Lithium ion secondary battery

Abstract

The present application relates to the technical field of battery, in particular to a kind of lithium ion secondary battery.It includes electrode assembly, electrode assembly includes positive plate, diaphragm and negative plate;Positive plate includes the first positive plate located in the outermost layer of electrode assembly;First positive electrode includes first positive electrode current collector, and first positive electrode current collector includes first surface and second surface;First positive plate contains convex part and recess part;Convex part is raised from second surface to the direction of far from the center of electrode assembly, and recess part is recessed from first surface to the direction of second surface.The height of convex part is h1, the thickness of first positive electrode current collector is h2, and the peeling force of first positive plate and diaphragm is N1, wherein, A=h1 / h2, 0.005≤A / N1≤0.5, 0.8≤N1 / N2≤2.The present application can improve the anti-curling ability of first positive plate and improve "purple stain" defect, significantly improve the cycle stability of battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a lithium-ion secondary battery. Background Technology

[0002] Stacked lithium-ion secondary batteries are widely used due to their advantages such as high space utilization and good rate performance. Their electrode components are usually formed by alternating stacking of single-sided positive electrode (with an active layer coated only on the current collector side), negative electrode, and double-sided positive electrode (with an active layer coated on both sides of the current collector).

[0003] However, this structure has inherent technical defects: because the single-sided positive electrode is coated with active material on only one side, the active material loading per unit area is relatively higher than that at its reaction interface. As a result, it can withstand a much higher local current density than the double-sided positive electrode under the same current, which accelerates the electrolyte consumption in this area. This can easily lead to the formation of "purple spots" on the negative electrode surface in the later stages of cycling due to electrolyte bridging. At the same time, the shrinkage and expansion of the active layer during drying and charge-discharge cycles will generate stress on the empty foil side, causing the single-sided positive electrode to curl towards the empty foil side. This results in poor bonding between the positive and negative electrodes, weakening of the interfacial adhesion between the positive electrode and the separator, and increased interfacial impedance. This further aggravates the battery capacity decay and causes a series of problems such as electrode component misalignment, uneven stress in the tab welding area, and decreased welding yield.

[0004] Therefore, it is very important to invent a lithium-ion secondary battery that can overcome the aforementioned inherent technical defects. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) can effectively suppress the curling deformation of the first positive electrode, improve the uneven distribution of current density, reduce the "purple spot" on the negative electrode, and enhance the interfacial adhesion between the first positive electrode and the separator, thereby effectively improving the cycle stability of the battery.

[0006] This invention discovered that increasing the thickness of the first current collector can improve the bending stiffness of the first positive electrode, thereby suppressing electrode curling deformation. However, this solution has a drawback: while thickening the first current collector can improve stiffness, excessive thickness can actually enhance its current carrying capacity, leading to an increase in local current density. This accelerates the consumption and decomposition of the electrolyte in that area, causing a broken bridge to form between the first positive and negative electrodes due to insufficient electrolyte filling. Under high charging current, the supply and demand of lithium ions become unbalanced, ultimately inducing the "purple spot" defect.

[0007] Based on the above problems, the inventors conducted extensive targeted research and discovered: First, by improving the first positive electrode sheet, it is made to include multiple protrusions and multiple concave portions corresponding to the protrusions (the protrusions extend from the second surface of the first positive electrode sheet away from the center of the electrode assembly, and the concave portions are recessed from the first surface of the first positive electrode sheet towards the second surface). The height h1 of the protrusions, the thickness h2 of the first positive current collector, the peel force N1 between the first positive electrode sheet and the separator, and the peel force N2 between the second positive electrode sheet and the separator are adjusted to satisfy specific relationships. This improves the anti-curling ability of the first positive electrode sheet while also mitigating the "purple spot" defect. The reason is that by setting protrusions on the second surface, the microstructure of the electrode sheet surface and near-surface layer can be reconstructed. The resulting micro-uneven structure introduces local plastic deformation and residual compressive stress at the interface of the electrode assembly. On the one hand, this balances the stress generated by the drying shrinkage of the active layer; on the other hand, it increases the stiffness of the surface layer of the first positive electrode sheet. Therefore, it is not necessary to excessively rely on increasing the thickness of the first positive current collector to suppress curling deformation, thereby improving the "purple spot" defect. However, the height h1 of the protrusion affects the peel force N1 between the first positive electrode and the separator. Since the protrusion faces the aluminum-plastic film of the battery, if h1 is too large, the contact area between the first positive electrode and the separator will be smaller, which will weaken N1. With repeated expansion and contraction during cycling, it is easy to cause delamination of the positive and negative electrode interfaces. This area cannot be replenished after the electrolyte is consumed, forming a broken bridge, resulting in insufficient lithium intercalation of the negative electrode and thus forming "purple spots". If h1 is too small, the amount of compressive plastic deformation is insufficient, making it difficult to effectively neutralize the bending stress generated by the single-sided active layer. At the same time, the height h1 of the protrusion also needs to be matched with the thickness h2 of the first positive electrode current collector. When h2 is large, h1 can be appropriately reduced to maintain the anti-curling ability with the macroscopic stiffness of the current collector, while avoiding the weakening of N1 due to the protrusion being too deep. When h2 is small, h1 needs to be appropriately increased to compensate for the insufficient bending stiffness of the current collector through the microscopic reconstruction effect of embossing, suppressing curling deformation while maintaining the interfacial adhesion force N1.

[0008] Secondly, this invention also regulates the ratio of N1 to N2 to meet a specific range, ensuring the interface consistency of the electrode assembly in the thickness direction. If N1 / N2 is too large (e.g., N1 is too large and N2 is too small), the difference between the interface impedance of the first positive electrode and the interface impedance of the separator will be too large, which will further aggravate the distortion of current density. The current will tend to flow to the area with lower impedance, thereby reducing the lithium-ion flux of the first positive electrode and causing the corresponding negative electrode area to have a "purple spot" problem. If N1 / N2 is too small (e.g., N1 is too small and N2 is too large), the interfacial adhesion between the first positive electrode and the separator will be too low. During cycling, the repeated expansion and contraction of the electrode will easily cause the interface of the first positive electrode to delaminate. This area will not be filled with electrolyte, forming a broken bridge, which will further aggravate the lithium plating and "purple spot" problems, affecting the cycle stability and service life of the battery.

[0009] Based on this, the inventors of this invention propose the following solution: This invention provides a lithium-ion secondary battery, including an electrode assembly comprising a positive electrode, a separator, and a negative electrode stacked sequentially. The positive electrode includes at least one first positive electrode and at least one second positive electrode. The first positive electrode is located on the outermost side of the electrode assembly and includes a first positive current collector. The first positive current collector includes a first surface and a second surface disposed opposite to each other along its thickness direction. The first surface is provided with a first positive active layer, the first surface being close to the center of the electrode assembly, and the second surface being away from the center of the electrode assembly. The second positive electrode includes a second positive current collector and a second positive active layer disposed on both sides of the second positive current collector. The first positive electrode includes a plurality of protrusions and a plurality of recesses disposed opposite to the plurality of protrusions. The protrusions protrude from the second surface in a direction away from the center of the electrode assembly, and the recesses are recessed from the first surface in a direction towards the second surface.

