Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises an electrode assembly of a winding structure, the electrode assembly comprises a positive pole piece and a negative pole piece, the negative pole piece comprises a negative active material, and the mass percentage content of Si element in the negative active material is w; the positive pole piece comprises a first positive pole material layer; the electrode assembly comprises a straight area and a bent area, a plurality of first bulges are arranged on the first positive electrode material layer in the straight area, a plurality of second bulges are arranged on the first positive electrode material layer in the bent area, the average height of the plurality of first bulges is H1 mm, the average height of the plurality of second bulges is H2 mm, 0 < = H1-0. 059 w0.2047 < = 0.003, and the average height of the plurality of second bulges is H2 mm along the thickness direction of the unfolded positive electrode plate. 5% < = w < = 100%, and H1 < H2. The secondary battery has relatively high energy density and also has good cycle performance and safety performance.

Description

TECHNICAL FIELD

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

[0002] With the continuous development of technology, people have higher requirements for the energy density of secondary batteries, and using high specific energy electrodes is the current effective way. Among them, silicon material has a high specific capacity, more than ten times that of graphite, is abundant in resources, and is low in price, which is a relatively ideal scheme as a negative active material. However, the volume of silicon material changes significantly during charging and discharging, which causes the negative current collector copper foil to be stretched and leads to the fracture of the negative current collector copper foil after long-term cycling. At present, the main ways to reduce the risk of copper foil fracture are to increase the thickness of the copper foil, reduce the coating weight of the negative electrode material layer, and reduce the design capacity of the secondary battery. However, the above methods only reduce the risk of copper foil fracture by reducing the energy density of the secondary battery, and the expansion of the secondary battery increases in the later cycle, which still has a high risk of copper foil fracture, affecting the cycle performance and safety performance of the secondary battery. SUMMARY

[0003] The purpose of the present application is to provide a secondary battery and an electronic device that can reduce the risk of negative current collector fracture and the cycle expansion rate of the secondary battery, thereby improving the cycle performance and safety performance of the secondary battery.

[0004] It should be noted that the present application uses lithium ion batteries as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to lithium ion batteries. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a secondary battery, comprising an electrode assembly in a roll structure, the electrode assembly comprising a positive electrode sheet and a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode material layer arranged on at least one surface of the negative current collector, the negative electrode material layer comprising a negative active material, the mass percentage content of Si element in the negative active material being w; the positive electrode sheet comprising a positive current collector and a first positive electrode material layer arranged on one surface of the positive current collector; the electrode assembly comprising a flat area and a bending area connected to the flat area, a plurality of first protrusions being arranged on the first positive electrode material layer in the flat area, and a plurality of second protrusions being arranged on the first positive electrode material layer in the bending area, the average height of the plurality of first protrusions being H1 mm in the thickness direction of the positive electrode sheet after being unfolded, the average height of the plurality of second protrusions being H2 mm, and 0≤|H1-0.0059w 0.2047|≤0.003, 5% ≤ w ≤ 100%, H1 < H2. By respectively providing a plurality of first protrusions and a plurality of second protrusions on the first positive electrode material layer in the straight region and the bent region, controlling w and H1 to satisfy the above relationship and making H1 < H2, when the mass percentage content of Si element in the negative electrode active material changes, different first protrusion heights can be matched to reserve expansion space for the negative electrode sheet in advance during the winding stage, thereby reducing the excessive stretching of the negative electrode current collector (such as copper foil) caused by the expansion of the negative electrode sheet during the cycling process, reducing the risk of fracture of the negative electrode current collector and the cycling expansion rate of the secondary battery, and improving the cycling performance and safety performance of the secondary battery; meanwhile, the gap between the electrode sheets located in the straight region is relatively moderate, which can reserve sufficient expansion space for the negative electrode sheet located in the straight region, while enabling the electrolyte to better infiltrate the electrode assembly and maintaining the tightness between the electrode sheets, reducing the impedance, and improving the cycling performance of the secondary battery; in addition, the relatively high second protrusions can reduce the influence of the stress in the bent region on the electrode assembly, so that the secondary battery has good cycling performance and safety performance while having a relatively high energy density.

[0006] In one or more embodiments of the present application, 0 ≤ |H2 - 0.017w 0.3139 |≤0.003. By controlling the values of H2 and w to satisfy the above relationship, it is beneficial to better match the average height H2 of the second protrusions located in the bent region with the mass percentage content w of the Si element in the negative electrode active material, and it is beneficial for the secondary battery to have good cycling performance and safety performance while having a relatively high energy density.

[0007] In one or more embodiments of the present application, 0.002 ≤ H2 - H1 ≤ 0.014. By controlling the value of H2 - H1 to be within the above range, the difference between the average height of the plurality of first protrusions and the average height of the plurality of second protrusions is relatively moderate, which is beneficial to improving the cycling performance of the secondary battery.

[0008] In one or more embodiments of the present application, the plurality of first protrusions are distributed in a dot pattern on the first positive electrode material layer located in the straight region, and the density of the first protrusions is x pieces / cm 2 , the plurality of second protrusions are distributed in a dot pattern on the first positive electrode material layer located in the bent region, and the density of the second protrusions is y pieces / cm 2 , 0.5 ≤ x / y ≤ 1, 5 ≤ x ≤ 9. By controlling the values of x / y and x to be within the above range, it is beneficial to better disperse the bending stress and expansion stress, reduce the cycling expansion rate of the secondary battery and the risk of fracture of the negative electrode current collector; and it is beneficial to improve the infiltration effect of the electrolyte on the electrode assembly and reduce the impedance; in addition, it is beneficial to reduce the loss of energy density, so that the secondary battery has good cycling performance and safety performance while having a relatively high energy density.

[0009] In one or more embodiments of this application, the maximum circumscribed circle diameter of a single first protrusion on the surface of the first positive electrode material layer is D1 μm, 1.5≤D1≤3; and / or, the maximum circumscribed circle diameter of a single second protrusion on the surface of the first positive electrode material layer is D2 μm, 2.5≤D2≤4. By adjusting the values ​​of D1 and / or D2 to be within the above ranges, it is beneficial to provide sufficient expansion space for the negative electrode sheet, while also improving the wetting effect of the electrolyte on the electrode sheet and reducing the risk of increased electrochemical impedance due to increased local aggregation side reactions of the electrolyte; in addition, it is beneficial to improve the structural stability of the positive electrode sheet, thereby giving the secondary battery higher cycle performance and safety performance.

[0010] In one or more embodiments of this application, along the thickness direction of the electrode assembly, the number of layers of the positive electrode sheet is N, 10≤N≤39. Along the length direction of the unfolded positive electrode sheet, the total length of the region containing the multiple first protrusions and the multiple second protrusions is L1 mm, and the length of the first positive electrode material layer is L2 mm. N, L1, and L2 satisfy: (1) 10≤N<19, 0.3≤L1 / L2<0.5; (2) 19≤N<29, 0.5≤L1 / L2<0.7; (3) 29≤N≤39, 0.7≤L1 / L2≤0.9. By adjusting the values ​​of N, L1, and L2 to satisfy the above relationships, it is beneficial to reserve sufficient expansion space for the negative electrode sheet while further reducing the loss of energy density of the secondary battery. Thus, the secondary battery has both high energy density and good cycle performance and safety performance.

[0011] In one or more embodiments of this application, in any ring with a first protrusion in the flat region, the elongation of the positive current collector in the flat region of the ring is e1, and the elongation of the negative current collector adjacent to the positive current collector in the ring along the thickness direction of the electrode assembly is e2. The length of the first positive electrode material layer in the flat region is d mm, and 8.5 ≤ |e2-e1|d / H2 ≤ 39. By controlling the value of |e2-e1|d / H2 within the above range, it is beneficial to reduce the risk of breakage of the negative and positive electrode sheets and the risk of electrolyte bridging between the layers of the electrode sheets, thereby reducing impedance and giving the secondary battery good cycle performance and safety performance.

[0012] In one or more embodiments of this application, in any loop with a second protrusion in the bending region, the elongation of the positive current collector in the bending region of the loop is e3, and the elongation of the negative current collector adjacent to the positive current collector of the loop along the thickness direction of the electrode assembly is e4. The radius of the inscribed circle of the negative current collector adjacent to the positive current collector of the loop and far from the winding center is r mm, and 3≤|e4-e3|r / H2≤12. By controlling the value of |e4-e3|r / H2 within the above range, it is beneficial to reduce the risk of breakage of the negative and positive electrode sheets and the risk of electrolyte bridging between the layers of the electrode sheets, thereby reducing impedance and giving the secondary battery good cycle performance and safety performance.

[0013] In one or more embodiments of this application, the positive electrode sheet further includes a second positive electrode material layer disposed on another surface of the positive electrode current collector. The second positive electrode material layer in the flat region has a plurality of first recesses, with each first recess corresponding to a first protrusion along the thickness direction of the electrode assembly; and / or, the second positive electrode material layer in the bent region has a plurality of second recesses, with each second recess corresponding to a second protrusion along the thickness direction of the unfolded positive electrode sheet and along the thickness direction of the electrode assembly. This configuration helps to improve the cycle performance and safety performance of the secondary battery while meeting mass production manufacturability requirements.

[0014] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0015] The beneficial effects of this application are:

[0016] This application provides a secondary battery and an electronic device. The secondary battery includes a wound electrode assembly with a flat region and a bent region. By setting multiple first protrusions on the positive electrode sheet in the flat region and multiple second protrusions on the positive electrode sheet in the bent region, and adjusting the values ​​of H1 and w to satisfy the above relationship and H1 < H2, when the Si content in the negative electrode active material changes, different first protrusion heights can be used to reserve expansion space for the negative electrode sheet in advance during the winding stage. This reduces the excessive stretching of the negative electrode current collector (such as copper foil) caused by the expansion of the negative electrode sheet during cycling, reduces the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery, and improves the cycle performance and safety performance of the secondary battery. At the same time, the gap between the electrode layers in the flat region is relatively moderate. While reserving sufficient expansion space for the negative electrode sheet in the flat region, it improves the wetting effect of the electrolyte on the electrode assembly and maintains the tightness between the electrode layers, reduces impedance, and improves the cycle performance of the secondary battery. In addition, the higher second protrusions can reduce the impact of stress in the bent region on the electrode assembly. Therefore, secondary batteries not only have high energy density, but also good cycle performance and safety performance.

