Secondary battery and electronic apparatus

Abstract

The present application provides a secondary battery and an electronic apparatus. The secondary battery comprises a positive electrode sheet, a separator, and a negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector, and a first coating and a second coating that are arranged on at least one surface of the negative electrode current collector; the second coating comprises a first active material layer; and in the thickness direction of the negative electrode sheet, the first active material layer is arranged between the negative electrode current collector and the first coating and is connected to the first coating. The thickness of the separator is T11, and 4 μm≤T11≤20 μm. The first active material layer comprises a first negative electrode active material, and the first negative electrode active material comprises a first silicon material. The first coating does not comprise the first silicon material, the thickness of the first coating is T20, and 3 μm≤T20≤15 μm. The secondary battery has good safety performance and rate performance, and also has high energy density.

Description

Secondary batteries and electronic devices 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 advantages such as high energy density, high open-circuit voltage, low self-discharge rate, long cycle life, and good safety, and are widely used in various fields such as energy storage, mobile electronic devices, electric vehicles, and aerospace. With the rapid development of mobile electronic devices and electric vehicles, the market is placing increasingly higher demands on the energy density and safety of secondary batteries.

[0003] As the negative electrode active material in secondary batteries, silicon possesses a high energy density advantage that traditional graphite cannot match, and significant progress has been made in recent years. However, it faces numerous problems such as poor conductivity and poor rate performance. During lithium delithiation and lithium insertion, the capacity contribution and volume expansion of silicon are almost linearly related, resulting in huge volume changes. Furthermore, silicon particles are relatively hard and have sharp surfaces, which may puncture the separator during charging and discharging, causing localized physical short circuits and self-discharge, thus posing a high safety risk to secondary batteries. Therefore, how to reduce the safety risk of secondary batteries has become a pressing technical problem for those skilled in the art.

[0004] Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and electronic device to reduce the safety risks of secondary batteries.

[0006] 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:

[0007] The first aspect of this application provides a secondary battery, comprising a positive electrode, a separator, and a negative electrode. The separator is disposed between the positive and negative electrode. The negative electrode includes a negative current collector and a first coating and a second coating disposed on at least one surface of the negative current collector. The second coating includes a first active material layer. Along the thickness direction of the negative electrode, the first active material layer is disposed between the negative current collector and the first coating and is connected to the first coating. The thickness of the separator is T. 11 4μm≤T 11 ≤20μm; the first active material layer includes a first negative electrode active material, which includes a first silicon material; the first coating does not include the first silicon material, and the thickness of the first coating is T. 20 3μm≤T 20≤15μm. The secondary battery of this application simultaneously forms a first coating without silicon material and a second coating containing silicon material in the negative electrode sheet, and the thickness of the first coating is controlled to be between 3μm and 15μm. This allows the first coating to isolate the separator from the silicon material particles in the second coating, thus reducing the probability of silicon material particles puncturing the separator and causing a localized physical short circuit during the charging and discharging process of the secondary battery. This reduces the safety risk of the secondary battery. Furthermore, the thinness of the first coating makes it extremely unlikely that the lithium-ion transport path in the negative electrode active material of the second coating will become longer. Therefore, the silicon material can fully utilize its high capacity, giving the secondary battery a high energy density. Thus, the secondary battery achieves high energy density while reducing safety risks.

[0008] In some embodiments of this application, 4μm≤T 11 ≤13μm, 9μm≤T 20 ≤15μm. Membrane thickness T 11 And the thickness T of the first coating 20 Within the above-mentioned range, the safety risks of secondary batteries are reduced, and secondary batteries also have high energy density.

[0009] In some embodiments of this application, 13 μm < T 11 ≤20μm, 3μm≤T 20 <9μm. Membrane thickness T 11 Satisfying 13μm < T 11 When the thickness is ≤20μm, the membrane itself has a certain resistance to puncture. Therefore, the thickness T of the first coating is... 20 Within the above-mentioned range, secondary batteries have lower safety risks and higher energy density.

[0010] In some embodiments of this application, the second coating further includes a second active material layer. Along the thickness direction of the negative electrode sheet, the second active material layer is disposed between the first active material layer and the negative electrode current collector. The second active material layer includes a second negative electrode active material, which includes a second silicon material. The mass percentage of the first silicon material in the first negative electrode active material is W1, and the mass percentage of the second silicon material in the second negative electrode active material is W2, where 0% ≤ W2 ≤ 40% and 3% ≤ W1 ≤ 99%. By further providing a second active material layer in the second coating of the negative electrode sheet, and controlling the mass percentage of the first silicon material W1 in the first negative electrode active material and the mass percentage of the second silicon material W2 in the second negative electrode active material within the aforementioned ranges, the content of the negative electrode active material in the negative electrode sheet can be increased, thereby improving the energy density of the secondary battery. Thus, the secondary battery can further improve its energy density while reducing its safety risks.

[0011] In some embodiments of this application, W1 > W2, and 3% ≤ W1 - W2 ≤ 99%. When the mass percentage W1 of the first silicon material in the first negative electrode active material is greater than the mass percentage W2 of the second silicon material in the second negative electrode active material, the difference between W1 and W2 is controlled within the above range, and the secondary battery has high energy density while having good safety performance.

[0012] In some embodiments of this application, W1 < W2, -10% ≤ W1 - W2 < 0%. When the mass percentage W1 of the first silicon material in the first negative electrode active material is less than the mass percentage W2 of the second silicon material in the second negative electrode active material, controlling the difference between W1 and W2 within the above range is beneficial to improving the cycle stability of the secondary battery while maintaining good safety performance and high energy density.

[0013] In some embodiments of this application, the thickness of the first active material layer is T. 21 The thickness of the second active material layer is T. 22 20μm≤T 21 ≤T 22 ≤200μm, 0.1≤T 21 / T 22 ≤1.0. The thickness T of the first active material layer... 21 The thickness T of the second active material layer 22 and the thickness T of the first active material layer 21 With respect to the thickness T of the second active material layer 22 The ratio T between them 21 / T 22 Within the above-mentioned range, secondary batteries possess high-rate performance while maintaining good safety performance and high energy density.

[0014] In some embodiments of this application, the thickness of the first active material layer is T. 21 The thickness of the second active material layer is T. 22 20μm≤T 22 <T 21 ≤200μm, 1.0<T 21 / T 22 ≤1.5. The thickness T of the first active material layer... 21 The thickness T of the second active material layer 22 and the thickness T of the first active material layer 21 With respect to the thickness T of the second active material layer 22 By adjusting the ratio between the two values ​​within the above range, the secondary battery can improve its cycle stability while maintaining good safety performance and high energy density.

[0015] In some embodiments of this application, 3% ≤ W1 < 20%, 100 μm ≤ T 21 ≤200μm, 3μm≤T 20 ≤8μm. When the mass percentage W1 of the first silicon material in the first negative electrode active material satisfies 3% ≤ W1 < 20% and the thickness T of the first active material layer... 21 Satisfying 100μm≤T 21 When the thickness is ≤200μm, adjust the thickness T of the first coating. 20 Within the range of 3μm to 8μm, the content of the first silicon material, the thickness of the first active material layer, and the thickness of the first coating are matched, which reduces the safety risks of the secondary battery and also enables the secondary battery to have high energy density.

[0016] In some embodiments of this application, 20% ≤ W1 ≤ 99%, 20 μm ≤ T 21 <100μm, 8μm<T 20 ≤15μm. When the mass percentage W1 of the first silicon material in the first negative electrode active material satisfies 20%≤W1≤99% and the thickness T of the first active material layer... 21 Satisfying 20μm≤T 21 When the thickness is <100μm, adjust the thickness T of the first coating. 20 Within the aforementioned range, the content of the first silicon material, the thickness of the first active material layer, and the thickness of the first coating layer are matched, thereby reducing the safety risks of the secondary battery and enabling the secondary battery to have high energy density.

[0017] In some embodiments of this application, the second coating further includes a third active material layer. Along the thickness direction of the negative electrode sheet, the third active material layer is disposed between the second active material layer and the negative electrode current collector. The third active material layer includes a third negative electrode active material, which includes a third silicon material. The mass percentage of the third silicon material in the third negative electrode active material is W3, where W3 ≤ W2. When the second coating of the negative electrode sheet further comprises a second active material layer and a third active material layer, and the mass percentage of the second silicon material in the second negative electrode active material W2 and the mass percentage of the third silicon material in the third negative electrode active material W3 satisfy the above-mentioned relationship, the secondary battery exhibits good safety and performance characteristics (such as rate performance and cycle performance) while maintaining high energy density.

[0018] In some embodiments of this application, the first, second, and third negative electrode active materials each independently include a carbon material; the carbon material includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, or hard carbon. Using the aforementioned types of carbon materials as the first, second, and third negative electrode active materials helps reduce the probability of silicon material expanding and puncturing the separator, resulting in lower safety risks in the secondary battery and enabling it to achieve higher capacity and energy density.

[0019] In some embodiments of this application, the first silicon material, the second silicon material, and the third silicon material each independently comprise at least one of pure silicon, silicon oxide, silicon alloy, or silicon carbon. Using materials of the above types as the first, second, and third silicon materials is advantageous for achieving high energy density in the secondary battery.

[0020] In some embodiments of this application, the first coating includes a third negative electrode material, which includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, hard carbon, lithium titanate, alumina, boehmite, silicon dioxide, or polyvinylidene fluoride. Using the above-mentioned third negative electrode materials in the first coating enables the secondary battery to have low safety risk, long cycle life, and high cycle stability.

[0021] In some embodiments of this application, the volume average particle size Dv50 of the third anode material -3 The volume average particle size Dv50 of the first negative electrode active material is smaller than that of the first negative electrode active material. -1 And 11μm≤Dv50 -1 ≤15μm. It can improve the rate performance of secondary batteries, reduce the probability of lithium plating on the first coating of the negative electrode, and also help improve the safety performance of secondary batteries.

