Lithium ion secondary battery

Abstract

The application relates to the technical field of batteries, in particular to a lithium ion secondary battery which comprises an electrode assembly and an electrolyte; the electrode assembly comprises a positive electrode sheet, the positive electrode sheet comprises a first positive electrode sheet and a second positive electrode sheet, the first positive electrode sheet comprises a first positive electrode current collector and a first positive electrode material, the average particle size of the first positive electrode material is D1 mu m; the second positive electrode sheet comprises a second positive electrode current collector and a second positive electrode material, the average particle size of the second positive electrode material is D2 mu m, and D1>D2; the thickness of the first positive electrode current collector is h1 mu m, the thickness of the second positive electrode current collector is h2 mu m, B=h1-h2 is defined, and 2 mu m<=B<=20 mu m; the electrolyte comprises a nitrile additive, the mass content of the nitrile additive in the electrolyte is C1, the unit is wt%, K=(D1 / D2)*C1, and 0.6 wt%<=K<=16 wt%. The application can significantly improve the structural stability of the positive electrode sheet and the cycle performance 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] Stacked lithium-ion batteries are widely used in consumer electronics and power batteries due to their advantages such as low internal resistance, high energy density, and good shape adaptability. Their electrode assembly typically consists of alternating layers of single-sided positive electrode sheets (with an active layer coated only on the current collector side), negative electrode sheets, and double-sided positive electrode sheets (with active layers coated on both sides of the current collector). Research has found that due to the special structure of stacked lithium-ion batteries, the negative electrode region corresponding to the single-sided positive electrode sheet is prone to developing "purple spots" (lithium dendrites or transition metal deposition) after cycling. Furthermore, during charging, the active material on only one side of the single-sided positive electrode sheet reacts, resulting in excessively high local current density and increased polarization overpotential. This can easily lead to the collapse of the lattice structure of the positive electrode material (such as lithium cobalt oxide), releasing oxygen and accelerating the oxidative decomposition of the electrolyte. Simultaneously, a large amount of transition metal ions dissolve, severely affecting the structural stability of the positive electrode sheet and the cycle performance of the battery. Summary of the Invention

[0003] 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) achieves synergistic optimization of the positive electrode and electrolyte, thus avoiding problems such as excessive delithiation, crystal structure collapse, dissolution of transition metal ions, excessively rapid electrolyte consumption, and "purple spots" on the negative electrode caused by high overpotential of the first positive electrode. This effectively improves the structural stability of the positive electrode material and the cycle performance of the battery.

[0004] The inventors of this invention discovered that by adjusting the average particle size D1 of the first positive electrode material to be larger than the average particle size D2 of the second positive electrode material, and by ensuring that D1, D2, and the mass content C1 of the nitrile additives in the electrolyte satisfy a specific relationship, the interfacial stability of the first positive electrode can always be maintained at an optimal level, thereby suppressing the dissolution of transition metal ions (such as Co and Ni ions) and the "purple spot" defect of the negative electrode. The reason for this is: First, the C≡N functional groups in nitrile additive molecules can undergo complexation reactions with transition metal ions such as Ni and Co in the cathode material, thereby forming a stable solid electrolyte interface (CEI) film on the surface of the high-voltage cathode. This prevents the dissolution of transition metal ions, alleviates their oxidative decomposition of the electrolyte, reduces the polarization overpotential on the electrode surface, protects the crystal structure of the cathode material, and avoids the appearance of "purple spots" on the anode.

[0005] Secondly, when the first positive electrode uses a positive electrode material with a larger average particle size D1, its specific surface area is significantly reduced, which reduces the number of side reaction sites that directly contact the electrolyte. Furthermore, the structure of large-diameter particles (such as single crystals or near-single crystals) is more stable and can resist the lattice strain caused by excessive local current density and increased polarization overpotential in the first positive electrode, thus helping to improve the structural stability of the material. Therefore, when D1 is greater than D2 and D1 is at a high level, the framework of the first cathode material is sufficient to resist the lattice stress under high overpotential, and the structural stability is significantly improved. At this time, the amount of nitrile additives (such as succinic anion and adiponitrile) in the electrolyte can be reduced accordingly to avoid side reactions such as reduction decomposition of high nitrile additives on the negative electrode surface, which would damage the negative electrode. If the particle size difference between D1 and D2 is too small or even equal, the stability of the first cathode material is insufficient, and problems such as the dissolution of transition metal ions and the appearance of "purple spots" on the negative electrode are likely to occur. At this time, it is necessary to appropriately increase the content of nitrile additives, and use the strong complexation between nitrile groups and transition metal ions to adsorb and form a protective film on the first cathode surface to make up for the structural defects, thereby ensuring the structural stability and cycle performance of the battery.

