Negative pole piece, secondary battery and electric device

Abstract

The invention provides a negative pole piece, a secondary battery and an electric device. The negative electrode piece comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, and the negative electrode film layer comprises a binder; wherein 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, and the first area is an area where the surface of one side, far away from the negative electrode current collector, of the negative electrode film layer extends towards the interior of the negative electrode film layer by a distance within h / 2, and the second area is an area where the surface of the other side extends towards the interior of the negative electrode film layer by a distance within h / 2. The second region is a region extending from the surface of one side, close to the negative current collector, of the negative electrode film layer to the first region by a distance within h / 2, and h represents the thickness of the negative electrode film layer.

Description

Technical Field

[0001] 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

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] The performance of the negative electrode sheet has a key impact on the performance of the secondary battery. At present, the negative electrode sheet has many defects and cannot meet the application needs of the new generation of electrochemical systems. Summary of the invention

[0004] 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 or equal to 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.

[0005] 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;

[0006] 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, and h represents the thickness of the negative electrode film layer.

[0007] The mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1-1.4 to reduce the aggregation degree of the binder in the first region, 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 released from the negative electrode plate, improve the kinetic performance of the battery, reduce the DC impedance of the battery, and enhance the cycle performance of the battery. For example, in a lithium secondary battery, the mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1-1.4, which is beneficial to the embedding and release of lithium ions in the negative electrode plate, improves the kinetic performance of the lithium secondary battery, reduces its DC impedance, and thus improves the lithium precipitation window on the surface of the negative electrode plate.

[0008] In any embodiment, the mass ratio of the binder in the first region to the binder in the second region is 0.6-1.

[0009] Further controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.6-1 can not only reduce the aggregation of the binder in the first region, but also further take into account the bonding performance of the negative electrode sheet and the DC impedance of the battery, thereby comprehensively improving the cycle performance of the battery.

[0010] In any embodiment, the negative electrode plate is a negative electrode plate that has been subjected to electromagnetic induction heating treatment.

[0011] Electromagnetic induction heating can effectively remove part of the binder in the first region, which can reduce the aggregation of the binder in the first region and thus reduce the DC impedance of the battery, and retain another part of the binder to maintain the bonding between the negative electrode active materials in the negative electrode film layer to form a complete negative electrode sheet structure.

[0012] In any embodiment, the binder includes a water-based binder, and the water-based binder includes one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate); optionally, the water-based binder includes styrene-butadiene rubber.

[0013] In any embodiment, 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%.

[0014] In any embodiment, 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 1% to 3%.

[0015] 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 plate; controlling the mass content of the binder in the second region within a suitable range can not only make the negative electrode plate have excellent bonding performance, but also control the impact of the introduction of the binder on the DC impedance of the battery.

[0016] 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.

[0017] The second aspect of the present application also provides a method for preparing a negative electrode sheet, comprising:

[0018] Preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0019] 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, and h represents the thickness of the negative electrode film layer.

[0020] The mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4 to reduce the aggregation of the binder in the first region, 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 plate, improve the battery's kinetic performance, reduce the battery's DC impedance, and enhance the battery's cycle performance. For example, in a lithium secondary battery, the mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4, which is beneficial to the embedding and extraction of lithium ions in the negative electrode plate, improves the kinetic performance of the lithium secondary battery, reduces its DC impedance, and thus improves the lithium precipitation window on the surface of the negative electrode plate.

[0021] In any embodiment, the step of preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet comprises:

[0022] Preparing the negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0023] The negative electrode plate is treated by electromagnetic induction heating to obtain the negative electrode plate treated by electromagnetic induction heating.

[0024] Electromagnetic induction heating can effectively remove part of the binder in the first region, so that the mass ratio of the binder in the first region to the binder in the second region is reduced from 1.5 to 10 to 0.1 to 1.4, which can not only reduce the aggregation of the binder in the first region and thus reduce the DC impedance of the battery, but also retain another part of the binder to maintain the bonding between the negative electrode active materials in the negative electrode film layer to form a complete negative electrode sheet structure.

[0025] In any embodiment, before the electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet is 1.5-10.

[0026] Before the negative electrode plate is treated with electromagnetic induction heating, the mass ratio of the binder in the first region to the binder in the second region is 1.5-10. 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 treated with electromagnetic induction heating, the mass ratio of the binder in the first region to the binder in the second region can be made to 0.1-1.4, 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.

[0027] In any embodiment, the method of treating the negative electrode plate by electromagnetic induction heating comprises:

[0028] The negative electrode plate is moved inside the energized coil.