[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, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0019] Figure 1 This is a schematic diagram of a wound electrode assembly in one embodiment of this application;

[0020] Figure 2 This is a partial structural diagram of the unfolded positive electrode sheet along its own thickness direction in one embodiment of this application;

[0021] Figure 3 This is a partial schematic diagram of the unfolded surface of the positive electrode sheet in one embodiment of this application;

[0022] Figure 4 This is a schematic diagram of the inscribed circle of the negative current collector adjacent to the positive current collector located in the bending region at any turn and far from the winding center in one embodiment of this application.

[0023] Reference numerals: electrode assembly 001; positive electrode tab 10; first positive electrode material layer 101; second positive electrode material layer 102; positive electrode current collector 103; negative electrode tab 20; negative electrode current collector 201; negative electrode material layer 202; separator 30; flat region 0011; bent region 0012; first protrusion 1011; first recess 1021; second protrusion 1012; second recess 1022. Detailed implementation manners

[0024] Next, the technical solutions in this application will be clearly and completely described in conjunction with the embodiments of this application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all of the embodiments. All other embodiments obtained by those skilled in the art based on this application belong to the scope of protection of this application.

[0025] It should be noted that in the specific implementation manners of this application, a lithium-ion battery is taken as an example of a secondary battery to explain this application. However, the secondary battery of this application is not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0026] In the first aspect of this application, a secondary battery is provided, including a wound electrode assembly. The electrode assembly includes a positive electrode tab and a negative electrode tab. The negative electrode tab includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material. Based on the mass of the negative electrode active material, the mass percentage content of Si element is w. The positive electrode tab includes a positive electrode current collector and a first positive electrode material layer provided on one surface of the positive electrode current collector. The electrode assembly includes a flat region and a bent region connected to the flat region. A plurality of first protrusions are provided on the first positive electrode material layer located in the flat region, and a plurality of second protrusions are provided on the first positive electrode material layer located in the bent region. Along the thickness direction after the positive electrode tab is unfolded, the average height of the plurality of first protrusions is H1 mm, and the average height of the plurality of second protrusions is H2 mm, 0 ≤ |H1 - 0.0059w 0.2047 | ≤ 0.003, 5% ≤ w ≤ 100%, H1 < H2. For example, the value of |H1 - 0.0059w 0.2047 | can be 0, 0.0005, 0.001, 0.0015, 0.002, 0.0025, 0.003 or a range composed of any two of these values, |H1 - 0.0059w 0.2047The value of | can be 0 to 0.003, 0.0005 to 0.0025, 0.001 to 0.002, and all of these ranges, as well as subranges; the value of w can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or any two of these values. The value of w can be 5% to 100%, 10% to 90%, 20% to 80%, 30% to 70%, 40% to 60%, and all of these ranges, as well as subranges.

[0027] For ease of understanding, we define the length direction of the electrode assembly in its unfolded state as the X direction, its width direction as the Y direction, and its 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. For example... Figure 1 As shown, the electrode assembly 001 includes a positive electrode 10, a negative electrode 20, and a separator 30; wherein, the positive electrode 10 includes a positive current collector 103 and a first positive electrode material layer 101 disposed on the surface of the positive current collector 103 near the winding center, and the negative electrode 20 includes a negative current collector 201 and a negative electrode material layer 202 disposed on both surfaces of the negative current collector 201. The electrode assembly 001 includes a flat region 0011 and two bent regions 0012 connected to the flat region 0011. A plurality of first protrusions 1011 are provided on the first positive electrode material layer 101 located in the flat region 0011, and a plurality of second protrusions 1012 are provided on the first positive electrode material layer 101 located in the bent regions 0012. Figure 2 As shown, along the thickness direction Z of the unfolded positive electrode sheet 10, the average height of the plurality of first protrusions 1011 in the flat region 0011 is H1 mm, and the average height of the plurality of second protrusions 1012 in the bent region 0012 is H2 mm. It is understood that the number, size, spacing, height of the first and second protrusions, the number of turns of the electrode assembly, and the number of layers of the positive electrode sheet are merely illustrative examples and are not intended to limit this application. In this application, "plural" refers to two or more.

[0028] When |H1-0.0059w 0.2047 When the value of | is too high, that is, higher than the upper limit of this application, there are two situations: H1-0.0059w 0.2047 The value is less than -0.003 or H1-0.0059w 0.2047 When the value is greater than 0.003, the mass percentage of Si in the negative electrode active material, w, cannot be well matched with the average height H1 of the first protrusion located in the flat region. When H1 = 0.0059w... 0.2047When the value is less than -0.003, the first protrusion cannot provide sufficient space for the volume expansion of the negative electrode during the cycle of the secondary battery. The expansion stress generated by the negative electrode during cycling cannot be adequately buffered, leading to an increased cycle expansion rate of the secondary battery, affecting its cycle performance. It also causes excessive stretching of the negative electrode current collector (such as copper foil), increasing the risk of breakage and impacting the safety performance of the secondary battery. When H1-0.0059w 0.2047When the value is greater than 0.003, the gaps between electrode layers in the flat and bent regions are relatively large. Electrolyte bridging is prone to occur between some electrode layers, leading to increased electrochemical impedance during battery cycling and greater resistance to ion conduction, thus reducing the battery's cycle performance. Simultaneously, an excessively high first protrusion also reduces the battery's energy density. When H1 ≥ H2, the second protrusion cannot provide sufficient space for the volume expansion of the negative electrode in the bent region. The expansion stress generated by the negative electrode during cycling cannot be adequately buffered, and the second protrusion cannot effectively disperse the bending stress in the bent region, increasing the battery's cycle expansion rate and the risk of negative electrode current collector breakage, resulting in decreased cycle performance and safety of the battery. By setting multiple first protrusions and multiple second protrusions on the first positive electrode material layer in the flat region and the bending region respectively, and adjusting the mass percentage content w of Si element in the negative electrode active material and the average height H1 of the first protrusion in the flat region to satisfy the above relationship, different first protrusion heights can be matched according to the mass percentage content of Si element in the negative electrode active material. Space for expansion of the negative electrode sheet can be reserved in advance during the winding stage, thereby reducing the excessive stretching of the negative electrode current collector copper foil caused by the expansion of the negative electrode sheet during cycling, reducing the risk of breakage of the negative electrode current collector copper foil and reducing the cycle expansion rate of the secondary battery. At the same time, the gap between the electrode layers is relatively moderate, which can make the electrolyte evenly distributed between the electrode layers, improve the wetting effect of the electrolyte on the electrode assembly, reduce the risk of electrolyte bridging between electrode layers, thereby reducing impedance and improving the cycle performance of the secondary battery. Furthermore, by adjusting H1 < H2, the average height H2 of the second protrusion in the bending region is greater than the average height H1 of the first protrusion in the straight region. Since the bending region of the electrode assembly experiences periodic bending stress compared to the straight region during charging and discharging, by setting a higher second protrusion on the first positive electrode material layer in the bending region, expansion space can be reserved for the negative electrode sheet in the bending region, and the impact of bending stress on the electrode assembly can be reduced. This further reduces the cycle expansion rate of the secondary battery and the risk of negative electrode current collector breakage, improving the cycle performance and safety performance of the secondary battery. Conversely, by setting a lower first protrusion on the first positive electrode material layer in the straight region, sufficient expansion space can be reserved for the negative electrode sheet in the straight region while maintaining the tightness between the electrode layers in the straight region, reducing interface resistance, and further improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application has both high energy density and good cycle performance and safety performance.

[0029] The aforementioned "negative electrode material layer disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its own thickness direction, or on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface of the negative electrode current collector, or only a portion thereof; this application has no particular limitation, as long as the purpose of this application is achieved. The aforementioned "first positive electrode material layer disposed on one surface of the positive electrode current collector" means that the first positive electrode material layer can be disposed on the surface of the positive electrode current collector away from the winding center, or on the surface of the positive electrode current collector close to the winding center. The "surface" here can be the entire surface of the positive electrode current collector, or only a portion thereof; this application has no particular limitation, as long as the purpose of this application is achieved.

[0030] In one or more embodiments of this application, 0.0002 ≤ H1 ≤ 0.0089. For example, the value of H1 can be 0.0002, 0.0005, 0.001, 0.002, 0.004, 0.006, 0.008, 0.0089, or a range consisting of any two of these values. The value range of H1 can be 0.0002 to 0.0089, 0.0005 to 0.008, 0.001 to 0.006, 0.002 to 0.004, and all such ranges and sub-ranges. By adjusting the value of H1 to be within the aforementioned range, it is beneficial to better match the average height H1 of the first protrusion located in the flat region with the mass percentage w of Si element in the negative electrode active material. This allows for sufficient expansion space to be reserved for the negative electrode sheet during the winding stage, further reducing the risk of copper foil breakage in the negative electrode current collector and lowering the cycle expansion rate of the secondary battery. At the same time, it helps to make the electrolyte distribution more uniform between the electrode layers, improving the wetting effect of the electrolyte on the electrode assembly and enhancing the cycle performance of the secondary battery. As a result, the secondary battery has good cycle performance and safety performance.

[0031] In one or more embodiments of this application, 0 ≤ |H2 - 0.017w 0.3139 |≤0.003. For example, |H2-0.017w 0.3139 The value of | can be 0, 0.0005, 0.001, 0.0015, 0.002, 0.0025, 0.003, or a range of any two of these values, |H2-0.017w 0.3139The value of | can range from 0 to 0.003, 0.0005 to 0.0025, 0.001 to 0.002, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of H2 and w to satisfy the above relationship, it is beneficial to make the average height H2 of the second protrusion located in the bending region better match the mass percentage of Si element w in the negative electrode active material. By matching different heights of the second protrusion according to the mass percentage of Si element in the negative electrode active material, space for expansion of the negative electrode sheet located in the bending region is reserved in advance, and the bending stress of the negative electrode sheet in the bending region is effectively dispersed. This reduces the excessive stretching of the negative electrode current collector (such as copper foil) caused by the expansion of the negative electrode sheet in the bending region during cycling, further reducing the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery. As a result, the secondary battery has good cycle performance and safety performance.

