Secondary batteries and electronic devices

Abstract

This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode, a separator, and a negative electrode. The positive electrode includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The negative electrode includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The first negative electrode material layer has a first end face at its terminal end, and the second negative electrode material layer has a second end face at its terminal end, extending beyond the first end face. The first positive electrode material layer includes a first region and a second region. The projection of the first end face onto the first positive electrode material layer is located within the first region. The length of the first region is greater than 0 mm and less than or equal to 10 mm. A groove is provided within the first region, and the average mass per unit area of ​​the first positive electrode material layer within the first region is m1 mg / mm². 2 The average mass per unit area of ​​the first cathode material layer in the second region is m²mg / mm². 2 0.1% ≤ (m2-m1) / m2×100% ≤ 100%, 0.039 ≤ m2 ≤ 0.584. With the above settings, the secondary battery has good resistance to lithium plating.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

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

[0003] However, in actual use, the expansion of the positive and / or negative electrode plates during cycling is restricted by the external packaging bag or shell, resulting in local differences in CB values ​​and stress concentration in the lithium-ion battery, such as at the junction of the two negative electrode material layers in the negative electrode plate. This leads to lithium plating problems and reduces the safety performance of the secondary battery. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and an electronic device to improve the lithium plating resistance of the secondary battery. The specific technical solution is as follows:

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] A first aspect of this application provides a secondary battery, comprising an electrode assembly including a positive electrode, a separator, and a negative electrode. The positive electrode includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The first positive electrode material layer is located on the surface of the positive current collector away from the winding center of the electrode assembly, and the second positive electrode material layer is located on the surface of the positive current collector facing the winding center of the electrode assembly. The negative electrode includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The first negative electrode material layer is located on the surface of the negative current collector away from the winding center of the electrode assembly, and the second negative electrode material layer is located on the surface of the negative current collector facing the winding center of the electrode assembly. Along the length direction of the unfolded electrode assembly, the end of the first negative electrode material layer includes a first end face, and the end of the second negative electrode material layer includes a second end face. The length of the second negative electrode material layer is greater than the length of the first negative electrode layer, and the second end face extends beyond the first end face. Along the length of the unfolded electrode assembly, the first positive electrode material layer includes a first region and a second region. Along the thickness of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer lies within the first region. The length of the first region is greater than 0 mm and less than or equal to 10 mm. The second region comprises all regions except the first region. Along the length of the unfolded electrode assembly, a groove is provided within the first region, extending along the width of the unfolded electrode assembly. The average mass per unit area of ​​the first positive electrode material layer within the first region is m1 mg / mm². 2 The average mass per unit area of ​​the first cathode material layer in the second region is m²mg / mm². 2 0.1% ≤ (m2-m1) / m2×100% ≤ 100%, 0.039 ≤ m2 ≤ 0.584. In some embodiments, 5% ≤ (m2-m1) / m2×100% ≤ 40%. During the charging and discharging process of a secondary battery, as lithium ions are inserted and extracted, the stress concentration problem at the first end face of the negative electrode sheet in the wound electrode assembly becomes more significant. In this application, the projection of the first end face of the first positive electrode material layer is located in the first region. By setting a groove in the first region and adjusting the values ​​of (m2-m1) / m2×100% and m2 within the above range, more expansion space can be provided at the first end face of the negative electrode sheet, effectively reducing the compressive stress on the first end face of the negative electrode sheet. At the same time, the CB value at the first end face is reduced, which can reduce the positive electrode polarization overpotential. The groove provides more space and channels for electrolyte wetting of the secondary battery, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery.

[0007] In one embodiment of this application, 0.1 ≤ m1 ≤ 0.29. By adjusting the value of m1 within the above range, a groove is provided in the first region, which helps to provide more expansion space at the first end face of the negative electrode, reduces the compressive stress on the first end face of the negative electrode and the CB value at the first end face, thereby improving the lithium plating resistance of the secondary battery.

[0008] In one embodiment of this application, the width of a single groove is d μm along the length direction of the unfolded electrode assembly, where 1 ≤ d ≤ 10000. In some embodiments, 50 ≤ d ≤ 800. By adjusting the value of d within the above range, it is beneficial to reduce the compressive stress at the first end face of the negative electrode and the CB value at the first end face, thereby improving the lithium plating resistance of the secondary battery.

[0009] In one embodiment of this application, a plurality of grooves are provided in the first region. The grooves are evenly distributed along the length of the unfolded electrode assembly within the first region, with a spacing of σμm, where 0 < σ ≤ 1500. In some embodiments, 300 ≤ σ ≤ 800. By adjusting the value of σ within the above range, it is beneficial to reduce the compressive stress at the first end face of the negative electrode and the CB value at the first end face, thereby improving the lithium plating resistance of the secondary battery.

[0010] In one embodiment of this application, along the thickness direction of the unfolded electrode assembly, the depth of the groove is h μm, and the thickness of the first positive electrode material layer is H μm, where 1 ≤ h ≤ H, and 10 ≤ H ≤ 200. In some embodiments, 15 ≤ h ≤ 45. By adjusting the values ​​of h and H within the above ranges, the depth of a single groove is less than or equal to the thickness of the first positive electrode material layer, which helps to reduce the compressive stress at the first end face of the negative electrode sheet and the CB value at the first end face, thereby improving the lithium plating resistance of the secondary battery.

[0011] In one embodiment of this application, along the length of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer coincides with the centerline of the first region. When the projection of the first end face onto the first positive electrode material layer coincides with the centerline of the first region, it helps to provide more expansion space at the first end face in the negative electrode sheet, reducing the compressive stress on the first end face and the CB value at the first end face. At the same time, the groove provides more space and channels for electrolyte wetting of the secondary battery, thereby improving the lithium plating resistance of the secondary battery.

[0012] In one embodiment of this application, along the length direction of the unfolded electrode assembly, the first region includes a third region located in the middle of the first region. Along the thickness direction of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer is located within the third region. Along the length direction of the unfolded electrode assembly, the length of the third region is greater than 0 mm and less than or equal to 2 mm. The average mass per unit area of ​​the first positive electrode material layer within the third region is m1', and 25% ≤ (m2-m1') / m2×100% ≤ 40%. By controlling the value of (m2-m1') / m2×100% within the above range, the compressive stress at the first end face of the negative electrode sheet and the CB value at the first end face are further reduced, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance performance of the secondary battery.

[0013] In one embodiment of this application, the first region includes a fourth region, which is any region other than the third region in the first region. The average mass per unit area of ​​the first positive electrode material layer in the fourth region is m1”, 5% ≤ (m2-m1”) / m2×100% ≤ 25%. By controlling the value of (m2-m1”) / m2×100% within the above range, the compressive stress at the first end face of the negative electrode sheet and the CB value at the first end face are further reduced, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery.