[0029] A magnetic field will be generated inside the energized coil, and the movement of the negative pole piece inside the coil is equivalent to cutting the magnetic induction lines in the magnetic field. Combining the electromagnetic induction theory and the skin effect, the current density on the surface of the negative pole piece is the largest. As the depth inside the negative pole piece increases, its current density decreases significantly. Therefore, current induction heating can effectively remove part of the binder in the first region to reduce the aggregation of the binder in the first region, reduce the DC impedance of the battery, and improve the electrical performance of the battery.

[0030] In any embodiment, when the negative electrode pole piece is treated by electromagnetic induction heating, the distance S between the coil and the negative electrode pole piece located at the center position inside the coil and the moving speed V of the negative electrode pole piece inside the energized coil satisfy: 50≤S×V≤900, optionally, 100≤S×V≤800, wherein S represents the shortest distance between the coil and the negative electrode pole piece located at the center position inside the coil along the thickness direction of the negative electrode pole piece, the unit of S is cm, and the unit of V is m / min.

[0031] The distance S between the control coil and the negative electrode pole piece located at the center of the coil and the moving speed V of the negative electrode pole piece inside the energized coil satisfy the above relationship, so that the surface of the negative electrode pole piece 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.

[0032] In any embodiment, the distance S between the coil and the negative electrode plate located at the center of the coil is 1 cm to 12 cm, and can be 5 cm to 10 cm.

[0033] In any embodiment, when the negative electrode plate is treated by electromagnetic induction heating, the movement speed V of the negative electrode plate inside the energized coil is 10 m / min to 180 m / min, and can be optionally 20 m / min to 150 m / min.

[0034] In any embodiment, when the negative electrode plate is treated by electromagnetic induction 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., and can be 400° C. to 700° C.

[0035] The distance S between the coil and the negative electrode pole piece located at the center of the coil, the moving speed V of the negative electrode pole piece inside the energized coil, and the surface temperature T of the negative electrode pole piece on the side away from the negative electrode current collector are controlled within appropriate ranges, thereby removing part of the binder in the first region so that the mass ratio of the binder in the first region to the binder in the second region is reduced to 0.1-1.4, and reducing the influence of electromagnetic induction heating on the negative electrode active material.

[0036] In any embodiment, the specific steps include:

[0037] 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 a negative electrode sheet; optionally, the raw materials further include sodium carboxymethyl cellulose;

[0038] The negative electrode sheet is moved inside the energized coil to obtain the negative electrode sheet after the electromagnetic induction heating treatment.

[0039] The mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate after electromagnetic induction 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 negative electrode plate is treated with the electromagnetic induction heating.

[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 negative electrode sheet is treated with the electromagnetic induction heating.

[0042] The step of treating the negative electrode plate with electromagnetic induction heating can be before or after the compaction step or the die-cutting step, and can ensure that after the electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate 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] Figure 1 is a schematic diagram of a negative electrode sheet according to an embodiment of the present application;

[0047] Figure 2 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0048] Figure 3 yes Figure 2 An exploded view of a secondary battery according to an embodiment of the present application is shown;

[0049] Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0050] Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0051] Figure 6 yes Figure 5 An exploded view of a battery pack according to an embodiment of the present application is shown;

[0052] Figure 7 FIG. 1 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0053] Description of reference numerals:

[0054] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate; 501 negative electrode film layer, 5011 first region, 5012 second region; 502 negative electrode current collector. DETAILED DESCRIPTION

[0055] Below, the embodiments of the negative electrode plate, secondary battery and electric device of the present application are specifically disclosed in detail with appropriate reference to the 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 structure 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 descriptions 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.

[0056] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a 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 a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present 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 real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it 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.

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

[0058] 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.

[0059] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means 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), which means 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.

[0060] If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0061] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0062] At present, the thickness of the negative electrode film layer in the negative electrode plate has the phenomenon of uneven distribution of binder. For example, the proportion of styrene butadiene rubber binder in the negative electrode film layer gradually increases from close to the negative electrode current collector to far away from the negative electrode current collector. Since the styrene butadiene rubber binder is concentrated on the side of the negative electrode plate away from the negative electrode current collector, the impedance increases, affecting the electrical performance of the battery. Therefore, it is necessary to provide a new negative electrode plate so that the battery can meet the needs of the new generation of electrochemical systems.

[0063] [Negative electrode]

[0064] Based on this, the present application proposes 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;

[0065] 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 to the inside of the negative electrode film layer within a distance of h / 2, and the second region is a 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. Binder In some embodiments, the binder includes an aqueous binder, and the aqueous binder includes one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate); optionally, the aqueous binder includes 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 sheet includes a negative electrode current collector and a negative electrode film layer located on one side of the negative electrode current collector. Figure 1 As shown in (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, and the negative electrode film layer includes a first region 5011 and a second region 5012, the first region 5011 is a region where the surface of the negative electrode film layer 501 on one side away from the negative electrode current collector 502 extends toward the inside of the negative electrode film layer 501 within a distance of h / 2, and the second region 5012 is a region where the surface of the negative electrode film layer 501 on one side close to the negative electrode current collector 502 extends toward the first region 5011 within a distance of h / 2.

