Secondary batteries and electrical devices

Abstract

This application provides a secondary battery and an electrical device. The secondary battery includes a negative electrode sheet, which includes a negative current collector and a first negative active layer and a second negative active layer located on opposite sides of the negative current collector. The first negative active layer includes a first silicon-based material, and the second negative active layer includes a first graphite material. The resistance ρ1 of the first negative active layer and the resistance ρ2 of the second negative active layer satisfy the condition: 1.8 ≤ ρ1 / ρ2 ≤ 5.4. Furthermore, in the fully discharged state, the porosity of the first negative active layer and the porosity of the second negative active layer satisfy the condition. The secondary battery provided by this application combines high energy density and excellent charging capability.

Description

Technical Field

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

[0002] In recent years, with the increasingly wide application of rechargeable batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. With the rapid development of rechargeable batteries, higher requirements have been placed on their energy density and charging capabilities.

[0003] Therefore, how to achieve both high energy density and excellent charging capability in secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electrical device, wherein the secondary battery prepared in this application can achieve both excellent energy density and charging capability.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a first negative electrode active layer and a second negative electrode active layer located on opposite sides of the negative current collector. The first negative electrode active layer includes a first silicon-based material, and the second negative electrode active layer includes a first graphite material. The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.8 ≤ ρ1 / ρ2 ≤ 5.4, and in the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy:

[0006] In this application, because the first negative electrode active layer includes a first silicon-based material, the specific capacity of the secondary battery anode is relatively high, which is beneficial to improving the energy density of the secondary battery. In this application, by setting the first silicon-based material and the first graphite material on opposite sides of the negative electrode current collector, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy the above-mentioned relationship, and the porosity of the first negative electrode active layer is set to a certain value under fully discharged conditions. Porosity of the second negative electrode active layer By satisfying the above relationship, the combined ability of the first and second negative electrode active layers to conduct electrons and ions is comparable, which can improve the charging capacity of the first negative electrode active layer to a level comparable to that of the second negative electrode active layer, thereby improving the charging capacity of the entire negative electrode. Therefore, the secondary battery in this application can achieve both high energy density and excellent charging capacity.

[0007] In some embodiments, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.87 ≤ ρ1 / ρ2 ≤ 4.6. In some embodiments, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 2.3 ≤ ρ1 / ρ2 ≤ 4.2, and in the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy: In some embodiments, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy the condition: 2.4 ≤ ρ1 / ρ2 ≤ 3.05. This makes it easier to ensure that the charging capabilities of the first and second negative electrode active layers are comparable, and better balances the energy density and charging capacity of the secondary battery.

[0008] In some embodiments, the resistance ρ1 of the first negative electrode active layer is 2.7mΩ to 6.9mΩ. This gives the first negative electrode active layer suitable conductivity, thereby improving its charging capability.

[0009] In some embodiments, the resistance ρ2 of the second negative electrode active layer is 0.65mΩ to 1.50mΩ. This gives the second negative electrode active layer suitable conductivity, which in turn improves its charging capability.

[0010] In some embodiments, the porosity of the first negative electrode active layer The porosity φ1 of the first negative electrode active layer is 18% to 53%; optionally, the porosity φ1 of the first negative electrode active layer is 26% to 40%. This gives the first negative electrode active layer suitable ion-conducting ability, which in turn helps to improve the charging capacity of the first negative electrode active layer.

[0011] In some embodiments, the porosity of the second negative electrode active layer The percentage is 16% to 20%. This gives the second negative electrode active layer a suitable ion-conducting ability, which in turn benefits the charging capability of the second negative electrode active layer.

[0012] In some embodiments, the unit area capacity A1 of the first negative electrode active layer and the unit area capacity A2 of the second negative electrode active layer satisfy the condition: 1.000 ≤ A1 / A2 ≤ 1.019. This ensures that the anode has sufficient active sites for lithium ion insertion while preventing lithium ions from depositing on the anode surface and forming lithium dendrites, thus promoting capacity matching between the anode and cathode.

[0013] In some embodiments, under fully discharged conditions, the tortuosity τ1 of the first negative electrode active layer and the tortuosity τ2 of the second negative electrode active layer satisfy the condition: 1.06 ≤ τ2 / τ1 ≤ 2.0. This helps to reduce the concentration polarization of lithium ions in the first and second negative electrode active layers, improving the ion-conducting capability of the first negative electrode active layer to a level comparable to that of the second negative electrode active layer, thereby improving the charging capability of the secondary battery.

[0014] In some embodiments, the tortuosity τ1 of the first negative electrode active layer is 2.0 to 3.0. This gives the first negative electrode active layer suitable ion-conducting capability, thereby improving its charging capacity.

[0015] In some embodiments, the tortuosity τ2 of the second negative electrode active layer is 3.2 to 4.0. This gives the second negative electrode active layer suitable ion-conducting capability, thereby improving its charging capacity.

[0016] In some embodiments, the average particle size of the first silicon-based material is 5 μm to 11 μm; and / or, the specific surface area of ​​the first silicon-based material is 1.5 m². 2 / g~3m 2 / g.

[0017] In some embodiments, the first silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

[0018] In some embodiments, the silicon-oxide material includes one or more of the following: non-lithium silicon-oxide compound, pre-lithium silicon-oxide compound, non-magnesium silicon-oxide compound, and pre-magnesium silicon-oxide compound.

[0019] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0020] (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; optionally, the porous carbon is hard carbon;

[0021] (2) The silicon-carbon composite also includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials;

[0022] (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass;

[0023] (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm, and optionally 7 μm to 11 μm;

[0024] (5) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0025] (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

[0026] This is beneficial to improving the structural stability and specific capacity of silicon-carbon composites, thereby improving the energy density, lifespan, and fast-charging performance of secondary batteries.

[0027] In some embodiments, the first negative electrode active layer further includes a second graphite material, which includes one or more of the following characteristics:

[0028] (1) The average particle size is 13μm to 18μm; (2) The specific surface area is 1.6m². 2 / g~2.3m 2 / g; (3) The degree of graphitization is 91% to 93%. Therefore, the use of the above-mentioned fast-charging graphite in the first negative electrode active layer can further improve the charging capability of the first negative electrode active layer.

[0029] In some embodiments, the mass percentage M1 of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is greater than or equal to 20% and less than 80%. This allows for space to be reserved within the first negative electrode active layer for the expansion of the first silicon-based material, thereby balancing the energy density and charging capacity of the secondary battery.

[0030] In some embodiments, the mass percentage (M1) of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is 40% to 60%. This is more conducive to reserving space for the expansion of the first silicon-based material within the first negative electrode active layer, thereby better balancing the energy density and charging capacity of the secondary battery.

[0031] In some embodiments, silicon accounts for 5.5% to 48% of the mass of the first negative electrode active layer. This is beneficial for increasing the specific capacity of the first negative electrode active layer, and thus for increasing the energy density of the secondary battery.

[0032] In some embodiments, the first graphite material satisfies one or more of the following characteristics:

[0033] (1) Average particle size is 9.5 μm to 13 μm; (2) Specific surface area is 0.8 m². 2 / g~1.0m 2 / g; (3) Graphitization degree is 93% to 96%; (4) Specific capacity is 350mAh / g to 390mAh / g. Therefore, the use of the above-mentioned high energy density graphite in the second negative electrode active layer can further improve the specific capacity of the anode, which is beneficial to improving the energy density of the secondary battery.

[0034] In some embodiments, the second negative electrode active layer further includes a second silicon-based material, wherein the mass percentage (M2) of the second silicon-based material in the total mass of the first graphite material and the second silicon-based material is 0% to 0.5%. This is beneficial for balancing the energy density and charging capability of the secondary battery.

[0035] In some embodiments, the second silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

[0036] In some embodiments, the mass percentage of silicon in the second negative electrode active layer is 0% to 0.35%. This is beneficial for increasing the specific capacity of the second negative electrode active layer, and consequently for increasing the energy density of the secondary battery.

[0037] In some embodiments, the coating weight W1 of the first negative electrode active layer and the coating weight W2 of the second negative electrode active layer satisfy the condition: 0.29 ≤ W1 / W2 ≤ 0.95. This ensures that the charging capacity of the first negative electrode active layer is at a comparable level to that of the second negative electrode active layer, thereby improving the overall charging capacity of the negative electrode.

[0038] In some embodiments, the coating weight W1 of the first negative electrode active layer is 2.3 mg / cm³. 2 ~9.5mg / cm 2 This gives the first negative electrode active layer suitable electron and ion conduction capabilities, thereby improving its charging capacity.

[0039] In some embodiments, the coating weight W2 of the second negative electrode active layer is 9 mg / cm³. 2 ~12mg / cm 2 Optionally, the coating weight W2 of the second negative electrode active layer is 10 mg / cm³. 2 ~12mg / cm 2 This gives the second negative electrode active layer suitable electron and ion conduction capabilities, thereby improving its charging capacity.

[0040] In some embodiments, under full discharge conditions, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy the condition: 0.78 ≤ D2 / D1 ≤ 1.9; optionally, under full discharge conditions, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy the condition: 0.78 ≤ D2 / D1 ≤ 1.14. This ensures that the charging capacity of the first negative electrode active layer and the second negative electrode active layer are at a comparable level, thereby improving the overall charging capacity of the negative electrode.

[0041] In some embodiments, the thickness D1 of the first negative electrode active layer is 30 μm to 70 μm. This gives the first negative electrode active layer suitable electron and ion conduction capabilities, thereby improving its charging capacity.

[0042] In some embodiments, the thickness D2 of the second negative electrode active layer is 50 μm to 80 μm; alternatively, the thickness D2 of the second negative electrode active layer is 60 μm to 80 μm. This allows the second negative electrode active layer to possess suitable electron and ion conduction capabilities, thereby improving its charging capability.

