Cylindrical secondary battery and electronic device

Abstract

The present application provides a cylindrical secondary battery and an electronic device. The cylindrical secondary battery comprises an electrode assembly. The electrode assembly comprises a positive electrode sheet, a negative electrode sheet, and a separator. The negative electrode sheet comprises a negative electrode current collector, a first negative electrode material layer, and a second negative electrode material layer. The positive electrode sheet comprises a positive electrode current collector and a first positive electrode material layer. The capacity per unit area of the first negative electrode material layer is C1 mAh / 1540.25 mm2, the capacity per unit area of the second negative electrode material layer is C2 mAh / 1540.25 mm2, and 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 of the first turn of the electrode assembly is L1 mm, the length of the first negative electrode material layer of the first turn of the electrode assembly is L2 mm, and the ratio of L1 to L2 is B, wherein 1.001≤A≤1.1, and 0.918≤A / B≤1.048. The present application is beneficial to improving the cycle performance of the cylindrical secondary battery.

Description

Cylindrical secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application No. 202411389223.1, filed on September 30, 2024, and entitled "Cylindrical secondary battery and electronic device", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the field of electrochemistry, in particular to a cylindrical secondary battery and an electronic device. BACKGROUND

[0003] Cylindrical secondary batteries, such as cylindrical lithium ion batteries, are applied in various high-rate discharge systems (such as discharge rate greater than 3C), have characteristics of high specific energy, high operating voltage, low self-discharge rate, small volume, light weight, etc., and have wide application in the field of consumer electronics.

[0004] Currently, the design of high-power cylindrical lithium ion batteries usually adopts full-tab design, i.e., the positive and negative tabs are stretched out from opposite directions, and full-tab flattening or rubbing technology is used for preparation. For cylindrical lithium ion batteries, the existing technology uses 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

[0005] The purpose of the present application is to provide a cylindrical secondary battery and an electronic device that can reduce the risk of lithium precipitation caused by insufficient CB value in the inner circle of the negative pole piece in the later stage of cycling, thereby improving the cycling performance of the secondary battery without affecting the production efficiency.

[0006] It should be noted that the lithium ion battery is used as an example of a cylindrical secondary battery in the summary of the present application, but the cylindrical secondary battery of the present application is not limited to a lithium ion battery. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a cylindrical secondary battery, which comprises an electrode assembly, the electrode assembly comprising a positive pole piece, a negative pole piece and a separator, the negative pole piece comprising a negative current collector, a first negative material layer and a second negative material layer, the first negative material layer being arranged on the surface of the negative current collector away from the winding center of the electrode assembly, and the second negative material layer being arranged on the surface of the negative current collector facing the winding center of the electrode assembly. The positive pole piece comprises a positive current collector and a first positive material layer, and the first positive material layer is arranged on the surface of the positive current collector facing the winding center of the electrode assembly. The unit area capacity of the first negative 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. In the winding direction of the electrode assembly, the length of the first positive electrode material layer in the first circle of the electrode assembly is L1 mm, the length of the first negative electrode material layer in the first circle of the electrode assembly is L2 mm, the ratio of L1 to L2 is B, 1.001≤A≤1.1, and optionally, 1.01≤A≤1.07; 0.918≤A / B≤1.048, and optionally, 0.93≤A / B≤1.02. By adjusting the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, the present application makes the spacing between layers in the inner circle of the electrode assembly moderate, the ion transport distance in the cycle process moderate, and facilitates the infiltration of electrolyte into the inner circle of the electrode assembly; in combination with the different unit area capacities of 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 caused by insufficient CB in the inner circle of the electrode assembly in the later cycle period, and thus the cylindrical secondary battery has good cycle performance.

[0008] In one or more embodiments, 1.05≤B≤1.09. By adjusting the value of B within the above range, the cycle performance of the cylindrical secondary battery is improved while reducing the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle period.

[0009] In one or more embodiments, 40≤C2≤70, and optionally, 46≤C2≤55; and / or, 8≤L2≤16, and optionally, 8≤L2≤12. By adjusting the value of C2 and / or the value of L2 within the above range, the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later cycle period is reduced, and the secondary battery has good cycle performance while taking into account high energy density and good mechanical safety performance.

[0010] In one or more embodiments, the curvature of the first circle of the second negative electrode material layer in the electrode assembly is R, and 0.3≤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.

[0011] In one or more embodiments, the first negative material layer comprises a first active material, the second negative material layer comprises a second negative active material, the first negative active material and the second negative active material both contain silicon elements; the mass percentage of the silicon elements in the first negative material layer is W1 based on the mass of the first negative material layer, the mass percentage of the silicon elements in the second negative material layer is W2 based on the mass of the second negative material layer, the ratio of W1 to W2 is F, 1.001≤F≤1.4; optionally, 1.03≤F≤1.1. By regulating the value of F within the above range, the risk of lithium precipitation due to insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle is reduced, while the mechanical safety performance and the cycle performance of the secondary battery are taken into account, and the energy density of the secondary battery is further improved.

[0012] In one or more embodiments, 0.5%≤W2≤20%. By regulating the value of W2 within the above range, the energy density and the cycle performance of the secondary battery are taken into account while the risk of lithium precipitation due to insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle is reduced.

[0013] In one or more embodiments, the first negative active material comprises a first silicon-based material, the second negative active material comprises 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 first silicon-based material and the second silicon-based material from the above categories, the secondary battery has a high energy density and good cycle performance.

[0014] In one or more embodiments, the mass percentage of the first silicon-based material is 1.2% to 32% based on the mass of the first negative material layer, and the mass percentage of the second silicon-based material is 1% to 30% based on the mass of the second negative material layer. 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 high energy density and good cycle performance.

[0015] The second aspect of the present application provides an electronic device comprising the cylindrical secondary battery in any of the foregoing embodiments. The cylindrical secondary battery of the present application has good cycle performance, and therefore, the electronic device of the present application has a long service life.

[0016] The beneficial effects of the embodiments of the present application are as follows:

[0017] The embodiment of the present application provides a cylindrical secondary battery and an electronic device, by regulating 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 transmission distance of ions in the cycle process is moderate, and the infiltration of the electrolyte to the inner circle of the electrode assembly is facilitated; in combination with the different unit area capacity of 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 caused by the insufficient CB of the inner circle of the electrode assembly in the later cycle, so that the cylindrical secondary battery has good cycle performance. In addition, the unit area capacity of different negative electrode material layers is different, the secondary battery has 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 also considered on the basis of not reducing the production efficiency.

