Cylindrical secondary battery and electronic device

Abstract

The invention provides a cylindrical secondary battery and an electronic device, the cylindrical secondary battery comprises an electrode assembly, the electrode assembly comprises a positive electrode piece, a negative electrode piece and a diaphragm, and the negative electrode piece comprises a negative electrode current collector, a first negative electrode material layer and a second negative electrode material layer. The positive pole piece comprises a positive current collector and a first positive material layer. The unit area capacity of the first negative electrode material layer is C1mAh / 1540.25 mm < 2 >, the unit area capacity of the second negative electrode material layer is C2mAh / 1540.25 mm < 2 >, and the ratio of C1 to C2 is A. In the winding direction of the electrode assembly, the length of the first circle of first positive electrode material layer in the electrode assembly is L1 mm, the length of the first circle of first negative electrode material layer in the electrode assembly is L2 mm, the ratio of L1 to L2 is B, 1.001 < = A < = 1.1, and 0.918 < = A / B < = 1.048. Through the arrangement, the improvement of the cycle performance of the cylindrical secondary battery is facilitated.

Description

Technical Field

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

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

[0003] At present, the design of high-power cylindrical lithium-ion batteries usually adopts a full-ear design, that is, the positive electrode ear and the negative electrode ear extend from opposite directions, and are prepared by flattening or kneading the full-ear design. For cylindrical lithium-ion batteries, the prior art adopts the method of setting grooves on the pole piece to improve the wetting effect of the pole piece, thereby improving the dynamic performance of the lithium-ion battery. Summary of the invention

[0004] The purpose of the present application is to provide a cylindrical secondary battery and electronic device, which can reduce the risk of lithium precipitation due to insufficient CB value in the inner circle of the negative electrode plate in the late cycle without affecting the production efficiency, thereby improving the cycle performance of the secondary battery.

[0005] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of cylindrical secondary batteries to explain this application, but the cylindrical secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0006] The first aspect of the present application provides a cylindrical secondary battery, which includes an electrode assembly, the electrode assembly includes a positive electrode sheet, a negative electrode sheet and a separator, the negative electrode sheet includes a negative electrode collector, a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer is arranged on the surface of the negative electrode collector away from the winding center of the electrode assembly, and the second negative electrode material layer is arranged on the surface of the negative electrode collector toward the winding center of the electrode assembly. The positive electrode sheet includes a positive electrode collector and a first positive electrode material layer, the first positive electrode material layer is arranged on the surface of the positive electrode collector toward the winding center of the electrode assembly. The unit area capacity of the first negative electrode material layer is C 1 mAh / 1540.25mm 2 , the unit area capacity of the second negative electrode material layer is C 2 mAh / 1540.25mm 2 , C 1 With C 2 The ratio is A. Along the winding direction of the electrode assembly, the length of the first positive electrode material layer in the first turn of the electrode assembly is L 1 mm, the length of the first negative electrode material layer in the first circle of the electrode assembly is L2 mm, L 1 With L 2 The ratio is B, 1.001≤A≤1.1, optionally, 1.01≤A≤1.07; 0.918≤A / B≤1.048, optionally, 0.93≤A / B≤1.02. 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 spacing between the layers in the inner circle of the electrode assembly is moderate, the ion transmission distance during the cycle is moderate, and it is conducive to the infiltration of the electrolyte into the inner circle of the electrode assembly; with the different unit area capacities in the two negative electrode material layers, the first negative electrode material layer has a larger unit area capacity, which can effectively reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, so the cylindrical secondary battery has good cycle performance.

[0007] In one or more embodiments, 1.05≤B≤1.09. By adjusting the value of B within the above range, the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle is reduced, and the cycle performance of the cylindrical secondary battery is improved.

[0008] In one or more embodiments, 40≤C 2 ≤70, optionally, 46≤C 2 ≤55; and / or, 8≤L 2 ≤16, optionally, 8≤L 2 ≤12. By regulating C 2 The value of and / or L 2 The value of is within the above range, 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. While taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0009] In one or more embodiments, the curvature of the first second negative electrode material layer in the electrode assembly is R, 0.3≤R≤0.9. By adjusting the value of R within the above range, the secondary battery has good cycle performance while taking into account high energy density and good processing performance.

[0010] In one or more embodiments, the first negative electrode material layer includes a first active material, the second negative electrode material layer includes a second negative electrode active material, and the first negative electrode active material and the second negative electrode active material both contain silicon. Based on the mass of the first negative electrode material layer, the mass percentage of the silicon element in the first negative electrode material layer is W. 1 Based on the mass of the second negative electrode material layer, the mass percentage of the silicon element in the second negative electrode material layer is W 2 , W 1 With W 2The ratio is F, 1.001≤F≤1.4; optionally, 1.03≤F≤1.1. By adjusting the value of F within the above range, the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle is reduced, while the mechanical safety performance and cycle performance of the secondary battery are taken into account, and the energy density of the secondary battery is further improved.

[0011] In one or more embodiments, 0.5%≤W 2 ≤20%. By adjusting W 2 The value is within the above range, which reduces the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, while taking into account the energy density and cycle performance of the secondary battery.

[0012] In one or more embodiments, the first negative electrode active material includes a first silicon-based material, the second negative electrode active material includes a second silicon-based material, and the first silicon-based material and the second silicon-based material are each independently selected from at least one of pure silicon, silicon-carbon, silicon-oxygen, silicon alloy, or nano-silicon. By selecting the above-mentioned first silicon-based material and second silicon-based material, the secondary battery has a higher energy density and good cycle performance.

[0013] In one or more embodiments, based on the mass of the first negative electrode material layer, the mass percentage of the first silicon-based material is 1.2% to 32%; based on the mass of the second negative electrode material layer, the mass percentage of the second silicon-based material is 1% to 30%. By regulating the mass percentage of the first silicon-based material and the mass percentage of the second silicon-based material within the above range, the secondary battery has a higher energy density and good cycle performance.

[0014] The second aspect of the present application provides an electronic device, which includes the cylindrical secondary battery in any of the above embodiments. The cylindrical secondary battery of the present application has good cycle performance, so the electronic device of the present application has a long service life.

[0015] Beneficial effects of the embodiments of the present application:

[0016] The embodiment of the present application provides a cylindrical secondary battery and electronic device. By adjusting 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, the spacing between the layers in the inner circle of the electrode assembly is moderate, the ion transmission distance during the cycle is moderate, and it is conducive to the infiltration of the electrolyte into the inner circle of the electrode assembly; with different unit area capacities in the two negative electrode material layers, the first negative electrode material layer has a larger unit area capacity, which can effectively 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, so the cylindrical secondary battery has good cycle performance. In addition, the unit area capacity of different negative electrode material layers is different, and the secondary battery has a high energy density without significantly increasing the actual production cost. In the actual process, the production operation steps of the secondary battery are simple, and the processing performance is taken into account without reducing the production efficiency.

[0017] Of course, implementing any product or method of the present application does not necessarily require achieving all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0019] Figure 1 A schematic diagram of a winding structure formed by an electrode assembly in one embodiment of the present application;

[0020] Figure 2 This is a computed tomography (CT) image of an electrode assembly in another embodiment of the present application.

