Negative electrode sheet, secondary battery and electrical apparatus

Abstract

Provided in the present application are a negative electrode sheet, a secondary battery and an electrical apparatus. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, the negative electrode film layer comprising a binder. The negative electrode film layer comprises a first area and a second area; the mass ratio of the binder in the first area to the binder in the second area is 0.1-1.4; the first area is an area in which the extending distance from the surface of the side of the negative electrode film layer away from the negative electrode current collector to the interior of the negative electrode film layer is within h / 2; and the second area is an area in which the extending distance from the surface of the side of the negative electrode film layer close to the negative electrode current collector to the first area is within h / 2, h representing the thickness of the negative electrode film layer. The negative electrode sheet is a negative electrode sheet subjected to flame heating treatment.

Description

Negative electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202310796987.1 filed on June 30, 2023, entitled “Negative Electrode Sheet, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a negative electrode plate, a secondary battery, and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] The performance of the negative electrode sheet has a critical impact on the performance of secondary batteries. Currently, the negative electrode sheet has many defects and cannot meet the application requirements of the new generation of electrochemical systems.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode plate, in which the mass ratio of the binder in the first region to the binder in the second region is less than 1.4, thereby reducing the degree of aggregation of the binder in the first region, which is beneficial to reducing the DC impedance of the battery and improving the electrical performance of the battery.

[0008] A first aspect of the present application provides a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, the negative electrode film layer comprising a binder;

[0009] In which, the negative electrode film layer includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is the region where the surface of the negative electrode film layer is away from the negative electrode current collector and extends to the inside of the negative electrode film layer within a distance of h / 2, and the second region is the region where the surface of the negative electrode film layer is close to the negative electrode current collector and extends to the first region within a distance of h / 2, h represents the thickness of the negative electrode film layer, and the negative electrode pole piece is a negative electrode pole piece after flame heating treatment.

[0010] By using flame heating to treat the negative electrode sheet, the mass ratio of the binder in the first region to the binder in the second region can be controlled to be 0.1 to 1.4, thereby reducing the degree of aggregation of the binder in the first region and thereby reducing the coating of the binder on the negative electrode active material in the first region, so that the metal ions in the secondary battery can be better embedded in and extracted from the negative electrode sheet, improving the battery's kinetic performance, reducing the battery's DC impedance, and enhancing the battery's cycle performance. For example, in a lithium secondary battery, controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.1 to 1.4 is conducive to the embedding and extraction of lithium ions in the negative electrode sheet, improving the kinetic performance of the lithium secondary battery, reducing its DC impedance, and thus improving the lithium deposition window on the surface of the negative electrode sheet. In addition, flame heating is low-cost, simple and easy to operate, and does not add excessive economic and labor costs to the production process.

[0011] In any embodiment, the mass ratio of the binder in the first region to the binder in the second region is 0.6-1.

[0012] The mass ratio of the binder in the first region to the binder in the second region is further controlled to be 0.6 to 1, which greatly reduces the aggregation of the binder in the first region, can further take into account the bonding performance of the negative electrode sheet and the DC impedance of the battery, and comprehensively improve the cycle performance of the battery.

[0013] In any embodiment, the binder comprises a water-based binder, and the water-based binder comprises one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate). In some embodiments, the water-based binder comprises styrene-butadiene rubber.

[0014] In any embodiment, the mass content of the binder in the first region is 0.18% to 2% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the first region is 0.3% to 1% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the second region is 0.32% to 3% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the second region is 0.7% to 2% based on the total mass of the negative electrode film layer.

[0015] In any embodiment, the sum of the mass contents of the binder in the first region and the binder in the second region is 0.5% to 5% based on the total mass of the negative electrode film layer. In some embodiments, the sum of the mass contents of the binder in the first region and the binder in the second region is 1% to 3% based on the total mass of the negative electrode film layer.

[0016] Controlling the mass content of the binder in the first region within a suitable range can not only make the battery have a lower DC impedance, but also bind the negative electrode active material to maintain the shape of the negative electrode sheet; controlling the mass content of the binder in the second region within a suitable range can not only make the negative electrode sheet have excellent bonding performance, but also control the impact of the introduction of the binder on the DC impedance of the battery.

[0017] In any embodiment, the negative electrode film layer further includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-carbon composite, lithium titanate, and silicon-oxygen composite.

[0018] A second aspect of the present application further provides a method for preparing a negative electrode sheet, comprising:

[0019] preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0020] Treating the negative electrode piece with flame heating to obtain a flame-heated negative electrode piece;

[0021] In which, the negative electrode film layer after flame heating treatment includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is the region where the surface of the negative electrode film layer is away from the negative electrode current collector and extends to the inside of the negative electrode film layer within a distance of h / 2, and the second region is the region where the surface of the negative electrode film layer is close to the negative electrode current collector and extends to the first region within a distance of h / 2, where h represents the thickness of the negative electrode film layer.

[0022] By using flame heating to treat the negative electrode sheet, the mass ratio of the binder in the first region to the binder in the second region can be controlled to be 0.1 to 1.4, thereby reducing the degree of aggregation of the binder in the first region, and thereby reducing the coating of the binder on the negative electrode active material in the first region, so that the metal ions in the secondary battery can be better embedded in and extracted from the negative electrode sheet, improving the battery's kinetic performance, reducing the battery's DC impedance, and enhancing the battery's cycle performance. For example, in a lithium secondary battery, controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.1 to 1.4 is conducive to the embedding and extraction of lithium ions in the negative electrode sheet, improving the kinetic performance of the lithium secondary battery, reducing its DC impedance, and thus improving the lithium deposition window on the surface of the negative electrode sheet. In addition, flame heating is low-cost, simple and easy to operate, and does not add excessive economic and labor costs to the production process.

[0023] In any embodiment, before the flame heating treatment of the negative electrode sheet, the mass ratio of the binder in the first region to the binder in the second region is 1.5-10.

