Negative pole piece, secondary battery and electric device

Abstract

The invention relates to the technical field of batteries, and provides a negative electrode piece, a secondary battery and an electric device, the negative electrode piece comprises a negative electrode current collector, a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer is arranged on at least one surface of the negative electrode current collector, and the second negative electrode film layer is arranged on at least one surface of the negative electrode current collector; the second negative electrode film layer is arranged on the surface, far away from the negative electrode current collector, of the first negative electrode film layer; the first negative electrode film layer contains a first silicon carbon material and first graphite, the second negative electrode film layer contains a second silicon carbon material and second graphite, and the Dv50 particle size of the first silicon carbon material is larger than that of the second silicon carbon material. By adopting the double-layer coating design, the dynamic performance of the small-particle-size silicon-carbon material can be fully played, so that the fast-charging performance of the battery is improved, the side reaction of the large-particle-size silicon-carbon material and electrolyte can be reduced, the irreversible consumption of active ions is reduced, and the service life of the battery is prolonged. And the negative pole piece with good cycle performance, long storage life, high energy density and good fast charging performance is obtained.

Description

Technical Field

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

[0002] In recent years, lithium-ion batteries have been used more and more widely, mainly in the consumer electronics and power battery sectors. At the same time, users have placed increasingly higher demands on the performance of lithium-ion batteries. Whether it's the need for longer battery life in consumer electronics or the need for greater driving range in power battery applications, both are placing higher demands on the energy density of lithium-ion batteries.

[0003] To improve the energy density of lithium-ion batteries, related technologies use silicon-based materials with a theoretical capacity limit of up to 4200 mAh / g as the negative electrode material. Common silicon-based materials include silicon-carbon materials and silicon-oxygen materials. However, silicon-carbon materials have poor kinetics, making it difficult to meet the requirements of fast charging performance. Moreover, they undergo significant volume changes during charging and discharging, which can easily cause silicon-carbon material particles to shatter or even detach from the negative electrode, severely affecting the cycle performance of lithium-ion batteries.

[0004] The above statements are for the purpose of providing background information in relation to this application only and do not necessarily constitute prior art. Summary of the Invention

[0005] The purpose of this application is to provide a negative electrode sheet, a secondary battery, and an electrical device, aiming to solve the problem that using silicon-carbon materials alone as negative electrode active materials easily leads to unsatisfactory fast-charging performance and cycle performance of the battery.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, embodiments of this application provide a negative electrode sheet, including a negative electrode current collector, a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is disposed on at least one surface of the negative electrode current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer away from the negative electrode current collector.

[0008] The first negative electrode film layer contains a first silicon-carbon material and a first graphite, and the second negative electrode film layer contains a second silicon-carbon material and a second graphite. The Dv50 particle size of the first silicon-carbon material is larger than the Dv50 particle size of the second silicon-carbon material.

[0009] The negative electrode sheet provided in this application embodiment contains a unique first negative electrode film layer and a second negative electrode film layer. Specifically, the first negative electrode film layer contains a first silicon-carbon material with a relatively large average particle size, while the second negative electrode film layer contains a second silicon-carbon material with a relatively small average particle size. This results in a smaller specific surface area for the first silicon-carbon material, which, when placed within the first negative electrode film layer, reduces contact between the first negative electrode film layer and the electrolyte, thereby reducing irreversible consumption of active ions and improving battery lifespan. Conversely, the larger specific surface area of ​​the second silicon-carbon material, when placed within the second negative electrode film layer, allows for better contact between the second negative electrode film layer and the electrolyte, effectively improving the diffusion kinetics of the second silicon-carbon material and resulting in excellent fast-charging performance. Furthermore, the graphite contained in both the first and second negative electrode film layers effectively reduces the volume change generated during the insertion / extraction of active ions in the silicon-containing negative electrode, thereby increasing the battery's cycle life.

[0010] In some embodiments, the Dv50 particle size of the first silicon carbide material is >8 μm, and the Dv50 particle size of the second silicon carbide material is ≤8 μm.

[0011] By selecting a first silicon-carbon material and a second silicon-carbon material with different Dv50 particle sizes—specifically, the first silicon-carbon material with a larger Dv50 particle size > 8 μm and the second silicon-carbon material with a smaller Dv50 particle size ≤ 8 μm—significant differences exist in the kinetic and compressive strength properties of the two silicon-carbon materials. Using the second silicon-carbon material with better kinetic performance in the second negative electrode film layer and the first silicon-carbon material with better compressive strength in the first negative electrode film layer can effectively improve the battery's fast charging performance, storage performance, and cycle performance.

[0012] In some embodiments, the Dv50 particle size of the first silicon carbide material is 9–12 μm.

[0013] The Dv50 particle size of the first silicon-carbon material is within the above-mentioned range, which means that its specific surface area is small. When placed in the first negative electrode film layer, the first silicon-carbon material has less contact with the electrolyte, thereby reducing the irreversible consumption of active ions and improving the battery's storage performance.

[0014] In some embodiments, the Dv50 particle size of the second silicon-carbon material is 4–7 μm.

[0015] The Dv50 particle size of the second silicon-carbon material is within the above range, which means that it has a large specific surface area. Placing it in the second negative electrode film layer can allow it to better contact with the electrolyte. In addition, its excellent diffusion kinetics can effectively enhance the fast charging performance of the battery.

[0016] In some embodiments, the specific surface area of ​​the first silicon-carbon material is <4m². 2 / g.

[0017] By controlling the specific surface area of ​​the first silicon-carbon material within the above-mentioned range, the direct contact between the silicon-carbon material and the electrolyte can be reduced, thereby reducing the consumption of active lithium by side reactions and improving the battery's storage life and cycle life.

[0018] In some embodiments, the specific surface area of ​​the first silicon-carbon material is 1–3 m². 2 / g.

[0019] By controlling the specific surface area of ​​the first silicon-carbon material within the aforementioned range and placing it in the first negative electrode film, the contact area between the first silicon-carbon material and the electrolyte can be reduced, thereby reducing the irreversible loss of active ions and achieving the goal of improving battery storage performance.

[0020] In some embodiments, the specific surface area of ​​the second silicon-carbon material is ≥4m². 2 / g.

[0021] By controlling the silicon content, tap density, and specific surface area of ​​the second silicon-carbon material within the aforementioned ranges, the second silicon-carbon material exhibits excellent kinetic properties, thereby effectively improving the fast-charging performance of the battery.

[0022] In some embodiments, the specific surface area of ​​the second silicon-carbon material is 4–7 m². 2 / g.

[0023] By controlling the specific surface area of ​​the second silicon-carbon material within the aforementioned range and placing it in the second negative electrode film, the second silicon-carbon material can be in full contact with the electrolyte, thereby maximizing its excellent kinetic properties to enhance the battery's fast-charging performance.

[0024] In some embodiments, the mass percentage of the first silicon-carbon material is 1-32% based on the total mass of the first silicon-carbon material and the first graphite.

[0025] In some embodiments, the mass percentage of the first silicon-carbon material is 1 to 20% based on the total mass of the first silicon-carbon material and the first graphite.

[0026] In some embodiments, based on the total mass of the first silicon-carbon material and the first graphite, the mass percentage of the first silicon-carbon material is greater than 10% and less than or equal to 20%.

[0027] By controlling the mass ratio of the first silicon-carbon material within the above range, the volume change of the silicon-containing negative electrode sheet during the deintercalation and deintercalation of active ions can be significantly reduced, thereby increasing the cycle life of the battery.

