Negative electrode sheet, secondary battery, and electric device

Abstract

This application provides a negative electrode sheet, 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 surface of the negative current collector. The negative electrode film layer includes a first film layer and a second film layer stacked together. The mass percentage of silicon in the second film layer is greater than that in the first film layer, and after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than that in the first film layer. The secondary battery of this application can balance energy density and fast charging performance.

Description

[0001] Cross-references to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411754616.8, filed on December 2, 2024, entitled “Negative Electrode Sheet, Secondary Battery and Electrical Device”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of lithium battery technology, and in particular to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0004] In recent years, as the application scope of secondary batteries has become increasingly wide, 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 many fields such as power tools, electric bicycles, electric motorcycles, electric cars, and aerospace.

[0005] Due to the significant advancements in rechargeable batteries, higher requirements have been placed on their energy density and fast-charging performance. Summary of the Invention

[0006] This application was made in view of the above-mentioned problems, and its purpose is to provide a negative electrode, a secondary battery, and an electrical device. The secondary battery of this application can balance energy density and fast charging performance.

[0007] To achieve the above objectives, a first aspect of this application provides a secondary battery. 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 surface of the negative current collector. The negative electrode film layer includes a first film layer and a second film layer stacked together. The mass percentage of silicon in the second film layer is greater than the mass percentage of silicon in the first film layer, and after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than the mass percentage of lithium in the first film layer.

[0008] In this application, silicon is added to the negative electrode film to improve the energy density of the secondary battery. Furthermore, by arranging more lithium ions in the second film, the problem of decreased conductivity caused by increased silicon content is addressed, thereby further optimizing the fast-charging performance of the secondary battery. Therefore, the secondary battery of this application balances both energy density and fast-charging performance.

[0009] In some embodiments, the second film layer is located on the side of the first film layer away from the negative electrode current collector. The second film layer has a high silicon content and more ion transport channels. By placing the second film layer on the side of the first film layer away from the negative electrode current collector, the contact opportunity between the second film layer and the electrolyte is increased, thereby further improving the fast-charging performance of the secondary battery.

[0010] In some embodiments, the mass percentage of silicon in the first film layer is 0% to 40%. Optionally, the mass percentage of silicon in the first film layer is 0% to 20%. More preferably, the mass percentage of silicon in the first film layer is 0% to 5%. This helps to increase the ion transport channels in the second film layer, thereby further improving the fast charging performance of the secondary battery.

[0011] In some embodiments, the silicon element in the second film layer accounts for 2.5% to 68% by mass. Optionally, the silicon element in the second film layer accounts for 5.5% to 48% by mass. More preferably, the silicon element in the second film layer accounts for 10% to 40% by mass. This helps to increase the ion transport channels within the second film layer, thereby further improving the fast charging performance of the secondary battery.

[0012] In some embodiments, after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the first film layer is greater than 0% and less than 0.48%, which helps to alleviate the problem of decreased conductivity of the second film layer due to increased silicon content, thereby further improving the fast charging performance of the secondary battery.

[0013] In some embodiments, after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is 1.2% to 2.4%. This helps to alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery.

[0014] In some embodiments, the first film layer comprises a silicon-based material, or both the first and second film layers independently comprise silicon-based materials. The silicon-based material comprises one or more of elemental silicon, silicon-oxygen compounds, and silicon-carbon compounds. Optionally, the silicon-based material comprises at least the silicon-carbon compound.

[0015] In some embodiments, the silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon, optionally the porous carbon being hard carbon. This helps to improve the charging capability of a single battery cell.

[0016] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of the porous carbon and / or silicon material. This improves the conductivity of the silicon-carbon composite, reduces the internal impedance of the battery cell, and effectively reduces the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby enhancing the chemical stability of the silicon-carbon composite.

[0017] In some embodiments, the silicon-carbon composite contains 30% to 70% silicon by mass. This allows for the effective utilization of silicon to significantly increase the specific capacity of the negative electrode active material. Furthermore, it facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0018] In some embodiments, the average particle size of the silicon-carbon composite is from 2 μm to 15 μm, optionally from 7 μm to 11 μm. This makes it easier to increase the compactness of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the battery cell.

[0019] In some embodiments, the silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa; the control of the powder resistivity improves the conductivity of the silicon-carbon composite, thereby increasing the charging rate of the battery cell.

[0020] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g to 6.7m 2 / g. This is beneficial for balancing energy density and fast charging performance.

[0021] In some embodiments, the first film layer includes a first dispersant, wherein the mass percentage of the first dispersant in the first film layer is greater than 0% and less than 4.5%, which is beneficial to further improve the fast charging performance of the secondary battery.

[0022] In some embodiments, the second film layer includes a second dispersant, wherein the mass percentage of the second dispersant in the second film layer is 0.5% to 4.5%. This is beneficial for further improving the fast-charging performance of the secondary battery.

[0023] In some embodiments, the first dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. Using the above substances as the first dispersant is beneficial for further improving the fast-charging performance of the secondary battery.

[0024] In some embodiments, the second dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. Using the above substances as the second dispersant is beneficial for further improving the fast-charging performance of the secondary battery.

[0025] In some embodiments, the total mass percentage of the first dispersant and the second dispersant in the negative electrode film layer is 0.5% to 1%. This is beneficial for further improving the fast charging performance of the battery.

[0026] In some embodiments, the mass percentage of the second dispersant in the second film layer is greater than the mass percentage of the first dispersant in the first film layer. This helps to further improve the fast-charging performance of the secondary battery.

[0027] In some embodiments, the mass ratio of the first dispersant in the first film layer to the mass ratio of the second dispersant in the second film layer is (1-4):(6-9). This is beneficial for further improving the fast charging performance of the battery.

[0028] In some embodiments, the first film layer includes a first binder, wherein the mass percentage of the first binder in the first film layer is greater than 0.5% and less than 4.5%. The first binder, by forming a conductive network between the negative electrode active material particles, helps to improve the overall conductivity of the first film layer, thereby further improving the fast-charging performance of the secondary battery.

[0029] In some embodiments, the second film layer includes a second binder, wherein the mass percentage of the second binder in the second film layer is 0.5% to 4.5%. The second binder, by forming a conductive network between the negative electrode active material particles, helps to improve the overall conductivity of the second film layer, thereby further improving the fast-charging performance of the secondary battery.

[0030] In some embodiments, the first binder includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan; by selecting the above materials as the first conductive agent, it is beneficial to further improve the conductivity of the first film layer, thereby further improving the fast charging performance of the secondary battery.

[0031] In some embodiments, the second binder includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan. Using these materials as the second binder helps to further improve the fast-charging performance of the secondary battery.

[0032] In some embodiments, the second binder comprises styrene-butadiene rubber and / or polyacrylic acid. Because these substances possess excellent adhesive properties, using them as a second binder in the second film layer helps alleviate the problem of second film layer cracking caused by increased silicon content, thereby improving the cycle performance of the secondary battery.

[0033] In some embodiments, the total mass percentage of the first binder and the second binder in the negative electrode film layer is 0.5% to 4.5%. This is beneficial for further improving the fast charging performance of the battery.

[0034] In some embodiments, the mass percentage of the second binder in the second film layer is greater than the mass percentage of the first binder in the first film layer. This helps alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery.

[0035] In some embodiments, the mass ratio of the first binder in the first film layer to the mass ratio of the second binder in the second film layer is (1-4):(6-9). This helps to further improve the fast charging performance of the battery.

[0036] In some embodiments, the first film layer includes a first conductive agent, wherein the mass percentage of the first conductive agent in the first film layer is 0.05% to 2%, which is beneficial to improving the overall conductivity of the first film layer, thereby further improving the fast charging performance of the secondary battery.

[0037] In some embodiments, the second film layer includes a second conductive agent, wherein the mass percentage of the second conductive agent in the second film layer is 0.05% to 2%. This improves the overall conductivity of the second film layer, thereby further enhancing the fast-charging performance of the secondary battery.

[0038] In some embodiments, the first conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. These substances have good electrical conductivity, and using them as the first conductive agent is beneficial to further improve the fast charging performance of the battery.

