Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device. The secondary battery comprises an electrode assembly, and the electrode assembly comprises a positive pole piece, a negative pole piece, a first diaphragm and a second diaphragm. The negative pole piece comprises a first negative pole material layer and a second negative pole material layer. The first diaphragm is located between the first positive electrode material layer and the second negative electrode material layer, and the second diaphragm is located between the second positive electrode material layer and the first negative electrode material layer. The first diaphragm comprises a first organic coating, the first organic coating is adjacent to the first positive electrode material layer, the coating weight of the first organic coating is M1 mg / dm < 2 >, the second diaphragm comprises a second organic coating, the second organic coating is adjacent to the second positive electrode material layer, the coating weight of the second organic coating is M2 mg / dm < 2 >, M1 is greater than M2, 5.0 < = M1 < = 16.0, and 2.0 < = M2 < = 8.0. Through the arrangement, the cycling stability and the safety performance of the secondary battery are improved while the energy density and the dynamic performance are considered.

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] Secondary batteries, such as lithium-ion batteries, have the characteristics of high specific energy, high operating voltage, low self-discharge rate, small size, and light weight, and are widely used in the consumer electronics field.

[0003] Electrode components in secondary batteries often use a wound structure. The corner areas of this structure have arc-shaped sections. Due to the difference in internal and external conditions, the negative electrode in these arc-shaped sections experiences greater compression on the side where the positive electrode surrounds the negative electrode. During cycling, a large amount of electrolyte is squeezed out from the negative electrode on the positive-to-negative-electrode side and from the separator, potentially leading to insufficient electrolyte and lithium deposition at the interface, posing a safety risk. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device to improve the lithium plating problem at the corner interface, thereby improving the cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

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

[0006] The first aspect of this application provides a secondary battery, including a wound electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, a first separator, and a second separator. The negative electrode includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The negative current collector includes a first surface and a second surface, with the first surface closer to the winding center of the electrode assembly than the second surface. The first negative electrode material layer is disposed on the first surface, and the second negative electrode material layer is disposed on the second surface. The positive electrode includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The positive current collector includes a third surface and a fourth surface, with the third surface closer to the winding center of the electrode assembly than the fourth surface. The first positive electrode material layer is disposed on the third surface, and the second positive electrode material layer is disposed on the fourth surface. The first separator is located between the first positive electrode material layer and the second negative electrode material layer, and the second separator is located between the second positive electrode material layer and the first negative electrode layer. The first separator includes a first organic coating and a first base film. The first organic coating is adjacent to the first positive electrode material layer, and the coating weight of the first organic coating is M1 mg / dm³. 2 The second separator includes a second organic coating and a second base membrane. The second organic coating is adjacent to the second positive electrode material layer, and the coating weight of the second organic coating is M2 mg / dm.2 M1 > M2, 5.0 ≤ M1 ≤ 16.0, 2.0 ≤ M2 ≤ 8.0. By employing two separators with different organic coating weights in the wound electrode assembly and controlling the coating weights of the two separators within the above range, expansion space is reserved for the second negative electrode material layer far from the winding center of the electrode assembly. During cycling, this alleviates the problem of electrolyte squeezing or insufficiency caused by electrode expansion, effectively improves the problem of lithium plating in the arc segment, and thus improves the cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0007] In one or more embodiments of this application, 1.00 < M1 / M2 ≤ 2.00. By adjusting the value of M1 / M2 within the above range, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while taking into account both energy density and kinetic performance.

[0008] In one or more embodiments of this application, the first organic coating comprises a first organic material, and the second organic coating comprises a second organic material. Each of the first and second organic materials independently comprises at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyamide, polyimide, or cellulose. By selecting the aforementioned first and second organic materials, it is beneficial to improve the membrane's affinity for the electrolyte and its electrolyte retention performance, while simultaneously enhancing the interfacial adhesion and mechanical support between the membrane and the electrode. Therefore, while considering energy density and kinetic performance, the secondary battery exhibits good cycle stability and safety performance.

[0009] In one or more embodiments of this application, the weight-average molecular weight of the first organic compound is Mw1 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw1≤5.0×10 5 ; and / or, the weight-average molecular weight of the second organic compound is Mw2 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw2≤5.0×10 5 By adjusting the values ​​of Mw1 and / or Mw2 within the above range, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while taking into account both energy density and kinetic performance.

[0010] In one or more embodiments of this application, the compressibility of the first separator is A1 μm, and the compressibility of the second separator is A2 μm, where A1 > A2. Through this configuration, the first and second separators work synergistically, which helps to further reduce the risk of lithium plating at the corners, improving the cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0011] In one or more embodiments of this application, 7.0 ≤ A1 ≤ 15.0; and / or, 4.0 ≤ A2 ≤ 9.0. By adjusting the values ​​of A1 and / or A2 within the above ranges, the first and second separators work synergistically to further reduce the risk of lithium plating at the corners, thereby improving the cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0012] In one or more embodiments of this application, the secondary battery is charged at a constant current of 1C to 4.35V, and then charged at a constant voltage of 4.35V to a current of 0.1C, with a second negative electrode material layer thickness of D1 μm; the secondary battery is discharged at a constant current of 0.5C to 2.5V, with a second negative electrode material layer thickness of D0 μm, satisfying: 1.05≤D1 / D0≤1.15, 7.0≤A1≤8.0; 1.15<D1 / D0≤1.35, 8.0<A1≤13.0; 1.35<D1 / D0≤1.70, 13.0<A1≤15.0. By adjusting the values ​​of D1 / D0 and A1 to satisfy the above relationships, and based on the expansion rate of the second negative electrode material layer during the secondary battery cycle, a first separator with different compressibility is selected, which helps to further reduce the risk of lithium plating at the corner, and improves the cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0013] In one or more embodiments of this application, the thickness of the first separator is T1 μm, 12.0 ≤ T1 ≤ 25.0; and / or, the thickness of the second separator is T2 μm, 9.0 ≤ T2 ≤ 16.0. By adjusting the values ​​of T1 and / or T2 within the above ranges, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0014] In one or more embodiments of this application, the permeability of the first separator is n1 secs / 100cc, and the permeability of the second separator is n2 secs / 100cc, where 0.5 ≤ n1 / n2 ≤ 0.9 and 40.0 ≤ n1 ≤ 120.0. By adjusting the values ​​of n1 / n2 and n1 within the above ranges, the first separator exhibits good ion conductivity, reducing the risk of lithium plating due to ion accumulation and facilitating the uniform embedding of lithium ions in the corner region. Therefore, the secondary battery possesses good cycle stability and safety performance.

[0015] In one or more embodiments of this application, the ratio of the areal capacity of the second negative electrode material layer to the areal capacity of the first positive electrode material layer is 1.03 to 1.20; and / or, the ratio of the areal capacity of the first negative electrode material layer to the areal capacity of the second positive electrode material layer is 1.03 to 1.20. With the above configuration, the capacity redundancy of the second negative electrode material layer and / or the first negative electrode material layer helps reduce the risk of lithium plating on the negative electrode, thereby ensuring good cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0016] In one or more embodiments of this application, the ratio of the areal capacity of the second negative electrode material layer to the areal capacity of the first negative electrode material layer is 1.00 to 1.05. This configuration helps to reduce the volume expansion of the second negative electrode material layer, alleviate the problem of electrolyte compression or insufficiency caused by electrode expansion, and reduce the risk of lithium plating. Therefore, while balancing energy density and kinetic performance, the secondary battery exhibits good cycle stability and safety performance.

[0017] In one or more embodiments of this application, the first separator further includes a first inorganic coating located between the first organic coating and the first base film. The first inorganic coating comprises a first inorganic material, which includes at least one of alumina or boehmite. And / or, the second separator further includes a second inorganic coating located between the second organic coating and the second base film. The second inorganic coating comprises a second inorganic material, which includes at least one of alumina or boehmite. This configuration improves the mechanical strength of the separator, effectively reducing the risk of thermal shrinkage and lithium dendrite puncture. Simultaneously, it further enhances the structural stability and electrolyte wettability of the separator, thereby improving the cycle stability and safety performance of the secondary battery while maintaining energy density and kinetic performance.

[0018] A second aspect of this application provides an electronic device including the secondary battery in any of the above embodiments. Therefore, the electronic device has good performance characteristics.

[0019] The beneficial effects of this application are:

[0020] This application provides a secondary battery and electronic device. By employing two separators with different organic coating weights in the wound electrode assembly and adjusting the coating weights of the two separators within the aforementioned range, expansion space is reserved for the second negative electrode material layer far from the winding center of the electrode assembly. During cycling, this alleviates the problem of electrolyte squeezing or insufficiency caused by electrode expansion, effectively improves the problem of lithium plating in the arc segment, and thus improves the cycle stability and safety performance of the secondary battery while maintaining energy density.

[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 an electrode assembly with a wound structure in one embodiment of this application.

[0024] Reference numerals: Electrode assembly 001; Negative electrode 10; Negative current collector 11; First negative electrode material layer 12; Second negative electrode material layer 13; Positive electrode 20; Positive current collector 21; First positive electrode material layer 22; Second positive electrode material layer 23; First separator 31; Second separator 32. Detailed Implementation

[0025] 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.

[0026] 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.

