A negative electrode sheet and a method for manufacturing the same

Abstract

This invention relates to the field of battery technology, specifically to a negative electrode sheet and its preparation method. The negative electrode sheet includes a negative electrode current collector and a negative electrode coating disposed on at least one side of the current collector. When the negative electrode sheet is in a wound state, the difference in thickness between the outer and inner rings of the negative electrode sheet is ≤2µm, the difference in curvature between the outer and inner rings is ≤0.6mm / m, the difference in peel force between the outer and inner rings is ≤3N / m, and the difference in moisture content between the outer and inner rings is ≤30ppm. Compared with the prior art, this invention improves the consistency of the negative electrode sheet's thickness, curvature, peel force, and moisture content by employing a process combining two baking cycles and rewinding before slitting.

Description

Technical Field

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

[0002] With the rapid development of the lithium battery industry, the issue of battery consistency has become increasingly prominent. How to effectively produce battery cells with high consistency has become a key focus for the industry. Improving cell consistency primarily involves improving the consistency of electrode properties, ensuring that electrode thickness, moisture content, peel strength, and curvature are at a consistent level.

[0003] The existing electrode production method mainly follows the traditional production process: coating-rolling-slitting-baking-winding. The problem with this production mode is that the internal stress of the electrode is not completely removed after processing. The release of internal stress during baking causes the electrode to expand, which is mainly manifested in: ① inconsistent stress release between the inner and outer rings, with the outer ring electrode being thicker than the inner ring; ② inconsistent moisture drying degree, with the outer ring electrode having lower moisture content than the inner ring; ③ inconsistent peel strength, with the outer ring electrode having higher peel strength than the inner ring; ④ the outer ring electrode having significantly worse curvature than the inner ring, resulting in more scrap of the outer ring electrode. Summary of the Invention

[0004] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a negative electrode sheet and its preparation method. By employing a process combining two baking cycles and rewinding before slitting, the uneven heating of the inner and outer rings of the negative electrode roll is effectively improved, promoting the uniform release of internal stress, thereby improving the consistency of the negative electrode sheet's thickness, curvature, peel strength, and moisture content.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode coating disposed on at least one side surface of the negative electrode current collector; When the negative electrode sheet is in a wound state, the difference in thickness between the outer ring and the inner ring of the negative electrode sheet is ≤2µm, the difference in curvature between the outer ring and the inner ring of the negative electrode sheet is ≤0.6mm / m, the difference in peel force between the outer ring and the inner ring of the negative electrode sheet is ≤3N / m, and the difference in moisture content between the outer ring and the inner ring of the negative electrode sheet is ≤30ppm.

[0006] According to another aspect of the present invention, a method for preparing a negative electrode sheet is provided, comprising the following steps: S1. The negative electrode slurry is coated on at least one side surface of the negative electrode current collector to obtain the first negative electrode; S2. The first negative electrode obtained in S1 is wound, cold-pressed, and baked once to obtain the second negative electrode; S3. The second negative electrode obtained in S2 is rewound and baked a second time to obtain the third negative electrode; S4. Cut the third negative electrode obtained in S3 to obtain the negative electrode sheet.

[0007] In some of these embodiments, in step S1, the negative electrode slurry includes a negative electrode active material, a conductive agent, a dispersant, and a binder.

[0008] In some embodiments, in step S2, the pressure of the cold pressing is 25t~55t, the temperature of the cold pressing is 20℃~25℃, and the speed of the cold pressing is 60m / min~100m / min.

[0009] In some embodiments, in step S2, the thickness of the first negative electrode after cold pressing is 91µm~95µm.

[0010] In some embodiments, step S2 includes a heating stage, a holding stage, and a cooling stage; the heating stage lasts for 1-2 hours; the holding stage lasts for 2-4 hours at a temperature of 110°C-130°C; and the cooling stage lasts for 1-2 hours.

[0011] In some of these embodiments, in step S2, the outer ring thickness of the second negative electrode is 94µm to 98µm.

[0012] In some of these embodiments, the outer ring curvature of the second negative electrode is 0.4 mm / m to 0.8 mm / m.

