Secondary battery and electric device

Abstract

This application provides a secondary battery and an electrical device. The secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes a silicon-carbon composite and a graphite material A. The mass percentage of silicon in the negative electrode film layer is 2.5% or more. The equivalent sphericity of the silicon-carbon composite is 0.8 or more, and the equivalent sphericity of the graphite material A is 0.7 or more. The secondary battery of this application has excellent fast-charging performance and safety performance.

Description

Technical Field

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

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

[0003] Therefore, improving the fast-charging performance and safety performance of secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electrical device, wherein the secondary battery of this application has excellent fast charging performance and safety performance.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one side surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, which comprises a silicon-carbon composite and a graphite material A. The mass percentage of silicon in the negative electrode film layer is 2.5% or more. The equivalent sphericity of the silicon-carbon composite is 0.8 or more, and the equivalent sphericity of the graphite material A is 0.7 or more. Therefore, the secondary battery of this application can simultaneously achieve fast charging performance and safety performance.

[0006] In some embodiments, the equivalent sphericity of the silicon-carbon composite is 0.8 to 1.0. When the equivalent sphericity of the silicon-carbon composite is within the above range, the material has a high degree of isotropy, which can effectively reduce expansion force and enhance the safety performance of the battery.

[0007] In some embodiments, the equivalent sphericity of graphite material A is 0.7 to 0.9. This is beneficial for improving fast charging capability and increasing battery safety and lifespan.

[0008] In some implementations, the silicon content in the negative electrode film is 2.5% to 65% by mass. This is beneficial for improving the fast-charging performance of the battery.

[0009] In some embodiments, the silicon-carbon composite comprises porous carbon and a silicon-containing material dispersed in the pores of the porous carbon. In some embodiments, the porous carbon comprises hard carbon, and / or the silicon-containing material comprises crystalline silicon. The aforementioned silicon-carbon composite exhibits high energy density and high initial coulombic efficiency, which is beneficial for improving cycling and rate capabilities.

[0010] In some embodiments, the silicon-carbon composite satisfies at least one of the following characteristics:

[0011] (1) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or the silicon-containing material;

[0012] (2) The mass percentage of silicon in the silicon-carbon composite is 30% to 70%;

[0013] (3) The average particle size of the silicon-carbon composite is 2 μm to 15 μm;

[0014] (4) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0015] (5) The BET specific surface area of ​​the silicon-carbon composite is 1 m². 2 / g~6.7m 2 / g.

[0016] This helps improve the battery's fast charging performance and safety performance.

[0017] In some embodiments, the graphite material A satisfies at least one of the following characteristics:

[0018] (1) The average particle size of the graphite material A is 3 μm to 20 μm;

[0019] (2) The BET specific surface area of ​​the graphite material A is 0.6 m². 2 / g~3.4m 2 / g;

[0020] (3) The graphitization degree of the graphite material A is 91% to 93%.

[0021] This is beneficial for improving the battery's energy density, dynamics, and fast-charging performance.

[0022] In some embodiments, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer being located between the negative electrode current collector and the second negative electrode film layer, the first negative electrode film layer including a first negative electrode active material, the first negative electrode active material including graphite material B, the second negative electrode film layer including a second negative electrode active material, the second negative electrode active material comprising the aforementioned silicon-carbon composite and the aforementioned graphite material A.

[0023] By adopting a layered electrode arrangement, the safety performance and lifespan of the battery can be further increased, which is beneficial to improving the fast charging dynamic performance.

[0024] In some embodiments, the equivalent sphericity of graphite material B is lower than that of graphite material A, and the equivalent sphericity of graphite material B is lower than that of the aforementioned silicon-carbon composite. By making the sphericity of the lower layer graphite material lower than that of the upper layer graphite material and the silicon-carbon composite graphite material, the lower layer graphite material has a higher compaction density and specific capacity, which can provide a higher energy density. When used in combination with the upper layer material, it is beneficial to integrate the compaction and fast charging capabilities of the first negative electrode film.

[0025] In some embodiments, the equivalent sphericity of graphite material B is 0.5 to 0.7. When the equivalent sphericity of graphite material B is within this range, the resulting first negative electrode film layer exhibits better kinetic and compaction properties, thereby improving the battery's energy density and fast-charging capability.

[0026] In some embodiments, the graphite material B satisfies at least one of the following characteristics:

[0027] (1) The average particle size of the graphite material B is 6 μm to 25 μm;

[0028] (2) The BET specific surface area of ​​the graphite material B is 1.5 m². 2 / g~3.0m 2 / g;

[0029] (3) The graphitization degree of the graphite material B is 90% to 96%.

[0030] This is beneficial for improving the battery's dynamic performance, energy density, and cycle stability.

[0031] In some embodiments, graphite material B includes at least one of artificial graphite, natural graphite, or mixtures thereof. In some embodiments, graphite material B includes artificial graphite B1 and artificial graphite B2 with different degrees of graphitization, wherein the equivalent sphericity of artificial graphite B1 is 0.5 to 0.7, and the equivalent sphericity of artificial graphite B2 is 0.5 to 0.7. By including graphite material with a lower equivalent sphericity in the first negative electrode film layer, higher compaction density and specific capacity are achieved, providing higher energy density. This allows for the simultaneous use of both the kinetic and compaction performance of the first negative electrode film layer.

[0032] In some embodiments, the mass ratio of artificial graphite B1 to artificial graphite B2 is 2:8 to 8:2. By adjusting the mass ratio of artificial graphite B1 and artificial graphite B2 with different degrees of graphitization, the first negative electrode film layer has better kinetic and compaction properties, which is beneficial to improving the energy density and fast charging capability of the battery.

[0033] In some embodiments, the mass percentage of silicon in the second negative electrode film is 2.5% to 65%. In some embodiments, the mass percentage of silicon in the second negative electrode film is 12% to 48%. A mass percentage of silicon in the second negative electrode film within the above range is beneficial for utilizing the high energy density of silicon, and silicon preferentially intercalates lithium compared to graphite, which can improve kinetic performance in the upper layer.

[0034] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:9 to 9:1.

[0035] In some embodiments, the thickness of the first negative electrode film is 10 μm to 70 μm. A thickness within this range can prevent current collector wrinkling during cycling, thus improving lifespan and safety performance.

[0036] In some embodiments, the thickness of the second negative electrode film is 6 μm to 40 μm. A thickness within this range is beneficial for the effective transport of lithium ions in the silicon-containing film, exhibiting good kinetic performance. It also helps maintain a perfect electrode structure after the film expands and rebounds, preventing abnormal problems such as powder shedding and cracking.

[0037] In some embodiments, the first negative electrode film layer further includes a first binder, wherein the mass percentage of the first binder in the first negative electrode film layer is 0.5% to 5.0%. By keeping the mass percentage of the first binder in the first negative electrode film layer within the above range, it is beneficial to achieve mutual adhesion between the active materials and between the active materials and the current collector, thereby forming a stable electrode structure.

[0038] In some embodiments, the second negative electrode film layer further includes a second binder, wherein the mass percentage of the second binder in the second negative electrode film layer is 1.0% to 5.0%. Thus, the addition of the binder enables the silicon-carbon composites in the second negative electrode film layer to interconnect, ensuring that the silicon-carbon composites can be tightly bound within the negative electrode film layer during battery charging and discharging.