[0021] Reference numerals: electrode assembly 001 ; positive electrode plate 10 ; positive electrode current collector 11 ; first positive electrode material layer 12 ; second positive electrode material layer 13 ; negative electrode plate 20 ; negative electrode current collector 21 ; first negative electrode material layer 22 ; second negative electrode material layer 23 ; separator 30 . DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0023] It should be noted that in the specific embodiments of the present application, a lithium-ion battery is used as an example of a cylindrical secondary battery to explain the present application, but the cylindrical secondary battery of the present application is not limited to a lithium-ion battery.

[0024] For high-power cylindrical secondary batteries, generally full-ear cylindrical lithium-ion batteries, in the late cycle, the electrolyte wetting effect is poor on the surface of the negative electrode sheet away from the winding center of the electrode assembly, especially near the winding center of the electrode assembly, and the negative electrode sheet is not well wetted; further, the curvature of the cylindrical electrode assembly will gradually increase with the decrease in the number of windings. Further, for the same circle of electrode assemblies, the length of the surface of the negative electrode sheet away from the winding center of the electrode assembly will be slightly less than the length of the surface of the positive electrode sheet facing the winding center of the electrode assembly, and starting from the winding center close to the electrode assembly, the length difference of 1 to 5 circles of the electrode assembly is more significant, resulting in insufficient CB in the inner circle of the electrode assembly in the late cycle, and lithium precipitation occurs. Based on the above reasons, the cycle performance of the lithium-ion battery decreases. In the prior art, grooves and improving electrolyte performance 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 materials in the electrode sheet, the secondary battery cannot take into account both high energy density and good cycle performance. Therefore, the present application provides a cylindrical secondary battery, which can effectively reduce the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle without affecting the actual production efficiency, and the secondary battery has good cycle performance.

[0025] The specific technical solutions are as follows:

[0026] The first aspect of the present application provides a cylindrical secondary battery, which includes an electrode assembly, the electrode assembly includes a positive electrode sheet, a negative electrode sheet and a separator, the negative electrode sheet includes a negative electrode collector, a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer is arranged on the surface of the negative electrode collector away from the winding center of the electrode assembly, and the second negative electrode material layer is arranged on the surface of the negative electrode collector toward the winding center of the electrode assembly. The positive electrode sheet includes a positive electrode collector and a first positive electrode material layer, the first positive electrode material layer is arranged on the surface of the positive electrode collector toward the winding center of the electrode assembly. The unit area capacity of the first negative electrode material layer is C 1 mAh / 1540.25mm 2 , the unit area capacity of the second negative electrode material layer is C 2 mAh / 1540.25mm 2 , C 1 With C 2 The ratio is A. Along the winding direction of the electrode assembly, the length of the first positive electrode material layer in the first turn of the electrode assembly is L 1 mm, the length of the first negative electrode material layer in the first circle of the electrode assembly is L 2mm, L 1 With L 2 The ratio is B, 1.001≤A≤1.1, optionally, 1.01≤A≤1.07, for example, the value of A can be 1.001, 1.003, 1.005, 1.008, 1.01, 1.015, 1.02, 1.025, 1.03, 1.035, 1.04, 1.045, 1.05, 1.055, 1.06, 1.065, 1.07, 1.075, 1.08, 1.085, 1.09, 1.095, 1.1 or a range consisting of any two values ​​therein. 0.918≤A / B≤1.048, optionally, 0.93≤A / B≤1.02, for example, the value of A / B can be 0.918, 0.92, 0.925, 0.93, 0.935, 0.94, 0.945, 0.95, 0.955, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1, 1.005, 1.01, 1.015, 1.02, 1.025, 1.03, 1.035, 1.04, 1.043, 1.045, 1.048 or a range consisting of any two of the values.

[0027] In this application, the winding direction of the electrode assembly is defined as the W direction. Figure 1 As shown, 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 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 arranged on the surface of the negative electrode collector 21 away from the winding center of the electrode assembly 001, and the second negative electrode material layer 23 is arranged on the surface of the negative electrode collector 21 toward the winding center of the electrode assembly 001. The positive electrode sheet 10 includes a positive electrode collector 11 and a first positive electrode material layer 12. The first positive electrode material layer 12 is arranged on the surface of the positive electrode collector 11 toward the winding center of the electrode assembly 001.

[0028] When the value of A is too small, that is, less than the lower limit of this application, the ratio of the unit area capacity of the first negative electrode material layer to the second negative electrode material layer is too small, and the risk of lithium precipitation caused by insufficient CB in the inner circle of the negative electrode sheet in the later stage of the cycle cannot be effectively reduced. When the value of A is too large, that is, greater than the upper limit of this application, the amount of active material supplemented in the coating of the negative electrode sheet is too much, and the excess active material causes an increase in cost in the actual production process, and causes a loss of energy density, and the kinetic performance of the secondary battery is reduced. When the value of A / B is too small, that is, less than the lower limit of this application, the amount of active material supplemented to the second negative electrode material layer in the negative electrode sheet is insufficient, and the risk of lithium precipitation caused by insufficient CB in the inner circle of the negative electrode sheet in the later stage of the cycle cannot be effectively reduced. When the value of A / B is too large, that is, greater than the upper limit of this application, the amount of active material supplemented to the second negative electrode material layer in the negative electrode sheet is too much, and the excess active material causes an increase in cost in the actual production process, and causes a loss of energy density, and the kinetic performance of the secondary battery is reduced. This 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 spacing between the layers in the inner circle of the electrode assembly is moderate, the ion transmission distance during the cycle is moderate, and it is conducive to the infiltration of the electrolyte into the inner circle of the electrode assembly; with different unit area capacities in the two negative electrode material layers, the first negative electrode material layer has a larger unit area capacity, which can effectively 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, so the cylindrical secondary battery has good cycle performance. In addition, the unit area capacity of different negative electrode material layers is different, and the secondary battery has a high energy density without significantly increasing the actual production cost. In the actual process, the production operation steps of the secondary battery are simple, and the processing performance is taken into account without reducing the production efficiency.

[0029] In this application, CB value refers to the ratio between the capacity of the negative electrode sheet per unit area and the capacity of the positive electrode sheet per unit area under the same conditions, for example, at an ambient temperature of 25°C and a discharge rate of 0.1C. CB = (gram capacity of negative electrode active material × mass of negative electrode active material per unit area of ​​negative electrode sheet) / (gram capacity of positive electrode active material × mass of positive electrode active material per unit area of ​​positive electrode sheet). The above unit area refers to 1mm 2. It can be understood that the starting point of the first circle of the 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 area on the positive electrode collector in the first circle where the first positive electrode material layer is provided, the separator and the area on the negative electrode collector where the first negative electrode material layer is provided begin to overlap along the thickness direction of the electrode assembly. The first circle of the electrode assembly refers to the electrode assembly winding around the starting end of the electrode assembly (i.e., the starting point) along the winding direction of the electrode assembly to the other end face of the electrode assembly (i.e., the ending point), 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 along the winding direction of the electrode assembly to the ending point 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 along the winding direction of the electrode assembly to the ending point of the electrode assembly. In addition, it should be noted that the "surface" in the present application can be the entire area of ​​the surface of the positive electrode collector and / or the negative electrode collector, or it can be a partial area of ​​the surface of the positive electrode collector and / or the negative electrode collector. The present application has no special restrictions as long as the purpose of the present application can be achieved.

