Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery comprises an electrode assembly that is of a stack structure; the electrode assembly comprises positive electrode sheets, a separator, and a negative electrode sheet; the positive electrode sheets comprise a double-sided positive electrode sheet and a single-sided positive electrode sheet located on the outermost side of the electrode assembly in a stack direction; the single-sided positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer provided on one surface of the positive electrode current collector, and the positive electrode material layer faces the double-sided positive electrode sheet; the positive electrode material layer comprises a bonding coating area and a non-coating area, the bonding coating area comprises a first binder, the non-coating area comprises a second binder, and the bonding coating area comprises a plurality of sub-areas arranged at intervals. The bonding coating area of the single-sided positive electrode sheet comprises the plurality of sub-areas, so that the problems of curling and wrinkling of the single-sided positive electrode sheet can be mitigated, the wettability of the single-sided positive electrode sheet is improved, the problems of purple spots, lithium precipitation and capacity attenuation of the secondary battery can be mitigated, and the influence on the energy density of the secondary battery is also small.

Description

Secondary battery and electronic device Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in consumer applications, power generation, energy storage, and other fields due to their high energy density, high dynamics, and long lifespan. They are currently the focus of much attention in the new energy sector. With the continuous advancement and innovation of technology, the demand for energy density and rate capability is becoming increasingly urgent. As a technological innovation, laminated lithium-ion batteries are becoming the future direction of lithium-ion battery technology development.

[0003] To reduce active material waste and maximize energy density, laminated lithium-ion batteries typically have a single-sided positive or negative electrode sheet on the outermost side of the electrode assembly, with a double-sided positive and negative electrode sheet in the middle. However, single-sided positive electrodes can warp and wrinkle due to differing internal stresses on both sides. While addressing this issue, new problems often arise, such as reduced liquid retention and wettability in the single-sided positive electrode area, leading to purple spots, lithium deposition, and capacity decay in secondary batteries.

[0004] Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and electronic device to improve the curling and wrinkling problems of single-sided positive electrode sheets, increase the liquid retention and wettability of the single-sided positive electrode sheet area, and thereby improve the purple spots, lithium plating, and capacity attenuation problems of the secondary battery.

[0006] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a secondary battery comprising an electrode assembly of a laminated structure, the electrode assembly comprising a positive electrode sheet, a separator, and a negative electrode sheet, the positive electrode sheet comprising a double-sided positive electrode sheet and a single-sided positive electrode sheet located on the outermost side of the electrode assembly in the direction of the laminate; the single-sided positive electrode sheet comprising a positive current collector and a positive electrode material layer disposed on one surface of the positive current collector, the positive electrode material layer facing the double-sided positive electrode sheet; the positive electrode material layer comprising an adhesive coating area and a non-coating area, the adhesive coating area comprising a first adhesive, the non-coating area comprising a second adhesive, and the adhesive coating area comprising a plurality of spaced-apart sub-areas. The adhesive coating area of ​​the single-sided positive electrode sheet comprises a plurality of sub-areas, which can improve the problem of different internal stresses on both sides of the single-sided positive electrode sheet, thereby improving the curling and wrinkling problems of the single-sided positive electrode sheet; furthermore, the provision of the adhesive coating area can also improve the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium deposition, and capacity fade problems of the secondary battery. In addition, the adhesive coating area only covers a portion of the surface of the positive electrode material layer, and its effect on the energy density of the secondary battery is also relatively small.

[0008] In some embodiments of the present application, the multiple sub-regions form a plurality of convex portions on the surface of the positive electrode material layer, and the mass ratio of the first binder to the second binder is X, 1<X≤5. The mass ratio X of the first binder to the second binder is within the above range, and the content of the first binder is greater than the content of the second binder, thereby improving the adhesion between the bonding coating area and the diaphragm, improving the liquid retention and wettability of the single-sided positive electrode plate area, and thus improving the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery. Moreover, the multiple convex portions can alleviate the problem of different internal stresses on both sides of the single-sided positive electrode plate, and improve the curling and wrinkling problems of the single-sided positive electrode plate. As a result, the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery are further improved.

[0009] In some embodiments of the present application, the multiple protrusions are distributed in a stripe shape, the height of a single protrusion is h1, 0μm

[0010] ​In some embodiments of the present application, the angle between the protrusion and the length direction of the single-sided positive electrode sheet is α, and the secondary battery satisfies the following conditions: 0μm

[0011] In some embodiments of the present application, the multiple protrusions are distributed in a dotted manner, and the shape of the protrusions includes at least one of a circle, an ellipse, or a polygon. The height of a single protrusion is h1, 0μm

[0012] In some embodiments of the present application, the multiple sub-regions form a plurality of recesses on the surface of the positive electrode material layer, and the mass ratio of the first binder to the second binder is X, 0.2≤X<1. The multiple recesses formed in the bonding coating area provide channels for the transmission of the electrolyte, thereby improving the liquid retention and wettability of the electrolyte in the negative electrode area close to the single-sided positive electrode sheet, thereby improving the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery. Moreover, the multiple recesses can alleviate the problem of different internal stresses on both sides of the single-sided positive electrode sheet, and improve the curling and wrinkling problems of the single-sided positive electrode sheet. As a result, the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery are further improved.

[0013] ​​In some embodiments of the present application, the multiple sub-regions are distributed in a stripe shape, the depth of a single recess is h2, 0μm<h2≤1μm, the minimum spacing d2 between two adjacent recesses is 0.5mm to 5mm, and the width L2 of a single recess is 50μm to 1000μm. The multiple recesses formed in the adhesive coating area are distributed in a stripe shape. By regulating h2, L2 and d2 within the above ranges, the height and distribution density of the multiple recesses are appropriate, thereby improving the curling and wrinkling problems of the positive electrode sheet, and improving the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery, while having little effect on the energy density of the secondary battery.

[0014] In some embodiments of the present application, the secondary battery satisfies the following conditions: 0μm<h2<0.5μm, L2<d2<5L2; or, 0.5μm≤h2≤1μm, 3L2<d2≤10L2. The heights of the different recesses and the minimum spacing between two adjacent recesses are matched to each other. This improves the curling and wrinkling of the positive electrode sheet, and improves the liquid retention and wettability of the single-sided positive electrode sheet area to improve the purple spot, lithium deposition, and capacity fade problems of the secondary battery. At the same time, it can reduce the impact on the energy density of the secondary battery and facilitate the winding of the single-sided positive electrode sheet during the production process.