[0043] In some embodiments, the negative electrode sheet further includes a buffer layer located between the negative electrode current collector and the first negative electrode active layer. The buffer layer includes a binder and a conductive agent, and the binder accounts for 50% to 80% of the mass of the buffer layer. This reduces the probability of the first negative electrode active layer delaminating and buffers the force exerted by silicon-based material particles on the negative electrode current collector during cold pressing. The buffer layer also reduces the risk of damage to the negative electrode current collector by silicon-based material particles, thereby improving the safety performance of the secondary battery.

[0044] In some embodiments, the thickness of the buffer layer is 0.3 μm to 1.5 μm. This helps to reduce the probability of the first negative electrode active layer delaminating, and also ensures that the overall negative electrode sheet maintains a suitable thickness, thereby improving the energy density of the secondary battery.

[0045] In some embodiments, the first negative electrode active layer includes a first sublayer and a second sublayer, the second sublayer being located on the surface of the first sublayer away from the negative electrode current collector, the first sublayer including a first graphite material and / or a second graphite material, and the second sublayer including a first silicon-based material.

[0046] In some embodiments, the second sublayer includes a second graphite material, and the mass percentage (M3) of the first silicon-based material in the negative electrode active material of the second sublayer is 30% to 60%. This is beneficial for balancing the energy density and charging capability of the secondary battery.

[0047] In some embodiments, the first sublayer includes a first silicon-based material, and the mass percentage (M4) of the first silicon-based material in the negative electrode active material of the first sublayer is 0% to 0.5%. This allows for a balance between the energy density and charging capability of the secondary battery.

[0048] In some embodiments, the first negative electrode active layer further includes a first binder, the first binder comprising 1% to 5% of the mass of the first negative electrode active layer. This benefits, on the one hand, by binding the expansion of silicon-based materials and graphite materials within the first negative electrode active layer; on the other hand, it reduces polarization, improves the fast-charging capability of the secondary battery; and furthermore, it helps reduce the probability of the first negative electrode active layer delaminating during battery cycling.

[0049] In some embodiments, the second negative electrode active layer further includes a second binder, the second binder comprising 0.5% to 5% of the mass of the second negative electrode active layer. This benefits the second negative electrode active layer by binding the expansion of silicon-based materials and graphite materials, reduces polarization, improves the fast-charging capability of the secondary battery, and also reduces the probability of the second negative electrode active layer delaminating during battery cycling.

[0050] In some embodiments, the first negative electrode active layer further includes a first conductive agent, which includes carbon nanotubes. Therefore, by introducing a highly conductive first conductive agent into the first negative electrode active layer, the conductivity of the first negative electrode active layer is improved, thereby benefiting the power performance of the secondary battery.

[0051] In some embodiments, the secondary battery further includes a positive electrode and a separator. The positive electrode, separator, and negative electrode are bent and wound along a winding axis to form an electrode assembly. In the electrode assembly, a first negative electrode active layer is located on the side of the negative electrode current collector closer to the winding axis. This reduces the expansion of the negative electrode active material in the first negative electrode active layer and also reduces the effect of the expansion of the negative electrode active material in the first negative electrode active layer on the elongation at break of the current collector.

[0052] In some embodiments, the positive electrode includes a positive current collector and a first positive active layer and a second positive active layer located on opposite sides of the positive current collector. The first positive active layer is disposed opposite to a first negative active layer, and the second positive active layer is disposed opposite to a second negative active layer. The unit area capacity A3 of the first positive active layer and the unit area capacity A4 of the second positive active layer satisfy: 1 ​​≤ A3 / A4 ≤ 1.019. This ensures that the anode has sufficient active sites for lithium ion insertion, and prevents lithium ions from depositing on the anode surface to form lithium dendrites, thus promoting capacity matching between the anode and cathode.

[0053] In some embodiments, the coating weight W3 of the first positive electrode active layer is 16.6 mg / cm³. 2 ~17.4 mg / cm 2 .

[0054] In some embodiments, the coating weight W4 of the second positive electrode active layer is 16.2 mg / cm³. 2~17.4 mg / cm 2 .

[0055] A second aspect of this application provides an electrical device including the secondary battery provided in the first aspect. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of the negative electrode sheet according to one embodiment of this application.

[0057] Figure 2 This is a schematic diagram of the negative electrode sheet according to another embodiment of this application.

[0058] Figure 3 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0059] Figure 4 yes Figure 3 An exploded view of a battery cell according to one embodiment of this application is shown.

[0060] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application.

[0061] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0062] Figure 7 yes Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown.

[0063] Figure 8 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0064] Explanation of reference numerals in the attached figures:

[0065] 10 Negative electrode sheet; 101 Negative current collector; 102 First negative electrode active layer; 103 Second negative electrode active layer; 104 Buffer layer; 105 First sub-layer; 106 Second sub-layer; 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0066] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the secondary battery and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0067] The "range" disclosed in this application 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 included. 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.

[0068] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0069] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0070] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0071] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0072] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0073] With the increasing popularity of electric vehicles, the range anxiety and charging anxiety inherent in power batteries are becoming increasingly prominent. Typical commuter electric vehicles have a capacity of 60kWh to 70kWh. For the anode, the specific capacity of pure graphite anodes has reached its limit. Silicon materials have a higher specific capacity; therefore, adding a small amount of high-specific-capacity silicon material to the anode can effectively increase the specific capacity of the anode to meet energy density requirements and alleviate range anxiety. However, the addition of silicon materials will worsen the battery's charging capability. Therefore, silicon-containing batteries struggle to balance energy density and charging capability.

[0074] In related technologies, the charging capability of the battery cell can be improved by reducing the coating weight of the negative electrode film or reducing the cold pressing density of the negative electrode sheet, but these methods will affect the energy density of the battery cell.

[0075] Based on this, this application provides a secondary battery and an electrical device. The secondary battery prepared in this application can achieve both high energy density and excellent charging capability. The following provides a more detailed description of this application and its optional embodiments.

[0076] Secondary batteries

[0077] The first aspect of this application provides a secondary battery, which includes a negative electrode, such as... Figure 1 As shown, the negative electrode 10 includes a negative current collector 101 and a first negative active layer 102 and a second negative active layer 103 located on opposite sides of the negative current collector 101. The first negative active layer 102 includes a first silicon-based material, and the second negative active layer 103 includes a first graphite material. The resistance ρ1 of the first negative active layer and the resistance ρ2 of the second negative active layer satisfy: 1.8 ≤ ρ1 / ρ2 ≤ 5.4, and in the fully discharged state, the porosity of the first negative active layer is... Porosity of the second negative electrode active layer satisfy: Optionally, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.87≤ρ1 / ρ2≤4.6.

[0078] In this application, because the first negative electrode active layer includes a first silicon-based material, the specific capacity of the secondary battery anode is relatively high, which is beneficial to improving the energy density of the secondary battery. The first silicon-based material and the first graphite material are respectively disposed on opposite sides of the negative electrode current collector. The resistance of the first negative electrode active layer and the resistance of the second negative electrode active layer reflect the conductivity of the first active material and the conductivity of the second active material, respectively. The lower the resistance of the active layer, the better its conductivity, and the smaller the resistance encountered when the current flows in the active material; the porosity of the first negative electrode active layer... Porosity of the second negative electrode active layer The porosity reflects the ion-conducting ability of the active layer; the higher the porosity, the better the ion-conducting performance. The ion-conducting and electron-conducting abilities can comprehensively reflect the fast-charging capabilities of the first and second active layers. In this application, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer are set to satisfy the above relationship, and the porosity of the first negative electrode active layer is set to be [value missing] under fully discharged conditions. Porosity of the second negative electrode active layer By satisfying the above relationship, the combined ability of the first and second negative electrode active layers to conduct electrons and ions is comparable, which can improve the charging capacity of the first negative electrode active layer to a level comparable to that of the second negative electrode active layer, thereby improving the charging capacity of the entire negative electrode. Therefore, the secondary battery in this application can achieve both high energy density and excellent charging capacity.

[0079] For example, the ratio of the resistance of the first negative electrode active layer to the resistance of the second negative electrode active layer is a value within a range of 1.8, 1.87, 2.0, 2.3, 2.4, 2.5, 3.0, 3.05, 3.5, 4.0, 4.13, 4.2, 4.5, 4.6, 5.0, 5.2, 5.4, or any combination thereof. The ratio of the porosity of the first negative electrode active layer to the porosity of the second negative electrode active layer is a value within a range of 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, 2.7, 2.9, or any combination thereof.

[0080] In some embodiments, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 2.3 ≤ ρ1 / ρ2 ≤ 4.2, and in the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy: This makes it easier to ensure that the combined electron and ion conduction capabilities of the first and second negative electrode active layers are comparable, thus making their charging capabilities comparable and better balancing the energy density and charging capacity of the secondary battery. Optionally, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 2.4 ≤ ρ1 / ρ2 ≤ 3.05.

[0081] In some embodiments, the resistance ρ1 of the first negative electrode active layer is 2.7 mΩ to 6.9 mΩ. A resistance within this range allows the first negative electrode active layer to have suitable electron conductivity, thereby improving its charging capability. For example, the resistance ρ1 of the first negative electrode active layer is a value within a range of 2.7 mΩ, 3.0 mΩ, 4.0 mΩ, 5.0 mΩ, 6.0 mΩ, 6.5 mΩ, 6.9 mΩ, or any combination thereof.

[0082] In some embodiments, the resistance ρ2 of the second negative electrode active layer is 0.65 mΩ to 1.50 mΩ. A resistance within this range allows the second negative electrode active layer to have suitable electron conductivity, thereby improving its charging capability. For example, the resistance ρ2 of the second negative electrode active layer is a value within a range of 0.65 mΩ, 0.70 mΩ, 0.90 mΩ, 1.00 mΩ, 1.30 mΩ, 1.50 mΩ, or any combination thereof.

