Secondary battery and electronic device

Abstract

The application provides a secondary battery, which comprises an electrode assembly in a winding structure, and the electrode assembly comprises a positive electrode sheet, and the positive electrode sheet comprises a positive electrode current collector, a first positive electrode material layer and a second positive electrode material layer. At least part of a first surface is provided with the first positive electrode material layer, and at least part of a second surface is provided with the second positive electrode material layer. The first positive electrode material layer comprises a first positive electrode active material, and the second positive electrode material layer comprises a second positive electrode active material. The thermal conductivity of the first positive electrode material layer is λ1 W / (m·K), the thermal conductivity of the second positive electrode material layer is λ2 W / (m·K), and λ1>λ2. The Dv50 of the first positive electrode active material is D1 μm, the Dv50 of the second positive electrode active material is D2 μm, 2≤D1≤6, and 8≤D2≤15. The secondary battery satisfies the above characteristics, can effectively improve the wettability and liquid retention capacity of the positive electrode sheet, and improve the cycle performance and rate performance of the secondary battery.

Description

Technical Field

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

[0002] With the rapid development of technology, lithium-ion batteries, which conform to the concept of green development, have been widely used and promoted in various fields. At the same time, the market has higher demands for the lifespan, charge and discharge speed, and other performance characteristics of lithium-ion batteries.

[0003] The existing conventional electrode designs are gradually failing to meet market demands for lithium-ion batteries in terms of lifespan, charge / discharge speed, and other performance characteristics. Therefore, it is of great significance to provide a lithium-ion battery with better cycle performance and rate performance. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that can effectively improve the wettability and liquid retention capacity of the positive electrode, thereby enhancing the cycle performance and rate performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of this application provides a secondary battery comprising a wound electrode assembly. The electrode assembly includes a positive electrode sheet, which includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. Along the thickness direction of the positive electrode sheet, the positive current collector includes opposing first and second surfaces. The surface of the positive current collector facing away from the winding center is the first surface, and the surface of the positive current collector facing the winding center is the second surface. At least a portion of the first surface is provided with the first positive electrode material layer, and at least a portion of the second surface is provided with the second positive electrode material layer. The first positive electrode material layer includes a first positive electrode active material, a first conductive agent, and a first binder. The second positive electrode material layer includes a second positive electrode active material, a second conductive agent, and a second binder. The thermal conductivity of the first positive electrode material layer is λ1 W / (m·K), and the thermal conductivity of the second positive electrode material layer is λ2 W / (m·K), where λ1 > λ2. The first positive electrode active material has a Dv50 of D1 μm, and the second positive electrode active material has a Dv50 of D2 μm, where 2 ≤ D1 ≤ 6 and 8 ≤ D2 ≤ 15. When a first positive electrode material layer is disposed on the surface of the positive electrode current collector facing away from the winding center, and a second positive electrode material layer is disposed on the surface of the positive electrode current collector facing the winding center, and the first positive electrode material layer includes a first positive electrode active material, by adjusting the relationship between the Dv50 of the first and second positive electrode active materials and the thermal conductivity of the first and second positive electrode material layers within the scope of this application, the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0006] In one or more embodiments of this application, 0.7 ≤ λ1 ≤ 1.2, and / or 0.4 ≤ λ2 ≤ 0.6.

[0007] In one or more embodiments of this application, the specific surface area of ​​the first positive electrode active material is B1 m². 2 / g, 0.4≤B1≤0.8, and / or, the specific surface area of ​​the second positive electrode active material is B2 m². 2 / g, 0.3≤B2≤0.5.

[0008] In one or more embodiments of this application, the powder conductivity of the first positive electrode active material is σ1S / m, 5.5≤σ1≤7.5, and / or the powder conductivity of the second positive electrode active material is σ2S / m, 2.5≤σ2≤5.

[0009] In one or more embodiments of this application, the first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, or lithium cobalt oxide, and the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, or lithium cobalt oxide. By selecting the aforementioned first and second positive electrode active materials, the secondary battery can exhibit better rate performance and cycle performance.

[0010] In one or more embodiments of this application, the porosity of the first positive electrode material layer is K1%, 15≤K1≤25, and / or the porosity of the second positive electrode material layer is K2%, 27≤K2≤40.

[0011] In one or more embodiments of this application, the liquid retention coefficient of the first positive electrode material layer is N1 mg / mAh, 5≤N1≤8, and / or the liquid retention coefficient of the second positive electrode material layer is N2 mg / mAh, 9≤N2≤12.

[0012] In one or more embodiments of this application, the coating weight of the first positive electrode material layer is CW1 mg / cm³. 2 10≤CW1≤20, and / or, the coating weight of the second cathode material layer is CW2 mg / cm³. 2 , 9≤CW2≤18.

[0013] In one or more embodiments of this application, the mass percentage of the first positive electrode active material is W, based on the mass of the first positive electrode material layer. 11 %, 95≤W 11 ≤97.8; and / or, based on the mass of the second cathode material layer, the mass percentage of the second cathode active material is W. 21 %, 95.5≤W 21 ≤97.7.

[0014] In one or more embodiments of this application, the first conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the first conductive agent is W based on the mass of the first positive electrode material layer. 12 %, 1.2≤W 12 ≤3; and / or, the second conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the second conductive agent is W based on the mass of the second positive electrode material layer. 22 %, 1.5≤W 22 ≤2.5.

[0015] In one or more embodiments of this application, W 12 >W 22 W 12 and W 22 By satisfying the above size relationship and constructing a relatively sufficient conductive network structure in the first and second positive electrode material layers, the DC impedance of the secondary battery can be reduced, and the high and low temperature performance, rate performance, cycle performance and storage performance of the secondary battery can be improved.

[0016] In one or more embodiments of this application, the first binder comprises at least one selected from polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the first binder is W based on the mass of the first positive electrode material layer. 13 %, 1≤W 13 ≤3; and / or, the second binder comprises at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the second binder is W based on the mass of the second cathode material layer. 23 %, 0.8≤W 23 ≤2.

[0017] In one or more embodiments of this application, the weight-average molecular weight of the first adhesive is Mw1, 700,000 ≤ Mw1 ≤ 1200,000, and / or the weight-average molecular weight of the second adhesive is Mw2, 700,000 ≤ Mw2 ≤ 1200,000.

[0018] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance characteristics.

[0019] The beneficial effects of this application are:

[0020] This application provides a secondary battery and an electronic device. The secondary battery includes a wound electrode assembly. The electrode assembly includes a positive electrode sheet, which includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. Along the thickness direction of the positive electrode sheet, the positive current collector includes a first surface and a second surface opposite to each other. The surface of the positive current collector facing away from the winding center is the first surface, and the surface of the positive current collector facing the winding center is the second surface. At least a portion of the first surface is provided with the first positive electrode material layer, and at least a portion of the second surface is provided with the second positive electrode material layer. The first positive electrode material layer includes a first positive electrode active material, a first conductive agent, and a first binder. The second positive electrode material layer includes a second positive electrode active material, a second conductive agent, and a second binder. The thermal conductivity of the first positive electrode material layer is λ1 W / (m·K), and the thermal conductivity of the second positive electrode material layer is λ2 W / (m·K), where λ1 > λ2. The Dv50 of the first positive electrode active material is D1μm, and the Dv50 of the second positive electrode active material is D2μm, where 2≤D1≤6 and 8≤D2≤15. The secondary battery meets these characteristics, effectively improving the wettability and liquid retention capacity of the positive electrode sheet, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0021] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0023] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet according to one embodiment of this application. Detailed Implementation

[0024] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0025] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0026] The first aspect of this application provides a secondary battery comprising a wound electrode assembly, the electrode assembly including a positive electrode sheet, such as... Figure 1As shown, the positive electrode 10 includes a positive current collector 100, a first positive electrode material layer 110, and a second positive electrode material layer 120. Along the thickness direction of the positive electrode, the positive current collector 100 includes a first surface 101 and a second surface 102 facing each other. The surface of the positive current collector facing away from the winding center is the first surface, and the surface facing the winding center is the second surface. At least a portion of the first surface 101 is provided with the first positive electrode material layer 110, and at least a portion of the second surface 102 is provided with the second positive electrode material layer 120. The first positive electrode material layer 110 includes a first positive electrode active material 111, a first conductive agent, and a first binder. The second positive electrode material layer 120 includes a second positive electrode active material 121, a second conductive agent, and a second binder. The thermal conductivity of the first positive electrode material layer is λ1 W / (m·K), and the thermal conductivity of the second positive electrode material layer is λ2 W / (m·K), where λ1 > λ2. The Dv50 of the first positive electrode active material is D1μm, and the Dv50 of the second positive electrode active material is D2μm, where 2≤D1≤6. For example, the value of D1 can be 2, 2.3, 2.5, 2.7, 3, 3.3, 3.5, 3.7, 4, 4.3, 4.5, 4.7, 5, 5.3, 5.5, 5.7, 6, or any range of any two of the above values; 8≤D2≤15. For example, the value of D2 can be 8, 8.3, 8.5, 8.7, 9, 9.3, 9.5, 9.7, 10, 10.3, 10.5, 10.7, 11, 11.3, 11.5, 11.7, 12, 13, 14, 15, or any range of any two of the above values. The aforementioned "at least a portion of the first surface is provided with a first positive electrode material layer" can mean that a portion of the first surface is provided with a first positive electrode material layer, or that the entire first surface is provided with a first positive electrode material layer. This application has no particular limitation, as long as the purpose of this application is achieved. Similarly, the aforementioned "at least a portion of the second surface is provided with a second positive electrode material layer" can mean that a portion of the second surface is provided with a second positive electrode material layer, or that the entire second surface is provided with a second positive electrode material layer. This application has no particular limitation, as long as the purpose of this application is achieved.