[0028] In some embodiments, the mass percentage of the second silicon-carbon material is 1-32% based on the total mass of the second silicon-carbon material and the second graphite.

[0029] In some embodiments, the mass percentage of the second silicon-carbon material is 1 to 20% based on the total mass of the second silicon-carbon material and the second graphite.

[0030] In some embodiments, based on the total mass of the second silicon-carbon material and the second graphite, the mass percentage of the second silicon-carbon material is greater than 10% and less than or equal to 20%.

[0031] By controlling the mass ratio of the second silicon-carbon material within the above range, the volume change of the silicon-containing negative electrode sheet during the deintercalation and deintercalation of active ions can be significantly reduced, thereby increasing the cycle life of the battery.

[0032] In some embodiments, the Dv50 particle size of the second graphite is 10 μm to 16 μm.

[0033] In some embodiments, the specific surface area of ​​the second graphite is 0.6–4.2 m². 2 / g.

[0034] By combining the second graphite with the second silicon-carbon material within the aforementioned particle size range and / or specific surface area range, the negative electrode sheet exhibits excellent fast-charging performance.

[0035] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:(0.5~2).

[0036] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:(0.75~1.5).

[0037] By controlling the thickness ratio of the first negative electrode film layer to the second negative electrode film layer within the above range, not only can the first negative electrode film layer effectively reduce the irreversible consumption of active ions and effectively improve the battery's storage life, but the second negative electrode film layer can also fully utilize its excellent kinetic performance and significantly improve the battery's fast charging performance.

[0038] In some embodiments, the first graphite includes at least one of artificial graphite and natural graphite.

[0039] Combining artificial graphite and / or natural graphite with a first silicon-carbon material results in a negative electrode with a long storage life.

[0040] In some embodiments, the compaction density of the negative electrode sheet is 1.5–1.8 g / cm³. 3 .

[0041] In some embodiments, the compaction density of the negative electrode sheet is 1.6–1.75 g / cm³. 3 .

[0042] The compaction density of the negative electrode sheet is within the above range, which means that the mass of active material contained in a unit volume of the negative electrode sheet is relatively high. Thus, the negative electrode sheet exhibits high volumetric capacity and energy density.

[0043] In some embodiments, the first silicon-carbon material is in the form of spherical or near-spherical particles.

[0044] The first silicon-carbon material is in the form of spherical or near-spherical particles. During charging and discharging, the stress generated by the expansion is more uniform, and it is less likely to cause particle crushing due to stress concentration. In addition, spherical or near-spherical particles have better compressive strength and are less likely to break under high pressure, which is more conducive to improving the compaction density of the negative electrode sheet.

[0045] In some embodiments, the second silicon-carbon material is in the form of spherical or near-spherical particles.

[0046] The second silicon-carbon material is in the form of spherical or near-spherical particles. During charging and discharging, the stress generated by expansion is more uniform, making it less likely to cause particle breakage due to stress concentration. In addition, spherical or near-spherical particles have better compressive strength, which is beneficial to improving the compaction density of the negative electrode sheet, thereby increasing the energy density of the battery.

[0047] In some embodiments, the first silicon-carbon material includes a first core and a first carbon coating layer covering the surface of the first core, wherein the first core includes a first porous carbon substrate and first silicon particles distributed in the pore structure of the first porous carbon substrate.

[0048] The first porous carbon substrate has a rich pore structure, and the first silicon particles can be uniformly distributed in the pores of the first porous carbon, forming a stable composite structure. This structure not only benefits the dispersion and stability of silicon particles, but also improves the mechanical strength and stability of silicon-carbon materials.

[0049] In some embodiments, the second silicon-carbon material includes a second core and a second carbon coating layer covering the surface of the second core, wherein the second core includes a second porous carbon substrate and second silicon particles distributed in the pore structure of the second porous carbon substrate.

[0050] The second porous carbon substrate has a rich pore structure, and the second silicon particles can be uniformly distributed in the pores of the second porous carbon, forming a stable composite structure. This structure not only benefits the dispersion and stability of silicon particles, but also improves the mechanical strength and stability of silicon-carbon materials.

[0051] Secondly, embodiments of this application provide a secondary battery, including the negative electrode sheet of the first aspect of embodiments of this application.

[0052] By using the negative electrode sheet provided in the embodiments of this application, the battery of the embodiments of this application has better fast charging performance, cycle performance and service life.

[0053] Thirdly, embodiments of this application provide an electrical device including the secondary battery described in the second aspect of this application.

[0054] By using the secondary battery provided in the embodiments of this application, such an electrical device has good charging and discharging performance, excellent fast charging performance, and can work more stably and for a longer period of time.

[0055] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, specific embodiments of this application are given below. Attached Figure Description

[0056] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0057] Figure 1 This is a schematic diagram of one embodiment of the negative electrode sheet provided in this application.

[0058] Figure 2 This is a schematic diagram of another embodiment of the negative electrode sheet provided in this application;

[0059] Figure 3 These are schematic diagrams of individual battery cells in some embodiments of this application;

[0060] Figure 4 These are exploded structural diagrams of battery cells in some embodiments of this application;

[0061] Figure 5 This is a schematic diagram of the battery module structure in some embodiments of this application;

[0062] Figure 6 This is an exploded view of the battery pack in some embodiments of this application;

[0063] Figure 7 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in the present application.

[0064] The following are the labeling elements in the figure:

[0065] 100. Negative electrode sheet; 10. Negative current collector; 20. First negative electrode film layer; 30. Second negative electrode film layer;

[0066] 3. Battery cell; 31. Casing; 32. Electrode assembly; 33. Cover plate;

[0067] 4. Battery module;

[0068] 5. Battery pack, 51. Housing, 52. Lower housing. Detailed Implementation

[0069] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0070] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0071] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0072] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0073] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0074] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0075] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0076] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0077] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0078] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0079] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0080] Traditional rechargeable batteries are increasingly failing to meet people's needs. To improve battery performance such as energy density and lifespan, traditional technologies mainly focus on improving active materials, such as using silicon anode materials. However, with the widespread application of rechargeable batteries in smartphones, tablets, smart wearables, power tools, and electric vehicles, people are also placing higher demands on the fast-charging performance of batteries.

[0081] Fast charging performance of a battery means that active ions are extracted and reach the negative electrode in a short time, generally requiring the negative electrode material to have rapid lithium intercalation capability. Currently, commercially available lithium-ion battery negative electrode materials are mainly graphite, while the specific capacity of carbon negative electrode materials is approaching the theoretical value (372 mAh / g), and the energy density is difficult to meet the ever-increasing demands. Silicon-based materials have a theoretical capacity limit of up to 4200 mAh / g, and with abundant silicon content and a low lithium intercalation potential, they are the most promising next-generation negative electrode material to replace graphite negative electrodes.

[0082] Common silicon-based materials include silicon-carbon materials and silicon-oxygen materials. However, silicon-carbon materials have the following problems: for example, silicon-carbon materials have poor kinetic properties, making it difficult to meet the requirements for fast charging performance; for example, silicon-carbon materials undergo large volume changes during charging and discharging, which can easily cause the silicon-based materials to crack, and even lead to the cracking of the negative electrode material, seriously affecting the energy density and cycle performance of lithium-ion batteries; for example, the amorphous carbon coating on the surface of silicon-carbon materials has many defects, which can easily cause side reactions with the electrolyte, which is not conducive to the high-temperature storage performance of the battery; for example, silicon-carbon materials have low compaction density, and space must be reserved for the expansion of silicon particles during cycling, resulting in a lower energy density of the battery.