[0083] In some embodiments, the porosity of the first negative electrode active layer The porosity is 18% to 53%; optionally, the porosity of the first negative electrode active layer is... The porosity is between 26% and 40%. When the porosity of the first negative electrode active layer is within this range, the first negative electrode active layer possesses suitable ion-conducting ability, thereby improving its charging capability. For example, the porosity of the first negative electrode active layer... The value is a range between 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 45%, 50%, 53%, or any combination thereof.

[0084] In some embodiments, the porosity of the second negative electrode active layer The porosity is between 16% and 20%. A porosity within this range allows the second negative electrode active layer to possess suitable ion-conducting capabilities, thereby improving its charging capacity. For example, the porosity of the second negative electrode active layer... The value is a range between 15%, 16%, 17%, 18%, 19%, 20%, or any combination thereof.

[0085] In this application, the resistance of the first negative electrode active layer and the second negative electrode active layer has a meaning known in the art and can be measured using instruments and methods known in the art. For example, the film resistance can be tested using a HIOKI BT3562 internal resistance tester. Specifically, a square test sample of 10cm×10cm is cut from the negative electrode sheet, and the upper and lower sides of the test sample are clamped between the two conductive terminals of the internal resistance tester and fixed with a certain pressure. The resistance R of the test sample is then tested, wherein the diameter of the conductive terminals is 14mm, the applied pressure is 15MPa~27MPa, and the sampling time ranges from 5s to 17s.

[0086] In this application, the porosity of the first negative electrode active layer and the second negative electrode active layer has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a true density meter according to GB / T 24586-2009. Specifically: the first negative electrode active layer and the second negative electrode active layer are cut into 3mm×3mm pieces respectively, and the apparent volume V0 of the sample is measured (the apparent volume of the sample is the thickness of the sample × the area of ​​the sample). Then, the true volume of the sample is measured using a true density meter. Specifically, the sample is placed in the sample test chamber, and nitrogen gas is introduced into the sample test chamber. The sample test chamber is connected to the reference chamber, and the pressure after stabilization is recorded. By detecting the pressure before the reference chamber and the sample chamber are connected and the pressure after the connection between the reference chamber and the sample chamber is stabilized, the pore volume is calculated according to Bohr's law PV=nRT. The porosity of the sample = pore volume / apparent volume.

[0087] In some embodiments, the unit area capacity A1 of the first negative electrode active layer and the unit area capacity A2 of the second negative electrode active layer satisfy the condition: 1.000 ≤ A1 / A2 ≤ 1.019. By ensuring that A1 and A2 satisfy the above relationship, on the one hand, the anode has sufficient active sites for lithium ion insertion, and on the other hand, lithium ions are prevented from depositing on the anode surface to form lithium dendrites, which is beneficial for promoting capacity matching between the anode and cathode. For example, A1 / A2 is a value within a range of 1.000, 1.003, 1.005, 1.008, 1.010, 1.013, 1.015, 1.019, or any combination thereof.

[0088] In this application, the unit area capacity of the first negative electrode active layer and the second negative electrode active layer has a meaning known in the art and can be measured using instruments and methods known in the art. For example, the unit area capacity A1 of the first negative electrode active layer can be measured by the following steps: punching the negative electrode sheet into a small disc with an area of ​​S, erasing the second negative electrode active layer on the small disc, preparing a coin cell with the small disc and a lithium sheet, and performing a capacity test on the coin cell to obtain the capacity C1 of the first negative electrode active layer of the small disc. Then, the unit area capacity A1 of the first negative electrode active layer is equal to C1 / S. For example, the unit area capacity A2 of the second negative electrode active layer can be measured by the following steps: punching the negative electrode sheet into a small disc with an area of ​​S, erasing the first negative electrode active layer on the small disc, preparing a coin cell with the small disc and a lithium sheet, and performing a capacity test on the coin cell to obtain the capacity C2 of the second negative electrode active layer of the small disc. Then, the unit area capacity A2 of the second negative electrode active layer is equal to C2 / S.

[0089] In some embodiments, under full discharge conditions, the tortuosity τ1 of the first negative electrode active layer and the tortuosity τ2 of the second negative electrode active layer satisfy the condition: 1.06 ≤ τ2 / τ1 ≤ 2.0. When τ1 and τ2 satisfy the above relationship, the tortuosity of the first negative electrode active layer is smaller than that of the second negative electrode active layer. This results in a shorter lithium ion transport path in the first negative electrode active layer compared to the second negative electrode active layer, thereby reducing the concentration polarization of lithium ions in the first and second negative electrode active layers. This improves the ion-conducting capability of the first negative electrode active layer to a level comparable to that of the second negative electrode active layer, thus enhancing the charging capability of the secondary battery. For example, τ1 / τ2 is a value within a range of 1.06, 1.1, 1.3, 1.5, 1.8, 2.0, or any combination thereof.

[0090] In some embodiments, the tortuosity τ1 of the first negative electrode active layer is 2.0 to 3.0. A tortuosity within this range gives the first negative electrode active layer suitable ion-conducting capability, thereby improving its charging capability. For example, τ1 is a value within a range of 2.0, 2.4, 2.6, 2.8, 3.0, or any combination thereof.

[0091] In some embodiments, the tortuosity τ2 of the second negative electrode active layer is 3.2 to 4.0. A tortuosity within this range gives the second negative electrode active layer suitable ion-conducting capability, thereby improving its charging capability. For example, τ2 is a value within a range of 3.2, 3.4, 3.6, 3.8, 4.0, or any combination thereof.

[0092] In this application, the fully discharged state refers to the state in which the secondary battery is discharged to 0% SOC.

[0093] In this application, the term "torque" refers to the degree of tortuosity. The flow of electrolyte in the negative electrode active material layer is not a straight line, but a tortuous flow. Torque reflects the degree of this tortuosity. Torque is equal to the ratio of the actual length of the electrolyte in the channel to the apparent length (macroscopic distance, i.e., the thickness of the negative electrode active material layer) through the negative electrode active material layer. In other words, it is the true length of the trajectory of the electrolyte particles in the channel when they travel a unit distance through the negative electrode active material layer.

[0094] In this application, the tortuosity of the first negative electrode active layer and the second negative electrode active layer has a meaning known in the art and can be determined using instruments and methods known in the art. For example, the tortuosity can be calculated according to the formula τ=(ε*k*Rion*A) / d, where d is the thickness of the electrode sheet, ε is the porosity of the electrode sheet, A is the reaction area, k is the conductivity of the electrolyte, and Rion is the electrode ion impedance. Specifically, the tortuosity τ1 of the first negative electrode active layer can be determined by the following steps: wiping off the second negative electrode active layer on the negative electrode sheet, assembling the wiped negative electrode sheet with a lithium sheet into a coin cell, performing an electrochemical impedance test on the coin cell, fitting the electrode ion impedance Rion, and calculating the tortuosity τ1 of the first negative electrode active layer according to the above formula. The tortuosity τ2 of the second negative electrode active layer can be determined by the following steps: wipe off the first negative electrode active layer on the negative electrode sheet, assemble the wiped negative electrode sheet with the lithium sheet into a coin cell, perform an electrochemical impedance test on the coin cell, fit the electrode ion impedance Rion, and calculate the tortuosity τ2 of the second negative electrode active layer according to the above formula.

[0095] In some embodiments, the average particle size of the first silicon-based material is 5 μm to 11 μm; and / or, the specific surface area of ​​the first silicon-based material is 1.5 m². 2 / g~3m 2 / g. Exemplarily, the average particle size of the first silicon-based material is a value within a range of 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, or any combination thereof, and the specific surface area of ​​the first silicon-based material is 1.5m². 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 2.8m 2 / g, 3.0m 2 / g or a range between the values ​​of either / g or any two of them.

[0096] In some embodiments, the first silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

[0097] In some embodiments, the first silicon-based material includes a silicon-carbon composite, which can be prepared using conventional silicon-carbon composites or conventional preparation methods, such as depositing nano-silicon materials on porous carbon by chemical vapor deposition, and can be further coated with carbon, such as using amorphous carbon coating.

[0098] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0099] (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; optionally, the porous carbon is hard carbon;

[0100] (2) The silicon-carbon composite also includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials;

[0101] (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass;

[0102] (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm, and optionally, it is 7 μm to 11 μm;

[0103] (5) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0104] (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

[0105] In some embodiments, the silicon-carbon composite includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support for the nanoscale silicon-containing material and, simultaneously, provides expansion space for the expansion of the silicon nanoparticles, effectively mitigating the stress compression caused by expansion during charging. Especially when the silicon-containing particles are in the nanometer range, the specific capacity is higher and dispersion in the pores of the porous carbon is more efficient. Furthermore, the buffering effect of the porous carbon's pores on expansion can be more fully utilized. When this silicon-carbon composite is applied in a wound electrode assembly, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0106] In some embodiments, the porous carbon may optionally be hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the battery cell.

[0107] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the battery cell.

[0108] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer that coats the surface of the core. This can improve the conductivity of the silicon-carbon composite and reduce the internal impedance of the battery cell, while also effectively reducing the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0109] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0110] In this application, the method for testing the silicon content in the silicon-carbon composite can be a method known in the art. As an example, the following method can be used for testing: a certain amount of silicon-carbon composite is taken, and the mass of silicon element in the silicon-carbon composite is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon element in the silicon-carbon composite can be calculated.

[0111] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in the silicon-carbon composite also form between the particles. To further improve the flow of lithium ions through the intraparticle and interparticle pores, in some embodiments, the average particle size of the silicon-carbon composite is 2 μm to 15 μm. Optionally, the average particle size of the silicon-carbon composite is 7 μm to 11 μm, or 5 μm to 10 μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite and the average particle size of the graphite material, which is more conducive to increasing the compaction of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the battery cell.