[0027] In this application, Dv50 refers to the particle size that reaches 50% of the volumetric accumulation in the particle size distribution of the material based on volume.

[0028] The inventors discovered that when a first positive electrode material layer is disposed on the surface of the positive electrode current collector facing away from the winding center, and a second positive electrode material layer is disposed on the surface of the positive electrode current collector facing the winding center, wherein the first positive electrode material layer includes a first positive electrode active material and the second positive electrode material layer includes a second positive electrode active material, by adjusting the relationship between the Dv50 of the first and second positive electrode active materials and the thermal conductivity of the first and second positive electrode material layers within the scope of this application, the first positive electrode active material has a smaller particle size and a larger specific surface area, and the first positive electrode material layer has a high packing density structure. The high packing density structure bears the main lithium intercalation stress, which can effectively reduce the anisotropy caused by expansion during the cycling process of the secondary battery. In addition, the small particle structure can also weaken the shear stress generated by the secondary battery during cycling, storage, and abuse through grain boundary slip, thereby reducing the expansion of the secondary battery electrode during cycling and storage, and thus improving the cycle performance and storage performance of the secondary battery. Furthermore, this structure is also beneficial to the stress release of the secondary battery in collision, drop, and puncture tests, and improves the pass rate of the secondary battery. Furthermore, the thermal conductivity of the first positive electrode material layer is greater than that of the second positive electrode material layer. The thermal expansion coefficient of the first positive electrode active material forms a gradient transition with the positive electrode current collector, which can quickly conduct the heat generated during the charging and discharging process of the secondary battery to the outside of the secondary battery, reduce the risk of thermal runaway caused by heat accumulation, and also reduce the temperature rise during the charging and discharging process, thereby improving the rate performance of the secondary battery. The second positive electrode active material has a larger particle size and higher porosity, while the first positive electrode active material has a smaller particle size and lower porosity. On one hand, the second positive electrode material layer can form a buffer design with the lower porosity of the first positive electrode material layer, effectively absorbing the expansion pressure of the electrolyte during the operation of the secondary battery. On the other hand, the higher porosity of the second positive electrode material layer facing the winding center facilitates rapid wetting of the electrolyte in the positive electrode sheet, improving the electrolyte retention capacity of the positive electrode sheet and improving the kinetics of the inner positive electrode sheet in the later stages of the secondary battery cycle. In addition, the smaller particle size and lower porosity of the first positive electrode active material in the first positive electrode material layer facing away from the winding center result in a relatively smaller contact area between the first positive electrode active material and the electrolyte, which can effectively reduce the occurrence of side reactions and improve the cycle performance of the secondary battery. Therefore, when a first positive electrode material layer is disposed on the surface of the positive electrode current collector facing away from the winding center, and a second positive electrode material layer is disposed on the surface of the positive electrode current collector facing the winding center, wherein the first positive electrode material layer includes a first positive electrode active material and the second positive electrode material layer includes a second positive electrode active material, by adjusting the relationship between the Dv50 of the first positive electrode active material and the second positive electrode active material and the thermal conductivity of the first positive electrode material layer and the second positive electrode material layer within the scope of this application, the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby improving the cycle performance and rate performance of the secondary battery.

[0029] In one or more embodiments of this application, 0.7 ≤ λ1 ≤ 1.2. Exemplarily, the value of λ1 can be 0.7, 0.8, 0.83, 0.85, 0.87, 0.9, 0.93, 0.95, 0.97, 1.0, 1.03, 1.05, 1.07, 1.1, 1.13, 1.15, 1.17, 1.2, or a range consisting of any two of the above values. By adjusting the value of λ1 within the above range, the thermal conductivity of the first positive electrode material layer is higher, and the thermal expansion coefficient of the first positive electrode active material forms a gradient transition with the positive electrode current collector. This allows for faster conduction of heat generated during the charging and discharging process of the secondary battery to the outside of the secondary battery, further reducing the risk of thermal runaway caused by heat accumulation, further reducing the temperature rise during charging and discharging, and further improving the rate performance of the secondary battery.

[0030] In one or more embodiments of this application, 0.4 ≤ λ2 ≤ 0.6. Exemplarily, the value of λ2 can be 0.4, 0.43, 0.45, 0.47, 0.49, 0.5, 0.53, 0.55, 0.57, 0.59, 0.6, or a range consisting of any two of the above values. By adjusting the value of λ2 within the above range, the thermal conductivity of the second positive electrode material layer is lower than that of the first positive electrode material layer, and the thermal conductivity of the first positive electrode material layer is higher. The thermal expansion coefficient of the first positive electrode active material is more likely to form a gradient transition with the positive electrode current collector, which is beneficial for more quickly conducting the heat generated during the charging and discharging process of the secondary battery to the outside of the secondary battery, further reducing the risk of thermal runaway caused by heat accumulation, and further reducing the temperature rise during the charging and discharging process, thereby further improving the rate performance of the secondary battery.

[0031] In one or more embodiments of this application, 0.7≤λ1≤1.2, and exemplarily, the value of λ1 can be 0.7, 0.8, 0.83, 0.85, 0.87, 0.9, 0.93, 0.95, 0.97, 1.0, 1.03, 1.05, 1.07, 1.1, 1.13, 1.15, 1.17, 1.2 or a range consisting of any two of the above values; and 0.4≤λ2≤0.6, and exemplarily, the value of λ2 can be 0.4, 0.43, 0.45, 0.47, 0.49, 0.5, 0.53, 0.55, 0.57, 0.59, 0.6 or a range consisting of any two of the above values. By adjusting the values ​​of λ1 and λ2 within the above range, the thermal conductivity of the first positive electrode material layer is relatively high, while the thermal conductivity of the second positive electrode material layer is relatively low. The thermal conductivity of the outer ring of the positive electrode sheet is higher than that of the inner ring. The thermal expansion coefficient of the first positive electrode active material is more likely to form a gradient transition with the positive electrode current collector, which is beneficial to conduct the heat generated during the charging and discharging process of the secondary battery to the outside of the secondary battery more quickly, further reducing the risk of thermal runaway caused by heat accumulation, further reducing the temperature rise during the charging and discharging process, and further improving the rate performance of the secondary battery.

[0032] In one or more embodiments of this application, the specific surface area of ​​the first positive electrode active material is B1 m². 2 / g, 0.4≤B1≤0.8. For example, the value of B1 can be 0.4, 0.43, 0.45, 0.47, 0.5, 0.53, 0.55, 0.57, 0.6, 0.63, 0.65, 0.67, 0.7, 0.73, 0.75, 0.77, 0.8, or a range consisting of any two of the above values. By adjusting the value of B1 within the above range, the particle size of the first positive electrode active material is smaller, the porosity of the first positive electrode material layer is lower, and the first positive electrode material layer can form a buffer design with the second positive electrode material layer with higher porosity, which is beneficial for further absorbing the electrolyte expansion pressure during the operation of the secondary battery; furthermore, the contact area between the first positive electrode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0033] In one or more embodiments of this application, the specific surface area of ​​the second positive electrode active material is B² m². 2 / g, 0.3≤B2≤0.5. For example, the value of B2 can be 0.3, 0.33, 0.35, 0.37, 0.4, 0.43, 0.45, 0.47, 0.5, or a range consisting of any two of the above values. By adjusting the value of B2 within the above range, the particle size of the second positive electrode active material is larger, and the porosity of the second positive electrode material layer is higher. The second positive electrode material layer can form a buffer design with the first positive electrode material layer, which has lower porosity, thus facilitating the absorption of electrolyte expansion pressure during secondary battery operation. Furthermore, it facilitates rapid wetting of the electrolyte in the positive electrode sheet, further improving the electrolyte retention capacity of the positive electrode sheet and further improving the kinetics of the inner positive electrode sheet in the later stages of secondary battery cycling.

[0034] In one or more embodiments of this application, the specific surface area of ​​the first positive electrode active material is B1 m². 2 / g, 0.4≤B1≤0.8, for example, the value of B1 can be 0.4, 0.43, 0.45, 0.47, 0.5, 0.53, 0.55, 0.57, 0.6, 0.63, 0.65, 0.67, 0.7, 0.73, 0.75, 0.77, 0.8 or a range of any two of the above values; and, the specific surface area of ​​the second positive electrode active material is B2 m². 2 / g, 0.3≤B2≤0.5, for example, the value of B2 can be 0.3, 0.33, 0.35, 0.37, 0.4, 0.43, 0.45, 0.47, 0.5 or a range of any two of the above values. By adjusting the values ​​of B1 and B2 within the above range, the particle size of the second positive electrode active material is larger, and the porosity of the second positive electrode material layer is higher. At the same time, the particle size of the first positive electrode active material is smaller, and the porosity of the first positive electrode material layer is lower. On the one hand, the second positive electrode material layer can form a buffer design with the first positive electrode material layer with lower porosity, which is beneficial to further absorb the electrolyte expansion pressure during the operation of the secondary battery. On the other hand, it is beneficial to the rapid wetting of the electrolyte in the positive electrode sheet, further improving the liquid retention capacity of the positive electrode sheet, which is beneficial to further improve the kinetics of the inner positive electrode sheet in the later stage of the secondary battery cycle. In addition, the contact area between the first positive electrode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0035] In one or more embodiments of this application, the powder conductivity of the first positive electrode active material is σ1S / m, where 5.5 ≤ σ1 ≤ 7.5. Exemplarily, the value of σ1 can be 5.5, 5.7, 6, 6.1, 6.3, 6.5, 6.7, 7, 7.1, 7.3, 7.5, or a range consisting of any two of the above values. By adjusting the value of σ1 within the above range, the electronic conductivity of the first positive electrode active material can be improved, the film resistance of the positive electrode sheet can be reduced, and the rate performance of the secondary battery can be enhanced.