[0039] In some embodiments, the second conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. These substances possess excellent electrical conductivity, and using them as the second conductive agent helps to further improve the fast-charging performance of the battery.

[0040] In some embodiments, the second conductive agent comprises carbon black and / or carbon nanotubes. This is beneficial for further improving the fast-charging performance of the battery.

[0041] In some embodiments, the total mass percentage of the first binder and the second binder in the negative electrode film layer is 0.05% to 2%. This is beneficial for further improving the fast charging performance of the battery.

[0042] In some embodiments, the mass percentage of the second conductive agent in the second film layer is greater than the mass percentage of the first conductive agent in the first film layer. This is beneficial for further improving the fast-charging performance of the secondary battery.

[0043] In some embodiments, the mass ratio of the first conductive agent in the first film layer to the mass ratio of the second conductive agent in the second film layer is (4-1):(6-9). This is beneficial for further improving the fast charging performance of the battery.

[0044] In some embodiments, the first film layer further includes a first graphite material, and the second film layer further includes a second graphite material, wherein the graphitization degree of the first graphite material is greater than that of the second graphite material. This facilitates slurry dispersion, thereby improving battery charging performance.

[0045] In some embodiments, the graphitization degree of the first graphite material is 91% to 96%; thereby, it is beneficial to improve the energy density of the secondary battery.

[0046] In some embodiments, the average particle size of the first graphite material is 5 μm to 25 μm; thereby, it is beneficial to improve the fast charging performance of the secondary battery while taking into account the energy density.

[0047] In some embodiments, the BET specific surface area of ​​the first graphite material is 0.8 m². 2 / g to 1.3m 2 / g; This is beneficial for improving the cycle performance and fast charging performance of secondary batteries.

[0048] In some embodiments, the tap density of the first graphite material is 1.1 g / cm³. 3 Up to 1.6 g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0049] In some embodiments, the compacted density of the first graphite material under a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 2.2 g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0050] In some embodiments, the graphitization degree of the second graphite material is 91% to 94%; thereby, the second graphite material has a high compaction density and specific capacity, thereby improving the energy density of the secondary battery.

[0051] In some embodiments, the average particle size of the second graphite material is 8 μm to 16 μm; thereby, it is beneficial to improve the fast charging performance of the secondary battery while taking into account the energy density.

[0052] In some embodiments, the BET specific surface area of ​​the second graphite material is 0.8 m². 2 / g to 1.4m 2 / g; This is beneficial for improving the cycle performance and fast charging performance of secondary batteries.

[0053] In some embodiments, the tap density of the second graphite material is 1.3 g / cm³. 3 Up to 1.5g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0054] In some embodiments, the compacted density of the second graphite material at a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 1.7 g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0055] In some embodiments, the compaction density of the negative electrode film is 1.55 g / cm³. 3 Up to 1.75 g / cm 3 This is beneficial for improving the fast-charging performance and lifespan of the negative electrode film, while also ensuring high energy density.

[0056] In some embodiments, the thickness of the negative electrode film is 34 μm to 66 μm; thus, the negative electrode film can achieve both high capacity, high ion and electron transport performance, which in turn helps the secondary battery to achieve both high energy density and fast charging performance.

[0057] In some embodiments, the adhesion force between the negative electrode film and the negative electrode current collector is 13 N / m to 20 N / m. This, on the one hand, helps reduce the risk of the negative electrode film detaching from the negative electrode current collector, thereby improving the cycle life of the battery; on the other hand, it facilitates the achievement of high energy density in the negative electrode film.

[0058] In some embodiments, the ratio of the thickness of the first film layer to the thickness of the second film layer is (55-87):(13-45). This is beneficial for the secondary battery to have excellent fast charging performance while maintaining energy density.

[0059] In some embodiments, the thickness of the first film layer is greater than the thickness of the second film layer. This allows silicon to be more concentratedly distributed in the second film layer, thereby improving the fast-charging performance of the battery.

[0060] In some embodiments, the thickness of the first film layer is 27.5 μm to 43.5 μm, which is beneficial for the secondary battery to balance energy density and fast charging performance.

[0061] In some embodiments, the thickness of the second film layer is 6.5 μm to 22.5 μm. This is beneficial for the secondary battery to balance energy density and fast charging performance.

[0062] A second aspect of this application provides a negative electrode sheet, including a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a first film layer and a second film layer stacked together. The mass percentage of silicon in the second film layer is greater than the mass percentage of silicon in the first film layer. After the voltage value of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than the mass percentage of lithium in the first film layer.

[0063] In some embodiments, the second membrane layer is located on the side of the first membrane layer away from the negative electrode current collector.

[0064] In some embodiments, the mass percentage of silicon in the first film layer is 0 to 40%. Optionally, the mass percentage of silicon in the first film layer is 0 to 20%. More preferably, the mass percentage of silicon in the first film layer is 0 to 5%.

[0065] In some embodiments, the silicon element in the second film layer accounts for 2.5% to 68% by mass. Optionally, the silicon element in the second film layer accounts for 5.5% to 48% by mass. More optionally, the silicon element in the second film layer accounts for 10% to 40% by mass.

[0066] In some embodiments, after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the first film layer is greater than 0% and less than 0.48%.

[0067] After the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is 0.05% to 0.48%.

[0068] In some embodiments, the first film layer includes a first dispersant, wherein the mass percentage of the first dispersant in the first film layer is 0.5% or more and less than 4.5%.

[0069] In some embodiments, the second film layer includes a second dispersant, wherein the mass percentage of the second dispersant in the second film layer is from 0.5% to 4.5%.

[0070] In some embodiments, the first dispersant comprises one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate.

[0071] In some embodiments, the second dispersant comprises one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate.

[0072] The secondary battery using this negative electrode can improve energy density while maintaining fast charging performance.

[0073] A third aspect of this application provides an electrical device comprising a secondary battery as described in the first aspect of this application, or a negative electrode as described in the second aspect. Since the electrical device of this application includes the secondary battery provided in this application, it possesses at least the same advantages as the secondary battery. Attached Figure Description

[0074] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0075] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0076] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0077] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0078] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0079] Figure 6 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.

[0080] Figure 7 This is an argon ion polishing (CP)-scanning electron microscope (SEM) image of the negative electrode obtained in Example 1 of this application.

[0081] Explanation of reference numerals in the attached figures:

[0082] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0083] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the negative electrode, secondary battery, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0084] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0085] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0086] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0087] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates 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.

[0088] Silicon-based anode materials (silicon-carbon composites) possess high specific capacity, and the industry has begun to improve the energy density of secondary batteries by incorporating them into the anode plates. However, silicon-based anode materials have low conductivity, which severely impacts the fast-charging performance of secondary batteries. Therefore, it is currently difficult to simultaneously improve energy density and fast-charging performance.

[0089] In view of the above, this application proposes a negative electrode, a secondary battery, and a power-consuming device. This secondary battery can balance energy density and fast-charging performance. The invention and its optional embodiments are described in more detail below.

[0090] Secondary batteries

[0091] The first aspect of this application provides a secondary battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer including a first film layer and a second film layer stacked together, the mass percentage of silicon element in the second film layer being greater than the mass percentage of silicon element in the first film layer, and after the voltage value of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium element in the second film layer being greater than the mass percentage of lithium element in the first film layer.

[0092] In this application, silicon is added to the negative electrode film to improve the energy density of the secondary battery. Furthermore, by designing the negative electrode film as a first and second film with different silicon mass ratios, the second film can concentrate more silicon while maintaining a fixed total silicon content. Due to the volume expansion characteristic of silicon, the second film with a higher silicon content provides more transport channels for ion transport, thereby enhancing the battery's fast-charging performance. Additionally, by arranging more lithium ions in the second film, the decrease in conductivity caused by the increased silicon content is mitigated, further optimizing the fast-charging performance of the secondary battery. Therefore, the secondary battery of this application balances both energy density and fast-charging performance.

