the second positive electrode active material layer are the same as those of the negative electrode plate. The above-mentioned "active layer disposed on at least one surface of the positive electrode collector" means that the active layer is disposed on one surface or two surfaces of the positive electrode collector, and the above-mentioned "surface" can be a partial surface of the positive electrode collector or the entire surface of the positive electrode collector. Figure 1 As shown, the positive electrode sheet 100 includes a positive electrode current collector 10 and an active layer 20, the active layer 20 is disposed on both surfaces of the positive electrode current collector 10, the active layer 20 includes a first positive electrode active material layer 11 and a second positive electrode active material layer 12, and the first positive electrode active material layer 11 is disposed between the positive electrode current collector 10 and the second positive electrode active material layer 12. Of course, it can be understood that in some embodiments of the present application, the active layer 20 is disposed on one surface of the positive electrode current collector 10.

[0027] For example, the particle sphericity B of the first positive electrode active material is 0.7, 0.72, 0.75, 0.76, 0.77, 0.79, 0.80, 0.82, 0.85, 0.86, 0.88, 0.90, 0.92, 0.95, 0.96, 0.98, 0.99 or any value between any two of the above numerical ranges. The particle sphericity B of the first positive electrode active material is less than 0.7. When the positive electrode sheet is cold-pressed, the first positive electrode active material in contact with the positive electrode current collector will damage the positive electrode active material, thereby reducing the elongation at break of the positive electrode sheet, and the safety performance of the secondary battery using the positive electrode sheet will be reduced.

[0028] For example, the compaction density A2 of the second positive electrode active material layer is 3.2 g / cm 3 、3.3g / cm3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 Or any value between any two numerical ranges mentioned above. The compaction density A2 of the second positive electrode active material layer is less than 3.2 g / cm 3 If the compaction density of the second positive electrode active material layer is too small, the energy density of the secondary battery will be affected; if the compaction density A2 of the second positive electrode active material layer is greater than 3.8 g / cm 3 The compaction density of the second positive electrode active material layer is too large, and the second positive electrode active material particles in the second positive electrode active material layer are easily broken, which affects the performance of the second positive electrode active material and thus affects the performance of the secondary battery, such as the cycle performance and rate performance.

[0029] In general, the positive electrode sheet provided in the first aspect of the present application, by regulating the particle sphericity B of the first positive electrode active material in the first positive electrode active material layer close to the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer on the surface of the positive electrode sheet within the scope of the present application, the second positive electrode active material layer has a higher compaction density, so that the positive electrode sheet as a whole can have a higher compaction density, thereby providing a higher energy density for the secondary battery; the higher particle sphericity of the first positive electrode active material makes the probability of the first positive electrode active material particles causing damage to the positive electrode collector during the cold pressing process of the positive electrode sheet smaller, so that the positive electrode sheet has a higher elongation at break. In this way, the positive electrode sheet has a higher elongation at break while having a higher compaction density. The secondary battery using the positive electrode sheet provided in the first aspect of the present application can have a high energy density while taking into account its safety performance.

[0030] The present application has no particular restrictions on the method for controlling the compaction density of the second positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, it can be achieved by adjusting the cold pressing pressure during the preparation of the positive electrode sheet, or by adjusting at least one of the particle size distribution, sphericity or type of the second positive electrode active material.

[0031] In some embodiments of the present application, 0.85≤B≤0.95. For example, the particle sphericity B of the first positive electrode active material is 0.85, 0.86, 0.87, 0.88, 0.90, 0.92, 0.93, 0.95 or any value between any two of the above numerical ranges. By regulating the particle sphericity B of the first positive electrode active material within the above range, the positive electrode sheet is conducive to further reducing the damage of the first positive electrode active material particles to the positive electrode collector during the cold pressing process, thereby further improving the elongation at break of the positive electrode sheet. In this way, the secondary battery can further improve its safety performance on the basis of having a higher energy density.

[0032] In some embodiments of the present application, 3.5 g / cm 3 ≤A2≤3.7g / cm 3 For example, the compaction density A2 of the second positive electrode active material layer is 3.5 g / cm 3 、3.6g / cm 3 、3.7g / cm 3 Or any value between any two of the above numerical ranges. By regulating the compaction density A2 of the second positive electrode active material layer within the above range, the second positive electrode active material layer has a higher compaction density when the second positive electrode active material particles in the second positive electrode active material layer are not squeezed and crushed, so that the positive electrode sheet as a whole has a higher compaction density. In this way, the secondary battery can further improve its energy density on the basis of having higher safety performance.

