Secondary batteries, battery modules, battery packs and electrical devices

Abstract

This application relates to secondary batteries, battery modules, battery packs, and electrical devices. The secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a positive current collector, a first positive active material layer distributed on one side of the positive current collector, and a second positive active material layer distributed on the other side of the positive current collector. The negative electrode includes a negative current collector, a first negative active material layer distributed on one side of the negative current collector and opposite to the second positive active material layer, and a second negative active material layer distributed on the other side of the negative current collector. The resistance of the first positive active material layer is R1, the resistance of the second positive active material layer is R2, the resistance of the first negative active material layer is R3, and the resistance of the second negative active material layer is R4, and they satisfy the following relationships: (1) R2 > R1, (2) R3 ≥ R4, (3) 0 < (R1 + R4) / (R2 + R3) ≤ 1, and (4) R1 / R2 = A × R4 / R3, where A is 0.8 - 1.2.

Description

Technical Field

[0001] This application relates to a secondary battery, specifically a secondary battery, such as a lithium-ion battery, and a battery module, battery pack, and electrical device containing the secondary battery. Background Technology

[0002] With the country's vigorous promotion of new energy electric vehicles, their market share is also increasing. While enjoying the benefits brought by clean and environmentally friendly energy, people are also paying attention to some shortcomings in their own development process and are constantly improving them. At present, the most commonly used energy storage batteries in new energy electric vehicles are lithium-ion secondary batteries, mainly including lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide and ternary materials containing nickel, cobalt and manganese elements. At present, the cycle life of secondary batteries is generally about 1000-2000 charge and discharge cycles, after which the capacity decays to about 80% of the initial capacity. If the battery continues to be used at this time, the probability of a safety accident will gradually increase. Therefore, in order to avoid the occurrence of safety accidents, the battery is usually disposed of. This not only causes a great waste of batteries, but also brings a series of problems such as environmental pollution. In the existing technology, in order to increase the service life of batteries, the general methods are: (1) improve the surface coating of active materials and reduce surface side reactions; (2) increase the content of conductive materials and reduce resistance; (3) reduce the battery usage range, etc. The existing technology has the problems of high cost increase and technical challenges. Therefore, the improvement of secondary batteries is still an urgent problem to be solved. Summary of the Invention

[0003] To address the problems existing in the prior art, this application addresses the issue of reducing battery degradation and extending battery life by pre-activating active ions (e.g., lithium ions) in the positive electrode and allowing them to be slowly released during use to replenish the consumption of active ions.

[0004] According to a first aspect of this application, a secondary battery is provided, the secondary battery including a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes a positive current collector, a first positive active material layer distributed on one side of the positive current collector, and a second positive active material layer distributed on the other side of the positive current collector.

[0005] The negative electrode sheet includes a negative electrode current collector, a first negative electrode active material layer distributed on one side of the negative electrode current collector and opposite to the second positive electrode active material layer, and a second negative electrode active material layer distributed on the other side of the negative electrode current collector.

[0006] Wherein, the resistance of the first positive electrode active material layer is R1, the resistance of the second positive electrode active material layer is R2, the resistance of the first negative electrode active material layer is R3, and the resistance of the second negative electrode active material layer is R4, and they satisfy the following relationship:

[0007] (1) R2 > R1,

[0008] (2) R3 ≥ R4,

[0009] (3) 0 < (R1 + R4) / (R2 + R3) ≤ 1, and

[0010] (4) R1 / R2=A×R4 / R3, where A is 0.8-1.2.

[0011] In the embodiments of this application, A is 0.95-1.05.

[0012] In the embodiments of this application, R1 is 0.1Ω-20Ω, R2 is 0.15Ω-22Ω, R3 is 0.7mΩ-40mΩ, and R4 is 0.5mΩ-35mΩ; or R1 is 0.5Ω-10Ω, R2 is 0.55Ω-12Ω, R3 is 1mΩ-30mΩ, and R4 is 1.1mΩ-30mΩ.

[0013] In the embodiments of this application, the coating weight per unit area of ​​the first positive electrode active material layer is CW1, and the coating weight per unit area of ​​the second positive electrode active material layer is CW2, and the positive electrode active material satisfies 0.5≤CW1 / CW2<1.

[0014] In the embodiments of this application, 6.5 mg / cm 2 ≤CW1≤32.5mg / cm 2 6.5 mg / cm 2 ≤CW2≤32.5mg / cm 2 Optionally, 13.0 mg / cm³ 2 ≤CW1≤26.0mg / cm 2 13.0 mg / cm 2 ≤CW2≤26.0mg / cm 2 .

[0015] In the embodiments of this application, the coating weight per unit area of ​​the first negative electrode active material layer is CW3, the coating weight per unit area of ​​the second negative electrode active material layer is CW4, and 1≤CW3 / CW4≤1.5 is satisfied.

[0016] In the embodiments of this application, 5.2 mg / cm 2 ≤CW3≤19.5mg / cm 2 5.2 mg / cm 2 ≤CW4≤19.5mg / cm 2 ;Optional, 8.4 mg / cm 2 ≤CW3≤16.9mg / cm 28.4 mg / cm 2 ≤CW4≤16.9mg / cm 2 .