[0024] Before the negative electrode plate is flame-heated, the mass ratio of the binder in the first region to the binder in the second region is 1.5 to 10. Due to the aggregation of the binder in the first region, the DC impedance of the battery is increased, affecting the electrical performance of the battery. After the negative electrode plate is flame-heated, the mass ratio of the binder in the first region to the binder in the second region can be made 0.1 to 1.4, which significantly reduces the aggregation of the binder in the first region, reduces the DC impedance of the battery, and improves the performance of the battery.

[0025] In any embodiment, the flame heating treatment of the negative electrode plate comprises:

[0026] Place the negative electrode piece under the flame and move it;

[0027] The surface of the negative electrode plate away from the negative electrode current collector is roasted by flame.

[0028] An independent gas supply system provides gas, which flows through a pipeline to a nozzle, forming a stable flame at the nozzle. This flame roasts the surface of the negative electrode plate away from the negative current collector, rapidly raising the temperature of the negative electrode plate away from the negative current collector, thereby removing some of the binder in the first region. Simultaneously, the negative electrode plate is controlled to move at a constant speed beneath the flame, reducing the impact of excessive surface temperature on the negative electrode active material within the negative electrode plate.

[0029] In any embodiment, when the flame is used to heat the negative electrode plate, the flame gas flow rate Q and the movement speed V of the negative electrode plate under the flame are controlled to satisfy: 0.05≤Q / V≤0.3. In some embodiments, Q and V are controlled to satisfy: 0.06≤Q / V≤0.25.

[0030] The flame gas flow rate Q is controlled to satisfy the above relationship with the movement speed V of the negative electrode plate under the flame, so that the surface of the negative electrode plate away from the negative electrode current collector has a suitable temperature, thereby effectively removing part of the binder in the first region, and then making the mass ratio of the binder in the first region to the binder in the second region 0.1 to 1.4, thereby reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0031] In any embodiment, when the flame is used to heat the negative electrode plate, the flame gas flow rate Q is controlled to be 3 L / min to 15 L / min. In some embodiments, the flame gas flow rate Q is controlled to be 4 L / min to 12 L / min.

[0032] In any embodiment, when the flame is used to heat the negative electrode sheet, the movement speed V of the negative electrode sheet under the flame is 10 m / min to 180 m / min. In some embodiments, the movement speed V of the negative electrode sheet under the flame is controlled to be 30 m / min to 150 m / min.

[0033] In any embodiment, when the negative electrode plate is subjected to flame heating, the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector is 300° C. to 700° C. In some embodiments, when the negative electrode plate is subjected to flame heating, the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector is 400° C. to 700° C.

[0034] By controlling the gas flow rate Q of the flame, the moving speed V of the negative electrode plate under the flame, or the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector within a suitable range, part of the binder in the first region can be removed, so that the mass ratio of the binder in the first region to the binder in the second region is reduced to 0.1 to 1.4, and the influence of flame heating on the negative electrode active material can be reduced.

[0035] In any embodiment, the specific steps include:

[0036] The raw materials including the negative electrode active material and the binder are uniformly mixed with deionized water to prepare a negative electrode slurry, and the negative electrode slurry is coated on the negative electrode current collector to prepare the negative electrode film layer to obtain the negative electrode sheet. In some embodiments, the raw materials further include sodium carboxymethyl cellulose;

[0037] Place the negative electrode piece under the flame and move it;

[0038] The surface of the negative electrode plate away from the negative electrode current collector is roasted by flame to obtain the flame-heated negative electrode plate.

[0039] The mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate after flame heating treatment is 0.1 to 1.4, which can effectively reduce the aggregation of the binder in the first region, reduce the DC impedance of the battery, and improve the performance of the battery.

[0040] In any embodiment, the preparation method further comprises a compaction treatment step, and the compaction treatment step is performed before or after the flame heating treatment of the negative electrode sheet.

[0041] In any embodiment, the preparation method further comprises a die-cutting step, wherein the die-cutting step is performed after the compacting step, and the die-cutting step is performed before or after the flame heating treatment of the negative electrode sheet.

[0042] The step of flame heating the negative electrode plate can be placed before or after the compaction step or the die-cutting step, and can ensure that the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate after the flame heating treatment is 0.1 to 1.4.

[0043] The third aspect of the present application provides a secondary battery, comprising the negative electrode sheet described in the first aspect of the present application or the negative electrode sheet prepared by the preparation method described in the second aspect of the present application.

[0044] In any embodiment, the secondary battery includes one or more of a lithium secondary battery, a sodium secondary battery, and a potassium secondary battery.

[0045] A fourth aspect of the present application provides an electrical device comprising the secondary battery described in the third aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG1 is a schematic diagram of a negative electrode sheet according to an embodiment of the present application;

[0047] FIG2 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0048] FIG3 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG2 ;

[0049] FIG4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0050] FIG5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0051] FIG6 is an exploded view of the battery pack shown in FIG5 according to an embodiment of the present application;

[0052] FIG. 7 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0053] Explanation of the reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate; 501 negative electrode film layer, 5011 first region, 5012 second region; 502 negative electrode current collector. DETAILED DESCRIPTION

[0054] Below, the embodiments of the negative electrode sheet, secondary battery, and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0055] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0057] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0058] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0059] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0060] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0061] Currently, the negative electrode film layer in the negative electrode sheet exhibits uneven binder distribution across its thickness. For example, the proportion of styrene-butadiene rubber binder in the negative electrode film increases gradually from closer to the negative electrode current collector to farther away from the negative electrode current collector. This concentration of styrene-butadiene rubber binder on the side of the negative electrode sheet away from the negative electrode current collector increases impedance, affecting the battery's electrical performance. Therefore, a new negative electrode sheet is needed to enable batteries to meet the requirements of the next generation of electrochemical systems.