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

[0071] In some embodiments, the negative electrode plate also includes a coating located between the negative electrode current collector and the negative electrode film layer, the coating including superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers at least one, the coating also includes styrene-butadiene rubber, the thickness of the coating can be selected as 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.95μm, 0.9μm, 0.95μm, 1μm, or a value in the range formed by any two of the above points.

[0072] In some embodiments, the mass ratio of the binder in the first region to the binder in the second region may 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 a range consisting of any two of the above points.

[0073] As used herein, the term "aqueous binder" refers to a binder dispersed in water using water as a dispersant, including but 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 test instrument is a STA449F3 differential scanning thermogravimetric analyzer. The specific test process is: 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 roll it three times in the same direction with a pressure roller, 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 tape to stick it); gently scrape off the negative electrode sheet with a scraper, and be careful not to hang on the copper foil. This part of the powder is counted as the second region powder. The upper and lower layers of powder were tested for the binder content in the upper and lower layers 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] The mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4 to reduce the aggregation degree of the binder in the first region, 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 released from the negative electrode plate, improve the kinetic performance of the battery, reduce the DC impedance of the battery, and enhance the cycle performance of the battery. For example, in a lithium secondary battery, the mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4, which is beneficial to the embedding and release of lithium ions in the negative electrode plate, improves the kinetic performance of the lithium secondary battery, reduces its DC impedance, and thus improves the lithium precipitation window on the surface of the negative electrode plate.

[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 0.6-1, which greatly reduces the aggregation of the binder in the first region, can further take into account the bonding performance of the negative electrode plate and the DC impedance of the battery, and comprehensively improve the cycle performance of the battery.

[0078] In some embodiments, the negative electrode plate is a negative electrode plate that has been subjected to electromagnetic induction heating treatment.

[0079] Electromagnetic induction heating can effectively remove part of the binder in the first region, which can reduce the aggregation of the binder in the first region and thus reduce the DC impedance of the battery, while retaining another part of the binder to maintain the bonding between the negative electrode active materials in the negative electrode film layer.

[0080] In some embodiments, 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%. In some embodiments, based on the total mass of the negative electrode film layer, 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 the range formed by any two of the above points, and 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 formed by any two of the above points.

[0081] In some embodiments, 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 1% to 3%. In some embodiments, 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 can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a value in the range consisting of any two of the above points.

[0082] 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 morphology 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 (bonding force between the negative electrode film layer and the negative electrode collector), but also control the effect of the introduction of the binder on the DC impedance of the battery.

[0083] The present application also provides a method for preparing a negative electrode sheet, comprising:

[0084] Preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0085] 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 a 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 a 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.

[0086] The mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4 to reduce the aggregation of the binder in the first region, 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 plate, improve the battery's kinetic performance, reduce the battery's DC impedance, and enhance the battery's cycle performance. For example, in a lithium secondary battery, the mass ratio of the binder in the first region to the binder in the second region is controlled to be 0.1 to 1.4, which is beneficial to the embedding and extraction of lithium ions in the negative electrode plate, improves the kinetic performance of the lithium secondary battery, reduces its DC impedance, and thus improves the lithium precipitation window on the surface of the negative electrode plate.

[0087] In some embodiments, the step of preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet includes:

[0088] Preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet;

[0089] The negative electrode piece is treated by electromagnetic induction heating to obtain the negative electrode piece treated by electromagnetic induction heating.

[0090] Electromagnetic induction heating can effectively remove part of the binder in the first region, so that the mass ratio of the binder in the first region to the binder in the second region is reduced from 1.5 to 10 to 0.1 to 1.4, which can not only reduce the aggregation of the binder in the first region and thus reduce the DC impedance of the battery, but also retain another part of the binder to maintain the bonding between the negative electrode active materials in the negative electrode film layer.

[0091] In some embodiments, before the negative electrode plate is subjected to electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region is 1.5 to 10. In some embodiments, before the negative electrode plate is subjected to electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region 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 the range formed by any two of the above points.

[0092] Before the negative electrode plate is treated with electromagnetic induction heating, the mass ratio of the binder in the first region to the binder in the second region is 1.5-10. 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 treated with electromagnetic induction heating, the mass ratio of the binder in the first region to the binder in the second region can be made to 0.1-1.4, 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.