[0022] In some embodiments of this application, 8μm≤T 20 ≤15μm, the porosity α of the first coating 20 Porosity α greater than that of the first active material layer 21 And 50% ≤ α 20 ≤60%. Secondary batteries can achieve high energy density while having low safety risks.

[0023] In some embodiments of this application, α 20 -α 21 ≥10%. Porosity α of the first coating. 20 The porosity α of the first active material layer 21 The difference α 20 -α 21With the value controlled within the above range, the secondary battery has good rate performance while having low safety risks and high energy density.

[0024] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0025] The beneficial effects of this application are:

[0026] This application provides a secondary battery and an electronic device. When the separator thickness of the secondary battery is in the range of 4μm to 20μm, by simultaneously forming a first coating without silicon material and a second coating containing silicon material in the negative electrode sheet, and adjusting the thickness of the first coating to 3μm to 15μm, the first coating isolates the separator from the silicon material particles in the second coating. This reduces the probability of silicon material particles puncturing the separator and causing a localized physical short circuit during the charging and discharging process of the secondary battery, thereby reducing the safety risk of the secondary battery. Furthermore, the thinness of the first coating makes it extremely unlikely that the lithium-ion transport path in the negative electrode active material of the second coating will become longer. Therefore, the silicon material can fully utilize its high capacity, giving the secondary battery a high energy density. Thus, the secondary battery has a high energy density while reducing safety risks. Attached Figure Description

[0027] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0028] Figure 1 is a schematic cross-sectional view of the negative electrode sheet along its thickness and length directions in some embodiments of this application.

[0029] Figure 2 is a schematic cross-sectional view of the negative electrode sheet along its thickness and length directions in some other embodiments of this application.

[0030] Figure 3 is a schematic cross-sectional view of the negative electrode sheet along its thickness and length directions in some embodiments of this application.

[0031] Reference numerals: 10-Negative electrode sheet; 11-First coating; 12-Second coating; 121-First active material layer; 13-Negative current collector; 13a-First surface; 13b-Second surface; 122-Second active material layer; 123-Third active material layer. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely 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.

[0033] 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.

[0034] The first aspect of this application provides a secondary battery, comprising a positive electrode, a separator, and a negative electrode. The separator is disposed between the positive and negative electrode. The negative electrode includes a negative current collector and a first coating and a second coating disposed on at least one surface of the negative current collector. The second coating includes a first active material layer. Along the thickness direction of the negative electrode, the first active material layer is disposed between the negative current collector and the first coating and is connected to the first coating. The thickness of the separator is T. 11 4μm≤T 11 ≤20μm. The first active material layer includes a first negative electrode active material, which includes a first silicon material. The first coating does not include the first silicon material, and the thickness of the first coating is T. 20 3μm≤T 20 ≤15μm.

[0035] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as X, and its thickness direction as Z. It should be understood that the above definitions of direction are for the purpose of conveniently describing this application, and the directions defined in this application can be understood based on the relative positions of the elements in the accompanying drawings and actual products. Furthermore, the length and thickness directions of the negative electrode current collector, the first coating, and the second coating are the same as those of the negative electrode sheet. The aforementioned "first coating and second coating disposed on at least one surface of the negative electrode current collector" means that the first coating and the second coating are disposed on one or both surfaces of the negative electrode current collector. The aforementioned "surface" can be a portion of the surface of the negative electrode current collector or the entire surface of the negative electrode current collector. As shown in Figure 1, the negative electrode sheet 10 includes a negative current collector 13, a first coating 11, and a second coating 12. The negative current collector 13 includes a first surface 13a and a second surface 13b. The first coating 11 and the second coating 12 are simultaneously disposed on the first surface 13a and the second surface 13b of the negative current collector 13. The second coating 12 includes a first active material layer 121. Along the thickness direction Z of the negative electrode sheet 10, the first active material layer 121 is disposed between the negative current collector 13 and the first coating 11 and is connected to the first coating 11. The thickness of the first coating 11 is given by T. 20As shown. In some embodiments of this application, the first coating 11 and the second coating 12 may also be disposed only on the first surface 13a or the second surface 13b of the negative electrode current collector 13.

[0036] For example, T 11 The thickness T of the diaphragm is 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, or any value between any two of the above ranges. 11 When the thickness is less than 4μm, the membrane thickness is too thin, limiting its function and affecting the normal use of the secondary battery; the membrane thickness T... 11 When the thickness is greater than 20μm, the membrane thickness is too thick, which will increase the volume of the secondary battery and reduce its volumetric energy density.

[0037] For example, T 20 The thickness T is 3μm, 5μm, 7μm, 8μm, 10μm, 11μm, 13μm, 15μm, or any value between any two of the above ranges. The first coating isolates the separator from the silicon material in the second coating, thus reducing the probability of silicon particles puncturing the separator and causing a localized physical short circuit during the charging and discharging process of the secondary battery. 20 If the thickness of the first coating is less than 3μm, and the silicon material in the second coating expands in volume, the thickness of the first coating will be insufficient to withstand the expanded silicon material. This increases the likelihood of the separator being punctured by the silicon material, thus increasing the safety risk of the secondary battery. The thickness T of the first coating... 20 If the thickness of the first coating is greater than 15μm, the transport path of lithium ions in the negative electrode becomes longer, which will negatively affect the capacity of the negative electrode active material and reduce the energy density of the secondary battery.

[0038] During lithium delithiation and lithium insertion, the capacity contribution and volume expansion of silicon materials are almost linearly related. During the charging and discharging of a secondary battery, the silicon material in the second coating often undergoes significant volume expansion during these processes. Furthermore, silicon particles are relatively hard and have sharp surfaces. Therefore, when the volume expansion occurs and the separator thickness is between 4 μm and 20 μm, the silicon particles can easily damage the separator, leading to safety issues. The secondary battery of this application simultaneously incorporates a silicon-free first coating and a silicon-containing second coating in the negative electrode, with the thickness of the first coating controlled to be between 3 μm and 15 μm. This allows the first coating to isolate the separator from the silicon particles in the second coating, reducing the probability of silicon particles puncturing the separator and causing a localized physical short circuit during charging and discharging. This reduces the safety risk of the secondary battery. In addition, the thinness of the first coating greatly reduces the probability of a longer lithium-ion transport path in the negative electrode active material of the second coating. Therefore, the silicon material can fully utilize its high capacity, resulting in a high energy density for the secondary battery. Thus, the secondary battery achieves high energy density while reducing safety risks.

[0039] In some embodiments of this application, 4μm≤T 11 ≤13μm, 9μm≤T 20 ≤15μm. For example, T 11 It is 4μm, 6μm, 7μm, 8μm, 9μm, 11μm, 12μm, 13μm, or any value between any two of the above ranges. T 20 The thickness T of the diaphragm is 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or any value between any two of the above ranges. 11 And the thickness T of the first coating 20 Within the aforementioned range, the first coating can isolate the separator from the compression of silicon particles, reducing the probability of the separator being punctured by silicon particles. The first coating can also restrain the expansion of the substances in the second coating, improving the overall expansion of the negative electrode. In this way, the safety risks of the secondary battery are reduced, and the secondary battery also has high energy density.

[0040] In some embodiments of this application, 13 μm < T 11 ≤20μm, 3μm≤T 20 <9μm. For example, T 11 It is 13.1μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or any value between any two of the above ranges. T 20 The thickness T of the diaphragm is 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 8.9μm, or any value between any two of the above ranges. 11Satisfying 13μm < T 11 When the thickness is ≤20μm, the membrane itself has a certain resistance to puncture. Therefore, the thickness T of the first coating is... 20 Within the aforementioned range, the first coating has a relatively thin thickness, which, while preventing silicon material particles from piercing the separator, further reduces the transport distance of lithium ions in the negative electrode sheet, allowing the capacity of the negative electrode active material to be better utilized, and enabling the secondary battery to have lower safety risks and higher energy density.

[0041] This application addresses the thickness T of the diaphragm. 11 There are no particular restrictions on the control method, as long as it achieves the purpose of this application. For example, it can be achieved by directly purchasing a diaphragm with a thickness within the range of this application. This application specifies the thickness T of the first coating. 20 There are no particular restrictions on the method of adjustment, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the coating weight of the first coating layer.

[0042] In some embodiments of this application, the second coating further includes a second active material layer, which is disposed between the first active material layer and the negative current collector along the thickness direction of the negative electrode sheet. As shown in FIG2, the negative electrode sheet 10 includes a negative current collector 13, a first coating 11, and a second coating 12. The second coating 12 includes a first active material layer 121 and a second active material layer 122. Along the thickness direction Z of the negative electrode sheet 10, the second active material layer 122 is disposed between the first active material layer 121 and the negative current collector 13. The second active material layer 122, the first active material layer 121, and the first coating 11 are respectively disposed on two surfaces of the negative current collector 13. It is understood that in some embodiments, the first coating and the second coating may also be disposed on only one surface of the negative current collector. The second active material layer includes a second negative electrode active material, which includes a second silicon material. The mass percentage of the first silicon material in the first negative electrode active material is W1, and the mass percentage of the second silicon material in the second negative electrode active material is W2, where 0% ≤ W2 ≤ 40% and 3% ≤ W1 ≤ 99%. For example, W2 can be 0%, 3%, 9%, 15%, 20%, 23%, 28%, 30%, 32%, 36%, 40%, or any value between any two of the above ranges. W1 can be 3%, 10%, 21%, 29%, 40%, 47%, 60%, 72%, 80%, 90%, 99%, or any value between any two of the above ranges. A second active material layer is further provided in the second coating of the negative electrode sheet, and the mass percentage W1 of the first silicon material in the first negative electrode active material and the mass percentage W2 of the second silicon material in the second negative electrode active material are controlled within the above-mentioned range. This increases the content of the negative electrode active material in the negative electrode sheet, thereby improving the energy density of the secondary battery. Thus, the energy density of the secondary battery can be further improved while reducing its safety risks.