[0015] In some embodiments of the present application, the multiple recesses are distributed in a dotted pattern, and the shape of the recesses includes at least one of a circle, an ellipse, and a polygon. The depth of a single recess is h2, 0μm<h2≤1μm, the diameter D2 of the maximum circumscribed circle of the outer contour of a single recess is 0.2mm to 4mm, and the minimum spacing d2 between two adjacent recesses is 0.5mm to 5mm. The multiple recesses formed in the bonding coating area are distributed in a dotted pattern. By regulating h2, D2 and d2 within the above range, the height and distribution density of the multiple recesses are appropriate, thereby improving the curling and wrinkling problems of the positive electrode sheet, and improving the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery, while having little effect on the energy density of the secondary battery. In addition, the dotted recesses can better alleviate the problem of different internal stresses on both sides of the single-sided positive electrode sheet, further improve the curling and wrinkling problems of the single-sided positive electrode sheet, and thus further improve the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery.

[0016] In some embodiments of the present application, along the thickness direction of the single-sided positive electrode sheet, the total area of ​​the orthographic projection of the bonding coating region is S1, the area of ​​the single-sided positive electrode sheet is S2, and 2% ≤ S1 / S2 ≤ 50%. By regulating the value of S1 / S2 within the above range, the curling and wrinkling problems of the positive electrode sheet are improved, and the liquid retention and wettability of the single-sided positive electrode sheet area are improved, thereby improving the purple spot, lithium deposition, and capacity fade problems of the secondary battery, while having a minimal impact on the energy density of the secondary battery.

[0017] In some embodiments of the present application, the positive electrode current collector is aluminum foil, with a thickness of 8 μm ≤ H1 ≤ 20 μm. By adjusting the thickness H1 of the positive electrode current collector to fall within the above range, the positive electrode current collector has an appropriate thickness, resulting in a secondary battery with high energy density and a high production process efficiency, which is conducive to industrialization.

[0018] In some embodiments of the present application, the first binder and the second binder each independently include at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate. The first binder and the second binder are highly compatible and can be adapted to the design of more secondary battery material systems.

[0019] In some embodiments of the present application, a ceramic coating is provided on the surface of the separator facing the positive electrode plate.

[0020] A second aspect of the present application provides an electronic device comprising the secondary battery according to any one of the aforementioned embodiments.

[0021] Beneficial effects of this application:

[0022] The present application provides a secondary battery and an electronic device. The secondary battery includes an electrode assembly having a laminated structure, the electrode assembly including a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet includes a double-sided positive electrode sheet and a single-sided positive electrode sheet located on the outermost side of the electrode assembly in the direction of the laminate. The single-sided positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on one surface of the positive current collector, the positive electrode material layer facing the double-sided positive electrode sheet. The positive electrode material layer includes an adhesive coating area and an uncoated area, the adhesive coating area including a first adhesive, the uncoated area including a second adhesive, and the adhesive coating area including a plurality of spaced-apart sub-areas. The adhesive coating area of ​​the single-sided positive electrode sheet includes a plurality of sub-areas. The adhesive coating area can partially eliminate internal stress in the positive electrode material layer during the installation process. The structural design of the multiple sub-areas can also alleviate the problem of different internal stresses on both sides of the single-sided positive electrode sheet, thereby improving curling and wrinkling of the single-sided positive electrode sheet. Furthermore, the provision of the adhesive coating area can improve the wettability of the single-sided positive electrode sheet, thereby improving the purple spots, lithium deposition, and capacity fade problems of the secondary battery. In addition, the adhesive coating area only covers a portion of the surface of the positive electrode material layer, and its effect on the energy density of the secondary battery is also relatively small. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings described herein are used to provide further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0024] FIG1 is a schematic diagram of a partial cross-sectional structure of a secondary battery along the thickness direction in one embodiment of the present application;

[0025] FIG2 is a schematic diagram of a cross-sectional structure of a single-sided positive electrode sheet along the thickness direction according to an embodiment of the present application;

[0026] FIG3 is a schematic structural diagram of a single-sided positive electrode sheet according to an embodiment of the present application;

[0027] FIG4 is a schematic structural diagram of a single-sided positive electrode sheet according to another embodiment of the present application;

[0028] FIG5 is a schematic structural diagram of a single-sided positive electrode sheet according to another embodiment of the present application;

[0029] FIG6 is a schematic diagram of a partial cross-sectional structure of a single-sided positive electrode sheet along the thickness direction according to another embodiment of the present application. DETAILED DESCRIPTION

[0030] To make the objectives, technical solutions, and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples. It is apparent that the described examples are only a portion of the embodiments of this application, rather than all of them. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0031] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0032] At present, facing the curling problem of single-sided positive electrode sheets, most of the curling problems are solved by increasing the thickness of the positive current collector to improve the stiffness of the positive current collector, or by applying a coating with greater bending strength on the other side of the positive current collector that is not coated with the positive electrode material layer to offset the curvature, but the above methods all have a relatively large impact on the energy density of the secondary battery. There is also a method of improving curling by applying a layer of solvent, such as N-methylpyrrolidone, on the surface of the single-sided positive electrode sheet to eliminate the residual stress after cold pressing, but this method will lead to the loss of positive active material of the single-sided positive electrode sheet, and the surface binder of the single-sided positive electrode sheet will be reduced, and the adhesion between the single-sided positive electrode sheet and the diaphragm will be poor, resulting in an increase in the distance between the single-sided positive electrode sheet and the diaphragm and the adjacent negative electrode sheet in the later stage of the secondary battery cycle, which will increase the polarization and cause problems such as purple spots, lithium precipitation, and capacity decay.

[0033] Based on the above problems, the present application provides a secondary battery and an electronic device, which improve the curling and wrinkling problems of the single-sided positive electrode sheet on the basis of having little impact on the energy density of the secondary battery, and improve the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery.

[0034] The first aspect of the present application provides a secondary battery, which includes an electrode assembly of a laminate structure, the electrode assembly including a positive electrode sheet, a separator and a negative electrode sheet, the positive electrode sheet including a double-sided positive electrode sheet and a single-sided positive electrode sheet located on the outermost side of the electrode assembly in the lamination direction. Specifically, as shown in Figures 1 to 3, the secondary battery includes a shell 100 and an electrode assembly 200 of a laminate structure, the outermost side of the electrode assembly 200 adjacent to the shell 100 is a single-sided positive electrode sheet 210, and the electrode assembly 200 also includes a double-sided positive electrode sheet 220, a separator 230 and a negative electrode sheet 240. The single-sided positive electrode sheet 210 includes a positive current collector 211 and a positive electrode material layer 212 provided on one surface of the positive current collector 211, the positive electrode material layer 212 includes an adhesive coating area 213 and a non-coating area 214, and the adhesive coating area 213 includes a plurality of sub-areas spaced apart. The double-sided positive electrode sheet 220 includes a double-sided positive electrode current collector 221 and a double-sided positive electrode material layer 222 disposed on both surfaces of the double-sided positive electrode current collector 221. The positive electrode material layer 212 in the single-sided positive electrode sheet 210 faces the negative electrode sheet 240. The negative electrode sheet 240 is a double-sided negative electrode sheet, which includes a negative electrode current collector 241 and a negative electrode material layer 242 disposed on both surfaces of the negative electrode current collector 241. In the secondary battery provided in this application, the adhesive coating area of ​​the single-sided positive electrode sheet includes multiple sub-areas. The adhesive coating area can eliminate some of the internal stress in the positive electrode material layer during the setting process. The structural design of the multiple sub-areas can also improve the problem of different internal stresses on both sides of the single-sided positive electrode sheet, thereby improving the curling and wrinkling problems of the single-sided positive electrode sheet. Moreover, the setting of the adhesive coating area and the non-coating area can also improve the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium deposition, and capacity fade problems of the secondary battery. In addition, the adhesive coating area only covers a portion of the surface of the positive electrode material layer, and its impact on the energy density of the secondary battery is also relatively small. It should be understood that the sizes and numbers of the double-sided positive electrode sheet, negative electrode sheet, and separator in Figure 1 are only examples, and the sizes, numbers, and shapes of the adhesive coating areas in Figures 2 and 3 are only examples and do not limit the scope of protection of this application.