[0017] In an embodiment of this application, the unit area capacity of the second positive electrode active material layer is CapA, and the unit area capacity of the first negative electrode active material layer is CapB, and both satisfy 0.300≤CapA / CapB≤1.00; optionally, 0.468≤CapA / CapB≤0.870.

[0018] In the embodiments of this application, the thickness of the first positive electrode active material layer is T1, the thickness of the second positive electrode active material layer is T2, and T1≤T2 is satisfied.

[0019] In the embodiments of this application, the thickness of the first negative electrode active material layer is T3, the thickness of the second negative electrode active material layer is T4, and T4≤T3 is satisfied.

[0020] In the embodiments of this application, 0 < (T1+T4) / (T2+T3) ≤ 1.

[0021] In an embodiment of this application, the compaction density of the first positive electrode active material layer and / or the second positive electrode active material layer is 2.0 g / cm³. 3 -3.6g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer is 0.5 g / cm³. 3 -2g / cm 3 Alternatively, the compaction density of the first positive electrode active material layer and / or the second positive electrode active material layer is 2.3 g / cm³. 3 -3.5g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer is 1.0 g / cm³. 3 -1.8g / cm 3 .

[0022] In embodiments of this application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain at least one of NCM ternary material, NCA ternary material, lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide, and oxides, polyanionic materials or Prussian blue materials containing active sodium ions.

[0023] In embodiments of this application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a conductive agent composed of graphite, carbon black, acetylene black, graphene, carbon nanotubes, and combinations thereof, as well as a binder selected from polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, polyimide, and combinations thereof.

[0024] In embodiments of this application, the first negative electrode active material layer and / or the second negative electrode active material layer each independently contain natural graphite, artificial graphite, graphene, carbon nanotubes, soft carbon, hard carbon, and combinations of two or more of them.

[0025] According to a second aspect of this application, a battery module is provided, including the secondary battery described in this application.

[0026] According to a third aspect of this application, a battery pack is provided, including the secondary battery or battery module described in this application.

[0027] According to a fourth aspect of this application, an electrical device is provided, including a secondary battery, battery module, or battery pack as described in this application, wherein the secondary battery, battery module, or battery pack serves as a power source for the electrical device or an energy storage unit for the electrical device.

[0028] Compared with existing technologies, by adjusting and matching the resistance of the active material layers on both sides of the current collector in the positive electrode and the active material layers on both sides of the current collector in the negative electrode, the battery life and energy density can be extended.

[0029] These and other features and advantages will become apparent from the following detailed description and with reference to the accompanying drawings. It should be understood that the foregoing general description and the following detailed description are illustrative only and do not limit the scope of the claims. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the positive and negative electrode plates of a secondary battery in some embodiments of this application.

[0031] Figure 2 These are battery cycle life curves for specific embodiments and comparative embodiments of this application.

[0032] Figure 3 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0033] Figure 4 yes Figure 3 The diagram shows an exploded view of the secondary battery.

[0034] Figure 5 This is a schematic diagram of one embodiment of the battery module of this application.

[0035] Figure 6 This is a schematic diagram of one embodiment of the battery pack of this application.

[0036] Figure 7 yes Figure 6 An exploded view of the battery pack shown.

[0037] Figure 8 This is a schematic diagram of one embodiment of a device that uses the secondary battery of this application as a power source.

[0038] Explanation of reference numerals in the attached figures:

[0039] 1. Battery pack

[0040] 2. Upper box

[0041] 3. Lower box

[0042] 4. Battery module

[0043] 5. Secondary batteries

[0044] 51. Shell

[0045] 52. Electrode Assembly

[0046] 53. Cover plate Detailed Implementation

[0047] The present application will now be described in detail with reference to the accompanying drawings, and its features will become even more apparent in the following detailed description.

[0048] The "range" disclosed herein is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0049] Unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions. Similarly, unless otherwise specified, all technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0050] In this application, unless otherwise specified, the terms "comprising" and "including" as used herein are open-ended or closed-ended. For example, "comprising" and "including" may mean that other components not listed may also be included, or that only the listed components may be included.

[0051] In this description, unless otherwise stated, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0052] In this application, the resistance of the first positive electrode active material layer is R1, the resistance of the second positive electrode active material layer is R2, the resistance of the first negative electrode active material layer is R3, and the resistance of the second negative electrode active material layer is R4, and they satisfy the following relationships: (1) R2>R1, (2) R3≥R4, (3) 0<(R1+R4) / (R2+R3)≤1, and (4) R1 / R2=A×R4 / R3, where A is 0.8-1.2.

[0053] Typically, when a secondary battery begins charging, active lithium ions simultaneously begin to be extracted from the positive electrode material, pass through the separator, and embed into the negative electrode graphite on both sides of the positive electrode current collector. Meanwhile, electrons transfer to the negative electrode through the positive electrode current collector and the external circuit. At this point, the potential difference between the active material layers on both sides of the positive electrode current collector is equal. In this application, the resistance of the active material layers on both sides of the positive electrode current collector is set to R2 > R1. During the charging process of the secondary battery, the rate of lithium ion extraction in the active material layer on the side with higher resistance (hereinafter referred to as the R2 surface) is relatively slower. When the battery reaches the charging cutoff voltage, some lithium remains in the active material layer on the R2 surface, which helps to pre-store some active lithium on one side of the positive electrode. This portion of active lithium is gradually released as the battery charges and discharges, effectively mitigating battery capacity decay. To ensure potential matching between the positive electrode active material layers on both sides of the positive electrode and the corresponding negative electrode active material layer, the resistance of the first negative electrode active material layer (also known as the R3 surface) facing the R2 surface must be greater than the resistance of the second negative electrode active material layer (also known as the R4 surface) facing the R1 surface. Based on this, this application further specifies that the positive and negative electrode sheets satisfy 0 < (R1 + R4) / (R2 + R3) ≤ 1, and R1 / R2 = A × R4 / R3, where A can range from 0.8 to 1.2. This ensures a sufficient amount of pre-active lithium ions in the secondary battery while maintaining low polarization and high energy density, thus extending the cycle life of the secondary battery and increasing its energy density.

