Negative pole piece, preparation method thereof and lithium ion battery

Abstract

The invention provides a negative pole piece, a preparation method thereof and a lithium ion battery. The negative pole piece comprises a negative current collector and a negative active material layer coated on the surface of the negative current collector; wherein the negative current collector is a copper foil, and the copper foil is a copper foil which is subjected to stretching pretreatment to reach a yield stage or a strengthening stage; the negative electrode active material layer comprises a graphite material, a silicon carbon material, a conductive agent and a binder, and the mass of the silicon carbon material accounts for 10-40% of the total mass of the negative electrode active material layer. According to the negative electrode plate provided by the invention, the copper foil with ultralow elongation is used as the negative electrode current collector, so that the negative electrode plate doped with a high-proportion silicon-carbon material has ultralow elongation rate, and the safety risk caused by extension can be improved while the energy density of the lithium battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a negative electrode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] The rapid development of electric vehicles has placed extremely high demands on battery energy density. High-energy areal density batteries can reduce the size and weight of battery packs while increasing vehicle range, which is crucial for the popularization of electric vehicles and the development of sustainable transportation. Silicon materials in lithium-ion batteries, with a theoretical specific capacity as high as 4200 mAh / g, more than 10 times that of traditional graphite anodes (theoretical specific capacity 372 mAh / g), have become an important technological means to improve battery energy density.

[0003] The conventional method for manufacturing lithium-ion battery anodes involves coating a copper current collector substrate with a slurry containing graphite anode active material. After drying, this slurry is used as the anode electrode for lithium-ion batteries. However, to achieve ultra-high energy density cells (>280Wh / kg), a high proportion (>10%) of silicon-carbon anode material needs to be added to the graphite anode. However, when silicon undergoes an alloying reaction with lithium, it produces a huge volume expansion, and this expansion increases with the increase of silicon content. This volume expansion generates huge mechanical stress, which exerts a large lateral tensile force on the anode electrode, causing the anode sheet to extend in the lateral direction.

[0004] As a metallic material, copper foil for negative electrode current collectors typically exhibits four stages in its tensile stress-strain curve under stress, as shown in the figure below: Stage 1 (stage a) is the elastic stage, characterized by a direct proportional relationship between stress and strain when the stress is low, satisfying Hooke's Law (σ = Eε), where σ is stress, ε is strain, and E is the elastic modulus. Deformation in this stage is reversible; the deformation fully recovers after the stress disappears. Stage 2 (stages b to c) is the yielding stage, characterized by plastic deformation occurring when the stress exceeds a certain limit, with a significant increase in strain. The deformation at this stage is irreversible, leaving residual deformation after the stress disappears. Stage 3 (stages c to d) is the strengthening stage, characterized by increased resistance to deformation with increasing strain, leading to a rise in stress but minimal deformation. This phenomenon is called work hardening or strain strengthening. Stage 4 (stage d) is the necking and fracture stage, characterized by localized necking of the specimen after the stress reaches the tensile strength, followed by a decrease in stress. Finally, the specimen fractures when the stress reaches the fracture strength.

[0005] Based on the characteristics of copper foil, when stress is generated in the silicon anode, it will cause the anode sheet to stretch laterally. Excessive stretching can cause the anode sheet to exceed the size of the separator, thus connecting with the positive electrode and forming a short circuit, which poses a safety risk.

[0006] To reduce the dimensional stretching of the negative electrode, existing technologies often modify silicon-carbon materials to avoid the volume expansion of silicon.

[0007] Some literature discloses a multidimensional lithium-ion battery anode sheet and its preparation method. This method sequentially prepares a graphene quantum dot layer, a silicon material layer, a carbon material layer, and a ceramic layer on the anode current collector. The resulting multidimensional lithium-ion battery anode can effectively suppress silicon expansion and improve the cycle performance of lithium-ion batteries. However, this method involves cumbersome steps and a relatively complex process, making it unsuitable for mass production.

