Lithium ion secondary battery

Abstract

The application relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises an electrode assembly, the electrode assembly comprises a positive electrode sheet, a diaphragm and a negative electrode sheet which are stacked; the positive electrode sheet comprises a first positive electrode sheet located at the outermost side of the electrode assembly; the first positive electrode sheet comprises a first positive electrode current collector, the first positive electrode current collector comprises a first surface and a second surface which are oppositely arranged along the thickness direction of the first positive electrode current collector; the first positive electrode sheet comprises a convex part and a concave part, the height h1 of the convex part is 3-60 mu m; the negative electrode sheet comprises silicon-carbon particles and graphite material; the cross-sectional profile of the silicon-carbon particles comprises a gully structure, the depth of the gully structure is d1, the length-diameter ratio of the silicon-carbon particles is d2, and 0.02<=d1 / d2<=0.6; the particle size Dv50 of the graphite material is 3-14 mu m. The application can effectively improve the uneven current density distribution of the first positive electrode sheet, reduce the problems of negative electrode purple stain and lithium precipitation, and effectively improve the cycle stability of the 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] As market demands for higher energy density and faster charging performance of lithium-ion batteries continue to increase, existing technologies often employ stacked battery designs with silicon-carbon particles introduced into the negative electrode. Silicon-carbon particles, due to their high theoretical specific capacity, are a key factor in improving energy density, while stacked batteries, with their lower impedance characteristics, facilitate rapid charging and discharging.

[0003] However, stacked battery structures have inherent technical defects: due to the tight stacking of electrodes, uneven local current density distribution is easily caused, affecting ion diffusion. In particular, the outermost positive electrode is only coated with active material on one side, resulting in a higher active material loading per unit area relative to its reaction interface. As a result, it can withstand a much higher local current density than the inner positive electrode under the same current, accelerating electrolyte consumption in this area. This can easily lead to the formation of "purple spots" (lithium dendrites or transition metal deposition) on the corresponding negative electrode surface in the later stages of cycling due to electrolyte bridging. This phenomenon is more pronounced, especially during high-temperature cycling at 45°C, accompanied by a significant volume expansion effect of silicon and carbon particles. As the number of cycles increases, the severity of the purple spots becomes more severe, and even white lithium plating may appear in the center of the purple spots, seriously affecting the cycle life of the battery.

[0004] Therefore, how to effectively improve the interfacial lithium plating and "purple spot" phenomena caused by uneven local current density distribution of electrodes and excessive electrolyte consumption in single-sided areas of silicon-doped stacked batteries has become an urgent technical problem to be solved. 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 and deformation of the first positive electrode, improve the uneven distribution of current density, and reduce problems such as "purple spots" on the negative electrode and lithium plating, thereby effectively improving the cycle stability of the battery.

[0006] Based on the above problems, the inventors conducted extensive targeted research, specifically: First, this invention improves the first positive electrode by providing multiple protrusions and corresponding recesses (the protrusions extend from the second surface of the first positive electrode away from the center of the electrode assembly, and the recesses are recessed from the first surface of the first positive electrode towards the second surface) and limiting the height h1 of the protrusions. This improves the anti-curling ability of the first positive electrode while also mitigating the "purple spot" defect. The reason is that the active material is only coated on one side of the first positive electrode, leading to an imbalance of stress on both sides. By providing a concave-convex structure on the first positive electrode, the microstructure of the electrode can be reconstructed, balancing the stress and increasing rigidity. This improves the current collector warping and deformation to some extent, thereby suppressing delamination between the first positive and negative electrodes and mitigating the purple spot problem caused by electrolyte bridging in the later stages of cycling. The value of h1 affects the peeling force between the first positive electrode and the separator. Since the protrusion extends from the second surface away from the center of the electrode assembly, if h1 is too large (e.g., greater than 60 μm), the gap between the positive electrode and the separator / negative electrode will be too large, weakening their bonding strength. With repeated expansion and contraction during cycling, delamination of the positive and negative electrode interfaces is likely to occur. This area cannot be replenished after electrolyte consumption, forming a broken bridge, which in turn causes insufficient lithium intercalation in the negative electrode, resulting in "purple spots." Conversely, if h1 is too small (e.g., less than 3 μm), it cannot improve problems such as current collector warping and deformation, and it is also difficult to store enough electrolyte to compensate for the rapid consumption of electrolyte in this area, resulting in minimal improvement in "purple spots." Therefore, by limiting h1 within a suitable range, this invention can effectively improve the structural stability of the positive electrode and alleviate the "purple spot" problem.

[0007] However, the protrusions also introduce new technical challenges. Specifically, the protrusion structure on the surface of the first positive electrode causes differences in lithium-ion transport paths between different regions. Under high-rate charge-discharge conditions, this can easily lead to uneven distribution of local lithium-ion flux and current density, resulting in uneven lithium intercalation in the corresponding negative electrode region and inducing local lithium plating. This is especially true for negative electrode systems using spherical silicon-carbon particles. The smooth surface of spherical particles, fewer contact points between particles, and weaker adhesion to the binder result in low overall electrode cohesion. During cycling, the repeated expansion and contraction of the silicon-carbon particles further disrupts the conductive network between particles, making the local current density differences more pronounced, thereby exacerbating uneven lithium intercalation and the risk of local lithium plating.