[0013] In some of these embodiments, the moisture content of the outer ring of the second negative electrode is 200 ppm to 250 ppm.

[0014] In some of these embodiments, the outer ring peeling force of the second negative electrode is 9 N / m to 11 N / m.

[0015] In some implementations, step S3 includes rewinding the second negative electrode by swapping the positions of the beginning and end of the winding.

[0016] In some embodiments, step S3 includes a heating stage, a holding stage, and a cooling stage; the heating stage lasts for 1-2 hours; the holding stage lasts for 2-4 hours at a temperature of 110°C-130°C; and the cooling stage lasts for 1-2 hours.

[0017] In some embodiments, in step S3, the outer ring thickness of the third negative electrode is 97µm~98µm.

[0018] In some embodiments, the outer ring curvature of the third negative electrode is 0.6 mm / m to 1.0 mm / m.

[0019] In some embodiments, the outer ring moisture content of the third negative electrode is 200 ppm to 240 ppm.

[0020] In some of these embodiments, the outer ring peel force of the third negative electrode is 9 N / m to 11 N / m.

[0021] The technical solution of the present invention has at least the following beneficial effects: The present invention optimizes the preparation method of the negative electrode sheet by adding winding and two baking after cold pressing, and introducing a rewinding step between the two baking steps to fully release the internal stress of the electrode sheet, so that the thickness, curvature, peeling force and moisture of the inner and outer rings of the negative electrode sheet are closer, thereby improving the consistency of the negative electrode sheet.

[0022] Additional aspects and advantages of the invention 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 the invention. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 This is a flowchart illustrating the preparation of the negative electrode sheet in Example 1 of the present invention. Detailed Implementation

[0025] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0026] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0027] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0028] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0029] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0030] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0031] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0032] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0033] With the rapid development of the lithium battery industry, the issue of battery consistency has become increasingly prominent. How to effectively produce battery cells with high consistency has become a key focus for the industry. Improving cell consistency primarily involves improving the consistency of electrode properties, ensuring that electrode thickness, moisture content, peel strength, and curvature are at a consistent level.

[0034] Current electrode production methods mainly follow the traditional production process: coating-rolling-slitting-baking-winding. The inventors discovered that the internal stress of the electrode after processing is not completely removed. During baking, the release of internal stress causes the electrode to expand, mainly manifested in: ① inconsistent stress release between the inner and outer rings, with the outer ring electrode being thicker than the inner ring; ② inconsistent moisture drying, with the outer ring electrode having lower moisture content than the inner ring; ③ inconsistent peel strength, with the outer ring electrode having higher peel strength than the inner ring; ④ significantly worse curvature of the outer ring electrode compared to the inner ring, resulting in more scrap of the outer ring electrode.

[0035] Based on this, the present invention optimizes the preparation method of the negative electrode sheet by adding winding, rewinding and two baking processes after traditional rolling, and eliminating the baking step after traditional slitting. This can effectively and uniformly release the internal stress of the negative electrode sheet, thereby improving the consistency of electrode sheet thickness, moisture content, peeling force and curvature.

[0036] Specifically, the present invention adopts the following technical solution: According to one aspect of the present invention, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode coating disposed on at least one side surface of the negative electrode current collector; When the negative electrode sheet is in a wound state, the difference in thickness between the outer ring and the inner ring of the negative electrode sheet is ≤2µm, the difference in curvature between the outer ring and the inner ring of the negative electrode sheet is ≤0.6mm / m, the difference in peel force between the outer ring and the inner ring of the negative electrode sheet is ≤3N / m, and the difference in moisture content between the outer ring and the inner ring of the negative electrode sheet is ≤30ppm.

[0037] In a specific embodiment of the present invention, the negative electrode coating preferably comprises a negative electrode active material, a conductive agent, a dispersant, and a binder. The negative electrode active material preferably comprises one or more of graphite, hard carbon, and activated carbon; the conductive agent preferably comprises one or more of conductive carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the dispersant preferably comprises one or more of N-methylpyrrolidone (NMP), polyvinylpyrrolidone (PVP), sodium dodecylbenzenesulfonate (SDBS), and sodium carboxymethyl cellulose (CMC); the binder preferably comprises one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The present invention does not impose any special restrictions on the source of the above-mentioned negative electrode active material, binder, conductive agent, and dispersant; commercially available products well known to those skilled in the art can be used.