[0062] [Negative electrode]

[0063] Based on this, the present application proposes a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, wherein the negative electrode film layer includes a binder;

[0064] Among them, the negative electrode film layer includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is the region where the surface of the negative electrode film layer away from the negative electrode current collector extends to the inside of the negative electrode film layer within a distance of h / 2, and the second region is the region where the surface of the negative electrode film layer close to the negative electrode current collector extends to the first region within a distance of h / 2, h represents the thickness of the negative electrode film layer, and the negative electrode pole piece is a negative electrode pole piece after flame heating treatment.

[0065] In some embodiments, the binder comprises a water-based binder, and the water-based binder comprises one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate). In some embodiments, the water-based binder comprises styrene-butadiene rubber.

[0066] In some embodiments, the negative electrode film layer further includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-carbon composite, lithium titanate, and silicon-oxygen composite.

[0067] In some embodiments, the negative electrode film layer may further include a conductive agent, which includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0068] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0069] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on one side of the negative electrode current collector. As shown in Figure 1(a), the negative electrode plate includes a negative electrode current collector 502 and a negative electrode film layer 501 located on one side of the negative electrode current collector 502. The negative electrode film layer includes a first region 5011 and a second region 5012. The first region 5011 is the region of the negative electrode film layer 501 extending within a distance h / 2 from the surface of the negative electrode film layer 501 away from the negative electrode current collector 502 to the interior of the negative electrode film layer 501. The second region 5012 is the region of the negative electrode film layer 501 extending within a distance h / 2 from the surface of the negative electrode film layer 501 close to the negative electrode current collector 502 to the first region 5011.

[0070] In some embodiments, the negative electrode plate includes a negative electrode current collector and negative electrode film layers located on opposite sides of the negative electrode current collector. As shown in Figure 1(b), the negative electrode plate includes a negative electrode current collector 502 and negative electrode film layers 501 located on opposite sides of the negative electrode current collector. The negative electrode film layer 501 includes a first region 5011 and a second region 5012. The first region 5011 is the region of the negative electrode film layer 501 on the side away from the negative electrode current collector 502 that extends within a distance h / 2 into the interior of the negative electrode film layer 501. The second region 5012 is the region of the negative electrode film layer 501 on the side close to the negative electrode current collector 502 that extends within a distance h / 2 into the first region 5011.

[0071] In some embodiments, the mass ratio of the binder in the first region to the binder in the second region can be selected as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.1, 1.2, 1.3, 1.4, or a value in the range consisting of any two of the above points.

[0072] In some embodiments, the negative electrode plate further includes a coating located between the negative electrode current collector and the negative electrode film layer. The coating includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the coating further includes styrene-butadiene rubber. The coating thickness can be 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, 0.8 μm, 0.95 μm, 0.9 μm, 0.95 μm, 1 μm, or a value within a range consisting of any two of the foregoing values.

[0073] As used herein, the term "aqueous binder" refers to a binder using water as a dispersant, wherein the binder is dispersed in water and includes but is not limited to styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate or poly(styrene-acrylate).

[0074] In this article, the mass ratio of the binder in the first region to the binder in the second region can be tested by any known method. As an example, a differential scanning thermogravimetric analyzer is used for testing. The testing instrument is a STA449F3 differential scanning thermogravimetric analyzer. The specific test process is as follows: measure the thickness of the negative electrode film layer in the negative electrode sheet; take a double-sided tape and stick it on the surface of the negative electrode sheet to be tested, then use a pressure roller to roll it three times in the same direction, tear off the double-sided tape, and the powder stuck on the tape is counted as the first region powder (Note: The thickness of the first region where the tape sticks should be 1 / 2 of the thickness of the negative electrode film layer. The specific operation is: measure the thickness of the film layer left on the negative electrode sheet. If the ratio of the thickness of the film layer left on the negative electrode sheet to the thickness of the negative electrode film layer is less than 1 / 2, continue to use the tape to stick it); use a scraper to gently scrape off the powder left on the negative electrode sheet, taking care not to get on the copper foil. This part of the powder is counted as the second region powder. The binder content in the upper and lower layers of powder was tested according to the following steps: weigh about 50 mg of sample in an Al2O3 crucible and shake it flat; parameter setting: nitrogen atmosphere, purge gas 60 mL / min, protective gas 20 mL / min; temperature rise program: 10℃ / min, 35℃~600℃, to ensure that the sample is completely dispersed, and then draw a thermogravimetric-temperature change graph. The weight loss range of the binder is 300℃~600℃, and the binder content in the first region / second region is obtained respectively, and then the binder mass ratio in the first region / second region is calculated.

[0075] By using flame heating to treat the negative electrode sheet, the mass ratio of the binder in the first region to the binder in the second region can be controlled to be 0.1 to 1.4, thereby reducing the degree of aggregation of the binder in the first region, and further reducing the coating of the binder on the negative electrode active material in the first region, so that the metal ions in the secondary battery can be better embedded in and extracted from the negative electrode sheet, improving the battery's kinetic performance, reducing the battery's DC impedance, and enhancing the battery's cycle performance. For example, in a lithium secondary battery, controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.1 to 1.4 is conducive to the embedding and extraction of lithium ions in the negative electrode sheet, improving the kinetic performance of the lithium secondary battery, reducing its DC impedance, and thus improving the lithium deposition window on the surface of the negative electrode sheet. In addition, flame heating is low-cost, simple and easy to operate, and does not add excessive economic and labor costs to the production process.

[0076] In some embodiments, the mass ratio of the binder in the first region to the binder in the second region is 0.6-1.

[0077] The mass ratio of the binder in the first region to the binder in the second region is further controlled to be 0.6 to 1, which greatly reduces the aggregation of the binder in the first region, can further take into account the bonding performance of the negative electrode sheet and the DC impedance of its battery, and comprehensively improve the cycle performance of the battery.