[0112] The average particle size of the aforementioned silicon-carbon composite can be tested using equipment and methods known in the art. For example, a scanning electron microscope (SEM) (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain SEM images of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0113] In some embodiments, the silicon-carbon composite powder resistivity at 8 MPa is 4 Ω·cm to 17 Ω·cm. Controlling the powder resistivity improves the conductivity of the silicon-carbon composite, thereby increasing the charging rate of the battery cell. Exemplarily, the silicon-carbon composite powder resistivity at 8 MPa is a value within the range of 4 Ω·cm, 5 Ω·cm, 7 Ω·cm, 10 Ω·cm, 13 Ω·cm, 15 Ω·cm, 17 Ω·cm, or any combination thereof.

[0114] In this application, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0115] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g. For example, the specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g, 2.0m 2 / g, 3.0m 2 / g, 4.0m 2 / g, 5.0m 2 / g, 6.0m 2 / g, 6.7m 2 / g or a range between the values ​​of either / g or any two of them.

[0116] In this application, the method for testing the BET specific surface area of ​​the silicon-carbon composite can be a method known in the art. As an example, referring to GB / T 19587-2017, a nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196℃, and the amount of nitrogen adsorbed on the surface of the solid sample at different pressures of 0.05 to 0.30 is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is obtained. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0117] In some embodiments, the silicon-oxygen material includes one or more of the following: non-lithium silicon-oxygen compounds, pre-lithium silicon-oxygen compounds, non-magnesium silicon-oxygen compounds, and pre-magnesium silicon-oxygen compounds. Specifically, the silicon-oxygen material includes materials with the chemical formula SiO. x The material is given by , where 0 < x < 2. Optionally, 0.5 ≤ x ≤ 1.5.

[0118] In some embodiments, the first negative electrode active layer further includes a second graphite material, which includes one or more of the following characteristics:

[0119] (1) The average particle size is 13 μm to 18 μm, for example, an average particle size within a range of 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or any combination thereof. (2) The specific surface area is 1.6 m². 2 / g~2.3m 2 / g, for example, has a specific surface area of ​​1.6m³. 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.3m 2 / g or a value within a range of any two of them. (3) The degree of graphitization is 91% to 93%, for example, the degree of graphitization is 91%, 92%, 93% or a value within a range of any two of them. The average particle size, specific surface area and degree of graphitization of the second graphite material are within the above range, reflecting that the second graphite material is a fast-charging graphite. Therefore, the second graphite material has more ion transport channels, which is beneficial to the insertion and extraction of lithium ions. The use of the above-mentioned fast-charging graphite in the first negative electrode active layer can further improve the charging capability of the first negative electrode active layer.

[0120] In some embodiments, the second graphite material is assembled with lithium metal into a coin cell, and the measured initial coulombic efficiency is greater than or equal to 93%. An initial coulombic efficiency of the second graphite material within this range can further reduce lithium-ion consumption and increase the energy density of the secondary battery. Exemplarily, the initial coulombic efficiency is a value within a range of 93%, 94%, 95%, 96%, 97%, 98%, 99%, or any combination thereof.

[0121] In some embodiments, the mass percentage M1 of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is greater than or equal to 20% and less than 80%, optionally, M1 is 40% to 60%. The mass percentage M1 of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material being within the above range allows for a reasonable ratio of the first silicon-based material and the second graphite material, which is beneficial for reserving space for the expansion of the first silicon-based material within the first negative electrode active layer, thereby balancing the energy density and charging capacity of the secondary battery. Exemplarily, M1 is a value within a range of 20%, 30%, 40%, 60%, 70%, 79%, or any combination thereof.

[0122] In some embodiments, the mass percentage of silicon in the first negative electrode active layer is between 5.5% and 48%. A mass percentage of silicon in the first negative electrode active layer within this range is beneficial for increasing the specific capacity of the first negative electrode active layer, and consequently for increasing the energy density of the secondary battery. Exemplarily, the mass percentage of silicon in the first negative electrode active layer is a value within the range of 5.5%, 10%, 20%, 30%, 40%, 48%, or any combination thereof.

[0123] In some embodiments, the first graphite material satisfies one or more of the following characteristics:

[0124] (1) The average particle size is 9.5 μm to 13 μm, for example, an average particle size within the range of 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, or any combination thereof. (2) The specific surface area is 0.8 m². 2 / g~1.0m 2 / g, for example, has a specific surface area of ​​0.8m³. 2 / g, 0.9m 2 / g, 1.0m 2 (3) The degree of graphitization is 93% to 96%, for example, the degree of graphitization is 93%, 94%, 95%, 96%, or any two of these values. (4) The specific capacity is 350 mAh / g to 390 mAh / g, for example, the specific capacity is 350 mAh / g, 360 mAh / g, 370 mAh / g, 380 mAh / g, 390 mAh / g, or any two of these values. The average particle size, specific surface area, degree of graphitization, and energy density of the first graphite material are within the above ranges, reflecting that the second graphite material is a high-energy-density graphite. The use of the above-mentioned high-energy-density graphite in the second negative electrode active layer can further improve the specific capacity of the anode, which is beneficial to improving the energy density of the secondary battery.

[0125] In some embodiments, a first graphite material is assembled with lithium metal to form a coin cell, and the measured initial coulombic efficiency is greater than or equal to 93%. An initial coulombic efficiency of the first graphite material within this range can reduce lithium-ion consumption, thus improving the energy density of the secondary battery. Exemplarily, the initial coulombic efficiency is a value within a range of 93%, 94%, 95%, 96%, 97%, 98%, 99%, or any combination thereof.

[0126] In this application, the method for testing the average particle size of graphite materials is as described above and will not be repeated here.

[0127] In this application, the specific surface area of ​​the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0128] In this application, the degree of graphitization of the material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using an X-ray diffractometer (such as a Bruker D8 Discover) according to JIS K 0131-1996 and JB / T 4220-2011. The measurement method can refer to the measurement of d... 002 The size is then determined according to the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002 This refers to the interlayer spacing in the graphite crystal structure, measured in nm. In X-ray diffraction analysis, Cu Kα rays were used as the radiation source, with the ray wavelength scanning 2θ angle range of 20° to 80° and the scanning rate of 4° / min.

[0129] In this application, the specific capacity of the first graphite material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, the specific capacity of the first graphite material can be measured by the following steps: A negative electrode sheet is punched into a small circular sheet with an area of ​​S; the first negative electrode active layer on the small circular sheet is wiped off; the weight of the small circular sheet sample is weighed and recorded as M1; the weight of the negative electrode current collector in the small circular sheet sample is weighed and recorded as M2; a coin cell is prepared by combining the small circular sheet with a lithium sheet; the obtained coin cell is allowed to stand for 12 hours, then discharged at a constant current of 0.05C to 0.005V; allowed to stand for 10 minutes; then discharged at a constant current of 50μA to 0.005V; and finally charged at a constant current of 0.1C to 2V, and the charging capacity C is recorded. The specific capacity of the first graphite material = C / (M1-M2).

[0130] In some embodiments, the second negative electrode active layer further includes a second silicon-based material, the second silicon-based material comprising a mass percentage (M2) of 0% to 0.5% of the total mass of the first graphite material and the second silicon-based material. Having the mass percentage (M2) of the second silicon-based material within the aforementioned range is beneficial for balancing the energy density and charging capability of the secondary battery. Exemplarily, M2 is a value within a range of 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any combination thereof.

[0131] In some embodiments, the second silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys; optionally, the second silicon-based material includes silicon-carbon composites.

[0132] In some embodiments, the mass percentage of silicon in the second negative electrode active layer is 0% to 0.35%. A mass percentage of silicon in the second negative electrode active layer within this range is beneficial for increasing the specific capacity of the second negative electrode active layer, and consequently for increasing the energy density of the secondary battery. Exemplarily, the mass percentage of silicon in the second negative electrode active layer is a value within a range of 0%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, or any combination thereof.

[0133] In this application, the mass percentage of silicon in the first and second negative electrode active layers can be determined by inductively coupled plasma optical emission spectrometry (ICP). Specifically, it can be determined by the following steps: Powder samples of the first and second negative electrode active layers are scraped off respectively. 0.2 g of the sample to be tested is weighed into a 100 mL beaker, and 10 mL of 10% w / w nitric acid solution is added. After digestion at 120 °C for 0.5 hours, the solution is diluted to volume with a 100 mL volumetric flask. Then, 1 mL is transferred to another 100 mL volumetric flask and diluted to volume again to obtain the test solution. The mass fraction of silicon in the test solution is determined using inductively coupled plasma optical emission spectrometry (ICP-OES).

[0134] In some embodiments, the coating weight W1 of the first negative electrode active layer and the coating weight W2 of the second negative electrode active layer satisfy the condition: 0.29 ≤ W1 / W2 ≤ 0.95. When W1 and W2 satisfy the above relationship, the lithium-ion transport capacity in the first and second negative electrode active layers is comparable, thus ensuring that the charging capacity of the first and second negative electrode active layers is at a comparable level, which is beneficial to improving the overall charging capacity of the negative electrode. For example, W1 / W2 is a value within a range of 0.29, 0.3, 0.5, 0.7, 0.9, 0.95, or any combination thereof.

[0135] In some embodiments, the coating weight W1 of the first negative electrode active layer is 2.3 mg / cm³. 2 ~9.5mg / cm 2 W1, within the aforementioned range, ensures that the first negative electrode active layer possesses suitable electron and ion conduction capabilities, thereby improving the charging capability of the first negative electrode active layer. For example, W1 is 2.3 mg / cm³. 2 2.5 mg / cm 2 3.0 mg / cm 2 5.0 mg / cm 27.0 mg / cm 2 9.0 mg / cm 2 9.5 mg / cm 2 Or the value between any two of them within a range.

[0136] In some embodiments, the coating weight W2 of the second negative electrode active layer is 9 mg / cm³. 2 ~12mg / cm 2 Optionally, the coating weight W2 of the second negative electrode active layer is 10 mg / cm³. 2 ~12mg / cm 2 W2, within the aforementioned range, ensures that the second negative electrode active layer possesses suitable electron and ion conduction capabilities, thereby improving the charging capability of the second negative electrode active layer. For example, W2 is 9 mg / cm³. 2 9.5 mg / cm 2 10mg / cm 2 10.5 mg / cm 2 11mg / cm 2 11.5 mg / cm 2 12mg / cm 2 Or the value between any two of them within a range.