[0093] In this application, the term "discharge cutoff voltage" has a conventional meaning in the art. Discharge cutoff voltage refers to the lower limit to which the voltage drops during battery discharge. The minimum permissible voltage for a battery in the battery product specification sheet is defined as the discharge cutoff voltage.

[0094] In this application, the morphology of the negative electrode film can be tested using methods known in the art. The negative electrode film to be tested can be a prepared negative electrode film or a negative electrode film obtained by disassembling the battery. The latter will be used as an example to describe the testing process below. Specifically, the negative electrode sheet is obtained by disassembling the battery, and then placed on a sample holder and locked in place. An argon ion cross-section polisher (such as the IB-09010 CP type argon ion cross-section polisher from JEOL Corporation of Japan) is used to cut a cross-section of the negative electrode sheet. A scanning electron microscope (HR-TEM Talos F200) is used to acquire a SEM image of the cross-section of the negative electrode sheet. From the cross-section SEM image, it can be seen that the negative electrode film includes a first film layer and a second film layer stacked together.

[0095] In this application, the mass percentage of lithium or silicon in the film layer (first or second film layer) can be tested using methods known in the art. The negative electrode sheet to be tested can be a pre-prepared negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. The following describes the lithium testing process using the latter as an example. Specifically, the secondary battery is discharged until the voltage reaches the discharge cutoff voltage, the battery is disassembled, and the negative electrode sheet is removed. The first and second film layers are scraped off separately. The mass of lithium in each film layer is tested according to GB / T11064.1-2018. Then, based on the ratio of the mass of lithium to the mass of the film layer, the lithium content in the film layer is determined.

[0096] Negative electrode sheet

[0097] In this application, the negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a first film layer and a second film layer arranged in a stacked manner.

[0098] In some embodiments, the second membrane layer is located between the first membrane layer and the negative electrode current collector.

[0099] In some embodiments, the second membrane layer is located on the side of the first membrane layer away from the negative electrode current collector. The second membrane layer has a higher mass percentage of silicon and more ion transport channels. By placing the second membrane layer on the side of the first membrane layer away from the negative electrode current collector, the contact opportunity between the second membrane layer and the electrolyte is increased, thereby further improving the fast-charging performance of the secondary battery.

[0100] In some embodiments, the mass percentage of silicon in the first film layer is 0-40%. A mass percentage of silicon in the first film layer within this range indicates that the mass percentage of silicon in the first film layer is controlled at a low level, and the silicon is mainly concentrated in the second film layer. This is beneficial for increasing the ion transport channels within the second film layer, thereby further improving the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of silicon in the first film layer is 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 5%, 10%, 20%, 30%, 40%, or a value within any two of these ranges. Optionally, the mass percentage of silicon in the first film layer is 0 to 20%, and more preferably, it is 0 to 5%.

[0101] In some embodiments, the mass percentage of silicon in the second film layer is between 2.5% and 68%. A mass percentage of silicon in the second film layer within this range indicates a high mass percentage, which is beneficial for increasing ion transport channels within the second film layer, thereby further improving the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of silicon in the second film layer is 5.5%, 10%, 20%, 30%, 40%, 48%, 50%, 60%, 68%, or any value within a range of two such values. Optionally, the mass percentage of silicon in the second film layer is between 5.5% and 48%. More preferably, the mass percentage of silicon in the second film layer is between 10% and 40%.

[0102] In some embodiments, the mass percentage of lithium in the first film layer is 0% or more and less than 0.48%. This range indicates that the mass percentage of lithium in the first film layer is controlled at a low level, and the lithium is mainly concentrated in the second film layer. This helps alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of lithium in the first film layer is 0%, 0.4%, 0.24%, 0.4%, or a value within a range of any two values.

[0103] In some embodiments, the mass percentage of lithium in the second film layer is between 0.05% and 0.48%. A mass percentage of lithium in the second film layer within this range indicates that the mass percentage of lithium in the second film layer is controlled at a high level. This helps to alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of lithium in the second film layer is 0.05%, 0.12%, 0.15%, 0.2%, 0.24%, 0.48%, or a value within a range of any two values.

[0104] In some embodiments, the first film layer comprises a silicon-based material, or both the first and second film layers independently comprise silicon-based materials, wherein the silicon-based material comprises one or more of elemental silicon, silicon-oxygen compounds, and silicon-carbon compounds. Optionally, the silicon-based material comprises at least the silicon-carbon compound.

[0105] In some embodiments, the silicon-oxygen complex includes at least one of unpre-lithium silicon-oxygen compound, pre-lithium silicon-oxygen compound, unpre-magnesium silicon-oxygen compound, and pre-magnesium silicon-oxygen compound.

[0106] In some embodiments, both the first and second films independently include silicon-carbon composites.

[0107] 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.

[0108] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0109] (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon, wherein the porous carbon is optionally hard carbon;

[0110] (2) The silicon-carbon composite also includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials;

[0111] (3) The silicon content in the silicon-carbon composite is 30%-70% by mass;

[0112] (4) The average particle size of the silicon-carbon composite is 2μm-15μm, and optionally 7μm-11μm;

[0113] (5) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm-17 Ω·cm;

[0114] (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0115] In some embodiments, the silicon-carbon composite includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support while its pores offer expansion space, effectively mitigating stress caused by expansion during charging. Especially when the silicon-containing material has a nanometer-scale particle size, it exhibits higher specific capacity and is better dispersed within the pores of the porous carbon. Furthermore, it allows for more efficient utilization of the porous carbon's buffering effect on expansion. When this silicon-carbon composite is used in wound electrode assemblies, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0116] 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 secondary battery.

[0117] 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 secondary battery.

[0118] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials. This can improve the conductivity of the silicon-carbon composite, reduce the internal impedance of the secondary battery, and effectively reduce the probability of direct contact between the silicon-containing materials in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0119] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0120] 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.

[0121] 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-15 μm. Optionally, the average particle size of the silicon-carbon composite is 7 μm-11 μm, or 5 μm-10 μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite and the average particle size of the graphite material, which is more conducive to increasing the compaction of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the secondary battery.

[0122] The number-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 number-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.

[0123] In some embodiments, the silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa. Exemplarily, this ranges from 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 secondary battery.

[0124] 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.

[0125] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0126] 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.

[0127] In some embodiments, the first film layer includes a first dispersant, wherein the mass percentage of the first dispersant in the first film layer is 0% or more and less than 4.5%. By adding the first dispersant to the first film layer, the dispersing effect of the first dispersant can reduce the contact resistance between particles (e.g., negative electrode active material) within the first film layer, improve the conductivity of the negative electrode sheet (first film layer), and thereby further enhance the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of the first dispersant is 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 4.5%, or a value within a range of any two values; optionally, the mass percentage of the first dispersant is from 0.2% to 2.5%.

[0128] In some embodiments, a second dispersant is included in the second film layer, and the mass percentage of the second dispersant in the second film layer is from 0.5% to 4.5%. By adding the second dispersant to the second film layer, the contact resistance between particles (e.g., negative electrode active material) within the second film layer can be reduced by utilizing the dispersing effect of the second dispersant, thereby improving the conductivity of the negative electrode sheet (second film layer), and further enhancing the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of the second dispersant is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or a value within a range of any two values; optionally, the mass percentage of the second dispersant is from 0.1% to 2%.

[0129] In some embodiments, the first dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. These substances have high ionic conductivity, and selecting them as the first dispersant is beneficial for further improving the fast-charging performance of the secondary battery.

[0130] In some embodiments, the second dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. These substances have high ionic conductivity, and selecting them as the second dispersant is beneficial for further improving the fast-charging performance of the secondary battery.

[0131] In some embodiments, the total mass percentage of the first dispersant and the second dispersant in the negative electrode film layer is greater than 0% and less than 4.5%. Maintaining the total mass percentage of the first dispersant and the second dispersant within this range further improves the fast-charging performance of the battery. For example, the total mass percentage of the first dispersant and the second dispersant in the negative electrode film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 8%, 0.9%, 1%, or a value within any two of these ranges. Optionally, the total mass percentage of the first dispersant and the second dispersant in the negative electrode film layer is between 0.1% and 2%.