[0054] In one embodiment of this application, A can be within a numerical range formed by any two of the following values ​​as endpoints: 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20. It should be particularly emphasized that although the above values ​​are listed side-by-side, it does not mean that any numerical range formed by any two of the above values ​​as endpoints will yield comparable or similar performance. This also applies to the numerical ranges mentioned below. The preferred embodiments of this application are selected solely based on the specific discussion below and specific experimental data. In an optional embodiment of this application, A is 0.95-1.05.

[0055] In one embodiment of this application, based on satisfying the above relationships (1)-(4), R1 can be within the numerical range formed by any two of the following values ​​as end values: 0.1Ω, 0.5Ω, 1Ω, 5Ω, 10Ω, 15Ω, 20Ω; R2 can be within the numerical range formed by any two of the following values ​​as end values: 0.15Ω, 0.55Ω, 1Ω, 3Ω, 6Ω, 9Ω, 12Ω, 16Ω, 19Ω, 22Ω; R3 can be within the numerical range formed by any two of the following values ​​as end values: 0.7mΩ, 1mΩ, 5mΩ, 10mΩ, 20mΩ, 30mΩ, 35mΩ, 40mΩ; and R4 can be within the numerical range formed by any two of the following values ​​as end values: 0.5mΩ, 1.1mΩ, 5mΩ, 10mΩ, 20mΩ, 30mΩ, 35mΩ. In this application, R1, R2, R3, and R4 ensure that when both sides of the positive electrode in the secondary battery are discharged simultaneously, the side with more transferable active material will retain some. As the secondary battery cycles and ages, the retained transferable active material will be gradually released, delaying battery aging and increasing cycle life. In optional embodiments of this application, R1 is 0.1Ω-20Ω, R2 is 0.15Ω-22Ω, R3 is 0.7mΩ-40mΩ, and R4 is 0.5mΩ-35mΩ; or R1 is 0.5Ω-10Ω, R2 is 0.55Ω-12Ω, R3 is 1mΩ-30mΩ, and R4 is 1.1mΩ-30mΩ.

[0056] In one embodiment of this application, the first positive electrode active material layer contains an active ionic compound of amount CW1, and the second positive electrode active material layer contains an active ionic compound of amount CW2, satisfying 0.5 ≤ CW1 / CW2 < 1. CW1 / CW2 can be within a numerical range formed by using any two of the following values ​​as endpoints: 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99. Within this CW1 / CW2 range, the side with more transferable active material (CW2) will retain some. As the secondary battery cycles and ages, the retained transferable active material will be gradually released, delaying battery aging and increasing cycle life.

[0057] In one embodiment of this application, based on the condition that 0.5 ≤ CW1 / CW2 < 1, CW1, calculated by weight of active ions per unit area, can be within a numerical range consisting of any two of the following values ​​as endpoints: 6.5 mg / cm² 2 9.5 mg / cm 2 13.0 mg / cm 2 18.5 mg / cm 2 26.0 mg / cm 232.5 mg / cm 2 CW2 can be within a range consisting of any two of the following values ​​as endpoints: 6.5 mg / cm² 2 9.5 mg / cm 2 13.0 mg / cm 2 18.5 mg / cm 2 26.0 mg / cm 2 32.5 mg / cm 2 In an optional embodiment of this application, the weight of active ions per unit area is 6.5 mg / cm². 2 ≤CW1≤32.5mg / cm 2 and 6.5 mg / cm 2 ≤CW2≤32.5mg / cm 2 ; or 13.0 mg / cm 2 ≤CW1≤26.0mg / cm 2 , and 13.0 mg / cm 2 ≤CW2≤26.0mg / cm 2 .

[0058] In one embodiment of this application, the first negative electrode active material layer contains active ion vacancy material of CW3, and the second negative electrode active material layer contains active ion vacancy material of CW4, satisfying 1 ≤ CW3 / CW4 ≤ 1.5. In this application, CW3 / CW4 can be within a numerical range formed by using any two of the following values ​​as endpoints: 1.0, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 13.5, 1.40, 1.45, 1.50. Within this CW3 / CW4 range, the energy density of the secondary battery of this application is improved.

