Negative electrode sheet, method for manufacturing the same, and lithium ion battery

Abstract

The present application relates to the technical field of lithium ion batteries, and particularly provides a negative electrode sheet, a preparation method thereof and a lithium ion battery, aiming to solve the problem that the existing negative electrode sheet is prone to wrinkling and affects the cycle performance of the battery. To this end, the negative electrode sheet comprises a negative electrode current collector, a negative electrode film and a separation layer, the negative electrode film is arranged on the surface of the negative electrode current collector, the separation layer is arranged on the surface of the negative electrode film away from the negative electrode current collector, and the separation layer comprises a plurality of mutually spaced coating portions. The negative electrode sheet of the present application is provided with a separation layer, the separation layer comprises a plurality of spaced coating portions, the arrangement of the separation layer avoids the direct contact of the negative electrode film with the battery separator, and the gap between the coating portions provides space for the expansion of the negative electrode sheet during the charging process, which can effectively alleviate the wrinkling of the negative electrode sheet, thereby improving the cycle performance of the lithium battery.

Description

Technical Field

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

[0002] With the increasing depletion of energy resources and growing environmental awareness, lithium-ion batteries have been widely used in the new energy field due to their advantages such as high energy density, no memory effect, and environmental friendliness.

[0003] High capacity and low cost have become the development trend of energy storage cells, and major companies are trying to increase the coating density to produce high-energy-density, high-capacity batteries. However, increasing the cell density means thicker electrode sheets and greater expansion thickness of the negative electrode sheet, which makes the interface wrinkling problem more likely to occur, thus affecting the cycle performance of the battery.

[0004] Accordingly, a new technical solution is needed in this field to solve the above-mentioned technical problems. Summary of the Invention

[0005] The present invention aims to solve the above-mentioned technical problems, namely, to solve the problem that existing negative electrode sheets are prone to wrinkling, which affects the cycle performance of batteries.

[0006] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, a negative electrode film and an isolation layer, the negative electrode film being disposed on the surface of the negative electrode current collector, the isolation layer being disposed on the surface of the negative electrode film away from the negative electrode current collector, and the isolation layer comprising a plurality of mutually spaced coating portions.

[0007] In the preferred embodiment of the above-mentioned negative electrode sheet, a plurality of the coating portions are arranged in an array.

[0008] In the preferred embodiment of the above-mentioned negative electrode sheet, a plurality of the coating portions are distributed in a rectangular array, and the spacing between two adjacent coating portions is 0.2 mm to 1 mm in the first direction and / or the second direction, wherein the first direction is perpendicular to the second direction.

[0009] In the preferred embodiment of the above-mentioned negative electrode sheet, the total area of ​​the contact surface between all the coating portions and the negative electrode film is 50% to 70% of the total area of ​​the side of the negative electrode film away from the negative electrode current collector.

[0010] In the preferred embodiment of the above-mentioned negative electrode sheet, the contact surface between the coating portion and the negative electrode film is circular, and the diameter of the circle is 0.2mm to 2.3mm; and / or, the coating portion is cylindrical or frustum-shaped.

[0011] In the preferred embodiment of the above-mentioned negative electrode sheet, the thickness of the coating portion is 2μm to 5μm.

[0012] In the preferred embodiment of the above-mentioned negative electrode, the separator layer comprises a metal oxide and a binder, wherein the mass ratio of the metal oxide to the binder is metal oxide:binder = (1-6):1, and preferably the mass ratio of the metal oxide to the binder is metal oxide:binder = (1-4):1.

[0013] In the preferred embodiment of the above-mentioned negative electrode, the metal oxide includes at least one of aluminum oxide, magnesium oxide, chromium oxide, and zirconium oxide; and / or, the binder is one or more of acrylate polymers, polyvinylidene fluoride, and polyimide.

[0014] In a second aspect, the present invention provides a method for preparing a negative electrode sheet, the method comprising the following steps: (1) coating a negative electrode slurry onto a negative electrode current collector and drying it to form a negative electrode film on the negative electrode current collector, thereby obtaining a preliminary negative electrode sheet; (2) coating a slurry of an isolation layer onto the surface of the negative electrode film of the preliminary negative electrode sheet using a gravure roller and drying it to obtain an isolation layer on the surface of the negative electrode film, thereby obtaining a negative electrode sheet.

[0015] In a third aspect, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a battery separator and the aforementioned negative electrode.

