Cylindrical secondary battery and electronic device

Abstract

A cylindrical secondary battery and an electronic device. The secondary battery comprises an electrode assembly (001); the electrode assembly (001) comprises a positive electrode sheet (10), a negative electrode sheet (20), and a separator (30); the negative electrode sheet (20) comprises a negative electrode current collector (21), a first negative electrode material layer (22), and a second negative electrode material layer (23); the positive electrode sheet (10) comprises a positive electrode current collector (11) and a first positive electrode material layer (12); the coating weight of the first negative electrode material layer (22) is CW1 mg / mm2, the coating weight of the second negative electrode material layer (23) is CW2 mg / mm2, and the ratio of CW1 to CW2 is A; in the winding direction of the electrode assembly, the length of the first positive electrode material layer (12) of the first turn in the electrode assembly (001) is L1 mm, the length of the first negative electrode material layer (22) of the first turn in the electrode assembly (001) is L2 mm, the ratio of L1 to L2 is B, 1.05≤B≤1.09 and 0.918≤A / B≤1.048. By means of the arrangement, the present application is conducive to improving the cycle performance of the cylindrical secondary battery.

Description

Cylindrical secondary battery and electronic device TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to a cylindrical secondary battery and an electronic device. BACKGROUND

[0002] Cylindrical secondary batteries, such as cylindrical lithium ion batteries, are applied in various high-rate discharge systems (such as a discharge rate greater than 3C), have characteristics such as high specific energy, high operating voltage, low self-discharge rate, small volume, light weight, and are widely used in the consumer electronics field.

[0003] Currently, the design of high-power cylindrical lithium ion batteries usually adopts a full-tab design, that is, the positive and negative tabs are extended from opposite directions, and a full-tab flattening or rubbing technology is used for preparation. For cylindrical lithium ion batteries, the prior art uses a groove provided on the pole piece to improve the wetting effect of the pole piece, thereby improving the dynamic performance of the lithium ion battery.

[0004] SUMMARY

[0005] The purpose of the present application is to provide a cylindrical secondary battery and an electronic device to reduce the risk of lithium precipitation due to insufficient CB value of the negative pole piece inner ring in the later stage of cycling, improve the wettability of the electrolyte to the negative pole piece, and improve the cycling performance of the secondary battery while taking into account low production cost and mechanical safety performance.

[0006] It should be noted that the present application uses lithium ion batteries as an example of cylindrical secondary batteries to explain the present application, but the cylindrical secondary batteries of the present 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 cylindrical secondary battery, which comprises an electrode assembly, the electrode assembly comprising a positive pole piece, a negative pole piece and a separator, the negative pole piece comprising a negative current collector, a first negative material layer and a second negative material layer, the first negative material layer being arranged on the surface of the negative current collector away from the winding center of the electrode assembly, and the second negative material layer being arranged on the surface of the negative current collector facing the winding center of the electrode assembly. The positive pole piece comprises a positive current collector and a first positive material layer, and the first positive material layer is arranged on the surface of the positive current collector facing the winding center of the electrode assembly; the coating weight of the first negative material layer is CW1 mg / mm 2 , and the coating weight of the second negative material layer is CW2 mg / mm 2, the ratio of CW1 to CW2 is A; in the winding direction of the electrode assembly, the length of the first positive electrode material layer in the first circle of the electrode assembly is L1 mm, the length of the first negative electrode material layer in the first circle of the electrode assembly is L2 mm, the ratio of L1 to L2 is B, 1.05≤B≤1.09, and 0.918≤A / B≤1.048. By adjusting the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, the spacing between the layers in the inner circle of the electrode assembly is moderate, the ion transport distance during the cycle process is moderate, and the electrolyte is beneficial to the infiltration of the inner circle of the electrode assembly; the two negative electrode material layers with different coating weights are matched, the active material is supplemented in the negative electrode plate coating, which can effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle, and therefore the cylindrical secondary battery has good cycle performance.

[0008] In one or more embodiments, 1.01≤A≤1.07; and / or, 0.93≤A / B≤1.02. By adjusting the value of A and / or A / B within the above range, the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle is reduced, thereby improving the cycle performance of the secondary battery.

[0009] In one or more embodiments, 4≤CW2≤15, optionally 7≤CW2≤10; and / or, 7≤L2≤16, optionally 8≤L2≤12. By adjusting the value of CW2 and / or the value of L2 within the above range, the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle is reduced, while taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0010] In one or more embodiments, the curvature of the first circle of the second negative electrode material layer in the electrode assembly is R, and 0.4≤R≤0.9. By adjusting the value of R within the above range, while taking into account high energy density and good processing performance, the secondary battery has good cycle performance.

[0011] In one or more embodiments, the thickness of the separator is 9μm to 15μm. By adjusting the thickness of the separator within the above range, the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle is reduced, while taking into account the energy density, the secondary battery has good cycle performance.

[0012] In one or more embodiments, the thickness of the first circle of the negative electrode plate in the electrode assembly is 50μm to 200μm. By adjusting the thickness of the first circle of the negative electrode plate in the electrode assembly within the above range, the secondary battery has good cycle performance, and at the same time, the secondary battery has high energy density.

[0013] In one or more embodiments, the coating weight of the first positive electrode material layer is CW3 mg / mm2 19≤ CW3≤ 21. By regulating the value of CW3 within the above range, the secondary battery has good cycle performance, and at the same time, the secondary battery has high energy density.

[0014] In one or more embodiments, the first negative electrode material layer and / or the second negative electrode material layer are each provided with a plurality of first stripes, the plurality of first stripes extending along the width direction of the unfolded negative electrode tab and being arranged at intervals along the length direction of the unfolded negative electrode tab. Through the above arrangement, the wettability of the electrolyte to the negative electrode tab is improved, thereby further improving the cycle performance of the secondary battery.

[0015] In one or more embodiments, along the thickness direction of the negative electrode tab, the average depth of the plurality of first stripes is H μm, 5≤ H≤ 25. By regulating the value of H within the above range, the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle is reduced, while the mechanical safety performance and energy density of the secondary battery are taken into account, thereby further improving the cycle performance of the secondary battery.

[0016] In one or more embodiments, along the width direction of the unfolded negative electrode tab, the ratio of the length of a single first stripe to the width of the first negative electrode material layer or the ratio of the length of a single first stripe to the width of the second negative electrode material layer is P, 0.2≤ P≤ 1. By regulating the value of P within the above range, the circulation of the electrolyte on the negative electrode material layer is facilitated, thereby further improving the cycle performance of the cylindrical secondary battery.

[0017] In one or more embodiments, along the length direction of the unfolded negative electrode tab, the width of a single first stripe is W μm, 10≤ W≤ 400. By regulating the value of W within the above range, the cycle performance of the cylindrical secondary battery is further improved while the energy density is taken into account.

[0018] In one or more embodiments, along the length direction of the unfolded negative electrode tab, the spacing between two adjacent first stripes is A mm, 0.5≤ A≤ 10. By regulating the value of A within the above range, the wettability of the electrolyte to the negative electrode tab is improved, thereby further improving the cycle performance of the cylindrical secondary battery while reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle.

[0019] In one or more embodiments, along the width direction of the unfolded negative electrode tab, the negative electrode current collector includes a hollow foil area connected to the first negative electrode material layer, and the hollow foil area is provided with a plurality of second stripes, the plurality of second stripes extending along the width direction of the unfolded negative electrode tab and being arranged at intervals along the length direction of the unfolded negative electrode tab. By providing the second stripes on the hollow foil area, more electrolyte flow channels are provided for the hollow foil area, especially the flattened part in the hollow foil area, thereby further improving the cycle performance of the cylindrical secondary battery.

[0020] In one or more embodiments, the ratio of the length of the single second stripe to the width of the empty-foil region is P' along the width direction of the negative electrode tab after being developed, and 0.1≤P'≤0.7. By regulating the value of P' within the above range, the cycle performance of the cylindrical secondary battery is further improved while the mechanical safety performance is taken into account.

[0021] In one or more embodiments, the thickness of the negative current collector is T0μm, the average depth of the plurality of second stripes is T1μm, 0.1≤T1 / T0≤0.8, and 3≤T0≤20. By regulating the value of T1 / T0 and T0 within the above range, the cycle performance of the cylindrical secondary battery is further improved while the mechanical safety performance is taken into account.

[0022] A second aspect of the present application provides an electronic device comprising the cylindrical secondary battery in any of the foregoing embodiments. The cylindrical secondary battery of the present application has good cycle performance, and therefore, the electronic device of the present application has a longer service life.

[0023] Advantages of the embodiments of the present application:

[0024] The embodiments of the present application regulate the ratio of the length of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, so that the spacing between the layers in the inner circle of the electrode assembly is moderate, the transmission distance of ions during the cycle process is moderate, and the impregnation of the electrolyte to the inner circle of the electrode assembly is facilitated. In combination with the ratio of the coating weights of the two negative electrode material layers, the active material is supplemented in the negative electrode tab coating, which can effectively reduce the risk of lithium precipitation caused by the lack of CB in the inner circle of the electrode assembly in the later cycle period, and therefore, the cylindrical secondary battery has good cycle performance. In addition, the coating weights of different negative electrode material layers are different, which can reduce the actual production cost while having high energy density, and also takes into account the processing performance. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings, which are included to provide a further understanding of the present application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and serve to explain the present application. Features of the present application, both as to organization and method of operation, together with an understanding of the same, can be best understood by reference to the following detailed description, taken in connection with the accompanying drawings in which:

[0026] FIG. 1 is a schematic view of a wound structure formed by an electrode assembly in an embodiment of the present application;

[0027] FIG. 2 is a partial front view of a negative electrode tab after the electrode assembly is opened in another embodiment of the present application;

[0028] FIG. 3 is a cross-sectional view of the negative electrode tab in FIG. 2 along the P-P direction;

[0029] FIG. 4 is a cross-sectional view of the negative electrode tab in FIG. 2 along the Q-Q direction;

[0030] FIG. 5 is a computed tomography (CT) image of an electrode assembly in another embodiment of the present application.