[0039] In some embodiments, the first adhesive and the second adhesive are independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid and carboxymethyl chitosan.

[0040] In some embodiments, the first negative electrode film layer further includes a first conductive agent, wherein the mass percentage of the conductive agent in the first negative electrode film layer is 0.05% to 1.0%. In some embodiments, the second negative electrode film layer further includes a second conductive agent, wherein the mass percentage of the second conductive agent in the second negative electrode film layer is 0.05% to 2.0%. By adding the above-mentioned conductive agents, a more stable conductive network can be formed, ensuring that the silicon-carbon composite can be stably maintained in the conductive network during charging and discharging.

[0041] In some embodiments, the first conductive agent and the second conductive agent are independently selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0042] In some embodiments, the second conductive agent comprises carbon black and carbon nanotubes, wherein the mass ratio of carbon black to carbon nanotubes is 6:4 to 9:1.

[0043] In some embodiments, the volumetric particle size Dv50 of the first negative electrode active material is 6 μm to 25 μm, and / or the BET specific surface area of ​​the first negative electrode active material is 1.5 m². 2 / g~3.0m 2 / g. This is beneficial to the energy density and kinetic performance of the electrode, as well as the structural stability of the electrode.

[0044] In some embodiments, the volumetric particle size Dv50 of the second negative electrode active material is 2 μm to 19.5 μm, and / or the BET specific surface area of ​​the second negative electrode active material is 0.64 m². 2 / g~6.7m 2 / g. This is beneficial to the energy density and kinetic performance of the electrode, as well as the structural stability of the electrode.

[0045] In some embodiments, a buffer layer is further included between the negative electrode current collector and the first negative electrode film layer. The buffer layer includes a third binder and a third conductive agent. This improves the conductivity of the current collector and the film layer, as well as the adhesion between them, thereby enhancing the fast-charging performance and safety performance of the secondary battery.

[0046] In some embodiments, the third binder comprises 70% to 90% by mass and the third conductive agent comprises 10% to 30% by mass in the buffer layer. A mass percentage of the third binder within these ranges is beneficial for enhancing the adhesion between the first negative electrode film and the negative electrode current collector. A mass percentage of the third conductive agent within these ranges is beneficial for forming a stable conductive network between the first negative electrode film and the negative electrode current collector.

[0047] In some embodiments, the thickness of the buffer layer is 0.2 μm to 2 μm. A buffer layer thickness within this range helps ensure good adhesion between the negative electrode film and the current collector, while having a relatively small impact on the energy density of the individual battery cells.

[0048] A second aspect of this application provides an electrical device that includes the secondary battery provided in the first aspect. Therefore, the electrical device of this application possesses at least the advantages of the secondary battery of this application. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of the negative electrode sheet according to one embodiment of this application.

[0050] Figure 2 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0051] Figure 3 yes Figure 2 An exploded view of a battery cell according to one embodiment of this application is shown.

[0052] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0053] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0054] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0055] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0056] Explanation of reference numerals in the attached figures:

[0057] 10 Negative electrode; 101 Negative electrode current collector; 102 First negative electrode film layer; 103 Second negative electrode film layer; 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0058] Hereinafter, embodiments of the secondary battery and power-consuming device of this application will be described in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0059] In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0060] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0061] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0062] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0063] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0064] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0065] Currently, silicon-based materials are widely used in the negative electrode of rechargeable batteries to achieve higher energy density. However, silicon-based materials are hard, and the cold pressing process can easily damage the current collector upon contact with it. Furthermore, the volume expansion of silicon-based materials during lithium intercalation severely affects battery safety. Additionally, the significant volume expansion of silicon-based materials causes orientation adjustments and disconnection between active material particles within the electrode film, leading to a decrease in kinetic performance and negatively impacting the battery's fast-charging performance. As the demand for faster charging and safer rechargeable batteries increases, improvements are needed to address these issues and enhance both fast-charging and safety performance.

[0066] Existing technologies have reportedly proposed methods to improve battery fast-charging safety by adjusting the binding effect of binders on active material particles, increasing the amount of binder added, and strengthening the surface coating of silicon-based materials to constrain the volume expansion of silicon-based materials. However, binders and coatings continuously expand and contract under long-term cycling, especially high-rate fast-charging cycles, eventually leading to excessive expansion forces, accelerated cell failure, and even safety risks such as bulging and valve opening. This makes it impossible to balance fast-charging performance and safety.

[0067] Based on this, this application provides a secondary battery and an electrical device, wherein the secondary battery of this application has excellent fast charging performance and safety performance.

[0068] Secondary batteries

[0069] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one side surface of the negative current collector, the negative electrode film layer including a negative electrode active material, the negative electrode active material including a silicon-carbon composite and a graphite material A, the mass percentage of silicon element in the negative electrode film layer being 2.5% or more, the equivalent sphericity of the silicon-carbon composite being 0.8 or more, and the equivalent sphericity of the graphite material A being 0.7 or more.

[0070] The inventors discovered that the expansion of silicon-carbon during charging leads to a rearrangement of the active material particles. During the charging expansion process, silicon-carbon undergoes a significant volume change, while graphite experiences a smaller volume change. Due to the difference in their expansion characteristics and the irregular particle morphology, the rearrangement caused by the mutual compression between particles is anisotropic, resulting in large-angle flipping of the active material particles. This leads to electrode rebound and a significant increase in expansion force. In this application, a highly rounded silicon-carbon composite is combined with highly rounded graphite. The high-rounded silicon-carbon composite exerts a more uniform force on surrounding active material particles during expansion, with some stresses canceling each other out. The highly rounded graphite avoids the large-scale angular flipping caused by the expansion compression of silicon-carbon, greatly mitigating the rearrangement caused by the interaction between particles during silicon-carbon composite expansion. This reduces electrode rebound during charging and the expansion force during cell charging, enhancing battery safety. Furthermore, highly rounded materials have more active sites, providing more ion transport channels, which helps improve the ion transport rate and thus enhances the fast-charging capability of the negative electrode film. In addition, the mass ratio of silicon element in the negative electrode film layer in this application is more than 2.5%. The lithium intercalation potential of silicon material is higher than that of graphite, and it intercalates lithium earlier than graphite, resulting in better kinetic performance. Therefore, introducing and adjusting the content of silicon element can improve the fast charging performance of the battery.

[0071] In this application, equivalent roundness represents the degree of roundness of the particles, with a larger value indicating higher roundness. The maximum value is 1.0.