[0016] Compared with the prior art, the negative electrode of this application has the following beneficial effects:

[0017] The negative electrode sheet of this application has an isolation layer on the side of the negative electrode film away from the current collector. The isolation layer includes a number of spaced coating portions. The isolation layer prevents the negative electrode film from directly contacting the battery separator, and the gaps between the coating portions provide space for the expansion of the negative electrode sheet during charging, which can effectively alleviate the wrinkling of the negative electrode sheet and thus improve the cycle performance of the lithium battery. Attached Figure Description

[0018] The preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0019] Figure 1 This is a side view of the negative electrode sheet of this application.

[0020] Figure 2 This is a top view of the negative electrode sheet of this application.

[0021] List of reference numerals in the attached diagram:

[0022] 1-Negative electrode current collector; 2-Negative electrode membrane; 3-Separation layer; 31-Coating part. Detailed Implementation

[0023] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0024] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0025] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0026] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0027] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0028] The mass of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the masses of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass mentioned in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0029] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used in the following examples are commercially available unless otherwise specified.

[0031] Based on the problem pointed out in the background art that existing negative electrode sheets are prone to wrinkling, which affects the battery cycle performance.

[0032] This application provides a negative electrode sheet, which has an isolation layer on the side of the negative electrode film away from the current collector. The isolation layer includes a plurality of spaced coating portions. The isolation layer prevents the negative electrode film from directly contacting the battery separator, and the gaps between the coating portions provide space for the expansion of the negative electrode sheet during charging, which can effectively alleviate the wrinkling of the negative electrode sheet and thus improve the cycle performance of the lithium battery.

[0033] Specifically, please also refer to Figure 1 and Figure 2 The negative electrode sheet of this application includes a negative current collector 1, a negative electrode film 2, and an isolation layer 3.

[0034] The negative electrode film 2 is disposed on the surface of the negative electrode current collector 1, and the isolation layer 3 is disposed on the surface of the negative electrode film 2 away from the negative electrode current collector 1. The isolation layer 3 includes a plurality of mutually spaced coating portions 31.

[0035] The main component of the negative electrode sheet is graphite. During full charging and lithium insertion, volume expansion occurs, resulting in a thicker coating on the negative electrode sheet after full charging compared to the thickness after rolling. This leads to insufficient space between the negative electrode sheet and the coating during the rebound after full charging, causing deformation in a direction parallel to the electrode sheet and resulting in wrinkling. The negative electrode sheet of this application has an isolation layer 3 on the surface of the negative electrode film 2. The isolation layer 3 includes multiple spaced coating portions 31. The isolation layer 3 prevents direct contact between the negative electrode film 2 and the battery separator. The spaced distribution of the multiple coating portions 31 creates a gap between adjacent coating portions 31, thus providing space for the expansion of the negative electrode sheet during charging. This effectively alleviates wrinkling of the negative electrode sheet and improves the cycle performance of the lithium battery.

[0036] In some embodiments, a plurality of coating portions 31 are arranged in an array.

[0037] The arrangement of several coating portions 31 in an array facilitates coating preparation, makes it easy to control the thickness of each coating portion 31, ensures coating consistency, and reduces the difficulty of preparation.

[0038] It should be noted that the array distribution in this application refers to arranging multiple coated portions 31 according to certain rules and spacing. No restrictions are placed on the type of array; the array can be a linear array, a rectangular array, or a ring array. Of course, a rectangular array is preferred.

[0039] In some embodiments, a plurality of coating portions 31 are arranged in a rectangular array, and the spacing between two adjacent coating portions 31 is 0.2 to 1 mm in a first direction and / or a second direction, wherein the first direction is perpendicular to the second direction. The first direction and the second direction are the transverse direction and the longitudinal direction of the rectangular array, respectively.

[0040] By controlling the spacing between two adjacent coating portions 31 and setting the spacing between two adjacent coating portions 31 to 0.2 mm to 1 mm in the first and second directions, the ratio of the total area of ​​the isolation layer 3 to the area of ​​the negative electrode film 2 can be adjusted, thereby improving the anti-wrinkling ability of the negative electrode sheet.

[0041] It should be noted that the spacing between two adjacent coating portions 31 is the straight-line distance between the two closest points on the adjacent edges of the contact surfaces of the two adjacent coating portions 31 and the negative electrode film 2, as shown in the attached figure. Figure 2 d1 is the distance between two adjacent coating portions in the first direction, and d2 is the distance between two coating portions 31 in the second direction.