[0031] Reference signs: electrode assembly 001; positive electrode tab 10; positive electrode current collector 11; first positive electrode material layer 12; second positive electrode material layer 13; negative electrode tab 20; negative electrode current collector 21; first negative electrode material layer 22; second negative electrode material layer 23; separator 30; first stripe 221; empty foil area 210; second stripe 211. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the present application clearer, further detailed descriptions will be made to the present application with reference to the drawings and examples. Obviously, the described examples are only some of the examples of the present application, but not all the examples. All other examples obtained by those skilled in the art based on the present application shall fall within the scope of protection of the present application.

[0033] It should be noted that in the specific embodiments of the present application, the present application is explained by taking lithium-ion batteries as examples of the cylindrical secondary batteries, but the cylindrical secondary batteries of the present application are not limited to lithium-ion batteries.

[0034] For high-power cylindrical secondary batteries, generally full-tab cylindrical lithium-ion batteries, the electrolyte impregnation effect is poor on the surface of the negative electrode tab away from the winding center of the electrode assembly, especially at the position close to the winding center of the electrode assembly, and the negative electrode tab is poorly impregnated; further, the curvature of the cylindrical electrode assembly gradually decreases with the increase of the winding number, and further, for the same electrode assembly, the length of the surface of the negative electrode tab away from the winding center of the electrode assembly is slightly smaller than that of the surface of the positive electrode tab opposite to it and facing the winding center of the electrode assembly, resulting in insufficient CB in the inner circle of the electrode assembly in the later cycle, and lithium precipitation occurs, and based on the above reasons, the cycle performance of the lithium-ion battery is reduced. In the prior art, grooving, improving the performance of electrolyte, and the like are used to improve the above problems, but the improvement effect of a single technical element on the secondary battery is limited, and due to the loss of active material in the tab, the secondary battery cannot balance high energy density and good cycle performance. Therefore, the present application provides a cylindrical secondary battery, which effectively reduces the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle, and the secondary battery has good cycle performance.

[0035] The specific technical solutions are as follows:

[0036] A cylindrical secondary battery according to a first aspect of the present application includes an electrode assembly including a positive electrode sheet, a negative electrode sheet, and a separator, the negative electrode sheet including a negative electrode current collector, a first negative electrode material layer, and a second negative electrode material layer, the first negative electrode material layer being disposed on a surface of the negative electrode current collector facing away from a winding center of the electrode assembly, the second negative electrode material layer being disposed on a surface of the negative electrode current collector facing toward the winding center of the electrode assembly. The positive electrode sheet includes a positive electrode current collector and a first positive electrode material layer, the first positive electrode material layer being disposed on a surface of the positive electrode current collector facing toward the winding center of the electrode assembly; a coating weight of the first negative electrode material layer is CW1 mg / mm 2 , a coating weight of the second negative electrode material layer is CW2 mg / mm 2 , a ratio of CW1 to CW2 is A; in a winding direction of the electrode assembly, a length of the first positive electrode material layer in a first turn of the electrode assembly is L1 mm, a length of the first negative electrode material layer in the first turn of the electrode assembly is L2 mm, a ratio of L1 to L2 is B, 1.05 ≤ B ≤ 1.09, for example, the value of B can be 1.05, 1.052, 1.055, 1.058, 1.06, 1.062, 1.065, 1.068, 1.07, 1.072, 1.075, 1.078, 1.08, 1.082, 1.085, 1.088, 1.09, or a range between any two of them; 0.918 ≤ A / B ≤ 1.048, optionally, 0.93 ≤ A / B ≤ 1.02, for example, the value of A / B can be 0.918, 0.920, 0.930, 0.940, 0.950, 0.960, 0.970, 0.980, 0.990, 1, 1.001, 1.005, 1.01, 1.02, 1.03, 1.04, 1.042, 1.045, 1.048, or a range between any two of them.

[0037] In the present application, the length direction of the electrode assembly in the unfolded state is defined as the X direction, the width direction of the electrode assembly in the unfolded state is defined as the Y direction, and the thickness direction of the electrode assembly in the unfolded state is defined as the Z direction. It can be understood that the length direction, the width direction, and the thickness direction of the negative electrode sheet, the positive electrode sheet, and the separator in the unfolded state are the same as those of the electrode assembly, and the winding direction of the electrode assembly is the W direction. As shown in FIG. 1, the electrode assembly 001 includes a positive electrode sheet 10, a negative electrode sheet 20, and a separator 30. The negative electrode sheet 20 includes a negative electrode current collector 21, a first negative electrode material layer 22, and a second negative electrode material layer 23. The first negative electrode material layer 22 is disposed on a surface of the negative electrode current collector 21 facing away from a winding center of the electrode assembly 001, and the second negative electrode material layer 23 is disposed on a surface of the negative electrode current collector 21 facing toward the winding center of the electrode assembly 001. The positive electrode sheet 10 includes a positive electrode current collector 11 and a first positive electrode material layer 12, and the first positive electrode material layer 12 is disposed on a surface of the positive electrode current collector 11 facing toward the winding center of the electrode assembly 001.

[0038] When the value of B is too small, i.e. less than the lower limit value of the present application, the layers in the inner circle of the electrode assembly are too close to each other, which is not conducive to the infiltration of the electrolyte into the electrode sheet, affecting the cycle performance of the secondary battery. When the value of B is too large, i.e. greater than the upper limit value of the present application, the gap between the first negative electrode material layer and the first positive electrode material layer in the first circle of the electrode assembly is too large, the ion transport path is increased during the cycle process, the dynamic performance of the secondary battery is poor, and the increased gap reduces the length ratio of the positive electrode (the surface facing the winding center of the electrode assembly) / negative electrode (the surface away from the winding center of the electrode assembly) in the same circle, which makes it easy to occur lithium precipitation during the cycle process. For actual production, the practical value is low. When the value of A / B is too small, i.e. less than the lower limit value of the present application, the amount of active material supplement in the negative electrode sheet coating is insufficient, which cannot effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the negative electrode sheet in the later cycle. When the value of A / B is too large, i.e. greater than the upper limit value of the present application, the amount of active material supplement in the negative electrode sheet coating is too much, and the excess active material increases the cost in the actual production process, and causes the loss of energy density and the decrease of the dynamic performance of the secondary battery. The present application adjusts the length ratio of the first positive electrode material layer and the first negative electrode material layer in the first circle of the electrode assembly, so that the distance between the layers in the inner circle of the electrode assembly is moderate, the ion transport distance in the cycle process is moderate, and the electrolyte is infiltrated into the inner circle of the electrode assembly. The different coating weights of the two negative electrode material layers supplement the active material in the negative electrode sheet coating, which can effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle, so that the cylindrical secondary battery has good cycle performance. In addition, the different coating weights of the different negative electrode material layers can reduce the actual production cost while maintaining high energy density, and also consider the processing performance.

[0039] In the present application, the value of CB refers to the ratio between the capacity per unit area of the negative electrode sheet and the capacity per unit area of the positive electrode sheet under the same conditions, such as at an ambient temperature of 25°C and a 0.1C discharge rate. CB = (gram capacity of negative electrode active material x mass of negative electrode active material per unit area of negative electrode sheet) / (gram capacity of positive electrode active material x mass of positive electrode active material per unit area of positive electrode sheet). The above unit area refers to 1 mm 2It can be understood that the starting point of the first electrode assembly is the starting end of the electrode assembly, and the starting point of the first circle of the electrode assembly is the position where the region provided with the first positive electrode material layer on the positive electrode current collector, the separator, and the region provided with the first negative electrode material layer on the negative electrode current collector begin to overlap in the thickness direction of the electrode assembly. The first circle of the electrode assembly refers to that the electrode assembly is wound from the starting end of the electrode assembly (i.e., the starting point) to the other end surface of the electrode assembly (i.e., the ending point) along the winding direction of the electrode assembly, and the starting point and the ending point are flush in the thickness direction of the electrode assembly. Then, the length of the first positive electrode material layer in the first circle of the electrode assembly refers to the length of the first positive electrode material layer in the electrode assembly from the starting point of the electrode assembly to the ending point of the electrode assembly along the winding direction of the electrode assembly; the length of the first negative electrode material layer in the first circle of the electrode assembly refers to the length of the first negative electrode material layer in the electrode assembly from the starting point of the electrode assembly to the ending point of the electrode assembly along the winding direction of the electrode assembly. In addition, it should be noted that the "surface" in the present application can be the entire region of the positive electrode current collector and / or the negative electrode current collector surface, or can be part of the region of the positive electrode current collector and / or the negative electrode current collector surface, and the present application does not have a specific limitation as long as the purpose of the present application can be achieved.

[0040] In one or more embodiments, 1.01≤A≤1.07, for example, the value of A can be 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, or a range composed of any two of the above values. By adjusting the value of A within the above range, it is beneficial to supplement appropriate active materials in the negative electrode sheet coating, so that the secondary battery has high energy density, and it is beneficial to reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, thereby improving the cycle performance of the secondary battery. In addition, by adjusting the value of A within the above range, it is beneficial to control the production cost in actual operation.

[0041] In one or more embodiments, 4≤CW2≤15, optionally, 7≤CW2≤10, for example, the value of CW2 can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or a range composed of any two of the above values. By adjusting the value of CW2 within the above range, the content of the active material of the second negative electrode material layer in the negative electrode sheet is moderate, which is beneficial to actual production, reduces the risk of local lithium precipitation and short circuit of the electrode sheet in the charging and discharging process due to metal exposure or scratches on the surface of the negative electrode sheet in the processing process, while taking into account the energy density of the secondary battery, and reducing the risk of the decline of the cycle performance of the secondary battery due to the intensification of the side reaction between the active material and the electrolyte caused by excessive active material, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, thereby improving the cycle performance of the secondary battery.