[0033] In some embodiments of the present application, 0.85≤B≤0.95; 3.5 g / cm 3 ≤A2≤3.7g / cm 3 For example, the particle sphericity B of the first positive electrode active material is 0.85, 0.86, 0.87, 0.88, 0.90, 0.92, 0.93, 0.95 or any value between any two of the above numerical ranges. The compaction density A2 of the second positive electrode active material layer is 3.5 g / cm 3 、3.6g / cm 3 、3.7g / cm 3 Or any value between any two of the above numerical ranges. By regulating the particle sphericity B of the first positive electrode active material and the compaction density A2 of the second positive electrode active material layer within the above range, the positive electrode sheet is further helped to reduce the damage of the first positive electrode active material particles to the positive electrode current collector during the cold pressing process, thereby further improving the elongation at break of the positive electrode sheet. When the second positive electrode active material particles in the second positive electrode active material layer are not squeezed and crushed, the second positive electrode active material layer has a higher compaction density, so that the positive electrode sheet as a whole has a higher compaction density. In this way, the energy density and safety performance of the secondary battery can be further improved.

[0034] In some embodiments of the present application, the compaction density of the first positive electrode active material layer is A1, and A1 and A2 satisfy: 0.9≤A1 / A2≤0.98. For example, the value of A1 / A2 is 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98 or any value between any two of the above numerical ranges. By regulating the ratio A1 / A2 between the compaction density A1 of the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer within the above range, the first positive electrode active material layer is less than the compaction density of the second positive electrode active material layer when it has a suitable compaction density, which is conducive to making the positive electrode sheet as a whole have a higher compaction density. During the cold pressing process of the positive electrode sheet, the damage of the first positive electrode active material particles to the positive electrode collector also has a higher elongation at break. In this way, the positive electrode sheet is applied to the secondary battery, and the secondary battery can have good safety performance while having a higher energy density.

[0035] In some embodiments of the present application, 3.2 g / cm 3 ≤A1≤3.7g / cm 3 For example, the compaction density A1 of the first positive electrode active material layer is 3.2 g / cm 3 、3.3g / cm 3 、3.4g / cm 3 、3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 Or any value between any two of the above numerical ranges. By regulating the compaction density A1 of the first positive electrode active material layer within the above range, the probability of the first positive electrode active material particles in the first positive electrode active material layer being embedded in the positive electrode current collector during the cold pressing of the positive electrode plate is reduced, which can reduce the damage to the positive electrode current collector, so that the positive electrode plate still has a high elongation at break after cold pressing. In this way, the secondary battery using the positive electrode plate has a higher safety performance while having a higher energy density.

[0036] The present application has no particular restrictions on the manner of regulating the compaction density of the first positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, it can be achieved by adjusting the cold pressing pressure during the preparation of the positive electrode sheet, or by adjusting at least one of the particle size distribution, sphericity or type of the first positive electrode active material.

[0037] In some embodiments of the present application, the volume average particle size of the first positive electrode active material is Dv50 -1 , 3μm≤Dv50 -1 ≤8μm; the volume average particle size of the second positive electrode active material is Dv50-2 , 3μm≤Dv50 -2 ≤8μm. For example, the volume average particle size Dv50 of the first positive electrode active material -1 The volume average particle size Dv50 of the second positive electrode active material is 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm or any value between any two of the above numerical ranges. -2 The volume average particle size Dv50 of the first positive electrode active material is 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm or any value between any two of the above numerical ranges. -1 and the volume average particle size Dv50 of the second positive electrode active material -2 When the first positive electrode active material particles are regulated within the above range, they are evenly distributed in the first positive electrode active material layer, and the second positive electrode active material particles are evenly distributed in the second positive electrode active material layer. The particles have good contact with each other, and the first positive electrode active material layer and the second positive electrode active material layer have appropriate compaction density. In this way, during the cold pressing process of the positive electrode sheet, the first positive electrode active material particles cause less damage to the positive electrode sheet, so that the positive electrode sheet has a higher elongation at break, and the positive electrode sheet as a whole also has a higher compaction density. As a result, the secondary battery has higher energy density and safety performance.

[0038] In this application, the volume average particle size Dv50 of the first positive electrode active material is -1 " means that the first positive electrode active material particles have a particle size that reaches 50% of the volume accumulation from the small particle size side in the volume-based particle size distribution. "The volume average particle size Dv50 of the second positive electrode active material -2 ” means that the second positive electrode active material particles have a particle size at which the volume accumulation is 50% from the smallest particle size side in the volume-based particle size distribution.

[0039] The present application has no particular limitation on the method for controlling the volume average particle size of the first positive electrode active material and the second positive electrode active material, as long as the purpose of the present application can be achieved, for example, by crushing or screening.

