A negative electrode sheet and a lithium ion battery

Abstract

The application relates to the new energy technology field, in particular to a negative electrode sheet and a lithium ion battery. The negative electrode sheet is designed in three layers. The carbon-based negative active material is arranged in the negative active layer close to the current collector. The carbon-based negative active material is arranged in the negative active layer close to the diaphragm. The carbon-based negative active material and the silicon-carbon negative active material are arranged in the negative active layer in the middle. The design can avoid the direct contact between the silicon-carbon negative active material and the diaphragm, avoid the risk of piercing the diaphragm, the silicon-carbon negative active material is arranged in the middle of the negative active layer, the migration path of lithium ions in the silicon-carbon during the charging and discharging process can be shortened to a certain extent, the polarization is reduced, the cycle life of the lithium battery is guaranteed, meanwhile, the limitation of the relationship between the silicon-carbon particle Dv95 and the thickness of the current collector and the particle size distribution of the silicon-carbon can effectively prevent the fracture of the current collector caused by the volume expansion of the silicon-carbon particles, and the safety and high-temperature performance are improved.

Description

Technical Field

[0001] This application relates to the field of new energy technology, specifically to a negative electrode and a lithium-ion battery. Background Technology

[0002] The booming development of the new energy industry has brought great convenience to people's lives. As the industry matures, how to further improve the range, lifespan, fast-charging performance, and safety performance of new energy batteries has gradually become a research hotspot in the power battery industry.

[0003] Currently, the battery design system used in the power battery industry mainly uses lithium iron phosphate or lithium nickel cobalt manganese oxide cathodes paired with artificial graphite anodes. This system suffers from limited improvement in battery energy density due to the low specific capacity of both cathode and anode materials. For lithium-ion battery cathode materials, lithium iron phosphate and lithium nickel cobalt manganese oxide offer relatively small capacity increases. However, for anode materials, silicon-containing anode active materials, compared to graphite, can significantly increase the anode-side capacity, thereby reducing the mass and thickness of the anode sheet and improving battery energy density. However, pure silicon anodes suffer from volume expansion during charging, leading to structural damage to the anode active material and poorer charge-discharge cycles, limiting their application scenarios. Depositing nano-silicon particles into the porous structure of hard carbon pores via chemical vapor deposition (silicon-carbon anode material) can solve the volume expansion problem of silicon anodes and improve the material's charge-discharge cycle capability. However, silicon-carbon material particles have high mechanical hardness and many sharp edges, easily puncturing the separator when in contact with it on the anode side, leading to short circuits, poor self-discharge performance, and potential safety hazards. The problem of silicon-carbon particles piercing the separator can be solved by using double-layer or multi-layer coating on the negative electrode, placing the silicon-carbon particles at the end closer to the current collector and pure graphite at the end closer to the separator. However, this negative electrode design results in an excessively long lithium-ion migration path during charging and discharging. Since the silicon-carbon negative electrode itself has poor electronic and ion conductivity, placing the silicon-carbon material in the lower layer of the negative electrode leads to significant polarization during charging and discharging, making it difficult for the silicon-carbon negative electrode to achieve its intrinsic capacity. This results in rapid capacity decay during lithium battery charge-discharge cycles, and the volume expansion of the silicon-carbon particles can cause excessive stretching and breakage of the current collector, compromising its lifespan. Summary of the Invention

[0004] This application provides a negative electrode sheet and a lithium-ion battery to solve the problems of silicon-containing negative electrodes causing excessive stretching of the current collector plane due to volume expansion during charging and discharging, resulting in breakage and affecting service life; silicon carbon particles piercing the separator, causing battery short circuits and poor self-discharge performance; and the inability to simultaneously maintain ion conduction performance.

[0005] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active layer disposed on at least one side surface of the current collector. The negative electrode active layer includes a first negative electrode active layer, a second negative electrode active layer and a third negative electrode active layer disposed sequentially. The third negative electrode active layer is disposed on the side close to the current collector, and the second negative electrode active layer is disposed between the first negative electrode active layer and the third negative electrode active layer. Wherein, the first negative electrode active layer includes a first carbon-based negative electrode active material, and the third negative electrode active layer includes a third carbon-based negative electrode active material; The second negative electrode active layer includes a second carbon-based negative electrode active material and a silicon-carbon negative electrode active material. The particle size distribution range of the silicon-carbon negative electrode active material is Span=(Dv90-Dv10) / Dv50, 0.5≤Span≤1.0; the particle size Dv95 of the silicon-carbon negative electrode active material is μm. The thickness of the current collector is W μm; and the following relationship exists between P and W: 1.0≤P / W≤3.0.

[0006] In one optional implementation, the values ​​of P and W satisfy the following ranges: 11≤P≤15, 5≤W≤11.

[0007] In one optional embodiment, the mass percentage of the silicon-carbon anode active material is X%, based on the total mass of the anode active layer. The thickness of the first negative electrode active layer is Y μm, the thickness of the second negative electrode active layer is Z μm, and the total thickness of the negative electrode active layer is M μm; The following relationship exists between X, Y, Z, and M: 0.1≤X / (Y+Z)≤0.2, 0.25≤Y / Z≤0.75, 0.625≤(Y+Z) / M≤1.0.

[0008] In one optional implementation, the values ​​of X, Y, Z, and M satisfy the following ranges: 5≤X≤30, 20≤Y≤60, 30≤Z≤80, and 80≤M≤200.

[0009] In one optional embodiment, the particle size Dv50 of the first carbon-based negative electrode active material is 5-10 μm; And / or, the particle size Dv50 of the second carbon-based anode active material is 10-15 μm, the particle size Dv50 of the silicon-carbon anode active material is 5-10 μm, and the mass percentage of silicon in the silicon-carbon anode active material is 40%-60%; And / or, the particle size Dv50 of the third carbon-based negative electrode active material is 10-20 μm; And / or, the first carbon-based negative electrode active material, the second carbon-based negative electrode active material, and the third carbon-based negative electrode active material independently include at least one of graphite, hard carbon, soft carbon, and mesophase carbon microspheres; optionally, the graphite includes at least one of artificial graphite and natural graphite. And / or, when the first negative electrode active layer, the second negative electrode active layer, and the third negative electrode active layer independently include graphite, the graphite in the first negative electrode active layer and the second negative electrode active layer includes a carbon coating layer, and the graphite in the third negative electrode active material layer does not include a carbon coating layer, or is a mixture of graphite without a carbon coating layer and graphite with a carbon coating layer.

[0010] In an optional embodiment, the XRD diffraction peak 004 of the first carbon-based anode active material has a crystal plane spacing of 0.1680-0.1683 nm, the XRD diffraction peak 004 of the second carbon-based anode active material has a crystal plane spacing of 0.1678-0.1681 nm, the crystal size of the XRD diffraction peak 111 of the silicon-carbon anode active material in the second anode active layer is 2-10 nm, and the XRD diffraction peak 002 of the third anode active material has a crystal plane spacing of 0.3358-0.3356 nm. And / or, the resistivity of the negative electrode is R1Ω·m, and satisfies 0.5≤R1≤1.5.