[0042] In one or more embodiments, 7≤L2≤16, optionally 8≤L2≤12, for example, the value of L2 can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or a range consisting of any two of them. By regulating the value of L2 in the above range, the length of the first negative pole piece of the electrode assembly is moderate, effectively reducing the risk of fracture of the first negative pole piece due to excessive curvature during charging and discharging, at the same time, the size of the center hole of the winding center of the electrode assembly is moderate, the secondary battery has a higher energy density, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle, while taking into account the high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0043] In one or more embodiments, 4≤CW2≤15, optionally 7≤CW2≤10, for example, the value of CW2 can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or a range consisting of any two of them, and 7≤L2≤16, optionally 8≤L2≤12, for example, the value of L2 can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or a range consisting of any two of them. By regulating the value of CW2 and the value of L2 in the above range, the content of active material in the second negative material layer of the negative pole piece and the length of the first negative pole piece of the electrode assembly are moderate, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle, while taking into account the high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0044] In one or more embodiments, 3.8556≤CW1≤17.1348, for example, the value of CW2 can be 3.8556, 3.9, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 17.1, 17.1348 or a range consisting of any two of them. By regulating the value of CW1 in the above range, the content of active material in the first negative material layer of the negative pole piece is moderate, which is beneficial to actual production, reduces the risk of local lithium precipitation short circuit of the pole piece during charging and discharging due to metal exposure or scratches on the surface of the negative pole piece in the processing process, at the same time, the energy density of the secondary battery is also taken into account, and the risk of cycle performance decline of the secondary battery due to the intensification of the side reaction between the active material and the electrolyte caused by excessive active material is also reduced, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle, thereby improving the cycle performance of the secondary battery.

[0045] In one or more embodiments, 7.35≤L1≤17.44, for example, the value of L1 can be 7.35, 7.4, 7.5, 7.8, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 17.2, 17.44, or a range formed by any two of the above values. By regulating the value of L1 within the above range, the length of the first positive electrode sheet of the electrode assembly is moderate, effectively reducing the risk of fracture caused by excessive curvature of the first positive electrode sheet during charging and discharging. At the same time, the size of the central hole of the winding center of the electrode assembly is moderate, and the secondary battery has a high energy density, which is beneficial to reduce the risk of lithium precipitation caused by insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle. While taking into account the high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0046] In one or more embodiments, the curvature of the first circle of the second negative electrode material layer in the electrode assembly is R, and 0.4≤R≤0.9, for example, the value of R can be 0.4, 0.42, 0.45, 0.48, 0.5, 0.52, 0.55, 0.58, 0.6, 0.62, 0.65, 0.68, 0.7, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, or a range formed by any two of the above values. By regulating the value of R within the above range, it is beneficial to regulate the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, thereby reducing the risk of lithium precipitation caused by insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle. While taking into account the actual production difficulty, at the same time, the size of the central hole of the winding center of the electrode assembly is moderate, and the secondary battery has a high energy density. Therefore, while taking into account the high energy density and good processing performance, the secondary battery has good cycle performance.

[0047] In one or more embodiments, the thickness of the separator is 9 μm to 15 μm, for example, the thickness of the separator can be 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or a range formed by any two of the above values. By regulating the thickness of the separator within the above range, it is beneficial to moderate the distance between layers in each circle of the electrode assembly, the transmission distance of ions in the cycle process is moderate, and it is beneficial to regulate the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, thereby reducing the risk of lithium precipitation caused by insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle. While taking into account the energy density, the secondary battery has good cycle performance. The regulation method of the thickness of the separator in the present application is not particularly limited as long as the purpose of the present application can be achieved, for example, a separator with different thickness can be selected, and the thickness of the separator can be determined by combining the test method of "L1, L2, thickness of the separator, and thickness of the negative electrode sheet" in the present application, and then the separator with the required thickness is selected.

[0048] In one or more embodiments, the thickness of the first negative electrode tab in the first circle of the electrode assembly is 50 μm to 200 μm, for example, the thickness of the first negative electrode tab in the first circle of the electrode assembly can be 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, 125 μm, 130 μm, 135 μm, 140 μm, 145 μm, 150 μm, 155 μm, 160 μm, 165 μm, 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm, 200 μm, or a range defined by any two of the above values. By adjusting the thickness of the first negative electrode tab in the first circle of the electrode assembly within the above range, the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly is adjusted, thereby reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly at the later stage of the cycle, and thus the secondary battery has good cycle performance, and at the same time, the secondary battery has a high energy density.

[0049] In the present application, it can be understood that when there is no second negative electrode material layer on the negative electrode tab in the first circle of the electrode assembly, the thickness of the first negative electrode tab in the first circle of the electrode assembly refers to the sum of the thickness of the first negative electrode material layer and the thickness of the negative electrode current collector; when there is a second negative electrode material layer on the negative electrode tab in the first circle of the electrode assembly, the actual starting end of the second negative electrode material layer in the first circle of the electrode assembly can be the same as or different from the actual starting end of the first negative electrode material layer, and the thickness of the first negative electrode tab in the first circle of the electrode assembly refers to the sum of the thickness of the first negative electrode material layer, the thickness of the second negative electrode material layer, and the thickness of the negative electrode current collector. The thickness of the first negative electrode material layer and / or the second negative electrode material layer is adjusted by means known to those skilled in the art, for example, when the negative electrode slurry is coated on the surface of the negative electrode current collector, under the condition that the solid content of the negative electrode slurry is constant, the thickness of the first negative electrode material layer and / or the second negative electrode material layer can be increased by increasing the coating weight of the first negative electrode material layer and / or the second negative electrode material layer, or the thickness of the first negative electrode material layer and / or the second negative electrode material layer can be reduced by reducing the coating weight of the first negative electrode material layer and / or the second negative electrode material layer; under other conditions, the thickness of the first negative electrode material layer and / or the second negative electrode material layer can also be reduced by increasing the cold pressing pressure when the negative electrode tab is cold pressed, or the thickness of the first negative electrode material layer and / or the second negative electrode material layer can be increased by reducing the cold pressing pressure.

[0050] In one or more embodiments, the coating weight of the first positive electrode material layer is CW3 mg / mm 2, 19≤CW3≤21, for example, the value of CW3 can be 19, 19.1, 19.2, 19.3, 19.4, 19.5, 19.6, 19.7, 19.8, 19.9, 20, 20.1, 20.2, 20.3, 20.4, 20.5, 20.6, 20.7, 20.8, 20.9, 21, or a range formed by any two of them. By regulating the value of CW3 in the above range, the CB value of the secondary battery during the cycle is moderate, which is conducive to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle, and thus the secondary battery has good cycle performance, and at the same time, the secondary battery has high energy density.

[0051] In one or more embodiments, the positive electrode tab further includes a second positive electrode material layer, the second positive electrode material layer is arranged on the surface of the positive electrode current collector away from the winding center of the electrode assembly. For example, as shown in FIG. 1, the positive electrode tab 10 can further include a second positive electrode material layer 13 arranged on the surface of the positive electrode current collector 11 away from the winding center of the electrode assembly 001. The coating weight of the second positive electrode material layer is CW4 mg / mm 2 , 19≤CW4≤21, for example, the value of CW4 can be 19, 19.1, 19.2, 19.3, 19.4, 19.5, 19.6, 19.7, 19.8, 19.9, 20, 20.1, 20.2, 20.3, 20.4, 20.5, 20.6, 20.7, 20.8, 20.9, 21, or a range formed by any two of them. By regulating the value of CW4 in the above range, the CB value of the secondary battery during the cycle is moderate, which is conducive to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle, and thus the secondary battery has good cycle performance, and at the same time, the secondary battery has high energy density.

[0052] In one or more embodiments, the first negative electrode material layer is provided with a plurality of first stripes, the plurality of first stripes extend along the width direction of the expanded negative electrode tab and are arranged at intervals along the length direction of the expanded negative electrode tab. As shown in FIG. 2, the first negative electrode material layer 22 is provided with a plurality of first stripes 221, the plurality of first stripes 221 extend along the width direction (Y direction) of the expanded negative electrode tab 20 and are arranged at intervals along the length direction (X direction) of the expanded negative electrode tab 20. Through the above arrangement, it is conducive to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle, and at the same time, improving the wetting effect of the electrolyte on the negative electrode tab, thereby further improving the cycle performance of the secondary battery.

[0053] In one or more embodiments, the second negative material layer is provided with a plurality of first stripes, and the plurality of first stripes extend along the width direction of the unfolded negative electrode tab and are arranged at intervals along the length direction of the unfolded negative electrode tab. Through the above arrangement, it is beneficial to reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, while improving the wetting effect of the electrolyte on the negative electrode tab, thereby further improving the cycle performance of the secondary battery.

[0054] In one or more embodiments, the first negative material layer and the second negative material layer are each provided with a plurality of first stripes, and the plurality of first stripes extend along the width direction of the unfolded negative electrode tab and are arranged at intervals along the length direction of the unfolded negative electrode tab. Through the above arrangement, it is beneficial to reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, while improving the wetting effect of the electrolyte on the negative electrode tab, thereby further improving the cycle performance of the secondary battery.

[0055] In one or more embodiments, along the thickness direction of the negative electrode tab, the average depth of the plurality of first stripes is H μm. As shown in FIG. 3, along the thickness direction (Z direction) of the negative electrode tab 20, the average depth of the plurality of first stripes 221 is H μm. 5≤H≤25, for example, the value of H can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or a range composed of any two of them. By adjusting the value of H within the above range, it is beneficial to improve the wetting effect of the electrolyte on the negative electrode tab, while reducing the risk of the negative material layer being punched by the first stripes when the first stripes are arranged on the negative material layer. In addition, the loss of negative active material is less, the energy density of the secondary battery is higher, and the mechanical safety performance and energy density of the secondary battery are taken into account while reducing the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, thereby further improving the cycle performance of the secondary battery.