[0027] Currently, in wound electrode assemblies, during cycling, a large amount of electrolyte is squeezed out from the negative electrode and the separator on the positive electrode side, potentially leading to insufficient electrolyte, lithium plating at the interface, and even safety risks. Current solutions mainly address this by controlling the porosity of the separator on different sides of the negative electrode. While this considers the electrolyte transport rate, it doesn't completely solve the problem of insufficient electrolyte on the electrode when expansion is excessive. Therefore, this application provides a secondary battery and electronic device that uses two different separators on both sides of the positive electrode in a wound electrode assembly. This reduces the compressive stress on the electrode during cycling, lowers the risk of lithium plating on the arc-shaped electrode segment due to electrolyte squeezing, and improves the cycle stability and safety performance of the secondary battery. The specific technical solution is as follows:

[0028] The first aspect of this application provides a secondary battery, including a wound electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, a first separator, and a second separator. The negative electrode includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The negative current collector includes a first surface and a second surface, with the first surface closer to the winding center of the electrode assembly than the second surface. The first negative electrode material layer is disposed on the first surface, and the second negative electrode material layer is disposed on the second surface. The positive electrode includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The positive current collector includes a third surface and a fourth surface, with the third surface closer to the winding center of the electrode assembly than the fourth surface. The first positive electrode material layer is disposed on the third surface, and the second positive electrode material layer is disposed on the fourth surface. The first separator is located between the first positive electrode material layer and the second negative electrode material layer, and the second separator is located between the second positive electrode material layer and the first negative electrode layer. The first separator includes a first organic coating and a first base film. The first organic coating is adjacent to the first positive electrode material layer, and the coating weight of the first organic coating is M1 mg / dm³. 2 The second separator includes a second organic coating and a second base membrane. The second organic coating is adjacent to the second positive electrode material layer, and the coating weight of the second organic coating is M2 mg / dm. 2 M1 > M2, 5.0 ≤ M1 ≤ 16.0, 2.0 ≤ M2 ≤ 8.0. For example, the value of M1 can be 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, or a range consisting of any two of these values. The range of M1 can be 5.0 to 16.0, 6.0 to 15.0, 7.0 to 14.0, 8.0 to 13.0, 9.0 to 12.0, 10.0 to 11.0, and all ranges therein. The value of M2 can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, or any two of these values. The value range of M2 can be 2.0 to 8.0, 2.5 to 7.5, 3.0 to 7.0, 3.5 to 6.5, 4.0 to 6.0, 4.5 to 5.5, and all of these ranges, as well as subranges.

[0029] In this application, the winding direction of the electrode assembly is defined as the W direction. For example, as shown... Figure 1As shown, the electrode assembly 001 includes a negative electrode 10, a positive electrode 20, a first separator 31, and a second separator 32. The negative electrode 10 includes a negative current collector 11, a first negative electrode material layer 12, and a second negative electrode material layer 13. The negative current collector 11 includes a first surface and a second surface, with the first surface closer to the winding center of the electrode assembly 001 than the second surface. The first negative electrode material layer 12 is disposed on the first surface, and the second negative electrode material layer 13 is disposed on the second surface. The positive electrode 20 includes a positive current collector 21, a first positive electrode material layer 22, and a second positive electrode material layer 23. The positive current collector 21 includes a third surface and a fourth surface, with the third surface closer to the winding center of the electrode assembly 001 than the fourth surface. The first positive electrode material layer 22 is disposed on the third surface, and the second positive electrode material layer 23 is disposed on the fourth surface. The first separator 31 is located between the first positive electrode material layer 22 and the second negative electrode material layer 13, and the second separator 32 is located between the second positive electrode material layer 23 and the first negative electrode material layer 12.

[0030] The inventors discovered that by using two different organic coating weights on both sides of the positive electrode sheet of a wound electrode assembly, with the first separator located between the first positive electrode material layer and the second negative electrode material layer, and the second negative electrode material layer being farther from the winding center and thus expanding more during cycling, the coating weight of the first organic coating can be controlled within the aforementioned range. A thicker first organic coating exhibits better compressibility and elasticity, reserving expansion space for the second negative electrode material layer, which is farther from the winding center of the electrode assembly. This alleviates the problem of electrolyte compression or insufficient electrolyte caused by electrode sheet expansion. Furthermore, the higher the coating weight of the first organic coating, the stronger the electrolyte adsorption and retention capacity. Sufficient electrolyte is present at the interface of the second negative electrode material layer, which helps maintain the ion conduction pathway during cycling and reduces the risk of lithium plating. The second separator is located between the second positive electrode material layer and the first negative electrode material layer. The first negative electrode material layer is close to the winding center and expands less during cycling. By controlling the coating weight of the second organic coating within the above range, the thinner second separator reduces the stacking thickness, alleviates the mechanical constraint of the inner arc of the electrode assembly, and provides space for the slight expansion of the first negative electrode material layer. Furthermore, a lower coating weight of the second organic coating facilitates the penetration and diffusion of electrolyte in the inner arc region, reducing the risk of local concentration polarization. When the coating weight of the first organic coating and / or the second organic coating is too large, for example, exceeding the upper limit of this application, the energy density of the secondary battery decreases, and the internal resistance of the separator increases, resulting in reduced kinetic performance of the secondary battery and increased cycle temperature rise. Therefore, this application, by employing two organic coatings with different weights of separators in the wound electrode assembly, alleviates the problem of electrolyte squeezing or insufficiency caused by electrode expansion during cycling, effectively improves the problem of lithium plating in the arc segment, and can improve the cycle stability and safety performance of the secondary battery while taking into account the energy density and kinetic performance of the secondary battery.

[0031] It should be noted that, as mentioned above, "the first negative electrode material layer is disposed on the first surface," the first negative electrode material layer can be disposed on the entire area of ​​the first surface of the negative electrode current collector, or it can be disposed on a portion of the first surface of the negative electrode current collector; as mentioned above, "the second negative electrode material layer is disposed on the second surface," the second negative electrode material layer can be disposed on the entire area of ​​the second surface of the negative electrode current collector, or it can be disposed on a portion of the second surface of the negative electrode current collector; as mentioned above, "the first positive electrode material layer is disposed on the third surface," the first positive electrode material layer can be disposed on the entire area of ​​the third surface of the positive electrode current collector, or it can be disposed on a portion of the third surface of the positive electrode current collector; as mentioned above, "the second positive electrode material layer is disposed on the fourth surface," the second positive electrode material layer can be disposed on the entire area of ​​the fourth surface of the positive electrode current collector, or it can be disposed on a portion of the fourth surface of the positive electrode current collector. This application has no particular limitations, as long as the purpose of this application can be achieved.

[0032] In one or more embodiments of this application, 1.00 < M1 / M2 ≤ 2.00. For example, the value of M1 / M2 can be 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, or a range consisting of any two values ​​therein. The value range of M1 / M2 can be 1.00 (excluding 1.00) to 2.00, 1.10 to 1.90, 1.20 to 1.80, 1.30 to 1.70, 1.40 to 1.60, and all such ranges and sub-ranges. By adjusting the value of M1 / M2 within the aforementioned range, the organic coating weight of the first separator is relatively large. This provides sufficient expansion buffer space and electrolyte retention for electrode expansion during cycling, reducing the risk of lithium plating in this area due to electrode expansion and electrolyte compression. Simultaneously, the moderate difference in organic coating thickness between the inner and outer separators balances the energy density of the secondary battery and reduces the risk of decreased liquid retention and kinetic performance at the interface between the inner electrode and separator due to an excessively thin organic coating. This balances the interfacial stability and ion transport efficiency of the inner and outer arc segments of the wound structure. Therefore, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while considering both energy density and kinetic performance.

