Negative electrode sheet, secondary battery, and electric device

Abstract

The application provides a negative electrode sheet, a secondary battery and a power utilization device. 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, and the negative electrode film layer comprises a binder; wherein the negative electrode film layer comprises 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 0.1-1.4, the first region is a region on the side of the negative electrode film layer away from the negative electrode current collector and extending to the inside of the negative electrode film layer within h / 2, the second region is a region on the side of the negative electrode film layer close to the negative electrode current collector and extending to the first region 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, in particular to a negative electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] In recent years, secondary batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

[0003] The performance of the negative electrode sheet has a crucial influence 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

[0004] The present application is made in view of the above-mentioned problems, and aims to provide a negative electrode sheet, 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, the aggregation degree of the binder in the first region is reduced, which is beneficial to reduce the direct current impedance of the battery and improve the electrical performance of the battery.

[0005] The first aspect of the present application provides a negative electrode sheet, which comprises a negative current collector and a negative film layer located on at least one side of the negative current collector, and the negative film layer comprises a binder.

[0006] The negative film layer comprises 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 0.1-1.4, the first region is a region of the negative film layer extending inward from the surface of the side of the negative film layer away from the negative current collector within a distance of h / 2, and the second region is a region of the negative film layer extending inward from the surface of the side of the negative film layer close to the negative current collector within a distance of h / 2, and h represents the thickness of the negative 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, and then reduce the coating of the binder on the negative active material in the first region, so that the metal ions in the secondary battery can better be inserted and extracted in the negative electrode sheet, improve the kinetic performance of the battery, reduce the direct current impedance of the battery, and improve 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 insertion and extraction of lithium ions in the negative electrode sheet, improves the kinetic performance of the lithium secondary battery, reduces the direct current impedance, and thus improves the lithium precipitation window on the surface of the negative electrode sheet.

[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 balance the adhesion performance of the negative electrode sheet and the direct current impedance of the battery, thereby comprehensively improving the cycle performance of the battery.

[0010] In any embodiment, the negative electrode sheet is a negative electrode sheet after electromagnetic induction heating treatment.

[0011] The electromagnetic induction heating can effectively remove part of the binder in the first region, which can not only reduce the aggregation of the binder in the first region and thereby reduce the direct current impedance of the battery, but also retain another part of the binder to maintain the adhesion between the negative 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, the mass content of the binder in the first region is 0.18%-2%, and the mass content of the binder in the second region is 0.32%-3%, based on the total mass of the negative electrode film layer.

[0014] In any embodiment, the sum of the mass content of the binder in the first region and the mass content of the binder in the second region is 0.5%-5%, and the sum is optionally 1%-3%, based on the total mass of the negative electrode film layer.

[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 direct current impedance, but also bind the negative active materials 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 adhesion performance, but also control the influence of the introduction of the binder on the direct current impedance of the battery.

[0016] In any embodiment, the negative electrode film layer further includes a negative active material, and the negative 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 preparation method of a negative electrode sheet, including:

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

[0019] wherein a mass ratio of the binder in the first region to the binder in the second region is 0.1-1.4, the first region is a region of the negative electrode film layer extending inward from a surface of the negative electrode film layer away from the negative electrode current collector by a distance within h / 2, and the second region is a region of the negative electrode film layer extending inward from a surface of the negative electrode film layer close to the negative electrode current collector by a distance within h / 2, and h represents a 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 0.1-1.4 to reduce the aggregation of the binder in the first region and further reduce 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 better be inserted and extracted in the negative electrode tab, improve the kinetic performance of the battery, reduce the direct current impedance of the battery, and improve the cycle performance of the battery. 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 0.1-1.4 is beneficial to the insertion and extraction of lithium ions in the negative electrode tab, improves the kinetic performance of the lithium secondary battery, reduces the direct current impedance, and thus improves the lithium precipitation window on the surface of the negative electrode tab.

[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 tab comprises:

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

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

[0024] The 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 1.5-10 to 0.1-1.4, which can not only reduce the aggregation of the binder in the first region and further reduce the direct current impedance of the battery, but also retain another part of the binder to maintain the adhesion between the negative electrode active materials in the negative electrode film layer to form a complete negative electrode tab structure.

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

[0026] The mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet before the electromagnetic induction heating treatment is 1.5-10. Due to the aggregation of the binder in the first region, the direct current impedance of the battery is increased, and the electrical performance of the battery is affected. After the electromagnetic induction 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 0.1-1.4. The aggregation of the binder in the first region is significantly reduced, the direct current impedance of the battery is reduced, and the performance of the battery is improved.

[0027] In any embodiment, the electromagnetic induction heating treatment of the negative electrode sheet comprises:

[0028] The negative electrode sheet is moved inside the coil with current.

[0029] A magnetic field is generated inside the coil with current. The movement of the negative electrode sheet inside the coil is equivalent to the movement of cutting the magnetic induction lines in the magnetic field. According to the electromagnetic induction theory and the skin effect, the current density on the surface of the negative electrode sheet is the largest, and the current density decreases significantly with the increase of the depth inside the negative electrode sheet. Therefore, the current induction heating can effectively remove part of the binder in the first region, reduce the aggregation of the binder in the first region, reduce the direct current impedance of the battery, and improve the electrical performance of the battery.

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

[0031] The distance S between the coil and the negative electrode sheet located at the center inside the coil and the moving speed V of the negative electrode sheet inside the coil with current satisfy the above relationship, so that the surface of the negative electrode sheet far from the negative current collector has a suitable temperature, thereby effectively removing part of the binder in the first region, and further reducing the mass ratio of the binder in the first region to the binder in the second region to 0.1-1.4, reducing the direct current 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 sheet located at the center inside the coil is 1-12 cm, and optionally, 5-10 cm.