[0036] In one or more embodiments of this application, the powder conductivity of the second positive electrode active material is σ²S / m, where 2.5 ≤ σ² ≤ 5. Exemplarily, the value of σ² can be 2.5, 2.7, 3, 3.5, 3.7, 4, 4.5, 4.7, 5, or a range consisting of any two of the above values. By adjusting the value of σ² within the above range, the electronic conductivity of the second positive electrode active material can be improved, the film resistance of the positive electrode sheet can be reduced, and the rate performance of the secondary battery can be enhanced.

[0037] In one or more embodiments of this application, the powder conductivity of the first positive electrode active material is σ1S / m, 5.5≤σ1≤7.5. Exemplarily, the value of σ1 can be 5.5, 5.7, 6, 6.1, 6.3, 6.5, 6.7, 7, 7.1, 7.3, 7.5, or a range consisting of any two of the above values; and the powder conductivity of the second positive electrode active material is σ2S / m, 2.5≤σ2≤5. Exemplarily, the value of σ2 can be 2.5, 2.7, 3, 3.5, 3.7, 4, 4.5, 4.7, 5, or a range consisting of any two of the above values. By adjusting the values ​​of σ1 and σ2 within the above ranges, the first and second positive electrode active materials exhibit high electronic conductivity, which is beneficial for further reducing the film resistance of the positive electrode sheet and further improving the rate performance of the secondary battery.

[0038] In one or more embodiments of this application, the first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, or lithium cobalt oxide (LiCoO2), and the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, or lithium cobalt oxide. In this application, lithium nickel cobalt manganese oxide includes LiNi... 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.9 Co 0.04 Mn 0.06 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn0.2 O2(NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 At least one of O2 (NCM111). The above-mentioned first positive electrode active material and second positive electrode active material are selected. The first positive electrode active material and the second positive electrode active material have high powder conductivity and high electronic conductivity, which enables the secondary battery to have good rate performance and cycle performance.

[0039] In one or more embodiments of this application, the porosity of the first positive electrode material layer is K1%, where 15 ≤ K1 ≤ 25. Exemplarily, the value of K1 can be 15, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, or a range consisting of any two of the above values. By adjusting the value of K1 within the above range, the porosity of the first positive electrode material layer is low, resulting in a high packing density structure. This high packing density structure bears the main lithium intercalation stress, effectively reducing anisotropy caused by expansion during secondary battery cycling. Furthermore, the first positive electrode material layer can form a buffer design with the second positive electrode material layer, which has a higher porosity, facilitating further absorption of electrolyte expansion pressure during secondary battery operation. Additionally, the relatively small contact area between the first positive electrode active material and the electrolyte further reduces side reactions and improves the cycle performance of the secondary battery.

[0040] In one or more embodiments of this application, the porosity of the second positive electrode material layer is K2%, where 27 ≤ K2 ≤ 40. Exemplarily, the value of K2 can be 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, or a range consisting of any two of the above values. By adjusting the value of K2 within the above range, the porosity of the second positive electrode material layer is higher. The second positive electrode material layer can form a buffer design with the first positive electrode material layer, which has lower porosity. This is beneficial for further absorbing the electrolyte expansion pressure during the operation of the secondary battery; furthermore, it facilitates the rapid wetting of the electrolyte in the positive electrode sheet, further improving the electrolyte retention capacity of the positive electrode sheet, and further improving the dynamics of the inner positive electrode sheet in the later stages of the secondary battery cycle.

[0041] In one or more embodiments of this application, the porosity of the first positive electrode material layer is K1%, 15≤K1≤25. Exemplarily, the value of K1 can be 15, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, or a range consisting of any two of the above values; and the porosity of the second positive electrode material layer is K2%, 27≤K2≤40. Exemplarily, the value of K2 can be 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, or a range consisting of any two of the above values. By adjusting the values ​​of K1 and K2 within the aforementioned range, the second cathode material layer has a higher porosity, while the first cathode material layer has a lower porosity. On the one hand, the second cathode material layer can form a buffer design with the lower porosity of the first cathode material layer, which is beneficial for further absorbing the electrolyte expansion pressure during the operation of the secondary battery. On the other hand, it is beneficial for the rapid wetting of the electrolyte in the cathode sheet, further improving the electrolyte retention capacity of the cathode sheet, and further improving the kinetics of the inner cathode sheet in the later stage of the secondary battery cycle. In addition, the contact area between the first cathode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0042] In one or more embodiments of this application, the electrolyte retention coefficient of the first positive electrode material layer is N1 mg / mAh, where 5 ≤ N1 ≤ 8. Exemplarily, the value of N1 can be 5, 5.3, 5.5, 5.7, 6, 6.3, 6.5, 6.7, 7, 7.3, 7.5, 7.7, 8, or a range of any two of the above values. The electrolyte retention coefficient refers to the ratio of electrolyte mass to the secondary battery capacity, representing the amount of electrolyte retained within the secondary battery. By adjusting the value of N1 within the above range, side reactions occurring during cycling or storage are reduced, while ensuring that the secondary battery still has a significant amount of electrolyte wetting the electrode in the later stages. This improves the cycle capacity retention rate and storage capacity recovery rate of the secondary battery, thereby enhancing its cycle performance and storage performance.

[0043] In one or more embodiments of this application, the liquid retention coefficient of the second positive electrode material layer is N2 mg / mAh, where 9 ≤ N2 ≤ 12. Exemplarily, the value of N2 can be 9, 9.3, 9.5, 9.7, 10, 10.3, 10.5, 10.7, 11, 11.3, 11.5, 11.7, 12, or a range consisting of any two of the above values. By adjusting the value of N2 within the above range, a larger liquid retention coefficient of the second positive electrode material layer indicates better wettability of the electrolyte in the positive electrode sheet, resulting in a higher electrolyte retention within the secondary battery. This is beneficial for lithium-ion transport and for improving the kinetics of the inner positive electrode sheet during the later stages of secondary battery cycling.

[0044] In one or more embodiments of this application, the liquid retention coefficient of the first positive electrode material layer is N1 mg / mAh, 5≤N1≤8. For example, the value of N1 can be 5, 5.3, 5.5, 5.7, 6, 6.3, 6.5, 6.7, 7, 7.3, 7.5, 7.7, 8, or a range consisting of any two of the above values; and the liquid retention coefficient of the second positive electrode material layer is N2 mg / mAh, 9≤N2≤12. For example, the value of N2 can be 9, 9.3, 9.5, 9.7, 10, 10.3, 10.5, 10.7, 11, 11.3, 11.5, 11.7, 12, or a range consisting of any two of the above values. By adjusting the values ​​of N1 and N2 within the aforementioned range, on the one hand, adjusting the appropriate electrolyte retention ensures that the secondary battery still has a sufficient amount of electrolyte under long-term testing conditions, which can improve the cycle capacity retention rate and storage capacity recovery rate of the secondary battery, thereby improving the cycle performance and storage performance of the secondary battery; on the other hand, the large electrolyte retention coefficient of the second positive electrode material layer indicates that the electrolyte has good wettability in the positive electrode sheet, and the electrolyte retention rate inside the secondary battery is high, which is beneficial to lithium-ion transport and to the improvement of the kinetics of the inner positive electrode sheet in the later stage of the secondary battery cycle.

[0045] In one or more embodiments of this application, the coating weight of the first positive electrode material layer is CW1 mg / cm³. 2 10 ≤ CW1 ≤ 20. For example, the value of CW1 can be 10, 12, 14, 16, 18, 20, or a range of any two of the above values. The coating weight of the positive electrode material layer affects the porosity of the positive electrode material layer. By adjusting the value of CW1 within the above range, the porosity of the first positive electrode material layer is lower, and the first positive electrode material layer has a high packing density structure. The high packing density structure bears the main lithium intercalation stress, which can effectively reduce the anisotropy caused by expansion during the secondary battery cycle. Furthermore, the first positive electrode material layer can form a buffer design with the second positive electrode material layer with higher porosity, which is beneficial for further absorbing the electrolyte expansion pressure during secondary battery operation. In addition, the contact area between the first positive electrode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0046] In one or more embodiments of this application, the coating weight of the second positive electrode material layer is CW2 mg / cm³. 29 ≤ CW2 ≤ 18. For example, the value of CW2 can be 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or a range consisting of any two of the above values. By adjusting the value of CW2 within the above range, the porosity of the second positive electrode material layer is higher. The second positive electrode material layer can form a buffer design with the first positive electrode material layer, which has lower porosity. This is beneficial for further absorbing the electrolyte expansion pressure during the operation of the secondary battery; furthermore, it facilitates the rapid wetting of the electrolyte in the positive electrode sheet, further improving the electrolyte retention capacity of the positive electrode sheet, and further improving the dynamics of the inner positive electrode sheet in the later stages of the secondary battery cycle.

[0047] In one or more embodiments of this application, the coating weight of the first positive electrode material layer is CW1 mg / cm³. 2 10 ≤ CW1 ≤ 20, for example, the value of CW1 can be 10, 12, 14, 16, 18, 20 or a range consisting of any two of the above values; and, the coating weight of the second positive electrode material layer is CW2 mg / cm³. 2 The values ​​of CW1 and CW2 are 9 ≤ CW2 ≤ 18. For example, the value of CW2 can be 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or any two of the above values. By adjusting the values ​​of CW1 and CW2 within the above range, the porosity of the second positive electrode material layer is higher, and the porosity of the first positive electrode material layer is lower. On the one hand, the second positive electrode material layer can form a buffer design with the first positive electrode material layer with lower porosity, which is beneficial to further absorb the electrolyte expansion pressure during the operation of the secondary battery. On the other hand, it is beneficial to the rapid wetting of the electrolyte in the positive electrode sheet, further improving the liquid retention capacity of the positive electrode sheet, which is beneficial to further improve the kinetics of the inner positive electrode sheet in the later stage of the secondary battery cycle. In addition, the contact area between the first positive electrode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0048] In one or more embodiments of this application, the mass percentage of the first positive electrode active material is W, based on the mass of the first positive electrode material layer. 11 %, 95≤W 11 ≤97.8. For example, W 11 The value can be 95, 95.8, 96, 96.2, 96.4, 96.6, 96.8, 97, 97.2, 97.4, 97.6, 97.8, or a range of any two of the above values. This can be achieved by adjusting W. 11Within the above range, the thermal conductivity of the first positive electrode material layer is relatively high, which can conduct the heat generated during the charging and discharging process of the secondary battery to the outside of the secondary battery more quickly, further reducing the risk of thermal runaway caused by heat accumulation, and further reducing the temperature rise during the charging and discharging process, thus further improving the rate performance of the secondary battery. At the same time, it can also make the porosity of the first positive electrode material layer lower, and the contact area between the first positive electrode active material and the electrolyte is relatively small, which can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery.