[0010] The height of the protrusion is h1, in μm; the thickness of the first positive current collector is h2, in μm; the peel force between the first positive electrode and the separator is N1, in N / m; the peel force between the second positive electrode and the separator is N2, in N / m; where A = h1 / h2, 0.005 ≤ A / N1 ≤ 0.5, 0.8 ≤ N1 / N2 ≤ 2.

[0011] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) The present invention improves the first positive electrode sheet so that the first positive electrode sheet includes multiple protrusions and multiple concave portions corresponding to the multiple protrusions, and adjusts the height h1 of the protrusions, the thickness h2 of the first positive current collector and the peeling force N1 between the first positive electrode sheet and the separator to satisfy a specific relationship, which can not only improve the anti-curling ability of the first positive electrode sheet, but also improve the "purple spot" defect. (2) By adjusting the peeling force N1 between the first positive electrode and the separator and the peeling force N2 between the second positive electrode and the separator, the present invention ensures that the ratio of the two forces meets a specific range, thereby ensuring the interface consistency of the electrode assembly in the thickness direction and effectively improving the structural stability of the positive electrode and the cycle performance of the battery.

[0012] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0013] Figure 1 The figure shown is a schematic diagram of the electrode assembly along the thickness direction in an embodiment of the present invention.

[0014] Figure 2 The diagram shown is a schematic diagram of the structure of the first positive electrode in an embodiment of the present invention.

[0015] Figure 3 The diagram shown is a schematic diagram of the structure of the second positive electrode in an embodiment of the present invention.

[0016] Figure 4 The diagram shown is a schematic representation of the convex and concave portions in an embodiment of the present invention.

[0017] Figure 5 The image shown is a front view of the first positive electrode in an embodiment of the present invention.

[0018] Figure 6 The image shown is a cross-sectional scanning electron microscope (SEM) image of silicon-carbon material in an example of the present invention.

[0019] Figure label: 1. Electrode assembly; 11. Positive electrode sheet; 111. First positive electrode sheet; 1111. First positive current collector; 1112. First positive active layer; 1113. Protrusion; 1114. Recess; 112. Second positive electrode sheet; 1121. Second positive current collector; 1122. Second positive active layer; 12. Negative electrode sheet; 121. Negative current collector; 122. Negative active layer; 13. Separator; 14. Tab. Detailed Implementation

[0020] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0021] This invention provides a lithium-ion secondary battery, including an electrode assembly comprising a positive electrode, a separator, and a negative electrode stacked sequentially. The positive electrode includes at least one first positive electrode and at least one second positive electrode. The first positive electrode is located on the outermost side of the electrode assembly and includes a first positive current collector. The first positive current collector includes a first surface and a second surface disposed opposite to each other along its thickness direction. The first surface is provided with a first positive active layer, and the second surface does not contain a positive active layer. The first surface is close to the center of the electrode assembly, and the second surface is away from the center of the electrode assembly. The second positive electrode includes a second positive current collector and a second positive active layer disposed on both sides of the second positive current collector. The first positive electrode includes a plurality of protrusions and a plurality of recesses corresponding to the plurality of protrusions. The protrusions protrude from the second surface in a direction away from the center of the electrode assembly, and the recesses are recessed from the first surface in a direction towards the second surface.

[0022] The height of the protrusion is h1, in μm; the thickness of the first positive current collector is h2, in μm; the peel force between the first positive electrode and the separator is N1, in N / m; the peel force between the second positive electrode and the separator is N2, in N / m; wherein, A = h1 / h2, 0.005 ≤ A / N1 ≤ 0.5 (for example, within the range of any two values ​​of 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.24, 0.25, 0.3, 0.4, 0.5 or above), and 0.8 ≤ N1 / N2 ≤ 2 (for example, within the range of any two values ​​of 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2 or above).

[0023] like Figure 1 The figure shows a schematic diagram of the electrode assembly along the thickness direction in an embodiment of the present invention. As can be seen from the figure, the electrode assembly 1 includes a positive electrode 11, a separator 13, and a negative electrode 12 stacked sequentially. The positive electrode 11 includes a first positive electrode 111 and a second positive electrode 112. Figure 2 The diagram shows a schematic representation of the structure of the first positive electrode in an embodiment of the present invention. As can be seen from the diagram, the first positive electrode 111 includes a first positive current collector 1111. The first positive current collector includes a first surface S1 and a second surface S2 disposed opposite to each other along the thickness direction. A first positive active layer 1112 is disposed on the first surface. The first positive electrode includes multiple protrusions 1113 and multiple recesses 1114 corresponding to the protrusions. The height of the protrusion is h1, the thickness of the first positive current collector is h2, the width of the protrusion is D, and the spacing between the protrusions is P. Figure 3The figure shows a schematic diagram of the structure of the second positive electrode in an embodiment of the present invention. As can be seen from the figure, the second positive electrode 112 includes a second positive current collector 1121 and a second positive active layer 1122 disposed on both sides of the second positive current collector.

[0024] In one instance, 0.01 ≤ A / N1 ≤ 0.24.

[0025] In one instance, 0.2 ≤ A ≤ 2.2 (for example, within the range of any two of the following values: 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.5, 1.6, 1.8, 2, 2.2).

[0026] In one instance, 0.4 ≤ A ≤ 1.5.

[0027] In one instance, h1 is 3μm-60μm (e.g., within the range of any two of the values ​​3μm, 5μm, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, or above).

[0028] In one instance, h1 is 5μm-30μm.

[0029] In one instance, h2 is 10μm-30μm (e.g., within the range of any two of the values ​​10μm, 12μm, 15μm, 20μm, 25μm, 30μm, or above).

[0030] In one instance, h2 is 12μm-20μm.

[0031] In one instance, N1 is 4N / m to 40N / m (e.g., within the range of 4N / m, 6N / m, 10N / m, 20N / m, 30N / m, 40N / m, or any two of the above values).

[0032] In one instance, N1 is 6 N / m - 30 N / m.

[0033] In one instance, N2 is 4N / m to 35N / m (e.g., within the range of 4N / m, 6N / m, 10N / m, 15N / m, 20N / m, 25N / m, 30N / m, 35N / m, or any two of the above values).

[0034] In one instance, N2 is 6 N / m - 30 N / m.

[0035] The height h1 of the protrusion needs to be matched with the thickness h2 of the first positive current collector and the peel force N1 between the first positive electrode and the separator. If A / N1 is too large (>0.5), the height of the protrusion is too large relative to the thickness of the first positive current collector, or the peel force between the first positive electrode and the separator is too small. The effective contact area between the first positive electrode and the separator becomes smaller, and during battery cycle expansion, delamination of the positive and negative electrodes is likely to occur, which is not conducive to improving purple spots. If A / N1 is too small (<0.005), the thickness of the first positive current collector is too large relative to the height of the protrusion, or the peel force between the first positive electrode and the separator is too large. The current collector has a strong flow capacity, and the electrolyte is consumed too quickly, which is also not conducive to improving purple spots.