[0030] In one or more embodiments, 1.05≤B≤1.09. For example, the value of B can be 1.05, 1.053, 1.055, 1.058, 1.06, 1.063, 1.065, 1.068, 1.07, 1.072, 1.075, 1.078, 1.08, 1.083, 1.085, 1.088, 1.09 or a range consisting of any two of these values. By regulating the value of B within the above range, the spacing between the layers in the inner circle of the electrode assembly is moderate, the ion transmission distance during the cycle is moderate, and it is conducive to the infiltration of the electrolyte into the inner circle of the electrode assembly without increasing the manufacturing cost of the secondary battery. The actual production value of the secondary battery is high, while reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, the cycle performance of the cylindrical secondary battery is improved.

[0031] In one or more embodiments, 40≤C 2 ≤70, optionally, 46≤C 2 ≤55, for example, C 2 The value of can be 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70 or a range consisting of any two of them. 2The value of is within the above range. At this time, the content of negative electrode active material in the second negative electrode material layer in the negative electrode plate is moderate, which is beneficial to actual production and reduces the risk of metal exposure or scratches on the surface of the negative electrode plate during processing, which may cause local lithium deposition and short circuit in the plate during charging and discharging. At the same time, the energy density of the secondary battery is taken into account, and the risk of a decrease in 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 is reduced. This is beneficial to reducing the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle, thereby improving the cycle performance of the secondary battery.

[0032] In one or more embodiments, 8≤L 2 ≤16, optionally, 8≤L 2 ≤12, for example, L 2 The value of can be 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16 or a range consisting of any two of them. 2 The value of is within the above range, so that the length of the negative electrode sheet of the first circle of the electrode assembly is moderate, which is beneficial to reducing the risk of breakage during charging and discharging due to excessive curvature of the negative electrode sheet of the first circle. At the same time, the center hole of the winding center of the electrode assembly is moderate in size, and the secondary battery has a high energy density, which is beneficial to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle. While taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0033] In one or more embodiments, 40≤C 2 ≤70, optionally, 46≤C 2 ≤55; and, 8≤L 2 ≤16, optionally, 8≤L 2 ≤12. For example, C 2 The value of can be 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70 or a range consisting of any two values ​​therein; 2 The value of can be 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16 or a range consisting of any two of them. 2 The value of L 2The value is within the above range, the content of the active material of the second negative electrode material layer in the negative electrode sheet and the length of the negative electrode sheet of the first circle of the electrode assembly are moderate, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle. While taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0034] In one or more embodiments, 40.04≤C 1 ≤74.9, e.g., C 1 The value of can be 40.04, 40.1, 42, 45, 50, 52, 55, 58, 60, 62, 65, 68, 70, 72, 74.9 or a range consisting of any two of them. 1 The value of is within the above range, so that the content of active material in the first negative electrode material layer in the negative electrode plate is moderate, which is beneficial to actual production and reduces the risk of metal exposure or scratches on the surface of the negative electrode plate during processing, which may cause local lithium deposition and short circuit in the plate during charging and discharging. At the same time, the energy density of the secondary battery is taken into account, and the risk of a decrease in 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 is reduced. It is beneficial to reduce the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle, thereby improving the cycle performance of the secondary battery.

[0035] In one or more embodiments, 7.6412≤L 1 ≤19.1721, for example, L 1 The value of can be 5.1795, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16.5, 17, 17.5, 18, 18.5, 19, 19.1721 or a range consisting of any two of the values. By adjusting L 1 The value of is within the above range, so that the length of the positive electrode sheet of the first circle of the electrode assembly is moderate, which effectively reduces the risk of breakage during charging and discharging due to excessive curvature of the first circle of the electrode sheet. At the same time, the center hole of the winding center of the electrode assembly is moderate in size, and the secondary battery has a high energy density, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle. While taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0036] In one or more embodiments, the curvature of the second negative electrode material layer in the first circle of the electrode assembly is R, 0.3≤R≤0.9. For example, the value of R can be 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9 or a range consisting of any two of these 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 helping 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, taking into account the actual production difficulty. At the same time, the center hole size of the winding center of the electrode assembly is moderate, and the secondary battery has a higher energy density. Therefore, while taking into account both high energy density and good processing performance, the secondary battery has good cycle performance.

[0037] In one or more embodiments, the first negative electrode material layer includes a first active material, the second negative electrode material layer includes a second negative electrode active material, and the first negative electrode active material and the second negative electrode active material both contain silicon. Based on the mass of the first negative electrode material layer, the mass percentage of the silicon element in the first negative electrode material layer is W. 1 Based on the mass of the second negative electrode material layer, the mass percentage of the silicon element in the second negative electrode material layer is W 2 , W 1 With W 2 The ratio is F, 1.001≤F≤1.4; optionally, 1.03≤F≤1.1. For example, the value of F can be 1.001, 1.003, 1.005, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, 1.21, 1.23, 1.25, 1.28, 1.3, 1.32, 1.35, 1.38, 1.4, or a range consisting of any two of the values. By regulating the value of F within the above range, it is beneficial to regulate the ratio of the capacity per unit area of ​​the first negative electrode material layer to that of the second negative electrode material layer, and the volume expansion of different negative electrode material layers in the negative electrode plate during the cycle of the secondary battery is relatively uniform, which is beneficial to reduce the risk of the plate breaking due to uneven expansion, causing the diaphragm to be punctured and then short-circuited, and when the negative electrode material layer contains silicon, the theoretical specific capacity of silicon is relatively high, while reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, the mechanical safety performance and cycle performance of the secondary battery are taken into account, and the energy density of the secondary battery is further improved.

[0038] In one or more embodiments, 0.5%≤W 2 ≤20%. For example, W 2The value of can be 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or a range consisting of any two values ​​thereof. By adjusting W 2 The value of is within the above range, so that the volume expansion of the negative electrode plate is moderate during the cycle, which is beneficial to adjust the unit area capacity of the second negative electrode material layer. While reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, it also takes into account the energy density and cycle performance of the secondary battery.

[0039] In one or more embodiments, 0.5005%≤W 1 ≤28%. For example, W 2 The value of can be 0.5005%, 0.0501%, 0.0503%, 0.0505%, 0.0508%, 0.051%, 0.055%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.5%, 0.8%, 1%, 1.01%, 1.1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 22%, 24%, 26%, 28% or a range consisting of any two of them. By regulating W 1 The value of is within the above range, so that the volume expansion of the negative electrode plate is moderate during the cycle, which is beneficial to adjust the unit area capacity of the first negative electrode material layer. While reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle, it also takes into account the energy density and cycle performance of the secondary battery.