[0049] In one or more embodiments of this application, the mass percentage of the second positive electrode active material is W, based on the mass of the second positive electrode material layer. 21 %, 95.5≤W 21 ≤97.7. For example, W 21 The value can be 95.5, 96, 96.2, 96.4, 96.6, 96.8, 97, 97.2, 97.4, 97.6, 97.7, or a range consisting of any two of the above values. This can be achieved by adjusting W. 21 Within the aforementioned range, the thermal conductivity of the second positive electrode material layer is lower than that of the first positive electrode material layer. The higher thermal conductivity of the first positive electrode material layer facilitates the faster conduction of heat generated during the charging and discharging process to the outside of the secondary battery, further reducing the risk of thermal runaway caused by heat accumulation. It also further reduces the temperature rise during the charging and discharging process, further improving the rate performance of the secondary battery. Simultaneously, it also allows for higher porosity of the second positive electrode material layer, which is beneficial for the rapid wetting of the electrolyte in the positive electrode sheet, further improving the electrolyte retention capacity of the positive electrode sheet, and further improving the dynamics of the inner positive electrode sheet in the later stages of the secondary battery cycle.

[0050] In one or more embodiments of this application, the mass percentage of the first positive electrode active material is W, based on the mass of the first positive electrode material layer. 11 %, 95≤W 11 ≤97.8, for example, W 11 The value can be 95, 95.8, 96, 96.2, 96.4, 96.6, 96.8, 97, 97.2, 97.4, 97.6, 97.8, or a range consisting of any two of the above values; and, based on the mass of the second positive electrode material layer, the mass percentage content of the second positive electrode active material is W. 21 %, 95.5≤W 21 ≤97.7, for example, W 21 The value can be 95.5, 96, 96.2, 96.4, 96.6, 96.8, 97, 97.2, 97.4, 97.6, 97.7, or a range consisting of any two of the above values. This can be achieved by adjusting W. 11 and W21 Within the aforementioned range, the thermal conductivity of the second positive electrode material layer is lower than that of the first positive electrode material layer. The higher thermal conductivity of the first positive electrode material layer facilitates the faster conduction of heat generated during the charging and discharging process to the outside of the secondary battery, further reducing the temperature rise during charging and discharging and improving the rate performance of the secondary battery. Simultaneously, the relatively small contact area between the first positive electrode active material and the electrolyte in the first positive electrode material layer can further reduce the occurrence of side reactions and further improve the cycle performance of the secondary battery. In addition, the good wettability of the electrolyte in the second positive electrode material layer is beneficial to further improve the kinetics of the inner positive electrode sheet in the later stage of the secondary battery cycle.

[0051] In one or more embodiments of this application, the first conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber. In this application, the conductive carbon black may include, but is not limited to, at least one of Super P, acetylene black, or Ketjen black; the carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes; the carbon fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or carbon nanofibers. Based on the mass of the first positive electrode material layer, the mass percentage of the first conductive agent is W. 12 %, 1.2≤W 12 ≤3. For example, W 12 The value can be 1.2, 1.4, 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.5, 2.6, 2.8, 3, or a range of any two of the above values. By adjusting the type and mass percentage of the first conductive agent within the above range, the first conductive agent has a suitable mass percentage, which can improve the electronic conductivity of the first positive electrode material layer, reduce the film resistance of the positive electrode sheet, and improve the rate performance of the secondary battery.

[0052] In one or more embodiments of this application, the second conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber. Based on the mass of the second positive electrode material layer, the mass percentage of the second conductive agent is W. 22 %, 1.5≤W 22 ≤2.5. For example, W 22 The value can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or a range of any two of the above values. By adjusting the type and mass percentage of the second conductive agent within the above range, the second conductive agent has a suitable mass percentage, which can improve the electronic conductivity of the second positive electrode material layer, reduce the film resistance of the positive electrode sheet, and improve the rate performance of the secondary battery.

[0053] In one or more embodiments of this application, the first conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the first conductive agent is W based on the mass of the first positive electrode material layer. 12 %, 1.2≤W 12 ≤3, for example, W 12 The value can be 1.2, 1.4, 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.5, 2.6, 2.8, 3, or a range consisting of any two of the above values; and, the second conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the second conductive agent is W based on the mass of the second positive electrode material layer. 22 %, 1.5≤W 22 ≤2.5, for example, W 22 The value can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or a range consisting of any two of the above values. By adjusting the types and mass percentages of the first and second conductive agents within the above ranges, the first and second conductive agents, with appropriate mass percentages, can improve the electronic conductivity of the first and second positive electrode material layers, further reduce the film resistance of the positive electrode sheet, and further enhance the rate performance of the secondary battery.

[0054] In one or more embodiments of this application, W 12 >W 22 W 12 and W 22 By satisfying the above size relationship and constructing a relatively sufficient conductive network structure in the first and second positive electrode material layers, the DC impedance of the secondary battery can be reduced, and the high and low temperature performance, rate performance, cycle performance and storage performance of the secondary battery can be improved.

[0055] In one or more embodiments of this application, the first binder comprises at least one selected from polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the first binder is W based on the mass of the first positive electrode material layer. 13 %, 1≤W 13 ≤3. For example, W 13The value can be 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, or a range of any two of the above values. By adjusting the type and mass percentage of the first binder within the above range, the adhesion between the first positive electrode material layer and the positive electrode current collector is improved, which can effectively reduce the anisotropy caused by expansion during the secondary battery cycle; and it can also shorten the lithium-ion transport path, which is beneficial to improving the cycle performance of the secondary battery.

[0056] In one or more embodiments of this application, the second binder comprises at least one selected from polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the second binder is W based on the mass of the second positive electrode material layer. 23 %, 0.8≤W 23 ≤2. For example, W 23 The value can be 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, or a range of any two of the above values. By adjusting the type and mass percentage of the second binder within the above range, the adhesion between the second positive electrode material layer and the positive electrode current collector is improved, which can effectively reduce the anisotropy caused by expansion during the secondary battery cycle; and it can also shorten the lithium-ion transport path, which is beneficial to improving the cycle performance of the secondary battery.

[0057] In one or more embodiments of this application, the first binder comprises at least one selected from polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the first binder is W based on the mass of the first positive electrode material layer. 13 %, 1≤W 13 ≤3, for example, W 13 The value can be 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, or a range of any two of the above values; and, the second binder includes at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the second binder is W based on the mass of the second positive electrode material layer. 23 %, 0.8≤W 23 ≤2, for example, W 23The value can be 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, or a range of any two of the above values. By adjusting the types and mass percentages of the first and second binders within the above ranges, the adhesion between the first and second positive electrode material layers and the positive electrode current collector is improved, effectively reducing the anisotropy caused by expansion during secondary battery cycling; furthermore, it can further shorten the lithium-ion transport path and further improve the cycle performance of the secondary battery.

[0058] In one or more embodiments of this application, the weight-average molecular weight of the first binder is Mw1, where 700,000 ≤ Mw1 ≤ 1,200,000. Exemplarily, the value of Mw1 can be 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1050,000, 1,100,000, 1150,000, 1,200,000, or a range consisting of any two of the above values. By adjusting the value of Mw1 within the above range, the first binder has a suitable weight-average molecular weight, resulting in better adhesion between the first positive electrode material layer and the positive electrode current collector. This effectively reduces the anisotropy caused by expansion during secondary battery cycling and shortens the lithium-ion transport path, thus improving the cycle performance of the secondary battery.

[0059] In one or more embodiments of this application, the weight-average molecular weight of the second binder is Mw2, where 700,000 ≤ Mw2 ≤ 1,200,000. Exemplarily, the value of Mw2 can be 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1050,000, 1,100,000, 1150,000, 1,200,000, or a range consisting of any two of the above values. By adjusting the value of Mw2 within the above range, the adhesion between the second positive electrode material layer and the positive electrode current collector is improved, effectively reducing anisotropy caused by expansion during secondary battery cycling; and it also shortens the lithium-ion transport path, which is beneficial for improving the cycle performance of the secondary battery.

[0060] In one or more embodiments of this application, the weight-average molecular weight of the first adhesive is Mw1, where 700,000 ≤ Mw1 ≤ 1,200,000. Exemplarily, the value of Mw1 can be 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1050,000, 1100,000, 1150,000, 1200,000, or any two of the above numbers. The values ​​are within the range specified above; and the weight-average molecular weight of the second binder is Mw2, 700,000 ≤ Mw2 ≤ 1,200,000. For example, the value of Mw2 can be 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, 1050,000, 1,100,000, 1150,000, 1200,000, or any two of the above values. By adjusting the values ​​of Mw1 and Mw2 within the above range, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is improved, which can effectively reduce the anisotropy caused by expansion during the secondary battery cycle; and can further shorten the lithium-ion transport path, further improving the cycle performance of the secondary battery.