[0033] In one or more embodiments of this application, the first organic coating comprises a first organic material, and the second organic coating comprises a second organic material. Each of the first and second organic materials independently comprises at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyamide, polyimide, or cellulose. By selecting the aforementioned first and second organic materials, it is beneficial to improve the membrane's affinity for the electrolyte and its electrolyte retention performance, reducing the risk of lithium plating at the corner electrode-membrane interface due to insufficient electrolyte during cycling; simultaneously, it enhances the interfacial adhesion and mechanical support between the membrane and the electrode. Therefore, it is advantageous to achieve good cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0034] In one or more embodiments of this application, the weight-average molecular weight of the first organic compound is Mw1 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw1≤5.0×10 5 For example, the value of Mw1 can be 5.0 × 10. 3 8.0×10 3 1.0×10 4 3.0×10 4 5.0×10 4 8.0×10 4 1.0×10 5 1.2×10 5 1.5×10 5 1.8×10 5 2.0×10 5 2.2×10 5 2.5×10 5 2.8×10 5 3.0×10 5 3.2×10 5 3.5×10 5 3.8×10 5 4.0×10 5 4.2×10 5 4.5×10 5 4.8×10 5 5.0×10 5 It can be any range consisting of any two of these values; the range of Mw1 can be 5.0 × 10⁻⁶. 3 Up to 5.0×10 5 1.0×10 4 Up to 4.5×10 5 5.0×10 4 Up to 4.0×10 5 1.0×10 5 Up to 3.5×105 1.5×10 5 Up to 3.0×10 5 2.0×10 5 Up to 2.5×10 5 And all of its ranges, and sub-ranges. By adjusting the value of Mw1 within the above range, it is beneficial to improve the affinity and electrolyte retention performance of the first separator, reducing the risk of lithium plating at the interface between the corner electrode and the first separator due to insufficient electrolyte during cycling; at the same time, it enhances the interfacial adhesion and mechanical support between the first separator and the electrode. Therefore, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0035] In one or more embodiments of this application, the weight-average molecular weight of the second organic compound is Mw2 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw2≤5.0×10 5 For example, the value of Mw2 can be 5.0 × 10. 3 8.0×10 3 1.0×10 4 3.0×10 4 5.0×10 4 8.0×10 4 1.0×10 5 1.2×10 5 1.5×10 5 1.8×10 5 2.0×10 5 2.2×10 5 2.5×10 5 2.8×10 5 3.0×10 5 3.2×10 5 3.5×10 5 3.8×10 5 4.0×10 5 4.2×10 5 4.5×10 5 4.8×10 5 5.0×10 5 It can be any range consisting of two of these values; the range of Mw2 can be 5.0 × 10⁻⁶. 3 Up to 5.0×10 5 1.0×10 4 Up to 4.5×10 5 5.0×10 4 Up to 4.0×10 5 1.0×10 5 Up to 3.5×105 1.5×10 5 Up to 3.0×10 5 2.0×10 5 Up to 2.5×10 5 And all of its ranges, and sub-ranges. By adjusting the value of Mw2 within the above range, it is beneficial to improve the affinity and electrolyte retention performance of the second separator, reducing the risk of lithium plating at the interface between the corner electrode and the second separator due to insufficient electrolyte during cycling; at the same time, it enhances the interfacial adhesion and mechanical support between the second separator and the electrode. Therefore, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0036] In one or more embodiments of this application, the weight-average molecular weight of the first organic compound is Mw1 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw1≤5.0×10 5 For example, the value of Mw1 can be 5.0 × 10. 3 8.0×10 3 1.0×10 4 3.0×10 4 5.0×10 4 8.0×10 4 1.0×10 5 1.2×10 5 1.5×10 5 1.8×10 5 2.0×10 5 2.2×10 5 2.5×10 5 2.8×10 5 3.0×10 5 3.2×10 5 3.5×10 5 3.8×10 5 4.0×10 5 4.2×10 5 4.5×10 5 4.8×10 5 5.0×10 5 It can be any range consisting of any two of these values; the range of Mw1 can be 5.0 × 10⁻⁶. 3 Up to 5.0×10 5 1.0×10 4 Up to 4.5×10 5 5.0×10 4 Up to 4.0×10 5 1.0×10 5 Up to 3.5×105 1.5×10 5 Up to 3.0×10 5 2.0×10 5 Up to 2.5×10 5 And all of its ranges, and sub-ranges; and / or, the weight-average molecular weight of the second organic compound is Mw²g / mol, 5.0 × 10⁻⁶. 3 ≤Mw2≤5.0×10 5 For example, the value of Mw2 can be 5.0 × 10. 3 8.0×10 3 1.0×10 4 3.0×10 4 5.0×10 4 8.0×10 4 1.0×10 5 1.2×10 5 1.5×10 5 1.8×10 5 2.0×10 5 2.2×10 5 2.5×10 5 2.8×10 5 3.0×10 5 3.2×10 5 3.5×10 5 3.8×10 5 4.0×10 5 4.2×10 5 4.5×10 5 4.8×10 5 5.0×10 5 It can be any range consisting of two of these values; the range of Mw2 can be 5.0 × 10⁻⁶. 3 Up to 5.0×10 5 1.0×10 4 Up to 4.5×10 5 5.0×10 4 Up to 4.0×10 5 1.0×10 5 Up to 3.5×10 5 1.5×10 5 Up to 3.0×10 5 2.0×10 5 Up to 2.5×10 5And all of these ranges, as well as sub-ranges. By adjusting the values ​​of Mw1 and / or Mw2 within the aforementioned ranges, it is beneficial to improve the membrane's affinity for the electrolyte and its electrolyte retention performance, reducing the risk of lithium plating at the corner electrode-membrane interface due to insufficient electrolyte during cycling; at the same time, it enhances the interfacial adhesion and mechanical support between the membrane and the electrode. Therefore, while balancing energy density and kinetic performance, the secondary battery exhibits good cycle stability and safety performance.

[0037] This application does not impose any particular restrictions on the source of the first organic compound and the second organic compound, as long as the purpose of this application can be achieved. For example, different commercially available first organic compounds and second organic compounds can be purchased according to the various parameters of the required first organic compound and second organic compound (such as type, weight average molecular weight, etc.).

[0038] In one or more embodiments of this application, the compressibility of the first separator is A1 μm, and the compressibility of the second separator is A2 μm, where A1 > A2. With this configuration, the first separator provides sufficient expansion space for the second negative electrode material layer, while the second separator provides relatively stable mechanical support, which helps ensure the integrity of the inner ion transport channels. The synergistic effect of the first and second separators further reduces the risk of lithium plating at the corners, improving the cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0039] In one or more embodiments of this application, 7.0 ≤ A1 ≤ 15.0. For example, the value of A1 can be 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0, or a range consisting of any two of these values. The value range of A1 can be 7.0 to 15.0, 8.0 to 14.0, 9.0 to 13.0, 10.0 to 12.0, and all of these ranges, as well as sub-ranges. By adjusting the value of A1 within the above range, the first separator provides sufficient expansion space for the second negative electrode material layer. Combined with the second separator, this helps to further reduce the risk of lithium plating at the corners, improving the cycle stability and safety performance of the secondary battery while maintaining energy density.

[0040] In one or more embodiments of this application, 4.0 ≤ A2 ≤ 9.0. For example, the value of A2 can be 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, or a range consisting of any two of these values. The value range of A2 can be 4.0 to 9.0, 4.5 to 8.5, 5.0 to 8.0, 5.5 to 7.5, 6.0 to 7.0, and all of these ranges, as well as sub-ranges. By adjusting the value of A2 within the above range, the second separator provides relatively stable mechanical support, which is beneficial to ensuring the integrity of the inner ion transport channel. Combined with the first separator, it is beneficial to further reduce the risk of lithium plating at the corners, and improve the cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0041] In one or more embodiments of this application, 7.0 ≤ A1 ≤ 15.0. For example, the value of A1 can be 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0, or a range consisting of any two of these values. The value range of A1 can be 7.0 to 15.0, 8.0 to 14.0, 9.0 to 13.0, etc. The values ​​of A1 and / or A2 are within the ranges of 10.0 to 12.0 and all subranges thereof; and / or, 4 ≤ A2 ≤ 9. For example, the value of A2 can be 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, or any two of these values. The range of A2 values ​​can be 4.0 to 9.0, 4.5 to 8.5, 5.0 to 8.0, 5.5 to 7.5, 6.0 to 7.0 and all subranges thereof. By adjusting the values ​​of A1 and / or A2 within the above ranges, the first separator provides sufficient expansion space for the second negative electrode material layer. At the same time, the second separator provides relatively stable mechanical support, which is beneficial to ensuring the integrity of the inner ion transport channel. The synergistic effect of the first and second separators helps to further reduce the risk of lithium plating at the corners, improving the cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0042] In one or more embodiments of this application, the secondary battery is charged at a constant current of 1C to 4.35V, and charged at a constant voltage of 4.35V to a current of 0.1C, with the thickness of the second negative electrode material layer being D1 μm; the secondary battery is discharged at a constant current of 0.5C to 2.5V, with the thickness of the second negative electrode material layer being D0 μm, satisfying: 1.05≤D1 / D0≤1.15, 7.0≤A1≤8.0; 1.15<D1 / D0≤1.35, 8.0<A1≤13.0; 1.35<D1 / D0≤1.70, 13.0<A1≤15.0. For example, when 1.05 ≤ D1 / D0 ≤ 1.15, the value of A1 can be 7.0, 7.2, 7.4, 7.6, 7.8, 8.0, or any two of these values ​​within a range. The value range of A1 can be 7.0 to 8.0, 7.2 to 7.8, 7.4 to 7.6, and all of these ranges, as well as subranges. When 1.15 < D1 / D0 ≤ 1.35, the value of A1 can be 8.1, 8.5, 9.0, 10.0, 11.0, 12.0, 13.0, or any two of these values ​​within a range. The value range of A1 can be 8.0 (excluding 8.0) to 13. 0, 8.1 to 12.0, 8.5 to 11.0, 9.0 to 10.0 and all of these ranges and subranges; when 1.35 < D1 / D0 ≤ 1.70, the value of A1 can be 13.1, 13.2, 13.5, 13.8, 14.0, 14.2, 14.5, 14.8, 15.0 or any two of these values. The value range of A1 can be 13.0 (excluding 13.0) to 15.0, 13.1 to 14.8, 13.2 to 14.5, 13.5 to 14.2, 13.8 to 14.0 and all of these ranges and subranges. By adjusting the values ​​of D1 / D0 and A1 to satisfy the above relationship, and based on the expansion rate of the second negative electrode material layer during the secondary battery cycle, a first separator with different compressibility is selected. This is beneficial to reserve sufficient expansion space for the second negative electrode material layer while reducing the loss of energy density in the secondary battery. Combined with the second separator, it is beneficial to further reduce the risk of lithium plating at the corner. While taking into account energy density and kinetic performance, it improves the cycle stability and safety performance of the secondary battery.

[0043] In one or more embodiments of this application, the thickness of the first separator is T1 μm, where 12.0 ≤ T1 ≤ 25.0. For example, the value of T1 can be 12.0, 15.0, 18.0, 20.0, 22.0, 25.0, or a range consisting of any two of these values. The value range of T1 can be 12.0 to 25.0, 15.0 to 22.0, 18.0 to 20.0, and all such ranges and sub-ranges. By adjusting the value of T1 within the above range, expansion space is reserved for the second negative electrode material layer, which is far from the winding center of the electrode assembly. This alleviates the problem of electrolyte compression or insufficiency caused by electrode expansion, ensuring sufficient electrolyte at the interface of the second negative electrode material layer. This is beneficial for maintaining the ion conduction pathway during cycling and reducing the risk of lithium plating. Therefore, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while balancing energy density and kinetic performance.

[0044] In one or more embodiments of this application, the thickness of the second separator is T2 μm, where 9.0 ≤ T2 ≤ 16.0. For example, the value of T2 can be 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, or a range consisting of any two of these values. The range of T2 can be 9.0 to 16.0, 10.0 to 15.0, 11.0 to 14.0, 12.0 to 13.0, and all of these ranges, as well as sub-ranges. By adjusting the value of T2 within the above range, the stacking thickness is reduced, the mechanical constraint of the inner arc of the electrode assembly is alleviated, space is provided for the slight expansion of the first negative electrode material layer, and the penetration and diffusion of the electrolyte in the inner arc region is facilitated, reducing the risk of local concentration polarization. Therefore, it is beneficial to achieve good cycle stability and safety performance of the secondary battery while taking into account energy density and kinetic performance.