[0008] It is evident that although existing technologies have conducted some research on improving the size extension of negative electrode sheets, there is still room for further exploration. Summary of the Invention

[0009] The problem the invention aims to solve

[0010] Based on the above problems, the present invention provides a negative electrode sheet with low elongation to solve the drawbacks of elongation technology in high-energy-density lithium batteries with ultra-high silicon content, thereby improving the safety risks caused by elongation while increasing the energy density of lithium batteries.

[0011] Solution for solving the problem

[0012] The present invention first provides a negative electrode sheet, wherein the negative electrode current collector and a negative electrode active material layer coated on the surface of the negative electrode current collector are included;

[0013] The negative electrode current collector is a copper foil, which is a copper foil that has undergone stretching pretreatment to reach the yield stage or the strengthening stage.

[0014] The negative electrode active material layer includes graphite material, silicon carbon material, conductive agent and binder, wherein the mass of silicon carbon material accounts for 10-40% of the total mass of the negative electrode active material layer.

[0015] According to the negative electrode sheet of the present invention, after the battery is fully charged, the width elongation of the negative electrode sheet is less than 2.5% and the height elongation is less than 3%.

[0016] According to the negative electrode sheet of the present invention, the tensile force in the stretching pretreatment is 50-300 MPa, and the tensile deformation rate of the copper foil is 0.1-2%.

[0017] According to the negative electrode sheet of the present invention, the thickness of the copper foil is 3 to 12 μm.

[0018] According to the negative electrode sheet of the present invention, the particle size D of the graphite material is... 50 Its diameter is 8–16 μm, and its specific surface area is 0.9–1.4 m². 2 / g.

[0019] According to the negative electrode sheet of the present invention, wherein,

[0020] The silicon-carbon material is vapor-deposited silicon-carbon; and / or,

[0021] The conductive agent includes a short-range conductive agent and a long-range conductive agent; the short-range conductive agent includes conductive carbon black SP; and the long-range conductive agent includes single-walled carbon nanotubes; and / or,

[0022] The adhesive comprises styrene-butadiene rubber and polyacrylic acid.

[0023] According to the negative electrode sheet of the present invention, the graphite material accounts for 30-90% of the total mass of the negative electrode active material layer; and / or,

[0024] The conductive agent comprises 0.01% to 10% of the total mass of the negative electrode active material layer; and / or,

[0025] The binder accounts for 0.6% to 10% of the total mass of the negative electrode active material layer.

[0026] According to the negative electrode sheet of the present invention, the negative electrode active material layer further includes a dispersant, wherein the mass of the dispersant accounts for 0.1-5% of the total mass of the negative electrode active material layer.

[0027] This invention also provides a method for preparing a negative electrode sheet according to the present invention, which includes the following steps:

[0028] S1: The copper foil is stretched and pretreated to reach the yielding stage or strengthening stage of the copper foil to obtain the negative electrode current collector;

[0029] S2: Mix graphite material, silicon carbon material, conductive agent, binder and other optional additives in a certain proportion to obtain a negative electrode active coating. Apply the negative electrode active coating to the negative electrode current collector and dry it to obtain a negative electrode sheet.

[0030] The present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises a negative electrode sheet according to the present invention.

[0031] The effects of the invention

[0032] 1. The negative electrode sheet provided by the present invention uses ultra-low elongation copper foil as the negative electrode current collector. The ultra-low elongation copper foil can be obtained by simple pretreatment of copper metal, which enables the negative electrode sheet doped with a high proportion of silicon-carbon material to have an ultra-low elongation. The width elongation of the electrode sheet can be controlled below 2.5%, and the height elongation can be controlled below 3%, thereby ensuring that the safety risks caused by elongation can be improved while increasing the energy density of lithium battery.