[0093] In some embodiments, the method of treating the negative electrode plate by electromagnetic induction heating includes:

[0094] Move the negative pole piece inside the energized coil.

[0095] In this article, the shape of the coil is square, and the number of the coil may be one.

[0096] It can be understood that a magnetic field will be generated inside a coil that is energized. For example, a changing magnetic field will be generated inside a coil that is energized with alternating current. The movement of the negative pole piece inside the coil is equivalent to further cutting the magnetic induction lines in the magnetic field. Combining the electromagnetic induction theory and the skin effect, the current density on the surface of the negative pole piece is the largest. As the depth inside the negative pole piece increases, the current density decreases significantly, that is, the temperature on the surface of the negative pole piece can rise rapidly, while the temperature inside the negative pole piece gradually decreases with the increase in depth. Therefore, current induction heating can effectively remove part of the binder in the first region, and the amount of binder removed in the second region by current induction heating is much less than the amount of binder removed in the first region, thereby achieving the effect of reducing the aggregation of binder in the first region, reducing the DC impedance of the battery, and improving the electrical performance of the battery.

[0097] In addition, when negative electrode film layers are provided on both sides of the negative electrode current collector, electromagnetic induction heating can act on the negative electrode film layers on both sides of the negative electrode pole piece at the same time, and the influence of electromagnetic induction heating on the negative electrode current collector can be ignored, which greatly saves processing time and cost.

[0098] In some embodiments, when electromagnetic induction heating is used to treat the negative electrode pole piece, the distance S between the coil and the negative electrode pole piece located at the center of the coil and the moving speed V of the negative electrode pole piece inside the energized coil satisfy: 50≤S×V≤900, optionally, 100≤S×V≤800, wherein S represents the shortest distance between the coil and the negative electrode pole piece located at the center of the coil along the thickness direction of the negative electrode pole piece, the unit of S is cm, and the unit of V is m / min. In some embodiments, when electromagnetic induction heating is used to treat the negative electrode pole piece, the distance S between the coil and the negative electrode pole piece located at the center of the coil and the moving speed V of the negative electrode pole piece inside the energized coil satisfy S×V, which can be selected as 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, or a value in the range formed by any two of the above points.

[0099] In this article, S×V refers to the product of the values ​​of S and V, where S is in cm and V is in m / min.

[0100] In this article, when electromagnetic induction heating is used for the negative electrode plate, the negative electrode plate is located at the center position inside the coil, the plane where the coil is located coincides with the thickness direction of the negative electrode plate, and the four sides of the coil are perpendicular or parallel to the thickness direction of the negative electrode plate, that is, the plane where the coil is located is perpendicular to the surface of the negative electrode plate (the surface of the negative electrode plate is the plane perpendicular to the thickness direction of the negative electrode plate), and the vertical distances from the two sides of the coil parallel to the surface of the negative electrode plate to the negative electrode plate are equal.

[0101] In this article, the term "the distance S between the coil and the negative pole piece located at the center of the coil" refers to the distance from the coil to the surface of the negative pole piece in the thickness direction of the negative pole piece when the negative pole piece is located at the center of the coil.

[0102] In this article, the term "the negative pole piece moving speed V inside the energized coil" refers to the negative pole piece being located at the center of the coil and moving in a direction perpendicular to the plane of the coil, that is, the plane of the coil is perpendicular to the moving direction of the negative pole piece inside the coil.

[0103] It can be understood that the greater the distance S between the coil and the negative electrode plate located at the center of the coil, the less favorable it is for the surface temperature of the negative electrode plate to rise on the side away from the negative electrode current collector; the greater the moving speed V of the negative electrode plate inside the energized coil, the less favorable it is for the surface temperature of the negative electrode plate to rise on the side away from the negative electrode current collector. The distance S between the coil and the negative electrode plate located at the center of the coil and the moving speed V of the negative electrode plate inside the energized coil are controlled to satisfy the above relationship, so that the surface of the negative electrode plate on the side away from the negative electrode current collector has a suitable temperature, thereby effectively removing part of the binder in the first region, and setting the mass ratio of the binder in the first region to the binder in the second region to 0.1 to 1.4.

[0104] In some embodiments, the distance S between the coil and the negative electrode piece located at the center of the coil is 1 cm to 12 cm, and can be 5 cm to 10 cm. In some embodiments, the distance S between the coil and the negative electrode piece located at the center of the coil can be 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, 10 cm, 11 cm, 12 cm, or a value in the range formed by any two of the above points.