[0018] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0020] Fig. 1 is a schematic diagram of the winding structure formed by the electrode assembly in an embodiment of the present application;

[0021] Fig. 2 is a computerized tomography (CT) diagram of the electrode assembly in another embodiment of the present application.

[0022] Reference signs: electrode assembly 001; positive electrode sheet 10; positive electrode current collector 11; first positive electrode material layer 12; second positive electrode material layer 13; negative electrode sheet 20; negative electrode current collector 21; first negative electrode material layer 22; second negative electrode material layer 23; separator 30. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all embodiments. All other embodiments obtained by those skilled in the art based on the present application are within the scope of protection of the present application.

[0024] It should be noted that in the specific embodiments of the present application, lithium ion batteries are used as examples of cylindrical secondary batteries to explain the present application, but the cylindrical secondary batteries of the present application are not limited to lithium ion batteries.

[0025] For high-power cylindrical secondary batteries, generally full-tab cylindrical lithium-ion batteries, at the later stage of the cycle, the negative electrode tab surface away from the winding center of the electrode assembly, especially near the winding center of the electrode assembly, the electrolyte infiltration effect is poor, the negative electrode tab is poorly infiltrated; further, the curvature of the cylindrical electrode assembly gradually increases as the number of winding decreases, further, for the same electrode assembly, the length of the negative electrode tab surface away from the winding center of the electrode assembly is slightly smaller than that of the positive electrode tab surface opposite to it toward the winding center of the electrode assembly, and from the winding center of the electrode assembly, the length difference of 1-5 circles of the electrode assembly is more significant, resulting in insufficient CB in the inner circle of the electrode assembly at the later stage of the cycle, causing lithium precipitation phenomenon, based on the above reasons, the cycle performance of the lithium-ion battery decreases. The prior art uses grooving, improving the performance of the electrolyte, etc. to improve the above problems, but the improvement effect of a single technical element on the secondary battery is limited, and due to the loss of active material in the tab, the secondary battery cannot balance high energy density and good cycle performance. Therefore, the present application provides a cylindrical secondary battery which can effectively reduce the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly at the later stage of the cycle without affecting the actual production efficiency, and the secondary battery has good cycle performance.

[0026] The specific technical solutions are as follows:

[0027] The first aspect of the present application provides a cylindrical secondary battery, the cylindrical secondary battery comprising an electrode assembly, the electrode assembly comprising a positive electrode tab, a negative electrode tab and a separator, the negative electrode tab comprising a negative electrode current collector, a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being arranged on the surface of the negative electrode current collector away from the winding center of the electrode assembly, and the second negative electrode material layer being arranged on the surface of the negative electrode current collector toward the winding center of the electrode assembly. The positive electrode tab comprises a positive electrode current collector and a first positive electrode material layer, and the first positive electrode material layer is arranged on the surface of the positive electrode current collector toward the winding center of the electrode assembly. The unit area capacity of the first negative electrode material layer is C1 mAh / 1540.25mm 2 , and 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. In the winding direction of the electrode assembly, the length of the first positive electrode material layer in the first coil of the electrode assembly is L1 mm, the length of the first negative electrode material layer in the first coil of the electrode assembly is L2 mm, the ratio of L1 to L2 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 between any two of them. 0.918≤A / B≤1.048, optionally, 0.93≤A / B≤1.02, for example, the value of A / B can be 0.918, 0.92, 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 between any two of them.

[0028] In this application, the winding direction of the electrode assembly is defined as the W direction. As shown in FIG. 1, the electrode assembly 001 includes a positive electrode tab 10, a negative electrode tab 20, and a separator 30. The negative electrode tab 20 includes a negative current collector 21, a first negative electrode material layer 22, and a second negative electrode material layer 23. The first negative electrode material layer 22 is disposed on the surface of the negative current collector 21 away from the winding center of the electrode assembly 001. The second negative electrode material layer 23 is disposed on the surface of the negative current collector 21 toward the winding center of the electrode assembly 001. The positive electrode tab 10 includes a positive current collector 11 and a first positive electrode material layer 12. The first positive electrode material layer 12 is disposed on the surface of the positive current collector 11 toward the winding center of the electrode assembly 001.

[0029] When the value of A is too small, i.e. less than the lower limit value of the present application, the ratio of the capacities per unit area of the first negative electrode material layer and the second negative electrode material layer is too small, which cannot effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the negative electrode jellyroll in the later stage of the cycle. When the value of A is too large, i.e. greater than the upper limit value of the present application, the amount of active material supplement in the negative electrode jellyroll coating is too much, the excess active material increases the cost in the actual production process, and causes loss of energy density and reduction of the kinetic performance of the secondary battery. When the value of A / B is too small, i.e. less than the lower limit value of the present application, the amount of active material supplement for the second negative electrode material layer in the negative electrode jellyroll is insufficient, which cannot effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the negative electrode jellyroll in the later stage of the cycle. When the value of A / B is too large, i.e. greater than the upper limit value of the present application, the amount of active material supplement for the second negative electrode material layer in the negative electrode jellyroll is too much, the excess active material increases the cost in the actual production process, and causes loss of energy density and reduction of the kinetic performance of the secondary battery. The present application adjusts the length ratio of the first positive electrode material layer and the first negative electrode material layer in the inner circle of the electrode assembly, so that the spacing between the layers in the inner circle of the electrode assembly is moderate, the transmission distance of ions in the cycle process is moderate, and the impregnation of the electrolyte into the inner circle of the electrode assembly is facilitated; in combination with the different capacities per unit area of the two negative electrode material layers, the first negative electrode material layer has a larger capacity per unit area, which can effectively reduce the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, so that the cylindrical secondary battery has good cycle performance. In addition, the capacities per unit area of the different negative electrode material layers are different, the secondary battery has high energy density without significantly increasing the actual production cost, and the production operation steps of the secondary battery in the actual process are simple, which does not reduce the production efficiency and also considers the processing performance.