[0083] Based on this, the first aspect of the present application provides a negative electrode sheet, which includes a negative current collector, a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is disposed on at least one surface of the negative current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer away from the negative current collector.

[0084] The first negative electrode film contains a first silicon-carbon material and a first graphite, and the second negative electrode film contains a second silicon-carbon material and a second graphite. The Dv50 particle size of the first silicon-carbon material is larger than the Dv50 particle size of the second silicon-carbon material.

[0085] The larger particle size of the first silicon-carbon material means a larger contact area between the large-diameter particles and the negative electrode current collector, with fewer sharp edges coming into contact with the current collector. This increases the compaction density of the negative electrode sheet, reducing the damage to the current collector caused by sharp edges, thus achieving a reduction in damage to the current collector due to increased compaction density. Furthermore, the smaller specific surface area of ​​the first silicon-carbon material, when placed within the first negative electrode film, further reduces the contact area between the film and the electrolyte, thereby minimizing irreversible consumption of active ions and effectively improving battery lifespan. Simultaneously, combining the first silicon-carbon material with first graphite effectively reduces the volume change during the insertion / extraction of active ions in the silicon-carbon-containing negative electrode, further increasing the battery's cycle life.

[0086] The smaller particle size of the second silicon-carbon material allows for superior diffusion kinetics. When placed within the second anode film, it enables better contact between the film and the electrolyte, resulting in excellent fast-charging performance. Furthermore, the smaller particle size of the second silicon-carbon material minimizes expansion, reducing the risk of particle detachment and improving the cycle life of the anode. In addition, combining the second silicon-carbon material with second graphite effectively reduces volume changes during the insertion / extraction of active ions in the silicon-containing anode, further increasing battery cycle life.

[0087] This application employs a double-layer coating design, placing a large-particle-size first silicon-carbon material in the first negative electrode film layer and a small-particle-size second silicon-carbon material in the second negative electrode film layer. That is, the first negative electrode film layer is located between the second negative electrode film layer and the negative electrode current collector. This not only fully utilizes the excellent kinetic properties of the small-particle-size second silicon-carbon material, thereby improving the fast-charging performance of the battery, but also minimizes the occurrence of side reactions between the large-particle-size first silicon-carbon material and the electrolyte, thereby reducing the irreversible consumption of active ions and improving the battery's storage life. As a result, a negative electrode sheet with good cycle performance, long storage life, high energy density, and excellent fast-charging performance is obtained.

[0088] It is understood that "Dv50" refers to the particle size that, in the particle size distribution, reaches 50% of the cumulative volume distribution percentage, starting from the smallest particle size. The method for determining the Dv50 particle size can refer to GB / T19077-2016 Particle Size Distribution Laser Diffraction Method, such as using a Malvern 2000 (Malvern Master Size 3000) laser particle size analyzer.

[0089] A current collector is a structure or component in a battery used to collect current. A negative electrode current collector refers to a structure or component in the battery used to collect current at the negative electrode. As an example, a negative electrode current collector can be a metal foil or a composite current collector. The metal foil can be copper foil. A composite current collector can be a polymer matrix material and a metal layer formed on at least one surface of the polymer matrix material. The composite current collector can be formed on the surface of the polymer matrix material using copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, or silver alloys. The polymer matrix material can be polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0090] A negative electrode film layer refers to a film layer disposed on a negative electrode current collector and containing a negative electrode active material. As an example, a film layer containing a first silicon carbide material and a first graphite as negative electrode active materials is called a first negative electrode film layer, and a film layer containing a second silicon carbide material and a second graphite as negative electrode active materials is called a second negative electrode film layer.

[0091] 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.

[0092] For example, see Figure 1 The negative electrode 100 includes a negative current collector 10, a first negative electrode film layer 20, and a second negative electrode film layer 30. At this time, a negative electrode film layer is disposed on one side surface of the negative current collector 10, specifically: the first negative electrode film layer 20 is disposed on one side surface of the negative current collector 10, and the second negative electrode film layer 30 is disposed on the surface of the first negative electrode film layer 20 and away from the negative current collector 10, that is, the first negative electrode film layer 20 is located between the negative current collector 10 and the second negative electrode film layer 30.

[0093] For example, see Figure 2 The negative electrode 100 includes a negative current collector 10, a first negative electrode film layer 20, and a second negative electrode film layer 30. At this time, negative electrode film layers are provided on both sides of the negative current collector 10.

[0094] In some embodiments, the Dv50 particle size of the first silicon carbide material is >8 μm, and the Dv50 particle size of the second silicon carbide material is ≤8 μm.

[0095] By selecting a first silicon-carbon material and a second silicon-carbon material with different Dv50 particle sizes—specifically, the first silicon-carbon material with a larger Dv50 particle size > 8 μm and the second silicon-carbon material with a smaller Dv50 particle size ≤ 8 μm—significant differences exist in the kinetic and storage properties of the two silicon-carbon materials. Using the second silicon-carbon material with better kinetic properties in the second negative electrode film layer and the first silicon-carbon material with better storage properties in the first negative electrode film layer can effectively improve the battery's fast charging performance, storage performance, and cycle performance.

[0096] In some embodiments, the Dv50 particle size of the first silicon carbide material is 9 to 12 μm. For example, the Dv50 particle size of the first silicon carbide material can be 9 μm, 10 μm, 11 μm, 12 μm, or within any two of the above values.

[0097] The first silicon-carbon material has a Dv50 particle size within the aforementioned range, meaning it has a small specific surface area. When placed in the first negative electrode film layer, the first silicon-carbon material has less contact with the electrolyte, thereby reducing the irreversible consumption of active ions and improving the battery's storage performance. At the same time, the larger particles of the first silicon-carbon material have a larger contact area with the negative electrode current collector. This reduces the contact area between sharp edges and the negative electrode current collector, minimizing damage to the negative electrode current collector caused by increased compaction density, thus contributing to improving the compaction density of the negative electrode sheet.

[0098] In some embodiments, the Dv50 particle size of the second silicon carbide material is 4 to 7 μm. For example, the Dv50 particle size of the second silicon carbide material can be 4 μm, 5 μm, 6 μm, 7 μm, or within any two of the above values.

[0099] The Dv50 particle size of the second silicon-carbon material is within the above range, which means that it has a large specific surface area. Placing it in the second negative electrode film layer can allow it to contact the electrolyte better. In addition, its excellent diffusion kinetics can effectively improve the fast charging performance of the battery.

[0100] In some embodiments, the specific surface area of ​​the first silicon-carbon material is <4m². 2 / g.

[0101] In some embodiments, the specific surface area of ​​the first silicon-carbon material is 1–3 m². 2 / g. For example, the specific surface area of ​​the first silicon-carbon material can be 1m². 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g or within the range of any two of the above values.

[0102] By controlling the specific surface area of ​​the first silicon-carbon material within the aforementioned range and placing it in the first negative electrode film, the contact area between the first silicon-carbon material and the electrolyte can be reduced, thereby reducing the irreversible loss of active ions and achieving the goal of improving battery storage performance.

[0103] In some embodiments, the specific surface area of ​​the second silicon-carbon material is ≥4m². 2 / g.