[0032] In one or more embodiments of this application, 0.004 ≤ H2 ≤ 0.020. For example, the value of H2 can be 0.004, 0.006, 0.008, 0.010, 0.012, 0.014, 0.016, 0.018, 0.020, or a range consisting of any two of these values. The value range of H2 can be 0.004 to 0.020, 0.006 to 0.018, 0.008 to 0.016, 0.010 to 0.014, and all such ranges and subranges. By adjusting the value of H2 to be within the above range, it is beneficial to better match the average height H2 of the second protrusion in the bending region with the mass percentage w of Si element in the negative electrode active material. This allows for sufficient expansion space to be reserved in advance for the negative electrode sheet in the bending region and better disperses the bending stress of the negative electrode sheet in the bending region. This further reduces the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery, thus giving the secondary battery good cycle performance and safety performance.

[0033] In one or more embodiments of this application, 0.002 ≤ H2 - H1 ≤ 0.014. For example, the value of H2 - H1 can be 0.002, 0.004, 0.006, 0.008, 0.010, 0.012, 0.014, or a range consisting of any two of these values. The value range of H2 - H1 can be 0.002 to 0.014, 0.004 to 0.012, 0.006 to 0.010, and all such ranges and sub-ranges. By adjusting the value of H2 - H1 to be within the above range, the average height H1 of the multiple first protrusions and the average height H2 of the multiple second protrusions are made to have a more moderate difference. This is beneficial to improving the wetting effect of the electrolyte on the electrode located in the bending region, and at the same time, it is beneficial to reduce the stress gradient between the bending region and the straight region, thereby reducing the risk of peeling between the first positive electrode material layer and the positive electrode current collector, and between the negative electrode material layer and the negative electrode current collector, which would affect the electron transport of the electrode. As a result, the secondary battery has good cycle performance.

[0034] In one or more embodiments of this application, a plurality of first protrusions are distributed in a dotted pattern on the first positive electrode material layer located in the flat region, and the density of the first protrusions is x protrusions / cm³. 2 Multiple second protrusions are distributed in a dot-like pattern on the first positive electrode material layer located in the bending region, with a density of y protrusions / cm³. 2 0.5≤x / y≤1, 5≤x≤9. For example, the value of x / y can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any two of these values. The range of x / y can be 0.5 to 1, 0.6 to 0.9, 0.7 to 0.8, and all of these ranges, as well as subranges. The value of x can be 5, 6, 7, 8, 9, or any two of these values. The range of x can be 5 to 9, 6 to 8, and all of these ranges, as well as subranges. By adjusting x / y and ensuring that the value of x is within the aforementioned range, bending stress can be better dispersed, reducing the risk of negative electrode current collector breakage and / or negative electrode material layer cracking caused by bending stress. Furthermore, it allows for sufficient expansion space to be reserved for the negative electrode sheet during the winding stage, better dispersing cyclic expansion stress and further reducing the cyclic expansion rate of the secondary battery and the risk of negative electrode current collector breakage. Simultaneously, it optimizes the distribution of electrolyte between electrode layers, further improving the electrolyte's wetting effect on the electrode assembly, reducing impedance, and improving the cycle performance of the secondary battery. In addition, a moderate gap between electrode layers helps reduce energy density loss for secondary batteries of the same specifications, thus enabling the secondary battery to have both high energy density and good cycle performance and safety.

[0035] In one or more embodiments of this application, 5 ≤ y ≤ 18. For example, the value of y can be 5, 6, 8, 10, 12, 14, 16, 18, or a range consisting of any two of these values. The range of y can be 5 to 18, 6 to 16, 8 to 14, 10 to 12, and all of these ranges, as well as sub-ranges. By adjusting the value of y to be within the above range, the number of second protrusions set in the bending area is moderate, which is beneficial for providing sufficient expansion space for the negative electrode sheet in this part, further reducing the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery; at the same time, it is beneficial for the flow and uniform distribution of electrolyte between the electrode layers, thereby improving the wetting effect of electrolyte on the bending area of ​​the electrode assembly. Therefore, the secondary battery has good cycle performance and safety performance.

[0036] In one or more embodiments of this application, the maximum outer circle diameter of a single first protrusion on the surface of the first positive electrode material layer is D1 μm, where 1.5 ≤ D1 ≤ 3. For example, the value of D1 can be 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, or a range consisting of any two of these values. The range of D1 can be 1.5 to 3, 1.6 to 2.8, 1.8 to 2.6, 2 to 2.4, and all such ranges, as well as sub-ranges. Figure 3 As shown, multiple first protrusions 1011 are provided on the surface of the first positive electrode material layer 101 located in the flat region 0011. The outer contour of the orthographic projection of a single first protrusion 1011 on the surface of the first positive electrode material layer 101 is circular. Therefore, the maximum circumscribed circle diameter D1 of a single first protrusion 1011 on the surface of the first positive electrode material layer 101 is the diameter of the outer contour of the orthographic projection of the first protrusion 1011. By adjusting the value of D1 to be within the above range, it is beneficial to provide sufficient expansion space for the negative electrode sheet located in the flat region, further reducing the risk of breakage of the negative electrode current collector and reducing the cycle expansion rate of the secondary battery. At the same time, the size of the first protrusions on the surface of the first positive electrode material layer is moderate, which is beneficial to improve the wetting effect of the electrolyte on the electrode sheet and reduce the risk of increased electrochemical impedance due to increased local aggregation of electrolyte side reactions, thereby improving the cycle performance of the secondary battery. In addition, it is beneficial to make the first protrusions have high support strength, thereby improving the structural stability of the positive electrode sheet located in the flat region, so that the secondary battery has high cycle performance and safety performance.

[0037] In one or more embodiments of this application, the maximum outer circle diameter of a single second protrusion on the surface of the first positive electrode material layer is D2 μm, where 2.5 ≤ D2 ≤ 4. For example, the value of D2 can be 2.5, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, or a range consisting of any two of these values. The range of D2 can be 2.5 to 4, 2.6 to 3.8, 2.8 to 3.6, 3 to 3.4, and all such ranges, as well as sub-ranges. Figure 3As shown, multiple second protrusions 1012 are provided on the surface of the first positive electrode material layer 101 located in the bending region 0012. The outer contour of the orthographic projection of a single second protrusion 1012 on the surface of the first positive electrode material layer 101 is circular. Therefore, the maximum circumscribed circle diameter D2 of a single second protrusion 1012 on the surface of the first positive electrode material layer 101 is the diameter of the outer contour of the orthographic projection of the second protrusion 1012. By adjusting the value of D2 to be within the above range, it is beneficial to provide sufficient expansion space for the negative electrode sheet located in the bending region, further reducing the risk of breakage of the negative electrode current collector and reducing the cycle expansion rate of the secondary battery. At the same time, the size of the second protrusions on the surface of the first positive electrode material layer is moderate, which is beneficial to improve the wetting effect of the electrolyte on the electrode sheet and reduce the risk of increased electrochemical impedance due to increased local aggregation of electrolyte side reactions, thereby improving the cycle performance of the secondary battery. In addition, it is beneficial to make the second protrusions have high support strength, thereby improving the structural stability of the positive electrode sheet located in the bending region, so that the secondary battery has high cycle performance and safety performance.

[0038] In one or more embodiments of this application, the maximum circumscribed circle diameter of a single first protrusion on the surface of the first positive electrode material layer is D1 μm, where 1.5 ≤ D1 ≤ 3. For example, the value of D1 can be 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, or a range consisting of any two of these values. The value range of D1 can be 1.5 to 3, 1.6 to 2.8, 1.8 to 2.6, 2 to 2.4, and all such ranges and sub-ranges; and / or, the maximum circumscribed circle diameter of a single second protrusion on the surface of the first positive electrode material layer is D2. μm, 2.5≤D2≤4, for example, the value of D2 can be 2.5, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4 or any two of these values. The range of D2 values ​​can be 2.5 to 4, 2.6 to 3.8, 2.8 to 3.6, 3 to 3.4, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of D1 and / or D2 to be within the above ranges, it is beneficial to provide sufficient expansion space for the negative electrode sheet located in the flat and bent regions, further reducing the risk of breakage of the negative electrode current collector and reducing the cycle expansion rate of the secondary battery. At the same time, the size of the first and second protrusions on the surface of the first positive electrode material layer is moderate, which is beneficial to improving the wetting effect of the electrolyte on the electrode sheet and reducing the risk of increased electrochemical impedance due to increased local aggregation side reactions of the electrolyte, thus improving the cycle performance of the secondary battery. In addition, it is beneficial to make the first and second protrusions have high support strength, thereby improving the overall structural stability of the positive electrode sheet, so that the secondary battery has high cycle performance and safety performance.

[0039] This application does not impose any particular limitation on the shape of the outer contour of the orthographic projection of the first protrusion and the second protrusion on the surface of the first positive electrode material layer, as long as the purpose of this application can be achieved. For example, the shape of the outer contour of the orthographic projection of the first protrusion and / or the second protrusion on the surface of the first positive electrode material layer can be circular, elliptical, or polygonal. It is understood that when the shape of the outer contour of the orthographic projection of the first protrusion and / or the second protrusion on the surface of the first positive electrode material layer is irregular, the maximum circumscribed circle diameter D1 of a single first protrusion on the surface of the first positive electrode material layer is the equivalent diameter of the outer contour of the orthographic projection of the first protrusion, and the maximum circumscribed circle diameter D2 of a single second protrusion on the surface of the first positive electrode material layer is the equivalent diameter of the outer contour of the orthographic projection of the second protrusion. The equivalent diameter is the distance between the two farthest points on the outer contour. It should be noted that the shapes of the outer contours of the orthographic projection of the first protrusion and the second protrusion on the surface of the first positive electrode material layer in this application can be the same or different.