[0078] In some embodiments, the mass content of the binder in the first region is 0.18% to 2% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the first region is 0.3% to 1% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the second region is 0.32% to 3% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the second region is 0.7% to 2% based on the total mass of the negative electrode film layer. In some embodiments, the mass content of the binder in the first region is 0.18%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, or a value in a range consisting of any two of the above, based on the total mass of the negative electrode film layer. The mass content of the binder in the second region is 0.32%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.6%, 2.8%, 3%, or a value in the range consisting of any two of the above points.

[0079] In some embodiments, the sum of the mass contents of the binder in the first region and the binder in the second region is 0.5% to 5% based on the total mass of the negative electrode film layer. In some embodiments, the sum of the mass contents of the binder in the first region and the binder in the second region is 1.0% to 3.0% based on the total mass of the negative electrode film layer. In some embodiments, the sum of the mass contents of the binder in the first region and the binder in the second region, based on the total mass of the negative electrode film layer, can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a value in a range consisting of any two of the above.

[0080] Controlling the mass content of the binder in the first region within an appropriate range can not only make the battery have a lower DC impedance, but also bind the negative electrode active material to maintain the morphology of the negative electrode sheet; controlling the mass content of the binder in the second region within an appropriate range can not only make the negative electrode sheet have excellent bonding performance (bonding force between the negative electrode film layer and the negative electrode current collector), but also control the impact of the introduction of the binder on the DC impedance of the battery.

[0081] The present application also provides a method for preparing a negative electrode sheet, comprising:

[0082] preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0083] Treating the negative electrode piece with flame heating to obtain a flame-heated negative electrode piece;

[0084] Among them, the negative electrode film layer after flame heating treatment includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is the region where the surface of the negative electrode film layer away from the negative electrode current collector extends to the inside of the negative electrode film layer within a distance of h / 2, and the second region is the region where the surface of the negative electrode film layer close to the negative electrode current collector extends to the first region within a distance of h / 2, and h represents the thickness of the negative electrode film layer.

[0085] By using flame heating to treat the negative electrode sheet, the mass ratio of the binder in the first region to the binder in the second region can be controlled to be 0.1 to 1.4, thereby reducing the degree of aggregation of the binder in the first region, and thereby reducing the coating of the binder on the negative electrode active material in the first region, so that the metal ions in the secondary battery can be better embedded in and extracted from the negative electrode sheet, improving the battery's kinetic performance, reducing its DC impedance, and improving the battery's cycle performance. For example, in a lithium secondary battery, controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.1 to 1.4 is conducive to the embedding and extraction of lithium ions in the negative electrode sheet, improving the kinetic performance of the lithium secondary battery, reducing its DC impedance, and thus improving the lithium deposition window on the surface of the negative electrode sheet. In addition, flame heating is low-cost, simple and easy to operate, and does not add excessive economic and labor costs to the production process.

[0086] In some embodiments, before the flame heat treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet is 1.5 to 10. In some embodiments, before the flame heat treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or a value in a range consisting of any two of the foregoing values.

[0087] Before the negative electrode plate is flame-heated, the mass ratio of the binder in the first region to the binder in the second region is 1.5 to 10. Due to the aggregation of the binder in the first region, the DC impedance of the battery is increased, affecting the electrical performance of the battery. After the negative electrode plate is flame-heated, the mass ratio of the binder in the first region to the binder in the second region can be made 0.1 to 1.4, which significantly reduces the aggregation of the binder in the first region, reduces the DC impedance of the battery, and improves the performance of the battery.

[0088] In some embodiments, the flame heating treatment of the negative electrode plate includes:

[0089] Place the negative electrode under the flame and move it;

[0090] The surface of the negative electrode sheet away from the negative electrode current collector is roasted with a flame.

[0091] It is understood that by providing gas through an independent gas supply system, the gas passes through a pipeline to the nozzle, forming a stable flame at the nozzle, roasting the surface of the negative electrode plate on the side away from the negative current collector, thereby increasing the temperature of the negative electrode plate. The temperature of the surface of the negative electrode plate on the side away from the negative current collector can rise rapidly, and the degree of temperature rise decreases as the depth of the negative electrode plate increases. Therefore, it can effectively remove part of the binder in the first region. The amount of binder removed in the second region by flame heating is much less than the amount of binder removed in the first region, achieving the effect of reducing binder aggregation in the first region, reducing the DC impedance of the battery, and improving the electrical performance of the battery. At the same time, controlling the negative electrode plate to move at a uniform speed under the flame can reduce the impact of excessive surface temperature of the negative electrode plate on the negative active material in the negative electrode plate.

[0092] In this article, the nozzle for forming the flame is in the shape of an elongated strip, and a plurality of side-by-side small holes for transmitting the combustible gas are arranged in the nozzle.

[0093] In some embodiments, when the negative electrode plate is treated by flame heating, the gas flow rate Q of the flame is controlled and the velocity V of the negative electrode plate under the flame satisfies the following relationship: 0.05≤Q / V≤0.3. In some embodiments, the gas flow rate Q of the flame is controlled and the velocity V of the negative electrode plate under the flame satisfies the following relationship: 0.06≤Q / V≤0.25. In some embodiments, when the negative electrode plate is treated by flame heating, the gas flow rate Q of the flame is controlled and the velocity V of the negative electrode plate under the flame satisfies the following relationship: Q / V can be 0.05, 0.06, 0.08, 0.1, 0.12, 0.14, 0.15, 0.16, 0.18, 0.2, 0.22, 0.24, 0.25, 0.26, 0.28, 0.3, or a value within a range consisting of any two of the foregoing values.

[0094] In this article, Q / V is a ratio of values, with the unit of Q being L / min and the unit of V being m / min.

[0095] Herein, the term "flame-controlling gas flow rate Q" refers to the total flow rate of gas for forming a stable flame.