[0033] In any embodiment, when the negative electrode tab is treated by the electromagnetic induction heating, the moving speed V of the negative electrode tab inside the coil with current is 10 m / min to 180 m / min, or 20 m / min to 150 m / min.

[0034] In any embodiment, when the negative electrode tab is treated by the electromagnetic induction heating, the surface temperature T of the negative electrode tab away from the negative electrode current collector is 300℃ to 700℃, or 400℃ to 700℃.

[0035] Controlling the distance S between the coil and the negative electrode tab at the center of the coil, the moving speed V of the negative electrode tab inside the coil with current, and the surface temperature T of the negative electrode tab away from the negative electrode current collector in a proper range can remove part of the binder in the first area, so that the mass ratio of the binder in the first area to the binder in the second area is reduced to 0.1 to 1.4, and can reduce the influence of the electromagnetic induction heating on the negative electrode active material.

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

[0037] Mixing raw materials including negative electrode active material and binder with deionized water to prepare negative electrode slurry, and coating the negative electrode slurry on the negative electrode current collector to prepare the negative electrode film layer, to obtain the negative electrode tab; optionally, the raw materials further include sodium carboxymethyl cellulose;

[0038] Moving the negative electrode tab inside the coil with current to obtain the negative electrode tab treated by the electromagnetic induction heating.

[0039] The mass ratio of the binder in the first area to the binder in the second area in the negative electrode tab treated by the electromagnetic induction heating is 0.1 to 1.4, which can effectively reduce the aggregation of the binder in the first area, reduce the direct current impedance of the battery, and improve the performance of the battery.

[0040] In any embodiment, the preparation method further includes a compaction treatment step, which is performed before or after the negative electrode tab is treated by the electromagnetic induction heating.

[0041] In any embodiment, the preparation method further includes a die cutting treatment step, which is performed after the compaction step, and the die cutting treatment step is performed before or after the negative electrode tab is treated by the electromagnetic induction heating.

[0042] The step of the electromagnetic induction heating treatment of the negative electrode sheet can be located before or after the compaction treatment step or the die cutting treatment step, and can make the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet after the electromagnetic induction heating treatment be 0.1-1.4.

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

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

[0045] The fourth aspect of the present application provides an electric device comprising the secondary battery of the third aspect of the present application. BRIEF DESCRIPTION OF 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 is a schematic diagram of a secondary battery according to an embodiment of the present application; Figure 2 is an exploded view of the secondary battery according to an embodiment of the present application;

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

[0052] Figure 7 is a schematic diagram of an electric device powered by a secondary battery according to an embodiment of the present application.

[0053] REFERENCE SIGNS:

[0054] 1 battery pack; 2 upper case; 3 lower case; 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] Hereinafter, specific embodiments of the negative electrode sheet, secondary battery, and electric device of the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0056] The ranges disclosed herein are defined by their lower and upper limits. Ranges that include both a lower limit and an upper limit are defined by selecting a lower limit and an upper limit. The ranges can be either inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if a range is listed as 60-120 and 80-110, it is understood that the ranges 60-110 and 80-120 are also contemplated. Furthermore, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" indicates a shorthand manner of describing all of the individual ranges that are within the range of a to b, wherein a and b are both real numbers. For instance, the numerical range "0-5" indicates that all of the real numbers between 0 and 5 have been listed herein, and "0-5" is merely a shorthand manner of describing these numerical combinations. Also, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like.

[0057] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0058] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0059] If not specifically stated, all steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any order. For example, the method can comprise steps (a), (b), and (c), or steps (a), (c), and (b), or steps (c), (a), and (b), and the like.

[0060] If not specifically stated, the "comprising" and "including" mentioned in the present application represent open-ended, and can also be closed-ended. For example, the "comprising" and "including" can represent that other components not listed can also be included, or only the listed components can be included.

[0061] If not specifically stated, in the present 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 satisfy 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 A and B are both true (or exist).

[0062] At present, the negative electrode film layer in the negative electrode sheet has the phenomenon of uneven distribution of the binder in thickness, for example, the proportion of the butadiene-styrene rubber binder in the negative electrode film layer gradually increases in the direction from the negative electrode current collector to the far negative electrode current collector. Since the butadiene-styrene rubber binder is gathered on the side of the negative electrode sheet far from the negative electrode current collector, the impedance is increased, which affects the electrical performance of the battery. Therefore, it is necessary to provide a new negative electrode sheet to enable the battery to meet the demand of the new generation of electrochemical system.

[0063] [Negative electrode sheet]

[0064] Based on this, the present application provides a negative electrode sheet, the negative electrode sheet 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] Wherein, the negative electrode film layer comprises 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 0.1-1.4, the first region is a region on the side of the negative electrode film layer far from the negative electrode current collector, the extension distance of the surface of the first region to the inside of the negative electrode film layer is within h / 2, the second region is a region on the side of the negative electrode film layer close to the negative electrode current collector, the extension distance of the surface of the second region to the first region is within h / 2, and h represents the thickness of the negative electrode film layer. In some embodiments, the binder comprises a water-based binder, the water-based binder comprises one or more of butadiene-styrene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate); optionally, the water-based binder comprises butadiene-styrene rubber.

[0066] In some embodiments, the negative electrode film layer further comprises a negative electrode active material, the negative electrode active material comprises 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 further optionally includes a conductive agent. The conductive agent includes at least one of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers.