[0043] In some embodiments of this application, W1 > W2, and 3% ≤ W1 - W2 ≤ 99%. W1 > W2, and the value of W1 - W2 is 3%, 10%, 21%, 29%, 40%, 47%, 60%, 72%, 80%, 90%, 99%, or any value between any two of the above ranges. When the mass percentage W1 of the first silicon material in the first negative electrode active material is greater than the mass percentage W2 of the second silicon material in the second negative electrode active material, controlling the difference between W1 and W2 within the above range is beneficial for the silicon material particles to exert their high capacity during the charging and discharging process of the secondary battery, while controlling the volume expansion of the negative electrode sheet. This results in the secondary battery having high energy density while maintaining good safety performance.

[0044] In some embodiments of this application, W1 < W2, -10% ≤ W1-W2 < 0%. The values ​​of W1 < W2 and W1-W2 are -10%, -9%, -8%, -7%, -6%, -5%, -4%, -3%, -2%, -1%, or any value between any two of the above ranges. When the mass percentage W1 of the first silicon material in the first negative electrode active material is less than the mass percentage W2 of the second silicon material in the second negative electrode active material, the difference between W1 and W2 is controlled within the above range. With the overall silicon content of the negative electrode sheet being the same, the silicon particles are relatively uniformly distributed between the layers, reducing the probability of excessive silicon particles accumulating in the first active material layer and compressing the separator, reducing the risk of the separator being punctured by silicon particles. Less silicon particles are exposed in the first active material layer, which can reduce side reactions and electrolyte consumption, thereby improving the cycle stability of the secondary battery while maintaining good safety performance and high energy density.

[0045] In some embodiments of this application, as shown in FIG2, the thickness of the first active material layer 121 is T. 21 The thickness of the second active material layer 122 is T. 22 20μm≤T 21 ≤T 22 ≤200μm, 0.1≤T 21 / T 22 ≤1.0. For example, T 21 T is 20μm, 50μm, 80μm, 100μm, 120μm, 150μm, 180μm, 200μm, or any value between any two of the above ranges. 22 T is 20μm, 50μm, 80μm, 100μm, 120μm, 150μm, 180μm, 200μm, or any value between any two of the above ranges. 21 / T 22 The value is 0.1, 0.3, 0.5, 0.6, 0.8, 1.0, or any value between any two of the above ranges. The thickness T of the first active material layer... 21 The thickness T of the second active material layer 22 and the thickness T of the first active material layer 21 With respect to the thickness T of the second active material layer 22 The ratio T between them 21 / T 22 By adjusting the parameters within the aforementioned range, a relatively short lithium-ion transport path is achieved within the silicon system of the negative electrode, enabling the secondary battery to achieve high rate performance while maintaining good safety performance and high energy density.

[0046] In some embodiments of this application, as shown in FIG2, the thickness of the first active material layer 121 is T. 21The thickness of the second active material layer 122 is T. 22 20μm≤T 22 <T 21 ≤200μm, 1.0<T 21 / T 22 ≤1.5. For example, T 21 T is 20μm, 50μm, 80μm, 100μm, 120μm, 150μm, 180μm, 200μm, or any value between any two of the above ranges. 22 The value can be 20μm, 50μm, 80μm, 100μm, 120μm, 150μm, 180μm, 200μm, or any value between any two of the above ranges. For example, T 21 / T 22 The value is 1.1, 1.2, 1.3, 1.4, 1.5, or any value between any two of the above ranges. The thickness T of the first active material layer... 21 The thickness T of the second active material layer 22 and the thickness T of the first active material layer 21 With respect to the thickness T of the second active material layer 22 When the ratio between the two is controlled within the above range, the binding effect of the first active material layer on the second active material layer is stronger, which inhibits the expansion of the second active material layer. This is beneficial to improving the cycle stability of the secondary battery while maintaining good safety performance and high energy density.

[0047] This application specifies the thickness T of the first active material layer. 21 The thickness T of the second active material layer 22 There are no particular restrictions on the method of adjustment, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the coating weight of the first active material layer and the second active material layer.

[0048] In some embodiments of this application, 3% ≤ W1 < 20%, 100 μm ≤ T 21 ≤200μm, 3μm≤T 20 ≤8μm. 3%≤W1<20% and 100μm≤T 21 When ≤200μm, T 20 The thickness is 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, or any value between two of the above ranges. The higher the silicon content of the second coating, the thicker the first active material layer, the greater the pressure on the separator, and the easier it is for the separator to be punctured by silicon particles, leading to localized physical short circuits and increasing the safety risk of the secondary battery. When the mass percentage W1 of the first silicon material in the first negative electrode active material satisfies 3% ≤ W1 < 20% and the thickness T of the first active material layer... 21 Satisfying 100μm≤T21 When the thickness is ≤200μm, adjust the thickness T of the first coating. 20 Within the range of 3μm to 8μm, the content of the first silicon material, the thickness of the first active material layer, and the first coating layer are matched to ensure that the first coating layer provides good isolation between the silicon material particles and the separator. This reduces the possibility of silicon material particles piercing the separator, thereby reducing the risk of localized physical short circuits caused by the separator being pierced by silicon material particles, thus lowering the safety risks of the secondary battery. It also provides a shorter transport path for lithium ions in the first negative electrode active material, allowing the capacity of the first negative electrode active material to be fully utilized, resulting in a high energy density for the secondary battery.

[0049] In some embodiments of this application, 20% ≤ W1 ≤ 99%, 20 μm ≤ T 21 <100μm, 8μm<T 20 ≤15μm. 20%≤W1≤99% and 20μm≤T 21 When <100μm, T 20 The value is 8.1 μm, 10 μm, 11 μm, 13 μm, 15 μm, or any value between any two of the above ranges. When the mass percentage W1 of the first silicon material in the first negative electrode active material satisfies 20% ≤ W1 ≤ 99% and the thickness T of the first active material layer... 21 Satisfying 20μm≤T 21 When the thickness is <100μm, adjust the thickness T of the first coating. 20 Within the aforementioned range, the content of the first silicon material, the thickness of the first active material layer, and the thickness of the first coating layer are matched, enabling the first coating layer to achieve a good barrier effect between the silicon material particles and the separator. This reduces the possibility of silicon material particles piercing the separator, thereby reducing the risk of localized physical short circuits caused by the separator being pierced by silicon material particles, thus lowering the safety risks of the secondary battery. It also ensures that lithium ions in the first negative electrode active material have a shorter transport path, allowing the capacity of the first negative electrode active material to be fully utilized, resulting in a high energy density for the secondary battery.

[0050] In some embodiments of this application, the second coating further includes a third active material layer, which is disposed between the second active material layer and the negative current collector along the thickness direction of the negative electrode sheet. As shown in FIG3, the negative electrode sheet 10 includes a negative current collector 13, a first coating 11 and a second coating 12. The second coating 12 includes a first active material layer 121, a second active material layer 122 and a third active material layer 123. Along the thickness direction Z of the negative electrode sheet 10, the third active material layer 123 is disposed between the second active material layer 122 and the negative current collector 13. The third active material layer 123, the second active material layer 122, the first active material layer 121 and the first coating 11 are sequentially disposed on the surface of the negative current collector 13. The thickness of the third active material layer 123 is T. 23 As shown. It is understood that in some embodiments, the first and second coatings may also be disposed only on one surface of the negative electrode current collector. The third active material layer includes a third negative electrode active material, which includes a third silicon material, wherein the mass percentage of the third silicon material in the third negative electrode active material is W3, where W3 ≤ W2. When the second coating of the negative electrode sheet further comprises a second active material layer and a third active material layer, and the mass percentage of the second silicon material in the second negative electrode active material W2 and the mass percentage of the third silicon material in the third negative electrode active material W3 satisfy the above relationship, the content of the negative electrode active material in the negative electrode sheet is increased, and the expansion of the silicon material in the third active material layer is less than the expansion of the silicon material in the second active material layer. This is beneficial for improving the volume expansion of the negative electrode sheet, thereby enabling the secondary battery to have good safety and performance characteristics (such as rate performance, cycle performance, etc.) while maintaining high energy density.

[0051] This application does not impose any particular limitation on the thickness of the third active material layer, as long as it achieves the purpose of this application. For example, the thickness of the third active material layer can be from 0 μm to 50 μm.

[0052] This application does not impose any particular limitation on the mass percentage W3 of the third silicon material in the third anode active material, as long as the purpose of this application can be achieved. For example, the mass percentage W3 of the third silicon material in the third anode active material can be 0% to 10%.

[0053] In some embodiments of this application, the first, second, and third negative electrode active materials each independently include a carbon material; the carbon material includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, or hard carbon. The aforementioned types of carbon materials exhibit almost no volume change during lithium insertion / extraction, which can suppress the expansion of silicon materials, improve the volume expansion of the negative electrode sheet, and also possess high capacity. Using the aforementioned types of carbon materials as the first, second, and third negative electrode active materials helps reduce the probability of silicon materials expanding and puncturing the separator, resulting in lower safety risks for the secondary battery and enabling it to achieve higher capacity and energy density.

[0054] In some embodiments of this application, the first silicon material, the second silicon material, and the third silicon material each independently include pure silicon, silicon oxide (SiO2), etc. x The above materials have high capacity, and using them as the first, second, and third silicon materials is beneficial for achieving high energy density in secondary batteries.