[0045] In one or more embodiments of this application, the thickness of the first diaphragm is T1 μm, 12.0 ≤ T1 ≤ 25.0. For example, the value of T1 can be 12.0, 15.0, 18.0, 20.0, 22.0, 25.0, or a range consisting of any two of these values. The value range of T1 can be 12.0 to 25.0, 15.0 to 22.0, 18.0 to 20.0, and all such ranges and sub-ranges; and / or, the thickness of the second diaphragm is T2. μm, 9.0≤T2≤16.0, for example, the value of T2 can be 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0 or any two of these values. The range of T2 can be 9.0 to 16.0, 10.0 to 15.0, 11.0 to 14.0, 12.0 to 13.0 and all of these ranges, as well as sub-ranges. By adjusting the values ​​of T1 and / or T2 within the above ranges, expansion space is reserved for the second negative electrode material layer far from the winding center of the electrode assembly, alleviating the problem of electrolyte compression or insufficiency caused by electrode expansion. This ensures sufficient electrolyte at the interface of the second negative electrode material layer, which is beneficial for maintaining the ion conduction pathway during cycling and reducing the risk of lithium plating. At the same time, the stacking thickness is reduced, alleviating the mechanical constraint of the inner arc of the electrode assembly, providing space for the slight expansion of the first negative electrode material layer, and facilitating the penetration and diffusion of electrolyte in the inner arc region, reducing the risk of local concentration polarization. Therefore, it is beneficial to ensure that secondary batteries have good cycle stability and safety performance while taking into account both energy density and kinetic performance.

[0046] In one or more embodiments of this application, the air permeability of the first diaphragm is n1 secs / 100cc, the air permeability of the second diaphragm is n2 secs / 100cc, 0.5≤n1 / n2≤0.9, and 40.0≤n1≤120.0. For example, the value of n1 / n2 can be 0.5, 0.6, 0.7, 0.8, 0.9, or any two of these values. The range of n1 / n2 can be from 0.5 to 0.9, from 0.6 to 0.8, and all of these ranges, as well as sub-ranges. The value of n1 can be 40.0, 50.0, 60.0, 70.0, 80.0, 90.0, 100.0, 110.0, 120.0, or any two of these values. The range of n1 can be from 40.0 to 120.0, 50.0 to 110.0, 60.0 to 100.0, 70.0 to 90.0, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of n1 / n2 and n1 within the above range, the first separator has good ion conductivity, which is conducive to establishing a high-speed ion transport channel at the interface between the second negative electrode material layer, which expands more, and the separator. This reduces the risk of lithium plating caused by ion accumulation and facilitates the uniform embedding of lithium ions in the corner region. Therefore, the secondary battery has good cycle stability and safety performance.

[0047] In one or more embodiments of this application, 70.0 ≤ n2 ≤ 150.0. For example, the value of n2 can be 70.0, 80.0, 90.0, 100.0, 110.0, 120.0, 130.0, 140.0, 150.0, or a range consisting of any two of these values. The value range of n2 can be 70.0 to 150.0, 80.0 to 140.0, 90.0 to 130.0, 100.0 to 120.0, and all of these ranges, as well as sub-ranges. By adjusting the value of n2 within the above range, the second separator has good ion conductivity, which is beneficial for establishing a high-speed ion transport channel at the interface between the first negative electrode material layer and the separator, reducing the risk of local concentration polarization. Therefore, while taking into account energy density and kinetic performance, the secondary battery has good cycle stability and safety performance.

[0048] In one or more embodiments of this application, the ratio X1 of the areal capacity of the second negative electrode material layer to the areal capacity of the first positive electrode material layer is 1.03 to 1.20. For example, the value of X1 can be 1.03, 1.05, 1.08, 1.10, 1.12, 1.15, 1.18, 1.20, or any range of two values ​​therein. The value range of X1 can be 1.03 to 1.20, 1.05 to 1.18, 1.08 to 1.15, 1.10 to 1.12, and all such ranges and sub-ranges. By adjusting the value of X1 within the above range, the capacity redundancy of the second negative electrode material layer is achieved. During the charging of the secondary battery, this helps to ensure that there are sufficient vacancies in the second negative electrode material layer to accommodate lithium ions released from the first positive electrode material layer, reducing the risk of lithium plating on the negative electrode. Therefore, while considering energy density and kinetic performance, the secondary battery has good cycle stability and safety performance.

[0049] In one or more embodiments of this application, the ratio X2 of the areal capacity of the first negative electrode material layer to the areal capacity of the second positive electrode material layer is 1.03 to 1.20. For example, the value of X2 can be 1.03, 1.05, 1.08, 1.10, 1.12, 1.15, 1.18, 1.20, or a range consisting of any two of these values. The value range of X2 can be 1.03 to 1.20, 1.05 to 1.18, 1.08 to 1.15, 1.10 to 1.12, and all such ranges and sub-ranges. By adjusting the value of X2 within the above range, the capacity redundancy of the first negative electrode material layer is achieved. During the charging of the secondary battery, this helps to ensure that there are sufficient vacancies in the first negative electrode material layer to accommodate lithium ions released from the second positive electrode material layer, reducing the risk of lithium plating on the negative electrode. Therefore, while considering energy density and kinetic performance, the secondary battery has good cycle stability and safety performance.

[0050] In one or more embodiments of this application, the ratio X1 of the areal capacity of the second negative electrode material layer to the areal capacity of the first positive electrode material layer is 1.03 to 1.20. For example, the value of X1 can be 1.03, 1.05, 1.08, 1.10, 1.12, 1.15, 1.18, 1.20, or a range consisting of any two of these values. The value range of X1 can be 1.03 to 1.20, 1.05 to 1.18, 1.08 to 1.15, 1.10 to 1.12, and all ranges therewith. And sub-ranges; and / or, the ratio X2 of the areal capacity of the first negative electrode material layer to the areal capacity of the second positive electrode material layer is 1.03 to 1.20, for example, the value of X2 can be 1.03, 1.05, 1.08, 1.10, 1.12, 1.15, 1.18, 1.20 or any two of these values, and the range of X2 can be 1.03 to 1.20, 1.05 to 1.18, 1.08 to 1.15, 1.10 to 1.12 and all of these ranges, and sub-ranges. By adjusting the values ​​of X1 and / or X2 within the aforementioned range, the capacity redundancy of the first negative electrode material layer and / or the second negative electrode material layer is achieved. During the charging of the secondary battery, this facilitates ensuring that the first negative electrode material layer has sufficient vacancies to accommodate lithium ions released from the second positive electrode material layer, and vice versa. This reduces the risk of lithium plating on the negative electrode. Therefore, while balancing energy density and kinetic performance, the secondary battery exhibits good cycle stability and safety performance.

[0051] In one or more embodiments of this application, the ratio X3 of the areal capacity of the second negative electrode material layer to the areal capacity of the first negative electrode material layer is 1.00 to 1.05. For example, the value of X3 can be 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, or a range consisting of any two of these values. The value range of X3 can be 1.00 to 1.05, 1.01 to 1.14, 1.02 to 1.03, and all of these ranges, as well as sub-ranges. By adjusting the value of X3 within the above range, it is beneficial to have more vacancies in the second negative electrode material layer to accommodate lithium ions released from the first positive electrode material layer, thereby reducing the utilization rate of the second negative electrode material layer, reducing the volume expansion of the second negative electrode material layer, alleviating the problem of electrolyte compression or insufficiency caused by electrode expansion, and reducing the risk of lithium plating. Therefore, while taking into account energy density and kinetic performance, the secondary battery has good cycle stability and safety performance.

[0052] In this application, the ratio of the areal capacity of the first negative electrode material layer to the areal capacity of the second positive electrode material layer can be controlled by adjusting the areal capacity of the first negative electrode material layer and / or the areal capacity of the second positive electrode material layer. Furthermore, the areal capacity of the first negative electrode material layer can be controlled by adjusting the type of negative electrode active material in the first negative electrode material layer, or by adjusting the coating weight of the first negative electrode material layer slurry during the preparation of the negative electrode sheet. When the negative electrode active material in the first negative electrode material layer includes silicon-containing materials, the areal capacity of the first negative electrode material layer can also be controlled by adjusting the mass percentage of silicon-containing materials and / or the type of silicon-containing materials in the negative electrode active material. Furthermore, the areal capacity of the second positive electrode material layer can be controlled by adjusting the type of positive electrode active material in the second positive electrode material layer, or by adjusting the coating weight of the second positive electrode material layer slurry during the preparation of the positive electrode sheet. The areal capacity of the second negative electrode material layer can be adjusted to control the ratio of its to the first positive electrode material layer by adjusting the areal capacity of the second negative electrode material layer and / or the areal capacity of the first positive electrode material layer. Furthermore, the areal capacity of the second negative electrode material layer can be adjusted by controlling the type of negative electrode active material in the second negative electrode material layer, or by adjusting the coating weight of the second negative electrode material layer slurry during the preparation of the negative electrode sheet. When the negative electrode active material in the second negative electrode material layer includes silicon-containing materials, the areal capacity of the second negative electrode material layer can also be adjusted by controlling the mass percentage of silicon-containing materials and / or the type of silicon-containing materials in the negative electrode active material. Similarly, the areal capacity of the first positive electrode material layer can be adjusted by controlling the type of positive electrode active material in the first positive electrode material layer, or by adjusting the coating weight of the first positive electrode material layer slurry during the preparation of the positive electrode sheet. The ratio of the areal capacity of the second negative electrode material layer to the areal capacity of the first negative electrode material layer can be adjusted by adjusting the areal capacity of the second negative electrode material layer and / or the areal capacity of the first negative electrode material layer.