[0056] In one or more embodiments, along the width direction of the unfolded negative electrode tab, the ratio of the length of a single first stripe to the width of the first negative material layer or the ratio of the length of a single first stripe to the width of the second negative material layer is P, and 0.2≤P≤1. For example, the value of P can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range composed of any two of them. By adjusting the value of P within the above range, it is beneficial to the flow of electrolyte on the negative material layer, improving the wetting effect of the electrolyte on the negative electrode tab, while reducing the processing difficulty in the actual production process, reducing the risk of excessive loss of the content of negative active material in the negative material layer leading to lithium precipitation at the negative interface, and further improving the cycle performance of the cylindrical secondary battery while reducing the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle.

[0057] In one or more embodiments, the width of a single first stripe is W pm along the length direction of the negative electrode tab after being unfolded, as shown in FIG. 2, the width of a single first stripe 221 is W pm along the length direction (X direction) of the negative electrode tab 20 after being unfolded. 10≤W≤400. For example, the value of W can be 10, 30, 50, 80, 100, 120, 150, 180, 200, 220, 250, 280, 300, 320, 350, 380, 400 or a range composed of any two of them. By adjusting the value of W within the above range, the uniform distribution of the first stripe on the surface of the negative electrode tab is facilitated, the infiltration effect of the electrolyte on the negative electrode tab is improved, the infiltration performance of the electrolyte on the negative electrode tab is effectively improved, the processing difficulty of the actual production process is reduced, the risk of lithium precipitation of the secondary battery caused by excessive loss of the content of the negative active material in the negative material layer is reduced, the risk of lithium precipitation caused by insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle is reduced, the cycle performance of the cylindrical secondary battery is further improved while the energy density is taken into account.

[0058] In one or more embodiments, the distance between two adjacent first stripes is A mm along the length direction of the negative electrode tab after being unfolded, as shown in FIG. 2, the distance between two adjacent first stripes 221 is A mm along the length direction (X direction) of the negative electrode tab 20 after being unfolded. 0.5≤A≤10, for example, the value of A can be 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range composed of any two of them. By adjusting the value of A within the above range, the distance between two adjacent first stripes is moderate, which is beneficial to reduce the risk of insufficient infiltration of the electrolyte on the negative electrode tab, while facilitating the reduction of the processing difficulty of the grooving process and the risk of local collapse of the material layer, improving the infiltration effect of the electrolyte on the negative electrode tab, and further improving the cycle performance of the cylindrical secondary battery while reducing the risk of lithium precipitation caused by insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle. In this application, the distance between two adjacent first stripes refers to the distance between the width centers of two adjacent first stripes along the length direction of the negative electrode tab after being unfolded.

[0059] In one or more embodiments, along the width direction of the negative electrode tab after being unfolded, the negative current collector comprises an empty foil area connected with the first negative material layer, and a plurality of second stripes are arranged on the empty foil area, the plurality of second stripes extend along the width direction of the negative electrode tab after being unfolded and are arranged at intervals along the length direction of the negative electrode tab after being unfolded. As shown in FIG. 2, along the width direction (Y direction) of the negative electrode tab 20 after being unfolded, the negative current collector 21 comprises an empty foil area 210 connected with the first negative material layer 22, and a plurality of second stripes 211 are arranged on the empty foil area 210, the plurality of second stripes 211 extend along the width direction (Y direction) of the negative electrode tab 20 after being unfolded and are arranged at intervals along the length direction (X direction) of the negative electrode tab 20 after being unfolded. By arranging the second stripes on the empty foil area, more electrolyte flow channels are provided for the empty foil area, especially for the flattened part in the empty foil area, and the diffusion efficiency of the electrolyte in the empty foil area is improved. In addition, arranging the second stripes on the empty foil area has little effect on the capacity of the secondary battery and can effectively reduce the electrochemical impedance. The first stripes and the second stripes are arranged in combination, and the first stripes and the second stripes achieve synergistic effect, effectively improving the infiltration efficiency and performance of the electrolyte to the negative electrode tab level, and further improving the internal resistance consistency of the secondary battery level, thereby further improving the cycle performance of the cylindrical secondary battery.

[0060] In one or more embodiments, along the length direction of the negative electrode tab after being unfolded, the center line of the part of the second stripes coincides with the center line of the plurality of first stripes. As shown in FIG. 2, along the length direction (X direction) of the negative electrode tab 20 after being unfolded, the center line of the plurality of second stripes 211 arranged on the empty foil area 210 coincides with the center line of the plurality of first stripes 221. By the above arrangement, the flow of the electrolyte on the negative electrode tab is accelerated, thereby further improving the infiltration effect of the electrolyte on the negative electrode tab, so that the cylindrical secondary battery has better cycle performance and lithium precipitation performance.

[0061] In one or more embodiments, along the length direction of the negative electrode tab after being unfolded, the center line of the second stripes does not coincide with the center line of the first stripes.

[0062] In one or more embodiments, along the width direction of the negative electrode tab after being unfolded, the ratio of the length of a single second stripe to the width of the empty foil area is P', 0.1≤P'≤0.7, for example, the value of P' can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or a range composed of any two of them. By adjusting the value of P' within the above range, the flow of the electrolyte on the empty foil area is facilitated, the infiltration effect of the electrolyte on the negative electrode tab is improved, and the safety risk of reducing the mechanical performance of the secondary battery due to the reduction of the strength of the empty foil area in the preparation process of the secondary battery is reduced. While taking into account the mechanical safety performance, the cycle performance of the cylindrical secondary battery is further improved.

[0063] In one or more embodiments, the thickness of the negative current collector is T0 pm, the average depth of the plurality of second stripes is T1 pm, as shown in FIG. 4, the thickness of the negative current collector 21 is T0 pm, and the average depth of the plurality of second stripes 211 is T1 pm. 0.1≤T1 / T0≤0.8, for example, the value of T1 / T0 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or a range formed by any two of them; 3≤T0≤20, for example, the value of T0 can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range formed by any two of them. By adjusting the values of T1 / T0 and T0 within the above range, the diffusion efficiency of the electrolyte to the empty foil area of the negative current collector is improved, the wetting effect of the electrolyte on the negative electrode sheet is improved, and at the same time, the risk of the empty foil area being punched by the second stripe when the second stripe is arranged in the empty foil area is reduced, the mechanical safety performance is taken into account, and the cycle performance of the cylindrical secondary battery is further improved. The regulation method of the negative current collector thickness T0 in the present application is not particularly limited as long as the purpose of the present application can be achieved, for example, commercially available current collectors with different thicknesses can be selected, and the thickness of the negative current collector can be determined by combining the test method of “test of P, P’, T0, T, H, W, W’, A, A’” in the present application, and then the negative current collector with the required thickness is selected.

[0064] In one or more embodiments, 0.3≤T1≤16. For example, the value of T1 can be 0.3, 0.5, 0.8, 1, 3, 5, 8, 10, 12, 15, 16, or a range formed by any two of them. By adjusting the value of T1 within the above range, the diffusion efficiency of the electrolyte to the empty foil area of the negative current collector is improved, the wetting effect of the electrolyte on the negative electrode sheet is improved, and at the same time, the risk of the empty foil area being punched by the second stripe when the second stripe is arranged in the empty foil area is reduced, the mechanical safety performance is taken into account, and the cycle performance of the cylindrical secondary battery is further improved.

[0065] In one or more embodiments, the width of a single second stripe is W' pm, 10≤W'≤400, along the length direction of the negative electrode tab after being unfolded. For example, the value of W' can be 10, 30, 50, 80, 100, 120, 150, 180, 200, 220, 250, 280, 300, 320, 350, 380, 400, or a range between any two of the values. By adjusting the value of W' within the above range, the uniform distribution of the stripes on the surface of the negative electrode tab is facilitated, the infiltration effect of the electrolyte on the negative electrode tab is improved, the infiltration performance of the electrolyte on the negative electrode tab is effectively improved, the processing difficulty in the actual production process is reduced, and the mechanical safety performance of the secondary battery caused by the reduction of the strength of the hollow foil area is reduced, while the mechanical safety performance is taken into account, the lithium precipitation performance and the cycle performance of the cylindrical secondary battery are further improved.

[0066] In one or more embodiments, the distance between two adjacent second stripes is A' mm, 0.5≤A'≤10, along the length direction of the negative electrode tab after being unfolded. For example, the value of A' can be 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or a range between any two of the values. By adjusting the value of A' within the above range, the distance between two adjacent second stripes is moderate, which is conducive to reducing the risk of insufficient infiltration of the electrolyte on the negative electrode tab, while reducing the processing difficulty of the grooving process and the risk of local collapse of the hollow foil area, improving the infiltration effect of the electrolyte on the negative electrode tab, and further improving the cycle performance of the cylindrical secondary battery. In this application, the distance between two adjacent second stripes refers to the distance between the width centers of two adjacent second stripes along the length direction of the negative electrode tab after being unfolded.

[0067] In this application, the cross section of a single first stripe and the cross section of a single second stripe refer to the plane formed by the length direction and the thickness direction of the first stripe and the second stripe after being unfolded, or the cross section obtained by cutting the first stripe and the second stripe along the length direction and the thickness direction after being unfolded. The cross section shape of a single first stripe and a single second stripe is not particularly limited in this application, as long as the purpose of this application can be achieved. For example, the cross section of a single first stripe and a single second stripe can each independently be selected from at least one of a triangle, an arc (with an area smaller than a semicircle with the same radius), a semicircle, a rectangle, a trapezoid, or a square.

[0068] The application does not have a particular restriction on the cross-sectional shape obtained by sectioning the first stripe and the second stripe in the length direction and the width direction after being developed along the negative electrode tab, as long as the purpose of the application can be achieved. For example, the cross-sectional shape obtained by sectioning the first stripe and the second stripe in the length direction and the width direction after being developed along the negative electrode tab can be linear, curved, or other specific shapes, etc. As shown in FIG. 2, the cross-sectional shape obtained by sectioning the first stripe and the second stripe in the length direction and the width direction after being developed along the negative electrode tab is linear.