[0042] In some embodiments, the total contact area between all the coating portions 31 and the negative electrode film 2 is 40% to 80% of the total area of ​​the side of the negative electrode film 2 away from the negative electrode current collector 1, and preferably the total contact area between all the coating portions 31 and the negative electrode film 2 is 50% to 70% of the total area of ​​the side of the negative electrode film 2 away from the negative electrode current collector 1.

[0043] The total area of ​​the contact surfaces of all the coating portions 31 with the negative electrode film 2 is set to 50% to 70% of the total area of ​​the side of the negative electrode film 2 away from the negative electrode current collector 1, so that the area of ​​the total gap reaches 30% to 50%, which can provide sufficient space for the expansion of the negative electrode sheet, maximize the anti-wrinkling ability of the negative electrode sheet, and not affect the electrolyte wetting and lithium ion transport, thereby maximizing the cycle performance of the lithium battery.

[0044] It should be noted that this application does not impose any restrictions on the shape of the coating portion 31. In practical applications, those skilled in the art can set the shape of the coating portion according to actual needs. For example, the coating portion 31 can be set as a pyramid shape, or it can be set as a cylinder, or it can be set as a rectangle, and so on. Adjustments and changes to the specific shape of the coating portion 31 do not deviate from the basic principles of this application and should be limited to the protection scope of this application.

[0045] In some embodiments, the coating portion 31 is cylindrical or frustum-shaped.

[0046] The coating part 31 is set to a cylindrical or frustum shape, so that the contact surface between the coating part 31 and the negative electrode film 2 is circular, which facilitates the coating preparation. Moreover, compared with the contact surface of rectangular, square and other shapes, the circular shape has smooth edges, which can better provide space for the expansion of the negative electrode sheet and improve the anti-wrinkling performance of the negative electrode sheet.

[0047] In some preferred embodiments, when the coating portion 31 is frustum-shaped, the top diameter of the frustum: bottom diameter = 1:(0.4~0.8), wherein the top of the frustum is the side of the coating portion 31 away from the negative electrode film 2, and the bottom of the frustum is the side of the coating portion 31 close to the negative electrode film 2.

[0048] The ratio of the top diameter to the bottom diameter of the truncated cone is 1:(0.4~0.8). This ratio range makes it easier to prepare the gravure roller to reduce production difficulty. At the same time, it ensures that the area of ​​the side of the coating layer 31 that contacts the battery separator is not too small, thereby avoiding scratching the battery separator and facilitating application.

[0049] In some embodiments, the contact surface between the coating portion 31 and the negative electrode film 2 is circular, with a diameter of no more than 3 mm, preferably 0.2 mm to 2.3 mm.

[0050] Preferably, the contact surface between the coating portion 31 and the negative electrode film 2 is circular, and the diameter of the circle is controlled between 0.2 mm and 2.3 mm. By controlling the cross-sectional area of ​​the coating portion 31, the multiple coating portions 31 are more evenly distributed, thereby further improving the anti-wrinkling performance of the negative electrode sheet.

[0051] In some embodiments, the thickness of the coating portion 31 is 1 μm to 6 μm, and preferably the thickness of the coating portion 31 is 2 μm to 5 μm.

[0052] Preferably, the thickness of the coating portion 31 is controlled between 2μm and 5μm to maximize the anti-wrinkling ability of the negative electrode sheet without affecting the rate performance of the battery and ensuring the battery energy density. If the thickness of the coating portion 31 is too small, its effect on improving wrinkling is minimal; if the thickness of the coating portion 31 is too large, the gap between the negative electrode sheet and the battery separator will be too large, the lithium-ion transport path will be longer, affecting the rate performance of the battery and reducing the battery energy density.

[0053] In some embodiments, the isolation layer 3 comprises a metal oxide and an adhesive, wherein the mass ratio of the metal oxide to the adhesive is metal oxide: adhesive = (1-6): 1.

[0054] The mass ratio of metal oxide to binder in the isolation layer 3 is controlled by controlling the mass percentage of metal oxide and binder in the slurry of the isolation layer. Specifically, the slurry of the isolation layer includes metal oxide, binder and solvent, wherein the mass percentage of metal oxide is 5% to 30%, the mass percentage of binder is 5% to 10%, and the balance is solvent.