[0035] In this application, the bonding coating area and the non-coating area refer to the area on the surface of the positive electrode material layer; the positive electrode current collector is also the positive electrode current collector in the single-sided positive electrode sheet, and the positive electrode material layer is also the positive electrode material layer in the single-sided positive electrode sheet; the double-sided positive electrode current collector is also the positive electrode current collector in the double-sided positive electrode sheet, and the double-sided positive electrode material layer is also the positive electrode material layer in the double-sided positive electrode sheet.

[0036] In some embodiments of the present application, as shown in FIG2 , a plurality of spaced-apart sub-regions of the bonding coating area 213 form a plurality of convex portions on the surface of the positive electrode material layer 212. The mass ratio of the first binder to the second binder is X, 1<X≤5. For example, X can be 1.1, 1.2, 1.3, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or a range consisting of any two values ​​therein. The mass ratio X of the first binder to the second binder is within the above range, and the content of the first binder is greater than the content of the second binder, so that the single-sided positive electrode sheet and the diaphragm have good adhesion, and the plurality of sub-regions form a plurality of convex portions on the surface of the positive electrode material layer, which as a supporting structure can provide a channel for the electrolyte, so that the electrolyte retention and wettability of the negative electrode near the single-sided positive electrode sheet area are improved, thereby improving the purple spots, lithium precipitation, and capacity attenuation problems of the secondary battery. Furthermore, the adhesive coating area can partially eliminate the internal stress in the positive electrode material layer during the installation process. The structural design of multiple protrusions can also alleviate the problem of different internal stresses on both sides of the single-sided positive electrode sheet, improving the curling and wrinkling problems of the single-sided positive electrode sheet. As a result, the purple spots, lithium deposition, and capacity decay problems of the secondary battery are further improved. In this application, the mass ratio of the first binder to the second binder refers to the ratio of the mass of the first binder in the adhesive coating area to the mass of the second binder in the non-coated area on the surface of the positive electrode material layer.

[0037] In some embodiments of the present application, as shown in Figures 2 and 3, the plurality of protrusions formed in the adhesive coating area 213 are distributed in a stripe shape, the height of a single protrusion is h1, 0μm

[0038] ​In some embodiments of the present application, as shown in FIG4 , the plurality of protrusions formed by the adhesive coating area 213 are distributed in a stripe shape, and the angle between the protrusions formed by the adhesive coating area 213 and the length direction of the single-sided positive electrode sheet is α, and the secondary battery satisfies: 0μm

[0039] In this application, the width L1 of a single protrusion is the maximum width of the orthographic projection of a single protrusion along the thickness direction of the single-sided positive electrode sheet. When multiple protrusions are distributed in a stripe-like manner, the width L1 of two adjacent protrusions can be the same or different, the length of the strips can be the same or different, and h1 and d1 can also be the same or different, as long as the purpose of this application can be achieved. This application does not specifically limit the length of the strips, and can be designed according to the size of the single-sided positive electrode sheet, as long as the purpose of this application can be achieved.

[0040] ​In some embodiments of the present application, the plurality of protrusions formed in the bonding coating area are distributed in a dotted manner, and the shape of the protrusions includes at least one of a circle, an ellipse, or a polygon. The height of a single protrusion is h1, 0μm

[0041] Specifically, as shown in FIG5 , the protrusions formed by the bonding coating area 213 are distributed in a dot-like array. The shapes of the protrusions formed by the bonding coating area 213 are circular and triangular. The maximum circumscribed circle diameter D1 of the circle is also the diameter of the circle, and the circumscribed circle diameter D1 of the triangle is also the diameter of the circumscribed circle. The minimum spacing between two adjacent circles or triangles is d1. It is understood that when the protrusions are distributed in a dot-like manner, they can also be distributed in a scattered point-like manner. The diameter D1 of the maximum circumscribed circle of the outer contour of a single protrusion and the minimum spacing d1 between two adjacent protrusions both meet the above ranges. It is understood that the size, number, and shape of the protrusions in FIG5 are only examples and do not limit the scope of protection of this application. ​

[0042] In the present application, the shape of the convex portion is a polygon, and illustratively, the polygon can be a triangle, square, rectangle, pentagon, hexagon, octagon, etc. When multiple convex portions are distributed in a point-like manner, the shapes of the orthographic projections of two adjacent convex portions can be the same or different, and h1, D1, and d1 can also be the same or different, as long as the purpose of the present application can be achieved.

[0043] The present application does not particularly limit the preparation method of a single-sided positive electrode sheet with a plurality of protrusions formed in the bonding coating area, as long as the purpose of the present application can be achieved. For example, the preparation method of the single-sided positive electrode sheet may include but is not limited to the following steps: (1) mixing the positive electrode active material, the second binder, and the conductive agent, adding them to a solvent to prepare a positive electrode slurry, and then setting the positive electrode slurry on one surface of the positive electrode current collector, drying, and cold pressing to obtain a sheet with a positive electrode active material layer set on one surface of the positive electrode current collector; (2) adding the first binder to the solvent to prepare a bonding coating area slurry, coating the bonding coating area slurry on the surface of the positive electrode active material layer, and drying to obtain a single-sided positive electrode sheet. The solid content W of the bonding coating area slurry is 1% to 5%. The high content of the binder in the bonding coating area slurry can form protrusions on the surface of the positive electrode material layer, and the solvent in the bonding coating area slurry can soften the second binder in the positive electrode active material layer, thereby eliminating some internal stress in the positive electrode active material layer. The present application has no particular limitation on the above-mentioned solvent, as long as the purpose of the present application can be achieved. For example, the above-mentioned solvent can be selected from conventional solvents used in the preparation of positive electrode slurry in the art, and the present application does not impose any limitation on this.

[0044] The present application does not particularly limit the method for regulating h1, L1, d1, and D1, as long as the purpose of the present application can be achieved. For example, the values ​​of h1, L1, d1, and D1 can be regulated by regulating the solid content and content of the slurry in the bonding coating area during the coating process, as well as the coating method.