[0069] The application does not have a particular restriction on the positive current collector, as long as the purpose of the application can be achieved. For example, the positive current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. The first positive electrode material layer of the application includes a first positive electrode active material, and the second positive electrode material layer includes a second positive electrode active material. The application does not have a particular restriction on the types of the first positive electrode active material and the second positive electrode active material, as long as the purpose of the application can be achieved. For example, the first positive electrode active material and the second positive electrode active material can each independently be selected from lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05at least one of O2(NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate (LiCoO2), lithium manganate, lithium manganese iron phosphate, or lithium titanate, etc. In the present application, the first positive electrode active material, the second positive electrode active material can also include a non-metal element, for example, the non-metal element includes at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In the present application, the thickness of the positive electrode current collector is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm. In the present application, the first positive electrode material layer can also include a first positive electrode binder and a first positive electrode conductive agent, and the second positive electrode material layer can also include a second positive electrode binder and a second positive electrode conductive agent. The present application does not particularly limit the type of the first positive electrode binder in the first positive electrode material layer, the second positive electrode binder in the second positive electrode material layer, as long as the purpose of the present application can be achieved, for example, the first positive electrode binder, the second positive electrode binder can each include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The present application does not particularly limit the type of the first positive electrode conductive agent in the first positive electrode material layer, the second positive electrode conductive agent in the second positive electrode material layer, as long as the purpose of the present application can be achieved, for example, the first positive electrode conductive agent, the second positive electrode conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, ketjen black, graphene, metal materials, conductive polymers, or sodium carboxymethyl cellulose. The above-mentioned carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal materials can include, but are not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymers can include, but are not limited to, at least one of polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not particularly limit the mass ratio of the first positive electrode active material, the first positive electrode conductive agent, the first positive electrode binder in the first positive electrode material layer, the mass ratio of the second positive electrode active material, the second positive electrode conductive agent, the second positive electrode binder in the second positive electrode material layer, and a person skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0070] The negative current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative current collector can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (e.g., 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.), etc. The first negative material layer of the present application includes a first negative active material, and the second negative material layer includes a second negative active material. The kinds of the first negative active material and the second negative active material are not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the first negative active material and the second negative active material can each independently be selected from at least one of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), a Li-Sn alloy, a Li-Sn-O alloy, Sn, SnO, SnO2, a lithium titanate Li4Ti5O 12 with a spinel structure, a Li-Al alloy, or metallic lithium. Optionally, the first negative material layer can further include a first negative conductive agent and a first negative binder, and the second negative material layer can further include a second negative conductive agent and a second negative binder. The kinds of the first negative conductive agent in the first negative material layer and the second negative conductive agent in the second negative material layer are not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the first negative conductive agent and the second negative conductive agent can be the same as the kinds of the first positive conductive agent in the first positive material layer and the second positive conductive agent in the second positive material layer described above. The kinds of the first negative binder in the first negative material layer and the second negative binder in the second negative material layer are not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the first negative binder and the second negative binder can be the same as the kinds of the first positive binder in the first positive material layer and the second positive binder in the second positive material layer described above. The mass ratio of the first negative active material, the first negative conductive agent, and the first negative binder in the first negative material layer, and the mass ratio of the second negative active material, the second negative conductive agent, and the second negative binder in the second negative material layer are not particularly limited in the present application, and a person skilled in the art can select them according to actual needs, as long as the object of the present application can be achieved.

[0071] The preparation method of the negative electrode sheet is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the negative electrode sheet includes but is not limited to the following steps: (1) mixing the first negative electrode active material, the first negative electrode binder and the first negative electrode conductive agent, adding a solvent and stirring uniformly to prepare a first negative electrode material layer slurry, mixing the second negative electrode active material, the second negative electrode binder and the second negative electrode conductive agent, adding a solvent and stirring uniformly to prepare a second negative electrode material layer slurry; (2) determining the surface of the negative electrode current collector away from the winding center of the electrode assembly and the starting position of the negative electrode current collector for winding, coating the first negative electrode material layer slurry on the surface of the negative electrode current collector away from the winding center of the electrode assembly, and drying to obtain a negative electrode sheet coated with the first negative electrode material layer; (3) coating the second negative electrode material layer slurry on the surface of the negative electrode current collector facing the winding center of the electrode assembly, and drying to obtain a negative electrode sheet coated with the first negative electrode material layer and the second negative electrode material layer; (4) after cold pressing and slitting, the negative electrode sheet is obtained.

[0072] In one or more embodiments, a first stripe is provided on the first negative electrode material layer and / or the second negative electrode material layer after cold pressing and slitting in step (4). In one or more embodiments, while the first stripe is provided on the first negative electrode material layer and / or the second negative electrode material layer after cold pressing and slitting in step (4), the empty foil area of the negative electrode current collector connected to the first negative electrode material layer is determined along the width direction of the negative electrode sheet after unfolding, and a second stripe is provided on the empty foil area.

[0073] The present application does not have a particular limitation on the solid content of the above-mentioned slurry, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the solvent used in the above-mentioned slurry, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the temperature and time of the above-mentioned drying, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the process parameters of the above-mentioned cold pressing and slitting, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the way of providing the first stripe and the second stripe, as long as the purpose of the present application can be achieved. For example, the first stripe and the second stripe can be provided by pulse laser etching; the average depth H of the plurality of first stripes, the average depth T1 of the plurality of second stripes, the width W of a single first stripe, and the width W' of a single second stripe can be controlled by the power and the defocusing amount of the pulse laser emitter; the ratio P of the length of a single first stripe to the width of the first negative electrode material layer can be controlled by adjusting the width of the first negative electrode material layer, the power and the defocusing amount of the pulse laser emitter; the ratio P' of the length of a single second stripe to the width of the empty foil area can be controlled by adjusting the width of the empty foil area, the power and the defocusing amount of the pulse laser emitter; the pitch A of adjacent two first stripes and the pitch A' of adjacent two second stripes can be controlled by adjusting the pitch between the pulse laser emitters or the laser emission frequency.

[0074] In the present application, the different features of the above-mentioned negative electrode tab can be combined, and the combinations are also within the scope of the present application.

[0075] The preparation method of the positive electrode tab is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode tab includes but is not limited to the following steps: (1) mixing the first positive electrode active material, the first positive electrode binder, and the first positive electrode conductive agent, adding a solvent and stirring uniformly to prepare a first positive electrode material layer slurry, mixing the second positive electrode active material, the second positive electrode binder, and the second positive electrode conductive agent, adding a solvent and stirring uniformly to prepare a second positive electrode material layer slurry; (2) determining in advance the surface of the positive current collector facing the center of the electrode assembly winding and the starting position of the positive current collector winding, coating the first positive electrode material layer slurry on the surface of the positive current collector facing the center of the electrode assembly winding, and drying to obtain a positive electrode tab coated with the first positive electrode material layer; (3) coating the second positive electrode material layer slurry on the surface of the positive current collector away from the center of the electrode assembly winding, and drying to obtain a positive electrode tab coated with the first positive electrode material layer and the second positive electrode material layer; (4) after cold pressing and dividing, the positive electrode tab is obtained.

[0076] The solid content of the above-mentioned slurry is not particularly limited in the present application, as long as the purpose of the present application can be achieved. The solvent used in the above-mentioned slurry is not particularly limited in the present application, as long as the purpose of the present application can be achieved. The temperature and time of the above-mentioned drying are not particularly limited in the present application, as long as the purpose of the present application can be achieved. The process parameters of the above-mentioned cold pressing and dividing are not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0077] In the present application, the separator is not particularly limited, as long as the purpose of the present application can be achieved. For example, the material of the separator can include but is not limited to at least one of polyethylene (PE), polyolefin (PO) mainly including polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include at least one of woven film, non-woven film, microporous film, composite film, calendered film, or spunlaced film. The separator of the present application can have a porous structure, and the size of the pore diameter of the porous structure of the separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the size of the pore diameter can be 0.01 μm to 1 μm.

[0078] The cylindrical secondary battery in the present application includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt can include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluorophosphate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the object of the present application is achieved. The present application does not particularly limit the non-aqueous solvent, as long as the object of the present application is achieved. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, or methyl ethyl carbonate. The cyclic carbonate can include, but is not limited to, at least one of vinylene carbonate, propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated vinylene carbonate, carbonic acid-1,2-difluoroethylene ester, carbonic acid-1,1-difluoroethylene ester, carbonic acid-1,1,2-trifluoroethylene ester, carbonic acid-1,1,2,2-tetrafluoroethylene ester, carbonic acid-1-fluoro-2-methyl ethylene ester, carbonic acid-1-fluoro-1-methyl ethylene ester, carbonic acid-1,2-difluoro-1-methyl ethylene ester, carbonic acid-1,1,2-trifluoro-2-methyl ethylene ester, or carbonic acid trifluoromethyl ethylene ester. The carboxylate compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can 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 other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0079] The cylindrical secondary battery of the present application further includes a case for accommodating the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte solution, and other components known in the art in the cylindrical secondary battery, which are not limited by the present application. The case is not particularly limited by the present application, and can be a case known in the art, as long as the purpose of the present application can be achieved.

[0080] The cylindrical secondary battery of the present application is not particularly limited, and can include any device in which electrochemical reaction occurs. In one or more embodiments, the cylindrical secondary battery can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0081] The method for preparing the cylindrical secondary battery of the present application is not particularly limited, and a method known in the art can be used, as long as the purpose of the present application can be achieved. For example, the method for preparing the cylindrical secondary battery includes, but is not limited to, the following steps: stacking the separator, the negative electrode sheet, the separator, and the positive electrode sheet in order, and winding, folding, etc. as needed to obtain an electrode assembly having a wound structure, placing the electrode assembly into the case, welding the current collector plate, assembling the insulating sheet, and then injecting the electrolyte solution into the case and sealing to obtain the cylindrical secondary battery.