[0055] In some preferred embodiments, the mass ratio of metal oxide to binder in the isolation layer 3 is metal oxide: binder = (1-4):1. Specifically, the slurry of the isolation layer includes metal oxide, binder, and solvent, wherein the mass percentage of metal oxide is 5%-20%, the mass percentage of binder is 5%-10%, and the balance is solvent. Preferably, in the slurry of the isolation layer, the mass percentage of metal oxide is 10%-20%, the mass percentage of binder is 5%-10%, and the balance is solvent.

[0056] Preferably, the mass percentage of binder in the slurry of the isolation layer is controlled between 5% and 10%. This ensures that the viscosity of the slurry remains within a certain range, improving the uniformity of the slurry coating. If the mass percentage of binder is too high, the slurry viscosity will be too high, resulting in poor flowability and coating uniformity. If the mass percentage of binder is too low, the slurry viscosity will be too low, making it prone to sedimentation and inconvenient to use. Controlling the mass percentage of metal oxides between 5% and 30%, preferably between 5% and 20%, and even more preferably between 10% and 20%, ensures ion transport performance and electrolyte wettability.

[0057] In some embodiments, the metal oxide includes at least one of aluminum oxide, magnesium oxide, chromium oxide, and zirconium oxide.

[0058] The metal oxide is selected from at least one of aluminum oxide, magnesium oxide, chromium oxide and zirconium oxide, and the metal oxide will not undergo redox reactions with the components of the electrolyte within the battery operating voltage range.

[0059] In some embodiments, the binder is one or more of acrylate polymers (e.g., polyacrylate), polyvinylidene fluoride (PVDF), and polyimide (PI).

[0060] In some embodiments, the solvent is one or more of water, triethyl sulfate, acetone, and ethylene glycol.

[0061] In a second aspect, this application provides a method for preparing a negative electrode sheet to prepare the negative electrode sheet provided in the first aspect.

[0062] Specifically, the preparation method of the negative electrode includes the following steps:

[0063] (1) A negative electrode slurry is coated on the negative electrode current collector 1 and dried to form a negative electrode film 2 on the negative electrode current collector 1, thus obtaining a preliminary negative electrode sheet.

[0064] (2) A slurry for the separator layer is coated on the surface of the negative electrode film 2 of the initial negative electrode sheet using a gravure roller and then dried to obtain the separator layer 3 on the surface of the negative electrode film, thus producing the negative electrode sheet.

[0065] The method for preparing the negative electrode sheet of this application is simple and easy to control. It uses a gravure roller to coat the isolation layer 3, which is convenient for coating. Furthermore, the distribution of each coating part 31 in the isolation layer 3 can be controlled by adjusting the distribution of the pits of the gravure roller, which is convenient for use.

[0066] Thirdly, this application provides a lithium-ion battery, which includes a positive electrode, a battery separator, and the negative electrode provided in the first aspect.

[0067] The following detailed embodiments illustrate the negative electrode sheet of this application and the beneficial effects it brings.

[0068] Example 1

[0069] The negative electrode sheet in this embodiment is prepared through the following steps:

[0070] (1) Preparation of preliminary negative electrode sheet: A negative electrode slurry is coated on the surface of the negative electrode current collector, and after drying, a negative electrode film is obtained on the surface of the negative electrode current collector, thus obtaining a preliminary negative electrode sheet. The composition of the negative electrode slurry is as follows: graphite: conductive carbon black: sodium carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 96%: 1.0%: 1.2%: 1.8%. The coating thickness of the negative electrode film is 120 μm, and the negative electrode current collector is a copper foil with a thickness of 6 μm.

[0071] (2) Preparation of negative electrode sheet: A slurry for coating the separator layer is applied to the surface of the negative electrode film of the preliminary negative electrode sheet using a gravure roller. After drying, a separator layer is obtained on the surface of the negative electrode film, thus obtaining the negative electrode sheet. The composition of the separator layer slurry is as follows: 15% alumina, 5% polyvinylidene fluoride (PVDF), and 80% water. The alumina used has a particle size of 0.5–1.2 μm, the thickness of each coating portion in the separator layer is 3 μm, the total contact area between the separator layer (i.e., all coating portions) and the negative electrode film is 60% of the total area of ​​the side of the negative electrode film away from the negative current collector, the coating portion is frustum-shaped, the diameter of the contact surface between the coating portion and the negative electrode film is 1.5 mm, and the ratio of the top diameter to the bottom diameter of the frustum is 1:0.5.