[0038] In a specific embodiment of the present invention, when the negative electrode sheet is in a wound state, the difference in thickness between the outer ring and the inner ring of the negative electrode sheet is ≤2µm, the difference in curvature between the outer ring and the inner ring of the negative electrode sheet is ≤0.6mm / m, the difference in peel force between the outer ring and the inner ring of the negative electrode sheet is ≤3N / m, and the difference in moisture content between the outer ring and the inner ring of the negative electrode sheet is ≤30ppm. The present invention uses the above-mentioned negative electrode sheet to effectively ensure the consistency of thickness, moisture content, peel force, and curvature between the inner and outer rings of the negative electrode sheet, thereby further improving the consistency of the battery cell.

[0039] like Figure 1 As shown, according to another aspect of the present invention, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1. The negative electrode slurry is coated on at least one side surface of the negative electrode current collector to obtain the first negative electrode; S2. The first negative electrode obtained in S1 is wound, cold-pressed, and baked once to obtain the second negative electrode; S3. The second negative electrode obtained in S2 is rewound and baked a second time to obtain the third negative electrode; S4. Cut the third negative electrode obtained in S3 to obtain the negative electrode sheet.

[0040] The present invention first coats the negative electrode slurry onto at least one side of the surface of the negative electrode current collector to obtain the first negative electrode.

[0041] In a specific embodiment of the present invention, in step S1, the negative electrode slurry preferably includes a negative electrode active material, a conductive agent, a dispersant, and a binder. Specifically, the first negative electrode can be prepared by the following method: firstly, the negative electrode active material, the conductive agent, the dispersant, and the binder are mixed in a solvent in a certain proportion to form a slurry, and then the negative electrode slurry is coated onto a negative electrode current collector, and dried and rolled to obtain the first negative electrode. The solvent is preferably water, and the negative electrode current collector is preferably copper foil.

[0042] After obtaining the first negative electrode, the first negative electrode is wound, cold-pressed, and baked once to obtain the second negative electrode.

[0043] In a specific embodiment of the present invention, in step S2, the pressure of the cold pressing is preferably 25t to 55t, specifically 25t, 30t, 35t, 40t, 45t, 50t, 55t, and any value between the two mentioned above; the temperature of the cold pressing is preferably 20℃ to 25℃, specifically 20℃, 21℃, 22℃, 23℃, 24℃, 25℃, and any value between the two mentioned above; the speed of the cold pressing is preferably 60m / min to 100m / min, specifically 60m / min, 70m / min, 80m / min, 90m / min, 100m / min, and any value between the two mentioned above. In this invention, the above-mentioned cold-pressing conditions are used to compact the electrode thickness. The thickness of the first negative electrode after cold pressing is preferably 91µm to 95µm, specifically 91µm, 91.5µm, 92µm, 92.5µm, 93µm, 93.5µm, 94µm, 94.5µm, 95µm, and any value between these two ranges. This invention controls the thickness of the first negative electrode after cold pressing within the above range to ensure that the subsequently obtained second negative electrode meets the performance requirements of this invention.

[0044] In a specific embodiment of the present invention, step S2, the primary baking includes a heating stage, a heat preservation stage, and a cooling stage; the heating stage is preferably 1-2 hours, specifically 1 hour, 1.5 hours, 2 hours, or any value between the two; the heat preservation stage is preferably 110°C-130°C, specifically 110°C, 115°C, 120°C, 125°C, 130°C, or any value between the two; the heat preservation stage is preferably 2 hours-4 hours, specifically 2 hours, 3 hours, 4 hours, or any value between the two; the cooling stage is preferably 1-2 hours, specifically 1 hour, 1.5 hours, 2 hours, or any value between the two. By selecting the above-mentioned suitable primary baking conditions, the present invention can release the internal stress accumulated on the outer ring of the electrode roll due to winding and cold pressing, thereby initially optimizing the performance of the outer ring of the electrode roll and preventing deformation or performance degradation due to sudden stress release during subsequent processing or battery use.