[0137] In some embodiments, in the fully discharged state, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy the condition: 0.78 ≤ D2 / D1 ≤ 1.9; optionally, in the fully discharged state, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy the condition: 0.78 ≤ D2 / D1 ≤ 1.14. When D1 and D2 satisfy the above relationship, the lithium ion transport capacity in the first and second negative electrode active layers is comparable, thus ensuring that the charging capacity of the first and second negative electrode active layers is at a comparable level, which is beneficial to improving the overall charging capacity of the negative electrode. For example, D2 / D1 is a value within a range of 0.78, 0.8, 0.9, 1.0, 1.1, 1.14, 1.2, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or any combination thereof.

[0138] In some embodiments, the thickness D1 of the first negative electrode active layer is 30 μm to 70 μm. A thickness within this range allows the first negative electrode active layer to possess suitable electron and ion conduction capabilities, thereby improving its charging capability. For example, the thickness of the first negative electrode active layer is a value within a range of 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or any combination thereof.

[0139] In some embodiments, the thickness D2 of the second negative electrode active layer is 50 μm to 80 μm; optionally, the thickness D2 of the second negative electrode active layer is 60 μm to 80 μm. A thickness within the above range allows the second negative electrode active layer to possess suitable electron and ion conduction capabilities, thereby improving its charging capability. Exemplarily, the thickness of the second negative electrode active layer is a value within a range of 50 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, or any combination thereof.

[0140] In some implementations, such as Figure 1 As shown, the negative electrode sheet 10 also includes a buffer layer 104 located between the negative electrode current collector 101 and the first negative electrode active layer 102. The buffer layer 104 includes a binder and a conductive agent, and the binder accounts for 50% to 80% of the mass of the buffer layer 104. Providing a buffer layer containing a binder and a conductive agent between the negative electrode current collector and the first negative electrode active layer can, on the one hand, maintain good adhesion between the first negative electrode active layer and the negative electrode current collector, reducing the probability of the first negative electrode active layer delaminating; on the other hand, it can buffer the force exerted by silicon-based material particles on the negative electrode current collector during cold pressing. Simultaneously, the buffer layer can also reduce the probability of direct contact between silicon-based material particles and the negative electrode current collector, thus reducing the risk of damage to the negative electrode current collector by silicon-based material particles. This, in turn, can maintain good structural stability of the negative electrode current collector, which is beneficial to improving the safety performance of the secondary battery. For example, the mass percentage of the binder in the buffer layer is a value within the range of 50%, 60%, 70%, 80%, or any combination thereof.

[0141] In some embodiments, the adhesive in the buffer layer includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, and polyvinyl alcohol.

[0142] In some embodiments, the conductive agent in the buffer layer includes one or more of conductive graphite, conductive carbon black, carbon nanotubes, carbon nanofibers, and graphene.

[0143] In some embodiments, the thickness of the buffer layer is 0.3 μm to 1.5 μm. A thickness within this range allows the buffer layer to have a better buffering effect, reducing the probability of the first negative electrode active layer delaminating. It also ensures that the overall thickness of the negative electrode sheet is maintained at a suitable level, thereby improving the energy density of the secondary battery. For example, the thickness of the buffer layer is a value within the range of 0.3 μm, 0.5 μm, 0.8 μm, 1.0 μm, 1.3 μm, 1.5 μm, or any combination thereof.

[0144] In this application, the thicknesses of the first negative electrode active layer, the second negative electrode active layer, and the buffer layer can be measured in the following manner: The first negative electrode active layer, the second negative electrode active layer, and the buffer layer are respectively peeled off from the negative electrode current collector; the peeled-off negative electrode active layer is fixed on the sample stage; the sample stage is installed into the sample holder and locked in place; the power of the argon ion cross-section polisher (e.g., the IB-09010CP argon ion cross-section polisher from JEOL Corporation, Japan) is turned on, and a vacuum is applied (e.g., 10...). -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, use a scanning electron microscope to measure the thickness of the cross-sections of the first negative electrode active layer, the second negative electrode active layer, and the buffer layer.

[0145] In some implementations, such as Figure 2 As shown, the first negative electrode active layer 102 includes a first sublayer 105 and a second sublayer 106. The second sublayer 106 is located on the surface of the first sublayer 105 away from the negative electrode current collector 101. The first sublayer 105 includes a first graphite material and / or a second graphite material, and the second sublayer 106 includes a first silicon-based material.

[0146] In some embodiments, the second sublayer includes a second graphite material, and the mass percentage (M3) of the first silicon-based material in the negative electrode active material of the second sublayer is 30% to 60%. The mass percentage (M3) of the first silicon-based material in the negative electrode active material of the second sublayer within the above range ensures a reasonable ratio of silicon-based material and graphite material, which is beneficial for reserving space for the expansion of the silicon-based material within the negative electrode sheet, thereby balancing the energy density and charging capacity of the secondary battery. Exemplarily, M3 is a value within a range of 30%, 40%, 50%, 60%, or any combination thereof.

[0147] In some embodiments, the first sublayer includes a first silicon-based material, and the mass percentage (M4) of the first silicon-based material in the negative electrode active material of the first sublayer is 0% to 0.5%. When the mass percentage (M4) of the first silicon-based material in the negative electrode active material of the first sublayer is within the above range, it can improve the specific capacity of the anode on the one hand, and on the other hand, it will not excessively affect the charging capability of the first sublayer, thus achieving a balance between the energy density and charging capability of the secondary battery. For example, M4 is a value within a range of 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any combination thereof.

[0148] In some embodiments, the mass percentage of silicon in the first sublayer is 0% to 0.5%. A mass percentage of silicon in the first sublayer within this range is beneficial for increasing the specific capacity of the first sublayer, and consequently for increasing the energy density of the secondary battery. Exemplarily, the mass percentage of silicon in the first sublayer is a value within a range of 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any combination thereof.

[0149] In some embodiments, the first negative electrode active layer further includes a first binder, the first binder comprising 1% to 5% of the mass of the first negative electrode active layer, optionally 1.5% to 4.5%. The mass percentage of the first binder in the first negative electrode active layer within the above range is beneficial in two ways: firstly, it helps to bind the expansion of the silicon-based material and graphite material in the first negative electrode active layer; secondly, it can reduce polarization and improve the fast-charging capability of the secondary battery; and thirdly, it helps to improve the adhesion between the first negative electrode active layer and the negative electrode current collector, reducing the probability of the first negative electrode active layer delaminating during battery cycling. Exemplarily, the mass percentage of the first binder in the first negative electrode active layer is a value within the range of 1%, 1.5%, 2%, 3%, 4%, 4.5%, 5%, or any combination thereof.

[0150] In some embodiments, the second negative electrode active layer further includes a second binder, the second binder comprising 0.5% to 5% of the mass of the second negative electrode active layer, optionally 1.5% to 3.5%. The mass percentage of the second binder in the second negative electrode active layer within the above range is beneficial in two ways: firstly, it helps to bind the expansion of the silicon-based material and graphite material in the second negative electrode active layer; secondly, it can reduce polarization and improve the fast-charging capability of the secondary battery; and thirdly, it helps to improve the adhesion between the second negative electrode active layer and the negative electrode current collector, reducing the probability of the second negative electrode active layer delaminating during battery cycling. Exemplarily, the mass percentage of the second binder in the second negative electrode active layer is a value within the range of 0.5%, 1%, 1.5%, 2%, 3%, 3.5%, 4%, 5%, or any combination thereof.

[0151] In some embodiments, the first adhesive and the second adhesive each independently include one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, and polyvinyl alcohol. Optionally, both the first adhesive and the second adhesive are styrene-butadiene rubber.

[0152] In some embodiments, the first negative electrode active layer further includes a first conductive agent, which includes carbon nanotubes. The addition of a first silicon-based material can degrade the conductivity of the first negative electrode active layer, resulting in poor charging capability. In this application, by introducing a highly conductive first conductive agent into the first negative electrode active layer, the conductivity of the first negative electrode active layer can be improved, thereby improving the power performance of the secondary battery.

[0153] In some embodiments, the first negative electrode active layer further includes a second conductive agent, which includes conductive carbon black. The combination of carbon nanotubes and carbon black can provide good conductivity, further enabling the first negative electrode active layer to have suitable electronic conductivity, thereby further improving the charging capability of the first negative electrode active layer.

[0154] In some embodiments, the second negative electrode active layer further includes a third conductive agent, which includes conductive carbon black or includes conductive carbon black and carbon nanotubes.

[0155] In some embodiments, the first negative electrode active layer and the second negative electrode active layer also independently include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0156] In some embodiments, the secondary battery further includes a positive electrode and a separator. The positive electrode, separator, and negative electrode are bent and wound along a winding axis to form an electrode assembly. In the electrode assembly, a first negative electrode active layer is located on the side of the negative electrode current collector closer to the winding axis. Therefore, on the one hand, the outer second negative electrode active layer can constrain the inner first negative electrode active layer, reducing the expansion of the negative electrode active material in the first negative electrode active layer; on the other hand, it can reduce the effect of the expansion of the negative electrode active material in the first negative electrode active layer on the elongation at break of the current collector.

[0157] In some embodiments, the positive electrode includes a positive current collector and a first positive active layer and a second positive active layer located on opposite sides of the positive current collector. The first positive active layer is disposed opposite to a first negative active layer, and the second positive active layer is disposed opposite to a second negative active layer. The unit area capacity A3 of the first positive active layer and the unit area capacity A4 of the second positive active layer satisfy the condition: 1 ≤ A3 / A4 ≤ 1.019. When A3 and A4 satisfy the above relationship, on the one hand, the anode has sufficient active sites for lithium ion insertion, and on the other hand, lithium ions will not deposit on the anode surface to form lithium dendrites, which is beneficial to promoting the matching of anode and cathode capacities. For example, A3 / A4 is a value within a range of 1, 1.005, 1.010, 1.015, 1.019, or any two of them.