[0105] In some embodiments, when the negative electrode plate is treated by electromagnetic induction heating, the movement speed V of the negative electrode plate inside the energized coil is 10m / min to 180m / min, and can be 20m / min to 150m / min. In some embodiments, when the negative electrode plate is treated by electromagnetic induction heating, the movement speed V of the negative electrode plate inside the energized coil can be 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 formed by any two of the above points.

[0106] In some embodiments, when the negative electrode sheet is treated by electromagnetic induction 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., and can be 400° C. to 700° C. In some embodiments, when the negative electrode sheet is treated by electromagnetic induction 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 in the range formed by any two of the above points.

[0107] It can be understood that since the negative electrode plate moves inside the coil after the alternating current is passed, the surface of the negative electrode plate has a large current density, and the negative electrode plate can be heated in a short time to rapidly increase the surface temperature of the negative electrode plate. The distance S between the coil and the negative electrode plate located at the center of the coil and the moving speed V of the negative electrode plate inside the energized coil can be controlled so that the surface temperature T of the negative electrode plate away from the negative electrode current collector is within a suitable range. The surface temperature of the negative electrode plate can be measured by a temperature sensor. In addition, the decomposition temperature and reaction temperature of the negative electrode active material are both higher than 800°C, so the surface temperature T of the negative electrode plate is less than or equal to 700°C, which can decompose the binder, but the effect on the negative electrode active material can be ignored.

[0108] The distance S between the coil and the negative electrode pole piece located at the center of the coil, the moving speed V of the negative electrode pole piece inside the energized coil, and the surface temperature T of the negative electrode pole piece on the side away from the negative electrode current collector are controlled within appropriate ranges, thereby removing part of the binder in the first region so that the mass ratio of the binder in the first region to the binder in the second region is reduced to 0.1-1.4, and reducing the influence of electromagnetic induction heating on the negative electrode active material.

[0109] In some embodiments, the specific steps include:

[0110] 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 a negative electrode film layer, and obtaining a negative electrode sheet; optionally, the raw materials further include sodium carboxymethyl cellulose;

[0111] The negative electrode piece is moved inside the energized coil to obtain the negative electrode piece after electromagnetic induction heating treatment.

[0112] It is understandable that in the process of coating the negative electrode slurry on the negative electrode current collector and drying it to prepare the negative electrode film layer, due to the affinity problem between the binder and the negative electrode active material, there will be a problem of the upward flow of the solvent driving the binder to float during the solvent evaporation process, resulting in the binder enriched on the surface of the negative electrode sheet away from the negative electrode current collector, thereby increasing the DC impedance of the battery; and the enrichment of the binder on the surface of the negative electrode sheet will also cause the negative electrode film layer on the negative electrode sheet to stick to the roller when the negative electrode sheet is rolled later, resulting in damage to the negative electrode sheet. In some embodiments of the present application, electromagnetic induction heating is used to treat the negative electrode sheet, which can effectively remove part 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 aggregation of the binder in the first region, reduce the DC impedance of the battery, improve the performance of the battery, and also help improve the processing technology of the negative electrode sheet.

[0113] In some embodiments, the preparation method further comprises a compaction treatment step, and the compaction treatment step is performed before or after the negative electrode sheet is treated with electromagnetic induction heating.

[0114] In some embodiments, the preparation method further comprises a die-cutting process step, and the die-cutting process step is performed after the compacting process, and the die-cutting process step is performed before or after the negative electrode sheet is treated with electromagnetic induction heating.

[0115] The step of treating the negative electrode plate with electromagnetic induction heating can be before or after the compaction step or the die-cutting step, and can ensure that after the electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate is 0.1 to 1.4.

[0116] [Positive electrode]

[0117] 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.

[0118] 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 any one or both of the two opposite surfaces of the positive electrode current collector.

[0119] 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.).

[0120] In some embodiments, the positive electrode active material may adopt a positive electrode active material for a battery that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium metal oxide, 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. Among them, 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, etc. Examples of lithium-containing phosphates with an olivine structure 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.

[0121] 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.

[0122] In some embodiments, the positive electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0123] [Electrolytes]

[0124] The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0125] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0126] In some embodiments, the electrolyte salt can 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.

[0127] 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, methyl propyl carbonate, ethyl propyl 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 sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0128] In some embodiments, the electrolyte may further include additives, such as 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, additives that improve battery high or low temperature performance, etc.

[0129] [Isolation film]

[0130] 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 stability and mechanical stability can be selected.

[0131] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

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

[0133] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0134] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0135] [Secondary battery]

[0136] In some embodiments, a secondary battery includes a positive electrode sheet, an electrolyte, a separator, and a negative electrode sheet in some embodiments.

[0137] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0138] In some embodiments, the secondary battery includes one or more of a lithium secondary battery, a sodium secondary battery, and a potassium secondary battery.