[0006] Based on this, the present invention further optimizes the thickness h1 of the first positive electrode current collector and the thickness h2 of the second positive electrode current collector. By controlling the difference between h1 and h2 within a reasonable range, it can ensure that the first positive electrode sheet has sufficient bending stiffness to suppress electrode curling during cycling, while avoiding the risk of local lithium plating and "purple spots" caused by excessive thickness of the first positive electrode current collector. If the thickness difference is too large (e.g., h1 is much larger than h2), although it can improve the bending stiffness to a certain extent, it will significantly increase the single-sided current flow area, leading to an increase in local current density, accelerating the transitional delithiation of the first positive electrode sheet and the consumption and decomposition of the electrolyte, which in turn induces lithium plating on the negative electrode and causes "purple spots". If the thickness difference is too small (h1 is close to or even smaller than h2), its bending stiffness is insufficient to balance the stress generated by the drying shrinkage and cyclic expansion and contraction of the first positive electrode active layer, resulting in problems such as curling deformation and poor interface adhesion of the positive electrode sheet, which will also exacerbate the risk of local purple spots.

[0007] Based on this, the inventors of this invention propose the following solution: This invention provides a lithium-ion secondary battery, characterized in that it includes an electrode assembly and an electrolyte. 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 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, which faces the center of the electrode assembly, while the second surface is away from the center of the electrode assembly. The first positive active layer includes a first positive electrode material with an average particle size of D1 (μm). 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 second positive active layer includes a second positive electrode material with an average particle size of D2 (μm). Wherein, D1 > D2. The thickness of the first positive electrode current collector is h1, in μm; the thickness of the second positive electrode current collector is h2, in μm; B = h1 - h2, 2μm ≤ B ≤ 20μm; the electrolyte includes nitrile additives, and the mass content of the nitrile additives in the electrolyte is C1, in wt%; wherein, K = (D1 / D2) × C1, 0.6wt% ≤ K ≤ 16wt%.

[0008] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) The present invention has synergistically optimized the positive electrode and the electrolyte, controlled the average particle size D1 of the first positive electrode material to be greater than the average particle size D2 of the second positive electrode material, and made D1, D2 and the mass content C1 of nitrile additives in the electrolyte satisfy a specific relationship by adjusting them, which can ensure that the interface stability of the first positive electrode is always at the best level, avoid a series of problems such as excessive delithiation, material structure collapse, cobalt dissolution and electrolyte consumption caused by high overpotential of the first positive electrode, thereby suppressing the dissolution of transition metal ions (such as Co and Ni ions) and the "purple spot" defect of the negative electrode, and effectively improving the structural stability of the positive electrode.

[0009] (2) The present invention adjusts the thickness h1 of the first positive current collector and the thickness h2 of the second positive current collector. By controlling the difference between h1 and h2 within 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 lithium plating and "purple spots" caused by excessive thickness of the first positive current collector.

[0010] 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

[0011] Figure 1 The figure shown is a schematic diagram of the electrode assembly along the thickness direction in an embodiment of the present invention.

[0012] Figure 2 The diagram shown is a schematic diagram of the structure of the first positive electrode in an embodiment of the present invention.

[0013] Figure 3 The diagram shown is a schematic diagram of the structure of the second positive electrode in an embodiment of the present invention.

[0014] 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

[0015] 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.

[0016] This invention provides a lithium-ion secondary battery, including an electrode assembly and an electrolyte. 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 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, which faces the center of the electrode assembly, while the second surface is away from the center of the electrode assembly. The first positive active layer includes a first positive electrode material with an average particle size of D1 (μm). 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 second positive active layer includes a second positive electrode material with an average particle size of D2 (μm). Wherein, D1 > D2.

[0017] In this invention, the thickness of the first positive current collector is h1, in μm, and the thickness of the second positive current collector is h2, in μm. B = h1 - h2, and 2μm ≤ B ≤ 20μm (for example, 2μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm or any two of the above values).

[0018] 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 thickness of the first positive current collector is h1. 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 first positive current collector is h2.

[0019] In this invention, the electrolyte includes a nitrile additive, and the mass content of the nitrile additive in the electrolyte is C1, in wt%; wherein, K = (D1 / D2) × C1, 0.6wt% ≤ K ≤ 16wt% (for example, 0.6wt%, 1wt%, 3wt%, 5wt%, 7wt%, 9wt%, 11wt%, 13wt%, 16wt% or any two of the above values).

[0020] In one instance, D1 is 12μm-20μm (e.g., within the range of any two of the following values: 12μm, 12.2μm, 12.5μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm).

[0021] In one instance, D2 is 10μm-18μm (e.g., within the range of 10μm, 10.1μm, 10.2μm, 10.5μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm or any two of the above values).

[0022] In one instance, 0.5μm ≤ D1 - D2 ≤ 10μm (e.g., within the range of any two of the following values: 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm).

[0023] In one instance, C1 is 0.4wt%-8.5wt% (e.g., within the range of any two of the following values: 0.4wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 8.5wt%).

[0024] In one instance, h1 is 12μm-26μm (e.g., 12μm, 12.1μm, 12.5μm, 13μm, 14μm, 15μm, 17μm, 19μm, 21μm, 23μm, 26μm or any two of the above values).

[0025] In one instance, h2 is 5μm-10μm (e.g., within the range of any two of the values ​​5μm, 5.1μm, 5.5μm, 6μm, 7μm, 8μm, 9μm, 10μm, or above).

[0026] In one instance, 0.8wt%≤K≤9wt%.

[0027] In one example, the nitrile additive includes at least one selected from benzonitrile, succinic anionyl, fluorobenzonitrile, adiponitrile, 1,3,6-hexanetrionitrile, glyceryl trionitrile, 1,4-dicyano-2-butene, and ethylene glycol bis(propionitrile) ether.