[0061] This application does not impose any particular limitation on the method of controlling the thermal conductivity of the first and second positive electrode material layers, as long as the purpose of this application can be achieved. For example, the thermal conductivity of the first and second positive electrode material layers can be controlled by adjusting the specific surface area of ​​the first and second positive electrode active materials. For instance, increasing the specific surface area of ​​the first positive electrode active material increases the thermal conductivity of the first positive electrode material layer; or decreasing the specific surface area of ​​the first positive electrode active material decreases the thermal conductivity of the first positive electrode material layer. Similarly, increasing the specific surface area of ​​the second positive electrode active material increases the thermal conductivity of the second positive electrode material layer; or decreasing the specific surface area of ​​the second positive electrode active material decreases the thermal conductivity of the second positive electrode material layer.

[0062] For example, the thermal conductivity of the first and second positive electrode material layers can be adjusted by regulating their thicknesses. For instance, increasing the thickness of the first positive electrode material layer decreases its thermal conductivity; or decreasing its thickness increases its thermal conductivity. Similarly, increasing the thickness of the second positive electrode material layer decreases its thermal conductivity; or decreasing its thickness increases its thermal conductivity.

[0063] For example, the thermal conductivity of the first and second positive electrode material layers can be adjusted by regulating their compaction densities. For instance, increasing the compaction density of the first positive electrode material layer decreases its thermal conductivity; or, decreasing the compaction density increases its thermal conductivity. Similarly, increasing the compaction density of the second positive electrode material layer decreases its thermal conductivity; or, decreasing the compaction density increases its thermal conductivity.

[0064] This application does not impose any particular limitation on the method of controlling the Dv50 and specific surface area of ​​the first positive electrode active material, as long as the purpose of this application can be achieved. For example, the Dv50 and specific surface area of ​​the first positive electrode active material can be controlled by grinding it. For instance, the Dv50 and specific surface area of ​​the first positive electrode active material can be controlled by adjusting the grinding time. For example, when other conditions remain unchanged, extending the grinding time decreases the Dv50 and increases the specific surface area of ​​the first positive electrode active material; or, shortening the grinding time increases the Dv50 and decreases the specific surface area of ​​the first positive electrode active material.

[0065] This application does not impose any particular limitation on the method of controlling the Dv50 and specific surface area of ​​the second positive electrode active material, as long as the purpose of this application can be achieved. For example, the Dv50 and specific surface area of ​​the second positive electrode active material can be controlled by grinding it. For instance, the Dv50 and specific surface area of ​​the second positive electrode active material can be controlled by adjusting the grinding time. For example, when other conditions remain unchanged, extending the grinding time decreases the Dv50 and increases the specific surface area of ​​the second positive electrode active material; or, shortening the grinding time increases the Dv50 and decreases the specific surface area of ​​the second positive electrode active material.

[0066] This application does not impose any particular limitation on the method of controlling the powder conductivity of the first positive electrode active material and the second positive electrode active material, as long as the purpose of this application can be achieved. For example, the powder conductivity of the first positive electrode active material can be controlled by controlling the type of the first positive electrode active material; the powder conductivity of the second positive electrode active material can be controlled by controlling the type of the second positive electrode active material.

[0067] This application does not impose any particular limitation on the method of controlling the porosity of the first positive electrode material layer and the second positive electrode material layer, as long as the purpose of this application can be achieved. For example, the porosity of the first positive electrode material layer can be controlled by adjusting the Dv50, specific surface area, and mass percentage of the first positive electrode active material, the first conductive agent, and the first binder; or, the porosity of the second positive electrode material layer can be controlled by adjusting the Dv50, specific surface area, and mass percentage of the second positive electrode active material, the second conductive agent, and the second binder.

[0068] This application does not impose any particular limitation on the method of adjusting the liquid retention coefficient of the first positive electrode material layer and the second positive electrode material layer, as long as the purpose of this application can be achieved. For example, the liquid retention coefficient of the first positive electrode material layer can be adjusted by adjusting the porosity of the first positive electrode material layer, as described above; or, the liquid retention coefficient of the second positive electrode material layer can be adjusted by adjusting the porosity of the second positive electrode material layer, as described above.

[0069] In this application, the coating weights of the first and second cathode material layers can be adjusted using methods known to those skilled in the art. For example, with a fixed solid content in the slurry of the first cathode material layer, increasing the coating amount of the slurry in the first cathode material layer can increase the coating weight of the first cathode material layer; or, with a fixed solid content in the slurry of the second cathode material layer, increasing the coating amount of the slurry in the second cathode material layer can increase the coating weight of the second cathode material layer. This application does not impose any particular limitations, as long as the purpose of this application can be achieved.

[0070] This application does not impose any particular limitation on the method of controlling the mass percentage content of the first positive electrode active material, the first binder, and the first conductive agent, as long as the purpose of this application can be achieved. For example, the mass percentage content of the first positive electrode active material can be controlled by controlling the mass of the added first positive electrode active material; or, the mass percentage content of the first binder can be controlled by controlling the mass of the added first binder; or, the mass percentage content of the first conductive agent can be controlled by controlling the mass of the added first conductive agent.

[0071] This application does not impose any particular limitation on the method of controlling the mass percentage content of the second positive electrode active material, the second binder, and the second conductive agent, as long as the purpose of this application can be achieved. For example, the mass percentage content of the second positive electrode active material can be controlled by controlling the mass of the added second positive electrode active material; or, the mass percentage content of the second binder can be controlled by controlling the mass of the added second binder; or, the mass percentage content of the second conductive agent can be controlled by controlling the mass of the added second conductive agent.

[0072] This application does not impose any particular restrictions on the method of adjusting the weight-average molecular weight of the first and second adhesives, as long as the purpose of this application can be achieved. For example, commercially available first adhesives with different weight-average molecular weights can be selected, and the weight-average molecular weight of the first adhesive can be tested using the "molecular weight test" method described in this application, and then the first adhesive with the desired weight-average molecular weight can be selected. Similarly, commercially available second adhesives with different weight-average molecular weights can be selected, and the weight-average molecular weight of the second adhesive can be tested using the "molecular weight test" method described in this application, and then the second adhesive with the desired weight-average molecular weight can be selected.

[0073] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0074] This application does not impose any particular limitation on the thickness of the positive electrode current collector, the first positive electrode material layer, and the second positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 8 μm to 15 μm, the thickness of the first positive electrode material layer is 50 μm to 150 μm, and the thickness of the second positive electrode material layer is 50 μm to 150 μm.

[0075] Optionally, the positive electrode may further include a conductive layer, which may be located between the positive current collector and the first positive electrode material layer, or between the positive current collector and the second positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not particularly limit the conductive agent and binder in the conductive layer; for example, it may be at least one of the aforementioned first conductive agent and first binder.

[0076] In this application, the electrode assembly further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application does not have any particular limitation, as long as the purpose of this application is achieved.

[0077] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0078] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0079] In some embodiments of this application, the negative electrode material layer may further include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, they may be at least one of the aforementioned first conductive agent and the aforementioned first binder. This application does not impose any particular restrictions on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of this application is achieved.

[0080] In some embodiments of this application, the negative electrode material layer may further include a conductive agent, a binder, and a thickener. This application does not particularly limit the types of conductive agents, binders, and thickeners, as long as they achieve the purpose of this application. For example, the conductive agent and binder may be at least one of the aforementioned first conductive agent and first binder. The thickener may include, but is not limited to, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose. This application does not particularly limit the mass ratio of the negative electrode active material, conductive agent, binder, and thickener in the negative electrode material layer; those skilled in the art can select them according to actual needs, as long as the purpose of this application is achieved.

[0081] This application does not impose any particular limitation on the thickness of the negative electrode material layer, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode material layer is 30 μm to 80 μm.

[0082] This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 10 μm.

[0083] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it may be at least one of the first conductive agent and the first binder described above.

[0084] In this application, the electrode assembly also includes a diaphragm. This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0085] In some embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0086] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0087] In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the first binders described above. In some embodiments of this application, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0088] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 20 μm.

[0089] In this application, the secondary battery also includes an electrolyte, which comprises lithium salts and non-aqueous solvents. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0090] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0091] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0092] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal or rigid plastic. This application does not limit the type of metal or rigid plastic; the rigid casing can be a metal casing known in the art, as long as it achieves the purpose of this application. The flexible casing can be a metal-plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0093] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. In this application, the secondary battery may include, but is not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries, etc.

[0094] The preparation process of the secondary battery in this application is well known to those skilled in the art, and this application has no particular limitations. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator and negative electrode in sequence, and winding, folding and other operations as needed to obtain a wound electrode assembly, placing the electrode assembly into the housing, injecting electrolyte into the housing and sealing it to obtain the secondary battery.

[0095] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance characteristics.

[0096] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0097] Example

[0098] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0099] Test methods and equipment:

[0100] Positive electrode sampling method:

[0101] The lithium-ion batteries prepared in each embodiment and comparative example were discharged to 3.0V at a constant current of 0.5C. The lithium-ion batteries were disassembled to obtain the positive electrode sheet. The positive electrode sheet was soaked in dimethyl carbonate (DMC) for 20 minutes, and then rinsed with dimethyl carbonate and acetone in sequence. After that, it was placed in an oven and baked at 80°C for 12 hours to obtain the dried positive electrode sheet.