[0139] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0140] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0141] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 2 The secondary battery 5 is a square structure as an example. The secondary battery may be a sodium ion battery, a magnesium ion battery, or a potassium ion battery.

[0142] In some embodiments, reference Figure 3 , the outer package may include a shell 51 and a cover plate 53. Among them, 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 isolation membrane can form 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.

[0143] [Battery module]

[0144] In some embodiments, secondary batteries may be assembled into a battery module. The number of secondary batteries contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0145] Figure 4 4 is an example of a battery module. Figure 4 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0146] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0147] [Battery Pack]

[0148] 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 according to the application and capacity of the battery pack.

[0149] Figure 5 and Figure 6 1 is a battery pack 1 as an example. Figure 5 and Figure 6 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0150] [Electrical devices]

[0151] In one embodiment of the present application, there is provided an electric device, comprising at least one of a secondary battery of any embodiment, a battery module of any embodiment, or a battery pack of any embodiment.

[0152] The electrical device includes at least one of the secondary battery, battery module, or battery pack provided in the present 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 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., but are not limited thereto.

[0153] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0154] Figure 7 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the high power and high energy density requirements of the power consumption device for the secondary battery, a battery pack or a battery module can be used.

[0155] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0156] Example

[0157] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0158] 1. Preparation method

[0159] Example 1

[0160] 1) Negative electrode

[0161] The negative electrode active material artificial graphite, thickener hydroxymethyl cellulose (CMC), and adhesive styrene butadiene rubber (SBR) were mixed and dissolved in deionized water at a weight ratio of 96.5:1.5:2.0 to form a 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. Then it was 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.

[0162] Electromagnetic induction heating is used to treat the negative electrode sheet. The prepared negative electrode sheet is passed through the inside of the electromagnetic induction coil and moved. The distance S from the electromagnetic induction coil to the negative electrode sheet located at the center of the coil is 5 cm, the moving speed V of the negative electrode sheet is 20 m / min, and the temperature T at which the surface of the negative electrode sheet is heated is 700°C, thereby obtaining the negative electrode sheet after electromagnetic induction heating treatment.

[0163] 2) Preparation of positive electrode

[0164] 1wt% polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and then 1wt% carbon black conductive agent and 98wt% ternary positive electrode active material are added and stirred to obtain positive electrode slurry. The slurry is evenly coated on the surface of the positive electrode current collector aluminum foil to prepare a positive electrode film layer, and then transferred to a vacuum drying oven for complete drying. The dried pole piece is rolled and punched to obtain a positive electrode piece.

[0165] 3) Electrolyte

[0166] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), 30% ethylene carbonate and 60% dimethyl carbonate were mixed evenly to obtain a mixed solvent. Then, 9% lithium hexafluorophosphate was added to the mixed solvent and dissolved, stirred evenly, and mixed to prepare an electrolyte.

[0167] 4) Isolation film

[0168] Polypropylene film is used as the isolation film.

[0169] 5) Preparation of batteries

[0170] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, the separator is placed between the positive and negative electrode sheets to play an isolating role, the bare cell is welded with the pole ear, and the bare cell is placed in an aluminum shell, and baked at 80°C to remove water, and then the above-mentioned electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery is then subjected to the processes of static, hot and cold pressing, formation, shaping, and capacity testing in sequence to obtain the lithium secondary battery product of Example 1.

[0171] Embodiments 2 to 17

[0172] The battery preparation methods in Examples 2 to 17 are basically similar to those in Example 1, but the moving speed V of the negative electrode plate or the distance S from the coil to the negative electrode plate located at the center of the coil during electromagnetic induction heating is adjusted. The specific parameters are shown in Table 1.

[0173] Comparative Example 1

[0174] The battery preparation method in Comparative Example 1 is basically similar to that in Example 1, but electromagnetic induction heating is not used to heat the negative electrode plate. The specific parameters are shown in Table 1.

[0175] Comparative Examples 2 to 5

[0176] The battery preparation methods in Comparative Examples 2 to 5 are basically similar to those in Example 1, but the distance S from the coil to the negative electrode plate located at the center of the coil and the moving speed V of the negative electrode plate during electromagnetic induction heating are controlled at the same time so that the S×V value is lower than 30 or higher than 900. The specific parameters are shown in Table 1.

[0177] 2. Performance Test

[0178] 1. Negative electrode performance test

[0179] 1) Test on the mass ratio of the binder in the first region to the binder in the second region

[0180] The test was conducted using a differential scanning thermogravimetric analyzer. The test 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 considered the first area powder (note: the thickness of the first area 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); gently scrape off the powder left on the negative electrode sheet with a scraper, and be careful not to hang on the copper foil. This part of the powder is considered the second area powder. The upper and lower layers of powder are tested for the binder content in the upper and lower layers 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, and then the binder mass ratio in the first region / second region is calculated.