[0040] In some embodiments of the present application, the total thickness of the active layer is H, the single layer thickness of the first positive electrode active material layer is H1, 0.1≤H1 / H≤0.8, 50μm≤H≤120μm. Figure 1As shown, the total thickness of the active layer 20 is shown as H, and the single layer thickness of the first positive electrode active material layer 11 is shown as H1. The total thickness H of the active layer 20 is the sum of the single layer thickness H1 of the first positive electrode active material layer 11 and the single layer thickness H2 of the second positive electrode active material layer 12. For example, the value of H1 / H is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or any value between any two of the above numerical ranges. H is 50μm, 59μm, 66μm, 70μm, 80μm, 85μm, 90μm, 98μm, 100μm, 110μm, 115μm, 120μm or any value between any two of the above numerical ranges. By adjusting the ratio H1 / H of the single layer thickness of the first positive electrode active material layer to the total thickness of the active layer and the total thickness H of the active layer within the above range, the positive electrode sheet is easier to prepare, the processing difficulty is less, and the positive electrode sheet as a whole has a higher compaction density. In this way, the secondary battery has a higher energy density on the basis of having good safety performance.

[0041] In some embodiments of the present application, 20μm≤H1≤95μm. For example, the single layer thickness H1 of the first positive electrode active material layer is 20μm, 26μm, 30μm, 40μm, 50μm, 60μm, 65μm, 70μm, 80μm, 83μm, 95μm or any value between any two of the above ranges. Regulating the single layer thickness H1 of the first positive electrode active material layer within the above range is conducive to improving the processing performance of the positive electrode sheet, so that the secondary battery can have good processing performance and low production cost on the basis of good safety performance and high energy density.

[0042] In some embodiments of the present application, a carbon material is disposed on at least part of the surface of the first positive electrode active material. Based on the mass of the first positive electrode active material, the mass percentage of the carbon material is C, 0.5%≤C≤3%; B and C satisfy: 15%≤C / B×10×100%≤30%. For example, C is 0.5%, 0.8%, 1.5%, 1.7%, 2.0%, 2.5%, 3% or any value between any two of the above numerical ranges. The value of C / B×10×100% is 15%, 17%, 18%, 20%, 22%, 23%, 25%, 27%, 30% or any value between any two of the above numerical ranges. Regulating the mass percentage C of the carbon material on the surface of the first positive electrode active material and the relationship between C and the particle sphericity B of the first positive electrode active material within the above range is conducive to improving the conductivity between the particles of the first positive electrode active material, so that a good conductive network is formed between the particles of the first positive electrode active material. In this way, the positive electrode sheet has good conductivity while having a high elongation at break. Therefore, the secondary battery using the positive electrode sheet has good safety performance and cycle performance on the basis of high energy density.

[0043] The present application does not particularly limit the method of setting the carbon material on the surface of the first positive electrode active material. Those skilled in the art can implement it by methods known in the art, as long as the purpose of the present application can be achieved. For example, it can be achieved by gas phase coating. Generally speaking, when preparing the carbon material on the surface of the first positive electrode active material by gas phase coating, the greater the carbon source gas flow rate, the greater the mass percentage of the carbon material; the smaller the carbon source gas flow rate, the smaller the mass percentage of the carbon material. The above-mentioned carbon source gas can be a carbon source gas known in the art, such as methane.

[0044] In some embodiments of the present application, the first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide; the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide. Selecting the above-mentioned types of materials as the first positive electrode active material and the second positive electrode active material is conducive to making the secondary battery have good performance, such as good cycle performance and safety performance, high energy density, etc.

[0045] The present application has no particular restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil or aluminum alloy foil. In the present application, there is no particular restriction 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.

[0046] In one embodiment of the present application, the first positive electrode active material layer and the second positive electrode active material layer may also independently include at least one of a positive electrode conductor or a positive electrode binder. The present application does not particularly limit the mass ratio of the first positive electrode active material, the positive electrode conductor, and the positive electrode binder in the first positive electrode active material layer, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved. For example, the mass ratio of the first positive electrode active material, the positive electrode conductor, and the positive electrode binder in the first positive electrode active material layer is (96.5-97.9): (0.8-1.7): (1.0-2.0). The present application does not particularly limit the mass ratio of the second positive electrode active material, the positive electrode conductor, and the positive electrode binder in the second positive electrode active material layer, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved. For example, the mass ratio of the second positive electrode active material, the positive electrode conductor, and the positive electrode binder in the second positive electrode active material layer is (96.5-97.9): (0.8-1.7): (1.0-2.0).