[0014] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery provided by this application exhibits good resistance to lithium plating; therefore, the electronic device of this application has a long service life.

[0015] The beneficial effects of the embodiments of this application are as follows:

[0016] In this embodiment, the projection of the first end face onto the first positive electrode material layer is located within the first region. By setting a groove in the first region and adjusting the values ​​of (m2-m1) / m2×100% and m2 within the aforementioned range, more expansion space can be provided at the first end face of the negative electrode sheet, effectively reducing the compressive stress at the first end face of the negative electrode sheet. At the same time, the CB value at the first end face is reduced, which can reduce the positive electrode polarization overpotential. The groove provides more space and channels for electrolyte wetting of the secondary battery, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery.

[0017] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0019] Figure 1 This is a schematic diagram of the winding structure formed by the electrode assembly according to one embodiment of this application;

[0020] Figure 2 for Figure 1 A partial structural diagram of the unfolded electrode assembly;

[0021] Figure 3 for Figure 2 A magnified view of region Q in the electrode assembly;

[0022] Figure 4 This is a partial structural diagram of the electrode assembly after deployment in another embodiment of this application;

[0023] Figure 5 This is a partial structural diagram of the electrode assembly after it has been deployed in another embodiment of this application.

[0024] Reference numerals: Electrode assembly 001; Positive electrode 10; First positive electrode material layer 11; Second positive electrode material layer 12; Positive electrode current collector 13; Negative electrode 20; First negative electrode material layer 21; Second negative electrode material layer 22; Negative electrode current collector 23; First end face 211; Second end face 221; Separator 30; Groove 40; First region 11a; Second region 11b; Third region 11c; Fourth region 11d. Detailed Implementation

[0025] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0026] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0027] Currently, the main solutions to the problem of lithium plating caused by stress concentration at the interface between the two negative electrode material layers in a secondary battery negative electrode sheet are as follows: ① Increasing the CB value, but this easily leads to a decrease in the energy density of the secondary battery. Simultaneously, the increased weight of the negative electrode coating leads to a greater depth of lithium delithiation from the positive electrode during cycling, resulting in a decrease in the cycle performance of the secondary battery; ② Reducing the electrode sheet thickness and compaction density to alleviate local stress concentration, but this easily leads to a decrease in the energy density or a decline in the kinetic performance of the secondary battery. Based on this, this application provides a secondary battery that can improve lithium plating resistance.

[0028] A first aspect of this application provides a secondary battery, comprising an electrode assembly including a positive electrode, a separator, and a negative electrode. The positive electrode includes a positive current collector, a first positive electrode material layer, and a second positive electrode material layer. The first positive electrode material layer is located on the surface of the positive current collector away from the winding center of the electrode assembly, and the second positive electrode material layer is located on the surface of the positive current collector facing the winding center of the electrode assembly. The negative electrode includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The first negative electrode material layer is located on the surface of the negative current collector away from the winding center of the electrode assembly, and the second negative electrode material layer is located on the surface of the negative current collector facing the winding center of the electrode assembly. Along the length direction of the unfolded electrode assembly, the end of the first negative electrode material layer includes a first end face, and the end of the second negative electrode material layer includes a second end face. The length of the second negative electrode material layer is greater than the length of the first negative electrode layer, and the second end face extends beyond the first end face. Along the length of the unfolded electrode assembly, the first positive electrode material layer includes a first region and a second region. Along the thickness of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer lies within the first region. The length of the first region is greater than 0 mm and less than or equal to 10 mm. The second region comprises all regions except the first region. Along the length of the unfolded electrode assembly, a groove is provided within the first region, extending along the width of the unfolded electrode assembly. The average mass per unit area of ​​the first positive electrode material layer within the first region is m1 mg / mm². 2 The average mass per unit area of ​​the first cathode material layer in the second region is m²mg / mm². 20.1% ≤ (m2-m1) / m2×100% ≤ 100%. In some embodiments, 5% ≤ (m2-m1) / m2×100% ≤ 40%. For example, the value of (m2-m1) / m2×100% can be 0.1%, 0.3%, 0.5%, 0.7%, 1%, 3%, 5%, 7%, 10%, 13%, 15%, 17%, 20%, 23%, 25%, 27%, 30%, 33%, 35%, 37%, 40%, 43%, 45%, 47%, 50%, 53%, 55%, 57%, 60%, 63%, 65%, 67%, 70%, 73%, 75%, 77%, 80%, 83%, 85%, 87%, 90%, 93%, 95%, 97%, 100%, or a range of any two of these values. 0.039 ≤ m2 ≤ 0.584, for example, the value of m2 can be 0.039, 0.050, 0.070, 0.1, 0.13, 0.15, 0.17, 0.2, 0.23, 0.25, 0.27, 0.3, 0.33, 0.35, 0.37, 0.4, 0.43, 0.45, 0.47, 0.5, 0.53, 0.55, 0.57, 0.575, 0.577, 0.58, 0.582, 0.584, or a range of any two of these values. In some embodiments, the length of the first region can be only the minimum width of a groove.

[0029] In this application, for ease of understanding, the length direction of the electrode assembly in its unfolded state is defined as the X direction, the width direction as the Y direction, and the thickness direction as the Z direction. It can be understood that the positive electrode, negative electrode, and separator in their unfolded state have the same length, width, and thickness directions as the electrode assembly, and the winding direction of the electrode assembly is the W direction. Figures 1 to 5As shown, the electrode assembly 001 is formed by stacking and winding a separator 30, a positive electrode 10, and a negative electrode 20. The positive electrode 10 includes a first positive electrode material layer 11, a second positive electrode material layer 12, and a positive electrode current collector 13. The first positive electrode material layer 11 is located on the surface of the positive electrode current collector 13 away from the winding center of the electrode assembly 001, and the second positive electrode material layer 12 is located on the surface of the positive electrode current collector 13 facing the winding center of the electrode assembly 001. The negative electrode 20 includes a first negative electrode material layer 21, a second negative electrode material layer 22, and a negative electrode current collector 23. The first negative electrode material layer 21 is located on the surface of the negative electrode current collector 23 away from the winding center of the electrode assembly 001, and the second negative electrode layer 22 is located on the surface of the negative electrode current collector 23 facing the winding center of the electrode assembly 001. Along the length direction (X direction) of the unfolded electrode assembly 001, the first negative electrode material layer 21 has a first end face 211 at its end, and the second negative electrode material layer 22 has a second end face 221 at its end. The length of the second negative electrode material layer 22 is greater than the length of the first negative electrode material layer 21, and the second end face 221 extends beyond the first end face 211. Along the length direction (X direction) of the unfolded electrode assembly 001, the first positive electrode material layer 11 includes a first region 11a and a second region 11b (not shown in the figure). Along the thickness direction (Z direction) of the unfolded electrode assembly 001, the projection of the first end face 211 onto the first positive electrode material layer 11 is located within the first region 11a. The length of the first region 11a is 10 mm, and the second region 11b is any region other than the first region 11a. Along the length direction (X direction) of the unfolded electrode assembly 001, a groove 40 is provided in the first region 11a, and the groove 40 extends along the width direction (Y direction) of the unfolded electrode assembly 001.