[0030] In the present application, the value of CB refers to the ratio between the capacity per unit area of the negative electrode jellyroll and the capacity per unit area of the positive electrode jellyroll under the same conditions, such as at an ambient temperature of 25°C and a 0.1C discharge rate. CB = (gram capacity of negative electrode active material x mass of negative electrode active material per unit area of negative electrode jellyroll) / (gram capacity of positive electrode active material x mass of positive electrode active material per unit area of positive electrode jellyroll). The above unit area refers to 1 mm 2It 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 region provided with the first positive electrode material layer on the positive electrode current collector, the separator and the region provided with the first negative electrode material layer on the negative electrode current collector begin to overlap in the thickness direction of the electrode assembly. The first circle of the electrode assembly refers to that the electrode assembly is wound one circle from the starting end of the electrode assembly (i.e. the starting point) to the other end surface of the electrode assembly (i.e. the ending point) along the winding direction of the electrode assembly, and the starting point and the ending point are flush in the thickness direction of the electrode assembly. Then, the length of the first positive electrode material layer in the first circle of the electrode assembly refers to the length of the first positive electrode material layer in the electrode assembly from the starting point of the electrode assembly to the ending point of the electrode assembly along the winding direction of the electrode assembly; the length of the first negative electrode material layer in the first circle of the electrode assembly refers to the length of the first negative electrode material layer in the electrode assembly from the starting point of the electrode assembly to the ending point of the electrode assembly along the winding direction of the electrode assembly. In addition, it should be noted that the "surface" in the present application can be the entire region of the surface of the positive electrode current collector and / or the negative electrode current collector, or can be part of the surface of the positive electrode current collector and / or the negative electrode current collector, and the present application does not have a special limitation as long as the purpose of the present application can be achieved.

[0031] 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 between any two of them. By adjusting the value of B within the above range, the spacing between the layers in the inner circle of the electrode assembly is moderate, the transmission distance of ions during the cycle process is moderate, and the electrolyte is beneficial to the infiltration of the inner circle of the electrode assembly without increasing the manufacturing cost of the secondary battery, and the actual production value of the secondary battery is higher. While reducing the risk of lithium precipitation caused by insufficient CB in the inner circle of the electrode assembly at the later stage of the cycle, the cycle performance of the cylindrical secondary battery is improved.

[0032] In one or more embodiments, 40≤C2≤70, optionally, 46≤C2≤55, for example, the value of C2may 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 formed by any two of them. By regulating the value of C2in the above range, the content of the negative active material in the second negative material layer in the negative pole piece is moderate, which is conducive to actual production, reduces the risk of metal exposure or scratches on the surface of the negative pole piece in the processing process, causing local lithium precipitation short circuit of the pole piece in the charging and discharging process, at the same time, the energy density of the secondary battery is also taken into account, and the risk of the secondary battery cycle performance decline caused by the excessive active material leading to the intensification of the side reaction between the active material and the electrolyte is also reduced, which is conducive to reducing the risk of lithium precipitation caused by the insufficient CB in the inner circle of the electrode assembly in the later cycle stage, thereby improving the cycle performance of the secondary battery.

[0033] In one or more embodiments, 8≤L2≤16, optionally, 8≤L2≤12, for example, the value of L2may 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 formed by any two of them. By regulating the value of L2in the above range, the length of the first circle negative pole piece of the electrode assembly is moderate, which is conducive to reducing the risk of fracture caused by excessive curvature of the first circle negative pole piece in the charging and discharging process, at the same time, the size of the center hole of the winding center of the electrode assembly is moderate, the secondary battery has a high energy density, which is conducive to reducing the risk of lithium precipitation caused by the insufficient CB in the inner circle of the electrode assembly in the later cycle stage, while taking into account the high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0034] In one or more embodiments, 40≤C2≤70, optionally, 46≤C2≤55; and, 8≤L2≤16, optionally, 8≤L2≤12. For example, the value of C2 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 between any two of them; the value of L2 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 between any two of them. By regulating the value of C2 and the value of L2 within the above range, the content of active material in the second negative material layer of the negative tab and the length of the first negative tab of the electrode assembly are moderate, which is conducive to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, while taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0035] In one or more embodiments, 40.04≤C1≤74.9, for example, the value of C1 can be 40.04, 40.1, 42, 45, 50, 52, 55, 58, 60, 62, 65, 68, 70, 72, 74.9, or a range between any two of them. By regulating the value of C1 within the above range, the content of active material in the first negative material layer of the negative tab is moderate, which is conducive to actual production, reduces the risk of local lithium precipitation and short circuit of the tab during charging and discharging due to metal exposure or scratches on the surface of the negative tab during processing, at the same time, the energy density of the secondary battery is also taken into account, and the risk of cycle performance degradation of the secondary battery due to the intensification of side reactions between active material and electrolyte caused by excessive active material is reduced, which is conducive to reducing the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later stage of the cycle, thereby improving the cycle performance of the secondary battery.

[0036] In one or more embodiments, 7.6412≤L1≤19.1721, for example, the value of L1may 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 defined by any two of them. By regulating the value of L1in the above range, the length of the first positive electrode sheet of the electrode assembly is moderate, effectively reducing the risk of fracture caused by excessive curvature of the first positive electrode sheet during charging and discharging. At the same time, the size of the central hole of the winding center of the electrode assembly is moderate, and the secondary battery has a high energy density, which is beneficial to reduce the risk of lithium precipitation caused by insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle. While taking into account high energy density and good mechanical safety performance, the secondary battery has good cycle performance.

[0037] In one or more embodiments, the curvature of the first circle of the second negative electrode material layer in the electrode assembly is R, and 0.3≤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 defined by any two of them. By regulating the value of R in the above range, it is beneficial to regulate the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first circle of the electrode assembly, thereby reducing the risk of lithium precipitation caused by insufficient CB of the inner circle of the electrode assembly in the later stage of the cycle. While taking into account the actual production difficulty, at the same time, the size of the central hole of the winding center of the electrode assembly is moderate, and the secondary battery has a high energy density. Therefore, while taking into account high energy density and good processing performance, the secondary battery has good cycle performance.

[0038] In one or more embodiments, the first negative material layer comprises a first active material, the second negative material layer comprises a second negative active material, the first negative active material and the second negative active material both contain silicon element; the mass percentage content of the silicon element in the first negative material layer is W1 based on the mass of the first negative material layer, the mass percentage content of the silicon element in the second negative material layer is W2 based on the mass of the second negative material layer, the ratio of W1 to W2 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 between any two of them. By adjusting the value of F within the above range, the ratio of the unit area capacity of the first negative material layer to the second negative material layer is adjusted, and the volume expansion of the different negative material layers in the negative electrode sheet during the cycle of the secondary battery is more uniform, which is beneficial to reduce the risk of the fracture of the electrode sheet caused by the uneven expansion of the electrode sheet, which leads to the puncture of the separator and short circuit, and when the negative material layer contains silicon element, the theoretical specific capacity of silicon is high, which reduces the risk of lithium precipitation caused by insufficient inner ring CB of the electrode assembly in the later cycle stage, while taking into account the mechanical safety performance and cycle performance of the secondary battery, and further improves the energy density of the secondary battery.