[0072] In this application, the equivalent circularity can be tested using equipment and methods known in the art. For example, it can be measured using a cross-section polisher and a scanning electron microscope. As an example, it can be tested as follows: ① Disassemble the secondary battery to obtain the negative electrode sheet, and peel the negative electrode film layer from the negative electrode current collector. Fix the peeled negative electrode film layer onto the sample stage, install the sample stage into the sample holder and lock it in place, turn on the power of the argon ion cross-section polisher (e.g., IB-09010CP type argon ion cross-section polisher) and perform vacuuming (e.g., 10...-7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to swing mode to begin polishing. ② After polishing, observe the cross-section using a scanning electron microscope (e.g., ZEISS Sigma 300). Randomly select a test sample with a length × width of 50 mm × 100 mm. Randomly select a test area within the test sample and, at a certain magnification (e.g., 1000x), read the area and perimeter of a certain number of n material particles in the test area. Calculate the equivalent roundness using the following formula: X1 = 4πS1 / C1 2 Where S1 and C1 are the area and perimeter of the material, respectively, and the equivalent circularity of the material X = (X1 + X2 + ... + X...) n To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0073] In this application, the equivalent sphericity of the silicon-carbon composite is 0.8 or higher. Exemplarily, the equivalent sphericity of the silicon-carbon material is a value within the range of 0.8, 0.85, 0.90, 0.95, 0.99, 1.0, or any combination thereof. In some embodiments, the equivalent sphericity of the silicon-carbon composite is 0.8 to 1.0, preferably 0.9 to 1.0. When the equivalent sphericity of the silicon-carbon composite is within the above range, the material exhibits high isotropy, resulting in a uniform effect on surrounding particles during expansion, partial stress cancellation, and minimal orientation adjustment during self-flipping. This effectively reduces expansion force and enhances battery safety performance.

[0074] In this application, the equivalent roundness of graphite material A is 0.7 or higher. Exemplarily, the equivalent roundness of graphite material A is a value within the range of 0.7, 0.72, 0.74, 0.76, 0.78, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.95, 1.0, or any combination thereof. In some embodiments, the equivalent roundness of graphite material A is 0.7 to 0.9. Graphite materials with high equivalent roundness have higher isotropy, exposing more of the graphite lithium-intercalation end facets, which is beneficial for improving fast charging capabilities; furthermore, graphite is relatively soft and less prone to damage to the current collector under high compaction. In addition, graphite materials with high equivalent sphericity have a low degree of orientation adjustment due to the flipping caused by the interparticle forces during orientation adjustment. This can alleviate the orientation rearrangement caused by the interaction between particles during material expansion, especially reducing the degree of angular flipping, thereby reducing the rebound of electrode charging and the expansion force during cell charging, and increasing the safety performance and life of the battery.

[0075] In this application, the mass percentage of silicon in the negative electrode film is 2.5% or more, preferably, the mass percentage of silicon in the negative electrode film is 2.5% to 65%. Exemplarily, the mass percentage of silicon in the negative electrode film is a value within the range of 2.5%, 3%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 65%, or any combination thereof. More preferably, the mass percentage of silicon in the negative electrode film is 5.5% to 48%.

[0076] The silicon-carbon composite of this application can be prepared using conventional silicon-carbon composites or conventional preparation methods, such as depositing nano-silicon materials on porous carbon by chemical vapor deposition, and further carbon coating, such as using amorphous carbon coating.

[0077] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0078] 1) The silicon-carbon composite includes a core, which includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon, wherein the porous carbon is optionally hard carbon;

[0079] 2) The silicon-carbon composite also includes a carbon-containing coating layer, which coats the surface of the core;

[0080] 3) The silicon content in the silicon-carbon composite is 30%–70% by mass;

[0081] 4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm, and optionally 7 μm to 11 μm;

[0082] 5) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0083] 6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

[0084] In some embodiments, the silicon-carbon composite includes a core comprising porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support for the nanoscale silicon-containing material and, simultaneously, provides expansion space for the expansion of the silicon nanoparticles, effectively mitigating the stress compression caused by expansion during charging. Especially when the silicon-containing particles are in the nanometer range, the specific capacity is higher and dispersion in the pores of the porous carbon is facilitated. Furthermore, the buffering effect of the porous carbon's pores on expansion can be more fully utilized. When this silicon-carbon composite is applied in a wound electrode assembly, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0085] In some embodiments, the porous carbon may optionally be hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the battery cell.

[0086] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the battery cell.

[0087] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer that coats the surface of the core. This can improve the conductivity of the silicon-carbon composite and reduce the internal impedance of the battery cell, while also effectively reducing the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0088] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass, for example, it can be a value within the range of 30%, 40%, 50%, 60%, 70%, or any combination thereof. This approach, while maximizing the specific capacity of the negative electrode active material using silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0089] In this application, the method for testing the silicon content in the silicon-carbon composite can be a method known in the art. As an example, the following method can be used for testing: a certain amount of silicon-carbon composite is taken, and the mass of silicon element in the silicon-carbon composite is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon element in the silicon-carbon composite can be calculated.

[0090] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in the silicon-carbon composite also form between the particles. To further improve the flow of lithium ions through the intraparticle and interparticle pores, in some embodiments, the average particle size of the silicon-carbon composite is 2 μm to 15 μm. For example, it can be a value within the range of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any combination thereof. Optionally, the average particle size of the silicon-carbon composite is 7 μm to 11 μm, or 5 μm to 10 μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite and the average particle size of the graphite material, thus facilitating the compaction of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the battery cell.

[0091] The average particle size of the aforementioned silicon-carbon composite can be tested using equipment and methods known in the art. For example, a scanning electron microscope (SEM) (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain SEM images of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0092] In some embodiments, the powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm, exemplary values ​​within the range of 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, or any combination thereof. By controlling the powder resistivity as described above, the conductivity of the silicon-carbon composite is improved, thereby increasing the charging rate of the battery cell.

[0093] In this application, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0094] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g. For example, the specific surface area of ​​the silicon-carbon composite is 1m². 2 / g, 1.1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.2m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g, 6.7m 2 / g or a range between the values ​​of either / g or any two of them.

[0095] In this application, the method for testing the BET specific surface area of ​​the silicon-carbon composite can be a method known in the art. As an example, referring to GB / T 19587-2017, a nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196℃, and the amount of nitrogen adsorbed on the surface of the solid sample at different pressures of 0.05 to 0.30 is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is obtained. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0096] In some embodiments, the average particle size of graphite material A is 3 μm to 20 μm. Exemplarily, the average particle size of graphite material A is a value within the range of 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or any combination thereof. In some embodiments, the average particle size of graphite material A is 4 μm to 18 μm. By keeping the average particle size of graphite material A within the above range, it is beneficial for the graphite material to exert its energy density, high pressure density, and kinetic properties. A suitable particle size facilitates the dispersion and coating processing of the material in the slurry.

[0097] In some embodiments, the specific surface area of ​​graphite material A is 0.6 m². 2 / g~3.4m 2 / g, for example, the specific surface area of ​​graphite material A is 0.6m². 2 / g, 1.0m 2 / g, 1.5m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g, 3.4m 2 / g or a value between any two of these ranges. In some embodiments, the specific surface area of ​​graphite material A is 0.7m². 2 / g~3.0m 2 / g. By ensuring that the specific surface area of ​​graphite material A is within the above range, it is beneficial to improve the initial coulombic efficiency, kinetic properties, and the stability of slurry processing performance.

[0098] In some embodiments, the graphitization degree of graphite material A is 91% to 93%. Exemplarily, the graphitization degree of graphite material A is a value within a range of 91.0%, 91.5%, 92.0%, 92.5%, 93.0%, or any combination thereof. By ensuring the graphitization degree of graphite material A is within the aforementioned range, it is beneficial to improve energy density and fast-charging kinetic performance.