[0181] 2) Relative adhesion test

[0182] At 25°C, the prepared negative electrode sheet is used as the electrode sheet to be tested. A sample with a width of 30mm and a length of 100-160mm is cut with a blade. A special double-sided tape is attached to the steel plate. The tape width is 20mm and the length is 90-150mm. The negative electrode film layer of the cut negative electrode sheet sample is attached to the double-sided tape, and then rolled three times in the same direction with a roller. A paper tape with a width equal to that of the electrode sheet and a length greater than that of the sample by 80-200mm is fixed to the negative electrode current collector and fixed with wrinkled glue. Turn on the power of the tensile machine, the indicator light is on, adjust the limit block to the appropriate position, fix the end of the steel plate without 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 machine to adjust the position of the upper clamp, turn on the dedicated computer connected to the tensile machine, double-click the desktop software icon, and test. The stretching 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 ordinate and the corresponding test distance is used as the abscissa to obtain the bonding force curve of the negative electrode. The force when the negative electrode is balanced 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 use the bonding force of Example 5 as the reference value (100%), and the other embodiments or comparative examples are all relative values ​​relative to the bonding force of Example 5.

[0183] 2. Battery performance test

[0184] 1) Relative DC resistance (DCR) test

[0185] At 25°C, the prepared battery was charged to 4.2V at 1C constant current, and then charged to 0.05C at 4.2V constant voltage. After 5 minutes of storage, it was discharged at 1C constant current for 12 minutes, and the voltage V1 was recorded. After 5 minutes of storage, it was placed at -10°C for 2 hours, and then discharged at 4C for 10 seconds, and the voltage V2 was recorded. Then (V2-V1) / 4C was used to obtain the DC resistance DCR of the battery. The data in Table 2 use the battery internal resistance DCR of Example 5 as the reference value (100%), and the other examples or comparative examples are all relative values ​​of the DC resistance DCR of the battery in Example 5.

[0186] 2) Battery cycle capacity retention rate test

[0187] 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 record the discharge capacity Cn of the battery after the nth cycle at the same time. Then, after each cycle, the battery capacity retention rate Pn=Cn / C0×100%, with the 300 point values ​​of P1, P2...300 as the ordinate, and the corresponding number of cycles as the abscissa, to obtain a curve chart of the battery capacity retention rate and the number of cycles. During the test, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and...the 300th cycle corresponds to n=300. The battery capacity retention rate data corresponding to the embodiment or comparative example in Table 2 is the data measured after 300 cycles under the above test conditions, that is, the value of P300. The test process of the comparative example and other embodiments is the same as above.

[0188] III. Analysis of test results of various embodiments and comparative examples

[0189] The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0190] Table 1

[0191]

[0192] Table 2

[0193]

[0194] According to the above results, the negative electrode sheets in Examples 1 to 17 include a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, and the negative electrode film layer includes a binder; wherein the negative electrode film layer includes a first region and a second region, and 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 to the inside of the negative electrode film layer within a distance of h / 2, and the second region is a 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.

[0195] From the comparison between Examples 1 to 17 and Comparative Example 1, it can be seen that the electromagnetic induction heating treatment of the negative electrode plate is beneficial to reducing the DC impedance of the battery and improving the cycle performance of the battery.

[0196] From the comparison between Examples 1 to 17 and Comparative Examples 2 to 5, it can be seen that controlling the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate to be 0.1 to 1.4 is beneficial to reducing the DC impedance of the battery and improving the cycle performance of the battery while taking into account the bonding performance of the negative electrode plate.

[0197] From the comparison between Examples 1 to 8 and Comparative Examples 2 to 3, it can be seen that when the distance S between the coil and the negative electrode sheet located at the center of the coil is 5 cm, the movement speed V of the negative electrode sheet inside the energized coil is too high or too low, which will cause the surface temperature of the negative electrode sheet to be lower than 300°C or higher than 730°C. The surface temperature of the negative electrode sheet lower than 300°C is not conducive to the removal of the styrene butadiene rubber SBR in the first region, and the surface temperature of the negative electrode sheet higher than 730°C may cause the decomposition of the negative electrode active material and affect the electrical performance of the battery. By controlling the movement speed of the negative electrode sheet within a suitable range, the styrene butadiene rubber SBR in the first region can be removed and the impact on the negative electrode active material can be reduced.