[0132] In some embodiments, the mass percentage of the second dispersant in the second film layer is greater than the mass percentage of the first dispersant in the first film layer. This helps to maintain a high mass percentage of the second dispersant in the second film layer, which helps alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery.

[0133] In some embodiments, the mass ratio γ of the first dispersant in the first film layer to the second dispersant in the second film layer is (1-4):(6-9). By ensuring that the mass ratio of the first dispersant in the first film layer to the second dispersant in the second film layer is within the above range, it is beneficial to further improve the fast charging performance of the battery. For example, γ is a value between 1:9, 1:8, 1:7, 1:6, 4:6, or any two of these values.

[0134] In some embodiments, the first film layer includes a first binder, the first binder comprising 0.5% to 4.5% by mass in the first film layer. The first binder, by forming a conductive network between the negative electrode active material particles, helps to improve the overall conductivity of the first film layer, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of the first binder in the first film layer is 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 4.5%, or a value within a range of any two of these values.

[0135] In some embodiments, the second film layer includes a second binder, the second binder comprising 0.5% to 4.5% by mass. The second binder, by forming a conductive network between the negative electrode active material particles, helps to improve the overall conductivity of the second film layer, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of the second binder in the second film layer is 0.5%, 1%, 2%, 3%, 4%, 4.5%, or a value within a range of any two of these values.

[0136] In some embodiments, the first binder includes one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). Using the above materials as the first binder is beneficial for further improving the fast-charging performance of the secondary battery.

[0137] In some embodiments, the second binder includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan. Using these materials as the second binder helps to further improve the fast-charging performance of the secondary battery.

[0138] In some embodiments, the second binder includes styrene-butadiene rubber and / or polyacrylic acid. Because these substances possess excellent adhesive properties, using them as a second binder in the second film layer helps alleviate the problem of second film layer cracking caused by increased silicon content, thereby improving the cycle performance of the secondary battery.

[0139] In some embodiments, the total mass percentage of the first binder and the second binder in the negative electrode film layer is between 0.5% and 4.5%. Maintaining the total mass percentage of the first binder and the second binder within this range is beneficial for further improving the fast-charging performance of the battery. For example, the total mass percentage of the first binder and the second binder is 0.5%, 1%, 2%, 3%, 4%, 4.5%, or a value within a range of any two of these values.

[0140] In some embodiments, the mass percentage of the second binder in the second film layer is greater than the mass percentage of the first binder in the first film layer. This helps to maintain a high mass percentage of the second binder in the second film layer, which helps alleviate the problem of decreased conductivity caused by increased silicon content in the second film layer, thereby further improving the fast-charging performance of the secondary battery.

[0141] In some embodiments, the ratio β of the mass percentage of the first binder in the first film layer to the mass percentage of the second binder in the second film layer is (1-4):(6-9). By ensuring that the ratio of the mass percentage of the first binder in the first film layer to the mass percentage of the second binder in the second film layer is within the above range, it helps to further improve the fast charging performance of the battery. For example, β is a value between 1:9, 1:7, 1:6, 4:6, or any two of these values.

[0142] In some embodiments, the first film layer includes a first conductive agent, and the mass percentage of the first conductive agent in the first film layer is 0.05% to 2%. Adding the first conductive agent to the first film layer improves the overall conductivity of the first film layer, thereby further enhancing the fast-charging performance of the secondary battery. For example, the mass percentage of the first conductive agent in the first film layer is 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, or a value within a range of any two values.

[0143] In some embodiments, the second film layer includes a second conductive agent, wherein the mass percentage of the second conductive agent in the second film layer is 0.05% to 2%. Adding a second conductive agent to the second film layer improves the overall conductivity of the second film layer, thereby further enhancing the fast-charging performance of the secondary battery. For example, the mass percentage of the second conductive agent in the second film layer is 0.05%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any value within a range of two such values.

[0144] In some embodiments, the first conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. These substances possess excellent electrical conductivity, and using them as the first conductive agent helps to further improve the fast-charging performance of the battery.

[0145] In some embodiments, the second conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. These substances possess excellent electrical conductivity, and using them as the second conductive agent can further improve the fast-charging performance of the battery.

[0146] In some embodiments, the second conductive agent comprises carbon black and carbon nanotubes. By selecting the above-mentioned substances as the second conductive agent, it is beneficial to further improve the fast charging performance of the battery. Optionally, the mass ratio of carbon black to carbon nanotubes is (6-9):(4-1), and more preferably, the mass ratio of carbon black to carbon nanotubes is (80-85):(20-15).

[0147] In some embodiments, the total mass percentage of the first binder and the second binder in the negative electrode film layer is between 0.05% and 2%. Maintaining the total mass percentage of the first binder and the second binder within this range further enhances the fast-charging performance of the battery. For example, the total mass percentage of the first binder and the second binder may be 0.05%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any value within a range of two such values.

[0148] In some embodiments, the mass percentage of the second conductive agent in the second film layer is greater than the mass percentage of the first conductive agent in the first film layer. This helps to maintain a high mass percentage of the second conductive agent in the second film layer, which helps alleviate the problem of decreased conductivity in the second film layer due to increased silicon content, thereby further improving the fast-charging performance of the secondary battery.

[0149] In some embodiments, the ratio δ of the mass percentage of the first conductive agent in the first film layer to the mass percentage of the second conductive agent in the second film layer is (4-1):(6-9). By ensuring that the ratio of the mass percentage of the first conductive agent in the first film layer to the mass percentage of the second conductive agent in the second film layer is within the above range, it is beneficial to further improve the fast-charging performance of the battery. For example, δ can be a value between 1:9, 1:8, 4:6, or any two of these values.

[0150] In some embodiments, the first film layer further includes a first graphite material, and the second film layer further includes a second graphite material, wherein the graphitization degree of the first graphite material is greater than that of the second graphite material. By adding a second graphite material with a high degree of graphitization to the first film layer, this helps to disperse the slurry and improve battery charging performance.

[0151] In some embodiments, the graphitization degree of the first graphite material is 91% to 96%. By keeping the graphitization degree of the first graphite material within the above range, it is beneficial for the first graphite material to have high compaction density and specific capacity, thereby improving the energy density of the secondary battery. Exemplarily, the graphitization degree of the first graphite material is 91%, 91.5%, 92%, 92.5%, 93%, 96%, or a value between any two of these values.

[0152] In some embodiments, the average particle size of the first graphite material is between 5 μm and 25 μm. By ensuring that the average particle size of the first graphite material is within this range, it is beneficial to improve the fast-charging performance of the secondary battery while maintaining energy density. For example, the average particle size of the first graphite material is a value between 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or any two of these values.

[0153] In some embodiments, the BET specific surface area of ​​the first graphite material is 0.8 m². 2 / g to 1.3m 2 / g. By ensuring the specific surface area of ​​the first graphite material is within the aforementioned range, it is beneficial to reduce the surface side reaction activity of the first graphite material, thereby reducing the consumption of active ions during SEI film formation and improving the cycle performance of the secondary battery. Furthermore, the first graphite material also possesses high active ion transport performance, enhancing the fast-charging performance of the secondary battery. For example, the specific surface area of ​​the first graphite material can be 0.8m². 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.3m 2 / g or a range of values ​​between any two of them.

[0154] In some embodiments, the tap density of the first graphite material is 1.1 g / cm³. 3 Up to 1.6 g / cm 3 By ensuring the tap density of the first graphite material is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the tap density of the first graphite material can be 1.1 g / cm³. 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 Or the value between any two of them within a range.

[0155] In some embodiments, the compacted density of the first graphite material powder under a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 2.2 g / cm 3 By ensuring the powder compaction density of the first graphite material is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the powder compaction density of the first graphite material at 50000 N can be 1.6 g / cm³. 3 1.65g / cm 3 1.7g / cm 3 2.2g / cm 3 Or the value between any two of them within a range.