[0099] In this application, the average particle size can be tested using equipment and methods known in the art. For example, it can be determined using a cross-sectional polisher and a scanning electron microscope. As an example, the test can be performed as follows: ① Disassemble the secondary battery to obtain the negative electrode sheet, and peel the negative electrode film layer from the negative electrode current collector. Fix the peeled negative electrode film layer on the sample stage, install the sample stage into the sample holder and lock it in place, turn on the power of the argon ion cross-sectional polisher (e.g., IB-09010CP type argon ion cross-sectional polisher) and perform vacuuming (e.g., 10-7 Pa), set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to the swing mode to start polishing. ② After polishing, observe the cross-section using a scanning electron microscope (e.g., ZEISS Sigma 300). Randomly select a test sample with dimensions of 50mm x 100mm. Within the sample, randomly select multiple test areas (e.g., 5 areas). At a certain magnification (e.g., 1000x), read the particle size of each material particle in each test area (i.e., take the distance between the two farthest points on the material particle as the particle size). Count the number and particle size values ​​in each test area, and take the arithmetic mean of the particle size in each test area. This is the average particle size of the material in the test sample. To ensure the accuracy of the test results, repeat the above test on multiple test samples (e.g., 10 samples), and take the average of all test samples as the final test result.

[0100] In this application, the specific surface area of ​​the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0101] In this application, the degree of graphitization of the material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be tested using an X-ray diffractometer (such as a Bruker D8 Discover) according to JIS K 0131-1996 and JB / T 4220-2011. The test can refer to the measurement of d... 002 The size is then determined according to the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002 This refers to the interlayer spacing in the graphite crystal structure, measured in nm. In X-ray diffraction analysis, Cu Kα rays were used as the radiation source, with the ray wavelength scanning 2θ angle range of 20° to 80° and the scanning rate of 4° / min.

[0102] In some implementations, such as Figure 1 As shown, the negative electrode 10 includes a negative current collector 101 and a negative electrode film layer located on at least one side surface of the negative current collector. The negative electrode film layer includes a first negative electrode film layer 102 and a second negative electrode film layer 103. The first negative electrode film layer 102 is located between the negative current collector 101 and the second negative electrode film layer 103. The first negative electrode film layer 102 includes a first negative electrode active material, which includes graphite material B. The second negative electrode film layer 103 includes a second negative electrode active material, which includes the aforementioned silicon-carbon composite and graphite material A.

[0103] By employing a layered electrode arrangement, placing the silicon material in the second negative electrode film layer, away from the current collector, damage to the current collector caused by the silicon material during cold pressing can be avoided, further increasing the battery's safety and lifespan. The silicon-carbon composite material is relatively hard and distributed in the second negative electrode film layer away from the current collector. During electrode cold pressing, it will not come into contact with the current collector, thus avoiding the safety risk of subsequent current collector cracking caused by current collector damage. The silicon-carbon composite has significant volume expansion; its distribution in the second negative electrode film layer away from the current collector prevents the graphite material in the first negative electrode film layer from being affected during electrode expansion during charging. This effectively avoids abnormal phenomena such as active material demolding and electrode wrinkling caused by silicon-carbon expansion on the current collector surface film layer, improving battery safety. When used as a negative electrode material, the silicon-carbon composite has a much higher energy density than graphite, and its lithium intercalation potential is higher than that of graphite, preferentially intercalating lithium into the electrode. Distributing silicon-carbon in the second negative electrode film layer, close to the positive electrode (the source of lithium ions), is beneficial for improving fast-charging dynamics.

[0104] In some embodiments, the equivalent sphericity of graphite material B is lower than that of graphite material A, and the equivalent sphericity of graphite material B is lower than that of the silicon-carbon composite. In some embodiments, the equivalent sphericity of graphite material B is 0.5 to 0.7. Exemplarily, the equivalent sphericity of graphite material B is a value within the range of 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, or any combination thereof. Graphite material A with high equivalent sphericity has high isotropy, exposes more lithium-intercalating facets, and has better kinetic performance; graphite material B with low equivalent sphericity has higher compaction density and specific capacity, and can provide higher energy density. By placing the material with high equivalent sphericity on the upper layer and the material with low equivalent sphericity on the lower layer, compaction and fast charging capability can be better balanced.

[0105] In some embodiments, the average particle size of graphite material B is 6 μm to 25 μm. Exemplarily, the average particle size of graphite material B is a value within the range of 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, or any combination thereof. In some embodiments, the average particle size of graphite material B is 10 μm to 19 μm. By keeping the average particle size of graphite material B within the above range, it is beneficial to improve energy density and kinetic performance. A suitable particle size facilitates the dispersion and coating processing of the material in the slurry.

[0106] In some embodiments, the BET specific surface area of ​​graphite material B is 1.5 m². 2 / g~3.0m 2 / g, for example, the specific surface area of ​​graphite material B is 1.5m². 2 / g, 1.6m 2 / g, 1.7m 2 / g, 1.8m 2 / g, 1.9m 2 / g, 2.0m 2 / g、2.1m 2 / g, 2.2m 2 / g, 2.3m 2 / g, 2.4m 2 / g, 2.5m 2 / g, 3.0m 2 / g or a value within a range of any two of them. In some embodiments, the specific surface area of ​​graphite material B is 1.6m². 2 / g~2.3m 2 / g. By ensuring that the specific surface area of ​​graphite material B is within the above range, it is beneficial to improve the initial coulombic efficiency, dynamic performance, and stability of slurry processing performance.

[0107] In some embodiments, the graphite degree of graphite material B is 90% to 96%. Exemplarily, the graphite degree of graphite material B is a value within a range of 90%, 91%, 92%, 93%, 94%, 95%, 96%, or any combination thereof. In some embodiments, the graphite degree of graphite material B is 91% to 95%. In some embodiments, the graphite degree of graphite material B is 92% to 94.5%. By keeping the graphite degree of graphite material B within the above ranges, it is beneficial to improve energy density and cycle stability.

[0108] In some embodiments, graphite material B includes at least one of artificial graphite, natural graphite, or mixtures thereof. In some embodiments, graphite material B includes artificial graphite B1 and artificial graphite B2 with different degrees of graphitization. The equivalent roundness of artificial graphite B1 is 0.5 to 0.7, and exemplaryly, the equivalent roundness of artificial graphite B1 is a value within the range of 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, or any combination thereof. The equivalent roundness of artificial graphite B2 is 0.5 to 0.7, and exemplaryly, the equivalent roundness of artificial graphite B2 is a value within the range of 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.7, or any combination thereof. Graphite material B with lower equivalent sphericity has higher compaction density and specific capacity. By setting the above-mentioned graphite material B in the first negative electrode film layer, higher energy density can be provided.

[0109] In some embodiments, the mass ratio of artificial graphite B1 to artificial graphite B2 is 2:8 to 8:2. Exemplarily, the mass ratio of artificial graphite B1 and artificial graphite B2 with different degrees of graphitization is a value within a range of 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, or any combination thereof. By adjusting the mass ratio of artificial graphite B1 to artificial graphite B2, the first negative electrode film layer exhibits better kinetic and compaction properties, thereby improving the battery's energy density and fast-charging capability.