[0011] In one optional embodiment, the first negative electrode active layer, the second negative electrode active layer, and the third negative electrode active layer independently include a conductive agent and a binder; Optionally, the adhesive includes at least one of polyvinylidene fluoride (PVDF), lithium polyacrylate (PAA-Li), polyimide (PI), styrene-butadiene rubber (SBR), and lithium carboxymethyl cellulose (CMC-Li); Optionally, the conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene. Further optionally, the binder in the first negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass percentage of the binder in the first negative electrode active layer is 2.0%-3.5% of the total mass of the first negative electrode active layer. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). Further optionally, the binder in the second negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass ratio of the binder in the second negative electrode active layer to the total mass of the second negative electrode active layer is 2.5%-5.0%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:3:4). The binder in the third negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber. The mass percentage of the binder in the third negative electrode active layer to the total mass of the first negative electrode active layer is 2.0%-3.5%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). Further optionally, the conductive agent in the second negative electrode active layer includes acetylene black and single-walled carbon nanotubes, wherein the mass percentage of acetylene black in the second negative electrode active layer is 0.5-1.5%, and the mass percentage of single-walled carbon nanotubes in the second negative electrode active layer is 0.05-0.5%.

[0012] In one optional embodiment, the current collector further includes a carbon coating layer on both sides; Optionally, the thickness of the carbon coating layer is 0.5-1.5 μm; Optionally, the carbon coating layer includes acetylene black; Alternatively, the current collector includes an insulating layer and a conductive layer, the conductive layer being located on at least one surface of the insulating layer, the material of the conductive layer being selected from at least one of copper, titanium, silver, and nickel-copper alloy; the material of the insulating layer is selected from at least one of organic polymer insulating materials, inorganic insulating materials, and organic-inorganic composite insulating materials.

[0013] Secondly, this application provides a lithium-ion battery, including a positive electrode, a separator, and a negative electrode, wherein the positive electrode and the negative electrode are stacked, the separator is disposed between the positive electrode and the negative electrode, and the negative electrode is the aforementioned negative electrode. And / or, the negative electrode has one more layer than the positive electrode.

[0014] In one alternative embodiment, an electrolyte is also included, the electrolyte comprising a solvent, a lithium salt, and additives; Optionally, the additive includes at least one of fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), vinylene sulfate (DTD), and methylene disulfonate (MMDS); Further optionally, the electrolyte comprises 0.1-1% by mass of fluoroethylene carbonate (FEC), 0.5-7% of lithium difluorophosphate (LiPO2F2), 0.1-1% of vinylene sulfate (DTD), and 0.1-1% of methylene disulfonate (MMDS).

[0015] The technical solution of this application has the following advantages: The negative electrode sheet provided in this application adopts a three-layer design. The negative electrode active layer near the current collector contains carbon-based negative electrode active material, the negative electrode active layer near the separator contains carbon-based negative electrode active material, and the negative electrode active layer in the middle contains both carbon-based negative electrode active material and silicon-carbon negative electrode active material. This design can avoid direct contact between the silicon-carbon negative electrode active material and the separator, avoiding the risk of puncturing the separator. The silicon-carbon negative electrode active material is located in the middle of the negative electrode active layer, which can shorten the migration path of lithium ions during charging and discharging to a certain extent, reduce polarization, and ensure the cycle life of the lithium battery. At the same time, with the limitation of the relationship between the silicon-carbon particle Dv95 and the current collector thickness, as well as the limitation of the silicon-carbon particle size distribution, it can effectively prevent the current collector from breaking due to excessive extension caused by the volume expansion of silicon-carbon particles, thus improving safety and high-temperature performance.

[0016] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the negative electrode sheet in Embodiment 1 of this application; Explanation of reference numerals in the attached figures: 1. Copper foil; 2. Carbon coating layer; 3. Third negative electrode active layer; 4. Second negative electrode active layer; 41. Carbon-based negative electrode active material; 42. Silicon-carbon negative electrode active material; 5. First negative electrode active layer; 6. Separator.

[0019] The numbering method of the attached figures is as follows: A device includes: Component A, which is represented by 1. Component A includes three parts, which are represented by 101, 102 and 103 respectively. If the part represented by the number 101 includes three more parts, it is represented by 1011, 1012 and 1013. Detailed Implementation

[0020] The following embodiments are provided to better understand this application. However, the following embodiments do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining the features of this application with other prior art, falls within the scope of protection of this application.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having” and any variations thereof in the text of this application are intended to cover non-exclusive inclusion.

[0022] In the description of the embodiments of this application, the technical terms "first", "second", etc. are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0023] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] 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 the specific 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. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to disclosing that the parameter can be, for example, integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0025] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0026] In the description of the embodiments of this application, the term "at least one" refers to one or more (including two).

[0027] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, but sequentially is preferred.

[0028] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0029] As described in the background section, existing negative electrode designs do not address the impact of silicon-carbon material volume expansion on the current collector's planar extension during charging. Larger silicon-carbon particles experience greater volume expansion, which in turn has a greater impact on the current collector's extension. Therefore, to avoid the influence of silicon-carbon particles on the current collector and separator, and to ensure their various electrical performance characteristics, this application provides the following technical solution.

[0030] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active layer disposed on at least one side surface of the current collector. The negative electrode active layer includes a first negative electrode active layer, a second negative electrode active layer and a third negative electrode active layer disposed sequentially. The third negative electrode active layer is disposed on the side close to the current collector, and the second negative electrode active layer is disposed between the first negative electrode active layer and the third negative electrode active layer. Wherein, the first negative electrode active layer includes a first carbon-based negative electrode active material, and the third negative electrode active layer includes a third carbon-based negative electrode active material; The second negative electrode active layer includes a second carbon-based negative electrode active material and a silicon-carbon negative electrode active material. The particle size distribution range of the silicon-carbon negative electrode active material is Span=(Dv90-Dv10) / Dv50, 0.5≤Span≤1.0; the particle size Dv95 of the silicon-carbon negative electrode active material is μm. The thickness of the current collector is W μm; and the following relationship exists between P and W: 1.0≤P / W≤3.0.

[0031] As an example, the particle size distribution range Span of the silicon-carbon anode active material can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or within any of the above values; the P / W ratio can be 1.0, 1.3, 1.5, 1.7, 2.0, 2.2, 2.5, 2.8, 3, or within any of the above values.