[0045] In some embodiments of the present application, as shown in FIG6 , the bonding coating area 213 has multiple sub-areas forming multiple recesses on the surface of the positive electrode material layer, and the mass ratio of the first binder to the second binder is X, 0.2≤X<1. For example, X can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.99 or a range consisting of any two of these values. The multiple recesses formed in the bonding coating area provide channels for the transmission of the electrolyte, thereby improving the electrolyte retention and wettability of the negative electrode near the single-sided positive electrode plate area, and there is still good adhesion between the non-coated area and the separator, thereby improving the purple spot, lithium deposition, and capacity fade problems of the secondary battery. Moreover, the bonding coating area can eliminate some of the internal stress in the positive electrode material layer during the setting process, and the structural design of the multiple recesses can also alleviate the problem of different internal stresses on both sides of the single-sided positive electrode plate, improving the curling and wrinkling problems of the single-sided positive electrode plate, thereby further improving the purple spot, lithium deposition, and capacity fade problems of the secondary battery. It is understood that the size, number and shape of the bonding coating areas in FIG6 are merely examples and do not limit the scope of protection of the present application.

[0046] In some embodiments of the present application, the plurality of sub-regions are distributed in a stripe shape, the depth of a single concave portion is h2, 0 μm < h2 ≤ 1 μm, the minimum spacing d2 between two adjacent concave portions is 0.5 mm to 5 mm, and the width L2 of a single concave portion is 50 μm to 1000 μm. For example, h2 may be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a range consisting of any two values ​​thereof. For example, L2 may be 50 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1000 μm, or a range consisting of any two values ​​thereof. For example, d2 can be 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, or a range consisting of any two of these values. The multiple recesses formed in the adhesive coating area are distributed in a stripe pattern. By regulating h2, L2, and d2 within the above ranges, the height and distribution density of the multiple recesses are appropriately adjusted, thereby improving the curling and wrinkling problems of the positive electrode sheet and increasing the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spot, lithium deposition, and capacity decay problems of the secondary battery while having little impact on the energy density of the secondary battery.

[0047] In some embodiments of the present application, the multiple recesses formed in the adhesive coating area are distributed in a stripe shape, and the secondary battery satisfies: 0μm<h2<0.5μm, L2<d2<5L2; for example, when h2 is 0.1μm, 0.15μm, 0.2μm, 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, 0.49μm or any value in a range consisting of any two of them, d2 can be 1.1L2, 1.2L2, 1.3L2, 1.5L2, 2L2, 2.5L2, 3L2, 3.5L2, 4L2, 4.5L2, 4.9L2 or a range consisting of any two of them. In some embodiments of the present application, the multiple recesses formed in the bonding coating area are distributed in stripes, and the secondary battery satisfies: 0μm<h2<0.5μm, 0.5mm≤d2≤5mm; for example, when h2 is 0.1μm, 0.15μm, 0.2μm, 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, 0.49μm or any value in a range consisting of any two of them, d2 can be 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm or a range consisting of any two of them. In some embodiments of the present application, the multiple recesses formed in the bonding coating area are distributed in a stripe shape, and the secondary battery satisfies: 0.5μm≤h2≤1μm, 3L2<d2≤10L2; for example, when h2 is 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.85μm, 0.9μm, 0.95μm, 1μm or any value in a range consisting of any two of them, d2 can be 3.1L2, 4L2, 5L2, 6L2, 7L2, 8L2, 9L2, 10L2 or a range consisting of any two of them. In some embodiments of the present application, the multiple recesses formed in the bonding coating area are distributed in stripes, and the secondary battery satisfies: 0.5μm≤h2≤1μm, 0.5mm≤d2≤5mm; for example, when h2 is 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.85μm, 0.9μm, 0.95μm, 1μm or any value in a range consisting of any two of them, d2 can be 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm or a range consisting of any two of them.The heights of different recesses match the minimum spacing between two adjacent recesses, which improves the curling and wrinkling problems of the positive electrode sheet, and improves the liquid retention and wettability of the single-sided positive electrode sheet area to improve the purple spots, lithium deposition, and capacity attenuation problems of the secondary battery. At the same time, on the one hand, it can reduce the impact on the energy density of the secondary battery, and on the other hand, it is beneficial to the winding of the single-sided positive electrode sheet during the production process.

[0048] In this application, the width L2 of a single recess is the maximum width of the orthographic projection of the single recess along the thickness direction of the single-sided positive electrode sheet. When multiple recesses are distributed in a stripe-like pattern, the widths L2 of two adjacent recesses can be the same or different, the lengths of the stripes can be the same or different, and h2 and d2 can also be the same or different, as long as the purpose of this application can be achieved. This application does not specifically limit the length of the stripes and can be designed according to the size of the single-sided positive electrode sheet, as long as the purpose of this application can be achieved.

[0049] In some embodiments of the present application, the plurality of recesses are distributed in a dotted pattern, and the shape of the recesses includes at least one of a circle, an ellipse, and a polygon. The depth of a single recess is h2, 0 μm < h2 ≤ 1 μm, the diameter D2 of the maximum circumscribed circle of the outer contour of the single recess is 0.2 mm to 4 mm, and the minimum spacing d2 between two adjacent recesses is 0.5 mm to 5 mm. For example, h2 can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a range consisting of any two values ​​therein. For example, D2 can be 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.7 mm, 0.9 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, or a range consisting of any two values ​​therein. For example, d2 can be 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm or a range consisting of any two of the values. The multiple recesses formed in the bonding coating area are distributed in a point-like manner. By regulating h2, D2 and d2 within the above range, the height and distribution density of the multiple recesses are appropriate, thereby improving the curling and wrinkling problems of the positive electrode sheet, and improving the liquid retention and wettability of the single-sided positive electrode sheet area, thereby improving the purple spots, lithium precipitation and capacity decay problems of the secondary battery, while having little effect on the energy density of the secondary battery. In addition, the point-like distribution of the recesses can better alleviate the problem of different internal stresses on both sides of the single-sided positive electrode sheet, further improve the curling and wrinkling problems of the single-sided positive electrode sheet, thereby further improving the purple spots, lithium precipitation and capacity decay problems of the secondary battery. In the present application, the shape of the recess is also the shape of the orthographic projection of the recess along the thickness direction of the single-sided positive electrode sheet; the outer contour of the orthographic projection of a single recess is also the outer contour of the orthographic projection along the thickness direction of the single-sided positive electrode sheet; the minimum spacing d2 between two adjacent recesses is also the minimum spacing d2 between the outer contours of the orthographic projections of two adjacent recesses.

[0050] In the present application, the multiple recesses formed in the bonding coating area are distributed in a dotted pattern. The multiple recesses can be distributed in a dotted array or in a scattered pattern. In the present application, the shape of the recess is a polygon. For example, the polygon can be a triangle, square, rectangle, pentagon, hexagon, octagon, etc. When the multiple recesses are distributed in a dotted pattern, the shapes of the orthographic projections of two adjacent recesses can be the same or different, and h2, D2, and d2 can also be the same or different, as long as the purpose of the present application is achieved.