[0030] During the charging and discharging process of a secondary battery, the stress concentration problem at the first end face of the negative electrode sheet in the wound electrode assembly becomes more significant due to the insertion and extraction of lithium ions. When a groove is set in the first region and the value of (m2-m1) / m2×100% is less than the lower limit value of this application, the reduction in the mass of the positive electrode material layer is small, resulting in a small improvement in the lithium plating resistance of the secondary battery. In this application, the projection of the first end face of the first positive electrode material layer is located in the first region. By setting a groove in the first region and controlling the values ​​of (m2-m1) / m2×100% and m2 within the above range, more expansion space can be provided at the first end face of the negative electrode sheet, effectively reducing the compressive stress on the first end face of the negative electrode sheet. At the same time, the CB value at the first end face is reduced, which can reduce the positive electrode polarization overpotential. The groove provides more space and channels for electrolyte wetting of the secondary battery, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery. Under the same secondary battery specifications, the secondary battery of this application has better lithium plating resistance. The average mass per unit area of ​​the first positive electrode material layer can be controlled by means known to those skilled in the art. For example, when coating the first positive electrode slurry onto the surface of the positive electrode current collector, the coating amount of the first positive electrode slurry can be increased to increase the average mass per unit area of ​​the first positive electrode material layer, provided that the solid content of the positive electrode slurry is constant. This application does not impose any particular restrictions, as long as the purpose of this application can be achieved.

[0031] In this application, the CB value refers to the ratio between the capacity of a negative electrode sheet per unit area and the capacity of a positive electrode sheet per unit area under the same conditions, such as an ambient temperature of 25°C and a discharge rate of 0.1C. CB = (Specific capacity of negative electrode active material × Mass of negative electrode active material per unit area of ​​negative electrode sheet) / (Specific capacity of positive electrode active material × Mass of positive electrode active material per unit area of ​​positive electrode sheet). The aforementioned unit area refers to 1 mm². 2 .

[0032] In one embodiment of this application, 0.1 ≤ m1 ≤ 0.29. For example, the value of m1 can be 0.1, 0.12, 0.15, 0.18, 0.2, 0.22, 0.25, 0.28, 0.29, or a range consisting of any two of these values. By adjusting the value of m1 within the above range, a groove is provided in the first region, which helps to provide more expansion space at the first end face of the negative electrode, reducing the compressive stress and CB value at the first end face of the negative electrode. At the same time, the groove provides more space and channels for electrolyte wetting of the secondary battery, reducing the risk of lithium plating at the first end face of the secondary battery, thereby improving the lithium plating resistance of the secondary battery.

[0033] In one embodiment of this application, such as Figure 2 and Figure 3 As shown, along the length direction (X direction) of the unfolded electrode assembly 001, the width of a single groove 40 is d μm, where 1 ≤ d ≤ 10000. In some embodiments, 50 ≤ d ≤ 800. For example, the value of d can be 1, 20, 50, 80, 100, 200, 500, 800, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, or a range of any two of these values. By adjusting the value of d within the above range, the positive electrode material layer in the first region is more uniformly distributed, and the etching amount of a single groove is moderate. This helps to reduce the compressive stress and CB value at the first end face of the negative electrode sheet, thereby reducing the risk of lithium plating at the first end face of the secondary battery, thus improving the lithium plating resistance performance of the secondary battery, while also reducing the processing difficulty and production cost of the secondary battery. In this application, it is understood that when d is 10000μm, the width of a single groove is equal to the length of the first region along the length direction after the electrode assembly is unfolded, that is, there is one and only one groove in the first region.

[0034] In one embodiment of this application, such as Figure 2 and Figure 3As shown, along the thickness direction Z of the electrode assembly 001 after unfolding, the depth of the groove 40 is h μm, and the thickness of the first positive electrode material layer 11 is H μm, where 1 ≤ h ≤ H and 10 ≤ H ≤ 200. In some embodiments, 15 ≤ h ≤ 45. For example, when the value of H is 200, the value of h can be 1, 3, 5, 7, 10, 13, 15, 17, 20, 23, 25, 27, 30, 33, 35, 37, 40, 43, 45, 47, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, or a range of any two of these values. The value of H can be 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 1880, 190, 200, or a range of any two of these values. By adjusting the values ​​of h and H within the aforementioned range, the depth of a single groove is less than or equal to the thickness of the first positive electrode material layer. The etching amount of a single groove is moderate, which helps reduce the compressive stress and CB value at the first end face of the negative electrode sheet, thus reducing the risk of lithium plating at the first end face of the secondary battery. This improves the lithium plating resistance of the secondary battery, while also reducing the processing difficulty and production cost. In this application, the cross-section of a single groove refers to the plane formed by the groove along the length direction and its own thickness direction after the electrode assembly is unfolded (or the cross-section obtained by viewing the groove along the length and thickness directions after the electrode assembly is unfolded). This application does not impose any particular limitation on the cross-sectional shape of the groove, as long as it achieves the purpose of this application. For example, the cross-section of the groove can be triangular, arc-shaped (with an area smaller than a semicircle with the same radius), semicircular, rectangular, trapezoidal, or square. In this application, the thickness of the first positive electrode material layer can be controlled by means known to those skilled in the art. For example, when coating the first positive electrode slurry onto the surface of the positive electrode current collector, the thickness of the first positive electrode material layer can be increased by increasing the coating weight and decreased by decreasing the coating weight, provided that the solid content of the first positive electrode slurry is constant. Under the condition that other parameters are constant, the thickness of the first positive electrode material layer can also be controlled by adjusting the cold pressing pressure.

[0035] In one embodiment of this application, such as Figure 2 and Figure 3As shown, a plurality of grooves 40 are provided in the first region 11a. The plurality of grooves 40 are evenly distributed in the first region 11a along the length direction (X direction) of the electrode assembly 001 after it is unfolded, with a spacing of σ μm, where 0 < σ ≤ 1500. In some embodiments, 300 ≤ σ ≤ 800. For example, the value of σ can be 0.1, 1, 3, 5, 8, 10, 30, 50, 70, 100, 130, 150, 170, 200, 230, 250, 270, 300, 330, 350, 370, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 or a range of any two of these values. By adjusting the value of σ within the above range, the positive electrode material layer in the first region is distributed more evenly, which helps to reduce the compressive stress and CB value at the first end face of the negative electrode sheet, thereby reducing the risk of lithium plating at the first end face of the secondary battery, thus improving the lithium plating resistance of the secondary battery, while reducing the processing difficulty of the secondary battery and reducing the production cost of the secondary battery.