[0032] Based on the aforementioned technical means, this application employs a three-layer coating process in the negative electrode design. The negative electrode active layer closest to the current collector contains carbon-based negative electrode active material, as does the negative electrode active layer near the separator. The middle negative electrode active layer includes both carbon-based and silicon-carbon negative electrode active materials. This design avoids direct contact between the silicon-carbon negative electrode active material and the separator, preventing the risk of separator puncture. Furthermore, the silicon-carbon negative electrode active material being located in the middle of the negative electrode active material layers can, to some extent, shorten the migration path of lithium ions during charging and discharging, reducing polarization and ensuring the cycle life of the lithium battery. However… Because silicon doping causes significant volume expansion and poor conductivity of the silicon-carbon anode active material during charging, it is necessary to control the thickness range of both the silicon-carbon anode active material and the current collector. The extension of the current collector is directly proportional to its thickness. This application effectively prevents current collector breakage due to excessive extension caused by the volume expansion of silicon-carbon particles by defining the relationship between the larger particle size Dv95 of the silicon-carbon particles and the thickness of the current collector. Silicon-carbon particles with larger Dv95 should use current collectors with larger thicknesses, while silicon-carbon particles with smaller particle sizes should use current collectors with smaller thicknesses. This effectively prevents electrode breakage caused by excessive extension of the current collector. The Span value represents the uniformity of particle size distribution. The smaller the value, the smaller the difference between particle sizes Dv10, Dv50, and Dv90, indicating a high degree of particle size uniformity. Conversely, a larger value indicates an uneven particle size distribution. Since silicon-carbon materials expand significantly during charging, the difference in particle size results in smaller particles expanding less and larger particles expanding more. Controlling the Span value can ensure a uniform size distribution of silicon-carbon particles, thereby ensuring a better uniformity of volume expansion of silicon-carbon particles in the negative electrode and improving the stability of battery charge-discharge cycles. If the P / W value is less than 1, it indicates that the silicon-carbon anode active material has a small particle size and a large current collector thickness. This leads to low anode capacity and a high proportion of current collector in the entire anode system, resulting in low cell energy density. Small silicon-carbon particles have many defects and charge / discharge side reactions, leading to poor battery cycle performance. If the P / W value is higher than 3, the silicon-carbon anode active material has a large particle size and a short current collector thickness. In this case, the silicon-carbon particles have a long bulk ion and electron transport distance, resulting in poor fast charging kinetics. The short current collector thickness further increases the charge / discharge polarization of the anode, leading to poor fast charging kinetics and affecting the battery's fast charging performance. A Span value less than 0.5 exceeds the limits of material processing equipment, making actual production difficult. A Span value higher than 1.0 indicates poor uniformity of silicon-carbon particle size distribution, which will worsen battery cycle performance.

[0033] It should be noted that the particle sizes Dv10, Dv50, Dv90, and Dv95 were all measured by laser method using Mastersize 3000 (Malvin 3000).

[0034] In one optional implementation, the values ​​of P and W satisfy the following ranges: 11≤P≤15, 5≤W≤11.

[0035] As an example, the particle size Dv95 of the silicon-carbon anode active material can be 11μm, 12μm, 13μm, 14μm, 15μm, or within any of the above values; the thickness of the current collector can be 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, or within any of the above values.

[0036] Based on the aforementioned technical methods, limiting the particle size Dv95 of the silicon-carbon anode active material to the range described above can achieve a better balance between battery cycle performance and high-temperature performance. The P value represents the larger particle size of the silicon-carbon particles; a larger value indicates larger silicon-carbon particles, resulting in greater volume expansion and affecting battery cycle performance. A smaller value indicates smaller silicon-carbon particles, leading to higher surface defects and affecting high-temperature performance. Similarly, by limiting the thickness of the current collector to the range described above, a better balance between energy density and fast-charging capability can be achieved. The W value represents the thickness of the current collector; a larger value indicates a higher proportion of its mass and volume in the cell, resulting in greater energy density loss. Conversely, a smaller value indicates a thinner current collector, leading to a corresponding decrease in the current density it can withstand and a reduction in the fast-charging capability of the anode.

[0037] In one optional embodiment, the mass percentage of the silicon-carbon anode active material is X%, based on the total mass of the anode active layer. The thickness of the first negative electrode active layer is Y μm, the thickness of the second negative electrode active layer is Z μm, and the total thickness of the negative electrode active layer is M μm; The following relationship exists between X, Y, Z, and M: 0.1≤X / (Y+Z)≤0.2, 0.25≤Y / Z≤0.75, 0.625≤(Y+Z) / M≤1.0.

[0038] As an example, X / (Y+Z) can take values ​​of 0.1, 0.12, 0.14, 0.15, 0.17, 0.19, 0.2, or any of the above values; Y / Z can take values ​​of 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, or any of the above values; and (Y+Z) / M can take values ​​of 0.625, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.75, 0.8, 0.85, 0.9, 1.0, or any of the above values.

[0039] According to the above technical means, when the content of silicon-carbon anode active material in the electrode is low, the coating thickness of the first and second anode active layers accounts for a low proportion of the overall anode thickness. This can also ensure that the silicon-carbon anode active material is evenly distributed in the second anode active layer. As the content of silicon-carbon anode active material in the electrode increases, the proportion of the coating thickness of the first and second anode active layers increases. This can ensure that more silicon-carbon anode active material can be evenly distributed in the coating, ensuring the cycle stability of the anode. However, the thickness of the third anode active layer should not be too low. If the silicon-carbon anode active material is too close to the current collector and too far from the separator, it will increase the charge and discharge polarization of the silicon-carbon anode active material, resulting in increased impedance and decreased fast charging dynamics of the anode.

[0040] In one optional implementation, the values ​​of X, Y, Z, and M satisfy the following ranges: 5≤X≤30, 20≤Y≤60, 30≤Z≤80, and 80≤M≤200.

[0041] As an example, based on the total mass of the negative electrode active layer, the mass percentage of the silicon-carbon negative electrode active material can be 5%, 10%, 15%, 20%, 25%, 28%, 30%, or within any range of the above values; the thickness (Y) of the first negative electrode active layer can be 20μm, 30μm, 40μm, 50μm, 60μm, or within any range of the above values; the thickness (Z) of the second negative electrode active layer can be 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, or within any range of the above values; the total thickness (M) of the negative electrode active layer can be 80μm, 100μm, 120μm, 150μm, 170μm, 190μm, 200μm, or within any range of the above values.

[0042] In this application, X represents the mass percentage of silicon-carbon material in the negative electrode sheet. A smaller X value indicates less silicon-carbon content, resulting in lower negative electrode capacity and limited improvement in battery energy density. A larger X value indicates a higher silicon-carbon content in the negative electrode sheet, leading to a greater improvement in battery energy density. However, due to the large volume expansion of silicon-carbon material during charging, excessive use of silicon-carbon can lead to poor structural stability of the negative electrode active material layer, resulting in poor battery cycle performance. Y represents the thickness of the first negative electrode active layer in the three-layer coating. A lower thickness indicates a smaller proportion of the first negative electrode active layer, reducing the battery's fast-charging capability and causing lithium plating. A larger Y value indicates a larger thickness of the first negative electrode active layer. The negative electrode active layer uses graphite particles with small particle sizes, resulting in lower capacity and compaction, and a higher degree of surface defects. This reduces the battery's energy density and deteriorates its high-temperature performance. The Z-value represents the thickness of the second negative electrode active layer in the three-layer coating. A thicker second negative electrode active layer worsens the battery's fast-charging performance, while a thinner layer leaves less space for silicon-carbon distribution within the coating, hindering uniform distribution of silicon-carbon and carbon-based active materials, thus affecting the battery's charge-discharge cycle performance. M represents the overall thickness of the negative electrode sheet. A larger thickness reduces the battery's fast-charging capability, while a smaller thickness is detrimental to improving energy density. The limitations imposed on these parameters in this application achieve a better balance between energy density, high-temperature performance, and fast-charging performance.