[0072] Example 2

[0073] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the thickness of the coating part is different in step (2).

[0074] Specifically, in this embodiment, the thickness of the coating portion is 2 μm.

[0075] Example 3

[0076] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the thickness of the coating part is different in step (2).

[0077] Specifically, the thickness of the coating in this embodiment is 4 μm.

[0078] Example 4

[0079] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the thickness of the coating part is different in step (2).

[0080] Specifically, in this embodiment, the thickness of the coating portion is 5 μm.

[0081] Example 5

[0082] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the thickness of the coating part is different in step (2).

[0083] Specifically, in this embodiment, the thickness of the coating portion is 1 μm.

[0084] Example 6

[0085] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the thickness of the coating part is different in step (2).

[0086] Specifically, in this embodiment, the thickness of the coating portion is 6 μm.

[0087] Example 7

[0088] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that in step (2), the total contact area between the isolation layer (i.e. all the coating parts) and the negative electrode film is different.

[0089] Specifically, in this embodiment, the total contact area between the isolation layer (i.e., all the coating portions) and the negative electrode film is 50% of the total area of ​​the side of the negative electrode film away from the negative electrode current collector.

[0090] Example 8

[0091] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that in step (2), the total contact area between the isolation layer (i.e. all the coating parts) and the negative electrode film is different.

[0092] Specifically, in this embodiment, the total contact area between the isolation layer (i.e., all the coating portions) and the negative electrode film is 70% of the total area of ​​the side of the negative electrode film away from the negative electrode current collector.

[0093] Example 9

[0094] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that in step (2), the total contact area between the isolation layer (i.e. all the coating parts) and the negative electrode film is different.

[0095] Specifically, in this embodiment, the total contact area between the isolation layer (i.e., all the coating portions) and the negative electrode film is 40% of the total area of ​​the side of the negative electrode film away from the negative electrode current collector.

[0096] Example 10

[0097] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that in step (2), the total contact area between the isolation layer (i.e. all the coating parts) and the negative electrode film is different.

[0098] Specifically, the shape and radius of the coating portion in this embodiment are the same as in Embodiment 1, and the total area of ​​the contact surface between the isolation layer (i.e., all the coating portions) and the negative electrode film is 80% of the total area of ​​the side of the negative electrode film away from the negative electrode current collector.

[0099] Example 11

[0100] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the diameter of the contact surface between the coating part and the negative electrode film is different in step (2).

[0101] Specifically, in this embodiment, the diameter of the contact surface between the coating portion and the negative electrode film is 0.5 mm.

[0102] Example 12

[0103] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the diameter of the contact surface between the coating part and the negative electrode film is different in step (2).

[0104] Specifically, in this embodiment, the diameter of the contact surface between the coating portion and the negative electrode film is 1 mm.

[0105] Example 13

[0106] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the diameter of the contact surface between the coating part and the negative electrode film is different in step (2).

[0107] Specifically, in this embodiment, the diameter of the contact surface between the coating portion and the negative electrode film is 2.3 mm.

[0108] Example 14

[0109] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the diameter of the contact surface between the coating part and the negative electrode film is different in step (2).

[0110] Specifically, in this embodiment, the diameter of the contact surface between the coating portion and the negative electrode film is 3mm.

[0111] Example 15

[0112] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the ratio of the slurry in the isolation layer is different in step (2).

[0113] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 10% alumina, 10% polyvinylidene fluoride (PVDF) and 80% water.

[0114] Example 16

[0115] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the ratio of the slurry in the isolation layer is different in step (2).

[0116] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 20% alumina, 10% polyvinylidene fluoride (PVDF) and 70% water.

[0117] Example 17

[0118] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the ratio of the slurry in the isolation layer is different in step (2).

[0119] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 20% alumina, 5% polyvinylidene fluoride (PVDF) and 75% water.

[0120] Example 18

[0121] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the ratio of the slurry in the isolation layer is different in step (2).

[0122] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 30% alumina, 5% polyvinylidene fluoride (PVDF) and 65% water.

[0123] Example 19

[0124] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the ratio of the slurry in the isolation layer is different in step (2).

[0125] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 5% alumina, 5% polyvinylidene fluoride (PVDF) and 90% water.

[0126] Example 20

[0127] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the composition of the slurry of the isolation layer is different in step (2).