[0047] The present application does not particularly limit the preparation method of the first positive electrode active material. Those skilled in the art can prepare it by a known method, as long as the purpose of the present application can be achieved. For example, the first positive electrode active material can be prepared by a sol-gel method or a spray drying method. The present application does not particularly limit the method for regulating the particle sphericity B of the first positive electrode active material, as long as the purpose of the present application can be achieved. For example, in some embodiments, the first positive electrode active material whose particle sphericity B is within the scope of the present application can be directly purchased, and the particle sphericity B of the first positive electrode active material can be determined by combining the test method of "testing the particle sphericity B of the first positive electrode active material" in the present application, and the first positive electrode active material with the required particle sphericity B can be selected. In other embodiments, when the first positive electrode active material is prepared by the sol-gel method, it can be achieved by regulating the time and temperature of gelation, the temperature and time of heat treatment. In some further embodiments, when the first positive electrode active material is prepared by the spray drying method, it is achieved by regulating the flow rate, pressure and nozzle type of the spray, or by regulating the drying temperature, time and air flow rate. Generally speaking, the higher the spray pressure, the higher the sphericity of the particles of the first positive electrode active material; the faster the drying airflow speed, the higher the sphericity of the particles of the first positive electrode active material. In some other embodiments, this can be achieved by screening, grading, etc. of the first positive electrode active material.

[0048] The present application does not particularly limit the preparation method of the positive electrode sheet, as long as the object of the present application can be achieved. For example, in some embodiments, the preparation method of the positive electrode sheet includes, but is not limited to, the following steps: (1) preparing a first positive electrode active material layer slurry and a second positive electrode active material layer slurry; (2) coating the first positive electrode active material layer slurry on the first surface of the positive electrode current collector, and after drying, obtaining a semi-finished positive electrode sheet coated with the first positive electrode active material layer. Then, coating the second positive electrode active material layer slurry on the surface of the first positive electrode active material layer away from the positive electrode current collector, and after drying, obtaining a semi-finished positive electrode sheet with a single-sided coating of the first positive electrode active material layer and the second positive electrode active material layer; (3) repeating step (2) on the second surface of the positive electrode current collector to obtain a semi-finished positive electrode sheet with a double-sided coating of the first positive electrode active material layer and the second positive electrode active material layer; (4) obtaining the positive electrode sheet after cold pressing and slitting. The present application does not particularly limit the solid content of the above-mentioned first positive electrode active material layer slurry and second positive electrode active material layer slurry, as long as the object of the present application can be achieved. The present application does not particularly limit the process parameters of the above-mentioned drying, cold pressing and slitting, as long as the object of the present application can be achieved.

[0049] The second aspect of the present application provides a secondary battery, and the secondary battery includes the positive electrode sheet described in any of the foregoing embodiments. Applying the positive electrode sheet described in any of the foregoing embodiments to the secondary battery can enable the secondary battery to have a high energy density while taking into account its safety performance.

[0050] In some embodiments of the present application, the secondary battery includes a negative electrode sheet. The present application does not particularly limit the negative electrode sheet, as long as the object of the present application can be achieved. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode active material layer is disposed on one surface or two surfaces of the negative electrode current collector. The above-mentioned "surface" can be a partial surface or the entire surface of the negative electrode current collector. The present application does not particularly limit the negative electrode current collector, as long as the object of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, etc. The negative electrode active material layer of the present application contains a negative electrode active material. The present application does not particularly limit the type of the negative electrode active material, as long as the object of the present application can be achieved. For example, the negative electrode active material 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, Li-Al alloy or lithium metal. In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6μm to 10μm, and the thickness of the negative electrode active material layer is 30μm to 130μm. Optionally, the negative electrode active material layer may also include at least one of a negative electrode conductor, a dispersant or a negative electrode binder. The present application does not particularly limit the types of negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer, as long as the purpose of the present application can be achieved. The present application does not particularly limit the mass ratio of negative electrode active materials, negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the mass ratio of negative electrode active materials, negative electrode conductors, dispersants and negative electrode binders in the negative electrode active material layer is (96-98): (0.5-1.5): (0-1.5): (1.0-2.9).

[0051] In some embodiments of the present application, the secondary battery includes a diaphragm, which is disposed between the positive electrode sheet and the negative electrode sheet. The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include, but is not limited to, polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) films), cellulose, polyimide (PI), polyamide (PA), spandex or at least one of aramid. The type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. The present application has no particular restrictions on the thickness of the diaphragm, as long as the purpose of the present application can be achieved.

[0052] In some embodiments of the present application, the secondary battery further includes an electrolyte. The present application has no particular limitation on the electrolyte, and those skilled in the art can select the electrolyte according to actual needs, as long as the purpose of the present application can be achieved.

[0053] In some embodiments of the present application, the secondary battery further includes an outer package. The present application has no particular restrictions on the type and shape of the outer package, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0054] The secondary battery of the present application is not particularly limited, and may include any device that undergoes an electrochemical reaction. For example, the secondary battery may include, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0055] The present application does not particularly limit the preparation method of the secondary battery, and the preparation method known in the art can be selected as long as the purpose of the present application can be achieved. For example, the preparation method of the secondary battery includes but is not limited to the following steps: stacking the diaphragm, the positive electrode sheet, the diaphragm and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly of a winding structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the diaphragm, the positive electrode sheet, the diaphragm and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure to obtain an electrode assembly of a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery.