[0104] In some embodiments, the specific surface area of ​​the second silicon-carbon material is 4–7 m². 2 / g. For example, the specific surface area of ​​the second silicon-carbon material can be 4m². 2 / g, 4.5m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g、7m 2 / g or within the range of any two of the above values.

[0105] By controlling the specific surface area of ​​the second silicon-carbon material within the aforementioned range and placing it in the second negative electrode film, the second silicon-carbon material can be in full contact with the electrolyte, thereby maximizing its excellent kinetic properties to enhance the battery's fast-charging performance.

[0106] It is understandable that "specific surface area" refers to the total surface area per unit mass of silicon-carbon material, with the international dimension being m. 2 The specific surface area (S / g) can be determined using instruments and methods known in the art. For example, the standard for determining the specific surface area of ​​solid materials can be found in GB / T19587-2017, using the gas adsorption BET method. The specific surface area is determined using the nitrogen adsorption analysis method and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be performed using a Tri Star II 3020 specific surface area and porosity analyzer from Micromeritics, Inc.

[0107] In some embodiments, the mass content of silicon in the first silicon-carbon material is 30% to 70%. For example, the mass content of silicon in the first silicon-carbon material can be 30%, 40%, 50%, 60%, 70%, or within any two of the above values.

[0108] Nano-silicon offers high capacity, but the lithium insertion / extraction process involves significant volume changes that damage the material structure. Porous carbon, with its abundant pores, serves as a substrate to support nano-silicon, while also reserving pores to reduce the adverse effects of volume changes during the lithium insertion / extraction process. Therefore, considering both capacity and expansion, the embodiments of this application control the mass content of silicon in the first silicon-carbon material within the aforementioned range, which can effectively improve the electrochemical performance of the battery.

[0109] In some embodiments, the silicon content in the second silicon-carbon material is 30% to 70% by mass. For example, the silicon content in the second silicon-carbon material can be 30%, 40%, 50%, 60%, 70%, or within any two of the above values.

[0110] In this application embodiment, silicon-carbon materials with silicon content within the above-mentioned range are selected, so that the silicon-carbon materials have high capacity and at the same time reduce the adverse effects of volume expansion.

[0111] In some embodiments, the tap density of the first silicon carbide material is 0.8–1.0 g / cm³. 3 For example, the tap density of the first silicon-carbon material can be 0.8 g / cm³. 3 0.85g / cm 3 0.9g / cm 3 0.95g / cm 3 1.0g / cm 3 Or it falls within the range formed by any two of the above values.

[0112] The embodiments of this application can effectively improve the energy density of the battery by adjusting the tap density of the first silicon-carbon material.

[0113] In some embodiments, the tap density of the second silicon carbide material is 0.8–1.0 g / cm³. 3 For example, the tap density of the second silicon-carbon material can be 0.8 g / cm³. 3 0.85g / cm 3 0.9g / cm 3 0.95g / cm 3 1.0g / cm 3 Or it falls within the range formed by any two of the above values.

[0114] The embodiments of this application select a second silicon-carbon material with a tap density within the above-mentioned range, which can effectively improve the energy density of the battery.

[0115] It is understood that "tap density" refers to the mass per unit volume of a material powder in a container after it has been tapped under specified conditions. Furthermore, "tap density" can be obtained using well-known methods commonly used in the field, such as referring to standard GB / T 5162-2006. In this application, referring to standard GB / T 5162-2006, the specific testing procedure is as follows: First, the powder is loaded into the sample tube, then the sample tube is placed in the instrument's working position. The start button is pressed, and without damaging the original morphology of the powder particles, the sample tube is repeatedly vibrated up and down for inertial motion. When the gap between the particles approaches its limit and the powder volume no longer decreases, the volume and weight data of the powder after vibration are input into the computer, and the tap density result of the powder is automatically calculated. Further, during the vibration process, the amplitude is 3.0 ± 0.1 mm, the vibration frequency is 250 ± 15 times / min, and the number of vibrations is 5000.

[0116] In some embodiments, the mass percentage of the first silicon-carbon material is 1% to 32% based on the total mass of the first silicon-carbon material and the first graphite. Exemplarily, the mass percentage of the first silicon-carbon material can be typical but not limiting values ​​such as 1%, 5%, 10%, 15%, 20%, 25%, 28%, 30%, and 32%. The negative electrode active material contained in the first negative electrode film layer includes the first silicon-carbon material and the first graphite, and the mass percentage of the first silicon-carbon material is 1% to 32%, the mass percentage of the first graphite is 68% to 99%, and the total mass of the first silicon-carbon material and the first graphite is 100%.

[0117] In some embodiments, the mass percentage of the first silicon-carbon material is 1-20% based on the total mass of the first silicon-carbon material and the first graphite. Exemplarily, the mass percentage of the first silicon-carbon material can be typical but not limiting values ​​such as 1%, 5%, 8%, 10%, 15%, and 20%. The negative electrode active material contained in the first negative electrode film layer includes the first silicon-carbon material and the first graphite, and the mass percentage of the first silicon-carbon material is 1-20%, the mass percentage of the first graphite is 80%-99%, and the total mass of the first silicon-carbon material and the first graphite is 100%.

[0118] By controlling the mass ratio of the first silicon-carbon material within the above range, the volume change of the silicon-containing negative electrode sheet during the deintercalation and deintercalation of active ions can be significantly reduced, thereby increasing the cycle life of the battery.

[0119] In some embodiments, based on the total mass of the first silicon-carbon material and the second graphite, the mass percentage of the first silicon-carbon material is greater than 10% and less than or equal to 20%. Exemplarily, the mass percentage of the first silicon-carbon material can be typical but not limiting values ​​such as 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0120] Controlling the mass ratio of the first silicon-carbon material within the above range can not only significantly reduce the volume change of the silicon-containing negative electrode during the insertion and extraction of active ions, thereby increasing the cycle life of the battery, but also effectively improve the fast charging performance of the battery.

[0121] In some embodiments, the mass percentage of the second silicon-carbon material is 1% to 32% based on the total mass of the second silicon-carbon material and the second graphite. Exemplarily, the mass percentage of the second silicon-carbon material can be typical but not limiting values ​​such as 1%, 5%, 10%, 15%, 20%, 25%, 28%, 30%, and 32%. The negative electrode active material contained in the second negative electrode film layer includes the second silicon-carbon material and the second graphite, and the mass percentage of the second silicon-carbon material is 1% to 32%, the mass percentage of the second graphite is 68% to 99%, and the total mass of the second silicon-carbon material and the second graphite is 100%.

[0122] In some embodiments, the mass percentage of the second silicon-carbon material is 1-20% based on the total mass of the second silicon-carbon material and the second graphite. Exemplarily, the mass percentage of the second silicon-carbon material can be typical but not limiting values ​​such as 1%, 5%, 8%, 10%, 15%, and 20%. The negative electrode active material contained in the second negative electrode film layer includes the second silicon-carbon material and the second graphite, and the mass percentage of the second silicon-carbon material is 1-20%, the mass percentage of the second graphite is 80%-99%, and the total mass of the second silicon-carbon material and the second graphite is 100%.

[0123] By controlling the mass ratio of the second silicon-carbon material within the aforementioned range, the volume change of the silicon-containing negative electrode sheet during the insertion / extraction of active ions can be significantly reduced, thereby increasing the cycle life of the battery.