[0082] In the present application, the length L2 of the first negative electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the curvature R of the second negative electrode material layer in the first turn of the electrode assembly during winding; the length L1 of the first positive electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the winding needle radius, the thickness of the separator, the thickness of the negative electrode sheet in the first turn of the electrode assembly, etc. during winding, and the curvature R of the second negative electrode material layer in the first turn of the electrode assembly can be controlled by the winding needle radius during winding.

[0083] The second aspect of the present application provides an electronic device including the cylindrical secondary battery of any of the preceding embodiments. The cylindrical secondary battery of the present application has good cycle performance, and thus the electronic device of the present application has a long service life.

[0084] The electronic device of the present application is not particularly limited, and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0085] Examples

[0086] Hereinafter, examples and comparative examples are cited to more specifically describe the embodiments of the present application. Various tests and evaluations are performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0087] Test methods and equipment:

[0088] Test of CW1, CW2, CW3 and CW4:

[0089] At an ambient temperature of 25°C, the lithium ion battery in each example and comparative example was disassembled after constant current discharge at 0.2C to 2.5V, and the negative electrode sheet and the positive electrode sheet were obtained. The material layer on the surface of the negative electrode current collector in the negative electrode sheet away from the winding center of the electrode assembly was the first negative electrode material layer, the material layer on the surface of the negative electrode current collector in the negative electrode sheet toward the winding center of the electrode assembly was the second negative electrode material layer, the material layer on the surface of the positive electrode current collector in the positive electrode sheet toward the winding center of the electrode assembly was the first positive electrode material layer, and the material layer on the surface of the positive electrode current collector in the positive electrode sheet away from the winding center of the electrode assembly was the second positive electrode material layer. After the positive electrode sheet and the negative electrode sheet were cleaned with dimethyl carbonate (DMC) and dried, the positive electrode sheet sample and the negative electrode sheet sample were obtained.

[0090] The negative electrode sheet sample was punched into 32 small discs with a radius of 22.14 mm (an area of 1540.25 mm 2 ), and the average value was calculated as W 总 ; 16 small discs were randomly selected, and the second negative electrode material layer in the small discs was wiped off with deionized water, and the average value was calculated as W1 after weighing; and the first negative electrode material layer in the remaining 16 small discs was wiped off with deionized water, and the average value was calculated as W2 after weighing. Then,

[0091] The coating weight CW1 of the first negative electrode material layer = (W 总 -W1) / 1540.25;

[0092] The coating weight CW2 of the second negative electrode material layer = (W 总 -W2) / 1540.25.

[0093] The coating weight CW3 of the first positive electrode material layer and the coating weight CW4 of the second positive electrode material layer can be obtained by replacing the negative electrode sheet sample with the positive electrode sheet sample and following the above test steps.

[0094] Test of L1, L2, thickness of the separator and thickness of the negative electrode sheet:

[0095] At an ambient temperature of 25°C, a lithium ion battery discharged at 0.2C constant current to 2.5V was disassembled, and the electrode assembly in a wound structure was taken out. The cross section of the electrode assembly in the wound structure was marked along the axial direction of the electrode assembly. The pole piece located in the innermost circle of the electrode assembly and having a copper foil as the current collector or mainly containing copper was the negative pole piece. The pole piece adjacent to the negative pole piece and away from the winding center of the electrode assembly was the positive pole piece. The material layer on the surface of the negative current collector in the negative pole piece away from the winding center of the electrode assembly was the first negative material layer. The material layer on the surface of the positive current collector in the positive pole piece toward the winding center of the electrode assembly was the first positive material layer. The position where the region provided with the first positive material layer on the positive current collector in the first circle of the electrode assembly, the separator, and the region provided with the first negative material layer on the negative current collector began to overlap in the thickness direction of the electrode assembly was the starting end of the first circle of the electrode assembly, which was marked as M. The winding of one circle from the starting end of the electrode assembly in the winding direction of the electrode assembly was the ending end of the first circle of the electrode assembly, which was marked as N.

[0096] The electrode assembly was disassembled, and the separator, the positive pole piece, and the negative pole piece were taken out. The distance from M to N on the first positive material layer in the positive pole piece was measured along the length direction of the unfolded pole piece, which was the length L1 of the first positive material layer in the first circle of the electrode assembly. The distance from M to N on the first negative material layer in the negative pole piece was measured, which was the length L2 of the first negative material layer in the first circle of the electrode assembly. The thickness of the negative pole piece in the first circle (M to N) of the electrode assembly was measured along the thickness direction of the pole piece. The thickness of the separator was measured along the thickness direction of the pole piece.

[0097] Test of the curvature R:

[0098] At an ambient temperature of 25°C, a lithium ion battery discharged at 0.2C constant current to 2.5V was disassembled, and the electrode assembly in a wound structure was taken out. The cross section of the electrode assembly in the wound structure was CT scanned and photographed along the axial direction of the electrode assembly using an industrial computer tomography (industrial CT, Zeiss Xradia 620 Versa). The pole piece located in the innermost circle of the electrode assembly was the negative pole piece. The material layer on the surface of the negative current collector in the negative pole piece toward the winding center of the electrode assembly was the second negative material layer. The diameter of the circle formed by the first circle of the second negative material layer in the electrode assembly was measured 10 times. The 10 measurement values were sorted from large to small, and the average of the first 5 values was taken as the diameter d of the circle formed by the first circle of the second negative material layer in the electrode assembly. Exemplarily, as shown in FIG. 5.

[0099] Then the curvature R of the first circle of the second negative material layer in the electrode assembly was 2 / d.

[0100] Test of P, P', T0, T, H, W, W', A, and A':

[0101] 25℃ of ambient temperature, the lithium ion battery was discharged at 0.2C to 2.5V, then disassembled, the negative electrode sheet was taken out, and the first negative electrode material layer and the second negative electrode material layer of the negative electrode sheet were determined according to the orientation of the negative electrode sheet in the electrode assembly. The negative electrode sheet was soaked in dimethyl carbonate (DMC) for 20 min, and then placed in an oven, dried at 80℃ for 12h to obtain a negative electrode sheet sample.

[0102] Along the width direction of the negative electrode sheet after unfolding, the interface area between the negative electrode current collector and the first negative electrode material layer was used to distinguish the empty foil area of the negative electrode current collector and the coating area of the negative electrode current collector provided with the first negative electrode material layer, and the boundary line between the empty foil area and the coating area was determined.

[0103] When the first stripe is provided on the first negative electrode material layer, along the width direction of the negative electrode sheet, the width of the first negative electrode material layer is measured, and optionally a single first stripe, the length of the stripe is measured, which is the length of a single first stripe. The length of a single first stripe is divided by the width of the first negative electrode material layer to obtain the ratio P of the length of a single first stripe to the width of the first negative electrode material layer. When the first stripe is provided on the second negative electrode material layer, the first negative electrode material layer is replaced by the second negative electrode material layer, and the ratio P of the length of a single first stripe to the width of the second negative electrode material layer can be obtained according to the above test method.

[0104] Along the width direction of the negative electrode sheet, the width of the empty foil area is measured, and optionally a single second stripe, the length of the stripe is measured, which is the length of a single second stripe. The length of a single second stripe is divided by the width of the empty foil area to obtain the ratio P' of the length of a single second stripe to the width of the empty foil area.

[0105] Along the thickness direction of the negative electrode sheet and the boundary line between the empty foil area and the coating area, the negative electrode sheet was cut to obtain the longitudinal section of the coating area, i.e. the longitudinal section of the area where the first negative electrode material layer and the second negative electrode material layer are located, and the longitudinal section of the empty foil area. Each longitudinal section was measured by a scanning electron microscope.

[0106] The longitudinal section of the region where the first negative electrode material layer and the second negative electrode material layer are located is subjected to ion polishing treatment, and is observed using an electron scanning microscope. When the first stripe is arranged on the first negative electrode material layer, a single first stripe on the first negative electrode material layer is selected, the width of the selected 5 positions on the stripe in the width direction after the negative electrode sheet is unfolded is measured, and the average value is taken, to obtain the width W of a single first stripe. The distance between the width center of a single first stripe and the width center of an adjacent first stripe in the length direction after the negative electrode sheet is unfolded is measured, 5 positions are selected for measurement once, and the average value is taken, that is, the pitch A of two adjacent first stripes. 5 first stripes on the first negative electrode material layer are selected, the two ends and the midpoint position of a single first stripe are identified in the width direction after the negative electrode sheet is unfolded, the distance between the surface of the first negative electrode material layer and the bottom surface at the two ends and the midpoint position of a single first stripe in the thickness direction of the negative electrode sheet is measured, and the average value is taken, that is, the depth of a single first stripe. The average depth H of the first stripe is obtained by taking the average value of the depths of the 5 first stripes measured.

[0107] When the first stripe is arranged on the second negative electrode material layer, the first negative electrode material layer is replaced by the second negative electrode material layer, and the H, W, and A of the first stripe arranged on the second negative electrode material layer can be obtained according to the above test method.

[0108] The longitudinal section of the empty foil area is subjected to ion polishing treatment, and the longitudinal section of the empty foil area is observed using an electron scanning microscope. The thickness T0 of the negative electrode current collector is measured in the thickness direction of the negative electrode sheet. A single second stripe on the empty foil area is selected, the width of the selected 5 positions on the stripe in the width direction after the negative electrode sheet is unfolded is measured, and the average value is taken, that is, the width W' of a single second stripe. The distance between the width center of a single second stripe and the width center of an adjacent second stripe in the length direction after the negative electrode sheet is unfolded is measured, 5 positions are selected for measurement once, and the average value is taken, that is, the pitch A' of two adjacent second stripes. 5 second stripes on the empty foil area are selected, the two ends and the midpoint position of a single second stripe are identified in the width direction after the negative electrode sheet is unfolded, the distance between the surface of the empty foil area and the bottom surface at the two ends and the midpoint position of a single second stripe in the thickness direction of the negative electrode sheet is measured, and the average value is taken, that is, the depth of a single second stripe. The average depth T of a single second stripe is obtained by taking the average value of the depths of the 5 second stripes measured.