[0055] In some embodiments of this application, the first coating includes a third negative electrode material, which includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, hard carbon, lithium titanate, alumina, boehmite, silicon dioxide, or polyvinylidene fluoride. These materials are inert and do not readily react with other materials in the secondary battery. Compared to silicon, these materials exhibit almost no volume change during lithium insertion / extraction, effectively suppressing the expansion of silicon in the second coating. This reduces the probability of silicon puncturing the separator and causing a localized physical short circuit, thus improving the overall volume expansion of the negative electrode sheet. Furthermore, these materials exhibit fewer side reactions with the electrolyte, which helps suppress the continuous consumption of electrolyte (solvent, additives) by silicon in the second coating, extending the battery's lifespan and improving its cycle stability. Therefore, using the aforementioned third negative electrode material in the first coating enables the secondary battery to have low safety risk, long cycle life, and high cycle stability.

[0056] In some embodiments of this application, the volume average particle size Dv50 of the third anode material -3 The volume average particle size Dv50 of the first negative electrode active material is smaller than that of the first negative electrode active material. -1 And 11μm≤Dv50 -1 ≤15μm. For example, the volume average particle size Dv50 of the first negative electrode active material. -1The particle size is 11 μm, 11.6 μm, 12 μm, 12.7 μm, 13.4 μm, 14 μm, 15 μm, or any value between any two of the above ranges. The volume average particle size Dv50 of the third anode material. -3 The volume average particle size Dv50 of the first negative electrode active material is smaller than that of the first negative electrode active material. -1 Furthermore, the volume average particle size Dv50 of the first negative electrode active material -1 When the control is within the above range, the third negative electrode material has a smaller volume average particle size and a larger specific surface area compared to the first negative electrode active material. This is beneficial for providing more active reaction sites, improving the rate performance of the secondary battery, and reducing the probability of lithium plating in the first coating of the negative electrode sheet. Moreover, the first coating composed of the third negative electrode material with a smaller volume average particle size is relatively more dense as a protective layer. When the first coating is squeezed by the second coating, the third negative electrode material can undergo particle rearrangement, which can reduce the probability of directly squeezing and puncturing the separator, thus improving the safety performance of the secondary battery.

[0057] In this application, "the volume average particle size Dv50 of the third anode material" -3 "This refers to the particle size distribution of the third anode material, which, starting from the smallest particle size, reaches 50% of the total volumetric particle size." "The volume average particle size Dv50 of the first anode active material..." -1 "This refers to the particle size distribution of the first negative electrode active material, which, starting from the smallest particle size, reaches 50% of the total volumetric particle size.

[0058] This application addresses the volume average particle size Dv50 of the third anode material. -3 There are no particular restrictions, as long as the purpose of this application can be achieved. For example, the volume average particle size Dv50 of the third anode material. -3 The size ranges from 8μm to 13μm.

[0059] This application does not impose any particular restrictions on the method of controlling the volume average particle size of the first and third negative electrode active materials, as long as the purpose of this application can be achieved. For example, it can be achieved through crushing, sieving, or other methods.

[0060] In some embodiments of this application, 8μm≤T 20 ≤15μm, the porosity α of the first coating 20 Porosity α greater than that of the first active material layer 21 And 50% ≤ α 20 ≤60%. For example, the porosity α of the first coating. 20 The thickness T of the first coating is 50%, 51%, 53%, 54%, 56%, 57%, 58%, 60%, or any value between any two of the above ranges. 20 Satisfying 8μm≤T20 When the porosity α of the first coating is ≤15μm, 20 Porosity α greater than that of the first active material layer 21 And the porosity α of the first coating 20 Within the aforementioned range, the first coating possesses suitable strength, and the probability of it adversely affecting ion transport in the second coating is low. Thus, while the first coating isolates the separator from the silicon particles in the second coating, lithium ions can freely pass through the first coating, allowing the silicon material to fully utilize its high capacity. Therefore, the secondary battery can achieve high energy density while maintaining low safety risks.

[0061] In some embodiments of this application, α 20 -α 21 ≥10%. For example, the porosity α of the first coating. 20 The porosity α of the first active material layer 21 The difference α 20 -α 21 The porosity α of the first coating is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 20%, or any value between any two of the above ranges. 20 The porosity α of the first active material layer 21 The difference α 20 -α 21 With the value controlled within the aforementioned range, both the first coating and the first active material layer possess suitable strength, and the first active material layer contains an appropriate amount of the first negative electrode active material. The probability of the first coating affecting the rate performance of the negative electrode sheet is low. Thus, while the first coating isolates the separator from the first silicon material particles in the first active material layer, lithium ions can freely pass through the first coating, allowing the silicon material to exert its high capacity. Therefore, the secondary battery exhibits good rate performance while possessing low safety risks and high energy density.

[0062] In some embodiments of this application, 10% ≤ α 20 -α 21 ≤15%. For example, the porosity α of the first coating. 20 The porosity α of the first active material layer 21 The difference α 20 -α 21 The porosity α of the first coating is 10%, 11%, 12%, 13%, 14%, 15%, or any value between any two of the above ranges. 20 The porosity α of the first active material layer 21 The difference α 20 -α 21With the value controlled within the above range, the secondary battery has good rate performance while having low safety risks and high energy density.

[0063] This application addresses the porosity α of the first active material layer. 21 There are no particular restrictions on the value of α, as long as it achieves the purpose of this application. For example, the porosity α of the first active material layer 21 It ranges from 40% to 55%.

[0064] In this application, "the porosity α of the first coating" 20 "Porosity α of the first active material layer" refers to the percentage of the volume of pores between various particles in the first coating to the total volume of the first coating. 21 "" refers to the percentage of the volume of the pores between various particles in the first active material layer to the total volume of the first active material layer.

[0065] This application addresses the porosity α of the first coating. 20 There are no particular restrictions on the method of adjustment, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting at least one of the particle size distribution of the third negative electrode material in the first coating or the compaction density of the negative electrode sheet. This application specifies the porosity α of the first active material layer. 21 There are no particular restrictions on the control method, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the particle size distribution of the first negative electrode active material in the first active material layer or the compaction density of the negative electrode sheet.

[0066] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors (such as lithium copper composite current collectors, carbon copper composite current collectors, nickel copper composite current collectors, titanium copper composite current collectors, etc.). In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector can be from 4 μm to 20 μm.

[0067] In some embodiments of this application, the first coating may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. This application does not impose any particular limitation on the mass percentage of the third negative electrode material, negative electrode conductive agent, dispersant, and negative electrode binder in the first coating, as long as the purpose of this application is achieved. For example, the mass ratio of the third negative electrode material, negative electrode conductive agent, dispersant, and negative electrode binder in the first coating may be (96.2 to 97.8):(0.3 to 0.7):(0.0 to 0.7):(1.5 to 2.5).

[0068] In some embodiments of this application, the first active material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. This application does not impose any particular limitation on the mass percentage of the first negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the first active material layer, as long as the purpose of this application is achieved. For example, the mass ratio of the first negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the first active material layer is (94.5 to 95.5):(0.5 to 1.0):(0.0 to 1.0):(3.0 to 4.0).

[0069] In some embodiments of this application, the second active material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. This application does not impose any particular limitation on the mass percentage of the second negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the second active material layer, as long as the purpose of this application is achieved. For example, the mass ratio of the second negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the second active material layer is (95.5 to 96.5):(0.5 to 0.8):(0.0 to 0.7):(2.5 to 3.0).

[0070] In some embodiments of this application, the third active material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. This application does not impose any particular limitation on the mass percentage of the third negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the third active material layer, as long as the purpose of this application is achieved. For example, the mass ratio of the third negative electrode active material, negative electrode conductive agent, dispersant, and negative electrode binder in the third active material layer is (95.5 to 96.5):(0.5 to 0.8):(0.0 to 0.7):(2.5 to 3.0).

[0071] This application does not impose any particular restrictions on the types of negative electrode conductive agents, dispersants, and negative electrode binders, as long as they can achieve the purpose of this application.

[0072] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) preparing a first coating slurry and a first active material layer slurry; (2) coating the first active material layer slurry onto the first surface of the negative electrode current collector, drying it to obtain a semi-finished negative electrode sheet coated with the first active material layer, coating the first coating slurry onto the surface of the first active material layer away from the negative electrode current collector, drying it to obtain a semi-finished negative electrode sheet with the first active material layer and the first coating layer coated on one side; (3) repeating step (2) on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with the first active material layer and the first coating layer coated on both sides; (4) obtaining the negative electrode sheet after cold pressing and slitting. In other embodiments, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) preparing a first coating slurry, a first active material layer slurry, and a second active material layer slurry; (2) coating the second active material layer slurry onto the first surface of the negative electrode current collector, and drying it to obtain a semi-finished negative electrode sheet coated with the second active material layer; coating the first active material layer slurry onto the surface of the second active material layer away from the negative electrode current collector, and drying it to obtain a semi-finished negative electrode sheet coated with the second active material layer and the first active material layer; coating the first coating slurry onto the surface of the first active material layer away from the negative electrode current collector, and drying it to obtain a semi-finished negative electrode sheet with a single-sided coating of the second active material layer, the first active material layer, and the first coating layer; (3) repeating step (2) on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with a double-sided coating of the second active material layer, the first active material layer, and the first coating layer; (4) obtaining the negative electrode sheet after cold pressing and slitting.In some embodiments, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) preparing a first coating slurry, a first active material layer slurry, a second active material layer slurry, and a third active material layer slurry; (2) coating the third active material layer slurry onto the first surface of the negative electrode current collector, and drying it to obtain a semi-finished negative electrode sheet coated with the third active material layer; coating the second active material layer slurry onto the surface of the third active material layer away from the negative electrode current collector, and drying it to obtain a semi-finished negative electrode sheet coated with the third active material layer and the second active material layer; coating the first active material layer slurry onto the surface of the second active material layer away from the negative electrode current collector. After drying, a semi-finished negative electrode sheet coated with a third active material layer, a second active material layer, and a first active material layer is obtained on the surface of the first active material layer away from the negative electrode current collector; after drying, a semi-finished negative electrode sheet coated with a third active material layer, a second active material layer, a first active material layer, and a first coating layer on one side is obtained; (3) Step (2) is repeated on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet coated with a third active material layer, a second active material layer, a first active material layer, and a first coating layer on both sides; (4) after cold pressing and slitting, a negative electrode sheet is obtained. This application does not impose any special restrictions on the solid content of the above-mentioned first coating slurry, first active material layer slurry, second active material layer slurry, and third active material layer slurry. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. This application does not impose any special restrictions on the above-mentioned drying, cold pressing, and slitting process parameters. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0073] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, in some embodiments, the diaphragm includes a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, the substrate layer can be any one of a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane. At least one surface of the substrate layer is provided with a surface treatment layer, which can be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The polymer layer comprises a polymer selected from at least one of polyamide, polyacrylonitrile, acrylate polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene). For example, in some embodiments, the diaphragm is a polypropylene diaphragm or a polyethylene diaphragm.