[0040] In one or more embodiments, the first negative electrode active material includes a first silicon-based material, the second negative electrode active material includes a second silicon-based material, and the first silicon-based material and the second silicon-based material are independently selected from at least one of pure silicon, silicon carbon, silicon oxygen, silicon alloy or nano silicon. By selecting the above-mentioned first silicon-based material and second silicon-based material, it is beneficial to regulate the unit area capacity of the first negative electrode material layer and the second negative electrode material layer, 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 late cycle, and the secondary battery has a higher energy density and good cycle performance.

[0041] In one or more embodiments, based on the mass of the first negative electrode material layer, the mass percentage of the first silicon-based material is 1.2% to 32%. For example, the mass percentage of the first negative electrode active material can be 1.2%, 1.5%, 1.8%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32% or any two thereof. Based on the mass of the second negative electrode material layer, the mass percentage of the second silicon-based material is 1% to 30%. For example, the mass percentage of the second negative electrode active material can be 1%, 1.2%, 1.5%, 1.8%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30% or a range consisting of any two of the values. By regulating the mass percentage of the first silicon-based material and the mass percentage of the second silicon-based material within the above range, it is beneficial to regulate the unit area capacity of the first negative electrode material layer and the second negative electrode material layer. The volume expansion of the negative electrode plate during the cycle is moderate, which is beneficial to reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the late cycle. The secondary battery has a high energy density and good cycle performance.

[0042] In one or more embodiments, the first negative electrode active material further comprises at least one of artificial graphite or natural graphite, and the second negative electrode active material further comprises at least one of artificial graphite or natural graphite. By selecting the first negative electrode active material and the second negative electrode active material, it is beneficial to improve the structural stability of the negative electrode sheet, so that the volume expansion of the negative electrode sheet during the cycle is moderate.

[0043] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or composite current collector (such as lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.). The present application has no particular restrictions on the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector may be 3μm to 20μm. The first negative electrode material layer and the second negative electrode material layer may also include a conductive agent and a negative electrode binder. The present application has no particular restrictions on the type of conductive agent in the first negative electrode material layer and the second negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fiber may include but is not limited to vapor grown carbon fiber (VGCF) and / or nano carbon fiber. The metal material may include but is not limited to metal powder and / or metal fiber. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The present application does not particularly limit the type of negative electrode binder in the first negative electrode material layer and the second negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The present application does not particularly limit the mass ratio of the negative electrode active material, the conductive agent and the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer, as long as the purpose of the present application can be achieved. In the present application, the first negative electrode material layer and the second negative electrode material layer also include a thickener. The present application has no particular restrictions on the type of thickener, as long as the purpose of the present application can be achieved. For example, the thickener may include but is not limited to at least one of carboxymethyl cellulose or sodium carboxymethyl cellulose.

[0044] In the present application, the positive electrode sheet may further include a second positive electrode material layer, and the second positive electrode material layer is disposed on the surface of the positive electrode current collector away from the winding center of the electrode assembly, for example, Figure 1As shown, the positive electrode sheet 10 may further include a second positive electrode material layer 13, and the second positive electrode material layer 13 is arranged on the surface of the positive electrode collector 11 away from the winding center of the electrode assembly 001. The present application has no special restrictions on the positive electrode collector, as long as the purpose of the present application can be achieved. For example, the positive electrode collector may include aluminum foil, 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 present application includes a first positive electrode active material, and the second positive electrode material layer includes a second positive electrode active material. The present application has no special restrictions on the types of the first positive electrode active material and the second positive electrode active material, as long as the purpose of the present application can be achieved. For example, the first positive electrode active material and the second positive electrode active material can be independently selected from lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O 2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO 2 ), lithium manganate, lithium iron manganese phosphate or lithium titanate, etc. In the present application, the first positive electrode active material and the second positive electrode active material may also contain non-metallic elements, for example, the non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive electrode collector, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode collector is 5μm to 20μm. In the present application, the first positive electrode material layer and the second positive electrode material layer may also include a positive electrode binder and a conductive agent. The present application does not particularly limit the type of the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer, as long as the purpose of the present application can be achieved. The present application does not particularly limit the type of the positive electrode conductive agent in the first positive electrode material layer and the second positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may be the same as the type of the conductive agent in the first negative electrode material layer and the second negative electrode material layer. The present application does not particularly limit the type of the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the positive electrode binder can be the same type as the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer. The present application has no particular restrictions on the mass ratio of the positive electrode active material, the conductive agent, and the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0045] The present application has no particular restrictions on the preparation method of the negative electrode sheet, 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) preparing the first negative electrode material layer slurry and preparing the second negative electrode material layer slurry; (2) predetermining the surface of the negative electrode collector away from the winding center of the electrode assembly and the starting position of the negative electrode collector to start winding, applying the first negative electrode material layer slurry on the surface of the negative electrode collector away from the winding center of the electrode assembly, and drying to obtain the negative electrode sheet coated with the first negative electrode material layer; (3) applying the second negative electrode material layer slurry on the surface of the negative electrode collector toward the winding center of the electrode assembly, and drying to obtain the negative electrode sheet coated with the first negative electrode material layer and the second negative electrode material layer; (4) cold pressing and slitting to obtain the negative electrode sheet.

[0046] The present application has no particular restrictions on the solid content of the above-mentioned slurry, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the solvent used in the above-mentioned slurry, as long as the purpose of the present application can be achieved. The present application has no particular restrictions 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 has no particular restrictions on the process parameters of the above-mentioned cold pressing and slitting, as long as the purpose of the present application can be achieved.

[0047] In the present application, the capacity per unit area C of the first negative electrode material layer is 1 The capacity per unit area of ​​the first negative electrode material layer can be controlled by the proportion of the first negative electrode active material in the first negative electrode material layer, the type of the first negative electrode active material, or the coating weight of the first negative electrode material layer. For example, by increasing the proportion of the first negative electrode active material in the first negative electrode material layer, the capacity per unit area of ​​the first negative electrode material layer increases at any time; by reducing the proportion of the first negative electrode active material in the first negative electrode material layer, the capacity per unit area of ​​the first negative electrode material layer increases at any time. In the present application, the capacity per unit area of ​​the second negative electrode material layer C 2 The control can be performed by the proportion of the second negative electrode active material in the second negative electrode material layer, the type of the second negative electrode active material or the coating weight of the second negative electrode material layer. For example, by increasing the proportion of the second negative electrode active material in the second negative electrode material layer, the unit area capacity of the second negative electrode material layer increases at any time; by reducing the proportion of the second negative electrode active material in the second negative electrode material layer, the unit area capacity of the second negative electrode material layer increases at any time. The mass percentage content W of silicon in the first negative electrode material layer 1The control can be performed by the type of the first silicon-based material and / or the mass percentage of the first silicon-based material in the first negative electrode material layer. For example, when the amount of the first silicon-based material added to the first negative electrode material layer is increased, that is, when the mass percentage of the first silicon-based material in the first negative electrode material layer is increased, the mass percentage of silicon in the first negative electrode material layer increases accordingly; when the amount of the first silicon-based material added to the first negative electrode material layer is reduced, that is, when the mass percentage of the first silicon-based material in the first negative electrode material layer is reduced, the mass percentage of silicon in the first negative electrode material layer decreases accordingly. The mass percentage of silicon in the second negative electrode material layer is W 2 The regulation can be performed by the type of the second silicon-based material and / or the mass percentage of the second silicon-based material in the second negative electrode material layer. For example, when the amount of the second silicon-based material added to the second negative electrode material layer is increased, that is, when the mass percentage of the second silicon-based material in the second negative electrode material layer is increased, the mass percentage of silicon in the second negative electrode material layer increases accordingly; when the amount of the second silicon-based material added to the second negative electrode material layer is reduced, that is, when the mass percentage of the second silicon-based material in the second negative electrode material layer is reduced, the mass percentage of silicon in the second negative electrode material layer decreases accordingly.