[0039] In one or more embodiments, 0.5%≤W2≤20%. For example, the value of W2 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 between any two of them. By adjusting the value of W2 within the above range, the volume expansion of the negative electrode sheet during the cycle is moderate, which is beneficial to adjust the unit area capacity of the second negative material layer, while taking into account the energy density and cycle performance of the secondary battery, which reduces the risk of lithium precipitation caused by insufficient inner ring CB of the electrode assembly in the later cycle stage.

[0040] In one or more embodiments, 0.5005%≤ W1≤ 28%. For example, the value of W2 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 between any two of them. By regulating the value of W1 within the above range, the volume expansion of the negative electrode tab during the cycle process is moderate, which is conducive to adjusting the capacity per unit area of the first negative electrode material layer, and at the same time, the risk of lithium precipitation due to insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle is reduced, and the energy density and cycle performance of the secondary battery are considered.

[0041] In one or more embodiments, the first negative active material comprises a first silicon-based material, and the second negative active material comprises 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 first silicon-based material and the second silicon-based material of the above types, it is conducive to regulating the capacity per unit area of the first negative electrode material layer and the second negative electrode material layer, and reducing the risk of lithium precipitation due to insufficient CB in the inner ring of the electrode assembly in the later stage of the cycle, and the secondary battery has a higher energy density and good cycle performance.

[0042] In one or more embodiments, the mass percentage of the first silicon-based material is 1.2% to 32% based on the mass of the first negative electrode material layer, 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 a range between any two of them; the mass percentage of the second silicon-based material is 1% to 30% based on the mass of the second negative electrode material layer, 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 between any two of them. By adjusting the mass percentage of the first silicon-based material and the mass percentage of the second silicon-based material within the above range, the unit area capacity of the first negative electrode material layer and the second negative electrode material layer can be adjusted, the volume expansion of the negative electrode sheet during the cycle process is moderate, which is beneficial to reduce the risk of lithium precipitation caused by insufficient CB in the inner ring of the electrode assembly in the later cycle, and the secondary battery has higher energy density and good cycle performance.

[0043] 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 above first negative electrode active material and second negative electrode active material, the structural stability of the negative electrode sheet can be improved, so that the volume expansion of the negative electrode sheet during the cycle process is moderate.

[0044] The negative current collector is not particularly limited as long as the object of the present application can be achieved. For example, the negative current collector can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (e.g., a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.), etc. The thickness of the negative current collector is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the negative current collector can be 3 µm to 20 µm. The first negative material layer and the second negative material layer can further include a conductive agent and a negative binder. The kind of the conductive agent in the first negative material layer and the second negative material layer is not particularly limited as long as the object of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, a metal material, or a conductive polymer. The carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCFs) and / or nanocarbon fibers. The metal material can include, but is not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The conductive polymer can include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The kind of the negative binder in the first negative material layer and the second negative material layer is not particularly limited as long as the object of the present application can be achieved. For example, the negative binder can include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, a polyacrylic acid salt, polyvinylpyrrolidone, a polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The mass ratio of the negative active material, the conductive agent, and the negative binder in the first negative material layer and the second negative material layer is not particularly limited as long as the object of the present application can be achieved. In the present application, the first negative material layer and the second negative material layer further include a thickening agent. The kind of the thickening agent is not particularly limited as long as the object of the present application can be achieved. For example, the thickening agent can include, but is not limited to, at least one of carboxymethyl cellulose or sodium carboxymethyl cellulose.

[0045] In the present application, the positive electrode tab can further include a second positive electrode material layer disposed on the surface of the positive current collector facing away from the winding center of the electrode assembly. For example, as shown in FIG. 1, the positive electrode tab 10 can further include a second positive electrode material layer 13 disposed on the surface of the positive current collector 11 facing away from the winding center of the electrode assembly 001. The positive current collector in the present application is not particularly limited as long as the purpose of the present application can be achieved. For example, the positive current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector), etc. The first positive electrode material layer in 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 types of the first positive electrode active material and the second positive electrode active material in the present application are not particularly limited 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 each independently be selected from at least one of lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2(NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), 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 can further include a non-metallic element, for example, the non-metallic element includes at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In the present application, the thickness of the positive current collector is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the positive current collector is 5 μm to 20 μm. In the present application, the first positive electrode material layer and the second positive electrode material layer can further include a positive electrode binder and a conductive agent. The type of the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer in the present application is not particularly limited as long as the purpose of the present application can be achieved. The type of the positive electrode conductive agent in the first positive electrode material layer and the second positive electrode material layer in the present application is not particularly limited as long as the purpose of the present application can be achieved. For example, the conductive agent can be the same as the type of the conductive agent in the first negative electrode material layer and the second negative electrode material layer described above. The type of the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer in the present application is not particularly limited as long as the purpose of the present application can be achieved. For example, the positive electrode binder can be the same as the type of the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer described above. 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 in the present application is not particularly limited, and a person skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0046] The preparation method of the negative electrode sheet is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the negative electrode sheet includes but is not limited to the following steps: (1) preparing a first negative electrode material layer slurry, and preparing a second negative electrode material layer slurry; (2) pre-determining the surface of the negative current collector away from the winding center of the electrode assembly and the starting position of the negative current collector for winding, coating the first negative electrode material layer slurry on the surface of the negative current collector away from the winding center of the electrode assembly, and drying to obtain a negative electrode sheet coated with the first negative electrode material layer; (3) coating the second negative electrode material layer slurry on the surface of the negative current collector facing the winding center of the electrode assembly, and drying to obtain a negative electrode sheet coated with the first negative electrode material layer and the second negative electrode material layer; (4) after cold pressing and slitting, the negative electrode sheet is obtained.

[0047] The solid content of the above-mentioned slurry is not particularly limited in the present application, as long as the purpose of the present application can be achieved. The solvent used in the above-mentioned slurry is not particularly limited in the present application, as long as the purpose of the present application can be achieved. The temperature and time of the above-mentioned drying are not particularly limited in the present application, as long as the purpose of the present application can be achieved. The process parameters of the above-mentioned cold pressing and slitting are not particularly limited in the present application, as long as the purpose of the present application can be achieved.