[0109] Cycle performance test:

[0110] The lithium ion battery in the examples and comparative examples was subjected to charge-discharge cycle test in a 25°C thermostat. The lithium ion battery was charged at 2C constant current to 4.2V, charged at 4.2V constant voltage to 0.05C, and then discharged at 6C constant current to 2.5V after 5 minutes of rest. This was the first cycle, and the discharge capacity C1 was recorded. After 600 cycles according to the above cycle process, the discharge capacity C of the lithium ion battery was recorded, and the cycle capacity retention rate at 600 cycles was calculated as an index for evaluating the infiltration effect of the negative electrode sheet and the cycle performance of the lithium ion battery. The formula is shown in formula (I). The lower the 600 cycle (cls) capacity retention rate, the worse the infiltration effect of the negative electrode sheet in the lithium ion battery, and the worse the cycle performance of the lithium ion battery. The higher the 600 cls capacity retention rate, the better the infiltration effect of the negative electrode sheet in the lithium ion battery, and the better the cycle performance of the lithium ion battery. 600 , and the cycle capacity retention rate at 600 cycles was calculated as an index for evaluating the infiltration effect of the negative electrode sheet and the cycle performance of the lithium ion battery. The formula is shown in formula (I). The lower the 600 cycle (cls) capacity retention rate, the worse the infiltration effect of the negative electrode sheet in the lithium ion battery, and the worse the cycle performance of the lithium ion battery. The higher the 600 cls capacity retention rate, the better the infiltration effect of the negative electrode sheet in the lithium ion battery, and the better the cycle performance of the lithium ion battery.

[0111] 600 cls capacity retention rate (%) = C 600 / C1x 100%. (I)

[0112] Lithium precipitation performance test:

[0113] The lithium ion battery in the examples and comparative examples was placed in a 10°C thermostat for 60 minutes, and then charged at 2C constant current to 4.2V, charged at 4.2V constant voltage to 0.05C, and then discharged at 0.5C constant current to 2.5V after 5 minutes of rest. This was one cycle. After 10 cycles according to the above charge-discharge process, the lithium ion battery was charged at 2C constant current to 4.2V, charged at 4.2V constant voltage to 0.05C, and then discharged at 0.5C constant current to 2.5V after 5 minutes of rest. The lithium ion battery was disassembled, and the lithium precipitation state on the surface of the first negative electrode material layer of the negative electrode sheet was observed. The area without lithium precipitation was golden yellow, and the area with lithium precipitation was grayish white.

[0114] The judgment standard for the degree of lithium precipitation of the lithium ion battery is as follows: 0% of the lithium precipitation area is no lithium precipitation, i.e. no lithium precipitation; more than 0 and less than or equal to 2% of the lithium precipitation area is mild lithium precipitation, i.e. mild lithium precipitation; more than 2% and less than or equal to 20% of the lithium precipitation area is moderate lithium precipitation, i.e. moderate lithium precipitation; and more than 20% and less than or equal to 100% of the lithium precipitation area is severe lithium precipitation, i.e. severe lithium precipitation. The percentage of the lithium precipitation area is calculated based on the total area of the first negative electrode material layer.

[0115] In the present application, those skilled in the art can understand that "C" refers to the rated capacity of the finished lithium ion battery when it leaves the factory. "1C" is the current value that completely discharges the capacity of the lithium ion battery in 1 hour, "0.1C" is the current value that completely discharges the capacity of the lithium ion battery in 10 hours, and other rates are calculated in the same way.

[0116] Example 1-1

[0117] Preparation of the negative electrode sheet

[0118] The first negative active material artificial graphite, the silicon oxide material SiO, the first negative conductive agent sodium carboxymethyl cellulose (CMC-Na), and the first negative binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 84:13:1.7:1.3, and then deionized water is added as a solvent, and the mixture is stirred and mixed uniformly to obtain a first negative material layer slurry with a solid content of 50wt%. The first negative material layer slurry is uniformly coated on one surface of the negative current collector copper foil with a thickness T0 of 12μm, and a certain size of empty foil area is reserved along the width direction of the developed negative electrode sheet for rubbing, and dried at 105℃ to obtain a negative electrode sheet coated with a first negative material layer on one side. The second negative active material artificial graphite, the silicon oxide material SiO, the second negative conductive agent sodium carboxymethyl cellulose (CMC-Na), and the second negative binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 84:13:1.7:1.3, and then deionized water is added as a solvent, and the mixture is stirred and mixed uniformly to obtain a second negative material layer slurry with a solid content of 50wt%. The second negative material layer slurry is coated on the other surface of the negative current collector copper foil to obtain a negative electrode sheet coated with a first negative material layer and a second negative material layer. Then, after cold pressing and slitting, a negative electrode sheet with a specification of 67.45mm×1436mm is obtained, wherein the coating weight CW1 of the first negative material layer is 8.32mg / mm 2 , the coating weight CW2 of the second negative material layer is 8mg / mm 2 , the thickness of the first negative material layer after cold pressing is 50.7μm, the thickness of the second negative material layer is 48.76μm, the width of the empty foil area along the width direction of the developed negative electrode sheet is 5.45mm, the width of the first negative material layer is 62mm, and the width of the second negative material layer is 62mm.

[0119] Preparation of the positive electrode sheet

[0120] The first positive active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1The first positive electrode active material lithium nickel cobalt manganese oxide (LiNi0.8Co0.1Mn0.1O2), the first positive electrode binder polyvinylidene fluoride (PVDF), and the first positive electrode conductive agent conductive carbon black are dispersed in N-methyl pyrrolidone (NMP) solvent at a mass ratio of 94.8:2.8:2.4, mixed thoroughly under stirring, to obtain a first positive electrode material layer slurry with a solid content of 72wt%. The first positive electrode material layer slurry is uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 15μm, and a certain size of empty foil area is reserved along the width direction of the positive electrode tab after unfolding for rubbing, and dried at 105℃ to obtain a positive electrode tab with a single-side coated first positive electrode material layer; then the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode tab with double-side coated positive electrode material layers. 0.8 Co 0.1 Mn 0.1 O2), the second positive electrode binder polyvinylidene fluoride (PVDF), and the second positive electrode conductive agent conductive carbon black are dispersed in N-methyl pyrrolidone (NMP) solvent at a mass ratio of 94.8:2.8:2.4, mixed thoroughly under stirring, to obtain a second positive electrode material layer slurry with a solid content of 72wt%. The second positive electrode material layer slurry is uniformly coated on the other surface of a positive electrode current collector aluminum foil with a thickness of 15μm, and dried at 105℃ to obtain a positive electrode tab with a first positive electrode material layer and a second positive electrode material layer. Then after cold pressing and slitting, the positive electrode tab is dried at 105℃ under vacuum for 4h to obtain a positive electrode tab with a specification of 64.5mm×1422mm for use. Among them, the coating weight CW3 of the first positive electrode material layer is 20mg / mm 2 , the coating weight CW4 of the second positive electrode material layer is 20mg / mm 2 , and the thickness of the first positive electrode material layer and the second positive electrode material layer after cold pressing is 125μm, the width of the first positive electrode material layer and the second positive electrode material layer is 60mm, and the width of the empty foil area of the positive electrode tab is 4.5mm.

[0121] <separator>

[0122] A polyethylene (PE) film with a thickness of 12μm is used as the separator.

[0123] <Preparation of electrolyte>

[0124] In a dry argon glove box, organic solvents ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a mass ratio of 30:50:20 to obtain a base solvent, and then lithium salt lithium hexafluorophosphate (LiPF6) is added to the above base solvent. After being mixed thoroughly and uniformly, an electrolyte is obtained. Based on the mass of the electrolyte, the mass percentage of LiPF6 is 12.5%, and the rest is the base solvent.

[0125] <Preparation of lithium ion battery>

[0126] The separator, the negative electrode sheet, the separator, and the positive electrode sheet prepared above are sequentially stacked in order, and pre-wound to ensure that the separator is between the negative electrode and the positive electrode, and to ensure that the first negative electrode material layer is away from the center of the electrode assembly formed by pre-winding, and the curvature of the first circle of the second negative electrode material layer in the electrode assembly is determined by a winding needle with a radius of 1.6 mm, the length L2 of the first circle of the first negative electrode material layer is 10.7 mm, and the length L1 of the first circle of the first positive electrode material layer is 10.7 mm. Then, after winding, flattening, tab welding, shell insertion, bottom penetration welding, code spraying, vacuum drying, electrolyte injection, potting, high-temperature standing, and formation capacity, a lithium ion battery is obtained. The upper limit of the formation voltage is 3.6 V, the formation temperature is 45°C, and the battery is allowed to stand at room temperature of 25°C for 24 hours after formation.

[0127] Examples 1-2 to 1-23

[0128] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. When the curvature R of the first circle of the second negative electrode material layer in the electrode assembly changes, the radius of the winding needle during winding is adjusted so that the R value is as shown in Table 1.

[0129] Example 1-24

[0130] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. When the curvature R of the first circle of the second negative electrode material layer in the electrode assembly changes, the radius of the winding needle during winding is adjusted so that the R value is as shown in Table 1.

[0131] Example 2-1

[0132] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. When the curvature R of the first circle of the second negative electrode material layer in the electrode assembly changes, the radius of the winding needle during winding is adjusted so that the R value is as shown in Table 1.

[0133] A first stripe is arranged on the first negative electrode material layer in the length direction after the negative electrode sheet is unfolded. The shape of the first stripe is specifically shown in FIG. 2. The average depth H of the first stripe is set to 15 μm, the ratio P of the length of a single first stripe to the width of the first negative electrode material layer in the width direction after the negative electrode sheet is unfolded is 0.6, the width W of the first stripe is 180 μm, and the pitch A of adjacent two first stripes in the length direction after the negative electrode sheet is unfolded is 5 mm. The first stripe is laser etched on the first negative electrode material layer according to the above parameters.