[0074] This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, in some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer, with the positive active material layer disposed on one or both surfaces of the positive current collector. The aforementioned "surface" can be a portion of the surface of the positive current collector or the entire surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector can contain aluminum foil or aluminum alloy foil, etc. The positive active material layer of this application contains a positive active material. This application does not impose any particular limitation on the type of positive active material, as long as it achieves the purpose of this application. For example, the positive active material can contain at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate, etc. In this application, the positive active material can also contain non-metallic elements, which can include at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. These elements can further improve the stability of the positive active material. In this application, there are no particular limitations on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm. The thickness of the single-layer positive electrode active material layer is 30 μm to 120 μm. Optionally, the positive electrode active material layer may also include at least one of a positive electrode conductive agent or a positive electrode binder. This application does not particularly limit the types of positive electrode conductive agents and positive electrode binders in the positive electrode active material layer, as long as the purpose of this application can be achieved. This application does not particularly limit the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode active material layer; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0075] The secondary battery of this application also includes an electrolyte. This application does not impose any particular limitation on the electrolyte, as long as it achieves the purpose of this application. For example, in some embodiments, the electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluoroborate. This application does not limit the content of lithium salt 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, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate, or vinylene carbonate. Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1,2-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.

[0076] The secondary battery of this application also includes a packaging bag for containing the positive electrode, negative electrode, separator, electrolyte, and other components known in secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application.

[0077] This application does not impose any particular limitation on secondary batteries, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0078] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the separator, positive electrode, separator and negative electrode in sequence, and winding, folding or other operations as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the separator, positive electrode, separator and negative electrode in sequence, and then fixing the four corners of the entire stacked structure to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery.

[0079] A second aspect of this application provides an electronic device comprising a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0080] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, 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, household large-capacity batteries, and lithium-ion capacitors.

[0081] Example

[0082] 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.

[0083] Test methods and equipment:

[0084] The thickness T of the diaphragm 11 The test:

[0085] The thickness was tested using a thickness gauge according to the national standard GB / T 6672-2001 "Mechanical Measurement Method for Determination of Thickness of Plastic Films and Sheets".

[0086] Thickness testing of the first coating, the first active material layer, and the second active material layer:

[0087] Argon ion polishing was performed on the negative electrode sheet to obtain its cross-section. The morphology of the cross-section along the thickness direction of the negative electrode sheet was observed and scanned electron microscopy (SEM) images were taken using a field emission scanning electron microscope (Philips XL-30). The thickness T of the first coating was measured using the SEM. 20 The thickness T of the first active material layer 21 The thickness T of the second active material layer 22 .

[0088] Test of volume average particle size:

[0089] The volume average particle size Dv50 of the first negative electrode active material was determined using a laser particle size analyzer. -1 The volume average particle size Dv50 of the third anode material -3 .

[0090] The porosity α of the first coating 20 The test:

[0091] Fifty negative electrode sheets with a radius of r1 were punched using the same die. The thickness h1 of each negative electrode sheet was measured using a micrometer, and they were placed in the sample chamber of an AccuPyc 1340 instrument. Helium (He) gas was used to fill the sealed sample chamber, and Bohr's law PV = nRT was used to determine the true volume V1 of the negative electrode sheet. After testing, the number of small discs was counted, and the apparent volume πr1 of the negative electrode sheet was calculated. 2 ×50×h1. Finally, the porosity α1 of the negative electrode sheet is obtained by the following formula: α1(%)=(1-V1 / (πr1) 2 ×50×h1))×100%.

[0092] The first coating on the surface of the negative electrode sheet was scraped off, and this sample was designated as sample A. Fifty samples A with a radius of r2 were punched out using the same die. The thickness h2 of each sample A was measured using a micrometer, and the samples were placed into the sample chamber of an AccuPyc 1340 instrument. The sealed sample chamber was filled with helium (He), and the true volume V2 of sample A was measured using Bohr's law PV = nRT. After the test, the number of small discs was counted, and the apparent volume πr2 of sample A was calculated. 2×50×h2. Finally, the porosity α2 of sample A is obtained by the following formula: α2(%)=(1-V2 / (πr2) 2 ×50×h2))×100%.

[0093] α 20 (%)=α1α2(V1-V2) / (V1α2-α1V2)×100%.

[0094] The porosity α of the first active material layer 21 The test:

[0095] The first coating on the surface of the negative electrode sheet was scraped off, and this sample was designated as sample A. Fifty samples A with a radius of r2 were punched out using the same die. The thickness h2 of each sample A was measured using a micrometer, and the samples were placed into the sample chamber of an AccuPyc 1340 instrument. The sealed sample chamber was filled with helium (He), and the true volume V2 of sample A was measured using Bohr's law PV = nRT. After the test, the number of small discs was counted, and the apparent volume πr2 of sample A was calculated. 2 ×50×h2. Finally, the porosity α2 of sample A is obtained by the following formula: α2(%)=(1-V2 / (πr2) 2 ×50×h2))×100%.

[0096] Continue scraping away the first active material layer on the surface of the negative electrode sheet, denoted as sample B. Fifty sample B sheets with a radius of r3 were punched out using the same die. The thickness h3 of each sample B sheet was measured using a micrometer, and the samples were placed into the sample chamber of an AccuPyc 1340 instrument. The sealed sample chamber was filled with helium (He), and the true volume V3 of sample B was measured using Bohr's law PV = nRT. After the test, the number of small discs was counted, and the apparent volume πr3 of sample B was calculated. 2 ×50×h3. Finally, the porosity α3 of sample B is obtained by the following formula: α3(%)=(1-V3 / (πr3) 2 ×50×h3))×100%.

[0097] α 21 (%)=α3α2(V2-V3) / (V2α3-α2V3)×100%.

[0098] Security risk testing:

[0099] (1) Test of self-discharge value:

[0100] Take the lithium-ion battery after formation and record the open circuit potential OCV1. After resting for one week, measure the open circuit potential OCV2 again. Calculate the self-discharge value (mV / d) according to the following formula: (OCV1-OCV2) / resting time.

[0101] (2) Test of recoverable capacity retention:

[0102] The lithium-ion battery was discharged to 2.5V at a constant current of 0.5C, then charged to 4.3V at a constant current of 0.5C, and then charged to 0.05C at a constant voltage of 4.3V. It was then discharged to 2.5V at a constant current of 0.5C, and recorded as the initial capacity C0 of the lithium-ion battery. It was then charged to 4.3V at a constant current of 0.5C, and then charged to 0.05C at a constant voltage of 4.3V. The initial thickness of the fully charged lithium-ion battery at this point was measured.

[0103] The lithium-ion battery was stored at 80°C for 24 hours, and the thickness change during the storage process was recorded. The test was performed every 6 hours.

[0104] After 24 hours of storage, the battery is discharged at 0.5C to 2.5V, and the residual capacity C1 of the lithium-ion battery is recorded. Then, it is charged at a constant current of 0.5C to 4.3V, charged at a constant voltage of 4.3V to 0.05C, and then discharged at a constant current of 0.5C to 0.5V, which is recorded as the recoverable capacity C2 of the lithium-ion battery.

[0105] The recoverable capacity retention rate (%) is calculated using the following formula: C2 / C0 × 100%.

[0106] The lower the self-discharge value and the higher the recoverable capacity retention rate, the lower the safety risk of the lithium-ion battery; conversely, the higher the self-discharge value and the lower the recoverable capacity retention rate, the greater the safety risk of the lithium-ion battery.

[0107] Energy density testing:

[0108] First, the lithium-ion battery is weighed to obtain its mass m.

[0109] Then, charge the lithium-ion battery according to the following procedure, and then discharge it to obtain the discharge capacity E of the lithium-ion battery:

[0110] Charging: Charge at a constant current of 2C to 4.3V, then charge at a constant voltage of 4.3V to 0.05C;

[0111] Discharge: Discharge at a constant current of 0.5C to 2.5V to obtain the discharge capacity E.

[0112] The energy density (ED) of a lithium-ion battery can be calculated using the following formula: ED (Wh / kg) = E / m.

[0113] Example 1-1

[0114] <Preparation of Negative Electrode Sheets>

[0115] The third anode material, artificial graphite, the anode conductive agent, conductive carbon black (Super P), and the anode binder, polyvinylidene fluoride (PVDF, weight average molecular weight 9000), were mixed at a mass ratio of 97:0.5:2.5. Deionized water was added as a solvent, and the mixture was stirred under vacuum until a first coating slurry with a solid content of 70 wt% and a homogeneous system was obtained. The volume average particle size (Dv50) of the third anode material was [not specified]. -3 =8μm.