[0045] In a specific embodiment of the present invention, in step S2, the negative electrode roll after winding and cold pressing is baked once to obtain the second negative electrode. The thickness of the second negative electrode is preferably 94µm to 98µm; specifically, it can be 94µm, 95µm, 96µm, 97µm, 98µm, or any value between any two of the above. The curvature of the second negative electrode is preferably 0.4mm / m to 0.8mm / m; specifically, it can be 0.4mm / m, 0.5mm / m, 0.6mm / m, 0.7mm / m, 0.8mm / m, or any value between any two of the above. The moisture content of the second negative electrode is preferably 200ppm to 250ppm; specifically, it can be 200ppm, 210ppm, 220ppm, 230ppm, 240ppm, 250ppm, or any value between any two of the above. The peeling force of the second negative electrode is preferably 9N / m to 11N / m, specifically 9N / m, 10N / m, 11N / m, and any value between the two mentioned above.

[0046] After obtaining the second negative electrode, the second negative electrode is rewound and baked a second time to obtain the third negative electrode.

[0047] In a specific embodiment of the present invention, step S3 preferably includes rewinding by swapping the positions of the first and last ends of the second negative electrode roll. The rewinding operation alters the layered structure of the wound negative electrode, causing the side originally located on the inner ring to become the outer ring, and vice versa. This disrupts the original stress distribution of the negative electrode roll, allowing the inner and outer ring electrodes to experience more uniform constraint and heating conditions during subsequent baking, thus creating conditions for further uniform release of internal stress.

[0048] In a specific embodiment of the present invention, step S3, the secondary baking includes a heating stage, a holding stage, and a cooling stage; the heating stage is preferably 1-2 hours, specifically 1 hour, 1.5 hours, 2 hours, or any value between the two; the holding stage is preferably 110°C-130°C, specifically 110°C, 115°C, 120°C, 125°C, 130°C, or any value between the two; the holding stage is preferably 2 hours-4 hours, specifically 2 hours, 3 hours, 4 hours, or any value between the two; the cooling stage is preferably 1-2 hours, specifically 1 hour, 1.5 hours, 2 hours, or any value between the two. The present invention uses the above-mentioned suitable secondary baking conditions to release the internal stress of the electrode sheet that becomes the outer ring after rewinding, ultimately ensuring that the internal stress of the entire electrode roll, both inner and outer rings, is fully and uniformly released.

[0049] In a specific embodiment of the present invention, in step S3, the thickness of the third negative electrode is 96µm to 98µm; specifically, it can be 96µm, 96.5µm, 97µm, 97.5µm, 98µm, or any value between any two of the above. The curvature of the third negative electrode is preferably 0.6mm / m to 1.0mm / m; specifically, it can be 0.6mm / m, 0.7mm / m, 0.8mm / m, 0.9mm / m, 1.0mm / m, or any value between any two of the above. The moisture content of the third negative electrode is preferably 200ppm to 240ppm; specifically, it can be 200ppm, 210ppm, 220ppm, 230ppm, 240ppm, or any value between any two of the above. The peeling force of the third negative electrode is preferably 9N / m to 11N / m; specifically, it can be 9N / m, 10N / m, 11N / m, or any value between any two of the above.

[0050] In a specific embodiment of the present invention, the negative electrode coil, which has undergone secondary baking and whose internal stress has been uniformly released, is slit and cut into the required size and shape. Since stress release has been completed before slitting, the slit electrode sheets have excellent consistency and can be directly used in subsequent cell assembly processes such as winding or stacking to form electrode assemblies.