[0048] In the present application, the unit area capacity C1 of the first negative electrode material layer can be regulated 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, increasing the proportion of the first negative electrode active material in the first negative electrode material layer, the unit area capacity of the first negative electrode material layer increases accordingly; decreasing the proportion of the first negative electrode active material in the first negative electrode material layer, the unit area capacity of the first negative electrode material layer decreases accordingly. In the present application, the unit area capacity C2 of the second negative electrode material layer can be regulated 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, 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 accordingly; decreasing 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 decreases accordingly. The mass percentage content W1 of silicon element in the first negative electrode material layer can be regulated by the type of the first silicon-based material in the first negative electrode material layer and / or the mass percentage content of the first silicon-based material. For example, increasing the amount of the first silicon-based material added in the first negative electrode material layer, i.e. increasing the mass percentage content of the first silicon-based material in the first negative electrode material layer, the mass percentage content of silicon element in the first negative electrode material layer increases accordingly; decreasing the amount of the first silicon-based material added in the first negative electrode material layer, i.e. decreasing the mass percentage content of the first silicon-based material in the first negative electrode material layer, the mass percentage content of silicon element in the first negative electrode material layer decreases accordingly. The mass percentage content W2 of silicon element in the second negative electrode material layer can be regulated by the type of the second silicon-based material in the second negative electrode material layer and / or the mass percentage content of the second silicon-based material. For example, increasing the amount of the second silicon-based material added in the second negative electrode material layer, i.e. increasing the mass percentage content of the second silicon-based material in the second negative electrode material layer, the mass percentage content of silicon element in the second negative electrode material layer increases accordingly; decreasing the amount of the second silicon-based material added in the second negative electrode material layer, i.e. decreasing the mass percentage content of the second silicon-based material in the second negative electrode material layer, the mass percentage content of silicon element in the second negative electrode material layer decreases accordingly.

[0049] In the present application, the separator is not particularly limited as long as the purpose of the present application can be achieved, for example, the material of the separator can include but is not limited to at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator can include at least one of woven film, non-woven film, microporous film, composite film, calendered film or spunlaced film. The separator of the present application can have a porous structure, and the present application does not particularly limit the size of the pore size of the porous structure of the separator as long as the purpose of the present application can be achieved. For example, the size of the pore size can be 0.01-1 μm.

[0050] The cylindrical secondary battery in the present application includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt can include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluoroborate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application is achieved. The present application does not particularly limit the non-aqueous solvent, as long as the purpose of the present application is achieved. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic ester compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, or methyl ethyl carbonate. The cyclic carbonate can include, but is not limited to, at least one of vinylene carbonate, propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated vinylene carbonate, carbonic acid-1,2-difluoroethylene ester, carbonic acid-1,1-difluoroethylene ester, carbonic acid-1,1,2-trifluoroethylene ester, carbonic acid-1,1,2,2-tetrafluoroethylene ester, carbonic acid-1-fluoro-2-methyl ethylene ester, carbonic acid-1-fluoro-1-methyl ethylene ester, carbonic acid-1,2-difluoro-1-methyl ethylene ester, carbonic acid-1,1,2-trifluoro-2-methyl ethylene ester, or carbonic acid trifluoromethyl ethylene ester. The carboxylic ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0051] The cylindrical secondary battery of the present application further comprises a case for accommodating the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte solution, and other components known in the art in the cylindrical secondary battery, which are not limited by the present application. The case is not particularly limited by the present application, and can be any case known in the art as long as the purpose of the present application can be achieved.

[0052] The cylindrical secondary battery of the present application is not particularly limited, and can include any device that undergoes electrochemical reactions. In one or more embodiments, the cylindrical secondary battery can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0053] The method for preparing the cylindrical secondary battery of the present application is not particularly limited, and can be any method known in the art as long as the purpose of the present application can be achieved. For example, the method for preparing the cylindrical secondary battery includes, but is not limited to, the following steps: stacking the separator, the negative electrode sheet, the separator, and the positive electrode sheet in order, and performing operations such as winding, folding, etc. as needed to obtain an electrode assembly in a wound structure, placing the electrode assembly into the case, welding the current collector plate, assembling the insulating sheet, and then injecting the electrolyte solution into the case and sealing it to obtain the cylindrical secondary battery.

[0054] In the present application, the length L2 of the first negative electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the curvature R of the second negative electrode material layer in the first turn of the electrode assembly during winding; the length L1 of the first positive electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the radius of the winding needle, the thickness of the separator, the thickness of the negative electrode sheet in the first turn of the electrode assembly, etc. during winding, and the curvature R of the second negative electrode material layer in the first turn of the electrode assembly can be controlled by adjusting the radius of the winding needle during winding.

[0055] The second aspect of the present application provides an electronic device comprising the cylindrical secondary battery of any of the preceding embodiments. The cylindrical secondary battery of the present application has good cycle performance, and therefore the electronic device of the present application has a longer service life.

[0056] The electronic device of the present application is not particularly limited, and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0057] Examples

[0058] Hereinafter, the embodiments of the present application will be described more specifically by citing examples and comparative examples. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0059] Test methods and equipment:

[0060] Test of C1 and C2:

[0061] The lithium ion battery in each example and comparative example was disassembled after constant current discharge at 0.2C to 2.5V at an ambient temperature of 25°C, and a negative electrode sheet was obtained. The material layer on the surface of the negative electrode current collector in the negative electrode sheet away from the center of the electrode assembly winding was a first negative electrode material layer, and the material layer on the surface of the negative electrode current collector in the negative electrode sheet toward the center of the electrode assembly winding was a second negative electrode material layer. The negative electrode sheet was washed with dimethyl carbonate (DMC) and dried to obtain a negative electrode sheet sample.

[0062] The negative electrode sheet sample was punched into 32 small discs with a radius of 22.14 mm (an area of 1540.25 mm 2 ), and the average value was calculated as W 总 ; 16 small discs were randomly selected, and the second negative electrode material layer in the small discs was removed with deionized water to obtain single-sided small discs with only the first negative electrode material layer;

[0063] The single-sided small discs with only the first negative electrode material layer to be tested were used as the working electrode, lithium was used as the counter electrode, and the separator was placed between the tested negative electrode sheet and the lithium to act as a barrier. Electrolyte was injected to assemble a button cell in a glove box. The button cell was tested as follows: (1) constant current discharge at 0.2mA to 0.02V, and the capacity was recorded; (2) stand for 10min; (3) 0.2mA direct current charging to 2V; (4) stand for 20min; (4) repeat steps (1) to (3) three times to obtain capacities C, C' and C", respectively, and the average value of the three is the capacity of the button cell. The capacities of the 16 button cells with only the first negative electrode material layer single-sided small discs as the working electrode were averaged to obtain the unit area capacity C1 of the first negative electrode material layer. In the test, the separator used was the same as in Example 1-1, and the electrolyte was prepared by adding 0.8mol / 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).