[0051] The present application does not particularly limit the preparation method of a single-sided positive electrode sheet with multiple recesses formed in the bonding coating area, as long as the purpose of the present application can be achieved. For example, the preparation method of a single-sided positive electrode sheet may include but is not limited to the following steps: (1) mixing a positive electrode active material, a second binder, and a conductive agent, adding the mixture to a solvent to form a positive electrode slurry, and then setting the positive electrode slurry on one surface of a positive electrode current collector, drying, and cold pressing to obtain a sheet with a positive electrode active material layer set on one surface of a positive electrode current collector; (2) adding a first binder to a solvent to form a bonding coating area slurry, coating the bonding coating area slurry on the surface of the positive electrode active material layer, and drying to obtain a single-sided positive electrode sheet. The solid content W of the bonding coating area slurry is greater than or equal to 0% and less than or equal to 0.8%. The content of the binder in the bonding coating area slurry is relatively low, so that recesses can be formed on the surface of the positive electrode material layer. The present application has no particular limitation on the above-mentioned solvent, as long as the purpose of the present application can be achieved. For example, the above-mentioned solvent can be selected from conventional solvents used in the preparation of positive electrode slurry in the art, and the present application does not impose any limitation on this.

[0052] The present application does not particularly limit the method for regulating h2, L2, d2, and D2, as long as the purpose of the present application can be achieved. For example, the values ​​of h2, L2, d2, and D2 can be regulated by regulating the solid content and content of the slurry in the bonding coating area during the coating process, as well as the coating method.

[0053] In some embodiments of the present application, along the thickness direction of the single-sided positive electrode sheet, the total area of ​​the positive projection of the bonding coating area is S1, the area of ​​the single-sided positive electrode sheet is S2, and 2% ≤ S1 / S2 ≤ 50%. For example, S1 / S2 can be 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or a range consisting of any two values ​​therein. By regulating the value of S1 / S2 within the above range, while improving the curling and wrinkling problems of the positive electrode sheet, and improving the liquid retention and wettability of the single-sided positive electrode sheet area, the purple spots, lithium precipitation, and capacity attenuation problems of the secondary battery are improved, while having little effect on the energy density of the secondary battery.

[0054] In some embodiments of the present application, the positive electrode current collector is aluminum foil, and 8μm ≤ H1 ≤ 20μm. For example, H1 can be 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or a range consisting of any two of these values. The positive electrode current collector is aluminum foil. By regulating the thickness H1 of the positive electrode current collector within the above range, the positive electrode current collector thickness is appropriate, the resulting secondary battery has high energy density, and the production process has a high yield, which is conducive to industrialization.

[0055] In some embodiments of the present application, the double-sided positive electrode current collector is aluminum foil, and 8 μm ≤ H2 ≤ 20 μm. For example, H2 can be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or a range consisting of any two of these values.

[0056] In the present application, positive electrode current collectors of different thicknesses can be purchased and measured with a micrometer to select a positive electrode current collector of the desired thickness.

[0057] In some embodiments of the present application, the first binder and the second binder each independently include at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate, and the first binder and the second binder may be the same or different. The first binder and the second binder are highly compatible and can be adapted to the design of a wider range of secondary battery material systems.

[0058] In the present application, the positive electrode material layer and the double-sided material layer each independently include a positive electrode active material. The present application has no special restrictions on the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include but is not limited to at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium iron manganese phosphate or lithium titanate.

[0059] The positive electrode material layer and the double-sided material layer may further include a conductive agent. The present application does not particularly limit the type of conductive agent, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers, and the conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymers may include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole.

[0060] The double-sided material layer may further include an adhesive, which may include at least one of the first adhesive and the second adhesive mentioned above, or at least one of sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide or styrene-butadiene rubber.

[0061] This application does not particularly limit the mass ratio of the positive electrode active material, the conductive agent, and the second binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. This application does not particularly limit the mass ratio of the positive electrode active material, the conductive agent, and the binder in the double-sided positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. This application does not particularly limit the thickness of the single-layer positive electrode material layer and the double-sided positive electrode material layer, as long as the purpose of this application can be achieved, for example, the thickness is 30μm to 120μm.

[0062] In this application, the positive electrode material layer includes an adhesive coating area and an uncoated area. The adhesive coating area of ​​a single-sided positive electrode sheet comprises multiple sub-areas, with the adhesive coating area forming convex or concave portions, such that the roughness Ra of the adhesive coating area is greater than the roughness Rb of the uncoated area. This can improve purple spots, lithium deposition, and capacity fade issues in secondary batteries, while also having a minimal impact on the energy density of the secondary battery.

[0063] In some embodiments of the present application, the separator includes a substrate layer and a ceramic coating disposed on one surface of the substrate layer, wherein the ceramic coating faces the positive electrode sheet.

[0064] In some embodiments of the present application, the ceramic coating includes an inorganic layer including inorganic particles and a binder. The present application has no particular restrictions on the inorganic particles. For example, the inorganic particles may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the binder. For example, the binder may be at least one of the above-mentioned binders. The present application has no particular restrictions on the mass of the inorganic particles and the binder, as long as the purpose of the present application can be achieved. For example, the mass ratio of the inorganic particles and the binder is 1: (0.05 to 0.12).

[0065] In some embodiments of the present application, the thickness of the ceramic coating is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the ceramic coating can be 2 μm to 6 μm. For example, the thickness of the ceramic coating can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or a range consisting of any two of these values.

[0066] The material of the substrate layer may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane, or a spun membrane. Exemplarily, the substrate layer may use a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0067] In the present application, the thickness of the separator is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the separator may be 3 μm to 30 μm.

[0068] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector. For example, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.

[0069] The negative electrode material layer includes a negative electrode active material. The present application has no particular limitation on the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include but is not limited to natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 or at least one of Li-Al alloys.

[0070] In some embodiments of the present application, the negative electrode material layer may further include a conductive agent and a binder. The present application does not particularly limit the types of the conductive agent and binder, as long as they can achieve the purpose of the present application. For example, they can be at least one of the above-mentioned conductive agents and binders. The present application does not particularly limit the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as they can achieve the purpose of the present application.

[0071] The present application does not particularly limit the thickness of the negative electrode material layer, as long as it can achieve the purpose of the present application. For example, the thickness of the negative electrode material layer is 30 μm to 120 μm. The present application does not particularly limit the thickness of the negative electrode current collector, as long as it can achieve the purpose of the present application. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0072] In the present application, the secondary battery further includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent.

[0073] The present application does not specifically limit the lithium salt, as long as the objectives of the present application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB), or lithium difluoroborate. The present application does not specifically limit the content of the lithium salt in the electrolyte, as long as the objectives of the present application can be achieved.

[0074] The present application has no particular limitation on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents.