[0053] In one or more embodiments of this application, the first separator further includes a first inorganic coating located between the first organic coating and the first base film. The first inorganic coating comprises a first inorganic material, including at least one of alumina or boehmite. This configuration improves the mechanical strength of the first separator, effectively reducing the risk of thermal shrinkage and lithium dendrite puncture. Simultaneously, it further enhances the structural stability and electrolyte wettability of the first separator. Therefore, while considering energy density and kinetic performance, it further improves the cycle stability and safety performance of the secondary battery.

[0054] In one or more embodiments of this application, the second separator further includes a second inorganic coating located between the second organic coating and the second base film. The second inorganic coating comprises a second inorganic material, including at least one of alumina or boehmite. This configuration improves the mechanical strength of the second separator, effectively reducing the risk of thermal shrinkage and lithium dendrite puncture. It also further enhances the structural stability and electrolyte wettability of the second separator. Therefore, while considering energy density and kinetic performance, it further improves the cycle stability and safety performance of the secondary battery.

[0055] In one or more embodiments of this application, the first separator further includes a first inorganic coating located between the first organic coating and the first base film. The first inorganic coating comprises a first inorganic material, which includes at least one of alumina or boehmite. And / or, the second separator further includes a second inorganic coating located between the second organic coating and the second base film. The second inorganic coating comprises a second inorganic material, which includes at least one of alumina or boehmite. This configuration improves the mechanical strength of the separator, effectively reducing the risk of thermal shrinkage and lithium dendrite puncture. Simultaneously, it further enhances the structural stability and electrolyte wettability of the separator. Therefore, while considering energy density and kinetic performance, it further improves the cycle stability and safety performance of the secondary battery.

[0056] In one or more embodiments of this application, the first separator includes a first adhesive layer, and the first adhesive layer and a first organic coating are respectively located on two surfaces of the first base film; the first adhesive layer includes a first adhesive, which includes at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyamide, polyimide, or cellulose. This configuration enhances the interfacial adhesion between the first separator and the electrode, improving the safety performance and cycle stability of the secondary battery.

[0057] In one or more embodiments of this application, the second separator includes a second adhesive layer, and the second adhesive layer and the second organic coating are respectively located on two surfaces of the second base membrane; the second adhesive layer includes a second adhesive, which includes at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyamide, polyimide, or cellulose. This configuration enhances the interfacial adhesion between the second separator and the electrode, improving the safety performance and cycle stability of the secondary battery.

[0058] In this application, the permeability of the first membrane can be controlled by adjusting the weight-average molecular weight Mw1 of the first organic compound, the pore size of the first base membrane, or the number of pores in the first base membrane. For example, when other conditions are constant, increasing the value of Mw1 decreases the permeability of the first membrane, and vice versa; or, when other conditions are constant, increasing the pore size of the first base membrane decreases the permeability of the first membrane, and vice versa; or, when other conditions are constant, increasing the number of pores in the first base membrane decreases the permeability of the first membrane, and vice versa. Similarly, the permeability of the second membrane can be controlled by adjusting the weight-average molecular weight Mw2 of the second organic compound, the pore size of the second base membrane, or the number of pores in the second base membrane. For example, when other conditions are constant, an increase in the value of Mw2 decreases the permeability of the second diaphragm, and vice versa; or, when other conditions are constant, an increase in the pore size in the second base membrane decreases the permeability of the second diaphragm, and vice versa; or, when other conditions are constant, an increase in the number of pores in the second base membrane decreases the permeability of the second diaphragm, and vice versa.

[0059] In this application, the type of the first organic compound also affects the permeability of the first membrane. When the type of the first organic compound changes, the permeability of the first membrane is also affected. Similarly, the type of the second organic compound also affects the permeability of the second membrane. When the type of the second organic compound changes, the permeability of the second membrane is also affected.

[0060] In this application, the compressibility of the first membrane can be controlled by adjusting the coating weight M1 of the first organic coating or the weight-average molecular weight Mw1 of the first organic compound. For example, when other conditions are constant, increasing the value of M1 increases the compressibility of the first membrane, and vice versa; when other conditions are constant, increasing the value of Mw1 increases the compressibility of the first membrane, and vice versa. Similarly, the compressibility of the second membrane can be controlled by adjusting the coating weight M2 of the second organic coating or the weight-average molecular weight Mw2 of the second organic compound. For example, when other conditions are constant, increasing the value of M2 increases the compressibility of the second membrane, and vice versa; when other conditions are constant, increasing the value of Mw2 increases the compressibility of the second membrane, and vice versa.

[0061] In this application, the type of the first organic compound also affects the compressibility of the first diaphragm. When the type of the first organic compound changes, the compressibility of the first diaphragm is also affected. Similarly, the type of the second organic compound also affects the compressibility of the second diaphragm. When the type of the second organic compound changes, the compressibility of the second diaphragm is also affected.

[0062] In this application, the thickness of the first separator can be controlled by adjusting the coating weight M1 of the first organic coating, the coating weight of the first inorganic coating, the thickness of the first base film, or the weight-average molecular weight Mw1 of the first organic compound. For example, when other conditions are constant, increasing the value of M1 increases the thickness of the first organic coating and the thickness of the first separator, and vice versa; or, when other conditions are constant, increasing the coating weight of the first inorganic coating increases the thickness of the first inorganic coating and the thickness of the first separator, and vice versa; or, when other conditions are constant, increasing the thickness of the first base film increases the thickness of the first separator, and vice versa; or, when other conditions are constant, increasing the value of Mw1 increases the thickness of the first organic coating and the thickness of the first separator, and vice versa. The thickness of the second separator can be controlled by adjusting the coating weight M2 of the second organic coating, the coating weight of the second inorganic coating, the thickness of the second base film, or the weight-average molecular weight Mw2 of the second organic compound. For example, when other conditions are constant, an increase in the value of M2 leads to an increase in the thickness of the second organic coating and the thickness of the second diaphragm, and vice versa; or, when other conditions are constant, an increase in the coating weight of the second inorganic coating leads to an increase in the thickness of the second inorganic coating and the thickness of the second diaphragm, and vice versa; or, when other conditions are constant, an increase in the thickness of the second base film leads to an increase in the thickness of the second diaphragm, and vice versa; or, when other conditions are constant, an increase in the value of Mw2 leads to an increase in the thickness of the second organic coating and the thickness of the second diaphragm, and vice versa.

[0063] This application does not impose any particular restrictions on the types of the first base film and the second base film, as long as they can achieve the purpose of this application. For example, the materials of the first base film and the second base film may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide.

[0064] This application does not impose any particular limitation on the preparation method of the first diaphragm, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the first diaphragm includes, but is not limited to, the following steps: (1) preparing a first organic coating slurry; (2) coating the first organic coating slurry on one surface of the first base film, and drying it to obtain the first diaphragm. In other embodiments, the preparation method of the first diaphragm includes, but is not limited to, the following steps: (1) preparing a first organic coating slurry and a first inorganic coating slurry; (2) coating the first inorganic coating slurry on one surface of the first base film, and drying it to obtain a first diaphragm with a first inorganic coating on one side; (3) coating the first organic coating slurry on the surface of the first inorganic coating away from the first base film, and drying it to obtain the first diaphragm. In still other embodiments, the preparation method of the first diaphragm includes, but is not limited to, the following steps: (1) preparing a first organic coating slurry and a first adhesive layer slurry; (2) coating the first organic coating slurry on one surface of the first base film, and drying it to obtain a first diaphragm with a first organic coating on one side; (3) coating the first adhesive layer slurry on the other surface of the first base film, and drying it to obtain the first diaphragm. In some other embodiments, the preparation method of the first diaphragm includes, but is not limited to, the following steps: (1) preparing a first organic coating slurry, a first inorganic coating slurry and a first adhesive layer slurry; (2) coating the first inorganic coating slurry on one surface of the first base film, and after drying, obtaining a first diaphragm with a first inorganic coating on one side; (3) coating the first organic coating slurry on the surface of the first inorganic coating away from the first base film, and after drying, obtaining a first diaphragm with a first inorganic coating and a first organic coating on one side; (4) coating the first adhesive layer slurry on the other surface of the first base film, and after drying, obtaining the first diaphragm.

[0065] This application does not impose any particular limitation on the preparation method of the second diaphragm, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the second diaphragm includes, but is not limited to, the following steps: (1) preparing a second organic coating slurry; (2) coating the second organic coating slurry on one surface of the second base membrane, and drying it to obtain the second diaphragm. In other embodiments, the preparation method of the second diaphragm includes, but is not limited to, the following steps: (1) preparing a second organic coating slurry and a second inorganic coating slurry; (2) coating the second inorganic coating slurry on one surface of the second base membrane, and drying it to obtain a second diaphragm with a second inorganic coating on one side; (3) coating the second organic coating slurry on the surface of the second inorganic coating away from the second base membrane, and drying it to obtain the second diaphragm. In still other embodiments, the preparation method of the second diaphragm includes, but is not limited to, the following steps: (1) preparing a second organic coating slurry and a second adhesive layer slurry; (2) coating the second organic coating slurry on one surface of the second base membrane, and drying it to obtain a second diaphragm with a second organic coating on one side; (3) coating the second adhesive layer slurry on the other surface of the second base membrane, and drying it to obtain the second diaphragm. In some other embodiments, the preparation method of the second diaphragm includes, but is not limited to, the following steps: (1) preparing a second organic coating slurry, a second inorganic coating slurry, and a second adhesive layer slurry; (2) coating a second inorganic coating slurry on one surface of the second base film, and drying it to obtain a second diaphragm with a second inorganic coating on one side; (3) coating a second organic coating slurry on the surface of the second inorganic coating away from the second base film, and drying it to obtain a second diaphragm with a second inorganic coating and a second organic coating on one side; (4) coating a second adhesive layer slurry on the other surface of the first base film, and drying it to obtain a second diaphragm.