[0102] Sampling methods for the first positive electrode active material, the first binder, the first conductive agent, the second positive electrode active material, the second binder, and the second conductive agent:

[0103] The dried positive electrode sheet was observed and measured using a scanning electron microscope (Thermo Fisher FEI-Apreo S) to distinguish the first and second positive electrode material layers. The side with the smaller positive electrode material size was the first positive electrode material layer, and the side with the larger positive electrode material size was the second positive electrode material layer. The second positive electrode material layer was scraped off, resulting in an electrode sheet coated with the first positive electrode material layer. The electrode sheet coated with the first positive electrode material layer was immersed in liquid nitrogen for 5 minutes to embrittle it, and the powder of the first positive electrode material layer was scraped off with a blade. The powder of the first positive electrode material layer was passed through a 400-mesh sieve (pore size 38 μm) to remove positive electrode current collector fragments. The sieved powder of the first positive electrode material layer was then vacuum dried at 60°C for 12 hours to remove electrolyte residue. Subsequently, bromoform (CHBr3, density 2.89 g / cm³) was used. 3 ) and n-hexane (C6H 14 Its density is 0.66 g / cm³. 3 Mixed solutions of different densities were obtained by mixing at mass ratios of 9:1 and 4:6. The mixed solutions of different densities were then injected into different centrifuge tubes. 20 mg of the dried first positive electrode material powder was dispersed in 1 mL of isopropanol and sonicated for 10 min (300 W) to obtain a suspension of the first positive electrode material powder. The suspension of the first positive electrode material powder was then injected into centrifuge tubes. Using a syringe, the aforementioned 9:1 mixed solution was slowly injected into the top of the first positive electrode material layer powder suspension in a centrifuge tube. After centrifugation at 2000 rpm for 120 min, based on the principle of density difference, the first positive electrode active material would be enriched in the lower layer of the centrifuge tube. The upper layer of solution (denoted as supernatant A, including the first binder and the first conductive agent) and the lower layer solution containing the first positive electrode active material were then aspirated into different centrifuge tubes. Next, the aforementioned 4:6 mixed solution was injected into the top of the supernatant A. After centrifugation at 2000 rpm for 120 min, the first binder was enriched in the lower layer of the centrifuge tube, and the first conductive agent was enriched in the upper layer. The upper layer solution containing the first conductive agent and the lower layer solution containing the first binder were then aspirated into different centrifuge tubes. By sequentially centrifuging in this way, solutions containing the first positive electrode active material, the first binder, and the first conductive agent were obtained, respectively. After vacuum drying, solid powders of the first positive electrode active material, the first binder, and the first conductive agent were obtained, respectively. Similarly, after scraping off the first positive electrode material layer, the electrode sheet can be used to obtain the second positive electrode active material, the second binder, and the second conductive agent in the same way as described above.

[0104] The above method was used to sample the first and second positive electrode active materials in the following specific surface area test and powder conductivity test.

[0105] In the following mass percentage tests of positive electrode active material, conductive agent, and binder, the first positive electrode active material, the first binder, the first conductive agent, the second positive electrode active material, the second binder, and the second conductive agent were sampled using the above method.

[0106] Thermal conductivity test:

[0107] The second positive electrode material layer in the dried positive electrode sheet is scraped off to obtain the electrode sheet to be tested coated with the first positive electrode material layer. The electrode sheet to be tested is cut into 30mm × 30mm pieces. The cut electrode sheets are assembled into a sample in the following order: heating plate - electrode sheet to be tested - cooling plate. The sample is pre-compressed to a contact pressure of 0.02MPa, and the electrode sheet to be tested is heated to a set temperature difference ΔT (the temperature difference here refers to the temperature difference between the two sides of the electrode sheet, in K) by the heating plate at a heating rate of ≤2℃ / min. The temperature of the electrode sheet to be tested is continuously monitored until the electrode sheet to be tested satisfies the relationship: ΔT / T-average < 0.5%, and then maintained for 30min. During this period, the electrode sheet to be tested must satisfy the aforementioned relationship; where T-average refers to the average temperature of the surface of the electrode sheet to be tested near the heating plate during the heating process and during the sustained time interval. Subsequently, the heat flux value Q of the electrode sheet to be tested after temperature stabilization can be measured by a heat flux sensor, in W. According to Fourier's law, the thermal conductivity of the first positive electrode material layer is calculated as: (Q×Δx) / (A×ΔT), with units of W / (m·K); where A is the effective heat transfer area (i.e., the actual contact area between the electrode under test and the heating plate), with units of m². 2 Δx represents the sample thickness in meters (m). Two cut electrode sheets were tested in each embodiment and comparative example, and the average value was taken as the thermal conductivity λ1 of the first positive electrode material layer.

[0108] Similarly, the thermal conductivity λ2 of the second positive electrode material layer can be tested on the electrode after the first positive electrode material layer has been scraped off, using the method described above.

[0109] Particle size Dv50 test:

[0110] Take the dried positive electrode sheet and scrape off the first positive electrode material layer powder with a blade. Soak the first positive electrode material layer powder in water for 1 hour, with a mass ratio of the first positive electrode material layer powder to water of 1:5. Disperse the powder using a single bar disperser at 1800 rpm for 3 hours to obtain the first slurry. Use a Mastersizer 3000 laser particle size analyzer to test the particle size Dv50 of the substances in the first slurry, which is the Dv50 of the first positive electrode active material. The test method refers to the national standard "Particle Size Distribution Laser Diffraction Method" (GB / T19077-2016).

[0111] Take the dried positive electrode sheet and scrape off the second positive electrode material layer powder with a blade. Soak the second positive electrode material layer powder in water for 1 hour, with a mass ratio of second positive electrode material layer powder to water of 1:5. Disperse the powder using a single bar disperser at 1800 rpm for 3 hours to obtain the second slurry. Use a Mastersizer 3000 laser particle size analyzer to test the particle size Dv50 of the material in the second slurry, which is the Dv50 of the second positive electrode active material. The test method refers to the national standard "Particle Size Distribution Laser Diffraction Method" (GB / T19077-2016).

[0112] Specific surface area test:

[0113] According to the national standard "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method" (GB / T 19587-2017), a specific surface area analyzer (model TristarⅡ3020M) was used to test the specific surface area of ​​the first positive electrode active material and the second positive electrode active material by gas adsorption method.

[0114] Powder conductivity test:

[0115] The powder conductivity of the positive electrode active material was tested using a resistivity meter and an electronic pressure testing machine. The correction factor F of the resistivity meter was set to 1. The powder conductivity test mold was placed on the platform of the electronic pressure testing machine, and the pressure was increased to 1000 kg before zeroing the displacement and deformation values. 0.5 g of the positive electrode active material powder was added to the test mold, vibrated for 10 seconds, and then the upper gasket was added. The test mold was placed on the platform of the electronic pressure testing machine and connected to the resistivity meter. The electronic pressure testing machine was pressurized to 6 MPa, held at that pressure for 60 seconds, and then depressurized to zero. The sample deformation height and the resistivity meter reading (representing the sample resistance) were recorded.

[0116] The electrical conductivity of powder is calculated using the following formula: σ = h / (S × R).

[0117] In the formula: σ is the powder conductivity of the positive electrode active material, in S / m; h is the sample deformation height, in m; S is the sample bottom area, which is 3.14 cm². 2 R is the sample resistance, in Ω.

[0118] The powder conductivity of the first and second positive electrode active materials was tested using the method described above.

[0119] Porosity test:

[0120] A diameter of Φ 15.8 mm (area 2 cm²) is punched into the positive electrode material layer using a precision die. 2 Small discs were baked to constant weight (mass change < 0.01 mg) under a pressure P < 100 Pa at 80℃ for 4 hours. Porosity was tested using the helium displacement method, with ≥10 cycles and a relative standard deviation (RSD) < 0.05%. The porosity of the positive electrode material layer was calculated as follows:

[0121] Porosity K(%) of the positive electrode material layer = (1-ρ1 / ρ2)×100%;

[0122] Where, apparent density ρ1=m / (a×t); m is the mass of the small disc, a is the area of ​​the small disc, and t is the thickness of the small disc;

[0123] True density ρ2=m / V He m is the mass of the small disc, V He This represents the volume of helium used during the measurement.

[0124] The second positive electrode material layer in the positive electrode sheet is scraped off to obtain an electrode sheet coated with the first positive electrode material layer. The electrode sheet coated with the first positive electrode material layer is immersed in liquid nitrogen for 5 minutes to embrittle it, and then mechanically peeled off to obtain the first positive electrode material layer. The porosity of the first positive electrode material layer is then tested according to the above method.

[0125] The first positive electrode material layer is scraped off from the positive electrode sheet to obtain an electrode sheet coated with a second positive electrode material layer. The electrode sheet coated with the second positive electrode material layer is immersed in liquid nitrogen for 5 minutes to embrittle it, and then mechanically peeled off to obtain the second positive electrode material layer. The porosity of the second positive electrode material layer is then tested according to the above method.

[0126] Liquid retention coefficient test:

[0127] Take the dried positive electrode sheet described above, scrape off the second positive electrode material layer from the positive electrode sheet to obtain an electrode sample coated with the first positive electrode material layer. Weigh the dried electrode sample in a dry environment and record the weight as Wd. Immerse the electrode sample completely in the electrolyte and soak it at a constant temperature of 25°C for 60 minutes to ensure the electrode sample is fully wetted. Then remove the electrode sample and gently wipe off the excess electrolyte on the surface with filter paper, while avoiding squeezing the internal electrolyte. Immediately weigh the wet electrode sample and record the weight as Ww. Calculate the liquid retention coefficient N1 of the first positive electrode material layer according to the following formula: Liquid retention coefficient N1 of the first positive electrode material layer = (Ww - Wd) / Capacity of the lithium-ion battery, in mg / mAh. The method for measuring the capacity of the lithium-ion battery is as follows: Charge the lithium-ion battery at a constant current of 0.5C to 4.3V, charge it at a constant voltage of 4.3V to 0.05C, let it stand for 5 minutes, and then discharge it at a constant current of 0.5C to 3.0V to obtain the capacity of the lithium-ion battery. The electrolyte described above is the same as the electrolyte used in Examples 1-1.

[0128] Similarly, the electrode after scraping off the first positive electrode material layer can be tested for the liquid retention coefficient N2 of the second positive electrode material layer using the method described above.