[0110] In some embodiments, the degree of graphitization of artificial graphite B1 is 90%–93%, and the degree of graphitization of artificial graphite B2 is 91%–96%. By using artificial graphite B1 and artificial graphite B2 with different degrees of graphitization, the first negative electrode film layer has better kinetic and compaction properties, which is beneficial to improving the energy density and fast charging capability of the battery.

[0111] In some embodiments, the mass percentage of silicon in the second negative electrode film is 2.5% to 65%. Exemplarily, the mass percentage of silicon in the second negative electrode film is a value within a range of 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 12%, 16%, 20%, 24%, 28%, 32%, 36%, 40%, 44%, 48%, 52%, 56%, 60%, 64%, 65%, or any combination thereof. In some embodiments, the mass percentage of silicon in the second negative electrode film is 12% to 48%. The mass percentage of silicon in the second negative electrode film is within the above range, which is conducive to leveraging the high energy density of silicon. Furthermore, silicon preferentially intercalates lithium compared to graphite, which can improve the kinetic performance in the upper layer. The silicon content within this range, combined with the highly rounded silicon-carbon and graphite structure in the upper layer, can reduce the electrode rebound and cell expansion force caused by silicon-carbon expansion, thereby improving the safety of the battery.

[0112] In this application, the mass percentage of silicon in the second negative electrode film can be determined by scanning electron microscopy (SEM-EDX). As an example, the test can be performed as follows: The cross-section of the negative electrode film is cut and polished using an argon ion cross-section polisher (e.g., IB-09010CP type argon ion cross-section polisher). The polished cross-section sample is then observed using a scanning electron microscope (e.g., ZEISS Sigma 300) to distinguish between the first and second negative electrode films. Electron energy dispersive spectroscopy (EDX) is used to perform a surface scan test on the mass percentage of silicon in the second negative electrode film region. To ensure the accuracy of the measurement results, 5 to 10 regions of the second negative electrode film can be randomly selected for silicon mass percentage testing, and the average value is calculated as the mass percentage of silicon in the second negative electrode film.

[0113] In some embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:9 to 9:1. Exemplarily, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is a value between 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, or any combination thereof.

[0114] In some embodiments, the thickness of the first negative electrode film is 10 μm to 70 μm. A thickness within this range is advantageous for covering the current collector surface with a graphite negative electrode film of a certain thickness, effectively preventing current collector damage during cold pressing. Furthermore, the lower graphite layer exhibits smaller volume changes during charging and discharging, resulting in less force on the current collector and preventing wrinkling during cycling, thus improving lifespan and safety performance. Exemplarily, the thickness of the first negative electrode film is a value within the range of 10 μm, 12 μm, 15 μm, 20 μm, 24 μm, 30 μm, 40 μm, 50 μm, 60 μm, 65 μm, 68 μm, 70 μm, or any combination thereof. In some embodiments, the thickness of the first negative electrode film is 12 μm to 68 μm. In some embodiments, the thickness of the first negative electrode film is 15 μm to 65 μm.

[0115] In some embodiments, the thickness of the second negative electrode film is 6 μm to 40 μm. A thickness within this range is beneficial for the effective transport of lithium ions in the silicon-containing film, exhibiting good kinetic performance, and also helps maintain a perfect electrode structure after film expansion and rebound, preventing abnormal problems such as powder shedding and cracking. Exemplarily, the thickness of the second negative electrode film is a value within the range of 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, 32 μm, 34 μm, 36 μm, 38 μm, 40 μm, or any combination thereof. In some embodiments, the thickness of the second negative electrode film is 6 μm to 38 μm. In some embodiments, the thickness of the second negative electrode film is 6 μm to 35 μm.

[0116] In this application, the thicknesses of the first negative electrode film layer, the buffer layer, and the second negative electrode film layer can be measured in the following manner: the negative electrode film layer is peeled off from the negative electrode current collector, the peeled-off negative electrode film layer is fixed on the sample stage, the sample stage is installed into the sample holder and locked in place, the power of the argon ion cross-section polisher (e.g., the IB-09010CP argon ion cross-section polisher from JEOL Corporation of Japan) is turned on and a vacuum is drawn (e.g., 10...). -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, use a scanning electron microscope to measure the thickness of the cross-sections of the first negative electrode film, the buffer layer, and the second negative electrode film.

[0117] In some embodiments, the first negative electrode film layer further includes a first binder, the mass percentage of which is 0.5% to 5.0%. For example, the mass percentage of the first binder in the first negative electrode film layer is a value within the range of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or any combination thereof. By keeping the mass percentage of the first binder in the first negative electrode film layer within the above range, it is beneficial to facilitate the adhesion between the active materials and between the active material and the current collector, forming a stable electrode structure. In some embodiments, the mass percentage of the first binder in the first negative electrode film layer is 1.5% to 3.5%.

[0118] In some embodiments, the second negative electrode film layer further includes a second binder, the mass percentage of which is 1.0% to 5.0%. Since the second negative electrode film layer includes a silicon-carbon composite, the addition of the binder enables the silicon-carbon composite to interconnect, ensuring that during battery charging and discharging, the silicon-carbon composite is tightly bound within the negative electrode film layer and does not lose its electrical connection to form electrochemical islands, thus preventing the silicon-carbon composite from losing its electrochemical activity. The mass percentage of the second binder in the second negative electrode film layer is a value within the range of 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or any combination thereof. In some embodiments, the mass percentage of the second binder in the second negative electrode film layer is 1.5% to 4.5%.

[0119] The first and second adhesives mentioned above are independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid and carboxymethyl chitosan.

[0120] In some embodiments, the first negative electrode film layer further includes a first conductive agent, the mass percentage of which is 0.05% to 1.0%. In some embodiments, the second negative electrode film layer further includes a second conductive agent, the mass percentage of which is 0.05% to 2.0%. Since the silicon-carbon composite undergoes volume expansion during battery charging and discharging, adding a conductive agent can form a more stable conductive network, ensuring that the silicon-carbon composite remains stably within the conductive network during charging and discharging, and preventing material loss due to loss of electrical connection between silicon particles and graphite particles during significant contraction or expansion. Exemplarily, the mass percentage of the first conductive agent in the first negative electrode film layer is a value within the range of 0.05%, 0.5%, 1.0%, or any combination thereof, and the mass percentage of the second conductive agent in the second negative electrode film layer is a value within the range of 0.05%, 0.5%, 1.0%, 1.5%, 2.0%, or any combination thereof.

[0121] The first and second conductive agents are independently selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the second conductive agent comprises carbon black and carbon nanotubes, with a carbon black to carbon nanotube mass ratio of 6:4 to 9:1. Exemplarily, the carbon black to carbon nanotube mass ratio is a value within a range of 6:4, 7:3, 8:2, 8.5:1.5, 9:1, or any combination thereof. In some embodiments, the carbon black to carbon nanotube mass ratio is 8:2 to 8.5:1.5.