[0124] In some embodiments, based on the total mass of the second silicon-carbon material and the second graphite, the mass percentage of the second silicon-carbon material is greater than 10% and less than or equal to 20%. Exemplarily, the mass percentage of the second silicon-carbon material can be typical but not limiting values ​​such as 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0125] Controlling the mass ratio of the second silicon-carbon material within the above range can not only significantly reduce the volume change of the silicon-containing negative electrode during the insertion and extraction of active ions, thereby increasing the cycle life of the battery, but also effectively improve the fast charging performance of the battery.

[0126] In the embodiments of this application, when the mass percentage of silicon-carbon material in the negative electrode active material is >10%, it is referred to as a high-silicon system; when the mass percentage of silicon-carbon material in the negative electrode active material is ≤10%, it is referred to as a low-silicon system.

[0127] In some embodiments, the Dv50 particle size of the second graphite is 10 μm to 16 μm. For example, the Dv50 particle size of the second graphite can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, or within any two of the above values.

[0128] The second graphite in the above-mentioned particle size range has excellent rate performance. When combined with the second silicon-carbon material, it can not only reduce the volume expansion problem caused by the second silicon-carbon material, but also effectively improve the fast charging performance of the battery.

[0129] In some embodiments, the specific surface area of ​​the second graphite is 0.6–4.2 m². 2 / g. For example, the specific surface area of ​​the second graphite can be 0.6m². 2 / g, 1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.2m 2 / g or within the range of any two of the above values.

[0130] By combining the second graphite with the second silicon-carbon material within the aforementioned specific surface area range, the second negative electrode film exhibits excellent fast-charging performance.

[0131] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:(0.5 to 2). For example, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer can be typical but not limiting values ​​such as 1:0.5, 1:0.75, 1:1, 1:1.5, 1:2, etc.

[0132] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:(0.75 to 1.5). For example, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer can be typical but not limiting values ​​such as 1:0.75, 1:0.85, 1:0.95, 1:1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc.

[0133] By controlling the thickness ratio of the first negative electrode film layer to the second negative electrode film layer within the above range, not only can the first negative electrode film layer effectively reduce the irreversible consumption of active ions and effectively improve the battery's storage life, but the second negative electrode film layer can also fully utilize its excellent kinetic performance and significantly improve the battery's fast charging performance.

[0134] In some embodiments, the first graphite includes at least one of artificial graphite and natural graphite.

[0135] By combining artificial graphite and / or natural graphite with the first silicon-carbon material, the first negative electrode film layer has a high storage life.

[0136] In some embodiments, the compaction density of the negative electrode sheet is 1.5–1.8 g / cm³. 3 .

[0137] In some embodiments, the compaction density of the negative electrode sheet is 1.6–1.75 g / cm³. 3 .

[0138] The compaction density of the negative electrode sheet is within the above range, which means that the mass of active material contained in a unit volume of the negative electrode sheet is relatively high. Thus, the negative electrode sheet exhibits high volumetric capacity and energy density.

[0139] In the embodiments of this application, the compaction density of the material has a meaning known in the art and can be tested using equipment and methods known in the art. For example, according to the test standard GB / T24533-2009, a certain amount of the above material is taken and added to a mold with a bottom area of ​​1.327 cm² in a UTM7305 electronic pressure testing machine, and pressure is applied to 2000 kg (equivalent to 20000 N), held for 30 s, then depressurized, held for 10 s, and then the compaction density of the material under a force of 20000 N is recorded and calculated.

[0140] In some embodiments, the first silicon-carbon material is in the form of spherical or near-spherical particles.

[0141] A spherical particle refers to a particle with a regular geometric shape. A sphere can be understood as the set of all points in space that are equidistant from a fixed point, and it has a high degree of symmetry.

[0142] Spherical particles refer to particles whose shape is close to that of a sphere, but not a perfect sphere. They can take various forms, such as elongated spherical, oblate spherical, and spherical. As an example, one form of spherical particle is an elongated or oblate spherical particle, which can be obtained by rotating an ellipse around its principal axis (major axis or minor axis) in three-dimensional space.

[0143] The first silicon-carbon material is in the form of spherical or near-spherical particles. On the one hand, the stress generated by expansion during charging and discharging is more uniform, making it less likely for particles to break due to stress concentration. On the other hand, the spherical or near-spherical particles have a smaller contact area with the negative electrode current collector, which can reduce the damage to the negative electrode current collector caused by the increased compaction density of the negative electrode sheet. In addition, the spherical or near-spherical particles have better compressive strength, and the material is less likely to break after increasing pressure, which is more conducive to increasing the compaction density of the negative electrode sheet, thereby improving the energy density of the battery.

[0144] In some embodiments, the second silicon-carbon material is in the form of spherical or near-spherical particles.

[0145] The second silicon-carbon material is in the form of spherical or near-spherical particles. On the one hand, the stress generated by expansion during charging and discharging is more uniform, and it is not easy for the particles to be crushed due to stress concentration. On the other hand, spherical or near-spherical particles have better compressive strength, and the material is not easy to break after increasing pressure, which is conducive to improving the compaction density of the negative electrode sheet, thereby improving the energy density of the battery.

[0146] In some embodiments, the first silicon-carbon material includes a first core and a first carbon coating layer covering the surface of the first core. The first core includes a first porous carbon substrate and first silicon particles distributed in the pore structure of the first porous carbon substrate.

[0147] The first porous carbon substrate has a rich pore structure, and the first silicon particles can be uniformly distributed in the pores of the first porous carbon, forming a stable composite structure. This structure not only benefits the dispersion and stability of silicon particles, but also improves the mechanical strength and stability of silicon-carbon materials.

[0148] In some embodiments, the second silicon-carbon material includes a second core and a second carbon coating layer covering the surface of the second core. The second core includes a second porous carbon substrate and second silicon particles distributed in the pore structure of the second porous carbon substrate.

[0149] The second porous carbon substrate has a rich pore structure, and the second silicon particles can be uniformly distributed in the pores of the second porous carbon, forming a stable composite structure. This structure not only benefits the dispersion and stability of silicon particles, but also improves the mechanical strength and stability of silicon-carbon materials.

[0150] In some embodiments, the first negative electrode film layer may optionally include a first conductive agent. The second negative electrode film layer may optionally include a second conductive agent. As an example, the first conductive agent and the second conductive agent are each independently selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0151] In some embodiments, the first negative electrode film layer may optionally include a first adhesive. The second negative electrode film layer may optionally include a second adhesive. As an example, the first adhesive and the second adhesive are each independently selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0152] In some embodiments, the first negative electrode film layer and / or the second negative electrode film layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0153] The first conductive agent, the second conductive agent, the first binder, the second binder, and other additives mentioned above, if present, are not subject to any particular restrictions on their types and amounts in this application. Those skilled in the art can select and determine them according to actual needs.

[0154] In some embodiments, the negative electrode sheet can be prepared by the following steps:

[0155] A first negative electrode film layer and a second negative electrode film layer are sequentially formed on the surface of the negative electrode current collector.

[0156] In some embodiments, the negative electrode sheet can be prepared in the following manner:

[0157] The components used to prepare the first negative electrode film layer, such as the first silicon carbon material, the first graphite, the first conductive agent, the first binder, and any other components, are dispersed in a solvent (e.g., deionized water) to form a first coating slurry; the first coating slurry is coated on the surface of the negative electrode current collector and dried to form the initial first negative electrode film layer;

[0158] The components used to prepare the second negative electrode film, such as the second silicon carbon material, the second graphite, the second conductive agent, the second binder, and any other components, are dispersed in a solvent (e.g., deionized water) to form a second coating slurry. The second coating slurry is coated on the surface of the initial first negative electrode film, and after drying, cold pressing, and other processes, the negative electrode sheet can be obtained.