[0075] Above-mentioned carbonate compound can include but not limited to at least one in linear carbonate compound, cyclic carbonate compound or fluorinated carbonate compound.Above-mentioned linear carbonate compound can include but not limited to at least one in dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methyl ethyl carbonate (MEC).Above-mentioned cyclic carbonate can include but not limited to at least one in ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include, but is not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The above-mentioned carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, or caprolactone. The above-mentioned ether compound may include but is not limited to at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate. The present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the purpose of the present application can be achieved.

[0076] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking a diaphragm, a negative electrode sheet, a diaphragm, a double-sided positive electrode sheet, a diaphragm, a negative electrode sheet, and a diaphragm in order, and setting a single-sided positive electrode sheet on the outermost sides with the first material layer facing the negative electrode sheet, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly of the stacked structure, placing the electrode assembly in a shell, injecting the electrolyte into the shell and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. can also be placed in the shell to prevent pressure rise and overcharge and discharge inside the secondary battery.

[0077] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the aforementioned embodiments. Therefore, the electronic device provided by the present application has good performance.

[0078] The present application does not particularly limit the type of electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0079] Example

[0080] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0081] Test methods and equipment:

[0082] The mass ratio X of the first binder to the second binder:

[0083] (1) Take a single-sided positive electrode sheet with a radius r of 10 mm and soak it in 99.9% pure dimethyl carbonate (DMC) for 24 hours to wash away the residual electrolyte. The DMC is replaced every 8 hours. After soaking, evacuate the vacuum box at room temperature and let it air-dry for 12 hours until no DMC residue is left on the surface.

[0084] (2) placing a single-sided positive electrode sheet in a scanning electron microscope (SEM) sample chamber, selecting a magnification of 100 to 2000 times, and observing and determining the coated area and non-coated area on the surface of the positive electrode material layer under the SEM field of view;

[0085] (3) Perform EDS distribution analysis on the area within the field of view in backscattering mode; select the coated area and the non-coated area to analyze the composition content of the specified elements;

[0086] (4) Taking PVDF as an example, the specified element is F, and the F element content data of the coating area and the specific value of the F element content of the non-coating area are output; if it is polyacrylonitrile, the specified element is N, and the N element content data of the coating area and the specific value of the N element content of the non-coating area are output;

[0087] (5) Calculating the mass ratio of the specified elements in the coating area and the non-coating area is the mass ratio X of the first binder to the second binder;

[0088] (6) Repeat steps (2) to (4) three times and take the average value as the final output result.

[0089] Tests for h1, L1, d1, D1, h2, L2, d2, D2:

[0090] Disassemble the lithium-ion battery and observe whether the coating area on the surface of the positive electrode material layer of the single-sided positive electrode sheet is a convex or concave part, as well as its distribution state. Ion-polish the cross section of the single-sided positive electrode sheet along its own thickness direction, observe the convex or concave part, and select one to measure its h1, L1, d1, h2, L2, and d2. Observe the surface of the positive electrode material layer under a scanning electron microscope, select a convex or concave part, measure D1 or D2, and calculate S1. Measure 10 of the above dimensions and take the average value as the final result. S2 = 55 × 91.5 = 5032.5 mm 2 , and then calculate S1 / S2.

[0091] Single-sided positive electrode curling test:

[0092] 1. Take the single-sided positive electrode sheet, and then cut the single-sided positive electrode sheet into a rectangular electrode sheet of 91.5mm×55mm;

[0093] 2. Lay the rectangular electrode flat on a smooth marble table with the first material layer facing downward in a naturally stretched state. Measure the dimensions of the rectangular electrode in the transverse TD and longitudinal MD directions in the stretched state, and record them as T1 and M1;

[0094] 3. Use a glass plate to flatten the rectangular electrode until it is flush with the tabletop, and measure the actual transverse TD and longitudinal MD dimensions of the rectangular electrode, which are recorded as T2 and M2;

[0095] 4. Calculate the curling rate of the single-sided positive electrode sheet: TD curling rate = (T2-T1) / T2×100%; MD curling rate = (M2-M1) / M2×100%;

[0096] The larger of the TD and MD curling ratios is used as the final result. A curling ratio greater than or equal to 2% indicates that the single-sided positive electrode sheet is severely curled, posing a high risk during processing. A curling ratio less than 2% indicates that the single-sided positive electrode sheet is not severely curled, posing little risk during processing.

[0097] Observation of purple spots and lithium precipitation:

[0098] Place the lithium-ion battery in an environment of 25°C, charge it at a constant current of 0.5C to a voltage of 4.5V, then charge it at a constant voltage of 4.5V to a cut-off current of 0.05C, let it stand for 5 minutes, discharge it at a constant current of 0.5C to a voltage of 3.0V, let it stand for 5 minutes. This is one charge and discharge cycle. Perform 500 cycles of charge and discharge in the same steps. Then charge it to 4.5V, then charge it at a constant voltage of 4.5V to a cut-off current of 0.05C, let it stand for 5 minutes. Disassemble the lithium-ion battery and observe whether there are purple spots and lithium precipitation on the surface of the negative electrode adjacent to the single-sided positive electrode. If so, record it as "yes", if not, record it as "no".

[0099] Cycle capacity retention test:

[0100] Place the lithium-ion battery in an environment of 25°C, charge it at a constant current of 1C to a voltage of 4.5V, then charge it at a constant voltage of 4.5V to a cutoff current of 0.2C, let it stand for 5 minutes, discharge it at a constant current of 0.5C to a voltage of 3.0V, let it stand for 5 minutes. This is one charge and discharge cycle. Record the discharge capacity at this time as C1. Perform x rounds of charge and discharge cycles with the same steps. Take the discharge capacity of the xth round as Cx. The battery capacity retention rate of the xth round is: Cx / C1×100%, x=500.

[0101] Example 1

[0102] <Preparation of positive electrode slurry>

[0103] The positive electrode active material LiCoO2, the conductive agent acetylene black, and the second binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75wt%. After vacuum stirring, the positive electrode slurry was obtained.

[0104] <Preparation of single-sided positive electrode sheet>

[0105] (1) The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness H1 of 10 μm, and dried at 85°C to obtain a positive electrode sheet with a single-side coating of a 100 μm thick material layer, which was then cold pressed for use.

[0106] (2) The first binder, PVDF, was added to NMP and dispersed at 1000 rpm for 60 minutes to form a bonding coating slurry with a solid content W of 2%. The bonding coating slurry was then applied to the surface of the material layer and dried at 100°C to form a plurality of protrusions with a stripe pattern, h1 = 0.2 μm, L1 = 100 μm, d1 = 0.8 mm, and α = 90°. The material was then vacuum dried at 85°C for 8 hours. After cutting and welding the tabs, a single-sided positive electrode sheet with a specification of 55 mm × 91.5 mm was obtained for use. The thickness of the positive electrode material layer was 53.7 μm.