[0048] In the present application, there is no particular restriction on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or at least one of aramid. The type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. The diaphragm of the present application may have a porous structure, and the present application does not particularly limit the size of the pore size of the porous structure of the diaphragm, as long as the purpose of the present application can be achieved. For example, the size of the pore size may be 0.01 μm to 1 μm.

[0049] The cylindrical secondary battery in the present application includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may include LiPF 6 、LiNO 3 , LiBF 4 、LiClO 4 、LiB(C 6 H 5 ) 4 、LiCH 3 SO 3 、LiCF 3 SO 3 、LiN(SO 2 CF 3 ) 2 、LiC(SO 2 CF3 ) 3 , Li 2 SiF 6 , lithium bis(oxalatoborate) (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium difluoroborate. The present application does not limit the content of lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no special restrictions on non-aqueous solvents, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. The above-mentioned carbonate compounds may include but are not limited to at least one of chain carbonate compounds, cyclic carbonate compounds or fluorocarbonate compounds. The above-mentioned chain carbonate compounds may include but are not limited to at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or methyl ethyl carbonate. The above-mentioned cyclic carbonate may include but is not limited to at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate or vinyl ethylene carbonate. The fluorinated carbonate compound may include, but is not limited to, at least one of fluorinated ethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The above-mentioned carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, or caprolactone. The above-mentioned ether compound may include but is not limited to at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, cyclopentane, methyl cyclopentane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0050] The cylindrical secondary battery of the present application also includes a shell for accommodating a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, as well as other components known in the art in the cylindrical secondary battery, and the present application does not limit the above other components. The present application does not specifically limit the shell, and it can be a shell known in the art, as long as it can achieve the purpose of the present application.

[0051] The cylindrical secondary battery of the present application is not particularly limited, and may include any device that generates an electrochemical reaction. In one or more embodiments, the cylindrical secondary battery may 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.

[0052] The present application has no particular restrictions on the preparation method of the cylindrical secondary battery, and a preparation method known in the art can be selected as long as the purpose of the present application can be achieved. For example, the preparation method of the cylindrical secondary battery includes but is not limited to the following steps: stacking the diaphragm, the negative electrode sheet, the diaphragm and the positive electrode sheet in order, and winding and folding them as needed to obtain an electrode assembly with a winding structure, placing the electrode assembly in a shell, welding the collector plate, assembling the insulating sheet, and then injecting the electrolyte into the shell and sealing it to obtain a cylindrical secondary battery.

[0053] In the present application, the length L of the first negative electrode material layer in the first turn of the electrode assembly is 2 The curvature R of the second negative electrode material layer in the first turn of the electrode assembly can be adjusted; the length L of the first positive electrode material layer in the first turn of the electrode assembly 1 It can be controlled by adjusting the radius of the winding needle, the thickness of the diaphragm, the thickness of the negative electrode plate in the first turn of the electrode assembly, etc. The curvature R of the second negative electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the radius of the winding needle during winding.

[0054] The second aspect of the present application provides an electronic device, which includes the cylindrical secondary battery in any of the above embodiments. The cylindrical secondary battery of the present application has good cycle performance, so the electronic device of the present application has a long service life.

[0055] The electronic device of the present application is not particularly limited, and it can be used for any electronic device known in the prior art. For example, the electronic device can include but is not limited to a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0056] Example

[0057] The following examples and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0058] Test methods and equipment:

[0059] C 1 and C 2 The test:

[0060] At an ambient temperature of 25°C, the lithium-ion batteries in the embodiments and comparative examples were discharged at a constant current of 0.2C to 2.5V and then disassembled to obtain negative electrode sheets, wherein the material layer on the surface of the negative electrode collector in the negative electrode sheet away from the winding center of the electrode assembly is the first negative electrode material layer, and the material layer on the surface of the negative electrode collector in the negative electrode sheet facing the winding center of the electrode assembly is the second negative electrode material layer. The negative electrode sheet was washed with dimethyl carbonate (DMC) and dried to obtain a negative electrode sheet sample.

[0061] The negative electrode sample was punched and broken into 32 small discs with a radius of 22.14 mm (area of ​​1540.25 mm 2 ), weigh them one by one and take the average value, which is calculated as W 总 ; Randomly select 16 small discs, and use deionized water to erase the second negative electrode material layer in the small discs to obtain single-sided small discs with only the first negative electrode material layer;

[0062] The tested single-sided small disc with only the first negative electrode material layer is used as the working electrode, and the lithium sheet is used as the counter electrode. The diaphragm is placed between the tested negative electrode and the lithium sheet to play an isolating role. The electrolyte is injected and assembled in a glove box to obtain a button battery. The button battery is tested as follows: (1) 0.2mA constant current discharge to 0.02V, record the capacity; (2) stand for 10min; (3) 0.2mA DC charge to 2V; (4) stand for 20min; (4) Steps (1) to (3) are repeated 3 times to obtain the capacities C, C' and C", respectively. The average of the three is the capacity of the button battery. The capacity of 16 button batteries with only the single-sided small disc with the first negative electrode material layer as the working electrode is averaged to obtain the unit area capacity C of the first negative electrode material layer. 1 The diaphragm used in the test is the same as that in Example 1-1, and the electrolyte used is prepared by adding 0.8 mol / L lithium hexafluorophosphate to an organic solvent composed of 40% ethylene carbonate (EC) +, 30% dimethyl carbonate (DMC), 29.5% diethyl carbonate (DEC) and 0.5% fluoroethylene carbonate (FEC).

[0063] Use deionized water to wipe the first negative electrode material layer of the remaining 16 small discs to obtain a single-sided small disc with only the second negative electrode material layer. Replace the single-sided small disc with only the first negative electrode material layer with a single-sided small disc with only the second negative electrode material layer and follow the above test steps to obtain the unit area capacity C of the second negative electrode material layer. 2.