[0159] It should be noted that the thickness of each layer in the negative electrode sheet, such as the first negative electrode film layer and the second negative electrode film layer, refers to the thickness of the corresponding layers in the negative electrode sheet after cold pressing and compaction and used for battery assembly.

[0160] In this application, the negative electrode sheet may include other additional functional layers besides the first negative electrode film layer and the second negative electrode film layer. For example, in some embodiments, the negative electrode sheet may also include a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative current collector and the first negative electrode film layer and disposed on the surface of the negative current collector; in some embodiments, the negative electrode sheet may also include a protective layer covering the surface of the second negative electrode film layer.

[0161] Secondly, embodiments of this application provide a secondary battery, including the negative electrode sheet of the first aspect of embodiments of this application.

[0162] By using the negative electrode sheet provided in the embodiments of this application, the battery of the embodiments of this application has better fast charging performance, cycle performance and service life.

[0163] The term "secondary battery" as used in this article refers to a single battery cell, battery module, or battery pack.

[0164] Typically, a secondary battery consists of 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.

[0165] [Positive electrode plate]

[0166] In some embodiments, the secondary battery further includes a positive electrode.

[0167] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material.

[0168] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0169] 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 matrix material and a metal layer formed on at least one surface of the polymer matrix material. 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 matrix material (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0170] In some embodiments, when the secondary battery 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 NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 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.

[0171] In some embodiments, when the secondary battery is a sodium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for sodium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0172] 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 cathode materials in this application, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0173] In the examples of cathode 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.

[0174] In some embodiments, the positive electrode active material 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.

[0175] In some embodiments, the positive electrode active material layer may optionally include a positive electrode conductive agent. As an example, the positive electrode 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.

[0176] 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.

[0177] Electrolyte

[0178] In some embodiments, the battery cell further includes an electrolyte.

[0179] During the charging and discharging process of a single battery cell, active ions repeatedly insert and extract between the positive and negative electrode plates, while the electrolyte acts as a conductor for these active ions. The embodiments of this application do not impose any particular restrictions on the type of electrolyte; it can be selected according to actual needs.

[0180] Electrolytes consist of electrolyte salts and solvents. The types of electrolyte salts and solvents are not specifically limited and can be selected according to actual needs.

[0181] When the secondary battery of this application is a lithium-ion battery, as an example, the electrolyte salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0182] When the secondary battery of this application is a sodium-ion battery, as an example, the electrolyte salt may include, but is not limited to, at least one of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium perchlorate (NaClO4), sodium hexafluoroarsenate (NaAsF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluorooxalate borate (NaDFOB), sodium dioxalate borate (NaBOB), sodium difluorophosphate (NaPO2F2), sodium difluorodioxalate phosphate (NaDFOP), and sodium tetrafluorooxalate phosphate (NaTFOP).

[0183] As an example, the solvent may include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl ester carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0184] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, additives that improve battery low-temperature power performance, etc.

[0185] [Isolation membrane]

[0186] In some embodiments, the secondary battery further includes a separator.

[0187] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0188] In some embodiments, the material of the separator may include at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation.

[0189] When the separator is a multilayer composite film, the materials of each layer can be the same or different, without any particular restrictions.

[0190] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0191] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0192] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the individual battery cells can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0193] 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 This is a square-structured battery cell 3, used as an example.

[0194] In some embodiments, refer to Figure 4 , Figure 4 This is an exploded structural diagram of a battery cell 3 provided in some embodiments of this application. The outer casing of the battery cell 3 may include a housing 31 and a cover plate 33. The housing 31 may include a bottom plate and a side plate connected to the bottom plate, the bottom plate and the side plate enclosing a receiving cavity. The housing 31 has an opening communicating with the receiving cavity, and the cover plate 33 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly 32 by a winding process and / or a stacking process. The electrode assembly 32 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 32. The number of electrode assemblies 32 contained in the battery cell 3 may be one or more, which can be adjusted according to actual needs. The negative electrode sheet includes the negative electrode sheet of the above embodiments.

[0195] In some embodiments, 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.

[0196] Reference Figure 5 , Figure 5 This is a schematic diagram of the battery module 4 provided in an embodiment of this application. In the battery module 4, multiple battery cells 3 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple battery cells 3 can be fixed by fasteners.

[0197] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 3 are housed.

[0198] 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.

[0199] Please refer to Figure 6 , Figure 6 This is an exploded view of the battery pack 5 provided in an embodiment of this application. The battery pack 5 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 51 and a lower box 52. The upper box 51 covers the lower box 52 and forms a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in the battery box in any manner.

[0200] Furthermore, a fourth aspect of this application provides an electrical device, including a secondary battery as described in the embodiments of this application. In the embodiments of this application, the secondary battery can be either the power source of the electrical device or the energy storage unit of the electrical device. Therefore, the electrical device described in the embodiments of this application has a long standby or battery life and good safety performance.

[0201] Some embodiments of this application provide an electrical device that uses a battery as a power source. This electrical device can be, but is not limited to, vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools. Vehicles can be, but are not limited to, gasoline-powered vehicles, natural gas-powered vehicles, or new energy vehicles. New energy vehicles can be, but are not limited to, pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles. Spacecraft include airplanes, rockets, space shuttles, and spacecraft. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0202] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0203] Please refer to Figure 7 , Figure 7 This is a schematic diagram of an electrical device provided in an embodiment of this application. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device, a battery pack or battery module can be used.

[0204] Example

[0205] 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.

[0206] High-silicon systems:

[0207] Example 1

[0208] This embodiment provides a negative electrode sheet and a secondary battery.

[0209] Negative electrode plate:

[0210] The first silicon-carbon material and the first graphite, with a mass ratio of 18:82, were used as the negative electrode active materials. The negative electrode active materials, conductive carbon nanotubes, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.5:1.3:1.2:2. Ionized water was added, and the mixture was stirred under vacuum until the system was homogeneous to obtain the first coating slurry.

[0211] The second silicon-carbon material and the second graphite, with a mass ratio of 18:82, were used as the negative electrode active materials. The negative electrode active materials, conductive carbon nanotubes, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.2:1.3:1.2:2.3. Deionized water was added, and the mixture was stirred under vacuum until the system was homogeneous to obtain the second coating slurry.

[0212] Negative current collector: copper foil.

[0213] Method for preparing negative electrode sheet:

[0214] The first coating slurry is coated onto the negative electrode current collector and dried to form the initial first negative electrode film layer.

[0215] The second coating slurry is applied to the surface of the initial first negative electrode film layer. After drying and cold pressing, the first and second negative electrode films are formed on the surface of the negative electrode current collector, resulting in a negative electrode sheet with a compaction density of 1.65 g / cm³. 3 The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:1.

[0216] The parameters of the first silicon carbide material, the second silicon carbide material, the first graphite, and the second graphite are shown in Table 1 below.

[0217] Table 1

[0218]

[0219] In Table 1, the first graphite is a mixture of natural graphite and artificial graphite in a mass ratio of 4:6; the second graphite is artificial graphite with a Dv50 particle size of 11.8 μm and a specific surface area of ​​0.9 m². 2 / g.

[0220] Secondary batteries:

[0221] [Preparation of the negative electrode sheet]

[0222] That is, the negative electrode sheet of Embodiment 1 of this application.