[0128] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 15% magnesium oxide, 5% polyimide (PI) and 80% ethylene glycol.

[0129] Example 21

[0130] The preparation method of the negative electrode sheet in this embodiment is the same as that in embodiment 1. The only difference between this embodiment and embodiment 1 is that the composition of the slurry of the isolation layer is different in step (2), and the shape of the coating part is different.

[0131] Specifically, the composition of the slurry in the isolation layer of this embodiment is as follows: 15% magnesium oxide, 5% polyacrylate and 80% ethylene glycol, and the coating part is cylindrical.

[0132] Comparative Example

[0133] The negative electrode sheet of this comparative example was prepared through the following steps:

[0134] A negative electrode slurry is coated onto the surface of the negative electrode current collector, and after drying, a negative electrode film is obtained on the surface of the negative electrode current collector, thus obtaining a preliminary negative electrode sheet. The composition of the negative electrode slurry is as follows: graphite: conductive carbon black: sodium carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 96%: 1.0%: 1.2%: 1.8%. The coating thickness of the negative electrode film is 120 μm, and the negative electrode current collector is a 6 μm thick copper foil.

[0135] Test Example

[0136] The negative electrode sheets prepared in the above embodiments and comparative examples are combined with the positive electrode sheet and the battery separator (polyethylene separator) to form a battery.

[0137] The positive electrode includes a positive current collector and a positive electrode film located on the positive current collector. The positive electrode film is composed of 97% LFP, 1% conductive carbon black and 2% PVDF.

[0138] The electrolyte consists of 12.5% ​​lithium salt (LiPF6), 3% VC and 84.5% solvent; the solvent is a mixture of EC, EMC and DMC, with EC:EMC:DMC = 30:40:30.

[0139] After the batteries of the above embodiments and comparative examples were formed and capacity tested, they were subjected to cycle tests at 25±3℃ and 0.5C / 0.5C for 1500 cycles to test their capacity retention rate. The test results are shown in Table 1.

[0140] The cycled battery was then fully charged at a rate of 0.33C, the cells were disassembled, and the wrinkling of the negative electrode was recorded. The test results are shown in Table 1.

[0141] Table 1. Test results of the examples and comparative examples. Item Rippling phenomenon of negative electrode sheet Capacity retention rate (%) after 1500 cycles Example 1 No rippling 93.40％ Example 2 No rippling 93.35％ Example 3 No rippling 93.12％ Example 4 No rippling 92.19％ Example 5 Slight rippling 91.62％ Example 6 No rippling 91.83％ Example 7 No rippling 93.39％ Example 8 No rippling 93.11％ Example 9 Slight rippling 91.61％ Example 10 No rippling 92.03％ Example 11 No rippling 93.38％ Example 12 No rippling 93.40％ Example 13 No rippling 93.31％ Example 14 No rippling 93.18％ Example 15 No rippling 93.33％ Example 16 No rippling 93.25％ Example 17 No rippling 93.27％ Example 18 Slight rippling 91.79％ Example 19 Slight rippling 91.71％ Example 20 No rippling 91.98％ Example 21 No rippling 93.22％ Comparative Example Rippling occurred at both the corner of winding and the outer circle of winding 89.58％

[0142] The test results in Table 1 show that:

[0143] 1. Comparing Examples 1 to 21 with the comparative examples, the wrinkling of the negative electrode sheets in Examples 1 to 21 is less than that in the comparative examples, and the capacity retention rate of the batteries in Examples 1 to 21 after 1500 cycles is higher than that in the comparative examples. Therefore, it can be seen that the technical solution of the negative electrode sheet in this application can improve the wrinkling of the negative electrode sheet to a certain extent by setting an isolation layer, thereby improving the cycle performance of the lithium battery.

[0144] 2. Comparing Examples 1-6, the negative electrode sheet of Example 5 exhibits slight wrinkling. The capacity retention rates of the batteries from Examples 5 and 6 after 1500 cycles are lower than those of Examples 1-4. This indicates that when the coating thickness is too small, its ability to improve negative electrode sheet wrinkling is poor; conversely, when the coating thickness is too large, the gap between the negative electrode sheet and the battery separator becomes too large, lengthening the lithium-ion transport path and thus affecting battery performance. Therefore, it is preferable to have a coating thickness of 2μm to 5μm, more preferably 2μm to 4μm, and most preferably 3μm.