[0036] In one embodiment of this application, such as Figure 4 As shown, along the length direction (X direction) of the unfolded electrode assembly 001, the projection of the first end face 211 onto the first positive electrode material layer 11 coincides with the center line of the first region 11a. When the projection of the first end face onto the first positive electrode material layer coincides with the center line of the first region, a groove is provided in the first region. This helps to provide more expansion space at the first end face of the negative electrode sheet, reducing the compressive stress and CB value at the first end face. At the same time, the groove provides more space and channels for electrolyte wetting of the secondary battery, reducing the risk of lithium plating at the first end face of the secondary battery, thereby improving the lithium plating resistance of the secondary battery.

[0037] In one embodiment of this application, such as Figure 5As shown, along the length direction (X direction) of the unfolded electrode assembly 001, the first region 11a includes a third region 11c located in the middle of the first region 11a. Along the thickness direction (Z direction) of the unfolded electrode assembly 001, the projection of the first end face 211 onto the first positive electrode material layer 11 is located within the third region 11c. Along the length direction (X direction) of the unfolded electrode assembly 001, the length of the third region 11c is greater than 0 mm and less than or equal to 2 mm. The average mass per unit area of ​​the first positive electrode material layer 11 within the third region 11c is m1', and 25% ≤ (m2 - m1') / m2 × 100% ≤ 40%. For example, the value of (m2 - m1') / m2 × 100% can be 25%, 27%, 30%, 33%, 35%, 37%, 40%, or a range consisting of any two of these values. The stress is non-uniform in the first region. By adjusting the value of (m2-m1') / m2×100% within the above range, the etching amount of the groove in the third region is increased, further reducing the compressive stress and CB value at the first end face of the negative electrode sheet, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery. In this application, 0.0234≤m1'≤0.438. For example, the value of m1' can be 0.0234, 0.025, 0.03, 0.05, 0.08, 0.1, 0.13, 0.15, 0.18, 0.2, 0.23, 0.25, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.438, or a range of any two of these values.

[0038] In one embodiment of this application, such as Figure 5As shown, the first region 11a includes a fourth region (not shown in the figure). The fourth region (not shown in the figure) is the region of the first region 11a other than the third region 11c. The average mass per unit area of ​​the first positive electrode material layer 11 in the fourth region (not shown in the figure) is m1”, 5% ≤ (m2-m1”) / m2×100% ≤ 25%. For example, the value of (m2-m1”) / m2×100% can be 5%, 7%, 10%, 13%, 15%, 17%, 20%, 23%, 25%, or a range of any two of these values. The stress is non-uniform in the first region. By adjusting the value of (m2-m1”) / m2×100% within the above range, the compressive stress at the first end face of the negative electrode sheet and the CB value at the first end face are further reduced, thereby reducing the risk of lithium plating at the first end face of the secondary battery and improving the lithium plating resistance of the secondary battery. In this application, 0.02925 ≤ m1” ≤ 0.5548. For example, the value of m1” can be 0.02925, 0.03, 0.05, 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, 0.22, 0.25, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.45, 0.48, 0.5, 0.52, 0.55, 0.5548 or a range of any two of these values.

[0039] In one embodiment of this application, a groove is provided in the first region along the length direction of the unfolded electrode assembly, while no groove is provided in the second region, resulting in a secondary battery with good lithium plating resistance. In another embodiment of this application, a groove is provided in both the first and second regions along the length direction of the electrode assembly, providing good wetting properties of the electrolyte on the negative electrode sheet, resulting in a secondary battery with good lithium plating resistance and cycle performance.

[0040] This application does not impose any particular restrictions on the shape of the groove or its position along the width direction after the electrode assembly is unfolded, as long as it meets the purpose of this application. For example, along the width direction after the electrode assembly is unfolded, the first positive electrode material layer has two opposing edges, and the groove can extend from either edge along the width direction after the electrode assembly is unfolded, or the groove can be located between the two edges. There can also be an angle between the groove and the length direction of the unfolded electrode assembly, with the angle being 30° to 150°. Through the above configuration, the lithium plating resistance of the secondary battery is improved while satisfying mass production manufacturability.

[0041] This application does not impose any particular limitation on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). The first positive electrode material layer of this application includes a first positive electrode active material, and the second positive electrode material layer includes a second positive electrode active material. This application does not impose any particular limitation on the types of the first and second positive electrode active materials, as long as they can achieve the purpose of this application. For example, the first and second positive electrode active materials can each be independently selected from lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05 At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. In this application, there is no particular limitation on the thickness of the positive electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The first and second positive electrode material layers may also include a positive electrode binder and a conductive agent. This application does not particularly limit the type of positive electrode binder in the first and second positive electrode material layers, as long as it achieves the purpose of this application. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. This application does not impose any particular restrictions on the types of conductive agents in the first and second positive electrode material layers, as long as they can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metallic materials, or conductive polymers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular restrictions on the mass ratio of the positive electrode active material, conductive agent, and positive electrode binder in the first and second positive electrode material layers. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0042] This application does not impose any particular restrictions on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: (1) mixing the first positive electrode active material, the positive electrode binder, and the conductive agent, adding a solvent and stirring evenly to prepare a first positive electrode slurry; mixing the second positive electrode active material, the positive electrode binder, and the conductive agent, adding a solvent and stirring evenly to prepare a second positive electrode slurry; (2) coating the first positive electrode slurry onto one surface of the positive electrode current collector, drying it to obtain a positive electrode sheet coated with the first positive electrode material layer, coating the second positive electrode slurry onto the other surface of the positive electrode current collector, drying it to obtain a positive electrode sheet coated with the first positive electrode material layer and the second positive electrode material layer; (3) cold pressing to determine the first region, and setting grooves at equal intervals along the length direction of the positive electrode sheet in the first region to obtain the positive electrode sheet.