[0036] In this invention, the second surface further includes a first flat section, and the protrusion includes a first outer surface. The height h1 of the protrusion refers to the vertical distance from the highest point on the first outer surface to the surface of the first flat section, which can be obtained by conventional methods in the art, such as testing with a 3D profiler. At least 10 protrusions are selected, the height of each protrusion is measured, and the average value is taken.

[0037] In this invention, the thickness h2 of the first positive current collector can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC (e.g., discharging the battery to 3V), disassembling and removing the first positive electrode sheet, scraping off the first positive electrode active layer, cutting the first positive electrode sheet with an argon ion milling machine using a CP laser, and then observing the first positive current collector with a scanning electron microscope (SEM), randomly selecting 10 test sites on its surface, measuring the thickness of each site, and taking the average value.

[0038] In this invention, the peel force N1 between the first positive electrode and the separator and the peel force N2 between the second positive electrode and the separator can be tested using conventional methods in the art. For example, the battery is placed in an environment of (25+2)℃ and left to stand for 2-3 hours. The battery is charged at a constant current of 0.7C with a cutoff current of 0.05C. When the battery terminal voltage reaches the charging limit voltage (4.55V), it is switched to constant voltage charging until the charging current is less than the cutoff current. Charging is then stopped and left to stand for 5 minutes. The fully charged (100% SOC) battery is then dissected and subjected to a 180° peel test according to the national standard GB / T2790-1995.

[0039] In this invention, the areal density of the protrusion on the second surface is ρ, where 1 ≤ ρ / A ≤ 12 (for example, within the range of any two values ​​of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or above). By limiting ρ / A to a specific range, this invention effectively balances the relationship between the distribution density ρ of the protrusion on the second surface and the relative height A of the protrusion, thereby enhancing the structural stability of the first positive electrode current collector and also improving purple spots. Specifically, the larger ρ is, the denser the protrusions per unit area. To avoid stress concentration leading to current collector damage, the relative height A of the protrusions needs to be reduced accordingly (e.g., lowering the height of the protrusions). Lowering the relative height of the protrusions can further improve the adhesion between the first positive electrode and the separator, thereby improving the purple spots on the negative electrode. Conversely, the smaller ρ is, the sparser the protrusions per unit area. The relative height A of the protrusions needs to be appropriately increased (e.g., increasing the height of the protrusions) to balance the bending stress generated by the single-sided active layer and improve the anti-curling ability of the first positive electrode. However, if the relative height of the protrusions is too large, it will weaken the adhesion between the first positive electrode and the separator, which is not conducive to improving the purple spots.

[0040] In one instance, 1 per cm 2 ≤ρ≤8 pieces / cm 2 (e.g., 1 per cm) 2 2 per cm 2 3 per cm 2 4 per cm 2 5 pieces / cm 2 6 pieces / cm 2 7 per cm 2 8 per cm 2 Or within the range of any two of the above values).

[0041] In one instance, 2 / cm 2 ≤ρ≤5 pieces / cm 2 .

[0042] In this invention, the areal density ρ of the protrusion on the second surface can be obtained by methods conventional in the art, such as randomly selecting 5 test areas on the second surface for parallel testing (the area of ​​each test area is 1 cm²). 2 (And it is necessary to avoid the tabs and damaged edge areas to ensure test accuracy) The test area is observed and clear images are taken using a scanning electron microscope. The protrusions in the images are identified and counted manually or using image analysis software. The total number of protrusions in the test area is counted. Then, the total number of protrusions is divided by the actual area of ​​the test area to calculate the number of protrusions per unit area. Finally, the average value of 5 test areas is taken as the areal density ρ of the protrusions on the second surface (unit: protrusions / cm²). 2If the protrusions are small or unevenly distributed, the number of test areas can be increased appropriately to improve test accuracy.

[0043] In this invention, the second surface includes a first straight section, the convex portion includes a first outer surface, and the concave portion includes a second outer surface (the first outer surface and the second outer surface are connected). The orthographic projection diameter of the first outer surface of the convex portion onto the second surface is r1, and the orthographic projection diameter of the second outer surface of the concave portion onto the first surface is r2. 40≤r2 / h1≤1070 (for example, within the range of any two values ​​of 40, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1070, or above). The unit of r2 is mm, and the unit of h1 is μm. If r2 is too small and h1 is too large, the protrusion will be too sharp, easily damaging the electrode and affecting the battery's safety performance. If r2 and h1 are too large, the protrusion will be too sharp, and the space of the concave portion will be too large, exceeding the electrode's tensile strength and insufficient to support the deformation of the protrusion, easily leading to electrode breakage. If r2 is too large and h1 is too small, the support of the protrusion will be insufficient, making it difficult to provide effective support during charge-discharge cycles and unable to balance the bending stress generated by the single-sided active layer. This will make the protrusion susceptible to excessive deformation under extrusion and tensile forces during the manufacturing process, tending towards flattening, thus resulting in poor improvement. If r2 is too small and h1 is too small, the support of the protrusion will be insufficient, and the space of the concave portion will be too small, resulting in poor improvement. Therefore, it is necessary to limit r2 and h1. By limiting r2 / h1 to an appropriate range, the sharpness and support of the protrusion and the space of the concave portion can be limited, maintaining a suitable gap between the electrode and the separator, providing sufficient resistance to deformation, and maintaining the stability of the electrode interface.

[0044] In this invention, the angle between the tangent at the connection point of the first outer surface and the first straight section and the horizontal line containing the first straight section is α, where 100°≤α≤160° (for example, within the range of any two values ​​of 100°, 110°, 120°, 130°, 140°, 150°, 160°, or above). Limiting the angle α effectively limits the tilt angle of the convex portion, further optimizing the convex structure and improving the structural stability of the first positive electrode. If the angle α is too small, the connection point between the convex portion and the first straight section is prone to microcracks or fractures due to stress concentration, affecting the stability of the current collector structure. If α is too large, the tilt angle of the convex portion is too large, the convex height is insufficient, making it difficult to provide effective support during charge-discharge cycles, unable to balance the bending stress generated by the single-sided active layer, and easily flattened by electrode expansion or external pressure, thereby weakening its advantage in improving interface stability. Therefore, by controlling 'a' within the above range, it can be ensured that the protrusion has a suitable tilt angle, and that the protrusion has sufficient height and support, thereby ensuring that the first positive electrode maintains a stable interface structure during long-term cycling, and improving the cycle stability and service life of the battery.

[0045] like Figure 4 The figure shows a schematic diagram of the structure of the convex and concave parts in an embodiment of the present invention. As can be seen from the figure, the second surface S2 includes the first straight section B, the convex part 1113 includes the first outer surface R1, and the concave part 1114 includes the second outer surface R2. The orthographic projection radius of the first outer surface of the convex part on the second surface is r1 / 2, and the orthographic projection radius of the second outer surface of the concave part on the first surface is r2 / 2. The angle between the tangent at the connection between the first outer surface and the first straight section and the horizontal line where the first straight section is located is α.