[0059] In one embodiment of this application, based on the condition that 1 ≤ CW3 / CW4 ≤ 1.5, CW3, calculated by weight of active ion vacancy material per unit area, can be within a numerical range consisting of any two of the following values ​​as endpoints: 5.2 mg / cm³ 2 8.4 mg / cm 2 12.0 mg / cm 2 14.6 mg / cm 2 16.9 mg / cm 2 19.5 mg / cm 2 CW4 can be within a range consisting of any two of the following values ​​as endpoints: 5.2 mg / cm³ 2 8.4 mg / cm 2 12.0 mg / cm 214.6 mg / cm 2 16.9 mg / cm 2 19.5 mg / cm 2 In an optional embodiment of this application, the weight of active ion vacancy material per unit area is 5.2 mg / cm². 2 ≤CW3≤19.5mg / cm 2 and 5.2 mg / cm 2 ≤CW4≤19.5mg / cm 2 ; or 8.4 mg / cm 2 ≤CW3≤16.9mg / cm 2 and 8.4 mg / cm 2 ≤CW4≤16.9mg / cm 2 .

[0060] In one embodiment of this application, the unit area capacity of the first positive electrode active material layer and the second positive electrode active material layer is CapA, and the unit area capacity of the first negative electrode active material layer and the second negative electrode active material layer is CapB, satisfying 0.300≤CapA / CapB≤0.933. In this application, CapA / CapB can be within a numerical range formed by using any two of the following values ​​as endpoints: 0.300, 0.368, 0.400, 0.450, 0.500, 0.550, 0.670, 0.933. Within the above CapA / CapB range, the secondary battery of this application can avoid excessive positive electrode active material (e.g., lithium), which would lead to insufficient negative electrode capacity and cause "lithium plating," thereby deteriorating the cycle life of the secondary battery.

[0061] In one embodiment of this application, the first positive electrode active material layer includes an active ionic compound layer with a thickness of T1, and the second positive electrode active material layer includes an active ionic compound layer with a thickness of T2, wherein T1 < T2. In this application, the coating thickness of the active ionic compound in the second positive electrode active material layer can be greater than the coating thickness of the active ionic compound in the first positive electrode active material layer. Therefore, the transferable active material (e.g., lithium) in the second positive electrode active material layer (also referred to as the R2 surface) is greater than that in the first positive electrode active material layer (also referred to as the R1 surface).

[0062] In one embodiment of this application, the first negative electrode active material layer includes an active ion vacancy material layer with a thickness of T3, and the second negative electrode active material layer includes an active ion vacancy material layer with a thickness of T4, wherein T4 < T3. In this application, the coating thickness of the active ion vacancy material in the first negative electrode active material layer (also referred to as the R3 surface) opposite to the R2 surface can be greater than the coating thickness of the active ion vacancy material in the second negative electrode active material layer (also referred to as the R4 surface) facing the R1 surface, so as to provide sufficient vacancies for lithium ions in the negative electrode.

[0063] In one embodiment of this application, 0 < (T1+T4) / (T2+T3)≤1, thereby ensuring that the side of the positive electrode with less transferable active material coated on it and the side of the corresponding negative electrode with less active ion vacancy mass has a small total resistance and small polarization, which can preferentially cause the transfer of transferable active material (e.g., lithium), and ultimately ensure that the side with a large coating amount can retain transferable active material.

[0064] In this application, the compaction density of the first and second positive electrode active material layers of the secondary battery is controlled to reduce the ion transport path and improve the cycle life of the secondary battery. Simultaneously, excessively high compaction density prevents particle breakage of the active material, which could lead to an increase in specific surface area (BET) and cause side reactions that would negatively impact the cycle life of the secondary battery. In one embodiment of this application, the compaction density of the first and / or second positive electrode active material layers can be within a numerical range formed by using any two of the following values ​​as endpoints: 2.0 g / cm³. 3 2.3g / cm 3 2.6g / cm 3 2.9g / cm 3 3.2g / cm 3 3.5g / cm 3 3.6g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer can be within a numerical range consisting of any two of the following values ​​as endpoints: 0.5 g / cm³ 3 0.8g / cm 3 1.0g / cm 3 1.3g / cm 3 1.5g / cm 3 1.8g / cm 3 2.0g / cm 3 In an optional embodiment of this application, the compaction density of the first positive electrode active material layer and / or the second positive electrode active material layer is 2.0 g / cm³. 3 -3.6g / cm3 Or 2.3g / cm 3 -3.5g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer is 0.5 g / cm³. 3 -2g / cm 3 Or 1.0g / cm 3 -1.8g / cm 3 .

[0065] In the following text, the performance improvement brought about by coating different amounts of transferable active material on both sides of the positive electrode of this application is mainly based on secondary batteries, especially lithium-ion secondary batteries. However, it should be emphasized that the differentiated coating design on both sides of the positive electrode of this application can be used in any electrical device including a carbon-based electrode, and the electrical device can benefit from it.

[0066] Secondary batteries

[0067] In one embodiment of this application, a secondary battery is provided, which can be a lithium-ion secondary battery, a potassium-ion secondary battery, a sodium-ion secondary battery, a lithium-sulfur battery, etc., with a lithium-ion secondary battery being particularly preferred. The secondary battery of this application includes a positive electrode (plate), a negative electrode (plate), a separator, and an electrolyte / liquid. During the charging and discharging process, active ions repeatedly insert and extract between the positive and negative electrode plates. The electrolyte acts as a conductor of ions between the positive and negative electrode plates.

[0068] [Positive electrode plate]

[0069] In the secondary battery of this application, the positive electrode sheet includes a positive current collector and a positive electrode film layer (or positive electrode active material layer) disposed on at least one surface of the positive current collector and comprising a positive electrode active material. For example, the positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector. In the secondary battery of this application, the positive current collector can be a metal foil or a composite current collector. For example, the metal foil can be aluminum foil, and the composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers, etc.).