[0008] Based on this, the present invention also makes synergistic improvements to the negative electrode. The negative electrode of the present invention adopts silicon-carbon particles with a trench structure on the surface and small-particle graphite material (particle size Dv50 between 3μm and 14μm), and the particle size of the graphite material, the depth d1 of the trench structure, and the major axis d2 of the silicon-carbon particles are controlled so that d1 / d2 and the graphite particle size meet specific ranges. This not only effectively improves the electrolyte storage capacity, but also provides space for the graphite material with a suitable particle size to be embedded, enhances the lithium ion transport capacity between particles, and thus effectively improves the problem of uneven local lithium intercalation of the electrode and the resulting interface lithium deposition caused by the protrusion of the first positive electrode, thereby improving the long-term cycle stability of the battery. If d1 / d2 is too small (e.g., less than 0.02) and the graphite particle size is too large (e.g., greater than 14μm), the groove structure on the surface of the silicon-carbon particles will be too shallow, resulting in insufficient electrolyte storage space. In the later stages of battery cycling, it will be unable to provide sufficient ion transport channels and electrolyte replenishment to the negative electrode side. Furthermore, the groove structure of large graphite particles is difficult to match with that of silicon-carbon materials, weakening the transport capacity of lithium ions between particles and making it difficult to effectively improve the lithium plating problem caused by uneven local lithium intercalation on the electrode. If d1 / d2 is too large (e.g., greater than 0.6), the groove structure on the surface of the silicon-carbon particles will be too deep, which will easily cause a decrease in the mechanical strength of the silicon-carbon particles. During cycling, as the particle volume repeatedly expands and contracts, stress concentration will occur inside the particles, making them prone to breakage and pulverization, damaging the negative electrode conductive network, aggravating uneven local current density, and thus aggravating the negative electrode "purple spots" and lithium plating defects. At the same time, if the graphite particle size is too small (e.g., less than 3μm), the specific surface area will be too large, which will aggravate the side reactions with the electrolyte and affect the cycle 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 together. 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. In the thickness direction of the electrode assembly, the first surface is closer to the center of the electrode assembly, and the second surface is farther from the center of the electrode assembly. The second positive electrode includes a second positive current collector and a second positive current collector disposed on both sides of the second positive current collector. The first positive electrode sheet includes a plurality of protrusions and a plurality of recesses opposite to the protrusions; the protrusions protrude from the second surface toward the direction away from the center of the electrode assembly, and the recesses are recessed from the first surface toward the second surface; the height h1 of the protrusions is 3μm-60μm; the negative electrode sheet includes a negative current collector and a negative active layer located on the surface of the negative current collector, the negative active layer includes silicon carbon particles and graphite material, the cross-sectional profile of the silicon carbon particles includes a trench structure, the depth of the trench structure is d1, the major axis of the silicon carbon particles is d2, wherein 0.02≤d1 / d2≤0.6; the particle size Dv50 of the graphite material is d3, and d3 is 3μm-14μm.

[0010] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) By improving the first positive electrode sheet, the present invention makes the first positive electrode sheet contain multiple concave and convex structures and limits the height h1 of the convex part, which can improve the anti-curling ability of the first positive electrode sheet and effectively improve defects such as "purple spots"; (2) The present invention also makes synergistic improvements to the negative electrode by using silicon carbon particles with trench structure and graphite material with appropriate particle size, and by controlling the particle size of graphite material, the depth d1 of trench structure and the major diameter d2 of silicon carbon particles, so that d1 / d2 and the particle size of graphite material meet specific ranges. This not only effectively improves the storage capacity of electrolyte on the negative electrode side, but also provides space for the graphite material with suitable particle size to be embedded, enhances the transport capacity of lithium ions between particles, thereby effectively improving the problem of uneven local lithium intercalation of the electrode after the first positive electrode is set with protrusion, which causes lithium plating at the interface and improves the long-term cycle stability of the battery.

[0011] 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

[0012] Figure 1 The diagram shown is a schematic diagram of the electrode assembly along the thickness direction in an embodiment of the present invention.

[0013] Figure 2 The diagram shown is a schematic diagram of the structure of the first positive electrode in an embodiment of the present invention.

[0014] Figure 3 The diagram shown is a schematic diagram of the structure of the second positive electrode in an embodiment of the present invention.

[0015] Figure 4 The diagram shown is a schematic outline of the cross-section of silicon-carbon particles in an embodiment of the present invention. Wherein, Figure 4 (a) is a schematic diagram of the second curve of the cross-section of silicon-carbon particles. Figure 4 (b) is a schematic diagram of the first curve of the cross-section of silicon-carbon particles.

[0016] 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. Detailed Implementation

[0017] 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.

[0018] This invention provides a lithium-ion secondary battery, including an electrode assembly comprising a positive electrode, a separator, and a negative electrode stacked together. 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. In the thickness direction of the electrode assembly, the first surface is closer to the center of the electrode assembly, and the second surface is farther away from the electrode. The center of the 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 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 h1 of the protrusions is 3μm-60μm (for example, within the range of any two of the following values: 3μm, 5μm, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm). Figure 1 The diagram shows a schematic representation of the electrode assembly along its thickness in an embodiment of the present invention. As can be seen from the diagram, 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. The first positive electrode includes a first positive current collector 1111 and a first positive active layer 1112. The second positive electrode includes a second positive current collector 1121 and a second positive active layer 1122. The negative electrode includes a negative current collector 121 and a negative active layer 122. Figure 2 The figure shows a schematic diagram of the structure of the first positive electrode in an embodiment of the present invention. As can be seen from the figure, 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 a plurality of protrusions 1113 and a plurality of concave portions 1114 disposed corresponding to the plurality of protrusions. The height of the protrusions is h1, and the thickness of the first positive current collector is h2.

[0019] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector. The negative electrode active layer includes silicon carbon particles and graphite material. The cross-sectional profile of the silicon carbon particles includes a trench structure, the depth of the trench structure is d1, and the major axis of the silicon carbon particles is d2, wherein 0.02≤d1 / d2≤0.6 (for example, within the range of any two values ​​of 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or above). The particle size Dv50 of the graphite material is d3, and d3 is 3μm-14μm (for example, within the range of any two values ​​of 3μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, or above). The presence of the trench structure allows graphite material to embed and fill within it, establishing more and more stable contact points between silicon-carbon particles and graphite. This effectively compensates for uneven local current density distribution in the positive electrode, enhances lithium-ion transport between particles, and helps to synergistically improve the problem of lithium plating in the negative electrode caused by uneven local lithium intercalation. If d3 is too small (e.g., less than 3 μm), its specific surface area increases significantly, exacerbating side reactions with the electrolyte and accelerating electrolyte consumption, thus affecting battery cycle performance. If d3 is too large (e.g., greater than 14 μm), the lithium-ion diffusion path inside the particles is prolonged, and excessively large graphite particles are difficult to embed into the trench structure on the surface of silicon-carbon particles, weakening the contact effect between particles, reducing ion transport kinetics, and worsening the lithium plating problem.