[0046] In one instance, 120° ≤ a ≤ 150°.

[0047] In one instance, r1 is 0.5mm-8mm (e.g., within the range of any two of the values ​​0.5mm, 1mm, 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm).

[0048] In one example, r1 is 1.5mm-4mm; In one instance, r2 is 0.5mm-8mm (e.g., within the range of any two of the values ​​0.5mm, 1mm, 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, or above).

[0049] In one example, r2 is 1.5mm-4mm.

[0050] In one instance, 100 ≤ r2 / h1 ≤ 300.

[0051] In one instance, 1.02 ≤ r1 / r2 ≤ 1.2 (for example, within the range of any two of the values ​​1.02, 1.05, 1.1, 1.15, 1.2, or above).

[0052] In this invention, the area of ​​the second surface is S1, and the sum of the projected areas of the protrusions on the second surface along the thickness direction of the first positive electrode is S2, where 0.2 ≤ S2 / S1 ≤ 0.7 (for example, within the range of any two of the values ​​above 0.2, 0.3, 0.4, 0.5, 0.6, 0.7). Adjusting S2 / S1 is equivalent to limiting the density of the protrusions on the second surface. By coordinating with the protrusion height and the included angle, it can better provide support stability, further effectively improving the bending resistance of the electrode and the negative electrode purple spots.

[0053] In one instance, 0.3 ≤ S2 / S1 ≤ 0.6.

[0054] In this invention, S1 can be obtained by conventional methods in the art, such as using a 2.5D tester to measure the size of the second surface and calculating the area S1 of the second surface. S2 can also be obtained by conventional methods in the art, such as using a 3D profilometer to acquire and simulate the three-dimensional profile of the protrusion, selecting multiple protrusions to measure their diameters and calculating the average diameter, obtaining the average projected area of ​​a single protrusion based on the average diameter, and then combining the total number of protrusions to calculate the sum of the projected areas S2 of the protrusions.

[0055] In this invention, the areal density of the first positive electrode active layer is X, and the areal density of the second positive electrode active layer is Y, wherein 0.94 ≤ X / Y ≤ 1 (for example, within the range of any two values ​​of 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, or above). The areal density of the first positive electrode active layer is lower than that of the second positive electrode active layer, which can reduce the curling degree of the first positive electrode sheet and the current density in that region, thus suppressing "purple spots" to a certain extent.

[0056] It is worth noting that the areal densities X and Y here both represent the areal densities of a single side. This can be understood as follows: when the current collector has an active layer on only one side, the areal density of the active layer is the areal density of the active layer on that side; when the current collector has active layers on both sides, the areal densities of the active layers on both sides of the current collector are the same, and the areal density of the active layer is the areal density of the active layer on either side.

[0057] In one instance, 0.96 ≤ X / Y ≤ 0.98.

[0058] In this invention, X and Y can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the first and second positive electrode sheets are disassembled and removed. After soaking in DMC solvent for 12 hours, the electrodes are rinsed with DMC solvent to remove the lithium salts adhering to them. Then, the surface residue of the electrodes is washed off with deionized water and dried. At least 20 sites are selected on the first and second positive electrode sheets respectively, and the thickness of the positive electrode sheet at each site is measured using a micrometer. The average value h (in μm) is taken. The first and second positive electrode sheets are punched into discs with a diameter of 44.3 mm using a punching die. Ten discs are taken, and the mass of each disc is weighed. The average value m (in mg) is taken. The areal density M1 is calculated as M1 = (m - m1) × 100 / 1540.25, where m1 is the mass of the first or second positive electrode current collector in the disc, in mg, and the areal density is in mg / cm³. 2 .

[0059] This invention obtains a structure with a concave portion on one side and a convex portion on the other side by embossing the surface of the first positive electrode. The shape of the orthographic projection of the concave portion and the convex portion onto the surface of the first positive electrode is not limited; it can be circular, near-circular, rectangular, near-rectangular, elliptical, linear (including straight lines or wavy lines), polygonal, or other shapes.

[0060] In this invention, the spacing P of the protrusions can be 0.5mm-8mm, for example, within the range of any two of the following values: 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm.

[0061] In one example, the spacing P of the protrusions is 2mm-6mm.

[0062] In this invention, the spacing between the protrusions refers to the shortest distance between the edges of the orthographic projections of two adjacent protrusions on the surface of the first positive electrode. The spacing between the protrusions can be tested using conventional methods in the art, such as using a 3D profilometer to select at least 10 groups of adjacent protrusions on the surface of the positive electrode, measuring the spacing, and taking the average value.

[0063] This invention, by limiting the spacing of the protrusions within a suitable range, ensures a moderate density of protrusions, thereby providing sufficient support for the electrode and enabling it to have sufficient deformation stress to suppress one-sided curling. When the spacing of the protrusions is too small, the protrusions are too dense, and the ductility of the electrode cannot meet the density of the protrusions. At the same time, the height of the protrusions is limited, which cannot effectively improve the anti-curling ability of the first positive electrode. When the spacing of the protrusions is too large, the protrusions are too sparse and the supporting area is insufficient, which also fails to achieve the effect of suppressing the curling of the first positive electrode and improving interface adhesion.

[0064] In this invention, the distance from the protrusion to the first edge of the first positive electrode sheet is L1, and the distance from the protrusion to the second edge of the first positive electrode sheet is L2, wherein 0mm ≤ L1 / L2 ≤ 4mm, for example, within the range of any two values ​​of 0mm, 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, or above. The protrusion is located at the third and fourth edges of the first positive electrode sheet. The first edge is the side where the tab is disposed, the second edge is the side opposite to the side where the tab is disposed, and the third and fourth edges are the two side edges of the first positive electrode sheet. By controlling L1 and L2, it is possible to ensure that the positive electrode does not shed powder, which is beneficial to improving the cycle performance of the battery.

[0065] In one instance, 1mm ≤ L1 / L2 ≤ 2mm.

[0066] like Figure 5 The figure shows a front view of the first positive electrode sheet in an embodiment of the present invention. As can be seen from the figure, the first positive electrode sheet includes a plurality of protrusions 1113. The distance from the protrusion to the first edge is L1, the distance from the protrusion to the second edge is L2, and the protrusion is located at the third edge and the fourth edge. The first edge is the side where the tab 14 is disposed, the second edge is opposite to the first edge, and the third and fourth edges are the two side edges of the first positive electrode sheet.

[0067] In this invention, the diaphragm comprises a carrier layer and an adhesive layer located on one or both surfaces of the carrier layer, wherein the coverage of the adhesive layer is 15-50% (e.g., within the range of any two values ​​of 15%, 20%, 30%, 40%, 50%, or higher). The coverage of the adhesive layer can be tested using conventional methods in the art, such as using a scanning electron microscope (SEM) combined with image analysis software.