[0056] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any one of the above embodiments. Therefore, the electronic device has good performance.

[0057] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device can include, but is not limited to: a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0058] Example

[0059] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

[0060] Test methods and equipment:

[0061] Test of particle sphericity B of the first positive electrode active material:

[0062] The scanning electron microscope (SEM) photo of the first positive electrode active material layer is measured, and the average of the ratio of the longest side to the shortest side of multiple particles (generally 400 to 800 particles) is obtained to obtain the particle sphericity B of the first positive electrode active material. Specifically, B is the SEM photo of the first positive electrode active material, and the longest side and the shortest side of each particle are measured using Image J software. The sphericity of each particle is defined as the ratio of the shortest side to the longest side, and the particle sphericity B of the first positive electrode active material is the average of the sphericity of all particles in the SEM photo. The closer B is to 1, the more the first positive electrode active material tends to be an ideal sphere.

[0063] Compacted density test:

[0064] After the voltage of the lithium-ion battery is controlled at 3.95V (if the voltage of the lithium-ion battery is lower than 3.95V, charge it to 3.95V at 0.2C; if the voltage of the lithium-ion battery is higher than 3.95V, discharge it to 3.95V at 0.2C), disassemble it to obtain the positive electrode sheet. Punch a small disc with an area of ​​S on the positive electrode sheet to obtain the mass and thickness of the small disc, which are recorded as m and h respectively; remove the second positive electrode active material layer on the small disc by etching to obtain the mass and thickness of the small disc with the second positive electrode active material layer peeled off, which are recorded as m1 and h1 respectively; remove the first positive electrode active material layer on the small disc by etching to obtain the mass and thickness of the negative electrode collector, which are recorded as m2 and h2 respectively.

[0065] The compaction density A1 of the first positive electrode active material layer = (m1 - m2) / ((h1 - h2) x S).

[0066] The compaction density of the second positive electrode active material layer is A2 = (m-m1) / ((h-h1)×S).

[0067] Volume average particle size test:

[0068] The particle size distribution was tested using a laser particle size analyzer MasterSizer 2000 to obtain the volume average particle diameters of the first positive electrode active material and the second positive electrode active material.

[0069] Test of total thickness of active layer:

[0070] The total thickness of the active layer was measured using a microscope.

[0071] Principle: Use a scanning electron microscope (SEM) to observe the cross-section of the positive electrode and directly measure the thickness of the active layer.

[0072] Steps: (1) Sample preparation: Cut the positive electrode sheet into thin slices to obtain a clear cross section. (2) Observation: Use SEM to observe the cross section and find the boundary of the active layer. (3) Measurement: Use the measurement function of SEM to directly measure the thickness of the active layer.

[0073] Test of single layer thickness of the first positive electrode active material layer:

[0074] The single layer thickness of the first positive electrode active material layer was measured by microscopy.

[0075] Principle: Use SEM to observe the cross section of the positive electrode sheet and directly measure the thickness of the first positive electrode active material layer.

[0076] Steps: (1) Sample preparation: Cut the positive electrode sheet into thin slices to obtain a clear cross section. (2) Observation: Use SEM to observe the cross section and find the boundary of the first positive electrode active material layer. (3) Measurement: Use the measurement function of SEM to directly measure the thickness of the first positive electrode active material layer.

[0077] Test of carbon material content C:

[0078] Thermogravimetric analysis: A method of determining the composition of a substance by measuring the change in mass during heating. Thermogravimetric analysis curves are usually used to calculate the amount of carbon dioxide generated in the system due to carbon oxidation, thereby estimating the carbon content.

[0079] The details are as follows: (1) Sample preparation: Weigh 10 mg of sample and record the initial mass. Place the sample in a dryer to remove surface moisture and impurities. (2) Instrument setup: Select a thermogravimetric analyzer and set the atmosphere to oxygen. Set the temperature range from room temperature to 1000°C. Set the heating rate to 10°C / min. (3) Experimental steps: Place the sample in the sample pan of the thermogravimetric analyzer. Start the thermogravimetric analyzer and record the mass change of the sample during heating. (4) Data analysis: Analyze the thermogravimetric curve to determine the mass loss of the sample during heating.

[0080] Calculate the carbon content: Carbon content (%) = mass loss (mg) / initial mass (mg) × 100%.

[0081] Energy density test:

[0082] First, the dimensions of the lithium-ion battery are measured to obtain the thickness T, width W and length L, and the volume V is calculated using the following formula: V = T × W × L.

[0083] Then charge the lithium-ion battery according to the following operating process, and then discharge it to obtain the discharge capacity E of the lithium-ion battery:

[0084] Charging: Charge at 2C constant current to 4.3V, then charge at 4.3V constant voltage to 0.05C;

[0085] Discharge: Discharge at a constant current of 0.5C to 2.5V, and obtain the discharge energy E.