[0116] The first negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed in a mass ratio of 96:0.5:3.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first active material slurry with a solid content of 70 wt% and a homogeneous system was formed. The first negative electrode active material comprises a first silicon material (pure silicon) and a carbon material (artificial graphite), with the first silicon material comprising 41.7% by mass (W1) = 41.7%. The volume average particle size of the first negative electrode active material is Dv50. -1 =13μm.

[0117] The first active material layer slurry is coated onto the first surface of the negative electrode current collector copper foil with a thickness of 8 μm, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the first active material layer. The first coating slurry is then coated onto the surface of the first active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet with a single-sided coating of the first active material layer and the first coating. The above steps are then repeated on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with a double-sided coating of the first active material layer and the first coating (structure shown in Figure 1, but not limited to Figure 1). After cold pressing, cutting, and welding of negative electrode tabs (copper tabs), a negative electrode sheet with dimensions of 116 mm × 1050 mm is obtained for use. The coating weight of the first active material layer is 8 mg / cm³. 2 The coating weight of the first coating is 0.4 mg / cm³. 2 The thickness T of the first active material layer 21 =120μm, the thickness T of the first coating 20 =7μm. The porosity α of the first coating 20 =55%, porosity α of the first active material layer 21 =43%. The compacted density of the negative electrode sheet is 1.4 g / cm³. 3 .

[0118] <Preparation of the positive electrode>

[0119] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.91 Co 0.06 Mn 0.03O2, positive electrode conductive agent: multi-walled carbon nanotubes (CNTs), positive electrode conductive agent: conductive carbon black (Super P), positive electrode binder: polyvinylidene fluoride (PVDF, weight average molecular weight 5×10⁻⁶). 5 The materials were mixed at a mass ratio of 97:0.6:1:1.4, with N-methylpyrrolidone (NMP) added as a solvent. The mixture was stirred under vacuum until a homogeneous positive electrode slurry with a solid content of 75 wt% was obtained. The positive electrode slurry was uniformly coated onto one surface of a 13 μm thick aluminum foil current collector and dried at 100°C to obtain a positive electrode sheet with a single-sided coating of the positive active material layer. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive active material layer. After cold pressing and slitting, positive electrode sheets with dimensions of 1450 mm × 63 mm were obtained for later use. The coating weight of the positive active material layer was 13 mg / cm³. 2 The compaction density of the positive electrode active material layer is 3.45 g / cm³. 3 .

[0120] <Preparation of the diaphragm>

[0121] A polypropylene (PP) film with a thickness of 16 μm is used.

[0122] <Preparation of Electrolyte>

[0123] In a dry argon atmosphere, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) are mixed in a mass ratio of 20:30:40:10 to obtain an organic solvent. Then, lithium salt LiPF6 is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1 mol / L.

[0124] <Preparation of Lithium-ion Batteries>

[0125] The separator, negative electrode, and positive electrode prepared above are stacked and wound in sequence to obtain a wound electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte (6g). After vacuum sealing, standing, formation, degassing, and edge trimming, a lithium-ion battery is obtained. Formation steps: constant current charging at 0.02C to 3.3V, and then constant current charging at 0.1C to 3.6V.

[0126] Examples 1-2 to Examples 1-4

[0127] In addition to adjusting the thickness T of the first coating according to Table 1 20 Except for the above, the rest is the same as in Example 1-1.

[0128] Examples 1-5

[0129] Except for adjusting the thickness of the diaphragm to 4 μm in the <Preparation of Diaphragm> section, the rest is the same as in Examples 1-3.

[0130] Examples 1-6

[0131] Except for adjusting the thickness of the diaphragm to 20 μm in the <Preparation of Diaphragm> section, the rest is the same as in Examples 1-3.

[0132] Examples 1-7

[0133] In addition to adjusting the thickness T of the first coating according to Table 1 20 Except for the above, the rest are the same as in Examples 1-5.

[0134] Examples 1-8

[0135] Except for adjusting the thickness of the diaphragm to 8 μm in the <Preparation of Diaphragm> section, the rest is the same as in Examples 1-7.

[0136] Examples 1-9

[0137] Except for adjusting the thickness of the diaphragm to 13 μm in the <Preparation of Diaphragm> section, the rest is the same as in Examples 1-7.

[0138] Examples 1-10 to Examples 1-12

[0139] In addition to adjusting the thickness T of the first coating according to Table 1 20 Except for the above, the rest are the same as in Examples 1-8.

[0140] Examples 1-13

[0141] Except for adjusting the thickness of the diaphragm to 14 μm in the <Preparation of Diaphragm> section, the rest is the same as in Example 1-1.

[0142] Examples 1-14

[0143] Except for adjusting the thickness of the diaphragm to 20 μm in the <Preparation of Diaphragm> section, the rest is the same as in Example 1-1.

[0144] Example 2-1

[0145] <Preparation of Negative Electrode Sheets>

[0146] The third anode material, artificial graphite, the anode conductive agent Super P, and the anode binder PVDF (weight average molecular weight 9000) were mixed at a mass ratio of 97:0.5:2.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first coating slurry with a solid content of 70 wt% and a homogeneous system was obtained. The volume average particle size of the third anode material was Dv50. -3 =8μm.

[0147] The first negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed in a mass ratio of 96:0.5:3.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first active material slurry with a solid content of 70 wt% and a homogeneous system was formed. The first negative electrode active material comprises a first silicon material (pure silicon) and a carbon material (artificial graphite), with the first silicon material comprising 41.7% by mass (W1) = 41.7%. The volume average particle size of the first negative electrode active material is Dv50. -1 =14μm.

[0148] The second negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed at a mass ratio of 97:0.5:2.5. Deionized water was added as a solvent, and the mixture was stirred under vacuum until a homogeneous second active material slurry with a solid content of 70 wt% was obtained. The volume average particle size of the second negative electrode active material was 16 μm. The second negative electrode active material included carbon-based artificial graphite, and the mass percentage (W2) of the second silicon material in the second negative electrode active material was 0%.

[0149] The second active material layer slurry is coated onto the first surface of an 8μm thick copper foil used as a negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the second active material layer. The first active material layer slurry is coated onto the surface of the second active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with both the second and first active material layers. The first coating slurry is coated onto the surface of the first active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet with a single-sided coating of the second active material layer, the first active material layer, and the first coating layer. The above steps are then repeated on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with a double-sided coating of the second active material layer, the first active material layer, and the first coating layer (the structure is shown in Figure 2, but is not limited to Figure 2). After cold pressing, cutting, and welding of negative electrode tabs (copper tabs), a negative electrode sheet with dimensions of 116mm × 1050mm is obtained for later use. The coating weight of the first active material layer is 8 mg / cm³. 2 The coating weight of the first coating is 0.4 mg / cm³. 2 The thickness T of the second active material layer 22 =90μm, the thickness T of the first active material layer 21 =90μm, the thickness T of the first coating 20 =5μm. The porosity α of the first coating. 20 =55%, porosity α of the first active material layer 21 =43%. The compacted density of the negative electrode sheet is 1.5 g / cm³.3 .

[0150] The preparation of the positive electrode, the separator, the electrolyte, and the lithium-ion battery are the same as in Examples 1-1.

[0151] Example 2-2

[0152] Except for the fact that in the <Preparation of the Negative Electrode Sheet>, the second negative electrode active material carbon material artificial graphite is replaced with the second silicon material pure silicon and the second carbon material artificial graphite, and the mass percentage of the second silicon material W2 in the second negative electrode active material is 20% and the mass percentage of the carbon material is 80%, the rest is the same as in Example 2-1.

[0153] Examples 2-3 and 2-4

[0154] Except for adjusting the mass percentage W2 of the second silicon material in the second negative electrode active material according to Table 2, the rest is the same as in Example 2-2.

[0155] When the mass percentage W2 of the second silicon material in the second negative electrode active material changes, the mass percentage of the carbon material in the second negative electrode active material also changes. The sum of the mass percentage of the second silicon material and the mass percentage of the carbon material in the second negative electrode active material is 100%.

[0156] Examples 2-5 to Examples 2-8

[0157] Except for adjusting the mass percentage W1 of the first silicon material in the first negative electrode active material according to Table 2, the rest is the same as in Example 2-1.

[0158] When the mass percentage W1 of the first silicon material in the first negative electrode active material changes, the mass percentage of the carbon material in the first negative electrode active material also changes. The sum of the mass percentage of the first silicon material in the first negative electrode active material and the mass percentage of the carbon material in the first negative electrode active material is 100%.

[0159] Examples 2-9 to 2-12

[0160] Except for adjusting the mass percentage W1 of the first silicon material in the first negative electrode active material according to Table 2, the rest is the same as in Example 2-2.

[0161] When the mass percentage W1 of the first silicon material in the first negative electrode active material changes, the mass percentage of the carbon material in the first negative electrode active material also changes. The sum of the mass percentage of the first silicon material in the first negative electrode active material and the mass percentage of the carbon material in the first negative electrode active material is 100%.

[0162] Example 3-1

[0163] In addition to the volume average particle size Dv50 of the first negative electrode active material in the <Preparation of Negative Electrode Sheet>, -1 The thickness of the first active material layer (T) was adjusted from 14 μm to 13 μm, and the thickness of the first active material layer (T) was adjusted according to Table 3. 21 Except for the above, the rest is the same as in Example 2-1.

[0164] Examples 3-2 to 3-10

[0165] In addition to adjusting the thickness T of the first active material layer according to Table 3 21 and / or the thickness T of the second active material layer 22 Except for the above, it is the same as in Example 3-1.