[0028] In this invention, D1 and D2 can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC (e.g., discharging the battery to 3V), the first and second positive electrode sheets are disassembled, soaked in dimethyl carbonate (DMC) solvent for 12 hours, rinsed with DMC solvent, and the positive electrode sheets are cut using an argon ion milling machine with a CP laser. SEM is then used for observation at 5K magnification. At least 10 particles of the first or second positive electrode material are randomly selected, and the longest diameter of each particle is measured, with the average value taken. If the number of particles at 5K magnification is less than 10, another microscopic image is taken until 10 particles are measured.

[0029] In this invention, h1 and h2 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, cutting the first and second positive electrode sheets with an argon ion milling machine using a CP laser, and then observing the first and second positive electrode current collectors with a scanning electron microscope (SEM), randomly selecting 10 test sites on their surface, measuring the thickness of each site, and taking the average value.

[0030] In this invention, C1 can be obtained by methods conventional in the art, such as gas chromatography (GC), gas chromatography-mass spectrometry (GCMS), or liquid chromatography (LC).

[0031] Because the first positive electrode is coated with active material on only one side, the current density in this area is too high, leading to an increased polarization overpotential. During charging, the first positive electrode is prone to excessive delithiation, causing lattice collapse of the positive electrode material (such as lithium cobalt oxide). Therefore, the stability requirements for the first positive electrode material used in the first positive electrode are higher. This invention effectively solves the problem of "purple spots" in the negative electrode region corresponding to the first positive electrode caused by the instability of the first positive electrode structure by controlling the first positive electrode material to be different from the second positive electrode material. Its core lies in controlling D1, D2, and C1 to satisfy a specific relationship. When D1 is greater than D2 and D1 is at a high level, the first cathode material has a stable framework, which is sufficient to resist lattice stress under high overpotential. Furthermore, the larger particle size of the first cathode material results in fewer active sites in contact with the electrolyte, reducing the occurrence of side reactions. In this case, the content of nitrile additives, C1, can be reduced accordingly to avoid side reactions such as reduction decomposition on the negative electrode surface caused by high nitrile additive content, which could damage the negative electrode. If the particle size difference between D1 and D2 is too small or even equal, the structural stability of the first cathode material is relatively insufficient. In this case, C1 needs to be increased to utilize the strong complexation between nitrile groups and transition metal ions to form a protective film on the surface of the first cathode material, thereby compensating for structural defects and inhibiting transition metal dissolution and "purple spot" formation. However, the content of nitrile additives is not always better the higher it is. Further research has found that excessively high nitrile additive content can lead to poor high-temperature storage performance of the battery and damage the solid electrolyte interphase (SEI) film on the negative electrode surface, affecting the high-temperature safety performance of the battery. Therefore, it is necessary to coordinate and regulate the relationship between D1, D2 and C1 so that their relationship meets a specific range, thereby significantly improving the structural stability of the positive electrode, suppressing the dissolution of transition metal ions, reducing the "purple spot" defect of the negative electrode, and significantly improving the cycle stability of the battery without affecting the high-temperature safety performance of the battery.

[0032] In this invention, the particle size distribution of the first positive electrode material is SPAN1, and the particle size distribution of the second positive electrode material is SPAN2, where SPAN1 < SPAN2, and SPAN = (Dv90 - Dv10) / Dv50.

[0033] In one example, the particle size Dv90 of the first cathode material is 22μm-30μm (e.g., within the range of any two values ​​of 22μm, 24μm, 26μm, 28μm, 30μm or more), Dv50 is 12μm-20μm (e.g., within the range of any two values ​​of 12μm, 14μm, 16μm, 18μm, 20μm or more), and Dv10 is 3μm-9μm (e.g., within the range of any two values ​​of 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or more).

[0034] In one example, the particle size Dv90 of the second cathode material is 20μm-28μm (e.g., within the range of any two values ​​of 20μm, 22μm, 24μm, 26μm, 28μm or more), Dv50 is 10μm-18μm (e.g., within the range of any two values ​​of 10μm, 12μm, 14μm, 16μm, 18μm or more), and Dv10 is 2μm-8μm (e.g., within the range of any two values ​​of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or more).

[0035] In one example, the particle size distribution SPAN1 of the first cathode material is 0.8-1.5 (e.g., 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4 or 1.5).

[0036] In one example, the particle size distribution SPAN2 of the second cathode material is 1-2 (e.g., 1, 1.2, 1.4, 1.6, 1.8 or 2).

[0037] By controlling the particle size distribution of the first and second cathode materials, ensuring that the particle size distribution SPAN1 of the first cathode material is smaller than that of the second cathode material SPAN2, a reasonable difference in porosity and lithium intercalation / deintercalation uniformity between the first and second cathode active layers can be achieved, thus realizing differentiated adaptation of their lithium intercalation / deintercalation behavior and liquid storage capacity. Specifically, when the SPAN1 of the first cathode sheet, which is a single-sided coated electrode, is controlled within a small range, it means there are almost no fine powders or ultra-large particles. This avoids localized overpotential concentration and excessive lithium deintercalation caused by the large specific surface area and high reactivity of fine powders, and suppresses cobalt dissolution and the formation of "purple spots" on the negative electrode side. Meanwhile, the relatively larger SPAN2 range of the second cathode sheet is beneficial for increasing its compaction density, enabling the battery to meet high energy density requirements.