[0068] In some embodiments, the negative electrode film layer further optionally includes other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc.

[0069] In some embodiments, the negative electrode tab 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 As shown in (a), the negative electrode tab 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 a region on the side of the negative electrode film layer 501 away from the negative electrode current collector 502, extending to the inside of the negative electrode film layer 501 within a distance of h / 2, and the second region 5012 is a region on the side of the negative electrode film layer 501 close to the negative electrode current collector 502, extending to the first region 5011 within a distance of h / 2.

[0070] In some embodiments, the negative electrode tab 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 As shown in (b), the negative electrode tab 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 layers 501 include a first region 5011 and a second region 5012, the first region 5011 is a region on the side of the negative electrode film layer 501 away from the negative electrode current collector 502, extending to the inside of the negative electrode film layer 501 within a distance of h / 2, and the second region 5012 is a region on the side of the negative electrode film layer 501 close to the negative electrode current collector 502, extending to the first region 5011 within a distance of h / 2.

[0071] In some embodiments, the negative electrode tab further includes a coating layer between the negative electrode current collector and the negative electrode film layer, the coating layer includes at least one of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, the coating layer further includes styrene butadiene rubber, and the thickness of the coating layer can be selected as 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 defined by any two of the above.

[0072] In some embodiments, the mass ratio of the binder in the first region to the binder in the second region can be selected from 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 within a range defined by any two of the aforementioned values.

[0073] In this context, the term "water-based binder" refers to a binder that is 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 context, the mass ratio of the binder in the first region to the binder in the second region can be tested using any known method. As an example, the test is performed using a differential scanning calorimetry analyzer. The testing instrument is a STA449F3 differential scanning calorimetry analyzer, and the specific testing procedure is as follows: measure the thickness of the negative electrode film layer in the negative electrode sheet; adhere double-sided tape to the surface of the negative electrode sheet to be tested, and then roll the tape three times in the same direction using a roller; tear the double-sided tape, and the first region powder adhered to the tape is collected (note: the thickness of the first region powder adhered to the tape should be half the thickness of the negative electrode film layer; to achieve this, measure the thickness of the film layer remaining on the negative electrode sheet, and if the ratio of the thickness of the film layer remaining on the negative electrode sheet to the thickness of the negative electrode film layer is less than 1 / 2, continue to adhere the tape); gently scrape the second region powder remaining on the negative electrode sheet with a spatula, taking care not to hang onto the copper foil; and test the binder content in the upper and lower layers according to the following steps: weigh about 50 mg of the sample into an Al2O3 crucible and level it off; set the parameters as follows: nitrogen atmosphere, purge gas 60 mL / min, and protective gas 20 mL / min; temperature increase program: 10°C / min, 35°C-600°C; ensure that the sample is completely dispersed, and then plot the thermogravimetric-temperature change graph; the weight loss range of the binder is 300°C-600°C, and the binder content in the first region / second region is obtained, respectively, and the mass ratio of the binder in the first region to the binder in the 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 0.1-1.4 to reduce the aggregation of the binder in the first region, thereby reducing the coating of the binder on the negative active material in the first region, so that the metal ions in the secondary battery can better be embedded and extracted in the negative electrode sheet, improve the kinetic performance of the battery, reduce the direct current impedance of the battery, and improve 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 extraction of lithium ions in the negative electrode sheet, improves the kinetic performance of the lithium secondary battery, reduces the direct current impedance, and thus improves the lithium precipitation window on the surface of the negative electrode sheet.

[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] Further controlling the mass ratio of the binder in the first region to the binder in the second region to be 0.6-1 greatly reduces the aggregation of the binder in the first region, and can further balance the adhesion performance of the negative electrode sheet and the direct current impedance of the battery, thereby comprehensively improving the cycle performance of the battery.

[0078] In some embodiments, the negative electrode sheet is a negative electrode sheet subjected to electromagnetic induction heating treatment.

[0079] The electromagnetic induction heating can effectively remove part of the binder in the first region, which can not only reduce the aggregation of the binder in the first region and thus reduce the direct current impedance of the battery, but also retain another part of the binder to maintain the adhesion between the negative active materials in the negative electrode film layer.

[0080] In some embodiments, the mass content of the binder in the first region is 0.18%-2%, and the mass content of the binder in the second region is 0.32%-3%, 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 within a range defined by any two of the above values, 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 within a range defined by any two of the above values.

[0081] In some embodiments, the sum of the mass content of the binder in the first region and the binder in the second region is 0.5% to 5%, optionally 1% to 3%, based on the total mass of the negative electrode film layer. In some embodiments, the sum of the mass content of the binder in the first region and the binder in the second region is optionally 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a value in a range defined by any two of the aforementioned values, based on the total mass of the negative electrode film layer.

[0082] Controlling the mass content of the binder in the first region within a suitable range can both make the battery have a lower direct current impedance and 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 both make the negative electrode sheet have excellent adhesion (the adhesion between the negative electrode film layer and the negative electrode current collector) and control the influence of the introduction of the binder on the direct current impedance of the battery.

[0083] The 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] The negative electrode film layer comprises 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 on the side of the negative electrode film layer away from the negative electrode current collector, extending inward from the surface of the negative electrode film layer to a distance of h / 2 or less. The second region is a region on the side of the negative electrode film layer close to the negative electrode current collector, extending from the surface of the negative electrode film layer to a distance of h / 2 or less from the first region. h represents the thickness of the negative electrode film layer.