[0066] In this application, there are no particular limitations 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 (e.g., aluminum-carbon composite current collector). Both the first positive electrode material layer and the second positive electrode material layer include positive electrode active materials. In this application, there are no particular limitations on the positive electrode active materials, as long as they can achieve the purpose of this application. For example, the positive electrode active materials may include, but are not limited to, at least one of lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. The first and second positive electrode material layers may each independently include a conductive agent and a binder. This application does not impose any particular limitation on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the first positive electrode material layer, or in the second positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved.

[0067] In this application, there are no particular limitations on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can 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 can be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc. Both the first negative electrode material layer and the second negative electrode material layer include negative electrode active materials. This application has no particular limitations on the negative electrode active materials, as long as they can achieve the purpose of this application. For example, the negative electrode active materials can include, but are not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon-containing materials, 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, wherein the silicon-containing material includes, but is not limited to, elemental silicon, silicon oxide (SiO2). b The first negative electrode material layer and the second negative electrode material layer may each independently include a conductive agent, a binder, and a thickener. This application does not impose any particular limitation on the types of conductive agents, binders, and thickeners, as long as they can achieve the purpose of this application. For example, the conductive agent and binder may be at least one of the conductive agents and binders in the first positive electrode material layer and the second positive electrode material layer described above. The thickener may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. This application does not impose any particular restrictions on the mass ratio of negative electrode active material, conductive agent, binder and thickener in the first negative electrode material layer, or on the mass ratio of negative electrode active material, conductive agent, binder and thickener in the second negative electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0068] 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 lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. 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 (MEC). The aforementioned cyclic carbonates 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). Fluorinated carbonate 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. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-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.

[0069] In this application, 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 any 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; any metal casing known in the art can be used, 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.

[0070] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the second separator, the negative electrode, the first separator, and the positive electrode in sequence and winding them as needed to obtain a wound electrode assembly; then placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Wherein, the first organic coating on the first separator is adjacent to the first positive electrode material layer of the positive electrode, and the second organic coating on the second separator is adjacent to the second positive electrode material layer of the positive electrode.

[0071] A second aspect of this application provides an electronic device including the secondary battery in any of the above embodiments. Therefore, the electronic device provided by this application has good performance.

[0072] 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.

[0073] Example

[0074] 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.

[0075] Test methods and equipment:

[0076] Sampling methods for the first and second septa:

[0077] The lithium-ion batteries in the tested examples and comparative examples were discharged to 2.5V at 0.5C and then disassembled. The first and second separators were removed and soaked in dimethyl carbonate (DMC) for 20 minutes to remove electrolyte residue. Then, the first and second separators were placed in an oven and dried at 60°C for 12 hours to obtain the first separator sample and the second separator sample.

[0078] Testing of coating weights M1 and M2 for organic coatings:

[0079] The cut area is S1 dm 2 The first separator sample was weighed on a balance and recorded as p1 mg. Then, the first organic coating on the first separator (i.e., the coating adjacent to the first positive electrode material layer, where the first positive electrode material layer is close to the winding center of the electrode assembly) was scraped off and weighed on a balance, recorded as p2 mg. The coating weight of the first organic coating is M1 = (p1 - p2) / S1, in mg / dm³. 2 .

[0080] Following the steps described above, the coating weight M2 mg / dm of the second organic coating (i.e., the coating adjacent to the second positive electrode material layer, wherein the second positive electrode material layer is far from the winding center of the electrode assembly) was determined through testing. 2 .

[0081] Tests of weight-average molecular weights Mw1 and Mw2:

[0082] The weight-average molecular weights of the first and second organic compounds were determined using an Agilent 1200 series liquid chromatograph. The first and second organic compounds were dissolved in deionized water to prepare solutions with a solid content of 6 wt%. After filtration through an aqueous filter membrane, the solutions were injected into the Agilent 1200 series liquid chromatograph for analysis, yielding the measured weight-average molecular weights Mw1 g / mol and Mw2 g / mol for the first and second organic compounds.

[0083] Testing of diaphragm thicknesses T1 and T2, and diaphragm compressibility A1 and A2:

[0084] During the disassembly of lithium-ion batteries, the overhang regions of the first and second separator samples can be observed with the naked eye. These overhang regions are the areas where the first separator extends beyond the negative and positive electrode plates along the width direction of the unfolded first and second separators. A portion of the separator from the overhang regions of the first and second separator samples is cut off, yielding sample 1-1 (corresponding to the first separator) and sample 2-1 (corresponding to the second separator). The cross-sectional morphology of samples 1-1 and 2-1 along the thickness direction is then observed using a field emission scanning electron microscope (Philips, XL-30 model), and scanning electron microscope images are taken. The thicknesses of samples 1-1 and 2-1 are measured using the scanning electron microscope, which are T1 μm and T2 μm, respectively. A portion of the separator was cut from the areas where the first and second separator samples overlapped with the positive and negative electrode sheets, respectively, to obtain sample 1-2 (corresponding to the first separator) and sample 2-2 (corresponding to the second separator). The cross-sectional morphology of samples 1-2 and 2-2 along the thickness direction was then observed using a field emission scanning electron microscope (SEM), and SEM images were taken. The thicknesses of samples 1-2 and 2-2 were measured using the SEM and denoted as T1' μm and T2' μm, respectively. Therefore, the compressibility of the first separator is A1 = T1 - T1', and the compressibility of the second separator is A2 = T2 - T2'.

[0085] Testing of the thicknesses D0 and D1 of the second negative electrode material layer:

[0086] The lithium-ion battery was charged at a constant current of 1C to 4.35V, and then charged at a constant voltage of 4.35V to a current of 0.1C. After disassembly, the thickness of the second negative electrode material layer was measured, and D1 μm was obtained.

[0087] The lithium-ion batteries in the same group were discharged at a constant current of 0.5C to 2.5V. After disassembly, the thickness of the second negative electrode material layer was measured, and D0 μm was obtained.

[0088] Membrane air permeability test:

[0089] The air permeability of the first and second diaphragm samples was tested using a Gurley air permeability meter. The air column volume of the air permeability meter was 100 cc, and the test area of ​​the diaphragm was 6.45 cm². 2 During the test, the separator membrane was kept absolutely flat, and the test was repeated 3 times. The average value was taken as the final air permeability value, and the air permeability of the first separator n1 secs / 100cc and the air permeability of the second separator n2 secs / 100cc were obtained.

[0090] Areal capacity testing of each material layer:

[0091] At 25°C, the lithium-ion battery was discharged to 0% SOC (discharge cutoff voltage was 2.5V) at a constant current of 0.5C. The discharged lithium-ion battery was then disassembled, and the positive and negative electrode sheets were removed. During removal, the first and second positive electrode material layers in the positive electrode sheet were marked, wherein the first positive electrode material layer was located on the surface of the positive current collector near the winding center of the electrode assembly, and the second positive electrode material layer was located on the surface of the positive current collector away from the winding center of the electrode assembly. At the same time, the first and second negative electrode material layers in the negative electrode sheet were marked, wherein the first negative electrode material layer was located on the surface of the negative current collector near the winding center of the electrode assembly, and the second negative electrode layer was located on the surface of the negative current collector away from the winding center of the electrode assembly. In a glove box, the positive and negative electrode sheets were cleaned with dimethyl carbonate (DMC) to remove electrolyte residue, and dried at 60°C for 12 hours to obtain the positive and negative electrode sheets.

[0092] Using a punching machine, cut two circular positive electrode samples (sample 1-1 and sample 1-2) and two circular negative electrode samples (sample 2-1 and sample 2-2) from the above-mentioned positive and negative electrode sheets, respectively, with an area of ​​S² cm². 2The second positive electrode material layer of sample 1-1, the first positive electrode material layer of sample 1-2, the second negative electrode material layer of sample 2-1, and the first negative electrode material layer of sample 2-2 were wiped off using N-methylpyrrolidone (NMP) solvent. Then, samples 1-1, 1-2, 2-1, and 2-2 were assembled with lithium metal sheets, a separator, and an electrolyte to form a CR2032 button battery. The electrolyte consisted of a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1, lithium hexafluorophosphate (LiPF6), and the additive vinylene carbonate (VC). The molar concentration of lithium salt in the electrolyte was 1 mol / L, and the mass percentage of VC was 2% based on the mass of the electrolyte. The separator was a 25 μm thick polypropylene (PP) film (Celgard 2500). Each assembled button cell was subjected to charge-discharge tests. The charge-discharge mechanism was as follows: the button cell was charged to 4.35V with a constant current of 0.1S2 mA, allowed to stand for 15 minutes, and then discharged to 2.5V with a constant current of 0.1S2 mA, allowed to stand for 30 minutes. This was repeated twice as one cycle to obtain the discharge capacity of each button cell, in mAh. The areal capacity of the first positive electrode material layer = discharge capacity of the button cell assembled from sample 1-1 / S2; the areal capacity of the second positive electrode material layer = discharge capacity of the button cell assembled from sample 1-2 / S2; the areal capacity of the first negative electrode material layer = discharge capacity of the button cell assembled from sample 2-1 / S2; and the areal capacity of the second negative electrode material layer = discharge capacity of the button cell assembled from sample 2-2 / S2. The unit of areal capacity is mAh / cm². 2 .

[0093] 25℃ Cyclic Lithium Plating Test:

[0094] At 25°C, the lithium-ion battery is charged to 4V with a constant current of 3C, then charged to 1C with a constant voltage of 4V, and then charged to 4.35V with a constant current of 1C, and then charged to 0.05C with a constant voltage of 4.35V. After resting for 15 minutes, it is discharged to 2.5V at a 3C rate and then rested for 30 minutes. This is one cycle, and the above process is repeated 300 times.