[0040] In one or more embodiments of this application, along the thickness direction of the electrode assembly, the number of layers of the positive electrode sheet is N, 10≤N≤39, along the length direction of the unfolded positive electrode sheet, the total length of the region where the multiple first protrusions and multiple second protrusions are located is L1 mm, the length of the first positive electrode material layer is L2 mm, and N, L1 and L2 satisfy: (1) 10≤N<19, 0.3≤L1 / L2<0.5, for example, when 10≤N<19, the value of L1 / L2 can be 0.3, 0.32, 0.34, 0.36, 0.38, 0.4, 0.42, 0.44, 0.46, 0.48, 0.49 or a range of any two of these values, and the value range of L1 / L2 can be from 0.3 to 0.5 (excluding 0). .5), 0.32 to 0.49, 0.34 to 0.48, 0.36 to 0.46, 0.38 to 0.44, 0.4 to 0.42 and all of these ranges and subranges; (2) 19 ≤ N < 29, 0.5 ≤ L1 / L2 < 0.7, for example, when 19 ≤ N < 29, the value of L1 / L2 can be 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0. 66, 0.68, 0.69 or any two of these values, the range of L1 / L2 can be 0.5 to 0.7 (excluding 0.7), 0.52 to 0.69, 0.54 to 0.68, 0.56 to 0.66, 0.58 to 0.64, 0.6 to 0.62 and all of these ranges and subranges; (3) 29≤N≤39, 0.7≤L1 / L2≤0.9, for example, when 29≤N≤3 At 9 o'clock, the value of L1 / L2 can be 0.7, 0.72, 0.74, 0.76, 0.78, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, or any two of these values. The range of L1 / L2 can be 0.7 to 0.9, 0.72 to 0.88, 0.74 to 0.86, 0.76 to 0.84, 0.78 to 0.82, and all such ranges and sub-ranges. By adjusting the values ​​of N, L1, and L2 to satisfy the above relationship, the size of the area containing multiple first protrusions and multiple second protrusions on the positive electrode sheet can be controlled according to the number of layers N of the positive electrode sheet in the electrode assembly. This helps to reserve sufficient expansion space for the negative electrode sheet while further reducing the loss of energy density in the secondary battery. Thus, the secondary battery has both high energy density and good cycle performance and safety performance.

[0041] This application does not impose any particular restrictions on the specific locations of the multiple first protrusions and multiple second protrusions in the positive electrode sheet, as long as the purpose of this application can be achieved. Optionally, multiple first protrusions and multiple second protrusions are provided along the length direction of the unfolded positive electrode sheet, starting from the winding start end of the positive electrode sheet.

[0042] It is understood that a single turn of the positive electrode in a wound electrode assembly refers to a turn of the positive electrode, starting from one end and ending at the end of one full rotation along the winding direction of the electrode assembly, with the starting and ending points aligned in the thickness direction of the electrode assembly; a single turn of the negative electrode can also be defined similarly. In this application, the starting point of the first turn of the positive electrode is the beginning of the first positive electrode material layer of the positive electrode, the starting point of the first turn of the negative electrode is the beginning of the negative electrode material layer of the negative electrode, and so on. In this application, a single turn of the positive electrode comprises two layers, meaning that along the thickness direction of the electrode assembly, the number of positive electrode layers in each turn of the positive electrode is two.

[0043] In one or more embodiments of this application, in any ring with a first protrusion in the flat region, the elongation of the positive current collector in the flat region of the ring is e1, and the elongation of the negative current collector adjacent to the positive current collector in the ring along the thickness direction of the electrode assembly is e2. The length of the first positive electrode material layer in the flat region is d mm, and 8.5 ≤ |e2-e1|d / H2 ≤ 39. For example, the value of |e2-e1|d / H2 can be 8.5, 10, 15, 20, 25, 30, 35, 39, or a range of any two of these values. The value range of |e2-e1|d / H2 can be 8.5 to 39, 10 to 35, 15 to 30, 20 to 25, and all of these ranges, as well as sub-ranges. By adjusting the value of |e2-e1|d / H2 within the above range, it is beneficial to reduce the risk of electrode breakage caused by excessive stretching of the negative and positive electrode sheets. At the same time, it is beneficial to ensure that the electrolyte better wets the electrode assembly and maintains the tightness between the electrode layers, reducing the risk of electrolyte bridging between the electrode layers, reducing impedance, and improving the cycle performance of the secondary battery. Thus, the secondary battery has good cycle performance and safety performance.

[0044] In one or more embodiments of this application, 1% ≤ e1 ≤ 2.5%. For example, the value of e1 can be 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, or a range consisting of any two of these values. The value range of e1 can be 1% to 2.5%, 1.2% to 2.4%, 1.4% to 2.2%, 1.6% to 2%, and all of these ranges, as well as sub-ranges. By controlling the value of e1 within the above range, it is beneficial to reduce the risk of positive electrode current collector breakage due to excessive stretching of the positive electrode sheet, and to reduce the risk of poor electrode contact during cycling, thereby giving the secondary battery good cycle performance and safety performance.

[0045] In one or more embodiments of this application, 1% ≤ e2 ≤ 4%. For example, the value of e2 can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or a range consisting of any two of these values. The value range of e2 can be 1% to 4%, 1.5% to 3.5%, 2% to 3%, and all of these ranges, as well as sub-ranges. By controlling the value of e2 within the above range, it is beneficial to reduce the risk of breakage of the negative electrode current collector due to excessive stretching of the negative electrode sheet, and to reduce the risk of poor electrode contact during cycling, thereby giving the secondary battery good cycle performance and safety performance.

[0046] In one or more embodiments of this application, 28 ≤ d ≤ 200. For example, the value of d can be 28, 40, 60, 80, 100, 120, 140, 160, 180, 200, or a range consisting of any two of these values. The value range of d can be 28 to 200, 40 to 180, 60 to 160, 80 to 140, 100 to 120, and all such ranges and sub-ranges. By adjusting the value of d to be within the above range, it is beneficial to make the stress distribution in the electrode assembly more uniform, thereby giving the secondary battery good cycle performance and safety performance.

[0047] In one or more embodiments of this application, in any loop with a second protrusion in the bending region, the elongation of the positive current collector in the bending region of the loop is e3, the elongation of the negative current collector adjacent to the positive current collector of the loop along the thickness direction of the electrode assembly is e4, the radius of the inscribed circle of the negative current collector adjacent to the positive current collector of the loop and far from the winding center is r mm, and 3≤|e4-e3|r / H2≤12. For example, the value of |e4-e3|r / H2 can be 3, 4, 6, 8, 10, 12 or a range of any two of these values, and the range of |e4-e3|r / H2 can be 3 to 12, 4 to 10, 6 to 8, and all of these ranges, as well as sub-ranges. Figure 4 As shown, at any loop in the bending region 0012 where the second protrusion 102 is provided, the radius of the inscribed circle of the negative electrode current collector adjacent to the positive electrode current collector of that loop and far from the winding center is r mm. By adjusting the value of |e4-e3|r / H2 within the above range, it is beneficial to reduce the risk of electrode breakage due to excessive stretching of the negative and positive electrode sheets. At the same time, it is beneficial to ensure better wetting of the electrode assembly by the electrolyte and maintain the tightness between the electrode layers, reducing the risk of electrolyte bridging between electrode layers, reducing impedance, and improving the cycle performance of the secondary battery. Thus, the secondary battery has good cycle performance and safety performance.

[0048] In one or more embodiments of this application, 1% ≤ e3 ≤ 3%. For example, the value of e3 can be 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or a range consisting of any two of these values. The value range of e3 can be 1% to 3%, 1.2% to 2.8%, 1.4% to 2.6%, 1.6% to 2.4%, 1.8% to 2.2%, and all such ranges and sub-ranges. By controlling the value of e3 within the above range, it is beneficial to reduce the risk of positive electrode current collector breakage due to excessive stretching of the positive electrode sheet, and to reduce the risk of poor electrode contact during cycling, thereby giving the secondary battery good cycle performance and safety performance.

[0049] In one or more embodiments of this application, 2% ≤ e4 ≤ 8%. For example, the value of e4 can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, or a range consisting of any two of these values. The value range of e4 can be 2% to 8%, 3% to 7%, 4% to 6%, and all of these ranges, as well as sub-ranges. By adjusting the value of e4 to be within the above range, it is beneficial to reduce the risk of breakage of the negative electrode current collector due to excessive stretching of the negative electrode sheet, and it is also beneficial to reduce the risk of poor electrode contact during cycling, thereby giving the secondary battery good cycle performance and safety performance.

[0050] In one or more embodiments of this application, 1 ≤ r ≤ 6. For example, the value of r can be 1, 2, 3, 4, 5, 6, or a range consisting of any two of these values. The range of r can be 1 to 6, 2 to 5, 3 to 4, and all such ranges, as well as sub-ranges. By adjusting the value of r to be within the above range, it is beneficial to improve the cycle performance and safety performance of the secondary battery while simultaneously increasing the manufacturing yield of the secondary battery.

[0051] This application does not impose any particular restrictions on the methods for adjusting the values ​​of e1, e2, e3, and e4, as long as the purpose of this application can be achieved. For example, the elongation of the current collector can be adjusted by controlling the thickness of the current collector. When other conditions remain constant, increasing the thickness of the positive electrode current collector increases e1 and e3, and vice versa; or, when other conditions remain constant, increasing the thickness of the negative electrode current collector increases e2 and e4, and vice versa.