[0043] In one optional embodiment, the particle size Dv50 of the first carbon-based negative electrode active material is 5-10 μm; In one optional embodiment, the particle size Dv50 of the second carbon-based anode active material is 10-15 μm, the particle size Dv50 of the silicon-carbon anode active material is 5-10 μm, and the mass percentage of silicon in the silicon-carbon anode active material is 40%-60%. In one optional embodiment, the particle size Dv50 of the third carbon-based negative electrode active material is 10-20 μm; As an example, the particle size Dv50 of the first carbon-based anode active material can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or within any range of the above values; the particle size Dv50 of the second carbon-based anode active material can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or within any range of the above values; the particle size Dv50 of the silicon-carbon anode active material can be 5 μm, 6 μm, ... The particle size of the third carbon-based anode active material can be 7μm, 8μm, 9μm, 10μm, or within any range of the above values; the mass percentage of silicon in the silicon-carbon anode active material can be 40%, 45%, 50%, 55%, 60%, or within any range of the above values; the particle size Dv50 of the third carbon-based anode active material can be 10μm, 12μm, 14μm, 15μm, 17μm, 19μm, 20μm, or within any range of the above values.

[0044] Based on the aforementioned technical means, this application has appropriately matched the particle sizes of the active materials in the three negative electrode active layers. In a lithium battery system, the first negative electrode active layer is located closest to the separator, and has the lowest lithium potential during charging, making it most prone to lithium plating and thus posing a safety risk. Therefore, the first negative electrode active layer needs to use carbon-based negative electrode active materials with smaller particle sizes. Simultaneously, the lithium ion transport path within the particles is short, resulting in better fast charging capability and reducing the likelihood of lithium plating. Larger particle sizes lead to poorer fast charging, while excessively small particle sizes result in excessively high reactivity and more side reactions, affecting the battery's high-temperature performance. The selection of carbon-based negative electrode active materials in the second negative electrode active layer requires the use of larger particle sizes. Since the second negative electrode active layer is a design combining carbon-based active materials with silicon-carbon, the conductivity of the silicon-carbon negative electrode active material itself... The first anode layer has poor fast-charging capability and requires a carbon-based anode active material with better fast-charging performance. Therefore, its particle size cannot be too large. Furthermore, when paired with silicon-carbon, silicon-carbon itself has low compaction density, and the carbon-based anode active material needs to have a certain compaction and capacity (the smaller the particle size of the carbon-based anode active material, the lower the capacity and compaction density). Therefore, the particle size of the carbon-based anode active material cannot be too small, otherwise it will affect the battery's energy density. The third anode active layer is located close to the current collector, and its potential is lowest during charging, making it least prone to lithium plating. Therefore, the carbon-based anode active material for the third anode active layer can be graphite with a larger particle size, which has higher capacity and compaction performance, as well as better high-temperature performance. Smaller particle sizes will result in a loss of battery energy density and worsened high-temperature performance, while excessively large particle sizes will also adversely affect the battery's fast-charging performance. In silicon-carbon anode active materials, the low silicon content results in lower specific capacity and lower anode energy density. Conversely, higher silicon content leads to poorer charge-discharge cycle stability.

[0045] In one optional embodiment, the first carbon-based negative electrode active material, the second carbon-based negative electrode active material, and the third carbon-based negative electrode active material independently include at least one of graphite, hard carbon, soft carbon, and mesophase carbon microspheres; optionally, the graphite includes at least one of artificial graphite and natural graphite. In an optional embodiment, when the first carbon-based negative electrode active material, the second negative electrode active layer, and the third negative electrode active layer independently include graphite, the graphite in the first negative electrode active material layer and the second negative electrode active layer includes a carbon coating layer, and the graphite in the third negative electrode active material layer does not include a carbon coating layer, or is a mixture of graphite without a carbon coating layer and graphite with a carbon coating layer.

[0046] Based on the above technical means, graphite materials have better fast charging performance compared to other carbon-based materials. The carbon coating layer of the active material in the first negative electrode active layer is conducive to improving the fast charging capability. In the second negative electrode active layer, due to the mixing of silicon and carbon, the conductivity of the silicon and carbon negative electrode active material itself is poor, resulting in poor fast charging capability. Therefore, carbon-coated graphite is also required to provide fast charging capability. The third negative electrode active layer does not have high requirements for fast charging capability and can use uncoated carbon-based materials, while improving the high-temperature performance of the overall negative electrode sheet.

[0047] In an optional embodiment, the XRD diffraction peak 004 of the first carbon-based anode active material has a crystal plane spacing of 0.1680-0.1683 nm, the XRD diffraction peak 004 of the second carbon-based anode active material has a crystal plane spacing of 0.1678-0.1681 nm, the crystal size of the XRD diffraction peak 111 of the silicon-carbon anode active material in the second anode active layer is 2-10 nm, and the XRD diffraction peak 002 of the third carbon-based anode active material has a crystal plane spacing of 0.3358-0.3356 nm. Based on the aforementioned technical methods, if the 004 crystal plane spacing of the first carbon-based anode active material is too large, there will be too many defects, affecting high-temperature performance; if the crystal plane spacing is too small, the fast-charging capability will be poor, and the battery will easily plaque lithium during charging. If the 004 crystal plane spacing of the second carbon-based anode active material is too large, its capacity compaction density will be low, resulting in energy density loss and deteriorating high-temperature performance. If the 004 crystal plane spacing is too small, the fast-charging performance will be poor, and the mixing with silicon and carbon will worsen the cycle performance. The grain size of the 111 XRD diffraction peak of the silicon-carbon anode active material represents the smallest unit of silicon particles in the silicon-carbon anode active material. If its size is large, the volume expansion during charging will be large, and the fast-charging capability will be poor, affecting the battery's fast-charging cycle performance. If the silicon grain size is small, its fast-charging performance will be better, and the volume expansion will be lower, but the degree of defects will be high, and the high-temperature stability will be poor, affecting the battery's high-temperature performance. If the 002 crystal plane spacing of the third carbon-based anode active material is too large, the capacity compaction will be low, and the energy density will be low. If the crystal plane spacing is too small, the fast-charging capability will be poor, and the battery's cycle performance will be deteriorated.

[0048] In one alternative embodiment, the resistivity of the negative electrode is R1 Ω·m, and satisfies 0.5 ≤ R1 ≤ 1.5. As an example, the resistivity of the negative electrode is 0.5 Ω·m, 0.7 Ω·m, 0.9 Ω·m, 1.0 Ω·m, 1.3 Ω·m, 1.5 Ω·m, or within any range of the above values.

[0049] Based on the above technical means, the resistivity of the entire negative electrode active layer should also be controlled within a certain range. If the resistivity is too small, the carbon-based active materials will be in close contact with each other and there will be fewer pores, which will affect the subsequent liquid absorption capacity of the negative electrode sheet and thus affect the fast charging performance of the battery. On the other hand, if the resistivity of the active layer is too large, it indicates that the contact between the carbon-based active materials is poor, and the conductivity and the distribution of the binder and silicon-carbon negative electrode in the electrode sheet are uneven, which will also affect the cycle performance and high-temperature performance of the battery.