[0051] The following detailed description of this application is based on specific embodiments, but the implementation and protection of this invention are not limited thereto. The following embodiments are only some embodiments of this application and are not intended to limit this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0052] Example 1 Graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were added to deionized water and thoroughly mixed to prepare a slurry. This slurry was then coated onto both sides of a copper foil to obtain the first negative electrode. The first negative electrode was wound and cold-pressed at 23°C with a pressing pressure of 38t and a pressing speed of 80m / min until the thickness of the first negative electrode reached 93μm. The cold-pressed negative electrode roll was then baked for the first time, with the temperature increased to 120°C at a rate of 1°C / min, held for 4 hours, and then cooled for 2 hours to obtain the second negative electrode. The positions of the first and last ends of the second negative electrode roll were reversed, and it was rewound. A second baking process was then performed, with the temperature increased to 120°C at a rate of 2°C / min, held for 4 hours, and then cooled for 2 hours to obtain the third negative electrode. The third negative electrode was then slit and processed to obtain the negative electrode sheet.

[0053] Example 2 Graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were added to deionized water and thoroughly mixed to prepare a slurry. This slurry was then coated onto both sides of a copper foil to obtain the first negative electrode. The first negative electrode was wound and cold-pressed at 23°C with a pressing pressure of 43t and a pressing speed of 80m / min until the thickness of the first negative electrode reached 95μm. The cold-pressed negative electrode roll was then baked for the first time, with the temperature increased to 120°C at a rate of 2°C / min, held for 3 hours, and then cooled for 2 hours to obtain the second negative electrode. The positions of the first and last ends of the second negative electrode roll were reversed, and it was rewound. A second baking process was then performed, with the temperature increased to 120°C at a rate of 2°C / min, held for 4 hours, and then cooled for 2 hours to obtain the third negative electrode. The third negative electrode was then slit and processed to obtain the negative electrode sheet.

[0054] Example 3 Graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were added to deionized water and thoroughly mixed to prepare a slurry. This slurry was then coated onto both sides of a copper foil to obtain the first negative electrode. The first negative electrode was wound and cold-pressed at 20°C with a pressing pressure of 55t and a pressing speed of 80m / min until the thickness of the first negative electrode reached 94μm. The cold-pressed negative electrode roll was then baked for the first time, with the temperature increased to 120°C at a rate of 2°C / min, held for 4 hours, and then cooled for 2 hours to obtain the second negative electrode. The positions of the first and last ends of the second negative electrode roll were reversed, and it was rewound. A second baking process was then performed, with the temperature increased to 120°C at a rate of 1°C / min, held for 4 hours, and then cooled for 2 hours to obtain the third negative electrode. The third negative electrode was then slit and processed to obtain the negative electrode sheet.

[0055] Comparative Example 1 Graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were added to deionized water and thoroughly mixed to prepare a slurry. This slurry was then coated onto both sides of a copper foil to obtain the first negative electrode. The first negative electrode was cold-pressed at 23°C with a pressing pressure of 38t and a pressing speed of 80m / min until its thickness reached 93μm. The cold-pressed first negative electrode was then slit and heated to 120°C at a rate of 1°C / min, held at that temperature for 4 hours, and then cooled for 2 hours to obtain the second negative electrode. The second negative electrode was then wound to obtain the negative electrode sheet.

[0056] Comparative Example 2 Graphite, conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were added to deionized water and thoroughly mixed to prepare a slurry. This slurry was then coated onto both sides of a copper foil to obtain the first negative electrode. The first negative electrode was wound and cold-pressed at 23°C with a pressing pressure of 38t and a pressing speed of 80m / min until the thickness of the first negative electrode reached 93μm. The cold-pressed negative electrode roll was then baked, heated to 120°C at a heating rate of 1°C / min, held at that temperature for 4 hours, and then cooled for 2 hours to obtain the second negative electrode. The second negative electrode was then slit and processed to obtain the negative electrode sheet.

[0057] Performance testing: The consistency of the negative electrode sheets prepared in the above embodiments and comparative examples was tested. Samples were taken from the outer ring of the negative electrode rolls after the first baking and the outer ring of the negative electrode rolls after the second baking in Examples 1-3. Samples were taken from the outer and inner rings of the negative electrode sheet in Comparative Example 1, and from the outer and inner rings of the second negative electrode in Comparative Example 2. The uniformity of internal stress release was evaluated by comparing the differences between the two sampling test data. The outer ring of the negative electrode rolls after the second baking in Examples 1-3 is designated as the outer ring, and the corresponding outer ring of the negative electrode rolls after the first baking is designated as the inner ring.