[0158] In some embodiments, the coating weight W3 of the first positive electrode active layer is 16.6 mg / cm³. 2~17.4 mg / cm 2 For example, W3 is 16.6 mg / cm³. 2 16.8 mg / cm 2 17.0 mg / cm 2 17.2 mg / cm 2 17.4 mg / cm 2 Or the value between any two of them within a range.

[0159] In some embodiments, the coating weight W4 of the second positive electrode active layer is 16.2 mg / cm³. 2 ~17.4 mg / cm 2 For example, W4 is 16.2 mg / cm³. 2 16.4 mg / cm 2 16.8 mg / cm 2 17.0 mg / cm 2 17.2 mg / cm 2 17.4 mg / cm 2 Or the value between any two of them within a range.

[0160] The secondary battery in this application can be prepared, for example, by the following method: S1, a first negative electrode slurry is coated on one side of the negative electrode current collector, and a second negative electrode slurry is coated on the other side of the negative electrode current collector. After drying and cold pressing, a first negative electrode active layer and a second negative electrode active layer are formed respectively, resulting in a negative electrode film layer, thereby forming a negative electrode sheet. The first negative electrode active layer includes a first silicon-based material, and the second negative electrode active layer includes a first graphite material. The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.8≤ρ1 / ρ2≤5.4; Optionally, the resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.87≤ρ1 / ρ2≤4.6; S2, the negative electrode sheet, the separator, and the positive electrode sheet are bent and wound to form an electrode assembly, and an electrolyte is injected to obtain a secondary battery. In the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy:

[0161] In some embodiments, the coating weight W1 of the first negative electrode active layer and the coating weight W2 of the second negative electrode active layer satisfy the condition: 0.29 ≤ W1 / W2 ≤ 0.95. This allows the charging capability of the first negative electrode active layer to be increased to a level comparable to that of the second negative electrode active layer, thereby improving the charging capability of the secondary battery.

[0162] In some embodiments, the coating weight W1 of the first negative electrode active layer is 2.3 mg / cm³. 2~9.5mg / cm 2 .

[0163] In some embodiments, the coating weight W2 of the second negative electrode active layer is 9 mg / cm³. 2 ~12mg / cm 2 Optionally, the coating weight W2 of the second negative electrode active layer is 10 mg / cm³. 2 ~12mg / cm 2 .

[0164] In some embodiments, the cold-pressed thickness of the first negative electrode active layer is 30 μm to 70 μm, and the cold-pressed thickness of the second negative electrode active layer is 50 μm to 80 μm. Optionally, the cold-pressed thickness of the second negative electrode active layer is 60 μm to 80 μm. The cold-pressed thicknesses of the first and second negative electrode active layers are within the above ranges, ensuring that both the first and second negative electrode active layers possess suitable electron and ion conduction capabilities, thereby improving the charging capacity of both layers. For example, the cold-pressed thickness of the first negative electrode active layer is a value within the range of 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or any combination thereof, and the cold-pressed thickness of the second negative electrode active layer is a value within the range of 50 μm, 60 μm, 70 μm, 80 μm, or any combination thereof.

[0165] In some embodiments, the first negative electrode slurry further includes a second graphite material, and the second negative electrode slurry further includes a second silicon-based material.

[0166] In some embodiments, the first silicon-based material and the second silicon-based material each independently include one or more of elemental silicon, silicon-oxygen materials, silicon-carbon composites, and silicon alloys.

[0167] In some embodiments, the first silicon-based material and the second silicon-based material comprise silicon-carbon composites.

[0168] In some embodiments, the mass percentage M1 of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is greater than or equal to 20% and less than 80%, and optionally, it is 40% to 60%.

[0169] In some embodiments, the mass percentage M2 of the second silicon-based material in the total mass of the first graphite material and the second silicon-based material is 0% to 0.5%.

[0170] In some embodiments, step S1 includes: S11, thoroughly mixing the binder and conductive agent in an appropriate amount of deionized water to form a buffer layer slurry; S12, coating the buffer layer slurry on one side of the negative electrode current collector, then drying it and coating the surface of the buffer layer slurry with a first negative electrode slurry, and coating the other side of the negative electrode current collector with a second negative electrode slurry, and then drying and cold pressing to form a buffer layer, a first negative electrode active layer and a second negative electrode active layer, thereby obtaining a negative electrode sheet, wherein the buffer layer is located between the negative electrode current collector and the first negative electrode active layer.

[0171] In some embodiments, the adhesive accounts for 50% to 80% of the mass of the buffer layer. When the mass percentage of the adhesive in the buffer layer is within this range, good adhesion is maintained between the first negative electrode active layer and the negative electrode current collector, which helps reduce the probability of the first negative electrode active layer delaminating.

[0172] In some embodiments, the coating weight ratio of the first negative electrode slurry, the buffer layer slurry, and the second negative electrode slurry is 1:(0.17-0.34):(0.19-1.15).

[0173] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0174] Typically, a single secondary battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0175] Negative electrode sheet

[0176] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, wherein the negative electrode film layer is the negative electrode film layer in the above embodiment.

[0177] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0178] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0179] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0180] Positive electrode sheet

[0181] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, which may be a positive electrode active material known in the art and is not particularly limited.

[0182] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0183] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0184] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: 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 battery positive electrode active materials 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 include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0185] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0186] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0187] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0188] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0189] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0190] electrolytes

[0191] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0192] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0193] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0194] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0195] In some embodiments, the electrolyte may optionally include additives. For example, additives may include 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 or low-temperature performance, etc.

[0196] Separating membrane

[0197] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0198] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0199] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0200] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0201] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0202] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 3 The example shown is a square-structured battery cell 5.

[0203] In some implementations, refer to Figure 4 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0204] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0205] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5 In battery module 4, multiple battery cells 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 battery cells 5 can be fixed in place using fasteners.

[0206] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0207] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0208] 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, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0209] Electrical appliances

[0210] The second aspect of this application provides an electrical device that includes the secondary battery provided in the first aspect of this application.

[0211] Secondary batteries can be used as a power source for electrical devices or as an energy storage unit for electrical devices. Electrical devices can include, but are not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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.

[0212] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0213] Figure 8This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0214] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0215] Example

[0216] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0217] Example 1

[0218] Preparation of secondary batteries:

[0219] S1, Preparing the negative electrode sheet, specifically including the following steps:

[0220] a. Preparation of the first negative electrode slurry: The first silicon-based material (silicon-carbon composite, silicon element accounts for 70 wt%), styrene-butadiene rubber, conductive carbon black, conductive carbon nanotubes, and sodium carboxymethyl cellulose are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 94.5:3:1:0.3:1.2 to form the first negative electrode slurry.

[0221] b. Preparation of the second negative electrode slurry: The first graphite material, styrene-butadiene rubber, conductive carbon black, and sodium carboxymethyl cellulose are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 95.8:2:1:1.2 to form the second negative electrode slurry;

[0222] c. Preparation of buffer layer slurry: Styrene-butadiene rubber and conductive carbon black are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 1:1 to form buffer layer slurry;

[0223] d. A buffer layer slurry is coated onto one side of the negative electrode current collector copper foil. After drying, a first negative electrode slurry is coated onto the buffer layer slurry, and a second negative electrode slurry is coated onto the other side of the negative electrode current collector copper foil. After drying and cold pressing, a buffer layer, a first negative electrode active layer, and a second negative electrode active layer are formed, respectively, to obtain the negative electrode sheet. The coating weight ratio of the first negative electrode slurry, the buffer layer slurry, and the second negative electrode slurry is 1:0.09:2.02, and the coating weight W1 of the first negative electrode active layer is 4.5 mg / cm³. 2The coating weight W2 of the second negative electrode active layer is 9.09 mg / cm³. 2 The coating weight of the buffer layer is 0.39 mg / cm³. 2 The thickness D1 of the first negative electrode active layer is 45.91 μm, the thickness D2 of the second negative electrode active layer is 55.09 μm, the thickness of the buffer layer is 0.3 μm, the mass percentage of the binder in the buffer layer is 50%, the mass percentage of the first binder in the first negative electrode active layer is 3%, and the mass percentage of the second binder in the second negative electrode active layer is 2%.

[0224] S2, Preparing a secondary battery, specifically includes the following steps:

[0225] e. Preparation of the positive electrode: The nickel-cobalt-manganese ternary positive electrode material LiNi... 0.8 Co 0.1 Mn 0.1 O2, conductive carbon black, and polyvinylidene fluoride are mixed in a weight ratio of 97:2:1 and then added to the solvent N-methylpyrrolidone. The mixture is stirred until homogeneous to form a positive electrode slurry. This slurry is coated onto both sides of the positive electrode current collector aluminum foil. After drying and cold pressing, a first and a second positive electrode active layer are formed, respectively. These two active layers constitute the positive electrode film, resulting in the positive electrode sheet. The coating weight W3 of the first positive electrode active layer is 17.0 mg / cm³. 2 The coating weight W4 of the second positive electrode active layer is 17.4 mg / cm³. 2 ;

[0226] f. Preparation of electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:2:1 to form an organic solvent. LiPF6 is dissolved in the organic solvent to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0227] g. Separating membrane: Polyethylene microporous film is used as the separating membrane substrate. Inorganic alumina powder, polyvinylpyrrolidone and acetone are mixed evenly in a weight ratio of 3:1.5:5.5 to form a slurry and coated on one side of the substrate. After drying, the separating membrane is obtained with a thickness of 11μm.

[0228] h. The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The first positive active layer and the first negative active layer are positioned opposite each other, and the second positive active layer and the second negative active layer are positioned opposite each other. The electrode assembly is then formed by bending and winding along a winding axis. In the electrode assembly, the first negative active layer is located on the side of the negative current collector closer to the winding axis. The electrode assembly is placed in an outer packaging, dried, and then injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping processes, a secondary battery is obtained.