[0050] It should be noted that the resistance can be adjusted by adjusting the particle size of the carbon-based anode active material in the active layer. The smaller the carbon-based anode active material particles, the higher its defect level and the higher its resistivity. The larger the carbon-based anode active material particles, the lower its defect level and the lower its resistivity.

[0051] In one optional embodiment, the first negative electrode active layer, the second negative electrode active layer, and the third negative electrode active layer independently include a conductive agent and a binder; Optionally, the adhesive includes at least one of polyvinylidene fluoride (PVDF), lithium polyacrylate (PAA-Li), polyimide (PI), styrene-butadiene rubber (SBR), and lithium carboxymethyl cellulose (CMC-Li); Optionally, the conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene. Further optionally, the binder in the first negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass percentage of the binder in the first negative electrode active layer is 2.0%-3.5% of the total mass of the first negative electrode active layer. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). Further optionally, the binder in the second negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass ratio of the binder in the second negative electrode active layer to the total mass of the second negative electrode active layer is 2.5%-5.0%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:3:4). The binder in the third negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber. The mass percentage of the binder in the third negative electrode active layer to the total mass of the first negative electrode active layer is 2.0%-3.5%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). This application employs a three-layer coating design for the negative electrode. Due to the significant differences in physical properties between carbon-based and silicon-carbon negative electrode active materials, different binder combinations can be selected during material formulation and processing. The first negative electrode active layer contains pure carbon-based negative electrode active material, with lithium polyacrylate as the primary binder, accounting for a relatively high proportion. Styrene-butadiene rubber (SBR) is used as an auxiliary binder to improve the solid content of the pure carbon-based negative electrode active material slurry. Since carbon-based negative electrode active material has better slurry processability and less volume expansion during charging compared to silicon-carbon negative electrode active material, a higher proportion of lithium polyacrylate can improve the fast-charging kinetics of the first negative electrode active layer coating. Conversely, a higher proportion of SBR would deteriorate the fast-charging performance. The second negative electrode active layer coating is a carbon-based negative electrode active material mixed with silicon-carbon. Because silicon-carbon undergoes significant volume expansion during charging, SBR with strong adhesion is required to suppress this expansion. Therefore, the binder for the second negative electrode active layer must be primarily SBR. However, styrene-butadiene rubber (SBR) has poor fast-charging capability, requiring the addition of a small amount of lithium polyacrylate to adjust its fast-charging performance. Simultaneously, the slurry properties must be similar to those of the first negative electrode active layer to facilitate improved processing performance during coating. If the amount of SBR is insufficient, it cannot suppress the volume expansion during silicon-carbon charging, leading to a discontinuity between the carbon-based active material and silicon-carbon, resulting in deteriorated battery cycle performance. Lithium carboxymethyl cellulose has a thickening effect, used to adjust the affinity of the carbon-based and / or silicon-carbon negative electrode active materials for water, which helps to ensure uniform dispersion and stability of both in the slurry. The third negative electrode active layer is made of pure carbon-based active material, and the same binder ratio as the first negative electrode active layer can be selected, achieving similar results.

[0052] Further optionally, the conductive agent in the first negative electrode active layer includes acetylene black, accounting for 0-2% of the mass of the first negative electrode active layer; as an example, the percentage of the conductive agent in the mass of the first negative electrode active layer can be 0%, 0.2%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.7%, 2%, or within any range of the above values. Further optionally, the conductive agent in the third negative electrode active layer includes acetylene black, accounting for 0-2% of the mass of the third negative electrode active layer; as an example, the percentage of the conductive agent in the third negative electrode active layer can be 0%, 0.2%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.7%, 2%, or within any range of the above values; Further optionally, the conductive agent in the second negative electrode active layer includes acetylene black and single-walled carbon nanotubes, wherein the mass percentage of acetylene black in the second negative electrode active layer is 0.5-1.5%, and the mass percentage of single-walled carbon nanotubes in the second negative electrode active layer is 0.05-0.5%. As an example, the mass percentage of acetylene black in the second negative electrode active layer can be 0.5%, 0.7%, 0.9%, 1%, 1.2%, 1.4%, 1.5%, or within any range of the above values; and the mass percentage of single-walled carbon nanotubes can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or within any range of the above values.

[0053] It should be noted that in the three-layer coating design, there are certain requirements for the amount of conductive agent added. When the first and third negative electrode active layers are designed with pure carbon-based materials, acetylene black is selected as the conductive agent, with an addition amount of 0-2%. Carbon-based negative electrode active materials themselves have conductivity, especially graphite, which has good conductivity. The content of acetylene black can be 0%, but too much will reduce the energy density of the negative electrode sheet and deteriorate the high-temperature performance of the lithium battery. When the second negative electrode active layer is a carbon-based material mixed with silicon and carbon, acetylene black and single-walled sodium carbon are selected as the conductive agents. The acetylene black content is 0.5-1.5%, and the single-walled carbon nanotube content is 0.05-0.5%. Due to the presence of silicon carbon particles, their conductivity is poor. Acetylene black and single-walled carbon nanotubes are used together. The single-walled carbon nanotube particles connect the silicon carbon particles and carbon-based particles, while the acetylene black can fill the gaps between the carbon-based particles and silicon carbon particles, thereby enhancing the conductivity of the active material layer. The amount of single-walled carbon nanotubes should not be excessive, as they are difficult to disperse evenly in the negative electrode slurry. Too much will cause the single-walled carbon nanotubes to agglomerate, resulting in poor slurry processing performance.

[0054] In one optional embodiment, the current collector further includes a carbon coating layer on both sides; Optionally, the thickness of the carbon coating layer is 0.5-1.5 μm; as an example, the thickness of the carbon coating layer can be 0.5 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm, 1.5 μm, or within any of the above values.

[0055] Optionally, the carbon coating layer includes acetylene black; According to the above technical means, the carbon coating layer can increase the adhesion between the current collector and the negative electrode active material layer, and improve the conductivity of the current collector and the active material layer.

[0056] In one optional embodiment, the current collector includes an insulating layer and a conductive layer, the conductive layer being located on at least one surface of the insulating layer, the material of the conductive layer being selected from at least one of copper, titanium, silver, and nickel-copper alloy; the material of the insulating layer is selected from at least one of organic polymer insulating materials, inorganic insulating materials, and composite materials.

[0057] Based on the above technical means, the current collector includes a composite structure of an insulating layer and a conductive layer, which significantly reduces the weight of the current collector while effectively improving the energy density of the battery.

[0058] Secondly, this application provides a lithium-ion battery, including a positive electrode, a separator, and a negative electrode, wherein the positive electrode and the negative electrode are stacked, the separator is disposed between the positive electrode and the negative electrode, and the negative electrode is the negative electrode provided in this application above. In one alternative implementation, the negative electrode has one more layer than the positive electrode.

[0059] Based on the above technical means, the stacked design is used, with one more layer of negative electrode than positive electrode. This is beneficial for making full use of the internal volume space in the hard-shell battery, thereby achieving a higher energy density.