[0129] Molecular weight test:

[0130] 0.01g of the first binder was dissolved in 5mL of solvent (N-methylpyrrolidone) to obtain a solution. After the solution was completely dissolved, impurities in the solution were filtered out using a filter head. Then, the weight-average molecular weight of the first binder was determined using a gel permeation chromatography (PL-GPC220) instrument. The weight-average molecular weight of the second binder was determined using the same method.

[0131] Mass percentage test of positive electrode active material, conductive agent, and binder:

[0132] By weighing, the mass of the first positive electrode active material is m1 mg, the mass of the first conductive agent is m2 mg, and the mass of the first binder is m3 mg. Then W 11 = (m1 / 20)×100%, W 12 = (m2 / 20)×100%, W 13 = (m3 / 20)×100%.

[0133] By weighing, the mass of the second positive electrode active material was found to be m4 mg, the mass of the second conductive agent was found to be m5 mg, and the mass of the second binder was found to be m6 mg. Therefore, W... 21 = (m4 / 20)×100%, W 22 = (m5 / 20)×100%, W 23 = (m6 / 20) × 100%.

[0134] Diaphragm resistance test:

[0135] Cut the dried positive electrode sample into 60mm × 80mm pieces and place them on the sample stage base. Cover the sample with the top cover to ensure it covers the test holes. Place the sample stage into the test chamber and close the protective door. Open the BER1200 film resistance meter software, select continuous test mode, and measure 12 parallel points for each positive electrode sample. Measure 3 positive electrode samples and take the average value as the film resistance of the positive electrode.

[0136] Adhesion test:

[0137] The dried positive electrode sheet was observed and measured using a scanning electron microscope (Thermo Fisher FEI-Apreo S) to distinguish the first and second positive electrode material layers. The side with the smaller positive electrode material size was the first positive electrode material layer, and the side with the larger positive electrode material size was the second positive electrode material layer. The positive electrode sheet was cut into samples with a length × width of 100 mm × 30 mm. Three-quarters of the sample was adhered to a steel plate with double-sided tape pre-applied, with one side of the first positive electrode material layer bonded to the double-sided tape. The remaining one-quarter of the sample was fixed together with a piece of A4 paper of the same size using wrinkle adhesive. The adhesion between the first positive electrode material layer and the positive current collector was tested using a high-speed rail tensile testing machine (GT-7010-EP). Four parallel samples were tested, and the average value was taken to obtain the final result. Then, the second positive electrode material layer was bonded to one side of the double-sided tape using the same method, and the adhesion between the second positive electrode layer and the positive current collector was tested.

[0138] DC internal resistance (DCR) test:

[0139] At 25°C, the lithium-ion battery is charged to 4.3V at a constant current of 0.5C, then charged to 0.05C at a constant voltage of 4.3V, and left to stand for 10 minutes. Then it is discharged at a constant current of 0.1C for 1 hour (at which point the lithium-ion battery is at 90% state of charge). Then it is discharged at a constant current of 1C for 1 second. The voltage before the 1C constant current discharge is V0, the voltage after the 1C constant current discharge is V1, and the current of the 1C constant current discharge is A. Calculate the DC impedance R corresponding to 90% state of charge (SOC) of the lithium-ion battery as (V0-V1) / A.

[0140] Ratio performance test:

[0141] The lithium-ion battery was placed in a constant temperature environment of 25°C and left to stand for 30 minutes to allow it to reach a constant temperature. The battery was then charged at a constant current of 0.5C to 4.3V, followed by constant voltage charging at 4.3V to 0.05C. After standing for 5 minutes, it was discharged at a constant current of 0.5C to 3.0V to obtain the 0.5C discharge capacity of the lithium-ion battery.

[0142] The lithium-ion battery was charged at a constant current of 0.5C to 4.3V, then charged at a constant voltage of 4.3V to 0.05C. After resting for 5 minutes, it was discharged at a constant current of 2C to 3.0V to obtain the 2C discharge capacity of the lithium-ion battery. Dividing the 2C discharge capacity by the 0.5C discharge capacity gives the 2C capacity retention rate.

[0143] 2C capacity retention rate (%) = 2C discharge capacity / 0.5C discharge capacity × 100%.

[0144] Temperature rise test:

[0145] (1) Take a lithium-ion battery and test its temperature rise at 25°C.

[0146] (2) Let stand for 10 minutes;

[0147] (3) Charge at a constant current of 2C to 4.2V, then charge at a constant voltage of 4.2V to 0.025C;

[0148] (4) Let stand for 30 minutes;

[0149] (5) Discharge at a constant current of 0.2C to 2.5V;

[0150] (6) Let stand for 15 minutes;

[0151] (7) Charge at a constant current of 2C to 4.2V, then charge at a constant voltage of 4.2V to 0.05C;

[0152] (8) Let stand for 60 minutes;

[0153] (9) Discharge at 10C constant current to 2.5V; (sampling time is 10s)

[0154] (10) Let stand for 60 minutes;

[0155] (11) At 25℃, a multi-channel thermometer (model LR8401-21, HIOKI) was used to test the temperature at the geometric center of the lithium-ion battery surface in step (9) and the temperature at the center point of the width direction near the tab end. The average of the highest point temperature recorded at the two locations was taken as the 10C discharge temperature rise at 25℃.

[0156] The 10s sampling method refers to the multi-channel temperature measuring instrument's testing system recording a set of temperature parameters of the tested lithium-ion battery every 10 seconds.

[0157] Cyclic performance test:

[0158] The lithium-ion battery was placed in a constant temperature environment of 25°C and allowed to stand for 30 minutes to reach a constant temperature. The battery was then charged at a constant current of 0.5C to 4.3V, followed by constant voltage charging at 4.3V to 0.05C. After standing for 5 minutes, it was discharged at a constant current of 1C to 3.0V. This constitutes one charge-discharge cycle, and the discharge capacity obtained at this point is recorded as the initial discharge capacity C0. Using the initial discharge capacity as 100%, the charge-discharge cycle was repeated. After 500 cycles, the test was stopped, and the discharge capacity of the lithium-ion battery at this point was recorded as the discharge capacity after 500 cycles, C1. The cycle capacity retention rate of the lithium-ion battery was calculated using the following formula.

[0159] Cyclic capacity retention (%) = C1 / C0 × 100%.

[0160] Example 1-1

[0161] <Preparation of the positive electrode>

[0162] The first positive electrode active material LiNi 0.9 Co 0.05 Mn 0.05 O2, the first binder polyvinylidene fluoride (PVDF), and the first conductive agent Super P are mixed in a weight ratio of 95:2:3. N-methylpyrrolidone (NMP) is added as a solvent and stirred until homogeneous to obtain the first positive electrode material layer slurry. The solid content of the first positive electrode material layer slurry is 68wt%, and the Dv50 (D1) of the first positive electrode active material is 5μm.

[0163] The second positive electrode active material LiNi 0.9 Co 0.04 Mn 0.06 O2, the second binder polyvinylidene fluoride (PVDF), and the second conductive agent Super P are mixed in a weight ratio of 96.5:1.5:2. N-methylpyrrolidone (NMP) is added as a solvent and the mixture is stirred and mixed evenly to obtain a second positive electrode material layer slurry. The solid content of the second positive electrode material layer slurry is 70wt%, and the Dv50 (D2) of the second positive electrode active material is 12μm.

[0164] A first positive electrode material slurry was uniformly coated onto the first surface of a 12 μm thick aluminum foil used as a positive electrode current collector. The foil was then dried at 120°C for 1 hour to obtain a positive electrode sheet with a 60 μm thick first positive electrode material layer on one side. A second positive electrode material slurry was then uniformly coated onto the second surface of the aluminum foil and dried at 120°C for 1 hour to obtain a positive electrode sheet with the first positive electrode material layer on one side and the second positive electrode material layer on the other side; the second positive electrode material layer has a thickness of 60 μm. After drying under vacuum at 120°C for 1 hour, the sheet was cold-pressed, cut, and slit to obtain a positive electrode sheet with dimensions of 74 mm × 867 mm. The compaction density during the cold pressing process was 3.7 g / cm³. 3 The surface tension of the positive current collector is 40 dyn / cm.

[0165] <Preparation of Negative Electrode Sheets>

[0166] Artificial graphite (anode active material), carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) were mixed in a weight ratio of 96:2.5:1.5. Deionized water was added as a solvent, and the mixture was stirred until homogeneous to obtain a cathode slurry with a solid content of 55 wt%. The cathode slurry was uniformly coated onto one surface of an 8 μm thick copper foil current collector and dried at 120°C to obtain a cathode sheet with a 40 μm thick cathode material layer on one side. The above steps were repeated on the other surface of the copper foil to obtain a cathode sheet with a double-sided cathode material layer. The cathode sheet was dried under vacuum at 120°C for 1 hour, and then cold-pressed, cut, and slit to obtain a cathode sheet with a size of 78 mm × 875 mm. The compaction density during the cold pressing process was 1.6 g / cm³. 3 .

[0167] <Preparation of Electrolyte>

[0168] In an argon-atmospheric glove box with a water content of less than 10 ppm, ethylene carbonate (EC) and propylene carbonate (PC) were mixed at a weight ratio of 1:1 to obtain a base solvent. Then, lithium hexafluorophosphate (LiPF6) was added and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the lithium salt content was 12.5% ​​by mass, with the remainder being the base solvent.

[0169] <Preparation of the diaphragm>

[0170] A polyethylene (PE) film with a thickness of 15 μm is used.

[0171] <Preparation of Lithium-ion Batteries>

[0172] The positive electrode, separator, negative electrode, and separator prepared above are stacked in sequence, with the separator positioned between the positive and negative electrode to act as a separator, and then wound to obtain the electrode assembly. The first positive electrode material layer faces away from the winding center, and the second positive electrode material layer faces the winding center. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and subjected to hot pressing, forming, top sealing, vacuum drying, electrolyte injection, high-temperature settling, degassing, formation, and capacity testing to obtain a lithium-ion battery.