[0043] This application does not impose any particular restrictions on the solid content of the first and second positive electrode slurries in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the solvent in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the drying time and temperature in step (2) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the cold pressing process parameters in step (3) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the method of setting the grooves in step (3) above, as long as the purpose of this application can be achieved. For example, the grooves can be set by pulsed laser etching. The depth h and width d of a single groove can be controlled by the power and defocusing amount of the pulsed laser emitter. The spacing σ between adjacent grooves can be controlled by adjusting the spacing between the pulsed laser emitters or the laser emission frequency. In one embodiment of this application, when the first region is confirmed after cold pressing in step (3), the third and fourth regions are simultaneously determined. Grooves are set at equal intervals in the third and fourth regions along the length direction of the positive electrode sheet to obtain the positive electrode sheet. In this application, the values ​​of m1, m1', and m1” can be adjusted by regulating the volume and number of grooves in each region. For example, when the length of a single groove is constant, the greater the depth h and width d of a single groove, the larger the volume of a single groove. When the number of grooves in the first region is constant, the larger the volume of a single groove, the smaller the value of m1; when the number of grooves in the first region is constant, the smaller the volume of a single groove, the larger the value of m1. When the volume of a single groove in the first region is constant, the more grooves there are, the smaller the value of m1; when the volume of a single groove in the first region is constant, the fewer grooves there are, the larger the value of m1. In this application, it is understood that the width d' of a single groove in the third region and the width d” of a single groove in the fourth region have the same value range as the width d of a single groove, and the depth h' of a single groove in the third region and the depth h” of a single groove in the fourth region have the same value range as the depth h of a single groove; these are only distinguished by different symbols.

[0044] This application places no particular restrictions on the negative electrode current collector, as long as the objectives of this application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors (such as lithium-copper composite current collectors, carbon-copper composite current collectors, nickel-copper composite current collectors, titanium-copper composite current collectors, etc.). The first negative electrode material layer of this application includes the first negative electrode active material, and the second negative electrode material layer includes the second negative electrode active material. This application places no particular restrictions on the types of the first negative electrode active material and the second negative electrode active material, as long as the objectives of this application can be achieved. For example, the first negative electrode active material and the second negative electrode active material can each independently be selected from natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy, or metallic lithium, etc. At least one of them. In this application, there is no particular restriction on the thickness of the negative electrode current collector, as long as the objectives of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm. The first negative electrode material layer and the second negative electrode material layer can also include a negative electrode binder and a conductive agent. This application places no particular restrictions on the type of the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer, as long as the objectives of this application can be achieved. For example, the negative electrode binder can be of the same type as the positive electrode binder in the above first positive electrode material layer and the second positive electrode material layer. This application places no particular restrictions on the type of the conductive agent in the first negative electrode material layer and the second negative electrode material layer, as long as the objectives of this application can be achieved. For example, the conductive agent can be of the same type as the conductive agent in the above first positive electrode material layer and the second positive electrode material layer.

[0045] This application places no particular restrictions on the preparation method of the negative electrode sheet, as long as the objectives of this application can be achieved. For example, the preparation method of the negative electrode sheet can include but is not limited to the following steps: (1) Mix the first negative electrode active material, the negative electrode binder, and the conductive agent, add a solvent and stir evenly to prepare the first negative electrode slurry; mix the second negative electrode active material, the negative electrode binder, and the conductive agent, add a solvent and stir evenly to prepare the second negative electrode slurry; (2) Coat the first negative electrode slurry on one surface of the negative electrode current collector, and after drying, obtain the negative electrode sheet coated with the first negative electrode material layer, and determine the end position of the first negative electrode material layer, that is, the position of the first end face; (3) Coat the second negative electrode slurry on the other surface of the negative electrode current collector, so that the length of the second negative electrode material layer is greater than the length of the first negative electrode material layer, and the end position of the second negative electrode material layer extends beyond the end position of the first negative electrode material layer, that is, the second end face extends beyond the first end face. After drying, obtain the negative electrode sheet coated with the first negative electrode material layer and the second negative electrode material layer, and after cold pressing, obtain the negative electrode sheet.

[0046] This application does not impose any particular restrictions on the solid content of the first and second negative electrode slurries in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the solvent in step (1) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the drying time and temperature in steps (2) and (3) above, as long as the purpose of this application can be achieved. This application does not impose any particular restrictions on the cold pressing process parameters in step (3) above, as long as the purpose of this application can be achieved.

[0047] This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. The diaphragm of this application may have a porous structure, and this application does not impose any particular limitation on the size of the pores in the porous structure of the diaphragm, as long as it achieves the purpose of this application. For example, the pore size may be from 0.01 μm to 1 μm. This application does not impose any particular limitation on the thickness of the diaphragm, as long as it achieves the purpose of this application. For example, the thickness of the diaphragm may be from 5 μm to 50 μm.

[0048] The electrolyte in the secondary battery of this application includes lithium salts and non-aqueous solvents. The lithium salt may 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. This application does not limit the content of lithium salts in the electrolyte, as long as the purpose of this application is achieved. This application does not particularly limit the non-aqueous solvent, as long as the purpose of this application is achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate (PC), butylene carbonate, or vinylene carbonate. Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1,2-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0049] The secondary battery of this application also includes a casing for housing the positive electrode, negative electrode, separator, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it can achieve the purpose of this application.

[0050] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of this application, the secondary battery may include, but is not limited to, a lithium-ion secondary battery, a lithium polymer secondary battery, or a lithium-ion polymer secondary battery.

[0051] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the negative electrode, separator, positive electrode, and separator in sequence; placing the projection of the first end face onto the first positive electrode material layer in the first region along the thickness direction of the unfolded electrode assembly; then winding and folding the stacked electrode assembly to obtain a wound electrode assembly; placing the electrode assembly into the housing; welding and assembling the current collector and insulating sheet; and finally injecting the electrolyte into the housing and sealing it to obtain the secondary battery.

[0052] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The secondary battery provided by this application exhibits good resistance to lithium plating; therefore, the electronic device of this application has a long service life.

[0053] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0054] Example

[0055] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0056] Test methods and equipment:

[0057] At an ambient temperature of 25℃, the lithium-ion battery was disassembled, and the positive electrode sheet was removed and soaked in dimethyl carbonate (DMC) for 20 minutes. Then, the positive electrode sheet was placed in an oven and dried at 80℃ for 12 hours to obtain the positive electrode sheet sample. The positive electrode sheet samples used in the following tests for m1, m2, m1', m1”, d, h, H, and σ were all sampled using the above method.

[0058] Tests of m1, m2, m1', m1”:

[0059] (1) m2 test:

[0060] Take the second region of the positive electrode sheet to be tested, and use a stamping die to cut 10 small circular pieces with a diameter of 2mm. Weigh the mass of the cut small circular pieces. Take another aluminum foil circular piece with the same diameter and weigh it. The value of m2 is obtained by dividing (mass of the cut small circular pieces - mass of the aluminum foil circular piece) by the area of ​​the cut small circular pieces. The test result of m2 is the average of the 10 test results.