[0060] In one alternative embodiment, an electrolyte is also included, the electrolyte comprising a solvent, a lithium salt, and additives; Optionally, the additive includes at least one of fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), vinylene sulfate (DTD), and methylene disulfonate (MMDS); Further optionally, the electrolyte comprises 0.5-7% FEC, 0.1-1% LiPO2F2, 0.1-1% DTD, and 0.1-1% MMDS by mass percentage.

[0061] Based on the above technical means, the selected F-type additives (FEC, LiPO2F2) can participate in film formation at the negative electrode, forming a LIF-rich SEI film, improving the mechanical strength of the SEI film, and preventing the SEI film from being damaged as the volume of silicon-carbon anode changes. A higher FEC content will lead to degradation in the high-temperature cycling performance of lithium batteries, while a lower FEC content will result in poor mechanical strength of the lithium battery SEI film and poor room-temperature cycling performance. If the LiPO2F2 content is high, the impedance of the lithium battery will increase, further deteriorating the kinetics of the silicon-carbon anode. In addition, electrolyte additives DTD and MMDS can also participate in film formation on the negative electrode side. DTD and MMDS form a uniform and dense SEI film with Li2SO3 as the framework on the negative electrode side. The lithium conductivity of Li2SO3 is higher than that of LIF and Li2CO3, which improves the kinetics of silicon-carbon anode.

[0062] Those skilled in the art will understand that the lithium-ion battery provided in this application, in addition to the electrolyte described above, also includes structural components such as a positive electrode, a negative electrode, a separator, and a casing. During the charging and discharging process, lithium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes, and the separator, disposed between the positive and negative electrodes, primarily serves to prevent short circuits between the positive and negative electrodes while allowing lithium ions to pass through.

[0063] As an example, the positive electrode sheet includes a positive current collector and a positive active layer. The positive current collector has two opposing surfaces in its own thickness direction, and the positive active layer is disposed on either or both of the opposing surfaces of the positive current collector. The materials, composition, and manufacturing methods of the positive electrode sheet used in the lithium-ion battery of this application may include any techniques disclosed in the prior art.

[0064] As an example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer. The negative electrode current collector has two opposing surfaces in its own thickness direction, and the negative electrode active layer is disposed on either or both of the opposing surfaces of the negative electrode current collector. The materials, composition, and manufacturing methods of the negative electrode sheet used in the lithium-ion battery of this application may include any techniques disclosed in the prior art.

[0065] The materials and shapes of the separators used in the lithium-ion batteries of this application are not particularly limited, and may include any techniques disclosed in the prior art.

[0066] Those skilled in the art will understand that the electrolyte plays a role in conducting ions between the positive and negative electrodes, and the electrolyte used in the lithium-ion battery of this application can include any technology disclosed in the prior art. As an example, the electrolyte may include lithium salts, solvents, etc. In some embodiments, the electrolyte may also 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.

[0067] This application does not specify a particular method for preparing lithium-ion batteries; conventional methods in the art can be used to prepare lithium-ion batteries. For example, a positive electrode, a separator, and a negative electrode can be stacked sequentially, with the separator positioned between the positive and negative electrodes. A cell can be obtained through a stacking or winding process, followed by baking, electrolyte injection, formation, and encapsulation to obtain the lithium-ion battery of this application.

[0068] It is understood that in the electrical equipment provided in this application, the lithium-ion battery can be used as a power source for the electrical equipment, or as an energy storage unit for the electrical equipment. The electrical equipment may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0069] The electrical equipment provided in this application has the same advantages as the lithium-ion battery mentioned above because it uses the lithium-ion battery provided in this application, which will not be repeated here.

[0070] The present application will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present application in any way.

[0071] Example 1 This embodiment provides a negative electrode sheet and a lithium-ion battery including the negative electrode sheet. A schematic diagram of the structure of the negative electrode sheet is shown below. Figure 1As shown, the battery includes a current collector, which comprises a copper foil 1 and carbon coating layers 2 located on both sides of the copper foil. The negative electrode also includes a negative electrode active layer disposed on at least one surface of the current collector. The negative electrode active layer comprises a first negative electrode active layer 5, a second negative electrode active layer 4, and a third negative electrode active layer 3 disposed sequentially. The third negative electrode active layer 3 is disposed near the current collector, and the second negative electrode active layer 4 is disposed between the first negative electrode active layer 5 and the third negative electrode active layer 3. The second negative electrode active layer includes a carbon-based negative electrode active material 41 and a silicon-carbon negative electrode active material 42. In the battery, the first negative electrode active layer 5 is disposed near the separator 6. The specific composition and preparation method of the negative electrode are as follows: Negative electrode preparation: The negative electrode uses a three-layer coating process. The active material in the first negative electrode active layer is artificial graphite with a carbon coating. The second negative electrode active layer is artificial graphite with a carbon coating mixed with silicon carbon. The active material in the third negative electrode active layer is artificial graphite without a carbon coating. The particle size of the artificial graphite in the first negative electrode active layer is Dv50 = 7.5 μm, the XRD diffraction peak 004 interplanar spacing is 0.1681 nm, the carbon coating content is 1% by mass, and the thickness is 50 nm. The particle size of the artificial graphite in the second negative electrode active layer is Dv50 = 12 μm. The interplanar spacing of the XRD diffraction peak 004 is 0.1678 nm, the carbon coating layer has a mass percentage of 2% and a thickness of 70 nm, and is composed of a mixture of hard carbon and soft carbon; the second anode active layer has silicon-carbon particle sizes Dv50=7.6 μm, Dv95=13.2 μm, the grain size of XRD diffraction peak 111 is 2.3 nm, the silicon-carbon particle size distribution Span=0.86, and the silicon content in the silicon-carbon is 48%; the third anode active layer has artificial graphite particle size Dv50=13.0 μm, and the interplanar spacing of the XRD diffraction peak 002 is 0.3357 nm. The first and third negative electrode active layers are made of pure graphite. Artificial graphite, acetylene black, and binder (the mass ratio of CMC-Li, PAA-Li, and SBR in the binder is 1:3:1) are added sequentially to deionized water in a mass ratio of 97.0:0.5:2.5, with a solid content of 50%. The mixture is then thoroughly mixed and dispersed to obtain the negative electrode slurry. The second negative electrode active layer is made of graphite mixed with silicon carbon. Artificial graphite, silicon carbon, acetylene black, single-walled carbon nanotubes, and binder (the mass ratio of CMC-Li, PAA-Li, and SBR in the binder is 1:2:3) are added sequentially to deionized water in a mass ratio of 66.5:29:0.9:0.1:3.5, with a solid content of 45%. The mixture is then thoroughly mixed and dispersed to obtain the negative electrode slurry. A three-layer negative electrode coating machine was used to simultaneously coat the first, second, and third negative electrode active layers with negative electrode slurry onto a 6μm thick double-sided carbon-coated copper foil. The carbon coating layer thickness was 0.6μm. After coating, the negative electrode sheets were dried in a forced-air oven. The dried negative electrode sheets were then calendered in a negative electrode roller press to obtain the negative electrode sheets. The thicknesses of the first, second, and third negative electrode active layers were controlled to be 30μm, 50μm, and 30μm, respectively. The negative electrode sheets were then sealed and stored for later use. The mass percentage of silicon-carbon negative electrode active material in the negative electrode active layer (X%) = mass percentage of silicon-carbon negative electrode active material in the second negative electrode active layer × thickness of the second negative electrode active layer / total thickness of the negative electrode active layers.