[0086] 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 can reduce the aggregation of the binder in the first region, and further reduce the coating of the binder in the first region to the negative electrode active material, so that the metal ions in the secondary battery can better be inserted and extracted in the negative electrode sheet, improve the kinetic performance of the battery, reduce the direct current impedance of the battery, and improve the cycle performance of the battery. 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 beneficial to the insertion and extraction of lithium ions in the negative electrode sheet, improves the kinetic performance of the lithium secondary battery, reduces the direct current impedance, and thus improves the lithium precipitation window on the surface of the negative electrode sheet.

[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 comprises:

[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 sheet is treated by electromagnetic induction heating to obtain a negative electrode sheet treated by electromagnetic induction heating.

[0090] The 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 1.5-10 to 0.1-1.4, which can not only reduce the aggregation of the binder in the first region and further reduce the direct current impedance of the battery, but also retain another part of the binder to maintain the adhesion between the negative active materials in the negative electrode film.

[0091] In some embodiments, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet before the electromagnetic induction heating treatment is 1.5-10. In some embodiments, the mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet before the electromagnetic induction heating treatment can be selected as 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 formed by any two of the above values.

[0092] The mass ratio of the binder in the first region to the binder in the second region of the negative electrode sheet before the electromagnetic induction heating treatment is 1.5-10, which increases the direct current impedance of the battery due to the aggregation of the binder in the first region, affecting the electrical performance of the battery; after the negative electrode sheet is treated by electromagnetic induction heating, the mass ratio of the binder in the first region to the binder in the second region is 0.1-1.4, which significantly reduces the aggregation of the binder in the first region, reduces the direct current impedance of the battery, and improves the performance of the battery.

[0093] In some embodiments, the negative electrode sheet is treated by electromagnetic induction heating, which includes:

[0094] The negative electrode sheet is moved inside the coil with current.

[0095] In this document, the shape of the coil is square, and the number of coils can be 1.

[0096] It can be understood that a magnetic field is generated inside the coil when the coil is energized, for example, a changing magnetic field is generated inside the coil when the coil is energized with alternating current, and the movement of the negative electrode tab inside the coil is equivalent to further cutting the magnetic induction line in the magnetic field. According to the electromagnetic induction theory and the skin effect, the current density on the surface of the negative electrode tab is the largest, and the current density decreases significantly with the increase of the depth inside the negative electrode tab, that is, the temperature on the surface of the negative electrode tab can rise rapidly, and the temperature inside the negative electrode tab increases gradually with the increase of the depth, and the temperature rise degree is gradually weakened, therefore, the current induction heating can effectively remove part of the binder in the first area, and the removal amount of the binder in the second area by the current induction heating is much smaller than that in the first area, thereby achieving the effect of reducing the binder aggregation in the first area, reducing the direct current impedance of the battery, and improving the electrical performance of the battery.

[0097] In addition, when the negative electrode film layers are arranged on both sides of the negative electrode current collector, the electromagnetic induction heating can simultaneously act on the negative electrode film layers on both sides of the negative electrode tab, and the influence of the electromagnetic induction heating on the negative electrode current collector can be ignored, thereby greatly saving the processing time and cost.

[0098] In some embodiments, when the negative electrode tab is treated by electromagnetic induction heating, the distance S between the coil and the negative electrode tab located at the center position inside the coil and the moving speed V of the negative electrode tab inside the coil satisfy: 50≤S×V≤900, and optionally, 100≤S×V≤800, wherein S represents the shortest distance between the coil and the negative electrode tab located at the center position inside the coil along the thickness direction of the negative electrode tab, and the unit of S is cm, and the unit of V is m / min. In some embodiments, when the negative electrode tab is treated by electromagnetic induction heating, the distance S between the coil and the negative electrode tab located at the center position inside the coil and the moving speed V of the negative electrode tab inside the coil satisfy S×V, which is optionally 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.

[0099] In this paper, S×V refers to the product of the values between S and V, the unit of S is cm, and the unit of V is m / min.

[0100] In this paper, when the negative electrode tab is treated by electromagnetic induction heating, the negative electrode tab 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 tab, and the four edges of the coil are perpendicular or parallel to the thickness direction of the negative electrode tab, that is, the plane where the coil is located is perpendicular to the surface of the negative electrode tab (the surface of the negative electrode tab is a plane perpendicular to the thickness direction of the negative electrode tab), and the vertical distance from the two edges of the coil parallel to the surface of the negative electrode tab to the negative electrode tab is equal.

[0101] In the present text, the term "distance S between the coil and the negative electrode tab located at the center position inside the coil" refers to the distance from the coil to the surface of the negative electrode tab located at the center position inside the coil in the thickness direction of the negative electrode tab.

[0102] In the present text, the term "moving speed V of the negative electrode tab inside the coil under current" refers to the moving speed of the negative electrode tab located at the center of the coil in the direction perpendicular to the plane where the coil is located, i.e., the plane where the coil is located is perpendicular to the moving direction of the negative electrode tab inside the coil.

[0103] It can be understood that the greater the distance S between the coil and the negative electrode tab located at the center position inside the coil, the less conducive to the rise of the surface temperature of the negative electrode tab away from the negative electrode current collector side; the greater the moving speed V of the negative electrode tab inside the coil under current, the less conducive to the rise of the surface temperature of the negative electrode tab away from the negative electrode current collector side. The distance S between the coil and the negative electrode tab located at the center position inside the coil and the moving speed V of the negative electrode tab inside the coil under current satisfy the above relationship, so that the surface of the negative electrode tab away from the negative electrode current collector side has a suitable temperature, thereby effectively removing part of the binder in the first area, and the mass ratio of the binder in the first area to the binder in the second area is 0.1-1.4.