[0223] [Preparation of the positive electrode sheet]

[0224] The positive electrode active material NCM811, conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) were mixed in a mass ratio of 98:1:1. NMP (N-methylpyrrolidone) solvent was added and stirred until the system was homogeneous, yielding a positive electrode slurry (60% solid content). The positive electrode slurry was then subjected to a reaction at 20 mg / cm³. 2 The load is evenly coated on both sides of the positive current collector aluminum foil, dried at room temperature, transferred to an oven for further drying, and then cut into positive electrode sheets with a specification of 40mm×50mm.

[0225] [Isolation membrane]

[0226] Separator membrane: porous polyethylene membrane, size 45mm×55mm.

[0227] Electrolyte

[0228] Ethyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1. LiPF6 was then uniformly dissolved in the mixed solvent to obtain an electrolyte in which the concentration of lithium salt was 1 mol / L.

[0229] [Preparation of Secondary Batteries]

[0230] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. This is then wrapped in an aluminum-plastic film bag to form a stacked dry cell. 0.6g of the electrolyte is injected, and the aluminum-plastic film bag is vacuum-sealed using heat pressing to assemble a pouch battery.

[0231] Examples 2-13

[0232] Examples 2-13 provide a negative electrode sheet and a secondary battery, wherein the main difference between the negative electrode sheet and that of Example 1 is that at least one of the following is different: the particle size, specific surface area, and mass content of the first silicon-carbon material in the first negative electrode film layer; the particle size, specific surface area, and mass content of the second silicon-carbon material in the second negative electrode film layer; the thickness ratio of the first negative electrode film layer to the second negative electrode film layer; and the compaction density of the negative electrode sheet. See Table 2 for details.

[0233] Table 2

[0234]

[0235] In Table 2:

[0236] D1 represents the Dv50 particle size of the first silicon-carbon material, and S1 represents the specific surface area of ​​the first silicon-carbon material.

[0237] D2 represents the Dv50 particle size of the second silicon-carbon material, and S2 represents the specific surface area of ​​the second silicon-carbon material.

[0238] W1 represents: the mass percentage of the first silicon-carbon material based on the total mass of the first silicon-carbon material and the first graphite; and the mass percentage of the second silicon-carbon material based on the total mass of the second silicon-carbon material and the second graphite.

[0239] W2 represents: the mass percentage of the first graphite based on the total mass of the first silicon carbide material and the first graphite; and the mass percentage of the second graphite based on the total mass of the second silicon carbide material and the second graphite.

[0240] For example, in Example 8, the mass ratio of the first silicon carbide material to the first graphite is 20:80, and the mass ratio of the second silicon carbide material to the second graphite is 20:80.

[0241] The thickness ratio represents the ratio of the thickness of the first negative electrode film layer to the thickness of the second negative electrode film layer. For example, in Example 10, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:0.5.

[0242] Comparative Example 1

[0243] This comparative example provides a negative electrode and a secondary battery. The difference from Example 1 is that the second silicon-carbon material has a Dv50 particle size of 9.5 μm and a specific surface area of ​​2.2 m². 2 / g. Everything else is the same. See Table 2 for details.

[0244] Comparative Example 2

[0245] This comparative example provides a negative electrode and a secondary battery. The difference from Example 1 is that the first silicon-carbon material has a Dv50 particle size of 6.5 μm and a specific surface area of ​​4.7 m². 2 / g. Everything else is the same. See Table 2 for details.

[0246] Low-silicon systems:

[0247] Examples 14-26

[0248] Example 14 provides a negative electrode sheet and a secondary battery, wherein the negative electrode sheet differs from that of Example 1 in that the mass ratio of the first silicon-carbon material to the first graphite is 5:95, and the mass ratio of the second silicon-carbon material to the second graphite is 5:95. See Table 3 for details.

[0249] Examples 15-26 provide a negative electrode sheet and a secondary battery, wherein the main difference between the negative electrode sheet and that of Example 14 is that at least one of the following is different: the particle size, specific surface area, and mass content of the first silicon-carbon material in the first negative electrode film layer; the particle size, specific surface area, and mass content of the second silicon-carbon material in the second negative electrode film layer; the thickness ratio of the first negative electrode film layer to the second negative electrode film layer; and the compaction density of the negative electrode sheet. See Table 3 for details.

[0250] Table 3

[0251]

[0252] In Table 3:

[0253] D1 represents the Dv50 particle size of the first silicon-carbon material, and S1 represents the specific surface area of ​​the first silicon-carbon material.

[0254] D2 represents the Dv50 particle size of the second silicon-carbon material, and S2 represents the specific surface area of ​​the second silicon-carbon material.

[0255] W1 represents: the mass percentage of the first silicon-carbon material based on the total mass of the first silicon-carbon material and the first graphite; and the mass percentage of the second silicon-carbon material based on the total mass of the second silicon-carbon material and the second graphite.

[0256] W2 represents: the mass percentage of the first graphite based on the total mass of the first silicon carbide material and the first graphite; and the mass percentage of the second graphite based on the total mass of the second silicon carbide material and the second graphite.

[0257] For example, in Example 14, the mass ratio of the first silicon carbide material to the first graphite is 5:95, and the mass ratio of the second silicon carbide material to the second graphite is 5:95.

[0258] The thickness ratio represents the ratio of the thickness of the first negative electrode film layer to the thickness of the second negative electrode film layer. For example, in Example 23, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:0.5.

[0259] Comparative Example 3

[0260] This comparative example provides a negative electrode and a secondary battery. The difference from Example 14 is that the second silicon-carbon material has a Dv50 particle size of 9.5 μm and a specific surface area of ​​2.2 m². 2 / g. Everything else is the same. See Table 3 for details.

[0261] Comparative Example 4

[0262] This comparative example provides a negative electrode and a secondary battery. The difference from Example 14 is that the first silicon-carbon material has a Dv50 particle size of 6.5 μm and a specific surface area of ​​4.7 m². 2 / g. Everything else is the same. See Table 3 for details.

[0263] Performance testing

[0264] (1) Charging time test

[0265] At 35°C, the batteries of the above embodiments and comparative examples were charged and discharged for the first time at a current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, the batteries were charged at a constant current rate of 1C to the charging cutoff voltage V1, then charged at a constant voltage until the current was ≤0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to the discharge cutoff voltage V2. The actual capacity was recorded as C0. Then, the battery is sequentially charged at constant currents of 2.8C0, 3C0, 3.2C0, 3.5C0, 3.8C0, 4.1C0, 4.4C0, 4.7C0, 5C0, 5.3C0, 5.6C0, and 5.9C0 until it reaches the full battery charging cutoff voltage V1 or the 0V negative terminal cutoff potential (whichever comes first). After each charging cycle, it is discharged at 1C0 until it reaches the full battery discharge cutoff voltage V2. The charging rates at different states of charge (SOC) to 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% are recorded. By plotting the negative electrode potential corresponding to the state of charge (SOC), charging rate-negative electrode potential curves are generated for different SOC states. Linear fitting yields the charging rate corresponding to a negative electrode potential of 0V at each SOC state. This charging rate is the charging window for that SOC state, denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC. The charging time T from 10% SOC to 80% SOC is calculated using the formula (60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%, in minutes. A shorter time indicates better fast-charging performance.

[0266] (2) Expansion test

[0267] After the negative electrode sheet is cold-pressed, the thickness of the sheet is measured at 5-8 points using a micrometer screw gauge. The average value is recorded as A1. The battery is charged to 4.25V, and then charged at a constant voltage until the current is ≤0.05C. After the battery is fully charged, the negative electrode sheet is removed, and 5-8 points are measured after removal. The average value is recorded as A2.