[0038] In this invention, the particle sizes Dv90, Dv50, and Dv10 of the first positive electrode material and the second positive electrode material can all be obtained by conventional methods in the art, such as by laser particle size analyzer.

[0039] In this invention, the surface of the first positive electrode current collector has a plurality of micropores, the pore diameter of which is 50μm-150μm, and the total area ratio of the micropores on the surface of the first positive electrode current collector is A, where 0.005≤A / B≤0.5 (for example, within the range of any two of the values ​​of 0.005, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, or above).

[0040] In one instance, 0.1% ≤ A ≤ 1% (e.g., within the range of any two values ​​of 0.1%, 0.3%, 0.5%, 0.7%, 1%, or above).

[0041] By setting several micropores on the surface of the first positive electrode current collector, the impedance of the first positive electrode can be increased and the current density of the first positive electrode can be reduced, thereby reducing the consumption of electrolyte in this area. At the same time, these micropores can also store a small amount of electrolyte, alleviating the local "dry area" phenomenon caused by severe side reactions of the first positive electrode, ensuring the continuous smooth flow of lithium-ion transport pathways, thereby synergistically improving the purple spot problem of the negative electrode, and thus improving the cycle stability and interface consistency of the battery.

[0042] This invention further defines the "total area ratio A of the surface micropores of the first positive electrode current collector" and the "thickness difference B between the first and second positive electrode current collectors". By adjusting A and B to meet a specific range, a better synergistic effect can be achieved. Specifically, when the thickness difference B between the first and second positive electrode current collectors is too large, it will significantly increase the single-sided current flow area of ​​the first positive electrode, leading to an increase in local current density, accelerating the consumption and decomposition of electrolyte in this area, and thus inducing the "purple spot" phenomenon of the negative electrode. At this time, by increasing the number of micropores accordingly, the interfacial impedance can be effectively increased and the local current density can be reduced. At the same time, the electrolyte stored in the microporous structure can also alleviate the "broken bridge" phenomenon formed between the first and negative electrode due to the lack of electrolyte filling, further mitigating the risk of local lithium plating and improving the structural stability of the positive electrode.

[0043] In this invention, the micropores can be obtained by conventional means in the art, such as, but not limited to, laser processing, mechanical processing (continuous drilling), chemical etching, and electrochemical etching. This invention does not specifically limit the method for preparing the micropores; any preparation method that can form a satisfactory micropore structure on the surface of the first positive electrode current collector and achieve the technical effects described in this invention falls within the protection scope of this invention.

[0044] In this invention, the total area ratio A of the micropores on the surface of the first positive current collector can be obtained by conventional methods in the art, such as taking a microscopic image of the surface of the first positive current collector by scanning electron microscopy (SEM) and calculating the total area ratio of the micropores by image analysis software.

[0045] In this invention, the first positive electrode sheet includes multiple protrusions and multiple 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 resulting micro-uneven structure introduces local plastic deformation and residual compressive stress field at the interface of the electrode assembly. On the one hand, this can balance the stress generated by the drying shrinkage of the active layer; on the other hand, it can improve the stiffness of the surface layer of the first positive electrode sheet, thereby effectively suppressing curling deformation without excessively increasing the thickness of the first positive current collector, thus improving the "purple spot". 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 recesses 1114 disposed corresponding to the plurality of protrusions. The width of the protrusion is D and the spacing between the protrusions is P.

[0046] 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. An appropriate amount of protrusions can provide better support stability, further effectively improving the bending resistance of the positive electrode and reducing purple spots.

[0047] In one instance, 0.3 ≤ S2 / S1 ≤ 0.6.

[0048] 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.

[0049] The present invention does not limit the shape of the orthographic projection of the concave portion and the convex portion on the surface of the first positive electrode sheet, and can be circular, quasi-circular, rectangular, quasi-rectangular, elliptical, linear (including straight line or wavy line), polygonal, etc.

[0050] In this invention, the first cathode material and the second cathode material each independently include at least one of lithium cobalt oxide, ternary cathode material, and lithium manganese oxide. The ternary cathode material refers to a cathode active material containing three transition metal elements (nickel, cobalt, and manganese, or nickel, cobalt, and aluminum).

[0051] In one example, the first cathode material and / or the second cathode material comprises lithium cobalt oxide, which contains Al, and the mass content of Al in the lithium cobalt oxide is 5000ppm-15000ppm (e.g., within the range of any two of the values ​​of 5000ppm, 7000ppm, 9000ppm, 11000ppm, 13000ppm, 15000ppm or above). 3+ With a small ionic radius and high charge density, it can effectively suppress the formation of oxygen vacancies and lattice oxygen evolution under high pressure by occupying cobalt sites, thereby suppressing lattice distortion under high pressure, reducing irreversible phase transformation, reducing the risk of particle microcracks and structural collapse, effectively suppressing the dissolution of transition metal ions, reducing the oxidative decomposition of electrolyte on the positive electrode side, reducing the generation of by-products, and further improving the structural stability of the positive electrode material.