[0096] In this document, the term "moving speed V of the negative electrode piece below the flame" refers to the moving speed of the negative electrode piece in the horizontal direction below the nozzle forming the flame.

[0097] It can be understood that a greater flame gas flow rate Q is more conducive to increasing the surface temperature of the negative electrode plate on the side away from the negative current collector; a greater velocity V of the negative electrode plate beneath the flame is less conducive to increasing the surface temperature of the negative electrode plate on the side away from the negative current collector. The flame gas flow rate Q and the velocity V of the negative electrode plate beneath the flame are controlled to satisfy the aforementioned relationship to maintain a suitable surface temperature on the side of the negative electrode plate away from the negative current collector, thereby effectively removing some of the binder in the first region and achieving a mass ratio of binder in the first region to binder in the second region of 0.1 to 1.4, thereby reducing the DC impedance of the battery and improving its cycling performance.

[0098] In some embodiments, when flame heating is used to treat the negative electrode plate, the flame-controlled gas flow rate Q is 3 L / min to 15 L / min. In some embodiments, the flame-controlled gas flow rate Q is 4 L / min to 12 L / min. In some embodiments, the flame-controlled gas flow rate Q can be selected from 3 L / min, 4 L / min, 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min, 10 L / min, 11 L / min, 12 L / min, 13 L / min, 14 L / min, 15 L / min, or a value within a range consisting of any two of the foregoing values.

[0099] In some embodiments, when the negative electrode sheet is treated by flame heating, the moving speed V of the negative electrode sheet under the flame is 10m / min to 180m / min. In some embodiments, the moving speed V of the negative electrode sheet under the flame is 30m / min to 150m / min. In some embodiments, when the negative electrode sheet is treated by flame heating, the moving speed V of the negative electrode sheet under the flame can be selected from 10m / min, 20m / min, 30m / min, 40m / min, 50m / min, 60m / min, 70m / min, 80m / min, 90m / min, 100m / min, 110m / min, 120m / min, 130m / min, 140m / min, 150m / min, 160m / min, 170m / min, 180m / min, or a value in the range consisting of any two of the above points.

[0100] In some embodiments, when the negative electrode sheet is treated by flame heating, the surface temperature T of the negative electrode sheet on the side away from the negative electrode current collector is 300°C to 700°C. In some embodiments, T is 400°C to 700°C. In some embodiments, when the negative electrode sheet is treated by flame heating, the surface temperature T of the negative electrode sheet on the side away from the negative electrode current collector can be 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, or a value within a range consisting of any two of the foregoing values.

[0101] It is understood that since the surface of the negative electrode plate is heated by a flame, the negative electrode plate can be heated in a short period of time, causing the surface temperature of the negative electrode plate to rise rapidly. By controlling the flame gas flow rate Q and the movement speed V of the negative electrode plate under the flame, the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector is kept within a suitable range. The surface temperature of the negative electrode plate can be measured using a temperature sensor. In addition, the decomposition temperature and reaction temperature of the negative electrode active material are both above 800°C. Therefore, if the surface temperature T of the negative electrode plate is less than or equal to 700°C, the binder can be decomposed, but the impact on the negative electrode active material is negligible.

[0102] By controlling the gas flow rate Q of the flame, the moving speed V of the negative electrode plate under the flame, or the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector within a suitable range, part of the binder in the first region can be removed, so that the mass ratio of the binder in the first region to the binder in the second region is reduced to 0.1 to 1.4, and the influence of flame heating on the negative electrode active material can be reduced.

[0103] In some embodiments, the specific steps include:

[0104] The raw materials including the negative electrode active material and the binder are uniformly mixed with deionized water to prepare a negative electrode slurry, and the negative electrode slurry is coated on the negative electrode current collector to prepare a negative electrode film layer to obtain a negative electrode sheet. In some embodiments, the raw materials further include sodium carboxymethyl cellulose;

[0105] Place the negative electrode under the flame and move it;

[0106] The surface of the negative electrode plate away from the negative electrode current collector is roasted with a flame to obtain the flame-heated negative electrode plate.

[0107] It is understood that during the process of applying the negative electrode slurry to the negative electrode current collector and drying it to prepare the negative electrode film layer, due to the affinity between the binder and the negative electrode active material, the upward flow of the solvent during solvent evaporation causes the binder to float upward, resulting in the binder being concentrated on the surface of the negative electrode sheet away from the negative electrode current collector, thereby increasing the DC impedance of the battery. Furthermore, the binder concentration on the surface of the negative electrode sheet can also cause the negative electrode film on the negative electrode sheet to stick to the roller during subsequent rolling of the negative electrode sheet, resulting in damage to the negative electrode sheet. In some embodiments of the present application, flame heating is used to treat the negative electrode sheet, which can effectively remove some of the binder in the first region. The mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode sheet is 0.1 to 1.4, which can effectively reduce the accumulation of binder in the first region, reduce the DC impedance of the battery, improve battery performance, and also facilitate the processing of the negative electrode sheet.

[0108] In some embodiments, the preparation method further comprises a compaction treatment step, and the compaction treatment step is performed before or after the flame heating treatment of the negative electrode sheet.

[0109] In some embodiments, the preparation method further includes a die-cutting step, which is performed after the compacting step, and the die-cutting step is performed before or after the flame heating treatment of the negative electrode sheet.

[0110] The step of flame heating the negative electrode plate can be placed before or after the compaction step or the die-cutting step, and can ensure that the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate after the flame heating treatment is 0.1 to 1.4.

[0111] [Positive electrode]

[0112] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0113] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0114] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0115] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0116] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0117] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0118] [Electrolytes]

[0119] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0120] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0121] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0122] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0123] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0124] [Isolation film]

[0125] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0126] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0127] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0128] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0129] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0130] [Secondary battery]

[0131] In some embodiments, a secondary battery includes a positive electrode sheet, an electrolyte, a separator, and in some embodiments, a negative electrode sheet.