[0166] Example 4-1

[0167] In addition to the volume average particle size Dv50 of the first negative electrode active material in the <Preparation of Negative Electrode Sheet>, -1 The thickness T of the first coating was adjusted from 14μm to 13μm, and the thickness T of the first coating was adjusted according to Table 4. 20 The thickness T of the first active material layer 21 The thickness T of the second active material layer 22 Except for the above, the rest are the same as in Examples 2-5.

[0168] Examples 4-2 to 4-4

[0169] In addition to adjusting the thickness T of the first coating according to Table 4 20 Except for the above, the rest are the same as in Examples 2-5.

[0170] Examples 4-5 to 4-8

[0171] In addition to adjusting the thickness T of the first coating according to Table 4 20 The thickness T of the first active material layer 21 The thickness T of the second active material layer 22 Except for adjusting the thickness of the diaphragm to 8 μm in the <Preparation of Diaphragm>, the rest is the same as in Example 2-1.

[0172] Example 5-1

[0173] <Preparation of Negative Electrode Sheets>

[0174] The third anode material, artificial graphite, the anode conductive agent Super P, and the anode binder PVDF (weight average molecular weight 9000) were mixed at a mass ratio of 97:0.5:2.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first coating slurry with a solid content of 70 wt% and a homogeneous system was obtained. The volume average particle size of the third anode material was Dv50.-3 =8μm.

[0175] The first negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed in a mass ratio of 96:0.5:3.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first active material slurry with a solid content of 70 wt% and a homogeneous system was formed. The first negative electrode active material comprises a first silicon material (pure silicon) and a carbon material (artificial graphite), with the first silicon material comprising 41.7% by mass (W1) = 41.7%. The volume average particle size of the first negative electrode active material is Dv50. -1 =13μm.

[0176] The second negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed at a mass ratio of 97:0.5:2.5. Deionized water was added as a solvent, and the mixture was stirred under vacuum until a second active material slurry with a solid content of 70 wt% and a homogeneous system was formed. The second negative electrode active material includes carbon material artificial graphite, and the mass percentage of the second silicon material in the second negative electrode active material, W2 = 0%.

[0177] The third anode active material, the anode conductive agent Super P, and the anode binder PVDF (weight average molecular weight 9000) were mixed at a mass ratio of 97:0.5:2.5. Deionized water was added as a solvent, and the mixture was stirred under vacuum until a homogeneous third active material slurry with a solid content of 70 wt% was obtained. The third anode active material includes hard carbon, and the mass percentage of the third silicon material in the third anode active material is W3 = 0%.

[0178] A third active material layer slurry is coated onto the first surface of an 8μm thick copper foil used as a negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the third active material layer. A second active material layer slurry is coated onto the surface of the third active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the third and second active material layers. A first active material layer slurry is coated onto the surface of the second active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the third, second, and first active material layers. A first coating slurry is coated onto the surface of the first active material layer away from the negative electrode current collector, and dried at 110°C to obtain a semi-finished negative electrode sheet coated with the third, second, first, and first coating layers on one side. Then, the above steps are repeated on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with a double-sided coating of a third active material layer, a second active material layer, a first active material layer, and a first coating layer (the structure is shown in Figure 3, but is not limited to Figure 3). After cold pressing, cutting, and welding of negative electrode tabs (copper tabs), a negative electrode sheet with dimensions of 116mm × 1050mm is obtained for use. The coating weight of the first active material layer is 8mg / cm³. 2 The coating weight of the first coating is 0.4 mg / cm³. 2 The thickness of the third active material layer is 90 μm, and the thickness of the second active material layer is T. 22 =90μm, the thickness T of the first active material layer 21 =90μm, the thickness T of the first coating 20 =5μm. The porosity α of the first coating. 20 =55%, porosity α of the first active material layer 21 =43%. The compacted density of the negative electrode sheet is 1.5 g / cm³. 3 .

[0179] The preparation of the positive electrode, the separator, the electrolyte, and the lithium-ion battery are the same as in Examples 1-1.

[0180] Example 5-2

[0181] Except for the fact that in the <Preparation of the Negative Electrode Sheet>, the second negative electrode active material carbon material artificial graphite is replaced with the second silicon material pure silicon and the carbon material hard carbon, and the mass percentage content of the second silicon material in the second negative electrode active material W2 = 20%, the rest is the same as in Example 5-1.

[0182] Example 5-3

[0183] Except for the fact that in the <Preparation of Negative Electrode Sheet>, the third negative electrode active material carbon material artificial graphite is replaced with the third silicon material pure silicon and the carbon material hard carbon, and the mass percentage of the third silicon material in the third negative electrode active material W3 = 10%, the rest is the same as in Example 5-2.

[0184] Examples 6-1 to 6-8

[0185] Except for adjusting the types of carbon and silicon materials in the first negative electrode active material, the types of carbon and silicon materials in the second negative electrode active material, the types of carbon and silicon materials in the third negative electrode active material, and the type of the third negative electrode material according to Table 6, the rest are the same as in Examples 5-3.

[0186] Examples 7-1 to 7-4

[0187] In addition to adjusting the volume average particle size Dv50 of the first negative electrode active material according to Table 7 -1 Except for the above, the rest are the same as in Examples 1-9.

[0188] Examples 8-1 to 8-4

[0189] In addition to adjusting the porosity α of the first coating according to Table 8 20 Except for the above, the rest are the same as in Examples 1-9.

[0190] Comparative Example 1-1

[0191] <Preparation of Negative Electrode Sheets>

[0192] The first negative electrode active material, the negative electrode conductive agent Super P, and the negative electrode binder PVDF (weight average molecular weight 9000) were mixed in a mass ratio of 96:0.5:3.5, with deionized water added as a solvent. The mixture was stirred under vacuum until a first active material slurry with a solid content of 70 wt% and a homogeneous system was formed. The first negative electrode active material comprises a first silicon material (pure silicon) and a carbon material (artificial graphite), with the first silicon material comprising 41.7% by mass (W1) = 41.7%. The volume average particle size of the first negative electrode active material is Dv50. -1 =13μm.

[0193] The first active material layer slurry is coated onto the first surface of the negative electrode current collector copper foil with a thickness of 8 μm, and dried at 110°C to obtain a semi-finished negative electrode sheet with a single-sided coating of the first active material layer. Then, the above steps are repeated on the second surface of the negative electrode current collector to obtain a semi-finished negative electrode sheet with a double-sided coating of the first active material layer. After cold pressing, cutting, and welding of negative electrode tabs (copper tabs), a negative electrode sheet with a size of 116 mm × 1050 mm is obtained for use. The coating weight of the first active material layer is 8 mg / cm³. 2The thickness T of the first active material layer 21 =120μm; Porosity α of the first active material layer 21 =43%. The compacted density of the negative electrode sheet is 1.5 g / cm³. 3 .

[0194] The preparation of the positive electrode, the separator, the electrolyte, and the lithium-ion battery are the same as in Examples 1-1.

[0195] Comparative Examples 1-2 and 1-3

[0196] In addition to adjusting the thickness T of the first coating according to Table 1 20 Except for the above, the rest is the same as in Example 1-1.

[0197] Comparative Examples 1-4

[0198] Except for adjusting the thickness of the diaphragm to 8 μm in the <Preparation of Diaphragm> section, the rest is the same as Comparative Example 1-1.

[0199] Comparative Example 2-1 and Comparative Example 2-2

[0200] In addition to adjusting the thickness T of the first coating according to Table 2 20 Except for the above, the rest are the same as in Examples 2-6.

[0201] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 8.

[0202] Table 1

[0203] Note: In Table 1, "\" indicates that there is no corresponding parameter.

[0204] As can be seen from Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-4, the secondary batteries of this application have negative electrode sheets containing silicon material and a separator thickness T. 11 When the thickness is in the range of 4μm to 20μm, a first coating is formed that does not include the first silicon material, and the thickness T of the first coating is set. 20 By adjusting the electrode thickness within the range of 3μm to 15μm, the secondary battery can exhibit both low self-discharge and high recoverable capacity retention while maintaining a high energy density. This indicates that the safety risk of the secondary battery in this embodiment has been reduced, and the secondary battery possesses a high energy density. Therefore, this demonstrates that the secondary battery in this embodiment achieves a high energy density while reducing safety risks. In contrast, the secondary battery in the comparative example has a negative electrode containing silicon material and a separator thickness T. 11 When the thickness is in the range of 4μm to 20μm, no first coating is applied, or the thickness T of the first coating is... 20The secondary batteries in the comparative examples, which are not within the 3μm to 15μm range, exhibit higher self-discharge values ​​and lower recoverable capacity retention rates, or lower energy densities. This indicates that the comparative secondary batteries have higher safety risks or lower energy densities, making it impossible to balance low safety risks and high energy density. Furthermore, the secondary batteries in comparative examples 1-3 cannot undergo high-rate charge-discharge operations during use, exhibiting poor charge-discharge kinetics.

[0205] Table 2

[0206] When the second coating of the negative electrode sheet includes a first active material layer and a second active material layer, the mass percentage W1 of the first silicon material in the first negative electrode active material and the mass percentage W2 of the second silicon material in the second negative electrode active material usually affect the safety performance and energy density of the secondary battery. As can be seen from Examples 2-1 to 2-12, secondary batteries using first silicon material with a mass percentage W1 in the first negative electrode active material and second silicon material with a mass percentage W2 in the second negative electrode active material within the scope of this application have lower self-discharge values, higher recoverable capacity retention rates, and higher energy densities, indicating that the secondary batteries have lower safety risks and higher energy densities.