[0020] It should be noted that: "the cross-sectional profile of the silicon carbide particle" refers to the cross-sectional profile obtained by taking a cross-section in the concave direction of the groove structure on the surface of the silicon carbide particle (from the surface of the silicon carbide particle to the interior of the silicon carbide particle). The plane containing this cross-sectional profile is the cross-sectional profile obtained by passing through the geometric center and major axis of the silicon carbide particle.

[0021] In one instance, h1 is 5μm-30μm.

[0022] In one instance, d1 is 0.2μm-8μm (e.g., within the range of 0.2μm, 0.3μm, 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or any two of the above values).

[0023] In one instance, d2 is 5μm-15μm (e.g., within the range of any two of the values ​​5μm, 7μm, 9μm, 11μm, 13μm, 15μm, or above).

[0024] In one example, the silicon-carbon particles comprise a porous carbon matrix and silicon particles located within pores of the porous carbon matrix.

[0025] In one instance, d3 is 4μm-10μm.

[0026] The present 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 sheet; the shape of the orthographic projection of the concave portion and the convex portion on the surface of the first positive electrode sheet is not limited, and can be circular, quasi-circular, rectangular, quasi-rectangular, elliptical, polygonal, etc.

[0027] In this invention, "the height h1 of the protrusion" refers to the vertical distance from the highest point of the protrusion to the surface of the positive electrode sheet, which can be obtained by conventional methods in the art, such as testing with a 3D profilometer, selecting at least 10 protrusions, measuring the height of each protrusion, and taking the average value.

[0028] In this invention, "the groove structure" refers to a groove with a certain depth and width formed on the cross-section of the silicon carbide particle, and its cross-sectional morphology can be observed by scanning electron microscopy (SEM); "the depth d1 of the groove structure" refers to the vertical distance between the tangents of the lowest point of the groove structure and the highest points of the contours on both sides of the groove structure in the SEM image of the cross-section of the silicon carbide particle; "the major axis d2 of the silicon carbide particle" refers to the length of the line connecting the two farthest points on the cross-sectional contour of the silicon carbide particle. 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 2.7V), disassembling the battery and removing the negative electrode, soaking it in dimethyl carbonate (DMC) solvent for 12 hours, then rinsing it with DMC to remove the lithium salt attached to the negative electrode, drying it, and then cutting the silicon-carbon particles using an argon ion cutter (e.g., NEC IB-19530CP argon ion section polisher). After cutting, the sample is quickly transferred to the scanning electron microscope sample chamber for observation. In the backscatter mode of the scanning electron microscope, the silicon-carbon particles show a brighter contrast, while the graphite particles are darker. The images of each bright particle in the SEM backscatter mode image of the negative electrode at a certain magnification (e.g., 5K) can be tested and calculated using image processing software (e.g., Image J).

[0029] In one example, the particle size Dv10 of the silicon-carbon particles is 2μm-8μm (e.g., within the range of any two values ​​of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or more), Dv50 is 4μm-15μm (e.g., within the range of any two values ​​of 4μm, 5μm, 6μm, 7μm, 8μm, 10μm, 15μm or more), and Dv90 is 7μm-20μm (e.g., within the range of any two values ​​of 7μm, 10μm, 12μm, 15μm, 18μm, 20μm or more). Dv10, Dv50, and Dv90 refer to the particle size corresponding to a cumulative volume distribution percentage of 10%, 50%, and 90% of the silicon-carbon particles, respectively, and can be obtained by laser particle size analyzer.

[0030] In this invention, the mass content w1 of silicon element in the silicon-carbon particles is 30%-80% (for example, within the range of any two values ​​of 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% or more).

[0031] In this invention, the mass content w2 of silicon element in the negative electrode active layer is 2%-50% (for example, within the range of any two values ​​of 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or above). When w2 is too low (for example, below 2%), the high capacity contribution of silicon material is negligible compared to pure graphite negative electrode, and it cannot achieve the effect of improving battery energy density; when w2 is too high (for example, above 50%), although it can bring higher specific capacity, a large number of silicon-carbon particles will cause drastic volume expansion after lithium intercalation, which can easily cause the electrode structure to collapse, the active material to fall off, affect the cycle stability of the battery, and worsen the lithium plating window of the battery.

[0032] In this invention, w1 can be obtained by conventional methods in the art, such as using an X-ray energy dispersive spectroscopy (EDS), specifically as follows: the battery is discharged to 0% SOC (e.g., the battery is discharged to 2.7V), the battery is disassembled and the negative electrode is removed. After soaking in dimethyl carbonate (DMC) solvent for 12 hours, the negative electrode is rinsed with DMC to remove the lithium salt attached to it. After drying, the silicon-carbon particles are cut using an argon ion cutter (e.g., a NEC IB-19530CP argon ion section polisher). The cut sample is quickly transferred to the scanning electron microscope sample chamber for observation. Using the EDS spot scan mode, at least 5 points on the surface of the silicon-carbon particles are measured to obtain the mass content of silicon at each point. The average value is calculated. At least 10 silicon-carbon particles are selected for surface scan testing, and the final average value is taken.

[0033] In this invention, w2 can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active layer can then be peeled off from the negative electrode current collector, and the negative electrode active layer is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes. This allows the non-silicon components in the negative electrode active layer to volatilize while the silicon is fully oxidized to silicon dioxide. Weigh the residual material. The silicon content in the negative electrode active layer can be obtained by the following formula: Silicon content in the negative electrode active layer = 7 × mass of residual material / (15 × mass of test sample).