[0052] In this application, setting different protrusions at different locations on the electrode (flat region and / or bent region) will also affect the elongation of the current collector at the corresponding location. When the parameters of the protrusions (such as average height, density, maximum circumscribed circle diameter of a single protrusion on the surface of the material layer, etc.) change, the elongation of the current collector will also be affected. For example, with other conditions fixed, if the value of H1 increases, the values ​​of e1 and e2 increase, and vice versa; if the value of H2 increases, the values ​​of e3 and e4 increase, and vice versa; if the value of x increases, the values ​​of e1 and e2 increase, and vice versa; if the value of y increases, the values ​​of e3 and e4 increase, and vice versa; if the value of D1 decreases, the values ​​of e1 and e2 increase, and vice versa; if the value of D2 decreases, the values ​​of e3 and e4 increase, and vice versa.

[0053] In one or more embodiments of this application, the positive electrode further includes a second positive electrode material layer disposed on another surface of the positive electrode current collector. A plurality of first recesses are disposed on the second positive electrode material layer located in the flat region, and along the thickness direction of the electrode assembly, the first recesses correspond one-to-one with the first protrusions. For example... Figure 1 As shown, the positive electrode 10 includes a positive current collector 103 and a first positive electrode material layer 101 and a second positive electrode material layer 102 respectively disposed on both sides of the positive current collector 103. The first positive electrode material layer 101 is disposed on the surface of the positive current collector 103 near the winding center, and the second positive electrode material layer 102 is disposed on the surface of the positive current collector 103 away from the winding center. A plurality of first protrusions 1011 are disposed on the first positive electrode material layer 101 located in the flat region 0011, and a plurality of first recesses 1021 are disposed on the second positive electrode material layer 102 located in the flat region 0011. Along the thickness direction Z of the electrode assembly 001, the positions of the first protrusions 1011 and the first recesses 1021 correspond one-to-one. The above-mentioned design improves the operability of setting the first protrusion and the first concave portion on the positive electrode sheet in the flat region. At the same time, it helps to reserve space for expansion of the negative electrode sheet in advance during the winding stage, reducing the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery. In addition, it helps to improve the wetting effect of the electrolyte on the electrode assembly, reduce impedance, and improve the cycle performance of the secondary battery, thereby improving the cycle performance and safety performance of the secondary battery while meeting the requirements for mass production manufacturability.

[0054] In one or more embodiments of this application, the positive electrode sheet further includes a second positive electrode material layer disposed on another surface of the positive electrode current collector. A plurality of second recesses are disposed on the second positive electrode material layer located in the bending region. Along the thickness direction of the unfolded positive electrode sheet and along the thickness direction of the electrode assembly, the second recesses correspond one-to-one with the second protrusions. For example... Figure 1As shown, the positive electrode sheet 10 includes a positive current collector 103 and a first positive electrode material layer 101 and a second positive electrode material layer 102 respectively disposed on both sides of the positive current collector 103. The first positive electrode material layer 101 is disposed on the surface of the positive current collector 103 near the winding center, and the second positive electrode material layer 102 is disposed on the surface of the positive current collector 103 away from the winding center. A plurality of second protrusions 1012 are disposed on the first positive electrode material layer 101 located in the bending region 0012, and a plurality of second recesses 1022 are disposed on the second positive electrode material layer 102 located in the bending region 0012. Along the thickness direction Z of the positive electrode sheet 10 after it is unfolded, the positions of the second protrusions 1012 and the second recesses 1022 correspond one-to-one. The above-mentioned design improves the operability of setting the second protrusion and the second concave portion on the positive electrode sheet in the bending area; at the same time, it helps to reserve space for expansion of the negative electrode sheet in advance during the winding stage, reducing the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery; in addition, it helps to improve the wetting effect of the electrolyte on the electrode assembly, reduce impedance, and improve the cycle performance of the secondary battery, thereby improving the cycle performance and safety performance of the secondary battery while meeting the requirements for mass production manufacturability.

[0055] In one or more embodiments of this application, the positive electrode sheet further includes a second positive electrode material layer disposed on another surface of the positive electrode current collector. The second positive electrode material layer in the flat region has a plurality of first recesses, with each first recess corresponding to a first protrusion along the thickness direction of the electrode assembly; and / or, the second positive electrode material layer in the bent region has a plurality of second recesses, with each second recess corresponding to a second protrusion along the thickness direction of the unfolded positive electrode sheet and along the thickness direction of the electrode assembly. This configuration improves the operability of providing first protrusions and first recesses on the positive electrode sheet in the flat region and second protrusions and second recesses on the positive electrode sheet in the bent region. Simultaneously, it facilitates reserving expansion space for the negative electrode sheet in advance during the winding stage, reducing the risk of breakage of the negative electrode current collector and the cycle expansion rate of the secondary battery. Furthermore, it improves the wetting effect of the electrolyte on the electrode assembly, reduces impedance, and improves the cycle performance of the secondary battery, thereby improving both the cycle performance and safety performance of the secondary battery while meeting mass production manufacturability requirements.

[0056] The "surface" in the above-mentioned "second positive electrode material layer disposed on another surface of the positive electrode current collector" can be the entire area of ​​the surface of the positive electrode current collector or a part of the surface of the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0057] This application does not impose any particular restrictions on the method of controlling the mass percentage w of Si element in the negative electrode active material, as long as the purpose of this application can be achieved. For example, if the negative electrode active material includes silicon-based materials, and the mass percentage of silicon-based materials in the negative electrode active material is fixed, the mass percentage w of Si element in the negative electrode active material can be controlled by selecting different types of silicon-based materials; or, if the type of silicon-based materials in the negative electrode active material is fixed, the mass percentage of Si element in the negative electrode active material can be controlled by controlling the mass percentage of silicon-based materials in the negative electrode active material; or, the mass percentage w of Si element in the negative electrode active material can be controlled by selecting different types of silicon-based materials and controlling the mass percentage of silicon-based materials in the negative electrode active material.

[0058] This application does not impose any particular limitation on the type of silicon-based material, as long as it can achieve the purpose of this application. For example, silicon-based materials include, but are not limited to, elemental silicon and silicon oxide (SiO2). m The silicon-carbon composite, silicon-metal composite, and silicon alloy are selected from the following: (0 < m < 2). The silicon-metal composite includes at least one of the following: Si-Ti composite, Si-Fe composite, Si-Ni composite, Si-Sn composite, Si-Al composite, Si-CuO composite, and Si-Fe2O3 composite. The silicon alloy includes at least one of the following: Mg2Si, TiSi2, Ti5Si3, NiSi2, FeSi2, SnSi, Sn4Si3, and AlSi2.

[0059] In addition to silicon-based materials, the negative electrode active materials in this application may also include other types of negative electrode active materials. For example, other types of negative electrode active materials may include natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, and spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloy or metallic lithium.

[0060] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector, wherein the composite current collector may be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 3.5 μm to 15 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 120 μm. Optionally, the negative electrode material layer may also include a conductive agent, a negative electrode binder, and a thickener. This application does not impose any particular limitation on the types of conductive agents, negative electrode binders, and thickeners, as long as they achieve the purpose of this application. For example, the conductive agent includes, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, scaly graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. 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 polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The negative 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, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The thickener may include, but is not limited to, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose. This application does not impose any particular restrictions on the mass ratio of negative electrode active material, conductive agent, negative electrode binder, and thickener in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0061] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves 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). In this application, there are no particular limitations on the thickness of the positive electrode current collector, the first positive electrode material layer, and the second positive electrode material layer, as long as they achieve the purpose of this application. For example, the thickness of the positive electrode current collector may be 5 μm to 20 μm, the thickness of the first positive electrode material layer may be 30 μm to 120 μm, and the thickness of the second positive electrode material layer may be 30 μm to 120 μm. The first and second positive electrode material layers of this application contain positive electrode active materials. This application does not impose any particular limitation on the type of positive electrode active material, as long as it achieves the purpose of this application. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (NCM955 (LiNi)). 0.90 Co 0.05 Mn 0.05 At least one of the following: lithium nickel cobalt aluminum oxide (NCM2), 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, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. It should be noted that the positive electrode active material in the first positive electrode material layer and the positive electrode active material in the second positive electrode material layer may be the same or different. In this application, 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 be the same type as the negative electrode binder in the aforementioned negative electrode material layer. 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 the purpose of this application can be achieved. For example, the conductive agent can be the same type as the conductive agent in the aforementioned negative electrode material layer. It should be noted that the positive electrode binder and conductive agent in the first and second positive electrode material layers can be the same or different. 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 choose according to actual needs, as long as the purpose of this application can be achieved.

[0062] 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 includes, but is not limited to, the following steps: (1) preparing a first positive electrode material layer slurry and a second positive electrode material layer slurry; (2) coating the first positive electrode material layer slurry on one surface of the positive electrode current collector, and drying it to obtain a positive electrode sheet with a first positive electrode material layer on one side; (3) coating the second positive electrode material layer slurry on the other surface of the positive electrode current collector, and drying it to obtain a positive electrode sheet with a first positive electrode material layer and a second positive electrode material layer; (4) after cold pressing and slitting, placing the positive electrode sheet, separator, negative electrode sheet and separator in sequence, pre-winding them, determining the straight area and bending area of ​​the electrode assembly and the number of winding turns, thereby determining the straight area and bending area of ​​the positive electrode sheet and the number of layers of the positive electrode sheet; setting a first protrusion on the first positive electrode material layer in the straight area according to the requirements, and setting a second protrusion on the first positive electrode material layer in the bending area according to the requirements, thereby obtaining the positive electrode sheet.

[0063] This application does not impose any particular limitation on the solid content of the first and second positive electrode material layer slurries, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the drying temperature and time, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the process parameters for cold pressing and slitting, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the method of setting the first and second protrusions on the first positive electrode material layer in the flat and bent regions, respectively. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. For example, an embossing process can be used, i.e., using embossing rollers and rubber rollers to set the first and second protrusions on the first positive electrode material layer in the flat and bent regions, respectively. In this case, the second positive electrode material layer in the flat region has multiple first recesses, with each first recess corresponding to a first protrusion along the thickness direction of the unfolded positive electrode sheet; similarly, the second positive electrode material layer in the bent region has multiple second recesses, with each second recess corresponding to a second protrusion along the thickness direction of the unfolded positive electrode sheet. Along the thickness direction of the positive electrode sheet, the shapes of the orthographic projections of the first and second protrusions onto the positive electrode sheet can be controlled by the shape of the stainless steel needles on the embossing roller. The average height H1 of the first protrusion and the average height H2 of the second protrusion can be controlled by the specifications of the stainless steel needles on the embossing roller. The maximum outer circle diameter D1 of a single first protrusion and the maximum outer circle diameter D2 of a single second protrusion on the surface of the first positive electrode material layer can be controlled by the specifications of the stainless steel needles on the embossing roller. The density x of the first protrusion and the density y of the second protrusion can be controlled by the distribution density of the stainless steel needles on the embossing roller.