[0122] In some embodiments, the volume distribution particle size Dv50 of the first negative electrode active material is 6 μm to 25 μm. Exemplarily, the volume distribution particle size Dv50 of the first negative electrode active material is a value within the range of 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, or any combination thereof. In some embodiments, the specific surface area of ​​the first negative electrode active material is 1.5 m². 2 / g~3.0m 2 / g, for example, the specific surface area of ​​the first negative electrode active material is 1.5m². 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g or a value within a range of any two of these. By ensuring that the volume distribution particle size Dv50 and specific surface area of ​​the first negative electrode active material are within the above range, it is beneficial to improve the energy density and kinetic performance of the battery, and to improve the structural stability of the electrode.

[0123] In some embodiments, the volume distribution particle size Dv50 of the first and second negative electrode active materials is 2 μm to 19.5 μm. Exemplarily, the volume distribution particle size Dv50 of the second negative electrode active material is a value within the range of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 19.5 μm, or any combination thereof. In some embodiments, the specific surface area of ​​the second negative electrode active material is 0.64 m². 2 / g~6.7m 2 / g, for example, the specific surface area of ​​the second negative electrode active material is 0.64m². 2 / g, 1.0m 2 / g, 2.0m2 / g, 3.0m 2 / g, 4.0m 2 / g, 5.0m 2 / g, 6.0m 2 / g, 6.7m 2 / g or a value within a range of any two of these. By ensuring that the volume distribution particle size Dv50 and specific surface area of ​​the second negative electrode active material are within the above range, it is beneficial to improve the energy density and kinetic performance of the battery, and to enhance the structural stability of the electrode.

[0124] In this application, the volume distribution particle size Dv50 of the first and second negative electrode active materials has a well-known meaning in the art and can be determined using instruments and methods known in the art. For example, particle size distribution-laser diffraction can be used, and the test operation refers to GB / T19077-2016 ("Particle Size Distribution Laser Diffraction Method", pages 4-10). Specifically, the test steps are as follows: The particle sample can be dispersed at a suitable concentration in a suitable liquid (e.g., deionized water) or gas through ultrasonic treatment, etc., and tested using a Malvern Mastersizer-3000 instrument: the sample is passed through a monochromatic beam (usually a laser). When the light encounters the particles, it is scattered at different angles. The scattered light is measured by a multi-element detector, and these values ​​related to the scattering pattern are stored for subsequent analysis. Through appropriate optical models and mathematical processes, these quantified scattering data are converted to obtain the percentage of particle volume relative to the total particle volume in a series of discrete particle size ranges, thereby obtaining the particle size volume distribution.

[0125] In some embodiments, a buffer layer may be included between the negative electrode current collector 101 and the first negative electrode film layer 102. The buffer layer includes a third binder and a third conductive agent. Thus, the buffer layer can, on the one hand, enhance the adhesion between the first negative electrode film layer and the negative electrode current collector, thereby maintaining the adhesion between the first negative electrode film layer and the negative electrode current collector throughout the entire service life, effectively preventing the first negative electrode active material from detaching, and improving battery safety and lifespan; on the other hand, it can form a stable conductive network between the first negative electrode film layer and the negative electrode current collector, improving the conductivity of the negative electrode current collector and the first negative electrode film layer, thereby improving the battery's fast-charging performance.

[0126] In some embodiments, the third binder comprises 70% to 90% by mass in the buffer layer, and the third conductive agent comprises 10% to 30% by mass. A mass percentage of the third binder within these ranges is beneficial for enhancing the adhesion between the first negative electrode film layer and the negative electrode current collector. A mass percentage of the third conductive agent within these ranges is beneficial for forming a stable conductive network between the first negative electrode film layer and the negative electrode current collector. For example, in the buffer layer, the mass percentage of the third binder is a value within the range of 70%, 80%, 90%, or any combination thereof, and the mass percentage of the third conductive agent is a value within the range of 10%, 20%, 30%, or any combination thereof.

[0127] In some embodiments, the thickness of the buffer layer is 0.2 μm to 2 μm. A buffer layer thickness within this range helps ensure good adhesion between the negative electrode film and the current collector, while reducing the impact on the energy density of the individual battery cells. For example, the thickness of the buffer layer is a value within a range of 0.2 μm, 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, or any combination thereof.

[0128] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0129] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0130] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0131] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0132] Typically, a single secondary battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0133] Positive electrode sheet

[0134] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes the positive electrode material of this application or the positive electrode material prepared according to the preparation method of this application.

[0135] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the opposite surfaces of the positive current collector. 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0136] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 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.

[0137] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0138] In the examples of positive electrode active materials in this application, the molar content of oxygen 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 oxygen will fluctuate.

[0139] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0140] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0141] 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.

[0142] electrolytes

[0143] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0144] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0145] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0146] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0147] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0148] Separating membrane

[0149] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0150] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0151] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0152] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0153] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0154] 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 2 The example shown is a square-structured battery cell 5.

[0155] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0156] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0157] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0158] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0159] 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.

[0160] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0161] Electrical appliances

[0162] The second aspect of this application provides an electrical device that includes the secondary battery provided in the first aspect of this application.

[0163] Secondary batteries can be used as a power source for electrical devices or as an energy storage unit for electrical devices. Electrical devices can include, but are not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0164] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0165] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0166] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0167] Example

[0168] 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.

[0169] Example 1

[0170] Preparation of secondary batteries:

[0171] S1, Preparing the negative electrode sheet, specifically including the following steps:

[0172] a. Preparation of negative electrode slurry: Graphite material A and silicon-carbon composite (the silicon-carbon composite includes a core and a carbon coating layer on the surface of the core, the core includes hard carbon and nano-silicon particles dispersed in the pores of the hard carbon, the silicon-carbon composite contains 50% silicon, has an average particle size of 9 μm, a powder resistivity of 6 Ω·cm at 8 MPa, and a BET specific surface area of ​​2 m²) are prepared. 2 The following ingredients are mixed in a weight ratio of 57:38:3:1:1 in an appropriate amount of deionized water to form a negative electrode slurry: (g), binder (styrene-butadiene rubber), conductive agent (carbon black: carbon nanotubes = 8.5:1.5), and dispersant (carboxymethyl cellulose).

[0173] b. Preparation of the buffer layer: The third binder (PVDF) and the third conductive agent (carbon black) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 80:20 to form a buffer layer slurry.

[0174] c. Using an extrusion coating device, the above-mentioned buffer layer slurry is coated onto the negative electrode current collector copper foil to form a buffer layer with a thickness of 0.5 μm. Then, the above-mentioned negative electrode slurry is coated onto the buffer layer, dried, and cold-pressed to form a negative electrode film, resulting in a negative electrode sheet. The coating weight of the negative electrode film is 110 mg / 1540 cm³. 2 The thickness of the negative electrode film is 46 μm;

[0175] S2, Preparing a secondary battery, specifically includes the following steps:

[0176] Preparation of the positive electrode: A nickel-cobalt-manganese ternary positive electrode material (LiNi) is prepared. 0.9 Co 0.05 Mn 0.05 O2), conductive acetylene black, and polyvinylidene fluoride are mixed in a weight ratio of 97:1.5:1.5 and then added to the solvent N-methylpyrrolidone. The mixture is stirred evenly to form a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil to form a positive electrode film. After drying and cold pressing, the positive electrode sheet is obtained.

[0177] Preparation of electrolyte: Ethyl carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) are mixed in a volume ratio of 1:1:1 to form an organic solvent. LiPF6 is dissolved in the organic solvent to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0178] Separator: A polyethylene film coated with a 1μm Al2O3 ceramic coating on both sides is used as the separator.