[0061] (2) Test of m1:

[0062] Take the first region of the positive electrode sheet to be tested, and use a stamping die to cut 10 small circular pieces with a diameter of 2mm. Weigh the mass of the cut small circular pieces. Take another aluminum foil circular piece with the same diameter and weigh it. The value of m1 is obtained by dividing the mass of the cut small circular pieces by the mass of the aluminum foil circular piece and the area of ​​the cut small circular pieces by m2. The test result of m1 is the average of the 10 test results.

[0063] (3) Test of m1':

[0064] Take the third region of the positive electrode sheet to be tested, and use a stamping die to cut 10 small circular pieces with a diameter of 2mm. Weigh the mass of the cut small circular pieces. Take another aluminum foil circular piece with the same diameter and weigh it. The value of m1' is obtained by dividing the mass of the cut small circular pieces by the mass of the aluminum foil circular piece and the area of ​​the cut small circular pieces by m2. The test result of m1' is the average of the 10 test results.

[0065] (4) Test of “m1”:

[0066] Take the fourth region of the positive electrode sheet to be tested, and use a stamping die to cut 10 small circular pieces with a diameter of 2mm. Weigh the mass of the cut small circular pieces. Take another aluminum foil circular piece with the same diameter and weigh it. The value of m1” is obtained by dividing the mass of the cut small circular pieces by the mass of the aluminum foil circular piece and the area of ​​the cut small circular piece by m2. The test result of m1” is the average of the 10 test results.

[0067] Tests of d, h, H and σ:

[0068] d: Using a measuring tool under a charge-coupled device (CCD), select 10 grooves and measure the width of the 10 grooves along the length of the unfolded electrode assembly. Take the average value and record it as the width d of the groove. (If the number of grooves is insufficient, the width of a single groove at different positions can be sampled repeatedly).

[0069] h: Select the depth measurement function of the CCD, choose any 10 grooves, and measure the depth from the bottom of the 10 grooves to the upper plane of the positive electrode. Take the average value, which is the depth h of the groove. (If the number of grooves is insufficient, the depth from the bottom of the groove to the upper plane of the positive electrode can be repeatedly sampled at different positions of a single groove).

[0070] H: Using a micrometer, measure the thickness of the positive electrode corresponding to 10 points evenly distributed along the length direction in the second region. Half of the value obtained by subtracting the thickness of the positive current collector foil from this thickness is the thickness H of the first positive electrode material layer.

[0071] σ: Using a measuring tool under CCD, measure the spacing between the grooves at 10 locations, take the average value, and record it as the spacing σ. (If the number of grooves is insufficient, the spacing between different locations of a few grooves can be sampled repeatedly).

[0072] Testing of resistance to lithium plating:

[0073] The lithium-ion batteries of each embodiment and comparative example were left to stand at 25°C for 30 minutes, charged at a constant current of 1C to 4.2V, charged at a constant voltage of 4.2V to 0.025C, left to stand for 5 minutes, and then discharged at a constant current of 0.2C to 2.5V. After standing for 120 minutes, the following test was performed: charged at a constant current of 1C to 4.2V, charged at a constant voltage of 4.2V to 0.025C, left to stand for 5 minutes, discharged at a constant current of 0.5C to 2.5V, and left to stand for 20 minutes. This constituted one cycle. The above charge-discharge cycle was repeated 10 times. Then, the batteries were charged at a constant current of 1C to 4.2V, charged at a constant voltage of 4.2V to 0.025C, and left to stand for 15 minutes. The lithium-ion batteries were then disassembled, and the lithium deposition on the surface of the negative electrode at the first end face was observed. The non-lithium-deposited area on the surface of the negative electrode was golden yellow, while the lithium-deposited area was grayish white. Simultaneously, scanning electron microscopy and energy dispersive spectroscopy (EDS) were used to detect and analyze the surface of the negative electrode at the first end face. Scanning electron microscopy revealed a non-negative electrode material layer in the lithium plating region, and EDS showed that the carbon content in the lithium plating region was lower than in other regions. The lithium plating rate boundary at 10℃ was determined by the result showing the minimum lithium plating rate.

[0074] If no lithium plating occurs on the surface of the negative electrode at the first end face, the lithium-ion battery prepared in the same way is placed at 25°C for 30 minutes, charged at a constant current of 1C to 4.2V, charged at a constant voltage of 4.2V to 0.025C, placed at rest for 5 minutes, and then discharged at a constant current of 0.2C to 2.5V. After placing at rest for 120 minutes, the battery is tested, with the charging rate increased by 0.5C. The remaining charging and discharging parameters are the same as the above charging and discharging steps, and the battery is cycled for 10 cycles. Then, it is charged at a constant current of 1.5C to 4.2V, charged at a constant voltage of 4.2V to 0.025C, placed at rest for 15 minutes, and the lithium-ion battery is disassembled to observe the lithium plating state on the surface of the negative electrode at the first end face. The charging rate is increased by 0.5C each time until lithium plating on the negative electrode at the first end face ends the test. The lithium plating rate boundary of the negative electrode at the first end face at 10°C is recorded. For example, if lithium does not deposit even when the charging rate is increased to 5C, then the lithium deposition rate boundary at 10°C for the negative electrode at the first end face of the corresponding lithium-ion battery is recorded as 5C.

[0075] The lithium plating resistance performance is characterized by the lithium plating rate boundary at 10℃ for the negative electrode at the first end face. The higher the lithium plating rate boundary at 10℃, the better the lithium plating resistance performance of the lithium-ion battery.

[0076] Example 1-1

[0077] <Preparation of Negative Electrode Sheets>

[0078] The first negative electrode active material, artificial graphite, the negative electrode binder, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC), were mixed at a mass ratio of 96:2.3:1.7. Deionized water was added as a solvent and the mixture was stirred until homogeneous, forming a first negative electrode slurry with a solid content of 50 wt%. The second negative electrode active material, artificial graphite, the negative electrode binder, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC), were mixed at a mass ratio of 96:2.3:1.7. Deionized water was added as a solvent and the mixture was stirred until homogeneous, forming a second negative electrode slurry with a solid content of 50 wt%. The first negative electrode slurry prepared above was uniformly coated on one surface of a 7 μm thick copper foil current collector and dried at 105 °C to obtain a negative electrode sheet with a first negative electrode material layer with a coating thickness of 60 μm. The position of the end of the first negative electrode material layer, i.e., the first end face, was determined. A second negative electrode slurry is coated onto the other surface of the negative electrode current collector, such that the length of the second negative electrode material layer is greater than the length of the first negative electrode material layer, and the end of the second negative electrode material layer extends beyond the end of the first negative electrode material layer, i.e., the second end face extends beyond the first end face. After drying, a negative electrode sheet coated with the first and second negative electrode material layers is obtained. After cold pressing and slitting, a negative electrode sheet with a specification of 62mm × 1464mm is obtained. The coating thickness of the second negative electrode material layer after drying is equal to the thickness of the first negative electrode material layer, and the thickness of both the first and second negative electrode material layers after cold pressing is 45μm.