[0064] The first negative material layer in the remaining 16 small discs was wiped off with deionized water to obtain single-sided small discs with only the second negative material layer. The single-sided small discs with only the first negative material layer were replaced with the single-sided small discs with only the second negative material layer, and the unit area capacity C2 of the second negative material layer could be obtained according to the above test steps.

[0065] Test of L1 and L2:

[0066] At an ambient temperature of 25°C, a lithium ion battery discharged at 0.2C constant current to 2.5V was disassembled, and the electrode assembly in a wound structure was taken out. The cross section of the electrode assembly in the wound structure was marked along the axial direction of the electrode assembly. The electrode tab located in the innermost circle of the electrode assembly and having a copper foil as the current collector or mainly containing copper was the negative electrode tab. The electrode tab adjacent to the negative electrode tab and away from the winding center of the electrode assembly was the positive electrode tab. The material layer on the surface of the negative current collector in the negative electrode tab away from the winding center of the electrode assembly was the first negative material layer. The material layer on the surface of the positive current collector in the positive electrode tab toward the winding center of the electrode assembly was the first positive material layer. The position where the region provided with the first positive material layer on the positive current collector, the separator, and the region provided with the first negative material layer on the negative current collector in the first circle of the electrode assembly began to overlap in the thickness direction of the electrode assembly was the starting end of the first circle of the electrode assembly, which was marked as M. The end of the first circle of the electrode assembly was one winding in the winding direction of the electrode assembly from the starting end of the electrode assembly, which was marked as N.

[0067] The electrode assembly was disassembled, and the separator, the positive electrode tab, and the negative electrode tab were taken out. The distance from M to N on the first positive material layer in the positive electrode tab was measured along the length direction of the unwound electrode tab, which was the length L1 of the first positive material layer in the first circle of the electrode assembly. The distance from M to N on the first negative material layer in the negative electrode tab was measured, which was the length L2 of the first negative material layer in the first circle of the electrode assembly.

[0068] Test of curvature R:

[0069] At an ambient temperature of 25°C, a lithium ion battery discharged at 0.2C constant current to 2.5V was disassembled, and the electrode assembly in a wound structure was taken out. The cross section of the electrode assembly in the wound structure was CT scanned and photographed along the axial direction of the electrode assembly using an industrial computed tomography (industrial CT, Zeiss Xradia 620 Versa). The electrode tab located in the innermost circle of the electrode assembly was the negative electrode tab. The material layer on the surface of the negative current collector in the negative electrode tab toward the winding center of the electrode assembly was the second negative material layer. The diameter of the formed circle of the first circle of the second negative material layer in the electrode assembly was measured 10 times. The 10 measurement values were sorted from large to small, and the average of the first 5 values was taken as the diameter d of the formed circle of the first circle of the second negative material layer in the electrode assembly.

[0070] Then the curvature R of the first circle of the second negative electrode material layer in the electrode assembly is 2 / d.

[0071] Test of W1 and W2:

[0072] The lithium ion batteries in the examples and the comparative examples were disassembled after constant current discharge at 0.1C to 2.5V, and the negative electrode sheets were taken out. The material layer on the surface of the negative electrode current collector in the negative electrode sheet away from the center of the electrode assembly winding was the first negative electrode material layer, and the material layer on the surface of the negative electrode current collector in the negative electrode sheet toward the center of the electrode assembly winding was the second negative electrode material layer. After the negative electrode sheet was cleaned with dimethyl carbonate (DMC) and dried, a negative electrode sheet sample was obtained.

[0073] The first negative electrode material layer on the negative electrode sheet was scraped off with a blade to obtain a powdery sample of the first negative electrode material layer. Inductively coupled plasma emission spectroscopy (ICP-AES) was used to test the mass percentage content of silicon elements in the first negative electrode material layer according to the method of GB / T 39527-2020 Determination of calcium, aluminum and silicon content in solid surface material products by chemical analysis.

[0074] The second negative electrode material layer on the negative electrode sheet was scraped off with a blade to obtain a powdery sample of the second negative electrode material layer. The powdery sample of the first negative electrode material layer was replaced with the powdery sample of the second negative electrode material layer, and the mass percentage content of silicon elements in the second negative electrode material layer was obtained according to the above test steps.

[0075] Test of the mass percentage content of the first silicon-based material and the mass percentage content of the second silicon-based material:

[0076] The lithium ion batteries in the examples and the comparative examples were disassembled after constant current discharge at 0.2C to 2.5V at an ambient temperature of 25℃, and the negative electrode sheets were obtained. The material layer on the surface of the negative electrode current collector in the negative electrode sheet away from the center of the electrode assembly winding was the first negative electrode material layer, and the material layer on the surface of the negative electrode current collector in the negative electrode sheet toward the center of the electrode assembly winding was the second negative electrode material layer. After the negative electrode sheet was cleaned with dimethyl carbonate (DMC) and dried, a negative electrode sheet sample was obtained.

[0077] The longitudinal section (a plane composed of the length direction and the thickness direction of the negative electrode sheet) of the first negative electrode material layer is observed by a scanning electron microscope, and the silicon-based material and the graphite particles in the longitudinal section of the first negative electrode material layer are distinguished and counted by a backscattering mode to obtain the proportion a1 of the silicon-based material in the first negative electrode material layer and the proportion b1 of the graphite particles in the first negative electrode material layer. The silicon-based material has a higher atomic number and a higher electron density, and the interaction between the silicon-based material and the test electron beam is stronger than the interaction between the graphite particles and the test electron beam, so that the silicon-based material region is brighter, and the graphite particle region is darker. The longitudinal section of the first negative electrode material layer is tested by an X-ray energy spectrometer (EDS) for element analysis, the carbon material and the silicon-based material are distinguished by the X-ray energy spectrometer (EDS), and the proportion a2 of the silicon-based material in the first negative electrode material layer and the proportion of the graphite particles in the first negative electrode material layer are determined. The proportion of the silicon-based material in the first negative electrode material layer is a = (a1 + a2) / 2, that is, the mass percentage content of the first silicon-based material based on the mass of the first negative electrode material layer.

[0078] The powder sample of the first negative electrode material layer is replaced by the powder sample of the second negative electrode material layer, and the mass percentage content b of the second silicon-based material based on the mass of the second negative electrode material layer can be obtained according to the above test steps.