[0086] The energy density (ED) of a lithium-ion battery can be calculated using the following formula: ED (Wh / L) = E / V.

[0087] Test of elongation at break of positive electrode:

[0088] (1) Sample preparation: The cold-pressed positive electrode sheet was cut into a rectangular shape with a length × width = 50 mm × 20 mm;

[0089] (2) Installing the sample: Use a clamp to clamp the positive electrode sample, ensuring that one end of the positive electrode sample is fixed and the other end can stretch freely;

[0090] (3) Applying loading: Using a high-speed rail tensile testing machine (equipment model: INSTRON 34TM-5), uniform tensile force is applied to the positive electrode sheet sample, so that the positive electrode sheet sample begins to extend;

[0091] (4) Measuring elongation: When the positive electrode sheet sample begins to break, use an extensometer to measure the original length of the sample (L0) and the final length of the sample after breaking (LE).

[0092] (5) Calculate the elongation at break: The elongation at break is calculated using the following formula: Elongation at Break = (LE-L0) / L0×100%; where Elongation at Break represents the elongation at break, LE represents the final length of the sample after fracture, and L0 represents the original length of the sample. The calculation processes of steps (4) and (5) can be automatically generated by a high-speed rail tensile machine (equipment model: INSTRON 34TM-5).

[0093] The safety performance of lithium-ion batteries is characterized by the elongation at break of the positive electrode. The higher the elongation at break, the better the safety performance of the lithium-ion battery.

[0094] Example 1-1

[0095] <Preparation of positive electrode sheet>

[0096] LiNi 0.91 Co 0.06 Mn 0.03O2 is placed in a sample boat of a chemical vapor deposition reactor, and nitrogen is introduced as an inert gas, and the inert gas flow rate is set to 110sccm; the reactor is heated to 850°C and kept at a constant temperature for 1 hour; methane is selected as a carbon source gas, and the carbon source gas flow rate is set to 60sccm, and the gas is kept introduced for 3 hours; the introduction of methane gas is stopped, and nitrogen is continued to be introduced for cooling. After cooling, the nickel cobalt manganese oxide lithium material containing a carbon layer is collected and collected to obtain a first positive electrode active material with a carbon material on the surface, and the mass percentage of the carbon material C=2.5%.

[0097] The first positive electrode active material prepared above, the positive electrode conductive agent multi-walled carbon nanotubes (CNT), the positive electrode conductive agent conductive carbon black (Super P), the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight 5×10 5 ) are mixed in a mass ratio of 97:0.6:1:1.4, N-methylpyrrolidone (NMP) is added as a solvent, and stirred under a vacuum mixer until a solid content of 75wt% and a uniform system is obtained to obtain a first positive electrode slurry. The particle sphericity B of the first positive electrode active material is 0.88, and the volume average particle size Dv50 of the first positive electrode active material is -1 =5μm.

[0098] The second positive electrode active material is lithium nickel cobalt manganese oxide LiNi 0.91 Co 0.06 Mn 0.03 O2, positive electrode conductive agent CNT, positive electrode conductive agent Super P, positive electrode binder PVDF (weight average molecular weight 5×10 5 ) are mixed in a mass ratio of 97:0.6:1:1.4, NMP is added as a solvent, and stirred in a vacuum mixer until a second positive electrode slurry with a solid content of 75wt% and a uniform system is obtained. The particle size of the second positive electrode active material is Dv50 -2 =5μm.

[0099] The first positive electrode slurry is coated on one surface of a positive electrode current collector aluminum foil with a thickness of 13 μm, and dried at 110°C to obtain a semi-finished positive electrode sheet coated with the first positive electrode active material layer; the second positive electrode slurry is coated on the surface of the first positive electrode active material layer away from the positive electrode current collector, and dried at 110°C to obtain a semi-finished positive electrode sheet coated with the first positive electrode active material layer and the second positive electrode active material layer on one side. After that, the above steps are repeated on the other surface of the positive electrode current collector to obtain a semi-finished positive electrode sheet coated with the first positive electrode active material layer and the second positive electrode active material layer on both sides. Cold press, then cut and weld the positive electrode ears (such as aluminum ears) to obtain a positive electrode sheet with a specification of 74mm×851mm for standby use. Among them, the compaction density A1 of the first positive electrode active material layer is 3.4g / cm 3, the compaction density of the second positive electrode active material layer A2 = 3.6 g / cm 3 , the total thickness of the active layer is H=80 μm, and the single layer thickness of the first positive electrode active material layer is H1=40 μm.