[0079] <Preparation of the positive electrode>

[0080] The first positive electrode active material LiNi 0.91 Co 0.04 Mn 0.05 O2, positive electrode binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black were mixed at a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent and stirred until homogeneous, forming a first positive electrode slurry with a solid content of 72 wt%. The second positive electrode active material, LiNi... 0.91 Co 0.04 Mn 0.05 O2, positive electrode binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black were mixed at a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent and stirred until homogeneous, forming a second positive electrode slurry with a solid content of 72 wt%. The first positive electrode slurry prepared above was uniformly coated on one surface of a 10 μm thick aluminum foil for the positive electrode current collector and dried at 105 °C to obtain a positive electrode sheet coated with the first positive electrode material layer. The coating weight of the first positive electrode material layer was 0.303 mg / mm². 2 The second positive electrode slurry is coated onto the other surface of the positive electrode current collector, and after drying, a positive electrode sheet coated with the first and second positive electrode material layers is obtained. The coating weight of the second positive electrode material layer is 0.303 mg / mm². 2 Then, after cold pressing and slitting, it is pre-wound with the negative electrode sheet to determine the first region of the first positive electrode material layer. Grooves are then laser-etched at equal intervals along the length of the positive electrode sheet within this first region to obtain a positive electrode sheet with dimensions of 60.3mm × 1400mm for later use. The length of the first region is 10mm, and m1 is 0.27mg / mm². 2 m2 is 0.3 mg / mm 2 The value of (m2-m1) / m2×100% is 10%. The thickness H of the first positive electrode material layer is 60μm, and the thickness H of the second positive electrode material layer is equal to the thickness H of the first positive electrode material layer. The width d of a single groove is 300μm, the depth h of a single groove is 20μm, and the spacing σ between adjacent grooves is 800μm.

[0081] <Septum>

[0082] A 7μm thick polypropylene (PP) film was used as the separator.

[0083] <Preparation of Electrolyte>

[0084] In a dry argon-atmospheric glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 30:50:20. Then, lithium hexafluorophosphate (LiPF6) was added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%.

[0085] <Preparation of Lithium-ion Batteries>

[0086] The prepared separator, negative electrode, and positive electrode are stacked sequentially, with the separator acting as an separator between the positive and negative electrodes. Simultaneously, along the thickness direction of the unfolded electrode assembly, the projection of the first end face of the first negative electrode material layer onto the first region of the first positive electrode material layer (the projection does not coincide with the center line of the first region) is arranged. Then, the assembly is wound, current collectors are welded, it is installed in the casing, marked, vacuum dried, electrolyte is injected, marked again, and after high-temperature settling, a formation capacity test is performed to obtain the lithium-ion battery. The upper limit of the formation voltage is 3.6V, the formation temperature is 45℃, and the formation settling time is 2 hours.

[0087] Examples 1-2 to Examples 1-19

[0088] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0089] Examples 1-20

[0090] Except for the fact that in the <Preparation of Lithium-ion Batteries>, the projection of the first end face of the first negative electrode material layer onto the first positive electrode material layer coincides with the center line of the first region of the first positive electrode material layer, the rest is the same as in Examples 1-13.

[0091] Example 2-1

[0092] Except for the preparation of the positive electrode sheet according to the following steps, the rest is the same as in Example 1-1.

[0093] <Preparation of the positive electrode>

[0094] The first positive electrode active material LiNi 0.91 Co 0.04 Mn 0.05 O2, positive electrode binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black were mixed at a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent and stirred until homogeneous, forming a first positive electrode slurry with a solid content of 72 wt%. The second positive electrode active material, LiNi... 0.91 Co 0.04 Mn 0.05O2, positive electrode binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black were mixed at a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent and stirred until homogeneous, forming a second positive electrode slurry with a solid content of 72 wt%. The first positive electrode slurry prepared above was uniformly coated on one surface of a 10 μm thick aluminum foil for the positive electrode current collector and dried at 105 °C to obtain a positive electrode sheet coated with the first positive electrode material layer. The second positive electrode slurry was coated on the other surface of the positive electrode current collector and dried to obtain a positive electrode sheet coated with both the first and second positive electrode material layers. After cold pressing and slitting, the positive electrode sheet was pre-wound with a negative electrode sheet to define the first, third, and fourth regions. Grooves were laser-etched at equal intervals in the third and fourth regions along the length of the positive electrode sheet to obtain a positive electrode sheet with dimensions of 60.3 mm × 1400 mm for later use. The length of the third region is 2 mm, and m1 is 0.27 mg / mm. 2 m2 is 0.3 mg / mm 2 (m2-m1) / m2×100% is 10%. m1' is 0.225mg / mm 2 (m2-m1') / m2×100% is 25%. m1” is 0.243mg / mm 2 The ratio (m2-m1”) / m2×100% is 18.75%. The thickness H of the first positive electrode material layer is 60μm, the thickness H of the second positive electrode material layer is equal to the thickness H of the first positive electrode material layer, and the spacing σ between adjacent grooves is 800μm. The width d' of a single groove in the third region is 250μm, and the depth h' of a single groove in the third region is 30μm. The width d” of a single groove in the fourth region is 250μm, and the depth h” of a single groove in the fourth region is 33μm.

[0095] Examples 2-2 to 2-6

[0096] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Example 2-1.

[0097] Comparative Example 1

[0098] Except for not providing a groove in the first region of the first positive electrode material layer, it is the same as in Example 1-1.

[0099] Comparative Examples 2 to 3

[0100] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0101] The relevant parameters and performance tests of each embodiment and comparative example are shown in Tables 1 and 2.

[0102] Table 1

[0103]

[0104]

[0105] Note: " / " in Table 1 indicates that there are no relevant preparation parameters.