[0033] 2. In the method for preparing the negative electrode sheet provided by the present invention, a negative current collector with ultra-low elongation can be obtained by simply pre-treating copper metal. The negative electrode sheet made using this copper foil can offset the initial elastic stage, i.e., the stage with high deformation, in advance, thereby enabling the negative electrode sheet doped with a high proportion of silicon-carbon material to have an ultra-low elongation.

[0034] 3. The lithium-ion battery provided by the present invention uses the negative electrode sheet described in the present invention, which solves the drawbacks of the extension technology of high energy density lithium batteries with ultra-high silicon content, and can provide a high-stability, high-safety ultra-high energy density lithium-ion battery. Attached Figure Description

[0035] Figure 1 The stress-strain curve of the copper foil is shown. Detailed Implementation

[0036] The present invention will now be described in detail. The descriptions of the technical features described below are based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0037] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0038] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0039] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0040] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0041] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0042] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" used refers to weight or mass percentage content.

[0043] In this specification, the terms "substantially" and "essentially" are used to indicate that the standard deviation from the theoretical model, theoretical data, or target data is within a range of 2%, preferably 1%, and more preferably 0.8%.

[0044] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0045] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0046] <First Aspect>

[0047] A first aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer coated on the surface of the negative electrode current collector; wherein the negative electrode current collector is a copper foil, and the copper foil is a copper foil that has undergone stretching pretreatment to reach the yield stage or the strengthening stage; the negative electrode active material layer comprises a graphite material, a silicon carbide material, a conductive agent and a binder, wherein the mass of the silicon carbide material accounts for 10-40% of the total mass of the active material layer.

[0048] In some specific embodiments, the negative electrode sheet of the present invention, after the battery is fully charged, has a width elongation of less than 2.5%, for example, 2.2%, 2%, 1.8%, 1.5%, 1.2%, 1%, 0.5%, etc.; and a height elongation of less than 3%, for example, 2.9%, 2.7%, 2.5%, 2.2%, 2%, 1.8%, 1.5%, 1.2%, 1%, 0.5%, etc. The negative electrode sheet of the present invention has a low elongation, which can suppress the dimensional elongation caused by the high silicon content in the negative electrode active material layer, thereby improving the battery's energy density while also improving battery safety.

[0049] (Negative electrode current collector)

[0050] The negative electrode current collector material of this invention is copper foil, which is an ultra-low elongation copper foil that has undergone stretching pretreatment to reach the yielding stage or the strengthening stage. The negative electrode sheet made of this copper foil can offset the initial elastic stage, i.e., the stage with high deformation, in advance, thereby enabling the negative electrode sheet doped with a high proportion of silicon-carbon material to have an ultra-low elongation.

[0051] The copper foil used in this invention is copper foil that has undergone tensile pretreatment. The pretreatment conditions are as follows: the copper foil to be used is subjected to tensile pretreatment with a tensile force of 50-300 MPa, for example, 100 MPa, 150 MPa, 200 MPa, 250 MPa, etc.; the copper foil is stretched to the yield stage or strengthening stage of the copper foil metal, that is, the tensile deformation is 0.1% to 2%, for example, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, etc. This invention can obtain ultra-low elongation copper foil through simple tensile pretreatment. The negative electrode sheet made using this copper foil can offset the initial elastic stage, that is, the stage with high deformation, in advance, thereby effectively improving the dimensional ductility of the negative electrode sheet.

[0052] In some specific implementations, the thickness of the copper foil can be 3–12 μm, for example, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, etc. When the copper foil is too thin, the production process may be difficult to control, and it may also result in high impedance, affecting battery performance. When the copper foil is too thick, its weight may be too great, affecting the battery's energy density. A copper foil thickness within the above-mentioned range is practically applicable.

[0053] (Negative electrode active material layer)

[0054] The negative electrode active material layer of this invention comprises graphite material, silicon-carbon material, a conductive agent, and a binder. The energy density of the battery is improved by doping the graphite negative electrode with silicon-carbon material. The silicon-carbon material accounts for 10-40% of the total mass of the negative electrode active material layer, for example, 15%, 20%, 25%, 30%, 35%, etc. In this invention, even with a high proportion of silicon-carbon material, the negative electrode sheet also has an ultra-low elongation due to the use of copper foil with ultra-low elongation as the negative electrode current collector.