[0100] <Preparation of negative electrode sheet>

[0101] The negative electrode active material artificial graphite, negative electrode conductive agent Super P, and negative electrode binder PVDF (weight average molecular weight 9000) are mixed in a mass ratio of 97:0.5:2.5, deionized water is added as a solvent, and stirred under the action of a vacuum mixer until the solid content is 70wt% and the system is uniform. The negative electrode slurry is coated on one surface of the negative electrode current collector copper foil with a thickness of 8μm, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with a negative electrode active material layer. After that, the above steps are repeated on the other surface of the negative electrode current collector to obtain a negative electrode sheet coated with a negative electrode active material layer on both sides. Cold pressing, cutting and welding of negative electrode tabs (such as copper tabs) to obtain a negative electrode sheet with a specification of 76mm×856mm for standby use.

[0102] <Preparation of Separator>

[0103] A polypropylene (PP) film with a thickness of 16 μm is used.

[0104] <Preparation of Electrolyte>

[0105] In a dry argon atmosphere, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 20:30:40:10 to obtain an organic solvent, and then lithium salt LiPF6 is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1 mol / L.

[0106] <Preparation of lithium-ion batteries>

[0107] The prepared separator, negative electrode sheet, separator and positive electrode sheet are stacked and wound in order to obtain an electrode assembly of a wound structure. The electrode assembly is placed in an aluminum-plastic film packaging bag, and after drying, an electrolyte (the amount of liquid injection is 6g) is injected, and a lithium-ion battery is obtained through vacuum packaging, standing, formation, degassing, trimming and other processes. Formation steps: charge to 3.3V at a constant current of 0.02C, and then charge to 3.6V at a constant current of 0.1C.

[0108] Example 1-2 to Example 1-13

[0109] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0110] Example 2-1 to Example 2-8

[0111] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0112] Example 3-1 to Example 3-10

[0113] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 1-1.

[0114] Example 4-1 to Example 4-9

[0115] Except for adjusting the relevant preparation parameters according to Table 4, the rest is the same as Example 1-1.

[0116] The change in the mass percentage of the carbon material is achieved by regulating the carbon source gas flow rate. When the carbon source gas flow rate is increased, the mass percentage C of the carbon material increases; when the carbon source gas flow rate is reduced, the mass percentage C of the carbon material decreases.

[0117] Example 5-1 and Example 5-2

[0118] Except for adjusting the relevant preparation parameters according to Table 5, the rest is the same as Example 1-1.

[0119] Comparative Examples 1 to 5

[0120] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0121] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 5.

[0122] Table 1

[0123]

[0124] It can be seen from Examples 1-1 to 1-9 and Comparative Examples 1 to 5 that the secondary battery of the embodiment of the present application, by regulating the particle sphericity B of the first positive electrode active material and the compaction density A2 of the second positive electrode active material layer within the scope of the present application, makes the positive electrode sheet have a higher elongation at break, and the secondary battery has a higher energy density, indicating that the secondary battery implemented in the present application can have high energy density while taking into account safety performance. In the secondary battery of the comparative example, the particle sphericity B of the first positive electrode active material and / or the compaction density A2 of the second positive electrode active material layer are not within the scope of the present application, and it has a lower elongation at break of the positive electrode sheet or has a lower energy density, indicating that the secondary battery of the comparative example cannot take into account its energy density and safety performance.

[0125] The particle sphericity B of the first positive electrode active material usually affects the energy density and safety performance of the secondary battery. From Examples 1-1 to 1-5 and Comparative Example 1, it can be seen that the secondary battery in which the particle sphericity B of the first positive electrode active material is within the range of the present application has a higher elongation at break and the secondary battery has a higher energy density, indicating that the secondary battery in the embodiment of the present application can have a high energy density while taking into account safety performance.

[0126] The compaction density A2 of the second positive electrode active material layer usually affects the energy density and safety performance of the secondary battery. It can be seen from Example 1-1, Example 1-6 to Example 1-9, Comparative Example 2 and Comparative Example 3 that the secondary battery whose compaction density A2 of the second positive electrode active material layer is within the scope of the present application has a higher elongation at break and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have a high energy density while taking into account safety performance.

[0127] The ratio A1 / A2 between the compaction density A1 of the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer usually affects the energy density and safety performance of the secondary battery. It can be seen from Examples 1-1, 1-6 to 1-13 that the secondary battery whose ratio A1 / A2 between the compaction density A1 of the first positive electrode active material layer and the compaction density A2 of the second positive electrode active material layer is within the scope of the present application has a higher elongation at break of the positive electrode sheet, and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have both high energy density and safety performance.

[0128] The compaction density A1 of the first positive electrode active material layer usually affects the energy density and safety performance of the secondary battery. It can be seen from Examples 1-1, 1-10 to 1-13 that the secondary battery whose compaction density A1 of the first positive electrode active material layer is within the scope of the present application has a higher elongation at break and a higher energy density of the secondary battery, indicating that the secondary battery of the present application embodiment can have both high energy density and safety performance.