[0106] As can be seen from Examples 1-1 to 1-20 and Comparative Examples 1 to 3, when the projection of the first end face onto the first positive electrode material layer is located within the first region, a groove is set in the first region. The values ​​of (m2-m1) / m2×100% and m2 are controlled within the scope of this application. The lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode at 10°C, meaning that lithium plating does not occur at a higher rate at the first end face of the negative electrode at 10°C. This demonstrates that, under the same lithium-ion battery specifications, the lithium-ion battery in this application has better anti-lithiation performance. In Comparative Example 1, no groove is set in the first region of the first positive electrode material layer. In Comparative Examples 2 to 3, the values ​​of (m2-m1) / m2×100% or m2 are not within the scope of this application. In these comparative examples, the lithium plating rate boundary at 10°C is relatively low at the first end face of the negative electrode, indicating that lithium plating occurs at a lower rate at the first end face of the negative electrode at 10°C. This demonstrates that the lithium-ion battery in this application has better anti-lithiation performance. In Examples 1-7 to 1-8, although the lithium plating rate boundary at 10°C is relatively high on the first end face of the negative electrode sheet, and the lithium-ion battery has good anti-lithiation performance, the processing difficulty of setting grooves in the first region of the first positive electrode material layer is increased due to the large value of (m2-m1) / m2×100% at this time, resulting in a lower energy density of the lithium-ion battery.

[0107] The value of the average mass per unit area m1 of the first positive electrode material layer in the first region usually affects the lithium plating resistance of the lithium-ion battery. As can be seen from Examples 1-1 to Examples 1-8, by adjusting the value of m1 within the range of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode, that is, at 10°C, lithium plating can be avoided at a higher rate at the first end face of the negative electrode, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0108] The width d of the groove typically affects the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-15, by adjusting the value of d within the range of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode, that is, lithium plating does not occur at a relatively high rate at the first end face of the negative electrode at 10°C, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0109] The spacing σ of the grooves typically affects the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-17, by adjusting the value of σ within the range of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode, that is, lithium plating does not occur at a relatively high rate at the first end face of the negative electrode at 10°C, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0110] The depth h of the groove and the thickness H of the first material layer typically affect the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-19, by adjusting the values ​​of h and H within the range of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode, that is, lithium plating does not occur at a relatively high rate at the first end face of the negative electrode at 10°C, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0111] The projection position of the first end face on the first positive electrode material layer typically affects the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-13 and 1-20, by adjusting the projection position of the first end face on the first positive electrode material layer within the scope of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode, that is, at 10°C, lithium plating can be avoided at a higher rate at the first end face of the negative electrode, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0112] Table 2

[0113]

[0114]

[0115] Note: " / " in Table 2 indicates that there are no relevant preparation parameters.

[0116] The value of (m2-m1') / m2×100% typically affects the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-3, by adjusting the value of (m2-m1') / m2×100% within the scope of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode. That is, at 10°C, lithium plating does not occur at a relatively high rate at the first end face of the negative electrode, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0117] The value of (m2-m1”) / m2×100% typically affects the lithium plating resistance of lithium-ion batteries. As can be seen from Examples 1-1, 2-4 to 2-6, by adjusting the value of (m2-m1”) / m2×100% within the scope of this application, the lithium plating rate boundary at 10°C is relatively high at the first end face of the negative electrode. That is, at 10°C, lithium plating does not occur at a relatively high rate at the first end face of the negative electrode, thus demonstrating that the lithium-ion battery has good lithium plating resistance.

[0118] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0119] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0120] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly including a positive electrode sheet, a separator and a negative electrode sheet, the positive electrode sheet including a positive current collector, a first positive electrode material layer and a second positive electrode material layer, the first positive electrode material layer being located on the surface of the positive current collector away from the winding center of the electrode assembly, and the second positive electrode material layer being located on the surface of the positive current collector facing the winding center of the electrode assembly; The negative electrode sheet includes a negative current collector, a first negative electrode material layer, and a second negative electrode material layer. The first negative electrode material layer is located on the surface of the negative current collector away from the winding center of the electrode assembly, and the second negative electrode material layer is located on the surface of the negative current collector facing the winding center of the electrode assembly. Along the length direction of the unfolded electrode assembly, the end of the first negative electrode material layer includes a first end face, the end of the second negative electrode material layer includes a second end face, the length of the second negative electrode material layer is greater than the length of the first negative electrode material layer, and the second end face extends beyond the first end face. Along the length direction of the unfolded electrode assembly, the first positive electrode material layer includes a first region and a second region. Along the thickness direction of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer is located within the first region. The length of the first region is greater than 0 mm and less than or equal to 10 mm. The second region is other regions besides the first region. Along the length direction of the unfolded electrode assembly, a groove is provided in the first region, and the groove extends along the width direction of the unfolded electrode assembly. The average mass per unit area of ​​the first positive electrode material layer in the first region is m1mg / mm 2 The average mass per unit area of ​​the first positive electrode material layer in the second region is m² mg / mm². 2 , 0.1%≤(m2-m1) / m2×100%≤100%, 0.039≤m2≤0.

584.

2. The secondary battery according to claim 1, wherein, 0.1≤m1≤0.29。 3. The secondary battery according to claim 1, wherein, 5%≤(m2-m1) / m2×100%≤40%.

4. The secondary battery according to claim 1, wherein, Along the length of the unfolded electrode assembly, the width of a single groove is dμm, where 1≤d≤10000.

5. The secondary battery according to claim 1, wherein, The first region is provided with a plurality of grooves, which are evenly distributed along the length of the electrode assembly after it is unfolded, and the spacing is σμm, where 0<σ≤1500.

6. The secondary battery according to claim 1, wherein, Along the thickness direction of the unfolded electrode assembly, the depth of the groove is h μm, the thickness of the first positive electrode material layer is H μm, 1≤h≤H, 10≤H≤200.

7. The secondary battery according to claim 1, wherein, Along the length of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer coincides with the center line of the first region.

8. The secondary battery according to claim 1, wherein, Along the length direction of the unfolded electrode assembly, the first region includes a third region located in the middle of the first region. Along the thickness direction of the unfolded electrode assembly, the projection of the first end face onto the first positive electrode material layer is located within the third region. Along the length direction of the unfolded electrode assembly, the length of the third region is greater than 0 mm and less than or equal to 2 mm. The average mass per unit area of ​​the first positive electrode material layer within the third region is m1', and 25% ≤ (m2 - m1') / m2 × 100% ≤ 40%.

9. The secondary battery according to claim 8, wherein, The first region includes a fourth region, which is the other regions of the first region besides the third region. The average mass per unit area of ​​the first positive electrode material layer in the fourth region is m1”, 5% ≤ (m2-m1”) / m2×100% ≤ 25%.

10. The secondary battery according to claim 1, wherein it satisfies at least one of the following characteristics: (1) Along the length direction of the unfolded electrode assembly, the width of a single groove is dμm, 50≤d≤800; (2) A plurality of grooves are provided in the first region, and the plurality of grooves are equally spaced along the length direction of the electrode assembly after it is unfolded in the first region, and the spacing is σμm, 300≤σ≤800; (3) Along the thickness direction of the electrode assembly after it is unfolded, the depth of the groove is hμm, 15≤h≤45.

11. An electronic device comprising a secondary battery as described in any one of claims 1 to 10.

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