[0156] In some embodiments, the graphitization degree of the second graphite material is 91% to 94%. By keeping the graphitization degree of the second graphite material within the above range, it is beneficial for the second graphite material to have high compaction density and specific capacity, thereby improving the energy density of the secondary battery. Exemplarily, the graphitization degree of the second graphite material is 91%, 91.5%, 92%, 93%, 94%, or a value between any two of these values.

[0157] In some embodiments, the average particle size of the second graphite material is between 8 μm and 16 μm. By ensuring that the average particle size of the second graphite material is within this range, it is beneficial to improve the fast-charging performance of the secondary battery while maintaining energy density. For example, the average particle size of the second graphite material is 8 μm, 10 μm, 15 μm, 16 μm, or a value within a range consisting of any two of these values.

[0158] In some embodiments, the BET specific surface area of ​​the second graphite material is 0.8 m². 2 / g to 1.4m 2 By ensuring the specific surface area of ​​the second graphite material is within the aforementioned range, it is beneficial to reduce the surface side reaction activity of the second graphite material, thereby reducing the consumption of active ions during SEI film formation and improving the cycle performance of the secondary battery. Furthermore, the second graphite material also possesses high active ion transport performance, enhancing the fast-charging performance of the secondary battery. For example, the specific surface area of ​​the second graphite material can be 0.8 m² / g. 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g or a range of values ​​between any two of them.

[0159] In some embodiments, the tap density of the second graphite material is 1.3 g / cm³. 3 Up to 1.5g / cm 3 By ensuring the tap density of the second graphite material is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the tap density of the second graphite material can be 1.3 g / cm³. 3 1.4g / cm 3 1.5g / cm 3 Or the value between any two of them within a range.

[0160] In some embodiments, the compacted density of the second graphite material at a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 1.7 g / cm 3By ensuring the powder compaction density of the second graphite material is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the powder compaction density of the second graphite material at 50000 N can be 1.6 g / cm³. 3 1.65g / cm 3 1.7g / cm 3 Or the value between any two of them within a range.

[0161] In some embodiments, the ratio of the thickness of the first film layer to the thickness of the second film layer is (55-87):(13-45). By keeping the ratio of the thickness of the first film layer to the thickness of the second film layer within the above range, it is beneficial for the secondary battery to have excellent fast-charging performance while maintaining energy density. For example, the ratio of the thickness of the first film layer to the thickness of the second film layer can be 87:45, 55:45, 87:13, or a value within a range of any two of these values.

[0162] In some embodiments, the thickness of the first film layer is between 27.5 μm and 43.5 μm. By controlling the thickness of the first film layer within this range, it is beneficial for the secondary battery to balance energy density and fast charging performance. For example, the thickness of the first film layer is a value between 27 μm, 28 μm, 28.5 μm, 30 μm, 40 μm, 43.5 μm, or any two of these values.

[0163] In some embodiments, the thickness of the second film layer is between 6.5 μm and 22.5 μm. By controlling the thickness of the second film layer within the above range, it is beneficial for the secondary battery to balance energy density and fast charging performance. Exemplarily, the thickness of the second film layer is 6.5 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22.5 μm, or a value within a range of any two of these values.

[0164] In some embodiments, the compaction density of the negative electrode film is 1.55 g / cm³. 3 Up to 1.75 g / cm 3 By maintaining the compaction density of the negative electrode film within the aforementioned range, it is beneficial to maintain a better pore structure, reduce the tortuosity of the negative electrode film, and shorten the active ion transport path, thereby improving the fast-charging performance and lifespan of the negative electrode film while also achieving high energy density. For example, the powder compaction density of the negative electrode film at a pressure of 50,000 N can be 1.55 g / cm³. 3 1.60g / cm 3 1.62g / cm 3 1.64 g / cm 3 1.66 g / cm 3 1.68g / cm 31.70g / cm 3 1.75g / cm 3 Or the value between any two of them within a range.

[0165] In some embodiments, the thickness of the negative electrode film is between 34 μm and 66 μm. By keeping the thickness of the negative electrode film within this range, it is advantageous for the negative electrode film to achieve both high capacity, high ion and electron transport performance, thereby benefiting the secondary battery in achieving both high energy density and fast charging performance. Exemplarily, the thickness of the negative electrode film can be 34 μm, 36 μm, 40 μm, 50 μm, 60 μm, 66 μm, or a value within a range of any two of these values.

[0166] In some embodiments, the adhesion force between the negative electrode film layer and the negative electrode current collector is between 13 N / m and 20 N / m. By keeping the adhesion force between the negative electrode film layer and the negative electrode current collector within this range, a moderate adhesion force is achieved. This helps reduce the risk of detachment between the negative electrode film layer and the negative electrode current collector, thereby improving the cycle life of the battery. Furthermore, it facilitates achieving a high energy density for the negative electrode film layer. For example, the adhesion force between the negative electrode film layer and the negative electrode current collector is a value between 13 N / m, 14 N / m, 15 N / m, 16 N / m, 17 N / m, 18 N / m, 19 N / m, 20 N / m, or any two of these values.

[0167] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0168] 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.

[0169] In this application, the negative electrode current collector can be a metal foil or a composite current collector. For example, copper foil can 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 (copper, copper 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.).

[0170] In this application, the thickness of the film layers (negative electrode film layer, first film layer, second film layer) has a meaning known in the art and can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. The latter is used as an example to describe the testing process below. Specifically, the negative electrode sheet is obtained by disassembling the battery, and then placed and locked in a sample holder. An argon ion cross-section polisher (such as the IB-09010CP argon ion cross-section polisher from JEOL Corporation of Japan) is turned on, and a cross-section of the negative electrode sheet in the stacking direction of the negative electrode film layer and the negative electrode current collector is taken. A scanning electron microscope (HR-TEM Talos F200) is used to acquire a SEM image of the cross-section of the negative electrode sheet. The thickness of each film layer can be measured using a scale bar.

[0171] In this application, the additives (first dispersant, first binder, second dispersant, and second binder) can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process below. Specifically, the battery is disassembled, and the negative electrode sheet is removed. The first and second film layers are scraped off respectively. The mass of the additives in the film layers is determined according to the infrared spectroscopy analysis method in GB / T 6040-2019. Based on the ratio of the mass of the additives to the mass of the film layers, the mass percentage of each additive in the film layers is determined.

[0172] In this application, the degree of graphitization of the graphite materials (first graphite material, second graphite material) is the proportion of carbon elements in the material existing in the form of a graphite structure, which can be tested 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), and the test can be performed with reference to JIS K 0131-1996 and JB / T4220-2011 to obtain the average interlayer spacing d002 of the C(002) crystal plane in the material's crystal structure. Then, the degree of graphitization is calculated according to the formula g = (0.344 - d002) / (0.344 - 0.3354) × 100%. In the above formula, d002 is the average interlayer spacing of the C(002) crystal plane in the material's crystal structure, expressed in nanometers (nm).

[0173] In this application, the average particle size of the graphite materials (first graphite material and second graphite material) can be tested using equipment and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain a scanning electron microscope (SEM) image of the negative electrode sheet. As an example, the test can be performed as follows: arbitrarily select a test sample with a length × width of 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) in the test sample, and at a certain magnification (e.g., 1000x when measuring a certain material), read the particle size of each material particle in each test area (i.e., take the distance between the two farthest points on a certain material particle as the particle size). Count the number and particle size values ​​of the material particles in each test area, and take the arithmetic mean of the material particles in each test area, which is the average particle size of a certain material particle in the test sample.

[0174] In this application, the specific surface area (BET) of the graphite materials (first graphite material and second graphite material) has a well-known meaning 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.

[0175] In this application, the tap density of the graphite materials (first graphite material and second graphite 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 determined using a powder tap density tester according to GB / T 5162-2006. The testing instrument can be Dandong Baite BT-301, with the following test parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, number of vibrations 5000 times, and a measuring cylinder of 25mL.

[0176] In this application, the compaction density of graphite materials (first graphite material, second graphite 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 determined using an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) in accordance with GB / T24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure is increased to 50,000 N, held for 30 seconds, then the pressure is released and held for 10 seconds. The compaction density of the powder under 50,000 N pressure is then recorded and calculated.