[0104] In some embodiments, the distance S between the coil and the negative electrode tab located at the center position inside the coil is 1-12 cm, which can be optionally 5-10 cm. In some embodiments, the distance S between the coil and the negative electrode tab located at the center position inside the coil can be optionally 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 constituted by any two of the above.

[0105] In some embodiments, when the negative electrode tab is treated by electromagnetic induction heating, the moving speed V of the negative electrode tab inside the coil under current is 10-180 m / min, which can be optionally 20-150 m / min. In some embodiments, when the negative electrode tab is treated by electromagnetic induction heating, the moving speed V of the negative electrode tab inside the coil under current can be optionally 10 m / min, 20 m / min, 30 m / min, 40 m / min, 50 m / min, 60 m / min, 70 m / min, 80 m / min, 90 m / min, 100 m / min, 110 m / min, 120 m / min, 130 m / min, 140 m / min, 150 m / min, 160 m / min, 170 m / min, 180 m / min, or a value in the range constituted by any two of the above.

[0106] In some embodiments, when the negative electrode tab is heated by electromagnetic induction, the surface temperature T of the negative electrode tab away from the negative electrode current collector is 300-700°C, and can be 400-700°C.

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

[0108] Controlling the distance S between the coil and the negative electrode tab at the center of the coil inside, the moving speed V of the negative electrode tab inside the coil, and the surface temperature T of the negative electrode tab away from the negative electrode current collector in a suitable range, respectively, can remove part of the binder in the first area to reduce the mass ratio of the binder in the first area to the binder in the second area to 0.1-1.4, and can also reduce the influence of electromagnetic induction heating on the negative electrode active material.

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

[0110] Mixing raw materials including the negative electrode active material and the binder with deionized water to prepare a negative electrode slurry, and coating the negative electrode slurry on the negative electrode current collector to prepare a negative electrode film layer, to obtain the negative electrode tab. Optionally, the raw materials further include sodium carboxymethyl cellulose.

[0111] Moving the negative electrode tab inside the coil to obtain the negative electrode tab after electromagnetic induction heating.

[0112] It can be understood that, in the process of applying the negative electrode slurry on the negative electrode current collector, drying to prepare the negative electrode film layer, due to the problem of mutual affinity between the binder and the negative electrode active material, solvent upward flow will cause the binder to float upward during the solvent evaporation process, resulting in the enrichment of the binder on the surface of the negative electrode tab away from the negative electrode current collector, and further resulting in the increase of the direct current impedance of the battery; and the enrichment of the binder on the surface of the negative electrode tab will also cause the negative electrode film layer on the negative electrode tab to be seriously stuck with the roller during subsequent rolling of the negative electrode tab, resulting in damage to the negative electrode tab. In some embodiments of the present application, the negative electrode tab is treated by electromagnetic induction heating, which can effectively remove part of the binder in the first area, and the mass ratio of the binder in the first area to the binder in the second area in the treated negative electrode tab is 0.1-1.4, which can effectively reduce the aggregation of the binder in the first area, reduce the direct current impedance of the battery, improve the performance of the battery, and also help to improve the processing technology of the negative electrode tab.

[0113] In some embodiments, the preparation method further comprises a compaction treatment step, which is performed before or after the treatment of the negative electrode tab by electromagnetic induction heating.

[0114] In some embodiments, the preparation method further comprises a die cutting treatment step, which is performed after the compaction step, and the die cutting treatment step is performed before or after the treatment of the negative electrode tab by electromagnetic induction heating.

[0115] The step of treating the negative electrode tab by electromagnetic induction heating can be performed before or after the compaction treatment step or the die cutting treatment step, and in both cases, the mass ratio of the binder in the first area to the binder in the second area in the negative electrode tab after the electromagnetic induction heating treatment is 0.1-1.4.

[0116] [Positive electrode tab]

[0117] The positive electrode tab comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer comprises a positive electrode active material.

[0118] For example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, 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 can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0120] In some embodiments, the positive electrode active material can employ a positive electrode active material for a battery known in the art. As an example, the positive electrode active material can include at least one of a lithium-containing phosphate of an olivine structure, a lithium metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode active material for a battery can also be used. These positive electrode active materials can be used alone only one or in combination of two or more. Among them, examples of the lithium metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also referred to simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also referred to simply as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05O2) and modified compounds thereof, etc. Examples of the lithium-containing phosphate of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to simply as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0121] In some embodiments, the positive electrode film layer can also optionally include a conductive agent. As an example, the conductive agent can include at least one of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0122] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., obtaining the positive electrode tab.

[0123] [Electrolyte]

[0124] The electrolyte functions to conduct ions between the positive electrode tab and the negative electrode tab. The type of electrolyte is not specifically limited in the present application and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0125] In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution 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 bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0127] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl 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, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0128] In some embodiments, the electrolyte can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, and the like.

[0129] [Separator]

[0130] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0131] In some embodiments, the separator can be made of at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0132] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to manufacture an electrode assembly by a winding process or a stacking process.

[0133] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0134] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, and the like. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, and the like can be listed.

[0135] [Secondary battery]

[0136] In some embodiments, the 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 can be used to manufacture an electrode assembly by a winding process or a stacking 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 can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0140] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0141] The shape of the secondary battery is not particularly limited in the present application, and it can be cylindrical, square, or any other shape. For example, Figure 2 is a square structure as an example of a secondary battery 5. The secondary battery can also be a sodium-ion battery, a magnesium-ion battery, or a potassium-ion battery.