[0064] In one or more embodiments of this application, the secondary battery includes a casing, an electrode assembly, and an electrolyte, wherein the electrode assembly and the electrolyte are housed in the casing; the electrode assembly includes a positive electrode, a separator, and a negative electrode, wherein the separator is disposed between the positive electrode and the negative electrode to separate the positive electrode and the negative electrode, prevent short circuits inside the secondary battery, allow electrolyte ions to pass freely, and does not affect the electrochemical charging and discharging process.

[0065] This application does not impose any particular restrictions on the diaphragm, as long as it can achieve the purpose of this application. For example, the material of the diaphragm may include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on 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.

[0066] In one or more embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have particular limitations on the inorganic particles; for example, the inorganic particles may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not impose any particular limitation on the binder; for example, the binder may be at least one of the above-mentioned positive electrode binders or negative electrode binders. The polymer layer comprises a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0067] 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 can be from 3 μm to 30 μm.

[0068] This application does not impose any particular restrictions on the electrolyte, as long as it can achieve the purpose of this application. For example, the electrolyte includes lithium salts and non-aqueous solvents.

[0069] This application does not impose any particular limitation on the type of lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalateborate)borate (LiBOB), or lithium difluorooxalateborate (LiDFOB). This application does not limit the content of lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0070] This application does not impose any particular limitation on the types of non-aqueous solvents mentioned above, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are 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 fluorinated carbonate 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 carbonate compounds may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate, butylene carbonate, or ethylene ethylene carbonate. The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The 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 ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. 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. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0071] This application does not impose any particular restrictions on the outer casing; any casing known in the art can be used, as long as it achieves the purpose of this application. For example, the outer casing can be a rigid casing or a flexible casing; wherein, the material of the rigid casing can be metal or rigid plastic, and this application does not limit the type of metal, and any metal casing known in the art can be used, as long as it achieves the purpose of this application; the flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0072] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0073] 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 positive electrode, separator, negative electrode, and separator in sequence, and winding them as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery.

[0074] A second aspect of this application provides an electronic device that includes a secondary battery from any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0075] 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: laptop computers, 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.

[0076] Example

[0077] 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.

[0078] Test methods and apparatus:

[0079] Tests for H1, H2, x, y, D1, D2, L1, L2, N, and d:

[0080] At an ambient temperature of 25°C, the lithium-ion batteries of each embodiment and comparative example were discharged at a constant current of 0.5C to 2.0V and then disassembled under an argon atmosphere to obtain electrode assemblies. The straight and bent regions of the electrode assemblies were observed with the naked eye, and the length of the straight region of the electrode assembly was measured to obtain d mm. At the same time, the positive electrode areas located in the straight and bent regions were determined.

[0081] Unwind the entire core along the winding direction, and record the number of positive electrode layers as N during the unwinding process. Remove the unwound positive electrode and soak it in dimethyl carbonate (DMC) for 20 minutes. Then place the positive electrode in an oven and dry it at 80°C for 12 hours to obtain a test sample of the positive electrode.

[0082] The surface of the positive electrode test sample was observed using a scanning electron microscope (SEM). Specifically, the observation was performed along the thickness direction of the positive electrode, observing the plane formed by the length and width directions of the unfolded positive electrode. Due to the height difference between the protrusion and the surface of the first positive electrode material layer, a noticeable color difference was observed on the surface of the positive electrode. The protrusions projected onto the surface of the positive electrode, which could be used to distinguish the protrusions on the positive electrode material layer. Specifically, the positive electrode material layer with protrusions is designated as the first positive electrode material layer, and the positive electrode material layer with recesses is designated as the second positive electrode material layer. Protrusions on the first positive electrode material layer in the flat region are designated as first protrusions, and protrusions on the first positive electrode material layer in the bent region are designated as second protrusions. Recesses on the second positive electrode material layer in the flat region are designated as first recesses, and recesses on the second positive electrode material layer in the bent region are designated as second recesses.

[0083] The total length of the region containing protrusions (including the first and second protrusions) in the positive electrode test sample is measured to obtain L1 mm. The total length of the region containing the first positive electrode material layer in the positive electrode test sample is measured to obtain L2 mm. L1 / L2 is then calculated. Along the length of the unfolded positive electrode, the number of first protrusions in the flat region is counted and denoted as M1. The area of ​​this flat region is measured and denoted as S1 cm². 2 Then the density of the first protrusion x (numbers / cm³) 2 = M1 / S1; Count the number of the second protrusion in the bend area, denoted as M2, and measure the area of ​​the bend area, denoted as S2. cm 2 Then the density y (numbers / cm²) of the second protrusion 2= M2 / S2. Along the thickness direction of the positive electrode test sample, the first protrusion has an orthographic projection on the surface of the positive electrode. Select a single first protrusion in the straight area and measure the maximum circumscribed circle diameter of the outer contour of the orthographic projection of the first protrusion to obtain the maximum circumscribed circle diameter D1 mm of the single first protrusion on the surface of the first positive electrode material layer. Along the thickness direction of the positive electrode test sample, the second protrusion has an orthographic projection on the surface of the positive electrode. Select a single second protrusion in the bent area and measure the maximum circumscribed circle diameter of the outer contour of the orthographic projection of the second protrusion to obtain the maximum circumscribed circle diameter D2 mm of the single second protrusion on the surface of the first positive electrode material layer. When the protrusion is irregular in shape, an equivalent diameter is used, i.e., the distance between the two farthest points on the outer contour is measured.

[0084] Along the length and thickness directions of the unfolded positive electrode test sample, the positive electrode test sample is subjected to ion polishing to obtain a cross-section of the positive electrode test sample. The cross-section is observed using a scanning electron microscope (SEM). Five first protrusions located on the first positive electrode material layer in the flat region are randomly selected. The maximum height from the surface of the first positive electrode material layer to the single first protrusion along the thickness direction of the positive electrode is measured, and the average value is taken as the average height H1 mm of the multiple first protrusions. Five second protrusions located on the first positive electrode material layer in the bending region are randomly selected. The maximum height from the surface of the first positive electrode material layer to the single second protrusion along the thickness direction of the positive electrode is measured, and the average value is taken as the average height H2 mm of the multiple second protrusions.

[0085] w test:

[0086] At an ambient temperature of 25°C, the lithium-ion batteries of each embodiment and comparative example were discharged at a constant current of 0.5C to 2.0V and then disassembled under an argon atmosphere. The negative electrode sheet was removed and soaked in dimethyl carbonate (DMC) for 20 min. The negative electrode sheet was then placed in an oven and dried at 80°C for 12 h to obtain test samples of the negative electrode sheet. The test samples of the negative electrode sheet were freeze-brittled, and the negative current collector was separated from the negative electrode material layer on its surface using a precision blade along the interface. The separated negative electrode material layer was dissolved in N-methylpyrrolidone (NMP) at 60°C for 2 h, the precipitate was centrifuged, and the residual conductive agent was washed with dilute hydrochloric acid to obtain the negative electrode active material sample. The above negative electrode active material sample was tested using inductively coupled plasma mass spectrometry (ICP-MS) to accurately determine the mass percentage w of Si element in the negative electrode active material. The instrument used for ICP-MS testing was a PerkinElmer NexlON 350X inductively coupled plasma mass spectrometer.

[0087] Tests for r, e1, e2, e3, and e4:

[0088] At an ambient temperature of 25°C, the lithium-ion batteries of each embodiment and comparative example were discharged at a constant current of 0.5C to 2.0V and then disassembled under an argon atmosphere to obtain electrode assemblies. A positive electrode and a negative electrode adjacent to the positive electrode were obtained from the electrode assemblies. The positive and negative electrode assemblies were immersed in dimethyl carbonate (DMC) for 20 minutes, then placed in an oven and dried at 80°C for 12 hours to obtain test samples of the positive and negative electrode assemblies. The number of rings of the first and second protrusions on the first positive electrode material layer of the positive electrode assembly was recorded as M. The test samples of the positive electrode and the negative electrode adjacent to the positive electrode assembly were freeze-brittled. Using a precision blade, the negative current collector was separated from its surface negative electrode material layer along the interface, and the positive current collector was separated from its surface first and second positive electrode material layers to obtain a test sample of the positive current collector and a test sample of the negative current collector adjacent to the positive current collector.

[0089] Using a sampler, the positive current collector located in the flat region of the 0.5M turn and the negative current collector adjacent to it are sampled along their unfolded length. The value of 0.5M is an integer rounded down. For example, with M=11, 0.5M=5.5, so 0.5M is rounded down to 5. It should be noted that in the wound electrode assembly, the negative current collectors located in the flat region and adjacent to the positive current collector include those near the winding center and those far from the winding center. The difference in elongation between the two negative current collectors is negligible, and either one can be selected for testing. The positive and negative current collector samples were tested using a universal tensile testing machine at a tensile speed of 5 mm / min until the samples broke. Based on the obtained data, the elongation of the positive current collector in the straight region of the same circle and the elongation of the negative current collector adjacent to the positive current collector in the same circle were calculated. Five samples were tested in parallel and the average value was taken to obtain the elongation e1 of the positive current collector in the straight region of the same circle and the elongation e2 of the negative current collector adjacent to the positive current collector in the same circle.