[0177] Positive electrode sheet

[0178] 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 including a positive electrode active material.

[0179] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0180] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0181] In some embodiments, when the battery cell is a secondary battery, the positive electrode active material may be a positive electrode active material known in the art for secondary 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 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0182] 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.

[0183] 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.

[0184] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0185] 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, or carbon nanofibers.

[0186] 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.

[0187] electrolytes

[0188] 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.

[0189] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0190] 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.

[0191] 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.

[0192] 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.

[0193] Separating membrane

[0194] 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.

[0195] 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.

[0196] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0197] 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.

[0198] 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.

[0199] 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 1 The example shown is a square-structured battery cell 5.

[0200] In some implementations, refer to Figure 2 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 be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by 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 the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0201] 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.

[0202] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 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.

[0203] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0204] 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.

[0205] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5The 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.

[0206] Electrical appliances

[0207] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is 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.

[0208] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0209] Figure 6 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.

[0210] 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.

[0211] Example

[0212] 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.

[0213] Preparation Example 1

[0214] The first graphite material (see Material 1-1 in Table 1) and the silicon-carbon composite (silicon content 70%, average particle size 8 μm, powder resistivity 16 Ω·cm, BET specific surface area 2.1 m²) were used. 2The first dispersant (carboxymethyl cellulose lithium salt, in which lithium element accounts for 10.7% by mass), the first binder (SBR), and the first conductive agent (carbon black) are mixed in a mass ratio of 74.7:20:1.1:3:1.2 to prepare a negative electrode slurry 1-1 (first negative electrode slurry).

[0215] Preparation Example 2

[0216] The second graphite material (see Material 2-1 in Table 1) and the silicon-carbon composite (silicon content 70%, average particle size 8 μm, powder resistivity 16 Ω·cm, BET specific surface area 2.1 m²) were used. 2 The negative electrode slurry 2-1 (second negative electrode slurry) is prepared by mixing the second dispersant (carboxymethyl cellulose lithium salt), the second binder (SBR), and the second conductive agent (carbon black) in a mass ratio of 73.8:23:1.2:1.2:0.8.

[0217] Preparation Example 3

[0218] The first graphite material (Material 1-1) and the silicon-carbon composite (silicon element mass ratio of 70%, average particle size of 8 μm, powder resistivity of 16 Ω·cm, BET specific surface area of ​​2.1 m²) were used. 2 The negative electrode slurry 1'-1 is prepared by mixing the first dispersant (carboxymethyl cellulose lithium salt), the first binder (SBR), and the first conductive agent (carbon black) in a mass ratio of 75.8:20:1.2:2.0:1.0.

[0219] Preparation Example 4

[0220] The second graphite material (Material 2-1) and the silicon-carbon composite (with a silicon element content of 70%, an average particle size of 8 μm, a powder resistivity of 16 Ω·cm, and a BET specific surface area of ​​2.1 m²) were used. 2 The negative electrode slurry 1'-2 is prepared by mixing the second dispersant (carboxymethyl cellulose lithium salt), the second binder (SBR), and the second conductive agent (carbon black) in a mass ratio of 73.9:23:1.1:1.0:1.0.

[0221] Preparation Example 5

[0222] The first graphite material (Material 1-1) and the silicon-carbon composite (silicon element mass ratio of 70%, average particle size of 8 μm, powder resistivity of 16 Ω·cm, BET specific surface area of ​​2.1 m²) were used. 2 The negative electrode slurry 2'-1 is prepared by mixing the first dispersant (carboxymethyl cellulose lithium salt), the first binder (SBR), and the first conductive agent (carbon black) in a mass ratio of 76.35:20:1.15:1.5:1.0.

[0223] Preparation Example 6

[0224] The second graphite material (Material 2-1) and the silicon-carbon composite (with a silicon element content of 70%, an average particle size of 8 μm, a powder resistivity of 16 Ω·cm, and a BET specific surface area of ​​2.1 m²) were used. 2 The negative electrode slurry 2'-2 is prepared by mixing the second dispersant (carboxymethyl cellulose lithium salt), the second binder (SBR), and the second conductive agent (carbon black) in a mass ratio of 73.65:23:1.15:1.0:1.2.

[0225] Table 1

[0226]

[0227]

[0228] Example 1

[0229] (1) Preparation of negative electrode sheet

[0230] Using a dual-cavity coating device, a first negative electrode slurry (negative electrode slurry 1-1) and a second negative electrode slurry (negative electrode slurry 1-2) are simultaneously extruded, with the mass ratio of the first negative electrode slurry to the second negative electrode slurry being [value missing]. After drying and cold pressing, a negative electrode sheet is obtained. The first negative electrode slurry is coated onto the negative electrode current collector (copper foil) to form a first film layer, i.e., the film layer adjacent to the negative electrode current collector is the first film layer. The second negative electrode slurry is coated onto the side of the first negative electrode slurry away from the negative electrode current collector to form a second film layer, i.e., the film layer away from the negative electrode current collector is the second film layer, meaning the second film layer is located on the side of the first film layer away from the negative electrode current collector.

[0231] (2) Preparation of positive electrode sheet

[0232] The positive electrode active material (LiNi) 0.9 Co 0.05 Mn 0.05 O2), binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black are dissolved in solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:1.5:1.5. After thorough mixing, a positive electrode slurry is prepared. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0233] (3) Separating membrane

[0234] Polypropylene film is used as the separator.

[0235] (4) Preparation of electrolyte

[0236] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0237] (5) Preparation of secondary batteries

[0238] The positive and negative electrode sheets prepared above are arranged in order, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0239] Testing of negative electrode morphology

[0240] The negative electrode sheet was installed and locked onto the sample holder. An argon ion cross-section polisher (e.g., the JEOL IB-09010CP argon ion cross-section polisher) was used to cut a cross-section of the negative electrode sheet. A scanning electron microscope (HR-TEMTalos F200) was used to acquire a SEM image of the negative electrode sheet cross-section. Figure 7 As shown in the cross-sectional SEM image, the negative electrode sheet includes a negative current collector and a negative electrode film layer. The negative electrode film layer includes a first film layer and a second film layer stacked together.

[0241] Electrical performance testing

[0242] (1) Fast charging performance test:

[0243] ① At 25℃, charge the secondary battery at a constant current of 0.33C to 4.25V, then charge it at a constant voltage to a current of 0.05C. After standing for 5 minutes, discharge the secondary battery at a constant current of 0.33C to 2.0V and record its actual capacity as C0.

[0244] ②Then charge the secondary battery sequentially with constant current at 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 to 3.8V or 0V negative electrode cutoff potential (whichever comes first). After each charge, discharge to 2.0V with 1C0. Record the negative electrode potential corresponding to 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% SOC (State of Charge) at different charging rates.

[0245] ③ Plot the charging rate-negative electrode potential curves under different SOC states. After linear fitting, obtain the charging rate corresponding to the negative electrode potential of 0V under different SOC states. This charging rate is the charging window under this SOC state, and is denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively.

[0246] ④ Calculate the charging time T (assuming no lithium plating) for the secondary battery from 10% SOC to 80% SOC using the following formula, in minutes.

[0247] (60 / C 20％SOC +60 / C 30％SOC +60 / C 40％SOC +60 / C 50％SOC +60 / C 60％SOC +60 / C 70％SOC +60 / C 80％SOC )×10%.

[0248] (2) Discharge capacity test:

[0249] ① Let stand for 5 minutes;

[0250] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.05C;

[0251] ③ Let stand for 5 minutes;

[0252] ④ Discharge at a constant current of 1C to 1.5V, and record the discharge capacity D1 of the first cycle;

[0253] The mass ratio of D1 to the positive electrode active material is used as the discharge capacity.