[0207] When the mass percentage W1 of the first silicon material in the first negative electrode active material is greater than the mass percentage W2 of the second silicon material in the second negative electrode active material, the difference W1-W2 between the mass percentage W1 of the first silicon material in the first negative electrode active material and the mass percentage W2 of the second silicon material in the second negative electrode active material usually affects the safety performance and energy density of the secondary battery. As can be seen from Examples 2-1 to 2-8, secondary batteries with W1-W2 values ​​within the range of this application have lower self-discharge values, higher recoverable capacity retention rates, and higher energy densities, indicating that the secondary batteries have lower safety risks and higher energy densities. However, changes in the mass percentage W1 of the first silicon material in the first negative electrode active material have a significant impact on the energy density of the secondary battery. As can be seen from Examples 2-6, Comparative Examples 2-1 and 2-2, when the mass percentage W1 of the first silicon material in the first negative electrode active material is the same, the thickness T of the separator in the secondary battery of this application embodiment... 11 And the thickness T of the first coating 20 Within the scope of this application, it can better balance the safety performance and energy density of the secondary battery. Furthermore, the secondary battery in Comparative Example 2-2 cannot be charged and discharged at high rates during use, exhibiting poor charge-discharge kinetics.

[0208] When the mass percentage W1 of the first silicon material in the first negative electrode active material is less than the mass percentage W2 of the second silicon material in the second negative electrode active material, the difference W1-W2 between the mass percentage W1 of the first silicon material in the first negative electrode active material and the mass percentage W2 of the second silicon material in the second negative electrode active material usually affects the safety performance and energy density of the secondary battery. As can be seen from Examples 2-9 to Examples 2-12, secondary batteries with W1-W2 values ​​within the range of this application have lower self-discharge values, higher recoverable capacity retention rates, and higher energy densities, indicating that the secondary batteries have lower safety risks and higher energy densities.

[0209] Table 3

[0210] The thickness T of the first active material layer 21 The thickness T of the second active material layer 22 And the ratio T between the two 21 / T 22 This typically affects the safety performance and energy density of secondary batteries. As can be seen from Examples 3-1 to 3-10, the thickness T of the first active material layer is selected... 21 The thickness T of the second active material layer 22 and the ratio T between the two 21 / T 22 The secondary batteries within the scope of this application have low self-discharge values, high recoverable capacity retention rates, and high energy density, indicating that the secondary batteries have low safety risks and high energy density.

[0211] Table 4

[0212] The mass percentage W1 of the first silicon material and the thickness T of the first active material in the first negative electrode active material. 21 Satisfying 3% ≤ W1 < 20% and 100μm ≤ T 21 When the thickness T of the first coating is ≤200μm, 20 This typically affects the safety performance and energy density of secondary batteries. As can be seen from Examples 4-1 to 4-4, the thickness T of the first coating is selected... 20 The secondary batteries within the scope of this application have low self-discharge values, high recoverable capacity retention rates, and high energy density, indicating that the secondary batteries have low safety risks and high energy density.

[0213] The mass percentage W1 of the first silicon material and the thickness T of the first active material in the first negative electrode active material. 21 Satisfying 20% ​​≤ W1 ≤ 99% and 20 μm ≤ T 21When the thickness T of the first coating is <100μm, 20 This typically affects the safety performance and energy density of secondary batteries. As can be seen from Examples 4-5 to 4-8, the thickness T of the first coating is selected... 20 The secondary batteries within the scope of this application have low self-discharge values, high recoverable capacity retention rates, and high energy density, indicating that the secondary batteries have low safety risks and high energy density.

[0214] Table 5

[0215] When the second coating of the negative electrode sheet includes a first active material layer, a second active material layer, and a third active material layer, the relationship between the mass percentage W2 of the second silicon material in the second negative electrode active material and the mass percentage W3 of the third silicon material in the third negative electrode active material often affects the safety performance and energy density of the secondary battery. As can be seen from Examples 5-1 to 5-3, secondary batteries with W3 ≤ W2 exhibit lower self-discharge values, higher recoverable capacity retention rates, and higher energy density, indicating that the secondary battery has lower safety risks and higher energy density.

[0216] Table 6

[0217] The types of the first, second, third, and fourth negative electrode active materials also typically affect the safety performance and energy density of a secondary battery. As can be seen from Examples 5-3, 6-1 to 6-8, secondary batteries using first, second, third, and fourth negative electrode active materials within the scope of this application exhibit lower self-discharge values, higher recoverable capacity retention rates, and higher energy density, indicating that the secondary batteries have lower safety risks and higher energy density.

[0218] Table 7

[0219] The volume average particle size of the third anode material is Dv50. -3 The volume average particle size Dv50 of the first negative electrode active material -1 This typically affects the safety performance and energy density of secondary batteries. As can be seen from Examples 1-9 and Examples 7-1 to 7-4, the volume average particle size Dv50 of the selected third anode material... -3 The volume average particle size Dv50 of the first negative electrode active material -1 The secondary batteries within the scope of this application have low self-discharge values, high recoverable capacity retention rates, and high energy density, indicating that the secondary batteries have low safety risks and high energy density.

[0220] Table 8

[0221] The porosity α of the first coating 20 and the porosity α of the first active material layer 21 This typically affects the safety performance and energy density of secondary batteries. As can be seen from Examples 1-9 and Examples 8-1 to 8-4, the porosity α of the first coating is selected... 20 and the porosity α of the first coating 20 and the porosity α of the first active material layer 21 The difference α between 20 -α 21 The secondary batteries within the scope of this application have low self-discharge values, high recoverable capacity retention rates, and high energy density, indicating that the secondary batteries have low safety risks and high energy density.

[0222] 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, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0223] 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.

[0224] 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 a positive electrode, a separator, and a negative electrode, wherein the separator is disposed between the positive electrode and the negative electrode, the negative electrode comprising a negative current collector and a first coating and a second coating disposed on at least one surface of the negative current collector, the second coating comprising a first active material layer, wherein the first active material layer is disposed between the negative current collector and the first coating and is connected to the first coating along the thickness direction of the negative electrode; in, The thickness of the diaphragm is T. 11 4μm≤T 11 ≤20μm; The first active material layer includes a first negative electrode active material, and the first negative electrode active material includes a first silicon material; The first coating does not include the first silicon material, and the thickness of the first coating is T. 20 3μm≤T 20 ≤15μm.

2. The secondary battery according to claim 1, wherein, 4μm≤T 11 ≤13μm,9μm≤T 20 ≤15μm。 3. The secondary battery according to claim 1 or 2, wherein, 13μm<T 11 ≤20μm,3μm≤T 20 <9μm。 4. The secondary battery according to any one of claims 1 to 3, wherein, The second coating further includes a second active material layer. Along the thickness direction of the negative electrode sheet, the second active material layer is disposed between the first active material layer and the negative current collector. The second active material layer includes a second negative electrode active material, which includes a second silicon material. The mass percentage of the first silicon material in the first negative electrode active material is W1, and the mass percentage of the second silicon material in the second negative electrode active material is W2, where 0% ≤ W2 ≤ 40% and 3% ≤ W1 ≤ 99%.

5. The secondary battery according to claim 4, wherein, W1>W2, 3%≤W1-W2≤99%.

6. The secondary battery according to claim 4, wherein, W1<W2, -10%≤W1-W2<0%.

7. The secondary battery according to any one of claims 4 to 6, wherein, The thickness of the first active material layer is T 21 The thickness of the second active material layer is T. 22 20μm≤T 21 ≤T 22 ≤200μm, 0.1≤T 21 / T 22 ≤1.

0.

8. The secondary battery according to any one of claims 4 to 6, wherein, The thickness of the first active material layer is T 21 The thickness of the second active material layer is T. 22 20μm≤T 22 <T 21 ≤200μm, 1.0<T 21 / T 22 ≤1.

5.

9. The secondary battery according to claim 7 or 8, wherein, 3%≤W1<20%,100μm≤T 21 ≤200μm,3μm≤T 20 ≤8μm。 10. The secondary battery according to claim 7 or 8, wherein, 20%≤W1≤99%,20μm≤T 21 <100μm, 8μm<T 20 ≤15μm.

11. The secondary battery according to any one of claims 4 to 10, wherein, The second coating further includes a third active material layer. Along the thickness direction of the negative electrode sheet, the third active material layer is disposed between the second active material layer and the negative current collector. The third active material layer includes a third negative electrode active material, which includes a third silicon material. The mass percentage of the third silicon material in the third negative electrode active material is W3, where W3 ≤ W2.

12. The secondary battery according to claim 11, wherein, The first negative electrode active material, the second negative electrode active material, and the third negative electrode active material each independently comprise a carbon material; The carbon material includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, or hard carbon.

13. The secondary battery according to claim 11 or 12, wherein, The first silicon material, the second silicon material, and the third silicon material each independently include at least one of pure silicon, silicon oxide, silicon alloy, or silicon carbon.

14. The secondary battery according to claim 1, wherein, The first coating includes a third negative electrode material, which includes at least one of natural graphite, artificial graphite, modified graphite, soft carbon, hard carbon, lithium titanate, alumina, boehmite, silicon dioxide, or polyvinylidene fluoride.

15. The secondary battery according to claim 8, wherein, The volume average particle size of the third negative electrode material is Dv50. -3 The volume average particle size Dv50 of the first negative electrode active material is smaller than that of the first negative electrode active material. -1 And 11μm≤Dv50 -1 ≤15μm.

16. The secondary battery according to any one of claims 1 to 15, wherein, 8μm≤T 20 ≤15μm, the porosity α of the first coating 20 The porosity α of the first active material layer is greater than that of the first active material layer. 21 And 50% ≤ α 20 ≤60%.

17. The secondary battery according to claim 16, wherein, α 20 -α 21 ≥10％。 18. An electronic device, wherein, The electronic device includes a secondary battery as described in any one of claims 1 to 17.

Images