[0107] <Preparation of double-sided positive electrode sheet>

[0108] The positive electrode slurry is evenly coated on one surface of a double-sided positive electrode current collector aluminum foil with a thickness of 10μm. Drying at 85°C produces a positive electrode sheet with a 100μm thick double-sided positive electrode material layer on one side. Repeat the above steps on the other surface of the aluminum foil to obtain a double-sided positive electrode sheet. This is then cold-pressed and vacuum-dried at 85°C for 8 hours. After cutting and welding the tabs, a double-sided positive electrode sheet with a size of 55mm x 91.5mm is obtained for future use. The thickness of the single double-sided positive electrode material layer is 53.7μm.

[0109] <Preparation of negative electrode sheet>

[0110] The negative electrode active material, artificial graphite, the binder, styrene-butadiene rubber, and the conductive agent, acetylene black, were mixed in a mass ratio of 96:2:2. Deionized water was added as a solvent to form a slurry with a solid content of 45 wt%. The mixture was then stirred evenly in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on one surface of a 6 μm thick copper foil, a negative electrode current collector, and dried at 85°C to obtain a negative electrode sheet coated on one side with a 100 μm thick negative electrode material layer. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet coated on both sides with a negative electrode material layer. After cold pressing, cutting, and welding the tabs, the sheet was vacuum dried at 120°C for 12 hours to obtain a negative electrode sheet measuring 56 mm x 93 mm for use. The thickness of the negative electrode material layer on one side was 66.5 μm.

[0111] <Preparation of Electrolyte>

[0112] In an environment with a water content of less than 10 ppm, ethylene carbonate, propylene carbonate, and diethyl carbonate are mixed in a mass ratio of 1:1:1 to obtain an organic solvent. An electrolyte salt, LiPF6, is then added to the organic solvent and mixed thoroughly to obtain an electrolyte solution. The molar concentration of the electrolyte salt is 1.15 mol / L based on the mass of the electrolyte solution.

[0113] <Diaphragm>

[0114] A 5μm-thick porous polyethylene film (provided by Celgard) was used as a base film. A ceramic coating slurry was applied to one surface of the base film to form a ceramic coating. The thickness of the single ceramic coating layer was 3μm. The ceramic coating slurry was prepared by mixing aluminum oxide and polyvinylidene fluoride in a mass ratio of 9:1, then adding NMP and mixing thoroughly. The solid content was 45wt%.

[0115] <Preparation of lithium-ion batteries>

[0116] The above-prepared separator, negative electrode sheet, separator, double-sided positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order, and single-sided positive electrode sheets are placed on the outermost sides with the first material layer facing the negative electrode sheet and the ceramic coating facing the positive electrode sheet. The four corners of the entire stack are then secured with tape to obtain a stacked electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The above-prepared electrolyte is injected and the lithium-ion battery is obtained through vacuum packaging, standing, formation, shaping, capacity, degassing, and trimming. The formation temperature is 80°C and the formation standing time is 2 hours.

[0117] Examples 2 to 37

[0118] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Example 1.

[0119] Comparative Example 1

[0120] Except for preparing the single-sided positive electrode sheet according to the following preparation method, the rest is the same as Example 1:

[0121] <Preparation of single-sided positive electrode sheet>

[0122] The positive electrode slurry was evenly coated on one surface of a 10μm thick positive electrode current collector aluminum foil and dried at 85°C to produce a positive electrode sheet with a 100μm thick positive electrode material layer on one side. This sheet was then cold pressed and vacuum dried at 85°C for 8 hours. After cutting and welding the tabs, a single-sided positive electrode sheet measuring 55mm x 91.5mm was obtained for future use. The positive electrode material layer was 53.7μm thick.

[0123] Comparative Example 2

[0124] Except for preparing the single-sided positive electrode sheet according to the following preparation method, the rest is the same as Example 1:

[0125] <Preparation of single-sided positive electrode sheet>

[0126] (1) The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness H1 of 10 μm, and dried at 85°C to obtain a positive electrode sheet with a single-side coating of a 100 μm thick positive electrode material layer.

[0127] (2) Boehmite with a D50 of 1 μm and PVDF as a binder were mixed in a mass ratio of 95:5, and deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was then stirred in a vacuum to obtain a coating slurry. The coating slurry was then applied to the other surface of the aluminum foil, dried, cold pressed, and vacuum dried at 85°C for 8 hours. After cutting and welding the tabs, a single-sided positive electrode sheet with a specification of 55 mm × 91.5 mm was obtained for use. The thickness of the positive electrode material layer was 53.7 μm, and the thickness of the coating was 10 μm.

[0128] Comparative Example 3

[0129] Except for preparing the single-sided positive electrode sheet according to the following preparation method, the rest is the same as Example 1:

[0130] <Preparation of single-sided positive electrode sheet>

[0131] (1) The positive electrode slurry was evenly coated on one surface of the positive electrode current collector aluminum foil with a thickness of H1 of 10 μm, and dried at 85 ° C to obtain a positive electrode sheet with a single-side coating of a 100 μm thick positive electrode material layer. After cold pressing, 30 mg / 1540.25 mm2 of positive electrode material was evenly coated on the surface of the positive electrode material layer. 2 The material was dried in NMP and then vacuum dried at 85°C for 8 hours. After cutting and welding the tabs, a single-sided positive electrode sheet with a size of 55mm×91.5mm was obtained for use. The thickness of the positive electrode material layer was 53.7μm.

[0132] The preparation parameters and performance tests of each embodiment and comparative example are shown in Table 1.

[0133] From Examples 1 to 37 and Comparative Examples 1 to 3, it can be seen that the lithium-ion batteries in the examples have a single-sided positive electrode sheet with a positive electrode material layer provided with an adhesive coating area and a non-coating area, and the adhesive coating area includes multiple sub-areas; the surface of the single-sided positive electrode sheet in Comparative Example 1 is not treated in any way, the other surface of the single-sided electrode sheet in Comparative Example 2 is provided with an anti-bending coating, and the single-sided positive electrode sheet in Comparative Example 3 is coated with a solvent on the surface of the positive electrode material layer after preparation. None of the above comparative examples have the aforementioned adhesive coating area and non-coating area at the same time. The single-sided positive electrode sheet obtained in Comparative Example 1 has a very high curling rate, and cannot be made into a lithium-ion battery due to abnormal processing; the single-sided positive electrode sheet obtained in Comparative Example 2 has improved curling by providing an anti-bending coating. Although the lithium-ion battery does not have the problems of black spots and lithium plating, the cycle capacity retention rate is low, and the coating setting also has a great impact on the energy density of the lithium-ion battery; in Comparative Example 3, the lithium-ion battery has both black spots and lithium plating problems, and the cycle capacity retention rate is low. The curling rate of the single-sided positive electrode sheets obtained in the embodiments of the present application is less than 2%, and there is basically no risk in the processing process. The lithium-ion batteries will not have both black spots and lithium plating problems, and the cycle capacity retention rate is high. This shows that the curling problem of the single-sided positive electrode sheets in the embodiments of the present application is improved. At the same time, the black spots, lithium plating and capacity attenuation problems of the lithium-ion batteries are improved compared with the lithium-ion batteries in the comparative examples, and they have better cycle performance. In addition, the existence of the adhesive coating area and the non-coating area has basically no effect on the energy density of the secondary battery.