[0254] Example 2

[0255] The battery preparation method in Example 2 is similar to that in Example 1, except that:

[0256] (1) In the preparation step of the negative electrode sheet, the first negative electrode slurry (negative electrode slurry 1-1) and the second negative electrode slurry (negative electrode slurry 1-2) are simultaneously extruded using a dual-cavity coating device, wherein the mass ratio of the first negative electrode slurry to the second negative electrode slurry is 55:45. The second negative electrode slurry is coated on the negative electrode current collector (copper foil) to form a second film layer, that is, the film layer adjacent to the negative electrode current collector is the second film layer, and the first negative electrode slurry is coated on the side of the second negative electrode slurry away from the negative electrode current collector to form a first film layer, that is, the film layer away from the negative electrode current collector is the first film layer, i.e., the first film layer is located on the side of the second film layer away from the negative electrode current collector. After drying and cold pressing, the negative electrode sheet is obtained.

[0257] Comparative Example 1

[0258] The battery preparation method of Comparative Example 1 is similar to that of Example 1. The difference is:

[0259] (1) In the preparation steps of the negative electrode sheet

[0260] A first negative electrode slurry (negative electrode slurry 1'-1) and a second negative electrode slurry (negative electrode slurry 1'-2) are simultaneously extruded using a dual-cavity coating apparatus. The mass ratio of the first negative electrode slurry to the second negative electrode slurry is 55:45. The first negative electrode slurry is coated onto the negative electrode current collector (copper foil) to form a film layer adjacent to the negative electrode current collector. The second negative electrode slurry is coated onto the side of the first negative electrode slurry away from the negative electrode current collector to form a film layer away from the negative electrode current collector. After drying and cold pressing, a negative electrode sheet is obtained.

[0261] Comparative Example 2

[0262] The battery preparation method of Comparative Example 2 is similar to that of Example 1. The difference is:

[0263] (1) In the preparation step of the negative electrode sheet, the first negative electrode slurry (negative electrode slurry 2'-1) and the second negative electrode slurry (negative electrode slurry 2'-2) are simultaneously extruded using a dual-cavity coating device, wherein the mass ratio of the first negative electrode slurry to the second negative electrode slurry is 55:45. The first negative electrode slurry is coated on the negative electrode current collector (copper foil) to form a film layer adjacent to the negative electrode current collector. The second negative electrode slurry is coated on the side of the first negative electrode slurry away from the negative electrode current collector to form a film layer away from the negative electrode current collector. After drying and cold pressing, the negative electrode sheet is obtained.

[0264] Comparative Example 3

[0265] The battery preparation method of Comparative Example 3 is similar to that of Example 1. The difference is:

[0266] (1) In the preparation step of the negative electrode sheet, the negative electrode slurry 1-1 is coated on both sides of the negative electrode current collector. The coating thickness of the negative electrode slurry 1-1 is 50 μm.

[0267] The electrical performance of the secondary batteries of Examples 2 and Comparative Examples 1 to 3 was tested in the same manner as in Example 1, and the test results are shown in Table 2-2.

[0268] Table 2-1

[0269] project Location of the first membrane layer The location of the second membrane layer Example 1 The side adjacent to the negative electrode current collector The side away from the negative electrode current collector Example 2 The side away from the negative electrode current collector The side adjacent to the negative electrode current collector

[0270] Table 2-2

[0271]

[0272]

[0273] The data in Tables 2-1 and 2-2 show that, compared to Comparative Examples 1 to 3, the mass percentage of silicon in the second film layer of the batteries in Examples 1 and 2 is greater than that in the first film layer. Furthermore, after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than that in the first film layer. The batteries in Examples 1 and 2 can balance energy density and fast charging performance.

[0274] Examples 3 to 6

[0275] The battery preparation methods in Examples 3 to 6 are similar to those in Example 1. The difference lies in the following: in the preparation step of the negative electrode sheet, the types of the first graphite material, the first dispersant, the first binder, and the first conductive agent in the first negative electrode slurry are adjusted according to Table 3-1, and at least one of the second graphite material, the second dispersant, the second binder, and the second conductive agent in the second negative electrode slurry is adjusted.

[0276] The electrical performance of the secondary batteries of Examples 3 to 6 was tested in the same manner as in Example 1, and the test results are shown in Table 3-2.

[0277] Table 3-1

[0278]

[0279] Table 3-2

[0280]

[0281]

[0282] The data in Tables 3-1 to 3-2 show that the batteries in Examples 3 to 6 can balance energy density and fast charging performance.

[0283] Examples 7 to 12

[0284] The battery preparation methods in Examples 7 to 12 are similar to those in Example 1. The difference is:

[0285] In the preparation step of the negative electrode sheet, the first negative electrode slurry is adjusted according to Table 4-1 so that the mass proportions of silicon, the first binder, the first conductive agent, and the first dispersant in the first film layer are as shown in Table 4-2. The second negative electrode slurry is adjusted according to Table 4-1 so that the mass proportions of the second binder, the second conductive agent, and the second dispersant in the first film layer are as shown in Table 4-2.

[0286] The electrical performance of the secondary batteries of Examples 7 to 12 was tested in the same manner as in Example 1, and the test results are shown in Tables 4-3.

[0287] Table 4-1

[0288]

[0289] Table 4-2

[0290]

[0291]

[0292] Table 4-3

[0293]

[0294] In Table 4-3, γ represents the ratio of the mass percentage of the first dispersant in the first film layer to the mass percentage of the second dispersant in the second film layer; β represents the ratio of the mass percentage of the first binder in the first film layer to the mass percentage of the second binder in the second film layer; and δ represents the ratio of the mass percentage of the first conductive agent in the first film layer to the mass percentage of the second conductive agent in the second film layer.

[0295] The data in Tables 4-1 to 4-3 show that when the mass percentage of silicon in the first film layer is 0% to 40% and the mass percentage of silicon in the second film layer is 2.5% to 68%, the battery can balance energy density and fast charging performance.

[0296] Examples 13 and 14

[0297] The battery preparation methods in Examples 13 and 14 are similar to those in Example 1. The difference lies in the preparation step of the negative electrode sheet, where the mass ratio of the first negative electrode slurry and the second negative electrode slurry is adjusted according to Table 5.

[0298] The electrical performance of the secondary batteries of Examples 13 and 14 was tested in the same manner as in Example 1, and the test results are shown in Table 5.

[0299] Table 5

[0300]

[0301] The data in Table 5 shows that when the ratio of the thickness of the first film layer to the thickness of the second film layer is (55-87):(13-45), the thickness of the first film layer is 27.5μm to 43.5μm, and the thickness of the second film layer is 6.5μm to 22.5μm, the secondary battery can balance fast charging performance and energy density.

[0302] 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, comprising a negative electrode, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer includes a first film layer and a second film layer arranged in a stacked manner. The mass percentage of silicon in the second film layer is greater than that in the first film layer, and after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than that in the first film layer.

2. The secondary battery according to claim 1, characterized in that, The second membrane layer is located on the side of the first membrane layer away from the negative electrode current collector.

3. The secondary battery according to claim 1 or 2, characterized in that, The silicon element in the first film layer has a mass percentage of 0% to 40%. And / or, The silicon element in the second film layer accounts for 2.5% to 68% of the total mass.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The mass percentage of silicon in the first film layer is 0% to 20%. And / or, The silicon element in the second film layer accounts for 5.5% to 48% of the total mass.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The mass percentage of silicon in the first film layer is 0% to 5%. And / or, The silicon element in the second film layer accounts for 10% to 40% of the total mass.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, After the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the first film layer is greater than 0% and less than 0.48%. And / or, After the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is 0.05% to 0.48%.

7. The secondary battery according to any one of claims 1 to 6, characterized in that, The first film layer comprises a silicon-based material, or both the first film layer and the second film layer independently comprise silicon-based materials.

8. The secondary battery according to claim 7, characterized in that, The silicon-based material includes one or more of elemental silicon, silicon-oxygen complexes, and silicon-carbon complexes.

9. The secondary battery according to claim 8, characterized in that, The silicon-based material includes at least the silicon-carbon composite.

10. The secondary battery according to claim 9, characterized in that, The silicon-carbon composite satisfies one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon; (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or silicon material; (3) The silicon element in the silicon-carbon composite has a mass ratio of 30% to 70%; (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm; (5) The silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g to 6.7m 2 / g.