[0268] Full charge expansion of the negative electrode = (A2 - A1) * 100% / A1

[0269] (3) Storage test

[0270] The prepared battery was discharged at a constant current rate of 0.33C to the cutoff voltage of 2.5V, allowed to stand for 30 minutes, and then charged at a constant current rate of 0.33C to the charging cutoff voltage of 4.25V. Afterward, it was charged at a constant voltage until the current ≤0.05C. After standing for 30 minutes, the initial capacity was recorded as C0. The battery was stored in a 60℃ constant temperature chamber for 60 days. After 60 days, the battery was removed and allowed to stand at room temperature for 60 minutes before capacity testing. The specific procedure was as follows: discharged at a constant current rate of 0.33C to the cutoff voltage of 2.5V, allowed to stand for 30 minutes, then charged at a constant current rate of 0.33C to the charging cutoff voltage of 4.25V, and then charged at a constant voltage until the current ≤0.05C. The capacity was recorded as C1, and the 60-day storage capacity retention rate was C1 / C0*100%.

[0271] The performance parameters of the negative electrode sheets and secondary batteries provided in Examples 1-13 and Comparative Examples 1-2 are shown in Table 4.

[0272] Table 4

[0273]

[0274]

[0275] As shown in Table 4, the specific double-layer coating design—with a large-particle-size first silicon-carbon material placed in the first negative electrode film layer and a small-particle-size second silicon-carbon material placed in the second negative electrode film layer—improves the battery's fast-charging performance while reducing the full-charge expansion of the negative electrode sheet, thus increasing the battery's energy density. Specifically, compared to Comparative Example 1, Example 1 shows a 15.6% improvement in fast-charging performance, a 1.6% improvement in full-charge expansion, and only a 1.7% deterioration in 30D high-temperature storage performance; compared to Comparative Example 2, Example 1 shows a 9.1% improvement in fast-charging performance, a 1.2% improvement in 30D high-temperature storage performance, and only a 1.8% deterioration in full-charge expansion. Therefore, it can be seen that in the high-silicon system, the fast-charging performance of the secondary battery provided in this application embodiment is greatly improved, with minimal impact on expansion performance and high-temperature storage performance.

[0276] The performance parameters of the negative electrode sheets and secondary batteries provided in Examples 14-26 and Comparative Examples 3-4 are shown in Table 5.

[0277] Table 5

[0278] Based on cold-press full-fill expansion (%) 30D storage capacity retention rate (%) Fast charging (min) Example 14 25.40％ 96.80％ 16.4min Example 15 25.30％ 96.80％ 16.3min Example 16 25.50％ 96.90％ 16.6min Example 17 25.80％ 97.00％ 16.8min Example 18 24.60％ 96.20％ 15.6min Example 19 25.80％ 96.70％ 16.6min Example 20 26.00％ 97.20％ 16.8min Example 21 33.30％ 96.40％ 15.2min Example 22 22.20％ 98.20％ 18.5min Example 23 25.60％ 97.10％ 16.9min Example 24 24.60％ 96.00％ 15.8min Example 25 24.60％ 96.00％ 16min Example 26 26.20％ 95.50％ 16.8min Comparative Example 3 26.50％ 97.80％ 18min Comparative Example 4 24.20％ 95.80％ 16.6min

[0279] As can be seen from Table 5, compared with Comparative Example 3, the fast charging performance of Example 14 was improved by 9.9%, while the full charge expansion of the negative electrode sheet was improved by 1.1%, and the 30D high-temperature storage performance deteriorated by only 1.4%; compared with Comparative Example 4, the fast charging performance of Example 14 was improved by 5.7%, while the high-temperature storage performance was also improved by 1%, and the full charge expansion of the negative electrode sheet deteriorated by only 1.2%.

[0280] Combining Tables 2 and 3, it can be found that in the low-silicon system, the improvement effect of reducing silicon content on fast charging performance is not as obvious as in the high-silicon system. At the same time, the degree of deterioration of other properties such as expansion and high-temperature storage performance is also reduced.

[0281] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector, a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is disposed on at least one surface of the negative electrode current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer away from the negative electrode current collector. The first negative electrode film layer contains a first silicon-carbon material and a first graphite, and the second negative electrode film layer contains a second silicon-carbon material and a second graphite. The Dv50 particle size of the first silicon-carbon material is larger than the Dv50 particle size of the second silicon-carbon material.

2. The negative electrode sheet as described in claim 1, characterized in that, The first silicon-carbon material has a Dv50 particle size > 8 μm, and the second silicon-carbon material has a Dv50 particle size ≤ 8 μm.

3. The negative electrode sheet as described in claim 1 or 2, characterized in that, The first silicon carbide material has a Dv50 particle size of 9–12 μm; and / or, the second silicon carbide material has a Dv50 particle size of 4–7 μm.

4. The negative electrode sheet as described in any one of claims 1 to 3, characterized in that, The specific surface area of ​​the first silicon-carbon material is <4m². 2 / g.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that, The specific surface area of ​​the second silicon-carbon material is ≥4m². 2 / g.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that, Based on the total mass of the first silicon-carbon material and the first graphite, the mass percentage of the first silicon-carbon material is 1-32%. And / or, based on the total mass of the second silicon-carbon material and the second graphite, the mass percentage of the second silicon-carbon material is 1% to 32%.

7. The negative electrode sheet as described in claim 6, characterized in that, Based on the total mass of the first silicon-carbon material and the first graphite, the mass percentage of the first silicon-carbon material is 1-20%. And / or, based on the total mass of the second silicon-carbon material and the second graphite, the mass percentage of the second silicon-carbon material is 1-20%.

8. The negative electrode sheet as described in claim 6 or 7, characterized in that, Based on the total mass of the first silicon-carbon material and the first graphite, the mass percentage of the first silicon-carbon material is greater than 10% and less than or equal to 20%. And / or, based on the total mass of the second silicon-carbon material and the second graphite, the mass percentage of the second silicon-carbon material is greater than 10% and less than or equal to 20%.

9. The negative electrode sheet according to any one of claims 1 to 8, characterized in that, The second graphite has a Dv50 particle size of 10 μm to 16 μm; and / or, the second graphite has a specific surface area of ​​0.6 to 4.2 m². 2 / g.

10. The negative electrode sheet according to any one of claims 1 to 9, characterized in that, The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:(0.5~2).

11. The negative electrode sheet according to any one of claims 1 to 10, characterized in that, The compaction density of the negative electrode sheet is 1.5–1.8 g / cm³. 3 .

12. The negative electrode sheet according to any one of claims 1 to 11, characterized in that, The first silicon-carbon material is in the form of spherical or near-spherical particles; And / or, the second silicon-carbon material is in the form of spherical or near-spherical particles.

13. The negative electrode sheet according to any one of claims 1 to 12, characterized in that, The first silicon-carbon material includes a first core and a first carbon coating layer covering the surface of the first core. The first core includes a first porous carbon substrate and first silicon particles distributed in the pore structure of the first porous carbon substrate. And / or, the second silicon-carbon material includes a second core and a second carbon coating layer covering the surface of the second core, the second core including a second porous carbon substrate and second silicon particles distributed in the pore structure of the second porous carbon substrate.

14. A secondary battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 13.

15. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 14.

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