[0079] Cycle performance test:

[0080] The lithium ion battery in the examples and comparative examples is tested for charge and discharge cycles in a 25°C constant temperature box, the lithium ion battery is charged at 2C constant current to 4.2V, and charged at 4.2V constant voltage to 0.05C, and then discharged at 6C constant current to 2.5V after standing for 5 minutes. At this time, it is the first cycle, and the first cycle discharge capacity C1 is recorded. After 600 cycles according to the above cycle process, the discharge capacity C 600 of the lithium ion battery is recorded, and the cycle capacity retention rate at the 600th cycle is calculated as an index for evaluating the infiltration effect of the negative electrode sheet and the cycle performance of the lithium ion battery, and the formula is shown as formula (I). When the 600 cycle (cls) capacity retention rate is lower, it indicates that the infiltration 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 600 cls capacity retention rate is higher, it indicates that the infiltration effect of the negative electrode sheet in the lithium ion battery is better, and the cycle performance of the lithium ion battery is better.

[0081] 600 cls capacity retention rate (%) = C 600 / C1 x 100%. (I)

[0082] Lithium precipitation performance test:

[0083] The lithium ion battery in the example and the comparative example was placed in a thermostat at 10°C, and after 60 minutes, charged at 2C constant current to 4.2V, charged at 4.2V constant voltage to a current of 0.05C, and after 5 minutes of standing, discharged at 0.5C constant current to 2.5V, which was one cycle. After 10 cycles of the above charging and discharging process, the lithium ion battery was charged at 2C constant current to 4.2V, charged at 4.2V constant voltage to a current of 0.05C, and after 5 minutes of standing, the lithium ion battery was disassembled, and the lithium deposition state on the surface of the first negative material layer of the negative electrode plate was observed. The surface area without lithium deposition was golden yellow, and the lithium deposition area was grayish white.

[0084] The judgment standard of the lithium deposition degree of the lithium ion battery is as follows: the lithium deposition area is 0% for no lithium deposition, i.e., the lithium deposition degree is none, the lithium deposition area is greater than 0% and less than or equal to 2% for mild lithium deposition, i.e., the lithium deposition degree is mild, the lithium deposition area is greater than 2% and less than or equal to 20% for moderate lithium deposition, i.e., the lithium deposition degree is moderate, and the lithium deposition area is greater than 20% and less than or equal to 100% for severe lithium deposition, i.e., the lithium deposition degree is severe, wherein the percentage of the lithium deposition area is calculated based on the total area of the first negative material layer.

[0085] In the present application, it is understood by those skilled in the art that "C" refers to the rated capacity of the finished lithium ion battery when it leaves the factory. "1C" is the current value that completely discharges the capacity of the lithium ion battery within 1 hour, "0.1C" is the current value that completely discharges the capacity of the lithium ion battery within 10 hours, and other rates are calculated in the same way.

[0086] Example 1

[0087] <Preparation of the negative electrode plate>

[0088] The first negative active material artificial graphite and the first silicon-based material SiO, the thickening agent sodium carboxymethyl cellulose (CMC-Na) and the negative electrode adhesive styrene-butadiene rubber (SBR) are mixed in a mass ratio of 84:13:1.7:1.3, then deionized water is added as a solvent, and stirring is performed to obtain a first negative electrode material layer slurry with a solid content of 50wt%. The first negative electrode material layer slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 10μm, and dried at 105°C to obtain a negative electrode sheet coated with a single first negative electrode material layer. The second negative active material artificial graphite and the second silicon-based material SiO, the thickening agent sodium carboxymethyl cellulose (CMC-Na) and the negative electrode adhesive styrene-butadiene rubber (SBR) are mixed in a mass ratio of 83.7:13.3:1.7:1.3, then deionized water is added as a solvent, and stirring is performed 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 a first negative electrode material layer and a second negative electrode material layer. Then, after cold pressing and slitting, a negative electrode sheet with a size 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 , the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer are both 45μm after cold pressing, and the width of the empty foil area is 5.45mm in the width direction after the negative electrode sheet is unfolded, and the width of the first negative electrode material layer and the second negative electrode material layer is both 62mm.

[0089] <Preparation of a positive electrode sheet>

[0090] The first positive active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), the first positive electrode binder polyvinylidene fluoride (PVDF) and the first positive electrode conductive agent conductive carbon black are dispersed in N-methyl pyrrolidone (NMP) solvent in a mass ratio of 94.8:2.8:2.4, and fully stirred to obtain a first positive electrode material layer slurry with a solid content of 72wt%. The first positive electrode material layer slurry is uniformly coated on one surface of an aluminum foil with a thickness of 13μm, and dried at 105°C to obtain a positive electrode sheet coated with a single first positive electrode material layer; then, the above steps are repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. The second positive active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1The second positive electrode material layer slurry with a solid content of 72wt% was obtained by dispersing the second positive electrode binder polyvinylidene fluoride (PVDF) and the second positive electrode conductive agent conductive carbon black in a mass ratio of 94.8:2.8:2.4 in N-methyl pyrrolidone (NMP) solvent, and mixing well under stirring. The second positive electrode material layer slurry was uniformly coated on the other surface of the positive electrode current collector aluminum foil with a thickness of 13μm, and dried at 105℃ to obtain the positive electrode sheet coated with the first positive electrode material layer and the second positive electrode material layer. Then, after cold pressing and slitting, the positive electrode sheet was dried at 105℃ under vacuum for 4h to obtain the positive electrode sheet with a size of 64.5mm×1422mm for use. Among them, the coating weight of the first positive electrode material layer and the second positive electrode material layer was 234mg / 1540.25mm 2 , the compaction density of the first positive electrode material layer and the second positive electrode material layer was 3.4g / cm 3 , the width of the first positive electrode material layer and the second positive electrode material layer was 60mm, and the width of the empty foil area of the positive electrode sheet was 4.5mm.

[0091] <separator>

[0092] A polyethylene (PE) film with a thickness of 12μm was used as the separator.

[0093] <Preparation of electrolyte>

[0094] In a dry argon atmosphere glove box, the organic solvents ethylene carbonate (EC), methyl ethyl 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 (LiPF6) was added to the above-mentioned base solvent. After mixing well and uniformly, the electrolyte was obtained. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, and the balance was the base solvent.