[0132] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0133] In some embodiments, the secondary battery includes one or more of a lithium secondary battery, a sodium secondary battery, and a potassium secondary battery.

[0134] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0135] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0136] The present application does not particularly limit the shape of the secondary battery; it can be cylindrical, square, or any other shape. For example, FIG2 shows a secondary battery 5 having a square structure as an example. The secondary battery can also be a sodium ion battery, a magnesium ion battery, or a potassium ion battery.

[0137] In some embodiments, referring to FIG3 , the outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0138] [Battery Module]

[0139] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0140] Figure 4 shows an example battery module 4. Referring to Figure 4 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0141] In some embodiments, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 may be received in the receiving space.

[0142] [Battery Pack]

[0143] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0144] Figures 5 and 6 illustrate an example battery pack 1. Referring to Figures 5 and 6 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0145] [Electrical devices]

[0146] In one embodiment of the present application, an electric device is provided, comprising at least one of a secondary battery according to any embodiment, a battery module according to any embodiment, or a battery pack according to any embodiment.

[0147] The electrical device includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0148] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0149] Figure 7 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0150] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0151] Example

[0152] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0153] 1. Preparation method

[0154] Example 1

[0155] 1) Negative electrode

[0156] The negative electrode active material artificial graphite, thickener sodium carboxymethyl cellulose, and binder styrene butadiene rubber (SBR) were mixed in a weight ratio of 96.5:1.5:2.0 and dissolved in deionized water to prepare the negative electrode slurry. The negative electrode slurry was obtained under the action of a vacuum mixer and then coated on the surface of the copper foil. It was then transferred to a vacuum drying oven for complete drying. The weight per side was 100mg / 1540.25mm 2 , and then the negative electrode is obtained by cold pressing.

[0157] The negative electrode sheet is treated by flame heating. The prepared negative electrode sheet is placed under the flame-generating nozzle and moved at a speed of 30 m / min. The flame gas flow rate Q is controlled at 8 L / min. The generated flame roasts the surface of the negative electrode sheet away from the current collector, and the surface temperature of the negative electrode sheet is heated to 689.5°C, resulting in the flame-heated negative electrode sheet.

[0158] 2) Preparation of positive electrode sheet

[0159] The binder polyvinylidene fluoride is fully dissolved in N-methylpyrrolidone, and then the conductive agent carbon black and the ternary positive electrode active material LiNi are added. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 was stirred and mixed uniformly. The weight ratio of polyvinylidene fluoride, carbon black, and ternary positive electrode active material was 1:1:98 to obtain a positive electrode slurry. The slurry was evenly coated on the surface of the current collector aluminum foil to form a positive electrode film. The film was then transferred to a vacuum drying oven for complete drying. The dried electrode was roll-pressed and punched to obtain the positive electrode sheet.

[0160] 3) Electrolyte

[0161] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), based on the total mass of the electrolyte, 30wt% of ethylene carbonate and 60wt% of dimethyl carbonate were mixed uniformly, and then 9wt% of lithium hexafluorophosphate and 1wt% of the additive vinylene carbonate were added and dissolved in the above mixed solvent, stirred and mixed to prepare an electrolyte.

[0162] 4) Isolation film

[0163] Polypropylene film is used as the isolation film.

[0164] 5) Battery Preparation

[0165] The positive electrode sheet, separator, and negative electrode sheet are stacked in order. The separator is placed between the positive and negative electrode sheets to act as an insulator. The bare cell is welded with tabs and placed in an aluminum shell. The bare cell is baked at 80°C to remove water. The above electrolyte is then injected and sealed to obtain an uncharged battery. The uncharged battery is then subjected to the following steps: standing, hot and cold pressing, formation, shaping, and capacity testing to obtain the lithium secondary battery product of Example 1. Examples 2 to 16

[0166] The battery preparation methods in Examples 2 to 16 are basically similar to those in Example 1, but the moving speed V of the negative electrode plate or the gas flow rate Q of the flame is adjusted. The specific parameters are shown in Table 1.

[0167] Comparative Example 1

[0168] The battery preparation method in Comparative Example 1 is basically similar to that in Example 1, but flame heating is not used to heat the negative electrode plate. The specific parameters are shown in Table 1.

[0169] Comparative Examples 2 to 5

[0170] The battery preparation methods in Comparative Examples 2 to 5 are basically similar to those in Example 1, but the flame gas flow rate Q and the movement speed V of the negative electrode plate are adjusted at the same time to make the Q / V value lower than 0.04 or higher than 0.4. The specific parameters are shown in Table 1.

[0171] 2. Performance Testing

[0172] 1. Negative electrode performance test

[0173] 1) Test of the mass ratio of the binder in the first region to the binder in the second region

[0174] The test was conducted using a differential scanning thermogravimetric analyzer (STA449F3). The specific test process is as follows: measure the thickness of the negative electrode film in the negative electrode; apply double-sided tape to the surface of the negative electrode to be tested, then roll it three times in the same direction with a roller. Remove the double-sided tape, and the powder removed from the tape is considered the first region (note: the thickness of the first region removed by the tape should be 1 / 2 the thickness of the negative electrode film. The specific procedure is: measure the thickness of the film remaining on the negative electrode. If the ratio of the film thickness remaining on the negative electrode to the negative electrode film thickness is less than 1 / 2, continue to apply the tape). Gently scrape the powder remaining on the negative electrode with a scraper, taking care not to get on the copper foil. This part of the powder is considered the second region. The binder content in the upper and lower layers of powder was tested according to the following steps: weigh about 50 mg of sample in an Al2O3 crucible and shake it flat; parameter setting: nitrogen atmosphere, purge gas 60 mL / min, protective gas 20 mL / min; temperature rise program: 10℃ / min, 35℃~600℃, to ensure that the sample is completely dispersed, and then draw a thermogravimetric-temperature change graph. The weight loss range of the binder is 300℃~600℃, and the binder content in the first region / second region is obtained respectively, and then the binder mass ratio in the first region / second region is calculated.