[0179] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer packaging, and the electrolyte prepared above is added. After processes such as encapsulation, standing, formation, and aging, a secondary battery is obtained.

[0180] Parameter testing:

[0181] Battery electrode sampling method: Disassemble the battery, obtain the electrode, and soak the negative electrode in dimethyl carbonate (DMC) for a certain period of time (e.g., 2-10 hours); then take out the negative electrode and dry it at 40℃-70℃ for 2-5 hours. After drying, take out the negative electrode and use it for the following tests.

[0182] (1) Equivalent circularity test

[0183] ① Peel the negative electrode film from the negative electrode current collector. Fix the peeled negative electrode film onto the sample stage, install the sample stage into the sample holder and lock it in place. Turn on the power of the argon ion cross-section polisher (e.g., IB-09010CP type argon ion cross-section polisher) and perform vacuuming (e.g., 10... -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (90 min), and adjust the sample stage to rocking mode to begin polishing.

[0184] ② After polishing, the cross-section was observed using a scanning electron microscope (ZEISS Sigma 300). A test sample with a length × width of 50mm × 100mm was randomly selected. A test area was randomly chosen within the sample, and the area and perimeter of 50 material particles in the test area were read at 1000x magnification. The equivalent roundness was then calculated using the following formula: X1 = 4πS1 / C1 2 Where S1 and C1 are the area and perimeter of the material, respectively, and the equivalent circularity of the material X = (X1 + X2 + ... + X...) 50 ) / 50. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0185] (2) Average particle size test

[0186] The cross-sectional sample of the negative electrode film was polished according to the equivalent roundness test method described above. After polishing, the cross-section was observed using a scanning electron microscope S-4800. A test sample with a length × width of 50 mm × 100 mm was randomly selected on the cross-sectional sample of the negative electrode film. Test areas were randomly selected in the test sample, and the particle size of each material particle in the test area was read at a magnification of 1000x (i.e., the distance between the two farthest points on the material particle was taken as the particle size). The number and particle size of each material particle in the test area were counted, and the arithmetic mean of each material particle in the test area was taken as the average particle size of the material particle in the test sample.

[0187] (3) Element content test

[0188] The negative electrode film was polished according to the equivalent roundness test method described above. After polishing, the cross-section was observed using a scanning electron microscope (S-4800). Electron energy dispersive spectroscopy (EDX) was used to perform a surface scan test on the mass percentage of silicon in the negative electrode film region. To ensure the accuracy of the measurement results, 5 to 10 regions of the negative electrode film were randomly selected for silicon mass percentage testing, and the average value was calculated as the mass percentage of silicon in the negative electrode film. The EDX test results showed that the mass percentage of silicon in the negative electrode film of Example 1 was 19%.

[0189] (4) Volume distribution particle size test

[0190] The volume distribution particle size of the anode material was tested using a Malvern 2000 (MasterSizer 2000) laser particle size analyzer, following the standard procedure GB / T19077-2016 / ISO 13320:2009.

[0191] (5) Specific surface area test

[0192] The specific surface area and pore size analyzer of the negative electrode material was tested using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA, following the standard procedure GB / T 19587-2017. The specific surface area was calculated using the BET method.

[0193] (6) Graphitization degree test

[0194] The degree of graphitization of the negative electrode material was tested using an X-ray diffractometer (Bruker D8 Discover) following standard procedures: JIS K 0131-1996 and JB / T4220-2011. In the X-ray diffraction analysis, Cu Kα rays were used as the radiation source, with the ray wavelength scanning 2θ angle range of 20°–80° and a scanning rate of 4° / min. The measured d... 002 The size of is determined by the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002 It is the interlayer spacing in a graphite crystal structure measured in nanometers.

[0195] Performance testing of secondary batteries:

[0196] Security performance test

[0197] Place the battery in the fixture and install the pressure sensor. Place the clamped battery in a constant temperature environment of 25℃. Charge at 2.5-4.25V: 0.5C to 2.5% SOC, 2.5C to 40% SOC, 1.8C to 70% SOC, 1C to 80% SOC, and 0.5C to 100% SOC. After charging, allow it to rest for 30 minutes. Discharge at 0.5C to 0% SOC, and allow it to rest for 30 minutes after discharging. Repeat this process, recording the sensor expansion force with each cycle. Record the cell expansion force data after 600 cycles. Disassemble the battery and observe whether there are cracks in the current collector and electrode plates.

[0198] Fast charging performance test

[0199] The battery was placed in a constant temperature environment at 25℃ and charged to 100% at 0.5C under a voltage range of 2.5-4.25V, and then discharged to 0% at 0.5C. The capacity C was recorded. 0.5 Then, a charge-discharge rate of 2.0C was used, and the discharge capacity C2 and C2 / C were recorded. 0.5 ×100% is the capacity retention rate at a 2C rate. The higher the capacity retention rate at higher rates, the better the battery's fast charging capability.

[0200] Examples 2 to 13

[0201] The secondary battery was prepared using the same method as in Example 1, except that the silicon-carbon composite and graphite material A had different properties and different mass ratios, as detailed in Table 1 below.

[0202] Comparative Example 1

[0203] The secondary battery was prepared using the same method as in Example 1, except that the equivalent sphericity of the silicon-carbon composite was 0.7.

[0204] Comparative Example 2

[0205] The secondary battery was prepared using the same method as in Example 1, except that the equivalent sphericity of graphite material A was 0.5.

[0206] Comparative Example 3

[0207] The secondary battery was prepared using the same method as in Example 1, except that the equivalent sphericity of the silicon-carbon composite was 0.7 and the equivalent sphericity of the graphite material A was 0.5.

[0208] Comparative Example 4

[0209] The secondary battery was prepared using the same method as in Example 4, except that the mass percentage of silicon in the negative electrode film was 2%.

[0210] Comparative Example 5

[0211] The secondary battery was prepared using the same method as in Example 1, except that the silicon-based material in the negative electrode film was a silicon-oxygen compound.

[0212] Examples 2 to 13 and Comparative Examples 1 to 4 were tested using the same testing method as in Example 1. The results are shown in Tables 1 and 2 below.

[0213] Table 1

[0214]

[0215] Table 2

[0216]

[0217]

[0218] As can be seen from Tables 1 and 2, this application utilizes a silicon-carbon composite with high equivalent sphericity and graphite material A, and ensures that the silicon content in the negative electrode film is within a specific range, enabling the secondary battery to possess both excellent fast-charging performance and safety performance.

[0219] Example 14

[0220] The secondary battery was prepared using the same method as in Example 4, except that the method for preparing the negative electrode film was different. Details are as follows:

[0221] Graphite material B (artificial graphite material B1:artificial graphite material B2 = 4:6, wherein the graphitization degree of artificial graphite material B1 is 90% and the graphitization degree of artificial graphite material B2 is 96%), first binder (styrene-butadiene rubber), first conductive agent (SP), and first dispersant (CMC) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 96 (38.4:57.6):2:1:1 to form the first negative electrode slurry.

[0222] The negative electrode slurry was prepared as the second negative electrode slurry in the same manner as step a in S1 of Example 4.