[0068] In this invention, the adhesive layer includes first particles and / or second particles, wherein the second particles include secondary particles formed by the aggregation of primary particles. Specifically, the first particles comprise primary particles that are dispersedly distributed; the second particles comprise aggregated particles, i.e., formed by the aggregation of two or more primary particles.

[0069] In one example, the average particle size d1 of the primary particles of the first particle is 0.3 μm to 1.5 μm (e.g., within the range of any two values ​​of 0.3 μm, 0.5 μm, 0.7 μm, 1 μm, 1.3 μm, 1.5 μm or above), and the average particle size d2 of the secondary particles of the second particle is 4 μm to 20 μm (e.g., within the range of any two values ​​of 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm or above). Controlling d1 and d2 within the above range ensures that the particles are highly uniformly dispersed and densely packed in the adhesive layer, thereby forming a denser adhesive layer with good adhesion. This not only ensures the pore-closing effect of the separator but also enhances the bonding strength between the separator and the electrode. It avoids problems such as delamination between the first positive electrode and the negative electrode due to repeated expansion and contraction during charge and discharge cycles, and the formation of ion conduction breaks due to lack of electrolyte filling. It also prevents insufficient local lithium insertion in the corresponding negative electrode area, suppresses the generation of "purple spots" on the negative electrode surface, and improves the interface stability of the electrode and the cycle performance of the battery.

[0070] In one example, the components of the first particle and the second particle each independently include one or more of fluoropolymers, acrylate polymers, polyimides, and modified polyimides.

[0071] In this invention, d1 and d2 can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC (e.g., discharging the battery to 3V), disassembling and removing the separator, observing the cross-section of the adhesive layer using SEM, randomly selecting at least 10 first particles or second particles, measuring the particle size of each particle, and taking the average value.

[0072] In one embodiment, the carrier layer includes a substrate layer and a heat-resistant coating located on at least one surface of the substrate layer, the heat-resistant coating comprising heat-resistant particles. The substrate layer is selected from conventional materials in the art, such as polypropylene (PP), polyethylene (PE), or a composite layer of PP and PE.

[0073] In a preferred embodiment, the heat-resistant particles comprise at least one of the following: alumina, boehmite, magnesium oxide, magnesium hydroxide, barium sulfate, barium titanate, zinc oxide, calcium oxide, silicon dioxide, silicon carbide, boron nitride, polyacrylonitrile, nitrile rubber, melamine cyanurate, 1,3,5-triazine-2,4,6-triamine, melamine thiocyanate, 2,4,6-tris(2-pyridyl)triazine, and 2,4,6-triphenyl-1.

[0074] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a silicon-carbon material, which comprises a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix. A coating layer is provided on the outer surface of the porous carbon matrix, and the coating layer comprises amorphous carbon and / or a solid electrolyte material. In a 5000x SEM image, the cross-sectional profile of the silicon-carbon material includes a first interior angle, the degree of which is greater than 180 degrees.

[0075] The "first interior angle" refers to the angle formed by two straight lines tangent to the cross-sectional profile of a silicon carbide particle in a cross-sectional SEM image. The angle between these two tangent lines is greater than 180 degrees. Figure 6 The image shown is a cross-sectional scanning electron microscope (SEM) image of the silicon-carbon material in an example of the present invention (A represents the first interior angle, and A1, A2, and A3 represent different first interior angles). The cross-sectional profile of the silicon-carbon material of the present invention includes such interior angles. The surface of the silicon-carbon material corresponding to the region where such interior angles (first interior angles) are located forms a groove-like structure. This groove-like structure can increase the specific surface area of ​​the silicon-carbon material, which is beneficial for storing electrolyte. During battery cycling, the first positive electrode plate will consume electrolyte faster due to its higher current density. However, the electrolyte stored on the negative electrode side of the silicon-carbon material can be continuously replenished through the pores, thereby avoiding the interruption of local lithium-ion transport channels and suppressing lithium plating and "purple spots" in the negative electrode region corresponding to the first positive electrode plate.

[0076] It should be noted that the silicon-carbon material in this invention is prepared by vapor deposition, where silicon particles (by introducing silane gas into a fluidized bed reactor) are deposited onto a porous carbon matrix (using methods well-known to those skilled in the art). The preparation of silicon-carbon differs from conventional methods in that it uses phenol and formaldehyde aqueous solutions as raw materials, reacting under sodium hydroxide catalysis to obtain phenolic resin microspheres. Using silica-supported sulfonic acid as a catalyst, the obtained phenolic resin is reacted with polyethylene glycol to obtain modified resin microspheres. This modified resin is dispersed in a mixed solvent of ethanol and water, and ammonia is added for reaction, followed by a hydrothermal reaction to obtain spherical phenolic resin microspheres. The obtained microspheres are mixed with potassium hydroxide, carbonized at a certain temperature under an inert atmosphere, and then heated to a high temperature and held for treatment to finally obtain a spherical porous carbon matrix material (reaction conditions can be designed according to actual needs).

[0077] In this invention, the lithium-ion secondary battery further includes an electrolyte comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a carboxylic acid ester solvent. Based on the total mass of the electrolyte, the mass content of lithium bis(trifluoromethanesulfonyl)imide is C1, and the mass content of the carboxylic acid ester solvent is C2, where 10 ≤ C2 / C1 ≤ 100 (for example, within the range of any two values ​​of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, or above).

[0078] In one example, the carboxylic acid ester solvent includes at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), and 2,2-difluoroethyl acetate (DFEA).

[0079] In one example, the carboxylic acid ester solvent includes at least one of EP, PP, and EB.

[0080] In one instance, C1 is 0.1%-5% (e.g., within the range of any two values ​​of 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or more).

[0081] In one instance, C2 is 3%-50% (e.g., within the range of any two values ​​of 3%, 5%, 10%, 20%, 30%, 40%, 50% or more).

[0082] This invention optimizes the conductivity and interfacial stability of the electrolyte by controlling the mass content of LiTFSI and carboxylic acid ester solvents in the electrolyte to achieve a C2 / C1 ratio within a specific range. While dual LiTFSI, as a lithium salt, helps improve the electrolyte's conductivity and enhances the battery's high-temperature cycle performance, excessive amounts can corrode the aluminum foil. Carboxylic acid ester solvents, with their low viscosity and high wettability, significantly improve the electrolyte's wetting ability on the separator and electrodes. Furthermore, the carbonyl functional groups in their molecular structure form a stable adsorption layer on the surface of the positive electrode current collector, effectively inhibiting LiTFSI corrosion of the current collector and preventing corrosion products from clogging the interface, causing lithium plating on the negative electrode, and resulting in "purple spots." If the C2 / C1 ratio is too high (e.g., greater than 100), meaning the relative content of carboxylic acid ester solvents is too high and the LiTFSI content is too low, the conductivity of the electrolyte will decrease significantly, making it difficult to meet the requirements of battery rate performance and high-temperature stability. At the same time, excessive carboxylic acid ester solvents may trigger other side reactions, leading to a deterioration of the interface protection effect. If the C2 / C1 ratio is too low (e.g., less than 10), the relative content of LiTFSI is too high and the carboxylic acid ester solvents are insufficient. In this case, the carboxylic acid esters cannot effectively inhibit the corrosion of the positive electrode current collector by LiTFSI. The corrosion products will gradually accumulate at the interface, blocking the ion transport channels, increasing the interface impedance, and thus inducing local lithium plating and purple spots on the negative electrode, affecting the cycle life of the battery.