[0175] 2) Relative adhesion test

[0176] At 25°C, use the prepared negative electrode sheet as the test piece. Use a razor blade to cut a sample with a width of 30mm and a length of 100-160mm. Apply special double-sided tape with a width of 20mm and a length of 90-150mm to a steel plate. Attach the negative electrode film layer of the cut negative electrode sheet sample to the double-sided tape and then roll it three times in the same direction with a roller. Secure a paper tape with a width equal to the electrode sheet and a length 80-200mm longer than the sample to the negative electrode current collector and secure it with crepe adhesive. Turn on the power of the tensile testing machine, the indicator light will light up, adjust the limit block to the appropriate position, fix the end of the steel plate that is not attached to the electrode with the lower clamp, fold the paper tape upwards, and fix it with the upper clamp. Use the "up" and "down" buttons on the manual controller attached to the tensile testing machine to adjust the position of the upper clamp, turn on the dedicated computer connected to the tensile testing machine, double-click the desktop software icon, and perform the test. The tensile rate is 50m / min, the test distance is 50mm, and the software takes a data point every 10s. These data point values ​​are used as the vertical coordinate and the corresponding test distance is used as the horizontal coordinate to obtain the bonding force curve of the negative electrode. The force when the negative electrode is under force balance is divided by the width of the tape as the bonding force of the negative electrode film layer per unit length to characterize the bonding strength between the negative electrode film layer and the negative electrode current collector. The data in Table 2 uses the bonding force of Example 5 as the benchmark value (100%), and the other embodiments or comparative examples are all relative values ​​relative to the bonding force of Example 5.

[0177] 2. Battery performance test

[0178] 1) Relative DC resistance (DCR) test

[0179] At 25°C, the prepared battery was charged at a constant current of 1C to 4.2V, then charged at a constant voltage of 4.2V to a current of 0.05C. After 5 minutes of stagnation, the battery was discharged at a constant current of 1C for 12 minutes, and the voltage V1 was recorded. After 5 minutes of stagnation, the battery was placed at -10°C for 2 hours, and then discharged at 4C for 10 seconds. The voltage V2 was recorded. The DC resistance (DCR) of the battery was calculated as (V2 - V1) / 4C. The data in Table 2 uses the internal resistance (DCR) of the battery in Example 5 as the reference value (100%). The values ​​of the other examples and comparative examples are relative to the DC resistance (DCR) of the battery in Example 5.

[0180] 2) Battery cycle capacity retention test

[0181] The battery capacity retention rate test process is as follows: at 25°C, the prepared battery is charged to 4.3V at a constant current of 1 / 3C, then charged to a current of 0.05C at a constant voltage of 4.3V, left for 5 minutes, and then discharged to 2.8V at 1 / 3C. The obtained capacity is recorded as the initial capacity C0. Repeat the above steps for the same battery mentioned above, and at the same time record the discharge capacity Cn of the battery after the nth cycle. The battery capacity retention rate after each cycle is Pn = Cn / C0 × 100%. The 500 point values ​​P1, P2...500 are used as the vertical coordinates, and the corresponding number of cycles is used as the horizontal coordinates to obtain a curve graph of the battery capacity retention rate and the number of cycles. During this test process, the first cycle corresponds to n = 1, the second cycle corresponds to n = 2, ... the 500th cycle corresponds to n = 500. The battery capacity retention rate data corresponding to the embodiment or comparative example in Table 2 are the data measured after 500 cycles under the above test conditions, that is, the value of P500. The test process of the comparative example and other embodiments is the same as above.

[0182] 3. Analysis of test results of various embodiments and comparative examples

[0183] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0184] Table 1

[0185] Table 2

[0186] According to the above results, the negative electrode sheets in Examples 1 to 16 include a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, the negative electrode film layer includes a second region and a first region in a direction from close to the negative electrode current collector to away from the negative electrode current collector, the thickness ratio of the first region to the negative electrode film layer is 1 / 2, and the thickness ratio of the second region to the negative electrode film layer is 1 / 2; wherein, both the first region and the second region include a binder, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, and the negative electrode sheet is a negative electrode sheet after flame heating treatment.

[0187] From the comparison between Examples 1 to 16 and Comparative Example 1, it can be seen that flame heating treatment of the negative electrode plates is beneficial to reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0188] Comparisons of Examples 1-16 with Comparative Examples 2-5 show that flame-heating the negative electrode sheet and controlling the mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode sheet to 0.1-1.4 not only improves the bonding performance of the negative electrode sheet, but also helps reduce the DC impedance of the battery and improves the battery's cycling performance. Comparisons of Examples 1-5 with Examples 6-7 show that further controlling the mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode sheet to 0.4-1.0 can also balance the bonding performance of the negative electrode sheet with the DC impedance of the battery, thereby comprehensively improving the battery's cycling performance to the greatest extent.

[0189] A comparison of Examples 1-7 with Comparative Examples 2-3 shows that when the gas flow rate Q is 8 L / min, excessively high or low negative electrode plate movement speeds can cause the negative electrode plate surface temperature to fall below 300°C or rise above 730°C. A negative electrode plate surface temperature below 300°C is detrimental to the removal of the styrene-butadiene rubber (SBR) in the first region, while a negative electrode plate surface temperature above 730°C may cause decomposition of the negative electrode active material, affecting the battery's electrical performance. Controlling the negative electrode plate movement speed within an appropriate range can remove the styrene-butadiene rubber (SBR) in the first region while minimizing the impact on the negative electrode active material.