[0070] In the secondary battery of this application, the positive electrode active material (substance) may be a positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include one or more of the following: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811)), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 One or more of lithium iron phosphates (O2) and their modified compounds. Examples of lithium-containing phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4(LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. In embodiments of this application, the second and third positive electrode active materials may be the same or different, and are selected from lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium nickel cobalt manganese oxide (NCM), lithium cobalt oxide (LCO), lithium nickel cobalt aluminum oxide (NCA), and oxides, polyanionic materials, or Prussian blue materials containing active sodium ions.

[0071] In some embodiments, the positive electrode film layer may optionally include a binder. Non-limiting examples of binders that can be used in the positive electrode film layer may include one or more of the following: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In embodiments of this application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a binder selected from polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, polyimide, and combinations thereof.

[0072] In some embodiments, the positive electrode film layer may optionally include a conductive agent. Examples of conductive agents used for the positive electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In embodiments of this application, the first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a conductive agent composed of graphite, carbon black, acetylene black, graphene, carbon nanotubes, and combinations thereof.

[0073] In one embodiment of this application, the positive electrode can be prepared by dispersing the above-mentioned components for preparing the positive electrode, such as positive electrode active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a uniform positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0074] [Negative electrode plate]

[0075] The secondary battery of this application includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer (or negative electrode active material layer) disposed on at least one surface of the negative current collector. In the embodiments of this application, the first negative electrode active material and the second negative electrode active material are the same or different, and each independently contains natural graphite, artificial graphite, graphene, carbon nanotubes, soft carbon, hard carbon and combinations of two or more of them.

[0076] In one embodiment of this application, the negative electrode film layer may include, in addition to the negative electrode active material described above, a certain amount of other commonly used negative electrode active materials, such as natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, or several of these. The silicon-based material may be selected from elemental silicon, silicon oxide, and silicon-carbon composites, or several of these. The tin-based material may be selected from elemental tin, tin oxide compounds, and tin alloys, or several of these.

[0077] In the secondary battery of this application, the negative electrode film comprises a negative electrode active material and optional binders, optional conductive agents, and other optional additives, and is typically formed by coating and drying a negative electrode slurry. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agents, and binders in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0078] As an example, conductive agents may include one or more of superconducting carbon, carbon black (e.g., acetylene black, Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0079] As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). Other optional additives include thickeners (such as sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials, etc.

[0080] Furthermore, in the secondary battery of this application, the negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application may also include a conductive undercoat layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the first negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application may also include a protective covering layer covering the surface of the second negative electrode film layer.

[0081] In the secondary battery of this application, the negative electrode current collector can be a metal foil or a composite current collector. For example, the metal foil can be a copper foil, silver foil, iron foil, or an alloy of the above metals. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. It can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base layer (such as a base layer made of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers, etc.).

[0082] [Electrolytes]

[0083] The electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions). In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent. In some embodiments, the electrolyte salt can be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate). In one embodiment of this application, the solvent may be selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). In one embodiment of this application, the solvent content is 60-99% by weight, for example 65-95% by weight, or 70-90% by weight, or 75-89% by weight, or 80-85% by weight, based on the total weight of the electrolyte. In another embodiment of this application, the electrolyte content is 1-40% by weight, for example 5-35% by weight, or 10-30% by weight, or 11-25% by weight, or 15-20% by weight, based on the total weight of the electrolyte.

[0084] In one embodiment of this application, the electrolyte may optionally contain additives. For example, additives may include one or more of the following: negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, additives that improve battery low-temperature performance, etc.

[0085] [Isolation membrane]

[0086] In one embodiment of this application, the secondary battery further includes a separator membrane that separates the anode side from the cathode side of the secondary battery, providing selective permeability or blocking for substances of different types, sizes, and charges within the system. For example, the separator membrane can provide electronic insulation, physically isolate the positive and negative electrode active materials of the secondary battery, prevent internal short circuits, and form an electric field in a certain direction, while allowing ions in the battery to pass through the separator membrane and move between the positive and negative electrodes. In one embodiment of this application, the material used to prepare the separator membrane may include one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator membrane can be a single-layer film or a multilayer composite film. When the separator membrane is a multilayer composite film, the materials of each layer can be the same or different. In an embodiment of this application, the separator membrane is selected from polyolefin separator membranes, polyester separator membranes, polyimide separator membranes, polyamide separator membranes, and cellulose separator membranes.

[0087] In one embodiment of this application, the above-mentioned positive electrode sheet, negative electrode sheet and separator can be manufactured into electrode assembly / bare cell by winding process or stacking process.

[0088] In one embodiment of this application, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode components and electrolyte. In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. In other embodiments, the outer packaging of the secondary battery may be a soft pack, such as a pouch. The material of the soft pack may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0089] In this application, as Figure 1 As shown, positive and negative electrode sheets are arranged face-to-face. The R1 face of the positive electrode sheet has a greater amount of transferable active material, or the coating of the active material layer is thicker. The R2 face of the positive electrode sheet has less transferable active material, or the coating of the active material layer is thinner. Similarly, the R3 face opposite the R1 face of the positive electrode sheet has a greater amount of active ion vacancy material, or the coating of the active ion vacancy material layer is thicker. Likewise, the R4 face of the negative electrode sheet has less active ion vacancy material, or the coating of the active ion vacancy material layer is thinner.