[0058] The specific tests are as follows: a Karl Fischer moisture meter was used to test the moisture content, a peel force tester was used to test the peel force, a micrometer was used to test the thickness, and a 1m steel ruler was used to test the curvature.

[0059] The test results are shown in Table 1 below.

[0060] Table 1 Test data for each set of examples and comparative examples Referring to Table 1, the differences between the electrode sheets after the first and second baking processes meet the following requirements: thickness within ±2μm, moisture content difference ≤30ppm, peel strength difference ≤3N / m, and curvature control ≤2mm / m. This demonstrates that the present invention, by optimizing the preparation method of the negative electrode sheet, fully and uniformly releases the internal stress of the negative electrode sheet and ensures that the physical properties of the inner and outer rings of the electrode roll are consistent, effectively improving the consistency of the electrode sheets.

[0061] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0062] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0063] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that, Includes a negative electrode current collector and a negative electrode coating disposed on at least one side surface of the negative electrode current collector; When the negative electrode sheet is in a wound state, the difference in thickness between the outer ring and the inner ring of the negative electrode sheet is ≤2µm, the difference in curvature between the outer ring and the inner ring of the negative electrode sheet is ≤0.6mm / m, the difference in peel force between the outer ring and the inner ring of the negative electrode sheet is ≤3N / m, and the difference in moisture content between the outer ring and the inner ring of the negative electrode sheet is ≤30ppm.

2. A method for preparing the negative electrode sheet according to claim 1, characterized in that, Includes the following steps: S1. The negative electrode slurry is coated on at least one side surface of the negative electrode current collector to obtain the first negative electrode; S2. The first negative electrode obtained in S1 is wound, cold-pressed, and baked once to obtain the second negative electrode; S3. The second negative electrode obtained in S2 is rewound and baked a second time to obtain the third negative electrode; S4. Cut the third negative electrode obtained in S3 to obtain the negative electrode sheet.

3. The preparation method according to claim 2, characterized in that, In step S1, the negative electrode slurry includes a negative electrode active material, a conductive agent, a dispersant, and a binder.

4. The preparation method according to claim 2, characterized in that, In step S2, the pressure of the cold pressing is 25t~55t, the temperature of the cold pressing is 20℃~25℃, and the speed of the cold pressing is 60m / min~100m / min.

5. The preparation method according to claim 2, characterized in that, In step S2, the thickness of the first negative electrode after cold pressing is 91µm~95µm.

6. The preparation method according to claim 2, characterized in that, In step S2, the baking process includes a heating stage, a holding stage, and a cooling stage; the heating stage lasts for 1 to 2 hours; the holding stage lasts for 2 to 4 hours at a temperature of 110°C to 130°C; and the cooling stage lasts for 1 to 2 hours.

7. The preparation method according to claim 2, characterized in that, In step S2, the outer ring thickness of the second negative electrode is 94µm~98µm; And / or, the outer ring curvature of the second negative electrode is 0.4 mm / m to 0.8 mm / m; And / or, the moisture content of the outer ring of the second negative electrode is 200ppm~250ppm; And / or, the outer ring peeling force of the second negative electrode is 9 N / m to 11 N / m.

8. The preparation method according to claim 2, characterized in that, In step S3, the rewinding includes: swapping the positions of the first and last ends of the second negative electrode roll.

9. The preparation method according to claim 2, characterized in that, In step S3, the secondary baking includes a heating stage, a heat preservation stage, and a cooling stage; the heating stage lasts for 1 to 2 hours; the heat preservation stage lasts for 2 to 4 hours at a temperature of 110°C to 130°C; and the cooling stage lasts for 1 to 2 hours.

10. The preparation method according to claim 2, characterized in that, In step S3, the outer ring thickness of the third negative electrode is 97µm~98µm; And / or, the outer ring curvature of the third negative electrode is 0.6 mm / m to 1.0 mm / m; And / or, the outer ring moisture content of the third negative electrode is 200ppm~240ppm; And / or, the outer ring peeling force of the third negative electrode is 9 N / m to 11 N / m.

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