[0052] In this invention, the separator includes a substrate layer, a first coating layer located on one side of the substrate layer, and a second coating layer located on the other side of the substrate layer; the first coating layer corresponds to the positive electrode sheet, and the second coating layer corresponds to the negative electrode sheet; the first coating layer includes a heat-resistant layer and a first adhesive layer, the heat-resistant layer being located on the surface of the substrate layer, and the first adhesive layer being located on the surface of the heat-resistant layer away from the substrate layer, the first adhesive layer including a first polymer, and the second coating layer including a second polymer; the first polymer includes at least one of first particles and second particles, the first particles being agglomerated polymer particles, and the second particles being dispersed polymer particles; the average particle size d3 of the primary particles of the second particles is 0.3 μm-1.5 μm (e.g., 0.3 μm). The first particle comprises secondary particles formed by the aggregation of primary particles. The average particle size d4 of the primary particles of the first particle is 0.15μm-0.25μm (e.g., within the range of any two values ​​of 0.15μm, 0.17μm, 0.19μm, 0.21μm, 0.23μm, 0.25μm or above), and the average particle size d5 of the secondary particles of the first particle is 4μm-20μm (e.g., within the range of any two values ​​of 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm or above). d3, d4, and d5 can all be obtained by conventional methods in the art, such as scanning electron microscopy (SEM).

[0053] Controlling d3, d4, and d5 within the aforementioned range ensures highly uniform dispersion and tight packing of particles in the adhesive layer, resulting in a denser adhesive layer with good adhesion. This not only guarantees the pore-closing effect of the separator but also enhances the bonding strength between the separator and the electrode. It prevents problems such as delamination between the first positive and negative electrode plates and the formation of ion conduction "broken bridges" due to lack of electrolyte filling caused by repeated expansion and contraction of the electrode assembly during charge and discharge cycles. It also prevents insufficient local lithium insertion in the corresponding negative electrode area, suppresses the formation of "purple spots" on the negative electrode surface, and improves the interfacial stability of the electrode and the cycle performance of the battery.

[0054] In one example, the first polymer and the second polymer each independently include at least one of fluoropolymers, acrylate polymers, polyimides, modified polyimides, poly(p-phenylene terephthalamide), and poly(m-phenylene isophthalamide).

[0055] In one example, the first adhesive layer covers 8%-50% of the surface of the heat-resistant layer (e.g., within the range of any two values ​​of 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or more).

[0056] In one example, the first adhesive layer has a coverage of 10%-35% on the surface of the heat-resistant layer.

[0057] In one example, the second coating has a coverage of 8%-50% on the surface of the substrate layer (e.g., within the range of any two values ​​of 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or more).

[0058] In one example, the coverage of the second coating on the surface of the substrate layer is 10%-35%. The coverage of the first adhesive layer on the surface of the heat-resistant layer and the coverage of the second coating on the surface of the substrate layer can both be tested using conventional methods in the art, such as SEM combined with image analysis software.

[0059] In this invention, 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. The silicon-carbon material includes 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. The mass content of silicon element in the negative active layer is 2%-50% (for example, within the range of any two values ​​of 2%, 5%, 10%, 20%, 30%, 40%, 50%, or more). The thickness of the coating layer is 2nm-20nm (for example, within the range of any two values ​​of 2nm, 5nm, 10nm, 15nm, 20nm, or more).

[0060] In one example, the sphericity of the silicon-carbon material is ≥0.85 (e.g., within the range of any two values ​​of 0.85, 0.9, 0.95, 1, or above). The silicon-carbon material of this invention has high sphericity, making it less prone to breakage during cyclic reactions and maintaining interface integrity. Simultaneously, it consumes less electrolyte, helping to suppress excessive electrolyte decomposition and synergistically improving the negative electrode "purple spot" problem.

[0061] In one example, the particle size Dv50 of the silicon-carbon material is 5.5 μm to 12.5 μm (e.g., within the range of 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 12.5 μm, or any two of the above values). This can be determined by a laser particle size analyzer. When the particle size Dv50 of silicon-carbon material is too small (e.g., less than 5.5 μm), the specific surface area of ​​silicon-carbon material increases, the effective contact area with electrolyte increases, side reactions intensify, and the thickness of SEI film formed on the surface of negative electrode increases, leading to an increase in the internal resistance of negative electrode, which will affect the kinetic performance and cycle performance of battery. When the particle size Dv50 of silicon-carbon material is too large (e.g., greater than 12.5 μm), the overall compressive strength of silicon-carbon material is poor, and the risk of breakage under external pressure increases. The repeated breakage and growth of SEI film on the surface of silicon-carbon material will continuously consume electrolyte and active lithium, resulting in poor structural stability of negative electrode and affecting the cycle stability of battery.

[0062] In one example, the particle size Dv50 of the silicon-carbon material is 6.5 μm-10 μm.

[0063] In this invention, the mass content of silicon in the negative electrode active layer 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 attached 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 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. The remaining substance is the ash of the negative electrode active layer. The mass content of silicon in the negative electrode active layer can be calculated based on the mass of ash. The calculation formula is as follows: Mass content of silicon in the negative electrode active layer = 7 × mass of ash / (15 × mass of test sample).