[0090] The secondary battery of this application can be cylindrical, square, or any other arbitrary shape. Figure 3 This is an example of a square-structured secondary battery 5. Figure 4 Showing Figure 3An exploded view of the secondary battery 5 shows that the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. This electrode assembly is encapsulated in the receiving cavity, and the electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52.

[0091] In one embodiment of this application, a number of secondary batteries can be assembled together to form a battery module. The battery module contains two or more secondary batteries, the specific number depending on the application of the battery module and the parameters of the individual battery module.

[0092] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0093] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0094] In one embodiment of this application, two or more of the above-described battery modules can be assembled into a battery pack. The number of battery modules contained in the battery pack depends on the application of the battery pack and the parameters of individual battery modules. The battery pack may include a battery box and multiple battery modules disposed within the battery box. The battery box includes an upper box and a lower box, the upper box being able to cover and fit snugly onto the lower box to form a closed space for accommodating the battery modules. Two or more battery modules can be arranged in the battery box in a desired manner.

[0095] Figure 6 and Figure 7 This is battery pack 1 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0096] Electrical appliances

[0097] In one embodiment of this application, the electrical device includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device includes, but is not limited to, mobile digital devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0098] Figure 8 This is an example device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0099] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0100] In the following text, the effects of secondary batteries manufactured according to the embodiments of this application on the performance of electrochemical devices are characterized based on specific embodiments. However, it should be noted that the scope of protection of this application is defined by the claims and is not limited to the specific embodiments described above.

[0101] Example

[0102] Unless otherwise stated, all raw materials used in this invention are of analytical grade, and all water is deionized water.

[0103] Example 1

[0104] Preparation of positive electrode

[0105] NCM523 positive electrode material, conductive carbon black (Super-P), and polyvinylidene fluoride (PVDF) were mixed with N-methylpyrrolidone in a weight ratio of 96.2:2.3:1.1 and stirred until homogeneous to obtain a slurry for coating the positive electrode sheet. The viscosity could be adjusted using N-methylpyrrolidone during stirring, and the process could be carried out under a vacuum of -80 kPa. The slurry was then uniformly coated across both sides of a 13 μm thick positive electrode current collector (aluminum foil), ensuring that the CW1 / CW2 ratio of the active material between the two sides was 0.5. The positive electrode sheet was then cold-pressed and sliced ​​(i.e., the electrode sheet was cut to the required size) to obtain the final positive electrode sheet.

[0106] Preparation of negative electrode

[0107] Artificial graphite, conductive carbon black (Super-P), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed with water in a weight ratio of 97:0.5:1:1.5 and stirred until homogeneous to obtain a slurry for coating the negative electrode. The viscosity could be adjusted with water during the stirring process. The slurry was then coated onto both sides of a 6μm thick negative electrode current collector (copper foil) with a specific width, ensuring that the ratio of active ion vacancy material (CW3 / CW4) on both sides was 1.3. The negative electrode was then cold-pressed and sliced ​​to obtain the final negative electrode sheet.

[0108] Preparation of lithium-ion secondary batteries

[0109] A 7μm polyethylene (PE) separator is used. The positive electrode, separator, and negative electrode are wound into a cell, and then subjected to electrode tab welding, aluminum shell packaging, electrolyte injection, encapsulation formation, and degassing to produce a lithium-ion secondary battery. The cell has a width of 148mm, a thickness of 28mm, a height of 98mm, and a capacity of 40Ah. The electrolyte is a solution of 1M LiPF6 ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:2).

[0110] Examples 2-5

[0111] Except for controlling CW1 / CW2 and CW3 / CW4 as described in Table 1 below, the lithium-ion secondary batteries of Examples 2-5 were prepared in accordance with the manner of Example 1.

[0112] Examples 6-11

[0113] Except for using LiFePO4 (hereinafter abbreviated as LFP) instead of NCM523 as the positive electrode material and controlling CapA / CapB as described in Table 1 below, the lithium-ion secondary batteries of Examples 6-11 were prepared in accordance with the method of Example 3.

[0114] Examples 12-16

[0115] Except for using LFP instead of NCM523 as the positive electrode material for the R1 side, using NCM811 instead of NCM523 as the positive electrode material for the R2 side, and controlling (R1+R4) / (R2+R3) as described in Table 1 below, the lithium-ion secondary batteries of Examples 12-16 were prepared in accordance with the manner of Example 3.

[0116] Example 17

[0117] Except for using NaFePO4 (hereinafter abbreviated as NaVP) instead of LFP as the positive electrode material of R1 surface and using NaVP instead of NCM811 as the positive electrode material of R2 surface, the lithium-ion secondary battery of Example 17 was prepared in accordance with the manner of Example 16.

[0118] Comparative Example 1

[0119] Except for controlling CW1 / CW2 and CW3 / CW4 to 0.2 and 1.5 respectively as described in Table 1 below, the lithium-ion secondary battery of Comparative Example 1 was prepared in accordance with the manner of Example 1.

[0120] Comparative Example 2

[0121] Except for controlling CapA / CapB to 0.21 as described in Table 1 below, the lithium-ion secondary battery of Comparative Example 2 was prepared according to the method of Example 6.