[0095] After 300 cycles, the lithium-ion battery was disassembled to check the interface condition of the second negative electrode material layer surface and determine the degree of lithium plating in the arc segment of the second negative electrode material layer. The percentage of lithium plating area in the arc segment of the second negative electrode material layer was calculated based on the total area of ​​the arc segment of the second negative electrode material layer. The criteria for judging the lithium plating state are as follows: lithium plating area less than or equal to 1% is no lithium plating; lithium plating area greater than 1% and less than or equal to 3% is slight lithium plating; lithium plating area greater than 3% and less than or equal to 5% is moderate lithium plating; and lithium plating area greater than 5% is severe lithium plating.

[0096] This application uses the degree of lithium plating in the arc segment of the lithium-ion battery to characterize the cycle stability of the lithium-ion battery. The less severe the lithium plating in the arc segment, the better the cycle stability of the lithium-ion battery.

[0097] Energy density test:

[0098] At 25℃, the lithium-ion battery was charged to 4.35V at a constant current of 0.5C, then charged to 0.05C at a constant voltage of 4.35V, left to stand for 15 minutes, and then discharged to 2.5V at a rate of 0.5C, left to stand for 30 minutes. The capacity C and energy value E released by the lithium-ion battery were recorded. The energy density (Wh / kg) of the lithium-ion battery = energy value E / weight of the lithium-ion battery.

[0099] Charging kinetics test:

[0100] At 5°C, the lithium-ion battery was charged at a constant current of 0.5C to 4.35V, then charged at a constant voltage of 4.35V to 0.05C, left to stand for 15 minutes, and then discharged at a constant current of 0.5C to 2.5V, left to stand for 15 minutes. This constitutes one cycle, and the process was repeated 10 times. The fully charged lithium-ion battery was then disassembled to obtain the negative electrode. If any part of the negative electrode was found to be ≥2mm... 2 If lithium plating occurs in a region, it is determined to be lithium plating on the negative electrode, with the unit being C. If it is determined that the negative electrode does not plating lithium, the charging rate is increased by 0.1C sequentially until lithium plating occurs on the negative electrode of the disassembled lithium-ion battery. The charging rate at which lithium plating does not occur is recorded as the critical lithium plating rate for that lithium-ion battery.

[0101] This application uses the critical lithium plating rate of lithium-ion batteries to characterize the charging kinetics performance of lithium-ion batteries. The higher the critical lithium plating rate, the better the charging kinetics performance of the lithium-ion battery.

[0102] Discharge temperature rise test:

[0103] At 25℃, a temperature sensing wire was attached to the center of the diagonal intersection of the lithium-ion battery surface to monitor the process temperature. The lithium-ion battery pack with the sensing wire attached was then tested in foam. The lithium-ion battery was charged to 4.35V at a constant current of 1C, and then charged at a constant voltage of 0.05C at 4.35V, followed by a resting period of 15 minutes. Subsequently, it was discharged at a rate of 4C to 2.5V and then rested for 30 minutes. The temperature of the lithium-ion battery during the discharge process was recorded; temperature rise T = highest lithium-ion battery temperature - ambient temperature.

[0104] This application uses the discharge temperature rise of lithium-ion batteries to characterize the discharge kinetics performance of lithium-ion batteries. The lower the discharge temperature rise, the better the discharge kinetics performance of the lithium-ion battery.

[0105] Example 1-1

[0106] <Preparation of the first diaphragm>

[0107] A 5μm thick nonwoven fabric, made of polyethylene, was used as the first base film, with an average pore size of 100nm. A first organic coating slurry with a solid content of 8wt% was obtained by mixing polymethyl methacrylate (Mw1=300000) with deionized water. This first organic coating slurry also served as the first adhesive layer slurry. A first inorganic coating slurry was obtained by mixing alumina (aluminum oxide), styrene-butadiene rubber (styrene-butadiene rubber), and deionized water (deionized water) in a mass ratio of 35:10:55.

[0108] An inorganic coating slurry is applied to one surface of a first base membrane and dried at 60°C to form a first inorganic coating. A first organic coating slurry is then applied to the surface of the first inorganic coating away from the first base membrane and dried at 60°C, forming both the first inorganic coating and the first organic coating on one surface of the first base membrane. A first adhesive layer slurry is then applied to the other surface of the first base membrane and dried at 60°C to obtain a first diaphragm. The coating weight M1 of the first organic coating is 10.0 mg / dm³. 2 The coating weight of the first adhesive layer slurry is 1.0 mg / dm³. 2 The coating weight of the first inorganic coating is 4.0 mg / dm³. 2 The thickness of the first diaphragm is 17.9 μm.

[0109] <Preparation of the Second Diaphragm>

[0110] A 5μm thick nonwoven fabric, made of polyethylene, was used as the second base film, with an average pore size of 100nm. A second organic coating slurry with a solid content of 8wt% was obtained by mixing polyvinylidene fluoride (Mw2=300000) with deionized water. This second organic coating slurry also served as the second adhesive layer slurry. A second inorganic coating slurry was obtained by mixing alumina (aluminum oxide), styrene-butadiene rubber (styrene-butadiene rubber), and deionized water in a mass ratio of 35:10:55.

[0111] A second inorganic coating slurry is applied to one surface of the second base membrane and dried at 60°C to form a second inorganic coating. A second organic coating slurry is then applied to the surface of the second inorganic coating away from the second base membrane and dried at 60°C to form both an inorganic coating and a second organic coating on one surface of the second base membrane. A second adhesive layer slurry is then applied to the other surface of the second base membrane and dried at 60°C to obtain the second diaphragm. The coating weight M2 of the second organic coating is 6.0 mg / dm³. 2 The coating weight of the second adhesive layer slurry is 1.0 mg / dm³. 2 The coating weight of the second inorganic coating is 4.0 mg / dm³. 2 The thickness of the second diaphragm is 13.7 μm.

[0112] <Preparation of the positive electrode>

[0113] The positive electrode active material NCM811 (chemical formula LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black (conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) solvent was added, and the mixture was thoroughly stirred and mixed to form a first positive electrode material layer slurry with a solid content of 75 wt%. This first positive electrode material layer slurry was also used as the second positive electrode material layer slurry. The first positive electrode material layer slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector and dried at 90°C to obtain a positive electrode sheet with the first positive electrode material layer. The second positive electrode material layer slurry was coated onto the other surface of the aluminum foil used as a positive electrode current collector and dried to obtain a positive electrode sheet with both the first and second positive electrode material layers. The coated positive electrode sheet was cold-pressed and then cut into 60 mm × 1000 mm specifications for later use. The single-sided coating weight of the first positive electrode material layer was 300 mg / 1540.25 mm. 2 The compaction density of the first cathode material layer is 3.4 g / cm³. 3 The single-sided coating weight of the second cathode material layer is 300 mg / 1540.25 mm.2 The compaction density of the second cathode material layer is 3.4 g / cm³. 3 The areal capacity of the first cathode material layer is 3.89 mAh / cm². 2 The areal capacity of the second cathode material layer is 3.89 mAh / cm². 2 .

[0114] <Preparation of Negative Electrode Sheets>

[0115] Artificial graphite and silicon-carbon composite material (with Si content of 50% by mass), conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 80.0:15.0:2.0:1.5:1.5. Deionized water was then added as a solvent, and the mixture was stirred until homogeneous, resulting in a first negative electrode material layer slurry with a solid content of 70 wt%. This first negative electrode material layer slurry was also used as the second negative electrode material layer slurry. The first negative electrode material layer slurry was coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 90°C to obtain a negative electrode sheet with the first negative electrode material layer. The second negative electrode material layer slurry was coated onto the other surface of the copper foil used as a negative electrode current collector and dried to obtain a negative electrode sheet with both the first and second negative electrode material layers. The coated negative electrode sheets were cold-pressed and then cut into 63 mm × 1200 mm dimensions for later use. The single-sided coating weight of the first negative electrode material layer is 122 mg / 1540.25 mm. 2 The compaction density of the first negative electrode material layer is 1.6 g / cm³. 3 The single-sided coating weight of the second negative electrode material layer is 122 mg / 1540.25 mm. 2 The compaction density of the second negative electrode material layer is 1.6 g / cm³. 3 The areal capacity of the first negative electrode material layer is 4.28 mAh / cm³. 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0116] <Preparation of Electrolyte>

[0117] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 20:30:40:10 to obtain an organic solvent. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of the lithium salt was 1 mol / L, and the remainder was the organic solvent.

[0118] <Preparation of Lithium-ion Batteries>

[0119] The first separator, negative electrode, second separator, and positive electrode prepared above are stacked in sequence, wherein the first positive electrode material layer of the positive electrode is adjacent to the first separator, and the second positive electrode material layer of the positive electrode is adjacent to the second separator. The electrode assembly is then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming processes to obtain a lithium-ion battery. The electrolyte injection volume is 6g.

[0120] Examples 1-2 to Examples 1-4

[0121] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1. In Examples 1-2 and 1-3, the negative electrode active material does not contain silicon-carbon composite material, and the mass ratio of artificial graphite, conductive agent, binder, and thickener is 95.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer is 174 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 174 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 In Examples 1-4, the mass ratio of artificial graphite, silicon-carbon composite material, conductive agent, binder, and thickener was 45.0:50.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer was 67 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 67 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0122] Examples 2-1 and 2-2

[0123] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0124] Examples 3-1 to 3-4

[0125] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Examples 1-1. The formulations, coating weights of the first and second anode material layers, and areal capacities in each example are as follows:

[0126] In Example 3-1, the mass ratio of artificial graphite, silicon-carbon composite material, conductive agent, binder, and thickener is 90.0:5.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer is 155 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 155 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0127] In Examples 3-2, the mass ratio of artificial graphite, silicon-carbon composite material, conductive agent, binder, and thickener was 83.0:12.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer was 129 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 129 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0128] In Examples 3-3, the mass ratio of artificial graphite, silicon-carbon composite material, conductive agent, binder, and thickener was 70.0:25.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer was 98 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 98 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0129] In Examples 3-4, the mass ratio of artificial graphite, silicon-carbon composite material, conductive agent, binder, and thickener was 60.0:35.0:2.0:1.5:1.5, and the single-sided coating weight of the first negative electrode material layer was 83 mg / 1540.25 mm. 2 The single-sided coating weight of the second negative electrode material layer is 83 mg / 1540.25 mm. 2 The areal capacity of the first anode material layer is 4.28 mAh / cm². 2 The areal capacity of the second negative electrode material layer is 4.28 mAh / cm². 2 .