[0090] The disassembled electrode assembly was scanned using Micro-CT to obtain an image of its internal structure. Image analysis software allowed for precise location of the positive current collector at the 0.5M turn. An inscribed circle was drawn on the inner surface of the negative current collector adjacent to the positive current collector in that turn and far from the winding center. The radius of this inscribed circle was measured as r mm. Here, 0.5M is a rounded-down integer; for example, with M=11, 0.5M=5.5, so 0.5M is rounded down to 5. It should be noted that the wound electrode assembly includes two bending regions. The difference in elongation between the positive current collectors in the two bending regions within the same turn is negligible, and either bending region's positive current collector can be selected for testing. Similarly, the difference in elongation between the negative current collectors in the two bending regions within the same turn is negligible, and either bending region's negative current collector can be selected. The negative current collectors adjacent to the positive current collector of this coil include the negative current collector near the winding center and the negative current collector far from the winding center. The difference in elongation between the two negative current collectors is negligible, and either negative current collector can be selected for testing. The negative current collectors adjacent to the positive current collector of this coil and far from the winding center include the negative current collector in one bending area and the negative current collector in another bending area. The difference in the inscribed circle radius between the two negative current collectors is negligible, and either negative current collector can be selected for testing. Using a sampler, samples were cut along the length of the positive current collector located in the bending zone of the aforementioned 0.5M ring and the negative current collector adjacent to it. A universal tensile testing machine was used to test the positive and negative current collector samples at a tensile speed of 5 mm / min until the samples broke. Based on the obtained data, the elongation of the positive current collector in the bending zone of the ring and the elongation of the negative current collector adjacent to it were calculated. Five samples were tested in parallel, and the average value was taken to obtain the elongation e3 of the positive current collector in the bending zone of the ring and the elongation e4 of the negative current collector adjacent to it.

[0091] Cyclic performance test:

[0092] The thickness of the lithium-ion battery was measured using a PPG soft-pack battery thickness gauge at 25℃ with a measurement pressure of 700gf. The initial thickness of the lithium-ion battery was recorded as T1. The lithium-ion battery was charged at a constant current of 1.0C to 4.25V, then charged at a constant voltage of 4.25V to a cutoff current of 0.05C, and allowed to stand for 10 minutes. Subsequently, it was discharged at a constant current of 4C to 2.0V and allowed to stand for 15 minutes. The above charge and discharge cycle was repeated for 400 cycles. The thickness of the lithium-ion battery after the cycle was measured using a PPG soft-pack battery thickness gauge with a measurement pressure of 700gf. The thickness of the lithium-ion battery after the cycle was recorded as T2. With the discharge capacity of the first cycle as Q1 and the discharge capacity of the 400th cycle as Q2, the capacity retention rate after 400 cycles = (Q2 / Q1)×100%, and the expansion rate after 400 cycles = (T2-T1) / T1×100%.

[0093] Security performance test:

[0094] Multiple lithium-ion batteries of the same batch and specifications were prepared for testing in each embodiment and comparative example. At 25°C, the lithium-ion batteries were charged at a constant current of 1.0C to 4.25V, then charged at a constant voltage of 4.25V to a cutoff current of 0.05C, and allowed to stand for 10 minutes. Subsequently, they were discharged at a constant current of 4C to 2.0V and allowed to stand for 15 minutes. This constituted one cycle. The lithium-ion batteries were subjected to charge-discharge cycle testing according to the above process. When the capacity of the lithium-ion battery decreased to 60% of its initial capacity, the discharged lithium-ion battery was disassembled to obtain the negative electrode. The breakage status of the negative electrode was checked. A breakage was considered to have occurred on the negative electrode; otherwise, the negative electrode was considered intact. If the negative electrode was determined not to be broken, the cycle continued until a breakage was observed in the disassembled lithium-ion battery. The number of cycles in which the negative electrode did not break in the last cycle was recorded. The higher the number of cycles in which the negative electrode did not break in the last cycle, the better the safety performance of the lithium-ion battery.

[0095] Example 1-1

[0096] <Preparation of negative electrode sheet>

[0097] The negative electrode active material SiC and artificial graphite (with a Si element mass percentage of 40%, i.e., w=40%), negative electrode binder styrene-butadiene rubber, thickener carboxymethyl cellulose (CMC), and conductive agent acetylene black were mixed in a mass ratio of 53.7:40.3:3:2:1. Deionized water was added as solvent to prepare a slurry with a solid content of 45wt%. After being stirred evenly in a vacuum mixer, a negative electrode material layer slurry was obtained. The negative electrode material layer slurry was uniformly coated on one surface of a 6μm thick negative electrode current collector copper foil and dried at 120℃ to obtain a negative electrode sheet with a single-sided coating of the negative electrode material layer. The single-sided coating weight of the negative electrode material layer was 142mg / 1540mm. 2 Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120°C, it is cold-pressed, cut, and then the tabs are welded to obtain a negative electrode sheet with a size of 78mm×879mm for use. The thickness of the single-sided negative electrode material layer is 54.5μm.

[0098] <Septum>

[0099] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0100] <Preparation of the positive electrode>

[0101] LiNi, the positive electrode active material 0.9 Co 0.05 Mn 0.05 O2, conductive agent Super P, and positive electrode binder polyvinylidene fluoride were mixed in a mass ratio of 97.1:1.3:1.6, and N-methylpyrrolidone (NMP) solvent was added to prepare a slurry with a solid content of 75 wt%. After vacuum stirring, a positive electrode material layer slurry was obtained. The positive electrode material layer slurry was uniformly coated on one surface of a 10 μm thick aluminum foil for the positive electrode current collector and dried at 120 °C to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The single-sided coating weight of the positive electrode material layer was 267.8 mg / 1540 mm². 2 Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120°C, it is cold-pressed, cut, and welded with tabs to obtain a positive electrode sheet with a size of 74mm×867mm. The length of the positive electrode material layer is 799mm (i.e., L2=799), and the thickness of the single-sided positive electrode material layer is 42μm.

[0102] The positive electrode sheet, separator, negative electrode sheet, and separator prepared above are placed in sequence and pre-wound. The positive electrode material layer closest to the winding center of the electrode assembly is defined as the first positive electrode material layer, and the positive electrode material layer farther away from the winding center of the electrode assembly is defined as the second positive electrode material layer. After winding, the straight and bent regions of the electrode assembly are determined, and then the straight and bent regions of the positive electrode sheet are determined. The number of layers of the positive electrode sheet is 25 (i.e., N=25), and the length of the first positive electrode material layer located in the straight region is 30 mm (i.e., d=30). Setting: Multiple first protrusions are formed on the first positive electrode material layer in the flat region, and the multiple first protrusions are distributed in a dot-like pattern. Multiple second protrusions are formed on the first positive electrode material layer in the bent region, and the multiple second protrusions are distributed in a dot-like pattern. Along the length direction of the unfolded positive electrode sheet, the total length of the area containing the multiple first protrusions and the multiple second protrusions is 479.4 mm (i.e., L1 = 479.4). The average height of the multiple first protrusions is 0.0060 mm (i.e., H1 = 0.0060), and the density of the first protrusions is 7 per cm³. 2 (i.e., x=7), along the thickness direction of the positive electrode sheet, a single first protrusion has an orthographic projection on the surface of the first positive electrode material layer. The outer contour of the orthographic projection is elliptical, with a minor axis of 1.65 mm and a major axis of 2.2 mm. The diameter of the largest circumcircle of the outer contour of a single orthographic projection is the same as the major axis of the ellipse, meaning the largest circumcircle diameter of a single first protrusion on the surface of the first positive electrode material layer is 2.2 mm (i.e., D1=2.2). The average height H2 of multiple second protrusions is 0.0140 mm (H2=0.0140), and the density of the second protrusions is 9 protrusions / cm³. 2 (i.e., y=9), along the thickness direction of the positive electrode sheet, a single second protrusion has an orthographic projection on the surface of the first positive electrode material layer. The shape of the outer contour of the orthographic projection is elliptical, with a minor axis length of 2.4mm and a major axis length of 3.2mm. The diameter of the largest circumscribed circle of the outer contour of a single orthographic projection is the length of the major axis of the ellipse, that is, the diameter of the largest circumscribed circle of a single second protrusion on the surface of the first positive electrode material layer is 3.2mm (i.e., D2=3.2).

[0103] Using embossing rollers of the corresponding specifications according to the above parameters, the positive electrode sheet is cold-pressed along the length direction of the positive electrode sheet, starting from the winding start end of the positive electrode sheet, to obtain a positive electrode sheet with a first protrusion, a second protrusion, a first concave portion, and a second concave portion. Along the thickness direction of the positive electrode sheet, the first concave portion corresponds one-to-one with the first protrusion, and the second concave portion corresponds one-to-one with the second protrusion.

[0104] <Preparation of Electrolyte>

[0105] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 20:30:40:10 to obtain a mixed solvent. Then, lithium hexafluorophosphate (LiPF6) and fluoroethylene carbonate (FEC) were added to the mixed solvent and mixed thoroughly to obtain the electrolyte. The concentration of lithium salt was 1 mol / L, FEC accounted for 8% of the organic solvent by mass, and the remainder was the mixed solvent.

[0106] <Preparation of Lithium-ion Batteries>

[0107] The positive electrode, separator, negative electrode, and separator are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator, and then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 100°C. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0108] Examples 1-2 to Examples 1-8

[0109] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Examples 1-1. Specifically, in Examples 1-2, the mass ratios of SiC and artificial graphite in the negative electrode active material were adjusted in the <Preparation of the Negative Electrode Sheet>, resulting in w values ​​as shown in Table 1. In Examples 1-3, SiC was replaced with elemental silicon in the <Preparation of the Negative Electrode Sheet>, and the mass ratio of elemental silicon, artificial graphite, styrene-butadiene rubber, CMC, and acetylene black was adjusted to 65.1:27.9:4:2:1, resulting in w values ​​as shown in Table 1. In Examples 1-4, elemental silicon was used as the negative electrode active material in the <Preparation of the Negative Electrode Sheet>, and flake graphite was added as a conductive agent. Simultaneously, the mass ratio of elemental silicon, styrene-butadiene rubber, CMC, acetylene black, and flake graphite was adjusted to 85:6:2:3:4, resulting in w values ​​as shown in Table 1.