[0034] In this invention, the thickness h2 of the first positive current collector and the thickness h3 of the second positive current collector satisfy: 1 ​​< h2 / h3 ≤ 4 (for example, within the range of any two values ​​of 1.1, 1.3, 1.5, 2, 2.5, 3, 3.5, 4, or above). Figure 3 The 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. The thickness of the second positive current collector is h3.

[0035] 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).

[0036] In one instance, h2 is 12μm-20μm.

[0037] In one instance, h3 is 6μm-18μm (e.g., within the range of 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or any two of the above values).

[0038] The first positive electrode, located on the outermost side of the electrode assembly, is the concentrated area for electron collection and conduction. Therefore, compared to the second positive electrode, the first positive electrode can carry a larger current per unit area. Appropriately increasing the thickness of the first positive current collector can effectively improve its anti-curling ability, preventing delamination of the positive and negative electrode interfaces caused by electrode warping during cycling, effectively preventing electrolyte bridging in the later stages of cycling, and further improving the negative electrode purple spot problem. Simultaneously, a thicker first positive current collector enhances the overall structural stability of the positive electrode, allowing it to adapt to smaller convex structures, reducing the difference in lithium-ion transport paths at uneven areas, and thus improving the lithium plating problem caused by uneven lithium intercalation at the negative electrode. Furthermore, since the first positive electrode is only coated with active material on its first surface, there is asymmetrical stress between the current collector and the active layer. Appropriately increasing the thickness of the first positive current collector can also provide stronger mechanical support, resisting the asymmetrical contraction and expansion stress of the active layer, and preventing the electrode from curling, deforming, or breaking.

[0039] Based on this, the present invention further optimizes the thickness h2 of the first positive current collector and the thickness h3 of the second positive current collector. By limiting h1 / h2 to a reasonable range, it can ensure that the first positive electrode has sufficient bending stiffness to suppress electrode curling during cycling, while avoiding the risk of local "purple spots" caused by excessive thickness of the first positive current collector. If h2 / h3 is too large (i.e., h2 / h3 > 4), it means that the thickness of the first positive electrode current collector is much larger than that of the second positive electrode current collector. Although it can improve the bending stiffness to a certain extent, it will significantly increase the single-sided flow area, leading to an increase in current density in this area, causing local heating and higher lithium-ion flux. This will accelerate the transitional delithiation of the first positive electrode and the consumption and decomposition of the electrolyte, which in turn induces lithium plating on the negative electrode and the "purple spot" phenomenon. If h2 / h3 is too small (i.e., h2 / h3 ≤ 1), it means that the thickness of the first positive electrode current collector is close to or even smaller than that of the second positive electrode current collector. Its bending stiffness is insufficient to balance the stress generated by the cyclic expansion and contraction of the first positive electrode active layer, which will cause the positive electrode to be prone to curling deformation, poor interface adhesion and other problems, which will also exacerbate the risk of local purple spots.

[0040] In this invention, h2 and h3 can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the first and second positive electrode sheets, scraping off the first and second positive electrode active layers on their surfaces, cutting the first and second positive electrode sheets with an argon ion milling machine using a CP laser, observing the first and second positive electrode current collectors (avoiding convex and concave areas) and the second positive electrode current collectors using a scanning electron microscope (SEM), randomly selecting 10 test sites on their surfaces, measuring the thickness of each site, and taking the average value.

[0041] In this invention, the OI value of the negative electrode is φ, 5≤φ≤80 (for example, within the range of any two values ​​of 5, 10, 20, 30, 40, 50, 60, 70, 80 or above).

[0042] In one instance, 20 ≤ φ ≤ 60.

[0043] The OI value of the negative electrode, or the orientation ratio, is a parameter characterizing the crystal orientation of the negative electrode material. This invention ensures that φ meets a specific range, indicating good anisotropy of the negative electrode material, which helps mitigate the extrusion or stretching effects on the electrode. Furthermore, due to the highly ordered layer arrangement, consistent expansion direction, and uniform stress distribution, it facilitates uniform diffusion and embedding of lithium ions between layers, reducing local stress concentration and thus suppressing irreversible expansion, further improving the battery's cycle performance. If the OI value of the negative electrode is too low (e.g., φ less than 5), the expansion and extrusion of graphite and silicon-carbon particles in the planar direction (XY direction) increases, leading to mutual compression and displacement between particles, which further damages the electrode structure, causing coating peeling and current collector breakage. Moreover, during repeated expansion / contraction, particle rearrangement and breakage are prone to occur, generating new active surfaces, exacerbating side reactions and the continuous growth of the solid electrolyte interphase (SEI) film, leading to increased internal resistance. If the OI value of the negative electrode is too high (e.g., φ greater than 80), it will result in excessive cyclic expansion rate of the electrode assembly in the thickness direction, adversely affecting the battery casing and cycle life.

[0044] In this invention, the OI value of the negative electrode can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, soaked in DMC solvent for 12 hours, then rinsed with DMC to remove the lithium salt adhering to the negative electrode, dried, and tested using an X-ray diffractometer to obtain the diffraction pattern. With the diffraction angle 2θ as the horizontal axis, the height of the diffraction peak of the {004} crystal plane in the range of 53.7° to 55.7° is taken as I1, and the height of the diffraction peak of the {110} crystal plane in the range of 76.4° to 78.4° is taken as I2. The OI value φ is the ratio of I1 / I2.

[0045] In this invention, the OI value φ of the negative electrode and the solidity α of the silicon-carbon particles satisfy: 1 ​​≤ 100 × α / φ ≤ 15 (for example, within the range of any two values ​​of 1, 1.2, 1.3, 1.5, 1.8, 2, 4, 6, 8, 10, 12, 14, 15, or above); where α refers to the ratio of the first curve of the cross-section of the silicon-carbon particles to the second curve of the cross-section of the silicon-carbon particles, the second curve of the cross-section of the silicon-carbon particles refers to the perimeter of the outline of the cross-section of the silicon-carbon particles, and the first curve of the cross-section of the silicon-carbon particles is formed by connecting the following two parts: 1) For regions with gully structures, the tangent line at the highest point of the contour on both sides of the gully structure is used as the measurement length of the region; 2) For the remaining areas without grooves, the actual perimeter of the outline is used as the measurement length.