[0229] Negative electrode film parameter testing:

[0230] (1) Resistance test

[0231] The resistance of the first negative electrode active layer and the second negative electrode active layer was tested using a HIOKI BT3562 internal resistance tester.

[0232] The second negative electrode active layer on the negative electrode sheet is wiped off. The wiped negative electrode sheet is cut into a square test sample of 10cm×10cm. The upper and lower sides of the test sample are clamped between the two conductive terminals of the internal resistance tester and a certain pressure is applied to fix it. The resistance R of the test sample is tested. The diameter of the conductive terminal is 14mm, the applied pressure is 15MPa~27MPa, and the sampling time range is 5s~17s.

[0233] The first negative electrode active layer on the negative electrode sheet is wiped off. The wiped negative electrode sheet is cut into a square test sample of 10cm×10cm. The upper and lower sides of the test sample are clamped between the two conductive terminals of the internal resistance tester and a certain pressure is applied to fix it. The resistance R of the test sample is tested. The diameter of the conductive terminal is 14mm, the applied pressure is 15MPa~27MPa, and the sampling time ranges from 5s to 17s.

[0234] In Example 1, the resistance ρ1 of the first negative electrode active layer is 4.2 mΩ, the resistance ρ2 of the second negative electrode active layer is 1.5 mΩ, and the ratio of ρ1 to ρ2 is 2.8.

[0235] (2) Porosity test

[0236] The porosity of the first and second negative electrode active layers was tested using a true density meter, referring to the standard procedure GB / T 24586-2009. The first and second negative electrode active layers were cut into 3mm × 3mm pieces, and the apparent volume V0 of the samples was measured (the apparent volume of the sample is the sample thickness × the sample area). Then, the true density meter was used to test the actual volume of the samples. Specifically, the samples were placed in the sample testing chamber, nitrogen gas was introduced into the chamber, and the sample testing chamber was connected to the reference chamber. The pressure after stabilization was recorded. By detecting the pressure before the reference chamber and sample chamber were connected and the pressure after the connection was stabilized, the pore volume was calculated according to Bohr's law PV = nRT. The porosity of the sample = pore volume / apparent volume.

[0237] Porosity of the first negative electrode active layer in Example 1 The porosity of the second negative electrode active layer is 30%. It is 18%. It is 1.67.

[0238] (3) Unit area capacity test

[0239] The negative electrode sheet is punched into a small circular sheet with an area of ​​S. The second negative electrode active layer on the small circular sheet is wiped off. The small circular sheet and lithium sheet are used to prepare a coin cell. The capacity of the coin cell is tested using a blue electric tester to obtain the capacity C1 of the first negative electrode active layer of the small circular sheet. Then, the unit area capacity of the first negative electrode active layer is A1 = C1 / S.

[0240] The negative electrode sheet is punched into a small circular sheet with an area of ​​S. The first negative electrode active layer on the small circular sheet is wiped off. The small circular sheet and lithium sheet are used to prepare a coin cell. The capacity of the coin cell is tested using a blue electric tester to obtain the capacity C2 of the second negative electrode active layer of the small circular sheet. Then, the unit area capacity of the second negative electrode active layer is A2 = C2 / S.

[0241] The positive electrode sheet is punched into a small circular sheet with an area of ​​S. The second positive electrode active layer on the small circular sheet is wiped off. The small circular sheet and lithium sheet are used to prepare a coin cell. The capacity of the coin cell is tested using a blue electric tester to obtain the capacity C3 of the first positive electrode active layer of the small circular sheet. Then, the unit area capacity of the first positive electrode active layer is A3 = C3 / S.

[0242] The positive electrode sheet is punched into a small circular sheet with an area of ​​S. The first positive electrode active layer on the small circular sheet is wiped off. The small circular sheet and lithium sheet are used to prepare a coin cell. The capacity of the coin cell is tested using a blue electric tester to obtain the capacity C4 of the second positive electrode active layer of the small circular sheet. Then, the unit area capacity of the second positive electrode active layer is A4 = C4 / S.

[0243] In Example 1, the unit area capacity A1 of the first negative electrode active layer is 4.25 mAh / cm². 2 The capacity A2 of the second negative electrode active layer is 4.25 mAh / cm². 2 The capacity per unit area A3 of the first positive electrode active layer is 4.03 mAh / cm³. 2 The capacity per unit area (A4) of the second positive electrode active layer is 4.03 mAh / cm². 2 A1 / A2 is 1, A3 / A4 is 1.

[0244] (4) Tortuousness test

[0245] The tortuosity calculation formula is τ=(ε*k*Rion*A) / d, where d is the thickness of the electrode, ε is the porosity of the electrode, A is the reaction area, k is the conductivity of the electrolyte, and Rion is the electrode ion impedance.

[0246] The second negative electrode active layer on the negative electrode sheet is wiped off, and the wiped negative electrode sheet is assembled with a lithium metal sheet to form a coin cell. The coin cell is subjected to electrochemical impedance testing, and the electrode ion impedance Rion is obtained by fitting. The tortuosity τ1 of the first negative electrode active layer is calculated according to the formula.

[0247] The first negative electrode active layer on the negative electrode sheet is wiped off, and the wiped negative electrode sheet is assembled with a lithium metal sheet to form a coin cell. The electrochemical impedance of the coin cell is tested, and the electrode ion impedance Rion is obtained by fitting. The tortuosity τ2 of the second negative electrode active layer is calculated according to the formula.

[0248] In Example 1, the tortuosity τ1 of the first negative electrode active layer is 2.2, the tortuosity τ2 of the second negative electrode active layer is 3.5, and the ratio of τ2 to τ1 is 1.59.

[0249] (5) Element content test

[0250] Disassemble the secondary battery to obtain the negative electrode sheet, then scrape off the first negative electrode active layer to obtain the powder to be tested. Weigh 0.2g of the powder to be tested into a 100mL beaker, add 10mL of 10% w / w nitric acid solution, heat and digest at 120℃ for 0.5 hours, then dilute to volume with a 100mL volumetric flask, and then use a pipette to transfer 1mL to a 100mL volumetric flask and dilute to volume again to obtain the solution to be tested.

[0251] Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to analyze the test solution using an EXPEC 6000 instrument. The ICP results showed that the mass percentage of silicon in the first negative electrode active layer of Example 1 was 48%.

[0252] Performance testing of secondary batteries:

[0253] (1) Charging capability test

[0254] ① At 25℃, charge the secondary battery with a constant current of 0.33C to 4.25V, then charge it with a constant voltage to a current of 0.05C. After standing for 5 minutes, discharge the secondary battery with a constant current of 0.33C to 2.5V and record its actual capacity as C0.

[0255] ②Then charge the secondary battery sequentially with constant current at 0.33C0, 0.5C0, 1.0C0, 1.5C0, 2.0C0, 2.5C0, and 3.0C0 to 4.25V or 0V negative electrode cutoff potential (whichever comes first). After each charge, discharge to 2.5V with 1C0. Record the negative electrode potential corresponding to 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% SOC (State of Charge) at different charging rates.

[0256] ③ Plot the charging rate-negative electrode potential curves under different SOC states. After linear fitting, obtain the charging rate corresponding to the negative electrode potential of 0V under different SOC states. This charging rate is the charging window under this SOC state, and is denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively.

[0257] ④ Calculate the charging time T (assuming no lithium plating) of the secondary battery from 10% SOC to 80% SOC using the following formula, in minutes.

[0258] (60 / C 20％SOC +60 / C 30％SOC +60 / C 40％SOC +60 / C 50％SOC +60 / C 60％SOC +60 / C 70％SOC +60 / C 80％SOC )×10%.

[0259] The test results are recorded in Table 2 below.

[0260] (2) Volumetric energy density test

[0261] ① At 25℃, the secondary battery is charged at a constant current of 0.33C to the charging cutoff voltage of 4.25V, and then charged at a constant voltage of 4.25V until the current is 0.05C. At this point, the secondary battery is fully charged. After the fully charged secondary battery is left to stand for 5 minutes, it is discharged at a constant current of 0.33C to 2.5V. The discharge capacity at this point is the actual capacity of the secondary battery at 0.33C, denoted as C0.

[0262] ②Then, the secondary battery was charged at a constant current of 0.33C0 to the cutoff voltage of 4.25V, and then charged at a constant voltage until the current reached 0.04C. At this point, the secondary battery was fully charged. After the fully charged secondary battery was left to stand for 5 minutes, it was discharged at a constant current of 0.33C0 to 2.5V, and the discharge energy Q of the secondary battery was obtained. The volumetric energy density of the secondary battery (Wh / L) = discharge energy Q of the secondary battery / volume V of the secondary battery. The test results are recorded in Table 2 below.

[0263] Examples 2 to 9

[0264] The secondary battery was prepared using the same method as in Example 1, with specific differences shown in Table 1.

[0265] Comparative Example 1

[0266] The secondary battery was prepared using the same method as in Example 1, except that in the preparation step of the negative electrode sheet, the first negative electrode slurry and the second negative electrode slurry were mixed evenly at a coating weight ratio of 1:2.02, the buffer layer slurry was coated onto the copper foil of the negative electrode current collector, and then the mixed negative electrode slurry was coated onto the buffer layer slurry. After drying and cold pressing, the buffer layer and the negative electrode film layer were formed respectively, and the negative electrode sheet was obtained.

[0267] Comparative Examples 2 to 6

[0268] The secondary battery was prepared using the same method as in Example 1, except that the resistance ρ1 and porosity of the first negative electrode active layer were adjusted according to Table 1 below. and the resistance ρ2 and porosity of the second negative electrode active layer

[0269] The negative electrode films prepared in Examples 2 to 9 and Comparative Examples 1 to 6 were subjected to parameter tests using the same test methods as in Example 1, and the secondary batteries prepared in Examples 2 to 9 and Comparative Examples 1 to 6 were subjected to performance tests using the same test methods as in Example 1.