[0130] Examples 4-1 to 4-3

[0131] Except for adjusting the relevant preparation parameters according to Table 4, the rest is the same as in Examples 1-1.

[0132] Examples 5-1 and 5-2

[0133] Except for adjusting the relevant preparation parameters according to Table 5, the rest is the same as in Examples 1-1.

[0134] Comparative Examples 1-1 to 1-5

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

[0136] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 5.

[0137] Table 1

[0138] Table 2

[0139] Table 3

[0140] Table 4

[0141] Table 5

[0142] Note: " / " in Table 5 indicates that no relevant preparation parameters are available.

[0143] As can be seen from Examples 1-1 to 1-4, Examples 2-1 and 2-2, Examples 3-1 to 3-4, Examples 4-1 to 4-3, Examples 5-1 and 5-2, and Comparative Examples 1-1 to 1-5, by using two separators with different organic coating weights in the wound electrode assembly and adjusting the coating weight of each organic coating within the scope of this application, the lithium-ion battery exhibits a relatively mild degree of lithium plating at 25°C, a high critical lithium plating rate and energy density at 5°C, and a low discharge temperature rise. This indicates that the lithium-ion battery of this application improves the problem of interface lithium plating at corners, and enhances the cycle stability and safety performance of the lithium-ion battery while balancing energy density and kinetic performance. In Comparative Examples 1-1 and 1-2, M1 is not within the scope of this application; in Comparative Example 1-3, M1 is not within the scope of this application and M1=M2; in Comparative Example 1-4, M1 and M2 are not within the scope of this application and M1=M2; in Comparative Example 1-5, M2 is not within the scope of this application and M1=M2. The severe lithium plating at 25°C indicates a significant problem with lithium plating at the interface at the corners of the lithium-ion battery, making it impossible to improve the cycle stability and safety performance of the lithium-ion battery while simultaneously considering energy density and kinetic performance. However, in the examples, the lithium-ion battery exhibits less severe lithium plating at 25°C, higher critical lithium plating rate and energy density at 5°C, and lower discharge temperature rise, demonstrating that this application can improve the cycle stability and safety performance of the lithium-ion battery while balancing energy density and kinetic performance.

[0144] As can be seen from Examples 1-1 to 1-4, the value of M1 / M2 affects the cycle stability and safety performance of lithium-ion batteries. When the value of M1 / M2 is within the range of this application, the lithium-ion battery exhibits a relatively mild degree of lithium plating at 25°C, a higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that this application can improve the cycle stability and safety performance of lithium-ion batteries while balancing energy density and kinetic performance.

[0145] As can be seen from Examples 1-1, 2-1, and 2-2, the values ​​of Mw1 and Mw2 affect the cycle stability and safety performance of lithium-ion batteries. When the values ​​of Mw1 and Mw2 are within the range of this application, the lithium-ion battery exhibits a relatively mild degree of lithium plating at 25°C, a higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that this application can improve the cycle stability and safety performance of lithium-ion batteries while balancing energy density and kinetic performance.

[0146] As can be seen from Examples 1-1 to 1-4 and Examples 3-1 to 3-4, the values ​​of A1 and D1 / D0 affect the cycle stability and safety performance of lithium-ion batteries. When the values ​​of A1 and D1 / D0 satisfy the relationship described in this application, the lithium-ion battery exhibits a relatively mild degree of lithium plating at 25°C, a higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that this application can improve the cycle stability and safety performance of lithium-ion batteries while balancing energy density and kinetic performance.

[0147] As can be seen from Examples 1-1, 4-1 to 4-3, the values ​​of X1, X2, and X3 affect the cycle stability and safety performance of lithium-ion batteries. When the values ​​of X1, X2, and X3 satisfy the relationship described in this application, the lithium-ion battery exhibits a relatively mild degree of lithium plating at 25°C, a higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that this application can improve the cycle stability and safety performance of lithium-ion batteries while balancing energy density and kinetic performance. However, when X1 and X2 are large, it can lead to a loss in the energy density of the lithium-ion battery and increase gas production during storage or cyclic charging and discharging at high temperatures.

[0148] As can be seen from Examples 1-1 and 5-1, the types of the first organic compound, the second organic compound, the first inorganic material, and the second inorganic material affect the cycle stability and safety performance of lithium-ion batteries. When the first organic compound, the second organic compound, the first inorganic material, and the second inorganic material of this application are selected, the lithium-ion battery exhibits a less severe degree of lithium plating at 25°C, higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that this application can improve the cycle stability and safety performance of lithium-ion batteries while balancing energy density and kinetic performance.

[0149] As can be seen from Examples 1-1 and 5-2, the placement of the first and second inorganic coatings affects the cycle stability and safety performance of the lithium-ion battery. When the first inorganic coating is placed between the first organic coating and the first base film, and the second inorganic coating is placed between the second organic coating and the second base film, the lithium-ion battery exhibits a less severe degree of lithium plating at 25°C, higher critical lithium plating rate and energy density at 5°C, and a lower discharge temperature rise. This indicates that the present application can improve the cycle stability and safety performance of the lithium-ion battery while balancing energy density and kinetic performance.

[0150] 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.

[0151] 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.

[0152] 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, a negative electrode, a first separator, and a second separator; The negative electrode sheet includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The negative current collector includes a first surface and a second surface. The first surface is closer to the winding center of the electrode assembly than the second surface. The first negative electrode material layer is disposed on the first surface, and the second negative electrode material layer is disposed on the second surface. The positive electrode sheet includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The positive current collector includes a third surface and a fourth surface. The third surface is closer to the winding center of the electrode assembly than the fourth surface. The first positive electrode material layer is disposed on the third surface, and the second positive electrode material layer is disposed on the fourth surface. The first separator is located between the first positive electrode material layer and the second negative electrode material layer, and the second separator is located between the second positive electrode material layer and the first negative electrode material layer; The first separator includes a first organic coating and a first base film. The first organic coating is adjacent to the first positive electrode material layer, and the coating weight of the first organic coating is M1 mg / dm³. 2 The second separator includes a second organic coating and a second base film. The second organic coating is adjacent to the second positive electrode material layer, and the coating weight of the second organic coating is M2 mg / dm. 2 , M1>M2, 5.0≤M1≤16.0, 2.0≤M2≤8.

0.

2. The secondary battery according to claim 1, wherein, 1.00 < M1 / M2 ≤ 2.

00.

3. The secondary battery according to claim 1, wherein, The first organic coating comprises a first organic compound, and the second organic coating comprises a second organic compound. The first organic compound and the second organic compound each independently comprise at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyamide, polyimide, or cellulose.

4. The secondary battery according to claim 3, wherein, The weight-average molecular weight of the first organic compound is Mw1 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw1≤5.0×10 5 ; and / or, the weight-average molecular weight of the second organic compound is Mw2 g / mol, 5.0 × 10⁻⁶. 3 ≤Mw2≤5.0×10 5 .

5. The secondary battery according to claim 1, wherein, The compressibility of the first diaphragm is A1 μm, and the compressibility of the second diaphragm is A2 μm, where A1 > A2.

6. The secondary battery according to claim 5, wherein, 7.0≤A1≤15.0; and / or, 4.0≤A2≤9.

0.

7. The secondary battery according to claim 5, wherein, The secondary battery is charged at a constant current of 1C to 4.35V, and then charged at a constant voltage of 4.35V to a current of 0.1C, with the thickness of the second negative electrode material layer being D1 μm; the secondary battery is discharged at a constant current of 0.5C to 2.5V, with the thickness of the second negative electrode material layer being D0 μm, satisfying the following: 1.05≤D1 / D0≤1.15, 7.0≤A1≤8.0; 1.15<D1 / D0≤1.35, 8.0<A1≤13.0; 1.35<D1 / D0≤1.70, 13.0<A1≤15.

0.

8. The secondary battery according to claim 1, wherein, The thickness of the first diaphragm is T1 μm, 12.0≤T1≤25.0; and / or, the thickness of the second diaphragm is T2 μm, 9.0≤T2≤16.

0.

9. The secondary battery according to claim 1, wherein, The air permeability of the first diaphragm is n1 secs / 100cc, and the air permeability of the second diaphragm is n2 secs / 100cc, where 0.5≤n1 / n2≤0.9 and 40.0≤n1≤120.

0.

10. The secondary battery according to claim 1, wherein, The ratio of the areal capacity of the second negative electrode material layer to the areal capacity of the first positive electrode material layer is 1.03 to 1.20; and / or, the ratio of the areal capacity of the first negative electrode material layer to the areal capacity of the second positive electrode material layer is 1.03 to 1.

20.

11. The secondary battery according to claim 1, wherein, The ratio of the areal capacity of the second negative electrode material layer to the areal capacity of the first negative electrode material layer is 1.00 to 1.

05.

12. The secondary battery according to claim 1, wherein, The first diaphragm further includes a first inorganic coating located between the first organic coating and the first base film, the first inorganic coating comprising a first inorganic material, the first inorganic material comprising at least one of alumina or boehmite; and / or, the second diaphragm further includes a second inorganic coating located between the second organic coating and the second base film, the second inorganic coating comprising a second inorganic material, the second inorganic material comprising at least one of alumina or boehmite.

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

Images