[0083] In this invention, C1 and C2 can be obtained by conventional methods in the art. For example, C1 can be obtained by inductively coupled plasma (ICP) testing, and C2 can be obtained by gas chromatography (GC), gas chromatography-mass spectrometry (GCMS), or liquid chromatography (LC) testing.

[0084] The batteries can all be assembled in accordance with conventional methods in the field.

[0085] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0086] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0087] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0088] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0089] Example 1 Batteries are prepared according to the following method. (1) Preparation of positive electrode Preparation of the first positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.50:0.8:0.7 and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated on one side of the first positive electrode current collector (thickness h2=16.1μm), and after drying, rolling, and die-cutting, the first positive electrode sheet was obtained.

[0090] Preparation of the second positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.5:0.8:0.7 and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated on both sides of a second positive electrode current collector (9 μm thick), and after drying, rolling, and die-cutting, a second positive electrode sheet was obtained.

[0091] (2) Preparation of negative electrode Artificial graphite, silicon carbide material (quasi-spherical silicon carbide, first interior angle A1=256°, A2=258°, A3=262°, with an amorphous carbon layer on the surface), polyacrylic acid (PAA), sodium carboxymethyl cellulose, and acetylene black were added to a vacuum mixer in a mass ratio of 57.6:38.4:2.7:0.65:0.65, along with an appropriate amount of deionized water. The mixture was thoroughly mixed under vacuum until a uniform, fluid negative electrode slurry was formed. The negative electrode slurry was then uniformly coated onto both sides of a copper foil. After drying, rolling, and slitting, the negative electrode sheet was obtained. The silicon content in the negative electrode active layer was 15% by weight.

[0092] (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of 1:1:1 as the base solvent. Lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and carboxylic acid ester solvents (EA, EP, EB, and DFEA mixed at a mass ratio of 1:1:1:0.5) were added sequentially. After passing moisture and free acid tests, the electrolyte was obtained. Based on the total mass of the electrolyte, the mass content of LiPF6 was 12.5%, the content of lithium bis(trifluoromethanesulfonyl)imide (C1) was 0.5%, the total mass of the carboxylic acid ester solvents (C2) was 35%, and the C2 / C1 ratio was 70.

[0093] (4) Preparation of the diaphragm Heat-resistant granules (alumina) were dispersed in deionized water, and polyacrylic acid was added. After stirring evenly, a mixed slurry with a solid content of 25% was obtained. The mixed slurry was continuously coated onto one side of the substrate layer (polyethylene) using a gravure roller, and then dried and shaped in a multi-section oven at 60°C to obtain a heat-resistant layer. Polymethyl acrylate (first granules) and polyvinylidene fluoride (second granules) were mixed and dispersed in water at a weight ratio of 60:40. After stirring evenly, a mixed slurry with a solid content of 20% was obtained. The mixed slurry was continuously coated onto the side of the heat-resistant layer away from the first substrate layer using a gravure roller (forming an adhesive layer on the heat-resistant layer). The mixed slurry of the first and second granules was continuously coated onto the other surface of the substrate layer using a gravure roller, and dried and shaped in a multi-section oven at 60°C to obtain the desired diaphragm. The average particle size of the primary particles of the first granules was 0.95 μm, the average particle size of the secondary particles of the second granules was 12.5 μm, and the coverage of the adhesive layer was 35%.

[0094] (5) Battery manufacturing The process of creating protrusions on the first positive electrode sheet specifically includes: rolling the prepared first positive electrode sheet using a roller to form specific protrusions on the electrode sheet surface (h1 is 17.5 μm, p is 4 mm, ρ is 4 protrusions / cm). 2 ρ / A is 3.7, r1 is 2.8mm, r2 is 2.5mm, r1 / r2 is 1.12, r2 / h1 is 143, included angle a is 135, S2 / S1 is 0.44), the convex direction of the protrusion is from the second surface of the first positive electrode sheet towards the direction away from the center of the electrode assembly.

[0095] The prepared first positive electrode, second positive electrode, negative electrode and separator are stacked in the following order to form an electrode assembly: the outermost electrode is the first positive electrode, and the middle is in the following cycle: separator, negative electrode, second positive electrode to obtain a bare cell; the bare cell is welded with tabs; the stacked cell is obtained, the obtained cell is put into an aluminum-plastic film of matching size and sealed, the electrolyte of step (3) is injected under vacuum conditions and vacuum sealed, and the battery is obtained through standing, formation and sorting processes.

[0096] Example 2 Batteries are prepared according to the following method. (1) Preparation of positive electrode Preparation of the first positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.50:0.8:0.7 and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated on one side of the first positive electrode current collector, and after drying, rolling, and die-cutting, the first positive electrode sheet was obtained.

[0097] Preparation of the second positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.5:0.8:0.7 and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated on both sides of the second positive electrode current collector, and after drying, rolling, and die-cutting, the second positive electrode sheet was obtained.

[0098] (2) Preparation of negative electrode Artificial graphite, silicon carbide material (quasi-spherical silicon carbide, first interior angle A1=256°, A2=258°, A3=262°, with an amorphous carbon layer on the surface), polyacrylic acid (PAA), sodium carboxymethyl cellulose, and acetylene black were added to a vacuum mixer in a mass ratio of 57.6:38.4:2.7:0.65:0.65, along with an appropriate amount of deionized water. The mixture was thoroughly mixed under vacuum until a uniform, fluid negative electrode slurry was formed. The negative electrode slurry was then uniformly coated onto both sides of a copper foil. After drying, rolling, and slitting, the negative electrode sheet was obtained. The silicon content in the negative electrode active layer was 15% by weight.

[0099] (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of 1:1:1 as the base solvent. Lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and carboxylic acid ester solvents (EA, EP, EB, and DFEA mixed at a mass ratio of 1:1:1:0.5) were added sequentially. After passing moisture and free acid tests, the electrolyte was obtained. Based on the total mass of the electrolyte, the mass content of LiPF6 was 12.5%, the content of lithium bis(trifluoromethanesulfonyl)imide (C1) was 0.25%, the total mass of the carboxylic acid ester solvents (C2) was 25%, and the C2 / C1 ratio was 100.