[0046] like Figure 4 The diagram shown is a schematic outline of the cross-section of silicon-carbon particles in an embodiment of the present invention. Wherein, Figure 4 (a) is a schematic diagram of the second curve of the cross-section of silicon-carbon particles. Figure 4 (b) is a schematic diagram of the first curve of the cross-section of silicon-carbon particles. It can be seen from the figure that the depth of the trench structure is d1 and the major axis of the silicon-carbon particles is d2.

[0047] In one instance, 0.7 ≤ α < 1 (for example, within the range of 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 0.98, 0.99, or any two of the above values).

[0048] The firmness α of silicon-carbon particles is a parameter characterizing the surface morphology of the particles. It can be understood that the closer the particle shape is to a perfect sphere and the smoother the surface, the closer its firmness α is to 1; the more grooves and complex the contours of the particle surface, the smaller its firmness α. The silicon-carbon particles of this invention have suitable firmness, which is not only beneficial for electrolyte storage but also for improving the interfacial contact between silicon-carbon particles and graphite, enhancing the uniformity of current density distribution, improving battery cycle performance, and further reducing the risk of negative electrode purple spots and lithium plating. If α is less than 0.7, the specific surface area increases, making it easier for excessive side reactions with the electrolyte to occur, resulting in low initial battery efficiency and deteriorated cycle life. Furthermore, excessively low firmness may lead to too many protruding sharp corners on the surface of the silicon-carbon particles, easily causing internal short circuits in the battery and affecting battery safety performance.

[0049] Furthermore, to achieve a better match between the morphology of the silicon-carbon particles and the orientation of the negative electrode, this invention also regulates the OI value φ of the negative electrode and the solidity α of the silicon-carbon particles to satisfy a specific relationship. Here, α affects the contact with graphite. When α is high, it means that the particle shape is regular, the surface grooves are few, and the particles tend to make point contact, limiting the contact area and electronic conductivity channels. In this case, it is necessary to increase the OI value of the negative electrode to enhance the efficient lithium-ion conduction of the electrode plane to compensate for the insufficient contact of the silicon-carbon particles themselves. Conversely, when α is low, it means that the surface groove structure of the silicon-carbon particles is rich, enabling multi-faceted and stable contact with the graphite material, but it also causes stronger compression of the surrounding graphite. In this case, appropriately reducing the OI value of the negative electrode results in a more random orientation of the graphite sheet structure, which is beneficial for lithium-ion transport in the XY direction. If 100×α / φ is less than 1, it means the OI value is relatively high, the silicon-carbon particles are too small, and the surface of the silicon-carbon particles has a rich trench structure. Although it can form a large area of ​​contact with graphite, it will also exert a strong squeezing effect on the surrounding graphite, destroy the stability of the electrode structure, hinder the uniform transport of lithium ions, and increase the risk of lithium plating. If 100×α / φ is greater than 15, it means that the solidity α is relatively too large or the OI value is too small. The silicon-carbon particles have a regular morphology and few surface defects. Most of the particles are in point contact mode, resulting in insufficient particle contact area. The low OI value of the negative electrode cannot provide efficient planar lithium-ion conduction capability, which is difficult to compensate for the poor contact performance of the silicon-carbon particles themselves. As a result, the silicon-carbon particles are easy to detach during the expansion process, destroying the surrounding conductive network. Under high rate conditions, local purple spots are likely to appear, affecting the cycle performance of the battery.

[0050] In this invention, the positive electrode further includes a positive tab, and the negative electrode further includes a negative tab. The distance between the center of the positive tab and the center of the negative tab is H, where 10mm ≤ H ≤ 50mm (for example, within the range of any two values ​​of 10mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 45mm, 50mm, or above). When H meets the above range, the uniformity of electron distribution between the positive and negative electrodes can be improved, thus mitigating purple spots and lithium plating problems inside the battery. If H < 10mm, the positive and negative tabs are too close. Although the electron transport path is shortened, it can cause excessively high local current density on the electrode during charging, leading to accelerated electrolyte decomposition, local overheating, and excessive flux of lithium ions on the negative electrode surface, thereby causing purple spots and lithium plating in the corresponding area. If H > 50mm, the current needs to travel a longer path to reach the tab, resulting in increased overall internal resistance and uneven internal temperature distribution, which can also easily lead to lithium plating problems.

[0051] 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 ≤ 0.9 (for example, within the range of any two of the values ​​above 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9). Adjusting S2 / S1 is equivalent to limiting the density of the protrusions on the second surface. Through coordination with the height of the protrusions, better support stability can be provided, further improving the bending resistance of the positive electrode and reducing the purple spots on the negative electrode.

[0052] In one instance, 0.3 ≤ S2 / S1 ≤ 0.6.

[0053] 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.

[0054] In this invention, the orthographic projection diameter of the protrusion on the second surface is 0.5mm-8mm (for example, within the range of any two values ​​of 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm or above).

[0055] In one example, the orthographic diameter of the protrusion on the second surface is 1.5 mm to 4 mm.

[0056] When the orthographic projection of the protrusion onto the second surface is a regular circle, its orthographic projection diameter is the diameter of that regular circle; when the orthographic projection of the protrusion onto the second surface is a non-regular circle (e.g., an ellipse or an irregular curved polygon), its orthographic projection diameter is the diameter of an equivalent circle with the same area as the non-regular circle. This can be obtained through conventional methods in the art, such as using a 3D profilometer to test the orthographic projection diameters of at least 10 protrusions on the second surface and taking the average value.

[0057] In this invention, the negative electrode active layer includes a first coating and a second coating, the first coating being located between the negative electrode current collector and the second coating; the graphite material includes a first graphite and a second graphite, the first coating including the first graphite, and the second coating including the second graphite.