[0064] L 1 and L 2 The test:

[0065] At an ambient temperature of 25°C, the lithium-ion battery discharged to 2.5V at a constant current of 0.2C was disassembled, and the electrode assembly of the wound structure was taken out. The cross section of the electrode assembly of the wound structure was marked along the axial direction of the electrode assembly. The electrode sheet located in the innermost circle of the electrode assembly and the current collector is copper foil or the main element is copper is the negative electrode sheet, and the electrode sheet adjacent to the negative electrode sheet and away from the winding center of the electrode assembly is the positive electrode sheet. The material layer on the surface of the negative electrode current collector located in the negative electrode sheet away from the winding center of the electrode assembly is the first negative electrode material layer, and the material layer on the surface of the positive electrode current collector located in the positive electrode sheet facing the winding center of the electrode assembly is the first positive electrode material layer. The area where the first positive electrode material layer is provided on the positive electrode current collector in the first circle of the electrode assembly, and the area where the first negative electrode material layer is provided on the separator and the negative electrode current collector in the thickness direction of the electrode assembly begin to overlap is the starting end of the first circle of the electrode assembly, marked M, and the end of the first circle of the electrode assembly is wound around the starting end of the electrode assembly along the winding direction of the electrode assembly, marked N.

[0066] Disassemble the electrode assembly, take out the diaphragm, positive electrode sheet and negative electrode sheet, and measure the distance from M to N on the first positive electrode material layer in the positive electrode sheet along the length direction of the unfolded electrode sheet, which is the length L of the first positive electrode material layer in the first circle of the electrode assembly. 1 ; Measure the distance from M to N on the first negative electrode material layer in the negative electrode sheet, which is the length L of the first negative electrode material layer in the first circle of the electrode assembly 2 .

[0067] Test of curvature R:

[0068] At an ambient temperature of 25°C, a lithium-ion battery discharged to 2.5V at a constant current of 0.2C was disassembled, and the electrode assembly of the wound structure was taken out. Industrial computed tomography (industrial CT, Zeiss Xradia 620Versa) was used to perform a CT scan of the cross section of the electrode assembly of the wound structure along the axial direction of the electrode assembly. The electrode sheet located in the innermost circle of the electrode assembly is the negative electrode sheet, and the material layer on the surface of the negative electrode current collector located in the negative electrode sheet toward the winding center of the electrode assembly is the second negative electrode material layer. The diameter of the circle formed by the second negative electrode material layer in the first circle of the electrode assembly is measured, and the measurement is performed 10 times. The 10 measured values ​​are sorted from large to small, and the average of the first 5 values ​​is taken, which is the diameter d of the circle formed by the second negative electrode material layer in the first circle of the electrode assembly. For example, Figure 2 shown.

[0069] Then the curvature of the first circle of the second negative electrode material layer in the electrode assembly is R=2 / d.

[0070] W 1 and W 2 The test:

[0071] The lithium-ion batteries in the examples and comparative examples were discharged at a constant current of 0.1C to 2.5V and then disassembled to take out the negative electrode sheet, wherein 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 is the first negative electrode material layer, and the material layer on the surface of the negative electrode current collector in the negative electrode sheet facing the winding center of the electrode assembly is the second negative electrode material layer. The negative electrode sheet was washed with dimethyl carbonate (DMC) and then dried to obtain a negative electrode sheet sample.

[0072] The first negative electrode material layer on the negative electrode sheet was scraped off with a blade to obtain a powdered sample of the first negative electrode material layer. The sample was tested by inductively coupled plasma emission spectroscopy (ICP-AES) and in accordance with the "Chemical Analysis Method for Determination of Calcium, Aluminum and Silicon Content in Solid Surface Materials Products GB / T 39527-2020" to obtain the mass percentage content W of silicon in the first negative electrode material layer. 1 .

[0073] Use a blade to scrape off the second negative electrode material layer on the negative electrode sheet to obtain a powder sample of the second negative electrode material layer. Replace the powder sample of the first negative electrode material layer with the powder sample of the second negative electrode material layer and follow the above test steps to obtain the mass percentage content W of the silicon element in the second negative electrode material layer. 2 .

[0074] Test of the mass percentage of the first silicon-based material and the mass percentage of the second silicon-based material:

[0075] At an ambient temperature of 25°C, the lithium-ion batteries in the embodiments and comparative examples were discharged at a constant current of 0.2C to 2.5V and then disassembled to obtain negative electrode sheets, wherein the material layer on the surface of the negative electrode collector in the negative electrode sheet away from the winding center of the electrode assembly is the first negative electrode material layer, and the material layer on the surface of the negative electrode collector in the negative electrode sheet facing the winding center of the electrode assembly is the second negative electrode material layer. The negative electrode sheet was washed with dimethyl carbonate (DMC) and dried to obtain a negative electrode sheet sample.

[0076] Prepare the longitudinal section of the first negative electrode material layer (a plane formed by the length direction of the unfolded negative electrode sheet and its own thickness direction), observe the longitudinal section of the first negative electrode material layer using a scanning electron microscope, and then distinguish and count the silicon-based material and graphite particles in the longitudinal section of the first negative electrode material layer using the backscattering mode to obtain the proportion of silicon-based materials in the first negative electrode material layer a1 and the proportion of graphite particles in the first negative electrode material layer b1. Among them, because of the higher atomic number and higher electron density of silicon-based materials, and the stronger interaction between silicon-based materials and the test electron beam than the interaction between graphite particles and the test electron beam, the silicon-based material area is brighter, and the graphite particle area is darker. And use an X-ray energy spectrometer (EDS) to perform elemental analysis on the longitudinal section of the first negative electrode material layer, use an X-ray energy spectrometer (EDS) to distinguish carbon materials and silicon-based materials, and determine the proportion of silicon-based materials in the first negative electrode material layer a2 and the proportion of graphite particles in the first negative electrode material layer. Then the proportion of the silicon-based material in the first negative electrode material layer, that is, based on the mass of the first negative electrode material layer, the mass percentage of the first silicon-based material is a=(a1+a2) / 2.

[0077] The powdered sample of the first negative electrode material layer is replaced by the powdered sample of the second negative electrode material layer and the mass percentage b of the second silicon-based material based on the mass of the second negative electrode material layer is obtained according to the above test steps.

[0078] Cycle performance test:

[0079] The lithium ion batteries in the embodiment and the comparative example were subjected to a charge and discharge cycle test in a 25°C constant temperature box. The lithium ion batteries were charged to 4.2V at a constant current of 2C, and charged to 0.05V at a constant voltage of 4.2V. After standing for 5 minutes, they were discharged to 2.5V at a constant current of 6C. This was the first cycle, and the first cycle discharge capacity C was recorded. 1 After 600 cycles according to the above cycle process, the discharge capacity C of the lithium-ion battery is recorded. 600 , the cycle capacity retention rate of the 600th cycle is calculated as an indicator for evaluating the wetting effect of the negative electrode sheet and the cycle performance of the lithium-ion battery, and the formula is shown in formula (I). When the 600-cycle (cls) capacity retention rate is lower, it indicates that the wetting effect of the negative electrode sheet in the lithium-ion battery is worse, and the cycle performance of the lithium-ion battery is worse; when the 600cls capacity retention rate is higher, it indicates that the wetting effect of the negative electrode sheet in the lithium-ion battery is better, and the cycle performance of the lithium-ion battery is better.