[0134] The form of the bonding coating area usually affects the cycle performance of the lithium-ion battery. It can be seen from Examples 1 to 37 that the bonding coating area exists in the form of a convex or concave portion, and the resulting single-sided positive electrode sheet has a low curling rate. The lithium-ion battery will not have both black spots and lithium plating problems, and the cycle capacity retention rate is high. This shows that while the curling problem of the single-sided positive electrode sheet is improved in the above embodiments, the black spots, lithium plating and capacity attenuation problems of the lithium-ion battery are also improved, and the battery has good cycle performance.

[0135] The size and distribution of the protrusions usually affect the cycle performance of the lithium-ion battery. From Examples 1 to 18, it can be seen that the protrusions are distributed in stripes or matrix points, and the resulting single-sided positive electrode sheet has a low curling rate. The lithium-ion battery will not have both black spots and lithium plating problems, and the cycle capacity retention rate is high. This shows that while the curling problem of the single-sided positive electrode sheet is improved in the above embodiments, the black spots, lithium plating and capacity attenuation problems of the lithium-ion battery are also improved, and the battery has good cycle performance. Moreover, when the height h1 of the protrusion, the minimum spacing d1 between two adjacent protrusions, the width L1 of the protrusion, the diameter D1 of the maximum circumscribed circle of the outer contour of a single protrusion, and the mass ratio X of the first binder to the second binder are within the scope of this application, the obtained single-sided positive electrode sheet has a low curvature, the lithium-ion battery does not have black spots and lithium plating problems, and the cycle capacity retention rate is high, which means that when h1, d1, L1, D1, and X are within the scope of this application, the curling problem of the single-sided positive electrode sheet is improved, and the black spots, lithium plating, and capacity attenuation problems of the lithium-ion battery are improved, and the cycle performance is better.

[0136] The size and distribution of the recesses usually affect the cycle performance of the lithium-ion battery. From Examples 19 to 34, it can be seen that the recesses are distributed in stripes or in matrix points, and the resulting single-sided positive electrode sheet has a low curling rate. The lithium-ion battery will not have both black spots and lithium plating problems, and the cycle capacity retention rate is high. This shows that while the curling problem of the single-sided positive electrode sheet in the above embodiments is improved, the black spots, lithium plating and capacity attenuation problems of the lithium-ion battery are also improved, and the battery has good cycle performance. Moreover, when the height h2 of the recess, the minimum spacing d2 between two adjacent recesses, the width L2 of the recess, the diameter D2 of the maximum circumscribed circle of the outer contour of a single recess, and the mass ratio X of the first binder to the second binder are within the scope of this application, the curling rate of the obtained single-sided positive electrode sheet is low, the lithium-ion battery does not have black spots and lithium plating problems, and the cycle capacity retention rate is high, which means that when h2, d2, L2, D2, and X are within the scope of this application, the curling problem of the single-sided positive electrode sheet is improved, and the black spots, lithium plating, and capacity attenuation problems of the lithium-ion battery are improved, and the cycle performance is better.

[0137] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery comprising an electrode assembly having a laminated structure, the electrode assembly comprising a positive electrode sheet, a separator, and a negative electrode sheet, the positive electrode sheet comprising a double-sided positive electrode sheet and a single-sided positive electrode sheet located on the outermost side of the electrode assembly in the direction of the laminated structure; The single-sided positive electrode sheet includes a positive electrode collector and a positive electrode material layer arranged on one surface of the positive electrode collector, and the positive electrode material layer faces the double-sided positive electrode sheet; the positive electrode material layer includes an adhesive coating area and a non-coating area, the adhesive coating area includes a first adhesive, the non-coating area includes a second adhesive, and the adhesive coating area includes a plurality of sub-areas arranged at intervals.

2. The secondary battery according to claim 1, wherein The multiple sub-regions form a plurality of protrusions on the surface of the positive electrode material layer, and the mass ratio of the first binder to the second binder is X, where 1<X≤5.

3. The secondary battery according to claim 2, wherein The plurality of convex portions are distributed in a stripe shape, the height of a single convex portion is h1, 0 μm<h1≤1 μm, the width L1 of a single convex portion is 50 μm to 1000 μm, and the minimum distance d1 between two adjacent convex portions is 0.5 mm to 5 mm.

4. The secondary battery according to claim 3, wherein The angle between the protrusion and the length direction of the single-sided positive electrode sheet is α, and the secondary battery satisfies: 0μm<h1<0.5μm, 0°≤α≤90°; or 0.5μm≤h1≤1μm, 20°≤α≤75°.

5. The secondary battery according to claim 2, wherein The multiple protrusions are distributed in a point-like manner, and the shape of the protrusions includes at least one of a circle, an ellipse or a polygon. The height of a single protrusion is h1, 0μm<h1≤1μm, the diameter D1 of the maximum circumscribed circle of the outer contour of a single protrusion is 0.2mm to 4mm, and the minimum spacing d1 between two adjacent protrusions is 0.5mm to 5mm.

6. The secondary battery according to claim 1, wherein The multiple sub-regions form a plurality of recesses on the surface of the positive electrode material layer, and a mass ratio of the first binder to the second binder is X, where 0.2≤X<1.

7. The secondary battery according to claim 6, wherein The multiple sub-regions are distributed in stripes, the depth of a single concave portion is h2, 0μm<h2≤1μm, the minimum spacing d2 between two adjacent concave portions is 0.5mm to 5mm, and the width L2 of a single concave portion is 50μm to 1000μm. 8 . The secondary battery according to claim 7 , which satisfies: 0 μm<h2<0.5 μm, L2<d2<5L2; or 0.5 μm≤h2≤1 μm, 3L2<d2≤10L2.

9. The secondary battery according to claim 6, wherein The multiple recesses are distributed in a point-like manner, and the shapes of the recesses include at least one of a circle, an ellipse, and a polygon. The depth of a single recess is h2, 0μm<h2≤1μm, the diameter D2 of the maximum circumscribed circle of the outer contour of a single recess is 0.2mm to 4mm, and the minimum spacing d2 between two adjacent recesses is 0.5mm to 5mm.

10. The secondary battery according to any one of claims 1 to 9, wherein The positive electrode current collector is aluminum foil, with a thickness of 8 μm ≤ H1 ≤ 20 μm.

11. The secondary battery according to any one of claims 1 to 9, wherein The first binder and the second binder each independently include at least one of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polymethyl methacrylate.

12. The secondary battery according to any one of claims 1 to 9, wherein A ceramic coating is provided on the surface of the separator facing the positive electrode plate. 13 . An electronic device comprising the secondary battery according to claim 1 .

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