[0095] <Preparation of lithium ion battery>

[0096] The above-prepared separator, negative electrode sheet, separator, positive electrode sheet were sequentially stacked in order, and pre-wound first to ensure that the separator was between the negative electrode and the positive electrode, and at the same time to ensure that the first negative electrode material layer was away from the center of the electrode assembly formed by pre-winding, and the first segment in the first negative electrode material layer was located close to the center of the electrode assembly formed by pre-winding. Among them, the winding was carried out with a winding needle with a radius of 1.6mm (to ensure the designed curvature), and then after rubbing, welding the current collector, entering the shell, bottom penetration welding, code spraying, vacuum drying, injecting electrolyte, potting, and high-temperature standing, the lithium ion battery was obtained after formation capacity. Among them, the upper limit of the formation voltage was 3.6V, the formation temperature was 45℃, and after formation, the lithium ion battery was statically placed at room temperature of 25℃ for 24h.

[0097] Examples 2 to 19

[0098] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 1.

[0099] When the mass percentage of the first silicon-based material changes, the mass percentages of the binder and the negative electrode binder in the first negative electrode material layer remain unchanged, and the mass percentage of the artificial graphite changes accordingly. When the mass percentage of the second silicon-based material changes, the mass percentages of the binder and the negative electrode binder in the second negative electrode material layer remain unchanged, and the mass percentage of the artificial graphite changes accordingly. When the curvature R of the first coil of the second negative electrode material layer in the electrode assembly changes, the radius of the winding needle during winding is adjusted so that the R value is as shown in Table 1.

[0100] The thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing in Examples 6, 8, 10, and 16 were all 30 μm; the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing in Examples 7 and 9 were all 60 μm; the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing in Example 17 were all 50 μm; the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing in Example 18 were all 70 μm; and the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer after cold pressing in Example 19 were all 80 μm.

[0101] Comparative Examples 1 to 3

[0102] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 1. When the mass percentage of the first silicon-based material changes, the mass percentages of the binder and the negative electrode binder in the first negative electrode material layer remain unchanged, and the mass percentage of the artificial graphite changes accordingly. When the mass percentage of the second silicon-based material changes, the mass percentages of the binder and the negative electrode binder in the second negative electrode material layer remain unchanged, and the mass percentage of the artificial graphite changes accordingly.

[0103] The preparation parameters and performance parameters of each example and comparative example are shown in Table 1.

[0104] As can be seen from Examples 1 to 19 and Comparative Examples 1 to 3, by adjusting the length ratio of the first positive electrode material layer to the first negative electrode material layer in the first turn of the electrode assembly, and the capacity ratio per unit area of the two negative electrode material layers, so that the values of A and A / B are within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, the capacity retention rate at 600 cls is improved, which indicates that the secondary battery of the present application effectively reduces the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle, and the lithium ion battery has good cycle performance. In Comparative Examples 1 to 3, the values of A and / or A / B are not within the scope of the present application; the degree of lithium precipitation of the lithium ion battery in Comparative Examples 1 to 3 is heavier; and / or, the capacity retention rate at 600 cls is lower. While the degree of lithium precipitation of the lithium ion battery in Examples 1 to 19 is lighter, and the capacity retention rate at 600 cls is higher, the secondary battery of the present application effectively reduces the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle, and the lithium ion battery has good cycle performance.

[0105] The value of B generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1, 6 to 10, when the value of B is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0106] The value of C2 generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1, 11 to 15, when the value of C2 is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0107] The value of L2 generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1, 16 to 19, when the value of L2 is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0108] The value of R generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1, 16 to 19, when the value of R is within the scope of the present application, the degree of lithium precipitation of the lithium ion battery is lighter, and the capacity retention rate at 600 cls is higher, which indicates that the risk of lithium precipitation caused by insufficient CB in the inner turn of the electrode assembly in the later stage of the cycle in the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0109] The value of F generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 15, when the value of F is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0110] The value of W2 generally affects the cycle performance of the lithium ion battery. As can be seen from Examples 1, 8 to 15, when the value of W2 is within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0111] The types of the first silicon-based material and the second silicon-based material generally affect the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 19, when the first silicon-based material and the second silicon-based material within the range of the present application are selected, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0112] The mass percentage content a of the first silicon-based material and the mass percentage content b of the second silicon-based material generally affect the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 19, when the values of a and b are within the range of the present application, the lithium ion battery has a lighter degree of lithium precipitation and a higher capacity retention rate at 600 cls, indicating that the risk of lithium precipitation due to insufficient CB in the inner circle of the electrode assembly in the later cycle stage of the secondary battery of the present application is lower, and the cycle performance of the lithium ion battery is better.

[0113] It should be noted that, in this document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0114] Each of the embodiments in the present specification is described in a related manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the difference from other embodiments.

[0115] The above description is only the preferred embodiment of the present application, and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should 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 tab, a negative electrode tab, and a separator, the negative electrode tab comprising a negative electrode current collector, a first negative electrode material layer, and a second negative electrode material layer, the first negative electrode material layer being disposed on a surface of the negative electrode current collector facing away from a center of winding of the electrode assembly, the second negative electrode material layer being disposed on a surface of the negative electrode current collector facing toward the center of winding of the electrode assembly; the positive electrode tab comprising a positive electrode current collector and a first positive electrode material layer, the first positive electrode material layer being disposed on a surface of the positive electrode current collector facing toward the center of winding of the electrode assembly; The unit area capacity of the first negative electrode material layer is C1 mAh / 15 40.25 mm 2 The unit area capacity of the second negative electrode material layer is C2 mAh / 15 40.25 mm 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, and 0.918≤A / B≤1.

048.

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

02.

3. The cylindrical secondary battery according to claim 1, 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 A curvature of the first negative electrode material layer in the first circle of the electrode assembly is R, 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 comprises a first active material, the second negative electrode material layer comprises a second negative electrode active material, the first negative electrode active material and the second negative electrode active material each independently comprise a silicon element; A mass percentage content of the silicon element in the first negative electrode material layer based on a mass of the first negative electrode material layer is W1, a mass percentage content of the silicon element in the second negative electrode material layer based on a mass of the second negative electrode material layer is W2, a 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 comprises a first silicon-based material, the second negative electrode active material comprises a second silicon-based material, the first silicon-based material, 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 A mass percentage content of the first silicon-based material based on a mass of the first negative electrode material layer is 1.2% to 32%, a mass percentage content of the second silicon-based material based on a mass of the second negative electrode material layer is 1% to 30%. 11.An electronic device comprising the cylindrical secondary battery of any one of claims 1 to 10.

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