[0080] 600cls capacity retention rate (%) = C 600 / C 1 ×100%. (I)

[0081] Lithium deposition performance test:

[0082] The lithium-ion battery in the embodiment and the comparative example was placed in a thermostat at 10°C. After 60 minutes, it was charged to 4.2V at 2C constant current, charged to 0.05C at 4.2V constant voltage, and left to stand for 5 minutes. Then, it was discharged to 2.5V at 0.5C constant current. This was one cycle. After 10 cycles of the above charge and discharge process, it was charged to 4.2V at 2C constant current, charged to 0.05C at 4.2V constant voltage, and left to stand for 5 minutes. The lithium-ion battery was disassembled to observe the lithium deposition state on the surface of the first negative electrode material layer of the negative electrode sheet. The surface area without lithium deposition was golden yellow, and the lithium deposition area was grayish white.

[0083] The criteria for judging the degree of lithium deposition in lithium-ion batteries are as follows: a lithium deposition area of ​​0% means no lithium deposition, i.e., the degree of lithium deposition is zero; a lithium deposition area greater than 0 and less than or equal to 2% means mild lithium deposition, i.e., the degree of lithium deposition is mild; a lithium deposition area greater than 2% and less than or equal to 20% means moderate lithium deposition, i.e., the degree of lithium deposition is moderate; a lithium deposition area greater than 20% and less than or equal to 100% means severe lithium deposition, i.e., the degree of lithium deposition is severe, wherein the percentage of the lithium deposition area is calculated based on the total area of ​​the first negative electrode material layer.

[0084] In this application, those skilled in the art will 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 fully discharges the capacity of the lithium-ion battery within 1 hour, "0.1C" is the current value that fully discharges the capacity of the lithium-ion battery within 10 hours, and other multiples are similar.

[0085] Example 1

[0086] <Preparation of negative electrode sheet>

[0087] The first negative electrode active material artificial graphite, the first silicon-based material SiO, the thickener sodium carboxymethyl cellulose (CMC-Na) and the negative electrode 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 to obtain a first negative electrode material layer slurry with a solid content of 50wt%; the first negative electrode material layer slurry is evenly coated on one surface of a negative electrode current collector copper foil with a current collector thickness of 10μm, and dried at 105°C to obtain a negative electrode sheet with a single-side coating of the first negative electrode material layer. The second negative electrode active material artificial graphite and the second silicon-based material SiO, the thickener sodium carboxymethyl cellulose (CMC-Na) and the negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 83.7:13.3:1.7:1.3, and then deionized water is added as a solvent, and the mixture is stirred and mixed evenly to obtain a second negative electrode material layer slurry with a solid content of 50wt%; the second negative electrode material layer slurry is coated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet coated with the first negative electrode material layer and the second negative electrode material layer. Then, after cold pressing and stripping, a negative electrode sheet with a specification of 67.45mm×1436mm is obtained. Among them, the coating weight of the first negative electrode material layer and the second negative electrode material layer is 110mg / 1540.25mm 2 After cold pressing, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer are both 45μm. Along the width direction of the unfolded negative electrode sheet, the width of the empty foil area is 5.45mm, and the width of the first negative electrode material layer and the second negative electrode material layer are both 62mm.

[0088] <Preparation of positive electrode sheet>

[0089] The first positive electrode active material is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), the first positive electrode binder polyvinylidene fluoride (PVDF) and the first positive electrode conductive agent conductive carbon black are dispersed in N-methylpyrrolidone (NMP) solvent in a mass ratio of 94.8:2.8:2.4, and are fully stirred and mixed to obtain a first positive electrode material layer slurry with a solid content of 72wt%. The first positive electrode material layer slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 13μm, and dried at 105°C to obtain a positive electrode sheet coated with the first positive electrode material layer on one side; then, the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. The second positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O 2), the second positive electrode binder polyvinylidene fluoride (PVDF) and the second positive electrode conductive agent conductive carbon black are dispersed in N-methylpyrrolidone (NMP) solvent in a mass ratio of 94.8:2.8:2.4, and are fully stirred and mixed to obtain a second positive electrode material layer slurry with a solid content of 72wt%. The second positive electrode material layer slurry is evenly coated on the other surface of the positive electrode current collector aluminum foil with a thickness of 13μm, and dried at 105°C to obtain a positive electrode sheet coated with the first positive electrode material layer and the second positive electrode material layer. Then after cold pressing and slitting, it is dried under vacuum conditions at 105°C for 4h to obtain a positive electrode sheet with a specification of 64.5mm×1422mm for standby use. Among them, the coating weight of the first positive electrode material layer and the second positive electrode material layer is 234mg / 1540.25mm 2 The compaction density of the first positive electrode material layer and the second positive electrode material layer is 3.4g / cm 3 The width of the first positive electrode material layer and the second positive electrode material layer is 60 mm, and the width of the empty foil area of ​​the positive electrode sheet is 4.5 mm.

[0090] <Diaphragm>

[0091] A polyethylene (PE) film with a thickness of 12 μm was used as the separator.

[0092] <Preparation of Electrolyte>

[0093] In a dry argon atmosphere glove box, organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 30:50:20 to obtain a base solvent, and then lithium salt lithium hexafluorophosphate (LiPF 6 ), and then mixed thoroughly to obtain an electrolyte. Based on the mass of the electrolyte, LiPF 6 The mass percentage is 12.5%, and the balance is the base solvent.

[0094] <Preparation of lithium-ion batteries>

[0095] The prepared diaphragm, negative electrode sheet, diaphragm, and positive electrode sheet are stacked in order, and pre-wound first to ensure that the diaphragm is between the negative electrode and the positive electrode, and at the same time ensure that the first negative electrode material layer is away from the center of the electrode assembly formed by pre-winding, and the first section of the first negative electrode material layer is located near the center of the electrode assembly formed by pre-winding. The winding adopts a winding needle with a radius of 1.6mm (to ensure the designed curvature), and then after flattening, current collecting plate welding, shelling, bottom penetration welding, coding, vacuum drying, electrolyte injection, sealing, high-temperature standing, and formation capacity, a lithium-ion battery can be obtained. Among them, the formation upper limit voltage is 3.6V, the formation temperature is 45°C, and after formation, it is left to stand at room temperature 25°C for 24h.

[0096] Example 2 to Example 19

[0097] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0098] Among them, when the mass percentage of the first silicon-based material changes, the mass percentage of the adhesive and the negative electrode binder in the first negative electrode material layer remains unchanged, and the mass percentage of the artificial graphite changes accordingly. When the mass percentage of the second silicon-based material changes, the mass percentage of the adhesive and the negative electrode binder in the second negative electrode material layer remains unchanged, and the mass percentage of the artificial graphite changes accordingly. When the curvature R of the first turn of the second negative electrode material layer in the electrode assembly changes, the winding needle radius during winding is adjusted so that the R value is as shown in Table 1.

[0099] In Examples 6, 8, 10, and 16, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing are both 30 μm; in Examples 7 and 9, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing are both 60 μm; in Example 17, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing are both 50 μm; in Example 18, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing are both 70 μm; in Example 19, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing are both 80 μm.