[0110] Examples 2-1 to 2-8

[0111] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0112] Examples 3-1 to 3-13

[0113] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Examples 1-1. Among them, in Examples 3-10 to 3-13, the thickness of the negative electrode current collector was adjusted in the <Preparation of Negative Electrode Sheet> so that the values ​​of e2 and e4 are as shown in Table 3.

[0114] Comparative Example 1

[0115] Except for the absence of a first and second protrusion on the positive electrode in the <Preparation of Lithium-ion Batteries>, the rest is the same as in Example 1-1.

[0116] Comparative Example 2

[0117] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0118] Comparative Example 3

[0119] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0120] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0121] Table 1

[0122] Table 2

[0123] Table 3

[0124] Note: " / " in Table 1 indicates that there are no relevant preparation parameters.

[0125] As can be seen from Examples 1-1 to 1-8, Examples 2-1 to 2-8, Examples 3-1 to 3-13, and Comparative Examples 1 to 3, by setting multiple first protrusions on the positive electrode sheet located in the flat region and multiple second protrusions on the positive electrode sheet located in the bent region, and by controlling H1 and w to satisfy the above relationship and H1 < H2, the lithium-ion battery exhibits a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the last cycle. This indicates that the lithium-ion battery of this application has good cycle performance and safety performance. In Comparative Example 1, no first or second protrusions are set on the positive electrode sheet; in Comparative Example 2, the average height H1 of the first protrusion on the positive electrode sheet is greater than the average height H2 of the second protrusion; in Comparative Example 3, the average height H1 of the first protrusion and the mass percentage content w of Si element in the negative electrode active material do not satisfy 0 ≤ |H1 - 0.0059w. 0.2047 The values ​​≤0.003 indicate that the lithium-ion batteries in Comparative Examples 1 to 3 exhibited low cycle capacity retention, high cycle expansion rates, and a limited number of cycles in which the negative electrode did not break during the final cycle, suggesting poor cycle performance and safety. In contrast, the lithium-ion batteries in the embodiments of this application exhibited high cycle capacity retention, low cycle expansion rates, and a large number of cycles in which the negative electrode did not break during the final cycle, indicating improved cycle performance and safety.

[0126] The value of w affects the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-4, when the value of w is within the range of this application, the lithium-ion battery exhibits a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the final cycle, indicating that the lithium-ion battery of this application has good cycle performance and safety performance. Among them, the lithium-ion battery in Example 1-2 has high cycle performance and safety performance, but its energy density is low.

[0127] |H2-0.017w 0.3139 The value of | affects the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-7, when |H2-0.017w 0.3139 When the value of | is within the range of this application, the lithium-ion battery has a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the last cycle, indicating that the lithium-ion battery of this application has good cycle performance and safety performance.

[0128] The values ​​of H2-H1 affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-2, 1-6 to 1-8, when the values ​​of H2-H1 are within the range of this application, the lithium-ion battery has a higher cycle capacity retention rate, a lower cycle expansion rate, and a greater number of cycles in which the negative electrode sheet does not break in the last cycle, indicating that the lithium-ion battery of this application has better cycle performance and safety performance.

[0129] The values ​​of x / y and x affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-4, when the values ​​of x / y and x are within the range of this application, the lithium-ion battery has a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the last cycle, indicating that the lithium-ion battery of this application has good cycle performance and safety performance.

[0130] The values ​​of D1 and D2 affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-5 to 2-8, when the values ​​of D1 and D2 are within the range of this application, the lithium-ion battery exhibits a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode does not break in the last cycle, indicating that the lithium-ion battery of this application has good cycle performance and safety performance. Specifically, the lithium-ion battery in Example 2-5 exhibits high cycle performance and safety performance; however, when D1 is too small, the positive electrode material layer at the first protrusion is prone to detachment during the processing of the positive electrode, and the first protrusion is too sharp during cycling, causing the separator to be punctured and resulting in a short circuit between the positive and negative electrodes. Similarly, the lithium-ion battery in Example 2-7 exhibits high cycle performance and safety performance; however, when D2 is too small, the positive electrode material layer at the second protrusion is prone to detachment during the processing of the positive electrode, and the second protrusion is too sharp during cycling, causing the separator to be punctured and resulting in a short circuit between the positive and negative electrodes.

[0131] N, L1, and L2 affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1, 3-1 to 3-9, when N, L1, and L2 meet the requirements of this application, the lithium-ion battery has a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the last cycle. This indicates that the lithium-ion battery of this application has good cycle performance and safety performance.

[0132] The values ​​of d, e1, and e2 affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1 and 1-2, Examples 1-5 to 1-8, Examples 2-1 and 2-2, Examples 2-5 and 2-6, Examples 3-10 and 3-11, when d, e1, and e2 meet the requirements of this application, the lithium-ion battery exhibits a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode sheet does not break in the final cycle. This indicates that the lithium-ion battery of this application has good cycle performance and safety performance.

[0133] The values ​​of r, e3, and e4 affect the cycle performance and safety performance of lithium-ion batteries. As can be seen from Examples 1-1 and 1-2, Examples 1-5 to 1-8, Examples 2-3 and 2-4, Examples 2-7 and 2-8, Examples 3-12 and 3-13, when r, e3, and e4 meet the requirements of this application, the lithium-ion battery exhibits a high cycle capacity retention rate, a low cycle expansion rate, and a large number of cycles in which the negative electrode does not break in the final cycle. This indicates that the lithium-ion battery of this application has good cycle performance and safety performance.

[0134] The placement of the first protrusion and first recess, and the second protrusion and second recess in the positive electrode sheet affects the cycle performance and safety performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-8, Examples 2-1 to 2-8, and Examples 3-1 to 3-13, when the placement of the first protrusion and first recess, and the second protrusion and second recess in the positive electrode sheet falls within the scope of this application, the lithium-ion battery exhibits a higher cycle capacity retention rate, a lower cycle expansion rate, and a greater number of cycles in which the negative electrode sheet does not break in the final cycle. This indicates that the lithium-ion battery of this application has better cycle performance and safety performance.

[0135] 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.

[0136] 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.

[0137] 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 a wound electrode assembly, the electrode assembly comprising a positive electrode and a negative electrode, the negative electrode comprising a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer comprising a negative electrode active material, wherein the mass percentage content of Si element is w based on the mass of the negative electrode active material; The positive electrode sheet includes a positive current collector and a first positive electrode material layer disposed on one surface of the positive current collector; The electrode assembly includes a flat region and a bent region connected to the flat region. A plurality of first protrusions are disposed on the first positive electrode material layer located in the flat region, and a plurality of second protrusions are disposed on the first positive electrode material layer located in the bent region. Along the thickness direction of the unfolded positive electrode sheet, the average height of the plurality of first protrusions is H1 mm, and the average height of the plurality of second protrusions is H2 mm, where 0 ≤ |H1 - 0.0059w 0.2047 |≤0.003, 5%≤w≤100%, H1<H2.

2. The secondary battery according to claim 1, wherein, 0≤|H2-0.017w 0.3139 |≤0.003。 3. The secondary battery according to claim 1, wherein, 0.002≤H2-H1≤0.

014.

4. The secondary battery according to claim 1, wherein, The plurality of first protrusions are distributed in a dotted pattern on the first positive electrode material layer located in the flat region, and the density of the first protrusions is x protrusions / cm³. 2 The plurality of second protrusions are distributed in a dotted pattern on the first positive electrode material layer located in the bending region, and the density of the second protrusions is y protrusions / cm³. 2 , 0.5≤x / y≤1, 5≤x≤9.

5. The secondary battery according to claim 1, wherein, The maximum circumscribed circle diameter of a single first protrusion on the surface of the first positive electrode material layer is D1 mm, 1.5≤D1≤3; and / or, the maximum circumscribed circle diameter of a single second protrusion on the surface of the first positive electrode material layer is D2 mm, 2.5≤D2≤4.

6. The secondary battery according to claim 1, wherein, Along the thickness direction of the electrode assembly, the number of layers of the positive electrode sheet is N, 10≤N≤39. Along the length direction of the unfolded positive electrode sheet, the total length of the region containing the plurality of first protrusions and the plurality of second protrusions is L1 mm, and the length of the first positive electrode material layer is L2 mm. N, L1, and L2 satisfy: (1) 10≤N<19, 0.3≤L1 / L2<0.5; (2) 19≤N<29, 0.5≤L1 / L2<0.7; (3) 29≤N≤39, 0.7≤L1 / L2≤0.

9.

7. The secondary battery according to claim 1, wherein, In any ring with the first protrusion in the flat region, the elongation of the positive current collector in the flat region of the ring is e1, the elongation of the negative current collector adjacent to the positive current collector in the thickness direction of the electrode assembly is e2, the length of the first positive electrode material layer in the flat region is d mm, and 8.5≤|e2-e1|d / H2≤39.

8. The secondary battery according to claim 1, wherein, In any loop where the second protrusion is provided in the bending region, the elongation of the positive current collector in the bending region of the loop is e3, the elongation of the negative current collector adjacent to the positive current collector of the loop along the thickness direction of the electrode assembly is e4, the radius of the inscribed circle of the negative current collector adjacent to the positive current collector of the loop and far from the winding center is r mm, and 3≤|e4-e3|r / H2≤12.

9. The secondary battery according to claim 1, wherein, The positive electrode further includes a second positive electrode material layer disposed on another surface of the positive current collector. The second positive electrode material layer located in the flat region has a plurality of first recesses, and along the thickness direction of the electrode assembly, each first recess corresponds to a first protrusion; and / or, A plurality of second recesses are provided on the second positive electrode material layer located in the bending region. Along the thickness direction of the positive electrode sheet after it is unfolded and along the thickness direction of the electrode assembly, the second recesses correspond one-to-one with the second protrusions.

10. An electronic device comprising a secondary battery according to any one of claims 1 to 9.

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