[0198] From the comparison of Examples 4, 10-16 and Comparative Examples 4-5, it can be seen that when the negative electrode plate moves at a speed V of 70 m / min inside the energized coil, if the distance S between the coil and the negative electrode plate located at the center of the coil is too high or too low, the surface temperature of the negative electrode plate will be lower than 300°C or higher than 730°C. The surface temperature of the negative electrode plate lower than 300°C is not conducive to the removal of the styrene butadiene rubber SBR in the first region, and the surface temperature of the negative electrode plate higher than 730°C may cause the decomposition of the negative electrode active material, affecting the electrical performance of the battery. By controlling the moving speed of the negative electrode plate within a suitable range, the styrene butadiene rubber SBR in the first region can be removed, and the impact on the negative electrode active material can be reduced.

[0199] From the comparison between Examples 1 to 17 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by electromagnetic induction heating, the distance S between the control coil and the negative electrode plate located at the center of the coil is 1 cm to 12 cm, 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.

[0200] From the comparison between Examples 1 to 17 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by electromagnetic induction heating, the movement speed V of the negative electrode plate inside the energized coil is 10m / min to 180m / 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.

[0201] From the comparison between Examples 1 to 17 and Comparative Examples 1 to 5, it can be seen that when the negative electrode plate is treated by electromagnetic induction 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.

[0202] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode plate, characterized in that: 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, wherein the negative electrode film layer comprises 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, and h represents the thickness of the negative electrode film layer.

2. The negative electrode sheet according to claim 1, characterized in that: 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, characterized in that: 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, characterized in that: The negative electrode plate is a negative electrode plate that has been subjected to electromagnetic induction heating treatment.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: 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%.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: 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.

7. A method for preparing a negative electrode sheet, characterized in that: include: Preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain the negative electrode plate; 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, and h represents the thickness of the negative electrode film layer.

8. The preparation method according to claim 7, characterized in that: The step of preparing a negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet comprises: Preparing the negative electrode film layer on at least one side of the negative electrode current collector to obtain a negative electrode sheet; The negative electrode plate is treated by electromagnetic induction heating to obtain the negative electrode plate treated by electromagnetic induction heating.

9. The preparation method according to claim 8, characterized in that: Before the electromagnetic induction heating treatment, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode plate is 1.5-10.

10. The preparation method according to claim 8 or 9, characterized in that: The method of using electromagnetic induction heating to process the negative electrode plate includes: The negative electrode plate is moved inside the energized coil.

11. The preparation method according to any one of claims 8 to 10, characterized in that: When the negative electrode plate is treated by electromagnetic induction heating, the distance S between the coil and the negative electrode plate located at the center position inside the coil and the moving speed V of the negative electrode plate inside the energized coil satisfy: 50≤S×V≤900, optionally, 100≤S×V≤800, wherein S represents the shortest distance between the coil and the negative electrode plate located at the center position inside the coil along the thickness direction of the negative electrode plate, the unit of S is cm, and the unit of V is m / min.

12. The preparation method according to claim 10 or 11, characterized in that: The distance S between the coil and the negative electrode plate located at the center of the coil is 1 cm to 12 cm, and can be optionally 5 cm to 10 cm.

13. The preparation method according to any one of claims 10 to 12, characterized in that: When the negative electrode plate is treated by electromagnetic induction heating, the moving speed V of the negative electrode plate inside the energized coil is 10 m / min to 180 m / min, and can be optionally 20 m / min to 150 m / min.

14. The preparation method according to any one of claims 8 to 13, characterized in that: When the negative electrode plate is treated by electromagnetic induction 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., and can be optionally 400° C. to 700° C.

15. The preparation method according to any one of claims 10 to 14, characterized in that: The specific steps 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 a negative electrode sheet; optionally, the raw materials further include sodium carboxymethyl cellulose; The negative electrode sheet is moved inside the energized coil to obtain the negative electrode sheet after the electromagnetic induction heating treatment.

16. The preparation method according to any one of claims 8 to 15, characterized in that: The preparation method further comprises a compaction treatment step, and the compaction treatment step is performed before or after the negative electrode plate is treated with the electromagnetic induction heating.

17. The preparation method according to any one of claims 8 to 16, characterized in that: The preparation method further comprises a die-cutting process step, which is performed after the compacting step, and the die-cutting process step is performed before or after the negative electrode sheet is treated with the electromagnetic induction heating.

18. A secondary battery, characterized in that: A negative electrode sheet comprising the negative electrode sheet according to any one of claims 1 to 6 or the negative electrode sheet prepared by the preparation method according to any one of claims 7 to 17.

19. The secondary battery according to claim 18, characterized in that: The secondary battery includes one or more of a lithium secondary battery, a sodium secondary battery, and a potassium secondary battery.

20. An electrical device, characterized in that: Includes the secondary battery according to claim 18 or 19.

Images