[0064] In this invention, the sphericity of the silicon-carbon material can be tested using conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, or the negative electrode is directly removed. The cross-section of the negative electrode is polished using an argon-ion polisher, and then observed using backscatter imaging mode on a scanning electron microscope (SEM). Silicon-carbon material particles with continuous and smooth contours are found. Any two points on the edge of the particle are connected to form a straight line segment inside the particle. The longest straight line segment inside the particle is selected, and its length is denoted as Z1. The midpoint of this longest straight line segment is taken, and a straight line is drawn through this midpoint to form a straight line segment with both ends at the edge of the particle. The shortest straight line segment is selected, and its length is denoted as Z2. The sphericity of the particle is then Z2 / Z1. At least 10 silicon-carbon material particles are selected, and the sphericity is measured and the average value is taken.

[0065] In this invention, the thickness of the coating layer can be tested using 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 solvent to remove lithium salts adhering to the negative electrode. After drying, the silicon-carbon material is cut using an argon ion mill (CP), and the location of the coating layer is determined by SEM observation. Ten test sites are randomly selected on the surface of the coating layer, and the thickness of each site is measured, with the average value taken. At least ten silicon-carbon material particles are selected for measurement, and the final average value is taken.

[0066] The batteries can all be assembled in accordance with conventional methods in the field.

[0067] 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.

[0068] 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.

[0069] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0070] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0071] Example 1 Batteries are prepared according to the following method. (1) Preparation of positive electrode Preparation of the first positive electrode Lithium cobalt oxide (average particle size D1 is 16.5 μm, particle size D) V10 is 6.1 μm, particle size D V 50 is 16.2 μm, particle size D V A positive electrode slurry was prepared by mixing N-methylpyrrolidone (NMP) with a mass ratio of 98.50:0.8:0.7, using a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), 26.5 μm of 90, 1.26 μm of SPAN1, and polyvinylidene fluoride (PVDF). The positive electrode slurry was uniformly coated on one side of the first positive electrode current collector (18.6 μm thick, with several micropores formed on the surface of the aluminum foil by laser drilling, the pore size of the micropores being 101 μm, A being 0.5%, and A / B being 0.046). After drying, rolling, and slitting, the first positive electrode sheet was obtained.

[0072] Preparation of the second positive electrode Lithium cobalt oxide (average particle size D2 is 13.3 μm, particle size D) V 10 is 5.2 μm, particle size D V 50 is 14μm, particle size D V A positive electrode slurry was prepared by mixing N-methylpyrrolidone (NMP) with 90 (25.5 μm) and SPAN2 (1.5 μm), a conductive agent (a mixture of single-walled and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) in a mass ratio of 98.5:0.8:0.7 and stirring until homogeneous. The positive electrode slurry was then uniformly coated on both sides of a second positive electrode current collector (7.8 μm thick), and after drying, rolling, and slitting, a second positive electrode sheet was obtained.

[0073] (2) Preparation of negative electrode A mixture of silicon-carbon artificial graphite, carbon nanotubes, styrene-butadiene rubber, sodium carboxymethyl cellulose, and PAA was placed in deionized water at a mass ratio of 96.1:0.5:2.1:0.2:1.1. The above slurry was stirred evenly to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto a negative electrode current collector, and then dried, rolled, and die-cut to obtain a negative electrode sheet. The weight content of silicon in the negative electrode active layer was 15%, the sphericity of the silicon-carbon material was 0.9, the particle size Dv50 of the silicon-carbon material was 8.2 μm, and an amorphous carbon coating layer with a thickness of 11.2 nm was provided on the outer surface of the porous carbon matrix.

[0074] (3) Preparation of electrolyte Under an argon atmosphere and in an environment with a water content of less than 10 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. Fluorinated ethylene carbonate (15% by mass of the total electrolyte) and succinate (3% by mass of the total electrolyte) were added. After stirring evenly, the electrolyte was obtained after passing the tests for moisture and free acid.

[0075] (4) Preparation of the diaphragm Heat-resistant granules (alumina) are dispersed in deionized water, polyacrylic acid is added, and the mixture is stirred evenly to obtain a slurry with a solid content of 25%. The slurry is 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 (second granules) and polyvinylidene fluoride (first granules) are mixed and dispersed in water at a weight ratio of 60:40 and stirred evenly to obtain a slurry with a solid content of 10%. The slurry is continuously coated onto the side of the heat-resistant layer away from the first substrate layer (first adhesive layer) using a gravure roller. The slurry of the first and second granules is continuously coated onto the other side of the substrate layer (second coating layer) using a gravure roller, and then 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 particle is 0.2 μm, the average particle size of the secondary particles of the first particle is 12.4 μm, the average particle size of the primary particles of the second particle is 0.9 μm, the coverage of the first adhesive layer on the surface of the heat-resistant layer is 20%, and the coverage of the second coating layer on the surface of the substrate layer is 20%.

[0076] (5) Battery manufacturing 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.