[0100] (4) Preparation of the diaphragm Heat-resistant granules (alumina) were dispersed in deionized water, and polyacrylic acid was added. After stirring evenly, a mixed slurry with a solid content of 25% was obtained. The mixed slurry was continuously coated onto one side of the substrate layer (polyethylene) using a gravure roller, and then dried and shaped in a multi-section oven at 60°C to obtain a heat-resistant layer. Polymethyl acrylate (first granules) and polyvinylidene fluoride (second granules) were mixed and dispersed in water at a weight ratio of 60:40. After stirring evenly, a mixed slurry with a solid content of 10% was obtained. The mixed slurry was continuously coated onto the side of the heat-resistant layer away from the first substrate layer using a gravure roller (forming an adhesive layer on the heat-resistant layer). The mixed slurry of the first and second granules was continuously coated onto the other surface of the substrate layer using a gravure roller, and dried and shaped in a multi-section oven at 60°C to obtain the desired diaphragm. The average particle size of the primary particles of the first granules was 0.32 μm, the average particle size of the secondary particles of the second granules was 4.1 μm, and the coverage of the adhesive layer was 15%.

[0101] (5) Battery manufacturing The process of creating protrusions on the first positive electrode sheet specifically includes: rolling the prepared first positive electrode sheet using a roller to form specific protrusions on the electrode sheet surface (h1 is 5.3 μm, p is 2 mm, and ρ is 5 protrusions / cm). 2 ρ / A is 11.4, r1 is 0.6mm, r2 is 0.58mm, r1 / r2 is 1.03, r2 / h1 is 109, included angle α is 123, S2 / S1 is 0.2), the convex part of the first positive electrode plate protrudes from the second surface of the first positive electrode plate in a direction away from the center of the electrode assembly.

[0102] The prepared first positive electrode, second positive electrode, negative electrode and separator are stacked in the following order to form an electrode assembly: the outermost electrode is the first positive electrode, and the middle is in the following cycle: separator, negative electrode, second positive electrode to obtain a bare cell; the bare cell is welded with tabs; the stacked cell is obtained, the obtained cell is put into an aluminum-plastic film of matching size and sealed, the electrolyte of step (3) is injected under vacuum conditions and vacuum sealed, and the battery is obtained through standing, formation and sorting processes.

[0103] Example 3 Batteries are prepared according to the following method. (1) Preparation of positive electrode Preparation of the first positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.50:0.8:0.7 and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated on one side of the first positive electrode current collector, and after drying, rolling, and slitting, the first positive electrode sheet was obtained.

[0104] Preparation of the second positive electrode: Lithium cobalt oxide, a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) were placed in N-methylpyrrolidone (NMP) at a mass ratio of 98.5:0.8:0.7 and stirred evenly to obtain a positive electrode slurry. The above positive electrode slurry was uniformly coated on both sides of the second positive electrode current collector, and after drying, rolling, and slitting, the second positive electrode sheet was obtained.

[0105] (2) Preparation of negative electrode Artificial graphite, silicon carbide material (spherical silicon carbide with first interior angles A1=256°, A2=258°, A3=262°, and an amorphous carbon layer on the surface), polyacrylic acid (PAA), sodium carboxymethyl cellulose, and acetylene black were added to a vacuum mixer in a mass ratio of 57.6:38.4:2.7:0.65:0.65, along with an appropriate amount of deionized water. The mixture was thoroughly mixed under vacuum until a uniform, fluid negative electrode slurry was formed. The negative electrode slurry was then uniformly coated onto both sides of a copper foil. After drying, rolling, and slitting, the negative electrode sheet was obtained. The silicon content in the negative electrode active layer was 15% by weight.

[0106] (3) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of 1:1:1 as the base solvent. Lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and carboxylic acid ester solvents (EA, EP, EB, and DFEA mixed at a mass ratio of 1:1:1:0.5) were added sequentially. After passing moisture and free acid tests, the electrolyte was obtained. Based on the total mass of the electrolyte, the mass content of LiPF6 was 12.5%, the content of lithium bis(trifluoromethanesulfonyl)imide (C1) was 5%, the total mass of the carboxylic acid ester solvents (C2) was 50%, and the C2 / C1 ratio was 10.

[0107] (4) Preparation of the diaphragm Heat-resistant granules (alumina) were dispersed in deionized water, and polyacrylic acid was added. After stirring evenly, a mixed slurry with a solid content of 25% was obtained. The mixed slurry was continuously coated onto one side of the substrate layer (polyethylene) using a gravure roller, and then dried and shaped in a multi-section oven at 60°C to obtain a heat-resistant layer. Polymethyl acrylate (first granules) and polyvinylidene fluoride (second granules) were mixed and dispersed in water at a weight ratio of 60:40. After stirring evenly, a mixed slurry with a solid content of 30% was obtained. The mixed slurry was continuously coated onto the side of the heat-resistant layer away from the first substrate layer using a gravure roller (forming an adhesive layer on the heat-resistant layer). The mixed slurry of the first and second granules was continuously coated onto the other surface of the substrate layer using a gravure roller, and dried and shaped in a multi-section oven at 60°C to obtain the desired diaphragm. The average particle size of the primary particles of the first granules was 1.46 μm, the average particle size of the secondary particles of the second granules was 18.6 μm, and the coverage of the adhesive layer was 50%.

[0108] (5) Battery manufacturing The first positive electrode sheet is provided with protrusions, specifically by: rolling the prepared first positive electrode sheet with a roller to form specific protrusions on the surface of the electrode sheet (h1 is 29.3 μm, p is 6 mm, ρ is 3 protrusions / cm). 2 ρ / A is 2, r1 is 8mm, r2 is 7.5mm, r1 / r2 is 1.07, r2 / h1 is 256, included angle a is 145, S2 / S1 is 0.7), the convex part of the first positive electrode plate protrudes from the second surface of the first positive electrode plate in a direction away from the center of the electrode assembly.

[0109] The prepared first positive electrode, second positive electrode, negative electrode and separator are stacked in the following order to form an electrode assembly: the outermost electrode is the first positive electrode, and the middle is in the following cycle: separator, negative electrode, second positive electrode to obtain a bare cell; the bare cell is welded with tabs; the stacked cell is obtained, the obtained cell is put into an aluminum-plastic film of matching size and sealed, the electrolyte of step (3) is injected under vacuum conditions and vacuum sealed, and the battery is obtained through standing, formation and sorting processes.

[0110] Examples 4-5 were carried out with reference to Examples 1-3, and the specific settings are shown in Tables 1-1 and 1-2.

[0111] Example 6 This embodiment is based on Embodiment 1, except that the spherical silicon carbide material is replaced with a commercially available spherical silicon carbide material (with a sphericity greater than 0.98), and the cross-sectional profile of this silicon carbide material does not have a first interior angle.

[0112] Example 7 group This set of embodiments follows the same procedure as Embodiment 1, except that C2 / C1 is adjusted by changing C1 and C2 to verify the effect of the change in C2 / C1, as follows: Example 7-1: C1 is 0.5%, C2 is 4.5%, and C2 / C1 is 9. Example 7-2: C1 is 0.48%, C2 is 50%, and C2 / C1 is 104.17.

[0113] Comparative Example 1 This comparative example is based on Example 1, except that no protrusion is provided on the first positive electrode plate, and h2 is 16.1 μm.