[0129] Table 2

[0130]

[0131] The volume average particle size Dv50 of the first positive electrode active material -1 It usually affects the energy density and safety performance of the secondary battery. From Examples 1-1, 2-1 to 2-4, it can be seen that the volume average particle size Dv50 of the first positive electrode active material is selected. -1The secondary battery within the scope of the present application has a positive electrode sheet with a higher elongation at break, and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have both high energy density and safety performance.

[0132] The volume average particle size Dv50 of the second positive electrode active material -2 It usually affects the energy density and safety performance of the secondary battery. From Examples 1-1, 2-5 to 2-8, it can be seen that the volume average particle size Dv50 of the second positive electrode active material is selected. -2 The secondary battery within the scope of the present application has a positive electrode sheet with a higher elongation at break, and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have both high energy density and safety performance.

[0133] Table 3

[0134]

[0135] The single layer thickness H1 of the first positive electrode active material layer, the total thickness H of the active layer, and the ratio H1 / H of the two usually affect the energy density and safety performance of the secondary battery. It can be seen from Examples 1-1, 3-1 to 3-10 that the secondary battery whose single layer thickness H1 of the first positive electrode active material layer, the total thickness H of the active layer, and the ratio H1 / H of the two are within the scope of the present application has a higher elongation at break of the positive electrode sheet, and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have a high energy density while taking into account safety performance.

[0136] Table 4

[0137]

[0138]

[0139] The value of the mass percentage C of the carbon material on the surface of the first positive electrode active material and the relationship between C and the particle sphericity B of the first positive electrode active material C / B×10×100% usually affect the energy density and safety performance of the secondary battery. It can be seen from Examples 1-1, 4-1 to 4-9 that the value of the mass percentage C of the carbon material on the surface of the first positive electrode active material and the relationship between C and the particle sphericity B of the first positive electrode active material C / B×10×100% is selected for the secondary battery within the scope of the present application, and its positive electrode sheet has a higher elongation at break, and the secondary battery has a higher energy density, indicating that the secondary battery of the embodiment of the present application can have a high energy density while taking into account safety performance.

[0140] Table 5

[0141]

[0142] The types of the first positive electrode active material and the second positive electrode active material usually affect the energy density and safety performance of the secondary battery. It can be seen from Examples 1-1, 5-1 and 5-2 that the secondary battery using the first positive electrode active material and the second positive electrode active material in the range of the present application has a higher elongation at break and a higher energy density, indicating that the secondary battery in the embodiments of the present application can have both high energy density and safety performance.

[0143] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0144] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0145] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and an active layer disposed on at least one surface of the positive electrode current collector, wherein the active layer comprises a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer is disposed between the positive electrode current collector and the second positive electrode active material layer; The first positive electrode active material layer includes a first positive electrode active material, and the particle sphericity of the first positive electrode active material is B, B≥0.7; The second positive electrode active material layer includes a second positive electrode active material, and the compaction density of the second positive electrode active material layer is A2,3.2 g / cm 3 ≤A2≤3.8g / cm 3 .

2. The positive electrode sheet according to claim 1, wherein: The positive electrode sheet satisfies at least one of the following characteristics: (1)0.85≤B≤0.95； (2)3.5g / cm 3 ≤A2≤3.7g / cm 3 。 3. The positive electrode sheet according to claim 1, wherein: The compaction density of the first positive electrode active material layer is A1, and A1 and A2 satisfy: 0.9≤A1 / A2≤0.

98.

4. The positive electrode sheet according to claim 1, wherein: 3.2g / cm 3 ≤A1≤3.7g / cm 3 。 5. The positive electrode sheet according to claim 1, wherein: The volume average particle size of the first positive electrode active material is Dv50 -1 , 3μm≤Dv50 -1 ≤8μm; The volume average particle size of the second positive electrode active material is Dv50 -2 , 3μm≤Dv50 -2 ≤8μm.

6. The positive electrode sheet according to claim 1, wherein: The total thickness of the active layer is H, the single layer thickness of the first positive electrode active material layer is H1, 0.1≤H1 / H≤0.8, 50 μm≤H≤120 μm.

7. The positive electrode sheet according to claim 6, wherein: 20μm≤H1≤95μm.

8. The positive electrode sheet according to claim 1, wherein: A carbon material is disposed on at least part of the surface of the first positive electrode active material, and the mass percentage of the carbon material is C, based on the mass of the first positive electrode active material, 0.5%≤C≤3%; B and C satisfy: 15% ≤ C / B × 10 × 100% ≤ 30%.

9. The positive electrode sheet according to claim 1, wherein: The first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide; the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate or lithium cobalt oxide.

10. A secondary battery, wherein: The secondary battery comprises the positive electrode sheet according to any one of claims 1 to 9.

11. An electronic device, wherein: The electronic device includes the secondary battery according to claim 10.

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