11. The secondary battery according to any one of claims 1 to 10, characterized in that, The first film layer includes a first dispersant, wherein the mass percentage of the first dispersant in the first film layer is greater than 0% and less than 4.5%. And / or, The second film layer includes a second dispersant, wherein the mass percentage of the second dispersant in the second film layer is 0.5% to 4.5%.

12. The secondary battery according to claim 11, characterized in that, The first dispersant comprises one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. And / or, The second dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate.

13. The secondary battery according to claim 11 or 12, characterized in that, In the negative electrode film layer, the total mass percentage of the first dispersant and the second dispersant is greater than 0% and less than 4.5%.

14. The secondary battery according to any one of claims 11 to 13, characterized in that, The mass percentage of the second dispersant in the second film layer is greater than the mass percentage of the first dispersant in the first film layer.

15. The secondary battery according to any one of claims 11 to 14, characterized in that, In the first film layer, the mass ratio of the first dispersant to the mass ratio of the second dispersant in the second film layer is (1-4):(6-9).

16. The secondary battery according to any one of claims 1 to 15, characterized in that, The first film layer includes a first adhesive, wherein the mass percentage of the first adhesive in the first film layer is 0.5% to 4.5%. And / or, The second film layer includes a second adhesive, wherein the mass percentage of the second adhesive in the second film layer is 0.5% to 4.5%.

17. The secondary battery according to claim 16, characterized in that, The first adhesive includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan; And / or, The second adhesive includes one or more of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan.

18. The secondary battery according to claim 16 or 17, characterized in that, The second adhesive comprises styrene-butadiene rubber and / or polyacrylic acid.

19. The secondary battery according to any one of claims 16 to 18, characterized in that, In the negative electrode film layer, the total mass percentage of the first binder and the second binder is 0.5% to 4.5%.

20. The secondary battery according to any one of claims 16 to 19, characterized in that, The mass percentage of the second adhesive in the second film layer is greater than the mass percentage of the first adhesive in the first film layer.

21. The secondary battery according to any one of claims 16 to 20, characterized in that, The ratio of the mass percentage of the first adhesive in the first film layer to the mass percentage of the second adhesive in the second film layer is (1-4):(6-9).

22. The secondary battery according to any one of claims 1 to 21, characterized in that, The first film layer includes a first conductive agent, wherein the mass percentage of the first conductive agent in the first film layer is 0.05% to 2%. And / or, The second film layer includes a second conductive agent, wherein the mass percentage of the second conductive agent in the second film layer is 0.05% to 2%.

23. The secondary battery according to claim 22, characterized in that, The first conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. And / or, The second conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

24. The secondary battery according to claim 22 or 23, characterized in that, The second conductive agent comprises carbon black and / or carbon nanotubes.

25. The secondary battery according to any one of claims 22 to 24, characterized in that, In the negative electrode film layer, the total mass percentage of the first binder and the second binder is 0.05% to 2%.

26. The secondary battery according to any one of claims 22 to 25, characterized in that, The mass percentage of the second conductive agent in the second film layer is greater than the mass percentage of the first conductive agent in the first film layer.

27. The secondary battery according to any one of claims 22 to 26, characterized in that, The ratio of the mass percentage of the first conductive agent in the first film layer to the mass percentage of the second conductive agent in the second film layer is (4-1):(6-9).

28. The secondary battery according to any one of claims 7 to 27, characterized in that, The first film layer further includes a first graphite material, and the second film layer further includes a second graphite material. The graphitization degree of the first graphite material is greater than that of the second graphite material.

29. The secondary battery according to claim 28, characterized in that, The first graphite material satisfies at least one of the following: (1) The graphitization degree of the first graphite material is 91% to 96%; (2) The average particle size of the first graphite material is 5 μm to 25 μm; (3) The BET specific surface area of ​​the first graphite material is 0.8 m². 2 / g to 1.3m 2 / g; (4) The tap density of the first graphite material is 1.1 g / cm³. 3 Up to 1.6 g / cm 3 ; (5) The compacted density of the first graphite material under a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 2.2 g / cm 3 .

30. The secondary battery according to claim 28 or 29, characterized in that, The second graphite material satisfies at least one of the following: (1) The graphitization degree of the second graphite material is 91% to 94%; (2) The average particle size of the second graphite material is 8 μm to 16 μm; (3) The BET specific surface area of ​​the second graphite material is 0.8 m². 2 / g to 1.4m 2 / g; (4) The tap density of the second graphite material is 1.3 g / cm³. 3 Up to 1.5g / cm 3 ; (5) The compacted density of the second graphite material under a pressure of 50,000 N is 1.6 g / cm³. 3 Up to 1.7 g / cm 3 .

31. The secondary battery according to any one of claims 1 to 30, characterized in that, The negative electrode film layer satisfies at least one of the following: (1) The compaction density of the negative electrode film is 1.55 g / cm³. 3 Up to 1.75 g / cm 3 ; (2) The thickness of the negative electrode film is 34 μm to 66 μm; (3) The adhesion between the negative electrode film layer and the negative electrode current collector is 13 N / m to 20 N / m.

32. The secondary battery according to any one of claims 1 to 31, characterized in that, The thickness of the first film layer is greater than the thickness of the second film layer.

33. The secondary battery according to any one of claims 1 to 32, characterized in that, The ratio of the thickness of the first film layer to the thickness of the second film layer is (55-87):(13-45).

34. The secondary battery according to any one of claims 1 to 33, characterized in that, The thickness of the first film layer is 27.5 μm to 43.5 μm. And / or, The thickness of the second film layer is 6.5 μm to 22.5 μm.

35. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a first film layer and a second film layer arranged in a stacked manner. The mass percentage of silicon in the second film layer is greater than that in the first film layer, and after the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is greater than that in the first film layer.

36. The secondary battery according to claim 35, characterized in that, The second membrane layer is located on the side of the first membrane layer away from the negative electrode current collector.

37. The negative electrode sheet according to claim 35 or 36, characterized in that, The silicon element in the first film layer has a mass percentage of 0% to 40%. And / or, The silicon element in the second film layer accounts for 2.5% to 68% of the total mass.

38. The negative electrode sheet according to any one of claims 35 to 37, characterized in that, The mass percentage of silicon in the first film layer is 0% to 20%. And / or, The silicon element in the second film layer accounts for 5.5% to 48% of the total mass.

39. The secondary battery according to any one of claims 35 to 38, characterized in that, The mass percentage of silicon in the first film layer is 0% to 5%. And / or, The silicon element in the second film layer accounts for 10% to 40% of the total mass.

40. The negative electrode sheet according to any one of claims 35 to 39, characterized in that, After the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the first film layer is greater than 0% and less than 0.48%, and / or, After the voltage of the secondary battery reaches the discharge cutoff voltage, the mass percentage of lithium in the second film layer is 0.05% to 0.48%.

41. The negative electrode sheet according to any one of claims 35 to 40, characterized in that, The first film layer comprises a silicon-carbon composite, or both the first and second film layers comprise silicon-carbon composites, wherein the silicon-carbon composite satisfies one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon. (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or silicon material; (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass; (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm; (5) The silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g to 6.7m 2 / g.

42. The negative electrode sheet according to any one of claims 35 to 41, characterized in that... The first film layer includes a first dispersant, wherein the mass percentage of the first dispersant in the first film layer is greater than 0.5% and less than 4.5%. And / or, The second film layer includes a second dispersant, wherein the mass percentage of the second dispersant in the second film layer is 0.5% to 4.5%.

43. The negative electrode sheet according to claim 42, characterized in that, The first dispersant comprises one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate. And / or, The second dispersant includes one or more of lithium carboxymethyl cellulose salt, poly(lithium acrylate), poly(lithium methacrylate), poly(lithium acrylamide), or lithium alkyl phosphate.

44. An electrical appliance, characterized in that, It includes the secondary battery according to any one of claims 1 to 34 or the negative electrode sheet according to any one of claims 35 to 43.

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