[0145] 3. Comparing Examples 1 and 7-10, the negative electrode sheet of Example 9 exhibits slight wrinkling. The capacity retention rates of the batteries from Examples 9 and 10 after 1500 cycles are lower than those of Examples 1, 7, and 8. This indicates that when the area ratio of the separator layer is too small, its ability to improve the wrinkling of the negative electrode sheet is poor, while when the area ratio of the separator layer is too large, it will affect the cycle performance of the battery. Therefore, it is preferable to have the area ratio of the separator layer be 50% to 70%.

[0146] 4. Comparing Examples 1 and 11-14, the capacity retention rate of Example 14 after 1500 cycles is slightly lower than that of Examples 1 and 11-13. This indicates that, given a fixed area ratio of the separator layer, an excessively large diameter of the contact surface between the coating and the negative electrode film will somewhat affect the battery's cycle performance. Therefore, it is preferable to have a contact surface diameter between the coating and the negative electrode film of 0.5-2.3 mm.

[0147] 5. Comparing Examples 1 and 15-19, the negative electrode sheets of Examples 18 and 19 showed slight wrinkling. The capacity retention rates of the batteries from Examples 18 and 19 after 1500 cycles were lower than those of Examples 1 and 15-17. This indicates that when the content of metal oxide is too high and the solid content of the slurry is too high, or when the content of metal oxide is too low and the solid content of the slurry is too low, the ability to improve the wrinkling of the negative electrode sheet is poor, and the ability to improve the cycle performance of the battery is also poor. Therefore, in the separator layer, it is preferable that the mass ratio of metal oxide to binder is 1-4:1. Simultaneously, in the slurry of the separator layer, it is preferable that the mass percentage of metal oxide is 10%-20%, the mass percentage of binder is 5%-10%, and the mass percentage of solvent is 70%-80%.

[0148] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector (1), a negative electrode film (2) and an isolation layer (3). The negative electrode film (2) is disposed on the surface of the negative current collector (1), and the isolation layer (3) is disposed on the surface of the negative electrode film (2) away from the negative current collector (1). The isolation layer (3) includes a plurality of mutually spaced coating portions (31).

2. The negative electrode sheet according to claim 1, characterized in that, Several of the coating portions (31) are arranged in an array.

3. The negative electrode sheet according to claim 2, characterized in that, A plurality of the coating portions (31) are arranged in a rectangular array, and the spacing between two adjacent coating portions (31) is 0.2 mm to 1 mm in a first direction and / or a second direction, wherein the first direction is perpendicular to the second direction.

4. The negative electrode sheet according to claim 1, characterized in that, The total contact area between all the coating portions (31) and the negative electrode film (2) is 50% to 70% of the total area of ​​the side of the negative electrode film (2) away from the negative electrode current collector (1).

5. The negative electrode sheet according to claim 4, characterized in that, The contact surface between the coating portion (31) and the negative electrode film (2) is circular, and the diameter of the circle is 0.2 mm to 2.3 mm. And / or, the coating portion (31) is cylindrical or frustum-shaped.

6. The negative electrode sheet according to claim 1, characterized in that, The thickness of the coating portion (31) is 2μm to 5μm.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The isolation layer (3) comprises a metal oxide and an adhesive, wherein the mass ratio of the metal oxide to the adhesive is metal oxide: adhesive = (1-6):1, preferably the mass ratio of the metal oxide to the adhesive is metal oxide: adhesive = (1-4):

1.

8. The negative electrode sheet according to claim 7, characterized in that, The metal oxide includes at least one of aluminum oxide, magnesium oxide, chromium oxide, and zirconium oxide; And / or, the adhesive is one or more of the following: acrylate polymers, polyvinylidene fluoride, and polyimide.

9. A method for preparing a negative electrode sheet, used to prepare the negative electrode sheet according to any one of claims 1 to 8, characterized in that, The preparation method includes the following steps: (1) A negative electrode slurry is coated on the negative electrode current collector (1) and dried to form a negative electrode film (2) on the negative electrode current collector (1) to obtain a preliminary negative electrode sheet; (2) A slurry for the isolation layer is coated on the surface of the negative electrode film (2) of the preliminary negative electrode sheet using a gravure roller and dried to obtain an isolation layer (3) on the surface of the negative electrode film, thereby obtaining a negative electrode sheet.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a battery separator, and a negative electrode as described in any one of claims 1 to 8.

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