[0055] The present invention does not specifically limit the type of silicon-carbon material, which can be a commonly used silicon-carbon material in the art, such as vapor-deposited silicon-carbon.

[0056] In some specific implementations, the particle size D of the graphite material 50 The thickness can be 8–16 μm, for example, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, etc.; the specific surface area of ​​the graphite material can be 0.9–1.4 m². 2 / g, for example, can be 1m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, etc. The specific surface area can be determined by using the BET specific surface area test method.

[0057] In some specific implementations, the graphite material accounts for 30% to 90% of the total mass of the negative electrode active material layer, for example, it can be 40%, 50%, 60%, 70%, 80%, etc.

[0058] The present invention does not impose any particular limitation on the type of conductive agent, and commonly used conductive agents in the art can be used. In some specific embodiments, the conductive agent may include short-range conductive agents and long-range conductive agents, preferably, a composite conductive agent of short-range and long-range conductive agents. The short-range conductive agent may include carbon black, conductive carbon black SP, and Ketjen black, preferably conductive carbon black SP; the long-range conductive agent may include single-walled carbon nanotubes, etc.

[0059] In some specific embodiments, the conductive agent accounts for 0.01% to 10% of the total mass of the negative electrode active material layer, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, etc. In some preferred embodiments, the conductive agent is a composite conductive system of conductive carbon black SP and single-walled carbon nanotubes. Specifically, the conductive carbon black SP accounts for 0.1% to 10% of the total mass of the negative electrode active material layer, for example, it can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, etc.; the single-walled carbon nanotubes account for 0.01% to 0.5% of the total mass of the negative electrode active material layer, for example, it can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, etc.

[0060] In some specific embodiments, the adhesive includes flexible aqueous adhesives such as styrene-butadiene rubber (SBR) and high-modulus polyacrylic acid (PAA) adhesives. The numerous carboxyl groups on the surface of the polyacrylic acid can form hydrogen bonds or covalent bonds with the hydroxyl groups on the silicon surface, significantly improving adhesion. The adhesive accounts for 0.6% to 10% of the total mass of the negative electrode active material layer, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, etc.

[0061] In some preferred embodiments, the binder comprises a combination of styrene-butadiene rubber (SBR) and polyacrylic acid (PAA). The SBR comprises 0.1% to 5% of the total mass of the negative electrode active material layer, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, etc.; the PAA comprises 0.5% to 5% of the total mass of the negative electrode active material layer, for example, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, etc.

[0062] Furthermore, without limitation, various optional functional additives may be used in the negative electrode active material layer, provided that the technical effects of the present invention are not impaired. In some specific embodiments, the functional additives may include dispersants. The dispersant may include sodium carboxymethyl cellulose (CMC), etc. Preferably, the mass of the dispersant accounts for 0.1% to 5% of the total mass of the active material layer, for example, 0.5%, 1%, 2%, 3%, 4%, etc.

[0063] In some specific embodiments, the negative electrode active material layer of the present invention is coated on both sides of the current collector, and the total thickness of the negative electrode active material layer on both sides can be 60-200 μm, for example, 80 μm, 100 μm, 120 μm, 140 μm, 150 μm, 160 μm, 180 μm, etc.; the thickness of the negative electrode active material layer on one side can be 30-100 μm, for example, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, etc.

[0064] <Second aspect>

[0065] A second aspect of the present invention provides a method for preparing a negative electrode sheet according to the first aspect, comprising the following steps:

[0066] S1: The copper foil is stretched to achieve the yield stage. Figure 1 (stage b to c) or reinforcement stage ( Figure 1 From stage c to stage d, the negative electrode current collector is obtained;

[0067] S2: Mix graphite material, silicon carbon material, conductive agent, binder and other optional additives in a certain proportion to obtain a negative electrode active coating. Apply the negative electrode active coating to the negative electrode current collector and dry it to obtain a negative electrode sheet.