[0114] Comparative Examples 2-3 were carried out with reference to Example 1, and the specific settings are shown in Tables 1-1 and 1-2.

[0115] Table 1-1 Table 1-2 Note: The " / " in Tables 1-1 and 1-2 indicates that the parameter does not exist.

[0116] Test case (1) Purple spot test The batteries prepared in the examples and comparative examples were subjected to purple spot testing. The specific testing methods are as follows: The battery was subjected to a 25°C room temperature cycle test. The cycle test procedure was as follows: charge at 2.0C to 4.1V, charge at 1.8C to 4.3V, charge at 1C to 4.4V, charge at 0.5C to 4.53V, stop at 0.05C, rest for 5 minutes, discharge at 0.7C to 3V, and repeat this cycle 600 times. The fully charged 600T cycled battery was disassembled to observe the degree of purple spots. The purple spot area was used to determine the severity: 0% purple spot area was no purple spot (Level 0), less than 5% purple spot area was very slight purple spot (Level 1), 5%~10% purple spot area was slight purple spot (Level 2), 10%~20% purple spot area was purple spot (Level 3), 20%~50% purple spot area was severe purple spot (Level 4), and more than 50% purple spot area was very severe purple spot (Level 5). Specific test results are shown in Table 2.

[0117] (2) Loop testing The batteries prepared in the examples and comparative examples were subjected to cycle tests, and the specific test methods are as follows: (1) Let stand at 25℃±2℃ for 5min, then discharge at 0.2C to the lower limit voltage (3V); (2) After standing for 5 minutes at 25℃±2℃, discharge at 0.7C to 3V and measure the discharge capacity at this time as Q1. After standing for 5 minutes, charge at 2.0C to 4.1V, 1.8C to 4.3V, 1C to 4.4V, and 0.5C to 4.53V. Cut off at 0.05C, stand for 5 minutes, and discharge at 0.7C to 3V. Repeat this process 600 times. Record the discharge capacity at 0.7C to 3V on the 600th cycle as Q2. The cycle capacity retention rate of the battery = (Q2 / Q1)×100%. See Table 2 for specific test results.

[0118] Table 2 As can be seen from Table 2, the battery prepared by the present invention can significantly improve the "purple spot" defect compared with the comparative example, and significantly improve the cycle stability of the battery.

[0119] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The electrode assembly includes a positive electrode, a separator, and a negative electrode stacked sequentially. The positive electrode includes at least one first positive electrode and at least one second positive electrode; the first positive electrode is located on the outermost side of the electrode assembly, and the first positive electrode includes a first positive current collector, the first positive current collector including a first surface and a second surface disposed opposite to each other along its thickness direction, the first surface being disposed of a first positive active layer, the first surface being close to the center of the electrode assembly, and the second surface being away from the center of the electrode assembly; the second positive electrode includes a second positive current collector and a second positive active layer disposed on both sides of the second positive current collector; the first positive electrode includes a plurality of protrusions and a plurality of recesses disposed opposite to the plurality of protrusions; the protrusions protrude from the second surface in a direction away from the center of the electrode assembly, and the recesses are recessed from the first surface in a direction towards the second surface; The height of the protrusion is h1, in μm; the thickness of the first positive current collector is h2, in μm. The peel force between the first positive electrode and the separator is N1, in N / m; the peel force between the second positive electrode and the separator is N2, in N / m. Where A = h1 / h2, 0.005 ≤ A / N1 ≤ 0.5, 0.8 ≤ N1 / N2 ≤ 2.

2. The lithium-ion secondary battery according to claim 1, wherein, 0.01≤A / N1≤0.24; And / or, 0.2 ≤ A ≤ 2.2; preferably, 0.4 ≤ A ≤ 1.5; And / or, h1 is 3μm-60μm; preferably 5μm-30μm; And / or, h2 is 10μm-30μm; preferably 12μm-20μm; And / or, N1 is 4N / m-40N / m; preferably 6N / m-30N / m; And / or, N2 is 4N / m-35N / m; preferably 6N / m-30N / m.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The surface density of the protrusion on the second surface is ρ, 1≤ρ / A≤12; Preferably, 1 per cm 2 ≤ρ≤8 pieces / cm 2 More preferably, 2 per cm 2 ≤ρ≤5 pieces / cm 2 .

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The second surface further includes a first straight section. The convex portion includes a first outer surface, and the concave portion includes a second outer surface. The orthographic projection diameter of the first outer surface of the convex portion onto the second surface is r1, and the orthographic projection diameter of the second outer surface of the concave portion onto the first surface is r2. 40≤r2 / h1≤1070, where r2 is in mm and h1 is in μm. And / or, the angle between the tangent at the junction of the first outer surface and the first straight segment and the horizontal line where the first straight segment is located is α, where 100°≤α≤160°; preferably, 120°≤α≤150°.

5. The lithium-ion secondary battery according to claim 4, wherein, r1 is 0.5mm-8mm; preferably 1.5mm-4mm; And / or, r2 is 0.5mm-8mm; preferably 1.5mm-4mm; And / or, 100≤r2 / h1≤300; And / or, 1.02≤r1 / r2≤1.

2.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, The area of ​​the second surface is S1, and the sum of the projected areas of the protrusions on the second surface along the thickness direction of the first positive electrode is S2, where 0.2≤S2 / S1≤0.

7. Preferably, 0.3 ≤ S2 / S1 ≤ 0.

6.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein, The areal density of the first positive electrode active layer is X, and the areal density of the second positive electrode active layer is Y, wherein 0.94≤X / Y≤1; Preferably, 0.96≤X / Y≤0.

98.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, The diaphragm includes a carrier layer and an adhesive layer located on one or both surfaces of the carrier layer. The adhesive layer has a coverage of 15%-50%. The adhesive layer includes first particles and / or second particles, wherein the second particles include secondary particles formed by the aggregation of primary particles. Preferably, the average particle size d1 of the primary particles of the first particle is 0.3 μm-1.5 μm, and the average particle size d2 of the secondary particles of the second particle is 4 μm-20 μm. Preferably, the components of the first particle and the second particle each independently include one or more of fluoropolymers, acrylate polymers, polyimides, and modified polyimides.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The negative electrode sheet includes a negative current collector and a negative active layer located on at least one side surface of the negative current collector. The negative active layer includes a silicon-carbon material, which includes a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix. The outer surface of the porous carbon matrix is ​​provided with a coating layer, which includes amorphous carbon and / or solid electrolyte material. In a 10Kx SEM image, the cross-sectional profile of the silicon-carbon material includes a first interior angle, the degree of which is greater than 180 degrees.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein, The lithium-ion secondary battery further includes an electrolyte, which comprises lithium bis(trifluoromethanesulfonyl)imide and a carboxylic acid ester solvent. Based on the total mass of the electrolyte, the mass content of lithium bis(trifluoromethanesulfonyl)imide is C1, and the mass content of the carboxylic acid ester solvent is C2, where 10 ≤ C2 / C1 ≤ 100. Preferably, C1 is 0.1%-5%; Preferably, C2 is 3%-50%.

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