[0134] Examples 2-2 to 2-9

[0135] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0136] Example 2-10

[0137] The rest is the same as Example 2-1 except that a second stripe is additionally provided in the empty foil area according to the following steps in the <preparation of negative electrode sheet>.

[0138] The second stripe is provided in the empty foil area, and the shape of the second stripe is specifically shown in FIG. 2. The average depth T1 of the second stripe is set to 6 pm, T1 / T0 is 0.5, the ratio P' of the length of a single second stripe to the width of the empty foil area in the width direction after the negative electrode sheet is developed is 0.6, the width W' of the second stripe is 180 pm, and the pitch A' of adjacent two second stripes in the length direction after the negative electrode sheet is developed is 5 mm. The second stripe is laser etched on the empty foil area according to the above parameters.

[0139] Examples 2-11 to 2-16

[0140] The rest is the same as Example 2-10 except that the relevant preparation parameters are adjusted according to Table 2.

[0141] Example 2-17

[0142] The rest is the same as Example 2-10 except that the first stripe is provided on both the first negative electrode material layer and the second negative electrode material layer in the <preparation of negative electrode sheet>, and the parameters of the first stripe are consistent.

[0143] Comparative Example 1

[0144] The rest is the same as Example 1-1 except that in the <preparation of negative electrode sheet>, when the second negative electrode slurry is coated, the starting end of the second negative electrode material layer is 10 mm behind the starting end of the first negative electrode in the pre-rolling direction of the negative electrode sheet, and the relevant preparation parameters are adjusted according to Table 1.

[0145] Comparative Examples 2 to 3

[0146] The rest is the same as Example 1-1 except that the relevant preparation parameters are adjusted according to Table 1.

[0147] The preparation parameters and performance parameters of each example and comparative example are shown in Table 1 and Table 2.

[0148] Table 1 Note: " / " in Table 1 means no relevant preparation parameter.

[0149] As can be seen from Examples 1-1 to 1-24, Comparative Examples 1 to 3, by adjusting the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first turn of the electrode assembly, and the ratio of the coating weights of the two negative electrode material layers, so that the values of B and A / B are within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, the capacity retention rate at 600 cls of the lithium ion battery is improved, which indicates that the lithium ion battery of the present application effectively reduces the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle, and the lithium ion battery has good cycle performance. In Comparative Examples 1 to 3, the values of B and / or A / B are not within the scope of the present application; the degree of lithium precipitation of the lithium ion battery in Comparative Examples 1 to 3 is heavier; and / or, the capacity retention rate at 600 cls is lower. While the degree of lithium precipitation of the lithium ion battery in Examples 1-1 to 1-24 is lighter, and the capacity retention rate at 600 cls is higher, the lithium ion battery of the present application effectively reduces the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle, and the lithium ion battery has good cycle performance.

[0150] The value of A generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1-1, 1-9 to 1-10, 1-17 to 1-19, when the value of A is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0151] The value of CW2 generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1-1, 1-11 to 1-16, when the value of CW2 is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0152] The value of L2 generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1-1, 1-4 to 1-8, when the value of L2 is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0153] The value of R generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-8, when the value of R is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0154] The thickness of the separator generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-21 to Example 1-24, when the thickness of the separator is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0155] The thickness of the first negative electrode tab in the electrode assembly generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-8, Example 1-17 to Example 1-24, when the thickness of the first negative electrode tab in the electrode assembly is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0156] The value of CW3 generally affects the cycle performance of the lithium ion battery. As can be seen from Example 1-1 to Example 1-3, Example 1-17 to Example 1-24, when the value of CW3 is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the lithium ion battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0157] Table 2 Note: " / " in Table 2 indicates no relevant preparation parameters.

[0158] The value of H generally affects the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-3, when the value of H is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the lithium ion battery of the present application is lower, and the electrolyte in the example has a better wetting effect on the negative electrode tab, and the cycle performance of the lithium ion battery is better.

[0159] The value of P generally affects the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, Example 2-4 to Example 2-5, when the value of P is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate of 600 cls, indicating that the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, and the electrolyte has a better wetting effect on the negative electrode sheet in the examples, and the lithium ion battery has better cycle performance.

[0160] The value of W generally affects the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, Example 2-6 to Example 2-7, when the value of W is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate of 600 cls, indicating that the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, and the electrolyte has a better wetting effect on the negative electrode sheet in the examples, and the lithium ion battery has better cycle performance.

[0161] The value of A generally affects the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, Example 2-8 to Example 2-9, when the value of A is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate of 600 cls, indicating that the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, and the electrolyte has a better wetting effect on the negative electrode sheet in the examples, and the lithium ion battery has better cycle performance.

[0162] The value of T0 and the value of T1 / T0 generally affect the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, Example 2-10 to Example 2-14, when the value of T0 and the value of T1 / T0 are within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate of 600 cls, indicating that the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, and the electrolyte has a better wetting effect on the negative electrode sheet in the examples, and the lithium ion battery has better cycle performance.

[0163] The value of P' generally affects the lithium precipitation performance and cycle performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1, Example 2-15 to Example 2-16, when the value of P' is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate of 600 cls, indicating that the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, and the electrolyte has a better wetting effect on the negative electrode sheet in the examples, and the lithium ion battery has better cycle performance.

[0164] The arrangement of the first stripe on the different negative material layers generally affects the lithium precipitation performance and the cycle performance of the lithium ion battery. As can be seen from Example 1-1 and Example 2-17, when the arrangement of the first stripe on the different negative material layers is within the scope of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher 600 cls capacity retention rate, indicating that the risk of lithium precipitation caused by insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle of the lithium ion battery of the present application is lower, the electrolyte has a better wetting effect on the negative electrode plate in the examples, and the lithium ion battery has better cycle performance.

[0165] It should be noted that, in this document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0166] Each of the embodiments in the specification is described in a relevant manner, and the same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the difference from other embodiments.

[0167] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A cylindrical secondary battery comprising an electrode assembly including a positive electrode tab, a negative electrode tab, and a separator, the negative electrode tab including a negative electrode current collector, a first negative electrode material layer disposed on a surface of the negative electrode current collector facing away from a winding center of the electrode assembly, and a second negative electrode material layer disposed on a surface of the negative electrode current collector facing toward the winding center of the electrode assembly; the positive electrode tab including a positive electrode current collector and a first positive electrode material layer disposed on a surface of the positive electrode current collector facing toward the winding center of the electrode assembly; a coating weight of the first negative electrode material layer is CW1 mg / mm 2 a coating weight of the second negative electrode material layer is CW2 mg / mm 2 a ratio of CW1 to CW2 is A; in a winding direction of the electrode assembly, a length of the first positive electrode material layer in a first turn of the electrode assembly is L1 mm, a length of the first negative electrode material layer in the first turn of the electrode assembly is L2 mm, a ratio of L1 to L2 is B, 1.05≤B≤1.09, and 0.918≤A / B≤1.

048.

2. The cylindrical secondary battery according to claim 1, wherein 1.01≤A≤1.07; and / or, 0.93≤A / B≤1.

02.

3. The cylindrical secondary battery according to claim 1 or 2, wherein 4≤CW2≤15, and / or, 7≤L2≤16.

4. The cylindrical secondary battery according to claim 3, wherein 7≤CW2≤10; and / or, 8≤L2≤12.

5. The cylindrical secondary battery according to any one of claims 1 to 4, wherein A curvature of a first turn of the second negative electrode material layer in the electrode assembly is R, and 0.4≤R≤0.

9.

6. The cylindrical secondary battery according to any one of claims 1 to 5, wherein A thickness of the separator is 9μm to 15μm.

7. The cylindrical secondary battery according to any one of claims 1 to 6, wherein A thickness of a first turn of the negative electrode tab in the electrode assembly is 50μm to 200μm.

8. The cylindrical secondary battery according to any one of claims 1 to 7, wherein The coating weight of the first positive electrode material layer is CW3 mg / mm 2 19≤ CW3≤ 21.

9. The cylindrical secondary battery according to any one of claims 1 to 8, wherein A plurality of first stripes are disposed on the first negative electrode material layer and / or the second negative electrode material layer, the plurality of first stripes extending in a width direction of the negative electrode tab after being unrolled and being disposed at intervals in a length direction of the negative electrode tab after being unrolled.

10. The cylindrical secondary battery according to claim 9, wherein An average depth of the plurality of first stripes in a thickness direction of the negative electrode tab is Hμm, and 5≤H≤25.

11. The cylindrical secondary battery according to claim 9 or 10, wherein A ratio of a length of a single first stripe to a width of the first negative electrode material layer or a ratio of a length of a single first stripe to a width of the second negative electrode material layer in the width direction of the negative electrode tab after being unrolled is P, and 0.2≤P≤1.

12. The cylindrical secondary battery according to any one of claims 9 to 11, wherein A width of a single first stripe in the length direction of the negative electrode tab after being unrolled is Wμm, and 10≤W≤400.

13. The cylindrical secondary battery according to any one of claims 9 to 12, wherein A pitch of adjacent two first stripes in the length direction of the negative electrode tab after being unrolled is A mm, and 0.5≤A≤10.

14. The cylindrical secondary battery according to any one of claims 1 to 13, wherein The negative electrode current collector includes a blank foil region connected to the first negative electrode material layer in the width direction of the negative electrode tab after being unrolled, a plurality of second stripes are disposed on the blank foil region, the plurality of second stripes extending in the width direction of the negative electrode tab after being unrolled and being disposed at intervals in the length direction of the negative electrode tab after being unrolled.

15. The cylindrical secondary battery according to claim 14, wherein A ratio of a length of a single second stripe to a width of the blank foil region in the width direction of the negative electrode tab after being unrolled is P', and 0.1≤P'≤0.

7.

16. The cylindrical secondary battery according to claim 14, wherein A thickness of the negative electrode current collector is T0μm, an average depth of the plurality of second stripes is Tμm, 0.1≤T / T0≤0.8, and 3≤T0≤20. 17.An electronic device comprising the cylindrical secondary battery according to any one of claims 1 to 16.

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