[0068] The conditions for the stretching pretreatment, as well as the types, specifications, and usage ratios of the copper foil, graphite material, silicon carbide material, conductive agent, and binder, are the same as in the first aspect and will not be repeated here.

[0069] Other additives may include various functional additives or solvents.

[0070] The types, specifications, and usage ratios of the functional additives are the same as in the first aspect, and will not be repeated here.

[0071] The present invention does not particularly limit the type of solvent, which can be selected as needed, such as distilled water. Regarding the amount of solvent used, the solid content in the negative electrode active coating can be 40-50% by mass.

[0072] The present invention does not impose any particular limitation on the coating method, and the method can be selected as needed, such as using a coating machine to perform the coating.

[0073] <Third aspect>

[0074] A third aspect of the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises a negative electrode sheet according to the first aspect. The lithium-ion battery of the present invention is a high-energy-density power battery.

[0075] (positive electrode)

[0076] The positive electrode of the present invention includes a positive electrode current collector and a positive electrode material system coated on the surface of the positive electrode current collector, wherein the positive electrode material system includes a positive electrode active material, a conductive agent, and a binder.

[0077] In some specific implementations, the positive electrode active material is one or more of lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium-rich manganese-based, sodium cobalt oxide (NCO), sodium iron phosphate (NFP), and sodium manganese oxide (NMO).

[0078] In some specific implementations, the positive electrode active material is used in the form of particles with a particle size of 5-100 μm.

[0079] In some specific implementations, the mass of the positive electrode active material accounts for 70% to 99.9% of the total mass of the positive electrode material system, for example, it can be 75%, 80%, 85%, 90%, 95%, etc.

[0080] In some specific implementations, the conductive agent is one or more of carbon nanotubes (CNTs), graphene, conductive graphite, conductive carbon black, Ketjen black ECP, and carbon fiber (VGCF); preferably, the mass of the conductive agent accounts for 0.1% to 15% of the total mass of the cathode material system, for example, it can be 1%, 5%, 8%, 10%, 12%, etc.

[0081] In some specific embodiments, the binder is polyvinylidene fluoride (PVDF). Preferably, the mass of the binder accounts for 0.1% to 15% of the total mass of the cathode material system, for example, 1%, 5%, 8%, 10%, 12%, etc.

[0082] (Septum)

[0083] The diaphragm of the present invention is not particularly limited, and diaphragms commonly used in the art can be used. For example, the diaphragm can be at least one of a single-layer PE film, a single-layer PP film, or a multilayer composite PE and PP film. The diaphragm can also be a film formed by coating the above-mentioned film with a functional coating, such as a ceramic diaphragm coated with an alumina layer, an adhesive diaphragm coated with a PVDF layer, and a composite diaphragm with a ceramic-adhesive mixed coating.

[0084] (electrolyte)

[0085] The electrolyte of the present invention includes an organic solvent, a lithium salt, and additives.

[0086] The present invention does not particularly limit the type of organic solvent, and it can be selected as needed. In some specific embodiments, the organic solvent is a mixture of ethylene carbonate, fluoroethylene carbonate, diethyl carbonate and methyl ethyl carbonate, and the mixing ratio is (10-30):(5-20):(10-30):(35-50) by mass.

[0087] The present invention does not particularly limit the type of lithium salt, and it can be selected as needed. In some specific embodiments, the lithium salt is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, and lithium di(trifluoromethanesulfonyl)imide.

[0088] The present invention does not particularly limit the type of additives, and they can be selected as needed. In some specific embodiments, the additives include one or more of ethylene sulfate, propylene sulfite, propylene sulfonate lactone, methylene disulfonate, vinylene carbonate, and succinate.