[0077] Example 2 Batteries are prepared according to the following method. (1) Preparation of positive electrode The positive electrode was prepared according to Example 1, except that the thickness of the first positive electrode current collector was 12.1 μm, the average particle size D1 of the first positive electrode material was 12.2 μm, and the particle size D... V 10 is 9μm, particle size D V50 is 13μm, particle size D V 90 is 28.5 μm, SPAN1 is 1.5; the thickness of the second positive electrode current collector is 9.9 μm, the average particle size D2 of the second positive electrode material is 11.7 μm, and the particle size D... V 10 is 7.5 μm, particle size D V 50 is 10.3 μm, particle size D V 90 is 27.6 μm, SPAN2 is 2; the pore size of the micropore is 51 μm, A is 0.9%, and A / B is 0.409.

[0078] (2) Preparation of negative electrode The negative electrode was prepared according to Example 1, except that the sphericity of the silicon-carbon material was 0.85, the particle size Dv50 of the silicon-carbon material was 5.5 μm, and an amorphous carbon coating layer with a thickness of 2.1 nm was provided on the outer surface of the porous carbon matrix.

[0079] (3) Preparation of electrolyte The electrolyte was prepared in accordance with Example 1, except that 8.5% succinic anhydride was added based on the total mass of the electrolyte.

[0080] (4) Preparation of the diaphragm The membrane was prepared according to Example 1, except that the average particle size of the primary particles of the first particle was 0.15 μm, the average particle size of the secondary particles of the first particle was 4.2 μm, the average particle size of the primary particles of the second particle was 0.3 μm, the coverage of the first adhesive layer on the surface of the heat-resistant layer was 50%, and the coverage of the second coating layer on the surface of the substrate layer was 50%.

[0081] (5) Battery manufacturing The battery was prepared according to Example 1.

[0082] Example 3 Batteries are prepared according to the following method. (1) Preparation of positive electrode The positive electrode sheet was prepared according to Example 1, except that the thickness of the first positive electrode current collector was 25 μm, the average particle size D1 of the first positive electrode material was 19.8 μm, and the particle size D... V 10 is 6.2 μm, particle size D V 50 is 19.8 μm, particle size D V 90 is 22.1 μm, SPAN1 is 0.8; the thickness of the second positive electrode current collector is 5.1 μm, the average particle size D2 of the second positive electrode material is 11.8 μm, and the particle size D... V 10 is 2.3 μm, particle size D V 50 is 17.9 μm, particle size D V90 is 20.5μm, SPAN2 is 1; the pore size of the micropore is 148μm, A is 0.2%, and A / B is 0.01.

[0083] (2) Preparation of negative electrode The negative electrode was prepared according to Example 1, except that the sphericity of the silicon-carbon material was 0.98, the particle size Dv50 of the silicon-carbon material was 12.5 μm, and an amorphous carbon coating layer with a thickness of 19.5 nm was provided on the outer surface of the porous carbon matrix.

[0084] (3) Preparation of electrolyte The electrolyte was prepared in accordance with Example 1, except that 0.5% succinate based on the total mass of the electrolyte was added.

[0085] (4) Preparation of the diaphragm The membrane was prepared according to Example 1, except that the average particle size of the primary particles of the first particle was 0.25 μm, the average particle size of the secondary particles of the first particle was 19.6 μm, the average particle size of the primary particles of the second particle was 1.5 μm, the coverage of the first adhesive layer on the surface of the heat-resistant layer was 8%, and the coverage of the second coating layer on the surface of the substrate layer was 8%.

[0086] (5) Battery manufacturing The battery was prepared according to Example 1.

[0087] Examples 4-5 and Comparative Examples 1-4 were performed in accordance with Examples 1-3, and the specific parameters are shown in Table 1-1.

[0088] Table 1-1 Note: “√” in Table 1-1 indicates that the feature is satisfied.

[0089] Example 6 group This set of embodiments refers to Embodiments 1-3. Specifically, Embodiment 6-1 refers to Embodiment 1, Embodiments 6-2 and 6-4 refer to Embodiment 2, and Embodiments 6-3 and 6-5 refer to Embodiment 3. The difference is that SPAN1 and SPAN2 are controlled by changing the particle size Dv10, particle size Dv50, and particle size Dv90 of the first and second cathode materials to verify the effect of changing SPAN1 and SPAN2. The specific parameters are shown in Table 1-2.

[0090] Table 1-2 Note: “√” in Table 1-2 indicates that the feature is satisfied.

[0091] Example 7 group This set of embodiments follows the examples 1-3, except that A / B is adjusted by changing A and B to verify the effect of changing A / B. In particular, the surface of the first positive current collector in Example 7-1 does not have micropores. Specific parameters are shown in Tables 1-3.

[0092] Table 1-3 Note: The " / " in Table 1-3 indicates that the parameter does not exist here.

[0093] Test case (1) Purple spot test and metal dissolution test The batteries prepared in the examples and comparative examples were subjected to purple spot tests and metal dissolution tests. The specific test methods are as follows: Purple spot test: 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.

[0094] Transition metal leaching test: The aforementioned cycled battery was discharged until it was completely empty. Then, the negative electrode powder was disassembled and inductively coupled plasma (ICP) tests were performed to detect Co ions (the positive electrode used lithium cobalt oxide material). Specific test results are shown in Table 2.

[0095] (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.