[0223] Using a dual-cavity coating device, the first negative electrode slurry and the second negative electrode slurry are coated on the surface of the buffer layer. After drying and cold pressing, a first negative electrode film and a second negative electrode film are formed respectively. The first negative electrode film is located between the second negative electrode film and the buffer layer. The thickness of the first negative electrode film is 26 μm, and the thickness of the second negative electrode film is 24 μm.

[0224] In addition, Example 14 was tested using the same test method as Example 4. The results are shown in Tables 3 and 4 below.

[0225] Examples 15 to 23

[0226] The secondary battery was prepared using the same method as in Example 14, except that the graphite materials B, B1, and B2 in the first negative electrode film layer had different properties, as detailed in Table 3 below.

[0227] The tests were conducted using the same method as in Example 4, and the results are shown in Table 4.

[0228] Table 3

[0229]

[0230] Table 4

[0231]

[0232] As can be seen from Tables 3 and 4, dividing the negative electrode film into two layers, placing the silicon-carbon composite and graphite material A in the second negative electrode film layer, and placing graphite material B in the first negative electrode film layer, can further reduce the electrode charging rebound and the expansion force during cell charging, resulting in a secondary battery with better safety and fast charging performance.

[0233] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, The negative electrode includes a negative current collector and a negative electrode film layer located on at least one side surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, which comprises a silicon-carbon composite and graphite material A. The mass percentage of silicon in the negative electrode film layer is 2.5% or more. The equivalent circularity of the silicon-carbon composite is greater than 0.

8. The equivalent circularity of the graphite material A is 0.7 or higher.

2. The secondary battery according to claim 1, characterized in that, The equivalent circularity of the silicon-carbon composite is 0.8 to 1.

0.

3. The secondary battery according to claim 1 or 2, characterized in that, The equivalent circularity of the graphite material A is 0.7 to 0.

9.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The mass percentage of silicon in the negative electrode film is 2.5% to 65%.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon.

6. The secondary battery according to claim 5, characterized in that, The porous carbon includes hard carbon, and / or the silicon-containing material includes crystalline silicon.

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

8. The secondary battery according to any one of claims 1 to 7, characterized in that, The graphite material A satisfies at least one of the following characteristics: (1) The average particle size of the graphite material A is 3 μm to 20 μm; (2) The BET specific surface area of ​​the graphite material A is 0.6 m². 2 / g~3.4m 2 / g; (3) The graphitization degree of the graphite material A is 91% to 93%.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is located between the negative electrode current collector and the second negative electrode film layer. The first negative electrode film layer includes a first negative electrode active material, which includes graphite material B. The second negative electrode film layer includes a second negative electrode active material, which includes the silicon-carbon composite and the graphite material A.

10. The secondary battery according to claim 9, characterized in that, The equivalent roundness of graphite material B is lower than that of graphite material A, and the equivalent roundness of graphite material B is lower than that of the silicon-carbon composite.

11. The secondary battery according to claim 9 or 10, characterized in that, The equivalent circularity of the graphite material B is 0.5 to 0.

7.

12. The secondary battery according to any one of claims 9 to 11, characterized in that, The graphite material B satisfies at least one of the following characteristics: (1) The average particle size of the graphite material B is 6 μm to 25 μm; (2) The BET specific surface area of ​​the graphite material B is 1.5 m². 2 / g~3.0m 2 / g; (3) The graphitization degree of the graphite material B is 90% to 96%.

13. The secondary battery according to any one of claims 9 to 12, characterized in that, The graphite material B includes at least one of artificial graphite, natural graphite, or a mixture thereof.

14. The secondary battery according to any one of claims 9 to 13, characterized in that, The graphite material B includes artificial graphite B1 and artificial graphite B2 with different degrees of graphitization. The equivalent roundness of artificial graphite B1 is 0.5 to 0.7, and the equivalent roundness of artificial graphite B2 is 0.5 to 0.

7.

15. The secondary battery according to claim 14, characterized in that, The mass ratio of the artificial graphite B1 to the artificial graphite B2 is 2:8 to 8:

2.

16. The secondary battery according to any one of claims 9 to 15, characterized in that, The mass percentage of silicon in the second negative electrode film is 2.5% to 65%.

17. The secondary battery according to any one of claims 9 to 16, characterized in that, The mass percentage of silicon in the second negative electrode film is 12% to 48%.

18. The secondary battery according to any one of claims 9 to 17, characterized in that, The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:9 to 9:

1.

19. The secondary battery according to any one of claims 9 to 18, characterized in that, The thickness of the first negative electrode film is 10 μm to 70 μm.

20. The secondary battery according to any one of claims 9 to 19, characterized in that, The thickness of the second negative electrode film is 6μm to 40μm.

21. The secondary battery according to any one of claims 9 to 20, characterized in that, The first negative electrode film layer further includes a first binder, wherein the mass percentage of the first binder in the first negative electrode film layer is 0.5% to 5.0%, and / or The second negative electrode film layer further includes a second binder, wherein the mass percentage of the second binder in the second negative electrode film layer is 1.0% to 5.0%.

22. The secondary battery according to claim 21, characterized in that, The first adhesive and the second adhesive are independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid and carboxymethyl chitosan.

23. The secondary battery according to any one of claims 9 to 22, characterized in that, The first negative electrode film layer further includes a first conductive agent, wherein the mass percentage of the first conductive agent in the first negative electrode film layer is 0.05% to 1.0%, and / or The second negative electrode film layer further includes a second conductive agent, and the mass percentage of the second conductive agent in the second negative electrode film layer is 0.05% to 2.0%.

24. The secondary battery according to claim 23, characterized in that, The first conductive agent and the second conductive agent are independently selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

25. The secondary battery according to claim 23 or 24, characterized in that, The second conductive agent comprises carbon black and carbon nanotubes, wherein the mass ratio of carbon black to carbon nanotubes is 6:4 to 9:

1.

26. The secondary battery according to any one of claims 9 to 25, characterized in that, In the first negative electrode film layer, the first negative electrode active material satisfies one or both of the following characteristics: (1) The volume distribution particle size Dv50 of the first negative electrode active material is 6μm to 25μm; (2) The BET specific surface area of ​​the first negative electrode active material is 1.5 m². 2 / g~3.0m 2 / g.

27. The secondary battery according to any one of claims 9 to 26, characterized in that, In the second negative electrode film layer, the second negative electrode active material satisfies one or both of the following characteristics: (1) The volume distribution particle size Dv50 of the second negative electrode active material is 2μm~19.5μm; (2) The BET specific surface area of ​​the second negative electrode active material is 0.64 m². 2 / g~6.7m 2 / g.

28. The secondary battery according to any one of claims 9 to 27, characterized in that, A buffer layer is further included between the negative electrode current collector and the first negative electrode film layer, the buffer layer comprising a third binder and a third conductive agent.

29. The secondary battery according to claim 28, characterized in that, In the buffer layer, the third adhesive accounts for 70% to 90% of the mass, and the third conductive agent accounts for 10% to 30% of the mass.

30. The secondary battery according to claim 28 or 29, characterized in that, The thickness of the buffer layer is 0.2μm to 2μm.

31. An electrical device, characterized in that, The secondary battery includes any one of claims 1 to 30.

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