[0089] (Methods for preparing lithium batteries)

[0090] The lithium-ion battery provided by the present invention can be produced by conventional methods, whereby the prepared positive electrode, negative electrode and separator can be stacked to form an electrode core, which is then packaged and then injected with liquid, packaged and activated.

[0091] Example

[0092] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0093] Comparative Example 1

[0094] Negative electrode sheet fabrication: 70% by weight of graphite material (conventional fast-charging graphite is used, D...) 50 Its diameter is 12 μm, and its specific surface area is 1.2 m². 2 The mixture comprises 25% silicon-carbon material (the silicon-carbon material is commercially available silicon-carbon material prepared by CVD process), 0.5% conductive carbon black (SP), 0.15% single-walled carbon nanotubes, 0.8% sodium carboxymethyl cellulose (CMC), 1.3% styrene-butadiene rubber (SBR), and 2.25% polyacrylic acid (PAA), with distilled water added as a solvent to prepare a slurry. The weight of the distilled water is the weight corresponding to a solid content of 45% by mass in the slurry. The slurry is coated on the surface of the copper foil of the negative electrode current collector. The copper foil is a conventional copper metal foil with a thickness of 6 μm. The coating thickness on both sides of the current collector is 50 μm. After drying, rolling, and die-cutting, the negative electrode sheet 1 is obtained.

[0095] Example 1

[0096] Negative electrode sheet fabrication: 70% by weight of graphite material (conventional fast-charging graphite is used, D...) 50 Its diameter is 12 μm, and its specific surface area is 1.2 m². 2 The composition comprises: 25% silicon-carbon material (prepared using a commercially available CVD process), 0.5% conductive carbon black (SP), 0.15% single-walled carbon nanotubes, 0.8% sodium carboxymethyl cellulose (CMC), 1.3% styrene-butadiene rubber (SBR), and 2.25% polyacrylic acid (PAA). Distilled water is added as a solvent to prepare a slurry, wherein the weight of distilled water corresponds to a solid content of 45% by mass. The slurry is coated onto the surface of a copper foil used as a negative electrode current collector. The copper foil is a pre-treated copper metal foil with a thickness of 6 μm. The pre-treatment process involves stretching the copper foil under a tensile force of 200 MPa until the tensile deformation rate reaches 1.0%, stretching it to the yield stage of the copper metal foil. Then, a negative electrode slurry with a thickness of 50μm is coated on both sides of the current collector, and the negative electrode sheet 2 is obtained by drying, rolling and die cutting.

[0097] Example 2

[0098] Negative electrode sheet fabrication: 70% by weight of graphite material (conventional fast-charging graphite is used, D...) 50 Its diameter is 12 μm, and its specific surface area is 1.2 m². 2The composition comprises: 25% silicon-carbon material (prepared using a commercially available CVD process), 0.5% conductive carbon black (SP), 0.15% single-walled carbon nanotubes, 0.8% sodium carboxymethyl cellulose (CMC), 1.3% styrene-butadiene rubber (SBR), and 2.25% polyacrylic acid (PAA). Distilled water is added as a solvent to prepare a slurry, wherein the weight of distilled water corresponds to a solid content of 45% by mass. The slurry is coated onto the surface of a copper foil used as a negative electrode current collector. The copper foil is a pre-treated copper metal foil with a thickness of 6 μm. The pre-treatment process involves stretching the copper foil under a tensile force of 220 MPa until the tensile deformation rate reaches 1.5%, indicating the strengthening stage of the copper metal foil. Then, a negative electrode slurry with a thickness of 50μm is coated on both sides of the current collector, and the negative electrode sheet 3 is obtained by drying, rolling and die cutting.

[0099] Furthermore, the negative electrode 1, negative electrode sheet 2, negative electrode sheet 3, positive electrode, separator, and electrolyte are respectively fabricated into soft-pack batteries 1, 2, and 3 with a capacity of 50Ah.