[0122] Comparative Example 3

[0123] Except for controlling (R1+R4) / (R2+R3) to 0.05 as described in Table 1 below, the lithium-ion secondary battery of Comparative Example 3 was prepared in accordance with the manner of Example 12.

[0124] Resistance test of active material layer

[0125] The resistivity per unit area of ​​the positive electrode sheet can be tested using methods known in the art. As an example, the following method can be used: The testing instrument is a Yuaneng Technology IEST BER1000 electrode resistance meter. Specific steps include: cutting a 10cm × 10cm square test sample from the positive electrode sheet (double-sided coated with a positive active material layer); clamping the top and bottom sides of the test sample between two conductive terminals of the internal resistance tester and applying a certain pressure to fix it; and testing the resistance R of the test sample. The diameter of the conductive terminals is 14mm, the applied pressure is 15MPa to 27MPa, and the sampling time ranges from 5s to 17s. The resistivity per unit area of ​​the positive electrode sheet is calculated using the formula r = R / S, where S is the contact area between the conductive terminals and the test sample (usually considered as the cross-sectional area of ​​the conductive terminals); and R is the resistance value of the sample read by the testing instrument. In the secondary battery obtained in the embodiments of this application, R1 is controlled between 0.1Ω and 20Ω, R2 is controlled between 0.15Ω and 22Ω, R3 is controlled between 0.7mΩ and 40mΩ, and R4 is controlled between 0.5mΩ and 35mΩ.

[0126] Electrode capacity test

[0127] The positive or negative electrode sheets were cut into small round pieces and assembled into button batteries in a glove box. The electrodes were made of lithium metal, the separator was Celgard C2400, and the electrolyte was a 1.3M solution of LiPF6EC and DMC (volume ratio 1:2). The assembled button batteries were then tested for capacity using a blue battery tester.

[0128] Energy density testing

[0129] The secondary battery was charged at a constant current rate of 0.1C to the rated voltage of 4.0V and held at that voltage for 30 minutes. Then it was discharged at a constant current rate of 0.1C to 2.5V. The energy released during the constant current discharge was recorded and divided by the total mass of the secondary battery to obtain the energy density of the secondary battery.

[0130] Cyclic performance test

[0131] Place the prepared secondary battery on the Xinwei machine. At 25°C, charge it to the rated voltage at a constant current rate of 0.5C. After resting for 5 minutes, discharge it to the cutoff voltage at a constant current rate of 0.5C. Record the discharge capacity, and then let it rest for another 5 minutes. Repeat this cycle until the energy density drops to approximately 80% of its initial value. Record the current number of cycles.

[0132]

[0133]

[0134] As shown in Table 1, in Examples 1-5, the cycle life of the secondary battery initially increases and then decreases as the CW1 / CW2 ratio increases. As described in Table 1, when the ratio is 0.62, the cycle life of the secondary battery reaches 2450 cycles. Compared to the secondary battery obtained in Example 5, the cycle life of the secondary battery is increased by 22.5%. Simultaneously, as the CW3 / CW4 ratio continuously approaches the lower limit of 1.0 from the upper limit of 1.3, the gravimetric energy density of the secondary battery gradually increases.

[0135] As shown in Table 1, in Examples 6-11, the cycle life of the secondary battery initially increases and then decreases as the CapA / CapB ratio increases. As described in Table 1, when CapA / CapB is 0.671, the cycle life of the secondary battery reaches 4680 cycles. Compared to the secondary battery obtained in Example 6, the cycle life of the secondary battery is increased by 15.6%. Simultaneously, as CapA / CapB continuously approaches 0.933 from 0.312, the gravimetric energy density of the secondary battery also gradually increases.

[0136] As shown in Table 1, in Examples 12-16, the cycle life of the secondary battery initially increases and then decreases as (R1+R4) / (R2+R3) increases. As described in Table 1, when (R1+R4) / (R2+R3) is 0.62, the cycle life of the secondary battery reaches 2600 cycles. Compared to the secondary battery obtained in Example 16, the cycle life of the secondary battery is increased by 15.6%. Simultaneously, as (R1+R4) / (R2+R3) continuously approaches 1.00 from 0.12, the gravimetric energy density of the secondary battery also gradually increases.

[0137] Comparing Example 1 with Comparative Example 1, it can be seen that controlling CW1 / CW2 (i.e., R2 > R1) and CW3 / CW4 (i.e., R3 ≥ R4) has a significant impact on the cycle life and gravimetric energy density of the secondary battery. Comparing Example 6 with Comparative Example 2, it can be seen that controlling CapA / CapB has a significant impact on the cycle life and gravimetric energy density of the secondary battery. Comparing Example 12 with Comparative Example 3, it can be seen that controlling (R1+R4) / (R2+R3) also has a significant impact on the cycle life and gravimetric energy density of the secondary battery.