[0142] In some embodiments, referring to Figure 3 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can 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 communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and the skilled person can select according to the specific actual needs.

[0143] [Battery module]

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

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

[0146] Optionally, the battery module 4 can also 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 above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by the skilled person according to the application and capacity of the battery pack.

[0149] Figure 5 andFigure 6 is a battery pack 1 as an example. Referring to Figure 5 and Figure 6 In the battery pack 1, a battery case and a plurality of battery modules 4 disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 and form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0150] [Power consuming device]

[0151] In one embodiment of the present application, a power consuming device is provided, which includes at least one of the secondary battery of any embodiment, the battery module of any embodiment, or the battery pack of any embodiment.

[0152] The power consuming device includes at least one of the secondary battery, the battery module, or the battery pack provided by the present application. The secondary battery, the battery module, or the battery pack can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0153] As the power consuming device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

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

[0155] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thin and light, and the secondary battery can be used as a power source.

[0156] Embodiment

[0157] Hereinafter, the embodiments of the present application are described. The embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not mentioned in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument is not mentioned by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0158] I. Preparation method

[0159] Example 1

[0160] 1) Negative electrode sheet

[0161] The negative active material artificial graphite, thickening agent hydroxymethyl cellulose (CMC), and adhesive styrene-butadiene rubber (SBR) were mixed in a weight ratio of 96.5:1.5:2.0, dissolved in deionized water to prepare a negative electrode slurry, and the negative electrode slurry was obtained under the action of a vacuum stirrer. The slurry was then coated on the surface of a copper foil. Then, it was transferred to a vacuum drying oven for complete drying, and the single-sided weight was 100 mg / 1540.25 mm 2 Afterwards, the negative electrode sheet was obtained by cold pressing.

[0162] The negative electrode sheet was treated by electromagnetic induction heating. The prepared negative electrode sheet was moved inside an electromagnetic induction coil, the distance S between the electromagnetic induction coil and the negative electrode sheet located at the center of the coil inside was 5 cm, the moving speed V of the negative electrode sheet was 20 m / min, and the temperature T of the surface of the negative electrode sheet heated was 700℃, to obtain the negative electrode sheet treated by electromagnetic induction heating.

[0163] 2) Preparation of positive electrode sheet

[0164] 1wt% polyvinylidene fluoride binder was fully dissolved in N-methyl pyrrolidone, 1wt% carbon black conductive agent was added, and 98wt% ternary positive electrode active material was stirred and mixed uniformly to obtain a positive electrode slurry. The slurry was uniformly coated on the surface of a 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 electrode sheet was roll-pressed and punched to obtain a positive electrode sheet.

[0165] 3) Electrolyte

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

[0167] 4) Separation film

[0168] A polypropylene film was used as a separation film.

[0169] 5) Preparation of battery

[0170] The positive electrode sheet, the separation film, and the negative electrode sheet were stacked in order, the separation film was between the positive electrode sheet and the negative electrode sheet to play a separation role, the bare cell was welded with tabs, and the bare cell was loaded into an aluminum shell and baked at 80℃ to remove water. Then, the above electrolyte was injected and sealed to obtain a non-charged battery. The non-charged battery was sequentially subjected to processes such as standing, hot and cold pressing, formation, shaping, and capacity testing to obtain the lithium secondary battery product of Example 1.

[0171] Examples 2-17

[0172] The battery preparation method in Examples 2-17 was substantially similar to that in Example 1, but the moving speed V of the negative electrode sheet or the distance S from the coil to the negative electrode sheet located at the center of the coil in the electromagnetic induction heating was adjusted, and the specific parameters are shown in Table 1.

[0173] Comparative Example 1

[0174] The battery preparation method in Comparative Example 1 was substantially similar to that in Example 1, but the negative electrode sheet was not heated by electromagnetic induction heating, and the specific parameters are shown in Table 1.

[0175] Comparative Examples 2-5

[0176] The battery preparation method in Comparative Examples 2-5 was substantially similar to that in Example 1, but the distance S from the coil to the negative electrode sheet located at the center of the coil in the electromagnetic induction heating and the moving speed V of the negative electrode sheet were simultaneously controlled to make the value of S x V less than 30 or more than 900, and the specific parameters are shown in Table 1.

[0177] II. Performance Test

[0178] 1. Negative electrode sheet performance test

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

[0180] A differential scanning calorimetric analyzer was used for the test. The test instrument was STA449F3 differential scanning calorimetric analyzer, and the specific test procedure was as follows: measuring the thickness of the negative electrode film layer in the negative electrode sheet; sticking double-sided tape on the surface of the negative electrode sheet to be tested, and then rolling three times in the same direction with a pressure roller, tearing off the double-sided tape, and the powder stuck on the tape was the first region powder (note: the thickness of the first region powder stuck on the tape should be 1 / 2 of the thickness of the negative electrode film layer, and the specific operation was as follows: measuring the thickness of the film layer remaining on the negative electrode sheet, and if the ratio of the thickness of the film layer remaining on the negative electrode sheet to the thickness of the negative electrode film layer was less than 1 / 2, the double-sided tape was continuously stuck); the powder remaining on the negative electrode sheet was gently scraped off with a scraper, and attention should be paid not to hang on the copper foil. This part of the powder was the second region powder. The upper and lower powders were tested for the binder content according to the following steps, and the test steps were as follows: weighing about 50 mg of sample in an Al2O3 crucible and shaking to flatten; parameter setting: nitrogen atmosphere, purge gas 60 mL / min, protection gas 20 mL / min; temperature rising program: 10 ℃ / min, 35 ℃-600 ℃, ensuring complete dispersion of the sample, and then drawing a thermogravimetric-temperature change graph. The weight loss range of the binder was 300 ℃-600 ℃, the binder content in the first region / second region was obtained, and the mass ratio of the binder in the first region / second region was calculated.