[0100] Performance testing

[0101] 1. Negative Electrode Elongation Test: Battery 1, Battery 2, and Battery 3 were fully charged to 4.35V using a constant current and constant voltage of 16A, with a charging cutoff current of 2.5A. After full charging, the batteries were disassembled, and the negative electrode sheets were measured. The elongation was calculated and compared with the dimensions of the die-cut negative electrode sheets. The specific conversion method is as follows: Width elongation: Width W1 of the disassembled negative electrode sheet, width W2 of the initial die-cut electrode sheet, elongation = (W1-W2) / W2×100%; Height elongation: Height H1 of the disassembled negative electrode sheet, width H2 of the initial die-cut electrode sheet, elongation = (H1-H2) / H2×100%. The test results are shown in Table 1.

[0102] Table 1. Elongation test results of the battery negative electrode sheets prepared in the comparative examples and embodiments.

[0103] Battery pack Negative electrode width elongation High elongation of negative electrode Battery 1 (Comparative Example 1) 3.22％ 4.51％ Battery 2 (Example 1) 1.62％ 2.63％ Battery 3 (Example 2) 1.08％ 2.01％

[0104] As can be seen from the test results in Table 1, the high-silicon content negative electrode made of untreated copper foil (Comparative Example 1) exhibits excessive elongation of the negative electrode sheet after the battery is fully charged. This can easily cause the electrode sheet size to exceed the separator and overlap with the positive electrode, creating a short circuit risk. In batteries 2 (Example 1) and 3 (Example 2), the copper foil underwent pretreatment, which preemptively offset the elastic phase of the copper foil. This resulted in a significant reduction in the elongation of the negative electrode sheet after battery fabrication, greatly improving the safety performance of the high-energy-density battery.

[0105] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0106] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer coated on the surface of the negative electrode current collector; The negative electrode current collector is a copper foil, which is a copper foil that has undergone stretching pretreatment to reach the yield stage or the strengthening stage. The negative electrode active material layer includes graphite material, silicon carbon material, conductive agent and binder, wherein the mass of silicon carbon material accounts for 10-40% of the total mass of the negative electrode active material layer.

2. The negative electrode sheet according to claim 1, characterized in that, After the battery is fully charged, the width elongation of the negative electrode sheet is less than 2.5%, and the height elongation is less than 3%.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The tensile force in the tensile pretreatment is 50–300 MPa, and the tensile deformation rate of the copper foil is 0.1–2%.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that, The thickness of the copper foil is 3–12 μm.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that, The particle size D of the graphite material 50 Its diameter is 8–16 μm, and its specific surface area is 0.9–1.4 m². 2 / g.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that, The silicon-carbon material is vapor-deposited silicon-carbon; and / or, The conductive agent includes a short-range conductive agent and a long-range conductive agent; the short-range conductive agent includes conductive carbon black SP; and the long-range conductive agent includes single-walled carbon nanotubes; and / or, The adhesive comprises styrene-butadiene rubber and polyacrylic acid.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The graphite material accounts for 30-90% of the total mass of the negative electrode active material layer; and / or, The conductive agent comprises 0.01% to 10% of the total mass of the negative electrode active material layer; and / or, The binder accounts for 0.6% to 10% of the total mass of the negative electrode active material layer.

8. The negative electrode sheet according to any one of claims 1 to 7, characterized in that, The negative electrode active material layer also includes a dispersant, the mass of which accounts for 0.1% to 5% of the total mass of the negative electrode active material layer.

9. A method for preparing a negative electrode sheet according to any one of claims 1 to 8, characterized in that, Includes the following steps: S1: The copper foil is stretched and pretreated to reach the yielding stage or strengthening stage of the copper foil to obtain the negative electrode current collector; S2: Mix graphite material, silicon carbon material, conductive agent, binder and other optional additives in a certain proportion to obtain a negative electrode active coating. Apply the negative electrode active coating to the negative electrode current collector and dry it to obtain a negative electrode sheet.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes a negative electrode sheet according to any one of claims 1 to 8.

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