[0270] Table 1 below shows the relevant parameters of the negative electrode films prepared in Examples 1 to 9 and Comparative Examples 1 to 6. Table 2 below shows the performance test results of the secondary batteries prepared in Examples 1 to 9 and Comparative Examples 1 to 6.

[0271] Table 1

[0272]

[0273]

[0274] In Table 1, " / " indicates that the symbol does not exist.

[0275] Table 2

[0276]

[0277]

[0278] As can be seen from Tables 1 and 2, compared with Comparative Example 1 (where the first negative electrode slurry and the second negative electrode slurry are mixed and then coated onto the buffer layer slurry), Comparative Example 2 (ρ1 / ρ2 is less than 1.8), Comparative Example 3 (ρ1 / ρ2 is greater than 5.4), and Comparative Example 4 (…), the results show that… Less than 1.1), Comparative Example 5 ( (greater than 2.9) and Comparative Example 6 (ρ1 / ρ2 less than 1.8 and Less than 1.1), Examples 1 to 9 control ρ1 / ρ2 to be 1.87 to 4.13 and With a value of 1.1 to 2.9, it can balance high energy density and excellent charging capability.

[0279] 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, characterized in that, It includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a first negative electrode active layer and a second negative electrode active layer located on opposite sides of the negative current collector, the first negative electrode active layer includes a first silicon-based material, and the second negative electrode active layer includes a first graphite material; The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 1.8 ≤ ρ1 / ρ2 ≤ 5.4, and in the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy:

2. The secondary battery according to claim 1, characterized in that, The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy the following condition: 1.87≤ρ1 / ρ2≤4.

6.

3. The secondary battery according to claim 1 or 2, characterized in that, The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy: 2.3 ≤ ρ1 / ρ2 ≤ 4.2, and in the fully discharged state, the porosity of the first negative electrode active layer is... Porosity of the second negative electrode active layer satisfy:

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The resistance ρ1 of the first negative electrode active layer and the resistance ρ2 of the second negative electrode active layer satisfy the following condition: 2.4≤ρ1 / ρ2≤3.

05.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The resistance ρ1 of the first negative electrode active layer is 2.7mΩ to 6.9mΩ.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The resistance ρ2 of the second negative electrode active layer is 0.65mΩ to 1.50mΩ.

7. The secondary battery according to any one of claims 1 to 6, characterized in that, Porosity of the first negative electrode active layer The porosity φ1 of the first negative electrode active layer is 18% to 53%; optionally, the porosity φ1 of the first negative electrode active layer is 26% to 40%.

8. The secondary battery according to any one of claims 1 to 7, characterized in that, Porosity of the second negative electrode active layer It ranges from 16% to 20%.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The unit area capacity A1 of the first negative electrode active layer and the unit area capacity A2 of the second negative electrode active layer satisfy: 1.000≤A1 / A2≤1.

019.

10. The secondary battery according to any one of claims 1 to 9, characterized in that, In the fully discharged state, the tortuosity τ1 of the first negative electrode active layer and the tortuosity τ2 of the second negative electrode active layer satisfy: 1.06≤τ2 / τ1≤2.

0.

11. The secondary battery according to claim 10, characterized in that, The tortuosity τ1 of the first negative electrode active layer is 2.0 to 3.

0.

12. The secondary battery according to claim 10 or 11, characterized in that, The tortuosity τ2 of the second negative electrode active layer is 3.2 to 4.

0.

13. The secondary battery according to any one of claims 1 to 12, characterized in that, The average particle size of the first silicon-based material is 5 μm to 11 μm; and / or, the specific surface area of ​​the first silicon-based material is 1.5 m². 2 / g~3m 2 / g.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The first silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

15. The secondary battery according to claim 14, characterized in that, The first silicon-based material includes the silicon-carbon composite, which satisfies one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon; (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or the silicon-containing material; (3) The mass percentage of silicon in the silicon-carbon composite is 30% to 70%; (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm; (5) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

16. The secondary battery according to any one of claims 1 to 15, characterized in that, The first negative electrode active layer further includes a second graphite material; the second graphite material includes one or more of the following characteristics: (1) The average particle size is 13μm~18μm; (2) Specific surface area is 1.6m² 2 / g~2.3m 2 / g; (3) The degree of graphitization is 91% to 93%.

17. The secondary battery according to claim 16, characterized in that, The mass percentage M1 of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is greater than or equal to 20% and less than 80%.

18. The secondary battery according to claim 16 or 17, characterized in that, The mass percentage (M1) of the first silicon-based material in the total mass of the second graphite material and the first silicon-based material is 40% to 60%.

19. The secondary battery according to any one of claims 1 to 18, characterized in that, The mass percentage of silicon in the first negative electrode active layer is 5.5% to 48%.

20. The secondary battery according to any one of claims 1 to 19, characterized in that, The first graphite material satisfies one or more of the following characteristics: (1) The average particle size is 9.5 μm to 13 μm; (2) Specific surface area is 0.8 m² 2 / g~1.0m 2 / g; (3) The degree of graphitization is 93% to 96%; (4) The capacity is 350mAh / g to 390mAh / g.

21. The secondary battery according to any one of claims 1 to 20, characterized in that, The second negative electrode active layer further includes a second silicon-based material, wherein the mass percentage (M2) of the second silicon-based material in the total mass of the first graphite material and the second silicon-based material is 0% to 0.5%.

22. The secondary battery according to claim 21, characterized in that, The second silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

23. The secondary battery according to any one of claims 1 to 22, characterized in that, The mass percentage of silicon in the second negative electrode active layer is 0% to 0.35%.

24. The secondary battery according to any one of claims 1 to 23, characterized in that, The coating weight W1 of the first negative electrode active layer and the coating weight W2 of the second negative electrode active layer satisfy the following condition: 0.29≤W1 / W2≤0.

95.

25. The secondary battery according to claim 24, characterized in that, The coating weight W1 of the first negative electrode active layer is 2.3 mg / cm³. 2 ~9.5mg / cm 2 .

26. The secondary battery according to claim 24 or 25, characterized in that, The coating weight W2 of the second negative electrode active layer is 9 mg / cm³. 2 ~12mg / cm 2 Optionally, the coating weight W2 of the second negative electrode active layer is 10 mg / cm³. 2 ~12mg / cm 2 .

27. The secondary battery according to any one of claims 1 to 26, characterized in that, In the fully discharged state, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy: 0.78≤D2 / D1≤1.9; optionally, in the fully discharged state, the thickness D1 of the first negative electrode active layer and the thickness D2 of the second negative electrode active layer satisfy: 0.78≤D2 / D1≤1.

14.

28. The secondary battery according to claim 27, characterized in that, The thickness D1 of the first negative electrode active layer is 30 μm to 70 μm.

29. The secondary battery according to claim 27 or 28, characterized in that, The thickness D2 of the second negative electrode active layer is 50 μm to 80 μm; optionally, the thickness D2 of the second negative electrode active layer is 60 μm to 80 μm.

30. The secondary battery according to any one of claims 1 to 29, characterized in that, The negative electrode sheet also includes a buffer layer located between the negative electrode current collector and the first negative electrode active layer; The buffer layer includes an adhesive and a conductive agent, and the adhesive accounts for 50% to 80% of the mass of the buffer layer.

31. The secondary battery according to claim 30, characterized in that, The thickness of the buffer layer is 0.3μm to 1.5μm.

32. The secondary battery according to any one of claims 1 to 31, characterized in that, The first negative electrode active layer includes a first sublayer and a second sublayer, wherein the second sublayer is located on the surface of the first sublayer away from the negative electrode current collector; The first sublayer comprises a first graphite material and / or a second graphite material, and the second sublayer comprises the first silicon-based material.

33. The secondary battery according to claim 32, characterized in that, The second sublayer includes the second graphite material, and the mass percentage (M3) of the first silicon-based material in the negative electrode active material of the second sublayer is 30% to 60%.

34. The secondary battery according to claim 32 or 33, characterized in that, The first sublayer includes the first silicon-based material, and the mass percentage (M4) of the first silicon-based material in the negative electrode active material of the first sublayer is 0% to 0.5%.

35. The secondary battery according to any one of claims 1 to 34, characterized in that, The first negative electrode active layer further includes a first binder, wherein the first binder accounts for 1% to 5% of the mass of the first negative electrode active layer.

36. The secondary battery according to any one of claims 1 to 35, characterized in that, The second negative electrode active layer further includes a second binder, the second binder comprising 0.5% to 5% of the mass of the second negative electrode active layer.

37. The secondary battery according to any one of claims 1 to 36, characterized in that, The first negative electrode active layer further includes a first conductive agent, which includes carbon nanotubes.

38. The secondary battery according to any one of claims 1 to 37, characterized in that, The secondary battery also includes a positive electrode and a separator, wherein the positive electrode, the separator, and the negative electrode are bent and wound along a winding axis to form an electrode assembly; In the electrode assembly, the first negative electrode active layer is located on the side of the negative electrode current collector closer to the winding shaft.

39. The secondary battery according to claim 38, characterized in that, The positive electrode includes a positive current collector and a first positive active layer and a second positive active layer located on opposite sides of the positive current collector. The first positive active layer is disposed opposite to the first negative active layer, and the second positive active layer is disposed opposite to the second negative active layer. The unit area capacity A3 of the first positive electrode active layer and the unit area capacity A4 of the second positive electrode active layer satisfy: 1≤A3 / A4≤1.

019.

40. The secondary battery according to claim 39, characterized in that, The coating weight W3 of the first positive electrode active layer is 16.6 mg / cm³. 2 ~17.4 mg / cm 2 .

41. The secondary battery according to claim 39 or 40, characterized in that, The coating weight W4 of the second positive electrode active layer is 16.2 mg / cm³. 2 ~17.4 mg / cm 2 .

42. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 41.

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