[0181] 2) Relative adhesion test

[0182] At 25°C, the prepared negative electrode sheet was taken as the electrode sheet to be tested, and a sample with a width of 30 mm and a length of 100-160 mm was cut off with a blade. A special double-sided tape was pasted on a steel plate, with a width of 20 mm and a length of 90-150 mm. The negative electrode film layer of the cut negative electrode sheet sample was pasted on the double-sided tape, and then a pressure roller was rolled in the same direction for three times. A paper tape with a width equal to that of the electrode sheet and a length of 80-200 mm longer than the sample length was fixed on the negative electrode current collector, and was fixed with a corrugated adhesive. The power supply of the tensile testing machine was turned on, and the indicator light was on. The limit block was adjusted to the appropriate position, and the end of the steel plate without the electrode sheet was fixed with the lower clamp. The paper tape was folded upwards, and the upper clamp was fixed. The position of the upper clamp was adjusted using the "up" and "down" buttons on the manual controller attached to the tensile testing machine. A special computer linked to the tensile testing machine was turned on, and the desktop software icon was double-clicked to start the test. The tensile rate was 50 m / min, and the test distance was 50 mm. The software took a data point every 10 s. The adhesion curve of the negative electrode sheet was obtained by taking the data point values as the vertical coordinate and the corresponding test distance as the horizontal coordinate. The adhesion of the negative electrode film layer per unit length was obtained by dividing the force at which the negative electrode sheet was in equilibrium by the width of the tape, so as to characterize the adhesion strength between the negative electrode film layer and the negative electrode current collector. The data in Table 2 took the adhesion of Example 5 as the reference value (100%), and the relative values of the other examples and comparative examples were relative to the adhesion of Example 5.

[0183] 2) Battery performance test

[0184] 1) Relative direct current resistance (DCR) test

[0185] At 25°C, the prepared battery was charged at 1C constant current to 4.2V, and then charged at 4.2V constant voltage to a current of 0.05C. After 5 min of standing, it was discharged at 1C constant current for 12 min, and the voltage V1 was recorded. After 5 min of standing, it was placed at -10°C for 2h, and then discharged at 4C for 10s, and the voltage V2 was recorded. Then (V2-V1) / 4C was obtained, and the direct current resistance DCR of the battery was obtained. The data in Table 2 took the battery resistance DCR of Example 5 as the reference value (100%), and the relative values of the other examples and comparative examples were relative to the battery direct current resistance DCR of Example 5.

[0186] 2) Battery cycle capacity retention rate test

[0187] The battery capacity retention test process is as follows: at 25℃, the prepared battery is charged at 1 / 3C constant current to 4.3V, then charged at 4.3V constant voltage to the current of 0.05C, and then rested for 5min, and then discharged at 1 / 3C to 2.8V, and the obtained capacity is recorded as the initial capacity C0. The above steps are repeated for the same battery, and the discharge capacity Cn of the battery after the nth cycle is recorded, then the battery capacity retention Pn after each cycle is Cn / C0x100%, and the battery capacity retention and cycle number curve is obtained with P1, P2……300 as the vertical coordinate and the corresponding cycle number as the horizontal coordinate. In the test process, 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 data corresponding to the examples or comparative examples in Table 2 is the data measured after 300 cycles under the above test conditions, i.e. the value of P300. The test process of the comparative examples and other examples is the same as above.

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

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

[0190] Table 1

[0191]

[0192] Table 2

[0193]

[0194] According to the above results, the negative electrode sheet in examples 1-17 includes 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, the mass ratio of the binder in the first region to the binder in the second region is 0.1-1.4, the first region is a region on the side of the negative electrode film layer away from the negative electrode current collector extending to the inside of the negative electrode film layer within h / 2, and the second region is a region on the side of the negative electrode film layer close to the negative electrode current collector extending to the first region within h / 2, and h represents the thickness of the negative electrode film layer.

[0195] From the comparison of examples 1-17 and comparative example 1, it can be seen that after the electromagnetic induction heating treatment of the negative electrode sheet, the direct current impedance of the battery is reduced, and the cycle performance of the battery is improved.

[0196] From the comparison of examples 1-17 and comparative examples 2-5, it can be seen that when the mass ratio of the binder in the first region to the binder in the second region in the negative electrode sheet is controlled to be 0.1-1.4, the adhesion performance of the negative electrode sheet is taken into account, and at the same time, the direct current impedance of the battery is reduced, and the cycle performance of the battery is improved.

[0197] From the comparison of examples 1-8 and comparative examples 2-3, it can be seen that when the distance S between the coil and the negative electrode sheet located at the center position inside the coil is 5 cm, if the moving speed V of the negative electrode sheet inside the electrified coil is too high or too low, the surface temperature of the negative electrode sheet will be lower than 300℃ or higher than 730℃, the surface temperature of the negative electrode sheet lower than 300℃ is not conducive to the removal of the SBR in the first region, and the surface temperature of the negative electrode sheet higher than 730℃ may cause decomposition of the negative active material, affecting the electrical performance of the battery. Controlling the moving speed of the negative electrode sheet within a suitable range can remove the SBR in the first region and reduce the impact on the negative active material.