[0058] In the Raman spectrum of the graphite material, at 1300 cm⁻¹ -1 -1360cm-1 A D peak exists, with a peak intensity of ID; at 1580 cm⁻¹ -1 -1585cm -1 There is a G peak with an intensity of IG; wherein, the ID / IG of the first graphite is less than the ID / IG of the second graphite.

[0059] The Raman spectra of the graphite material can be tested using a scanning electron microscope-Raman combined system. For example, the following method can be used: discharge the battery to 0% SOC, disassemble and remove the negative electrode, or directly remove the negative electrode and longitudinally cut it along the thickness direction using an argon-ion milling instrument to obtain a flat and clear cross-section. SEM cross-sectional images are obtained in the scanning electron microscope-Raman combined system, and silicon-carbon or graphite particles are identified using SEM-EDS. Then, a Thermo Fisher Raman spectrometer is used to perform Raman surface scanning tests on the first and second graphite materials, with a wavenumber range of 400 cm⁻¹. -1 -4000cm -1 This will give you the average ID / IG value for each individual.

[0060] The degree of order in the arrangement of carbon atoms in graphite materials can be characterized by Raman spectroscopy. In its Raman spectrum, at approximately 1350 cm⁻¹... -1 Nearby, there exists a D peak caused by structural defects and disordered carbon, denoted as ID; its peak intensity is approximately 1580 cm⁻¹. -1 The vicinity contains sp in the graphite lattice 2 The G peak caused by the in-plane stretching vibration of carbon atoms is denoted as IG. The ratio of the two peak intensities, ID / IG, is widely used as a key parameter to evaluate the degree of orderliness of carbon atom arrangement or the density of structural defects in graphite materials. The higher the ID / IG, the more defects or disordered structures there are in the material, and the lower the degree of graphitization; conversely, the lower the ID / IG, the more regular the carbon atom arrangement and the higher the lattice integrity.

[0061] During lithium-ion battery charging, polarization mainly occurs in the second coating of the negative electrode, i.e., the side closest to the separator. Therefore, this invention controls the second graphite contained in the second coating to have a relatively large ID / IG value, meaning that there are many structural defect sites in the graphite structure, which is beneficial for electrolyte wetting, providing rapid insertion and extraction channels for lithium ions, resulting in excellent ion transport performance. This effectively alleviates polarization and improves the battery's cycle purple spots and lithium plating problems. The first coating, which directly contacts the negative electrode current collector, needs to withstand greater pressure during electrode rolling. Therefore, the first graphite contained in the first coating has a smaller ID / IG value, indicating a high degree of graphitization, highly ordered lattice, perfect layered structure, and strong interlayer bonding, enabling it to withstand high rolling pressure without lamellar slippage or breakage. During repeated charge and discharge, the ordered structure of the first coating can resist the volume expansion and stress caused by lithium ion insertion / extraction, thereby extending the battery's cycle life. In summary, by applying a layered coating to the negative electrode, this invention avoids the contradiction that a single type of graphite cannot simultaneously satisfy high ionic conductivity and high compressive strength. This enables the battery to achieve excellent kinetic and cycle performance, further improves ion transport rate, and alleviates battery problems such as purple spots and lithium plating.

[0062] In one example, the ID / IG of the first graphite is 0.06-0.7 (e.g., within the range of any two values ​​of 0.06, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7). If the ID / IG of the first graphite is too small (e.g., less than 0.06), although the degree of graphitization is high and the structural strength is good, its ion transport performance is too poor, which cannot effectively alleviate the polarization of the second coating, resulting in increased internal resistance of the battery and easy triggering of lithium plating. If the ID / IG of the first graphite is too large (e.g., greater than 0.7), the defects in the graphite are too numerous, the side reactions with the electrolyte increase, affecting the initial coulombic efficiency of the battery, resulting in a decrease in battery energy density, and under high temperature conditions, highly active defect sites will catalyze the decomposition of the electrolyte, generating a large amount of gas, affecting the high-temperature cycle performance of the battery.

[0063] In one example, the ID / IG of the second graphite is 0.2-0.9 (e.g., within the range of any two values ​​of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9). If the ID / IG of the second graphite is too small (e.g., less than 0.2), the degree of graphitization is high, and the diffusion of lithium ions between layers is extremely slow, resulting in excessive DC internal resistance of the battery and easily inducing purple spots and lithium plating. If the ID / IG of the second graphite is too large (e.g., greater than 0.9), the degree of graphitization of this graphite layer is too low, the initial coulombic efficiency is too low, and the side reactions with the electrolyte increase, leading to a decrease in the energy density and cycle stability of the battery.

[0064] In one example, the specific surface area of ​​the graphite material is 0.9 m².2 / g-3m 2 / g (e.g., 0.9m) 2 / g, 1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g or any two of the above values ​​within the range).

[0065] In one example, the compacted density of the first graphite at 5000 N was 1.8 g / cm³. 3 -2.1g / cm 3 (For example, 1.8g / cm) 3 1.9g / cm 3 2g / cm 3 2.1g / cm 3 (or within the range of any two of the above values). Graphite materials with high compaction density typically have a more regular and smooth surface morphology. First-grade graphite has a high compaction density, resulting in smaller volume changes and less compressive stress on the surrounding silicon-carbon particles due to its smooth surface. This reduces the internal stress generated by the silicon-carbon particles, thereby preventing particle cracking and further extending the cycle life of the battery.

[0066] In one example, the compacted density of the second graphite at 5000 N was 1.7 g / cm³. 3 -2.0g / cm 3 (For example, 1.7g / cm) 3 1.8g / cm 3 1.9g / cm 3 2.0g / cm 3 Or within the range of any two of the above values).

[0067] The batteries can all be assembled in accordance with conventional methods in the field.

[0068] 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.

[0069] 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.