[0138] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, the secondary battery comprising a positive electrode and a negative electrode, wherein, The positive electrode sheet includes a positive current collector, a first positive active material layer distributed on one side of the positive current collector, and a second positive active material layer distributed on the other side of the positive current collector; The negative electrode sheet includes a negative electrode current collector, a first negative electrode active material layer distributed on one side of the negative electrode current collector and opposite to the second positive electrode active material layer, and a second negative electrode active material layer distributed on the other side of the negative electrode current collector. Wherein, the resistance of the first positive electrode active material layer is R1, the resistance of the second positive electrode active material layer is R2, the resistance of the first negative electrode active material layer is R3, and the resistance of the second negative electrode active material layer is R4, and they satisfy the following relationship: (1) R2 > R1, (2) R3 ≥ R4, (3) 0 < (R1+R4) / (R2+R3) ≤ 1, and (4) R1 / R2 = A×R4 / R3, where A is 0.8-1.2; R1 is 0.1Ω-20Ω, R2 is 0.15Ω-22Ω, R3 is 0.7mΩ-40mΩ, and R4 is 0.5mΩ-35mΩ; the resistance is tested by an IEST BER1000 electrode resistance meter, and the test pressure is 15MPa~27MPa. The positive electrode active material layer has a coating weight of CW1 per unit area and a coating weight of CW2 per unit area for the second positive electrode active material layer, and satisfies 0.5 ≤ CW1 / CW2 < 1. 6.5mg / cm 2 ≤ CW1 ≤ 32.5mg / cm 2 ,6.5mg / cm 2 ≤ CW2 ≤ 32.5mg / cm 2 。 2. The secondary battery according to claim 1, wherein, A is 0.95-1.

05.

3. The secondary battery according to claim 1, wherein, R1 is 0.5Ω-10Ω, R2 is 0.55Ω-12Ω, R3 is 1mΩ-30mΩ, and R4 is 1.1mΩ-30mΩ.

4. The secondary battery according to claim 1, wherein, 13.0mg / cm 2 ≤ CW1 ≤ 26.0mg / cm 2 ,13.0mg / cm 2 ≤ CW2 ≤ 26.0mg / cm 2 。 5. The secondary battery according to claim 1, wherein, The coating weight per unit area of ​​the first negative electrode active material layer is CW3, and the coating weight per unit area of ​​the second negative electrode active material layer is CW4, and satisfies 1 ≤ CW3 / CW4 ≤ 1.

5.

6. The secondary battery according to claim 5, wherein, 5.2mg / cm 2 ≤ CW3 ≤ 19.5mg / cm 2 ,5.2mg / cm 2 ≤ CW4 ≤ 19.5mg / cm 2 。 7. The secondary battery according to claim 6, wherein, 8.4mg / cm 2 ≤ CW3 ≤ 16.9mg / cm 2 ,8.4mg / cm 2 ≤ CW4 ≤ 16.9mg / cm 2 。 8. The secondary battery according to any one of claims 1-7, wherein, The unit area capacity of the second positive electrode active material layer is CapA, and the unit area capacity of the first negative electrode active material layer is CapB, and both satisfy 0.300 ≤ CapA / CapB ≤ 1.

00.

9. The secondary battery according to claim 8, wherein, 0.468 ≤ CapA / CapB ≤ 0.

870.

10. The secondary battery according to any one of claims 1-7, wherein, The thickness of the first positive electrode active material layer is T1, and the thickness of the second positive electrode active material layer is T2, and T1≤T2.

11. The secondary battery according to claim 10, wherein, The thickness of the first negative electrode active material layer is T3, and the thickness of the second negative electrode active material layer is T4, and T4≤T3.

12. The secondary battery according to claim 11, wherein, 0 < (T1 + T4) / (T2 + T3) ≤ 1.

13. The secondary battery according to any one of claims 1-7, wherein, The compaction density of the first positive electrode active material layer and / or the second positive electrode active material layer is 2.0 g / cm³. 3 -3.6 g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer is 0.5 g / cm³. 3 -2 g / cm 3 .

14. The secondary battery according to any one of claims 1-7, wherein, The compaction density of the first positive electrode active material layer and / or the second positive electrode active material layer is 2.3 g / cm³. 3 -3.5 g / cm 3 The compaction density of the first negative electrode active material layer and / or the second negative electrode active material layer is 1.0 g / cm³. 3 -1.8 g / cm 3 .

15. The secondary battery according to any one of claims 1-7, wherein, The first positive electrode active material layer and / or the second positive electrode active material layer each independently contain at least one of NCM ternary material, NCA ternary material, lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide, and oxides, polyanionic materials or Prussian blue materials containing active sodium ions.

16. The secondary battery according to any one of claims 1-7, wherein, The first positive electrode active material layer and / or the second positive electrode active material layer each independently contain a conductive agent consisting of graphite, carbon black, acetylene black, graphene, carbon nanotubes, and combinations thereof, as well as a binder selected from polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, polyimide, and combinations thereof.

17. The secondary battery according to any one of claims 1-7, wherein, The first negative electrode active material layer and / or the second negative electrode active material layer each independently contain natural graphite, artificial graphite, graphene, carbon nanotubes, soft carbon, hard carbon, and combinations of two or more of them.

18. A battery module, wherein, Includes the secondary battery according to any one of claims 1-17.

19. A battery pack, wherein, Includes the secondary battery according to any one of claims 1-17 or the battery module according to claim 18.

20. An electrical appliance, wherein, Includes a secondary battery according to any one of claims 1-17, or a battery module according to claim 18, or a battery pack according to claim 19, wherein the secondary battery, the battery module, or the battery pack serves as a power source for the electrical device or an energy storage unit for the electrical device.

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