[0198] From the comparison of examples 4, 10-16 and comparative examples 4-5, it can be seen that when the moving speed V of the negative electrode sheet inside the electrified coil is 70 m / min, if the distance S between the coil and the negative electrode sheet located at the center position inside the coil is too high or too low, the surface temperature of the negative electrode sheet will be lower than 300℃ or higher than 730℃, the surface temperature of the negative electrode sheet lower than 300℃ is not conducive to the removal of the SBR in the first region, and the surface temperature of the negative electrode sheet higher than 730℃ may cause decomposition of the negative active material, affecting the electrical performance of the battery. Controlling the moving speed of the negative electrode sheet within a suitable range can remove the SBR in the first region and reduce the impact on the negative active material.

[0199] From the comparison of examples 1-17 and comparative examples 1-5, it can be seen that when the negative electrode sheet is treated by electromagnetic induction heating, controlling the distance S between the coil and the negative electrode sheet located at the center position inside the coil to be 1-12 cm can achieve a mass ratio of the binder in the first region to the binder in the second region in the treated negative electrode sheet of 0.1-1.4, reduce the direct current impedance of the battery, and improve the cycle performance of the battery.

[0200] From the comparison of examples 1-17 and comparative examples 1-5, it can be seen that when the negative electrode sheet is treated by electromagnetic induction heating, the moving speed V of the negative electrode sheet inside the electrified coil is 10-180 m / min, which can achieve a mass ratio of the binder in the first region to the binder in the second region in the treated negative electrode sheet of 0.1-1.4, reduce the direct current impedance of the battery, and improve the cycle performance of the battery.

[0201] From the comparison of Examples 1 to 17 and Comparative Examples 1 to 5, it can be seen that when the negative electrode sheet is treated by electromagnetic induction heating, the surface temperature T on the side of the negative electrode sheet away from the negative electrode current collector is controlled to be 300 to 700°C, the mass ratio of the binder in the first region to the binder in the second region in the treated negative electrode sheet can be 0.1 to 1.4, the direct current impedance of the battery is reduced, and the cycle performance of the battery is improved.

[0202] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration and exerting the same effects as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. Furthermore, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the configuration elements of the embodiments are also included in the scope of the present application.

Claims

1. 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 of the negative electrode film layer that extends vertically within a distance h / 2 from the surface of the negative electrode film layer on the side away from the negative electrode current collector to the inside of the negative electrode film layer, and the second region is a region of the negative electrode film layer that extends vertically within a distance h / 2 from the surface of the negative electrode film layer on the side close to the negative electrode current collector to the first region, where h represents the thickness of the negative electrode film layer; 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; Treating the negative electrode piece with electromagnetic induction heating to obtain the negative electrode piece after electromagnetic induction heating; The method of using electromagnetic induction heating to treat the negative electrode plate includes: moving the negative electrode plate inside a powered coil; The distance S between the coil and the negative pole piece located at the center position inside the coil and the moving speed V of the negative pole piece inside the energized coil satisfy the following conditions: 50≤S×V≤900, wherein S represents the shortest distance between the coil and the negative pole piece located at the center position inside the coil along the thickness direction of the negative pole piece, the unit of S is cm, and the unit of V is m / min.

2. The preparation method 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 preparation method according to claim 1, characterized in that The adhesive includes a water-based adhesive, and the water-based adhesive includes one or more of styrene-butadiene rubber, polyamide, poly(acrylonitrile-acrylate), polyacrylate, and poly(styrene-acrylate).

4. The preparation method according to claim 3, characterized in that The water-based adhesive includes styrene-butadiene rubber.

5. The preparation method according to claim 1, 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%, and the mass content of the binder in the second region is 0.32% to 3%; 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%.

6. The preparation method according to claim 5, 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 0.3% to 1%.

7. The preparation method according to claim 5, characterized in that Based on the total mass of the negative electrode film layer, the mass content of the binder in the second region is 0.7% to 2%.

8. The preparation method according to claim 5, characterized in that The sum of the mass contents of the binder in the first region and the binder in the second region is 1% to 3%.

9. The preparation method according to claim 1, characterized in that 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.

10. The preparation method according to claim 1, characterized in that The distance S between the coil and the negative electrode piece located at the center of the coil and the moving speed V of the negative electrode piece inside the energized coil satisfy the following relationship: 100≤S×V≤800.

11. The preparation method according to claim 1, 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.

12. The preparation method according to claim 1, 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.

13. The preparation method according to claim 12, characterized in that The distance S between the coil and the negative electrode plate located at the center of the coil is 5 cm to 10 cm.

14. The preparation method according to claim 1, 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.

15. The preparation method according to claim 14, 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 20 m / min to 150 m / min.

16. The preparation method according to claim 1, 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.

17. The preparation method according to claim 16, 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 400° C. to 700° C.

18. The preparation method according to claim 1, characterized in that The specific steps include: uniformly mixing raw materials including a negative electrode active material and a binder with deionized water to prepare a negative electrode slurry, and coating the negative electrode slurry on a negative electrode current collector to prepare the negative electrode film layer, thereby obtaining a negative electrode sheet; The negative electrode sheet is moved inside the energized coil to obtain the negative electrode sheet after the electromagnetic induction heating treatment.

19. The preparation method according to claim 18, characterized in that The raw materials also include sodium carboxymethyl cellulose.

20. The preparation method according to any one of claims 1 to 19, characterized in that The preparation method further includes a compaction treatment step, which is performed before or after the negative electrode plate is treated with electromagnetic induction heating.

21. The preparation method according to claim 20, characterized in that 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 negative electrode sheet is treated with electromagnetic induction heating.

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