[0070] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0071] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0072] Example 1 Batteries are prepared according to the following method. (1) Preparation of positive electrode Preparation of the first positive electrode Lithium cobalt oxide, carbon nanotubes, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:0.5:1.5:2 and placed in N-methylpyrrolidone (NMP). The mixture was stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto the first surface of a first positive electrode current collector (an aluminum foil with a thickness h2 of 16 μm). After drying, rolling, and die-cutting, a first positive electrode sheet was obtained. The cut first positive electrode sheet was then embossed using a special roller containing protrusions. The height h1 of the protrusions was 17.5 μm; the projected area ratio of the protrusions on the second surface S2 / S1 was 0.5; and the orthogonal projection diameter of the protrusions on the second surface was 2.8 mm. Preparation of the second positive electrode Lithium cobalt oxide, carbon nanotubes, acetylene black, and PVDF were mixed in a mass ratio of 96:0.5:1.5:2 and placed in NMP. The mixture was stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of a second positive electrode current collector (an aluminum foil with a thickness h3 of 10 μm). After drying, rolling, and die-cutting, a second positive electrode sheet was obtained. A square empty foil of a fixed size protruded at a certain position of the positive electrode sheet was formed. Nickel tabs were ultrasonically welded together to form the square empty foils of all the positive electrode sheets to obtain the positive electrode tab. (2) Preparation of negative electrode A negative electrode material (a mixture of silicon carbon particles and first graphite in a mass ratio of 3:7), conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a mass ratio of 96:0.5:1.5:2 is mixed and placed in deionized water. The mixture is stirred evenly to obtain a first negative electrode slurry. A second negative electrode slurry is obtained by uniformly coating the first negative electrode slurry onto both sides of a copper foil and drying it. Then, the second negative electrode slurry is uniformly coated onto the dried surfaces of the first negative electrode slurry-coated foil. After drying, rolling, die-cutting, and sheet forming, a negative electrode sheet is obtained. A negative electrode tab groove and a negative electrode embedding groove are formed on the negative electrode active layer using laser technology. Copper-plated nickel tabs are ultrasonically welded into the negative electrode tab groove to obtain the negative electrode tabs. The cross-sectional profile of the silicon-carbon particles includes a groove structure with a groove depth d1 of 2.6 μm, a major axis d2 of 10.4 μm, a d1 / d2 ratio of 0.25, a solidity α of 0.82, and a particle size d3 of 7.3 μm for the artificial graphite. The silicon-carbon particles comprise a porous carbon matrix and silicon particles located within the pores of the porous carbon matrix. The particle sizes Dv10 and Dv50 of the silicon-carbon particles are 4.5 μm and 9.1 μm, respectively. The particle size distribution (Dv90) is 12.8 μm; the silicon content (w1) in the silicon-carbon particles is 50%, and the silicon content (w2) in the negative electrode active layer is 15%; the OI value (φ) of the negative electrode sheet is 45.55, and 100×α / φ is 1.8; the distance H between the centers of the positive and negative electrode tabs is 30.5 mm; the ID / IG ratio of the first graphite is 0.25, and that of the second graphite is 0.45; the specific surface area of ​​the graphite material is 1.8 m². 2 / g, the compacted density of the first graphite at 5000N is 2g / cm³. 3 The compaction density of the second graphite at 5000 N is 1.8 g / cm³. 3 ; (3) Preparation of the diaphragm A polyethylene film with a thickness of 8 μm is coated with a boehmite-containing ceramic layer with a thickness of 2 μm on one side of the polyethylene film. Then, a polyvinylidene fluoride adhesive layer with a thickness of 1 μm is coated on the surface of the ceramic layer away from the polyethylene film and on the other side of the polyethylene film. After drying and shaping, a diaphragm is obtained. (4) Preparation of electrolyte In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), lithium hexafluorophosphate was mixed with a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): propyl propionate (PP): diethyl carbonate (DEC) = 1:1:1:1, mass percentage) to prepare an electrolyte with a lithium salt concentration of 1.05 mol / L. 15% fluoroethylene carbonate and 2.5% succinate based on the total mass of the electrolyte were added. After stirring evenly, the electrolyte was obtained after passing the tests for moisture and free acid.

[0073] (5) Battery manufacturing The first positive electrode, the second positive electrode, the negative electrode, and the separator prepared above are stacked in the following order to form an electrode assembly: the outermost electrode is the first positive electrode, and the middle electrode is in the following cycle: separator, negative electrode, second positive electrode, to obtain a stacked cell. The obtained cell is placed in an aluminum-plastic film of matching size and sealed. The electrolyte from step (4) is injected under vacuum conditions and vacuum sealed. The battery is obtained through standing, formation and sorting processes.

[0074] Examples 2-6 and Comparative Examples 2-4 were performed in accordance with Example 1, and the specific parameters are shown in Table 1.

[0075] Table 1 Example 7 group This set of embodiments is based on Embodiment 1, except that the ID / IG ratio of the first graphite and the ID / IG ratio of the second graphite are changed, as follows: Example 7-1: The ID / IG ratio of the first graphite is 0.06, and the ID / IG ratio of the second graphite is 0.21. In Example 7-2, the ID / IG ratio of the first graphite is 0.7, and the ID / IG ratio of the second graphite is 0.89. In Example 7-3, the ID / IG ratio of the first graphite is 0.25, and the ID / IG ratio of the second graphite is 0.25.

[0076] Comparative Example 1 This comparative example is based on Example 1, except that the silicon carbide particles are replaced with spherical silicon carbide particles of the same mass content, i.e., the silicon carbide particles do not contain groove structures.

[0077] 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 underwent a 45°C cycle test. The cycle test regime was as follows: 3.5C charging to 4.1V, 2C charging to 4.3V, 1C charging to 4.4V, 0.5C charging to 4.55V, cut off at 0.05C, rest for 5 minutes, and discharge at 0.7C to 2.8V. This was repeated 600 times. The fully charged battery with 600T of batteries 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.