[0096] Table 2 As can be seen from Table 2, the battery of the present invention can significantly improve the "purple spot" defect compared with the comparative example, greatly reduce the dissolution of transition metal ions (Co ions), and improve the cycle stability of the battery.

[0097] 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 an electrode assembly and an electrolyte, wherein the electrode assembly comprises a positive electrode, a separator, and a negative electrode stacked in sequence. 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, which faces the center of the electrode assembly. The second surface is away from the center of the electrode assembly. The first positive active layer includes a first positive electrode material with an average particle size of D1 in μm. 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 second positive active layer includes a second positive electrode material with an average particle size of D2 in μm. Wherein, D1 > D2. The thickness of the first positive electrode current collector is h1, in μm, and the thickness of the second positive electrode current collector is h2, in μm. B = h1 - h2, and 2μm ≤ B ≤ 20μm. The electrolyte includes nitrile additives, and the nitrile additives in the electrolyte have a mass content of C1, in wt%; Where K = (D1 / D2) × C1, 0.6wt% ≤ K ≤ 16wt%.

2. The lithium-ion secondary battery according to claim 1, wherein, D1 is 12μm-20μm; And / or, D2 is 10μm-18μm; Preferably, 0.5μm≤D1-D2≤10μm; And / or, C1 is 0.4wt%-8.5wt%; And / or, h1 is 12μm-26μm; And / or, h2 is 5μm-10μm; And / or, 0.8wt%≤K≤9wt%.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The nitrile additives include at least one of benzonitrile, succinic anionyl, fluorobenzonitrile, adiponitrile, 1,3,6-hexanetrionitrile, glyceryltrionitrile, 1,4-dicyano-2-butene, and ethylene glycol bis(propionitrile) ether.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, The particle size distribution of the first cathode material is SPAN1, and the particle size distribution of the second cathode material is SPAN2, where SPAN1 < SPAN2, and SPAN = (Dv90 - Dv10) / Dv50.

5. The lithium-ion secondary battery according to claim 4, wherein, The particle size of the first cathode material is Dv90 of 22μm-30μm, Dv50 of 12μm-20μm, and Dv10 of 3μm-9μm; And / or, the particle size of the second positive electrode material is 20μm-28μm for Dv90, 10μm-18μm for Dv50, and 2μm-8μm for Dv10; Preferably, the particle size distribution SPAN1 of the first positive electrode material is 0.8-1.5; Preferably, the particle size distribution SPAN2 of the second positive electrode material is 1-2.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, The surface of the first positive electrode current collector has a plurality of micropores, the pore size of which is 50μm-150μm, and the total area of ​​the micropores on the surface of the first positive electrode current collector is A, where 0.005≤A / B≤0.5; Preferably, 0.1% ≤ A ≤ 1%.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein, The diaphragm includes a substrate layer, a first coating layer located on one side of the substrate layer, and a second coating layer located on the other side of the substrate layer; The first coating corresponds to the positive electrode sheet, and the second coating corresponds to the negative electrode sheet; the first coating includes a heat-resistant layer and a first adhesive layer, the heat-resistant layer is located on the surface of the substrate layer, and the first adhesive layer is located on the surface of the heat-resistant layer away from the substrate layer; the first adhesive layer includes a first polymer, and the second coating includes a second polymer; the first polymer includes at least one of first particles and second particles, the first particles are agglomerated polymer particles, and the second particles are dispersed polymer particles; the average particle size of the primary particles of the second particles is 0.3 μm-1.5 μm; the first particles include secondary particles formed by the agglomeration of primary particles, the average particle size of the primary particles of the first particles is 0.15 μm-0.25 μm, and the average particle size of the secondary particles of the first particles is 4 μm-20 μm; Preferably, the first polymer and the second polymer each independently include at least one of fluoropolymers, acrylate polymers, polyimides, modified polyimides, poly(p-phenylene terephthalamide) and poly(m-phenylene isophthalamide); Preferably, the first adhesive layer has a coverage of 8%-50% on the surface of the heat-resistant layer, more preferably 10%-35%; the second coating layer has a coverage of 8%-50% on the surface of the substrate layer, more preferably 10%-35%.

8. The lithium-ion secondary battery according to claim 7, wherein, 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 area of ​​the second surface is S1, and along the thickness direction of the first positive electrode, the sum of the projected areas of the protrusions on the second surface is S2, 0.2≤S2 / S1≤0.7; Preferably, 0.3 ≤ S2 / S1 ≤ 0.

6.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector. The negative electrode active layer includes a silicon-carbon material. The silicon-carbon material includes 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. The mass content of silicon element in the negative electrode active layer is 2%-50%, and the thickness of the coating layer is 2nm-20nm. Preferably, the sphericity of the silicon-carbon material is ≥0.85; Preferably, the particle size Dv50 of the silicon-carbon material is 5.5μm-12.5μm, more preferably 6.5μm-10μm.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein, The first cathode material and the second cathode material each independently include at least one of lithium cobalt oxide, ternary cathode material and lithium manganese oxide; Preferably, the first cathode material and / or the second cathode material comprises lithium cobalt oxide, wherein the lithium cobalt oxide contains Al, and the mass content of Al in the lithium cobalt oxide is 5000ppm-15000ppm.

Images