[0173] Examples 1-2 to Examples 1-18

[0174] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0175] Examples 2-1 to 2-4

[0176] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-2.

[0177] Comparative Examples 1 to 9

[0178] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0179] The preparation parameters and electrical performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0180]

[0181]

[0182]

[0183]

[0184] As can be seen from Examples 1-1 to 1-18 and Comparative Examples 1 to 9, by adjusting the relationship between the Dv50 of the first positive electrode active material and the second positive electrode active material, and the thermal conductivity of the first positive electrode material layer and the second positive electrode material layer within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is higher, the DC impedance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This shows that it can effectively improve the wettability and liquid retention capacity of the positive electrode sheet, and improve the cycle performance and rate performance of the secondary battery. The Dv50 of the first positive electrode active material and / or the second positive electrode active material in Comparative Examples 1 to 5 and Comparative Examples 7 to 9 is not within the scope of this application. The relationship between the thermal conductivity of the first positive electrode material layer and the second positive electrode material layer in Comparative Example 6 is not within the scope of this application. The film resistance of the positive electrode sheet is low, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is low, the DC impedance of the lithium-ion battery is high, the 2C capacity retention rate is low, the temperature rise is high, and the cycle capacity retention rate is low, indicating that the cycle performance and rate performance of the lithium-ion battery are poor.

[0185] The specific surface area of ​​the first positive electrode active material affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1-2, 1-8 to 1-10, by adjusting the specific surface area of ​​the first positive electrode active material within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is higher, the DC resistance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0186] The specific surface area of ​​the second positive electrode active material affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1-2, 1-13 to 1-16, by adjusting the specific surface area of ​​the second positive electrode active material within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, resulting in lower DC resistance, higher 2C capacity retention, lower temperature rise, and higher cycle capacity retention of the lithium-ion battery. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0187] The powder conductivity of the first positive electrode active material affects the rate performance and cycle performance of lithium-ion batteries. As can be seen from Examples 1-2, 1-8 to 1-10, by adjusting the powder conductivity of the first positive electrode active material within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, resulting in lower DC impedance, higher 2C capacity retention, lower temperature rise, and higher cycle capacity retention of the lithium-ion battery. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0188] The powder conductivity of the second positive electrode active material affects the rate performance and cycle performance of lithium-ion batteries. As can be seen from Examples 1-2, 1-13 to 1-16, by adjusting the powder conductivity of the second positive electrode active material within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, resulting in lower DC impedance, higher 2C capacity retention, lower temperature rise, and higher cycle capacity retention of the lithium-ion battery. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0189] The porosity of the first and second positive electrode material layers affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-18, by adjusting the porosity of the first positive electrode material layer within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first and second positive electrode material layers and the positive electrode current collector is higher, and the DC resistance, 2C capacity retention, temperature rise, and cycle capacity retention of the lithium-ion battery are lower. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0190] The liquid retention coefficients of the first and second positive electrode material layers affect the cycle performance, storage performance, and rate performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-18, by adjusting the liquid retention coefficient of the first positive electrode material layer within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first and second positive electrode material layers and the positive current collector is higher, and the DC resistance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0191] The coating weight of the first positive electrode material layer affects the cycle performance and rate performance of the lithium-ion battery. As can be seen from Examples 1-2, 1-11 to 1-12, by adjusting the coating weight of the first positive electrode material layer within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is higher, the DC resistance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby improving the cycle performance and rate performance of the secondary battery.

[0192] The coating weight of the second positive electrode material layer affects the cycle performance and rate performance of the lithium-ion battery. As can be seen from Examples 1-2, 1-17 to 1-18, by adjusting the coating weight of the second positive electrode material layer within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is higher, the DC resistance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby improving the cycle performance and rate performance of the secondary battery.

[0193] The mass percentage of the first positive electrode active material, the first conductive agent, and the first binder affects the rate performance and cycle performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-4, by adjusting the mass percentage of the first positive electrode active material, the first conductive agent, and the first binder within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, and the DC impedance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0194] The mass percentage of the second positive electrode active material, the second conductive agent, and the second binder affects the rate performance and cycle performance of the lithium-ion battery. As can be seen from Examples 1-2, 1-5 to 1-7, by adjusting the mass percentage of the second positive electrode active material, the second conductive agent, and the second binder within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, and the DC impedance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0195] Table 2

[0196]

[0197] Note: In Table 2, the weight-average molecular weight Mw1 of the first adhesive in Examples 1-2 is "80W", which means that the weight-average molecular weight Mw1 of the first adhesive is 800,000. The same applies to other examples.

[0198] The weight-average molecular weight of the first binder affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1-2 and 2-1 to 2-2, by adjusting the weight-average molecular weight of the first binder within the range of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer, and the positive electrode current collector is higher, resulting in lower DC impedance, higher 2C capacity retention, lower temperature rise, and higher cycle capacity retention of the lithium-ion battery. This indicates that the wettability and liquid retention capacity of the positive electrode sheet can be effectively improved, thereby enhancing the cycle performance and rate performance of the secondary battery.

[0199] The weight-average molecular weight of the second binder affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1-2, 2-3 to 2-4, by adjusting the weight-average molecular weight of the second binder within the scope of this application, the film resistance of the positive electrode sheet is lower, the adhesion between the first positive electrode material layer, the second positive electrode material layer and the positive electrode current collector is higher, the DC impedance of the lithium-ion battery is lower, the 2C capacity retention rate is higher, the temperature rise is lower, and the cycle capacity retention rate is higher. This indicates that it can effectively improve the wettability and liquid retention capacity of the positive electrode sheet, and improve the cycle performance and rate performance of the secondary battery.

[0200] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0201] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0202] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising an electrode assembly with a wound structure, the electrode assembly comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector, a first positive electrode material layer and a second positive electrode material layer, wherein along the thickness direction of the positive electrode sheet, the positive current collector comprises a first surface and a second surface opposite to each other, the surface of the positive current collector facing away from the winding center is the first surface, and the surface of the positive current collector facing the winding center is the second surface; At least a portion of the first surface is provided with the first positive electrode material layer, and at least a portion of the second surface is provided with the second positive electrode material layer. The first positive electrode material layer includes a first positive electrode active material, a first conductive agent, and a first binder. The second positive electrode material layer includes a second positive electrode active material, a second conductive agent, and a second binder. in, The thermal conductivity of the first positive electrode material layer is λ1 W / (m·K), and the thermal conductivity of the second positive electrode material layer is λ2 W / (m·K), where λ1 > λ2; 0.7≤λ1≤1.2, and / or, 0.4≤λ2≤0.6; The Dv50 of the first positive electrode active material is D1μm, and the Dv50 of the second positive electrode active material is D2μm, where 2≤D1≤6 and 8≤D2≤15.

2. The secondary battery according to claim 1, wherein, The specific surface area of ​​the first positive electrode active material is B1m. 2 / g, 0.4≤B1≤0.8, and / or, the specific surface area of ​​the second positive electrode active material is B2 m². 2 / g, 0.3≤B2≤0.

5.

3. The secondary battery according to claim 1, wherein, The powder conductivity of the first positive electrode active material is σ1S / m, 5.5≤σ1≤7.5, and / or the powder conductivity of the second positive electrode active material is σ2S / m, 2.5≤σ2≤5.

4. The secondary battery 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 iron manganese phosphate, or lithium cobalt oxide, and the second positive electrode active material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, or lithium cobalt oxide.

5. The secondary battery according to claim 1, wherein, The porosity of the first positive electrode material layer is K1%, 15≤K1≤25, and / or the porosity of the second positive electrode material layer is K2%, 27≤K2≤40.

6. The secondary battery according to claim 1, wherein, The liquid retention coefficient of the first positive electrode material layer is N1 mg / mAh, 5≤N1≤8, and / or the liquid retention coefficient of the second positive electrode material layer is N2 mg / mAh, 9≤N2≤12.

7. The secondary battery according to claim 1, wherein, The coating weight of the first positive electrode material layer is CW1 mg / cm³. 2 10≤CW1≤20, and / or, the coating weight of the second positive electrode material layer is CW2 mg / cm³. 2 , 9≤CW2≤18.

8. The secondary battery according to claim 1, wherein, Based on the mass of the first positive electrode material layer, the mass percentage of the first positive electrode active material is W. 11 %, 95≤W 11 ≤97.8; and / or, Based on the mass of the second positive electrode material layer, the mass percentage of the second positive electrode active material is W. 21 %, 95.5≤W 21 ≤97.

7.

9. The secondary battery according to claim 1, wherein, The first conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the first conductive agent is W based on the mass of the first positive electrode material layer. 12 %, 1.2≤W 12 ≤3; and / or, The second conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, or carbon fiber, and the mass percentage of the second conductive agent is W based on the mass of the second positive electrode material layer. 22 %, 1.5≤W 22 ≤2.

5.

10. The secondary battery according to claim 9, wherein, IN 12 >W 22 。 11. The secondary battery according to claim 1, wherein, The first binder comprises at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the first binder is W based on the mass of the first positive electrode material layer. 13 %, 1≤W 13 ≤3; and / or, The second binder comprises at least one of polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyvinylidene fluoride, or sodium alginate, and the mass percentage of the second binder is W based on the mass of the second positive electrode material layer. 23 %, 0.8≤W 23 ≤2.

12. The secondary battery according to claim 1, wherein, The first adhesive has a weight-average molecular weight of Mw1, 700,000 ≤ Mw1 ≤ 1200,000, and / or the second adhesive has a weight-average molecular weight of Mw2, 700,000 ≤ Mw2 ≤ 1200,000.

13. An electronic device comprising a secondary battery according to any one of claims 1 to 12.

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