[0078] (2) Lithium plating test The batteries prepared in the examples and comparative examples were subjected to lithium plating tests. The specific test methods are as follows: The resulting batteries were charged at 45°C at a 5C rate to a cutoff voltage of 4.55V and a cutoff current of 0.05C. After resting for 5 minutes, they were discharged at a 5C rate to a cutoff voltage of 2.8V. This constituted one charge-discharge cycle. After 20 cycles, the batteries were disassembled, and the lithium plating state on the surface of the negative electrode was observed. The evaluation criteria for lithium plating on the negative electrode were 0-3: "0" represented by no lithium plating; "1" represented by slight lithium plating when the lithium plating area accounted for less than 10% of the total negative electrode area; "2" represented by moderate lithium plating when the lithium plating area accounted for 10%-30% of the total negative electrode area; and "3" represented by severe lithium plating when the lithium plating area accounted for more than 30% of the total negative electrode area. The test results are recorded in Table 2.

[0079] (3) Loop testing The batteries prepared in the examples and comparative examples were subjected to cycle tests, and the specific test methods are as follows: After standing for 5 minutes at 45℃±2℃, the battery was discharged at 0.7C to 2.8V, and the discharge capacity at this point was measured as Q1. After standing for 5 minutes, the battery was charged at 2.5C to 4.1V, 2C to 4.3V, 1C to 4.4V, and 0.5C to 4.55V. The charge was then stopped at 0.05C, stood for 5 minutes, and discharged at 0.7C to 2.8V. This process was repeated 500 times. The discharge capacity at 0.7C to 2.8V on the 500th cycle was recorded as Q2. The cycle capacity retention rate of the battery is calculated as (Q2 / Q1) × 100%. Specific test results are shown in Table 2.

[0080] Table 2 As can be seen from Table 2, the battery prepared by the present invention can significantly improve the "purple spots" and lithium plating defects compared with the comparative example, and significantly improve the cycle stability of the battery.

[0081] 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 together. 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. The first positive electrode includes a first positive current collector, which 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. In the thickness direction of the electrode assembly, the first surface is close to the center of the electrode assembly, and the second surface is far 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 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 h1 of the protrusions is 3μm-60μm. The negative electrode sheet includes a negative current collector and a negative active layer located on the surface of the negative current collector. The negative active layer includes silicon carbon particles and graphite material. The cross-sectional profile of the silicon carbon particles includes a trench structure with a depth of d1 and a major axis of d2, wherein 0.02≤d1 / d2≤0.

6. The particle size Dv50 of the graphite material is d3, and d3 is 3μm-14μm.

2. The lithium-ion secondary battery according to claim 1, wherein, h1 is 5μm-30μm; And / or, d1 is 0.2μm-8μm; And / or, d2 is 5μm-15μm; And / or, the silicon-carbon particles comprise a porous carbon matrix and silicon particles located in the internal channels of the porous carbon matrix; And / or, d3 is 4μm-10μm.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The particle size of the silicon carbide particles is Dv10 of 2μm-8μm, Dv50 of 4μm-15μm, and Dv90 of 7μm-20μm; And / or, the mass content w1 of silicon element in the silicon-carbon particles is 30%-80%; And / or, the mass content w2 of silicon element in the negative electrode active layer is 2%-50%.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The thickness h2 of the first positive electrode current collector and the thickness h3 of the second positive electrode current collector satisfy: 1 ​​< h2 / h3 ≤ 4; Preferably, h2 is 10μm-30μm; more preferably, it is 12μm-20μm. Preferably, h3 is 6μm-18μm.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The OI value of the negative electrode is φ, where 5 ≤ φ ≤ 80; preferably, 20 ≤ φ ≤ 60.

6. The lithium-ion secondary battery according to claim 5, wherein, The OI value φ of the negative electrode and the solidity α of the silicon-carbon particles satisfy the following condition: 1 ≤ 100 × α / φ ≤ 15; where α refers to the ratio of the first curve of the cross-section of the silicon-carbon particles to the second curve of the cross-section of the silicon-carbon particles, the second curve of the cross-section of the silicon-carbon particles refers to the perimeter of the outline of the cross-section of the silicon-carbon particles, and the first curve of the cross-section of the silicon-carbon particles is formed by connecting the following two parts: 1) For regions with gully structures, the tangent line at the highest point of the contour on both sides of the gully structure is used as the measurement length of the region; 2) For the remaining areas without groove structures, the actual perimeter of the outline is used as the measurement length; Preferably, 0.7 ≤ α < 1.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein, The positive electrode plate also includes a positive electrode tab, and the negative electrode plate also includes a negative electrode tab. The distance between the center of the positive electrode tab and the center of the negative electrode tab is H, where 10mm ≤ H ≤ 50mm.

8. The lithium-ion secondary battery according to claim 7, 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.

9. And / or, the diameter of the protrusion projected onto the second surface is 0.5mm-8mm, preferably 1.5mm-4mm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The negative electrode active layer includes a first coating and a second coating, with the first coating located between the negative electrode current collector and the second coating; the graphite material includes a first graphite and a second graphite, with the first coating including the first graphite and the second coating including the second graphite; In the Raman spectrum of the graphite material, at 1300 cm⁻¹ -1 -1360cm -1 A D peak exists, with a peak intensity of ID; at 1580 cm⁻¹ -1 -1585cm -1 There is a G peak with an intensity of IG; Wherein, the ID / IG of the first graphite is less than the ID / IG of the second graphite.

10. The lithium-ion secondary battery according to claim 9, wherein, The ID / IG ratio of the first graphite is 0.06-0.7; And / or, the ID / IG of the second graphite is 0.2-0.9; And / or, the specific surface area of ​​the graphite material is 0.9 m². 2 / g-3m 2 / g; And / or, the compaction density of the first graphite at 5000 N is 1.8 g / cm³. 3 -2.1g / cm 3 ; And / or, the compacted density of the second graphite at 5000 N is 1.7 g / cm³. 3 -2.0g / cm 3 .

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