[0190] A comparison of Examples 3, 9-15 with Comparative Examples 4-5 shows that when the negative electrode plate's travel speed is 60 m / min, excessively high or low gas flow rate Q can cause the negative electrode plate's surface temperature to fall below 300°C or rise above 730°C. A negative electrode plate surface temperature below 300°C is detrimental to the removal of the styrene-butadiene rubber (SBR) in the first region, while a negative electrode plate surface temperature above 730°C may cause decomposition of the negative electrode active material, affecting the battery's electrical performance. Controlling the negative electrode plate's travel speed within an appropriate range can remove the styrene-butadiene rubber (SBR) in the first region while minimizing the impact on the negative electrode active material.

[0191] From the comparison of Examples 1 to 16 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by flame heating, the flame gas flow rate Q is controlled to be 3L / min to 15L / min, and the mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode plate can be 0.1 to 1.4, thereby reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0192] From the comparison between Examples 1 to 16 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by flame heating, the moving speed V of the negative electrode plate under the flame is controlled to be 10 m / min to 180 m / min, and the mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode plate can be 0.1 to 1.4, thereby reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0193] From the comparison of Examples 1 to 16 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by flame heating, the surface temperature T of the negative electrode plate on the side away from the negative electrode current collector is controlled to be 300°C to 700°C, and the mass ratio of the binder in the first region to the binder in the second region of the treated negative electrode plate can be 0.1 to 1.4, thereby reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0194] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode sheet, wherein: The negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector. The negative electrode film layer includes a binder; The negative electrode film layer includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is a region where the surface of the negative electrode film layer away from the negative electrode current collector extends within a distance of h / 2 to the inside of the negative electrode film layer, and the second region is a region where the surface of the negative electrode film layer close to the negative electrode current collector extends within a distance of h / 2 to the first region, h represents the thickness of the negative electrode film layer, and the negative electrode pole piece is a negative electrode pole piece after flame heating treatment.

2. The negative electrode sheet according to claim 1, wherein: The mass ratio of the binder in the first region to the binder in the second region is 0.6-1.

3. The negative electrode sheet according to claim 1 or 2, wherein: The adhesive comprises a water-based adhesive, and the water-based adhesive comprises one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate); optionally, the water-based adhesive comprises styrene-butadiene rubber.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: Based on the total mass of the negative electrode film layer, the mass content of the binder in the first region is 0.18% to 2%, optionally 0.3% to 1%, and the mass content of the binder in the second region is 0.32% to 3%, optionally 0.7% to 2%; and / or Based on the total mass of the negative electrode film layer, the sum of the mass contents of the binder in the first region and the binder in the second region is 0.5% to 5%, and can be optionally 1% to 3%.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The negative electrode film layer also includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-carbon composite, lithium titanate, and silicon-oxygen composite.

6. A method for preparing a negative electrode sheet, wherein: include: Preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain the negative electrode plate; Treating the negative electrode sheet by flame heating to obtain the negative electrode sheet after flame heating; Among them, the negative electrode film layer after the flame heating treatment includes a first region and a second region, the mass ratio of the binder in the first region to the binder in the second region is 0.1 to 1.4, the first region is the region where the surface of the negative electrode film layer away from the negative electrode current collector extends to the inside of the negative electrode film layer within a distance of h / 2, and the second region is the region where the surface of the negative electrode film layer close to the negative electrode current collector extends to the first region within a distance of h / 2, and h represents the thickness of the negative electrode film layer.

7. The preparation method according to claim 6, wherein: Before the flame heating treatment of the negative electrode plate, the mass ratio of the binder in the first region to the binder in the second region is 1.5-10.

8. The preparation method according to claim 7, wherein: The flame heating treatment of the negative electrode plate comprises: Place the negative electrode sheet under the flame and move it; The surface of the negative electrode sheet which is away from the negative electrode current collector is roasted by flame.

9. The preparation method according to claim 8, wherein: When the flame is used to heat the negative electrode plate, the gas flow Q of the flame is controlled to satisfy the following relationship with the moving speed V of the negative electrode plate under the flame: 0.05≤Q / V≤0.3, which can be optionally 0.06≤Q / V≤0.

25.

10. The preparation method according to claim 6 or 9, wherein: When the flame is used to heat the negative electrode plate, the gas flow rate Q of the flame is controlled to be 3L / min to 15L / min, and can be optionally 4L / min to 12L / min; and / or The moving speed V of the negative electrode sheet under the flame is 10m / min to 180m / min. The speed can be selected from 30m / min to 150m / min.

11. The preparation method according to any one of claims 6 to 10, wherein When the negative electrode plate is treated by flame heating, the surface temperature T of the negative electrode plate at the side away from the negative electrode current collector is 300° C. to 700° C., and can be 400° C. to 700° C.

12. The preparation method according to any one of claims 6 to 11, wherein include: Uniformly mixing raw materials including negative electrode active materials and binders with deionized water to prepare negative electrode slurry, coating the negative electrode slurry on a negative electrode current collector to prepare the negative electrode film layer, and obtaining the negative electrode sheet; optionally, the raw materials further include sodium carboxymethyl cellulose; Place the negative electrode sheet under the flame and move it; The surface of the negative electrode sheet away from the negative electrode current collector is roasted by flame to obtain the negative electrode sheet after flame heating.

13. The preparation method according to any one of claims 6 to 12, wherein The preparation method further comprises a compaction treatment step, which is performed before or after the flame heating treatment of the negative electrode sheet; Optionally, the preparation method further comprises a die-cutting step, wherein the die-cutting step is performed after the compacting step, and the die-cutting step is performed before or after the flame heating treatment of the negative electrode sheet.

14. A secondary battery, wherein: A negative electrode sheet comprising any one of claims 1 to 5 or a negative electrode sheet prepared by the preparation method of any one of claims 6 to 13; optionally, the secondary battery comprises one or more of a lithium secondary battery, a sodium secondary battery, and a potassium secondary battery.

15. An electrical device, wherein: Includes the secondary battery as claimed in claim 14.

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