[0100] Comparative Example 1 to Comparative Example 3

[0101] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1. Among them, when the mass percentage of the first silicon-based material changes, the mass percentage of the binder and the negative electrode binder in the first negative electrode material layer remains unchanged, and the mass percentage of the artificial graphite changes accordingly. When the mass percentage of the second silicon-based material changes, the mass percentage of the binder and the negative electrode binder in the second negative electrode material layer remains unchanged, and the mass percentage of the artificial graphite changes accordingly.

[0102] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1.

[0103]

[0104]

[0105] It can be seen from Examples 1 to 19 and Comparative Examples 1 to 3 that 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, and coordinating the ratio of the capacity per unit area in the two negative electrode material layers, the values ​​of A and A / B are within the scope of this application, the lithium ion battery has a relatively light lithium deposition degree, and the 600cls capacity retention rate of the lithium ion battery is improved, indicating that the secondary battery of this application effectively reduces the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle, and the lithium ion battery has good cycle performance. In Comparative Examples 1 to 3, the value of A and / or the value of A / B are not within the scope of this application; the lithium ion battery in Comparative Examples 1 to 3 has a heavier lithium deposition degree; and / or, the 600cls capacity retention rate is lower. The lithium ion battery in Examples 1 to 19 has a relatively light lithium deposition degree, and the 600cls capacity retention rate is relatively high. The secondary battery of this application effectively reduces the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle, and the lithium ion battery has good cycle performance.

[0106] The value of B usually affects the cycle performance of lithium-ion batteries. It can be seen from Examples 1, 6 to 10 that when the value of B is within the range of this application, the lithium deposition of the lithium-ion battery is relatively mild, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium deposition due to insufficient CB in the inner ring of the electrode assembly in the late cycle of the secondary battery of this application is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0107] C 2 The value of usually affects the cycle performance of lithium-ion batteries. From Examples 1, 11 to 15, it can be seen that when C 2 When the value is within the range of this application, the degree of lithium plating of the lithium-ion battery is relatively light, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium plating due to insufficient CB in the inner circle of the electrode assembly in the secondary battery of this application in the late cycle is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0108] L 2 The value of usually affects the cycle performance of lithium-ion batteries. From Examples 1, 16 to 19, it can be seen that when L 2 When the value is within the range of this application, the degree of lithium plating of the lithium-ion battery is relatively light, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium plating due to insufficient CB in the inner circle of the electrode assembly in the secondary battery of this application in the late cycle is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0109] The value of R usually affects the cycle performance of lithium-ion batteries. It can be seen from Examples 1 and 16 to 19 that when the value of R is within the range of this application, the lithium deposition of the lithium-ion battery is relatively mild, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium deposition in the secondary battery of this application due to insufficient CB in the inner ring of the electrode assembly in the late cycle is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0110] The value of F usually affects the cycle performance of lithium-ion batteries. It can be seen from Examples 1 to 15 that when the value of F is within the range of this application, the lithium deposition of the lithium-ion battery is relatively mild, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium deposition due to insufficient CB in the inner ring of the electrode assembly in the late cycle of the secondary battery of this application is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0111] W 2 The value of usually affects the cycle performance of lithium-ion batteries. It can be seen from Examples 1, 8 to 15 that when W 2 When the value is within the range of this application, the degree of lithium plating of the lithium-ion battery is relatively light, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium plating due to insufficient CB in the inner circle of the electrode assembly in the secondary battery of this application in the late cycle is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0112] The types of the first silicon-based material and the second silicon-based material usually affect the cycle performance of the lithium-ion battery. It can be seen from Examples 1 to 19 that when the first silicon-based material and the second silicon-based material within the scope of the present application are selected, the lithium deposition of the lithium-ion battery is relatively light, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium deposition due to insufficient CB in the inner circle of the electrode assembly in the late cycle of the secondary battery of the present application is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0113] The mass percentage a of the first silicon-based material and the mass percentage b of the second silicon-based material usually affect the cycle performance of the lithium-ion battery. It can be seen from Examples 1 to 19 that when the values ​​of a and b are within the range of this application, the lithium deposition of the lithium-ion battery is relatively light, and the 600cls capacity retention rate is relatively high, indicating that the risk of lithium deposition in the secondary battery of this application due to insufficient CB in the inner ring of the electrode assembly in the late cycle is relatively low, and the cycle performance of the lithium-ion battery is relatively good.

[0114] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method or article including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method or article.

[0115] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0116] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A cylindrical secondary battery, comprising an electrode assembly, the electrode assembly comprising a positive electrode sheet, a negative electrode sheet and a separator, the negative electrode sheet comprising a negative electrode collector, a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being arranged on a surface of the negative electrode collector away from a winding center of the electrode assembly, and the second negative electrode material layer being arranged on a surface of the negative electrode collector toward a winding center of the electrode assembly; The positive electrode sheet comprises a positive electrode current collector and a first positive electrode material layer, wherein the first positive electrode material layer is arranged on a surface of the positive electrode current collector facing the winding center of the electrode assembly; The unit area capacity of the first negative electrode material layer is C1 mAh / 1540.25mm 2 The unit area capacity of the second negative electrode material layer is C2 mAh / 1540.25mm 2 , the ratio of C1 to C2 is A; along 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, 1.001≤A≤1.1, 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, wherein: 1.05≤B≤1.09。 4. The cylindrical secondary battery according to any one of claims 1 to 3, wherein: 40≤C2≤70, and / or, 8≤L2≤16.

5. The cylindrical secondary battery according to claim 4, wherein: 46≤C2≤55; and / or, 8≤L2≤12.

6. The cylindrical secondary battery according to any one of claims 1 to 5, wherein: The curvature of the first circle of the second negative electrode material layer in the electrode assembly is R, and 0.3≤R≤0.

9.

7. The cylindrical secondary battery according to any one of claims 1 to 6, wherein: The first negative electrode material layer includes a first active material, the second negative electrode material layer includes a second negative electrode active material, and the first negative electrode active material and the second negative electrode active material both contain silicon; Based on the mass of the first negative electrode material layer, the mass percentage of the silicon element in the first negative electrode material layer is W1, and based on the mass of the second negative electrode material layer, the mass percentage of the silicon element in the second negative electrode material layer is W2. The ratio of W1 to W2 is F, 1.001≤F≤1.

4.

8. The cylindrical secondary battery according to claim 7, wherein: 1.03≤F≤1.1; and / or, 0.5%≤W2≤20%.

9. The cylindrical secondary battery according to claim 7, wherein: The first negative electrode active material includes a first silicon-based material, and the second negative electrode active material includes a second silicon-based material. The first silicon-based material and the second silicon-based material are each independently selected from at least one of pure silicon, silicon-carbon, silicon-oxygen, silicon alloy or nano-silicon.

10. The cylindrical secondary battery according to claim 9, wherein: Based on the mass of the first negative electrode material layer, the mass percentage of the first silicon-based material is 1.2% to 32%; based on the mass of the second negative electrode material layer, the mass percentage of the second silicon-based material is 1% to 30%. 11 . An electronic device comprising the cylindrical secondary battery according to claim 1 .

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