[0072] Positive electrode preparation: Lithium iron phosphate powder was used as the positive electrode. It was added to N-methylpyrrolidone in a mass ratio of 97.0:0.5:0.5:2.0 with conductive carbon black, multi-walled carbon nanotubes, and polyvinylidene fluoride, setting the solid content to 65%. The mixture was thoroughly mixed and dispersed to obtain a positive electrode slurry. The slurry was then coated onto a 12μm thick double-sided carbon-coated aluminum foil (coating thickness 1μm) using a positive electrode coating machine, with the coating surface density controlled at 24mg / cm². 2 The electrode is coated on both sides and then dried in a forced-air oven. The dried electrode is then calendered in a positive electrode roller press with a double-sided roller pressing thickness of 195μm to obtain the positive electrode for later use.

[0073] Lithium-ion battery assembly: The designed lithium-ion battery type is a square aluminum-cased battery with a length of 208mm, a width of 50mm, and a height of 103mm. The internal cells are Z-shaped stacked cores, consisting of two stacked cores connected in parallel. The positive and negative electrode sheets and separator are cut according to the dimensions specified for lithium-ion batteries. The separator uses an oil-based adhesive separator with a total thickness of 11μm, a substrate thickness of 7μm, a single-sided coating of burlite with a thickness of 2μm, and a double-sided coating of oil-based PVDF with a thickness of 1μm. The separator is located between the positive and negative electrodes. The die-cut positive and negative electrode sheets and the separator are assembled together on a lithium-ion battery stacking machine using a Z-shaped stacking core. The square aluminum-cased battery was then assembled, followed by electrolyte injection and encapsulation. The electrolyte contained 3.5% FEC, 0.6% LiPO2F2, 1% DTD, 0.5% MMDS, 3% lithium bis(fluorosulfonyl)imide (LiFSI), and 11% lithium salt LiPF6. The solvent was ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) in a mass ratio of 3:2:3. The resulting battery was then filled with electrolyte.

[0074] Lithium-ion battery formation and sorting: After electrolyte injection, the batteries are charged and formed in a lithium-ion battery formation cabinet, with an upper limit voltage of 3.8V. Following this, a sorting process is performed, where the formed batteries are first discharged, then charged, and then discharged again, with a lower limit voltage of 2.0V. The sorting capacity is determined by the capacity of the second discharge, and the average voltage during the second discharge is the battery's nominal voltage. The battery's volumetric and gravimetric energy densities can then be derived from this process.

[0075] Examples 2-5 The difference from Example 1 is that the parameters of the silicon-carbon anode active material and the composition of the current collector in the second anode active layer are different, as shown in the table below.

[0076] Table 1

[0077] Examples 6-12 The difference from Example 1 is that the proportion of silicon-carbon anode active material in the second anode active layer is different (adaptively increasing or decreasing the content of carbon-based anode active material in the layer so that the sum of the mass percentages of each component is 100%), or the thickness of each active layer is different, as shown in the table below.

[0078] Table 2

[0079] Examples 13-27 The difference from Example 1 is that the silicon-carbon anode active material (graphite) in the active layer and the parameters of the silicon-carbon anode active material are different, as shown in the table below.

[0080] Table 3

[0081] Examples 28-29 The difference from Example 1 is that the silicon content in the silicon-carbon anode active material in the second anode active layer is different, at 40% and 60% respectively.

[0082] Examples 30-32 The difference from Example 1 is that the resistivity of the negative electrode active layer is different, as shown in the table below.

[0083] Table 4

[0084] Examples 33-35 The difference from Example 1 is that the raw material ratio of the negative electrode active layer is different (the proportion of silicon-carbon negative electrode active material is adaptively adjusted so that the sum of the mass percentages of each component is 100%) and the thickness of the current collector carbon coating layer is different, as shown in the table below.

[0085] Table 5

[0086] Examples 36-38 The difference from Example 1 is that the amounts of FEC, LiPO2F2, DTD, and MMDS added to the electrolyte are different, as shown in the table below.

[0087] Table 6

[0088] Comparative Examples 1-4 The difference from Example 1 is that the parameters of the silicon-carbon anode active material in the second anode active layer are different, as shown in the table below.

[0089] Table 7

[0090] Experimental Example 1 The lithium-ion batteries provided in each embodiment and comparative example were subjected to various performance tests. The specific test items and test methods are as follows: Lithium-ion battery cycle test: The battery is charged and discharged using a Blue Electric test cabinet. The charging time is controlled at 15 minutes for 10%-80% SOC (SOC is the battery's measured state of charge; a fully charged battery is 100% SOC, and an empty battery is 0% SOC), and the discharging time is 60 minutes. The battery is then charged and discharged using this charge and discharge cycle test. One charge and one discharge is called one cycle, and the fast charging cycle performance of the battery is tested accordingly.

[0091] Lithium-ion battery self-discharge test: Place the above lithium-ion battery in a 25℃ environment for 2 hours, charge it to 3.8V at a current density of 0.33C, then switch to constant voltage charging, cut-off current density is 0.05C, let it stand for 30 minutes, and discharge it to 33% SOC at a current density of 0.33C. Record the battery voltage at this time as V1 (unit: mV). Then let it stand in this environment for 7 days and record the battery voltage at this time as V2 (mV). The self-discharge K value of the battery is calculated as K=(V1-V2) / 7 / 24 (unit: mV / h). The larger the K value, the greater the self-discharge of the battery and the worse the performance.

[0092] Lithium-ion battery calendar life test at 60℃ and 100% SOC: The lithium-ion battery was placed in a 25℃ environment for 2 hours, charged to 3.8V at a current density of 0.33C, then switched to constant voltage charging with a cutoff current density of 0.05C. After resting for 30 minutes, it was discharged to 2.0V at a current density of 0.33C, and its discharge capacity was recorded as X1 (unit: Ah). After resting for 30 minutes, it was charged to 3.8V at a current density of 0.33C, then switched to constant voltage charging with a cutoff current density of 0.05C. The battery was then stored in a 60℃ environment for 180 days, after which the battery was removed. The battery was left to stand at 25℃ for 2 hours, then discharged to 2.0V at a current density of 0.33C, and the discharge capacity was recorded as X2 (unit: Ah). After standing for 30 minutes, it was charged to 3.8V at a current density of 0.33C, then switched to constant voltage charging with a cutoff current density of 0.05C, left to stand for 30 minutes, and then discharged to 2.0V at a current density of 0.33C, and the discharge capacity was recorded as X3 (unit: Ah). The residual capacity retention rate after 180 days of storage at 60℃ and 100% SOC is X2 / X1×100%, and the recovered capacity retention rate is X3 / X1×100%.

[0093] The specific test results are shown in the table below: Table 8

[0094] The test results above show that the negative electrode sheet provided in this application adopts a three-layer structure design, with the silicon-carbon negative electrode active material located in the middle layer. This design can avoid direct contact between the silicon-carbon negative electrode active material and the separator, thus avoiding the risk of puncturing the separator. The silicon-carbon negative electrode active material is located in the middle of the negative electrode active layer, which can shorten the migration path of lithium ions during charging and discharging to a certain extent, reduce polarization, and ensure the cycle life of the lithium battery. At the same time, with the limitation of the relationship between the silicon-carbon particle Dv95 and the current collector thickness, as well as the limitation of the silicon-carbon particle size distribution, it can effectively prevent the current collector from breaking due to excessive extension caused by the volume expansion of silicon-carbon particles, thereby improving safety and high-temperature performance.

[0095] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A negative electrode sheet, characterized in that, The present invention includes a current collector and a negative electrode active layer disposed on at least one side surface of the current collector. The negative electrode active layer includes a first negative electrode active layer, a second negative electrode active layer and a third negative electrode active layer disposed sequentially. The third negative electrode active layer is disposed on the side close to the current collector, and the second negative electrode active layer is disposed between the first negative electrode active layer and the third negative electrode active layer. Wherein, the first negative electrode active layer includes a first carbon-based negative electrode active material, and the third negative electrode active layer includes a third carbon-based negative electrode active material; The second negative electrode active layer includes a second carbon-based negative electrode active material and a silicon-carbon negative electrode active material. The particle size distribution range of the silicon-carbon negative electrode active material is Span=(Dv90-Dv10) / Dv50, 0.5≤Span≤1.0; the particle size Dv95 of the silicon-carbon negative electrode active material is μm. The thickness of the current collector is W μm; and the following relationship exists between P and W: 1.0≤P / W≤3.

0.

2. The negative electrode sheet according to claim 1, characterized in that, The values ​​of P and W are subject to the following conditions: 11≤P≤15, 5≤W≤11.

3. The negative electrode sheet according to claim 1, characterized in that, The mass percentage of the silicon-carbon anode active material is X%, based on the total mass of the anode active layer. The thickness of the first negative electrode active layer is Y μm, the thickness of the second negative electrode active layer is Z μm, and the total thickness of the negative electrode active layer is M μm; The following relationship exists between X, Y, Z, and M: 0.1≤X / (Y+Z)≤0.2, 0.25≤Y / Z≤0.75, 0.625≤(Y+Z) / M≤1.

0.

4. The negative electrode sheet according to claim 3, characterized in that, The values ​​of X, Y, Z, and M are within the following ranges: 5≤X≤30, 20≤Y≤60, 30≤Z≤80, and 80≤M≤200.

5. The negative electrode sheet according to claim 1, characterized in that, The particle size Dv50 of the first carbon-based anode active material is 5-10 μm. And / or, the particle size Dv50 of the second carbon-based anode active material is 10-15 μm, the particle size Dv50 of the silicon-carbon anode active material is 5-10 μm, and the mass percentage of silicon in the silicon-carbon anode active material is 40%-60%; And / or, the particle size Dv50 of the third carbon-based negative electrode active material is 10-20 μm; And / or, the first carbon-based negative electrode active material, the second carbon-based negative electrode active material, and the third carbon-based negative electrode active material independently include at least one of graphite, hard carbon, soft carbon, and mesophase carbon microspheres; optionally, the graphite includes at least one of artificial graphite and natural graphite. And / or, when the first carbon-based negative electrode active material, the second carbon-based negative electrode active material, and the third carbon-based negative electrode active material independently include graphite, the first carbon-based negative electrode active material and the second carbon-based negative electrode active material include a carbon coating layer, and the third carbon-based negative electrode active material does not include a carbon coating layer, or is a mixture of graphite without a carbon coating layer and graphite including a carbon coating layer.

6. The negative electrode sheet according to claim 5, characterized in that, The XRD diffraction peak 004 of the first carbon-based anode active material has a crystal plane spacing of 0.1680-0.1683 nm, the XRD diffraction peak 004 of the second carbon-based anode active material has a crystal plane spacing of 0.1678-0.1681 nm, the crystal size of the XRD diffraction peak 111 of the silicon-carbon anode active material in the second anode active layer is 2-10 nm, and the XRD diffraction peak 002 of the third carbon-based anode active material has a crystal plane spacing of 0.3358-0.3356 nm. And / or, the resistivity of the negative electrode is R1Ω·m, and satisfies 0.5≤R1≤1.

5.

7. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active layer, the second negative electrode active layer, and the third negative electrode active layer each independently include a conductive agent and a binder; Optionally, the adhesive includes at least one of polyvinylidene fluoride, lithium polyacrylate, polyimide, styrene-butadiene rubber, and lithium carboxymethyl cellulose; Optionally, the conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene. Further optionally, the binder in the first negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass percentage of the binder in the first negative electrode active layer is 2.0%-3.5% of the total mass of the first negative electrode active layer. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). Further optionally, the binder in the second negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber, and the mass ratio of the binder in the second negative electrode active layer to the total mass of the second negative electrode active layer is 2.5%-5.0%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:3:4). The binder in the third negative electrode active layer includes lithium polyacrylate, lithium carboxymethyl cellulose and styrene-butadiene rubber. The mass percentage of the binder in the third negative electrode active layer to the total mass of the first negative electrode active layer is 2.0%-3.5%. Optionally, the mass ratio of lithium carboxymethyl cellulose, lithium polyacrylate and styrene-butadiene rubber is (1:1:1)-(1:4:3). Further optionally, the conductive agent in the second negative electrode active layer includes acetylene black and single-walled carbon nanotubes, wherein the mass percentage of acetylene black in the second negative electrode active layer is 0.5-1.5%, and the mass percentage of single-walled carbon nanotubes in the second negative electrode active layer is 0.05-0.5%.

8. The negative electrode sheet according to claim 1, characterized in that, The current collector also includes a carbon coating layer on both sides; Optionally, the thickness of the carbon coating layer is 0.5-1.5 μm; Optionally, the carbon coating layer includes acetylene black; Alternatively, the current collector includes an insulating layer and a conductive layer, the conductive layer being located on at least one surface of the insulating layer, the material of the conductive layer being selected from at least one of copper, titanium, silver, and nickel-copper alloy; the material of the insulating layer is selected from at least one of organic polymer insulating materials, inorganic insulating materials, and organic-inorganic composite insulating materials.

9. A lithium-ion battery, comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode and the negative electrode are stacked, and the separator is disposed between the positive electrode and the negative electrode, characterized in that, The negative electrode is the negative electrode as described in any one of claims 1-8; And / or, the negative electrode has one more layer than the positive electrode.

10. The lithium-ion battery according to claim 9, characterized in that, It also includes an electrolyte, which comprises a solvent, a lithium salt, and additives; Optionally, the additive includes at least one of fluoroethylene carbonate, lithium difluorophosphate, vinylene sulfate, and methylene disulfonate; Further optionally, the electrolyte comprises 0.1-1% fluoroethylene carbonate, 0.5-7% lithium difluorophosphate, 0.1-1% vinylene sulfate, and 0.1-1% methylene disulfonate by mass.

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